Fibrillary glomerulonephritis, first described in 1977, is characterized by the accumulation of non-amyloid fibrils, predominantly within the glomeruli, the microscopic filtering units of the kidney. Each glomerulus contains a sophisticated filtration barrier composed of fenestrated endothelial cells, the glomerular basement membrane, and podocytes, highly specialized epithelial cells whose interdigitating foot processes are joined by slit diaphragms built from proteins such as nephrin, podocin, and CD2-associated protein. When fibrillary deposits infiltrate the mesangium and capillary walls, this barrier is breached, and plasma proteins leak into the urine. The result is typically nephrotic-range proteinuria, microscopic hematuria, hypertension, and progressive loss of kidney function.

The patient arrived at the Department of Nephrology at Charles University and General University Hospital in Prague with bilateral leg swelling, blood in her urine, and an unintentional weight loss of 15 kilograms over two years. Her history included well-controlled hypertension, hypothyroidism, and rheumatoid arthritis diagnosed roughly five years earlier. Laboratory testing told a familiar story of glomerular injury: serum creatinine of 108 micromoles per liter, indicating mildly impaired kidney function; 24-hour urine protein excretion of 4.6 grams, well into the nephrotic range; serum albumin depressed to 30 grams per liter; and dysmorphic red blood cells in the urinary sediment, a signature of glomerular bleeding.

The kidney biopsy, performed on the fifth day of hospitalization, showed focal segmental mesangial expansion and increased mesangial cellularity, with 11 of 31 glomeruli globally sclerotic and two containing fibrocellular crescents. Interstitial fibrosis and tubular atrophy involved about a third of the renal cortex. Critically, Congo red and Saturn red stains were negative, ruling out amyloidosis, the great mimic of fibrillary disease. Immunofluorescence microscopy then revealed diffuse granular staining for IgG and complement C3 in the mesangium and along capillary walls, with moderate staining for lambda light chain and none for kappa, an apparent lambda-restricted pattern that immediately raised a diagnostic alarm.

Here the case becomes technically fascinating. Light-chain restriction on routine frozen-section immunofluorescence alone does not prove that deposits are truly monotypic. Unequal staining sensitivity or masking of immunoglobulin epitopes can produce a falsely restricted pattern, concealing a mixed population of antibodies beneath an apparently single one. To resolve the ambiguity, the team performed two additional investigations. IgG-subclass staining showed isolated IgG1 positivity, with IgG2, IgG3, and IgG4 all negative. Immunofluorescence on pronase-digested paraffin-embedded tissue, a technique that strips away masked epitopes, confirmed strong IgG and lambda staining with persistent absence of kappa. Concordant restriction to a single IgG subclass and a single light chain established a true monotypic IgG1-lambda phenotype.

Electron microscopy sealed the morphological diagnosis, revealing randomly oriented, non-branching fibrils measuring approximately 20 nanometers, accompanied by podocyte foot-process effacement, the structural correlate of heavy proteinuria. These dimensions distinguish fibrillary glomerulonephritis from immunotactoid glomerulopathy, which features larger microtubular deposits with hollow centers and more organized arrangement. Meanwhile, DNAJB9 immunohistochemistry showed strong, diffuse staining of the glomerular deposits. DNAJB9, a member of the DnaJ heat-shock protein family that functions as an endoplasmic-reticulum co-chaperone involved in protein folding, was identified through proteomic studies as an abundant component of fibrillary deposits. As a diagnostic biomarker it boasts roughly 98 percent sensitivity and over 99 percent specificity, making it indispensable for separating fibrillary glomerulonephritis from amyloidosis, immunotactoid glomerulopathy, and other organized-deposit diseases.

Monotypy carries serious clinical implications, because it raises the specter of monoclonal gammopathy of renal significance, a condition in which a hidden clone of B cells or plasma cells produces the very immunoglobulin that damages the kidney. Yet the systemic workup came back clean. Serum immunofixation detected no monoclonal protein, serum free light-chain concentrations and the kappa-to-lambda ratio were within reference ranges, hepatitis B and C were excluded serologically, and no hematological or other malignancy was identified. Bone marrow biopsy was deemed unnecessary given the absence of any circulating monoclonal protein. The authors are careful, however, to describe their findings as monotypic disease without demonstrable monoclonal gammopathy rather than claiming a clonal disorder was definitively excluded, since a small or non-secretory clone cannot be absolutely ruled out without marrow examination.

The association with rheumatoid arthritis adds another layer of rarity. In a multi-institutional cohort of 226 patients with fibrillary glomerulonephritis reported by Andeen and colleagues, only three had rheumatoid arthritis, roughly 1.3 percent, and none of 66 patients in the Mayo Clinic cohort had the disease. The mechanism linking chronic autoimmune inflammation to fibrillary glomerular injury remains uncertain. Persistent B-cell activation and dysregulated immunoglobulin production in autoimmune disorders could theoretically promote immunoglobulin-associated glomerular damage, and the presence of IgG1 and DNAJB9 within the deposits has fueled speculation that DNAJB9 may act as a putative autoantigen in an IgG-mediated autoimmune response. The authors emphasize, though, that fibrillary glomerulonephritis is not considered a conventional immune-complex complication of rheumatoid arthritis, and the coexistence should be interpreted as a rare clinicopathological association rather than proof of causation.

Treatment posed its own dilemma, since no standardized or definitively effective therapy for fibrillary glomerulonephritis exists. After discussion at a clinicopathological seminar, the team selected rituximab, a B-cell-depleting monoclonal antibody, based on limited evidence suggesting potential benefit when an immune-mediated process is suspected. The patient received one gram intravenously at baseline, a second gram two weeks later, and a third dose six months after initiation. The response was clinically meaningful: at 14 months, proteinuria had fallen from 4.6 grams to approximately 1.0 gram per 24 hours, and serum albumin had climbed from 30 to 38 grams per liter. Serum creatinine rose modestly to 133 micromoles per liter, indicating persistent kidney dysfunction without progression to kidney failure. The authors caution that this uncontrolled observation cannot establish causality or general efficacy.

The story ends somberly. The patient subsequently developed COVID-19 and died, with the infection documented as the direct cause of death, truncating follow-up at 14 months. The limitations of a single-patient report, incomplete intermediate laboratory data, absence of bone marrow examination and repeat biopsy, and restricted observation window mean that causality between rheumatoid arthritis and the kidney disease, the possible presence of a small hematological clone, and rituximab’s efficacy all remain open questions. Nevertheless, the case delivers a durable diagnostic lesson: establishing a truly monotypic fibrillary glomerulonephritis demands the integration of routine immunofluorescence, IgG-subclass assessment, pronase-digested paraffin-embedded immunofluorescence, DNAJB9 immunohistochemistry, and electron microscopy. Light-chain restriction alone is never enough, and monotypic deposits warrant careful hematological evaluation, even when, as here, the hunt for a culprit clone comes up empty.

Subject of Research: Monotypic IgG1-lambda fibrillary glomerulonephritis associated with rheumatoid arthritis without monoclonal gammopathy

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

Article References: Nedoroscik, A., Moravcova, J., Manakova, L., Zakiyanov, O., Rysava, R., Tesar, V., & Kollar, M. (2026). Case Report: Monotypic IgG1-λ fibrillary glomerulonephritis associated with rheumatoid arthritis. Frontiers in Nephrology, 6, Article 1932311. https://doi.org/10.3389/fneph.2026.1932311

Image Credits: AI Generated

DOI: 10.3389/fneph.2026.1932311

Keywords: fibrillary glomerulonephritis, rheumatoid arthritis, monotypic deposits, DNAJB9, kidney biopsy, rituximab, monoclonal gammopathy, nephrotic syndrome, light-chain restriction, immunofluorescence, electron microscopy, podocytes