A Rare Case of Monotypic IgG1-Kappa Fibrillary Glomerulonephritis

Introduction: Fibrillary glomerulonephritis (FGN) accounts for 0.5–1% of native kidney biopsies. It is characterized by glomerular immune deposits with non-amyloid fibrillary substructure, and positive glomerular immunohistochemical stain for DNAJB9. Although the etiology of FGN remains poorly understood, association with malignancies, autoimmune diseases, hepatitis C infection, and dysproteinemia is reported. Case Description: A 74-year-old female with CKD G3a presented with increasing proteinuria (UPCR 442 mg/g). She had a history of systemic lupus erythematosus, diagnosed eight years earlier, being treated with hydroxychloroquine and intravenous rituximab infusions. Kidney biopsy showed focal mild mesangial expansion and mild tubulointerstitial scarring. Congo red stain was negative. Immunofluorescence (IF) showed focal smudgy mesangial and glomerular capillary wall staining for IgG (2+) and kappa light chain (3+), but negative for lambda (confirmed on deparaffinized sections). Staining for IgG subclasses was positive only for IgG1 (2-3+) [Fig.1]. Electron microscopy showed randomly oriented, non-branching fibrils (average diameter 15 nm), within the mesangium and glomerular basement membranes. DNAJB9 immunohistochemistry was positive, confirming the diagnosis of FGN [Fig.2]. Serum and urine immunofixation, and bone marrow biopsy were performed, but no monoclonal gammopathy (MG) was identified. Discussion: Monotypic FGN represents a rare entity within the spectrum of FGN. Its diagnosis relies on IF stain of IgG subclasses and confirmation of light chain restriction by IF on deparaffinized tissue sections. Of interest, in this patient no evidence of MG or hematological abnormalities were revealed by extensive work up, similar to few cases reported in the literature. Further studies will be necessary to understand the etiology of this rare condition, its potential association with MG, risk of progression and optimal management.