Mechanisms that regulate the function of the selectins and their ligands

Divergent effects of platelet-endothelial cell adhesion molecule-1 and beta 3 integrin blockade on leukocyte transmigration in vivo

The final stage in the migration of leukocytes to sites of inflammation involves movement of leukocytes through the endothelial cell layer and the perivascular basement membrane. Both platelet-endothelial cell adhesion molecule-1 (PECAM-1/CD31) and the integrin alphavbeta3 have been implicated in this process, and in vitro studies have identified alphavbeta3 as a heterotypic ligand for PECAM-1. In the present study we have addressed the roles of these molecules by investigating and comparing the effects of PECAM-1 and alphavbeta3 blockade on leukocyte migration in vivo. For this purpose we have examined the effects of neutralizing Abs directed against PECAM-1 (domain 1-specific, mAb 37) and beta3 integrins (mAbs 7E3 and F11) on leukocyte responses in the mesenteric microcirculation of anesthetized rats using intravital microscopy. The anti-PECAM-1 mAb suppressed leukocyte extravasation, but not leukocyte rolling or firm adhesion, elicited by IL-1beta in a dose-dependent manner (e.g., 67% inhibition at 10 mg/kg 37 Fab), but had no effect on FMLP-induced leukocyte responses. Analysis by electron microscopy suggested that this suppression was due to an inhibition of neutrophil migration through the endothelial cell barrier. By contrast, both anti-beta3 integrin mAbs, 7E3 F(ab')2 (5 mg/kg) and F11 F(ab')2 (5 mg/kg), selectively reduced leukocyte extravasation induced by FMLP (38 and 46%, respectively), but neither mAb had an effect on IL-1beta-induced leukocyte responses. These findings indicate roles for both PECAM-1 and beta3 integrins in leukocyte extravasation, but do not support the concept that these molecules act as counter-receptors in mediating leukocyte transmigration.

Transfer of tissue factor from leukocytes to platelets is mediated by CD15 and tissue factor

We describe thrombogenic tissue factor (TF) on leukocyte-derived microparticles and their incorporation into spontaneous human thrombi. Polymorphonuclear leukocytes and monocytes transfer TF+particles to platelets, thereby making them capable of triggering and propagating thrombosis. This phenomenon calls into question the original dogma that vessel wall injury and exposure of TF within the vasculature to blood is sufficient for the occurrence of arterial thrombosis. The transfer of TF+ leukocyte-derived particles is dependent on the interaction of CD15 and TF with platelets. Both the inhibition of TF transfer to platelets by antagonizing the interaction CD15 with P-selectin and the direct interaction of TF itself suggest a novel therapeutic approach to prevent thrombosis.

Specialized contributions by alpha(1,3)-fucosyltransferase-IV and FucT-VII during leukocyte rolling in dermal microvessels

Noninflamed skin venules support constitutive leukocyte rolling. P-selectin controls the rolling frequency, whereas E-selectin dictates rolling velocity (Vroll). Fucosylated selectin ligands are essential for all interactions, as rolling was absent in mice doubly deficient in alpha1,3-fucosyltransferase (FucT)-IV and FucT-VII. The rolling fraction was reduced in FucT-VII-/- animals but normal in FucT-IV-/- mice. However, Vroll was markedly increased in both strains. P-selectin ligands generated by FucT-VII are crucial for initial leukocyte tethering, whereas E-selectin ligands that permit maximum slowing of Vroll require simultaneous expression of FucT-IV and FucT-VII. These results demonstrate a role for FucT-IV in selectin-dependent adhesion and suggest that the endothelial selectins and FucTs have distinct but overlapping functions in the immunosurveillance of the skin.

E-selectin expression and stimulation by inflammatory mediators are developmentally regulated during embryogenesis

Leukocyte recruitment during inflammation is specified, in part, by the spatial distribution and temporal regulation of endothelial adhesion molecules. In this study we investigated the developmental onset of E-selectin and intercellular adhesion molecule-1 (ICAM-1) basal expression and inducibility by inflammatory mediators as indices of lineage-restricted endothelial adhesion molecule expression. We studied both murine embryos and embryoid bodies (EB), derived from differentiated embryonic stem cells, to examine a broad range of endothelial ontogeny. Our results reveal that E-selectin and ICAM-1 are differentially regulated during development and that three stages define the ontogeny of the E-selectin-inducible response. The earliest endothelial lineage cells in Day 4 and Day 5 EB did not express E-selectin in the basal state or after stimulation. A second stage, observed between embryonic Day 9.5 (E9.5) and E11.5 to E12.5 in cultured embryo cells and transiently at Day 6 of EB differentiation, was characterized by basal expression that was not stimulated by inflammatory mediators. A third stage was characterized by both basal and inducible expression of E-selectin and was observed beginning at E12.5 to E13.5 in cultured embryo cells and at Day 7 in EB. In contrast ICAM-1 was stimulated at all of the embryonic stages examined and before the onset of E-selectin inducibility in both embryos and EB. E-selectin expression in embryos was also stimulated by introducing endotoxin into the embryonic, but not the maternal, peritoneum. This suggests that embryos are protected from inflammatory insults present in the maternal circulation. The developmentally regulated acquisition of E-selectin inducibility during embryogenesis likely involves changes in signal transduction cascades, transcription factors, and/or chromatin accessibility that specify inducible expression within the endothelial lineage and further restrict inducibility to particular endothelial subpopulations.

Discrete interactions in cell adhesion measured by single-molecule force spectroscopy

Cell-cell adhesion mediated by specific cell-surface molecules is essential for multicellular development. Here we quantify de-adhesion forces at the resolution of individual cell-adhesion molecules, by controlling the interactions between single cells and combining single-molecule force spectroscopy with genetic manipulation. Our measurements are focused on a glycoprotein, contact site A (csA), as a prototype of cell-adhesion proteins. csA is expressed in aggregating cells of Dictyostelium discoideum, which are engaged in development of a multicellular organism. Adhesion between two adjacent cell surfaces involves discrete interactions characterized by an unbinding force of 23 +/- 8 pN, measured at a rupture rate of 2.5 +/- 0.5 microm s-1.

Leukocyte-endothelium interactions in pial venules during the early and late reperfusion period after global cerebral ischemia in gerbils

The contribution of leukocytes to secondary brain damage after cerebral ischemia is still under discussion. The purpose of the present study was to examine the pial microcirculation after global cerebral ischemia while focusing on leukocyte-endothelium interactions during the early and late reperfusion period of up to 4 days. A closed cranial window technique that leaves the dura mater intact was used. Global cerebral ischemia of 15 minutes' duration was induced in male Mongolian gerbils (n = 91). Pial microcirculation was observed by intravital fluorescence microscopy. Leukocyte-endothelium interactions (LEIs) in pial venules, vessel diameters, capillary density, and regional microvascular blood flow measured by laser Doppler flowmetry were quantified during 3 hours of reperfusion and in intervals up to 4 days after ischemia. Within 3 hours of reperfusion, the number of leukocytes (cells/100 microm x minute) rolling along or adhering to the venular endothelium increased from 0.1 +/- 0.2 to 28.4 +/- 17.4 (P < 0.01 vs. control) and from 0.2 +/- 0.2 to 4.0 +/- 3.8 (P < 0.05), respectively. There was no capillary plugging by leukocytes; capillary density remained unchanged. In the late reperfusion period, at 7 hours after ischemia, LEIs had returned to baseline values. Furthermore, from 12 hours to 4 days after ischemia, no LEIs were observed. Changes in regional microvascular blood flow did not correlate with LEIs. Global cerebral ischemia of 15 minutes' duration induces transient LEIs that reach a maximum within 3 hours of reperfusion and return to baseline at 7 hours after ischemia. LEIs are not related to changes in microvascular perfusion, which suggests mainly that the expression of adhesion receptors is necessary to induce LEIs rather than rheologic factors. It seems unlikely that this short-lasting activation of leukocytes can play a role in the development of secondary brain damage.

P-Selectin glycoprotein ligand 1 (PSGL-1) is expressed on platelets and can mediate platelet-endothelial interactions in vivo

The platelet plays a pivotal role in maintaining vascular integrity. In a manner similar to leukocytes, platelets interact with selectins expressed on activated endothelium. P-selectin glycoprotein ligand 1 (PSGL-1) is the main P-selectin ligand expressed on leukocytes. Searching for platelet ligand(s), we used a P-selectin-immunoglobulin G (IgG) chimera to affinity purify surface-biotinylated proteins from platelet lysates. P-selectin-bound ligands were eluted with ethylenediaminetetraacetic acid. An approximately 210-kD biotinylated protein was isolated from both human neutrophil and platelet preparations. A band of the same size was also immunopurified from human platelets using a monoclonal anti-human PSGL-1 antibody and could be blotted with P-selectin-IgG. Under reducing conditions, both the predicted PSGL-1 approximately 210-kD dimer and the approximately 120-kD monomer were isolated from platelets. Comparative immunoelectron microscopy and Western blotting experiments suggested that platelet PSGL-1 expression is 25-100-fold lower than that of leukocytes. However, patients with chronic idiopathic thrombocytopenic purpura who harbor predominantly young platelets displayed greater expression, indicating that PSGL-1 expression may be decreased during platelet aging. By flow cytometry, thrombin-activated platelets from normal individuals exhibited greater expression than those unstimulated. An inhibitory anti-PSGL-1 antibody significantly reduced platelet rolling in mesenteric venules, as observed by intravital microscopy. Our results indicate that functional PSGL-1 is expressed on platelets, and suggest an additional mechanism by which selectins and their ligands participate in inflammatory and/or hemostatic responses.

Inhibition by nitric oxide-releasing compounds of E-selectin expression in and neutrophil adhesion to human endothelial cells

The effects of two chemically unrelated nitric oxide (NO)-releasing compounds were studied on adhesion molecule expression in and neutrophil adhesion to human umbilical vein endothelial cells. Incubation of confluent monolayers of endothelial cells with increasing concentrations of lipopolysaccharide stimulated the adhesion of polymorphonuclear leukocytes to endothelial cells. Flow cytometric analysis showed that lipopolysaccharide treatment upregulated the expression of adhesion molecules E-selectin and intercellular adhesion molecule-1 (ICAM-1) in human umbilical vein endothelial cells. A novel NO-releasing compound GEA 3175 (1,2,3, 4-oxatriazolium, -3-(3-chloro-2-methylphenyl)-5-[[(4-methylphenyl)sulfonyl]amino]-, hydroxide inner salt) inhibited lipopolysaccharide-induced adhesion being more potent than the earlier known NO donor S-nitroso-N-acetylpenicillamine. The increased E-selectin expression induced by lipopolysaccharide was significantly attenuated by the two NO donors tested whereas ICAM-1 expression remained unaltered. The present data show that NO donors inhibit E-selectin expression in and neutrophil adhesion to lipopolysaccharide-stimulated vascular endothelial cells. Thus, by inhibiting leukocyte adhesion NO donors may reduce leukocyte infiltration and leukocyte-mediated tissue injury in inflammation and ischemia-reperfusion injury.

The acute influence of tobacco smoking on adhesion molecule expression on monocytes and neutrophils and on circulating adhesion molecule levels in vivo

Soluble adhesion molecules have been reported as risk markers of a wide range of human diseases and specific adhesion molecules may play a direct role in pathological processes. Serum soluble intercellular adhesion molecule-1 (sICAM-1) is known to be significantly elevated in smokers compared to non-smokers. We examined the acute effects of smoking a standard 2R1 research cigarette on the serum concentration of sICAM-1 and other circulating adhesion molecules (sP-selectin, sE-selectin, sL-selectin, sVCAM-1 and sPECAM-1) in heavy smokers (serum cotinine >/= 100 ng/ml), light smokers (serum cotinine </= 60 ng/ml) and non-smokers (serum cotinine </= 10 ng/ml) by ELISA. Adhesion molecule expression on the cell surface of monocytes and neutrophils in peripheral blood was examined by flow cytometry. The sICAM-1 concentration directly correlated to serum cotinine concentration (p= 0.047) and nicotine load (p= 0.033) in smokers and was significantly elevated compared to non-smokers (p= 0.037). Other than a decrease in the concentration of sP-selectin over 1 hour regardless of smoking, no significant temporal alterations of any adhesion molecule were observed following the smoking experience or in the non-smoking control group. No significant difference in surface expression of ICAM-1, CD18, PECAM-1 or L-selectin on peripheral monocytes or neutrophils was observed over a 1-hour period following smoking. These data suggest that the elevated concentration of sICAM-1 in smokers is not due to an immediate effect of smoking.

Expression of carbohydrate antigens in the goat uterus during early pregnancy and on steroid-treated polarized uterine epithelial cells in vitro

Our objectives were to determine whether specific fucosylated carbohydrate antigens, associated with uterine receptivity in rodents, are expressed in pregnant caprine uterine tissues and polarized uterine luminal epithelial (ULE) cells in culture. Immunofluorescence microscopy on frozen endometrium revealed that expression of the H-type 1 antigen, confined to epithelial cells, was regulated during early pregnancy. Staining was high on Day 5 and low on Days 11 and 13. Strong, uniform apical staining was characteristic of ULE cells between Days 15 and 19 but declined markedly by Day 25. Immunofluorescence analysis of the apical surface of polarized ULE cells cultured in steroid-free medium revealed weak and diffuse staining for the H-type 1 antigen, while progesterone (P(4)) treatment resulted in the formation of aggregates of punctate staining along the apical surface. Domain-specific biotinylation of polarized ULE cells, coupled with streptavidin precipitation and Western blotting, revealed that six apical surface proteins (31, 33, 42, 55, 60, and 70 kDa) carry the H-type 1 antigen. Therefore, H-type 1 antigen expression is up-regulated in vivo during the periimplantation period, stimulated by P(4) on polarized ULE cells in culture, and may be a useful marker for uterine receptivity in this species.

Molecular mechanisms that control endothelial cell contacts

Endothelial cell contacts control the permeability of the blood vessel wall. This allows the endothelium to form a barrier for solutes, macromolecules, and leukocytes between the vessel lumen and the interstitial space. Loss of this barrier function in pathophysiological situations can lead to extracellular oedema. The ability of leukocytes to enter tissue at sites of inflammation is dependent on molecular mechanisms that allow leukocytes to adhere to the endothelium and to migrate through the endothelial cell layer and the underlying basal lamina. It is a commonly accepted working hypothesis that inter-endothelial cell contacts are actively opened and closed during this process. Angiogenesis is another important process that requires well-controlled regulation of inter-endothelial cell contacts. The formation of new blood vessels by sprouting from pre-existing vessels depends on the loosening of established endothelial cell contacts and the migration of endothelial cells that form the outgrowing sprouts. This review focuses on the molecular composition of endothelial cell surface proteins and proteins of the cytoskeletal undercoat of the plasma membrane at sites of inter-endothelial cell contacts and discusses the current knowledge about the potential role of such molecules in the regulation of endothelial cell contacts.

Blocking P-selectin protects from ischemia/reperfusion-induced acute renal failure

Acute renal failure (ARF) in response to ischemia-reperfusion is thought to be associated with neutrophil infiltration. Neutrophil recruitment depends on adhesion molecules, including P-selectin. Our study sought to characterize the role of P-selectin in ischemia-reperfusion (I/R) -induced acute renal failure (ARF). In wild-type (wt) and P-selectin-deficient (P-/-) mice (both C57BL/6), ARF was induced by 32 min bilateral renal ischemia, followed by reperfusion (I/R). Wt showed a 12- and 20-fold increase in creatinine at 24 and 48 h after I/R, respectively. Similar changes were seen in blood urea nitrogen (BUN). By contrast, in P-/- creatinine and BUN increased only moderately (fourfold over sham). In wt, renal myeloperoxidase activity, indicating neutrophil infiltration, peaked after 24 h (19-fold over sham). This was significantly attenuated in P-/- (fivefold over sham). Western blot analysis revealed maximum P-selectin expression 12 h after I/R in wt. Immunostaining detected P-selectin in glomerular endothelium and in platelets adherent in glomerular and peritubular vessels. Postischemic injection of P-selectin antibody at 10 min after reperfusion, but not isotype control antibody, protected wt from ARF similar to the protection seen in P-/-. We conclude that blocking P-selectin even after onset of reperfusion protects mice from I/R-induced ARF, suggesting potential therapeutic strategies aimed at blocking P-selectin.

Integrin cell adhesion receptors and the concept of agonism

Most cells are adherent and rely on adhesive interactions to regulate their shape, motility and growth. These interactions are critical for tissue integrity and homeostasis but they also contribute to many of the most common diseases in humans. The integrins are a key family of cell-surface receptors that mediate the downstream consequences of cell adhesion and are therefore prime targets for the development of therapeutic agents. In addition to their adhesive activity, integrins also exhibit several other classical features of signalling receptors. Sufficient evidence is now available to pose the question of whether integrins should be classified as true signalling receptors; this article both reviews this evidence and attempts to identify remaining gaps.

Visualization of carbohydrate-binding molecules expressed by myelomono- and erythropoietic cells derived from human bone marrow: an immunoenzymatic double-staining study

Interactions between human haematopoietic and bone marrow stromal cells are governed by complex carbohydrate-mediated adhesion processes. In order to evaluate corresponding carbohydrate-binding sites on human myelo- and erythropoietic cells which were able to react with mono- or oligosaccharides, we established an immunocytochemical double-staining assay. In a first step, cell lineages were visualized using Lewis(x) (CD15) or glycophorin C-specific monoclonal antibodies. The second step included polyacrylamide-conjugated carbohydrate structures. According to our results, the carbohydrate-binding potential of granulopoietic cells increased during the process of maturation, contrasting a reduction of carbohydrate-binding sites on erythroid precursor cells during differentiation. With respect to previous in vitro studies, these findings shed some light on certain aspects of bone marrow homing as well as on the trafficking of mature cellular elements into circulation. It is tempting to speculate that carbohydrate-mediated adhesion mechanisms may be involved in the various functional defects of progenitor cells in chronic myelogenic leukaemia, especially regarding their complex interactions with the marrow microenvironment.

Correction of Leukocyte Adhesion Deficiency Type II With Oral Fucose

We describe a simple, noninvasive, and effective therapy for leukocyte adhesion deficiency type II (LAD II), a rare inherited disorder of fucose metabolism. This disorder leads to an immunodeficiency caused by the absence of carbohydrate-based selectin ligands on the surface of neutrophils as well as to severe psychomotor and mental retardation. The fucosylation defect in LAD II fibroblasts can be corrected by addition of L-fucose to the culture medium. This prompted us to initiate dietary fucose therapy on a patient with LAD II. Oral supplementation of fucose in this patient induced the expression of fucosylated selectin ligands on neutrophils and core fucosylation of serum glycoproteins. During 9 months of treatment, infections and fever disappeared, elevated neutrophil counts returned to normal, and psychomotor capabilities improved.

Correction of Leukocyte Adhesion Deficiency Type II With Oral Fucose

We describe a simple, noninvasive, and effective therapy for leukocyte adhesion deficiency type II (LAD II), a rare inherited disorder of fucose metabolism. This disorder leads to an immunodeficiency caused by the absence of carbohydrate-based selectin ligands on the surface of neutrophils as well as to severe psychomotor and mental retardation. The fucosylation defect in LAD II fibroblasts can be corrected by addition of L-fucose to the culture medium. This prompted us to initiate dietary fucose therapy on a patient with LAD II. Oral supplementation of fucose in this patient induced the expression of fucosylated selectin ligands on neutrophils and core fucosylation of serum glycoproteins. During 9 months of treatment, infections and fever disappeared, elevated neutrophil counts returned to normal, and psychomotor capabilities improved.

Multiple, targeted deficiencies in selectins reveal a predominant role for P-selectin in leukocyte recruitment

We extend our previous analyses of mice deficient in selectins by describing the generation and comparative phenotype of mice lacking one, two, or three selectins after sequential ablation of the murine genes encoding P-, E-, and L-selectins. All mice deficient in selectins are viable and fertile as homozygotes. However, mice missing both P- and E-selectins (PE(-/-)), and mice missing all three selectins (ELP(-/-)) develop mucocutaneous infections that eventually lead to death. Mice deficient in multiple selectins display varying degrees of leukocytosis, resulting in part from alterations in leukocyte rolling and recruitment. PE(-/-) mice, ELP(-/-) mice, and mice missing both P- and L-selectins (PL(-/-)) show drastic reductions in leukocyte rolling and in extravasation of neutrophils in thioglycollate-induced peritonitis. In a separate inflammatory model (ragweed-induced peritoneal eosinophilia), we demonstrate P-selectin to be both necessary and sufficient for the recruitment of eosinophils. The phenotype of mice missing both E- and L-selectins (EL(-/-)) is less severe than those seen in the other double knockouts. Comparisons among the double knockouts suggest that P-selectin normally cooperates with both E- and L-selectins. Our results indicate a preeminent role for P-selectin in regulating leukocyte behavior in mice. Data from the ELP(-/-) mice indicate, however, that all three selectins are important to leukocyte homeostasis and efficient neutrophil recruitment.

Function of alpha3beta1-tetraspanin protein complexes in tumor cell invasion. Evidence for the role of the complexes in production of matrix metalloproteinase 2 (MMP-2)

Tumor cell migration through the three- dimensional extracellular matrix (ECM) environment is an important part of the metastatic process. We have analyzed a role played by the integrin-tetraspanin protein complexes in invasive migration by culturing MDA-MB-231 cells within Matrigel. Using time-lapse video recording, we demonstrated that the Matrigel-embedded cells remain round and exhibit only limited ability for migration by extending short, highly dynamic pseudopodia. The alpha3beta1-tetraspanin protein complexes were clustered on the thin microvilli-like protrusions extending from both the main cell body and pseudopodia. Ligation of the alpha3beta1-tetraspanin protein complexes with monoclonal antibodies specifically stimulates production of matrix metalloproteinase 2 (MMP-2) and induces formation of long invasive protrusions within Matrigel. Accordingly, treatment with the monoclonal antibodies to various tetraspanin proteins and to the alpha3 integrin subunit increases invasive potential of the MDA-MB-231 cells in the Matrigel-penetration assay. A specific inhibitor of phosphoinositide 3-kinase (PI3K), LY294002, negated the effect of the monoclonal antibodies on the morphology of the Matrigel-embedded cells and on production of MMP-2. Interestingly, broad-spectrum inhibitors of protein tyrosine kinases (genistein) and protein tyrosine phosphatases (orthovanadate), and actin filament stabilizing compound (jasplakinolide), also block protrusive activity of the Matrigel-embedded cells but have no effect on the production of MMP-2. These results indicate that alpha3beta1-tetraspanin protein complexes may control invasive migration of tumor cells by using at least two PI3K-dependent signaling mechanisms: through rearrangement of the actin cytoskeleton and by modulating the MMP-2 production.

Leukocyte adhesion: High-speed cells with ABS

In order to decide where to exit blood vessels and enter tissues, leukocytes roll along endothelial surfaces. Recent studies suggest that an 'automatic braking system' (ABS), involving selectin cell-adhesion molecules, enables leukocytes to roll at a fairly constant velocity despite large variations in blood flow rate.