Monoclonal IgG1κ anti-glomerular basement membrane disease: a case report.

We report a case of anti-glomerular basement membrane (anti-GBM) nephritis with indolent course, monoclonal IgG1κ (immunoglobulin G, subclass 1, κ light chain) linear staining of the GBM, and multifocal GBM breaks but without crescents or detectable serum anti-GBM antibody in a patient followed over 9 years. Atypically, anti-GBM nephritis follows an indolent course. A very small fraction of patients with anti-GBM nephritis lack detectable circulating anti-GBM antibodies, and rare reports of monoclonal anti-GBM nephritis exist. We report what is to our knowledge the first case manifesting all 3 of these rare variations. Our patient initially presented with asymptomatic decreased kidney function following an upper respiratory tract infection. He was found to have microhematuria and subnephrotic proteinuria with mild diffuse endocapillary proliferative and exudative glomerulonephritis with linear IgG1κ staining of the GBM. He was treated with an induction regimen of intravenous cyclophosphamide and corticosteroids followed by maintenance monotherapy with mycophenolic acid. Nine years later, repeat kidney biopsy for worsening kidney function after an upper respiratory tract infection showed persistent monoclonal staining of the GBM and acute glomerulonephritis with increased chronicity, including a single fibrocellular crescent. Despite extensive clinical investigations spanning nearly a decade, no circulating anti-GBM antibody or monoclonal protein has been detected. In this case report, we explore the unique features of this monoclonal IgG1κ-associated anti-GBM nephritis.