Low incidence of IgA isotype of HLA antibodies in alloantigen exposed individuals

Human leukocyte antigen (HLA) antibodies are induced by pregnancy, transfusion, or transplantation. Serum from transplant recipients is regularly screened for IgG HLA antibodies because of their clinical relevance for transplant outcome. While other isotypes of HLA antibodies, such as IgA may also contribute to the alloimmune response, validated detection assays for IgA HLA antibody detection are lacking. Therefore, we modified the commonly used luminex screening assay for IgG HLA antibody detection (IgG‐LMX) into an IgA HLA antibody screening assay (IgA‐LMX). Optimization and validation was performed with IgG, IgA1, and IgA2 isotype variants of HLA‐specific human recombinant monoclonal antibodies (mAbs). Reactivity patterns of IgA1 and IgA2 isotype HLA‐specific mAbs in IgA‐LMX were identical to those of the IgG isotype. Cross‐reactivity with IgG and IgM antibodies and nonspecific binding to the beads were excluded. Further assay validation showed the absence of IgA HLA antibodies in serum from individuals without alloantigen exposure (n = 18). When the IgA‐LMX assay was applied to sera from 289 individuals with known alloantigen exposure through pregnancy (n = 91) or kidney transplantation (n = 198), IgA HLA antibodies were detected in 3.5% of individuals; eight patients on the kidney retransplant waitlist and two women immunized through pregnancy. The majority (90%) of IgA HLA antibodies were directed against HLA class II and were always present in conjunction with IgG HLA antibodies. Results of this study show that this validated IgA‐LMX method can serve as a screening assay for IgA HLA antibodies and that the incidence of IgA HLA antibodies in alloantigen exposed individuals is low.