Low T cell receptor expression and thermal fluctuations contribute to formation of dynamic multifocal synapses in thymocytes

The interplay between membrane topology and mechanical forces in regulating T cell receptor activity

T cells are critically important for host defense against infections. T cell activation is specific because signal initiation requires T cell receptor (TCR) recognition of foreign antigen peptides presented by major histocompatibility complexes (pMHC) on antigen presenting cells (APCs). Recent advances reveal that the TCR acts as a mechanoreceptor, but it remains unclear how pMHC/TCR engagement generates mechanical forces that are converted to intracellular signals. Here we propose a TCR Bending Mechanosignal (TBM) model, in which local bending of the T cell membrane on the nanometer scale allows sustained contact of relatively small pMHC/TCR complexes interspersed among large surface receptors and adhesion molecules on the opposing surfaces of T cells and APCs. Localized T cell membrane bending is suggested to increase accessibility of TCR signaling domains to phosphorylation, facilitate selective recognition of agonists that form catch bonds, and reduce noise signals associated with slip bonds.

Al-Aghbar et al propose a TCR bending mechanosignal model that demonstrates how local mechanical membrane bending may influence T cell receptor binding events and thus T-cell activation.

Plasma membrane sphingomyelin modulates thymocyte development by inhibiting TCR-induced apoptosis

Sphingomyelin (SM) in combination with cholesterol forms specialized membrane lipid microdomains in which specific receptors and signaling molecules are localized or recruited to mediate intracellular signaling. SM-microdomain levels in mouse thymus were low in the early CD4+CD8+ double-positive (DP) stage prior to thymic selection and increased >10-fold during late selection. T-cell receptor (TCR) signal strength is a key factor determining whether DP thymocytes undergo positive or negative selection. We examined the role of SM-microdomains in thymocyte development and related TCR signaling, using SM synthase 1 (SMS1)-deficient (SMS1-/-) mice which display low SM expression in all thymocyte populations. SMS1 deficiency caused reduced cell numbers after late DP stages in TCR transgenic models. TCR-dependent apoptosis induced by anti-CD3 treatment was enhanced in SMS1-/- DP thymocytes both in vivo and in vitro. SMS1-/- DP thymocytes, relative to controls, showed increased phosphorylation of TCR-proximal kinase ZAP-70 and increased expression of Bim and Nur77 proteins involved in negative selection following TCR stimulation. Addition of SM to cultured normal DP thymocytes led to greatly increased surface expression of SM-microdomains, with associated reduction of TCR signaling and TCR-induced apoptosis. Our findings indicate that SM-microdomains are increased in late DP stages, function as negative regulators of TCR signaling and modulate the efficiency of TCR-proximal signaling to promote thymic selection events leading to subsequent developmental stages.

In vitro reconstitution of T cell receptor-mediated segregation of the CD45 phosphatase

T cell signaling initiates upon the binding of peptide-loaded MHC (pMHC) on an antigen-presenting cell to the T cell receptor (TCR) on a T cell. TCR phosphorylation in response to pMHC binding is accompanied by segregation of the transmembrane phosphatase CD45 away from TCR-pMHC complexes. The kinetic segregation hypothesis proposes that CD45 exclusion shifts the local kinase-phosphatase balance to favor TCR phosphorylation. Spatial partitioning may arise from the size difference between the large CD45 extracellular domain and the smaller TCR-pMHC complex, although parsing potential contributions of extracellular protein size, actin activity, and lipid domains is difficult in living cells. Here, we reconstitute segregation of CD45 from bound receptor-ligand pairs using purified proteins on model membranes. Using a model receptor-ligand pair (FRB-FKBP), we first test physical and computational predictions for protein organization at membrane interfaces. We then show that the TCR-pMHC interaction causes partial exclusion of CD45. Comparing two developmentally regulated isoforms of CD45, the larger R_ABC variant is excluded more rapidly and efficiently (∼50%) than the smaller R₀ isoform (∼20%), suggesting that CD45 isotypes could regulate signaling thresholds in different T cell subtypes. Similar to the sensitivity of T cell signaling, TCR-pMHC interactions with K_ds of ≤15 µM were needed to exclude CD45. We further show that the coreceptor PD-1 with its ligand PD-L1, immunotherapy targets that inhibit T cell signaling, also exclude CD45. These results demonstrate that the binding energies of physiological receptor-ligand pairs on the T cell are sufficient to create spatial organization at membrane-membrane interfaces.

Impact of lipid rafts on the T-cell-receptor and peptide-major-histocompatibility-complex interactions under different measurement conditions

The interactions between T-cell receptor (TCR) and peptide-major-histocompatibility complex (pMHC), which enable T-cell development and initiate adaptive immune responses, have been intensively studied. However, a central issue of how lipid rafts affect the TCR-pMHC interactions remains unclear. Here, by using a statistical-mechanical membrane model, we show that the binding affinity of TCR and pMHC anchored on two apposing cell membranes is significantly enhanced because of the lipid raft-induced signaling protein aggregation. This finding may provide an alternative insight into the mechanism of T-cell activation triggered by very low densities of pMHC. In the case of cell-substrate adhesion, our results indicate that the loss of lateral mobility of the proteins on the solid substrate leads to the inhibitory effect of lipid rafts on TCR-pMHC interactions. Our findings help to understand why different experimental methods for measuring the impact of lipid rafts on the receptor-ligand interactions have led to contradictory conclusions.

T cell activation requires force generation

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

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

The synaptic recruitment of lipid rafts is dependent on CD19-PI3K module and cytoskeleton remodeling molecules

Sphingolipid- and cholesterol-rich lipid raft microdomains are important in the initiation of BCR signaling. Although it is known that lipid rafts promote the coclustering of BCR and Lyn kinase microclusters within the B cell IS, the molecular mechanism of the recruitment of lipid rafts into the B cell IS is not understood completely. Here, we report that the synaptic recruitment of lipid rafts is dependent on the cytoskeleton-remodeling proteins, RhoA and Vav. Such an event is also efficiently regulated by motor proteins, myosin IIA and dynein. Further evidence suggests the synaptic recruitment of lipid rafts is, by principle, an event triggered by BCR signaling molecules and second messenger molecules. BCR-activating coreceptor CD19 potently enhances such an event depending on its cytoplasmic Tyr421 and Tyr482 residues. The enhancing function of the CD19-PI3K module in synaptic recruitment of lipid rafts is also confirmed in human peripheral blood B cells. Thus, these results improve our understanding of the molecular mechanism of the recruitment of lipid raft microdomains in B cell IS.

Specific adhesion of membranes simultaneously supports dual heterogeneities in lipids and proteins

Membrane adhesion mediated by one protein species simultaneously stabilizes both ordered-phase and disordered-phase heterogeneities, distinct from the non-adhered membrane.

Microspectroscopy reveals mechanisms of lymphocyte activation

The immunological synapse (IS) regulates immune responses by integrating extracellular stimuli into intracellular signalling networks, which causes leukocyte differentiation and effector functions. The dynamic spatial organisation of molecules at the IS was initially characterised by wide-field fluorescence microscopy of cell conjugates and cells interacting with planar lipid bilayers. These methods showed stable supramolecular clusters of several microns in size, which were proposed to be responsible for sustained signalling and cell-cell adhesion. The recent emergence of microspectroscopy techniques with higher spatial and temporal resolution nonetheless reveals the complex dynamics of molecular reactions that mediate IS assembly and function. This review describes microspectroscopy-based in vitro experimental approaches for imaging the molecular dynamics at the IS, as well as their contributions and open questions in the field. It also describes experimental methods to obtain quantitative parameters of dynamic biochemical reactions in living cells, and discusses about the important role of quantitative imaging and theoretical science in our understanding of molecular mechanisms underlying lymphocyte activation.

Diversity in immunological synapse structure

Immunological synapses (ISs) are formed at the T cell-antigen-presenting cell (APC) interface during antigen recognition, and play a central role in T-cell activation and in the delivery of effector functions. ISs were originally described as a peripheral ring of adhesion molecules surrounding a central accumulation of T-cell receptor (TCR)-peptide major histocompatibility complex (pMHC) interactions. Although the structure of these 'classical' ISs has been the subject of intense study, non-classical ISs have also been observed under a variety of conditions. Multifocal ISs, characterized by adhesion molecules dispersed among numerous small accumulations of TCR-pMHC, and motile 'immunological kinapses' have both been described. In this review, we discuss the conditions under which non-classical ISs are formed. Specifically, we explore the profound effect that the phenotypes of both T cells and APCs have on IS structure. We also comment on the role that IS structure may play in T-cell function.

Perspectives for computer modeling in the study of T cell activation

The T cell receptor (TCR) is responsible for discriminating between self- and foreign-derived peptides, translating minute differences in amino-acid sequence into large differences in response. Because of the great variability in the TCR and its ligands, activation of T cells by foreign peptides is a quantitative process, dependent on a mix of upstream signals and downstream integration. Accordingly, quantitative data and computational models have shed light on many important aspects of this process: molecular noise in ligand recognition, spatial dynamics in T cell-APC (antigen presenting cell) interactions, graded versus all-or-none decision making by the TCR apparatus, mechanisms of peptide antagonism and synergism, and the tunability and robustness of activation thresholds. Though diverse in their formalism, these studies together paint a picture of how modeling has shaped and will continue to shape understanding of T cell immunobiology.

Cutting Edge: mechanical forces acting on T cells immobilized via the TCR complex can trigger TCR signaling

Engagement of the TCR by antigenic peptides presented by the MHC activates specific T cells to control infections. Recent theoretical considerations have suggested that mechanical forces acting on the TCR may be important for receptor triggering. In this study, we directly tested the hypothesis that physical forces acting on the TCR can initiate signaling in T cells by micromanipulation of individual T cells bound to artificial APCs expressing engineered TCR ligands. We find that mechanical forces acting on T cells bound to APCs via the TCR complex but not other surface receptors can initiate signaling in T cells in an Src kinase-dependent fashion. Our data indicate that T cells are mechanically sensitive when coupled to APCs by the TCR and indicates that the TCR may act as a mechanosensor. Our data provide new insight into TCR function.

Anergic CD4+ T cells form mature immunological synapses with enhanced accumulation of c-Cbl and Cbl-b

CD4(+) T cell recognition of MHC:peptide complexes in the context of a costimulatory signal results in the large-scale redistribution of molecules at the T cell-APC interface to form the immunological synapse. The immunological synapse is the location of sustained TCR signaling and delivery of a subset of effector functions. T cells activated in the absence of costimulation are rendered anergic and are hyporesponsive when presented with Ag in the presence of optimal costimulation. Several previous studies have looked at aspects of immunological synapses formed by anergic T cells, but it remains unclear whether there are differences in the formation or composition of anergic immunological synapses. In this study, we energized primary murine CD4(+) T cells by incubation of costimulation-deficient, transfected fibroblast APCs. Using a combination of TCR, MHC:peptide, and ICAM-1 staining, we found that anergic T cells make mature immunological synapses with characteristic central and peripheral supramolecular activation cluster domains that were indistinguishable from control synapses. There were small increases in total phosphotyrosine at the anergic synapse along with significant decreases in phosphorylated ERK 1/2 accumulation. Most striking, there was specific accumulation of c-Cbl and Cbl-b to the anergic synapses. Cbl-b, previously shown to be essential in anergy induction, was found in both the central and the peripheral supramolecular activation clusters of the anergic synapse. This Cbl-b (and c-Cbl) accumulation at the anergic synapse may play an important role in anergy maintenance, induction, or both.

Nucleation of membrane adhesions

Recent experimental and theoretical studies of biomimetic membrane adhesions [Bruinsma, Phys. Rev. E 61, 4253 (2000); Boulbitch, Biophys. J. 81, 2743 (2001)] suggested that adhesion mediated by receptor interactions is due to the interplay between membrane undulations and a double-well adhesion potential, and should be a first-order transition. We study the nucleation of membrane adhesion by finding the minimum-energy path on the free energy surface constructed from the bending free energy of the membrane and the double-well adhesion potential. We find a nucleation free energy barrier around 20k(B)T for adhesion of flexible membranes, which corresponds to fast nucleation kinetics with a time scale of the order of seconds. For cell membranes with a larger bending rigidity due to the actin network, the nucleation barrier is higher and may require active processes such as the reorganization of the cortex network to overcome this barrier. Our scaling analysis suggests that the geometry of the membrane shapes of the adhesion contact is controlled by the adhesion length that is determined by the membrane rigidity, the barrier height, and the length scale of the double-well potential, while the energetics of adhesion is determined by the depths of the adhesion potential. These results are verified by numerical calculations.

Mechanistic insight into lymphocyte activation through quantitative imaging and theoretical modelling

Increasingly, it is apparent that in order to understand the complexity of immunosurveillance at the cell-cell junction, quantitative analysis at the single cell level is necessary. The visualisation of the large-scale rearrangement of proteins characterising what is known as the immunological synapse (IS) was an important discovery shaping our understanding of the events occurring during immune recognition. The use of supported planar bilayers and geometrically designed substrates combined with advanced imaging techniques such as total internal reflection fluorescence (TIRF) and Förster resonance energy transfer (FRET) has provided insight into the spatio-temporal dynamics of receptor signalling and the role of receptor trafficking in regulating cell signalling. Theoretical modelling will play a key role in the integration of such quantitative data providing mechanistic insight into lymphocyte activation.

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.

Segregation of HLA-C from ICAM-1 at NK cell immune synapses is controlled by its cell surface density

NK cell activity is controlled by the integration of signals from numerous activating and inhibitory receptors at the immunological synapse (IS). However, the importance of segregation and patterning of proteins at the NK cell IS is unknown. In this study, we report that the level of expression of HLA-C on target cells determined its supramolecular organization and segregation from ICAM-1 at the NK cell IS, as well as its capacity to inhibit NK cell cytotoxicity. At YTS NK cell synapses formed with target cells expressing low levels of HLA-C (i.e., 10(4)/cell surface), a multifocal patterning of MHC class I protein predominated, whereas for higher levels of expression (10(5)/cell surface), clusters of HLA-C were more commonly homogeneous, ring-shaped, or containing multiple exclusions. This correlation of protein density with its patterning at the IS was independent of ATP- or actin-driven processes. Importantly, ICAM-1 and HLA-C segregated only at synapses involving target cells expressing high levels of MHC protein. For peripheral blood NK clones, there were specific thresholds in the level of target cell HLA-C needed to inhibit cytotoxicity and to cause segregation of HLA-C from ICAM-1 at the synapse. Thus, the synapse organization of HLA-C, determined by its level of expression, could directly influence NK cell inhibition, e.g., by regulating the proximity of activating and inhibitory receptors. For the first time, this suggests an important function for the assembly of an inhibitory NK cell IS. More broadly, segregation of proteins at intercellular contacts could transmit information about protein expression levels between cells.

Micropatterned supported membranes as tools for quantitative studies of the immunological synapse

In living cells, membrane receptors transduce ligand binding into signals that initiate proliferation, specialization, and secretion of signaling molecules. Spatial organization of such receptors regulates signaling in several key immune cell interactions. In the most extensively studied of these, a T cell recognizes membrane-bound antigen presented by another cell, and forms a complex junction called the "immunological synapse" (IS). The importance of spatial organization at the IS and the quantification of its effect on signaling remain controversial topics. Researchers have successfully investigated the IS using lipid bilayers supported on solid substrates as model antigen-presenting membranes. Recent technical developments have enabled micron- and nanometre-scale patterning of supported lipid bilayers (SLBs) and their application to immune cell studies with provocative results, including spatial mutation of the IS. In this tutorial review, we introduce the IS; discuss SLB techniques, including micropatterning; and discuss various methods used to perturb and quantify the IS.

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.

Curvature and spatial organization in biological membranes

Cellular membranes bend and curve into a multitude of shapes as they perform various functions. These deformations make use of the remarkable material properties of biological membranes inherent in their nature as two-dimensional fluids. The curvature of membranes is controlled by the constituent proteins and lipids, but conversely, curvature itself provides mechanisms for organizing mobile membrane molecules. In this article we survey recent experiments that have uncovered intriguing connections between mechanics and biochemistry at membranes, focusing on the influence of molecular shape on curvature, links between phase separation and curvature, and membrane bending at inter-cellular contacts. We describe the concepts that emerge from these studies, especially the existence of long-range, curvature-mediated mechanisms for spatial organization in membranes, and highlight open areas for future research.

'KMC-TDGL'-a coarse-grained methodology for simulating interfacial dynamics in complex fluids: application to protein-mediated membrane processes

In this article, we describe a new multiscale simulation algorithm (which we term the 'KMC-TDGL' method) applicable for the description of equilibrium and dynamic processes associated with a particular class of complex fluids with nanoscale inclusions, namely, biological membranes mediated by membrane-associating and membrane-bound proteins. We adopt a novel strategy of integrating two different phenomenological approaches, namely, a field theoretic (continuum) description for the membrane dynamics given by the time-dependent Ginzburg-Landau equation and a random walk on a discretized lattice description for protein diffusion dynamics. We illustrate that this integrated approach results in a unified description of protein-mediated membrane dynamics.

Yeast surface display of a noncovalent MHC class II heterodimer complexed with antigenic peptide

Microbial protein display technologies have enabled directed molecular evolution of binding and stability properties in numerous protein systems. In particular, dramatic improvements to antibody binding affinity and kinetics have been accomplished using these tools in recent years. Examples of successful application of display technologies to other immunological proteins have been limited to date. Herein, we describe the expression of human class II major histocompatibility complex allele (MHCII) HLA-DR4 on the surface of Saccharomyces cerevisiae as a noncovalently associated heterodimer. The yeast-displayed MHCII is fully native as assessed by binding of conformationally specific monoclonal antibodies; failure of antibodies specific for empty HLA-DR4 to bind yeast-displayed protein indicates antigenic peptide is bound. This report represents the first example of a noncovalent protein dimer displayed on yeast and of successful display of wild-type MHCII. Results further point to the potential for using yeast surface display for engineering and analyzing the antigen binding properties of MHCII.

Cold-induced coalescence of T-cell plasma membrane microdomains activates signalling pathways

The plasma membranes of eukaryotic cells are hypothesised to contain microdomains with distinct lipid and protein composition known as lipid rafts. In T cells, cross-linking of lipid raft components triggers signalling cascades. We show that the T-cell antigen receptor (TCR) and a protein tyrosine kinase, Lck, have a patchy plasma membrane distribution in Jurkat T cells at reduced temperatures, although they have a continuous distribution at physiological temperature (37 degrees C). GM1 displays a patchy distribution at reduced temperature after Triton X-100 extraction. The archetypal non-lipid raft marker, the transferrin receptor, displays a more continuous plasma membrane distribution uncorrelated with that of Lck at 0 degrees C. Cold-induced aggregation of the lipid raft-partitioning proteins is accompanied by increased tyrosine phosphorylation and ERK activation, peaking at 10-20 degrees C. Tyrosine phosphorylation is further greatly increased by ligating the TCR with anti-CD3 at 10-20 degrees C. The tyrosine phosphorylation mainly occurred at the plasma membrane, was dependent on Lck and on the surface expression of the TCR. The activation of tyrosine phosphorylation and ERK by TCR ligation at reduced temperature also occurred in human primary T cells. These results support the concept that lipid rafts can form in membranes of live cells and that their coalescence stimulates signalling.

Molecular Organization and Signal Transduction at Intermembrane Junctions

Surfaces create an environment in which multiple forces conspire together to yield a wealth of complex chemical processes. This is especially true of cell membranes, whose fluidity and flexibility enables responsive feedback with surface chemical interactions in ways not generally seen with inorganic materials. Spatial pattern formation of cell-surface proteins at intermembrane junctions provides many beautiful examples of these phenomena, and is also emerging as a functional aspect of intercellular signaling. Correspondingly, the study of interactions of cell-membrane surfaces is attracting significant attention from cell biologists and physical chemists alike. This convergence is fueled be recent, exquisite observations of protein pattern formation events within living immunological synapses along with parallel advances in membrane reconstitution, manipulation, and imaging technologies.

Structure and dynamics of supported intermembrane junctions

Supported intermembrane junctions, formed by rupture of giant unilamellar vesicles onto conventional supported lipid membranes, have recently emerged as model systems for the study of biochemical processes at membrane interfaces. Using intermembrane fluorescence resonance energy transfer and optical standing wave fluorescence interferometry, we characterize the nanometer-scale topography of supported intermembrane junctions and find two distinct association states. In one state, the two membranes adhere in close apposition, with intermembrane separations of a few nanometers. In the second state, large intermembrane spacings of approximately 50 nm are maintained by a balance between Helfrich (entropic) repulsion and occasional sites of tight adhesion that pin the two membranes together. Reversible transitions between these two states can be triggered with temperature changes. We further examine the physical properties of membranes in each state using a membrane mixture near its miscibility phase transition temperature. Thermodynamic characteristics of the phase transition and diffusive mobility of individual lipids are comparable. However, collective Brownian motion of phase-separated domains and compositional fluctuations are substantially modulated by intermembrane spacing. The scaling properties of diffusion coefficient with particle size are determined from detailed analysis of domain motion in the different junction types. The results provide experimental verification of a theoretical model for two-dimensional mobility in membranes, which includes frictional coupling across an interstitial water layer.

Pattern formation during T-cell adhesion

T cells form intriguing patterns during adhesion to antigen-presenting cells. The patterns are composed of two types of domains, which either contain short TCR/MHCp receptor-ligand complexes or the longer LFA-1/ICAM-1 complexes. The final pattern consists of a central TCR/MHCp domain surrounded by a ring-shaped LFA-1/ICAM-1 domain, whereas the characteristic pattern formed at intermediate times is inverted with TCR/MHCp complexes at the periphery of the contact zone and LFA-1/ICAM-1 complexes in the center. Several mechanisms have been proposed to explain the T-cell pattern formation. Whereas biologists have emphasized the role of active cytoskeletal processes, previous theoretical studies suggest that the pattern evolution may be caused by spontaneous self-assembly processes alone. Some of these studies focus on circularly symmetric patterns and propose a pivot mechanism for the formation of the intermediate inverted pattern. Here, we present a statistical-mechanical model which includes thermal fluctuations and the full range of spatial patterns. We confirm the observation that the intermediate inverted pattern may be formed by spontaneous self-assembly. However, we find a different self-assembly mechanism in which numerous TCR/MHCp microdomains initially nucleate throughout the contact zone. The diffusion of free receptors and ligands into the contact zone subsequently leads to faster growth of peripheral TCR/MHCp microdomains and to a closed ring for sufficiently large TCR/MHCp concentrations. At smaller TCR/MHCp concentrations, we observe a second regime of pattern formation with characteristic multifocal intermediates, which resemble patterns observed during adhesion of immature T cells or thymozytes. In contrast to other theoretical models, we find that the final T-cell pattern with a central TCR/MHCp domain is only obtained in the presence of active cytoskeletal transport processes.

Real-time imaging of T-cell development

The cellular and molecular dynamics underlying intrathymic T-cell development are still poorly understood. Dynamic imaging techniques offer new opportunities for dissecting the complex journey of developing T cells in the thymus. In particular, two-photon laser scanning microscopy has recently demonstrated great promise in tracking lymphocyte behavior in tissue environments. Resolving thymocyte behavior at the single-cell level should help to clarify the mode of trafficking in the thymus and identify the spatio-temporal aspects of thymic selection events.

The protean immune cell synapse: a supramolecular structure with many functions

Heterogeneity in the supramolecular organization of immunological synapses arises from the involvement of different cells, distinct environmental stimuli, and varying levels of protein expression. There may also be heterogeneity in the types and amounts of cell surface proteins and lipids that transfer between lymphocytes during immune surveillance. In addition, immune cells can be involved in the assembly of a 'viral synapse', such that micrometer-scale organization of proteins at intercellular contacts occurs during transmission of a virus between T cells. Thus, while there may be unity in molecular mechanisms underlying the organization of cell surface receptors at immune cell synapses, there is diversity in their function.