Detection and cellular localization of human C4 gene expression in the renal tubular epithelial cells and other extrahepatic epithelial sources.

Although the liver is the major source of most complement proteins, recent reports have characterized extrahepatic expression of the genes for some of these components in a variety of tissues. In most cases, however, the specific cell type responsible for the extrahepatic complement expression has not been determined in situ. The authors studied expression of the fourth component of complement (C4) in a variety of human tissues by Northern analysis and by in situ hybridization. The C4 gene was found to be expressed at high levels in liver and both normal and diseased kidneys. In addition, evidence of C4 gene expression was found in the small intestine and brain. By in situ hybridization, the renal C4 gene expression was exclusively localized to tubular epithelial cells. C4 expression was also identified in hepatocytes, thyroid follicular epithelial cells, and ductal epithelial cells of the submandibular salivary gland. Although it is unlikely that local expression of the C4 gene contributes to glomerulonephritis, these results indicate that some components of complement may have a physiologic role in epithelial cell function.