In recent years T cell biology has been enriched and enlivened

by the description of two further subsets. Interleukin (IL)-17-producing T cells were identified as important drivers of autoimmune pathology, forcing the re-evaluation of the role of Th1 cells in models of autoimmunity [2–4]. Elucidation of the factors promoting development of these Th17 cells [transforming growth factor (TGF)-β, IL-6 and IL-21][5–8] and the regulators of their transcriptional profile (RORγt and RORα[9,10]) established Th17 cells as a third effector T cell subset (reviewed in [11]). The Sirolimus concentration three effector subsets appear to have evolved to cope with the threat posed by distinct classes of pathogen. Th1 cells are associated classically with intracellular bacteria and viral infections, Th2 responses are elicited by parasitic helminths, MK-8669 clinical trial while Th17 responses are protective against certain extracellular bacterial and fungal infections [11]. Dysregulated Th2 responses promote the development of allergy and asthma, while uncontrolled Th1 and Th17 responses can result in autoimmune inflammation; therefore, the actions of these effector CD4+ cells need to be controlled strictly. The

identification of a minor subpopulation of CD4+ cells capable of preventing the development of autoimmunity [12,13] revolutionized our concept of T cell regulation. Identification of forkhead box P3 (FoxP3) as the lineage-specific transcriptional Montelukast Sodium regulator determining this suppressive

phenotype [14,15] confirmed the status of FoxP3+ regulatory T cells (Tregs), as distinct from previously described effector subsets [16]. In the scurfy mouse, a frameshift mutation in FoxP3 results in production of non-functional product and a lethal lymphoproliferative disorder [17,18] caused by over-activation of CD4+ T cells [19]. Similarly, the human condition immune dysregulation, polyendocrinopathy, enteropathy, X-linked syndrome (IPEX) is caused by mutations of FoxP3 [20]. ‘Natural’ Treg (nTreg) provide the thymically derived FoxP3+ cells that prevent spontaneous inflammatory disease and provide the Treg population that are assessed in vitro when using naive mice [21]. In addition, T cell receptor (TCR) stimulation of naive T cells in the presence of TGF-β can drive de novo expression of FoxP3 in uncommitted naive T cells, providing a population of ‘induced’ Tregs (iTregs). Antigenic stimulation, therefore, can drive the polarization of naive T cells to become Th1, Th2, Th17 and/or iTreg cells, in addition to the activation of antigen-responsive nTregs. The balance of (and timing in the appearance of) these different populations is dependent upon the nature of the antigen presentation and the cytokine milieu.