Histocompatibility
testing and screening for presensitisatlon
HLA molecules are encoded by the major histocompatibility complex (MHC),
a cluster of genes situated on the short arm of chromosome 6 (Fig.
11.8). 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
(Fig. 11.9). HLA matching has a relatively small but definite beneficial effect
on renal allograft survival (HLA-DR
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
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.