The αβTCR mechanosensor exploits dynamic ectodomain allostery to optimize its ligand recognition site

Force-Induced Site-Specific Enzymatic Cleavage Probes Reveal That Serial Mechanical Engagement Boosts T Cell Activation

The T cell membrane is studded with >104 T cell receptors (TCRs) that are used to scan target cells to identify short peptide fragments associated with viral infection or cancerous mutation. These peptides are presented as peptide-major-histocompatibility complexes (pMHCs) on the surface of virtually all nucleated cells. The TCR-pMHC complex forms at cell-cell junctions, is highly transient, and experiences mechanical forces. An important question in this area pertains to the role of the force duration in immune activation. Herein, we report the development of force probes that autonomously terminate tension within a time window following mechanical triggering. Force-induced site-specific enzymatic cleavage (FUSE) probes tune the tension duration by controlling the rate of a force-triggered endonuclease hydrolysis reaction. This new capability provides a method to study how the accumulated force duration contributes to T cell activation. We screened DNA sequences and identified FUSE probes that disrupt mechanical interactions with F > 7.1 piconewtons (pN) between TCRs and pMHCs. This rate of disruption, or force lifetime (τF), is tunable from tens of minutes down to 1.9 min. T cells challenged with FUSE probes with F > 7.1 pN presenting cognate antigens showed up to a 23% decrease in markers of early activation. FUSE probes with F > 17.0 pN showed weaker influence on T cell triggering further showing that TCR-pMHC with F > 17.0 pN are less frequent compared to F > 7.1 pN. Taken together, FUSE probes allow a new strategy to investigate the role of force dynamics in mechanotransduction broadly and specifically suggest a model of serial mechanical engagement boosting TCR activation.

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.

Biomaterials-mediated ligation of immune cell surface receptors for immunoengineering

A wide variety of cell surface receptors found on immune cells are essential to the body's immunological defense mechanisms. Cell surface receptors enable immune cells to sense extracellular stimuli and identify pathogens, transmitting activating or inhibitory signals that regulate the immune cell state and coordinate immunological responses. These receptors can dynamically aggregate or disperse due to the fluidity of the cell membrane, particularly during interactions between cells or between cells and pathogens. At the contact surface, cell surface receptors form microclusters, facilitating the recruitment and amplification of downstream signals. The strength of the immune signal is influenced by both the quantity and the specific types of participating receptors. Generally, receptor cross-linking, meaning multivalent ligation of receptors on one cell, leads to greater interface connectivity and more robust signaling. However, intercellular interactions are often spatially restricted by other cellular structures. Therefore, it is essential to comprehend these receptors' features for developing effective immunoengineering approaches. Biomaterials can stimulate and simulate interactions between immune cells and their targets. Biomaterials can activate immune cells to act against pathogenic organisms or cancer cells, thereby offering a valuable immunoengineering toolset for vaccination and immunotherapy. In this review, we systematically summarize biomaterial-based immunoengineering strategies that consider the biology of diverse immune cell surface receptors and the structural attributes of pathogens. By combining this knowledge, we aim to advance the development of rational and effective approaches for immune modulation and therapeutic interventions.

Asymmetric framework motion of TCRαβ controls load-dependent peptide discrimination

Mechanical force is critical for the interaction between an αβ T cell receptor (TCR) and a peptide-bound major histocompatibility complex (pMHC) molecule to initiate productive T-cell activation. However, the underlying mechanism remains unclear. We use all-atom molecular dynamics simulations to examine the A6 TCR bound to HLA-A*02:01 presenting agonist or antagonist peptides under different extensions to simulate the effects of applied load on the complex, elucidating their divergent biological responses. We found that TCR α and β chains move asymmetrically, which impacts the interface with pMHC, in particular the peptide-sensing CDR3 loops. For the wild-type agonist, the complex stabilizes in a load-dependent manner while antagonists destabilize it. Simulations of the Cβ FG-loop deletion, which reduces the catch bond response, and simulations with in silico mutant peptides further support the observed behaviors. The present results highlight the combined role of interdomain motion, fluctuating forces, and interfacial contacts in determining the mechanical response and fine peptide discrimination by a TCR, thereby resolving the conundrum of nearly identical crystal structures of TCRαβ-pMHC agonist and antagonist complexes.

Parsing digital or analogue TCR performance through piconewton forces

αβ T-cell receptors (TCRs) recognize aberrant peptides bound to major histocompatibility complex molecules (pMHCs) on unhealthy cells, amplifying specificity and sensitivity through physical load placed on the TCR-pMHC bond during immunosurveillance. To understand this mechanobiology, TCRs stimulated by abundantly and sparsely arrayed epitopes (NP 366-374 /D b and PA 224-233 /D b , respectively) following in vivo influenza A virus infection were studied with optical tweezers. While certain NP repertoire CD8 T lymphocytes require many ligands for activation, others are digital, needing just few. Conversely, all PA TCRs perform digitally, exhibiting pronounced bond lifetime increases through sustained, energizing volleys of structural transitioning. Optimal digital performance is superior in vivo, correlating with ERK phosphorylation, CD3 loss, and activation marker upregulation in vitro . Given neoantigen array paucity, digital TCRs are likely critical for immunotherapies.

One Sentence Summary

Quality of ligand recognition in a T-cell repertoire is revealed through application of physical load on clonal T-cell receptor (TCR)-pMHC bonds.

Dichotomy in TCR V-domain dynamics binding the opposed inclined planes of pMHC-II and pMHC-I α-helices

Ligand recognition by the human α/β T-cell antigen receptor (TCR) heterodimer protein, unlike the surface immunoglobulin (sIg) B-cell receptor, is not governed by relative binding affinity. Its interaction with the peptide (p) plus major histocompatibility complex (MHC) protein (abbrev. pMHC) likely involves some different molecular mechanism linking pMHC binding to T-cell functions. Recent analytical geometry of TCR:pMHC-II solved crystallographic structures (n = 40) revealed that each variable (V)-domain is bound in similar, yet mathematically unique orientations to its target pMHC groove. The relative position of the central cysteine of each V-domain was examined by multivariable calculus in spherical coordinates, where a simple volume element (dV) was found to describe clonotypic geometry with pMHC-II. Here, the study was expanded to include TCR:pMHC-I structures, and to model a physical mechanism, specifically involving the two directionally opposed inclined planes (IP) manifest by the two major α-helices prominent in both MHC-I and MHC-II proteins. Calculations for rotational torque of each V-domain, together with acceleration up and down the slopes of both MHC α-helices were used to estimate the time a given V-domain spends sliding down its cognate MHC IP. This V-domain rotation/sliding mechanism appears to be quantitatively unique for each TCR:pMHC V-domain (n = 40). However, there is an apparent and common dichotomy between the mobility of each V-domain with respect to the two classes of MHC proteins. Evolutionary motifs in the MHC helices support that the V-domains negotiate the opposed inclined planes of pMHC ligands in clonotypic fashion. Thus, this model is useful in understanding how mechanical forces are linked to TCR function.

Asymmetric framework motion of TCRαβ controls load-dependent peptide discrimination

Mechanical force is critical for the interaction between an αβT cell receptor (TCR) and a peptide-bound major histocompatibility complex (pMHC) molecule to initiate productive T-cell activation. However, the underlying mechanism remains unclear. We use all-atom molecular dynamics simulations to examine the A6 TCR bound to HLA-A*02:01 presenting agonist or antagonist peptides under different extensions to simulate the effects of applied load on the complex, elucidating their divergent biological responses. We found that TCR α and β chains move asymmetrically, which impacts the interface with pMHC, in particular the peptide-sensing CDR3 loops. For the wild-type agonist, the complex stabilizes in a load-dependent manner while antagonists destabilize it. Simulations of the Cβ FG-loop deletion, which reduces the catch bond response, and simulations with in silico mutant peptides further support the observed behaviors. The present results highlight the combined role of interdomain motion, fluctuating forces, and interfacial contacts in determining the mechanical response and fine peptide discrimination by a TCR, thereby resolving the conundrum of nearly identical crystal structures of TCRαβ-pMHC agonist and antagonist complexes.

Quantitative approaches for decoding the specificity of the human T cell repertoire

T cell receptor (TCR)-peptide-major histocompatibility complex (pMHC) interactions play a vital role in initiating immune responses against pathogens, and the specificity of TCRpMHC interactions is crucial for developing optimized therapeutic strategies. The advent of high-throughput immunological and structural evaluation of TCR and pMHC has provided an abundance of data for computational approaches that aim to predict favorable TCR-pMHC interactions. Current models are constructed using information on protein sequence, structures, or a combination of both, and utilize a variety of statistical learning-based approaches for identifying the rules governing specificity. This review examines the current theoretical, computational, and deep learning approaches for identifying TCR-pMHC recognition pairs, placing emphasis on each method's mathematical approach, predictive performance, and limitations.

Force-induced site-specific enzymatic cleavage probes reveal that serial mechanical engagement boosts T cell activation

The surface of T cells is studded with T cell receptors (TCRs) that are used to scan target cells to identify peptide-major histocompatibility complexes (pMHCs) signatures of viral infection or cancerous mutation. It is now established that the TCR-pMHC complex is highly transient and experiences mechanical forces that augment the fidelity of T cell activation. An important question in this area pertains to the role of force duration in immune activation. Herein, we report the development of force probes that autonomously terminate tension within a time window following mechanical triggering. Force-induced site-specific enzymatic cleavage (FUSE) probes tune tension duration by controlling the rate of a force-triggered endonuclease hydrolysis reaction. This new capability provides a method to study how accumulated force duration contributes to T cell activation. We screened DNA sequences and identified FUSE probes that disrupt mechanical interactions with F >7.1 piconewtons (pN) between TCRs and pMHCs. Force lifetimes (τF) are tunable from tens of min down to 1.9 min. T cells challenged with FUSE probes presenting cognate antigens with τF of 1.9 min demonstrated dampened markers of early activation, thus demonstrating that repeated mechanical sampling boosts TCR activation. Repeated mechanical sampling F >7.1 pN was found to be particularly critical at lower pMHC antigen densities, wherein the T cell activation declined by 23% with τF of 1.9 min. FUSE probes with F >17.0 pN response showed weaker influence on T cell triggering further showing that TCR-pMHC with F >17.0 pN are less frequent compared to F >7.1 pN. Taken together, FUSE probes allow a new strategy to investigate the role of force dynamics in mechanotransduction broadly and specifically suggest a model of serial mechanical engagement in antigen recognition.

The Energetic Landscape of Catch Bonds in TCR Interfaces

Recognition of peptide/MHC complexes by αβ TCRs has traditionally been viewed through the lens of conventional receptor-ligand theory. Recent work, however, has shown that TCR recognition and T cell signaling can be profoundly influenced and tuned by mechanical forces. One outcome of applied force is the catch bond, where TCR dissociation rates decrease (half-lives increase) when limited force is applied. Although catch bond behavior is believed to be widespread in biology, its counterintuitive nature coupled with the difficulties of describing mechanisms at the structural level have resulted in considerable mystique. In this review, we demonstrate that viewing catch bonds through the lens of energy landscapes, barriers, and the ensuing reaction rates can help demystify catch bonding and provide a foundation on which atomic-level TCR catch bond mechanisms can be built.

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.

Catch bond models may explain how force amplifies TCR signaling and antigen discrimination

The TCR integrates forces in its triggering process upon interaction with pMHC. Force elicits TCR catch-slip bonds with strong pMHCs but slip-only bonds with weak pMHCs. We develop two models and apply them to analyze 55 datasets, demonstrating the models' ability to quantitatively integrate and classify a broad range of bond behaviors and biological activities. Comparing to a generic two-state model, our models can distinguish class I from class II MHCs and correlate their structural parameters with the TCR/pMHC's potency to trigger T cell activation. The models are tested by mutagenesis using an MHC and a TCR mutated to alter conformation changes. The extensive comparisons between theory and experiment provide model validation and testable hypothesis regarding specific conformational changes that control bond profiles, thereby suggesting structural mechanisms for the inner workings of the TCR mechanosensing machinery and plausible explanations of why and how force may amplify TCR signaling and antigen discrimination.

Physical-Chemical Regulation of Membrane Receptors Dynamics in Viral Invasion and Immune Defense

Mechanical cues dynamically regulate membrane receptors functions to trigger various physiological and pathological processes from viral invasion to immune defense. These cues mainly include various types of dynamic mechanical forces and the spatial confinement of plasma membrane. However, the molecular mechanisms of how they couple with biochemical cues in regulating membrane receptors functions still remain mysterious. Here, we review recent advances in methodologies of single-molecule biomechanical techniques and in novel biomechanical regulatory mechanisms of critical ligand recognition of viral and immune receptors including SARS-CoV-2 spike protein, T cell receptor (TCR) and other co-stimulatory immune receptors. Furthermore, we provide our perspectives of the general principle of how force-dependent kinetics determine the dynamic functions of membrane receptors and of biomechanical-mechanism-driven SARS-CoV-2 neutralizing antibody design and TCR engineering for T-cell-based therapies.

The Enigmatic Nature of the TCR-pMHC Interaction: Implications for CAR-T and TCR-T Engineering

The interaction of the T-cell receptor (TCR) with a peptide in the major histocompatibility complex (pMHC) plays a central role in the adaptive immunity of higher chordates. Due to the high specificity and sensitivity of this process, the immune system quickly recognizes and efficiently responds to the appearance of foreign and altered self-antigens. This is important for ensuring anti-infectious and antitumor immunity, in addition to maintaining self-tolerance. The most common parameter used for assessing the specificity of TCR-pMHC interaction is affinity. This thermodynamic characteristic is widely used not only in various theoretical aspects, but also in practice, for example, in the engineering of various T-cell products with a chimeric (CAR-T) or artificial (TCR-engineered T-cell) antigen receptor. However, increasing data reveal the fact that, in addition to the thermodynamic component, the specificity of antigen recognition is based on the kinetics and mechanics of the process, having even greater influence on the selectivity of the process and T lymphocyte activation than affinity. Therefore, the kinetic and mechanical aspects of antigen recognition should be taken into account when designing artificial antigen receptors, especially those that recognize antigens in the MHC complex. This review describes the current understanding of the nature of the TCR-pMHC interaction, in addition to the thermodynamic, kinetic, and mechanical principles underlying the specificity and high sensitivity of this interaction.

Mechanical Stabilization of a Bacterial Adhesion Complex

The adhesions between Gram-positive bacteria and their hosts are exposed to varying magnitudes of tensile forces. Here, using an ultrastable magnetic tweezer-based single-molecule approach, we show the catch-bond kinetics of the prototypical adhesion complex of SD-repeat protein G (SdrG) to a peptide from fibrinogen β (Fgβ) over a physiologically important force range from piconewton (pN) to tens of pN, which was not technologically accessible to previous studies. At 37 °C, the lifetime of the complex exponentially increases from seconds at several pN to ∼1000 s as the force reaches 30 pN, leading to mechanical stabilization of the adhesion. The dissociation transition pathway is determined as the unbinding of a critical β-strand peptide (“latch” strand of SdrG that secures the entire adhesion complex) away from its binding cleft, leading to the dissociation of the Fgβ ligand. Similar mechanical stabilization behavior is also observed in several homologous adhesions, suggesting the generality of catch-bond kinetics in such bacterial adhesions. We reason that such mechanical stabilization confers multiple advantages in the pathogenesis and adaptation of bacteria.

A Computational Algorithm to Assess the Physiochemical Determinants of T Cell Receptor Dissociation Kinetics

The rational design of T Cell Receptors (TCRs) for immunotherapy has stagnated due to a limited understanding of the dynamic physiochemical features of the TCR that elicit an immunogenic response. The physiochemical features of the TCR-peptide major histocompatibility complex (pMHC) bond dictate bond lifetime which, in turn, correlates with immunogenicity. Here, we: i) characterize the force-dependent dissociation kinetics of the bond between a TCR and a set of pMHC ligands using Steered Molecular Dynamics (SMD); and ii) implement a machine learning algorithm to identify which physiochemical features of the TCR govern dissociation kinetics. Our results demonstrate that the total number of hydrogen bonds between the CDR2β-MHC⍺(β), CDR1α-Peptide, and CDR3β-Peptide are critical features that determine bond lifetime.

Tuning T cell receptor sensitivity through catch bond engineering

Adoptive cell therapy using engineered T cell receptors (TCRs) is a promising approach for targeting cancer antigens, but tumor-reactive TCRs are often weakly responsive to their target ligands, peptide-major histocompatibility complexes (pMHCs). Affinity-matured TCRs can enhance the efficacy of TCR-T cell therapy but can also cross-react with off-target antigens, resulting in organ immunopathology. We developed an alternative strategy to isolate TCR mutants that exhibited high activation signals coupled with low-affinity pMHC binding through the acquisition of catch bonds. Engineered analogs of a tumor antigen MAGE-A3-specific TCR maintained physiological affinities while exhibiting enhanced target killing potency and undetectable cross-reactivity, compared with a high-affinity clinically tested TCR that exhibited lethal cross-reactivity with a cardiac antigen. Catch bond engineering is a biophysically based strategy to tune high-sensitivity TCRs for T cell therapy with reduced potential for adverse cross-reactivity.

Is There a Need for a More Precise Description of Biomolecule Interactions to Understand Cell Function?

An important goal of biological research is to explain and hopefully predict cell behavior from the molecular properties of cellular components. Accordingly, much work was done to build extensive “omic” datasets and develop theoretical methods, including computer simulation and network analysis to process as quantitatively as possible the parameters contained in these resources. Furthermore, substantial effort was made to standardize data presentation and make experimental results accessible to data scientists. However, the power and complexity of current experimental and theoretical tools make it more and more difficult to assess the capacity of gathered parameters to support optimal progress in our understanding of cell function. The purpose of this review is to focus on biomolecule interactions, the interactome, as a specific and important example, and examine the limitations of the explanatory and predictive power of parameters that are considered as suitable descriptors of molecular interactions. Recent experimental studies on important cell functions, such as adhesion and processing of environmental cues for decision-making, support the suggestion that it should be rewarding to complement standard binding properties such as affinity and kinetic constants, or even force dependence, with less frequently used parameters such as conformational flexibility or size of binding molecules.

Application of phage display for T-cell receptor discovery

The immune system is tasked to keep our body unharmed and healthy. In the immune system, B- and T-lymphocytes are the two main components working together to stop and eliminate invading threats like virus particles, bacteria, fungi and parasite from attacking our healthy cells. The function of antibodies is relatively more direct in target recognition as compared to T-cell receptors (TCR) which recognizes antigenic peptides being presented on the major histocompatibility complex (MHC). Although phage display has been widely applied for antibody presentation, this is the opposite in the case of TCR. The cell surface TCR is a relatively large and complex molecule, making presentation on phage surfaces challenging. Even so, recombinant versions and modifications have been introduced to allow the growing development of TCR in phage display. In addition, the increasing application of TCR for immunotherapy has made it an important binding motif to be developed by phage display. This review will emphasize on the application of phage display for TCR discovery as well as the engineering aspect of TCR for improved characteristics.

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.

Height, but not binding epitope, affects the potency of synthetic TCR agonists

Under physiological conditions, peptide-major histocompatibility complex (pMHC) molecules can trigger T cell receptors (TCRs) as monovalent ligands that are sparsely distributed on the plasma membrane of an antigen-presenting cell. TCRs can also be triggered by artificial clustering, such as with pMHC tetramers or antibodies; however, these strategies circumvent many of the natural ligand discrimination mechanisms of the T cell and can elicit nonphysiological signaling activity. We have recently introduced a synthetic TCR agonist composed of an anti-TCRβ Fab′ antibody fragment covalently bound to a DNA oligonucleotide, which serves as a membrane anchor. This Fab′-DNA ligand efficiently triggers TCR as a monomer when membrane associated and exhibits a potency and activation profile resembling agonist pMHC. In this report, we explore the geometric requirements for efficient TCR triggering and cellular activation by Fab′-DNA ligands. We find that T cells are insensitive to the ligand binding epitope on the TCR complex but that length of the DNA tether is important. Increasing, the intermembrane distance spanned by Fab′-DNA:TCR complexes decreases TCR triggering efficiency and T cell activation potency, consistent with the kinetic-segregation model of TCR triggering. These results establish design parameters for constructing synthetic TCR agonists that are able to activate polyclonal T cell populations, such as T cells from a human patient, in a similar manner as the native pMHC ligand.

Structural variability and concerted motions of the T cell receptor - CD3 complex

We investigate the structural and orientational variability of the membrane-embedded T cell receptor (TCR) – CD3 complex in extensive atomistic molecular dynamics simulations based on the recent cryo-EM structure determined by Dong et al., 2019. We find that the TCR extracellular (EC) domain is highly variable in its orientation by attaining tilt angles relative to the membrane normal that range from 15° to 55°. The tilt angle of the TCR EC domain is both coupled to a rotation of the domain and to characteristic changes throughout the TCR – CD3 complex, in particular in the EC interactions of the Cβ FG loop of the TCR, as well as in the orientation of transmembrane helices. The concerted motions of the membrane-embedded TCR – CD3 complex revealed in our simulations provide atomistic insights on conformational changes of the complex in response to tilt-inducing forces on antigen-bound TCRs.

Single Molecule Force Spectroscopy Reveals Distinctions in Key Biophysical Parameters of αβ T-Cell Receptors Compared with Chimeric Antigen Receptors Directed at the Same Ligand

Chimeric antigen receptor (CAR) T-cell therapies exploit facile antibody-mediated targeting to elicit useful immune responses in patients. This work directly compares binding profiles of CAR and αβ T-cell receptors (TCR) with single cell and single molecule optical trap measurements against a shared ligand. DNA-tethered measurements of peptide-major histocompatibility complex (pMHC) ligand interaction in both CAR and TCR exhibit catch bonds with specific peptide agonist peaking at 25 and 14 pN, respectively. While a conformational transition is regularly seen in TCR-pMHC systems, that of CAR-pMHC systems is dissimilar, being infrequent, of lower magnitude, and irreversible. Slip bonds are observed with CD19-specific CAR T-cells and with a monoclonal antibody mapping to the MHC α2 helix but indifferent to the bound peptide. Collectively, these findings suggest that the CAR-pMHC interface underpins the CAR catch bond response to pMHC ligands in contradistinction to slip bonds for CARs targeting canonical ligands.

A Unifying Framework for Understanding Biological Structures and Functions Across Levels of Biological Organization

The relationship between structure and function is a major constituent of the rules of life. Structures and functions occur across all levels of biological organization. Current efforts to integrate conceptual frameworks and approaches to address new and old questions promise to allow a more holistic and robust understanding of how different biological functions are achieved across levels of biological organization. Here, we provide unifying and generalizable definitions of both structure and function that can be applied across all levels of biological organization. However, we find differences in the nature of structures at the organismal level and below as compared to above the level of the organism. We term these intrinsic and emergent structures, respectively. Intrinsic structures are directly under selection, contributing to the overall performance (fitness) of the individual organism. Emergent structures involve interactions among aggregations of organisms and are not directly under selection. Given this distinction, we argue that while the functions of many intrinsic structures remain unknown, functions of emergent structures are the result of the aggregate of processes of individual organisms. We then provide a detailed and unified framework of the structure-function relationship for intrinsic structures to explore how their unknown functions can be defined. We provide examples of how these scalable definitions applied to intrinsic structures provide a framework to address questions on structure-function relationships that can be approached simultaneously from all subdisciplines of biology. We propose that this will produce a more holistic and robust understanding of how different biological functions are achieved across levels of biological organization.

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.

Molecular design of the γδT cell receptor ectodomain encodes biologically fit ligand recognition in the absence of mechanosensing

TCR mechanosensing is thought necessary for digital sensitivity of αβT cell response to scant pMHC antigens. We use bioinformatic analysis, molecular dynamics, single-molecule optical tweezers techniques, cellular activation, and RNA-seq analysis to explore this paradigm in the γδT cell lineage. We find that, in keeping with its role in recognizing abundant cell-surface ligands, the γδTCR lacks force-dependent hallmarks of mechanosensing in αβT cells.

High-acuity αβT cell receptor (TCR) recognition of peptides bound to major histocompatibility complex molecules (pMHCs) requires mechanosensing, a process whereby piconewton (pN) bioforces exert physical load on αβTCR–pMHC bonds to dynamically alter their lifetimes and foster digital sensitivity cellular signaling. While mechanotransduction is operative for both αβTCRs and pre-TCRs within the αβT lineage, its role in γδT cells is unknown. Here, we show that the human DP10.7 γδTCR specific for the sulfoglycolipid sulfatide bound to CD1d only sustains a significant load and undergoes force-induced structural transitions when the binding interface-distal γδ constant domain (C) module is replaced with that of αβ. The chimeric γδ–αβTCR also signals more robustly than does the wild-type (WT) γδTCR, as revealed by RNA-sequencing (RNA-seq) analysis of TCR-transduced Rag2^−/− thymocytes, consistent with structural, single-molecule, and molecular dynamics studies reflective of γδTCRs as mediating recognition via a more canonical immunoglobulin-like receptor interaction. Absence of robust, force-related catch bonds, as well as γδTCR structural transitions, implies that γδT cells do not use mechanosensing for ligand recognition. This distinction is consonant with the fact that their innate-type ligands, including markers of cellular stress, are expressed at a high copy number relative to the sparse pMHC ligands of αβT cells arrayed on activating target cells. We posit that mechanosensing emerged over ∼200 million years of vertebrate evolution to fulfill indispensable adaptive immune recognition requirements for pMHC in the αβT cell lineage that are unnecessary for the γδT cell lineage mechanism of non-pMHC ligand detection.

Affinity Selection in Germinal Centers: Cautionary Tales and New Opportunities

Our current quantitative knowledge of the kinetics of antibody-mediated immunity is partly based on idealized experiments throughout the last decades. However, new experimental techniques often render contradictory quantitative outcomes that shake previously uncontroversial assumptions. This has been the case in the field of T-cell receptors, where recent techniques for measuring the 2-dimensional rate constants of T-cell receptor–ligand interactions exposed results contradictory to those obtained with techniques measuring 3-dimensional interactions. Recently, we have developed a mathematical framework to rationalize those discrepancies, focusing on the proper fine-grained description of the underlying kinetic steps involved in the immune synapse. In this perspective article, we apply this approach to unveil potential blind spots in the case of B-cell receptors (BCR) and to rethink the interactions between B cells and follicular dendritic cells (FDC) during the germinal center (GC) reaction. Also, we elaborate on the concept of “catch bonds” and on the recent observations that B-cell synapses retract and pull antigen generating a “retracting force”, and propose some testable predictions that can lead to future research.

Quantitative Methodologies to Dissect Immune Cell Mechanobiology

Mechanobiology seeks to understand how cells integrate their biomechanics into their function and behavior. Unravelling the mechanisms underlying these mechanobiological processes is particularly important for immune cells in the context of the dynamic and complex tissue microenvironment. However, it remains largely unknown how cellular mechanical force generation and mechanical properties are regulated and integrated by immune cells, primarily due to a profound lack of technologies with sufficient sensitivity to quantify immune cell mechanics. In this review, we discuss the biological significance of mechanics for immune cells across length and time scales, and highlight several experimental methodologies for quantifying the mechanics of immune cells. Finally, we discuss the importance of quantifying the appropriate mechanical readout to accelerate insights into the mechanobiology of the immune response.

Functionalized Bead Assay to Measure Three-dimensional Traction Forces during T-cell Activation

When T-cells probe their environment for antigens, the bond between the T-cell receptor (TCR) and the peptide-loaded major histocompatibility complex (MHC) is put under tension, thereby influencing the antigen discrimination. Yet, the quantification of such forces in the context of T-cell signaling is technically challenging. Here, we developed a traction force microscopy platform which allows for quantifying the pulls and pushes exerted via T-cell microvilli, in both tangential and normal directions, during T-cell activation. We immobilized specific T-cell activating antibodies on the marker beads used to read out the hydrogel deformation. Microvilli targeted the functionalized beads, as confirmed by superresolution microscopy of the local actin organization. Moreover, we found that cellular components, such as actin, TCR, and CD45 reorganize upon interaction with the beads, such that actin forms a vortex-like ring structure around the beads and TCR is enriched at the bead surface, whereas CD45 is excluded from bead-microvilli contacts.

A general chemical crosslinking strategy for structural analyses of weakly interacting proteins applied to preTCR-pMHC complexes

T lymphocytes discriminate between healthy and infected or cancerous cells via T-cell receptor-mediated recognition of peptides bound and presented by cell-surface-expressed major histocompatibility complex molecules (MHCs). Pre-T-cell receptors (preTCRs) on thymocytes foster development of αβT lymphocytes through their β chain interaction with MHC displaying self-peptides on thymic epithelia. The specific binding of a preTCR with a peptide-MHC complex (pMHC) has been identified previously as forming a weak affinity complex with a distinct interface from that of mature αβTCR. However, a lack of appropriate tools has limited prior efforts to investigate this unique interface. Here we designed a small-scale linkage screening protocol using bismaleimide linkers for determining residue-specific distance constraints between transiently interacting protein pairs in solution. Employing linkage distance restraint-guided molecular modeling, we report the oriented solution docking geometry of a preTCRβ-pMHC interaction. The linkage model of preTCRβ-pMHC complex was independently verified with paramagnetic pseudocontact chemical shift (PCS) NMR of the unlinked protein mixtures. Using linkage screens, we show that the preTCR binds with differing affinities to peptides presented by MHC in solution. Moreover, the C-terminal peptide segment is a key determinant in preTCR-pMHC recognition. We also describe the process for future large-scale production and purification of the linked constructs for NMR, X-ray crystallography, and single-molecule electron microscopy studies.

Regulations of T Cell Activation by Membrane and Cytoskeleton

Among various types of membrane proteins that are regulated by cytoskeleton, the T cell receptor (TCR) greatly benefits from these cellular machineries for its function. The T cell is activated by the ligation of TCR to its target agonist peptide. However, the binding affinity of the two is not very strong, while the T cell needs to discriminate agonist from many nonagonist peptides. Moreover, the strength and duration of the activation signaling need to be tuned for immunological functions. Many years of investigations revealed that dynamic acto-myosin cytoskeletons and plasma membranes in T cells facilitate such regulations by modulating the spatiotemporal distributions of proteins in plasma membranes and by applying mechanical loads on proteins. In these processes, protein dynamics in multiple scales are involved, ranging from collective molecular motions and macroscopic molecular organizations at the cell–cell interface to microscopic changes in distances between receptor and ligand molecules. In this review, details of how cytoskeletons and membranes regulate these processes are discussed, with the emphasis on how all these processes are coordinated to occur within a single cell system.