Spatial relationships of microtubule-organizing centers and the contact area of cytotoxic T lymphocytes and target cells

Killing machines: three pore-forming proteins of the immune system

The evolution of early multicellular eukaryotes 400-500 million years ago required a defensive strategy against microbial invasion. Pore-forming proteins containing the membrane-attack-complex-perforin (MACPF) domain were selected as the most efficient means to destroy bacteria or virally infected cells. The mechanism of pore formation by the MACPF domain is distinctive in that pore formation is purely physical and unspecific. The MACPF domain polymerizes, refolds, and inserts itself into bilayer membranes or bacterial outer cell walls. The displacement of surface lipid/carbohydrate molecules by the polymerizing MACPF domain creates clusters of large, water-filled holes that destabilize the barrier function and provide access for additional anti-bacterial or anti-viral effectors to sensitive sites that complete the destruction of the invader via enzymatic or chemical attack. The highly efficient mechanism of anti-microbial defense by a combined physical and chemical strategy using pore-forming MACPF-proteins has been retargeted during evolution of vertebrates and mammals for three purposes: (1) to kill extracellular bacteria C9/polyC9 evolved in conjunction with complement, (2) to kill virus infected and cancer cells perforin-1/polyperforin-1 CTL evolved targeted by NK and CTL, and (3) to kill intracellular bacteria transmembrane perforin-2/putative polyperforin-2 evolved targeted by phagocytic and nonphagocytic cells. Our laboratory has been involved in the discovery and description of each of the three pore-formers that will be reviewed here.

From lipid second messengers to molecular motors: microtubule-organizing center reorientation in T cells

In T lymphocytes, polarization of the microtubule-organizing center (MTOC) to the immunological synapse enables the directional secretion of cytokines, cytolytic factors, and other soluble molecules toward the antigen-presenting cell. This is likely to be crucial for maintaining the specificity of T-cell effector responses. Here, we review recent advances in our understanding of MTOC reorientation in T cells, focusing first on the importance of diacylglycerol and protein kinase C isozymes and then on the molecular motor proteins that function downstream to drive MTOC movement.

The role of the cytoskeleton at the immunological synapse

It has been over 30 years since the reorganization of both the microtubule network and a 'peculiar actin polarization' was reported at the contact area of cytotoxic T lymphocytes interacting with target cells. Since that time, hundreds of studies have been published in an effort to elucidate the structure and function of the microtubule network and the actin cytoskeleton in T-cell activation, migration, and effector function at the interface between a T cell and its cognate antigen-presenting cell or target cell. This interface has become known as the immunological synapse, and this review examines some of the roles played by the cytoskeleton at the synapse.

Modulation of T cell signaling by the actin cytoskeleton

The actin cytoskeleton provides a dynamic framework to support membrane organization and cellular signaling events. The importance of actin in T cell function has long been recognized to go well beyond the maintenance of cell morphology and transport of proteins. Over the past several years, our understanding of actin in T cell activation has expanded tremendously, in part owing to the development of methods and techniques to probe the complex interplay between actin and T cell signaling. On the one hand, biochemical methods have led to the identification of many key cytoskeleton regulators and new signaling pathways, whereas, on the other, the combination of advanced imaging techniques and physical characterization tools has allowed the spatiotemporal investigation of actin in T cell signaling. All those studies have made a profound impact on our understanding of the actin cytoskeleton in T cell activation. Many previous reviews have focused on the biochemical aspects of the actin cytoskeleton. However, here we will summarize recent studies from a biophysical perspective to explain the mechanistic role of actin in modulating T cell activation. We will discuss how actin modulates T cell activation on multiple time and length scales. Specifically, we will reveal the distinct roles of the actin filaments in facilitating TCR triggering, orchestrating 'signalosome' assembly and transport, and establishing protein spatial organization in the immunological synapse.

Centrosome repositioning in T cells is biphasic and driven by microtubule end-on capture-shrinkage

T cells rapidly reposition their centrosome to the center of the immunological synapse (IS) to drive polarized secretion in the direction of the bound target cell. Using an optical trap for spatial and temporal control over target presentation, we show that centrosome repositioning in Jurkat T cells exhibited kinetically distinct polarization and docking phases and required calcium flux and signaling through both the T cell receptor and integrin to be robust. In "frustrated" conjugates where the centrosome is stuck behind the nucleus, the center of the IS invaginated dramatically to approach the centrosome. Consistently, imaging of microtubules during normal repositioning revealed a microtubule end-on capture-shrinkage mechanism operating at the center of the IS. In agreement with this mechanism, centrosome repositioning was impaired by inhibiting microtubule depolymerization or dynein. We conclude that dynein drives centrosome repositioning in T cells via microtubule end-on capture-shrinkage operating at the center of the IS and not cortical sliding at the IS periphery, as previously thought.

Do signalling endosomes play a role in T cell activation?

Signalling endosomes represent a general mechanism for modulating and compartmentalizing cell signalling, which is achieved by delineating specific spatial environments and connecting the plasma membrane with intracellular events. The molecular composition of vesicles, together with their targeting mechanisms and endocytic routes, contributes to the outcome of signalling pathways that are initiated either at the plasma membrane or within endosomes themselves. In T cell signalling, it is now accepted that the spatial distribution of signalling proteins is central to T cell activation not only at the immunological synapse, but also in endosomes travelling to and from the plasma membrane. In addition, there is a global rearrangement of the endosome machinery upon T cell activation, and emerging experimental evidence suggests that vesicles in T cells contain key T cell signalling proteins. We review the various mechanisms by which endosomes contribute to signalling pathways and consider whether signalling endosomes play a role in T cell signalling.

Regulation of Lipid Signaling by Diacylglycerol Kinases during T Cell Development and Function

Diacylglycerol (DAG) and phosphatidic acid (PA) are bioactive lipids synthesized when the T cell receptor binds to a cognate peptide-MHC complex. DAG triggers signaling by recruiting Ras guanyl-releasing protein 1, PKCθ, and other effectors, whereas PA binds to effector molecules that include mechanistic target of rapamycin, Src homology region 2 domain-containing phosphatase 1, and Raf1. While DAG-mediated pathways have been shown to play vital roles in T cell development and function, the importance of PA-mediated signals remains less clear. The diacylglycerol kinase (DGK) family of enzymes phosphorylates DAG to produce PA, serving as a molecular switch that regulates the relative levels of these critical second messengers. Two DGK isoforms, α and ζ, are predominantly expressed in T lineage cells and play an important role in conventional αβ T cell development. In mature T cells, the activity of these DGK isoforms aids in the maintenance of self-tolerance by preventing T cell hyper-activation and promoting T cell anergy. In this review, we discuss the roles of DAG-mediated pathways, PA-effectors, and DGKs in T cell development and function. We also highlight recent work that has uncovered previously unappreciated roles for DGK activity, for instance in invariant NKT cell development, anti-tumor and anti-viral CD8 responses, and the directional secretion of soluble effectors.

How the immune system talks to itself: the varied role of synapses

Using an elaborately evolved language of cytokines and chemokines as well as cell-cell interactions, the different components of the immune system communicate with each other and orchestrate a response (or wind one down). Immunological synapses are a key feature of the system in the ways in which they can facilitate and direct these responses. Studies analyzing the structure of an immune synapse as it forms between two cells have provided insight into how the stability and kinetics of this interaction ultimately affect the sensitivity, potency, and magnitude of a given response. Furthermore, we have gained an appreciation of how the immunological synapse provides directionality and contextual cues for downstream signaling and cellular decision-making. In this review, we discuss how using a variety of techniques, developed over the last decade, have allowed us to visualize and quantify key aspects of the dynamic synaptic interface and have furthered our understanding of their function. We describe some of the many characteristics of the immunological synapse that make it a vital part of intercellular communication and some of the questions that remain to be answered.

CD103 or LFA-1 engagement at the immune synapse between cytotoxic T cells and tumor cells promotes maturation and regulates T-cell effector functions

T-cell adhesion/costimulatory molecules and their cognate receptors on target cells play a major role in T-cell receptor (TCR)-mediated activities. Here, we compared the involvement of CD103 and LFA-1, and their respective ligands, in the maturation of the cytotoxic immune synapse (cIS) and in the activation of CTL effector functions. Our results indicate that cytotoxicity toward cancer cells and, to a lesser extent, cytokine production by specific CTL require, together with TCR engagement, the interaction of either CD103 with E-cadherin or LFA-1 with ICAM-1. Flow-based adhesion assay showed that engagement of CD103 or LFA-1, together with TCR, enhances the strength of the T-cell/target cell interaction. Moreover, electron microscopic analyses showed that integrin-dependent mature cIS (mcIS) displays a cohesive ultrastructure, with tight membrane contacts separated by extensive clefts. In contrast, immature cIS (icIS), which is unable to trigger target cell lysis, is loose, with multiple protrusions in the effector cell membrane. Experiments using confocal microscopy revealed polarized cytokine release and degranulation at the mcIS associated with target cell killing, whereas icIS is characterized by failure of IFN-γ and granzyme B relocalization. Thus, interactive forces between CTL and epithelial tumor cells, mainly regulated by integrin engagement, correlate with maturity and the ultrastructure of the cIS and influence CTL effector functions. These results provide new insights into molecular mechanisms regulating antitumor CTL responses and may lead to the development of more efficient cancer immunotherapy strategies.

Cytotoxic T lymphocyte effector function is independent of nucleus-centrosome dissociation

Cytotoxic T lymphocytes (CTLs) kill tumorigenic and virally infected cells by targeted secretion of lytic granule contents. The precise point at which secretion occurs is directed by the centrosome docking at the immunological synapse (IS). The centrosome is highly dynamic in CTLs, lagging behind the nucleus in the uropod of migrating CTLs, but translocating across the entire length of the cell to dock at the IS when a target cell is recognized. While in most cell types, the centrosome is always closely associated with the nuclear membrane, in CTLs, it often appears to be dissociated from the nucleus, both in migrating cells and when forming an IS. We asked whether this dissociation is required for CTL killing, by expressing GFP-BICD2-NT-nesprin-3, which tethers the centrosome to the nucleus irreversibly. Immunofluorescence microscopy revealed that the centrosome polarized successfully to the central supramolecular activation complex (cSMAC) of the synapse in GFP-BICD2-NT-nesprin-3-expressing CTLs, with the centrosome and nucleus migrating together to the IS. CTLs in which the centrosome was "glued" to the nucleus were able to dock and release granules at the IS as effectively as mock-treated cells. These data demonstrate that CTL cytotoxicity is independent of centrosomal dissociation from the nuclear envelope.

HIV-1 Virological Synapse is not Simply a Copycat of the Immunological Synapse

The virological synapse (VS) is a tight adhesive junction between an HIV-infected cell and an uninfected target cell, across which virus can be efficiently transferred from cell to cell in the absence of cell-cell fusion. The VS has been postulated to resemble, in its morphology, the well-studied immunological synapse (IS). This review article discusses the structural similarities between IS and VS and the shared T cell receptor (TCR) signaling components that are found in the VS. However, the IS and the VS display distinct kinetics in disassembly and intracellular signaling events, possibly leading to different biological outcomes. Hence, HIV-1 exploits molecular components of IS and TCR signaling machinery to trigger unique changes in cellular morphology, migration, and activation that facilitate its transmission and cell-to-cell spread.

Microtubule-organizing center polarity and the immunological synapse: protein kinase C and beyond

Cytoskeletal polarization is crucial for many aspects of immune function, ranging from neutrophil migration to the sampling of gut flora by intestinal dendritic cells. It also plays a key role during lymphocyte cell–cell interactions, the most conspicuous of which is perhaps the immunological synapse (IS) formed between a T cell and an antigen-presenting cell (APC). IS formation is associated with the reorientation of the T cell’s microtubule-organizing center (MTOC) to a position just beneath the cell–cell interface. This cytoskeletal remodeling event aligns secretory organelles inside the T cell with the IS, enabling the directional release of cytokines and cytolytic factors toward the APC. MTOC polarization is therefore crucial for maintaining the specificity of a T cell’s secretory and cytotoxic responses. It has been known for some time that T cell receptor (TCR) stimulation activates the MTOC polarization response. It has been difficult, however, to identify the machinery that couples early TCR signaling to cytoskeletal remodeling. Over the past few years, considerable progress has been made in this area. This review will present an overview of recent advances, touching on both the mechanisms that drive MTOC polarization and the effector responses that require it. Particular attention will be paid to both novel and atypical members of the protein kinase C family, which are now known to play important roles in both the establishment and the maintenance of the polarized state.

Casein kinase I delta controls centrosome positioning during T cell activation

CK1delta binds and phosphorylates the microtubule plus-end–binding protein EB1 and promotes centrosome translocation to the immunological synapse in T cells.

Although termed central body, the centrosome is located off-center in many polarized cells. T cell receptor (TCR) engagement by antigens induces a polarity switch in T cells. This leads to the recruitment of the centrosome to the immunological synapse (IS), a specialized cell–cell junction. Despite much recent progress, how TCR signaling triggers centrosome repositioning remains poorly understood. In this paper, we uncover a critical requirement for the centrosomal casein kinase I delta (CKIδ) in centrosome translocation to the IS. CKIδ binds and phosphorylates the microtubule plus-end–binding protein EB1. Moreover, a putative EB1-binding motif at the C terminus of CKIδ is required for centrosome translocation to the IS. We find that depletion of CKIδ in T lymphocytes and inhibition of CKI in epithelial cells reduce microtubule growth. Therefore, we propose that CKIδ–EB1 complexes contribute to the increase in microtubule growth speeds observed in polarized T cells, a mechanism that might serve to generate long-stable microtubules necessary for centrosome translocation.

Cytoskeletal stabilization of inhibitory interactions in immunologic synapses of mature human dendritic cells with natural killer cells

Human mature dendritic cells (DCs) can efficiently stimulate natural killer (NK)-cell responses without being targeted by their cytotoxicity. To understand this important regulatory crosstalk, we characterized the development of the immunologic synapse between mature DCs and resting NK cells. Conjugates between these 2 innate leukocyte populations formed rapidly, persisted for prolonged time periods and matured with DC-derived f-actin polymerization at the synapse. Polarization of IL-12 and IL-12R to the synapse coincided with f-actin polymerization, while other activating and inhibitory molecules were enriched at the interface between DCs and NK cells earlier. Functional assays revealed that inhibition of f-actin polymerization in mature synapses led to an increase of IFN-γ secretion and cytotoxicity by NK cells. This elevated NK-cell reactivity resulted from decreased inhibitory signaling in the absence of MHC class I polarization at the interface, which was observed on inhibition of f-actin polymerization in DCs. Thus, inhibitory signaling is stabilized by f-actin at the synapse between mature DCs and resting NK cells.

Dynein-driven transport of T cell receptor microclusters regulates immune synapse formation and T cell activation

When T cells recognize a peptide-major histocompatibility complex on antigen-presenting cells (APCs), T cell receptor microclusters (TCR-MCs) are generated and move to the center of the T cell-APC interface to form the central supramolecular activation cluster (cSMAC). cSMAC formation depends on stimulation strength and regulates T cell activation. We demonstrate that the dynein motor complex colocalized and coimmunoprecipitated with the TCR complex and that TCR-MCs moved along microtubules (MTs) toward the center of the immune synapse in a dynein-dependent manner to form cSMAC. MTs are located in close proximity to the plasma membrane at the activation site. TCR-MC velocity and cSMAC formation were impaired by dynein or MT inhibitors or by ablation of dynein expression. T cells with impaired cSMAC formation exhibited enhanced cellular activation including protein phosphorylation and interleukin-2 production. These results indicate that cSMAC formation by TCR-MC movement depends on dynein and MTs, and the movement regulates T cell activation.

Systems biomechanics of centrosome positioning: A conserved complexity

Positioning of centrosomes within cells determines the directionality of cell division, as well as directionality of cellular activities in the interphase. This brief review focuses on similarities (and differences) of centrosome positioning during early divisions in the Caenorhabditis embryo and during the interaction of T lymphocytes with other cells in the course of immune response. In the study of the two phenomena, a synergy of experimentation and numerical mechanical analysis has recently been achieved. The picture that emerges from these studies is one in which simple physical forces under the constraints of the basic cell structure lead to complex, "life-like" mechanical behavior. This behavior includes instability of equilibria, irreversibility of structural transitions and multidimensional, multiperiodic oscillations. This new picture of cell mechanics may form an interesting paradigm for future research.

Lymphocyte polarity, the immunological synapse and the scope of biological analogy

Lymphocytes such as T cells, B cells and natural killer (NK) cells form specialized contacts, called immunological synapses, with other cells in order to engage in specific intercellular communication and killing. Synapse formation is associated with the polarization of the microtubule-organizing center (MTOC) toward the contact site, which enables the directional secretion of cytokines and lytic factors. Although MTOC reorientation to the synapse is crucial for lymphocyte function, it has been difficult to study because of technical constraints. We have developed a photoactivation and imaging strategy that enables high-resolution analysis of cytoskeletal dynamics in individual T cells. Using this approach, we have demonstrated that the lipid second messenger diacylglycerol plays a crucial role in promoting MTOC reorientation by recruiting three members of the protein kinase C family to the synapse. Here, I will discuss these results along with studies from other labs, which have explored the role of polarity-inducing protein complexes after synapse formation. I will also propose a two-step model for MTOC reorientation in lymphocytes that reflects what we now know about the subject. Finally, I will consider the extent to which lymphocyte polarity resembles analogous cell polarity systems in other cell types.

Centriole polarisation to the immunological synapse directs secretion from cytolytic cells of both the innate and adaptive immune systems

Background

Cytolytic cells of the immune system destroy pathogen-infected cells by polarised exocytosis of secretory lysosomes containing the pore-forming protein perforin. Precise delivery of this lethal hit is essential to ensuring that only the target cell is destroyed. In cytotoxic T lymphocytes (CTLs), this is accomplished by an unusual movement of the centrosome to contact the plasma membrane at the centre of the immunological synapse formed between killer and target cells. Secretory lysosomes are directed towards the centrosome along microtubules and delivered precisely to the point of target cell recognition within the immunological synapse, identified by the centrosome. We asked whether this mechanism of directing secretory lysosome release is unique to CTL or whether natural killer (NK) and invariant NKT (iNKT) cytolytic cells of the innate immune system use a similar mechanism to focus perforin-bearing lysosome release.

Results

NK cells were conjugated with B-cell targets lacking major histocompatibility complex class I 721.221 cells, and iNKT cells were conjugated with glycolipid-pulsed CD1-bearing targets, then prepared for thin-section electron microscopy. High-resolution electron micrographs of the immunological synapse formed between NK and iNKT cytolytic cells with their targets revealed that in both NK and iNKT cells, the centrioles could be found associated (or 'docked') with the plasma membrane within the immunological synapse. Secretory clefts were visible within the synapses formed by both NK and iNKT cells, and secretory lysosomes were polarised along microtubules leading towards the docked centrosome. The Golgi apparatus and recycling endosomes were also polarised towards the centrosome at the plasma membrane within the synapse.

Conclusions

These results reveal that, like CTLs of the adaptive immune system, the centrosomes of NK and iNKT cells (cytolytic cells of the innate immune system) direct secretory lysosomes to the immunological synapse. Morphologically, the overall structure of the immunological synapses formed by NK and iNKT cells are very similar to those formed by CTLs, with both exocytic and endocytic organelles polarised towards the centrosome at the plasma membrane, which forms a focal point for exocytosis and endocytosis within the immunological synapse. We conclude that centrosomal polarisation provides a rapid, responsive and precise mechanism for secretory lysosome delivery to the immunological synapse in CTLs, NK cells and iNKT cells.

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.

Centrosome docking at the immunological synapse is controlled by Lck signaling

Docking of the centrosome at the plasma membrane directs lytic granules to the immunological synapse. To identify signals controlling centrosome docking at the synapse, we have studied cytotoxic T lymphocytes (CTLs) in which expression of the T cell receptor-activated tyrosine kinase Lck is ablated. In the absence of Lck, the centrosome is able to translocate around the nucleus toward the immunological synapse but is unable to dock at the plasma membrane. Lytic granules fail to polarize and release their contents, and target cells are not killed. In CTLs deficient in both Lck and the related tyrosine kinase Fyn, centrosome translocation is impaired, and the centrosome remains on the distal side of the nucleus relative to the synapse. These results show that repositioning of the centrosome in CTLs involves at least two distinct steps, with Lck signaling required for the centrosome to dock at the plasma membrane.

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.

T cell polarity at the immunological synapse is required for CD154-dependent IL-12 secretion by dendritic cells

Ag-specific interaction between T lymphocytes and dendritic cells (DCs) leads to both T cell and DC activation. CD154 (CD40 ligand)/CD40 interactions have been shown to play a major, although not exclusive, role in this functional cross-talk. Interactions between T cells and DCs are structured by an immunological synapse (IS), characterized by polarization of the T cell microtubule cytoskeleton toward the interacting DCs. Yet the role T cell polarization may play in T cell-induced DC activation is mostly unknown. In this study, we address the role of T cell polarity in CD154-dependent activation of DCs in a human model, using two different tools to block T cell polarity (i.e., a microtubule depolymerizing drug and an inhibitor of atypical protein kinase C). We show that CD154 is recruited and concentrated at the IS formed between human primary T cells and autologous DCs and that this recruitment requires T cell polarity at the IS. Moreover, we show that T cell polarization at the IS controls T cell-dependent CD154-CD40 signaling in DCs as well as CD154-dependent IL-12 secretion by DCs. This study shows that T cell polarity at the IS plays a key role in CD154/CD40-dependent cross-talk between CD4(+) T cells and DCs.

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.

Understanding the structure and function of the immunological synapse

The immunological synapse has been an area of very active scientific interest over the last decade. Surprisingly, much about the synapse remains unknown or is controversial. Here we review some of these current issues in the field: how the synapse is defined, its potential role in T-cell function, and our current understanding about how the synapse is formed.

Protein trafficking in immune cells

The majority of cells of the immune system are specialized secretory cells, whose function depends on regulated exocytosis. The latter is mediated by vesicular transport involving the sorting of specialized cargo into the secretory granules (SGs), thereby generating the transport vesicles; their transport along the microtubules and eventually their signal-dependent fusion with the plasma membrane. Each of these steps is tightly controlled by mechanisms, which involve the participation of specific sorting signals on the cargo proteins and their recognition by cognate adaptor proteins, posttranslational modifications of the cargo proteins and multiple GTPases and SNARE proteins. In some of the cells (i.e. mast cells, T killer cells) an intimate connection exists between the secretory system and the endocytic one, whereby the SGs are lysosome related organelles (LROs) also referred to as secretory lysosomes. Herein, we discuss these mechanisms in health and disease states.

Photochemical approaches to T-cell activation

Despite decades of intensive research, T-cell activation has remained mysterious because of both the dizzying diversity of antigen recognition and the speed and comprehensiveness of the T-cell-receptor signalling network. Further progress will require new approaches and reagents that provide added levels of control. Photochemistry allows specific biochemical processes to be controlled with light and is well suited to mechanistic studies in complex cellular environments. In recent years, several laboratories have adopted approaches based on photoreactive peptide-major histocompatibility complex reagents in order to study T-cell activation and function with high precision. Here, I review these efforts and outline future directions for this exciting area of research.

Functional anatomy of T cell activation and synapse formation

T cell activation and function require a structured engagement of antigen-presenting cells. These cell contacts are characterized by two distinct dynamics in vivo: transient contacts resulting from promigratory junctions called immunological kinapses or prolonged contacts from stable junctions called immunological synapses. Kinapses operate in the steady state to allow referencing to self-peptide-MHC (pMHC) and searching for pathogen-derived pMHC. Synapses are induced by T cell receptor (TCR) interactions with agonist pMHC under specific conditions and correlate with robust immune responses that generate effector and memory T cells. High-resolution imaging has revealed that the synapse is highly coordinated, integrating cell adhesion, TCR recognition of pMHC complexes, and an array of activating and inhibitory ligands to promote or prevent T cell signaling. In this review, we examine the molecular components, geometry, and timing underlying kinapses and synapses. We integrate recent molecular and physiological data to provide a synthesis and suggest ways forward.

Cytotoxic immunological synapses

One of the most fundamental activities of the adaptive immune system is to kill infected cells and tumor cells. Two distinct pathways mediate this process, both of which are facilitated by a cytotoxic immunological synapse. While traditionally thought of as innate immune cells, natural killer (NK) cells are now appreciated to have the capacity for long-term adaptation to chemical and viral insults. These cells integrate multiple positive and negative signals through NK cell cytotoxic or inhibitory synapses. The traditional CD8(+)alphabeta T-cell receptor-positive cells are among the best models for the concept of an immunological synapse, in which vectoral signaling is linked to directed secretion in a stable interface to induce apoptotic cell death in an infected cell. Large-scale molecular organization in synapses generated a number of hypotheses. Studies in the past 5 years have started to provide clear answers regarding the validity of these models. In vivo imaging approaches have provided some hints as to the physiologic relevance of these processes with great promise for the future. This review provides an overview of work on cytotoxic immunological synapses and suggests pathways forward in applying this information to the development of therapeutic agents.

Integrin and CD3/TCR activation are regulated by the scaffold protein AKAP450

During antigen recognition by T cells, membrane receptors and cytoskeletal molecules form a specialized structure at the T cell-antigen-presenting cell junction called the immune synapse (IS). We report a role for the scaffolding protein A-kinase anchoring protein-450 (AKAP450), a member of the A-kinase anchoring protein family, in IS formation and T-cell signaling in antigen- and superantigen-dependent T-cell activation. Suppression of AKAP450 by overexpression of a dominant-negative form or siRNA knockdown disrupted the positioning and conformational activation of lymphocyte function-associated antigen 1 at the IS and impaired associated signaling events, including phosphorylation of phospholipase C-gamma1 and protein kinase C-. AKAP450 was also required for correct activation and phosphorylation of CD3, LAT, and Vav1, key T-cell receptor-activated intracellular signaling molecules. Consistently, antigen-triggered reorientation of the microtubule-organizing center at the IS and interleukin-2 secretion were diminished in AKAP450-disrupted T cells. These results indicate key roles for AKAP450 in the organization and activation of receptor molecules at the IS during T-cell signaling events.

Subcellular dynamics of T cell immunological synapses and kinapses in lymph nodes

In vitro studies have revealed that T cell activation occurs during the formation of either dynamic or stable interactions with antigen-presenting cells (APC), and the respective cell junctions have been referred to as immunological kinapses and synapses. However, the relevance and molecular dynamics of kinapses and synapses remain to be established in vivo. Using two-photon imaging, we tracked the distribution of LAT-EGFP molecules during antigen recognition by activated CD4(+) T cells in lymph nodes. At steady state, LAT-EGFP molecules were preferentially found at the uropod of rapidly migrating T cells. In contrast to naïve T cells that fully stopped upon systemic antigen delivery, recently activated T cells decelerated and formed kinapses, characterized by continuous extension of membrane protrusions and by the absence of persistent LAT-EGFP clustering. On the other hand, activated CD4(+) T cells formed stable immunological synapses with antigen-loaded B cells and displayed sustained accumulation of LAT-EGFP fluorescence at the contact zone. Our results show that the state of T cell activation and the type of APC largely influence T cell-APC contact dynamics in lymph nodes. Furthermore, we provide a dynamic look at immunological kinapses and synapses in lymph nodes and suggest the existence of distinct patterns of LAT redistribution during antigen recognition.

Vesicle traffic to the immunological synapse: a multifunctional process targeted by lymphotropic viruses

The site of contact between T lymphocytes and antigen-presenting cells becomes, upon antigen recognition, an organized junction named the immunological synapse. Various T cell organelles polarize, together with microtubules, toward the antigen-presenting cell. Among them, intracellular vesicular compartments, such as the Golgi apparatus, the recycling endosomal compartment, or cytotoxic granules help to build the immunological synapse and ensure effector functions, such as polarized secretion of cytokines by helper T cells, or exocytosis of lytic granules by cytotoxic T cells. Lymphotropic retroviruses, such as the human immunodeficiency virus type 1, the human T cell leukemia virus type 1, or the Herpesvirus saimiri, can subvert some of the vesicle traffic mechanisms impeding the generation and function of the immunological synapses. This review focuses on the polarization of vesicle traffic, its regulation, and its role in maintaining the structure and function of the immunological synapse. We discuss how some lymphotropic viruses target the vesicle traffic in T lymphocytes, inhibiting the formation of immunological synapses and modulating the response of infected T cells.

Kinetics of early T cell receptor signaling regulate the pathway of lytic granule delivery to the secretory domain

Cytolytic granules mediate killing of virus-infected cells by cytotoxic T lymphocytes. We show here that the granules can take long or short paths to the secretory domain. Both paths utilized the same intracellular molecular events, which have different spatial and temporal arrangements and are regulated by the kinetics of Ca(2+)-mediated signaling. Rapid signaling caused swift granule concentration near the microtubule-organizing center (MTOC) and subsequent delivery by the polarized MTOC directly to the secretory domain-the shortest path. Indolent signaling led to late recruitment of granules that moved along microtubules to the periphery of the synapse and then moved tangentially to fuse at the outer edge of the secretory domain-a longer path. The short pathway is associated with faster granule release and more efficient killing than the long pathway. Thus, the kinetics of early signaling regulates the quality of the T cell cytolytic response.

Human immunodeficiency virus type 1 envelope gp120-induced partial T-cell receptor signaling creates an F-actin-depleted zone in the virological synapse

Cell-to-cell transmission of human immunodeficiency virus type 1 (HIV-1) occurs via a virological synapse (VS), a tight cell-cell junction formed between HIV-infected cells and target cells in which the HIV-1-infected cell polarizes and releases virions toward the noninfected target cell in a gp120- and intercellular adhesion molecule 1 (ICAM-1)-dependent process. The response of the target cell has been less studied. We utilized supported planar bilayers presenting gp120 and ICAM-1 as a reductionist model for the infected-cell membrane and investigated its effect on the target CD4 T cell. This study shows that HIV-1 gp120 interaction with its receptors is initially organized into microclusters that undergo F-actin-dependent consolidation into a central supramolecular activation complex (cSMAC). Src kinases are active in both gp120 microclusters and in the VS cSMAC. The early T-cell receptor (TCR) signaling machinery is partially activated at the VS, and signaling does not propagate to trigger Ca(2+) elevation or increase CD69 expression. However, these partial TCR signals act locally to create an F-actin-depleted zone. We propose a model in which the F-actin-depleted zone formed within the target CD4 T cell enhances the reception of virions by releasing the physical barrier for HIV-1 entry and facilitating postentry events.

Mechanisms and functions for the duration of intercellular contacts made by lymphocytes

Communication across intercellular contacts is central to establishing appropriate innate and adaptive immune responses. Recent imaging of lymphocyte interactions suggests that a complex orchestration of cell-cell contact times is a key correlate to establishing appropriate immune responses. Here I review the molecular and cellular processes that influence the duration of intercellular contacts, including integrin activation and dynamic changes in membrane morphology. I discuss how these processes can be regulated, for example, by the balance of activating and inhibitory receptor signals, and how they can establish the appropriate outcome for individual cell-cell interactions.

HTLV-1-Tax and ICAM-1 act on T-cell signal pathways to polarize the microtubule-organizing center at the virological synapse

Human T-lymphotropic virus type 1 (HTLV-1) spreads directly between lymphocytes and other cells via a specialized cell-cell contact, termed the virological synapse. The formation of the virological synapse is accompanied by the orientation of the microtubule-organizing center (MTOC) in the infected T cell toward the cell contact region with the noninfected target cell. We previously demonstrated that the combination of intracellular Tax protein expression and the stimulation of the intercellular adhesion molecule-1 (ICAM-1) on the cell surface is sufficient to trigger MTOC polarization in the HTLV-1-infected T cell. However, the mechanism by which Tax and ICAM-1 cause the MTOC polarization is not fully understood. Here we show that the presence of Tax at the MTOC region and its ability to stimulate cyclic AMP-binding protein-dependent pathways are both required for MTOC polarization in the HTLV-1-infected T cell at the virological synapse. Furthermore, we show that the MTOC polarization induced by ICAM-1 engagement depends on activation of the Ras-MEK-ERK signaling pathway. Our findings indicate that efficient MTOC polarization at the virological synapse requires Tax-mediated stimulation of T-cell activation pathways in synergy with ICAM-1 cross-linking. The results also reveal differences in the signaling pathways used to trigger MTOC polarization between the immunologic synapse and the virological synapse.

Comparative analysis of cell parameter groups for breast cancer detection

We present a method for the comparative analysis of parameter groups according to their correlation to disease. The theoretical basis of the proposed method is Information Theory and Nonparametric Statistics. Normalized mutual information is used as the measure of correlation between parameters, and statistical conclusions are based on ranking. The fluorescence polarization (FP) parameter is considered as the principal diagnostic characteristic. The FP was measured in fluorescein diacetate (FDA)-stained individual peripheral blood mononuclear cells (PBMC), derived from healthy subjects and breast cancer (BC) patients, under different stimulation conditions: by tumor tissue, the mitogen phytohemagglutinin (PHA) or without the stimulants. The FP parameters were grouped according to their correlation with breast cancer. It was established that the greatest difference between cells of BC patients and healthy subjects is found in the PHA test (parameter P1).

The cellular context of T cell signaling

Classical alphabeta T cells protect the host by monitoring intracellular and extracellular proteins in a two-step process. The first step is protein degradation and combination with a major histocompatibility complex (MHC) molecule, leading to surface expression of this amalgam (antigen processing). The second step is the interaction of the T cell receptor with the MHC-peptide complex, leading to signaling in the T cells (antigen recognition). The context for this interaction is a T cell-antigen presenting cell junction, known as an immunological synapse if symmetric and stable and as a kinapse if asymmetric and mobile. The physiological recognition of a ligand takes place most efficiently in the F-actin-rich lamellipodium and is F-actin dependent in stages of formation and triggering and myosin II dependent for signal amplification. This review discusses how these concepts emerged from early studies on adhesion, signaling, and cell biology of T cells.

Intracellular versus extracellular granzyme B in immunity and disease: challenging the dogma

The cytotoxic granzyme B (GrB)/perforin pathway has been traditionally viewed as a primary mechanism that is used by cytotoxic lymphocytes to eliminate allogeneic, virally infected and/or transformed cells. Although originally proposed to have intracellular and extracellular functions, upon the discovery that perforin, in combination with GrB, could induce apoptosis, other potential functions for this protease were, for the most part, disregarded. As there are 5 granzymes in humans and 11 granzymes in mice, many studies used perforin knockout mice as an initial screen to evaluate the role of granzymes in disease. However, in recent years, emerging clinical and biochemical evidence has shown that the latter approach may have overlooked a critical perforin-independent, pathogenic role for these proteases in disease. This review focuses on GrB, the most characterized of the granzyme family, in disease. Long known to be a pro-apoptotic protease expressed by cytotoxic lymphocytes and natural killer cells, it is now accepted that GrB can be expressed in other cell types of immune and nonimmune origin. To the latter, an emerging immune-independent role for GrB has been forwarded due to recent discoveries that GrB may be expressed in nonimmune cells such as smooth muscle cells, keratinocytes, and chondrocytes in certain disease states. Given that GrB retains its activity in the blood, can cleave extracellular matrix, and its levels are often elevated in chronic inflammatory diseases, this protease may be an important contributor to certain pathologies. The implications of sustained elevations of intracellular and extracellular GrB in chronic vascular, dermatological, and neurological diseases, among others, are developing. This review examines, for the first time, the multiple roles of GrB in disease pathogenesis.

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.

Hunter to gatherer and back: immunological synapses and kinapses as variations on the theme of amoeboid locomotion

The immunological synapse was initially defined as a stable cell-cell junction composed of three concentric supramolecular activation clusters (SMACs) enriched in particular components: a central SMAC with clustered antigen receptors and kinases, a peripheral SMAC rich in beta2 integrin adhesion molecule LFA-1, and a distal SMAC marked by a critical tyrosine phosphatase. In the past year the SMACs have each been identified with functional modules of amoeboid motility, and the stability of the immunological synapse has been revealed as a reconfiguration of the motile apparatus from an asymmetric hunting mode, a kinapse, to a symmetric gathering mode, the synapse. The genetic control of this process involves actinomyosin regulators PKCtheta and WASp. Crtam is involved in postsynaptic polarity in early kinapses prior to cell division. It is unlikely that the immune system is unique in using symmetrization to stop migration without inactivating motile machinery.

An experimental and computational study of effects of microtubule stabilization on T-cell polarity

T-killer cells eliminate infected and cancerous cells with precision by positioning their centrosome near the interface (immunological synapse) with the target cell. The mechanism of centrosome positioning has remained controversial, in particular the role of microtubule dynamics in it. We re-examined the issue in the experimental model of Jurkat cells presented with a T cell receptor-binding artificial substrate, which permits controlled stimulation and reproducible measurements. Neither 1-µM taxol nor 100-nM nocodazole inhibited the centrosome positioning at the “synapse” with the biomimetic substrate. At the same time, in micromolar taxol but not in nanomolar nocodazole the centrosome adopted a distinct peripheral rather than the normally central position within the synapse. This effect was reproduced in a computational energy-minimization model that assumed no microtubule dynamics, but only a taxol-induced increase in the length of the microtubules. Together, the experimental and computational results indicate that microtubule dynamics are not essential for the centrosome positioning, but that the fit of the microtubule array in the deformed body of the conjugated T cell is a major factor. The possibility of modulating the T-cell centrosome position with well-studied drugs and of predicting their effects in silico appears attractive for designing anti-cancer and antiviral therapies.

Shouts, whispers and the kiss of death: directional secretion in T cells

T cells use secreted soluble factors for highly specific intercellular communication and targeted cell killing. This specificity is achieved first through T cell receptor-mediated recognition of complexes of peptide and major histocompatibility complex displayed by appropriate antigen-presenting cells and then by the directed secretion of cytokines and lytic factors into the immunological synapse between the T cell and antigen-presenting cell. Studies have begun to probe the molecular basis for this synaptic secretion and have also shown that T cells release chemokines and certain inflammatory factors through a multidirectional pathway directed away from the synapse. Thus, the mode of secretion seems to be tailored to the intended function of the secreted molecule.

Human T-lymphotropic virus-1 visualized at the virological synapse by electron tomography

Human T-lymphotropic virus 1 (HTLV-1) is transmitted directly between cells via an organized cell-cell contact called a virological synapse (VS). The VS has been studied by light microscopy, but the ultrastructure of the VS and the nature of the transmitted viral particle have remained unknown. Cell-free enveloped virions of HTLV-1 are undetectable in the serum of individuals infected with the human T-lymphotropic virus 1 (HTLV-1) and during in vitro culture of naturally infected lymphocytes. However, the viral envelope protein is required for infectivity of HTLV-1, suggesting that complete, enveloped HTLV-1 virions are transferred across the synapse. Here, we use electron tomography combined with immunostaining of viral protein to demonstrate the presence of enveloped HTLV-1 particles within the VS formed between naturally infected lymphocytes. We show in 3D that HTLV-1 particles can be detected in multiple synaptic clefts at different locations simultaneously within the same VS. The synaptic clefts are surrounded by the tightly apposed plasma membranes of the two cells. HTLV-1 virions can contact the recipient cell membrane before detaching from the infected cell. The results show that the HTLV-1 virological synapse that forms spontaneously between lymphocytes of HTLV-1 infected individuals allows direct cell-cell transmission of the virus by triggered, directional release of enveloped HTLV-1 particles into confined intercellular spaces.

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.

Retractile processes in T lymphocyte orientation on a stimulatory substrate: morphology and dynamics

T cells of the immune system target infected and tumor cells in crowded tissues with high precision by coming into direct contact with the intended target and orienting the intracellular Golgi apparatus and the associated organelles to the area of the cell-cell contact. The mechanism of this orientation remains largely unknown. To further elucidate it we used three-dimensional microscopy of living T cells presented with an artificial substrate mimicking the target cell surface. The data indicate that long, finger-like processes emanate from the T cell surface next to the intracellular Golgi apparatus. These processes come in contact with the substrate and retract. The retraction accompanies the reorientation of the T cell body which brings the Golgi apparatus closer to the stimulatory substrate. Numerical modeling indicates that considering the forces involved the retraction of a process attached with one end to the cell body near the Golgi apparatus and with the other end to the substrate can bring the Golgi apparatus to the substrate by moving the entire cell body. The dynamic scenarios that are predicted by the quantitative model explain features of the reorientation movements that we measured but could not explain previously. We propose that retraction of the surface processes is a force-generating mechanism contributing to the functional orientation of T lymphocytes.