Tolerance Induction by Exopolysaccharide from the Probiotic Bacillus subtilis

Abstract

Abstract Intestinal microbiota can lessen the severity of inflammatory diseases, although the mechanisms by which this occur are poorly understood. We showed that exopolysaccharide (EPS) from the probiotic Bacillus subtilis induces anti-inflammatory macrophages and protects mice from enteric infection and sepsis. Here, we examined the molecular mechanisms by which EPS induces an anti-inflammatory response. In co-cultures of OVA-loaded bone marrow-derived dendritic cells (BMDC) and OT-II T cells, EPS inhibits T cell activation in a TLR4-dependent manner. We hypothesize that EPS engages TLR4 and activates the noncanonical NF-kB signaling pathway, which up-regulates the expression of indoleamine 2,3-dioxygenase (IDO). IDO degrades the essential amino acid tryptophan into kynurenine. The subsequent tryptophan depletion and kynurenine formation leads to suppression of T-cell activation and induction of tolerance. Western blot analysis of BMDC treated with EPS showed nuclear translocation of the transcriptional factor RelB, which is central to the noncanonical pathway, and upregulation of IDO. Inhibition of IDO using the inhibitor, 1-methyl tryptophan and deletion of IDO in BMDCs reverses the effect of EPS. We conclude that EPS inhibition of T cell activation and proliferation is mediated by IDO.