HIS14 Transplantation Immunology Flashcards
Transplantation
- Taking of cells, tissues, organs (graft) from one to another
- Donor vs Recipient
- Orthotopic: transplanted into normal anatomical location (e.g. liver transplant)
- Heterotopic: grafted into a different anatomical location (e.g. kidney transplant in iliac fossa)
Transplantation immunology
- Tolerance to foreign antigens (in transplantation in end stage organ failure)
- Attack tumour antigens (tumour biology)
Autograft, Isograft, Allograft, Xenograft
Autograft: Graft from one part of body to another (same body)
Isograft: Graft between genetically identical individuals (monozygotic twins)
Allograft: Graft between different members of same species
Xenograft: Graft between members of different species (monkey to man)
2nd set response - 1st input in transplantation
1st time: skin B on mouse A —> rejected between days 10-15
2nd time: skin B on mouse A —> rejected in days 5-8 (accelerated fashion)
1st time: skin C on mouse A —> rejected between days 10-15
Conclusion: something in mouse recognise specific strain of skin from different mouse
Human leukocyte antigens (HLA)
Chromosome 6
HLA-A, HLA-B, HLA-C region genes: MHC class I molecules —> ALL nucleated cells
HLA-D (DP, DQ, DR) region genes: MHC class II molecules —> APC
Well matched HLA predicts better transplant survival (less important in liver)
Specific immune response
Dendritic cells / Macrophage (MHC class II)
—> CD4 Helper T cell
—> **IFNγ + **IL-2
—> B cell —> Ab AND —> CD8 Cytotoxic T cell AND —> Macrophage (although belong to innate)
Allograft rejection
- Cell-mediated immunity (T cells)
- Humoral immunity (B cells)
- NK cells
—> Hard to target a specific type of cell and induce tolerance to Allograft
T cell selection —> Discriminate between Self and Non-self
HSC in BM
—> ***Thymocytes in BM
—> Maturation in Thymus
—> Positive selection (T cells able to bind self MHC molecules)
—> Negative selection (T cells not reactive but recognise self-MHC/antigen)
—> Mature T cell (CD4+ / CD8+, able to recognise foreign antigens/self-MHC complex)
System NOT absolutely perfect —> some Self-reacting T cells remain
MHC class II-Peptide TCR interaction
Signal 1:
- TCR: recognise foreign peptide + α1, β1 subunit of MHC
- CD4 co-receptor: recognise β2 subunit of MHC
Different foreign peptide —> Requires different TCR shape
Dendritic cells
Most potent APC
—> Immature DC circulating in bloodstream
—> Antigen capture
—> Mature DC migrate into Secondary lymphoid organ (e.g. LN)
—> Present Ag to T cell in Secondary lymphoid organ
Immature DC:
- High intracellular MHC-II
- High endocytosis/phagocytosis
- Low CD40, CD80, CD86
- Highly mobile
Mature DC:
- ***High surface MHC-II
- Low endocytosis/phagocytosis
- ***High CD40, CD80, CD86
- Stationary in LN
Antigen recognition by T cell
MHC restricted!!!
—> TCR “A” can only recognise MHC “A”
—> TCR “A” cannot recognise MHC “B”
2 signals of T cell activation (safety mechanism)
Signal 1: MHC / Peptide complex
Signal 2: Co-stimulatory signal (recognised by Co-receptor on T cell)
T cells cannot be activated with Signal 1 alone, Signal 2 also required
Signal 1 only: Anergy / cell death
Signal 2 only: No effects
Signal 1 + 2: Activation of T cells
Co-stimulatory pathways:
- B7 (on APC) - CD28/CTLA4 (on T cells)
- CD40-CD40L
- ICOS-B7RP1
B7 - CD28/CTLA4 pathway
Without B7 co-stimulation:
- Limited expansion
- Minimal cytokines
- Requires high concentration of Ag to activate T cell
- Non-sustained responses
- Anergy / apoptosis
Together with B7 co-stimulation:
- Robust expansion
- Maximal cytokines
- Responds to low Ag concentration
- Sustained responses
- T cells primed for re-challenge (i.e. Memory T cells)
Negative feedback loop of CTLA4 signaling:
—> B7 / CD28 stimulatory signaling
—> induce gene transcription of CTLA4 in T cell at the same time
—> B7 / CTLA4 bind together (CTLA4 compete with CD28)
—> Negative feedback
1. Blockage of IL-2R expression on T cell
2. Suppression of IL-2 production on T cell
3. Cell cycle arrest
4. Dominant inhibition of both CD28 and TCR mediated pathway
5. Induction of anergy / apoptosis
—> T cell response ***dampen down
—> Ensure no sustained T cell response (prevent severe inflammatory reaction)
***Direct + Indirect pathways in Ag presentation in organ transplantation
Indirect pathway (Non-self Peptide + Self MHC) (Predominant):
Donor Ag
—> picked up by Recipient APC
—> presented on Recipient MHC
—> recognised and activation of Recipient T cells
Direct pathway (Non-self Peptide + Non-self MHC) (Relatively minor):
Donor APC
—> present Donor MHC
—> recognised and activation of Recipient T cells
BOTH pathways need to be suppressed to ensure no rejection
***Tempo of rejection after organ transplantation
- Hyperacute rejection (用Ab)
- **Pre-existing Recipient Ab to Donor tissue (7 mins) (Rarely seen) (e.g. ABO blood group incompatibility)
- Rapid **thrombotic occlusion of graft vessels within minutes after vascular anastomosis
- Pre-existing Ab bind to graft endothelium —> activate ***Complement (Classical pathway) - Acute rejection (用Cellular immunity)
- **CD4 controlled CD8 mediated (8-11 days) (Mostly seen)
- CD4 + CD8 activation
- Acute cellular rejection
—> **Cytotoxic T cell-mediated lysis, Macrophage-meditated lysis, NK-mediated lysis
—> Necrosis of parenchymal cells, lymphocyte, macrophage infiltration - Chronic rejection
- ***Mixed CD4 and Ab (3 months - 10 years) (even patients are on immunosuppressants)
—> Perivascular inflammation, Fibrosis, Arteriosclerosis, Macrophages + Smooth muscle cells predominant - Xenograft rejection (Hyperacute)
- Pre-existing Ab to Donor tissue (7 mins)
***Immunological components of rejection
APC
—> CD4 Th cell
- IL-2, IFNγ —> ***CD8 (activated by graft cells directly as well, ∵ recognise foreign MHC) —> Cell mediated cytotoxicity
- IL-2, IL-4, IL-5 —> ***B cells —> Ab —> ADCC by NK cells, Complement activation, Lytic damage vascular occlusion
- Lymphotoxin, IFNγ —> ***Macrophage —> Inflammatory mediators
另外:
***Complement —> Lytic damage vascular occlusion, Inflammatory mediators
***Preventing rejection
- Pre-transplant - Laboratory tissue matching
- ABO antigens testing
- Lymphocytotoxicity
- Molecular HLA typing - Non-specific immunosuppression
- Cyclosporin A
- Tacrolimus
- Monoclonal Ab targeting CD / other lymphocyte surface molecules (more specific)
Prevent rejection —> 1. Pre-transplant - Laboratory tissue matching
- ABO antigens testing
- ABO antigens present on vascular endothelium of graft apart from RBC
- Prevent Hyperacute rejection - Lymphocytotoxicity
- **Serological detection of MHC-I and MHC-II by using Ab for **both Recipient and Donor
—> 用Ab去睇下Recipient / Donor既cell上面有咩類型的MHC
—> match MHC between them
—> better graft survival
- Mixing patient’s lymphocyte with Ab (specific for a particular MHC) —> incubate —> add Complement and Trypan blue after
- Performed while Donor organ is preserved on ice - Molecular HLA-typing
- PCR-based
- Detection of genomic HLA in donor + recipient using sequence-specific primers
- Very useful esp. in ***Bone marrow transplantation (HLA details are needed)
- Not routinely done in organ transplantation
- Good HLA matching —> Better graft survival
- ABO antigen mismatch not accepted
- HLA mismatch not accepted in BM transplantation
- HLA mismatch may be accepted in solid organ transplantation
***Prevent rejection —> 2. Non-specific immunosuppression
Non-specific immunosuppression abolish activity of immune system regardless of the antigen
—> leave graft recipient susceptible to infections
- Cyclosporin A
- **IL-2 transcription inhibitor
- natural metabolite in fungus
- common for current use
- **Narrow therapeutic window
- **Renal damage
MOA:
Binds to and inhibit **Calcineurin
—> inhibit NFAT
—> inhibit transcription of IL-2 (important for T cell expansion) - Tacrolimus (FK506)
- fungal metabolite, novel macrolide, structurally unrelated to Cyclosporin A
- **100x potent than Cyclosporin A
- **Less Renal damage
- Commonly used
MOA:
Binds to FKBP
—> inhibit ***Calcineurin
—> inhibit NFAT
—> inhibit transcription of IL-2 (important for T cell expansion) - New non-specific but more selective agents
MOA:
- Monoclonal Ab against lymphocyte surface molecules (esp. CD3, CD4, CD8, IL2R (Anti-CD25))
—> Eliminate cells / Block their function
- Rescue therapy in patients refractory to steroid therapy after acute rejection - Experimental: CTLA4-Ig (compete with CD28 on T cell for co-stimulatory signal —> block co-stimulatory signal to T cell), Anti-CD40
***Problems with long term immunosuppression
- Opportunistic infections
- by low virulent bacteria / fungi - Risk of malignancy
- lymphoma, post-transplant lymphoproliferative disease (PTLD) - Pharmacological SE
- hypertension, hirsutism, renal damage
EBV reactivation in immunosuppressed individuals
EBV reactivation
—> Transform B cells in vitro / in vivo
—> induce expression of **LMP-1 on B cell surface
—> **mimics CD40 in B cells (involved in ***B cell activation pathway)
—> monoclonal band of B cells (B cells express limited viral + cellular protein)
—> PTLD (cancer)
EBV viral load ↑ in PTLD
—> ∴ used to monitor transplant patients
—> initiate preemptive therapy
***Treatment of EBV PTLD
Against EBV:
- Reduce immunosuppression (to fight against EBV) (early, polymorphic lesions often responsive)
- IFNα
- Autologous OR HLA-matched, EBV-specific, ***CD8 T cell infusion (for solid organ transplant recipients)
Against B cells:
4. Anti-CD20 monoclonal Ab (Rituximab) —> may also be used for RA, SLE (Lecture 9)
Calcineurin inhibitors (Cyclosporin, Tacrolimus) and PTLD
Original intention:
- Inhibit generation of T cell cytotoxic activity
However:
- Induce expression of **IL-6 and **TGF-β that supports B cell activation and proliferation
- Enhance survival of EBV-transformed cells in vitro by ***protecting from Fas-mediated apoptosis
Therefore:
- Lower doses of Cyclosporin allow T cell responses to EBV (in vitro)
—> associated with lower rates of Lymphoma than higher doses
- In children Tacrolimus is associated with higher risk of PTLD than Cyclosporin in some studies
Holy Grail
Donor specific tolerance (can still recognise other foreign antigens) without need of immunosuppression
***4 Immunological mechanisms of tolerance
Central tolerance
1. Clonal Deletion - essential in central tolerance (i.e. tolerance to self)
Peripheral tolerance
1. Clonal Anergy - co-stimulatory blockage (remove signal 2 e.g. CTLA4-Ig)
- Activation-induced cell death
- negative feedback to T cells after a while of activation (e.g. CTLA4 signaling)
—> apoptosis - Sequestrated antigens / Ignorance
- immunologically privileged sites (e.g. anatomically isolated: Testis, Eye not attacked by WBC)
- precluded from contact with lymphocytes
- lack of Ag presentation - Treg cells (special Th cells): CD4+, CD25+
- production of suppressive cytokines TGFβ and IL-10
—> actively suppressing autoreactive T cells
Evidence of regulatory cells
- ***Infectious tolerance (CD4+ Treg cells將tolerance傳比其他T cells instead of suppressing donor T cells because once recipient Treg cells removed, recipient T cells can still tolerate skin graft)
- ***Linked suppression (將tolerated同intolerated skin crossover —> 溫水煮蛙, 慢慢接受graft)
Regulatory T cells (Treg) facts:
- CD4+, CD25+, FoXP3+ cells
- Expression of CTLA4 (negative regulator of other T cells)
- Existence of indigenous (native) regulatory cells to suppress “escaped” T cells against self Ag (e.g. deletion of CD25+ cells causes inflammatory bowel disease in mouse model)
Questions:
- Can we “induce” regulatory cells for transplantation tolerance?
- Presence of CD8+ T reg?
Unresolved clinical problem - Chronic rejection
Significant problem long-term
- **Histological hallmarks:
1. Perivascular inflammation
2. Fibrosis
3. Arteriosclerosis
4. Macrophages + Smooth muscle cells predominant - Antigen dependent and independent mechanisms
Possibilities of xenotransplantation?
- Lack of organs for transplantation
- Pig-human xenotransplantation:
- Hyperacute Xenograft rejection
—> Pre-existing human Ab against animals proteins
—> Construct transgenic pigs expressing human proteins that inhibit complement activation - Delayed Xenograft rejection
—> Acute vascular rejection (incompletely understood) - T cell-mediated Xenograft rejection
Concerns:
- Barrier
- ***Viruses?
Future directions in solid organ transplantation immunology
- Study into donor specific tolerance —> no need generalised immunosuppression
- Characterisation of regulatory cells (T-reg)
- Solution to chronic rejection
- Xenotransplantation
