Immuno: Transplantation Flashcards
Which organ is most commonly transplanted?
- Kidneys
- Liver
autologous vs allogenic stem cell transplant
autologous - patients own stem cells
allogenenic - donor stem cell from HLA matched donor
then patient BM will be ablated
What is the average half-life of a transplanted kidney?
12 years
higher in living than deceased donor
Survival rate with dialysis vs kidney transplant
better survival with transplant (apart from initial post-op periond)
Allograft
Autograft
Xenograft
allograft - transplant of tissue between same species (both hymans - not identical twins)
autograft - transplant from patient’s own body body
xenograft - transplant between different species e.g. pig to human
What are the three phases of an immune response to a graft?
- Phase 1: recognition of foreign antigens
- Phase 2: activation of antigen-specific lymphocytes
- Phase 3: effect phase of graft rejection
What are the most relevant cellular proteins that can determine compatibility?
ABO
HLA
Which chromosome is HLA encoded on?
Chromosome 6
What are the two major components of rejection?
- T cell-mediated rejection
- Antibody-mediated rejection
Describe the basic structure of HLA Class I and Class II.
- Class I: have three alpha domains and a beta-2 microglobulin domain, has one transmembrane domain
- Class II: has two alpha and two beta domains, had two transmembrane domains
Which alleles encode HLA Class I and Class II?
- Class I: A, B and C
- Class II: DP, DQ, DR
Where are HLA Class I and Class II expressed?
MHC = HLA
- Class I: all cells
- Class II: antigen-presenting cells (can be upregulated at times of stress)
CD4 bind MHC II
CD8 bind MHC I
What is the benefit of having high variability in the peptide-binding groove of MHC?
Can present a wide variety of antigens
What is the disadvantage of the variability in the peptide-binding groove of MHC with regards to transplants?
This means that the host immune system can react with the slightly different MHC of the donor leading to rejection.
HLA of donor presented to recipient T cells
Which HLA alleles are most immunogenic?
A, B and DR
How does HLA disparity cause rejection
T cell mediated:
1. donor HLA presented to recipient T cells by recipient APCs
2. CD4+ and CD8+ activation
3. Effector phase:
* CD8+ –> kill cells by granzymes, perforin, FasL
* CD4+ –> produces inflammatory cytokines IL-2, IL-15 activate B cells
* Macrophages/Neutrophil —> Phagocytosis, ROS
Antibody mediated
1. B cells recognise foreign HLA via APC
2. Proliferation and maturation of B cells with anti-HLA antibody production
3. effector phase; antibodies bind to graft endothelium causing intra-vascular disease
* complement activation
* phagocytosis
Key histological difference between T cell and antibody mediated rejection
T cell - tubuloinstertital inflammation
Antibody mediated - vascular inflammation
Where do the antigen-presenting cells that interact with host T cells come from?
From the recipient and the donor (the donor organ will contain many APCs)
NOTE: a lot of these interactions will happen in lymph nodes
Which test is used to give a definitive diagnosis of graft rejection?
Biopsy
Describe the effector phase of T-cell mediated graft rejection.
- T cells tether, roll and arrest on the endothelial cell surface
- They will migrate across into the interstitium and start attacking the tubular epithelium
CD8 cause cell death by either granzyme, perforin or FasL
- Macrophages (recruited by T cells) may also be seen in the interstitium
What are the typical histological features of T-cell mediated rejection of kidney
- Lymphocytic interstitial infiltration
- Ruptured tubular basement membrane
- Tubulitis - tubuloinsteritial infiltrate (inflammatory cells within the tubular epithelium)
What other explanation might there be for graft failure other than rejection?
Immunosuppressive drugs may be nephrotoxic
What are the three phases of antibody-mediated rejection?
- Phase 1: eposure to foreign antigen
- Phase 2: proliferation and maturation of B cells with antibody production
- Phase 3: effector phase - antibodies bind to graft endothelium
What is a key difference between the production of anti-AB (blood type) and anti-HLA antibodies?
- Anti-AB antibodies are naturally occuring (pre-formed)
- Anti-HLA antibodies are not naturally occuring but can be pre-formed due to previous exposure to epitopes (e.g. previous transplant, pregnancy) or post-formed (after transplantation)
Outline the pathophysiology of antibody-mediated transplant rejection.
- Antibodies bind to HLA on the endothelium of blood vessels within the transplanted organ
- The antibodies fix complement which leads to formation of MAC and endothelial cell lysis
- Binding of complement also recruits inflammatory cells
- This leads to inflammation of the microcirculation (capillaritis)
- This leads to procoagulant tendencies and closure of the microcirculation leading to graft fibrosis
- Antibodies can also cross-link MHC molecules and activate them
What are the main histological features of antibody-mediated transplant rejection?
- Presence of inflammatory cells within the capillaries of the graft (HALLMARK)
- Immunohistochemistry can show fixation of complement fragments on the endothelial cell surface
What are the three main approaches to preventing graft rejection?
- AB/HLA typing
- Screening for antibodies
- Overcoming organ mismatch tissues
Which technology is used for AB/HLA typing?
DNA sequencing using PCR
At what stages of transplantation will screening for antibodies be performed?
- Before transplantation
- At the time of transplant (once an organ has been assigned)
- After transplantation (check for new antibody formation)
Name and describe three assays for anti-HLA antibodies.
- Cytotoxic Assays: tests whether patient serum binds donor lymphocytes in the presence of complement
- Flow Cytometry: tests whether patient serum binds donor lymphocytes irrespective of complement
- Solid Phase Assays: beads containing all the possible HLA epitopes are mixed with the patient’s serum. This determines which HLA types the patient has antibodies against. Having many antibodies against different HLA epitopes suggests that the patient is highly sensitised.
How can organ mismatch issues be overcome?
- Improve transplantation across tissue barriers
- More donors
- Organ exchange programmes
- Xenotransplantation and stem cell research
What T cell pathway is the main target for immunosuppressive drugs used in transplants?
- The main signal is between MHC and TCR
- Downstream, there are a number of pathways that involve calcineurin which result in cell proliferation
- Once activated, T cells will release IL2 which has autocrine and paracine effects on Th2 cells
- These are all targets for immunosuppression
Why is it important to differentiate between T cell mediated and antibody mediate rejection
Different treatments
T cell mediated - steroids
antibody mediated - IVIG, plasma exchange
Biochemical signs of kidney transplant rejection
other differentials
rise in creatinine
+ve antibody screen
need to do a biopsy
other differentials:
infection due to immunosuppression
toxicity reaction to immunosuppressive drugs
recurrence of original disease - especially diabetes/HTN still poorly controlled, IgA nephropathy
What is the aim of induction before transplant
T cell depletion
Name two calcineurin inhibitors.
What are they used for in transplants
Tacrolimus
Ciclosporin
To provide baseline immunosuppression
Name two cell cycle inhibitors
Mycofenolate mofetil
Azathioprine
Name two drugs that target TCR.
Anti-CD3 antibody (OKT3)
Anti-thymocyte globulin
Name an anti-CD52 antibody and state its effect.
Alemtuzumab - causes lysis of T cells
Name an anti-CD25 antibody and state its effect.
Daclizumab - targets cytokine signalling
What are the main immunosuppressive targets of B-cell mediated graft rejection?
- B cell activation
- Secretion of antibodies by plasma cells
- Effects of antibodies on endothelium
What is rituximab?
Anti-CD20 - causes depletion of B cells
How do BAFF inhibitors work?
Target cytokines (BAFF) that promotes B cell activation and growth
Name a proteasome inhibitor and describe how it works.
- Bortezomib
- Blocks the production of antibodies by plasma cells
Name a complement inhibitor.
Eculizumab
Outline the components of modern transplant immunosuppression regimes.
- Induction agent (e.g. OKT3, anti-CD52, anti-CD25)
- Baseline immunosuppression (e.g. calcineurin inhibitor, mycofenolate mofetil, azathioprine, steroids)
- Treatment of acute rejection
- Cellular: steroids, OKT3
- Antibody-mediated: IVIG, plasma exchange, anti-CD20
Briefly describe the pathophysiology of graft-versus-host-dsiease.
- During SCT, the host immune system is eradicated and replaced by autologous or allogeneic bone marrow
- In allogeneic stem cell transplant, the donor lymphocytes can react with host tissues
NOTE: if there is a malignancy, the graft can kill these cells
How can GVHD be prevented?
- Methotrexate/ciclosporin
List some symptoms of GVHD.
- Rash
- Nausea and vomiting
- Abdominal pain
- Diarrhoea/bloody stools
- Jaundice
List some opportunistic infections that are more common in transplant recipients.
- CMV
- BK virus
- PCP
List some malignancies that are more common in transplant recipients.
- Kaposi sarcoma (HHV8)
- Lymphoproliferative disease (EBV)
- Skin cancer - squamous
