22-OR : FALSE-NEGATIVE VIRTUAL CROSSMATCH DUE TO UNOS’ UNACCEPTABLE ANTIGEN DATABASE LIMITATIONS: THE SAGA CONTINUES

Aim Strong allele-specific antibodies are frequently detected by Luminex single antigen bead (SAB) assays. However, these cannot be reported as unacceptable antigens in the UNOS database. The problem compounds when a patient expresses the same antigen and the non-self-allele specific antibody cannot be documented in the system. We report herein a case of a 60 year-old Hispanic female patient listed in UNOS for a deceased kidney donor who expressed the B62 antigen and whose sera were positive for strong anti-B*15:01(B62) antibody detected by positive B*15:01 beads in both Luminex PRA and SAB assays. Methods HLA typing was performed by SSP (Olerup). HLA antibodies were detected by Labscreen PRA and SAB (One Lambda). Flow cytometry and standard CDC crossmatches were performed. Results Strong anti-B62 antibody against her “self” B62 antigen was detected by both PRA and SAB panels in the patient’s pre-transplant sera. High resolution typing of the patient revealed B*15:15(B62), while B62 antigens beads on both PRA and SAB panels are B*15:01. B*15:15, a common allele in the US, is not represented in the PRA and SAB panels. Thus, non-self allele-specific B*15:01(B62) antibody was present. A deceased donor shared A, B, DR and DQ antigens with the patient was offered. However, strong positive T- and B-cell flow cytometry crossmatches and positive B-cell CDC crossmatches against this donor were observed. Auto-crossmatches were negative. The positive allo-crossmatch resulted from the strong allele donor-specific antibody against B*15:01which differs from the recipient’s B*15:15 at one amino acid at position 63 (B*15:01 = E63 vs. B*15:15 = N63). Conclusions High resolution typing is necessary for some HLA antigens in solid organ transplantation. The ability to report allele-specific unacceptable antibodies in the UNOS will further assist organ allocation and prevent unnecessary delays. In addition, lack of beads representing common alleles in the SAB assays can result in false-negative virtual crossmatch results.