PU.1 expression delineates heterogeneity in primary Th2 cells

GATA-binding protein-3 regulates T helper type 2 cytokine and ifng loci through interaction with metastasis-associated protein 2

GATA-binding protein-3 (GATA-3) regulates the T helper type 2 (Th2) cytokine locus through induction of chromatin remodelling. However, the molecular mechanism for this is poorly understood. To understand this mechanism better, we screened GATA-3 interacting proteins using affinity purification and mass spectrometry. We found that GATA-3 bound to metastasis-associated protein 2 (MTA-2), a component of the NuRD chromatin remodelling complex. GATA-3 and MTA-2 in turn bound to several regulatory regions of the Th2 cytokine locus and the ifng promoter. Cell transfection assay showed that MTA-2 acted as an antagonist with GATA-3 in the expression of Th2 cytokines, but co-operated with GATA-3 in the repression of the ifng gene expression. These results suggest that GATA-3 interacts with MTA-2 to co-ordinately regulate Th2 cytokine and ifng loci during T helper cell differentiation.

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

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 T_H 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 T_H2 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 T_H9 phenotype and in the development of allergic inflammation.

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.

Downregulation of PU.1 leads to decreased expression of Dectin-1 in alveolar macrophages during Pneumocystis pneumonia

Dectin-1 is an important macrophage phagocytic receptor recognizing fungal beta-glucans. In this study, the mRNA levels of the Dectin-1 gene were found to be decreased by 61% in alveolar macrophages (AMs) from Pneumocystis-infected mice. The expression of Dectin-1 protein on the surface of these cells was also significantly decreased. By fluorescence in situ hybridization, mRNA expression levels of the transcription factor PU.1 were also found to be significantly reduced in AMs from Pneumocystis-infected mice. Electrophoretic mobility shift assay showed that PU.1 protein bound Dectin-1 gene promoter. With a luciferase reporter gene driven by the Dectin-1 gene promoter, the expression of the PU.1 gene in NIH 3T3 cells was found to enhance the luciferase activity in a dose-dependent manner. PU.1 expression knockdown by small interfering RNA (siRNA) caused a 63% decrease in Dectin-1 mRNA level and 40% decrease in protein level in AMs. Results of this study indicate that downregulation of PU.1 during Pneumocystis pneumonia leads to decreased expression of Dectin-1 in AMs.

Heterogeneity and plasticity of T helper cells

CD4 T helper (Th) cells play critical roles in adaptive immune responses. They recruit and activate other immune cells including B cells, CD8 T cells, macrophages, mast cells, neutrophils, eosinophils and basophils. Based on their functions, their pattern of cytokine secretion and their expression of specific transcription factors, Th cells, differentiated from naïve CD4 T cells, are classified into four major lineages, Th1, Th2, Th17 and T regulatory (Treg) cells, although other Th lineages may exist. Subsets of the same lineage may express different effector cytokines, reside at different locations or give rise to cells with different fates, whereas cells from different lineages may secrete common cytokines, such as IL-2, IL-9 and IL-10, resulting in massive heterogeneity of the Th cell population. In addition, the pattern of cytokine secretion may switch from that of one lineage toward another under certain circumstances, suggesting that Th cells are plastic. Tregs are also more heterogeneous and plastic than were originally thought. In this review, we summarize recent reports on heterogeneity and plasticity of Th cells, and discuss potential mechanisms and implications of such features that Th cells display.

Differentiation of effector CD4 T cell populations (*)

CD4 T cells play critical roles in mediating adaptive immunity to a variety of pathogens. They are also involved in autoimmunity, asthma, and allergic responses as well as in tumor immunity. During TCR activation in a particular cytokine milieu, naive CD4 T cells may differentiate into one of several lineages of T helper (Th) cells, including Th1, Th2, Th17, and iTreg, as defined by their pattern of cytokine production and function. In this review, we summarize the discovery, functions, and relationships among Th cells; the cytokine and signaling requirements for their development; the networks of transcription factors involved in their differentiation; the epigenetic regulation of their key cytokines and transcription factors; and human diseases involving defective CD4 T cell differentiation.

GATA3 in development and cancer differentiation: cells GATA have it!

There is increasing evidence that the numerous mechanisms that regulate cell differentiation during normal development are also involved in tumorigenesis. In breast cancer, differentiation markers expressed by the primary tumor are routinely profiled to guide clinical decisions. Indeed, numerous studies have shown that the differentiation profile correlates with the metastatic potential of tumors. The transcription factor GATA3 has emerged recently as a strong predictor of clinical outcome in human luminal breast cancer. In the mammary gland, GATA3 is required for luminal epithelial cell differentiation and commitment, and its expression is progressively lost during luminal breast cancer progression as cancer cells acquire a stem cell-like phenotype. Importantly, expression of GATA3 in GATA3-negative, undifferentiated breast carcinoma cells is sufficient to induce tumor differentiation and inhibits tumor dissemination in a mouse model. These findings demonstrate the exquisite ability of a differentiation factor to affect malignant properties, and raise the possibility that GATA3 or its downstream genes could be used in treating luminal breast cancer. This review highlights our recent understanding of GATA3 in both normal mammary development and tumor differentiation.

PU.1 regulates TCR expression by modulating GATA-3 activity

The Ets transcription factor PU.1 is a master regulator for the development of multiple lineages during hematopoiesis. The expression pattern of PU.1 is dynamically regulated during early T lineage development in the thymus. We previously revealed that PU.1 delineates heterogeneity of effector Th2 populations. In this study, we further define the function of PU.1 on the Th2 phenotype using mice that specifically lack PU.1 in T cells using an lck-Cre transgene with a conditional Sfpi1 allele (Sfpi1(lck-/-)). Although deletion of PU.1 by the lck-Cre transgene does not affect T cell development, Sfpi1(lck-/-) T cells have a lower activation threshold than wild-type T cells. When TCR engagement is limiting, Sfpi1(lck-/-) T cells cultured in Th2 polarizing conditions secrete higher levels of Th2 cytokines and have greater cytokine homogeneity than wild-type cells. We show that PU.1 modulates the levels of TCR expression in CD4(+) T cells by regulating the DNA-binding activity of GATA-3 and limiting GATA-3 regulation of TCR gene expression. GATA-3-dependent regulation of TCR expression is also observed in Th1 and Th2 cells. In CD4(+) T cells, PU.1 expression segregates into subpopulations of cells that have lower levels of surface TCR, suggesting that PU.1 contributes to the heterogeneity of TCR expression. Thus, we have identified a mechanism whereby increased GATA-3 function in the absence of the antagonizing activity of PU.1 leads to increased TCR expression, a reduced activation threshold, and increased homogeneity in Th2 populations.

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

T-cell responses to a cytokine milieu instruct the development of multiple effector phenotypes. While transforming growth factor-beta(1) (TGF-beta(1)) 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-beta(1)-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.

Negative regulation of Stat3 by activating PTPN11 mutants contributes to the pathogenesis of Noonan syndrome and juvenile myelomonocytic leukemia

Noonan syndrome (NS) is an autosomal dominant congenital disorder characterized by multiple birth defects including heart defects and myeloproliferative disease (MPD). Approximately 50% of NS patients have germline gain-of-function mutations in PTPN11, which encodes the protein-tyrosine phosphatase, Shp2. We provide evidence that conditional ablation of Stat3 in hematopoietic cells and cardiac valvular tissues leads to myeloid progenitor hyperplasia and pulmonary stenosis due to the leaflet thickening, respectively. Consistently, STAT3 activation is significantly compromised in peripheral blood cells from NS patients bearing Shp2-activating mutations. Biochemical and functional analyses demonstrate that activated Shp2 is able to down-regulate Tyr(P)-Stat3 and that constitutively active Stat3 rescues activating mutant Shp2-induced granulocyte-macrophage colony-stimulating factor hypersensitivity in bone marrow cells. Collectively, our work demonstrates that Stat3 is an essential signaling component potentially contributing to the pathogenesis of NS and juvenile myelomonocytic leukemia caused by PTPN11 gain-of-function mutations.

BCL6 cooperates with CD40 stimulation and loss of p53 function to rapidly transform primary B cells

The BCL6 transcriptional repressor protein has been shown to promote B-cell lymphoma in transgenic mouse models. The mechanism by which BCL6 transforms primary B cells is unclear, although repression of the p53 tumor suppressor is thought to play a role. Here, we showed that BCL6 has critical oncogene functions that are independent of p53 repression. We found that BCL6 cooperates with constitutive CD40 signaling to rapidly transform p53-deficient primary mouse B cells in vitro. Constitutive CD40 signaling alone does not transform p53-deficient B cells, indicating that BCL6 acts specifically as an immortalizing oncogene in this system. The BCL6 transformed B cells are polyclonal and form polyclonal tumors. At the initiation of the cultures, BCL6 does not significantly alter cell cycle progression, but it does promote increased cell survival. Early cultures of BCL6-expressing B cells exhibited marked repression of ATR and p27kip1 but not other BCL6 target genes, suggesting that the ATR and p27kip1 genes have key early roles in mediating BCL6 transformation function. BCL6-transformed cell lines exhibited further decreases of ATR and p27kip1 expression plus strong decreases in Blimp1 and PDCD2 expression. Our study provides important clues about the critical target genes used by BCL6 to transform primary B cells and indicates that the CD40 signaling pathway can collaborate with BCL6 in the transformation of primary B cells. Thus, our study demonstrates a rapid in vitro system to analyze the transformation function of BCL6.

IFN regulatory factor 4 regulates the expression of a subset of Th2 cytokines

Th2 cells can be subdivided into subpopulations depending on the level of a cytokine and the subsets of cytokines they produce. We have recently identified the ETS family transcription factor PU.1 as regulating heterogeneity in Th2 populations. To define additional factors that might contribute to Th2 heterogeneity, we examined the PU.1 interacting protein IFN-regulatory factor (IRF)4. When Th2 cells are separated based on levels of IL-10 secretion, IRF4 expression segregates into the subset of Th2 cells expressing high levels of IL-10. Infection of total Th2 cells, and IL-10 nonsecreting cells, with retrovirus-expressing IRF4, resulted in increased IL-4 and IL-10 expression, no change in IL-5 or IL-13 production and decreased Il9 transcription. Transfection of an IRF4-specific small interfering RNA into Th2 cells decreases IL-10 production. IRF4 directly binds the Il10 gene as evidenced by chromatin immunoprecipitation assay, and regulates Il10 control elements in a reporter assay. IRF4 interacts with PU.1, and in PU.1-deficient T cells there was an increase in IRF4 binding to the Il10 gene, and in the ability of IRF4 to induce IL-10 production compared with wild-type cells and Il10 promoter activity in a reporter assay. Further heterogeneity of IRF4 expression was observed in Th2 cells analyzed for expression of multiple Th2 cytokines. Thus, IRF4 promotes the expression of a subset of Th2 cytokines and contributes to Th2 heterogeneity.

Increased c-Jun expression and reduced GATA2 expression promote aberrant monocytic differentiation induced by activating PTPN11 mutants

Juvenile myelomonocytic leukemia (JMML) is characterized by myelomonocytic cell overproduction and commonly bears activating mutations in PTPN11. Murine hematopoietic progenitors expressing activating Shp2 undergo myelomonocytic differentiation, despite being subjected to conditions that normally support only mast cells. Evaluation of hematopoietic-specific transcription factor expression indicates reduced GATA2 and elevated c-Jun in mutant Shp2-expressing progenitors. We hypothesized that mutant Shp2-induced Ras hyperactivation promotes c-Jun phosphorylation and constitutive c-Jun expression, permitting, as a coactivator of PU.1, excessive monocytic differentiation and reduced GATA2. Hematopoietic progenitors expressing activating Shp2 demonstrate enhanced macrophage CFU (CFU-M) compared to that of wild-type Shp2-expressing cells. Treatment with the JNK inhibitor SP600125 or cotransduction with GATA2 normalizes activating Shp2-generated CFU-M. However, cotransduction of DeltaGATA2 (lacking the C-terminal zinc finger, needed to bind PU.1) fails to normalize CFU-M. NIH 3T3 cells expressing Shp2E76K produce higher levels of luciferase expression directed by the macrophage colony-stimulating factor receptor (MCSFR) promoter, which utilizes c-Jun as a coactivator of PU.1. Coimmunoprecipitation demonstrates increased c-Jun-PU.1 complexes in mutant Shp2-expressing hematopoietic progenitors, while chromatin immunoprecipitation demonstrates increased c-Jun binding to the c-Jun promoter and an increased c-Jun-PU.1 complex at the Mcsfr promoter. Furthermore, JMML progenitors express higher levels of c-JUN than healthy controls, substantiating the disease relevance of these mechanistic findings.

Impaired development of human Th1 cells in patients with deficient expression of STAT4

IL-12 activates STAT4, which is a critical regulator of inflammation and T helper type I (Th1) lineage development in murine systems. The requirement for STAT4 in the generation of human Th1 cells has not been examined thoroughly. Compared with control Th1 cultures, expression of the Th1 genes IFNgamma, IL-12Rbeta2, and TNFalpha is greatly reduced in Th1 cultures of CD4 T cells isolated from lymphoma patients after autologous stem cell transplantation who have acquired STAT4 deficiency. Moreover, IL-4 and IL-5 production is increased in patient Th1 cultures though there are no defects in the development of Th2 cells. Reconstitution of STAT4 in patient T cells allowed recovery of IFNgamma and IL-12Rbeta2 expression, whereas ectopic expression of IL-12Rbeta2 did not rescue STAT4 expression, and increased IFNgamma production only to levels intermediate between control and patient samples. These results demonstrate that, as in murine systems, STAT4 is required for optimal human Th1 lineage development.

T helper cytokine patterns: defined subsets, random expression, and external modulation

Although T cell effector subsets, defined by cytokine patterns, have been recognized for more than 20 years, the functional cytokine expression patterns in vivo are still in considerable doubt, particularly for human T cells. At least three new subsets have been recently identified, but the committed cytokine pattern of a T cell (e.g., Th1 cells produce IL-2, interferon-gamma, and lymphotoxin) may differ from the expression pattern of one cell on one occasion, which may be a subset of its full potential. Recent advances in flow cytometry allowed detailed cytokine patterns of antigen-stimulated cells to be identified directly ex vivo. These patterns are clearly more diverse than the major subsets identified as committed phenotypes. Additional contributions to diversity may include new committed subsets, random expression of only part of the committed pattern, and modification of the expression patterns by cytokines and other mediators.

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.

IL4 blockade of inducible regulatory T cell differentiation: the role of Th2 cells, Gata3 and PU.1

Naive CD4 T cells differentiate into functionally distinct T helper (Th) cells subsets or into regulatory T (Treg) cells in response to the cytokine milieu in which they encounter antigen. A recurring theme in post-thymic CD4 T cell differentiation is the cross-regulation of lineage choice by cytokines and transcription factors that are expressed in alternative lineages. For example, TGFbeta induces the de novo expression of the Treg cell signature transcription factor Foxp3, but iTreg differentiation is blocked by high concentrations of the Th2 cytokine IL4. However, whether IL4 can antagonise Foxp3 induction in more physiological settings remains to be addressed. Here we use a co-culture system to demonstrate that IL4 provided by Th2 cells in vitro is sufficient to block Foxp3 induction in naive CD4 T cells. In addition, we find that Foxp3 induction is efficiently blocked not only by the Th2 transcription factor Gata3, but also by PU.1, which is transiently induced during Th2 differentiation. These data suggest that iTreg differentiation may be affected by the polarity of immune responses.

Transcription factor-dependent chromatin remodeling of Il18r1 during Th1 and Th2 differentiation

The IL-18Ralpha-chain is expressed on Th1 but not Th2 cells. We have recently shown that Stat4 is an important component of programming the Il18r1 locus (encoding IL-18Ralpha) for maximal expression in Th1 cells. Il18r1 is reciprocally repressed during Th2 development. In this report, we demonstrate the establishment of DH patterns that are distinct among undifferentiated CD4 T, Th1, and Th2 cells. Stat6 is required for the repression of Il18r1 expression and in Stat6-deficient Th2 cultures, mRNA levels, histone acetylation, and H3K4 methylation levels are intermediate between levels observed in Th1 and Th2 cells. Despite the repressive effects of IL-4 during Th2 differentiation, we observed only modest binding of Stat6 to the Il18r1 locus. In contrast, we observed robust GATA-3 binding to a central region of the locus where DNase hypersensitivity sites overlapped with conserved non-coding sequences in Il18r1 introns. Ectopic expression of GATA-3 in differentiated Th1 cells repressed Il18r1 mRNA and surface expression of IL-18Ralpha. These data provide further mechanistic insight into transcription factor-dependent establishment of Th subset-specific patterns of gene expression.

Competition and collaboration: GATA-3, PU.1, and Notch signaling in early T-cell fate determination

T-cell precursors remain developmentally plastic for multiple cell generations after entering the thymus, preserving access to developmental alternatives of macrophage, dendritic-cell, and even mast-cell fates. The underlying regulatory basis of this plasticity is that early T-cell differentiation depends on transcription factors which can also promote alternative developmental programs. Interfactor competition, together with environmental signals, keep these diversions under control. Here the pathways leading to several lineage alternatives for early pro-T-cells are reviewed, with close focus on the mechanisms of action of three vital factors, GATA-3, PU.1, and Notch-Delta signals, whose counterbalance appears to be essential for T-cell specification.

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.

Lymphoid enhancer factor interacts with GATA-3 and controls its function in T helper type 2 cells

GATA-3 is the master transcription factor for T helper 2 (Th2) cell differentiation and is critical for the expression of Th2 cytokines. Little is known, however, about the nature of the functional molecular complexes of GATA-3. We identified a high-mobility group (HMG)-box type transcription factor, lymphoid enhancer factor 1 (LEF-1), in the GATA-3 complex present in Th2 cells using a Flag-calmodulin-binding peptide (CBP)-tag based proteomics method. The interaction between GATA-3 and LEF-1 was confirmed by co-immunoprecipitation experiments using LEF-1-introduced T-cell lineage TG40 cells. The HMG-box domain of LEF-1 and two zinc finger domains of GATA-3 were found to be important for the physical association. The introduction of LEF-1 into developing Th2 cells resulted in the suppression of Th2 cytokine production. The suppression was significantly lower in the cells into which a HMG-box-deleted LEF-1 mutant was introduced. Moreover, LEF-1 inhibited the binding activity of GATA-3 to the interleukin (IL)-5 promoter. These results suggest that LEF-1 is involved in the GATA-3 complex, while also regulating the GATA-3 function, such as the induction of Th2 cytokine expression via the inhibition of the DNA-binding activity of GATA-3.

Interaction between GATA-3 and the transcriptional coregulator Pias1 is important for the regulation of Th2 immune responses

Th2 cytokine expression is dependent on the transcription factor GATA-3. However, the molecular interactions of GATA-3 leading to Th2 cytokine gene activation have not been well characterized. Here, we reported a number of GATA-3 associated proteins in Th2 cells, and one of such proteins Pias1 functioned as a positive transcriptional coregulator for GATA-3. When overexpressed in Th2 cells, Pias1 enhanced the expression of IL-13, and to lesser degrees, IL-4 and -5. Conversely, Pias1 siRNA down-regulated the Th2 cytokine expression. In Leishmania major infection, manipulating Pias1 expression in parasite-reactive CD4 T cells altered severity of disease caused by Th2 responses. Mechanistically, Pias1 markedly potentiated GATA-3-mediated activation of the IL-13 promoter by facilitating the recruitment of GATA-3 to the promoter. In contrast, IL-5 promoter was modestly enhanced by Pias1 and no effect was observed on IL-4 promoter. Thus, both promoter activation and additional mechanisms are responsible for regulation by Pias1.

BOB.1/OBF.1 controls the balance of TH1 and TH2 immune responses

BOB.1/OBF.1 is a transcriptional coactivator essential at several stages of B-cell development. In T cells, BOB.1/OBF.1 expression is inducible by co-stimulation. However, a defined role of BOB.1/OBF.1 for T-cell function had not been discovered so far. Here, we show that BOB.1/OBF.1 is critical for T helper cell function. BOB.1/OBF.1(-/-) mice showed imbalanced immune responses, resulting in increased susceptibility to Leishmania major infection. Functional analyses revealed specific defects in TH1 and TH2 cells. Whereas expression levels of TH1 cytokines were reduced, the secretion of TH2 cytokines was increased. BOB.1/OBF.1 directly contributes to the IFNgamma and IL2 promoter activities. In contrast, increased TH2 cytokine production is controlled indirectly, probably via the transcription factor PU.1, the expression of which is regulated by BOB.1/OBF.1. Thus, BOB.1/OBF.1 regulates the balance of TH1 versus TH2 mediated immunity.

IL-4-stimulated NF-kappaB activity is required for Stat6 DNA binding

IL-4 is a critical cytokine in the regulation of immune responses. In B lymphocytes, IL-4 signaling promotes the Stat6-dependent cell surface expression of several proteins including MHC Class II and CD86. However, the requirement for other transcription factors in IL-4-induced B cell gene expression has not been studied extensively. Here, we show that IL-4 induces NF-kappaB p100 processing to NF-kappaB p52 in B cells but not in T cells or macrophages. IL-4 induced NF-kappaB p52 production requires PI-3K activity and correlates with IkappaB kinase phosphorylation and TNF receptor-associated factor 3 degradation. Blocking NF-kappaB activity eliminates IL-4-stimulated gene expression in B cells by reducing IL-4-induced DNA binding but not phosphorylation or nuclear localization of Stat6. These results describe a novel role for NF-kappaB in IL-4-induced signaling and gene expression.

Reprogramming of committed T cell progenitors to macrophages and dendritic cells by C/EBP alpha and PU.1 transcription factors

The differentiation potential of T lineage cells becomes restricted soon after entry of multipotent precursors into the thymus and is accompanied by a downregulation of the transcription factors C/EBP alpha and PU.1. To investigate this restriction point, we have expressed C/EBP alpha and PU.1 in fully committed pre-T cells and found that C/EBP alpha (and C/EBP beta) induced the formation of functional macrophages. In contrast, PU.1 converted them into myeloid dendritic cells under identical culture conditions. C/EBP alpha-induced reprogramming is complex because upregulation of some but not all myelomonocytic markers required endogenous PU.1. Notch signaling partially inhibited C/EBP alpha-induced macrophage formation and completely blocked PU.1-induced dendritic cell formation. Likewise, expression of intracellular Notch or the transcription factor GATA-3 inhibited C/EBP alpha-induced lineage conversion. Our data show that committed T cell progenitors remain susceptible to the lineage instructive effects of myeloid transcription factors and suggest that Notch signaling induces T lineage restriction by downregulating C/EBP alpha and PU.1 in multilineage precursors.

A failure to launch: fuelling cytokine secretion in iNKT cells

In this issue of Immunity, Bezbradica et al., (2006) uncover an unsuspected role for the cytokine GM-CSF in the thymic development of invariant NKT cells, a role that licenses these cells to secrete effector cytokines upon activation in the periphery.

T helper cell effector fates — who, how and where?

CD4 helper T cells functionally organize the host immune response by elaborating cytokines, often in patterns that have overlapping effects on other cells. Much interest centers on understanding how these stereotyped cytokine patterns become elaborated and what mechanisms underlie the generation of distinct helper T cell subsets. The past two years have seen advances in understanding of additional subsets, including T helper follicular cells and IL-17-producing T helper cells. Progress has also been achieved in resolving some of the crosstalk that regulates effector fate at the level of distinct transcription factors and chromatin reorganization of the cytokine genes, and a crucial role for gene silencing has been exposed. Finally, the role of innate cells in influencing these processes has become increasingly realized.

At the crossroads: diverse roles of early thymocyte transcriptional regulators

Transcriptional regulation of T-cell development involves successive interactions between complexes of transcriptional regulators and their binding sites within the regulatory regions of each gene. The regulatory modules that control expression of T-lineage genes frequently include binding sites for a core set of regulators that set the T-cell-specific background for signal-dependent control, including GATA-3, Notch/CSL, c-myb, TCF-1, Ikaros, HEB/E2A, Ets, and Runx factors. Additional regulators in early thymocytes include PU.1, Id-2, SCL, Spi-B, Erg, Gfi-1, and Gli. Many of these factors are involved in simultaneous regulation of non-T-lineage genes, T-lineage genes, and genes involved in cell cycle control, apoptosis, or survival. Potential and known interactions between early thymic transcription factors such as GATA-3, SCL, PU.1, Erg, and Spi-B are explored. Regulatory modules involved in the expression of several critical T-lineage genes are described, and models are presented for shifting occupancy of the DNA-binding sites in the regulatory modules of pre-Talpha, T-cell receptor beta (TCRbeta), recombinase activating genes 1 and 2 (Rag-1/2), and CD4 during T-cell development. Finally, evidence is presented that c-kit, Erg, Hes-1, and HEBAlt are expressed differently in Rag-2(-/-) thymocytes versus normal early thymocytes, which provide insight into potential regulatory interactions that occur during normal T-cell development.

Regulation of Th2 Cytokine Expression in NKT Cells: Unconventional Use of Stat6, GATA-3, and NFAT2

NKT cells are unique in that they can produce high levels of both Th1 and Th2 cytokines, yet little is known about how NKT cells control the transcription of Th2 cytokines. The expression of IL-4 by NKT cells is independent of the Th2-associated transcription factor Stat6. We have found that Stat6 is critical for the expression of IL-5, IL-10, and IL-13 by NKT cells. However, the Th2 cell-associated transcription factor GATA-3, normally induced by Stat6 activation, is expressed at low levels in NKT cells. CD4+ NKT cells are highly enriched for Th2 cytokine expression compared with CD4- NKT cells, and we searched for transcription factors that are up-regulated in CD4+ NKT cells that could control Th2 cytokine expression. We found that the NFAT family member NFAT2 is selectively increased in CD4+ NKT cells. We tested the roles of NFAT2 and also GATA-3 in Th2 cytokine expression by retrovirus-mediated gene transduction into NKT cells and nonpolarized conventional T cells. Expression of NFAT2 increased the expression of IL-4 in both NKT cells and conventional T cells, and NFAT2 activated IL-10 in conventional T cells but not in NKT cells. GATA-3 strongly activated IL-4, IL-5, and IL-13 expression in conventional T cells but had comparatively weak effects on these cytokines in NKT cells. Thus, NFAT2, GATA-3, and Stat6 have surprisingly different roles in NKT cells than in conventional T cells. We propose that one mechanism by which CD4+ NKT cells express IL-4 independent of Stat6 is via increased NFAT2 activity.