Membranous Nephropathy Associated with Syphilis: Correlative Light and Electron Microscopy Revealing the Localization of Neuron-derived Neurotrophic Factor within Subepithelial Electron Dense Deposits

A 50-year-old man presented to our hospital with a fever, edema, and a rash. The clinical diagnosis was renal dysfunction, nephrotic syndrome, and syphilis. The patient was treated with benzylpenicillin, and his symptoms improved. A renal biopsy revealed membranous nephropathy (MN). Recently, neuron-derived neurotrophic factor (NDNF) was reported to be an antigen corresponding to syphilis-associated MN. In the present patient, immunofluorescence staining and immunoelectron microscopy revealed granular NDNF-positive findings within subepithelial deposits, suggesting the presence of NDNF-IgG immune complexes. Although the mechanism by which NDNF serves as a target antigen remains unclear, NDNF was found to colocalize within subepithelial immune complexes in syphilis-associated MN.