Tcr activation signaling

Tumor-specific antigen of murine T-lymphoma defined with monoclonal antibody. J Immunol. Isolation with a monoclonal antibody. J Exp Med. Clonotypic structures involved in antigen-specific human T cell function. Relationship to the T3 molecular complex. Transfer of specificity by murine alpha and beta T-cell receptor genes.

Saito T, Germain RN. Predictable acquisition of a new MHC recognition specificity following expression of a transfected T-cell receptor beta-chain gene. Weiss A, Stobo JD. Requirement for the coexpression of T3 and the T cell antigen receptor on a malignant human T cell line. Abraham RT, Weiss A. Jurkat T cells and development of the T-cell receptor signalling paradigm. Nat Rev Immunol. Weiss A, Imboden JB. Cell surface molecules and early events involved in human T lymphocyte activation.

Adv Immunol. Transmembrane signalling by the T cell antigen receptor. Perturbation of the T3-antigen receptor complex generates inositol phosphates and releases calcium ions from intracellular stores. Antigen activation of murine T cells induces tyrosine phosphorylation of a polypeptide associated with the T cell antigen receptor. Association of the fyn protein-tyrosine kinase with the T-cell antigen receptor.

The CD4 and CD8 T cell surface antigens are associated with the internal membrane tyrosine-protein kinase p56lck. Inhibition of tyrosine phosphorylation prevents T-cell receptor-mediated signal transduction. Reth M. Antigen receptor tail clue. Irving BA, Weiss A. The cytoplasmic domain of the T cell receptor zeta chain is sufficient to couple to receptor-associated signal transduction pathways. Sequential interactions of the TCR with two distinct cytoplasmic tyrosine kinases. Aivazian D, Stern LJ.

Phosphorylation of T cell receptor zeta is regulated by a lipid dependent folding transition. Nat Struct Biol. Recruitment of Nck by CD3 epsilon reveals a ligand-induced conformational change essential for T cell receptor signaling and synapse formation. Nat Immunol. The CD3epsilon proline-rich sequence, and its interaction with Nck, is not required for T cell development and function. A role for CD8 in the developmental tuning of antigen recognition and CD3 conformational change.

Structural and functional evidence that Nck interaction with CD3epsilon regulates T-cell receptor activity. J Mol Biol. Mechanisms contributing to T cell receptor signaling and assembly revealed by the solution structure of an ectodomain fragment of the CD3 epsilon gamma heterodimer.

The receptor deformation model of TCR triggering. Faseb J. Minguet S, Schamel WW. Trends Biochem Sci. T-cell antigen-receptor stoichiometry: pre-clustering for sensitivity.

EMBO Rep. The kinetic-segregation model: TCR triggering and beyond. Varma R. Sci Signal. J Biol Chem. LAT: a T lymphocyte adapter protein that couples the antigen receptor to downstream signaling pathways. Essential role of LAT in T cell development.

Curr Biol. Vav1 transduces T cell receptor signals to the activation of phospholipase C-gamma1 via phosphoinositide 3-kinase-dependent and -independent pathways. Qi Q, August A. Sci STKE. Vav1 acidic region tyrosine is required for the formation of T cell receptor-induced microclusters and is essential in T cell development and activation.

Mutation of the phospholipase C-gamma1-binding site of LAT affects both positive and negative thymocyte selection. Complementation in trans of altered thymocyte development in mice expressing mutant forms of the adaptor molecule SLP Molecular details of Itk activation by prolyl isomerization and phospholigand binding: the NMR structure of the Itk SH2 domain bound to a phosphopeptide. T cell receptor-initiated calcium release is uncoupled from capacitative calcium entry in Itk-deficient T cells.

Tec family kinases in T lymphocyte development and function. Shan X, Wange RL. Biochemical interactions integrating Itk with the T cell receptor-initiated signaling cascade. Eur J Immunol. Genot E, Cantrell DA. Ras regulation and function in lymphocytes. Curr Opin Immunol. Involvement of p21ras activation in T cell CD69 expression. Association of Sos Ras exchange protein with Grb2 is implicated in tyrosine kinase signal transduction and transformation.

RasGRP, a Ras guanyl nucleotide- releasing protein with calcium- and diacylglycerol-binding motifs. Mol Cell Biol. Hayashi K, Altman A. Pharmacol Res. Mechanism of diacylglycerol-induced membrane targeting and activation of protein kinase Ctheta. Schulze-Luehrmann J, Ghosh S. Antigen-receptor signaling to nuclear factor kappa B.

Mol Cell. Oh-Hora M, Rao A. Calcium signaling in lymphocytes. J Cell Biol. Calcium-dependent transcription of cytokine genes in T lymphocytes. Pflugers Arch. The actin cytoskeleton in T cell activation. Antigen-T lymphocyte interactions: inhibition by cytochalasin B. Defects in actin-cap formation in Vav-deficient mice implicate an actin requirement for lymphocyte signal transduction.

A single class II myosin modulates T cell motility and stopping, but not synapse formation. Dynamin 2 regulates T cell activation by controlling actin polymerization at the immunological synapse. The specific direct interaction of helper T cells and antigen-presenting B cells. Reorientation of the microtubule organizing center and reorganization of the membrane-associated cytoskeleton inside the bound helper T cells.

Recruitment of dynein to the Jurkat immunological synapse. Three-dimensional segregation of supramolecular activation clusters in T cells. The immunological synapse: a molecular machine controlling T cell activation. Dustin ML. T-cell activation through immunological synapses and kinapses.

Immunol Rev. T cell receptor ligation induces the formation of dynamically regulated signaling assemblies. Newly generated T cell receptor microclusters initiate and sustain T cell activation by recruitment of Zap70 and SLP T cell receptor-proximal signals are sustained in peripheral microclusters and terminated in the central supramolecular activation cluster.

Actin and agonist MHC-peptide complex-dependent T cell receptor microclusters as scaffolds for signaling. T cell costimulation via the integrin VLA-4 inhibits the actin-dependent centralization of signaling microclusters containing the adaptor SLP Asymmetric T lymphocyte division in the initiation of adaptive immune responses. ERM-dependent movement of CD43 defines a novel protein complex distal to the immunological synapse.

Int Immunol. Regulation of T-cell activation by the cytoskeleton. Regulation of T-cell antigen receptor-mediated inside-out signaling by cytosolic adapter proteins and Rap1 effector molecules. Rap1 is a potent activation signal for leukocyte function-associated antigen 1 distinct from protein kinase C and phosphatidylinositolOH kinase.

Rap1A positively regulates T cells via integrin activation rather than inhibiting lymphocyte signaling. Rap1A-deficient T and B cells show impaired integrin-mediated cell adhesion. Dev Cell. RIAM is identified a key adapter in Rap1 plasma membrane localization and integrin activation in T cells. Protein kinase D1 and the beta 1 integrin cytoplasmic domain control beta 1 integrin function via regulation of Rap1 activation. Reconstructing and deconstructing agonist-induced activation of integrin alphaIIbbeta3.

Acuto O, Michel F. CDmediated co-stimulation: a quantitative support for TCR signalling. Nuclear export of NF-ATc enhanced by glycogen synthase kinase The CD28 signaling pathway regulates glucose metabolism.

Glucose uptake is limiting in T cell activation and requires CDmediated Akt-dependent and independent pathways. Lck phosphorylates the activation loop tyrosine of the Itk kinase domain and activates Itk kinase activity. CD28 costimulatory signal induces protein arginine methylation in T cells. Induction of autoimmunity in the absence of CD28 costimulation. The inducible costimulator plays the major costimulatory role in humoral immune responses in the absence of CD Watts TH.

J Cell Sci. Cytokine Growth Factor Rev. CD a critical regulator of signaling thresholds in immune cells. Motheaten and viable motheaten mice have mutations in the haematopoietic cell phosphatase gene. Nat Genet. Dok-1 and Dok-2 are negative regulators of T cell receptor signaling.

T cell receptor for antigen induces linker for activation of T cell-dependent activation of a negative signaling complex involving Dok-2, SHIP-1, and Grb Hematopoietic progenitor kinase 1 negatively regulates T cell receptor signaling and T cell-mediated immune responses.

A phosphatase activity of Sts-1 contributes to the suppression of TCR signaling. TULA proteins regulate activity of the protein tyrosine kinase Syk. Antigen recognition by the T cell stimulates a burst of actin polymerization at the immunological synapse, generating a lamellipodial sheet structure that spreads over the surface of the APC. Negative regulation of TCR signaling is also important to keep the hyperactivation of immune response associated with the pathway, which is achieved through the intervention of several proteins and receptors.

Phosphoprotein associated with glycosphingolipid microdomains PAG , a transmembrane adaptor molecule in resting human T-cells, is tyrosine phosphorylated and associated with C-Src Tyrosine Kinase CSK , an inhibitor of Scr-related protein tyrosine kinases. Additionally, cytotoxic T-lymphocyte antigen-4 CLTA4 , a transmembrane protein, serves as a natural inhibitor.

T cells are especially significant in cell-mediated immunity, the defense against tumor cells, pathogenic organisms inside cells, and the rejection reactions. Deregulation of T-cell function, whether by defect or by excess, results in serious consequences for the organism including immunodeficiency and autoimmunity.

TCR is an extremely sensitive system. TCR signaling pathway helps us have a deep understanding of what faults in immune regulation leads to immune related diseases and of how the immune system could be better manipulated to overcome afflictions such as cancer, infection and autoimmune diseases. Conformational changes within the TCR complex may cause the dimerization or aggregation of TCR complexes [ 68 , 69 ] which could expose the ITAM residues by altering the lipid environment [ 48 ].

Overall, however, a structural understanding of the changes that occur within the bound TCR complex that facilitates its phosphorylation remains a major challenge. Lck is critical to the initiation of TCR signaling because it converts an extracellular recognition event, pMHC binding, into an intracellular biochemical signal by phosphorylating the TCR complex and Zap Importantly, models for the initiation of TCR signaling need to account for the availability of active Lck for phosphorylation of the TCR.

Lck activity is controlled through the conformation of its catalytic kinase domain which is regulated by phosphorylation [ 72 , 73 ] FIGURE 3A. It has been observed that a substantial amount of Lck is basally active in resting unstimulated T cells. To estimate the abundance of active Lck, specific Lck phosphoforms were quantified using immunodepletion [ 74 ]. Recent studies using an Lck FRET reporter have confirmed a pre-existing pool of Lck that is an open conformation and presumably active [ 76 ].

It was also noted that a subpopulation of Lck molecules does undergo a conformational change consistent with its activation upon TCR stimulation. A subsequent study using an improved FRET reporter confirmed these findings and also suggested that Lck autophosphorylation is required [ 77 ]. Mice that express a series of CD45 mutant alleles were used to titrate CD45 expression amounts on thymocytes and T cells. Altering the amount of CD45 caused changes in the amount of activated Lck, as assessed by changes in regulatory phosphorylation sites.

In these experiments, TCR responsiveness was best correlated with phosphorylation of the Lck activation loop which peaked at intermediate amounts of CD45 in resting cells, indicating that the initial amount of active Lck is important [ 78 ]. In a more recent study, the initial pool of Lck was manipulated by inhibiting the cytoplasmic kinase Csk, a negative regulator of Lck, and the impact of increasing the abundance of active Lck on antigen discrimination was assessed.

CD8 T cell responses to lower affinity antigens were potentiated while activation caused by high affinity antigens remained unchanged [ 22 ]. An important question emerges from these findings: how does a T cell control the abundance of active Lck molecules to enforce antigen discrimination? Quantitative mass spectrometry revealed that inhibition of the Lck substrate and downstream kinase, Zap70, causes an apparent increase in Lck activity [ 79 ].

Negative feedback pathways were therefore predicted to regulate active Lck abundance. A Zapdependent phosphorylation site, Y, within the SH2 domain of Lck was found to control its association and activation by CD45, which dephosphorylates the C-terminal negative regulatory tyrosine [ 80 ].

Interestingly, this site is conserved amongst other SFKs and its phosphorylation is reported in several hematopoetic malignancies [ 80 , 81 ]. In addition to regulating Lck activation by CD45, other regulatory loops are predicted to act concert to control the amount of active Lck. Adaptor proteins, such as PAG, can recruit Csk to the plasma membrane where it can inhibit Lck by phosphorylating its inhibitory C terminal tail [ 82 ] Figure 3D.

In many cases the loss of these adaptors in mice causes only subtle phenotypes which suggests that their loss can be compensated for.

However, it has recently been observed that deletion of PAG can alter the reactivity of effector T cells [ 83 ]. Therefore, looking at the loss of negative regulatory pathways in specific contexts may be important to unravelling their functions. T cells require fewer than 10 agonist pMHC interactions to trigger a full T cell response [ 2 , 44 , 65 ]. How can T cells exhibit such sensitivity toward foreign antigens while maintaining their quiescence toward self-pMHC? To address this question, it is necessary to consider the intracellular effectors employed by the TCR to propagate signaling events.

Because the kinases Lck and Zap70 control the initiation of proximal TCR signaling, their activities together must be tightly coordinated to enforce the exquisite sensitivity and discrimination demonstrated by the TCR [ 84 , 85 ]. Although other non-receptor tyrosine kinases, such as Syk, can play initiating roles in ITAM-containing receptor signaling in other cells of the immune system, this does not occur in T cells [ 86 ]. How then is a signaling hierarchy enforced in T cells?

First, it is basally active and associates with the cytoplasmic tails of CD4 and CD8 coreceptors. In contrast, Zap70 resides within the cytoplasm in its autoinhibited conformation until recruited to phosphorylated ITAM residues.

Specifically, Lck phosphorylates Zap70 on Y and Y which destabilizes the Zap70 autoinhibited conformation and promotes its adoption of an open active conformation. A Lck and Zap70 exist in a strict signaling hierarchy. C Zap70 contains a highly basic region that is important for its rigid substrate specificity. Meanwhile, additional Zap70 molecules are able to bind the TCR complex and become activated suggesting a mode of TCR signal amplification.

In addition to their specific mechanisms of activation, it is also necessary to consider how Lck and Zap70 recognize their substrates. In contrast to the related and more ubiquitously expressed Syk kinase, Zap70 is unable to phosphorylate ITAM residues and cannot phosphorylate additional Zap70 molecules.

Additionally, despite residing at the plasma membrane with Lck, the critical adaptor LAT is a poor Lck substrate and requires Zap70 to phosphorylate it. Because Zap70 expression is restricted to T and NK cells, these observations suggest it has evolved unique properties critical for T cell function. Recent analysis of the substrate bias of Lck and Zap70 using bacterial display libraries has revealed the unique features of Zap70 substrate recognition [ 88 ]. Specifically, Zap70 only efficiently phosphorylated peptides that contained tyrosines flanked by acidic residues.

The presence of nearby basic residues prevented phosphorylation and were largely absent in regions surrounding known Zap70 phosphorylation sites. This basic region acts as an electrostatic filter for potential Zap70 substrates. This electrostatic mechanism also prevents Zap70 from trans -autophosphorylating its activation loop or phosphorylating ITAM motifs within the TCR complex. Thus, the substrate preferences for Zap70 and the broader Lck specificity and its basal activity provide a logic for the ordered and sequential roles that these kinases play in TCR signaling.

The strict localization of active Zap70 could present a hurdle to initiating and amplifying a TCR signal when a bona-fide agonist pMHC is encountered. Katz el al. The released-yet-still-active Zap70 was observed to be mobile. Interestingly, the release of Zap70 from ITAMs is proposed to occur through phosphorylation of Y which is located within the linker that connects the SH2 domains of Zap Phosphorylation of this site was identified by mass spectrometry analysis and is reported to trigger the release of activated Zap70 from phosphorylated ITAM motifs [ 89 ].

In principle, this allows other not-yet-activated Zap70 molecules to bind and become activated. However, the translocation of Zap70 from TCR complexes to neighboring LAT clusters requires confirmation by microscopic imaging studies. If Zap70 is released from the TCR complex how far can it meander before it is inactivated by phosphatases or ubiquitin ligases, for instance?

Such parameters would be critical constraints for TCR signaling because continued ligation of the TCR is required for T cell responses. Recently, proximal TCR signaling, including assembly of a LAT signalosome, have been reconstituted in vitro using supported lipid bilayers [ 90 , 91 ]. Importantly, these clusters were found to be dynamic and could be disassembled through dephosphorylation of LAT.

Moreover, these findings highlight the unique properties of proteins that comprise the LAT signalosome which allow it to act as a hub of TCR signaling. These clusters are dynamic and can be reversed through dephosphorylation by protein tyrosine phosphatases.

Interestingly, a recent study suggested clusters of LAT in T cells may have heterogeneous compositions and by extension form distinct signaling hubs [ 92 ]. These latter clusters appeared to play a role in terminating conjugate formation between a T cell and an antigen-presenting cell by inactivating the integrin LFA The presence of two distinct LAT-based signalosomes suggests interesting possibilities for the regulation and function of the T cell-APC synapse.

It is interesting to speculate whether the composition of a LAT signalosome is dependent upon its proximity to an engaged TCR complex or whether it is influenced by additional co-inhibitory or co-stimulatory signals.

Early insights into TCR signaling have shaped our understanding of the T cell response and made possible the emerging use of T cells therapeutically. The successful design of cancer-targeting chimeric antigen receptors CARs , as an example, were derived from understanding the signaling motifs which recruit Zap70 to evoke TCR signaling and therefore elicit a T cell response [ 93 ]. We have highlighted recent advances in understanding how the TCR, and the proximal signaling proteins to which it is coupled, can detect antigens and drive a T cell response; however, many questions remain.

For example, the relative contributions of TCR proximal signaling events to models of kinetic proofreading remain poorly defined.

Such efforts to define how specific signaling events quantitatively contribute to the remarkable sensitivity and specificity of the TCR remain ongoing.

Efforts to understanding how T cells signal in different contexts and integrate multiple cues will also be critical to therapeutically exploiting the emerging role of T cells in maintaining tissue homeostasis [ 94 ]. How these signals induce responses at the single cell and population levels will be crucial. Many questions surround how observed heterogeneity in single cell T cell responses translates to a population based response in vivo [ 28 , 95 ]. During antigen detection, mechanical force may be important for antigen discrimination.

Active Lck abundance and its coupling to the TCR during antigen encounter is able to influence affinity discrimination. A hallmark of TCR signaling is its sensitivity and specificity. How the strict TCR signaling hierarchy is enforced is continuing to emerge through structural insight into the substrate selectivity of Lck and Zap70 and how the activation of Zap70 is controlled.

The emerging properties of the LAT signalosome include its assembly into liquid-like phase separated clusters that contain phospho-LAT and its binding partners which can enhance actin polymerization. Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication.

As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. National Center for Biotechnology Information , U.

Trends Biochem Sci. Author manuscript; available in PMC Feb 1. Adam H. Author information Copyright and License information Disclaimer. Copyright notice.

The publisher's final edited version of this article is available at Trends Biochem Sci. See other articles in PMC that cite the published article. Abstract The mechanisms by which a T cell detects antigen using its T cell antigen receptor TCR are crucial to our understanding of immunity and harnessing T cells therapeutically.

Introduction The specificity of a T cell for antigen is defined by its T cell antigen receptor TCR which acts as an antigen detector. Open in a separate window.

Models and observations: The initiation of TCR signaling Despite years of intensive study, the mechanism by which ligand recognition triggers TCR signaling remains enigmatic. A molecular handshake: Zap70 activation and assembly of the LAT signalosome T cells require fewer than 10 agonist pMHC interactions to trigger a full T cell response [ 2 , 44 , 65 ].

Concluding remarks Early insights into TCR signaling have shaped our understanding of the T cell response and made possible the emerging use of T cells therapeutically. Acknowledgments A. Produce cytokines to coordinate the adaptive immune response Antigen presenting cells Professional antigen presenting cells include dendritic cells, B cells, and macrophages.

Express CD8 and recognize pMHC class I complexes Coreceptor A receptor that does not typically signal on its own but influences the engagement or signaling of other receptors e. Those MHC complexes presenting peptides derived from the self proteins are referred to as self pMHC, whereas those MHC complexes presenting peptide derived from foreign antigens and able to trigger immune responses are agonist pMHC antigens Signaling effector Typically intracellular enzymes e.

Footnotes Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. References 1. Sewell AK. Why must T cells be cross-reactive? Nat Rev Immunol. Juang J, et al. Peptide-MHC heterodimers show that thymic positive selection requires a more restricted set of self-peptides than negative selection. J Exp Med. Balagopalan L, et al. The LAT story: a tale of cooperativity, coordination, and choreography. Cold Spring Harb Perspect Biol.

Wange RL. LAT, the linker for activation of T cells: a bridge between T cell-specific and general signaling pathways. Sci STKE. Zhang W, et al. Effect of LAT tyrosine mutations on T cell angigen receptor-mediated signaling. J Biol Chem. Liu SK, et al. Curr Biol. Yablonski D, et al.

Andreotti AH, et al. T-cell signaling regulated by the Tec family kinase, Itk. Das J, et al. Digital signaling and hysteresis characterize ras activation in lymphoid cells. Zehn D, et al. Complete but curtailed T-cell response to very low-affinity antigen. Davis MM, et al. Ligand recognition by alpha beta T cell receptors.

Annu Rev Immunol. McKeithan TW. Kinetic proofreading in T-cell receptor signal transduction. Lever M, et al.