The complex of immunoglobulin A and uromodulin as a diagnostic marker for immunoglobulin A nephropathy.

BACKGROUND: The only tool to diagnose immunoglobulinn A nephropathy (IgAN) is renal biopsy which requires hospitalization; moreover, renal biopsy has a risk of critical bleeding. Therefore, a non-invasive method for accurate diagnosis of IgAN is desirable and a must-to-have tool for the clinics. For this purpose, we evaluated the diagnostic value of the IgA-uromodulin complex in the urine of patients with IgAN for its feasibility and adequacy. METHOD: We determined the IgA-uromodulin complex as a candidate for a diagnostic marker of IgAN by immunoprecipitation, liquid chromatography-mass spectrometry (LC-MS) and Western blot analysis. The enzyme-linked immunosorbent assay (ELISA) for the IgA-uromodulin complex was developed and applied to urine samples obtained from various kidney disease patients. RESULT: One hundred and three of 126 urine samples (81.7%) from IgAN patients were positive for the IgA-uromodulin complex, while only 25 out of 94 urine samples (26.6%) in other kidney disease patients were positive. Sensitivity was 81.7%, specificity was 73.4%, and diagnosis efficiency was 78.2%. The complex was negative in eight urine samples obtained from patients with Alport syndrome which is almost impossible to discriminate from IgAN by routine urinalysis. CONCLUSION: Detection of the urinary IgA-uromodulin complex by ELISA is a useful non-invasive method to diagnose IgAN.

A novel immunoglobulin-immunoglobulin interaction in autoimmunity.

Well over six decades since its first description, the Rheumatoid Factor (RF)-autoantibodies recognizing Fc (constant) portion of IgG through their own Fab (antigen binding variable segments)-is believed to have come of age. Autoimmune pancreatitis is a unique form of pancreatitis, biologically characterized by an elevated serum IgG4 concentration. Given the fact that IgG4 myeloma proteins can act as RF, we initially hypothesized that IgG4 in autoimmune pancreatitis might do likewise, hence potentially contributing to disease pathogenesis. Indeed Western blotting clearly showed that IgG4 binds to IgG1 kappa, IgG2 kappa, IgG3 kappa myeloma proteins, as well as to IgG Fc, in line with a typical RF activity. Further experiments however unraveled the unexpected fact that unlike hitherto known RF, IgG4 does not engage IgG Fc through its Fab, but its very own Fc. These data therefore collectively describe a Novel RF (NRF) in autoimmune pancreatitis. In the future, the relevance of NRF, beyond autoimmune pancreatitis, in both diagnosis/prognosis as well as pathophysiology of autoimmune and other systemic diseases where IgG4's role seems paramount, needs to be systematically assessed.