C3, C4, factor B and HLA-DR alpha mRNA expression in renal biopsy specimens from patients with IgA nephropathy.

The deposition of complement in the kidney mesangium is a constant finding associated with renal injury in IgA nephropathy, even though IgA does not bind complement. We have previously reported that complement gene expression in the kidney increases concurrently with the progression of immune complex disease in murine lupus nephritis. We have now studied the expression of C3, C4, factor B and HLA-DR alpha mRNA by in situ hybridization in renal biopsy specimens of patients with IgA nephropathy and compared these findings to those in patients with other immune-mediated diseases of the kidney, hereditary nephritis and normal kidney. In IgA nephropathy, C3 and factor B mRNA were expressed in the renal tubular epithelial cells, while no expression of either C3 or factor B mRNA was apparent in the glomerulus. Specimens from patients with other immune-mediated forms of chronic glomerulonephritis also showed a similar pattern of expression of C3 and factor B mRNA only in the tubules, but not in the glomerules. However, C3 and factor B mRNA were not found in normal kidney tissue or biopsy specimens from patients with hereditary nephritis. C4 mRNA was expressed in the tubular epithelial cells in all specimens examined, indicating that C4 mRNA is constitutively expressed in the human kidney. In IgA nephropathy HLA-DR alpha mRNA was observed in the interstitium, but not the tubules or glomerular cells. In contrast, HLA-DR alpha mRNA was present in the glomerulus and scattered in the interstitium in other immune-mediated kidney diseases. There was no expression of HLA-DR alpha mRNA in hereditary nephritis or normal kidney. Our findings, which reflect the immunopathogenic events in vivo, provide new insights as to the interpretation of the molecular immunology of this immune complex disease.