Transplantation Flashcards
# 1. Define allograft 2. Types of allograft
- Allograft: The transplant of an organ or tissue from one individual to another of the same species with a different genotype. For example, a transplant from one person to another, but not an identical twin, is an allograft.
- Types/examples:
- Solid organs (kidney, liver, heart, lung, pancreas)
- Small bowel
- Free cells (bone marrow stem cells, pancreas islets)
- Temporary: blood, skin (burns)
- Privileged sites: cornea
- Framework: bone, cartilage, tendons, nerves
- Composite: hands, face
What is the most common transplant performed?
Kidney
How can transplant outcomes be improved?
- Improved surgical technique
- Improved pre- and post-transplant patient management and monitoring
–Drug levels
–Infections, cardiovascular disease, diabetes,…
•Better understanding of transplant immunology
–>Better immunosuppressive agents
–> Prevention, diagnosis and treatment of graft rejection
What are the 3 phases of the immune response to a transplanted graft?
- Phase 1: recognition of foreign antigens
- Phase 2: activation of antigen-specific lymphocytes
- Phase 3: effector phase of graft rejection
What are the most relevant protein variations in clinical transplantation?
- ABO blood group
- HLA (human leukocyte antigens) coded in chromosome 6 by Major histocompatibility complex (MHC)
Other determinants - minor histocompatibility genes
What are the two major components to rejection?
–T cell-mediated rejection
–Antibody-mediated rejection (B cells)
- What are HLA (Human leukocyte antigens)
- what do they do?
- What are their classes and variability?
- Cell surface proteins
- Presentation of foreign antigens on HLA molecules to T cells is central to T cell activation
- Classes:
- HLA Class I (A,B,C)– expressed on all cells
- HLA Class II (DR, DQ, DP) – expressed on antigen-presenting cells but also can be upregulated on other cells under stress
- Highly polymorphic – hundreds of alleles for each locus (for example: A1, A2, A3 – A372 and rising…)
Why does everyone have a variety of HLA proteins?
To maximise diversity against infections
•The variability in HLA molecules in the population provides a source for immunisation against the transplanted organ
Describe the different MHC/HLA combinations
What are the differences between MHC Class 1 and 2?
MHC Class 1: MHC class 1 are a class of major histocompatibility complex molecules found on the surface of all nucleated cells in mammals.
MHC Class 2: MHC class 2 are a class of major histocompatibility complex molecules mainly found on antigen presenting cells such as macrophages, dendritic cells, and B cells
When comparing HLA differences, what is the maximum amount of mismatches allowed across HLA-A, HLA-B and HLA-DR?
6
How many mismatches are in this example?
Remember HLA are proteins, so they come with two alleles, that may code for different versions of that HLA protein.
What is T cell mediated rejection?
T cells require presentation of the foreign HLA antigens by a professional antigen presenting cell (APC), in the context of HLA, to initiate activation of alloreactive T-cells
What is the role of alloreactive T cells? Describe the direct and indirect pathways that lead to transplant rejection
T-cell alloreactivity drives transplant rejection. Alloreactive recognition is believed to proceed with limited specificity, accounting for the high numbers of alloreactive T cells in humans
Describe the three signals that activate T cells after a transplant
Describe how graft infiltration occurs with the help of cytotoxic T cells and macrophages
Graft infiltration by alloreactive CD4+ cells
“Cytotoxic” T cells
- Release of toxins to kill target
- Granzyme B
- Punch holes in target cells
- Perforin
- Apoptotic cell death
- Fas -Ligand
Macrophages
- Phagocytosis
- Release of proteolytic enzymes
- Production of cytokines
- Production of oxygen radicals and nitrogen radicals
On a graft biopsy, what can be seen when acute cellular rejection has occurred?
Interstitial inflammation and tubulitis
What are the 3 phases of antibody-mediated rejection?
Phase 1 – exposure to foreign antigen
Phase 2 - proliferation and maturation of B cells with antibody production
Phase 3 – effector phase; antibodies bind to graft endothelium (capillaries of glomerulus and around tubules, arterial)
Describe the effectors in anti-body mediated rejection and whether they are naturally occuring or not
–Anti-A or anti-B antibodies are naturally occurring
–anti-HLA antibodies are not naturally occurring
- Pre-formed – previous exposure to epitopes (previous transplantation, pregnancy, transfusion)
- Post-formed - arise after transplantation
Describe the action of antibodies in infection
B cells produce specific antibodies against the cell surface proteins on donor tissue, in an attempt to destroy it as it is considered a foreign article.
Complement is then activated allowing for cell adhesion, and attraction of other cells to cause cell necrosis to the cells of the donor tissue
Describe how antibodies act to reject donor organs in transplant
B cells produce specific antibodies against the cell surface proteins on donor tissue, in an attempt to destroy it as it is considered a foreign article.
Complement is then activated allowing for cell adhesion, and attraction of other cells to cause cell necrosis to the cells of the donor tissue
What makes the ABO blood groups?
- A and B glycoproteins on red blood cells but also endothelial lining of blood vessels in transplanted organ
- Naturally occurring anti-A and anti-B antibodies
Describe what
- antibodies in plasma
- antigens on red blood cells
will be for blood groups A, B, AB and O
Describe tissue rejection
- T-cell mediated, antibody-mediated or combined
- Both cause graft dysfunction (e.g. raised creatinine, raised LFT)
- Graft biopsy: management and outcome are different
Describe the prevention and treatment of graft rejection
•Preventing rejection:
- A. AB/HLA matching
- B. Screening for anti-HLA antibodies
- C. Immunosuppression: dampen the immune system of the recipient
•Treating rejection:
- More immunosuppression
•Always balance the need for immunosuppression with the risk of infection/malignancy/drug toxicity
- Describe AB/HLA typing in transplantation
- When in HLA matching important in organ transplantation?
1.
- Part of the organ allocation procedure
- Encourage living donation from “blood” relatives
2.
•HLA matching is an important part of organ allocation procedure
- Bone marrow
- Kidney
•HLA matching not as important
- Heart
- Lung
•Liver - ?
How is donor and receipient HLA type determined?
•PCR-based DNA sequence analysis determines the individuals genotype
When should antibodies be screened for during transplantation proceedings?
- Before transplantation
- At time of transplantation: when a specific deceased donor kidney has been assigned to the patient
- After transplantation, repeat measurements to check for new antibody production
What 3 main types of assays are used to screen for anti-HLA antibodies?
–Cytotoxicity assays - does the recipient serum kill the donor’s lymphocytes in the presence of complement? – detection of cell death using vital dyes
–Flow cytometry - does the recipient’s serum bind to the donor’s lymphocytes (bound antibody detected by fluorescently-labelled anti-human Ig)
–Solid phase assays - does the recipient’s serum bind to recombinant single HLA molecules attached to a solid support such as beads (bound antibody detected by fluorescently-labelled anti-human Ig)
How is transplant rejection managed?
More immunosuppression
- Targeting T cell activation
- Mycophenolate mofentil - affect nucleotide synthesis
- Daclizumab - Anti-CD25 mAB
- Alemtuzumab - Anti-CD52 mAB
- Azathioprine - affects the cell cycle
- Tacrolimus - Calcineurin inhibitors
- Targeting antibody-mediated damage
- Anti-CD20 antibodies
- BAFF inhibitors
- Proteasome inhibitors
- Complement inhibitors
- CD28/B7 blockade
- Anti-CD40
Describe some modern transplant immunosuppression
- Induction agent ex. OKT3/ATG, anti-CD52, anti-CD25 (anti-IL2R)
- Base-line immunosuppression: CNI inhibitor + MMF or Aza, with or without steroids
- Treatment of episodes of acute rejection:
- Cellular: steroids (MP IV 3x 60mg/kg then oral), ATG/OKT3
- Antibody-mediated: IVIG, plasma exchange, anti-C5, anti-CD20
What is haematopoietic stem cell transplants used for?
- Haematological and lymphoid cancers
- Acquired (autoimmune) or inherited deficiencies in marrow cells such as errors of metabolism or immunodeficiencies
How does graft vs host disease occur in haemotopoietic stem cell transplants?
- Eliminate hosts immune system (total body irradiation; cyclophophamide; other drugs)
- Replace with own (autologous) or HLA-matched donor (allogeneic) bone marrow
- Allogeneic HSCT leads to reaction of donor lymphocytes against host tissues
- Related to degree of HLA-incompatibility
- Also graft-versus-tumour effect
- GVHD prophylaxis: Methotrexate/Cyclosporine
- What is graft vs host disease?
- What happens?
- Treatments
- Injury induced by preparative regime before HSCT
2.
- Skin: rash
- Gut: nausea, vomiting, abdominal pain, diarrheoa, bloody stool
- Liver: jaundice
- Treat with corticosteroids
Describe post-transplantation infections
•Increased risk for conventional infections
–Bacterial, viral, fungal
•Opportunistic infections – normally relatively harmless infectious agents give severe infections because of immune compromise
–Cytomegalovirus
–BK virus
–Pneumocytis carinii
Describe post transplantation malignancy
–Viral associated (x 100)
- Kaposi’s sarcoma (HHV8)
- Lymphoproliferative disease (EBV)
–Skin Cancer (x20)
–Risk of other cancers eg lung, colon also increased (x 2-3)
