|
Patients who have circulating antibodies that recognize HLA antigens on a donor organ or tissue may experience rapid and irreversible destruction of the graft upon transplantation (hyperacute rejection). Hyperacute rejections are rare today because lymphocytes from organ donors are routinely tested for reactivity with antibodies in the serum of potential transplant recipients prior to transplantation to insure that such reactions do not occur. Even low levels of antibodies may damage the grafts, so more sensitive antibody tests are used for crossmatching and for identifying the risk of immune antibody-mediated damage.
Crossmatch tests are used primarily for transplant candidates to assess the suitability of a potential donor. They are also used for platelet refractory patients and bone marrow candidates with aplastic anemia who may have developed anti-HLA antibodies.
A positive lymphocytotoxic T-cell crossmatch result is a contraindication for renal transplant with the donor, and is a relative contraindication for heart and lung transplant as well. A positive B-cell crossmatch, when it is due to anti-HLA antibodies, also carries a high risk of antibody-mediated damage and transplantation would not be recommended. Positive crossmatches detected only by flow cytometry suggest the presence of preformed anti-donor HLA antibodies and carry a risk of accelerated rejection and damage to the graft. These sensitive crossmatches should be interpreted in terms of the patient's sensitization history and the quality of the donor organ.
Lymphocytotoxicity crossmatches are reported as positive or negative depending upon the percentage of donor cells killed by the recipient serum in the presence of complement and a vital dye. Flow cytometry results are reported as positive or negative based upon the median channel shift caused by the binding of a specific antibody. Positive values have been determined empirically through retrospective analysis for the different solid organ transplants and for other applications. What is considered positive may differ according to the application of the observed sensitivity of the organ to antibody. Generally, a median channel shift above 50 (out of 1024 channels) indicates that antibody is present and above 150 indicates a very high risk and a contraindication to renal transplant except in exceptional circumstances. Channel shifts above 300 usually correlate with a positive cytotoxic crossmatch.
Crossmatch tests use the complement-dependent cytotoxicty test and flow cytometry to detect antibodies that react with donor HLA antigens prior to transplantation. Tests are performed separately using donor T- and B-cells to indicate the likely target of the reaction as HLA-A, -B, -C or HLA-DR, -DQ differences, respectively, between the donor and recipient.
Complement-dependent lymphocytotoxicity identifies the most important antibodies in the crossmatch test - those responsible for hyperacute rejection of grafts. A positive crossmatch due to IgG antibodies directed against HLA-A, -B, -C, -DR and -DQ antigens is a clear contraindication to transplantation because of the high risk of hyperacute rejection.
Patients exhibiting anti-HLA antibodies can be tested in vitro for potential response to IVIG therapy. Patient sera are incubated with IVIG and subsequently tested for residual lymphocytotoxic antibody activity. Positive in vitro inhibition indicates probable in vivo responsiveness to IVIG therapy.
Flow cytometry is the most sensitive test for the detection of antibody even when present at levels that are not detected by lymphocytotoxicity tests. The flow cytometry test detects IgG antibodies.
Crossmatch results can now be accurately predicted when the patient's antibody specificities have been identified using recombinant single HLA antigen bead technologies and the potential donor HLA type is known. Thus, offers of deceased donor kidneys and other organs can be avoided when a positive crossmatch can be predicted. This permits importation of organs over greater distances without the fear of a positive crossmatch and provides an advantage to those disadvantaged by sensitization.
Endothelial cells constitute the first contact point between the transplanted organ and the recipient's immune system. Endothelial cell crossmatching (ECXM) provides testing for antibodies to non-HLA antigens only expressed on endothelial cells prior to transplantation. Antibodies reactive with donor endothelial cell antigens have been implicated in cases of humoral rejection when no anti-HLA antibodies can be detected. The endothelial cell crossmatch can identify antibodies that may increase the risk of antibody-mediated rejection before or after transplantation.
|
