Cutting edge: Regulatory T cells induce CD4(+)CD25(-)Foxp3(-) T cells or are self-induced to become Th17 cells in the absence of exogenous TGF-beta

Recent studies have shown that TGF-beta together with IL-6 induce the differentiation of IL-17-producing T cells (Th17) T cells. We therefore examined whether CD4(+)CD25(+)Foxp3(+) regulatory T cells, i.e., cells previously shown to produce TGF-beta, serve as Th 17 inducers. We found that upon activation purified CD25(+) T cells (or sorted GFP(+) T cells obtained from Foxp3-GFP knockin mice) produce high amounts of soluble TGF-beta and when cultured with CD4(+) CD25(+)FoxP3(-) T cells in the presence of IL-6 induce the latter to differentiate into Th17 cells. Perhaps more importantly, upon activation, CD4+ CD25(+)Foxp3(+) (GFP(+)) T cells themselves differentiate into Th17 cells in the presence of IL-6 (and in the absence of exogenous TGF-beta). These results indicate that CD4+ CD25+Foxp3+ regulatory T cells can function as inducers of Th17 cells and can differentiate into Th17 cells. They thus have important implications to our understanding of regulatory T cell function and their possible therapeutic use.