Verbatim): IgA nephropathy (IgAN), the most common glomerulonephritis in the world is characterized by elevated levels of IgA1 in its polymeric form in circulation, presence of circulating immune complexes (CIC) and deposition of IgA1, frequently with C3 component of complement and IgG and/or IgM in glomerular mesangium. Earlier studies have shown that IgA1 in IgAN patients displays aberrant structural features in the heavy chain hinge region glycans. Carbohydrate analysis and lectin-binding studies have shown that the hinge region glycans bound by O-glycosidic bonds to Ser or Thr residues are deficient in galactose (Gal), resulting in an increased exposure of N-acetylgalactosamine (GalNAc). Furthermore, our earlier studies have shown that Gal deficient IgA1 molecules are present mainly in high molecular mass CIC that also contain IgG molecules. Our preliminary studies indicate that the interaction of Gal-deficient IgA1 with IgG, and probably other major isotypes, is based on antigen (IgA1) and antibody (IgG) recognition. Experimental approaches proposed in this application are designed to test the following hypothesis: An altered glycosylation of IgA1 hinge region results, due to the Gal deficiency, in exposure of GalNAc-Ser/Thr associated antigenic determinant(s) which are recognized by ubiquitous, naturally occurring antibodies, predominantly of the IgG isotype, which are involved in the formation of CIC and mesangial depositions. The following specific aims are proposed to test this hypothesis: 1) Determine the localization of antigenic determinants in IgA1 with aberrantly glycosylated hinge region glycans, involved in the formation of CIC; 2) Examine whether the Gal-deficient molecules are of systemic or mucosal origin; 3) Characterize the specificities and molecular properties of IgG, IgA, and IgM antibodies that bind to epitopes in the aberrantly glycosylated IgA1 hinge region; 4) Investigate the biological consequences of the formation of CIC containing Gal-deficient IgA1 and IgG. With GalNAc-Ser/Thr specificity; 5) Determine whether immune complexes composed of Gal-deficient IgA1-IgG with GalNAc-Ser/Thr binding activity are present in mesangial deposits of patients with IgAN; and 6) Correlate levels of Gal-deficient serum or secretory IgA1 and levels of IgG, IgA1, or IgM antibodies with GalNAc-Ser/Thr specificity with disease activity. Results of these studies may lead to the elucidation of molecular defects of IgA1 in IgAN and provide experimental basis for rational approach to the treatment.
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