Characterization of Membranous Nephropathy with Microspherular Deposits

INTRODUCTION: Membranous nephropathy (MN) is a common cause of adult nephrotic syndrome in the USA. The typical ultrastructural finding is of global uniformly dense subepithelial electron-dense immune complex deposits along glomerular basement membranes. However, early reports described deposits with a unique microspherular substructure. There was variability in what was identified as microspherular, sometimes overlapping with other entities such as podocyte infolding glomerulopathy. Currently, the nature, composition, and clinical significance of these microspherular deposits (MSDs) remain unknown. METHOD: We report the clinicopathologic features of a series of MN cases with MSD, with detailed ultrastructural characterization as well as PLA2R and THSD7A immunohistochemical and IgG subclass-staining characteristics. The proportion of MSD to overall deposits is segregated into two groups: global MSD with >50% MSD (n = 14) and segmental MSD with <50% (n = 5). RESULTS: The size and appearance of the microspherules were nearly identical in global and segmental MSD groups (mean diameter of 77.9 nm and 77.2 nm, respectively), with subepithelial (n = 19) or intramembranous (n = 12) distributions in all cases. Mesangial MSDs (n = 5) were only found in the global MSD group. The majority of biopsies (86% of global MSD and 100% of segmental MSD) were Ehrenreich-Churg stage 2 or above; early stage 1 was only observed in the global MSD group. All but 3 cases were PLA2R/THSD7A double negative; 1 THSD7A positive in global MSD and 2 PLA2R positive in segmental MSD. IgG1 was the dominant subclass in the global MSD group, and IgG4 was dominant in the segmental MSD group, including the 2 PLA2R-positive cases. CONCLUSION: The findings suggest that MSDs are more commonly associated with secondary MN. This case series is the largest to date, and the findings may yield etiologic and prognostic information on this rare but unique subset of MN and provide a well-characterized cohort of cases for future studies.