T cell receptor engagement by peptide-MHC ligands induces a conformational change in the CD3 complex of thymocytes

A conserved CXXC motif in CD3epsilon is critical for T cell development and TCR signaling

Structural integrity of the extracellular membrane-proximal stalk region of CD3ε is required for efficient signaling by the T cell antigen receptor complex. The results in this article suggest that receptor aggregation may not be sufficient for a complete T cell receptor signal and that some type of direct allosteric signal may be involved.

Virtually all T cell development and functions depend on its antigen receptor. The T cell receptor (TCR) is a multi-protein complex, comprised of a ligand binding module and a signal transmission module. The signal transmission module includes proteins from CD3 family (CD3ε, CD3δ, CD3γ) as well as the ζ chain protein. The CD3 proteins have a short extracellular stalk connecting their Ig-like domains to their transmembrane regions. These stalks contain a highly evolutionarily conserved CXXC motif, whose function is unknown. To understand the function of these two conserved cysteines, we generated mice that lacked endogenous CD3ε but expressed a transgenic CD3ε molecule in which these cysteines were mutated to serines. Our results show that the mutated CD3ε could incorporate into the TCR complex and rescue surface TCR expression in CD3ε null mice. In the CD3ε mutant mice, all stages of T cell development and activation that are TCR-dependent were impaired, but not eliminated, including activation of mature naïve T cells with the MHCII presented superantigen, staphylococcal enterotoxin B, or with a strong TCR cross-linking antibody specific for either TCR-Cβ or CD3ε. These results argue against a simple aggregation model for TCR signaling and suggest that the stalks of the CD3 proteins may be critical in transmitting part of the activation signal directly through the membrane.

The T cells of the immune system have surface receptors that detect unique features (called antigens) of foreign invaders such as viruses, bacteria and toxins. An encounter between an antigen and the T cell receptor sets off a chain of events that activates the T cell to proliferate and thus call to action the various arms of the immune response that ultimately eliminate the invader. A set of proteins, called CD3, associates with the T cell receptor, spanning the cell membrane. Their function is to deliver a signal to the inside of T cell that its receptor has encountered antigen on the outside of the cell. Two general ideas have been proposed to explain how the CD3 proteins accomplish this: That the engagement of the T cell receptor outside the cell directly causes a change in conformation in the intracellular portion of the associated CD3 proteins that is recognized by the intracellular signaling machinery; and that engagement of the T cell receptor causes clustering of multiple receptor and CD3 proteins such that interactions among the cytoplasmic portions of the many CD3 proteins now attract other proteins to start the chain of intercellular signaling. These two ideas are not mutually exclusive. We show here that mutations in a highly conserved extracellular portion of one of the CD3 proteins can impair the transmission of the activation signal without preventing receptor clustering. These results suggest that direct transmission of a conformational change across the membrane may constitute part of the CD3-mediated activation signal.

Prorenin has high affinity multiple binding sites for (pro)renin receptor

An important role of the decoy peptide sequence has recently been suggested in vitro for the binding of prorenin to the (pro)renin receptor [PRR]. In this study, other prospective crucial regions in renin and prorenin responsible for their interaction with PRR were investigated using various kinds of peptides, e.g., the "hinge" S149QGVLKEDVF158 designed from the structure of renin also common to prorenin, L1PPTDTTTF8P, L1PPTDTTTFKRIFLKR15P and the decoy (R10PIFLKRMPSI19P) designed from the predicted structure of prorenin. For the kinetic analysis, the recombinant hPRR was immobilized on the biosensor surface through a specific anti-PRR antibody. In case of the equilibrium state analysis, the PRR was directly adsorbed on plastic wells for observing the bindings of renin/prorenin. The dissociation constants (KD) for the bindings of renin and prorenin to the pre-adsorbed receptors were 4.5 and 1.0 nM, respectively, similar to those stated in the kinetic study by BIAcore assay. The "hinge" region peptide bound to PRR in a dose-dependent manner with a KD estimated 17.0 nM which was five times higher than that of the decoy. The KD values for L1PPTDTTTF8P and L1PPTDTTTFKRIFLKR15P were 52 and 7.6 nM, respectively. The "hinge" peptide, as the decoy, inhibited the bindings of renin and prorenin to PRR. The inhibition constant (Ki) for the binding of renin and prorenin by the decoy and "hinge" were 16.7 and 15.1, and 37.1 and 30.7 nM, respectively. These in vitro studies suggest that renin has a single and prorenin has at least two high affinity binding sites for the PRR.

Early T-cell activation biophysics

The T-cell is one of the main players in the mammalian immune response. It ensures antigen recognition at the surface of antigen-presenting cells in a complex and highly sensitive and specific process, in which the encounter of the T-cell receptor with the agonist peptide associated with the major histocompatibility complex triggers T-cell activation. While signaling pathways have been elucidated in increasing detail, the mechanism of TCR triggering remains highly controversial despite active research published in the past 10 years. In this paper, we present a short overview of pending questions on critical initial events associated with T-cell triggering. In particular, we examine biophysical approaches already in use, as well as future directions. We suggest that the most recent advances in fluorescence super-resolution imaging, coupled with the new classes of genetic fluorescent probes, will play an important role in elucidation of the T-cell triggering mechanism. Beyond this aspect, we predict that exploration of mechanical cues in the triggering process will provide new clues leading to clarification of the entire mechanism.

Visualization of multiprotein complexes by flow cytometry

Multiprotein complexes and other protein-protein interactions play important roles in virtually all cellular processes. Analysis of coimmunoprecipitation of protein complexes by flow cytometry (IP-FCM, or "the fly-p" method) provides a sensitive means to measure these interactions in the native/nondenatured state. First, immunoprecipitating antibodies are covalently coupled to polystyrene latex beads whose low autofluorescence is compatible with flow cytometry. These antibody-coupled beads are used to immunoprecipitate a specific protein (primary analyte) present in cell lysates. Finally, the protein complexes associated with the beads are probed with fluorochrome-conjugated antibodies specific for interaction partners, or secondary analytes, that may be associated with the primary analyte. The use of quantitative flow cytometric methodology can allow the semiquantitative fluorescence data generated to be converted into estimated numbers of co-associated molecules on the beads. The method represents a robust technique to assess native protein-protein interactions without requiring genetic engineering or large sample sizes.

Cooperativity between T cell receptor complexes revealed by conformational mutants of CD3epsilon

The CD3epsilon subunit of the T cell receptor (TCR) complex undergoes a conformational change upon ligand binding that is thought to be important for the activation of T cells. To study this process, we built a molecular dynamics model of the transmission of the conformational change within the ectodomains of CD3. The model showed that the CD3 dimers underwent a stiffening effect that was funneled to the base of the CD3epsilon subunit. Mutation of two relevant amino acid residues blocked transmission of the conformational change and the differentiation and activation of T cells. Furthermore, this inhibition occurred even in the presence of excess endogenous CD3epsilon subunits. These results emphasize the importance of the conformational change in CD3epsilon for the activation of T cells and suggest the existence of unforeseen cooperativity between TCR complexes.

N-terminal negatively charged residues in CD3varepsilon chains as a phylogenetically conserved trait potentially yielding isoforms with different isoelectric points: analysis of human CD3varepsilon chains

CD3varepsilon chains are essential to the structure, expression and signaling of T cell receptors. Here, we extend to human CD3varepsilon our previous data in mouse CD3varepsilon showing that, in T cells, proteolytic processing of the acidic N-terminal sequence of CD3varepsilon chains generate distinct polypeptide species that can be identified by two-dimension (IEF-SDS PAGE) electrophoresis and immunoblot. This was shown first by showing the processing of a fusion protein of GFP and the extracellular domain of mouse CD3varepsilon (mCD3GFP) expressed in Jurkat cells. Secondly, pI heterogeneity was also found in human CD3varepsilon chains immunoprecipitated from the surface of Jurkat cells or PHA blasts of human blood T lymphocytes. Comparison of CD3varepsilon chains from 27 different species shows that their N-terminal sequences share a strong acidic nature, despite the large differences in terms of length and composition, even among closely related species. Our results suggest that generation of CD3varepsilon chain isoforms with different N-terminal sequence and pI is a general phenomenon. Thus, as previously observed in the mouse, the relative abundance of CD3varepsilon chain species might regulate TCR/CD3 structure and function, including the strength of the interactions between CD3 dimers and the TCR clonotypic receptors, as well as TCR/CD3 activation thresholds. Interestingly, CD3varepsilon chains from 7 out of 27 species studied have putative N-glycosylation (NxS or NxT) motifs in their Ig extracellular domain. Their location, plus the conservation of residues involved in domain organization, the interactions with other CD3 chains, or the TCR, and signal triggering add new data useful to establish a permissive topology for the interaction between CD3 dimers and the TCR chains.

Relationship between CD8-dependent antigen recognition, T cell functional avidity, and tumor cell recognition

Effective immunotherapy using T cell receptor (TCR) gene-modified T cells requires an understanding of the relationship between TCR affinity and functional avidity of T cells. In this study, we evaluate the relative affinity of two TCRs isolated from HLA-A2-restricted, gp100-reactive T cell clones with extremely high functional avidity. Furthermore, one of these T cell clones, was CD4- CD8- indicating that antigen recognition by this clone was CD8 independent. However, when these TCRs were expressed in CD8- Jurkat cells, the resulting Jurkat cells recognized gp100:209-217 peptide loaded T2 cells and had high functional avidity, but could not recognize HLA-A2+ melanoma cells expressing gp100. Tumor cell recognition by Jurkat cells expressing these TCRs could not be induced by exogenously loading the tumor cells with the native gp100:209-217 peptide. These results indicate that functional avidity of a T cell does not necessarily correlate with TCR affinity and CD8-independent antigen recognition by a T cell does not always mean its TCR will transfer CD8-independence to other effector cells. The implications of these findings are that T cells can modulate their functional avidity independent of the affinity of their TCRs.

Natural immunity has significant impact on immune responses against cancer

The immune system defends the host against pathogenic attacks by micro-organisms and their products. It does not react against self-components due to the relatively efficient negative selection of developing T lymphocytes in the thymus. This process does permit T cells with low avidity against self to be present in the T cell repertoire. Such cells play an important physiological role as the host needs so-called autoimmune reactions in order to eliminate dying cells or transformed tumour cells. One of the mysteries in immunology is how the host maintains beneficial autoimmune reactions and avoids pathogenic autoimmune reactions. Activation of the adaptive T lymphocytes is mediated by the low avidity interaction between T-cell antigen receptors and antigenic peptides associated with major histocompatibility complex class I or class II molecules. This interaction is strengthened by T-cell co-receptors such as CD2, CD4, CD8, CD28 and CD154, which react with ligands expressed by cells of the innate immune system. In recent years, the importance of pre-activation of the innate immune system for initiation of adaptive T-cell immune responses has been appreciated. In the present review, I will summarize our work on how natural immunity plays an important role in determining the level of beneficial autoimmune reactions against cancer.

Central tolerance: what have we learned from mice?

Producing a healthy immune system capable of defending against pathogens, while avoiding autoimmunity, is dependent on thymic selection. Positive selection yields functional T cells that have the potential to recognize both self and foreign antigens. Therefore, negative selection exists to manage potentially self-reactive cells. Negative selection results from the induction of anergy, receptor editing, clonal diversion (agonist selection), and/or clonal deletion (apoptosis) in self-reactive clones. Clonal deletion has been inherently difficult to study because the cells of interest are undergoing apoptosis and being eliminated quickly. Furthermore, analysis of clonal deletion in humans has proved even more difficult due to availability of samples and lack of reagents. Mouse models have thus been instrumental in achieving our current understanding of central tolerance, and the evolution of elegant model systems has led to an explosion of new data to be assimilated. This review will focus on recent advances in the field of clonal deletion with respect to three aspects: the development of physiological model systems, signaling pathways that lead to apoptosis, and antigen presenting cell types involved in the induction of clonal deletion.

The protein tyrosine phosphatase PTPN4/PTP-MEG1, an enzyme capable of dephosphorylating the TCR ITAMs and regulating NF-kappaB, is dispensable for T cell development and/or T cell effector functions

T cell receptor signaling processes are controlled by the integrated actions of families of protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPases). Several distinct cytosolic protein tyrosine phosphatases have been described that are able to negatively regulate TCR signaling pathways, including SHP-1, SHP-2, PTPH1, and PEP. Using PTPase substrate-trapping mutants and wild type enzymes, we determined that PTPN4/PTP-MEG1, a PTPH1-family member, could complex and dephosphorylate the ITAMs of the TCR zeta subunit. In addition, the substrate-trapping derivative augmented basal and TCR-induced activation of NF-kappaB in T cells. To characterize the contribution of this PTPase in T cells, we developed PTPN4-deficient mice. T cell development and TCR signaling events were comparable between wild type and PTPN4-deficient animals. The magnitude and duration of TCR-regulated ITAM phosphorylation, as well as overall protein phosphorylation, was unaltered in the absence of PTPN4. Finally, Th1- and Th2-derived cytokines and in vivo immune responses to Listeria monocytogenes were equivalent between wild type and PTPN4-deficient mice. These findings suggest that additional PTPases are involved in controlling ITAM phosphorylations.

Structural and functional evidence that Nck interaction with CD3epsilon regulates T-cell receptor activity

Recruitment of signaling molecules to the cytoplasmic domains of the CD3 subunits of the T-cell receptor (TCR) is crucial for early T-cell activation. These transient associations either do or do not require tyrosine phosphorylation of CD3 immune tyrosine activation motifs (ITAMs). Here we show that the non-ITAM-requiring adaptor protein Nck forms a complex with an atypical PxxDY motif of the CD3epsilon tail, which encompasses Tyr166 within the ITAM and a TCR endocytosis signal. As suggested by the structure of the complex, we find that Nck binding inhibits phosphorylation of the CD3epsilon ITAM by Fyn and Lck kinases in vitro. Moreover, the CD3epsilon-Nck interaction downregulates TCR surface expression upon physiological stimulation in mouse primary lymph node cells. This indicates that Nck performs an important regulatory function in T lymphocytes by inhibiting ITAM phosphorylation and/or removing cell surface TCR via CD3epsilon interaction.

The proline-rich sequence of CD3epsilon as an amplifier of low-avidity TCR signaling

Engagement of peptide-MHC by the TCR induces a conformational change in CD3epsilon that exposes a proline-rich sequence (PRS) and recruits the cytoskeletal adaptor Nck. This event, which precedes phosphorylation of the CD3epsilon ITAM, has been implicated in synapse formation and T cell function. However, there is compelling evidence that responsiveness to TCR ligation is CD3epsilon PRS independent. In this study, we show that the CD3epsilon PRS is necessary for peptide-MHC-induced phosphorylation of CD3epsilon and for recruitment of protein kinase Ctheta to the immune synapse in differentiated CD8+ T lymphocytes. However, whereas these two events are dispensable for functional T cell responsiveness to high-avidity ligands, they are required for responsiveness to low-avidity ones. Thus, in at least certain T cell clonotypes, the CD3epsilon PRS amplifies weak TCR signals by promoting synapse formation and CD3epsilon phosphorylation.

A role for CD8 in the developmental tuning of antigen recognition and CD3 conformational change

TCR engagement by peptide-MHC class I (pMHC) ligands induces a conformational change (Deltac) in CD3 (CD3Deltac) that contributes to T cell signaling. We found that when this interaction took place between primary T lineage cells and APCs, the CD8 coreceptor was required to generate CD3Deltac. Interestingly, neither enhancement of Ag binding strength nor Src kinase signaling explained this coreceptor activity. Furthermore, Ag-induced CD3Deltac was developmentally attenuated by the increase in sialylation that accompanies T cell maturation and limits CD8 activity. Thus, both weak and strong ligands induced CD3Deltac in preselection thymocytes, but only strong ligands were effective in mature T cells. We propose that CD8 participation in the TCR/pMHC interaction can physically regulate CD3Deltac induction by "translating" productive Ag encounter from the TCR to the CD3 complex. This suggests one mechanism by which the developmentally regulated variation in CD8 sialylation may contribute to the developmental tuning of T cell sensitivity.

Signal initiation in T-cell receptor microclusters

Although dynamic imaging technologies have provided important insights into the underlying processes responsible for T-cell activation, the processes that link antigen recognition to downstream signaling remain poorly defined. Converging lines of inquiry indicate that T-cell receptor (TCR) microclusters are the minimal structures capable of directing effective TCR signaling. Furthermore, imaging studies have determined that these structures trigger the assembly of oligomeric signaling scaffolds that contain the adapters and effectors required for T-cell activation. Existing models of T-cell activation accurately explain the sensitivity and selectivity of antigen recognition. However, these models do not account for important properties of microclusters, including their peripheral formation, size, and movement on the actin cytoskeleton. Here we examine how lipid rafts, galectin lattices, and protein scaffolds contribute to the assembly, function, and fate of TCR microclusters within immune synapses. Finally, we propose a 'mechanical segregation' model of signal initiation in which cytoskeletal forces contribute to the lateral segregation of molecules and cytoskeletal scaffolds provide a template for microclusters assembly.

The proline-rich sequence of CD3epsilon controls T cell antigen receptor expression on and signaling potency in preselection CD4+CD8+ thymocytes

Antigen recognition by T cell antigen receptors (TCRs) is thought to 'unmask' a proline-rich sequence (PRS) present in the CD3epsilon cytosolic segment, which allows it to trigger T cell activation. Using 'knock-in' mice with deletion of the PRS, we demonstrate here that elimination of the CD3epsilon PRS had no effect on mature T cell responsiveness. In contrast, in preselection CD4+CD8+ thymocytes, the CD3epsilon PRS acted together with the adaptor protein SLAP to promote CD3zeta degradation, thereby contributing to downregulation of TCR expression on the cell surface. In addition, analysis of CD4+CD8+ thymocytes of TCR-transgenic mice showed that the CD3epsilon PRS enhanced TCR sensitivity to weak ligands. Our results identify previously unknown functions for the evolutionarily conserved CD3epsilon PRS at the CD4+CD8+ developmental stage and suggest a rather limited function in mature T cells.

The receptor deformation model of TCR triggering

Through T cell receptors (TCRs), T cells can detect and respond to very small numbers of foreign peptides among a huge number of self-peptides presented by major histocompatibility complexes (pMHCs) on the surface of antigen-presenting cells (APCs). How T cells achieve such remarkable sensitivity and specificity through pMHC-TCR binding is an intensively pursued issue in immunology today; the key question is how pMHC-TCR binding initiates, or triggers, a signal from TCRs. Multiple competing models have been proposed, none of which fully explains the sensitivity and specificity of TCR triggering. What has been omitted from existing theories is that the pMHC-TCR interaction at the T cell/APC interface must be under constant mechanical stress, due to the dynamic nature of cell-cell interaction. Taking this condition into consideration, we propose the receptor deformation model of TCR triggering. In this model, TCR signaling is initiated by conformational changes of the TCR/CD3 complex, induced by a pulling force originating from the cytoskeleton and transmitted through pMHC-TCR binding interactions with enough strength to resist rupture. By introducing mechanical force into a model of T cell signal initiation, the receptor deformation model provides potential mechanistic solutions to the sensitivity and specificity of TCR triggering.

T cell receptor engagement triggers its CD3epsilon and CD3zeta subunits to adopt a compact, locked conformation

How the T cell antigen receptor (TCR) discriminates between molecularly related peptide/Major Histocompatibility Complex (pMHC) ligands and converts this information into different possible signaling outcomes is still not understood. One current model proposes that strong pMHC ligands, but not weak ones, induce a conformational change in the TCR. Evidence supporting this comes from a pull-down assay that detects ligand-induced binding of the TCR to the N-terminal SH3 domain of the adapter protein Nck, and also from studies with a neoepitope-specific antibody. Both methods rely on the exposure of a polyproline sequence in the CD3ε subunit of the TCR, and neither indicates whether the conformational change is transmitted to other CD3 subunits. Using a protease-sensitivity assay, we now show that the cytoplasmic tails of CD3ε and CD3ζ subunits become fully protected from degradation upon TCR triggering. These results suggest that the TCR conformational change is transmitted to the tails of CD3ε and CD3ζ, and perhaps all CD3 subunits. Furthermore, the resistance to protease digestion suggests that CD3 cytoplasmic tails adopt a compact structure in the triggered TCR. These results are consistent with a model in which transduction of the conformational change induced upon TCR triggering promotes condensation and shielding of the CD3 cytoplasmic tails.

Surface-anchored monomeric agonist pMHCs alone trigger TCR with high sensitivity

At the interface between T cell and antigen-presenting cell (APC), peptide antigen presented by MHC (pMHC) binds to the T cell receptor (TCR) and initiates signaling. The mechanism of TCR signal initiation, or triggering, remains unclear. An interesting aspect of this puzzle is that although soluble agonist pMHCs cannot trigger TCR even at high concentrations, the same ligands trigger TCR very efficiently on the surface of APCs. Here, using lipid bilayers or plastic-based artificial APCs with defined components, we identify the critical APC-associated factors that confer agonist pMHCs with such potency. We found that CD4+ T cells are triggered by very low numbers of monomeric agonist pMHCs anchored on fluid lipid bilayers or fixed plastic surfaces, in the absence of any other APC surface molecules. Importantly, on bilayers, plastic surfaces, or real APCs, endogenous pMHCs did not enhance TCR triggering. TCR triggering, however, critically depended upon the adhesiveness of the surface and an intact T cell actin cytoskeleton. Based on these observations, we propose the receptor deformation model of TCR triggering to explain the remarkable sensitivity and specificity of TCR triggering.

Hypothesis: TCR signal transduction--A novel tri-modular signaling system

Antigenic peptides initiate an immune response in T cells via the T cell receptor (TCR). The TCR itself is widely regarded as one of the most complex receptors in nature, as it is comprised of at least six different subunits, the antigen recognizing TCRalpha and beta chains, and the signal transmitting CD3deltavarepsilon, gammaepsilon, and zeta2 dimers. In order for a signal to be transmitted from the TCR to the cytoplasm, the CD3 chains must "sense" that an antigenic peptide has been presented to the TCRalpha and beta subunits. After accomplishing this, there are a total of 10 different immunoreceptor tyrosine activation motifs (ITAMs) present within the CD3 chains which effectively activate the T cell and hence the immune response. The importance of each CD3 chain and subsequently each ITAM has been the focus of intense research. However, the precise role(s) played by each CD3 chain has remained elusive. Using the immunomodulatory peptide termed core peptide (CP), which is proposed to inhibit TCR activation by disrupting TCR-CD3 interactions, a tri-modular signaling system for T cell activation is proposed. By contrast to the existing two distinct signaling model (zeta2, CD3epsilongamma/epsilondelta), in this model each of the three dimers, CD3gammaepsilon, deltaepsilon, and zeta2, are proposed to act as three separate and distinct signaling modules, performing both specific and redundant functions.

Reciprocal regulation of SH3 and SH2 domain binding via tyrosine phosphorylation of a common site in CD3epsilon

Recruitment of cellular signaling proteins by the CD3 polypeptides of the TCR complex mediates T cell activation. We have screened a human Src homology 3 (SH3) domain phage display library for proteins that can bind to the proline-rich region of CD3epsilon. This screening identified Eps8L1 (epidermal growth factor receptor pathway substrate 8-like 1) together with the N-terminal SH3 domain of Nck1 and Nck2 as its preferred SH3 partners. Studies with recombinant proteins confirmed strong binding of CD3epsilon to Eps8L1 and Nck SH3 domains. CD3epsilon bound well also to Eps8 and Eps8L3, and modestly to Eps8L2, but not detectably to other SH3 domains tested. Interestingly, binding of Nck and Eps8L1 SH3 domains was mapped to a PxxDY motif that shared its tyrosine residue (Y166) with the ITAM of CD3epsilon. Phosphorylation of this residue abolished binding of Eps/Nck SH3 domains in peptide spot filter assays, as well as in cells cotransfected with a dominantly active Lck kinase. TCR ligation-induced binding and phosphorylation-dependent loss of binding were also demonstrated between Eps8L1 and endogenous CD3epsilon in Jurkat T cells. Thus, phosphorylation of Y166 serves as a molecular switch during T cell activation that determines the capacity of CD3epsilon to interact with either SH3 or SH2 domain-containing proteins.

A role for "self" in T-cell activation

The mechanisms by which alphabeta T-cells are selected in the thymus and then recognize peptide MHC (pMHC) complexes in the periphery remain an enigma. Recent work particularly with respect to quantification of T-cell sensitivity and the role of self-ligands in T-cell activation has provided some important clues to the details of how TCR signaling might be initiated. Here, we highlight recent experimental data that provides insights into the initiation of T-cell activation and also discuss the main controversies and uncertainties in this area.

Molecular mechanisms involved in T cell receptor triggering

Despite intensive investigation we still do not understand how the T cell antigen receptor (TCR) tranduces signals across the plasma membrane, a process referred to as TCR triggering. Three basic mechanisms have been proposed, involving aggregation, conformational change, or segregation of the TCR upon binding pMHC ligand. Given the low density of pMHC ligand it remains doubtful that TCR aggregation initiates triggering, although it is likely to enhance subsequent signalling. Structural studies to date have not provided definitive evidence for or against a conformational change mechanism, but they have ruled out certain types of conformational change. Size-induced segregation of the bound TCR from inhibitory membrane tyrosine phosphatases seems to be required, but is probably not the only mechanism. Current evidence suggests that TCR triggering is initiated by a combination of segregation and conformational change, with subsequent aggregation contributing to amplification of the signal.

Allosteric Changes in the TCR/CD3 Structure Upon Interaction With Extra- or Intra-cellular Ligands

T lymphocytes are activated by the interaction between the T-cell antigen receptor (TCR) and peptides presented by major histocompatibility complex (MHC) molecules. The avidity of this TCR-pMHC interaction is very low. Therefore, several hypotheses have been put forward to explain how T cells become specifically activated despite this handicap: conformational change model, aggregation model, kinetic segregation model, sequential interaction model and permissive geometry model. In the present paper, we conducted experiments to distinguish between the TCR-aggregation model and the TCR-conformational change model. The results obtained using a TCR capture ELISA with Brij 98-solubilized TCR molecules from normal or activated T cells showed that the ligand-TCR interaction causes structural changes in the CD3 epsilon cytoplasmic tail as well as in the extracellular TCR beta FG loop region. Size-fractionation experiments with Brij 98-solubilized TCR/CD3/co-receptor complexes from naïve or activated CD4(+) or CD8(+) T cells demonstrated that such complexes are found as either dimers or tetramers. No monomers or multimers were detected. We propose that: (1) ligand-TCR interaction results in conformational changes in the CD3 epsilon cytoplasmic tail leading to T-cell activation; (2) CD3 epsilon cytoplasmic tail interaction with intracellular proteins may dissociate pMHC and co-receptors (CD4 or CD8) from TCR/CD3 complexes, thus leading to the arrest of T-cell activation; and (3) T-cell activation appears to occur among dimers or tetramers of TCR/CD3/co-receptor complexes interacting with self and non-self (foreign) peptide-MHC complexes.

Loss of N-terminal charged residues of mouse CD3 epsilon chains generates isoforms modulating antigen T cell receptor-mediated signals and T cell receptor-CD3 interactions

The antigen T cell receptor (TCR)-CD3 complexes present on the cell surface of CD4(+) T lymphocytes and T cell lines express CD3 epsilon chain isoforms with different isoelectric points (pI), with important structural and functional consequences. The pI values of the isoforms fit the predicted pI values of CD3 epsilon chains lacking one, two, and three negatively charged amino acid residues present in the N-terminal region. Different T cells have different ratios of CD3 epsilon chain isoforms. At a high pI, degraded CD3 epsilon isoforms can be better recognized by certain anti-CD3 monoclonal antibodies such as YCD3-1, the ability of which to bind to the TCR-CD3 complex is directly correlated with the pI of CD3 epsilon. The abundance of CD3 epsilon isoforms can be modified by treatment of T cells with the proteinase inhibitor phenanthroline. In addition, these CD3 epsilon isoforms have functional importance. This is shown, first, by the different structure of TCR-CD3 complexes in cells possessing different amounts of isoforms (as observed in surface biotinylation experiments), by their different antigen responses, and by the stronger interaction between low pI CD3 epsilon isoforms and the TCR. Second, incubation of cells with phenanthroline diminished the proportion of degraded high pI CD3 epsilon isoforms, but also the ability of the cells to deliver early TCR activation signals. Third, cells expressing mutant CD3 epsilon chains lacking N-terminal acid residues showed facilitated recognition by antibody YCD3-1 and enhanced TCR-mediated activation. Furthermore, the binding avidity of antibody YCD3-1 was different in distinct thymus populations. These results suggest that changes in CD3 epsilon N-terminal chains might help to fine-tune the response of the TCR to its ligands in distinct activation situations or in thymus selection.

High-sensitivity detection and quantitative analysis of native protein-protein interactions and multiprotein complexes by flow cytometry

Most mechanisms of cell development, physiology, and signal transduction are controlled by protein-protein interactions. Immunoprecipitation of multiprotein complexes detected by flow cytometry (IP-FCM) is a means to quantitatively measure these interactions. The high sensitivity of this method makes it useful even when very little biomaterial is available for analysis, as in the case of rare primary cell subsets or patient samples. Detection of the T cell antigen receptor associated with the CD3 multiprotein complex from as few as 300 primary murine T cells is presented as an example. The method is compatible with quantitative flow cytometry techniques, making it possible to estimate the number of coimmunoprecipitated molecules. Both constitutive and inducible protein-protein interactions can be analyzed, as illustrated in related methodology using glutathione S-transferase-fusion protein pull-down experiments. IP-FCM represents a robust, quantitative, biochemical technique to assess native protein-protein interactions, without requiring genetic engineering or large sample sizes.

Stochasticity and spatial heterogeneity in T-cell activation

Stochastic and spatial aspects are becoming increasingly recognized as an important factor in T-cell activation. Activation occurs in an intrinsically noisy environment, requiring only a handful of agonist peptide-major histocompatibility complex molecules, thus making consideration of signal to noise of prime importance in understanding sensitivity and specificity. Furthermore, it is widely established that surface-bound ligands are more effective at activation than soluble forms, while surface patternation has highlighted the role of spatial relocation in activation. Here we consider the results of a number of models of T-cell activation, from a realistic model of kinetic segregation-induced T-cell receptor (TCR) triggering through to simple queuing theory models. These studies highlight the constraints on cell activation by a surface receptor that recruits kinases. Our analysis shows that TCR triggering based on trapping of bound TCRs in regions of close proximity that exclude large ectodomain-containing molecules, such as the phosphatases CD45 and CD148, can effectively reproduce known signaling characteristics and is a viable 'signal transduction' mechanism distinct from oligomerization and conformation-based mechanisms. A queuing theory analysis shows the interrelation between sensitivity and specificity, emphasizing that these are properties of individual cell functions and need not be, nor are likely to be, uniform across different functions. In fact, threshold-based mechanisms of detection are shown to be poor at ligand discrimination because, although they can be highly specific, that specificity is limited to a small range of peptide densities. Time integration mechanisms however are able to control noise effectively, while kinetic proofreading mechanisms endow them with good specificity properties. Thus, threshold mechanisms are likely to be important for rapidly detecting minimal signaling requirements, thus achieving efficient scanning of antigen-presenting cells. However, for good specificity, time integration on a scale of hours is required.

Is the activity of partially agonistic MHC:peptide ligands dependent on the quality of immunological help?

CD8(+) cytotoxic T lymphocytes (CTL) are important for the immunological control of infections and tumours. Engagement of the T-cell receptor (TCR) with major histocompatibility complex (MHC) class I/peptide complexes on antigen-presenting cells (APC) is the key interaction, which initiates the process of T-cell activation. Depending on the affinity of this interaction, different arrays of signalling pathways and functional outcomes can be activated in the specific T cells. Molecular alterations in the peptide bound to the MHC class I can lead to a lower affinity of the MHC:TCR interaction resulting in incomplete or qualitatively different T-cell responses. Altered peptide ligands (APL) exhibiting such activity are referred to as partial agonists and often occur naturally through genetic instability, which affects T-cell epitopes derived from rapidly mutating viruses or tumour-associated cellular antigens. Partial agonists are usually viewed as peptide variants, which escape efficient CTL recognition. Our recent data suggest that APL can not only trigger incomplete activation but also induce and modulate intrinsic T-cell programmes leading to the shut-off of specific CTL responses. This APL-induced suppression appears to be more prominent in the absence of immunological help, suggesting that under conditions of immune deregulation APL may actively inhibit CTL responses against infectious agents or tumours. In this review, we discuss experimental data supporting this model and possible role of APL-induced immunosuppression in different pathological conditions.

Full Activation of the T Cell Receptor Requires Both Clustering and Conformational Changes at CD3

T cell receptor (TCR-CD3) triggering involves both receptor clustering and conformational changes at the cytoplasmic tails of the CD3 subunits. The mechanism by which TCRalphabeta ligand binding confers conformational changes to CD3 is unknown. By using well-defined ligands, we showed that induction of the conformational change requires both multivalent engagement and the mobility restriction of the TCR-CD3 imposed by the plasma membrane. The conformational change is elicited by cooperative rearrangements of two TCR-CD3 complexes and does not require accompanying changes in the structure of the TCRalphabeta ectodomains. This conformational change at CD3 reverts upon ligand dissociation and is required for T cell activation. Thus, our permissive geometry model provides a molecular mechanism that rationalizes how the information of ligand binding to TCRalphabeta is transmitted to the CD3 subunits and to the intracellular signaling machinery.

Ligand detection and discrimination by spatial relocalization: A kinase-phosphatase segregation model of TCR activation

We develop a model of tyrosine phosphorylation and activation of the T-cell receptor (TCR) by localization to regions of close membrane-membrane proximity (close contact) that physically exclude tyrosine phosphatases such as CD45. Phosphatase exclusion generates regions of low phosphatase and high kinase activity and thus our model provides a framework to examine the kinetic segregation model of TCR activation. We incorporate a sequence of activation steps modeling the construction of the signalosome with a final sequestered, or high-stability, signaling state. The residence time of unbound TCRs in tyrosine kinase-rich domains is shown to be too short for accumulation of activation steps, whereas binding to an agonist lengthens the localization time and leads to generation of fully active TCRs. Agonist detection depends only on this localization, and therefore kinetic segregation represents a viable ligand detection mechanism, or signal transduction mechanism across membranes, distinct from receptor oligomerization and conformational change. We examine the degree of discrimination of agonists from a background of null (self) peptides, and from weak agonists achievable by this mechanism.

The CD3 gamma epsilon/delta epsilon signaling module provides normal T cell functions in the absence of the TCR zeta immunoreceptor tyrosine-based activation motifs

T cell receptor (TCR) signal transduction is mediated by the immunoreceptor tyrosine-based activation motifs (ITAM). The ten ITAM in the TCR complex are distributed in two distinct signaling modules termed TCR zetazeta and CD3 gammaepsilon/deltaepsilon. To delineate the specific role of the zeta ITAM in T cell development and TCR signal transmission, we compared the properties of T cells from different TCR zeta-transgenic lines wherein tyrosine-to-phenylalanine substitutions had been introduced in the zeta subunit. These lines lack selected phosphorylated forms of TCR zeta including just p23, both p21 and p23, or all phospho-zeta derivatives. We report herein that the efficiency of positive selection in HY TCR-transgenic female mice was directly related to the number of zeta ITAM in the TCR. In contrast, TCR-mediated signal transmission and T cell proliferative responses following agonist peptide stimulation were similar and independent of the zeta ITAM. Only the duration of MAPK activation was affected by multiple zeta ITAM substitutions. These results strongly suggest that the ITAM in the CD3 gammaepsilon/deltaepsilon module can provide normal TCR signal transmission, with zeta ITAM providing a secondary function facilitating MAPK activation and positive selection.

A conformational change senses the strength of T cell receptor-ligand interaction during thymic selection

T cell antigen receptor (TCR) triggering determines the fate of immature thymocytes. The affinity of the TCR for its endogenous peptide/MHC ligands serves as a signal for positive or negative selection through mechanisms that are still little understood. We have used a conformation-specific antibody to demonstrate that recognition of TCR ligands that lead to negative selection induces a conformational change in the TCR in situ. In contrast, this conformational change is elicited in only a small percentage of immature thymocytes during positive selection. Using a TUNEL assay, we demonstrate that the conformational change in the TCR is strongly linked to activation of programmed cell death in conditions leading to negative selection. Furthermore, the few conformational change-positive thymocytes detected in conditions that preferably lead to positive selection are also TUNEL-positive. Thus, the conformational change in the TCR may underlie the discrimination of ligands leading to positive and negative selection.

A conformation- and avidity-based proofreading mechanism for the TCR–CD3 complex

During antigen recognition, T cells show high sensitivity and specificity, and a wide dynamic range. Paradoxically, these characteristics are based on low-affinity receptor-ligand interactions [between the T-cell antigen receptor (TCR-CD3) complex and the antigen peptide bound to MHC]. Recent evidence indicates that the TCR-CD3 is expressed as multivalent complexes in the membrane of non-stimulated T cells and that conformational changes in the TCR-CD3 can be induced by strong but not weak agonists. Here, we propose a thermodynamic model whereby the specificity of the TCR-CD3-pMHC interaction is explained by its multivalent nature. We also propose that the free energy barriers involved in the change in conformation of the receptor impose a response threshold and determine the kinetic properties of recognition. Finally, we suggest that multivalent TCR-CD3s can amplify signals by spreading them from pMHC-engaged TCR-CD3s to unengaged complexes as a consequence of the cooperativity in the system.

Deconstructing the form and function of the TCR/CD3 complex

When T cells encounter antigens via the T cell antigen receptor (TCR), information about the quantity and quality of antigen engagement is relayed to the intracellular signal transduction machinery. This process is poorly understood. The TCR itself lacks a significant intracellular domain. Instead, it is associated with CD3 molecules that contain intracellular signaling domains that couple the TCR/CD3 complex to the downstream signaling machinery. The earliest events in TCR signaling must involve the transfer of information from the antigen binding TCR subunit to the CD3 signaling subunits of the TCR/CD3 complex. Elucidating the structural organization of the TCR with the associated CD3 signaling molecules is necessary for understanding the mechanism by which TCR engagement is coupled to activation. Here, we review the current state of our understanding of the structure and organization of the TCR/CD3 complex.

The why and how of thymocyte negative selection

The generation of T cell receptor (TCR) sequence diversity is the strength of adaptive immunity, yet is also the Achilles' heel. To purge highly self-reactive T cells from the immune system, generation of diversity has coevolved with a mechanism of negative selection. Recent studies have revealed new insights addressing the why and how of negative selection by examining situations in which negative selection has failed in human and animals models of autoimmunity. Both thymocyte extrinsic and intrinsic mechanisms are required to restrict the TCR repertoire to a non-autoreactive set. Negative selection also ensures that T cells emerge with receptors that are focussed on the peptide moiety of MHC-peptide complexes.

Thymocyte Negative Selection Is Mediated by Protein Kinase C- and Ca2+-Dependent Transcriptional Induction of Bim

The processes of positive and negative selection in the thymus both determine the population of T cells that will enter the peripheral immune system and eliminate self-reactive T cells by apoptosis. Substantial evidence indicates that TCR signal intensity mediates this cell fate choice: low-intensity signals lead to survival and differentiation, whereas high-intensity signals generated by self-Ag lead to cell death. The molecular mechanism by which these graded signals are converted to discrete outcomes is not understood. Positive selection requires the Ca(2+)-dependent phosphatase calcineurin, whereas negative selection requires the proapoptotic Bcl-2 family member Bcl-2-interacting mediator of cell death (Bim). In this study, we investigated the regulation of Bim expression and the role of Ca(2+) in mediating negative selection. Our results show that transcription is necessary for both negative selection and Bim induction. Surprisingly, we also found that Ca(2+) is necessary for Bim induction. Induction of bim transcription appears to involve protein kinase C, but not calcineurin, JNK, p38 MAPK, or MEK. These results localize the decision point in positive vs negative selection to a step downstream of Ca(2+) signaling and suggest that negative selection signals induce Ca(2+)-dependent bim transcription through PKC.

Modulation of T cell function by TCR/pMHC binding kinetics

The interaction between the T cell receptor (TCR) and the peptide-MHC complex (pMHC) at the interface between the T cell and the antigen presenting cell (APC) is the main event controlling the specificity of antigen recognition by T cells. It is thought that TCR/pMHC binding kinetics are critical for the selection of the T cell repertoire in the thymus, as well as the activation of mature T cells in the periphery. One of the binding parameters that conditions T cell activation by pMHC ligands is the half-life of the TCR/pMHC interaction. This kinetic parameter is highly significant for the regulation of T cell activation and therefore determines the capacity of T cells to respond against pathogen- and tumor-derived antigens, avoiding self-reactivity. Several studies support the notion that T cells are activated only by TCR/pMHC interactions that are above a threshold of half-life. pMHC complexes that bind TCRs with half-lives below that threshold behave as null or antagonistic ligands. However, since prolonged half-lives can also impair T cell activation, there seems to be a ceiling for the TCR/pMHC half life that leads to efficient activation of T cells. According to these observations, efficient T cell activation would require an optimal half-life of TCR/pMHC interaction. These kinetic restrictions for T cell activation are important to generate a protective adaptive immune response minimizing cross-reactivity against self-constituents. The nature of the TCR/pMHC interaction defines in the thymus whether a thymocyte develops into a mature T cell or is eliminated by apoptosis. In addition, the kinetics of TCR/pMHC binding can determine the type of response shown by mature T cells in the periphery. Although several studies have focused on the modulation of T cell function by the affinity of the TCR/pMHC interaction, the binding kinetics rules governing T cell activation remain poorly understood. Here we review recent data and propose a new model for the regulation of T cell function by TCR/pMHC binding kinetics.

Central tolerance: learning self-control in the thymus

In the past few years, there has been a flurry of discoveries and advancements in our understanding of how the thymus prepares T cells to exist at peace in normal healthy tissue: that is, to be self-tolerant. In the thymus, one of the main mechanisms of T-cell central tolerance is clonal deletion, although the selection of regulatory T cells is also important and is gaining enormous interest. In this Review, we discuss the emerging consensus about which models of clonal deletion are most physiological, and we review recent data that define the molecular mechanisms of central tolerance.

Immunology: how do T cells recognize antigen?

T cells recognize small fragments of microorganisms (antigens) on the surface of other cells using T cell antigen receptors. The mechanism by which these receptors signal into T cells is controversial, but two recent studies provide important new clues.

The CD3ε Proline-Rich Sequence, and Its Interaction with Nck, Is Not Required for T Cell Development and Function

The CD3epsilon proline-rich sequence (PRS) binds to the cytosolic adaptor molecule Nck after TCR ligation. It has been proposed that this interaction is essential for immunological synapse formation and T cell activation. To assess the physiological importance of the CD3epsilon PRS, we have generated mice that lack this motif (CD3epsilon.PRS(M)). Pull-down experiments demonstrated the inability of Nck to bind to the CD3epsilon PRS in thymocytes from mutant mice after TCR ligation. Surprisingly, no differences were observed in the number and percentage of T cell subsets in the thymus and spleen, and there was no apparent defect in positive or negative selection. Furthermore, the proliferative response of CD3epsilon.PRS(M) T cells to staphylococcal enterotoxin B and anti-CD3 Ab was normal. TCR surface expression, constitutive internalization, and Ag-induced down-modulation were also normal. These data suggest that the interaction between the CD3epsilon PRS and Nck, or any other Src homology 3 domain-containing molecule, is not essential for T cell development and function.

Twisting tails exposed: the evidence for TCR conformational change

The mechanism by which the ligand occupancy state of the T cell receptor complex is converted into intracellular signaling information has been a controversial topic. Although the majority of structural studies argue against a conformational change, recent studies support the possibility for such a change within the CD3 components of the TCR complex. In this commentary, the evidence for TCR conformational change is reviewed and potential mechanisms for its initiation are explored.

Ligand-induced conformational change in the T-cell receptor associated with productive immune synapses

Triggering of the T-cell receptor (TCR) can produce very different responses, depending on the nature of the major histocompatibility complex/antigen peptide (MHCp) ligand. The molecular mechanisms that permit such fine discrimination are still unknown. We show here that an epitope in the cytoplasmic tail of the TCR CD3epsilon subunit, recognized by antibody APA1/1, is only detected when the TCR is fully activated. Exposure of the APA1/1 epitope is shown to be fast and independent of tyrosine kinase activity and that it takes place even when T cells are stimulated at 0 degrees C. These results suggest that APA1/1 detects a conformational change in the TCR. APA1/1 staining concentrates in a restricted area of the immunologic synapse. Most important, we show that full agonist, but not partial agonist, peptides induce exposure of the APA1/1 epitope, indicating a correlation between the induction of the conformational change in the TCR and full T-cell activation. Finally, the conformational change is shown to occur in T cells that are being stimulated by antigen in vivo. Therefore, these results demonstrate that the TCR undergoes a conformational change on MHCp binding in vitro and in vivo, and they establish a molecular correlate for productive TCR engagement.