Antigen quantity is responsible for the discrepancy between phenotype and single antigen beads for the detection of human leukocyte antigen DQ antibody: Potential clinical implications

De novo donor-specific human leukocyte antigen DQ antibodies detected by single antigen beads (SAB) are significantly associated with chronic antibody-mediated rejection and lower overall graft survival. However, some DQ antibodies identified by SAB cannot be confirmed by phenotype antigen-bearing class II bead assays, raising concerns about the validity of SAB data. The inability to detect these antibodies on phenotype antigen beads could be due to a lower quantity of DQ antigens present on the surface of the beads compared to DR antigens. In this study, we demonstrate that DQ-enriched phenotype antigens exhibit the same reactivity with DQ antibodies detected by SAB, confirming the hypothesis that it is antigen quantity and not structural differences that account for discrepancies in human leukocyte antigen DQ antibody detection between phenotype antigen beads and SAB. In addition, we show that the expression of individual DQ antigens in heterozygous cells can vary significantly, further confounding correlation studies. Therefore, the common clinical practice of using the phenotype antigen beads as a screening assay, reflexing to SAB testing only when positive, may inadvertently fail to detect DQ-specific antibodies. Such errors could impact organ acceptance practices, immunosuppression treatment decisions, and/or the need for additional diagnostic testing to rule out antibody-mediated rejection.