Apical colonization of polarized epithelia by Salmonella typhimurium results in translocation of flagellin to the basolateral membrane

The clinical manifestations of human Salmonellosis result largely from the innate immune response elicited by this pathogen. Specifically, Salmonella typhimurium colonization of the intestinal epithelium induces the epithelium to secrete chemokines that recruit immune cells, especially polymorphonuclear leukocytes (McCormick et al., 1998a; McCormick et al., 1998b; McCormick et al., 1993). Such an innate immune inflammatory response generally results in clearance of the infection albeit resulting in substantial suffering in the host (Gewirtz et al., 2002a). A primary means by which this pathogen is recognized by intestinal epithelial cells is via the detection of flagellin monomers by toll-like receptor 5 (TLR5) (Gewirtz et al., 2001a). Although flagellin monomers with the potential to activate TLR5 are released by both commensal Escherichia coli strains and pathogens such as S. typhimurium (Gewirtz et al., 2001b; Moors et al., 2001), TLR5 expression appears to be limited to the basolateral surface of the intestinal epithelium; therefore only those bacteria with flagellin crossing the epithelium will activate this pro-inflammatory signaling pathway. We have observed that S. typhimurium, but not commensal E. coli can transcytose flagellin in this manner across the epithelium thus activating TLR5-mediated proinflammatory gene expression and thereby causing, at least in part, the gastroenteritis induced by this microbe (Gewirtz et al., 2001b). The goal of this study was to elucidate the mechanism by which S. typhimurium is able to transcytose flagellin across polarized epithelia. Using a variety of biochemical, cell biological and physiological techniques we show here that flagellin transcytosis occurs by a transcellular pathway mediated by vesicular trafficking and is dependent upon the function of the Salmonella pathogenicity island (SPI)-2.