Leukocyte activation induces surface redistribution of P‐selectin glycoprotein ligand‐1

Binding of P‐selectin on activated endothelium to P‐selectin glycoprotein ligand‐1 (PSGL‐1) on neutrophils mediates the initial tethering and rolling of neutrophils on the vessel wall at inflammatory sites. Upon activation of rolling cells by locally expressed signaling molecules, integrin‐dependent adhesion mechanisms are engaged and transendothelial migration proceeds. P‐selectin binding sites are uniformly distributed on the surface of quiescent neutrophils, but are redistributed to the uropod of activated neutrophils. It is unclear whether this activation‐induced change in the surface topography of P‐selectin binding sites is due to surface redistribution of PSGL‐1, shedding of PSGL‐1 from the lamellapod, and/or movement of PSGL‐1 from an intracellular compartment to the uropod of the polarized cell. With the use of immunogold electron microscopy we previously demonstrated that PSGL‐1 was localized to the tips of microvilli on neutrophils. Here we document a similar localization for PSGL‐1 on eosinophils, basophils, monocytes, and lymphocytes. On quiescent neutrophils, 80% of the PSGL‐1 label was on tips of microvilli, which are randomly distributed around the cell circumference. On activated, polarized neutrophils, the PSGL‐1 label was restricted to a segment of 42% of the cell circumference even though total labeling decreased by only 26%. Latex microbeads coated with anti‐PSGL‐1 mAb bound preferentially to the uropod of activated neutrophils. Subcellular fractionation and immunogold analysis of frozen thin sections of neutrophils failed to detect PSGL‐1 in any intracellular compartment. Taken together, these data indicate that the activation‐induced change in the surface topography of PSGL‐1 is due to surface redistribution of PSGL‐1. This process may facilitate transendothelial migration by disrupting bonds between P‐selectin and PSGL‐1 at the leading edge of migrating cells. J. Leukoc. Biol. 61: 489–499; 1997.