Transplantation & Immunosuppressive Drugs Flashcards
What is transplantation?
Transplantation is the introduction of biological material (eg organs, tissue, cells) into an organism
How does the immune system hinder transplantation?
The immune system has evolved to remove non-self
What are the different types of donor/recipient relationships?
-Autologous
-Syngeneic
(In these 2 donor/recipient relationships , donor and recipient is genetically identical = no immunological reactions)
-Allogeneic
What is an autologous transplantation?
Transplant biological material from one part of the organism into the same organism
e.g. skin graft - take skin from 1 part of the body and graft it onto another part of the body
Why is an immune response against autologous transplants unlikely?
May be inflammatory responses but no expected immune response as it is self transplant e.g. skin transplant
What is a syngeneic transplant?
Donor biological material is transplanted into recipient when donor and recipient are genetically identical e.g. twins
- no immunological reaction
What is an allogeneic donor/recipient relationship?
Donor and recipient are from the same species but are genetically different e.g. relatives: close genetic match (e.g. brother/sister)
= immune system reacts to the donated biological material
What is a xenogeneic donor/recipient relationship?
Donor and recipient are different species
e.g. pigs/bovine heart valves into humans in heart valve transplantation
What is histocompatibility?
Histocompatibility = tissue compatibility (compatibility b/w donor tissue and the recipient)
MHC = major histocompatibility antigens
Why do immune responses occur against transplants?
Immune responses to transplant are caused by genetic differences (histocompatibility differences) between the donor and the recipient
What is the major cause of transplant rejection?
MHC incompatibility
What is the human MHC?
Human MHC = Human Leukocyte Antigen
Describe the diversity of HLA classes
3 class I HLA alleles: HLA A, B & C
3 Class II HLA alleles (dimers)
- Thousands of Class I + Class II HLA alleles in a region of chr 6
- Each individual has 2 HLA A alleles + 2 HLA B alleles, + 2 HLA C alleles
Which cells express the different MHC molecules?
All nucleated cells express MHC Class I but only immune cells express MHC II molecules
Describe HLA Class I expression frequency
Even most common (A2) HLA can be classified into dozens of subtypes - lots of variability despite same HLA
What epitopes are present on donor MHC molecules?
B-cell epitopes on donor MHC
T-cell epitopes derived from donor MHC
1000’s of HLA alleles but perhaps only 100’s of epitopes
What technique is used to identify differences b/w donor HLA(MHC) and recipient HLA(MHC) alleles?
Next generation sequencing
What is the role of T cells in MHC Interaction?
T cells recognise foreign (non-self) peptides that are bound to self-MHC
CD8+ T-cells (cytotoxic T-cell) TCR interacts w MHC I(HLA) + the epitope it is expressing
CD4+ T-cells (helper T-cells) TCR interacts w MHC II (HLA) + the epitope in its binding groove
How do APCs express MHC molecules?
APC will express MHC (I/II) molecules where peptides bind in their variable region grooves
How do T cells recognise MHC molecules?
TCR detects both peptide and MHC complex
What cells do MHC I molecules activate?
MHC I activates TCR CD8+ cells
What cells do MHC II activate?
MHC II activates TCR CD4+ cells
What molecules do T cells recognise by MHC I presentation?
T cells recognise short peptide fragments that are presented to them by MHC) proteins (intracellular pathogens) e.g. viral infection
How are viral proteins processed?
Viral proteins are degraded by proteasome into peptides
How do MHC bind to viral peptides?
Peptides attract and bind MHC molecules that are then transported to cells surface
CD8 T cells can now interact
How are external pathogens cross presented?
Professional APCs (dendritic) can internalise external peptides and cross present onto MHC Class I pathway => CD8+ activation
Where does MHC II loading occur?
Only on professional APCs & WBCs
- immune cells
Describe the process of MHC II loading
- External antigens processed in phagolysosome into peptides
- Peptides interact with vesicles containing MHC and CLIP
- Vesicular complex transferred to surface
- CD4+ T cells activated
What is the role of the CLIP protein in MHC II loading?
Maintains HLA shape until peptide is ready to bind
What molecules do MHC I bind?
Fragments of intracellular proteins
Which molecules do MHC II bind?
Fragments of proteins which have been taken up by phagocytosis (APC)
What is the role of T helper cells (CD4+ T-cells)?
Helper T cells (CD4+ T-cells) are required to produce antibody and cytotoxic T-cell responses
CD4+ T-helper cells orchestrate the nature of the immune response
- Provide information about the nature of the infection by presenting epitopes (peptides) on HLA II
- Provides support to other immune cells via cytokine production = activates/inhibits B-cell responses, activates/inhibits cytotoxic T-cell responses (depending on cytokine production - IL2 / IFNgamma / IL10)
What is the role of cytotoxic T cells?
Highly specific killer cells
Lyse cells that present epitope (peptide) on their HLA I that the T-cell is specific to
Cytotoxic T-cells kill virally-infected cells/cancer cells/transplant(foreign) cells
What is the ‘foreign’ proteins are detected in transplant rejection?
In transplants, both the MHC protein and the peptide in its binding groove may be foreign
detected by TCR or antibody binding = various mechanisms by which the recipient’s immune system can reject donor transplantation
Describe how recipient cells may induce T cell activation
Recipient Cell:
No T-cell activation
- Self HLA + self peptide
T-cell activation
- Self HLA + non self peptide
How may donor cells induce T cell activation?
Donor cell:
No T-cell activation
- Matched HLA + peptide
T-cell activation
- Unmatched HLA + peptide
How are donors matched to recipients?
Usually try to match 4/6 MHC class II loci, reduces likelihood of future transplants and problems with future transplants - HLA mismatch reduces graft survival
What is the inflammation state of transplant recipients?
Recipients will have a history of disease which will have resulted in a degree of inflammation
What is the caution of using deceased donors?
Organs from deceased donors are also likely to be in inflamed condition due to ischemia
How is transplant success affected by live / deceased donors?
Transplant success is less sensitive to MHC mismatch for live donors
What are the 3 types of transplant rejection?
- Hyperacute rejection
- Acute rejection
- Chronic rejection
How soon after transplant does a hyperacute rejection occur?
Within a few hours of transplant
When is hyperacute rejection commonly seen?
Most commonly seen for highly vascularised organs (e.g. kidney)
(highly vascularised = greater immune system access)
What causes a hyperacute rejection?
Pre-existing antibodies, usually to ABO blood group antigens or MHC-I proteins
(ABO antigens are expressed on endothelial cells of blood vessels in the transplanted tissue)
Where do MHC antibodies come from?
Antibodies to MHC can arise from pregnancy, blood transfusion or previous transplants
How do antibodies cause damage to transplanted tissues?
Antibody binds to the non-self MHC/ABO antigen = Fc antibody region(not bound) is recognised by Complement/NK cells
Recognition of Fc region leading to -
- Complement activation
- Antibody-dependent cytotoxicity (Fc Receptors on NK cells)
- Phagocytosis (Fc Receptors on macrophages)
Describe how a hyperacute rejection occurs
- Antibodies bind to endothelial cells
- Complement fixation
- Accumulation of innate immune cells (NK cells, phagocytes) - phagocytose + lyse endothelial cells in transplanted tissue
- Endothelial damage, platelets accumulate, thrombi develop = tissue death, transplant failure
What is acute rejection?
Inflammation activates the transplant organ’s resident dendritic cells
DCs induce a T cell response due to MHC mismatch
Outline how direct allorecognition of a foreign MHC occurs
- Kidney graft dendritic cells activated
- DC migrate to spleen and activate effector T cells
- Effector T cells migrate to graft via blood
- Graft destroyed by macrophages + CTLs
- Inflammation activates transplant organ’s DCs
- DCs migrate to secondary lymphoid tissue + encounter recipient T-cells
- T-cells recognise foreign MHC = induces direct allorecognition
- T-cells + macrophages target transplanted tissue + destroy transplant
When does chronic rejection occur?
Occurs months/years after transplant
What are the effects of chronic rejection?
Blood vessel walls thickened, lumina narrowed – loss of blood supply
Correlates with presence of antibodies to MHC-I
How does chronic rejection arise?
Chronic rejection results from indirect allorecognition of foreign MHC/HLA
Describe how indirect allorecognition occurs
- Donor-derived cells die
- Membrane fragments containing donor MHC are taken up by host DC
- Donor MHC is presented into peptides presented by host MHC
- T cell response generated to peptide derived from processed donor MHC
What is HSCT?
Haematopoietic Stem Cell Transfer (HSCT)
Previously called bone marrow transplant, now renamed as source is often blood.
Often autologous
What are the advantages of HSCT?
HSCs find their way to bone marrow after infusion and regenerate there
HSCs can be cryopreserved with little damage
What is GVHD?
Graft Vs. Host Disease
When transplanted tissue is immune cells themselves, there is the risk of donor immune cells attacking the host
The host (transplant recipient) is seen as non-self = GVHD. Transplant is targeting the host (NOT THE HOST/RECIPIENT TARGETING THE TRANSPLANT)
How to reduce GVHD?
Can be lethal – best approach is prevention
Remove T cells from transplant or Suppress T-cell function using immunosuppressive drugs
What is GVL?
Graft Vs. Leukaemia
Why is GVL sometimes a good thing?
Sometimes mismatch and donor leukocytes can be beneficial - removing original leukemia
Development of GVL may prevent disease relapse
(the graft can identify + kill leukaemia cells, which necessitated the graft in the first place)
graft sees leukaemia as non-self.
recipient’s original immune response partially sees leukaemia as self = weaker immune response against tumour cells.
What is the significance of immunosuppression for transplants/?
Essential to maintain non-autologous (allogeneic) transplant.
3 phases of treatment:
- Induction - Antibody induction therapy + Triple drug regimen(calcineurin inhibitor + antiproliferative agent + corticosteroid)(high doses)
- Maintenance - Triple drug regimen at lower doses
- Rescue - T-cell mediated rejection = treat with ATG + high dose steroids. B-cell mediated rejection = treat with IV Ig(derived from other ppl) OR anti-CD20 monoclonal antibodies(depletes B-cells by binding to CD20 on B-cells) + high dose corticosteroids(for both)
rescue phase if transplant rejection occurs
What immunosuppressors are used for transplants?
General immune inhibitors:
- e.g. corticosteroids
Cytotoxic = specific:
- kill proliferating lymphocytes
- e.g. mycophenolic acid, cyclophosphamide, methotrexate
Inhibit T-cell activation:
- cyclosporin, tacrolimus, rapamycin
What is a cost-effective drawback of using immunosuppressives?
Immunosuppressives may need to be maintained indefinitely
What is cyclosporin?
Inhibits recipient immune response to donor transplant.
Blocks T cell proliferation and differentiation.
Breakthrough drug for transplant - enables transplants to survive for longer in recipients.
Next generation therapies developed are less toxic and effective at lower doses.
T-cells and MHC I
Cell processes the protein and cell’s MHC I presents the peptide to T-cells
- T-cells recognise short peptide fragments that are presented to them by cell
- In the cell, the protein is internalised in the cell - acts as a viral protein/is MHC protein itself
- Proteasome degrades the protein into peptides
- ER transports these peptides to MHC I
- MHC I travels to cell surface
- TCR recognises MHC I + the peptide it presents
2-6 occur in the cell
T-cells and MHC II
- Antigen is internalised from extracellular spaces by phagolysosome
- Protein is lysed into peptides
- These peptides are transferred to MHC II
- MHC II-peptide complex is presented on APC cell surface
MHC II , CD4 , Extracellular antigens (e.g. antibody-bound viral particles/dead cells
What is allorecognition?
Allorecognition = T-cells are activated to respond to transplanted material
- Direct allorecognition
- Indirect allorecognition
Recipient Cell
Self-HLA presents self-peptide to self-TCR = no immune response.
If immune response occurs here = autoimmunity
Self-HLA + self-peptide = no T-cell activation
Self-HLA presents foreign peptide to self-TCR.
Self-HLA + non-self peptide = T-cell activation.
Indirect allorecognition
Partial matching b/w donor + recipient HLA
1 HLA which does match (self) could present a foreign peptide which does not match. Immune system recognises as foreign.
The T-cells activated to this self-HLA, non-self peptide then find this same combination on transplanted cells
Donor cells
If the donor cell has matched HLA + self-peptide , no T-cell activation
If matched donor HLA presents a foreign peptide(from an unmatched HLA) = targeting of transplant in indirect allorecognition
Unmatched donor HLA presents non-self epitope from the HLA itself = TCR binding/antibody binding of unmatched HLA = causes direct allorecognition
The more donor/recipient HLA mismatches =
The more donor/recipient HLA mismatches = the less successful the transplant will be, lower graft survival
What happens in chronic rejection?
Alloantibodies bind to antigens + endothelial cells of the transplanted organ and recruit other immune effector cells - macrophages, cytotoxic T-cells, which induce damage + reduce blood supply to the organ.
Alloantibodies to MHC I of the transplanted organ
For chronic rejection, the MHC protein + its peptide in the binding groove may be foreign
In chronic rejection, the Anti-transplant immune responses are induced from a type of indirect allorecognition
Donor-derived cells die (e.g. donor HLA) + the dying cells are internalised by recipients’ DCs = donor HLA is presented on recipient HLA = recipient HLA induces T-cell + antibody responses generated from the peptides derived from donor’s MHC
What do HSCs do when they reach the bone marrow after infusion?
produce common precursors for lymphocytes = B-cells, T-cells, activated effector T-cells, plasma cells that produce Abs, granulocyte progenitors, basophils, neutrophils, eosinophils, monocytes, DCs, macrophages, mast cells, RBCs
Combination immunosuppressive regimes
1 - Steroids
2 - Cytotoxic
3 - Immunosuppressive specific for T-cells
What does calcineurin inhibitor inhibit?
Calcineurin inhibitor inhibits IL2 production
What do antiproliferatives do?
antiproliferatives inhibit B-cell/T-cell proliferation
what do corticosteroids do?
corticosteroids = anti-inflammatory
what do monoclonal antibodies do?
monoclonal antibodies = costimulation blockers
what do antithymocyte globulins do?
antithymocyte globulins inhibit/deplete T-cells
What are the negative consequences of immunosuppression?
Immunosuppression to prevent transplant rejection also reduces recipient ability to fight infection/tumorigenesis.
-Transplant patients are more susceptible to CMV infection
Immunosuppressive drug toxicity causes organ failure e.g. cyclosporin induces nephrotoxicity in kidney transplant
What new area has been found to be involved in transplant immunity?
The microbiome of the intestine is involved in regulating adaptive immune responses
= may be useful in transplantation outcomes
Summary
• Transplant rejection results from genetic differences
between donor and recipient – variability between MHC
proteins is the major genetic difference in transplant
rejection
• Host attacks transplant – rejection (hyperacute, acute,
chronic)
• Transplant attacks host – GVHD
• Immunosuppression can prolong the survival of
transplanted organs
