The virtual crossmatch – A screening tool for sensitized pediatric heart transplant recipients

Abstract

Heart transplantation in the setting of human leukocyte antigen (HLA) sensitization is challenging, as a time-consuming prospective crossmatch (XM) may be required, severely limiting the number of potential donors. We evaluated a 'virtual XM', defining a positive virtual XM as the presence of recipient pre-formed anti-HLA antibodies to the prospective donor HLA type, and compared the virtual XM to a standard direct XM. Bead-based flow cytometric analysis was used to identify anti-HLA antibody (Ab) present in a child listed for heart transplantation. Using recipient serum, direct-flow cytometric T- and B-cell XM were run for potential donors against whose HLA type the recipient had specific antibodies (group 1, n = 7) and for potential donors with predicted compatible HLA types by virtual XM (group 2, n = 7). Results were expressed as median channel difference (MCD) between the control and recipient serum. A positive T-cell XM was defined as MCD > 50, whereas MCD > 100 constituted a positive B-cell result. The rate of T-cell reactivity was significantly less in group 2 than in group 1 (29% vs. 100%, p = 0.02); similarly, B-cell reactivity was also less for group 2 (14% vs. 100%, p = 0.005). The virtual XM was 100% sensitive in detecting positive flow cytometric XM results for T and B cells. Although only 72% specific in predicting a negative T-cell XM, and 86% specific for negative B-cell XM, the false negatives were weakly positive and would probably have been clinically acceptable. Currently, potentially suitable donor organs are often declined for lack of a prospective XM; these organs may ultimately be allocated to more distant recipients or perhaps not used at all. While further studies are needed, virtual XM has the potential to improve availability of organs for sensitized patients and improve the overall allocation process.

Pediatric Heart Transplantation in Human Leukocyte Antigen–Sensitized Patients

“…15 Patients with a reaction to Ͼ10% of antigens (to either Class I or II) are generally considered to be allosensitized. 13,14,16 However, this is allosensitivity to a 'random' donor; having a positive crossmatch with the actual donor (eg, HLA antibody toward donor alloantigens) at the time of transplant has been clearly demonstrated to increase the risk for poor outcome after transplant. 14,17 Donor-directed HLA antibodies increase the risk for post-transplant complications in the cardiac allograft.…”

“…7 Determinations of Panel Reactive Antibody (PRA) are done to delineate a patient's potential for sensitization to donor HLA antigens. 13 Antibodies to HLA are assessed using lymphocytotoxic assays of preformed reactive antibodies 14 and more recently using solid-phase assays with microbeads coated with purified HLA antigens. 15 Patients with a reaction to Ͼ10% of antigens (to either Class I or II) are generally considered to be allosensitized.…”

Pretransplant Risk Assessment in Renal Allograft Recipients Using Virtual Crossmatching

“…Furthermore, in patients without DSA defined by virtualXM a pretransplant CDCXM or FCXM could be omitted, which will reduce cold ischemic time and the incidence of delayed graft function (22). These potential benefits will make the virtualXM approach of particular interest in lung and heart transplantation (6,23). However, responsible use of virtualXM implies thorough assessment of presensitizing events and careful determination of HLA-antibody specificities in current and remote sera.…”

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“…The specificity of virtual crossmatch ranged from 93% to 100% of flow T-cell crossmatch and 91% to 100% for flow B-cell crossmatch. [6][7][8] The other important advantage of the Luminex platform is introducing a single-antigen identification.…”