Distinct kinetic and mechanical properties govern selectin-leukocyte interactions

Leukocytes are recruited from the bloodstream to sites of inflammation by the selectin family of adhesion receptors. In vivo and in vitro studies reveal distinctive rolling velocities of polymorphonuclear leukocytes over E-, P- and L-selectin substrates. The kinetic and mechanical properties of the selectin-ligand bonds responsible for these differences at the single-molecule level are not well understood. Using single-molecule force spectroscopy, we probe in situ the rupture force, unstressed off-rate and reactive compliance of single selectin receptors to single ligands on whole human polymorphonuclear leukocytes (PMNs) under conditions that preserve the proper orientation and post-translational modifications of the selectin ligands. Single L-selectin bonds to PMNs were more labile than either E- or P-selectin in the presence of an applied force. This outcome, along with a higher unstressed off-rate and a higher reactive compliance, explain the faster L-selectin-mediated rolling. By quantifying binding frequency in the presence of a specific blocking monoclonal antibody or following enzyme treatment, we determined that P-selectin glycoprotein ligand-1 is a high-affinity ligand for E-selectin on PMNs under force. The rupture force spectra and corresponding unstressed off-rate and reactive compliance of selectin-ligand bonds provide mechanistic insights that might help to explain the variable rolling of leukocytes over different selectin substrates.