Summary Of The Last Section

Types of pathogens & antigens Cytosolic – “endogenous” Endocytic/vesicular – “exogenous”

Two classical pathways for Ag processing Endogenous MHC Class I pathway CD8+ T cells Exogenous MHC Class II pathway CD4+ T cells

Ag processing, presentation & clinical relevance MHC deficiencies Mechanisms used by pathogens to evade host immunity Considerations in vaccine design

Cell interactions & co-operation Lymphocyte re-circulation & homing Cell interactions: membrane-bound & soluble molecules

Dendritic cells (DC)& the initiation of immune responses

• Lymphocyte activation– Ag recognition (Signal 1)– Co-stimulations (Signal 2)

• T-B cell cooperation– The original model– The modified model– The contemporary model

• DC – the TRUE professional APC– Activator of naïve T cells– Special Ag processing machine– Ag cross-presentation

• Immune initiation & the ‘Danger model’

Ag recognition alone (signal 1)does not initiate an immune response

“signal 2” is needed

Question 1: How is an (adaptive) immune response initiated?

• Importance of thymus in immune responses– Miller & Good (1961)

• Phenomenon of T-B cell co-operation– AJS Davies (1964)– Claman & Mitchell (1967)

• ‘Linked recognition’ – the ‘hapten-carrier effect’– N. A. Mitchison (1970)

T-B cell cooperation

The “original” 2-signal model(Bretscher & cohn, 1970)

THB

2nd signal (‘co-stimulation’)

(1)

(2)

Activated

Finding:

T cell Ag recognition is MHC-restricted

Question 2

How could T and B cells recognize the same Ag if T cells recognized only processed Ag presented by MHC?

Linked recognition - the ‘carrier-priming’ experiment (N. A. Mitchison)

TNP-C1

C2

TNP-C2B

TH

Anti-TNP response

TNP: A hapten of trinitrophenyl group

The “modified” model

TH B

Cytokines etc.

(1)

(2)

Activated

TNP

C2

Another problem:

Resting B cells do not activate naïve T cells

Question 3

How can naive T cells be activated?

• Naïve lymphocytes Cells that have never encountered specific antigen

• Armed effector lymphocytesActivated & differentiated cells that may respond to antigen binding alone to produce effector functions

• Memory lymphocytesCells that have experienced specific antigen previously but need to be triggered to differentiate again to become effector cells

Dendritic cell (DC)

- The activator of naïve T cells

R. M. Steinman

(Rockefeller)

Photograph copyright:G.G. MacPherson (Oxford, 1997)

Dendritic cells & follicular dendritic cells are two very different cell types

DC FDC

Origin: Bone marrow-derived ?Haematopoietic

Where: T areas, B follicles& peripheral tissues

Nature: endocytic & migratory resident

Life-span: days years

Present Ag: as peptide to T cells as IC to B cellsby MHC by Ig-Fc

DC Biology

• Sentinel (constant surveillance)– Distribution throughout peripheral tissues

• Endocytic (Ag uptake)– Phagocytosis– Micropinocytosis– Macropinocytosis

• Migratory (Ag transport)– from peripheral tissues to secondary lymphoid organs

AL: Afferent LymphaticsSCS: Subcapsular SinusPCV: Post-Capillary Venule(HEV: High Endothelial Venule)

The site of lymphocyte activation

+ DC

GC

(B + FDC)F

T: T cell areaB: B cell area

F: B cell follicleGC: germinal centre

Organizeddistribution

DC: An unique Ag processing machine

(immature DC)

• Attenuated lysosomal potential for Ag degradation– Ag sequestered from lysosome for extended period

• Regulated cathepsin S activity– delayed cleavage of MHC II-associated Ii chain favouring MHC II

transport to lysosome

(Mellman I & Steinman RM, Cell. 2001; 106:255-8)

The invariant chain

CLIP

Ii

Two CLASSICAL pathwaysfor Ag processing

• “MHC class I pathway” CD8+ T cells (Endogenous/cytosolic/TAP-dependent pathway)

• “MHC class II pathway” CD4+ T cells(Exogenous/endocytic/TAP-independent pathway)

Ag ‘cross-presentation’ - DC breaking the rules

• Ag “cross-presentation”:– Endocytic/exogenous Ag Class I pathway (TC)

(proteasomal proteolysis, TAP-dependent)

• “Indirect” Ag presentation:– Cytosolic/endogenous Ags Class II pathway (TH)

Cytosolic/Endogenous Ag

Endocytic/Exogenous Ag

CD8+ T

MHC Class I

CD4+ T

MHC Class II

Cross-presentation Indirect presentation

Prot

easo

me

(TAP

-dep

ende

nt)

Lyso

som

e

CRAIG RR. Nature 425, 351-52 (2003)

Antigens cross-presented

• Virus-infected apoptotic cells

• Apoptotic tumor cells

• Other cell death due to normal cell turnover

• Transplantation Ags

• Endocytosed Ag: small fragments (3-12 KD)

DC – the TRUE professional APC

• Sentinel position• Endocytic• Migratory

• Unique location in the secondary lymphoid organs• Special Ag processing machine

• High MHC Class I, Class II (Ag presentation)• Constitutive expression of B7 (co-stimulation)

Co-stimulations- cell interactions other than Ag specific stimulation

Adhesion molecules

Cytokines & cytokine receptors

B7/CD28, CTLA-4:

-B7:CD28 interaction delivers a positive signal to T cells- B7:CTLA-4 interaction delivers a negative signal to T cells

CD40/CD40L:

- crucial for B cell growth & differentiation

The contemporary model

Naïve TH

Cytokines etc

(1)

(2)

DC

Activated

(1)(2)

CD28

B7Co-stimulations

B

T-B cell cooperation

Effector TH B

CD40L:CD40

(1)(2)

Abs:IgMIgG1, IgEIgAIgG2a, IgG3(isotype switch)

Cytokines:IL-4IL-5IFNg

Further questions:

Do DCs need to be activated?Do DCs need to be activated?

What then activates DCs?What then activates DCs?

Vaccination: why adjuvant?Vaccination: why adjuvant?

Immune system turned on Immune system turned on by “non-self” or by “Danger” ?by “non-self” or by “Danger” ?