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

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

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

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

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

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

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

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

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

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

Global analysis of IL-2 target genes: identification of chromosomal clusters of expressed genes

T lymphocytes play a central role in controlling adaptive immune responses. IL-2 critically regulates both T cell growth and death and is involved in maintaining peripheral tolerance, but the molecules involved in these and other IL-2 actions are only partially known. We now provide a comprehensive compendium of the genes expressed in T cells and of those regulated by IL-2 based on a combination of DNA microarrays and serial analysis of gene expression (SAGE). The newly identified IL-2 target genes include many genes previously linked to apoptosis in other cellular systems that may contribute to IL-2-dependent survival functions. We also studied the mRNA expression of known regulators of signaling pathways for their induction in response to IL-2 in order to identify potential novel positive and/or negative feedback regulators of IL-2 signaling. We show that IL-2 regulates only a limited number of these genes. These include suppressors of cytokine signaling (SOCS) 1, SOCS2, dual-specificity phosphatase (DUSP) 5, DUSP6 and non-receptor type phosphatase-7 (PTPN7). Additionally, we provide evidence that many genes expressed in T cells locate in chromosomal clusters, and that select IL-2-regulated genes are located in at least two clusters, one at 5q31, a known cytokine gene cluster, and the other at 6p21.3, a region that contains genes encoding the tumor necrosis factor (TNF) superfamily members TNF, LT-alpha and LT-beta.

Calcium-dependent activation of interleukin-21 gene expression in T cells

Interleukin (IL)-21 is a gamma(c)-dependent cytokine produced by activated T cells with important actions for T, B, and NK cells. The IL-21 gene is adjacent to the IL-2 gene, and like IL-2, IL-21 is strongly induced at the transcriptional level after T cell activation. Interestingly, however, in contrast to the IL-2 gene, a calcium ionophore alone was sufficient to induce IL-21 gene expression in preactivated T cells. Two DNase I hypersensitivity sites were found in the IL-21 gene, corresponding to nucleotide sequences that are conserved in humans and mice. One site is located at the IL-21 promoter region and conferred T cell receptor-mediated IL-21 gene transcription. TCR-induced IL-21 gene expression was inhibited by cyclosporin A and FK506. Correspondingly, the IL-21 5'-regulatory region contains three NFAT binding sites, and induction of IL-21 promoter activity was impaired when these sites were mutated or following treatment with cyclosporin A. Thus, our studies reveal that in contrast to IL-2, a calcium signal alone is sufficient to mediate induction of the IL-21 in preactivated T lymphocytes and that this induction appears to result from specific NFAT binding.

Cytokines and immunodeficiency diseases: critical roles of the gamma(c)-dependent cytokines interleukins 2, 4, 7, 9, 15, and 21, and their signaling pathways

In this review, we discuss the role of cytokines and their signaling pathways in immunodeficiency. We focus primarily on severe combined immunodeficiency (SCID) diseases as the most severe forms of primary immunodeficiencies, reviewing the different genetic causes of these diseases. We focus in particular on the range of forms of SCID that result from defects in cytokine-signaling pathways. The most common form of SCID, X-linked SCID, results from mutations in the common cytokine receptor gamma-chain, which is shared by the receptors for interleukin (IL)-2, IL-4, IL-7, IL-9, IL-15, and IL-21, underscoring that X-linked SCID is indeed a disease of defective cytokine signaling. We also review the signaling pathways used by these cytokines and the phenotypes in humans and mice with defects in the cytokines or signaling pathways. We also briefly discuss other cytokines, such as interferon-gamma and IL-12, where mutations in the ligand or receptor or signaling components also cause clinical disease in humans.

Defective B cell responses in the absence of SH2D1A

More than half of patients with X-linked lympho-proliferative disease, which is caused by a defect in the intracellular adapter protein SH2D1A, suffer from an extreme susceptibility to Epstein-Barr virus. One-third of these patients, however, develop dysgammaglobulenemia without an episode of severe mononucleosis. Here we show that in SH2D1A(-/-) mice, both primary and secondary responses of all Ig subclasses are severely impaired in response to specific antigens. Because germinal centers were absent in SH2D1A(-/-) mice upon primary immunization, and because SH2D1A was detectable in wt germinal center B cells, we examined whether SH2D1A(-/-) B cell functions were impaired. Using the adoptive cotransfer of B lymphocytes from hapten-primed SH2D1A(-/-) mice with CD4(+) T cells from primed wt mice into irradiated wt mice provided evidence that signal transduction events controlled by SH2D1A are essential for B cell activities resulting in antigen specific IgG production. Defects in naive SH2D1A(-/-) B cells became evident upon cotransfer with non-primed wt CD4(+) cells into Rag2(-/-) recipients. Thus, both defective T and B cells exist in the absence of SH2D1A, which may explain the progressive dysgammaglobulinemia in a subset of X-linked lympho-proliferative disease patients without involvement of Epstein-Barr virus.

Synergy of IL-21 and IL-15 in regulating CD8+ T cell expansion and function

Interleukin (IL)-21 is the most recently recognized of the cytokines that share the common cytokine receptor γ chain (γ_c), which is mutated in humans with X-linked severe combined immunodeficiency. We now report that IL-21 synergistically acts with IL-15 to potently promote the proliferation of both memory (CD44^high) and naive (CD44^low) phenotype CD8⁺ T cells and augment interferon-γ production in vitro. IL-21 also cooperated, albeit more weakly, with IL-7, but not with IL-2. Correspondingly, the expansion and cytotoxicity of CD8⁺ T cells were impaired in IL-21R^−/− mice. Moreover, in vivo administration of IL-21 in combination with IL-15 boosted antigen-specific CD8⁺ T cell numbers and resulted in a cooperative effect on tumor regression, with apparent cures of large, established B16 melanomas. Thus, our studies reveal that IL-21 potently regulates CD8⁺ T cell expansion and effector function, primarily in a synergistic context with IL-15.

Interleukin-7 deficiency in rheumatoid arthritis

Interleukin-7 (IL-7) is a stromal factor that is crucial for the development of T lymphocytes in humans and mice, and also B lymphocytes in mice. IL-7 can act as a T cell growth factor as well as a critical anti-apoptotic survival factor. The essential non-redundant role of this cytokine for T cell development in vivo is indicated by the phenotype of murine knockout models as well as by humans with a T^-B⁺NK⁺ form of severe combined immunodeficiency (SCID) resulting from mutations in IL-7 receptor α chain. IL-7 deficiency has now been found in patients with rheumatoid arthritis, a finding that relates not only to the T-lymphocyte status in this disease but also to the ability of patients with rheumatoid arthritis to recover from therapy-induced lymphopenia.

Regulation of B Cell Differentiation and Plasma Cell Generation by IL-21, a Novel Inducer of Blimp-1 and Bcl-6

IL-21 is a type I cytokine whose receptor is expressed on T, B, and NK cells. Within the B cell lineage, IL-21 regulates IgG1 production and cooperates with IL-4 for the production of multiple Ab classes in vivo. Using IL-21-transgenic mice and hydrodynamics-based gene delivery of IL-21 plasmid DNA into wild-type mice as well as in vitro studies, we demonstrate that although IL-21 induces death of resting B cells, it promotes differentiation of B cells into postswitch and plasma cells. Thus, IL-21 differentially influences B cell fate depending on the signaling context, explaining how IL-21 can be proapoptotic for B cells in vitro yet critical for Ag-specific Ig production in vivo. Moreover, we demonstrate that IL-21 unexpectedly induces expression of both Blimp-1 and Bcl-6, indicating mechanisms as to how IL-21 can serve as a complex regulator of B cell maturation and terminal differentiation. Finally, BXSB-Yaa mice, which develop a systemic lupus erythematosus-like disease, have greatly elevated IL-21, suggesting a role for IL-21 in the development of autoimmune disease.

GA binding protein regulates interleukin 7 receptor alpha-chain gene expression in T cells

The interleukin 7 receptor alpha-chain (IL-7Ralpha) is essential for T cell development in both humans and mice and for B cell development in mice. Whereas the transcription factor PU.1 regulates IL-7Ralpha expression in mouse pro-B cells via a GGAA motif, we demonstrate here that GA binding protein (GABP) bound to this site and was essential in the regulation of IL-7Ralpha expression in T cells, where PU.1 is not expressed. Moreover, IL-7Ralpha expression was diminished substantially in thymocytes but was normal on B220(+) fetal liver cells from mouse embryos with diminished expression of GABPalpha. Thus, GABP is essential for the regulation of IL-7Ralpha expression in T cells, and the differential regulation of IL-7Ralpha in distinct lymphoid lineages is achieved at least in part by differential recruitment of factors to the same GGAA motif.

Viral FLIP impairs survival of activated T cells and generation of CD8+ T cell memory

Viral FLIPs (vFLIPs) interfere with apoptosis signaling by death-domain-containing receptors in the TNFR superfamily (death receptors). In this study, we show that T cell-specific transgenic expression of MC159-vFLIP from the human Molluscum contagiosum virus blocks CD95-induced apoptosis in thymocytes and peripheral T cells, but also impairs postactivation survival of in vitro activated primary T cells despite normal early activation parameters. MC159 vFLIP impairs T cell development to a lesser extent than does Fas-associated death domain protein deficiency or another viral FLIP, E8. In the periphery, vFLIP expression leads to a specific deficit of functional memory CD8(+) T cells. After immunization with a protein Ag, Ag-specific CD8(+) T cells initially proliferate, but quickly disappear and fail to produce Ag-specific memory CD8(+) T cells. Viral FLIP transgenic mice exhibit impaired CD8(+) T cell responses to lymphocytic choriomeningitis virus and Trypanosoma cruzi infections, and a specific defect in CD8(+) T cell recall responses to influenza virus was seen. These results suggest that vFLIP expression in T cells blocks signals necessary for the sustained survival of CD8(+) T cells and the generation of CD8(+) T cell memory. Through this mechanism, vFLIP proteins expressed by T cell tropic viruses may impair the CD8(+) T cell immune responses directed against them.

A role for thymic stromal lymphopoietin in CD4(+) T cell development

Thymic stromal lymphopoietin (TSLP) signals via a receptor comprising the interleukin (IL)-7 receptor α chain and a distinctive subunit, TSLP receptor (TSLPR), which is most related to the common cytokine receptor γ chain, γ_c. We have generated TSLPR knockout (KO) mice and found that although these mice had normal lymphocyte numbers, γ_c/TSLPR double KO mice had a greater lymphoid defect than γ_c KO mice. This indicates that TSLP contributes to lymphoid development and accounts for some of the residual lymphoid development in γ_c KO mice and presumably in patients with X-linked severe combined immunodeficiency. Injection of TSLP into γ_c KO mice induced the expansion of T and B cells. Moreover, sublethally irradiated TSLPR KO mice showed weaker recovery of lymphocyte populations than wild-type (WT) littermates, even when neutralizing anti–IL-7 antibodies were injected. Interestingly, TSLP preferentially stimulated the proliferation and survival of CD4⁺ single positive thymocytes and peripheral T cells in vitro. Additionally, CD4⁺ T cells from TSLPR KO mice expanded less efficiently than WT CD4⁺ T cells in irradiated hosts, and TSLP preferentially expanded CD4⁺ T cells both in vitro and in vivo. Thus, as compared with other known cytokines, TSLP is distinctive in exhibiting a lineage preference for the expansion and survival of CD4⁺ T cells.

A distal region in the interferon-gamma gene is a site of epigenetic remodeling and transcriptional regulation by interleukin-2

Interferon-gamma (IFN-gamma) is a multifunctional cytokine that defines the development of Th1 cells and is critical for host defense against intracellular pathogens. IL-2 is another key immunoregulatory cytokine that is involved in T helper differentiation and is known to induce IFN-gamma expression in natural killer (NK) and T cells. Despite concerted efforts to identify the one or more transcriptional control mechanisms by which IL-2 induces IFN-gamma mRNA expression, no such genomic regulatory regions have been described. We have identified a DNase I hypersensitivity site approximately 3.5-4.0 kb upstream of the transcriptional start site. Using chromatin immunoprecipitation assays we found constitutive histone H3 acetylation in this distal region in primary human NK cells, which is enhanced by IL-2 treatment. This distal region is also preferentially acetylated on histones H3 and H4 in primary Th1 cells as compared with Th2 cells. Within this distal region we found a Stat5-like motif, and in vitro DNA binding assays as well as in vivo chromosomal immunoprecipitation assays showed IL-2-induced binding of both Stat5a and Stat5b to this distal element in the IFNG gene. We examined the function of this Stat5-binding motif by transfecting human peripheral blood mononuclear cells with -3.6 kb of IFNG-luciferase constructs and found that phorbol 12-myristate 13-acetate/ionomycin-induced transcription was augmented by IL-2 treatment. The effect of IL-2 was lost when the Stat5 motif was disrupted. These data led us to conclude that this distal region serves as both a target of chromatin remodeling in the IFNG locus as well as an IL-2-induced transcriptional enhancer that binds Stat5 proteins.

In vivo antitumor activity of interleukin 21 mediated by natural killer cells

Immunotherapy with high-dose interleukin (IL) 2 has been shown to successfully treat tumors in animal models and cause dramatic tumor regressions in some patients with metastatic melanoma, renal cell carcinoma, and non-Hodgkin's lymphoma. However, toxicity associated with IL-2 administration has compromised its widespread use in the clinic. IL-21 is a more recently discovered cytokine produced by activated CD4(+) T cells that shares significant sequence homology to IL-2, IL-4, and IL-15. Because IL-21 and IL-2 and their receptors share significant sequence similarities and both cytokines can stimulate T and natural killer (NK) cells, we sought to study whether IL-21, like IL-2, exhibits antitumor effects in vivo. In this study, we treated established s.c. tumor in mice by systemically administering plasmid DNA encoding murine IL-21 using a hydrodynamics-based gene delivery technique. Administration of IL-21 plasmid DNA resulted in high levels of circulating IL-21 in vivo. Treatment of tumor-bearing mice with IL-21 plasmid DNA significantly inhibited the growth of B16 melanoma and MCA205 fibrosarcoma in a dose-dependent manner without significant toxicity and increased the survival rate, compared with mice treated with control plasmid DNA. In vivo depletion of either CD4(+) or CD8(+) T cells did not affect IL-21-mediated antitumor activity. However, depletion of NK cells completely abolished IL-21-induced tumor inhibition. Consistent with this, the antitumor activity of IL-21 seemed to be mediated through enhanced cytolytic activity of NK cells. Our study suggests that IL-21 has significant antitumor activity and may have therapeutic potentials as an antitumor agent in the clinic.

Immune deficiencies due to defects in cytokine signaling

Severe combined immunodeficiency (SCID) represents a syndrome comprising the most severe forms of inherited immunodeficiencies. Defects in cytokine signaling pathways can result in impaired development of lymphoid cells and/or defective functioning of these cells, and most cases of SCID result from defective signaling through the common cytokine receptor g chain (g(c)) or associated molecules and signaling pathways. Studies of these patients and the analysis of gene-targeted mice provide insight into the underlying signaling defects in inherited immunodeficiencies. The identification of the genetic defects in humans with SCID provides the basis for future therapies for these patients. More subtle deficiencies in cytokine signaling have also been found as causes of other forms of immunodeficiency, and the knowledge learned could lead to novel approaches to antimicrobial therapy.

Stat5 synergizes with T cell receptor/antigen stimulation in the development of lymphoblastic lymphoma

Signal transducer and activator of transcription (STAT) proteins are latent transcription factors that mediate a wide range of actions induced by cytokines, interferons, and growth factors. We now report the development of thymic T cell lymphoblastic lymphomas in transgenic mice in which Stat5a or Stat5b is overexpressed within the lymphoid compartment. The rate of lymphoma induction was markedly enhanced by immunization or by the introduction of TCR transgenes. Remarkably, the Stat5 transgene potently induced development of CD8+ T cells, even in mice expressing a class II-restricted TCR transgene, with resulting CD8+ T cell lymphomas. These data demonstrate the oncogenic potential of dysregulated expression of a STAT protein that is not constitutively activated, and that TCR stimulation can contribute to this process.

Analysis of gamma c-family cytokine target genes. Identification of dual-specificity phosphatase 5 (DUSP5) as a regulator of mitogen-activated protein kinase activity in interleukin-2 signaling

Interleukin (IL)-2, IL-4, IL-7, IL-9, IL-15, and IL-21 form a family of cytokines based on their sharing the common cytokine receptor gamma chain, gamma(c), which is mutated in X-linked severe combined immunodeficiency (SCID). As a step toward further elucidating the mechanism of action of these cytokines in T-cell biology, we compared the gene expression profiles of IL-2, IL-4, IL-7, and IL-15 in T cells using cDNA microarrays. IL-2, IL-7, and IL-15 each induced a highly similar set of genes, whereas IL-4 induced distinct genes correlating with differential STAT protein activation by this cytokine. One gene induced by IL-2, IL-7, and IL-15 but not IL-4 was dual-specificity phosphatase 5 (DUSP5). In IL-2-dependent CTLL-2 cells, we show that IL-2-induced ERK-1/2 activity was inhibited by wild type DUSP5 but markedly increased by an inactive form of DUSP5, suggesting a negative feedback role for DUSP5 in IL-2 signaling. Our findings provide insights into the shared versus distinctive actions by different members of the gamma(c) family of cytokines. Moreover, we have identified a DUSP5-dependent negative regulatory pathway for MAPK activity in T cells.

A role for Stat5 in CD8+ T cell homeostasis

Cytokine signals are known to contribute to CD8+ memory T cell homeostasis, but an exact understanding of the mechanism(s) has remained elusive. We have now investigated the role of Stat5 proteins in this process. Whereas Stat5a and Stat5b KO mice have decreased numbers of CD8+ T cells, Stat5-transgenic mice have an increased number of these cells. Stat5b-transgenic mice exhibit increased Ag-induced cell death of CD4+ T cells and augmented proliferation and Bcl-2 expression in CD8+ T cells, providing a basis for this finding. Moreover, CD8+ memory T cells are substantially affected by Stat5 levels. These findings identify Stat5 proteins as critical signaling mediators used by cytokines to regulate CD8+ T cell homeostasis.

A critical role for IL-21 in regulating immunoglobulin production

The cytokine interleukin-21 (IL-21) is closely related to IL-2 and IL-15, and their receptors all share the common cytokine receptor gamma chain, gammac, which is mutated in humans with X-linked severe combined immunodeficiency disease (XSCID). We demonstrate that, although mice deficient in the receptor for IL-21 (IL-21R) have normal lymphoid development, after immunization, these animals have higher production of the immunoglobulin IgE, but lower IgG1, than wild-type animals. Mice lacking both IL-4 and IL-21R exhibited a significantly more pronounced phenotype, with dysgammaglobulinemia, characterized primarily by a severely impaired IgG response. Thus, IL-21 has a significant influence on the regulation of B cell function in vivo and cooperates with IL-4. This suggests that these gammac-dependent cytokines may be those whose inactivation is primarily responsible for the B cell defect in humans with XSCID.

Serine phosphorylation of Stat5 proteins in lymphocytes stimulated with IL-2

Tyrosine phosphorylation regulates cytokine-induced dimerization of STAT proteins. Serine phosphorylation has also been found to occur in a number of STAT proteins, including Stat1, Sat3, Stat4, Stat5a, Stat5b and Stat6, and was shown to be important for maximal transcriptional activation mediated by Stat1, Stat3 and Stat4, but not for Stat5a or Stat5b. As these latter proteins were studied in transiently transfected COS-7 cells stimulated with prolactin, we sought to further investigate the significance of their serine phosphorylation in a more physiologically based system in response to IL-2. Both Stat5a and Stat5b were rapidly phosphorylated on serine in response to IL-2 and the phosphorylation site in Stat5a was mapped to Ser780, which is not conserved in Stat5b. In vitro studies with reporter constructs, and experiments in which wild-type and mutant Stat5a retroviruses were used to transduce Stat5a-deficient splenocytes revealed that the serine mutant constructs were not diminished in their ability to mediate IL-2 signaling and if anything exhibited augmented proliferative capability. Thus, in contrast to the apparent importance of serine phosphorylation for transcriptional activation by Stat1, Stat3 and Stat4 in response to IFN, IL-6 and IL-12 respectively, serine phosphorylation of Stat5a does not enhance Stat5a-mediated signaling in response to IL-2.