T Cell Receptor Microclusters Research Articles

Clathrin mediates both internalization and vesicular release of triggered T cell receptor at the immunological synapse

Ligation of T cell receptor (TCR) to peptide-MHC (pMHC) complexes initiates signaling leading to T cell activation and TCR ubiquitination. Ubiquitinated TCR is then either internalized by the T cell or released toward the antigen-presenting cell (APC) in extracellular vesicles. How these distinct fates are orchestrated is unknown. Here, we show that clathrin is first recruited to TCR microclusters by HRS and STAM2 to initiate release of TCR in extracellular vesicles through clathrin- and ESCRT-mediated ectocytosis directly from the plasma membrane. Subsequently, EPN1 recruits clathrin to remaining TCR microclusters to enable trans-endocytosis of pMHC-TCR conjugates from the APC. With these results, we demonstrate how clathrin governs bidirectional membrane exchange at the immunological synapse through two topologically opposite processes coordinated by the sequential recruitment of ecto- and endocytic adaptors. This provides a scaffold for direct two-way communication between T cells and APCs.

Wiskott-Aldrich Syndrome Protein: Roles in Signal Transduction in T Cells.

Signal transduction regulates the proper function of T cells in an immune response. Upon binding to its specific ligand associated with major histocompatibility complex (MHC) molecules on an antigen presenting cell, the T cell receptor (TCR) initiates intracellular signaling that leads to extensive actin polymerization. Wiskott-Aldrich syndrome protein (WASp) is one of the actin nucleation factors that is recruited to TCR microclusters, where it is activated and regulates actin network formation. Here we highlight the research that has focused on WASp-deficient T cells from both human and mice in TCR-mediated signal transduction. We discuss the role of WASp in proximal TCR signaling as well as in the Ras/Rac-MAPK (mitogen-activated protein kinase), PKC (protein kinase C) and Ca2+-mediated signaling pathways.

Single-Molecule, Super-Resolution, and Functional Analysis of G Protein-Coupled Receptor Behavior Within the T Cell Immunological Synapse.

A central process in immunity is the activation of T cells through interaction of T cell receptors (TCRs) with agonistic peptide-major histocompatibility complexes (pMHC) on the surface of antigen presenting cells (APCs). TCR-pMHC binding triggers the formation of an extensive contact between the two cells termed the immunological synapse, which acts as a platform for integration of multiple signals determining cellular outcomes, including those from multiple co-stimulatory/inhibitory receptors. Contributors to this include a number of chemokine receptors, notably CXC-chemokine receptor 4 (CXCR4), and other members of the G protein-coupled receptor (GPCR) family. Although best characterized as mediators of ligand-dependent chemotaxis, some chemokine receptors are also recruited to the synapse and contribute to signaling in the absence of ligation. How these and other GPCRs integrate within the dynamic structure of the synapse is unknown, as is how their normally migratory Gαi-coupled signaling is terminated upon recruitment. Here, we report the spatiotemporal organization of several GPCRs, focusing on CXCR4, and the G protein Gαi2 within the synapse of primary human CD4+ T cells on supported lipid bilayers, using standard- and super-resolution fluorescence microscopy. We find that CXCR4 undergoes orchestrated phases of reorganization, culminating in recruitment to the TCR-enriched center. This appears to be dependent on CXCR4 ubiquitination, and does not involve stable interactions with TCR microclusters, as viewed at the nanoscale. Disruption of this process by mutation impairs CXCR4 contributions to cellular activation. Gαi2 undergoes active exclusion from the synapse, partitioning from centrally-accumulated CXCR4. Using a CRISPR-Cas9 knockout screen, we identify several diverse GPCRs with contributions to T cell activation, most significantly the sphingosine-1-phosphate receptor S1PR1, and the oxysterol receptor GPR183. These, and other GPCRs, undergo organization similar to CXCR4; including initial exclusion, centripetal transport, and lack of receptor-TCR interactions. These constitute the first observations of GPCR dynamics within the synapse, and give insights into how these receptors may contribute to T cell activation. The observation of broad GPCR contributions to T cell activation also opens the possibility that modulating GPCR expression in response to cell status or environment may directly regulate responsiveness to pMHC.

Polyubiquitin-dependent recruitment of NEMO/IKKγ into T cell receptor signaling microclusters is controlled by IKKβ kinase activity

Abstract The IkB kinase (IKK) complex coordinates inflammatory responses by activating canonical NFkB downstream of immune receptors. This complex consists of the kinases IKKa/β and the dimeric adaptor subunit NEMO (NFkB essential modulator protein). NEMO mutations cause immunodeficiencies and impair antigen receptor function. Canonical NFkB activation by immunoreceptors, such as the T cell receptor (TCR), requires the assembly of the Carma1/Bcl10/Malt1 (CBM) signalosome. While, functional studies and in vitro interactions, place the IKK complex downstream of the CBM signalosome, the spatial relationship between these complexes has never been observed in intact immune cells. We observed that NEMO is recruited into TCR microclusters and membrane compartments within ~70 seconds of TCR engagement. Point mutations impacting the K63-linked and linear polyubiquitin ubiquitin-binding domains of NEMO selectively impair NEMO recruitment into TCR microclusters. We verified the involvement of these polyubiquitin chains by showing that inhibitors of either the K63-specific Uev1A/Ubc13 ubiquitin E2 ligase or the linear ubiquitin activating complex (LUBAC) prevent NEMO from entering TCR microclusters. To examine whether the entire IKK complex is recruited to the TCR, we co-expressed IKKb chimeras with NEMO. We observed that wild-type (WT), but not kinase-dead, IKKb caused NEMO microclusters to disappear. Similarly, the treatment of cells expressing WT IKKb with inhibitors of IKKβ or its upstream activator, TAK1, restored the recruitment of IKKβ and NEMO into microclusters. These results suggest that the IKK complex is recruited to the TCR via linear- and K63-linked polyubiquitin chains and then dissociates from the TCR via IKKβ activity.

Lattice light-sheet microscopy Multi-Dimensional Analyses (LaMDA) of T-cell receptor dynamics predict T-cell signaling states

Abstract Cell surface receptors dynamically change in response to cell signaling, but a direct linkage between receptor dynamics and cell state has not been established. Here we report the development of lattice light-sheet microscopy multi-dimensional analyses (LaMDA), a pipeline that combines high spatiotemporal-resolution four-dimensional lattice light-sheet microscopy, machine learning, and diffusion maps to analyze T-cell receptor (TCR) dynamics and predict T-cell signaling states without the need for complex biochemical measurements. LaMDA images thousands of TCR microclusters on the surface of live primary cells to collect high-dimensional dynamic data for machine learning, which extracts key dynamic features to build predictive diffusion maps. LaMDA spatiotemporally reveals global changes of TCRs across the 3D cell surface, accurately differentiates stimulated cells from unstimulated cells, precisely predicts attenuated T-cell signaling after CD4 and CD28 receptor blockades, and reliably discriminates between structurally similar TCR ligands. We anticipate broad usage of this approach for other receptors and cells, as well as for guiding the design and development of future immunotherapies for cancer, infection, and autoimmunity.

Lattice Light-Sheet Microscopy Multi-Dimensional Analyses (LaMDA) of T-Cell Receptor Dynamics Predict T-Cell Signaling States

Cell surface receptors dynamically change in response to cell signaling, but a direct linkage between receptor dynamics and cell state has not been established. Here we report the development of lattice light-sheet microscopy multi-dimensional analyses (LaMDA), a pipeline that combines high spatiotemporal-resolution four-dimensional lattice light-sheet microscopy, machine learning, and diffusion maps to analyze T-cell receptor (TCR) dynamics and predict T-cell signaling states without the need for complex biochemical measurements. LaMDA images thousands of TCR microclusters on the surface of live primary cells to collect high-dimensional dynamic data for machine learning, which extracts key dynamic features to build predictive diffusion maps. LaMDA spatiotemporally reveals global changes of TCRs across the 3D cell surface, accurately differentiates stimulated cells from unstimulated cells, precisely predicts attenuated T-cell signaling after CD4 and CD28 receptor blockades, and reliably discriminates between structurally similar TCR ligands. We anticipate broad usage of this approach for other receptors and cells, as well as for guiding the design and development of future immunotherapies for cancer, infection, and autoimmunity.

Recruitment of NEMO/IKKγ to TCR microclusters during T cell activation

Abstract The IκB kinase (IKK) complex mediates the activation of canonical NFκB isoforms following T cell receptor (TCR) ligation. This complex consists of the kinases IKKα and IKKβ and an essential adaptor subunit, the NFκB essential modulator protein (NEMO). Most models suggest that the IKK complex is activated within oligomeric Carma1/Bcl10/Malt1 (CBM) signalosomes. However, we observed that NEMO enters TCR microclusters before CBM complexes are assembled, within ~70 seconds of TCR engagement. NEMO also entered mobile vesicles and in larger membrane-bounded structures (hereafter ‘macroclusters’). The recruitment of NEMO into TCR microclusters is prevented by Src kinase inhibitors and by the catalytic inactivation of ZAP-70, but occurs in the absence of either SLP-76 or Carma1. Further, NEMO fails to co-localize with TCR-induced CBM polymers. Thus, the recruitment of NEMO to the TCR occurs via a CBM-independent mechanism. The deletion the zinc-finger (ZnF) domain of NEMO disables NFκB signaling and eliminates NEMO from microclusters, while preserving NEMO macroclusters. Since the ZnF domain interacts with polyubiquitin chains, we generated point mutations impacting the ubiquitin-binding site in the ZnF domain and two independent sites within the NEMO ubiquitin-binding ‘NUB’ domain. These mutations impair the ability of NEMO to capture K63-linked and/or linear polymers, hinder NFκB signaling, and eliminate NEMO from microclusters without disrupting NEMO macroclusters. These findings suggest that NEMO is rapidly recruited to polyubiquitin chains associated with the TCR, rather than the CBM complex, and that the CBM complex augments IKK-dependent NFκB signaling via a distinct, recruitment-independent mechanism.

TCR microclusters form spatially segregated domains and sequentially assemble in calcium-dependent kinetic steps

Engagement of the T cell receptor (TCR) by stimulatory ligand results in the rapid formation of microclusters at sites of T cell activation. Whereas microclusters have been studied extensively using confocal microscopy, the spatial and kinetic relationships of their signaling components have not been well characterized due to limits in image resolution and acquisition speed. Here we show, using TIRF-SIM to examine the organization of microclusters at sub-diffraction resolution, the presence of two spatially distinct domains composed of ZAP70-bound TCR and LAT-associated signaling complex. Kinetic analysis of microcluster assembly reveal surprising delays between the stepwise recruitment of ZAP70 and signaling proteins to the TCR, as well as distinct patterns in their disassociation. These delays are regulated by intracellular calcium flux downstream of T cell activation. Our results reveal novel insights into the spatial and kinetic regulation of TCR microcluster formation and T cell activation.

The Actin Cytoskeleton: A Mechanical Intermediate for Signal Integration at the Immunological Synapse.

The immunological synapse (IS) is a specialized structure that serves as a platform for cell-cell communication between a T cell and an antigen-presenting cell (APC). Engagement of the T cell receptor (TCR) with cognate peptide-MHC complexes on the APC activates the T cell and instructs its differentiation. Proper T cell activation also requires engagement of additional receptor-ligand pairs, which promote sustained adhesion and deliver costimulatory signals. These events are orchestrated by T cell actin dynamics, which organize IS components and facilitate their signaling. The actin network flows from the edge of the cell inward, driving the centralization of TCR microclusters and providing the force to activate the integrin LFA-1. We recently showed that engagement of LFA-1 slows actin flow, and that this affects TCR signaling. This study highlights the physical nature of the IS, and contributes to a growing appreciation in the field that mechanosensing and mechanotransduction are essential for IS function. Additionally, it is becoming clear that there are multiple types of actin structures at the IS that promote signaling in distinct ways. How the different actin structures contribute to force production and mechanotransduction is just beginning to be explored. In this Perspective, we will feature recent work from our lab and others, that collectively points toward a model in which actin dynamics drive mechanical signaling and receptor crosstalk during T cell activation.

T Cells on Engineered Substrates: The Impact of TCR Clustering Is Enhanced by LFA-1 Engagement.

We created APC-mimetic synthetic substrates to study the impact of ligand clustering on T cell activation and spreading. The substrates exhibit antibodies directed against the TCR-complex in the form of a patterned array of sub micrometric dots surrounded by a fluid supported lipid bilayer (SLB) which may itself be functionalized with another bio-molecule. We show that for T cell adhesion mediated by T cell receptor (TCR) alone, in the patterned, but not in the corresponding homogeneous controls, the TCR, ZAP-70 and actin are present in the form of clusters or patches that co-localize with the ligand-dots. However, global cell scale parameters like cell area and actin distribution are only weakly impacted by ligand clustering. In presence of ICAM-1 - the ligand of the T cell integrin LFA-1 - on the SLB, the TCR is still clustered due to the patterning of its ligands, but now global parameters are also impacted. The actin organization changes to a peripheral ring, resembling the classical actin distribution seen on homogeneous substrates, the patterned membrane topography disappears and the membrane is flat, whereas the cell area increases significantly. These observations taken together point to a possible pivotal role for LFA-1 in amplifying the effect of TCR-clustering. No such effect is evident for co-engagement of CD28, affected via its ligand B7.2. Unlike on ICAM-1, on B7.2 cell spreading and actin organization are similar for homogeneous and patterned substrates. However, TCR and ZAP-70 clusters are still formed in the patterned case. These results indicate complementary role for LFA-1 and CD28 in the regulation and putative coupling of TCR micro-clusters to actin. The engineered substrates presented here clearly have the potential to act as platform for fundamental research in immune cell biology, as well as translational analyses in immunotherapy, for example to screen molecules for their role in T cell adhesion/activation.

Abstract 5699: A new method for evaluating T cell response after stimulation on gastrointestinal cancer patients

Abstract Background: In anti-tumor immunity, T cells play a central role to eliminate cancer cells. Immune responses of T cells are initiated by communication with antigen presenting cells (APCs). It became clear that an immunological synapse (IS) is formed at the T cell-APC interface and is made up of a supramolecular activation cluster (SMAC) containing the T cell receptors (TCR) microclusters (MCs) and CD28 MCs. TCR MCs consist of TCR, kinases, and adaptor molecules, and are the minimal unit of T cell signal transduction. CD28 MCs also function as the signalosomes of CD28 activation. By using total internal reflection fluorescence microscopy to observe the interaction of T cell and planar bilayer, it was elucidated that TCR activation is regulated by the amount of MCs aggregation, which shows the extent of IS formation. Based on this finding, we hypothesized that ex vivo T cells could be examined IS formation and that exhausted T cells of advanced cancer patients should be low in response to a stimulus. Method: To simplify IS formation of T cells, we used human monoclonal T cells by αCD3 and/or αCD28 antibody and replaced to evaluate the aggregation of MCs by detecting the CD3 or CD28 molecules on the surface of T cells. This phenomenon was defined as IS-like formation. In this method, IS-like formation was detected by confirming an image of each cell with imaging cytometry. After these conditions was established, it was verified whether it is possible to measure IS-like formation rate of T cells. Additionally, the correlation between IS-like formation and interferon-γ cytokine production after this activation was examined. In the same manner as above, IS - like formation rates of T cells derived from PBMCs of healthy donors or esophageal and gastric cancer patients were measured. Result: We have succeeded in observing that CD3 or CD28 molecules on a T cell aggregated in one area after stimulation as IS-like formation by imaging cytometry. In dot plot study, positive area of CD3 or CD28 decreased after the stimulation, which showed the aggregation of these molecules. Accordingly, we could measure IS-like formation rate of a population of T cell clones. In addition, the rate of IS-like formation showed a positive correlation with interferon-γ production. Similar results were obtained T cell from PBMC in healthy donors. Moreover, IS-like formation rate of T cell from cancer patients decreased compared with healthy donors. Conclusion: IS-like formation of T cell succeeded to observe by CD3 and/or CD28 antibody stimulation. We established the evaluation method of IS-like formation degree by imaging cytometry. Moreover, IS-like formation rate correlated with cytokine production of IFN-g. We established new method to evaluate the strength of T cell after activation. Citation Format: Tomoko Tanaka, Kimihiro Yamashita, Yutaka Sugita, Eiji Fukuoka, Akira Arimoto, Yoshihiro Kakeji. A new method for evaluating T cell response after stimulation on gastrointestinal cancer patients [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 5699.

High-Affinity Ligands Can Trigger T Cell Receptor Signaling Without CD45 Segregation.

How T cell receptors (TCRs) are triggered to start signaling is still not fully understood. It has been proposed that segregation of the large membrane tyrosine phosphatase CD45 from engaged TCRs initiates signaling by favoring phosphorylation of immunoreceptor tyrosine-based activation motifs (ITAMs) in the cytoplasmic domains of CD3 molecules. However, whether CD45 segregation is important to initiate triggering is still uncertain. We examined CD45 segregation from TCRs engaged to anti-CD3 scFv with high or low affinity and with defined molecular lengths on glass-supported lipid bilayers using total internal reflection microscopy. Both short and elongated high-affinity anti-CD3 scFv effectively induced similar calcium mobilization, Zap70 phosphorylation, and cytokine secretion in Jurkat T cells but CD45 segregated from activated TCR microclusters significantly less for elongated versus short anti-CD3 ligands. In addition, at early times, triggering cells with both high and low affinity elongated anti-CD3 scFv resulted in similar degrees of CD3 co-localization with CD45, but only the high-affinity scFv induced T cell activation. The lack of correlation between CD45 segregation and early markers of T cell activation suggests that segregation of CD45 from engaged TCRs is not mandatory for initial triggering of TCR signaling by elongated high-affinity ligands.

Visualizing dynamic microvillar search and stabilization during ligand detection by T cells.

During immune surveillance, T cells survey the surface of antigen-presenting cells. In searching for peptide-loaded major histocompatibility complexes (pMHCs), they must solve a classic trade-off between speed and sensitivity. It has long been supposed that microvilli on T cells act as sensory organs to enable search, but their strategy has been unknown. We used lattice light-sheet and quantum dot-enabled synaptic contact mapping microscopy to show that anomalous diffusion and fractal organization of microvilli survey the majority of opposing surfaces within 1 minute. Individual dwell times were long enough to discriminate pMHC half-lives and T cell receptor (TCR) accumulation selectively stabilized microvilli. Stabilization was independent of tyrosine kinase signaling and the actin cytoskeleton, suggesting selection for avid TCR microclusters. This work defines the efficient cellular search process against which ligand detection takes place.

Actin polymerization-dependent activation of Cas-L promotes immunological synapse stability.

The immunological synapse formed between a T‐cell and an antigen‐presenting cell is important for cell–cell communication during T‐cell‐mediated immune responses. Immunological synapse formation begins with stimulation of the T‐cell receptor (TCR). TCR microclusters are assembled and transported to the center of the immunological synapse in an actin polymerization‐dependent process. However, the physical link between TCR and actin remains elusive. Here we show that lymphocyte‐specific Crk‐associated substrate (Cas‐L), a member of a force sensing protein family, is required for transport of TCR microclusters and for establishing synapse stability. We found that Cas‐L is phosphorylated at TCR microclusters in an actin polymerization‐dependent fashion. Furthermore, Cas‐L participates in a positive feedback loop leading to amplification of Ca2+ signaling, inside–out integrin activation, and actomyosin contraction. We propose a new role for Cas‐L in T‐cell activation as a mechanical transducer linking TCR microclusters to the underlying actin network and coordinating multiple actin‐dependent structures in the immunological synapse. Our studies highlight the importance of mechanotransduction processes in T‐cell‐mediated immune responses.

Dynamic Regulation of TCR-Microclusters and the Microsynapse for T Cell Activation.

The interaction between a T cell and an antigen-presenting cell is the initiating event in T cell-mediated adaptive immunity. The Immunological Synapse (IS) is formed at the interface between these two cell types, and is the site where antigen (Ag)-specific recognition and activation are induced through the T cell receptor (TCR). This occurs at the center of the IS, and cell adhesion is supported through integrins in the area surrounding the TCR. Recently, this model has been revised based on data indicating that the initial Ag-specific activation signal is triggered prior to IS formation at TCR–microclusters (MCs), sites where TCR, kinases and adaptors of TCR proximal downstream signaling molecules accumulate as an activation signaling cluster. TCR–MCs then move into the center of the cell–cell interface to generate the cSMAC. This translocation of TCR–MCs is mediated initially by the actin cytoskeleton and then by dynein-induced movement along microtubules. The translocation of TCR–MCs and cSMAC formation is induced upon strong TCR stimulation through the assembly of a TCR–dynein super complex with microtubules. The Ag-specific activation signal is induced at TCR–MCs, but the adhesion signal is now shown to be induced by generating a “microsynapse,” which is composed of a core of TCR–MCs and the surrounding adhesion ring of integrin and focal adhesion molecules. Since the microsynapse is critical for activation, particularly under weak TCR stimulation, this structure supports a weak TCR signal through a cell–cell adhesion signal. The microsynapse has a structure similar to the IS but on a micro-scale and regulates Ag-specific activation as well as cell–cell adhesion. We describe here the dynamic regulation of TCR–MCs, responsible for inducing Ag-specific activation signals, and the microsynapse, responsible for adhesion signals critical for cell–cell interactions, and their interrelationship.

Micro-adhesion rings surrounding TCR microclusters are essential for T cell activation.

The immunological synapse (IS) formed at the interface between T cells and antigen-presenting cells represents a hallmark of initiation of acquired immunity. T cell activation is initiated at T cell receptor (TCR) microclusters (MCs), in which TCRs and signaling molecules assemble at the interface before IS formation. We found that each TCR-MC was transiently bordered by a ring structure made of integrin and focal adhesion molecules in the early phase of activation, which is similar in structure to the IS in microscale. The micro-adhesion ring is composed of LFA-1, focal adhesion molecules paxillin and Pyk2, and myosin II (MyoII) and is supported by F-actin core and MyoII activity through LFA-1 outside-in signals. The formation of the micro-adhesion ring was transient but especially sustained upon weak TCR stimulation to recruit linker for activation of T cells (LAT) and SLP76. Perturbation of the micro-adhesion ring induced impairment of TCR-MC development and resulted in impaired cellular signaling and cell functions. Thus, the synapse-like structure composed of the core TCR-MC and surrounding micro-adhesion ring is a critical structure for initial T cell activation through integrin outside-in signals.

Dynamic Behavior of TCR Microclusters on a Live Cell Investigated by a Gold Nanoparticle Array

T cell receptor (TCR) microclusters play an important role as they are the sites of regulation of early T cell signaling. TCR microclusters are, however, difficult to study because their size is too small for conventional fluorescence microscopy and too large for physicochemical methods that depend on proximity effect such as FRET. Despite the recent advancement in superresolution microscopies, their use is still limited to fixed cells or slow processes in the cell. We developed a new platform for probing and manipulating TCR microclusters on a live cell by mechanical means.

Nanometric Protein-Patch Arrays on Glass and Polydimethylsiloxane for Cell Adhesion Studies

We present a simple cost-effective benchtop protocol to functionalize glass and polydimethylsiloxane (PDMS) with nanometric protein patches for cell adhesion studies. Evaporation masks, covering macroscopic areas on glass, were made using improved strategies for self-assembly of colloidal microbeads which then served as templates for creating the protein patch arrays via the intermediate steps of organo-aminosilane deposition and polyethylene-glycol grafting. The diameter of the patches could be varied down to about 80 nm. The glass substrates were used for advanced optical imaging of T-lymphocytes to explore adhesion by reflection interference contrast microscopy and the possible colocalization of T-cell receptor microclusters and the activating protein patches by total internal reflection fluorescence microscopy. The selectively functionalized glass could also serve as template for transferring the protein nanopatches to the surface of a soft elastomer. We demonstrated successful reverse contact printing onto the surface of thin layers of PDMS with stiffness ranging from 30 KPa to 3 MPa.

Investigating the Dynamic Behavior of TCR Microclusters by a Gold Nanoparticle Array

T cell receptor (TCR) microclusters play an important role in early signaling in T cells, and their properties and dynamic behavior are the active target of research. TCR microclusters are, however, difficult to study because their size is too small for conventional fluorescence microscopy and too large for physicochemical methods that depend on proximity effect such as FRET. We developed a new platform for probing and manipulating TCR microclusters by mechanical means. A two-dimensional array of gold nanoparticles is embedded on the glass substrate. A supported membrane, which acts as a surrogate cell membrane that has laterally fluid ligand molecules for cells, was deposited on a glass substrate with a regular array of gold nanoparticles. The supported membrane interacted with T cells and the behavior of TCR microclusters on the T cell surface was effectively altered by the gold nanoparticles. This platform offers many opportunities for studying this poorly understood size regime of protein assemblies.View Large Image | View Hi-Res Image | Download PowerPoint Slide

Revisiting the putative TCR Cα dimerization model through structural analysis.

Despite major advances in T cell receptor (TCR) biology and structure, how peptide–MHC complex (pMHC) ligands trigger αβ TCR activation remains unresolved. Two views exist. One model postulates that monomeric TCR–pMHC ligation events are sufficient while a second proposes that TCR–TCR dimerization in cis via Cα domain interaction plus pMHC binding is critical. We scrutinized 22 known TCR/pMHC complex crystal structures, and did not find any predicted molecular Cα–Cα contacts in these crystals that would allow for physiological TCR dimerization. Moreover, the presence of conserved glycan adducts on the outer face of the Cα domain preclude the hypothesized TCR dimerization through the Cα domain. Observed functional consequences of Cα mutations are likely indirect, with TCR microclusters at the immunological synapse driven by TCR transmembrane/cytoplasmic interactions via signaling molecules, scaffold proteins, and/or cytoskeletal elements.