TCR dynamics on the surface of living T cells

INPP5E regulates CD3ζ enrichment at the immune synapse by phosphoinositide distribution control

The immune synapse, a highly organized structure formed at the interface between T lymphocytes and antigen-presenting cells (APCs), is essential for T cell activation and the adaptive immune response. It has been shown that this interface shares similarities with the primary cilium, a sensory organelle in eukaryotic cells, although the roles of ciliary proteins on the immune synapse remain elusive. Here, we find that inositol polyphosphate-5-phosphatase E (INPP5E), a cilium-enriched protein responsible for regulating phosphoinositide localization, is enriched at the immune synapse in Jurkat T-cells during superantigen-mediated conjugation or antibody-mediated crosslinking of TCR complexes, and forms a complex with CD3ζ, ZAP-70, and Lck. Silencing INPP5E in Jurkat T-cells impairs the polarized distribution of CD3ζ at the immune synapse and correlates with a failure of PI(4,5)P2 clearance at the center of the synapse. Moreover, INPP5E silencing decreases proximal TCR signaling, including phosphorylation of CD3ζ and ZAP-70, and ultimately attenuates IL-2 secretion. Our results suggest that INPP5E is a new player in phosphoinositide manipulation at the synapse, controlling the TCR signaling cascade.

CD4 and CD8 co-receptors modulate functional avidity of CD1b-restricted T cells

T cells recognize mycobacterial glycolipid (mycolipid) antigens presented by CD1b molecules, but the role of CD4 and CD8 co-receptors in mycolipid recognition is unknown. Here we show CD1b-mycolipid tetramers reveal a hierarchy in which circulating T cells expressing CD4 or CD8 co-receptor stain with a higher tetramer mean fluorescence intensity than CD4-CD8- T cells. CD4+ primary T cells transduced with mycolipid-specific T cell receptors bind CD1b-mycolipid tetramer with a higher fluorescence intensity than CD8+ primary T cells. The presence of either CD4 or CD8 also decreases the threshold for interferon-γ secretion. Co-receptor expression increases surface expression of CD3ε, suggesting a mechanism for increased tetramer binding and activation. Targeted transcriptional profiling of mycolipid-specific T cells from individuals with active tuberculosis reveals canonical markers associated with cytotoxicity among CD8+ compared to CD4+ T cells. Thus, expression of co-receptors modulates T cell receptor avidity for mycobacterial lipids, leading to in vivo functional diversity during tuberculosis disease.

Tuning cellular uptake of nanoparticles via ligand density: Contribution of configurational entropy

The bioactivity of nanoparticles (NPs) crucially depends on their ability to cross biological membranes. A fundamental understanding of cell-NP interaction is hence essential to improve the performance of the NP-based biomedical applications. Although extensive studies of cellular uptake have converged upon the idea that the uptake process is mainly regulated by the elastic deformation of the cell membrane or NP, recent experimental observations indicate the ligand density as another critical factor in modulating NP uptake into cells. In this study, we propose a theoretical model of the wrapping of an elastic vesicle NP by a finite lipid membrane to depict the relevant energetic and morphological evolutions during the wrapping process driven by forming receptor-ligand bonds. In this model, the deformations of the membrane and the vesicle NP are assumed to follow the continuum Canham-Helfrich framework, whereas the change of configurational entropy of receptors is described from statistical thermodynamics. Results show that the ligand density strongly affects the binding energy and configurational entropy of free receptors, thereby altering the morphology of the vesicle-membrane system in the steady wrapping state. For the wrapping process by the finite lipid membrane, we also find that there exists optimal ligand density for the maximum wrapping degree. These predictions are consistent with relevant experimental observations reported in the literature. We have further observed that there are transitions of various wrapping phases (no wrapping, partial wrapping, and full wrapping) in terms of ligand density, membrane tension, and molecular binding energy. In particular, the ligand and receptor shortage regimes for the small and high ligand density are, respectively, identified. These results may provide guidelines for the rational design of nanocarriers for drug delivery.

ERM-Dependent Assembly of T Cell Receptor Signaling and Co-stimulatory Molecules on Microvilli prior to Activation

T cell surfaces are covered with microvilli, actin-rich and flexible protrusions. We use super-resolution microscopy to show that ≥90% of T cell receptor (TCR) complex molecules TCRαβ and TCRζ, as well as the co-receptor CD4 (cluster of differentiation 4) and the co-stimulatory molecule CD2, reside on microvilli of resting human T cells. Furthermore, TCR proximal signaling molecules involved in the initial stages of the immune response, including the protein tyrosine kinase Lck (lymphocyte-specific protein tyrosine kinase) and the key adaptor LAT (linker for activation of T cells), are also enriched on microvilli. Notably, phosphorylated proteins of the ERM (ezrin, radixin, and moesin) family colocalize with TCRαβ as well as with actin filaments, implying a role for one or more ERMs in linking the TCR complex to the actin cytoskeleton within microvilli. Our results establish microvilli as key signaling hubs, in which the TCR complex and its proximal signaling molecules and adaptors are preassembled prior to activation in an ERM-dependent manner, facilitating initial antigen sensing.

Quantitative Bio-Imaging Tools to Dissect the Interplay of Membrane and Cytoskeletal Actin Dynamics in Immune Cells

Cellular function is reliant on the dynamic interplay between the plasma membrane and the actin cytoskeleton. This critical relationship is of particular importance in immune cells, where both the cytoskeleton and the plasma membrane work in concert to organize and potentiate immune signaling events. Despite their importance, there remains a critical gap in understanding how these respective dynamics are coupled, and how this coupling in turn may influence immune cell function from the bottom up. In this review, we highlight recent optical technologies that could provide strategies to investigate the simultaneous dynamics of both the cytoskeleton and membrane as well as their interplay, focusing on current and future applications in immune cells. We provide a guide of the spatio-temporal scale of each technique as well as highlighting novel probes and labels that have the potential to provide insights into membrane and cytoskeletal dynamics. The quantitative biophysical tools presented here provide a new and exciting route to uncover the relationship between plasma membrane and cytoskeletal dynamics that underlies immune cell function.

Soft Polydimethylsiloxane-Supported Lipid Bilayers for Studying T Cell Interactions

Much of what we know about the early stages of T cell activation has been obtained from studies of T cells interacting with glass-supported lipid bilayers that favor imaging but are orders of magnitude stiffer than typical cells. We developed a method for attaching lipid bilayers to polydimethylsiloxane polymer supports, producing "soft bilayers" with physiological levels of mechanical resistance (Young's modulus of 4 kPa). Comparisons of T cell behavior on soft and glass-supported bilayers revealed that whereas late stages of T cell activation are thought to be substrate-stiffness dependent, early calcium signaling was unaffected by substrate rigidity, implying that early steps in T cell receptor triggering are not mechanosensitive. The exclusion of large receptor-type phosphatases was observed on the soft bilayers, however, even though it is yet to be demonstrated at authentic cell-cell contacts. This work sets the stage for an imaging-based exploration of receptor signaling under conditions closely mimicking physiological cell-cell contact.

n-3 Polyunsaturated Fatty Acids Impede the TCR Mobility and the TCR-pMHC Interaction of Anti-Viral CD8+ T Cells

The immune-suppressive effects of omega-3 (n-3) polyunsaturated fatty acids (PUFAs) on T cells have been observed via multiple in vitro and in vivo models. However, the precise mechanism that causes these effects is still undefined. In this study, we investigated whether n-3 PUFAs regulated T cell receptor (TCR) and peptide-major histocompatibility complex (pMHC) interactions. The expansion of anti-viral CD8⁺ T cells that endogenously synthesize n-3 PUFAs (FAT-1) dramatically decreased upon lymphocytic choriomeningitis virus (LCMV) infection in vivo. This decrease was not caused by the considerable reduction of TCR expression or the impaired chemotactic activity of T cells. Interestingly, a highly inclined and laminated optical sheet (HILO) microscopic analysis revealed that the TCR motility was notably reduced on the surface of the FAT-1 CD8⁺ T cells compared to the wild type (WT) CD8⁺ T cells. Importantly, the adhesion strength of the FAT-1 CD8⁺ T cells to the peptide-MHC was significantly lower than that of the WT CD8⁺T cells. Consistent with this result, treatment with docosahexaenoic acid (DHA), one type of n-3 PUFA, significantly decreased CD8⁺ T cell adhesion to the pMHC. Collectively, our results reveal a novel mechanism through which n-3 PUFAs decrease TCR-pMHC interactions by modulating TCR mobility on CD8⁺ T cell surfaces.

A numerical mechanical model integrating actin treadmilling and receptor recycling to explain selective disengagement of immune cells

T-killer cells of the immune system eliminate virus-infected cells by releasing toxic granules through a direct cell to cell surface interface (synapse). The release of toxic granules only through the cell surface interface assures the specificity of the immune response. The toxic granule releasing apparatus, however, may not be aligned toward the synapse at the moment of the synapse formation. Therefore, mechanisms for reorienting the killing apparatus inside the T-killer cell to the interface with the target is required. Numerous research works were reported to suggest the mechanisms with direct and indirect evidence, but the most adversary situation, that is when the cell's initial orientation is the complete opposite of the desired direction, either remained answered or brought skepticism toward the suggested mechanisms. To address this issue, a computational mechanical model of T-killer cell synapse formation is constructed to test previously suggested models in a more realistic setting and at the same time to test previously neglected component in the model, namely the actin network formation on the synapse. In this model, by capturing the mechanical interaction between T-killer cell surface receptor dynamics and mechanical properties of synapse formation, it is shown that T-killer cells can selectively engage or disengage from the target cell depending on the killing apparatus alignment with the target. The mechanism works as a safeguard measure ensuring target-cell killing and specificity, so it will be effective when T-killer cells are stranded in situations where the alignment of the killing apparatus is challenging.

Mechanical feedback enables catch bonds to selectively stabilize scanning microvilli at T-cell surfaces

T-cells use microvilli to search the surfaces of antigen-presenting cells for antigenic ligands. The active motion of scanning microvilli provides a force-generating mechanism that is intriguing in light of single-molecule experiments showing that applied forces increase the lifetimes of stimulatory receptor–ligand bonds (catch-bond behavior). In this work, we introduce a theoretical framework to explore the motion of a microvillar tip above an antigen-presenting surface when receptors on the tip stochastically bind to ligands on the surface and dissociate from them in a force-dependent manner. Forces on receptor-ligand bonds impact the motion of the microvillus, leading to feedback between binding and microvillar motion. We use computer simulations to show that the average microvillar velocity varies in a ligand-dependent manner; that catch bonds generate responses in which some microvilli almost completely stop, while others move with a broad distribution of velocities; and that the frequency of stopping depends on the concentration of stimulatory ligands. Typically, a small number of catch bonds initially immobilize the microvillus, after which additional bonds accumulate and increase the cumulative receptor-engagement time. Our results demonstrate that catch bonds can selectively slow and stabilize scanning microvilli, suggesting a physical mechanism that may contribute to antigen discrimination by T-cells.

Single-Molecule Light-Sheet Imaging of Suspended T Cells

Adaptive immune responses are initiated by triggering of the T cell receptor. Single-molecule imaging based on total internal reflection fluorescence microscopy at coverslip/basal cell interfaces is commonly used to study this process. These experiments have suggested, unexpectedly, that the diffusional behavior and organization of signaling proteins and receptors may be constrained before activation. However, it is unclear to what extent the molecular behavior and cell state is affected by the imaging conditions, i.e., by the presence of a supporting surface. In this study, we implemented single-molecule light-sheet microscopy, which enables single receptors to be directly visualized at any plane in a cell to study protein dynamics and organization in live, resting T cells. The light sheet enabled the acquisition of high-quality single-molecule fluorescence images that were comparable to those of total internal reflection fluorescence microscopy. By comparing the apical and basal surfaces of surface-contacting T cells using single-molecule light-sheet microscopy, we found that most coated-glass surfaces and supported lipid bilayers profoundly affected the diffusion of membrane proteins (T cell receptor and CD45) and that all the surfaces induced calcium influx to various degrees. Our results suggest that, when studying resting T cells, surfaces are best avoided, which we achieve here by suspending cells in agarose.

Interplay between membrane elasticity and active cytoskeleton forces regulates the aggregation dynamics of the immunological synapse

Adhesion between a T cell and an antigen presenting cell is achieved by TCR-pMHC and LFA1-ICAM1 protein complexes. These segregate to form a special pattern, known as the immunological synapse (IS), consisting of a central quasi-circular domain of TCR-pMHC bonds surrounded by a peripheral domain of LFA1-ICAM1 complexes. Insights gained from imaging studies had led to the conclusion that the formation of the central adhesion domain in the IS is driven by active (ATP-driven) mechanisms. Recent studies, however, suggested that passive (thermodynamic) mechanisms may also play an important role in this process. Here, we present a simple physical model, taking into account the membrane-mediated thermodynamic attraction between the TCR-pMHC bonds and the effective forces that they experience due to ATP-driven actin retrograde flow and transport by dynein motor proteins. Monte Carlo simulations of the model exhibit a good spatio-temporal agreement with the experimentally observed pattern evolution of the TCR-pMHC microclusters. The agreement is lost when one of the aggregation mechanisms is "muted", which helps to identify their respective roles in the process. We conclude that actin retrograde flow drives the centripetal motion of TCR-pMHC bonds, while the membrane-mediated interactions facilitate microcluster formation and growth. In the absence of dynein motors, the system evolves into a ring-shaped pattern, which highlights the role of dynein motors in the formation of the final concentric pattern. The interplay between the passive and active mechanisms regulates the rate of the accumulation process, which in the absence of one them proceeds either too quickly or slowly.

T cell receptor repertoire usage in cancer as a surrogate marker for immune responses

Characterizing the interaction of cancer cells with the host adaptive immune system is critical for understanding tumor immunology and the modus operandi of immunotherapeutic interventions to treat cancer. As the key cellular effectors of adaptive immunity, T cells are endowed with specialized receptors (the T cell receptor; TCR), to recognize and to eliminate cancer cells. The diversity of the TCR repertoire results from specialized genetic diversification mechanisms that generate an incredible variability allowing recognizing extensive collections of antigens. Based on the attainment and function of the TCR, the TCR repertoire is a mirror of the human immune response, and the dynamic changes of its usage can be assumed as a promising biomarker to monitor immunomodulatory therapies. Recent advances in multiplexed PCR amplification and massive parallel sequencing technologies have facilitated the characterization of TCR repertoires at high resolution even when only biomaterial of limited quantity and quality, such as formalin-fixed paraffin-embedded (FFPE) archived tissues, is available. Here, we review the concept framework and current experimental approaches to characterize the TCR repertoire usage in cancer including inherent technical and biological challenges.

Endocytosis of Cytotoxic Granules Is Essential for Multiple Killing of Target Cells by T Lymphocytes

CTLs are serial killers that kill multiple target cells via exocytosis of cytotoxic granules (CGs). CG exocytosis is tightly regulated and has been investigated in great detail; however, whether CG proteins are endocytosed following exocytosis and contribute to serial killing remains unknown. By using primary CTLs derived from a knock-in mouse of the CG membrane protein Synaptobrevin2, we show that CGs are endocytosed in a clathrin- and dynamin-dependent manner. Following acidification, endocytosed CGs are recycled through early and late, but not recycling endosomes. CGs are refilled with granzyme B at the late endosome stage and polarize to subsequent synapses formed between the CTL and new target cells. Importantly, inhibiting CG endocytosis in CTLs results in a significant reduction of their cytotoxic activity. Thus, our data demonstrate that continuous endocytosis of CG membrane proteins is a prerequisite for efficient serial killing of CTLs and identify key events in this process.

Usp12 stabilizes the T-cell receptor complex at the cell surface during signaling

Posttranslational modifications are central to the spatial and temporal regulation of protein function. Among others, phosphorylation and ubiquitylation are known to regulate proximal T-cell receptor (TCR) signaling. Here we used a systematic and unbiased approach to uncover deubiquitylating enzymes (DUBs) that participate during TCR signaling in primary mouse T lymphocytes. Using a C-terminally modified vinyl methyl ester variant of ubiquitin (HA-Ub-VME), we captured DUBs that are differentially recruited to the cytosol on TCR activation. We identified ubiquitin-specific peptidase (Usp) 12 and Usp46, which had not been previously described in this pathway. Stimulation with anti-CD3 resulted in phosphorylation and time-dependent translocation of Usp12 from the nucleus to the cytosol. Usp12(-/-) Jurkat cells displayed defective NFκB, NFAT, and MAPK activities owing to attenuated surface expression of TCR, which were rescued on reconstitution of wild type Usp12. Proximity-based labeling with BirA-Usp12 revealed several TCR adaptor proteins acting as interactors in stimulated cells, of which LAT and Trat1 displayed reduced expression in Usp12(-/-) cells. We demonstrate that Usp12 deubiquitylates and prevents lysosomal degradation of LAT and Trat1 to maintain the proximal TCR complex for the duration of signaling. Our approach benefits from the use of activity-based probes in primary cells without any previous genome modification, and underscores the importance of ubiquitin-mediated regulation to refine signaling cascades.

Elastohydrodynamics and Kinetics of Protein Patterning in the Immunological Synapse

We propose a minimal mathematical model for the physical basis of membrane protein patterning in the immunological synapse (IS), which encompass membrane mechanics, protein binding kinetics and motion, and fluid flow in the synaptic cleft. Our theory leads to simple predictions for the spatial and temporal scales of protein cluster formation, growth and arrest as a function of membrane stiffness, rigidity and kinetics of the adhesive proteins, and the fluid flow in the synaptic cleft. Numerical simulations complement these scaling laws by quantifying the nucleation, growth and stabilization of proteins domains on the size of the cell. Direct comparison with experiment shows that passive elastohydrodynamics and kinetics of protein binding in the synaptic cleft can describe the short-time formation and organization of protein clusters, without evoking any active processes in the cytoskeleton. Despite the apparent complexity of the process, our analysis shows that just two dimensionless parameters characterize the spatial and temporal evolution of the protein pattern: a ratio of membrane elasticity to protein stiffness, and the ratio of a hydrodynamic time scale for fluid flow relative to the protein binding rate. A simple phase diagram encompasses the variety of patterns that can arise.

The coreceptor CD4 is expressed in distinct nanoclusters and does not colocalize with T-cell receptor and active protein tyrosine kinase p56lck

CD4 molecules on the surface of T lymphocytes greatly augment the sensitivity and activation process of these cells, but how it functions is not fully understood. Here we studied the spatial organization of CD4, and its relationship to T-cell antigen receptor (TCR) and the active form of Src kinase p56lck (Lck) using single and dual-color photoactivated localization microscopy (PALM) and direct stochastic optical reconstruction microscopy (dSTORM). In nonactivated T cells, CD4 molecules are clustered in small protein islands, as are TCR and Lck. By dual-color imaging, we find that CD4, TCR, and Lck are localized in their separate clusters with limited interactions in the interfaces between them. Upon T-cell activation, the TCR and CD4 begin clustering together, developing into microclusters, and undergo a larger scale redistribution to form supramolecluar activation clusters (SMACs). CD4 and Lck localize in the inner TCR region of the SMAC, but this redistribution of disparate cluster structures results in enhanced segregation from each other. In nonactivated cells these preclustered structures and the limited interactions between them may serve to limit spontaneous and random activation events. However, the small sizes of these island structures also ensure large interfacial surfaces for potential interactions and signal amplification when activation is initiated. In the later activation stages, the increasingly larger clusters and their segregation from each other reduce the interfacial surfaces and could have a dampening effect. These highly differentiated spatial distributions of TCR, CD4, and Lck and their changes during activation suggest that there is a more complex hierarchy than previously thought.

The C-terminus SH3-binding domain of Kv1.3 is required for the actin-mediated immobilization of the channel via cortactin

Polarization of Kv1.3 channels is a necessary step in T-cell activation and motility. Nonetheless, the mechanisms regulating the Kv1.3 channel's membrane movement are not understood. This study provides evidence that cortactin, an actin-binding protein, controls Kv1.3 membrane motility via the channel's SH3 domain.

Kv1.3 channels play a pivotal role in the activation and migration of T-lymphocytes. These functions are accompanied by the channels' polarization, which is essential for associated downstream events. However, the mechanisms that govern the membrane movement of Kv1.3 channels remain unclear. F-actin polymerization occurs concomitantly to channel polarization, implicating the actin cytoskeleton in this process. Here we show that cortactin, a factor initiating the actin network, controls the membrane mobilization of Kv1.3 channels. FRAP with EGFP-tagged Kv1.3 channels demonstrates that knocking down cortactin decreases the actin-based immobilization of the channels. Using various deletion and mutation constructs, we show that the SH3 motif of Kv1.3 mediates the channel immobilization. Proximity ligation assays indicate that deletion or mutation of the SH3 motif also disrupts interaction of the channel with cortactin. In T-lymphocytes, the interaction between HS1 (the cortactin homologue) and Kv1.3 occurs at the immune synapse and requires the channel's C-terminal domain. These results show that actin dynamics regulates the membrane motility of Kv1.3 channels. They also provide evidence that the SH3 motif of the channel and cortactin plays key roles in this process.

Dynamics of natural killer cell receptor revealed by quantitative analysis of photoswitchable protein

Natural Killer (NK) cell activation is dynamically regulated by numerous activating and inhibitory surface receptors that accumulate at the immune synapse. Quantitative analysis of receptor dynamics has been limited by methodologies that rely on indirect measurements such as fluorescence recovery after photobleaching. Here, we report an apparently novel approach to study how proteins traffic to and from the immune synapse using NK cell receptors tagged with the photoswitchable fluorescent protein tdEosFP, which can be irreversibly photoswitched from a green to red fluorescent state by ultraviolet light. Thus, after a localized switching event, the movement of the photoswitched molecules can be temporally and spatially resolved by monitoring fluorescence in two regions of interest. By comparing images with mathematical models, we evaluated the diffusion coefficient of the receptor KIR2DL1 (0.23 ± 0.06 μm(2) s(-1)) and assessed how synapse formation affects receptor dynamics. Our data conclude that the inhibitory NK cell receptor KIR2DL1 is continually trafficked into the synapse, and remains surprisingly stable there. Unexpectedly, however, in NK cells forming synapses with multiple target cells simultaneously, KIR2DL1 at one synapse can relocate to another synapse. Thus, our results reveal a previously undetected intersynaptic exchange of protein.

Orchestrating cytoskeleton and intracellular vesicle traffic to build functional immunological synapses

Immunological synapses are specialized cell-cell contacts formed between T lymphocytes and antigen-presenting cells. They are induced upon antigen recognition and are crucial for T-cell activation and effector functions. The generation and function of immunological synapses depend on an active T-cell polarization process, which results from a finely orchestrated crosstalk between the antigen receptor signal transduction machinery, the actin and microtubule cytoskeletons, and controlled vesicle traffic. Although we understand how some of these particular events are regulated, we still lack knowledge on how these multiple cellular elements are harmonized to ensure appropriate T-cell responses. We discuss here our view on how T-cell receptor signal transduction initially commands cytoskeletal and vesicle traffic polarization, which in turn sets the immunological synapse molecular design that regulates T-cell activation. We also discuss how the human immunodeficiency virus (HIV-1) hijacks some of these processes impairing immunological synapse generation and function.

Relaxation estimation of RMSD in molecular dynamics immunosimulations

Molecular dynamics simulations have to be sufficiently long to draw reliable conclusions. However, no method exists to prove that a simulation has converged. We suggest the method of "lagged RMSD-analysis" as a tool to judge if an MD simulation has not yet run long enough. The analysis is based on RMSD values between pairs of configurations separated by variable time intervals Δt. Unless RMSD(Δt) has reached a stationary shape, the simulation has not yet converged.

Formation of BCR oligomers provides a mechanism for B cell affinity discrimination

B cells encounter antigen over a wide affinity range, from K(A)=10(5) M(-1) to K(A)=10(10) M(-1). The strength of B cell antigen receptor (BCR) signaling in response to antigen increases with affinity, a process known as "affinity discrimination". In this work, we use a computational simulation of B cell surface dynamics and membrane-proximal signaling to show that affinity discrimination can arise from the formation of BCR oligomers. It is known that BCRs form oligomers upon encountering antigen, and that the size and rate of formation of these oligomers both increase with affinity. In our simulation, we have introduced a requirement that only BCR-antigen complexes that are part of an oligomer can engage cytoplasmic signaling molecules such as Src-family kinases. Our simulation shows that as affinity increases, BCR signaling activity increases in addition to the number of collected antigen. Our results are also consistent with the existence of an experimentally-observed threshold affinity of activation at K(A)=10(5)-10(6) M(-1) (no signaling activity below this affinity value) and affinity discrimination ceiling of K(A)=10(10) M(-1) (no affinity discrimination above this affinity value). Comparison with experiments shows that the time scale of BCR oligomer formation predicted by our model (less than 10 s) is well within the time scale of experimentally observed association of BCR with Src-family kinases (10-20 s).

Dissecting T-cell activation with high-resolution live-cell microscopy

Results from live-cell microscopy suggest that the behaviour of isolated components of the T-cell activation machinery in vitro does not represent the reality inside cells. Understanding the cellular-scale dynamics of microcluster migration can only be accomplished by in situ observation. Developments in 'super-resolution' microscopy have permitted investigators to move beyond tracking the movements of individual molecules, allowing the recognition of protein islands and nanodomains present in quiescent and active T cells. Many high-resolution techniques have their own susceptibilities to artefacts, so it is important to take a multifaceted approach to confirm results. A major challenge for the future will be to integrate all the new information into a coherent model of antigen recognition and T-cell activation.

Discrimination of membrane antigen affinity by B cells requires dominance of kinetic proofreading over serial engagement

B-cell receptor signaling in response to membrane-bound antigen increases with antigen affinity, a process known as affinity discrimination. We use computational modeling to show that B-cell affinity discrimination requires that kinetic proofreading predominate over serial engagement. We find that if B-cell receptors become signaling-capable immediately upon antigen binding, which results in decreasing serial engagement as affinity increases, then increasing affinity can lead to weaker signaling. Rather, antigen must stay bound to B-cell receptors for a threshold time of several seconds before becoming signaling-capable, a process similar to kinetic proofreading. This process overcomes the loss in serial engagement due to increasing antigen affinity, and replicates the monotonic increase in B-cell signaling with increasing affinity that has been observed in B-cell activation experiments. This finding matches well with the experimentally observed time (∼20 s) required for the B-cell receptor signaling domains to undergo antigen and lipid raft-mediated conformational changes that lead to Src-family kinase recruitment. We hypothesize that the physical basis for a threshold time of antigen binding might lie in the formation timescale of B-cell receptor dimers. The time required for dimer formation decreases with increasing antigen affinity, thereby resulting in shorter threshold antigen binding times as affinity increases. Such an affinity-dependent kinetic proofreading requirement results in affinity discrimination very similar to that observed in biological experiments. B-cell affinity discrimination is critical to the process of affinity maturation and the production of high-affinity antibodies, and thus our results have important implications in applications such as vaccine design.

Measuring immune receptor mobility by fluorescence recovery after photobleaching

The coordinated effort of cells in the immune system relies heavily on surface receptor interactions. Immune receptor mobility provides vital information on the function and responses of immune cells, and these measurements shed light on their interactions with other membrane, cytosolic, and extracellular matrix proteins. These measurements can be obtained using the fluorescence recovery after photobleaching (FRAP) technique in living cells. We describe here general approaches for FRAP using green fluorescent protein fusion proteins.

Monte Carlo study of B-cell receptor clustering mediated by antigen crosslinking and directed transport

It is known from experiments that in the presence of soluble antigen, B-cell receptors (BCRs) assemble into microclusters and then collect into a macrocluster known as a 'cap'. However, the mechanisms of BCR cluster formation during recognition of soluble antigens remain unclear. In previous work, we demonstrated that effective intrinsic attractions among BCRs can lead to the formation of small microclusters of BCR molecules. The effective intrinsic attractions could be caused by multivalent antigen binding, association with lipid rafts, or other biochemical factors. In the present study, we have developed and studied a Monte Carlo model of BCR clustering mediated by explicit binding and crosslinking of soluble bivalent antigens. Antigen crosslinking is shown to microcluster BCRs in an affinity-dependent manner and also in a biologically relevant timescale; however, antigen crosslinking alone does not appear to be sufficient for the formation of a single macrocluster of receptor molecules. We show that directed transport of BCRs is needed to drive the formation of large macroclusters. We constructed a simple model of directed transport, where BCR molecules diffuse towards the largest cluster or towards a random BCR microcluster, which results in a single macrocluster of receptor molecules. The mechanisms for both types of directed transport are compared using network-based metrics. We also develop and use appropriate network measures to analyze the effect of BCR and antigen concentration on BCR clustering, the stability of the formed clusters over time and the size of BCR-antigen crosslinked chains.

Syntaxin7 is required for lytic granule release from cytotoxic T lymphocytes

SNARE proteins are essential fusion mediators for many intracellular trafficking events. Here, we investigate the role of Syntaxin7 (Stx7) in the release of lytic granules from cytotoxic T lymphocytes (CTLs). We show that Stx7 is expressed in CTLs and is preferentially localized to the region of lytic granule release, the immunological synapse (IS). Interference of Stx7 function by expression of a dominant-negative Stx7 construct or by small interfering RNA leads to a dramatic reduction of CTL-mediated killing of target cells. Real-time visualization of individual lytic granules at the IS by evanescent wave microscopy reveals that lytic granules in Stx7-deprived CTLs not only fail to fuse with the plasma membrane but even fail to accumulate at the IS. Surprisingly, the accumulation defect is not caused by an overall reduction in lytic granule number, but by a defect in the trafficking of T cell receptors (TCRs) through endosomes. Subsequent high-resolution nanoscopy shows that Stx7 colocalizes with Rab7 on late endosomes. We conclude from these data that the accumulation of recycling TCRs at the IS is a SNARE-dependent process and that Stx7-mediated processing of recycling TCRs through endosomes is a prerequisite for the cytolytic function of CTLs.

Recycling and LFA-1-dependent trafficking of ICAM-1 to the immunological synapse

Little is known about how adhesion molecules on APCs accumulate at immunological synapses. We show here that ICAM-1 on APCs is continuously internalized and rapidly recycled back to the interface after antigen-priming T-cell contact. The internalization rate is high in APCs, including Raji B cells and dendritic cells, but low in endothelial cells. Internalization is significantly reduced by inhibitors of Na(+)/H(+) exchangers (NHEs), suggesting that members of the NHE-family regulate this process. Once internalized, ICAM-1 is co-localized with MHC class II in the polarized recycling compartment. Surprisingly, not only ICAM-1, but also MHC class II, is targeted to the immunological synapse through LFA-1-dependent adhesion. Cytosolic ICAM-1 is highly mobile and forms a tubular structure. Inhibitors of microtubule or actin polymerization can reduce ICAM-1 mobility, and thereby block accumulation at immunological synapses. Membrane ICAM-1 also moves to the T-cell contact zone, presumably through an active, cytoskeleton-dependent mechanism. Collectively, these results demonstrate that ICAM-1 can be transported to the immunological synapse through the recycling compartment. Furthermore, the high-affinity state of LFA-1 on T cells is critical to induce targeted movements of both ICAM-1 and MHC class II to the immunological synapse on APCs.

A mathematical framework for analyzing T cell receptor scanning of peptides

T cells continuously search for antigenic peptides presented on major histocompatibility complexes expressed on nearly all nucleated cells. Because only a few antigenic peptides are presented in a sea of thousands of self-peptides, the T cells have a critical task in discriminating between self- and nonself-peptides. This search process for antigens must be performed with sufficient speed in order to induce a fast response against invading pathogens. This study presents a mathematical framework for analyzing the scanning process of peptides. The framework includes analytic expressions for calculating the sampling rate as well as continuous-systems- and stochastic-agent-based models. The results show that the scanning of self-peptides is a very fast process due to fast off-rates. The simulations also predict the existence of an optimal sampling rate for a certain range of on-rates based on the recently proposed confinement time model. Calculations reveal that most of the self-peptides located within a microdomain are scanned within just a few seconds, and that the T cell receptors have kinetics for self-peptides, facilitating fast scanning. The derived mathematical expressions within this study provide conceptual calculations for further investigations of how the T cell discriminates between self- and nonself-peptides.

Variability of CD3 membrane expression and T cell activation capacity

BACKGROUND

AlphabetaT cells have a wide distribution of CD3 membrane density. The aim of this article was to evaluate the significance of the CD3 differential expression on T cell subsets. Analysis was performed on healthy donors and renal transplant patients by flow cytometry. The results obtained are: (1) CD3 expression was widely distributed (CV = 38.3 +/- 3.1 to 43 +/- 2.3%). (2) The CD4, CD8, CD45 and forward scatter were similarly distributed. (3) The diversity of CD3 expression was directly related to the clonotypes: gamma9, non gamma9 from gammadeltaT cells and Vbeta clonotype from alphabetaT cells (e.g., Vbeta3FITC 7,980 +/- 1,628 Vbeta8PE: Vbeta20-FITC 11,768 +/- 1,510). (4) Using a computer simulation, we could confirm differential kinetics of T cell activation according to the initial parameters. Finally, in vitro activation was significantly higher on Vbeta8 and Vbeta9 (high CD3) compared with Vbeta2 and Vbeta3 (low CD3, P = 0.040-0.0003). In conclusion, T cells have highly heterogeneous CD3 expression, possibly predetermined and with clear functional significance.

Monte Carlo investigation of diffusion of receptors and ligands that bind across opposing surfaces

Studies of receptor diffusion on a cell surface show a variety of behaviors, such as diffusive, sub-diffusive, or super-diffusive motion. However, most studies to date focus on receptor molecules diffusing on a single cell surface. We have previously studied receptor diffusion to probe the molecular mechanism of receptor clustering at the cell–cell junction between two opposing cell surfaces. Here, we characterize the diffusion of receptors and ligands that bind to each other across two opposing cell surfaces, as in cell–cell and cell–bilayer interactions. We use a Monte Carlo method, where receptors and ligands are simulated as independent agents that bind and diffuse probabilistically. We vary receptor–ligand binding affinity and plot the molecule-averaged mean square displacement (MSD) of ligand molecules as a function of time. Our results show that MSD plots are qualitatively different for flat and curved interfaces, as well as between the cases of presence and absence of directed transport of receptor–ligand complexes toward a specific location on the interface. Receptor–ligand binding across two opposing surfaces leads to transient sub-diffusive motion at early times provided the interface is flat. This effect is entirely absent if the interface is curved, however, in this instance we observe sub-diffusive motion. In addition, a decrease in the equilibrium value of the MSD occurs as affinity increases, something which is absent for a flat interface. In the presence of directed transport of receptor–ligand complexes, we observe super-diffusive motion at early times for a flat interface. Super-diffusive motion is absent for a curved interface, however, in this case we observe a transient decrease in MSD with time prior to equilibration for high-affinity values.

Modeling of B cell Synapse Formation by Monte Carlo Simulation Shows That Directed Transport of Receptor Molecules Is a Potential Formation Mechanism

The formation of the protein segregation structure known as the "immunological synapse" in the contact region between B cells and antigen presenting cells appears to precede antigen (Ag) uptake by B cells. The mature B cell synapse consists of a central cluster of B cell receptor/Antigen (BCR/Ag) complexes surrounded by a ring of LFA-1/ICAM-1 complexes. In this study, we used an in silico model to investigate whether cytoskeletally driven transport of molecules toward the center of the contact zone is a potential mechanism of immunological synapse formation in B cells. We modeled directed transport by the cytoskeleton in an effective manner, by biasing the diffusion of molecules toward the center of the contact zone. Our results clearly show that biased diffusion of BCR/Ag complexes on the B cell surface is sufficient to produce patterns similar to experimentally observed immunological synapses. This is true even in the presence of significant membrane deformation as a result of receptor-ligand binding, which in previous work we showed had a detrimental effect on synapse formation at high antigen affinity values. Comparison of our model's results to those of experiments shows that our model produces synapses for realistic length, time, and affinity scales. Our results also show that strong biased diffusion of free molecules has a negative effect on synapse formation by excluding BCR/Ag complexes from the center of the contact zone. However, synapses may still form provided the bias in diffusion of free molecules is an order-of-magnitude weaker than that of BCR/Ag complexes. We also show how diffusion trajectories obtained from single-molecule tracking experiments can generate insight into the mechanism of synapse formation.

T cell receptor triggering by force

Antigen recognition through the interaction between the T cell receptor (TCR) and peptide presented by major histocompatibility complex (pMHC) is the first step in T cell-mediated immune responses. How this interaction triggers TCR signalling that leads to T cell activation is still unclear. Taking into account the mechanical stress exerted on the pMHC-TCR interaction at the dynamic interface between T cells and antigen presenting cells (APCs), we propose the so-called receptor deformation model of TCR triggering. In this model, TCR conformational change induced by mechanical forces initiates TCR signalling. The receptor deformation model, for the first time, explains all three aspects of the TCR triggering puzzle: mechanism, specificity, and sensitivity.

Mechanisms of cellular communication through intercellular protein transfer

In a multicellular system, cellular communication is a must for orchestration and coordination of cellular events. Advent of the latest analytical and imaging tools has allowed us to enhance our understanding of the intercellular communication. An intercellular exchange of proteins or intact membrane patches is a ubiquitous phenomenon, and has been the subject of renewed interest, particularly in the context of immune cells. Recent evidence implicates that intercellular protein transfers, including trogocytosis is an important mechanism of the immune system to modulate immune responses and transferred proteins can also contribute to pathology. It has been demonstrated that intercellular protein transfer can be through the internalization/pathway, dissociation-associated pathway, uptake of exosomes and membrane nanotube formations. Exchange of membrane molecules/antigens between immune cells has been observed for a long time, but the mechanisms and functional consequences of these transfers remain unclear. In this review, we will discuss the important findings concerning intercellular protein transfers, possible mechanisms and highlight their physiological relevance to the immune system, with special reference to T cells such as the stimulatory or suppressive immune responses derived from T cells with acquired dendritic cell membrane molecules.

Localization of Kv1.3 channels in the immunological synapse modulates the calcium response to antigen stimulation in T lymphocytes

The immunological synapse (IS), a highly organized structure that forms at the point of contact between a T cell and an APC, is essential for the proper development of signaling events, including the Ca(2+) response. Kv1.3 channels control Ca(2+) homeostasis in human T cells and move into the IS upon Ag presentation. However, the process involved in channel accumulation in the IS and the functional implications of this localization are not yet known. Here we define the movement of Kv1.3 into the IS and study whether Kv1.3 localization into the IS influences Ca(2+) signaling in Jurkat T cells. Crosslinking of the channel protein with an extracellular Ab limits Kv1.3 mobility and accumulation at the IS. Moreover, Kv1.3 recruitment to the IS does not involve the transport of newly synthesized channels and it does not occur through recycling of membrane channels. Kv1.3 localization in the IS modulates the Ca(2+) response. Blockade of Kv1.3 movement into the IS by crosslinking significantly increases the amplitude of the Ca(2+) response triggered by anti-CD3/anti-CD28-coated beads, which induce the formation of the IS. On the contrary, the Ca(2+) response induced by TCR stimulation without the formation of the IS with soluble anti-CD3/anti-CD28 Abs is unaltered. The results presented herein indicate that, upon Ag presentation, membrane-incorporated Kv1.3 channels move along the plasma membrane to localize in the IS. This localization is important to control the amplitude of the Ca(2+) response, and disruption of this process can account for alterations of downstream Ca(2+)-dependent signaling events.

Fibroblast growth factor receptor-1 (FGFR1) nuclear dynamics reveal a novel mechanism in transcription control

Nuclear FGFR1 acts as a developmental gene regulator in cooperation with FGF-2, RSK1, and CREB-binding protein (CBP). FRAP analysis revealed three nuclear FGFR1 populations: i) a fast mobile, ii) a slower mobile population reflecting chromatin-bound FGFR1, and iii) an immobile FGFR1 population associated with the nuclear matrix. Factors (cAMP, CBP) that induce FGFR1-mediated gene activation shifted FGFR1 from the nuclear matrix (immobile) to chromatin (slow) and reduced the movement rate of the chromatin-bound population. Transcription inhibitors accelerated FGFR1 movement; the content of the chromatin-bound slow FGFR1 decreased, whereas the fast population increased. The transcriptional activation appears to involve conversion of the immobile matrix-bound and the fast nuclear FGFR1 into a slow chromatin-binding population through FGFR1's interaction with CBP, RSK1, and the high-molecular-weight form of FGF-2. Our findings support a general mechanism in which gene activation is governed by protein movement and collisions with other proteins and nuclear structures.

Effects of intracellular calcium and actin cytoskeleton on TCR mobility measured by fluorescence recovery

BACKGROUND

The activation of T lymphocytes by specific antigen is accompanied by the formation of a specialized signaling region termed the immunological synapse, characterized by the clustering and segregation of surface molecules and, in particular, by T cell receptor (TCR) clustering.

METHODOLOGY/PRINCIPAL FINDINGS

To better understand TCR motion during cellular activation, we used confocal microscopy and photo-bleaching recovery techniques to investigate the lateral mobility of TCR on the surface of human T lymphocytes under various pharmacological treatments. Using drugs that cause an increase in intracellular calcium, we observed a decrease in TCR mobility that was dependent on a functional actin cytoskeleton. In parallel experiments measurement of filamentous actin by FACS analysis showed that raising intracellular calcium also causes increased polymerization of the actin cytoskeleton. These in vitro results were analyzed using a mathematical model that revealed effective binding parameters between TCR and the actin cytoskeleton.

CONCLUSION/SIGNIFICANCE

We propose, based on our results, that increase in intracellular calcium levels leads to actin polymerization and increases TCR/cytoskeleton interactions that reduce the overall mobility of the TCR. In a physiological setting, this may contribute to TCR re-positioning at the immunological synapse.

Monte Carlo study of single molecule diffusion can elucidate the mechanism of B cell synapse formation

B cell receptors have been shown to cluster at the intercellular junction between a B cell and an antigen-presenting cell in the form of a segregated pattern of B cell receptor/antigen complexes known as an immunological synapse. We use random walk-based theoretical arguments and Monte Carlo simulations to study the effect of diffusion of surface-bound molecules on B cell synapse formation. Our results show that B cell synapse formation is optimal for a limited range of receptor-ligand complex diffusion coefficient values, typically one-to-two orders of magnitude lower than the diffusion coefficient of free receptors. Such lower mobility of receptor-ligand complexes can significantly affect the diffusion of a tagged receptor or ligand in an affinity dependent manner, as the binding/unbinding of such receptor or ligand molecules crucially depends on affinity. Our work shows how single molecule tracking experiments can be used to estimate the order of magnitude of the diffusion coefficient of receptor-ligand complexes, which is difficult to measure directly in experiments due to the finite lifetime of receptor-ligand bonds. We also show how such antigen movement data at the single molecule level can provide insight into the B cell synapse formation mechanism. Thus, our results can guide further single molecule tracking experiments to elucidate the synapse formation mechanism in B cells, and potentially in other immune cells.

Rab35 and its GAP EPI64C in T cells regulate receptor recycling and immunological synapse formation

Upon antigen recognition, T-cell receptor (TCR/CD3) and other signaling molecules become enriched in a specialized contact site between the T cell and antigen-presenting cell, i.e. the immunological synapse (IS). Enrichment occurs via mechanisms that include polarized secretion from recycling endosomes, but the Rabs and RabGAPs that regulate this are unknown. EPI64C (TBC1D10C) is an uncharacterized candidate RabGAP we identified by mass spectrometry as abundant in human peripheral blood T cells that is preferentially expressed in hematopoietic cells. EPI64C is a Rab35-GAP based both on in vitro Rab35-specific GAP activity and findings in transfection assays. EPI64C and Rab35 dominant negative (DN) constructs each impaired transferrin export from a recycling pathway in Jurkat T-cells and induced large vacuoles marked by transferrin receptor, TCR, and SNAREs implicated in TCR-polarized secretion. Rab35 localized to the plasma membrane and to intracellular vesicles where it substantially colocalized with TfR and with TCR. Rab35 was strongly recruited to the IS. Conjugate formation was impaired by transfection with Rab35-DN or EPI64C and by EPI64C knock down. TCR enrichment at the IS was impaired by Rab35-DN. Thus, EPI64C and Rab35 regulate a recycling pathway in T cells and contribute to IS formation, most likely by participating in TCR transport to the IS.

Analysis of membrane-localized binding kinetics with FRAP

Interactions between plasma membrane-associated proteins on interacting cells are critical for many important biological processes. Few experimental techniques, however, can accurately determine the association and the dissociation rates between such interacting pairs when the two molecules diffuse on apposing membranes or lipid bilayers. In this study, we give a theoretical description of how and when fluorescence recovery after photobleaching (FRAP) experiments can be used to quantify these reaction rates. We analyze the effect of binding on FRAP recovery curves with a reaction-diffusion model and systematically identify different regimes in the parameter space of the association and the dissociation constants for which the full model simplifies into equivalent one-parameter models. Based on this analysis, we propose an experimental protocol that may be used to identify the kinetic parameters of binding in the appropriate parameter regime. We present simulated experiments illustrating our protocol and lay down guidelines for parameter estimation.

Single-molecule level analysis of the subunit composition of the T cell receptor on live T cells

The T cell receptor (TCR) expressed on most T cells is a protein complex consisting of TCRalphabeta heterodimers that bind antigen and cluster of differentiation (CD) 3epsilondelta, epsilongamma, and zetazeta dimers that initiate signaling. A long-standing controversy concerns whether there is one, or more than one, alphabeta heterodimer per complex. We used a form of single-molecule spectroscopy to investigate this question on live T cell hybridomas. The method relies on detecting coincident fluorescence from single molecules labeled with two different fluorophores, as the molecules diffuse through a confocal volume. The fraction of events that are coincident above the statistical background is defined as the "association quotient," Q. In control experiments, Q was significantly higher for cells incubated with wheat germ agglutinin dual-labeled with Alexa488 and Alexa647 than for cells incubated with singly labeled wheat germ agglutinin. Similarly, cells expressing the homodimer, CD28, gave larger values of Q than cells expressing the monomer, CD86, when incubated with mixtures of Alexa488- and Alexa647-labeled antibody Fab fragments. T cell hybridomas incubated with mixtures of anti-TCRbeta Fab fragments labeled with each fluorophore gave a Q value indistinguishable from the Q value for CD86, indicating that the dominant form of the TCR comprises single alphabeta heterodimers. The values of Q obtained for CD86 and the TCR were low but nonzero, suggesting that there is transient or nonrandom confinement, or diffuse clustering of molecules at the T cell surface. This general method for analyzing the subunit composition of protein complexes could be extended to other cell surface or intracellular complexes, and other living cells.

Real-time detection of lymphocytes binding on an antibody chip using SPR imaging

We demonstrate the use of SPR imaging for the detection of site-specific binding of either B or T lymphocyte populations on an electrochemically-grafted antibody array.

A model for the interplay of receptor recycling and receptor-mediated contact in T cells

Orientation of organelles inside T cells (TC) toward antigen-presenting cells (APC) ensures that the immune response is properly directed, but the orientation mechanisms remain largely unknown. Structural dynamics of TC are coupled to dynamics of T-cell receptor (TCR), which recognizes antigen on the APC surface. Engagement of the TCR triggers its internalization followed by delayed polarized recycling to the plasma membrane through the submembrane recycling compartment (RC), which organelle shares intracellular location with the TC effector apparatus. TCR engagement also triggers TC-APC interface expansion enabling further receptor engagement. To analyze the interplay of the cell-cell contact and receptor dynamics, we constructed a new numerical model. The new model displays the experimentally observed selective stabilization of the contact initiated next to the RC, and only transient formation of contact diametrically opposed to the RC. In the general case wherein the TC-APC contact is initiated in an arbitrary orientation to the RC, the modeling predicts that the contact dynamics and receptor recycling can interact, resulting effectively in migration of the contact to the TC surface domain adjacent to the submembrane RC. Using three-dimensional live-cell confocal microscopy, we obtain data consistent with this unexpected behavior. We conclude that a TC can stabilize its contact with an APC by aligning it with the polarized intracellular traffic of TCR. The results also suggest that the orientation of TC organelles, such as the RC and the effector apparatus, toward the APC can be achieved without any intracellular translocation of the organelles.

Improving parameter estimation for cell surface FRAP data

Fluorescence Recovery After Photobleaching (FRAP) using the confocal laser scanning microscope has become a standard method used to determine the diffusion coefficient and mobile fraction of cell surface proteins. A common experimental approach is to bleach a stripe on the cell surface and fit the ensuing FRAP curve to a 1D diffusion model. This model is derived from the time course of recovery to an infinitely long stripe bleached on an infinite flat plane. This choice of model dictates the use of a long bleach stripe. We demonstrate that, in the case of a long bleach stripe, the finite extent of the cell leads to significant errors in parameter estimation. We further show that these errors are reduced when a relatively small stripe is bleached. Unfortunately, diffusion to such a region is fundamentally two dimensional and therefore applying the 1D model of diffusion leads to significant errors. We derive an equation suitable for fitting to FRAP data acquired from small bleach regions and analyze its accuracy using simulated data. We propose that the use of a small bleach region along with a two dimensional diffusion model is the ideal protocol for cell surface FRAP.

Mechanisms of B-cell synapse formation predicted by Monte Carlo simulation

The clustering of B-cell receptor (BCR) molecules and the formation of the protein segregation structure known as the "immunological synapse" at the contact region between B cells and antigen presenting cells appears to precede antigen (Ag) uptake by B cells. The mature B-cell synapse is characterized by a central cluster of BCR/Ag molecular complexes surrounded by a ring of LFA-1/ICAM-1 complexes. In this study, we investigate the biophysical mechanisms that drive immunological synapse formation in B cells by means of Monte Carlo simulation. Our approach simulates individual reaction and diffusion events on cell surfaces in a probabilistic manner with a clearly defined mapping between our model's probabilistic parameters and their physical equivalents. Our model incorporates the bivalent nature of the BCR as well as changes in membrane shape due to receptor-ligand binding. We find that differences in affinity and bond stiffness between BCR/Ag and LFA-1/ICAM-1 are sufficient to drive synapse formation in the absence of membrane deformation. When significant membrane deformation occurs as a result of receptor-ligand binding, our model predicts the affinity-dependent mechanism needs to be complemented by a BCR signaling-driven shift in LFA-1 affinity from low to high in order for synapses to form.

Geometrically repatterned immunological synapses uncover formation mechanisms

The interaction of T cells and antigen-presenting cells is central to adaptive immunity and involves the formation of immunological synapses in many cases. The surface molecules of the cells form a characteristic spatial pattern whose formation mechanisms and function are largely unknown. We perform computer simulations of recent experiments on geometrically repatterned immunological synapses and explain the emerging structure as well as the formation dynamics. Only the combination of in vitro experiments and computer simulations has the potential to pinpoint the kind of interactions involved. The presented simulations make clear predictions for the structure of the immunological synapse and elucidate the role of a self-organizing attraction between complexes of T cell receptor and peptide–MHC molecule, versus a centrally directed motion of these complexes.

Adaptive immunity is a response of the immune system that involves the activation of lymphocytes and that is most effective in defending against virus-infected cells, cancer cells, fungi, and intracellular bacteria. Central to this response is the interaction between a T cell and an antigen-presenting cell, and in particular the communication of information mediated by the T cell receptor and co-receptors. The contact zone between the cells is a highly organized interface, which is termed the immunological synapse, where both the spatial and the temporal organization of the bound receptors contribute to the generated activation signal on antigen recognition. Although a considerable amount of experimental and theoretical studies have dealt with the immunological synapse, the mechanisms that control its formation are still under discussion. In 2005, Mossman et al. conducted ingenious experiments using nanometer-scale structures to geometrically repattern the immunological synapse. These experiments are reproduced by Figge and Meyer-Hermann applying computer simulations, based on an agent-based model approach, to uncover the emerging structures as well as the underlying formation mechanisms. Clear predictions for the structure of proposed geometrically repatterned immunological synapses are obtained that will further elucidate the role of the involved formation mechanisms.

Quantitative analysis of the role of receptor recycling in T cell polarization

Activation of T cells of the immune system involves recognition of the antigen by the T cell receptor and subsequent internalization and recycling of this receptor. We present a numerical model for this process that accounts for the polarity of the intracellular traffic determined by the polarization of the microtubule-organizing center to the immunological synapse. Unexpectedly, the model explains the observed accumulation of receptors at the immunological synapse mainly as dynamic maintenance of the receptor density there, while the surface receptors everywhere else are depleted, even though the internalization occurs primarily at the synapse. In the case of an unsuccessful polarization of the microtubule-organizing center, which alters the polarity of the receptor trafficking, the model explains the absence of receptor accumulation as a dynamic downregulation at the synapse. The experiment shows that in this case the interaction of the T cell with its target is aborted. Disruption of recycling leads in the experiment to accumulation of the incompletely polarized cells. We propose that receptor recycling is a mechanism whereby the cell can sense its internal structure and detect polarity errors, analogous to checkpoint signaling mechanisms that ensure fidelity of cell division.