Interleukin-2 transcription is regulated in vivo at the level of coordinated binding of both constitutive and regulated factors

TRPC-mediated Ca2+ signaling and control of cellular functions

Canonical members of the TRP superfamily of ion channels have long been recognized as key elements of Ca²⁺ handling in a plethora of cell types. The emerging role of TRPC channels in human physiopathology has generated considerable interest in their pharmacological targeting, which requires detailed understanding of their molecular function. Although consent has been reached that receptor-phospholipase C (PLC) pathways and generation of lipid mediators constitute the prominent upstream signaling process that governs channel activity, multimodal sensing features of TRPC complexes have been demonstrated repeatedly. Downstream signaling by TRPC channels is similarly complex and involves the generation of local and global cellular Ca²⁺ rises, which are well-defined in space and time to govern specific cellular functions. These TRPC-mediated Ca²⁺ signals rely in part on Ca²⁺ permeation through the channels, but are essentially complemented by secondary mechanisms such as Ca²⁺ mobilization from storage sites and Na⁺/Ca²⁺ exchange, which involve coordinated interaction with signaling partners. Consequently, the control of cell functions by TRPC molecules is critically determined by dynamic assembly and subcellular targeting of the TRPC complexes. The very recent availability of high-resolution structure information on TRPC channel complexes has paved the way towards a comprehensive understanding of signal transduction by TRPC channels. Here, we summarize current concepts of cation permeation in TRPC complexes, TRPC-mediated shaping of cellular Ca²⁺ signals and the associated control of specific cell functions.

Encounters across networks: Windows into principles of genomic regulation

Gene regulatory networks account for the ability of the genome to program development in complex multi-cellular organisms. Such networks are based on principles of gene regulation by combinations of transcription factors that bind to specific cis-regulatory DNA sites to activate transcription. These cis-regulatory regions mediate logic processing at each network node, enabling progressive increases in organismal complexity with development. Gene regulatory network explanations of development have been shown to account for patterning and cell type diversification in fly and sea urchin embryonic systems, where networks are characterized by fast coupling between transcriptional inputs and changes in target gene transcription rates, and crucial cis-regulatory elements are concentrated relatively close to the protein coding sequences of the target genes, thus facilitating their identification. Stem cell-based development in post-embryonic mammalian systems also depends on gene networks, but differs from the fly and sea urchin systems. First, the number of regulatory elements per gene and the distances between regulatory elements and the genes they control are considerably larger, forcing searches via genome-wide transcription factor binding surveys rather than functional assays. Second, the intrinsic timing of network state transitions can be slowed considerably by the need to undo stem-cell chromatin configurations, which presumably add stability to stem-cell states but retard responses to transcription factor changes during differentiation. The dispersed, partially redundant cis-regulatory systems controlling gene expression and the slow state transition kinetics in these systems already reveal new insights and opportunities to extend understanding of the repertoire of gene networks and regulatory system logic.

STIM1 and Orai1 regulate Ca2+ microdomains for activation of transcription

Since calcium (Ca²⁺) regulates a large variety of cellular signaling processes in a cell's life, precise control of Ca²⁺ concentrations within the cell is essential. This enables the transduction of information via Ca²⁺ changes in a time-dependent and spatially defined manner. Here, we review molecular and functional aspects of how the store-operated Ca²⁺ channel Orai1 creates spatiotemporal Ca²⁺ microdomains. The architecture of this channel is unique, with a long helical pore and a six-fold symmetry. Energetic barriers within the Ca²⁺ channel pathway limit permeation to allow an extensive local Ca²⁺ increase in close proximity to the channel. The precise timing of the Orai1 channel function is controlled by direct binding to STIM proteins upon Ca²⁺ depletion in the endoplasmic reticulum. These induced Ca²⁺ microdomains are tailored to, and sufficient for, triggering long-term activation processes, such as transcription factor activation and subsequent gene regulation. We describe the principles of spatiotemporal activation of the transcription factor NFAT and compare its signaling characteristics to those of the autophagy regulating transcription factors, MITF and TFEB.

Constitutive and inducible in vivo protein-DNA interactions at the tumor necrosis factor-alpha promoter in primary human T lymphocytes

Tumor necrosis factor-alpha (TNF-alpha) is a key cytokine of lymphocytes with major regulatory functions in immunomodulation, chronic inflammation, and septic shock. However, only limited information on TNF promoter regulation in vivo in primary lymphocytes is available. To determine and compare protein-DNA interactions at the native TNF locus in primary lymphocytes, we analyzed the human TNF-alpha promoter by ligation-mediated polymerase chain reaction (LM-PCR) techniques. Accordingly, primary CD4+ T lymphocytes from peripheral blood were cultured in the presence of various stimuli and analyzed by LM-PCR. Inducible in vivo protein-DNA interactions at the TNF promoter were detected between -120 and -70 bp of the human TNF promoter relative to the transcriptional start site. This area includes binding sites for transcription factors such as ETS-1, NFAT, ATF-2/c-jun, SP-1/Egr-1, and NF-kappaB. In contrast, no protein-DNA interactions were observed at various binding sites with reported regulatory function in tumor cell lines such as the k2 element, the NFAT site at -160, the AP1 site at -50, and the SP1 site at -65. Additional mutagenesis and transfection studies demonstrated that NF-kappaB and CREB/AP-1 are important regulators of inducible TNF promoter activity in primary human T lymphocytes. These results provide novel insights into the complex regulation of TNF gene transcription in primary T lymphocytes in vivo by constitutive and inducible protein-DNA interactions that appear to be at least partially different compared to previously characterized tumor cell lines.

Oct1 and OCA-B are selectively required for CD4 memory T cell function

Shakya et al. identify the transcription factor Oct1 and its cofactor OCA-B as central mediators for generating memory T cell responses in mice.

Epigenetic changes are crucial for the generation of immunological memory. Failure to generate or maintain these changes will result in poor memory responses. Similarly, augmenting or stabilizing the correct epigenetic states offers a potential method of enhancing memory. Yet the transcription factors that regulate these processes are poorly defined. We find that the transcription factor Oct1 and its cofactor OCA-B are selectively required for the in vivo generation of CD4⁺ memory T cells. More importantly, the memory cells that are formed do not respond properly to antigen reencounter. In vitro, both proteins are required to maintain a poised state at the Il2 target locus in resting but previously stimulated CD4⁺ T cells. OCA-B is also required for the robust reexpression of multiple other genes including Ifng. ChIPseq identifies ∼50 differentially expressed direct Oct1 and OCA-B targets. We identify an underlying mechanism involving OCA-B recruitment of the histone lysine demethylase Jmjd1a to targets such as Il2, Ifng, and Zbtb32. The findings pinpoint Oct1 and OCA-B as central mediators of CD4⁺ T cell memory.

Epigenetic repression of interleukin 2 expression in senescent CD4+ T cells during chronic HIV type 1 infection

The molecular mechanisms for IL2 gene-specific dysregulation during chronic human immunodeficiency virus type 1 (HIV-1) infection are unknown. Here, we investigated the role of DNA methylation in suppressing interleukin 2 (IL-2) expression in memory CD4(+) T cells during chronic HIV-1 infection. We observed that CpG sites in the IL2 promoter of CD4(+) T cells were fully methylated in naive CD4(+) T cells and significantly demethylated in the memory populations. Interestingly, we found that the memory cells that had a terminally differentiated phenotype and expressed CD57 had increased IL2 promoter methylation relative to less differentiated memory cells in healthy individuals. Importantly, early effector memory subsets from HIV-1-infected subjects expressed high levels of CD57 and were highly methylated at the IL2 locus. Furthermore, the increased CD57 expression on memory CD4(+) T cells was inversely correlated with IL-2 production. These data suggest that DNA methylation at the IL2 locus in CD4(+) T cells is coupled to immunosenescence and plays a critical role in the broad dysfunction that occurs in polyclonal T cells during HIV-1 infection.

Implications of purinergic receptor-mediated intracellular calcium transients in neural differentiation

Purinergic receptors participate, in almost every cell type, in controlling metabolic activities and many physiological functions including signal transmission, proliferation and differentiation. While most of P2Y receptors induce transient elevations of intracellular calcium concentration by activation of intracellular calcium pools and forward these signals as waves which can also be transmitted into neighboring cells, P2X receptors produce calcium spikes which also include activation of voltage-operating calcium channels. P2Y and P2X receptors induce calcium transients that activate transcription factors responsible for the progress of differentiation through mediators including calmodulin and calcineurin. Expression of P2X2 as well as of P2X7 receptors increases in differentiating neurons and glial cells, respectively. Gene expression silencing assays indicate that these receptors are important for the progress of differentiation and neuronal or glial fate determination. Metabotropic receptors, mostly P2Y1 and P2Y2 subtypes, act on embryonic cells or cells at the neural progenitor stage by inducing proliferation as well as by regulation of neural differentiation through NFAT translocation. The scope of this review is to discuss the roles of purinergic receptor-induced calcium spike and wave activity and its codification in neurodevelopmental and neurodifferentiation processes.

Two-step binding of transcription factors causes sequential chromatin structural changes at the activated IL-2 promoter

Most gene promoters have multiple binding sequences for many transcription factors, but the contribution of each of these factors to chromatin remodeling is still unclear. Although we previously found a dynamic change in the arrangement of nucleosome arrays at the Il2 promoter during T cell activation, its timing preceded that of a decrease in nucleosome occupancy at the promoter. In this article, we show that the initial nucleosome rearrangement was temporally correlated with the binding of NFAT1 and AP-1 (Fos/Jun), whereas the second step occurred in parallel with the recruitment of other transcription factors and RNA polymerase II. Pharmacologic inhibitors for activation of NFAT1 or induction of Fos blocked the initial phase in the sequential changes. This step was not affected, however, by inhibition of c-Jun phosphorylation, which instead blocked the binding of the late transcription factors, the recruitment of CREB-binding protein, and the acetylation of histone H3 at lysine 27. Thus, the sequential recruitment of transcription factors appears to facilitate two separate steps in chromatin remodeling at the Il2 locus.

Enhanced chromatin accessibility and recruitment of JUNB mediate the sustained IL-4 expression in NFAT1 deficient T helper 2 cells

Nuclear factor of activated T cells (NFAT) is a family of transcription factors composed of five proteins. Among them, NFAT1 is a predominant NFAT protein in CD4(+) T cells. NFAT1 positively regulates transcription of a large number of inducible cytokine genes including IL-2, IL-4, IL-5 and other cytokines. However, disruption of NFAT1 results in an unexpected increase of IL-4. In this study, we have investigated the role of NFAT1 in regulation of IL-4 gene expression in T helper 2 cells (Th2) from an epigenetic viewpoint. NFAT1 deficient Th2 cells showed a sustained IL-4 expression while wild type (WT) cells reduced its expression. We tested whether epigenetic maintenance and changes in the chromatin architecture of IL-4 promoter locus play a role in differential IL-4 transcription between in WT and NFAT1 deficient Th2 cells. Compared with WT, NFAT1 deficient CD4(+) Th2 cells exhibited enhanced chromatin accessibility with permissive histone modification and DNA demethylation in the IL-4 promoter region. Transcription factors bound to IL-4 promoter region in the absence of NFAT1 were identified by Micro-LC/LC-MS/MS analysis. Among the candidates, preferential recruitment of JUNB to the IL-4 promoter was confirmed by chromatin immunoprecipitation analysis. Overexpression of JUNB together with SATB1 synergistically upregulated IL-4 promoter activity, while knockdown JUNB significantly reduced IL-4 expression. Our results suggest that the prolonged IL-4 expression in NFAT1 deficient Th2 cells is mediated by preferential binding of JUNB/SATB1 to the IL-4 promoter with permissive chromatin architecture.

Multilayered specification of the T-cell lineage fate

T-cell development from stem cells has provided a highly accessible and detailed view of the regulatory processes that can go into the choice of a cell fate in a postembryonic, stem cell-based system. But it has been a view from the outside. The problems in understanding the regulatory basis for this lineage choice begin with the fact that too many transcription factors are needed to provide crucial input: without any one of them, T-cell development fails. Furthermore, almost all the factors known to provide crucial functions during the climax of T-lineage commitment itself are also vital for earlier functions that establish the pool of multilineage precursors that would normally feed into the T-cell specification process. When the regulatory genes that encode them are mutated, the confounding effects on earlier stages make it difficult to dissect T-cell specification genetically. Yet both the positive and the negative regulatory events involved in the choice of a T-cell fate are actually a mosaic of distinct functions. New evidence has emerged recently that finally provides a way to separate the major components that fit together to drive this process. Here, we review insights into T-cell specification and commitment that emerge from a combination of molecular, cellular, and systems biology approaches. The results reveal the regulatory structure underlying this lineage decision.

T-cell identity and epigenetic memory

T-cell development endows cells with a flexible range of effector differentiation options, superimposed on a stable core of lineage-specific gene expression that is maintained while access to alternative hematopoietic lineages is permanently renounced. This combination of features could be explained by environmentally responsive transcription factor mobilization overlaying an epigenetically stabilized base gene expression state. For example, "poising" of promoters could offer preferential access to T-cell genes, while repressive histone modifications and DNA methylation of non-T regulatory genes could be responsible for keeping non-T developmental options closed. Here, we critically review the evidence for the actual deployment of epigenetic marking to support the stable aspects of T-cell identity. Much of epigenetic marking is dynamically maintained or subject to rapid modification by local action of transcription factors. Repressive histone marks are used in gene-specific ways that do not fit a simple, developmental lineage-exclusion hierarchy. We argue that epigenetic analysis may achieve its greatest impact for illuminating regulatory biology when it is used to locate cis-regulatory elements by catching them in the act of mediating regulatory change.

The SNF2H chromatin remodeling enzyme has opposing effects on cytokine gene expression

Cytokine gene expression is a key control point in the function of the immune system. Cytokine gene regulation is linked to changes in chromatin structure; however, little is known about the remodeling enzymes mediating these changes. Here we investigated the role of the ATP-dependent chromatin remodeling enzyme SNF2H in mouse T cells; to date, SNF2H has not been investigated in T cells. We found that SNF2H repressed expression of IL-2 and other cytokines in activated cells. By contrast, SNF2H activated expression of IL-3. The ISWI components SNF2H and ACF1 bound to the tested loci, suggesting the regulation was direct. SNF2H decreased accessibility at some binding sites within the IL2 locus, and increased accessibility within some IL3 binding sites. The changes in gene expression positively correlated with accessibility changes, suggesting a simple model that accessibility enables transcription. We also found that loss of the ISWI ATPase SNF2H reduced binding to target genes and protein expression of ACF1, a binding partner for SNF2H, suggesting complex formation stabilized ACF1. Together, these findings reveal a direct role for SNF2H in both repression and activation of cytokine genes.

Evaluation of clinical biomaterial surface effects on T lymphocyte activation

Previous in vitro studies in our laboratory have shown that lymphocytes can influence macrophage adhesion and fusion on biomaterial surfaces. However, few studies have evaluated how material adherent macrophages can influence lymphocyte behavior, specifically T cells. In this study, we cultured human peripheral blood mononuclear cells from healthy donors on three synthetic nonbiodegradable biomedical polymers: elasthane 80A (PEU), silicone rubber (SR), or polyethylene terephthalate (PET) and tissue culture polystyrene (TCPS). Upregulation of T cell surface activation markers (CD69 and CD25), lymphocyte proliferation, and interleukin-2 (IL-2) and interferon-gamma (IFNgamma) concentrations were evaluated by flow cytometry, carboxy-fluorescein diacetate, succinimydyl ester (CFSE) incorporation, and multiplex cytokine immunoassay, respectively, to assess T cell activation. Following 3 and 7 days of culture, CD4+ helper T cells from cultures of any of the material groups did not express the activation markers CD69 and CD25 and lymphocyte proliferation was not present. IL-2 and IFNgamma levels were produced, but dependent on donor. These data indicate that T cells are not activated in response to clinically relevant synthetic biomaterials. The data also suggest that lymphocyte subsets exclusive of T cells are the source of the lymphokines, IL-2 and IFN-gamma, in certain donors.

An emerging player in the adaptive immune response: microRNA-146a is a modulator of IL-2 expression and activation-induced cell death in T lymphocytes

Activation of the T cell-mediated immune response has been associated with changes in the expression of specific microRNAs (miRNAs). However, the role of miRNAs in the development of an effective immune response is just beginning to be explored. This study focuses on the functional role of miR-146a in T lymphocyte-mediated immune response and provides interesting clues on the transcriptional regulation of miR-146a during T-cell activation. We show that miR-146a is low in human naive T cells and is abundantly expressed in human memory T cells; consistently, miR-146a is induced in human primary T lymphocytes upon T-cell receptor (TCR) stimulation. Moreover, we identified NF-kB and c-ETS binding sites as required for the induction of miR-146a transcription upon TCR engagement. Our results demonstrate that several signaling pathways, other than inflammation, are influenced by miR-146a. In particular, we provide experimental evidence that miR-146a modulates activation-induced cell death (AICD), acting as an antiapoptotic factor, and that Fas-associated death domain (FADD) is a target of miR-146a. Furthermore, miR-146a enforced expression impairs both activator protein 1 (AP-1) activity and interleukin-2 (IL-2) production induced by TCR engagement, thus suggesting a role of this miRNA in the modulation of adaptive immunity.

Modeling the dynamics of transcriptional gene regulatory networks for animal development

The dynamic process of cell fate specification is regulated by networks of regulatory genes. The architecture of the network defines the temporal order of specification events. To understand the dynamic control of the developmental process, the kinetics of mRNA and protein synthesis and the response of the cis-regulatory modules to transcription factor concentration must be considered. Here we review mathematical models for mRNA and protein synthesis kinetics which are based on experimental measurements of the rates of the relevant processes. The model comprises the response functions of cis-regulatory modules to their transcription factor inputs, by incorporating binding site occupancy and its dependence on biologically measurable quantities. We use this model to simulate gene expression, to distinguish between cis-regulatory execution of "AND" and "OR" logic functions, rationalize the oscillatory behavior of certain transcriptional auto-repressors and to show how linked subcircuits can be dealt with. Model simulations display the effects of mutation of binding sites, or perturbation of upstream gene expression. The model is a generally useful tool for understanding gene regulation and the dynamics of cell fate specification.

3,4-Dichloropropionanilide (DCPA) inhibits T-cell activation by altering the intracellular calcium concentration following store depletion

Stimulation of T cells through the T-cell receptor results in the activation of a series of signaling pathways that leads to the secretion of interleukin (IL)-2 and cell proliferation. Influx of calcium (Ca(2+)) from the extracellular environment, following internal Ca(2+) store depletion, provides the elevated and sustained intracellular calcium concentration ([Ca(2+)](i)) critical for optimal T-cell activation. Our laboratory has documented that exposure to the herbicide 3,4-dichloropropionanilide (DCPA) inhibits intracellular signaling events that have one or more Ca(2+) dependent steps. Herein we report that DCPA attenuates the normal elevated and sustained [Ca(2+)](i) that follows internal store depletion in the human leukemic Jurkat T cell line and primary mouse T cells. DCPA did not alter the depletion of internal Ca(2+) stores when stimulated by anti-CD3 or thapsigargin demonstrating that early inositol 1,4,5-triphosphate-mediated signaling and depletion of Ca(2+) stores were unaffected. 2-Aminoethyldiphenol borate (2-APB) is known to alter the store-operated Ca(2+) (SOC) influx that follows Ca(2+) store depletion. Exposure of Jurkat cells to either DCPA or 50 microM 2-APB attenuated the increase in [Ca(2+)](i) following thapsigargin or anti-CD3 induced store depletion in a similar manner. At low concentrations, 2-APB enhances SOC influx but this enhancement is abrogated in the presence of DCPA. This alteration in [Ca(2+)](i), when exposed to DCPA, significantly reduces nuclear levels of nuclear factor of activated T cells (NFAT) and IL-2 secretion. The plasma membrane polarization profile is not altered by DCPA exposure. Taken together, these data indicate that DCPA inhibits T-cell activation by altering Ca(2+) homeostasis following store depletion.

Negative feedback regulation of T cells via interleukin-2 and FOXP3 reciprocity

As interleukin-2 (IL2) is central to the clonal expansion of antigen-selected T cells, we investigated the relationship between IL2 and the negative regulatory transcription factor FOXP3. We found IL2 to be responsible for T cell antigen receptor (TCR)-activated FOXP3 expression by both CD4+ and CD8+ human T cells, and as anticipated, FOXP3 expression restricted TCR-stimulated IL2 expression. However, no evidence could be found that FOXP3+ cells actively suppress IL2 expression by FOXP3- cells. These data are consistent with an IL2/FOXP3-dependent negative feedback loop that normally regulates the T cell immune response. It follows that a defect in this negative feedback loop as a result of a deficiency of either IL2 or FOXP3 will lead to a hyperproliferative autoimmune syndrome, without the necessity of invoking an active suppressive function for FOXP3+ T cells.

Digital NFATc2 activation per cell transforms graded T cell receptor activation into an all-or-none IL-2 expression

The expression of interleukin-2 (IL-2) is a key event in T helper (Th) lymphocyte activation, controlling both, the expansion and differentiation of effector Th cells as well as the activation of regulatory T cells. We demonstrate that the strength of TCR stimulation is translated into the frequency of memory Th cells expressing IL-2 but not into the amount of IL-2 per cell. This molecular switch decision for IL-2 expression per cell is located downstream of the cytosolic Ca²⁺ level. Here we show that in a single activated Th cell, NFATc2 activation is digital but NF-κB activation is graded after graded T cell receptor (TCR) signaling. Subsequently, NFATc2 translocates into the nucleus in an all-or-none fashion per cell, transforming the strength of TCR-stimulation into the number of nuclei positive for NFATc2 and IL-2 transcription. Thus, the described NFATc2 switch regulates the number of Th cells actively participating in an immune response.

Functional Role for IκBNS in T Cell Cytokine Regulation As Revealed by Targeted Gene Disruption

Triggering of the TCR by cognate peptide/MHC ligands induces expression of I kappa BNS, a member of the I kappa B family of NF-kappaB inhibitors whose expression is associated with apoptosis of immature thymocytes. To understand the role of I kappa BNS in TCR triggering, we created a targeted disruption of the I kappa BNS gene. Surprisingly, mice lacking I kappa BNS show normal thymic progression but both thymocytes and T cells manifest reduced TCR-stimulated proliferation. Moreover, I kappa BNS knockout thymocytes and T cells produce significantly less IL-2 and IFN-gamma than wild-type cells. Transfection analysis demonstrates that I kappa BNS and c-Rel individually increase IL-2 promoter activity. The effect of I kappa BNS on the IL-2 promoter, unlike c-Rel, is dependent on the NF-kappaB rather than the CD28RE site; mutation of the NF-kappaB site extinguishes the induction of transcription by I kappa BNS in transfectants and prevents association of I kappa BNS with IL-2 promoter DNA. Microarray analyses confirm the reduction in IL-2 production and some IFN-gamma-linked transcripts in I kappa BNS knockout T cells. Collectively, our findings demonstrate that I kappa BNS regulates production of IL-2 and other cytokines induced via "strong" TCR ligation.

NF90 regulates inducible IL-2 gene expression in T cells

Activation of T cells induces the production of T cell growth and survival factor interleukin (IL) 2. Regulatory T cells intrinsically fail to induce IL-2 expression upon activation and can suppress IL-2 production in conventional T cells. Thus, the control of IL-2 expression is critically important to T cell immune responses, yet the mechanisms remain incompletely understood. Nuclear factor (NF) 90 is a zinc-finger DNA- and double-stranded RNA-binding protein subunit that binds specifically to the antigen receptor response element (ARRE)/NF of activated T cells target sequence in the IL-2 proximal promoter. Inducible binding of NF90 to the IL-2 promoter in vivo is shown by chromatin immunoprecipitation. NF90 gene-targeted mice exhibit perinatal lethality. Compared with newborn NF90(+/+) mice, newborn NF90(-/-) mice demonstrate severe impairment of IL-2 expression. Compared with wild-type cells, T cells deficient in NF90 are impaired in ARRE and IL-2 transcriptional activation and IL-2 mRNA stabilization. Fetal liver cells from NF90 gene-targeted mice were transplanted into irradiated adult recombination activating gene (RAG)-2(-/-) and IL-2Rgamma(-/-) mice deficient in T cells, B cells, and natural killer cells. NF90(+/+)- and NF90(-/-)-RAG chimeric mice showed grossly normal repopulation of the thymus and spleen, but only NF90(-/-) T cells were severely impaired in IL-2 gene expression. Compared with littermates, NF90(-/-) RAG chimeric mice exhibited profound T cell lymphocytopenia in the peripheral circulation. Thus, NF90 regulates inducible IL-2 transcription, mRNA stability, and gene expression in T cells and represents a novel therapeutic target for the modulation of T cell immune responses.

Molecular mechanisms for adaptive tolerance and other T cell anergy models

Since the original description of T cell anergy in CD4 clones from mice and humans, a number of different unresponsive states have been described, both in vivo and in vitro, that have been called anergic. While initial attempts were made to understand the similarities between the different models, it has now become clear from biochemical experiments that many of them have different molecular mechanisms underlying their unresponsiveness. In this review we will detail our own work on the in vivo model referred to as adaptive tolerance and then attempt to compare this biochemical state to the multitude of other states that have been described in the literature.

Grb2 and Gads exhibit different interactions with CD28 and play distinct roles in CD28-mediated costimulation

Although both CD28 and ICOS bind PI3K and provide stimulatory signal for T cell activation, unlike CD28, ICOS does not costimulate IL-2 secretion. CD28 binds both PI3K and Grb2, whereas ICOS binds only PI3K. We have generated an ICOS mutant, which can bind Grb2 by replacement of its PI3K binding motif YMFM with the CD28 YMNM motif, and shown that it induces significant activation of the IL-2 promoter. However, this mutant ICOS was insufficient to activate the NF-kappaB pathway. In this study, we show that Gads, but not Grb2, is essential for CD28-mediated NF-kappaB activation, and its binding to CD28 requires the whole CD28 cytoplasmic domain in addition to the YMNM motif. Mutagenesis experiments have indicated that mutations in the N-terminal and/or C-terminal PXXP motif(s) of CD28 significantly reduce their association with Gads, whereas their associations with Grb2 are maintained. They induced strong activity of the NFAT/AP-1 reporter comparable with the CD28 wild type, but weak activity of the NF-kappaB reporter. Grb2- and Gads-dominant-negative mutants had a strong effect on NFAT/AP-1 reporter, but only Gads-dominant-negative significantly inhibited NF-kappaB reporter. Our data suggest that, in addition to the PI3K binding motif, the PXXP motif in the CD28 cytoplasmic domain may also define a functional difference between the CD28- and ICOS-mediated costimulatory signals by binding to Gads.

The quantal theory of immunity

Exactly how the immune system discriminates between all environmental antigens to which it reacts vs. all self-antigens to which it does not, is a principal unanswered question in immunology. As set forth in this review, because of the advances in our understanding of the immune system that have occurred in the last 50 years, for the first time it is possible to formulate a new theory, termed the "Quantal Theory of Immunity", which reduces the problem from the immune system as a whole, to the individual cells comprising the system, and finally to a molecular explanation as to how the system behaves as it does.

Low and high levels of α-tocopherol exert opposite effects on IL-2 possibly through the modulation of PPAR-γ, IκBα, and apoptotic pathway in activated splenocytes

OBJECTIVE

We previously demonstrated that a high dose of alpha-tocopheryl succinate inhibits interleukin-2 (IL-2) mRNA and production in autoimmune-prone MRL/lpr mice. In the present study, we investigated the regulation of alpha-tocopherol (alphaTOC) on IL-2 gene expression by examining the mRNA of IL-2, inhibitor kappaBalpha (IkappaBalpha), and peroxisome proliferator-activated receptor-gamma (PPARgamma).

METHODS

Messenger RNA expression in active splenocytes of BALB/c mice was investigated with reverse transcriptase polymerase chain reaction.

RESULTS

Levels of IL-2 mRNA in phorbol 12-myristate 13-acetate/ionomycin activated splenocytes and cytokine in T-helper-1 cells were increased by 50 microM of alphaTOC but decreased by 1 mM of alphaTOC. In addition, the IkappaBalpha gene expression significantly increased by the high dose (>or=500 microM) of alphaTOC, suggesting an inhibition on nuclear factor-kappaB pathway for activation of IL-2 expression. PPARgamma mRNA level in activated splenocytes was upregulated by 1 mM of alphaTOC. PPARgamma mRNA level in unstimulated splenocytes was upregulated by alphaTOC in a dose-dependent manner, suggesting that alphaTOC might enhance the PPARgamma signaling pathway. High-dose alphaTOC induced apoptosis of splenocytes and inhibited phytohemagglutinin-stimulated T-cell proliferation. Conversely, the proliferative response of splenocytes was enhanced by 5 microM of alphaTOC. Low-dose (50 microM) alphaTOC increased IL-2 expression, which may have been due to the absence of downregulation of PPARgamma and IkappaBalpha on the IL-2 gene.

CONCLUSION

The results indicated that low and high doses of alphaTOC exert opposite effects on IL-2, possibly through modulation of PPARgamma, IkappaBalpha, and apoptosis pathways. The present findings support our previous observation of opposite effects of low- and high-dose vitamin E on survival of MRL/lpr mice.

Adaptive tolerance and clonal anergy are distinct biochemical states

Adaptive tolerance is a process by which T cells become desensitized when Ag stimulation persists following an initial immune response in vivo. To examine the biochemical changes in TCR signaling present in this state, we used a mouse model in which Rag2(-/-) TCR-transgenic CD4(+) T cells were transferred into CD3epsilon(-/-) recipients expressing their cognate Ag. Compared with naive T cells, adaptively tolerant T cells had normal levels of TCR and slightly increased levels of CD4. Following activation with anti-TCR and anti-CD4 mAbs, the predominant signaling block in the tolerant cells was at the level of Zap70 kinase activity, which was decreased 75% in vitro. Phosphorylations of the Zap70 substrates (linker of activated T cells and phospholipase Cgamma1 were also profoundly diminished. This proximal defect impacted mostly on the calcium/NFAT and NF-kappaB pathways, with only a modest decrease in ERK1/2 phosphorylation. This state was contrasted with T cell clonal anergy in which the RAS/MAPK pathway was preferentially impaired and there was much less inhibition of Zap70 kinase activity. Both hyporesponsive states manifested a block in IkappaB degradation. These results demonstrate that T cell adaptive tolerance and clonal anergy are distinct biochemical states, possibly providing T cells with two molecular mechanisms to curtail responsiveness in different biological circumstances.

A specific CpG site demethylation in the human interleukin 2 gene promoter is an epigenetic memory

DNA demethylation plays a critical role in transcriptional regulation in differentiated somatic cells. However, there is no experimental evidence that CpG methylation in a small region of a genome restricts gene expression. Here, we show that the anti-CD3repsilon/CD28 antibody stimulation of human CD4+ T cells induces IL2 expression following epigenetic changes, including active demethylation of a specific CpG site, recruitment of Oct-1, and changes in histone modifications. When the stimulatory signal is withdrawn, Oct-1 remains on the enhancer region as a stable marker of the stimulation, causing the second induction to be faster and stronger. Our observations indicate that Oct-1-binding followed by CpG demethylation are key events in the epigenetic regulation of IL2 expression and may act as a memory of the regulatory event.

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.

Control of interleukin-2 gene transcription: a paradigm for inducible, tissue-specific gene expression

Interleukin-2 (IL-2) is a key cytokine that controls immune cell function, in particular the adaptive arm of the immune system, through its ability to control the clonal expansion and homeostasis of peripheral T cells. IL-2 is produced almost exclusively by T cells in response to antigenic stimulation and thus provides an excellent example of a cell-specific inducible gene. The mechanisms that control IL-2 gene transcription have been studied in detail for the past 20 years and our current understanding of the nature of the inducible and tissue-specific controls will be discussed.

T-cell specific enhancement of histone H3 acetylation in 5' flanking region of the IL-2 gene

The IL-2 gene expression is highly T-cell specific in an activation-dependent manner. However, little is known about how T-cell specific expression is regulated, although related transcription factors have been identified. To address this issue, we examined the chromatin structure change of the IL-2 gene by analyzing histone acetylation status in the upstream of IL-2 gene transcription initiation site. Interestingly, the histone acetylation level was found to be higher in various sites on the widespread upstream region in resting T-cells than resting B-cells. After T-cell stimulation with PMA and ionomycin, the same regions were further acetylated on histone H3. Particularly, the distal enhancer region showed prompt enhancement of histone acetylation, followed by the IL-2 mRNA expression. These results suggest that the 5' flanking region including the distal enhancer region of the IL-2 gene is already accessible in T cells with constitutive acetylation of histone H3,which may serve for T-cell specific IL-2 expression.

Crucial Role for Nuclear Factor of Activated T Cells in T Cell Receptor-mediated Regulation of Human Interleukin-17

The biological activities of the inflammatory cytokine interleukin (IL)-17 have been widely studied. However, comparatively little is known about how IL-17 expression is controlled. Here, we examined the basis for transcriptional regulation of the human IL-17 gene. IL-17 secretion was induced in peripheral blood mononuclear cells following anti-CD3 cross-linking to activate the T cell receptor (TCR), and costimulatory signaling through CD28 strongly enhanced CD3-induced IL-17 production. To define cis-acting elements important for IL-17 gene regulation, we cloned 1.25 kb of genomic sequence upstream of the transcriptional start site. This putative promoter was active in Jurkat T cells following CD3 and CD28 cross-linking, and its activity was inhibited by cyclosporin A and MAPK inhibitors. The promoter was also active in Hut102 T cells, which we have shown to secrete IL-17 constitutively. Overexpression of nuclear factor of activated T cells (NFAT) or Ras enhanced IL-17 promoter activity, and studies in Jurkat lines deficient in specific TCR signaling pathways provided supporting evidence for a role for NFAT. To delineate the IL-17 minimal promoter, we created a series of 5' truncations and identified a region between -232 and -159 that was sufficient for inducible promoter activity. Interestingly, two NFAT sites were located within this region, which bound to NFATc1 and NFATc2 in nuclear extracts from Hut102 and Jurkat cells. Moreover, mutations of these sites dramatically reduced both specific DNA binding and reporter gene activity, and chromatin immunoprecipitation assays showed occupancy of NFAT at this region in vivo. Together, these data show that NFAT is the crucial sensor of TCR signaling in the IL-17 promoter.

Preferential activation of an IL-2 regulatory sequence transgene in TCR gamma delta and NKT cells: subset-specific differences in IL-2 regulation

A transgene with 8.4-kb of regulatory sequence from the murine IL-2 gene drives consistent expression of a green fluorescent protein (GFP) reporter gene in all cell types that normally express IL-2. However, quantitative analysis of this expression shows that different T cell subsets within the same mouse show divergent abilities to express the transgene as compared with endogenous IL-2 genes. TCR gamma delta cells, as well as alpha beta TCR-NKT cells, exhibit higher in vivo transgene expression levels than TCR alpha beta cells. This deviates from patterns of normal IL-2 expression and from expression of an IL-2-GFP knock-in. Peripheral TCR gamma delta cells accumulate GFP RNA faster than endogenous IL-2 RNA upon stimulation, whereas TCR alpha beta cells express more IL-2 than GFP RNA. In TCR gamma delta cells, IL-2-producing cells are a subset of the GFP-expressing cells, whereas in TCR alpha beta cells, endogenous IL-2 is more likely to be expressed without GFP. These results are seen in multiple independent transgenic lines and thus reflect functional properties of the transgene sequences, rather than copy number or integration site effects. The high ratio of GFP: endogenous IL-2 gene expression in transgenic TCR gamma delta cells may be explained by subset-specific IL-2 gene regulatory elements mapping outside of the 8.4-kb transgene regulatory sequence, as well as accelerated kinetics of endogenous IL-2 RNA degradation in TCR gamma delta cells. The high levels and percentages of transgene expression in thymic and splenic TCR gamma delta and NKT cells, as well as skin TCR gamma delta-dendritic epidermal T cells, indicate that the IL-2-GFP-transgenic mice may provide valuable tracers for detecting developmental and activation events in these lineages.

Targeting combinatorial transcriptional complex assembly at specific modules within the interleukin-2 promoter by the immunosuppressant SB203580

The proximal promoter sequence of the interleukin-2 (IL-2) gene contains a series of composite sites or modules that controls much of its responsiveness to environmental stimuli. The integrated targeting of these modules is therefore a major mode of regulation. This report describes how multiple functional hierarchies, required for the recruitment of the p300 co-activator to the CD28RE/AP1 (TRE) module of the IL-2 promoter, are selectively disrupted in human T-cells by the immunosuppressive and anti-inflammatory actions of the p38 mitogen-activated protein kinase inhibitor (MAPK), SB203580. The molecular hierarchies targeted by SB203580 include the combinatorial interaction of NF-kappaB and CREB at the CD28RE/AP1 element coupled with the subsequent dynamic co-assembly and activation of p300. Several aspects of this targeting are linked to the ability of SB203580 to inhibit p38 MAPK-controlled pathways. Together, these results provide the molecular basis through which the combinatorial structure and context of the composite elements of the IL-2 promoter dictates mitogen responsiveness and drug susceptibility that are quantitatively and qualitatively distinct from the isolated action of single consensus sequences and/or transcriptional motifs.

Immunosuppressive and anti-inflammatory mechanisms of triptolide, the principal active diterpenoid from the Chinese medicinal herb Tripterygium wilfordii Hook. f

Extracts of Tripterygium wilfordii hook. f. (leigong teng, Thundergod vine) are effective in traditional Chinese medicine for treatment of immune inflammatory diseases including rheumatoid arthritis, systemic lupus erythematosus, nephritis and asthma. Characterisation of the terpenoids present in extracts of Tripterygium identified triptolide, a diterpenoid triepoxide, as responsible for most of the immunosuppressive, anti-inflammatory and antiproliferative effects observed in vitro. Triptolide inhibits lymphocyte activation and T-cell expression of interleukin-2 at the level of transcription. In all cell types examined, triptolide inhibits nuclear factor-kappaB transcriptional activation at a unique step in the nucleus after binding to DNA. Further characterisation of the molecular mechanisms of triptolide action will serve to elucidate pathways of immune system regulation.

Selective, stable demethylation of the interleukin-2 gene enhances transcription by an active process

A role for DNA demethylation in transcriptional regulation of genes expressed in differentiated somatic cells remains controversial. Here, we define a small region in the promoter-enhancer of the interleukin-2 (Il2) gene that demethylates in T lymphocytes following activation, and remains demethylated thereafter. This epigenetic change was necessary and sufficient to enhance transcription in reporter plasmids. The demethylation process started as early as 20 minutes after stimulation and was not prevented by a G1 to S phase cell cycle inhibitor that blocks DNA replication. These results imply that this demethylation process proceeds by an active enzymatic mechanism.

Effects of tacrolimus on ischemia-reperfusion injury

In addition to efficacious immunosuppression for the benefit of organ transplantation, tacrolimus has diverse actions that result in amelioration of ischemia-reperfusion injury. Knowledge is accumulating rapidly on the mechanisms through which tacrolimus exerts these cytoprotective effects, including alterations in microcirculation, free radical metabolism, calcium-activated pathways, inflammatory cascades, mitochondrial stability, apoptosis, stress-response proteins, and tissue recovery. Within the nucleus, actions mediating the effects of tacrolimus appear to be dominantly influenced by interactions with the transcription factor, nuclear factor-kappaB. Because tacrolimus is a cornerstone agent in immunosuppression regimens throughout the world and knowledge of its cellular mechanisms is evolving, it is important to update the clinical literature with this information. We reviewed the published literature with intent to portray the interactions of tacrolimus in the intricate cellular mechanisms initiated by ischemia and reperfusion.

Correlation of transcriptional repression by p21(SNFT) with changes in DNA.NF-AT complex interactions

p21(SNFT) (21-kDa small nuclear factor isolated from T cells) is a novel human protein of the basic leucine zipper family. The overexpression of p21(SNFT) leads to the significant and specific repression of transcription from the interleukin-2 promoter as well as from several essential activator protein 1 (AP-1)-driven composite promoter elements. One example is the distal nuclear factor of activated T cells (NF-AT)/AP-1 element where the AP-1 (Fos/Jun) basic leucine zipper heterodimer interacts with members of the NF-AT family. p21(SNFT) has been shown to replace Fos in dimerization with Jun on a consensus AP-1 binding site (12-O-tetradecanolyphorbol-13-acetate response element (TRE)) and to interact with Jun and NF-AT at the distal NF-AT/AP-1 enhancer element. A detailed biochemical analysis presented here compares interactions involving p21(SNFT) with those involving Fos. The results demonstrate that a p21(SNFT)/Jun dimer binds a TRE similarly to AP-1 and like AP-1 binds cooperatively with NF-AT at the NF-AT/AP-1 composite element. However, Fos interacts significantly more efficiently than p21(SNFT) with Jun and NF-AT, and the replacement of Fos by p21(SNFT) in the trimolecular complex drastically alters protein-DNA contacts. The data suggest that p21(SNFT) may repress transcriptional activity by inducing a unique conformation in the transcription factor complex.

The human IL-2 gene promoter can assemble a positioned nucleosome that becomes remodeled upon T cell activation

Controlled production of the cytokine IL-2 plays a key role in the mammalian immune system. Expression from the gene is tightly regulated with no detectable expression in resting T cells and a strong induction following T cell activation. The IL-2 proximal promoter (+1 to -300) contains many well-defined transcriptional activation elements that respond to T cell stimulation. To determine the role of chromatin structure in the regulation of interleukin-2 gene transcription, nucleosome assembly across the IL-2 promoter region was examined using in vitro chromatin reconstitution assays. The IL-2 promoter assembles a nucleosome that is both translationally and rotationally positioned, spanning some of the major functional control elements. The binding of transcription factors to these elements, with the exception of the architectural protein HMGA1, was occluded by the presence of the nucleosome. Analysis of the chromatin architecture of the IL-2 gene in Jurkat T cells provided evidence for the presence of a similarly positioned nucleosome in vivo. The region encompassed by this nucleosome becomes remodeled following activation of Jurkat T cells. These observations suggest that the presence of a positioned nucleosome across the IL-2 proximal promoter may play an important role in maintaining an inactive gene in resting T cells and that remodeling of this nucleosome is important for gene activation.

Molecular mechanisms of IL-2 gene regulation following costimulation through LFA-1

The integrin LFA-1 serves as an accessory molecule in T cell activation. In addition to its well-known role as an adhesion molecule, LFA-1 can contribute to T cell activation and up-regulation of IL-2 gene expression. However, the specific mechanisms by which LFA-1 influences T cell activation have not been elucidated. Therefore, we examined the impact of LFA-1:ICAM-1 interactions on transcriptional and posttranscriptional IL-2 gene regulation, using a costimulation-negative cell line transfected with MHC class II alone, or in combination with ICAM-1 or B7-1. IL-2 transcription was assessed utilizing transgenic mice expressing an IL-2 promoter luciferase reporter construct crossed to DO11.10 TCR-transgenic mice, and IL-2 mRNA stability was evaluated by real-time RT-PCR. Comparison of naive and previously activated T cells demonstrates a dramatic increase in IL-2-luciferase transcription in activated T cells that can, in part, be attributed to downstream signaling events. Costimulation through LFA-1 enhances transcription of the transgenic reporter construct across a wide Ag dose range, but does not affect IL-2 mRNA stability. In contrast, CD28 costimulation is clearly mediated through up-regulation of IL-2 transcription and through enhancement of mRNA stability. These results indicate that the primary pathway whereby engagement of LFA-1 through its ligand ICAM-1 up-regulates IL-2 gene expression is through enhanced IL-2 transcription, in the absence of any effect on IL-2 mRNA stabilization.

Alterations in transcription factor binding at the IL-2 promoter region in anergized human CD4+ T lymphocytes

BACKGROUND

The mechanisms responsible for the induction of clonal anergy are not well understood. We have utilized an in vitro model of human T cell anergy to explore the perturbations in cell signaling at the level of interleukin (IL)-2 gene transcription and to define the contribution of other cytokines to this effect.

METHODS

An in vitro model of clonal anergy was established by using CD4+ T lymphocytes from healthy human donors. Cells were anergized by prestimulation with an anti-CD3 monoclonal antibody (mAb) followed by restimulation 72 hr later with anti-CD3 mAb with or without anti-CD28.

RESULTS

CD4+ T cells, anergized with anti-CD3 monoclonal antibody (OKT3) prestimulation, displayed a marked reduction in proliferation (P=0.0036) and IL-2 production (P<0.0001). Co-incubation with IL-10 reduced cellular proliferation in OKT3/CD28 pretreated cells by 19% (P=NS) and reduced IL-2 production by 40% (P=0.0024). Anergized T cells demonstrated a reduced binding activity of the AP-1 complex to the IL-2 promoter. Supershift experiments and Western blots confirmed that the binding of c-Fos, JunB, and JunD, but not of FosB, was reduced in anergized cells. At the sis-inducible element (SIE)-binding region of the c-Fos promoter, Stat3 binding was reduced.

CONCLUSIONS

T cell anergy, induced by prestimulation with OKT3, is characterized by reduced proliferation and a profound decrease in IL-2 production. Anergy can be prevented by co-incubation with anti-CD28 and partially re-established by IL-10. Anergy is accompanied by a reduction in AP-1 binding to the IL-2 promoter, with selective reduction in binding of c-Fos, JunB, and JunD. Defective binding for Stat3 at the c-Fos promoter suggests an involvement of the Jak-Stat pathway.

Chromatin remodeling, measured by a novel real-time polymerase chain reaction assay, across the proximal promoter region of the IL-2 gene

The structure of chromatin and its remodeling following activation are important aspects of the control of inducible gene transcription. The IL-2 gene is induced in a cell specific-manner in T cells following an antigenic stimulus. We show, using a novel real-time PCR assay, that significant chromatin remodeling of the IL-2 proximal promoter region occurred upon stimulation of both the murine EL-4 T cell line and primary CD4(+) T cells. Chromatin remodeling appears to be limited to the first 300 bp of the proximal promoter region as measured by micrococcal nuclease and restriction enzyme accessibility. Time course studies indicated that chromatin remodeling was observed at 1.5 h postinduction and was maintained for up to 16 h. The remodeling is reversible upon removal of the stimulus. The region immediately upstream from the transcription start site, however, remains accessible for up to 16 h. Upon restimulation, remodeling occurs much more rapidly, consistent with a more rapid rise in IL-2 mRNA levels. Using a number of pharmacological inhibitors we show that remodeling is dependent on the presence of specific transcription factors, but not on the modification of histones. The development of this novel chromatin accessibility assay based on real-time PCR has allowed rapid, sensitive, and quantitative measurements on the IL-2 gene following cellular activation in both T cell lines and primary cells.

The effect of HIV-1 regulatory proteins on cellular genes: derepression of the IL-2 promoter by Tat

In HIV-infected individuals dysregulation of the immune system is characterized by severe disorders of the cytokine network. Increase secretion of IL-2, the major T cell growth and differentiation cytokine, may play a decisive role in sensitization of T cells for activation induced apoptosis and indirect death of activated T cells through augmented virus replication. We investigated the cause of enhanced IL-2 secretion and found that the HIV Tat induces this effect. We demonstrate that increased IL-2 secretion is due to Tat-enhanced IL-2 promoter activation. Tat derepresses and activates the distal AP-1 site (position -185 to -177) in the IL-2 promoter. In nonstimulated T cells a repressor complex containing NF-IL6, JunB, c-Fos and Fra-1 is formed on the AP-1(IL-2/d) site and represses IL-2 promoter activity. After T cell activation, a heterodimeric activator containing p65 and c-Jun binds to the AP-1(IL-2/d) site. HIV Tat enhances activation of NF-kappaB and consequently, activates the AP-1(IL-2/d) site. Our data provide evidence for a novel mechanism by which HIV Tat dysregulates IL-2 production and therefore may contribute to the HIV-1 infection in a way yet to be clarified.

Protein kinase C-theta participates in the activation of cyclic AMP-responsive element-binding protein and its subsequent binding to the -180 site of the IL-2 promoter in normal human T lymphocytes

IL-2 gene expression is regulated by the cooperative binding of discrete transcription factors to the IL-2 promoter/enhancer and is predominantly controlled at the transcriptional level. In this study, we show that in normal T cells, the -180 site (-164/-189) of the IL-2 promoter/enhancer is a p-cAMP-responsive element-binding protein (p-CREB) binding site. Following activation of the T cells through various membrane-initiated and membrane-independent pathways, protein kinase C (PKC)-theta phosphorylates CREB, which subsequently binds to the -180 site and associates with the transcriptional coactivator p300. Rottlerin, a specific PKC-theta inhibitor, diminished p-CREB protein levels when normal T cells were treated with it. Rottlerin also prevented the formation of p-CREB/p300 complexes and the DNA-CREB protein binding. Cotransfection of fresh normal T cells with luciferase reporter construct driven by two tandem -180 sites and a PKC-theta construct caused a significant increase in the transcription of the reporter gene, indicating that this site is functional and regulated by PKC-theta. Cotransfection of T cells with a luciferase construct driven by the -575/+57 region of the IL-2 promoter/enhancer and a PKC-theta construct caused a similar increase in the reporter gene transcription, which was significantly limited when two bases within the -180 site were mutated. These findings show that CREB plays a major role in the transcriptional regulation of IL-2 and that a major pathway for the activation of CREB and its subsequent binding to the IL-2 promoter/enhancer in normal T cells is mediated by PKC-theta.

HIV-1 Tat inhibits IL-2 gene transcription through qualitative and quantitative alterations of the cooperative Rel/AP1 complex bound to the CD28RE/AP1 composite element of the IL-2 promoter

Dysregulation of cytokine secretion plays an important role in AIDS pathogenesis. Here, we demonstrate that expression of HIV-1 Tat protein in Jurkat cells induces a severe impairment of IL-2 but not TNF gene transcription. Interestingly, this inhibition correlates with the effect of the viral protein on the transactivation of the CD28RE/AP1 composite element (-164/-154), but not with that observed on the NFAT/AP1 site of the IL-2 gene promoter, neither with the effect on NF-kappa B- nor AP1-independent binding sites. Endogenous expression of Tat induced a decrease in the amount of the specific protein complex bound to the CD28RE/AP1 probe after PMA plus calcium ionophore stimulation. This effect was accompanied by qualitative alterations of the AP1 complex. Thus, in wild-type Jurkat cells, c-jun was absent from the complex, whereas in Tat-expressing cells, c-jun was increasingly recruited overtime. By contrast, similar amounts of c-rel and a small amount of NFAT1 were detected both in wild type and in Jurkat Tat(+) cells. Furthermore, Tat not only induced the participation of c-jun in the cooperative complex but also a decrease in its transactivation activity alone or in combination with c-rel. Thus, the interaction of Tat with the components of this rel/AP1 cooperative complex seems to induce quantitative and qualitative alterations of this complex as activation progresses, resulting in a decrease of IL-2 gene transcription. Altogether our results suggest the existence of tuned mechanisms that allow the viral protein to specifically affect cooperative interactions between transcription factors.

Evidence that Galpha(q)-coupled receptor-induced interleukin-6 mRNA in vascular smooth muscle cells involves the nuclear factor of activated T cells

The immunosuppressant cyclosporin A inhibits transcription mediated by the nuclear factor of activated T-cells (NFAT), a key regulator of cytokine gene expression in lymphocytes that integrates phospholipase C signaling. NFAT is also expressed in vascular smooth muscle cells, but the genes it regulates there are unknown. Here we show that Galpha(q)-coupled P2Y nucleotide receptor signaling in rat vascular smooth muscle cells increases NFAT-mediated luciferase reporter expression. It also induces interleukin (IL)-6 gene expression but not other cytokine mRNAs including IL-1, IL-2, IL-3, IL-4, IL-10, gamma-interferon, tumor necrosis factor-alpha, or tumor necrosis factor-beta. IL-6 mRNA induction by UTP is more rapid and transient then that caused by IL-1beta stimulation and is partially blocked by cyclosporin A or by expression of a trans-dominant NFAT inhibitor. Expression of recombinant NFATc1 markedly augments IL-6 mRNA induction by these and other agonists, which is partially attributable to NFAT-regulated paracrine mediators. However, trans-dominant NFkappaB inhibitors strongly interfere with IL-6 mRNA induction both by IL-1beta and by UTP, which synergistically evoke IL-6 mRNA expression. These findings suggest that NFAT is among the cofactors involved in NFkappaB-dependent IL-6 gene induction by Ca(2+)-mobilizing receptors in vascular smooth muscle cells.

The role of high-mobility group I(Y) proteins in expression of IL-2 and T cell proliferation

The high-mobility group I(Y) (HMGI(Y)) family of proteins plays an important architectural role in chromatin and have been implicated in the control of inducible gene expression. We have previously shown that expression of HMGI antisense RNA in Jurkat T cells inhibits the activity of the IL-2 promoter. Here we have investigated the role of HMGI(Y) in controlling IL-2 promoter-reporter constructs as well as the endogenous IL-2 gene in both Jurkat T cells and human PBL. We found that the IL-2 promoter has numerous binding sites for HMGI(Y), which overlap or are adjacent to the known transcription factor binding sites. HMGI(Y) modulates binding to the IL-2 promoter of at least three transcription factor families, AP-1, NF-AT and NF-kappaB. By using a mutant HMGI that cannot bind to DNA but can still interact with the transcription factors, we found that DNA binding by HMGI was not essential for the promotion of transcription factor binding. However, the non-DNA binding mutant acts as a dominant negative protein in transfection assays, suggesting that the formation of functional HMGI(Y)-containing complexes requires DNA binding as well as protein:protein interactions. The alteration of HMGI(Y) levels affects IL-2 promoter activity not only in Jurkat T cells but also in PBL. Importantly, we also show here that expression of the endogenous IL-2 gene as well as proliferation of PBL are affected by changes in HMGI(Y) levels. These results demonstrate a major role for HMGI(Y) in IL-2 expression and hence T cell proliferation.

On the stochastic regulation of interleukin-2 transcription

Interleukin-2 (IL-2) is a growth and differentiation factor critical for clonal T cell expansion and function. Produced exclusively in T cells, IL-2 transcription and synthesis occurs only after appropriate cellular activation via the clonotypic antigen-receptor and co-stimulatory molecules. IL-2 gene expression is initiated by the cooperative binding of different transcription factors and is predominantly controlled at the transcriptional level. Recently, it has been demonstrated that IL-2 transcriptional activity is normally confined to a single, randomly chosen allele. This monoallelic expression of a non-receptor gene product encoded at a non-imprinted, autosomal locus represents an unusual regulatory mode. Although the molecular mechanisms operational for IL-2 transcription have yet to be defined, allele-specific expression of the IL-2 locus constitutes an important expansion to the concept of stochastic gene expression.