BACKGROUND: We have previously reported that TGF-β signaling in dendritic cells (DCs) in the intestine contributes to goblet cell differentiation and the homeostasis of commensal bacteria. In this study, we used the mouse enteric pathogen Citrobacter rodentium infection model to investigate the role of DC TGF-β signaling in the host immune response to pathogenic bacteria.
METHODS: CD11c-Cre;Tgfbr2^fl/+ mice (HT) and Cre-negative control mice were infected with 50 million C. rodentium by oral gavage. We then collected feces and blood from each mouse on day 7, 14, 21, and 28, and collected the colon tissues on day 28. Bacterial colonies of C. rodentium in feces and spleens were counted. The serum titers of anti-C. rodentium IgG and IgM antibodies, and serum TGF-β1 levels were measured by ELISA. Colonic crypt depth and goblet cell numbers were determined by HE and UEA-1 staining. The localization of C. rodentium and cell population of immune cells, such as CD11c⁺ DCs and Foxp3⁺ regulatory T cells (Tregs), in the colon were analyzed by immunohistochemistry and flow cytometry. RNA was extracted from the colon tissues, and cytokine expressions were compared by quantitative PCR.
RESULTS: We found that TGFβR2 expression and the downstream signaling in DCs were reduced in HT mice compared to Cre-negative mice (J Immunol 2016). Without infection, there was no histological difference in the colon between Cre-negative and HT mice. After infection, bacterial numbers of C. rodentium in feces of HT mice were significantly higher than that of Cre-negative mice from day 7 through 28. Moreover, HT mice had significantly greater systemic infection in spleen at day 14. Cre-negative mice had cleared infection by day 28, whereas clearance did not occur until day 28 after infection in HT mice. The pathological findings were severer in HT mice with hyperplasia of the intestinal epithelium, decreased goblet cells, and infiltration of inflammatory cells, suggesting the exacerbation of colitis in HT mice. Immunological analysis revealed that, at day 14 after infection, HT mice had deeper invasion of C. rodentium, significantly higher numbers of CD11c⁺ DCs, higher levels of serum anti-C. rodentium IgG and IgM titers, and higher TGF-β1 levels in serum and the colon tissues, but lower numbers of Foxp3⁺ Tregs, compared to Cre-negative mice. These results suggest that Impaired TGF-β signaling in DCs was more susceptible to infection with C. rodentium, partially due to loss of Treg induction.
CONCLUSIONS: TGF-β signaling in DCs inhibits the growth of intestinal pathogens and prevents the exacerbation of infectious colitis, by orchestrating the innate and adaptive immune systems.

Mitsuhiro Fujishiro

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