Immuno 4 - Transplantation Flashcards
HLA classes
Expression
HLA Class I (A, B, C) - expressed on all cells
thought to be the most immunogenic
HLA Class II (DR, DQ, DP) - expressed on APC but also upregulated on other cells under stress
3 commonest transplanted organs
1 - kidney
2 - liver
3 - heart and lung
Phases of T cell mediated Immune Response to Transplanted Graft
+ where do they occur
Phase 1_ recognition of foreign antigens
Lymph node
Phase 2_ activation of antigen-specific lymphocytes
Lymph node
Phase 3_ effector phase of graft rejection
In the graft
Which are the most relevant protein variations in clinical transplantation?
ABO blood group
HLA - coded by the MHC on chr 6
o Two types of rejection
Direct
• Donor APC presenting antigen and/or MHC to recipient T cells
• acute rejection mainly involves direct presentation
Indirect
• Recipient APC presenting antigen to recipient T-cells – i.e. the immune system working normally as it would for an infection
• Chronic rejection mainly involves indirect presentation
Most important HLAs to match and why
o Most important to match = DR > B > A
o These are the HLA molecules with the most variation and therefore the most immunogenic
Major determinant of the risk of rejection
number of mismatches
Number of mismatches in siblings
25% 6MM
50% 3MM
25% 0MM
Parent to child - matches
> = 3/6 matched
how do we express differences between recipients and the HLA donor ?
o HLA-A: HLA-B: HLA-DR
How do we determine an individuals HLA genotype?
PCR based DNA sequence analysis for HLA alleles
How do alloreactive T cells get activated?
Presentation of foreign HLA antigens by APCs (i.e. the donor’s HLA molecule) both donor and host APC cells are involved
Co-stimulatory signals – particularly of the IL-2 receptor in T cells through the release of IL-2
what happens to the T cell during phase 2 of the transplant rejection?
Phase 2_ activation of antigen-specific lymphocytes
o Proliferation
o Product cytokines (IL2 is important)
o Provide help to CD8+ cells
o Provide help for antibody production by B cells
o Recruit phagocytic cells (monocyte/macrophage lineage cells)
Targets for immunosuppressive drugs targeting T cell/cellular activation
Most drugs target T cell activation
calcineurin
MTOR pathways
co-stimulatory pathways
targeting the 3 signals that activate T cells:
APC MHC - TCR
APC CD80/CD86 - T cell CD28
Cytokine IL-2 - T cell CD25
Key points of the phase 3 -effector response in rejection
graft infiltration by alloreactive CD4+ cells
and start attacking the tubules which express the donor HLA molecules
recruitment of CD8+ T cells + monocytes
o Cytotoxic T cells:
Granzyme B (toxin)
Perforin (punch holes)
Fas-ligand (apoptosis)
o Macrophages: Phagocytosis Proteolytic enzymes Cytokine release O2 and N2 radicals
o Antibodies bind to graft endothelium
Histological features of acute T cell mediated rejection
o Lymphocytic interstitial infiltration
Immunohistochemistry – some are CD4+ cells, some are CD8+ cells, others are macrophages lineage
o Arteritis
(also see tubulitis in kidney graft rejection - inflammatory cells within the tubule)
DDx for rejection in a transplanted kidney
• Calcineurin inhibitor toxicity
o Presentation - raised creatinine
o Nephortoxic
o Mx – reduce immunosuppressive drug
• Viral infections (reactivation of latent viruses e.g. polyoma viruses)
o BK nephrotpathy
o Mx – reduce immunosuppressive drugs
• Vascular disease o Arterial intermural thickening o Small lumen o Hypertension o Mx -BP control, vascular stent if larger arteries are affected
• Post transplant lymphoproliferative disease
o Some of this is virally driven
o Mx - immunosuppressive drug
o Depending on the malignancy of the infiltrate they might need chemo
• Recurrent
glomerulonephritis
o Depends on nature GN
Phases of antibody mediated Immune Response to Transplanted Graft + where do they occur
o Phase 1: B cell exposure to foreign antigen/HLA epitope
Lymph nodes
o Phase 2: proliferation and maturation of B cells with antibody production + start producing anti-HLA ab
Lymph nodes
o Phase 3: effector phase – antibodies in the circulation fix on the endothelial cell surface (bind to graft endothelium) + cause damage + recruit inflammatory cells to the endothelium (capillaries of glomerulus and around tubules)
Graft
Main difference between T cell mediated rejection + antibody mediated rejection
T cells – crawl out of the circulation + go to interstitium + tubules of the kidney
B cell mediated rejection = intra-vascular disease
T cells interstitial damage
Antibodies endothelial damage
Anti-ABO ab
Anti-HLA ab
naturally occurring or not
Anti-ABO ab - naturally occurring
Anti-HLA ab - not naturally occurring
Action of antibodies in infection/ transplant rejection
o Neutralise toxins
o Opsonise (aid phagocytosis)
o Antibody-dependant cellular cytotoxicity
o Recruitment of inflammatory cells through complement independent mechanisms
o Complement activation: (complement fixes on the antibodies and then gets activated)
MAC lysis
Phagocytosis of microbes opsonized wit h complement fragments (e.g. C3b)
Inflammation
• Antibody mediated rejection histopathology
o Inflammatory cell infiltrate
o Complement fragments on the endothelial cell surface activation of the complement
o capillaritis Inflammatory cells in the microcirculation – a cardinal feature of antibody mediated rejection
Describe phase 3 of the antibody mediated rejection
o Antibodies bind to antigens (HLA) on the endothelium of the blood vessels in the transplanted organ
o Antibodies fix/activate complement which assembles to:
Form MAC endothelial cell lysis
Recruit inflammatory cells to the microcirculation
o Antibodies can
Crosslink the MHC molecules activating them
Directly recruit mononuclear cells, NK cells and neutrophils capillaritis
how can mismatch positive transplantation take place?
requires a lot of preparation
Plasma exchange to remove ab
IVIG
*IVIG reduce antibody production through feedback and displaces troublesome ABs so they cannot exert their harmful effects
Which 3 assays can be used before transplantation to minimise rejection?
o Cytotoxicity assays [CDC – Complement depended cytotoxicity]:
Inspects if recipient’s serum will kill the lymphocytes of the donor, in the presence of complement
• No antibodies lymphocytes remain viable = negative crossmatch
• Antibodies present lymphocytes are killed = positive crossmatch
Positive crossmatch suggests that there is cell lysis (i.e. +ve is a bad thing) = contraindication for transplantation
o Flow cytometry [FACS]
Inspects if recipient’s serum binds to the donor’s lymphocytes
Uses fluorescently labelled anti-human immunoglobulin
Detection of whether there is human Ig from the recipients serum bound on the donor lymphocytes
Negative flow crossmatch donor specific antibody not detected
Positive flow crossmatch donor specific antibody detected
o Solid phase assays (uses a series of synthetic beads containing all the possible HLA epitopes) [Luminex]:
Does the recipient’s serum bind to recombinant single HLA molecules attached to a solid support such as beads
Recipient’s serum is mixed with beads and fluorescently labelled immunoglobulin is used to determine which HLA epitopes the antibodies bind to
Patients that have antibodies to lots of different types of antibodies are regarded as highly sensitised
T cell mediated transplant rejection prevention
Pre- transplant induction agents
Inhibitors of cell surface receptors
Alemtuzumab
- Anti- CD52
- Causes lysis of T cells + T cell depletion
- Dampens down immune response
Basiliximab
- Anti-CD25
- Targets IL-2 R
- Prevents proliferation
OKT3
- Anti-CD3 ab
ATG - anti-thymocyte globulin
T cell mediated transplant rejection prevention
Post- transplant drugs
- Steroids
Prevent general T cell mediated rejection - Calcineurin inhibitors (tacrolimus, cyclosporin)
Inhibit cell signalling - Anti-proliferative agents (mycophenolate mofetil, azahioprine)
Act downstream of the MTOR activation + block T cell proliferation in response to antigen
Episodes of acute rejection mx
Steroids
OKT3
- Anti-CD3 ab
ATG - anti-thymocyte globulin
B cell mediated rejection prevention (7)
Rituximab (anti-CD20) B cell depletion
BAFF inhibitors target cytokines that promote B cell activation and growth
Proteasome inhibitors (i.e. bortezomib) block production of antibodies by plasma cells
Complement inhibitors (i.e. eculizumab, anti-C5) block complement binding to endothelial cells
Corticosteroids
IVIG
Plasma exchange
GvHD
prophylaxis
sx
mx
prophylaxis
- Methotrexate
- Cyclosporin
Sx
- Skin (rash)
- GI (N, V, D, abdo pain, bloody stool)
- Liver (jaundice)
Mx
- CS
Post-transplantation complications
• Infection
o Increased risk of conventional infections – bacterial, viral, fungal
o Opportunistic infections – CMV, BK virus, Pneumocystis carinii
• Malignancy
o Viral associated (x100) – Kaposi’s sarcoma (HHV8), Lymphoproliferative disease (EBV)
o Skin cancer (x20)
o Other cancers e.g. lung, colon (x2-3)
• Atherosclerosis
o HTN, hyperlipidaemia
o X20 increase risk in death from MI compared to age-matched general population
rejection prevention by tissue typing
o ABO/HLA tissue matching
o Screening for anti-HLA antibodies
Post kidney transplant patient presenting with raised creatinine mx
o Rejection increase
immunosuppresive drugs
o Nephrotoxicity decrease immunosuppressive drugs
• The most important antigenic determinant of rejection in current clinical practice for kidney transplantation is
o Human leucocyte antigens/MHC
• A potential donor is described as being 1:1:0 MM what does this mean
o 1MM A locus, 1MM B locus, 0 DR locus
(• The way we express differences between recipients and the HLA donor
o HLA-A: HLA-B: HLA-DR)
• A patient has an episode of acute T cell-mediated rejection 2 months post transplantation. What additional drugs would most commonly be administered?
o Steroids
o (tacrolimus, cyclosporin, azathioprine are used for baseline immunosuppression)
What type of hypersensitivity reaction is acute cellular rejection of the transplant by T cells?
Type IV
Hyperacute vs acute vs chronic rejection + cause
Hyperacute - mins hrs
Pre formed ab activates complement
Acute- -<6m
antibody or t cell mediated
Chronic >6m
immune and non immune mechanisms
