Abstract

The United Network for Organ Sharing (UNOS) implemented the virtual crossmatch system in UNet as a way to improve the likelihood of a negative crossmatch when kidneys are shared with HLA-sensitized candidates across donor service area (DSA) boundaries. The role of HLA C in that process is not universally appreciated. We recently experienced an unexpected positive flow T and B cell crossmatch for an imported, HLA zero-mismatched kidney because of donor-specific HLA C antibodies and transplanted it into the backup candidate. HLA C locus antigens were not typed by the OPO's laboratory that sent the kidney so the UNet virtual crossmatch could not "strike" our candidate from the UNOS match run. HLA C locus typing data of donors for kidneys our DSA imported from other DSAs revealed that C typing was not performed in 23% (14/60) and was discrepant with our molecular type for 10% (6/60) and was concordant in 67% (40/60) of cases. The rate of positive donor-specific crossmatches was higher (83%) for HLA C discrepantly typed donors than for concordantly typed donors (44%). Sensitization for HLA C (42%) is less frequent than for A (80%) or B (83%) locus antigens but the immunogenicity of C locus antigens in patients who make C locus antibodies is equivalent in black and white patients. Finally, the transplant rate of imported kidneys into class I-sensitized candidates was 24%, and C locus-sensitized candidates comprised 55% of those transplanted.