Micromechanical tests of adhesion dynamics between neutrophils and immobilized ICAM-1

Strong, integrin-mediated adhesion of neutrophils to endothelium during inflammation is a dynamic process, requiring a conformational change in the integrin molecule to increase its affinity for its endothelial counterreceptors. To avoid general activation of the cell, Mg(2+) was used to induce the high-affinity integrin conformation, and micromechanical methods were used to determine adhesion probability to beads coated with the endothelial ligand ICAM-1. Neutrophils in Mg(2+) bind to the beads with much greater frequency and strength than in the presence of Ca(2+). An increase in adhesion strength and frequency was observed with both increasing temperature and contact duration (from 2 s to 1 min, 21 or 37 degrees C). The dependence of adhesion probability on contact time or receptor density yielded estimates of the effective reverse rate constant, k(r), and the equilibrium association constant, K(a), for binding of neutrophils to ICAM-1 coated surfaces in Mg(2+): k(r) approximately 0.7 s(-1) and the product K(a)rho(c) approximately 2.4 x 10(-4), where rho(c) is the density of integrin on the cell surface.