Detection of myeloperoxidase in membranous nephropathy-like deposits in patients with anti-neutrophil cytoplasmic antibody-associated glomerulonephritis.

Anti-neutrophil cytoplasmic antibody-associated glomerulonephritis is usually classified as a pauci-immune type. However, it sometimes shows immune complex deposition of unknown origin. We examined the glomerular localization of myeloperoxidase by double immunofluorescence and immunoelectron microscopy in cases of anti-neutrophil cytoplasmic antibody-associated glomerulonephritis with membranous nephropathy-like immunoglobulin G deposition to investigate the immune complex antigens in these cases. Six (35%) of the biopsy samples from 17 cases with anti-neutrophil cytoplasmic antibody-associated glomerulonephritis showed granular deposition of immunoglobulin G along the glomerular capillary walls. Light microscopy revealed necrotizing crescentic glomerulonephritis with segmental thickening of the glomerular basement membrane. Electron microscopy showed electron-dense deposits in intramembranous and mesangial areas. However, the size and distribution of the deposits were irregular and segmental in the examined cases, unlike typical global and subepithelial lesions of membranous nephropathy. Double immunofluorescence using Alexa Fluor 594-labeled anti-myeloperoxidase antibody and fluorescein isothiocyanate-labeled anti-immunoglobulin G antibody, as well as immunoelectron microscopy using anti-myeloperoxidase antibody labeled with 25-nm gold particles revealed partial colocalization of myeloperoxidase and immunoglobulin G within the glomerular basement membrane and mesangium. In some cases of anti-neutrophil cytoplasmic antibody-associated glomerulonephritis, myeloperoxidase may form immune complexes and develop membranous nephropathy-like lesions.