18: Supported Lipid Bilayers and Evanescent Wave Fields
Fluorescence emitted at water-glass interface provides the opportunity to study molecules localized at this interface by a cell-surface contact. This is possible when the interface is the source of an evanescent optical wave field. (267, 304). A particularly clear example of the evanescence wave technique was provided by the triggering of rat basophil leukemia cells by mobile lipid haptens in a supported bilayer lipid membrane. Here the lipid hapten was a monovalent nitro phenyl lipid. The other components of the system were fluorescent labeled specific IgE antibodies, and the leukemia cells. What was very clear in the optical microscope is that the cells became attached to the surface and the antibodies quickly became clustered, into large clusters of many molecules, patches at the cell surface interface, in a region a few hundreds of Angstroms thick (267). The physical chemistry in this experiment was well defined and was related to many years of work by Henry Metzger and collaborators at the NIH. Thus the experiment can serve as a model system, though this was not fully appreciated at the time. Many subsequent experiments with entirely different chemical and biological components have shown similar results (267). This technique of course takes advantage of supported bilayer membranes (304).
An interesting related experiment was carried out with specific T-helper cells, a labeled specific peptide antigen, and specific MHC incorporated in a bilayer membrane. In the wave field excitation region, there was strong emission from the cell-surface interface. In the absence of the helper cells there was not. One interpretation is that T-cell receptors and peptide-MHC complexes migrated to the cell-surface interface. (Motivation for the experiment might have been that ternary complexes between MHC, peptide and T-cell receptor could be especially strong. This reflects the extent of our knowledge at the time (312)). For a study of this “immunological synapse” using supported membranes, see Dustin and Depoil.
Many students over the years contributed to the development of the optical techniques used in these experiments, including Herman Gaub, Wally Parce, Barton Smith, Bob Weiss (312). There were also innumerable experiments supporting the validation of the use of bilayers as supports for immune response studies.