Host defence against infection requires a range of innate and adaptive immune responses that may lead to tissue damage. Such immune-mediated pathologies can be controlled with appropriate T regulatory (Treg) activity. The aim of the present study was to determine the influence of gut microbiota composition on Treg cellular activity and NF-κB activation associated with infection. Mice consumed the commensal microbe Bifidobacterium infantis 35624 followed by infection with Salmonella typhimurium or injection with LPS. In vivo NF-κB activation was quantified using biophotonic imaging. CD4+CD25+Foxp3+ T cell phenotypes and cytokine levels were assessed using flow cytometry while CD4+ T cells were isolated using magnetic beads for adoptive transfer to naïve animals. In vivo imaging revealed profound inhibition of infection and LPS induced NF-κB activity that preceded a reduction in S. typhimurium numbers and murine sickness behaviour scores in B. infantis–fed mice. In addition, pro-inflammatory cytokine secretion, T cell proliferation, and dendritic cell co-stimulatory molecule expression were significantly reduced. In contrast, CD4+CD25+Foxp3+ T cell numbers were significantly increased in the mucosa and spleen of mice fed B. infantis. Adoptive transfer of CD4+CD25+ T cells transferred the NF-κB inhibitory activity. Consumption of a single commensal micro-organism drives the generation and function of Treg cells which control excessive NF-κB activation in vivo. These cellular interactions provide the basis for a more complete understanding of the commensal-host-pathogen trilogue that contribute to host homeostatic mechanisms underpinning protection against aberrant activation of the innate immune system in response to a translocating pathogen or systemic LPS.

Summary Intestinal epithelial cells (IECs) and dendritic cells (DCs) play a pivotal role in antigen sampling and the maintenance of gut homeostasis. However, the interaction of commensal bacteria with the intestinal surface remains incompletely understood. Here we investigated immune cell responses to commensal and pathogenic bacteria. HT‐29 human IECs were incubated with Bifidobacterium infantis 35624, Lactobacillus salivarius UCC118 or Salmonella typhimurium UK1 for varying times, or were pretreated with a probiotic for 2 hr prior to stimulation with S. typhimurium or flagellin. Gene arrays were used to examine inflammatory gene expression. Nuclear factor (NF)‐κB activation, interleukin (IL)‐8 secretion, pathogen adherence to IECs, and mucin‐3 (MUC3) and E‐cadherin gene expression were assayed by TransAM assay, enzyme‐linked immunosorbent assay (ELISA), fluorescence, and real‐time reverse transcriptase–polymerase chain reaction (RT‐PCR), respectively. IL‐10 and tumour necrosis factor (TNF)‐α secretion by bacteria‐treated peripheral blood‐derived DCs were measured using ELISA. S. typhimurium increased expression of 36 of the 847 immune‐related genes assayed, including NF‐κB and IL‐8. The commensal bacteria did not alter expression levels of any of the 847 genes. However, B. infantis and L. salivarius attenuated both IL‐8 secretion at baseline and S. typhimurium‐induced pro‐inflammatory responses. B. infantis also limited flagellin‐induced IL‐8 protein secretion. The commensal bacteria did not increase MUC3or E‐cadherin expression, or interfere with pathogen binding to HT‐29 cells, but they did stimulate IL‐10 and TNF‐α secretion by DCs. The data demonstrate that, although the intestinal epithelium is immunologically quiescent when it encounters B. infantis or L. salivarius, these commensal bacteria exert immunomodulatory effects on intestinal immune cells that mediate host responses to flagellin and enteric pathogens.

Bacterial strain-specific induction of Foxp3(+) T regulatory cells in vivo is associated with protection from respiratory and oral allergy.

The host cell environment can alter bacterial pathogenicity. We employed a combination of cellular and molecular techniques to study the expression of Campylobacter jejuni polysaccharides cocultured with HCT-8 epithelial cells. After two passages, the amount of membrane-bound high-molecular-weight polysaccharide was considerably reduced. Microarray profiling confirmed significant downregulation of capsular polysaccharide (CPS) locus genes. Experiments using conditioned media showed that sugar depletion occurred only when the bacterial and epithelial cells were cocultured. CPS depletion occurred when C. jejuni organisms were exposed to conditioned media from a different C. jejuni strain but not when exposed to conditioned media from other bacterial species. Proteinase K or heat treatment of conditioned media under coculture conditions abrogated the effect on the sugars, as did formaldehyde fixation and cycloheximide treatment of host cells or chloramphenicol treatment of the bacteria. However, sugar depletion was not affected in flagellar export (fliQ) and quorum-sensing (luxS) gene mutants. Passaged C. jejuni showed reduced invasiveness and increased serum sensitivity in vitro. C. jejuni alters its surface polysaccharides when cocultured with epithelial cells, suggesting the existence of a cross talk mechanism that modulates CPS expression during infection.The importance of the host cell environment in bacterial pathogenicity is an emerging paradigm in the study of bacterial infection. For example, human colonization creates a hyperinfectious state in Vibrio cholerae that appears to contribute to the epidemic spread of this intestinal pathogen (29). Earlier work with Campylobacter jejuni demonstrated that coculture conditions altered the protein synthesis and virulence of this organism (23). Stintzi et al. demonstrated major effects in gene expression among C. jejuni organisms exposed to pathogenic conditions (36). However, how the host cell environment alters Campylobacter pathogenesis remains largely unknown. C. jejuni is now known to produce capsular polysaccharide (CPS) (4, 21, 31). Although relatively little is known about the contribution of CPS to the pathogenicity of the organism during bacterial interaction with mucosal surfaces, existing data suggest that CPS in general plays a complex and dynamic role in the infection process (2, 33).In a study that was carried out before CPS was identified in C. jejuni, Babakhani and Joens reported that C. jejuni cocultured with primary swine intestinal cells showed increased mucoidy when regrown on solid medium (1). In addition, passaged C. jejuni showed enhanced invasiveness in vitro. We hypothesized that the change in mucoidy observed in these experiments reflected altered CPS expression by bacteria exposed to pathogenic conditions in vitro. We further speculated that altered CPS under these conditions might have relevance to pathogenic behavior among these organisms during infection. In this study, we show that coculture of C. jejuni with human intestinal epithel...