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Infect. Immun. doi:10.1128/IAI.01662-06
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Surface expression of TLR9 is upregulated on intestinal epithelial cells in response to pathogenic bacterial DNA

Division of Gastroenterology Surgical-Medical Research Institute, University of Alberta Edmonton, Alberta, Canada University of Regensburg, Regensburg Germany; Dept of Animal Science, University of Manitoba

^* To whom correspondence should be addressed. Email: karen.madsen{at}ualberta.ca.

	Abstract

Colonic epithelial cells are constantly exposed to high levels of bacterial DNA in the intestinal lumen and must recognize and respond appropriately to pathogens, while maintaining a tolerance to non-pathogenic commensal bacterial strains. Bacterial DNA is recognized by toll-like receptor 9 (TLR9). The aim of this study was to investigate TLR9 expression and localization in colonic epithelial cells under basal conditions and in response to bacterial DNA. HT-29 cells were exposed to DNA from various strains of commensal and pathogenic microbes. TLR9 mRNA expression was determined by real-time RT-PCR and IL-8 secretion measured by ELISA. Localization of TLR9 was determined by flow cytometry in HT-29 cells, and immunofluorescence in HT-29 cells and mouse colonic tissue. Immunofluorescence and flow cytometric analyses demonstrated intracellular and surface expression of TLR9 in HT-29 cells under basal conditions. Exposure of cells to DNA from pathogenic strains of Salmonella and E. coli resulted in a significant increase in TLR9 mRNA expression. S. dublin DNA increased surface TLR9 protein and IL-8 secretion. There was no change in mRNA levels or localization of TLR9 in response to Bifidobacteria breve. Chloroquine did not block IL-8 secretion in response to S. dublin DNA. TLR9 was expressed on the colonic apical surface in wildtype mice, but not in germfree mice. These results demonstrate that intestinal epithelial cells recognize pathogenic bacterial DNA and respond by increasing surface localization and expression of TLR9 suggesting that the epithelial inflammatory response to pathogenic DNA is mediated at least in part by increased TLR9 expression.