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Flagellin-Dependent and -Independent Inflammatory Responses following Infection by Enteropathogenic Escherichia coli and Citrobacter rodentium

Division of Gastroenterology, BC Children's Hospital, Vancouver, British Columbia, Canada,¹ Institute of Systems Biology, Seattle, Washington,² Division of Infectious Disease and Vancouver General Hospital, University of British Columbia, Vancouver, British Columbia, Canada³

Received 17 August 2007/ Returned for modification 23 November 2007/ Accepted 15 January 2008

Enteropathogenic Escherichia coli (EPEC) and the murine pathogen Citrobacter rodentium belong to the attaching and effacing (A/E) family of bacterial pathogens. These noninvasive bacteria infect intestinal enterocytes using a type 3 secretion system (T3SS), leading to diarrheal disease and intestinal inflammation. While flagellin, the secreted product of the EPEC fliC gene, causes the release of interleukin 8 (IL-8) from epithelial cells, it is unclear whether A/E bacteria also trigger epithelial inflammatory responses that are FliC independent. The aims of this study were to characterize the FliC dependence or independence of epithelial inflammatory responses to direct infection by EPEC or C. rodentium. Following infection of Caco-2 intestinal epithelial cells by wild-type and fliC EPEC, a rapid activation of several proinflammatory genes, including those encoding IL-8, monocyte chemoattractant protein 1, macrophage inflammatory protein 3 (MIP3), and β-defensin 2, occurred in a FliC-dependent manner. These responses were accompanied by mitogen-activated protein kinase activation, as well as the Toll-like receptor 5 (TLR5)-dependent activation of NF-B. At later infection time points, a subset of these proinflammatory genes (IL-8 and MIP3) was also induced in cells infected with fliC EPEC. The nonmotile A/E pathogen C. rodentium also triggered similar innate responses through a TLR5-independent but partially NF-B-dependent mechanism. Moreover, the EPEC FliC-independent responses were increased in the absence of the locus of enterocyte effacement-encoded T3SS, suggesting that translocated bacterial effectors suppress rather than cause the FliC-independent inflammatory response. Thus, we demonstrate that infection of intestinal epithelial cells by A/E pathogens can trigger an array of proinflammatory responses from epithelial cells through both FliC-dependent and -independent pathways, expanding our understanding of the innate epithelial response to infection by these pathogens.

Published ahead of print on 28 January 2008.

Editor: V. J. DiRita