Differential T-cell antigen receptor signaling mediated by the Src family kinases Lck and Fyn

Regulated expression of galectin-1 during T-cell activation involves Lck and Fyn kinases and signaling through MEK1/ERK, p38 MAP kinase and p70S6 kinase

Recent evidence has implicated galectins and their carbohydrate ligands as novel regulators of T-cell homeostasis. Galectin-1 (Gal-1), a member of this family, inhibits clonal expansion, induces apoptosis of antigen-primed T lymphocytes and suppresses the development of T-cell-mediated autoimmune diseases in vivo. Because the beta-galactoside-binding protein is expressed in activated but not resting T cells, it has been hypothesized that Gal-1-induced apoptosis may constitute an autocrine suicide mechanism to eliminate activated T cells contributing to the termination of an effector immune response. We undertook this study to investigate the signals and intracellular pathways leading to Gal-1 expression during T-cell activation. When T cells were stimulated either with anti-CD3 or anti-CD28 monoclonal antibody plus PMA in the presence of accessory cells, a sustained up-regulation of Gal-1 was observed, reaching a plateau between days 3 and 5 following CD3 engagement or costimulation through CD28. Investigation of the signal transduction events involved in this process revealed a role for Lck and Fyn kinases, since the Src kinase inhibitor PP1 inhibited the up-regulated expression of Gal-1 following T-cell activation. Downstream signaling routes involve mitogen-activated protein kinase (MAPK) kinase (MEK)1/extracellular signal-regulated kinase (ERK) and p38 MAPK, as Gal-1 expression was prevented by U0126 and SB202190. In addition, expression of Gal-1 involves interleukin (IL)-2-dependent signaling routes triggered by p70S6 kinase, as it could be inhibited by rapamycin. This is the first demonstration of the intracellular pathways that control activation-induced expression of Gal-1, which may reveal potential targets for immune intervention to modulate expression of this beta-galactoside-binding protein in pathological disorders.

Nonsteroidal anti-inflammatory drugs inhibit a Fyn-dependent pathway coupled to Rac and stress kinase activation in TCR signaling

In addition to their anti-inflammatory properties, nonsteroidal anti-inflammatory drugs (NSAIDs) harbor immunosuppressive activities related to their capacity both to inhibit cyclooxygenases (COXs) and to act as peroxisome proliferator-activated receptor (PPAR) ligands. We have previously shown that the stress-activated kinase p38 is a selective target of NSAIDs in T cells. Here we have investigated the effect of NSAIDs on the signaling pathway triggered by the T-cell antigen receptor (TCR) and leading to stress kinase activation. The results show that nonselective and COX-1-selective NSAIDs also block activation of the stress kinase c-Jun N-terminal kinase (JNK) and that prostaglandin-E2 (PGE2) reverses this block and enhances TCR-dependent JNK activation. Analysis of the activation state of the components upstream of p38 and JNK showed that NSAIDs inhibit the serine-threonine kinase p21-activated protein kinase 1 (Pak1) and the small guanosine 5'-triphosphatase (GTPase) Rac, as well as the Rac-specific guanine nucleotide exchanger, Vav. Furthermore, activation of Fyn, which controls Vav phosphorylation, is inhibited by NSAIDs, whereas activation of lymphocyte-specific protein tyrosine kinase (Lck) and of the Lck-dependent tyrosine kinase cascade is unaffected. Accordingly, constitutively active Fyn reverses the NSAID-dependent stress kinase inhibition. The data identify COX-1 as an important early modulator of TCR signaling and highlight a TCR proximal pathway selectively coupling the TCR to stress kinase activation.

Activation of naïve CD4 T cells by anti-CD3 reveals an important role for Fyn in Lck-mediated signaling

Although there was no impairment in IL-2 secretion and proliferation of Fyn-deficient naïve CD4 cells after stimulation with antigen and antigen-presenting cells, stimulation of these cells with anti-CD3 and anti-CD28 revealed profound defects. Crosslinking of purified wild-type naïve CD4 cells with anti-CD3 activated Lck and initiated the signaling cascade downstream of Lck, including phosphorylation of ZAP-70, LAT, and PLC-gamma1; calcium flux; and dephosphorylation and nuclear translocation of the nuclear factor of activated T cells (NFAT)p. All of these signaling events were diminished severely in Fyn-deficient naïve cells activated by CD3 crosslinking. Coaggregation of CD3 and CD4 reconstituted this Lck-dependent signaling pathway in Fyn(-/-) T cells. These results suggest that when signaling of naïve T cells is restricted to the T cell antigen receptor, Fyn plays an essential role by positive regulation of Lck activity.

The adapter molecule Sin regulates T-cell-receptor-mediated signal transduction by modulating signaling substrate availability

Engagement of the T-cell receptor (TCR) results in the activation of a multitude of signaling events that regulate the function of T lymphocytes. These signaling events are in turn modulated by adapter molecules, which control the final functional output through the formation of multiprotein complexes. In this report, we identified the adapter molecule Sin as a new regulator of T-cell activation. We found that the expression of Sin in transgenic T lymphocytes and Jurkat T cells inhibited interleukin-2 expression and T-cell proliferation. This inhibitory effect was specific and was due to defective phospholipase C-gamma (PLC-gamma) phosphorylation and activation. In contrast to other adapters that become phosphorylated upon TCR stimulation, Sin was constitutively phosphorylated in resting cells by the Src kinase Fyn and bound to signaling intermediates, including PLC-gamma. In stimulated cells, Sin was transiently dephosphorylated, which coincided with transient dissociation of Fyn and PLC-gamma. Downregulation of Sin expression using Sin-specific short interfering RNA oligonucleotides inhibited transcriptional activation in response to TCR stimulation. Our results suggest that endogenous Sin influences T-lymphocyte signaling by sequestering signaling substrates and regulating their availability and/or activity in resting cells, while Sin is required for targeting these intermediates to the TCR for fast signal transmission during stimulation.

Impaired T cell proliferation and zeta chain phosphorylation after stimulation with staphylococcal enterotoxin-B in hemodialysis patients

BACKGROUND

Patients on regular hemodialysis treatment are in an immunodeficiency state. Several studies have shown defective T cell proliferation after stimulation with various agents. Staphylococcal enterotoxin B (SEB) is a MHC-dependent superantigen that triggers proliferation of a large proportion of T cells. T cell activation after stimulation with SEB parallels normal T cell signal transduction. An important and early event in this transduction pathway is the phosphorylation of the zeta chain. In this study, T cell proliferation and zeta chain phosphorylation after stimulation with SEB were evaluated.

METHODS

Peripheral blood mononuclear cells (PBMCs) from 24 patients and 14 healthy individuals were isolated and cultured with or without stimulation with SEB (1 ng/ml). Cell proliferation was estimated by immunoenzymatic measurement of bromodeoxyuridine uptake. PBMCs from 8 patients and 6 healthy individuals were isolated and pulsed for 2 min with or without SEB (10 microg/ml). Zeta chain phosphorylation was estimated by immunoprecipitation and immunoblotting with antiphosphotyrosine antibody.

RESULTS

Lymphocyte proliferation index after SEB stimulation was lower in hemodialyzed patients. Stimulation of T cells with SEB also resulted in a lower zeta chain phosphorylation in hemodialyzed patients.

CONCLUSIONS

Lymphocyte proliferation after MHC-dependent stimulation is impaired in hemodialyzed patients. This proliferation defect is due to impaired zeta chain phosphorylation.

Superantigen stimulation reveals the contribution of Lck to negative regulation of T cell activation

The conventional paradigm of T cell activation through the TCR states that Lck plays a critical activating role in this signaling process. However, the T cell response to bacterial superantigens does not require Lck. In this study we report that not only is Lck dispensable for T cell activation by superantigens, but it actively inhibits this signaling pathway. Disruption of Lck function, either by repression of Lck gene expression or by selective pharmacologic inhibitors of Lck, led to increased IL-2 production in response to superantigen stimulation. This negative regulatory effect of Lck on superantigen-induced T cell responses required the kinase activity of Lck and correlated with early TCR signaling, but was independent of immunological synapse formation and TCR internalization. Our data demonstrate that the multistage role of Lck in T cell signaling includes the activation of a negative regulatory pathway of T cell activation.

Dopamine inhibits cytokine release and expression of tyrosine kinases, Lck and Fyn in activated T cells

The effect of dopamine (DA) on the release of cytokines from activated human T cells has been evaluated to analyze the mechanism by which physiological concentration of dopamine inhibits T cell proliferation. Dopamine inhibited anti-CD3 mAb-induced release of both Th1 and Th2 cytokines, IL2, IFN-gamma and IL4 from T cells by specific class of dopamine receptors. This action of dopamine was mediated by a new mechanism. Dopamine suppressed non-receptor tyrosine kinases, Lck and Fyn expression which are the initial and pivotal signaling steps in T cell receptor (TCR) mediated different down stream signaling cascades, leading to cytokine release and subsequent clonal expansion of these immune effector cells.

Up-regulated expression of MICA on activated T lymphocytes involves Lck and Fyn kinases and signaling through MEK1/ERK, p38 MAP kinase, and calcineurin

Major histocompatibility complex class I-related chain (MICA) is a cell stress-regulated molecule recognized by cytotoxic cells expressing the NKG2D molecule. MICA can be induced on T cells after CD3 or CD28 engagement. Here, we investigated the intracellular pathways leading to activation-induced expression of MICA. The Src kinase inhibitor PP1 inhibited up-regulated expression of MICA on anti-CD3-stimulated T cells. Downstream signaling routes involved mitogen-activated protein kinase (MAPK) kinase (MEK)1/extracellular signal-regulated kinase (ERK), p38 MAPK, and calcineurin, as MICA expression was prevented by U0126, SB202190, cyclosporin A, and FK506. Also, Lck and Fyn as well as MEK1/ERK and p38 MAPK were found to regulate MICA expression in anti-CD28/phorbol 12-myristate 13-acetate-stimulated T cells. Expression of MICA on activated T cells involved interleukin-2-dependent signaling routes triggered by Janus tyrosine kinases/signal transducer and activators of transcription and p70(S)(6) kinase, as it could be inhibited by AG490 and rapamycin. This is the first demonstration of the intracellular pathways involved in activation-induced expression of MICA, which may reveal potential targets for immune intervention to modulate MICA expression in pathological disorders.

The TCR ADAPts to integrin-mediated cell adhesion

Adapter proteins regulate leukocyte signal transduction through recruitment of effector molecules to multiprotein complexes. Recent studies in Adhesion and Degranulation promoting Adapter Protein (ADAP)-deficient mice have established that the cytoplasmic phosphoprotein ADAP is required for optimal, mature T-cell proliferation. Furthermore, ADAP plays a key role in T-cell antigen receptor (TCR)-mediated 'inside out' signaling leading to integrin activation and to enhanced cellular adhesion to integrin ligands. ADAP associates physically with molecules known to play roles in the regulation of TCR-stimulated actin polymerization. These associations support the hypothesis that ADAP functions in actin cytoskeletal reorganization leading to cellular adhesion and activation.

T cell receptor can be recruited to a subset of plasma membrane rafts, independently of cell signaling and attendantly to raft clustering

The constitutive/inducible association of the T cell receptor (TCR) with isolated detergent-resistant, lipid raft-derived membranes has been studied in Jurkat T lymphocytes. Membranes resistant to 1% Triton X-100 contained virtually no CD3epsilon, part of the TCR complex, irrespective of cell stimulation. On the other hand, membranes resistant either to a lower Triton X-100 concentration (i.e. 0.2%) or to the less hydrophobic detergent Brij 58 (1%) contained (i) a low CD3epsilon amount (approximate 2.7% of total) in resting cells and (ii) a several times higher amount of the TCR component, after T cell stimulation with either antigen-presenting cells or with phytohemagglutinin. It appeared that CD3/TCR was constitutively associated with and recruited to a raft-derived membrane subset because (i) all three membrane preparations contained a similar amount of the raft marker tyrosine kinase Lck but no detectable amounts of the conventional membrane markers, CD45 phosphatase and transferrin receptor; (ii) a larger amount of particulate membranes were resistant to solubilization with 0.2% Triton X-100 and Brij 58 than to solubilization with 1% Triton X-100; and (iii) higher cholesterol levels were present in membranes resistant to either the lower Triton X-100 concentration or to Brij 58, as compared with those resistant to 1% Triton X-100. The recruitment of CD3 to the raft-derived membrane subset appeared (i) to occur independently of cell signaling events, such as protein-tyrosine phosphorylation and Ca(2+) mobilization/influx, and (ii) to be associated with clustering of plasma membrane rafts induced by multiple cross-linking of either TCR or the raft component, ganglioside GM(1). We suggest that during T cell stimulation a lateral reorganization of rafts into polarized larger domains can determine the recruitment of TCR into these domains, which favors a polarization of the signaling cascade.

The influence of the src-family kinases, Lck and Fyn, on T cell differentiation, survival and activation

The src-family kinases p56lck (Lck) and p59fyn (Fyn) are expressed in T cells and are among the first signaling molecules to be activated downstream of the T cell receptor (TCR). Evidence is emerging that although closely related, these signaling molecules have discrete functions during development, maintenance and activation of peripheral T cells. For example, during thymopoiesis Lck is uniquely able to provide all the signals required for pre-TCRbeta selection, although Fyn can substitute for a subset of these. Positive selection of CD4 single-positive (SP) cells is also critically dependent on the expression of Lck but not Fyn, while differentiation of CD8 SP cells proceeds relatively efficiently in the absence of Lck. In naïve peripheral T cells either Lck or Fyn can transmit TCR-mediated survival signals, and yet only Lck is able to trigger TCR-mediated expansion signals under conditions of lymphopenia. Stimulation of naïve T cells by antigenic stimuli is also severely compromised in the absence of Lck, but more subtly impaired by the absence of Fyn. We discuss recent experiments addressing how these two src-kinase family members interface with downstream signaling pathways to regulate these diverse aspects of T cell behavior.

The formation and functions of the 21- and 23-kDa tyrosine-phosphorylated TCR zeta subunits

The interaction between the T cell receptor (TCR) and its cognate antigen/major histocompatibility complex (MHC) complex activates a cascade of intracellular protein phosphorylations within the T cell. The signals are initiated by the specific phosphorylation of two tyrosine residues located in a conserved sequence motif termed an ITAM (immune receptor-based tyrosine activation motif). There are 10 ITAMs in the TCR complex, and 6 of these ITAMs are present in the TCR zeta homodimer. Following TCR stimulation, the TCR zeta subunit forms two tyrosine-phosphorylated intermediates of 21- and 23-kDa, respectively. The dramatic and diverse biological responses of T cells are proposed to be partly regulated by the relative ratios of the 21- vs. 23-kDa phosphorylated forms of TCR zeta that are induced following TCR ligation. In this review, we describe a stepwise model of zeta phosphorylation required for the formation of these two phosphorylated derivatives. We describe the kinases and phosphatases controlling these phosphorylation processes. In addition, we present some preliminary findings from ongoing studies that discuss the contributions of each phosphorylated form of zeta on T cell development, TCR signaling, T cell anergy induction, and T cell survival.

Integrin-mediated tyrosine phosphorylation of Shc in T cells is regulated by protein kinase C-dependent phosphorylations of Lck

Integrin receptor signals are costimulatory for mitogenesis with the T-cell receptor during T-cell activation. A subset of integrin receptors can link to the adapter protein Shc and provide a mitogenic stimulus. Using a combination of genetic and pharmacological approaches, we show herein that integrin signaling to Shc in T cells requires the receptor tyrosine phosphatase CD45, the Src family kinase member Lck, and protein kinase C. Our results suggest a model in which integrin-dependent serine phosphorylation of Lck is the critical step that determines the efficiency of Shc tyrosine phosphorylation in T cells. Serine phosphorylation of Lck is dependent on PKC and is also linked to CD45 dephosphorylation. Mutants of Lck that cannot be phosphorylated on the critical serine residues do not signal efficiently to Shc and have greatly reduced kinase activity. This signaling from integrins to Lck may be an important step in the costimulation with the T-cell receptor during lymphocyte activation.

HIV Nef inhibits T cell migration

Nef is a viral regulatory protein of the human immunodeficiency virus (HIV) that has been shown to contribute to disease progression. Among its putative effects on T cell functions are the down-regulation of CD4 and major histocompatibility class I surface molecules. These effects occur in part via Nef interactions with intracellular signaling molecules. We sought to better characterize the effects of HIV Nef on T cell function by examining chemotaxis in response to stromal cell-derived factor-1alpha (SDF-1alpha) as well as CXCR4 signaling molecules. Here, we report the novel observation that HIV Nef inhibited chemotaxis in response to SDF-1alpha in both Jurkat T cells and primary peripheral CD4+ T lymphocytes. Our data indicate that HIV Nef altered critical downstream molecules in the CXCR4 pathway, including focal adhesion kinases. These findings suggest that HIV Nef may blunt the T cell response to chemokines. Because T lymphocyte migration is an integral component of host defense, HIV Nef may thereby contribute to the pathogenesis of AIDS.

P59(fyn) is upregulated in anergic CD8+ T cells

The ultimate goal in clinical transplantation is achievement of graft tolerance. Despite long-term immunosuppression, alloantigens on transplants elicit alloresponses that can initiate organ rejection. Acute rejection is mediated by CD8(+) cytotoxic T cells, whereas chronic rejection is a result of many factors including non-immunological events. The aim of this study was to examine the molecular requirements of T cell anergy, a cellular state that is an integral component of tolerance in vivo. In vitro, the tolerant state is usually best represented by T cell anergy, which is defined by loss of the ability of T cells to produce and secrete interleukin-2 upon restimulation. In the literature, molecular changes in anergic CD4(+) T cells have been studied in great detail, but only little is known about functional and biochemical characteristics of anergic CD8(+) T lymphocytes. In this study, we demonstrate, that CD8(+) T cells are rendered anergic by TCR stimulation without costimulation. They exhibit impaired interleukin-2 production and tyrosine-phosphorylation, but markedly upregulated p59(fyn) expression, which could be shown to be an early event during anergization. Anergic CD8(+) T lymphocytes show elevated surface expression of early activation markers as well as costimulatory molecules, especially that of CTLA4. These results, are an important component for the discovery of potential molecular targets, which contribute to the development and maintenance of tolerance.

Novel mode of interference with nuclear factor of activated T-cells regulation in T-cells by the bacterial metabolite n-butyrate

The transcription factor nuclear factor of activated T-cells (NF-AT) plays an essential role in the activation of many early immune response genes. A dynamic equilibrium between calcineurin and cellular kinases controls its phosphorylation and thus regulates its activity by determining its subcellular localization. Here, we demonstrate that T-cell activation in the presence of the bacterial metabolite n-butyrate, which leads to inhibition of interleukin-2 transcription, is characterized by the maintenance of the activity of counter-regulatory kinases glycogen synthase kinase 3 and protein kinase A as well as persistence of intracellular cAMP levels, whereas calcium response and mitogen-activated protein kinase activation were indistinguishable from cells stimulated in the absence of n-butyrate. Nuclear binding of NF-AT was decreased but other transcription factors implicated in interleukin-2 expression such as AP1 and nuclear factor kappaB were unaffected. The effect on NF-AT binding appeared to be the result of increased nuclear export because the export inhibitor leptomycin B completely restored nuclear binding of NF-AT. We, therefore, provide first evidence for interference with NF-AT regulation alternative to the currently understood inhibition of nuclear import. This mechanism might represent a bacterial strategy to subvert host defense, which could be of particular clinical importance in the gastrointestinal tract where high amounts of n-butyrate are physiologically present.

The Src family kinase Fyn mediates signals induced by TCR antagonists

FcR nonbinding anti-CD3 epsilon mAbs elicit partial TCR signaling that leads to T cell unresponsiveness and tolerance in vivo. In this study, the membrane-proximal events that promote T cell inactivation by FcR nonbinding anti-CD3 mAbs were examined. In the context of FcR nonbinding anti-CD3, TCR complexes did not aggregate and failed to translocate into glycolipid-enriched membrane microdomains. Furthermore, FcR nonbinding anti-CD3 mAbs induced tyrosine phosphorylation of the Fyn substrate Cbl, but not the ZAP-70 substrate linker for activation of T cells. Overexpression of Fyn, but not Lck, restored the mitogenicity of FcR nonbinding anti-CD3 in primary T cells. Taken together, these results suggest that Fyn mediates the partial signaling induced by TCR antagonists.

Mutational analysis of immunoreceptor tyrosine-based inhibition motifs of the Ig-like transcript 2 (CD85j) leukocyte receptor

The inhibitory receptor Ig-like transcript (ILT)2 (leukocyte Ig-like receptor or CD85j) is a type I transmembrane protein expressed by different leukocyte lineages. The extracellular region of ILT2 binds HLA class I molecules, and its cytoplasmic domain displays four immunoreceptor tyrosine-based inhibition motifs. Upon tyrosine phosphorylation ILT2 recruits the Src homology 2 domain-containing protein tyrosine phosphatase 1 (SHP-1) that is involved in negative signaling. To address the structural basis of ILT2-mediated inhibitory signaling, deletion and single tyrosine mutants were generated and transfected in the COS-7 and rat basophilic leukemia cell lines; their abilities to bind SHP-1 and to inhibit FcepsilonR-induced serotonin release in rat basophilic leukemia cells were studied. Both biochemical and functional analyses revealed tyrosines 644 (SIYATL) and 614 (VTYAQL) as the SHP-1 docking sites required for ILT2 inhibitory function. Substitution of tyrosine 562 (VTYAEV) did not alter receptor function. By contrast, mutation of tyrosine 533 (NLYAAV) interfered with ILT2 tyrosine phosphorylation and the subsequent SHP-1 recruitment, thus supporting a regulatory role for this motif.

Signalling pathways induced by protease-activated receptors and integrins in T cells

Recent characterization of the thrombin receptor indicates that it plays a role in T-cell signalling pathways. However, little is known regarding the signalling events following stimulation of additional members of the protease-activated receptor (PAR) family, i.e. PAR2 and PAR3. Most of the postligand cascades are largely unknown. Here, we illustrate that in Jurkat T-leukaemic cells, activation of PAR1, PAR2 and PAR3 induce tyrosine phosphorylation of Vav1. This response was impaired in Jurkat T cells deficient in p56lck (JCaM1.6). Activation of PARs also led to an increase in tyrosine phosphorylation of ZAP-70 and SLP-76, two key proteins in T-cell receptor (TCR) signalling. We also demonstrated that p56lck is meaningful for integrin signalling. Thus, JCaM1.6 cells exhibited a marked reduction in their adherence to fibronectin-coated plates, as compared to the level of adherence of Jurkat T cells. While the phosphorylation of Vav1 in T cells is augmented following adhesion, no additional increase was noted following treatment of the adhered cells with PARs. Altogether, we have identified key components in the postligand-signalling cascade of PARs and integrins. Furthermore, we have identified Lck as a critical and possibly upstream component of PAR-induced Vav1 phosphorylation, as well as integrin activation, in Jurkat T cells.

T cell regulation of p62(dok) (Dok1) association with Crk-L

In addition to engagement of the T cell receptor-CD3 complex, T lymphocytes can be activated by a variety of cell surface molecules including the approximately 50-kDa surface receptor CD2. While the majority of biochemical signaling elements are triggered by either CD2 or TcR-CD3 receptors, a small number of proteins are engaged by only one receptor. Recently, p62(dok) (Dok1), a member of the Dok family of adapter molecules, has been reported to be activated by CD2 and not by CD3 engagement. Here we have examined the role of p62(dok) in CD2-dependent signaling in Jurkat T cells. As previously reported, we find that ligation of the CD2 molecule by mitogenic pairs of anti-CD2 mAbs led to phosphorylation of p62(dok). While CD2-induced p62(dok) tyrosine phosphorylation was independent of both the p36/38 membrane adapter protein linker of activated T cells (LAT) and the ZAP70/Syk family of kinases, it was dependent upon the Src family of kinases including Lck and Fyn. We find further that CD2 engagement induced the association of tyrosine-phosphorylated p62(dok) to Crk-L. The CD2-dependent association of p62(dok) to Crk-L was independent of expression of the ZAP70/Syk family of kinases. Of note, while T cell receptor-CD3 engagement did not induce either p62(dok) phosphorylation or Crk-L association in Jurkat T cells, it did inhibit CD2-dependent p62(dok)-Crk-L complexes; this TcR-CD3-mediated regulation was dependent upon ZAP70 kinase activity. Our data suggest that phosphorylation of p62(dok) and its interaction with other signaling proteins may depend upon integrated signals emanating from the CD2 receptor, utilizing a ZAP70/LAT-independent pathway, and the TcR-CD3 receptor, which is ZAP70/Syk-dependent.

Superantigen-induced T cell:B cell conjugation is mediated by LFA-1 and requires signaling through Lck, but not ZAP-70

The formation of a conjugate between a T cell and an APC requires the activation of integrins on the T cell surface and remodeling of cytoskeletal elements at the cell-cell contact site via inside-out signaling. The early events in this signaling pathway are not well understood, and may differ from the events involved in adhesion to immobilized ligands. We find that conjugate formation between Jurkat T cells and EBV-B cells presenting superantigen is mediated by LFA-1 and absolutely requires Lck. Mutations in the Lck kinase, Src homology 2 or 3 domains, or the myristoylation site all inhibit conjugation to background levels, and adhesion cannot be restored by the expression of Fyn. However, ZAP-70-deficient cells conjugate normally, indicating that Lck is required for LFA-1-dependent adhesion via other downstream pathways. Several drugs that inhibit T cell adhesion to ICAM-1 immobilized on plastic, including inhibitors of mitogen-activated protein/extracellular signal-related kinase kinase, phosphatidylinositol-3 kinase, and calpain, do not inhibit conjugation. Inhibitors of phospholipase C and protein kinase C block conjugation of both wild-type and ZAP-70-deficient cells, suggesting that a phospholipase C that does not depend on ZAP-70 for its activation is involved. These results are not restricted to Jurkat T cells; Ag-specific primary T cell blasts behave similarly. Although the way in which Lck signals to enhance LFA-1-dependent adhesion is not clear, we find that cells lacking functional Lck fail to recruit F-actin and LFA-1 to the T cell:APC contact site, whereas ZAP-70-deficient cells show a milder phenotype characterized by disorganized actin and LFA-1 at the contact site.

Zap-70-independent Ca(2+) mobilization and Erk activation in Jurkat T cells in response to T-cell antigen receptor ligation

The tyrosine kinase ZAP-70 has been implicated as a critical intermediary between T-cell antigen receptor (TCR) stimulation and Erk activation on the basis of the ability of dominant negative ZAP-70 to inhibit TCR-stimulated Erk activation, and the reported inability of anti-CD3 antibodies to activate Erk in ZAP-70-negative Jurkat cells. However, Erk is activated in T cells receiving a partial agonist signal, despite failing to activate ZAP-70. This discrepancy led us to reanalyze the ZAP-70-negative Jurkat T-cell line P116 for its ability to support Erk activation in response to TCR/CD3 stimulation. Erk was activated by CD3 cross-linking in P116 cells. However, this response required a higher concentration of anti-CD3 antibody and was delayed and transient compared to that in Jurkat T cells. Activation of Raf-1 and MEK-1 was coincident with Erk activation. Remarkably, the time course of Ras activation was comparable in the two cell lines, despite proceeding in the absence of LAT tyrosine phosphorylation in the P116 cells. CD3 stimulation of P116 cells also induced tyrosine phosphorylation of phospholipase C-gamma1 (PLCgamma1) and increased the intracellular Ca(2+) concentration. Protein kinase C (PKC) inhibitors blocked CD3-stimulated Erk activation in P116 cells, while parental Jurkat cells were refractory to PKC inhibition. The physiologic relevance of these signaling events is further supported by the finding of PLCgamma1 tyrosine phosphorylation, Erk activation, and CD69 upregulation in P116 cells on stimulation with superantigen and antigen-presenting cells. These results demonstrate the existence of two pathways leading to TCR-stimulated Erk activation in Jurkat T cells: a ZAP-70-independent pathway requiring PKC and a ZAP-70-dependent pathway that is PKC independent.

Cleavage of Fyn and Lyn in their N-terminal unique regions during induction of apoptosis: a new mechanism for Src kinase regulation

The members of the Src kinase family are expressed in a wide variety of tissues, but some of them such as Blk, Hck, Fgr, Lck and Lyn are found primarily in hematopoietic cells. In the present study, we have undertaken experiments to test whether Src kinase cleavage and relocation is a general mechanism during induction of apoptosis. Our results indicate that Fyn and Lyn are efficiently cleaved in their unique region in hematopoietic cells undergoing apoptosis. Fyn cleavage occurred in Fas-stimulated Jurkat T cells but Fyn and Lyn were also processed in the SKW6.4 B cell line. Inhibition of caspases by Z-VAD-fmk or Ac-DEVD-CHO totally prevented Fyn and Lyn cleavage in both intact cells and in vitro. Fyn and Lyn but not Lck, Src and Hck were processed in vitro by human recombinant caspase 3 and by cellular extracts prepared from Fas-stimulated cells. Single mutation of Asp 19 or Asp 18 in the unique N-terminal domains of Fyn and Lyn respectively abolished their cleavage and relocation into the cytoplasm of apoptotic cells. When immunoprecipitated from COS cells N-terminal deleted Src kinases exhibited increased enzymatic kinase activity toward enolase. Thus, cleavage of Fyn and Lyn during induction of apoptosis represents a new mechanism for the regulation of Src kinases that may have important functional and physiological consequences.

Requirements for activation and RAFT localization of the T-lymphocyte kinase Rlk/Txk

BACKGROUND

The Tec family kinases are implicated in signaling from lymphocyte antigen receptors and are activated following phosphorylation by Src kinases. For most Tec kinases, this activation requires an interaction between their pleckstrin homology (PH) domains and the products of phosphoinositide 3-Kinase, which localizes Tec kinases to membrane RAFTs. Rlk/Txk is a Tec related kinase expressed in T cells that lacks a pleckstrin homology domain, having instead a palmitoylated cysteine-string motif. To evaluate Rlk's function in T cell receptor signaling cascades, we examined the requirements for Rlk localization and activation by Src family kinases.

RESULTS

We demonstrate that Rlk is also associated with RAFTs, despite its lack of a pleckstrin homology domain. Rlk RAFT association requires the cysteine-string motif and is independent of PI3 Kinase activity. We further demonstrate that Rlk can be phosphorylated and activated by Src kinases, leading to a decrease in its half-life. A specific tyrosine in the activation loop of Rlk, Y420, is required for phosphorylation and activation, as well as for decreased stability, but is not required for lipid RAFT association. Mutation of this tyrosine also prevents increased tyrosine phosphorylation of Rlk after stimulation of the T cell receptor, suggesting that Rlk is phosphorylated by Src family kinases in response to T cell receptor engagement.

CONCLUSIONS

Like the other related Tec kinases, Rlk is associated with lipid RAFTs and can be phosphorylated and activated by Src family kinases, supporting a role for Rlk in signaling downstream of Src kinases in T cell activation.

A novel function for the Tec family tyrosine kinase Itk in activation of beta 1 integrins by the T-cell receptor

Stimulation of T cells via the CD3--T-cell receptor (TCR) complex results in rapid increases in beta 1 integrin-mediated adhesion via poorly defined intracellular signaling events. We demonstrate that TCR-mediated activation of beta 1 integrins requires activation of the Tec family tyrosine kinase Itk and phosphatidylinositol 3-kinase (PI 3-K)-dependent recruitment of Itk to detergent-insoluble glycosphingolipid-enriched microdomains (DIGs) via binding of the pleckstrin homology domain of Itk to the PI 3-K product PI(3,4,5)-P(3). Activation of PI 3-K and the src family kinase Lck, via stimulation of the CD4 co-receptor, can initiate beta 1 integrin activation that is dependent on Itk function. Targeting of Itk specifically to DIGs, coupled with CD4 stimulation, can also activate beta 1 integrin function independently of TCR stimulation. Changes in beta 1 integrin function mediated by TCR activation of Itk are also accompanied by Itk-dependent modulation of the actin cytoskeleton. Thus, TCR-mediated activation of beta 1 integrins involves membrane relocalization and activation of Itk via coordinate action of PI 3-K and a src family tyrosine kinase.

p59fyn (Fyn) promotes the survival of anergic CD4-CD8- alpha beta TCR+ cells but negatively regulates their proliferative response to antigen stimulation

T cell anergy is characterized by alterations in TCR signaling that may play a role in controlling the unresponsiveness of the anergic cell. We have addressed questions regarding the importance of the Src kinase p59(fyn) (Fyn) in this process by using Fyn null mice. We demonstrate that a mature population of CD4(-)CD8(-) alphabeta TCR(+) anergic T cells lacking Fyn have a substantial recovery of their proliferation defect in response to Ag stimulation. This recovery cannot be explained by ameliorated production of IL-2, and the improved proliferation correlates with an enhanced ability of the Fyn(-/-) anergic T cells to up-regulate the high affinity IL-2 receptor. We also observe that anergic CD4(-)CD8(-) alphabeta TCR(+) T cells have a heightened survival ability that is partially dependent on the elevated levels of Fyn and IL-2 receptor beta-chain expressed by these cells. The enhanced survival correlates with an increased capacity of the anergic cells to respond to IL-15. We conclude that Fyn plays an important role in aspects of T cell anergy pertaining to TCR signaling and to cell survival.

IL-2 independent transformation of a unique human T cell line, TY8-3, and its subclones by HTLV-I and -II

Human T cell leukemia virus type I (HTLV-I) is etiologically associated with adult T cell leukemia (ATL) and chronic neurological disease, tropical spastic paraparesis (TSP). In our study, a unique IL-2 dependent human T cell line, designated TY8-3, was established from a thymoma obtained from a myasthenia gravis patient. The cells were heterogeneous and mainly consisted of those with CD4 , CD8 as well as activation markers and adhesion molecules including IL-2Ralpha,beta,gamma, CD45RO, Tf-R, HLA-DR, LFA-1alpha,beta, LFA-3, ICAM-1 and OX40 but without CD3 surface markers. Furthermore, these cells underwent an efficient and reproducible IL-2 independent transformation upon cocultivation with HTLV-I/II producing cell lines. Interestingly, although the infected cells became IL-2 independent, the growth rate of infected cells was significantly lower than those of parental TY8-3 cells. Clonal HTLV-I proviral DNA and viral particles were detected in the cells. Down-regulation of the lck and fyn genes and activation of the lyn gene was demonstrated in the IL-2 independent HTLV-positive TY8-3 cells. Subclones of TY8-3 cells were again able to be efficiently transformed and became IL-2 independent several months after coculture. Our results thus exhibit that TY8-3 cells and its subclones provide us with a very unique model whereby IL-2 independent transformation events of human T cells by HTLV-I/II in vitro can be studied at a clonal level.

CD28 and the tyrosine kinase lck stimulate mitogen-activated protein kinase activity in T cells via inhibition of the small G protein Rap1

Proliferation of T cells via activation of the T-cell receptor (TCR) requires concurrent engagement of accessory costimulatory molecules to achieve full activation. The best-studied costimulatory molecule, CD28, achieves these effects, in part, by augmenting signals from the TCR to the mitogen-activated protein (MAP) kinase cascade. We show here that TCR-mediated stimulation of MAP kinase extracellular-signal-regulated kinases (ERKs) is limited by activation of the Ras antagonist Rap1. CD28 increases ERK signaling by blocking Rap1 action. CD28 inhibits Rap1 activation because it selectively stimulates an extrinsic Rap1 GTPase activity. The ability of CD28 to stimulate Rap1 GTPase activity was dependent on the tyrosine kinase Lck. Our results suggest that CD28-mediated Rap1 GTPase-activating protein activation can help explain the augmentation of ERKs during CD28 costimulation.

T-cell receptor antagonists induce Vav phosphorylation by selective activation of Fyn kinase

T cell receptor (TCR) antagonists inhibit antigen-induced T cell activation and by themselves fail to induce phenotypic changes associated with T cell activation. However, we have recently shown that TCR antagonists are inducers of antigen-presenting cell (APC)-T cell conjugates. The signaling pathway associated with this cytoskeleton-dependent event appears to involve tyrosine phosphorylation and activation of Vav. In this study, we investigated the role played by the protein tyrosine kinases Fyn, Lck, and ZAP-70 in antagonist-induced signaling pathway. Antagonist stimulation increased tyrosine phosphorylation and kinase activity of Fyn severalfold, whereas little or no increase in Lck and ZAP-70 activity was observed. Second, TCR stimulation of Lck(-), Fyn(hi) Jurkat cells induced strong tyrosine phosphorylation of Vav. In contrast, minimal increase in tyrosine phosphorylation of Vav was observed in Lck(hi), Fyn(lo) Jurkat cells. Finally, study of T cells from a Fyn-deficient TCR transgenic mouse also showed that Fyn was required for tyrosine phosphorylation and activation of Vav induced by both antagonist and agonist peptides. The deficiency in Vav phosphorylation in Fyn-deficient T cells was associated with a defect in the formation of APC-T cell conjugates when T cells were stimulated with either agonist or antagonist peptide. We conclude from these results that Vav is a selective substrate for Fyn, especially under conditions of low-affinity TCR-mediated signaling, and that this signaling pathway involving Fyn, Vav, and Rac-1 is required for the cytoskeletal reorganization that leads to T cell-APC conjugates and the formation of the immunologic synapse.

Signaling via LAT (linker for T-cell activation) and Syk/ZAP70 is required for ERK activation and NFAT transcriptional activation following CD2 stimulation

Activation of T cells can be initiated through cell surface molecules in addition to the T-cell receptor-CD3 (TCR-CD3) complex. In human T cells, ligation of the CD2 molecule by mitogenic pairs of anti-CD2 monoclonal antibodies activates T cells via biochemical signaling pathways similar but not identical to those elicited on TCR engagement. This study describes a key role for the p36/38 membrane adapter protein linker for T cell activation (LAT) in CD2-mediated T-cell activation. Following ligation of CD2 on the surface of the Jurkat T-cell line and human purified T cells, LAT was tyrosine phosphorylated and shown to associate in vivo with a number of other tyrosine phosphorylated proteins including PLCγ-1, Grb-2, and SLP-76. Using Jurkat cell lines deficient in ZAP70/Syk (P116) or LAT (ANJ3) expression, CD2-dependent PLCγ-1 and SLP-76 tyrosine phosphorylation required expression both of ZAP70 or Syk and of LAT. As predicted, the absence of either LAT or ZAP70/Syk kinases correlated with a defect in the induction of nuclear factor of activated T cells (NFAT) transcriptional activity, activation of the interleukin-2 promoter, and ERK phosphorylation following CD2 stimulation. These data suggest that LAT is an adapter protein important for the regulation of CD2-mediated T-cell activation.

Signaling via LAT (linker for T-cell activation) and Syk/ZAP70 is required for ERK activation and NFAT transcriptional activation following CD2 stimulation

Activation of T cells can be initiated through cell surface molecules in addition to the T-cell receptor-CD3 (TCR-CD3) complex. In human T cells, ligation of the CD2 molecule by mitogenic pairs of anti-CD2 monoclonal antibodies activates T cells via biochemical signaling pathways similar but not identical to those elicited on TCR engagement. This study describes a key role for the p36/38 membrane adapter protein linker for T cell activation (LAT) in CD2-mediated T-cell activation. Following ligation of CD2 on the surface of the Jurkat T-cell line and human purified T cells, LAT was tyrosine phosphorylated and shown to associate in vivo with a number of other tyrosine phosphorylated proteins including PLCγ-1, Grb-2, and SLP-76. Using Jurkat cell lines deficient in ZAP70/Syk (P116) or LAT (ANJ3) expression, CD2-dependent PLCγ-1 and SLP-76 tyrosine phosphorylation required expression both of ZAP70 or Syk and of LAT. As predicted, the absence of either LAT or ZAP70/Syk kinases correlated with a defect in the induction of nuclear factor of activated T cells (NFAT) transcriptional activity, activation of the interleukin-2 promoter, and ERK phosphorylation following CD2 stimulation. These data suggest that LAT is an adapter protein important for the regulation of CD2-mediated T-cell activation.

Both src-dependent and -independent mechanisms mediate phosphatidylinositol 3-kinase regulation of colony-stimulating factor 1-activated mitogen-activated protein kinases in myeloid progenitors

Colony-stimulating factor 1 (CSF-1) supports the proliferation, survival, and differentiation of bone marrow-derived cells of the monocytic lineage. In the myeloid progenitor 32D cell line expressing CSF-1 receptor (CSF-1R), CSF-1 activation of the extracellular signal-regulated kinase (ERK) pathway is both Ras and phosphatidylinositol 3-kinase (PI3-kinase) dependent. PI3-kinase inhibition did not influence events leading to Ras activation. Using the activity of the PI3-kinase effector, Akt, as readout, studies with dominant-negative and oncogenic Ras failed to place PI3-kinase downstream of Ras. Thus, PI3-kinase appears to act in parallel to Ras. PI3-kinase inhibitors enhanced CSF-1-stimulated A-Raf and c-Raf-1 activities, and dominant-negative A-Raf but not dominant-negative c-Raf-1 reduced CSF-1-provoked ERK activation, suggesting that A-Raf mediates a part of the stimulatory signal from Ras to MEK/ERK, acting in parallel to PI3-kinase. Unexpectedly, a CSF-1R lacking the PI3-kinase binding site (DeltaKI) remained capable of activating MEK/ERK in a PI3-kinase-dependent manner. To determine if Src family kinases (SFKs) are involved, we demonstrated that CSF-1 activated Fyn and Lyn in cells expressing wild-type (WT) or DeltaKI receptors. Moreover, CSF-1-induced Akt activity in cells expressing DeltaKI is SFK dependent since Akt activation was prevented by pharmacological or genetic inhibition of SFK activity. The docking protein Gab2 may link SFK to PI3-kinase. CSF-1 induced Gab2 tyrosyl phosphorylation and association with PI3-kinase in cells expressing WT or DeltaKI receptors. However, only in DeltaKI cells are these events prevented by PP1. Thus in myeloid progenitors, CSF-1 can activate the PI3-kinase/Akt pathway by at least two mechanisms, one involving direct receptor binding and one involving SFKs.