Linking single integrin–ligand bond properties to cell adhesiveness under external forces exemplified by the VLA-4–VCAM-1 bond

Abstract

Cell adhesion is a complex event dictated by the properties of individual adhesion molecules. It is desirable to link their individual properties to the adhesive behavior of a whole cell. Here, we examine with atomic force microscopic (AFM) and flow chamber experiments how the exogenous activation of the major T cell integrin VLA-4 affects the adhesiveness of T-lymphocytes to the endothelial VLA-4 ligand, VCAM-1. The atomic force measurements are performed on substrates with different VCAM-1 densities to compare the properties of single adhesive bonds on low-density ligands with the effect of multiple bonds on higher ligand densities. We determine the basal off-rate and the potential width of the single adhesive VLA-4–VCAM-1 bond by Monte-Carlo simulations. We show that activating VLA-4 with magnesium increases the potential width of the bond from 0.18 nm to 0.44 nm, but hardly affects the basal off-rate of single VLA-4–VCAM-1 bonds measured at low VCAM-1 density, lowering it only from 1.2 s⁻¹ to 0.9 s⁻¹. At higher VCAM-1 densities, the apparent off-rate of high avidity VLA-4–VCAM-1 interactions is significantly reduced by magnesium to <0.1 s⁻¹. In the flow chamber, both the frequency of T cell attachments to VCAM-1 and their lifetime rise after VLA-4 activation by magnesium. By extrapolating the single-molecule properties to whole cells, we suggest that exogenous stimulation with magnesium strongly increases the rate of VLA-4 rebinding to VCAM-1 and only slightly decreases the off-rate of single VLA-4-VCAM-1 bonds.