Transplantation and Immunosuppressive Drugs

Question 1

What is a transplantation?

Answer 1

Transplantation is the introduction of biological material (e.g. organs, tissue, cells) into an organism

Question 2

What is the function of the immune system in regards to transplantation?

Answer 2

The immune system has evolved to remove anything it regards as non-self

Question 3

Define Autologous

Answer 3

Autologous- transplantation of tissue from one part of the organism into another part of the same organism

• Even though there may be inflammatory responses there wouldn’t expect an immune response, as its self-transplanted into self
• Skin graft from one part of the patient’s body to another part

Question 4

Define Syngeneic

Answer 4

Syngeneic- is similar but it is donor material transplanted into the recipient but when the donor and recipient are genetically identical

• So again, self to self-transplantation, doesn’t usually generate immunological responses because there is no genetic difference between donor and recipient
• E.g. identical twins
• Typically, no immune response in transplantation as they are genetically identical
• Donors and recipients are genetically identical, do not usually generate any immunological problems

Question 5

Define allogenic

Answer 5

Donors and recipients are from the same species but genetically different.

• Donors and recipients are from the same species but genetically different
• The most common type of transplantation is allogenic
• Allogenic- from one individual the donor, transplanted by material into the recipient
• Immune responses described as the donor transplant and the recipient immune response
• With the exception of when white blood cells are transferred then we are talking about the donor immune response and the recipient as target

Question 6

Give an example of an allogenic transplant in humans

Answer 6

• Example, brother and sister, siblings, relatives

* We want as close of a genetic match as possible

Question 7

Define xenogeneic and give an example

Answer 7

donor and recipient are different species.

• Donor and recipient are different species
• Use of material from pigs and cows to humans
• Humans closest biological relative are chimpanzees but they are endangered

Question 8

Why do get an immune response against a transplant?

Answer 8

• The most important are differences between the major histocompatibility antigens
• The reason why we get immune responses against transplant is because there are genetic differences between the donor and the recipient, and the most important differences are in the MHC

Question 9

What is another name for MHC?

Answer 9

HLA (Human Leukocyte Antigen)

Question 10

Describe the diversity of HLA molecules

Why are there more HLA class 2 molecules than HLA class 1?

Answer 10

• 2 types of HLA
o Class 1 – A, B, C
o Class 2 – DR, DP, DQ
• Most diverse area of the genome an on chromosome 6

• Class 1 HLA comprises of a single protein which is linked to a molecule called beta 2m and a surface of nucleated cells
• Class 2 HLA composed of 2 proteins, A and B subunit. Typically expressed on immune cells but not normal somatic cells

Question 11

Describe the importance of epitopes on donor MHC

Answer 11

• 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.
• Next generation sequencing required.

Question 12

Describe how T-cells recognise HLA molecules

Answer 12

HLA antigens are vital in the activation of cellular adaptive immunity and they activate different T cells
• HLA class 1 – interacts with TCR of CD8 T cells
o called CD8 because of the CD8 co receptor interacting with the MHC.
o CD8 T cells are involved in cytotoxic T cell immunity, so clearance of infected or tumorigenic cells
• MHC class 1 will activate TCRs and CD8 T cells, so APC but also any nucleated cell

• HA class 2 – interact with TCR of CD4+ T cells in association with CD4 receptor
o these activate CD4 T cell immunity which is helper immunity and induce TH1 cytotoxic responses or TH2 antibody responses.
o Both of these responses are vital for viral infections and anti-tumor.
• Just for APCs, MHC class 2 with peptide will activate CD4 T cells with the correct TCR

Question 13

Summarise MHC binding

Answer 13

On image

Question 14

Describe Direct and indirect T-cell activation

Answer 14

On image

• Direct – where HLA is recognised as foreign
• Indirect – where peptide presented on host HLA is regarded as foreign by immune system

Question 15

Describe Live vs dead donors:

Answer 15

• Transplants can come from live or dead donors
• Less material can be taken from a live donor than you can take from a dead donor
• Recipients will have a history of disease which will have resulted in a degree of inflammation
• Organs from deceased donors are also likely to be in inflamed condition due to ischemia
• Inflammation can activate innate and adaptive immune responses and can break tolerance to transplants
• Pairing this with the fact that recipients who may have a history of disease which is why they need the transplant also have a degree of chronic inflammation, this inflammation can activate and exacerbate immune responses to mismatches with however many exist
• Transplant success is less sensitive to MHC mismatch for live donors

Question 16

What are the 3 types of graft rejection mechanisms?

Answer 16

• Hyperacute rejection
• Acute rejection
• Chronic rejection

Question 17

What is Hyperacute rejection?

Answer 17

• Within a few hours of transplant
• Most commonly seen for highly vascularised organs (e.g. kidney)
• Because host immune response has access to tissues very quickly through this
• Requires pre-existing antibodies, usually to ABO blood group antigens or MHC-I proteins
• (ABO antigens are expressed on endothelial cells of blood vessels)
• Antibodies to MHC can arise from pregnancy, blood transfusion or previous transplants.

Question 18

How can antibodies cause damage to transplanted tissue? (hyperacute rejection)

Answer 18

• Antibodies are quick as they rapidly identify differences in the transplanted material
• Once they’ve bound the antigen like ABO blood group à activate innate immune effectors à phagocytosis and uptake of donated tissue
• Recognition of Fc region leading to:
o The variable region of the antibody binds to the antigen, the Fc region is less variable and can bind to fc receptors on innate immune cells
o So, this is an innate immune response using adaptive antibody binding and the binding of this Fc region is compliment activation
o Complement activation
o Antibody dependent cellular cytotoxicity
 (Fc Receptors on NK cells)
o Phagocytosis
 (Fc Receptors on macrophages and Dendritic cells)
• This is all very quick as the antibodies are already there it’s an innate immune response to the Fc region binding, so it’s very quick unlike an adaptive immune response

• Antibodies bind to endothelial cells
• Activate complement fixation
• While this kills individual cells it also causes a build-up of inflammatory cells in the endothelium which results damage to the endothelial cells, the tissue is therefore not adequality vascularised and the tissue can indirectly die from this breakdown in the endothelial cells caused by the antibody, ADCC and phagocytosis
• Lead to accumulation of innate immune cells and eventually adaptive immune cells
• Endothelial damage, platelets accumulate, thrombi develop

Question 19

What is acute rejection and describe the mechanism for it?

Answer 19

• Second mechanism of rejection
• Inflammation results in activation of organ’s resident dendritic cells in the donor tissue
• Dendritic cells from transplant have foreign HLA molecules which activate host recipient T cells
• Inflammation activates innate immune responses which causes T cell responses
• T cell response develops as a result of MHC mismatch
• Inflammation results in activation of organ’s resident dendritic cells
• DC migrate to secondary lymphoid tissue where they encounter circulating effector T cells
• The donor DC react with the recipient T cells and there’s an MHC mismatch here as the donor DCs have donor HLA and the recipient T cells will identify it as foreign
• These donor DCs will have MHC class1 and class 2 because they’re professional immune cells so they’ll activate CD8 T cells, which will activate CD4 T cells
• These T cells will proliferate and migrate to the donor organ which may have an inflammatory environment which helps the migration and then those CD4s and CD8s will target every cell which express HLA in the donor tissue, which is why it is quite quick
• Macrophages and CTL increase inflammation and destroy transplant

Question 20

What is chronic rejection?

Answer 20

• Can occur months or years after transplant
• Blood vessel walls thickened, lumina narrowed – loss of blood supply
• Correlates with presence of antibodies to MHC-I
• The MHC mismatch is less pronounced than direct allo recognition

 Antibodies regarded as alloantibodies, identify the transplant material and bind and recruit immune cells which recruit other cells like T cells and gradually increases and causes the thickening of blood vessel walls and reduced blood supply
 This correlates with our antibodies which are identifying donor HLA as they recognize them as non-self
 The migration of effector cells to these sites cause more damage and inflammation and aids in the breaking of tolerance in the host T cells to the transplant
 Even if MHC mismatch isn’t too severe, in the context of inflammation it can break tolerance that exists

• Membrane fragments containing donor MHC are taken up by host DC
• Donor MHC is presented into peptides which are presented by host MHC
• T cell response is generated
• Donor-derived cells die, those cells taken up by recipient antigen presenting cells - have HLA cells which are foreign to the host
• They break down the fragment of these donor cells and present the non-self foreign HLA as peptides
• DC activate T cells in lymph nodes which are then reactive to foreign peptide.
• They will then target wherever they see that foreign peptide even if there are HLA similarities if there’s HLA present in that peptide the activated T cells will kill it

Question 21

What is HSCT?

Answer 21

• The donor’s immune response removes, and the stem self-transfer provides a new immune system
• In HSCT the situation is flipped around, instead of having the recipient’s immune system targeting the donor we now have the donor transplant targeting the recipient
• Sometimes it’s using stem cells from the same patient as there won’t be an immune response
• Previously called bone marrow transplant, now renamed as source is often blood
• Often autologous
• Can induce graft versus host disease because the donor immune system attacks the recipient
• Until 1980 only HLA identical siblings could be used as donors due to the risk of rejection or graft versus host disease
• HSCs can find their way to bone marrow after infusion and regenerate there.
• They can be cryopreserved with little damage

• From these cells you can get all the leukocytes involved in an immune response so this will include:
o lymphoid progenitors which eventually produce B and T cells
o myeloid progenitors which eventually make monocytes, macrophages, dendritic cells, neutrophils, etc
• Before recipient immune response would target the foreign tumor
• now because the immune cells have been grafted, the graft attacks the host because the new immune system has been transplanted into the recipient, identifies the recipient itself as foreign using many of the same mechanisms discussed and it can be lethal

Question 22

What is Graft Versus Host Disease?

What does removing T-cells from the transplant do?

Answer 22

• When transplanted tissue is immune cells themselves, there is the risk of donor immune cells attacking the host – GVHD
• Once that immune system has kicked off it continues to develop and gets worse, when immune system is activated has a snow ball effect in terms of T cells proliferating and responding to antigen and proliferating more
• If we have an autoimmune disease it will get worse the more it progresses, so best thing to do is to stop it at that activation stage
• One way to do this is to get rid of the T cells, but then you don’t have an immune response
• Can be lethal – best approach is prevention and using immunosuppressive therapy to control
• Removing T cells from transplant reduces GVHD

Question 23

Describe Graft vs Leukaemia

Answer 23

• But sometimes mismatch and donor leukocytes can be beneficial - removing original leukaemia
o Patients’ immune response sees leukemia as self but with grafted transplanted HSCs, foreign immune response, the leukemia is seen as foreign and will target it and is better at clearing leukemia
• Example- grafting a donor immune system to a recipient could be advantageous, typically used for patients who have radiation therapy for leukaemia’s which destroys their immune response, so need a new one provided by a graft
• There could be residual tumorigenic cells from the leukaemia
• The befit of having a foreign donor immune system is that it can target that cancer in a way that the hosts immune system typically can’t
• Problem with the immune response against cancer is that the cancer derives from self and so the immune system struggles to identify it as it would with a virus
• But then if you have the donor’s immunes systems implanted won’t have this problem because of the HLA mismatch
• Graft versus leukaemia response
• Development of GVL may prevent disease relapse

Question 24

Why is immunosuppression needed and the 3 phases of it?

Answer 24

• Essential to maintain non-autologous transplant

1. Induction phase – chances of generating response against the transplantation are highest due to inflammatory environment. To try and prevent that build-up of that immune response
2. Maintenance phase – recipient of transplant has to take immunosuppressant on a regular basis which can be toxic and has other issues involved, if drugs fail/become too toxic/immune response to transplant initiated then. May require change in medication or lower does depending upon side effects and how effective it is
3. Rescue phase – more suppressive and powerful to stop transplant from being rejected entirely. If it starts to be less functional, if the immune response starts to mount towards the transplant you have this rescue phase

Question 25

Give some examples of immunosuppression

Answer 25

• Cyclosporin- Breakthrough drug for transplant. Blocks T cell proliferation and differentiation through inhibiting production of IL-2.
• Next generation therapies less toxic and effective at lower doses.
• General immune inhibitors (e.g. corticosteroids- General anti-inflammatory effects).
• Cytotoxic – kill proliferating lymphocytes (e.g. mycophenolic acid- Blocks lymphocyte proliferation through inhibition of DNA synthesis in T and B cells-, cyclophosphamide, methotrexate).
• inhibit T-cell activation (cyclosporin, tacrolimus, rapamycin- Blocks lymphocyte proliferation by inhibiting IL-2 signalling).

Question 26

Combination immunosuppressive regimes

Answer 26

• Combinations are more effective than single drugs
• May also allow you to lower the dose of both of them and lower the side effects

1. Steroids – e.g. prednisolone
2. Cytotoxic – e.g. mycophenolate motefil
3. Immunosuppressive specific for T cells – e.g. cyclosporin A, FK506,

Question 27

Describe Intestinal microbiome

Answer 27

• The microbiome, particularly of the intestine, is involved in regulating adaptive immune responses.
• Immunosuppressed patients (e.g. cancer patients) can take FMT – faecal material transplant – in order to promote effective anti-cancer immune responses.
• May be implicated in transplantation outcomes.