Membrane mobility of beta2 integrins and rolling associated adhesion molecules in resting neutrophils

The mobilities of transmembrane adhesion proteins are key underlying physical factors that contribute to neutrophil adhesion and arrest during inflammation. Here we present a novel (to our knowledge) fluorescence recovery after photobleaching system and a complementary analytical model to measure the mobility of the four key receptors involved in the adhesion cascade: L-selectin, PSGL-1, Mac-1, and LFA-1 for resting, spherical, and human neutrophils. In general, we find that β₂ integrins (Mac-1, LFA-1) have mobilities 3-7 times faster than rolling associated molecules (L-selectin; PSGL-1), but that the mobilities within each of these groups are indistinguishable. Increasing temperature (room temperature versus 37°C) results in increased mobility, in all cases, and the use of a bivalent antibody label (mAb versus Fab) decreases mobility, except in the case of rolling associated molecules at room temperature. Disrupting the actin cytoskeleton increased mobility except that the highest mobilities measured for integrins (D = 1.2 × 10⁻⁹ cm²/s; 37°C, Fab) are not affected by actin poisons and approach the expected value for free diffusion. Although evidence of cytoskeletal hindrance of integrin mobility has been found in other systems, our data suggest such hindrance does not limit bulk integrin diffusion in resting neutrophils over distances and times important for adhesive plaque formation.