Formation of IL-7Ralphahigh and IL-7Ralphalow CD8 T cells during infection is regulated by the opposing functions of GABPalpha and Gfi-1

IL-7 is essential for the survival of naive and memory T cells, and IL-7 receptor alpha-chain (IL-7Ralpha) expression is dynamically regulated in activated CD8 T cells during acute viral and bacterial infections. Most virus-specific CD8 T cells become IL-7Ralpha(low) and are relatively short-lived, but some escape IL-7Ralpha repression (referred to as IL-7Ralpha(high) memory precursor effector cells) and preferentially enter the memory CD8 T cell pool. How antiviral effector CD8 T cells regulate IL-7Ralpha expression in an "on and off" fashion remains to be characterized. During lymphocytic choriomeningitis virus infection, we found that opposing actions of the transcription factors GABPalpha (GA binding protein alpha) and Gfi-1 (growth factor independence 1) control IL-7Ralpha expression in effector CD8 T cells. Specifically, GABPalpha was required for IL-7Ralpha expression in memory precursor effector cells, and this correlated with hyperacetylation of the Il7ra promoter. In contrast, Gfi-1 was required for stable IL-7Ralpha repression in effector CD8 T cells and acted by antagonizing GABPalpha binding and recruiting histone deacetylase 1, which deacetylated the Il7ra promoter. Thus, Il7ra promoter acetylation and activity was dependent on the reciprocal binding of GABPalpha and Gfi-1, and these data provide a biochemical mechanism for the generation of stable IL-7Ralpha(high) and IL-7Ralpha(low) states in virus-specific effector CD8 T cells.

Interleukin-21: basic biology and implications for cancer and autoimmunity

Interleukin-21 (IL-21), a potent immunomodulatory four-alpha-helical-bundle type I cytokine, is produced by NKT and CD4(+) T cells and has pleiotropic effects on both innate and adaptive immune responses. These actions include positive effects such as enhanced proliferation of lymphoid cells, increased cytotoxicity of CD8(+) T cells and natural killer (NK) cells, and differentiation of B cells into plasma cells. Conversely, IL-21 also has direct inhibitory effects on the antigen-presenting function of dendritic cells and can be proapoptotic for B cells and NK cells. IL-21 is also produced by Th17 cells and is a critical regulator of Th17 development. The regulatory activity of IL-21 is modulated by the differentiation state of its target cells as well as by other cytokines or costimulatory molecules. IL-21 has potent antitumor activity but is also associated with the development of autoimmune disease. IL-21 transcription is dependent on a calcium signal and NFAT sites, and IL-21 requires Stat3 for its signaling. The key to harnessing the power of IL-21 will depend on better understanding its range of biological actions, its mechanism of action, and the molecular basis of regulation of expression of IL-21 and its receptor.

Interleukin 21: a cytokine/cytokine receptor system that has come of age

Interleukin-21 (IL-21) and its receptor represent the sixth cytokine system whose actions were recognized to require the common cytokine receptor gamma chain. IL-21 is produced by activated CD4+ T cells, natural killer T cells, and follicular T helper cells and has actions on a range of lymphohematopoietic lineages. Among its many effects, IL-21 serves a critical role for immunoglobulin production and terminal B cell differentiation, acts as a T cell comitogen and can drive the expansion of CD8+ T cells, can negatively regulate dendritic cell function and plays an essential role in the differentiation of Th17 cells. Importantly, IL-21 is implicated in the pathogenesis of autoimmunity and exhibits potent actions as an antitumor agent. The ability to regulate and manipulate the actions of IL-21, therefore, has important implications for immunoregulation and the therapy of human disease.

Thymic stromal lymphopoietin: a new cytokine in asthma

Airway epithelial cells provide mechanical and immune protection against pathogens and allergens. Following activation, these cells produce a wide range of cytokines including thymic stromal lymphopoietin (TSLP). Recently it was established that a high level of TSLP is associated with asthma in mice and in humans. These findings suggest that interfering with the ability of cells to respond to TSLP might prevent the development of airway inflammation. Our review presents current knowledge on mediators that induce TSLP production and on the actions of TSLP on different populations of cells that are related to airway inflammation. TSLP affects dendritic cells, T cells, NKT cells, and mast cells, indicative of the broad role of TSLP in the regulation of inflammatory/allergic processes.

Measurement of interleukin-21

This unit describes three procedures for measurement of interleukin-21 (IL-21). The first employs the use of an antibody sandwich ELISA. An alternative procedure measures proliferative responses of T cells to a combination of IL-21 and IL-15 using CFSE. Finally, a method to assess IL-21-induced tyrosine phosphorylation of Stat3 in splenic CD8(+) T cells using a flow cytometry-based analysis is described.

T-cell development and function are modulated by dual specificity phosphatase DUSP5

Interleukin-2 (IL-2) is a pleiotropic cytokine that regulates lymphocyte proliferation and peripheral tolerance. IL-2 activates mitogen-activated protein kinase (MAPK), phosphatidylinositol 3-kinase, and signal transducer and activator of transcription (STAT) pathways and modulates expression of target genes. Systematic analysis of IL-2 target genes has revealed regulation of potential feedback inhibitors of IL-2 signaling, including several suppressor of cytokine signaling (SOCS) family members as well as MAPK pathway-regulating dual specificity phosphatases (DUSPs). Here we have evaluated the in vivo actions of DUSP5, an extracellular signal-regulated kinase 1/2 (ERK1/2)-specific phosphatase, by generating transgenic mice overexpressing DUSP5 within the lymphoid compartment. We show that transgenic DUSP5 expression results in a block in thymocyte development at the double positive stage. We also demonstrate that DUSP5-expressing mature T cells exhibit decreased IL-2-dependent proliferation and defective IL-2-mediated induction of genes. Finally, DUSP5 transgenic mice develop autoimmune symptoms, suggesting a role for the MAPK pathway in the regulation of tolerance. Thus, proper regulation of DUSP5 activity is critical for normal immune system development, IL-2 actions, and tolerance.

A Stat5b transgene is capable of inducing CD8+ lymphoblastic lymphoma in the absence of normal TCR/MHC signaling

Stat5 proteins are critical signaling molecules activated by many cytokines. Within the immune system, Stat5 plays important roles related to the development of thymocytes and proliferation of T cells. Stat5 has been implicated in malignant transformation, and moreover, the activated tyrosine phosphorylated form of Stat5 is frequently observed in human lymphomas. We previously demonstrated the oncogenic potential of Stat5, with thymic lymphoblastic lymphomas developing in a significant proportion of transgenic (TG) mice overexpressing Stat5a or Stat5b in lymphocytes. In addition, immunization or expression of a T-cell receptor (TCR) transgene augmented the rate of tumor formation. Here, we investigate the mechanism of Stat5-mediated lymphomagenesis by exploring the contributions of major histocompatibility complex (MHC)/TCR and pre-TCR signals. We present data demonstrating that Stat5b TG mice unexpectedly develop CD8(+) lymphoma even in the absence of either pre-TCR signaling or normal thymic selection. Indeed, acceleration of Stat5b transgene-mediated lymphoma occurred on TCRalpha(-/-) and pre-TCRalpha(-/-) backgrounds. In light of these data, we propose a model in which alterations in T-cell development at the double-negative/double-positive (DN/DP) stages cooperate with cytokine-mediated pathways in immature thymocytes to give rise to lymphoblastic T-cell lymphomas in Stat5b TG mice.

GA-binding protein regulates KIS gene expression, cell migration, and cell cycle progression

The cyclin-dependent kinase inhibitor p27(Kip1) arrests cell cycle progression through G1/S phases and is regulated by phosphorylation of serine/threonine residues. Recently, we identified the serine/threonine kinase, KIS, which phosphorylates p27(Kip1) on serine 10 leading to nuclear export of p27(Kip1) and protein degradation. However, the molecular mechanisms of transcriptional activation of the human KIS gene and its biological activity are not known. We mapped the transcription initiation site approximately 116 bp 5' to the translation start site, and sequences extending to -141 were sufficient for maximal promoter activity. Mutation in either of two Ets-binding sites in this region resulted in an approximately 75-80% decrease in promoter activity. These sites form at least 3 specific complexes, which contained GA-binding protein (GABP). Knocking down GABPalpha by siRNA in vascular smooth muscle cells (VSMCs) diminished KIS gene expression and reduced cell migration. Correspondingly, in serum stimulated GABPalpha-deficient mouse embryonic fibroblasts (MEFs), KIS gene expression was also significantly reduced, which was associated with an increase in p27(Kip1) protein levels and a decreased percentage of cells in S-phase. Consistent with these findings, following vascular injury in vivo, GABPalpha-heterozygous mice demonstrated reduced KIS gene expression within arterial lesions and these lesions were significantly smaller compared to GABP+/+ mice. In summary, serum-responsive GABP binding to Ets-binding sites activates the KIS promoter, leading to KIS gene expression, cell migration, and cell cycle progression.

Cutting edge: direct action of thymic stromal lymphopoietin on activated human CD4+ T cells

Thymic stromal lymphopoietin (TSLP) is a cytokine that promotes CD4(+) T cell homeostasis and contributes to allergic and inflammatory responses. TSLP can act directly on mouse CD4(+) T cells, but in humans, the available data have indicated that TSLP receptors are not expressed on CD4(+) T cells and that TSLP instead activates dendritic cells, which in turn promote the proliferation and differentiation of CD4(+) T cells. We now unexpectedly demonstrate the presence of TSLP receptors on activated human CD4(+) T cells. Strikingly, whereas freshly isolated peripheral blood human T cells show little if any response to TSLP, TCR stimulation allows a potent response to this cytokine. Moreover, TSLP increases the sensitivity of human CD4(+) T cells to low doses of IL-2, augmenting responsiveness of these cells to TCR engagement. Our results establish that human CD4(+) T cells are direct targets for TSLP.

CREB/ATF-dependent T cell receptor-induced FoxP3 gene expression: a role for DNA methylation

Regulatory T cells (T reg cells) are a population of CD4⁺ T cells that limit immune responses. FoxP3 is a master control transcription factor for development and function of these cells, but its regulation is poorly understood. We have identified a T cell receptor–responsive enhancer in the FoxP3 first intron that is dependent on a cyclic-AMP response element binding protein (CREB)/activating transcription factor (ATF) site overlapping a CpG island. Methylation of this island inversely correlates with CREB binding and FoxP3 expression. Interestingly, transforming growth factor-β, which induces T reg cell formation, decreases methylation of the CpG island and increases FoxP3 expression. Similarly, inhibiting methylation with 5-azacytidine or knocking down the DNA methyltransferase Dnmt1 also induces FoxP3 expression. Conversely, methylation of the CpG island, which decreases CREB binding or expression of dominant-negative CREB, decreases FoxP3 gene expression. Thus, T cell receptor–induced FoxP3 expression in T reg cells is controlled both by sequence-specific binding of CREB/ATF and by DNA methylation of a CpG island.

IL-6 programs T(H)-17 cell differentiation by promoting sequential engagement of the IL-21 and IL-23 pathways

T helper cells that produce interleukin 17 (IL-17; 'T(H)-17 cells') are a distinct subset of proinflammatory cells whose in vivo function requires IL-23 but whose in vitro differentiation requires only IL-6 and transforming growth factor-beta (TGF-beta). We demonstrate here that IL-6 induced expression of IL-21 that amplified an autocrine loop to induce more IL-21 and IL-23 receptor in naive CD4(+) T cells. Both IL-21 and IL-23, along with TGF-beta, induced IL-17 expression independently of IL-6. The effects of IL-6 and IL-21 depended on STAT3, a transcription factor required for the differentiation of T(H)-17 cells in vivo. IL-21 and IL-23 induced the orphan nuclear receptor RORgammat, which in synergy with STAT3 promoted IL-17 expression. IL-6 therefore orchestrates a series of 'downstream' cytokine-dependent signaling pathways that, in concert with TGF-beta, amplify RORgammat-dependent differentiation of T(H)-17 cells.

The transcription factor GABP is a critical regulator of B lymphocyte development

GA binding protein (GABP) is a ubiquitously expressed Ets-family transcription factor that critically regulates the expression of the interleukin-7 receptor alpha chain (IL-7Ralpha) in T cells, whereas it is dispensable for IL-7Ralpha expression in fetal liver B cells. Here we showed that deficiency of GABPalpha, the DNA-binding subunit of GABP, resulted in profoundly defective B cell development and a compromised humoral immune response, in addition to thymic developmental defects. Furthermore, the expression of Pax5 and Pax5 target genes such as Cd79a was greatly diminished in GABPalpha-deficient B cell progenitors, pro-B, and mature B cells. GABP could bind to the regulatory regions of Pax5 and Cd79a in vivo. Thus, GABP is a key regulator of B cell development, maturation, and function.

The molecular basis of IL-21-mediated proliferation

Interleukin-21 (IL-21) is a type I cytokine that modulates functions of T, B, natural killer (NK), and myeloid cells. The IL-21 receptor (IL-21R) is closely related to the IL-2 receptor beta chain and is capable of transducing signals through its dimerization with the common cytokine receptor gamma chain (gamma(c)), the protein whose expression is defective in humans with X-linked severe combined immunodeficiency. To clarify the molecular basis of IL-21 actions, we investigated the role of tyrosine residues in the IL-21R cytoplasmic domain. Simultaneous mutation of all 6 tyrosines greatly diminished IL-21-mediated proliferation, whereas retention of tyrosine 510 (Y510) allowed full proliferation. Y510 efficiently mediated IL-21-induced phosphorylation of Stat1 and Stat3, but not of Stat5, and CD8(+) T cells from Stat1/Stat3 double knock-out mice exhibited decreased proliferation in response to IL-21 + IL-15. In addition, IL-21 weakly induced phosphorylation of Shc and Akt, and consistent with this, specific inhibitors of the MAPK and PI3K pathways inhibited IL-21-mediated proliferation. Collectively, these data indicate the involvement of the Jak-STAT, MAPK, and PI3K pathways in IL-21 signaling.

Overexpression of Interleukin 21 Induces Expansion of Hematopoietic Progenitor Cells

The interleukin 21 (IL-21) receptor is expressed on T-cells, B-cells, and natural killer cells, and IL-21 is critical for regulating immunoglobulin production in vivo in cooperation with IL-4. So far, however, little is known about a role for IL-21 outside the immune system. We investigated the effect of IL-21 on hematopoiesis in vivo by using the hydrodynamics gene-delivery method. Overexpression of IL-21 increases Sca-1+ cells in the periphery and spleen. It also increases the numbers of c-Kit+, Sca-1+, and lineage-/low (KSL) cells and colony-forming units-granulocyte-macrophage (CFU-GM) in the spleen, indicating the expansion of hematopoietic progenitor cells. We found that even in RAG2-/- mice, which lack mature T-cells and B-cells, IL-21 induced an increase in KSL cells and CFU-GM in the spleen. These results demonstrate that IL-21 can induce the expansion of hematopoietic progenitor cells in vivo, even in the absence of mature T-cells and B-cells.

Maturation stage-specific regulation of megakaryopoiesis by pointed-domain Ets proteins

Numerous megakaryocyte-specific genes contain signature Ets-binding sites in their regulatory regions. Fli-1 (friend leukemia integration 1), an Ets transcription factor, is required for the normal maturation of megakaryocytes and controls the expression of multiple megakaryocyte-specific genes. However, in Fli-1-/- mice, early megakaryopoiesis persists, and the expression of the early megakaryocyte-specific genes, alphaIIb and cMpl, is maintained, consistent with functional compensation by a related Ets factor(s). Here we identify the Ets protein GABPalpha (GA-binding protein alpha) as a regulator of early megakaryocyte-specific genes. Notably, GABPalpha preferentially occupies Ets elements of early megakaryocyte-specific genes in vitro and in vivo, whereas Fli-1 binds both early and late megakaryocyte-specific genes. Moreover, the ratio of GABPalpha/Fli-1 expression declines throughout megakaryocyte maturation. Consistent with this expression pattern, primary fetal liver-derived megakaryocytes from Fli-1-deficient murine embryos exhibit reduced expression of genes associated with late stages of maturation (glycoprotein [GP] Ibalpha, GPIX, and platelet factor 4 [PF4]), whereas GABPalpha-deficient megakaryocytes were mostly impaired in the expression of early megakaryocyte-specific genes (alphaIIb and cMpl). Finally, mechanistic experiments revealed that GABPalpha, like Fli-1, can impart transcriptional synergy between the hematopoietic transcription factor GATA-1 and its cofactor FOG-1 (friend of GATA-1). In concert, these data reveal disparate, but overlapping, functions of Ets transcription factors at distinct stages of megakaryocyte maturation.

Both integrated and differential regulation of components of the IL-2/IL-2 receptor system

Interleukin-2 was discovered in 1976 as a T-cell growth factor. It was the first type I cytokine cloned and the first for which a receptor component was cloned. Its importance includes its multiple actions, therapeutic potential, and lessons for receptor biology, with three components differentially combining to form high, intermediate, and low-affinity receptors. IL-2Ralpha and IL-2Rbeta, respectively, are markers for double-negative thymocytes and regulatory T-cells versus memory cells. gamma(c), which is shared by six cytokines, is mutated in patients with X-linked severe-combined immunodeficiency. We now cover an under-reviewed area-the regulation of genes encoding IL-2 and IL-2R components, with an effort to integrate/explain this knowledge.

IL-21 induces differentiation of human naive and memory B cells into antibody-secreting plasma cells

IL-21 is a type I cytokine that influences the function of T cells, NK cells, and B cells. In this study, we report that IL-21 plays a major role in stimulating the differentiation of human B cells. When human B cells were stimulated through the BCR, IL-21 induced minimal proliferation, IgD down-modulation, and small numbers of plasma cells. In contrast, after CD40 engagement, IL-21 induced extensive proliferation, class switch recombination (CSR), and plasma cell differentiation. Upon cross-linking both BCR and CD40, IL-21 induced the largest numbers of plasma cells. IL-21 drove both postswitch memory cells as well as poorly responsive naive cord blood B cells to differentiate into plasma cells. The effect of IL-21 was more potent than the combination of IL-2 and IL-10, especially when responsiveness of cord blood B cells was examined. IL-21 costimulation potently induced the expression of both B lymphocyte-induced maturation protein-1 (BLIMP-1) and activation-induced cytidine deaminase as well as the production of large amounts of IgG from B cells. Despite the induction of activation-induced cytidine deaminase and CSR, IL-21 did not induce somatic hypermutation. Finally, IL-2 enhanced the effects of IL-21, whereas IL-4 inhibited IL-21-induced plasma cell differentiation. Taken together, our data show that IL-21 plays a central role in CSR and plasma cell differentiation during T cell-dependent B cell responses.

Interleukin-21 receptor gene induction in human T cells is mediated by T-cell receptor-induced Sp1 activity

Interleukin-21 (IL-21) plays important roles in regulating the immune response. IL-21 receptor (IL-21R) mRNA is expressed at a low level in human resting T cells but is rapidly induced by mitogenic stimulation. We now investigate the basis for IL21R gene regulation in T cells. We found that the -80 to -20 region critically regulates IL-21R promoter activity and corresponds to a major DNase I-hypersensitive site. Electrophoretic mobility shift assays, DNA affinity chromatography followed by mass spectrometry, and chromatin immunoprecipitation assays revealed that Sp1 binds to this region in vitro and in vivo. Moreover, mutation of the Sp1 motif markedly reduced IL-21R promoter activity, and Sp1 small interfering RNAs effectively diminished IL-21R expression in activated T cells. Interestingly, upon T-cell receptor (TCR) stimulation, T cells increased IL-21R expression and Sp1 protein levels while decreasing Sp1 phosphorylation. Moreover, phosphatase inhibitors that increased phosphorylation of Sp1 diminished IL-21R transcription. These data indicate that TCR-induced IL-21R expression is driven by TCR-mediated augmentation of Sp1 protein levels and may partly depend on the dephosphorylation of Sp1.

A role for TSLP in the development of inflammation in an asthma model

Thymic stromal lymphopoietin (TSLP) is a cytokine that promotes CD4⁺ T cell homeostasis. We now demonstrate that TSLP is required to mount a normal CD4⁺ T cell–mediated inflammatory response. TSLP acts directly on naive, but not, memory CD4⁺ T cells, and promotes their proliferation in response to antigen. In addition, TSLP exerts an effect indirectly through DCs to promote Th2 differentiation of CD4⁺ T cells. Correspondingly, TSLP receptor (TSLPR) knockout (KO) mice exhibit strong Th1 responses, with high levels of interleukin (IL)-12, interferon-γ, and immunoglobulin (Ig) G2a, but low production of IL-4, -5, -10, -13, and IgE; moreover, CD4⁺ T cells from these animals proliferate less well in response to antigen. Furthermore, TSLPR KO mice fail to develop an inflammatory lung response to inhaled antigen unless supplemented with wild-type CD4⁺ T cells. This underscores an important role for this cytokine in the development of inflammatory and/or allergic responses in vivo.

Interleukin-21: a modulator of lymphoid proliferation, apoptosis and differentiation

The interleukin-21 (IL-21)-IL-21-receptor system was discovered in 2000. It was immediately of great interest because of the homology of IL-21 to IL-2, IL-4 and IL-15, and of the IL-21-receptor subunit IL-21R to the beta-subunit of the IL-2 receptor, and because the IL-21 receptor also contains the common cytokine-receptor gamma-chain, the protein that is mutated in X-linked severe combined immunodeficiency. As we discuss, IL-21 has pleiotropic actions, from augmenting the proliferation of T cells and driving the differentiation of B cells into memory cells and terminally differentiated plasma cells to augmenting the activity of natural killer cells. Moreover, it has antitumour activity and might have a role in the development of autoimmunity, so these findings have implications for the treatment of cancer and autoimmune diseases.

Smad-dependent cooperative regulation of interleukin 2 receptor alpha chain gene expression by T cell receptor and transforming growth factor-beta

The interleukin 2 receptor alpha chain (IL-2Ralpha) is a component of high affinity IL-2 receptors and thus critically regulates T cell growth and other lymphoid functions. Five positive regulatory regions together control lineage-restricted and activation-dependent IL-2Ralpha induction in response to antigen and IL-2. We now show that TGF-beta cooperates with T cell receptor (TCR) signaling to increase IL-2Ralpha gene expression. Moreover, we identify a sixth positive regulatory region that regulates IL-2Ralpha expression in cells treated with anti-CD3 + anti-CD28 as well as TGF-beta and show that this region contains binding sites for Smad3, AP-1, and cAMP-responsive element-binding protein/ATF proteins. The importance of Smad complexes is indicated by impaired IL-2Ralpha induction by TGF-beta in CD4+ T cells from both Smad3-/- and Smad4-/- mice. Thus, we have identified a novel positive regulatory region in the IL-2Ralpha gene that mediates TGF-beta-dependent induction of the gene. These findings have implications related to IL-2Ralpha expression on activated T cells and regulatory T cells.