In solid-organ transplantation, human leukocyte antigen (HLA) donor-specific antibodies (DSA) are strongly associated with graft rejection, graft loss, and patient death. The predominant tests used for detecting HLA DSA before and after solid-organ transplantation are HLA single antigen bead (SAB) assays. However, SAB assays may not detect antibodies directed against HLA epitopes that are not represented in the SAB. The prevalence and potential impact of unrepresented HLA epitopes are expected to vary by ethnicity, but have not been thoroughly investigated. To address this knowledge gap, HLA allele frequencies from seven ethnic populations were compared with HLA proteins present in SAB products from two manufacturers to determine unrepresented HLA proteins.
Allele frequencies were obtained from the Common, Intermediate, and Well Documented HLA catalog v3.0, and frequencies of unrepresented HLA types were calculated. Next-generation sequencing was used to determine HLA types of 60 deceased solid-organ donors, and results were used to determine if their HLA-A, -B, -C, and -DRB1 proteins were not present in SAB reagents from two vendors. Unrepresented HLA proteins were compared with the most similar protein in SAB assays from either vendor and then visualized using modeling software to assess potential HLA epitopes.
For the seven ethnic populations, 0.5% to 11.8% of each population had HLA proteins not included in SAB assays from one vendor. Non-European populations had greater numbers of unrepresented alleles. Among the deceased donors, 26.7% (16/60) had at least one unrepresented HLA-A, -B, -C, or -DRB1 protein. Structural modeling demonstrated that a subset of these had potential HLA epitopes that are solvent accessible amino acid mismatches and are likely to be accessible to B cell receptors.
In conclusion, SAB assays cannot completely rule out the presence of HLA DSA. HLA epitopes not represented in those assays vary by ethnicity and should not be overlooked, especially in non-European populations. Allele-level HLA typing can help determine the potential for HLA antibodies that could evade detection.