Lck-independent triggering of T-cell antigen receptor signal transduction by staphylococcal enterotoxins

Integrin linked kinase and threonine tyrosine kinase modulate TCR signaling

T cell activation is critical for adaptive immunity, helping to protect the body from infection and tumors. A key step in this activation is signal transduction downstream of the T cell antigen receptor. This signaling involves several steps, with early ones occurring at the plasma membrane and others that occur later, after TCR internalization. The late steps in TCR signaling remain poorly understood. Since the TCR can signal after its internalization, we postulated that kinases abundantly expressed in T cells may regulate TCR signaling. This study focuses on two such enzymes: integrin-linked kinase (ILKs) and threonine-tyrosine kinase (TTKs), whose involvement in TCR signaling has not been previously studied. Using specific depletion of TTK and ILK by lentiviral shRNA, we show that in the absence of ILK and TTK, the early steps of TCR signaling are strongly enhanced, while IL-2 production by activated T cells is strongly decreased. These findings are relevant because TTK and ILK are both important targets in oncology, and our results show that their inhibition affects the activation of T cells, which play an essential role in anti-tumor defense.

Novel insights into the immune response to bacterial T cell superantigens

Bacterial T cell superantigens (SAgs) are a family of microbial exotoxins that function to activate large numbers of T cells simultaneously. SAgs activate T cells by direct binding and crosslinking of the lateral regions of MHC class II molecules on antigen-presenting cells with T cell receptors (TCRs) on T cells; these interactions alter the normal TCR-peptide-MHC class II architecture to activate T cells in a manner that is independent of the antigen specificity of the TCR. SAgs have well-recognized, central roles in human diseases such as toxic shock syndrome and scarlet fever through their quantitative effects on the T cell response; in addition, numerous other consequences of SAg-driven T cell activation are now being recognized, including direct roles in the pathogenesis of endocarditis, bloodstream infections, skin disease and pharyngitis. In this Review, we summarize the expanding family of bacterial SAgs and how these toxins can engage highly diverse adaptive immune receptors. We highlight recent findings regarding how SAg-driven manipulation of the adaptive immune response may operate in multiple human diseases, as well as contributing to the biology and life cycle of SAg-producing bacterial pathogens.

Combined Immunodeficiency Caused by a Novel Nonsense Mutation in LCK

Mutations affecting T-cell receptor (TCR) signaling typically cause combined immunodeficiency (CID) due to varying degrees of disturbed T-cell homeostasis and differentiation. Here, we describe two cousins with CID due to a novel nonsense mutation in LCK and investigate the effect of this novel nonsense mutation on TCR signaling, T-cell function, and differentiation. Patients underwent clinical, genetic, and immunological investigations. The effect was addressed in primary cells and LCK-deficient T-cell lines after expression of mutated LCK. RESULTS: Both patients primarily presented with infections in early infancy. The LCK mutation led to reduced expression of a truncated LCK protein lacking a substantial part of the kinase domain and two critical regulatory tyrosine residues. T cells were oligoclonal, and especially naïve CD4 and CD8 T-cell counts were reduced, but regulatory and memory including circulating follicular helper T cells were less severely affected. A diagnostic hallmark of this immunodeficiency is the reduced surface expression of CD4. Despite severely impaired TCR signaling mTOR activation was partially preserved in patients' T cells. LCK-deficient T-cell lines reconstituted with mutant LCK corroborated partially preserved signaling. Despite detectable differentiation of memory and effector T cells, their function was severely disturbed. NK cell cytotoxicity was unaffected. Residual TCR signaling in LCK deficiency allows for reduced, but detectable T-cell differentiation, while T-cell function is severely disturbed. Our findings expand the previous report on one single patient on the central role of LCK in human T-cell development and function.

Myricetin inhibits interferon-γ-induced PD-L1 and IDO1 expression in lung cancer cells

Programmed death ligand-1 (PD-L1) and indoleamine 2, 3-dioxygenase 1 (IDO1) are immune checkpoints induced by interferon-γ (IFN-γ) in the tumor microenvironment, leading to immune escape of tumors. Myricetin (MY) is a flavonoid distributed in many edible and medicinal plants. In this study, MY was identified to inhibit IFN-γ-induced PD-L1 expression in human lung cancer cells. It also reduced the expression of IDO1 and the production of kynurenine which is the product catalyzed by IDO1, while didn't show obvious effect on the expression of major histocompatibility complex-I (MHC-I), a crucial molecule for antigen presentation. In addition, the function of T cells was evaluated using a co-culture system consist of lung cancer cells and the Jurkat-PD-1 T cell line overexpressing PD-1. MY restored the survival, proliferation, CD69 expression and interleukin-2 (IL-2) secretion of Jurkat-PD-1 T cells suppressed by IFN-γ-treated lung cancer cells. Mechanistically, IFN-γ up-regulated PD-L1 and IDO1 at the transcriptional level through the JAK-STAT-IRF1 axis, which was targeted and inhibited by MY. Together, our research revealed a new mechanism of MY mediated anti-tumor activity and highlighted the potential implications of MY in tumor immunotherapy.

Superantigen Recognition and Interactions: Functions, Mechanisms and Applications

Superantigens are unconventional antigens which recognise immune receptors outside their usual recognition sites e.g. complementary determining regions (CDRs), to elicit a response within the target cell. T-cell superantigens crosslink T-cell receptors and MHC Class II molecules on antigen-presenting cells, leading to lymphocyte recruitment, induction of cytokine storms and T-cell anergy or apoptosis among many other effects. B-cell superantigens, on the other hand, bind immunoglobulins on B-cells, affecting opsonisation, IgG-mediated phagocytosis, and driving apoptosis. Here, through a review of the structural basis for recognition of immune receptors by superantigens, we show that their binding interfaces share specific physicochemical characteristics when compared with other protein-protein interaction complexes. Given that antibody-binding superantigens have been exploited extensively in industrial antibody purification, these observations could facilitate further protein engineering to optimize the use of superantigens in this and other areas of biotechnology.

The Innate Biologies of Adaptive Antigen Receptors

Nonclonal innate immune responses mediated by germ line-encoded receptors, such as Toll-like receptors or natural killer receptors, are commonly contrasted with diverse, clonotypic adaptive responses of lymphocyte antigen receptors generated by somatic recombination. However, the Variable (V) regions of antigen receptors include germ line-encoded motifs unaltered by somatic recombination, and theoretically available to mediate nonclonal, innate responses, that are independent of or largely override clonotypic responses. Recent evidence demonstrates that such responses exist, underpinning the associations of particular γδ T cell receptors (TCRs) with specific anatomical sites. Thus, TCRγδ can make innate and adaptive responses with distinct functional outcomes. Given that αβ T cells and B cells can also make nonclonal responses, we consider that innate responses of antigen receptor V-regions may be more widespread, for example, inducing states of preparedness from which adaptive clones are better selected. We likewise consider that potent, nonclonal T cell responses to microbial superantigens may reflect subversion of physiologic innate responses of TCRα/β chains.

Enhanced interaction between SEC2 mutant and TCR Vβ induces MHC II-independent activation of T cells via PKCθ/NF-κB and IL-2R/STAT5 signaling pathways

SEC2, a major histocompatibility complex class II (MHC II)-dependent T-cell mitogen, binds MHC II and T-cell receptor (TCR) Vβs to induce effective co-stimulating signals for clonal T-cell expansion. We previously characterized a SEC2 mutant with increased recognition of TCR Vβs, ST-4, which could intensify NF-κB signaling transduction, leading to IL-2 production and T-cell activation. In this study, we found that in contrast to SEC2, ST-4 could induce murine CD4⁺ T-cell proliferation in a Vβ8.2- and Vβ8.3-specific manner in the absence of MHC II⁺ antigen-presenting cells (APCs). Furthermore, although IL-2 secretion in response to either SEC2 or ST-4 stimulation was accompanied by up-regulation of protein kinase Cθ (PKCθ), inhibitor of κB (IκB), α and β IκB kinase (IKKα/β), IκBα, and NF-κB in mouse splenocytes, only ST-4 could activate CD4⁺ T cells in the absence of MHC II⁺ APCs through the PKCθ/NF-κB signaling pathway. The PKCθ inhibitor AEB071 significantly suppressed SEC2/ST-4-induced T-cell proliferation, CD69 and CD25 expression, and IL-2 secretion with or without MHC II⁺ APCs. Further, SEC2/ST-4-induced changes in PKCθ/NF-κB signaling were significantly relieved by AEB071 in a dose-dependent manner. Using Lck siRNA, we found that Lck controlled SEC2/ST-4-induced phosphorylation of PKCθ. We also demonstrated that the IL-2R/STAT5 pathway is essential for SEC2/ST-4-induced T-cell activation. Collectively, our data demonstrate that an enhanced ST-4-TCR interaction can compensate for lack of MHC II and stimulate MHC II-free CD4⁺ T-cell proliferation via PKCθ/NF-κB and IL-2R/STAT5 signaling pathways. Compared with SEC2, intensified PKCθ/NF-κB and IL-2R/STAT5 signals induced by ST-4 lead to enhanced T-cell activation. The results of this study will facilitate better understanding of TCR-based immunotherapies for cancer.

Staphylococcal Superantigens Use LAMA2 as a Coreceptor To Activate T Cells

Canonical Ag-dependent TCR signaling relies on activation of the src-family tyrosine kinase LCK. However, staphylococcal superantigens can trigger TCR signaling by activating an alternative pathway that is independent of LCK and utilizes a Gα11-containing G protein-coupled receptor (GPCR) leading to PLCβ activation. The molecules linking the superantigen to GPCR signaling are unknown. Using the ligand-receptor capture technology LRC-TriCEPS, we identified LAMA2, the α2 subunit of the extracellular matrix protein laminin, as the coreceptor for staphylococcal superantigens. Complementary binding assays (ELISA, pull-downs, and surface plasmon resonance) provided direct evidence of the interaction between staphylococcal enterotoxin E and LAMA2. Through its G4 domain, LAMA2 mediated the LCK-independent T cell activation by these toxins. Such a coreceptor role of LAMA2 involved a GPCR of the calcium-sensing receptor type because the selective antagonist NPS 2143 inhibited superantigen-induced T cell activation in vitro and delayed the effects of toxic shock syndrome in vivo. Collectively, our data identify LAMA2 as a target of antagonists of staphylococcal superantigens to treat toxic shock syndrome.

LFA-1 Engagement Triggers T Cell Polarization at the HIV-1 Virological Synapse

HIV-1 efficiently disseminates by cell-cell spread at intercellular contacts called virological synapses (VS), where the virus preferentially assembles and buds. Cell-cell contact triggers active polarization of organelles and viral proteins within infected cells to the contact site to support efficient VS formation and HIV-1 spread; critically, however, which cell surface protein triggers contact-induced polarization at the VS remains unclear. Additionally, the mechanism by which the HIV-1 envelope glycoprotein (Env) is recruited to the VS remains ill defined. Here, we use a reductionist bead-coupled antibody assay as a model of the VS and show that cross-linking the integrin LFA-1 alone is sufficient to induce active T cell polarization and recruitment of the microtubule organizing center (MTOC) in HIV-1-infected cells. Mutant cell lines coupled with inhibitors demonstrated that LFA-1-induced polarization was dependent on the T cell kinase ZAP70. Notably, immunofluorescent staining of viral proteins revealed an accumulation of surface Env at sites of LFA-1 engagement, with intracellular Env localized to a Golgi compartment proximal to the polarized MTOC. Furthermore, blocking LFA-1-induced MTOC polarization through ZAP70 inhibition prevented intracellular Env polarization. Taken together, these data reveal that LFA-1 is a key determinant in inducing dynamic T cell remodeling to the VS and suggest a model in which LFA-1 engagement triggers active polarization of the MTOC and the associated Env-containing secretory apparatus to sites of cell-cell contact to support polarized viral assembly and egress for efficient cell-cell spread.

IMPORTANCE

HIV-1 causes AIDS by spreading within immune cells and depletion of CD4 T lymphocytes. Rapid spread between these cells occurs by highly efficient cell-cell transmission that takes place at virological synapses (VS). VS are characterized by striking T cell remodeling that is spatially associated with polarized virus assembly and budding at sites of cell contact. Here, we show that the integrin LFA-1 triggers organelle polarization and viral protein recruitment, facilitating formation of the VS, and that this requires the T cell kinase ZAP70. Taken together, these data suggest a mechanism by which HIV-1-infected T cells sense and respond to cell contact to polarize viral egress and promote cell-cell spread. Understanding how cell-cell spread is regulated may help reveal therapeutic targets to specifically block this mode of HIV-1 dissemination.

Staphylococcal superantigens in colonization and disease

Superantigens (SAgs) are a family of potent immunostimulatory exotoxins known to be produced by only a few bacterial pathogens, including Staphylococcus aureus. More than 20 distinct SAgs have been characterized from different S. aureus strains and at least 80% of clinical strains harbor at least one SAg gene, although most strains encode many. SAgs have been classically associated with food poisoning and toxic shock syndrome (TSS), for which these toxins are the causative agent. TSS is a potentially fatal disease whereby SAg-mediated activation of T cells results in overproduction of cytokines and results in systemic inflammation and shock. Numerous studies have also shown a possible role for SAgs in other diseases such as Kawasaki disease (KD), atopic dermatitis (AD), and chronic rhinosinusitis (CRS). There is also now a rich understanding of the mechanisms of action of SAgs, as well as their structures and function. However, we have yet to discover what purpose SAgs play in the life cycle of S. aureus, and why such a wide array of these toxins exists. This review will focus on recent developments within the SAg field in terms of the molecular biology of these toxins and their role in both colonization and disease.

DHHC2 is a protein S-acyltransferase for Lck

Lck is a non-receptor tyrosine kinase of the Src family that is essential for T cell activation. Dual N-terminal acylation of Lck with myristate (N-acylation) and palmitate (S-acylation) is essential for its membrane association and function. Reversible S-acylation of Lck is observed in vivo and may function as a control mechanism. Here we identify the DHHC family protein S-acyltransferase DHHC2 as an enzyme capable of palmitoylating of Lck in T cells. Reducing the DHHC2 level in Jurkat T cells using siRNA causes decreased Lck S-acylation and partial dislocation from membranes, and conversely overexpression of DHHC2 increases S-acylation of an Lck surrogate, LckN10-GFP. DHHC2 localizes primarily to the endoplasmic reticulum and Golgi apparatus suggesting that it is involved in S-acylation of newly-synthesized or recycling Lck involved in T cell signalling.

Dendritic Cells Sensitize TCRs through Self-MHC-Mediated Src Family Kinase Activation

It is unclear whether peptide-MHC class II (pMHC) complexes on distinct types of APCs differ in their capacity to trigger TCRs. In this study, we show that individual cognate pMHC complexes displayed by dendritic cells (DCs), as compared with nonprofessional APCs, are far better in productively triggering Ag-specific TCRs independently of conventional costimulation. As we further show, this is accomplished by the unique ability of DCs to robustly activate the Src family kinases (SFKs) Lck and Fyn in T cells even in the absence of cognate peptide. Instead, this form of SFK activation depends on interactions of DC-displayed MHC with TCRs of appropriate restriction, suggesting a central role of self-pMHC recognition. DC-mediated SFK activation leads to "TCR licensing," a process that dramatically increases sensitivity and magnitude of the TCR response to cognate pMHC. Thus, TCR licensing, besides costimulation, is a main mechanism of DCs to present Ag effectively.

Superantigens: supersignalers?

Some bacterial and viral proteins are potent activators of the immune response, earning them the title of superantigens (SAgs). Infection with pathogens containing these proteins can produce massive T cell activation and can result in various potentially fatal conditions, such as toxic shock and food poisoning. Unlike conventional peptide antigens, SAgs bind promiscuously to the external faces of class II major histocompatibility complex (MHC) molecules and families of T cell receptors (TCRs), thereby activating large numbers of T cells simultaneously. The manner in which SAgs bind MHC and TCR differs from the way in which peptide antigens interact with these structures. Nevertheless, because they simultaneously engage MHC and TCR, SAgs were assumed to activate T cells through the canonical signaling pathway that has been described for T cell activation by TCR engagement of peptide-MHC complexes. However, recent research shows that SAgs also activate an alternative signaling pathway in T cells. This study shows that SAgs can stimulate T cells in the absence of the Src family kinase, Lck, by activating a heterotrimeric guanine nucleotide-binding protein (G protein), Galpha(11). Galpha(11) activates phospholipase C-beta (PLC-beta), rather than the more abundant PLC-gamma1, and, by this means, links SAg signaling to the phosphatidylinositol and protein kinase C signaling pathways. The discovery of a signaling pathway specifically activated by SAgs, and not by conventional peptide antigens, opens the possibility of developing therapeutic reagents that may help control diseases caused by these agents.

Bacterial Superantigens Bypass Lck-Dependent T Cell Receptor Signaling by Activating a Gα11-Dependent, PLC-β-Mediated Pathway

The paradigm to explain antigen-dependent T cell receptor (TCR) signaling is based on the activation of the CD4 or CD8 coreceptor-associated kinase Lck. It is widely assumed that this paradigm is also applicable to signaling by bacterial superantigens. However, these bacterial toxins can activate human T cells lacking Lck, suggesting the existence of an additional pathway of TCR signaling. Here we showed that this alternative pathway operates in the absence of Lck-dependent tyrosine-phosphorylation events and was initiated by the TCR-dependent activation of raft-enriched heterotrimeric Galpha11 proteins. This event, in turn, activated a phospholipase C-beta and protein kinase C-mediated cascade that turned on the mitogen-activated protein kinases ERK-1 and ERK-2, triggered Ca(2+) influx, and translocated the transcription factors NF-AT and NF-kappaB to the nucleus, ultimately inducing the production of interleukin-2 in Lck-deficient T cells. The triggering of this alternative pathway by superantigens suggests that these toxins use a G protein-coupled receptor as a coreceptor on T cells.

Role of Fyn in the rearrangement of tubulin cytoskeleton induced through TCR

The translocation of the microtubule-organizing center (MTOC), its associated signaling complex, and the secretory apparatus is the most characteristic early event that involves the tubulin cytoskeleton of T or NK cells after their interaction with APC or target cells. Our results show that Fyn kinase activity is essential for MTOC reorientation in an Ag-dependent system. Moreover, T cells from Fyn-deficient mice are unable to rearrange their tubulin cytoskeleton in response to anti-CD3-coated beads. Analysis of conjugates of T cells from transgenic OT-I mice with dendritic cells revealed that an antagonist peptide induces translocation of the MTOC, and that this process is impaired in T cells from Fyn(-/-) OT-I mice. In addition, Fyn deficiency significantly affects the MTOC relocation mediated by agonist peptide stimulation. These results reveal Fyn to be a key regulator of tubulin cytoskeleton reorganization in T cells.

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.

Gads/Grb2-mediated association with LAT is critical for the inhibitory function of Gab2 in T cells

A docking protein, Gab2, is recruited to the vicinity of the TCR complex and inhibits downstream signaling by interaction with negative regulators. However, the molecular mechanisms of this recruitment remain unclear. We have found that Gab2 associates with LAT upon TCR stimulation and that LAT is essential for Gab2 phosphorylation. By analysis of several Gab2 mutants, the c-Met binding domain (MBD) of Gab2 was found to be both necessary and sufficient for stimulation-induced LAT binding. Within the MBD domain, a novel Grb2 SH3 binding motif, PXXXR, is critical for constitutive association with Gads/Grb2. Through this association, Gab2 is recruited to the lipid raft after TCR ligation and exerts inhibitory function. The in vivo significance of this association is illustrated by the fact that T-cell responses are impaired in transgenic mice expressing wild-type Gab2 but not in mice expressing mutant Gab2 lacking the motif. Furthermore, T cells from Gab2-deficient mice showed enhanced proliferative responses upon TCR stimulation. These results indicate that Gads/Grb2-mediated LAT association is critical for the inhibitory function of Gab2, implying that Gab2 induced in stimulated T cells may exert an efficient negative feedback loop by recruiting inhibitory molecules to the lipid raft and competing with SLP-76 through Gads binding.

Strong induction of tyrosine phosphorylation, intracellular calcium, nuclear transcription factors and interferongamma, but weak induction of IL-2 in naïve T cells stimulated by bacterial superantigen

The outcome of T cell receptor (TCR) engagement is controlled by the differential recruitment of a variety of pathways, depending on the nature of the TCR ligand. Studies on superantigens (SAGs) were among the first describing such differential signaling; however, reported results are inconsistent. We took a quantitative approach to reinvestigate this question. Using nai;ve T cells from TCR transgenic mice, we found that compared to the antigenic peptide from pigeon cytochrome c, the SAG staphylococcal enterotoxin A very efficiently (100-2000-fold more sensitive on a weight basis) induced tyrosine kinase activity, intracellular calcium increase, and interferon (IFN)gamma production. Up-regulation of CD25 and CD69 and proliferation were less efficiently induced (20-30-fold more sensitive), and interleukin (IL)-2 production was induced least efficiently (only 2-fold more sensitive). This differential activation profile that varies with the activation event analyzed is discussed with respect to the propensity for SAG to induce anergy.

Lack of evidence for aggregation-dependent enhancement of p56lck in the signal transduction upon major histocompatibility complex recognition by mature T cells

The kinase activity of lymphocyte-specific tyrosine kinase p56lck (Lck) upon physiological major histocompatibility complex (MHC) recognition by normal mature T cells was examined. Recognition of the target MHC molecules by T cells induced phosphorylation of the zeta-chain without obvious enhancement of the background Lck activity. There was no sign of enhancement of Lck through putative T-cell receptor (TCR)-independent class II MHC/CD4 interactions either. As has been reported, cross-linking of CD4 molecules by antibodies induced a marked enhancement of Lck activity. However, it did not have an immediate relevance to TCR-mediated signal transduction, as judged from the lack of detectable de novo phosphorylation of zeta-chain and the absence of functional responses of T cells. These results strongly favour the model in which TCR-mediated signal transduction does not involve aggregation-dependent enhancement of Lck, suggesting that the signal can be triggered simply by the recruitment of already active Lck with basal kinase activity through the formation of a TCR/MHC/CD4 ternary complex.

F-actin dynamics control segregation of the TCR signaling cascade to clustered lipid rafts

Following ligand binding the TCR segregates to plasma membrane microdomains, termed lipid rafts, characterized by a highly ordered lipid structure favoring partitioning of glycosyl phosphatidyl inositol-linked costimulatory receptors and acylated signaling molecules. Here we show that the inducible association of the TCR and key signaling proteins with lipid rafts is dependent on the actin cytoskeleton through a mechanism involving raft coalescence. Although lipid rafts are required for full activation of the TCR-dependent tyrosine phosphorylation cascade and sustained signaling, triggering of TCR-proximal events, including Fyn activation and a first wave of Vav phosphorylation, is independent of lipid rafts, while a second wave of raft-dependent Vav phosphorylation occurs after raft coalescence, as also supported by the finding that Vav is phosphorylated in response to lipid raft clustering by GM1 aggregation. The constitutive association found between Vav and the CD3zeta chain suggests a model whereby the TCR-associated signaling machinery initiates raft aggregation by promoting F-actin reorganization, which permits full activation of the tyrosine phosphorylation cascade, further reorganization of the actin cytoskeleton and sustained signaling, leading to cell activation.

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.

Defective signaling to Fyn by a T cell antigen receptor lacking the alpha -chain connecting peptide motif

A key role in the communication between the alphabetaTCR and the CD3/zeta complex is played by a specific motif within the connecting peptide domain of the TCR alpha chain (alpha-CPM). T cell hybridomas expressing an alpha-CPM-mutated TCR show a dramatic impairment in antigen-driven interleukin-2 production. This defect can be complemented by a calcium ionophore, indicating that activation of the calcium pathway is impaired. Several lines of evidence implicate Fyn in the regulation of calcium mobilization, at least in part through the activation of phospholipase Cgamma. Here we have investigated the potential involvement of Fyn in the TCR alpha-CPM signaling defect. Using T cell hybridomas expressing either a wild-type TCR or an alpha-CPM mutant, we show that Fyn fails to be activated by the mutant receptor following SEB binding and fails to generate tyrosine-phosphorylated Pyk2, a member of the focal adhesion kinase family. This defect correlated with an impairment in phospholipase Cgamma phosphorylation. Production of interlukin-2 and activation of the transcription factor NF-AT in response to triggering of the TCR alpha-CPM mutant with SEB were fully restored in the presence of constitutively active Fyn. Hence the signaling defect generated by the TCR alpha-CPM mutation results at least in part from an impaired coupling of the TCR.CD3 complex to Fyn activation.

Synchronous deletion of Mtv-superantigen-reactive thymocytes in the CD3(medium/high) CD4(+)CD8(+) subset

Multiple model systems have demonstrated that negatively selected thymocytes can be deleted during the immature CD4(+)CD8(+) CD3(low) stage after high affinity interaction of T-cell receptors (TCRs) with antigen:major histocompatibility complex (MHC) complexes. Superantigens (SAGs) derived from endogenous mammary tumour viruses (Mtv) induce negative selection of Mtv-SAG-reactive thymocytes regardless of which peptide antigen is presented by MHC molecules. In this study, the timing of deletion of multiple subsets of Mtv-SAG-reactive CD4(+)CD8(+) thymocytes was investigated by a 4 colour flow cytometry in SJL x CBA/J cross-bred mice. Deletion of V beta 3(+), V beta 5(+), V beta 11(+), and V beta 17(+) Mtv-SAG-reactive thymocytes was found to occur synchronously in the most mature CD3(medium) and early CD3(high) subsets of CD4(+)CD8(+) thymocytes, in contrast with reports showing that the deletion of Mtv-SAG-reactive thymocytes can occur at different stages in particular model systems.

The Extracellular Domain of the ζ-Chain Is Essential for TCR Function

The ζ-chain homodimer is a key component in the TCR complex and exerts its function through its cytoplasmic immunoreceptor-tyrosine activation motif (1). The ζ-chain extracellular (EC) domain is highly conserved; however, its functional and structural contributions to the TCR signaling have not been elucidated. We show that the EC domain of the ζ homodimer is essential for TCR surface expression. To gain a more detailed structural and functional information about the ζ-chain EC domain, we applied a cysteine scanning mutagenesis to conserved amino acids of the short domain. The results showed that the interchain disulfide bridge can be displaced by seven or eight amino acids along the EC domain. The TCR signaling efficacy was dramatically reduced during peptide/MHC engagement in the ζ mutants containing the displaced disulfide bond. These signaling defective ζ mutants produced an unconventional early tyrosine phosphorylation pattern. While the tyrosine phosphorylated forms of ζ (p21 and p23) could be observed during Ag stimulation, downstream signaling events such as the generation of phospho-p36, higher m.w. forms of phospho-ζ, and phospho-ζ/ZAP-70 complexes were impaired. Together these results suggest an important function of the phylogenetically conserved ζ-EC domain.

Two Distinct Pathways Exist for Down-Regulation of the TCR

TCR down-regulation plays an important role in modulating T cell responses both during T cell development and in mature T cells. Down-regulation of the TCR is induced by engagement of the TCR by specific ligands and/or by activation of protein kinase C (PKC). We report here that ligand- and PKC-induced TCR down-regulation is mediated by two distinct, independent mechanisms. Ligand-induced TCR down-regulation is dependent on the protein tyrosine kinases p56lck and p59fyn but independent of PKC and the CD3γ leucine-based (L-based) internalization motif. In contrast, PKC-induced TCR down-regulation is dependent on the CD3γ L-based internalization motif but independent of p56lck and p59fyn. Finally, our data indicate that in the absence of TCR ligation, TCR expression levels can be finely regulated via the CD3γ L-based motif by the balance between PKC and serine/threonine protein phosphatase activities. Such a TCR ligation-independent regulation of TCR expression levels could probably be important in determining the activation threshold of T cells in their encounter with APC.