TCR Microclusters pre-exist and contain molecules necessary for TCR signal transduction

T Cell Microvilli: Finger-Shaped External Structures Linked to the Fate of T Cells

Microvilli are outer membrane organelles that contain cross-linked filamentous actin. Unlike well-characterized epithelial microvilli, T-cell microvilli are dynamic similar to those of filopodia, which grow and shrink intermittently via the alternate actin-assembly and -disassembly. T-cell microvilli are specialized for sensing Ags on the surface of Ag-presenting cells (APCs). Thus, these finger-shaped microprotrusions contain many signaling-related proteins and can serve as a signaling platforms that induce intracellular signals. However, they are not limited to sensing external information but can provide sites for parts of the cell-body to tear away from the cell. Cells are known to produce many types of extracellular vesicles (EVs), such as exosomes, microvesicles, and membrane particles. T cells also produce EVs, but little is known about under what conditions T cells generate EVs and which types of EVs are released. We discovered that T cells produce few exosomes but release large amounsts of microvilli-derived particles during physical interaction with APCs. Although much is unanswered as to why T cells use the same organelles to sense Ags or to produce EVs, these events can significantly affect T cell fate, including clonal expansion and death. Since TCRs are localized at microvilli tips, this membrane event also raises a new question regarding long-standing paradigm in T cell biology; i.e., surface TCR downmodulation following T cell activation. Since T-cell microvilli particles carry T-cell message to their cognate partner, these particles are termed T-cell immunological synaptosomes (TISs). We discuss the potential physiological role of TISs and their application to immunotherapies.

T Cell Immunological Synaptosomes: Definition and Isolation

In addition to microvilli's role as structural scaffold for TCR clustering, we recently discovered a novel function as message senders. We found that microvilli are separated from the T cell body shortly upon TCR stimulation and vesiculated to form T cell microvilli particles (TMPs), a new type of membrane vesicles. TMPs and synaptic ectosomes, which bud from the synaptic cleft, constitute "T cell immunological synaptosomes (TISs)" and act as conveyors of T cell messages or traits to cognate antigen-presenting cells. In practice, it is almost impossible to distinguish between TMPs and synaptic ectosomes. Here, we describe a newly developed protocol to isolate TISs from activated T cells using antibody-immobilized agarose beads and density gradient ultracentrifugation. We further describe the methods for TIS quantification with flow cytometry and to evaluate TIS efficacy on dendritic cells.

Discrete LAT condensates encode antigen information from single pMHC:TCR binding events

LAT assembly into a two-dimensional protein condensate is a prominent feature of antigen discrimination by T cells. Here, we use single-molecule imaging techniques to resolve the spatial position and temporal duration of each pMHC:TCR molecular binding event while simultaneously monitoring LAT condensation at the membrane. An individual binding event is sufficient to trigger a LAT condensate, which is self-limiting, and neither its size nor lifetime is correlated with the duration of the originating pMHC:TCR binding event. Only the probability of the LAT condensate forming is related to the pMHC:TCR binding dwell time. LAT condenses abruptly, but after an extended delay from the originating binding event. A LAT mutation that facilitates phosphorylation at the PLC-γ1 recruitment site shortens the delay time to LAT condensation and alters T cell antigen specificity. These results identify a function for the LAT protein condensation phase transition in setting antigen discrimination thresholds in T cells.

Tetraspanin-5-mediated MHC class I clustering is required for optimal CD8 T cell activation

MHC molecules are not randomly distributed on the plasma membrane but instead are present in discrete nanoclusters. The mechanisms that control formation of MHC I nanoclusters and the importance of such structures are incompletely understood. Here, we report a molecular association between tetraspanin-5 (Tspan5) and MHC I molecules that started in the endoplasmic reticulum and was maintained on the plasma membrane. This association was observed both in mouse dendritic cells and in human cancer cell lines. Loss of Tspan5 reduced the size of MHC I clusters without affecting MHC I peptide loading, delivery of complexes to the plasma membrane, or overall surface MHC I levels. Functionally, CD8 T cell responses to antigen presented by Tspan5-deficient dendritic cells were impaired but were restored by antibody-induced reclustering of MHC I molecules. In contrast, Tspan5 did not associate with two other plasma membrane proteins, Flotillin1 and CD55, with or the endoplasmic reticulum proteins Tapasin and TAP. Thus, our findings identify a mechanism underlying the clustering of MHC I molecules that is important for optimal T cell responses.

The interplay between membrane topology and mechanical forces in regulating T cell receptor activity

T cells are critically important for host defense against infections. T cell activation is specific because signal initiation requires T cell receptor (TCR) recognition of foreign antigen peptides presented by major histocompatibility complexes (pMHC) on antigen presenting cells (APCs). Recent advances reveal that the TCR acts as a mechanoreceptor, but it remains unclear how pMHC/TCR engagement generates mechanical forces that are converted to intracellular signals. Here we propose a TCR Bending Mechanosignal (TBM) model, in which local bending of the T cell membrane on the nanometer scale allows sustained contact of relatively small pMHC/TCR complexes interspersed among large surface receptors and adhesion molecules on the opposing surfaces of T cells and APCs. Localized T cell membrane bending is suggested to increase accessibility of TCR signaling domains to phosphorylation, facilitate selective recognition of agonists that form catch bonds, and reduce noise signals associated with slip bonds.

Al-Aghbar et al propose a TCR bending mechanosignal model that demonstrates how local mechanical membrane bending may influence T cell receptor binding events and thus T-cell activation.

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

Lattice light-sheet microscopy provides large amounts of high-dimensional, high-spatiotemporal resolution imaging data of cell surface receptors across the 3D surface of live cells, but user-friendly analysis pipelines are lacking. Here, we introduce lattice light-sheet microscopy multi-dimensional analyses (LaMDA), an end-to-end pipeline comprised of publicly available software packages that combines machine learning, dimensionality reduction, and diffusion maps to analyze surface receptor dynamics and classify cellular signaling states without the need for complex biochemical measurements or other prior information. We use LaMDA to analyze images of T-cell receptor (TCR) microclusters on the surface of live primary T cells under resting and stimulated conditions. We observe global spatial and temporal changes of TCRs across the 3D cell surface, accurately differentiate stimulated cells from unstimulated cells, precisely predict attenuated T-cell signaling after CD4 and CD28 receptor blockades, and reliably discriminate between structurally similar TCR ligands. All instructions needed to implement LaMDA are included in this paper.

Microclusters as T Cell Signaling Hubs: Structure, Kinetics, and Regulation

When T cell receptors (TCRs) engage with stimulatory ligands, one of the first microscopically visible events is the formation of microclusters at the site of T cell activation. Since the discovery of these structures almost 20 years ago, they have been studied extensively in live cells using confocal and total internal reflection fluorescence (TIRF) microscopy. However, due to limits in image resolution and acquisition speed, the spatial relationships of signaling components within microclusters, the kinetics of their assembly and disassembly, and the role of vesicular trafficking in microcluster formation and maintenance were not finely characterized. In this review, we will summarize how new microscopy techniques have revealed novel insights into the assembly of these structures. The sub-diffraction organization of microclusters as well as the finely dissected kinetics of recruitment and disassociation of molecules from microclusters will be discussed. The role of cell surface molecules in microcluster formation and the kinetics of molecular recruitment via intracellular vesicular trafficking to microclusters is described. Finally, the role of post-translational modifications such as ubiquitination in the downregulation of cell surface signaling molecules is also discussed. These results will be related to the role of these structures and processes in T cell activation.

Multiple actin networks coordinate mechanotransduction at the immunological synapse

Activation of naive T cells by antigen-presenting cells (APCs) is an essential step in mounting an adaptive immune response. It is known that antigen recognition and T cell receptor (TCR) signaling depend on forces applied by the T cell actin cytoskeleton, but until recently, the underlying mechanisms have been poorly defined. Here, we review recent advances in the field, which show that specific actin-dependent structures contribute to the process in distinct ways. In essence, T cell priming involves a tug-of-war between the cytoskeletons of the T cell and the APC, where the actin cytoskeleton serves as a mechanical intermediate that integrates force-dependent signals. We consider each of the relevant actin-rich T cell structures separately and address how they work together at the topologically and temporally complex cell-cell interface. In addition, we address how this mechanobiology can be incorporated into canonical immunological models to improve how these models explain T cell sensitivity and antigenic specificity.

Membrane Organization and Physical Regulation of Lymphocyte Antigen Receptors: A Biophysicist's Perspective

Receptors at the membrane of immune cells are the central players of innate and adaptative immunity, providing effective defence mechanisms against pathogens or cancer cells. Their function is intimately linked to their position at and within the membrane which provides accessibility, mobility as well as membrane proximal cytoskeleton anchoring, all of these elements playing important roles in the final function and links to cellular actions. Understanding how immune cells integrate the specific signals received at their membrane to take a decision remains an immense challenge and a very active field of fundamental and applied research. Recent progress in imaging and micromanipulation techniques have led to an unprecedented refinement in the description of molecular structures and supramolecular assemblies at the immune cell membrane, and provided a glimpse into their dynamics and regulation by force. Several key elements have been scrutinized such as the roles of relative sizes of molecules, lateral organisation, motion in the membrane of the receptors, but also physical cues such as forces, mediated by cellular substrates of different rigidities or applied by the cell itself, in conjunction with its partner cell. We review here these recent discoveries associated with a description of the biophysical methods used. While a conclusive picture integrating all of these components is still lacking, mainly due to the implication of diverse and different mechanisms and spatio-temporal scales involved, the amount of quantitative data available opens the way for physical modelling and numerical simulations and new avenues for experimental research.

Orchestration of Immunological Synapse Assembly by Vesicular Trafficking

Ligation of the T-cell antigen receptor (TCR) by cognate peptide bound to the Major Histocompatibility Complex on the surface of an antigen-presenting cell (APC) leads to the spatial reorganization of the TCR and accessory receptors to form a specialized area of intimate contact between T cell and APC, known as the immunological synapse (IS), where signals are deciphered, coordinated, and integrated to promote T cell activation. With the discovery that an endosomal TCR pool contributes to IS assembly and function by undergoing polarized recycling to the IS, recent years have witnessed a shift from a plasma membrane-centric view of the IS to the vesicular trafficking events that occur at this location following the TCR-dependent translocation of the centrosome toward the synaptic membrane. Here we will summarize our current understanding of the trafficking pathways that are responsible for the steady delivery of endosomal TCRs, kinases, and adapters to the IS to sustain signaling, as well as of the endocytic pathways responsible for signal termination. We will also discuss recent evidence highlighting a role for endosomes in sustaining TCR signaling after its internalization at the IS and identifying the IS as a site of formation and release of extracellular vesicles that allow for transcellular communication with the APC.

Dual Role of CD4 in Peripheral T Lymphocytes

The interaction of T-cell receptors (TCRs) with self- and non-self-peptides in the major histocompatibility complex (MHC) stimulates crucial signaling events, which in turn can activate T lymphocytes. A variety of accessory molecules further modulate T-cell signaling. Of these, the CD4 and CD8 coreceptors make the most critical contributions to T cell sensitivity in vivo. Whereas, CD4 function in T cell development is well-characterized, its role in peripheral T cells remains incompletely understood. It was originally suggested that CD4 stabilizes weak interactions between TCRs and peptides in the MHC and delivers Lck kinases to that complex. The results of numerous experiments support the latter role, indicating that the CD4-Lck complex accelerates TCR-triggered signaling and controls the availability of the kinase for TCR in the absence of the ligand. On the other hand, extremely low affinity of CD4 for MHC rules out its ability to stabilize the receptor-ligand complex. In this review, we summarize the current knowledge on CD4 in T cells, with a special emphasis on the spatio-temporal organization of early signaling events and the relevance for CD4 function. We further highlight the capacity of CD4 to interact with the MHC in the absence of TCR. It drives the adhesion of T cells to the cells that express the MHC. This process is facilitated by the CD4 accumulation in the tips of microvilli on the surface of unstimulated T cells. Based on these observations, we suggest an alternative model of CD4 role in T-cell activation.

NK cells specifically TCR-dressed to kill cancer cells

Background

Adoptive T-cell transfer of therapeutic TCR holds great promise to specifically kill cancer cells, but relies on modifying the patient's own T cells ex vivo before injection. The manufacturing of T cells in a tailor-made setting is a long and expensive process which could be resolved by the use of universal cells. Currently, only the Natural Killer (NK) cell line NK-92 is FDA approved for universal use. In order to expand their recognition ability, they were equipped with Chimeric Antigen Receptors (CARs). However, unlike CARs, T-cell receptors (TCRs) can recognize all cellular proteins, which expand NK-92 recognition to the whole proteome.

Methods

We herein genetically engineered NK-92 to express the CD3 signaling complex, and showed that it rendered them able to express a functional TCR. Functional assays and in vivo efficacy were used to validate these cells.

Findings

This is the first demonstration that a non-T cell can exploit TCRs. This TCR-redirected cell line, termed TCR-NK-92, mimicked primary T cells phenotypically, metabolically and functionally, but retained its NK cell effector functions. Our results demonstrate a unique manner to indefinitely produce TCR-redirected lymphocytes at lower cost and with similar therapeutic efficacy as redirected T cells.

Interpretation

These results suggest that an NK cell line could be the basis for an off-the-shelf TCR-based cancer immunotherapy solution.

Fund

This work was supported by the Research Council of Norway (#254817), South-Eastern Norway Regional Health Authority (#14/00500-79), by OUS-Radiumhospitalet (Gene Therapy program) and the department of Oncology at the University of Lausanne.

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.

Cutting Edge: Quantitative Determination of CD40L Threshold for IL-12 and IL-23 Production from Dendritic Cells

Early secretion of IL-12 by mouse dendritic cells (DCs) instructs T cells to make IFN-γ. However, only activated, but not naive T cells are able to license DCs for IL-12 production. We hypothesized that it might be due to different levels of CD40L expression on the surface of these cells, as CD40 signals are required for IL-12 production. Using quantitative cell-free systems incorporating CD40L in lipid bilayers combined with total internal reflection fluorescence microscopy and flow cytometry, we show that as low as ∼200 CD40L molecules/μm2 in combination with IL-4 is sufficient to induce IL-12 production by DCs. Remarkably, CD40L alone is adequate to induce IL-23 secretion by DCs. Thus, although activated T cells have somewhat higher levels of CD40L, it is the combination of CD40L and the cytokines they secrete that licenses DCs and influences the effector class of the immune response.

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.

Growth of B Cell Receptor Microclusters Is Regulated by PIP2 and PIP3 Equilibrium and Dock2 Recruitment and Activation

The growth of B cell receptor (BCR) microclusters upon antigen stimulation drives B cell activation. Here, we show that PI3K-mediated PIP₃ production is required for the growth of BCR microclusters. This growth is likely inhibited by PTEN and dependent on its plasma membrane binding and lipid phosphatase activities. Mechanistically, we find that PIP₃-dependent recruitment and activation of a guanine nucleotide exchange factor, Dock2, is required for the sustained growth of BCR microclusters through remodeling of the F-actin cytoskeleton. As a consequence, Dock2 deficiency significantly disrupts the structure of the B cell immunological synapse. Finally, we find that primary B cells from systemic lupus erythematosus (SLE) patients exhibit more prominent BCR and PI3K microclusters than B cells from healthy controls. These results demonstrate the importance of a PI3K- and PTEN-governed PIP₂ and PIP₃ equilibrium in regulating the activation of B cells through Dock2-controlled growth of BCR microclusters.

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.

A hierarchy of affinities between cytokine receptors and the common gamma chain leads to pathway cross-talk

Cytokines belonging to the common gamma chain (γ_c) family depend on the shared γ_c receptor subunit for signaling. We report the existence of a fast, cytokine-induced pathway cross-talk acting at the receptor level, resulting from a limiting amount of γ_c on the surface of T cells. We found that this limited abundance of γ_c reduced interleukin-4 (IL-4) and IL-21 responses after IL-7 preexposure but not vice versa. Computational modeling combined with quantitative experimental assays indicated that the asymmetric cross-talk resulted from the ability of the "private" IL-7 receptor subunits (IL-7Rα) to bind to many of the γ_c molecules even before stimulation with cytokine. Upon exposure of T cells to IL-7, the high affinity of the IL-7Rα:IL-7 complex for γ_c further reduced the amount of free γ_c in a manner dependent on the concentration of IL-7. Measurements of bioluminescence resonance energy transfer (BRET) between IL-4Rα and γ_c were reduced when IL-7Rα was overexpressed. Furthermore, in a system expressing IL-7Rα, IL-4Rα, and γ_c, BRET between IL-4Rα and γ_c increased after IL-4 binding and decreased when cells were preexposed to IL-7, supporting the assumption that IL-7Rα and the IL-7Rα:IL-7 complex limit the accessibility of γ_c for other cytokine receptor complexes. We propose that in complex inflammatory environments, such asymmetric cross-talk establishes a hierarchy of cytokine responsiveness.

The Affinity of Elongated Membrane-Tethered Ligands Determines Potency of T Cell Receptor Triggering

T lymphocytes are important mediators of adoptive immunity but the mechanism of T cell receptor (TCR) triggering remains uncertain. The interspatial distance between engaged T cells and antigen-presenting cells (APCs) is believed to be important for topological rearrangement of membrane tyrosine phosphatases and initiation of TCR signaling. We investigated the relationship between ligand topology and affinity by generating a series of artificial APCs that express membrane-tethered anti-CD3 scFv with different affinities (OKT3, BC3, and 2C11) in addition to recombinant class I and II pMHC molecules. The dimensions of membrane-tethered anti-CD3 and pMHC molecules were progressively increased by insertion of different extracellular domains. In agreement with previous studies, elongation of pMHC molecules or low-affinity anti-CD3 scFv caused progressive loss of T cell activation. However, elongation of high-affinity ligands (BC3 and OKT3 scFv) did not abolish TCR phosphorylation and T cell activation. Mutation of key amino acids in OKT3 to reduce binding affinity to CD3 resulted in restoration of topological dependence on T cell activation. Our results show that high-affinity TCR ligands can effectively induce TCR triggering even at large interspatial distances between T cells and APCs.

Ca2+ Microdomains in T-Lymphocytes

Early Ca²⁺ signaling is characterized by occurrence of Ca²⁺ microdomains formed by opening of single or clusters of Ca²⁺ channels, thereby initiating first signaling and subsequently activating global Ca²⁺ signaling mechanisms. However, only few data are available focusing on the first seconds and minutes of Ca²⁺ microdomain formation and related signaling pathways in activated T-lymphocytes. In this review, we condense current knowledge on Ca²⁺ microdomain formation in T-lymphocytes and early Ca²⁺ signaling, function of Ca²⁺ microdomains, and microdomain organization. Interestingly, considering the first seconds of T cell activation, a triphasic Ca²⁺ signal is becoming apparent: (i) initial Ca²⁺ microdomains occurring in the first second of T cell activation, (ii) amplification of Ca²⁺ microdomains by recruitment of further channels in the next 5-10 s, and (iii) a transition to global Ca²⁺ increase. Apparently, the second messenger nicotinic acid adenine dinucleotide phosphate is the first second messenger involved in initiation of Ca²⁺ microdomains. Ryanodine receptors type 1 act as initial Ca²⁺ release channels in CD4⁺ T-lymphocytes. Regarding the temporal correlation of Ca²⁺ microdomains with other molecular events of T cell activation, T cell receptor-dependent microdomain organization of signaling molecules Grb2 and Src homology [SH2] domain-containing leukocyte protein of 65 kDa was observed within the first 20 s. In addition, fast cytoskeletal changes are initiated. Furthermore, the involvement of additional Ca²⁺ channels and organelles, such as the Ca²⁺ buffering mitochondria, is discussed. Future research developments will comprise analysis of the causal relation between these temporally coordinated signaling events. Taken together, high-resolution Ca²⁺ imaging techniques applied to T cell activation in the past years paved the way to detailed molecular understanding of initial Ca²⁺ signaling mechanisms in non-excitable cells.

Nano-clustering of ligands on surrogate antigen presenting cells modulates T cell membrane adhesion and organization

We investigate the adhesion and molecular organization of the plasma membrane of T lymphocytes interacting with a surrogate antigen presenting cell comprising glass supported ordered arrays of antibody (α-CD3) nano-dots dispersed in a non-adhesive matrix of polyethylene glycol (PEG). The local membrane adhesion and topography, as well as the distribution of the T cell receptors (TCRs) and the kinase ZAP-70, are influenced by dot-geometry, whereas the cell spreading area is determined by the overall average density of the ligands rather than specific characteristics of the dots. TCR clusters are recruited preferentially to the nano-dots and the TCR cluster size distribution has a weak dot-size dependence. On the patterns, the clusters are larger, more numerous, and more enriched in TCRs, as compared to the homogeneously distributed ligands at comparable concentrations. These observations support the idea that non-ligated TCRs residing in the non-adhered parts of the proximal membrane are able to diffuse and enrich the existing clusters at the ligand dots. However, long distance transport is impaired and cluster centralization in the form of a central supramolecular cluster (cSMAC) is not observed. Time-lapse imaging of early cell-surface contacts indicates that the ZAP-70 microclusters are directly recruited to the site of the antibody dots and this process is concomitant with membrane adhesion. These results together point to a complex interplay of adhesion, molecular organization and activation in response to spatially modulated stimulation.

HIV Envelope gp120 Alters T Cell Receptor Mobilization in the Immunological Synapse of Uninfected CD4 T Cells and Augments T Cell Activation

HIV is transmitted most efficiently from cell to cell, and productive infection occurs mainly in activated CD4 T cells. It is postulated that HIV exploits immunological synapses formed between CD4 T cells and antigen-presenting cells to facilitate the targeting and infection of activated CD4 T cells. This study sought to evaluate how the presence of the HIV envelope (Env) in the CD4 T cell immunological synapse affects synapse formation and intracellular signaling to impact the downstream T cell activation events. CD4 T cells were applied to supported lipid bilayers that were reconstituted with HIV Env gp120, anti-T cell receptor (anti-TCR) monoclonal antibody, and ICAM-1 to represent the surface of HIV Env-bearing antigen-presenting cells. The results showed that the HIV Env did not disrupt immunological synapse formation. Instead, the HIV Env accumulated with TCR at the center of the synapse, altered the kinetics of TCR recruitment to the synapse and affected synapse morphology over time. The HIV Env also prolonged Lck phosphorylation at the synapse and enhanced TCR-induced CD69 upregulation, interleukin-2 secretion, and proliferation to promote virus infection. These results suggest that HIV uses the immunological synapse as a conduit not only for selective virus transmission to activated CD4 T cells but also for boosting the T cell activation state, thereby increasing its likelihood of undergoing productive replication in targeted CD4 T cells.

IMPORTANCE

There are about two million new HIV infections every year. A better understanding of how HIV is transmitted to susceptible cells is critical to devise effective strategies to prevent HIV infection. Activated CD4 T cells are preferentially infected by HIV, although how this is accomplished is not fully understood. This study examined whether HIV co-opts the normal T cell activation process through the so-called immunological synapse. We found that the HIV envelope is recruited to the center of the immunological synapse together with the T cell receptor and enhances the T cell receptor-induced activation of CD4 T cells. Heightened cellular activation promotes the capacity of CD4 T cells to support productive HIV replication. This study provides evidence of the exploitation of the normal immunological synapse and T cell activation process by HIV to boost the activation state of targeted CD4 T cells and promote the infection of these cells.

Three-dimensional localization of T-cell receptors in relation to microvilli using a combination of superresolution microscopies

Leukocyte microvilli are flexible projections enriched with adhesion molecules. The role of these cellular projections in the ability of T cells to probe antigen-presenting cells has been elusive. In this study, we probe the spatial relation of microvilli and T-cell receptors (TCRs), the major molecules responsible for antigen recognition on the T-cell membrane. To this end, an effective and robust methodology for mapping membrane protein distribution in relation to the 3D surface structure of cells is introduced, based on two complementary superresolution microscopies. Strikingly, TCRs are found to be highly localized on microvilli, in both peripheral blood human T cells and differentiated effector T cells, and are barely found on the cell body. This is a decisive demonstration that different types of T cells universally localize their TCRs to microvilli, immediately pointing to these surface projections as effective sensors for antigenic moieties. This finding also suggests how previously reported membrane clusters might form, with microvilli serving as anchors for specific T-cell surface molecules.

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

Saito et al. describe a ring of focal adhesion molecules that surrounds T cell receptor microclusters and is essential for early 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.

Action and Traction: Cytoskeletal Control of Receptor Triggering at the Immunological Synapse

It is well known that F-actin dynamics drive the micron-scale cell shape changes required for migration and immunological synapse (IS) formation. In addition, recent evidence points to a more intimate role for the actin cytoskeleton in promoting T cell activation. Mechanotransduction, the conversion of mechanical input into intracellular biochemical changes, is thought to play a critical role in several aspects of immunoreceptor triggering and downstream signal transduction. Multiple molecules associated with signaling events at the IS have been shown to respond to physical force, including the TCR, costimulatory molecules, adhesion molecules, and several downstream adapters. In at least some cases, it is clear that the relevant forces are exerted by dynamics of the T cell actomyosin cytoskeleton. Interestingly, there is evidence that the cytoskeleton of the antigen-presenting cell also plays an active role in T cell activation, by countering the molecular forces exerted by the T cell at the IS. Since actin polymerization is itself driven by TCR and costimulatory signaling pathways, a complex relationship exists between actin dynamics and receptor activation. This review will focus on recent advances in our understanding of the mechanosensitive aspects of T cell activation, paying specific attention to how F-actin-directed forces applied from both sides of the IS fit into current models of receptor triggering and activation.

Function and Dynamics of Tetraspanins during Antigen Recognition and Immunological Synapse Formation

Tetraspanin-enriched microdomains (TEMs) are specialized membrane platforms driven by protein–protein interactions that integrate membrane receptors and adhesion molecules. Tetraspanins participate in antigen recognition and presentation by antigen-­presenting cells (APCs) through the organization of pattern-recognition receptors (PRRs) and their downstream-induced signaling, as well as the regulation of MHC-II–peptide trafficking. T lymphocyte activation is triggered upon specific recognition of antigens present on the APC surface during immunological synapse (IS) formation. This dynamic process is characterized by a defined spatial organization involving the compartmentalization of receptors and adhesion molecules in specialized membrane domains that are connected to the underlying cytoskeleton and signaling molecules. Tetraspanins contribute to the spatial organization and maturation of the IS by controlling receptor clustering and local accumulation of adhesion receptors and integrins, their downstream signaling, and linkage to the actin cytoskeleton. This review offers a perspective on the important role of TEMs in the regulation of antigen recognition and presentation and in the dynamics of IS architectural organization.

Membrane nanodomains in T-cell antigen receptor signalling

The organization of the T-cell's plasma membrane continues to nourish the curiosity of immunologists, cell biologists and biophysicists. The main reason is the biological and biomedical interest to understand the workings of the cell-cell communication network activated by T-cells during an immune response. The molecular armamentarium of the T-cell plasma membrane helps to identify with high sensitivity, specificity and rapidity antigens from invading microbial pathogens and prepare adequate countermeasures to fend them off, while protecting from attacks against our normal tissues. Many T-cell membrane proteins act as receptors to carry out and finely tune these complex tasks. However, the TCR (T-cell receptor) holds a decisive hegemony for its crucial contribution in steering T-cell function and fate. An emerging notion is that TCR proximal signalling occurs at submicrometre-scale membrane domains. In the present chapter, we discuss the current knowledge on the TCR structure and the associated signal transduction machinery and how the notion of membrane nanodomains has decisively contributed to further understand the molecular basis of T-cell activation.

Supported Lipid Bilayer Technology for the Study of Cellular Interfaces

Glass-supported lipid bilayers presenting freely diffusing proteins have served as a powerful tool for studying cell-cell interfaces, in particular, T cell-antigen presenting cell (APC) interactions, using optical microscopy. Here we expand upon existing protocols and describe the preparation of liposomes by an extrusion method, and describe how this system can be used to study immune synapse formation by Jurkat cells. We also present a method for forming such lipid bilayers on silica beads for the study of signaling responses by population methods, such as western blotting, flow cytometry, and gene-expression analysis. Finally, we describe how to design and prepare transmembrane-anchored protein-laden liposomes, following expression in suspension CHO (CHOs) cells, a mammalian expression system alternative to insect and bacterial cell lines, which do not produce mammalian glycosylation patterns. Such transmembrane-anchored proteins may have many novel applications in cell biology and immunology.

The cellular environment regulates in situ kinetics of T-cell receptor interaction with peptide major histocompatibility complex

T cells recognize antigens at the two-dimensional (2D) interface with antigen-presenting cells (APCs), which trigger T-cell effector functions. T-cell functional outcomes correlate with 2D kinetics of membrane-embedded T-cell receptors (TCRs) binding to surface-tethered peptide-major histocompatibility complex molecules (pMHCs). However, most studies have measured TCR-pMHC kinetics for recombinant TCRs in 3D by surface plasmon resonance, which differs drastically from 2D measurements. Here, we compared pMHC dissociation from native TCR on the T-cell surface to recombinant TCR immobilized on glass surface or in solution. Force on TCR-pMHC bonds regulated their lifetimes differently for native than recombinant TCRs. Perturbing the cellular environment suppressed 2D on-rates but had no effect on 2D off-rate regardless of whether force was applied. In contrast, for the TCR interacting with its monoclonal antibody, the 2D on-rate was insensitive to cellular perturbations and the force-dependent off-rates were indistinguishable for native and recombinant TCRs. These data present novel features of TCR-pMHC kinetics that are regulated by the cellular environment, underscoring the limitations of 3D kinetics in predicting T-cell functions and calling for further elucidation of the underlying molecular and cellular mechanisms that regulate 2D kinetics in physiological settings.

The T cell receptor resides in ordered plasma membrane nanodomains that aggregate upon patching of the receptor

Two related models for T cell signalling initiation suggest either that T cell receptor (TCR) engagement leads to its recruitment to ordered membrane domains, often referred to as lipid rafts, where signalling molecules are enriched or that ordered TCR-containing membrane nanodomains coalesce upon TCR engagement. That ordered domains form upon TCR engagement, as they do upon lipid raft marker patching, has not been considered. The target of this study was to differentiate between those three options. Plasma membrane order was followed in live T cells at 37 °C using laurdan to report on lipid packing. Patching of the TCR that elicits a signalling response resulted in aggregation, not formation, of ordered plasma membrane domains in both Jurkat and primary T cells. The TCR colocalised with actin filaments at the plasma membrane in unstimulated Jurkat T cells, consistent with it being localised to ordered membrane domains. The colocalisation was most prominent in cells in G1 phase when the cells are ready to commit to proliferation. At other cell cycle phases the TCR was mainly found at perinuclear membranes. Our study suggests that the TCR resides in ordered plasma membrane domains that are linked to actin filaments and aggregate upon TCR engagement.

The nanoscale organization of signaling domains at the plasma membrane

In this chapter, we present an overview of the role of the nanoscale organization of signaling domains in regulating key cellular processes. In particular, we illustrate the importance of protein and lipid nanodomains as triggers and mediators of cell signaling. As particular examples, we summarize the state of the art of understanding the role of nanodomains in the mounting of an immune response, cellular adhesion, intercellular communication, and cell proliferation. Thus, this chapter underlines the essential role the nanoscale organization of key signaling proteins and lipid domains. We will also see how nanodomains play an important role in the lifecycle of many pathogens relevant to human disease and therefore illustrate how these structures may become future therapeutic targets.

GRB2 Nucleates T Cell Receptor-Mediated LAT Clusters That Control PLC-γ1 Activation and Cytokine Production

GRB2 is a ubiquitously expressed adaptor protein required for signaling downstream of multiple receptors. To address the role of GRB2 in receptor-mediated signaling, the expression of GRB2 was suppressed in human CD4+ T cells and its role downstream of the T cell receptor (TCR) was examined. Interestingly, GRB2 deficient T cells had enhanced signaling from complexes containing the TCR. However, GRB2 deficient T cells had substantially reduced production of IL-2 and IFN-γ. This defect was attributed to diminished formation of linker for activation of T cells (LAT) signaling clusters, which resulted in reduced MAP kinase activation, calcium flux, and PLC-γ1 recruitment to LAT signaling clusters. Add back of wild-type GRB2, but not a novel N-terminal SH3 domain mutant, rescued LAT microcluster formation, calcium mobilization, and cytokine release, providing the first direct evidence that GRB2, and its ability to bind to SH3 domain ligands, is required for establishing LAT microclusters. Our data demonstrate that the ability of GRB2 to facilitate protein clusters is equally important in regulating TCR-mediated functions as its capacity to recruit effector proteins. This highlights that GRB2 regulates signaling downstream of adaptors and receptors by both recruiting effector proteins and regulating the formation of signaling complexes.

Ligand-mediated friction determines morphodynamics of spreading T cells

Spreading of T cells on antigen presenting cells is a crucial initial step in immune response. Spreading occurs through rapid morphological changes concomitant with the reorganization of surface receptors and of the cytoskeleton. Ligand mobility and frictional coupling of receptors to the cytoskeleton were separately recognized as important factors but a systematic study to explore their biophysical role in spreading was hitherto missing. To explore the impact of ligand mobility, we prepared chemically identical substrates on which molecules of anti-CD3 (capable of binding and activating the T cell receptor complex), were either immobilized or able to diffuse. We quantified the T cell spreading area and cell edge dynamics using quantitative reflection interference contrast microscopy, and imaged the actin distribution. On mobile ligands, as compared to fixed ligands, the cells spread much less, the actin is centrally, rather than peripherally distributed and the edge dynamics is largely altered. Blocking myosin-II or adding molecules of ICAM1 on the substrate largely abrogates these differences. We explain these observations by building a model based on the balance of forces between activation-dependent actin polymerization and actomyosin-generated tension on one hand, and on the frictional coupling of the ligand-receptor complexes with the actin cytoskeleton, the membrane and the substrate, on the other hand. Introducing the measured edge velocities in the model, we estimate the coefficient of frictional coupling between T Cell receptors or LFA-1 and the actin cytoskeleton. Our results provide for the first time, to our knowledge, a quantitative framework bridging T cell-specific biology with concepts developed for integrin-based mechanisms of spreading.

The immunological synapse

The molecular interactions underlying regulation of the immune response take place in a nanoscale gap between T cells and antigen-presenting cells, termed the immunological synapse. If these interactions are regulated appropriately, the host is defended against a wide range of pathogens and deranged host cells. If these interactions are disregulated, the host is susceptible to pathogens or tumor escape at one extreme and autoimmunity at the other. Strategies targeting the synapse have helped to establish immunotherapy as a mainstream element in cancer treatment. This Masters' primer will cover the basics of the immunological synapse and some of the applications to tumor immunology.