Monoclonal immunoglobulin G1 κ-type atypical antiglomerular basement membrane disease accompanied by necrotizing glomerulonephritis

Patients without detectable serum antiglomerular basement membrane (GBM) antibodies but with GBM staining for immunoglobulins (Ig), absence of a crescentic phenotype, mild renal insufficiency, and absence of pulmonary hemorrhage have atypical anti-GBM diseases. We report the case of a 64-year-old man with slowly progressive glomerulonephritis. A 64-year-old Peruvian man presented with persistent microscopic hematuria, proteinuria of 2.1 g/g creatinine (Cr), serum Cr 1.00 mg/dL, and C-reactive protein 0.80 mg/dL. Renal biopsy revealed necrotizing glomerulonephritis with 39% cellular crescent formation and diffuse segmental endocapillary proliferation. He had linear staining of monoclonal IgG1-κ in the capillary walls but no detectable serum anti-GBM antibodies. Because renal dysfunction was slowly progressing, steroid monotherapy was initiated, and serum Cr level decreased from 1.48 to 1.13 mg/dL. However, serum Cr increased again to 1.35 mg/dL owing to active glomerular damage with crescent formation and endocapillary proliferation, confirmed by the second renal biopsy at 9 months after therapy. Renal function improved after cyclophosphamide therapy. We described an atypical variant of anti-GBM disease due to monoclonal IgG1-κ. Unlike usual atypical anti-GBM disease cases, we observed crescent formation in our patient. Further investigations are needed to identify the cause of nondetectable serum anti-GBM antibodies and to describe the causal relationships between clinicopathological features and the pattern of IgG subclass and light chain in atypical anti-GBM disease.