Tissue (or HLA) typing
Tissue typing, or HLA typing, is the process which identifies the HLA molecules expressed on the surface of cells in the human body. HLA genes are extremely variable and this results in many different HLA antigens or molecules.
The HLA antigens are divided up into groups denoted by letters. For example, HLA-A, -B and –C are HLA class I molecules and are found on all nucleated cells including T lymphocytes.
HLA-DR, and –DQ are class II molecules which are distributed on fewer cell types, for example, B lymphocytes, macrophages and endothelial cells. The subdivisions of each group (the individual gene specificity) are denoted by numbers e.g. HLA-A 1, or HLA-DR 4. It is the combination of molecules on the surface of the cells, determined genetically, which gives an HLA type.
We inherit one HLA antigen from each group, from each of our parents, so an individual HLA type might be, for example, HLA-A 1, A25; B 8, B44; Cw5, Cw7; DR 4, DR 17; DQ 2, DQ 7.
These HLA molecules enable the cells of the immune system to differentiate between ‘self’ and ‘non-self’ so that an immune response can be specifically directed against ‘non-self’ (foreign) organisms, in the case of infection. In the transplant situation, the graft will display donor HLA molecules which are recognised as being foreign ('non-self’). Cells of the immune system will react against them as if they were an infecting agent, which could lead to rejection of the graft.
We carry out HLA typing of recipient and donor cells to identify how well the HLA molecules are matched, in theory giving an indication of the severity of possible rejection processes. In thoracic organ transplantation, HLA matching has been shown to improve graft survival, with most importance being placed on matching for HLA-DR antigens (ref: Smith et al. Lancet 1995; 346: 1318).
HLA antibody screening
Any exposure to non-self HLA antigens, such as through transplantation, transfusion or pregnancy, can stimulate the production of HLA-specific antibodies. These can vary in their potency and persistence depending on the nature and number of stimulating events but represent a significant risk of graft failure. All patients on a transplant waiting list should be monitored regularly for the presence of HLA-specific antibodies. HLA-specific sensitisation is best investigated by serological analysis for antibodies.
Much of our work involves testing patient serum for the presence of any HLA specific antibodies. It is our policy always to determine if a patient has preformed HLA antibodies prior to transplantation which could lead to early rejection of a graft (Smith et al Transplant Immunology 1993; 1: 60-65, Smith et al American Journal of Transplantation 2007 7:2809-2815). For prospective cardiothoracic transplant patients, the recommendation is that each patient should be tested at least three-monthly and after each potential sensitising event (BSHI/BTS Guidelines for the detection and characterisation of clinically relevant antibodies in allotransplantation, 2014). All antibody positive sera will be characterised for specificity for known HLA A, B, C, DR, DQ and DP antigens. For some sera (i.e. those from highly sensitised patients, reacting with over 80% of the donor population) this may require successive testing by increasingly sensitive and specific techniques. In such cases, the completion of testing may take significantly longer than for less complex cases.
Patient serum is also monitored for HLA specific antibodies after transplant. For any transplant, if rejection is suspected, a test for donor-specific antibodies can aid a diagnosis of rejection and indicate a course of management. We can perform this testing on demand during normal working hours or by prior arrangement with the laboratory.
If present at a high concentration, patient antibodies corresponding to donor mismatched HLA will cause immediate and irreversible rejection of all forms of organ transplants, with the exception of the liver. Performing a prospective serological crossmatch between donor and recipient obviates the risk of such hyperacute rejection.
For cardiothoracic transplants, where extended cold ischaemia time has an unacceptable influence on transplant outcome, we perform the crossmatch retrospectively.
For sensitised patients, it is essential that we perform either a prospective crossmatch or a ‘virtual crossmatch’ before transplantation. For a virtual crossmatch, we must obtain the donor HLA type from the Organ Donation and Transplantation Directorate of NHSBT. We can then determine if the recipient has previously produced HLA antibodies to those donor HLA antigens which could adversely affect the transplant. In some cases, the results of crossmatch tests are complex, particularly in patients with historically high levels of antibodies which have since decreased. Specialised interpretation of these results is necessary to determine their clinical significance. Our senior scientists can provide advice on specific cases as required.
Post-transplant antibody removal is known as desensitisation. Desensitisation is achieved by extracorporeal antibody removal using various techniques, such as plasma exchange or immunoadsorption. During the desensitisation process, antibody removal should be monitored so that the effectiveness of the process can be assessed.
View the Tissue Typing user guide (PDF, 454KB).