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, approximately 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 approximately 42% of the cell circumference even though total labeling decreased by only approximately 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.