Activation of the interleukin-4 gene is controlled by the unique calcineurin-dependent transcriptional factor NF(P)

Generation of reporter mice for detecting the transcriptional activity of nuclear factor of activated T cells

Nuclear factor of activated T cells (NFAT) is a transcription factor essential for immunological and other biological responses. To develop analyzing system for NFAT activity in vitro and in vivo, we generated reporter mouse lines introduced with NFAT-driven enhanced green fluorescent protein (EGFP) expressing gene construct. Six tandem repeats of -286 to -265 of the human IL2 gene to which NFAT binds in association with its co-transcription factor, activator protein (AP)-1, was conjunct with thymidine kinase minimum promoter and following EGFP coding sequence. Upon introduction of the resulting reporter cassette into C57BL/6 fertilized eggs, the transgenic mice were obtained. Among 7 transgene-positive mice in 110 mice bone, 2 mice showed the designated reporter mouse character. Thus, the EGFP fluorescence of CD4+ and CD8+ T cells in these mice was enhanced by stimulation through CD3 and CD28. Each of phorbol 12-myristate 13-acetate (PMA) and ionomycin (IOM) stimulation weakly but their combined stimulation strongly enhanced EGFP expression. The stimulation-induced EGFP upregulation was also observed following T cell subset differentiation in a different manner. The EGFP induction by PMA + IOM stimulation was more potent than that by CD3/CD28 stimulation in helper T (Th)1, Th2, Th9, and regulatory T cells, while both stimulation conditions displayed the equivalent EGFP induction in Th17 cells. Our NFAT reporter mouse lines are useful for analyzing stimulation-induced transcriptional activation mediated by NFAT in cooperation with AP-1 in T cells.

Severely blunted allergen-induced pulmonary Th2 cell response and lung hyperresponsiveness in type 1 transient receptor potential channel-deficient mice

Transient receptor potential channels (TRPCs) are widely expressed and regulate Ca²⁺ entry in the cells that participate in the pathophysiology of airway hyperreactivity, inflammation, and remodeling. In vitro studies point to a role for TRPC1-mediated Ca²⁺ signaling in several of these cell types; however, physiological evidence is lacking. Here we identify TRPC1 signaling as proinflammatory and a regulator of lung hyperresponsiveness during allergen-induced pulmonary response. TRPC1-deficient (Trpc1(-/-)) mice are hyposensitive to methacholine challenge and have significantly reduced allergen-induced pulmonary leukocyte infiltration coupled with an attenuated T helper type 2 (Th2) cell response. Upon in vitro allergen exposure, Trpc1(-/-) splenocytes show impaired proliferation and T cell receptor-induced IL-2 production. A high number of germinal centers in spleens of Trpc1(-/-) mice and elevated levels of immunoglobulins in their serum are indicative of dysregulated B cell function and homeostasis. Thus we propose that TRPC1 signaling is necessary in lymphocyte biology and in regulation of allergen-induced lung hyperresponsiveness, making TRPC1 a potential target for treatment of immune diseases and asthma.

Regulation of the Il4 gene is independently controlled by proximal and distal 3' enhancers in mast cells and basophils

Mast cells and basophils are known to be a critical interleukin 4 (IL-4) source for establishing Th2 protective responses to parasitic infections. Chromatin structure and histone modification patterns in the Il13/Il4 locus of mast cells were similar to those of IL-4-producing type 2 helper T cells. However, using a transgenic approach, we found that Il4 gene expression was distinctly regulated by individual cis regulatory elements in cell types of different lineages. The distal 3' element contained conserved noncoding sequence 2 (CNS-2), which was a common enhancer for memory phenotype T cells, NKT cells, mast cells, and basophils. Targeted deletion of CNS-2 compromised production of IL-4 and several Th2 cytokines in connective-tissue-type and immature-type mast cells but not in basophils. Interestingly, the proximal 3' element containing DNase I-hypersensitive site 4 (HS4), which controls Il4 gene silencing in T-lineage cells, exhibited selective enhancer activity in basophils. These results indicate that CNS-2 is an essential enhancer for Il4 gene transcription in mast cell but not in basophils. The transcription of the Il4 gene in mast cells and basophils is independently regulated by CNS-2 and HS4 elements that may be critical for lineage-specific Il4 gene regulation in these cell types.

The cGMP/protein kinase G pathway contributes to dihydropyridine-sensitive calcium response and cytokine production in TH2 lymphocytes

Th2 lymphocytes differ from other CD4+ T lymphocytes not only by their effector tasks but also by their T cell receptor (TCR)-dependent signaling pathways. We previously showed that dihydropyridine receptors (DHPR) involved in TCR-induced calcium inflow were selectively expressed in Th2 cells. In this report, we studied whether cGMP-dependent protein kinase G (PKG) activation was implicated in the regulation of DHPR-dependent calcium response and cytokine production in Th2 lymphocytes. The contribution of cGMP in Th2 signaling was supported by the following results: 1) TCR activation elicited cGMP production, which triggered calcium increase responsible for nuclear factor of activated T cell translocation and Il4 gene expression; 2) guanylate cyclase activation by nitric oxide donors increased intracellular cGMP concentration and induced calcium inflow and IL-4 production; 3) reciprocally, guanylate cyclase inhibition reduced calcium response and Th2 cytokine production associated with TCR activation. In addition, DHPR blockade abolished cGMP-induced [Ca2+]i increase, indicating that TCR-induced DHP-sensitive calcium inflow is dependent on cGMP in Th2 cells. Th2 lymphocytes from PKG1-deficient mice displayed impaired calcium signaling and IL-4 production, as did wild-type Th2 cells treated with PKG inhibitors. Altogether, our data indicate that, in Th2 cells, cGMP is produced upon TCR engagement and activates PKG, which controls DHP-sensitive calcium inflow and Th2 cytokine production.

A cytokine promoter/yellow fluorescent protein reporter transgene serves as an early activation marker of lymphocyte subsets

A mouse containing an IL-4 promoter linked to the yellow fluorescent protein (YFP) reporter transgene was created to follow aspects of lymphocyte development and function. Following stimulation with phorbol 12-myristate 13-acetate and ionomycin, anti-CD3/CD28, antigen-specific peptide, or allogeneic cells, both CD4 and CD8 T cells expressed the transgene within 24h in a manner that was consistent with cellular activation markers. Transgene induction was inhibited by cyclosporine and FK506, suggesting that its activation occurs in an NFAT-dependent manner. B lymphocytes were also able to express the transgene when stimulated with LPS. This induction was inhibited in part by rapamycin. The results suggest that this transgene can function as an indicator of lymphocyte activation. Because YFP is not toxic and requires no preparation of the cells to view the reporter gene, this system provides a unique tool to follow lymphocyte activation in a number of model systems, such as those involving transplantation, allergy, and vaccine development.

Transcriptional repressor DREAM regulates T-lymphocyte proliferation and cytokine gene expression

Downstream Regulatory Element Antagonist Modulator (DREAM) is a Ca2+-dependent transcriptional repressor expressed in the brain, thyroid gland and thymus. Here, we analyzed the function of DREAM and the related protein KChIP-2 in the immune system using transgenic (tg) mice expressing a cross-dominant active mutant (EFmDREAM) for DREAM and KChIPs Ca2+-dependent transcriptional derepression. EFmDREAM tg mice showed reduced T-cell proliferation. Tg T cells exhibited decreased interleukin (IL)-2, -4 and interferon (IFN)gamma production after polyclonal activation and following antigen-specific response. Chromatin immunoprecipitation and transfection assays showed that DREAM binds to and represses transcription from these cytokine promoters. Importantly, specific transient knockdown of DREAM or KChIP-2 induced basal expression of IL-2 and IFNgamma in wild-type splenocytes. These data propose DREAM and KChIP-2 as Ca2+-dependent repressors of the immune response.

Nuclear factor of activated T cells balances angiogenesis activation and inhibition

It has been demonstrated that vascular endothelial cell growth factor (VEGF) induction of angiogenesis requires activation of the nuclear factor of activated T cells (NFAT). We show that NFATc2 is also activated by basic fibroblast growth factor and blocked by the inhibitor of angiogenesis pigment epithelial–derived factor (PEDF). This suggests a pivotal role for this transcription factor as a convergence point between stimulatory and inhibitory signals in the regulation of angiogenesis.

We identified c-Jun NH₂-terminal kinases (JNKs) as essential upstream regulators of NFAT activity in angiogenesis. We distinguished JNK-2 as responsible for NFATc2 cytoplasmic retention by PEDF and JNK-1 and JNK-2 as mediators of PEDF-driven NFAT nuclear export.

We identified a novel NFAT target, caspase-8 inhibitor cellular Fas-associated death domain–like interleukin 1β–converting enzyme inhibitory protein (c-FLIP), whose expression was coregulated by VEGF and PEDF. Chromatin immunoprecipitation showed VEGF-dependent increase of NFATc2 binding to the c-FLIP promoter in vivo, which was attenuated by PEDF. We propose that one possible mechanism of concerted angiogenesis regulation by activators and inhibitors may be modulation of the endothelial cell apoptosis via c-FLIP controlled by NFAT and its upstream regulator JNK.

Dihydropyridine Receptors Are Selective Markers of Th2 Cells and Can Be Targeted to Prevent Th2-Dependent Immunopathological Disorders

Th1 cells that produce IFN-gamma are essential in the elimination of intracellular pathogens, and Th2 cells that synthetize IL-4 control the eradication of helminths. However, highly polarized Th1 or Th2 responses may be harmful and even lethal. Thus, the development of strategies to selectively down-modulate Th1 or Th2 responses is of therapeutic importance. Herein, we demonstrate that dihydropyridine receptors (DHPR) are expressed on Th2 and not on Th1 murine cells. By using selective agonists and antagonists of DHPR, we show that DHPR are involved in TCR-dependent calcium response in Th2 cells as well as in IL-4, IL-5, and IL-10 synthesis. Nicardipine, an inhibitor of DHPR, is beneficial in experimental models of Th2-dependent pathologies in rats. It strongly inhibits the Th2-mediated autoimmune glomerulonephritis induced by injecting Brown Norway (BN) rats with heavy metals. This drug also prevents the chronic graft vs host reaction induced by injecting CD4(+) T cells from BN rats into (LEW x BN)F(1) hybrids. By contrast, treatment with nicardipine has no effect on the Th1-dependent experimental autoimmune encephalomyelitis triggered in LEW rats immunized with myelin. These data indicate that 1) DHPR are a selective marker of Th2 cells, 2) these calcium channels contribute to calcium signaling in Th2 cells, and 3) blockers of these channels are beneficial in the treatment of Th2-mediated pathologies.

Resting T cells negatively regulate osteoclast generation from peripheral blood monocytes

There is accumulating evidence that T cells may be involved in osteoclastogenesis in a variety of murine systems. However, the precise role of human T cells in the regulation of osteoclast generation is still unclear. To address this issue, we investigated the effect of resting peripheral T cells on receptor activator of NF-kappaB ligand (RANKL)-induced osteoclast generation from human peripheral monocytes. Although osteoclasts were not generated in the culture of human peripheral blood mononuclear cells (PBMC) in the presence of RANKL and macrophage colony-stimulating factor (M-CSF), the addition of cyclosporine A (CsA), a potent inhibitor of T-cell function, resulted in the formation of an increasing number of lacunae resorption on dentine, suggesting T cells may inhibit osteoclast formation. In a coculture of T cells and monocytes, which were isolated from PBMC, T cells inhibited the osteoclast generation from monocytes, as determined by tartrate-resistant acid phosphatase (TRAP) staining and a pit assay using dentine. This inhibition of osteoclast generation by T cells was also observed in a culture of the parathyroid hormone-stimulated SaOS4/3 osteoblast cell line and monocytes. The culture in Transwell plates revealed that the cell-to-cell interaction was not required for the inhibition, suggesting that T-cell cytokines may be responsible for the inhibition. Among inhibitory T-cell cytokines on osteoclastogenesis, granulocyte-macrophage colony-stimulating factor (GM-CSF) and interferon-gamma (IFN-gamma) were actively produced by CD4 T cells but not CD8 T cells in the coculture of T cells with monocytes, and the neutralizing antibodies to these cytokines partially rescued the T-cell-induced inhibition of osteoclast formation. Although CsA did not affect RANKL-induced osteoclast generation in the culture of monocytes alone, it completely rescued the T-cell-induced inhibition of osteoclast formation and strongly inhibited the production of GM-CSF and IFN-gamma. Thus, we demonstrate that resting T cells negatively regulate the osteoclast generation via production of GM-CSF and IFN-gamma by CD4 T cells and that CsA stimulates the osteoclast generation through the inhibition of the production of these cytokines. These findings provide new insight into therapeutic strategies for immunosuppression-induced bone loss in transplant and other diseases.

Friend of GATA is expressed in naive Th cells and functions as a repressor of GATA-3-mediated Th2 cell development

The commitment of naive T cells to polarized Th cells requires specific changes in their transcription factors. Retrovirally overexpressed GATA-3 has been reported to induce the Th2 cytokine profile in developing Th1 cells. In this study, we examined the role of the N-terminal finger (Nf) of GATA-3 in Th2 cell development. The Nf, as well as the C-terminal finger and the transactivation domain, is critical for the induction of the Th2 phenotype. Using the GATA-3-Nf as a bait, our yeast two-hybrid screening identified friend of GATA (FOG) in the Th2 cell-specific library. Naive T cells express significant levels of FOG mRNA, which was rapidly down-regulated upon commitment to both Th1 and Th2 lineages. In reporter assays, FOG blocked the GATA-3-mediated activation of several cytokine promoters. Finally, retroviral expression of FOG in developing Th2 cells suppressed both IL-4 and IL-5 and allowed for IFN-gamma production, which was accompanied by a significant level of T-bet mRNA expression. Serial deletion mutation analysis indicated that the N-terminal region, but not the consensus C-terminal binding protein-binding motif, of FOG is critical for the effects. Our results clearly indicate that 1) FOG is a repressor of GATA-3 in naive T cells and 2) the down-regulation of FOG induces Th2 cell differentiation by releasing GATA-3 from its repression.

Alveolar macrophages and T cells from sarcoid, but not normal lung, are permissive to adenovirus infection and allow analysis of NF-kappa b-dependent signaling pathways

Adenovirus (Adv)-mediated gene transfer requires efficient infection of target cells. The objective of this study was to establish whether alveolar macrophages (AM) and T cells (AT) from sarcoid patients were permissive to infection with Adv vectors and if this property could be used to investigate cytokine gene regulation. Sarcoid and normal bronchoalveolar lavage (BAL) specimens infected with Adv vectors expressing either beta-galactosidase or a green fluorescent protein were analyzed for transgene expression by fluorescence-activated cell sorter (FACS) and direct immunofluorescence, respectively. Expression of surface antigens previously associated with Adv infection, the coxsackie/adenovirus receptor (CAR), alpha v beta 3, and alpha v beta 5 integrins, was also assessed using FACS analysis. Sarcoid AM and AT were found to efficiently express Adv transgenes, unlike AM from normal volunteers, peripheral blood monocytes, and peripheral blood T cells. Cells permissive to Adv infection expressed the CAR and alpha v beta 5 integrin (also alpha v beta 3 integrin for AM). The data indicate that the upregulation of Adv receptors and the ability to infect sarcoid AM and AT are related to the inflammatory environment within the lung. Having demonstrated efficient Adv-mediated transgene delivery to sarcoid AM and AT, a construct encoding porcine I kappa B alpha was then used to investigate the requirement for nuclear factor (NF)-kappa B in the regulation of cytokine gene expression in pulmonary sarcoidosis. Overexpression of I kappa B alpha in sarcoid BAL specimens indicated that tumor necrosis factor-alpha and interleukin (IL)-6 production by AM and interferon (IFN)-gamma production by AT is NF-kappa B dependent, whereas IL-4 production by AT is NF-kappa B independent. This is the first occasion that the requirement for NF-kappa B in IFN-gamma gene expression within primary human T cells has been demonstrated. The results of this study have implications for the future investigation of molecular pathways in inflammatory lung disease.

Weak TCR stimulation induces a calcium signal that triggers IL-4 synthesis, stronger TCR stimulation induces MAP kinases that control IFN-gamma production

Th1 and Th2 cells produce different cytokines and have distinct functions. Th1/Th2 cell differentiation is influenced, among other factors, by the nature of TCR-MHC interactions. However, how the TCR transduces a signal resulting in IFN-gamma or IL-4 production is a matter of debate. For example, some authors reported a loss of calcium signaling pathway in Th2 cells. We used a T cell hybridoma producing IL-4 upon weak TCR stimulation and both IL-4 and IFN-gamma for strong TCR engagement as a model to study how TCR signaling pathways are differentially activated in both conditions of stimulation and how this influences the production of cytokines. We show that: (1) the calcium response is identical following weak and strong TCR stimulation; (2) mitogen-activated protein kinase(MAPK) activation is a gradual phenomenon depending upon the strength of TCR activation; (3) a calcium response, even weak, triggers IL-4 expression; (4) IFN-gamma synthesis requires not only a calcium response but also MAPK activation. The MAPK pathway is dispensable for IL-4 production, although it amplifies IL-4 synthesis upon strong TCR stimulation; (5) TCR-induced IL-4 production also depends on calcium signaling in Th2 cells, while IFN-gamma synthesis is dependent, in addition, on MAPK activation in Th1 cells.

Effects of a constitutively active form of calcineurin on T cell activation and thymic selection

Calcineurin is a calcium/calmodulin-dependent phosphatase whose activity is required for the induction of T cell lymphokine production and proliferation. Although its specific role in T cell development is less well defined, studies with the immunosuppressive drugs cyclosporin A and FK-506 suggest that it is involved in both positive and negative selection of immature thymocytes. To more completely characterize a role for calcineurin in T cell development in vivo, we have generated transgenic mice that express an activated form of this enzyme in thymocytes and peripheral T cells. We find that the transgene causes a block in early thymic development, resulting in a reduction in the steady-state number of CD4 and CD8 double positives, but not on the number of mature T cells. We also find that thymocytes and mature T cells expressing this transgene are more sensitive to signals through their TCR. In thymocytes this sensitivity difference is manifested as an increase in positive selection, although negative selection seems to remain unaffected. Therefore, these studies confirm and extend past reports that suggested a role for calcineurin in thymic development and selection.

Differences in regulatory pathways identify subgroups of T cell-derived Th2 cytokines

We analysed regulatory mechanisms involved in the production of Th2 cytokines by freshly isolated human T cells. We used an in vitro culture system in which the primary signal was provided by a cross-linking anti-CD3 MoAb presented on the Fc receptors of P815 cells. Both CD80 and CD86, expressed on transfected P815 cells, were able to provide efficient costimulation for the production of IL-4, IL-5 and IL-13. IL-2 was also highly important for induction of all three Th2 cytokines. However, differences between IL-4 on the one hand and IL-5 and IL-13 on the other hand were observed when sensitivity to cyclosporin A (CsA) was studied. CsA (an inhibitor of calcineurin phosphatase activity) strongly inhibited IL-4 production, but it did either not affect or even increased IL-5 and IL-13 production. In accordance with this, CD80 and phorbol myristate acetate (PMA) (without anti-CD3 or calcium ionophore) were sufficient to induce production of IL-5 and IL-13, but not of IL-4. The subgrouping of Th2 cytokines was further confirmed at another level on the basis of differences in cell sources: IL-4 was predominantly produced by CD4+ T cells, while IL-5 and IL-13 were produced by both CD4+ and CD8+ T cells. Thus, differences in cell sources and in the requirement of the calcium/calcineurin-signalling pathway allowed us to identify two subgroups (IL-4 and IL-5/IL-13) among human Th2-type T cell cytokines.

Ganglioside control over IL-4 priming and cytokine production in activated T cells

Our previous studies have shown that the enzymatic activities of Neu-1, an endogenous sialidase encoded in the murine MHC, are involved in promoting IL-4 synthesis by naive CD4(+)T cells. Our present studies have characterized responsible sialoconjugate targets of Neu-1 and questioned possible biochemical mechanisms responsible for their regulatory influences on IL-4 gene expression. These studies determined that treatment of T cells with the naturally occurring ganglioside GM3 inhibited the production of IL-4 without affecting the production of IL-2. An analysis of IL-4-primed CD4(+)T cells further demonstrated that GM3 treatment specifically inhibited the restimulated production of IL-4, IL-5 and IL-13, without inhibiting the production of IL-2 and IFN-gamma. The inhibitory effects of GM3 could be overcome by treatment with thapsigargin or ionomycin, suggesting ganglioside regulation occurs upstream of activation-induced calcium mobilization. GM3 treatment attenuated the level of calcium influx following CD3epsilon crosslinking, and CD4(+)T cells from Neu-1-deficient B10.SM strain mice (neu-1(a)and IL-4-deficient) expressed reduced levels of intracellular calcium following activation. Our results indicate that activities by membrane gangliosides can influence the cytokine programs in CD4(+)T cells, possibly through the modulation of calcium responses induced by T cell activation.

Local regulation of immune responses: corneal endothelial cells alter t cell activation and cytokine production

Corneal endothelial cells (CE cells) inhibit antigen- and mitogen-activated lymphocyte proliferation assays, although interleukin 2 receptor (IL-2R) expression and responsiveness to exogenous IL-2 are unaffected. To examine this activity further, co-cultures of CE cells and T cell clones were studied. CE cells inhibited IL-2 and IL-4 production by T cells stimulated with Ag and APC, but not IL-5 or IL-6 production. CE cells also inhibited NFAT-driven lacZ reporter gene production following Ag stimulation of transfected KZO T hybridoma cells. Conversely, stimulation of IL-2 production by ionomycin, with or without PMA, was unaffected by the CE cells. Preincubation of KZO hybridoma or Jurkat cells with CE cells, or CE cell-conditioned culture supernatant, inhibited the intracellular calcium ([Ca(2+)](i)) increase induced by TCR ligation, but not the [Ca(2+)](i)increase induced by ionomycin or thapsigargin. The inhibitory effect was independent of APC and did not act by blocking costimulation, since IL-2 production stimulated by immobilized anti-CD3 alone was also inhibited by CE cells. The supernatant factor was heat labile. This novel activity is unlike other immunoregulatory molecules, including transforming growth factor beta (TGF-beta) and may contribute to local immune privilege.

Glucocorticoids inhibit calcium- and calcineurin-dependent activation of the human IL-4 promoter

The mechanism by which glucocorticoids (GC) inhibit IL-4 gene expression is currently unknown. In T lymphocytes, IL-4 gene expression is regulated at the level of transcription by increases in intracellular calcium concentration and by the calcium-activated phosphatase calcineurin. In this paper we report that dexamethasone (Dex) inhibits calcium ionophore-induced activation of the human IL-4 promoter in transiently transfected Jurkat T cells. Inhibition of the promoter by Dex is dependent on expression of the GC receptor (GR), because it does not occur in GR-deficient cells. Dex also represses activation of the promoter induced by cotransfecting cells with a constitutively active mutant of calcineurin. Using a series of deletion constructs, we show that the proximal 95 bp of the IL-4 promoter contain a Dex-sensitive regulatory element. This region contains the P1 sequence, a proximal binding site for NF-AT. A calcium-induced but Dex-inhibited nuclear complex containing NF-AT binds to the P1 element in EMSA. Using immunoprecipitation under nondenaturing conditions, we found that the GRalpha isoform coprecipitates with NF-ATc in nuclear extracts of calcium ionophore- and Dex-treated cells. Taken together, our results show that GC inhibit IL-4 gene expression by interfering with NF-AT-dependent transactivation of the proximal human IL-4 promoter.

Role of nuclear factor of activated T cells (NFAT) in the expression of interleukin-5 and other cytokines involved in the regulation of hemopoetic cells

NFAT (nuclear factor of activated T cells) is a transcription factor that plays a role in the regulation of various cytokines, including those involved in the regulation of hemopoetic cells such as granulocyte-macrophage colony stimulating factor (GM-CSF), interleukin-4 (IL4), interleukin-3 (IL3), interleukin-13 (IL13) and interleukin-5 (IL5). In this report we provide a summary of the various locations in the promoters of each of these cytokines where NFAT has been shown or suggested to bind, and at which sites NFAT has been shown to be involved in transcriptional regulation. We also provide experimental data to show that the binding of NFAT to the nucleotides GAA at positions -113 to -111 of the human IL5 promoter is associated with functional activity in human T cells.

Calcineurin. Structure, function, and inhibition

Calcineurin is a serine-threonine specific Ca(2+)-calmodulin-activated protein phosphatase that is conserved from yeast to humans. Remarkably, this enzyme is the common target for two novel and structurally unrelated immunosuppressive antifungal drugs, cyclosporin A and FK506. Both drugs form complexes with abundant intracellular binding proteins, cyclosporin A with cyclophilin A and FK506 with FKBP 12, which bind to and inhibit calcineurin. The X-ray structure of an FKPB12-FK506-calcineurin AB ternary complex reveals that FKBP12-FK506 binds in a hydophobic groove between the calcineurin A catalytic and the regulatory B subunit, in accord with biochemical and genetic studies on inhibitor action. Calcineurin plays a key role in regulating the transcription factor NF-AT during T-cell activation, and in mediating responses of microorganisms to cation stress. These findings highlight the potential of yeast genetic studies to define novel drug targets and elucidate conserved elements of signal transduction cascades.

T Cell Priming Enhances IL-4 Gene Expression by Increasing Nuclear Factor of Activated T Cells

The repetitive activation of T cells (priming) enhances the expression of many cytokines, such as IL-4, but not others, such as IL-2. Molecular mechanisms underlying selective expression of cytokines by T cells remain poorly understood. Here we show that priming of CD4 T cells selectively enhances IL-4 expression relative to IL-2 expression by a transcriptional mechanism involving nuclear factor of activated T cells (NFAT) proteins. As detected by in vivo footprinting, priming markedly increases the activation-dependent engagement of the P0 and P1 NFAT-binding elements of the IL-4 promoter. Moreover, each proximal P element is essential for optimal IL-4 promoter activity. Activated primed CD4 T cells contain more NFAT1 and support greater NFAT-directed transcription than unprimed CD4 T cells, while activator protein 1 binding and activator protein 1-mediated transcription by both cell types is similar. Increased expression of wild-type NFAT1 substantially increases IL-4 promoter activity in unprimed CD4 T cells, suggesting NFAT1 may be limiting for IL-4 gene expression in this cell type. Furthermore, a truncated form of NFAT1 acts as a dominant-negative, reducing IL-4 promoter activity in primed CD4 T cells and confirming the importance of endogenous NFAT to increased IL-4 gene expression by effector T cells. NFAT1 appears to be the major NFAT family member responsible for the initial increased expression of IL-4 by primed CD4 T cells.

Distinct NFAT Family Proteins Are Involved in the Nuclear NFAT-DNA Binding Complexes from Human Thymocyte Subsets

The nuclear factor of activated T cells (NFAT) is involved in the transcriptional induction of cytokine and other immunoregulatory genes during an immune response. Among four distinct NFAT family members identified to date, mRNAs of NFAT1, NFATc, and NFATx are expressed in the thymus. Here, we report the distribution of these three NFAT family members in human fetal thymocyte subsets and in peripheral mature T cells. We show that NFATx mRNA was expressed in all T lymphocyte subsets tested and was highest in CD4+CD8+ double positive (DP) thymocytes. Conversely, NFAT1 mRNA was preferentially expressed in the mature CD4+ single positive (SP) populations. NFATc mRNA was present at low levels in all subsets but strongly induced upon treatment with phorbol ester and calcium ionophore. Interestingly, we detected NFAT-DNA binding complexes in DP thymocytes, albeit at lower levels than in CD4 SP cells. Corresponding to the mRNA expression, we observed that NFATx was responsible for the NFAT-DNA binding in DP thymocytes. Moreover, this DNA binding was inhibited by cyclosporin A, indicating that NFATx nuclear translocation was regulated by the calcineurin phosphatase in DP thymocytes. For the CD4 SP populations, NFAT1 and NFATc, and to some extent NFATx, were responsible for the NFAT-DNA binding complexes. These results indicate that NFAT family members are differentially regulated during the development of T cells, and that NFATx may play a distinct role in calcineurin-dependent signaling in DP thymocytes.

HMG I(Y) interferes with the DNA binding of NF-AT factors and the induction of the interleukin 4 promoter in T cells

HMG I(Y) proteins bind to double-stranded A + T oligonucleotides longer than three base pairs. Such motifs form part of numerous NF-AT-binding sites of lymphokine promoters, including the interleukin 4 (IL-4) promoter. NF-AT factors share short homologous peptide sequences in their DNA-binding domain with NF-kappa B factors and bind to certain NF-kappa B sites. It has been shown that HMG I(Y) proteins enhance NF-kappa B binding to the interferon beta promoter and virus-mediated interferon beta promoter induction. We show that HMG I(Y) proteins exert an opposite effect on the DNA binding of NF-AT factors and the induction of the IL-4 promoter in T lymphocytes. Introduction of mutations into a high-affinity HMG I(Y)-binding site of the IL-4 promoter, which decreased HMG I(Y)-binding to a NF-AT-binding sequence, the Pu-bB (or P) site, distinctly increased the induction of the IL-4 promoter in Jurkat T leukemia cells. High concentrations of HMG I(Y) proteins are able to displace NF-ATp from its binding to the Pu-bB site. High HMG I(Y) concentrations are typical for Jurkat cells and peripheral blood T lymphocytes, whereas E14 T lymphoma cells and certain T helper type 2 cell clones contain relatively low HMG I(Y) concentrations. Our results indicate that HMG I(Y) proteins do not cooperate, but instead compete with NF-AT factors for the binding to DNA even though NF-AT factors share some DNA-binding to DNA even though NF-AT factors share some DNA-binding properties with NF-kB factors. This competition between HMG I(Y) and NF-AT proteins for DNA binding might be due to common contacts with minor groove nucleotides of DNA and may be one mechanism contributing to the selective IL-4 expression in certain T lymphocyte populations, such as T helper type 2 cells.

Repression of interleukin-2 and interleukin-4 promoters by tumor suppressor protein p53

Interleukin 2 (IL-2) and interleukin 4 (IL-4) secreted by activated but not by resting mature T cells are pleiotropic cytokines affecting growth and differentiation of diverse cell types, such as T cells, B cells, and mast cells. There is little information about the molecular basis for the constitutive repression of IL-2 and IL-4 gene expression in unstimulated T cells. We investigated the possibility that wild-type (wt) p53, a nuclear tumor suppressor protein, might serve to repress IL-2 and IL-4 gene expression in murine E14 T lymphoma and in human Jurkat cells. We transiently cotransfected these cells with constitutive simian virus 40 (SV 40) early promoter expression plasmids overproducing wt or mutant murine p53 and with appropriate luciferase (luc) reporter plasmids containing the promoter elements of murine IL-2 and IL-4 genes to evaluate the effect of various p53 species on these promoters. Murine wt p53 derived from pSG5p53cD strongly repressed the IL-2 and IL-4 promoters in both cell lines induced by the phorbol ester TPA and the Ca2+ ionophore ionomycin but not, however, in uninduced cells. In similar transient transfection experiments with lymphoma cells, overexpression of deletion mutant species of murine p53 revealed that the N-terminal and C-terminal domains are crucial for inhibition of both IL-2 and IL-4 gene expression. These parts of p53 comprise the transactivation domain at the amino terminal side, which has previously also been shown to interact with the TATA-box binding-protein TBP and the carboxy-terminal oligomerization domain. Additionally, it was shown that a previously described inhibitory protein, the high-mobility-group protein HMG-I/Y, does not functionally interact with p53. Cotransfection of expression plasmids for both p53 and HMG-I/Y did not alter the extent of inhibition by the individual proteins. These data suggest that p53 can downmodulate both IL-2 and IL-4 gene expression and that both the transactivation and oligomerization domains of the tumor suppressor protein are essential for this transcriptional repression.

The CD40L promoter contains nuclear factor of activated T cells-binding motifs which require AP-1 binding for activation of transcription

Four nuclear factor of activated T cells (NF-AT) binding motifs were found in the murine CD40 ligand promoter. Electrophoretic mobility shift assays using 18-base pair (bp) long oligonucleotides corresponding to the proximal site and nuclear extracts from activated T cells revealed two complexes which were inhibited by cyclosporin A and contained NF-ATc and NF-ATp. Neither complex contained AP-1 proteins. Multimers of the 18-bp oligonucleotides were not active in transient transfection assays using luciferase reporter gene constructs. In contrast, a 30-bp long oligonucleotide bound AP-1 proteins in addition to NF-AT proteins and its multimers strongly induced luciferase gene expression. These results suggested that NF-AT proteins play an important role in the expression of the CD40L gene and that their transcriptional activity requires AP-1 binding.

Identification of an ionomycin/cyclosporin A-responsive element within the human T cell receptor gamma enhancer

Activation through the Ca2+/calcineurin pathway is essential to the transcription of many cytokine genes. The conserved cis-acting sequence, GGAAAA, and transcription factors binding to this sequence are involved in the response to increased intracellular Ca2+ concentrations. Here we report the identification and importance of the same sequence in a non-cytokine gene, the human T cell receptor gamma (TCRG) enhancer. Results from site-directed mutations and electrophoretic mobility shift assays strongly suggest that this sequence mediates the ionomycin-induced activation of the TCRG enhancer. Our studies provide an explanation for a previous observation that TCRG mRNA levels, but not mRNA levels for T cell receptor alpha and -beta, are increased by ionomycin treatment.