A supramolecular basis for CD45 tyrosine phosphatase regulation in sustained T cell activation

Visualization of Cell-Cell Interaction Contacts: Synapses and Kinapses

T-cell activation requires interactions of T-cell antigen receptors (TCR) and peptides presented by major histocompatibility complex molecules (MHCp) in an adhesive junction between the T-cell and antigen-presenting cell (APC). Stable junctions with bull's eye supramolecular activation clusters (SMACs) have been defined as immunological synapses. The term synapse works in this case because it joins roots for "same" and "fasten," which could be translated as "fasten in the same place." These structures maintain T-cell-APC interaction and allow directed secretion. We have proposed that SMACs are not really clusters, but are analogous to higher order membrane-cytoskeleton zones involved in amoeboid locomotion including a substrate testing lamellipodium, an adhesive lamella and anti-adhesive uropod. Since T-cells can also integrate signaling during locomotion over antigen presenting cells, it is important to consider adhesive junctions maintained as cells move past each other. This combination of movement (kine-) and fastening (-apse) can be described as a kinapse or moving junction. Synapses and kinapses operate in different stages of T-cell priming. Optimal effector functions may also depend upon cyclical use of synapses and kinapses. Visualization of these structures in vitro and in vivo presents many distinct challenges that will be discussed in this paper.

Imaging techniques for assaying lymphocyte activation in action

Imaging techniques have greatly improved our understanding of lymphocyte activation. Technical advances in spatial and temporal resolution and new labelling tools have enabled researchers to directly observe the activation process. Consequently, research using imaging approaches to study lymphocyte activation has expanded, providing an unprecedented level of cellular and molecular detail in the field. As a result, certain models of lymphocyte activation have been verified, others have been revised and yet others have been replaced with new concepts. In this article, we review the current imaging techniques that are used to assess lymphocyte activation in different contexts, from whole animals to single molecules, and discuss the advantages and potential limitations of these methods.

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.

Signaling microdomains in T cells

Sub-micron scale signaling domains induced in the plasma membrane of cells are thought to play important roles in signal transduction. In T cells, agonist MHC-peptide complexes induce small diffraction-limited domains enriched in T cell receptor (TCR) and signaling molecules. These microclusters serve as transient platforms for signal initiation and are required for sustained signaling in T cells, although each microcluster functions for only a couple of minutes. How they are formed, and what mechanisms promote and regulate signaling within TCR microclusters is largely unknown, although it is clear that TCR engagement and dynamic reorganization of cortical actin are involved. Here, we review current understanding of signaling within microclusters in T cells, and speculate on how these structures may form, initiate biochemical signals, and serve as sites of both signal integration and amplification, while also facilitating appropriate termination of TCR and related signaling.

Modular design of immunological synapses and kinapses

The concept of an immunological synapse goes back to the early 1980s with the discovery of the relationship between T-cell antigen receptor mediated Ca(2+) signaling, adhesion, and directed secretion. However, this concept did not gain traction until images were published starting in 1998 that revealed a specific molecular pattern in the interface between T cells and model antigen-presenting cells or supported planar bilayers. The dominant pattern, a ring of adhesion molecules surrounding a central cluster of antigen receptors, was observed in both model systems. Analysis of the origins of this pattern over the past 10 years has presented a solution for a difficult problem in lymphocyte biology--how a highly motile cell can suddenly stop when it encounters a signal delivered by just a few antigenic ligands on the surface of another cell without disabling the sensory machinery of the motile cell. The T lymphocyte actively assembles the immunological synapse pattern following a modular design with roots in actin-myosin-based motility.

CD148 enhances platelet responsiveness to collagen by maintaining a pool of active Src family kinases

SUMMARY BACKGROUND

We have previously shown that the receptor-like protein tyrosine phosphatase (PTP) CD148 is essential for initiating glycoprotein VI (GPVI) signaling in platelets. We proposed that CD148 does so by dephosphorylating the C-terminal inhibitory tyrosine of Src family kinases (SFKs). However, this mechanism is complicated by CD148-deficient mouse platelets having a concomitant reduction in GPVI expression.

OBJECTIVES

To investigate the effect of CD148 on GPVI signaling independent of the decrease in GPVI expression and to further establish the molecular basis of the activatory effect of CD148 and downregulation of GPVI.

METHODS

CD148-deficient mouse platelets were investigated for functional and biochemical defects. The DT40/NFAT-lucifierase reporter assay was used to analyze the effect of CD148 on GPVI signaling. CD148-SFK interactions and dephosphorylation were quantified using biochemical assays.

RESULTS

CD148-deficient mouse platelets exhibited reduced collagen-mediated aggregation, secretion and spreading in association with reduced expression of GPVI and FcR gamma-chain and reduced tyrosine phosphorylation. The phosphorylation status of SFKs suggested a global reduction in SFK activity in resting CD148-deficient platelets. Studies in a cell model confirmed that CD148 inhibits GPVI signaling independent of a change in receptor expression and through a mechanism dependent on tyrosine dephosphorylation. Recombinant CD148 dephosphorylated the inhibitory tyrosines of Fyn, Lyn and Src in vitro, although paradoxically it also dephosphorylated the activation loop of SFKs.

CONCLUSIONS

CD148 plays a critical role in regulating GPVI/FcR gamma-chain expression and maintains a pool of active SFKs in platelets by directly dephosphorylating the C-terminal inhibitory tyrosines of SFKs that is essential for platelet activation.

An enigmatic tail of CD28 signaling

CD28 costimulation regulates a wide range of cellular processes, from proliferation and survival to promoting the differentiation of specialized T-cell subsets. Since first being identified over 20 years ago, CD28 has remained a subject of intense study because of its profound consequences on T cell function and its potential for therapeutic manipulation. In this review we highlight the signaling cascades initiated by the major signaling motifs in CD28, focusing on PI-3 kinase-dependent and -independent pathways and how these are linked to specific cellular outcomes. Recent studies using gene targeted knockin mice have clarified the relative importance of these motifs on in vivo immune responses; however, much remains to be elucidated. Understanding the mechanism behind costimulation holds great potential for development of new clinically relevant reagents, a fact beginning to be realized with the advent of drugs that prevent CD28 ligation and signaling.

Laquinimod suppress antigen presentation in relapsing-remitting multiple sclerosis: in-vitro high-throughput gene expression study

Laquinimod (LAQ) is a new immunomodulatory drug shown to be effective in the treatment of relapsing-remitting multiple sclerosis (RRMS); however, its molecular target pathways are not well recognized. In this study we characterized in-vitro the molecular effects of LAQ in peripheral blood mononuclear cells (PBMC) of healthy subjects and RRMS patients by gene expression microarrays. We demonstrated that LAQ induced suppression of genes related to antigen presentation and corresponding inflammatory pathways. These findings were demonstrated mainly via the NFkB pathway. Analysis of PBMC subpopulations identified activation of Th2 response in CD14+ and CD4+ cells and suppression of proliferation in CD8+ cells.

Modulation of immune cell signalling by the leukocyte common tyrosine phosphatase, CD45

CD45 is a leukocyte specific transmembrane glycoprotein and a receptor-like protein tyrosine phosphatase (PTP). CD45 can be expressed as several alternatively spliced isoforms that differ in the extracellular domain. The isoforms are regulated in a cell type and activation state-dependent manner, yet their function has remained elusive. The Src family kinase members Lck and Lyn are key substrates for CD45 in T and B lymphocytes, respectively. CD45 lowers the threshold of antigen receptor signalling, which impacts T and B cell activation and development. CD45 also regulates antigen triggered Fc receptor signalling in mast cells and Toll-like receptor (TLR) signalling in dendritic cells, thus broadening the role of CD45 to other recognition receptors involved in adaptive and innate immunity. In addition, CD45 can affect immune cell adhesion and migration and can modulate cytokine production and signalling. Here we review what is known about the substrate specificity and regulation of CD45 and summarise its effect on immune cell signalling pathways, from its established role in T and B antigen receptor signalling to its emerging role regulating innate immune cell recognition and cytokine production.

T cell signal regulation by the actin cytoskeleton

T cells form an immunological synapse (IS) that sustains and regulates signals for cell stimulation. Enriched in the IS is the Src family kinase Lck. Conversely, the membrane phosphatase CD45, which activates Src family kinases, is excluded, and this is necessary to avoid quenching of T cell receptor phosphosignals. Data suggest that this arrangement occurs by an enrichment of cholesterol-dependent rafts in the IS. However, the role of rafts in structuring the IS remains unclear. To address this question, we used fluorescence resonance energy transfer (FRET) to interrogate the nanoscopic structure of the IS. The FRET probes consisted of membrane-anchored fluorescent proteins with distinct affinities for rafts. Both the raft and nonraft probes exhibited clustering in the IS. However, co-clustering of raft donor-acceptor pairs was 10-fold greater than co-clustering of raft-nonraft pairs. We measured the effect of disrupting rafts in the IS on CD45 localization and Lck regulation by treating stimulated T cells with filipin. The filipin specifically disrupted co-clustering of the raft FRET pairs in the IS and allowed targeting of CD45 to the IS and dephosphorylation of the regulatory tyrosine of Lck. Clustering of the raft probes was also sensitive to latrunctulin B, which disrupts actin filaments. Strikingly, enriching the cortical cytoskeleton using jasplakinolide maintained raft probe co-clustering, CD45 exclusion, and Lck regulation in the IS following the addition of filipin. These data show the actin cytoskeleton maintains a membrane raft environment in the IS that promotes Lck regulation by excluding CD45.

Multiple microclusters: diverse compartments within the immune synapse

The activation of classical alphabeta T cells is initiated when the T cell receptor (TCR) recognizes peptide antigens presented by major histocompatibility complex (pMHC) molecules. This recognition always occurs at the junction of a T cell and antigen-presenting cell (APC). Existing models of T-cell activation accurately explain the sensitivity and selectivity of antigen recognition within the immunological synapse. However, these models have not fully incorporated the diverse microcluster types revealed by current imaging technologies. It is increasingly clear that a better understanding of T-cell activation will require an appreciation of the diverse signaling assemblies arising within the immune synapse, the interrelationships between these structures, and the mechanisms by which underlying cytoskeletal systems govern their assembly and fate. Here, we will provide a brief framework for understanding these issues, review our contributions to current knowledge, and provide perspectives on the future of this rapidly advancing field.

The immunological synapse, TCR microclusters, and T cell activation

T cell activation begins with the interaction between an antigen-specific T cell and an antigen-presenting cell (APC). This interaction results in the formation of the immunological synapse, which had been considered to be responsible for antigen recognition and T cell activation. Recent advances in imaging analysis have provided new insights into T cell activation. The T cell receptor (TCR) microclusters, TCRs, kinases, and adaptors are generated upon antigen recognition at the interfaces between the T cells and the APCs and serve as a fundamental signaling unit for T cell activation. CD28-mediated costimulation is also found to be regulated by the formation of microclusters. Therefore, the dynamic regulations of TCR and CD28 microcluster formation, migration, and interaction are the key events for the initiation of T cell-mediated immune responses. Comprehensive analyses of the composition and characteristics of the TCR microcluster have identified its dynamic features. This review will outline new discoveries of the microclusters and its related concept in T cell activation.

Two pathways of costimulation through CD28

CD28 is recognized as the primary costimulatory molecule involved in the activation of naïve T cells. However, the biochemical signaling pathways that are activated by CD28 and how these pathways are integrated with TCR signaling are still not understood. We have recently shown that there are at least two independent activation pathways induced by CD28 costimulation. One is integrated with TCR signaling in the context of the immunological synapse and is mediated through transcriptional enhancement and the second is mediated through the induction of mRNA stability. Here, we review the immunological consequences and biochemical mechanisms associated with CD28 costimulation and discuss the major questions that need to be resolved to understand the molecular mechanisms that transduce CD28 costimulation.

Visualizing intermolecular interactions in T cells

The use of appropriate fluorescent proteins has allowed the use of FRET microscopy for investigation of intermolecular interactions in living cells. This method has the advantage of both being dynamic and of working at the subcellular level, so that the time and place where proteins interact can be visualized. We have used FRET microscopy to analyze the interactions between the T cell antigen receptor and the coreceptors CD4 and CD8. This chapter reviews data on how these coreceptors are recruited to the immunological synapse, and how they interact when the T cell is stimulated by different ligands.

Signals and sequences that control CD28 localization to the central region of the immunological synapse

During T cell interaction with APC, CD28 is recruited to the central region (cSMAC) of the immunological synapse. CD28-mediated signaling through PI3K results in the recruitment of protein kinase C-theta (PKCtheta) to the cSMAC, activation of NF-kappaB, and up-regulation of IL-2 transcription. However, the mechanism that mediates CD28 localization to the cSMAC and the functional consequences of CD28 localization to the cSMAC are not understood. In this report, we show that CD28 recruitment and persistence at the immunological synapse requires TCR signals and CD80 engagement. Addition of mAb to either MHC class II or CD80 results in the rapid displacement of CD28 from the immunological synapse. Ligand binding is not sufficient for CD28 localization to the immunological synapse, as truncation of the cytosolic tail of CD28 disrupts synapse localization without effecting the ability of CD28 to bind CD80. Furthermore, a single point mutation in the CD28 cytosolic tail (tyrosine 188) interferes with the ability of CD28 to preferentially accumulate at the cSMAC. PKCtheta distribution at the immunological synapse mirrors the distribution of tyrosine 188-mutated CD28, indicating that CD28 drives the localization of PKCtheta even when CD28 is not localized to the cSMAC. Mutation of tyrosine 188 also results in diminished activation of NF-kappaB, suggesting that CD28-mediated localization of PKCtheta to the cSMAC is important for efficient signal transduction. These data reinforce the importance of the interplay of signals between TCR and CD28 and suggest that CD28 signaling through PCKtheta may be mediated through localization to the cSMAC region of the immunological synapse.

Visualization of cell-cell interaction contacts-synapses and kinapses

T-cell activation requires interactions of T-cell antigen receptors (TCR) and peptides presented by major histocompatibility complex molecules (MHCp) in an adhesive junction between the T-cell and antigen-presenting cell (APC). Stable junctions with bull's eye supramolecular activation clusters (SMACs) have been defined as immunological synapses. The term synapse works in this case because it joins roots for "same" and "fasten", which could be translated as "fasten in the same place". These structures maintain T-cell-APC interaction and allow directed secretion. We have proposed that SMACs are not really clusters, but are analogous to higher order membrane-cytoskeleton zones involved in amoeboid locomotion including a substrate testing lamellipodium, an adhesive lamella and anti-adhesive uropod. Since T-cells can also integrate signaling during locomotion over antigen presenting cells, it is important to consider adhesive junctions maintained as cells move past each other. This combination of movement (kine-) and fastening (-apse) can be described as a kinapse or moving junction. Synapses and kinapses operate in different stages of T-cell priming. Optimal effector functions may also depend upon cyclical use of synapses and kinapses. Visualization of these structures in vitro and in vivo presents many distinct challenges that will be discussed in this chapter.

CD45 down-regulates Lck-mediated CD44 signaling and modulates actin rearrangement in T cells

The tyrosine phosphatase CD45 dephosphorylates the negative regulatory tyrosine of the Src family kinase Lck and plays a positive role in TCR signaling. In this study we demonstrate a negative regulatory role for CD45 in CD44 signaling leading to actin rearrangement and cell spreading in activated thymocytes and T cells. In BW5147 T cells, CD44 ligation led to CD44 and Lck clustering, which generated a reduced tyrosine phosphorylation signal in CD45(+) T cells and a more sustained, robust tyrosine phosphorylation signal in CD45(-) T cells. This signal resulted in F-actin ring formation and round spreading in the CD45(+) cells and polarized, elongated cell spreading in CD45(-) cells. The enhanced signal in the CD45(-) cells was consistent with enhanced Lck Y394 phosphorylation compared with the CD45(+) cells where CD45 was recruited to the CD44 clusters. This enhanced Src family kinase-dependent activity in the CD45(-) cells led to PI3K and phospholipase C activation, both of which were required for elongated cell spreading. We conclude that CD45 induces the dephosphorylation of Lck at Y394, thereby preventing sustained Lck activation and propose that the amplitude of the Src family kinase-dependent signal regulates the outcome of CD44-mediated signaling to the actin cytoskeleton and T cell spreading.

Activation of c-Src and Fyn kinases by protein-tyrosine phosphatase RPTPalpha is substrate-specific and compatible with lipid raft localization

Src family tyrosine kinases (SFKs) function in multiple signaling pathways, raising the question of how appropriate regulation and substrate choice are achieved. SFK activity is modulated by several protein-tyrosine phosphatases, among which RPTPalpha and SHP2 are the best established. We studied how RPTPalpha affects substrate specificity and regulation of c-Src and Fyn in response to epidermal growth factor and platelet-derived growth factor. We find that RPTPalpha, in a growth factor-specific manner, directs the specificity of these kinases toward a specific subset of SFK substrates, particularly the focal adhesion protein Paxillin and the lipid raft scaffolding protein Cbp/PAG. A significant fraction of RPTPalpha is present in lipid rafts, where its targets Fyn and Cbp/PAG reside, and growth factor-mediated SFK activation within this compartment is strictly dependent on RPTPalpha. Forced concentration of RPTPalpha into lipid rafts is compatible with activation of Fyn. Finally, RPTPalpha-induced phosphorylation of Paxillin and Cbp/PAG induces recruitment of the SFK inhibitory kinase Csk, indicative of negative feedback loops limiting SFK activation by RPTPalpha. Our findings indicate that individual SFK-controlling PTPs play important and specific roles in dictating SFK substrate specificity and regulatory mechanism.

Th1 and Th2 cells form morphologically distinct immunological synapses

The arrangement of molecules at the interface between T cells and APCs is known as the immunological synapse (IS). We conducted experiments with supported planar bilayers and transfected fibroblast APC to examine the IS formed by polarized Th1 and Th2 cells. Th1 cells formed typical "bull's-eye" IS with a ring of adhesion molecules surrounding MHC/TCR interactions at all Ag concentrations tested, while Th2 cells formed multifocal IS at high concentrations of Ag. At low Ag concentrations, the majority of Th2 cells formed IS with a compact, central accumulation of MHC/TCR, but ICAM-1 was not excluded from the center of the IS. Additionally, CD45 was excluded from the center of the interface between Th1 cells and APC, while CD45 was found at the center of the multifocal IS formed by Th2 cells. Finally, phosphorylated signaling molecules colocalized with MHC/TCR to a greater extent in Th2 IS. Together, our results indicate that the IS formed by Th1 and Th2 cells are distinct in structure, with Th2 cells failing to form bull's-eye IS.

T-cell activation through immunological synapses and kinapses

T-cell activation requires 'contact' with antigen-presenting cells (APCs) to bring the T-cell receptor (TCR) and antigenic major histocompatibility complex (MHC)-peptide complex together. Contact is defined by the size of the TCR and MHC-peptide complex, which at approximately 13 nm requires extensive interdigitation of the glycocalyx of the T cell and APC. T cells may be activated through formation of a stable T cell-APC junction, referred to as an immunological synapse. It has also been shown in vitro that T cells can integrate signals from APCs without a stable interaction. In vivo imaging studies supported the importance of both motile and stable T cell-APC interactions in T-cell priming. We have found that stability depends not upon turning off motile machinery but by symmetrization of force-generating structures to balance forces and hold the cell in place. Motility is induced by breaking this symmetry, which may be necessary to maintain the differentiation potential of the T cell. Recently, we also discovered a mode of T-cell signaling leading to tolerance in vivo based purely on motile interactions. Because this entire process takes place in a state of continuous T-cell kinesis, I propose the term 'kinapse' for motile T cell-APC contacts leading to signaling. Synapses and kinapses are inter-convertible by symmetrization/symmetry breaking processes, and both modes appear to be involved in normal T-cell priming. Imbalance of synapse/kinapse states may lead to immunopathology.

Signal initiation in T-cell receptor microclusters

Although dynamic imaging technologies have provided important insights into the underlying processes responsible for T-cell activation, the processes that link antigen recognition to downstream signaling remain poorly defined. Converging lines of inquiry indicate that T-cell receptor (TCR) microclusters are the minimal structures capable of directing effective TCR signaling. Furthermore, imaging studies have determined that these structures trigger the assembly of oligomeric signaling scaffolds that contain the adapters and effectors required for T-cell activation. Existing models of T-cell activation accurately explain the sensitivity and selectivity of antigen recognition. However, these models do not account for important properties of microclusters, including their peripheral formation, size, and movement on the actin cytoskeleton. Here we examine how lipid rafts, galectin lattices, and protein scaffolds contribute to the assembly, function, and fate of TCR microclusters within immune synapses. Finally, we propose a 'mechanical segregation' model of signal initiation in which cytoskeletal forces contribute to the lateral segregation of molecules and cytoskeletal scaffolds provide a template for microclusters assembly.

From T-cell activation signals to signaling control of anti-cancer immunity

The activation of resting T cells is crucial to most immune processes. Recognition of foreign antigen by T-cell receptors has to be correctly translated into signal transduction events necessary for the induction of an effective immune response. In this review, we discuss the essential signals, molecules, and processes necessary to achieve full T-cell activation. In addition to describing these key biological events, we also discuss how T-cell receptor signaling may be harnessed to yield new therapeutic targets for a next generation of anti-cancer drugs.

Interrogating the T cell synapse with patterned surfaces and photoactivated proteins

The immunological synapse is a site rich in spatially modulated signaling cascades. The importance of spatial organization in intercellular signal transduction has prompted much recent interest in techniques to control the localization of cell-surface signaling molecules to investigate synaptic signaling. Photoactivation, patterning, and mechanical constraint of surface-associated molecules are three prominent examples of such techniques. Recent results have demonstrated the effectiveness of these techniques as tools to investigate the mechanisms of immune synapse assembly and synaptic signaling.

LFA-1-mediated T cell costimulation through increased localization of TCR/class II complexes to the central supramolecular activation cluster and exclusion of CD45 from the immunological synapse

T cell activation is associated with a dramatic reorganization of cell surface proteins and associated signaling components into discrete subdomains within the immunological synapse in T cell:APC conjugates. However, the signals that direct the localization of these proteins and the functional significance of this organization have not been established. In this study, we have used wild-type and LFA-1-deficient, DO11.10 TCR transgenic T cells to examine the role of LFA-1 in the formation of the immunological synapse. We found that coengagement of LFA-1 is not required for the formation of the central supramolecular activation cluster (cSMAC) region, but does increase the accumulation of TCR/class II complexes within the cSMAC. In addition, LFA-1 is required for the recruitment and localization of talin into the peripheral supramolecular activation cluster region and exclusion of CD45 from the synapse. The ability of LFA-1 to increase the amount of TCR engaged during synapse formation and segregate the phosphatase, CD45, from the synapse suggests that LFA-1 might enhance proximal TCR signaling. To test this, we combined flow cytometry-based cell adhesion and calcium-signaling assays and found that coengagement of LFA-1 significantly increased the magnitude of the intracellular calcium response following Ag presentation. These data support the idea that in addition to its important role on regulating T cell:APC adhesion, coengagement of LFA-1 can enhance T cell signaling, and suggest that this may be accomplished in part through the organization of proteins within the immunological synapse.

Lipid rafts in T cell signalling and disease

Lipid rafts is a blanket term used to describe distinct areas in the plasma membrane rich in certain lipids and proteins and which are thought to perform diverse functions. A large number of studies report on lipid rafts having a key role in receptor signalling and activation of lymphocytes. In T cells, lipid raft involvement was demonstrated in the early steps during T cell receptor (TCR) stimulation. Interestingly, recent evidence has shown that signalling in these domains differs in T cells isolated from patients with autoimmune diseases such as systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA). Here, we discuss these findings and explore the potential of lipid rafts as targets for the development of a new class of agents to downmodulate immune responses and for the treatment of autoimmune diseases.

The role of integrins and coreceptors in refining thresholds for B-cell responses

Despite compelling evidence that a large proportion of antigens encountered in vivo by B cells are membrane bound, the general view is that B cells are mainly activated by soluble antigens. This notion may have been biased somewhat over the years because the high affinity of the B-cell receptor (BCR) for soluble intact ligands allows efficient B-cell stimulation in vitro. In vivo, however, even soluble antigens are likely to be deposited on the surface of antigen-presenting cells, either by complement or Fc receptors in the form of immune complexes, thus becoming more potent stimulators of B-cell activation. In this framework, the BCR works in a complex environment of integrins and coreceptors, as well as the B-cell cytoskeleton. Over the last few years, we have focused on B-cell membrane-bound antigen recognition. Here, we discuss some of our findings in the context of what is currently known in this exciting new field.

Phase segregation on different length scales in a model cell membrane system

Lipid rafts are sphingolipid- and cholesterol-enriched domains on cell membranes that have been implicated in many biological functions, especially in T lymphocytes. We used a field theory to examine the forces underlying raft formation on resting living cell membranes. We find that it is difficult to reconcile the observed size of rafts on living cell membranes ( approximately 100 nm) with a mechanism that involves coupling between spontaneous curvature differences and concentration fluctuations. Such a mechanism seems to predict raft domain sizes that are larger and commensurate with those observed on synthetic membranes. Therefore, using a Poisson-Boltzmann approach, we explore whether electrostatic forces originating from transmembrane proteins and net negative charges on cell membranes could play a role in determining the raft size in living cell membranes. We find that a balance among the intrinsic tendency of raft components to segregate, the line tension, and the effective dipolar interactions among membrane constituents leads to a stable phase with a characteristic length scale commensurate with the observed size of rafts on living cell membranes. We calculate the phase diagram of a system in which these three types of forces are important. In a certain region of the parameter space, an interesting phase with mosaic-like morphology consisting of an intertwined pattern of raft and nonraft domains is predicted. Experiments that could further assess the importance of dipolar interactions for lateral organization of the components on multiple length scales in membranes are suggested.

Altered lipid raft-associated proximal signaling and translocation of CD45 tyrosine phosphatase in B lymphocytes from patients with systemic lupus erythematosus

OBJECTIVE

B lymphocytes from patients with systemic lupus erythematosus (SLE) exhibit defective intracellular signaling, hyperactivity, and autoantibody production. Recent evidence indicates a reduced expression of Lyn kinase, a negative regulator of B cell signaling, and reduced translocation of Lyn into membrane signaling domains in SLE. The present study was undertaken to investigate the causes of this altered regulation of Lyn by assessing the expression levels of regulatory molecules and their translocation into the signaling domains of SLE B lymphocytes.

METHODS

Blood was obtained from 48 patients with SLE and 28 healthy controls for the assessment of B lymphocytes. Levels and intracellular distribution of Lyn, CD45, COOH-terminal Src kinase (Csk), and c-Cbl were studied by Western blotting, confocal microscopy, and flow cytometry. The kinetics of signaling molecule translocation to the B cell receptor (BCR)-antigen synapse were investigated by confocal microscopy.

RESULTS

A profound alteration in the expression and translocation of regulatory signaling molecules in membrane domains of B cells from patients with SLE was observed. B lymphocytes from SLE patients, but not those from healthy controls, expressed a low molecular weight isoform of CD45 in lipid raft signaling microdomains. Kinetic studies revealed that translocation of Lyn, CD45, Csk, and c-Cbl led to increased recruitment and retention of Lyn and CD45 in the BCR-antigen synapse in SLE B cells.

CONCLUSION

The results provide evidence of altered expression and translocation/interaction of kinases and phosphatases in membrane signaling microdomains of B cells from patients with SLE. Altered translocation of CD45 correlated with reduced expression of Lyn, indicating that Lyn is a key molecule in the regulation of BCR-mediated signaling.

Changes in the Role of the CD45 Protein Tyrosine Phosphatase in Regulating Lck Tyrosine Phosphorylation during Thymic Development

CD45-dependent dephosphorylation of the negative regulatory C-terminal tyrosine of the Src family kinase Lck, promotes efficient TCR signal transduction. However, despite the role of CD45 in positively regulating Lck activity, the distinct phenotypes of CD45 and Lck/Fyn-deficient mice suggest that the role of CD45 in promoting Lck activity may be differentially regulated during thymocyte development. In this study, we have found that the C-terminal tyrosine of Lck (Y505) is markedly hyperphosphorylated in total thymocytes from CD45-deficient mice compared with control animals. In contrast, regulation of the Lck Y505 phosphorylation in purified, double-negative thymocytes is relatively unaffected in CD45-deficient cells. These changes in the role of CD45 in regulating Lck phosphorylation during thymocyte development correlate with changes in coreceptor expression and the presence of coreceptor-associated Lck. Biochemical analysis of coreceptor-associated and nonassociated Lck in thymocytes, and in cell lines varying in CD4 and CD45 expression, indicate that CD45-dependent regulation of Lck Y505 phosphorylation is most evident within the fraction of Lck that is coreceptor associated. In contrast, Lck Y505 phosphorylation that is not coreceptor associated is less affected by the absence of CD45. These data define distinct pools of Lck that are differentially regulated by CD45 during T cell development.

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.

Defective activation of protein kinase C and Ras-ERK pathways limits IL-2 production and proliferation by CD4+CD25+ regulatory T cells

Naturally occurring CD4+CD25+ regulatory T cells (Tregs), which play an important role in the maintenance of self-tolerance, proliferate poorly and fail to produce IL-2 following stimulation in vitro with peptide-pulsed or anti-CD3-treated APCs. When TCR proximal and distal signaling events were examined in Tregs, we observed impairments in the amplitude and duration of tyrosine phosphorylation when compared with the response of CD4+CD25- T cells. Defects were also seen in the activity of phospholipase C-gamma and in signals downstream of this enzyme including calcium mobilization, NFAT, NF-kappaB, and Ras-ERK-AP-1 activation. Enhanced stimulation of diacylglycerol-dependent pathways by inhibition of diacylglycerol metabolism could overcome the "anergic state" and support the ability of Tregs to up-regulate CD69, produce IL-2, and proliferate. Our results demonstrate that Tregs maintain their hyporesponsive state by suppressing the induction and propagation of TCR-initiated signals to control the accumulation of second messengers necessary for IL-2 production and proliferation.

The kinetic-segregation model: TCR triggering and beyond

How the T cell receptor engages antigen is known, but not how that 'triggers' intracellular signaling. The first direct support for a mechanism based on the spatial reorganization of signaling proteins, proposed 10 years ago and referred to as the 'kinetic-segregation' model, is now beginning to emerge, along with indications that it may also apply to the triggering of nonclonotypic receptors. We describe here the development of the model, review new data and suggest how the model fits a broader conceptual framework for receptor triggering. We also consider the capacity of the model, versus that of other proposals, to account for the established features of TCR triggering.

T cell receptor-proximal signals are sustained in peripheral microclusters and terminated in the central supramolecular activation cluster

T cell receptor (TCR) signaling is initiated and sustained in microclusters; however, it's not known whether signaling also occurs in the TCR-rich central supramolecular activation cluster (cSMAC). We showed that the cSMAC formed by fusion of microclusters contained more CD45 than microclusters and is a site enriched in lysobisphosphatidic acid, a lipid involved in sorting ubiquitinated membrane proteins for degradation. Calcium signaling via TCR was blocked within 2 min by anti-MHCp treatment and 1 min by latrunculin-A treatment. TCR-MHCp interactions in the cSMAC survived these perturbations for 10 min and hence were not sufficient to sustain signaling. TCR microclusters were also resistant to disruption by anti-MHCp and latrunculin-A treatments. We propose that TCR signaling is sustained by stabilized microclusters and is terminated in the cSMAC, a structure from which TCR are sorted for degradation. Our studies reveal a role for F-actin in TCR signaling beyond microcluster formation.

Immunological synapse and microclusters: the site for recognition and activation of T cells

An immunological synapse (IS) is formed at the interface between antigen-presenting cells and T cells, and is believed to be the structure responsible for antigen recognition and T-cell activation. However, recent imaging analyses reveal that T-cell receptor microclusters (MCs) formed prior to IS are the site for antigen recognition and T-cell activation. MCs are continuously generated at the periphery of the interface, even after IS formation, and induce sustained activation signals. MC formation is not accompanied by lipid-raft clustering. Central supramolecular activation cluster is considered functional in recycling and degradation of T-cell receptors, directional secretion of cytokines and cytolytic granules, generation of sustained signals, or maintenance of the cell-cell conjugation.

CD45: all is not yet crystal clear

CD45 has been recognized as an important player in regulating signalling in lymphocytes. However, compared with tyrosine kinases, phosphatases are still poorly understood in terms of the details of their specificity and regulation. Here, the recent progress in understanding the biology of the first recognized receptor tyrosine phosphatase, CD45, is reviewed.

CD80 cytoplasmic domain controls localization of CD28, CTLA-4, and protein kinase Ctheta in the immunological synapse

The binding of costimulatory ligand CD80 to CD28 or CTLA-4 on T cells plays an important role in the regulation of the T cell response. We have examined the role of the cytoplasmic domain of CD80 in murine T cell costimulation and its organization in the immunological synapse (IS). Removal of CD80 cytoplasmic tail decreased its effectiveness in costimulating T cell proliferative response and early IL-2 production in response to agonist MHC-peptide complexes. Immunofluorescent study showed a decreased tailless CD80 accumulation in the IS of naive T cells. The two forms of CD80 accumulated differently at the IS; the tailless CD80 was colocalized with the TCR whereas the full-length CD80 was segregated from the TCR. In addition, we showed that CD80, CD28, and protein kinase Ctheta colocalized in the presence or absence of the CD80 cytoplasmic tail. Thus, the cytoplasmic tail of CD80 regulates its spatial localization at the IS and that of its receptors and T cell signaling molecules such as protein kinase Ctheta, and thereby facilitates full T cell activation.

The phenotype and survival of antigen-stimulated transgenic CD4 T cells in vivo: the influence of persisting antigen

Naive and primed/memory CD4 T cells are distinguished by changes in the expression of activation/adhesion molecules that correspond with an altered function. Adoptively transferred TCR transgenic (tg) CD4 T cells specific for ovalbumin peptide (OVA-pep) were analysed for changing phenotype and the speed of change in vivo following antigen challenge with alum-precipitated (ap) OVA-pep, a conjugate that stimulated a Th2-type cytokine response. The change of CD45RB in relation to number of divisions showed that the transition from CD45RB(hi) (naive) to CD45RB(low) (primed/memory) was incremental; with each cell cycle the number of CD45RB(hi) molecules on the cell surface was diluted by approximately half and replaced by the low-weight isoform. Similarly, the change to CD44(hi) expression increased gradually during four rounds of proliferation. The loss of CD62L expression occurred early and was independent of cell division. CD69 was up-regulated quickly within 1-2 cycles, but down-regulated after about seven divisions. The expression of CD49d was not altered during the early rounds of division, although it was up-regulated on 30-60% of tg T cells dividing repeatedly (>or=8 cycles). When analysed on day 3 following stimulation, CD25 was no longer up-regulated. The intra-peritoneal injection of ap-OVA-pep stimulated tg T cells in the spleen and mesenteric lymph node one day in advance of those in more distant peripheral lymph nodes. Evidence indicated that residual antigen persisted for at least 4 weeks and was able to stimulate naive tg T cells. However, residual antigen had no net effect on extending or reducing survival of the transferred population.

Impact of the immunological synapse on T cell signaling

T cell activation requires interactions of T cell antigen receptors and peptides presented by major histocompatibility complex molecules in an adhesive junction between the T cell and antigen-presenting cell (APC). Stable junctions with bull's-eye supramolecular activation clusters have been defined as immunological synapses (IS). These structures maintain T cell-APC interaction and allow directed secretion. T cells can also be activated by asymmetric hemisynapses (HS) that allow migration during signal integration. IS and HS dominate in different stages of T cell priming. Optimal effector functions may also depend upon cyclical use of IS and HS.

A dynamic view of the immunological synapse

T cell activation requires interactions of T cell antigen receptors (TCR) and peptides presented by major histocompatibility complex molecules (MHCp) in an adhesive junction between the T cell and antigen-presenting cell. Stable junctions with bull's eye supramolecular activation clusters (SMACs) have been defined as immunological synapses (IS). These structures maintain T cell-APC interaction and allow directed secretion. T cells can also be activated by asymmetric hemi-synapses (HS) that allow migration during signal integration. IS and HS operate in different stages of T cell priming. Optimal effector functions may also depend upon cyclical use of IS and HS.

Actin and agonist MHC-peptide complex-dependent T cell receptor microclusters as scaffolds for signaling

T cell receptor (TCR) microclusters form within seconds of T cell contact with supported planar bilayers containing intercellular adhesion molecule-1 and agonist major histocompatibility complex (MHC)–peptide complexes, and elevation of cytoplasmic Ca²⁺ is observed within seconds of the first detectable microclusters. At 0–30 s after contact, TCR microclusters are colocalized with activated forms of Lck, ZAP-70, and the linker for activation of T cells. By 2 min, activated kinases are reduced in the older central microclusters, but are abundant in younger peripheral microclusters. By 5 min, TCR in the central supramolecular activation cluster have reduced activated kinases, whereas faint peripheral TCR microclusters efficiently generated activated Lck and ZAP-70. TCR microcluster formation is resistant to inhibition by Src family kinase inhibitor PP2, but is abrogated by actin polymerization inhibitor latrunculin A. We propose that Src kinase–independent formation of TCR microclusters in response to agonist MHC–peptide provides an actin-dependent scaffold for signal amplification.

Single-molecule microscopy reveals plasma membrane microdomains created by protein-protein networks that exclude or trap signaling molecules in T cells

Membrane subdomains have been implicated in T cell signaling, although their properties and mechanisms of formation remain controversial. Here, we have used single-molecule and scanning confocal imaging to characterize the behavior of GFP-tagged signaling proteins in Jurkat T cells. We show that the coreceptor CD2, the adaptor protein LAT, and tyrosine kinase Lck cocluster in discrete microdomains in the plasma membrane of signaling T cells. These microdomains require protein-protein interactions mediated through phosphorylation of LAT and are not maintained by interactions with actin or lipid rafts. Using a two color imaging approach that allows tracking of single molecules relative to the CD2/LAT/Lck clusters, we demonstrate that these microdomains exclude and limit the free diffusion of molecules in the membrane but also can trap and immobilize specific proteins. Our data suggest that diffusional trapping through protein-protein interactions creates microdomains that concentrate or exclude cell surface proteins to facilitate T cell signaling.

CD45 Signals outside of Lipid Rafts to Promote ERK Activation, Synaptic Raft Clustering, and IL-2 Production

CD45 is dynamically repositioned within lipid rafts and the immune synapse during T cell activation, although the molecular consequences of CD45 repositioning remain unclear. In this study we examine the role of CD45 membrane compartmentalization in regulating murine T cell activation. We find that raft-localized CD45 antagonizes IL-2 production by opposing processive TCR signals, whereas raft-excluded CD45 promotes ERK-dependent polarized synaptic lipid raft clustering and IL-2 production. We propose that these dual CD45 activities ensure that only robust TCR signals proceed, whereas signals meeting threshold requirements are potentiated. Our findings highlight membrane compartmentalization as a key regulator of CD45 function and elucidate a novel signal transduction pathway by which raft-excluded CD45 positively regulates T cell activation.

The Immunological Synapse and Rho GTPases

Rho GTPases are molecular switches controlling a broad range of cellular processes including lymphocyte activation. Not surprisingly, Rho GTPases are now recognized as pivotal regulators of antigen-specific T cell activation by APCs and immunological synapse formation. This review summarizes recent advances in our understanding of how Rho GTPase-dependent pathways control T lymphocyte motility, polarization and activation.

CD45 isoforms in T cell signalling and development

The CD45 phosphotyrosine phosphatase is expressed on T cells as multiple isoforms due to alternative splicing. The panoply of isoforms expressed is tightly regulated during T cell development and on mature peripheral T cell subsets following activation. We describe the analysis of comparative CD45 isoform expression levels on thymic and T cell subsets from the C57BL/6 mouse. Only four isoforms were expressed at significant protein levels: CD45R0, CD45RB, CD45RBC and CD45RABC, although trace amounts of others may be present. The expression of CD45RBC was about nine-fold higher on CD8(+) than on CD4(+) peripheral T cells, whereas CD45R0 expression was higher on CD4(+) T cells. We provide a general overview of the current models that have been proposed to explain the molecular actions of the different CD45 isoforms. Achieving a thorough understanding of the biological reasons for the existence and tight regulation of CD45 isoform expression in immune cells remains one of the outstanding challenges in the CD45 research field.

Equilibrium thermodynamics of cell-cell adhesion mediated by multiple ligand-receptor pairs

In many situations, cell-cell adhesion is mediated by multiple ligand-receptor pairs. For example, the interaction between T cells and antigen-presenting cells of the immune system is mediated not only by T cell receptors and their ligands (peptide-major histocompatibility complex) but also by binding of intracellular adhesion molecules. Interestingly, these binding pairs have different resting lengths. Fluorescent labeling reveals segregation of the longer adhesion molecules from the shorter T cell receptors in this case. Here, we explore the thermal equilibrium of a general cell-cell interaction mediated by two ligand-receptor pairs to examine competition between the elasticity of the cell wall, nonspecific intercellular repulsion, and bond formation, leading to segregation of bonds of different lengths at equilibrium. We make detailed predictions concerning the relationship between physical properties of the membrane and ligand-receptor pairs and equilibrium pattern formation, and suggest experiments to refine our understanding of the system. We demonstrate our model by application to the T cell/antigen-presenting-cell system and outline applications to natural killer cell adhesion.

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.

Regulated Expression of the Receptor-Like Tyrosine Phosphatase CD148 on Hemopoietic Cells

CD148 is a receptor-like protein tyrosine phosphatase expressed on a wide variety of cell types. Through the use flow cytometry and immunofluorescence microscopy on tissue sections, we examined the expression of CD148 on multiple murine hemopoietic cell lineages. We found that CD148 is moderately expressed during all stages of B cell development in the bone marrow, as well as peripheral mature B cells. In contrast, CD148 expression on thymocytes and mature T cells is substantially lower. However, stimulation of peripheral T cells through the TCR leads to an increase of CD148 expression. This up-regulation on T cells can be partially inhibited by reagents that block the activity of src family kinases, calcineurin, MEK, or PI3K. Interestingly, CD148 levels are elevated on freshly isolated T cells from MRL lpr/lpr and CTLA-4-deficient mice, two murine models of autoimmunity. Together, these expression data along with previous biochemical data suggest that CD148 may play an important regulatory role to control an immune response.

Activation-induced polarized recycling targets T cell antigen receptors to the immunological synapse; involvement of SNARE complexes

The mechanism by which T cell antigen receptors (TCR) accumulate at the immunological synapse has not been fully elucidated. Since TCRs are continuously internalized and recycled back to the cell surface, we investigated the role of polarized recycling in TCR targeting to the immunological synapse. We show here that the recycling endosomal compartment of T cells encountering activatory antigen-presenting cells (APCs) polarizes towards the T cell-APC contact site. Moreover, TCRs in transit through recycling endosomes are targeted to the immunological synapse. Inhibition of T cell polarity, constitutive TCR endocytosis, or recycling reduces TCR accumulation at the immunological synapse. Conversely, increasing the amount of TCRs in recycling endosomes before synapse formation enhanced their accumulation. Finally, we show that exocytic t-SNAREs from T cells cluster at the APC contact site and that tetanus toxin inhibits TCR accumulation at the immunological synapse, indicating that vesicle fusion mediated by SNARE complexes is involved in TCR targeting to the immunological synapse.