Autoimmunity in Membranous Nephropathy Targets Aldose Reductase and SOD2

Glomerular targets of autoimmunity in human membranous nephropathy are poorly understood. Here, we used a combined proteomic approach to identify specific antibodies against podocyte proteins in both serum and glomeruli of patients with membranous nephropathy (MN). We detected specific anti–aldose reductase (AR) and anti–manganese superoxide dismutase (SOD2) IgG₄ in sera of patients with MN. We also eluted high titers of anti-AR and anti-SOD2 IgG₄ from microdissected glomeruli of three biopsies of MN kidneys but not from biopsies of other glomerulonephritides characterized by IgG deposition (five lupus nephritis and two membranoproliferative glomerulonephritis). We identified both antigens in MN biopsies but not in other renal pathologies or normal kidney. Confocal and immunoelectron microscopy (IEM) showed co-localization of anti-AR and anti-SOD2 with IgG₄ and C5b-9 in electron-dense podocyte immune deposits. Preliminary in vitro experiments showed an increase of SOD2 expression on podocyte plasma membrane after treatment with hydrogen peroxide. In conclusion, our data support AR and SOD2 as renal antigens of human MN and suggest that oxidative stress may drive glomerular SOD2 expression.Primary membranous nephropathy (MN) is a common glomerular disease in humans with no universally effective clinical therapy. Treatments are entirely empirical, and the disease evolves toward renal failure in a significant number of patients.^1,2 The presence of glomerular subepithelial immune deposits is the distinctive pathologic feature of MN, thus supporting the concept of an immunologic origin. It is also known that inflammatory compounds such as complement, oxygen radicals,^3,4 or intracellular protein kinase Cβ⁵ may participate, having a key role in disease progression.In the past few decades, studies of experimental models, with a particular emphasis on the Heymann nephritis (HN) model,^6–8 have led to the identification of antigens of the autoantibody response in rats (megalin),⁷ mice (aminopeptidase A),⁹ and rabbits (neutral endopeptidase [NEP]),^10–12 but limited data are available for humans. Moreover, megalin, which is the target antigen in HN, is absent in human glomeruli, and the LDL receptor, its human homolog, is only partially co-localized with MN IgG deposits.^5,13–15 Seminal studies by Debiec et al.^11,12 support the formation of immune deposits against NEP, in particular, cases with metallomembrane endopeptidase mutations that lead to NEP deficiency and alloimmunization during pregnancy. More recently, Beck et al.¹⁶ reported the presence of specific IgG₄ against the M-type phospholipase A2 receptor (PLA2R) in glomerular eluates and in plasma of a significant percentage of patients with MN, suggesting PLA2R is a major antigen in this disease.Unequivocal identification of coexisting antigens in the podocyte membrane and in subepithelial deposits is essential for any progression in the understanding of the mechanisms of MN in humans. The aim of this study was to identify podocyte proteins recognized by circulating autoantibodies in patients with MN, to define their expression in glomeruli, and to quantify the levels of specific antibodies in sera and in renal biopsies. Results provide first evidence for de novo expression of specific autoantibodies against aldose reductase (AR) and superoxide dismutase 2 (SOD2) in sera and glomeruli of patients with MN.