Expression and regulation of complement factors H and I in rat and human cells: some critical notes

The complement factors I (FI) and H (FH) are complement regulatory proteins. FI, a highly glycosylated serine protease of 88 kDa cleaves the alpha-chains of both complement components C3b and C4b, thereby inactivating them. Complement FH, a glycoprotein of 150 kDa which is composed of 20 short consensus repeats synergizes with FI by increasing the affinity of FI for C3b in the C3b/FH complex by about 15-fold as compared to free C3b. Furthermore, FH prevents factor B from binding to C3b and promotes the dissociation of the C3bBb complex. Both, FI and FH are mainly synthesized in the liver. According to the quantification of specific mRNA of both factors, various amounts are produced by different liver cell types, i.e. hepatocytes (HC) and Kupffer cells (KC). Investigations of cultured primary HC and KC from rat liver showed that FI is exclusively synthesized and secreted by HC whereas FH is synthesized by both HC and KC. Using quantitative-competitive PCR for the quantification of FH-specific mRNA, its constitutive rate of synthesis was found to be nearly ten times higher in KC than in HC. An extrahepatic source of both proteins are human umbilical vein endothelial cells (HUVEC) in which the synthesis of FI is upregulated by IL-6 which is in accord with the upregulation observed in rat HC and two rat hepatoma cell lines (FAO and H4IIE). Three other proinflammatory cytokines, IL-1beta, IFN-gamma and TNF-alpha, were alone or in combination, without any effect on the regulation of FI. This demonstrates that the regulation of FI is similar in HUVEC and HC. These results are in contrast to a previously described IFN-gamma-mediated upregulation of FI in HUVEC and suggest, in accordance with other investigations on extrahepatic sources of FI (e.g. myoblasts), that IFN-gamma has probably no prominent role in the regulation of FI. Instead, IL-6 appears to be the main upregulating cytokine of FI mRNA and of FI protein synthesis in HC as well as in rat and human hepatoma cells and in HUVEC. Of note are experiments by others and us who could not identify FI-specific mRNA in peripheral blood-derived monocytes, granulocytes, or B- and T-cells of man or rat and in rat peritoneal macrophages. FI-specific mRNA could also not be detected in B- or T-cell lymphoma cells, whereas FH-specific mRNA was easily detectable in both human and rat monocytes, and in rat peritoneal macrophages. These data support the notion that FI in contrast to FH is not expressed by cells of the monocyte-macrophage lineage or by other leukocytes of peripheral blood, at least in the absence of additional stimulants.