Transplantation and Immunosuppressive Drugs

Question 1

Define transplantation.

Challenge with transplantation

Answer 1

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

The problem with that is that the immune system has evolved to remove anything it regards as non-self.

Question 2

What are the different kinds of transplants?

Answer 2

• autologous
• syngeneic
• allogeneic
• xenogeneic

Question 3

Describe autologous donor/recipient relationships.

Answer 3

transplantation of tissue from one part of an organism into another part of the same organism

• no immune response
• e.g. skin transplant

Question 4

Describe syngeneic donor/recipient relationships.

Answer 4

transplantation of tissue from donor into recipient who is genetically identical

• no immune response because genetically identical
• e.g. between identical twins

Question 5

Describe allogeneic donor/recipient relationships.

Answer 5

transplantation of tissue from donor into recipient who is genetically different

• e.g. siblings/relatives
• most common

Question 6

Describe xenogeneic donor/recipient relationships.

Answer 6

This is the transfer of tissue from an organism of one species to another.

This is not common; there have only been a few cases where this happens with success.

Question 7

Describe the importance of MHCs in transplants.

Answer 7

MHCS are major histocompatibility antigens.
The human MHC is called HLA, Human Leukocyte antigen. It defines tissue compatibility.

The most important are differences between the antigens forming the major histocompatibility complex (MHC).

Question 8

Describe HLA diversity (where it is found in the genome).

Answer 8

The HLA gene is found on chromosome 6.

Almost all nucleated cells present HLA Class I, while immune cells present both Class I and Class II.

Question 9

Classifications of HLA

Answer 9

The alleles are split up into Class I (consisting of 3 alleles) and II (consisting of 6 alleles). The Class II molecules are heterodimers, while Class I are monomers.

Class I
-HLA-A, HLA-B, HLA-C

Class II
-HLA-DRA, HLA-DRB, HLA-DPA, HLA-DPB, HLA-DQA, HLA-DQB

Question 10

Where are MHC I and II expressed?

Answer 10

MHC I
-all nucleated cells

MHC II
-APCs (which also express MHC I)

Question 11

How can we match donor and recipient MHC?

Answer 11

via next generation sequencing

Question 12

What is the importance of epitopes on donor MHC?

Answer 12

At the current time, HLA are typed, and the donor and recipient are matched based on typing information. There are many ways of doing it, sequencing being the best one. The trouble is that there are 1000s of HLA.

NGS can be used to identify the epitopes on the HLA, rather than the HLA themselves. There could be two HLAs that have genetic differences, but don’t have any different epitopes. It’s possible that the actual number of epitopes that are important in rejection is far fewer than the HL alleles. In terms of matching, we are moving from matching HLA to matching alleles on HLA.

This could be much better, but NGS is more expensive so cannot be done routinely.

Question 13

Describe how APCs activate different T-cells.

Answer 13

Antigen-presenting cells will express MHC Class I and MHC Class II; this is where the variation is in the molecule.

The TCR detects a combination of both the peptide and the MHC, so it is a peptide-MHC or peptide-HLA complex that the TCR is detecting.

MHC Class I will activate TCR in CD8 T Cells, and MHC Class II will activate TCR in CD4 T Cells.

Question 14

Describe MHC II loading in cells.

Answer 14

This is only on professional APCs.
These cells are good at taking up external material, which they will process in the phagolysosome into peptides. These peptides interact with the vesicle containing MHC and CLIP (maintains the shape of HLA until the peptides arrive).

When the peptides bind, the complex goes to the cell surface to activate CD4 T cells.

Question 15

What is the key point about the differences in which cells have which class of MHC?

Answer 15

All cells have the MHC Class I, which means that if they are infected by a virus or bacteria, they can alert the immune system of it so that they are targeted or removed by CD8 cytotoxic cells.

MHC Class II is only on immune cells, so that when those cells have got to the site of infection and internalised the infectious material, they can present them to CD4 T cells to orchestrate a more robust and long-lasting immune response. Thus, they need more regulation.

Question 16

As a recap, what is the difference between helper T cells and cytotoxic T cells?

Answer 16

Helper T cells – information and support for other immune cells via cytokine production

(helper T cells are required to produce antibody and cytotoxic T cell response)

Cytotoxic T cells – highly specific killer cells

Question 17

What are the reasons for transplant rejection?

Answer 17

Either:

• MHC protein is foreign
• Peptide in the binding groove is foreign

Question 18

Allorecognition

Answer 18

Activation of T cells to react against transplant

Question 19

What are the two ways in which foreign bodies can be recognised?

Answer 19

There can be direct recognition of the HLA, or indirect recognition of the peptide as foreign.

Question 20

Indirect allorecognition

Answer 20

Indirect allorecognition:
Recipient T cells recognise a self-MHC molecule on a recipient cell with a bound peptide derived from the donor/foreign MHC molecule, causing T cell activation

Question 21

Direct allorecognition

Answer 21

Recipient T cells recognise an intact allogenic (unmatched/foreign) MHC molecule expressed by a donor cell, causing T cell activation

Question 22

Why is is so important to match HLA?

Answer 22

HLA mismatch correlated to the graft survival.

Even with no mismatches, the 10-year estimate of survival is only 33%.
This isn’t even with taking into account the immunosuppressive medicine, and the complications arising from that.

Question 23

Dead donor transplant

Answer 23

More sensitive to MHC mismatch, therefore more complications

• organs from deceased donors likely to be in inflamed condition due to ischaemia
• the donor inflammation can exacerbate the immune response to mismatches.

Question 24

What are the three types of graft rejection?

Answer 24

• hyper-acute rejection
• acute rejection
• chronic rejection

Question 25

Describe hyper-acute rejection.

Answer 25

Antibodies to MHC can arise from pregnancy, blood transfusion or previous transplants.

Pre-existing anti-donor antibodies rapidly bind antigens on donor tissue and result in:

• complement activation
• antibody dependent cellular cytotoxicity (Fc receptors on NK cells)
• phagocytosis (Fc receptors on macrophages)

Question 26

Onset of hyperacute rejection

Answer 26

within a few hours

Question 27

Which antigens do pre-existing anti-donor antibodies bind to in hyperacute rejection?

Answer 27

• Usually to ABO blood group antigens expressed on endothelial cells of blood vessels
• MHC I proteins

Question 28

Where is hyperacute rejection most commonly seen?

Answer 28

commonly seen in organs like kidneys because the immune response can easily access highly-vascularised tissue.

Question 29

Effector mechanisms of preexisting anti-donor antibodies in hyperacute rejection

Answer 29

1) Antibodies bind to endothelial cells
2) Complement fixation
3) Accumulation of innate immune cells
4) Endothelial damage, platelets accumulate, thrombi develop

Question 30

How can antibodies cause damage to transplanted tissue?

Answer 30

This works by the pre-existing antibodies detecting the antigen on the donor tissue.
The Fc region (which is normally conserved) on the antibody binds to the antigen. This can also bind to Fc receptors on immune cells.

This is an innate immune response using adaptive antibody binding.

The binding of the Fc region allows for complement activation.
This could be antibody-dependent cellular cytotoxicity, where cells such as NK cells kill the cell bound to the antibody.
It could also be phagocytosis, by cells such as macrophages.

The reason this happens so quickly is due to the pre-existing antibodies and the complement fixation.

Whilst it kills individual cells, it also causes a build-up of inflammatory cells in endothelium, which results in damage to the endothelial cells, so the tissue is not sufficiently vascularised, and the tissue can die.

Question 31

Describe acute rejection.

Answer 31

Direct allorecognition of foreign MHC:

• donor dendritic cells migrate to secondary lymphoid tissue where they encounter effector T cells → MHC mismatch
• dendritic cells express both MHC I and II, meaning they activate both CD8+ and CD4+ T cells
• CTL and macrophages increase inflammation and destroy transplant

Question 32

Describe chronic rejection.

Answer 32

Indirect allorecognition
1) Donor cells die

2) Membrane fragments containing donor MHC are taken up by recipient/host dendritic cells
3) Donor MHC is processed into peptides which are presented by host MHC
4) T cell response is generated to the peptide derived from the processed donor MHC

Question 33

Onset of chronic rejection

Answer 33

can occur months or years after transplant

Question 34

Effector mechanisms of chronic rejection

Answer 34

· Blood vessel walls thickened, lumina narrowed - loss of blood supply

· Correlates with presence of antibodies to MHC I

Question 35

Describe HSCT.

Answer 35

HSCT is Haematopoietic Stem Cell Transfer.

• stem cells find their way to the bone marrow after infusion and regenerate there where they are cryopreserved with little damage
• often autologous

Question 36

What is the risk of transplanting immune cells?

Answer 36

GVHD is Graft-Versus-Host Disease.

• risk of donor immune cells attacking the host
• can be lethal - it’s best to stop it at the early activation stage, when it isn’t too prevalent.

One way to do that is to remove the T cells or suppress their function, except then you don’t have an immune response.

Question 37

When can GVHD be advantageous?

Answer 37

There is one scenario where GVHD can be advantageous.

Sometimes mismatch and donor leukocytes are beneficial

We can use the stem cell transfer to remove any remnants of original leukaemia in a patient.

The donor immune system will be able to target the cancer in a way that the host immune system can’t.

Cancer cell are derived from self, and the immune system struggles to identify it as a foreign cell. However, if you have a donor’s immune system transplanted, it will be able to recognise the cancer cells as foreign and kill them because of the HLA mismatch.

Question 38

How can we maintain a non-autologous transplant?

Answer 38

Immunosuppression drugs are essential to maintain a non-autologous transplant

Question 39

What are the different phases of immunosuppression?

Answer 39

It’s essential to maintain non-autologous transplants.

There are various phases:

• INDUCTION: to try and prevent the build-up of an immune response
• MAINTENANCE: may require changing medication or lower doses depending on the side effects experienced
• RESCUE: if the immune response starts to mount towards the transplant, you have a rescue phase

Immunosuppressives may need to be maintained indefinitely.

Question 40

List some examples of immunosuppressants.

Answer 40

General immune inhibitors (e.g. corticosteroids)

Cytotoxic – kill proliferating lymphocytes (e.g. mycophenolic acid, cyclophosphamide, methotrexate)

Inhibit T-cell activation (cyclosporin, tacrolimus, rapamycin)

Question 41

MOA of cyclosporin

Disadvantage of cyclosporin

Answer 41

It blocks T cell proliferation and differentiation. Even if there was a nascent immune response against the transplant, it cannot develop into an effective cytotoxic response.

More toxic than other next generation therapies which are less toxic and effective at lower doses

Question 42

List some examples of next generation therapies in immunosuppression and their mechanism of action.

Answer 42

• Cyclosporin and FK506 (Tacrolimus): inhibits production of IL-2 (IL-2 stimulates proliferation in T-cells).
• Mycophenolic acid: blocks lymphocyte proliferation through inhibition of DNA synthesis in T and B cells
• Rapamycin: blocks lymphocyte proliferation by inhibiting IL-2 signalling
• Steroids: they stop general anti-inflammatory effects from taking hold
• Anti-CD3 monoclonal antibody: depletes T cells by targeting them for destruction
• Anti-IL-2 receptor antibody: inhibits T cell proliferation by blocking IL-2 binding; it may also promote phagocytosis and complement activation

Question 43

Why are combination therapies preferred over single-drug therapies?

Answer 43

Combinations are normally more effective than taking a single drug.

It will also allow you to lower the dose of both of them, and reduce the side-effects.

· Steroids e.g. prednisolone

· Cytotoxic e.g. mycophenolate motefil

· Immunosuppressive specific for T cells e.g. cyclosporine A, FK506

Question 44

Why is immunosuppressive therapy monitoring important?

Answer 44

An immune response to the transplant may still develop.

Other immune responses may be affected

Transplant patients are more susceptible to infection and malignancy, and so we need to make sure that the patient’s immune response is not too suppressed
- Immediate risk e.g. CMV

Immunosuppressive drug toxicity can lead to organ failure
- E.g. cyclosporine nephrotoxicity in kidney transplant

Question 45

What is the role of the intestinal microbiome in transplants?

Answer 45

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.

Thus, it may be implicated in transplantation outcomes.

Question 46

What are immune responses to transplant caused by?

Answer 46

Genetic differences between donor and recipient