The alphabeta T cell receptor is an anisotropic mechanosensor

Isolation of a Structural Mechanism for Uncoupling T Cell Receptor Signaling from Peptide-MHC Binding

TCR-signaling strength generally correlates with peptide-MHC binding affinity; however, exceptions exist. We find high-affinity, yet non-stimulatory, interactions occur with high frequency in the human T cell repertoire. Here, we studied human TCRs that are refractory to activation by pMHC ligands despite robust binding. Analysis of 3D affinity, 2D dwell time, and crystal structures of stimulatory versus non-stimulatory TCR-pMHC interactions failed to account for their different signaling outcomes. Using yeast pMHC display, we identified peptide agonists of a formerly non-responsive TCR. Single-molecule force measurements demonstrated the emergence of catch bonds in the activating TCR-pMHC interactions, correlating with exclusion of CD45 from the TCR-APC contact site. Molecular dynamics simulations of TCR-pMHC disengagement distinguished agonist from non-agonist ligands based on the acquisition of catch bonds within the TCR-pMHC interface. The isolation of catch bonds as a parameter mediating the coupling of TCR binding and signaling has important implications for TCR and antigen engineering for immunotherapy.

αβ T Cell Receptor Mechanosensing Forces out Serial Engagement

T lymphocytes use αβ T cell receptors (TCRs) to recognize sparse antigenic peptides bound to MHC molecules (pMHCs) arrayed on antigen-presenting cells (APCs). Contrary to conventional receptor–ligand associations exemplified by antigen-antibody interactions, forces play a crucial role in nonequilibrium mechanosensor-based T cell activation. Both T cell motility and local cytoskeleton machinery exert forces (i.e., generate loads) on TCR–pMHC bonds. We review biological features of the load-dependent activation process as revealed by optical tweezers single molecule/single cell and other biophysical measurements. The findings link pMHC-triggered TCRs to single cytoskeletal motors; define the importance of energized anisotropic (i.e., force direction dependent) activation; and characterize immunological synapse formation as digital, revealing no serial requirement. The emerging picture suggests new approaches for the monitoring and design of cytotoxic T lymphocyte (CTL)-based immunotherapy.

Review: Bioengineering strategies to probe T cell mechanobiology

T cells play a major role in adaptive immune response, and T cell dysfunction can lead to the progression of several diseases that are often associated with changes in the mechanical properties of tissues. However, the concept that mechanical forces play a vital role in T cell activation and signaling is relatively new. The endogenous T cell microenvironment is highly complex and dynamic, involving multiple, simultaneous cell-cell and cell-matrix interactions. This native complexity has made it a challenge to isolate the effects of mechanical stimuli on T cell activation. In response, researchers have begun developing engineered platforms that recapitulate key aspects of the native microenvironment to dissect these complex interactions in order to gain a better understanding of T cell mechanotransduction. In this review, we first describe some of the unique characteristics of T cells and the mounting research that has shown they are mechanosensitive. We then detail the specific bioengineering strategies that have been used to date to measure and perturb the mechanical forces at play during T cell activation. In addition, we look at engineering strategies that have been used successfully in mechanotransduction studies for other cell types and describe adaptations that may make them suitable for use with T cells. These engineering strategies can be classified as 2D, so-called 2.5D, or 3D culture systems. In the future, findings from this emerging field will lead to an optimization of culture environments for T cell expansion and the development of new T cell immunotherapies for cancer and other immune diseases.

Perspective: Biophysical regulation of cancerous and normal blood cell lineages in hematopoietic malignancies

It is increasingly appreciated that physical forces play important roles in cancer biology, in terms of progression, invasiveness, and drug resistance. Clinical progress in treating hematological malignancy and in developing cancer immunotherapy highlights the role of the hematopoietic system as a key model in devising new therapeutic strategies against cancer. Understanding mechanobiology of the hematopoietic system in the context of cancer will thus yield valuable fundamental insights that can information about novel cancer therapeutics. In this perspective, biophysical insights related to blood cancer are defined and detailed. The interactions with immune cells relevant to immunotherapy against cancer are considered and expounded, followed by speculation of potential regulatory roles of mesenchymal stromal cells (MSCs) in this complex network. Finally, a perspective is presented as to how insights from these complex interactions between matrices, blood cancer cells, immune cells, and MSCs can be leveraged to influence and engineer the treatment of blood cancers in the clinic.

PI(4,5)P2 determines the threshold of mechanical force-induced B cell activation

B lymphocytes use B cell receptors (BCRs) to sense the chemical and physical features of antigens. The activation of isotype-switched IgG-BCR by mechanical force exhibits a distinct sensitivity and threshold in comparison with IgM-BCR. However, molecular mechanisms governing these differences remain to be identified. In this study, we report that the low threshold of IgG-BCR activation by mechanical force is highly dependent on tethering of the cytoplasmic tail of the IgG-BCR heavy chain (IgG-tail) to the plasma membrane. Mechanistically, we show that the positively charged residues in the IgG-tail play a crucial role by highly enriching phosphatidylinositol (4,5)-biphosphate (PI(4,5)P2) into the membrane microdomains of IgG-BCRs. Indeed, manipulating the amounts of PI(4,5)P2 within IgG-BCR membrane microdomains significantly altered the threshold and sensitivity of IgG-BCR activation. Our results reveal a lipid-dependent mechanism for determining the threshold of IgG-BCR activation by mechanical force.

Monomeric TCRs drive T cell antigen recognition

T cell antigen recognition requires T cell antigen receptors (TCRs) engaging MHC-embedded antigenic peptides (pMHCs) within the contact region of a T cell with its conjugated antigen-presenting cell. Despite micromolar TCR:pMHC affinities, T cells respond to even a single antigenic pMHC, and higher-order TCRs have been postulated to maintain high antigen sensitivity and trigger signaling. We interrogated the stoichiometry of TCRs and their associated CD3 subunits on the surface of living T cells through single-molecule brightness and single-molecule coincidence analysis, photon-antibunching-based fluorescence correlation spectroscopy and Förster resonance energy transfer measurements. We found exclusively monomeric TCR-CD3 complexes driving the recognition of antigenic pMHCs, which underscores the exceptional capacity of single TCR-CD3 complexes to elicit robust intracellular signaling.

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.

CD8+ T Cells and NK Cells: Parallel and Complementary Soldiers of Immunotherapy

CD8+ T cells and NK cells are both cytotoxic effector cells of the immune system, but the recognition, specificity, sensitivity, and memory mechanisms are drastically different. While many of these topics have been extensively studied in CD8+ T cells, very little is known about NK cells. Current cancer immunotherapies mainly focus on CD8+ T cells, but have many issues of toxicity and efficacy. Given the heterogeneous nature of cancer, personalized cancer immunotherapy that integrates the power of both CD8+ T cells in adaptive immunity and NK cells in innate immunity might be the future direction, along with precision targeting and effective delivery of tumor-specific, memory CD8+ T cells and NK cells.

Rewiring T-cell responses to soluble factors with chimeric antigen receptors

Chimeric antigen receptor (CAR)-expressing T cells targeting surface-bound tumor antigens have yielded promising clinical outcomes, with two CD19 CAR-T cell therapies recently receiving FDA approval for the treatment of B-cell malignancies. The adoption of CARs for the recognition of soluble ligands, a distinct class of biomarkers in physiology and disease, could considerably broaden the utility of CARs in disease treatment. In this study, we demonstrate that CAR-T cells can be engineered to respond robustly to diverse soluble ligands, including the CD19 ectodomain, GFP variants, and transforming growth factor beta (TGF-β). We additionally show that CAR signaling in response to soluble ligands relies on ligand-mediated CAR dimerization and that CAR responsiveness to soluble ligands can be fine-tuned by adjusting the mechanical coupling between the CAR's ligand-binding and signaling domains. Our results support a role for mechanotransduction in CAR signaling and demonstrate an approach for systematically engineering immune-cell responses to soluble, extracellular ligands.

Enhanced activation and expansion of T cells using mechanically soft elastomer fibers

Practical deployment of cellular therapies requires effective platforms for producing clinically relevant numbers of high-quality cells. This report introduces a materials-based approach to improving activation and expansion of T cells, which are rapidly emerging as an agent for treating cancer and a range of other diseases. Electrospinning is used to create a mesh of poly(ε-caprolactone) fibers, which is used to present activating ligands to CD3 and CD28, which activate T cells for expansion. Incorporation of poly(dimethyl siloxane) elastomer into the fibers reduces substrate rigidity and enhances expansion of mixed populations of human CD4⁺ and CD8⁺ T cells. Intriguingly, this platform also rescues expansion of T cells isolated from CLL patients, which often show limited responsiveness and other features resembling exhaustion. By simplifying the process of cell expansion, compared to current bead-based platforms, and improving T cell expansion, the system introduced here may accelerate development of cellular immunotherapy.

Cutting Edge: Piezo1 Mechanosensors Optimize Human T Cell Activation

TCRs recognize peptides on MHC molecules and induce downstream signaling, leading to activation and clonal expansion. In addition to the strength of the interaction of TCRs with peptides on MHC molecules, mechanical forces contribute to optimal T cell activation, as reflected by the superior efficiency of immobilized TCR-cross-linking Abs compared with soluble Abs in TCR triggering, although a dedicated mechanotransduction module is not identified. We found that the professional mechanosensor protein Piezo1 is critically involved in human T cell activation. Although a deficiency in Piezo1 attenuates downstream events on ex vivo TCR triggering, a Piezo1 agonist can obviate the need to immobilize TCR-cross-linking Abs. Piezo1-driven Ca²⁺ influx, leading to calpain activation and organization of cortical actin scaffold, links this mechanosensor to optimal TCR signaling. Thus, we discovered a hitherto unknown regulatory mechanism for human T cell activation and provide the first evidence, to our knowledge, for the involvement of Piezo1 mechanosensors in immune regulation.

Platelet integrins exhibit anisotropic mechanosensing and harness piconewton forces to mediate platelet aggregation

Platelet aggregation at the site of vascular injury is essential in clotting. During this process, platelets are bridged by soluble fibrinogen that binds surface integrin receptors. One mystery in the mechanism of platelet aggregation pertains to how resting platelets ignore soluble fibrinogen, the third most abundant protein in the bloodstream, and yet avidly bind immobile fibrinogen on the surface of other platelets at the primary injury site. We speculate that platelet integrins are mechanosensors that test their ligands across the platelet-platelet synapse. To investigate this model, we interrogate human platelets using approaches that include the supported lipid bilayer platform as well as DNA tension sensor technologies. Experiments suggest that platelet integrins require lateral forces to mediate platelet-platelet interactions. Mechanically labile ligands dampen platelet activation, and the onset of piconewton integrin tension coincides with calcium flux. Activated platelets display immobilized fibrinogen on their surface, thus mediating further recruitment of resting platelets. The distribution of integrin tension was shown to be spatially regulated through two myosin-signaling pathways, myosin light chain kinase and Rho-associated kinase. Finally, we discovered that the termination of integrin tension is coupled with the exposure of phosphatidylserine. Our work reveals the highest spatial and temporal resolution maps of platelet integrin mechanics and its role in platelet aggregation, suggesting that platelets are physical substrates for one another that establish mechanical feedback loops of activation. The results are reminiscent of mechanical regulation of the T-cell receptor, E-cadherin, and Notch pathways, suggesting a common feature for signaling at cell junctions.

The role of CaMK4 in immune responses

Calcium/calmodulin-dependent protein kinase IV (CaMK4) is a multifunctional serine/threonine kinase that regulates gene expression by activating transcription factors in a wide range of immune cells including T cells and antigen-presenting cells. The function of CaMK4 is suggested to be abnormal mainly in systemic lupus erythematosus (SLE), which is characterized by autoantibody production, immune complex formation, and immune dysregulation. Although accumulating evidence indicates that CaMK4 plays important roles in the immune responses, the precise molecular mechanisms underlying the development of autoimmune diseases and inflammatory disorders have not been established. In this review, we briefly summarize the role of CaMK4 in immune responses. We also discuss T-cell signaling pathways that control interleukin (IL)-17 production in patients with lupus nephritis and in glomerulonephritis in lupus-prone mice. A better understanding of the signaling and gene regulation of CaMK4 will lead to the identification of novel therapeutic targets in Th17 driven-autoimmune diseases.

Cell Biology of T Cell Receptor Expression and Regulation

T cell receptors (TCRs) are protein complexes formed by six different polypeptides. In most T cells, TCRs are composed of αβ subunits displaying immunoglobulin-like variable domains that recognize peptide antigens associated with major histocompatibility complex molecules expressed on the surface of antigen-presenting cells. TCRαβ subunits are associated with the CD3 complex formed by the γ, δ, ε, and ζ subunits, which are invariable and ensure signal transduction. Here, we review how the expression and function of TCR complexes are orchestrated by several fine-tuned cellular processes that encompass (a) synthesis of the subunits and their correct assembly and expression at the plasma membrane as a single functional complex, (b) TCR membrane localization and dynamics at the plasma membrane and in endosomal compartments, (c) TCR signal transduction leading to T cell activation, and (d) TCR degradation. These processes balance each other to ensure efficient T cell responses to a variety of antigenic stimuli while preventing autoimmunity.

TCR Signaling: Mechanisms of Initiation and Propagation

The mechanisms by which a T cell detects antigen using its T cell antigen receptor (TCR) are crucial to our understanding of immunity and the harnessing of T cells therapeutically. A hallmark of the T cell response is the ability of T cells to quantitatively respond to antigenic ligands derived from pathogens while remaining inert to similar ligands derived from host tissues. Recent studies have revealed exciting properties of the TCR and the behaviors of its signaling effectors that are used to detect and discriminate between antigens. Here we highlight these recent findings, focusing on the proximal TCR signaling molecules Zap70, Lck, and LAT, to provide mechanistic models and insights into the exquisite sensitivity and specificity of the TCR.

A minimally invasive optical trapping system to understand cellular interactions at onset of an immune response

T-cells and antigen presenting cells are an essential part of the adaptive immune response system and how they interact is crucial in how the body effectively fights infection or responds to vaccines. Much of the experimental work studying interaction forces between cells has looked at the average properties of bulk samples of cells or applied microscopy to image the dynamic contact between these cells. In this paper we present a novel optical trapping technique for interrogating the force of this interaction and measuring relative interaction forces at the single-cell level. A triple-spot optical trap is used to directly manipulate the cells of interest without introducing foreign bodies such as beads to the system. The optical trap is used to directly control the initiation of cell-cell contact and, subsequently to terminate the interaction at a defined time point. The laser beam power required to separate immune cell pairs is determined and correlates with the force applied by the optical trap. As proof of concept, the antigen-specific increase in interaction force between a dendritic cell and a specific T-cell is demonstrated. Furthermore, it is demonstrated that this interaction force is completely abrogated when T-cell signalling is blocked. As a result the potential of using optical trapping to interrogate cellular interactions at the single cell level without the need to introduce foreign bodies such as beads is clearly demonstrated.

NMR-directed design of pre-TCRβ and pMHC molecules implies a distinct geometry for pre-TCR relative to αβTCR recognition of pMHC

The pre-T cell receptor (pre-TCR) guides early thymocytes through maturation processes within the thymus via interaction with self-ligands displayed on thymic epithelial cells. The pre-TCR is a disulfide-linked heterodimer composed of an invariant pre-TCR α (pTα) subunit and a variable β subunit, the latter of which is incorporated into the mature TCR in subsequent developmental progression. This interaction of pre-TCR with peptide-major histocompatibility complex (pMHC) molecules has recently been shown to drive robust pre-TCR signaling and thymocyte maturation. Although the native sequences of β are properly folded and suitable for NMR studies in isolation, a tendency to self-associate rendered binding studies with physiological ligands difficult to interpret. Consequently, to structurally define this critical interaction, we have re-engineered the extracellular regions of β, designated as β-c1, for prokaryotic production to be used in NMR spectroscopy. Given the large size of the full extracellular domain of class I MHC molecules such as H-K^b, we produced a truncated form termed K^b-t harboring properties favorable for NMR measurements. This system has enabled robust measurement of a pre-TCR-pMHC interaction directly analogous to that of TCRαβ-pMHC. Binding surface analysis identified a contact surface comparable in size to that of the TCRαβ-pMHC but potentially with a rather distinct binding orientation. A tilting of the pre-TCRβ when bound to the pMHC ligand recognition surface versus the upright orientation of TCRαβ would alter the direction of force application between pre-TCR and TCR mechanosensors, impacting signal initiation.

Receptor-mediated cell mechanosensing

Mechanosensing depicts the ability of a cell to sense mechanical cues, which under some circumstances is mediated by the surface receptors. In this review, a four-step model is described for receptor-mediated mechanosensing. Platelet GPIb, T-cell receptor, and integrins are used as examples to illustrate the key concepts and players in this process.

Mechanosensing describes the ability of a cell to sense mechanical cues of its microenvironment, including not only all components of force, stress, and strain but also substrate rigidity, topology, and adhesiveness. This ability is crucial for the cell to respond to the surrounding mechanical cues and adapt to the changing environment. Examples of responses and adaptation include (de)activation, proliferation/apoptosis, and (de)differentiation. Receptor-mediated cell mechanosensing is a multistep process that is initiated by binding of cell surface receptors to their ligands on the extracellular matrix or the surface of adjacent cells. Mechanical cues are presented by the ligand and received by the receptor at the binding interface; but their transmission over space and time and their conversion into biochemical signals may involve other domains and additional molecules. In this review, a four-step model is described for the receptor-mediated cell mechanosensing process. Platelet glycoprotein Ib, T-cell receptor, and integrins are used as examples to illustrate the key concepts and players in this process.

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.

Catch Bonds at T Cell Interfaces: Impact of Surface Reorganization and Membrane Fluctuations

Catch bonds are characterized by average lifetimes that initially increase with increasing tensile force. Recently, they have been implicated in T cell activation, where small numbers of antigenic receptor-ligand bonds at a cell-cell interface can stimulate a T cell. Here, we use computational methods to investigate small numbers of bonds at the interface between two membranes. We characterize the time-dependent forces on the bonds in response to changes in the membrane shape and the organization of other surface molecules. We then determine the distributions of bond lifetimes using recent force-dependent lifetime data for T cell receptors bound to various ligands. Strong agonists, which exhibit catch bond behavior, are markedly more likely to remain intact than an antagonist whose average lifetime decreases with increasing force. Thermal fluctuations of the membrane shape enhance the decay of the average force on a bond, but also lead to fluctuations of the force. These fluctuations promote bond rupture, but the effect is buffered by catch bonds. When more than one bond is present, the bonds experience reduced average forces that depend on their relative positions, leading to changes in bond lifetimes. Our results highlight the importance of force-dependent binding kinetics when bonds experience time-dependent and fluctuating forces, as well as potential consequences of collective bond behavior relevant to T cell activation.

Mechanosensing in the immune response

Cells have a remarkable ability to sense and respond to the mechanical properties of their environment. Mechanosensing is essential for many phenomena, ranging from cell movements and tissue rearrangements to cell differentiation and the immune response. Cells of the immune system get activated when membrane receptors bind to cognate antigen on the surface of antigen presenting cells. Both T and B lymphocyte signaling has been shown to be responsive to physical forces and mechanical cues. Cytoskeletal forces exerted by cells likely mediate this mechanical modulation. Here, we discuss recent advances in the field of immune cell mechanobiology at the molecular and cellular scale.

Contractile actomyosin arcs promote the activation of primary mouse T cells in a ligand-dependent manner

Mechano-transduction is an emerging but still poorly understood component of T cell activation. Here we investigated the ligand-dependent contribution made by contractile actomyosin arcs populating the peripheral supramolecular activation cluster (pSMAC) region of the immunological synapse (IS) to T cell receptor (TCR) microcluster transport and proximal signaling in primary mouse T cells. Using super resolution microscopy, OT1-CD8+ mouse T cells, and two ovalbumin (OVA) peptides with different affinities for the TCR, we show that the generation of organized actomyosin arcs depends on ligand potency and the ability of myosin 2 to contract actin filaments. While weak ligands induce disorganized actomyosin arcs, strong ligands result in organized actomyosin arcs that correlate well with tension-sensitive CasL phosphorylation and the accumulation of ligands at the IS center. Blocking myosin 2 contractility greatly reduces the difference in the extent of Src and LAT phosphorylation observed between the strong and the weak ligand, arguing that myosin 2-dependent force generation within actin arcs contributes to ligand discrimination. Together, our data are consistent with the idea that actomyosin arcs in the pSMAC region of the IS promote a mechano-chemical feedback mechanism that amplifies the accumulation of critical signaling molecules at the IS.

Mechanosensing drives acuity of αβ T-cell recognition

T lymphocytes use surface [Formula: see text] T-cell receptors (TCRs) to recognize peptides bound to MHC molecules (pMHCs) on antigen-presenting cells (APCs). How the exquisite specificity of high-avidity T cells is achieved is unknown but essential, given the paucity of foreign pMHC ligands relative to the ubiquitous self-pMHC array on an APC. Using optical traps, we determine physicochemical triggering thresholds based on load and force direction. Strikingly, chemical thresholds in the absence of external load require orders of magnitude higher pMHC numbers than observed physiologically. In contrast, force applied in the shear direction ([Formula: see text]10 pN per TCR molecule) triggers T-cell Ca²⁺ flux with as few as two pMHC molecules at the interacting surface interface with rapid positional relaxation associated with similarly directed motor-dependent transport via [Formula: see text]8-nm steps, behaviors inconsistent with serial engagement during initial TCR triggering. These synergistic directional forces generated during cell motility are essential for adaptive T-cell immunity against infectious pathogens and cancers.

Mechanical forces in the immune system

Leukocytes can completely reorganize their cytoskeletal architecture within minutes. This structural plasticity, which facilitates their migration and communicative function, also enables them to exert a substantial amount of mechanical force against the extracellular matrix and the surfaces of interacting cells. In recent years, it has become increasingly clear that these forces have crucial roles in immune cell activation and subsequent effector responses. Here, I review our current understanding of how mechanical force regulates cell-surface receptor activation, cell migration, intracellular signalling and intercellular communication, highlighting the biological ramifications of these effects in various immune cell types.

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.

Different TCR-induced T lymphocyte responses are potentiated by stiffness with variable sensitivity

T cells are mechanosensitive but the effect of stiffness on their functions is still debated. We characterize herein how human primary CD4⁺ T cell functions are affected by stiffness within the physiological Young’s modulus range of 0.5 kPa to 100 kPa. Stiffness modulates T lymphocyte migration and morphological changes induced by TCR/CD3 triggering. Stiffness also increases TCR-induced immune system, metabolism and cell-cycle-related genes. Yet, upon TCR/CD3 stimulation, while cytokine production increases within a wide range of stiffness, from hundreds of Pa to hundreds of kPa, T cell metabolic properties and cell cycle progression are only increased by the highest stiffness tested (100 kPa). Finally, mechanical properties of adherent antigen-presenting cells modulate cytokine production by T cells. Together, these results reveal that T cells discriminate between the wide range of stiffness values found in the body and adapt their responses accordingly.

DOI:http://dx.doi.org/10.7554/eLife.23190.001

Our immune system contains many cells that play various roles in defending the body against infection, cancer and other threats. For example, T cells constantly patrol the body ready to detect and respond to dangers. They do so by gathering cues from their surroundings, which can be specific chemical signals or physical properties such as the stiffness of tissues. Once the T cells are active they respond in several different ways including releasing hormones and dividing to produce more T cells.

Tissue stiffness varies considerably between different organs. Furthermore, disease can lead to changes in tissue stiffness. For example, tissues become more rigid when they are inflamed. The stiffness and other physical properties of the surfaces that T cells interact with affect how the cells respond when they detect a threat, but few details are known about exactly how these cues tune T cell responses. Saitakis et al. studied how human T cells respond to artificial surfaces of varying stiffness that mimic the range found in the body.

The experiments show that T cells that interact with stiff surfaces become more active than T cells that interact with softer surfaces. However, some responses are more sensitive to the stiffness of the surface than others. For example, the ability of the T cells to release hormones was affected by the whole range of stiffnesses tested in the experiments, whereas only very stiff surfaces stimulated the T cells to divide.

These findings show that T cells can detect the stiffness of surfaces in the body and use this to adapt how they respond to threats. Future challenges will be to find out how T cells sense the physical properties of their surroundings and investigate whether cell and tissue stiffness affects immune responses in the body. This will help us to understand how T cells fight infections and other threats, and could be used to develop new ways of boosting these cells to fight cancer and other diseases.

DOI:http://dx.doi.org/10.7554/eLife.23190.002

Tapping out a mechanical code for T cell triggering

Mechanical forces play increasingly recognized roles in T cell receptor (TCR) signal transduction. Hu and Butte (2016. J. Cell Biol.http://dx.doi.org/10.1083/jcb.201511053) demonstrate that actin is required for T cells to generate forces at the TCR and that exogenous application of force can emulate these cytoskeletal forces and trigger T cell activation.

Surface engineering for lymphocyte programming

The once nascent field of immunoengineering has recently blossomed to include approaches to deliver and present biomolecules to program diverse populations of lymphocytes to fight disease. Building upon improved understanding of the molecular and physical mechanics of lymphocyte activation, varied strategies for engineering surfaces to activate and deactivate T-Cells, B-Cells and natural killer cells are in preclinical and clinical development. Surfaces have been engineered at the molecular level in terms of the presence of specific biological factors, their arrangement on a surface, and their diffusivity to elicit specific lymphocyte fates. In addition, the physical and mechanical characteristics of the surface including shape, anisotropy, and rigidity of particles for lymphocyte activation have been fine-tuned. Utilizing these strategies, acellular systems have been engineered for the expansion of T-Cells and natural killer cells to clinically relevant levels for cancer therapies as well as engineered to program B-Cells to better combat infectious diseases.

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.

Modular Activating Receptors in Innate and Adaptive Immunity

Triggering of cell-mediated immunity is largely dependent on the recognition of foreign or abnormal molecules by a myriad of cell surface-bound receptors. Many activating immune receptors do not possess any intrinsic signaling capacity but instead form noncovalent complexes with one or more dimeric signaling modules that communicate with a common set of kinases to initiate intracellular information-transfer pathways. This modular architecture, where the ligand binding and signaling functions are detached from one another, is a common theme that is widely employed throughout the innate and adaptive arms of immune systems. The evolutionary advantages of this highly adaptable platform for molecular recognition are visible in the variety of ligand-receptor interactions that can be linked to common signaling pathways, the diversification of receptor modules in response to pathogen challenges, and the amplification of cellular responses through incorporation of multiple signaling motifs. Here we provide an overview of the major classes of modular activating immune receptors and outline the current state of knowledge regarding how these receptors assemble, recognize their ligands, and ultimately trigger intracellular signal transduction pathways that activate immune cell effector functions.

A Perspective on the Role of Computational Models in Immunology

This is an exciting time for immunology because the future promises to be replete with exciting new discoveries that can be translated to improve health and treat disease in novel ways. Immunologists are attempting to answer increasingly complex questions concerning phenomena that range from the genetic, molecular, and cellular scales to that of organs, whole animals or humans, and populations of humans and pathogens. An important goal is to understand how the many different components involved interact with each other within and across these scales for immune responses to emerge, and how aberrant regulation of these processes causes disease. To aid this quest, large amounts of data can be collected using high-throughput instrumentation. The nonlinear, cooperative, and stochastic character of the interactions between components of the immune system as well as the overwhelming amounts of data can make it difficult to intuit patterns in the data or a mechanistic understanding of the phenomena being studied. Computational models are increasingly important in confronting and overcoming these challenges. I first describe an iterative paradigm of research that integrates laboratory experiments, clinical data, computational inference, and mechanistic computational models. I then illustrate this paradigm with a few examples from the recent literature that make vivid the power of bringing together diverse types of computational models with experimental and clinical studies to fruitfully interrogate the immune system.

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.

A conserved αβ transmembrane interface forms the core of a compact T-cell receptor-CD3 structure within the membrane

The T-cell antigen receptor (TCR) is an assembly of eight type I single-pass membrane proteins that occupies a central position in adaptive immunity. Many TCR-triggering models invoke an alteration in receptor complex structure as the initiating event, but both the precise subunit organization and the pathway by which ligand-induced alterations are transferred to the cytoplasmic signaling domains are unknown. Here, we show that the receptor complex transmembrane (TM) domains form an intimately associated eight-helix bundle organized by a specific interhelical TCR TM interface. The salient features of this core structure are absolutely conserved between αβ and γδ TCR sequences and throughout vertebrate evolution, and mutations at key interface residues caused defects in the formation of stable TCRαβ:CD3δε:CD3γε:ζζ complexes. These findings demonstrate that the eight TCR-CD3 subunits form a compact and precisely organized structure within the membrane and provide a structural basis for further investigation of conformationally regulated models of transbilayer TCR signaling.

The intercell dynamics of T cells and dendritic cells in a lymph node-on-a-chip flow device

T cells play a central role in immunity towards cancer and infectious diseases. T cell responses are initiated in the T cell zone of the lymph node (LN), where resident antigen-bearing dendritic cells (DCs) prime and activate antigen-specific T cells passing by. In the present study, we investigated the T cell : DC interaction in a microfluidic device to understand the intercellular dynamics and physiological conditions in the LN. We show random migration of antigen-specific T cells onto the antigen-presenting DC monolayer independent of the flow direction with a mean T cell : DC dwell time of 12.8 min and a mean velocity of 6 μm min(-1). Furthermore, we investigated the antigen specific vs. unspecific attachment and detachment of CD8(+) and CD4(+) T cells to DCs under varying shear stress. In our system, CD4(+) T cells showed long stable contacts with APCs, whereas CD8(+) T cells presented transient interactions with DCs. By varying the shear stress from 0.01 to 100 Dyn cm(-2), it was also evident that there was a much stronger attachment of antigen-specific than unspecific T cells to stationary DCs up to 1-12 Dyn cm(-2). The mechanical force of the cell : cell interaction associated with the pMHC-TCR match under controlled tangential shear force was estimated to be in the range of 0.25-4.8 nN. Finally, upon performing attachment & detachment tests, there was a steady accumulation of antigen specific CD8(+) T cells and CD4(+) T cells on DCs at low shear stresses, which were released at a stress of 12 Dyn cm(-2). This microphysiological model provides new possibilities to recreate a controlled mechanical force threshold of pMHC-TCR binding, allowing the investigation of intercellular signalling of immune synapses and therapeutic targets for immunotherapy.

Pre-T Cell Receptors (Pre-TCRs) Leverage Vβ Complementarity Determining Regions (CDRs) and Hydrophobic Patch in Mechanosensing Thymic Self-ligands

The pre-T cell receptor (pre-TCR) is a pTα-β heterodimer functioning in early αβ T cell development. Although once thought to be ligand-autonomous, recent studies show that pre-TCRs participate in thymic repertoire formation through recognition of peptides bound to major histocompatibility molecules (pMHC). Using optical tweezers, we probe pre-TCR bonding with pMHC at the single molecule level. Like the αβTCR, the pre-TCR is a mechanosensor undergoing force-based structural transitions that dynamically enhance bond lifetimes and exploiting allosteric control regulated via the Cβ FG loop region. The pre-TCR structural transitions exhibit greater reversibility than TCRαβ and ordered force-bond lifetime curves. Higher piconewton force requires binding through both complementarity determining region loops and hydrophobic Vβ patch apposition. This patch functions in the pre-TCR as a surrogate Vα domain, fostering ligand promiscuity to favor development of β chains with self-reactivity but is occluded by α subunit replacement of pTα upon αβTCR formation. At the double negative 3 thymocyte stage where the pre-TCR is first expressed, pre-TCR interaction with self-pMHC ligands imparts growth and survival advantages as revealed in thymic stromal cultures, imprinting fundamental self-reactivity in the T cell repertoire. Collectively, our data imply the existence of sequential mechanosensor αβTCR repertoire tuning via the pre-TCR.

Biophysical Tools for Cellular and Subcellular Mechanical Actuation of Cell Signaling

The ability to spatially control cell signaling can help resolve fundamental biological questions. Optogenetic and chemical dimerization techniques along with fluorescent biosensors to report cell signaling activities have enabled researchers to both visualize and perturb biochemistry in living cells. A number of approaches based on mechanical actuation using force-field gradients have emerged as complementary technologies to manipulate cell signaling in real time. This review covers several technologies, including optical, magnetic, and acoustic control of cell signaling and behavior and highlights some studies that have led to novel insights. I will also discuss some future direction on repurposing mechanosensitive channel for mechanical actuation of spatial cell signaling.

Ratiometric Tension Probes for Mapping Receptor Forces and Clustering at Intermembrane Junctions

Short-range communication between cells is required for the survival of multicellular organisms. One mechanism of chemical signaling between adjacent cells employs surface displayed ligands and receptors that only bind when two cells make physical contact. Ligand-receptor complexes that form at the cell-cell junction and physically bridge two cells likely experience mechanical forces. A fundamental challenge in this area pertains to mapping the mechanical forces experienced by ligand-receptor complexes within such a fluid intermembrane junction. Herein, we describe the development of ratiometric tension probes for direct imaging of receptor tension, clustering, and lateral transport within a model cell-cell junction. These probes employ two fluorescent reporters that quantify both the ligand density and the ligand tension and thus generate a tension signal independent of clustering. As a proof-of-concept, we applied the ratiometric tension probes to map the forces experienced by the T-cell receptor (TCR) during activation and showed the first direct evidence that the TCR-ligand complex experiences sustained pN forces within a fluid membrane junction. We envision that the ratiometric tension probes will be broadly useful for investigating mechanotransduction in juxtacrine signaling pathways.

T cell activation requires force generation

The T cell receptor requires force for triggering. Here, Hu and Butte show that T cells generate pushing and pulling forces against an antigen-coated AFM cantilever in an actin-dependent fashion. Exogenous, oscillating forces delivered by the cantilever rescued T cell receptor signaling in the absence of an intact F-actin cytoskeleton. These findings highlight the importance of mechanical forces in T cell activation.

Triggering of the T cell receptor (TCR) integrates both binding kinetics and mechanical forces. To understand the contribution of the T cell cytoskeleton to these forces, we triggered T cells using a novel application of atomic force microscopy (AFM). We presented antigenic stimulation using the AFM cantilever while simultaneously imaging with optical microscopy and measuring forces on the cantilever. T cells respond forcefully to antigen after calcium flux. All forces and calcium responses were abrogated upon treatment with an F-actin inhibitor. When we emulated the forces of the T cell using the AFM cantilever, even these actin-inhibited T cells became activated. Purely mechanical stimulation was not sufficient; the exogenous forces had to couple through the TCR. These studies suggest a mechanical–chemical feedback loop in which TCR-triggered T cells generate forceful contacts with antigen-presenting cells to improve access to antigen.

DNA-based nanoparticle tension sensors reveal that T-cell receptors transmit defined pN forces to their antigens for enhanced fidelity

T cells are triggered when the T-cell receptor (TCR) encounters its antigenic ligand, the peptide-major histocompatibility complex (pMHC), on the surface of antigen presenting cells (APCs). Because T cells are highly migratory and antigen recognition occurs at an intermembrane junction where the T cell physically contacts the APC, there are long-standing questions of whether T cells transmit defined forces to their TCR complex and whether chemomechanical coupling influences immune function. Here we develop DNA-based gold nanoparticle tension sensors to provide, to our knowledge, the first pN tension maps of individual TCR-pMHC complexes during T-cell activation. We show that naïve T cells harness cytoskeletal coupling to transmit 12-19 pN of force to their TCRs within seconds of ligand binding and preceding initial calcium signaling. CD8 coreceptor binding and lymphocyte-specific kinase signaling are required for antigen-mediated cell spreading and force generation. Lymphocyte function-associated antigen 1 (LFA-1) mediated adhesion modulates TCR-pMHC tension by intensifying its magnitude to values >19 pN and spatially reorganizes the location of TCR forces to the kinapse, the zone located at the trailing edge of migrating T cells, thus demonstrating chemomechanical crosstalk between TCR and LFA-1 receptor signaling. Finally, T cells display a dampened and poorly specific response to antigen agonists when TCR forces are chemically abolished or physically "filtered" to a level below ∼12 pN using mechanically labile DNA tethers. Therefore, we conclude that T cells tune TCR mechanics with pN resolution to create a checkpoint of agonist quality necessary for specific immune response.

Differential utilization of binding loop flexibility in T cell receptor ligand selection and cross-reactivity

Complementarity determining region (CDR) loop flexibility has been suggested to play an important role in the selection and binding of ligands by T cell receptors (TCRs) of the cellular immune system. However, questions remain regarding the role of loop motion in TCR binding, and crystallographic structures have raised questions about the extent to which generalizations can be made. Here we studied the flexibility of two structurally well characterized αβ TCRs, A6 and DMF5. We found that the two receptors utilize loop motion very differently in ligand binding and cross-reactivity. While the loops of A6 move rapidly in an uncorrelated fashion, those of DMF5 are substantially less mobile. Accordingly, the mechanisms of binding and cross-reactivity are very different between the two TCRs: whereas A6 relies on conformational selection to select and bind different ligands, DMF5 uses a more rigid, permissive architecture with greater reliance on slower motions or induced-fit. In addition to binding site flexibility, we also explored whether ligand-binding resulted in common dynamical changes in A6 and DMF5 that could contribute to TCR triggering. Although binding-linked motional changes propagated throughout both receptors, no common features were observed, suggesting that changes in nanosecond-level TCR structural dynamics do not contribute to T cell signaling.

Backbone resonance assignment of N15, N30 and D10 T cell receptor β subunits

The αβT Cell receptor (TCR) governs T cell immunity through its interaction with peptide bound to major histocompatibility complex molecules (pMHC). Previously, soluble ectodomain constructs have been used to elucidate the binding mode of the TCR for the MHC. However, the full heterodimeric αβTCR has proven difficult to produce reproducibly in recombinant systems to the extent seen in the routine production of novel antibodies. Particularly, the route of production in E. coli, which is most convenient for isotopic labeling of proteins, is challenging for a wide range of αβTCR, including N15αβ, N30αβ, but not D10αβ. With the aim of understanding the TCR-pMHC interaction through the use of dynamic binding measurements, we set out to produce TCRβ subunits with which we could investigate binding with pMHC. The TCRβ constructs are more readily produced and refolded than their αβ counterparts and have proven to be an effective model of preTCR in pMHC binding studies. As a first step towards characterizing potential interactions with protein ligands, we have assigned the backbone resonances of three TCRβ subunits, N15β, N30β and D10β.

Structural Model of the Extracellular Assembly of the TCR-CD3 Complex

Antigen recognition of peptide-major histocompatibility complexes (pMHCs) by T cells, a key step in initiating adaptive immune responses, is performed by the T cell receptor (TCR) bound to CD3 heterodimers. However, the biophysical basis of the transmission of TCR-CD3 extracellular interaction into a productive intracellular signaling sequence remains incomplete. Here we used nuclear magnetic resonance (NMR) spectroscopy combined with mutational analysis and computational docking to derive a structural model of the extracellular TCR-CD3 assembly. In the inactivated state, CD3γε interacts with the helix 3 and helix 4-F strand regions of the TCR Cβ subunit, whereas CD3δε interacts with the F and C strand regions of the TCR Cα subunit in this model, placing the CD3 subunits on opposing sides of the TCR. This work identifies the molecular contacts between the TCR and CD3 subunits, identifying a physical basis for transmitting an activating signal through the complex.

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.

Human Primary Immune Cells Exhibit Distinct Mechanical Properties that Are Modified by Inflammation

T lymphocytes are key modulators of the immune response. Their activation requires cell-cell interaction with different myeloid cell populations of the immune system called antigen-presenting cells (APCs). Although T lymphocytes have recently been shown to respond to mechanical cues, in particular to the stiffness of their environment, little is known about the rigidity of APCs. In this study, single-cell microplate assays were performed to measure the viscoelastic moduli of different human myeloid primary APCs, i.e., monocytes (Ms, storage modulus of 520 +90/-80 Pa), dendritic cells (DCs, 440 +110/-90 Pa), and macrophages (MPHs, 900 +110/-100 Pa). Inflammatory conditions modulated these properties, with storage moduli ranging from 190 Pa to 1450 Pa. The effect of inflammation on the mechanical properties was independent of the induction of expression of commonly used APC maturation markers, making myeloid APC rigidity an additional feature of inflammation. In addition, the rigidity of human T lymphocytes was lower than that of all myeloid cells tested and among the lowest reported (Young's modulus of 85 ± 5 Pa). Finally, the viscoelastic properties of myeloid cells were dependent on both their filamentous actin content and myosin IIA activity, although the relative contribution of these parameters varied within cell types. These results indicate that T lymphocytes face different cell rigidities when interacting with myeloid APCs in vivo and that this mechanical landscape changes under inflammation.

Biophysical Aspects of T Lymphocyte Activation at the Immune Synapse

T lymphocyte activation is a pivotal step of the adaptive immune response. It requires the recognition by T-cell receptors (TCR) of peptides presented in the context of major histocompatibility complex molecules (pMHC) present at the surface of antigen-presenting cells (APCs). T lymphocyte activation also involves engagement of costimulatory receptors and adhesion molecules recognizing ligands on the APC. Integration of these different signals requires the formation of a specialized dynamic structure: the immune synapse. While the biochemical and molecular aspects of this cell–cell communication have been extensively studied, its mechanical features have only recently been addressed. Yet, the immune synapse is also the place of exchange of mechanical signals. Receptors engaged on the T lymphocyte surface are submitted to many tensile and traction forces. These forces are generated by various phenomena: membrane undulation/protrusion/retraction, cell mobility or spreading, and dynamic remodeling of the actomyosin cytoskeleton inside the T lymphocyte. Moreover, the TCR can both induce force development, following triggering, and sense and convert forces into biochemical signals, as a bona fide mechanotransducer. Other costimulatory molecules, such as LFA-1, engaged during immune synapse formation, also display these features. Moreover, T lymphocytes themselves are mechanosensitive, since substrate stiffness can modulate their response. In this review, we will summarize recent studies from a biophysical perspective to explain how mechanical cues can affect T lymphocyte activation. We will particularly discuss how forces are generated during immune synapse formation; how these forces affect various aspects of T lymphocyte biology; and what are the key features of T lymphocyte response to stiffness.