Ligand-mediated friction determines morphodynamics of spreading T cells

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.

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.

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.

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.

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.

Mechanosurveillance: Tiptoeing T Cells

Efficient scanning of tissue that T cells encounter during their migratory life is pivotal to protective adaptive immunity. In fact, T cells can detect even a single antigenic peptide/MHC complex (pMHC) among thousands of structurally similar yet non-stimulatory endogenous pMHCs on the surface of antigen-presenting cells (APCs) or target cells. Of note, the glycocalyx of target cells, being composed of proteoglycans and bulky proteins, is bound to affect and even modulate antigen recognition by posing as a physical barrier. T cell-resident microvilli are actin-rich membrane protrusions that puncture through such barriers and thereby actively place the considerably smaller T-cell antigen receptors (TCRs) in close enough proximity to APC-presented pMHCs so that productive interactions may occur efficiently yet under force. We here review our current understanding of how the plasticity of T-cell microvilli and physicochemical properties of the glycocalyx may affect early events in T-cell activation. We assess insights gained from studies on T-cell plasma membrane ultrastructure and provide an update on current efforts to integrate biophysical aspects such as the amplitude and directionality of TCR-imposed mechanical forces and the distribution and lateral mobility of plasma membrane-resident signaling molecules into a more comprehensive view on sensitized T-cell antigen recognition.

Ligand Nanocluster Array Enables Artificial-Intelligence-Based Detection of Hidden Features in T-Cell Architecture

Protein patterning has emerged as a powerful means to interrogate adhering cells. However, the tools to apply a sub-micrometer periodic stimulus and the analysis of the response are still being standardized. We propose a technique combining electron beam lithography and surface functionalization to fabricate nanopatterns compatible with advanced imaging. The repetitive pattern enables a deep-learning algorithm to reveal that T cells organize their membrane and actin network differently depending upon whether the ligands are clustered or homogeneously distributed, an effect invisible to the unassisted human eye even after extensive image analysis. This fabrication and analysis toolbox should be useful, both together and separately, for exploring general correlation between a spatially structured subcellular stimulation and a subtle cellular response.

Molecular Biomechanics Controls Protein Mixing and Segregation in Adherent Membranes

Cells interact with their environment by forming complex structures involving a multitude of proteins within assemblies in the plasma membrane. Despite the omnipresence of these assemblies, a number of questions about the correlations between the organisation of domains and the biomechanical properties of the involved proteins, namely their length, flexibility and affinity, as well as about the coupling to the elastic, fluctuating membrane, remain open. Here we address these issues by developing an effective Kinetic Monte Carlo simulation to model membrane adhesion. We apply this model to a typical experiment in which a cell binds to a functionalized solid supported bilayer and use two ligand-receptor pairs to study these couplings. We find that differences in affinity and length of proteins forming adhesive contacts result in several characteristic features in the calculated phase diagrams. One such feature is mixed states occurring even with proteins with length differences of 10 nm. Another feature are stable nanodomains with segregated proteins appearing on time scales of cell experiments, and for biologically relevant parameters. Furthermore, we show that macroscopic ring-like patterns can spontaneously form as a consequence of emergent protein fluxes. The capacity to form domains is captured by an order parameter that is founded on the virial coefficients for the membrane mediated interactions between bonds, which allow us to collapse all the data. These findings show that taking into account the role of the membrane allows us to recover a number of experimentally observed patterns. This is an important perspective in the context of explicit biological systems, which can now be studied in significant detail.

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.

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.

Extracellular Matrix Geometry and Initial Adhesive Position Determine Stress Fiber Network Organization during Cell Spreading

Three-dimensional matrices often contain highly structured adhesive tracks that require cells to turn corners and bridge non-adhesive areas. Here, we investigate these complex processes using micropatterned cell adhesive frames. Spreading kinetics on these matrices depend strongly on initial adhesive position and are predicted by a cellular Potts model (CPM), which reflects a balance between adhesion and intracellular tension. As cells spread, new stress fibers (SFs) assemble periodically and parallel to the leading edge, with spatial intervals of ∼2.5 μm, temporal intervals of ∼15 min, and characteristic lifetimes of ∼50 min. By incorporating these rules into the CPM, we can successfully predict SF network architecture. Moreover, we observe broadly similar behavior when we culture cells on arrays of discrete collagen fibers. Our findings show that ECM geometry and initial cell position strongly determine cell spreading and that cells encode a memory of their spreading history through SF network organization.

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.

Missing-in-Metastasis/Metastasis Suppressor 1 Regulates B Cell Receptor Signaling, B Cell Metabolic Potential, and T Cell-Independent Immune Responses

Efficient generation of antibodies by B cells is one of the prerequisites of protective immunity. B cell activation by cognate antigens via B cell receptors (BCRs), or pathogen-associated molecules through pattern-recognition receptors, such as Toll-like receptors (TLRs), leads to transcriptional and metabolic changes that ultimately transform B cells into antibody-producing plasma cells or memory cells. BCR signaling and a number of steps downstream of it rely on coordinated action of cellular membranes and the actin cytoskeleton, tightly controlled by concerted action of multiple regulatory proteins, some of them exclusive to B cells. Here, we dissect the role of Missing-In-Metastasis (MIM), or Metastasis suppressor 1 (MTSS1), a cancer-associated membrane and actin cytoskeleton regulating protein, in B cell-mediated immunity by taking advantage of MIM knockout mouse strain. We show undisturbed B cell development and largely normal composition of B cell compartments in the periphery. Interestingly, we found that MIM-/- B cells are defected in BCR signaling in response to surface-bound antigens but, on the other hand, show increased metabolic activity after stimulation with LPS or CpG. In vivo, MIM knockout animals exhibit impaired IgM antibody responses to immunization with T cell-independent antigen. This study provides the first comprehensive characterization of MIM in B cells, demonstrates its regulatory role for B cell-mediated immunity, as well as proposes new functions for MIM in tuning receptor signaling and cellular metabolism, processes, which may also contribute to the poorly understood functions of MIM in cancer.

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.

Nanobody-CD16 Catch Bond Reveals NK Cell Mechanosensitivity

Antibodies are key tools in biomedical research and medicine. Their binding properties are classically measured in solution and characterized by an affinity. However, in physiological conditions, antibodies can bridge an immune effector cell and an antigen-presenting cell, implying that mechanical forces may apply to the bonds. For example, in antibody-dependent cell cytotoxicity-a major mode of action of therapeutic monoclonal antibodies-the Fab domains bind the antigens on the target cell, whereas the Fc domain binds to the activating receptor CD16 (also known as FcgRIII) of an immune effector cell, in a quasi-bidimensional environment (2D). Therefore, there is a strong need to investigate antigen/antibody binding under force (2D) to better understand and predict antibody activity in vivo. We used two anti-CD16 nanobodies targeting two different epitopes and laminar flow chamber assay to measure the association and dissociation of single bonds formed between microsphere-bound CD16 antigens and surface-bound anti-CD16 nanobodies (or single-domain antibodies), simulating 2D encounters. The two nanobodies exhibit similar 2D association kinetics, characterized by a strong dependence on the molecular encounter duration. However, their 2D dissociation kinetics strongly differ as a function of applied force: one exhibits a slip bond behavior in which off rate increases with force, and the other exhibits a catch-bond behavior in which off rate decreases with force. This is the first time, to our knowledge, that catch-bond behavior was reported for antigen-antibody bond. Quantification of natural killer cells spreading on surfaces coated with the nanobodies provides a comparison between 2D and three-dimensional adhesion in a cellular context, supporting the hypothesis of natural killer cell mechanosensitivity. Our results may also have strong implications for the design of efficient bispecific antibodies for therapeutic applications.

Simultaneous defeat of MCF7 and MDA-MB-231 resistances by a hypericin PDT-tamoxifen hybrid therapy

Currently the greatest challenge in oncology is the lack of homogeneity of the lesions where different cell components respond differently to treatment. There is growing consensus that monotherapies are insufficient to eradicate the disease and there is an unmet need for more potent combinatorial treatments. We have previously shown that hypericin photodynamic therapy (HYP-PDT) triggers electron transport chain (ETC) inhibition in cell mitochondria. We have also shown that tamoxifen (TAM) enhances cytotoxicity in cells with high respiration, when combined with ETC inhibitors. Herein we introduce a synergistic treatment based on TAM chemotherapy and HYP-PDT. We tested this novel combinatorial treatment (HYPERTAM) in two metabolically different breast cancer cell lines, the triple-negative MDA-MB-231 and the estrogen-receptor-positive MCF7, the former being quite sensitive to HYP-PDT while the latter very responsive to TAM treatment. In addition, we investigated the mode of death, effect of lipid peroxidation, and the effect on cell metabolism. The results were quite astounding. HYPERTAM exhibited over 90% cytotoxicity in both cell lines. This cytotoxicity was in the form of both necrosis and autophagy, while high levels of lipid peroxidation were observed in both cell lines. We, consequently, translated our research to an in vivo pilot study encompassing the MDA-MB-231 and MCF7 tumor models in NOD SCID-γ immunocompromised mice. Both treatment cohorts responded very positively to HYPERTRAM, which significantly prolonged mice survival. HYPERTAM is a potent, synergistic modality, which may lay the foundations for a novel, composite anticancer treatment, effective in diverse tumor types.

Biphasic mechanosensitivity of T cell receptor-mediated spreading of lymphocytes

Mechanosensing by T cells through the T cell receptor (TCR) is at the heart of immune recognition. While the mechanobiology of the TCR at the molecular level is increasingly well documented, its link to cell-scale response is poorly understood. Here we explore T cell spreading response as a function of substrate rigidity and show that remarkably, depending on the surface receptors stimulated, the cellular response may be either biphasic or monotonous. When adhering solely via the TCR complex, T cells respond to environmental stiffness in an unusual fashion, attaining maximal spreading on an optimal substrate stiffness comparable to that of professional antigen-presenting cells. However, in the presence of additional ligands for the integrin LFA-1, this biphasic response is abrogated and the cell spreading increases monotonously with stiffness up to a saturation value. This ligand-specific mechanosensing is effected through an actin-polymerization-dependent mechanism. We construct a mesoscale semianalytical model based on force-dependent bond rupture and show that cell-scale biphasic or monotonous behavior emerges from molecular parameters. As the substrate stiffness is increased, there is a competition between increasing effective stiffness of the bonds, which leads to increased cell spreading and increasing bond breakage, which leads to decreased spreading. We hypothesize that the link between actin and the receptors (TCR or LFA-1), rather than the ligand/receptor linkage, is the site of this mechanosensing.

NK cells specifically TCR-dressed to kill cancer cells

Background

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

Methods

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

Findings

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

Interpretation

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

Fund

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

Lamellipod Reconstruction by Three-Dimensional Reflection Interference Contrast Nanoscopy (3D-RICN)

There are very few techniques to reconstruct the shape of a cell at nanometric resolution, and those that exist are almost exclusively based on fluorescence, implying limitations due to staining constraints and artifacts. Reflection interference contrast microscopy (RICM), a label-free technique, permits the measurement of nanometric distances between refractive objects. However, its quantitative application to cells has been largely limited due to the complex interferometric pattern caused by multiple reflections on internal or thin structures like lamellipodia. Here we introduce 3D reflection interference contrast nanoscopy, 3D-RICN, which combines information from multiple illumination wavelengths and aperture angles to characterize the lamellipodial region of an adherent cell in terms of its distance from the surface and its thickness. We validate this new method by comparing data obtained on fixed cells imaged with atomic force microscopy and quantitative phase imaging. We show that as expected, cells adhering to micropatterns exhibit a radial symmetry for the lamellipodial thickness. We demonstrate that the substrate-lamellipod distance may be as high as 100 nm. We also show how the method applies to living cells, opening the way for label-free dynamical study of cell structures with nanometric resolution.

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

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

Printing Functional Protein Nanodots on Soft Elastomers: From Transfer Mechanism to Cell Mechanosensing

Living cells sense the physical and chemical nature of their micro/nano environment with exquisite sensitivity. In this context, there is a growing need to functionalize soft materials with micro/nanoscale biochemical patterns for applications in mechanobiology. This, however, is still an engineering challenge. Here a new method is proposed, where submicronic protein-patterns are first formed on glass and are then printed on to an elastomer. The degree of transfer is shown to be governed mainly by hydrophobic interactions and to be influenced by grafting an appropriate fluorophore onto the core protein of interest. The transfer mechanism is probed by measuring the forces of adhesion/cohesion using atomic force microscopy. The transfer of functional arrays of dots with size down to about 400 nm, on elastomers with stiffness ranging from 3 kPa to 7 MPa, is demonstrated. Pilot studies on adhesion of T lymphocytes on such soft patterned substrates are reported.

Ligand Nano-cluster Arrays in a Supported Lipid Bilayer

Currently there is considerable interest in creating ordered arrays of adhesive protein islands in a sea of passivated surface for cell biological studies. In the past years, it has become increasingly clear that living cells respond, not only to the biochemical nature of the molecules presented to them but also to the way these molecules are presented. Creating protein micro-patterns is therefore now standard in many biology laboratories; nano-patterns are also more accessible. However, in the context of cell-cell interactions, there is a need to pattern not only proteins but also lipid bilayers. Such dual proteo-lipidic patterning has so far not been easily accessible. We offer a facile technique to create protein nano-dots supported on glass and propose a method to backfill the inter-dot space with a supported lipid bilayer (SLB). From photo-bleaching of tracer fluorescent lipids included in the SLB, we demonstrate that the bilayer exhibits considerable in-plane fluidity. Functionalizing the protein dots with fluorescent groups allows us to image them and to show that they are ordered in a regular hexagonal lattice. The typical dot size is about 800 nm and the spacing demonstrated here is 2 microns. These substrates are expected to serve as useful platforms for cell adhesion, migration and mechano-sensing studies.

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

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

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.

Contribution of myosin II activity to cell spreading dynamics

Myosin II activity and actin polymerization at the leading edge of the cell are known to be essential sources of cellular stress. However, a quantitative account of their separate contributions is still lacking; so is the influence of the coupling between the two phenomena on cell spreading dynamics. We present a simple analytic elastic theory of cell spreading dynamics that quantitatively demonstrates how actin polymerization and myosin activity cooperate in the generation of cellular stress during spreading. Consistent with experiments, myosin activity is assumed to polarize in response to the stresses generated during spreading. The characteristic response time and the overall spreading time are predicted to determine different evolution profiles of cell spreading dynamics. These include, a (regular) monotonic increase of cell projected area with time, a non-monotonic (overshooting) profile with a maximum, and damped oscillatory modes. In addition, two populations of myosin II motors are distinguished based on their location in the lamella; those located above the major adhesion zone at the cell periphery are shown to facilitate spreading whereas those in deeper regions of the lamella are shown to oppose spreading. We demonstrate that the attenuation of myosin activity in the two regions may result in reciprocal effects on spreading. These findings provide important new insight into the function of myosin II motors in the course of spreading.

Distinct impacts of substrate elasticity and ligand affinity on traction force evolution

Cell adhesion is regulated by the mechanical characteristics of the cell environment. The influences of different parameters of the adhesive substrates are convoluted in the cell response leading to questions on the underlying mechanisms, like biochemical signaling on the level of adhesion molecules, or viscoelastic properties of substrates and cell. By a time-resolved analysis of traction force generation during early cell adhesion, we wanted to elucidate the contributions of substrate mechanics to the adhesion process, in particular the impact of substrate elasticity and the molecular friction of adhesion ligands on the substrate surface. Both parameters were independently adjusted by (i) an elastic polyacrylamide hydrogel of variable crosslinking degree and (ii) a thin polymer coating of the hydrogel surface controlling the affinity (and the correlated substrate-ligand friction) of the adhesion ligand fibronectin. Our analysis showed two sequential regimes of considerable force generation, whose occurrence was found to be independent of substrate properties. The first regime is characterized by spreading of the cell and a succeeding force increase. After spreading cells enter the second regime with saturated forces. Substrate elasticity and viscosity, namely hydrogel elasticity and ligand affinity, were both found to affect the kinetics and absolute levels of traction force quantities. A faster increase and a higher saturation level of traction forces were observed for a higher substrate stiffness and a higher ligand affinity. The results complement recent modeling approaches on the evolution of forces in cell spreading and contribute to a better understanding of the dynamics of cell adhesion on viscoelastic substrates.

Supported Lipid Bilayer Technology for the Study of Cellular Interfaces

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

Size-Tunable Organic Nanodot Arrays: A Versatile Platform for Manipulating and Imaging Cells

Arrays of protein nanodots with dot-size tuned independently of spacing (e.g., ∼100 to 600 nm diameter for 900 nm spacing) are fabricated. The mechanism of size control is demonstrated, by numerical simulations, to arise from shadow effects during deposition of a sacrificial metal mask. We functionalize the nanodots with antibodies and embed them in a polymer-cushion or in lipid-bilayers or transfer them to soft elastomers. Their ability to influence cell architecture and local membrane organization is demonstrated in T-lymphocytes, using reflection interference contrast and total internal reflection fluorescence microscopy.

Crowding of receptors induces ring-like adhesions in model membranes

The dynamics of formation of macromolecular structures in adherent membranes is a key to a number of cellular processes. However, the interplay between protein reaction kinetics, diffusion and the morphology of the growing domains, governed by membrane mediated interactions, is still poorly understood. Here we show, experimentally and in simulations, that a rich phase diagram emerges from the competition between binding, cooperativity, molecular crowding and membrane spreading. In the cellular context, the spontaneously-occurring organization of adhesion domains in ring-like morphologies is particularly interesting. These are stabilized by the crowding of bulky proteins, and the membrane-transmitted correlations between bonds. Depending on the density of the receptors, this phase may be circumvented, and instead, the adhesions may grow homogeneously in the contact zone between two membranes. If the development of adhesion occurs simultaneously with membrane spreading, much higher accumulation of binders can be achieved depending on the velocity of spreading. The mechanisms identified here, in the context of our mimetic model, may shed light on the structuring of adhesions in the contact zones between two living cells. This article is part of a Special Issue entitled: Mechanobiology.