There has been much interest in the differentiation of Th17 cells from naive precursors and it is now understood that

Th17 commitment is linked reciprocally to that of Tregs. While transforming growth factor (TGF)-β differentiates murine naive CD4+ T cells to Tregs, the presence of IL-6, in addition to TGF-β, skews the commitment towards Th17 [62–64]. There is greater debate regarding human Th17 differentiation. These pathways of differentiation are discussed Kinase Inhibitor Library in more depth in the review by de Jong and Lord in this series [65]. However, it is important to note that the evidence indicates that Th17 cells are unstable or that the phenotype may be an intermediately differentiated state. In particular, bulk CD4+ T cells primed to produce IL-17 by polyclonal activation (anti-CD3 and anti-CD28) in the presence of IL-23 can be redirected away from IL-17 production towards a Th1 phenotype by subsequent TCR activation in the absence of IL-23 or by induction of the Th1 specifying transcription factor, T-bet, suggesting that the Th17 state may be either unstable or a non-terminally

differentiated one [66]. This is corroborated by in vivo murine data demonstrating that the adoptive transfer of highly purified islet-specific Th17 cells, devoid of IFN-γ producing populations, causes type 1 diabetes mellitus in recipient mice through the conversion of the check details Th17 population to a Th1 phenotype (as characterized by cytokine and transcription factor profiles) [67]. This is also observed in experimental autoimmune encephalomyelitis (EAE) models, where fate-mapping of adoptively transferred Th17-skewed cells reveals a significant conversion in vivo to the Th1 lineage [68]. All these findings suggest that there is considerably more plasticity among ‘skewed, lineage-committed’ Th17 cells than thought previously, and contrasts

with Th1 and Th2 lineages which Farnesyltransferase are resistant to further differentiation as a result of epigenetic modifications of gene loci associated with the reciprocal lineage [69], ensuring that Th1 and Th2 phenotypes remain stably expressed. A number of groups, including our own, have investigated the subversion of Tregs by inflammatory cytokines in both mouse and man and found that, in addition to reduced suppressive activity on target cells, inflammatory cytokines direct Tregs to differentiate into the Th17 lineage and produce IL-17. That this conversion is not the result of outgrowth of a contaminating Th17 precommitted population is indicated by the demonstration of double-positive cells for the Treg transcription factor FoxP3 and IL-17 (our unpublished observations), which is suggestive of an intermediate, transitional, stage. The conversion of Tregs to Th17 cells has now been reported by a number of groups, in both mouse and human, as shown in Table 1[70–79], albeit with a very interesting difference.