Mechanism by Which Commensal Bacteria Limit Inflammation

Trillions of bacteria live within the gastrointestinal tract and are critical for maintaining intestinal homeostasis; however, the mechanisms utilized by specific bacterial molecules to contribute to homeostasis are not well understood. We utilize a mouse model in which a single oral dose of the probiotic, Bacillus subtilis, protects mice from acute colitis induced by the enteric pathogen Citrobacter rodentium. Our goal is to elucidate the mechanism by which B. subtilis prevents inflammation. We identified exopolysaccharides (EPS) to be the active molecule of B. subtilis, and a single dose of EPS protects mice from disease. EPS binds F4/80+CD11b+ peritoneal macrophages, and adoptive transfer of macrophage-rich peritoneal cells from EPS-treated mice confers protection from disease to recipient mice. Following EPS treatment, macrophages increase expression of CD206, arginase-1, YM-1, FIZZ-1, and IL-4Rα, markers indicative of anti-inflammatory M2 macrophages. EPS does not protect TLR4-deficient mice from C. rodentium-induced disease, and as expected, M2 macrophages do not develop in TLR4-/- mice following EPS treatment. CD4+ T cells drive much of the inflammation associated with C. rodentium infection, and we hypothesized that EPS-induced M2 macrophages inhibit CD4+ T cell responses in vivo. Accordingly, we measured levels of IFN-γ (Th1), IL-17 (Th17), and IL-13 (Th2) in splenic T cells following EPS treatment and found decreased levels of these cytokines. In vitro, EPS-induced M2 macrophages inhibit activation and proliferation of both CD4+ and CD8+ T cells. The inhibition of CD4+ T cells is dependent on TGF-β, whereas inhibition of CD8+ T cells is dependent on both TGF-β and PD-L1. We suggest that administration of B. subtilis EPS can be utilized to broadly inhibit T cell activation and thus control T cell-mediated immune responses in numerous inflammatory diseases

Divergent metabolic programmes control two populations of MAIT cells that protect the lung.

Although mucosal-associated invariant T (MAIT) cells provide rapid, innate-like responses, they are not pre-set, and memory-like responses have been described for MAIT cells following infections. The importance of metabolism for controlling these responses, however, is unknown. Here, following pulmonary immunization with a Salmonella vaccine strain, mouse MAIT cells expanded as separate CD127-Klrg1+ and CD127+Klrg1- antigen-adapted populations that differed in terms of their transcriptome, function and localization in lung tissue. These populations remained altered from steady state for months as stable, separate MAIT cell lineages with enhanced effector programmes and divergent metabolism. CD127+ MAIT cells engaged in an energetic, mitochondrial metabolic programme, which was critical for their maintenance and IL-17A synthesis. This programme was supported by high fatty acid uptake and mitochondrial oxidation and relied on highly polarized mitochondria and autophagy. After vaccination, CD127+ MAIT cells protected mice against Streptococcus pneumoniae infection. In contrast, Klrg1+ MAIT cells had dormant but ready-to-respond mitochondria and depended instead on Hif1a-driven glycolysis to survive and produce IFN-γ. They responded antigen independently and participated in protection from influenza virus. These metabolic dependencies may enable tuning of memory-like MAIT cell responses for vaccination and immunotherapies

Gut CD4+ T cell phenotypes are a continuum molded by microbes, not by TH archetypes

CD4 effector lymphocytes (T ) are traditionally classified by the cytokines they produce. To determine the states that T cells actually adopt in frontline tissues in vivo, we applied single-cell transcriptome and chromatin analyses to colonic T cells in germ-free or conventional mice or in mice after challenge with a range of phenotypically biasing microbes. Unexpected subsets were marked by the expression of the interferon (IFN) signature or myeloid-specific transcripts, but transcriptome or chromatin structure could not resolve discrete clusters fitting classic helper T cell (T ) subsets. At baseline or at different times of infection, transcripts encoding cytokines or proteins commonly used as T markers were distributed in a polarized continuum, which was functionally validated. Clones derived from single progenitors gave rise to both IFN-γ- and interleukin (IL)-17-producing cells. Most of the transcriptional variance was tied to the infecting agent, independent of the cytokines produced, and chromatin variance primarily reflected activities of activator protein (AP)-1 and IFN-regulatory factor (IRF) transcription factor (TF) families, not the canonical subset master regulators T-bet, GATA3 or RORγ. + eff eff eff H

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