OTHER ARTICLES PUBLISHED IN THIS MINI-REVIEW SERIES ON Th17 CELLS
Function and regulation of human T helper 17 cells in health and disease. Clin Exp Immunol 2009; doi:10.1111/j.1365-2249.2009.04037.x
Are T helper 17 cells really pathogenic in autoimmunity? Clin Exp Immunol 2009; doi:10.1111/j.1365-2249.2009.04039.x
CD4⁺ T helper cells: functional plasticity and differential sensitivity to regulatory T cell-mediated regulation. Clin Exp Immunol 2009; doi:10.1111/j.1365-2249.2009.04040.x
Development of mouse and human T helper 17 cells. Clin Exp Immunol 2009; doi:10.1111/j.1365-2249.2009.04041.x

Uncommitted (naive) CD4⁺ T helper cells (Thp) can be induced to differentiate to specific lineages according to the local cytokine milieu, towards T helper type 1 (Th1), Th2, Th17 and regulatory T cell (T_reg) phenotypes in a mutually exclusive manner. Each phenotype is characterized by unique signalling pathways and expression of specific transcription factors, notably T-bet for Th1, GATA-3 for Th2, forkhead box P3 (FoxP3) for T_regs and receptor-related orphan receptor (ROR)α and RORγt for Th17 cells. T_regs and Th17 cells have been demonstrated to arise from common precursors in a reciprocal manner based on exposure to transforming growth factor (TGF)-β or TGF-β plus interleukin (IL)-6 and carry out diametrically opposing functions, namely suppression or propagation of inflammation, respectively. However, while epigenetic modifications in Th1 and Th2 differentiated cells prevents their conversion to other phenotypes, Th17 cells generated in vitro using TGF-β and IL-6 are unstable and can convert to other phenotypes, especially Th1, both in vitro and in vivo. T_regs are generated from naive precursors both in the thymus (natural, nT_regs) and in the periphery (induced, iT_regs). The highly suppressive function of T_regs enables them to control many inflammatory diseases in animals and makes them particularly attractive candidates for immunotherapy in humans. The stability of the T_reg phenotype is therefore of paramount importance in this context. Recent descriptions of T_reg biology have suggested that components of pathogens or inflammatory mediators may subvert the suppressive function of T_regs in order to allow propagation of adequate immune responses. Unexpectedly, however, a number of groups have now described conversion of T_regs to the Th17 phenotype induced by appropriate inflammatory stimuli. These observations are particularly relevant in the context of cell therapy but may also explain some of the dysregulation seen in autoimmune diseases. In this paper, we review T_reg to Th17 conversion and propose some potential mechanisms for this phenomenon.