Innate lymphoid cells (ILCs) and CD4+ T cells produce IL-22, which is critical for intestinal immunity. The microbiota is central to IL-22 production in the intestines; however, the factors that regulate IL-22 production by CD4+ T cells and ILCs are not clear. Here, we show that microbiota-derived short-chain fatty acids (SCFAs) promote IL-22 production by CD4+ T cells and ILCs through G-protein receptor 41 (GPR41) and inhibiting histone deacetylase (HDAC). SCFAs upregulate IL-22 production by promoting aryl hydrocarbon receptor (AhR) and hypoxia-inducible factor 1α (HIF1α) expression, which are differentially regulated by mTOR and Stat3. HIF1α binds directly to the Il22 promoter, and SCFAs increase HIF1α binding to the Il22 promoter through histone modification. SCFA supplementation enhances IL-22 production, which protects intestines from inflammation. SCFAs promote human CD4+ T cell IL-22 production. These findings establish the roles of SCFAs in inducing IL-22 production in CD4+ T cells and ILCs to maintain intestinal homeostasis.

T-cells are crucial in maintanence of intestinal homeostasis, however, it is still unclear how microbiota metabolites regulate T-effector cells. Here we show gut microbiota-derived short-chain fatty acids (SCFAs) promote microbiota antigen-specific Th1 cell IL-10 production, mediated by G-protein coupled receptors 43 (GPR43). Microbiota antigen-specific Gpr43−/− CBir1 transgenic (Tg) Th1 cells, specific for microbiota antigen CBir1 flagellin, induce more severe colitis compared with wide type (WT) CBir1 Tg Th1 cells in Rag−/− recipient mice. Treatment with SCFAs limits colitis induction by promoting IL-10 production, and administration of anti-IL-10R antibody promotes colitis development. Mechanistically, SCFAs activate Th1 cell STAT3 and mTOR, and consequently upregulate transcription factor B lymphocyte-induced maturation protein 1 (Blimp-1), which mediates SCFA-induction of IL-10. SCFA-treated Blimp1−/− Th1 cells produce less IL-10 and induce more severe colitis compared to SCFA-treated WT Th1 cells. Our studies, thus, provide insight into how microbiota metabolites regulate Th1 cell functions to maintain intestinal homeostasis.

The antimicrobial peptides (AMP) produced by intestinal epithelial cells (IEC) play crucial roles in the regulation of intestinal homeostasis by controlling microbiota. Gut microbiota has been shown to promote IEC expression of RegIIIγ and certain defensins. However, the mechanisms involved are still not completely understood. In this report, we found that IEC expression of RegIIIγ and β-defensins 1, 3, and 4 was lower in G protein-coupled receptor (GPR)43−/−mice compared to that of wild-type (WT) mice. Oral feeding with short chain fatty acids (SCFA) promoted IEC production of RegIIIγ and defensins in mice. Furthermore, SCFA induced RegIIIγ and β-defensins in intestinal epithelial enteroids generated from WT but not GPR43−/−mice. Mechanistically, SCFA activated mTOR and STAT3 in IEC, and knockdown of mTOR and STAT3 impaired SCFA induction of AMP production. Our studies thus demonstrated that microbiota metabolites SCFA promoted IEC RegIIIγ and β-defensins in a GPR43-dependent manner. The data thereby provides a novel pathway by which microbiota regulates IEC expression of AMP and intestinal homeostasis.

Intestinal IgA, which is regulated by gut microbiota, plays a crucial role in maintenance of intestinal homeostasis and in protecting the intestines from inflammation. However, the means by which microbiota promotes intestinal IgA responses remain unclear. Emerging evidence suggests that the host can sense gut bacterial metabolites in addition to pathogen-associated molecular patterns and that recognition of these small molecules influences host immune response in the intestines and beyond. We reported here that microbiota metabolite short-chain fatty acid acetate promoted intestinal IgA responses, which was mediated by “metabolite-sensing” GPR43. GPR43−/− mice demonstrated lower levels of intestinal IgA and IgA+ gut bacteria compared to those in WT mice. Feeding WT but not GPR43−/− mice acetate but not butyrate promoted intestinal IgA response independent of T cells. Acetate promoted B cell IgA class switching and IgA production in vitro in the presence of WT but not GPR43−/− dendritic cells (DC). Mechanistically, acetate induced DC expression of Aldh1a2, which converts Vitamin A into its metabolite retinoic acid (RA). Moreover, blockade of RA signaling inhibited the acetate induction of B cell IgA production. Our studies thus identified a new pathway by which microbiota promotes intestinal IgA response through its metabolites.

Although enriched in normal intestines, the role of CD4+ Th17 cells in regulation of the host response to microbiota, and whether and how they contribute to intestinal homeostasis is still largely unknown. It is also unclear whether Th17 cells regulate intestinal IgA production, which is also abundant in the intestinal lumen and plays a crucial role as the first defense line in host response to microbiota. In this study, we found that intestinal polymeric Ig receptor (pIgR) and IgA production was impaired in T cell-deficient TCRβxδ−/− mice. Repletion of TCRβxδ−/− mice with Th17 cells from CBir1 flagellin TCR transgenic mice, which are specific for a commensal antigen, increased intestinal pIgR and IgA. The levels of intestinal pIgR and IgA in B6.IL-17 receptor (IL-17R−/−) mice were lower than wild-type mice. Treatment of colonic epithelial HT-29 cells with IL-17 increased pIgR expression. IL-17R−/− mice demonstrated systemic anti-microflora antibody response. Consistently, administering dextran sulfate sodium (DSS) to C57BL/6 mice after treatment with IL-17-neutralizing antibody resulted in more severe intestinal inflammation as compared to control antibody. Administering DSS to IL-17R−/− mice resulted in increased weight loss and more severe intestinal inflammation compared to wild-type mice, indicating a protective role of Th17 cells in intestinal inflammation. Individual mice with lower levels of pIgR and intestinal secreted IgA correlated with increased weight loss at the end of DSS administration. Collectively, our data reveal that microbiota-specific Th17 cells contribute to intestinal homeostasis by regulating intestinal pIgR expression and IgA secretion.