ACTIVATION OF POLYMORPHONUCLEAR LEUKOCYTES REDUCES THEIR ADHESION TO P-SELECTIN AND CAUSES REDISTRIBUTION OF LIGANDS FOR P-SELECTIN ON THEIR SURFACES

In acute inflammatory responses, selectins mediate initial rolling of neutrophils (PMNs) along the endothelial surface. This is followed by tight adhesion that requires activation-dependent up-regulation of CD1 1/CD18 integrins on PMNs. For emigration to occur, the initial bonds t hat are established at the endothelial surface must be disengaged. We show that activation of PMNs results in their detachment from P-select in, a glycoprotein expressed at the surface of inflamed endothelium th at mediates initial tethering of PMNs. Loosening of the bond occurs wh en PMNs are activated by platelet-activating factor, which is coexpres sed with P-selectin, or by other signaling molecules. The time course of reduced adhesion to P-selectin, when compared to up-regulation of C D11/CD18 integrins, suggests that ''bond trading'' may occur as activa ted PMNs transmigrate in vivo. Activation of PMNs did not alter bindin g of fluid-phase P-selectin, indicating that the ligand(s) for P-selec tin is not shed or internalized. Using microspheres coated with P-sele ctin, we found that ligands for P-selectin were randomly distributed o ver the surfaces of rounded, unactivated PMNs. An antibody against P-s electin glycoprotein ligand-1 (PSGL-1) completely inhibited binding of P-selectin-coated beads suggesting that P-selectin glycoprotein ligan d-1 is the critical binding site in this assay, In contrast to the dis persed pattern on unactivated PMNs, the ligands for P-selectin were lo calized on the uropods of activated, polarized cells. Pretreating PMNs with cytochalasin D before activation prevented the change in cell sh ape, the redistribution of binding sites for P-selectin-coated beads, and the decrease in cellular adhesiveness for P-selectin. These experi ments indicate that the distribution of ligands for P-selectin is infl uenced by cellular activation and by cytoskeletal interactions, and th at redistribution of these ligands may influence adhesive interactions . Activation of PMNs may cause loosening or disengagement of bonds bet ween P-selectin and its ligands, facilitating transendothelial migrati on.