In line with this, we found that the combination of IL-12, IL-6 PD98059 clinical trial and TGF-β is able to induce Th1, Th17 and IFN-γ/IL-17A double-positive cells. One might easily envisage that these distinct cytokines are expressed under inflammatory conditions and induce the typical picture of distinct T helper effector lineages in vivo. The data described here show that plasticity, at

least on a population level, is common to Th17 and Th1 cells. Whether this plasticity occurs during natural conditions such as infections or autoimmunity needs to be defined. The data by O’Connor et al. 15 suggested that Th17-transfer EAE can only be found under circumstances where a part of the transferred population shifts toward IFN-γ-producing cells. This was not the case for Th1-transfer EAE. Our finding that in some of the highly pure transferred Th1 cell population expression of IL-17A was induced indicates that also a Th1–Th17 shift may play a role in Th1-transfer EAE. Future experiments using either IL-17A/F knockout

Th1 high throughput screening assay cells or IFN-γ or T-bet knockout Th17 cells for transfer EAE should clarify the role of the cytokine shift in EAE development. In a model for airway hyperresponsiveness, another group recently showed that a shift to IFN-γ expression is necessary to induce airway hyperresponsiveness, whereas IL-17A expression was necessary for neutrophil infiltration 39. In light of the beneficial effects of IFN-γ in EAE one might speculate whether the cytokine shift to IFN-γ expression may even have a certain protective role. Our finding that also highly pure Th1 cells are able to shift to cells that express both IFN-γ and IL-17A is new. We found these cells particularly in the mLN. Together with the finding that also Th17 cells recovered from the mLN contained

a large fraction of double-expressing cells, this indicates that the gut immune system creates Adenosine triphosphate a specific local milieu, which favors this Th1/Th17 dichotomous response. Potential mechanisms for the bias to coexpress IL-17 might be the local presence of CD103+ and CD103− mLN DC, which may favor under certain conditions the development of Th17 cells 40, 41. In our transfer experiments, the driving force of trans-differentiation in the lymphopenic environment might be homeostatic proliferation of the transferred cells. Evidence against that is a recent report demonstrating that shifting of Th17 cells to IFN-γ expression was independent of IL-7 blockage 33, which largely inhibited proliferation of the injected cells. Whether, and which, other factors present in the lymphocyte-deficient lymphoid compartments trigger the reprogramming of Th17 cell populations needs to be determined. In transfers to RAG1−/−, and more strikingly in transfer experiments using WT mice, we found a strong downregulation of cytokine expression of the donor cells.