Regulation of IL-6 Synthesis in Human Peripheral Blood Mononuclear Cells by C3a and C3adesArg

Abstract

The anaphylatoxin C3a has been reported to have immunomodulatory effects on a number of different cell types. In this study we investigated the effects of C3a and C3adesArg on gene expression and protein secretion of IL-6 in human PBMCs, either alone or in combination with LPS or IL-1β. C3a or C3adesArg alone exhibited no effect on the expression or secretion of IL-6. However, when PBMC were stimulated with LPS or IL-1β, both C3a and C3adesArg were found to enhance IL-6 release by PBMC in a dose-dependent manner. Since C3a has been shown to induce PGE2 production by monocytes, and PGE2 has been shown to influence cytokine production, we investigated the potential role of PGE2 in C3a-mediated enhancement of LPS- and IL-1β-induced IL-6 production. Indomethacin blocked PGE2 release, but had no influence on the observed effects of C3a, suggesting that the effects of C3a on IL-6 production are independent of PGE2 formation by monocytes. Northern blot analysis showed that C3a as well as C3adesArg enhanced LPS-induced mRNA levels for IL-6. Pretreatment of PBMCs with pertussis toxin blocked the functions of C3a and C3adesArg, indicating that the actions of these two molecules are mediated by a G protein-coupled pathway. Furthermore, we investigated the effects of C3a and C3adesArg on induction of NF-κB and activating protein-1 binding. Both molecules enhanced LPS-induced NF-κB and activating protein-1 binding activity. These results demonstrate the capacity of intact C3a and its circulating des-Arg form to exert immunmodulatory effects in vitro.