Humoral immunity against the proline-rich peptide epitope of the IgA1 hinge region in IgA nephropathy

Background. The human IgAl hinge region is a unique mucin-like O-linked proline-rich glycopeptide, and its core peptide was found to be exposed aberrantly by the underglycosylation in IgA nephropathy (IgAN). We describe here the presence of humoral immunity against the IgAl hinge peptide epitope in IgAN and evaluate the relationship between the underglycosylation of the IgAl hinge region and humoral immunity. Method. The serum anti-IgAl hinge peptide antibody (anti-alpha 1HP ab) titre was measured and compared between the IgAN (n = 37) and control groups (n = 34) by enzyme-linked immunosorbent assay (ELISA) using a synthetic peptide corresponding to the human IgAl hinge region, PVPSTPPTPSPSTPPTPSPS, as an antigen. Next, to evaluate the relationship between the underglycosylation of the IgAl hinge region and the humoral immunity, the reactivity of the serum IgG from the patients with IgAN against monoclonal IgAl which had been digested enzymatically to remove the carbohydrates from the IgAl hinge region was measured by ELISA. Results. The anti-alpha 1HP ab titre was significantly higher in the IgAN group than in the control group (OD value: IgG class, 0.564+/-0.344 vs 0.331+/-0.154, P=0.0014, IgM class, 0.272+/-0.148 vs 0.141+/-0.072, P < 0.0001) and it was positive in similar to 40% of the patients with IgAN. In addition, the reactivity of the serum IgG from the IgAN patients against the monoclonal IgAl was found to be increased as the carbohydrates were enzymatically removed from the IgAl hinge region (when native = 100; asialo, 122+/-9.5; agalacto, 167+/11.5, naked, 188+/-3.9). Conclusion, These results suggested that the peptide epitope of the IgAl hinge region which was aberrantly exposed by underglycosylation could induce the humoral immune response in IgAN.