Involvement of platelet-endothelial cell adhesion molecule-1 in neutrophil recruitment in vivo

Lymphatic endothelium expresses PECAM-1

The expression of adhesion molecules on the lymphatic endothelium of human small intestine and submandibular lymph node was studied immunohistochemically with the antibodies for selectin family and Ig superfamily members. In both small intestine and submandibular lymph node, lymphatic endothelium did not express intercellular adhesion molecule-1 and endothelial cell-selectin but expressed platelet-endothelial cell adhesion molecule-1 (PECAM-1). Though lymphatic vessels may not have a positive function in leukocyte rolling and adhesion, lymphatic endothelium may interact with leukocytes, with PECAM-1 playing a role.

Ligation of CD31/PECAM-1 modulates the function of lymphocytes, monocytes and neutrophils

CD31 or platelet/endothelial cell adhesion molecule (PECAM-1) is a 130-kDa glycoprotein expressed on endothelial cells, granulocytes, a subset of lymphocytes and platelets. In this study, we examined the ability of four monoclonal antibodies (mAb) against different domains of CD31 to modulate the function of T lymphocytes, monocytes and neutrophils. Engagement of CD31 on T lymphocytes results in co-stimulation of T lymphocyte proliferation to suboptimal doses of anti-CD31 mAb. This proliferation is accompanied by secretion of numerous cytokines and chemokines, up-regulation of CD25 and an increase in cell size. Purification of T lymphocytes into CD45RO and CD45RA subsets showed that only naive CD45RA T lymphocytes are co-stimulated by anti-CD31 mAb. Further studies on neutrophils show that engagement of CD31 results in down-regulation of CD62L and up-regulation of CD11b/CD18 as well as oxidative burst, as assessed by superoxide release. In addition, ligation of CD31 on monocytes results in TNF-alpha secretion, and studies with various cell signaling inhibitors indicate that tyrosine kinases and cAMP-dependent kinases are involved in monocyte activation via CD31. Of the four mAb used in this study, only two activated human leukocytes. These mAb were PECAM-1.3 and hec7, which bind to domains 1 and 2 of CD31. We conclude that engagement of domains 1 and 2 of CD31 results in outside-in signaling in leukocytes.

Cell surface glycosaminoglycans do not serve as ligands for PECAM-1. PECAM-1 is not a heparin-binding protein

Previous studies have suggested that PECAM-1 mediates cellular interactions via both homophilic and heterophilic adhesive mechanisms. Cell surface glycoaminoglycans have been implicated as one of the heterophilic ligands for PECAM-1. To determine whether PECAM-1 is capable of interacting directly with glycosaminoglycans, we examined the adhesive properties of multiple monovalent and multivalent forms of this adhesion molecule. We found that the binding of a bivalent PECAM-1/IgG chimeric protein or multivalent PECAM-1-containing proteoliposomes to multiple different cell lines was 1) strictly dependent upon cell surface expression of PECAM-1 and 2) unaffected by the presence of excess heparin or heparan sulfate. The extracellular domain of PECAM-1 failed to interact specifically with heparin-Sepharose, 3H-labeled heparin, or a heparin-bovine serum albumin conjugate. In addition, an amino acid sequence motif inadvertently created by the juxtaposition of PECAM-1 and IgG sequences within the hinge region of certain PECAM-1/IgG chimeric constructs was found to confer glycosaminoglycan binding properties not normally present within the extracellular domain of the native molecule. Together, these data suggest that the mechanism by which heparin is able to affect PECAM-1-dependent cell-cell adhesion is indirect and occurs via inhibition of events that occur downstream from PECAM-1 engagement.

Role of PECAM-1 (CD31) in neutrophil transmigration in murine models of liver and peritoneal inflammation

Platelet endothelial cell adhesion molecule-1 (PECAM-1) is thought to be critical for transendothelial migration of leukocytes, including neutrophils. Because neutrophil-mediated liver injury during endotoxemia is dependent on transmigration, we investigated the role of PECAM-1 in the pathophysiology of endotoxin-induced liver injury. Male C3Heb/FeJ mice were treated with galactosamine (Gal) and endotoxin (ET) (700 mg/kg Gal/100 micrograms/kg ET), and liver sections were stained for PECAM-1 expression. Control livers showed the presence of PECAM-1 on endothelial cells of large vessels but not in sinusoids. Gal/ET treatment did not change the expression pattern of PECAM-1. Gal/ET-induced liver injury (area of necrosis: 38 +/- 3%) was not attenuated by treatment with 3 mg/kg of the antimurine PECAM-1 antibody 2H8. The antibody had no effect on sequestration and transmigration of neutrophils in sinusoids or the margination of neutrophils in large vessels. In contrast, 2H8 inhibited glycogen-induced neutrophil migration into the peritoneum by 74%; this effect correlated with PECAM-1 expression in the intestinal vasculature. Thus PECAM-1 is neither expressed nor inducible in hepatic sinusoids and is consequently not involved in neutrophil transmigration in the liver during endotoxemia. On the other hand, expression of PECAM-1 in mesenteric veins is critical for peritoneal neutrophil accumulation.

Vascular endothelium of human dental pulp expresses diverse adhesion molecules for leukocyte emigration

The expression of adhesion molecules on the vascular endothelium of healthy and inflamed human dental pulp was studied immunohistochemically using antibodies for selectin family and immunoglobulin super-family members. In healthy dental pulp, some vessels reacted very weakly with major histocompatibility complex (MHC) class II, and E- and P-selectin, and many vessels reacted with platelet-endothelial cell adhesion molecule-1 (PECAM-1), but no vessels reacted with intercellular adhesion molecule-1 (ICAM-1), ICAM-3 and vascular cell adhesion molecule-1 (VCAM-1). In inflamed pulp, a large number of vessels reacted strongly with MHC class II, E- and P-selectin, PECAM-1, ICAM-1, ICAM-3, and VCAM-1. This indicates that the vascular endothelium of the inflamed human dental pulp expresses diverse adhesion molecules for leukocyte emigration from the blood stream into tissue. Dental pulp is surrounded by dentin and isolated from other tissue, like an in vitro system, so dental pulp is an attractive model for studying the roles of adhesion molecules on the vascular endothelium in transendothelial migration of leukocytes.

Tyrosine phosphorylation of platelet endothelial cell adhesion molecule-1 induced by lysophosphatidylcholine in cultured endothelial cells

Lysophosphatidylcholine (lyso-PC), a biologically active phospholipid, appears to modulate various endothelial cell functions through tyrosine kinase-dependent signaling pathways. In cultured bovine aortic endothelial cells (BAEC), we have found that a 130 kDa protein (p130) was rapidly tyrosine phosphorylated within 2 min and sustained for, at least, 1 hr in response to 10 mumol/L of lyso-PC but not to phorbol myristate acetate (PMA). Prolonged preexposure to PMA did not affect lyso-PC-induced p130 tyrosine phosphorylation, suggesting that mechanisms independent of protein kinase C may be involved. Fractionation of the cell lysates revealed that p130 was detectable in the membrane fraction but not in the cytosolic fraction. Immunoprecipitation followed by immunoblotting of lyso-PC-treated BAEC identified p130 as bovine PECAM-1. Tyrosine phosphorylation of PECAM-1 appears to be one of the earliest events elicited by lyso-PC, and may play a role in lyso-PC-induced modulation of endothelial functions.

Requirements for αd in IgG Immune Complex-Induced Rat Lung Injury

αd is a newly cloned adhesion molecule that forms a heterodimer with CD18. The requirement for αd in IgG immune complex-induced lung injury in rats has been evaluated by the use of blocking polyclonal and monoclonal antibodies to rat αd. Using whole lung extracts, Northern and Western blot analyses have revealed up-regulation of mRNA and αd protein in inflamed lungs. Immunostaining has revealed the presence of αd in lung tissue and in alveolar macrophages as early as 1 h after initiation of the inflammatory reaction. When polyclonal rabbit Ab to rat αd was coinstilled into lung together with Ab to BSA, lung injury (as determined by leakage of [125I]albumin into lung parenchyma) was significantly diminished. In parallel, there was reduced accumulation of neutrophils recoverable in bronchoalveolar lavage (BAL) fluids. These findings were associated with reduced levels of TNF-α as well as NO2−/NO3− in BAL fluids. A hamster mAb to rat αd was also protective in this lung injury model. Anti-αd inhibited in vitro production of NO2−/NO3− by rat alveolar macrophages (stimulated with LPS and IFN-γ) by approximately 60%. These data suggest that, in the lung inflammatory model employed, αd up-regulation occurs in lung macrophages and is necessary for expression of TNF-α, recruitment of neutrophils, and full development of lung injury.

Platelet/Endothelial Cell Adhesion Molecule-1 Serves as a Costimulatory Agonist Receptor That Modulates Integrin-Dependent Adhesion and Aggregation of Human Platelets

Platelet/endothelial cell adhesion molecule-1 (PECAM-1) is a 130-kD member of the Ig gene superfamily that is expressed on the surface of circulating platelets, monocytes, neutrophils, and selective T-cell subsets. It is also a major component of the endothelial cell intercellular junction. Previous studies have shown that cross-linking PECAM-1 on the surface of leukocytes results in the activation of adhesion molecules of both the β1 and β2integrin family. In addition, the process of leukocyte transendothelial migration appears to be mediated, at least in part, by homophilic adhesive interactions that take place between leukocyte and endothelial cell junctional PECAM-1 molecules. However, little is known about the functional role of this membrane glycoprotein in human platelets. In the present study, we examined the effects of PECAM-1 engagement on integrin-mediated platelet-extracellular matrix or platelet-platelet interactions. Bivalent, but not monovalent, anti–PECAM-1 monoclonal antibodies (MoAbs) specific for membrane-proximal Ig-homology domain 6 significantly augmented platelet deposition (increased surface coverage) and aggregation (increased average size) onto extracellular matrix, under both oscillatory or defined low shear flow conditions (200 s−1) in a modified cone and plate viscometer. Moreover, bivalent anti-domain 6 MoAbs were capable of serving as costimulatory agonists to markedly enhance both adenosine diphosphate (ADP)- and platelet activating factor (PAF)-induced platelet aggregation responses. These antibodies appeared to act via outside-in signal transduction through PECAM-1, as evidenced by the fact that their binding (1) led to conformational changes in the αIIbβ3 integrin complex, (2) induced surface expression of P-selectin, and (3) resulted in the tyrosine phosphorylation of PECAM-1. Together, these data support a role for PECAM-1 in cellular activation and suggest that PECAM-1 may serve as a costimulatory agonist receptor capable of modulating integrin function in human platelets during adhesion and aggregation.

Platelet/Endothelial Cell Adhesion Molecule-1 Serves as a Costimulatory Agonist Receptor That Modulates Integrin-Dependent Adhesion and Aggregation of Human Platelets

Platelet/endothelial cell adhesion molecule-1 (PECAM-1) is a 130-kD member of the Ig gene superfamily that is expressed on the surface of circulating platelets, monocytes, neutrophils, and selective T-cell subsets. It is also a major component of the endothelial cell intercellular junction. Previous studies have shown that cross-linking PECAM-1 on the surface of leukocytes results in the activation of adhesion molecules of both the β1 and β2integrin family. In addition, the process of leukocyte transendothelial migration appears to be mediated, at least in part, by homophilic adhesive interactions that take place between leukocyte and endothelial cell junctional PECAM-1 molecules. However, little is known about the functional role of this membrane glycoprotein in human platelets. In the present study, we examined the effects of PECAM-1 engagement on integrin-mediated platelet-extracellular matrix or platelet-platelet interactions. Bivalent, but not monovalent, anti–PECAM-1 monoclonal antibodies (MoAbs) specific for membrane-proximal Ig-homology domain 6 significantly augmented platelet deposition (increased surface coverage) and aggregation (increased average size) onto extracellular matrix, under both oscillatory or defined low shear flow conditions (200 s−1) in a modified cone and plate viscometer. Moreover, bivalent anti-domain 6 MoAbs were capable of serving as costimulatory agonists to markedly enhance both adenosine diphosphate (ADP)- and platelet activating factor (PAF)-induced platelet aggregation responses. These antibodies appeared to act via outside-in signal transduction through PECAM-1, as evidenced by the fact that their binding (1) led to conformational changes in the αIIbβ3 integrin complex, (2) induced surface expression of P-selectin, and (3) resulted in the tyrosine phosphorylation of PECAM-1. Together, these data support a role for PECAM-1 in cellular activation and suggest that PECAM-1 may serve as a costimulatory agonist receptor capable of modulating integrin function in human platelets during adhesion and aggregation.

Engagement of human PECAM-1 (CD31) on human endothelial cells increases intracellular calcium ion concentration and stimulates prostacyclin release

Platelet-endothelial cell adhesion molecule-1 (PECAM-1) is a member of the immunoglobulin superfamily that plays a role in a number of endothelial cell (EC) functions including migration, angiogenesis, and transmigration of leukocytes across endothelium. We postulated that one way PECAM-1 might exert its effects was by regulating intracellular EC levels of calcium. Using single-cell fluorometry, we found that engagement of PECAM-1 by mAbs induced a slow but sustained increase in intracellular calcium, both in EC and in an adherent PECAM-1-transfected cell line that models endothelium. Generation of this signal was specific for certain anti-PECAM-1 antibodies, required the presence of the cytoplasmic domain, depended on extracellular calcium and on tyrosine phosphorylation, but did not require cross-linking; in fact, calcium increases were stimulated by certain Fab fragments. Activation of EC by PECAM-1 also caused a time-dependent increase in prostacyclin release. Given the importance of intracellular calcium and prostacyclin release as signaling molecules, engagement of PECAM-1 during cell-cell interactions may alter a number of EC functions including secretion of vasoactive mediators.

The protein-tyrosine phosphatase SHP-2 associates with tyrosine-phosphorylated adhesion molecule PECAM-1 (CD31)

Aggregation of many cell-surface receptors results in tyrosine phosphorylation of numerous proteins. We previously observed the tyrosine phosphorylation of the platelet/endothelial cell adhesion molecule, PECAM-1 (CD31), after FcepsilonRI stimulation in rat basophilic leukemia RBL-2H3 cells. Here we found that PECAM-1 was also transiently tyrosine-phosphoryated after adherence of these cells to fibronectin. Similarly aggregation of the T cell receptor on Jurkat cells also induced this tyrosine phosphorylation. The protein-tyrosine phosphatase SHP-2 is a widely expressed cytosolic enzyme with two Src homology 2 (SH2) domains. SHP-2, but not the related protein-tyrosine phosphatase SHP-1, associated with PECAM-1. This association of the two proteins correlated with the extent of the tyrosine phosphorylation of PECAM-1. A fusion protein containing the two SH2 domains of SHP-2 precipitated PECAM-1 from cell lysates and also directly bound to phosphorylated PECAM-1. In immune precipitate phosphatase assays, there was tyrosine dephosphorylation of PECAM-1. Therefore, integrin and immune receptor activation results in tyrosine phosphorylation of PECAM-1 and the binding of the protein-tyrosine phosphatase SHP-2, which could regulate receptor-mediated signaling in cells.

Tyrosine residue in exon 14 of the cytoplasmic domain of platelet endothelial cell adhesion molecule-1 (PECAM-1/CD31) regulates ligand binding specificity

Platelet/endothelial cell adhesion molecule (PECAM-1) is a cell adhesion molecule of the immunoglobulin superfamily that plays a role in a number of vascular processes including leukocyte transmigration through endothelium. The presence of a specific 19- amino acid exon within the cytoplasmic domain of PECAM-1 regulates the binding specificity of the molecule; specifically, isoforms containing exon 14 mediate heterophilic cell-cell aggregation while those variants missing exon 14 mediate homophilic cell-cell aggregation. To more precisely identify the region of exon 14 responsible for ligand specificity, a series of deletion mutants were created in which smaller regions of exon 14 were removed. After transfection into L cells, they were tested for their ability to mediate aggregation. For heterophilic aggregation to occur, a conserved 5-amino acid region (VYSEI in the murine sequence or VYSEV in the human sequence) in the mid-portion of the exon was required. A final construct, in which this tyrosine was mutated into a phenylalanine, aggregated in a homophilic manner when transfected into L cells. Inhibition of phosphatase activity by exposure of cells expressing wild type or mutant forms of PECAM-1 to sodium orthovanadate resulted in high levels of cytoplasmic tyrosine phosphorylation and led to a switch from heterophilic to homophilic aggregation. Our data thus indicate either loss of this tyrosine from exon 14 or its phosphorylation results in a change in ligand specificity from heterophilic to homophilic binding. Vascular cells could thus determine whether PECAM-1 functions as a heterophilic or homophilic adhesion molecule by processes such as alternative splicing or by regulation of the balance between tyrosine phosphorylation or dephosphorylation. Defining the conditions under which these changes occur will be important in understanding the biology of PECAM-1 in transmigration, angiogenesis, development, and other processes in which this molecule plays a role.

Oxidant stress-induced transendothelial migration of monocytes is linked to phosphorylation of PECAM-1

Reactive oxygen species (ROS) are believed to cause vascular injury in the pathophysiology of atherosclerosis, diabetes, and vasoocclusion in sickle cell disease. Studies have shown that ROS causes increased adhesion of monocytes and neutrophils to the endothelium. We investigated the effects of tert-butylhydroperoxide (t-BuOOH), an inducer of oxidant stress, to determine the cellular signaling pathway leading to the transendothelial migration of polymorphonuclear leukocytes. Our studies revealed that signaling by t-BuOOH in human umbilical vein endothelial cells (HUVECs) causes a twofold increase in the transendothelial migration of monocyte-like HL-60 cells and a fivefold increase in platelet endothelial cell adhesion molecule-1 (PECAM-1) phosphorylation. The transmigration induced by t-BuOOH was inhibited by an antibody to PECAM-1. These events were inhibited by antioxidants and inhibitors of protein kinase C, p21ras and glutathione synthesis. However, treatment of HUVECs with the phosphatase inhibitor calyculin A augmented the t-BuOOH-mediated transendothelial migration of monocytes and PECAM-1 phosphorylation. Our results suggest that oxidative stress can induce the transendothelial migration of monocytes as a result of phosphorylation of PECAM-1, a crucial event in the diapedesis of leukocytes during pathophysiology of vascular diseases.

Residues on both faces of the first immunoglobulin fold contribute to homophilic binding sites of PECAM-1/CD31

CD31 (PECAM-1) is a member of the immunoglobulin superfamily whose extracellular domain is comprised of six immunoglobulin-like domains. It is widely expressed on endothelium, platelets, around 50% of lymphocytes, and cells of myeloid lineage. CD31 has been shown to be involved in interendothelial adhesion and leukocyte-endothelial interactions, particularly during transmigration. CD31-mediated adhesion is complex, because CD31 is capable of mediating both homophilic and multiple heterophilic adhesive interactions. Here we show that the NH2-terminal (membrane-distal) immunoglobulin domain of CD31 is necessary but not sufficient to support stable homophilic adhesion. Key residues forming the binding site within this domain have been identified by analysis of 26 single point mutations, representing the most systematic analysis of a fully homophilic interaction between immunoglobulin superfamily family members to date. This revealed five mutations that affect homophilic binding. Uniquely, the residues involved are exposed on both faces of the immunoglobulin fold, leading us to propose a novel mechanism for CD31 homophilic adhesion.

Diabetic RBC-induced oxidant stress leads to transendothelial migration of monocyte-like HL-60 cells

Red blood cells (RBC) from patients with diabetes mellitus exhibit an increased propensity to adhere to cultured human umbilical vein endothelial cells (HUVEC) as a result of interaction of advanced glycation end products with their counter receptors, contributing to the pathogenesis of vascular complications. We determined whether the interaction of diabetic RBC with HUVEC induced cellular oxidant stress that would culminate in adherence and diapedesis of monocytes, these being initiating events in endothelial injury and atherogenesis. We show that the adherence of diabetic RBC (2% hematocrit), but not normal RBC, to HUVEC results in a fourfold increase in the production of lipid peroxides. Furthermore, diabetic RBC-induced oxidant stress causes a sixfold increase in platelet endothelial cell adhesion molecule-1 (PECAM-1) phosphorylation and doubles transendothelial migration of monocyte-like HL-60 cells; both are blocked by antioxidants and protein kinase C (PKC) inhibitors. Our results show that the adherence of diabetic RBC to endothelial cells initiates a cascade of cellular events resulting in PKC activation, causing PECAM-1 phosphorylation and concomitant transendothelial migration of monocytes. The increased diapedesis of monocytes, brought about by the interaction of diabetic RBC across vascular endothelium, may play an important role in accelerated atherosclerosis and cardiovascular disease in diabetics.

The leukocyte cell adhesion cascade and its role in myocardial ischemia-reperfusion injury

Cell-cell and cell-matrix interactions are known to be mediated by specific cell adhesion receptors expressed on the cell surface. The characterization of these cell adhesion molecules has allowed researchers to examine their roles in a variety of physiologic and pathophysiologic conditions. Numerous studies have demonstrated that myocardial ischemia-reperfusion injury is an acute inflammatory process in which leukocytes are intimately involved. In this review, we summarize the current data on the leukocyte cell adhesion cascade, focus upon studies which have demonstrated specific cell adhesion molecule interactions which mediate the leukocyte involvement in myocardial ischemia-reperfusion injury and suggest future avenues of exploration and possible clinical implications of the studies reviewed.

Molecular Mechanisms of Monocyte Adhesion to Interleukin-1β–Stimulated Endothelial Cells Under Physiologic Flow Conditions

This study identifies multiple pathways used by monocytes to adhere to 4-hour interleukin-1β stimulated human umbilical vein endothelial cells under flow conditions. Physiologic shear stresses were simulated in a flow chamber with parallel plate geometry; quantitation of primary adhesion, secondary adhesion, and transmigration was performed using phase contrast videomicroscopy. Neuraminidase treatment of monocytes reduced primary interaction by 50%, whereas blocking L-selectin or very late antigen-4 showed significant but smaller effects (∼30% inhibition). However, a combined treatment against all three pathways was able to reduce interaction by 80%. Blocking β2 and α4 integrin pathways together inhibited secondary/firm adhesion by 75%. Only 40% of firmly adherent monocytes transmigrated across the endothelial monolayer with significantly increased transmigration times when both β2 and α4 integrins were blocked. These results demonstrate that monocytes can use multiple receptors to interact with endothelial cells at both primary and secondary adhesion stages, and that these pathways have to be blocked simultaneously for maximum inhibition.

Aggregation of the high affinity IgE receptor results in the tyrosine phosphorylation of the surface adhesion protein PECAM-1 (CD31)

One of the earliest events after aggregation of the high affinity receptor for IgE (FcepsilonRI) on mast cells is the activation of protein tyrosine kinases resulting in tyrosine phosphorylation of numerous proteins. Using a monoclonal antibody raised against the rat basophilic leukemia RBL-2H3 cells, we identified that platelet/endothelial cell adhesion molecule 1 (PECAM-1 or CD31) was tyrosine phosphorylated in these cells. Aggregation of PECAM-1 did not induce a detectable increase in its tyrosine phosphorylation, nor did it result in degranulation. However, the minimal tyrosine phosphorylation of PECAM-1 in nonstimulated cells was dramatically increased after FcepsilonRI aggregation. This receptor-induced tyrosine phosphorylation of PECAM-1 was an early event, independent of Ca2+ influx or of the activation of protein kinase C and of cell adhesion. PECAM-1 is an adhesion molecule that is required for the transmigration of leukocytes across the endothelium into sites of inflammation. Therefore tyrosine phosphorylation of PECAM-1 may modulate its interaction with other molecules, thereby regulating the migration of basophils into inflammatory sites.

Role of cell adhesion molecules in inflammatory bowel diseases

Infiltration of leukocytes into the bowel wall is a landmark of the inflammatory bowel diseases (IBD) ulcerative colitis (UC) and Crohn's disease (CD). The leukocyte movement is dependent on physical contact (adhesion) between the leukocytes and activated endothelial cells and can be divided into capturing, rolling, leukocyte flattening, and extra-vasation. The molecules shown to form the basis of leukocyte-endothelial binding are referred to as cell adhesion molecules (CAMs). Several of these molecules have additional properties, including interaction between leukocytes and proteins in the extracellular matrix, collaen in basement membranes, and stromal cells in lymphoid tissue and bone marrow. Furthermore, studies have indicated that CAMs interfere with the tumor cell's ability to metastasize. This paper will focus on a description of those CAMs that are either known or believed to be involved in the pathogenesis of IBD. Investigations of the presence and functions of these CAMs in IBD is reviewed, and potential new treatments are discussed.

Soluble domain 1 of platelet-endothelial cell adhesion molecule (PECAM) is sufficient to block transendothelial migration in vitro and in vivo

The inflammatory response involves sequential adhesive interactions between cell adhesion molecules of leukocytes and the endothelium. Unlike the several adhesive steps that precede it, transendothelial migration (diapedesis), the step in which leukocytes migrate between apposed endothelial cells, appears to involve primarily one adhesion molecule, platelet-endothelial cell adhesion molecule (PECAM, CD31). Therefore, we have focused on PECAM as a target for antiinflammatory therapy. We demonstrate that soluble chimeras made of the entire extracellular portion of PECAM, or of only the first immunoglobulin domain of PECAM, fused to the Fc portion of IgG, block diapedesis in vitro and in vivo. Furthermore, the truncated form of the PECAM-IgG chimera does not bind stably to its cellular ligand. This raises the possibility of selective anti-PECAM therapies that would not have the untoward opsonic or cell-activating properties of antibodies directed against PECAM.

The protein-tyrosine phosphatase SHP-2 binds platelet/endothelial cell adhesion molecule-1 (PECAM-1) and forms a distinct signaling complex during platelet aggregation. Evidence for a mechanistic link between PECAM-1- and integrin-mediated cellular signaling

Platelet/endothelial cell adhesion molecule-1 (PECAM-1) is a homophilic adhesion receptor that mediates leukocyte/endothelial cell interactions that take place during transendothelial migration. Recent reports have shown that the binding of certain anti-PECAM-1 antibodies results in up-regulation of integrin function on the surface of leukocytes and platelets, suggesting that PECAM-1 may be capable of transmitting information into the cell following its engagement. PECAM-1 isolated from resting or activated but nonaggregated platelets was phosphorylated predominantly on serine residues; however, PECAM-1 derived from activated, aggregated platelets was strongly phosphorylated on tyrosine. Synthetic tyrosine-phosphorylated peptides derived from five different regions within the cytoplasmic domain of PECAM-1 were screened for their ability to associate with cytoplasmic signaling molecules. The protein-tyrosine phosphatase SHP-2 was found to interact specifically with two different PECAM-1 phosphopeptides containing highly conserved phosphatase-binding motifs on PECAM-1 with the sequences VQpY663TEV and TVpY686SEV. More important, SHP-2 bound not only PECAM-1 phosphopeptides, but also became associated with full-length cellular PECAM-1 during the platelet aggregation process, and this interaction was mediated by the amino-terminal Src homology 2 domains of the phosphatase. Since SHP-2 normally serves as a positive regulator of signal transduction, its association with activated PECAM-1 suggests a number of potential mechanisms by which PECAM-1 engagement might be coupled to integrin activation in vascular cells.

VE-cadherin antibody accelerates neutrophil recruitment in vivo

Neutrophils enter sites of inflammation by crossing the endothelial lining of the blood vessel wall. VE-cadherin is an endothelial specific, homophilic adhesion molecule located at the lateral cell surface. We have generated a monoclonal antibody against mouse VE-cadherin which inhibits electrical resistance of endothelial cell monolayers in vitro as well as aggregation of VE-cadherin transfected cells. In vivo, this antibody was found to increase vascular permeability and to accelerate the entry of neutrophils into chemically inflamed mouse peritoneum. Thus, VE-cadherin is essential for the integrity of the endothelial barrier in vivo. Our data suggest that opening of VE-cadherin mediated endothelial cell contacts may be a relevant step during neutrophil extravasation.

Characterization of the Human Platelet/Endothelial Cell Adhesion Molecule-1 Promoter: Identification of a GATA-2 Binding Element Required for Optimal Transcriptional Activity

Platelet/endothelial cell adhesion molecule-1 (PECAM-1) is a 130-kD member of the Ig gene superfamily that is expressed on platelets, endothelial cells, and certain leukocyte subsets. To examine the factors controlling vascular-specific expression of PECAM-1, we cloned the 5′-flanking region of the PECAM-1 gene and analyzed its transcriptional activity. 5′-Rapid amplification of cDNA ends (5′-RACE) analysis showed that transcription initiation occurred at several closely spaced nearby sites originating approximately 204 bp upstream from the translation start site. Analysis of the sequence immediately upstream from the transcription initiation site (TIS) showed no canonical TATA or CAAT elements, however an initiator element commonly found in TATA-less promoters encompassed the TIS. 5′-serially truncated PECAM-1 promoter segments cloned in front of a luciferase reporter drove transcription in both a lineage- and orientation-specific manner. Putative cis-acting control elements present within a 300-bp core promoter included two ets sites, an Sp1 site, tandem E-box domains, two GATA-associated sites (CACCC), an AP-2 binding site, and a GATA element at −24. Mutational analysis showed that optimal transcriptional activity required the GATA sequence at position −24, and gel-shift assays further showed that the GATA-2 transcription factor, but not GATA-1, bound to this region of the PECAM-1 promoter. Understanding the cis- and trans-acting factors that regulate the tissue-specific expression of PECAM-1 should increase our understanding of the mechanisms by which vascular-specific gene expression is achieved.

Administration of a CD31-Derived Peptide Delays the Onset and Significantly Increases Survival From Lethal Graft-Versus-Host Disease

The CD31 monoclonal antibody, LYP21, binds to the CD31 domain 6 and inhibits the human mixed-lymphocyte reaction (MLR) in a specific and dose-dependent fashion. A synthetic CD31 peptide based on human CD31 epitope (amino acids 551 to 574) recognized by LYP21 is equally effective in inhibiting the MLR. In this study, we used the murine homolog of CD31 peptide 551 to 574 and a control peptide to study the role of CD31 molecule on T-cell activation. In vitro, CD31 peptide inhibited the MLR across several major and minor histocompatibility differences in a specific and dose-dependent fashion, similar to the results observed in the human system. Maximal inhibition was achieved at a dose of 200 μg/mL. In the cytotoxic T-lymphocyte (CTL) assay, CD31 peptide inhibited CTL responses by 97%. To study the in vivo effect of this peptide, graft-versus-host disease (GVHD) across minor histocompatibility barriers was induced in the B10.D2 (H-2d) → BALB/c (H-2d) model. BALB/c recipients received CD31 peptide (100 μg/d), or phosphate-buffered saline (PBS), or control peptide (100 μg/d) intraperitoneally (IP) for the first 5 weeks. CD31 peptide delayed onset of graft-versus-host disease and significantly increased long-term survival. Twelve of 14 mice receiving CD31 peptide survived more than 100 days after transplantation, as compared with none of 10 mice receiving PBS and none of five mice receiving control peptide (P = .0001). Long-term engraftment of allogeneic bone marrow was documented in all transplanted mice by polymerase chain reaction (PCR) analysis of microsatellite region in the interleukin (IL)-1β gene. Our data suggest that the CD31 molecule has an important functional role in T-cell activation in vitro and in vivo.

Angiogenesis as a biologic and prognostic indicator in human breast carcinoma

In this review we describe angiogenesis pathways involved in the development of breast carcinoma. Different assessment techniques for angiogenesis and their optimisation are discussed. Angiogenesis is an important factor for prognosis and will be increasingly important in therapeutic decisions.

Cell adhesion molecules--update

Cell adhesion molecules are glycoproteins expressed on the cell surface and play an important role in inflammatory as well as neoplastic diseases. There are four main groups: the integrin family, the immunoglobulin superfamily, selectins, and cadherins. The integrin family has eight subfamilies, designated as beta 1 through beta 8. The most widely studied subfamilies are beta 1 (CD29, very late activation [VLA] members), beta 2 (leukocyte integrins such as CD11a/CD18, CD11b/CD18, CD11c/CD18, and alpha d beta 2), beta 3 (CD61, cytoadhesions), and beta 7 (alpha 4 beta 7 and alpha E beta 7). The immunoglobulin superfamily includes leukocyte function antigen-2 (LFA-2 or CD2), leukocyte function antigen-3 (LFA-3 or CD58), intercellular adhesion molecules (ICAMs), vascular adhesion molecule-1 (VCAM-1), platelet-endothelial cell adhesion molecule-1 (PE-CAM-1), and mucosal addressin cell adhesion molecule-1 (MAdCAM-1). The selectin family includes E-selectin (CD62E), P-selectin (CD62P), and L-selectin (CD62L). Cadherins are major cell-cell adhesion molecules and include epithelial (E), placental (P), and neural (N) subclasses. The binding sites (ligands/receptors) are different for each of these cell adhesion molecules (e.g., ICAM binds to CD11/CD18; VCAM-1 binds to VLA-4). The specific cell adhesion molecules and their ligands that may be involved in pathologic conditions and potential therapeutic strategies by modulating the expression of these molecules will be discussed.

Antibody to platelet/endothelial cell adhesion molecule-1 reduces myocardial infarct size in a rat model of ischemia-reperfusion injury

BACKGROUND

Antibodies to selected neutrophil or endothelial cell adhesion molecules decrease myocardial infarct size in vivo. Platelet/endothelial cell adhesion molecule-1 (PECAM-1) is an immunoglobulin gene superfamily member expressed constitutively on neutrophils and endothelium. F(ab')2 fragments of antibody against PECAM-1 inhibit transendothelial migration of neutrophils in several in vivo models of acute inflammation. Therefore, we examined the effect of F(ab')2 fragments of anti-PECAM-1 antibody in a rat model of myocardial infarction.

METHODS AND RESULTS

F(ab')2 fragments of the anti-PECAM-1 antibody SEW16 and control normal rabbit IgG (NRIgG) were administered at 5 mg/kg to male Wistar rats, and the rats were subjected to a 30-minute coronary artery occlusion followed by 2 hours of reperfusion. At the completion of each experiment, the area at risk, infarct size (IS), and myeloperoxidase (MPO) activity were determined. Compared with untreated (n = 8; IS, 57 +/- 5%) or NRIgG-treated (n = 10; IS, 62 +/- 3%) control rats, SEW16-treated rats (n = 15; IS, 28.5 +/- 4%) displayed a 54% decrease in myocardial infarct size (P < .001). Hemodynamic parameters, leukocyte counts, total left ventricular weight, and area-at-risk weights did not differ significantly between the treatment groups. However, measurement of MPO activity revealed that neutrophil accumulation was reduced 83% (NRIgG, 975 +/- 55 mU/g; SEW16, 167 +/- 62 mU/g).

CONCLUSIONS

These results demonstrate that blocking PECAM-1 exerts a significant protective effect in a rat model of myocardial ischemia-reperfusion injury via blockade of neutrophil accumulation in the myocardium.

Effects of pulse methylprednisolone on cell adhesion molecules in the synovial membrane in rheumatoid arthritis. Reduced E-selectin and intercellular adhesion molecule 1 expression

OBJECTIVE

To investigate the effects of a 1,000-mg intravenous pulse of methylprednisolone succinate (MP) on cell adhesion molecule expression on the synovial vascular endothelium in patients with rheumatoid arthritis (RA).

METHODS

Sequential arthroscopic biopsy samples were taken before and 24 hours after MP administration (10 patients) and at the time of RA flare (2 patients) and after retreatment with MP (1 patient). Immunoperoxidase staining for E-selectin (CD62E), P-selectin (CD62P), intercellular adhesion molecule 1 (ICAM-1; CD54) and platelet-endothelial cell adhesion molecule (PECAM; CD31) was performed, and the staining was quantified by color video image analysis.

RESULTS

MP caused a rapid (within 24 hours) and substantial decrease in the expression of E-selectin on the synovial vascular endothelium, with a smaller reduction in ICAM-1 expression on synovial vascular endothelium and the synovial lining. There were no similar effects on synovial membrane P-selectin or PECAM expression.

CONCLUSION

A potential mechanism by which MP impairs neutrophil trafficking into inflamed RA joints might be by reducing E-selectin, and possibly, ICAM-1, expression in the synovial membrane.

Role of complement, chemokines, and regulatory cytokines in acute lung injury

The roles of complement, proinflammatory cytokines and regulatory cytokines in lung inflammatory injury are becoming defined. Like the proinflammatory cytokines (TNF alpha and IL-1), complement activation products (C5a and/or the membrane attack complex, C5b-9) can directly activate endothelial cells to cause upregulation of adhesion molecules (P-selectin) or can function in a synergistic manner with TNF alpha to cause enhanced upregulation of ICAM-1 and E-selectin. The beta chemokine, MIP-1 alpha, appears to function in vivo as an autocrine activator, enhancing TNF alpha production by pulmonary macrophages, which, in turn, enhances the inflammatory response. Finally, IL-4 and IL-10 have strong regulatory effects by suppressing in vivo production of TNF alpha. There is now compelling evidence to suggest that, in IgG immune-complex-induced lung inflammation in rats, endogenous IL-10 is produced and regulates the intensity of the inflammatory response. Blocking of endogenous IL-10 substantially increases lung TNF alpha production, the recruitment of neutrophils, and the intensity of lung inflammatory injury. Accordingly, the network of cytokines carefully regulates lung inflammatory responses.

Glucose-induced transmigration of monocytes is linked to phosphorylation of PECAM-1 in cultured endothelial cells

The adherence of circulating monocytes to the endothelium, their migration into the subendothelium, and the subsequent formation of foam cells are initial events in the pathogenesis of atherosclerosis. However, the effect of hyperglycemia on the transendothelial migration of monocytes is not known. Exposure of human umbilical vein endothelial cells (HUVEC) cultured in a Transwell chamber to 25 mM D-glucose (a concentration representing a hyperglycemic state) for 2 h resulted in a twofold increase in the migration of vitamin D3-differentiated monocyte-like HL-60 cells. The migration was inhibited by addition of either an antibody to platelet-endothelial cell adhesion molecule-1 (PECAM-1) or a protein kinase C inhibitor, GF-109203X. In HUVEC, high concentrations of D-glucose (25 mM), but not of other sugars such as L-glucose, 2-deoxyglucose, D-galactose, or D-mannitol, caused a sevenfold increase in the phosphorylation of PECAM-1 as a result of activation of protein kinase C. The 25 mM D-glucose-induced PECAM-1 phosphorylation and transmigration of monocyte-like HL-60 cells were further increased by treatment of HUVEC with the phosphatase inhibitor calyculin A. These results suggest that direct phosphorylation of PECAM-1 in response to elevated glucose promotes transendothelial migration of monocytes, contributing to accelerated atherogenesis in diabetics.

Interferon gamma regulates platelet endothelial cell adhesion molecule 1 expression and neutrophil infiltration into herpes simplex virus-infected mouse corneas

In a mouse model of herpes simplex virus (HSV) 1 corneal infection, tissue destruction results from a CD4+ T cell-mediated chronic inflammation, in which interleukin 2 and interferon (IFN) gamma are requisite inflammatory mediators and polymorphonuclear leukocytes (PMN) are the predominant infiltrating cells. In vivo neutralization of IFN-gamma relieved inflammation at least in part through a specific block of PMN extravasation into HSV-1-infected corneas. Intercellular adhesion molecule (ICAM) 1 and platelet endothelial cell adhesion molecule (PECAM) 1 were upregulated on the vascular endothelium of inflamed corneas. Reduced PMN extravasation in anti-IFN-gamma-treated mice was associated with a dramatic reduction of PECAM-1 but not ICAM-1 expression on vascular endothelium. PMN accumulated in the lumen of corneal vessels after in vivo IFN-gamma neutralization. PECAM-1 was readily detectable on PMN inside the vessels but was not detectable on PMN that extravasated into the infected cornea. Moreover, flow cytometric analysis revealed reduced PECAM-1 expression but elevated major histocompatibility complex class I expression on PMN that recently extravasated into the peritoneal cavity when compared with PMN in the peripheral blood. We conclude that IFN-gamma contributes to HSV-1-induced corneal inflammation by facilitating PMN infiltration; this appears to be accomplished through upregulation of PECAM-1 expression on the vascular endothelium; and PMN downregulate PECAM-1 expression during the process of extravasation.

Platelet endothelial cell adhesion molecule-1 (PECAM-1) homophilic adhesion is mediated by immunoglobulin-like domains 1 and 2 and depends on the cytoplasmic domain and the level of surface expression

PECAM-1/CD31 is vascular cell adhesion and signaling molecule of the Ig superfamily that plays a role in neutrophil recruitment at inflammatory sites and may be involved the release of leukocytes from the bone marrow and in cardiovascular development. The interactions of PECAM-1 with its ligands are complex in that it is able to bind both with itself (homophilic adhesion) or with non-PECAM-1 ligands (heterophilic adhesion). Although the factors that regulate ligand binding are not fully understood, these interactions are regulated in part by its large cytoplasmic domain, a region of 118 amino acids encoded by 8 exons of its gene (exons 9-16). The purpose of this work was to better define the mechanisms of PECAM-1-dependent homophilic adhesion by analyzing the binding interactions of L-cells expressing full-length and selectively mutated forms of human, murine, and human/murine chimeric PECAM-1 molecules in an established aggregation assay. These studies demonstrate that 1) the minimal length of the cytoplasmic domain required for cellular aggregation is represented within the sequences encoded by exons 9 and 10, 2) removal or addition of the sequences encoded by exon 14 from the cytoplasmic domain can determine whether the mechanism of aggregation is a heterophilic calcium-dependent process or a homophilic calcium-independent process, 3) high levels of surface expression of PECAM-1 on the cell surface change the mechanism of aggregation from heterophilic to homophilic, and 4) PECAM-1-dependent homophilic binding appears to involve the direct interaction of only the first two extracellular Ig-like domains. These data suggest that PECAM-1-ligand interactions can be regulated through multiple pathways including alterations of the cytoplasmic domain and the level of surface expression.

Cytokine-induced increases in endothelial permeability occur after adhesion molecule expression

BACKGROUND

Transmigration of neutrophils (PMNs) through endothelial cell tight junctions is a critical stage in the tissue injury of ischemia-reperfusion (I/R). Although cytokines are released in I/R, it is unclear whether cytokines directly increase permeability or this phenomenon requires both expression of cell adhesion molecules and PMN adhesion-activation.

METHODS

We exposed confluent monolayers of human umbilical vein endothelial cells to physiologic concentrations of interleukin-1 (10 pg/ml) and tumor necrosis factor-alpha (10 pg/ml) in the absence of PMNs. Tight junction permeability was quantified with both transendothelial electrical resistance and albumin flux, whereas expression of endothelial-leukocyte adhesion molecule-1 was measured by flow cytometry (t test p < 0.05).

RESULTS

Stimulation with tumor necrosis factor-alpha or interleukin-1 produced maximal transendothelial electrical resistance decreases at 12 hours with return to baseline at 24 hours. Increases in albumin flux began at 6 hours, with maximum effects at 24 hours. These changes occurred soon after maximal expression of endothelial-leukocyte adhesion molecule-1 at 4 hours.

CONCLUSIONS

Cytokines induced increases in both cell adhesion molecule expression and endothelial permeability. This sequence of events is consistent with direct cytokine effects on cytoarchitecture, because it occurred without the adhesion-activation of PMNs.

An anti-platelet-endothelial cell adhesion molecule-1 antibody inhibits leukocyte extravasation from mesenteric microvessels in vivo by blocking the passage through the basement membrane

Platelet-endothelial cell adhesion molecule-1 (PECAM-1, CD31) plays an active role in the process of leukocyte migration through cultured endothelial cells in vitro and anti-PECAM-1 antibodies (Abs) inhibit accumulation of leukocytes into sites of inflammation in vivo. Despite the latter, it is still not clear at which stage of leukocyte emigration in vivo PECAM-1 is involved. To address this point directly, we studied the effect of an anti-PECAM-1 Ab, recognizing rat PECAM-1, on leukocyte responses within rat mesenteric microvessels using intravital microscopy. In mesenteric preparations activated by interleukin (IL)-1 beta, the anti-PECAM-1 Ab had no significant effect on the rolling or adhesion of leukocytes, but inhibited their migration into the surrounding extravascular tissue in a dose-dependent manner. Although in some vessel segments these leukocytes had come to a halt within the vascular lumen, often the leukocytes appeared to be trapped within the vessel wall. Analysis of these sections by electron microscopy revealed that the leukocytes had passed through endothelial cell junctions but not the basement membrane. In contrast to the effect of the Ab in mesenteric preparations treated with IL-1 beta, leukocyte extravasation induced by topical or intraperitoneal administration of the chemotactic peptide formyl-methionyl-leucyl-phenylalanine was not inhibited by the anti-PECAM-1 Ab. These results directly demonstrate a role for PECAM-1 in leukocyte extravasation in vivo and indicate that this involvement is selective for leukocyte extravasation elicited by certain inflammatory mediators. Further, our findings provide the first in vivo indication that PECAM-1 may have an important role in triggering the passage of leukocytes through the perivascular basement membrane.

Interaction of CD31 with a heterophilic counterreceptor involved in downregulation of human T cell responses

CD31 is a 130-kD glycoprotein of the immunoglobulin (Ig) superfamily expressed on the surface of endothelial cells, platelets, and several leukocyte subsets. Previous reports indicated that CD31 can mediate intercellular adhesion via both homophilic and heterophilic interaction mechanisms. Using a soluble recombinant CD31-Ig fusion protein (CD31 receptor globulin [Rg]), we demonstrate here that human CD31- T lymphocytes and CD4+CD31- T cell clones express a heterophilic CD31 ligand that is upregulated 18 h after activation. Interaction of CD31Rg with CD31- T helper cell (Th) clones was divalent cation independent but could be blocked by heparin, thus indicating that the CD31 counterreceptor on T cells can be distinguished from the ligands identified on other cell types. Moreover, a single chain protein of 120 kD was precipitated by CD31Rg from the lysates of CD31- Th clones. CD31Rg completely downregulated the proliferative response and cytokine production (interleukin-4, interferon-gamma, and tumor necrosis factor-alpha) of CD31- Th clones when the cells were maximally stimulated via immobilized CD3 monoclonal antibody. These results suggest that interaction of CD31 with a heterophilic counterreceptor on T lymphocytes can interfere with a positive regulatory pathway of T cell activation, or directly signal T cells to downregulate immune function.

Drug-induced in vitro inhibition of neutrophil-endothelial cell adhesion

1. Leukocyte-endothelial cell interactions play an important role during ischaemia-reperfusion events. Adhesion molecules are specifically implicated in this interaction process. 2. Since defibrotide has been shown to be an efficient drug in reducing damage due to ischaemia-reperfusion in many experimental models, we analysed the effect of defibrotide in vitro on leukocyte adhesion to endothelial cells in basal conditions and after their stimulation. 3. In basal conditions, defibrotide (1000 micrograms ml-1) partially inhibited leukocyte adhesion to endothelial cells by 17.3% +/- 3.6 (P < 0.05), and after endothelial cell stimulation (TNF-alpha, 500 u ml-1) or after leukocyte stimulation (fMLP, 10(-7) M), it inhibited leukocyte adhesion by 26.5% +/- 3.4 and 32.4% +/- 1.8, respectively (P < 0.05). 4. In adhesion blockage experiments, the use of the monoclonal antibody anti-CD31 (5 micrograms ml-1) did not demonstrate a significant inhibitory effect whereas use of the monoclonal antibody anti-LFA-1 (5 micrograms ml-1) significantly interfered with the effect of defibrotide. 5. This result was confirmed in NIH/3T3-ICAM-1 transfected cells. 6. We conclude that defibrotide is able to interfere with leukocyte adhesion to endothelial cells mainly in activated conditions and that the ICAM-1/LFA-1 adhesion system is involved in the defibrotide mechanism of action.

Lipoxygenase metabolites induced expression of adhesion molecules and transendothelial migration of monocyte-like HL-60 cells is linked to protein kinase C activation

Studies have shown that, among lipoxygenase metabolites examined, 15(S)-hydroperoxy-5,8,11,13-eicosa-tetraenoic acid (15[S]-HPETE), at micromolar concentrations, selectively causes injury to cultured endothelial cells. We investigated whether physiologically relevant concentrations of lipoxygenase metabolites affected the expression of cell adhesion molecules (CAMs) involved in the adhesion of leukocytes and/or the accumulation of leukocytes in the vascular endothelium, these being the initial events in endothelial cell injury. Among lipoxygenase metabolites, 15(S)-HPETE and 12(S)-HETE, at nanomolar concentrations, induced surface expression of a subset of cell adhesion molecules (CAM), ICAM-1, ELAM-1, and VCAM-1, in human umbilical vein endothelial cells (HUVEC), which is associated with an increased binding activity of the transcription factor, NF-kappa B, to the consensus motif common to the CAM genes in the HUVEC nuclear extracts. Furthermore, 15(S)-HPETE (1 nM) caused a threefold increase in the rate of transendothelial migration of vitamin D3-differentiated HL-60 monocyte-like cells and showed a thirtyfold increase in the phosphorylation of PECAM-1, an adhesion molecule involved in endothelial cell-cell adhesion. Both an antibody to PECAM-1 and the protein kinase C inhibitor, GF 109203X, reduced 15(S)-HPETE-induced transmigration of monocyte-like HL-60 cells by approximately 75% and 85%, respectively. Treatment of HUVEC with a phosphatase inhibitor, calyculin A, augmented both the phosphorylation of PECAM-1 and transmigration of monocyte-like HL-60 cells induced by 15(S)-HPETE. Our results show that 15(S)-HPETE, at physiological concentrations, induced activation of protein kinase C in HUVEC and leads to the phosphorylation of PECAM-1, thus facilitating the migration of monocyte-like HL-60 cells across the endothelial cell monolayer. It is suggested that phosphorylation/dephosphorylation events in PECAM-1 are important in regulating the trafficking of monocytes across the endothelial cell monolayer.

Nitric oxide production and perivascular nitration in brain after carbon monoxide poisoning in the rat

Nitric oxide is a short-lived free radical and physiological mediator which has the potential to cause cytotoxicity. Studies were conducted to investigate whether nitric oxide, and the potent oxidant peroxynitrite, were generated in brain during experimental carbon monoxide (CO) poisoning in the rat. Nitric oxide production was documented by electron paramagnetic resonance spectroscopy, and found to be increased by ninefold immediately after CO poisoning. Evidence that peroxynitrite was generated was sought by looking for nitrotyrosine in the brains of CO-poisoned rats. Nitrotyrosine was found deposited in vascular walls, and also diffusely throughout the parenchyma in inummocytochemical studies. The affinity and specificity of an anti-nitrotyrosine antibody was investigated and a solid phase immunoradiochemical assay was developed to quantity nitrotyrosine in brain homogenates. A 10-fold increase in nitrotyrosine was found in the brains of CO-poisoned rats. Platelets were involved with production of nitrotyrosine in the early phase of exposure to CO. However, nitrotyrosine formation and leukocyte sequestration were not decreased in thrombocytopenic rats poisoned with CO according to the standard model. When rats were pre-treated with the nitric oxide synthase inhibitor, L-nitroarginine methyl ester, formation of both nitric oxide and nitrotyrosine in response to CO poisoning were abolished, as well as leukocyte sequestration in the microvasculature, endothelial xanthine dehydrogenase conversion to xanthine oxidase, and brain lipid peroxidation. We conclude that perivascular reactions mediated by peroxynitrite are important in the cascade of events which lead to brain oxidative stress in CO poisoning.

Individually distinct Ig homology domains in PECAM-1 regulate homophilic binding and modulate receptor affinity

PECAM-1 (CD31) is a 130-kDa member of the immunoglobulin (Ig) gene superfamily that is constitutively expressed at high concentration at endothelial cell intercellular junctions and at moderate density on the surface of circulating leukocytes and platelets. Recent in vitro and in vivo studies have shown the PECAM-1 plays a central role in mediating the extravasation of leukocytes from the vessel wall in response to inflammatory mediators. To study the binding characteristics of PECAM-1, phospholipid vesicles were prepared and examined by flow cytometry and immunofluorescence microscopy for their ability to associate with each other and with cells. Proteoliposomes containing high concentrations of PECAM-1 interacted homophilically with each other, forming large self-aggregates. PECAM-1 proteoliposomes, as well as soluble bivalent PECAM-1 in the form of a PECAM-1/IgG immunoadhesin, associated homophilically with cells expressing human, but not murine, PECAM-1. This binding could be completely inhibited by monoclonal antibody Fab fragments specific for Ig homology Domain 1 or Domains 1 + 2. Binding studies using cells expressing human PECAM-1 deletion mutants and murine/human chimeras confirmed that both Ig Domains 1 and 2 were both necessary and sufficient for homophilic binding. In contrast, engagement of membrane-proximal Domain 6 with monoclonal antibody Fab fragments had the opposite effect and augmented the binding of PECAM-1 proteoliposomes to cells. Thus, PECAM-1, like certain integrins, appears to be capable of antibody-induced conformational changes that alter affinity for its ligand. Similar changes induced by physiologic stimuli could be important in regulating the function of PECAM-1 in vascular cells.

Thrombin stimulates production of interleukin-8 in human umbilical vein endothelial cells

Interleukin-8 (IL-8) is regarded as an important mediator of inflammation because of its potent and specific chemotactic activity on neutrophils. In the present investigation, human umbilical vein endothelial cells (HUVEC) stimulated with thrombin were found to produce IL-8, in a dose- and time-dependent manner. After stimulation with 10 U/ml thrombin for 24 hr, the level of IL-8 in the conditioned medium was 14 ng/ml, or enough to elicit PMN chemotaxis in vitro. Northern blot analysis revealed that thrombin as well as IL-1 beta elevates the level of IL-8 mRNA preceding the formation of IL-8 protein. A synthetic peptide SFLLRN [human thrombin receptor-activating peptide (TRAP)] was found to mimic the action of thrombin. Preincubation with anti-thrombin compounds such as hirudin and antithrombin-III-heparin almost completely suppressed the action of thrombin without affecting the actions of other stimuli including IL-1 beta, phorbol 12-myristate 13-acetate (PMA) and TRAP. Diisopropylfluorophosphate-treated thrombin did not stimulate IL-8 production. Calphostin-C, a protein kinase C (PKC) inhibitor, attenuated the production of IL-8 by thrombin, TRAP and PMA, but left the action of IL-1 beta unchanged. These results strongly suggest that catalytic activation of thrombin receptor by thrombin results in PKC-dependent IL-8 production accompanied by an increase in IL-8 mRNA level.

CD31/PECAM-1-driven chemokine-independent transmigration of human T lymphocytes

We assessed the relative contribution of CD31/PECAM-1 (platelet-endothelial cell adhesion molecule-1) to T lymphocyte transmigration by the use of transfected murine fibroblasts stably expressing either the human CD31/PECAM-1 or the intercellular adhesion molecule-1 (CD54/ICAM-1). Unlike CD54/ICAM-1, CD31/PECAM-1 supported migration of activated T cells in the absence of chemokines: most of the migrating lymphocytes were CD31+ and displayed a phenotype corresponding to the naive subpopulation (LFA-1 dull and CD45RA+). Migration of activated T lymphocytes through CD54/ICAM-1+ transfected monolayers could be induced by creating a chemotactic gradient with the chemokine monocyte chemotactic protein-1, and the migrating cells mainly displayed a memory phenotype (LFA-1 bright CD45RO+) under these conditions. Furthermore, we found that in transfected cells CD54/ICAM-1 is uniformly distributed along the apical surface of the cells, while CD31/PECAM-1 is concentrated at the intercellular junctions, suggesting the existence of a haptotactic gradient (i.e. a gradient of substrate- or cell-bound molecules) responsible for T cell migration. This was also confirmed by the finding that monolayers of murine fibroblasts transfected with a CD31/PECAM-1 mutant lacking the cytoplasmic domain (CD31/PECAM-1-delta cyto), which has a reduced tendency to localize at cell-cell contact areas, supported efficient adhesion but were unable to induce migration of activated T cells unless a chemotactic gradient was created. We propose that in lymphocytes, homophilic CD31/PECAM-1 adhesion may be primarily involved in transmigration of naive T cells and that its role is complementary to that of CD54/ICAM-1.

The role of selectins and CD18 in leukotriene B4-mediated white blood cell emigration in human skin grafts transplanted on SCID mice

The purpose of this study was to examine the role of selectins and CD18 cell adhesion molecules (CAMs) in inflammation induced by injection of leukotriene B4 (LTB4) into human skin. To accomplish this, the expression of CAMs and the ability of specific antibodies against CAMs to block white blood cell (WBC) transmigration were studied in an in vivo model consisting of human skin transplanted onto mice with the severe combined immune deficiency (SCID) mutation. The results indicate that LTB4-induced WBC transmigration in the human/SCID model is rapid and pronounced; however, it is not accompanied by a significant upregulation of the baseline expression of endothelial P-selectin, E-selectin, ICAM-1 or VCAM-1. An anti-murine CD18 mAb markedly inhibited white cell infiltration (89% inhibition) confirming the importance of beta 2 integrins in the process. The role of selectins was also examined. MEL-14, a bioactive antibody against murine L-selectin inhibited transmigration by 66%. A significant, but smaller, effect (39% inhibition) was observed by blocking E-selectin function. These results indicate that LTB4-induced inflammation does not require upregulation of endothelial CAM expression and, in contrast to TNF alpha-induced transmigration, is only partially blocked by anti-E-selectin antibodies.

Identification of alpha v beta 3 as a heterotypic ligand for CD31/PECAM-1

CD31 (PECAM-1) is a member of the immunoglobulin gene superfamily (IgSF) and has an important role in a number of endothelial cell functions including angiogenesis, inflammation, integrin activation and cell-cell adhesion. CD31 has both homotypic and heterotypic adhesive properties and in common with other IgSF members contains multiple functional domains. Using chimaeric fusion proteins of CD31 and a panel of haematopoietic cell lines we show that CD31 can bind cells in a predominantly homotypic or heterotypic manner depending on the cell line used. Heterotypic binding was found to be cation and temperature dependent and enhanced by Mn2+: all features of integrin mediated binding. Using a panel of anti-CD31 and anti-integrin antibodies we show that alpha v beta 3 is a ligand for CD31 on the monocytic cell line U937. The specificity of the interaction between alpha v beta 3 and CD31 was further confirmed by solid phase binding assays and the use of alpha v beta 3 transfected cells which bound CD31 specifically. Furthermore, we have mapped the binding site for alpha v beta 3 to domains 1 and 2 of CD31. The interaction of CD31 with alpha v beta 3 may be important in many aspects of endothelial function including leukocyte-endothelial transmigration and angiogenesis.