MHC and Transplantation

Question 1

What actually is the MHC complex?

Answer 1

It is a gene complex on the short arm of chromosome 6

The proteins coded for by the MHC genes are HLA proteins

Question 2

What are the different types of HLA antigens in humans?

Answer 2

Class I - A, B and C

Class II - DR, DQ and DP

Question 3

Where are class I HLA antigens found?

Answer 3

They consist of 3 alpha helices and are a heterodimer

they are found on all nucleated cells

they are not present on red blood cells

Question 4

WHat is meant by the HLA molecules being a heterodimer?

Answer 4

They are completed by something that is non-polymorphic (same in everyone)

This is B₂u in humans

Question 5

How are class II antigens different to class I?

Answer 5

They have 2 alpha helices and 2 beta pleat sheets

They are expressed more selectively on antigen presenting cells

This includes dendritic cells, langerhans cells and Kuppfer cells

Question 6

Where are the MHC genes found?

Answer 6

On the short arm of chromosome 6

A = alpha chain

B = beta chain

Question 7

What is meant by HLA molecules being highly polymorphic?

Answer 7

They vary between different people depending on their genetic backgrounds

Variation is around the peptide groove area as different peptides will fit in the grooves

Question 8

How has the number of defined HLA alleles changed over time?

Answer 8

There has been a rapid increase in defined HLA alleles

This is because humans are genetically outbred - everyone is very genetically different so everyone has different HLA antigens

Question 9

What is the chance that 2 siblings will have the same HLA antigens?

Answer 9

A sibling has a 1 in 4 chance of inheriting the same antigens from their parents

It can be hard to find a match for a bone marrow transplant if a parent or sibling is not a match, due to the extent of diversity

Question 10

What is the difference in function between class I and class II MHC molecules?

Answer 10

Class I are important in handling intracellular foreign proteins

Class II are important in handling extracellular proteins that have been endocytosed

Question 11

Complete the table for the roles of MHC I and II in different cell types

Answer 11

Question 12

What is the relationship between HLA and infectious disease?

Answer 12

There are associations of protection

e.g. If you have HLA-B53, you are protected from contracting malaria

Genetic diversity prevents the whole population from dying out as some people will be able to develop an immune response and eradicate the infection

Question 13

What is the association between autoimmune diseases and HLA?

Answer 13

Autoimmune diseases are associated with HLA polymorphisms

e.g. DR2 has an association with Goodpastures nephritis - you are 16x more likely to get the disease

Question 14

What is the difference between positive and negative selection?

Answer 14

Positive selection:

• involves selection of T cells with an affinity for self antigens

Negative selection:

• involves selecting against T cells that have too much affinity for self-antigens
• this would lead to autoimmune disease

Question 15

What is meant by allorecognition?

Answer 15

Anyone with a different genetic background to you that appears foreign to your immune system

Question 16

How is rejection mediated in an allogeneic transplantation?

Answer 16

Rejection is mediated by lymphocytes

This is against HLA molecules

Question 17

Why does rejection occur in allogeneic transplantation?

Answer 17

APC migration

Foreign cells from the donor migrate to the inguinal lymph nodes and are recognised by lymphocytes in the recipient as foreign

There is a reaction between APCs and CD4+ T cells

This excites a vigorous immune response

Question 18

What molecules are missing from the immunological synapse?

Answer 18

Costimulation is needed for full immunological activation

Question 19

What are the effector functions of the allogeneic APC?

Answer 19

• CD4 T cell gets activated and generates B cells that generate antibodies
• CD8 T cells will destroy any cells with a class I molecule on them
• CD4 produce delayed type hypersensitivity reaction

Question 20

Answer 20

Question 21

What is the purpose of tissue typing?

What is meant by haplotype?

Answer 21

Matching HLA types avoids rejection

You receive one haplotype from the mother and one from the father

a haplotype is a copy of the short arm of chromosome 6

there is a possibility of 4 different combinations

Question 22

What are the 2 different types of HLA typing?

Answer 22

Serological:

• cell based

Molecular:

• extraction of DNA
• amplification
• detection of sequence polymorphisms

Question 23

Why do people do HLA typing?

Answer 23

• Less rejection episodes
• better graft survival
• less sensitisation
• to establish relationships - e.g. paternity testing as one haplotype must come from the father

Question 24

What are the missing stages in the renal transplant pathway?

Answer 24

Every 3 months on dialysis, the patient is screened to make sure they do not have HLA antibodies

Question 25

Why may someone acquire HLA antibodies whilst waiting for a transplant?

Answer 25

1. If they have had a previous transplant 2. In pregnancy as the women is exposed to the HLA antigens of the male so may generate antibodies 3. In a blood transfusion as white cells contaminate blood and perform the sensitisation

Question 26

What will happen if the patient has HLA antibodies at the time of transfusion?

Answer 26

Hyperacute reaction the transplant turns black and is rejected immediately

Question 27

Why is antibody detection important and carried out?

Answer 27

1. Prevents hyperacute reaction **General antibody detection:** * this is against many HLA types **Specific antibody detection:** * this is against the donor * pretransplant cross match either living or cadaveric 2. Avoids aborted transplant

Question 28

What is meant by complement dependent cytotoxicity test?

Answer 28

It detects complement fixing IgG/IgM HLA and non-HLAs 1. Recipient serum is added to a Petri dish containing donor T cells (with class I antigens on the surface) 2. If the recipient has antibodies, they will bind and complement is added 3. There is cell lysis if antibodies are present

Question 29

What are the disadvantages of complement dependent cytotoxicity test?

Answer 29

1. Limited sensitivity 2. Subjective 3. Non-complement fixing antibodies 4. Non-HLA antibody interference

Question 30

What is a more modern way of performing CDC?

Answer 30

**Luminex screening** Beads have recombinant HLA molecules on their surface if there is fluorescence, this shows that the patient has antibodies to the HLA molecule

Question 31

What are the 3 types of rejection?

Answer 31

1. Acute antibody mediated rejection 2. Acute cellular rejection 3. Chronic antibody mediated rejection

Question 32

When does hyperacute rejection occur?

Answer 32

It happens if you have pre-existing antibodies at the point the transplant goes in blood flows in from the renal artery, antibodies bind and activate clotting cascade and complement

Question 33

What are the 3 important types of pre-formed antibodies?

Answer 33

**Blood group:** * if you have blood group A, you will have anti-blood group B antibodies **HLA molecules:** * high levels can lead to immediate rejection * this comes from transfusion, pregnancy or previous transplant **Animals of lower species:** * this prevents pseudotransplantation * all humans have antibodies against carbohydrates from lower mammals

Question 34

How does hyperacute rejection occur?

Answer 34

Anti-HLA antibodies bind to endothelial cells and activate them they may bind the antigen and activate the complement cascade any cell with Fc portion will bind and set off inflammation

Question 35

What are the properties of acute cellular rejection?

Answer 35

It is T cell dependent It typically occurs 7-10 days after transplant the reaction is directed against foreign HLA molecules as a result of HLA mismatch

Question 36

What is the treatment for acute cellular rejection?

Answer 36

Invasion of lymphocytes means that patients are kept on immunosuppressant drugs and steroids a large dose of Methylprednisolone is used to treat rejection

Question 37

Why are antibody tests performed after transplant?

Answer 37

Antibodies are a bio marker of poor outcome