Comparison of PSGL-1 microbead and neutrophil rolling: microvillus elongation stabilizes P-selectin bond clusters.

A cell-scaled microbead system was used to analyze the force-dependent kinetics of P-selectin adhesive bonds independent of micromechanical properties of the neutrophil's surface microvilli, an elastic structure on which P-selectin ligand glycoprotein-1 (PSGL-1) is localized. Microvillus extension has been hypothesized in contributing to the dynamic range of leukocyte rolling observed in vivo during inflammatory processes. To evaluate PSGL-1/P-selectin bond kinetics of microbeads and neutrophils, rolling and tethering on P-selectin-coated substrates were compared in a parallel-plate flow chamber. The dissociation rates for PSGL-1 microbeads on P-selectin were briefer than those of neutrophils for any wall shear stress, and increased more rapidly with increasing flow. The microvillus length necessary to reconcile dissociation constants of PSGL-1 microbeads and neutrophils on P-selectin was 0.21 microm at 0.4 dyn/cm2, and increased to 1.58 microm at 2 dyn/cm2. The apparent elastic spring constant of the microvillus ranged from 1340 to 152 pN/microm at 0.4 and 2.0 dyn/cm2 wall shear stress. Scanning electron micrographs of neutrophils rolling on P-selectin confirmed the existence of micrometer-scaled tethers. Fixation of neutrophils to abrogate microvillus elasticity resulted in rolling behavior similar to PSGL-1 microbeads. Our results suggest that microvillus extension during transient PSGL-1/P-selectin bonding may enhance the robustness of neutrophil rolling interactions.