P-selectin glycoprotein ligand-1–deficient mice have impaired leukocyte tethering to E-selectin under flow

Abstract

Introduction Leukocyte recruitment during inflammation is a multistep process involving tethering and rolling of leukocytes on vascular surfaces, followed by firm adhesion and transmigration into the affected tissues (1). Interactions of selectins with glycoconjugate ligands primarily mediate the initial tethering and rolling adhesion of leukocytes (2, 3). L-selectin, expressed on leukocytes, binds to ligands on other leukocytes and on activated endothelial cells. P-selectin, expressed on activated platelets and endothelial cells, and E-selectin, expressed on activated endothelial cells, bind to ligands on leukocytes. Studies with selectin-deficient mice and blocking mAb's indicate that the selectins have both unique and overlapping functions (4, 5). In the microcirculation of the cremaster muscle, P-selectin mediates most tethering and rolling of leukocytes early after trauma-induced inflammation (6), whereas L-selectin supports significant tethering and rolling 6-8 hours after exposure to TNF-α (7). E-selectin supports slow rolling of leukocytes on venules 2-4 hours after stimulation with TNF-α (8). Selectin ligands require α2,3-sialylation and α1,3fucosylation on capping structures such as sialyl Lewis x (sLe x), which bind to the C-type lectin domains of the selectins (9). In addition, sulfation is a critical requirement of ligands that bind to P-and L-selectin. A subset of glycoproteins binds with high affinity or avidity to the selectins (2). Of these, only P-selectin glycoprotein ligand-1 (PSGL-1) has a clearly documented function in mediating selectin-dependent cell adhesion under flow (3, 10). Human and murine PSGL-1 are extended homodimeric sialomucins that are expressed on most leukocytes. PSGL-1 binds to all three selectins in vitro. P-and L-selectin bind to the N-terminal region of PSGL-1 (11-13), whereas E-selectin binds to one or more additional sites on PSGL-1 (14). Human PSGL-1 binds to P-selectin with specific stereochemical requirements that include optimal orientation of three tyrosine sulfates, amino acids, and a core-2 O-glycan capped with sLe x (15-17). PSGL-1 does not require tyrosine sulfation to bind to E-selectin, but expression of sLe x on core-2 O-glycans enhances binding (18). mAb's against the N-terminal region of PSGL-1 markedly inhibit rolling of human and murine leukocytes on P-selectin in vitro (11, 12), rolling of human neutrophils on P-selectin in rat venules in vivo (19), and rolling of murine leukocytes on P-selectin in murine venules in vivo (12, 20). These data demonstrate that

Endoglycan, a Member of the CD34 Family, Functions as an L-selectin Ligand through Modification with Tyrosine Sulfation and Sialyl Lewis x

“…If it is found that P-selectin and E-selectin can utilize endoglycan as a ligand, as is strongly suspected from its parallels to PSGL-1, the range of potential in vivo functions for this molecule would be further broadened since these selectins are found on platelets and/or activated vascular endothelium. Gene targeting approaches (52,53) and antibody blockade studies (49) have been essential in establishing the importance of PSGL-1 as a selectin ligand. Similar approaches will be necessary to gain a further understanding of endoglycan.…”

Mechanisms and Consequences of Neutrophil Interaction with the Endothelium

“…10 Antibody blockade of the N-terminal region of PSGL-1 inhibits rolling of leukocytes on P-selectin and L-selectin in vivo. 12,13 Elimination of PSGL-1 by gene targeting and homologous recombination reduces the number of neutrophils interacting with the endothelium, 14,15 alters the rolling velocity on P-selectin and on E-selectin, 14,16 and diminishes neutrophil recruitment into inflamed tissue. 15 In addition to its adhesive function, PSGL-1 is also known to initiate intracellular signaling events upon ligand engagement.…”

Clustering endothelial E-selectin in clathrin-coated pits and lipid rafts enhances leukocyte adhesion under flow

“…The leukocyte mucin P-selectin glycoprotein ligand-1 (PSGL-1) is the dominant ligand for P-and L-selectin and an important ligand for E-selectin. 3,4 The manner in which selectins or their ligands distribute on cell surfaces has major influence on rolling behavior. 5 For example, L-selectin and PSGL-1 are located on tips of microvilli, 6,7 which enhances the initial tethering of leukocytes.…”