Tec Family Of Kinases Research Articles

The C-type Lectin Receptors CLEC-2 and Dectin-1, but Not DC-SIGN, Signal via a Novel YXXL-dependent Signaling Cascade

The two lectin receptors, CLEC-2 and Dectin-1, have been shown to signal through a Syk-dependent pathway, despite the presence of only a single YXXL in their cytosolic tails. In this study, we show that stimulation of CLEC-2 in platelets and in two mutant cell lines is dependent on the YXXL motif and on proteins that participate in signaling by immunoreceptor tyrosine-based activation motif receptors, including Src, Syk, and Tec family kinases, and on phospholipase Cgamma. Strikingly, mutation of either Src homology (SH) 2 domain of Syk blocks signaling by CLEC-2 despite the fact that it has only a single YXXL motif. Furthermore, signaling by CLEC-2 is only partially dependent on the BLNK/SLP-76 family of adapter proteins in contrast to that of immunoreceptor tyrosine-based activation motif receptors. The C-type lectin receptor, Dectin-1, which contains a YXXL motif preceded by the same four amino acids as for CLEC-2 (DEDG), signals like CLEC-2 and also requires the two SH2 domains of Syk and is only partially dependent on the BLNK/SLP-76 family of adapters. In marked contrast, the C-type lectin receptor, DC-SIGN, which has a distinct series of amino acids preceding a single YXXL, signals independent of this motif. A mutational analysis of the DEDG sequence of CLEC-2 revealed that the glycine residue directly upstream of the YXXL tyrosine is important for CLEC-2 signaling. These results demonstrate that CLEC-2 and Dectin-1 signal through a single YXXL motif that requires the tandem SH2 domains of Syk but is only partially dependent on the SLP-76/BLNK family of adapters.

Reduction of contact sensitivity responses to FITC, but not DNFB in mice lacking the tyrosine kinase, Itk (B103)

Abstract Itk (inducible T cell kinase) is a member of the Tec kinase family of nonreceptor protein tyrosine kinases that are expressed in T cells, NK cells, and mast cells. Defects in Itk protein lead to impaired T cell activation and effector function. Although mice lacking Itk are impaired in their ability to generate protective Th1 and Th2 responses in vivo, LCMV clearance requiring CD8+ T cells appear to be intact. Contact sensitivity (CS) is a type IV hypersensitivity reaction resulting from secondary skin exposure to a reactive hapten. Murine models of CS using chemical haptens such as DNFB are shown to be predominantly CD8+ T cell mediated with CD4+ T cells acting as modulators. Recently, data from CS models using FITC suggest a dominant role of Th2-type CD4+ T cells. The purpose of our studies was to determine whether Itk plays a role in the development of contact sensitivity responses to FITC or DNFB using Itk knockout mice. Indeed mice lacking Itk have significantly reduced ear swelling and leukocyte infiltration following FITC ear challenge compared to WT mice, which correlates with the lack of FITC-specific cytokine secretion from primed Itk KO LNC in vitro. In contrast, Itk−/− mice were able to generate ear swelling responses comparable to WT mice following DNFB challenge. Together, these data further support the effector role of CD4+ T cells in FITC-CS and the inability of Itk−/− mice to generate effective Th2 responses in vivo.

Generation of peripheral B cells occurs via two spatially and temporally distinct pathways

AbstractWe have identified a population of newly formed bone marrow (BM) B cells that shares multiple characteristics with late transitional B cells in the spleen. Both late splenic transitional B cells and cells within this uncharacterized BM population expressed the cell-surface phenotype AA4+ CD23+, yet the developmental kinetics and the renewal rate of AA4+ CD23+ BM B cells mirrored recently formed BM B cells. Further, unlike the least mature B cells in the BM and spleen, AA4+ CD23+ BM B cells expressed the homing receptor CD62L, were dependent on the antiapoptotic cytokine receptor BR3 and the tec family kinase Btk, and proliferated in response to IL-4 plus CD40 stimulation. Finally, frequencies of λ light chain-positive B cells declined among AA4+ CD23+ B cells in both the BM and spleen, suggesting that V-gene selection events correlate with CD23 expression in both compartments. These observations indicate that the first step in B-cell maturation occurs in both the BM and the periphery and suggest that recently formed B cells exit the BM as a heterogeneous pool of immature and semimature B cells.

Activation Mechanism and Steady State Kinetics of Bruton's Tyrosine Kinase

Bruton's tyrosine kinase (BTK) is a member of the Tec non-receptor tyrosine kinase family that is involved in regulating B cell proliferation. To better understand the enzymatic mechanism of the Tec family of kinases, the kinetics of BTK substrate phosphorylation were characterized using a radioactive enzyme assay. We first examined whether autophosphorylation regulates BTK activity. Western blotting with a phosphospecific antibody revealed that BTK rapidly autophosphorylates at Tyr(551) within its activation loop in vitro. Examination of a Y551F BTK mutant indicated that phosphorylation of Tyr(551) causes a 10-fold increase in BTK activity. We then proceeded to characterize the steady state kinetic mechanism of BTK. Varying the concentrations of ATP and S1 peptide (biotin-Aca-AAAEEIY-GEI-NH2) revealed that BTK employs a ternary complex mechanism with KmATP = 84 +/- 20 microM and KmS1 = 37 +/- 8 microM. Inhibition studies were also performed to examine the order of substrate binding. The inhibitors ADP and staurosporine were both found to be competitive with ATP and non-competitive with S1, indicating binding of ATP and S1 to BTK is either random or ordered with ATP binding first. Negative cooperativity was also found between the S1 and ATP binding sites. Unlike ATP site inhibitors, substrate analog inhibitors did not inhibit BTK at concentrations less than 1 mm, suggesting that BTK may employ a "substrate clamping" type of kinetic mechanism whereby the substrate Kd is weaker than Km. This investigation of BTK provides the first detailed kinetic characterization of a Tec family kinase.

The Role of Tec Family Kinases in Inflammatory Processes

A variety of different cell lineages of the hematopoietic system are activated during inflammatory processes. These cells not only contribute to the beneficial outcome of an immune response, but can also cause pathology such as autoimmunity as a consequence of extended or uncontrolled reactions. Therefore, an understanding of the basic mechanisms that lead to immune cell activation and the identification of key molecular players will also lead to strategies towards therapeutic manipulation of extended immune reactions. Members of the Tec kinase family (Bmx, Btk, Itk, Rlk and Tec) constitute an important class of non-receptor protein tyrosine kinases that are primarily expressed in the hematopoietic system. They are activated upon a variety of signals and are important participants of major signal transduction pathways in immunological processes. Hence, deficiencies in Tec family kinases cause several immunological defects, both in man and mice. Since Tec family kinases have been shown to function as modulators of immune cell activation, they may provide attractive drug targets for the manipulation of the immune response. In this review, we summarize recent data from studies about the activities of Tec family kinases in inflammatory cells and their role in in vivo models of infection, inflammation and autoimmune diseases. Keywords: Tec family kinases, inflammation, mouse models, immunity, Th1/Th2

Btk regulates multiple stages in the development and survival of B-1 cells

B-1 cells are important players in the first line of defense against pathogens. According to current models for the origin of B-1 cells, they either represent a separate lineage from conventional B-2 cells or differentiate from conventional B-2 cells via an intermediate, B-1int, in response to positive selection by antigen. Here we show that Btk, a Tec family kinase that mediates B cell antigen receptor (BCR) signaling, is required at multiple stages of B-1 cell development. VH12 anti-phosphatidylcholine (PtC) IgH transgenic mice provide a model for the induced differentiation of B-1 cells. This transgene selects for PtC-reactive cells and induces them to adopt a B-1 phenotype. Both processes have been shown to depend on Btk. To determine whether this is secondary to a requirement for Btk in the development of mature B-2 cells, we crossed VH12 transgenic mice to mice expressing low levels of Btk. B-2 cell development occurs normally in Btklo mice despite reduced responsiveness to BCR crosslinking. Analysis of VH12.Btklo mice reveals that Btk regulates the B-1int to B-1 transition and/or the survival of splenic B-1 cells, in part via a mechanism independent of its role in BCR signaling. We also show that Btk mediates the survival of, and expression of IL-10 by, those B-1 cells that do develop and migrate to the peritoneum. Multiple roles for Btk in B-1 cell development and maintenance may explain the particular sensitivity of this population to mutations in components of Btk signaling pathways.

An Improved Mouse Model of Atopic Dermatitis and Suppression of Skin Lesions by an Inhibitor of Tec Family Kinases

Atopic dermatitis is a chronic or chronically relapsing, pruritic inflammatory skin disease. The incidence of atopic dermatitis has dramatically increased during the past three decades in industrialized countries. We attempted to develop an improved method to induce an animal model of atopic dermatitis and to use it to evaluate the efficacy of a Tec family kinase inhibitor. We treated dermatitis-prone inbred mice, NC/Nga, by repetitive epicutaneous applications of a house dust mite allergen and staphylococcal enterotoxin B to induce atopic dermatitis-like skin lesions. We established a highly efficient protocol to induce skin lesions in NC/Nga mice, which were histologically and immunologically similar to human atopic dermatitis. Similar to human patients, serum IgE levels were increased in dermatitis-induced mice. Consistent with the proposed roles of infiltrated immune cells in the pathogenesis of human atopic dermatitis, skin lesions were treatable with terreic acid, an inhibitor of Tec family kinases, as well as dexamethasone. We established a highly efficient, highly reproducible protocol to induce skin lesions in NC/Nga mice and successfully applied it to show the efficacy of terreic acid in treating skin lesions. This mouse model of atopic dermatitis will be useful to study the pathogenetic processes of atopic dermatitis and to evaluate the efficacy of drug candidates.

Characterization of the Tec tyrosine kinase signaling network in the murine myocardium

The Tec family of non-receptor tyrosine kinases (Tec, Btk, Bmx, Emt and Txk) have well established roles in cell growth and differentiation. These proteins are known to be activated by growth factors and cytokines and are critically involved in T-cell activation. Recent studies have suggested a role for Bmx in ischemic injury to mouse heart and skeletal muscle, however the actions of the Tec family in cardiac signal transduction remain largely unknown. In the present study, we conducted comprehensive analyses of Tec family expression in whole heart and isolated cardiac myocytes under normal and ischemic conditions. Immunoblotting for Tec kinases revealed expression of Bmx and Tec in both whole heart and myocyte lysates. Interestingly, we reproducibly observe two distinct bands when blotting for Tec in myocyte lysates and only one band in whole heart lysate, suggesting the existence of two Tec isoforms in myocytes. We have optimized immunoprecipitations for isolation of Tec-interacting proteins (including preclearing with IgG control to reduce nonspecific contamination) and are using this approach to evaluate the Tec signaling network in cardiac cells. These Tec-associated proteins have been electrophoresed, trypsinized and are being identified by LC/MS/MS. These studies are an important first step to understanding the mechanistic role of Tec family kinases in myocardial ischemia. Supported by AHA & NIH/NHLBI.

A FLIPR-based Assay to Assess Potency of Inhibitors of the TEC Family Kinases Itk and Btk

A FLIPR-based Assay to Assess Potency of Inhibitors of the TEC Family Kinases Itk and Btk

The Gα13-Rho Signaling Axis Is Required for SDF-1-induced Migration through CXCR4

The CXC chemokine stromal cell-derived factor-1alpha (SDF-1) binds to CXCR4, a seven-transmembrane G protein-coupled receptor that plays a critical role in many physiological processes that involve cell migration and cell fate decisions, ranging from stem cell homing, angiogenesis, and neuronal development to immune cell trafficking. CXCR4 is also implicated in various pathological conditions, including metastatic spread and human immunodeficiency virus infection. Although SDF-1-induced cell migration in CXCR4-expressing cells is sensitive to pertussis toxin treatment, hence involving heterotrimeric G proteins of the G(i) family, whether other G proteins participate in the chemotactic response to SDF-1 is still unknown. In this study, we took advantage of the potent chemotactic activity of SDF-1 in Jurkat T-cells to examine the nature of the heterotrimeric G protein subunits contributing to CXCR4-mediated cell migration. We observed that whereas G(i) and Gbetagamma subunits are involved in SDF-1-induced Rac activation and cell migration, CXCR4 can also stimulate Rho potently leading to the phosphorylation of myosin light chain through the Rho effector, Rho kinase, but independently of G(i). Furthermore, we found that Galpha(13) mediates the activation of Rho by CXCR4 and that the functional activity of both Galpha(13) and Rho is required for directional cell migration in response to SDF-1. Collectively, our data indicate that signaling by CXCR4 to Rho through Galpha(13) contributes to cell migration when stimulated by SDF-1, thus identifying the Galpha(13)-Rho signaling axis as a potential pharmacological target in many human diseases that involve the aberrant function of CXCR4.

Vav1 Acidic Region Tyrosine 174 Is Required for the Formation of T Cell Receptor-induced Microclusters and Is Essential in T Cell Development and Activation

Vav proteins are multidomain signaling molecules critical for mediating signals downstream of several surface receptors, including the antigen receptors of T and B lymphocytes. The catalytic guanine nucleotide exchange factor (GEF) activity of the Vav Dbl homology (DH) domain is thought to be controlled by an intramolecular autoinhibitory mechanism involving an N-terminal extension and phosphorylation of tyrosine residues in the acidic region (AC). Here, we report that the sequences surrounding the Vav1 AC: Tyr(142), Tyr(160), and Tyr(174) are evolutionarily conserved, conform to consensus SH2 domain binding motifs, and bind several proteins implicated in TCR signaling, including Lck, PI3K p85alpha, and PLCgamma1, through direct interactions with their SH2 domains. In addition, the AC tyrosines regulate tyrosine phosphorylation of Vav1. We also show that Tyr(174) is required for the maintenance of TCR-signaling microclusters and for normal T cell development and activation. In this regard, our data demonstrate that while Vav1 Tyr(174) is essential for maintaining the inhibitory constraint of the DH domain in both developing and mature T cells, constitutively activated Vav GEF disrupts TCR-signaling microclusters and leads to defective T cell development and proliferation.

Different roles for non-receptor tyrosine kinases in arachidonate release induced by zymosan and Staphylococcus aureus in macrophages

BackgroundYeast and bacteria elicit arachidonate release in macrophages, leading to the formation of leukotrienes and prostaglandins, important mediators of inflammation. Receptors recognising various microbes have been identified, but the signalling pathways are not entirely understood. Cytosolic phospholipase A2 is a major down-stream target and this enzyme is regulated by both phosphorylation and an increase in intracellular Ca2+. Potential signal components are MAP kinases, phosphatidylinositol 3-kinase and phospholipase Cγ2. The latter can undergo tyrosine phosphorylation, and Src family kinases might carry out this phosphorylation. Btk, a Tec family kinase, could also be important. Our aim was to further elucidate the role of Src family kinases and Btk.MethodsArachidonate release from murine peritoneal macrophages was measured by prior radiolabeling. Furthermore, immunoprecipitation and Western blotting were used to monitor changes in activity/phosphorylation of intermediate signal components. To determine the role of Src family kinases two different inhibitors with broad specificity (PP2 and the Src kinase inhibitor 1, SKI-1) were used as well as the Btk inhibitor LFM-A13.ResultsArachidonate release initiated by either Staphylococcus aureus or yeast-derived zymosan beads was shown to depend on members of the Src kinase family as well as Btk. Src kinases were found to act upstream of Btk, phosphatidylinositol 3-kinase, phospholipase Cγ2 and the MAP kinases ERK and p38, thereby affecting all branches of the signalling investigated. In contrast, Btk was not involved in the activation of the MAP-kinases. Since the cytosolic phospholipase A2 in macrophages is regulated by both phosphorylation (via ERK and p38) and an increase in intracellular Ca2+, we propose that members of the Src kinase family are involved in both types of regulation, while the role of Btk may be restricted to the latter type.ConclusionArachidonate release induced by either Staphylococcus aureus or zymosan was found to depend on Src family kinases as well as Btk. While members of the Src kinase family were shown to act upstream of Btk and the MAP kinases, Btk plays another role independent of MAP kinases, but down-stream of the Src family kinases.

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, NFκB 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 NFκB 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.

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.

Signalling by the Platelet C-Type Lectin Receptor CLEC-2 Is Mediated by a Novel Mechanism Involving Syk and a Single YxxL Motif.

We have recently identified the C-type lectin-like receptor CLEC-2 as a novel receptor for the snake toxin rhodocytin on platelets. CLEC-2 is a 32 kDa type II transmembrane protein with a single cytoplasmic tyrosine residue in a YxxL consensus sequence. It is the first C-type lectin receptor that has been shown to activate platelets. The aim of the present study was to investigate the mechanism of signalling by CLEC-2 using model cell lines and to establish the role of the YxxL motif in this process.We have expressed CLEC-2 in wild type DT40 B cells and Jurkat T cells and in mutant derivatives of these cell lines that lack key signalling proteins. We have assayed PLCγ activation using an NFAT-luciferase reporter assay. Site directed mutagenesis of the cytoplasmic tyrosine residue of CLEC-2 has been used to assess its role in CLEC-2 signalling. We have compared the signalling pathway of CLEC-2 with that of the ITAM-coupled collagen receptor GPVI by expressing GPVI in the mutant cell lines and monitoring responses to the GPVI specific agonist convulxin as above.Cells transfected with CLEC-2 respond to rhodocytin. Mutation of the CLEC-2 YxxL motif to FxxL abrogates CLEC-2 signalling. CLEC-2 signalling is abrogated in cells deficient in Src or Syk family kinases or PLCγ . CLEC-2 signalling is partially dependent on Tec family kinases and the adaptors LAT and SLP-76/BLNK. Interestingly, GPVI signalling and CLEC-2 signalling show a differential requirement for the SLP-76 family adaptor proteins SLP-76 or Blnk; GPVI signalling is entirely dependent on SLP-76/Blnk, whereas CLEC-2 signalling is only partially dependent on these proteins.These observations are consistent with a model whereby CLEC-2 recruits Syk via its YxxL motif and initiates a signalling cascade similar to the major platelet glycoproteins GPVI and integrin α IIbβ 3. CLEC-2 is the first platelet receptor shown to regulate Syk through a single YxxL motif. Importantly, despite the similarity in the signalling proteins involved in signalling by the ITAM receptor, the integrin receptor and the C-type lectin receptor, the signalling pathways are organised and initiated differently.

Estimating Immunoregulatory Gene Networks in Human Herpesvirus Type 6 (HHV-6)-Infected T Cells.

The immune response to viral infection involves complex network of dynamic gene and protein interactions. Comprehensive gene expression analysis of the host immune response against viruses has been extensively studied, however, the mechanism of virus-induced immune response is not completely understood. This might be due in part to the difficulty of finding pathologically relevant genes, despite the fact that DNA microarray technology can simultaneously monitor the expression of thousands of genes. Likewise, it is hard to estimate how each gene interferes during the viral infection. Thus, construction of gene networks from microarray gene expression data is becoming an important challenge in the post-genome era. Human herpesvirus 6 (HHV-6) is a β-herpesvirus that is closely related to human cytomegalovirus. HHV-6 shows a predominant tropism for CD4 T lymphocytes, on which it exerts marked cytopathic effects. Understanding of the clinical spectrum of HHV-6 is still evolving, however, in vitro interactions between HHV-6 and other viruses, such as the human immunodeficiency virus (HIV), and their relevance to the in vivo situation has become increasingly apparent. We present here the dynamic gene network of the host immune response during human herpesvirus type 6 (HHV-6) infection in an adult T cell leukemia (ATL) cell line. Using a pathway-focused oligonucleotide DNA microarray, we found a possible association between chemokine genes regulating Th1/Th2 balance and genes regulating T-cell proliferation during HHV-6B infection. Gene network analysis using an integrated comprehensive workbench, VoyaGene® revealed that a gene encoding a TEC-family kinase, ITK, might be a putative modulator in the host immune response against HHV-6B infection. We conclude that Th2-dominated inflammatory reaction in host cells may play an important role in HHV-6B infected T cells, thereby suggesting the possibility that ITK might be a therapeutic target in diseases related to dysregulation of Th1/Th2 balance. This study describes a novel approach to find genes related with the complex host-virus interaction using microarray data employing the Bayesian statistical framework. [Display omitted]

Estimating immunoregulatory gene networks in human herpesvirus type 6-infected T cells

The immune response to viral infection involves complex network of dynamic gene and protein interactions. We present here the dynamic gene network of the host immune response during human herpesvirus type 6 (HHV-6) infection in an adult T-cell leukemia cell line. Using a pathway-focused oligonucleotide DNA microarray, we found a possible association between chemokine genes regulating Th1/Th2 balance and genes regulating T-cell proliferation during HHV-6B infection. Gene network analysis using an integrated comprehensive workbench, VoyaGene, revealed that a gene encoding a TEC-family kinase, ITK, might be a putative modulator in the host immune response against HHV-6B infection. We conclude that Th2-dominated inflammatory reaction in host cells may play an important role in HHV-6B-infected T cells, thereby suggesting the possibility that ITK might be a therapeutic target in diseases related to dysregulation of Th1/Th2 balance. This study describes a novel approach to find genes related with the complex host–virus interaction using microarray data employing the Bayesian statistical framework.

Dynamic Regulation of Tec Kinase Localization in Membrane-proximal Vesicles of a T Cell Clone Revealed by Total Internal Reflection Fluorescence and Confocal Microscopy

Tec family tyrosine kinases are key regulators of lymphocyte activation and effector function. Several Tec family kinases (Tec, Itk, Rlk/Txk) are expressed in T cells, but it is still not clear to what degree these are redundant or have unique functions. We recently demonstrated that Tec alone, among the Tec kinase family members examined, can induce nuclear factor of activated T cell-dependent transcription. This unique functional characteristic correlated with a unique pattern of subcellular localization, as Tec (but not other family members) was found in small vesicles, the appearance of which requires signaling through the T cell receptor for antigen. Here we report on our studies of these Tec-containing structures in live T cells, using total internal reflection fluorescence microscopy. With this technique, we showed that, in live T cells, the Tec vesicles are located at the plasma membrane, the vesicles are unique to Tec (and not the related kinase Itk), and their formation and maintenance require T cell receptor signaling through Src family kinases and PI 3-kinase. Finally, we have imaged isolated T cell membranes by confocal microscopy, confirming the membrane-proximal location of Tec vesicles, as well as demonstrating overlap of these vesicles with the tyrosine kinase Lck, the Tec substrate PLC-gamma1, and the early endosomal antigen 1 marker EEA1.

TEC FAMILY KINASES IN T LYMPHOCYTE DEVELOPMENT AND FUNCTION

The Tec family tyrosine kinases are now recognized as important mediators of antigen receptor signaling in lymphocytes. Three members of this family, Itk, Rlk, and Tec, are expressed in T cells and activated in response to T cell receptor (TCR) engagement. Although initial studies demonstrated a role for these proteins in TCR-mediated activation of phospholipase C-gamma, recent data indicate that Tec family kinases also regulate actin cytoskeletal reorganization and cellular adhesion following TCR stimulation. In addition, Tec family kinases are activated downstream of G protein-coupled chemokine receptors, where they play parallel roles in the regulation of Rho GTPases, cell polarization, adhesion, and migration. In all these systems, however, Tec family kinases are not essential signaling components, but instead function to modulate or amplify signaling pathways. Although they quantitatively reduce proximal signaling, mutations that eliminate Tec family kinases in T cells nonetheless qualitatively alter T cell development and differentiation.

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.