Chemokines and the immunological synapse.

Abstract

The critical event which initiates an adaptive immune response is the interaction of T cells through the T-cell antigen receptor (TCR) with major histocompatibility complex (MHC)–peptide (MHCp) complexes on the surface of antigen-presenting cells (APCs). This interaction requires close cell-to-cell contact for a prolonged period of time to permit full activation of the T cell. The interface between the two cells is referred to as an immunological synapse (IS) and has become an intense focus of recent research. The IS is characterized by the ordered organization of proteins at the interface. The TCR is clustered in the centre, along with the costimulatory receptor, CD28, in a structure known as the central supramolecular activation cluster (cSMAC).1–3 This, in turn, is surrounded by a ring of the β2 integrin lymphocyte function-associated antigen-1 (LFA-1), in an area known as the peripheral supramolecular activation cluster (pSMAC). This arrangement of proteins is in turn mirrored on the APC, with a central cluster of MHCp and CD80 (the ligand for CD28) surrounded by a ring of intracellular adhesion molecule-1 (ICAM-1) (the ligand for LFA-1). The distribution of a number of intracellular molecules has also been shown to segregate. Whereas the Lck and PKCθ kinases are located in the cSMAC, talin is found in the pSMAC.2 The function of the IS is currently unclear and controversial.4,5 Original suggestions that its purpose was to enhance or sustain TCR triggering seem unlikely as TCR triggering and activation of proximal tyrosine kinases such as Lck and Zap-70 occurs within seconds, whereas a mature IS takes minutes to assemble.6 Instead, the function of the IS may be to concentrate secondary signaling molecules, such as CD28, into the interface, which in turn would amplify TCR signalling pathways,7 or to polarize signaling molecules (such as kinases) away from inhibitors of TCR signaling (such as phosphatases). Alternatively, it has been proposed that the IS may serve to direct secretion from the T cell to the APC, thereby avoiding bystander activation.5,8,9 Irrespective of its precise function, much interest has focused on the mechanism by which an IS is formed. In order to initiate formation of an IS, naïve T cells must survey large numbers of APCs for the presence of cognate antigen. The most relevant APC for the stimulation of naïve T cells is the dendritic cell (DC), which can form at least short-lived interactions with T cells in the absence of antigen. These interactions result in some of the events characteristic of an IS, even in the absence of antigen.10,11 For example, CD3 and the CD4 and CD8 co-receptors were found to polarize towards the DC, and the T cells show a rise in intracellular calcium. How T cells can form such conjugates with DCs is not known but may involve interactions between LFA-1 and ICAM-1, CD2 and CD48, and DC-SIGN and ICAM-3. An interesting possibility is that this interaction is in part enhanced by chemokines, several of which are produced by DCs. Chemokines have been shown to induce a rapid increase in LFA-1 affinity and mobility and may thus enhance the LFA-1/ICAM-1 binding, leading to stabilization of T-cell : DC conjugates.12 In addition, chemokines have other effects on T cells. They induce chemotaxis of T lymphocytes and at the same time cause a polarization of the cell, with a leading edge and a trailing uropod as well as polarization of various cell-surface molecules. Most interestingly, this results in an increased sensitivity to TCR stimulation at the leading edge.13,14 Thus it seems probable that chemokines might play an important role in the formation of the IS between naïve T cells and DCs. The study of Bromley & Dustin, published in the current issue of Immunology, addresses the effect of chemokines on adhesion and formation of the IS between naïve T cells and DCs.15 The authors first used a simplified system in which naïve CD4+ T cells bearing a monoclonal TCR were allowed to interact with planar lipid bilayers containing fluorescently labelled MHC, ICAM-1 and CD80 molecules. If the MHC molecules are loaded with agonist peptide, then an IS forms with a central cluster of MHC and CD80 surrounded by a ring of ICAM-1. Increasing the concentration of MHC molecules bearing agonist peptide resulted in an increase in the percentage of cells adhering to the bilayer and in an increase in the density of ICAM-1 and CD80. In contrast, addition of either CXCL12 [stromal-cell-derived factor 1α (SDF-1α)] or CCL21 [secondary lymphoid tissue chemokine (SLC)] chemokines to this system resulted in an increase in the percentage of adherent T cells. However, the formation of the IS was unaffected, both in terms of the fraction of cells showing an IS and in the density of the MHC, ICAM-1 and CD80 molecules. Therefore, the effects of signalling through the TCR and through chemokine receptors are distinct. Both receptors can induce increased adhesion to ICAM-1-containing bilayers, presumably by activating LFA-1, but only TCR signals can drive the clustering of ICAM-1, MHC and CD80 at the conjugate interface. Bromley & Dustin then extended their work to study the effect of chemokines on the interaction of naïve T cells with DCs. In the absence of antigen they observed a basal level of conjugate formation, as previously described, which is further enhanced by CXCL12. In contrast, when the DCs were loaded with antigen, the basal level of conjugate formation was increased and this could no longer be augmented by chemokines. Both the basal and the antigen-induced conjugate formation was insensitive to pertussis toxin, an inhibitor of some G protein-coupled receptors such as the chemokine receptors. Thus, while chemokines can enhance the binding of T cells to DCs, this study shows that there must also exist chemokine-independent mechanisms which initiate this adhesion event. Possible candidates that may mediate such an interaction are DC-SIGN/ICAM-3 and CD2/CD48. Further work is needed to elucidate the molecular details of the antigen-independent interaction of T cells and DCs, a key event at the initiation of the immune response.