Activation of the Lck tyrosine kinase targets cell surface T cell antigen receptors for lysosomal degradation

Short-interfering RNA-mediated Lck knockdown results in augmented downstream T cell responses

The Src family kinase Lck is essential for T cell Ag receptor-mediated signaling. In this study, we report the effects of acute elimination of Lck in Jurkat TAg and primary T cells using RNA interference mediated by short-interfering RNAs. In cells with Lck knockdown (kd), proximal TCR signaling was strongly suppressed as indicated by reduced zeta-chain phosphorylation and intracellular calcium mobilization. However, we observed sustained and elevated phosphorylation of ERK1/2 in Lck kd cells 30 min to 2 h after stimulation. Downstream effects on immune function as determined by activation of a NFAT-AP-1 reporter, and TCR/CD28-stimulated IL-2 secretion were strongly augmented in Jurkat and primary T cells, respectively. As expected, overexpression of SHP-1 in Jurkat cells inhibited TCR-induced NFAT-AP-1 activation, but this effect could be overcome by simultaneous kd of Lck. Furthermore, acute elimination of Lck also suppressed TCR-mediated activation of SHP-1, suggesting the possible role of SHP-1 in a negative feedback loop originating from Lck. This report underscores Lck as an important mediator of proximal TCR signaling, but also indicates a suppressive role on downstream immune function.

Attenuation of gammadeltaTCR signaling efficiently diverts thymocytes to the alphabeta lineage

The role of the T cell antigen receptor complex (TCR) in alphabeta/gammadelta lineage commitment remains controversial, in particular whether different TCR isoforms intrinsically favor adoption of a certain lineage. Here, we demonstrate that impairing the signaling capacity of a gammadeltaTCR complex enables it to efficiently direct thymocytes to the alphabeta lineage. In the presence of a ligand, a transgenic gammadeltaTCR mediates almost exclusive adoption of the gammadelta lineage, while in the absence of ligand, the same gammadeltaTCR promotes alphabeta lineage development with efficiency comparable to the pre-TCR. Importantly, attenuating gammadeltaTCR signaling through Lck deficiency causes reduced ERK1/2 activation and Egr expression and diverts thymocytes to the alphabeta lineage even in the presence of ligand. Conversely, ectopic Egr overexpression favors gammadelta T cell development. Our data support a model whereby gammadelta versus alphabeta lineage commitment is controlled by TCR signal strength, which depends critically on the ERK MAPK-Egr pathway.

T cells in G1 provide a memory-like response to secondary stimulation

The commitment of naive T cells to proliferate is a function of the strength and duration of stimuli mediated by the TCR and coreceptors. Ranges of 2-20 h of stimulation have been reported as necessary in vitro. Whether T cells actually experience uninterrupted stimulation for such long periods under physiological conditions is controversial. Here we ask whether commitment to proliferate requires continuous stimulation, or can T cells integrate intermittent periods of stimulation. T cells were stimulated for two short-term (subthreshold) periods (5-7 h) either sequentially or separated by an interval of rest. Naive lymph node T cells were able to integrate interrupted stimulation, even when the duration of rest was as long as 2 days. Furthermore, when short-term-stimulated T cells were separated by density, three populations were observed: low density blasts, intermediate density G(1) cells, and high density G(0) cells. Low density cells progressed to division without further stimulation, whereas G(0) and G(1) cells remained undivided. However, after a period of rest, a second subthreshold stimulation caused the G(1) but not the G(0) fraction to quickly proceed through the cell cycle. We conclude that noncycling T cells in the G(1) phase of the cell cycle remain in a state of readiness for prolonged periods of time, and may represent a population of memory-like effectors capable of responding rapidly to antigenic challenge.

Masking of the CD3 gamma di-leucine-based motif by zeta is required for efficient T-cell receptor expression

The T-cell receptor (TCR) is a multimeric receptor composed of the Ti alpha beta heterodimer and the noncovalently associated CD3 gamma delta epsilon and zeta(2) chains. All of the TCR chains are required for efficient cell surface expression of the TCR. Previous studies on chimeric molecules containing the di-leucine-based endocytosis motif of the TCR subunit CD3 gamma have indicated that the zeta chain can mask this motif. In this study, we show that successive truncations of the cytoplasmic tail of zeta led to reduced surface expression levels of completely assembled TCR complexes. The reduced TCR expression levels were caused by an increase in the TCR endocytic rate constant in combination with an unaffected exocytic rate constant. Furthermore, the TCR degradation rate constant was increased in cells with truncated zeta. Introduction of a CD3 gamma chain with a disrupted di-leucine-based endocytosis motif partially restored TCR expression in cells with truncated zeta chains, indicating that the zeta chain masks the endocytosis motif in CD3 gamma and thereby stabilizes TCR cell surface expression.

Exclusion and inclusion of TCR alpha proteins during T cell development in TCR-transgenic and normal mice

Allelic exclusion of immune receptor genes (and molecules) is incompletely understood. With regard to TCRalphabeta lineage T cells, exclusion at the tcr-b, but not tcr-a, locus seems to be strictly controlled at the locus rearrangement level. Consequently, while nearly all developing TCRalphabeta thymocytes express a single TCRbeta protein, many thymocytes rearrange and express two different TCRalpha chains and, thus, display two alphabetaTCRs on the cell surface. Of interest, the number of such dual TCR-expressing cells is appreciably lower among the mature T cells. To understand the details of TCR chain regulation at various stages of T cell development, we analyzed TCR expression in mice transgenic for two rearranged alphabetaTCR. We discovered that in such TCR double-transgenic (TCRdTg) mice peripheral T cells were functionally monospecific. Molecularly, this monospecificity was due to TCRalpha exclusion: one transgenic TCRalpha protein was selectively down-regulated from the thymocyte and T cell surface. In searching for the mechanism(s) governing this selective TCRalpha down-regulation, we present evidence for the role of protein tyrosine kinase signaling and coreceptor involvement. This mechanism may be operating in normal thymocytes.

Engaged and bystander T cell receptors are down-modulated by different endocytotic pathways

T cell antigen receptor (TCR) engagement by stimulatory antibodies or its major histocompatibility complex-antigen ligand results in its down-modulation from the cell surface, a phenomenon that is thought to play a role in T cell desensitization. However, TCR engagement results in the down-modulation not only of the engaged receptors but also of non-engaged bystander TCRs. We have investigated the mechanisms that mediate the down-modulation of engaged and bystander receptors and show that co-modulation of the bystander TCRs requires protein-tyrosine kinase activity and is mediated by clathrin-coated pits. In contrast, the down-modulation of engaged TCRs is independent of protein-tyrosine kinases and clathrin pits, suggesting that this process is mediated by an alternate mechanism. Indeed, down-modulation of engaged TCRs appears to depend upon lipid rafts, because cholesterol depletion with methyl-beta-cyclodextrin completely blocks this process. Thus, two independent pathways of internalization are involved in TCR down-modulation and act differentially on directly engaged and bystander receptors. Finally, we propose that although both mechanisms coexist, the predominance of one or the other mechanisms will depend on the dose of ligand.

TCR zeta-chain downregulation: curtailing an excessive inflammatory immune response

The T-cell receptor (TCR) functions in both antigen recognition and signal transduction, which are crucial initial steps of antigen-specific immune responses. TCR integrity is vital for the induction of optimal and efficient immune responses, including the routine elimination of invading pathogens and the elimination of modified cells and molecules. Of the TCR subunits, the zeta-chain has a key role in receptor assembly, expression and signalling. Downregulation of TCR zeta-chain expression and impairment of T-cell function have been shown for T cells isolated from hosts with various chronic pathologies, including cancer, and autoimmune and infectious diseases. This review summarizes studies of the various pathologies that show this phenomenon and provides new insights into the mechanism responsible for downregulation of zeta-chain expression, its relevance and its clinical implications.

T cell receptor engagement leads to phosphorylation of clathrin heavy chain during receptor internalization

T cell receptor (TCR) internalization by clathrin-coated vesicles after encounter with antigen has been implicated in the regulation of T cell responses. We demonstrate that TCR internalization after receptor engagement and TCR signaling involves inducible phosphorylation of clathrin heavy chain (CHC) in both CD4⁺ and CD8⁺ human T cells. Studies with mutant Jurkat T cells implicate the Src family kinase Lck as the responsible enzyme and its activity in this process is influenced by the functional integrity of the downstream signaling molecule ZAP-70. CHC phosphorylation positively correlates with ligand-induced TCR internalization in both CD4⁺ and CD8⁺ T cells, and CHC phosphorylation as a result of basal Lck activity is also implicated in constitutive TCR endocytosis by CD4⁺ T cells. Remarkably, irreversible CHC phosphorylation in the presence of pervanadate reduced both constitutive and ligand-induced TCR internalization in CD4⁺ T cells, and immunofluorescence studies revealed that this inhibition affected the early stages of TCR endocytosis from the plasma membrane. Thus, we propose that CHC phosphorylation and dephosphorylation are involved in TCR internalization and that this is a regulatory mechanism linking TCR signaling to endocytosis.

Constitutive and Ligand-Induced TCR Degradation

Modulation of TCR expression levels is a central event during T cell development and activation, and it probably plays an important role in adjusting T cell responsiveness. Conflicting data have been published on down-regulation and degradation rates of the individual TCR subunits, and several divergent models for TCR down-regulation and degradation have been suggested. The aims of this study were to determine the rate constants for constitutive and ligand-induced TCR degradation and to determine whether the TCR subunits segregate or are processed as an intact unit during TCR down-regulation and degradation. We found that the TCR subunits in nonstimulated Jurkat cells were degraded with rate constants of approximately 0.0011 min(-1), resulting in a half-life of approximately 10.5 h. Triggering of the TCR by anti-TCR Abs resulted in a 3-fold increase in the degradation rate constants to approximately 0.0033 min(-1), resulting in a half-life of approximately 3.5 h. The subunits of the TCR complex were down-regulated from the cell surface and degraded with identical kinetics, and most likely remained associated during the passage throughout the endocytic pathway from the cell surface to the lysosomes. Similar results were obtained in studies of primary human Vbeta8+ T cells stimulated with superantigen. Based on these results, the simplest model for TCR internalization, sorting, and degradation is proposed.

Tyrosine kinase inhibitors: a new approach for asthma

The pathogenesis of allergic asthma involves the interplay of inflammatory cells and airway-resident cells, and of their secreted mediators including cytokines, chemokines, growth factors and inflammatory mediators. Receptor tyrosine kinases are important for the pathogenesis of airway remodeling. Activation of epidermal growth factor (EGF) receptor kinase and platelet-derived growth factor (PDGF) receptor kinase leads to hyperplasia of airway smooth muscle cells, epithelial cells and goblet cells. Stimulation of non-receptor tyrosine kinases (e.g. Lyn, Lck, Syk, ZAP-70, Fyn, Btk, Itk) is the earliest detectable signaling response upon antigen-induced immunoreceptor activation in inflammatory cells. Cytokine receptor dimerization upon ligand stimulation induces activation of Janus tyrosine kinases (JAKs), leading to recruitment and phosphorylation of signal transducer and activator of transcription (STAT) for selective gene expression regulation. Activation of chemokine receptors can trigger JAK-STAT pathway, Lck, Fyn, Lyn, Fgr, and Syk/Zap-70 to induce chemotaxis of inflammatory cells. Inhibitors of tyrosine kinases have been shown in vitro to block growth factor-induced hyperplasia of airway-resident cells; antigen-induced inflammatory cell activation and cytokine synthesis; cytokine-mediated pro-inflammatory gene expression in inflammatory and airway cells; and chemokine-induced chemotaxis of inflammatory cells. Recently, anti-inflammatory effects of tyrosine kinase inhibitors (e.g. genistein, tyrphostin AG213, piceatannol, tyrphostin AG490, WHI-P97, WHI-P131, Syk antisense) in animal models of allergic asthma have been reported. Therefore, development of inhibitors of tyrosine kinases can be a very attractive strategy for the treatment of asthma.

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.

Architectural Changes in the TCR:CD3 Complex Induced by MHC:Peptide Ligation

A hallmark of T cell activation is the ligation-induced down-modulation of the TCR:CD3 complex. However, little is known about the molecular events that drive this process. The CD3 zeta-chain has been shown to play a unique role in regulating the assembly, transport, and cell surface expression of the TCR:CD3 complex. In this study we have investigated the relationship between CD3zeta and the TCRalphabetaCD3epsilondeltagamma complex after ligation by MHC:peptide complexes. Our results show that there is a significant increase in free surface CD3zeta, which is not associated with the TCR:CD3 complex, after T cell stimulation. This may reflect dissociation of CD3zeta from the TCRalphabetaCD3epsilondeltagamma complex or transport of intracellular CD3zeta directly to the cell surface. We also show that MHC:peptide ligation also results in exposure of the TCR-associated CD3zeta NH2 terminus, which is ordinarily buried in the complex. These observations appears to be dependent on Src family protein tyrosine kinases, which are known to be critical for efficient T cell activation. These data suggest a mechanism by which ligated TCR may be differentiated from unligated TCR and selectively down-modulated.

Surface T-cell antigen receptor expression and availability for long-term antigenic signaling

It is important to understand how T-cell antigen receptor (TCR) engagement and signaling are regulated throughout an immune response. This review examines the dynamics of surface TCR expression and signaling capacity during thymic and effector T-cell development. Although the TCR can undergo vast changes in surface expression, T cells remain capable of sustaining TCR engagement for long periods of time. This may be achieved by a combination of mechanisms that involve (a) controlling the quantity of surface TCR available for ligand interaction and (b) controlling the quality of surface TCR expression during T-cell activation. TCR signaling itself appears to be one of the main quantitative modulators of surface TCR expression, and it can cause both downregulation and upregulation at different times of T-cell activation. Recent studies indicate that the degree of upregulation is tunable by the strength of antigenic stimulation. There is evidence that qualitatively distinct forms of the TCR exist, and their potential role in sustained antigenic signaling is also discussed. A goal of future studies will be to better characterize these modulations in surface TCR expression and to clarify their impact on the regulation of immune responses.

Regulation of membrane trafficking and subcellular organization of endocytic compartments revealed with FM1-43 in resting and activated human T cells

FM1-43, a fluorescent styryl dye that penetrates into and stains membranes, was used to investigate kinetics of constitutive endocytosis and to visualize the fate of endocytic organelles in resting and activated human T lymphocytes. The rate of dye accumulation was strongly temperature dependent and approximately 10-fold higher in activated than in resting T cells. Elevation of cytosolic free Ca2+ concentration with thapsigargin or ionomycin further accelerated the rate of FM1-43 accumulation associated with cytosolic actin polymerization. Direct modulation of actin polymerization affected membrane trafficking. Actin condensation beneath the plasma membrane with calyculin A abolished FM1-43 internalization, whereas actin depolymerization with cytochalasin D had no effect. Photoconversion of DAB by FM1-43 revealed altered endocytic compartment targeting associated with T cell activation. Internalized cargo was carried to lysosome-like compartments in resting T cells and to multivesicular bodies (MVB) in activated T cells. Externalization of exosomes from MVB occurred commonly in activated but not in resting T cells. T cell exosomes contained raft-associated CD3 proteins, GM1 glycosphingolipids, and phosphatidylserine at the outer membrane leaflet. The present study demonstrates the utility of FM1-43 as a marker of membrane trafficking in T cells and reveals possible mechanisms of its modulation during T cell activation.

Sustained exposure to bacterial antigen induces interferon-γ-dependent T cell receptor ζ down-regulation and impaired T cell function

T cell antigen receptor zeta chain down-regulation and impaired in vitro T cell function have been described in cancer and autoimmune and infectious diseases. However, the immunological basis for this phenomenon is unknown. Sustained exposure to antigen and chronic systemic inflammation, factors shared by the various pathologies, might account for this phenomenon. We developed an in vivo experimental system that mimics these conditions and show that sustained exposure of mice to bacterial antigens was sufficient to induce T cell antigen receptor zeta chain down-regulation and impair T cell function, provided an interferon-gamma-dependent T helper type 1 immune response developed. This indicates zeta chain down-regulation could be a physiological response that attenuates an exacerbated immune response. However, it can act as a 'double-edged sword', impairing immune responses to chronic diseases.

TCR comodulation of nonengaged TCR takes place by a protein kinase C and CD3 gamma di-leucine-based motif-dependent mechanism

One of the earliest events following TCR triggering is TCR down-regulation. However, the mechanisms behind TCR down-regulation are still not fully known. Some studies have suggested that only directly triggered TCR are internalized, whereas others studies have indicated that, in addition to triggered receptors, nonengaged TCR are also internalized (comodulated). In this study, we used transfected T cells expressing two different TCR to analyze whether comodulation took place. We show that TCR triggering by anti-TCR mAb and peptide-MHC complexes clearly induced internalization of nonengaged TCR. By using a panel of mAb against the Ti beta chain, we demonstrate that the comodulation kinetics depended on the affinity of the ligand. Thus, high-affinity mAb (K(D) = 2.3 nM) induced a rapid but reversible comodulation, whereas low-affinity mAb (K(D) = 6200 nM) induced a slower but more permanent type of comodulation. Like internalization of engaged TCR, comodulation was dependent on protein tyrosine kinase activity. Finally, we found that in contrast to internalization of engaged TCR, comodulation was highly dependent on protein kinase C activity and the CD3 gamma di-leucine-based motif. Based on these observations, a physiological role of comodulation is proposed and the plausibility of the TCR serial triggering model is discussed.

Signaling molecules as therapeutic targets in allergic diseases

A molecular understanding of physiologic and pathologic processes requires complete knowledge about the signal transduction mechanism of involved cells. Signal transduction research is a rapidly growing field in basic science. Unlike intercellular inflammatory mediators, signaling molecules show less functional redundancy. This allows inhibition of multiple cytokines/mediators by blocking one common signaling molecule. Interference with signaling pathways has shown significant potential for inhibition of fundamental processes as well as clinical phenotype of allergic diseases. The purpose of this review was to provide a theoretical classification of signaling molecules based on their function and to analyze various strategies for developing effective signaling inhibitors for allergic diseases.

Distinct roles of cellular Lck and p80 proteins in herpesvirus saimiri Tip function on lipid rafts

Lipid rafts are proposed to function as platforms for both receptor signaling and trafficking. Following interaction with antigenic peptides, the T-cell receptor (TCR) rapidly translocates to lipid rafts, where it transmits signals and subsequently undergoes endocytosis. The Tip protein of herpesvirus saimiri (HVS), which is a T-lymphotropic tumor virus, interacts with cellular Lck tyrosine kinase and p80, a WD domain-containing endosomal protein. Interaction of Tip with p80 induces enlarged vesicles and recruits Lck and TCR complex into these vesicles for trafficking. We report here that Tip is constitutively present in lipid rafts and that Tip interaction with p80 but not with Lck is necessary for its efficient localization in lipid rafts. The Tip-Lck interaction was required for recruitment of the TCR complex to lipid rafts, and the Tip-p80 interaction was critical for the aggregation and internalization of lipid rafts. These results suggest the potential mechanism for Tip-mediated TCR downregulation: Tip interacts with Lck to recruit TCR complex to lipid rafts, and it subsequently interacts with p80 to initiate the aggregation and internalization of the lipid raft domain and thereby downregulate the TCR complex. Thus, the signaling and targeting functions of HVS Tip rely on two functionally and genetically separable mechanisms that independently target cellular Lck tyrosine kinase and p80 endosomal protein.

Allogeneic bone marrow inhibits T-cell activation and clonal expansion in vitro

BACKGROUND

Donor bone marrow infusion has long been used to enhance graft survival or induce tolerance in T cell depleted solid organ allograft recipients. However, the mechanisms through which bone marrow cells affect tolerance remain obscure. We studied the affect of allogeneic bone marrow cells on the activation of allospecific T cells in vitro.

METHODS

Carboxyfluorescein-diacetate succinimidyl ester-labeled CBA/Ca strain CD8+ splenocytes, bearing T-cell receptor alpha and beta transgenes from the BM3.3 T-cell clone specific for the major histocompatibility complex class I antigen Kb, were placed in culture with irradiated C57BL/6J stimulator cells in the presence of increasing numbers of C57BL/6J or Balb/cJ bone marrow cells for 1 to 3 days. Responder cells were individually analyzed for proliferative history, expression of activation-associated antigens, and intracellular cytokine production.

RESULTS

Allogeneic bone marrow cells exert a dose-dependent inhibitory effect on proliferation of allospecific CD8+ T cells in mixed lymphocyte culture. However, the inhibited T-cell subpopulations show physiologic changes associate with the early stages of T-cell activation, including expression of CD69 and early decrease of surface T-cell expression. Unlike cells not co-cultured with bone marrow, these cells fail to reexpress the T-cell receptor (TCR) by 72 hr of culture. The observed inhibitory effect is also associated with a decrease in the proportion of CD8+ cells expressing interleukin-2 and interferon-gamma.

CONCLUSIONS

Collectively, these results suggest that peripheral allospecific T cells undergo the initial stages of activation on exposure to antigen in the presence of bone marrow cells, but the cell cycle is arrested and TCR reexpression is inhibited. We speculate that bone marrow cells effect this inhibition through a receptor-ligand interaction that modulates the transmembrane signal pathway for the TCR.

Allelic exclusion of the TCR alpha-chain is an active process requiring TCR-mediated signaling and c-Cbl

Phenotypic allelic exclusion at the TCRalpha locus is developmentally regulated in thymocytes. Many immature thymocytes express two cell surface alpha-chain species. Following positive selection, the vast majority of mature thymocytes and peripheral T cells display a single cell surface alpha-chain. A posttranslational mechanism occurring at the same time as positive selection and TCR up-regulation leads to this phenotypic allelic exclusion. Different models have been proposed to explain the posttranslational regulation of the alpha-chain allelic exclusion. In this study, we report that allelic exclusion is not regulated by competition between distinct alpha-chains for a single beta-chain, as proposed by the dueling alpha-chain model, nor by limiting CD3 zeta-chain in mature TCR(high) thymocytes. Our data instead favor the selective retention model where the positive selection signal through the TCR leads to phenotypic allelic exclusion by specifically maintaining cell surface expression of the selected alpha-chain while the nonselected alpha-chain is internalized. The use of inhibitors specific for Lck and/or other Src kinases indicates a role for these protein tyrosine kinases in the signaling events leading to the down-regulation of the nonselectable alpha-chain. Loss of the ubiquitin ligase/TCR signaling adapter molecule c-Cbl, which is important in TCR down-modulation and is a negative regulator of T cell signaling, leads to increased dual alpha-chain expression on the cell surface of double-positive thymocytes. Thus, not only is there an important role for TCR signaling in causing alpha-chain allelic exclusion, but differential ubiquitination by c-Cbl may be an important factor in causing only the nonselected alpha-chain to be down-modulated.

A role for the cytoplasmic tail of the pre-T cell receptor (TCR) alpha chain in promoting constitutive internalization and degradation of the pre-TCR

Engagement of the alpha beta T cell receptor (TCR) by its ligand results in the down-modulation of TCR cell surface expression, which is thought to be a central event in T cell activation. On the other hand, pre-TCR signaling is a key process in alpha beta T cell development, which appears to proceed in a constitutive and ligand-independent manner. Here, comparative analyses on the dynamics of pre-TCR and TCR cell surface expression show that unligated pre-TCR complexes expressed on human pre-T cells behave as engaged TCR complexes, i.e. they are rapidly internalized and degraded in lysosomes and proteasomes but do not recycle back to the cell surface. Thus, pre-TCR down-regulation takes place constitutively without the need for extracellular ligation. By using TCR alpha/p Tau alpha chain chimeras, we demonstrate that prevention of recycling and induction of degradation are unique pre-TCR properties conferred by the cytoplasmic domain of the pT alpha chain. Finally, we show that pre-TCR internalization is a protein kinase C-independent process that involves the combination of src kinase-dependent and -independent pathways. These data suggest that constitutive pre-TCR down-modulation regulates pre-TCR surface expression levels and hence the extent of ligand-independent signaling through the pre-TCR.

Identification of three NFAT binding motifs in the 5'-upstream region of the human CD3gamma gene that differentially bind NFATc1, NFATc2, and NF-kappa B p50

Human immunodeficiency virus, type 1 (HIV-1) infection of CD4(+) T cells progressively abrogates T cell receptor (TCR).CD3 function and surface expression by specifically interfering with CD3gamma gene transcription. Our data show that the loss of CD3gamma transcripts begins very early after infection and accumulates to a >90% deficiency before a significant effect on surface receptor density is apparent. Blocking TCR.CD3-directed NFAT activation with cyclosporin A provokes a partial re-expression of CD3gamma gene transcripts and surface complexes in a time- and dose-dependent manner. We have identified three NFAT consensus sequences (5'-GGAAA-3') in the 5'-upstream region of the human CD3gamma gene at: -124 to -120 (NFAT(gamma1)), -384 to -380 (NFAT(gamma2)), and +450 to +454 (NFAT(gamma3)) from the first transcription initiation site. Using electrophoretic mobility shift and supershift assays, we show that NFATc2 alone binds to the NFAT(gamma2) motif; however, complexes containing either NFATc2 or NFATc1 plus NF-kappaB p50 bind to the NFAT(gamma1) and NFAT(gamma3) sites. We further demonstrate that NFATc1 and NF-kappaB p50 bind in the same protein.DNA complex and that a fourth Ala added to the core sequence (5'-GGAAAA-3') in NFAT(gamma1), and NFAT(gamma3) is critical for their binding. Finally, we have shown that an increase in the binding of nuclear NFATc2, NFATc1, and NF-kappaB p50 to these three motifs is correlated with a progressive loss of CD3gamma transcripts after HIV-1 infection.

Regulation of constitutive TCR internalization by the zeta-chain

The ability of a T cell to be activated is critically regulated by the number of TCRs expressed on the plasma membrane. Cell surface TCR expression is influenced by dynamic processes such as synthesis and transport of newly assembled receptors, endocytosis of surface TCR, and recycling to the plasma membrane of internalized receptors. In this study, the internalization of fluorescently labeled anti-TCR Abs was used to analyze constitutive endocytosis of TCRs on T cells, and to investigate the role of the zeta-chain in this process. We found that cell surface TCRs lacking zeta were endocytosed more rapidly than completely assembled receptors, and that reexpression of full-length zeta led to a dose-dependent decrease in the rate of TCR internalization. Rapid TCR internalization was also observed with CD4(+)CD8(+) thymocytes from zeta-deficient mice, whereas TCR internalization on thymocytes from CD3-delta deficient animals was slow, similar to that of wild-type thymocytes. This identifies a specific role for zeta in the regulation of constitutive receptor internalization. Furthermore, chimeric zeta molecules containing non-native intracellular amino acid sequences also led to high levels of TCR expression and reduced TCR cycling. These effects were dependent solely on the length of the intracellular tail, ruling out a role for intracellular zeta-specific interactions with other molecules as a mechanism for regulating TCR internalization. Rather, these findings strongly support a model in which the zeta-chain stabilizes TCR residency on the cell surface, and functions to maintain cell surface receptor expression by sterically blocking internalization sequences in other TCR components.

Optimal colocalization of TCR and CD8 as a novel mechanism for the control of functional avidity

The improved efficacy of high avidity CTL for clearance of virus has been well-documented. Thus, elucidation of the mechanisms that confer the increased sensitivity to peptide ligand demonstrated by high avidity CTL is critical. Using CTL lines of high and low avidity generated from a TCR transgenic mouse, we have found that functional avidity can be controlled by the expression of CD8alphaalpha vs CD8alphabeta and the ability of CTLs to colocalize the TCR and CD8 in the membrane. Colocalization of these molecules was mediated by lipid rafts and importantly, raft disruption resulted in the conversion of high avidity CTL into a lower functional avidity phenotype. These novel findings provide insights into the control of functional avidity in response to viral infection.

Regulation of T cell receptor-induced activation of the Ras-ERK pathway by diacylglycerol kinase zeta

T cell development in the thymus and activation of mature T cells in the periphery depend on signals stimulated by engagement of the T cell antigen receptor (TCR). Among the second messenger cascades initiated by TCR ligation include the phosphatidylinositol pathway where the membrane phospholipid, phosphatidylinositol 4,5-bisphosphate, is hydrolyzed to inositol 1,4,5-trisphosphate and diacylglycerol (DAG). Inositol 1,4,5-trisphosphate signals a rise in intracellular free calcium, leading to translocation of nuclear factor of activated T cells into the nucleus. DAG activates RasGRP and protein kinase C theta. Because both RasGRP and protein kinase C theta are essential for thymocyte and T cell function, it is critical to understand how DAG is regulated. In this report, we demonstrate expression of DAG kinase zeta (DGKzeta, the enzyme that catalyzes the conversion of DAG to phosphatidic acid) in multiple lymphoid organs, with highest expression observed within the T cell compartment. Overexpression studies in Jurkat T cells indicate that DGKzeta interferes with TCR-induced Ras and ERK activation, AP-1 induction, and expression of the activation marker CD69. In contrast, TCR-stimulated calcium influx is not altered. Mutational analysis indicates that the kinase and DAG binding domains, but not the ankyrin repeats of DGKzeta, are required for its inhibitory effects. Collectively these studies demonstrate a potential role of DGKzeta to function as a selective negative regulator of DAG signaling on T cell activation and provide the first structure/function analysis of this enzyme in T cells.

TCR/CD3 down-modulation and zeta degradation are regulated by ZAP-70

TCR down-modulation following binding to MHC/peptide complexes is considered to be instrumental for T cell activation because it allows serial triggering of receptors and the desensitization of stimulated cells. We studied CD3/TCR down-modulation and zeta degradation in T cells from two ZAP-70-immunodeficient patients. We show that, at high occupancy of the TCR, down-modulation of the CD3/TCR is comparable whether T cells express or do not express ZAP-70. However, if TCR occupancy was low, we found that CD3/TCR was down-regulated to a lesser extent in ZAP-70-negative than in ZAP-70-positive T cells. We studied CD3/TCR down-modulation in P116 (a ZAP-70-negative Jurkat cell-derived clone) and in P116 transfected with genes encoding the wild-type or a kinase-dead form of ZAP-70. Down-modulation of the TCR at high occupancy did not require ZAP-70, whereas at low TCR occupancy down-modulation was markedly reduced in the absence of ZAP-70 and in cells expressing a dead kinase mutant of ZAP-70. Thus, the presence of ZAP-70 alone is not sufficient for down-modulation; the kinase activity of this molecule is also required. The degradation of zeta induced by TCR triggering is also severely impaired in T cells from ZAP-70-deficient patients, P116 cells, and P116 cells expressing a kinase-dead form of ZAP-70. This defect in TCR-induced zeta degradation is observed at low and high levels of TCR occupancy. Our results identify ZAP-70, a tyrosine kinase known to be crucial for T cell activation, as a key player in TCR down-modulation and zeta degradation.

Herpesviral protein targets a cellular WD repeat endosomal protein to downregulate T lymphocyte receptor expression

Herpesvirus saimiri Tip associates with Lck and downregulates Lck signal transduction. Here we demonstrate that Tip targets a lysosomal protein p80, which consists of an N-terminal WD repeat domain and a C-terminal coiled-coil domain. Interaction of Tip with p80 facilitated lysosomal vesicle formation and subsequent recruitment of Lck into the lysosomes for degradation. Consequently, Tip interactions with Lck and p80 result in downregulation of T cell receptor (TCR) and CD4 surface expression. Remarkably, these actions of Tip are functionally and genetically separable: the N-terminal p80 interaction is responsible for TCR downregulation and the C-terminal Lck interaction is responsible for CD4 downregulation. Thus, lymphotropic herpesvirus has evolved an elaborate mechanism to deregulate lymphocyte receptor expression to disarm host immune control.

Constitutive endocytosis and degradation of the pre-T cell receptor

The pre-T cell receptor (TCR) signals constitutively in the absence of putative ligands on thymic stroma and signal transduction correlates with translocation of the pre-TCR into glycolipid-enriched microdomains (rafts) in the plasma membrane. Here, we show that the pre-TCR is constitutively routed to lysosomes after reaching the cell surface. The cell-autonomous down-regulation of the pre-TCR requires activation of the src-like kinase p56(lck), actin polymerization, and dynamin. Constitutive signaling and degradation represents a feature of the pre-TCR because the gammadeltaTCR expressed in the same cell line does not exhibit these features. This is also evident by the observation that the protein adaptor/ubiquitin ligase c-Cbl is phosphorylated and selectively translocated into rafts in pre-TCR- but not gammadeltaTCR-expressing cells. A role of c-Cbl-mediated ubiquitination in pre-TCR degradation is supported by the reduction of degradation through pharmacological inhibition of the proteasome and through a dominant-negative c-Cbl ubiquitin ligase as well as by increased pre-TCR surface expression on immature thymocytes in c-Cbl-deficient mice. The pre-TCR internalization contributes significantly to the low surface level of the receptor on developing T cells, and may in fact be a requirement for optimal pre-TCR function.

Ligand-induced TCR down-regulation is not dependent on constitutive TCR cycling

TCR internalization takes place both in resting T cells as part of constitutive TCR cycling, after PKC activation, and during TCR triggering. It is still a matter of debate whether these pathways represent distinct pathways. Thus, some studies have indicated that ligand-induced TCR internalization is regulated by mechanisms distinct from those involved in constitutive internalization, whereas other studies have suggested that the ligand-induced TCR internalization pathway is identical with the constitutive pathway. To resolve this question, we first identified requirements for constitutive TCR cycling. We found that in contrast to PKC-induced TCR internalization where both CD3gamma-S(126) and the CD3gamma leucine-based internalization motif are required, constitutive TCR cycling required neither PKC nor CD3gamma-S(126) but only the CD3gamma leucine-based motif. Having identified these requirements, we next studied ligand-induced internalization in cells with abolished constitutive TCR cycling. We found that ligand-induced TCR internalization was not dependent on constitutive TCR internalization. Likewise, constitutive internalization and recycling of the TCR were independent of an intact ligand-induced internalization of the TCR. In conclusion, ligand-induced TCR internalization and constitutive cycling of the TCR represents two independent pathways regulated by different mechanisms.

The CD3 gamma leucine-based receptor-sorting motif is required for efficient ligand-mediated TCR down-regulation

TCR down-regulation plays an important role in modulating T cell responses both during T cell development and in mature T cells. At least two distinct pathways exist for down-regulation of the TCR. One pathway is activated following TCR ligation and is dependent on tyrosine phosphorylation. The other pathway is dependent on protein kinase C (PKC)-mediated activation of the CD3 gamma di-leucine-based receptor-sorting motif. Previous studies have failed to demonstrate a connection between ligand- and PKC-induced TCR down-regulation. Thus, although an apparent paradox, the dogma has been that ligand- and PKC-induced TCR down-regulations are not interrelated. By analyses of a newly developed CD3 gamma-negative T cell variant, freshly isolated and PHA-activated PBMC, and a mouse T cell line, we challenged this dogma and demonstrate in this work that PKC activation and the CD3 gamma di-leucine-based motif are indeed required for efficient ligand-induced TCR down-regulation.

The Src-like adaptor protein downregulates the T cell receptor on CD4+CD8+ thymocytes and regulates positive selection

In this report, we show that the Src-like adaptor protein (SLAP) plays an important role in thymocyte development. SLAP expression is developmentally regulated; it is low in CD4-CD8- thymocytes, it peaks in the CD4+CD8+ subset, and it decreases to low levels in more mature cells. Disruption of the SLAP gene leads to a marked upregulation of TCR and CD5 expression at the CD4+CD8+ stage. The absence of SLAP was also developmentally significant because it enhanced positive selection in mice expressing the DO11.10 transgenic T cell receptor. Moreover, SLAP deletion at least partially rescued the development of ZAP-70-deficient thymocytes. These results demonstrate that SLAP participates in a novel mechanism of TCR downregulation at the CD4+CD8+ stage and regulates positive selection.

Immunotoxin FN18-CRM9 induces stronger T cell signaling than unconjugated monoclonal antibody FN18

BACKGROUND

The T-cell receptor (TCR)/CD3 complex is the target of therapeutic strategies aimed at prolonging allograft survival. The immunotoxin FN18-CRM9, composed of the anti-CD3 monoclonal antibody FN18 and the mutated diphtheria toxin CRM9, is useful for prolonging allograft survival in preclinical models of transplantation. To explore the influence of conjugation of the mutated diphtheria toxin on functional activation of the TCR/CD3 complex, we compared the effects of FN18-CRM9 and unconjugated FN18 on protein tyrosine phosphorylation and ligand/receptor internalization in purified monkey peripheral blood T cells.

METHODS

Purified normal rhesus monkey T cells were incubated with unconjugated FN18 or conjugated FN18-CRM9 and examined for differences in antibody binding, tyrosine phosphorylation, and CD3 internalization.

RESULTS

Binding cross-inhibition studies demonstrated that both compounds were able to inhibit fluorescein isothiocyanate-FN18 binding to CD3 with similar efficacy and potency. However, FN18-CRM9 was more potent than FN18 in triggering the phosphorylation of several proteins on tyrosine residues and in inducing CD3 internalization. The tyrosine kinase inhibitor genistein blocked FN18-CRM9-induced protein tyrosine phosphorylation and CD3 internalization, suggesting that tyrosine phosphorylation is involved in the internalization of the immunotoxin. Interestingly, in FN18-CRM9- but not FN18-treated cells, there was a gradual decrease in cellular CD3 protein levels within 24 and 48 hr; such a decrease was not observed with the control protein Csk.

CONCLUSIONS

Our findings suggest that the conjugation of the mutated diphtheria toxin CRM9 to FN18 modulates the monoclonal antibody-mediated cross-linking of the TCR/CD3 complex, leading to a stronger protein tyrosine phosphorylation and CD3 internalization. This may in turn contribute to the greater efficacy of the immunotoxin in prolonging allograft survival.

Antigen decoding by T lymphocytes: from synapses to fate determination

Naïve T lymphocytes sense foreign antigens by establishing contacts with dendritic cells (DCs). At the immunological synapse between the T cell and a DC, T cell receptors (TCRs) are serially engaged and triggered by specific ligands. The amount and duration of TCR triggering and the efficiency of signal amplification determine T cell commitment to proliferation and differentiation. The nature and availability of DCs bearing antigen and costimulatory molecules shape the T cell response, giving rise to distinct functional outputs such as effector and memory T cell generation or T cell tolerance.

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.

Autonomous maturation of alpha/beta T lineage cells in the absence of COOH-terminal Src kinase (Csk)

The deletion of COOH-terminal Src kinase (Csk), a negative regulator of Src family protein tyrosine kinases (PTKs), in immature thymocytes results in the development of alpha/beta T lineage cells in T cell receptor (TCR) beta-deficient or recombination activating gene (rag)-1-deficient mice. The function of Csk as a repressor of Lck and Fyn activity suggests activation of these PTKs is solely responsible for the phenotype observed in csk-deficient T lineage cells. We provide genetic evidence for this notion as alpha/beta T cell development is blocked in lck(-/)-fyn(-/)- csk-deficient mice. It remains unclear whether activation of Lck and Fyn in the absence of Csk uncouples alpha/beta T cell development entirely from engagement of surface-expressed receptors. We show that in mice expressing the alpha/beta TCR on csk-deficient thymocytes, positive selection is biased towards the CD4 lineage and does not require the presence of major histocompatibility complex (MHC) class I and II. Furthermore, the introduction of an MHC class I-restricted transgenic TCR into a csk-deficient background results in the development of mainly CD4 T cells carrying the transgenic TCR both in selecting and nonselecting MHC background. Thus, TCR-MHC interactions have no impact on positive selection and commitment to the CD4 lineage in the absence of Csk. However, TCR-mediated negative selection of csk-deficient, TCR transgenic cells is normal. These data suggest a differential involvement of the Csk-mediated regulation of Src family PTKs in positive and negative selection of developing thymocytes.

Glucocorticoids attenuate T cell receptor signaling

Glucocorticoids (GCs) affect peripheral immune responses by inhibiting T cell immunity at several stages of the activation cascade, causing impaired cytokine production and effector function. The recent demonstration that the thymic epithelium and possibly thymocytes themselves produce steroids suggests that endogenous GCs also play a role in the control of T cell development. As both peripheral responsiveness and thymic differentiation appear to be regulated by the quantity and quality of intracellular signals issued by antigen-major histocompatibility complex-engaged T cell receptor (TCR) complexes, we investigated the effects of GCs on the signaling properties of T cells stimulated by anti-CD3 monoclonal antibodies or agonist peptides. We demonstrate in this work that dexamethasone, a synthetic GC, inhibits the early signaling events initiated upon TCR ligation, such as tyrosine phosphorylation of several TCR-associated substrates including the zeta chain, the ZAP70 kinase, and the transmembrane adapter molecule linker for activation of T cells. Hypophosphorylation was not a consequence of reduced kinase activity of src protein tyrosine kinases, but was correlated with an altered- membrane compartmentalization of these molecules. These observations indicate that in addition to their well-described ability to interfere with the transcription of molecules involved in peripheral responses, GCs inhibit T cell activation by affecting the early phosphorylating events induced after TCR ligation.

Raft-partitioning of the ubiquitin ligases Cbl and Nedd4 upon IgE-triggered cell signaling

The high affinity receptor for IgE, FcepsilonRI on mast cells and basophils plays an essential role in immunological defense. Upon multivalent antigen binding, FcepsilonRI becomes phoshorylated by the protein-tyrosine kinase Lyn, as a result of receptor clustering in lipid rafts. FcepsilonRI has been shown to be ubiquitinated. Ubiquitination can lead to degradation by proteasomes, but it can also act as a sorting signal to internalize proteins destined to the endosomal/lysosomal pathway. We have analyzed whether FcepsilonRI ubiquitination takes place within rafts. We report biochemical and imaging evidence in rat basoleukemia cells for the presence of ubiquitinated FcepsilonRI in clustered rafts upon receptor activation. Moreover, we demonstrated that the ubiquitin ligases Cbl and Nedd4 colocalize with FcepsilonRI patches and showed that both ligases become associated with lipid rafts after activation of IgE signaling. Because Cbl is known to interact with the FcepsilonRI signaling complex, ubiquitination is likely to be an important parameter regulating IgE-triggered signaling occurring in rafts.

A redundant role of the CD3 gamma-immunoreceptor tyrosine-based activation motif in mature T cell function

At least four different CD3 polypeptide chains are contained within the mature TCR complex, each encompassing one (CD3gamma, CD3delta, and CD3epsilon) or three (CD3zeta) immunoreceptor tyrosine-based activation motifs (ITAMs) within their cytoplasmic domains. Why so many ITAMs are required is unresolved: it has been speculated that the different ITAMs function in signal specification, but they may also serve in signal amplification. Because the CD3zeta chains do not contribute unique signaling functions to the TCR, and because the ITAMs of the CD3-gammadeltaepsilon module alone can endow the TCR with normal signaling capacity, it thus becomes important to examine how the CD3gamma-, delta-, and epsilon-ITAMs regulate TCR signaling. We here report on the role of the CD3gamma chain and the CD3gamma-ITAM in peripheral T cell activation and differentiation to effector function. All T cell responses were reduced or abrogated in T cells derived from CD3gamma null-mutant mice, probably because of decreased expression levels of the mature TCR complex lacking CD3gamma. Consistent with this explanation, T cell responses proceed undisturbed in the absence of a functional CD3gamma-ITAM. Loss of integrity of the CD3gamma-ITAM only slightly impaired the regulation of expression of activation markers, suggesting a quantitative contribution of the CD3gamma-ITAM in this process. Nevertheless, the induction of an in vivo T cell response in influenza A virus-infected CD3gamma-ITAM-deficient mice proceeds normally. Therefore, if ITAMs can function in signal specification, it is likely that either the CD3delta and/or the CD3epsilon chains endow the TCR with qualitatively unique signaling functions.

Activated p56lck directs maturation of both CD4 and CD8 single-positive thymocytes

p56(lck) is a protein tyrosine kinase expressed throughout T cell development. It associates noncovalently with the cytoplasmic domains of the CD4 and CD8 coreceptor molecules and has been implicated in TCR signaling in mature T cells. Its role in early thymocyte differentiation has been demonstrated in vivo, both by targeted gene disruption and by transgene expression. Previously, we showed that expression of a dominant-negative form of p56(lck) in double-positive thymocytes inhibits positive selection. We now demonstrate that expression of constitutively activated p56(lck) (p56(lck)F505) accelerates the transition from the double-positive to the single-positive stage. Importantly, p56(lck)F505 drives survival and lineage commitment of thymocytes in the absence of TCR engagement by appropriate MHC molecules. These results indicate that activation of p56(lck) constitutes an early step in conveying maturational signals after TCR ligation by a positively selecting ligand. Our study provides direct in vivo evidence for the role of p56(lck) in regulating TCR signaling.

A mathematical analysis of TCR serial triggering and down-regulation

Despite the increasing knowledge on the pathways involved in TCR signal transduction and T cell activation, the molecular mechanism of TCR triggering by ligand, MHC-peptide complexes, is still elusive and controversial. The present paper addresses the controversy on the early events of TCR engagement and triggering. Mathematical modelling techniques are applied to experimental data to infer plausible molecular mechanisms of TCR triggering and down-regulation. A similar approach has been followed by Bachmann et al. (Eur. J. Immunol. 1998, 28: 2571 - 2579), who concluded that the TCR triggering requires the formation of MHC-TCR dimers or trimers. We report here the failure to generalize this conclusion to the data reported by Valitutti et al. (Nature 1995, 375: 148 - 151). We show that there are several kinetic features in these experimental curves of TCR down-regulation that cannot be explained by the simple model proposed by Bachmann et al. unless some phenomenological extensions are considered. These extensions are: (1) a ligand independent turnover of the TCR; (2) a transient accumulation of triggered TCR; (3) a high order of TCR triggering kinetics; and (4) two pools of membrane TCR in dynamic equilibrium.

On the dynamics of TCR:CD3 complex cell surface expression and downmodulation

TCR downmodulation following ligation by MHC:peptide complexes is considered to be a pivotal event in T cell activation. Here, we analyzed the dynamics of TCR:CD3 cell surface expression on resting and antigen-activated T cells. We show that the TCR:CD3 complex is very stable and is rapidly internalized and recycled in resting T cells. Surprisingly, the internalization rate is not increased following TCR ligation by MHC:peptide complexes, despite significant TCR downmodulation, suggesting that constitutive internalization rather than ligation-induced downmodulation serves as the force that drives serial ligation. Furthermore, TCR downmodulation is mediated by the intracellular retention of ligated complexes and degradation by lysosomes and proteasomes. Thus, our data demonstrate that ligation induces TCR downmodulation by preventing recycling rather than inducing internalization.

The CD45 tyrosine phosphatase: a positive and negative regulator of immune cell function

The CD45 phosphotyrosine phosphatase is one of the most abundant glycoproteins expressed on immune cells. Previously, the serpentine twists and turns of the CD45 research field have tended to draw attention to CD45 either as a positive or negative regulator of immune cell function. This review draws heavily on CD45 knockout mouse data to emphasize that CD45 has both positive and negative actions in regulating receptor thresholds, and these roles vary according to cell lineage and developmental stage. Previously conflicting results are reconciled in a model suggesting how CD45 regulates the p56(lck)tyrosine kinase in T cell signalling and development.

A role for the region encompassing the c" strand of a TCR V alpha domain in T cell activation events

The distinct strand topology of TCR V alpha domains results in a flatter surface in the region encompassing the c" strand than the corresponding region in Ig V domains. In the current study a possible role for this region in T cell activation has been investigated by inserting a potential glycosylation site at V alpha residue 82. This residue is in proximity to the c" strand and distal to the putative interaction site for cognate peptide:MHC ligand. An additional N-linked carbohydrate at this position would create a protrusion on the V alpha domain surface, and this may interfere with TCR aggregation and/or recruitment of signaling molecules. The modified TCR has been expressed in transfected T cells, and the phenotype following stimulation has been compared with that of cells expressing the wild-type TCR. The mutation has significant effects on activation-induced cell death and TCR internalization, but, unexpectedly, does not affect IL-2 secretion. Furthermore, analyses with tetrameric, peptide:MHC class II complexes suggest that the mutation decreases the ability of the TCR to aggregate into a configuration compatible with avid binding by these multivalent ligands.

Enhanced surface TCR replenishment mediated by CD28 leads to greater TCR engagement during primary stimulation

When T cells are stimulated with high concentrations of strong TCR agonist, engaged TCR are internalized and degraded, resulting in greatly reduced surface TCR levels for up to several days post-stimulation. It has been noted that surface TCR levels rise subsequently, even in the presence of continuing stimulation, but the role of CD28 co-stimulation in surface TCR replenishment has not been investigated. Here, we have examined the return of surface TCR following activation, the availability of these TCR for antigenic engagement and the role of CD28 in that process. We report that within 24 h of stimulation, the level of surface TCR expression becomes dependent on the degree of CD28 signaling provided during T cell activation. In addition, when cells are removed from stimulus after 24 h, surface TCR expression recovers to a stable level which exceeds that of unstimulated cells and is proportional to the degree of CD28 co-stimulation. TCR that replenish the plasma membrane during T cell activation can be down-regulated by receptor occupancy with the same efficiency as TCR on freshly stimulated cells. Thus, as a result of enhanced surface TCR replenishment, CD28-co-stimulated cells can engage and down-regulate more TCR than co-stimulation-deprived cells in the face of ongoing stimulation. Furthermore, engagement of newly expressed TCR on activated T cells re-induces CD69, suggesting participation of these replenishing TCR in continued T cell signaling. These data identify the augmentation of surface TCR replenishment during activation as a novel mechanism that likely contributes to the enhanced antigenic sensitivity of CD28-co-stimulated T cells.

Sequential requirements of the N-terminal palmitoylation site and SH2 domain of Src family kinases in the initiation and progression of FcepsilonRI signaling

Initial biochemical signaling originating from high-affinity immunoglobulin E receptor (FcepsilonRI) has been ascribed to Src family kinases. To understand the mechanisms by which individual kinases drive the signaling, we conducted reconstitution experiments: FcepsilonRI signaling in RBL2H3 cells was first suppressed by a membrane-anchored, gain-of-function C-terminal Src kinase and then reconstructed with Src family kinases whose C-terminal negative regulatory sequence was replaced with a c-myc epitope. Those constructs derived from Lyn and Fyn, which are associated with detergent-resistant membranes (DRMs), physically interacted with resting FcepsilonRI and reconstructed clustering-induced signaling that leads to calcium mobilization and ERK1 and -2 activation. c-Src-derived construct, which was excluded from DRMs, failed to interact with FcepsilonRI and to restore the signaling, whereas creation of palmitoylatable Cys3 enabled it to interact with DRMs and with FcepsilonRI and to restore the signaling. Deletion of Src homology 3 (SH3) domain from the Lyn-derived construct did not alter its ability to transduce the series of signaling. Deletion of SH2 domain did not affect its association with DRMs and with FcepsilonRI nor clustering-induced tyrosine phosphorylation of FcepsilonRI beta and gamma subunits, but it almost abrogated the next step of tyrosine phosphorylation of Syk and its recruitment to FcepsilonRI. These findings suggest that Lyn and Fyn could, but c-Src could not, drive FcepsilonRI signaling and that N-terminal palmitoylation and SH2 domain are required in sequence for the initial interaction with FcepsilonRI and for the signal progression to the molecular assembly.

Receptor engagement transiently diverts the T cell receptor heterodimer from a constitutive degradation pathway

In the absence of ligand, the T cell receptor (TCR)/CD3 complex is continuously internalized and recycled to the cell surface, whereas receptor engagement results in its down-regulation. The present study shows that the TCR and CD3 components follow different fates accompanying their constitutive internalization. Although the CD3 moiety is recycled to the cell surface, the TCR heterodimer is degraded and replaced by newly synthesized chains. Since the TCR heterodimer cannot reach the cell membrane on its own, we propose a model in which recycling CD3 is transported along a retrograde pathway to the endoplasmic reticulum, where it associates with newly made TCR. Interestingly, engagement of the TCR.CD3 complex by superantigen resulted not only in the down-regulation of the TCR and CD3 components but also caused a transient stabilization of the TCR heterodimer. This suggests that TCR engagement diverts the TCR heterodimer from a degradation to a recycling pathway. Contrary to CD3, the intracellular fate of the TCR heterodimer is thus regulated, providing a mechanism for rapidly replacing nonfunctional TCR during intrathymic development of T cells.

Antigen-induced TCR-CD3 down-modulation does not require CD3delta or CD3gamma cytoplasmic domains, necessary in response to anti-CD3 antibody

We studied cytotoxic T lymphocyte (CTL) clones expressing cytoplasmic domain-deleted CD3delta and CD3gamma chains. These cells retained efficient antigen-specific cytolysis. Because the cytoplasmic domains of native CD3delta and CD3gamma chains contain a dileucine-based and a tyrosine-based motif thought to be important for receptor endocytosis, we compared TCR-CD3 down-modulation on the CTL clones expressing or not these domains. We found that antigen-induced TCR-CD3 down-modulation was not dependent on either the CD3delta or CD3gamma cytoplasmic domains. This contrasts with phorbol ester- and anti-CD3 mAb (soluble or plastic-coated)-induced TCR-CD3 down-modulation, that are respectively dependent on CD3gamma and on either CD3delta or CD3gamma cytoplasmic domains, suggesting that differences may exist between the mechanisms of TCR-CD3 down-modulation in response to the three stimuli. TCR-CD3 down-modulation in response to antigen was demonstrated by confocal microscopy to be associated with TCRbeta chain internalization, whether CD3delta and CD3gamma were native or truncated. Inhibition by the protein tyrosine kinase inhibitor PP1 of TCR-CD3 down-modulation in response to antigen was also similar whether CD3delta and CD3gamma cytoplasmic domains were present or not. These properties of receptor down-modulation are discussed with respect to the requirements for TCR engagement on antigen-presenting cells.

Dysregulation of the protein tyrosine kinase LCK in lymphoproliferative disorders and in other neoplasias

Initially identified as a T-cell specific member of the Src family of protein tyrosine kinases, Lck has become the object of intensive investigations which have revealed a key role for this kinase in the central processes controlling T-cell development, activation, proliferation and survival. Experimental evidence of the oncogenic potential of Lck, together with the identification of defects in the regulation of Lck expression or activity in T-cell leukemias, suggests that dysregulation of Lck might play a role in neoplastic transformation. Here we review the data documenting a potential role for this kinase in the initiation and maintenance of the transformed state in human cancers.

The oncogenic 70Z Cbl mutation blocks the phosphotyrosine binding domain-dependent negative regulation of ZAP-70 by c-Cbl in Jurkat T cells

T-cell receptor (TCR) engagement results in the activation of Src family (Lck and Fyn) and ZAP-70 protein tyrosine kinases, leading to tyrosine phosphorylation of multiple cellular substrates including the complex adapter protein c-Cbl. Moreover, Cbl is tyrosine phosphorylated upon engagement of growth factor receptors, cytokine receptors, and immunoreceptors and functions as a negative regulator of tyrosine kinase signalling pathways. Cbl associates via its phosphotyrosine binding (PTB) domain to the ZAP-70 pY292 negative regulatory phosphotyrosine. We recently demonstrated that the oncogenic Cbl mutant, 70Z Cbl, requires its PTB domain to upregulate NFAT in unstimulated Jurkat T cells. Here, we demonstrate that kinase-dead but not wild-type forms of Fyn, Lck, and ZAP-70 block 70Z Cbl-mediated NFAT activation. Moreover, 70Z Cbl does not upregulate NFAT in the ZAP-70-deficient P116 Jurkat T-cell line. The requirement for Fyn, Lck, and ZAP-70 is not due to tyrosine phosphorylation of 70Z Cbl, as mutation of all tyrosines in, or deletion of, the C-terminal region of 70Z Cbl (amino acids 655 to 906) blocks 70Z Cbl tyrosine phosphorylation but enhances 70Z Cbl-mediated NFAT activation. Further, 70Z Cbl does not cooperate with ZAP-70 Y292F to upregulate NFAT, indicating that 70Z Cbl and ZAP-70 do not activate parallel signalling pathways. Finally, the upregulation of NFAT observed upon ZAP-70 overexpression is blocked by Cbl in a PTB domain-dependent manner. We conclude that oncogenic 70Z Cbl acts as a dominant negative to block the PTB domain-dependent negative regulatory role of endogenous Cbl on ZAP-70, leading to constitutive ZAP-70 signalling and activation of transcription factors.