Reprogramming the signalling requirement for AP-1 (activator protein-1) activation during differentiation of precursor CD4+ T-cells into effector Th1 and Th2 cells

JNK and p38 MAP kinases in CD4+ and CD8+ T cells

The c-Jun aminoterminal kinase (JNK) and p38 mitogen-activated protein (MAP) kinase signaling pathways have been associated with cell death, differentiation and proliferation. CD4+ and CD8+ T cells have different effector functions after antigen stimulation and control specific aspects of the immune response. The studies carried out in our group indicate that the role of JNK and p38 MAP kinases in CD4+ T cells is different from their role in CD8+ T cells. Moreover, these two pathways are not redundant in either T cell population. We have also shown that p38 MAP kinase regulates early stages of T cell development in the thymus. It is therefore important to consider the specific function of these kinases in each T cell population when pharmacological inhibitors of JNK and p38 MAP kinases are used for therapeutic purposes to control the immune response.

Role of TCR-induced extracellular signal-regulated kinase activation in the regulation of early IL-4 expression in naive CD4+ T cells

Although extracellular signal-regulated kinase (Erk) activation influences IL-4 production in various experimental systems, its role during Th differentiation is unclear. In this study, we show that Erk plays a critical role in IL-4 expression during TCR-induced Th differentiation of naive CD4(+) T cells. Stimulation of CD4(+) T cells with a high affinity peptide resulted in sustained Erk activation and Th1 differentiation. However, reduction of Erk activity led to a dramatic increase in IL-4 production and Th2 generation. Analysis of RNA and nuclear proteins of CD4(+) T cells 48 h after stimulation revealed that this was due to early IL-4 expression. Interestingly, transient Erk activation resulted in altered AP-1 DNA binding activity and the induction of an AP-1 complex that was devoid of Fos protein and consisted of Jun-Jun dimers. These data show that in the presence of a strong TCR signal, IL-4 expression can be induced in naive CD4(+) T cells by altering the strength of Erk activation. In addition, these data suggest that TCR-induced Erk activation is involved in the regulation of IL-4 expression by altering the composition of the AP-1 complex and its subsequent DNA binding activity.

Amplification and overexpression of JUNB is associated with primary cutaneous T-cell lymphomas

Primary cutaneous lymphomas (PCLs) represent a heterogeneous group of extranodal T- and B-cell malignancies. The underlying molecular pathogenesis of this malignancy remains unclear. This study aimed to characterize oncogene abnormalities in PCLs. Using genomic microarray, we detected oncogene copy number gains of RAF1 (3p25), CTSB (8p22), PAK1 (11q13), and JUNB (19p13) in 5 of 7 cases of mycosis fungoides (MF)/Sezary syndrome (SS) (71%), gains of FGFR1 (8p11), PTPN (20q13), and BCR (22q11) in 4 cases (57%), and gains of MYCL1 (1p34), PIK3CA (3q26), HRAS (11p15), MYBL2 (20q13), and ZNF217 (20q13) in 3 cases (43%). Amplification of JUNB was studied in 104 DNA samples from 78 PCL cases using real-time polymerase chain reaction. Twenty-four percent of cases, including 7 of 10 cases of primary cutaneous CD30(+) anaplastic large-cell lymphoma (C-ALCL), 4 of 14 MF, 4 of 22 SS, and 2 of 23 primary cutaneous B-cell lymphoma (PCBCL) showed amplification of JUNB, and high-level amplification of this oncogene was present in 3 C-ALCL and 2 MF cases. JUNB protein expression was analyzed in tissue sections from 69 PCL cases, and 44% of cases, consisting of 21 of 23 SS, 6 of 8 C-ALCL, 5 of 10 MF, and 9 of 21 PCBCL, demonstrated nuclear expression of JUNB by tumor cells. Overexpression of JUNB also was detected in 5 C-ALCL and 2 SS cases. These results have revealed, for the first time, amplification and expression patterns of JUNB in PCL, suggesting that JUNB may be critical in the pathogenesis of primary cutaneous T-cell lymphomas.

Phosphorylation of activating transcription factor in murine splenocytes through delta opioid receptors

Delta opioid receptors (DORs) modulate TCR signaling through the mitogen-activated protein kinases (MAPKs), ERKs 1 and 2. These studies determined whether a DOR agonist alone ([D-Ala(2)-D-Leu(5)]enkephalin; DADLE) affects phosphorylation of the activating transcription factor (ATF-2) and its interaction with the MAPK, c-Jun NH(2)-terminal kinase (JNK). DOR expression was induced on murine splenocytes by anti-CD3 and then quiescent cells were treated with DADLE. DADLE, itself, dose-dependently induced maximal phosphorylation of ATF-2 within 5-10min; naltrindole, a specific antagonist, abolished this. Anti-ATF-2 immunoprecipitates from control and DADLE-treated splenocytes showed a dominant 59kDa phosphorylated band and a 71kDa band. DADLE stimulated phosphorylation of both bands, although the 71kDa band was selectively immunoprecipitated by anti-JNK. Thus, DADLE stimulated phosphorylation of 71kDa ATF-2 and its association with JNK, suggesting that JNK is activated through DORs. Along with previous observations, these studies suggest that lymphocyte DORs can affect the activation of MAPKs by TCR-independent stimulation (e.g., JNK) or indirectly by modulating TCR-dependent stimulation (e.g., ERK).

Inhibition of the p38 pathway upregulates macrophage JNK and ERK activities, and the ERK, JNK, and p38 MAP kinase pathways are reprogrammed during differentiation of the murine myeloid M1 cell line

Mitogen-activated protein (MAP) kinases have been implicated as important mediators of the inflammatory response. Here we report that c-Jun NH(2)-terminal kinase (JNK), extracellular signal-regulated kinase (ERK), and p38 MAP kinase activities are reprogrammed during the IL-6 induced macrophage-like differentiation of the murine myeloid M1 cell line. Moreover, p38 inhibition upregulates JNK and ERK activity in M1 cells and in thioglycollate-elicited peritoneal exudate macrophages. IL-6-induced M1 differentiation also induces expression of the anti-inflammatory cytokine IL-10, and p38 inhibition potentiates this increase in IL-10 expression in an ERK-dependent manner. Thus, we speculate that during inflammatory conditions in vivo macrophage p38 may regulate JNK and ERK activity and inhibit IL-10 expression. These data highlight the importance of p38 in the molecular mechanisms of macrophage function.

Th2 cell-specific cytokine expression and allergen-induced airway inflammation depend on JunB

Naïve CD4+ T cells differentiate into effector T helper 1 (Th1) or Th2 cells, which are classified by their specific set of cytokines. Here we demonstrate that loss of JunB in in vitro polarized Th2 cells led to a dysregulated expression of the Th2-specific cytokines IL-4 and IL-5. These cells produce IFN-gamma and express T-bet, the key regulator of Th1 cells. In line with the essential role of Th2 cells in the pathogenesis of allergic asthma, mice with JunB-deficient CD4+ T cells exhibited an impaired allergen-induced airway inflammation. This study demonstrates novel functions of JunB in the development of Th2 effector cells, for a normal Th2 cytokine expression pattern and for a complete Th2-dependent immune response in mice.

Interleukin-6 promotes the production of interleukin-4 and interleukin-5 by interleukin-2-dependent and -independent mechanisms in freshly isolated human T cells

T helper 2 (Th2) cytokines [interleukin (IL)-4 and IL-5] play a central role in the development of allergic immune responses. After allergen provocation, the expression of Th2 cytokines is rapidly up-regulated in atopy and asthma. IL-6 is a multifunctional cytokine that is able to direct Th2 immune responses and is secreted by multiple tissue cell types. This study shows that IL-6 induces up-regulation of IL-4 and IL-5 after short (5 min) preincubation periods in freshly isolated, alpha-CD3/alpha-CD28-stimulated T cells. After longer preincubation periods with IL-6 (12 and 24 hr), the priming effect on IL-4 production gradually disappears, whereas the effect on IL-5 becomes more pronounced. In contrast, a small but significant inhibitory effect is found on the production of the Th1 cytokine interferon-gamma. Additional experiments indicate that the long-term priming effect of IL-6 on IL-5 production is dependent on IL-2 signalling. This is not the case for the short-term IL-6 effect on IL-5 secretion, where the p38 mitogen-activated protein kinase-dependent induction of activator protein-1 DNA-binding activity is involved, independent of signal transducer and activator of transcription 3 phosphorylation. In summary, these data demonstrate that the short-term and long-term priming effects of IL-6 on Th2 cytokine production are regulated by different mechanisms.

Repression of AP-1 function: a mechanism for the regulation of Blimp-1 expression and B lymphocyte differentiation by the B cell lymphoma-6 protooncogene

The B cell lymphoma-6 (BCL-6) transcriptional repressor protein is an important regulator of B cell differentiation and is strongly implicated in the development of B cell lymphoma. Expression of the Blimp-1 transcription factor, which is critical for promoting B cell differentiation into plasma cells, is repressed by BCL-6. We have investigated the mechanism for how BCL-6 represses Blimp-1 transcription, and have found that BCL-6 regulates the Blimp-1 promoter through a novel mechanism involving AP-1 elements. Specifically, BCL-6 is a potent repressor of transcriptional activity mediated by AP-1 factors. We found that the zinc-finger region of BCL-6 interacts with c-Jun, JunB, and JunD proteins but does not bind c-Fos or Fra-2 proteins. An estrogen receptor ligand binding domain fusion with the BCL-6 zinc finger domain can act as a estrogen-inducible dominant negative protein and increase AP-1 activity in BCL-6(+) cells but not in BCL-6(-) cells, indicating that endogenous BCL-6 represses AP-1 activity. Additionally, we have confirmed a specific interaction between c-Jun and the zinc finger domain of BCL-6 in vivo using a mammalian two-hybrid assay. Repression of AP-1 function by BCL-6 may be a key mechanism for how BCL-6 regulates gene expression to control inflammation, lymphocyte differentiation, and lymphomagenesis.

Gammadelta T cells promote a Th1 response during coxsackievirus B3 infection in vivo: role of Fas and Fas ligand

Fas/Fas ligand (FasL) interactions regulate disease outcome in coxsackievirus B3 (CVB3)-induced myocarditis. MRL(+/+) mice infected with CVB3 develop severe myocarditis, a dominant CD4(+) Th1 (gamma interferon [IFN-gamma(+)]) response to the virus, and a predominance of gammadelta T cells in the myocardial infiltrates. MRL lpr/lpr and MRL gld/gld mice, which lack normal expression of Fas and express a mutated FasL, respectively, have minimal myocarditis and show a dominant CD4(+) Th2 (interleukin-4 [IL-4(+)]) phenotype to CVB3. Spleen cells from virus-infected wild-type, lpr, and gld animals proliferate equally to virus in vitro. Adoptive transfer of gammadelta T cells from hearts of CVB3-infected MRL(+/+) mice (FasL(+)) into infected MRL gld/gld recipients (FasL(-)/Fas(+)) restores both disease susceptibility and Th1 cell phenotype. However, transfer of these cells into MRL lpr/lpr recipients (FasL(+)/Fas(-)) did not promote myocarditis and the viral response remained Th2 biased. This paralleled the expression of very high surface levels of FasL by myocardial gammadelta T cells, as well as their propensity to selectively lyse Th2 virus-specific CD4(+) T cells. These results demonstrate that Fas/FasL interactions conferred by gammadelta T cells on lymphocyte subpopulations may regulate the cytokine response to CVB3 infection and pathogenicity.

T-cell activation through the antigen receptor. Part 2: role of signaling cascades in T-cell differentiation, anergy, immune senescence, and development of immunotherapy

Part 2 of this review on cellular activation by the T-cell antigen receptor (TCR) will highlight how TCR signaling pathways are adapted to achieve specific biologic outcomes, including different states of T-cell differentiation and the induction of T-cell tolerance. We will also explore how treatment with altered peptide ligands affects TCR signaling to change T-cell differentiation or to induce an anergy state. These changes are accomplished through alteration of protein tyrosine kinase activity, the stoichiometry of phosphorylation of immunoreceptor tyrosine-based activation motifs, intracellular free ionized calcium flux, mitogen-activated protein kinase activity, and transcriptional activation of key cytokine promoters. The CTLA-4 plays an important role in the induction and maintenance of anergy. The second theme will highlight how altered TCR signal transduction, including changes in the compartmentalization of signaling components at the TCR synapse, contributes to decreased T-cell activation during immune senescence. Finally, we will illustrate how the molecular details of TCR activation can be used to modify the function of the immune system. This includes a description of the mechanism of action of altered peptide ligands, CTLA-4Ig, and pharmacologic inhibitors of mitogen-activated protein kinases, nuclear factor kappaB, and protein kinase C cascades.

c-Jun NH(2)-terminal kinase (JNK)1 and JNK2 have distinct roles in CD8(+) T cell activation

The c-Jun NH(2)-terminal kinase (JNK) signaling pathway is induced by cytokines and stress stimuli and is implicated in cell death and differentiation, but the specific function of this pathway depends on the cell type. Here we examined the role of JNK1 and JNK2 in CD8(+) T cells. Unlike CD4(+) T cells, the absence of JNK2 causes increased interleukin (IL)-2 production and proliferation of CD8(+) T cells. In contrast, JNK1-deficient CD8(+) T cells are unable to undergo antigen-stimulated expansion in vitro, even in the presence of exogenous IL-2. The hypoproliferation of these cells is associated with impaired IL-2 receptor alpha chain (CD25) gene and cell surface expression. The reduced level of nuclear activating protein 1 (AP-1) complexes in activated JNK1-deficient CD8(+) T cells can account for the impaired IL-2 receptor alpha chain gene expression. Thus, JNK1 and JNK2 play different roles during CD8(+) T cell activation and these roles differ from those in CD4(+) T cells.

Overactivation of IL-4-induced activator protein-1 in atopic dermatitis

Atopic dermatitis is regarded as mediated by Th2-type immunity. In fact, it frequently coincides with the elevation of immunoglobulin (Ig)-E in patients' sera. Due to the pivotal role of interleukin (IL)-4 in regulation of IgE, we hypothesized if atopic dermatitis represents a hyper-reactive condition in response to IL-4 when it coincides the higher serum level of IgE. To address this possibility, peripheral blood mononuclear cells (PBMC) isolated from patients with atopic dermatitis with the high serum IgE level, from those with psoriasis or from healthy volunteers were stimulated with recombinant IL-4 and analyzed for activation of transcription factors including activator protein (AP)-1 or signal transducers and activators of transcription (STAT)-6 by employing electrophoretic mobility shift assays. Although no significant difference between atopy patients and other groups was observed in the STAT-6 binding activity in IL-4-stimulated PBMC, it over-activated the binding of AP-1 in PBMC of the patients with atopic dermatitis. The AP-1 binding was interfered by the use of an antibody directed against JunB. This is the indication that IL-4-overactivated AP-1 is composed of JunB. Furthermore, semi-quantitative RT-PCR analyses revealed marked down-modulation of a Th1 cytokine, interferon (IFN)-gamma, in IL-4-stimulated PBMC derived from atopy patients, but not that from healthy individuals. Together, our present study indicates that AP-1 is over-activated by IL-4 in PBMC of the atopic patients with the higher IgE level, thereby implying that IL-4-induced over-activation of AP-1 might be one of pathogenic factors in atopic dermatitis.

Signaling by the JNK group of MAP kinases. c-jun N-terminal Kinase

c-Jun N-terminal kinase (JNK) is one of the several main MAP kinase groups identified in mammals. Original studies by use of Jurkat T cells implicated JNK in T cell activation and interleukin (IL-2) expression. Recent advances using mouse genetic approaches have revealed novel functions of this pathway in primary mouse T cells. JNK is not essential for T-cell activation; instead, it is required for helper T differentiation into effector cells and their cytokine production. In this review, we summarize these advances in understanding the expression, function, and regulation of the JNK pathway in T-lymphocyte activation and differentiation.

Transcriptional reprogramming during T helper cell differentiation

Our laboratory employs reporter transgenic mice as model systems to study the transcriptional reprogramming that accompanies T helper cell differentiation. These studies demonstrate that changes in the activity of simple transcriptional elements associated with the IFN-gamma gene can recapitulate alterations in gene expression. In addition, our studies have revealed a key role for the transcription factor, CAMP response element binding protein (CREB), in the protection of differentiating T cells from apoptosis. Together, these findings further our understanding of the logic employed by T cells to alter gene expression profiles in response to differentiation signals.

MAP kinases in the immune response

MAP kinases are among the most ancient signal transduction pathways and are widely used throughout evolution in many physiological processes. In mammalian species, MAP kinases are involved in all aspects of immune responses, from the initiation phase of innate immunity, to activation of adaptive immunity, and to cell death when immune function is complete. In this review, we summarize recent progress in understanding the function and regulation of MAP kinase pathways in these phases of immune responses.

Mixed lineage kinase-dependent JNK activation is governed by interactions of scaffold protein JIP with MAPK module components

It has been proposed that JNK-interacting proteins (JIP) facilitate mixed lineage kinase-dependent signal transduction to JNK by aggregating the three components of a JNK module. A new model for the assembly and regulation of these modules is proposed based on several observations. First, artificially induced dimerization of dual leucine zipper-bearing kinase (DLK) confirmed that DLK dimerization is sufficient to induce DLK activation. Secondly, under basal conditions, DLK associated with JIP is held in a monomeric, unphosphorylated and catalytically inactive state. Thirdly, JNK recruitment to JIP coincided with significantly decreased affinity of JIP and DLK. JNK promoted the dimerization, phosphorylation and activation of JIP-associated DLK. Similarly, treatment of cells with okadaic acid inhibited DLK association with JIP and resulted in DLK dimerization in the presence of JIP. In summary, JIP maintains DLK in a monomeric, unphosphorylated, inactive state. Upon stimulation, JNK-JIP binding affinity increases while JIP-DLK interaction affinity is attenuated. Dissociation of DLK from JIP results in subsequent DLK dimerization, autophosphorylation and module activation. Evidence is provided that this model holds for other MLK-dependent JNK modules.

Signaling by the JNK group of MAP kinases. c-jun N-terminal Kinase

c-Jun N-terminal kinase (JNK) is one of the several main MAP kinase groups identified in mammals. Original studies by use of Jurkat T cells implicated JNK in T cell activation and interleukin (IL-2) expression. Recent advances using mouse genetic approaches have revealed novel functions of this pathway in primary mouse T cells. JNK is not essential for T-cell activation; instead, it is required for helper T differentiation into effector cells and their cytokine production. In this review, we summarize these advances in understanding the expression, function, and regulation of the JNK pathway in T-lymphocyte activation and differentiation.

Structure and specificity of GATA proteins in Th2 development

Development of Th2 subset of CD4+ T cells involves the interleukin-4 (IL-4)- and Stat6-dependent increase in GATA-3 expression during primary activation. Recently we reported that the phenotypic stability and factor independence of Th2 cells involves acquisition of an intracellular pathway that maintains GATA-3 expression. Evidence from retroviral expression studies implied that this pathway involved an autoactivation of GATA-3 expression, since Stat6-deficient T cells induced endogenous GATA-3 when infected with GATA-3-expressing retroviruses. That study left unresolved the issue of whether GATA-3 autoactivation was direct or indirect. Several other Th2-specific transcription factors have been described, including c-Maf and JunB. We therefore examined the ability of these other transcription factors to induce GATA-3 expression and promote Th2 development. Neither c-Maf nor JunB induced Th2 development in Stat6-deficient CD4+ T cells, in contrast to GATA-3. Consistent with this indication of a possible direct autoactivation pathway, we also observed that heterologous GATA family proteins GATA-1, GATA-2, and GATA-4 were also capable of inducing GATA-3 expression in developing Stat6-deficient T cells and promote Th2 development. Mutational analysis revealed evidence for two distinct mechanisms of GATA-3 action. IL-4 induction by GATA-3 required each of the functional domains to be present, whereas repression of gamma interferon could occur even when mutants of GATA-3 lacking the second transactivation domain, TA2, were expressed. The GATA-dependent induction of the GATA-3 but not the other GATA genes in T cells suggests that T-cell-specific cis elements within the GATA-3 locus likely cooperate with a general GATA recognition motif to allow GATA-3-dependent autoactivation.

Regulation of activator protein-1 and NF-kappa B in CD8+ T cells exposed to peripheral self-antigens

The transcriptional events that control T cell tolerance to peripheral self Ags are still unknown. In this study, we analyzed the regulation of AP-1- and NF-kappa B-mediated transcription during in vivo induction of tolerance to a self Ag expressed exclusively on hepatocytes. Naive CD8(+)Désiré (Des)(+) T cells isolated from the Des TCR-transgenic mice that are specific for the H-2K(b) class I Ag were transferred into Alb-K(b)-transgenic mice that express the H-2K(b) Ag on hepatocytes only. Tolerance develops in these mice. We found that the self-reactive CD8(+)Des(+) T cells were transiently activated, then became unresponsive and were further deleted. In contrast to CD8(+)Des(+) T cells activated in vivo with APCs, which express high AP-1 and high NF-kappa B transcriptional activity, the unresponsive CD8(+)Des(+) T cells expressed no AP-1 and only weak NF-kappa B transcriptional activity. The differences in NF-kappa B transcriptional activity correlated with the generation of distinct NF-kappa B complexes. Indeed, in vivo primed T cells predominantly express p50/p50 and p65/p50 dimers, whereas these p50-containing complexes are barely detectable in tolerant T cells that express p65- and c-Rel-containing complexes. These observations suggest that fine regulation of NF-kappa B complex formation may determine T cell fate.

Jun N-terminal kinase 2 modulates thymocyte apoptosis and T cell activation through c-Jun and nuclear factor of activated T cell (NF-AT)

The Jun N-terminal kinases (JNKs) recently have been shown to be required for thymocyte apoptosis and T cell differentiation and/or proliferation. To investigate the molecular targets of JNK signaling in lymphoid cells, we used mice in which the serines phosphorylated by JNK in c-Jun were replaced by homologous recombination with alanines (junAA mice). Lymphocytes from these mice showed no phosphorylation of c-Jun in response to activation stimuli, whereas c-Jun was rapidly phosphorylated in wild-type cells. Despite the fact that c-jun is essential for early development, junAA mice develop normally; however, c-Jun N-terminal phosphorylation was required for efficient T cell receptor-induced and tumor necrosis factor-alpha-induced thymocyte apoptosis. In contrast, c-Jun phosphorylation by JNK is not required for T cell proliferation or differentiation. Because jnk2-/- T cells display a proliferation defect, we concluded that JNK2 must have other substrates required for lymphocyte function. Surprisingly, jnk2-/- T cells showed reduced NF-AT DNA-binding activity after activation. Furthermore, overexpression of JNK2 in Jurkat T cells strongly enhanced NF-AT-dependent transcription. These results demonstrate that JNK signaling differentially uses c-Jun and NF-AT as molecular effectors during thymocyte apoptosis and T cell proliferation.

Control of T helper cell differentiation--in search of master genes

Naïve T helper (T(H)0) cells can differentiate into one of two distinct populations: T(H)1 and T(H)2. Each population is characterized by the expression of specific cytokines and their ability to participate in cell-mediated or humoral immune responses. Recent efforts at identifying the molecular mechanisms through which T(H)0 cells become T(H)1 or T(H)2 cells have been promising. A number of transcription factors, including GATA-3 and T-bet, have been identified that promote the differentiation of T(H)0 cells and the maintenance of the differentiated cell phenotype. Dong and Flavell review recent findings on proteins that control the fate of T(H)0 differentiation, whether by promotion or inhibition, and discuss the role of epigenesis in the differentiation process.

Defective Th function induced by a dominant-negative cAMP response element binding protein mutation is reversed by Bcl-2

cAMP response element binding protein (CREB) is a critical regulator of diverse stimulus-dependent transcriptional events. Following TCR stimulation, CREB is rapidly induced in CD4+ Th cell precursors, but not in effector Th cells. However, its role in mature T cell function is incompletely defined. Transgenic mice expressing a CREB dominant-negative (dn) mutation in the T cell lineage exhibited normal T cell development in the thymus, normal T cell homeostasis in the periphery, and normal T cell clonal expansion following Ag challenge. However, this mutation caused selective inhibition of Th cell function in vitro and in vivo, and increased susceptibility of Th cells to activation-induced cell death. Th cells expressing the CREB-dn mutation contained reduced levels of the inhibitor of programmed cell death, BCL-2; overexpression of BCL-2 in transgenic mice reversed both susceptibility to activation-induced cell death in CREB-dn T cells and the defect in effector cytokine production. Thus, CREB plays a critical role in Th cell function and development of Th cell-mediated adaptive immune responses, at least in part, by inhibiting stimulus-dependent cell death.

Signaling and transcription in T helper development

The recognition of polarized T cell subsets defined by cytokine production was followed by a search to define the factors controlling this phenomenon. Suitable in vitro systems allowed the development of cytokine "recipes" that induced rapid polarization of naïve T cells into Th1 or Th2 populations. The next phase of work over the past several years has begun to define the intracellular processes set into motion during Th1/Th2 development, particularly by the strongly polarizing cytokines IL-12 and IL-4. Although somewhat incomplete, what has emerged is a richly detailed tapestry of signaling and transcription, controlling an important T cell developmental switch. In addition several new mediators of control have emerged, including IL-18, the intriguing Th2-selective T1/ST2 product, and heterogeneity in dendritic cells capable of directing cytokine-independent Th development.

GATA-3 induces T helper cell type 2 (Th2) cytokine expression and chromatin remodeling in committed Th1 cells

Committed T helper type 1 (Th1) and Th2 effector cells, resulting from chronic antigenic stimulation in interleukin (IL)-12 and IL-4, are implicated in the pathology of autoimmune and allergic diseases. Committed Th1 cells cannot be induced to change their cytokine profiles in response to antigenic stimulation and Th2 cytokine-inducing conditions. Here, we report that ectopic expression of GATA-3 induced Th2-specific cytokine expression not only in developing Th1 cells but also in otherwise irreversibly committed Th1 cells and a Th1 clone, HDK1. Moreover, cAMP, an inhibitor of cytokine production by Th1 cells, markedly augmented Th2 cytokine production in GATA-3-expressing Th1 cells. Ectopic expression of GATA-3 in developing Th1 cells, but not in Th1 clone HDK1, induced endogenous GATA-3, suggesting an autoregulatory mechanism for maintenance of GATA-3 expression in Th2 cells. Structure-function analyses of GATA-3 revealed that the NH(2)-terminal transactivation domain and the COOH-terminal zinc finger domain of GATA-3 were critical, whereas the NH(2)-terminal zinc finger domain was dispensable for the induction of IL-4. Both zinc fingers, however, were required for IL-5 induction. A Th2-specific DNaseI-hypersensitive site of the IL-4 locus was detected in GATA-3-expressing Th1 cells. Thus, GATA-3 can change the phenotype of committed Th1 cells, previously considered to be irreversible.

Inhibition of NF-kappaB activity and enhancement of apoptosis by the neuropeptide calcitonin gene-related peptide

Calcitonin gene-related peptide (CGRP) is a neuropeptide produced by the central and peripheral nervous systems and by endocrine cells. CGRP exerts diverse biological effects on the cardiovascular, gastrointestinal, respiratory, central nervous and immune systems. Little is known, however, about the molecular mechanisms that mediate CGRP effects. Using the NFkappaB-luciferase reporter transgenic mice, here we show that CGRP selectively inhibits NF-kappaB-mediated transcription in thymocytes in vitro and in vivo. In contrast, CGRP does not affect transcription mediated by the AP-1 and NFAT transcription factors. CGRP inhibits the accumulation of NF-kappaB complexes in the nucleus by preventing phosphorylation and degradation of the NF-kappaB inhibitor IkappaB. Inhibition of NF-kappaB activity is associated with the induction of apoptosis by CGRP in thymocytes. Together these results demonstrate for the first time the selective implication of the NF-kappaB signaling pathway in the regulatory function of the neuropeptide CGRP. Our study suggests a potential molecular mechanism by which CGRP can induce cell death in thymocytes.

The JNK and P38 MAP kinase signaling pathways in T cell-mediated immune responses

The mitogen-activated protein (MAP) kinase family members, which include the extracellular response kinases (ERK), p38, and c-Jun amino terminal kinases (JNK), play a role in mediating signals triggered by cytokines, growth factors, and environmental stress. JNK and p38 MAP kinases have been involved in inflammatory processes induced by a variety of stimuli, such as oxidative stress. Here, we describe the role of the JNK and p38 MAP kinase signaling pathways in the development of T cells in the thymus, and activation and differentiation of T cells in the peripheral immune system.

Conference highlight: do T cells care about the mitogen-activated protein kinase signalling pathways?

Mitogen-activated protein (MAP) kinases, which include the extracellular response kinases, p38 and c-Jun amino terminal kinases (JNK), play a significant role in mediating signals triggered by cytokines, growth factors and environmental stress. The JNK and p38 MAP kinases have been involved in growth, differentiation and cell death in different cell types. In the present paper, we describe how the JNK and p38 MAP kinase signalling pathways are regulated and their role during thymocyte development and the activation and differentiation of T cells in the peripheral immune system. The results from these studies demonstrate that the JNK and p38 MAP kinase signalling pathways regulate different aspects of T-cell mediated immune responses.

Activation of p38 mitogen-activated protein kinase in vivo selectively induces apoptosis of CD8(+) but not CD4(+) T cells

CD4(+) and CD8(+) T cells play specific roles during an immune response. Different molecular mechanisms could regulate the proliferation, death, and effector functions of these two subsets of T cells. The p38 mitogen-activated protein (MAP) kinase pathway is induced by cytokines and environmental stress and has been associated with cell death and cytokine expression. Here we report that activation of the p38 MAP kinase pathway in vivo causes a selective loss of CD8(+) T cells due to the induction of apoptosis. In contrast, activation of p38 MAP kinase does not induce CD4(+) T-cell death. The apoptosis of CD8(+) T cells is associated with decreased expression of the antiapoptotic protein Bcl-2. Regulation of the p38 MAP kinase pathway in T cells is therefore essential for the maintenance of CD4/CD8 homeostasis in the peripheral immune system. Unlike cell death, gamma interferon production is regulated by the p38 MAP kinase pathway in both CD4(+) and CD8(+) T cells. Thus, specific aspects of CD4(+) and CD8(+) T-cell function are differentially controlled by the p38 MAP kinase signaling pathway.

Activation of the p38 mitogen-activated protein kinase pathway arrests cell cycle progression and differentiation of immature thymocytes in vivo

The development of T cells in the thymus is coordinated by cell-specific gene expression programs that involve multiple transcription factors and signaling pathways. Here, we show that the p38 mitogen-activated protein (MAP) kinase signaling pathway is strictly regulated during the differentiation of CD4(-)CD8(-) thymocytes. Persistent activation of p38 MAP kinase blocks fetal thymocyte development at the CD25(+)CD44(-) stage in vivo, and results in the lack of T cells in the peripheral immune system of adult mice. Inactivation of p38 MAP kinase is required for further differentiation of these cells into CD4(+)CD8(+) thymocytes. The arrest of cell cycle in mitosis is partially responsible for the blockade of differentiation. Therefore, the p38 MAP kinase pathway is a critical regulatory element of differentiation and proliferation during the early stages of in vivo thymocyte development.

Regulation of c-Jun NH(2)-terminal kinase (Jnk) gene expression during T cell activation

The c-Jun NH(2)-terminal kinases (JNKs) are a group of mitogen-activated protein (MAP) kinases that participate in signal transduction events mediating specific cellular functions. Activation of JNK is regulated by phosphorylation in response to cellular stress and inflammatory cytokines. Here, we demonstrate that JNK is regulated by a second, novel mechanism. Induction of Jnk gene expression is required in specific tissues before activation of this signaling pathway. The in vivo and in vitro ligation of the T cell receptor (TCR) leads to induction of JNK gene and protein expression. TCR signals are sufficient to induce JNK expression, whereas JNK phosphorylation also requires CD28-mediated costimulatory signals. Therefore, both expression and activation contribute to the regulation of the JNK pathway to ensure proper control during the course of an immune response.

Cell fate decision: T-helper 1 and 2 subsets in immune responses

After activation CD4(+) helper T cells differentiate into T-helper (Th) 1 or Th2 effector cells. These two subsets are characterized by their distinct cytokine expression pattern and the immune function they mediate. Over the past years, a number of factors have been identified to affect helper T cell lineage determination, including antigen receptor, coreceptors and, most importantly, cytokine environment. In this review, we also summarize recent advancement in understanding of transcriptional and signaling regulation of the differentiation process. This knowledge will become important in the future to develop means in treating immune disorders.

Stat6-independent GATA-3 autoactivation directs IL-4-independent Th2 development and commitment

The initial source of IL-4-inducing Th2 development and the mechanism of stable Th2 commitment remain obscure. We found the reduced level of IL-4 production in Stat6-deficient T cells to be significantly higher than in Th1 controls. Using a novel cell surface affinity matrix technique, we found that IL-4-secreting Stat6-deficient T cells stably expressed GATA-3 and Th2 phenotype. Introducing GATA-3 into Stat6-deficient T cells completely restored Th2 development, inducing c-Maf, Th2-specific DNase I hypersensitive sites in the IL-4 locus, and Th2 cytokine expression. The fact that GATA-3 fully reconstitutes Th2 development in Stat6-deficient T cells indicates it is a master switch in Th2 development. Finally, GATA-3 exerts Stat6-independent autoactivation, creating a feedback pathway stabilizing Th2 commitment.

Triggering of T cell proliferation through CD28 induces GATA-3 and promotes T helper type 2 differentiation in vitro and in vivo

The relative contribution of T cell receptor-versus CD28-mediated signals in co-stimulation of resting CD4 T cells is thought to influence their functional differentiation towards T helper (Th) 1 versus Th2 subsets. We have used a conventional and a mitogenic CD28-specific monoclonal antibody to assess the effect of polyclonal T cell activation through CD28 alone on CD4 subset differentiation. In vivo, mitogenic but not conventional anti-CD28 induces massive lymphocytosis, the Th2 cytokines interleukin (IL)-4 and IL-10, and Th2-dependent immunoglobulin isotypes, most notably IgE. In vitro, it is shown that mitogenic anti-CD28 primes for IL-4-dependent induction of IL-4 expression much more efficiently than conventional co-stimulation. At the molecular level, we show for the first time that the activation of the "Th2 promoting" transcription factor GATA-3 requires co-stimulation by CD28 and is also induced by mitogenic anti-CD28 alone. We suggest that CD28-dependent induction of GATA-3 in concert with other transcription factors, which are preferentially induced by strong CD28-signals, primes CD4 T cells for IL-4-dependent Th2 differentiaton.

The molecular basis of T cell differentiation

Our laboratory has studied the molecular basis of T helper cell differentiation. We have used reporter transgenic mice, selective hybridization techniques, and studies of cell signaling to show that a complex pattern of gene expression is reprogrammed as the decision is made to become either a Th1 or Th2 cell. Many of these components have been identified, and their mechanisms of action elucidated. Understanding these mechanisms is likely to lead in the long-term to ways to intervene in these processes and, therefore, to direct immune response in therapeutically useful directions.

CTLA4 ligation attenuates AP-1, NFAT and NF-kappaB activity in activated T cells

CTLA4 is currently viewed as a late-appearing T cell surface receptor which is able to inhibit the proliferation of activated T cells. We sought to identify how CTLA4 ligation exerts these anti-proliferative effects by studying its influence on the activities of the relevant nuclear transcription factors AP-1, NFAT and NF-kappaB. We found that cross-linking CTLA4 on activated T cells completely blocks AP-1 and NFAT transcription factor activity before any effects on T cell proliferation can be observed, with NF-kappaB activity affected to a lesser degree. The suppression of AP-1 and NFAT transcriptional activity correlates with reduced levels of AP-1 and NFAT DNA binding as early as 10 h after T cell activation, prior to detectable up-regulation of CTLA4 on the T cell surface. Additionally, inhibitory effects on T cell proliferation only occurred when CTLA4 molecules were ligated in proximity to signaling TCR complexes, and inhibition of transcription factor DNA binding and activity was observed in the absence of CD28 stimulation. CTLA4 can thus act early during T cell activation to reduce the activity of several key nuclear transcription factors important for continued T cell proliferation and differentiation.

Regulation of IL-4 expression by the transcription factor JunB during T helper cell differentiation

The molecular basis for restricted cytokine expression by T helper 1 (Th1) and T helper 2 (Th2) cells is unclear. Previous studies found that P1, an element of the interleukin 4 (IL-4) promoter that binds AP-1, is important for Th2-restricted IL-4 expression. Here we show that JunB, but not the other Jun family members, was selectively induced in Th2 cells and not in Th1 cells during differentiation. JunB has previously been considered to be a negative regulator of transcription. However, we show that JunB binds directly to the P1 site and synergizes with c-Maf to activate an IL-4 luciferase reporter gene. JunB-control of IL-4 expression is mediated by the phosphorylation of JunB at Thr102 and -104 by JNK MAP kinase. The synergy between c-Maf and JunB can be attributed to cooperative DNA binding, which is facilitated by JunB phosphorylation. In transgenic mice, elevated JunB levels caused increased expression of several Th2 cytokines in developing Th1 cells. JunB also upregulated IL-4 expression in response to immunization. Thus, the early increase of JunB protein in Th2 cells can provide the specificity for c-Maf in IL-4 expression during T cell development and directs thereby Th2 differentiation.

Defective T cell differentiation in the absence of Jnk1

The c-Jun NH2-terminal kinase (JNK) signaling pathway has been implicated in the immune response that is mediated by the activation and differentiation of CD4 helper T (TH) cells into TH1 and TH2 effector cells. JNK activity observed in wild-type activated TH cells was severely reduced in TH cells from Jnk1-/- mice. The Jnk1-/- T cells hyperproliferated, exhibited decreased activation-induced cell death, and preferentially differentiated to TH2 cells. The enhanced production of TH2 cytokines by Jnk1-/- cells was associated with increased nuclear accumulation of the transcription factor NFATc. Thus, the JNK1 signaling pathway plays a key role in T cell receptor-initiated TH cell proliferation, apoptosis, and differentiation.

Regulation of the Activity of IFN-γ Promoter Elements During Th Cell Differentiation

Before they can deliver their effector functions, CD4+ Th cells must differentiate into Th1 or Th2 subsets. We have prepared reporter transgenic mice that express the luciferase gene under the control of proximal (prox.IFN-γ) and distal (dist.IFN-γ) regulatory elements from the IFN-γ promoter to permit investigation of mechanisms that regulate IFN-γ gene transcription during Th cell differentiation. Precursor Th cells (pTh) contain high levels of cAMP response element binding protein-activation transcription factor-1 (CREB-ATF1) proteins that bind these promoter elements from the IFN-γ gene, and these cells fail to express promoter activity. Restimulated effector Th (eTh) cells have reduced levels of CREB-ATF1 proteins, their nuclear extracts exhibit reduced CREB-ATF1 binding and greater Jun and Jun-ATF2 binding to dist.IFN-γ, and eTh cells express promoter activity. CREB directly competes with effector T cell nuclear proteins for dist.IFN-γ binding, and overexpression of CREB inhibits both prox.IFN-γ- and dist.IFN-γ-directed transcription in Jurkat T cells. IL-12-stimulated Th1 differentiation increases dist.IFN-γ activity in restimulated eTh1 cells; eTh1 nuclear extracts form increased levels of Jun-ATF2-dist.IFN-γ complexes. Taken together, these data suggest that both de-repression and trans-activation contribute to the induction of IFN-γ gene transcription during Th1 differentiation.

The JNK pathway regulates the In vivo deletion of immature CD4(+)CD8(+) thymocytes

The extracellular signal-regulated kinase (ERK), the c-Jun NH2-terminal kinase (JNK), and p38 MAP kinase pathways are triggered upon ligation of the antigen-specific T cell receptor (TCR). During the development of T cells in the thymus, the ERK pathway is required for differentiation of CD4(-)CD8(-) into CD4(+)CD8(+) double positive (DP) thymocytes, positive selection of DP cells, and their maturation into CD4(+) cells. However, the ERK pathway is not required for negative selection. Here, we show that JNK is activated in DP thymocytes in vivo in response to signals that initiate negative selection. The activation of JNK in these cells appears to be mediated by the MAP kinase kinase MKK7 since high levels of MKK7 and low levels of Sek-1/MKK4 gene expression were detected in thymocytes. Using dominant negative JNK transgenic mice, we show that inhibition of the JNK pathway reduces the in vivo deletion of DP thymocytes. In addition, the increased resistance of DP thymocytes to cell death in these mice produces an accelerated reconstitution of normal thymic populations upon in vivo DP elimination. Together, these data indicate that the JNK pathway contributes to the deletion of DP thymocytes by apoptosis in response to TCR-derived and other thymic environment- mediated signals.

Differentiation of CD4+ T cells to Th1 cells requires MAP kinase JNK2

Precursor CD4+ T cells develop into effector Th1 and Th2 cells that play a central role in the immune response. We show that the JNK MAP kinase pathway is induced in Th1 but not in Th2 effector cells upon antigen stimulation. Further, the differentiation of precursor CD4+ T cells into effector Th1 but not Th2 cells is impaired in JNK2-deficient mice. The inability of IL-12 to differentiate JNK2-deficient CD4+ T cells fully into effector Th1 cells is caused by a defect in IFNgamma production during the early stages of differentiation. The addition of exogenous IFNgamma during differentiation restores IL-12-mediated Th1 polarization in the JNK2-deficient mice. The JNK MAP kinase signaling pathway, therefore, plays an important role in the balance of Th1 and Th2 immune responses.

Interferon-gamma expression by Th1 effector T cells mediated by the p38 MAP kinase signaling pathway

Signal transduction via MAP kinase pathways plays a key role in a variety of cellular responses, including growth factor-induced proliferation, differentiation and cell death. In mammalian cells, p38 MAP kinase can be activated by multiple stimuli, such as pro-inflammatory cytokines and environmental stress. Although p38 MAP kinase is implicated in the control of inflammatory responses, the molecular mechanisms remain unclear. Upon activation, CD4+ T cells differentiate into Th2 cells, which potentiate the humoral immune response or pro-inflammatory Th1 cells. Here, we show that pyridinyl imidazole compounds (specific inhibitors of p38 MAP kinase) block the production of interferon-gamma (IFNgamma) by Th1 cells without affecting IL-4 production by Th2 cells. These drugs also inhibit transcription driven by the IFNgamma promoter. In transgenic mice, inhibition of the p38 MAP kinase pathway by the expression of dominant-negative p38 MAP kinase results in selective impairment of Th1 responses. In contrast, activation of the p38 MAP kinase pathway by the expression of constitutivelyactivated MAP kinase kinase 6 in transgenic mice caused increased production of IFNgamma during the differentiation and activation of Th1 cells. Together, these data demonstrate that the p38 MAP kinase is relevant for Th1 cells, not Th2 cells, and that inhibition of p38 MAP kinase represents a possible site of therapeutic intervention in diseases where a predominant Th1 immune response leads to a pathological outcome. Moreover, our study provides an additional mechanism by which the p38 MAP kinase pathway controls inflammatory responses.

Signal transduction by the c-Jun N-terminal kinase (JNK)--from inflammation to development

The c-Jun amino-terminal kinase (JNK) group of MAP kinases has been identified in mammals and insects. JNK is activated by exposure of cells to cytokines or environmental stress, indicating that this signaling pathway may contribute to inflammatory responses. Genetic and biochemical studies demonstrate that this signaling pathway also regulates cellular proliferation, apoptosis, and tissue morphogenesis. A functional role for JNK is therefore established in both the cellular response to stress and in many normal physiological processes.

Down-regulation of IL-4 gene transcription and control of Th2 cell differentiation by a mechanism involving NFAT1

Transcription factors of the NFAT family play a critical role in the immune response by activating the expression of cytokines and other inducible genes in antigen-stimulated cells. Here we show that a member of this family, NFAT1, is involved in down-regulating the late phase of IL-4 gene transcription, thus inhibiting T helper 2 responses. Whereas stimulated T cells from wild-type mice show a transient increase and then a rapid decline in the steady-state levels of IL-4 mRNA in vitro, the levels of IL-4 gene transcripts in NFAT1-deficient T cells are maintained at high levels under the same conditions. Consistent with this observation, NFAT1-/- mice are more susceptible to infection with Leishmania major. This report provides evidence that NFAT proteins regulate not only the initiation but also the termination of gene transcription.

T-cell subsets: transcriptional control in the Th1/Th2 decision

Transcriptional mechanisms in CD4(+) T cells and antigen-presenting cells determine the activation or differentiation of Th1 and Th2 helper cell subsets, and also form attractive targets for therapeutic intervention in the balance of Th1/Th2 responses.

c-Jun amino-terminal kinase is regulated by Galpha12/Galpha13 and obligate for differentiation of P19 embryonal carcinoma cells by retinoic acid

Retinoic acid induces P19 mouse embryonal carcinoma cells to differentiate to endoderm and increases expression of the heterotrimeric G-protein subunits Galpha12 and Galpha13. Retinoic acid was found to induce differentiation and sustained activation of c-Jun amino-terminal kinase, but not of ERK1,2 or of p38 mitogen-activated protein kinases. Much like retinoic acid, expression of constitutively active forms of Galpha12 and Galpha13 induced differentiation and constitutive activation of c-Jun amino-terminal kinase. Expression of the dominant negative form of c-Jun amino-terminal kinase 1 blocked both the activation of c-Jun amino-terminal kinase and the induction of endodermal differentiation in the presence of retinoic acid. These data implicate c-Jun amino-terminal kinase as a downstream element of activation of Galpha12 or Galpha13 obligate for retinoic acid-induced differentiation.