MPO-ANCA-associated glomerulonephritis with immune complex deposition and membranous features in a patient with Sjögren's syndrome: a case report.

Antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) is classically characterized by pauci-immune necrotizing crescentic glomerulonephritis; however, immune complex deposition is increasingly recognized in a subset of patients, and its clinical significance remains incompletely defined. Coexistence with membranous nephropathy (MN)-like features is uncommon and may complicate diagnosis and treatment. We report a 37-year-old woman with persistent urinary abnormalities detected during routine health screening. Laboratory evaluation showed proteinuria (1.83 g/gCr), microscopic hematuria (> 50 red blood cells/high-power field), mildly impaired kidney function [estimated glomerular filtration rate (eGFR) 51.4 mL/min/1.73 m2], elevated myeloperoxidase (MPO)-ANCA levels, and no hypocomplementemia. Kidney biopsy revealed crescentic glomerulonephritis with granular IgG, C3, and C1q deposition in mesangial and capillary wall regions, together with subepithelial electron-dense deposits suggestive of MN-like features. IgG subclass staining was positive for IgG1-3 but negative for IgG4. Glomerular staining for PLA2R, THSD7A, and NELL1 was negative, whereas glomerular EXT1 staining was positive and EXT2 staining was negative. Overall, the clinicopathological profile was not typical of primary MN. Further evaluation demonstrated anti-SSA antibody positivity and objective ocular findings consistent with Sjögren's syndrome (SjS). Following treatment with high-dose corticosteroids, rituximab, and avacopan, kidney function and urinary abnormalities gradually improved, although proteinuria declined relatively slowly. This case was characterized by MPO-ANCA-associated crescentic glomerulonephritis with immune complex deposition and membranous features in the setting of SjS. The findings highlight the diagnostic difficulty of attributing the membranous component to a single disease process and suggest the involvement of complex and overlapping autoimmune mechanisms.