Seminars in Immunology 17 (2005) 400–410 Review A dynamic view of the immunological synapse Michael L. Dustin ∗ Program in Molecular Pathogenesis, Skirball Institute of Biomolecular Medicine, and Department of Pathology, New York University School of Medicine, 540 1st Ave, New York, NY 10016, USA Abstract 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. © 2005 Published by Elsevier Ltd. Keywords: Intravital microscopy; Immunological synapse; Total internal reflection fluorescence microscopy; T cell receptor; Signalling 1. Introduction in vitro and in vivo systems have emphasized the ability of T cells to integrate signals without the formation of a long-lived The partnership between dendritic cells (DC) and T lym- IS. Therefore, a challenge for in vitro molecular imaging is to phocytes (T cells) defends the body against microbes, parasites, provide insight into how T cells integrate signals from IS and abnormal cells and environmental toxins that breach the barrier more dynamic interactions. function of skin and epithelial surfaces [1]. Diverse tools includ- One of the basic biological questions in immunology is what ing those of biochemistry, cell biology, genetics and imaging distinguishes T cell responses to DC that lead to tolerance or have been employed to understand the mechanistic basis of this priming. One concept is that the outcome of antigen presentation partnership. In recent years imaging approaches have become depends upon the activation status of the DC [4]. Immature DCs increasingly useful as molecular technologies for labeling cells patrol the tissue spaces and boundaries of the body and gather and proteins and imaging hardware and software have improved. antigenic structures, both self and foreign. Induced or sponta- In vitro imaging led to the initial definition of the immunological neous maturation of DC triggers their migration to the lymph synapse (IS) based on cell polarity and segregation of adhesion node and concurrent processing of antigens to generate pep- molecules and T cell antigen receptors (TCR) in the interface tides that bind to major histocompatibility complex molecules between T cells and antigen presenting cells [2]. In vivo imaging (MHCp) that are then presented at the cell surface. DC migrate has led to a basic understanding of the dynamics of T cell-APC to the lymph node via the lymphatics and then migrate in the interactions in the lymph node and the effect of antigen, which parenchyma and join DC networks in the T cell zones where they causes formation of long lived T cell-APC contacts that may be encounter many T cells. The level of costimulatory molecules the in vivo counterparts of immunological synapses [3]. Both expressed by the DC is determined by the level of cytokines like TNF produced in response to various endogenous or exogenous activators of innate immunity [5]. This level of innate stimu- Abbreviations: APC, antigen presenting cell; CFP, cyan fluorescent protein; lation appears to control whether the antigen dependent T-DC DC, dendritic cell; eGFP, enhanced green fluorescent protein; HS, hemisynapse; interactions lead to tolerance or priming of an immune response ICAM, intercellular adhesion molecule; IS, immunological synapse; LFA, lym- over a period of 5–7 days [6]. phocyte function associated; MHCp, major histocompatibility complex-peptide Once T cells are primed they can either exit the lymph node complex; SMAC, supra molecular activation cluster; TCR, T cell receptor; to migrate to sites of inflammation or remain within the lymph TIRFM, total internal reflection fluorescence microscopy; YFP, yellow fluo- rescent protein node to help B cells. While there has been no work to date on ∗ Tel.: +1 212 263 3207; fax: +1 212 263 5711. antigen specific effector T cells in peripheral effector sites, we E-mail address: [email protected]. will discuss a recent paper on initial stages of T cell help. There 1044-5323/$ – see front matter © 2005 Published by Elsevier Ltd. doi:10.1016/j.smim.2005.09.002 M.L. Dustin / Seminars in Immunology 17 (2005) 400–410 401 has also been little work on memory T cell interactions with DC ment [16]. It was later found that the primary role of perforin during secondary stimulation, however, the availability of mice was to induce the target cell to take a “poison pill” by intro- with stably integrated fluorescent proteins that do not dilute out ducing granzyme A or B into the cytoplasm, which initiates a during cell division should enable such studies. pro-apoptotic caspase cascade [17]. Evidence that cell T cell Peripheral tissue scanning by DC is only one mode of innate polarity was related to directed secretion was provided by sem- immune surveillance of tissues. Two striking examples are the inal studies of Geiger and Kupfer showing that the microtubule surveillance of the brain by the dynamic processes of microglial organizing center and Golgi apparatus reorients toward the target cells [7] and the active patrolling of liver sinusoids by natural cell for killing [18,19]. Kupfer continued with a series of studies killer T cells, an innate like T cell [8]. on molecular makeup of the T cell-B cell interface with the first In this review we will summarize a new view of sustained description of CD4, LFA-1, IL-4, talin and protein kinase C-␪ T cells activation through the IS and the implications of how polarization to the interface between T cells and B cells [20–24]. migrating cells integrate signals through a “hemi-synapse” (HS). All of these studies were performed with fixed cells so temporal Then how the IS and HS work together in T cell tolerance and information was deduced from populations of images for cells immune surveillance will be discussed. fixed at different times. In 1998 Kupfer published a paper on the organization of LFA- 2. New model for sustained signaling through the IS 1, talin, TCR and protein kinase C-␪ in the interface between antigen specific T cells and antigen presenting B cells [25]. Studies on the IS bring together three parallel lines of experi- LFA-1 and talin were shown to form a ring in the interface and mentation in immunology through high-resolution fluorescence TCR and protein kinase C-␪ were shown to cluster in the mid- microscopy: TCR signal transduction, T cell adhesion and dle. These structures were defined as supramolecular activation directed secretion mechanisms involved in T cell effector func- clusters (SMAC). The TCR cluster marked the cSMAC, while tion. TCR signaling is based on a tyrosine kinase cascades the LFA-1 ring marked the pSMAC. It was implied that TCR that leads to rapid activation of phospholipase C ␥ [9]. The signaling was initiated and sustained by the cSMAC. My col- key tyrosine kinases are Lck, which initiates phosphorylation laborators and I published a paper in parallel in which live T of immunotyrosine activation motifs (ITAMs) in the cytoplas- cells interacting with supported planar bilayers were imaged in mic domain of the TCR, ZAP-70, which is recruited to phos- real time to visualize segregated adhesive domains composed phorylated ITAMs and phosphorylates LAT and ITK, which of LFA-1-ICAM-1 and CD2–CD58 interactions [26]. It was phosphorylates phospholipase C ␥ that is recruited to phospho- posited that the segregation of the adhesion molecules was driven rylated LAT. Phospholipase C ␥ activation leads to generation by the different topology of the LFA-1-ICAM-1 (40 nm domain) of inositol-1,4,5-triphosphate leading to Ca2+ mobilization and and CD2–CD58 (15 nm domain) interactions [27]. The antigen diacylglycerol leading to activation of protein kinase C and Ras dependent organization of these domains into a bull’s eye pat- exchange factors [10]. The triggering of the cascade is based on tern, similar to that reported in several international meetings recruitment of Lck associated co-receptors to the TCR and on by Kupfer, was an active process. We proposed the definition of TCR oligomer formation. “immunological synapse” for the bull’s eye pattern described by Members of the integrin and immunoglobulin families medi- Kupfer and colleagues and our studies with adhesion molecules, ate T cell adhesion to APCs. These interactions greatly extend linking a specific molecular pattern to the widely discussed con- the sensitivity of TCR to small numbers of MHCp bearing cept [28,29]. Taking these two studies together the IS was defined agonist peptides [11]. Costimulatory molecules also are con- as a specialized cell–cell junction composed of a cSMAC and a figured as adhesion molecules, but have regulatory features that pSMAC [26]. IS has subsequently been applied to a more diverse make them less effective as adhesion molecules [12]. By def- array of structures, but here we will adhere to this original defi- inition, adhesion enhances the physical interaction of T cells nition. with APC and the interaction of TCR and MHCp, while cos- The formation of the cSMAC was first evaluated in live T cell- timulation enhances TCR signaling or produces independent supported planar bilayer models [30]. It was shown that TCR are signals that integrate with the TCR signal to influence T cell engaged first in the periphery within 30 s and then these TCR activation. However, the major T cell adhesion molecules have clusters translocate to the center of the IS to form the cSMAC some co-stimulatory activity.