The adaptive immune response is initiated by the interaction of T cell antigen receptors with major histocompatibility complex molecule-peptide complexes in the nanometer scale gap between a T cell and an antigen-presenting cell, referred to as an immunological synapse. In this review we focus on the concept of immunological synapse formation as it relates to membrane structure, T cell polarity, signaling pathways, and the antigen-presenting cell. Membrane domains provide an organizational principle for compartmentalization within the immunological synapse. T cell polarization by chemokines increases T cell sensitivity to antigen. The current model is that signaling and formation of the immunological synapse are tightly interwoven in mature T cells. We also extend this model to natural killer cell activation, where the inhibitory NK synapse provides a striking example in which inhibition of signaling leaves the synapse in its nascent, inverted state. The APC may also play an active role in immunological synapse formation, particularly for activation of naïve T cells.

Lymphocyte function-associated antigen-1 (LFA-1) interaction with intercellular adhesion molecules (ICAMs) facilitates T cell antigen receptor (TCR)-mediated killing. To dissect TCR and LFA-1 contributions, we evaluated cytolytic activity and granule release by cytotoxic T lymphocytes (CTL) as well as intracellular granule redistribution and morphology of CTL stimulated with natural TCR ligand in the presence or absence of LFA-1 engagement. Although other adhesion mechanisms, e.g., CD2-CD58 interaction, could substitute for LFA-1 to trigger CTL degranulation, productive LFA-1 ligation was indispensable for effective target cell lysis by the released granules. LFA-1-mediated adhesion to glass-supported bilayers containing intercellular adhesion molecule-1 was characterized by a much larger junction area, marked by LFA-1 segregation, and a more compact cell shape compared with those observed for CD2-mediated adhesion to bilayers containing CD58. A larger contact induced by intercellular adhesion molecule 1 determined a unique positioning of granules near the interface. These data provide evidence that LFA-1 delivers a distinct signal essential for directing released cytolytic granules to the surface of antigenbearing target cells to mediate the effective destruction of these cells by CTL.cytolytic granules ͉ immunological synapse ͉ T cell receptor K illing of virus-infected cells by cytotoxic T lymphocytes (CTL) is triggered by interactions of T cell antigen receptor (TCR) with viral peptides presented by MHC class I proteins on the surface of infected cells and can be mediated by cytotoxic granule exocytosis or FasL-Fas interaction (1, 2). This is a sensitive response often requiring less than a dozen cognate peptide-MHC [complex of antigenic peptide with MHC protein (pMHC)] complexes on the target cell (3, 4). Although productive TCR engagement is necessary and essential to induce CTL cytolytic activity, other accessory and costimulatory molecules are thought to play a role in mediating CTL degranulation and effective cytolytic activity of released granules. For example, both lymphocyte function-associated antigen-1 (LFA-1) and CD2 contribute to CTL adhesion and killing of target cells (5). Ligation of LFA-1 on CTL by high densities of intercellular adhesion molecule-1 (ICAM-1) is sufficient to initiate large-scale molecular segregation and formation of peripheral supramolecular-activating cluster (6), which typically requires antigen for helper T cells (7,8) and CTL precursors (9). In contrast, ligation of CD2 with its natural ligand CD58 (5) mediates formation of a very small adhesion area by CTL (10). However, how specifically LFA-1, CD2, and other adhesion molecules mediate granuleinduced cytotoxicity has not been defined.Here we have investigated the role of productive LFA-1 engagement for antigen-induced granule release and target cell lysis in vitro as well as for granule polarization in CTL exposed to the glasssupported bilayer. We have found that, although blocking of LFA-1-ICAM-1 interaction abrogates ...

Conflict of interest:The authors have declared that no conflict of interest exists. Nonstandard abbreviations used: MHC-peptide (MHCp); immunological synapse (IS); lymphocyte function associated antigen-1 (LFA-1); peripheral supramolecular activation cluster (pSMAC); T cell receptor (TCR); central supramolecular activation cluster (cSMAC); glycophosphatidylinositol (GPI); 6-histidine-tagged (His6).