The immunological synapse: the more you look the less you know

Before T cells of the immune system can recognize pathogens, antigen presenting cells (APCs) must process pathogen-derived peptides and present them together with major histocompatibility complex molecules (MHC) to T lymphocytes. T lymphocytes then scan the surface of APCs and antigen-specific activation of the T cell will happen after interaction of T cell antigen receptor (TCR) with MHC-peptide complexes expressed at the membrane of APCs. This interaction takes place in a nanometer scale gap between the two cells, referred to as an immunological synapse. Recent three-dimensional fluorescence analysis of this synapse revealed a dynamic spatial organization of membrane receptors, cytoskeleton and intracellular signaling complexes on the T cell side showing specific patterns, which depend on the nature of the T cell:APC pair. Although it is obvious that establishment of an intimate contact between T cells and APCs will facilitate cell:cell communication it is not clear what is the role, if any, of this receptors patterning. This molecular reorganization has long been thought to enhance and/or sustain TCR signaling and thus T cell activation, but this is now a matter of controversy. Moreover, mechanisms controlling immunological synapse formation are still unraveled. Segregation of proteins may occur spontaneously as proposed by mathematical modeling taking into account membrane fluidity, protein size and receptor/ligand affinity. Alternatively patterning of the molecules at the cell:cell interface could be driven by active processes involving T cell signaling and/or specific features of the APC. These different questions will be discussed herein.