Interleukin 2-induced activation of JAK3: possible involvement in signal transduction for c-myc induction and cell proliferation

Effect of serum interleukin 21 on the development of coronary artery disease

There has been more and more evidence to confirm the essential role of inflammatory processes in the development of coronary artery disease (CAD). Interleukin-21 (IL-21), the most recently discovered CD132-dependent cytokine, plays a key role in regulating inflammation. The aim of the study was to understand the effect of peripheral IL-21 on the pathogenesis and progression of CAD. Serum level of IL-21 in 92 CAD patients and 73 controls was measured by the enzyme-linked immunosorbent assay. Data showed that IL-21 expression was significantly increased in CAD than in controls (p < 0.001). Interestingly, when comparing IL-21 level with different genders, male subjects revealed higher IL-21 than female subjects (p = 0.024). Also, we observed that patients with hypertension had upregulated level of serum IL-21 (p = 0.002). Moreover, serum level of IL-21 was positively correlated with total cholesterol level (p = 0.015) or low-density lipoprotein cholesterol (p = 0.0009) of CAD cases. In addition, we analyzed IL-21 level with the severity of CAD, and identified that cases with 3-vessel affected had significantly elevated level of IL-21 than those with 1-vessel or 2-vessel affected. These data suggested that serum level of IL-21 may be closely associated with the development and progression of CAD.

Polymorphism of stromal cell-derived factor-1 selectively upregulates gene expression and is associated with increased susceptibility to coronary artery disease

Stromal cell-derived factor-1 (SDF-1) plays critical roles in vascular development and hematopoiesis. Here, we investigated the function of SDF-1 rs1801157G/A polymorphism in various immune cells and examined its association with susceptibility to coronary artery disease (CAD). Protein and mRNA levels of SDF-1 were tested in peripheral CD4+ T cell, CD8+ T cells, monocytes, and natural killer (NK) T cells from healthy donors with different genotypes of rs1801157G/A polymorphism. Prevalence of the polymorphism was compared between CAD patients and healthy controls. Data revealed that SDF-1 mRNA and protein were detectable in CD4+ T cells, CD8+ T cells, monocytes and NK T cells. Interestingly, both protein level and mRNA level of SDF-1 were significantly increased in the monocytes with rs1801157AA genotype, whereas the same phenomenon was not observed in the other three cell types. Blockage of CD14 completely inhibited the upregulation of SDF-1 in the monocytes with rs1801157AA genotype. Association analysis showed that frequencies of the rs1801157AA genotype and A allele were significantly higher in CAD cases than in controls (odds ratio [OR]=2.28, 95% confidence interval [CI], 1.50-3.29, p<0.0001, and OR=1.46, 95% CI, 1.21-3.73, p<0.0001, respectively). Also, prevalence of rs1801157AA genotype was further increased in cases with ST-elevation myocardial infarction (OR=1.65, 95% CI, 1.04-2.56, p=0.028). Our data suggest a novel pathway for regulating SDF-1 and a new risk factor for CAD.

Interleukin-21: updated review of Phase I and II clinical trials in metastatic renal cell carcinoma, metastatic melanoma and relapsed/refractory indolent non-Hodgkin's lymphoma

IMPORTANCE TO THE FIELD

In advanced renal cell cancer and malignant melanoma, the current FDA approved immune modulators, such as IL-2, are the only agents which provide a durable complete remission. These responses, however, occur in < 10% of treated patients and their applicability is limited to selected patients because of their toxicity. The identification of new immunotherapeutic agents with an improved response rate and toxicity profile would represent a significant advancement in the treatment of these malignancies.

AREAS COVERED IN THIS REVIEW

This is a comprehensive review of IL-21 including its pharmacology and current developmental status. A literature review was performed using all PubMed listed publications involving IL-21, including original research articles, reviews and abstracts. It also includes a review of current ongoing trials and information from the official product website.

WHAT THE READER WILL GAIN

Recombinant IL-21 (rIL-21) is a new immune modulator currently undergoing Phase I and II testing. It is a cytokine with a four helix structure that has structural and sequence homology to IL-2 and -15, but also possesses many unique biological properties. In this review, we evaluate the development, pharmacologic properties, safety profile and current clinical efficacy of rIL-21.

TAKE HOME MESSAGE

rIL-21 has an acceptable safety profile and encouraging single agent activity in early phase renal cell carcinoma and melanoma clinical trials.

Impaired IL-7 signaling may explain a case of atypical JAK3-SCID

Janus kinase 3-severe combined immunodeficiency (JAK3-SCID) is an autosomal recessive immunodeficiency disease caused by various mutations in the JAK3 gene. Typical JAK3-SCID is characterized by a phenotype in which B cells are present but T and NK cells are not, the T(-)B(+)NK(-) phenotype, and by impaired signaling through cytokine receptors that use the common gamma chain (gammac) subunit. An atypical JAK3-SCID case carrying a single glutamate to glycine substitution mutation (E481G) in the JH3 domain of one JAK3 allele, and a deletion mutation (del482-596) in the JH3 and JH2 domains of the other allele was reported previously. Although this patient had CD4(+) T cells and NK cells unlike typical cases, the CD4(+) T cells were functionally impaired. We report here that the JAK3-E481G mutant transduced IL-2-, IL-4-, IL-15-, and IL-21-induced signals as efficiently as wild-type JAK3. However, this mutant failed to respond to IL-7 by phosphorylating JAK1, JAK3, or STAT5. The other mutant JAK3, JAK3-del482-596, was non-functional. Thus, an impaired IL-7 signal may cause SCID and compromise T-cell differentiation, even if the IL-15 signal is preserved and supports NK-cell development, as in this patient.

Phage phiC31 integrase-mediated genomic integration of the common cytokine receptor gamma chain in human T-cell lines

BACKGROUND

X-linked severe combined immunodeficiency (SCID-X1, X-SCID) is a life-threatening disease caused by a mutated common cytokine receptor gamma chain (gammac) gene. Although ex vivo gene therapy, i.e., transduction of the gammac gene into autologous CD34(+) cells, has been successful for treating SCID-X1, the retrovirus vector-mediated transfer allowed dysregulated integration, causing leukemias. Here, to explore an alternative gene transfer methodology that may offer less risk of insertional mutagenesis, we employed the phiC31 integrase-based integration system using human T-cell lines, including the gammac-deficient ED40515(-).

METHODS

A phiC31 integrase and a neo(r) gene expression plasmid containing the phiC31 attB sequence were co-delivered by electroporation into Jurkat cells. After G418 selection, integration site analyses were performed using linear amplification mediated-polymerase chain reaction (LAM-PCR). ED40515(-) cells were also transfected with a gammac expression plasmid containing attB, and the integration sites were determined. IL-2 stimulation was used to assess the functionality of the transduced gammac in an ED40515(-)-derived clone.

RESULTS

Following co-introduction of the phiC31 integrase expression plasmid and the plasmid carrying attB, the efficiency of integration into the unmodified human genome was assessed. Several integration sites were characterized, including new integration sites in intergenic regions on chromosomes 13 and 18 that may be preferred in hematopoietic cells. An ED40515(-) line bearing the integrated gammac gene exhibited stable expression of the gammac protein, with normal IL-2 signaling, as assessed by STAT5 activation.

CONCLUSIONS

This study supports the possible future use of this phiC31 integrase-mediated genomic integration strategy as an alternative gene therapy approach for treating SCID-X1.

Human bronchial epithelium controls TH2 responses by TH1-induced, nitric oxide-mediated STAT5 dephosphorylation: implications for the pathogenesis of asthma

Increased levels of NO in exhaled air in association with increased NO synthetase (NOS)2 expression in bronchial epithelial are hallmark features of asthma. It has been suggested that NO contributes to asthma pathogenesis by selective down-regulation of TH1 responses. We demonstrate, however, that NO can reversibly limit in vitro expansion of both human TH1 and TH2 CD4+ T cells. Mechanistically, NO induces cGMP-mediated reversible STAT5 dephosphorylation and therefore interferes with the IL-2R activation cascade. Human bronchial epithelial cells (HBEC) up-regulate NOS2 after stimulation with IFN-gamma secreted by TH1 CD4+ T cells and release NO, which inhibits both TH1 and TH2 cell proliferation. This reversible T cell growth arrest depends on NO because T cell proliferation is completely restored after in vitro blocking of NOS2 on HBEC. HBEC thus drive the effector end of a TH1-controlled feedback loop, which protects airway mucosal tissues at the potential lesional site in asthma from overwhelming CD4+ TH2 (and potentially TH1) responses following allergen exposure. Variations in the efficiency of this feedback loop provides a plausible mechanism to explain why only a subset of atopics sensitized to ubiquitous aeroallergens progress to expression of clinically relevant levels of airways inflammation.

Ovarian carcinoma cells inhibit T cell proliferation: suppression of IL-2 receptor beta and gamma expression and their JAK-STAT signaling pathway

Deficient T cell immune function and intracellular signaling in cancer patients may result from effects of tumors or their products on lymphocytes. Recently, it was demonstrated that several ovarian carcinoma cell lines could produce soluble factors that inhibited T cell proliferation. The aim of this study is to assess the effect of supernatants from 3 ovarian carcinoma cell lines (OVCAR3, CAOV3, SKOV3) on signal transduction elements that are linked to the IL-2R and its JAK-STAT pathway. A profound inhibition of proliferation, lower level of IFN-gamma and higher level of IL-10 gene expression were observed when CD8+ T cells were co-cultured with supernatants from 3 ovarian carcinoma cell lines. Cell cycle studies on inhibited CD8+ T cells showed most of them were growth arrested in G0/G1 phase. Western blot analysis showed that tumor supernatants suppressed expression of JAK3 and tyrosine phosphorylation of STAT5. JAK1 was not altered and the inhibition of STAT3 only appeared in OVCAR3 cells. Tumor supernatants also partially blocked induction of IL-2R beta and gamma chains expression. These findings suggest that ovarian carcinoma cells may suppress T cell proliferation through inhibition IL-2 dependent signaling pathways, which may be a mechanism of ovarian carcinoma induced immunosuppression.

Jak3 expression and genomic sequence in pediatric acute lymphoblastic leukemia

Janus tyrosine kinase 3 (JAK3) is one of several key regulatory enzymes in B-cell precursors which is highly conserved between multiple species. The gene for Jak3 has been mapped to human chromosome 19p12-13.1 and encompasses 23 exons. Constitutively high levels of JAK3 activity may contribute to drug resistance and enhanced clonogenicity of leukemic B-cell precursors from children and infants with acute lymphoblastic leukemia (ALL). As part of a systematic effort to accurately determine the genomic sequence of Jak3 gene in normal and leukemic B-cell precursors, we sequenced a relatively short region of Jak3 spanning two introns, originally termed introns 10 and 11. This genomic sequence appeared in certain RT-PCR products from our analysis of Jak3 gene expression in pediatric, as well as infant, primary ALL cells. Unexpectedly, a gap in the original Jak3 genomic sequence was found in intron 10 across the sequence matching to an Alu element. Furthermore, the sequence obtained from intron 11 did not match at all to that previously reported, and the length of the intron was much larger than expected at 1.1 kb. Homology to Alu elements (three regions, 699 bp total) and a LINE2 element (one region, 189 bp total) were seen across the entire region covering exons 10-12 (2.1 kb total). Two potential single nucleotide polymorphisms (SNPs) were observed in intron 11. No apparent genomic mutation was found across this region in leukemic B-cell precursors from any of the ALL patients examined. This newly described sequence corrects the previous published genomic sequence from this region rather than identifying an insertion or translocation specific to these ALL cases. Our results significantly extend previous efforts to determine the genomic sequence of Jak3 and analyze its expression in childhood pro-B ALL and other forms of ALL.

Carbohydrate recognition site of interleukin-2 in relation to cell proliferation

Interleukin-2 (IL-2) is a cytokine with important roles in the immune system. IL-2 initially binds a high mannose-type glycan and a specific peptide sequence of the IL-2 receptor alpha-subunit and sequentially forms a high affinity complex of IL-2.IL-2 receptor alpha-, beta-, and gamma-subunits. This formation induces cellular signaling and cell proliferation (Fukushima, K., and Yamashita, K. (2001) J. Biol. Chem. 276, 7351-7356). To determine the carbohydrate-binding site of IL-2, we prepared wild-type and point-mutated (35)S-IL-2 by an in vitro transcription and translation method. We found that wild-type (35)S-IL-2 tends to form a dimer spontaneously, and the dimeric form has both carbohydrate recognition activity and cell proliferation activity. Moreover, substitution of Asn-26 in IL-2 with Gln or Asp conserved the dimeric form and affected the carbohydrate recognition activities in correspondence with the cell proliferation activities, suggesting that Asn-26 in IL-2 is involved in the carbohydrate recognition site. These results suggest that the carbohydrate recognition of IL-2 dimer triggers formation of high affinity complex (IL-2.IL-2Ralpha, -beta, -gamma)(2), and the hetero-octamer stimulates IL-2-dependent T-cell proliferation by intensifying cellular signaling.

Cutting edge: the common gamma-chain is an indispensable subunit of the IL-21 receptor complex

The common gamma-chain (gamma(c)) is an indispensable subunit of the functional receptor complexes for IL-4, IL-7, IL-9, and IL-15 as well as IL-2. Here we show that the gamma(c) is also shared with the IL-21R complex. Although IL-21 binds to the IL-21R expressed on gamma(c)-deficient ED40515(-) cells, IL-21 is unable to transduce any intracytoplasmic signals. However, in EDgamma-16 cells, a gamma(c)-transfected ED40515(-) cell line, IL-21 binds to the IL-21R and can activate Janus kinase (JAK)1, JAK3, STAT1, and STAT3. The chemical cross-linking study reveals the direct binding of IL-21 to the gamma(c). These data clearly demonstrate that the gamma(c) is an indispensable subunit of the functional IL-21R complex.

Differential requirement of the cytoplasmic subregions of gamma c chain in T cell development and function

The common cytokine receptor gamma chain (gammac), a shared component of the receptors for IL-2, IL-4, IL-7, IL-9, and IL-15, is critical for the development and function of lymphocytes. The cytoplasmic domain of gammac consists of 85 aa, in which the carboxyl-terminal 48 aa are essential for its interaction with and activation of the Janus kinase, Jak3. Evidence has been provided that Jak3-independent signals might be transmitted via the residual membrane-proximal region; however, its role in vivo remains totally unknown. In the present study, we expressed mutant forms of gammac, which lack either most of the cytoplasmic domain or only the membrane-distal Jak3-binding region, on a gammac null background. We demonstrate that, unlike gammac or Jak3 null mice, expression of the latter, but not the former mutant, restores T lymphopoiesis in vivo, accompanied by strong expression of Bcl-2. On the other hand, the in vitro functions of the restored T cells still remained impaired. These results not only reveal the hitherto unknown role of the gammac membrane-proximal region, but also suggest the differential requirement of the cytoplasmic subregions of gammac in T cell development and function.

Constitutive expression of a chimeric receptor that delivers IL-2/IL-15 signals allows antigen-independent proliferation of CD8+CD44high but not other T cells

We have prepared transgenic mice whose T cells constitutively express a chimeric receptor combining extracellular human IL-4R and intracellular IL-2Rbeta segments. This receptor can transmit IL-2/IL-15-like signals in response to human, but not mouse, IL-4. We used these animals to explore to what extent functional IL-2R/IL-15R expression controls the capacity of T cells to proliferate in response to IL-2/IL-15-like signals. After activation with Con A, naive transgenic CD8+ and CD4+ T cells respond to human IL-4 as well as to IL-2. Without prior activation, they failed to proliferate in response to human IL-4, although human IL-4 did prolong their survival. Thus, IL-2-induced proliferation of activated T cells requires at least one other Ag-induced change apart from the induction of a functional IL-2R. However, a fraction of CD8+CD44high T cells proliferate in human IL-4 without antigenic stimulation or syngeneic feeder cells. In contrast, CD4+CD44high T cells are not constitutively responsive to human IL-4. We conclude that although all transgenic T cells express a functional chimeric receptor, only some CD8+CD44high T cells contain all molecules required for entry into the cell cycle in response to human IL-4 or IL-15.

A novel protein MAJN binds to Jak3 and inhibits apoptosis induced by IL-2 deprival

To find a possible signal interacting with the Jak3 N-terminal, we screened the human peripheral blood cDNA library through both a two-hybrid system and a tyrosine-phosphorylation-modified two-hybrid system using the N-terminal of Jak3 as bait. Results showed that one new homologue of myosin heavy chain, designated MAJN (molecule associated with Jak3 N-terminal), could bind to Jak3 in a tyrosine-phosphorylation-independent manner. The interaction between Jak3 and MAJN was further confirmed by immunoprecipitation in BAF-B03 beta cells. To investigate the function of MAJN, we have constructed the BAF-B03 beta/MAJN cell line that stably expresses MAJN and found that overexpression of MAJN can partially inhibit the apoptosis induced by interleukin-2 deprival. Further studies are needed to elucidate how MAJN executes its function to antagonize BAF-B03beta cell death in the absence of IL-2.

Possible involvement of a novel STAM-associated molecule "AMSH" in intracellular signal transduction mediated by cytokines

STAM containing an SH3 (Src homology 3) domain and an immunoreceptor tyrosine-based activation motif was previously revealed to be implicated in signaling pathways immediately downstream of Jak2 and Jak3 tyrosine kinases associated with cytokine receptors. We molecularly cloned a novel molecule interacting with the SH3 domain of STAM, which was named AMSH (associated molecule with the SH3 domain of STAM). AMSH contains a putative bipartite nuclear localization signal and a homologous region of a c-Jun activation domain-binding protein 1 (JAB1) subdomain in addition to a binding site for the SH3 domain of STAM. AMSH mutant deleted of the C-terminal half conferred dominant negative effects on signaling for DNA synthesis and c-myc induction mediated by interleukin 2 and granulocyte macrophage-colony-stimulating factor. These results suggest that AMSH plays a critical role in the cytokine-mediated intracellular signal transduction downstream of the Jak2/Jak3.STAM complex.

Critical role of the membrane-proximal, proline-rich motif of the interleukin-2 receptor gammac chain in the Jak3-independent signal transduction

BACKGROUND

The interleukin-2 receptor (IL-2R) consists of three subunits, the IL-2Ralpha, IL-2Rbetac, and IL-2Rgammac chains. The essential role of the IL-2Rgammac cytoplasmic domain, consisting of 86 amino acids, in signal transmission has been well documented. Particularly, the carboxyl ter-minal region containing 48 amino acids. is essential for the association with and activation of the Jak3 protein tyrosine kinase. On the other hand, little is known about the role of the rest of the IL-2Rgammac cytoplasmic region consisting of the membrane-proximal 38 amino acids.

RESULTS

We show that a truncated mutant form of IL-2Rgammac which lacks the membrane-distal 48 amino acids is still capable of inducing the activation of Jak1 and Stat3/Stat5 in the absence of Jak3 activation. This membrane-proximal region can also mediate the IL-2-induced tyrosine phosphorylation of the p85 subunit of phosphatidylinositol-3-kinase (PI3K). Furthermore, these signalling events are completely abrogated when mutations are introduced into the proline-rich motif in this region.

CONCLUSIONS

In this study, we identified a Jak3-independent signalling pathway(s) from the membrane-proximal region of IL-2Rgammac. Our results indicate that the proline-rich motif in this region plays a critical role in this signalling pathway(s). The present study may provide further insight into the mechanism of cellular responses mediated by IL-2 and other cytokines which utilize the IL-2Rgammac for their signal transmission.

Downregulation of JAK3 Protein Levels in T Lymphocytes by Prostaglandin E2 and Other Cyclic Adenosine Monophosphate-Elevating Agents: Impact on Interleukin-2 Receptor Signaling Pathway

The Janus kinase, JAK3 plays an important role in interleukin-2 (IL-2)–dependent signal transduction and proliferation of T lymphocytes. Our findings show that prostaglandin E2(PGE2) can inhibit upregulation of JAK3 protein in naive T cells and can downregulate its expression in primed cells. Reduction in JAK3 was selective because expression of other tyrosine kinases (JAK1, p56lck, and p59fyn) and signal transducer and activator of transcription (STAT)5, which are linked to IL-2 receptor (IL-2R) signaling pathway, were not affected. Inhibition of JAK3 may be controlled by intracellular cyclic adenosine monophosphate (cAMP) levels, as forskolin, a direct activator of adenylate cyclase and dibutyryl cAMP (dbcAMP), a membrane permeable analogue of cAMP suppressed JAK3 expression. Moreover, 3-isobutyl-1-methylxanthine (IBMX), an inhibitor of cAMP phosphodiesterase, potentiated PGE2-induced suppression of JAK3. In naive T cells, but not primed T cells, PGE2 and other cAMP elevating agents also caused a modest reduction in surface expression of the common gamma chain (γc) that associates with JAK3. The absence of JAK3, but not IL-2R in T cells correlated with impaired IL-2–dependent signal transduction and proliferation. The alteration in IL-2 signaling included decreased tyrosine phosphorylation and DNA binding activity of STAT5 and poor induction of the c-Myc and c-Jun pathways. In contrast, IL-2–dependent induction of Bcl-2 was unaffected. These findings suggest that suppression of JAK3 levels may represent one mechanism by which PGE2 and other cAMP elevating agents can inhibit T-cell proliferation.

Downregulation of JAK3 Protein Levels in T Lymphocytes by Prostaglandin E2 and Other Cyclic Adenosine Monophosphate-Elevating Agents: Impact on Interleukin-2 Receptor Signaling Pathway

The Janus kinase, JAK3 plays an important role in interleukin-2 (IL-2)–dependent signal transduction and proliferation of T lymphocytes. Our findings show that prostaglandin E2(PGE2) can inhibit upregulation of JAK3 protein in naive T cells and can downregulate its expression in primed cells. Reduction in JAK3 was selective because expression of other tyrosine kinases (JAK1, p56lck, and p59fyn) and signal transducer and activator of transcription (STAT)5, which are linked to IL-2 receptor (IL-2R) signaling pathway, were not affected. Inhibition of JAK3 may be controlled by intracellular cyclic adenosine monophosphate (cAMP) levels, as forskolin, a direct activator of adenylate cyclase and dibutyryl cAMP (dbcAMP), a membrane permeable analogue of cAMP suppressed JAK3 expression. Moreover, 3-isobutyl-1-methylxanthine (IBMX), an inhibitor of cAMP phosphodiesterase, potentiated PGE2-induced suppression of JAK3. In naive T cells, but not primed T cells, PGE2 and other cAMP elevating agents also caused a modest reduction in surface expression of the common gamma chain (γc) that associates with JAK3. The absence of JAK3, but not IL-2R in T cells correlated with impaired IL-2–dependent signal transduction and proliferation. The alteration in IL-2 signaling included decreased tyrosine phosphorylation and DNA binding activity of STAT5 and poor induction of the c-Myc and c-Jun pathways. In contrast, IL-2–dependent induction of Bcl-2 was unaffected. These findings suggest that suppression of JAK3 levels may represent one mechanism by which PGE2 and other cAMP elevating agents can inhibit T-cell proliferation.

Interleukin-7: physiological roles and mechanisms of action

Interleukin-7 (IL-7), a product of stromal cells, provides critical signals to lymphoid cells at early stages in their development. Two types of cellular responses to IL-7 have been identified in lymphoid progenitors: (1) a trophic effect and (2) an effect supporting V(D)J recombination. The IL-7 receptor is comprised of two chains, IL-7R alpha and gamma(c). Following receptor crosslinking, rapid activation of several classes of kinases occurs, including members of the Janus and Src families and PI3-kinase. A number of transcription factors are subsequently activated including STATs, c-myc, NFAT and AP-1. However, it remains to be determined which, if any, previously identified pathway leads to the trophic or V(D)J endpoints. The trophic response to IL-7 involves protecting lymphoid progenitors from a death process that resembles apoptosis. This protection is partly mediated by IL-7 induction of Bcl-2, however other IL-7-induced events are probably also involved in the trophic response. The V(D)J response to IL-7 is partly mediated through increased production of Rag proteins (which cleave the target locus) and partly by increasing the accessibility of a target locus to cleavage through chromatin remodeling.

Urokinase is required for T lymphocyte proliferation and activation in vitro

We have previously demonstrated that urokinase-deficient (uPA-/-) mice do not increase lung T lymphocyte number and fail to mount protective immune responses during pulmonary Cryptococcus neoformans infection. These observations suggest a previously unconsidered role for urokinase-type plasminogen activator (uPA) in T lymphocyte-mediated immune responses. Accordingly, we sought to determine whether uPA is required for T cell receptor-mediated (TCR-mediated) lymphocyte proliferation and activation. Splenocytes from uPA-/- and uPA+/+ mice were stimulated with concanavalin A (Con A). The uPA-/- mice had diminished T cell proliferation as compared with uPA+/+ mice. Coculturing uPA-/- T cells with uPA+/+ accessory cells led to the restoration of proliferation. Similarly, T cell proliferation induced by CD3 cross-linking was diminished in uPA-/- mice as compared with uPA+/+ mice. T lymphocyte activation, defined as the induced expression of antigens and the elaboration of cytokines, was determined. The expression of CD69 and that of CD49d were diminished in response to Con A stimulation in uPA-/- mice as compared with uPA+/+ mice. The elaboration of cytokines in response to Con A was also altered in the uPA-/- mice. The production of the Th1 cytokines interferon-gamma and interleukin-12 was diminished in uPA-/- mice as compared with uPA+/+ mice. The uPA-/- mice produced increased amounts of interleukin-10, a Th2 cytokine. We conclude that the lack of uPA results in impaired T cell activation and proliferation in response to TCR-mediated signaling and the expression of a less Th1-polarized profile of cytokines. These findings suggest that the inability of uPA-/- mice to combat Cryptococcus neoformans infection may be caused by the impairment of T lymphocyte immune responses in the absence of uPA.

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

Role of interleukin-7 in T-cell development from hematopoietic stem cells

All lymphocytes are derived from hematopoietic stem cells (HSC). The interleukin-7 receptor (IL-7R) transduces non-redundant signals for both T and B-cell development from HSC. The upregulation of the IL-7R occurs at the stage of the clonogenic common lymphoid progenitor, a recently identified population that can give rise to all lymphoid lineages (T, B and natural killer cells) at a single cell level. The IL-7R plays a critical role in the rearrangement of immunoglobulin heavy chain genes required for B-cell development. IL-7R expression is critically regulated in developing thymocytes; thymocytes that fail the positive selection process downregulate the IL-7R, but those undergoing positive selection upregulate or maintain IL-7R expression. Recent data indicate that IL-7 signaling enhances the survival of developing thymocytes and mature T cells, presumably by its upregulating Bcl-2. Detailed analysis of the signaling cascades activated by the IL-7R may help to reveal the differential roles of IL-7 signaling in T and B-cell development.

Macrophage-Derived Nitric Oxide Regulates T Cell Activation via Reversible Disruption of the Jak3/STAT5 Signaling Pathway

Nitric oxide (NO) has been invoked as an important pathogenic factor in a wide range of immunologically mediated diseases. The present study demonstrates that macrophage-derived NO may conversely function to fine tune T cell-mediated inflammation via reversible dephosphorylation of intracellular signaling molecules, which are involved in the control of T cell proliferation. Thus, T cells activated in the presence of alveolar macrophages are unable to proliferate despite expression of IL-2R and secretion of IL-2. This process is reproduced by the NO generator S-nitroso-N-acetylpenicillamine and is inhibitable by the NO synthase inhibitor NG-methyl-l-arginine. Analysis of T cell lysates by immunoprecipitation with specific Abs and subsequent immunoblotting indicated marked reduction of tyrosine phosphorylation of Jak3 and STAT5 mediated by NO. Further studies indicated that NO-mediated T cell suppression was reversible by the guanylate cyclase inhibitors methylene blue and LY-83583 and was reproduced by a cell-permeable analogue of cyclic GMP, implicating guanylate cyclase activation as a key step in the inhibition of T cell activation by NO.

STAM, signal transducing adaptor molecule, is associated with Janus kinases and involved in signaling for cell growth and c-myc induction

We previously identified a putative signal transducing adaptor molecule, named STAM, that contains an Src homology 3 (SH3) domain and immunoreceptor tyrosine-based activation motif (ITAM). In this report, we demonstrate the functional significance of STAM in cytokine-mediated signal transduction. STAM is associated with Jak3 and Jak2 tyrosine kinases via its ITAM region and phosphorylated by Jak3 and Jak2 upon stimulation with IL-2 and GM-CSF, respectively. An SH3 deletion mutant of STAM confers a dominant-negative effect on DNA synthesis mediated by IL-2 and GM-CSF. Furthermore, the wild-type STAM, but not STAM mutants deleted of SH3 and ITAM, significantly enhances c-myc induction mediated by IL-2 and GM-CSF. These results strongly implicate STAM in the signaling pathways for cell growth and c-myc induction immediately downstream of the Jaks associated with the cytokine receptors.

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.

CD40-mediated regulation of interleukin-4 signaling pathways in B lymphocytes

The importance of cytokines in controlling immunoglobulin isotype switching is well known. Given the defect in switching to IgG, IgA and IgE isotypes in mice and humans that carry mutations in the CD40 and CD40 ligand genes, we have investigated the role of CD40 ligation in controlling B cell responses to interleukin (IL)-4. We have found that CD40-mediated signals cause a fivefold upregulation of IL-4 receptor (IL-4R) on the B cell surface and that this is associated with a 100-1000-fold increase in the cells' responsiveness to the cytokine. While we found no evidence of increased affinity or structural change of the receptor, we do find that prestimulation of B cells with anti-CD40 antibodies brings about several changes in the IL-4 signaling pathways. Subsequent delivery of IL-4 to CD40-prestimulated cells provokes intracellular signals distinct from those induced in resting B cells in response to IL-4. While resting B cells phosphorylate Jak3 kinase shortly after IL-4 activation, cells pre-incubated with anti-CD40 exhibit active dephosphorylation of this molecule and phosphorylation of proteins of around 45 kDa upon addition of IL-4. The common gamma chain, Jak3 and Jak1 can all be immunoprecipitated in normal amounts with the IL-4R chain after CD40 prestimulation. We show that the observed dephosphorylation of Jak3 may be due to a stable association with the src-homology protein tyrosine phosphatase SH-PTP2. In contrast, the enzyme appears to be inactive and to dissociate very quickly from the signaling complex in cells that are stimulated with IL-4 alone.

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.

Interferon-independent activation of (2′-5′) oligoadenylate synthetase in Friend erythroleukemia cell variants exposed to HMBA

To provide evidence for the implication of interferon (IFN)-induced proteins in the regulation of cell growth during differentiation, the activation of (2′-5′) oligoadenylate synthetase (2-5A synthetase) as well as of PKR, two IFN-induced proteins, during differentiation of Friend erythroleukemia cells, was studied. Two cell variants were used. The first (FL) was completely susceptible to hexamethylene bis-acetamide (HMBA)-treatment and responded in both growth-retardation and hemoglobin synthesis. The second (R1) failed to synthesize hemoglobin in response to HMBA although cell growth was still inhibited. In both cell variants, 2–5A synthetase enzyme activity was induced in a similar fashion, reaching a peak at 26 hours after treatment with HMBA. However, the down regulation of activity thereafter was not identical in both cases. In R1 cells, the reduction was much slower compared to FL cells. A similar pattern was observed with the appearance of the 43 kDa isoform of 2–5A synthetase in immunoblots. An analysis of 2–5A synthetase gene expression revealed the presence of 1.7 kb transcripts which peaked at 16 hours after HMBA-treatment in both cell variants. Again, the down-regulation in expression was slower in R1 than in FL cells. Addition of anti-murin alpha/beta-IFN antibodies did not reduce the level of either 2–5A synthetase expression or enzyme activity in either cell variant. Interestingly, the presence of antibodies also did not affect the pattern of pRb phosphorylation in the cell variants exposed to HMBA. In both cell variants, an increase in the amount of the phosphorylated form (ppRb) was observed in immunoblots after 4 hours. This form was gradually transformed to the underphosphorylated molecule (pRb) with time in culture, even in the presence of antibodies. This further substantiates the notion that IFN-induced regulation of pRb phosphorylation is mediated by IFN-induced proteins. The basal level of either expression or ezymatic activity of PKR detected in untreated FL or R1 cells, was relatively high. Treatment with HMBA did not result in further induction of PKR in either cell variant.

Dispensability of Jak1 tyrosine kinase for interleukin-2-induced cell growth signaling in a human T cell line

The tyrosine kinases Jak1 and Jak3 are known to be associated with the beta and gamma chains of interleukin-2 receptor (IL-2R). They are activated by stimulation with IL-2, IL-4, IL-7, IL-9, or IL-15, receptors of which share the gamma chain of the IL-2R. We have obtained direct evidence of Jak1 association with the alpha chains of receptors for IL-4, IL-7 and IL-9 and with the beta chain of IL-2R, which is also common to the IL-15R. Furthermore, we have prepared mutant IL-2R beta chains with a mutation in the box 1 region, which is conserved among the IL-2R beta chain and the alpha chains of the other cytokine receptors sharing the IL-2R gamma chain. Using MOLT-4 transfectants with the mutant beta chains, we found that two conserved proline residues within the box 1 region are essentially involved in association with Jak1. The MOLT-4 transfectants with the mutant beta chains lacking Jak1 association showed IL-2 responsiveness, in terms of activation of Jak3 and Stat5 and induction of cell growth, indicating that Jak1 is dispensable for IL-2-mediated cell growth signaling and that Jak1 activation is not required for activation of Jak3 and Stat5 in the MOLT-4 transfectants.

The interleukin-2 receptor gamma chain: its role in the multiple cytokine receptor complexes and T cell development in XSCID

Interleukin 2 (IL-2), a T cell-derived cytokine, targets a variety of cells to induce their growth, differentiation, and functional activation. IL-2 inserts signals into the cells through IL-2 receptors expressed on cell surfaces to induce such actions. In humans, the functional IL-2 receptor consists of the subunit complexes of the alpha, beta and gamma chains, or the beta and gamma chains. The third component, the gamma chain, of IL-2 receptor plays a pivotal role in formation of the full-fledged IL-2 receptor, together with the beta chain, the gamma chain participates in increasing the IL-2 binding affinity and intracellular signal transduction. Moreover, the cytokine receptors for at least IL-2, IL-4, IL-7, IL-9, and IL-15 utilize the same gamma chain as an essential subunit. Interestingly, mutations of the gamma chain gene cause human X-linked severe combined immunodeficiency (XSCID) characterized by a complete or profound T cell defect. Among the cytokines sharing the gamma chain, at least IL-7 is essentially involved in early T cell development in the mouse organ culture system. The molecular identification of the gamma chain brought a grasp of the structures and functions of the cytokine receptor and an in-depth understanding of the cause of human XSCID. To investigate the mechanism of XSCID and development of gene therapy for XSCID, knockout mice for the gamma chain gene were produced that showed similar but not exactly the same phenotypes as human XSCID.

Signal transduction pathway of interleukin-4 and interleukin-13 in human B cells derived from X-linked severe combined immunodeficiency patients

Interleukin-4 (IL-4) and IL-13 are functionally similar cytokines. The functional IL-4 receptor (IL-4R) consists of the IL-4R alpha chain (IL-4R alpha) and the IL-2R gamma chain (gamma c), which is shared by the IL-2, IL-7, IL-9, and IL-15 receptors. The functional IL-13R is thought to involve the IL-4R alpha but not gamma c. In this study, we have analyzed activation of members of the Janus tyrosine kinase (Jak) family and signal transducers and activators of transcription (STAT) 6 induced by IL-4 and IL-13 in Epstein-Barr virus-transformed B cells derived from two patients of X-linked severe combined immunodeficiency, who have mutations of the gamma c gene in the extracellular and intracellular domains. In these B cells, IL-4 failed to induce tyrosine phosphorylation of Jak3 and activation of STAT6, or activation of these molecules was significantly decreased compared with Epstein-Barr virus-transformed normal B cells. In contrast, IL-13 activated STAT6 in these cells as well as normal B cells. However, Jak3 was not activated by IL-13, even in normal B cells. These results clearly indicated that gamma c is essential for activation of Jak3 and STAT6 in the signal transduction pathway of IL-4 in human B cells and that IL-13 does not utilize gamma c but activates STAT6 through an alternative pathway, which is not impaired in B cells of X-linked severe combined immunodeficiency patients.

A membrane-proximal region of the interleukin-2 receptor gamma c chain sufficient for Jak kinase activation and induction of proliferation in T cells

The interleukin-2 (IL-2) receptor (IL-2R) consists of three distinct subunits (alpha, beta, and gamma c) and regulates proliferation of T lymphocytes. Intracellular signalling results from ligand-mediated heterodimerization of the cytoplasmic domains of the beta and gamma c chains. To identify the residues of gamma c critical to this process, mutations were introduced into the cytoplasmic domain, and the effects on signalling were analyzed in the IL-2-dependent T-cell line CTLL2 and T-helper clone D10, using chimeric IL-2R chains that bind and are activated by granulocyte-macrophage colony-stimulating factor. Whereas previous studies of fibroblasts and transformed T cells have suggested that signalling by gamma c requires both membrane-proximal and C-terminal subdomains, our results for IL-2-dependent T cells demonstrate that the membrane-proximal 52 amino acids are sufficient to mediate a normal proliferative response, including induction of the proto-oncogenes c-myc and c-fos. Although gamma c is phosphorylated on tyrosine upon receptor activation and could potentially interact with downstream molecules containing SH2 domains, cytoplasmic tyrosine residues were dispensable for mitogenic signalling. However, deletion of a membrane-proximal region conserved among other cytokine receptors (cytoplasmic residues 5 to 37) or an adjacent region unique to gamma c (residues 40 to 52) abrogated functional interaction of the receptor chain with the tyrosine kinase Jak3. This correlated with a loss of all signalling events analyzed, including phosphorylation of the IL-2R beta-associated kinase Jak1, expression of c-myc and c-fos, and induction of the proliferative response. Thus, it appears in T cells that Jak3 is a critical mediator of mitogenic signaling by the gamma c chain.

Molecular mechanisms regulating the hyaluronan binding activity of the adhesion protein CD44

In the present study, we describe the isolation and characterization of a cDNA clone designated B6F1.3, that appears to 'activate' the hyaluronan-binding capacity of CD44 upon transfection into the murine fibroblastoid cell line MOP8. Sequence analysis indicates that the putative regulatory molecule encoded by this clone is identical to the murine interleukin-2 receptor gamma chain (mIL-2R gamma), a recently described type 1 transmembrane protein that constitutes an integral component of the cell surface receptors that bind a number of cytokines including IL-2, IL-4, IL-7, IL-9, IL-15 and perhaps also IL-13. Mutations in this molecule have been shown to be responsible for X-linked severe combined immunodeficiency (XSCID) in humans. With the exception of bone marrow, the mIL-2R gamma chain was found to be expressed at high levels on all hemopoietic cell lines and tissue types examined. Non-hemopoietic tissues are generally negative. FACS analysis and Western blot analysis indicated respectively that B6F1.3 does not mediate its effects by upregulating the expression of CD44 or by altering the alternative splicing of the molecule. Removal of the cytoplasmic tail of the mIL-2R gamma chain, including a Src homology region 2 (SH2) subdomain, abolished its ability to enhance CD44-mediated binding to hyaluronan suggesting the involvement of signal transduction events triggered via the cytoplasmic domain in the 'activation' process. Determining whether activating molecules such as B6F1.3 are co-expressed within tumor cells may help improve the potential value of CD44 as a diagnostic marker of metastatic disease.

Differences in the interleukin-2 (IL-2) receptor system in human and mouse: alpha chain is required for formation of the functional mouse IL-2 receptor

Reconstitution with mouse interleukin-2 (IL-2) receptor subunits demonstrated that the mouse IL-2 receptor complex was different from the human complex in the alpha chain requirement for the functional mouse receptor complex. The heterotrimeric complex of the mouse exogenous alpha and beta chains and the endogenous gamma chain on mouse lymphoid BW5147 cells showed the ability to bind IL-2 with high affinity, resulting in IL-2-induced tyrosine phosphorylation of a cytosolic tyrosine kinase, JAK3, which is involved in IL-2-dependent signals. Exogenous introduction of the beta chain with the endogenous gamma chain, however, could neither confer appreciable IL-2 binding nor IL-2-induced signal transduction on BW5147 cells, unlike the human beta gamma heterodimer. Mouse spleen CD8+ cells, not having the alpha chain initially, showed IL-2-dependent cell proliferation only when expression of the alpha chain was induced. Collectively, these results illustrate that the functional mouse IL-2 receptor complex necessarily includes the alpha chain, and that the regulation of CD8+ T cell growth during immune reaction depends upon alpha chain expression.

Critical role of the interleukin 2 (IL-2) receptor gamma-chain-associated Jak3 in the IL-2-induced c-fos and c-myc, but not bcl-2, gene induction

The interleukin 2 receptor (IL-2R) consists of three subunits, the IL-2R alpha, IL-2R beta c, and IL-2R gamma c chains. Two Janus family protein tyrosine kinases (PTKs), Jak1 and Jak3, were shown to associate with IL-2R beta c and IL-2R gamma c, respectively, and their PTK activities are increased after IL-2 stimulation. A Jak3 mutant with truncation of the C-terminal PTK domain lacks its intrinsic kinase activity but can still associate with IL-2R gamma c. In a hematopoietic cell line, F7, that responds to either IL-2 or IL-3, overexpression of this Jak3 mutant results in selective inhibition of the IL-2-induced activation of Jak1/Jak3 PTKs and of cell proliferation. Of the three target nuclear protooncogenes of the IL-2 signaling, c-fos and c-myc genes, but not the bcl-2 gene, were found to be impaired. On the other hand, overexpression of the dominant negative form of the IL-2R gamma c chain, which lacks most of its cytoplasmic domain, in F7 cells resulted in the inhibition of all three protooncogenes. These results provide a further molecular basis for the critical role of Jak3 in IL-2 signaling and also suggest a Jak PTK-independent signaling pathway(s) for the bcl-2 gene induction by IL-2R.

Activation of JAK3, but not JAK1, is critical to interleukin-4 (IL4) stimulated proliferation and requires a membrane-proximal region of IL4 receptor alpha

The tyrosine kinases JAK1 and JAK3 have been shown to undergo tyrosine phosphorylation in response to interleukin-2 (IL), IL4, IL7, and IL9, cytokines which share the common IL2 receptor gamma-chain (IL2R gamma), and evidence has been found for a preferential coupling of JAK3 to IL2R gamma and JAK1 to IL2R beta. Here we show, using human premyeloid TF-1 cells, that IL4 stimulates JAK3 to a larger extent than JAK1, based upon three different evaluation criteria. These include a more vigorous tyrosine phosphorylation of JAK3 as measured by anti-phosphotyrosine immunoblotting, a more marked activation of JAK3 as determined by in vitro tyrosine kinase assays and a more manifest presence of JAK3 in activated IL4-receptor complexes. These observations suggest that IL4 receptor signal transduction does not depend on equimolar heterodimerization of JAK1 and JAK3 following IL4-induced heterodimerization of IL4R alpha and IL2R gamma. Indeed, when human IL4R alpha was stably expressed in mouse BA/F3 cells, robust IL4-induced proliferation and JAK3 activation occurred without detectable involvement of JAK1, JAK2, or TYK2. The present study suggests that JAK1 plays a subordinate role in IL4 receptor signaling, and that in certain cells exclusive JAK3 activation may mediate IL4-induced cell growth. Moreover, mutational analysis of human IL4R alpha showed that a membrane-proximal cytoplasmic region was critical for JAK3 activation, while the I4R motif was not, which is compatible with a role of JAK3 upstream of the recruitment of the insulin receptor substrate-1/4PS signaling proteins by IL4 receptors.

Cytokine signaling through nonreceptor protein tyrosine kinases

Cytokines are a family of soluble mediators of cell-to-cell communication that includes interleukins, interferons, and colony-stimulating factors. The characteristic features of cytokines lie in their functional redundancy and pleiotropy. Most of the cytokine receptors that constitute distinct superfamilies do not possess intrinsic protein tyrosine kinase (PTK) domains, yet receptor stimulation usually invokes rapid tyrosine phosphorylation of intracellular proteins, including the receptors themselves. It is now clear that these receptors are capable of recruiting or activating (or both) a variety of nonreceptor PTKs to induce downstream signaling pathways. Thus, the intracytoplasmic structure of cytokine receptors has evolved so as to allow the combined action of different PTK family members expressed in different cell types, which may ultimately determine the activity of cytokines.

Jaks and Stats in signaling by the cytokine receptor superfamily

Many cytokines mediate their biological effects through interaction with a distinct family of receptors termed the cytokine receptor superfamily. Although members of this family lack catalytic domains, they couple ligand binding to tyrosine phosphorylation. Recent studies have shown that a novel family of cytoplasmic protein tyrosine kinases, termed the Janus kinases (Jaks), associate with the cytokine receptors and are catalytically activated after ligand binding. The activated Jaks phosphorylate and activate members of a novel family of transcription factors termed signal transducers and activators of transcription (Stats). In addition, many cytokines induce the phosphorylation of SHC, Vav and the p85 subunit of PI-3 kinase. The region of the receptors proximal to the cytoplasmic membrane is required for Jak association, mitogenesis, Stat activation and Vav phosphorylation. The membrane-distal region, which contains the major sites of tyrosine phosphorylation, is required for phosphorylation of SHC and p85, not for mitogenesis, thus allowing functional dissection of the signaling pathways activated by cytokines.