IgG4-Related Kidney Disease With Hypocomplementemia and Circulating Immune Complex Containing IgG3 Presenting as Rapidly Progressive Glomerulonephritis: A Case Report.

A 76-year-old man presented with gross hematuria and rapid elevation in the serum creatinine levels over ∼6 months. Although rapidly progressive glomerulonephritis was initially suspected, serological test results for autoantibodies, including anti-neutrophil cytoplasmic antibody, were negative. In contrast, marked elevations in the serum IgG and IgG4 levels, severe hypocomplementemia, and a significant increase in the circulating immune complexes, predominantly of the immunoglobulin (Ig)G3 subclass, were observed. Kidney biopsy revealed tubulointerstitial nephritis characterized by dense lymphoplasmacytic infiltration with numerous IgG4-positive plasma cells and surrounding fibrosis. Immunofluorescence staining demonstrated tubular deposition of complement C1q, C4d, C3, and C5b-9. While most glomeruli appeared histologically unremarkable, rupture of the Bowman's capsule and infiltration of IgG4-positive plasma cells were noted in one glomerulus. Based on these findings, a diagnosis of IgG4-related kidney disease (IgG4-RKD) was established. Corticosteroid therapy was initiated, resulting in kidney function improvement and normalization of the serological and urinary abnormalities. Although tubulointerstitial nephritis is the most common histopathological manifestation of IgG4-RKD, this case highlights that patients may present with clinical features mimicking rapidly progressive glomerulonephritis when accompanied by atypical pathologic findings. Additionally, circulating immune complexes containing IgG3 may play a pathogenic role in certain patients with IgG4-RKD by activating the classical complement pathway.