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Uncommitted (naive) murine CD4+ T helper cells (Thp) can be induced to differentiate towards T helper 1 (Th1), Th2, Th17 and regulatory (Treg) phenotypes according to the local cytokine milieu. This can be demonstrated most readily both in vitro and in vivo in murine CD4+ T cells. The presence of interleukin (IL)-12 [signalling through signal transduction and activator of transcription (STAT)-4] skews towards Th1, IL-4 (signalling through STAT-6) towards Th2, transforming growth factor (TGF)-β towards Treg and IL-6 and TGF-β towards Th17. The committed cells are characterized by expression of specific transcription factors, T-bet for Th1, GATA-3 for Th2, forkhead box P3 (FoxP3) for Tregs and RORγt for Th17 cells. Recently, it has been demonstrated that the skewing of murine Thp towards Th17 and Treg is mutually exclusive. Although human Thp can also be skewed towards Th1 and Th2 phenotypes there is as yet no direct evidence for the existence of discrete Th17 cells in humans nor of mutually antagonistic development of Th17 cells and Tregs. There is considerable evidence, however, both in humans and in mice for the importance of interferon (IFN)-γ and IL-17 in the development and progression of inflammatory and autoimmune diseases (AD). Unexpectedly, some models of autoimmunity thought traditionally to be solely Th1-dependent have been demonstrated subsequently to have a non-redundant requirement for Th17 cells, notably experimental allergic encephalomyelitis and collagen-induced arthritis. In contrast, Tregs have anti-inflammatory properties and can cause quiescence of autoimmune diseases and prolongation of transplant function. As a result, it can be proposed that skewing of responses towards Th17 or Th1 and away from Treg may be responsible for the development and/or progression of AD or acute transplant rejection in humans. Blocking critical cytokines in vivo, notably IL-6, may result in a shift from a Th17 towards a regulatory phenotype and induce quiescence of AD or prevent transplant rejection. In this paper we review Th17/IL-17 and Treg biology and expand on this hypothesis. [ABSTRACT FROM AUTHOR]

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 (Treg) 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 Tregs and receptor-related orphan receptor (ROR)α and RORγt for Th17 cells. Tregs 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. Tregs are generated from naive precursors both in the thymus (natural, nTregs) and in the periphery (induced, iTregs). The highly suppressive function of Tregs enables them to control many inflammatory diseases in animals and makes them particularly attractive candidates for immunotherapy in humans. The stability of the Treg phenotype is therefore of paramount importance in this context. Recent descriptions of Treg biology have suggested that components of pathogens or inflammatory mediators may subvert the suppressive function of Tregs in order to allow propagation of adequate immune responses. Unexpectedly, however, a number of groups have now described conversion of Tregs 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 Treg to Th17 conversion and propose some potential mechanisms for this phenomenon. [ABSTRACT FROM AUTHOR]

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 Induction of interleukin-17 production by regulatory T cells. Clin Exp Immunol 2009; doi:10.1111/j.1365-2249.2009.04038.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 There has been a considerable amount of interest in the immunological community about new phenotypic subsets of CD4+ T cells, particularly cells that produce the cytokine interleukin (IL)-17 [named T helper type 17 (Th17) cells]. While the initial discovery of Th17 cells and the pathways that controlled their development was in the mouse, recent attention has shifted to the existence of these cells and the relevant upstream cytokine signals in humans. While it is clear that CD4+ T cells producing IL-17 exist in vivo, their relevance to disease pathogenesis is only just being understood. In this paper, we review the data regarding the generation of human Th17 cells in vitro and the evidence that this effector population is important in human disease states. [ABSTRACT FROM AUTHOR]