Requirement for Tec kinases Rlk and Itk in T cell receptor signaling and immunity

Selective expression rather than specific function of Txk and Itk regulate Th1 and Th2 responses

Itk and Txk/Rlk are Tec family kinases expressed in T cells. Itk is expressed in both Th1 and Th2 cells. By contrast, Txk is preferentially expressed in Th1 cells. Although Itk is required for Th2 responses in vivo and Txk is suggested to regulate IFN-gamma expression and Th1 responses, it remains unclear whether these kinases have distinct roles in Th cell differentiation/function. We demonstrate here that Txk-null CD4(+) T cells are capable of producing both Th1 and Th2 cytokines similar to those produced by wild-type CD4(+) T cells. To further examine whether Itk and Txk play distinct roles in Th cell differentiation and function, we examined Itk-null mice carrying a transgene that expresses Txk at levels similar to the expression of Itk in Th2 cells. Using two Th2 model systems, allergic asthma and schistosome egg-induced lung granulomas, we found that the Txk transgene rescued Th2 cytokine production and all Th2 symptoms without notable enhancement of IFN-gamma expression. These results suggest that Txk is not a specific regulator of Th1 responses. Importantly, they suggest that Itk and Txk exert their effects on Th cell differentiation/function at the level of expression.

Itk kinase inhibitors: initial efforts to improve the metabolical stability and the cell activity of the benzimidazole lead

Previously, we reported a series of novel benzimidazole based Itk inhibitors that exhibited excellent enzymatic potency and selectivity but low microsomal stability. Employing a structure based approach a new series of inhibitors with comparable potency and selectivity to the original series and with a potential for improved microsome stability was identified.

The Tec kinases Itk and Rlk regulate NKT cell maturation, cytokine production, and survival

The Tec kinases Itk and Rlk are required for efficient positive selection of conventional CD4+ and CD8+ T cells in the thymus. In contrast, recent studies have shown that these Tec kinases are dispensable for the development of CD8+ T cells with characteristics of innate T cells. These findings raise questions about the potential role of Itk and Rlk in NKT cell development, because NKT cells represent a subset of innate T cells. To address this issue, we examined invariant NKT cells in Itk-/- and Itk/Rlk-/- mice. We find, as has been reported previously, that Itk-/- mice have reduced numbers of NKT cells with a predominantly immature phenotype. We further show that this defect is greatly exacerbated in the absence of both Itk and Rlk, leading to a 7-fold reduction in invariant NKT cell numbers in the thymus of Itk/Rlk-/- mice and a more severe block in NKT cell maturation. Splenic Itk-/- and Itk/Rlk-/- NKT cells are also functionally defective, because they produce little to no cytokine following in vivo activation. Tec kinase-deficient NKT cells also show enhanced cell death in the spleen. These defects correlate with greatly diminished expression of CD122, the IL-2R/IL-15R beta-chain, and impaired expression of the T-box transcription factor, T-bet. These data indicate that the Tec kinases Itk and Rlk provide important signals for terminal maturation, efficient cytokine production, and peripheral survival of NKT cells.

T-cell-targeted signaling inhibitors

Recent advances in our understanding of the mechanisms of T-cell activation, migration to inflammatory sites, and pathologic disease processes triggered the development of a wide variety of T-cell-targeted signaling inhibitors, which have different targets and modes of action. Depending on the distribution and the role of targets in disease processes, T-cell inhibitors exhibit different levels of efficacy and potential side effects. This review outlines target molecules to which T-cell inhibitors have been developed, their efficacy, and potential safety concerns of T-cell inhibitors.

The Tec family kinase, IL-2-inducible T cell kinase, differentially controls mast cell responses

The Tec family tyrosine kinase, IL-2-inducible T cell kinase (Itk), is expressed in T cells and mast cells. Mice lacking Itk exhibit impaired Th2 cytokine secretion; however, they have increased circulating serum IgE, but exhibit few immunological symptoms of allergic airway responses. We have examined the role of Itk in mast cell function and FcepsilonRI signaling. We report in this study that Itk null mice have reduced allergen/IgE-induced histamine release, as well as early airway hyperresponsiveness in vivo. This is due to the increased levels of IgE in the serum of these mice, because the transfer of Itk null bone marrow-derived cultured mast cells into mast cell-deficient W/W(v) animals is able to fully rescue histamine release in the W/W(v) mice. Further analysis of Itk null bone marrow-derived cultured mast cells in vitro revealed that whereas they have normal degranulation responses, they secrete elevated levels of cytokines, including IL-13 and TNF-alpha, particularly in response to unliganded IgE. Analysis of biochemical events downstream of the FcepsilonRI revealed little difference in overall tyrosine phosphorylation of specific substrates or calcium responses; however, these cells express elevated levels of NFAT, which was largely nuclear. Our results suggest that the reduced mast cell response in vivo in Itk null mice is due to elevated levels of IgE in these mice. Our results also suggest that Itk differentially modulates mast cell degranulation and cytokine production in part by regulating expression and activation of NFAT proteins in these cells.

Preferential development of CD4 and CD8 T regulatory cells in RasGRP1-deficient mice

RasGRP1 and Sos are two Ras-guanyl-nucleotide exchange factors that link TCR signal transduction to Ras and MAPK activation. Recent studies demonstrate positive selection of developing thymocytes is crucially dependent on RasGRP1, whereas negative selection of autoreactive thymocytes appears to be RasGRP1 independent. However, the role of RasGRP1 in T regulatory (Treg) cell development and function is unknown. In this study, we characterized the development and function of CD4(+)CD25(+)Foxp3(+) and CD8(+)CD44(high)CD122(+) Treg lineages in RasGRP1(-/-) mice. Despite impaired CD4 Treg cell development in the thymus, the periphery of RasGRP1(-/-) mice contained significantly increased frequencies of CD4(+)Foxp3(+) Treg cells that possessed a more activated cell surface phenotype. Furthermore, on a per cell basis, CD4(+)Foxp3(+) Treg cells from mutant mice are more suppressive than their wild-type counterparts. Our data also suggest that the lymphopenic environment in the mutant mice plays a dominant role of favored peripheral development of CD4 Treg cells. These studies suggest that whereas RasGRP1 is crucial for the intrathymic development of CD4 Treg cells, it is not required for their peripheral expansion and function. By contrast to CD4(+)CD25(+)Foxp3(+) T cells, intrathymic development of CD8(+)CD44(high)CD122(+) Treg cells is unaffected by the RasGRP1(-/-) mutation. Moreover, RasGRP1(-/-) mice contained greater numbers of CD8(+)CD44(high)CD122(+) T cells in the spleen, relative to wild-type mice. Activated CD8 Treg cells from RasGRP1(-/-) mice retained their ability to synthesize IL-10 and suppress the proliferation of wild-type CD8(+)CD122(-) T cells, albeit at a much lower efficiency than wild-type CD8 Treg cells.

Naive and innate memory phenotype CD4+ T cells have different requirements for active Itk for their development

The Tec family kinase Itk regulates the development of conventional and innate CD8(+) T cells. However, little is known about the role of Itk in the development of CD4(+) T cell lineages, although the role of Itk in the T cell activation and function is well defined. We show in this study that Itk null mice have increased percentage of CD62L(low)CD44(high) memory phenotype CD4(+) T cells compared with wild-type mice. These cells arise directly in the thymus, express high levels of transcripts for the T-bet and IFN-gamma and are able to produce IFN-gamma directly ex vivo in response to stimulation. Itk deficiency greatly decreases the number of CD4(+) T cells with CD62L(high)CD44(low) naive phenotype, but has no effect on the number of memory phenotype CD4(+) T cells, indicating that the development of memory phenotype CD4(+) T cells is Itk-independent. We further show that the development of the naive phenotype CD4(+) T cells is dependent on active Itk signals and can be rescued by expression of Itk specifically in T cells. Our data also show that Itk is required for functional TCR signaling in these cells, but not for the innate function in response to IL-12/IL-18 or Listeria monocytogenes stimulation. These results indicate that CD62L(high)CD44(low) "naive" CD4(+) and CD62L(low)CD44(high) "innate memory phenotype" CD4(+) T cells may be independent populations that differ in their requirement for Itk signals for development. Our data also suggest that CD4(+)CD62L(low)CD44(high) memory phenotype T cells have innate immune function.

Tumor-induced impairment of TCR signaling results in compromised functionality of tumor-infiltrating regulatory T cells

This study demonstrates, for the first time, that murine regulatory T (Treg) cells in the tumor microenvironment display both enhanced proliferation and reduced functionality. This enhanced proliferation, combined with decreased apoptosis, leads to an intratumoral accumulation of Treg cells with a unique phenotype: CD4(+)CD25(+)FoxP3(+)GITR(high)CD27(low)CD62L(-). The loss of functionality is associated with down-regulation of the TCR signaling complex, including IL-2-inducible T cell kinase. It is also demonstrated that tumor-infiltrating Treg cells have impaired TCR-mediated signaling and calcium influx. Based on these findings, this study supports the hypothesis that 1) tumor-infiltrating Treg cells lose functionality due to their diminished ability to become effectively activated and 2) intratumoral accumulation of Treg cells may compensate for the impaired functionality, thus maintaining immune tolerance to the tumor.

Differential sensitivity to Itk kinase signals for T helper 2 cytokine production and chemokine-mediated migration

Allergic asthma is dependent on chemokine-mediated Th2 cell migration and Th2 cytokine secretion into the lungs. The inducible T cell tyrosine kinase Itk regulates the production of Th2 cytokines as well as migration in response to chemokine gradients. Mice lacking Itk are resistant to developing allergic asthma. However, the role of kinase activity of Itk in the development of this disease is unclear. In addition, whether distinct Itk-derived signals lead to T cell migration and secretion of Th2 cytokines is also unknown. Using transgenic mice specifically lacking Itk kinase activity, we show that active kinase signaling is required for control of Th2 responses and development of allergic asthma. Moreover, dominant suppression of kinase Itk activity led to normal Th2 responses, but significantly reduced chemokine-mediated migration, resulting in prevention of allergic asthma. These observations indicate that signals required for Th2 responses and migration are differentially sensitive to Itk activity. Manipulation of Itk's activity can thus provide a new strategy to treat allergic asthma by differentially affecting migration of T cells into the lungs, leaving Th2 responses intact.

Tyrosine kinases Btk and Tec regulate osteoclast differentiation by linking RANK and ITAM signals

Certain autoimmune diseases result in abnormal bone homeostasis, but association of immunodeficiency with bone is poorly understood. Osteoclasts, which derive from bone marrow cells, are under the control of the immune system. Differentiation of osteoclasts is mainly regulated by signaling pathways activated by RANK and immune receptors linked to ITAM-harboring adaptors. However, it is unclear how the two signals merge to cooperate in osteoclast differentiation. Here we report that mice lacking the tyrosine kinases Btk and Tec show severe osteopetrosis caused by a defect in bone resorption. RANK and ITAM signaling results in formation of a Btk(Tec)/BLNK(SLP-76)-containing complex and PLCgamma-mediated activation of an essential calcium signal. Furthermore, Tec kinase inhibition reduces osteoclastic bone resorption in models of osteoporosis and inflammation-induced bone destruction. Thus, this study reveals the importance of the osteoclastogenic signaling complex composed of tyrosine kinases, which may provide the molecular basis for a new therapeutic strategy.

Complementation in trans of altered thymocyte development in mice expressing mutant forms of the adaptor molecule SLP76

The adaptor protein SLP76 directs signaling downstream of the T cell receptor (TCR) and is essential for thymocyte development. SLP76 contains three N-terminal tyrosines that are critical for its function. To define the role of these residues in thymocyte development, we generated two lines of "knock-in" mice, one expressing a mutation in tyrosine 145 (Y145F) and a second harboring two point mutations at tyrosines 112 and 128 (Y112-128F). We show here that although thymocyte development requires both Y145- and Y112-128-generated signals, selection was more dependent upon Y145. Although several proximal TCR signaling events were defective in both mutant mice, phosphorylation of the guanine nucleotide exchange factor, Vav1, and activation of Itk-dependent pathways were differentially affected by mutations at Y112-128 and Y145, respectively. Analysis of mice expressing one Y145F and one Y112-128F allele revealed that these mutants could complement one another in trans, demonstrating cooperativity between two or more SLP76 molecules. Thus, the N-terminal tyrosines of SLP76 are required for thymocyte selection but can function on separate molecules to support TCR signaling.

Role of Txk, a member of the Tec family of tyrosine kinases, in immune-inflammatory diseases

Txk/Rlk, a member of the Tec family of tyrosine kinases, is an important signaling mediator. We previously reported that human Txk is expressed in Th1/Th0 cells, and Txk translocates from cytoplasm into nuclei upon activation. Txk regulates specifically interferon-gamma gene transcription. Txk, poly(ADP-ribose) polymerase 1, and elongation factor 1alpha make a complex to bind to interferon-gamma gene promoter region-53/-39 (Txk responsive element) to exert positive effects on transcription as a Th1 cell-associated transcription factor. Txk expression is enhanced in rheumatoid arthritis and Behçet's disease, where Th1 dominant immunity occurs. In bronchial asthma and atopic dermatitis, typical Th2 diseases, Txk expression is reduced. Modulation of Txk expression by gene transfer or similar modality may lead to the correction of aberrant immunity and, consequently, disease treatment.

Subtle defects in pre-TCR signaling in the absence of the Tec kinase Itk

alphabeta T cell development in the thymus is dependent on signaling through the TCR. The first of these signals is mediated by the pre-TCR, which is responsible for promoting pre-T cell proliferation and the differentiation of CD4(-)8(-)3(-) (DN) thymocytes into CD4(+)8(+)3(+) (DP) cells. In many cases, T cell signaling proteins known to be essential for TCR signaling in mature T cells are also required for pre-TCR signaling in DN thymocytes. Therefore, it came as a surprise to discover that mice lacking the Tec kinases Itk and Rlk, enzymes required for efficient activation of phospholipase C-gamma1 in mature T cells, showed no obvious defects in pre-TCR-dependent selection events in the thymus. In this report, we demonstrate that DN thymocytes lacking Itk, or Itk and Rlk, are impaired in their ability to generate normal numbers of DP thymocytes, especially when placed in direct competition with WT DN thymocytes. We also show that Itk is required for maximal pre-TCR signaling in DN thymocytes. These data demonstrate that the Tec kinases Itk and Rlk are involved in, but are not essential for, pre-TCR signaling in the thymus, suggesting that there is an alternative mechanism for activating phospholipase C-gamma1 in DN thymocytes that is not operating in DP thymocytes and mature T cells.

Regulation of interferon-gamma during innate and adaptive immune responses

Interferon-gamma (IFN-gamma) is crucial for immunity against intracellular pathogens and for tumor control. However, aberrant IFN-gamma expression has been associated with a number of autoinflammatory and autoimmune diseases. This cytokine is produced predominantly by natural killer (NK) and natural killer T (NKT) cells as part of the innate immune response, and by Th1 CD4 and CD8 cytotoxic T lymphocyte (CTL) effector T cells once antigen-specific immunity develops. Herein, we briefly review the functions of IFN-gamma, the cells that produce it, the cell extrinsic signals that induce its production and influence the differentiation of naïve T cells into IFN-gamma-producing effector T cells, and the signaling pathways and transcription factors that facilitate, induce, or repress production of this cytokine. We then review and discuss recent insights regarding the molecular regulation of IFN-gamma, focusing on work that has led to the identification and characterization of distal regulatory elements and epigenetic modifications with the IFN-gamma locus (Ifng) that govern its expression. The epigenetic modifications and three-dimensional structure of the Ifng locus in naive CD4 T cells, and the modifications they undergo as these cells differentiate into effector T cells, suggest a model whereby the chromatin architecture of Ifng is poised to facilitate either rapid opening or silencing during Th1 or Th2 differentiation, respectively.

Memory phenotype CD8+ T cells with innate function selectively develop in the absence of active Itk

T cells with a memory-like phenotype and possessing innate immune function have been previously identified as CD8(+)CD44(hi) cells. These cells rapidly secrete IFN-gamma upon stimulation with IL-12/IL-18 and are involved in innate responses to infection with Listeria monocytogenes. The signals regulating these cells are unclear. The Tec kinase Itk regulates T cell activation and we report here that a majority of the CD8(+) T cells in Itk null mice have a phenotype of CD44(hi) similar to memory-like innate T cells. These cells are observed in mice carrying an Itk mutant lacking the kinase domain, indicating that active Tec kinase signaling suppresses their presence. These cells carry preformed message for and are able to rapidly produce IFN-gamma upon stimulation in vitro with IL-12/IL-18, and endow Itk null mice the ability to effectively respond to infection with L. monocytogenes or exposure to lipopolysaccharides by secretion of IFN-gamma. Transfer of these cells rescues the ability of IFN-gamma null mice to reduce bacterial burden following L. monocytogenes infection, indicating that these cells are functional CD8(+)CD44(hi) T cells previously detected in vivo. These results indicate that active signals from Tec kinases regulate the development of memory-like CD8(+) T cells with innate function.

Tec kinases, actin, and cell adhesion

The Tec family non-receptor tyrosine kinases have been recognized for their roles in the regulation of phospholipase C-gamma and Ca(2+) mobilization downstream from antigen receptors on lymphocytes. Recent data, however, show that the Tec family kinase interleukin-2-inducible T-cell kinase (Itk) also participates in pathways regulating the actin cytoskeleton and 'inside-out' signaling to integrins downstream from the T-cell antigen receptor. Data suggest that Itk may function in a kinase-independent fashion to regulate proper recruitment of the Vav1 guanine nucleotide exchange factor. By enhancing actin cytoskeleton reorganization, recruitment of signaling molecules to the immune synapse, and integrin clustering in response to both antigen and chemokine receptors, the Tec kinases serve as modulators or amplifiers that can increase the duration of T-cell signaling and regulate T-cell functional responses.

T-cell receptor signaling to integrins

Integrin adhesion receptors are critical for antigen recognition by T cells and for regulated recirculation and trafficking into and through various tissues in the body. T-cell receptor (TCR) signaling induces rapid increases in integrin function that facilitate T-cell activation by promoting stable contact with antigen-presenting cells and extracellular proteins in the environment. In this review, we outline the molecular mechanisms by which the TCR signals to integrins and present a model that highlights four key events: (i) initiation of proximal TCR signals nucleated by the linker for activated T cells (LAT) adapter protein and involving Itk, phospholipase C-gamma1, Vav1, and Src homology 2 domain-containing leukocyte-specific phosphoprotein of 76 kDa; (ii) transmission of integrin activation signals from the LAT signalosome to integrins by protein kinase (PK) C and the adapter protein, adhesion and degranulation-promoting adapter protein; (iii) assembly of integrin-associated signaling complexes that include PKD, the guanosine triphosphatase Rap1 and its effectors, and talin; and (iv) reorganization of the actin cytoskeleton by WAVE2 and other actin-remodeling proteins. These events coordinate changes in integrin conformation and clustering that result in enhanced integrin functional activity following TCR stimulation.

A key role for Itk in both IFN gamma and IL-4 production by NKT cells

NKT cells rapidly secrete cytokines upon TCR stimulation and thus may modulate the acquired immune response. Recent studies suggest that signaling for development and effector function in NKT cells may differ from conventional T cells. The tyrosine kinase Itk is activated downstream of the TCR, and its absence in CD4(+) T cells results in impaired Th2, but not Th1 responses. In this study, we investigated NKT cell function in the absence of Itk as impaired type 2 responses in vivo could be manifest through IL-4 defects in a number of cell types. We show that Itk-deficient NKT cells up-regulate IL-4 mRNA in the thymus and express constitutive IL-4 and IFN-gamma transcripts in peripheral organs. Thus, Itk is not required for the developmental activation of cytokine loci in NKT cells. Nevertheless, Itk-deficient NKT cells are severely impaired in IL-4 protein production. Strikingly, unlike conventional CD4(+) T cells, Itk-deficient NKT cells also have profound defects in IFN-gamma production. Furthermore, both IL-4 and IFN-gamma production were markedly impaired following in vivo challenge with alpha-galactosyl ceramide. Function can be restored in Itk-deficient NKT cells by provision of calcium signals using ionomycin. These results suggest that NKT cells are highly dependent on Itk for IL-4- and IFN-gamma-mediated effector function. Thus, the pattern of cytokine genes that are affected by Itk deficiency appears to be cell lineage-specific, likely reflecting differences in activation threshold between immune effectors. The severe defect in NKT cell function may underlie a number of the Th1 and Th2 immune defects in Itk-deficient mice.

A logical model provides insights into T cell receptor signaling

Cellular decisions are determined by complex molecular interaction networks. Large-scale signaling networks are currently being reconstructed, but the kinetic parameters and quantitative data that would allow for dynamic modeling are still scarce. Therefore, computational studies based upon the structure of these networks are of great interest. Here, a methodology relying on a logical formalism is applied to the functional analysis of the complex signaling network governing the activation of T cells via the T cell receptor, the CD4/CD8 co-receptors, and the accessory signaling receptor CD28. Our large-scale Boolean model, which comprises 94 nodes and 123 interactions and is based upon well-established qualitative knowledge from primary T cells, reveals important structural features (e.g., feedback loops and network-wide dependencies) and recapitulates the global behavior of this network for an array of published data on T cell activation in wild-type and knock-out conditions. More importantly, the model predicted unexpected signaling events after antibody-mediated perturbation of CD28 and after genetic knockout of the kinase Fyn that were subsequently experimentally validated. Finally, we show that the logical model reveals key elements and potential failure modes in network functioning and provides candidates for missing links. In summary, our large-scale logical model for T cell activation proved to be a promising in silico tool, and it inspires immunologists to ask new questions. We think that it holds valuable potential in foreseeing the effects of drugs and network modifications.

The tyrosine phosphatase SHP-2 regulates differentiation and apoptosis of individual primary T lymphocytes

Although phosphatases are key players of intracellular processes, not much is known about the phosphatase SHP-2 during T cell differentiation. Here we show that ectopic over-expression of SHP-2 in primary T helper cells directly reduced the frequency of individual lymphocytes expressing pro-inflammatory cytokines after antigen-specific stimulation by a mechanism impairing activation of protein kinase C. In addition we demonstrate that SHP-2 mediates enhanced migration upon CXCR4 signaling in a G-protein-dependent manner. Most strikingly, SHP-2 mediated a dramatic increase in apoptosis by highly enhanced activation of caspases. Co-immunoprecipitations of SHP-2 and c-Cbl from primary T helper cells demonstrated that SHP-2 strongly interacts with the ubiquitin ligase c-Cbl, indicating that c-Cbl could mediate the negative signals of SHP-2. Our results show that SHP-2 signal transduction regulates central checkpoints of T cell differentiation by the activation of distinct signaling cascades.

Differential regulation of human NK cell-mediated cytotoxicity by the tyrosine kinase Itk

NK cells are effector lymphocytes that can recognize and eliminate virally infected and transformed cells. NK cells express distinct activating receptors, including an ITAM-containing FcR complex that recognizes Ab-coated targets, and the DNAX-activating protein of 10 kDa-containing NKG2D receptor complex that recognizes stress-induced ligands. The regulatory role of specific tyrosine kinases in these pathways is incompletely understood. In this study, we show that, in activated human NK cells, the tyrosine kinase IL-2-inducible T cell kinase (Itk), differentially regulates distinct NK-activating receptors. Enhanced expression of Itk leads to increases in calcium mobilization, granule release, and cytotoxicity upon stimulation of the ITAM-containing FcR, suggesting that Itk positively regulates FcR-initiated cytotoxicity. In contrast, enhanced Itk expression decreases cytotoxicity and granule release downstream of the DNAX-activating protein of 10 kDa-containing NKG2D receptor, suggesting that Itk is involved in a pathway of negative regulation of NKG2D-initiated granule-mediated killing. Using a kinase mutant, we show that the catalytic activity of Itk is required for both the positive and negative regulation of these pathways. Complementary experiments where Itk expression was suppressed also showed differential regulation of the two pathways. These findings suggest that Itk plays a complex role in regulating the functions initiated by distinct NK cell-activating receptors. Moreover, understanding how these pathways may be differentially regulated has relevance in the setting of autoimmune diseases and antitumor immune responses where NK cells play key regulatory roles.

Mitogenic CD28 Signals Require the Exchange Factor Vav1 to Enhance TCR Signaling at the SLP-76-Vav-Itk Signalosome

Almost all physiological T cell responses require costimulation-engagement of the clonotypic TCR with MHC/Ag and CD28 by its ligands CD80/86. Whether CD28 provides signals that are qualitatively unique or quantitatively amplify TCR signaling is poorly understood. In this study, we use superagonistic CD28 Abs, which induce T cell proliferation without TCR coligation, to determine how CD28 contributes to mitogenic responses. We show that mitogenic CD28 signals require but do not activate the proximal TCR components TCRzeta and Zap-70 kinase. In cell lines lacking proximal TCR signaling, an early defect in the CD28 pathway is in phosphorylation of the adaptor molecule SLP-76, which we show is essential for recruitment of the exchange factor Vav leading to Ca(2+) flux and IL-2 production. Point mutations in CD28 that result in diminished Vav phosphorylation also result in defective Ca(2+) flux, IL-2 production, and Tec-kinase phosphorylation. Using Vav1-deficient mice, we further demonstrate the importance of Vav1 for efficient proliferation, IL-2 production, and Ca(2+) flux. Our results indicate that CD28 signals feed into the TCR signaling pathway at the level of the SLP-76 signalosome.

Tec kinases in T cell and mast cell signaling

The Tec family of tyrosine kinases consists of five members (Itk, Rlk, Tec, Btk, and Bmx) that are expressed predominantly in hematopoietic cells. The exceptions, Tec and Bmx, are also found in endothelial cells. Tec kinases constitute the second largest family of cytoplasmic protein tyrosine kinases. While B cells express Btk and Tec, and T cells express Itk, Rlk, and Tec, all four of these kinases (Btk, Itk, Rlk, and Tec) can be detected in mast cells. This chapter will focus on the biochemical and cell biological data that have been accumulated regarding Itk, Rlk, Btk, and Tec. In particular, distinctions between the different Tec kinase family members will be highlighted, with a goal of providing insight into the unique functions of each kinase. The known functions of Tec kinases in T cell and mast cell signaling will then be described, with a particular focus on T cell receptor and mast cell Fc epsilon RI signaling pathways.

The human and mouse complement of SH2 domain proteins-establishing the boundaries of phosphotyrosine signaling

SH2 domains are interaction modules uniquely dedicated to the recognition of phosphotyrosine sites and are embedded in proteins that couple protein-tyrosine kinases to intracellular signaling pathways. Here, we report a comprehensive bioinformatics, structural, and functional view of the human and mouse complement of SH2 domain proteins. This information delimits the set of SH2-containing effectors available for PTK signaling and will facilitate the systems-level analysis of pTyr-dependent protein-protein interactions and PTK-mediated signal transduction. The domain-based architecture of SH2-containing proteins is of more general relevance for understanding the large family of protein interaction domains and the modular organization of the majority of human proteins.

Itk and Th2 responses: action but no reaction

The Tec family tyrosine kinase, Itk, was initially characterized as a crucial component of T-cell receptor signaling pathways resulting in phospholipase C-gamma1 activation and actin polymerization. In 1999, a seminal report by Fowell, Locksley and colleagues demonstrated that, in CD4+ T cells, Itk-dependent signals are differentially required for T-helper (Th)2 versus Th1 differentiation and effector function. These findings launched a series of in vitro and in vivo studies addressing the molecular defects of Itk-/- CD4+ T cells, and the impaired immune responses of intact Itk-deficient mice. While demonstrating a bias against Th2 differentiation, overall these experiments have indicated that the most significant failing is an inability of Itk-/- CD4+ T cells to produce Th2 cytokines in a recall response, rather than an absolute defect in Th2 differentiation by T cells lacking Itk. In this review, we discuss the pathways by which Itk might impact the differentiation of Th cells.

Altered development of CD8+ T cell lineages in mice deficient for the Tec kinases Itk and Rlk

Mutations affecting the Tec kinases Itk and Rlk decrease T cell receptor-induced Ca(2+) mobilization and Erk kinase activation and impair both positive and negative thymic selection. Itk(-/-) and Rlk(-/-)Itk(-/-) mice also have decreased CD4:8 T cell ratios, suggestive of altered CD4:8 lineage commitment. Nonetheless, we find that CD8 single-positive (SP) thymocytes and peripheral CD8(+) T cells in these mice do not resemble conventional CD8(+) T cells. Instead, these cells express memory markers, rapidly produce interferon-gamma, and can be selected on hematopoietically derived cells, similar to MHC class Ib-restricted "innate-type" lymphocytes. Itk deficiency also greatly increases the number of cells selected by MHC class Ib. Expression of a hypersensitive Erk2 mutant partially corrects the CD8(+) T cell phenotypes in Itk(-/-) mice, arguing that altered signaling permits development of this innate-type CD8(+) cell population. Our results suggest that Tec kinases differentially regulate development of conventional versus nonconventional lymphocytes.

Phosphatidylinositol 3-kinase improves the efficiency of positive selection

We have generated transgenic mice expressing the amino-terminal fragment of the phosphatidylinositol 3-kinase (PI3K) catalytic subunit (p110ABD) in thymocytes. Expression of p110ABD results in constitutive activation of PI3K and in significant increases in the numbers of mature, single-positive thymocytes. We previously reported that the increase in mature cells was in part due to a defect in thymic emigration. In this study we identify another component to this phenotype. Expression of p110ABD results in an enhancement of positive selection, without alterations in thymocyte lifespan or negative selection. Since PI3K can affect activation of Btk, which in turn potentiates calcium fluxes, during B cell development, our results suggest that PI3K could play a role in the regulation of Itk kinases in T cells, and that both cell types share a common signaling network to modulate calcium responses downstream of their antigen receptor.

A kinase independent function for Tec kinase ITK in regulating antigen receptor induced serum response factor activation

The Tec family kinases are critical downstream regulators of antigen receptor signals in lymphocytes. As kinases, they act on critical substrates to regulate signals such as calcium increase leading to activation of transcription factors such as NFAT, NFkappaB and SRF. We now show here that ITK, a member of the Tec family of tyrosine kinases, has a kinase independent function. Mutants of ITK that lack kinase activity or a kinase domain can rescue cells lacking Tec family kinases for antigen receptor induced SRF activation, but not for NFAT, AP-1 or NFkappaB activation. Furthermore, expression of these mutants in WT cells enhanced SRF activation. This kinase independent function required the SH2 domain since a mutant lacking both the kinase and SH2 domains was much less effective at rescuing SRF activation. This kinase-deleted mutant could partially rescue ERK activation, and interact with multiple tyrosine phosphorylated proteins during antigen receptor signaling, suggesting that ITK uses a scaffolding function that regulates signals leading to specific regulation of SRF activation.

Discovery and SAR of 2-amino-5-(thioaryl)thiazoles as potent and selective Itk inhibitors

A series of structurally novel aminothiazole based small molecule inhibitors of Itk were prepared to elucidate their structure-activity relationships (SARs), selectivity, and cell activity in inhibiting IL-2 secretion in a Jurkat T-cell assay. Compound 3 is identified as a potent and selective Itk inhibitor which inhibits anti-TCR antibody induced IL-2 production in mice in vivo and was previously reported to reduce lung inflammation in a mouse model of ovalbumin induced allergy/asthma.

Cutting edge: Itk-dependent signals required for CD4+ T cells to exert, but not gain, Th2 effector function

The TCR signals for the release of CD4 effector function are poorly understood. Itk plays an essential role in Th2, but not Th1, responses. However, when Itk is required during Th2 development is unclear. We followed the fate of Itk-deficient T cells during Th2 development in vitro and in vivo using an IL-4/GFP reporter. Surprisingly, a similar frequency of itk(-/-) CD4(+) cells differentiated and committed to the Th2 lineage as wild-type cells. However, Itk-deficient Th2 cells failed to exert effector function upon TCR triggering. Loss of function was marked by defective transcriptional enhancement of Th2 cytokines and GATA3. IL-4 production in itk(-/-) Th2s could be rescued by the expression of kinase-active Itk. Thus, Itk is necessary for the release, but not gain, of Th2 function. We suggest that the liberation of effector function is tightly controlled through qualitative changes in TCR signals, facilitating postdifferentiation regulation of cytokine responses.

Discovery and SAR of 2-amino-5-[(thiomethyl)aryl]thiazoles as potent and selective Itk inhibitors

A series of structurally novel aminothiazole based small molecule inhibitors of Itk were prepared to elucidate their structure-activity relationships (SARs), selectivity and cell activity in inhibiting IL-2 secretion in a Jurkat T-cell assay. Compound 2 is identified as a potent and selective Itk inhibitor which inhibits anti-TCR antibody induced IL-2 production in mice in vivo.

Tec kinases Itk and Rlk are required for CD8+ T cell responses to virus infection independent of their role in CD4+ T cell help

Itk and Rlk are members of the Tec kinase family of nonreceptor protein tyrosine kinases that are expressed in T cells, NK cells, and mast cells. These proteins are involved in the regulation of signaling processes downstream of the TCR in CD4(+) T cells, particularly in the phosphorylation of phospholipase C-gamma1 after TCR activation; furthermore, both Itk and Rlk are important in CD4(+) T cell development, differentiation, function, and homeostasis. However, few studies have addressed the roles of these kinases in CD8(+) T cell signaling and function. Using Itk(-/-) and Itk(-/-)Rlk(-/-) mice, we examined the roles of these Tec family kinases in CD8(+) T cells, both in vitro and in vivo. These studies demonstrate that the loss of Itk and Rlk impairs TCR-dependent signaling, causing defects in phospholipase C-gamma1, p38, and ERK activation as well as defects in calcium flux and cytokine production in vitro and expansion and effector cytokine production by CD8(+) T cells in response to viral infection. These defects cannot be rescued by providing virus-specific CD4(+) T cell help, thereby substantiating the important role of Tec kinases in CD8(+) T cell signaling.

Novel t(5;9)(q33;q22) fuses ITK to SYK in unspecified peripheral T-cell lymphoma

Among peripheral T-cell lymphomas (PTCL), the heterogeneous category of unspecified PTCL represents the most common subtype. Nevertheless, recurrent chromosomal translocations are unknown in this aggressive type of lymphoma. Here we describe a novel t(5;9)(q33;q22) in unspecified PTCL. Molecular analyses delineated the breakpoints to ITK and SYK resulting in a previously undescribed expression of the Syk tyrosine kinase by Itk. ITK-SYK transcripts were detected in five of 30 (17%) unspecified PTCL, but not in cases of angioimmunoblastic T-cell lymphoma (n=9) and anaplastic lymphoma kinase-negative anaplastic large-cell lymphoma (n=7). In all five translocation-positive cases, the breakpoints were identical fusing the N-terminal pleckstrin homology domain and proline-rich region of ITK to the tyrosine kinase domain of SYK. Three of the five t(5;9)(q33;q22)+ unspecified PTCL shared a very similar histological pattern with predominant involvement of lymphoid follicles and the same CD3+CD5+CD4+bcl-6+CD10+ immunophenotype. These results demonstrate the presence of a recurrent t(5;9)(q33;q22) in a subset of unspecified PTCL, which may represent a novel distinct subgroup of PTCL.

Functional hierarchy of the N-terminal tyrosines of SLP-76

The adaptor protein Src homology 2 domain-containing leukocyte phosphoprotein of 76 kDa (SLP-76) plays a central role in T cell activation and T cell development. SLP-76 has three functional modules: an acidic domain with three key tyrosines, a central proline-rich domain, and a C-terminal Src homology 2 domain. Of these, mutation of the three N-terminal tyrosines (Y112, Y128, and Y145) results in the most profound effects on T cell development and function. Y112 and Y128 associate with Vav and Nck, two proteins shown to be important for TCR-induced phosphorylation of proximal signaling substrates, Ca(2+) flux, and actin reorganization. Y145 has been shown to be important for optimal association of SLP-76 with inducible tyrosine kinase, a key regulator of T cell function. To investigate further the role of the phosphorylatable tyrosines of SLP-76 in TCR signaling, cell lines and primary T cells expressing SLP-76 with mutations in individual or paired tyrosine residues were analyzed. These studies show that Tyr(145) of SLP-76 is the most critical tyrosine for both T cell function in vitro and T cell development in vivo.

Recruitment and activation of PLCgamma1 in T cells: a new insight into old domains

Engagement of the T-cell antigen receptor leads to recruitment of phospholipase Cgamma1 (PLCgamma1) to the LAT-nucleated signaling complex and to PLCgamma1 activation in a tyrosine phosphorylation-dependent manner. The mechanism of PLCgamma1 recruitment and the role of PLCgamma1 Src homology (SH) domains in this process remain incompletely understood. Using a combination of biochemical methods and real-time fluorescent imaging, we show here that the N-terminal SH2 domain of PLCgamma1 is necessary but not sufficient for its recruitment. Either the SH3 or C-terminal SH2 domain of PLCgamma1, with the participation of Vav1, c-Cbl and Slp76, are required to stabilize PLCgamma1 recruitment. All three PLCgamma1 SH domains are required for phosphorylation of PLCgamma1 Y783, which is critical for enzyme activation. These novel findings entailed revision of the currently accepted model of PLCgamma1 recruitment and activation in T lymphocytes.

A methodology for the structural and functional analysis of signaling and regulatory networks

Background

Structural analysis of cellular interaction networks contributes to a deeper understanding of network-wide interdependencies, causal relationships, and basic functional capabilities. While the structural analysis of metabolic networks is a well-established field, similar methodologies have been scarcely developed and applied to signaling and regulatory networks.

Results

We propose formalisms and methods, relying on adapted and partially newly introduced approaches, which facilitate a structural analysis of signaling and regulatory networks with focus on functional aspects. We use two different formalisms to represent and analyze interaction networks: interaction graphs and (logical) interaction hypergraphs. We show that, in interaction graphs, the determination of feedback cycles and of all the signaling paths between any pair of species is equivalent to the computation of elementary modes known from metabolic networks. Knowledge on the set of signaling paths and feedback loops facilitates the computation of intervention strategies and the classification of compounds into activators, inhibitors, ambivalent factors, and non-affecting factors with respect to a certain species. In some cases, qualitative effects induced by perturbations can be unambiguously predicted from the network scheme. Interaction graphs however, are not able to capture AND relationships which do frequently occur in interaction networks. The consequent logical concatenation of all the arcs pointing into a species leads to Boolean networks. For a Boolean representation of cellular interaction networks we propose a formalism based on logical (or signed) interaction hypergraphs, which facilitates in particular a logical steady state analysis (LSSA). LSSA enables studies on the logical processing of signals and the identification of optimal intervention points (targets) in cellular networks. LSSA also reveals network regions whose parametrization and initial states are crucial for the dynamic behavior.

We have implemented these methods in our software tool CellNetAnalyzer (successor of FluxAnalyzer) and illustrate their applicability using a logical model of T-Cell receptor signaling providing non-intuitive results regarding feedback loops, essential elements, and (logical) signal processing upon different stimuli.

Conclusion

The methods and formalisms we propose herein are another step towards the comprehensive functional analysis of cellular interaction networks. Their potential, shown on a realistic T-cell signaling model, makes them a promising tool.

A methodology for the structural and functional analysis of signaling and regulatory networks

BACKGROUND

Structural analysis of cellular interaction networks contributes to a deeper understanding of network-wide interdependencies, causal relationships, and basic functional capabilities. While the structural analysis of metabolic networks is a well-established field, similar methodologies have been scarcely developed and applied to signaling and regulatory networks.

RESULTS

We propose formalisms and methods, relying on adapted and partially newly introduced approaches, which facilitate a structural analysis of signaling and regulatory networks with focus on functional aspects. We use two different formalisms to represent and analyze interaction networks: interaction graphs and (logical) interaction hypergraphs. We show that, in interaction graphs, the determination of feedback cycles and of all the signaling paths between any pair of species is equivalent to the computation of elementary modes known from metabolic networks. Knowledge on the set of signaling paths and feedback loops facilitates the computation of intervention strategies and the classification of compounds into activators, inhibitors, ambivalent factors, and non-affecting factors with respect to a certain species. In some cases, qualitative effects induced by perturbations can be unambiguously predicted from the network scheme. Interaction graphs however, are not able to capture AND relationships which do frequently occur in interaction networks. The consequent logical concatenation of all the arcs pointing into a species leads to Boolean networks. For a Boolean representation of cellular interaction networks we propose a formalism based on logical (or signed) interaction hypergraphs, which facilitates in particular a logical steady state analysis (LSSA). LSSA enables studies on the logical processing of signals and the identification of optimal intervention points (targets) in cellular networks. LSSA also reveals network regions whose parametrization and initial states are crucial for the dynamic behavior. We have implemented these methods in our software tool CellNetAnalyzer (successor of FluxAnalyzer) and illustrate their applicability using a logical model of T-Cell receptor signaling providing non-intuitive results regarding feedback loops, essential elements, and (logical) signal processing upon different stimuli.

CONCLUSION

The methods and formalisms we propose herein are another step towards the comprehensive functional analysis of cellular interaction networks. Their potential, shown on a realistic T-cell signaling model, makes them a promising tool.

T-bet, a Th1 transcription factor, is up-regulated in T cells from patients with aplastic anemia

In aplastic anemia, immune destruction of hematopoietic cells results in bone marrow failure. Type 1 cytokines, especially IFN-gamma, have been implicated in the pathophysiology of T-cell-mediated, Fas-mediated stem cell apoptosis of hematopoietic cells. Here, we show that the transcription factor T-bet (T-box expressed in T cells) is increased in T cells from patients with aplastic anemia. Patients' T-bet bound directly to the proximal site of the IFN-gamma promoter without any prior stimulation, in contrast to healthy controls. Increased levels of Itk kinase participated in T-bet up-regulation and active transcription of the IFN-gamma gene observed in these patients. Blocking PKC-, a kinase that lies downstream of Itk kinase, decreased T-bet protein and IFN-gamma intracellular levels. These data suggest that the increased IFN-gamma levels observed in aplastic anemia patients are the result of active transcription of the IFN-gamma gene by T-bet. Blocking the transcription of the IFN-gamma gene with kinase inhibitors might lead to the development of novel therapeutic agents for patients with aplastic anemia and other autoimmune diseases.

Early phosphorylation kinetics of proteins involved in proximal TCR-mediated signaling pathways

Activation of T cells via the stimulation of the TCR plays a central role in the adaptive immunological response. Although much is known about TCR-stimulated signaling pathways, there are still gaps in our knowledge about the kinetics and sequence of events during early activation and about the in vivo specificity of kinases involved in these proximal signaling pathways. This information is important not only for understanding the activation of signaling pathways important for T cell function but also for the development of drug targets and computer-based molecular models. In this study, phospho-specific Abs directed toward individual sites on signaling proteins were used to investigate the early phosphorylation kinetics of proteins involved in proximal TCR-induced pathways. These studies indicate that linker for activation of T cells' tyrosines have substantially different phosphorylation kinetics and that Src homology 2 domain-containing leukocyte protein of 76 kDa has rapid, transient phosphorylation kinetics compared to other proteins. In additions, we provide evidence that ZAP-70 is the primary in vivo kinase for LAT tyrosine 191 and that Itk plays a role in the phosphorylation of tyrosine 783 on phospholipase C-gamma1. In total, these studies give new insight into the sequence, kinetics and specificity of early TCR-mediated signaling events that are vital for T cell activation.

Differential roles of PKC-theta in the regulation of intracellular calcium concentration in primary T cells

Activation of T lymphocytes requires protein kinase C theta (PKC-theta) and an appropriately elevated free intracellular Ca2+ concentration ([Ca2+]i). Here, we show that phorbol 12 myristate 13-acetate (PMA) inhibited Ca2+ influx in wild-type but not PKC-theta-/- T cells, suggesting that PKC-theta plays a role in PMA-mediated inhibition of Ca2+ influx. In contrast, T cell receptor (TCR) crosslinking in the same PKC-theta-/- T cells did result in significantly decreased [Ca2+]i compared to wild-type T cells, suggesting a positive role for PKC-theta in TCR-mediated Ca2+ mobilization. In PKC-theta-/- mice, peripheral mature T cells, but not developing thymocytes, displayed significantly decreased TCR-induced Ca2+ influx and nuclear factor of activated T cells (NFAT) translocation upon sub-optimal TCR crosslinking. The decreased intracellular free Ca2+ was due to changes in Ca2+ influx but not efflux, as observed in extracellular and intracellular Ca2+ mobilization studies. However, these differences in Ca2+ influx and nuclear factor of activated T cells (NFAT) translocation disappeared with increasing intensity of TCR crosslinking. The enhancing effect of PKC-theta on Ca2+ influx is not only dependent on the strength of TCR crosslinking but also on the developmental stage of T cells. The underlying mechanism involved phospholipase Cgamma1 activation and inositol triphosphate production. Furthermore, knockdown of endogenous PKC-theta expression in Jurkat cells resulted in significant inhibition of TCR-induced activation of NFAT, as evidenced from NFAT reporter studies. Forced expression of a constitutively active form of calcineurin in PKC-theta-/- Jurkat cells could readily overcome the above inhibition. Thus, PKC-theta can both positively and negatively regulate the Ca2+ influx that is critical for NFAT activity.

Tec Kinases Regulate TCR-Mediated Recruitment of Signaling Molecules and Integrin-Dependent Cell Adhesion

T cells deficient in the Tec kinases Itk or Itk and Rlk exhibit defective TCR-stimulated proliferation, IL-2 production, and activation of phospholipase C-gamma. Evidence also implicates Tec kinases in actin cytoskeleton regulation, which is necessary for cell adhesion and formation of the immune synapse in T lymphocytes. In this study we show that Tec kinases are required for TCR-mediated up-regulation of adhesion via the LFA-1 integrin. We also demonstrate that the defect in adhesion is associated with defective clustering of LFA-1 and talin at the site of interaction of Rlk-/-Itk-/- and Itk-/- T cells with anti-TCR-coated beads. Defective recruitment of Vav1, protein kinase Ctheta, and Pyk2 was also observed in Rlk-/-Itk-/- and Itk-/- T cells. Stimulation with ICAM-2 in conjunction with anti-TCR-coated beads enhanced polarization of Vav1, protein kinase Ctheta, and Pyk2 in wild-type cells, demonstrating a role for integrins in potentiating the recruitment of signaling molecules in T cells. Increased recruitment of signaling molecules was most pronounced under conditions of low TCR stimulation. Under these suboptimal TCR stimulation conditions, ICAM-2 could also enhance the recruitment of signaling molecules in Itk-/-, but not Rlk-/-Itk-/- T cells. Thus, Tec kinases play key roles in regulating TCR-mediated polarization of integrins and signaling molecules to the site of TCR stimulation as well as the up-regulation of integrin adhesion.

HIP-55 is important for T-cell proliferation, cytokine production, and immune responses

Engagement of the T-cell receptor (TCR) triggers a series of signaling events that lead to the activation of T cells. HIP-55 (SH3P7 or mAbp1), an actin-binding adaptor protein, interacts with and is tyrosine phosphorylated by ZAP-70, which is a crucial proximal protein tyrosine kinase for TCR signaling. HIP-55 is important for JNK and HPK1 activation induced by TCR signaling. In this study, we report the generation and characterization of HIP-55 knockout mice. We found that HIP-55 knockout mice were viable and fertile but showed decreased body weight and increased occurrence of death within the first 4 weeks after birth. The lymphoid organs in HIP-55 knockout mice showed cellularity and T-cell development comparable to that of the wild-type mice. HIP-55 knockout T cells displayed defective T-cell proliferation, decreased cytokine production, and decreased up-regulation of the activation markers induced by TCR stimulation. TCR internalization was slightly increased in HIP-55 knockout T cells. These phenotypes were accompanied by reduced immune responses, including antigen-specific antibody production and T-cell proliferation in HIP-55 knockout mice. The TCR-induced signaling events, including LAT/phospholipase Cgamma1 phosphorylation and HPK1/JNK activation, were partially defective in HIP-55 knockout T cells. These results demonstrate the importance of HIP-55 as an adaptor protein in the TCR signaling and immune system.

A role for the Tec family kinase ITK in regulating SEB-induced interleukin-2 production in vivo via c-jun phosphorylation

Background

Exposure to Staphylococcal Enterotoxin B (SEB), a bacterial superantigen secreted by the Gram-positive bacteria Staphyloccocus aureus, results in the expansion and eventual clonal deletion and anergy of Vβ8⁺ T cells, as well as massive cytokine release, including Interleukin-2 (IL-2). This IL-2 is rapidly secreted following exposure to SEB and may contribute to the symptoms seen following exposure to this bacterial toxin. The Tec family kinase ITK has been shown to be important for the production of IL-2 by T cells stimulated in vitro and may represent a good target for blocking the production of this cytokine in vivo. In order to determine if ITK represents such a target, mice lacking ITK were analyzed for their response to SEB exposure.

Results

It was found that T cells from mice lacking ITK exhibited significantly reduced proliferative responses to SEB exposure in vitro, as well as in vivo. Examination of IL-2 production revealed that ITK null mice produced reduced levels of this cytokine in vitro, and more dramatically, in vivo. In vivo analysis of c-jun phosphorylation, previously shown to be critical for regulating IL-2 production, revealed that this pathway was specifically activated in SEB reactive Vβ8⁺ (but not non-reactive Vβ6⁺) T cells from WT mice, but not in Vβ8⁺ T cells from ITK null mice. However, toxicity analysis indicated that both WT and ITK null animals were similarly affected by SEB exposure.

Conclusion

These data show that ITK is required for IL-2 production induced by SEB in vivo, and may regulate signals leading IL-2 production, in part by regulating phosphorylation of c-jun. The data also suggest that perturbing T cell activation pathways leading to IL-2 does not necessarily lead to improved responses to SEB toxicity.

TEC-family kinases: regulators of T-helper-cell differentiation

The TEC-family protein tyrosine kinases ITK, RLK and TEC have been identified as key components of T-cell-receptor signalling that contribute to the regulation of phospholipase C-gamma, the mobilization of Ca(2+) and the activation of mitogen-activated protein kinases. Recent data also show that TEC kinases contribute to T-cell-receptor-driven actin reorganization and cell polarization, which are required for productive T-cell activation. Functional studies have implicated TEC kinases as important mediators of pathways that control the differentiation of CD4(+) T helper cells. Here, we review studies of signalling pathways that involve TEC kinases and how these pathways might contribute to the regulation of T-helper-cell differentiation and function.