| Abstract: | Abnormal expression of CEACAM6 is observed at the apical surface of the ileal epithelium in Crohn''s disease (CD) patients, and CD ileal lesions are colonized by pathogenic adherent-invasive Escherichia coli (AIEC). We investigated the ability of AIEC reference strain LF82 to colonize the intestinal mucosa and to induce inflammation in CEABAC10 transgenic mice expressing human CEACAMs. AIEC LF82 virulent bacteria, but not nonpathogenic E. coli K-12, were able to persist in the gut of CEABAC10 transgenic mice and to induce severe colitis with reduced survival rate, marked weight loss, increased rectal bleeding, presence of erosive lesions, mucosal inflammation, and increased proinflammatory cytokine expression. The colitis depended on type 1 pili expression by AIEC bacteria and on intestinal CEACAM expression because no sign of colitis was observed in transgenic mice infected with type 1 pili–negative LF82-ΔfimH isogenic mutant or in wild-type mice infected with AIEC LF82 bacteria. These findings strongly support the hypothesis that in CD patients having an abnormal intestinal expression of CEACAM6, AIEC bacteria via type 1 pili expression can colonize the intestinal mucosa and induce gut inflammation. Thus, targeting AIEC adhesion to gut mucosa represents a new strategy for clinicians to prevent and/or to treat ileal CD.Inflammatory bowel disease (IBD) mainly consists of two disorders, ulcerative colitis and Crohn''s disease (CD), with a combined prevalence of ∼150–200 cases per 100,000 in Western countries (Shanahan, 2002; Loftus, 2004). The abnormal inflammatory response observed in IBD requires interplay between host genetic factors and the intestinal microbiota (Podolsky, 2002; for review see Strober et al., 2007). The role of the microbiota in IBD development is highlighted with the following observations: in CD patients, postsurgical exposure of the terminal ileum to luminal contents is associated with increased inflammation, and diversion of the fecal stream is associated with improvement (Rutgeerts et al., 1991); some IBD patients improve upon antibiotic treatment (Sartor, 2004; Sands, 2007); the severity of colitis in multiple animal models is decreased by the administration of antibiotics; and no sign of colitis is observed when those animals are in germ-free conditions (for review see Sartor, 2008).Two broad hypotheses have arisen regarding the role of the intestinal microbiota in the pathogenesis of IBD. Several lines of evidence support the notion that IBD results from an excessive immune response to gut commensal organisms (for review see Strober et al., 2007). However, the disease could result from a problem in the composition of the microflora leading to generalized or localized dysbiosis. Thus, a breakdown in the balance between putative species of “protective” versus “harmful” intestinal bacteria has been reported and may promote inflammation. A low proportion of Faecalibacterium prausnitzii on resected ileal Crohn mucosa is associated with endoscopic recurrence at 6 mo, and this bacteria has antiinflammatory properties (Tamboli et al., 2004; Sokol et al., 2008). In addition, host-mediated inflammation in response to a pathogen infection can disrupt the intestinal microbiota and shift the balance between the protective microbiota and the pathogen in favor of the pathogen, as seen with Citrobacter rodentium infection promoting the overgrowth of Enterobacteriaceae (Lupp et al., 2007) and with Salmonella Typhimurium infection (Stecher et al., 2007). In patients with CD and ulcerative colitis, high concentrations of bacteria forming a biofilm on the surface of the gut mucosa (Swidsinski et al., 2002) and increased numbers of mucosa-associated Escherichia coli are observed (Darfeuille-Michaud et al., 1998; Martin et al., 2004; Conte et al., 2006; Baumgart et al., 2007; Kotlowski et al., 2007; Sasaki et al., 2007). This overgrowth of E. coli can result from host-mediated inflammation or abnormal expression of molecules acting as receptors for bacterial adhesion. In CD patients with ileal involvement of the disease, we recently reported an abnormal ileal expression of carcinoembryonic antigen-related cell adhesion molecules (CEACAMs) 5 and 6 (Barnich et al., 2007) and showed that only CEACAM6 acts as a receptor for pathogenic E. coli. These bacteria, called adherent-invasive E. coli (AIEC), colonize the ileal mucosa of CD patients (Darfeuille-Michaud et al., 2004). They are able to adhere to and invade intestinal epithelial cells and to survive and highly replicate within macrophages, leading to the secretion of high amounts of TNF (Boudeau et al., 1999; Glasser et al., 2001). Interestingly, in vitro studies demonstrated that CEACAM6 expression is increased in cultured intestinal epithelial cells not only after IFN-γ or TNF stimulation (Fahlgren et al., 2003) but also after infection with AIEC bacteria, indicating that AIEC could promote their own colonization in CD patients (Barnich et al., 2007). In the present paper, using transgenic CEABAC10 mice harboring a bacterial artificial chromosome that contains part of the human CEA family gene cluster, including complete human CEACAM3, CEACAM5 (CEA), CEACAM6, and CEACAM7 genes (Chan and Stanners, 2004), we investigated whether LF82 bacteria isolated from a CD patient can colonize the intestinal mucosa as a result of CEACAM expression and whether AIEC LF82 colonization could lead to the development of gut inflammation. |
