Honoring 50 Years of Clinical Heart Transplantation in Circulation: In-Depth State-of-the-Art Review

Josef Stehlik, MD, MPH; Jon Kobashigawa, MD; Sharon A. Hunt, MD; Hermann Reichenspurner, MD, PhD; James K. Kirklin, MD


Circulation. 2018;137(1):71-87. 

In This Article


In 1969, a pivotal study by Patel and Terasaki[51] demonstrated poor outcomes in kidney transplant recipients who had preformed antibodies against donor HLA antigens. In the modern era, the presence of circulating antibodies against the allograft (allosensitization) remains challenging and is associated with worse outcomes for transplantation candidates and transplant recipients.[52] Risk factors for allosensitization include pregnancy, blood product transfusions, previous organ or tissue transplantation, and the use of mechanical circulatory support (MCS) devices.[37] Sensitized transplantation candidates are less likely to find an immunologically compatible donor, spend longer on the wait list, and are at increased risk of AMR after transplantation.[52] Screening for anti-HLA antibodies is routine for all heart transplantation candidates. If clinically significant antibodies are missed, the graft may be subjected to an aggressive humoral response. However, because all screened antibodies do not constitute the same allograft threat, restricting the potential donor pool on the basis of weak or likely irrelevant antibodies may reduce the organ opportunities for a given recipient.[52]

The original approach used for antibody detection was the cell-based complement-dependent cytotoxicity assay, which detects complement-fixing antibodies causing cell injury and death. Alloantibody screening entails adding the transplantation candidate serum to wells containing lymphocytes from a sample of donors. Alloantibodies in the transplantation candidate's serum will bind to corresponding HLA antigens on the donor's lymphocytes and activate the complement cascade, causing lysis of the lymphocytes. In the absence of reactive antibodies, this reaction will not take place. The results are reported as percent panel reactive antibody, a proportion of wells with cells injured by the recipient serum. This test does not differentiate between antibodies against HLA class I and II antigens. The sensitivity of the complement-dependent cytotoxicity assay was later improved with the addition of anti-human globulin to the reaction. More recently, the cell-based flow cytometry cross-match has been used, which is more sensitive than complement-dependent cytotoxicity, quantifies antibody-binding strength, and differentiates between antibodies against class I and II HLAs.

The introduction of solid-phase assays overcame some of the limitations of cell-based assays by increasing the sensitivity and specificity of the testing and providing semiquantitative information on the strength of the antibody. Rather than the use of cells, solubilized HLA antigens are fixed to color-coded microparticle beads identified by variations in fluorescence, and bound antibodies are identified with a flow cytometer or a Luminex platform (One Lambda, Thermo Fisher Scientific, Canoga Park, CA).[53] Single-antigen bead assays, which contain beads individually coated with a specific HLA molecule, have the highest sensitivity and specificity and can identify the HLA subtype against which an antibody is directed.[53] With the use of the information on antibody specificities, a calculated panel reactive antibody can be reported with a calculator containing the frequency of antigens in the donor population.[54] Similar to the original panel reactive antibody assessment, calculated panel reactive antibody provides an estimate of the proportion of the donors against which the transplantation candidate has antibodies.

In the past, sensitized patients required a prospective direct cross-match between donor cells and recipient serum before proceeding to transplantation. This required transport of donor lymph nodes or serum to the transplanting center and consequently limited the geographic area of potential donors. Now that the specificities of the prospective recipients' alloantibodies are known, a virtual cross-match (assessment of compatibility by comparing the donor HLA type with the recipient alloantibody specificities) usually obviates the need for prospective cross-match and expands the donor pool for sensitized patients.[55]

Highly sensitized patients require treatment to reduce the antibody burden and to prevent a humoral response against the allograft after transplantation. Antibodies, B cells, plasma cells, and the complement system are all targets for desensitization therapies. Desensitization strategies include combinations of intravenous immunoglobulin, plasmapheresis, rituximab, and bortezomib.[37] The generally accepted goal of desensitization is to achieve a negative cytotoxic cross-match.[56]

Patients who are not sensitized before transplantation can still produce anti-HLA antibodies against the allograft after transplantation, which are known as de novo donor-specific antibodies. Almost half of all patients will develop anti-HLA antibodies within 15 years after transplantation.[57] De novo donor-specific antibodies, especially when detected >1 year after transplantation, are a risk factor for rejection, CAV, graft dysfunction, and mortality. Most centers will treat de novo donor-specific antibodies only if there is evidence of graft dysfunction.