Sarita Sehra

The transcription factor PU.1 is required for the development of IL-9-producing T cells and allergic inflammation

CD4+ helper T 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 helper T cells. Decreasing PU.1 expression either by conditional deletion in mouse T cells or the use of small interfering RNA in human T cells impaired IL-9 production, whereas ectopic PU.1 expression promoted IL-9 production. Mice with PU.1-deficient T cells developed normal T helper type 2 (TH2) responses in vivo but showed attenuated allergic pulmonary inflammation that corresponded to lower expression of Il9 and chemokines in peripheral T cells and in lungs than that of wild-type mice. Together our data suggest a critical role for PU.1 in generating the IL-9-producing (TH9) phenotype and in the development of allergic inflammation.

Signal Transducer and Activator of Transcription 4 Is Required for the Transcription Factor T-bet to Promote T Helper 1 Cell-Fate Determination

Transcriptional regulatory networks direct the development of specialized cell types. The transcription factors signal tranducer and activator of transcription 4 (Stat4) and T-bet are required for the interleukin-12 (IL-12)-stimulated development of T helper 1 (Th1) cells, although the hierarchy of activity by these factors has not been clearly defined. In this report, we show that these factors did not function in a linear pathway and that each factor played 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 was not able to transactivate expression of Stat4-dependent genes in the absence of endogenous Stat4 expression. Thus, T-bet requires Stat4 to achieve complete IL-12-dependent Th1 cell-fate determination. These data provide a basis for understanding how transiently activated and lineage-specific transcription factors cooperate in promoting cellular differentiation.

Regulation of IL-10 gene expression in Th2 cells by Jun proteins.

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.

IL-4 Regulates Skin Homeostasis and the Predisposition toward Allergic Skin Inflammation

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.

TH9 cells are required for tissue mast cell accumulation during allergic inflammation.

BACKGROUND IL-9 is important for the growth and survival of mast cells. IL-9 is produced by T cells, natural killer T cells, mast cells, eosinophils, and innate lymphoid cells, although the cells required for mast cell accumulation during allergic inflammation remain undefined. OBJECTIVE We sought to elucidate the role of TH9 cells in promoting mast cell accumulation in models of allergic lung inflammation. METHODS Adoptive transfer of ovalbumin-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 compared with control or TH2 recipients. Mast cell accumulation was dependent on IL-9, but not IL-13, a 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 natural killer T cells or innate lymphoid cells, suggesting a TH cell-dependent phenotype. Rag1(-/-) mice subjected to a chronic model of allergic inflammation displayed reduced mast cell infiltration comparable with 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.

Bcl6 Controls the Th2 Inflammatory Activity of Regulatory T Cells by Repressing Gata3 Function

The transcriptional repressor Bcl6 is a critical arbiter of Th cell fate, promoting the follicular Th lineage while repressing other Th cell lineages. Bcl6-deficient (Bcl6−/−) mice develop a spontaneous and severe Th2-type inflammatory disease, thus warranting assessment of Bcl6 in regulatory T cell (Treg) function. Bcl6−/− Tregs were competent at suppressing T cell proliferation in vitro and Th1-type colitogenic T cell responses in vivo. In contrast, Bcl6−/− Tregs strongly exacerbated lung inflammation in a model of allergic airway disease and promoted higher Th2 responses, including systemic upregulation 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 that is required for specific suppression of Gata3 and Th2 effector responses by Tregs.

Periostin Regulates Goblet Cell Metaplasia in a Model of Allergic Airway Inflammation

Periostin is a 90-kDa member of the fasciclin-containing family and functions as part of the extracellular matrix. Periostin is expressed in a variety of tissues and expression is increased in airway epithelial cells from asthmatic patients. Recent studies have implicated a role for periostin in allergic eosinophilic esophagitis. To further define a role for periostin in Th2-mediated inflammatory diseases such as asthma, we studied the development of allergic pulmonary inflammation in periostin-deficient mice. Sensitization and challenge of periostin-deficient mice with OVA resulted in increased peripheral Th2 responses compared with control mice. In the lungs, periostin deficiency resulted in increased airway resistance and significantly enhanced mucus production by goblet cells concomitant with increased expression of Gob5 and Muc5ac compared with wild type littermates. Periostin also inhibited the expression of Gob5, a putative calcium-activated chloride channel involved in the regulation of mucus production, in primary murine airway epithelial cells. Our studies suggest that periostin may be part of a negative-feedback loop regulating allergic inflammation that could be therapeutic in the treatment of atopic disease.

IL-4 Is a Critical Determinant in the Generation of Allergic Inflammation Initiated by a Constitutively Active Stat6

IL-4 is required for the pathogenesis of atopic diseases and immune regulation. Stat6 is critical for IL-4-induced gene expression and Th cell differentiation. Recently, we have generated mice expressing a mutant Stat6 (Stat6VT) under control of the CD2 locus control region that is transcriptionally active independent of IL-4 stimulation. To determine whether active Stat6 in T cells is sufficient to alter immune regulation in vivo, we mated Stat6VT transgenic mice to IL-4-deficient mice. Stat6VT expression in IL-4-deficient lymphocytes was sufficient to alter lymphocyte homeostasis and promote Th2 differentiation in vitro. HyperTh2 levels in Stat6 transgenic mice correlated with an atopic phenotype that manifested as blepharitis and pulmonary inflammation with a high level of eosinophilic infiltration. In the absence of endogenous IL-4, Stat6VT transgenic mice were protected from allergic inflammation. Thus, in mice with hyperTh2 immune responses in vivo, IL-4 is a critical effector cytokine.

Signal transducer and activator of transcription 4 limits the development of adaptive regulatory T cells

Summary T‐cell responses to a cytokine milieu instruct the development of multiple effector phenotypes. While transforming growth factor‐β1 (TGF‐β1) inhibits the development of T helper type 1 (Th1) and Th2 cells, we demonstrate that like interleukin‐6 (IL‐6) and IL‐4, IL‐12 can inhibit the development of TGF‐β1‐induced Foxp3‐expressing adaptive T regulatory (aTreg) cells. Signal transducer and activator of transcription 4 (STAT4) is critical for the response to IL‐12, although there is a parallel pathway involving T box expressed in T cells (T‐bet), and cells from mice double‐deficient in STAT4 and T‐bet are refractory to the inhibition of aTreg‐cell development by IL‐12. While the ability of these cytokines to promote Th differentiation may contribute to this effect, we observe that culture with IL‐12, or other instructive cytokines, results in an increase in repressive chromatin modifications at the Foxp3 locus that limit STAT5 binding to Foxp3, without observed effects on IL‐2 signalling pathways. In a model of allergic lung inflammation there are increased percentages of Treg cells in the lungs of Stat4−/− mice, compared with wild‐type mice, and increases in Treg cells correlate with decreased allergic inflammation. Overall, these results suggest an important role for STAT4 in regulating Treg‐cell development.

Airway IgG counteracts specific and bystander allergen-triggered pulmonary inflammation by a mechanism dependent on Fc gamma R and IFN-gamma.

Besides IgE, the Ab isotype that gives rise to sensitization and allergic asthma, the immune response to common inhalant allergens also includes IgG. Increased serum titers of allergen-specific IgG, induced spontaneously or by allergen vaccination, have been implicated in protection against asthma. To verify the interference of topical IgG with the allergen-triggered eosinophilic airway inflammation that underlies asthma, sensitized mice were treated by intranasal instillation of specific IgG, followed by allergen challenge. This treatment strongly reduced eosinophilic inflammation and goblet cell metaplasia, and increased Th1 reactivity and IFN-γ levels in bronchoalveolar lavage fluid. In contrast, inflammatory responses were unaffected in IFN-γ-deficient mice or when applying F(ab′)2. Although dependent on specific allergen-IgG interaction, inflammation triggered by bystander allergens was similarly repressed. Perseverance of inflammation repression, apparent after secondary allergen challenge, and increased allergen capture by alveolar macrophages further characterized the consequences of topical IgG application. These results assign a novel protective function to anti-allergen IgG namely at the local level interference with the inflammatory cascade, resulting in repression of allergic inflammation through an FcγR- and IFN-γ-dependent mechanism. Furthermore, these results provide a basis for topical immunotherapy of asthma by direct delivery of anti-allergen IgG to the airways.