Identification of the Salmonella enterica serotype Typhimurium SipA domain responsible for inducing neutrophil recruitment across the intestinal epithelium

In human intestinal disease induced by Salmonella enterica serotype Typhimurium (S. typhimurium) transepithelial migration of polymorphonuclear leukocytes (PMNs) rapidly follows attachment of the bacteria to the epithelial apical membrane. Previously, we have shown that the S. typhimurium effector protein, SipA, plays a pivotal role in signalling epithelial cell responses that lead to the transepithelial migration of PMNs. Thus, the objective of this study was to determine the functional domain of SipA that regulates this signalling event. SipA was divided into two fragments: the SipAb C-terminal fragment(426-684) (259 AA), which binds actin, and the SipAa fragment(2-425) (424 AA), which a role has yet to be described. In both in vitro and in vivo models of S. typhimurium-induced intestinal inflammation the SipAa fragment exhibited a profound ability to induce PMN transmigration, whereas the SipAb actin-binding domain failed to induce PMN transmigration. Subsequent mapping of the SipAa domain identified a 131-amino-acid region (SipAa3(294-424)) responsible for modulating PMN transepithelial migration. Interestingly, neither intracellular translocation nor actin association of SipA was necessary for its ability to induce PMN transepithelial migration. As these results indicate SipA has at least two separate functional domains, we speculate that during infection S. typhimurium requires delivery of SipA to both extracellular and intracellular spaces to maximize pro-inflammatory responses and mechanisms of bacterial invasion.