Distinct Glycophenotypes with Abnormal Signaling Define a Subpopulation of B cells Responsible for Production of Galactose-Deficient IgA1, the Main Autoantigen in IgA Nephropathy

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Project Summary IgA nephropathy (IgAN) is the most common primary glomerulonephritis in the world, with 30-50% of patients progressing to end-stage kidney disease. Diagnosis is biopsy-based, with routine immunofluorescence showing IgA (co)dominant immunodeposits usually with C3 and often with IgG co-deposits. IgA in the immunodeposits is of IgA1 subclass and enriched for galactose-deficient IgA1 glycoforms (Gd-IgA1). There is currently no disease specific therapy for IgAN due to our limited understanding of the underlying mechanisms of disease progression. Recent data indicate that IgG is present in immunodeposits of virtually all IgAN patients and that it is enriched for Gd-IgA1-specific autoantibodies. Furthermore, Gd-IgA1 and anti-Gd-IgA1 IgG autoantibodies are elevated in the circulation of IgAN patients and their levels predict disease progression. Based on these, and other data, we have proposed a multi-hit hypothesis explaining the autoimmune nature of IgAN: Gd-IgA1 is elevated in circulation of IgAN patients and is recognized by anti-Gd-IgA1 IgG autoantibodies, resulting in the formation of circulating immune complexes, some of which deposit in the glomeruli, inciting renal injury. The origin of the autoantigen and the characteristics of specific cell population(s) producing Gd-IgA1 remain unknown. Recently, we showed that immortalized IgA1-secreting cells from IgAN patients produce more Gd-IgA1 autoantigen compared to the cells from healthy controls. Furthermore, only these IgAN-derived cells increase Gd-IgA1 production in response to cytokine stimulation (e.g., IL-6). Follow up analysis found enhanced STAT3 and STAT1 activation only in IgAN B cells, and that it was necessary for Gd-IgA1 production following cytokine stimulation. Single-cell transcriptome analysis of cytokine stimulated B cells from IgAN patients found IgHA1-subpopulations with abnormal expression of genes responsible for regulation of multiple cytokine signaling pathways. These IgHA1-subpopulations also exhibited abnormal alterations in the expression level of glycosyltransferase enzymes relevant to IgA1 glycosylation. Transcriptional data alone does not identify Gd-IgA1 producers. To address this, we developed a novel glycophenotyping method using lectins and recombinant anti-Gd-IgA1 IgG that can target cell surface presentation of Gd-IgA1. This process enriched for low and high Gd-IgA1-producing subpopulations, which had differential activation of transcriptional factors before and after cytokine stimulation. Using this glycophenotyping method with nucleotide conjugation, we can target Gd-IgA1-producing cells for transcriptome analysis, intracellular signaling staining, chromatin remodeling, and specific autoantigen production rates. Together, these studies will elucidate the mechanisms of autoantigen production within specific subpopulations of IgA1-producing cells and potentially lead to the development of new disease therapies for IgAN.