The role of Stat5a and Stat5b in signaling by IL-2 family cytokines

Molecular signatures induced by interleukin-2 on peripheral blood mononuclear cells and T cell subsets

Experimentally, interleukin-2 (IL-2) exerts complex immunological functions promoting the proliferation, survival and activation of T cells on one hand and inducing immune regulatory mechanisms on the other. This complexity results from a cross talk among immune cells which sways the effects of IL-2 according to the experimental or clinical condition tested. Recombinant IL-2 (rIL-2) stimulation of peripheral blood mononuclear cells (PBMC) from 47 donors of different genetic background induced generalized T cell activation and anti-apoptotic effects. Most effects were dependent upon interactions among immune cells. Specialized functions of CD4 and CD8 T cells were less dependent upon and often dampened by the presence of other PBMC populations. In particular, cytotoxic T cell effector function was variably affected with a component strictly dependent upon the direct stimulation of CD8 T cells in the absence of other PBMC. This observation may provide a roadmap for the interpretation of the discrepant biological activities of rIL-2 observed in distinct pathological conditions or treatment modalities.

Sex-Dependent Liver Gene Expression Is Extensive and Largely Dependent upon Signal Transducer and Activator of Transcription 5b (STAT5b): STAT5b-Dependent Activation of Male Genes and Repression of Female Genes Revealed by Microarray Analysis

Sexual dimorphism in mammalian liver contributes to sex differences in physiology, homeostasis, and steroid and foreign compound metabolism. Many sex-dependent liver genes are regulated by sex differences in pituitary GH secretion, with the transcription factor, signal transducer and activator of transcription (STAT5b), proposed to mediate signaling by the pulsatile, male plasma GH profile. Presently, a large-scale gene expression study was conducted using male and female mice, wild type and Stat5b inactivated, to characterize sex differences in liver gene expression and their dependence on STAT5b. The relative abundance of individual liver RNAs was determined for each sex-genotype combination by competitive hybridization to 23,574-feature oligonucleotide microarrays. Significant sex differences in hepatic expression were seen for 1603 mouse genes. Of 850 genes showing higher expression in males, 767 (90%) were down-regulated in STAT5b-deficient males. Moreover, of 753 genes showing female-predominant expression, 461 (61%) were up-regulated in STAT5b-deficient males. In contrast, approximately 90% of the sex-dependent genes were unaffected by STAT5b deficiency in females. Thus: 1) STAT5b is essential for sex-dependent liver gene expression, a characteristic of approximately 1600 mouse genes (4% of the genome); 2) male-predominant liver gene expression requires STAT5b, or STAT5b-dependent factors, which act in a positive manner; and 3) many female-predominant liver genes are repressed in males in a STAT5b-dependent manner. Several of the STAT5b-dependent male genes encode transcriptional repressors; these may include direct STAT5b targets that repress female-predominant genes in male liver. Several female-predominant repressors are elevated in STAT5b-deficient males; these may contribute to the major loss of male gene expression seen in the absence of STAT5b.

A role for STAT5A/B in protection of peripheral T-lymphocytes from postactivation apoptosis: Insights from gene expression profiling

Activation of the transcription factors STAT5A and STAT5B by JAK1 and JAK3 tyrosine kinases is a key event in downstream signaling of gammac (common gamma chain)-family cytokine receptors in lymphoid cells. STAT5A/B-deficiency in mice causes, among other consequences, a reduced size and altered composition of the peripheral T-cell pool with predominance of an activated or memory-like population (CD4(+)/CD44(high)/CD62L(low)) and a proliferative deficiency following antigenic stimulation and subsequent IL-2 treatment. To further elucidate the critical function of STAT5A/B in homeostasis and activation of murine naïve peripheral T-lymphocytes, we have analyzed global gene expression of STAT5A/B-deficient versus wild-type splenic T-cells by profiling with high-density oligonucleotide arrays (Affymetrix). Comparison of (1) basal gene expression of untreated peripheral STAT5A/B-deficient and control T-cells and (2) immediate early gene induction upon in vitro stimulation of either population with IL-2 has revealed differential expression of a broad range of genes potentially contributing to the defects of STAT5A/B deficient T-cells. In the context of enhanced apoptotic rates of STAT5A/B(-/-)-T-cells in vivo and upon TCR-stimulation in culture our data suggest a role for STAT5 in post-activation survival beyond regulation of antiapoptotic Bcl-2 proteins and hence provide new insights into the nature of the late proliferative block in the T-cell compartment caused by STAT5-deficiency.

Phospho-proteomic immune analysis by flow cytometry: from mechanism to translational medicine at the single-cell level

Understanding a molecular basis for cellular function is a common goal of biomedicine. The complex and dynamic cellular processes underlying physiological processes become subtly or grossly perturbed in human disease. A primary objective is to demystify this complexity by creating and establishing relevant model systems to study important aspects of human disease. Although significant technological advancements over the last decade in both genomic and proteomic arenas have enabled progress, accessing the complexity of cellular interactions that occur in vivo has been a difficult arena in which to make progress. Moreover, there are extensive challenges in translating research tools to clinical applications. Flow cytometry, over the course of the last 40 years, has revolutionized the field of immunology, in both the basic science and clinical settings, as well as having been instrumental to new and exciting areas of discovery such as stem cell biology. Multiparameter machinery and systems exist now to access the heterogeneity of cellular subsets and enable phenotypic characterization and functional assays to be performed on material from both animal models and humans. This review focuses primarily on the development and application of using activation-state readouts of intracellular activity for phospho-epitopes. We present recent work on how a flow cytometric platform is used to obtain mechanistic insight into cellular processes as well as highlight the clinical applications that our laboratory has explored. Furthermore, this review discusses the challenges faced with processing high-content multidimensional and multivariate data sets. Flow cytometry, as a platform that is well situated in both the research and clinical settings, can contribute to drug discovery as well as having utility for both biomarker and patient-stratification.

Murine mammary carcinoma exosomes promote tumor growth by suppression of NK cell function

Many tumor cells shed specialized membrane vesicles known as exosomes. In this study, we show that pretreatment of mice with exosomes produced by TS/A or 4T.1 murine mammary tumor cells resulted in accelerated growth of implanted tumor cells in both syngeneic BALB/c mice and nude mice. As implanted TS/A tumor cells grew more rapidly in mice that had been depleted of NK cells, we analyzed the effects of the tumor-derived exosomes on NK cells. The tumor-derived exosomes inhibit NK cell cytotoxic activity ex vivo and in vitro as demonstrated by chromium release assays. The treatment of mice with TS/A tumor exosomes also led to a reduction in the percentages of NK cells, as determined by FACS analysis, in the lungs and spleens. Key features of NK cell activity were inhibited, including release of perforin but not granzyme B, as well as the expression of cyclin D3 and activation of the Jak3-mediated pathways. Human tumor cell lines also were found to produce exosomes that were capable of inhibiting IL-2-stimulated NK cell proliferation. Exosomes produced by dendritic cells or B cells did not. The presentation of tumor Ags by exosomes is under consideration as a cancer vaccine strategy; however, we found that pretreatment of mice with tumor exosomes blunted the protective effect of syngeneic dendritic cells pulsed ex vivo with tumor exosomes. We propose that tumor exosomes contribute to the growth of tumors by blocking IL-2-mediated activation of NK cells and their cytotoxic response to tumor cells.

Cytokine receptor signalling and aging

With ageing the immune system is deregulated and this leads to the development of immunosenescence mainly affecting the adaptive immune response. There is much knowledge accumulated concerning various receptor functions and signalling with ageing such as TCR, FcRs, TLRs. Cytokines are playing a major role in haematopoietic cell functions and in the harmonious and integrated coordination of the innate and adaptive immune response. There exists a large amount of data on cytokine production changes with ageing, as IL-2 production is decreasing, while IL-6 production is increasing. In contrast, there is only scarce knowledge concerning the cytokine receptors and their signalling in ageing. However, there is some evidence that the signalling of IL-2 receptors is altered in T cells and macrophages, mainly in relation to the JAK/STAT pathway. We present here evidence that the IL-6 induced signalling is also altered in T cells with ageing. An alteration in the JAKs and STATs activations in T cells and macrophages was demonstrated. The exact cause of these altered activations is not known and future studies are needed to elucidate them. In this review we summarise our present knowledge on cytokine signalling with ageing, mainly focusing on IL-2 and IL-6 receptors signalling.

Sensitization of IL-2 Signaling through TLR-7 Enhances B Lymphoma Cell Immunogenicity

The innate ability of B lymphoma cells to escape control by tumor-reactive T cells must be overcome to develop effective immunotherapies for these diseases. Because signals from both the innate and adaptive immune systems direct the acquisition of strong immunogenicity by professional APCs, the effects of IL-2 and the TLR-7 agonist, S28690, on the immunogenic properties of chronic lymphocytic leukemia (CLL) B cells were studied. IL-2 with S28690 caused CLL cells to proliferate and increased their expression of B7-family members, production of TNF-alpha and IL-10, and levels of tyrosine-phosphorylated STAT-1 and STAT-3 proteins. S28690 increased CD25 expression on CLL cells and sensitized them to IL-2 signaling. However, IL-2 did not change TLR-7 expression or signaling in CLL cells. The ability to stimulate T cell proliferation required additional activation of protein kinase C, which inhibited tumor cell proliferation, "switched off" IL-10 production, and caused essentially all CLL cells (regardless of clinical stage) to acquire a CD83(high)CD80(high)CD86(high)CD54(high) surface phenotype marked by the activation of STAT-1 without STAT-3. These findings suggest that TLR-7 "licenses" human B cells to respond to cytokines of the adaptive immune system (such as IL-2) and provide a strategy to increase the immunogenicity of lymphoma cells for therapeutic purposes.

Clarifying the role of Stat5 in lymphoid development and Abelson-induced transformation

The Stat5 transcription factors Stat5a and Stat5b have been implicated in lymphoid development and transformation. Most studies have employed Stat5a/b-deficient mice where gene targeting disrupted the first protein-coding exon, resulting in the expression of N-terminally truncated forms of Stat5a/b (Stat5a/b(DeltaN/DeltaN) mice). We have now reanalyzed lymphoid development in Stat5a/b(null/null) mice having a complete deletion of the Stat5a/b gene locus. The few surviving Stat5a/b(null/null) mice lacked CD8(+) T lymphocytes. A massive reduction of CD8(+) T cells was also found in Stat5a/b(fl/fl) lck-cre transgenic animals. While gammadelta T-cell receptor-positive (gammadeltaTCR(+)) cells were expressed at normal levels in Stat5a/b(DeltaN/DeltaN) mice, they were completely absent in Stat5a/b(null/null) animals. Moreover, B-cell maturation was abrogated at the pre-pro-B-cell stage in Stat5a/b(null/null) mice, whereas Stat5a/b(DeltaN/DeltaN) B-lymphoid cells developed to the early pro-B-cell stage. In vitro assays using fetal liver-cell cultures confirmed this observation. Most strikingly, Stat5a/b(null/null) cells were resistant to transformation and leukemia development induced by Abelson oncogenes, whereas Stat5a/b(DeltaN/DeltaN)-derived cells readily transformed. These findings show distinct lymphoid defects for Stat5a/b(DeltaN/DeltaN) and Stat5a/b(null/null) mice and define a novel functional role for the N-termini of Stat5a/b in B-lymphoid transformation.

Allograft rejection requires STAT5a/b-regulated antiapoptotic activity in T cells but not B cells

STATs play key roles in immune function. We examined the role of STAT5a/b in allograft rejection. STAT5a/b-deficient mice showed a 4-fold increased survival time of heart allografts (p < 0.01). Unlike wild type, purified STAT5a/b-/- T cells transferred to Rag1-/- recipients failed to mediate heart allograft rejection until supplemented with STAT5a/b-/- B cells. In vitro, STAT5a/b-/- T cells did not proliferate in response to Con A or alloantigens but entered apoptosis within 48 h (95%). Activated STAT5a/b-/- T cells showed increased expression of proapoptotic (caspases, DNA repair genes, TNF/TNFR-associated factor family genes) and decreased antiapoptotic mRNAs in microarrays, while Western blots confirmed reduced antiapoptotic Bcl-2 and elevated proapoptotic Bax protein expression. Interestingly, at 24 h postactivation, STAT5a/b+/+ and STAT5a/b-/- T cells produced similar levels of IL-2, IL-4, IL-10, and IFN-gamma mRNA; ELISPOT assay showed an equivalent number of IL-4- and IFN-gamma-producing T cells in both STAT5a/b+/+ and STAT5a/b-/- splenic populations. Sera from STAT5a/b+/+ and STAT5a/b-/- rejectors had donor-specific IgM, IgG1, IgG2a, and IgG2b Ab, while STAT5a/b deficiency had no impact on B cell survival or proliferation in response to LPS. Compared with allografts from STAT5a/b+/+ recipients, heart allografts from STAT5a/b-/- recipients had markedly reduced infiltration by CD4 and CD8 T cells but increased infiltration by B cells and dense endothelial deposition of C4d, a marker of humoral rejection. Thus, activated STAT5a/b-/- T cells produce cytokines prior to entering apoptosis, thereby promoting differentiation of B cells yielding donor-specific IgM and IgG Ab that mediate allograft rejection.

Stat5a/b are essential for normal lymphoid development and differentiation

Cytokines that use the common gamma chain gammac are critical for lymphoid development and function. Mutations of the IL-7 receptor, gammac, or its associated kinase, Jak3, are the major cause of human severe combined immunodeficiency. Although activated by IL-7, Stat5a/b (Stat, signal transducer and activator of transcription) have been thought to play limited roles in lymphoid development. However, we now show that mice completely deficient in Stat5a/b have severely impaired lymphoid development and differentiation. Absence of Stat5 also abrogates T cell receptor gamma rearrangement and survival of peripheral CD8(+) T cells. Thus, deficiency of Stat5 results in severe combined immunodeficiency, similar in many respects to deficiency of IL-7R, gammac, and Jak3.

GATA-3 promotes Th2 responses through three different mechanisms: induction of Th2 cytokine production, selective growth of Th2 cells and inhibition of Th1 cell-specific factors

Naïve CD4 T cells can differentiate into at least two different types of T helpers, Th1 and Th2 cells. Th2 cells, capable of producing IL-4, IL-5 and IL-13, are involved in humoral immunity against extracellular pathogens and in the induction of asthma and other allergic diseases. In this review, we summarize recent reports regarding the transcription factors involved in Th2 differentiation and cell expansion, including Stat5, Gfi-1 and GATA-3. Stat5 activation is necessary and sufficient for IL-2-mediated function in Th2 differentiation. Enhanced Stat5 signaling induces Th2 differentiation independent of IL-4 signaling; although it does not up-regulate GATA-3 expression, it does require the presence of GATA-3 for its action. Gfi-1, induced by IL-4, promotes the expansion of GATA-3-expressing cells. Analysis of conditional Gata3 knockout mice confirmed the critical role of GATA-3 in Th2 cell differentiation (both IL-4 dependent and IL-4 independent) and in Th2 cell proliferation and also showed the importance of basal GATA-3 expression in inhibiting Th1 differentiation.

Effect of docosahexaenoic acid on interleukin-2 receptor signaling pathway in lipid rafts

Recent studies have shown that polyunsaturated fatty acids (PUFA) regulated the functions of membrane receptors in T cells and suppressed T cell -mediated immune responses. But the molecular mechanisms of immune regulation are not yet elucidated. Lipid rafts are plasma membrane microdomains, in which many receptors localized. The purpose of this study was to investigate the effect of DHA on IL-2R signaling pathway in lipid rafts. We isolated lipid rafts by discontinuous sucrose density gradient ultracentrifugation, and found that DHA could change the composition of lipid rafts and alter the distribution of key molecules of IL-2R signaling pathway, which transferred from lipid rafts to detergent-soluble membrane fractions. These results revealed that DHA treatment increased the proportion of polyunsaturated fatty acids especially n-3 polyunsaturated fatty acids in lipid rafts and changed the lipid environment of membrane microdomains in T cells. Compared with controls, DHA changed the localization of IL-2R, STAT5a and STAT5b in lipid rafts and suppressed the expression of JAK1, JAK3 and tyrosine phosphotyrosine in soluble membrane fractions. Summarily, this study concluded the effects of DHA on IL-2R signaling pathway in lipid rafts and explained the regulation of PUFAs in T cell-mediated immune responses.

Interleukin-2: from T cell growth and homeostasis to immune reconstitution of HIV patients

Interleukin (IL)-2 was initially described as a major stimulant of T lymphocytes in vitro. Later, the characterization of IL-2 knockout animals showed that the ability to stimulate T cells could be replaced by other cytokines. In vivo, IL-2 plays a unique role in controlling lymphoproliferation. This is partly explained by its role in the generation and maintenance of T regulatory cells (Treg). In HIV-infected patients, the IL-2/IL-2 receptor (IL-2R) system is dysregulated. The fact that IL-2 is underproduced along with defective IL-2R signaling detected in patient lymphocytes, may explain the progressive impairment of the immune system that occurs during chronic infection with this virus. These defects are partly reversed by highly active antiretroviral therapy (HAART). However, in some patients IL-2R defects persist and the CD4 counts remain low despite good control of the viral load. These patients benefit from HAART given in conjunction with IL-2 therapy.

IL-2 Regulates Perforin and Granzyme Gene Expression in CD8+T Cells Independently of Its Effects on Survival and Proliferation

Perforin and the serine protease granzymes are key effectors of CD8+ T cell granule-mediated cytotoxicity, but the requirements for their expression remain largely undefined. We show in this study that IL-2 increased the expression of perforin and granzyme A, B, and C mRNA; intracellular granzyme B protein levels; and cytolytic function in a dose-dependent manner during primary activation of murine CD8+ T cells in vitro. Two approaches showed that these responses were not a consequence of the effects of IL-2 on cell survival and proliferation. First, IL-2 enhancement of perforin and granzyme expression was equivalent in CD8+ T cells from wild-type and bcl-2 transgenic mice, although only the latter cells survived in low concentrations or the absence of added IL-2. This property of bcl-2 transgenic T cells also allowed the demonstration that induction of granzyme A, B, and C mRNA and granzyme B protein required exogenous IL-2, whereas induction of perforin and IFN-gamma expression did not. Second, analysis of perforin and granzyme mRNA levels in cells separated according to division number using the dye CFSE showed that the effects of IL-2 were unrelated to division number. Together, these findings indicate that IL-2 can directly regulate perforin and granzyme gene expression in CD8+ T cells independently of its effects on cell survival and proliferation.

The Mannich base NC1153 promotes long-term allograft survival and spares the recipient from multiple toxicities

JAK3 is a cytoplasmic tyrosine kinase with limited tissue expression but is readily found in activated T cells. Patients lacking JAK3 are immune compromised, suggesting that JAK3 represents a therapeutic target for immunosuppression. Herein, we show that a Mannich base, NC1153, blocked IL-2-induced activation of JAK3 and its downstream substrates STAT5a/b more effectively than activation of the closely related prolactin-induced JAK2 or TNF-alpha-driven NF-kappaB. In addition, NC1153 failed to inhibit several other enzymes, including growth factor receptor tyrosine kinases, Src family members, and serine/threonine protein kinases. Although NC1153 inhibited proliferation of normal human T cells challenged with IL-2, IL-4, or IL-7, it did not block T cells void of JAK3. In vivo, a 14-day oral therapy with NC1153 significantly extended survival of MHC/non-MHC mismatched rat kidney allografts, whereas a 90-day therapy induced transplantation tolerance (>200 days). Although NC1153 acted synergistically with cyclosporin A (CsA) to prolong allograft survival, it was not nephrotoxic, myelotoxic, or lipotoxic and did not increase CsA-induced nephrotoxicity. In contrast to CsA, NC1153 was not metabolized by cytochrome P450 3A4. Thus, NC1153 prolongs allograft survival without several toxic effects associated with current immunosuppressive drugs.

Multiple cytokines regulate the NK gene complex-encoded receptor repertoire of mature NK cells and T cells

Mature NK cells comprise a highly diverse population of lymphocytes that express different permutations of receptors to facilitate recognition of diseased cells and perhaps pathogens themselves. Many of these receptors, such as those belonging to the NKRP1, NKG2, and Ly49 families are encoded in the NK gene complex (NKC). It is generally thought that these NKC-encoded receptors are acquired by a poorly understood stochastic mechanism, which operates exclusively during NK cell development, and that following maturation the repertoire is fixed. However, we report a series of observations that demonstrates that the mature NK cell repertoire in mice can in fact be radically remodeled by multiple cytokines. Thus, both IL-2 and IL-15 selectively induce the de novo expression of Ly49E on the majority of mature NK cells. By contrast, IL-4 not only blocks this IL-2-induced acquisition of Ly49E, but reduces the proportion of mature NK cells that expresses pre-existing Ly49 receptors and abrogates the expression of NKG2 receptors while leaving the expression of several NKRP1 receptors unaltered. IL-21 also abrogates NKG2 expression on mature NK cells and selectively down-regulates Ly49F. IL-4 and IL-21 additionally cause dramatic and selective alterations in the NKC-encoded receptor repertoire of IL-2-activated T cells but these are quite different to the changes induced on NK cells. Collectively these findings reveal an unexpected aspect of NKC receptor expression that has important implications for our understanding of the function of these receptors and of the genetic mechanisms that control their expression.

Deregulation of cell-signaling pathways in HTLV-1 infection

Human T-lymphotropic virus type 1 (HTLV-1) infection is associated with the clonal expansion and transformation of mature T lymphocytes. While the mechanisms involved are incompletely understood the viral regulatory protein Tax plays a central role in these processes. Recent studies employing genomic and proteomic approaches have demonstrated the marked complexity of gene deregulation associated with Tax expression and confirmed the remarkable pleiotropism of this protein as evidenced by the numerous Tax-cellular protein interactions in infected cells. In this review, we summarize the role of Tax in the deregulation of selected cellular-signaling pathways. Specifically, this has focused on the influence and interaction of Tax with the AP-1 and NF-AT transcription factors, PDZ domain-containing proteins, Rho-GTPases, and the Janus kinase/signal transducer and activator of transcription and transforming growth factor-beta-signaling pathways. In addition to identifying the deregulation of events within these pathways, attempts have been made to highlight differences between HTLV-1 and -2, which may relate to differences in their pathogenic properties.

Docosahexaenoic acid changes lipid composition and interleukin-2 receptor signaling in membrane rafts

Polyunsaturated fatty acids, including docosahexaenoic acid (DHA, 22:6n-3), modulate immune responses and exert beneficial immunosuppressive effects, but the molecular mechanisms inhibiting T-cell activation are not yet elucidated. Lipid rafts have been shown to play an important role in the compartmentalization and modulation of cell signaling. We investigated the role of DHA in modulating the lipid composition in lipid rafts and membrane subdomain distribution of interleukin-2 (IL-2) receptor signaling molecules. We found that DHA altered lipid components of rafts and modified the IL-2-induced Janus kinase-signal transducer and activator of transcription (STAT) signaling pathway by partially displacing IL-2 receptors from lipid rafts. We fractionated plasma membrane subcellular compartments and discovered that certain amounts of STAT5a and STAT5b existed in detergent-resistant plasma membrane fractions of T-cells. After DHA treatment, STAT5a and STAT5b were not detected in lipid raft fractions and were located in detergent-soluble fractions. These data demonstrate for the first time that DHA alters the lipid composition of membrane microdomains and suppresses IL-2 receptor signaling in T-cells. Thus, our data provide evidence for a functional modification in lipid rafts by DHA treatment and explain PUFA-mediated immunosuppressive effects.

IL-2- and STAT5-regulated cytokine gene expression in cells expressing the Tax protein of HTLV-1

Interleukin-2 (IL-2) mediates cell cycle progression and antiapoptosis in human T cells via several signal transduction pathways. The Tax protein of the human T-cell leukemia virus type I (HTLV-1) deregulates cell growth and alters the role of IL-2 in infected cells. However, Tax-immortalized cells stay dependent on IL-2, suggesting that events besides HTLV-1 gene expression are required for leukemia to develop. Here, IL-2-dependent and -independent events were analysed in a human T cell line immortalized by Tax. These studies show that, of the signaling pathways evaluated, only STAT5 remains dependent. Microarray analyses revealed several genes, including il-5, il-9 and il-13, are uniquely upregulated by IL-2 in the presence of Tax. Bioinformatics and supporting molecular biology show that some of these genes are STAT5 targets, explaining their IL-2 upregulation. These results suggest that IL-2 and viral proteins work together to induce gene expression, promoting the hypothesis that deregulation via the constitutive activation of STAT5 may lead to the IL-2-independent phenotype of HTLV-1-transformed cells.

Activation of STAT5-dependent transcription by the neurotrophin receptor Trk

Neurotrophins exert many of their biological effects via the Trk receptor tyrosine kinases and require the regulated activation of distinct transcriptional and post-translational cellular events. Here we provide evidence for a novel signaling cascade from activated Trks to the transcription factor STAT5. Utilizing the STAT5 responsive element derived from the p21(WAF1/Cip1) promoter to modulate luciferase expression, neurotrophin-dependent activation of Trk A, B, and C was found to induce STAT5-mediated transcriptional response. Structure-function analysis using Trk A mutants in heterologous cells further revealed that the kinase activity and an intact phospholipase C-gamma binding site are required for STAT5 activation. In most cytokine responsive cell systems, STAT5 function is modulated by JAK2-dependent tyrosine phosphorylation. However, reconstitution studies using a JAK2 deficient cell line indicate that neurotrophin-induced STAT5 activation does not require the cognate upstream kinase JAK2. In contrast, the Src kinase inhibitor PP1 significantly abolishes STAT5-dependent transcription in Trk A expressing 293T cells and in BDNF-treated primary cortical neurons. Together these results suggest that neurotrophins may regulate neuronal gene expression via STAT5 in a JAK2 independent manner.

IL-2 responsiveness of CD4 and CD8 lymphocytes: further investigations with human IL-2Rbeta transgenic mice

Responsiveness to IL-2 varies from one lympho-mononuclear subset to another. NK lymphocytes and monocytes spontaneously respond to IL-2 whereas it is generally accepted that T and B lymphocytes need to be activated to fully acquire this competence. To further investigate this phenomenon, we studied human IL-2Rbeta (hIL-2Rbeta) transgenic mice constitutively expressing heterospecific, intermediate-affinity IL-2R (hIL-2Rbeta/mouse IL-2Rgamma(c)). We noted that the B lymphocytes and monocytes from spleens of these hIL-2Rbeta transgenic animals failed to grow when cultured in IL-2-containing medium. Under the same experimental conditions, CD4 lymphocytes survived, again without growth, whereas CD8 lymphocytes and NK cells were able to proliferate and develop potent LAK cytotoxicity. The properties of these CD4 and CD8 lymphocytes were then compared after purification. Both subsets expressed functional IL-2R able to induce global protein phosphorylation and, more precisely, signal transducer and activation of transcription 5 and Erk phosphorylation. Therefore, the differential growth potential of these CD4 and CD8 lymphocytes cannot be explained by the lack of IL-2R-dependent early signaling events. When the entrance of purified CD4 and CD8 lymphocytes into the cell cycle was analyzed, we found that the CD4 lymphocytes were unable to enter the G1 phase in the absence of anti-CD3 stimulation. This correlates with the effect of IL-2 on cyclin-dependent kinase inhibitor p27(kip1). In CD4 lymphocytes, IL-2 does not affect p27(kip1) expression. But in CD8 lymphocytes, IL-2 down-modulates p27(kip1). These results indicate that, aside from IL-2R expression and function, IL-2 responsiveness is also controlled by lineage-specific mechanisms.

Regulation of STAT signalling by proteolytic processing

Interaction of cytokines with their cognate receptors leads to the activation of latent transcription factors, the signal transducer and activator of transcription (STAT) proteins. Numerous studies have identified the critical roles played by STAT proteins in regulating cell proliferation, differentiation and survival. Consequently, the activity of STAT proteins is negatively regulated by a variety of different mechanisms, which include alternative splicing, covalent modifications, protein-protein interactions with negative regulatory proteins and proteolytic processing by proteases. Cleavage of STAT proteins by proteases results in the generation of C-terminally truncated proteins, called STATgamma, which lack the transactivation domain and behave as functional dominant-negative proteins. Currently, STATgamma isoforms have been identified for Stat3, Stat5a, Stat5b and Stat6 in different cellular contexts and biological processes. Evidence is mounting for the role of as yet unidentified serine proteases in the proteolytic processing of STAT proteins, although at least one cysteine protease, calpain is also known to cleave these STATs in platelets and mast cells. Recently, studies of acute myeloid leukaemia and cutaneous T cell lymphoma patients have revealed important roles for the aberrant expression of Stat3gamma and Stat5gamma proteins in the pathology of these diseases. Together, these findings indicate that proteolytic processing is an important mechanism in the regulation of STAT protein biological activity and provides a fertile area for future studies.

Stat5a is essential for the proliferation and survival of murine mast cells

The regulatory role of signal transducer and activator of transcription (Stat) 5a in the proliferation and survival of mast cells was determined using Stat5a-deficient (Stat5a(-/-)) mice. First, although the mast cells in Stat5a(-/-) mice were morphologically indistinguishable from those in wild-type (WT) mice, the number of peritoneal mast cells was significantly decreased in Stat5a(-/-) mice as compared with that in WT mice. Furthermore, the interleukin-3 (IL-3)-dependent development of bone marrow-derived mast cells (BMMCs) was markedly decreased in Stat5a(-/-) mice. Second, IL-3-induced but not stem cell factor (SCF)-induced proliferation of BMMCs was significantly diminished in Stat5a(-/-) mice as compared with that in WT mice. Moreover, survival rates of both peritoneal mast cells and BMMCs were significantly decreased with increased apoptotic cells in Stat5a(-/-) mice as compared with those in WT mice. Finally, mRNA of Bcl-x(L) was induced after IL-3 stimulation in WT BMMCs but not in Stat5a(-/-) BMMCs, which may account for the accelerated apoptosis in Stat5a(-/-) mast cells. These results indicate that Stat5a plays an important role in mast cell development, proliferation, and survival.

Haploinsufficiency identifies STAT5 as a modifier of IL-7-induced lymphomas

The requirement for receptor components and the signalling effector, signal transducer and activator of transcription (STAT) 5A/5B, was assessed genetically in a lymphoma development model induced by interleukin-7 (IL-7). This growth factor for T- and B-cell progenitors and mature lymphocytes activates survival and proliferative pathways including Bcl-2, phosphatidylinositol-3 kinase and STAT5. Overexpression of IL-7 in vivo causes early mortality from lymphoma development. Mice overexpressing IL-7 that were heterozygous for the IL-7Ralpha subunit showed improved survival compared to wild-type mice. In addition, STAT5A/5B+/- compound heterozygous mice with one targeted allele each of STAT5A and STAT5B showed striking amelioration of IL-7-induced mortality and disease development. STAT5A/5B+/- compound heterozygous mice were otherwise normal in stem cell and lymphocyte development and cellularity. Lower STAT5 protein levels accompanied the reduction in STAT5A/5B copy number, which suggests that STAT5 haploinsufficiency is a modifier of IL-7 signal strength.

Signaling pathways in Th2 development

In order for an immune response to be successful, it must be of the appropriate type and magnitude. Intracellular residing pathogens require a cell-mediated immune response, whereas extracellular pathogens evoke a humoral immune response. T-helper (Th) cells orchestrate the immune response and are divided into two subsets, Th1 and Th2 cells. Here, we discuss the mechanisms of Th2 development with a focus on signal transduction pathways that influence Th2 differentiation.

Stat5a Inhibits IL-12-Induced Th1 Cell Differentiation through the Induction of Suppressor of Cytokine Signaling 3 Expression

In previous studies, we have shown that Th2 cell differentiation is diminished but Th1 cell differentiation is increased in Stat5a-deficient (Stat5a(-/-)) CD4(+) T cells. In the present study, we clarified the molecular mechanisms of Stat5a-mediated Th cell differentiation. We found that enhanced Th1 cell differentiation and the resultant IFN-gamma production played a dominant inhibitory role in the down-regulation of IL-4-induced Th2 cell differentiation of Stat5a(-/-) CD4(+) T cells. We also found that IL-12-induced Stat4 phosphorylation and Th1 cell differentiation were augmented in Stat5a(-/-) CD4(+) T cells. Importantly, the expression of suppressor of cytokine signaling (SOCS)3, a potent inhibitor of IL-12-induced Stat4 activation, was decreased in Stat5a(-/-) CD4(+) T cells. Moreover, a reporter assay showed that a constitutively active form of Stat5a but not Stat6 activated the SOCS3 promoter. Furthermore, chromatin immunoprecipitation assays revealed that Stat5a binds to the SOCS3 promoter in CD4(+) T cells. Finally, the retrovirus-mediated expression of SOCS3 restored the impaired Th cell differentiation of Stat5a(-/-) CD4(+) T cells. These results suggest that Stat5a forces the Th1/Th2 balance toward a Th2-type by preventing IL-12-induced Th1 cell differentiation through the induction of SOCS3.

Indispensable Role of Stat5a in Stat6-Independent Th2 Cell Differentiation and Allergic Airway Inflammation

It is well-recognized that Stat6 plays a critical role in Th2 cell differentiation and the induction of allergic inflammation. We have previously shown that Stat5a is also required for Th2 cell differentiation and allergic airway inflammation. However, it is the relative importance and redundancy of Stat6 and Stat5a in Th2 cell differentiation and allergic airway inflammation are unknown. In this study we addressed these issues by comparing Stat5a-deficient (Stat5a(-/-)) mice, Stat6(-/-) mice, and Stat5a- and Stat6 double-deficient (Stat5a(-/-) Stat6(-/-)) mice on the same genetic background. Th2 cell differentiation was severely decreased in Stat6(-/-)CD4+ T cells, but Stat6-independent Th2 cell differentiation was still significantly observed in Stat6(-/-)CD4+ T cells. However, even in the Th2-polarizing condition (IL-4 plus anti-IFN-gamma mAb), no Th2 cells developed in Stat5a(-/-)Stat6(-/-) CD4+ T cells. Moreover, Ag-induced eosinophil and lymphocyte recruitment in the airways was severely decreased in Stat5a(-/-)Stat6(-/-) mice compared with that in Stat6(-/-) mice. These results indicate that Stat5a plays an indispensable role in Stat6-independent Th2 cell differentiation and subsequent Th2 cell-mediated allergic airway inflammation.

Jak3, severe combined immunodeficiency, and a new class of immunosuppressive drugs

The recent elucidation of the multiple molecular mechanisms underlying severe combined immunodeficiency (SCID) is an impressive example of the power of molecular medicine. Analysis of patients and the concomitant generation of animal models mimicking these disorders have quickly provided great insights into the pathophysiology of these potentially devastating illnesses. In this review, we summarize the discoveries that led to the understanding of the role of cytokine receptors and a specific tyrosine kinase, Janus kinase 3 (Jak3), in the pathogenesis of SCID. We discuss how the identification of mutations of Jak3 in autosomal recessive SCID has facilitated the diagnosis of these disorders, offered new insights into the biology of this kinase, permitted new avenues for therapy, and provided the rationale for a generation of a new class of immunosuppressants.

Differential STAT5 signaling by ligand-dependent and constitutively active cytokine receptors

Many leukemia and cancer cells exhibit constitutive activation of STAT5, which was suggested to provide an anti-apoptotic advantage. Transformation of cytokine-dependent hematopoietic cells, such as Ba/F3 cells to autonomous growth and tumorigenicity equally results in selection for constitutive activation of STAT5. We compared STAT5 signaling between erythropoietin(Epo)-dependent cells and cells that were transformed by oncogenic activation of the erythropoietin receptor (EpoR) by coexpression of the gp55-P envelope protein of the spleen focus forming virus or by expression of the R129C constitutively active EpoR mutant. In transformed cells it was mainly STAT5B that was constitutively activated. In contrast, Epo stimulation activated both STAT5A and STAT5B. In transformed cells, chromatin immunoprecipitation (ChIP) showed STAT5 to be physically bound to promoters of STAT5 target genes, such as Bcl(XL), and to be able to promote transactivation of the Bcl(XL) promoter in a constitutive fashion. Sequencing of native sequences after ChIP with anti-STAT5 antibodies in Epo-dependent and -transformed cells indicated that in gp55-transformed cells, STAT5B bound in the chromatin not only to N3 high affinity, but also to low affinity N4 GAS sites. Transactivation for N3 GAS sites in luciferase reporters was specific to gp55 transformation. Because we also found preferential constitutive STAT5B activation after transformation of cells by a truncated form of the G-CSF-R that produces severe neutropenia (Kostmann syndrome) and favors leukemia in humans, we discuss the potential role of STAT5B in oncogenic transformation of hematopoietic cells.

Cloning, sequencing and characterization of lentiviral-mediated expression of rhesus macaque (Macaca mulatta) interleukin-2 receptor alpha cDNA

The rhesus macaque CD25 (RhCD25) cDNA isolated from rhesus PBMCs was found to share 95.5 and 91.9% homology with the human orthologue at the nucleotide and amino acid levels, respectively. Comparative sequence analyses suggest that both human CD25 (HuCD25) and RhCD25 share identity for most of the critical amino acids previously identified to be essential for viable folding and IL-2 ligand binding. The human leukemic cell line, HH, deficient for IL-2Ralpha was transduced with a lentiviral vector (LV) engineered to express RhCD25 (HH-RhCD25). RhCD25 was characterized for expression by flow cytometric analyses, ELISA, Western blotting, functional signalling, and biological assays in comparison to HuCD25. In summary, vectors expressing the RhCD25 cDNA can be used as a tool to aid in the characterization of soluble CD25 in non-human primate studies, and to provide a tempting alternative as an autologous cell surface marker in rhesus macaque gene therapy and bone marrow transplantation studies.

Evidence of a novel IL-2/15R beta-targeted cytokine involved in homeostatic proliferation of memory CD8+ T cells

The homeostasis of memory CD8+ T cells is regulated by cytokines. IL-15 is shown to promote the proliferation of memory CD8+ T cells, while IL-2 suppresses their division in vivo. This inhibitory effect of IL-2 appears to occur indirectly, through other cell populations including CD25+CD4+ T cells; however, the details of this mechanism remain unclear. In this study, we show that 1) both Ag-experienced and memory phenotype CD8+ T cells divided after the depletion of IL-2 in vivo; 2) this division occurred normally and CD44(high)IL-2/15Rbeta(high) CD8+ T cells generated after IL-2 depletion in IL-15 knockout (KO) and in IL-7-depleted IL-15 KO mice; 3) surprisingly, the blockade of IL-2/15Rbeta signaling in IL-2-depleted IL-15 KO mice completely abolished the division of memory CD8+ T cells, although the only cytokines known to act through IL-2/15Rbeta are IL-2 and IL-15; and 4) the expression of IL-2/15Rbeta molecules on memory CD8+ T cells was required for their division induced by IL-2 depletion. These results demonstrate that the depletion of IL-2 in vivo induced memory CD8+ T cell division by an IL-15-independent but by an IL-2/15Rbeta-dependent mechanism, suggesting the existence of a novel IL-2/15Rbeta-utilizing cytokine that acts directly on memory CD8+ T cells to promote cell division.

Activity of the suppressor of cytokine signaling-3 promoter in human non-small-cell lung cancer

The Janus kinase (JAKs)/signal transducers and activators of transcription (STAT) signaling pathway is controlled by a classical feedback loop through suppressors of cytokine signaling (SOCS/JAB/SSI). Suppressors of cytokine signaling proteins are induced rapidly by activated STATs upon phosphorylation and act to block the cytokine signal. Abnormalities of the JAK/STAT pathway are associated with cancer. Recently, we cloned the functional 5' promoter region of the human SOCS-3 gene and showed that this region is highly conserved in murine and rat SOCS-3 promoters. In addition, we found that the wild type SOCS-3 promoter construct has significantly greater activity in human non-small-cell lung cancer (NSCLC) cell lines than in normal cells in accordance with STAT3 deregulation in these cells. Furthermore, we have confirmed that frequent hypermethylation of the functional SOCS-3 promoter correlates with its transcription silencing in NSCLC cell lines and primary lung cancer tissue samples. Restoration of SOCS-3 in lung cancer cells in which SOCS-3 has been methylation-silenced induces apoptosis and suppresses growth. Therefore, methylation silencing of SOCS-3 may be used as a marker for early detection of NSCLC. Suppressor of cytokine signaling-3 therapy may be useful for the treatment of lung cancer.

Cutting edge: IL-5 primes Th2 cytokine-producing capacity in eosinophils through a STAT5-dependent mechanism

Both type-2 CD4(+) Th cells (CD4(+)Th2) and type-2 innate effector cells play critical roles in generating type-2 immunity that can either be protective against parasitic infection or cause tissue damage in allergy and asthma. How innate effector cells acquire the capacity to produce Th2 cytokines is not entirely known. We previously showed that IL-4 induced differentiation of Th2 cytokine-producing eosinophils. To determine whether other Th2 cytokines can also induce Th2 cytokine-producing capacity in innate effector cells, we cultured bone marrow progenitor cells in the presence of various Th2 cytokines. IL-5, but not IL-13 or IL-25, primed bone marrow progenitor cells to differentiate into robust IL-4-producing cells. The majority of IL-4-producing cells induced by IL-5 were eosinophils. Importantly, IL-5 completely depended on STAT5 to promote IL-4-producing capacity in eosinophils. Thus, our study demonstrates that IL-5 functions as a potent factor that drives bone marrow progenitor cells into IL-4-producing eosinophils.

Phosphorylation of Grb2-Associated Binder 2 on Serine 623 by ERK MAPK Regulates Its Association with the Phosphatase SHP-2 and Decreases STAT5 Activation

IL-2 stimulation of T lymphocytes induces the tyrosine phosphorylation and adaptor function of the insulin receptor substrate/Grb2-associated binder (Gab) family member, Gab2. In addition, Gab2 undergoes a marked decrease in its mobility in SDS-PAGE, characteristic of migration shifts induced by serine/threonine phosphorylations in many proteins. This migration shift was strongly diminished by treating cells with the MEK inhibitor U0126, indicating a possible role for ERK in Gab2 phosphorylation. Indeed, ERK phosphorylated Gab2 on a consensus phosphorylation site at serine 623, a residue located between tyrosine 614 and tyrosine 643 that are responsible for Gab2/Src homology 2 domain-containing tyrosine phosphatase (SHP)-2 interaction. We report that pretreatment of Kit 225 cells with U0126 increased Gab2/SHP-2 association and tyrosine phosphorylation of SHP-2 in response to IL-2, suggesting that ERK phosphorylation of serine 623 regulates the interaction between Gab2 and SHP-2, and consequently the activity of SHP-2. This hypothesis was confirmed by biochemical analysis of cells expressing Gab2 WT, Gab2 serine 623A or Gab2 tyrosine 614F, a mutant that cannot interact with SHP-2 in response to IL-2. Activation of the ERK pathway was indeed blocked by Gab2 tyrosine 614F and slightly increased by Gab2 serine 623A. In contrast, STAT5 activation was strongly enhanced by Gab2 tyrosine 614F, slightly reduced by Gab2 WT and strongly inhibited by Gab2 serine 623A. Analysis of the rate of proliferation of cells expressing these mutants of Gab2 demonstrated that tyrosine 614F mutation enhanced proliferation whereas serine 623A diminished it. These results demonstrate that ERK-mediated phosphorylation of Gab2 serine 623 is involved in fine tuning the proliferative response of T lymphocytes to IL-2.

Interaction of the tyrosine phosphatase SHP-2 with Gab2 regulates Rho-dependent activation of the c-fos serum response element by interleukin-2

Gab2 (Grb2-associated binder-2), a member of the IRS (insulin receptor substrate)/Gab family of adapter proteins, undergoes tyrosine phosphorylation in response to cytokine or growth factor stimulation and serves as a docking platform for many signal transduction effectors, including the tyrosine phosphatase SHP-2 [SH2 (Src homology 2)-domain-containing tyrosine phosphatase]. Here, we report that, following IL-2 (interleukin-2) stimulation of human T lymphocytes, SHP-2 binds tyrosine residues 614 and 643 of human Gab2 through its N- and C-terminal SH2 domains respectively. However, the sole mutation of Tyr-614 into phenylalanine is sufficient to prevent Gab2 from recruiting SHP-2. Expression of the Gab2 Tyr-614-->Phe (Y614F) mutant, defective in SHP-2 association, prevents ERK (extracellular-signal-regulated kinase) activation and expression of a luciferase reporter plasmid driven by the c-fos SRE (serum response element), indicating that interaction of SHP-2 with Gab2 is required for ERK activation in response to IL-2. Further investigation of IL-2-dependent induction of SRE showed that expression of a constitutively active mutant of the RhoA GTPase synergizes with IL-2 for SRE-driven transcription, whereas a dominant-negative mutant reduces the IL-2 response. Thus, in response to IL-2, full induction of the SRE requires ERK-dependent as well as Rho-dependent signals that target the Ets-box and the CArG-box respectively. We also report that the synergy between Gab2/SHP-2 and RhoA for IL-2-dependent CArG-box-driven transcription depends upon MEK (mitogen-activated protein kinase/ERK kinase) activation, and is likely to involve regulation of the serum response factor co-activator MAL. Our studies thus provide new insights into the role of Gab2 and SHP-2 in IL-2 signal transduction.

Proliferation and Differentiation of CD8+ T Cells in the Absence of IL-2/15 Receptor β-Chain Expression or STAT5 Activation

Major gains in the efficacy of T cell-based therapies for cancer and infectious diseases could be realized through improved understanding of the signals that control expansion and differentiation of CD8(+) cytolytic T cells. IL-2, IL-15, and the downstream transcription factor STAT5 have all been implicated as important regulators of these processes, yet there are conflicting data regarding their contribution to in vivo T cell responses. We used a murine adoptive T cell transfer model to examine the contribution of IL-2 and IL-15 signaling to the proliferation and differentiation of naive, CD8(+) T cells bearing an OVA-specific TCR transgene (OT-I). OT-I T cells failed to express the high affinity IL-2R (CD25) while proliferating in vivo, irrespective of the mode of Ag delivery. Moreover, OT-I T cells rendered genetically deficient in the shared IL-2/IL-15Rbeta subunit (IL-2Rbeta) demonstrated normal Ag-induced proliferation and cytolytic activity in vivo. Accordingly, activation of STAT5 was not detected in proliferating IL-2Rbeta-deficient OT-I T cells, thus implicating a STAT5-independent cytokine or costimulatory pathway in this process. Even though IL-2 and IL-15 were dispensable for CD8(+) T cell proliferation, systemic infusion of IL-2 nevertheless promoted the expansion of OT-I T cells in vivo. Thus, IL-2 and IL-15 signals are not essential for CD8(+) T cell proliferation or differentiation, but IL-2 can promote supraphysiological expansion when supplied exogenously. These findings challenge current models that place CD8(+) T cell proliferation under the control of STAT5-dependent cytokines and suggest new approaches to the therapeutic manipulation of T cell numbers in vivo.

Chromatin acetylation and remodeling at the Cis promoter during STAT5-induced transcription

The signal transducer and activator of transcription STAT5 plays a major role in cytokine-induced expression of genes involved in cell proliferation and survival. Although several STAT5 partners have been identified, the molecular events taking place at the promoter level upon STAT5 recruitment have not yet been characterized in great detail. Using chromatin immunoprecipitation and accessibility assays, we characterized histone acetylation and chromatin remodeling events occurring during transcriptional activation of the endogenous murine Cis gene, a STAT5 target gene, in response to IL-3. We found that STAT5 binding in vivo is associated with low histone H3 and H4 acetylation levels in the proximity of the STAT5 binding sites. STAT5 recruitment also results in chromatin reorganization over that promoter region. These events (STAT5 binding, histone acetylation and chromatin remodeling) are not sufficient for transcriptional activation, which requires a non-histone protein deacetylase. These data reveal novel implications of STAT5 in chromatin regulation during cytokine-induced transcription, thus contributing to a better understanding of the mechanism of transcriptional activation by STAT5.

Evaluation of the host transcriptional response to human cytomegalovirus infection

Gene expression data from human cytomegalovirus (HCMV)-infected cells were analyzed using DNA-Chip Analyzer (dChip) followed by singular value decomposition (SVD) and compared with a previous analysis of the same data that employed GeneChip software and a fold change filtering approach. dChip and SVD analysis revealed two clusters of coexpressed human genes responding differently to HCMV infection: one containing some genes identified previously, and another that was largely unique to this analysis. Annotating these genes, we identified several functional categories important to host cell responses to HCMV infection. These categories included genes involved in transcriptional regulation, oncogenesis, and cell cycle regulation, which were more prevalent in cluster 1, and genes involved in immune system regulation, signal transduction, and cell adhesion, which were more prevalent in cluster 2. Within these categories, we found genes involved in the host response to HCMV infection (mainly in cluster 1), as well as genes targeted by HCMV’s immune evasion strategies (mainly in cluster 2). As the second group of genes identified by the dChip and SVD approach was statistically and biologically significant, our results point out the advantages of using different methods to analyze gene expression data.

Aging of innate immunity: functional comparisons of NK/LAK cells obtained from bulk cultures of young and aged mouse spleen cells in high concentrations of interleukin-2

The technique of bulk cultivation of aged mouse spleen cells in high concentration of IL-2 was employed to obtain NK/LAK cells in sufficient number and enrichment for studies on the effects of aging on their functions. The yield and enrichment were equivalent to that of young mouse spleen cells. The aged and young mouse NK/LAK cells were equivalent also in their functional competence to proliferate, kill target cells and produce IFNgamma; i.e. they did not display age-associated defects typical of freshly-isolated NK/LAK cells. In two respects, however, the NK/LAK cells derived from aged mouse spleen were altered: (a) in the efficiency of nuclear translocation of transcription factors STAT 5A and 5B, and (b) in the deficiency in production of mRNA transcripts representing several chemokines. We recommend caution in the use of bulk cultivation in IL-2 to obtain NK/LAK cells for studies on aging. However, it does appear from this study that aging may severely affect chemokine production, at least in the case of NK/LAK cells.

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Amplifying cancer vaccine responses by modifying pathogenic gene programs in tumor cells

Immunosuppressive factors, such as vascular endothelial growth factor, transforming growth factor-beta, prostaglandin E2, interleukin (IL)-10, and IL-6, are made frequently by cancer cells. These factors, along with others, can inhibit the development and function of tumor-reactive effector T cells and the clinical results of cancer vaccines. Production of these factors by tumor cells is associated with disease progression and may represent an active immune surveillance escape mechanism. However, a number of factors appear to be made directly in response to signaling molecules, such as RAS, AKT, and signal transducer and activator of transcription 3, which are activated as a result of genetic events that occur during oncogenesis. Methods to overcome the negative effects of immunosuppressive factors, which are "hard wired" into gene programs of cancer cells, might then improve the results of cancer vaccines. For example, specific blocking antibodies, which recognize such factors, or kinase inhibitors, which block the signaling pathways that lead to their production, could potentially be used as vaccine adjuvants. The effects of immunosuppressive factors may also be "turned off" by cytokines with tumor suppressor properties. The enhanced clinical and immunological effects of melanoma vaccines observed after the administration of high doses of interferon-alpha2b provide a "proof of principle" in human patients, that agents which counter the gene programs of cancer cells, causing them to intrinsically resist tumor-reactive T cells, may improve significantly the efficacy of cancer vaccines.

Enhancement of MEK/ERK signaling promotes glucocorticoid resistance in CD4+ T cells

Glucocorticoids have potent immunosuppressive properties, but their effects are often modulated by the conditions prevailing in the local immune milieu. In this study we determined whether the action of glucocorticoids is influenced by the degree of signaling during T cell activation. We found that dexamethasone (Dex) effectively suppressed T cell receptor-induced (TCR-induced) proliferation of naive CD4+ T cells, through a mechanism involving downregulation of c-Fos expression and inhibition of activator protein-1 (AP-1), nuclear factor of activated T cells (NF-AT), and NF-kappaB transcriptional activity. However, enhancement of TCR signaling by CD28- or IL-2-mediated costimulation abrogated the suppressive effect of Dex on c-Fos expression and AP-1 function and restored cellular proliferation. The amount of signaling through the MAPK pathway was critical in determining the effect of Dex on T cell activation. In particular, costimulatory signaling via MAPK kinase (MEK) and extracellular signal-regulated kinase (ERK) was essential for the development of T cell resistance to Dex. Selective blockade of MEK/ERK signal transduction abolished the costimulation-induced resistance. In contrast, transmission of IL-2 signals via STAT5 and CD28 signals via NF-kappaB remained inhibited by Dex. These results imply that the immune system, by regulating the degree of local costimulation through MEK/ERK, can modify the effect of glucocorticoids on T cells. Moreover, these findings suggest that MAPK inhibitors may offer a therapeutic solution for glucocorticoid resistance.

A mechanism of induction of the mouse zinc-fingers and homeoboxes 1 (ZHX1) gene expression by interleukin-2

The effect of IL-2 on the expression of the mouse zinc-fingers and homeoboxes 1 (ZHX1) gene was investigated in a mouse cytotoxic T cell line, CTLL-2 cells. IL-2 specifically induced the expression of ZHX1 mRNA. The level of ZHX1 mRNA was decreased in the absence of IL-2. These alterations were in parallel with the status of cell proliferation. The signaling pathways involved in the induction were examined. AG-490, wortmannin, and LY294002 blocked the induction by IL-2. Nuclear run-on assays and a mRNA stability analysis revealed that the half-life of ZHX1 mRNA but not the transcription rate of the gene was increased by IL-2. Thus, we conclude that IL-2 induces the expression of the mouse ZHX1 gene in CTLL-2 cells, that both Janus kinase 3/signal transducer and activator of transcription 5 and phosphoinositide 3-kinase pathways are involved in the induction, and that the increased mRNA stability results in the induction.

Interleukin 2 plays a central role in Th2 differentiation

Differentiation of naïve CD4 T cells into T helper (Th) 2 cells requires signaling through the T cell receptor and an appropriate cytokine environment. IL-4 is critical for such Th2 differentiation. We show that IL-2 plays a central role in this process. The effect of IL-2 on Th2 generation does not depend on its cell growth or survival effects. Stat5a(-/-) cells show diminished differentiation to IL-4 production, and forced expression of a constitutively active form of Stat5a replaces the need for IL-2. In vivo IL-2 neutralization inhibits IL-4 production in two models. Studies of restriction enzyme accessibility and binding of Stat5 to chromatin indicate that IL-2 mediates its effect by stabilizing the accessibility of the Il4 gene. Thus, IL-2 plays a critical role in the polarization of naive CD4 T cells to the Th2 phenotype.

Novel peptidomimetic inhibitors of signal transducer and activator of transcription 3 dimerization and biological activity

The critical role of signal transducer and activator of transcription 3 (Stat3) in the growth and survival of human tumor cells identifies it as a promising target for cancer drug discovery. We previously identified a Stat3 SH2 domain-binding phosphopeptide, PY*LKTK, and its tripeptide derivatives, PY*L and AY*L (where Y* represents phosphotyrosine), which inhibit Stat3 biochemical activity and biological function. Here, we report novel peptidomimetic compounds based on PY*L (or AY*L) with substitution of the Y-1 residue by benzyl, pyridyl, or pyrazinyl derivatives that are selective and greater than 5-fold more potent in disrupting Stat3 activity in vitro than lead tripeptides. The biological activities of these derivatives mirror that originally observed for peptides. In this context, the representative peptidomimetic ISS 610 with 4-cyanobenzoate substitution inhibits constitutive Stat3 activity in Src-transformed mouse fibroblasts and human breast and lung carcinoma cells. This effect is not evident with the non-phosphorylated counterpart, ISS 610NP, consistent with interaction of peptidomimetics with the SH2 domain of Stat3. Moreover, ISS 610 induces cell growth inhibition and apoptosis of Src-transformed fibroblasts that contain persistently active Stat3. We present the first report of a peptidomimetic approach to design of small-molecule inhibitors of Stat3 that are also among the first examples of disruptors of transcription factor dimerization with the potential for novel cancer therapy.

Defective interleukin-2-dependent STAT5 signalling in CD8 T lymphocytes from HIV-positive patients: restoration by antiretroviral therapy

BACKGROUND

CD8 T lymphocytes are critical in the control of HIV replication and disease progression. Our previous studies demonstrated that CD8 T cells from chronically infected patients with high virus load proliferated poorly in response to interleukin-2 (IL-2), a cytokine critical in CD8 T cell growth and differentiation, even though relatively high levels of IL-2 receptor (IL-2R) were expressed. This suggested that signal transduction defects in response to IL-2 might be involved. The STAT5 transcription factor is important in IL-2-dependent biological responses and it is known that there are defects in unstimulated CD3 and CD4 cells in HIV-infected patients.

OBJECTIVE

To investigate whether the induction of STAT5 by IL-2 is altered in the CD8 T cells from HIV-positive individuals and the impact of highly active antiretroviral therapy (HAART) on its status.

METHODS

CD8 T lymphocytes were purified from the peripheral blood mononuclear cells of HIV-positive patients before and after HAART. Ex vivo IL-2-induced STAT5 activation was evaluated by immunoblotting and electrophoretic mobility shift assays.

RESULTS

CD8 T cells from a subset of untreated HIV-positive patients were unable to activate STAT5a and STAT5b proteins functionally in response to IL-2. This defect was not a result of alterations in IL-2R expression but correlated with an impaired activation of the upstream kinase Jak-3, known to mediate STAT5 activation. Overall, HAART restored Jak/STAT signalling in such patients.

CONCLUSIONS

These results further uncover a potential mechanism by which CD8 T cell function is impaired in HIV-infected patients.

Loss of tolerance and autoimmunity affecting multiple organs in STAT5A/5B-deficient mice

STAT5 has previously been reported to be dispensable for the maintenance of tolerance in vivo. However, in examining hemopoiesis in mice lacking both isoforms of STAT5, STAT5A, and STAT5B, we noted that a subset of these mice demonstrated dramatic alterations in several bone marrow progenitor populations concomitant with lymphocytic infiltration of the bone marrow. In addition, cellular infiltration affecting the colon, liver, and kidney was observed in these mice. Survival analysis revealed that STAT5A/5B(-/-) mice exhibited early death. The increased mortality and the pathology affecting multiple organs observed in these mice were abrogated on the recombination-activating gene 1(-/-) background. In light of the similarities between STAT5A/5B-deficient mice and mice unable to signal through the IL-2R, we hypothesized that the tolerizing role of STAT5A/5B was triggered via activation of the IL-2R. In agreement with this, we found that IL-2Rbeta chain-deficient mice exhibited similar hemopoietic abnormalities. Because IL-2 signaling is thought to contribute to tolerance through maintenance of a CD4(+)CD25(+) regulatory T cell population, we examined these cells and observed a numerical reduction in STAT5A/5B(-/-) mice along with a higher rate of apoptosis. These data provide strong evidence for a requirement for STAT5 in the maintenance of tolerance in vivo.

Geldanamycin, a heat shock protein 90-binding agent, disrupts Stat5 activation in IL-2-stimulated cells

The 90-kDa heat shock protein (Hsp90) is the most abundant molecular chaperone in eukaryotic cells. Hsp90 plays a critical role in regulating signal transduction pathways that control cell proliferation since its chaperone function is restricted to a subset of proteins including some signal molecules. Improper function of these proteins can be induced by an anti-tumor agent geldanamycin (GA) which is the specific inhibitor of Hsp90. In this study, it was demonstrated that GA interferes with IL-2-stimulated proliferation of murine CTLL-2 cells. As to the signaling mechanisms underlying this inhibitory effect, we discovered GA disrupts the IL-2-stimulated activation and phosphorylation of the transcription factor Stat5, indicating the proper function of Hsp90 is indispensable for Stat5 activation. This conclusion is validated by the observation that Hsp90 interacts with Stat5 in the immunoprecipitation assay and GA interrupts their interaction. Furthermore, by constructing deletion mutants, we identified the c-terminal half of Stat5 coiled-coil region is responsible for binding with Hsp90.