CD4+ T helper cells acquire effector phenotypes that promote specialized inflammatory responses. We show that the ETS family transcription factor, PU.1 was required for the development of an interleukin 9 (IL-9)-secreting subset of TH cells. Decreasing PU.1 expression either by conditional deletion in murine T cells or siRNA in human T cells impaired IL-9 production, while ectopic PU.1 expression promoted IL-9 production. Mice with PU.1-deficient T cells developed normal TH2 responses in vivo, but exhibited attenuated allergic pulmonary inflammation corresponding to decreased Il9 and chemokine expression in peripheral T cells and in lungs as compared to wild-type mice. Together, these data suggest a critical role for PU.1 in generating the TH9 phenotype and in the development of allergic inflammation.

Summary Transcriptional regulatory networks direct the development of specialized cell types. The transcription factors Stat4 and T-bet are required for the development of T helper 1 cells, although the hierarchy of activity by these factors has not been clearly defined. In this report we show that these factors are not in a linear pathway and that each factor plays a unique role in programming chromatin architecture for Th1 gene expression, with subsets of genes depending on Stat4, T-bet, or both for expression in Th1 cells. T-bet is not able to transactivate expression of Stat4-dependent genes in the absence of endogenous Stat4 expression. Thus, T-bet requires Stat4 to achieve complete Th1 fate determination.

IL-4 promotes the development of Th2 cells and allergic inflammation. In atopic dermatitis lesions, IL-4 decreases the expression of multiple genes associated with innate defense, including genes in the epidermal differentiation complex (EDC) that regulate epidermal barrier function. However, it is not clear whether IL-4 also contributes to homeostatic control of EDC genes. In this report, we demonstrate that expression of EDC genes and barrier function is increased in the absence of endogenous IL-4. Mice that express a constitutively active Stat6 (Stat6VT) are prone to the development of allergic skin inflammation and have decreased expression of EDC genes. IL-4 deficiency protects Stat6VT transgenic mice from the development of allergic skin inflammation and decreased recovery time in barrier function following skin irritation, with a concomitant increase in EDC gene expression. These data suggest that IL-4 plays an important role in regulating epidermal homeostasis and innate barrier function.

Signal Transducer and Activator of Transcription (STAT) family members direct the differentiation of T helper cells, with specific STAT proteins promoting distinct effector subsets. STAT6 is required for the development of T helper 2 (Th2) cells, whereas STAT3 promotes differentiation of Th17 and follicular helper T cell subsets. We demonstrated that STAT3 was also activated during Th2 cell development and was required for the expression of Th2-cell associated cytokines and transcription factors. STAT3 bound directly to Th2-cell associated gene loci and was required for the ability of STAT6 to bind target genes. In vivo, STAT3-deficiency in T cells eliminated the allergic inflammation in mice sensitized and challenged with ovalbumin, or transgenic for constitutively active STAT6. Thus, STAT3 cooperates with STAT6 in promoting Th2 cell development. These results demonstrate that differentiating T helper cells integrate multiple STAT protein signals during Th2 cell development.

IL-10 is a key regulatory cytokine produced by T lymphocytes. Although Th2 cells are a major source of IL-10, little is known about IL-10 gene regulation in Th2 cells. High levels of IL-10 mRNA transcription are induced in the Th2 clone D10 after PMA plus ionomycin (P/I) stimulation; however we found that the IL-10 promoter was not inducible by P/I in D10 cells. We therefore sought regulatory regions in the IL-10 gene that could promote P/I-activated transcription in Th2 cells. Two strong DNase I-hypersensitive sites (DHSSs) were identified in the IL-10 gene in mouse T cells, and conserved noncoding sequences (CNSs) between the mouse and human IL-10 genes were also identified. One IL-10 DHSS maps within or next to a highly conserved CNS region, CNS-3. The CNS-3 region contains an AP-1 site that binds JunB and c-Jun proteins specifically in Th2 cells and not in Th1 cells. The CNS-3 element activates transcription from the IL-10 promoter after P/I stimulation and is responsive to c-Jun and JunB. Retroviral mediated-expression of either c-Jun or JunB in primary T cells led to a large increase in IL-10 expression, and inhibition of AP-1 activity by a dominant negative form of c-Jun in primary T cells strongly repressed IL-10 expression. IFN-γ was relatively unaffected by modulations in AP-1 activity. These data indicate that we have identified a novel regulatory element that can specifically activate transcription of the IL-10 gene in Th2 cells via the AP-1/Jun pathway.

Background IL-9 is important for the growth and survival of mast cells. IL-9 is produced by T cells, NKT cells, mast cells, eosinophils, and innate lymphoid cells, although the cells required for mast cell accumulation during allergic inflammation remain undefined. Objective To elucidate the role of Th9 cells in promoting mast cell accumulation in models of allergic lung inflammation. Methods Adoptive transfer of OVA-specific Th2 and Th9 cells was used to assess the ability of each subset to mediate mast cell accumulation in tissues. Mast cell accumulation was assessed in wild type mice and mice with PU.1-deficient T cells subjected to acute and chronic models of allergic inflammation. Results Adoptive transfer experiments demonstrated that recipients of Th9 cells had significantly higher mast cell accumulation and expression of mast cell proteases as compared to control or Th2 recipients. Mast cell accumulation was dependent on IL-9, but not IL-13, cytokine required for many aspects of allergic inflammation. In models of acute and chronic allergic inflammation, decreased IL-9 levels in mice with PU.1-deficient T cells corresponded to diminished tissue mast cell numbers and expression of mast cell proteases. Mice with PU.1-deficient T cells have defects in IL-9 production from CD4+ T cells, but not NKT cells or innate lymphoid cells, suggesting a T helper cell-dependent phenotype. Rag1−/− mice subjected to a chronic model of allergic inflammation displayed reduced mast cell infiltration comparable to accumulation in mice with PU.1-deficient T cells, emphasizing the importance of IL-9 produced by T cells in mast cell recruitment. Conclusion Th9 cells are a major source of IL-9 in models of allergic inflammation and play an important role in mast cell accumulation and activation.

The transcriptional repressor Bcl6 is a critical arbiter of T helper cell fate, promoting the follicular helper (Tfh) lineage while repressing other T helper cell lineages. Bcl6-deficient (Bcl6-/-) mice develop a spontaneous and severe Th2-type inflammatory disease, thus warranting assessment of Bcl6 in Treg cell function. Bcl6-/- Tregs were competent at suppressing T cell proliferation in vitro and Th1-type colitogenic T cell responses in vivo. In contrast, Bcl6-/- Treg cells strongly exacerbated lung inflammation in a model of allergic airway disease, and promoted higher Th2 responses, including systemic up-regulation of microRNA-21. Further, Bcl6-/- Tregs were selectively impaired at controlling Th2 responses but not Th1 and Th17 responses, in mixed chimeras of Bcl6-/- bone marrow with Foxp3-/- bone marrow. Bcl6-/- Tregs displayed increased levels of the Th2 transcription factor Gata3 and other Th2 and Treg genes. Bcl6 potently repressed Gata3 transcriptional transactivation, providing a mechanism for the increased expression of Th2 genes by Bcl6-/- Tregs. Gata3 has a critical role in regulating Foxp3 expression and functional fitness of Tregs, however, the signal that regulates Gata3 and restricts its transactivation of Th2 cytokines in Tregs has remained unexplored. Our results identify Bcl6 as an essential transcription factor regulating Gata3 activity in Tregs. Thus, Bcl6 represents a crucial regulatory layer in the Treg functional program, required for specific suppression of Gata3 and Th2 effector responses by Tregs.

Recently, a new developmental pathway for CD4 T cells that is mediated by major histocompatibility complex class II–positive thymocytes was identified (Choi, E.Y., K.C. Jung, H.J. Park, D.H. Chung, J.S. Song, S.D. Yang, E. Simpson, and S.H. Park. 2005. Immunity. 23:387–396; Li, W., M.G. Kim, T.S. Gourley, B.P. McCarthy, D.B. Sant'angelo, and C.H. Chang. 2005. Immunity. 23:375–386). We demonstrate that thymocyte-selected CD4 (T-CD4) T cells can rapidly produce interferon γ and interleukin (IL) 4 upon in vivo and in vitro T cell receptor stimulation. These T-CD4 T cells appear to be effector cells producing both T helper type 1 (Th1) and Th2 cytokines, and they maintain a potential to produce Th2 cytokines under Th1-skewing conditions in a signal transducer and activator of transcription 6–independent manner. The IL-4 mRNA level is high in CD4 single-positive thymocytes if they are selected on thymocytes, which is at least partly caused by enhanced histone acetylation of the IL-4 locus. However, mice that can generate T-CD4 T cells showed attenuated immune responses in an allergen-induced airway inflammation model, suggesting a protective role for T-CD4 T cells during an airway challenge. Our results imply that this thymic selection pathway plays an important role in determining the effector function of the resulting CD4 cells and in regulating immune response.