Analysis of the repertoire and characteristics of Th1 enhancers in the absence of STAT1 or STAT4 revealed these interleukin-12 (IL-12) and interferon-γ cytokine receptor-activated ERFs to be required for almost 60% of Th1 enhancer activation. Notably, while TBET regulated the expression of a number

of Th1 genes, the levels of p300 at associated enhancers were largely independent of TBET. However, 17% of Th1 enhancer activation (p300 recruitment) was dependent on TBET. These data raise interesting questions about TBET’s mechanism of action at target GSI-IX regulatory DNA. Elegant studies from Weinmann and colleagues have demonstrated the potential for TBET to act through at least two separable mechanisms mapped to distinct protein domains – recruitment of an H3K4me2 methyltransferase and direct transactivation.[32] Therefore, it will be interesting to determine if those few Th1 enhancers that require TBET for activation rely primarily on the chromatin-modifying potential of TBET, whereas the genes whose expression is augmented by TBET, independent of extensive modification of enhancer characteristics,

rely more heavily on the transactivation domain and increased recruitment of the general transcription machinery. As in Th1 cells, it appears that Th2 cell enhancer activation is heavily reliant on ERFs, namely BAY 80-6946 price STAT6 downstream of IL-4R signalling. STAT6 was required for the activation of 77% of all Th2-specific enhancers.[13] Although, like TBET, GATA3 plays a minor role in enhancer activation, when over-expressed, it is sufficient for enhancer activation at about half of STAT6-dependent enhancers. In this context, it is interesting

to consider potential GATA3 dosage effects in chromatin regulation and target gene expression, and the possibility for GATA3 to function as a ‘pioneer’-like factor in some settings. In fact, during early T-cell development, GATA3 and PU.1 binding can precede full enhancer activation and gene expression in developing PRKACG thymocytes.[33] However, during the initial events of Th cell polarization, GATA3 and TBET play a less substantial role in nucleating chromatin alterations, activating enhancers, and influencing gene expression compared with STATs. Although representing a minority, it will be interesting to better understand the enhancers and genes dependent on MRFs for activation, both in terms of their potentially distinct chromatin characteristics and functional roles. Considering the relative function of ERFs and MRFs in Th cell differentiation, a study from Littman and colleagues thoroughly explored the transcriptional programme of Th17 cells as defined by five key transcription factors: basic leucine zipper transcription factor (BATF), IRF4, STAT3, cellular musculoaponeurotic fibrosarcoma oncogene homolog (cMAF) and RORγt.