Clinically irrelevant circulating human leukocyte antigen antibodies in the presence of ventricular assist devices

Identification of anti-human leukocyte antigen (HLA) antibodies by single-antigen beads (SAB) allows for prediction of donor-specific crossmatches (virtual crossmatches), thus facilitating the allocation of organs from deceased donors. However, the clinical relevance of HLA antibodies identified by SAB has been less than clear. This study demonstrates that sera from cardiac transplant candidates with a ventricular assist device (VAD) or infection may contain clinically irrelevant antibodies that bind to the beads but not to lymphocytes.

Investigated were 5 cardiac transplant candidates (3 with VAD, all with infections, and 1 retransplant) with positive HLA antibodies detected by SAB, but negative by cytotoxicity. To determine clinical relevance of the antibodies, flow cytometric crossmatches (FCXM) were performed. Untreated beads and elution buffer-treated beads to dissociate the β-2 microglobulin and the peptide from the heavy chain were used.

The virtual crossmatch data were compared with data from actual FCXMs. Of 40 T-cell and B-cell FCXM, SAB-identified HLA antibodies were predictive for only 1 T-cell and 9 B-cell FCXM outcomes. Patients' sera contained a mixture of antibodies directed against cryptic epitopes on the heavy chain and exposed epitopes. The mean fluorescence intensity of antibodies varied from 1,040 to 11,000.

Sera from cardiac transplant candidates with or without VAD may contain natural antibodies that do not bind to intact antigens on the cell surface. Therefore, great care must be exercised before denying a life-saving transplant to these patients simply on the basis of SAB results.