IgA nephropathy (IgAN), the most common glomerulonephritis worldwide, leads to end-stage renal disease in 20-40% of patients and can reduce life expectancy by up to 10 years, as there is no known cure or disease- specific treatment. Most IgAN patients, regardless of age and ethnicity, have immunologic defects resulting in generation of pathogenic IgA1-containing immune complexes, which ultimately deposit in the kidneys to induce renal injury. These renal immunodeposits likely originate from circulating immune complexes consisting of IgA1 with hinge-region galactose-deficient O-glycans (Gd-IgA1) bound by Gd-IgA1-specific IgG autoantibodies. The long-term goal of this project is to define the underlying mechanisms that lead to the formation of pathogenic immune complexes, so that IgAN-specific treatments can be developed. Our hypothesis is that a molecular- level characterization of Gd-IgA1-specific IgG autoantibodies from IgAN patients coupled with an atomic-level characterization of autoantibodies in immune complexes will significantly advance our understanding of immune-complex formation in IgAN. This information will in-turn provide a basis for development of new disease-specific treatments. Over the past three years, the laboratories of the investigators have utilized biochemical, molecular, structural, and clinical studies to begin characterization of IgAN-specific autoantibodies. We have shown that IgG autoantibodies from patients with IgAN harbor a sequence (amino acids YCSR/K) at the junction of framework 3 and the CDR3 in the variable part of the heavy chain (VH), wherein the serine residue is essential for Gd-IgA1 binding. This serine residue originates from a somatic hypermutation (Ala->Ser) and not from a genetic mutation of a VH germline gene. Our crystallographic studies with an IgAN-derived (YCSK) and a germline-reverted (YCAK) recombinant IgG autoantibody revealed that this seemingly minor difference in the amino-acid sequence had allosteric effects on elements surrounding the serine residue, generating a new surface juxtaposed to the CDR loops. This surface is a potential binding site for part of the Gd-IgA1 hinge-region glycopeptide and is a potential target for the design of IgG autoantibody inhibitors. In this proposal, we will determine the population- and individual-level variability of IgG autoantibodies in IgAN based on VH/VL sequences and Gd-IgA1 binding (Aim 1), determine the structural features of representative IgG autoantibodies and the molecular mechanism of Gd-IgA1 recognition (Aim 2), and develop approaches to block the binding of IgG autoantibodies to Gd-IgA1 (Aim 3). By leveraging our access to biobanked clinical samples, new patients, the new high-throughput approaches for cloning and expression of IgG autoantibodies specific Gd-IgA1, high-resolution methods for structural analyses, and high- throughput testing of inhibitors, our studies have progressed to a stage where molecular-level assessments of the autoantibodies will advance our understanding of the mechanisms that drive IgAN disease. The results of our studies will define disease-specific targets to prevent pathogenic immune-complex formation in IgAN.

IgA nephropathy, the most common glomerulonephritis worldwide, leads to end-stage renal disease in up to 40% of the patients and reduces life expectancy by 10 years. A key feature of the disease is the development of pathogenic immune complexes formed between IgG autoantibodies and aberrantly glycosylated IgA1. In this proposal, we investigate the diversity and structure of these pathogenic autoantibodies with a long-term goal to provide a strategy for the development of disease-specific therapies.