Immunology of Transplantation Flashcards
What are the normal immune functions?
- Recognition of ‘non-self’ or ‘abnormal self’
- Protection from pathogens (bacteria, viruses etc)
- Surveillance for tumours
What are the components of the innate immune system?
- Macrophages
- Neutrophils
- Complement & natural antibodies (IgM)
What are the components of the adaptive immune system?
- Dendritic cells (antigen presentation)- regulate
- T cells (helper and cytotoxic T cells)
- Natural Killer (NK) cells - cytotoxic
- B cells (antibody generation & memory)
What are the MHC molecules?
- Major Histocompatibility complex
- MHC in humans called Histocompatibility Locus Antigen (HLA)
- These molecules imprint ‘individuality’ on cells and are pivotal in the generation of immune responses
- HLA genes are very polymorphic i.e. there are many different variations possible at each gene locus
Describe HLA Class 1
-Class I molecules: HLA-A, -B and -C
-Expressed by most somatic cells of body
-Used to present peptides from internally processed proteins (check cells are healthy)- peptide groove
= If Class 1 HLA molecule is associated with virus-derived protein then the cell is recognised as infected
=Infected cell will be killed by cytotoxic T cells
Describe HLA Class 2
-Class II: HLA-DP, -DQ and –DR
-Expressed by Antigen Presenting Cells (DCs etc) that constantly ’sample’ their microenvironment
-Used to present antigenic peptides derived from digested material by Antigen Presenting Cells (including pathogens, abnormal or foreign cells)
=Cell surface expression of a peptide derived from a pathogen or foreign cell will stimulate a T cell immune response
Describe T ell receptor complex and co-stimulation
- T cell synapse
- Other molecules engaged have to be strong enough to stimulate T cell to be active
- Range of receptors that have to be brought together
How does the cytotoxic T cell kill?
-Cell that is abnormal (allogenic cell)
=Fas ligand (induce cell death)
=TNF
=Perforin and granzyme B punches through membrane
Describe briefly how T cell activation occurs
- DC finds abnormal pathogen/ cell
- Take up material, process and APC
- T cell response that is antigen specific
- IL-2= proliferation of T cells= clonal expansion
- Circulate through body, exert function
- Die through cell death but memory when infection cleared
What are the key principles of Transplant Immunology?
- Rejection of Tx is directed at specific proteins called antigens
- Rejection is donor specific
- Rejection may be both Cell or Antibody mediated
- Rejection exhibits ‘memory’ i.e. a 2nd similar Tx is rejected MORE RAPIDLY and this results from the rapid generation of cytotoxic antibodies that recognise the Tx
What is HLA profiling?
-Performed using molecular biological and serological techniques
=HLA-A1 and A3,
=HLA-B44 and B44
=HLA-DR7 and DR15
-The HLA tissue types of all patients on the Kidney Transplant waiting list is held on a central UK database and the ‘best match’ chosen when kidneys become available
How is HLA Profiling used?
-Used to allocate kidneys but less important for other organs such as liver (less immunogenic)
-If all HLA-A, -B and –DR loci are the same the it is a 0-0-0 mismatch
-If they are all different then it is a 2-2-2 mismatch
=less long-term survival
What are the immunosuppression treatments?
-Corticosteroids
=Kill lymphocytes
=Interfere with T cell activation and gene transcription
=Powerful anti-inflammatory agents
-Calcineurin inhibitors (CNI) – Tacrolimus/ cyclosporine
=Inhibit T cell activation by interfering with intracellular signalling pathways
-Anti-proliferative agents - mycophenolate mofetil (MMF)
=Inhibit clonal expansion of T cells
-Various monoclonal and polyclonal antibodies directed against:
=IL-2 receptor blockers (IL-2 stimulates clonal expansion of T cells)
=T cells (cytotoxic complement fixing Abs)
=Co-stimulatory molecules
What are the Transplantable organs/ tissues?
- Kidney
- Pancreas (complete organ or pancreatic islets)
- Liver
- Lung
- Heart
- Small Bowel
- Cornea
- Faces, arms etc
What is included in patient assessment for transplant?
- Age important (biological vs chronological!)
- Primary cause of renal failure e.g. polycystic kidneys versus conditions which can recur in a Tx (e.g. aHUS, FSGS)
- Comorbid disease e.g. cardiovascular disease (IHD, PVD), diabetes etc
- History of infections (immunosuppressant)
- History of tumours (need tumour free period)
- Urological disease e.g. bladder dysfunction
What investigations are included in patient assessment for transplant?
- CARDIAC - exercise ECG, myocardial perfusion studies
- Angiography (need decent vessels for the anastomosis)
- Urodynamic studies
- Tumour markers, imaging etc
What are the types of transplantation?
-Cadaveric Tx (commonest) e.g. subarachnoid haemorrhage
=DCD = donated after cardiac death
=DBD = donation after brain death
-Living related donor Tx
=Sibling, spouse, altruistic
=Typically a kidney Tx
=better long term outcome
Describe the surgery of transplantation
-Plumbed in pelvis
-Attached to iliac vessels and bladder
-Assessment of vessels important
=How many vessels?
What is the criteria for a kidney transplant to go ahead?
-Blood group (ABO) compatible (RIE now has an ABO incompatible programme)
-Immunological ‘X-match negative’
=assay to see what antibodies are present that would react with donor organ
=serum from potential recipient + donor cells +vital dye and complement
=flow cytometry
What is sensitisation/ production of HLA antibodies?
- If a person is exposed to a foreign HLA molecule, they can produce an HLA antibody against this e.g. patient is A1, A24 and with exposure to A31 generating an anti-A31 antibody
- Approximately 30% patients on renal waiting list have anti-HLA antibodies
- The ‘specificity’ of these antibodies has to be defined (e.g. A31, B8, DR3 etc)
Where are the HLA antigens in sensitisation derived from?
-Highly sensitised patients exhibit high levels of cytotoxic Abs to many HLA antigens that may be derived from:
=Previous transfusions (WBC filtered)
=Pregnancies (paternal antigens in foetus)
=Previous Transplantation
How do we assess the presence of antibodies and level of antibodies in potential recipients?
- Hundreds of microbeads with different molecules
- Mix beads with patient serum and wash
- If antibody present, add anti-human immunoglobin= fluorescent signal
- Graph= how much antibody against antigens
Why do anti-HLA antibodies matter?
- Make a successful X-match less likely (long wait for Tx)
- Can lead to antibody mediated rejection
- Consider desensitisation/antibody removal or paired exchange Tx
What are the types of rejection?
- Hyperacute rejection (should not happen)
- Acute rejection
- Chronic rejection
Describe Hyperacute rejection
- Occurs when the Tx carries antigens to which the recipient is already sensitised
- Cytotoxic antibodies bind endothelial cells and induces complement activation, platelet aggregation and intravascular thrombus formation
- The Tx is often destroyed ‘on the operating table’
What are the features of acute rejection?
Cell or antibody mediated
- Rise in creatinine (often only indication)
- Reduced urine output
- Tender transplant
- Fever
What features need to be excluded to diagnose acute rejection?
- Dehydration (clinical examination, BP, weight)
- Renal obstruction (ultrasound)
- Vascular catastrophe (Doppler)
- Drug toxicity (tacrolimus levels in immunosuppressance)
How do we look at antibody rejection?
-When antibody binds endothelium =activates complement =deposits C4d =Footprint (indicates where previous antibody has bound) =capillaritis
What is the treatment for acute rejection?
- High dose methyl prednisolone (anti-inflammatory, kills lymphocyte etc)
- Change to more potent immunosuppressive agent or an increased dose
- ‘Anti-T cell’ antibody (increased risk of infection, tumours)
- Plasma exchange (severe acute Ab mediated rejection)
What are the features of chronic rejection?
- Progressive renal dysfunction
- Interstitial fibrosis (fibrillar collagen) and vascular disease on renal biopsy (smouldering lymphocyte infiltration, antibody deposition)
What do we need to exclude to diagnose chronic rejection?
- Recurrent disease (membranous, MCGN)
- Obstruction (ultrasound)
- Renal artery stenosis (Doppler of renal artery +/- MRI angiography)
- Recurrent crescentic nephritis
- Recurrent diabetic nephropathy
Describe the pathogenesis of chronic rejection
Multifactorial: immune and non-immune mechanisms:
=Increased HLA mismatch (1-2-1 vs 0-0-1)
=Previous acute rejection
=Poor drug compliance (low tacrolimus levels)
=Prolonged cold ischaemia time of kidney prior to surgery (CIT of living donor «_space;cadaveric donor)
What factors promote graft failure?
- Delayed graft function (doesn’t work for several days)
- Cytomegalovirus (CMV) infection
- Age of donor and ‘donor disease’
- Poor blood pressure control
- Proteinuria
How is chronic rejection managed?
- No specific treatment available
- Most patients will eventually require dialysis and potentially a further Tx
- Optimise immunosuppression
- Proactive treatment of BP, lipids, proteinuria etc
What are the infective risks of immunosuppression?
-Bacteria
=urinary tract infection, chest infection
=prophylactic cotrimoxazole
-Viral
=CMV, herpes virus, parvo virus, BK virus (causes renal dysfunction= reduce immunosuppression)
=prophylactic valganciclovir if recipient CMV –ve and donor CMV +ve
*TB (may require prophylactic treatment)
What are the tumour risks of immunosuppression?
-Tumours
=Incidence of all cancers increased
-Skin cancers common (UV block, avoid sun, skin surveillance)
-Post Tx Lymphoproliferative Disorder (PTLD) – secondary to infection with Epstein Barr virus
(reduce immunosuppression, may need chemotherapy)
What are the side effects of immunosuppressive drugs?
- Calcineurin inhibitors are nephrotoxic! Plasma levels of tacrolimus are measured regularly
- Increased risk of diabetes (steroids and tacrolimus)
- Hypertension (steroids and CNI)
- Osteoporosis (steroids)
What could be future treatments?
- Xenotransplantation using genetically engineered (humanised) pigs
- Tolerance induction – much better than immune suppression (interest in regulatory T cells)
- Artificially engineered kidneys using stem cell technology, organoids and biological scaffolds
