Case Report: IgG4-rich tubulointerstitial inflammation in MPO-ANCA-associated glomerulonephritis: a case-based review.

Immunoglobulin G4-related disease (IgG4-RD) is a systemic, immune-mediated fibro-inflammatory disorder with the potential to affect multiple organs, with the kidney being the most commonly involved organ, typically presenting as IgG4-related tubulointerstitial nephritis (IgG4-TIN). Anti-neutrophil cytoplasmic antibody (ANCA)-associated glomerulonephritis (ANCA-GN), in contrast, is a pauci-immune necrotizing crescentic glomerulonephritis characterized by acute renal dysfunction and systemic inflammation. While early reports have interpreted IgG4-positive plasma cell infiltration in ANCA-associated vasculitis as evidence of overlap with IgG4-TIN, histologic resemblance in varying disease contexts can make differentiation challenging. In this context, we present the case of an 81-year-old woman diagnosed with both ANCA-GN and IgG4-TIN, informed by a comprehensive review of all previously reported cases. Renal biopsy confirmed this rare overlap, revealing pauci-immune crescentic glomerulonephritis alongside dense IgG4-positive plasma cell infiltration and storiform fibrosis. This case illustrates the diagnostic complexity posed by IgG4-rich inflammatory infiltrates in ANCA-associated disease and underscores the importance of careful clinicopathologic correlation to guide accurate diagnosis, appropriate immunosuppressive therapy, and improved understanding of potentially overlapping autoimmune mechanisms.