Restoration of lymphoid populations in a murine model of X-linked severe combined immunodeficiency by a gene-therapy approach

X-linked severe combined immunodeficiency (XSCID) is a life-threatening syndrome in which both cellular and humoral immunity are profoundly compromised. This disease results from mutations in the IL2RG gene, which encodes the common cytokine receptor gamma chain, gamma(c). Previously, we generated gamma(c)-deficient mice as a murine model of XSCID. We have now used lethally irradiated gamma(c)-deficient mice to evaluate a gene therapeutic approach for treatment of this disease. Transfer of the human gamma(c) gene to repopulating hematopoietic stem cells using an ecotropic retrovirus resulted in an increase in T cells, B cells, natural killer (NK) cells, and intestinal intraepithelial lymphocytes, as well as normalization of the CD4:CD8 T-cell ratio and of serum Ig levels. In addition, the restored cells could proliferate in response to interleukin-2 (IL-2). Thus, our results provide added support that gene therapy is a feasible therapeutic strategy for XSCID. Moreover, because we used a vector directing expression of human gamma(c) to correct a defect in gamma(c)-deficient mice, these data also indicate that human gamma(c) can cooperate with the distinctive cytokine receptor chains such as IL-2Rbeta and IL-7Ralpha to mediate responses to murine cytokines in vivo.

CIS associates with the interleukin-2 receptor beta chain and inhibits interleukin-2-dependent signaling

CIS is a cytokine-induced SH2-containing protein that was originally cloned as an interleukin (IL)-3-inducible gene. CIS is known to associate with the IL-3 receptor beta chain and erythropoietin receptor and to inhibit signaling mediated by IL-3 and erythropoietin. We now demonstrate that CIS also interacts with the IL-2 receptor beta chain (IL-2Rbeta). This interaction requires the A region of IL-2Rbeta (residues 313-382), which also mediates the association of IL-2Rbeta with Lck and Jak3. Correspondingly, CIS inhibits functions associated with both of these kinases: Lck-mediated phosphorylation of IL-2Rbeta and IL-2-mediated activation of Stat5. Thus, we demonstrate that CIS can negatively control at least two independent IL-2 signaling pathways. Although a functional SH2 binding domain of CIS was not required for its interaction with IL-2Rbeta in vitro, its phosphotyrosine binding capability was essential for the inhibitory action of CIS. On this basis, we have generated a mutant form of CIS protein with an altered SH2 domain that acts as a dominant negative and should prove useful in further understanding CIS action.

The significance of tetramerization in promoter recruitment by Stat5

Stat5a and Stat5b are rapidly activated by a wide range of cytokines and growth factors, including interleukin-2 (IL-2). We have previously shown that these signal transducers and activators of transcription (STAT proteins) are key regulatory proteins that bind to two tandem gamma interferon-activated site (GAS) motifs within an IL-2 response element (positive regulatory region III [PRRIII]) in the human IL-2Ralpha promoter. In this study, we demonstrate cooperative binding of Stat5 to PRRIII and explore the molecular basis underlying this cooperativity. We demonstrate that formation of a tetrameric Stat5 complex is essential for the IL-2-inducible activation of PRRIII. Stable tetramer formation of Stat5 is mediated through protein-protein interactions involving a tryptophan residue conserved in all STATs and a lysine residue in the Stat5 N-terminal domain (N domain). The functional importance of tetramer formation is shown by the decreased levels of transcriptional activation associated with mutations in these residues. Moreover, the requirement for STAT protein-protein interactions for gene activation from a promoter with tandemly linked GAS motifs can be relieved by strengthening the avidity of protein-DNA interactions for the individual binding sites. Taken together, these studies demonstrate that a dimeric but tetramerization-deficient Stat5 protein can activate only a subset of target sites. For functional activity on a wider range of potential recognition sites, N-domain-mediated oligomerization is essential.

Role of Bcl-2 in alpha beta T cell development in mice deficient in the common cytokine receptor gamma-chain: the requirement for Bcl-2 differs depending on the TCR/MHC affinity

Mice lacking the common cytokine receptor gamma-chain (gamma c) exhibit severely compromised T cell development, with diminished Bcl-2 expression in mature (CD4+ or CD8+) thymocytes and peripheral T cells. Enforced expression of Bcl-2 in these mice partially rescued alpha beta T cell development but not gamma delta T cell development. Transgenic expression of the OVA-specific DO11.10 (DO10) TCR also could modestly increase thymocyte numbers, and T cells expressing the transgenic TCR (KJ1-26+ T cells) were found in the periphery. Interestingly, the presence of KJ1-26+ T cells was dependent on the MHC background and was seen in the moderate affinity H-2d/d background but not in the higher affinity H-2d/b background in gamma c-deficient mice. In contrast, KJ1-26+ T cells exist in the periphery in both the H-2d/d and H-2d/b backgrounds in DO10 transgenic gamma c wild-type mice. These results suggest that the importance of gamma c-dependent signals for T cell development differs depending on the affinity of TCR for MHC. Moreover, enforced expression of Bcl-2 had a much greater effect on the development of gamma c-deficient T cells expressing the DO10 TCR in the high affinity H-2d/b background than in the H-2d/d background, suggesting that gamma c-dependent Bcl-2 expression influences T cell development in a TCR/MHC-dependent manner.

Functional association of Nmi with Stat5 and Stat1 in IL-2- and IFNgamma-mediated signaling

Using the coiled-coil region of Stat5b as the bait in a yeast two-hybrid screen, we identified the association of Nmi, a protein of unknown function previously reported as an N-Myc interactor. We further show that Nmi interacts with all STATs except Stat2. We evaluated two cytokine systems, IL-2 and IFNgamma, and demonstrate that Nmi augments STAT-mediated transcription in response to these cytokines. Interestingly, Nmi lacks an intrinsic transcriptional activation domain; instead, Nmi enhances the association of CBP/p300 coactivator proteins with Stat1 and Stat5, and together with CBP/p300 can augment IL-2- and IFNgamma-dependent transcription. Therefore, our data not only reveal that Nmi can potentiate STAT-dependent transcription, but also suggest that it can augment coactivator protein recruitment to at least some members of a group of sequence-specific transcription factors.

Interleukin 7 receptor control of T cell receptor gamma gene rearrangement: role of receptor-associated chains and locus accessibility

VDJ recombination of T cell receptor and immunoglobulin loci occurs in immature lymphoid cells. Although the molecular mechanisms of DNA cleavage and ligation have become more clear, it is not understood what controls which target loci undergo rearrangement. In interleukin 7 receptor (IL-7R)alpha-/- murine thymocytes, it has been shown that rearrangement of the T cell receptor (TCR)-gamma locus is virtually abrogated, whereas other rearranging loci are less severely affected. By examining different strains of mice with targeted mutations, we now observe that the signaling pathway leading from IL-7Ralpha to rearrangement of the TCR-gamma locus requires the gammac receptor chain and the gammac-associated Janus kinase Jak3. Production of sterile transcripts from the TCR-gamma locus, a process that generally precedes rearrangement of a locus, was greatly repressed in IL-7Ralpha-/- thymocytes. The repressed transcription was not due to a lack in transcription factors since the three transcription factors known to regulate this locus were readily detected in IL-7Ralpha-/- thymocytes. Instead, the TCR-gamma locus was shown to be methylated in IL-7Ralpha-/- thymocytes. Treatment of IL-7Ralpha-/- precursor T cells with the specific histone deacetylase inhibitor trichostatin A released the block of TCR-gamma gene rearrangement. This data supports the model that IL-7R promotes TCR-gamma gene rearrangement by regulating accessibility of the locus via demethylation and histone acetylation of the locus.

Stat5b is essential for natural killer cell-mediated proliferation and cytolytic activity

We have analyzed the immune system in Stat5-deficient mice. Although Stat5a-/- splenocytes have a partial defect in anti-CD3-induced proliferation that can be overcome by high dose interleukin (IL)-2, we now demonstrate that defective proliferation in Stat5b-/- splenocytes cannot be corrected by this treatment. Interestingly, this finding may be at least partially explained by diminished expression of the IL-2 receptor beta chain (IL-2Rbeta), which is a component of the receptors for both IL-2 and IL-15, although other defects may also exist. Similar to the defect in proliferation in activated splenocytes, freshly isolated splenocytes from Stat5b-/- mice exhibited greatly diminished proliferation in response to IL-2 and IL-15. This results from both a decrease in the number and responsiveness of natural killer (NK) cells. Corresponding to the diminished proliferation, basal as well as IL-2- and IL-15-mediated boosting of NK cytolytic activity was also greatly diminished. These data indicate an essential nonredundant role for Stat5b for potent NK cell-mediated proliferation and cytolytic activity.

Defective IL7R expression in T(-)B(+)NK(+) severe combined immunodeficiency

Severe combined immunodeficiency (SCID) is caused by multiple genetic defects. The most common form of SCID, X-linked SCID (XSCID), results from mutations in IL2RG (ref. 4), which encodes the common cytokine receptor gamma chain (gamma(c)) that is shared by the IL-2, IL-4, IL-7, IL-9 and IL-15 receptors. In XSCID and SCID resulting from mutations in JAK3, which encodes a Janus family tyrosine kinase that couples to gamma(c) and is required for gamma(c)-dependent signalling, T- and natural killer (NK)-cells are decreased but B-cell numbers are normal (T(-)B(+)NK(-)SCID). Some SCID patients lack T cells but retain NK cells. Given diminished T-cell development in Il7- or Il7r-deficient mice and that Il/7r-deficient mice have NK cells, we hypothesized that T(-)B(+)NK(+) SCID might result from defective IL-7 signalling, although apparent differences in the role of the IL-7/IL-7R pathway in humans and mice in T-cell and B-cell development have been suggested. We now demonstrate that defective IL7R expression causes T(-)B(+)NK(+) SCID, indicating that the T-cell, but not the NK-cell, defect in XSCID results from inactivation of IL-7Ralpha signalling.

Functional cooperation of the interleukin-2 receptor beta chain and Jak1 in phosphatidylinositol 3-kinase recruitment and phosphorylation

Phosphatidylinositol 3-kinase (PI 3-K) plays an important role in signaling via a wide range of receptors such as those for antigen, growth factors, and a number of cytokines, including interleukin-2 (IL-2). PI 3-K has been implicated in both IL-2-induced proliferation and prevention of apoptosis. A number of potential mechanisms for the recruitment of PI 3-K to the IL-2 receptor have been proposed. We now have found that tyrosine residues in the IL-2 receptor beta chain (IL-2Rbeta) are unexpectedly not required for the recruitment of the p85 component of PI 3-K. Instead, we find that Jak1, which associates with membrane-proximal regions of the IL-2Rbeta cytoplasmic domain, is essential for efficient IL-2Rbeta-p85 interaction, although some IL-2Rbeta-p85 association can be seen in the absence of Jak1. We also found that Jak1 interacts with p85 in the absence of IL-2Rbeta and that IL-2Rbeta and Jak1 cooperate for the efficient recruitment and tyrosine phosphorylation of p85. This is the first report of a PI 3-K-Jak1 interaction, and it implicates Jak1 in an essential IL-2 signaling pathway distinct from the activation of STAT proteins.

Jaks and STATs: biological implications

Cytokines and interferons are molecules that play central roles in the regulation of a wide array of cellular functions in the lympho-hematopoietic system. These factors stimulate proliferation, differentiation, and survival signals, as well as specialized functions in host resistance to pathogens. Although cytokines are known to activate multiple signaling pathways that together mediate these important functions, one of these pathways, the Jak-STAT pathway, is the focus of this chapter. This pathway is triggered by both cytokines and interferons, and it very rapidly allows the transduction of an extracellular signal into the nucleus. The pathway uses a novel mechanism in which cytosolic latent transcription factors, known as signal transducers and activators of transcription (STATs), are tyrosine phosphorylated by Janus family tyrosine kinases (Jaks), allowing STAT protein dimerization and nuclear translocation. STATs then can modulate the expression of target genes. The basic biology of this system, including the range of known Jaks and STATs, is discussed, as are the defects in animals and humans lacking some of these signaling molecules.

Human and simian T-cell leukemia viruses type 2 (HTLV-2 and STLV-2(pan-p)) transform T cells independently of Jak/STAT activation

Human T-lymphotropic virus type 1 (HTLV-1) and HTLV-2 differ in pathogenicity in vivo. HTLV-1 causes leukemia and neurologic and inflammatory diseases, whereas HTLV-2 is less clearly associated with human disease. Both retroviruses transform human T cells in vitro, and transformation by HTLV-1 was found to be associated with the constitutive activation of the Jak/STAT pathway. To assess whether HTLV-2 transformation may also result in constitutive activation of the Jak/STAT pathway, six interleukin-2-independent, HTLV-2-transformed T-cell lines were analyzed for the presence of activated Jak and STAT proteins by electrophoretic mobility shift assay. In addition, the phosphorylation status of Jak and STAT proteins was assessed directly by immunoprecipitation and immunoblotting with an antiphosphotyrosine antibody. Jak/STAT proteins were not found to be constitutively activated in any of the T-cell lines infected by the type 2 human and nonhuman primate viruses, suggesting that HTLV-2 and the cognate virus simian T-lymphotropic virus type 2 from Pan paniscus transform T cells in vitro by mechanisms at least partially different from those used by HTLV-1.

Delineation of the regions of interleukin-2 (IL-2) receptor beta chain important for association of Jak1 and Jak3. Jak1-independent functional recruitment of Jak3 to Il-2Rbeta

Interleukin-2 (IL-2) induces heterodimerization of the IL-2 receptor beta (IL-2Rbeta) and gammac chains of its receptor and activates the Janus family tyrosine kinases, Jak1 and Jak3. Whereas Jak1 associates with IL-2Rbeta, Jak3 associates primarily with gammac but also with IL-2Rbeta. We analyzed four IL-2Rbeta mutations that diminish IL-2-induced proliferation and found that each also decreased IL-2-induced signal transducer and activator of transcription (STAT) activation. For this reason, and because the mutations were in the IL-2Rbeta membrane-proximal region, we investigated and found that each mutation diminished IL-2Rbeta association with both Jak1 and Jak3. This suggested that these Jaks might interact with the same region of IL-2Rbeta; however, certain IL-2Rbeta internal deletions and C-terminal truncations differentially affected the association of Jak1 and Jak3. Interestingly, just as Jak1-IL-2Rbeta association is Jak3-independent and functionally important, we show that Jak3-IL-2Rbeta association is Jak1-independent and implicate this association as being important for IL-2-induced Stat5 activation. Moreover, Jak1 and Jak3 could associate only in the presence of IL-2Rbeta, suggesting that these kinases can simultaneously bind to IL-2Rbeta. Thus, our data not only demonstrate that somewhat more distal as well as membrane-proximal cytoplasmic regions of a type I cytokine receptor are important for Jak kinase association but also suggest that two IL-2Rbeta-Jak kinase interactions are important for IL-2 signaling.

Infection of mice lacking the common cytokine receptor gamma-chain (gamma(c)) reveals an unexpected role for CD4+ T lymphocytes in early IFN-gamma-dependent resistance to Toxoplasma gondii

Mice lacking the common cytokine receptor gamma-chain (gamma(c)) gene exhibit defective development of NK cells and CD8+ T cells and greatly diminished production of IFN-gamma. Because resistance of SCID mice to Toxoplasma gondii requires IL-12-dependent IFN-gamma production by NK cells, we expected that gamma(c)-deficient mice would succumb rapidly to the parasite. Surprisingly, however, most gamma(c)-deficient mice survived the acute phase of T. gondii infection. As in wild-type mice, this resistance required IL-12 and IFN-gamma; nevertheless, whereas wild-type mice depleted of CD4+ T cells survived, anti-CD4+ treated gamma(c)-deficient mice displayed diminished production of IFN-gamma and all succumbed to acute infection. These data not only reveal a role for CD4+ T lymphocytes in IFN-gamma-dependent host defense but also establish SCID and gamma(c)-deficient mice as powerful complementary tools for assessing the function of NK vs CD4+ T cells in immunopathophysiologic responses.

Proliferation of adult T cell leukemia/lymphoma cells is associated with the constitutive activation of JAK/STAT proteins

Human T cell leukemia/lymphotropic virus type I (HTLV-I) induces adult T cell leukemia/lymphoma (ATLL). The mechanism of HTLV-I oncogenesis in T cells remains partly elusive. In vitro, HTLV-I induces ligand-independent transformation of human CD4+ T cells, an event that correlates with acquisition of constitutive phosphorylation of Janus kinases (JAK) and signal transducers and activators of transcription (STAT) proteins. However, it is unclear whether the in vitro model of HTLV-I transformation has relevance to viral leukemogenesis in vivo. Here we tested the status of JAK/STAT phosphorylation and DNA-binding activity of STAT proteins in cell extracts of uncultured leukemic cells from 12 patients with ATLL by either DNA-binding assays, using DNA oligonucleotides specific for STAT-1 and STAT-3, STAT-5 and STAT-6 or, more directly, by immunoprecipitation and immunoblotting with anti-phosphotyrosine antibody for JAK and STAT proteins. Leukemic cells from 8 of 12 patients studied displayed constitutive DNA-binding activity of one or more STAT proteins, and the constitutive activation of the JAK/STAT pathway was found to persist over time in the 2 patients followed longitudinally. Furthermore, an association between JAK3 and STAT-1, STAT-3, and STAT-5 activation and cell-cycle progression was demonstrated by both propidium iodide staining and bromodeoxyuridine incorporation in cells of four patients tested. These results imply that JAK/STAT activation is associated with replication of leukemic cells and that therapeutic approaches aimed at JAK/STAT inhibition may be considered to halt neoplastic growth.

Signaling from the IL-2 receptor to the nucleus

Interleukin-2 has pleiotropic actions on the immune system and plays a vital role in the modulation of immune responses. Our current understanding of IL-2 signaling has resulted from in vitro studies that have identified the signaling pathways activated by IL-2, including the Jak-STAT pathways, and from in vivo studies that have analyzed mice in which IL-2, each chain of the receptor, as well a number of signaling molecules have been individually targeted by homologous recombination. Moreover, mutations in IL-2Ralpha, gamma(c) and Jak3 have been found in patients with severe combined immunodeficiency. In addition, with the discovery that two components of the receptor, IL-2Rbeta and gamma(c), are shared by other cytokine receptors, we have an enhanced appreciation of the contributions of these molecules towards cytokine specificity, pleiotropy and redundancy.

Cytokine signaling: cytokine-inducible signaling inhibitors

Seven members of a family of cytokine-inducible proteins have been identified, a number of which have been shown to act as negative regulators of cytokine action. All of these proteins contain a central Src homology 2 domain, as well as conserved motifs in their carboxy-terminal regions.

Impaired peripheral deletion of activated T cells in mice lacking the common cytokine receptor gamma-chain: defective Fas ligand expression in gamma-chain-deficient mice

Mice lacking the common cytokine receptor gamma-chain (gamma c) exhibit severely compromised lymphoid development. T cells that develop in these mice exhibit decreased Bcl-2 levels and accelerated apoptosis; nevertheless, these mice exhibit an age-dependent accumulation of activated CD4+ T cells. To investigate the basis for this accumulation, we analyzed both thymic and peripheral deletion in these mice. Gamma c-deficient mice had increased numbers of V beta 11+ T cells, consistent with a possible defect in Mtv-9-induced deletion; however, the deletion of V beta 5+ T cells by Mtv-9 and that of V beta 6+ T cells by Mls-1a were normal. Moreover, antigenic peptide could induce wild-type levels of deletion of CD4+ CD8+ thymocytes in TCR-transgenic gamma c-deficient mice. In contrast to this relatively normal deletion of thymocytes, bacterial superantigen (staphylococcal enterotoxin B)-induced elimination of peripheral T cells was greatly impaired, suggesting that defective peripheral deletion contributed to the accumulation of activated T cells. Interestingly, despite CD4+ T cell accumulation, these cells exhibited increased sensitivity to Fas-mediated death in vitro. Correction of the defect in Bcl-2 expression by mating to Bcl-2 transgenic mice augmented the splenic T cell accumulation and substantially enhanced the survival of gamma c-deficient T cells; however, these cells still exhibited significant Fas-mediated death, indicating that the increased Fas-mediated death was not simply due to diminished Bcl-2 expression. Moreover, these T cells exhibit decreased expression of Fas ligand, suggesting that Fas-bearing cells cannot be effectively eliminated in vivo in gamma c-deficient mice. Thus, gamma c-dependent signals play a role in peripheral T cell deletion, presumably by inducing Fas ligand on activated T cells.

Impaired peripheral deletion of activated T cells in mice lacking the common cytokine receptor gamma-chain: defective Fas ligand expression in gamma-chain-deficient mice

Mice lacking the common cytokine receptor gamma-chain (gamma c) exhibit severely compromised lymphoid development. T cells that develop in these mice exhibit decreased Bcl-2 levels and accelerated apoptosis; nevertheless, these mice exhibit an age-dependent accumulation of activated CD4+ T cells. To investigate the basis for this accumulation, we analyzed both thymic and peripheral deletion in these mice. Gamma c-deficient mice had increased numbers of V beta 11+ T cells, consistent with a possible defect in Mtv-9-induced deletion; however, the deletion of V beta 5+ T cells by Mtv-9 and that of V beta 6+ T cells by Mls-1a were normal. Moreover, antigenic peptide could induce wild-type levels of deletion of CD4+ CD8+ thymocytes in TCR-transgenic gamma c-deficient mice. In contrast to this relatively normal deletion of thymocytes, bacterial superantigen (staphylococcal enterotoxin B)-induced elimination of peripheral T cells was greatly impaired, suggesting that defective peripheral deletion contributed to the accumulation of activated T cells. Interestingly, despite CD4+ T cell accumulation, these cells exhibited increased sensitivity to Fas-mediated death in vitro. Correction of the defect in Bcl-2 expression by mating to Bcl-2 transgenic mice augmented the splenic T cell accumulation and substantially enhanced the survival of gamma c-deficient T cells; however, these cells still exhibited significant Fas-mediated death, indicating that the increased Fas-mediated death was not simply due to diminished Bcl-2 expression. Moreover, these T cells exhibit decreased expression of Fas ligand, suggesting that Fas-bearing cells cannot be effectively eliminated in vivo in gamma c-deficient mice. Thus, gamma c-dependent signals play a role in peripheral T cell deletion, presumably by inducing Fas ligand on activated T cells.

An indirect effect of Stat5a in IL-2-induced proliferation: a critical role for Stat5a in IL-2-mediated IL-2 receptor alpha chain induction

Stat5a was identified as a prolactin-induced transcription factor but also is activated by other cytokines, including interleukin-2 (IL-2) and IL-7. We have now analyzed the immune system of Stat5a-deficient mice. Stat5a-/- splenocytes exhibited defective IL-2-induced expression of the IL-2 receptor alpha chain (IL-2R alpha), a protein that together with IL-2R beta and the common cytokine receptor gamma chain (gamma(c)) mediates high-affinity IL-2 binding. Correspondingly, Stat5a-/- splenocytes exhibited markedly decreased proliferation to IL-2, although maximal proliferation was still achieved at IL-2 concentrations high enough to titrate intermediate-affinity IL-2R beta/gamma(c) receptors. Thus, defective Stat5a expression results in diminished proliferation by an indirect mechanism, resulting from defective receptor expression. Correspondingly, we show that Stat5a is essential for maximal responsiveness to antigenic stimuli in vivo, underscoring the physiological importance of IL-2-induced IL-2R alpha expression.

Functional cleavage of the common cytokine receptor gamma chain (gammac) by calpain

The small subunit of calpain, a calcium-dependent cysteine protease, was found to interact with the cytoplasmic domain of the common cytokine receptor gamma chain (gammac) in a yeast two-hybrid interaction trap assay. This interaction was functional as demonstrated by the ability of calpain to cleave in vitro-translated wild-type gammac, but not gammac containing a mutation in the PEST (proline, glutamate, serine, and threonine) sequence in its cytoplasmic domain, as well as by the ability of endogenous calpain to mediate cleavage of gammac in a calcium-dependent fashion. In T cell receptor-stimulated murine thymocytes, calpain inhibitors decreased cleavage of gammac. Moreover, in single positive CD4(+) thymocytes, not only did a calpain inhibitor augment CD3-induced proliferation, but antibodies to gammac blocked this effect. Finally, treatment of cells with ionomycin could inhibit interleukin 2-induced STAT protein activation, but this inhibition could be reversed by calpain inhibitors. Together, these data suggest that calpain-mediated cleavage of gammac represents a mechanism by which gammac-dependent signaling can be controlled.

STAT5A-deficient mice demonstrate a defect in granulocyte-macrophage colony-stimulating factor-induced proliferation and gene expression

Responses of cells to cytokines typically involve the activation of a family of latent DNA binding proteins, referred to as signal transducers and activators of transcription (STAT) proteins, which are critical for the expression of early response genes. Of the seven known STAT proteins, STAT5 (originally called mammary gland factor) has been shown to be activated by several cytokines, such as granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3), and IL-5, which are known to play important roles in growth and differentiation of hematopoietic precursors. In this report we have used mice that are deficient in STAT5A (one of two homologues of STAT5) to study the role of STAT5A in GM-CSF stimulation of cells. When bone marrow-derived macrophages were generated by differentiation with macrophage-CSF (M-CSF), exposure of cells from wild-type mice to GM-CSF resulted in a typical pattern of assembly of DNA binding proteins specific for the gamma activation sequence (GAS) element within the beta-casein promoter. However, in cells from the STAT5A null mouse one of the shifted bands was absent. Immunoblotting analysis in the null mice showed that lack of STAT5A protein resulted in no alteration in activation of STAT5B by tyrosine phosphorylation. Proliferation experiments revealed that, when exposed to increasing concentrations of GM-CSF, cells derived from the null mice grew considerably more slowly than cells derived from the wild-type mice. Moreover, expression of GM-CSF-dependent genes, CIS and A1, was markedly inhibited in cells derived from null mice as compared with those of wild-type mice. The decreased expression observed with A1, a bcl-2 like gene, may account in part for the suppression of growth in cells from the null mice. These data suggest that the presence of STAT5A during the GM-CSF-induced assembly of STAT5 dimers is critical for the formation of competent transcription factors that are required for both gene expression and cell proliferation.

Distinct roles for signals relayed through the common cytokine receptor gamma chain and interleukin 7 receptor alpha chain in natural T cell development

The commitment, differentiation, and expansion of mainstream alpha/beta T cells during ontogeny depend on the highly controlled interplay of signals relayed by cytokines through their receptors on progenitor cells. The role of cytokines in the development of natural killer (NK)1(+) natural T cells is less clearly understood. In an approach to define the role of cytokines in the commitment, differentiation, and expansion of NK1(+) T cells, their development was studied in common cytokine receptor gamma chain (gammac) and interleukin (IL)-7 receptor alpha (IL-7Ralpha)-deficient mice. These mutations block mainstream alpha/beta T cell ontogeny at an early prethymocyte stage. Natural T cells do not develop in gammac-deficient mice; they are absent in the thymus and peripheral lymphoid organs such as the liver and the spleen. In contrast, NK1(+) T cells develop in IL-7Ralpha-deficient mice in the thymus, and they are present in the liver and in the spleen. However, the absolute number of NK1(+) T cells in the thymus of IL-7Ralpha-deficient mice is reduced to approximately 10%, compared to natural T cell number in the wild-type thymus. Additional data revealed that NK1(+) T cell ontogeny is not impaired in IL-2- or IL-4-deficient mice, suggesting that neither IL-2, IL-4, nor IL-7 are required for their development. From these data, we conclude that commitment and/or differentiation to the NK1(+) natural T cell lineage requires signal transduction through the gammac, and once committed, their expansion requires signals relayed through the IL-7Ralpha.

The immediate-early gene product Egr-1 regulates the human interleukin-2 receptor beta-chain promoter through noncanonical Egr and Sp1 binding sites

The interleukin-2 IL-2 receptor beta-chain (IL-2Rbeta) is an essential component of the receptors for IL-2 and IL-15. Although IL-2Rbeta is constitutively expressed by lymphocytes, its expression can be further induced by a number of stimuli, including phorbol 12-myristate 13-acetate (PMA). We have now characterized factors that bind to an enhancer region located between nucleotides -170 and -139 of the human IL-2Rbeta promoter. Both Sp1 and Sp3 bound to the 5' portion of this region, whereas a PMA-inducible factor (PIF) mainly bound to its 3' portion and bound to the Sp binding motifs as well. In Jurkat T cells, induction of PIF DNA binding activity was rapidly induced, required de novo protein synthesis, and was sustained at a high level for at least 23 h. Interestingly, PIF was constitutively activated in human T-cell leukemia virus type 1-transformed MT-2 cells. In this paper, we demonstrate that PIF is Egr-1 based on its recognition by anti-Egr-1 antisera in gel mobility shift assays, even though the IL-2Rbeta DNA binding motif differed substantially from the canonical Egr-1 binding site. In addition, Egr-1 bound to the Sp binding site. In Jurkat cells, both sites were required for maximal IL-2Rbeta promoter activity, and in HeLaS3 cells, transfection of Egr-1 could drive activity of a reporter construct containing both sites. Moreover, Sp1 and Egr-1 could form a complex with kinetics that correlated with the production of Egr-1 in Jurkat cells upon PMA stimulation. Thus, Sp1 and Egr-1 physically and functionally cooperate to mediate maximal IL-2Rbeta promoter activity.

Interleukin-2 (IL-2)-mediated induction of the IL-2 receptor alpha chain gene. Critical role of two functionally redundant tyrosine residues in the IL-2 receptor beta chain cytoplasmic domain and suggestion that these residues mediate more than Stat5 activation

The interleukin-2 receptor alpha chain (IL-2Ralpha) is potently induced by antigens, mitogens, and certain cytokines that include IL-2 itself. This induction leads to the formation of high affinity IL-2 receptors when IL-2Ralpha is co-expressed with the beta (IL-2Rbeta) and gamma (gammac) chains of this receptor. We investigated the signaling pathways mediating IL-2-induced IL-2Ralpha mRNA expression using 32D myeloid progenitor cells stably transfected with either wild type IL-2Rbeta or mutants of IL-2Rbeta containing tyrosine to phenylalanine substitutions. Of the six cytoplasmic tyrosines in IL-2Rbeta, we have found that only the two tyrosines that mediate Stat5 activation (Tyr-392 and Tyr-510) contribute to IL-2-induced IL-2Ralpha gene expression and that either tyrosine alone is sufficient for this process. Interestingly, the IL-7 receptor contains a tyrosine (Tyr-429)-based sequence resembling the motifs encompassing Tyr-392 and Tyr-510 of IL-2Rbeta. Further paralleling the IL-2 system, IL-7 could activate Stat5 and drive expression of IL-2Ralpha mRNA in 32D cells transfected with the human IL-7R. However, IL-3 could not induce IL-2Ralpha mRNA in 32D cells, despite its ability to activate Stat5 via the endogenous IL-3 receptor. Moreover, the combination of IL-3 and IL-2 could not "rescue" IL-2Ralpha mRNA expression in cells containing an IL-2Rbeta mutant with phenylalanine substitutions at Tyr-392 and Tyr-510. These data suggest that Tyr-392 and Tyr-510 couple to an additional signaling pathway beyond STAT protein activation in IL-2-mediated induction of the IL-2Ralpha gene.