Contribution of Anti-type IV Collagen Antibodies From Kidney Basement Membrane to Increased Level of Urinary Immunoglobulin G in Nephritis Patients: A Comparison of Nephritic and Healthy Urine.

BACKGROUND: Abnormality in the NC1 domain (NC1) of collagen IV is often associated with nephritis. As the α5 chain is common in heterotrimeric collagen IV within kidney basement membranes, in this study, we attempted to elucidate the nephritis-associated alteration of urinary levels of anti-α5 chain NC1 antibodies in relation to those of immunoglobulin G (IgG). METHODS: Urine samples from 19 patients (seven diabetic nephritis, six IgA nephropathy, four nephrotic syndrome, one membranous nephropathy, and one chronic glomerulonephritis) were compared with urine samples from 19 controls (healthy subjects with normal ranges for urinary protein, urinary glucose, urinary occult blood, serum creatinine, serum urea nitrogen, serum uric acid, and estimated glomerular filtration rate (eGFR)) by enzyme-linked immunosorbent assay (ELISA) (A450nm). Using ELISA, antibodies in urine samples were detected by coating the plate with urinary proteins for IgG, purified bovine NC1 for anti-NC1 antibodies (NC1Abs), and a synthesized partial peptide in the α5 chain NC1 (yp08) for anti-yp08 antibodies (yp08Abs). RESULTS: The levels of all measured antibodies were higher in urine samples from the nephritis group (A450nm (mean ± SD): IgG: 1.154 ± 0.298, NC1Abs: 0.479 ± 0.402, yp08Abs: 0.808 ± 0.569) than from the control group (IgG: 0.728 ± 0.236, NC1Abs: 0.051 ± 0.036, yp08Abs: 0.147 ± 0.107) (p < 0.01). In addition, the proportions of NC1Abs and yp08Abs in urinary IgG were higher in the nephritis group (NC1Abs/IgG: 0.370 ± 0.292, yp08Abs/IgG: 0.627 ± 0.396) than in the control group (NC1Abs/IgG: 0.071 ± 0.044, yp08Abs/IgG: 0.197 ± 0.127) (p < 0.01). In the nephritis group, the level of IgG correlated well with that of NC1Abs and yp08Abs (IgG versus NC1Abs: p < 0.001, r = 0.7754; IgG versus yp08Abs: p < 0.001, r = 0.7860), whereas this correlation was not present in the control group. CONCLUSION: Our results showed that NC1Abs and yp08Abs levels are raised in nephritis patients, accounting for the observed increase in IgG levels. Furthermore, antibodies against the α5 chain of collagen IV, which compose the basement membranes of the renal glomeruli and tubules, are responsible for the increased urine levels of IgG in nephritis patients.