Despite comparably low levels GSK126 in vivo in Th1 cells, SOCS3 and SOCS5 also regulate Th1 differentiation. Indeed through binding to the IL-12Rβ2 chain, SOCS3 prevents STAT4 activation (Fig. 2) and constitutive expression of SOCS3 in CD4+

T cells was shown to hinder Th1 polarization.33 Consistent with these findings, up-regulation of SOCS3 by IL-2 was found to prevent acute graft-versus-host disease by inhibiting the Th1 response.34 However, SOCS3 deletion in T cells also resulted in decreased Th1 differentiation, although this was proposed to be indirect. Indeed, increased IL-10 and transforming growth factor (TGF-β) secretion was also observed in these cells, perhaps suggesting that SOCS3 may limit Treg see more cell development.35 The role of SOCS5 is more controversial. Indeed, despite being highly expressed in Th1 cells,36 disruption of the socs5 gene does not affect the ability of cells

to differentiate either towards Th1 or Th2.37 Over-expression of SOCS5 in T cells is associated with increased levels of IL-12, IFN-γ and tumour necrosis factor-α in a mouse model of septic peritonitis,38 but this could be indirectly the result of enhanced macrophage activity, possibly through increased IFN-γ secretion by T cells.36,39 Finally, Th1 differentiation does not seem to be affected by higher levels of SOCS5,36 and so the exact role of SOCS5 in Th1 differentiation remains unclear. By regulating IL-12-mediated STAT4 activation and IFN-γ-mediated STAT1 signals, SOCS1, VEGFR inhibitor SOCS3 and SOCS5 certainly modulate the development

of Th1 cells, although the role of individual SOCS is, even at this point, far from clear. Our current understanding is summarized in Table 2. The Th2 cells secrete large amounts IL-4, IL-5, IL-9 and IL-13, and consequently promote the humoral response but also drive IgE class switching and allergic disease.40 The commitment of Th2 cells is essentially driven by IL-4, which activates both JAK1 and JAK3 and the transcription factor STAT641 (Fig. 3). Not surprisingly, STAT6 plays a key role in the acquisition of the Th2 phenotype. In particular STAT6 directly controls the expression of Th2 lineage master regulator, GATA3,42 and enforced expression of STAT6 in Th1 cells re-establishes their ability to secrete IL-4 and IL-5, while repressing IFN-γ and IL-12Rβ2 expression.42 STAT6-deficient T cells fail to polarize towards Th2 in vitro and in vivo,43–45 but the absence of STAT6 does not affect the emergence of Th2 cells in response to Nippostrongylus brasiliensis or Schistosoma mansoni challenge,46–48 which probably reflects the fact that STAT6 does not directly regulate the il4 gene. Instead, induction of IL-4 is controlled by GATA-3, which suggests that STAT6 essentially acts by up-regulating GATA-3 levels, although STAT6 seems to modify the chromatin structure of the Rad50 gene, which may allow optimal transcription of the il4 and il13 genes.