Immunology of membranous nephropathy: from animal models to humans

Abstract

Membranous nephropathy (MN), the leading cause of nephrotic syndrome in adults, is characterized by the deposition of subepithelial immune deposits that consist mainly of immunoglobulin (Ig)G and complement. Most of the cases are primary or idiopathic (iMN), while only approximately 25% of the cases are secondary to some known disease such as systemic lupus erythematosus, hepatitis B, drugs and malignancies. Most of our knowledge on the pathogenesis of iMN has relied upon old experimental models (i.e. Heymann nephritis) that have shown that immune deposits are formed in situ by the reaction of autoantibodies against the respective podocyte antigen. Recent findings indicate that podocyte proteins also act as an autoantigen in human iMN. The M-type phospholipase A2 receptor (PLA2R) has been identified as the main target antigen, as it can be found in approximately 70% of iMN patients but only rarely in other glomerulonephritides. Podocytes damage in the experimental model of Heymann nephritis is complement-mediated. In humans, the presence of complement within the subepithelial deposits is well established, but IgG4, which does not activate complement by classical or alternative pathways, represents the predominant subclass of IgG anti-PLA2R. Some evidence suggests that IgG4 anti-PLA2R autoantibodies can bind mannan-binding lectin (MBL) and activate the lectin complement pathway. A genetic background for iMN has been demonstrated by genome-wide association studies that have shown highly significant associations of the PLA2R1 and the human leucocyte antigen (HLA)-DQA1 loci with iMN. In addition to their diagnostic value, anti-PLA2R antibodies may be useful to monitor disease activity and predict response to treatment.

Keywords: anti-PLA2R antibody; membranous nephropathy; podocyte; subepithelial deposits IgG4.

Figure 1

Granular subepithelial deposits of immunoglobulin (Ig)G in a case of idiopathic membranous nephropathy (original magnification ×400).

Figure 2

Mechanism of in‐situ subepithelial immune complex formation in early childhood idiopathic membranous nephropathy. Because of its size and charge, modified, the cationic form of bovine serum albumin (BSA) reaches anionic glomerular subepithelial structures and serves as a planted antigen with subsequent formation of immune complexes in situ. The functional impairment represented by proteinuria is the result of formation of the membrane attack complex (C5b‐C9, MAC), which leads to sublethal podocyte injury resulting in the activation of transcription factors encoding for mediators of fibrosis and cytoskeletal podocyte rearrangement. It also increases production of potentially nephritogenic molecules such as reactive oxygen species (ROS), proinflammatory cytokines, proteases and vasoactive molecules.

Figure 3

Mechanism of circulating immune complex deposition in membranous nephropathy. Preformed small‐sized circulating immune complexes may traverse the glomerular basement membrane (GBM) and deposit beneath the podocyte. The functional impairment represented by proteinuria is the result of formation of the membrane attack complex (C5b‐C9, MAC), which leads to sublethal podocyte injury resulting in the activation of transcription factors encoding for mediators of fibrosis and cytoskeletal podocyte rearrangement. It also increases production of potentially nephritogenic molecules such as reactive oxygen species (ROS), proinflammatory cytokines, proteases and vasoactive molecules.

Figure 4

Mechanism of anti‐podocyte autoantibody‐mediated disease in membranous nephropathy. Circulating autoantibodies can target surface‐exposed intrinsic podocyte proteins to form in‐situ immune deposits. The functional impairment represented by proteinuria is the result of formation of the membrane attack complex (C5b‐C9, MAC), which leads to sublethal podocyte injury resulting in the activation of transcription factors encoding for mediators of fibrosis and cytoskeletal podocyte rearrangement. It also increases production of potentially nephritogenic molecules such as reactive oxygen species (ROS), proinflammatory cytokines, proteases and vasoactive molecules.