Glomerular Immunodeposits of Patients with IgA Nephropathy Are Enriched for IgG Autoantibodies Specific for Galactose-Deficient IgA1

Significance Statement IgA nephropathy (IgAN) is the leading primary GN worldwide. The disease is thought to result from glomerular deposition of circulating immune complexes of IgG bound to galactose-deficient IgA1 (Gd-IgA1). However, routine immunofluorescence microscopy fails to detect IgG in many kidney biopsies from patients with IgAN and the specificity of IgG in immunodeposits has not been tested. The authors show that IgG specific for Gd-IgA1 was extracted from remnant IgAN kidney-biopsy specimens, even when IgG was not detected by routine immunofluorescence. Using confocal microscopy, the authors confirmed that glomerular IgA and IgG colocalize in biopsies, including those negative for IgG by routine immunofluorescence microscopy, suggesting the two form a complex. The results highlight the pivotal role of IgG autoantibodies in IgAN, and bolster the hypothesis that Gd-IgA1–specific IgG autoantibodies are involved in the pathogenesis of the disease. Background IgA nephropathy (IgAN) is the leading primary GN worldwide. The disease is thought to result from glomerular deposition of circulating immune complexes of IgG bound to galactose-deficient IgA1 (Gd-IgA1). However, routine immunofluorescence microscopy fails to detect IgG in many kidney biopsies from patients with IgAN and the specificity of IgG in immunodeposits has not been tested. Methods We used remnant frozen kidney-biopsy specimens from 34 patients with IgAN; 14 were IgG-positive and 20 were IgG-negative by routine immunofluorescence microscopy. Six patients with primary membranous nephropathy (MN) and eight with lupus nephritis (LN) served as controls. IgG in the kidney tissue was extracted and its amount determined by ELISA. IgG molecular integrity was assessed by SDS-PAGE immunoblotting. Antigenic specificity of extracted IgG was determined by ELISA using phospholipase A2 receptor (PLA2R) or Gd-IgA1 as antigen. In addition, ten other IgAN cases, six IgG-positive and four IgG-negative by routine immunofluorescence, were used for colocalization studies by confocal microscopy. Results IgG extracted from MN but not IgAN immunodeposits reacted with PLA2R. Conversely, IgG extracted from IgAN but not MN or LN immunodeposits reacted with Gd-IgA1. Even IgAN kidney-biopsy specimens without IgG by routine immunofluorescence microscopy had IgG specific for Gd-IgA1. Confocal microscopy confirmed the presence of IgG in the IgAN biopsies with colocalization of glomerular IgA and IgG. Conclusions These results reveal for the first time that IgAN kidney biopsies, with or without IgG by routine immunofluorescence, contain Gd-IgA1–specific IgG autoantibodies. These findings support the importance of these autoantibodies in the pathogenesis of IgAN.