Mechanism by which Commensal Bacteria Prevent Colitis Trillions of bacteria live in homeostasis within the gastrointestinal tract and provide a variety of benefits to the host immune system. Many of these commensal bacteria have been shown to limit colitis, however, little is known about the mechanisms by which this occurs. We utilize a mouse model in which a single dose of the commensal bacterium, 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 protects from inflammation caused by C. rodentium infection. We identified exopolysaccharides (EPS) to be the active molecule of B. subtilis, and a single i.p. dose of EPS protect mice from disease. EPS bind F4/80+CD11b+ peritoneal macrophages, and after i.p. injection of EPS, we identified F4/80+CD11b+ cells that are MHCII+CD206+Arg1+YM-1+, indicative of alternatively-activated macrophages (AAM) or M2 macrophages (M2 MF). We further show that peritoneal macrophages from EPS-treated mice suppress T cell proliferation in vitro, suggesting that these cells contribute to EPS-mediated protection from inflammation. We hypothesize that EPS induce M2 MF and that these cells mediate protection from acute colitis induced by C. rodentium.
In aim 1, we will determine if B. subtilis spores also induce M2 MF, test if M2 MF protect mice from C. rodentium infection, and if these cells traffic to the site of inflammation during infection. M2 MF are induced in the presence of IL-4 and IL-13, usually derived from Th2 cells. Interestingly, F4/80+CD11b+ macrophages purified from EPS-treated mice expressed increased levels of these cytokines compared to untreated mice. We hypothesize that EPS bind to peritoneal macrophages, upregulating IL-4 and IL-13, which act in an autocrine fashion to upregulate MHC class II, CD206, arginase and YM-1. Since EPS-mediated protection requires TLR4 signaling, we hypothesize that this induction requires TLR4 and PPAR?, a DNA-binding protein downstream of TLR4 that induces M2 MF genes (Aim 2). CD4+ T cells drive much of the inflammation associated with C. rodentium infection. This inflammation is suppressed when mice are pretreated with EPS prior to infection.
In aim 3, we will use IL-2cre EGFP mice to test if EPS suppress T cell responses during infection and if M2 MF mediate this suppression. Taken together, these experiments will determine the mechanism by which EPS suppress inflammation leading to intestinal homeostasis. Gaining knowledge of the mechanisms by which commensals protect from colitis will ultimately help to design rational therapeutics that can be used to reduce inflammation and limit human disease.

Public Health Relevance

Probiotics are a commonly used therapeutic approach to treat inflammatory diseases of the gastrointestinal tract, including ulcerative colitis, Crohn's disease, and celiac disease; however, many questions remain as to how commensal bacteria contribute to intestinal homeostasis. We have shown that exopolysaccharides isolated from the commensal Bacillus subtilis prevent acute colitis induced by an enteric pathogen and potentially induce a protective macrophage population (alternatively-activated macrophages). Our goal is to determine the mechanism by which the commensal exopolysaccharides induce alternatively-activated macrophages and if these macrophages suppress colitis.

National Institute of Health (NIH)
National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK)
Predoctoral Individual National Research Service Award (F31)
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Special Emphasis Panel (ZDK1)
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Densmore, Christine L
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Loyola University Chicago
Schools of Medicine
United States
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Paynich, Mallory L; Jones-Burrage, Sara E; Knight, Katherine L (2017) Exopolysaccharide from Bacillus subtilis Induces Anti-Inflammatory M2 Macrophages That Prevent T Cell-Mediated Disease. J Immunol 198:2689-2698