Transplantation Flashcards
What is an allograft?
Transplant of an organ, tissue, or cells from one individual to another individual of the same species who is not an identical twin.
Which parts of the body can be transplanted in an allograft?
Solid organs: Kidney, liver, heart, lung, pancreas
Small bowel
Free cells: BM stem cells, pancreas islets
Temporary: Blood, skin (burns)
Privileged sites: Cornea
Framework: Bone, cartilage, tendons, nerves
Composite: Hands, face
How can transplant outcomes be improved?
Patient survival + graft survival
Improved surgical technique
Improved pre- + post-transplant patient management: Drug levels, Infections, cardiovascular disease, diabetes
Better understanding of transplant immunology: Prevention, dx + tx of graft rejection
What are the different stages of immune response to a transplanted graft?
Phase 1: Recognition of foreign antigens.
Phase 2: Activation of antigen-specific lymphocytes.
Phase 3: Effector phase- all cells that will cause injury to graft have coordinated recruitment
What are the relevant protein variations in clinical transplantation?
HLA (human leukocyte antigens).
ABO blood group (for ABO-incompatible transplantation)
Others: minor histocompatibility genes.
What is the immunology of transplantation?
The immune system recognises someone else’s organ as foreign.
2 major components to rejection (may occur simultaneously):
- T cell-mediated rejection
- Antibody-mediated rejection (B cells)
What is HLA?
Major Histocompatibility complex (MHC) (chr 6).
Discovered after 1st attempts at transplantation (animal models + humans).
Cell surface proteins.
- 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 up-regulated on other cells under stress.
Both have peptide binding groove to present antigen
What is special about HLA?
Highly polymorphic: 100s of alleles for each locus (e.g. A1, A2, A3–A372…).
Presentation of foreign antigens on HLA molecules to T cells is central to T cell activation.
How does HLA contribute to infections and neoplasia?
To maximise diversity in defense against infections/ neoplasia, each individual has a variety of HLA.
Each individual’s HLA are derived from a large pool of population varieties.
How does HLA affect transplantation?
The variability in HLA in the population provides a source for immunisation against the transplanted organ.
“Mismatches”
What is the nomenclature for HLA mismatches?
Work out number of mismatches based on differences
What is the relationship between HLA mismatches and transplant outcome?
Minimising HLA differences between donor + recipient improves transplant outcome.
The more mismatches, the worse the graft outcome
How is tissue typing (determining HLA in individuals) conducted?
PCR-based DNA sequence analysis for HLA alleles determines the individuals genotype
What is T cell mediated rejection?
Phase 1: Presentation of donor HLA by a professional antigen presenting cell (APC), in the context of recipient HLA.
Phase 2: T-cell activation, inflammatory cell recruitment.
Phase 3: Effector phase (organ damage).
Explain T cell activation.
Proliferation
Production of cytokines (IL-2, IL-15)- autocrine effect increasing activation
Activation of CD4 helper + CD8 cytotoxic T cells
Help for antibody production by B cells
Recruit monocytes/macrophages
What cells are involved in the effector phase of T cell mediated rejection?
Cause inflammation in interstitium
“Cytotoxic” T cells:
- Release of toxins to kill target: Granzyme B
- Punch holes in target cells: Perforin
- Apoptotic cell death: Fas -Ligand
Monocyte/macrophages:
- Phagocytosis
- Release of proteolytic enzymes
- Production of cytokines
- Production of oxygen radicals + nitrogen radicals
What can result from T cell mediated rejection?
Interstitial inflammation + tubulitis
Arteritis
What drugs can be used to manage T cell mediated rejection?
Target T cell activation pathway
Corticosteroids: anti proliferative
Daclizumab: Anti-CD25 monoclonal antibody
Mycophenolate mofetil: MPA inhibitor
Alemtuzumab: Anti-CD52 monoclonal antibody
OKT3, ATG: Anti-CD3 monoclonal antibody
Calcineurin inhibitors: Cyclosporine, tacrolimus
What are the phases of antibody mediated rejection?
Phase 1: B cells recognise foreign HLA.
Phase 2: Proliferation + maturation of B cells with anti-HLA antibody production.
Phase 3: Effector phase; antibodies bind to graft endothelium: intra-vascular disease.
How are anti-HLA antibodies formed?
Anti-HLA antibodies are not naturally occurring.
- Pre-formed: Transplantation, pregnancy, transfusion.
- Post-formed: Arise after transplantation.
Other antibodies:
- Anti-A or anti-B antibodies (naturally occurring)
- Non HLA antibodies
What are ABO blood groups?
A + B glycoproteins on RBCs but also endothelial lining of blood vessels in transplanted organ.
Naturally occurring anti-A + anti-B antibodies.
When is screening for anti-HLA antibodies conducted?
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 are three types of assay to test for anti-HLA antibodies?
Cytotoxicity assays
Flow cytometry
Solid phase assays
What is being tested for in a cytotoxic crossmatch?
does the recipient serum kill the donor’s lymphocytes in the presence of complement? – detection of cell death using vital dyes.
What is being tested for in flow cytometry?
Does the recipient’s serum bind to the donor’s lymphocytes (bound antibody detected by fluorescently-labelled anti-human Ig)?
What is being tested for in 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 can antibody mediated rejection be managed?
Remove antibodies with plasma exchange
Intravenous Ig
B cell depletion: Rituximab
Complement inhibitors
How is rejection detected?
Monitor transplant function (creatinine) + screen for antibodies.
If creatinine goes up: take a biopsy to confirm and classify rejection.
What does this show?
Drug toxicity
Calcineurin inhibitors at high doses can cause tubular injury
Decrease immunosuppressive drug
May present with rise in creatinine
What does this show?
Viral infections
Opportunistic infections related to baseline immunosuppression
Decrease immunosuppressive drug
BK nephropathy is a frequent complication
What does this show?
Vascular disease
BP control + Vascular stent
What does this show?
Post-transplant Lymphoproliferative Disease
Reduce immunosuppressive drug
Chemotherapy
What does this show?
Recent glomerulonephritis
What is the most important antigenic determinant of rejection in a patient for kidney transplantation?
HLA/ MHC
Why is ABO blood group now much less a concern for rejection?
Techniques can remove naturally occurring antibodies before + after transplantation
Why are previous mismatches recorded in a transplant recipient patient?
If seen foreign HLA before, will have memory cells which can be rapidly + vigorously activated if exposed again
Describe antigen presentation in organ transplantation
Both host + donor APCs can present antigens to T cells
APCs circulate in secondary lymphoid organs with HLA molecules they have picked up from the transplanted organ
Naive + memory T cells also circulate there
If appropriate match occurs- T cell activation
T cells recirculate to graft for effector phase
Describe the effector phase of antibody mediated rejection
Intravascular
Antibodies fix onto HLA molecule on endothelial surface, leads to complement activation, creation of holes in endothelium, coagulation, necrosis
Recruitment of mononuclear + NK cells
What is a defining histological feature of antibody mediated rejection?
Glomerulitis
Capillaries stuffed with inflammatory cells + swollen endothelial cells that are damaged
Which type of rejection has greater impact on organ function acutely?
Both present with rise in creatinine
T cell: tubular-interstitial infiltrate causes tubular dysfunction
Antibody: cells in glomerulus clog glomerulus- reduce GFR
Need biopsy to determine T cell, antibody or combination
Which type of rejection has greater impact on organ function in the long term?
T cell: responds quite well to therapy, creatinine returns to normal
Antibody: hard to treat, often doesn’t completely go away, a/w slow degradation
What footprint signature of antibody mediated rejection is sometimes identifiable with a stain?
Complement activation on endothelial cell surface with stain for C4d
Describe the antibodies and antigens present in each blood group
A: A antigen + Anti-B antibody in plasma
B: B antigen + Anti-A antibody in plasma
AB: A+B antigens, no antibodies
O: No antigens, anti-A + anti-B antibodies in plasma
What can be given prior to transplant to reduce risk of rejection?
Induction agent
Stronger agent of immunosuppression
T cell depleting e.g. OKT3/ATG or anti-CD52
Cytokine blocking agent e.g. anti-CD25
What should be monitored in a patient post kidney transplant?
Creatinine + screen for antibody development
If creatine rises or antibodies detected- biopsy
A patient has an episode of acute T cell mediated rejection 2 months post transplantation. What additional drug would most commonly be administered?
Methylprednisolone on 3 consecutive days
Which is the main cell that is injured in the effector phase of antibody mediated rejection?
Endothelial cell
