Introduction to transplantation immunology

Question 1

What is the difference between an allograft and a xenograft?

What is the different between an isograft and an autograft? When would they be used?

Answer 1

In an allograft the donor is of the same species, in a xenograft the donor is of a different species

An isograft occurs between genetically identical individuals, indicated in homozygous twins. In an autgraft, the donor and recipient are the same person, different sites, indicated after burns.

Question 2

Consider a renal transplantation

What adverse events can occur?

Answer 2

During operation: trauma, cold and warm ischaemia

Reperfusion of ischaemic organ

Inflammation/wound healing

Immune response against graft

Question 3

Outline the timescales of the three types of graft rejection

Answer 3

Hyperacute rejection occurs in minutes/hours/days

Acute rejection occurs in days-weeks

Chronic rejection occurs in (weeks-)months, years

Question 4

Describe the antigens and antibodies found in each blood group

Answer 4

GROUP A- A antigens, B antibodies

GROUP B- B antigens, A antibodies

GROUP AB- A and B antigens, no antibodies

GROUP O- A and B antibodies, no antigens

Question 5

Outline the blood groups that can donate and accept blood from A, B , AB and O

Answer 5

GROUP A

• can donate to A and AB
• can accept A, O

GROUP B

• can donate to B and AB
• can accept B, O

GROUP AB

• can donate to AB
• universal acceptor

GROUP O

• universal donor
• can accept from O

Question 6

Which kind of cells express HLA antigens?

Which kind of cells express ABO antigens?

Answer 6

Only nucleated cells (therefore not RBCs)

All stromal cells express ABO antigens. Stromal cells are connective tissue cells of any organ, primarily fibroblasts and pericytes.

Question 7

How is bedside blood typing conducted?

Answer 7

• Blood sample taken and mixed with Anti-A and Anti-B individually.
• Check for agglutination (antigen-antibody complex)
• Back typing: plasma mixed with known type a/type b blood to detect isoagglutinins
• Type O: no agglutination as no antibodies; Type A: agglutination with anti-A; Type B: agglutination with anti-B; Type AB: agglutination with Anti-A and Anti-B

Question 8

Consider a scenario where a patient with blood group O receives transplant from blood group B

Describe how a hyperacute rejection would occur and its consequences

Answer 8

• A and B antibodies in O plasma bind to inside of blood vessels in graft which would be lined with endothelial cells expressing A and B
• Complement binds to antigen-antibody complex –> attack on endothelium

–> coagulation of blood in affected areas occluding the blood vessels suppling graft with oxygen

Question 9

How do HLA antibodies cause graft injury?

Answer 9

By inducing phenotypic changes in the donor vasculature

• causes endothelial cell activation promoting leukocyte recruitment and CD4 T cell proliferation in response to alloantigen HLA class 2 on endothelial cell (EC)
• complement activation via classical pathway
• monocytes, neutrophils and NK cells also express Fc receptors which can interact with heavy chain of HLA antibodies in donor EC

Question 10

What is the function of Fc receptors?

Answer 10

Increase leukocyte recruitment and mediate phagocytosis and antibody-dependent cellular cytotoxicity

Question 11

Why is hyperacute rejection of renal grafts rare nowadays`?

Answer 11

Crossmatch techniques to assess compatibility of patient and donors

• crossmathcing detects donor specific alloantibodies in serum of recipients which may have developed in response to foreign HLA molecules encountered from pregnancy, transfusion and prior allografts
• Crossmatching uses PCR. The most important parts of the HLA molecule are those that differ from one another. This is where peptides are bound in binding groove

Question 12

State a hallmark characteristic of antibody-mediated rejection

Answer 12

Microvascular inflammation

Question 13

Briefly describe the process of cross matching

Answer 13

Incubation of washed donor cells with recipient serum, antibody binding detected by mouse-anti-human AB stain of recipient cells or cytotoxicity

Suitable detection system

• The number of positive antibody responses against a panel of HLA antigens predicts whether the cross match will be positive or negative

Question 14

What causes differences in minor H antigens between donor and recipient?

Answer 14

Polymorphic self proteins that differ in amino acid sequences

Question 15

Describe the mechanism of acute rejection

How does interference of this work?

Answer 15

If a non self APC presents antigen TCR/MHC recognition can focus on

1. presented peptide (PEPTIDE DOMINANT BINDING) OR;
2. presenting MHC (MHC DOMINANT BINDING)

Interference of this mechanism can occur at the level of 1. receptor/ligand interaction 2. signal transduction 3. gene expression and cell cycle control

Question 16

Drugs preventing acute rejection aim to….

Give two examples and their MOA

Answer 16

Limit T cell activation and immune cell proliferation

1. CYCLOSPORIN A, TACROLIMUS
   - Calcineurinine inhibitor, inhibitor of cytokine synthesis (IL2, IFN-y)
2. AZATHIOPRINE, MMF
   - Antiproliferation (inhibits clonal expansion)

Question 17

What are the factors that contribute towards the development of chronic graft rejection?

Answer 17

• depends on damage done to graft before removal from donor and being reperfused in recipient
• other contributing factors: minor histocompatibility antigens, infection, atherosclerosis
• short ischaemic time: in living donor; long ischaemic time in cadaveric donor

Question 18

Comment on why therapeutic methods to reduce/eliminate chronic graft rejection are weak

Answer 18

Multifactorial, difficult to target exact cause and ‘treat’

Question 19

Why are corticosteroids used in transplant patients?

Answer 19

• good at decreasing inflammation caused by transplantation as they block NFkB activation (2nd messenger system that helps regulate cytokine synthesis in acute inflammation
• achieve inhibition/reduction of ischaemia/reperfusion injury
• decrease activation of APC

Question 20

Which three drugs are considered clinical standard for immunosuppression?

Answer 20

1. Cyclosporin
2. Azathioprine
3. Corticosteroids

Question 21

How does the amount of therapeutic immunosuppression differ during early and late phases of transplantation?

Answer 21

• In initial phase, more immunosuppression is needed because of ‘passenger leukocytes’. Donor cells provide non-self MHC and reactivity against these is very strong
• In later phase recipient leukocytes (always present) require weaker suppression

The risk of rejection must be balanced in immunosuppression.

Question 22

What is the modern/danger theory?

Answer 22

• The immune sytem discriminates between ‘dangerous’ and ‘not dangerous’ as opposed to ‘self /non self’ (conventional view)
• Bacterial anitgens can activate APCs via TLRs but so can tissue damage. Inflammation is a necessary response to tissue damage as it begins tissue repair

Surgery is TRAUMATIC and therefore provides danger signals: trauma, inflammation, ischaemia/reperfusion

• Tissue injury due to hypoxia
• Cytokines (TNF, IL1)
• microbial products

Question 23

What is the difference between cold and warm ischaemia?

Answer 23

Cold ischaemia time: In surgery, the time between the chilling of a tissueafter its blood supply has been reduced or cut off and the time it is warmed by having its blood supply restored.

Warm ischaemia time: In surgery, the time a tissue remains at body temperature after its blood supply has been reduced or cut off but before it is cooled or reconnected to a blood supply.