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Why is an organ rejected?Why is an organ rejected?
The mammalian immune system is an complex system
developed in response to evolutionary stressors provided by
co-existence with micro-organisms over millions of years.
The system is divided into:
Natural immunity: the nonspecific immune response
Macrophages, neutrophils, natural killer cells, cytokines, certain
cellular receptors, and complement components
Adaptive immunity: the response to a specific antigen.
B cells, T cells
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Why is an organ rejected?Why is an organ rejected?
In organ Tx, the principal target of the immune
response to the graft are the Major
Histocompatibility Complex (MHC) molecules
expressed on the surface of donor cells (allo-MHC).
This is a form of adaptive immunity.
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T cells recognize antigen in the form of
peptide bound to MHC proteins.
B cells have IG receptors that can
recognize the antigenic portions of intact
molecules.
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T CellsT Cells
T cells regulate the activities of B cells, T cells & other
cells participating in immune responses.
They provide help for Ab production by B cells,
They are effectors of Ag-specific cell-mediated immunity
(CMI).
CMI is important in the elimination of Cells infected with intracellular pathogens (viruses,
mycobacteria, some bacteria)
Cells exhibiting aberrant differentiation (eg, neoplasms).
Allogeneic cells (graft rejection).
5 Bretscher P, Cohn M: A theory of self-nonself discrimination. Science 169:1042-49, 1970
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Activated T cells →
Clonal expansion →
Effector Mechanisms
Activated T cells →
Clonal expansion →
Effector Mechanisms
Mitogenic growth & differentiation factors. e.g IL-2.
Mitogenic growth & differentiation factors. e.g IL-2.
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CostimulationAllorocognition
T cell activation
Clonal Expansion ofAlloreactive Cells
Effector Mechanisms
CD8 + T cellsCD4 + T cells
Apoptosis Induction
Cell Lysis
MacrophagesB cells
Allo-Ab Production DTH
IL-2
Memory
T cells
Th1Th1Th2Th2 IL2
IFN
IL2
IFNIL4, 5, 10, 13IL4, 5, 10, 13
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MHC class IMHC class I
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MHC class I molecules are found on almost all cell types except for RBCs
MHC class I molecules are found on almost all cell types except for RBCs
Intra-cytoplasmic peptides are presented together
with class I MHC on the cell surface to CD8 T cells.
CD8 positive T cells induce cell lysis (by inducing
apoptosis or active killing of cells)
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MHC class IIMHC class II
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Class II MHC is expressed only on interstitial dendritic cells, macrophages, & B cells and activated endothelial cells.
Class II MHC is expressed only on interstitial dendritic cells, macrophages, & B cells and activated endothelial cells.
MHC II molecules bind peptides derived from
extracellular (exogenous) proteins.
Exogenous proteins that have been endocytosed and
fragmented are transferred to the cell surface and
presented to CD4 T cells together with class II MHC
molecules.
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High levels of class II MHC are expressed on APCsHigh levels of class II MHC are expressed on APCs
1. Dendritic cells: are the major "professional" APCs.
2. Monocyte/Macrophages: play a partial role in Ag
presentation
3. B cells: express MHC II, required to receive T cell help.
Although they do not activate resting naive T cells, they
may stimulate memory T cells.
4. Vascular endothelial cells.
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Other antigens mediating rejection
Minor Histocompatibility Ags
Other antigens mediating rejection
Minor Histocompatibility AgsNIH GUIDE, Volume 26, Number 24, July 25, 1997NIH GUIDE, Volume 26, Number 24, July 25, 1997
These antigens appear to be MHC class I bound small peptides presented to - -T cells in an MHC-restricted manner.
Thus, mHAgs are recognized with the same degree of specificity as the major histocompatibility antigens
Most mHAgs result from nonsynonymous Single Nucleotide Polymorphisms (SNP), accounting for around 90% of sequence differences (Collins et al, 1998).
They elicit most commonly, MHC I restricted responses. They donnot activate Ab production, though they need CD4+ activation for
CD8+ helpABO AntigensABO Antigens
Monocyte endothelial Antigens:Monocyte endothelial Antigens: Rarely cause hypercute rejection despite ABO compabilityRarely cause hypercute rejection despite ABO compability
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Pathways of Pathways of AllorcognitionAllorcognition
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NormalNormal
pathwaypathway
Early Acute Early Acute
rejectionrejection
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Ben M. Illigens et al, Human Immunology:2002, 63, 912–925
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Signal
1
Signal
1
Signal 2
Signal 2
Bretscher P, Cohn M: A theory of self-nonself discrimination. Science 169:1042-49, 1970
T cell activation and function
T cell activation and function
TK p
651c
k
TK p
651c
k
Ph. of IC
proteins
Ph. of IC
proteins
Activation of calcineurin,
NFAT, NF-kB
Activation of calcineurin,
NFAT, NF-kB
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ZAP-70
Fyn
Lck
Cellular and Molecular Immunology (5th Ed.) Abbas AK, & Lichtman, Editor: Saunders, Philadelphia, 2005
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Resting T cells express CD28, but resting antigen-presenting cells (APCs) do not express B7 molecules. Within six hours after activation, B7-2 is expressed by antigen-presenting cells and is available to bind to CD28, transmitting a costimulatory signal to the T cell. By 48 to 72 hours after activation, antigen-presenting cells also express B7-1, whereas T cells express the CTLA-4 inhibitory receptor. Both B7-1 and B7-2 can bind to either CD28 or CTLA-4, providing continued costimulation or a new inhibitory signal, respectively. Because CTLA-4 binds B7 molecules with a higher affinity than does CD28, its inhibitory interaction eventually predominates, leading to the termination of the immune response. The fusion protein CTLA-4–Ig can compete with CD28 and CTLA-4 for B7 binding, thus preventing costimulatory interactions.
Resting T cells express CD28, but resting antigen-presenting cells (APCs) do not express B7 molecules. Within six hours after activation, B7-2 is expressed by antigen-presenting cells and is available to bind to CD28, transmitting a costimulatory signal to the T cell. By 48 to 72 hours after activation, antigen-presenting cells also express B7-1, whereas T cells express the CTLA-4 inhibitory receptor. Both B7-1 and B7-2 can bind to either CD28 or CTLA-4, providing continued costimulation or a new inhibitory signal, respectively. Because CTLA-4 binds B7 molecules with a higher affinity than does CD28, its inhibitory interaction eventually predominates, leading to the termination of the immune response. The fusion protein CTLA-4–Ig can compete with CD28 and CTLA-4 for B7 binding, thus preventing costimulatory interactions.
Sayegh MH, Turka LA NEJM, 1998, 338: 1813-21
6 hrs
IL-2 & other cytokines
Anergy or apoptosis
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Resting antigen-presenting cells (APCs), which include B cells, macrophages, and dendritic cells, express CD40. When activated, T cells express CD40 ligand (CD40L). Interactions between CD40 ligand and CD40 are important in providing B-cell help to prevent apoptosis and induce immunoglobulin production and isotype switching. Activation of CD40 on APCs provides a signal for the induction of B7 molecules, particularly B7-2. CD40 ligand may act in T-cell costimulation by directly providing costimulation, by inducing B7, or by inducing other costimulatory ligands.
Sayegh MH, Turka LA NEJM, 1998, 338: 1813-21
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Accessory molecules of T cellsAccessory molecules of T cells
CD 28- B7: Production of IL-2 and other cytokines CTLA4- B7: Inhibition of T cell stimulation [Abatacept (CTLA4-Ig)] LFA-1- ICAM-1 (2, 3): Adhesion of T cells to APCs, endothelial cells and
extracellular matrix proteins and adhesion-dependent lymphocyte functions L-Selectin-CH ligands on HEV: Regulate migration of leukocytes to various
tissues CD44-matrix proteins: Retention of T cells to endothelium at sites of
inflammation CD40L- CD40:
important signals for Ab production Activation of macrophages to destroy phagocytosed microbes B7 expression on all APCs (role in co-stimulation) Production of adhesion molecules & inflammatory cytokines by APCs →
T- cell activation Fas ligand(D95)- Fas: T cell apoptosis and elimination
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Accessory molecules of T cellsAccessory molecules of T cells
CD 28- B7: Production of IL-2 and other cytokinesCD 28- B7: Production of IL-2 and other cytokines CTLA4- B7: Inhibition of T cell stimulation [Abatacept (CTLA4-Ig)] LFA-1- ICAM-1 (2, 3): Adhesion of T cells to APCs, endothelial cells and
extracellular matrix proteins and adhesion-dependent lymphocyte functions L-Selectin-CH ligands on HEV: Regulate migration of leukocytes to various
tissues CD44-matrix proteins: Retention of T cells to endothelium at sites of
inflammation CD40L- CD40:
important signals for Ab production Activation of macrophages to destroy phagocytosed microbes B7 expression on all APCs (role in co-stimulation) Production of adhesion molecules & inflammatory cytokines by APCs →
T- cell activation Fas ligand(D95)- Fas: T cell apoptosis and elimination
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Accessory molecules of T cellsAccessory molecules of T cells
CD 28- B7: Production of IL-2 and other cytokines CTLA4- B7: Inhibition of T cell stimulation [Abatacept (CTLA4-Ig)]CTLA4- B7: Inhibition of T cell stimulation [Abatacept (CTLA4-Ig)] LFA-1- ICAM-1 (2, 3): Adhesion of T cells to APCs, endothelial cells and
extracellular matrix proteins and adhesion-dependent lymphocyte functions L-Selectin-CH ligands on HEV: Regulate migration of leukocytes to various
tissues CD44-matrix proteins: Retention of T cells to endothelium at sites of
inflammation CD40L- CD40:
important signals for Ab production Activation of macrophages to destroy phagocytosed microbes B7 expression on all APCs (role in co-stimulation) Production of adhesion molecules & inflammatory cytokines by APCs →
T- cell activation Fas ligand(D95)- Fas: T cell apoptosis and elimination
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Accessory molecules of T cellsAccessory molecules of T cells
CD 28- B7: Production of IL-2 and other cytokines CTLA4- B7: Inhibition of T cell stimulation [Abatacept (CTLA4-Ig)] LFA-1- ICAM-1 (2, 3): Adhesion of T cells to APCs, endothelial cells and LFA-1- ICAM-1 (2, 3): Adhesion of T cells to APCs, endothelial cells and
extracellular matrix proteins & adhesion-dependent lymphocyte functionsextracellular matrix proteins & adhesion-dependent lymphocyte functions L-Selectin-CH ligands on HEV: Regulate migration of leukocytes to various
tissues CD44-matrix proteins: Retention of T cells to endothelium at sites of
inflammation CD40L- CD40:
important signals for Ab production Activation of macrophages to destroy phagocytosed microbes B7 expression on all APCs (role in co-stimulation) Production of adhesion molecules & inflammatory cytokines by APCs → T-
cell activation Fas ligand(D95)- Fas: T cell apoptosis and elimination
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Accessory molecules of T cellsAccessory molecules of T cells
CD 28- B7: Production of IL-2 and other cytokines CTLA4- B7: Inhibition of T cell stimulation [Abatacept (CTLA4-Ig)] LFA-1- ICAM-1 (2, 3): Adhesion of T cells to APCs, endothelial cells and
extracellular matrix proteins and adhesion-dependent lymphocyte functions L-Selectin-CH ligands on HEV: Regulate migration of leukocytes to L-Selectin-CH ligands on HEV: Regulate migration of leukocytes to
various tissuesvarious tissues CD44-matrix proteins: Retention of T cells to endothelium at sites of
inflammation CD40L- CD40:
important signals for Ab production Activation of macrophages to destroy phagocytosed microbes B7 expression on all APCs (role in co-stimulation) Production of adhesion molecules & inflammatory cytokines by APCs →
T- cell activation Fas ligand(D95)- Fas: T cell apoptosis and elimination
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Accessory molecules of T cellsAccessory molecules of T cells
CD 28- B7: Production of IL-2 and other cytokines CTLA4- B7: Inhibition of T cell stimulation [Abatacept (CTLA4-Ig)] LFA-1- ICAM-1 (2, 3): Adhesion of T cells to APCs, endothelial cells and
extracellular matrix proteins and adhesion-dependent lymphocyte functions L-Selectin-CH ligands on HEV: Regulate migration of leukocytes to various
tissues CD44-matrix proteins: Retention of T cells to endothelium at sites of CD44-matrix proteins: Retention of T cells to endothelium at sites of
inflammationinflammation CD40L- CD40:
important signals for Ab production Activation of macrophages to destroy phagocytosed microbes B7 expression on all APCs (role in co-stimulation) Production of adhesion molecules & inflammatory cytokines by APCs →
T- cell activation Fas ligand(D95)- Fas: T cell apoptosis and elimination
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Accessory molecules of T cellsAccessory molecules of T cells
CD 28- B7: Production of IL-2 and other cytokines CTLA4- B7: Inhibition of T cell stimulation [Abatacept (CTLA4-Ig)] LFA-1- ICAM-1 (2, 3): Adhesion of T cells to APCs, endothelial cells and
extracellular matrix proteins and adhesion-dependent lymphocyte functions L-Selectin-CH ligands on HEV: Regulate migration of leukocytes to various
tissues CD44-matrix proteins: Retention of T cells to endothelium at sites of
inflammation CD40L- CD40: CD40L- CD40:
important signals for Ab productionimportant signals for Ab production Activation of macrophages to destroy phagocytosed microbes Activation of macrophages to destroy phagocytosed microbes B7 expression on all APCs (role in co-stimulation)B7 expression on all APCs (role in co-stimulation) Production of adhesion molecules & inflammatory cytokines by Production of adhesion molecules & inflammatory cytokines by
APCs → T- cell activationAPCs → T- cell activation Fas ligand(D95)- Fas: T cell apoptosis and elimination
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Accessory molecules of T cellsAccessory molecules of T cells
CD 28- B7: Production of IL-2 and other cytokines CTLA4- B7: Inhibition of T cell stimulation [Abatacept (CTLA4-Ig)] LFA-1- ICAM-1 (2, 3): Adhesion of T cells to APCs, endothelial cells and
extracellular matrix proteins and adhesion-dependent lymphocyte functions L-Selectin-CH ligands on HEV: Regulate migration of leukocytes to various
tissues CD44-matrix proteins: Retention of T cells to endothelium at sites of
inflammation CD40L- CD40:
important signals for Ab production Activation of macrophages to destroy phagocytosed microbes B7 expression on all APCs (role in co-stimulation) Production of adhesion molecules & inflammatory cytokines by APCs →
T- cell activation Fas ligand (D95)- Fas: T cell apoptosis and eliminationFas ligand (D95)- Fas: T cell apoptosis and elimination
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Principle accessory molecules of T LymphocytesPrinciple accessory molecules of T LymphocytesNameGene
familyCellular expressionLigandRole in T cell
activationAdhesionSignaling
CD4IgClass II T cellsClass II MHC±+
CD8IgClass I T cellsClass I MHC±+
CD28Ig>90% CD4+ T cells, ~50% D8+ T cells
B7-1 (CD 80) B7-2 (CD 86)
-+
CTLA-4
(CD 152)
IgActivated T cellsB7-1
B7-2
-+
CD45RLeukocytesUnknown-+
CD2
(LFA-2)
Ig>90% T cells (human), NK cells
LFA-3++
LFA-1IntegrinLeukocytes, plateletsICAM-1
ICAM-2
ICAm-3
+±
L-selectinSelectinLeukocytesCarbohydrate ligands on HEV
+-
CD44Lymphocytes, granulocytes
Matrix proteins+±
Cellular and Molecular Immunology (5th Ed.) Abbas AK, & Lichtman, Editor: Saunders, Philadelphia, 2005.
33
Salama AD, Sayegh MH. Am J Transplant 2003; 3: 509–511
(B7-1)(B7-2)
Alternative T-Cell Costimulatory Pathways inAlternative T-Cell Costimulatory Pathways inTransplant Rejection and Tolerance InductionTransplant Rejection and Tolerance Induction
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Intracellular events
Intracellular events
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Immunoreceptor Tyrosine Based Activating Motifs
ζζ
Isakov N - J Leukoc Biol 1997 Jan;61(1):6-16.
CD4,
CD8
NF-қB
Lck
Fyn
ZAP-70
CD
3
LA
T
SLP-76
Grb2
Shc
Sos
Ras
MAP-K
AP-1Jun
Fos
LA
T
SLP-76
Vav
p110
p85
Rho Family
GTPase
Regulatoroy Function
for Cytoskelton
PI-3 K
LA
T
SLP-76
PLC-γ1
IP-3 DAG
PLC-γ1
Ca2+
calcineurin
NF-ATp
Adaptor proteins: Protein-Protein
interaction•SLP-76
•Lat•GRb•VAV
dephosphorylate
TCR complex
Itams
38
1, Cyclosporine 2, Tacrolimus (FK506)3, Rapamycin4, Corticosteroid
1 2
3
4
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L
spleen,lymph nodes,
Peyer's patches
T
Scan information about new
immunological events
peripheral tissues
T
T
T
T
T
T
T
DToll-like
recepto
r
40
ToleranceTolerance Central (Negative Selection):Central (Negative Selection):
Immature lymphocytes in the generative lymphoid organs (thymus for T cells and bone marrow for B cells) encounter only self Ags in high concentration.
These lymphyctes are killed or deleted. Some of the T cells may develop into regulatory cells
→ Inhibition of immune response Peripheral:Peripheral:
Important for maintaining unresponsiveness to those self Ags that are expressed in peripheral lymphoid tissues (spleen, lymph nodes, mucosal lymphoid)
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Mechanisms of ToleranceMechanisms of Tolerance1.1. Deletion: Apoptotic cell deathDeletion: Apoptotic cell death
Activation induced cell death: Fas-FasL interaction Binding if FAAD Activation of caspase 8 Activation of effector caspases Apoptosis
Passive cell death (death by neglect) Loss of survival stimuli e.g. growth factors or costimulators Increase mitochondrial permeability Release of cytochrome c and then Apaf Activation of caspase 9 Activation of effector caspases Apoptosis
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Mechanisms of Tolerance…Mechanisms of Tolerance…
2.2. Anergy: Functional inactivation without cell deathAnergy: Functional inactivation without cell death
a. Ag presentation by APCs without costimulation
(Signal 2 or B7-CD28)
b. Engagement of inhibitory receptors (CTLA-4 instead
of CD28)
CTLA-4 Ig: Abatacept, LEA29Y (belatacept)
Antibodies against CD40L
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Mechanisms of Tolerance…Mechanisms of Tolerance…
3.3. Supression: by regulatory lymphocytesSupression: by regulatory lymphocytes
These are CD4+ T cells expressing high levels of IL-2-R
(CD25) but not other markers of activation
Inhibition of these cells may cause autoimmune disease
4.4. Clonal ignorance:Clonal ignorance: Self Ags may be ignored by the
immune system, i.e. lymphocytes encounter the Ag
but fail to respond
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Auchincloss H, Am J Transplant 2001;1(6-12)Auchincloss H, Am J Transplant 2001;1(6-12)
The Thymic Deletional MechanismThe Thymic Deletional Mechanism
Exposure to donor antigens during fetal development
achieved the same thymic deletional tolerance that occurs
for many self antigens.
Engraftment of donor stem cells
Mixed bone marrow chimeras
Thymic deletion of T cells reactive to donor APCs
Re-poplulation of a new T cell repertoire
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Auchincloss H, Am J Transplant 2001;1(6-12)Auchincloss H, Am J Transplant 2001;1(6-12)
Peripheral Deletional MechanismsPeripheral Deletional Mechanisms Removal donor-specific T cells after they have matured. Mechanisms:
Veto phenomenon: involves the ability of a population of CD8 T cells to destroy attacking T cells that would otherwise kill them, via attachment to their MHC I molecules.
Cell death mediated through Fas Ligand, Other forms of activation-inducedcell death (AICD) Withdrawal of growth factors such as IL-2.
Peripheral Nondepleting MechanismsPeripheral Nondepleting Mechanisms An active regulation of T cell responses by other
(suppressor) T cells Change in the character of the donor specific response
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Although multiple mechanisms of tolerance induction
may be combined, there are two hallmarks of a
tolerance induction strategy that will ultimately be
successful in clinical practice:
It will include the use of donor bone marrow (or stem cells)
somewhere in the protocol,
It will eventually lead to easily measurable macrochimerism.
Auchincloss H, Am J Transplant 2001;1(6-12)Auchincloss H, Am J Transplant 2001;1(6-12)Auchincloss H, Am J Transplant 2001;1(6-12)Auchincloss H, Am J Transplant 2001;1(6-12)
47
Graft rejection is usually initiated by CD4 helper T cells (TH) that bind peptides in complexes with MHC class II molecules on antigen-presenting cells. In the direct pathway of recognition, an MHC molecule on a foreign (allogeneic) cell, such as an antigen-presenting cell (allogeneic APC), binds to the helper T cell. In the indirect pathway, the foreign MHC molecule is processed into peptides that are presented to the helper T cell by one of the body's own antigen-presenting cells (self APC). In either case, activated CD4 helper T cells proliferate and secrete a variety of cytokines that serve as growth and activation factors for CD8 cytotoxic T cells (TC), B cells, and macrophages, which cause destruction of the graft by direct lysis of target cells, antibody production, and delayed-type hypersensitivity mechanisms, respectively.
Sayegh MH, Turka LA NEJM, 1998, 338: 1813-21
