Most Tregs are born in the thymus and probably reflect a developmental pathway that can be taken when maturing thymocytes are activated by particular self-pMHC. Additionally, Tregs can be generated peripherally by stimulating the cells with high levels of cytokine TGFbeta. Research on natural (thymus-derived) and induced Treg cells has been hampered by the lack of a reliable surface marker uniquely identifying
Tregs. Currently, the transcription factor FoxP3 is the only reliable marker for Tregs [10, 12]. Mapping the target genes of FoxP3 indicated that this transcription factor fixes the phenotype of the cell by enforcing Treg-specific epigenetic this website changes [13, 14]. Mutations in the FoxP3 gene are associated with generalized autoimmunity, causing the scurfy phenotype in mice and IPEX syndrome in humans [15, 16]. Over the past decade, several other Th-cell phenotypes have been described (Figure 1). Th17 cells produce enhanced levels of IL17 and are implicated in many autoimmune diseases as well as antimicrobial defence [17, 18]. Several master transcription factors have been suggested for this Th-cell phenotype, including Rorgt, Rora, Ahr and Batf [19-22]. Th22 cells produce IL22 that is thought to play a role in epidermal and mucosal immunity [23, 24]. Th22 cells have been suggested find more to resemble Th17 and perhaps Th1 cells, but are typically considered
to be a separate Th-cell phenotype [25, 26]. IL9-producing Th9 cells have been implicated in allergy and are sometimes considered to be related to Th2 cells due to the fact that both of these phenotypes produce IL4 and share Gata3 as a master transcription factor [27-30]. Additionally, RBPj and Smad have been associated with Th9 cells and IL9 expression [31, 32]. Th9 and Th17 can induce pathology in the experimental autoimmune encephalitis, the mouse model for multiple sclerosis  and respiratory syncytial virus (RSV) infection . Furthermore,
hyper IgE (Job’s) syndrome in humans is associated with a lack of Th17 cells . Follicular helper T cells are a subset of helper cells that specifically provide costimulation to B cells in Mannose-binding protein-associated serine protease germinal centres. Although they do not produce the characteristic cytokines of the other Th-cell phenotypes, they produce IL21 as a growth factor for B cells [36, 37]. Surprisingly, there is evidence that Th2 cells can convert to Tfh cells when they enter germinal centres , suggesting that Th-cell phenotypes are not stable and can be modified by the local tissue environment . Transcriptional repressor Bcl6 is associated with Tfh cells . When the phenotype-driving master transcription factors are expressed, the relevant cytokine genes are derepressed by epigenetic modification such as DNA demethylation. Cell division has been suggested to play an important role in derepressing cytokine loci, because the duplication of the DNA has a ‘thinning’ effect on the density of epigenetic marks.