Toll-like receptor 2 plays a critical role in maintaining mucosal integrity during Citrobacter rodentium-induced colitis Deanna L. Gibson, 1 Caixia Ma, 1 Carrie M. Rosenberger, 2 Kirk S. B. Bergstrom, 1 Yanet Valdez, 3 Jingtian T. Huang, 1 Mohammed A. Khan 1 and Bruce A. Vallance 1 * 1 Division of Gastroenterology, BC Children’s Hospital, and 3 Michael Smith Laboratories, University of British Columbia, Vancouver, British Columbia, Canada. 2 Institute for Systems Biology, Seattle, Washington, USA. Summary Inflammatory bowel diseases and infectious gastro- enteritis likely occur when the integrity of intestinal barriers is disrupted allowing luminal bacterial prod- ucts to cross into the intestinal mucosa, stimulating immune cells and triggering inflammation. While spe- cific Toll-like receptors (TLR) are involved in the gen- eration of inflammatory responses against enteric bacteria, their contributions to the maintenance of intestinal mucosal integrity are less clear. These studies investigated the role of TLR2 in a model of murine colitis induced by the bacterial pathogen Cit- robacter rodentium. C. rodentium supernatants spe- cifically activated TLR2 in vitro while infected TLR2–/– mice suffered a lethal colitis coincident with colonic mucosal ulcerations, bleeding and increased cell death but not increased pathogen burden. TLR2–/– mice suffered impaired epithelial barrier function mediated via zonula occludens (ZO)-1 in naïve mice and claudin-3 in infected mice, suggesting this could underlie their susceptibility. TLR2 deficiency was also associated with impaired production of IL-6 by bone marrow-derived macrophages and infected colons cultured ex vivo. As IL-6 has antiapoptotic and epi- thelial repair capabilities, its reduced expression could contribute to the impaired mucosal integrity. These studies report for the first time that TLR2 plays a critical role in maintaining intestinal mucosal integrity during infection by a bacterial pathogen. Introduction Toll-like receptors (TLRs) play a central role in the initia- tion of innate cellular immune responses and in the devel- opment of subsequent adaptive immune responses to invading microbial pathogens. TLRs are activated by spe- cific microbial ligands such as lipopolysaccharide (LPS) and flagellin, and with the exception of TLR3, lead to an association with Toll-interleukin-1-related (TIR) domain- containing myeloid differentiation factor (MyD)88, mediat- ing a signalling cascade that activates the NF-kB transcription factor (Akira and Takeda, 2004). This activa- tion results in the subsequent upregulation of pro- inflammatory cytokines and chemokines that are required for protective inflammatory responses against many types of bacterial pathogens. If these innate responses are inadvertently triggered by commensal bacterial products, they can also result in maladaptive inflammation and tissue damage observed during inflammatory bowel dis- eases (IBD) (Canto et al., 2006; Sartor, 2006; Toiyama et al., 2006). Aside from their well-characterized roles in host inflam- matory responses, recent studies have shown that TLR activation by commensal bacteria also plays an essential role in maintaining colonic homeostasis. Mice lacking the adaptor protein MyD88 challenged with dextran sodium sulfate (DSS) were unable to control the resulting colitis, which was associated with the loss of mucosal integrity, development of ulcerations, pronounced colonic bleeding and high mortality rates (Rakoff-Nahoum et al., 2004). While both TLR2–/– and TLR4–/– mice showed modestly increased susceptibility to DSS colitis, the contribution of different TLRs to this susceptibility, as well as the mecha- nisms by which MyD88 prevented the injury, remains unclear. However it is possible that the activation of the innate immune system induces responses that protect the colonic epithelium from damage and/or promote epithelial repair of the damage induced by DSS. In fact, new reports are now revealing that activation of NF-kB within the colonic epithelium is essential for the regulation of epithe- lial integrity and for the prevention of chronic intestinal inflammation (Nenci et al., 2007). Curiously, despite this new role for innate immunity in the gastrointestinal (GI) tract, no studies have yet Received 23 July, 2007; revised 4 September, 2007; accepted 6 September, 2007. *For correspondence. E-mail bvallance@cw.bc.ca; Tel. (+1) 604 875 2343 ext 5118; Fax (+1) 604 875 3244. Cellular Microbiology (2008) 10(2), 388–403 doi:10.1111/j.1462-5822.2007.01052.x First published online 2 October 2007 © 2007 The Authors Journal compilation © 2007 Blackwell Publishing Ltd