VISUALIZATION OF CD2 INTERACTION WITH LFA-3 AND DETERMINATION OF THE 2-DIMENSIONAL DISSOCIATION-CONSTANT FOR ADHESION RECEPTORS IN A CONTACT AREA

Many adhesion receptors have high three-dimensional dissociation const ants (K-d) for counterreceptors compared to the K(d)s of receptors for soluble extracellular ligands such as cytokines and hormones. Interac tion of the T lymphocyte adhesion receptor CD2 with its counter-recept or, LFA-3, has a high solution-phase K-d (16 mu M at 37 degrees C), ye t the CD2/LFA-3 interaction serves as an effective adhesion mechanism. We have studied the interaction of CD2 with LFA-3 in the contact area between Jurkat T lymphoblasts and planar phospholipid bilayers contai ning purified, fluorescently labeled LFA-3. Redistribution and lateral mobility of LFA-3 were measured in contact areas as functions of the initial LFA-3 surface density and of time after contact of the cells w ith the bilayers. LFA-3 accumulated at sites of contact with a half-ti me of similar to 15 min, consistent with the previously determined kin etics of adhesion strengthening. The two-dimensional K-d for the CD2/L FA-3 interaction was 21 molecules/mu m(2), which is lower than the sur face densities of CD2 on T cells and LFA-3 on most target or stimulato r cells. Thus, formation of CD2/LFA-3 complexes should be highly favor ed in physiological interactions. Comparison of the two-dimensional (m embrane-bound) and three-dimensional (solution-phase) K(d)s suggest th at cell-cell contact favors CD2/LFA-3 interaction to a greater extent than that predicted by the three-dimensional K-d and the intermembrane distance at the site of contact. LFA-3 molecules in the contact site were capable of lateral diffusion in the plane of the phospholipid bil ayer and did not appear to be irreversibly trapped in the contact area , consistent with a rapid off-rate. These data provide insights into t he function of low affinity interactions in adhesion.