**P 0

**P 0.01, *** 0.001. We also verified the upregulation of these genes in IECs from GF mice infected with InlAm by quantitative reverse transcription PCR (qRT-PCR) ( Figure 4D ). late phase of infection is not capable of infiltrating through the intestinal barrier. established the commensalism through the reversible down regulation of virulence gene expression. CD8+ T cells were found to be sufficient for the commensalism of contributed to the down-regulation of virulence AG-014699 (Rucaparib) gene expression. Our data provide important insights into the host-microbe interaction and have implications for developing therapeutics against immune disorders induced by intestinal pathogens or pathobionts. (in humans, can establish commensalism (5). Commensalism of in GF AG-014699 (Rucaparib) mice can be established by the immune-mediated clearance of virulent bacteria. IgG specific to virulence factors, which are encoded by the locus of enterocyte effacement (LEE), eliminates virulent bacteria in the gut (9). However, it is still unclear whether other intestinal pathogens can establish commensalism within the host and whether host-derived factors, other than antibodies, can promote commensalism. interacts with epithelial cadherin (E-cad) expressed on the small intestinal epithelial cells (IECs). This interaction leads to the traversal of through the small intestinal barrier and its subsequent spreading into internal organs (11). infection results in high mortality in immunocompromised individuals, pregnant women, neonates, and elderly individuals (12). However, in healthy individuals or in immunocompetent mice, only induces acute AG-014699 (Rucaparib) infection and transient symptoms due to the induction of protective immunity such as infection; this phenomenon is called colonization resistance (14, 16). In this regard, orally-infected disappears in the late phase of infection (14, 17). GF mice are more susceptible to infection. Higher bacteremia occurs in GF mice upon oral infection than in specific pathogen-free (SPF) mice (14, 18). However, can be carried asymptomatically in various animals as well as in humans (19), although the underlying mechanisms are not clearly understood. It remains elusive whether inter-species interactions between and other microbial species in the gut are required or whether the host-derived AG-014699 (Rucaparib) factors are sufficient for the asymptomatic carriage of infection in GF mice and whether can transit from the pathogenicity to commensalism can persist in the lumen of GF mice without inducing chronic immunopathology and establish the commensalism through the reversible downregulation of virulence gene expression. in GF mice by promoting the downregulation of certain virulence gene expression. Materials and Methods Mice SPF C57BL/6 (B6), mice AG-014699 (Rucaparib) were kindly provided by Drs. A. Macpherson and K. McCoy (University of Bern, Switzerland) and maintained in sterile flexible film isolators (Class Biological Clean Ltd.) by feeding autoclaved Teklad global 18% protein rodent diets (2018S; Envigo, USA). Age-matched 9- to 12-week-old mice bred in our facility were used for all experiments. All animal experiments were conducted in accordance with the guidelines of the Institutional Animal Care and Use Committee of POSTECH. Oral Infection of strain “type”:”entrez-protein”,”attrs”:”text”:”S10403″,”term_id”:”87443″,”term_text”:”pirS10403 genetically modified to express mutated internalin A (S192N and Y369S) and ovalbumin (designated as InlAm was grown to OD600 at about 0.8, then washed with sterile PBS. Mice were infected by gavage with 0.2ml PBS containing 5 x 108 CFU InlAm in small intestinal epithelial cells (IECs), GF mice were intraperitoneally (i.p.) injected with LPS (20g/mouse) prepared from Escherichia coli O26:B6 (Sigma-Aldrich, USA) for 3 consecutive days. Enumeration of mice and 5 x 105 cells were intravenously (i.v.) injected into recipient SPF and GF mice. To facilitate the repopulation of donor OT-I cells in the small intestine, OT-I transferred mice were gavaged with ovalbumin (OVA, Sigam-Aldrich, USA, grade V, 0.2mm filtered, 20mg/mouse) every other day for 10 days. CD8+ T Cell Depletion For depletion of donor OT-I T cells mice were i.v. injected with 50g of anti-CD8 antibody (clone YTS 169.4, BioXcell) per mouse every other day for 10 days. RNA Extraction and Quantitative Reverse-Transcription PCR (qRT-PCR) For bacterial KLF15 antibody RNA preparation, the cecal contents of InlAm infected GF B6 and mice were harvested and bacterial RNAs were extracted by RNeasy PowerMicrobiome Kit (Qiagen, USA). For the preparation of RNA from.