Testing the organization of the immunological synapse

Abstract

Upon encounter with antigen-bearing presenting cells, T cells initiate the formation of a unique junction termed the immunological synapse (IS). The morphology of this junction, characterized in EM as a series of discrete contacts into which are interspersed synaptic spaces [1], has multiple similarities to neuronal synapses. The closely apposed membranes permit engagement of T cell receptors (TCR) on T cells with peptide-major histocompatibility complexes (pMHC) as well as distinct contacts for other receptor-ligand pairs such as integrins. The form of this synapse has also been extensively characterized at the level of molecular organization. When TCRs or pMHC complexes are attached to fluorophores, it is found that they first form dynamic and relatively small structures (clusters or 'microclusters') all over the contact area and perhaps predominantly at the outskirts of the synapse [2][3][4]. These ultimately coalesce [3,4] to a central spot and this spot corresponds to a structure first characterized by Kupfer and co-workers in fixed couples and termed the central 'supramolecular activating cluster' or cSMAC [5].The name cSMAC is perhaps misleading since early calcium imaging together with TCR visualization as well as antibody-staining for phosphotyrosine has since shown that activation of signaling actually coincides with the appearance of the smaller clusters, prior to their centralization [3,6]. As the cSMAC is the last spot from which TCRs are likely internalized, it is also the point at which TCR signaling likely ceases [7]. The cSMAC is contrasted by a slightly externalized distribution of CD4 [3] and a larger annulus of integrins such as LFA-1 in the peripheral region (the p-SMAC). The multiple zones of proteins in the IS define distinct membrane domains and the nature of the domains and their function is the overall theme of this issue.The IS involves events that are at once dynamic (involving the cessation of motility, repolarization and coalescence of receptor clusters) as well as having relatively stable aspects. In the last year, Vale and co-workers used a model system in which T cells were triggered by immobilized antibodies to demonstrate that TCRs are confined to relatively stable zones that behave as if they are bounded by a diffusion barrier [8]. This suggests that, while the pathway from an unstimulated cell surface to a highly ordered cSMAC/pSMAC array is highly dynamic, it is also highly aided by stabilizing forces in the membrane or in the cytoskeleton. As it is tested more, it becomes clear that the dynamic assembly of the IS relies upon a confluence of biophysical processes within the membrane, and cell biological processes deriving from other proteins in the membrane or within the cytosol and recent efforts have focused upon all of these. The foremost structure being studied is the lipid bilayer and the proteins arrays that assemble within this unique environment.

New Insights into the Early Molecular Events Underlying B Cell Activation

“…However, although these studies can provide a useful foundation for investigation of the mechanisms of B cell activation, a molecular dissection necessitates the characterization of pathways and interactions within their correct cellular context (Krummel, 2007;Treanor and Batista, 2007). Such dissections have been made feasible through recent advances that allow for the spatiotemporal dynamics of particular molecules to be monitored within cells in real time.…”

A method for prolonged imaging of motile lymphocytes

“…T cells are activated by the presentation of antigen by professional antigen‐presenting cells. This process involves morphological rearrangements including the formation of a structure at the interface between the two cells referred to as the immunological synapse 5 . In vivo imaging of T‐cell activation has revealed that T cells can remain in contact with antigen‐presenting cells for hours, 6 and can still interact at the point of cell division more than 20 h after activation 7 , 8 .…”

“…This process involves morphological rearrangements including the formation of a structure at the interface between the two cells referred to as the immunological synapse. 5 In vivo imaging of T-cell activation has revealed that T cells can remain in contact with antigen-presenting cells for hours, 6 and can still interact at the point of cell division more than 20 h after activation. 7,8 With recent observations that T cells can undergo asymmetric cell division to dictate cell fate, 9 there is strong incentive to develop new approaches to in vitro imaging of T-cell interactions with dendritic cells over hours and even days.…”