Exopolysaccharide from Bacillus subtilis Induces Anti-Inflammatory M2 Macrophages That Prevent T Cell-Mediated Disease.

Commensal bacteria contribute to immune homeostasis in the gastrointestinal tract; however, the underlying mechanisms for this are not well understood. A single dose of exopolysaccharide (EPS) from the probiotic spore-forming bacterium Bacillus subtilis protects mice from acute colitis induced by the enteric pathogen Citrobacter rodentium Adoptive transfer of macrophage-rich peritoneal cells from EPS-treated mice confers protection from disease to recipient mice. In vivo, EPS induces development of anti-inflammatory M2 macrophages in a TLR4-dependent manner, and these cells inhibit T cell activation in vitro and in C. rodentium-infected mice. In vitro, M2 macrophages inhibit CD4+ and CD8+ T cells. The inhibition of CD4+ T cells is dependent on TGF-ß, whereas inhibition of CD8+ T cells is dependent on TGF-ß and PD-L1. We suggest that administration of B. subtilis EPS can be used to broadly inhibit T cell activation and, thus, control T cell-mediated immune responses in numerous inflammatory diseases.

Protection from intestinal inflammation by bacterial exopolysaccharides.

Host inflammatory responses against pathogenic organisms can be abrogated by commensals; however, the molecular mechanisms by which pathogenesis is prevented are still poorly understood. Previous studies demonstrated that administration of a single dose of Bacillus subtilis prevented disease and inflammation by the enteric mouse pathogen Citrobacter rodentium, which causes disease similar to the human pathogen enteropathogenic Escherichia coli. No protection was observed when an exopolysaccharide (EPS)-deficient mutant of B. subtilis was used, suggesting that EPS are the protective factor. In this study, we isolated and characterized EPS and showed that they also prevent C. rodentium-associated intestinal disease after a single injection. Protection is TLR4 dependent because EPS-treated TLR4 knockout mice developed disease. Furthermore, protection could be conveyed to wild-type mice by adoptive transfer of macrophage-rich peritoneal cells from EPS-treated mice. We found that EPS specifically bind peritoneal macrophages, and because mice lacking MyD88 signaling in myeloid cells were not protected by EPS, we conclude that bacterial EPS prevent colitis in a TLR4-dependent manner that requires myeloid cells. These studies provide a simple means of preventing intestinal inflammation caused by enteric pathogens.