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A G E ‐ R E L A T E D MA C U L A R D E G E N E R A T I O N :
H I S T O P A T H O L O G I C A L A N D S E R UM
A U T O A N T I B O D Y S T U D I E S
Svetlana Cherepanoff
Submitted to the Faculty of Medicine
in fulfillment of the requirements of for the degree
Doctor of Philosophy (Medicine)
Department of Clinical Ophthalmology & Eye Health
University of Sydney
AUGUST 2007
For Rinjani and Ambika
i
A C K N O W L E D G E M E N T S
First and foremost, I would like to thank Soren Soengkoeng, my husband, for his
endless patience, unwavering support, and willingness to do far more than his fair
share of child rearing and housekeeping during the course of my candidature.
Without him this thesis would not have been possible.
To my parents, Mei Lan, Irina and Leonid, who have always encouraged me to
pursue my interests, and who arrive each week to look after the girls and perform
endless chores without question – I thank each of you for your kindness,
generosity and tireless support.
I thank Mark Gillies, my supervisor, for his enthusiastic support of all ideas and
potential projects, for introducing me to ophthalmology research and for
graciously putting up with a “firebrand” student.
Thanks must go especially to Shirley Sarks, who has been an extraordinary teacher
and mentor, and who has utterly changed the way I understand this disease. I
thank both Shirley and John Sarks for their generosity in sharing their tissue
resources, their unique insights and their unrivalled expertise.
Finally, a big thank you to all the people who enable the smooth running of both
Save Sight Institute and the Prince of Wales Medical Research Institute – without
you the experimental and written work could not have been produced.
The National Health and Medical Research Council (Canberra, ACT Australia)
provided financial support in the form of a PhD scholarship.
ii
D E C L A R A T I O N
I hereby declare that this thesis is my own work, except where due
acknowledgement is specified below. To the best of my knowledge, the thesis
contains no material previously published or written by another person, nor
material which to a substantial extent has been accepted for the award of any
other degree or diploma of a university or other institute of higher learning. All
experiments described in this thesis were performed at the Save Sight Institute
laboratories (University of Sydney, Sydney Eye Hospital Campus, Sydney NSW
Australia) and at the Prince of Wales Medical Research Institute laboratories
(Randwick, NSW Australia), between 2001 and 2006.
The histopathological observations in Chapter 3 were made by Dr Shirley Sarks
(Prince of Wales Medical Research Institute, Randwick NSW Australia), and the
electron micrographs in the same chapter were produced by A/Prof Murray
Killingsworth (South Western Area Pathology Services, Liverpool NSW
Australia). Dr Shirley Sarks produced the line drawings in Figure 2.2‐1 and Figure
3.1‐5. Ms Emma Kettle (Children’s Medical Research Institute, Westmead, NSW
Australia) provided assistance with the immunohistochemical studies. Dr Karen
Byth Wilson (NHMRC Clinical Trials Centre, University of Sydney, NSW
Australia) performed the linear mixed effects modelling in Chapter 4.
Svetlana Cherepanoff
August, 2007
iii
A B S T R A C T
BACKGROUND : The accumulation of abnormal extracellular deposits beneath
the retinal pigment epithelium characterises the pathology of early age‐related
macular degeneration. However, the histopathological threshold at which age‐
related changes become early AMD is not defined, and the effect of each of the
deposits (basal laminar deposit and membranous debris) on disease progression is
poorly understood. Evidence suggests that macrophages play a key role in the
development of AMD lesions, but the influence of basal laminar deposit (BLamD)
and membranous debris on the recruitment and programming of local
macrophages has not been explored. Although evidence also suggests that
inflammation and innate immunity are involved in AMD, the significance of anti‐
retinal autoantibodies to disesase pathogenesis is not known.
A IMS : (i) To determine the histopathological threshold that distinguishes normal
ageing from early AMD; (ii) to determine the influence of BLamD and
membranous debris on disease progression; (iii) to examine whether distinct early
AMD phenotypes exist based on clinicopathological evidence; (iv) to determine
the histopathological context in which Bruch’s membrane macrophages first
found; (v) to examine the relationship between Bruch’s membrane macrophages
and subclinical neovascularisation; (vi) to determine if the progressive
accumulation of BLamD and membranous debris alters the immunophenotype of
Bruch’s membrane macrophages and/or resident choroidal macrophages; (vii) to
determine if the anti‐retinal autoantibody profile differs significantly between
normal individuals and those with early AMD, neovascular AMD or geographic
atrophy; (viii) to examine whether baseline anti‐retinal autoantibodies can predict
progression to advanced AMD in individuals with early AMD; and (ix) to examine
iv
whether baseline anti‐retinal autoantibodies can predict vision loss in individuals
with neovascular AMD.
METHODS :Clinicopathological studies were performed to correlate progressive
accumulation of BLamD and membranous debris to fundus characteristics and
visual acuity, as well as to sub‐macular Bruch’s membrane macrophage count.
Immunohistochemical studies were perfomed to determine whether the presence
of BLamD and membranous debris altered the programming of Bruch’s
membrane or resident choroidal macrophages. The presence of serum anti‐retinal
autoantibodies was determined by western blotting, and the association with
disease progression examined in early and neovascular AMD.
RE SU L T S : The presence of both basal linear deposit (BLinD) and a continuous
layer of BLamD represents threshold early AMD histopathologically, which was
seen clinically as a normal fundus in the majority of cases. Membranous debris
accumulation appeared to influence the pathway of progression from early AMD
to advanced AMD. Bruch’s membrane macrophages were first noted when a
continuous layer of BLamD and clinical evidence of early AMD were present, and
increased with the amount of membranous debris in eyes with thin BLamD. Eyes
with subclinical CNV had high macrophage counts and there was some evidence
of altered resident choroidal macrophage programming in the presence of BLamD
and membranous debris. Serum anti‐retinal autoantibodies were found in a higher
proportion of early AMD participants compared with both controls and
participants with neovascular AMD, and in a higher proportion of individuals
with atrophic AMD compared to those with neovascular AMD. The presence of
baseline anti‐retinal autoantibodies in participants with early AMD was not
associated with progression to advanced AMD. Participants with neovascular
AMD lost more vision over 24 months if they had IgG autoantibodies at baseline
compared to autoantibody negative participants.
v
CONC LU S I ON S : The finding that eyes with threshold early AMD appear
clinically normal underscores the need to utilise more sophisticated tests to enable
earlier disease detection. Clinicopathological evidence suggests two distinct early
AMD phenotypes, which follow two pathways of AMD progression. Macrophage
recruitment and programming may be altered by the presence of BLamD and
membranous debris, highlighting the need to further characterise the biology of
human resident choroidal macropahges. Anti‐retinal autoantibodies can be found
in both control and AMD sera, and future approaches that allow the examination
of subtle changes in complex repertoires will determine whether they are involved
in AMD disease pathogenesis.
vi
P U B L I C A T I O N S
Original Articles
Cherepanoff S, Mitchell P, Wang JJ, and Gillies MC. Retinal autoantibody profile
in early age‐related macular degeneration: Preliminary findings from the Blue
Mountains Eye Study. Clin Exp Ophthalmol, 2006. 34(6): p. 590‐5.
Cherepanoff, S and Gillies, MC, Circulating antiretinal autoantibodies and age‐
related macular degeneration: What is the link? Exp Rev Ophthalmol, 2007. 2(1): p.
27‐31.
Sarks S, Cherepanoff S, Killingsworth M, and Sarks J. Relationship of basal
laminar deposit and membranous debris to the clinical presentation of early age‐
related macular degeneration. Invest Ophthalmol Vis Sci, 2007. 48(3): p. 968‐77.
Published Abstracts
Cherepanoff S, Sarks J, Killingsworth M, and Sarks S. The Life Cycle of Soft
Drusen. Invest Ophthalmol Vis Sci. 2007 48: E‐Abstract 3026.
Cherepanoff S, Gillies M, Luo W, Chua W, Penfold PL. Anti‐Retinal Auto‐
Antibodies in Neovascular Age Related Macular Degeneration ‐ A Useful
Prognostic Marker? Invest Ophthalmol Vis Sci. 2001; 42(4): S801. Abstract 4294
Gillies MC, Luo W, Chua W, Cherepanoff S, Billson FA, Mitchell P, Simpson J.
The Efficacy of a Single Injection of Triamcinolone for Neovascular Age Related
Macular Degeneration. One Year Results of a Randomised Clinical Trial: IVTAS.
Invest Ophthalmol Vis Sci. 2001; 42(4): S522. Abstract 2808
Jordan B, Cvejic S, Trapaidze N, Cherepanoff S, Unterwald E, Devi LA. Agonist
mediated internalization of the μ, κ and δ opioid receptor types. Society of
Neuroscience Abstracts 1997; 2: 691.6.
vii
T A B L E O F C O N T E N T S
ACKNOWLEDGEMENTS............................................................................................................................ i
DECLARATION............................................................................................................................................ii
ABSTRACT................................................................................................................................................... iii
PUBLICATIONS...........................................................................................................................................vi
TABLE OF CONTENTS .............................................................................................................................vii
LIST OF FIGURES ......................................................................................................................................xii
LIST OF TABLES....................................................................................................................................... xiii
LIST OF TABLES....................................................................................................................................... xiii
ABBREVIATIONS .....................................................................................................................................xiv
ABBREVIATIONS .....................................................................................................................................xiv
1 INTRODUCTION..................................................................................................................................... 1
2 BACKGROUND........................................................................................................................................ 2
2.1 AGE‐RELATED MACULAR DEGENERATION: CLINICAL DEFINITIONS AND EPIDEMIOLOGY........... 3
2.1.1 Clinical definition...................................................................................................................... 3
2.1.2 Disease classification ................................................................................................................. 5
2.1.3 Prevalence and incidence........................................................................................................... 6
2.1.4 Natural history.......................................................................................................................... 8
2.1.5 Risk factors .............................................................................................................................. 10 2.1.5.1 Smoking....................................................................................................................................... 10 2.1.5.2 Cardiovascular disease and cardiovascular risk factors ....................................................... 11 2.1.5.3 Inflammatory markers............................................................................................................... 12 2.1.5.4 Dietary factors............................................................................................................................. 13 2.1.5.5 Genetic factors ............................................................................................................................ 13
2.1.6 Estimated costs and disease burden......................................................................................... 16
2.1.7 Therapeutics ............................................................................................................................ 17 2.1.7.1 Preventative interventions ........................................................................................................ 17 2.1.7.2 Palliative interventions.............................................................................................................. 18
viii
2.2 THE “ANATOMY” OF AMD......................................................................................................... 19
2.2.1 The macula in health and ageing ............................................................................................. 19 2.2.1.1 Anatomy & embryology of the human eye ............................................................................ 19 2.2.1.2 The retinal pigment epithelium................................................................................................ 21 2.2.1.3 Age‐related changes in the RPE ............................................................................................... 22 2.2.1.4 The choroid and Bruch’s membrane........................................................................................ 23 2.2.1.5 Age‐related changes in the choroid ......................................................................................... 25 2.2.1.6 Age‐related changes in Bruch’s membrane ............................................................................ 25 2.2.1.7 Immunological specialisations of the retina and choroid ..................................................... 28
2.2.2 The pathology of endstage AMD............................................................................................. 29 2.2.2.1 Geographic atrophy of the RPE: the natural endstage of AMD........................................... 29 2.2.2.2 Choroidal neovascularisation and disciform scarring........................................................... 30
2.2.3 The basal deposits and early AMD.......................................................................................... 31 2.2.3.1 Basal laminar deposit is a marker of RPE degeneration ....................................................... 31 2.2.3.2 Basal linear deposit and soft drusen are the specific lesions of early AMD...................... 32 2.2.3.3 Drusen classification .................................................................................................................. 35 2.2.3.4 Drusen biogensis ........................................................................................................................ 36 2.2.3.5 Histopathological classification schemes ................................................................................ 38
2.3 AMD DISEASE PATHOGENESIS: SUMMARY OF CURRENT CONCEPTS .......................................... 41
2.3.1 Oxidative stress ....................................................................................................................... 41
2.3.2 Ischaemia ................................................................................................................................. 43
2.3.3 Inflammation ........................................................................................................................... 43
2.4 MACROPHAGES AND IMMUNE COMPETENT CELLS IN AMD...................................................... 44
2.4.1 Morphological evidence ........................................................................................................... 44 2.4.1.1 Macrophages, Bruch’s membrane breaks and early AMD................................................... 44 2.4.1.2 Macrophages in advanced AMD lesions................................................................................. 44
2.4.2 Macrophages and choroidal neovascularisation ...................................................................... 45
2.4.3 Choroidal macrophage recruitment and turnover and AMD.................................................. 47 2.4.3.1 Ccl‐2, Ccr‐2 and Cx3Cr1 knockout mice and AMD‐like lesions .......................................... 47 2.4.3.2 IL‐10 knockout mice and laser‐induced CNV ........................................................................ 49 2.4.3.3 Polymorphisms in leukocyte recruitment and turnover genes............................................ 49 2.4.3.4 Serum myeoloid cells and AMD .............................................................................................. 50 2.4.3.5 Macrophages in an immune‐privileged compartment.......................................................... 50
2.4.4 Dendritic cells and drusen biogenesis ..................................................................................... 52
2.5 INFLAMMATION AND AMD........................................................................................................ 54
2.5.1 Tissue evidence of inflammatory proteins ............................................................................... 54 2.5.1.1 Evidence of inflammation‐associated proteins in drusen..................................................... 54
ix
2.5.1.1.1 Immunoglobulins and complement in drusen and Bruch’s membrane ........................ 55 2.5.1.1.2 Amyloid, fibrinogen and other inflammatory proteins found in drusen...................... 57
2.5.2 Genetic associations................................................................................................................. 59 2.5.2.1 Single nucleotide polymorphisms in complement cascade and complement regulatory
genes 59 2.5.3 Serum inflammatory markers in AMD................................................................................... 61
2.6 SERUM ANTI‐RETINAL AUTOANTIBODIES IN AMD..................................................................... 63
2.6.1 Autoantibodies and autoimmunity.......................................................................................... 63
2.6.2 Autoantibodies and ocular disease........................................................................................... 64 2.6.2.1 Anti‐recoverin autoantibodies, CAR and autoimmune retinopathy................................... 64 2.6.2.2 Retinal degenerations, autoimmune retinopathy and glaucoma......................................... 66
2.6.3 Autoantibodies in AMD.......................................................................................................... 66
2.7 THESIS AIMS ................................................................................................................................. 74
3 HISTOPATHOLOGICAL STUDIES ................................................................................................... 76
3.1 RELATIONSHIP OF BASAL LAMINAR DEPOSIT AND MEMBRANOUS DEBRIS TO THE CLINICAL
PRESENTATION OF EARLY AGE‐RELATED MACULAR DEGENERATION....................................................... 77
3.1.1 Introduction............................................................................................................................. 77
3.1.2 Methods ................................................................................................................................... 78 3.1.2.1 Patients and Eyes........................................................................................................................ 78 3.1.2.2 Histopathological Methods and Definitions........................................................................... 78 3.1.2.3 Clinical Parameters .................................................................................................................... 81 3.1.2.4 Statistical Methods ..................................................................................................................... 81
3.1.3 Results ..................................................................................................................................... 82 3.1.3.1 Basal laminar deposit and RPE changes.................................................................................. 82 3.1.3.2 BLamD – clinical correlations ................................................................................................... 91 3.1.3.3 Membranous debris‐ localization and correlation with BLamD.......................................... 91 3.1.3.4 Membranous debris – clinical correlations ............................................................................. 92
3.1.4 Discussion ............................................................................................................................... 98
3.2 CHOROIDAL AND BRUCH’S MEMBRANE MACROPHAGES IN EARLY AND ADVANCED AMD... 105
3.2.1 Introduction........................................................................................................................... 105
3.2.2 Methods ................................................................................................................................. 108 3.2.2.1 Eyes ............................................................................................................................................ 108 3.2.2.2 Clinical definitions ................................................................................................................... 108 3.2.2.3 Histopathological definitions and grading: BLamD............................................................ 109 3.2.2.4 Histopathological definitions and grading: membranous debris ...................................... 109 3.2.2.5 BrM macrophage counts ......................................................................................................... 109 3.2.2.6 Immunohistochemistry ........................................................................................................... 110
x
3.2.2.7 Statistics ..................................................................................................................................... 111 3.2.3 Results ................................................................................................................................... 112
3.2.3.1 Groups I & II‐ normal and normal aged eyes....................................................................... 112 3.2.3.2 Groups III & IV– early AMD .................................................................................................. 112 3.2.3.3 Groups III & IV: BrM macrophages and subclinical CNV.................................................. 120 3.2.3.4 Group V – Geographic atrophy.............................................................................................. 125 3.2.3.5 Group VI – Disciform scarring ............................................................................................... 125
3.2.4 Discussion ............................................................................................................................. 128
4 SERUM AUTOANTIBODY STUDIES.............................................................................................. 131
4.1 COMPARISON OF THE ANTI‐RETINAL AUTOANTIBODY PROFILE IN EARLY, NEOVASCULAR &
ATROPHIC AMD ...................................................................................................................................... 132
4.1.1 Introduction........................................................................................................................... 132
4.1.2 Methods ................................................................................................................................. 133 4.1.2.1 Study serum.............................................................................................................................. 133
4.1.2.1.1 Blue Mountains Eye Study................................................................................................. 133 4.1.2.1.2 Intravitreal Triamcinolone Study...................................................................................... 134 4.1.2.1.3 Serum used in autoantibody assays ................................................................................. 135
4.1.2.2 Autoantibody assay ................................................................................................................. 135 4.1.2.2.1 Preparation of human retinal proteins for electrophoresis ........................................... 135 4.1.2.2.2 Electrophoresis and western blotting............................................................................... 136 4.1.2.2.3 Detection of serum anti‐retinal autoantibodies............................................................... 136 4.1.2.2.4 Antibodies used to detect serum autoantibodies............................................................ 137
4.1.2.3 Statistics ..................................................................................................................................... 138 4.1.3 Results ................................................................................................................................... 139
4.1.4 Discussion ............................................................................................................................. 149
4.2 ANTI‐RETINAL AUTOANTIBODIES AND DISEASE PROGRESSION IN EARLY AMD ..................... 152
4.2.1 Introduction........................................................................................................................... 152
4.2.2 Methods ................................................................................................................................. 152 4.2.2.1 Participants ............................................................................................................................... 152 4.2.2.2 Autoantibody assay ................................................................................................................. 152 4.2.2.3 Statistics ..................................................................................................................................... 153
4.2.3 Results ................................................................................................................................... 154
4.2.4 Discussion ............................................................................................................................. 156
xi
4.3 ANTI‐RETINAL AUTOANTIBODIES AND VISION LOSS IN NEOVASCULAR AMD........................ 157
4.3.1 Introduction........................................................................................................................... 157
4.3.2 Methods ................................................................................................................................. 158 4.3.2.1 Participants ............................................................................................................................... 158 4.3.2.2 Autoantibody assay ................................................................................................................. 158 4.3.2.3 Statistics ..................................................................................................................................... 158
4.3.3 Results ................................................................................................................................... 160
4.3.4 Discussion ............................................................................................................................. 164
5 SUMMARY & CONCLUSIONS......................................................................................................... 165
REFERENCES............................................................................................................................................. 168
APPENDICES............................................................................................................................................A‐H
xii
L I S T O F F I G U R E S
Figure 2.2‐1 Schematic diagram of BLamD and membranous debris ...................33
Figure 2.5‐1 The complement cascade........................................................................56
Figure 3.1‐1 Basal Laminar Deposit ............................................................................84
Figure 3.1‐2 Continuous Late Basal Laminar Deposit..............................................86
Figure 3.1‐3 High magnification membranous debris .............................................88
Figure 3.1‐4 Basal mounds ..........................................................................................89
Figure 3.1‐5 Membranous debris: basal mounds and soft drusen .........................94
Figure 3.1‐6 Influence of basal deposits on the progression of AMD.................102
Figure 3.2‐1 Morphological and immunohistochemical features of normal (group
I) and normal aged (group II) eyes ................................................................116
Figure 3.2‐2 Morphological and immunohistochemical features of eyes with early
AMD (groups III and IV) .................................................................................118
Figure 3.2‐3 Morphological features of eyes with subclinical CNV.....................121
Figure 3.2‐4 CD68 and iNOS Immunohistochemistry in eyes with geographic
atrophy (group V) or disciform scarring (group VI) ...................................126
Figure 4.1‐1 Western blot‐detected anti‐retinal autoantibodies in normal controls
and participants with geographic atrophy ...................................................141
Figure 4.1‐2 Western blot‐detected anti‐retinal autoantibodies in participants with
early AMD .........................................................................................................143
Figure 4.1‐3 Western blot‐detected anti‐retinal autoantibodies in participants with
neovascular AMD.............................................................................................145
Figure 4.3‐1 Vision loss over 24 months in anti‐retinal autoantibody positive
participants vs. anti‐retinal autoantibody negative participants (anti‐
IgGAM detected autoantibodies) ...................................................................162
Figure 4.3‐2 Vision loss over 24 months in anti‐retinal autoantibody positive
participants vs. anti‐retinal autoantibody negative participants (anti‐IgG
detected autoantibodies) .................................................................................163
xiii
L I S T O F T A B L E S
Table 2.2‐1 Histopathological grading scheme suggested by Sarks (1976)...........40
Table 2.2‐2 Histopathological grading scheme suggested by Curcio et al (1998) 40
Table 2.6‐1 Autoantibodies in AMD: summary of studies .......................................70
Table 3.1‐1 Clinical and histopathological characteristics of study eyes................83
Table 3.1‐2 Basal laminar deposit – clinical and histopathological correlations .96
Table 3.1‐3 Membranous debris – clinical and histopathological correlations.....97
Table 3.1‐4 Basal laminar deposit ‐ a summary......................................................103
Table 3.1‐5 Membranous debris – a summary ........................................................104
Table 3.2‐1 Clinical and histopathological features of study eyes........................114
Table 3.2‐2 BLamD, membranous debris and macrophages in early AMD .......115
Table 3.2‐3 BrM macrophages, membranous debris and subclinical CNV.........123
Table 3.2‐4 BrM macrophages and subclinical CNV in the fellow eye................124
Table 4.1‐1 Serum anti‐retinal autoantibodies in controls and in early, neovascular
and atrophic AMD: summary of findings ....................................................140
Table 4.1‐2 Comparison of Anti‐IgGAM and anti‐IgG detected autoantibodies in
control, early AMD, neovascular AMD and atrophic AMD participants 148
Table 4.2‐1 Baseline serum anti‐retinal autoantibodies and progression to
advanced AMD at 5 and 10 years...................................................................155
Table 4.3‐1 Participant characteristics and baseline anti‐retinal autoantibody
status...................................................................................................................161
xiv
A B B R E V I A T I O N S
AMD age‐related macular degeneration
APC antigen presenting cell
ApoE apolipoprotein E
BF factor B
BLamD basal laminar deposit
BLinD basal linear deposit
CAR cancer‐associated retinopathy
CC choriocapillaris
CFH complement factor H
CNV choroidal neovascularisation
CRP C‐reactive protein
FasL fas ligand
GA geographic atrophy
GFAP glial acidic fibrillary protein
HLA human leukocyte antigen
IL‐1 interleukin‐1
IL‐10 interleukin‐10
iNOS inducible nitric oxide synthase
MAC membrane attack complex
MCP‐1 macrophage chemoattractant protein‐1
MHC major histocompatibility complex
MMP membrane metalloproteinases
PDT photodynamic therapy
PEDF pigment epithelium‐derived factor
ROI reactive oxygen intermediate
RPE retinal pigment epithelium
TGFβ transforming growth factor beta
TIMP tissue inhibitors of metalloproteinases
TLR‐4 toll like receptor‐4
VEGF vascular endothelial growth factor
1
I N T R O D U C T I O N
The macula is a part of the retina found at the posterior pole of the eye. It has the
highest concentration of cone photoreceptors, and is responsible for high acuity
colour vision. In age‐related macular degeneration (AMD), loss of the retinal
pigment epithelium and the photoreceptors they support lead to loss of central
vision, with devastating consequences for the affected individual. AMD is the
leading cause of irreversible vision loss in industrialised populations, with the
numbers of cases expected to grow due to population ageing.
1
2
B A C K G R O U N D
2
3
2 . 1 A G E - R E L A T E D M A C U L A R D E G E N E R A T I O N : C L I N I C A L
D E F I N I T I O N S A N D E P I D E M I O L O G Y
2.1.1 Clinical definition
Drusen, atrophy of the retinal pigment epithelium (RPE) and disciform lesions
were recognised clinical entities by the late 19th century. Donders described the
clinical appearance of drusen in 1855 (reviewed in 1), while “symmetrical central
chorio‐retinal disease occurring in senile persons” was described by Hutchinson
and Tay in 1874 (reviewed in 2). In 1885, Haab described pigmentary and atrophic
changes in the macular of patients over the age of 50, which was associated with
central vision loss (reviewed in 3). However, Gass was first to suggest that drusen,
senile macular degeneration and senile disciform degeneration were
manifestations of a single disease entity 4. This entity has been variously named
“senile macular degeneration”, “disciform degeneration” and “maculopathy”, but
is currently known as “age‐related macular degeneration” (AMD).
Clinically, AMD can be classified as early or advanced. The advanced disease is
further categorised as neovascular (“wet” or “exudative”) or atrophic
(“geographic atrophy”). The term “dry” AMD has been used by some
investigators to describe both early AMD and geographic atrophy, i.e. the non‐
exudative forms of AMD.
Geographic atrophy (GA) of the RPE is considered the natural endstage of the
disease when choriodal neovascularisation (CNV) does not intervene 5. GA
appears as discreet areas of pallor, due to loss of RPE and reduced retinal
thickness. It commonly begins in the parafoveal region, and the foveal centre is
often spared until late in the disease process 6‐8. Visual acuity may thus be
preserved, although scotomas surrounding the point of fixation can be significant
9. GA may follow pigmentary change and attenuation or may occur subsequent to
drusen regression. GA may also follow involution of CNV, and CNV can develop
4
in an eye with GA 10. Sunness et al. found that the 4‐year cumulative incidence of
CNV was 11% for eyes with GA, and 34% for eyes with GA if the fellow eye had
CNV 11.
Neovascular AMD results from the growth of new vessels from the choroid into
the sub‐RPE or sub‐retinal space. These vessels are abnormally permeable, leading
to the leakage of fluid or blood into the sub‐RPE (“occult” CNV) or sub‐retinal
(“classic” CNV) space (reviewed in 12. Clinically, neovascular AMD can present as
sub‐retinal fluid or haemorrhage, lipid exudates, pigment epithelial detachment
or, in the later stages, as a fibrovascular “disciform” scar. Neovascular AMD is
responsible for an estimated 75‐90% of the severe vision loss attributable to AMD
13.
Early AMD is defined as the presence of soft (or large) macular drusen with or
without pigmentary changes. Soft drusen appear as ill‐defined yellow deposits
while pigmentary abnormalities can include hypo‐ or hyperpigmentation. There is
less agreement on whether pigmentary changes without drusen should be
considered early AMD (see Section 2.1.4). Patients with early AMD often have
good vision. There may be subtle deficits, such as reduced dark adaptation, which
are revealed by psychophysical or electrophysiological testing.
There is, thus, a wide range of clinical presentations of AMD, even within families
14. Various definitions (of early AMD in particular) can lead to different prevalence
estimates in population‐based studies. This “phenotypic heterogeneity” has lead
to the development of a number of systems of disease classification.
5
2.1.2 Disease classification
The Wisconsin Age‐Related Maculopathy Grading Scheme was developed to
quantify retinal changes in the Beaver Dam Study, one of the largest and earliest
population‐based studies of age‐related eye disease 15. This system uses grids with
a radius of 3000μm, centred on the fovea, to define subfields in the macula of
fundus photographs. Templates are then used to estimate drusen size and the area
involved by drusen or RPE abnormalities. Drusen are graded on each of three
criteria: size, type and area of macula involved. RPE changes are graded according
to type of abnormality and area of macula involved. Early AMD was defined as: (i)
the presence of hard or soft drusen with pigmentary changes; or (ii) the presence
of soft, indistinct drusen only, in the absence of GA or exudative lesions. Despite
achieving high agreement between trained graders, the Wisconsin system was not
universally adopted.
In the hope of creating common disease definitions that would allow comparisons
between studies, the International Age‐Related Maculopathy Study Group
brought together a number of leading investigators to develop the International
Classification System 16. This system incorporated the Wisconsin Grading Scheme
with other commonly used grading schemes. Early disease was termed “age‐
related maculopathy” (ARM), while advanced disease was termed “age‐related
macular degeneration” (AMD). ARM was defined as: (i) the presence of drusen,
“discrete whitish‐yellow spots” external to the neuroretina or RPE; or (ii) areas of
hyperpigmentation associated with drusen; or (iii) areas of hypopigmentation
associated with drusen. “Dry” AMD, or geographic atrophy, was defined as
sharply demarcated areas of hypopigmentation, at least 175μm in diameter, in
which the choroidal vessels are more visible than surrounding areas. “Wet” AMD,
or “neovascular”, “disciform” or “exudative” AMD, was defined as: (i) RPE
detachments, which may be associated with neurosensory retinal detachment; or
(ii) subretinal or sub‐RPE neovascular membranes; or (iii) epiretinal, intraretinal,
6
subretinal or sub‐RPE scar/glial tissue/fibrin‐like deposits; or (iv) subretinal
haemorrhages not related to other retinal vascular disease; or (v) hard (lipid)
exudates related to other ARM findings, and unrelated to other vascular diseases.
Of the longer‐running, large population‐based studies so far conducted, the
Wisconsin Grading Scheme is used by the Beaver Dam Study (US) 17, the Blue
Mountains Eye Study (Australia) 18 and the Melbourne Visual Impairment Project
(Australia) 19. The Vitamin E, Cataract and Age‐related Maculopathy (VACT)
Study (Australia) 20 and the Rotterdam Study (Netherlands) 21 utilise the
International Classification System. The Age‐Related Eye Diseases Study (US) uses
its own classification system 22, in which fundus features are classified into four
categories based on drusen type and number and, unlike the other studies,
incorporates a visual acuity score.
A unified classification system has yet to be formulated. This is partly because
new insights continue to be generated by ongoing epidemiological and
clinicopathological studies. The findings from recent genetic studies are also likely
to influence classification of disease subgroups.
2.1.3 Prevalence and incidence
AMD is the commonest cause of blindness in industrialised populations 23. In 2005,
Taylor and colleagues 24 reviewed the findings of the Melbourne Visual
Impairment Project (VIP) and the Blue Mountains Eye Study (BMES), involving a
total of 8909 participants, and estimated that 24 200 Australians are blind due to
AMD, making it by far the commonest cause of blindness in people aged 40 and
over. AMD was also found to be the most common cause of untreatable low
vision, affecting an estimated 48 300 individuals. When findings from the Beaver
Dam Study, the BMES and the Rotterdam study were pooled, AMD (early and
7
advanced) was present in 0.2% of 55‐64 year olds, rising to 13% of those aged 85 or
older 25. Geographic atrophy increased from 0.04% to 4.2% for these age groups,
while neovascular AMD increased from 0.17% to 5.8%. In general, population‐
based studies have shown that AMD is rare before age 55 and becomes more
common after age 75 26. The Age‐Related Eye Diseases Study estimated that AMD
affected over 8 million people in the US, with the advanced form affecting 1.75
million 27. This number is projected to increase 50% by 2020.
Racial differences in both AMD prevalence and incidence exist. The frequency of
neovascular AMD was higher in the Rotterdam population than the Beaver Dam
or the BMES populations 25. However, in all three studies, the incidence and
prevalence of neovascular CNV was higher in whites when compared to blacks 26.
Although the frequency of early AMD in blacks appears to be similar to whites,
the risk of progression to neovascular AMD is higher in whites.
The early form is more commonly found than the late form, with both types
increasing in frequency with age (17. The 5‐year overall incidence of advanced
AMD (geographic atrophy and neovascular AMD) was 0.4% in the Beaver Dam
Study, 0.7% in the BMES, and 1.1% in the Rotterdam study 28. In the longest
running population‐based study (Beaver Dam), the 15‐year cumulative incidence
for early AMD was 14.3% for early AMD and 3.1% for advanced AMD 29.
AMD frequently affects both eyes. In the BMES cohort, 80% of participants with
AMD (early and advanced) had bilateral disease 30. Once neovascular AMD is
present in one eye, involvement of the second eye occurs in 82% of cases within
four years in an Icelandic population 31.
8
2.1.4 Natural history
Gass noted that drusen size, area and detectability altered over time, and that
increased drusen area increased the risk of progression to advanced AMD 6. These
observations were verified by later, large population‐based studies. Using the
Wisconsin Grading System to grade retinal features, the Beaver Dam Study found
that 14% of eyes with large (>125μm) drusen developed advanced AMD over 10
years 32. The Blue Mountains Eye Study, also using the Wisconsin Grading System,
found that eyes with large drusen were six times as likely as eyes without large
drusen to develop advanced AMD 33. The BMES also found that eyes with large,
soft drusen were more likely to progress to neovascular AMD than atrophic AMD
33. The Rotterdam Study, using the International Classification System 16, found
that ten or more large drusen at baseline best predicted development of advanced
AMD six and a half years later 34. Using these findings and their own, the Age‐
Related Eye Diseases Study (AREDS) investigators developed a severity scale for
assessing AMD risk 35, which could produce a simplified score based on the
presence of large drusen or pigmentary changes in each eye 36.
The presence of large drusen is thus an established risk factor for progression to
advanced AMD. However, it is not known whether there is an orderly progression
from small, to intermediate and later, large drusen in the natural history of AMD.
Klein et al. found that approximately 25% of large drusen found at baseline faded,
or regressed over the same period, without progression to more advanced lesions
in the Beaver Dam Study 32. The smaller, Chesapeake Bay Waterman study found
that large drusen disappeared in 34% of eyes over a 5‐year period 37.
While AREDS recognised intermediate drusen as a feature of early AMD 22,
intermediate drusen were not significantly associated with progression from early
to advanced AMD in the Cardiovascular Health and Age‐related Maculopathy
(CHARM) Study 38. It should also be noted that while drusen size appears to be
9
the most consistently found predictor of progression, drusen appearance may also
influence risk of progression. Tikellis et al. 38 found that eyes with soft indistinct
drusen were more likely to progress to advanced AMD compared to eyes with
soft, distinct drusen in the CHARM study.
Small drusen (<65μm) are present in 90% of the population, are not considered
AMD and do not increase the risk of developing AMD 32. However, the
Chesapeake Bay study found that large numbers of small, hard drusen increased
the risk of developing larger drusen over a 5‐year period 37. Over a 15‐year period,
the Beaver Dam Study found that eyes with more than 8 small, hard drusen were
more likely to develop large, indistinct drusen and/or pigmentary abnormalities
compared to eyes with fewer than 8 small hard drusen (16.3% vs. 4.7%) 29. Thus it
appears that large drusen do not irreversibly progress to advanced AMD, and that
while a few small, hard drusen are not considered AMD, the presence of
numerous small hard drusen increases the risk of developing larger drusen or
pigmentary changes. The effect fading or regressing drusen has on the long‐term
risk of AMD progression is unknown. It is also unclear whether numerous small
hard drusen coalesce to form larger drusen, or whether the larger drusen arise
separately in these eyes.
The presence of pigmentary abnormalitites (hyperpigmentation or
hypopigmentation) alone, without drusen, is not included as a definition of early
AMD by all investigators, notably the International Classification System 16.
However, in the three largest and longest running population‐based surveys eyes
with pigmentary changes alone were included in the early AMD or age‐related
maculopathy diagnostic group. Both the BMES 33 and AREDS 35 found that the risk
of progression to advanced AMD is lower in eyes with hyperpigmentation alone
group compared to eyes with large drusen. The BMES also found that eyes with
hyperpigmentation alone did not show a strong predisposition for progession to
10
one or the other form of advanced AMD 33. However, in Beaver Dam Study, the 5‐
and 10‐ year incidence of advanced AMD is almost the same for large indistinct
drusen and pigment changes 32, 39. The Rotterdam Study 34 also found that the most
important predictors of progression to late AMD were either the presence of
drusen involving more than 10% of macula area or the presence of retinal pigment
changes 34. However, the BMES, Beaver Dam Study, Rotterdam Study and AREDS
have all found that the risk of progression to advanced AMD is highest when
abnormalities of the pigment epithelium are present in addition to large, soft
drusen 29, 33‐35.
Eyes with neovascular AMD are at highest risk of rapid vision loss, with 80‐90% of
eyes losing at least 2 lines of vision within 2 years (reviewed in 12). Once
neovascular AMD is present in one eye, the Macular Photocoagulation Study
Group found that 42% of individuals with neovascular AMD will have second eye
involvement within 5 years 40. Even after disciform scar formation, patients can
continue to lose vision due to the development of expanding areas of GA around
the scars 41.
In eyes with GA, vision loss can also continue due to enlargement of the area of
involvement. Sunness et al found that the median expansion rate for GA was
2.1mm2 per annum 42.
2.1.5 Risk factors
2 . 1 . 5 . 1 Smo k i n g
Smoking is the most consistently found modifiable risk factor and is associated
with the prevalence of both early and advanced AMD (and to a lesser extent,
incidence) 43‐49. Pooled data from the Beaver Dam Study, the BMES and the
Rotterdam Study suggest that smokers were approximately 3‐4 times more likely
11
to have any form of AMD compared to non‐smokers 25. Current smokers were also
2‐3 times more likely to develop AMD over a 5‐6 year period compared to non‐
smokers 50. The association between smoking and early AMD appears to be less
strong. The Beaver Dam Study found that smoking was associated with the
development of soft drusen, but not pigment abnormalities, over a 5‐year period
51.
It has been postulated that smoking creates additional metabolic or biological
stressors that contribute to the development of AMD. These include reductions in
macular carotenoids, choroidal blood flow, serum antioxidant levels, and RPE
anti‐oxidant defence systems 26. It appears however, that smoking cessation does
not reverse the damage. In the Beaver Dam Study, participants who quit between
baseline and the 5‐year follow up were no less likely to develop AMD than those
who continued smoking 32.
2 . 1 . 5 . 2 Ca r d i o v a s c u l a r d i s e a s e a n d c a r d i o v a s c u l a r r i s k f a c t o r s
The Rotterdam Study reported a 4.5‐fold increase in the risk of advanced AMD in
participants with carotid bifurcation plaques (determined by ultrasound) 52. A 2.0‐
2.5 fold increased risk was also found for plaques in common carotid and lower
extremities. However, when other studies have examined the association with
stroke or myocardial infarction and AMD, the results have been less conclusive 53.
The Beaver Dam Study found that higher pulse pressure and systolic blood
pressure were associated with pigmentary abnormalities and neovascular AMD
over a 10‐year period, although no association was found at the 5‐year follow up
54. The Framingham, National Health and Nutrition Examination Survey and
Rotterdam studies also found positive associations between hypertension and
AMD (reviewed in 53). However, other studies have found no association between
12
blood pressure and AMD (reviewed in 26). Significantly, there is no current
evidence suggesting that antihypertensive medications reduce the risk of AMD.
The relationship between serum lipids and AMD risk is appears to be complex.
While some studies have been unable to find an association (reviewed in 26), the
Beaver Dam & Rotterdam studies both found that increased serum HDL‐
cholesterol or low LDL‐cholesterol was associated with prevalent and incident
AMD. Additionally, the Beaver Dam Study also found an increased risk of early
AMD with low total serum cholesterol and lower cholesterol/HDL‐cholesterol
ratios 54. Surprisingly, these observations are the inverse of the relationship
between lipids and cardiovascular risk. The use of lipid‐lowering agents does not
appear to have a significant effect on AMD risk 26.
Genetic studies have found associations between allelic variations in the
apolipoprotein E (ApoE) gene and AMD. Apolipoproteins are involved in the
transport and metabolism of lipids, and ApoE is particularly important in the
central nervous system. Individuals with the epsilon 2 variant appear to have an
increased risk of developing AMD while those with the epsilon 4 variant are
protected 55‐60. While these observations have not been consistently found by all
studies 61, 62, they are again the opposite of the relationship between ApoE genetic
variants and cardiovascular disease.
2 . 1 . 5 . 3 I n f l amma t o r y ma r k e r s
The relationship between systemic markers of inflammation, such as C‐reactive
protein (CRP), fibrinogen, cytokines and white cell count have been examined by a
number of studies 63‐70. The results have been somewhat mixed, and are discussed
in more detail in Section 2.5.3.
13
2 . 1 . 5 . 4 D i e t a r y f a c t o r s
There does not appear to be a strong association between dietary fat and the risk
of AMD (reviewed in 53), although the BMES found that high omega‐3 fatty acid
intake may protect against early and advanced AMD 71. The BMES also found that
the use of lipid lowering drugs (statins) was associated with a decreased 10‐year
risk of soft drusen 72.
Oral supplementation with antioxidants (vitamins A, C and E) and zinc reduced
the risk of AMD progression and vision loss in the AREDS cohort 73. Diets high in
antioxidant‐rich fruits and vegetables also appear to be associated with a lower
risk of exudative AMD (reviewed in 53). The Nurse’s Health Study found that a
higher intake of high glycaemic index carbohydrates was associated with
increased risk of pigmentary changes over a 10‐year period 74.
2 . 1 . 5 . 5 Gen e t i c f a c t o r s
AMD is not a single gene disease. Its late onset (not typically seen until the
seventh decade) and phenotypic heterogeneity suggest that complex interactions
between multiple genes and environmental exposures are likely to be involved.
Twin studies have shown that the concordance rate for monozygotes is about
twice that for heterozygotes, supporting a genetic component for the large drusen
or multiple small drusen phenotypes in particular 75, 76. Familial aggregation
studies suggest that a first degree relative of an individual with AMD is up to five
times more likely to develop AMD than a family member of a control 77, 78,
although there appears to be a large degree of variability in the genetic risk among
families 79. Multiple sequence variations in many genes, either singly or more
likely, in combination, are likely to exert subtle effects on the development of the
disease.
14
Recently, candidate gene screening and genetic association (or linkage) studies
have found several single‐nucleotide polymorphisms that are moderately or
strongly associated with various AMD phenotypes. It is important to note that
polymorphisms are present in the normal population, and only confer
susceptibility. This is in contrast to gene mutations which, when inherited, always
lead to disease.
Technological advances have allowed the entire genomes of individuals to be
rapidly scanned. Linkage studies compare the genomes of normal individuals
with diseased individuals to identify regions which are sufficiently different and
thus potentially implicated in disease pathogenesis. In AMD, the majority of
studies support a linkage to the 1q31 region (reviewed in 80). A number of genes
potentially important to AMD reside in this region. These include the genes for
hemicentin, laminin C1 and C2, regulator of G‐protein signalling 16, oculomedin,
glutaredoxin 2, proline arginine‐rich end leucine‐rich repeat protein and
complement factor H (CFH). The next region of interest linkage studies identified
was 10q26 (reviewed in 80). Within this region resides HTRA1, the gene for a serine
protease involved in the degradation of extracellular matrix proteoglycans, and
loc387715, whose function and gene product are unknown.
Candidate gene screening studies select genes based on: (i) their functional
importance in the pathobiology of a disease; (ii) their presence in susceptibility loci
found in linkage studies; or (iii) their association with hereditary diseases that
share similarities with the disease of interest. The sequences of the selected genes
in normal individuals are compared with those with AMD to identify differences.
Screening of genes involved in rare, hereditary degenerative maculopathies have
yielded mixed results. The ABCA4 gene encodes a transporter protein involved in
the visual cycle. While ABCA4 mutations give rise to Stargardt disease, the gene
15
does not appear to play a role in the majority of AMD cases (reviewed in 81).
Deletions in the ELOVL4 gene, which is involved in fatty acid biosynthesis, result
in Stargardt‐like macular dystrophy. A single nucleotide polymorphism in the
ELOVL4 gene was associated with AMD in one population 80 but not another 82. A
missense mutation in the fibulin‐5 gene is responsible for all cases of Malattia
Leventinese, another rare hereditary maculopathy. The same mutation however,
was present in only 1.7% of patients with AMD 83, and no associations with
fibulin‐5 polymorphisms have been found.
Candidate screening of genes based on their role in other diseases has identified
associations between AMD and the ApoE gene, as previously discussed (Section
2.1.5.2). This approach has also identified: (i) two at‐risk polymorphisms in the
fraktalkine (CX3CR1) gene 84; (ii) one at‐risk polymorphism in the toll‐like receptor
4 (TLR‐4) gene 85; (iii) and one at‐risk polymorphism and two protective
polymorphisms in genes encoding human leukocyte antigens (HLA) 86. These
genes encode proteins involved in inflammatory cell recruitment (CX3CR1) and
phagocytosis (TLR‐4 and HLA). The possible functional implications of these
allelic variants for AMD pathogenesis are discussed in Section 2.4.3.3.
Screening of genes in the AMD susceptibility loci have found the strongest disease
associations to date. Of these, a single nucleotide polymorphism at position 402 in
the CFH gene has been robustly associated with both early and advanced AMD in
a number of studies 82, 87‐94. CFH is a regulatory protein of the complement
pathway, which prevents uncontrolled activation of complement cascades. These
observations have lead to candidate gene screening of other members of the
complement cascade. Both at‐risk and protective single nucleotide polymorphisms
have been identified for complement factor B (BF), and complement component 2
(C2) 95. The pathobiological implications of complement pathway gene variants are
discussed in Section 2.5.
16
Candidate studies of genes in the 10q26 region have found strong associations
between a single nucleotide polymorphism in the promoter region of HTRA1 and
AMD risk 96‐98. The potential functional implications of HTRA1 polymorphisms at
the tissue level are discussed in Sections 2.2.3.1. Candidate gene studies of
loc387715, the other gene located at 10q26, have also identified a single nucleotide
polymorphism that is strongly associated with both early and advanced AMD 82, 99‐
103. Although loc387715 mRNA is weakly expressed in retina, the gene product is
yet to be identified. Because the loc387715 and HTRA1 genes are only 6bp apart, it
is possible that the associations are due to one gene and not the other.
While associations between a gene single nucleotide polymorphism and a disease
do not necessarily infer causality, genetic studies have provided valuable insights,
which will continue to inform research into pathogenesis. There is already some
evidence that both the CFH and loc387715 polymorphisms are associated with
AMD progression 94, 103, and it has been estimated that individuals homozygous for
the at risk variants of CFH, loc 287715 and BF have a 250‐fold higher risk of
developing AMD 104. It is thus likely that genetic testing in some form will be used
in the near future, both to identify individuals most at risk of developing AMD
and those most at risk of progression.
In summary, the strongest risk factors thus identified for AMD are age, smoking
and polymorphisms in the complement factor H gene.
2.1.6 Estimated costs and disease burden
AMD has a significant impact on quality of life. Patients with mild AMD are
estimated to lose an average 17% in quality of life, similar to the reduction
experienced by patients with moderate cardiac angina or those with symptomatic
17
HIV. Those with moderate AMD have a 40% reduction in quality of life, akin to
patients with severe angina or on renal dialysis. Severe AMD causes a 63%
reduction in quality of life, similar to patients with end‐stage prostate cancer or
those who have suffered a catastrophic stroke (reviewed in 105).
The prevalence of AMD is expected to rise. In the US, for example, the population
over 85 years is set to increase 107% by 2020 53. In Australia, AMD is estimated to
cost $2.6 billion annually, projected to grow to $6.5 billion by 2025 (reviewed in
106). The direct financial costs of blindness due to AMD was estimated at up to $22
507 per individual per year in 2000 107. Estimates of the economic burden of AMD
using a value‐based medicine approach suggest it may have a $30 billion negative
impact annually in the US 108 and $2.6 billion in Canada 109. The Australian study
predicted a saving of $5.7 billion over 20 years if AMD progression could be
reduced by as little as 10%. The search for therapeutic interventions is thus an
urgent priority in industrialised countries.
2.1.7 Therapeutics
2 . 1 . 7 . 1 P r e v e n t a t i v e i n t e r v e n t i o n s
The only modifiable risk factor for AMD is smoking. However, cessation of
smoking has not been consistently shown to reverse risk 46, 47. The use of high dose
antioxidant and zinc supplementation is not yet a part of standard clinical
practice, since AREDS is the only prospective study that has found a reduction in
the risk of AMD progression 110, 111. Ongoing population‐based studies and
intervention trials will better define which nutrients confer the greatest protection
and the best formulations or routes of administration for their use as supplements.
18
2 . 1 . 7 . 2 P a l l i a t i v e i n t e r v e n t i o n s
For patients with neovascular AMD, a number of recent innovations have offered
hope of slowing progression and stabilising, or even improving, vision.
Photodynamic therapy (PDT) uses verteporfin, an intravenously administered
photosensitising dye to target subfoveal CNV, and can be used concomitantly
with intravitreal injection of triamcinolone, a corticosteroid. However, PDT
treatment (with or without triamcinolone) only leads to modest reductions in
vision loss and does not generally result in vision gain in the long term (reviewed
in 112).
The most promising therapeutic strategy to emerge in recent years is the use of
engineered antibodies that target vascular endothelial growth factor (VEGF) and
inhibit CNV. The dosing schedule and long term effects of a number of anti‐VEGF
therapies are currently being evaluated, although several have been shown to slow
disease progression and stabilise vision 112. To date, ranibizumab, a recombinant
monoclonal antibody which recognises all isoforms of VEGF, is the only anti‐
VEGF therapy shown to reverse vision loss. The rate of vision loss was slowed in
over 90% of participants over two years in a randomised trial, with up to a third of
participants experiencing vision gain which was maintained over two years 113.
There is currently no palliative therapy for GA and preventative interventions for
all forms of AMD are limited. Advances in gene therapy and stem cell therapy
have the potential to alleviate vision loss and the development of AMD lesions in
the near future. However, AMD is a complex disease and a thorough appreciation
of pathogenic mechanisms will necessarily underpin new therapies. The following
sections overview normal macular anatomy and AMD histopathology, and
summarise the current understanding of disease pathogenesis as well as its
limitations.
19
2 . 2 T H E “ A N A T O M Y ” O F A M D
2.2.1 The macula in health and ageing
2 . 2 . 1 . 1 An a t om y & em b r y o l o g y o f t h e h um a n e y e
The eyeball lies in the bony orbit, and consists of three layers of tissue: (i) the
outermost fibrous tunic, composed of the sclera and cornea; (ii) the uveal tract,
which forms the middle layer; and (iii) the retina, the innermost layer. The eye is
divided into the anterior and posterior chambers by the iris and lens. The sclera
and cornea together form an elastic envelope that gives the eye its shape. The
uveal tract is composed of the iris, ciliary body and choroid. It carries the principle
nerves and blood vessels of the eye, upon which the intraocular structures depend
for their nourishment.
The retina is an extension of the central nervous system, and extends from the
optic nerve to the ora serrata. The retina and optic nerve are forebrain derivatives,
with the optic nerve connecting the retina to the brain via the optic chiasm and
optic tracts. In life, the retina is completely transparent, and in contact with
Bruch’s membrane externally and the vitreous body internally. The macula, or
macula lutea (“yellow spot”), is a shallow depression that lies in the central retina,
3.5mm lateral to the edge of the optic disc. High concentrations of the carotenoids
lutein and zeaxanthin, known as macular pigment, give rise to the yellow colour
of the macula. Although the macula area measures 5‐6 mm in diameter, it accounts
for almost 10% of the visual field due to the concentration of photoreceptors.
Microscopically, the retina has ten well‐defined layers. From the outermost to
innermost, these are: (i) retinal pigment epithelium (RPE); (ii) photoreceptor outer
segments; (iii) outer limiting membrane; (iv) outer nuclear layer, comprised of
photoreceptor cell bodies; (v) outer plexiform layer; (vi) inner nuclear layer; (vii)
inner plexiform layer; (viii) ganglion cell layer; (ix) nerve fibre layer; and (x) outer
20
limiting membrane. The rod and cone cells contain photopigments essential for
the transduction of light into chemical and then electrical energy.
Within the macula lies the fovea (1.5mm in diameter) and foveola or fovea centralis
(0.35mm in diameter). The photoreceptor layer in the foveal region contains only
cones ‐ approximately 100 000 cones within the total foveal area of 1.75mm2 114. The
nerve fiber layer, ganglion cell layer, inner plexiform layer and inner nuclear layer
are all absent in the fovea.
Embryologically, the eyeball begins to form with the invagination of the optic
vesicle into the optic cup at week four. Two neuroectodermal layers, in contact
with each other at their apices, will have differentiated into the neurosensory
retina internally and the RPE externally by week four. A thin capillary network
appears and covers the entire optic cup after its formation. Primitive rod and cone
inner, and later outer, segments appear in the seventh week, with the neuroblastic
layers of the macula differentiating first. Pigmentation of the cells of the outer cup
increases during the eighth week. The anterior margin of the cup grows forward
during the tenth week to form the future ciliary body and iris. The inner plexiform
layer forms in the 12th week, the end of embryonic life. During the fourth month,
the outer plexiform layer is discernable and bipolar, amacrine, Muller and
horizontal cells differentiate. The first retinal vessels also appear in the fourth
month, and develop their adult pattern by the fifth month after birth. The RPE
cells enlarge from the fourth to the eight month. The final arrangement of retinal
cells and their processes is complete by the sixth month, with some further retinal
differentiation after birth, particularly in the macular region. RPE cells, however,
continue to increase in density between birth and two years of age 114.
21
2 . 2 . 1 . 2 Th e r e t i n a l p i gm e n t e p i t h e l i um
Approximately 4.2 ‐ 6.1 million RPE cells are present in the adult eye.
Pigmentation varies depending on the region of retina, with the highest density of
pigmentation at the macular region. RPE cells at the macula measure 14μm wide
by 10‐14μm high, compared to flatter cells at the ora serrata that measure up to
60μm wide. RPE cells of the ora serrata are often multinucleate. It is generally
accepted that in situ, RPE cells are post mitotic, and that adjacent cells slide in to
replace cells that have died 114.
The primary function of the RPE is to support the photoreceptors. They secrete
and maintain both the inter‐photoreceptor matrix and Bruch’s membrane,
regenerate bleached visual pigments, provide metabolic support by facilitating the
movement of fluid and ions between the choroid and the photoreceptors, and
phagocytose spent photoreceptor outer segments. The phagocytic load is great: in
the rhesus monkey, for example, each RPE cell supports around 30
photoreceptors, which shed a combined 3000 membrane disks every day 115. In
humans, each RPE cell supports approximately 45 photoreceptors, and a rod outer
segment is fully phagocytosed and replenished every 10 days (reviewed in 116). To
accomplish this, RPE cells have a highly developed lysosomal system. Outer
segments are engulfed by phagosomes, which fuse with lysosomes to allow
degradative enzymes to break down their contents. Undigested material becomes
residual bodies, which become the substrate for lipofuscin in iron‐catalysed
reactions in lysosomes. This autofluorescent lipid‐protein aggregate accumulates
in the post‐mitotic RPE cells from the age of 16 months (reviewed in 2). In addition,
the RPE releases cytoplasmic contents into Bruch’s membrane, to meet the
demands of the high phagocytic load (reviewed in 117.
The RPE secretes and maintains its own basement membrane and intercellular
membrane, as well as the intercellular matrix of the photoreceptors and
22
choriocapillaris. These processes depend on a balance of RPE‐derived membrane
metalloproteinases (MMPs) and tissue inhibitors of metalloproteinases (TIMPs) 118,
and possibly other proteolytic enzymes 119. As well as providing the local cellular
scaffolding, RPE cells regulate their own survival, together with that of the
choriocapillaris, by the production of VEGF and pigment epithelial derived
growth factor (PEDF), such that loss of RPE results in loss of not only the
overlying photoreceptors, but the choriocapillaris as well 120.
The RPE also regulates the choroidal vasculature, most likely by producing a
balance of pro‐ angiogenic factors such as VEGF121, 122, and anti‐angiogenic factors,
such as PEDF 123 and Fas ligand (FasL) 124. Additionally, RPE cells secrete a
number of immunomodulatory factors that maintain outer retinal immune
privilege and the immunological specialisations of the choroid (see Section 2.2.1.7).
2 . 2 . 1 . 3 Ag e ‐ r e l a t e d c h a n g e s i n t h e RPE
A degree of polymorphism with regard to RPE cell shape, size, nuclei and degree
of pigmentation occurs with advancing age. Cell nuclei become smaller and more
basophilic 114. In the macular region, RPE cells become taller and narrower with
normal ageing, and accumulate lipid and granular material, which is deposited in
the inner collagenous zone of Bruch’s membrane. Lipofuscin, consisting of
partially degraded cell membranes in lysosomes, accumulates and increases from
around 1% of the cytoplasm during the first decade of life, to 19% during the ninth
decade 125. Lipofuscin accumulates in cells that do not have a mechanism to deal
with oxidised lipoproteins. These cells can be professional lipoprotein scavengers
like macrophages, or non‐professional phagocytes such as central nervous system
neurons and RPE. In RPE, it is most concentrated in the central retina, where
photoreceptor cell density, and hence metabolic demand, is highest. Increasing
lipofuscin content not only significantly reduces the volume of functional RPE
cytoplasm, its generation also produces damaging reactive oxygen species
23
(reviewed in 116). Death of any critically damaged RPE cells results in increased
demands on the remaining cells.
Lipofuscin can slowly disappear once the overlying photoreceptors are lost. The
residual bodies that give rise to lipofuscin retain degradative enzyme activity,
implying that once the phagocytic demands on RPE are reduced, lipofuscin is
eventually degraded, recycled or shed into Bruch’s membrane (reviewed in 117).
Several histopathological studies have shown that an abnormal, thickened RPE
basement membrane, together with loss of RPE basal infoldings, occurs in eyes
after the seventh decade 10, 126, 127. Not only are these features indicative of cellular
stress and loss of function, they were also found to be commonly associated with
choroidal neovascularisation (see Section 2.2.2). Not surprisingly, RPE failure is
considered a central event in the pathogenesis of AMD.
2 . 2 . 1 . 4 Th e c h o r o i d a n d B r u c h ’ s memb r a n e
The retina has one of the highest metabolic demands of any tissue. While the inner
retina derives its nutrition from the retinal vessels, the choroid functions
principally to nourish the outer retina. It also acts as a conduit for vessels that
supply the anterior eye. The choroidal stroma contains a significant number of
pigmented cells – choroidal melanocytes – which are concentrated in the
outermost layers 128.
Microscopically, the choroid consists of: (i) Bruch’s membrane; (ii) choriocapillaris;
(iii) stroma; and (iv) suprachoroidea. In ultrastructure, Bruch’s membrane is
comprised of the basement membrane of the RPE, an inner collagenous layer, an
elastic layer, an outer collagenous layer, and incorporates the basement membrane
of choriocapillaris endothelial cells. It thus comprises the outer limit of the retina,
and acts not only as a stabilising and anchoring structure, but also as a barrier and
a filter 128.
24
Bruch’s membrane separates the photoreceptors and RPE from their primary
source of nourishment, the choriocapillaris, and forms a permeable filter through
which metabolic exchange can take place. Glycosaminoglycans, laminin,
fibronectin and collagens I, III, IV, V, and VI are found in various combinations
within the five layers of Bruch’s membrane 129. The composition and turnover of
these extracellular matrix components can alter the structural integrity and
permeability of Bruch’s membrane. Age‐related Bruch’s membrane thickening is
attributed to compositional changes (Section 2.2.1.6), and abnormal extracellular
Bruch’s membrane deposits are a feature of early AMD (see Section 2.2.3).
Some of the material shed from RPE into Bruch’s membrane is probably cleared by
the choriocapillaris by passive diffusion. Guymer et al suggest that choroidal
endothelial cells or pericytes may also actively phagocytose RPE‐derived debris 117.
Recent evidence points to a significant role for choroidal macrophages, a
population of dedicated phagocytes, in keeping Bruch’s membrane debris‐free
(discussed in Section 2.3).
Embryologically, endothelial cells in the mesoderm adjacent to the RPE of the
optic cup begin to differentiate during the fourth week. By the fifth week, the
vascular net extends along the entire optic cup, coinciding with heavier
pigmentation of the RPE. The RPE basement membrane and inner collagenous
layer of Bruch’s membrane develop by the sixth week. Also at this time,
rudimentary vortex veins appear and the choroidal capillary network becomes
more developed. The elastic layer of Bruch’s membrane appears in the twelfth
week. The elastic layer is one third (or less) as thick at the fovea than the periphery
in the adult 130. Most of the choroidal vessel systems develop during the third and
fourth months. Pigment in the choriodal melanocytes first appears in the fifth
month. The choroidal melanocytes, unlike the RPE, derive from the neural crest.
25
The unique capillary system of the choroid resides in a single plane and is highly
fenestrated on the Bruch’s membrane side. The blood flow rate through the
choroid is relatively fast, resulting in lower oxygen exchange per millilitre of
blood, and a relatively oxygen‐rich outer retina. In the macaque monkey, 90% of
outer‐retinal oxygen is consumed by the photoreceptors 131. A rich nerve supply
exists in the choroid, probably to control ocular blood flow, which also depends
on intraocular as well as systemic blood pressure. Loss of the choroid, either
experimental or disease‐related, results in atrophy of the overlying retina 128. A
capillary‐free zone of 0.4mm exists within the macula, making it particularly
vulnerable to age‐related reductions in choroidal blood flow 132.
2 . 2 . 1 . 5 Ag e ‐ r e l a t e d c h a n g e s i n t h e c h o r o i d
There is a decrease in choroidal thickness from 200μm at birth to 80μm by age 90,
due to a decrease in both choriocapillaris density and lumen diameter 5, 133 as well
as increase in intercapillary pillar width 134. This is accompanied by an increase in
Bruch’s membrane thickness 5, 133. The normal lobular arrangement, where a
central arteriole feeds lobules of capillaries surrounded by venules 135, becomes
tubular (reviewed in 117, thus reducing the surface area available for exchange.
There is also a reduction of choriocapillary fenestrations as the overlying RPE
becomes damaged or lost with age 5, 120. Correspondingly, there is a drop in
macular choroidal blood flow (as measured by laser Doppler flowmetry), with
delayed indocyanine green choriodal filling in normal individuals over 50 years
136.
2 . 2 . 1 . 6 Ag e ‐ r e l a t e d c h a n g e s i n B r u c h ’ s memb r a n e
Bruch’s membrane measures 2μm in thickness at birth and increases to 4 to 6μm
by the tenth decade 133, 137, 138. Between 10 and 90 years, Bruch’s membrane
26
thickness doubles. These changes have been attributed to increased or altered
protein content, increased lipid content and increased glycosaminoglycan size. On
light and electron microscopy, there is accumulation of vesicular, granular,
membranous and filamentous material, most prominently between the RPE
basement membrane and the inner collagenous layer 129, 137, 139.
There is increased collagen cross‐linking and possibly, glycosylation, which result
in resistance to breakdown by RPE collagenases. Correspondingly, there is
impaired collagen turnover and increased Bruch’s membrane collagen content
(reviewed in 116). Two types of collagen accumulate: the normally present 64nm‐
periodicity collagen, and a 100‐140nm‐periodicity collagen described as “long‐
spacing collagen” 137, 140. Although long‐spacing collagen is also found in young,
normal eyes in the outer collagenous zone, outer‐collagenous zone deposits
steadily increase with age 140. Laminin, an important basement membrane
structural component that facilitates RPE anchoring and choriocapillaris
remodelling, is also increasingly found deposited in the inner collagenous zone,
and between the outer collagenous zone and choriocapillaris basement membrane
(reviewed by 117). Guo et al found reduced active forms of matrix
metalloproteinases (MMPs) and increased inactive forms in the Bruch’s membrane
of aged eyes, suggesting impaired extracellular matrix turnover 118.
Bruch’s membrane hydroxyproline content decreases proportionate to tyrosine,
methionine and phenylalanine in the macular region 141. Material resembling RPE
phagosomal contents can be found in the inner collagenous layer, and later the
elastic layer 142. There is also evidence of an age‐related increase in advanced
glycation end products (products of non‐enzymatic glycosylation of long‐lived
protein) in Bruch’s membrane 143.
27
Bruch’s membrane lipid content is negligible before the age of 30 by rises to
220mg/m2 by age 100, and consists primarily of phospholipids, triglycerides, fatty
acids and free cholesterol, which suggest a cellular source 134, 144‐146. Peroxidised
lipid content also increases with age 147, particularly at the macula, and appear to
derive from the long‐chain polyunsaturated fatty acids found in outer segments.
These age‐related changes in Bruch’s membrane composition and thickness alter
its permeability. Fluid movement is significantly impeded by the fifth decade 148,
particularly at the macula. Maximal resistance was found in the inner collagenous
layer, and thought to be due to the entrapment of both lipid and visculo‐granular
material 149. Diffusion of macromolecules 150 and amino acids 151 across Bruch’s
membrane also follow a significant age‐dependent decline. The expression of
vascular endothelial growth factor (VEGF) is increased in RPE, photoreceptor
outer segments and the outer plexiform layer with increased Bruch’s membrane
lipid content 152. VEGF is a potent promoter of angiogenesis, and its production is
stimulated by hypoxia and ischaemia 153.
Taken together, age‐related changes in the RPE, choriocapillaris and Bruch’s
membrane result in reduced blood flow, impaired oxygen, fluid, amino acid and
macromolecular exchange, dysregulated extracellular matrix turnover and
alterations in RPE‐derived trophic factors. These are normal age‐related changes
and do not by themselves constitute AMD. However, in susceptible individuals, it
is within this compromised context that AMD develops.
Inflammation is increasingly recognised as a significant factor in the pathogenesis
of AMD (Sections 2.4 & 2.5). Before describing the pathological changes in early
and advanced AMD (Sections 2.2.2 & 2.2.3), it is important to consider the
specialised local immunological environment in which inflammation occurs.
28
2 . 2 . 1 . 7 Immun o l o g i c a l s p e c i a l i s a t i o n s o f t h e r e t i n a a n d c h o r o i d
Immune reactions within ocular tissues, while sharing many features with
immune responses in other tissues, are atypical. This was first demonstrated when
allografts transplanted into the anterior chamber of the eye survived better than
those transplanted in skin 154. It is now understood that “ocular immune privilege”
is the result of a complex network of interactions which allow the control of sight‐
threatening inflammation while affording immunity. This is made possible, in
part, by the relative lack of lymphatics draining intraocular tissues,and the
existence of the blood‐retina barrier (maintained by tight junctions between retinal
vascular epithelium internally and the RPE externally). However, it is clear that
immune privilege results from active as well as passive processes.
Intraocular tissues express and secrete a number of molecules that suppress
inflammation. Expression of major histocompatibility (MHC) molecules is limited
to low levels of class I and virtually no class II in the anterior chamber, vitreous
cavity and subretinal space 155, 156. Parenchymal ocular tissues express FasL,
inducing apoptosis in invading activated T cells 157. Despite the presence of the
blood‐retina barrier, ocular antigens can leave the eye compartment inside ocular‐
derived antigen presenting cells via the circulation 158. While the APCs travel to the
spleen, they do not activate helper T‐cells. Rather, ocular‐derived APCs are
programmed by a local microenvironment rich in immunosuppressive cytokines,
such as TGFβ, vasoactive intestinal peptide and somatostatin (reviewed in 159). The
end result is the production of CD8 suppressor cells and the suppression of both
delayed type hypersensitivity and complement‐fixing antibody producton 160.
While the location of the uveal tract is external to the blood‐retina barrier, several
lines of evidence suggest that local immune reactions are also “atypical”. In the
choroid, the local immune microenvironment appears to depend to a large degree
29
on RPE‐derived immunosuppressive factors. The RPE separates the retina from
the highly fenestrated capillary network of the choriocapillaris. Due to the large
amount of photoreceptor outer segments phagocytosed by RPE daily, and the
constant release of cytoplasmic contents into Bruch’s membrane (Section 2.2.1.2),
retinal antigens are readily available for antigen presentation in the choroid. It is
thus important that inappropriate retinal antigen presentation resulting in
sensitisation of the RPE or retina to autoimmune attack is prevented.
RPE cells do not express MHC II molecules in the quiescent eye 161. While RPE can
express pro‐inflammatory cytokines when stimulated, they constitutively express
the anti‐inflammatory molecules transforming growth factor‐β1 (TGFβ1) 162,
interleukine‐1 (IL‐1) receptor antagonist 163, somatostation, interleukin‐10 (IL‐10)
and PEDF 164. These molecules are thought to modulate the immune
microenvironment of Bruch’s membrane and the choriocapillaris, such that
resident dendritic cells and APCs become tolerogenic. RPE compromise may thus
have significant immunomodulatory consequences for the choroid and Bruch’s
membrane (discussed further in Sections 2.4 and 2.5).
2.2.2 The pathology of endstage AMD
2 . 2 . 2 . 1 Ge o g r a p h i c a t r o p h y o f t h e RPE : t h e n a t u r a l e n d s t a g e o f AMD
Gass used “geographic atrophy” to describe the slowly enlarging or coalescing
areas of RPE atrophy, resulting in lesions with irregular boundaries 6. Within an
area of atrophy, there is an absence of RPE cells, and loss of the overlying
photoreceptors. The underlying choriocapillaris is atrophic or absent, highlighting
its dependence on RPE‐derived maintenance factors 120. RPE cells at the edge of the
atrophic area form a heaped‐up, hyperpigmented edge, which consists of large
cells filled with lipofuscin and membrane‐bound bodies 165, with distortion of the
30
few remaining microvilli. Photoreceptors overlying the edge of atrophy are
grossly abnormal, if present at all 8.
2 . 2 . 2 . 2 Ch o r o i d a l n e o v a s c u l a r i s a t i o n a n d d i s c i f o rm s c a r r i n g
In the early stages of choroidal neovascularisation (CNV), capillary‐like vessels
extend from the choroid into the inner Bruch’s membrane and the sub‐RPE space 7,
166. Initially, the new vessels grow within the space normally occupied by basal
linear deposit and drusen, i.e. along the inner layers of Bruchʹs membrane 167.
Endothelial sprouts continuous with pre‐existing choroidal vessels are surrounded
by enlarged, activated pericytes, eventually forming lumina 167. The points of
origin can vary from one to 12, with an average of 2.2 per eye 168. These vessels
later become larger veins and arteries 126.
Early CNV is rarely clinically discernable. Sarks found sub‐clinical CNV in a
surprisingly high proportion (41.7%) of eyes with GA 10, suggesting that although
a common occurrence, early CNV does not readily progress to neovascular AMD.
Ultrastructural evidence supports two different phases of new vessel growth 167. A
“low‐turnover” phase characterises the initial intra‐choroidal neovascularisation,
while substantial sub‐RPE neovascularisation is accompanied by extensive
endothelial cell proliferation and migration in a “high‐turnover” phase. In rare
instances, CNV may encroach into the sub‐retinal space and join with retinal
vessels 126.
As discussed earlier (Section 2.1.1), once CNV is established in the sub‐RPE or sub‐
retinal space, vessels can leak or bleed, causing serous or haemorrhagic
detachments of the RPE. Resolution of these events results in the proliferation of
fibrous tissue and the formation of a sub‐RPE fibrovascular scar. Sarks noted that
the choroid underlying disciform scars was thicker than that underlying areas of
GA 10, although both are thin compared to normal 169.
31
The histopathological features of GA, CNV and disciform scars are thus well
defined. However, there is less agreement regarding the histopathological
definition of early AMD. This is partly because the earliest histopathological
changes are not clinically evident. It was Gass who first proposed that
neovascularisation originating from the choroid may occur secondary to
accumulation of abnormal deposits beneath the RPE 6.
2.2.3 The basal deposits and early AMD
2 . 2 . 3 . 1 B a s a l l am i n a r d e p o s i t i s a ma r k e r o f RPE d e g e n e r a t i o n
In 1976, Sarks described a deposit found between the RPE plasma membrane and
its basement membrane (Figure 2.2‐1), with a blue, striated appearance on picro‐
Mallory staining, which eventually came to be known as basal laminar deposit
(BLamD) 10 (Figure 3.1‐2). The first systematic clinicopathological review of its
kind, the study found that in 378 eyes, submacular BLamD correlated well with
age, Bruch’s membrane hyalinisation and importantly, with the degree of RPE
degeneration 10. When eyes in this series were organised by the appearance of
BLamD, a histopathological grading system, comprised of five groups, emerged
(detailed in Section 2.2.3.5).
In 1986 Loffler et al confirmed that the BLamD consisted of long‐spacing collagen
in a granular matrix, and was composed, in part, of normal RPE basement
membrane molecules, such as collagen IV, laminin and heparin‐sulphate
proteoglycan 170. On EM, the earliest evidence of BLamD had a remarkably similar
appearance to normal RPE basement membrane 170. A late form of BLamD, found
internal to the early form was noted subsequently, and found to correspond to
severe RPE degeneration 8 (Figure 3.1‐2). Ultrastructurally, the late BLamD is
32
amorphous, with the appearance of having been laid down in waves (Figure 3.1‐
2).
The similarity of BLamD to RPE basement membrane suggests it is perhaps an
aberrant type of basement membrane material, secreted by distressed RPE. Hirata
et al examined RPE secretion of radiolabelled proline in vitro and proposed that
BLamD accumulation may be the result of aberrant degradation, rather than
enhanced synthesis 171. There is some evidence of age‐related reduction in active
MMP‐2 and increase in inactive degradative enzymes, suggesting reduced Bruch’s
membrane extracellular matrix turnover 118. As discussed earlier, a single gene
polymorphism in the promoter region of the HTRA1 gene has been associated
with AMD risk (Section 2.1.5.5). The HTRA1 gene product is a member of the
serine protease family, which includes proteins which facilitate the activity of
matrix‐degrading enzymes (including MMPs) 172. The functional impact of the
polymorphism appears to be HTRA1 over‐expression 97, although it is not yet clear
how this impacts extracellular matrix turnover within Bruch’s membrane and
BLamD deposition.
Because material similar to BLamD can be found in the peripheral retina 173, and in
the trabecular meshwork and cornea 140, it is not considered specific for AMD.
However, it is the commonest histopathological finding in eyes with AMD 126, 169,
174, and remains the most reliable marker of RPE degeneration 8, 175, 176.
2 . 2 . 3 . 2 B a s a l l i n e a r d e p o s i t a n d s o f t d r u s e n a r e t h e s p e c i f i c l e s i o n s o f e a r l y AMD
Membranous material accumulates in the inner collagenous layer of Bruch’s
membrane from the second decade onwards 177. Ultrastructurally, this material
resembles coiled phospholipid membranes, and can be found in the RPE basement
33
membrane and between strands of early BLamD 3. Early investigators postulated
that it derived from RPE, either as partially digested phagosomes 177 or selectively
sequestered aliquots of cytoplasm shed into Bruch’s membrane 139, 177. Thin layer
chromatography studies have shown that lipids of cellular origin, such as
phospholipids, triglycerides, fatty acids and free cholesterol preferentially
accumulated in macular Bruch’s membrane 144.
Figure 2.2‐1 Schematic diagram of BLamD and membranous debris
When membranous material collects in a continuous layer external to the RPE
basement membrane, it is known as basal linear deposit (BLinD) (Figure 2.2‐1).
This deposit is only visible on electron microscopy (EM). In contrast to BLamD,
BLinD disappears when RPE is atrophied and overlying photoreceptors absent 178,
suggesting that it is a product of the RPE‐photoreceptor complex. Curcio and
Millican found that not only is BLinD specific for early AMD, but soft drusen, the
hallmark lesions of early AMD, are composed of the same membranous material
34
176 or “membranous debris”179, 180. Thus, soft drusen can be understood as focal
accumulations of BLinD external to the RPE basement membrane 175, 176, 179 (Figure
2.2‐1). Smaller focal collections of membranous debris can also accumulate internal
to the RPE basement membrane, and are known as basal mounds (Figure 2.2‐1).
The biochemical constituents of membranous debris are not fully characterised.
Earlier histochemical and ultrastructural studies identified neutral lipids and
cerebrosides/gangliosides 142 in drusen. Pauleikhoff and colleagues later showed
that drusen also contained phospholipids 181. Ruberti et al used quick freeze/deep
etch electron microscopy to find that lipids, including cholesterol, preferentially
accumulated with age in a thin layer external to the RPE basement membrane. The
authors proposed that lipid deposition in this layer may be the pre‐cursor event to
BLinD formation (and later, soft membranous drusen) 146. Recently, Curcio and
colleagues have shown that Bruch’s membrane and drusen contain solid lipid
particles 182, esterified and unesterified cholesterol 145, 183 and lipoproteins 184, 185.
Because they were also able to demonstrate the presence of microsomal
triglyceride transport protein 186 and apolipoproteins C‐I, C‐II, E, and J 185 in RPE,
these investigators have proposed that dysregulated lipid metabolism by RPE may
be responsible for the accumulation of membranous debris in AMD.
Although RPE apolipoprotein biochemistry is not well understood, genetic
studies support a role for ApoE in AMD pathogenesis. In the central nervous
system, ApoE modulates phospholipid and cholesterol transport as part of normal
neuronal metabolism, as well as during neuronal remodelling. Certain ApoE gene
polymorphisms, particularly the E4 allele, are linked to Alzheimer’s disease and
atherosclerosis. Interestingly, the E4 allele of ApoE appears to be associated with a
decreased risk of neovascular AMD 55, 58‐60, 187 (discussed in Sections 2.1.5.2 & 2.1.5.1).
35
Another mechanism by which membranous debris may form is by the non‐
apoptotic blebbing of damaged RPE cells, discussed further in Section 2.2.3.4.
2 . 2 . 3 . 3 Dru s e n c l a s s i f i c a t i o n
Drusen are seen as discrete spots in the fundus clinically (see Section 2.1.2).
Histopathologically, they consist of focal, discrete aggregates of abnormal
extracellular material external to the RPE basement membrane. However, drusen
can vary widely in their appearance in terms of size, shape, elevation, distribution
and contents, and drusen nomenclature remains confusing. To a large degree, this
is the result of the lack of availability of post mortem eyes which have been
photographed during life, thus limiting clinicopathological correlations.
Clinically, drusen have been described as: hard or soft; distinct or indistinct; large,
intermediate or small; as well as by their fluorescein staining characteristics.
Histopathologically, drusen have been described as: small hard (or hyalinised);
soft extensive (or membranous); as well as by their ultrastructural and histological
staining characteristics (with terms such as papillary drusen and basal laminar
drusen). Hard drusen have a discrete, hyalinisted light‐microscopic appearance,
while soft drusen are usually larger, have sloping margins, and membranous or
granular contents.
Large, population‐based studies have contributed a great deal to the
understanding of the natural history of drusen. It is clear that the natural histories
of small hard drusen and large soft drusen are distinct, and the associated AMD
risk is also significantly different. Small, hard drusen are present in 94% of the
population 29 and 80% of post mortem eyes 188. Over a 5 to 15 year period, eyes
with fewer than 8 small hard drusen are no more at risk of developing AMD than
normal eyes 29, 32‐34, 37, 39. Eyes with soft, indistinct drusen (>125μm), on the other
36
hand, are almost 18 times more likely to develop late AMD over 5 and 15 years 29,
37.
Unfortunately, studies investigating drusen contents often do not discriminate
between small hard drusen and large soft drusen, while the results are generalised
to explain AMD pathogenesis. This may be partly due to the vulnerability of soft
drusen contents to loss during tissue processing, and partly because small hard
drusen are much more commonly found in donor eyes. However, given the
differences in clinical behaviour, there has been a surprising lack of large‐scale
studies comparing the contents of hard and soft drusen. Hageman and Mullins
found that the extracellular matrix protein vitronectin, and carbohydrate moieties
stained by wheat germ agglutinin and limax flavus agglutinin were present in
several drusen phenotypes 189, 190. This, and their reports of the presence of
immunoglobulin G and complement components in both hard and soft drusen 191,
192 lead them to propose that phenotypic descriptions of drusen as “hard” or
“soft”, “small” or “large” were meaningless. While this approach simplifies
drusen nomenclature, it fails to address the clinical reality that a few small hard
drusen are considered normal while large soft drusen are the hallmarks of early
AMD, and carry a significant risk of progressing to advanced AMD. However,
Hageman and Mullins point out that different drusen phenotypes may result not
from differences in protein or carbohydrate composition, but from the amount and
type of lipids present 189.
2 . 2 . 3 . 4 Dru s e n b i o g e n s i s
Evidence to date suggests the RPE plays a significant role in the biogenesis of both
hard and soft drusen. Observations that RPE overlying drusen are cytologically
abnormal in shape, size and pigment characteristics were made at least as early as
1905 193. Ultrastructural studies later showed that pigmentation, intercellular
junctions, and organelle content and organelle distribution are altered in RPE
37
adjacent to drusen 142, 194, 195. As RPE becomes increasingly degenerate
histopathologically, soft drusen regress and collapse, and hard drusen may
disappear 196. Farkas and colleagues demonstrated the presence of degenerate
organelles and possibly lysosomes in drusen 197, implicating the RPE as a source of
drusen components. Although lysosomal activity was not demonstrated in a study
by Burns and Feeney‐Burns 195, cytoplasmic debris was found in small drusen. In
a later ultrastructural study, Ishibashi et al found that RPE cells in aged human
eyes with drusen shed portions of their basal cytoplasm into Bruch’s membrane
198. These “buds” of RPE cytoplasm contained many organelles, including
secondary lysosymes, mitochondria, vesicles and vacuoles. In conditions of stress,
mammalian cells may sequester non‐functional or toxic cytoplasmic components
in discrete packets of their cytoplasm to be shed as membrane blebs 199, 200. There is
in vitro evidence that oxidative damage causes increased RPE membrane blebbing
201. Interestingly, RPE MMP‐2 activity is correspondingly reduced under the same
conditions, suggesting a possible mechanism for increased membranous debris
production and decreased degradation.
A recent proteomic analysis of primarily hard drusen found increased
carboxyethylpyrrole adducts (CEP protein adducts), products of
docosahexaenoate lipid oxidation 202. Since docosahexanoate is the most abundant
fatty acid in photoreceptor outer segments 203, the finding of CEP adducts in
drusen suggests that drusen may contain indigestible, RPE‐derived photoreceptor
elements. However, it is evident that apart from RPE derived components, hard
drusen also contain molecules involved in extracellular matrix turnover, as well as
acute phase reactants and other molecules of the innate immune system 202
(discussed further in Section 2.5.1.2). The possible involvement of choroidal
dendritic cells in drusen biogenesis is discussed in Section 2.4.4.
38
2 . 2 . 3 . 5 H i s t o p a t h o l o g i c a l c l a s s i f i c a t i o n s c h em e s
As with clinical classifications, there is no universally accepted classification
system for AMD histopathology. In particular, histopathological definitions of
early AMD have varied. These include: (i) the type or number of drusen; (ii) the
presence of basal deposits; (iii) RPE changes; and (iv) degree of photoreceptor
degeneration 126, 133, 169, 175, 204.
Histopathological grading is only meaningful when pathological features can be
correlated to clinical (particularly fundus) features. Two large‐scale studies have
attempted to do this. Sarks surveyed 378 eyes, all with clinical data, including
fundus appearance, and in some cases, fluorescein angiography 10. Eyes with no
BLamD were considered normal (group I). A normal fundus was seen in all eyes
in this group. Eyes with patchy BLamD represented normal ageing (group II). The
majority of eyes in this group had a normal fundus. Since approximately half of
the eyes with a thin continuous layer of BLamD (group III), and all eyes with thick
continuous BLamD (group IV) had clinical fundus changes (soft drusen and/or
pigmentary disturbance), they were considered to have early AMD. Eyes with
geographic atrophy (Group V) and disciform scarring (Group VI) were classified
irrespective of BLamD appearance, although BLamD was incorporated into scar
tissue in disciform lesions and persisted in areas of GA even in the absence of RPE.
The Alabama Age‐Related Macular Degeneration Grading System for donor eyes
was devised by Curcio and colleagues, and surveyed 34 eyes, 30 of which had
accompanying clinical data 205. Clinicopathological correlations were made using 5
eyes. The grading in this study was based on the extent of the “basal deposits, a
combination of BLamD and BLinD” (since BLinD can only be distinguished on
EM). In both studies, normal and early AMD eyes were defined
histopathologically. Tables 2.2‐1 and 2.2‐2 compare the two grading schemes.
Although similar in many respects, Sarks’ grading scheme was used for the
39
histopathological studies in Chapter 3 due to its larger sample size and more
extensive clinico‐pathological correlations.
Despite the availability of clinical and histopathological classification systems,
there is no universal definition of early AMD. Chapter 3.1 presents findings of a
clinicopathological study that examined the influence of BLamD and membranous
debris on the progression of AMD. The histopathological threshold at which
ageing becomes early AMD is discussed.
40
Table 2‐1 Histopathological grading scheme suggested by Sarks (1976) 10
Group I II III IV V VI
BLamD Absent Patchy Thin continuousThick
continuousPersists after
loss of pigmentIncorporated into scar
RPE Normal3.6% with fine
pigment disturbance
46.7% fine pigment
11.7% coarse pigment
16.7% fine pigment
78.6% coarse pigment
GAIncorporated into scar
CNV Nil Nil Nil 14.3%* 41.7%* 100%
* subclinical CNV
Table 2‐2 Histopathological grading scheme suggested by Curcio et al (1998) 205
Grade 0 1 2 3 4
Basal deposits
None PatchyThin
continuousThick
continuousN/A
RPE Uniform Non‐uniformHeaped or sloughed
Anterior migration
GA
CNV None Present N/A N/A N/A
41
2 . 3 A M D D I S E A S E P A T H O G E N E S I S : S U M M A R Y O F C U R R E N T
C O N C E P T S
Although the pathogenesis of AMD is incompletely understood, histopathological
& epidemiological studies, together with newer molecular and genetic insights,
show that AMD results from age‐related changes plus additional pathological
events. Models of AMD pathogenesis fall into one of three main categories. The
mechanisms described in each are not mutually exclusive, and all centre on
interactions between photoreceptors, RPE, Bruch’s membrane, and choroid.
2.3.1 Oxidative stress
Reactive oxygen intermediates (ROIs), are released under physiological conditions
by mitochondrial oxidative phosphorylation and liver cytochrome p450‐related
reactions. ROIs can be quenched or removed by antioxidant defence systems,
which depend on the enzymes glutathione, superoxide dismutase and catalase,
and on vitamins A, C and E, the carotenoids, bioflavinoids, selenium and zinc.
Oxidative damage results if antioxidant defences are exceeded by the production
of ROIs.
Advancing age is accompanied by increased oxidative damage, due to an age‐
related reduction in antioxidant defences. For example, total plasma glutathione,
oxidised glutathione and peroxidised lipid increase with age, while total serum
vitamin C and E decrease (reviewed in 116). The retina is susceptible to oxidative
damage due to the amount of light irradiation it receives, the relatively high
concentration of local oxygen, the presence of peroxidation‐prone polyunsaturated
fatty acids (in photoreceptor outer segments), the presence of photosensitising
pigments and the high RPE phagocytic load. There is an age‐related decline in
macular pigment (lutein and zeaxanthin) 206, RPE cell density, and RPE cell
vitamin E 116. Conversely, there is a corresponding increase in RPE lipofuscin. In a
42
study of AMD donor eyes graded by post‐mortem fundus features, Decanini et al
found that RPE expression of catalase, copper‐zinc superoxide dismutase and
manganese superoxide dismutase increased with increasing AMD pathology 207,
along with proteins associated with secondary defence against oxidative damage.
These findings suggest oxidative defence is upregulated in response to increasing
oxidative stress. However, even though protein levels are increased, there is
evidence that at the activity of least one antioxidant enzyme, catalase, diminishes
with age 208. Other ROI‐quenching enzymes, such as metallothionein 209 and heme
oxygenase 210 appear to decrease in RPE with age.
Data from epidemiologic studies tend to support a role for oxidative stress in
AMD. Lower plasma glutathione reductase 211, and higher glutathione peroxidise
212 are associated with AMD. A number of population‐based studies have shown
increased risk of AMD with low dietary intake of carotenoids (reviewed in 206), and
AREDS (see Section 2.1.5) has reported a reduction in risk of progression to
advanced AMD with dietary antioxidant supplementation 73. In addition, cigarette
smoking, known to increase ROIs and reduce antioxidant defence systems, is a
well‐established risk factor for AMD (see Section 2.1.3).
Oxidative damage can result in destruction of cytoplasmic and nuclear structures,
leading eventually to cell death. Apoptotic cells themselves release cytochrome c
from their disintegrating mitochondria, leading to higher local levels of ROIs
(reviewed in 213). Non‐lethal oxidative damage to RPE cells may result in cellular
blebbing, which may contribute to the deposition of sub‐RPE membranous debris
214. Oxidative damage may also result in the production of an abnormal
extracellular matrix and thickened Bruch’s membrane, due to modifications in
matrix proteins or their degradative enzymes.
43
2.3.2 Ischaemia
The age‐related increase in lipid and protein content reduces Bruch’s membrane
permeability to water soluble plasma molecules and amino acids, resulting in
reduced metabolic exchange (Section 2.2.1). Increased Bruch’s membrane
thickness results in a greater distance between the fenestrated choriocapillaris and
the RPE, thus also impairing oxygen exchange. Age‐related reductions in
choroidal blood flow, due to loss of choriocapillaris density and choroidal
thinning, further exacerbate these changes. Even small reductions in the
diffusional capacity of Bruch’s membrane or choriocapillaris blood flow can
significantly affect photoreceptors and RPE, since the photoreceptors consume 90‐
100% of the oxygen delivered by the choriocapillaris 131. Reductions in gas
exchange and in the bidirectional flow of molecules and fluid between the choroid
and RPE, further promote the production of ROIs.
2.3.3 Inflammation
Inflammation is a tightly controlled response to injury or infection, essential in
restoring the balance between health and disease. It is becoming increasingly
apparent that a breakdown in the regulation of the inflammatory response
contributes to a number of chronic, degenerative diseases. The following sections
review the growing evidence implicating inflammatory and immune‐mediated
processes in AMD disease pathogenesis.
44
2 . 4 M A C R O P H A G E S A N D I M M U N E C O M P E T E N T C E L L S I N A M D
2.4.1 Morphological evidence
2 . 4 . 1 . 1 Ma c r o p h a g e s , B r u c h ’ s memb r a n e b r e a k s a n d e a r l y AMD
Sarks noted that in eyes with early AMD, macrophages were found adjacent to
breaks in Bruch’s membrane and in association with subclinical CNV, the earliest
evidence of neovascularisation 10, 179. These observations were reinforced by similar
follow‐up findings 127, 215. Stromal and choroidal leukocytes, including
macrophages, were found in increased numbers by Penfold et al 216 once eyes
developed a continuous layer of BLD. Mean macrophage counts were significantly
higher in eyes with continuous BLD compared to normal aged eyes 217. Later
reports found that macrophages appeared to be attracted to membranous debris
deposition within Bruch’s membrane in early AMD 218. Macrophages were also
found accompanying both “active” and “inactive” sublinical CNV 219. These
morphological observations suggested a link between macrophage infiltration and
the earliest stages of the neovascular process, possibly by causing breaks in
Bruch’s membrane which allow the in‐growth of newly formed vessels. This
hypothesis was strengthened by the known angiogenic influence of macrophages
in other chronic inflammatory and fibroproliferative states, including rheumatoid
arthritis and solid tumour growth 220, 221.
2 . 4 . 1 . 2 Ma c r o p h a g e s i n a d v a n c e d AMD l e s i o n s
An ultrastructural study of membranes from eyes with neovascular AMD found
leukocytes and macrophages within neovascular structures, at breaks in Bruch’s
membrane, and in contact with activated pericytes 222, further reinforcing the view
that macrophages induced new vessel growth. Macrophage association with the
neovascular process appeared to persist in burnt‐out disciform lesions 7. The most
recent large histopathological survey of surgically excised subfoveal CNV lesions
45
confirmed that macrophages were the most frequently found cell type after RPE in
both neovascular membranes and disciform scars 174.
Interestingly, giant cells appear to the be commonest type of leukocyte found in
eyes with GA 217, 223, whereas the breaks in Bruch’s membrane commonly found in
eyes with disciform scars are absent. This observation further emphasises the
association of Bruch’s membrane breaks with the in‐growth of neovascular tissue.
Sarks proposed that macrophages and giant cells are attracted to Bruch’s
membrane when the phagocytic capacity of RPE is exceeded 3. This idea has been
supported by a number of recent observations on choroidal macrophage
recruitment and turnover (see Section 2.4.3).
2.4.2 Macrophages and choroidal neovascularisation
With compelling morphological data that macrophages were intimately involved
with AMD lesions, and particularly the suggestion that they may facilitate
neovascularisation, investigators sought evidence of macrophage expression of
angiogenic mediators. Oh et al found that macrophages in surgically excised
neovascular membranes, identified by CD68 labelling, expressed the pro‐
inflammatory cytokines interleukin‐1β (IL‐1β) and tumour necrosis factor alpha
(TNFα), while RPE cells admixed in the same lesion expressed vascular
endothelial growth factor (VEGF) 224. The authors suggested that the presence of
activated, pro‐inflammatory macrophages may induce VEGF production by RPE,
promoting choroidal neovascularisation. Grossniklaus et al also examined
surgically excised CNV membranes, and observed that CD68‐positive
macrophages expressed VEGF and tissue factor, while RPE cells expressed
monocyte chemoattractant protein (MCP) 121. These authors concluded that RPE
might be important for macrophage recruitment, and that recruited macrophages
expressed two growth factors essential for neovascularisation.
46
Further evidence of the ability of macrophages to induce neovascularisation was
provided by animal model studies. The animal model most commonly used to
approximate the CNV found in AMD uses laser photocoagulation to cause breaks
in Bruch’s membrane. Neovascular lesions typically form within one week of
photocoagulation 225. Choroidal vascular endothelial cells migrated to into the
subretinal space via the laser‐induced defects in Bruch’s membrane in monkey
eyes three days after photocoagulation, and macrophage infiltration was observed
three to seven days after photocoagulation 226. In the same study, VEGF expression
was first found in macrophages and later in RPE and Muller cells. Similarly, in a
rat model of laser‐induced CNV, macrophages in both the subretinal space and the
choroid adjacent to the site of laser injury expressed VEGF three to seven days
after photocoagulation 227.
Tsutsumi and colleagues explored this relationship further when they compared
macrophage infiltration and extent of CNV after laser photocoagulation in a Ccr‐2
knockout mouse model 228. Ccr‐2 is the receptor for macrophage chemoattractant
protein‐1 (MCP‐1), is normally expressed by macrophages and is essential for
macrophage trafficking. Both the number of infiltrating macrophages and the
extent of CNV was significantly less in the knockout mice compared to wild type
mice. These observations suggested that the neovascularisation process depended
much more on recruited, and not resident, macrophages. Further strengthening
this observation, Sakurai et al found that depletion of circulating monocytes using
intravenous clodronate reduced both CNV lesion volume and leakage (as
measured by fluorescein angiography) 229.
To demonstrate that the recruited macrophages were derived from circulating
monocytes, Caicedo et al used fluorescently‐labelled monocytes transplanted into
the bone marrow of irradiated mice 230. Care was taken to reduce the laser intensity
47
and duration to limit its effect to Bruch’s membrane and minimise retinal injury.
Blood‐derived monocytes in this study infiltrated the retina overlying CNV three
days after laser photocoagulation. Activation of the overlying Muller cells
(evidenced by their expression of c‐fos and phosphorylated extracellular signal‐
regulated kinase) occurred secondary to macrophage infiltration. Muller cell
activation was abolished after depletion of circulating monocytes using
clodronate.
In summary, the laser‐induced model of CNV has produced a number of
important observations. Firstly, it is likely that circulating monocytes, and not
resident immune cells, are responsible for the inflammatory damage immediately
after Bruch’s membrane disruption. Indeed, infiltrating macrophages may activate
local cells 230. Secondly, recruited macrophages express VEGF and the extent of
neovascularisation and leakage depends on the extent of macrophage trafficking
and infiltration. However, laser photocoagulation tends to destroy the outer
retina, along within Bruch’s membrane and choroid. In any case it is a significant,
acute traumatic event very unlike the chronic, but sub‐lethal cellular changes that
take place in AMD. Not surprisingly, the first cells to infiltrate laser‐induced CNV
lesions are neutrophils, which are conspicuously absent in AMD lesions. As
discussed below, the Ccr‐2 knockout mice used to illustrate the dependence of
CNV on macrophage recruitment turn out to behave very differently when left to
age without intervention.
2.4.3 Choroidal macrophage recruitment and turnover and AMD
2 . 4 . 3 . 1 C c l ‐ 2 , C c r ‐ 2 a n d Cx 3C r 1 k n o c k o u t m i c e a n d AMD ‐
l i k e l e s i o n s
In 2003, Ambati et al reported that knockout mice deficient in either MCP‐1 (Ccl‐2)
or its receptor Ccr‐2, developed AMD‐like lesions when left to age beyond 9
48
months 231. These included RPE lipofuscin accumulation, RPE degeneration,
photoreceptor fall out and the development of drusen and choroidal
neovscularisation; features not seen in wild type mice even beyond 24 months of
age. Evidence of IgG and C3c in choroidal vessel walls of the knockout mice
suggested immune complex deposition. C5, serum amyloid P protein and
advanced glycation endproducts were also immunolocalised on RPE or the
choroids of knockout mice. In wild‐type mice, an age‐dependent increase in Ccl‐2
expression by RPE cells, together with an age‐dependent increase in choroidal
macrophages, suggested that Ccl‐2 – Ccr‐2 interaction was critical to normal
choroidal macrophage recruitment. This study also found that choroidal
macrophages from wild type mice were able to degrade C5 and IgG deposited on
the choroids or RPE of knockout mice. Together, data from this report suggests
that resident choroidal macrophages play a critical role in the elimination
opsonised debris in Bruch’s membrane. Macrophages expressing scavenger
receptors for oxidised lipoproteins are seen found in human AMD eyes 232.
Disruption of the normal recruitment or turnover of choroidal macrophages may
lead to persistence of both the debris and the opsonising molecules C5 and IgG,
resulting in lesions similar to those found in AMD. AMD‐like lesions were also
found in mice deficient in Ccl‐2 and CXC3R1 (fractalkine, a chemokine receptor
involved in leukocyte trafficking) 233, further reinforcing this view.
These reports highlight the differences between age‐related pathological changes
and those that result from laser disruption of Bruch’s membrane in mouse models.
While CNV is reduced in Ccl‐2 (MCP‐1) knockout mice after laser
photocoagulation, it develops spontaneously when the same mice are left to age
beyond 9 months.
49
2 . 4 . 3 . 2 I L ‐ 1 0 k n o c k o u t m i c e a n d l a s e r ‐ i n d u c e d CNV
An interesting, but counterintuitive finding in interleukin‐10 (IL‐10) knockout
mice was recently reported by Apte et al 234. IL‐10 is a major immunosuppressive
cytokine that programs macrophages along the non‐inflammatory, “repair”
pathway (discussed in Section 2.4.3.4). These authors reported that the knockout
mice developed less extensive laser‐induced CNV compared to wild type mice.
Lack of IL‐10 would be expected to permit macrophage programming along the
pro‐inflammatory pathway, resulting in more CNV. The authors explain their
unexpected observations by proposing that it is the inhibition of normal
macrophage recruitment (e.g. by local IL‐10) that promotes CNV. That is, a lack of
macrophage recruitment results in increased debris deposition in Bruch’s
membrane. This in turn results in the increased deposition of acute phase reactants
and other opsonising components of the innate immune system, leading to further
activation of damaging inflammatory cascades.
2 . 4 . 3 . 3 P o l ym o r p h i sm s i n l e u k o c y t e r e c r u i tm e n t a n d t u r n o v e r g e n e s
A number of genetic studies have supported the observation that normal
macrophage recruitment and function is important in the maintenance of a healthy
and debris‐free Bruch’s membrane. Goverdhan et al examined polymorphisms in
the human leukocyte antigen gene and AMD risk. The authors found one at‐risk
allele and two protective alleles, as well as differential expression of HLA antigens
in the choroid 86. HLAs are important molecules for macrophage recognition of
antigen. The HLA genes are the most polymorphic in the human genome, and the
mechanism by which the confer susceptibility to AMD is not yet clear. These
findings, however, provide further evidence for the involvement of immune or
inflammatory processes in AMD pathogenesis.
50
A single gene polymorphism in the CX3CR1 gene was also found to be associated
with AMD 84. Donor eyes with the “at risk” allele had reduced fractalkine
transcripts in the macula region. Since fraktalkine is an important chemokine in
monocyte transendothelial migration 235, the authors concluded that aberrant
macrophage recruitment may be implicated in AMD pathogenesis. Finally, a
single gene polymorphism of the toll‐like receptor 4 (TLR‐4) gene was also
associated with AMD 85. TLR‐4 plays an essential role in innate immunity, and
mediates macrophage recognition and phagocytosis of microbial invaders 236.
Interestingly, TLR‐4 appears to also involved in RPE photoreceptor phagocytosis
and in cholesterol transport 237, 238.
2 . 4 . 3 . 4 S e r um my e o l o i d c e l l s a n d AMD
There is some epidemiological evidence implicating circulating myeloid cells in
AMD, suggesting a higher systemic inflammatory “set point” A higher white cell
count was found in patients with neovascular AMD and disciform scars compared
to controls in a small case‐control study in 1986 239. High white cell count was also
associated with neovascular AMD 240, and with the 10‐year incidence of large
(>125μm) drusen in the Beaver Dam Study 241. Finally, monocytes isolated from
patients with neovascular AMD expressed more TNFα when stimulated with RPE
blebs in vitro compared to controls 242.
2 . 4 . 3 . 5 Ma c r o p h a g e s i n a n i mmun e ‐ p r i v i l e g e d c om p a r tm e n t
Cells of the macrophage/monocyte lineage can vary widely in function,
morphology and immuno‐phenotype depending on the tissue microenvironment
in which they are found. In broad terms, macrophages can be activated along a
“classical” or “alternate” pathway (reviewed in 243). Classical activation follows
stimulation with bacterial lipopolysaccharide and IFNγ or with TNFα. These
macrophages express the enzyme inducible nitric oxide synthase (iNOS), which
51
allows them to convert arginine to nitric oxide, producing nitrite, peroxynitrites
and superoxides. These powerful oxidants cause lipid peroxidation of affected cell
membranes, leading to cell death. Alternate activation of macrophages switches on
a different metabolic program, which is TGFβ1‐dependent. Macrophages thus
programmed express the enzyme arginase, which converts arginine to ornithine,
promoting cell division and repair.
A rich network of macrophages and dendritic cells has been found in the choroid
of mice 244. The dendritic cells appear to be in the “immature” phase of their life
cycle, similar to the Langerhans cells of skin, suggesting that their function in the
normal choroid is antigen capture and not antigen presentation. Although
definitive data is not available for human eyes, it is likely that similar networks
also exist. Resident choroidal dendritic cells and macrophages appear to be pre‐
programmed to resist classical activation, thus limiting inflammation and
promoting repair. In culture, dendritic cells and macrophages isolated from rat
and mouse choroid failed to present antigen 245. And, although the immunological
specialisations of the choroid are less well characterised than the retina and other
ocular compartments, there is evidence that local inflammation‐suppressing
immune modulation exists, primarily via RPE cells (see Section 2.2.1.7).
In culture, RPE produce of thrombospondin‐1 and TGFβ, and can directly
suppress T‐cell activation 246. Mouse eye cup RPE cells produced PEDF and IL‐10,
potent immunosuppressive cytokines. The supernatants from these RPE cells
inhibited activated macrophages by reducing their production of IL‐12 (a pro‐
inflammatory cytokine) and increasing their production of IL‐10 (an
immunosuppressive cytokine) 164. This effect was neutralised by anti‐PEDF
antibodies. Ingestion of oxidised photoreceptor outer segments by the human RPE
cell line ARPE‐19 in culture resulted in increased expression of MCP‐1, but not
TNFα 122, suggesting that RPE can recruit macrophages without activating them.
52
In pathological circumstances, it appears that choroidal macrophages can be
classically activated. Hattenbach et al found that macrophages in the eye of a 70
year old woman with neovascular AMD expressed iNOS 247. Endothelial cells
within the neovascular membranes of the other 6 CNV specimens in this study
also expressed iNOS. Although the macrophages in this study were identified by
morphology alone, the results suggest that CNV‐associated macrophages in
human eyes are phenotypically different to resident choroidal macrophages. Their
ability to produce nitric oxide can not only damage local cells, but may also
stimulate angiogenesis. Ando et al found that VEGF‐mediated retinal and
choroidal neovascularisation in mice depended on local nitric oxide production 248.
And finally, in the experimental autoimmune uveitis mouse model, choroidal
macrophages (identified by ED1 labelling) expressing iNOS were seen 11 days
after sensitisation with retinal S antigen, coinciding with the peak of inflammation
249. Thus, iNOS‐expressing choroidal macrophages may be associated with both
inflammation and neovascularisation.
In the context of AMD, the influence of the progressive accumulation of
extracellular deposits on local populations of macrophages, both recruited and
resident, is not well defined. Section 3.2 examines the influence of BLamD and
membranous debris on macrophages found within Bruch’s membrane and in the
choroid.
2.4.4 Dendritic cells and drusen biogenesis
Although dendritic cell networks are likely to be present in the normal human
choroid, the functional role of choroidal dendritic cells is at present poorly
characterised. Hageman and associates have proposed that sub‐RPE debris can
attract the processes of cells on the choroid side of Bruch’s membrane and that
these cells share immunophenotypic features with dendritic cells 250. Unfortunately
53
they reported CD68 and HLA‐DR immunoreactivity in drusen, an
immunophenotype more typical of macrophages. These authors propose that
dendritic cell processes are attracted to injured RPE or diffuse sub‐RPE debris,
becoming a focus for deposition of inflammatory proteins which eventually results
in the formation of drusen. While an interesting pathogenic mechanism, there has
so far been insufficient evidence to support it.
54
2 . 5 I N F L A M M A T I O N A N D A M D
2.5.1 Tissue evidence of inflammatory proteins
2 . 5 . 1 . 1 Ev i d e n c e o f i n f l amma t i o n ‐ a s s o c i a t e d p r o t e i n s i n d r u s e n
As discussed in Sections 2.2.3.2 and 2.2.3.4, drusen contain lipids and protein.
The protein constituents of drusen so far identified can be roughly divided,
with some overlap, into three groups: (i) cytoskeletal and basement
membrane proteins, such as crystallins, collagens I‐V, laminin, fibronectin,
vitronectin and proteoglycans 251, 252; (ii) proteins associated with lipid
metabolism (apolipoproteins) 184, 253; and (iii) inflammation‐associated
proteins, such as immunoglobulins, HLA‐DR, C‐reactive protein (CRP),
vitronectin, complement cascade components, clusterin, crystallins, amyloid,
serum amyloid P, albumin, factor X, thrombin and fibrinogen (see below).
Cytoskeletal proteins and those associated with lipid turnover may result
from extrusion by damaged RPE cells unable to meet the metabolic demands
of constant photoreceptor outer segment turnover (see Section 2.2.3.4).
Similarly, the production of excess basement membrane can be expected
from epithelial cells under stress. However, it is the inflammation‐associated
proteins that have the most obvious potential to influence the course of
disease, due to their capacity to activate damaging inflammatory cascades.
The following sections review the inflammation‐associated proteins found in
drusen and Bruch’s membrane to date. It should be emphasised that
interpretation and comparison of such studies is complicated by differences
in drusen phenotype (see Section 2.2.3.3). Where defined, the majority of
studies involve hard drusen, and often those found in the peripheral retina.
Nonetheless, the findings have contributed to a growing understanding of
the inflammatory pathways elicited by the presence of extracellular debris
within the Bruch’s membrane‐choroid complex.
55
2.5.1.1.1 Immunoglobulins and complement in drusen and Bruch’s membrane
Immunoreactivity to IgM was found to be strong in macular hard drusen by
Newsome et al 251, while IgG immunoreactivity was prominent in Bruch’s
membrane but less strong in drusen. Johnson et al 191 also demonstrated IgG
immunoreactivity in small, hard drusen in normal aged eyes, as well as in
the RPE that overly or flank these drusen. Interestingly, both the complement
component C5 and the terminal complement complex (membrane attack
complex) C5b9 were found in drusen and adjacent RPE, leading the authors
to propose that antibody‐mediated complement attack of RPE may result in
drusen formation.
The complement system consists of at least 20 proteins, found in serum in
inactive form (Figure 2‐2). The system is involved in both innate and
adaptive immunity, and in the removal of both microbial invaders and
extracellular debris. Formation of the final product, the membrane attack
complex (C5‐9) is triggered by activation of the critical element, C3, which
can occur in three main ways: (i) the classical pathway, by C1q binding to the
Fc region of IgM or IgG; (ii) the alternate pathway, by microbial elements,
aggregated immunoglobulins, polysaccharides and others; and (iii) the lectin
pathway, by collectin binding to viral or bacterial carbohydrate‐containing
proteins. The membrane attack complex (MAC) forms transmembrane
channels when it binds to target cells, leading to cell lysis. Proteolytic
fragments of the complement cascade, particularly those of C3 and C5, are
potent inflammatory mediators, and can be activated by proteolytic enzymes
within inflammatory exudates, independently of the complement cascade.
C3a and C5a cause vasodilation and increased vascular permeability via
mast cell degranulation. C3b and C3bi are opsonins, facilitating phagocytosis
of their targets by macrophages and neurtrophils, which possess cell surface
56
C3b receptors. Additionally, C5a is a powerful promoter of leukocyte
adhesion, chemotaxis and activation. To prevent self‐perpetuating
inflammatory damage, the complement system is closely regulated by
inhibitory proteins. These act primarily as inhibitors of C3 and C5
convertases, although regulatory proteins that prevent C1 binding to
immune complexes or MAC formation also exist 254.
Figure 2.5‐1 The complement cascade
Adapted from Cellular and Molecular Immunology 254
The presence of terminal complement components and IgG in drusen
provided tissue evidence that the complement system may trigger
inflammatory sequelae important in AMD pathogenesis via the classical
C1
C4 + C2 C4b2a C4b2a3b
C5-9Membrane Attack Complex (MAC)
C3 C3b C3bBb C3bBb3b
C3 C3b C5 C5b
Activated C1
IgG or IGM immune complex Acute phase proteins
+C6 +C7 +C8 +C9
Classical Pathway
Alternative PathwayMicrobial surfaces
Lipopolysaccharides
Factor BFactor B
Factor HFactor H
Accelerates decay of C3bBb
57
pathway (i.e. immune complexes). This hypothesis was further supported
when two complement inhibitory proteins were found in aged normal eyes
and eyes with GA: clusterin (in hard drusen) and vitronectin (in RPE
surrounding hard drusen and sub‐RPE deposits resembling basal mounds)
192. In a separate study, the same author group co‐localised IgG and C5b‐9
immunoreactivity within hard drusen (and on the internal aspect of
choroidal capillary walls) 255.C5b‐9 immunoreactivity was also found on the
basal RPE membrane, suggesting that RPE cells were targets of MAC‐
mediated attack. Interestingly, only C5 immunoreactivity was demonstrated
in the one soft druse presented in this study.
2.5.1.1.2 Amyloid, fibrinogen and other inflammatory proteins found in drusen
Immunofluorescence studies identified the coagulation proteins, thrombin,
fibrinogen and factor X in drusen in 253. RNA transcripts for fibrinogen and
factor X were amplified in the in RPE/choroid of AMD eyes in this study,
suggesting local production. Factor X, thrombin and fibrinogen are proteases
involved in the final common pathway of the clotting cascade. Since there is
a well‐recognised association between vascular injury, inflammation and
coagulation/fibrinolysis, these findings support a role for choroidal vascular
dysfunction (in addition to inflammation) in AMD pathogenesis.
C‐reactive protein, amyloid P component, α1‐antitrypsin and vitronectin are
acute phase proteins, and have all, to various degrees, been localised in
drusen by immunofluorescence techniques 190, 253. Acute phase proteins are
inflammatory mediators released in response to both microbial invasion and
tissue injury. One of their functions is to recognise molecules released by
apoptotic or necrotic cells, such as nucleic acids, mitochondrial elements,
membrane proteins, modified lipids, cholesterol, cytoskeletal proteins and
proteins associated with cellular stress 256. Acute phase protein binding to
58
microbial or cellular elements activates the complement cascade by the
classical pathway 257, 258. This process is called “opsonisation”, and allows the
rapid removal of potentially immunogenic debris from the extracellular
compartment by phagocytic cells. The presence of HLA‐DR, an MHC class II
molecule required in the capture of opsonised antigen by macrophages, has
also been demonstrated in drusen and sub‐RPE debris 253.
The abnormal extracellular deposits in found in early AMD (BLamD and
membranous debris), together with damaged RPE, are obvious candidates
for binding by acute phase proteins, immunoglobulins and complement.
That these deposits persist in AMD eyes despite the presence of opsonisation
molecules, HLA‐DR and macrophages suggests dysfunction somewhere
along the opsonisation‐phagocytosis axis. This may be in: (i) the control of
the acute phase response or complement cascade; (ii) the strength of the
acute phase response proportionate to the amount of debris present; (iii)
failure of phagocytosis; or (iv) failure of local recruitment or turnover of
phagocytes. As described in the previously (Section 2.4.2), human genetic
evidence and evidence from transgenic mice implicate dysfunctional
macrophage recruitment and turnover in AMD and the development of
AMD‐like lesions. Human genetic evidence has also strongly implicated the
complement cascade in AMD pathogenesis (Section 2.1.5.5). Interestingly, the
genetic evidence implicates defective regulation of complement activation
via the alternate pathway, and not the classical pathway. The next section
discusses the functional consequences of polymorphisms in complement
proteins.
59
2.5.2 Genetic associations
2 . 5 . 2 . 1 S i n g l e nu c l e o t i d e p o l ym o r p h i sm s i n c om p l em e n t c a s c a d e a n d c om p l em e n t r e g u l a t o r y g e n e s
In 2005, four author groups independently published data reporting an
association between AMD risk and a single nucleotide polymorphism in
complement factor H (CFH) gene 87‐90. These observations are particularly
strong, considering the participants were derived from four separate
(American Caucasian) populations, and distinct methods of gene screening
were used. The studies included participants with early, neovascular or
atrophic AMD.
CFH is the major soluble inhibitor of the alternate complement pathway,
preventing C3 activation by proteolytic cleavage of C3b. Single nucleotide
polymorphisms are variations in a single nucleotide within the genome, and
are commonly found (approximately 1 in 1250 base pairs). The Tyr402His
variant of CFH, in which histidine takes the place of tyrosine at amino acid
402, was found to increase an individual’s risk of AMD 2‐4 fold in
heterozygotes and 5‐7 fold in homozygotes. The tyrosine‐histidine
substitution is purported to affect the heparin and CRP binding site of CFH,
diminishing its ability to keep the complement cascade in check. Association
of the Tyr402His variant was found to be strongest for individuals with early
and neovascular AMD.
Studies in other ethnic groups have not corroborated closely the findings
from the Caucasian populations. No association was found between the CFH
Tyr402His polymorphism and Japanese patients with atrophic AMD 259,
neovascular AMD or disciform scars 260. Additionally, the Tyr402His allele
was associated with bilateral, but not unilateral early AMD in a US‐based
Latino/Hispanic population 261. Even within Caucasian populations, disease
60
risk for the allelic variants has not yet been adequately tested in large
prospective studies.
More recently, analysis of polymorphisms in the Factor B (FB) and C2 genes
found two “protective” and one “risk” haplotype 95. BF binds C3b, and its
cleavage ultimately results in the formation of C3 convertase (C3Bb), while
C2 is an activator of the classical pathway. The BF and C2 genes are situated
only 500bp apart within the major histocomaptibility (MHC) class III locus,
and the gene products have almost identical molecular structures, both
regulating the production of C3 (the critical complement cascade
component). Thus it is as yet unclear whether the association with AMD is
due to the BF or C2 polymorphisms. The authors suggest however, that the
BF polymorphism may be causal, since one of the protective alleles produces
a gene product with reduced haemolytic activity, and BF immunoreactivity
within retina was similar to that of CFH, C3 and the MAC 95.
The genetic evidence appears to implicate the alternative complement
pathway while earlier immunohistochemical and immunofluorescence
studies implicated the classical pathway (which is activated by acute phase
proteins such as β‐amyloid, IgG, CRP, serum amyloid P). As discussed
earlier, CFH also binds CRP 262 and a recent study provides some clues on the
way CFH‐CRP interactions may influence the development of AMD. When
the distribution of CFH protein in the RPE, Bruch’s membrane and choroid
was assessed in donor eyes with the “at risk” CFH Tyr402His variant, no
differences were found compared to normal donor eyes 263. However, the
Tyr402His homozygotes did have significantly more CRP protein deposition
in the choroidal stroma than normals. Since the Tyr402His variant affects a
CRP binding site, these findings suggest the associated increased AMD risk
may be due to failure of CFH to bind and inactivate or remove CRP.
61
Excessive extracellular CRP deposition would act as a persistent
inflammatory stimulus, and an activator of the classical complement
pathway.
Some have suggested that genetic evidence implicating the alternative
pathway points to the possible involvement of pathogens, such as Chlamydia
pneumoniae, in disease pathogenesis. There is some evidence that previous
infection with c.pneumomiae is associated with AMD 264 and AMD
progression 265, and c.pneumonia is more frequently found in neovascular
lesions compared to normal eyes 266. Current evidence therefore, suggests
both the classical and alternative pathways of complement activation may be
implicated in AMD pathogenesis.
2.5.3 Serum inflammatory markers in AMD
A number of population‐based studies and case‐control studies have
examined the association between serum markers of inflammation and
AMD. These studies were undertaken partly because of the strong evidence
implicating serum markers of inflammation, such as CRP and homocysteine,
in cardiovascular disease (reviewed in 267), and partly because of the evidence
for acute phase protein deposition in Bruch’s membrane and drusen.
While Smith et al found that serum fibrinogen levels greater than 4.5g/L were
associated with both neovascular and atrophic late AMD in an Australian
population‐based study 268, Dasch et al failed to find any association between
early or late AMD and serum fibrinogen in a German population‐based
study, once cardiovascular risk factors were adjusted for 65. A small study
suggests reduced serum leptin, a protein involved in control of inflammation
and lipid metabolism, was associated with early and advanced AMD 269.
62
Seddon and colleagues found a significant association between CRP and both
“intermediate” and advanced AMD 67 in the AREDS study, and progression
to advanced AMD in a cohort of participants with non‐exudative AMD 68.
Apart from a small case‐control study in which participants with drusen,
neovascular or atrophic AMD were found to have higher CRP and
homocysteine levels than controls 270, others have not been able to replicate
these findings. In the Cardiovascular Health Study, no association between
CRP and either early 64, or advanced AMD 270 could be found. Dasch and
colleagues also failed to find an association between CRP and either early or
advanced AMD in the Muenster Ageing and Retina Study 65. A case‐control
study of participants from the Beaver Dam population could not find an
association between prevalent or incident early AMD and several
inflammatory markers, including CRP, IL‐6, TNFα or amyloid A and 63. Wu
et al found a small association between serum intercellular adhesion
molecule‐1 levels and late AMD, but found no association between early or
advanced AMD and CRP, and interleukin‐6, white cell count (WCC),
fibrinogen, homocysteine, plasminogen activator inhibitor‐1 or von
Willebrand factor in the Blue Mountains Eye Study population 70. It may be
that factors associated with cardiovascular disease have an inverse, or at least
more complex, association with AMD. Certainly polymorphisms in the ApoE
(a lipoprotein), ABCA1 (a cholesterol transporter) and TLR4 (a receptor
important in innate immunity and cholesterol efflux) genes which are
associated with increased risk of atherosclerosis are protective for AMD 85.
Autoantibodies, found in serum and directed against retinal and other
proteins, have also been associated with AMD. However, their putative role
in AMD pathogenesis remains controversial.
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2 . 6 S E R U M A N T I - R E T I N A L A U T O A N T I B O D I E S I N A M D
2.6.1 Autoantibodies and autoimmunity
Autoimmunity is classically described as the failure of self‐tolerance. The
ability of the immune system to distinguish “self” from “non‐self” is
essential to its normal function. Antibodies are produced by B cells as part of
the acquired (or specific) immune response, after exposure to antigen.
Antibody binding to the antigen (immune complex formation) activates the
complement cascade, leading to complement‐mediated lysis of affected cells,
and recruitment and activation of inflammatory cells (neutrophils and
monocytes) 271. Autoimmune diseases arise when cells or antibodies of the
immune system recognise and bind self‐antigens.
Immunoglobulins that recognise self antigens are known as “autoantibodies”
and are typically considered pathological. They can cause tissue damage in
two recognised ways. Autoantibodies may form complexes with circulating
antigens, which eventually deposit in vessel walls and/or tissues. An
example of this occurs in systemic lupus erythematosus, whereby
autoantibodies bound to DNA, nucleoproteins and other proteins deposit to
cause nephritis, disseminated vasculitis and arthritis. Autoantibodies can
also directly bind antigens in tissues to cause damage, such as when anti‐
type IV collagen antibodies cause pnuemonitis and glomerulonephritis in
Goodpasture’s syndrome. Both mechanisms lead to the same downstream
consequences, i.e. cell lysis, inflammatory cell recruitment and opsonisation
and phagocytosis of affected cells.
Direct binding to a tissue antigen can also alter the functional properties of a
target protein. In myasthenia gravis, autoantibodies cause paralysis by
binding acetylcholine receptors, preventing neurotransmission at muscle
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endplates. In Grave’s disease, autoantibodies bind and activate receptors for
thyroid stimulating hormone, leading to thyrotoxicosis.
More recently however, autoantibodies have been found in association with
a number of degenerative diseases, such as Alzheimer’s disease 272 and
atherosclerosis 273. In the strictest definition of an autoimmune disease, the
following criteria, postulated by Witebsky 274 should be satisfied: (i) presence
of circulating autoantibodies; (ii) identification of the corresponding
antigen(s); (iii) induction of autoreactivity against these antigens in an
experimental model, leading to (iv) tissue pathology similar to the human
disease; and (v) initiation of disease following adoptive transfer of the self‐
reactive antibodies or lymphocytes. Autoantibodies found in association
with degenerative diseases do not of course, fulfill these criteria.
Degenerative diseases cannot thus be considered primary autoimmune
diseases. However, the findings do raise questions about definitions of
autoimmunity, and about the involvement of the immune system in the
setting of chronic degenerative disease.
Autoantibodies have also been found in association with AMD. Before
reviewing this evidence, autoantibody associations with several other ocular
diseases will be discussed.
2.6.2 Autoantibodies and ocular disease
2 . 6 . 2 . 1 An t i ‐ r e c o v e r i n a u t o a n t i b o d i e s , CAR a n d a u t o immun e r e t i n o p a t h y
Probably the best known autoantibody in ocular disease is the anti‐recoverin
autoantibody, which is well described in cancer‐associated retinopathy
(CAR). This is a paraneoplastic syndrome in which autoantibodies against
the 23kD protein recoverin cause unexplained vision loss in patients with
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diagnosed or occult tumours 275, 276, primarily small cell lung cancers.
Recoverin is a photoreceptor protein involved in the visual cycle. Tumour
expression of recoverin was thought to elicit the production of anti‐recoverin
antibodies which cross‐reacted with those found in photoreceptors, causing
cell death 277‐279. Typically, CAR patients have profoundly abnormal
electroretinograms.
Anti‐recoverin autoantibodies were thus thought to be specific for CAR.
However, later studies showed that anti‐recoverin autoantibodies could be
found in other ocular diseases, and that autoantibodies against other proteins
were found in CAR. Whitcup et al found anti‐recoverin autoantibodies in
patients without cancers but with symptoms clinically similar to CAR 280.
This entity became recognised as “recoverin‐associated retinopathy” or
“autoimmune retinopathy” 281. Anti‐recoverin autoantibodies were also
found in patients with retinitis pigmentosa, a hereditary retinal degenerative
disorder 282. CAR patients were later found to have autoantibodies against
enolase 283, 284 and heat shock protein 70 (hsp70) 285 (anti‐hsp 70 autoantibodies
induced CAR‐like electroretinogram changes when were injected into the
vitreous of rats 286). Up to 15 or more autoantigens have now been described
in CAR 287.
Melanoma‐associated retinopathy (MAR) is another paraneoplastic
syndrome whereby tumour expression of aberrant proteins leads to the
development of autoantibodies against retinal antigens 287. ERG changes
differ to that of CAR, although multiple autoantigens have been reported,
including a 35kD Muller cell antigen and a 22kD neuronal antigen 287, as well
as putative antigens retinal transducin, rhodopsin, visual mitofilin and titin
288.
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2 . 6 . 2 . 2 R e t i n a l d e g e n e r a t i o n s , a u t o immun e r e t i n o p a t h y a n d g l a u c om a
Immunohistochemical studies have shown increased serum reactivity
against normal human retinae for a number of retinal diseases, including
retinitis pigmentosa, diabetic retinopathy 289, 290 and cystoid macular oedema
291. Autoantibodies against enolase were found to be associated with
glaucoma 292‐294 and autoimmune retinopathy 295. More recent studies of
serum from glaucoma patients have also revealed complex patterns of
reactivity against retinal and optic nerve antigens 296, 297. These studies were
the first to show that the same complex reactivities were present in up to
100% of normal controls.
It is thus increasingly recognised that the presence of serum autoantibodies is
not necessarily a sign of pathological autoimmunity. However, the
significance of autoantibody associations in ocular diseases is still being
clarified.
2.6.3 Autoantibodies in AMD
Using direct immunofluorescence, Chant and colleagues noted in 1987 298,
that IgM autoantibodies from two patients with “macular degeneration”
bound to human retinal tissue. A larger study by Penfold et al 299 showed that
autoantibody binding to human tissue was present in 20% of normal
controls, 29% of patients with geographic atrophy, 52% of patients with
disciform lesions, 62% of patients with drusen and 78% of patients with
pigmentary disturbance. The autoantibody staining pattern on retinal tissue
in this study resembled that of glial acidic fibrillary protein (GFAP), a
cytoskeletal protein found in neurons, astrocytes and Muller cells. By
contrast, a later study 300 found that 47% of patients with macular
degeneration (including patients with neovascular AMD, atrophic AMD and
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bilateral drusen) had autoantibodies directed mainly to photoreceptor outer
segments. Western blot experiments by the same group showed that the
autoantibodies reacted with a protein between 58‐62kD, and cross‐reacted
with a bovine neurofilament protein. In 1993, Chen and co‐workers reported
that serum anti‐retinal autoantibodies detected by western blotting were
found in 66% of patients with “AMD”, compared with 18% of controls. The
autoantibodies reacted with peptides and proteins occurring in five different
molecular weight bands (Table 2.6‐1).
Gu and colleagues used ELISA to test autoantibody binding to
carboxyethylpyrrole protein (CEP) adducts 301, a product of lipid
peroxidation which, although not specific to retina, was identified as a
drusen component by earlier proteomic studies 202. They found that 53% of
“AMD” patients (compared to 21% of controls) had high titres of anti‐CEP
autoantibodies. Patel et al correlated immunohistochemical staining patterns
of serum autoantibodies with AMD disease classification 302. Autoantibodies
reacted with multiple retinal elements, including the inner and outer nuclear
layers, the inner and outer plexiform layers and the ganglion cell layer (with
some autoantibodies targeting membranes, others nucleoli and still others
nuclear membranes or photoreceptor inner segments). Autoantibodies also
reacted with RPE, Bruch’s membrane and choroid. The highest proportion of
autoantibody reactivity was seen in patients with large drusen and
pigmentary abnormalities.
Joachim et al recently found complex anti‐retinal autoantibody binding
patterns on western blots in both control and neovascular AMD sera 303.
Using statistical software to discriminate western blot densitometry
differences, AMD sera was found to have densitometry peaks at 46 and
52kD, and troughs at 18 and 36kD. Tandem liquid choromatography/mass
68
spectrometry was used by this author group to identify the 18kD antigen as
α‐crystallin, the 46kD antigen as α‐enolase and the 52kD antigen as GFAP.
Interpretation of these studies is complicated by several factors: (i) the
different detection methods used (immunofluorescence, western blotting and
ELISA); (ii) the different serum dilutions assayed (1:20 – 1:1000); and (iii) the
various clinical AMD phenotypes (some poorly defined) involved in
different studies (summarised in Table 2.6‐1). Most frustratingly, there
appears to be not one or two, but many, retinal antigens implicated. Only a
few of these are characterised or partially characterised 300, 301, 303. Many, if not
all, of the antibodies/antigens so far identified have not been shown to be
specific to AMD. Most perplexing of all, the very same autoantibodies are
found in a large percentage of (and in some cases, all) normal control sera,
making it difficult to argue that they are pathological.
One explanation is that only high autoantibody titres are associated with
disease. The evidence for anti‐CEP autoantibodies appears to support this
view 301. However, Joachim et al found that certain autoantibodies were
found in significantly lower amounts (based on western blot densitometry)
in AMD sera compared to controls 303. This observation can only be explained
by accepting that a least some of the autoantibodies detected are naturally
occurring autoantibodies, or “natural autoantibodies”.
Natural autoantibodies are present in all normal individuals 304, 305. They are
encoded by germline B cell genes, and are produced in the absence of
immunisation with antigen 306. Thus, they are considered part of the innate
immune system and react with a large number of self‐epitopes 307. An
individual’s natural autoantibody repertoire appears to be stable over time
308.
69
Some natural autoantibodies recognise highly conserved pathogen‐
associated molecular patterns, such as phosphorylcholine, which allow them
to neutralise invading pathogens rapidly. However they also react with
cytoskeletal proteins, nuclear proteins and DNA 309, suggesting that they are
involved in the removal of apoptotic cells, cellular debris and other altered
self antigens. In fact, immunoadsorbent assays show that the majority of
antibodies in human sera are self‐reactive 310. Mono‐reactive, highly specific
antibodies generated by adaptive immunity appear to represent a very small
proportion of total serum immunoglobulin. This is consistent with the
emerging view that innate immunity evolved not only as first line defence
against pathogens, but for the equally critical role of “housekeeping”, or
keeping the extracellular compartment “debris‐free”.
70
Table 2‐3 Autoantibodies in AMD: summary of studies
Year Authors AMD phenotype Method(s) Serum dilution Autoantibody Ig
classAutoantibody targets % Postive
(disease sera)% Positive (control
sera)
1985 Chant et al "macular degeneration" (n=2) Direct immunofluorescence using human retinae not specified IgG, IgM Rod outer segments [1] 50% 2%
1990 Penfold et al
Drusen (n=13) Pigmentary disturbance (n=40) Geographic atrophy (n=21) Disciform scar (n=44)
Direct immunofluorescence on human, rat and mouse
retinae1:10 not specified Staining similar to GFAP labelling,
targeting predominantly astrocytes 29-78% [2] 20%
Indirect immunofluorescence on human and monkey
retinae
1:2 1:4 IgG
Photoreceptor outer segments (prominent); outer plexiform and
outer nuclear layers (weak)
Western blotting using human, bovine and monkey
retinal homogenates
1:200 1:1000 IgG
Between 58-62kD. Cross reaction with 68kD bovine neurofilament
protein
1993 Chen et al "AMD" (n=41) Western blotting Not available [3] Not available 28-32kD, 38-42kD, 48-52kD, 62-65kD or 110-130kD 66% 18%
ELISA using synthesised carboxyethylpyrrole (CEP)
protein adducts [4]1:20 IgG CEP adducts 53% 21% [5]
Immunohistochemistry (to localise CEP in mouse and
human retinae)
CEP localised strongly to photoreceptor outer segments and RPE and weakly to inner plexiform
layer
Indirect immunofluorescence using murine retinal tissue
1:10 1:100 IgG
Choroid, Bruch's membrane, RPE, photoreceptors, inner nuclear layer,
ganglion cell layer83-94% [6] 9%
Western blotting using murine retinal homogenates 1:500 IgG 28-49kD
Western blotting using bovine retinal homogenates 1:40 IgG Complex staining, multiple
antigens in 10-180kD range 100% 100%
Liquid choromatography tandem mass spectrometry
alpha-crystallin (18kD); alpha-enolase (46kD); GFAP (52kD)
Joachim et al 2006
"weak or none"
2003 Gu et al
1991 Gurne et al
Wet AMD (n=39)
47%
2005 Patel et al
Bilateral drusen (n=5) Atrophic AMD (n=6) Exudative AMD (n=19)
"AMD" (n=19)
ARM (n=64) CNV (n=51)
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Table 2.6.1. Autoantibodies in AMD: summary of studies
[1] It is not clear whether this was the staining pattern for the two macular degeneration
patients or for the retinitis pigmentosa patients in this study
[2] Highest for pigmentary disturbance, lowest for geographic atrophy
[3] Chinese language journal
[4] Product of lipid peroxidation
[5] All controls had anti‐CEP autoantibodies. These percentages indicate high titre anti‐CEP
autoantibodies
[6] highest for bilateral drusen
72
In the context of AMD pathology, it is reasonable to postulate that the
presence of abnormal extracellular debris (in the form of BLamD, BLinD and
soft drusen), and cell signalling by damaged RPE, may lead to an increased
production of natural autoantibodies, in tandem with other opsonising
molecules of the innate immune system.
The antigenic targets so far characterised for AMD seem to support this view.
GFAP is a cytoskeletal protein normally expressed by astrocytes, and by
Muller cells after retinal injury as part of the neuroprotective gliosis reaction
Astrocyte and Muller cell GFAP expression has been shown to increase with
age, and particularly in the presence of drusen 311. Alpha‐crytallin is a
member of the small heat shock protein family, with molecular chaperone‐
like properties that prevent the precipitation of denatured proteins 312. Alpha‐
crystallin was one of the most frequently found proteins in drusen 202 and
appears to accumulate in Bruch’s membrane and the choroid in AMD eyes
313. Alpha‐enolase is an important glycolytic enzyme, as well as a heat shock
protein, produced in response to hypoxic stress 314. Interestingly, higher titres
of autoantibodies to α‐enolase are found in a number of autoimmune,
degenerative and inflammatory conditions 315. And, as already discussed,
CEP adducts are products of lipid peroxidation. The extracellular lipids
found in drusen are vulnerable to oxidation, and CEP adducts have been
found in drusen 202.
Immunoglobulins themselves have also been found in drusen 191, 251, 253,
Bruch’s membrane 251 and the abnormal RPE overlying drusen 191 in AMD
eyes. If they are indeed natural autoantibodies, they may have a protective
role in the removal of extracellular debris. Natural autoantibody‐mediated
clearance of debris may be particularly relevant under pathological
conditions that involve accumulation of stress‐induced structures, such as
73
atherosclerosis, where autoantibodies against heat shock proteins have been
shown to prevent plaque formation (reviewed in 213).
However, the setting of rapidly accumulating extracellular debris and an
altered immune microenvironment may also favour the development of
pathological antibodies. There is some evidence that pathological
autoantibodies (such as rheumatoid factor) may derive from the natural
autoantibody pool 316. The difficulty remains in determining which
autoantibodies are pathological, and which are protective. One way is to
examine whether there is “subclass restriction”, since restriction of
autoantibodies to one IgG subclass is a feature of pathological autoimmunity
317. Another way to determine whether autoantibodies are pathological is to
determine their association with disease progression. Chapter 4 summarises
preliminary studies that examine the autoantibody profile in normal controls
and in individuals with early, neovascular and atrophic AMD. Autoantibody
association with disease progression in early and neovascular AMD is also
examined.
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2 . 7 T H E S I S A I M S
The past two decades have brought valuable insights to AMD disease
pathogenesis. However, many questions remain. The definition of AMD
phenotypes, particularly early in the disease process, requires clarification.
The first part of Chapter 3 is a clinicopathological study of eyes with early
AMD, which sought to answer the following questions:
(i) What is the histopathological threshold that distinguishes normal
ageing from early AMD?
(ii) How do BLamD and membranous debris influence disease
progression and what are their clinical correlates?
(iii) Is there clinicopathological evidence for distinct early AMD
phenotypes?
An emerging theme in the pathogenesis of AMD is the contribution of
macrophages to the maintenance of choroidal/Bruch’s membrane health. The
second part of Chapter 3 examines the influence of BLamD and membranous
debris accumulation on both resident choroidal macrophages and Bruch’s
membrane macrophages. The following questions were addressed:
(i) In what histopathological context are Bruch’s membrane
macrophages first found?
(ii) What is the clinical fundus appearance when Bruch’s membrane
macrophages are present?
(iii) What is relationship between Bruch’s membrane macrophages and
subclinical neovascularisation?
(iv) Does the progressive accumulation of BLamD and membranous
debris alter the immunophenotype of Bruch’s membrane
macrophages and/or resident choroidal macrophages?
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A further emerging theme is the recognition of the significant role
inflammation plays in AMD pathogenesis. While there is growing evidence
implicating altered innate immunity in AMD, the relationship between anti‐
retinal autoantibodies and AMD remains poorly defined. Chapter 4
summarises preliminary studies which addressed the following questions:
(i) Does the anti‐retinal autoantibody profile differ significantly
between normal individuals and those with early AMD,
neovascular AMD or geographic atrophy?
(ii) Can baseline anti‐retinal autoantibodies predict progression to
advanced AMD in individuals with early AMD?
(iii) Can baseline anti‐retinal autoantibodies predict vision loss in
individuals with neovascular AMD?
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H I S T O P A T H O L O G I C A L S T U D I E S
3
77
3 . 1 R E L A T I O N S H I P O F B A S A L L A M I N A R D E P O S I T A N D
M E M B R A N O U S D E B R I S T O T H E C L I N I C A L P R E S E N T A T I O N O F
E A R L Y A G E - R E L A T E D M A C U L A R D E G E N E R A T I O N
3.1.1 Introduction
The development of interventional and preventative strategies in AMD will
depend on an understanding of the early stages of the disease. It is important,
therefore, to describe the evolution of the two deposits pathologically significant
to AMD, BLamD and membranous debris, in order to better understand the
threshold at which aging becomes AMD and to correlate their further
accumulation with changes in fundus appearance and visual acuity.
The basal deposits are not directly visible on fundoscopy and hence their
relationship to the clinical evolution of AMD is not well documented. The present
study reviews a clinicopathologic series of 132 eyes in which diffuse basal deposits
had been identified previously 10. Although membranous debris in the BLinD can
be demonstrated only by electron microscopy (EM), the focal basal mounds and
soft drusen can be identified by light microscopy, so that the respective influence
each exerts on the progress of degeneration can also be assessed in histologic
specimens. The type and thickness of BLamD are correlated with the sites of
membranous debris accumulation and with the clinical findings. The threshold at
which aging becomes early AMD is discussed.
78
3.1.2 Methods
3 . 1 . 2 . 1 P a t i e n t s a n d Ey e s
In earlier studies a large clinicopathologic collection of aged eyes had been
divided into histopathologic groups according to the appearance of the BLamD
under the macula 10. Briefly, eyes with no BLamD (group I) and patchy BLamD
(group II) were considered normal. The eyes chosen for the present study were
those with a thin (group III) or thick (group IV) continuous layer of BLamD,
considered to represent the pathologic changes preceding advanced AMD (groups
V and VI). A total of 132 eyes belonging to 75 patients (69 males and 6 females)
aged 67 to 97 years at death (mean 82 +/‐8 years) met the inclusion criteria (Table
3.1.1).
All the patients had been examined clinically, and follow up ranged from 0 to 120
months (mean 24 +/‐32 months). Examination included best corrected visual
acuity, direct fundoscopy and, unless unobtainable, fundus photography using the
Zeiss 30° fundus camera. Fluorescein angiography was carried out where
appropriate. Eyes in which the fundus could not be adequately visualized, or
demonstrated other pathology, were excluded. The last examination ranged from
two weeks to 75 months prior to death (mean 19 +/‐17 months).
The study was conducted in keeping with the tenets of the Declaration of Helsinki
and approved by the University of New South Wales Human Research Ethics
Committee. Written consent was obtained from all patients.
3 . 1 . 2 . 2 H i s t o p a t h o l o g i c a l Me t h o d s a n d De f i n i t i o n s
A review of archival material, including slides of 107 eyes prepared for light
microscopy (LM) and electronmicrographs of 25 eyes, was conducted. Preparation
79
of eye tissue is described in detail elsewhere 10, 179. Briefly, for LM, serial sections
8μm thick were cut horizontally through the disc and macula and every 10th
section stained and examined. Three sections closest to the centre of the fovea and
80μm apart were examined with a Leitz Dialux microscope. Objectives used were
the APO 25x (NA=0.65) and PL APO 40x (NA‐0.75). The microscope was
calibrated using a standard 0.01mm objective micrometer graticule (Olympus,
Japan) and measurements were obtained with an eyepiece graticule, one division
equaling 2.8μm with the 40x objective and 4.5μm with the 25x objective.
Histologically BLamD was distinguished most readily with the picro‐Mallory
method, an early type showing faint antero‐posterior striations and staining blue,
while a later type was hyalinized and stained red. These staining characteristics
are the result of the acid fuchsin (red staining) and aniline blue (blue staining)
components of the picro‐Mallory stain. Acid fuchsin is a smaller molecule than
aniline blue. Early type BLamD has a looser arrangement of molecules than late
type, allowing the smaller acid fuchsin to wash away while trapping aniline blue.
Late type BLamD is composed of more condensed material, imperveant to the
large aniline blue molecule while trapping acid fuchsin. BLamD was then graded
according to 2 parameters: thickness and staining characteristics. Maximum
BLamD thickness was recorded as thin if it was less than half the height of the
normal RPE (<7μm) and thick if greater (>7μm) 318. BLamD staining was graded in
the following degree of progression: i) blue‐staining early type only (Figure 3.1‐
1A); ii) patchy late BLamD appearing either as small, rounded inclusions
approximately 4μm in diameter lying within the early BLamD (Figure 3.1‐1C), or
as larger nodular elevations occurring singly or in rows on the internal surface of
the early BLamD, each nodule being overlain by a single RPE cell (Figure 3.1‐1D);
(iii) continuous late BLamD comprising segments ≥ 250μm in length (Figure 3.1‐
2B,C). On electron microscopy (EM) the earliest form of BLamD was fibrillar and
was continuous with the original basement membrane of the RPE, but the
80
predominant constituent of early BLamD was the banded form. This consists
primarily of long‐spacing collagen (Figure 3.1‐1B) accounting for the striations
seen on light microscopy. Late BLamD had a more condensed structure and
appeared to be produced in waves as the overlying RPE retracted (Figure 3.1‐2
inset).
Membranous debris was defined on EM as coiled membranes with a trilaminar
appearance (Figure 3.1‐3) and was assessed by EM in 25 eyes (from 18 patients,
mean age 79 +/‐7.14 years). The membranes often had a vesicular outline and may
have contained lipids lost during processing 182, so that the term “membranous” is
used herein as an ultrastructural description only. BLinD was defined as a layer of
this membranous debris lying between the RPE basement membrane and the
inner collagenous zone of Bruch’s membrane 126, 176, 179. The maximum number of
layers of BLinD was recorded for each eye examined by EM. Basal mounds
(Figure 3.1‐4) were defined on EM as pockets of membranous debris 176 lying
internal to the RPE basement membrane and early BLamD. In histologic sections,
they appeared a washed‐out pale blue with Picro‐Mallory staining, were at least
half the height of an RPE cell, and were mostly 1‐2 RPE cells wide. For each eye
the maximum number of mounds in any one of the 3 sections through the macula
was recorded.
Soft drusen were defined as focal accumulations of the BLinD with sloping
margins measuring over half the height of the normal RPE, up to 350μm wide
when confluent (Figure 3.1‐4), and up to 500μm wide in two cases with drusenoid
detachment. Like basal mounds they stain blue with the picro‐Mallory method,
but unlike basal mounds, soft drusen lay below the RPE basement membrane.
Drusen contents were recorded as predominantly membranous or predominantly
granular, those with granular contents being regarded as regressing.
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Hard drusen (or “nodular drusen” 126) had a globular, hyalinised appearance and
stained red with picro‐Mallory method. A few hard drusen are not considered
part of AMD. One to two per section were seen in 85 study eyes.
Histologic abnormalities of the RPE at the macula were graded as mild, moderate
or severe depending on the degree of irregularity, hypertrophy and
hyperpigmentation, or attenuation.
3 . 1 . 2 . 3 C l i n i c a l P a r am e t e r s
The best corrected visual acuity was converted to logMar for statistical purposes.
The fundoscopic appearance of the macula was graded as: (i) normal, which
included a few (< 5) small (<63μm) drusen; (ii) multiple small drusen (<63μm)
involving an area >125μm; (iii) intermediate drusen (63 – 124μm) with or without
pigment; (iv) large drusen ( ≥125μm) with or without pigment; or (v) pigment
abnormalities alone.
3 . 1 . 2 . 4 S t a t i s t i c a l Me t h o d s
Statistics were performed using SPSS for Windows (v15.0; SPSS Inc, Chicago, IL,
USA). Contingency tables of histopathological versus clinical parameters were
evaluated by the χ2 test. Comparison of mean age and visual acuities was
evaluated using the student’s t‐test. Correlations between BLamD type, BLamD
thickness, and histopathologic RPE abnormalities were performed using
Spearman’s correlation for non‐parametric data. A p ≤ 0.05 was considered
statistically significant.
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3.1.3 Results
The clinical and histopathologic features of the study eyes are summarised in
Table 1.
3 . 1 . 3 . 1 B a s a l l am i n a r d e p o s i t a n d RPE c h a n g e s
The RPE was not normal even in eyes in which BLamD was exclusively of the
early type, exhibiting a loss of uniformity and increase in pigmentation (Figure
3.1‐1A). However, it was the amount of late type BLamD that correlated
positively with increasingly severe histological RPE abnormalities (r = 0.435, p<
0.001), first appearing as patchy late BLamD in the form of inclusions (Figure 3.1‐
1C) and then nodules (or “nodular excrescences” 319) (Figure 3.1‐1D). In the
presence of severe RPE abnormality, late BLamD formed an unbroken layer
exceeding 250μm in length (Figure 3.1‐2B&C) – termed “continuous late BLamD”
here and also described as “diffuse thickening of the internal aspect of Bruch’s
membrane” 204. The late form always lay on or near the internal aspect of the early
type, closest to the base of the retracting RPE.
BLamD in Group III eyes formed a thin layer, ranging in thickness from 1.0 –
7.0μm (3.2 +/‐ 1.6μm) and in 66 of 95 eyes (69%), was exclusively of the early type.
Patchy late BLamD was present in the remaining 29 Group III eyes (31%). By
contrast, continuous late BLamD was found in all 37 group IV eyes (100%), and
ranged in thickness from 8 – 25μm (10.7 +/‐ 4.2μm). Overall, there was a strong
positive correlation between BLamD thickness and the proportion of late type
BLamD (r = 0.650, p < 0.001) (Table 3.1‐2).
83
Table 3‐1 Clinical and histopathological characteristics of study eyes
III IV
(thin continous BLamD) (thick continuous BLamD)Number of eyes/patients 95/53 37/22
Mean +/- SD 81 +/-8 84 +/- 6
Range 67 - 97 74 - 92
Sex M/F 49/4 20/2
Mean +/- SD 21 +/- 18 12 +/- 8
Range 0 - 36 1 - 28
Mean +/- SD 23 +/- 32 28 +/- 31
Range 0 - 120 0 - 99
Mean +/- SD 0.276 +/-0.228 0.611 +/- 0.401
Range 0.000 - 1.000 0.000 - 1.300
Normal 50 0
Pigment Only 16 20
Drusen* Only 18 0
Drusen* + Pigment 11 17
Number of eyes examined by EM 21 4
BLamD thickness (microns) 3.2 +/-1.6 10.7 +/- 4.2
Early Only 66 0
Patchy late 29 0
Continuous Late 0 37
*Clinical drusen defined as intermediate or large drusen
Visual Acuity (logMar)
Fundus Appearance
BLamD type
Histopathological Group
Age at death (years)
Interval between last visit and death (months)
Total clinical follow up
84
Figure 3.1‐1 Basal Laminar Deposit
85
Figure 3.1‐1 Basal Laminar Deposit
(A) Early type BLamD forming a blue‐staining continuous layer beneath the RPE
(asterisk). Note it is less than half the height of an RPE cell. Fundus at age 67 had shown
a few small soft drusen. Patient died aged 71. Bar marker 25μm.
(B) Electron micrograph illustrating changes between RPE and choriocapillaris (CC).
Early BLamD (bracket) lies internal to RPE basement membrane (vertical arrows) and
comprises mostly banded material resembling long‐spacing collagen. Other phenotypes
comprise a darker and denser material with an enveloping rim of pale fibrillar material.
Between the clumps of BLamD lie membrane fragments (horizontal arrow) that also form
a layer external to the basement membrane, the basal linear deposit (asterisk), and can be
traced even into Bruch’s membrane. Bar marker 1μm.
(C) Early type BLamD containing small hyalinised clumps of late type (arrows). These
stain red with picro‐Mallory and are found close to the RPE. Fundus at age 75 had
shown early focal hyperpigmentation related to small soft drusen, patient died age 76. Bar
marker 20μm.
(D) Further build up of late BLamD forms nodular excrescences on internal surface of
early type, each nodule overlain by single RPE cell. Fundus had shown pigment changes
and small soft distinct drusen 4 years before death at age 85.Bar marker 50μm
86
Figure 3.1‐2 Continuous Late Basal Laminar Deposit
87
Figure 3.1‐2. Continuous Late Basal Laminar Deposit
(A) Eye of 79‐year‐old man photographed 2 years before death, showing ring of pigment
clumps around foveal perimeter (arrow).
(B). Section through fovea of eye illustrated in (A), showing thick layer of late type
BLamD. Pigment clumps in fundus correspond to large, hyperpigmented RPE cells
(arrow).
Bar marker 200μm Picro‐Mallory stain.
(C). Higher magnification of (B). A thick confluent layer of late BLamD lies on internal
surface of early type.
Bar marker 30μm Picro‐Mallory stain
INSET. Electron micrograph of late type, showing amorphous structure apparently laid
down in waves as grossly abnormal RPE retracts from Bruch’s membrane.
Magnification x 1200.
88
Figure 3.1‐3 High magnification membranous debris
Electron micrograph of membranous debris (approx. X31 000) in the eye of an 81‐year‐old
woman with normal fundus and 20/30 vision. Higher magnification (inset) shows two
dark laminae enclosing a single electron lucent lamina (arrow), giving rise to a
ʺtrilaminarʺ appearance (approx. X 100 000).
89
Figure 3.1‐4 Basal mounds
90
Figure 3.1‐4 Basal mounds
(A, B, C) Section through macula of clinically normal eye of 79‐year‐old man. Thin
BLamD (arrowheads) contains several basal mounds that can be recognized histologically
as unstained spaces (arrows).
Bar markers A 60μm, B 50μm, C 50μm Picro‐Mallory stain
(D) Semithin section from eye of 80‐year‐old man with basal mounds (asterisk). Basal
linear deposit can be recognized as a narrow interval beneath the RPE and BLamD
(arrow).
Bar marker 30μm. Methylene blue and basic fuchsin.
E) Electron micrograph of basal mound internal to RPE basement membrane (arrows).
Patient aged 83, fundus had shown small drusen. Asterisk = BLamD. Bar marker 5μm
91
3 . 1 . 3 . 2 BL amD – c l i n i c a l c o r r e l a t i o n s
Clinically, 41 of 66 eyes (62%) with early BLamD had a normal fundus at the last
clinical examination. Eyes with early BLamD alone were 10.4 times (95% CI 4.4 –
24.6, p < 0.001) more likely to have a normal fundus than eyes showing any late
BLamD, clinical pigment changes being present in all those in which late BLamD
was continuous. Only 10 eyes had clinical pigment changes without evidence of
late BLamD, and 4 of these eyes were subsequently reclassified as adult vitelliform
lesions. Increasing formation of late BLamD was associated with poorer vision, the
mean logMar visual acuity for eyes with continuous late BLamD being 0.335 units
worse than the other eyes (95% CI 0.194 – 0.477, p < 0.001). Continuous late
BLamD also occurred in eyes that were on average 3 years older (95% CI 0.3 – 6
years, p = 0.013) than eyes with early BLamD (Table 3.1‐2).
3 . 1 . 3 . 3 Memb r a n o u s d e b r i s ‐ l o c a l i z a t i o n a n d c o r r e l a t i o n w i t h BL amD
Coiled membrane fragments were found at several levels reflecting their
presumed transit, i.e., blebbing from the basolateral RPE surface, scattered within
early BLamD, traversing the RPE basement membrane to form BLinD, and in the
inner and outer collagenous layers of Bruch’s membrane (Figure 3.1‐1B). BLinD
was present in all eyes examined by EM and ranged from 2 to 11 layers in
maximum thickness.
A greater quantity of debris formed basal mounds and soft drusen (Figure 3.1‐5).
Since basal mounds were found only in the presence of BLinD, and since they can
be recognized in histological sections, the mounds were considered a surrogate
marker for the presence of BLinD at the light microscopic level. Membranous
debris accumulation was limited to basal mounds in 44 eyes while in 50 eyes it
extended to soft drusen formation. Although basal mounds were seen in the
absence of soft drusen, soft drusen never occurred in the absence of basal mounds.
92
Basal mounds increased in number as early type BLamD thickened, but then
plateaued with the formation of late BLamD. On EM there was also loss of some
of their membranous contents. Additionally, soft drusen in eyes with continuous
late BLamD were predominantly of the granular type (12/15, 80%). This gradual
reduction in membranous debris mirrored the progressive degeneration of the
RPE and fallout of photoreceptors.
3 . 1 . 3 . 4 Memb r a n o u s d e b r i s – c l i n i c a l c o r r e l a t i o n s
Although histopathologically, soft drusen as small as 20μm were found, they
never occurred in the absence of larger drusen visible clinically. Thus,
histopathological soft drusen correlated very well with clinically observed
intermediate and large drusen (r = 0.953, p < 0.001). Only in 4 eyes (2 patients)
were soft drusen (intermediate‐sized) found on histological examination when the
fundus was clinically normal. The interval between last examination and death
was 5 and 3 years in these cases. There were also 5 eyes from 3 patients with
multiple small drusen clinically, that were found to be soft drusen on
histopathology.
In group III, the fundus appeared normal in 28 of 38 eyes (74%) in which
membranous debris was limited to BLinD, confirming that BLinD in conjunction
with early BLamD represents threshold AMD. Basal mounds did not appear until
early BLamD had thickened sufficiently, indicating that membranous debris and
early BLamD continue to develop together. However, in 18 of 24 (75%) of Group
III eyes with basal mounds alone, the fundus remained normal. It was not until
patchy late BLamD appeared that the majority of fundi exhibited abnormality.
In Group IV, all eyes had continuous late BLamD and there was little further
increase in membranous debris. In eyes in which the debris remained confined to
93
basal mounds, the mean segment length of late‐continuous BLamD was 413 μm
longer (95% CI 87 – 740μm, p = 0.015) than in eyes with soft drusen. These eyes
belonged to patients who were on average 6 years older (95% CI 2 – 9 years, p<
0.001) with poorer visual acuities (mean difference 0.402 logMar units; 95% CI
0.167 – 0.637, p< 0.001). Table 3.1‐3 summarizes the clinical findings of eyes in
groups III and IV according to the amount of membranous debris present.
94
Figure 3.1‐5 Membranous debris: basal mounds and soft drusen
95
Figure 3.1‐5 Membranous drusen
(A) Semithin section from 75‐year‐old man, with clinical large drusen. Mounds of
membranous debris (basal mounds) (m) are seen above BLamD (asterisk). Basal linear
deposit has built up into soft drusen (d).
Bar marker 100μm. Methylene blue and basic fuchsin.
(B) Electron micrograph of subclinical soft druse from eye illustrated in (A) above,
demonstrating membranous contents. Asterisk indicates late BLamD overlying early
BLamD.
Bar marker 5μm.
(C) Fundus of fellow eye of same patient at age 71, showing soft drusen measuring up to 2
vein widths (250μm).
96
Table 3‐2 Basal laminar deposit – clinical and histopathological correlations
Early Only Patchy Late Continuous Late
No. of Eyes 66 29 37*Clinical drusen defined as intermediate or large drusen**Demonstrated on EM
0.0%Drusen* only 16.7% 24.1%
BLamD
Pigment changes only
Soft Drusen
Mean Visual Acuity (logMar)
RPE Abnormalities
Membranous Debris
Normal
Drusen* with pigment changes
Mean Age (years)
62.1%
Membranous contents Contents becoming increasingly granular
Pathological
Fundus Appearance
Clinical
31.0% 0.0%
15.2% 20.7% 54.1%
BLinD** (no. layers)
Basal Mounds 1.7 +/- 2.5 3.1 +/- 1.92.7 +/- 1.1
Mild Moderate Severe
7.0 +/- 2.3 6.0 +/-0.0 5.8 +/- 2.1
0.275 +/- 0.242 0.280 +/- 0.197 0.611 +/- 0.401
6.1% 24.1% 45.9%
80 +/-8 82 +/-8 84 +/-6
97
Table 3‐3 Membranous debris – clinical and histopathological correlations
Membranous Debris Number of eyes
BLamD thickness (microns)
BLamD Mean Age (years)
Mean VA (logMar)
Approx Snellen
EquivalentNormal 28Pigment Only 10Drusen 0Drusen + Pigment 0Normal 18Pigment Only 6Drusen 0Drusen + Pigment 0Normal* 4Pigment Only 0Drusen 18Drusen + Pigment 11Normal 0Pigment Only 20Drusen 0Drusen + Pigment 0Normal 0Pigment Only 0Drusen 0Drusen + Pigment 17
*Clinical drusen defined as intermediate or large drusen** From 2 patients. Eyes were normal at last clinical examination. Found to have soft drusen on histopathological examination 3 and 5 years later
Group III: Thin Continuous
BLamD
BLinD onlyEarly Only 38 Patchy Late 0 Continuous Late 0
Basal mounds onlyEarly Only 12 Patchy Late 12 Continuous Late 0
Basal mounds + soft drusen
80 +/-8 0.245 +/- 0.206 6/10 20/302.5 +/- 1.2
6/12 20/404.5 +/- 1.6
83 +/-6 0.273 +/- 0.114 6/12 20/402.9 +/- 1.4
Early Only 16 Patchy Late 17 Continuous Late 0
81 +/-8 0.311 +/- 0.298
Group IV: Thick Continuous
BLamD
Basal mounds only 20Early Only 0 Patchy Late 0 Continuous Late 20
Basal mounds + soft drusen 17
Early Only 0 Patchy Late 0 Continuous Late 17
86 +/-5 0.796 +/- 0.362 6/37.5 20/12511.3 +/- 5.3
81 +/-5 0.394 +/- 0.337 6/15 20/509.9 +/- 2.1
Fundus Appearance*
38
24
33
98
3.1.4 Discussion
Zarbin proposed that the biological changes associated with tissue aging, whilst
present in AMD eyes, do not inevitably lead to AMD 116. The present study was
undertaken to trace the evolution of BLamD and membranous debris in order to
determine their relationship to aging and AMD.
The study found that the first appearance of BLinD coincides with a continuous
layer of early BLamD. Since membranous debris is not found in eyes without
BLamD (group I) and occurs only as occasional, isolated fragments in eyes with
patchy BLamD (group II) 196, the presence of both BLinD and continuous early
BLamD represents threshold early AMD, and can be considered a continuum of
normal aging. Also described as “incipient AMD” 168, 176, the findings confirm that
the majority of these eyes have a normal fundus and good vision.
The appearance of late BLamD signals severe RPE abnormality, and corresponds
to clinical pigment changes. Once produced, BLamD is remarkably resilient,
persisting even in areas of geographic atrophy and in disciform scars 8. BLamD is
not lost during processing, remaining detectable using routine H&E and PAS
staining and, although not specific for AMD, it is found in all eyes with the
disease. These properties make it a useful and reliable histopathologic marker for
AMD. In terms of the disease process, however, BLamD appears inert. Rather, it is
the degree of membranous debris accumulation that appears to influence the
course of disease. In the current study, the number of basal mounds increased as
BLamD progressively thickened but, once RPE cells lost their ability to support the
overlying photoreceptors, membrane production ceased. This was heralded by the
appearance of continuous late BLamD and a loss of the membranous contents of
the mounds. Soft drusen likewise became less membranous and more granular,
interpreted as the onset of drusen regression.
99
However, in some eyes membranous debris production did not progress beyond
the formation of basal mounds. Even in the ninth decade these eyes show no
histopathological evidence of soft drusen, instead developing pigment changes
associated with poor vision. Continuous late BLamD is always present in these
eyes, and in longer segments than in eyes with soft drusen, suggesting
longstanding RPE dysfunction. By contrast, soft drusen can occur in relatively
young eyes with good vision, prior to the appearance of continuous late BLamD.
Clinically, intermediate and large drusen are known to increase in size, number
and confluence comparatively rapidly 178, implying a rapid outpouring of
membranes. However, the resulting thickening of BLinD is not uniform, causing it
to develop undulations, the larger of which become visible as soft drusen (Figure
3.1‐4). In these eyes, soft drusen do not appear to develop from an earlier stage
when only basal mounds are present, i.e. soft drusen and basal mounds appear
concurrently.
Epidemiologic data support the concept of two pathways leading to the
development of advanced AMD, depending on the amount of accumulated
membranous debris. Eyes with a large amount of membranous debris, in the form
of large drusen, are at highest risk of developing advanced AMD over a 5‐ or 10‐
year period 33, 320, particularly the blinding effects of CNV 37, 39, 321, 322. Eyes with
pigment changes alone are at lower risk of developing advanced AMD over the
same period 33, 320, consistent with an alternate and slower course of disease.
This clinicopathologic study represents an overview of the basal deposits in the
evolution of AMD, but makes no attempt to correlate with the clinical fundus
grades or severity scales used in recent epidemiologic studies. It has two obvious
limitations. Firstly, the sometimes long interval between the last examination and
death, with drusen a particular concern since the clinical appearance can change
relatively rapidly. With a few exceptions, however, there was good correlation
100
between histopathological soft drusen and clinical intermediate and large drusen,
and the main study findings were unchanged by the exclusion of eyes with an
interval longer than 36 months. Secondly, differences in the post‐fixation method
for EM prevented comparison of membranous debris with other recent studies of
this material 182. It should thus be emphasized that the terms “BLinD”, “basal
mounds” and “soft drusen” are pathological descriptors of membranous debris
location, and that the biochemical nature of this material has not been completely
defined.
BLamD and membranous debris may be products of two distinct cell‐survival
strategies of RPE under stress. Early BLamD can be thought of as excess basement
membrane secreted by the RPE, a common strategy employed by cells attempting
to recover from injury, allowing them to remain attached to a tissue’s
“scaffolding”. As BLamD thickens it progressively separates the basal RPE
surface from its original basement membrane and choroidal blood supply,
exacerbating the metabolic insufficiency caused by decreasing Bruch’s membrane
permeability 148, 150, 151, 323. The production of late type BLamD signals a critical point
in RPE damage at which the basement membrane material becomes more
condensed and, once this forms continuous segments, the RPE cells become
hyperpigmented, enlarge, lose their microvilli and round off. These phenotypic
changes are initially accompanied by expression of the cytoskeletal protein
vimentin 324. The RPE at this stage is severely compromised and can no longer
support the photoreceptors. Eventually, affected RPE cells lose their anchoring to
both the basement membrane and to adjacent cells and are shed into the sub‐
retinal space 3.
The production of membranous debris, on the other hand, may reflect another
survival mechanism adopted by RPE under stress. Ultrastructurally, the earliest
membranes appear to bleb from the basolateral surface of the RPE. Non‐apoptotic
101
membrane blebbing has been observed in cultured RPE cells subjected to oxidative
stress 201, and is considered a non‐specific cellular response to ischaemic, oxidative
or other injury which allows the expulsion of damaged cellular constituents 199, 200.
Other studies show that membranous debris contains solid lipid particles 182, and
that dysregulated lipid trafficking by abnormal RPE may also contribute to its
formation 185, 325. Since the appearance of BLinD coincides with a continuous layer
of early BLamD, a threshold level of RPE injury (perhaps ischaemic) may trigger
the production of membranous debris.
In summary, late BLamD occurs in eyes with clinical pigment changes and
indicates a severely compromised RPE. It is membranous debris, however, that
appears to influence the course of the disease (Table 3.1‐4). Eyes in which
membranous debris accumulation is limited to basal mounds present with
pigment abnormalities alone. These eyes eventually develop “primary” or
“drusen‐unrelated” 3, 8 geographic atrophy. If there is a more rapid accumulation
of debris, early AMD presents with intermediate or large drusen, is at greater risk
of CNV 37, 39, 321, 322 and earlier vision loss, and eventually progresses to “drusen‐
related” geographic atrophy 3. These two pathways share a common threshold
stage when both BLinD and a continuous layer of early BLamD are present (Figure
3.1‐6). However, since the majority of these eyes will be normal in terms of fundus
appearance and visual acuity, identifying “at‐risk” eyes will come to rely on more
sophisticated clinical tests, possibly combined with genetic screening.
102
Figure 3.1‐6 Influence of basal deposits on the progression of AMD
Continuous early BLamD together with BLinD represent the threshold for onset of early
AMD. The pathway then followed depends on the degree of membranous debris
production. A large build up manifests as intermediate or large drusen, whereas if
limited to BLinD and basal mounds, the fundus initially remains normal. At the same
time, BLamD reflects the degree of RPE damage, clinical pigment abnormalities becoming
apparent when late BLamD appears. Both pathways then progress towards geographic
atrophy.
MD – membranous debris
Continuous early BLamD+/- Patchy late BLamD
Basal mounds
BLinD + Soft drusen
Cont thin early BLamD
BLinD
Continuous late BLamDBasal mounds
BLinD
Continuous late BLamDBasal mounds
BLinD + Granular drusen
MD production remains limitedOlder group, poorer vision
Excess production MD(High risk CNV)
Drusen-unrelatedGeographic atrophy
= RPE basement membrane.
None or patchy early BLamD
No BLinD
Continuous early BLamD+/- Patchy late BLamD
Basal mounds
BLinD + Soft drusen
Drusen-relatedGeographic atrophy
Normal Aging
Threshold early AMDFundus normal 74%
Pigment changes (25%)
Pigment changes (100%)
Intermediate or large drusen
Intermediate or large drusen,may show signs of regression
Pigment changes (100%)
103
Table 3‐4 Basal laminar deposit ‐ a summary
BLamD Type Site LM Appearance EM Appearance Clinical Correlation Significance
Early
Basement membrane‐like extracelluar material between RPE basal plasma membrane and its BM
Stains blue on P‐M. Shows feint vertical stirations and may contain small unstained spaces. May be absent, patchy or continous. When continuous, may be associated with early AMD. Reaches maximum height of 7mm.
Exists in 3 forms: fibrillar, amorphous (light and dark) and polymerised (banded). The polymerised form is similar to long‐spacing collagen. The light amorphous form is continuous with the RPE BM.
Not seen. May be compatible with a normal fundus.
The presence of fibronectin, laminin and glycoproteins imply it is abnormal basement membrane material. Considered inert, but reflects the escalating damage to RPE.
All late BLamD stains red on P‐M.
Late BLamD has a more compact appearance than early BLamD. It has a wave‐like appearance, described previously as ʺflocculentʺ or ʺmultilaminarʺ.
(i) Inclusions of late BLamD: small, globular bodies in the order of 4mm lying within early BLamD.
(i) Inclusions are seen beneath a segment or portion of RPE plasma membrane which becomes indented. May appear within membranous debris.
Not seen. May be compatible with a normal fundus.
(ii) Nodular late BLamD: are nodular excrecenses up to 7mm (1 RPE cell) in size. The overlying RPE cell becomes hypertrophied and hyperpigmented initially, and eventually attenuates.
(ii) Appear multilaminar. Distinct from hard drusen.
May be compatible with a normal fundus or seen as focal hyperpigmentation.
Results from contiued production of late BLamD by a single RPE cell.
(iii) Continous (confluent) BLamD: also known as ʺdiffuse thickening of the internal aspect of Bruchʹs membraneʺ. Seen as a confluent layer internal to early BLamD, up to 7mm in thickness.
(iii) Produced by several RPE cells.
Not compatible with a normal fundus. Likely to have pigmentary changes without soft drusen.Usually seen as hyperpigmentation, followed by hypopigmentation and atrophy.
A sign of severe RPE dysfunction. Pre‐atrophy.
Late
Being a later development, lies above (internal to) the early form. Occurs in 3 stages in order of severity:
104
Table 3‐5 Membranous debris – a summary
Relationship to
RPE BMEntity Site LM Appearance EM Appearance Clinical Correlation Significance
Membrane Fragments
Membrano‐vesicular material between RPE basal plasma membrane and its BM
Not seenBleb from basal surface of RPE cells. Found between strands of early BLamD
Not seen. Compatible with a normal fundus
Basal Membrane Mounds
Rounded collections of membranous debris between the RPE basal plasma membrane and its BM.
Small, unstained spaces with rounded margins between attenuated RPE cells internally, and early BLamD externally. RPE cells overlying mounds are grossly abnormal in shaped becoming attenuated over mounds but attached by their tight junctions. The smallest mound is at least half the height of an RPE cell. Mounds beneach adjacent cells may fuse, the largest up to 7 RPE cells wide and one RPE cell in height.
**
Patients with basal mounds less likely to have normal fundus. Increasing numbers of mounds correlates with poorer visual acuity. If over 30mm in diameter, may account for dot‐like drusen.
WithinMembrane Fragments
Membrano‐vesicular material apparently transiting through RPE BM
Not seenVesicles are smaller and more compact than at other sites.
Not seen. Compatible with a normal fundus
Basal Linear Deposit
Forms a layer lying between the RPE BM and the inner collagenous zone of Bruchʹs membrane
Not seen in histological sections. In semithin sections, appears as an unstained narrow interval between the RPE and Bruchʹs membrane.
A layer of membranous debris, thickest when adjacent to soft drusen.
Not seen. May be compatible with a normal fundus.
Forms a cleavage plane in which new vessels can spread.
Soft Drusen
Focal accumulation of BLinD between the RPE BM and the inner collagenous zone of Bruchʹs membrane
Seen between the RPE and Bruchʹs membrane. May be all sizes , the smallest defined as at least half the height of an RPE cell. Regressing drusen may become granular.
Composed of membranous debris.
Soft Drusen
Membrane Fragments
Withing the outer collagenous zone of Bruchʹs membrane
Not seen. Not seen.
Contribute to the increasing thickness of Bruchʹs membrane seen with age. May attract macrophages.
ApicalApical Membrane Mounds
Found at apex of RPE, within the subretinal space
Only seen in well‐preserved eyes as empty spaces between shortened photoreceptor outer segments and apical surface of RPE
Mounds of debris overlying RPE cells which have lost their microvilli.
If over 30mm may account for dot‐like drusen in patients with pigment clumps or adult vitelliform degeneration.
External
Internal
105
3 . 2 C H O R O I D A L A N D B R U C H ’ S M E M B R A N E M A C R O P H A G E S I N
E A R L Y A N D A D V A N C E D A M D
3.2.1 Introduction
Macrophages were first implicated in the pathogenesis of age‐related macular
degeneration (AMD) when early morphological studies found them within drusen
179, 215, 218 and choroidal neovascular membranes 219. Significantly, macrophages
were also found adjacent to breaks in Bruch’s membrane (BrM), through which
subclinical new vessels grew from the choroid 10, 179, 222. Later studies supported the
idea that macrophages facilitated choroidal neovascularisation (CNV) by
interrupting BrM and releasing pro‐angiogenic factors such as vascular
endothelium‐derived growth factor (VEGF) 121, 174, 226, 227, 229, 326, 327. Thus, macrophage
infiltration of BrM or the subretinal space is considered pathological. However,
another local population of macrophages can be found residing in the choroid of
normal eyes.
Extensive networks of resident choroidal macrophages have been demonstrated in
normal mouse 244 and rat 328 eyes. Although their human equivalents are not well
characterised, this population of macrophages appear to be programmed for
phagocytosis without eliciting inflammation. Indeed, recent animal models of
AMD suggests that the normal recruitment and turnover of resident choroidal
macrophages may prevent the development of AMD‐like lesions 231, 233,
presumably by facilitating the efficient removal of BrM debris.
106
It is not clear, however, how these two populations of macrophages – resident
choroidal macrophages and macrophages that infiltrate BrM – are influenced by
the progressive accumulation of the characteristic basal deposits found in the
macula of human AMD eyes – basal laminar deposit (BLamD) and membranous
debris.
BLamD, an abnormal basement membrane‐like material, is found between the
plasma membrane of the retinal pigment epithelium (RPE) and its basement
membrane. Its accumulation correlates with progressive RPE abnormalities 5 and it
is found in all eyes with early or advanced AMD 8, 126, 169, 174‐176. Membranous debris
appears to be composed of membrane fragments 179, 180 and lipid 142, 181, and is
specific for AMD 176. It is first found in eyes with early AMD as basal linear deposit
(BLinD), a thin layer external to the RPE basement membrane 126, 176, 179.
Membranous debris can also accumulate as small focal collections internal to the
RPE basement membrane, forming basal mounds 329. Larger collections external to
the RPE basement membrane become soft drusen 176.
Macrophages are a highly heterogenous group of cells. Early inflammatory
macrophages, for example, are phenotypically and functionally distinct from
resident tissue macrophages. Pro‐inflammatory (“classically activated”)
macrophages recruited to sites of injury or infection express inducible nitric oxide
107
synthase (iNOS), whereas resident macrophages do not 330. This enzyme allows
macrophages to convert L‐arginine to nitric oxide, a damaging reactive nitrogen
species with cytotoxic 331, 332 and pro‐angiogenic properties 220, 248. In the context of
AMD, we hypothesise that: (i) resident choroidal macrophages do not express
iNOS in normal eyes; (ii) the progressive accumulation of BLamD or membranous
debris attracts macrophages to BrM and may favour the recruitment of classically
activated, iNOS‐ expressing macrophages; (iii) the presence of BrM breaks and
subclinical CNV is associated with a high BrM macrophage count; and (iv) iNOS‐
expressing macrophages are likely to be found where active neovascular processes
are taking place.
In this study, macrophages were identified using CD68 immunohistochemistry in
a total of 16 human eyes, representing normal (2), normal aged (2), early AMD (7),
geographic atrophy (2) and disciform scars (2). iNOS immunohistochemistry was
used to determine whether macrophages were classically activated in these eyes.
A submacular BrM macrophage count was undertaken in a further 125 eyes,
representing normal (18), normal aged (20), early AMD (78) and geographic
atrophy (9). CD68/iNOS immunohistochemistry and BrM macrophage counts
were correlated with BLamD and membranous debris accumulation, and with
subclinical CNV. Results were also correlated with clinical data, including age,
visual acuity and fundus appearance.
108
3.2.2 Methods
3 . 2 . 2 . 1 Ey e s
Archival slides of human eyes were selected from a large clinicopathologic
collection of over 600. Eyes were previously graded according to the appearance of
BLamD 10 (see below). All of the eyes were examined (best corrected visual acuity
and direct fundoscopy) and most photographed (Zeiss 30° fundus camera) during
life. Eyes were paraffin embedded with 8μm serial sections cut horizontally
through the disc and macula, and every 10th section stained using the picro‐
Mallory method 10.
For histopathology and BrM macrophage counts, the best sub‐foveal sections of
125 eyes, representing histopathological groups I to V, were examined with a Leitz
Dialux microscope, using the PL APO 40x (NA‐0.75) objective. For CD68 and
iNOS immunohistochemistry, unstained sections of 16 eyes, belonging to
histopathological groups I to VI, were used. Informed consent was obtained before
the collection of all eyes. The study was carried out in keeping with the
Declaration of Helsinki and was approved by the Human Research Ethics
Committee of the University of NSW.
3 . 2 . 2 . 2 C l i n i c a l d e f i n i t i o n s
Clinical data from the last follow up prior to death was used. Best corrected visual
acuity was converted from Snellen to logMar for statistical purposes. The fundus
was graded as: (i) normal, which included up to 5 small (<63mm) hard drusen; (ii)
extensive small hard drusen; (iii) soft drusen (≥63μm) without pigment
abnormalities; (iv) soft drusen (≥63μm) with pigment abnormalities; and (v)
pigment abnormalities alone.
109
3 . 2 . 2 . 3 H i s t o p a t h o l o g i c a l d e f i n i t i o n s a n d g r a d i n g : BL amD
Eyes were graded based on the extent of submacular BLamD present, according to
the system proposed by Sarks 10, modified by the inclusion of BLamD type. Early
type BLamD stains blue with the picro‐Mallory method (Figure 3.2‐2a), while the
late type stains red and is found internal to the early type 329 (Figure 3.2‐2b). Group
I represented normal eyes and had no BLamD. Group II represented normal aged
eyes in which there was patchy early BLamD. Groups III and IV represented early
AMD. In group III, BLamD was present in a thin continuous layer, and consisted
of early or patchy late type. In group IV, BLamD was present in a thick continuous
layer, and consisted of continuous segments (>250μm) of the late type. Group V
eyes had geographic atrophy and Group VI eyes had disciform scars.
3 . 2 . 2 . 4 H i s t o p a t h o l o g i c a l d e f i n i t i o n s a n d g r a d i n g : memb r a n o u s d e b r i s
Due to the exclusive use of light microscopy in this study, the presence of BLinD,
an ulstrastructural finding, was not determined. However, BLinD is found in all
eyes in which there is a thin continuous layer of BLamD 329. Since soft drusen are
large focal collections of membranous debris, and basal mounds are small focal
collections, in eyes with early AMD (groups III and IV), the amount of submacular
membranous debris was thus graded: soft drusen > basal mounds > no basal
mounds (presumed BLinD).
3 . 2 . 2 . 5 B rM ma c r o p h a g e c o u n t s
BrM macrophages were defined morphologically by their abundant pale pink‐
staining cytoplasm, large oval or kidney‐shaped nuclei and granular chromatin,
and by their presence within the zone bounded by the inner margin of the inner
collagenous zone and the outer margin of the choroidal capillaries/intercapillary
pillars (Figure 3.2‐1b). Counts were not performed on group VI eyes, since the
110
large numbers of macrophages, fibroblasts and other cells within disciform scars is
well established.
An eyepiece graticule which equalled 280μm in total with the 40X objective was
used to examine the foveal field and one graticule length on either side, giving a
total of three fields (840μm). In eyes with hard drusen, soft drusen or RPE
changes, additional fields (up to five, all within the inner macula) were examined.
Cell counts were divided by the number of fields examined, multiplied by three,
and expressed as mean count per three 40X fields.
3 . 2 . 2 . 6 Immun o h i s t o c h em i s t r y
CD68 is a glycosylated protein (part of the scavenger receptor superfamily) found
in all cells from the monocyte/macrophage lineage 333. CD68
immunohistochemistry is commonly used to identify macrophages/histiocytes in
paraffin sections, and stains intracytoplasmic granules.
Detailed methods for CD68 and iNOS immunohistochemistry, and the antibodies
use, as described in the Appendices (A‐C). Briefly, sections were deparaffinised in
xylene, rehydrated and treated in 1% hydrogen peroxide in alcohol for 30 min to
block endogenous peroxidase. Non‐specific binding was blocked by incubation in
10% normal horse serum for 20 min. For iNOS detection, sections were incubated
with monoclonal mouse anti‐human iNOS (Transduction Laboratories BD
Biosciences, San Jose, CA, USA), diluted 1:400, for 1hr at 37oC. . For CD68
detection, antigen retrieval with pepsin digestion for 5 min at 37oC preceded the
blocking step, which was followed by incubation with monoclonal mouse anti‐
human CD68 antibody (Zymed Invitrogen, Carlsbad, CA, USA), neat, for 1hr at
37oC. After washing with tris, sections were incubated with biotinylated anti‐
111
mouse IgG antibody (Vector Laboratories, Burlingame, CA, USA), diluted 1:200,
for 30min at 37oC. Sections were then washed with tris, incubated with 1:500 ABC
(Vector Laboratories) for 30 min at room temperature. For iNOS detection, this
was followed by 3 changes of nickel ammonium phosphate tris solution and final
incubation with DAB (Sigma‐Aldrich, St Louis, MO, USA) in nickel ammonium
sulphate and peroxide solution for 10 min at room temperature, resulting in a dark
grey colour product. For CD68 detection, incubation with ABC was followed by
washing with tris and incubation with Vector SC Blue (Vector Laboratories) for 10
min at room temperature, resulting in a dark blue colour product. All sections
were then washed with tris followed by distilled water, dehydrated and
coverslipped in DPX (ProSciTech, Kirwan, QLD, Australia).
3 . 2 . 2 . 7 S t a t i s t i c s
Statistical analysis was performed using SPSS for Windows (v15.0; SPSS Inc,
Chicago, IL, USA). The student t test was used to compare mean BrM
macrophage counts and visual acuity measurements between histopathological
groups. Contingency tables of histopathological group versus the presence of
subclinical CNV were evaluated by the χ2 and Fisher’s exact tests. A p value ≤ 0.05
was considered statistically significant.
112
3.2.3 Results
3 . 2 . 3 . 1 Gr o u p s I & I I ‐ n o rm a l a n d n o rm a l a g e d e y e s
The fundus was normal in all 18 eyes from group I (Table 3‐2.1).
Histopathologically, the RPE appeared normal and BrM hyalinisation ranged from
non‐existent (Figure 3.2‐a) to extending a short distance down the intercapillary
pillars. Group II eyes were also fundoscopically normal, with the exception of two
eyes with pigment abnormalities in which extensive hard drusen of intermediate
size were present (Table 3.2‐1). BrM hyalinisation extended down the
intercapillary pillars to reach the base of the choroidal capillaries, resulting in
widened intercapillary spaces (Figure 3.2‐1b).
BrM macrophages were absent in group I and II eyes (confirmed by CD68
immunohistochemistry), except in the two group II eyes with pigment changes, in
which occasional macrophages were found beneath small hard drusen.
Occassionally, cells with small nuclei and scanty cytoplasm were found in the
place of choroidal capillaries (Figure 3.2‐1b).
CD68 immunohistochemistry revealed numerous ramified cells in the choroid,
within the choroidal melanocyte network, in all group I and II eyes (Figures 3.2‐1c
& d). Expression of iNOS was not found in either BrM or choroidal macrophages,
but was occasionally seen in the basal aspect of RPE cells (Figure 3.2‐1e).
3 . 2 . 3 . 2 Gr o u p s I I I & I V – e a r l y AMD
In group III eyes, the fundus was normal in 15 (79%) eyes without basal mounds
(presumed BLinD), and 6 (40%) eyes with basal mounds, while all group IV eyes
had clinically evident pigment changes, with or without soft drusen. Resident
choroidal macrophages ‐ CD68‐positive ramified cells among the choroidal
melanocytes‐were found in all group III and IV eyes (Table 3.2‐2). BrM
113
macrophages were also seen in group III and IV eyes, and were found in the place
of a choroidal capillary (Figure 3.2‐2a & c), or an intercapillary pillar (Figure 3.2‐
2b). Interestingly, BrM macrophages were only found in group III eyes with
clinical pigment changes or soft drusen, apart from three eyes with normal fundus
(which belonged to older patients).
iNOS expression was again seen in the basal aspect of RPE cells. However,
choroidal ramified cells also expressed iNOS (Figure 3.2‐2d) in group III and IV
eyes, as did choroidal vascular endothelial cells (3.2‐2e), and perivascular ramified
cells (3.2‐2f).
114
Table 3‐6 Clinical and histopathological features of study eyes
Light
microscopyImmunohisto-
chemistryNormal Drusen*
alonePigment
aloneDrusen* + Pigment
I No BLamD 18 2 64.5 +/-9.6 0.080 +/-0.105
18 (100%) 0 0 0
II Patchy BLamD 20 3 74.4 +/-8.9 0.110 +/-0.120
18 (90%) 0 2
(10%) 0
III Thin continuous BLamD 49 4 81.8 +/-7.4 0.277 +/-
0.21321
(43%)13
(27%)10
(20%)5
(10%)
IV Thick continuous BLamD 29 3 84.1 +/-6.3 0.593 +/-
1.164 0 0 17 (59%)
12 (41%)
V Geographic atrophy 9 2 80.9 +/-5.9 1.096 +/-
1.004 0 - - -
VI Disciform scar 0 2 78.5 +/-2.1 2.000 +/- 0.000 0 - - -
BLamD basal laminar deposit; VA visual acuity* intermediate or large drusen
logMar VA Fundus appearanceGroup Pathology
Number of Eyes Age at Death (yrs)
115
Table 3‐7 BLamD, membranous debris and macrophages in early AMD
Cell count* CD68 immunohistochemistry iNOS immunohistochemistry CD68 immunohistochemistry iNOS immunohistochemistry
I No BLamD 0 None None [occasional basal RPE expresson]
Ramified cells intermixed with chroidal melanocytes
None
II Patchy BLamD 0.03 +/-0.14 None None [basal RPE expresson]
Ramified cells intermixed with chroidal melanocytes
None
III Thin continuous BLamD
3.14 +/-5.82 Round to epitheliod cells within BrM
None [basal RPE expresson]
Ramified cells intermixed with chroidal melanocytes
Occassional ramified cells intermixed with choroidal melanocytes
IV Thick continuous BLamD
7.14 +/-5.90 Round to epitheliod cells within BrM
None [basal RPE expresson]
Ramified cells intermixed with chroidal melanocytes [some large & plump]
Frequent ramified cells intermixed with choroidal melanocytes, or in
perivascular region [choroidal endothelial cell expression]
V Geographic atrophy
2.72 +/-3.03 At edges of atrophy None Few cells beneath area of atrophy None
VI Disciform scar - Numerous, within fibrovascular membrane/scar in both active CNV
and disciform scar
Little within CNV or disciform scar Numerous, within fibrovascular membrane/scar in both active CNV
and disciform scar
Perivascular ramified cells and endothelial cell iNOS expression in active CNV. Little in disciform scar.
BLamD basal laminar deposit; BrM Bruch's membrane* Mean macrophage count per 3 (X40) fields
Group Pathology Bruch's membrane macrophages Choroidal macrophages
116
Figure 3.2‐1 Morphological and immunohistochemical features of normal (group I) and
normal aged (group II) eyes
ONL
RPECC
CCRPE
CCRPE
*
a
b
c d
e
ONL
RPECC
CCRPE
CCRPE
*
ONLONL
RPERPECCCC
CCRPECCCCRPERPE
CCCCRPERPE
*
a
b
c d
e
117
Figure 3.2‐1 Morphological and immunohistochemical features of normal (group I) and
normal aged (group II) eyes
(a) A group I eye from a 73 year old patient with a normal fundus. The RPE is normal,
there is no BLamD and little BrM hyalinisation.
(b) In group II eye from a 72 year old patient with a normal fundus, BrM hyalinisation
extends down the intercapillary pillars (black arrows). Fibrous tissue (asterix) or small
round cells (white arrow) appear in the place of lost choroidal capillaries. The black
arrows also define the area within which BrM macrophage were counted.
(c) Ramified cells with CD68 positive granules in the choroid of a group I eye from a 62
year old patient with a normal fundus, among choroidal melanocytes.
(d) A group I eye from the same patient as (a), with numerous CD68‐positive choroidal
cells intermixed with melanocytes (black arrows). An intravascular monocytoid CD68
positive cell is seen within a choroidal vessel (white arrow).
(e) Occassionally, iNOS expression was seen in the basal aspect of RPE cells (arrows), such
as in this group II eye from a 75 year old patient with a normal fundus. Neither BrM or
choroidal cell iNOS expression was found in group I and II eyes.
a‐b: picro‐Mallory stain; c‐e: CD68 and iNOS immunohistochemistry. The CD68 colour
product is dark blue and the iNOS colour product is dark grey.
Bar markers: approx. 100μm
ONL – outer nuclear layer; RPE – retinal pigment epithelium; CC – choriocapillaris; iNOS –
inducible nitric oxide synthase; BrM – Bruch’s membrane
118
Figure 3.2‐2 Morphological and immunohistochemical features of eyes with early
AMD (groups III and IV)
CC
RPE
CCRPE
CCRPE
CCRPE
CCRPE
*
a
b
c
d
e f
CC
RPE
CCCC
RPERPE
CCRPECCCCRPERPE
CCRPECCCCRPERPE
CCRPECCCCRPERPE
CCRPECCCCRPERPE
*
a
b
c
d
e f
119
Figure 3.2‐2 Morphological and immunohistochemical features of eyes with early AMD
(groups III and IV)
(a) BrM macrophages (black arrows) in the place of a lost choroidal capillary or eroding
intercapillary pillars in a group III eye from an 88 year old patient with pigment changes
and soft drusen. There is mild RPE disorganisation and thin continuous BLamD of the
early (pale blue staining) type (square brackets). A soft druse (asterix) appears empty due
to loss of lipid contents during processing. A pigment laden cell (white arrow) is also
found replacing a choroidal capillary.
(b) A group IV eye from an 89 year old patient with pigment changes only. The RPE
appears more disorganised compared to the eye in (a) and there is thick continuous
BLamD, with continuous segments of the late, red staining type (square brackets) type
overlying the early type. A BrM macrophage is found apparently eroding an
intercapillary pillar (arrow).
(c) CD68 immunohistochemistry in a group IV eye from an 80 year old patient with soft
drusen and pigment changes, confirming the presence of BrM macrophages (arrows).
Note again the network of CD68 positive ramified cells in the choroid.
(d) Ramified iNOS expressing cells in the choroid (white arrow), intermixed with
melanocytes, in the same eye as in (c). Basal expression of iNOS by the RPE cells (black
arrows) was also found in group III and IV eyes.
(e) Choroidal vascular endothelial cell iNOS expression (arrow) in a group IV eye from a
74 year old patient with soft drusen and pigment changes.
(f) iNOS expression by ramified cells found between choroidal melanocytes (white
arrow) and near a choroidal vessel in the choroid (arrows) of the same eye as in (c).
Bar marker: approx. 100μm
120
3 . 2 . 3 . 3 Gr o u p s I I I & I V : B rM ma c r o p h a g e s a n d s u b c l i n i c a l CNV
Sixteen of the 125 eyes examined histologically had subclinical CNV. With the
exception of two eyes from group V, the remainder belonged to groups III and IV.
Overall, eyes with subclinical CNV had significantly more macrophages on
average (p <0.001; mean difference 10.30 +/‐1.93; 95% CI = 6.19‐14.41) than eyes
without (Table 3.2‐4). Macrophages in eyes with subclinical CNV were found
eroding intercapillary pillars or BrM (Figure 3.2‐3a). Growth of fibrovascular
tissue into the sub‐RPE space was seen in eyes with complete breaks in BrM
(Figure 3.2‐3b). A lack of unstained tissue in these eyes prevented
immunohistochemical characterisation of the macrophages.
Subclinical CNV was associated with the amount of membranous debris present
in eyes with thin continuous BLamD (group III), since it was only found in eyes
with soft drusen (Table 3.2‐3). However, this relationship did not hold once thick
continuous late BLamD was present (group IV) (Table 3.2‐3).
Of the 23 pairs of eyes present in the study, when subclinical CNV was present in
one eye, greater numbers of BrM macrophages were also found in the fellow eye
(mean difference 9.18 +/‐2.52; 95% CI = 3.96‐14.41) compared to the fellow eyes of
eyes without subclinical CNV (Table 3.2‐4).
121
Figure 3.2‐3 Morphological features of eyes with subclinical CNV
CCRPE
CC
RPE
*
*
a
b
CCRPE
CC
RPE
*
*
a
b
CCRPECCCCRPERPE
CC
RPE
CCCC
RPERPE
*
*
a
b
122
Figure 3.2‐3 Morphological features of eyes with subclinical CNV
(a) A group III eye from an 88 year old patient with soft drusen and pigment changes.
Thin continuous BLamD of the early type (asterix) is overlain by mildly disorganised RPE.
BrM macrophages are seen eroding BrM (arrow) beneath a soft druse (contents emptied
by processing). Elsewhere in the same eye, fibrovascular tissue is found invading the sub‐
RPE space (not shown).
(b) A group III eye from an 81 year old patient with soft drusen only. Fibrovascular tissue
is seen invading the sub‐RPE space (asterix) via a focal break in BrM (arrow).
Bar marker: approx. 100μm
123
Table 3‐8 BrM macrophages, membranous debris and subclinical CNV
Extent Type
No basal mounds* 19 80 +/-8 0.185 +/-
0.179 0.8 +/-2.5 0
Basal mounds only 15 84 +/-5 0.262 +/-
0.940 0.5 +/- 1.0 0
Soft Drusen 15 82 +/-8 0.374 +/-0.272 8.7 +/-7.7 6
Basal mounds only 17 87 +/-5 0.935 +/-
0.160 8.0 +/-6.6 5
Soft Drusen 12 80 +/-5 0.250 +/-0.117 5.9 +/-4.8 1
BLamD basal laminar deposit ; VA visual acuity ; CNV choroidal neovascularisation*presumed BLinD ** Mean macrophage count per 3 (X40) fields ***Fisher's exact p
p***pGroup Membranous
DebrisNo. eyes Age (yrs)BLamD logMar VA BrM
macrophage count**
Subclinical CNV
0.354
III
IV Thick continuous 0.359Continuous
late
Thin continuous
Early + Patchy late
0.0170.001
124
Table 3‐9 BrM macrophages and subclinical CNV in the fellow eye
Number of eyes BrM macrophage count**
p
Subclinical CNV absent* 64 2.78 +/-4.07
Subclinical CNV present* 14 13.08 +/-6.97
Fellow eye subclinical CNV absent 20 4.24 +/-4.18
Fellow eye subclinical CNV present 3 13.42 +/-2.81CNV choroidal neovascularisation* all eyes from groups III and IV ** Mean macrophage count per 3 (X40) fields
<0.001
0.002
125
3 . 2 . 3 . 4 Gr o u p V – Ge o g r a p h i c a t r o p h y
Compared to groups I – IV, the resident choroidal macrophage network appeared
sparse immediately beneath areas of atrophy. BrM macrophages however, were
found at the edges of areas of atrophy in group V eyes, aligned with the still‐viable
RPE cells (Figure 3.2‐4a). iNOS expression was not found in BrM or the choroid of
these eyes (Figure 3.2‐4b).
3 . 2 . 3 . 5 Gr o u p V I – D i s c i f o rm s c a r r i n g
Immunohistochemistry performed on the 2 eyes with disciform scarring showed
differences in CD68 and iNOS staining between active and inactive scars. In a
clinically flat disciform lesion without haemorrhage or exudates, subretinal
fibrous tissue surrounding a large feeder vessel was present. Numerous CD68
positive cells were present within the scar (Figure 3.2‐4c) but iNOS staining was
minimal (Figure 3.2‐4d). By contrast, in an exudative disciform lesion, a
fibrovascular membrane consisting of numerous small vessels and cells, some
pigmented, was seen. CD68 positive cells were less numerous in this eye
compared to the inactive scar (Figure 3.2‐4e), while plump ramified CD68 positive
cells were noted in the choroid. However, large iNOS positive cells were found in
the perivascular region of choroidal vessels (Figure 3.2‐4f). Choroidal endothelial
cells in this region were also iNOS positive (Figure 3.2‐4f).
126
Figure 3.2‐4 CD68 and iNOS Immunohistochemistry in eyes with geographic atrophy
(group V) or disciform scarring (group VI)
CC
RPE
CC
RPE
CC
RPE
CCCC
RPERPE
CC
RPE
CCCC
RPERPE
127
Figure 3‐2.4 CD68 and iNOS Immunohistochemistry in eyes with geographic atrophy
(group V) or disciform scarring (group VI)
(a) CD68 positive cells were seen at the edges of areas of atrophy, in line with the
remnant RPE (arrow) in a group V eye from an 84 year old man. Few ramified CD68
positive cells were found in the choroid immediately beneath the atrophic region.
(b) The same eye in (a). iNOS expressing cells were not found within BrM or choroid.
(c) A group VI eye from an 80 year old man with an “inactive” disciform scar. Numerous
plump CD68 positive cells are present within the scar (arrows).
(d) The same eye in (c). iNOS positive cells are absent.
(e) A group VI eye from 76 year old man with exudative neovascular AMD, in which
strongly CD68 positive cells are found within the scar (arrows) and in the choroid, in
perivascular locations (arrowhead).
(f) The same eye in (e). Choroidal perivascular cells (arrows) and endothelial cells express
iNOS.
Bar markers: approx. 100μm
128
3.2.4 Discussion
The relationship between macrophages and the development of lesions specific to
age‐related macular degeneration, particularly choroidal neovascularisation, is
likely to be complex. Emerging evidence suggests that normal recruitment and
turnover of the resident choroidal macrophage population may, in fact, protect
against the development of AMD‐like lesions 231, 233.
The current study suggests that resident choroidal macrophages are present in all
eyes and, since they do not express iNOS, are phenotypically distinct from
classically activated, inflammatory macrophages. The choroidal immune
microenvironment appears to be tightly controlled. RPE cells express macrophage
chemoattractant protein‐1 334, which facilitates macrophage recruitment. However,
RPE cells also express immunomodulatory molecules such as complement factor
H 335, transforming growth factor‐β1 162, interleukin‐1 receptor antagonist 163,
pigment epithelium‐derived factor, somatostatin and interleukin‐10 164, preventing
newly recruited macrophages from developing an inflammatory program. RPE
cells can also suppress activated T cells 336 and initiate Fas‐Fas ligand mediated
apoptosis of infiltrating immune cells 157.
BrM macrophages were not found in significant numbers until eyes developed at
least a thin, continuous layer of BLamD, together with a large amount of
membranous debris (soft drusen). This suggests that while the RPE is relatively
intact, local control of macrophage recruitment and programming is not
significantly altered until a large amount of extracellular membranous/lipid
material accumulates. However, once moderate to severe RPE abnormalities
develop (thick continuous late BLamD), BrM macrophages increase irrespective of
the amount of membranous debris present. Surprisingly, iNOS expression was not
a feature of BrM macrophages in any eye. This suggests that although some
129
immunomodulatory capacity is lost once RPE are compromised, the mechanisms
that prevent infiltration of BrM by classically activated macrophages appear
robust. The same does not appear to hold for resident choroidal, or newly
recruited choroidal macrophages, evidenced by their expression of iNOS once RPE
abnormalities (continuous BLamD) and membranous debris are present.
The presence of extracellular debris and dead or compromised cells (RPE) attracts
components of the innate immune system, including acute phase proteins, natural
autoantiabodies and complement components 337. Complement components and
other acute phase proteins have been found in drusen 263 and BLamD 338. An excess
build up of debris or inadequate regulation of innate immune activation may thus
initiate damaging inflammatory cascades. The presence of iNOS expressing
macrophages in the choroid of eyes with continuous BLamD and membranous
debris suggests alteration of normal immune regulation within the choroid in
early AMD.
Macrophage nitric oxide production has cytotoxic amd proangiogenic effects in
the choroid and retina 248 and is involved in pathological ocular neovascularisation
339. Both iNOS and vascular endothelial growth factor expression have been noted
in macrophages and endothelial cells in excised neovascular membranes 121, 247. The
observation that a high BrM macrophage count is associated with subclinical CNV
suggests that BrM macrophages may overcome local RPE immunomodulation and
express iNOS in these eyes. Although this could not be confirmed in the current
study, the presence of iNOS expressing cells in the choroid and scar of the active
disciform lesion supports the idea that pro‐inflammatory macrophages are
involved in active neovascular processes.
Subclinical CNV is found in 41.7% of eyes with geographic atrophy 10, suggesting
that neovascularisation is a self‐limiting process in many AMD eyes. However,
130
individuals may be predisposed to progression to neovascular membrane
formation due to environmental and/or genetic risk factors. That the fellow eye of
eyes with subclinical CNV also had high BrM macrophage counts lends support to
this, and is also consistent with the clinical observation that these eyes are at
higher risk of developing CNV 340.
The significance of iNOS expression by RPE cells in normal aged eyes is unclear.
In vitro, RPE cells express iNOS after exposure to ischaemic 341 or inflammatory 342
stimuli. As a part of normal ageing, ischaemia due to choroidal capillary fallout
may stimulate RPE iNOS expression.
Ideally, CD68 and iNOS double‐immunohistochemistry should be used for
phenotypic studies. However, finding colour products easily co‐visualised in the
presence of both RPE and melanocyte pigment proved challenging. Pigment also
interferes with fluorophores, so that use of immunofluorescence double‐labelling
can be equally problematic. It is possible some of the choroidal CD68 expressing
cells in this study are activated dendritic cells and not macrophages, and that the
perivascular iNOS expressing cells are pericytes. Further immunophentyping of
the normal human resident choroidal macrophage population is needed to
understand their role in homeostasis and AMD.
In summary, while resident choroidal macrophages are present in all eyes, BrM
macrophages are associated with accumulation of BLamD and membranous
debris, corresponding to clinical evidence of early AMD. Choroidal macrophages
in these eyes developed a pro‐inflammatory phenotype, suggesting an altered
choroidal immune microenvironment. Macrophages are implicated in the
neovascular process, in a mechanism possibly involving a switch in programming
evidenced by iNOS expression.
131
S E R U M A U T O A N T I B O D Y S T U D I E S
4
132
4 . 1 C O M P A R I S O N O F T H E A N T I - R E T I N A L A U T O A N T I B O D Y P R O F I L E
I N E A R L Y , N E O V A S C U L A R & A T R O P H I C A M D
4.1.1 Introduction
A number of previous studies (reviewed in Section 2.6) have identified an
association between serum anti‐retinal autoantibodies and early, neovascular and
atrophic AMD. It appears more than one autoantibody may be involved, although
their significance to AMD pathogenesis is still uncertain.
This study used a single detection method, western blotting, to identify anti‐
retinal autoantibodies in the serum of normal individuals as well as those with
early, neovascular or atrophic AMD. Since restriction of autoantibodies to one IgG
subclass is a feature of pathological autoimmunity 317, the IgG subclass of
autoantibodies was identified.
133
4.1.2 Methods
4 . 1 . 2 . 1 S t u d y s e r um
4.1.2.1.1 Blue Mountains Eye Study
Control, early AMD and GA serum came from the Blue Mountains Eye Study
(BMES), a population‐based survey conducted in a geographically defined area
west of Sydney, Australia 18. Non‐institutionalised, permanent residents of two
postcode areas, aged 49 years or older, were invited to participate between 1992
and 1994. A total of 3654 (82.4% of the eligible) individuals were enrolled. Ocular
examination and detailed grading of retinal photographs was conducted at
baseline, and 5 343 and 10 years 344 later. The study was approved by the Western
Sydney Area Health Service Human Research Ethics Committee and written
consent was obtained from all participants.
The Wisconsin Age‐related Maculopathy Grading System 15, with slight
modifications, was used to define early AMD as either the presence of soft,
indistinct or reticular drusen, or the presence of both RPE abnormalities and soft
distinct drusen in the macula area (within a 3000 μm radius of the fovea). A total
of 191 participants had signs of early AMD at baseline.
The International Classification and Grading System 16 was used to define
geographic atrophy as a discrete area of retinal de‐pigmentation characterised by
sharp borders and the presence of visible choroidal vessels, occupying an area at
least 175μm in diameter. A total of 39 participants had geographic atrophy at
baseline.
Normal controls were aged 60 years or over and had an absence of ocular disease.
134
4.1.2.1.2 Intravitreal Triamcinolone Study
Serum for neovascular AMD came from the Intravitreal Triamcinolone Study
(IVTAS), a randomised trial of a single dose of intravitreal triamcinolone acetonide
(4mg) for neovascular AMD 345. Patients referred or presenting to the Retina Unit
of the Sydney Eye Hospital (Sydney, Australia) were invited to participate if they
met the following criteria: (1) newly diagnosed neovascular AMD with any classic
component (≤3.5 Macular Photocoagulation Study disc areas, subfoveal or within
199μm from the foveal centre) 346; (2) aged 60 years or older; (3) had visual
symptoms less than a year; (4) visual acuity 20/200 or better; (5) with clear media;
(6) were offered and declined laser photocoagulation. The exclusion criteria were:
(1) other serious eye disease, including diabetic retinopathy, hypertensive
retinopathy, macular dystrophy, angioid streaks, high myopia (>8 dioptres),
glaucoma (with glaucomatous field loss), epiretinal membranes, macular hole and
nystagmus; (2) the use of systemic corticosteroids (prednisone ≥5mg/d) or any
drug that affects the macula, including chloroquine, hydroxycholoroquine
sulphate, thioridazine and cholorpromazine; (3) any condition, physical, mental or
social, that would affect regular follow up; and (4) any condition that would
prevent photographic or angiographic documentation.
A total of 139 participants were enrolled in the study. Informed consent was
obtained from each participant. The study was conducted in accordance with the
principles of the Declaration of Helsinki and was approved by the Southern
Eastern Sydney Area Health Service Research Ethics Committee.
Baseline serum was collected before administration of triamcinolone or placebo.
135
4.1.2.1.3 Serum used in autoantibody assays
Further inclusion criteria for all participants, from the BMES or IVTAS cohorts,
included: (i) absence of autoimmune disease or history of autoimmune disease; (ii)
serum that had undergone fewer than two freeze‐thaw cycles (to ensure integrity
of serum immunoglobulins); and (iii) serum without evidence of haemolysis (to
prevent interference with autoantibody binding).
Available serum that fulfilled all the selection criteria totaled 47 participants with
early AMD and 14 participants with GA from the BMES, and 74 participants with
neovascular AMD from the IVTAS. A total of 16 normal controls from the BMES
were also included. All serum was stored after collection at ‐80oC until use.
4 . 1 . 2 . 2 Au t o a n t i b o d y a s s a y
Detailed descriptions of protein isolation and quantification, electrophoresis,
western blotting, detection antibodies, chemiluminescence reagents and associated
buffers can be found in the Appendices (A&D‐G).
4.1.2.2.1 Preparation of human retinal proteins for electrophoresis
Isolated human retinae within 12 hours post mortem, from 4 healthy donors
(mean age 40.25 +/‐ 7.89 years) without eye disease, were homogenised in
phosphate‐buffered saline (PBS) and lysis buffer containing salt buffer, sterile
water, Mini‐Complete protease inhibitor cocktail (Boehringer Mannheim,
Germany) and NP40. Homogenates were placed on an orbital shaker on ice at
50rpm for 20 minutes and centrifuged at 4000rpm for 5 minutes at 4oC.
Supernatants were collected, pooled and stored at ‐20oC until use. Protein
concentration was determined as 7mg/ml using Bio‐Rad’s protein assay (Bio‐Rad
Laboratories, Hercules, California, USA), in microtitre plates with bovine serum
albumin as standard. Retinal homogenates were diluted 1:2 in SDS‐glycine buffer
136
containing 0.02% β‐mercaptoethanol. Samples were heated for 2 minutes at 100 oC
and clarified by centrifugation at 3000rpm for 3 minutes.
4.1.2.2.2 Electrophoresis and western blotting
Approximately 3μg of protein was applied per 1 mm of SDS‐polyacrylamide gel.
For the early experiments, a 12% resolving gel (4% stacking) was poured and set
prior to electrophoresis. Because of the entire range of retinal proteins is present in
whole retinal homogenates, these gels failed to separate the higher molecular
weight proteins well. Later experiments used a 4‐20% linear gradient SDS‐
polyacrylamide gel (Ready Gel from Bio‐Rad Laboratories). All gels used had a
single small well for loading the protein standard (Kaleidoscope precision plus
protein standard, Bio‐Rad Laboratories) and a large “prep” well that ran almost
the entire length of the gel which allowed retinal proteins from the same aliquot to
be applied across the whole gel. Proteins were separated by electrophoresis at
200V, 100mA and at room temperature for 30‐40min (depending on ambient
temperature). Gels were electro‐blotted onto Hybond P membrane (Amersham
Pharmacia Biotech, Buckinghamshire, England) at 60V, 200mA for 4 hours on ice
at 4oC.
4.1.2.2.3 Detection of serum anti‐retinal autoantibodies
Transblots were incubated in 5% tween tris‐buffered saline (TTBS), ph 7.4, for 1
hour at room temperature to block non‐specific binding. Serum from each
participant was reacted with the transblots overnight at room temperature. A
multiscreen apparatus (Bio‐Rad Laboratories, see Appendix G) allowed the large
area of blotted protein to be sequestered into individual chambers, such that a
single blot could be used to test serum from up to 18 individuals. Each well had a
total volume of 600μL, which also permitted significant savings in detection
antibodies and reagents. From this step onwards, blots were kept in the
multiscreen apparatus until the final wash step was complete.
137
An optimal serum dilution of 1:200 (in 0.05% TTBS) was found after trials to
minimise background reactivity. After overnight incubation in serum, transblots
were washed twice for 10 minutes with 0.05% TTBS, and once for 10 minutes with
TBS. Transblots were incubated with the detection antibody (see below) for 1 hour
at room temperature, followed by two 10‐minute washes in 0.05% TTBS, and a 10‐
minute wash in TBS. Blots were removed from the multiscreen apparatus at this
point and incubated in enhanced chemiluminescence (ECL) western blotting
detection reagent (Amersham Pharmacia Biotech) for 2 minutes in a dark room.
Transblots were then placed in plastic sleeves and exposed to Hyperfilm ECL
(Amersham Pharmacia Biotech) for 1‐3 minutes to detect autoantibody binding.
Initially, the 3 minute exposure time was used. It was later found that a 1 minute
exposure resulted in adequate films. Thus earlier blots appear much darker than
later blots (Figures 4.1‐4.3). Equivocal ECL film results were adjudicated by two
masked assessors. Dilution buffer without serum acted as a negative control in
each experiment, and serum from a patient with known autoimmune retinopathy
acted as a positive control.
4.1.2.2.4 Antibodies used to detect serum autoantibodies
To detect autoantibody binding, transblots that had been incubated in participant
serum overnight were incubated with a HRP‐conjugated secondary antibody.
Initially, serum from each participant was tested with a goat anti‐human IgGAM
(Zymed Invitrogen) detection antibody. This is a polyclonal antibody that
recognises both heavy and light chains of human IgG, IgA and IgM. All tests were
repeated at least once. Serum testing positive for anti‐retinal autoantibodies with
the IgGAM detection antibody was then tested in 4 separate experiments, using
fresh blots, with one of the following monoclonal antibodies: anti‐human IgG1,
anti‐human IgG2, anti‐human IgG3 and anti‐human IgG4 (all HRP‐conjugated,
mouse anti‐human, Zymed Invitrogen). Based on past reports, it was anticipated
138
that the majority of autoantibodies would belong to the IgG class. However, an
unexpectedly small proportion of IgGAM‐detected autoantibodies were found to
belong to one of the four IgG subclasses. It was subsequently decided that serum
testing positive with the anti‐IgGAM detection antibody, but negative using anti‐
IgG1, IgG2, IgG3 and IgG4, would be re‐tested using HRP‐conjugated monoclonal
mouse anti‐human IgM (Zymed Invitrogen). All detection antibodies were diluted
1:10 000 in TBS.
4 . 1 . 2 . 3 S t a t i s t i c s
Contingency tables comparing the proportions of participants with serum
autoantibodies in the following groups: (i) controls vs. early AMD; (ii) controls vs.
GA; (iii) controls vs. neovascular AMD; (iv) early AMD vs. neovascular AMD; (v)
early AMD vs. GA; and (vi) neovascular AMD vs. GA, were evaluated by the χ2
test, using SPSS (v15.0; SPSS Inc, Chicago, IL, USA).
139
4.1.3 Results
Anti‐retinal autoantibodies were found in all groups, including 25% of controls,
when the anti‐IgGAM detection antibody was used (Table 4.1‐1). Multiple bands
of reactivity were frequently found in the same individual (Figures 4.1‐1 – 4.1‐3).
The complexity of the reactivity patterns made comparisons between groups
difficult. Autoantibodies reacted against antigens in the following molecular
weight ranges: 70‐90kD, 40‐50kD and around 30kD. High molecular weight
(>120kD) antigens were rare.
Only a small proportion of serum tested positive for IgG autoantibodies with the
monoclonal anti‐IgG1‐4 detection antibodies (Figures 4.1‐1 – 4.1‐3). IgG4
autoantibodies were rarest, found only in one participant with early AMD.
Antigens were similar to those targeted by anti‐IgGAM detected autoantibodies.
Of note, participants with early AMD and GA had the highest proportion of IgG
autoantibodies and the largest numbers of participants with IgG autoantibodies
belonging to more than one subclass (Table 4.1‐1).
IgM autoantibodies were more frequently found in participants with neovascular
AMD. However, it should be noted that IgM autoantibodies may be more
prevalent than suggested in Table 4.1‐1, since only IgGAM positive and IgG1‐4
negative serum was tested.
140
Table 4‐1 Serum anti‐retinal autoantibodies in controls and in early, neovascular and atrophic AMD: summary of findings
IgG1 IgG2 IgG3 IgG4
Control (n=16) 66.1 +/- 5.8 6/10 4/16
25%2
(13%) 0 0 2 0 0 2
Early AMD (n=47) 72.9 +/- 6.8 17/30 27/47
(49%)16
(34%) 5 8 11 1 1 10 (21%)
Neovascular AMD (n=74) 76.9 +/- 7.2 31/43 26/74
(35%)8
(11%) 2 7 0 0 7 11 (15%)
GA (n=14) 85.3 +/- 6.1 4/10 7/14
(50%)6
(43%) 4 4 5 0 1 0 (0%)
IgG autoantibodies by subclassMean Age (years +/-SD)
Sex (M/F)
Anti-IgGAM detected autoantibodies
IgM Non-IgG Non-IgM
Anti-IgG detected autoantibodies (all
subclasses)
Number of participants in each group with IgG autoantibodies belonging to more than one subclass:
Control= 0; Early AMD = 8; Neovascular AMD = 2; Geographic atrophy =
141
Figure 4.1‐1 Western blot‐detected anti‐retinal autoantibodies in normal controls and
participants with geographic atrophy
Anti-IgGAM detection antibody
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
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Anti-IgG1 detection antibody
Anti-IgG2 detection antibody
Anti-IgG3 detection antibody
Anti-IgG4 detection antibody
142
Figure 4.1‐1 Western blot‐detected anti‐retinal autoantibodies in normal controls and
participants with geographic atrophy
Rows 1‐8: blots of serum from normal controls. Rows 1‐4 represent serum with anti‐
IgGAM detected autoantibodies. Rows 5‐8 represent serum without anti‐IgGAM detected
autoantibodies
Rows 9‐20: blots of serum from GA participants. Rows 9‐16 represent serum with anti‐
IgGAM detected autoantibodies. Rows 17‐20 represent serum without anti‐IgGAM
detected autoantibodies
Each column represents western blot results from the same individual
143
Figure 4.1‐2 Western blot‐detected anti‐retinal autoantibodies in participants with early
AMD
Anti-IgGAM detection antibody
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 311 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
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Anti-IgG1 detection antibody
Anti-IgG2 detection antibody
Anti-IgG3 detection antibody
Anti-IgG4 detection antibody
144
Figure 4.1‐2 Western blot‐detected anti‐retinal autoantibodies in participants with early
AMD
Rows 1‐27: early AMD participants with anti‐IgGAM detected autoantibodies
Rows 28‐31: early AMD participants without anti‐IgGAM detected autoantibodies
Each column represents western blot results from the same individual
145
Figure 4.1‐3 Western blot‐detected anti‐retinal autoantibodies in participants with
neovascular AMD
Anti-IgGAM detection antibody
Anti-IgG1 detection antibody
Anti-IgG2 detection antibody
Anti-IgG3 detection antibody
Anti-IgG4 detection antibody
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146
Figure 4.1‐3 Western blot‐detected anti‐retinal autoantibodies in participants with
neovascular AMD
Rows 1‐26: neovascular AMD participants with anti‐IgGAM detected autoantibodies
Row 27: positive control – patient with known autoimmune retinopathy
Rows 28‐31: neovascular AMD participants without anti‐IgGAM detected autoantibodies
Each column represents western blot results from the same individual
147
Participants with early AMD were four times more likely to have anti‐IGAM
detected autoantibodies compared to controls (p= 0.0025; χ2=5.028; 95% CI = 1.1‐
14.4). Participants with neovascular or atrophic AMD were no more likely to have
anti‐IgGAM or anti‐IgG(1‐4) detected autoantibodies than controls (Table 3.1‐2).
Participants with GA were seven times more likely to have anti‐IgG(1‐4) detected
autoantibodies compared to participants with neovascular AMD (Fisher’s exact p
= 0.005; 95% CI = 1.9 – 26.3).
Participants with early AMD were 1.6 times more likely to have anti‐IgGAM
detected autoantibodies compared to participants with neovascular AMD (p =
0.016, χ2 = 5.813, 95% CI = 1.6 – 2.4), and three times more likely to have anti‐IgG(1‐
4) detected autoantibodies (p = 0.001, χ2 =, 95% CI = 1.6 – 7.1).
148
Table 4‐2 Comparison of Anti‐IgGAM and anti‐IgG detected autoantibodies in control,
early AMD, neovascular AMD and atrophic AMD participants
IgGAM autoantibodies
absent
IgGAM autoantibodies
presentp
IgG autoantibodies
absent
IgG autoantibodies
presentp
Control 12 4 14 2
Early AMD 20 27 29 18
Control 12 4 14 2
Neovascular AMD 48 26 66 8
Control 12 4 14 2
GA 6 7 7 6
Neovascular AMD 48 26 66 8
GA 6 7 7 6
Early AMD 20 27 30 17
GA 6 7 7 6
Early AMD 20 27 30 17
Neovascular AMD 48 26 66 8
0.092
0.005*
0.535
0.001*
0.143
0.227
0.817
0.016*
0.025* 0.115
10.436
149
4.1.4 Discussion
The study confirms that complex repertoires of autoantibodies against retinal
antigens can be found in all individuals, including those without evidence of eye
disease. Despite this, individuals with early AMD had significantly more anti‐
retinal autoantibodies compared to normal controls and to participants with
neovascular AMD.
In early AMD, it is possible that anti‐retinal autoantibodies arise as a consequence
of the accumulation of extracellular debris in the form of BLinD, BLamD and soft
drusen. At least some of the constituents of these deposits have been shown to be
retinal in origin (see Sections 2.2.3). Although retinal antigens regularly leave the
ocular compartment via the blood inside ocular‐derived APCs 158, autoimmune
responses are not mounted against them 160. This is because ocular‐derived APCs
are programmed by a local microenvironment rich in immunosuppressive
cytokines 159, to which RPE cells contribute significantly (see Section 2.2.1.7). RPE
abnormalities are present in early AMD, and this may compromise their
immunomodulatory capacity, favouring the formation of autoantibodies which, in
this context could be considered pathological.
Another explanation could be that the autoantibodies are part of the natural
autoantibody repertoire, deployed to remove the build‐up of extracellular debris.
In this context the autoantibodies may be protective, since the efficient removal of
extracellular debris can prevent uncontrolled activation of inflammatory cascades
(see Section 2.6).
It is not clear why anti‐IgG detected autoantibodies were more frequently found in
participants with GA compared to participants with neovascular AMD. The only
previous study examining this group found the opposite – that patients with GA
150
were less likely to have immunofluorescence‐detected autoantibodies compared to
those with early or neovascular AMD 299 (see Section 2.6.3). In GA, Muller cell end
processes replace the absent RPE 311 and maintain the blood‐retina barrier. There is
some remodelling at the edges of areas of atrophy 8 and it is possible that these
liberate cellular debris. However, it is difficult to argue that the amount of
remodelling in GA could exceed that in neovascular AMD, a group in which
autoantibodies were found in surprisingly few individuals.
One possible explanation for this observation is that perhaps the majority of
autoantibodies in neovascular AMD belong to the IgM class. However, results
from this study cannot determine if this is the case, since not all IgGAM‐detected
autoantibodies were retested with the IgM detection antibody. The results do,
however, highlight the need to assess not just the autoantibody target antigen, but
the autoantibody immunoglobulin class and subclass in future studies. Changes in
the whole autoantibody repertoire will also need to be assessed, since it may be
complex, even in normal controls
The numbers of individuals with IgG autoantibodies in this sample were small
compared with previous studies (see Table 2.6‐1, Section 2.6). This may be due to
the highly specific nature of the monoclonal antibodies used, and the relatively
low concentration of serum tested. Because IgG autoantibodies were less common
than expected, and because they targeted more than one antigen, it was difficult to
assess subclass restriction. The significance of a larger number of participants with
IgG autoantibodies belonging to more than one class in the GA and early AMD
groups is not known and requires further investigation.
This study is limited by the small control group, which is not age‐matched. The
use of a large control group to establish normal autoantibody patterns of reactivity
will be important in future studies. A number of limitations are inherent in the use
151
of Western blotting as a detection method. Firstly, proteins are separated under
reducing conditions, such that their tertiary structure is lost, leading to loss of
conformational epitopes. Secondly, when using retinal proteins from whole retinal
homogenates, the molecular weight of autoantibody target proteins can only be
approximated, since the number of proteins in each gel and transblot is large.
Related to this limitation is the inability to measure autoantibody titres, since the
identity of the target antigen is not known. Finally, it is likely that autoantibodies
against oxidatively modified proteins and glycosylated proteins may be of more
importance in AMD, and the concentrations of these proteins in normal retinae
may be too minimal to allow autoantibody binding. The results of the following
two sections should thus be interpreted with these limitations in mind.
152
4 . 2 A N T I - R E T I N A L A U T O A N T I B O D I E S A N D D I S E A S E P R O G R E S S I O N
I N E A R L Y A M D
4.2.1 Introduction
Although no anti‐retinal autoantibodies were found to be specific for AMD in the
previous study, they were more frequently found in individuals with early AMD
than controls. This study examined whether the presence of baseline anti‐retinal
autoantibodies in participants with early AMD was associated with progression to
advanced AMD 5 and 10 years later.
4.2.2 Methods
4 . 2 . 2 . 1 P a r t i c i p a n t s
Participants with early AMD came from the BMES cohort as described in Section
4.1.2. Follow up examinations took place 5 and 10 years from date of enrolment, at
which ocular examination and fundus photography was carried out. Photographs
were graded using the Wisconsin Grading System by masked graders, and side‐
by‐side comparisons between baseline and 5‐ and 10‐ year photographs were
made.
4 . 2 . 2 . 2 Au t o a n t i b o d y a s s a y
Autoantibodies were detected as described in Section 4.1.2. Detailed descriptions
can also be found in the Appendices (A & D‐G). Since relatively few participants
had IgG anti‐retinal autoantibodies, the relationship between IgGAM‐detected
baseline autoantibodies and progression to advanced AMD was examined.
153
4 . 2 . 2 . 3 S t a t i s t i c s
The χ2 test was used to evaluate the association between baseline autoantibodies
and progression to advanced AMD at the 5‐ and 10‐year examinations, using SPSS
(v15.0; SPSS Inc, Chicago, IL, USA). A p value of < 0.05 was considered significant.
154
4.2.3 Results
A total of 11 participants (23.4%) with early AMD progressed to advanced AMD
by the time the 10‐year follow up examinations were conducted, including 5
participants (10.6%) who developed advanced AMD 5 years after baseline (Table
4.2.1). Three of these 5 participants developed unilateral neovascular AMD, and
two developed atrophic AMD in one eye and neovascular AMD in the fellow eye.
No association as found between the presence of anti‐IgGAM detected
autoantibodies at baseline and progression to advanced AMD at 5 years (χ2 =
0.015, Fisher’s exact p = 1.00). A further four participants (12.7%) progressed to
advanced AMD at the 10‐year follow up examination. Two of these participants
developed neovascular AMD and two developed atrophic AMD (Table 4.2.1).
Additionally, 2 participants with neovascular AMD at the 5 year examination
developed neovascular AMD in the fellow eye at the 10 year examination. There
was no association between baseline autoantibodies and progression to advanced
AMD over the 10 year‐period (χ2 = 0.156, Fisher’s exact p = 1.00).
155
Table 4‐3 Baseline serum anti‐retinal autoantibodies and progression to advanced
AMD at 5 and 10 years.
No Yes No Yes
RR
95% CI
chi square
Fisher's exact p
0.30 - 6.00
0.156
1.00
0.16 - 4.90
0.150
1.00
Autoantibodies Absent
Autoantibodies Present
Advanced AMD at 5 years Advanced AMD at 10 years
18
24 24
3
3
0.90
2
3
17
1.35
156
4.2.4 Discussion
This study could not find an association between the presence of baseline anti‐
retinal autoantibodies and progression to advanced AMD over a 5 to 10 year
period. There are several possible explanations for these findings. Firstly, although
more participants with early AMD had anti‐retinal autoantibodies than controls, it
is possible that these are a mix of both natural and pathological autoantibodies (as
discussed in Section 4.1.4). Secondly, it is likely that any autoantibody‐AMD
disease associations can only be found by examining changes in the whole
repertoire, which may be subtle and would require larger sample sizes and more
sophisticated tests. Finally, the autoantibody repertoire in each individual is likely
to be dynamic, and may have changed many times in the 5 to 10 years between
follow up visits.
The study is limited by the small sample size, and larger studies with more
frequent autoantibody assays between follow up visits are necessary to confirm
the findings. It should also be noted that participants with pigmentary changes
alone were not included in the original BMES definition of early AMD. Since this
group may represent a separate early AMD phenotype (see Section 3.1), its
inclusion in future comparative studies is important.
157
4 . 3 A N T I - R E T I N A L A U T O A N T I B O D I E S A N D V I S I O N L O S S I N
N E O V A S C U L A R A M D
4.3.1 Introduction
Patients with neovascular AMD are at high risk of rapid vision loss (Section 2.1.4).
Identifying eyes most at‐risk of rapid deterioration is thus an important part of
assessment and management. Although the previous study found that
neovascular AMD participants were no more likely than controls to have anti‐
retinal autoantibodies, this study examined whether the presence of baseline anti‐
retinal autoantibodies was associated with disease progression over a 24‐month
period. Ideally, this association should be studied in a cohort of individuals with
neovascular AMD, followed up for 12 to 24 months, without any treatment or
interventions. However, given the availability of palliative treatments for
neovascular AMD, such a study would be unethical to undertake and difficult to
recruit for.
The study cohort was thus selected from the IVTAS. Vision loss was chosen as the
outcome measure because it is a continuous variable which allowed the use of
linear mixed effects modelling to compare autoantibody‐positive and
autoantibody‐negative participants. This was the best available statistical method
that could account for all of the following potential interactions, for each
individual participant, at each follow‐up time point: (i) triamcinolone (or placebo)
treatment and vision; (ii) anti‐retinal autoantibodies and treatment; and (iii) anti‐
retinal autoantibodies and vision.
158
4.3.2 Methods
4 . 3 . 2 . 1 P a r t i c i p a n t s
Participants with neovascular AMD came from the IVTAS, as described in Section
4.1.2. The study took place over 12 months and was later extended to 24 months.
Participants were randomised to receive an injection of intravitreal triamcinolone
or subconjunctival isotonic saline, given after baseline visual acuity (VA)
measurements. Treatment assignment and masking protocols have been described
in detail elsewhere 345. Best‐corrected visual acuity was measured by trained
examiners using a back‐lit, self‐calibrating logMAR chart (Lighthouse
International, LaDalle, IL, USA) at 2.40, 0.95 and 0.60 metres. VA was evaluated at
baseline, 3, 6, 12, 18 and 24 months after entry into the study. Angiographic
grading of CNV as classic or occult was based on Macular Photocoagulation Study
guidelines 346.
Serum was collected prior to the administration of triamcinolone or placebo. Of
the 139 participants (151 eyes) enrolled in the trial, baseline serum from 74
participants (82 eyes) had undergone fewer than two freeze‐thaw cycles, was not
haemolysed, and was thus included in this study.
4 . 3 . 2 . 2 Au t o a n t i b o d y a s s a y
Autoantibodies were detected as described in Section 4.1.2. Detailed descriptions
can also be found in Appendices (A&D‐G).
4 . 3 . 2 . 3 S t a t i s t i c s
S‐Plus (v6.2, Insightful Corporation, Seatle, WA, USA) was used to fit linear mixed
effect models to VA. Eye identification number was incorporated as a random
effect, treatment group and baseline autoantibody status as fixed effects, and
month as a fixed covariate. In all cases, both two and three‐way interactions were
159
fitted and dropped from the final models if not significant. Two‐tailed tests with a
5% level of significance were used throughout.
160
4.3.3 Results
Participant characteristics and baseline autoantibody status are summarised in
Table 4.3‐1. Classic neovascular lesions were present in 59 eyes, and occult lesions
in 23 eyes. There was no difference in gender, mean age (t72 = ‐1.21, p = 0.23) or rate
of vision loss over 24 months (t286 = ‐1.05, p = 0.30) between the triamcinolone
(n=32) and placebo (n=42) treated groups.
There were no significant differences in age (p = 0.504) or sex (p = 0.266) between
autoantibody‐positive and autoantibody‐negative participants. Autoantibodies
appeared to be more frequently found in participants with classic neovascular
lesions (12%) compared to those with occult lesions (4%), although this was not
statistically significant (p = 0.431).
Linear mixed effects modelling showed that anti‐IgGAM detected baseline anti‐
retinal autoantibodies were not associated with vision loss over a 24 month period
(t288 = ‐0.12, p = 0.91) (Figure 4.3.1). However, participants with baseline anti‐IgG(1‐
4) detected autoantibodies lost, on average, 0.7 more letters of vision on a logMAR
chart per month (or 8.4 more letters per year) than other participants (t289 = ‐2.17, p
= 0.03), irrespective of treatment group (Figure 4.3‐2)
161
Table 4‐4 Participant characteristics and baseline anti‐retinal autoantibody status
IgG1 IgG2 IgG3 IgG4 Total IgG
Placebo 42/42 18/24 77.4 +/- 6.6 16/42 (38%) 0 2 0 0 2 (5%)
Triamcinolone 40/32 13/19 75.3 +/- 7.8 10/32 (31%) 2 5 0 0 6 (9%)
Number of Patients/Eyes M/F Mean age
(years)IgGAM-detected autoantibodies
IgG1-4 detected autoantibodies
162
Figure 4.3‐1 Vision loss over 24 months in anti‐retinal autoantibody positive
participants vs. anti‐retinal autoantibody negative participants (anti‐IgGAM detected
autoantibodies)
0
10
20
30
40
50
60
0 3 6 12 18 24
Month
Mea
n Lo
gMar
Acu
ity
Anti-IgGAM detected autoantibodies absent
Anti-IgGAM detected autoantibodies present
p = 0.91
163
Figure 4.3‐2 Vision loss over 24 months in anti‐retinal autoantibody positive
participants vs. anti‐retinal autoantibody negative participants (anti‐IgG detected
autoantibodies)
0
10
20
30
40
50
60
0 3 6 12 18 24
Month
Mea
n Lo
gMar
Acu
ity
Anti-IgG detected autoantibodies absent
Anti-IgG detected autoantibodies present
p = 0.03
164
4.3.4 Discussion
That baseline IgG anti‐retinal autoantibodies were associated with more rapid
vision loss over 24 months in participants with neovascular AMD is surprising.
Few participants with neovascular AMD had IgG anti‐retinal compared to those
with early AMD or GA (see Section 4.1). The finding may thus be an indication
that in neovascular AMD, the anti‐retinal autoantibodies are pathological and not
a part of the natural repertoire. It is also interesting that this association was not
observed with anti‐IgGAM detected autoantibodies, which may have belonged to
any of three immunoglobulin classes, and were thus less specific.
The neovascular disease process involves the growth of abnormal and highly
permeable new vessels into the sub‐RPE or sub‐retinal space. The high rate of
extracellular matrix turnover and vascular remodelling in neovascular AMD
lesions may liberate significant cellular debris. There is disruption of the external
blood‐retina barrier, and the presence of local pro‐inflammatory cytokines 224. All
of these factors may produce circumstances favourable to the initiation of
autoimmune events. These explanations are at this stage, speculative, since the
number of participants with IgG autoantibodies in this cohort was small and the
findings will need to be confirmed in larger studies. However, the recruitment of
participants with neovascular AMD into a study without interventions will
continue to prove challenging.
It should also be noted that the main outcome measure, visual acuity, may be
influenced by other factors and may not be the most reliable indicator of disease
progression. Outcome measures such as changes in lesion size and vessel leakage
should be included in future studies where possible.
165
S U M M A R Y & C O N C L U S I O N S
5
166
The histopathological threshold at which normal ageing becomes early AMD is
defined by the presence of both BLinD and a continuous layer of BLamD. Because
these eyes appear clinically normal in the majority of cases, more sophisticated
clinical tests will be required for early detection. Evidence for distinct early AMD
phenotypes and two separate pathways along which early AMD can progress can
be used to inform future clinical classifications systems, as well as genetic and
epidemiological studies.
BLamD and membranous debris accumulation may increase the recruitment of
macrophages to sub‐macular Bruch’s membrane and alter the programming of
resident choroidal macrophages. This observation requires confirmation,
particularly in the context of subclinical CNV. Animal models that allow
macrophage‐associated genes to be knocked out are likely to further elaborate
normal resident choroidal macrophage function. However, interpretation of data
from animal models will require better characterisation of human resident
choroidal macrophages, about which little is known.
The relationship between anti‐retinal autoantibodies and AMD is probably
complex. It is unlikely that one or two autoantibodies will be specifically
associated with AMD. Rather, future studies will need to assess the whole
autoantibody repertoire to discern subtle changes in the reaction patterns. For
these studies to be meaningful, a large control group is necessary, so that normal
patterns of reactivity can be established. A number of approaches allow the
examination of complex autoantibody repertoires each with strengths and
limitations. Software designed to assess densitometry differences on western blots
have been used 296. While this approach does not allow identification of the target
antigens, it can be used to direct proteomic studies. A combination of 3‐D gel
electrophoresis and liquid chromatography/mass spectrometry can then
determine the amino acid sequence of the target proteins. This approach,
167
however, does not allow rapid screening of a large number of serum samples, a
problem that can be overcome by the use of protein microarrays. Made‐to‐order
arrays with thousands of purified proteins are becoming increasingly available
and affordable. The difficulty with this approach is in determining the appropriate
proteins for an autoantibody assay. It is likely that a combination of all three
methods will determine the significance of anti‐retinal autoantibodies in AMD
pathogenesis.
168
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194
A P P E N D I C E S
A
APPENDIX A : ANTIBODIES
Immunohistochemistry – primary antibodies
Antibody Clone Raised in Company Catalog
number
Monoclonal anti-human CD68 KP1 Mouse Zymed Invitrogen, Carlsbad, CA, USA 08-0125
Monoclonal anti-human iNOS 6 Mouse BD Transduction Laboratories, San Jose, CA, USA 610328
Immunohistochemistry – secondary antibody
Antibody Clone Raised in Company Catalog
numberBiotinylated monoclonal anti-mouse
IgG (H+L) N0403 Horse Vector Laboratories, Burlingame, CA, USA BA-2000
Western blotting – detection antibodies
Antibody Clone Raised in Company Catalog
numberPeroxidase polyclonal anti-human
IgG+A+M (H+L) N/A Goat Zymed Invitrogen, Carlsbad, CA, USA 62-8320
Peroxidase monoclonal anti-human IgG1 HP6069 Mouse Zymed Invitrogen, Carlsbad, CA, USA 05-3320
Peroxidase monoclonal anti-human IgG2 HP6014 Mouse Zymed Invitrogen, Carlsbad, CA, USA 05-0520
Peroxidase monoclonal anti-human IgG3 HP6047 Mouse Zymed Invitrogen, Carlsbad, CA, USA 05-3620
Peroxidase monoclonal anti-human IgG4 HP6025 Mouse Zymed Invitrogen, Carlsbad, CA, USA 05-3820
Peroxidase monoclonal anti-human IgM HP6083 Mouse Zymed Invitrogen, Carlsbad, CA, USA 05-4900
B
APPENDIX B : CD68 IMMUNOHISTOCHEMISTRY
1. Xylene 2 x 3 min 2. 100% alcohol 2 x 3 min 3. 95% alcohol 3 min 4. 70% alcohol 3 min 5. Tapwater rinse 6. 1% peroxide solution 30 min ( 0.5ml peroxide in 49.5ml 50% alcohol) 7. Distilled water rinse 8. Tris rinse 9. Pepsin digestion at 37oC 5 min 10. Tris rinse 11. 10% normal horse serum 20 min (1ml normal horse serum in 9 ml Tris) 12. Primary antibody : mouse anti‐human CD68 Neat 1 hr at 37oC (diluted in 1% normal horse serum) 13. Tris 3 x 5 min 14. Secondary antibody : Biotinylated anti‐mouse IgG 30 min at 37oC
1:200 15. Tris 3 x 5 min 16. ABC 1:500 30 min at RT 17. Tris 3 x 5 min 18. Vector SG Blue 10 min 19. Tris 3 x 5 min 20. Distilled water rinse 21. 70% alcohol 3 min 22. 95% alcohol 3 min 23. 100% alcohol 2 x 3 min 24. Xylene 2 x 3 min 25. Coverslip in DPX
C
APPENDIX C : iNOS IMMUNOHISTOCHEMISTRY
1. Xylene 2 x 3 min 2. 100% alcohol 2 x 3 min 3. 95% alcohol 3 min 4. 70% alcohol 3 min 5. Tapwater rinse 6. 1% peroxide solution 30 min ( 0.5ml peroxide in 49.5ml 50% alcohol) 7. Distilled water rinse 8. Tris rinse 9. 10% normal horse serum 20 min (1ml normal horse serum in 9 ml Tris) 10. Primary antibody : mouse anti‐human iNOS 1:400 1 hr at 37oC (diluted in 1% normal horse serum) 11. Tris 3 x 5 min 12. Secondary antibody : Biotinylated anti‐mouse IgG 30 min at 37oC 1:200 13. Tris 3 x 5 min 14. ABC 1:500 30 min at RT 15. Nickel ammonium sulphate Tris 3 x 5 min (100mg nickel ammonium sulphate in 250ml Tris) 16. DAB made in nickel ammonium sulphate 10 min
+ 70μl H2O2 (1 DAB tablet dissolved in 15ml of nickel ammonium sulphate in Tris and filter)
17. Tris 3 x 5 min 18. Distilled water rinse 19. 70% alcohol 3 min 20. 95% alcohol 3 min 21. 100% alcohol 2 x 3 min 22. Xylene 2 x 3 min 23. Coverslip in DPX
D
APPENDIX D : PREPARAT ION OF RET INAL PROTE IN FOR
ELECTROPHORES I S
Dissection: 1 X human eye 1 X sterile dissection kit dissection tray PBS ~20mL 6 X 1mL eppendorfs for storage Perform in vacuum extraction hood Remove cornea and anterior chamber using curved scissors Remove vitreous Add PBS to the cup formed by remaining eye Tease out retina using fine forceps and cut at optic disc Divide retina into approx 6 pieces Lysis: Lysis buffer ~10mL Sterile 10mL syringe Sterile drawing up or 19G needle 2 X sterile 10mL centrifuge tubes per piece of retina 10 X 1mL sterile eppendorfs per piece of retina for microfuge 10 X 1mL sterile eppendorfs per piece of retina for storage human retina ice in styrofoam container 1000μL pipetteman & sterile tips In vacuum extraction hood: Draw up 5mL lysis buffer in syringe Add retina Homogenise by drawing up and down into 10mL centrifuge tube until homogenate macroscopically evenly mixed Place on orbital shaker on ice @ 50rpm for 20min Place in large centrifuge after balancing Spin @ 4000rpm for 5min @ 4oC Transfer supernatant into 10 eppendorf tubes Place in microfuge and spin @ 13000rpm for 5min @ RToC Transfer supernatant into 5‐10 eppendorf tubes Label and store at –20oC.
E
APPENDIX E : ELECTROPHORES I S & WESTERN BLOTT ING
Electrophoresis
1. Dilute retinal homogenates 1:2 in loading buffer (2X) 2. Heat samples for 2 minutes at 100 oC and clarify by centrifugation at
3000rpm for 3 minutes. 3. Assemble MiniProtean cell (BioRad Laboratories) for electrophoresis & fill
with running buffer 4. Load samples in the prep well of a 4‐20% Tris‐Glycine polyacrylamide get
in a Load Kaleidoscope precision plus protein standard (BioRad Laboratories) in the standards well
5. Run samples at 200V, 100mA and at room temperature for 30‐40min (depending on ambient temperature)
Western blotting
1. Pre‐wet Hybond P membrane (Amersham Pharmacia Biotech) in methanol and allow to equilibrate for 10min in transfer buffer
2. Equilibrate gels in transfer buffer after electrophoresis 3. Assemble the Mini‐Protean cell is assembled for blotting & fill with transfer
buffer and ice block 4. Blot at 60V, 200mA for 4hrs at 4 oC
Autoantibody detection
1. Block transblots for 1hr in blocking buffer (5% tween tris‐buffered saline) at room temperature
2. Wash transblots in wash buffer 3x10min 3. Assemble multiscreen apparatus and insert transblot 4. Load each multiscreen well with serum diluted 1:200 in TBS and incubate
overnight at room temperature 5. Wash in wash buffer 2x10min 6. Remove transblots from multiscreen apparatus and wash in TBS 1x10min 7. Mix together 10mls each of ECL solutions 1&2 (Amersham Pharmacia
Biotech) 8. In dark room, incubate tranblot in ECL mixture for 2 min 9. Remove and blot with filter paper before inserting into plastic sleeves 10. Expose to ECL film for 1‐3min 11. Develop ECL film in photographic developer until a good signal can be
seen and 12. Place film in photographic fixer for 5 sec 13. Rinse and dry film
F
APPENDIX F : BUFFERS
Lysis Buffer: 1ml 10X Salt buffer 9ml sterile H2O 1 X tablet miniComplete protease inhibitor cocktail 100μL NP40
Loading Buffer:
4 ml 10% (w/v) SDS 2 ml glycerol 1 ml 0.1% (w/v) bromophenol blue 2.5 ml 0.5M Tris‐HCl (pH 6.8) 0.2 ml β‐mercaptoethanol
10 ml deionised water Running buffer (10X): 29g Tris base 144g Glycine 10g SDS miliQ H2O to 1000ml Transfer buffer:
3g tris base 14.4g glycine 200mL methanol miliQ H2O to 1000ml Tris‐buffered saline (10X):
6.05g Tris base 43.9g NaCl milliQ H2O to 500ml
Blocking buffer: 5ml Tween 20
1X TBS to 100ml Wash buffer: 400μL Tween 20 1X TBS to 800ml
G
APPENDIX G : MULT ISCREEN APPARATUS
An ECL film of a typical blot produced using the multiscreen apparatus. Protein
standards at left.
Photograph of apparatus courtesy of Bio‐Rad Laboratories. Hercules, California USA