Flagellin, a novel mediator of Salmonella-induced epithelial activation and systemic inflammation: IκBα degradation, induction of nitric oxide synthase, induction of proinflammatory mediators, and cardiovascular dysfunction

Gram-negative sepsis is mediated by the actions of proinflammatory genes induced in response to microbes and their products. We report that flagellin, the monomeric subunit of flagella, is a potent proinflammatory species released by Salmonella. Flagellin (1 μg/ml) induces IκBα degradation, NF-κB nuclear translocation, and inducible NO synthase expression in cultured intestinal epithelial cells (IEC). Aflagellic Salmonella mutants do not induce NF-κB activation or NO production by cultured IEC. Antiserum to flagellin blocks NO production in IEC induced by medium conditioned by a variety of motile Gram-negative enteric pathogens (Escherichia coli, Salmonella muenchen, Serratia marcescens, Proteus mirabilis, and Proteus vulgaris). Flagellin, when injected systemically (∼10 μg/mouse), induces systemic inflammation characterized by the systemic expression of a range of proinflammatory cytokines and chemokines and of inducible NO synthase. At higher doses (∼300 μg/mouse), flagellin induces shock, chaeacterized by hypotension, reduced vascular contractility in mice, and death. The effects of flagellin do not diminish in C3H/HeJ LPS-resistant mice, indicating that the Toll-like receptor-4 receptor is not involved in flagellin's actions. In LPS-resistant mice, i.p. injection of S. dublin flagellin or medium conditioned by wild-type S. dublin induces serum IFN-γ and TNF-α, whereas medium conditioned by aflagellic mutants has no effect. Flagellin can be detected in the blood of rats with septic shock induced by live bacteria at approximately 1 μg/ml. We propose that flagellin released by Gram-negative pathogens may contribute to the inflammatory response by an LPS- and Toll-like receptor-4-independent pathway.