Long-range transcriptional regulation of cytokine gene expression

Targeted disruption of Stat6 DNA binding activity by an oligonucleotide decoy blocks IL-4–driven TH2 cell response

The transcription factor, signal transducer and activator of transcription (Stat) 6, regulates TH2-lymphocyte activity by controlling the expression and responsiveness to interleukin (IL)–4, which plays a key role in numerous allergic maladies. Therefore, we sought to use a phosphorothiolate cis-element decoy to target disruption of Stat6 transcriptional activity. Here we showed that the Stat6 decoy potently ablated the messenger RNA expression and production of IL-4, but not of several other cytokines. The Stat6 decoy functionally disrupted IL-4–inducible cell proliferation of murine TH2 cells and primary human CD4+ T lymphocytes. Specificity of the decoy was demonstrated by its ability to directly block Stat6 binding to a cis-element probe and transactivation, but not affect Stat6 tyrosine phosphorylation or expression of the IL-4 receptor chains. Moreover, the decoy failed to inhibit non–Stat6-dependent signaling pathways since IL-2 was competent to induce cell proliferation and activation of Stats 1, 3, and 5a/b. With the use of laser scanning confocal microscopy, fluorescently tagged Stat6 decoy was detectable in the cytoplasm and nucleus; however, greater levels of oligonucleotide were present in the latter following IL-4 treatment. Taken together, these data suggest that IL-4–driven TH2 cell activity can be preferentially restricted via targeted disruption of Stat6 by a novel and specific decoy strategy that may possess gene therapeutic potential.

Differential Ca2+ influx, KCa channel activity, and Ca2+ clearance distinguish Th1 and Th2 lymphocytes

In Th1 and Th2 lymphocytes, activation begins with identical stimuli but results in the production of different cytokines. The expression of some cytokine genes is differentially induced according to the amplitude and pattern of Ca2+ signaling. Using fura- 2 Ca2+ imaging of murine Th1 and Th2 clones, we observed that the Ca2+ rise elicited following store depletion with thapsigargin is significantly lower in Th2 cells than in Th1 cells. Maximal Ca2+ influx rates and whole-cell Ca2+ currents showed that both Th1 and Th2 cells express indistinguishable Ca2+-release-activated Ca2+ channels. Therefore, we investigated other mechanisms controlling the concentration of intracellular Ca2+, including K+ channels and Ca2+ clearance from the cytosol. Whole-cell recording demonstrated that there is no distinction in the amplitudes of voltage-gated K+ currents in the two cell types. Ca2+-activated K+ (KCa) currents, however, were significantly smaller in Th2 cells than in Th1 cells. Pharmacological equalization of Ca2+-activated K+ currents in the two cell types reduced but did not completely eliminate the difference between Th1 and Th2 Ca2+ responses, suggesting divergence in an additional Ca2+ regulatory mechanism. Therefore, we analyzed Ca2+ clearance from the cytosol of both cell types and found that Th2 cells extrude Ca2+ more quickly than Th1 cells. The combination of a faster Ca2+ clearance mechanism and smaller Ca2+-activated K+ currents in Th2 cells accounts for the lower Ca2+ response of Th2 cells compared with Th1 cells.

Ectopic expression of activated Stat6 induces the expression of Th2-specific cytokines and transcription factors in developing Th1 cells

Stat6 is critical for IL-4-mediated Th2 cell development, but its molecular mechanism remains unclear. Here we constructed Stat6:ER, a Stat6-estrogen receptor fusion protein that can be activated by 4-hydroxy-tamoxifen, independently of IL-4 and endogenous Stat6. Retrovirus-mediated introduction of Stat6:ER into developing Th1 cells induced Th2-specific cytokines and suppressed IFNgamma production in a 4-HT-dependent manner and in the absence of IL-4. It also induced GATA-3 and c-maf expression and downregulated IL-12Rbeta2 chain expression. Its decreased ability to induce the Th2 phenotype with progressing Th1 cell commitment correlated with a decreased induction of GATA-3 and c-maf. This study indicates that Stat6 functions upstream of GATA-3 and c-Maf to induce Th2 development.

FasL promoter activation by IL-2 through SP1 and NFAT but not Egr-2 and Egr-3

Recently activated peripheral T cells treated with IL-2 for 4 days expressed Fas ligand (FasL)-mediated cytotoxicity. These IL-2-treated T cells had high nuclear expression of SP1 and NFAT, but lacked the Egr-2 and Egr-3 that could be induced by anti-CD3 stimulation and had been implicated in FasL gene activation. A minimal promoter region that responded to IL-2 was identified by transient transfection assays using deletion mutants. The data suggests that the GGGCGGAAA site present in the 5' end of the minimal FasL promoter is critical to IL-2-induced FasL gene activation. The GGGCGGAAA sequence contains an overlapping site used by two transcription factor families, one (GGGCGG) for the SP1 family and the other (GGAAA) for the NFAT family. FasL promoter activity was partially but statistically significantly reduced with constructs mutated at either site. More activity was lost with a construct mutated at both sites. In contrast, mutation at the Egr site had no effect on IL-2-induced FasL promoter activity. Our study identified a new FasL promoter site responding to IL-2-induced SP1 and NFAT factors. Furthermore, the nuclei of IL-2-treated cells express SP1 and NFAT, but not Egr-2 and Egr-3, for FasL gene activation.

Interleukin-2 expression by a subpopulation of primary T cells is linked to enhanced memory/effector function

Single cell studies have identified intraclonal heterogeneity of cytokine production by activated T cells. To investigate implications of cytokine heterogeneity for cell fate, an interleukin (IL)-2 promoter-green fluorescent protein (GFP) reporter transgenic model was developed to track IL-2+ and IL-2- T cells during differentiation from naive precursors. Antigen-activated IL-2+ and IL-2- cells had comparable proliferative capacities in primary responses. However, T cells that expressed IL-2 in primary responses demonstrated enhanced antigenic sensitivity and increased expression of effector cytokines in secondary responses in vitro and in vivo. Thus, heterogeneity of activation during a primary response translates into heterogeneous secondary responses, in which enhanced memory/effector function is linked to cells that previously exceeded an activation threshold associated with IL-2 gene transcription.

Chromatin-based regulatory mechanisms governing cytokine gene transcription

On initial contact with antigen, naive T cells differentiate and acquire effector characteristics, including the ability to transcribe specific cytokine genes rapidly and at high levels on subsequent exposure to antigen. Several effector T-cell subsets showing distinct patterns of cytokine gene transcription have been described. The patterns of cytokine expression in response to pathogenic challenges have a significant impact on the outcome of immune and inflammatory reactions. Here we review recent studies suggesting that the ability of naive T cells to differentiate into specific cytokine-expressing cells is regulated by epigenetic changes in the accessibility and chromatin structure of cytokine genetic loci. Antigen and cytokine stimulation of naive T cells activates diverse intracellular signaling pathways, which result in chromatin remodeling and demethylation of cytokine genes. These changes are likely to increase, in a stable and heritable fashion, the accessibility of these genes to the basal transcriptional machinery. Chromatin-based regulatory mechanisms may explain several features of cytokine gene expression in effector versus naive T cells, including their monoallelic expression, coordinate regulation, and stable maintenance in memory T cells. The hypothesis of epigenetic changes occurring during T-cell differentiation provides a framework for a comprehensive understanding of cytokine expression by T cells.

Expression of Murine IL-12 Is Regulated by Translational Control of the p35 Subunit

IL-12 is a heterodimer of two subunits, p35 and p40, encoded by separate genes that are regulated independently. To investigate the mechanisms underlying the regulation of the p35 gene, we characterized murine p35 expression in the B cell lymphoma line A20 and in bone marrow-derived dendritic cells. Multiple transcription start sites were identified in both cell types, resulting in four p35 mRNA isoforms (types I–IV) that differ in the number and position of upstream ATGs in their 5′ untranslated regions. In nonstimulated cells, the predominant forms of p35 message (types II and IV) contained an additional upstream ATG, whose presence was shown to inhibit the downstream translation of the p35 subunit. After LPS stimulation, however, transcription initiated from alternate positions, so that the proportion of transcripts not containing this upstream ATG (types I and III) was significantly increased in the population of p35 mRNA. These type I and type III transcripts readily supported translation of the p35 subunit and its incorporation into bioactive IL-12. Furthermore, p35 mRNA levels were substantially up-regulated after LPS stimulation in both cell types. Thus, our results show that p35 gene expression is highly regulated by both transcriptional and translational mechanisms.

Molecular regulation of cytokine gene expression during the immune response

Cytokine expression by immune system cells plays an important role in the regulation of the immune response. On first encounter with antigen, naive CD4+ T helper (Th) cells differentiate into cytokine-producing effector cells. Two types of effector cells characterized by their distinct expression of cytokine profiles have been described. Th1 cells produce IL-2 and IFN-gamma, whereas Th2 cells produce IL-4, IL-5, IL-6, IL-10, and IL-13. In many pathological situations, the balance between Th1 and Th2 immune responses determines the outcome of diverse immunologically mediated clinical syndromes including infectious, autoimmune, and allergic diseases. However, the molecular basis for the tissue-specific expression of Th1/Th2-like cytokines has remained elusive. In this review we evaluate the possible in vivo role of different transcription factors and transcriptional mechanisms in T cell differentiation and the immune response.

Modulation of chromatin structure regulates cytokine gene expression during T cell differentiation

Differentiating cells undergo programmed alterations in their patterns of gene expression, which are often regulated by structural changes in chromatin. Here we demonstrate that T cell differentiation results in long-range changes in the chromatin structure of effector cytokine genes, which persist in resting Th1 and Th2 cells in the absence of further stimulation. Differentiation of naive T helper cells into mature Th2 cells is associated with chromatin remodeling of the IL-4 and IL-13 genes, whereas differentiation into Th1 cells evokes remodeling of the IFNgamma but not IL-4 or IL-13 genes. IL-4 locus remodeling is accompanied by demethylation and requires both antigen stimulation and STAT6 activation. We propose that chromatin remodeling of cytokine gene loci is functionally associated with productive T cell differentiation and may explain the coordinate regulation of Th2 cytokine genes.