IFN-gamma mediates stimulation of complement C4 biosynthesis in human proximal tubular epithelial cells.

The liver has been presumed to be the main source of complement deposited in inflammatory lesions such as in glomerulonephritis. In a previous study, however, it was demonstrated that renal tubular cells synthesize C3 in vitro. Furthermore, it was shown by others that C4 gene transcripts were detectable in situ in renal tubular cells. Therefore studies were initiated to investigate the synthesis of C4 by proximal tubular epithelial cells (PTEC) in vitro. Biosynthetic labeling experiments showed de novo synthesis of C4 by PTEC. The synthesis of C4 by PTEC and its regulation by IFN-gamma was fully inhibitable by the addition of cycloheximide, indicating that protein synthesis is required for an increase in C4 secretion. Addition of increasing concentrations of IFN-gamma enhanced the production of C4 by PTEC in a dose dependent fashion, with a 2.5-fold maximum. Kinetic experiments demonstrated higher levels of C4 production when stimulated with IFN-gamma for up to 72 hours. The hemolytic activity of C4 present in culture supernatants of PTEC decreased during the culture period as assessed by hemolytic titration. Northern blot analysis showed no enhancement of C4 mRNA in IFN-gamma treated PTEC, indicating that IFN-gamma regulates C4 production at a post-transcriptional level. Antibody blocking experiments confirmed that regulation of C4 production was directly mediated by IFN-gamma. From this study it was concluded that renal cells are able to synthesize complement components that could possibly play a role in inflammatory responses evolving in the kidney.