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

Gram-negative sepsis is mediated by the actions of proinflammatory genes in duced in response to microbes and their products. We report that flagellin, the monomeric subunit of flagella, is a potent proinflammatory species rel eased by Salmonella. Flagellin (1 mug/ml) induces I kappaB alpha degradatio n, NF-kappaB nuclear translocation, and inducible NO synthase expression in cultured intestinal epithelial cells (IEC). Affagellic Salmonella mutants do not induce NF-kappaB activation or NO production by cultured IEC. Antise rum to flagellin blocks NO production in IEC induced by medium conditioned by a variety of motile Gram-negative enteric pathogens (Escherichia coli, S almonella muenchen, Serratia marcescens. Proteus mirabilis, and Proteus vul garis). Flagellin, when injected systemically (similar to 10 mug/mouse), in duces systemic inflammation characterized by the systemic expression of a r ange of proinflammatory cytokines and chemokines and of inducible NO syntha se. At higher doses (similar to 300 mug/mouse), flagellin induces shock, ch aracterized by hypotension, reduced vascular contractility in mice, and dea th. 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 flage llin's actions. In LPS-resistant mice, i.p. injection of S. dublin flagelli n or medium conditioned by wild-type S. dublin induces serum IFN-gamma and TNF-alpha, 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 mug/ml. We propose that flagellin release d by Gram-negative pathogens may contribute to the inflammatory response by an LPS- and Toll-like receptor-4-independent pathway.