Histocompatibility testing and screening for presensitisation
HLA molecules are encoded by the major histocompatibility complex (MHC), a cluster of genes situated on the short arm of chromosome 6 . The HLA class I antigens comprise HLA-A, -B and -C, and the HLA class II antigens comprise HLA-DR, -DP, and -DQ. Expression of MHC genes is codomsnant, i.e. the genes on both the maternally derived and the paternally derived chromosomes are expressed. Consequently, an individual may express between six and 12 different HLAs, depending on the degree of homozygosity (shared genes) at individual loci. Determination of the HLA or tissue type of an individual was traditionally performed on I- and B-lymphocytes by serological methods using a panel of antisera directed against the different HLA specificities in a microcytotoxicity assay, as described by Terasaki in 1965. However, increasing reliance is now placed on DNA-typing techniques to determine tissue type. These include techniques such as polymerase chain reaction (PCR) analysis using HLA sequence-specific primers.
In renal transplantation, attempts are made to match the donor and recipient histocompatibility antigens for as many of the relevant HLAs as possible. In addition to reducing the risk of graft loss from rejection, a well-matched kidney allograft that subsequently fails is less likely to cause sensitisation to the HLAs that it expresses. It is particularly important in children and young adults to avoid, where possible, grafts that are mismatched for common HLAs because, if retransplantation is required subsequently, it may be difficult to find an organ donor that does not express the antigens to which the recipient has become sensitised. In terms of organ transplantation, HLA-A, -B and -DR are the most important antigens to take into account when attempting to match donor and recipient in an attempt to reduce the risk of graft rejection. HLA matching has a relatively small but definite beneficial effect on renal allograft survival (HLA-DR> HLA-B > HLA-A). Recipients who receive well-matched renal allografts may require less intensive immunosuppression and also have less trouble from rejection episodes. It is common practice to express the degree of HLA matching between the donor and recipient in terms of whether or not there are mismatches at each locus for HLA-A, -B and -DR. A ‘000 mismatch’ is a full-house or complete match, whereas a ‘012 mismatch’ is matched at the HLA-A locus, has one mismatched HLA-B antigen and is mismatched for both DR antigens. Cadaveric kidneys are allocated in some countries, including the UK, by a points system which optimises HLA matching but also takes into account other factors, such as time on the waiting list and the relationship between the donor and recipient age. Allocation of organs for transplantation must also take into account the relative size of donor and recipient. This is not an issue in renal transplantation as adult kidneys can be readily used for paediatric recipients (and vice versa). However, for heart, lung, liver and small bowel transplantation, it is important to consider size compatibility between the donor and recipient.
HLA matching does not appear to confer an advantage for liver transplants and, although it is beneficial in cardiac transplantation, it is not practicable because of the relatively small size of the recipient poo1 and the short permissible cold ischaemic time.
As already noted, however, it is essential for all types of organ graft to ensure blood group compatibility. Permissible transplants are:
• group 0 donor to group 0, A, B or AB recipient;
• group A donor to group A or AB recipient;
• group B donor to group B or AB recipient;
• group AB donor to group AB recipient.
There is no need to take account of rhesus (Rh) antigen compatibility in organ transplantation. Interestingly, after an ABO mismatched but permissible liver transplant (0 donor into non-O recipient and A or B liver into an AB recipient) about 50 per cent of recipients develop an early and transient episode of haemolysis due to antibodies produced by donor lymphocytes in the transplanted liver.
In contrast to HLA matching, which is very desirable but not vital to success, the detection of sensitisation to HLA antigens in prospective renal allograft recipients is absolutely essential in order to avoid hyperacute rejection. Immediately before renal transplantation, a cross-match is performed by testing recipient sera against donor I-cells (which express HLA class I but not class II). When the cross-match test is positive transplantation should not proceed as anti-HLA class I antibodies cause hyperacute rejection. An additional cross-match test using donor B cells (which express both HLA class I and class II antigens) is also performed to detect antibodies directed against HLA class II antigens. A negative I-cell but positive B-cell cross-match may indicate the presence of HLAspecific anti-class II antibodies, and these are associated with an increased likelihood of acute rejection and a poor clinical outcome. Patients on the renal transplant waiting list should be screened for the development of HLA antibodies on a regular basis and especially after potential priming to HLAs by blood transfusion. Sensitisation is particularly common after blood transfusion in women who have been previously primed to paternal HLAs during pregnancy. Recipient sera are screened against an HLA-typed panel representing a broad range of HLA types found within the general population so that the specificity of recipient HLA antibodies can be determined. Highly sensitised recipients are arbitrarily defined as those whose sera contain immunoglobin G (IgG) HLA-specific antibodies reacting with > 85 per cent of the donor cell panel. Traditionally, cross-matching is performed by complement-dependent lymphocytotoxity, but flow cytometric cross-matching is becoming more widespread. Flow cytometric cross-matching is more sensitive than cytotoxicity, and is particularly valuable for screening highly sensitised recipients and patients undergoing retransplantation.
Patients awaiting heart transplantation are also screened for the presence of panel-reactive antibodies. Relatively few patients are highly sensitised (< 10 per cent) but in those who are, a prospective cross-match using donor lymphocytes should be performed. Although heart allografts rarely undergo hyperacute rejection, cardiac transplantation in the presence of a positive cross-match is associated with a high incidence of graft loss from accelerated acute rejection. In practice, highly sensitised heart transplant recipients are difficult to transplant because prospective cross-matching is problematic owing to the time constraints imposed by the short donor cold ischaemia time.
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