Positive flow cytometry crossmatch with discrepant antibody testing results following COVID-19 vaccination

The impact of COVID‐19 vaccination on the alloimmunity of transplant candidates is unknown. We report a case of positive B cell flow cytometry crossmatch in a patient waiting for second kidney transplantation, 37 days after receiving the COVID‐19 vaccine. The preliminary crossmatch, using sample collected before COVID‐19 vaccination, was negative. The antibodies to mismatched donor HLA‐DR7 were detected only with multi‐antigen beads but not with single‐antigen beads, excluding possible prozone effects in solid‐phase antibody assays. The crossmatches were positive with HLA‐DR7–positive surrogates (n = 2) while negative with HLA‐DR7–negative surrogates (n = 3), which confirms the HLA‐DR7 alloreactivity. The antigen configurations on B lymphocytes are similar to that on the multi‐antigen beads while distinct from the single‐antigen beads. HLA‐DR7 was the repeating mismatched antigen with the failing first kidney allograft. The newly emerged antibody to HLA‐DR7 probably is the consequence of bystander activation of memory response by the COVID‐19 vaccination. This case highlights the importance of verifying allo‐sensitization history and utilizing multiple assays, including cell‐based crossmatch and solid‐phase assays with multi‐antigens. COVID‐19 immunization may deserve special attention when assessing the immunological risk before and after organ transplantation.