Heterodimerization of the IL-2 receptor beta- and gamma-chain cytoplasmic domains is required for signalling

Jeg-3 human choriocarcinoma-induced immunosuppression: downregulation of interleukin-2, interleukin-2 receptor alpha-chain, and its Jak/Stat signaling pathway

PROBLEM

The mechanisms of the immunosuppressive and immunosuppression-inducing capacities of Jeg-3 human choriocarcinoma cell line supernatants (HCSs) are not yet completely understood. The influence on interleukin (IL)-2, IL-4 and interferon (IFN)-gamma production; IL-2 receptor (IL-2R) alpha-, beta-, and gamma-chain; and the signaling pathway molecules Janus kinase (Jak)1, Jak3, signal transducers and activators of transcription (Stat)1, Stat3, and Stat5 should be investigated.

METHOD OF STUDY

For assessment of IL production, whole peripheral venous blood from healthy donors was stimulated with phorbol-myristate-acetate and ionomycine. Secretion of ILs was blocked with monensine. Intracellular ILs were analyzed by flow cytometry. For IL-2R and signaling pathway molecule analysis, peripheral blood lymphocytes were stimulated with phytohemagglutinin (PHA). IL-2R chains were measured by flow cytometry, and Jaks/Stats by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and Western blot.

RESULTS

Phorbol-myristate-acetate and ionomycine strongly increase the percent-age of IL-2+ cells; an additional 50% HCSs significantly suppresses the percentage to, or below the level of unstimulated cells. IFN-gamma production is strongly decreased by HCSs in some cases, but not in others. PHA stimulates IL-2R alpha-, beta-, and gamma-chain expression and their signaling pathway molecules Jak1, Jak3, Stat1, Stat3, and Stat5. 50% HCS downregulates the alpha-chain and slightly upregulates the beta-chain. Jak1, Jak3, Stat1, Stat3, and Stat5 expression is suppressed approximately to, or below the level of unstimulated cells.

CONCLUSIONS

HCS forcefully blocks the production of IL-2; the IL-2R alpha-chain; and Jak1, Jak3, Stat1, Stat3, and Stat5 expression. The observed phenomena might be caused by downregulation of an IL-2R regulation gene, and might play a key role in the expansion of choriocarcinoma, and possibly in the survival of the fetal allograft.

Functional dissection of the cytoplasmic subregions of the IL-2 receptor betac chain in primary lymphocyte populations

The interleukin 2 (IL-2) receptor betac chain (IL-2Rbetac) is known to regulate the development and function of distinct lymphocyte populations. Thus far, the functions of the IL-2Rbetac cytoplasmic subregions have been studied extensively by using cultured cell lines; however, this approach has limitations with respect to their functions in distinct primary lymphocyte populations. In the present study, we generated mice each expressing a mutant form of an IL-2Rbetac transgene, lacking the cytoplasmic A- or H-region, on an IL-2Rbetac null background. We show that lack of the H-region, which mediates activation of the Stat5/Stat3 transcription factors, selectively affects the development of natural killer cells and T cells bearing the gamma delta T cell receptor. This region is also required for the IL-2-induced proliferation of T cells in vitro, by upregulating IL-2Ralpha expression. In contrast, the A-region, which mediates activation of the Src family protein tyrosine kinase (PTK) members, contributes to downregulation of the T cell proliferation function. The IL-2Rbetac null mutant mice develop severe autoimmune symptoms; these are all suppressed following the expression of either of the mutants, suggesting that neither the Stats nor the Src PTK members are required. Thus, our present approach offers new insights into the functions of these cytoplasmic subregions of the IL-2Rbetac chain.

IL-2Rα on One Cell Can Present IL-2 to IL-2Rβ/γc on Another Cell to Augment IL-2 Signaling

IL-2Rα augments IL-2 signaling. Although this is generally believed to occur only when the three known components of IL-2R are associated within a single cell membrane, we demonstrate here an intercellular interaction. Cocultivation of cells individually expressing chimerae incorporating the extracellular domain of IL-2Rα alone with cells expressing chimerae of IL-2Rβ alone permitted IL-2 dose-dependent oligomerization of the chimerae. Likewise, native IL-2Rα-bearing cells augmented the IL-2 proliferative response of ex vivo large granular lymphocytic leukemia cells expressing IL-2Rβ/γc but lacking IL-2Rα. In both cases, the response was inhibitable by an Ab to IL-2Rα. Intercellular augmentation of cytokine effects, acting in trans, has important implications for biology and medicine.

Biology of the interleukin-2 receptor

Studies of the biology of the IL-2 receptor have played a major part in establishing several of the fundamental principles that govern our current understanding of immunology. Chief among these is the contribution made by lymphokines to regulation of the interactions among vast numbers of lymphocytes, comprising a number of functionally distinct lineages. These soluble mediators likely act locally, within the context of the microanatomic organization of the primary and secondary lymphoid organs, where, in combination with signals generated by direct membrane-membrane interactions, a wide spectrum of cell fate decisions is influenced. The properties of IL-2 as a T-cell growth factor spawned the view that IL-2 worked in vivo to promote clonal T-cell expansion during immune responses. Over time, this singular view has suffered from increasing appreciation that the biologic effects of IL-2R signals are much more complex than simply mediating T-cell growth: depending on the set of conditions, IL-2R signals may also promote cell survival, effector function, and apoptosis. These sometimes contradictory effects underscore the fact that a diversity of intracellular signaling pathways are potentially activated by IL-2R. Furthermore, cell fate decisions are based on the integration of multiple signals received by a lymphocyte from the environment; IL-2R signals can thus be regarded as one input to this integration process. In part because IL-2 was first identified as a T-cell growth factor, the major focus of investigation in IL-R2 signaling has been on the mechanism of mitogenic effects in cultured cell lines. Three critical events have been identified in the generation of the IL-2R signal for cell cycle progression, including heterodimerization of the cytoplasmic domains of the IL-2R beta and gamma(c) chains, activation of the tyrosine kinase Jak3, and phosphorylation of tyrosine residues on the IL-2R beta chain. These proximal events led to the creation of an activated receptor complex, to which various cytoplasmic signaling molecules are recruited and become substrates for regulatory enzymes (especially tyrosine kinases) that are associated with the receptor. One intriguing outcome of the IL-2R signaling studies performed in cell lines is the apparent functional redundancy of the A and H regions of IL-2R beta, and their corresponding downstream pathways, with respect to the proliferative response. Why should the receptor complex induce cell proliferation through more than one mechanism or pathway? One possibility is that this redundancy is an unusual property of cultured cell lines and that primary lymphocytes require signals from both the A and the H regions of IL-2R beta for optimal proliferative responses in vivo. An alternative possibility is that the A and H regions of IL-2R beta are only redundant with respect to proliferation and that each region plays a unique and essential role in regulating other aspects of lymphocyte physiology. As examples, the A or H region could prove to be important for regulating the sensitivity of lymphocytes to AICD or for promoting the development of NK cells. These issues may be resolved by reconstituting IL-2R beta-/-mice with A-and H-deleted forms of the receptor chain and analyzing the effect on lymphocyte development and function in vivo. In addition to the redundant nature of the A and H regions, there remains a large number of biochemical activities mediated by the IL-2R for which no clear physiological role has been identified. Therefore, the circumstances are ripe for discovering new connections between molecular signaling events activated by the IL-2R and the regulation of immune physiology. Translating biochemical studies of Il-2R function into an understanding of how these signals regulate the immune system has been facilitated by the identification of natural mutations in IL-2R components in humans with immunodeficiency and by the generation of mice with targeted mutations in these gen

Molecular Mapping with Functional Antibodies Localizes Critical Sites on the Human IL Receptor Common γ (γc) Chain

The IL receptor common γ (γc) chain is required for the formation of high affinity cytokine receptor complexes for IL-2, IL-4, IL-7, IL-9, and IL-15, and for signals regulating cell survival, growth, and differentiation. Our current understanding of how γc chain associates with multiple ligands and receptor subunits is drawn largely from its structural homology to the human growth hormone (hGH) receptor and known structure of the hGH/hGH receptor complex. These receptors share distinct features in their extracellular portions and are believed to function by a mechanism of ligand-induced association of receptor subunits. Here, we report the first directed mutational analysis of the human γc chain by alanine scanning conducted across seven regions likely to contain residues required for intermolecular contact. Functionally distinct, neutralizing anti-γc mAbs were employed to define critical residues. One particular mAb, CP.B8, unique in its ability to inhibit IL-2-, IL-4-, IL-7-, and IL-15-induced proliferation and high affinity cytokine binding of normal T cells as an intact mAb and as a Fab fragment, localized critical residues to four noncontinuous stretches, namely residues in loops AB and EF of domain 1, in the interdomain segment, and in loop FG of domain 2. Notably, these residues form a contiguous patch on the γc chain surface in a three-dimensional structural model. These results provide functional evidence for the location of contact points on γc chain required for its association with multiple ligands.

Recent advances in the understanding of interleukin-2 signal transduction

Interleukin-2 is one of the critical cytokines that control the proliferation and differentiation of cells of the immune system. The present article briefly reviews the current and recently established knowledge on the intracellular signaling events that convert the initial interaction of IL-2 with its receptor into pathways leading to the various biological functions. A first step in IL-2 signaling is the activation of several protein tyrosine kinases that phosphorylate a large array of intracellular substrates including the receptor complex. Phosphorylated tyrosine residues within the receptor then serve as docking sites for multimolecular signaling complexes that initiate three major pathways: the Jak-STAT pathway controlling gene transcription, the Ras-MAPK pathway leading to cell proliferation and gene transcription as well, and the PI3-kinase pathway involved in antiapoptotic signaling and organization of the cytoskeleton. Finally, other recently identified and presumably important tyrosine kinase substrates, whose significance is not yet fully understood, are described.

Endocytosis of the common cytokine receptor gammac chain. Identification of sequences involved in internalization and degradation

The common cytokine receptor gammac, shared by interleukin 2, 4, 7, 9, and 15 receptors, has a major role in lymphocyte proliferation and differentiation, leading, when mutated, to a genetic disease, X-linked severe combined immunodeficiency. In this study, we report that gammac is internalized and degraded in lymphoid cells. To identify gammac regions involved in sorting along the endocytic pathway, we have studied a chimeric protein composed of the extracellular part of interleukin 2-receptor alpha and transmembrane and intracellular part of gammac, alpha gamma gammawt. When transfected in Jurkat T cells, alpha gamma gammawt is as efficiently internalized and degraded as gammac, demonstrating that the transmembrane and cytosolic tail of gammac carry sequences involved in this process. To identify these motifs, we have analyzed the trafficking of chimeric proteins with serial truncations in their cytosolic tail. Internalization studies showed that the cytosolic tail of gammac contains three regions located between cytosolic amino acids 1-35, 35-40, and 40-65 involved in gammac endocytosis. Successive deletions of these motifs result in reduced endocytosis. One region containing the 5 cytosolic amino acids 36-40 is essential to direct gammac to the degradation pathway. These sorting sequences, by participating in the fine tuning of cell surface gammac expression, might somewhat regulate the cell responsiveness to interleukins whose receptors share this component.

Cytokine receptors: structures and signal transduction

The characteristic features of cytokines are functional pleiotropy and redundancy. Each cytokine is produced by a variety of cell types and acts on a wide range of target cells and tissues. Many cytokines have overlapping biological activities in the same cells. It was originally thought that each cytokine has a specific receptor and a unique signal transduction system. However, extensive studies on cytokines and their receptors revealed that many cytokines share receptor subunits and signal transduction system, and that biological functions of a single cytokine can vary depending on the status of the cells. Therefore, it is important to know the structure and function of cytokine receptors to understand the pleiotropy and redundancy as well as specificity of cytokines. Among signal transduction pathways, recently identified Jak/STAT pathway, which connects activation of the receptor complexes and transcription of various genes directly, would give us further insights in the mechanisms of cytokine action.

Transcriptional control of cytokine genes

With antigen stimulation, cytokine genes are coordinately expressed in T cells. However, the optimal signals or helper T cell subset specificity for each cytokine gene expression differ. Mechanisms probably exist that transmit extracellular signals into the nucleus, thereby coordinately turning on and off transcriptional machinery in a signal-specific or cell type-specific manner. We compare here the regulation of cytokine genes and discuss possible mechanisms of coordinated and differential regulation of cytokine genes.

Involvement of phosphoinositide 3-kinase and Rac in membrane ruffling induced by IL-2 in T cells

IL-2 is known to play a critical role in regulating T lymphocyte proliferation. We show here that IL-2 also provokes an instantaneous and sustained membrane ruffling in cloned human or murine T cells as well as in lectin-activated peripheral blood lymphocytes. In the IL-2-induced lamellipodia, tubulin is depolymerized whereas actin is strongly polymerized, forming caps. IL-2-induced membrane ruffling is protein kinase C (PKC) independent, as judged by the absence of effects of bisindolylmaleimide, an efficient inhibitor of all PKC isoforms. The formation of lamellipodia by IL-2 is blocked by wortmannin and LY294002, two inhibitors of phosphoinositide 3-kinase (PI3-kinase). Moreover, expression in murine T cells of an inactive form of P13-kinase inhibits IL-2-induced membrane ruffling, whereas expression of a constitutively active p110 increases the basal membrane ruffling. Rac is also involved in IL-2-induced membrane ruffling since an inactive form of Rac (N17rac) blocks the IL-2-induced lamellipodia, whereas the constitutive form of Rac (Val12rac) can also lead to membrane ruffling. In the signaling cascade, Rac is downstream of PI3-kinase since constitutive membrane ruffling in Val12rac cells is insensitive to wortmannin. Thus, through a signaling cascade involving PI3-kinase and Rac, IL-2 can induce profound alterations of the T cell cytoskeleton, a phenomenon which might be of importance for T cell physiology.

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.

Activation of lymphocytes by HHV-6 antigen in normal children and adults

We investigated the immune reaction of lymphocytes in response to human herpes virus-6 (HHV-6) in normal children and adults. Cell proliferation was assayed by measurement of the incorporation of 5-bromo-2'-deoxyuridine (BrdU) by enzyme-linked immunosorbent assay (ELISA), and changes in expression of interleukin-2 receptors (IL-2Rs) (alpha-, beta3-, and gamma-chain) were assayed by flow cytometry. Incorporation of BrdU and expression of IL-2Rs (alpha-, beta-, and gamma-chain) in CD4+, CD8+, and CD45RO+ lymphocytes were increased when peripheral blood mononuclear cells (PBMC) from HHV-6 seropositive children aged 3 to 12 years and adults were cultured with HHV-6 antigen compared with control antigen. In contrast, cord blood mononuclear cells (CBMC) and PBMC from seronegative children did not show cell proliferation and changes in expression of IL-2Rs. In seropositive children less than 2 years of age, the magnitude of cell proliferation was low and IL-2Rs (alpha-, beta-, and gamma-chain) in CD8+ cells and IL-2Rs (alpha-chain) in CD45RO+ cells were increased. These data suggest that children below the age of 2 had immature lymphocytic response to HHV-6 antigen. Deletion of monocytes from PBMC and the addition of a mixture of anti-IL-2Rs (alpha-, beta-, and gamma-chain) antibodies reduced cell proliferation in response to HHV-6, suggesting the requirement of the presence of monocytes and expression of IL-2Rs.

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.

Identification of a Cys motif in the common beta chain of the interleukin 3, granulocyte-macrophage colony-stimulating factor, and interleukin 5 receptors essential for disulfide-linked receptor heterodimerization and activation of all three receptors

The human interleukin 3 (IL-3) and granulocyte-macrophage colony-stimulating factor (GM-CSF) receptors undergo covalent dimerization of the respective specific alpha chains with the common beta subunit (betac) in the presence of the cognate ligand. We have now performed alanine substitutions of individual Cys residues in betac to identify the Cys residues involved and their contribution to activation of the IL-3, GM-CSF, and IL-5 receptors. We found that substitution of Cys-86, Cys-91, and Cys-96 in betac but not of Cys-100 or Cys-234 abrogated disulfide-linked IL-3 receptor dimerization. However, although Cys-86 and Cys-91 betac mutants retained their ability to form non-disulfide-linked dimers with IL-3Ralpha, substitution of Cys-96 eliminated this interaction. Binding studies demonstrated that all betac mutants with the exception of C96A supported high affinity binding of IL-3 and GM-CSF. In receptor activation experiments, we found that betac mutants C86A, C91A, and C96A but not C100A or C234A abolished phosphorylation of betac in response to IL-3, GM-CSF, or IL-5. These data show that although Cys-96 is important for the structural integrity of betac, Cys-86 and Cys-91 participate in disulfide-linked receptor heterodimerization and that this linkage is essential for tyrosine phosphorylation of betac. Sequence alignment of betac with other cytokine receptor signaling subunits in light of these data shows that Cys-86 and Cys-91 represent a motif restricted to human and mouse beta chains, suggesting a unique mechanism of activation utilized by the IL-3, GM-CSF, and IL-5 receptors.

Structural and mechanistic aspects of Janus kinases: how the two-faced god wields a double-edged sword

The Janus family of protein-tyrosine kinases has long been known to function in signal transduction pathways initiated by a host of cytokines. A brief overview of the role of Janus kinases (Jaks) in both cytokine and noncytokine signaling pathways highlights the broad physiologic importance of this kinase family. New insights into the structural and mechanistic regulatory aspects of Janus kinases are rapidly emerging. Recent mutational analyses allow the dissection of Jaks into three distinct structural domains governing receptor affiliation, autoregulation, and catalysis. A fourth domain determining substrate specificity is as yet poorly defined and is, therefore, discussed in the context of known substrates and inhibitors, a collection of molecules that have been expanded recently to include Stam and Jab. The proposed mechanism of the interconversion of Janus kinases from inactive to fully active enzymes involves three states of enzymatic activity. Additional layers of regulation can be independently superimposed on this multistate model, providing a simplified description of the behavior of Janus kinases under normal and pathologic circumstances.

Further Analysis of Interleukin-2 Receptor Subunit Expression on the Different Human Peripheral Blood Mononuclear Cell Subsets

We have investigated the expression of the three components of the interleukin-2 receptor (IL-2Rα, IL-2Rβ, and IL-2Rγ) on the surface of the various peripheral blood mononuclear cell (PBMC) subsets by flow cytometry analysis. The PBMC were immediately isolated (ficoll) from blood collected on heparin as anticoagulant. The three IL-2R components are absent or only marginally detectable on CD4 T lymphocytes. No expression of the IL-2R chains is found for the B lymphocytes. In most donors, the three chains are not detectable on CD8 T lymphocytes, but for a few of them, IL-2Rβ or IL-2Rγ are clearly expressed. CD56 high (IL-2Rα+) and CD56 low (IL-2Rα−) natural killer (NK) cells express IL-2Rβ, but not IL-2Rγ. IL-2Rγ is expressed by monocytes of all donors although with variable intensity. When blood is collected on other anticoagulants or when cells are isolated 1 day after collection, IL-2Rα, IL-2Rβ, and IL-2Rγ are largely expressed on the surface of most PBMC. This observation provides a possible explanation for divergent data previously reported on IL-2R expression. Finally, we show that IL-2Rγ, which is not detectable on the cell surface of lymphocytes, is nevertheless expressed and stored as an intracellular component. This result is in agreement with the constitutive expression of the IL-2Rγ gene and suggests a specific regulatory mechanism for IL-2Rγ membrane translocation.

Further Analysis of Interleukin-2 Receptor Subunit Expression on the Different Human Peripheral Blood Mononuclear Cell Subsets

We have investigated the expression of the three components of the interleukin-2 receptor (IL-2Rα, IL-2Rβ, and IL-2Rγ) on the surface of the various peripheral blood mononuclear cell (PBMC) subsets by flow cytometry analysis. The PBMC were immediately isolated (ficoll) from blood collected on heparin as anticoagulant. The three IL-2R components are absent or only marginally detectable on CD4 T lymphocytes. No expression of the IL-2R chains is found for the B lymphocytes. In most donors, the three chains are not detectable on CD8 T lymphocytes, but for a few of them, IL-2Rβ or IL-2Rγ are clearly expressed. CD56 high (IL-2Rα+) and CD56 low (IL-2Rα−) natural killer (NK) cells express IL-2Rβ, but not IL-2Rγ. IL-2Rγ is expressed by monocytes of all donors although with variable intensity. When blood is collected on other anticoagulants or when cells are isolated 1 day after collection, IL-2Rα, IL-2Rβ, and IL-2Rγ are largely expressed on the surface of most PBMC. This observation provides a possible explanation for divergent data previously reported on IL-2R expression. Finally, we show that IL-2Rγ, which is not detectable on the cell surface of lymphocytes, is nevertheless expressed and stored as an intracellular component. This result is in agreement with the constitutive expression of the IL-2Rγ gene and suggests a specific regulatory mechanism for IL-2Rγ membrane translocation.

An interleukin-2 receptor gamma chain mutation with normal thymus morphology

One of the most common human immunodeficiencies is an X-linked condition arising from mutations of the gamma subunit of the interleukin-2 receptor (IL-2Rgamma). The IL-2Rgamma protein is one chain of the heterotrimeric (alpha, beta, gamma) IL-2 receptor, but also participates in the formation of the IL-4, 7, 9, and 15 receptor complexes. The diagnosis of X-linked SCID is usually relatively simple due to the distinctive immunological presentation; IL-2Rgamma-deficient patients typically lacking mature T lymphocytes (T-B+). However, it is becoming clear that this merely represents one extreme of a potential range of clinical presentations. We describe here a novel mutation of the human IL-2Rgamma chain (R222C) resulting in an unusual immunological phenotype. Although clinically immunodeficient, this patient has normal numbers of peripheral T and B cells, responds normally to mitogenic stimuli, and unusually, has a normal thymus gland. This IL-2Rgamma mutation is distinctive in that the protein is sufficiently stable to be expressed at the cell surface. While the T cell receptor repertoire appears complete, suggesting normal T cell differentiation occurs, patient T cells demonstrate a reduced ability to bind IL-2 and this appears sufficient to cause a deficiency in their ability to participate in antigenic responses. Early clinical recognition of this phenotype is critical as a delay in diagnosis may result in a fatal infection.

Distinct tyrosine phosphorylation sites in JAK3 kinase domain positively and negatively regulate its enzymatic activity

Cytokines are critically important for the growth and development of a variety of cells. Janus kinases (JAKs) associate with cytokine receptors and are essential for transmitting downstream cytokine signals. However, the regulation of the enzymatic activity of the JAKs is not well understood. Here, we investigated the role of tyrosine phosphorylation of JAK3 in regulating its kinase activity by analyzing mutations of tyrosine residues within the putative activation loop of the kinase domain. Specifically, tyrosine residues 980 and 981 of JAK3 were mutated to phenylalanine individually or doubly. We found that JAK3 is autophosphorylated on multiple sites including Y980 and Y981. Compared with the activity of wild-type (WT) JAK3, mutant Y980F demonstrated markedly decreased kinase activity, and optimal phosphorylation of JAK3 on other sites was dependent on Y980 phosphorylation. The mutant Y980F also exhibited reduced phosphorylation of its substrates, gammac and STAT5A. In contrast, mutant Y981F had greatly increased kinase activity, whereas the double mutant, YY980/981FF, had intermediate activity. These results indicate that Y980 positively regulates JAK3 kinase activity whereas Y981 negatively regulates JAK3 kinase activity. These observations in JAK3 are similar to the findings in the kinase that is closely related to the JAK family, ZAP-70; mutations of tyrosine residues within the putative activation loop of ZAP-70 also have opposing actions. Thus, it will be important to determine whether this feature of regulation is unique to JAK3 or if it is also a feature of other JAKs. Given the importance of JAKs and particularly JAK3, it will be critical to fully dissect the positive and negative regulatory function of these and other tyrosine residues in the control of kinase activity and hence cytokine signaling.

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.

Preassembly of interleukin 2 (IL-2) receptor subunits on resting Kit 225 K6 T cells and their modulation by IL-2, IL-7, and IL-15: a fluorescence resonance energy transfer study

Assembly and mutual proximities of alpha, beta, and gamma(c) subunits of the interleukin 2 receptors (IL-2R) in plasma membranes of Kit 225 K6 T lymphoma cells were investigated by fluorescence resonance energy transfer (FRET) using fluorescein isothiocyanate- and Cy3-conjugated monoclonal antibodies (mAbs) that were directed against the IL-2R alpha, IL-2R beta, and gamma(c) subunits of IL-2R. The cell-surface distribution of subunits was analyzed at the nanometer scale (2-10 nm) by FRET on a cell-by-cell basis. The cells were probed in resting phase and after coculture with saturating concentrations of IL-2, IL-7, and IL-15. FRET data from donor- and acceptor-labeled IL-2R beta-alpha, gamma-alpha, and gamma-beta pairs demonstrated close proximity of all subunits to each other in the plasma membrane of resting T cells. These mutual proximities do not appear to represent mAb-induced microaggregation, because FRET measurements with Fab fragments of the mAbs gave similar results. The relative proximities were meaningfully modulated by binding of IL-2, IL-7, and IL-15. Based on FRET analysis the topology of the three subunits at the surface of resting cells can be best described by a "triangular model" in the absence of added interleukins. IL-2 strengthens the bridges between the subunits, making the triangle more compact. IL-7 and IL-15 act in the opposite direction by opening the triangle possibly because they associate their private specific alpha receptors with the beta and/or gamma(c) subunits of the IL-2R complex. These data suggest that IL-2R subunits are already colocalized in resting T cells and do not require cytokine-induced redistribution. This colocalization is significantly modulated by binding of relevant interleukins in a cytokine-specific manner.

Janus kinases in interleukin-2-mediated signaling: JAK1 and JAK3 are differentially regulated by tyrosine phosphorylation

BACKGROUND

Cytokines mediate a variety of effector cell functions, including cellular proliferation, differentiation, and modulation of the immune response. Many cytokines activate receptor-associated Janus kinases (JAKs) that promote tyrosine phosphorylation of signal transducers and activators of transcription (STAT) factors. Although JAK activation has been correlated with phosphorylation, the role of this tyrosine phosphorylation in the regulation of JAK1 and JAK3 remains unclear. Furthermore, the relative roles of JAK1 and JAK3 in the activation of STAT5 by interleukin-2 (IL-2) remain poorly understood.

RESULTS

We targeted two conserved tyrosine residues within the activation loop of the JAK1 and JAK3 kinase domains for substitution with phenylalanines. In an overexpression system, the catalytic function of JAK1 strictly required the presence of the first of these tyrosines, Y1033. In contrast, JAK3 retained catalytic activity when either or both of these activation-loop tyrosines were mutated. Analysis of JAK1/3 chimeras demonstrated that JAK activity was also controlled by intramolecular interactions involving the amino-terminal domain of the JAK as well as by the inherent signaling properties of the kinase domain. Finally, we have reconstituted IL-2-dependent STAT5 induction in a cell line that lacks detectable expression of JAK1 and JAK3. Catalytically active versions of both JAK1 and JAK3 must be present for effective induction of STAT5.

CONCLUSIONS

JAK1 and JAK3 are differentially regulated by specific tyrosines within their respective activation loops. Additionally, the amino-terminal domain of JAK3 appears to contain regulatory sequences that modify the function of the kinase domain. Finally, both JAK1 and JAK3 must retain catalytic function for IL-2-induced STAT5 activation.

Advances in the understanding of cytokine signal transduction: the role of Jaks and STATs in immunoregulation and the pathogenesis of immunodeficiency

Cytokines are of great importance in the growth and differentiation of hematopoietic and other cells. Moreover, they are also crucial in immunoregulation and in host defense. Although our understanding of the molecular basis of cytokine action is far from complete, recent advances have substantially improved our knowledge of cytokine-dependent signal transduction. The delineation of the structure of cytokine receptors and the signaling pathways they utilize has provided clues as to how the strikingly specific effects of cytokines are achieved. Additionally, the basis of some of the pleiotropic and redundant effects of cytokines has also become clear. The discovery of the Janus family of protein tyrosine kinases (Jaks) and the STATs (signal transducers and activators of transcription) has also provided key insights into the mechanism by which intracellular signals are transduced. The following paradigm has emerged: cytokines induce dimerization of receptor subunits that are constitutively associated with Jaks. This activates the Jaks, which then phosphorylate the receptors. The phosphorylated receptors are bound by SH2-containing proteins, one class of which is the STATs. Activated STATs, then, translocate to the nucleus to effect gene transcription. Though the Jaks do not explain much in terms of specificity in signaling, the function of the STATs does. The discovery of patients with autosomal recessive severe combined immunodeficiency due to mutations of a particular Jak, Jak3, and the phenotype of knockout mice lacking Jak3 and various STATs demonstrate the specific and critical roles of these molecules in the development and function of the immune system.

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.

Cytokine receptor signal transduction through Jak tyrosine kinases and Stat transcription factors

Cytokines are the principal regulators of cell proliferation and differentiation of hematopoietic cells and these responses are initiated through activation of hematopoietic cytokine receptors. Although the receptor intracellular domains lack any kinase domains, activation of cytokine receptors lead to rapid induction of tyrosine phosphorylation. Recently, cytokine receptors have been shown to associate with and activate members of the cytoplasmic Jak tyrosine kinase family. Activation of Jak kinases leads to phosphorylation of several signaling proteins and thereby couples ligand-mediated receptor stimulation to activation of intracellular signaling pathways. The best characterized substrates for Jaks are the Stat transcription factors, which are crucial mediators of cytokine-mediated gene responses, and, particularly, central determinants for the specificity in cytokine responses.

Phosphatidylinositol 3-kinase links the interleukin-2 receptor to protein kinase B and p70 S6 kinase

Phosphatidylinositol 3-kinase (PI 3-kinase) is activated by the cytokine interleukin-2 (IL-2). We have used a constitutively active PI 3-kinase to identify IL-2-mediated signal transduction pathways directly regulated by PI 3-kinase in lymphoid cells. The serine/threonine protein kinase B (PKB)/Akt can act as a powerful oncogene in T cells, but its positioning in normal T cell responses has not been explored. Herein, we demonstrate that PKB is activated by IL-2 in a PI 3-kinase-dependent fashion. Importantly, PI 3-kinase signals are sufficient for PKB activation in IL-2-dependent T cells, and PKB is a target for PI 3-kinase signals in IL-2 activation pathways. The present study establishes also that PI 3-kinase signals or PKB signals are sufficient for activation of p70 S6 kinase in T cells. PI 3-kinase can contribute to, but is not sufficient for, activation of extracellular signal-regulated kinases (Erks) and Erk effector pathways. Therefore, PI 3-kinase is a selective regulator of serine/threonine kinase signal transduction pathways in T lymphocytes, and this enzyme provides a crucial link between the interleukin-2 receptor, the protooncogene PKB, and p70 S6 kinase.

Intermediate affinity interleukin-2 receptor mediates survival via a phosphatidylinositol 3-kinase-dependent pathway

Peripheral blood T lymphocytes require two signals to enter and progress along the cell cycle from their natural quiescent state. The first activation signal is provided by the stimulation through the T cell receptor, which induces the synthesis of cyclins and the expression of the high affinity interleukin-2 receptor. The second signal, required to enter the S phase, is generated upon binding of interleukin-2 to the high affinity alphabetagamma interleukin-2 receptor. However, resting T cells already express intermediate affinity betagamma interleukin-2 receptors. As shown here, T cell stimulation through intermediate affinity receptors is capable of inducing cell rescue from the apoptosis suffered in the absence of stimulation. Characterization of the signaling pathways utilized by betagamma interleukin-2 receptors in resting T cells, indicated that pp56(lck), but not Jak1 or Jak3, is activated upon receptor triggering. Compelling evidence is presented indicating that phosphatidylinositol 3-kinase associates with the intermediate affinity interleukin-2 receptor and is activated upon interleukin-2 addition. Bcl-xL gene was also found to be induced upon betagamma interleukin-2 receptor stimulation. Finally, pharmacological inhibition of phosphatidylinositol 3-kinase blocked both interleukin-2-mediated bcl-xL induction and cell survival. We conclude that betagamma interleukin-2 receptor mediates T-cell survival via a phosphatidylinositol 3-kinase-dependent pathway, possibly involving pp56(lck) and bcl-xL as upstream and downstream effectors, respectively.

126Gln is the residue of human IL-2 binding to IL-2R gamma subunit

The 126Gln of human interleukin-2 (IL-2) is a conserved amino acid residue. After substitution of 126Gln with Asp, the binding abilities of this mutant to different composites of IL-2 receptor (R) subunits have been determined. Results show that 126AspIL-2 has higher affinity to IL-2R alpha beta gamma complex and normal affinity to IL-2R alpha beta complex, but loses its binding ability to IL-2R beta gamma complex, demonstrating that the 126Gln is the residue of human IL-2 which binds to IL-2R gamma subunit.

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.

Requirement for an initial signal from the membrane-proximal region of the interleukin 2 receptor gamma(c) chain for Janus kinase activation leading to T cell proliferation

The interleukin 2 receptor (IL-2R) generates proliferative signals in T lymphocytes by ligand-induced heterodimerization of two chains, IL-2Rbeta and gamma(c), which associate with the tyrosine kinases Jak1 and Jak3, respectively. Genetic and molecular studies have demonstrated that Jak3 is essential for mitogenic signaling by the gamma(c) chain; because it is also the only molecule known to associate with gamma(c), we speculated that Jak3 might be sufficient for signaling by this chain. Therefore, fusion proteins were constructed in which all or part of the cytoplasmic domain of gamma(c) was replaced by Jak3. Signaling was evaluated in the IL-2-dependent T cell line CTLL-2 using chimeric IL-2Rbeta and gamma(c) chains that bind and are activated by the cytokine granulocyte-macrophage colony-stimulating factor. Chimeric gamma(c) chains containing only Jak3 in the cytoplasmic domain failed to mediate proliferation of CTLL-2 cells, but addition of a conserved membrane-proximal (PROX) domain of gamma(c) in tandem with Jak3 fully reconstituted gamma(c) function. The requirement for the PROX domain reflected an essential role in the activation of Jak3 in vivo. Despite lacking defined catalytic motifs, PROX induced an early Jak-independent signal, including tyrosine phosphorylation of IL-2Rbeta and the tyrosine phosphatase SHP-2. The results define the minimal signaling components of gamma(c) and suggest a new mechanism by which the IL-2R initiates signaling in response to ligand.

Functional expression on human trophoblasts of interleukin 4 and interleukin 7 receptor complexes with a common gamma chain

The gamma chain in the high-affinity receptor complex for interleukin 2(IL-2) is used in receptor complexes for IL-4 and IL-7. We examined expression and function of the gamma chain in trophoblast cells. Flow cytometric studies demonstrated that the human placental cell line and choriocarcinoma cell lines expressed the IL-2R gamma chain. None of these cell lines expressed the IL-2R alpha and beta chains, whereas the alpha chains of IL-4R and IL-7R were present. The gamma chain and IL-4R and IL-7R mRNA were detected in human placental trophoblasts by the in situ hybridization method. The release of human chorionic gonadotropin by trophoblasts was accelerated by IL-4 and IL-7 in a concentration-dependent manner. These findings indicate that IL-4 and IL-7 are involved in trophoblast function.

Protein kinase C in IL-2 signal transduction

Interleukin-2 (IL-2), secreted principally by activated helper T-cells, plays a pivotal role in the generation and regulation of the immune response. The various biologic functions of IL-2 have been the focus of intensive study over the years and have been well worked out. By contrast, an understanding of the intracellular signals coupled to the IL-2 receptor and responsible for mediating IL-2 effects in T-cells is far less developed, and the role that protein kinase C (PKC) may play in the various cellular responses to IL-2 receptor activation is unclear. In this article we will discuss IL-2, its receptors, and IL-2 signal transduction in relation to the physiological roles PKC activation may play in IL-2-mediated activation of T-cells and other hematopoietic cells.

Kinetic analysis of ligand binding to interleukin-2 receptor complexes created on an optical biosensor surface

The interleukin-2 receptor (IL-2R) is composed of at least three cell surface subunits, IL-2R alpha, IL-2R beta, and IL-2R gamma c. On activated T-cells, the alpha- and beta-subunits exist as a preformed heterodimer that simultaneously captures the IL-2 ligand as the initial event in formation of the signaling complex. We used BIAcore to compare the binding of IL-2 to biosensor surfaces containing either the alpha-subunit, the beta-subunit, or both subunits together. The receptor ectodomains were immobilized in an oriented fashion on the dextran matrix through unique solvent-exposed thiols. Equilibrium analysis of the binding data established IL-2 dissociation constants for the individual alpha- and beta-subunits of 37 and 480 nM, respectively. Surfaces with both subunits immobilized, however, contained a receptor site of much higher affinity, suggesting the ligand was bound in a ternary complex with the alpha- and beta-subunits, similar to that reported for the pseudo-high-affinity receptor on cells. Because the binding responses had the additional complexity of being mass transport limited, obtaining accurate estimates for the kinetic rate constants required global fitting of the data sets from multiple surface densities of the receptors. A detailed kinetic analysis indicated that the higher-affinity binding sites detected on surfaces containing both alpha- and beta-subunits resulted from capture of IL-2 by a preformed complex of these subunits. Therefore, the biosensor analysis closely mimicked the recognition properties reported for these subunits on the cell surface, providing a convenient and powerful tool to assess the structure-function relationships of this and other multiple subunit receptor systems.

Elf-1 and Stat5 bind to a critical element in a new enhancer of the human interleukin-2 receptor alpha gene

The interleukin 2 receptor alpha-chain (IL-2R alpha) gene is a key regulator of lymphocyte proliferation. IL-2R alpha is rapidly and potently induced in T cells in response to mitogenic stimuli. Interleukin 2 (IL-2) stimulates IL-2R alpha. transcription, thereby amplifying expression of its own high-affinity receptor. IL-2R alpha transcription is at least in part controlled by two positive regulatory regions, PRRI and PRRII. PRRI is an inducible proximal enhancer, located between nucleotides -276 and -244, which contains NF-kappaB and SRE/CArG motifs. PRRII is a T-cell-specific enhancer, located between nucleotides -137 and -64, which binds the T-cell-specific Ets protein Elf-1 and HMG-I(Y) proteins. However, none of these proximal regions account for the induction of IL-2R alpha transcription by IL-2. To find new regulatory regions of the IL-2R alpha gene, 8.5 kb of the 5' end noncoding sequence of the IL-2R alpha gene have been sequenced. We identified an 86-nucleotide fragment that is 90% identical to the recently characterized murine IL-2-responsive element (mIL-2rE). This putative human IL-2rE, designated PRRIII, confers IL-2 responsiveness on a heterologous promoter. PRRIII contains a Stat protein binding site that overlaps with an EBS motif (GASd/EBSd). These are essential for IL-2 inducibility of PRRIII/CAT reporter constructs. IL-2 induced the binding of Stat5a and b proteins to the human GASd element. To confirm the physiological relevance of these findings, we carried out in vivo footprinting experiments which showed that stimulation of IL-2R alpha expression correlated with occupancy of the GASd element. Our data demonstrate a major role of the GASd/EBSd element in IL-2R alpha regulation and suggest that the T-cell-specific Elf-1 factor can serve as a transcriptional repressor.

Distinct tyrosine residues within the interleukin-2 receptor beta chain drive signal transduction specificity, redundancy, and diversity

To explore the basis for interleukin (IL)-2 receptor (IL-2R) signaling specificity, the roles of tyrosine-based sequences located within the cytoplasmic tails of the beta and gammac chains were examined in the murine helper T cell line HT-2. Activation of the Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathway, cellular proliferation, and the induction of various genes were monitored. All four of the cytoplasmic tyrosine residues as well as the distal portion of the gammac proved dispensable for the entire spectrum of IL-2R signaling responses studied. Conversely, select tyrosine residues within the beta chain were essential and differentially required for various signaling events. Specifically, activation of c-fos gene expression was found to occur exclusively through the most membrane proximal tyrosine, Tyr-338, whereas proliferation and the activation of STAT-5 were induced either through Tyr-338 or through the two C-terminal tyrosine residues, Tyr-392 and Tyr-510. These tyrosine residues mediated the induction of two different STAT-5 isoforms, which were found to form heterodimers upon receptor activation. In contrast to the tyrosine dependence of c-fos and STAT-5 induction, bcl-2 gene induction proceeded independently of all IL-2Rbeta tyrosine residues. Thus, the tyrosine-based modules present within the IL-2Rbeta cytoplasmic tail play a critical role in IL-2R signaling, mediating specificity, redundancy, and multifunctionality.

A point mutation in interleukin-2 that alters ligand internalization

In previous studies, we have identified an interleukin-2 (IL-2) analog containing a point mutation at position 51 (T51P) that expresses nearly wild-type bioactivity, yet has approximately 10-fold lower receptor binding affinity. Since ligand-dependent receptor internalization may be the rate-limiting step controlling the duration of IL-2 receptor signaling, a reduction in the receptor internalization rate could contribute to the observed response enhancement for this analog. To evaluate this possibility, we compared the internalization of IL-2 and T51P in three separate assays. While the internalization rate for IL-2 agreed with values determined by others, the internalization of T51P was markedly reduced. The receptor binding rate constants for this analog were only slightly different; thus, altered binding kinetics could not explain the decreased internalization rate. The effects of reduced internalization were also observable in bioassays, where T51P maintained T-cell proliferation for a longer period compared with IL-2. These results indicate that the T51P point mutation reduces the receptor internalization rate compared with IL-2 in a fashion that is independent of the dissociation rate. This analog may represent a new approach to the preparation of cytokine analogs with potentiated agonist and antagonist properties.

The human T-cell leukemia/lymphotropic virus type 1 p12I proteins bind the interleukin-2 receptor beta and gammac chains and affects their expression on the cell surface

p12I is a small hydrophobic protein encoded by the human T-cell leukemia/lymphotropic virus type 1 (HTLV-1) that interacts with the 16-kDa component of the H+ vacuolar ATPase and cooperates with bovine papillomavirus 1 E5 oncoprotein in cell transformation. Just as an important step in E5 action appears to be its binding to the platelet-derived growth factor receptor, it was found that p12I binds specifically to both the beta and gamma(c) chains of the interleukin-2 receptor (IL-2R). The IL-2R beta and gamma(c) chains associated with p12I are endoglycosidase-H sensitive, suggesting that their interaction occurs in a pre-Golgi compartment. p12I stabilizes the immature forms of the IL-2R beta and gamma(c) chains and decreases their cell surface expression. The interactions of p12I with IL-2R beta and gamma(c) may have important implications in the immunosuppressive effect of HTLV-1 in vivo as well as in the ligand-independent HTLV-1-mediated T-cell proliferation.

Cloning and chromosomal mapping of bovine interleukin-2 receptor gamma gene

Interleukin-2 receptor (IL-2R) gamma chain, a member of the cytokine receptor superfamily, forms a high-affinity receptor with IL-2R alpha and beta chains that plays an important role in interleukin-2 (IL-2) signal transduction. We have cloned and characterized the bovine IL-2Rgamma gene and corresponding cDNA. Bovine IL-2Rgamma is a single-copy gene that contains 8 exons and spans approximately 3.8 kb. The promoter region lacks conventional TATA and CCAAT consensus sites, but contains several regulatory elements that are recognition sites for the GATA binding proteins, AP-1 and AP-2. Physical assignment by fluorescence in situ hybridization (FISH) placed the bovine IL-2Rgamma gene on chromosome Xq23.

Evidence for a role for the phosphotyrosine-binding domain of Shc in interleukin 2 signaling

Stimulation via the T-cell growth factor interleukin 2 (IL-2) leads to tyrosine phosphorylation of Shc, the interaction of Shc with Grb2, and the Ras GTP/GDP exchange factor, mSOS. Shc also coprecipitates with the IL-2 receptor (IL-2R), and therefore, may link IL-2R to Ras activation. We have further characterized the Shc-IL-2R interaction and have made the following observations. (i) Among the two phosphotyrosine-interaction domains present in Shc, the phosphotyrosine-binding (PTB) domain, rather than its SH2 domain, interacts with the tyrosine-phosphorylated IL-2R beta chain. Moreover, the Shc-PTB domain binds a phosphopeptide derived from the IL-2R beta chain (corresponding to residues surrounding Y338, SCFTNQGpYFF) with high affinity. (ii) In vivo, mutant IL-2R beta chains lacking the acidic region of IL-2Rbeta (which contains Y338) fail to phosphorylate Shc. Furthermore, when wild type or mutant Shc proteins that lack the PTB domain were expressed in the IL-2-dependent CTLL-20 cell line, an intact Shc-PTB domain was required for Shc phosphorylation by the IL-2R, which provides further support for a Shc-PTB-IL-2R interaction in vivo. (iii) PTB and SH2 domains of Shc associate with different proteins in IL-2- and T-cell-receptor-stimulated lysates, suggesting that Shc, through the concurrent use of its two different phosphotyrosine-binding domains, could assemble multiple protein complexes. Taken together, our in vivo and in vitro observations suggest that the PTB domain of Shc interacts with Y338 of the IL-2R and provide evidence for a functional role for the Shc-PTB domain in IL-2 signaling.

Characterization of the receptor binding sites of human leukemia inhibitory factor and creation of antagonists

Residues in human leukemia inhibitory factor (hLIF) crucial for binding to both the human LIF receptor (R) and gp130 were identified by analysis of alanine scanning mutants of hLIF in assays for both receptor binding and bioactivity. The region of hLIF most important for binding to the hLIF-R is composed of residues from the amino terminus of the D-helix, carboxyl terminus of the B-helix, and C-D loop. This site forms a distinct surface at the end of the four-helix bundle in the tertiary structure of the closely related murine LIF. The two residues of hLIF that contribute the majority of free energy for hLIF-R binding, Phe-156 and Lys-159 are surrounded by other residues which have only a moderate impact. This arrangement of a few key residues surrounded by less important ones is analogous to the functional binding epitope of human growth hormone for its receptor. A second region of hLIF that includes residues from the carboxyl terminus of the D-helix and A-B loop also had a weak influence on hLIF-R binding. Residues in hLIF from both the A- and C-helices are involved in binding the gp130 co-receptor. Abolition of the gp130 binding site in hLIF created antagonists of LIF action.

gp130 transducing receptor cross-linking is sufficient to induce interleukin-6 type responses

gP130 transducing receptor is involved in the formation of high affinity receptors for the cytokines of the interleukin-6 (IL-6) family. Recruitment of gp130 by IL-6 associated to its receptor leads to the dimerization of the transducing component. In the present study we did characterize the B-S12 monoclonal antibody raised against gp130 and able to elicit IL-6 type biological activities. B-S12 antibody triggered strongly the proliferation of TF1 and XGI hematopoietic cell lines and was able to increase the synthesis of acute phase proteins in HepG2 hepatoma cell line. B-S12 also behaved as a synergistic factor with granulocyte-macrophage colony-stimulating factor for both proliferation and differentiation of CD34-positive hematopoietic cell progenitors. By using a symmetric enzyme-linked immunosorbent assay, allowing the detection of dimeric forms of soluble gp130, we found that addition of B-S12 to gp130 led to its dimerization. Analysis of the tyrosine phosphorylation events in gp130 and Jak kinase family members revealed that B-S12 quickly induced the phosphorylation of gp130 in a neural derived cell line, and that Jak1 and Jak2 were also recruited. In conclusion, we show that gp130 cross-linking with the B-S12 monoclonal antibody was sufficient to generate functional IL-6 type responses in hematopoietic, neural, and hepatic cells.