Interleukin-12 Converts Foxp3+ Regulatory T Cells to Interferon-γ-Producing Foxp3+ T Cells That Inhibit Colitis

BACKGROUND & AIMS: Regulatory T (Treg) cells are plastic, but the in vivo mechanisms by which they are converted into foxhead box p3 (Foxp3(+)) interferon (IFN)-gamma(+) T cells and whether these converted cells retain the ability to inhibit colitis are not clear. METHODS: Foxp3(+) Treg cells were generated by culture of naive CD4(+) T cells from Foxp3(GFP) CBir1 T-cell receptor (TCR) transgenic (Tg) (CBir1-Tg) mice, which are specific for CBir1 flagellin (an immunodominant microbiota antigen), with transforming growth factor-beta. Foxp3(GFP+) CBir1-Tg Treg cells were isolated by fluorescence-activated cell sorting and transferred into TCR beta x delta(-)/(-) mice. Colitis was induced by transfer of naive CBir1-Tg CD4(+) T cells into immunodeficient mice. RESULTS: Microbiota antigen-specific Foxp3(+) Treg cells were converted, in the intestine, to IFN-gamma(+) T-helper (Th)1 cells, interleukin (IL)-17(+) Th17 cells, and Foxp3(+) T cells that coexpress IFN-gamma and/or IL-17. Conversion of Treg cells into IFN-gamma-producing Th1 cells and Foxp3(+) IFN-gamma(+) T cells required innate cell production of IL-12 in the intestine; blocking IL-12 with an antibody inhibited their conversion to Th1 and Foxp3(+) IFN-gamma(+) T cells in the intestines of mice that were recipients of Treg cells. Addition of IL-12, but not IL-23, promoted conversion of Treg cells into Th1 and Foxp3(+) IFN-gamma(+) T cells, in vitro. Foxp3(+) IFN-gamma(+) T cells had regulatory activity because they suppressed proliferation of naive T cells, in vitro, and inhibited induction of colitis by microbiota antigen-specific T cells. IFN-gamma(+) Th1 cells were not converted into Treg cells; Foxp3(+)IFN-gamma(+) T cells differentiated into IFN-gamma(+) but not Foxp3(+) T cells. CONCLUSIONS: IL-12 promotes conversion of Treg cells into IFN-gamma-expressing cells; Foxp3(+)IFN-gamma(+) T cells retain their regulatory functions and develop during the transition of Foxp3(+) Treg cells into IFN-gamma(+) Th1 cells.