Biphasic mechanosensitivity of T cell receptor-mediated spreading of lymphocytes

Morphodynamics of T-lymphocytes: Scanning to spreading

Binding of the T cell receptor complex to its ligand, the subsequent molecular rearrangement, and the concomitant cell-scale shape changes represent the very first steps of adaptive immune recognition. The first minutes of the interaction of T cells and antigen presenting cells have been extensively scrutinized; yet, gaps remain in our understanding of how the biophysical properties of the environment may impact the sequence of events. In particular, many pioneering experiments were done on immobilized ligands and gave major insights into the process of T cell activation, whereas later experiments have indicated that ligand mobility was of paramount importance, especially to enable the formation of T cell receptor clusters. Systematic experiments to compare and reconcile the two schools are still lacking. Furthermore, recent investigations using compliant substrates have elucidated other intriguing aspects of T cell mechanics. Here we review experiments on interaction of T cells with planar artificial antigen presenting cells to explore the impact of mechanics on adhesion and actin morphodynamics during the spreading process. We enumerate a sequence tracing first contact to final spread state that is consistent with current understanding. Finally, we interpret the presented experimental results in light of a mechanical model that captures all the different morphodynamic states.

Mechanical control of antigen detection and discrimination by T and B cell receptors

The adaptive immune response is orchestrated by just two cell types, T cells and B cells. Both cells possess the remarkable ability to recognize virtually any antigen through their respective antigen receptors-the T cell receptor (TCR) and B cell receptor (BCR). Despite extensive investigations into the biochemical signaling events triggered by antigen recognition in these cells, our ability to predict or control the outcome of T and B cell activation remains elusive. This challenge is compounded by the sensitivity of T and B cells to the biophysical properties of antigens and the cells presenting them-a phenomenon we are just beginning to understand. Recent insights underscore the central role of mechanical forces in this process, governing the conformation, signaling activity, and spatial organization of TCRs and BCRs within the cell membrane, ultimately eliciting distinct cellular responses. Traditionally, T cells and B cells have been studied independently, with researchers working in parallel to decipher the mechanisms of activation. While these investigations have unveiled many overlaps in how these cell types sense and respond to antigens, notable differences exist. To fully grasp their biology and harness it for therapeutic purposes, these distinctions must be considered. This review compares and contrasts the TCR and BCR, placing emphasis on the role of mechanical force in regulating the activity of both receptors to shape cellular and humoral adaptive immune responses.

Quantifying T cell receptor mechanics at membrane junctions using DNA origami tension sensors

The T cell receptor (TCR) is thought to be a mechanosensor, meaning that it transmits mechanical force to its antigen and leverages the force to amplify the specificity and magnitude of TCR signalling. Although a variety of molecular probes have been proposed to quantify TCR mechanics, these probes are immobilized on hard substrates, and thus fail to reveal fluid TCR-antigen interactions in the physiological context of cell membranes. Here we developed DNA origami tension sensors (DOTS) which bear force sensors on a DNA origami breadboard and allow mapping of TCR mechanotransduction at dynamic intermembrane junctions. We quantified the mechanical forces at fluid TCR-antigen bonds and observed their dependence on cell state, antigen mobility, antigen potency, antigen height and F-actin activity. The programmability of DOTS allows us to tether these to microparticles to mechanically screen antigens in high throughput using flow cytometry. Additionally, DOTS were anchored onto live B cells, allowing quantification of TCR mechanics at immune cell-cell junctions.

Mechanical regulation of lymphocyte activation and function

Mechanosensing, or how cells sense and respond to the physical environment, is crucial for many aspects of biological function, ranging from cell movement during development to cancer metastasis, the immune response and gene expression driving cell fate determination. Relevant physical stimuli include the stiffness of the extracellular matrix, contractile forces, shear flows in blood vessels, complex topography of the cellular microenvironment and membrane protein mobility. Although mechanosensing has been more widely studied in non-immune cells, it has become increasingly clear that physical cues profoundly affect the signaling function of cells of the immune system. In this Review, we summarize recent studies on mechanical regulation of immune cells, specifically lymphocytes, and explore how the force-generating cytoskeletal machinery might mediate mechanosensing. We discuss general principles governing mechanical regulation of lymphocyte function, spanning from the molecular scale of receptor activation to cellular responses to mechanical stimuli.

Improving regulatory T cell production through mechanosensing

Induced Tregs (iTregs) have great promise in adoptive immunotherapy for treatment of autoimmune diseases. This report investigates the impacts of substrate stiffness on human Treg induction, providing a powerful yet simple approach to improving production of these cells. Conventional CD4+ human T cells were activated on materials of different elastic modulus and cultured under suppressive conditions. Enhanced Treg induction was observed on softer materials as early as 3 days following activation and persisted for multiple weeks. Substrate stiffness also affected epigenetic modification of Treg specific genes and Treg suppressive capacity. Tregs induced on substrates of an optimal stiffness balance quantity and suppressive quality.

Optimization of polydimethylsiloxane (PDMS) surface chemical modification and formulation for improved T cell activation and expansion

Adoptive T cell therapy has undergone remarkable advancements in recent decades; nevertheless, the rapid and effective ex vivo expansion of tumor-reactive T cells remains a formidable challenge, limiting their clinical application. Artificial antigen-presenting substrates represent a promising avenue for enhancing the efficiency of adoptive immunotherapy and fostering T cell expansion. These substrates offer significant potential by providing flexibility and modularity in the design of tailored stimulatory environments. Polydimethylsiloxane (PDMS) silicone elastomer stands as a widely utilized biomaterial for exploring the varying sensitivity of T cell activation to substrate properties. This paper explores the optimization of PDMS surface modification and formulation to create customized stimulatory surfaces with the goal of enhancing T cell expansion. By employing soft PDMS elastomer functionalized through silanization and activating agent, coupled with site-directed protein immobilization techniques, a novel T cell stimulatory platform is introduced, facilitating T cell activation and proliferation. Notably, our findings underscore that softer modified elastomers (Young' modulus E∼300 kPa) exhibit superior efficacy in stimulating and activating mouse CD4+ T cells compared to their stiffer counterparts (E∼3 MPa). Furthermore, softened modified PDMS substrates demonstrate enhanced capabilities in T cell expansion and Th1 differentiation, offering promising insights for the advancement of T cell-based immunotherapy.

Surface chemical modification of poly(dimethylsiloxane) for stabilizing antibody immobilization and T cell cultures

Advances in cell immunotherapy underscore the need for effective methods to produce large populations of effector T cells, driving growing interest in T-cell bioprocessing and immunoengineering. Research suggests that T cells demonstrate enhanced expansion and differentiation on soft matrices in contrast to rigid ones. Nevertheless, the influence of antibody conjugation chemistry on these processes remains largely unexplored. In this study, we examined the effect of antibody conjugation chemistry on T cell activation, expansion and differentiation using a soft and biocompatible polydimethylsiloxane (PDMS) platform. We rigorously evaluated three distinct immobilization methods, beginning with the use of amino-silane (PDMS-NH2-Ab), followed by glutaraldehyde (PDMS-CHO-Ab) or succinic acid anhydride (PDMS-COOH-Ab) activation, in addition to the conventional physical adsorption (PDMS-Ab). By employing both stable amide bonds and reducible Schiff bases, antibody conjugation significantly enhanced antibody loading and density compared to physical adsorption. Furthermore, we discovered that the PDMS-COOH-Ab surface significantly promoted IL-2 secretion, CD69 expression, and T cell expansion compared to the other groups. Moreover, we observed that both PDMS-COOH-Ab and PDMS-NH2-Ab surfaces exhibited a tendency to induce the differentiation of naïve CD4+ T cells into Th1 cells, whereas the PDMS-Ab surface elicited a Th2-biased immunological response. These findings highlight the importance of antibody conjugation chemistry in the design and development of T cell culture biomaterials. They also indicate that PDMS holds promise as a material for constructing culture platforms to modulate T cell activation, proliferation, and differentiation.

Mechanical forces amplify TCR mechanotransduction in T cell activation and function

Adoptive T cell immunotherapies, including engineered T cell receptor (eTCR) and chimeric antigen receptor (CAR) T cell immunotherapies, have shown efficacy in treating a subset of hematologic malignancies, exhibit promise in solid tumors, and have many other potential applications, such as in fibrosis, autoimmunity, and regenerative medicine. While immunoengineering has focused on designing biomaterials to present biochemical cues to manipulate T cells ex vivo and in vivo, mechanical cues that regulate their biology have been largely underappreciated. This review highlights the contributions of mechanical force to several receptor-ligand interactions critical to T cell function, with central focus on the TCR-peptide-loaded major histocompatibility complex (pMHC). We then emphasize the role of mechanical forces in (i) allosteric strengthening of the TCR-pMHC interaction in amplifying ligand discrimination during T cell antigen recognition prior to activation and (ii) T cell interactions with the extracellular matrix. We then describe approaches to design eTCRs, CARs, and biomaterials to exploit TCR mechanosensitivity in order to potentiate T cell manufacturing and function in adoptive T cell immunotherapy.

Myosin-independent stiffness sensing by fibroblasts is regulated by the viscoelasticity of flowing actin

The stiffness of the extracellular matrix induces differential tension within integrin-based adhesions, triggering differential mechanoresponses. However, it has been unclear if the stiffness-dependent differential tension is induced solely by myosin activity. Here, we report that in the absence of myosin contractility, 3T3 fibroblasts still transmit stiffness-dependent differential levels of traction. This myosin-independent differential traction is regulated by polymerizing actin assisted by actin nucleators Arp2/3 and formin where formin has a stronger contribution than Arp2/3 to both traction and actin flow. Intriguingly, despite only slight changes in F-actin flow speed observed in cells with the combined inhibition of Arp2/3 and myosin compared to cells with sole myosin inhibition, they show a 4-times reduction in traction than cells with myosin-only inhibition. Our analyses indicate that traditional models based on rigid F-actin are inadequate for capturing such dramatic force reduction with similar actin flow. Instead, incorporating the F-actin network's viscoelastic properties is crucial. Our new model including the F-actin viscoelasticity reveals that Arp2/3 and formin enhance stiffness sensitivity by mechanically reinforcing the F-actin network, thereby facilitating more effective transmission of flow-induced forces. This model is validated by cell stiffness measurement with atomic force microscopy and experimental observation of model-predicted stiffness-dependent actin flow fluctuation.

T Cell Antigen Recognition and Discrimination by Electrochemiluminescence Imaging

Adoptive T lymphocyte (T cell) transfer and tumour-specific peptide vaccines are innovative cancer therapies. An accurate assessment of the specific reactivity of T cell receptors (TCRs) to tumour antigens is required because of the high heterogeneity of tumour cells and the immunosuppressive tumour microenvironment. In this study, we report a label-free electrochemiluminescence (ECL) imaging approach for recognising and discriminating between TCRs and tumour-specific antigens by imaging the immune synapses of T cells. Various T cell stimuli, including agonistic antibodies, auxiliary molecules, and tumour-specific antigens, were modified on the electrode's surface to allow for their interaction with T cells bearing different TCRs. The formation of immune synapses activated by specific stimuli produced a negative (shadow) ECL image, from which T cell antigen recognition and discrimination were evaluated by analysing the spreading area and the recognition intensity of T cells. This approach provides an easy way to assess TCR-antigen specificity and screen both of them for immunotherapies.

Probing mechanical interaction of immune receptors and cytoskeleton by membrane nanotube extraction

The role of force application in immune cell recognition is now well established, the force being transmitted between the actin cytoskeleton to the anchoring ligands through receptors such as integrins. In this chain, the mechanics of the cytoskeleton to receptor link, though clearly crucial, remains poorly understood. To probe this link, we combine mechanical extraction of membrane tubes from T cells using optical tweezers, and fitting of the resulting force curves with a viscoelastic model taking into account the cell and relevant molecules. We solicit this link using four different antibodies against various membrane bound receptors: antiCD3 to target the T Cell Receptor (TCR) complex, antiCD45 for the long sugar CD45, and two clones of antiCD11 targeting open or closed conformation of LFA1 integrins. Upon disruption of the cytoskeleton, the stiffness of the link changes for two of the receptors, exposing the existence of a receptor to cytoskeleton link-namely TCR-complex and open LFA1, and does not change for the other two where a weaker link was expected. Our integrated approach allows us to probe, for the first time, the mechanics of the intracellular receptor-cytoskeleton link in immune cells.

What's the Catch? The Significance of Catch Bonds in T Cell Activation

One of the main goals in T cell biology has been to investigate how TCR recognition of peptide:MHC (pMHC) determines T cell phenotype and fate. Ag recognition is required to facilitate survival, expansion, and effector function of T cells. Historically, TCR affinity for pMHC has been used as a predictor for T cell fate and responsiveness, but there have now been several examples of nonfunctional high-affinity clones and low-affinity highly functional clones. Recently, more attention has been paid to the TCR being a mechanoreceptor where the key biophysical determinant is TCR bond lifetime under force. As outlined in this review, the fundamental parameters between the TCR and pMHC that control Ag recognition and T cell triggering are affinity, bond lifetime, and the amount of force at which the peak lifetime occurs.

T Cell engineering for cancer immunotherapy by manipulating mechanosensitive force-bearing receptors

T cell immune responses are critical for in both physiological and pathological processes. While biochemical cues are important, mechanical cues arising from the microenvironment have also been found to act a significant role in regulating various T cell immune responses, including activation, cytokine production, metabolism, proliferation, and migration. The immune synapse contains force-sensitive receptors that convert these mechanical cues into biochemical signals. This phenomenon is accepted in the emerging research field of immunomechanobiology. In this review, we provide insights into immunomechanobiology, with a specific focus on how mechanosensitive receptors are bound and triggered, and ultimately resulting T cell immune responses.

DNA Origami Tension Sensors (DOTS) to study T cell receptor mechanics at membrane junctions

The T cell receptor (TCR) is thought to be a mechanosensor, meaning that it transmits mechanical force to its antigen and leverages the force to amplify the specificity and magnitude of TCR signaling. The past decade has witnessed the development of molecular probes which have revealed many aspects of receptor mechanotransduction. However, most force probes are immobilized on hard substrates, thus failing to reveal mechanics in the physiological context of cell membranes. In this report, we developed DNA origami tension sensors (DOTS) which bear force sensors on a DNA origami breadboard and allow mapping of TCR mechanotransduction at dynamic intermembrane junctions. We demonstrate that TCR-antigen bonds experience 5-10 pN forces, and the mechanical events are dependent on cell state, antigen mobility, antigen potency, antigen height and F-actin activity. We tethered DOTS onto a microparticle to mechanically screen antigen in high throughput using flow cytometry. Finally, DOTS were anchored onto live B cell membranes thus producing the first quantification of TCR mechanics at authentic immune cell-cell junctions.

Oxygen-Glucose Deprivation in Organotypic Hippocampal Cultures Leads to Cytoskeleton Rearrangement and Immune Activation: Link to the Potential Pathomechanism of Ischaemic Stroke

Ischaemic stroke is characterized by a sudden loss of blood circulation to an area of the brain, resulting in a corresponding loss of neurologic function. As a result of this process, neurons in the ischaemic core are deprived of oxygen and trophic substances and are consequently destroyed. Tissue damage in brain ischaemia results from a complex pathophysiological cascade comprising various distinct pathological events. Ischaemia leads to brain damage by stimulating many processes, such as excitotoxicity, oxidative stress, inflammation, acidotoxicity, and apoptosis. Nevertheless, less attention has been given to biophysical factors, including the organization of the cytoskeleton and the mechanical properties of cells. Therefore, in the present study, we sought to evaluate whether the oxygen-glucose deprivation (OGD) procedure, which is a commonly accepted experimental model of ischaemia, could affect cytoskeleton organization and the paracrine immune response. The abovementioned aspects were examined ex vivo in organotypic hippocampal cultures (OHCs) subjected to the OGD procedure. We measured cell death/viability, nitric oxide (NO) release, and hypoxia-inducible factor 1α (HIF-1α) levels. Next, the impact of the OGD procedure on cytoskeletal organization was evaluated using combined confocal fluorescence microscopy (CFM) and atomic force microscopy (AFM). Concurrently, to find whether there is a correlation between biophysical properties and the immune response, we examined the impact of OGD on the levels of crucial ischaemia cytokines (IL-1β, IL-6, IL-18, TNF-α, IL-10, IL-4) and chemokines (CCL3, CCL5, CXCL10) in OHCs and calculated Pearsons' and Spearman's rank correlation coefficients. The results of the current study demonstrated that the OGD procedure intensified cell death and nitric oxide release and led to the potentiation of HIF-1α release in OHCs. Moreover, we presented significant disturbances in the organization of the cytoskeleton (actin fibers, microtubular network) and cytoskeleton-associated protein 2 (MAP-2), which is a neuronal marker. Simultaneously, our study provided new evidence that the OGD procedure leads to the stiffening of OHCs and a malfunction in immune homeostasis. A negative linear correlation between tissue stiffness and branched IBA1 positive cells after the OGD procedure suggests the pro-inflammatory polarization of microglia. Moreover, the negative correlation of pro- and positive anti-inflammatory factors with actin fibers density indicates an opposing effect of the immune mediators on the rearrangement of cytoskeleton induced by OGD procedure in OHCs. Our study constitutes a basis for further research and provides a rationale for integrating biomechanical and biochemical methods in studying the pathomechanism of stroke-related brain damage. Furthermore, presented data pointed out the interesting direction of proof-of-concept studies, in which follow-up may establish new targets for brain ischemia therapy.

Mechano-modulation of T cells for cancer immunotherapy

Immunotherapy, despite its promise for future anti-cancer approach, faces significant challenges, such as off-tumor side effects, innate or acquired resistance, and limited infiltration of immune cells into stiffened extracellular matrix (ECM). Recent studies have highlighted the importance of mechano-modulation/-activation of immune cells (mainly T cells) for effective caner immunotherapy. Immune cells are highly sensitive to the applied physical forces and matrix mechanics, and reciprocally shape the tumor microenvironment. Engineering T cells with tuned properties of materials (e.g., chemistry, topography, and stiffness) can improve their expansion and activation ex vivo, and their ability to mechano-sensing the tumor specific ECM in vivo where they perform cytotoxic effects. T cells can also be exploited to secrete enzymes that soften ECM, thus increasing tumor infiltration and cellular therapies. Furthermore, T cells, such as chimeric antigen receptor (CAR)-T cells, genomic engineered to be spatiotemporally controllable by physical stimuli (e.g., ultrasound, heat, or light), can mitigate adverse off-tumor effects. In this review, we communicate these recent cutting-edge endeavors devoted to mechano-modulating/-activating T cells for effective cancer immunotherapy, and discuss future prospects and challenges in this field.

Tuning immunity through tissue mechanotransduction

Immune responses are governed by signals from the tissue microenvironment, and in addition to biochemical signals, mechanical cues and forces arising from the tissue, its extracellular matrix and its constituent cells shape immune cell function. Indeed, changes in biophysical properties of tissue alter the mechanical signals experienced by cells in many disease conditions, in inflammatory states and in the context of ageing. These mechanical cues are converted into biochemical signals through the process of mechanotransduction, and multiple pathways of mechanotransduction have been identified in immune cells. Such pathways impact important cellular functions including cell activation, cytokine production, metabolism, proliferation and trafficking. Changes in tissue mechanics may also represent a new form of 'danger signal' that alerts the innate and adaptive immune systems to the possibility of injury or infection. Tissue mechanics can change temporally during an infection or inflammatory response, offering a novel layer of dynamic immune regulation. Here, we review the emerging field of mechanoimmunology, focusing on how mechanical cues at the scale of the tissue environment regulate immune cell behaviours to initiate, propagate and resolve the immune response.

May the force be with your (immune) cells: an introduction to traction force microscopy in Immunology

For more than a couple of decades now, “force” has been recognized as an important physical parameter that cells employ to adapt to their microenvironment. Whether it is externally applied, or internally generated, cells use force to modulate their various actions, from adhesion and migration to differentiation and immune function. T lymphocytes use such mechano-sensitivity to decipher signals when recognizing cognate antigens presented on the surface of antigen presenting cells (APCs), a critical process in the adaptive immune response. As such, many techniques have been developed and used to measure the forces felt/exerted by these small, solitary and extremely reactive cells to decipher their influence on diverse T cell functions, primarily activation. Here, we focus on traction force microscopy (TFM), in which a deformable substrate, coated with the appropriate molecules, acts as a force sensor on the cellular scale. This technique has recently become a center of interest for many groups in the “ImmunoBiophysics” community and, as a consequence, has been subjected to refinements for its application to immune cells. Here, we present an overview of TFM, the precautions and pitfalls, and the most recent developments in the context of T cell immunology.

Immunoengineering strategies to enhance vascularization and tissue regeneration

Immune cells have emerged as powerful regulators of regenerative as well as pathological processes. The vast majority of regenerative immunoengineering efforts have focused on macrophages; however, growing evidence suggests that other cells of both the innate and adaptive immune system are as important for successful revascularization and tissue repair. Moreover, spatiotemporal regulation of immune cells and their signaling have a significant impact on the regeneration speed and the extent of functional recovery. In this review, we summarize the contribution of different types of immune cells to the healing process and discuss ways to manipulate and control immune cells in favor of vascularization and tissue regeneration. In addition to cell delivery and cell-free therapies using extracellular vesicles, we discuss in situ strategies and engineering approaches to attract specific types of immune cells and modulate their phenotypes. This field is making advances to uncover the extraordinary potential of immune cells and their secretome in the regulation of vascularization and tissue remodeling. Understanding the principles of immunoregulation will help us design advanced immunoengineering platforms to harness their power for tissue regeneration.

Cytotoxic T Lymphocyte Activation Signals Modulate Cytoskeletal Dynamics and Mechanical Force Generation

Cytotoxic T lymphocytes (CTLs) play an integral role in the adaptive immune response by killing infected cells. Antigen presenting cells (APCs), such as dendritic cells, present pathogenic peptides to the T cell receptor on the CTL surface and co-stimulatory signals required for complete activation. Activated CTLs secrete lytic granules containing enzymes that trigger target cell death at the CTL-target contact, also known as the immune synapse (IS). The actin and microtubule cytoskeletons are instrumental in the killing of CTL targets. Lytic granules are transported along microtubules to the IS, where granule secretion is facilitated by actin depletion and recovery. Furthermore, actomyosin contractility promotes target cell death by mediating mechanical force exertion at the IS. Recent studies have shown that inflammatory cytokines produced by APCs, such as interleukin-12 (IL-12), act as a third signal for CTL activation and enhance CTL proliferation and effector function. However, the biophysical mechanisms mediating such enhanced effector function remain unclear. We hypothesized that the third signal for CTL activation, IL-12, modulates cytoskeletal dynamics and force exertion at the IS, thus potentiating CTL effector function. Here, we used live cell total internal reflection fluorescence (TIRF) microscopy to study actomyosin and microtubule dynamics at the IS of murine primary CTLs activated in the presence of peptide-MHC and co-stimulation alone (two signals), or additionally with IL-12 (three signals). We found that three signal-activated CTLs have altered actin flows, myosin dynamics and microtubule growth rates as compared to two signal-activated CTLs. We further showed that lytic granules in three-signal activated CTLs are less clustered and have lower velocities than in two-signal activated CTLs. Finally, we used traction force microscopy to show that three signal-activated CTLs exert greater traction forces than two signal-activated CTLs. Our results demonstrate that activation of CTLs in the presence of IL-12 leads to differential modulation of the cytoskeleton, thereby augmenting the mechanical response of CTLs to their targets. This indicates a potential physical mechanism via which the third signal can enhance the CTL response.

Biphasic effect of mechanical stress on lymphocyte activation

Mechanical forces can modulate the immune response, mostly described as promoting the activation of immune cells, but the role and mechanism of pathological levels of mechanical stress in lymphocyte activation have not been focused on before. By an ex vivo experimental approach, we observed that mechanical stressing of murine spleen lymphocytes with 50 mmHg for 3 h induced the nuclear localization of NFAT1, increased C-Jun, and increased the expression of early activation marker CD69 in resting CD8+ cells. Interestingly, 50 mmHg mechanical stressing induced the nuclear localization of NFAT1; but conversely decreased C-Jun and inhibited the expression of CD69 in lymphocytes under lipopolysaccharide or phorbol 12-myristate 13-acetate/ionomycin stimulation. Additionally, we observed similar changes trends when comparing RNA-seq data of hypertensive and normotensive COVID-19 patients. Our results indicate a biphasic effect of mechanical stress on lymphocyte activation, which provides insight into the variety of immune responses in pathologies involving elevated mechanical stress.

Ion channel mediated mechanotransduction in immune cells

The immune system performs critical functions to defend against invading pathogens and maintain tissue homeostasis. Immune cells reside within or are recruited to a host of mechanically active tissues throughout the body and, as a result, are exposed to varying types and degrees of mechanical stimuli. Despite their abundance in such tissues, the role of mechanical stimuli in influencing immune cell function and the molecular mechanisms responsible for mechanics-mediated changes are still poorly understood. The recent emergence of mechanically-gated ion channels, particularly Piezo1, has provided an exciting avenue of research within the fields of mechanobiology and immunology. Numerous studies have identified roles for mechanically-gated ion channels in mechanotransduction within various different cell types, with a few recent studies in immune cells. These initial studies provide strong evidence that mechanically-gated ion channels play pivotal roles in regulating the immune system. In this review, we discuss characteristics of ion channel mediated force transduction, review the current techniques used to quantify and visualize ion channel activity in response to mechanical stimuli, and finally we provide an overview of recent studies examining the role of mechanically-gated ion channels in modulating immune cell function.

Mechanotransduction as a major driver of cell behaviour: mechanisms, and relevance to cell organization and future research

How do cells process environmental cues to make decisions? This simple question is still generating much experimental and theoretical work, at the border of physics, chemistry and biology, with strong implications in medicine. The purpose of mechanobiology is to understand how biochemical and physical cues are turned into signals through mechanotransduction. Here, we review recent evidence showing that (i) mechanotransduction plays a major role in triggering signalling cascades following cell-neighbourhood interaction; (ii) the cell capacity to continually generate forces, and biomolecule properties to undergo conformational changes in response to piconewton forces, provide a molecular basis for understanding mechanotransduction; and (iii) mechanotransduction shapes the guidance cues retrieved by living cells and the information flow they generate. This includes the temporal and spatial properties of intracellular signalling cascades. In conclusion, it is suggested that the described concepts may provide guidelines to define experimentally accessible parameters to describe cell structure and dynamics, as a prerequisite to take advantage of recent progress in high-throughput data gathering, computer simulation and artificial intelligence, in order to build a workable, hopefully predictive, account of cell signalling networks.

Probing T Cell 3D Mechanosensing With Magnetically-Actuated Structures

The ability of cells to recognize and respond to the mechanical properties of their environment is of increasing importance in T cell physiology. However, initial studies in this direction focused on planar hydrogel and elastomer surfaces, presenting several challenges in interpretation including difficulties in separating mechanical stiffness from changes in chemistry needed to modulate this property. We introduce here the use of magnetic fields to change the structural rigidity of microscale elastomer pillars loaded with superparamagnetic nanoparticles, independent of substrate chemistry. This magnetic modulation of rigidity, embodied as the pillar spring constant, changed the interaction of mouse naïve CD4⁺ T cells from a contractile morphology to one involving deep embedding into the array. Furthermore, increasing spring constant was associated with higher IL-2 secretion, showing a functional impact on mechanosensing. The system introduced here thus separates local substrate stiffness and long-range structural rigidity, revealing new facets of T cell interaction with their environment.

Journey to the Center of the Cell: Cytoplasmic and Nuclear Actin in Immune Cell Functions

Actin cytoskeletal dynamics drive cellular shape changes, linking numerous cell functions to physiological and pathological cues. Mutations in actin regulators that are differentially expressed or enriched in immune cells cause severe human diseases known as primary immunodeficiencies underscoring the importance of efficienct actin remodeling in immune cell homeostasis. Here we discuss recent findings on how immune cells sense the mechanical properties of their environement. Moreover, while the organization and biochemical regulation of cytoplasmic actin have been extensively studied, nuclear actin reorganization is a rapidly emerging field that has only begun to be explored in immune cells. Based on the critical and multifaceted contributions of cytoplasmic actin in immune cell functionality, nuclear actin regulation is anticipated to have a large impact on our understanding of immune cell development and functionality.

T cell stiffness is enhanced upon formation of immunological synapse

T cells are activated by target cells via an intimate contact, termed immunological synapse (IS). Cellular mechanical properties, especially stiffness, are essential to regulate cell functions. However, T cell stiffness at a subcellular level at the IS still remains largely elusive. In this work, we established an atomic force microscopy (AFM)-based elasticity mapping method on whole T cells to obtain an overview of the stiffness with a resolution of ~60 nm. Using primary human CD4⁺ T cells, we show that when T cells form IS with stimulating antibody-coated surfaces, the lamellipodia are stiffer than the cell body. Upon IS formation, T cell stiffness is enhanced both at the lamellipodia and on the cell body. Chelation of intracellular Ca²⁺ abolishes IS-induced stiffening at the lamellipodia but has no influence on cell-body-stiffening, suggesting different regulatory mechanisms of IS-induced stiffening at the lamellipodia and the cell body.

Programmable DNA-augmented hydrogels for controlled activation of human lymphocytes

Contractile forces within the planar interface between T cell and antigen-presenting surface mechanically stimulate T cell receptors (TCR) in the mature immune synapses. However, the origin of mechanical stimulation during the initial, i.e., presynaptic, microvilli-based TCR activation in the course of immune surveillance remains unknown and new tools to help address this problem are needed. In this work, we develop nucleic acid nanoassembly (NAN)-based technology for functionalization of hydrogels using isothermal toehold-mediated reassociation of RNA/DNA heteroduplexes. Resulting platform allows for regulation with NAN linkers of 3D force momentum along the TCR mechanical axis, whereas hydrogels contribute to modulation of 2D shear modulus. By utilizing different lengths of NAN linkers conjugated to polyacrylamide gels of different shear moduli, we demonstrate an efficient capture of human T lymphocytes and tunable activation of TCR, as confirmed by T-cell spreading and pY foci.

Integrin cross-talk modulates stiffness-independent motility of CD4+ T lymphocytes

To carry out their physiological responsibilities, CD4+ T lymphocytes interact with various tissues of different mechanical properties. Recent studies suggest that T cells migrate upstream on surfaces expressing intracellular adhesion molecule-1 (ICAM-1) through interaction with leukocyte function-associated antigen-1 (α_Lβ₂) (LFA-1) integrins. LFA-1 likely behaves as a mechanosensor, and thus we hypothesized that substrate mechanics might affect the ability of LFA-1 to support upstream migration of T cells under flow. Here we measured motility of CD4+ T lymphocytes on polyacrylamide gels with predetermined stiffnesses containing ICAM-1, vascular cell adhesion molecule-1 (VCAM-1), or a 1:1 mixture of VCAM-1/ICAM-1. Under static conditions, we found that CD4+ T cells exhibit an increase in motility on ICAM-1, but not on VCAM-1 or VCAM-1/ICAM-1 mixed, surfaces as a function of matrix stiffness. The mechanosensitivity of T-cell motility on ICAM-1 is overcome when VLA-4 (very late antigen-4 [α₄β₁]) is ligated with soluble VCAM-1. Last, we observed that CD4+ T cells migrate upstream under flow on ICAM-1-functionalized hydrogels, independent of substrate stiffness. In summary, we show that CD4+ T cells under no flow respond to matrix stiffness through LFA-1, and that the cross-talk of VLA-4 and LFA-1 can compensate for deformable substrates. Interestingly, CD4+ T lymphocytes migrated upstream on ICAM-1 regardless of the substrate stiffness, suggesting that flow can compensate for substrate stiffness.

Mechanobiology of T Cell Activation: To Catch a Bond

T cell activation is a critical event in the adaptive immune response, indispensable for cell-mediated and humoral immunity as well as for immune regulation. Recent years have witnessed an emerging trend emphasizing the essential role that physical force and mechanical properties play at the T cell interface. In this review, we integrate current knowledge of T cell antigen recognition and the different models of T cell activation from the perspective of mechanobiology, focusing on the interaction between the T cell receptor (TCR) and the peptide-major histocompatibility complex (pMHC) antigen. We address the shortcomings of TCR affinity alone in explaining T cell functional outcomes and the rising status of force-regulated TCR bond lifetimes, most notably the TCR catch bond. Ultimately, T cell activation and the ensuing physiological responses result from mechanical interaction between TCRs and the pMHC. Expected final online publication date for the Annual Review of Cell and Developmental Biology, Volume 37 is October 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Biphasic response of T cell activation to substrate stiffness

T cell activation is sensitive to the mechanical properties of an activating substrate. However, there are also contrasting results on how substrate stiffness affects T cell activation, including differences between T cells of mouse and human origin. Towards reconciling these differences, this report examines the response of primary human T cells to polyacrylamide gels with stiffness between 5 and 110 kPa presenting activating antibodies to CD3 and CD28. T cell proliferation and IL-2 secretion exhibited a biphasic functional response to substrate stiffness, which can be shifted by changing density of activating antibodies and abrogated by inhibition of cellular contractility. T cell morphology was modulated by stiffness at early time points. RNA-seq indicates that T cells show differing monotonic trends in upregulated genes and pathways towards both ends of the stiffness spectrum. These studies provide a framework of T cell mechanosensing and suggest an effect of ligand density that may reconcile different, contrasting patterns of stiffness sensing seen in previous studies.

Controlling T cells spreading, mechanics and activation by micropatterning

We designed a strategy, based on a careful examination of the activation capabilities of proteins and antibodies used as substrates for adhering T cells, coupled to protein microstamping to control at the same time the position, shape, spreading, mechanics and activation state of T cells. Once adhered on patterns, we examined the capacities of T cells to be activated with soluble anti CD3, in comparison to T cells adhered to a continuously decorated substrate with the same density of ligands. We show that, in our hand, adhering onto an anti CD45 antibody decorated surface was not affecting T cell calcium fluxes, even adhered on variable size micro-patterns. Aside, we analyzed the T cell mechanics, when spread on pattern or not, using Atomic Force Microscopy indentation. By expressing MEGF10 as a non immune adhesion receptor in T cells we measured the very same spreading area on PLL substrates and Young modulus than non modified cells, immobilized on anti CD45 antibodies, while retaining similar activation capabilities using soluble anti CD3 antibodies or through model APC contacts. We propose that our system is a way to test activation or anergy of T cells with defined adhesion and mechanical characteristics, and may allow to dissect fine details of these mechanisms since it allows to observe homogenized populations in standardized T cell activation assays.

Determination by Relaxation Tests of the Mechanical Properties of Soft Polyacrylamide Gels Made for Mechanobiology Studies

Following the general aim of recapitulating the native mechanical properties of tissues and organs in vitro, the field of materials science and engineering has benefited from recent progress in developing compliant substrates with physical and chemical properties similar to those of biological materials. In particular, in the field of mechanobiology, soft hydrogels can now reproduce the precise range of stiffnesses of healthy and pathological tissues to study the mechanisms behind cell responses to mechanics. However, it was shown that biological tissues are not only elastic but also relax at different timescales. Cells can, indeed, perceive this dissipation and actually need it because it is a critical signal integrated with other signals to define adhesion, spreading and even more complicated functions. The mechanical characterization of hydrogels used in mechanobiology is, however, commonly limited to the elastic stiffness (Young's modulus) and this value is known to depend greatly on the measurement conditions that are rarely reported in great detail. Here, we report that a simple relaxation test performed under well-defined conditions can provide all the necessary information for characterizing soft materials mechanically, by fitting the dissipation behavior with a generalized Maxwell model (GMM). The simple method was validated using soft polyacrylamide hydrogels and proved to be very useful to readily unveil precise mechanical properties of gels that cells can sense and offer a set of characteristic values that can be compared with what is typically reported from microindentation tests.

A Multimodal Platform for Simultaneous T-Cell Imaging, Defined Activation, and Mechanobiological Characterization

T-cell antigen recognition is accompanied by extensive morphological rearrangements of the contact zone between the T-cell and the antigen-presenting cell (APC). This process involves binding of the T-cell receptor (TCR) complex to antigenic peptides presented via MHC on the APC surface, the interaction of costimulatory and adhesion proteins, remodeling of the actin cytoskeleton, and the initiation of downstream signaling processes such as the release of intracellular calcium. However, multiparametric time-resolved analysis of these processes is hampered by the difficulty in recording the different readout modalities at high quality in parallel. In this study, we present a platform for simultaneous quantification of TCR distribution via total internal reflection fluorescence microscopy, of intracellular calcium levels, and of T-cell-exerted forces via atomic force microscopy (AFM). In our method, AFM cantilevers were used to bring single T-cells into contact with the activating surface. We designed the platform specifically to enable the study of T-cell triggering via functionalized fluid-supported lipid bilayers, which represent a widely accepted model system to stimulate T-cells in an antigen-specific manner. In this paper, we showcase the possibilities of this platform using primary transgenic T-cells triggered specifically via their cognate antigen presented by MHCII.

Micropatterned soft hydrogels to study the interplay of receptors and forces in T cell activation

The analysis of T cell responses to mechanical properties of antigen presenting cells (APC) is experimentally challenging at T cell-APC interfaces. Soft hydrogels with adjustable mechanical properties and biofunctionalization are useful reductionist models to address this problem. Here, we report a methodology to fabricate micropatterned soft hydrogels with defined stiffness to form spatially confined T cell/hydrogel contact interfaces at micrometer scale. Using automatized microcontact printing we prepared arrays of anti-CD3 microdots on poly(acrylamide) hydrogels with Young's Modulus in the range of 2 to 50 kPa. We optimized the printing process to obtain anti-CD3 microdots with constant area (50 µm², corresponding to 8 µm diameter) and comparable anti-CD3 density on hydrogels of different stiffness. The anti-CD3 arrays were recognized by T cells and restricted cell attachment to the printed areas. To test functionality of the hydrogel-T cell contact, we analyzed several key events downstream of T cell receptor (TCR) activation. Anti-CD3 arrays on hydrogels activated calcium influx, induced rearrangement of the actin cytoskeleton, and led to Zeta-chain-associated protein kinase 70 (ZAP70) phosphorylation. Interestingly, upon increase in the stiffness, ZAP70 phosphorylation was enhanced, whereas the rearrangements of F-actin (F-actin clearance) and phosphorylated ZAP70 (ZAP70/pY centralization) were unaffected. Our results show that micropatterned hydrogels allow tuning of stiffness and receptor presentation to analyze TCR mediated T cell activation as function of mechanical, biochemical, and geometrical parameters.

Mechanical Immunoengineering of T cells for Therapeutic Applications

T cells, a key component in adaptive immunity, are central to many immunotherapeutic modalities aimed at treating various diseases including cancer, infectious diseases, and autoimmune disorders. The past decade has witnessed tremendous progress in immunotherapy, which aims at activation or suppression of the immune responses for disease treatments. Most strikingly, cancer immunotherapy has led to curative responses in a fraction of patients with relapsed or refractory cancers. However, extending those clinical benefits to a majority of cancer patients remains challenging. In order to improve both efficacy and safety of T cell-based immunotherapies, significant effort has been devoted to modulating biochemical signals to enhance T cell proliferation, effector functions, and longevity. Such strategies include discovery of new immune checkpoints, design of armored chimeric antigen receptor (CAR) T cells, and targeted delivery of stimulatory cytokines and so on.Despite the intense global research effort in developing novel cancer immunotherapies, a major dimension of the interactions between cancer and the immune system, its biomechanical aspect, has been largely underappreciated. Throughout their lifecycle, T cells constantly survey a multitude of organs and tissues and experience diverse biomechanical environments, such as shear force in the blood flow and a broad range of tissue stiffness. Furthermore, biomechanical properties of tissues or cells may be altered in disease and inflammation. Biomechanical cues, including both passive mechanical cues and active mechanical forces, have been shown to govern T cell development, activation, migration, differentiation, and effector functions. In other words, T cells can sense, respond to, and adapt to both passive mechanical cues and active mechanical forces.Biomechanical cues have been intensively studied at a fundamental level but are yet to be extensively incorporated in the design of immunotherapies. Nonetheless, the growing knowledge of T cell mechanobiology has formed the basis for the development of novel engineering strategies to mechanically modulate T cell immunity, a nascent field that we termed "mechanical immunoengineering". Mechanical immunoengineering exploits biomechanical cues (e.g., stiffness and external forces) to modulate T cell differentiation, proliferation, effector functions, etc., for diagnostic or therapeutic applications. It provides an additional dimension, complementary to traditional modulation of biochemical cues (e.g., antigen density and co-stimulatory signals), to tailor T cell immune responses and enhance therapeutic outcomes. For example, stiff antigen-presenting matrices have been shown to enhance T cell proliferation independently of the intensity of biochemical stimulatory signals. Current strategies of mechanical immunoengineering of T cells can be categorized into two major fields including passive mechanical cue-oriented and active force-oriented strategies. In this Account, we first present a brief overview of T cell mechanobiology. Next, we summarize recent advances in mechanical immunoengineering, discuss the roles of chemistry and material science in the development of these engineering strategies, and highlight potential therapeutic applications. Finally, we present our perspective on the future directions in mechanical immunoengineering and critical steps to translate mechanical immunoengineering strategies into therapeutic applications in the clinic.

A Hydrogel-Integrated Culture Device to Interrogate T Cell Activation with Physicochemical Cues

The recent rise of adoptive T cell therapy (ATCT) as a promising cancer immunotherapy has triggered increased interest in therapeutic T cell bioprocessing. T cell activation is a critical processing step and is known to be modulated by physical parameters, such as substrate stiffness. Nevertheless, relatively little is known about how biophysical factors regulate immune cells, such as T cells. Understanding how T cell activation is modulated by physical and biochemical cues may offer novel methods to control cell behavior for therapeutic cell processing. Inspired by T cell mechanosensitivity, we developed a multiwell, reusable, customizable, two-dimensional (2D) polyacrylamide (PA) hydrogel-integrated culture device to study the physicochemical stimulation of Jurkat T cells. Substrate stiffness and ligand density were tuned by concentrations of the hydrogel cross-linker and antibody in the coating solution, respectively. We cultured Jurkat T cells on 2D hydrogels of different stiffnesses that presented surface-immobilized stimulatory antibodies against CD3 and CD28 and demonstrated that Jurkat T cells stimulated by stiff hydrogels (50.6 ± 15.1 kPa) exhibited significantly higher interleukin-2 (IL-2) secretion, but lower proliferation, than those stimulated by softer hydrogels (7.1 ± 0.4 kPa). In addition, we found that increasing anti-CD3 concentration from 10 to 30 μg/mL led to a significant increase in IL-2 secretion from cells stimulated on 7.1 ± 0.4 and 9.3 ± 2.4 kPa gels. Simultaneous tuning of substrate stiffness and stimulatory ligand density showed that the two parameters synergize (two-way ANOVA interaction effect: p < 0.001) to enhance IL-2 secretion. Our results demonstrate the importance of physical parameters in immune cell stimulation and highlight the potential of designing future immunostimulatory biomaterials that are mechanically tailored to balance stimulatory strength and downstream proliferative capacity of therapeutic T cells.

T Cell Membrane Heterogeneity Aids Antigen Recognition and T Cell Activation

T cells are critical for co-ordinating the immune response. T cells are activated when their surface T cell receptors (TCRs) engage cognate antigens in the form of peptide-major histocompatibility complexes (pMHC) presented on the surface of antigen presenting cells (APCs). Large changes in the contact interface between T cells and APCs occur over the course of tens of minutes from the initial contact to the formation of a large-scale junction between the two cells. The mature junction between a T cell and APC is known as the immunological synapse, and this specialized plasma membrane structure is the major platform for TCR signaling. It has long been known that the complex organization of signaling molecules at the synapse is critical for appropriate activation of T cells, but within the last decade advances in microscopy have opened up investigation into the dynamics of T cell surface topology in the immune synapse. From mechanisms mediating the initial contact between T cells and APCs to roles in the organization of molecules in the mature synapse, these studies have made it increasingly clear that local membrane topology has a large impact on signaling processes. This review focuses on the functional consequences of the T cells' highly dynamic and heterogeneous membrane, in particular, how membrane topology leads to the reorganization of membrane proteins on the T cell surface.

Colorectal cysts as a validating tool for CAR therapy

Background

Treatment of cancers has largely benefited from the development of immunotherapy. In particular, Chimeric Antigen Receptor (CAR) redirected T cells have demonstrated impressive efficacy against B-cell malignancies and continuous efforts are made to adapt this new therapy to solid tumors, where the immunosuppressive tumor microenvironment is a barrier for delivery. CAR T-cell validation relies on in vitro functional assays using monolayer or suspension cells and in vivo xenograft models in immunodeficient animals. However, the efficacy of CAR therapies remains difficult to predict with these systems, in particular when challenged against 3D organized solid tumors with highly intricate microenvironment. An increasing number of reports have now included an additional step in the development process in which redirected T cells are tested against tumor spheres.

Results

Here, we report a method to produce 3D structures, or cysts, out of a colorectal cancer cell line, Caco-2, which has the ability to form polarized spheroids as a validation tool for adoptive cell therapy in general. We used CD19CAR T cells to explore this method and we show that it can be adapted to various platforms including high resolution microscopy, bioluminescence assays and high-throughput live cell imaging systems.

Conclusion

We developed an affordable, reliable and practical method to produce cysts to validate therapeutic CAR T cells. The integration of this additional layer between in vitro and in vivo studies could be an important tool in the pre-clinical workflow of cell-based immunotherapy.

Artificial Biosystem for Modulation of Interactions between Antigen-Presenting Cells and T Cells

T cell activation is triggered by signal molecules on the surface of antigen-presenting cells (APC) and subsequent exertion of cellular forces. Deciphering the biomechanical and biochemical signals in this complex process is of interest and will contribute to an improvement in immunotherapy strategies. To address underlying questions, coculture and biomimetic models are established. Mature dendritic cells (mDC) are first treated with cytochalasin B (CytoB), a cytoskeletal disruption agent known to lower apparent cellular stiffness and reduction in T cell proliferation is observed. It is attempted to mimic mDC and T cell interactions using polyacrylamide (PA) gels with defined stiffness corresponding to mDC (0.2-25 kPa). Different ratios of anti-CD3 (aCD3) and anti-CD28 (aCD28) antibodies are immobilized onto PA gels. The results show T cell proliferation is triggered by both aCD3 and aCD28 in a stiffness-dependent manner. Cells cultured on aCD3 immobilized on gels has significantly enhanced proliferation and IL-2 secretion, compared to aCD28. Furthermore, ZAP70 phosphorylation is enhanced in stiffer substrate a in a aCD3-dependent manner. The biosystem provides an approach to study the reduction of T cell proliferation observed on CytoB-treated mDC. Overall, the biosystem allows distinguishing the impact of biophysical and biochemical signals of APC and T cell interactions in vitro.

T-cell activation is modulated by the 3D mechanical microenvironment

T cells recognize mechanical forces through a variety of cellular pathways, including mechanical triggering of both the T-cell receptor (TCR) and integrin LFA-1. Here we show that T cells can recognize forces arising from the mechanical rigidity of the microenvironment. We fabricated 3D scaffold matrices with mechanical stiffness tuned to the range 4-40 kPa and engineered them to be microporous, independently of stiffness. We cultured T cells and antigen presenting cells within the matrices and studied T-cell activation by flow cytometry and live-cell imaging. We found that there was an augmentation of T-cell activation, proliferation, and migration speed in the context of mechanically stiffer 3D matrices as compared to softer materials. These results show that T cells can sense their 3D mechanical environment and alter both their potential for activation and their effector responses in different mechanical environments. A 3D scaffold of tunable stiffness and consistent microporosity offers a biomaterial advancement for both translational applications and reductionist studies on the impact of tissue microenvironmental factors on cellular behavior.

Augmentation of T-Cell Activation by Oscillatory Forces and Engineered Antigen-Presenting Cells

Activation of T cells by antigen presenting cells (APCs) initiates their proliferation, cytokine production, and killing of infected or cancerous cells. We and others have shown that T-cell receptors require mechanical forces for triggering, and these forces arise during the interaction of T cells with APCs. Efficient activation of T cells in vitro is necessary for clinical applications. In this paper, we studied the impact of combining mechanical, oscillatory movements provided by an orbital shaker with soft, biocompatible, artificial APCs (aAPCs) of various sizes and amounts of antigen. We showed that these aAPCs allow for testing the strength of signal delivered to T cells, and enabled us to confirm that that absolute amounts of antigen engaged by the T cell are more important for activation than the density of antigen. We also found that when our aAPCs interact with T cells in the context of an oscillatory mechanoenvironment, they roughly double antigenic signal strength, compared to conventional, static culture. Combining these effects, our aAPCs significantly outperformed the commonly used Dynabeads. We finally demonstrated that tuning the signal strength down to a submaximal "sweet spot" allows for robust expansion of induced regulatory T cells. In conclusion, augmenting engineered aAPCs with mechanical forces offers a novel approach for tuning of T-cell activation and differentiation.

Mechanosensing through immunoreceptors

The immune response is orchestrated by a variety of immune cells. The function of each cell is determined by the collective signals from various immunoreceptors, whose expression and activity depend on the developmental stages of the cell and its environmental context. Recent studies have highlighted the presence of mechanical force on several immunoreceptor-ligand pairs and the important role of force in regulating their interaction and function. In this Perspective, we use the T cell antigen receptor as an example with which to review the current understanding of the mechanosensing properties of immunoreceptors. We discuss the types of forces that immunoreceptors may encounter and the effects of force on ligand bonding, conformational change and the triggering of immunoreceptors, as well as the effects of force on the downstream signal transduction, cell-fate decisions and effector function of immune cells.

T cell activation and immune synapse organization respond to the microscale mechanics of structured surfaces

Cells have the remarkable ability to sense the mechanical stiffness of their surroundings. This has been studied extensively in the context of cells interacting with planar surfaces, a conceptually elegant model that also has application in biomaterial design. However, physiological interfaces are spatially complex, exhibiting topographical features that are described over multiple scales. This report explores mechanosensing of microstructured elastomer surfaces by CD4⁺ T cells, key mediators of the adaptive immune response. We show that T cells form complex interactions with elastomer micropillar arrays, extending processes into spaces between structures and forming local areas of contraction and expansion dictated by the layout of microtubules within this interface. Conversely, cytoskeletal reorganization and intracellular signaling are sensitive to the pillar dimensions and flexibility. Unexpectedly, these measures show different responses to substrate rigidity, suggesting competing processes in overall T cell mechanosensing. The results of this study demonstrate that T cells sense the local rigidity of their environment, leading to strategies for biomaterial design.

Distinct actin cytoskeleton behaviour in primary and immortalised T-cells

Cytoskeletal actin dynamics are crucial for the activation of T-cells. Immortalised Jurkat T-cells have been the model system of choice to examine and correlate the dynamics of the actin cytoskeleton and the immunological synapse leading to T-cell activation. However, it has remained unclear whether immortalised cellular systems, such as Jurkat T-cells can recapitulate the cytoskeletal behaviour of primary T-cells. Studies delineating the cytoskeletal behaviour of Jurkat T-cells in comparison to primary T-cells are lacking. Here, we employ live-cell super-resolution microscopy to investigate the cytoskeletal actin organisation and dynamics of living primary and immortalised Jurkat T-cells at the appropriate spatiotemporal resolution. Under comparable activation conditions, we found differences in the architectural organisation and dynamics of Jurkat and primary mouse and human T-cells. Although the three main actin network architectures in Jurkat T-cells were reminiscent of primary T-cells, there were differences in the organisation and molecular mechanisms underlying these networks. Our results highlight mechanistic distinctions in the T-cell model system most utilised to study cytoskeletal actin dynamics.

Summary: The emerging idea that the cytoskeletal and biophysical principles are preserved in primary cells and transformed cell lines, and the two can be used to interchangeably examine synaptic actin characteristics, needs careful reconsideration.

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.