The 155-kDa complement regulator factor H (FH) is the predominant soluble regulatory protein of the complement system. It acts as a cofactor for the factor I-mediated conversion of the component C3b to iC3b, competes with factor B for a binding site on C3b and C3(H 2 O) and promotes the dissociation of the C3bBb complex. The primary site of synthesis is the liver, i.e. FH-specific mRNA and protein were identified in both hepatocytes (HC) and Kupffer cells (KC). Previous studies in rat primary HC and KC had shown that the proinflammatory cytokine IFN-c influences the balance between activation and inhibition of the complement system through up-regulation of the inhibitory FH. In this study we show that C5a, as a product of complement activation, stimulates the expression of FH-specific mRNA and protein in KC and thus induces a negative feedback. Quantitative-competitive RT-PCR showed an approximate threefold C5a-induced up-regulation of FH. ELISA analyses revealed a corresponding increase in FH protein in the supernatants of KC. The up-regulation of FH was completely inhibited by the C5a-blocking monoclonal antibody 6-9F. Furthermore, an involvement of LPS and IFN-c was excluded, which strongly indicates a direct effect of C5a on the expression of FH in KC.

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