Abstract. Neutrophils roll on P-selectin expressed by activated platelets or endothelial cells under the shear stresses in the microcirculation. P-selectin glycoprotein ligand-1 (PSGL-1) is a high affinity ligand for P-selectin on myeloid cells. However, it has not been demonstrated that PSGL-1 contributes to the rolling of neutrophils on P-selectin. We developed two IgG mAbs, PL1 and PL2, that appear to recognize proteindependent epitopes on human PSGL-1. The mAbs bound to PSGL-1 on all leukocytes as well as on heterologous cells transfected with PSGL-1 cDNA. PL1, but not PL2, blocked binding of '25I-PSGL-1 to immobilized P-selectin, binding of fluid-phase P-selectin to myeloid and lymphoid leukocytes, adhesion of neutrophils to immobilized P-selectin under static conditions, and rolling of neutrophils on P-selectinexpressing CHO cells under a range of shear stresses. PSGL-1 was localized to microvilli on neutrophils, a topography that may facilitate its adhesive function. These data indicate that (a) PSGL-1 accounts for the high affinity binding sites for P-selectin on leukocytes, and (b) PSGL-1 must interact with P-selectin in order for neutrophils to roll on P-selectin at physiological shear stresses.
TH~ selectins are a family of three Ca2+-dependent membrane-bound lectins that initiate the rolling adhesion of leukocytes to platelets or endothelial cells under the shear forces found in the venular circulation (6,25,33). L-selectin, expressed on leukocytes, binds to constitutive or inducible ligands on endothelial cells. E-selectin, expressed by cytokine-activated endothelial cells, and P-selectin, expressed by thrombin-activated platelets and endothelial cells, bind to ligands on myeloid cells and subsets of lymphocytes. Although the selectins interact weakly with small sialylated, fucosylated oligosaccharides such as sialyl Lewis x (Galfll-4[Fuccd-3lGlcNAc) (Le~) ~ (17, 50), they bind with higher affinity to glycans displayed on a limited number of glycoproteins (3,5,20,26,28,29,39,52) or proteoglycans (41). A major unresolved issue is whether selectins must bind to these higher affinity, but less abundant, ligands in order for leukocytes to roll on the vessel wall un-
