Transplantation and Immunosupressive Drugs

Question 1

what is transplantation?

Answer 1

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

Question 2

what has the immune system evolved to do?

Answer 2

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

Question 3

why was the eye transplant so successful and done so early on?

Answer 3

because it’s an immunologically privileged site

Question 4

Donor/recipient relationships - diff types?

Answer 4

Autologous
Syngeneic

Allogenic
Xenogenic

Question 5

Autologous vs Syngeneic

Answer 5

for both of these, you wouldn’t expect an immune response

Autologous: Transplantation of tissue from 1 part of the organism into another part of the same organism. May be inflammatory responses, but no immune response as it is “self transplanting into self”

Syngeneic: Donor material transplanted into a recipient - donor and recipient are genetically identical.

Question 6

Allogenic vs Xenogenic

Answer 6

both are genetically different, immune response may be generated

Allogenic: Donors and recipients are from the same species but genetically different

Xenogenic: Donor and recipient are different species

Question 7

what are Immune responses to transplant caused by?

Answer 7

genetic differences between the donor and the recipient

• most important are differences between the antigens forming the major histocompatibility complex (MHC)
• MHC = biggest site of variability in human genome, which is why immune responses are raised against it

Question 8

what is the human antigen called?

Answer 8

HLA (human leukocyte antigen)

Question 9

HLA diversity

Answer 9

can be split into class I and class II:

Class I has 3 alleles
Class II are dimers and there are 6 alleles

Almost all mutated cells express HLA class I
Only WBC’s/professional APC's express HLA class I and HLA class II – important for rejection

Question 10

Importance of epitopes on donor MHC

Answer 10

there are 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 - this means that even if people have different HLA’s, they may still have the same epitopes, therefore rejection won’t occur.

So, we are moving from matching recipient and donor HLA’s to matching recipient and donor alleles on the HLA, but NGS is too expensive for this to become a regular thing right now

Question 11

what do T cells need to recognise?

Answer 11

foreign peptides bound to self-MHC

• peptide is bound into the binding groove
• TCR detects the combination of peptide and MHC

Question 12

T cells and MHC class I

Answer 12

CD8 T cells recognise short peptide fragments from intracellular proteins that are presented to them by major histocompatibility (MHC) proteins

eg. a cell infected by a virus, viral proteins are processed by the proteasome into peptides

Question 13

T cells and MHC class II

Answer 13

MHC class II only on WBC’s and professional APC’s

Good at taking up external material into a phagolysosome and breaking it down into peptides

Peptides interact with vesicle containing MHC

CLIP maintains the shape of HLA until the peptides are ready to bind.

MHC/peptide —-> APC surface, activates CD4 T cells

Question 14

types of T cells?

Answer 14

Cytotoxic T cells – highly specific killer cells

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

Question 15

in transplants, what may be foreign?

Answer 15

In transplants, both the MHC protein and the peptide in its binding groove may be foreign

so, donor HLA could be detected as foreign by the recipient immune system, or the peptide, OR both!

Question 16

Direct and indirect T-cell activation

Answer 16

Direct allo recognition = recipient TCRs detecting donor/non-self HLA

Indirect allo recognition = recipient TCRs detecting donor/non self peptides

Self HLA + self peptide = no T-cell activation
Self HLA + non self peptide = T-cell activation

Matched HLA + peptide = no T-cell activation
Unmatched HLA + peptide = T-cell activation

Question 17

Live vs dead donors

Answer 17

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

Transplant success is less sensitive to MHC mismatch for live donors

Question 18

Types of graft rejection

Answer 18

Hyperacute rejection

Acute rejection

Chronic rejection

Question 19

Hyperacute rejection

Answer 19

Within a few hours of transplant

Most commonly seen for highly vascularised organs (e.g. kidney) because it is easier for an immune response to target highly vascularised tissue

Requires pre-existing antibodies, usually to ABO blood group antigens expressed on blood vessel endothelial cells, or to MHC class I - Antibodies to MHC can arise from pregnancy, blood transfusion or previous transplants

Antibodies bind to endothelial cells, complement fixation. Accumulation of innate immune cells, endothelial damage, platelets accumulate, thrombi develop

Question 20

How can antibodies cause damage to transplanted tissue?

Answer 20

The pre existing antibodies detect their antigen on the donor tissue, bind via the Fab-like fragment (variable)

The Fc region on antibodies can bind to Fc receptors on immune cells, initiating a quick innate immune response.

Recognition of Fc region leads to complement activation:

1. Antibody dependent cellular cytotoxicity
   (Fc Receptors on NK cells)
2. Phagocytosis
   (Fc Receptors on macrophages)

Question 21

Acute rejection

Answer 21

Inflammation results in activation of organ’s resident dendritic cells (donor DC’s, NOT recipient)

T cell response develops as a result of MHC mismatch

Question 22

Direct allorecognition of foreign MHC

Answer 22

eg. kidney transplant

inflammation activates recipient DC’s (in this case, they are in the kidney)

DC’s migrate to secondary lymphoid tissue. They are professional APC’s so they have MHC class I/II. Activate effector CD4 and CD8 T cells, which proliferate and migrate to the donor organ.

CD4’s and CD8’s target every cell that expresses the “foreign” HLA in the tissue and destroy the kidney transplant.

Question 23

Chronic rejection

Answer 23

results from indirect allorecognition of foreign MHC/HLA

Donor-derived cells die, and cells taken up and broken down by host DC’s and presented as peptides on host MHC. DC’s activate T cells in the lymph node, which are sensitive to the foreign peptide and kill any cell presenting it.

Can occur months or years after transplant - this MHC mismatch effect is less pronounced so takes longer to develop.

Blood vessel walls thickened, lumina narrowed – loss of blood supply

Question 24

Haematopoietic Stem cell transfer (HSCT)

Answer 24

The donor immune response is removed, and the stem cell transfer provides a new immune system

HSCs can find their way to bone marrow after infusion and regenerate there
They can be cryopreserved with little damage

Question 25

Graft Versus Host Disease

Answer 25

When transplanted tissue is immune cells themselves, there is the risk of donor immune cells attacking the host – GVHD

Can be lethal – best approach is prevention

Removing T cells from transplant or suppressing their function reduces GVHD

Question 26

Graft versus Leukemia

Answer 26

But sometimes mismatch and donor leukocytes can be benificial - removing original leukemia
Graft versus leukemia response
Development of GVL may prevent disease relapse

Typically happens with radiation therapy with leukaemia, where the patients immune system is destroyed, so they need a transplant of immune cells
There could be residual tumerogenic cells – but the new immune cells can target it in a way that the host immune system cannot – why? It’s because the cancer cells are self, so the immune system struggles to recognise it, so adding in these donor immune cells and creating a whole new immune system is a benefit.

Question 27

Immunosuppression

Answer 27

Essential to maintain non-autologous transplant, even if there’s good HLA matching

Immunosuppressants for transplant can be -
General immune inhibitors (e.g. corticosteroids)
Cytotoxic – kill proliferating lymphocytes (e.g. mycophenolic acid, cyclophosphamide, methotrexate)
Inhibit T-cell activation (cyclosporin, tacrolimus, rapamycin)

Immunosuppressives may need to be maintained indefinitely

Question 28

autologous transplant

Answer 28

uses a person’s own stem cells

Question 29

Cyclosporin

Answer 29

Breakthrough drug for transplant

Blocks T cell proliferation and differentiation, Inhibit production of IL-2

Next generation therapies less toxic and effective at lower doses

Question 30

Combination immunosuppressive regimes

Answer 30

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

Combination is typically more effective than single drugs, and it may also allow you to lower the dose of them and reduce side effects

Question 31

Immunosuppressive therapy monitoring

Answer 31

There is currently no immunosuppressive that will prevent transplant rejection whilst maintaining other immune responses

Transplant patients more susceptible to infection and malignancy
Immediate risk e.g. CMV

Immunosuppressive drug toxicity can lead to organ failure eg cyclosporin nephrotoxicity in kidney transplant.

Needs to be monitored as an immune response against the transplant may happen quietly in the background

Question 32

Intestinal microbiome

Answer 32

The microbiome, particularly of the intestine, is involved in regulating adaptive immune responses
-releases metabolites that reduce or increase the functioning of the immune system in a general way

Immunosuppressed/cancer patients who don’t respond to immunotherapy can take FMT (faecal material transplant) and it allows them to start responding – positive treatment effect.

May be implicated in transplantation outcomes