 enriched in CD45, although this will be discussed further below. The distal pole complex was described by Burkhardt and may be important for sequestering specific negative regulators like SHP-1 (Cullinan et al., 2002). TCR microclusters will be discussed at length. Table I outlines the compartments associated with stable, antigen specific T cell-B cell conjugates.
Parallel studies with supported planar bilayers presenting adhesion molecules revealed similar phenomena in which a single adhesion system would establish a cooperative supamolecular cluster and mixed systems would segregate into spatially discrete SMACs. We examined the organization of the CD2-CD58 and LFA-1-ICAM-1 interactions in contacts in Jurkat T cells on planar bilayers containing ICAM-1 and CD58 (Dustin et al., 1998). We proposed that the “specialized junction, cell polarization, and positional stability” of the symmetric contacts were similar to a neural synapse and proposed that stable T-APC interfaces should be defined as “immunological synapses”. A planar bilayer based system using MHC-peptide complexes and LFA-1 fully recapitulated Kupfer’s findings and provided insight into the dynamics of these structures, which were not formed en bloc, but evolved from very distinct early intermediates by an F-actin dependent transport process (Grakoui et al., 1999)(Fig 1). The immunological synapse has an axis of radial symmetry with its center at the cSMAC (Fig 1). At the same time, we can view the synapse as having a direction of polarity along the length of this axis with a proximal and distal pole relative to the APC containing distinct molecules (Fig 1). The function of the immunological synapse pattern is most transparent for cytotoxic T cells where the F-actin free cSMAC serves as a secretory domain and the pSMAC serves as a retaining wall (Beal et al., 2008; Stinchcombe et al., 2006). The importance of directed secretion is supported by studies with human patients with deficiencies in granule transport to the synapse that lead to defects in killing (Baetz et al., 1995). Stabilization of the pSMAC of CD4+ cytotoxic T lymphocyte (CTL) with a PKC-θ inhibitor quantified increased killing, the first evidence of for an advantage of an intact pSMAC (Beal et al., 2008). Structures similar to pSMACs also form between T cells in homotypic aggregates and are important for intensification of cytokine signals during T cell expansion (Sabatos et al., 2008). The symmetric immunological synapse may serve equally important functions in tolerance induction, priming and differentiation as discussed below.
Between chemokine dependent extravasation and MHC-peptide dependent immunological synapse formation T cells rapidly migrate in DC networks of T cell zones (Bajenoff et al., 2006). Rapid migration by T cells can be fully reconstituted with solid phase CCL21, a CCR7 ligand, in vitro (Woolf et al., 2007). This involved polarization and formation of a leading edge that takes the form of a flattened lamellipodium on CCL21 and ICAM-1 coated surfaces (Huang and Dustin, unpublished data). LFA-1 is not required for mobility in T cell zones (Woolf et al., 2007), but LFA-1 increases sensitivity to MHC-peptide complexes up to 100-fold (Bachmann et al., 1997) and ICAM-1 is required for antigen specific arrest in vivo (Scholer et al., 2008). The lamellipodium is a sensory structure and has been shown to be the most sensitive part of a T cell by ~40-fold in elegant studies with laser traps and anti-CD3 coated beads (Negulescu et al., 1996; Wei et al., 1999). Valitutti has also described the probing behavior of lymphocytes undergoing sustained signaling on APCs (Valitutti et al., 1995a). Another important experiment is the demonstration that TCR signaling is continually being renewed by new TCR-MHC-peptide interactions such that blocking the access to new MHC-peptide ligands with MHC antibodies immediately block signaling (Valitutti et al., 1995a).
Lamellipodia are coordinated by small G-proteins including Rac-1, which activates the WAVE2 complexes leading to dynamic actin in the perip