Case Report: Monotypic IgG1-λ fibrillary glomerulonephritis associated with rheumatoid arthritis

Background Fibrillary glomerulonephritis (FGN) is a rare glomerular disease characterized by non-amyloid fibrillary deposits and is identified in fewer than 1% of native kidney biopsies. Most cases demonstrate polytypic immunoglobulin G deposition, whereas confirmed monotypic FGN is exceptionally uncommon and may raise concern for an underlying monoclonal gammopathy. Case presentation A 72-year-old woman with rheumatoid arthritis presented with nephrotic-range proteinuria (4.6 g/24 h), microscopic hematuria, hypoalbuminemia, lower-extremity edema, and mildly impaired kidney function. Kidney biopsy demonstrated mesangial expansion and glomerular deposits staining for IgG, C3, and lambda light chain. The deposits were Congo red negative and showed strong, diffuse glomerular DNAJB9 positivity. Electron microscopy revealed randomly oriented, non-branching fibrils measuring approximately 20 nm, accompanied by podocyte foot-process effacement. Isolated IgG1 staining and lambda restriction, confirmed by immunofluorescence performed on pronase-digested paraffin-embedded tissue, supported a true monotypic IgG1-lambda phenotype. Serum immunofixation and serum free light-chain testing did not demonstrate a monoclonal protein, and no underlying hematological malignancy, other malignancy, or infection was identified. The patient received rituximab. At 14 months, proteinuria had decreased to approximately 1.0 g/24 h and serum albumin had increased from 30 to 38 g/L. Conclusion Confirmed monotypic FGN is exceptionally rare and requires integration of routine immunofluorescence, IgG-subclass assessment, paraffin-embedded immunofluorescence, DNAJB9 immunohistochemistry, and electron microscopy. Light-chain restriction alone is insufficient to establish a truly monotypic process, and monotypic deposits warrant careful evaluation for an underlying clonal disorder. This case documents DNAJB9-positive monotypic IgG1-lambda FGN occurring in the setting of rheumatoid arthritis without demonstrable monoclonal gammopathy.