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Salmonella enterica serotype Typhimurium causes acute inflammatory diarrhea in humans. Flagella contribute to intestinal inflammation, but the mechanism remains unclear since most mutations abrogating pattern recognition of flagellin also prevent motility and reduce bacterial invasion. To determine the contribution of flagellin pattern recognition to the generation of innate immune responses, we compared in two animal models a nonmotile, but flagellin-expressing and -secreting serotype Typhimurium strain (flgK mutant) to a nonmotile, non-flagellin-expressing strain (flgK fliC fljB mutant). In vitro, caspase-1 can be activated by cytosolic delivery of flagellin, resulting in release of the interferon gamma inducing factor interleukin-18 (IL-18). Experiments with streptomycin-pretreated caspase-1-deficient mice suggested that induction of gamma interferon expression in the murine cecum early (12 h) after serotype Typhimurium infection was caspase-1 dependent but independent of flagellin pattern recognition. In addition, mRNA levels of the CXC chemokines macrophage inflammatory protein 2 and keratinocyte-derived chemokine were markedly increased early after serotype Typhimurium infection of streptomycin-pretreated wild-type mice regardless of flagellin expression. In contrast, in bovine ligated ileal loops, flagellin pattern recognition contributed to increased mRNA levels of macrophage inflammatory protein 3␣ and more fluid accumulation at 2 h after infection. Collectively, our data suggest that pattern recognition of flagellin contributes to early innate host responses in the bovine ileal mucosa but not in the murine cecal mucosa.Salmonella enterica serotype Typhimurium is a major cause of gastroenteritis in humans, which is characterized by acute intestinal inflammation and diarrhea (11,36). One of the serotype Typhimurium virulence factors contributing to intestinal inflammation are flagella. Nonflagellated serotype Typhimurium mutants have been shown to cause less inflammation than their isogenic parents do after infection of bovine ligated ileal loops (59), streptomycin-pretreated mice (65, 74), and chickens (24).Several possible mechanisms by which flagella may contribute to eliciting proinflammatory responses have been proposed. Flagella are surface appendages of serotype Typhimurium that are required for motility and chemotaxis. Motility contributes to serotype Typhimurium invasion of intestinal epithelial cell lines by increasing bacterial contact with host cells (26,27). The invasion-associated type III secretion system (T3SS-1) is important for inducing intestinal inflammation in animal models (1,20,70,81). Nonmotile serotype Typhimurium mutants may thus cause reduced intestinal inflammation in vivo because the efficiency of T3SS-1-mediated invasion is reduced.In addition to its role in motility and invasion, the proteinaceous monomer of the flagellar filament, flagellin, has been shown to be a potent activator of the innate immune response in tissue culture models. Flagellin is an agonist of Toll-like ...

SummarySalmonella phosphothreonine lyase SpvC inactivates the dual-phosphorylated host mitogenactivated protein kinases (MAPK) through b-elimination. While SpvC can be secreted in vitro by both Salmonella pathogenicity island (SPI)-1 and SPI-2 type III secretion systems (T3SSs), translocation of this protein into the host cell cytosol has only been demonstrated by SPI-2 T3SS. In this study, we show that SpvC can be delivered into the host cell cytoplasm by both SPI-1 and SPI-2 T3SSs. Dephosphorylation of the extracellular signal-regulated protein kinases (ERK) was detected in an SPI-1 T3SS-dependent manner 2 h post infection. Using a mouse model for Salmonella enterocolitis, which was treated with streptomycin prior to infection, we observed that mice infected with Salmonella enterica serovar Typhimurium strains lacking the spvC gene showed pronounced colitis when compared with mice infected with the wild-type strain 1 day after infection. The effect of SpvC on the development of colitis was characterized by reduced mRNA levels of the pro-inflammatory cytokines and chemokines, and reduced inflammation with less infiltration of neutrophils. Furthermore, the reduction in inflammation by SpvC resulted in increased bacterial dissemination in spleen of mice infected with Salmonella. Collectively, our findings suggest that SpvC exerts as an antiinflammatory effector and the attenuation of intestinal inflammatory response by SpvC is involved in systemic infection of Salmonella.