Primary membranoproliferative glomerulonephritis: natural history, pathogenesis, and treatment.

Primary membranoproliferative glomerulonephritis (MPGN) is an ultrarare disease characterized by immunofluorescence microscopy as either immune-complex mediated (IC-MPGN) or C3 glomerulopathy (C3), the latter subdivided by electron microscopy to C3 glomerulonephritis (C3GN) and dense deposit disease (DDD). Both IC-MPGN and C3G typically have obvious C3 staining differentiating them from other causes of MPGN histology. Secondary causes must be excluded, including infections, autoimmune disease, and neoplasia. Clinical presentations are variable, including urinary sediment abnormalities, nephrotic syndrome, or a rapidly progressive course. The prognosis is unfavorable with about 50% reaching kidney failure by 10 years. Recurrence following transplantation is frequent, and allograft survival is shortened. The pathogenesis involves dysregulation of the alternate pathway (AP) of complement. Possibly 20% of patients harbor pathogenic mutations in AP proteins or their regulators, and up to 80% have autoantibodies impairing normal regulation. Paraproteins are found in 20 - 40% of otherwise primary MPGN, either directly detectable on biopsy (IC-MPGN) or as dysregulators of the AP. Therapy of MPGN begins with supportive care as for all glomerulopathies. Paraproteins require clone-directed therapy. When immunosuppression is considered, complement inhibition should be first line. Two agents are now FDA approved for C3G, the oral Factor B inhibitor iptacopan and the subcutaneous C3-inhibitor pegcetacoplan, the latter also approved for IC-MPGN. If complement inhibition is unavailable, MMF/steroids may be considered. Following transplantation, protocol biopsies are needed to detect early recurrence with the intent of complement inhibition.