Commensal A4 bacteria inhibit intestinal Th2-cell responses through induction of dendritic cell TGF-ß production.

It has been shown that while commensal bacteria promote Th1, Th17 and Treg cells in lamina propria (LP) in steady-state conditions, they suppress mucosal Th2 cells. However, it is still unclear whether there are specific commensal organisms down-regulating Th2 responses, and the mechanism involved. Here we demonstrate that commensal A4 bacteria, a member of the Lachnospiraceae family, which produce an immunodominant microbiota CBir1 antigen, inhibits LP Th2-cell development. When transferred into the intestines of RAG(-/-) mice, CBir1-specific T cells developed predominately towards Th1 cells and Th17 cells, but to a lesser extent into Th2 cells. The addition of A4 bacterial lysates to CD4(+) T-cell cultures inhibited production of IL-4. A4 bacteria stimulated dendritic cell production of TGF-ß, and blockade of TGF-ß abrogated A4 bacteria inhibition of Th2-cell development in vitro and in vivo. Collectively, our data show that A4 bacteria inhibit Th2-cell differentiation by inducing dendritic cell production of TGF-ß.

TGF-ß converts Th1 cells into Th17 cells through stimulation of Runx1 expression.

Differentiated CD4(+) T cells preserve plasticity under various conditions. However, the stability of Th1 cells is unclear, as is whether Th1 cells can convert into Th17 cells and thereby contribute to the generation of IFN-γ(+) IL-17(+) CD4(+) T cells, the number of which correlates with severity of colitis. We investigated whether IFN-γ(+) Th1 cells can convert into Th17 cells under intestinal inflammation and the mechanisms involved. IFN-γ(Thy1.1+) Th1 cells were generated by culturing naïve CD4(+) T cells from IFN-γ(Thy1.1) CBir1 TCR-Tg reporter mice, whose TCR is specific for an immunodominant microbiota antigen, CBir1 flagellin, under Th1 polarizing conditions. IFN-γ(Thy1.1+) Th1 cells induced colitis in Rag(-/-) mice after adoptive transfer and converted into IL-17(+) Th17, but not Foxp3(+) Treg cells in the inflamed intestines. TGF-ß and IL-6, but not IL-1ß and IL-23, regulated Th1 conversion into Th17 cells. TGF-ß induction of transcriptional factor Runx1 is crucial for the conversion, since silencing Runx1 by siRNA inhibited Th1 conversion into Th17 cells. Furthermore, TGF-ß enhanced histone H3K9 acetylation but inhibited H3K9 trimethylation of Runx1- and ROR-γt-binding sites on il-17 or rorc gene in Th1 cells. We conclude that Th1 cells convert into Th17 cells under inflammatory conditions in intestines, which is possibly mediated by TGF-ß induction of Runx1.