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

Phospho-mimetic CD3ε variants prevent TCR and CAR signaling

Introduction

Antigen binding to the T cell antigen receptor (TCR) leads to the phosphorylation of the immunoreceptor tyrosine-based activation motifs (ITAMs) of the CD3 complex, and thereby to T cell activation. The CD3ε subunit plays a unique role in TCR activation by recruiting the kinase LCK and the adaptor protein NCK prior to ITAM phosphorylation. Here, we aimed to investigate how phosphorylation of the individual CD3ε ITAM tyrosines impacts the CD3ε signalosome.

Methods

We mimicked irreversible tyrosine phosphorylation by substituting glutamic acid for the tyrosine residues in the CD3ε ITAM.

Results

Integrating CD3ε phospho-mimetic variants into the complete TCR-CD3 complex resulted in reduced TCR signal transduction, which was partially compensated by the involvement of the other TCR-CD3 ITAMs. By using novel CD3ε phospho-mimetic Chimeric Antigen Receptor (CAR) variants, we avoided any compensatory effects of other ITAMs in the TCR-CD3 complex. We demonstrated that irreversible CD3ε phosphorylation prevented signal transduction upon CAR engagement. Mechanistically, we demonstrated that glutamic acid substitution at the N-terminal tyrosine residue of the CD3ε ITAM (Y39E) significantly reduces NCK binding to the TCR. In contrast, mutation at the C-terminal tyrosine of the CD3ε ITAM (Y50E) abolished LCK recruitment to the TCR, while increasing NCK binding. Double mutation at the C- and N-terminal tyrosines (Y39/50E) allowed ZAP70 to bind, but reduced the interaction with LCK and NCK.

Conclusions

The data demonstrate that the dynamic phosphorylation of the CD3ε ITAM tyrosines is essential for CD3ε to orchestrate optimal TCR and CAR signaling and highlights the key role of CD3ε signalosome to tune signal transduction.

Ligand-induced segregation from large cell-surface phosphatases is a critical step in γδ TCR triggering

Gamma/delta (γδ) T cells are unconventional adaptive lymphocytes that recognize structurally diverse ligands via somatically-recombined antigen receptors (γδ TCRs). The molecular mechanism by which ligand recognition initiates γδ TCR signaling, a process known as TCR triggering, remains elusive. Unlike αβ TCRs, γδ TCRs are not mechanosensitive, and do not require coreceptors or typical binding-induced conformational changes for triggering. Here, we show that γδ TCR triggering by nonclassical MHC class Ib antigens, a major class of ligands recognized by γδ T cells, requires steric segregation of the large cell-surface phosphatases CD45 and CD148 from engaged TCRs at synaptic close contact zones. Increasing access of these inhibitory phosphatases to sites of TCR engagement, by elongating MHC class Ib ligands or truncating CD45/148 ectodomains, abrogates TCR triggering and T cell activation. Our results identify a critical step in γδ TCR triggering and provide insight into the core triggering mechanism of endogenous and synthetic tyrosine-phosphorylated immunoreceptors.

A Story of Kinases and Adaptors: The Role of Lck, ZAP-70 and LAT in Switch Panel Governing T-Cell Development and Activation

Specific antigen recognition is one of the immune system's features that allows it to mount intense yet controlled responses to an infinity of potential threats. T cells play a relevant role in the host defense and the clearance of pathogens by means of the specific recognition of peptide antigens presented by antigen-presenting cells (APCs), and, to do so, they are equipped with a clonally distributed antigen receptor called the T-cell receptor (TCR). Upon the specific engagement of the TCR, multiple intracellular signals are triggered, which lead to the activation, proliferation and differentiation of T lymphocytes into effector cells. In addition, this signaling cascade also operates during T-cell development, allowing for the generation of cells that can be helpful in the defense against threats, as well as preventing the generation of autoreactive cells. Early TCR signals include phosphorylation events in which the tyrosine kinases Lck and ZAP70 are involved. The sequential activation of these kinases leads to the phosphorylation of the transmembrane adaptor LAT, which constitutes a signaling hub for the generation of a signalosome, finally resulting in T-cell activation. These early signals play a relevant role in triggering the development, activation, proliferation and apoptosis of T cells, and the negative regulation of these signals is key to avoid aberrant processes that could generate inappropriate cellular responses and disease. In this review, we will examine and discuss the roles of the tyrosine kinases Lck and ZAP70 and the membrane adaptor LAT in these cellular processes.

A set point in the selection of the αβTCR T cell repertoire imposed by pre-TCR signaling strength

SignificanceThe ability of the T cell receptor (TCR) to convey signals of different intensity is essential for the generation of a diverse, protecting, and self-tolerant T cell repertoire. We provide evidence that pre-TCR signaling during the first stage of T cell differentiation, thought to only check for in-frame rearrangement of TCRβ gene segments, determines the degree of diversity in a signaling intensity-dependent manner and controls the diversity of the TCR repertoire available for subsequent thymic positive and negative selection. Pre-TCR signaling intensity is regulated by the transmembrane region of its associated CD3ζ chains, possibly by organizing pre-TCRs into nanoclusters. Our data provide insights into immune receptor signaling mechanisms and reveal an additional checkpoint of T cell repertoire diversity.

Fluctuations in T cell receptor and pMHC interactions regulate T cell activation

Adaptive immune responses depend on interactions between T cell receptors (TCRs) and peptide major histocompatibility complex (pMHC) ligands located on the surface of T cells and antigen presenting cells (APCs), respectively. As TCRs and pMHCs are often only present at low copy numbers their interactions are inherently stochastic, yet the role of stochastic fluctuations on T cell function is unclear. Here, we introduce a minimal stochastic model of T cell activation that accounts for serial TCR-pMHC engagement, reversible TCR conformational change and TCR aggregation. Analysis of this model indicates that it is not the strength of binding between the T cell and the APC cell per se that elicits an immune response, but rather the information imparted to the T cell from the encounter, as assessed by the entropy rate of the TCR-pMHC binding dynamics. This view provides an information-theoretic interpretation of T cell activation that explains a range of experimental observations. Based on this analysis, we propose that effective T cell therapeutics may be enhanced by optimizing the inherent stochasticity of TCR-pMHC binding dynamics.

SIRPα - CD47 axis regulates dendritic cell-T cell interactions and TCR activation during T cell priming in spleen

The SIRPα-CD47 axis plays an important role in T cell recruitment to sites of immune reaction and inflammation but its role in T cell antigen priming is incompletely understood. Employing OTII TCR transgenic mice bred to Cd47-/- (Cd47KO) or SKI mice, a knock-in transgenic animal expressing non-signaling cytoplasmic-truncated SIRPα, we investigated how the SIRPα-CD47 axis contributes to antigen priming. Here we show that adoptive transfer of Cd47KO or SKI Ova-specific CD4+ T cells (OTII) into Cd47KO and SKI recipients, followed by Ova immunization, elicited reduced T cell division and proliferation indices, increased apoptosis, and reduced expansion compared to transfer into WT mice. We confirmed prior reports that splenic T cell zone, CD4+ conventional dendritic cells (cDCs) and CD4+ T cell numbers were reduced in Cd47KO and SKI mice. We report that in vitro derived DCs from Cd47KO and SKI mice exhibited impaired migration in vivo and exhibited reduced CD11c+ DC proximity to OTII T cells in T cell zones after Ag immunization, which correlates with reduced TCR activation in transferred OTII T cells. These findings suggest that reduced numbers of CD4+ cDCs and their impaired migration contributes to reduced T cell-DC proximity in splenic T cell zone and reduced T cell TCR activation, cell division and proliferation, and indirectly increased T cell apoptosis.

Cooperative Interaction of Nck and Lck Orchestrates Optimal TCR Signaling

The T cell antigen receptor (TCR) is expressed on T cells, which orchestrate adaptive immune responses. It is composed of the ligand-binding clonotypic TCRαβ heterodimer and the non-covalently bound invariant signal-transducing CD3 complex. Among the CD3 subunits, the CD3ε cytoplasmic tail contains binding motifs for the Src family kinase, Lck, and the adaptor protein, Nck. Lck binds to a receptor kinase (RK) motif and Nck binds to a proline-rich sequence (PRS). Both motifs only become accessible upon ligand binding to the TCR and facilitate the recruitment of Lck and Nck independently of phosphorylation of the TCR. Mutations in each of these motifs cause defects in TCR signaling and T cell activation. Here, we investigated the role of Nck in proximal TCR signaling by silencing both Nck isoforms, Nck1 and Nck2. In the absence of Nck, TCR phosphorylation, ZAP70 recruitment, and ZAP70 phosphorylation was impaired. Mechanistically, this is explained by loss of Lck recruitment to the stimulated TCR in cells lacking Nck. Hence, our data uncover a previously unknown cooperative interaction between Lck and Nck to promote optimal TCR signaling.

Direct Regulation of the T Cell Antigen Receptor's Activity by Cholesterol

Biological membranes consist of hundreds of different lipids that together with the embedded transmembrane (TM) proteins organize themselves into small nanodomains. In addition to this function of lipids, TM regions of proteins bind to lipids in a very specific manner, but the function of these TM region-lipid interactions is mostly unknown. In this review, we focus on the role of plasma membrane cholesterol, which directly binds to the αβ T cell antigen receptor (TCR), and has at least two opposing functions in αβ TCR activation. On the one hand, cholesterol binding to the TM domain of the TCRβ subunit keeps the TCR in an inactive, non-signaling conformation by stabilizing this conformation. This assures that the αβ T cell remains quiescent in the absence of antigenic peptide-MHC (the TCR's ligand) and decreases the sensitivity of the T cell toward stimulation. On the other hand, cholesterol binding to TCRβ leads to an increased formation of TCR nanoclusters, increasing the avidity of the TCRs toward the antigen, thus increasing the sensitivity of the αβ T cell. In mouse models, pharmacological increase of the cholesterol concentration in T cells caused an increase in TCR clustering, and thereby enhanced anti-tumor responses. In contrast, the γδ TCR does not bind to cholesterol and might be regulated in a different manner. The goal of this review is to put these seemingly controversial findings on the impact of cholesterol on the αβ TCR into perspective.

The TCR is an allosterically regulated macromolecular machinery changing its conformation while working

The αβ T-cell receptor (TCR) is a multiprotein complex controlling the activation of T cells. Although the structure of the complete TCR is not known, cumulative evidence supports that the TCR cycles between different conformational states that are promoted either by thermal motion or by force. These structural transitions determine whether the TCR engages intracellular effectors or not, regulating TCR phosphorylation and signaling. As for other membrane receptors, ligand binding selects and stabilizes the TCR in active conformations, and/or switches the TCR to activating states that were not visited before ligand engagement. Here we review the main models of TCR allostery, that is, ligand binding at TCRαβ changes the structure at CD3 and ζ. (a) The ITAM and proline-rich sequence exposure model, in which the TCR's cytoplasmic tails shield each other and ligand binding exposes them for phosphorylation. (b) The membrane-ITAM model, in which the CD3ε and ζ tails are sequestered inside the membrane and again ligand binding exposes them. (c) The mechanosensor model in which ligand binding exerts force on the TCR, inducing structural changes that allow signaling. Since these models are complementary rather than competing, we propose a unified model that aims to incorporate all existing data.

The structural basis of T-cell receptor (TCR) activation: An enduring enigma

T cells are critical for protective immune responses to pathogens and tumors. The T-cell receptor (TCR)-CD3 complex is composed of a diverse αβ TCR heterodimer noncovalently associated with the invariant CD3 dimers CD3ϵγ, CD3ϵδ, and CD3ζζ. The TCR mediates recognition of antigenic peptides bound to MHC molecules (pMHC), whereas the CD3 molecules transduce activation signals to the T cell. Whereas much is known about downstream T-cell signaling pathways, the mechanism whereby TCR engagement by pMHC is first communicated to the CD3 signaling apparatus, a process termed early T-cell activation, remains largely a mystery. In this review, we examine the molecular basis for TCR activation in light of the recently determined cryoEM structure of a complete TCR-CD3 complex. This structure provides an unprecedented opportunity to assess various signaling models that have been proposed for the TCR. We review evidence from single-molecule and structural studies for force-induced conformational changes in the TCR-CD3 complex, for dynamically-driven TCR allostery, and for pMHC-induced structural changes in the transmembrane and cytoplasmic regions of CD3 subunits. We identify major knowledge gaps that must be filled in order to arrive at a comprehensive model of TCR activation that explains, at the molecular level, how pMHC-specific information is transmitted across the T-cell membrane to initiate intracellular signaling. An in-depth understanding of this process will accelerate the rational design of immunotherapeutic agents targeting the TCR-CD3 complex.

Anti-CD3 Fab Fragments Enhance Tumor Killing by Human γδ T Cells Independent of Nck Recruitment to the γδ T Cell Antigen Receptor

T lymphocytes expressing the γδ T cell receptor (γδ TCR) can recognize antigens expressed by tumor cells and subsequently kill these cells. γδ T cells are indeed used in cancer immunotherapy clinical trials. The anti-CD3ε antibody UCHT1 enhanced the in vitro tumor killing activity of human γδ T cells by an unknown molecular mechanism. Here, we demonstrate that Fab fragments of UCHT1, which only bind monovalently to the γδ TCR, also enhanced tumor killing by expanded human Vγ9Vδ2 γδ T cells or pan-γδ T cells of the peripheral blood. The Fab fragments induced Nck recruitment to the γδ TCR, suggesting that they stabilized the γδ TCR in an active CD3ε conformation. However, blocking the Nck-CD3ε interaction in γδ T cells using the small molecule inhibitor AX-024 neither reduced the γδ T cells' natural nor the Fab-enhanced tumor killing activity. Likewise, Nck recruitment to CD3ε was not required for intracellular signaling, CD69 and CD25 up-regulation, or cytokine secretion by γδ T cells. Thus, the Nck-CD3ε interaction seems to be dispensable in γδ T cells.

A window of opportunity for cooperativity in the T Cell Receptor

The T-cell antigen receptor (TCR) is pre-organised in oligomers, known as nanoclusters. Nanoclusters could provide a framework for inter-TCR cooperativity upon peptide antigen-major histocompatibility complex (pMHC) binding. Here we have used soluble pMHC oligomers in search for cooperativity effects along the plasma membrane plane. We find that initial binding events favour subsequent pMHC binding to additional TCRs, during a narrow temporal window. This behaviour can be explained by a 3-state model of TCR transition from Resting to Active, to a final Inhibited state. By disrupting nanoclusters and hampering the Active conformation, we show that TCR cooperativity is consistent with TCR nanoclusters adopting the Active state in a coordinated manner. Preferential binding of pMHC to the Active TCR at the immunological synapse suggests that there is a transient time frame for signal amplification in the TCR, allowing the T cells to keep track of antigen quantity and binding time.

T cells can be activated by a small, two-digit, number of antigen peptide molecules even though the receptor for antigen (TCR) is of low affinity. Here the authors present evidence that all TCRs within a nanocluster can become activated when only a subset is bound to antigen.

A Cholesterol-Based Allostery Model of T Cell Receptor Phosphorylation

Signaling through the T cell receptor (TCR) controls adaptive immune responses. Antigen binding to TCRαβ transmits signals through the plasma membrane to induce phosphorylation of the CD3 cytoplasmic tails by incompletely understood mechanisms. Here we show that cholesterol bound to the TCRβ transmembrane region keeps the TCR in a resting, inactive conformation that cannot be phosphorylated by active kinases. Only TCRs that spontaneously detached from cholesterol could switch to the active conformation (termed primed TCRs) and then be phosphorylated. Indeed, by modulating cholesterol binding genetically or enzymatically, we could switch the TCR between the resting and primed states. The active conformation was stabilized by binding to peptide-MHC, which thus controlled TCR signaling. These data are explained by a model of reciprocal allosteric regulation of TCR phosphorylation by cholesterol and ligand binding. Our results provide both a molecular mechanism and a conceptual framework for how lipid-receptor interactions regulate signal transduction.

The Allostery Model of TCR Regulation

The activity of the αβ TCR is controlled by conformational switches. In the resting conformation, the TCR is not phosphorylated and is inactive. Binding of multivalent peptide-MHC to the TCR stabilizes the active conformation, leading to TCR signaling. These two conformations allow the TCRs to be allosterically regulated. We review recent data on heterotropic allostery where peptide-MHC and membrane cholesterol serve opposing functions as positive and negative allosteric regulators, respectively. In resting T cells cholesterol keeps TCRs in the resting conformation that otherwise would become spontaneously active. This regulation is well described by the classical Monod-Wyman-Changeux model of allostery. Moreover, the observation that TCRs assemble into nanoclusters might allow for homotropic allostery, in which individual TCRs could positively cooperate and thus enhance the sensitivity of T cell activation. This new view of TCR regulation will contribute to a better understanding of TCR functioning.

Ionic CD3-Lck interaction regulates the initiation of T-cell receptor signaling

Antigen-triggered T-cell receptor (TCR) phosphorylation is the first signaling event in T cells to elicit adaptive immunity against invading pathogens and tumor cells. Despite its physiological importance, the underlying mechanism of TCR phosphorylation remains elusive. Here, we report a key mechanism regulating the initiation of TCR phosphorylation. The major TCR kinase Lck shows high selectivity on the four CD3 signaling proteins of TCR. CD3ε is the only CD3 chain that can efficiently interact with Lck, mainly through the ionic interactions between CD3ε basic residue-rich sequence (BRS) and acidic residues in the Unique domain of Lck. We applied a TCR reconstitution system to explicitly study the initiation of TCR phosphorylation. The ionic CD3ε-Lck interaction controls the phosphorylation level of the whole TCR upon antigen stimulation. CD3ε BRS is sequestered in the membrane, and antigen stimulation can unlock this motif. Dynamic opening of CD3ε BRS and its subsequent recruitment of Lck thus can serve as an important switch of the initiation of TCR phosphorylation.

Lipid-dependent conformational dynamics underlie the functional versatility of T-cell receptor

T-cell receptor-CD3 complex (TCR) is a versatile signaling machine that can initiate antigen-specific immune responses based on various biochemical changes of CD3 cytoplasmic domains, but the underlying structural basis remains elusive. Here we developed biophysical approaches to study the conformational dynamics of CD3ε cytoplasmic domain (CD3ε_CD). At the single-molecule level, we found that CD3ε_CD could have multiple conformational states with different openness of three functional motifs, i.e., ITAM, BRS and PRS. These conformations were generated because different regions of CD3ε_CD had heterogeneous lipid-binding properties and therefore had heterogeneous dynamics. Live-cell imaging experiments demonstrated that different antigen stimulations could stabilize CD3ε_CD at different conformations. Lipid-dependent conformational dynamics thus provide structural basis for the versatile signaling property of TCR.

IL-7 Induces an Epitope Masking of γc Protein in IL-7 Receptor Signaling Complex

IL-7 signaling via IL-7Rα and common γ-chain (γc) is necessary for the development and homeostasis of T cells. Although the delicate mechanism in which IL-7Rα downregulation allows the homeostasis of T cell with limited IL-7 has been well known, the exact mechanism behind the interaction between IL-7Rα and γc in the absence or presence of IL-7 remains unclear. Additionally, we are still uncertain as to how only IL-7Rα is separately downregulated by the binding of IL-7 from the IL-7Rα/γc complex. We demonstrate here that 4G3, TUGm2, and 3E12 epitope masking of γc protein are induced in the presence of IL-7, indicating that the epitope alteration is induced by IL-7 binding to the preassembled receptor core. Moreover, the epitope masking of γc protein is inversely correlated with the expression of IL-7Rα upon IL-7 binding, implying that the structural alteration of γc might be involved in the regulation of IL-7Rα expression. The conformational change in γc upon IL-7 binding may contribute not only to forming the functional IL-7 signaling complex but also to optimally regulating the expression of IL-7Rα.

Nck Binds to the T Cell Antigen Receptor Using Its SH3.1 and SH2 Domains in a Cooperative Manner, Promoting TCR Functioning

Ligand binding to the TCR causes a conformational change at the CD3 subunits to expose the CD3ε cytoplasmic proline-rich sequence (PRS). It was suggested that the PRS is important for TCR signaling and T cell activation. It has been shown that the purified, recombinant SH3.1 domain of the adaptor molecule noncatalytic region of tyrosine kinase (Nck) can bind to the exposed PRS of CD3ε, but the molecular mechanism of how full-length Nck binds to the TCR in cells has not been investigated so far. Using the in situ proximity ligation assay and copurifications, we show that the binding of Nck to the TCR requires partial phosphorylation of CD3ε, as it is based on two cooperating interactions. First, the SH3.1(Nck) domain has to bind to the nonphosphorylated and exposed PRS, that is, the first ITAM tyrosine has to be in the unphosphorylated state. Second, the SH2(Nck) domain has to bind to the second ITAM tyrosine in the phosphorylated state. Likewise, mutations of the SH3.1 and SH2 domains in Nck1 resulted in the loss of Nck1 binding to the TCR. Furthermore, expression of an SH3.1-mutated Nck impaired TCR signaling and T cell activation. Our data suggest that the exact pattern of CD3ε phosphorylation is critical for TCR functioning.

Co-potentiation of antigen recognition: A mechanism to boost weak T cell responses and provide immunotherapy in vivo

Adaptive immunity is mediated by antigen receptors that can induce weak or strong immune responses depending on the nature of the antigen that is bound. In T lymphocytes, antigen recognition triggers signal transduction by clustering T cell receptor (TCR)/CD3 multiprotein complexes. In addition, it hypothesized that biophysical changes induced in TCR/CD3 that accompany receptor engagement may contribute to signal intensity. Nonclustering monovalent TCR/CD3 engagement is functionally inert despite the fact that it may induce changes in conformational arrangement or in the flexibility of receptor subunits. We report that the intrinsically inert monovalent engagement of TCR/CD3 can specifically enhance physiologic T cell responses to weak antigens in vitro and in vivo without stimulating antigen-unengaged T cells and without interrupting T cell responses to strong antigens, an effect that we term as "co-potentiation." We identified Mono-7D6-Fab, which biophysically altered TCR/CD3 when bound and functionally enhanced immune reactivity to several weak antigens in vitro, including a gp100-derived peptide associated with melanoma. In vivo, Mono-7D6-Fab induced T cell antigen-dependent therapeutic responses against melanoma lung metastases, an effect that synergized with other anti-melanoma immunotherapies to significantly improve outcome and survival. We conclude that Mono-7D6-Fab directly co-potentiated TCR/CD3 engagement by weak antigens and that such concept can be translated into an immunotherapeutic design. The co-potentiation principle may be applicable to other receptors that could be regulated by otherwise inert compounds whose latent potency is only invoked in concert with specific physiologic ligands.

Sequence-function relationships in folding upon binding

Folding coupled to binding is ubiquitous in biology. Nevertheless, the relationship of sequence to function for protein segments that undergo coupled binding and folding remains to be determined. Specifically, it is not known if the well-established rules that govern protein folding and stability are relevant to ligand-linked folding transitions. Upon small ligand biotinoyl-5'-AMP (bio-5'-AMP) binding the Escherichia coli protein BirA undergoes a disorder-to-order transition that results in formation of a network of packed hydrophobic side chains. Ligand binding is also allosterically coupled to protein association, with bio-5'-AMP binding enhancing the dimerization free energy by -4.0 kcal/mol. Previous studies indicated that single alanine replacements in a three residue hydrophobic cluster that contributes to the larger network disrupt cluster formation, ligand binding, and allosteric activation of protein association. In this work, combined equilibrium and kinetic measurements of BirA variants with alanine substitutions in the entire hydrophobic network reveal large functional perturbations resulting from any single substitution and highly non-additive effects of multiple substitutions. These substitutions also disrupt ligand-linked folding. The combined results suggest that, analogous to protein folding, functional disorder-to-order linked to binding requires optimal packing of the relevant hydrophobic side chains that contribute to the transition. The potential for many combinations of residues to satisfy this requirement implies that, although functionally important, segments of homologous proteins that undergo folding linked to binding can exhibit sequence divergence.

Structure of the chicken CD3εδ/γ heterodimer and its assembly with the αβT cell receptor

In mammals, the αβT cell receptor (TCR) signaling complex is composed of a TCRαβ heterodimer that is noncovalently coupled to three dimeric signaling molecules, CD3εδ, CD3εγ, and CD3ζζ. The nature of the TCR signaling complex and subunit arrangement in different species remains unclear however. Here we present a structural and biochemical analysis of the more primitive ancestral form of the TCR signaling complex found in chickens. In contrast to mammals, chickens do not express separate CD3δ and CD3γ chains but instead encode a single hybrid chain, termed CD3δ/γ, that is capable of pairing with CD3ε. The NMR structure of the chicken CD3εδ/γ heterodimer revealed a unique dimer interface that results in a heterodimer with considerable deviation from the distinct side-by-side architecture found in human and murine CD3εδ and CD3εγ. The chicken CD3εδ/γ heterodimer also contains a unique molecular surface, with the vast majority of surface-exposed, nonconserved residues being clustered to a single face of the heterodimer. Using an in vitro biochemical assay, we demonstrate that CD3εδ/γ can assemble with both chicken TCRα and TCRβ via conserved polar transmembrane sites. Moreover, analogous to the human TCR signaling complex, the presence of two copies of CD3εδ/γ is required for ζζ assembly. These data provide insight into the evolution of this critical receptor signaling apparatus.

Characteristics of TCR/CD3 complex CD3{varepsilon} chains of regulatory CD4+ T (Treg) lymphocytes: role in Treg differentiation in vitro and impact on Treg in vivo

Tregs are anergic CD4(+)CD25(+)Foxp3(+) T lymphocytes exerting active suppression to control immune and autoimmune responses. However, the factors in TCR recognition underlying Treg differentiation are unclear. Based on our previous data, we hypothesized that Treg TCR/CD3 antigen receptor complexes might differ from those of CD4(+)CD25(-) Tconv. Expression levels of TCR/CD3, CD3ε,ζ chains, or other molecules involved in antigen signaling and the characteristics of CD3ε chains were analyzed in thymus or spleen Treg cells from normal mice. Tregs had quantitative and qualitatively distinct TCR/CD3 complexes and CD3ε chains. They expressed significantly lower levels of the TCR/CD3 antigen receptor, CD3ε chains, TCR-ζ chain, or the CD4 coreceptor than Tconv. Levels of kinases, adaptor molecules involved in TCR signaling, and early downstream activation pathways were also lower in Tregs than in Tconv. Furthermore, TCR/CD3 complexes in Tregs were enriched in CD3ε chains conserving their N-terminal, negatively charged amino acid residues; this trait is linked to a higher activation threshold. Transfection of mutant CD3ε chains lacking these residues inhibited the differentiation of mature CD4(+)Foxp3(-) T lymphocytes into CD4(+)Foxp3(+) Tregs, and differences in CD3ε chain recognition by antibodies could be used to enrich for Tregs in vivo. Our results show quantitative and qualitative differences in the TCR/CD3 complex, supporting the hyporesponsive phenotype of Tregs concerning TCR/CD3 signals. These differences might reconcile avidity and flexible threshold models of Treg differentiation and be used to implement therapeutic approaches involving Treg manipulation.

Trans-induced cis interaction in the tripartite NGL-1, netrin-G1 and LAR adhesion complex promotes development of excitatory synapses

The initial contact between axons and dendrites at early neuronal synapses is mediated by surface adhesion molecules and is thought to induce synaptic maturation through the recruitment of additional synaptic proteins. The initiation of synaptic maturation should be tightly regulated to ensure that synaptic maturation occurs selectively at subcellular sites of axo-dendritic adhesion. However, the underlying mechanism is poorly understood. Here, we report that the initial trans-synaptic adhesion mediated by presynaptic netrin-G1 and postsynaptic NGL-1 (netrin-G1 ligand-1) induces a cis interaction between netrin-G1 and the receptor protein tyrosine phosphatase LAR (leukocyte antigen-related), and that this promotes presynaptic differentiation. We propose that trans-synaptic adhesions at early neuronal synapses trigger recruitment of neighboring adhesion molecules in a cis manner in order to couple initial axo-dendritic adhesion with synaptic differentiation.

T cell triggering: insights from 2D kinetics analysis of molecular interactions

Interaction of the T cell receptor (TCR) with pathogen-derived peptide presented by the major histocompatibility complex (pMHC) molecule is central to adaptive immunity as it initiates intracellular signaling to trigger T cell response to infection. Kinetic parameters of this interaction have been under intensive investigation for more than two decades using soluble pMHCs and/or TCRs with at least one of them in the solution (three-dimensional (3D) methods). Recently, several techniques have been developed to enable kinetic analysis on live T cells with pMHCs presented by surrogate antigen presenting cells (APCs) or supported planar lipid bilayers (two-dimensional (2D) methods). Comparison of 2D versus 3D parameters reveals drastic differences with broader ranges of 2D affinities and on-rates and orders of magnitude faster 2D off-rates for functionally distinct pMHCs. Here we review new 2D data and discuss how it may impact previously developed models of T cell discrimination between pMHCs of different potencies.

T cell antigen recognition at the cell membrane

T cell antigen receptors (TCRs) on the surface of T cells bind specifically to particular peptide bound major histocompatibility complexes (pMHCs) presented on the surface of antigen presenting cells (APCs). This interaction is a key event in T cell antigen recognition and activation. Most studies have used surface plasmon resonance (SPR) to measure the in vitro binding kinetics of TCR-pMHC interactions in solution using purified proteins. However, these measurements are not physiologically precise, as both TCRs and pMHCs are membrane-associated molecules which are regulated by their cellular environments. Recently, single-molecule förster resonance energy transfer (FRET) and single-molecule mechanical assays were used to measure the in situ binding kinetics of TCR-pMHC interactions on the surface of live T cells. These studies have provided exciting insights into the biochemical basis of T cell antigen recognition and suggest that TCRs serially engage with a small number of antigens with very fast kinetics in order to maximize TCR signaling and sensitivity.

TCR Nanoclusters as the Framework for Transmission of Conformational Changes and Cooperativity

Increasing evidence favors the notion that, before triggering, the T cell antigen receptor (TCR) forms nanometer-scale oligomers that are called nanoclusters. The organization of the TCR in pre-existing oligomers cannot be ignored when analyzing the properties of ligand (pMHC) recognition and signal transduction. As with other membrane receptors, the existence of TCR oligomers points out to cooperativity phenomena. We review the data in support of conformational changes in the TCR as the basic principle to transduce the activation signal to the cytoplasm and the incipient data suggesting cooperativity within nanoclusters.

Basal and antigen-induced exposure of the proline-rich sequence in CD3ε

The CD3ε cytoplasmic tail contains a conserved proline-rich sequence (PRS) that influences TCR-CD3 expression and signaling. Although the PRS can bind the SH3.1 domain of the cytosolic adapter Nck, whether the PRS is constitutively available for Nck binding or instead represents a cryptic motif that is exposed via conformational change upon TCR-CD3 engagement (CD3Δc) is currently unresolved. Furthermore, the extent to which a cis-acting CD3ε basic amino acid-rich stretch (BRS), with its unique phosphoinositide-binding capability, might impact PRS accessibility is not clear. In this study, we found that freshly harvested primary thymocytes expressed low to moderate basal levels of Nck-accessible PRS ("open-CD3"), although most TCR-CD3 complexes were inaccessible to Nck ("closed-CD3"). Ag presentation in vivo induced open-CD3, accounting for half of the basal level found in thymocytes from MHC(+) mice. Additional stimulation with either anti-CD3 Abs or peptide-MHC ligands further elevated open-CD3 above basal levels, consistent with a model wherein antigenic engagement induces maximum PRS exposure. We also found that the open-CD3 conformation induced by APCs outlasted the time of ligand occupancy, marking receptors that had been engaged. Finally, CD3ε BRS-phosphoinositide interactions played no role in either adoption of the initial closed-CD3 conformation or induction of open-CD3 by Ab stimulation. Thus, a basal level of open-CD3 is succeeded by a higher, induced level upon TCR-CD3 engagement, involving CD3Δc and prolonged accessibility of the CD3ε PRS to Nck.

Rapid formation of extended processes and engagement of Theiler's virus-infected neurons by CNS-infiltrating CD8 T cells

A fundamental question in neuroimmunology is the extent to which CD8 T cells actively engage virus-infected neurons. In the Theiler's murine encephalomyelitis virus (TMEV) model of multiple sclerosis, an effective central nervous system (CNS)-infiltrating antiviral CD8 T cell response offers protection from this demyelinating disease. However, the specific CNS cell types engaged by these protective CD8 T cells in TMEV-resistant strains remains unknown. We used confocal microscopy to visualize the morphology, migration, and specific cellular interactions between adoptively transferred CD8 T cells and specific CNS cell types. Adoptively transferred GFP+ CD8+ splenocytes migrated to the brain and became 93% specific for the immunodominant virus epitope D(b):VP2(121-130). These CD8 T cells also polarized T cell receptor, CD8 protein, and granzyme B toward target neurons. Furthermore, we observed CD8 T cells forming cytoplasmic processes up to 45 μm in length. Using live tissue imaging, we determined that these T cell-extended processes (TCEPs) could be rapidly formed and were associated with migratory behavior through CNS tissues. These studies provide evidence that antiviral CD8 T cells have the capacity to engage virus-infected neurons in vivo and are the first to document and measure the rapid formation of TCEPs on these brain-infiltrating lymphocytes using live tissue imaging.

Integrin and CD3/TCR activation are regulated by the scaffold protein AKAP450

During antigen recognition by T cells, membrane receptors and cytoskeletal molecules form a specialized structure at the T cell-antigen-presenting cell junction called the immune synapse (IS). We report a role for the scaffolding protein A-kinase anchoring protein-450 (AKAP450), a member of the A-kinase anchoring protein family, in IS formation and T-cell signaling in antigen- and superantigen-dependent T-cell activation. Suppression of AKAP450 by overexpression of a dominant-negative form or siRNA knockdown disrupted the positioning and conformational activation of lymphocyte function-associated antigen 1 at the IS and impaired associated signaling events, including phosphorylation of phospholipase C-gamma1 and protein kinase C-. AKAP450 was also required for correct activation and phosphorylation of CD3, LAT, and Vav1, key T-cell receptor-activated intracellular signaling molecules. Consistently, antigen-triggered reorientation of the microtubule-organizing center at the IS and interleukin-2 secretion were diminished in AKAP450-disrupted T cells. These results indicate key roles for AKAP450 in the organization and activation of receptor molecules at the IS during T-cell signaling events.

T cell receptor triggering by force

Antigen recognition through the interaction between the T cell receptor (TCR) and peptide presented by major histocompatibility complex (pMHC) is the first step in T cell-mediated immune responses. How this interaction triggers TCR signalling that leads to T cell activation is still unclear. Taking into account the mechanical stress exerted on the pMHC-TCR interaction at the dynamic interface between T cells and antigen presenting cells (APCs), we propose the so-called receptor deformation model of TCR triggering. In this model, TCR conformational change induced by mechanical forces initiates TCR signalling. The receptor deformation model, for the first time, explains all three aspects of the TCR triggering puzzle: mechanism, specificity, and sensitivity.

The SCHOOL of nature: I. Transmembrane signaling

Receptor-mediated transmembrane signaling plays an important role in health and disease. Recent significant advances in our understanding of the molecular mechanisms linking ligand binding to receptor activation revealed previously unrecognized striking similarities in the basic structural principles of function of numerous cell surface receptors. In this work, I demonstrate that the Signaling Chain Homooligomerization (SCHOOL)-based mechanism represents a general biological mechanism of transmembrane signal transduction mediated by a variety of functionally unrelated single- and multichain activating receptors. within the SCHOOL platform, ligand binding-induced receptor clustering is translated across the membrane into protein oligomerization in cytoplasmic milieu. This platform resolves a long-standing puzzle in transmembrane signal transduction and reveals the major driving forces coupling recognition and activation functions at the level of protein-protein interactions-biochemical processes that can be influenced and controlled. The basic principles of transmembrane signaling learned from the SCHOOL model can be used in different fields of immunology, virology, molecular and cell biology and others to describe, explain and predict various phenomena and processes mediated by a variety of functionally diverse and unrelated receptors. Beyond providing novel perspectives for fundamental research, the platform opens new avenues for drug discovery and development.

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.

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.

The cytoplasmic tail of the T cell receptor CD3 epsilon subunit contains a phospholipid-binding motif that regulates T cell functions

The CD3 epsilon subunit of the TCR complex contains two defined signaling domains, a proline-rich sequence and an ITAM. We identified a third signaling sequence in CD3 epsilon, termed the basic-rich stretch (BRS). Herein, we show that the positively charged residues of the BRS enable this region of CD3 epsilon to complex a subset of acidic phospholipids, including PI(3)P, PI(4)P, PI(5)P, PI(3,4,5)P(3), and PI(4,5)P(2). Transgenic mice containing mutations of the BRS exhibited varying developmental defects, ranging from reduced thymic cellularity to a complete block in T cell development. Peripheral T cells from BRS-modified mice also exhibited several defects, including decreased TCR surface expression, reduced TCR-mediated signaling responses to agonist peptide-loaded APCs, and delayed CD3 epsilon localization to the immunological synapse. Overall, these findings demonstrate a functional role for the CD3 epsilon lipid-binding domain in T cell biology.

Macrophage oxygen sensing modulates antigen presentation and phagocytic functions involving IFN-gamma production through the HIF-1 alpha transcription factor

Low oxygen tension areas are found in inflamed or diseased tissues where hypoxic cells induce survival pathways by regulating the hypoxia-inducible transcription factor (HIF). Macrophages are essential regulators of inflammation and, therefore, we have analyzed their response to hypoxia. Murine peritoneal elicited macrophages cultured under hypoxia produced higher levels of IFN-gamma and IL-12 mRNA and protein than those cultured under normoxia. A similar IFN-gamma increment was obtained with in vivo models using macrophages from mice exposed to atmospheric hypoxia. Our studies showed that IFN-gamma induction was mediated through HIF-1alpha binding to its promoter on a new functional hypoxia response element. The requirement of HIF-alpha in the IFN-gamma induction was confirmed in RAW264.7 cells, where HIF-1alpha was knocked down, as well as in resident HIF-1alpha null macrophages. Moreover, Ag presentation capacity was enhanced in hypoxia through the up-regulation of costimulatory and Ag-presenting receptor expression. Hypoxic macrophages generated productive immune synapses with CD8 T cells that were more efficient for activation of TCR/CD3epsilon, CD3zeta and linker for activation of T cell phosphorylation, and T cell cytokine production. In addition, hypoxic macrophages bound opsonized particles with a higher efficiency, increasing their phagocytic uptake, through the up-regulated expression of phagocytic receptors. These hypoxia-increased immune responses were markedly reduced in HIF-1alpha- and in IFN-gamma-silenced macrophages, indicating a link between HIF-1alpha and IFN-gamma in the functional responses of macrophages to hypoxia. Our data underscore an important role of hypoxia in the activation of macrophage cytokine production, Ag-presenting activity, and phagocytic activity due to an HIF-1alpha-mediated increase in IFN-gamma levels.

Nck adapter proteins: functional versatility in T cells

Nck is a ubiquitously expressed adapter protein that is almost exclusively built of one SH2 domain and three SH3 domains. The two isoproteins of Nck are functionally redundant in many aspects and differ in only few amino acids that are mostly located in the linker regions between the interaction modules. Nck proteins connect receptor and non-receptor tyrosine kinases to the machinery of actin reorganisation. Thereby, Nck regulates activation-dependent processes during cell polarisation and migration and plays a crucial role in the signal transduction of a variety of receptors including for instance PDGF-, HGF-, VEGF- and Ephrin receptors. In most cases, the SH2 domain mediates binding to the phosphorylated receptor or associated phosphoproteins, while SH3 domain interactions lead to the formation of larger protein complexes. In T lymphocytes, Nck plays a pivotal role in the T cell receptor (TCR)-induced reorganisation of the actin cytoskeleton and the formation of the immunological synapse. However, in this context, two different mechanisms and adapter complexes are discussed. In the first scenario, dependent on an activation-induced conformational change in the CD3epsilon subunits, a direct binding of Nck to components of the TCR/CD3 complex was shown. In the second scenario, Nck is recruited to the TCR complex via phosphorylated Slp76, another central constituent of the membrane proximal activation complex. Over the past years, a large number of putative Nck interactors have been identified in different cellular systems that point to diverse additional functions of the adapter protein, e.g. in the control of gene expression and proliferation.

Permissive geometry model

Ligand binding to the T-cell antigen receptor (TCR) evokes receptor triggering and subsequent T-lymphocyte activation. Although TCR signal transduction pathways have been extensively studied, a satisfactory mechanism that rationalizes how the information of ligand binding to the receptor is transmitted into the cell remains elusive. Models proposed for TCR triggering can be grouped into two main conceptual categories: receptor clustering by ligand binding and induction of conformational changes within the TCR. None of these models or their variations (see Chapter 6 for details) can satisfactorily account for the diverse experimental observations regarding TCR triggering. Clustering models are not compatible with the presence of preformed oligomeric receptors on the surface of resting cells. Models based on conformational changes induced as a direct effect of ligand binding, are not consistent with the requirement for multivalent ligand to initiate TCR signaling. In this chapter, we discuss the permissive geometry model. This model integrates receptor clustering and conformational change models, together with the existence of preformed oligomeric receptors, providing a mechanism to explain TCR signal initiation.