A down-regulatable E-selectin ligand is functionally important for PSGL-1-independent leukocyte-endothelial cell interactions

P-selectin glycoprotein-1 (PSGL-1) supports P-selectin-dependent rolling in vivo and in vitro. However, controversy exists regarding the importance of PSGL-1-dependent and -independent E-selectin rolling. Using antibodies against PSGL-1 and PSGL-1(-/-) mice, we demonstrated abolition of P-selectin-dependent rolling but only partial inhibition of E-selectin-mediated rolling in the cremaster microcirculation following local administration of tumor necrosis factor alpha (TNF-alpha). In vitro studies demonstrated that binding of recombinant mouse E-selectin chimera to PSGL-1(-/-) neutrophils was dramatically decreased in mice treated systemically but not locally with TNF-alpha. Further, PSGL-1 blockade abolished E-selectin-dependent rolling in wild-type mice following systemic TNF-alpha administration but not local TNF-alpha administration. Together, these data support an E-selectin ligand present on PSGL-1(-/-) neutrophils that is down-regulatable upon systemic but not local activation. To determine whether the PSGL-1-independent E-selectin ligand was physiologically important, we used a P- and E-selectin-dependent cutaneous contact hypersensitivity model. Binding studies showed no E-selectin ligand down-regulation in this model. The few cells that rolled on E-selectin ligand following PSGL-1 antibody administration or in PSGL-1 deficiency were sufficient to induce profound contact hypersensitivity. In conclusion, E-selectin mediates PSGL-1-dependent and independent rolling and the latter can be down-regulated by systemic activation and can replace PSGL-1 to support the development of inflammation.     

An L-selectin ligand distinct from P-selectin glycoprotein ligand-1 is expressed on endothelial cells and promotes neutrophil rolling in inflammation

Neutrophils recruited from the blood are key players in the innate immune response. Selectins play critical roles in neutrophil recruitment by mediating their tethering and rolling in inflamed venules. While the roles of P- and E-selectin in this process are well established, the mechanisms of L-selectin-mediated neutrophil recruitment remain elusive. One proposal is that tethering is mediated by L-selectin on flowing neutrophils interacting with P-selectin glycoprotein ligand-1 (PSGL-1) on adherent neutrophils. To clarify whether L-selectin-mediated neutrophil recruitment depends entirely on PSGL-1, we examined the impact of L-selectin deficiency in mice with a PSGL-1-deficient background. L-selectin and PSGL-1 double-knockout mice exhibited a higher increase in their peripheral blood neutrophil count and a worse defect in neutrophil recruitment into the inflamed peritoneum than PSGL-1-deficient mice. Intravital microscopy of inflamed cremaster muscle venules showed that L-selectindeficiency or antibody blockade of L-selectin reduced the residual leukocyte rolling in PSGL-1-deficient mice. Flow cytometric analyses showed that the endothelial cells from the cremaster muscle bound L-selectin in a PSGL-1-independent manner. These results provide evidence for the existence of an L-selectin ligand distinct from PSGL-1 in inflammation and indicate that such a ligand is expressed on endothelial cells, promoting neutrophil rolling in vivo.     

Core2 beta-1,6-N-acetylglucosaminyltransferase enzyme activity is critical for P-selectin glycoprotein ligand-1 binding to P-selectin

P-selectin glycoprotein ligand-1 (PSGL-1) is a high-affinity counterreceptor for P-selectin on myeloid cells and activated T-cells. In addition, PSGL-1 can serve, both in vitro and in vivo, as an E- selectin ligand. Appropriate glycosylation of PSGL-1 is crucial for binding to P-selectin. Functional PSGL-1 is known to bear sialyl lewis X (SLex) or a closely related oligosaccharide. In this study, we show that Chinese hamster ovary (CHO) cells expressing PSGL-1 and fucosyltransferase show a dramatic increase in binding to P-selectin when transfected with "core2" transferase, the enzyme that initiates branching of O-linked glycans. Moreover, only PSGL-1 from core2 transfectant CHO cells can be affinity-captured with P-selectin, suggesting that branched O-linked glycans are required for high- affinity binding to P-selectin. Analysis of PSGL-1-derived O-linked oligosaccharides produced in core2 transfected cells shows the presence of more elaborated glycans. Interestingly, transfection of core2 in these cells does not alter binding to E-selectin.     

Mouse P-selectin glycoprotein ligand-1: molecular cloning, chromosomal localization, and expression of a functional P-selectin receptor

A mouse homolog of P-selectin glycoprotein ligand-1 (PSGL-1), a P- selectin receptor on myeloid cells, has been cloned using the human cDNA sequence to probe a cDNA library prepared from the mouse WEHI-3 monocytic cell line and a genomic DNA library prepared from 129/SvJ mouse tissue. The gene flanking the entire open reading frame of 397 amino acids is composed of a single exon. Mouse and human PSGL-1 show an overall similarity of 67% and an identity of 50% and contain a similar domain organization. However, there are 10 threonine/serine- rich decameric repeats in mouse PSGL-1 as compared with 15 threonine- rich repeats in human PSGL-1. When the mouse PSGL-1 cDNA is coexpressed with an alpha 1,3/1,4 fucosyltransferase cDNA in COS cells, a functional protein is expressed on the COS cell surface mediating binding to human P-selectin. The mouse PSGL-1 gene, Selpl, was mapped to a position on mouse chromosome 5 (Chr 5). Northern blot analyses of mouse tissues showed moderate expression of a PSGL-1 mRNA species in most tissues including heart, kidney, liver, muscle, ovary, and stomach and high levels of expression in blood, bone marrow, brain, adipose tissue, spleen, and thymus. Whereas certain mouse myeloid cell lines including PU5-1.8, WEHI-3B, and 32DC13 express high levels of PSGL-1 mRNA, only WEHI-3B and 32DC13 bind to P-selectin; this interaction is blocked by anti-PSGL-1 antibody. WEHI-3B cells bind significantly better to P-selectin than to E-selectin. Although comparable P-selectin binding is observed in 32DC13 cells, these cells bind better to E- selectin. Binding of 32DC13 cells to E-selectin is not blocked by anti- PSGL-1 antibody. Treatment of WEHI-3B cells with trypsin or neuraminidase abolished their ability to interact with P-selectin. These results indicate that mouse PSGL-1 has structural and functional homology to human PSGL-1 but is characterized by differences in the composition and number of the decameric repeats. PSGL-1 on mouse myeloid cells is critical for high-affinity binding to P-selectin but not E-selectin.     

The N-terminal peptide of PSGL-1 can mediate adhesion to trauma-activated endothelium via P-selectin in vivo

P-selectin glycoprotein ligand-1 (PSGL-1) is present on leukocytes and is the major ligand for endothelial expressed P-selectin. A variety of studies strongly suggests that the N-terminal region of PSGL-1 contains the binding site for P-selectin. We hypothesized that this relatively small N-terminal peptide of PSGL-1 is sufficient to support adhesion to P-selectin in vivo. To test this hypothesis, we coated 2 microm-diameter microspheres with a recombinant PSGL-1 construct, termed 19.ek.Fc. The 19.ek.Fc construct consists of the first 19 N-terminal amino acids of mature PSGL-1 linked to an enterokinase cleavage site that, in turn, is linked to human immunoglobulin G Fc. The 19.ek.Fc-coated microspheres were injected into the jugular vein of mice. Intravital microscopy of postcapillary venules within the cremaster muscle of mice revealed that a significantly greater number of 19.ek.Fc microspheres rolled compared with control microspheres. The number of rolling 19.ek.Fc microspheres was significantly diminished by pretreatment of the mice with a monoclonal antibody to P-selectin or by pretreatment of the 19.ek.Fc microspheres with a monoclonal antibody to PSGL-1. Combined, the results indicate that the N-terminal peptide of PSGL-1 can mediate adhesion to trauma-activated microvascular endothelium via P-selectin in vivo.     

Attachment of the PSGL-1 cytoplasmic domain to the actin cytoskeleton is essential for leukocyte rolling on P-selectin

P-selectin glycoprotein ligand-1 (PSGL-1) serves as the leukocyte ligand for P-selectin, and many of the structural features of its ectodomain required for interactions with P-selectin have been uncovered. In contrast, the function of the highly conserved PSGL-1 cytoplasmic domain has not been explored. Stable transfectants expressing similar levels of either wild-type PSGL-1 or truncated PSGL-1 in which only 4 cytoplasmic residues were retained (designated PSGL-1 Delta cyto), were analyzed. Transfectants expressing full-length PSGL-1 rolled well on P-selectin. In contrast, rolling was almost completely absent in cells transfected with PSGL-1 Delta cyto, even at low shear. Importantly, cells expressing truncated PSGL-1 were able to bind soluble P-selectin and to bind COS cells overexpressing P-selectin, demonstrating that the P-selectin binding site on the PSGL-1 Delta cyto transfectants was intact and was capable of recognizing P-selectin. Impaired rolling by PSGL-1 Delta cyto transfectants was not due to alterations in subcellular localization because both wild-type and truncated PSGL-1 had similar surface distributions on K562 transfectants. Treatment of cells expressing native PSGL-1 with actin cytoskeletal toxins inhibited adhesion in a dose-dependent way. PSGL-1 was associated with the actin cytoskeleton, and this interaction was greatly impaired in PSGL-1 Delta cyto- expressing cells. The PSGL-1 cytoplasmic domain interacted selectively with the ezrin/radixin/moesin (ERM) protein moesin, but not with other ERM proteins or several other cytoskeletal linker proteins. Pharmacologic disruption of interactions between moesin and F-actin in cells expressing PSGL-1 resulted in a dose-dependent inhibition of rolling on P-selectin. Thus, attachment of PSGL-1 to the leukocyte cortical cytoskeleton is essential for leukocyte rolling on P-selectin.     

Lipid raft adhesion receptors and Syk regulate selectin-dependent rolling under flow conditions

Selectins and their ligand P-selectin glycoprotein ligand-1 (PSGL-1) mediate leukocyte rolling along inflamed vessels. Cell rolling is modulated by selectin interactions with their ligands and by topographic requirements including L-selectin and PSGL-1 clustering on tips of leukocyte microvilli. Lipid rafts are cell membrane microdomains reported to function as signaling platforms. Here, we show that disruption of leukocyte lipid rafts with cholesterol chelating agents depleted raft-associated PSGL-1 and L-selectin and strongly reduced L-, P-, and E-selectin-dependent rolling. Cholesterol repletion reversed inhibition of cell rolling. Importantly, leukocyte rolling on P-selectin induced the recruitment of spleen tyrosine kinase (Syk), a tyrosine kinase associated to lipid raft PSGL-1. Furthermore, inhibition of Syk activity or expression, with pharmacologic inhibitors or by RNA interference, strongly reduced leukocyte rolling on P-selectin, but not on E-selectin or PSGL-1. These observations identify novel regulatory mechanisms of leukocyte rolling on selectins with a strong dependency on lipid raft integrity and Syk activity.     

P-selectin mediates spontaneous leukocyte rolling in vivo

Rolling represents the initial step leading to leukocyte extravasation from blood vessels during an inflammatory reaction. In vitro studies indicate that P-selectin could be one of the ligands on endothelium involved in the rolling phenomenon, although the molecular determinants responsible for this transient attachment in vivo are still undefined. Our objectives were to develop a blocking monoclonal antibody against canine P-selectin and to use it to investigate the role of P-selectin in leukocyte rolling in vivo using the technique of intravital microscopy. P-selectin was immunoaffinity purified from canine platelets and used for the production of monoclonal antibodies. One of the hybridomas generated, MD6, was shown by enzyme-linked immunosorbent assay and by flow cytometry to bind preferentially to stimulated platelets and to completely prevent binding of stimulated platelets to neutrophils. Visualization of canine mesenteric venules by intravital microscopy showed that administration of MD6 resulted in a marked inhibition in the number of rolling leukocytes (18.96 +/- 9.92 v 156.40 +/- 19.50 leukocytes/min, P < .05; 88.3% +/- 6.0% inhibition). Control antibody MD3 (which recognizes a nonfunctional epitope of canine P- selectin) had no effect on the number of rolling leukocytes or on their rolling velocity. These results show for the first time that P-selectin plays an essential role in leukocyte rolling in vivo, and therefore may be a key participant of the inflammatory response.     

Differential requirements for the O-linked branching enzyme core 2 beta1-6-N-glucosaminyltransferase in biosynthesis of ligands for E-selectin and P-selectin

Selectins are carbohydrate-binding adhesion molecules that play important roles in control of leukocyte traffic. Glycosyltransferases involved in selectin ligand biosynthesis include the alpha1,3-fucosyltransferases FucT-VII and FucT-IV, one or more sialyltransferases, and at least one O-linked branching enzyme. Previous studies have shown that core 2 beta1-6-N-glucosaminyltransferase (C2GlcNAcT-I; EC 2.4.1.102) is required for functional modification of PSGL-1, the leukocyte P-selectin ligand, but have been ambiguous on whether this enzyme is involved in E-selectin ligand formation. Using an attachment and rolling assay under defined shear flow in vitro, this study shows that C2GlcNAcT-I(-) lymphoid cells stably transfected with FucT-VII complementary DNA attach and roll well on E-selectin at 1.5 dynes/cm.(2) Further, attachment and rolling on P-selectin of neutrophils is sharply reduced and that of short- term polarized Th1 cells is virtually abolished, with leukocytes from C2GlcNAcT-I(-/-) mice. In contrast, both neutrophils and Th1 cells from C2GlcNAcT-I(-/-) mice attach and roll as well as wild-type cells on E-selectin. These results show that C2GlcNAcT-I is selectively required for biosynthesis of ligands for P-selectin, but is not essential for at least some E-selectin ligands. Distinct requirements for C2GlcNAcT-I in the formation of ligands for E-selectin versus P-selectin represents a novel level of regulation of expression of selectin ligands and lymphocyte traffic. (Blood. 2001;97:3806-3811)     

Separable requirements for cytoplasmic domain of PSGL-1 in leukocyte rolling and signaling under flow

In inflamed venules, leukocytes use P-selectin glycoprotein ligand-1 (PSGL-1) to roll on P-selectin and E-selectin and to activate integrin alphaLbeta2 (lymphocyte function-associated antigen-1, LFA-1) to slow rolling on intercellular adhesion molecule-1 (ICAM-1). Studies in cell lines have suggested that PSGL-1 requires its cytoplasmic domain to localize in membrane domains, to support rolling on P-selectin, and to signal through spleen tyrosine kinase (Syk). We generated "DeltaCD" mice that express PSGL-1 without the cytoplasmic domain. Unexpectedly, neutrophils from these mice localized PSGL-1 normally in microvilli, uropods, and lipid rafts. DeltaCD neutrophils expressed less PSGL-1 on their surfaces because of inefficient export from the endoplasmic reticulum. Limited digestion of wild-type neutrophils with O-sialoglycoprotein endopeptidase was used to reduce the PSGL-1 density to that on DeltaCD neutrophils. At matched PSGL-1 densities, both DeltaCD and wild-type neutrophils rolled similarly on P-selectin. However, DeltaCD neutrophils rolling on P-selectin did not trigger Syk-dependent activation of LFA-1 to slow rolling on ICAM-1. These data demonstrate that the PSGL-1 cytoplasmic domain is dispensable for leukocyte rolling on P-selectin but is essential to activate beta2 integrins to slow rolling on ICAM-1.     

Core 1-derived O-glycans are essential E-selectin ligands on neutrophils

Neutrophils roll on E-selectin in inflamed venules through interactions with cell-surface glycoconjugates. The identification of physiologic E-selectin ligands on neutrophils has been elusive. Current evidence suggests that P-selectin glycoprotein ligand-1 (PSGL-1), E-selectin ligand-1 (ESL-1), and CD44 encompass all glycoprotein ligands for E-selectin; that ESL-1 and CD44 use N-glycans to bind to E-selectin; and that neutrophils lacking core 2 O-glycans have partially defective interactions with E-selectin. These data imply that N-glycans on ESL-1 and CD44 and O-glycans on PSGL-1 constitute all E-selectin ligands, with neither glycan subset having a dominant role. The enzyme T-synthase transfers Gal to GalNAcα1-Ser/Thr to form the core 1 structure Galβ1–3GalNAcα1-Ser/Thr, a precursor for core 2 and extended core 1 O-glycans that might serve as selectin ligands. Here, using mice lacking T-synthase in endothelial and hematopoietic cells, we found that E-selectin bound to CD44 and ESL-1 in lysates of T-synthase–deficient neutrophils. However, the cells exhibited markedly impaired rolling on E-selectin in vitro and in vivo, failed to activate β2 integrins while rolling, and did not emigrate into inflamed tissues. These defects were more severe than those of neutrophils lacking PSGL-1, CD44, and the mucin CD43. Our results demonstrate that core 1-derived O-glycans are essential E-selectin ligands; that some of these O-glycans are on protein(s) other than PSGL-1, CD44, and CD43; and that PSGL-1, CD44, and ESL-1 do not constitute all glycoprotein ligands for E-selectin.     

Regulation of P-selectin binding to the neutrophil P-selectin counter-receptor P-selectin glycoprotein ligand-1 by neutrophil elastase and cathepsin G

In the inflammatory response, leukocyte rolling before adhesion and transmigration through the blood vessel wall is mediated by specific cell surface adhesion receptors. Neutrophil rolling involves the interaction of P-selectin expressed on activated endothelium and its counter-receptor on neutrophils, P-selectin glycoprotein ligand-1 (PSGL-1). Here, it is reported that P-selectin binding to neutrophils is lost under conditions that cause the release of proteinases from neutrophil primary granules. Treatment of neutrophils with the purified neutrophil granule proteinases, cathepsin G and elastase, rapidly abolished their capacity to bind P-selectin. This inactivation corresponded to loss of the N-terminal domain of PSGL-1, as assessed by Western blot analysis. A loss of intact PSGL-1 protein from the surfaces of neutrophils after the induction of degranulation was also detected by Western blot analysis. Cathepsin G initially cleaved near the PSGL-1 N-terminus, whereas neutrophil elastase predominantly cleaved at a more C-terminal site within the protein mucin core. Consistent with this, cathepsin G cleaved a synthetic peptide based on the PSGL-1 N-terminus between Tyr-7/Leu-8. Under conditions producing neutrophil degranulation in incubations containing mixtures of platelets and neutrophils, the loss of PSGL-1, but not P-selectin, from platelet-neutrophil lysates was detected. Cathepsin G- or neutrophil elastase-mediated PSGL-1 proteolysis may constitute a potential autocrine mechanism for down-regulation of neutrophil adhesion to P-selectin.     

Desmopressin induces endothelial P-selectin expression and leukocyte rolling in postcapillary venules

Desmopressin, (DDAVP; 1-desamino-8-D-arginine vasopressin) increases the release and activity of von Willebrand factor (vWF); however, its effects on the other major constituent of endothelial Weibel-Palade bodies, P-selectin, has not been investigated. DDAVP-induced P-selectin expression may explain DDAVP's efficacy in bleeding disorders in which vWF levels are normal. Therefore, the objective of this study is to assess the effect of DDAVP on P-selectin expression on endothelial cells of postcapillary venules in vivo and on human umbilical vein endothelium in vitro, and to determine whether DDAVP has direct effects on leukocyte behavior in postcapillary venules. DDAVP (0.1 and 1.0 microgram/mL) induced a significant but transient increase in P- selectin expression on human umbilical vein endothelial cells as well as on rat and human platelets. Immunohistochemical analysis of rat postcapillary venules showed that in contrast to saline, DDAVP injection (1 microgram/kg, intravenous) induced significant endothelial P-selectin expression. DDAVP administration also induced a rapid and significant increase in leukocyte rolling in rat mesenteric venules in vivo. This response was entirely dependent on P-selectin, as an anti-P- selectin antibody rapidly reversed the DDAVP-induced increase in leukocyte rolling. DDAVP induced leukocyte rolling in medium (20 to 40 microns) and large (> 40 microns), but not small (< 20 microns), postcapillary venules. In animals that were treated with DDAVP, there was a steady and significant increase in leukocyte adhesion. This study shows that DDAVP can directly induce P-selectin expression on endothelium in vitro and in vivo and that the latter response is capable of supporting prolonged leukocyte rolling in rat postcapillary venules.     

Adhesion to E-selectin promotes growth inhibition and apoptosis of human and murine hematopoietic progenitor cells independent of PSGL-1

Although both P- and E-selectin are constitutively expressed on bone marrow endothelial cells, their role in the regulation of hematopoiesis has only recently been investigated. We have previously shown that P-selectin glycoprotein ligand-l (PSGL-1/CD162) is expressed by primitive human bone marrow CD34+ cells, mediates their adhesion to P-selectin, and, more importantly, inhibits their proliferation. We now demonstrate that adhesion to E-selectin inhibits the proliferation of human CD34+ cells isolated either from human umbilical cord blood, adult mobilized blood, or steady-state bone marrow. Furthermore, a subpopulation, which does not contain the most primitive hematopoietic progenitor cells, undergoes apoptosis following E-selectin-mediated adhesion. The same phenomenon was observed in cells isolated from mouse bone marrow. Using lineage-negative Sca-1+ c-KIT+ bone marrow cells from PSGL-1(-/-) and wild-type mice, we establish that PSGL-1 is not the ligand involved in E-selectin-mediated growth inhibition and apoptosis. Moreover, stable transfection of the human myeloid cell line K562 (which does not express PSGL-1) with alpha(1,3) fucosyltransferase VII alone was sufficient to recapitulate the E-selectin-mediated growth inhibition and apoptosis observed in hematopoietic progenitor cells. These data demonstrate that an E-selectin ligand(s) other than PSGL-1 transduces growth inhibitory and proapoptotic signals and requires posttranslational fucosylation to be functional.     

Contrasting effects of P-selectin and E-selectin on the differentiation of murine hematopoietic progenitor cells

OBJECTIVE: The two endothelial selectins, P- and E-selectin, are critically important for adhesion and homing of hematopoietic progenitor cells (HPC) into the bone marrow. Little is known, however, about the roles of these two selectins in hematopoiesis. Here, we demonstrate that the most primitive HPC capable of long-term in vivo repopulation express P-selectin glycoprotein ligand-1/CD162 (PSGL-1), a receptor common to both P- and E-selectin. In addition, we demonstrate that P-selectin delays the differentiation of HPC whereas E-selectin enhances their differentiation along the monocyte/granulocyte pathway, describing different roles for these selectins in the regulation of hematopoiesis. MATERIALS AND METHODS: Murine bone marrow HPC were isolated according to their expression of c-kit and PSGL-1, transplanted into lethally irradiated congenic recipients, and chimerism analyzed 6 months posttransplant. Bone marrow lineage-negative (Lin(-)) Sca-1(+)c-kit(+) cells were then cultured on immobilized P- or E-selectin for 4 weeks in the presence of cytokines. Hematopoietic potential was assessed using in vitro phenotyping and colony-forming assays and in vivo spleen colony-forming unit (CFU-S) and long-term competitive repopulation assays. RESULTS: Long-term competitive repopulating HSCs were Lin(-)c-kit(bright) and expressed intermediate levels of PSGL-1. Both P- and E-selectin slowed the proliferation of Lin(-)Sca-1(+)c-kit(+) cells during the first two weeks of liquid culture. After two weeks, however, cells cultured on immobilized P-selectin showed increased proliferation with increased production of both colony-forming cells (CFC) and CFU-S(12) compared to the other cultures. In contrast, E-selectin enhanced the differentiation of Lin(-)Sca-1(+)c-kit(+) cells into cells that expressed the granulocyte maturation marker, Gr-1, accompanied by loss of CFC potential from these cultured cells. Finally, the long-term repopulation potential of these cells was not maintained following culture on either selectin. CONCLUSION: These results suggest that the two endothelial selectins, E-selectin and P-selectin, have very different effects on HPC. E-selectin accelerates the differentiation of maturing HPC towards granulocyte and monocyte lineages while maintaining the production of more immature CFU-S(12) in ex vivo liquid suspension culture. In marked contrast, P-selectin delays the differentiation of Lin(-)Sca-1(+)c-kit(+) cells, allowing enhanced ex vivo expansion of CFC and CFU-S(12) but not HSCs.     

Severe impairment of leukocyte rolling in venules of core 2 glucosaminyltransferase-deficient mice

Leukocyte capture and rolling are mediated by selectins expressed on leukocytes (L-selectin) and the vascular endothelium (P- and E-selectin). To investigate the role of core 2 beta1-6-N-glucosaminyltransferase (C2GlcNAcT-I) for synthesis of functional selectin ligands in vivo, leukocyte rolling flux and velocity were studied in venules of untreated and tumor necrosis factor-alpha (TNFalpha)-pretreated autoperfused cremaster muscles of C2GlcNAcT-I-deficient (core 2(-/-)) and littermate control mice. In untreated core 2(-/-) mice, leukocyte rolling was dramatically reduced with markedly increased rolling velocities (81 +/- 4 microm/s vs 44 +/- 3 microm/s). The reduced rolling in core 2(-/-) mice was due mainly to severely impaired binding of P-selectin to P-selectin glycoprotein ligand-1 (PSGL-1). Some rolling remained after blocking PSGL-1 in controls but not in core 2(-/-) mice. In TNFalpha-pretreated mice, rolling was markedly reduced in core 2(-/-) mice owing to impaired P-selectin- and E-selectin-mediated rolling. Rolling velocities in core 2(-/-) mice treated with an E-selectin-blocking monoclonal antibody (59 +/- 4 microm/s) were significantly higher than in controls (14 +/- 1 microm/s), which provides further evidence for the severe impairment in P-selectin-mediated rolling. In conclusion, P-selectin ligands including PSGL-1 are largely C2GlcNAcT-I dependent. In addition, E-selectin-mediated rolling in vivo is partially dependent on the targeted C2GlcNAcT-I. (Blood. 2001;97:3812-3819)     

Leukocyte rolling in vivo is mediated by P-selectin glycoprotein ligand- 1

Leukocyte rolling, an early and important step in the inflammatory response, is mediated by the selectin family of adhesion molecules. The selectins bind with low affinity to sialylated and fucosylated glycans such as sialyl Lewisx (sLex), but bind with high affinity to only a few specific glycoproteins on cell surfaces. One such glycoprotein is P- selectin glycoprotein ligand-1 (PSGL-1). The relative contributions of low- and high-affinity ligands to leukocyte rolling in vivo are unknown. We show here that a monoclonal antibody to PSGL-1 (PL1) dramatically reduces rolling of human polymorphonuclear neutrophils (PMN) and promyelocytic HL-60 cells in venules of acutely exteriorized rat mesentery. Control PMN and HL-60 cell rolling flux fractions were 39% +/- 3% and 33% +/- 5%, which were reduced by PL1 to 7% +/- 2% and 6% +/- 2%, respectively. Similar reductions were seen with F(ab) fragments of PL1. PL1-treated PMN rolled at significantly higher mean velocities than untreated PMN owing to intermittent rather that continuous interactions. These findings show that interaction of P- selectin with PSGL-1 is required for rolling of myeloid cells in mesenteric venules at physiologic shear stress in vivo.     

Platelets roll on stimulated endothelium in vivo: an interaction mediated by endothelial P-selectin.

P-selectin, found in storage granules of platelets and endothelial cells, can be rapidly expressed upon stimulation. Mice lacking this membrane receptor exhibit a severe impairment of leukocyte rolling. We observed that, in addition to leukocytes, platelets were rolling in mesenteric venules of wild-type mice. To investigate the role of P-selectin in this process, resting or activated platelets from wild-type or P-selectin-deficient mice were fluorescently labeled and transfused into recipients of either genotype. Platelet-endothelial interactions were monitored by intravital microscopy. We observed rolling of either wild-type or P-selectin-deficient resting platelets on wild-type endothelium. Endothelial stimulation with the calcium ionophore A23187 increased the number of platelets rolling 4-fold. Activated P-selectin-deficient platelets behaved similarly, whereas activated wild-type platelets bound to leukocytes and were seen rolling together. Platelets of either genotype, resting or activated, interacted minimally with mutant endothelium even after A23187 treatment. The velocity of platelet rolling was 6- to 9-fold greater than that of leukocytes. Our results demonstrate that (i) platelets roll on endothelium in vivo, (ii) this interaction requires endothelial but not platelet P-selectin, and (iii) platelet rolling appears to be independent of platelet activation, indicating constitutive expression of a P-selectin ligand(s) on platelets. We have therefore observed an interesting parallel between platelets and leukocytes in that both of these blood cell types roll on stimulated vessel wall and that this process is dependent on the expression of endothelial P-selectin.     

N-terminal residues in murine P-selectin glycoprotein ligand-1 required for binding to murine P-selectin

P-selectin binds to the N-terminal region of human P-selectin glycoprotein ligand-1 (PSGL-1). For optimal binding, this region requires sulfation on 3 tyrosines and specific core-2 O-glycosylation on a threonine. P-selectin is also thought to bind to the N terminus of murine PSGL-1, although it has a very different amino acid sequence than human PSGL-1. Murine PSGL-1 has potential sites for sulfation at Tyr13 and Tyr15 and for O-glycosylation at Thr14 and Thr17. We expressed murine PSGL-1 or constructs with substitutions of these residues in transfected Chinese hamster ovary cells that coexpressed the glycosyltransferases required for binding to P-selectin. The cells were assayed for binding to fluid-phase P-selectin and for tethering and rolling on P-selectin under flow. In both assays, substitution of Tyr13 or Thr17 markedly diminished, but did not eliminate, binding to P-selectin. In contrast, substitution of Tyr15 or Thr14 did not affect binding. Substitution of all 4 residues eliminated binding. Treatment of cells with chlorate, an inhibitor of sulfation, markedly reduced binding of wild-type PSGL-1 to P-selectin but did not further decrease binding of PSGL-1 with substitutions of both tyrosines. These data suggest that sulfation of Tyr13 and O-glycosylation of Thr17 are necessary for murine PSGL-1 to bind optimally to P-selectin. Because it uses only one tyrosine, murine PSGL-1 may rely more on other peptide components and O-glycosylation to bind to P-selectin than does human PSGL-1.     

P-selectin binding to P-selectin glycoprotein ligand-1 induces an intermediate state of alphaMbeta2 activation and acts cooperatively with extracellular stimuli to support maximal adhesion of human neutrophils

P-selectin glycoprotein ligand 1 (PSGL-1, CD162) and integrin alphaMbeta2 (Mac-1, CD11bCD18) are leukocyte adhesion molecules essential for innate immunity and inflammation. The interaction of PSGL-1 with P-selectin (CD62P) mediates tethering, rolling, and weak adhesion of leukocytes, during which they become sufficiently activated in situ by locally released or displayed cytokines and chemoattractants for integrin-mediated firm adhesion. However, communication between P-selectin and the integrin, whether P-selectin can trigger beta2-integrin activation, remains controversial. We found that P-selectin immunoglobulin chimera and PSGL-1 monoclonal antibodies (mAbs) increased adhesion of human neutrophils to immobilized, but not soluble, fibrinogen. This intermediate state of neutrophil adhesion was defined by moderate clustering of integrin alphaMbeta2, no increase in CBRM1/5 (a mAb specific for the activation epitope on the alphaM subunit) recognition, and no increase in surface expression of alphaMbeta2, whereas phorbol myristate acetate (PMA) induced extensive changes in these 3 parameters. Furthermore, platelet-activating factor or interleukin 8 acted in concert with P-selectin for further enhancing the activation of alphaMbeta2. We thus propose a model in which P-selectin induces an intermediate state of integrin activation and then cooperates with other extracellular stimuli to support maximal adhesion of human neutrophils.