HLA and Transplantation Flashcards
1
Q
What is transplantation?
A
Transplantation is the act of transferring cells, tissues, or organs from one site to another.
2
Q
Types of transplants
A
- Autograft
-> Tissue is derived from ‘self’, can be transplanted back to the same place or another site
-> Collection -> processing -> cryopreservation -> reinfusion - Isograft
-> Between genetically identical twins - Allograft
-> Tissue transferred from one individual to another (genetically non-identical, same species)
-> By far the most common type of transplant and includes: kidney, heart, pancreas, lung, liver, bowel, islet, bone, cornea, skin, tendon, cartilage, stem cell - Xenograft
-> Tissue transferred from one species to another (heart valves)
-> Whole organ xenografting limited by potential for hyperacute rejection
3
Q
Immunological privileged sites
A
- Sites where grafts are not rejected (cornea, brain, eyes, testis and foetus)
- Successful transplant of corneal allograft from cadaveric donor - requires no assessment of HLA type and no administration of immunosuppressive drugs
- Lack of rejection is due to naturally immune-suppressive environment in anterior chamber of eye and lack of blood vessels in the cornea
- HLA is a major barrier to overcome in transplant
4
Q
Human leukocyte antigens (HLA)
A
- 6 classical HLA loci: Class I (A, B, C) and class II (DR, DQ, DP) each encoded by separate genes
- These molecules allow tissues to be recognised as ‘self’ or ‘non-self’ by the host immune system and therefore, determines histocompatibility
5
Q
HLA role and expression
A
- The primary function of these antigens is to serve as recognition molecules in the initiation of an immune response so they are very polymorphic
- HLA antigens present peptides from foreign substances to effector cells of the immune system (mainly T-cells)
- HLA Class I are expressed on nearly all cells and recognise pathogens that reside inside the cells (viruses)
- HLA Class II are only found in immune cells and recognise pathogens that reside outside the cell (bacteria)
- MHC Class I - All nucleated cells - A, B and C
- MHC Class II - Antigen-presenting cells - DR, DQ and DP
6
Q
HLA nomenclature
A
- HLA-A defines the locus
- HLA-A24 shows the serologically defined antigen
- HLA-A*24 asterik denotes that the allele has been defined by molecular methods (low resolution)
- HLA-A*24:01 shows higher resolution, specific allele required for HSCT
7
Q
HLA typing: serology
A
- All partial organ recipients and donors must have their major HLA loci determined to minimise the chance of rejection
- Also applies to bone marrow transplantation
- Terasaki trays are used which are plates with serum containing anti-HLA antibodies, patient cells and complement added, death occurs in wells where antibody reacts with patient sample
- Dyes show live cells (green) and dead cells (red)
8
Q
HLA typing: molecular methods
A
- All molecular methods require the extraction of high quality genomic DNA
- In NHS laboratories this is achieved using a semi-automated system that extracts genomic DNA from whole blood
- Also possible to isolate DNA from buccal swabs, saliva samples and fingernails
- Sequence specific primer (SSP) PCR is often the first step in determining the HLA type
- SSP tests consist of multiple different PCR primers specific for known HLA polymorphisms
- Specific amplicons are produced if the primers are complementary to the DNA sample
9
Q
HLA tissue typing: B27 and B57
A
- Individuals who are HLA-B27 have an increased risk for ankylosing spondylitis and other inflammatory disorders
- Suspected AS cases will be screened for HLA-B27 as 95% of AS sufferers are B27 positive
- HLA-B57 is associated with drug-induced inflammatory disorder
- All HIV positive patients in the UK are screened for HLA-B57 prior to beginning Abacavir (reverse transcriptase inhibitor) treatment
10
Q
Anti-HLA antibody identification
A
- Recipients may have antibodies to antigens expressed on donor cells and this is a major risk factor for hyper-acute rejection
- They may arise from pregnancy, blood transfusion or previous transplantation
- CDC (Cytokine Dependent Cytotoxicity) cross match assays have been used successfully for several years, recipient sera is incubated with donor lymphocytes in presence of complement
11
Q
Sources of donors
A
- Living
-> Family members/friends (subject to ABO/HLA compatibility)
-> Altruistic donation
-> Paired/pooled donations - Cadaveric
-> Those who are deemed to be ‘brain stem dead’ following testing
-> Donation after circulatory death (DCD) or non-heartbeating donors, usually occurs after admittance to A&E
12
Q
Issues around consent
A
- 90% of the UK population support organ donation
- In practice, only 68% provide consent in the event of a family member being declared brain dead, possibly due to shock
- Opt-in and opt-out system
- Even with ‘deemed consent’, medics allow family input
13
Q
Transplant allocation
A
- For kidney and pancreas, allocation is based on blood group match and HLA-A, B and DR, 000 mismatch are given priority
- Paediatric patients are always prioritised, then sensitisation, waiting time, age match and location are also considered
- These factors should prevent a patient waiting for many years, and older organs being given to younger patients and also reduce ischaemic time
- One HLA-A mismatch is considered, as is one HLA-B mismatch
- For heart and lung, the main factors are ABO match and HLA-DR
- There are far fewer of these transplants and short CIT is essential
- The size of the heart in relation to the donor is also important
14
Q
Pre-transplant crossmatch
A
- For all transplants, a crossmatch is always performed immediately prior to surgery
- Recipients are screened prior to entry on the waiting list, but their antibody status may have changed (hyper-acute risk)
- For heart and lung, logistics may prevent the pre-transplant crossmatch, so a virtual crossmatch is performed instead
15
Q
Graft rejection
A
- Can be hyper-acute (should never happen)
- Acute (sometimes happens despite best medical care)
- Chronic (always happens to some degree with a solid organ)
