Galactose-Deficient IgA1-Specific Antibody Recognizes GalNAc-Modified Unique Epitope on Hinge Region of IgA1

Galactose-deficient IgA1 (Gd-IgA1) that exposes GalNAc or sialylated GalNAc has been shown to be associated with disease activity of IgA nephropathy (IgAN). In a previous report, we established an enzyme-linked immunosorbent assay that measures human Gd-IgA1 using a specific monoclonal antibody KM55 (KM55 mAb), and showed that patients with IgAN contain a higher level of serum Gd-IgA1 than other types of renal diseases. Recently, we also found that the KM55 mAb specifically recognized the glomerular-deposited Gd-IgA1 in renal biopsy. In this study, we aimed to analyze the epitope of KM55 mAb using synthesized peptides corresponding to the hinge region of IgA1 with GalNAc moiety on putative glycosylated Ser/Thr residues, which are Thr225, Thr228, Ser230, Ser232, and Thr236. Binding analysis to single GalNAc-modified hinge region peptide of IgA1 showed that Thr225 with GalNAc is required for recognition of KM55. PST(GalNAC)PP motif was required for KM55 mAb to recognize hinge region peptide of IgA1 which is shown by binding assay with deletion peptide. This result was confirmed by binding of KM55 mAb against peptide with GalNAc at Thr233, which resulted in containing another PST(GalNAC)PP motif. Taken together, we concluded that the epitope of Gd-IgA1-specific KM55 mAb is PST(GalNAc)PP motif.