Recent evidence suggests that similar mechanisms may regulate the commitment of Thp between Treg and Th17. In human cells, FoxP3 exists in two separate but equally expressed isoforms: one (FoxP3), which is encoded by a full length mRNA and the other a truncated form lacking exon 2 (FoxP3Δ2), which is coded by a splice variant mRNA [104,109]. Tregs, perhaps unexpectedly, also express Th17-specifying transcription factors, notably RORα[110] and RORγt [111]. However, co-immunoprecipitation experiments have shown that FoxP3 binds to RORα and RORγt and inhibits their biological activity

in a dose-dependent fashion [110,111]. This interaction is mediated through a (LxxLL) motif in the FoxP3 second exon; as expected, the FoxP3Δ2 isoform is unable to bind RORα or Sirolimus Belnacasan RORγt [110,111]. A similar interaction has subsequently been described, by the same group and others, in murine cells. Specifically, both FoxP3 and RORγt are co-expressed

in naive CD4+ T cells exposed to TGF-β, where FoxP3 inhibits RORγt directly through a physical interaction, repressing the Th17 programme [111]. In these experiments exposure of Thp to TGF-β leads to rapid induction of RORγt [92], but the binding of RORγt to the IL-17 promoter is suppressed by interaction with FoxP3 [112]. Upon addition of exogenous IL-6 or IL-21, the inhibitory effect of FoxP3 on IL-17 induction is circumvented [111] and FoxP3 levels are reduced [112]. The interaction between FoxP3 and RORγt

in murine cells is also dependent upon the second exon of FoxP3 [111,112]. These observations have also been confirmed by another, independent group [74]. These interactions can, in part, explain the conversion of Tregs to Th17, at least in mice. While TGF-β induces both FoxP3 and RORγt expression, IL-6 does not alter expression of RORγt but inhibits FoxP3. As a result, exposure of Tregs to IL-6 down-modulates FoxP3 preferentially and reduces the PDK4 physical inhibition of RORγt, permitting binding to the IL-17 gene promoter. In addition, very recent murine data suggest that IL-1 regulates expression of RORγt [79]. The Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathway is a receptor-coupled signal transduction mechanism linking cytokine–receptor interactions to gene expression. There are seven STAT (STAT1-4, 5A, 5B and 6) and four JAK [JAK1-3 and TYK2 (tyrosine kinase 2)] proteins in humans (reviewed in [113]). Specific JAKs are associated with the cytoplasmic tails of multimeric cytokine receptors, and are activated upon ligand-induced receptor oligomerization [113,114]. Activated JAKs phosphorylate specific tyrosine residues on cytoplasmic tails of their associated cytokine receptors, creating docking sites for the SH2 (Src-homology-2) domain of STAT proteins, and then activate the docked STATs through tyrosine phosphorylation.