Pathogenesis of IgA nephropathy as a tissue-specific autoimmune disease

Glomerulonephritis (GN) is a group of heterogeneous immune-mediated kidney diseases that causes inflammation within the glomerulus. Autoantibodies (auto-Abs) are considered to be central effectors in the pathogenesis of several types of GN. Immunoglobulin A nephropathy (IgAN) is the most common GN worldwide and is characterized by the deposition of IgA in the glomerular mesangium of the kidneys, which is thought to be mediated by immune complexes containing non-specific IgA. However, we recently reported that IgA auto-Abs specific to mesangial cells (anti-mesangium IgA) were found in the sera of gddY mice, a spontaneous IgAN model, and patients with IgAN. We identified two autoantigens (β2-spectrin and CBX3) that are selectively expressed on the mesangial cell surface and targeted by anti-mesangial IgA. Our findings redefined IgAN as a tissue-specific autoimmune disease. Regarding the mechanisms of production of anti-mesangium IgA, studies using gddY mice have revealed that the production of anti-CBX3 IgA is induced by particular strains of commensal bacteria in the oral cavity, possibly through their molecular mimicry to CBX3. Here, we discuss a new concept of IgAN pathogenesis from the perspective of this disease as autoimmune GN caused by tissue-specific auto-Abs.