Date of Award


Degree Type


Degree Name

Doctor of Philosophy (PhD)


Microbiology and Immunology


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.

Creative Commons License

Creative Commons License
This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 License.