Molecular dynamics of the transition from L-selectin- to β2-integrin- dependent neutrophil adhesion under defined hydrodynamic shear

Homotypic adhesion of neutrophils stimulated with chemoattractant is analogous to capture on vascular endothelium in that both processes depend on L-selectin and β₂-integrin adhesion receptors. Under hydrodynamic shear, cell adhesion requires that receptors bind sufficient ligand over the duration of intercellular contact to withstand hydrodynamic stresses. Using cone-plate viscometry to apply a uniform linear shear field to suspensions of neutrophils, we conducted a detailed examination of the effect of shear rate and shear stress on the kinetics of cell aggregation. A collisional analysis based on Smoluchowski's flocculation theory was employed to fit the kinetics of aggregation with an adhesion efficiency. Adhesion efficiency increased with shear rate from ~20% at 100 s^-1 to ~80% at 400 s^-1. The increase in adhesion efficiency with shear was dependent on L-selectin, and peak efficiency was maintained over a relatively narrow range of shear rates (400- 800 s^-1) and shear stresses (4-7 dyn/cm²). When L-selectin was blocked with antibody, β₂-integrin (CD11a, b) supported adhesion at low shear rates (< 400 s^-1). The binding kinetics of selectin and integrin appear to be optimized to function within discrete ranges of shear rate and stress, providing an intrinsic mechanism for the transition from neutrophil tethering to stable adhesion.