Insights into function of the immunological synapse from studies with supported planar bilayers

Abstract

Innate and adaptive immunity is dependent upon reliable cell-cell communication mediated by direct interactions of cell surface receptors with ligands integrated into the surface of apposing cells or bound directly to the surface as in complement deposition or antibody mediated recognition through Fc receptors. Supported lipid bilayers formed on glass surfaces offer a useful model system in which to explore some basic features of molecular interactions in immunological relevant contacts, which include signal integration and effector functions through immunological synapses and kinapses. We have exploited that lateral mobility of molecules in the supported planar bilayers and fluorescence microscopy to develop a system for measurement of two-dimensional affinities and kinetic rates in the contact area, which is of immunological interest. Affinity measurements are based on a modified Scatchard analysis. Measurements of kinetic rates are based on fluorescence photo bleaching after recovery at the level of the entire contact area. This has been coupled to a reaction-diffusion equation that allows calculation of on- and off-rates. We have found that mixtures of ligands in supported planar bilayers can effectively activate T lymphocytes and simultaneously allow monitoring of the immunological synapse. Recent studies in planar bilayers have provided additional insights into organization principles of cell-cell interfaces. Perennial problems in understanding cell-cell communication are yielding quantitative measurements based on planar bilayers in areas of ligand-driven receptor clustering and the role of the actin cytoskeleton in immune cell activation. A major goal for the field is determining quantitative rules involved in signaling complex formation by innate and adaptive receptor systems.