PECAM-1 is required for transendothelial migration of leukocytes

T lymphocyte chemotaxis and skin diseases

Recent advances in our understanding of the mechanisms of T lymphocyte motility and chemotaxis, particularly in aspects of lymphocyte-endothelial adhesion, transendothelial cell migration, and T-lymphocyte response to chemotactic gradients have contributed to our knowledge of how T lymphocytes accumulate during the initiation, the development and the control of inflammatory skin responses. In this review, we will summarize the present situation of studies on T lymphocyte adhesion and chemotaxis. The 3 major steps in T lymphocyte chemotaxis, e.g., recognition of extracellular chemotactic gradients, transduction into appropriate intracellular signals, and generation of motion, will be outlined. Skin-homing T lymphocytes, chemokines and other chemoattractants will also be discussed in relation to skin diseases.

Peripheral monocyte and naive T-cell recruitment and activation in Crohn's disease

BACKGROUND & AIMS

Transmural perivascular mononuclear cell infiltrates are a feature of Crohn's disease. The aim of this study was a molecular characterization of the mechanisms leading to the formation of these infiltrates.

METHODS

Endothelial cell and leukocyte expression of the adhesion molecules directing leukocyte transendothelial migration were studied in situ by immunohistochemical analysis of 10 samples from patients with Crohn's disease and 10 samples from normal controls. Double-staining methods were used to characterize the cells forming the infiltrates.

RESULTS

CD11a+ and L-selectin-positive mononuclear cells seemed to be the major component of perivascular infiltrates. The vast majority of these cells were CD68+, CD31+ monocytes/macrophages surrounded by CD3+, L-selectin-positive, CD31+, CD45RA+, and/or CD45RO+ T lymphocytes. T lymphocytes within the vessels expressed both CD45RA and CD45RO markers. Endothelial cells were intercellular adhesion molecule 1 positive and mostly CD34+. Strong adhesion between L-selectin-positive and CD11a+ intravascular mononuclear cells and CD34+ and intercellular adhesion molecule 1-positive endothelial cells were observed.

CONCLUSIONS

Data indicate that peripheral mononuclear cells are actively recruited in the submucosa of Crohn's disease tissue; endothelial cells express adhesion molecules highly permissive for transendothelial migration of monocytes and both naive and memory T cells contributing to infiltrates generation; and close membrane contact between migrated macrophages and naive T cells leads to the T-cell transition from naive to memory phenotype within Crohn's disease.

Hydrogen peroxide-induced endothelial retraction is accompanied by a loss of the normal spatial organization of endothelial cell adhesion molecules

Treatment of confluent monolayers of human umbilical vein endothelial cells with sublethal concentrations of hydrogen peroxide (H2O2) produces reversible cell retraction that opens gaps between adjacent cells. Despite the retraction, adjacent cells remain in contact through a network of dendrite-like processes. Retraction depends on cellular metabolism but not new protein synthesis or protein kinase C. Shape changes induced by H2O2 are accompanied by partial redistribution of actin filaments from the cell periphery in resting endothelial cells to a tangled network of centrally located filaments in H2O2-treated endothelial cells. This change in actin organization is associated with a loss of the normal distribution pattern of surface protein expression. Specifically, beta 1 and beta 3 integrins partly escape from focal adhesion plaques and migrate to the lateral and apical surface of the cell; PECAM-1 redistributes from the lateral borders to the basal surface; and ICAM-1 and ICAM-2 spread from apical caps to the basal surface and to the dendrite-like processes. The likely consequence of endothelial retraction accompanied by abnormal membrane protein distribution is a loss of normal endothelial cell functions. These changes are best considered manifestations of H2O2-induced sublethal injury that may cause endothelial dysfunction.

Adhesion molecule expression on murine cerebral endothelium following the injection of a proinflammagen or during acute neuronal degeneration

The acute inflammatory response in the murine CNS is different from that observed in other tissues. Few polymorphonuclear leukocytes are recruited to the brain parenchyma and there is a delay in the recruitment of monocytes. Leukocyte recruitment to sites of inflammation is dependent on adhesion molecules expressed on the endothelium. The atypical kinetics of leukocyte recruitment to the CNS may be the result of deficient or delayed adhesion molecule expression on the cerebral endothelium. Using immunohistochemistry, the present study demonstrates that following the intracranial injection of a proinflammagen, lipopolysaccharide, or following acute neuronal degeneration elicited with kainic acid, the adhesion molecules ICAM-1 and VCAM were readily upregulated on cerebral endothelium in a time course comparable with that demonstrated on non-CNS endothelium. Both molecules were expressed on vessels, irrespective of their size, at 24 h after kainic acid or 6 h after lipopolysaccharide injection but leukocyte recruitment was negligible. The expression of ICAM-1 was demonstrated not only on endothelium but also on microglia especially in response to nerve terminal degeneration. PECAM was constitutively expressed at high levels on cerebral endothelium and did not change during brain injury. However, PECAM was induced on astrocytes after lipopolysaccharide injection or during acute neuronal degeneration, the latter providing a particularly strong stimulus. This study indicates that the expression of these adhesion molecules on CNS endothelium is neither deficient or delayed and that they are unlikely to be limiting factors in leukocyte recruitment to the CNS.

Functional properties of human vascular endothelial cadherin (7B4/cadherin-5), an endothelium-specific cadherin

Human vascular endothelial cadherin (VE-cadherin, 7B4/cadherin-5) is an endothelial-specific cadherin localized at the intercellular junctions. To directly investigate the functional role of this molecule we cloned the full-length cDNA from human endothelial cells and transfected its coding region into Chinese hamster ovary cells. The product of the transfected cDNA had the same molecular weight as the natural VE-cadherin in human endothelial cells, and reacted with several VE-cadherin mouse monoclonal antibodies. Furthermore, it selectively concentrated at intercellular junctions, where it codistributed with alpha-catenin. VE-cadherin conferred adhesive properties to transfected cells. It mediated homophilic, calcium-dependent aggregation and cell-to-cell adhesion. In addition, it decreased intercellular permeability to high-molecular weight molecules and reduced cell migration rate across a wounded area. Thus, VE-cadherin may exert a relevant role in endothelial cell biology through control of the cohesion and organization of the intercellular junctions.

CD31/PECAM-1 is a ligand for alpha v beta 3 integrin involved in adhesion of leukocytes to endothelium

To protect the body efficiently from infectious organisms, leukocytes circulate as nonadherent cells in the blood and lymph, and migrate as adherent cells into tissues. Circulating leukocytes in the blood have first to adhere to and then to cross the endothelial lining. CD31/PECAM-1 is an adhesion molecule expressed by vascular endothelial cells, platelets, monocytes, neutrophils, and naive T lymphocytes. It is a transmembrane glycoprotein of the immunoglobulin gene superfamily (IgSF), with six Ig-like homology units mediating leukocyte-endothelial interactions. The adhesive interactions mediated by CD31 are complex and include homophilic (CD31-CD31) or heterophilic (CD31-X) contacts. Soluble, recombinant forms of CD31 allowed us to study the heterophilic interactions in leukocyte adhesion assays. We show that the adhesion molecule alpha v beta 3 integrin is a ligand for CD31. The leukocytes revealed adhesion mediated by the second Ig-like domain of CD31, and this binding was inhibited by alpha v beta 3 integrin-specific antibodies. Moreover alpha v beta 3 was precipitated by recombinant CD31 from cell lysates. These data establish a third IgSF-integrin pair of adhesion molecules, CD31-alpha v beta 3 in addition to VCAM-1, MadCAM-1/alpha 4 integrins, and ICAM/beta 2 integrins, which are major components mediating leukocyte-endothelial adhesion. Identification of a further versatile adhesion pair broadens our current understanding of leukocyte-endothelial interactions and may provide the basis for the treatment of inflammatory disorders and metastasis formation.

Interleukin-8 increases endothelial permeability independent of neutrophils

Interleukin-8 (IL-8) has been associated with a variety of hyperinflammatory states and adverse clinical events. Circulating IL-8 levels correlate with the severity of tissue trauma, and excessive elevations of IL-8 are associated with postinjury adult respiratory distress syndrome and multiple organ failure. While IL-8 is a potent neutrophil (PMN) chemoattractant and activator and enhances PMN transendothelial migration, it also acts to inhibit PMN adhesion to stimulated endothelial cells (ECs). We hypothesized that IL-8 could interact directly with ECs to increase permeability independent of PMNs. Human umbilical vein ECs (HUVECs) were cultured on collagen-coated micropore filters, and integrity of the EC monolayer measured by albumin flux across the filter. Cytochalasin D was used as a positive control. IL-8 induced increased permeability at a concentration of 1000 ng/mL. This effect was abrogated by preincubation of HUVECs with a protein synthesis inhibitor (cycloheximide). These data suggest a role for IL-8 in promoting endothelial leak independent of PMNs, via a mechanism involving protein synthesis.

Lymphocyte migration. A new spin on lymphocyte homing

New evidence confirms the multi-step model for leukocyte migration through endothelia, but with a twist. The selectins are not always required, and some lymphocyte subsets rely heavily on integrins for their migration to tissues.

Polymorphonuclear leucocyte migration through human dermal fibroblast monolayers is dependent on both beta 2-integrin (CD11/CD18) and beta 1-integrin (CD29) mechanisms

Accumulation of leucocytes in inflammation involves their migration through vascular endothelium and then in the connective tissue. We investigated human polymorphonuclear leucocyte (PMNL) migration through a biological barrier of human dermal fibroblasts grown on microporous filters, as a model of PMNL migration in the connective tissue. PMNL did not migrate through a fibroblast monolayer unless a chemotactic factor, e.g. C5a, interleukin-8 (IL-8) or zymosan-activated plasma (ZAP; C5adesArg), was added. This migration was partially inhibited (35-70%, depending on the stimulus) by treatment of PMNL with monoclonal antibody (mAb) to CD18 (beta 2-integrins). Most of the CD18-independent migration was inhibited by mAb to beta 1-integrins (CD29). Inhibition by mAb to beta 1 was observed when the PMNL, but not the fibroblasts, were treated with mAb. The role of beta 1-integrins in PMNL transfibroblast migration was detectable only when the function of the CD11-CD18 complex was blocked, because mAb to beta 1-integrin alone had no significant effect on PMNL migration. Migration induced by C5a was more CD18-independent compared to IL-8 or C5adesArg. The CD18-independent migration was also inhibited by mAb to the beta 1-integrin subunits alpha 5 (of very late antigens-5; VLA-5) and alpha 6 (of VLA-6). Treatment of the fibroblasts (4 hr) with tumour necrosis factor-alpha (TNF-alpha) or IL-1 alpha enhanced C5a-induced PMNL transfibroblast migration and increased the proportion of migration utilizing the CD11-CD18 mechanism. However, TNF-alpha treatment had no effect on the degree of beta 1-integrin-dependent migration. These findings suggest that in response to the chemotactic factors C5a, IL-8 and C5adesArg, PMNL migration in the connective tissue is mediated by both CD11-CD18 (beta 2) and beta 1-integrins on the PMNL. The VLA-5 and VLA-6 members of beta 1-integrins are involved in this process. This is in contrast to PMNL migration across endothelium in this system, which is virtually all CD18 dependent with no significant role for beta 1-integrins.

CD11b blockade prevents lung injury despite neutrophil priming after gut ischemia/reperfusion

Gut ischemia/reperfusion (I/R) provokes lung injury via a mechanism that involves neutrophils [polymorphonuclear neutrophils (PMNs)]. CD11b/CD18 (alpha mB2) is the integrin receptor on PMNs critical for adhesion-dependent oxidative burst. The purpose of this study was to investigate the mechanistic role of CD11b in the process of gut I/R-induced lung injury. Sprague-Dawley rats underwent 45 minutes of superior mesenteric artery (SMA) occlusion with and without CD11b monoclonal antibody treatment (IB6) (1 mg/kg, i.v.), before SMA clamping. At 2-hour reperfusion, PMN presence in tissue was quantitated by myeloperoxidase activity and circulating PMN priming determined by the difference in superoxide production with and without N-formyl-methionyl-leucyl-phenylalanine, whereas lung leak was assessed by 125I-albumin lung/blood ratio. In sum, CD11b blockade prevented gut I/R-induced lung leak, but did not attenuate gut I/R-induced PMN priming or tissue PMN accumulation. In conclusion, gut I/R promotes PMN priming and PMN adhesion in both local and distant beds via receptors other than CD11b, but this B2 integrin receptor is critical for PMN-mediated endothelial injury.

Activation of polymorphonuclear leukocytes reduces their adhesion to P-selectin and causes redistribution of ligands for P-selectin on their surfaces

In acute inflammatory responses, selectins mediate initial rolling of neutrophils (PMNs) along the endothelial surface. This is followed by tight adhesion that requires activation-dependent up-regulation of CD11/CD18 integrins on PMNs. For emigration to occur, the initial bonds that are established at the endothelial surface must be disengaged. We show that activation of PMNs results in their detachment from P-selectin, a glycoprotein expressed at the surface of inflamed endothelium that mediates initial tethering of PMNs. Loosening of the bond occurs when PMNs are activated by platelet-activating factor, which is coexpressed with P-selectin, or by other signaling molecules. The time course of reduced adhesion to P-selectin, when compared to up-regulation of CD11/CD18 integrins, suggests that "bond trading" may occur as activated PMNs transmigrate in vivo. Activation of PMNs did not alter binding of fluid-phase P-selectin, indicating that the ligand(s) for P-selectin is not shed or internalized. Using microspheres coated with P-selectin, we found that ligands for P-selectin were randomly distributed over the surfaces of rounded, unactivated PMNs. An antibody against P-selectin glycoprotein ligand-1 (PSGL-1) completely inhibited binding of P-selectin-coated beads suggesting that P-selectin glycoprotein ligand-1 is the critical binding site in this assay. In contrast to the dispersed pattern on unactivated PMNs, the ligands for P-selectin were localized on the uropods of activated, polarized cells. Pretreating PMNs with cytochalasin D before activation prevented the change in cell shape, the redistribution of binding sites for P-selectin-coated beads, and the decrease in cellular adhesiveness for P-selectin. These experiments indicate that the distribution of ligands for P-selectin is influenced by cellular activation and by cytoskeletal interactions, and that redistribution of these ligands may influence adhesive interactions. Activation of PMNs may cause loosening or disengagement of bonds between P-selectin and its ligands, facilitating transendothelial migration.

Interaction of acute leukemia cells with the bone marrow microenvironment: implications for control of minimal residual disease

There is increasing evidence for an interaction between acute leukemia cells and the microenvironment of the bone marrow. Blast cells from cases of acute myeloid leukemia (AML) and acute lymphoblastic leukemia (ALL) bind to cellular and extracellular matrix components of the bone marrow stroma. In AML, adhesion to stroma is mediated by the combined action of beta 1 (principally VLA-4) and beta 2 integrins, while in precursor-B ALL VLA-4 and VLA-5 integrins play a major role. Adhesion molecules such as CD31, CD44, non-beta 1, beta 2 integrins, growth factor receptors such as c-kit, and other molecules are also likely to play a role. Binding of acute leukemia blasts to ligands on stroma has several pathophysiological consequences. Stromal contact is able to inhibit programmed cell death (apoptosis) in a proportion of cases of both AML and ALL. In ALL, diffusible molecules derived from stroma appear to contribute. Marrow stroma also plays a part in regulating leukemic cell proliferation. While this is partly due to stromal production of hemopoietic growth factors, in soluble or transmembrane form or bound to extracellular matrix, signalling mediated directly by binding of adhesion molecules on leukemic cells may also have a role. Contact of ALL blasts with marrow fibroblasts is followed by migration of leukemic cells, utilizing VLA-4 and VLA-5 integrins, potentially allowing homing of blasts to favourable microenvironmental sites, or controlling egress into the circulation. AML cells compete for stromal binding sites with natural killer cells and cytotoxic lymphocytes, which are known to inhibit their clonogenic growth. We speculate that these complex interactions between leukemic blasts, cellular and matrix components of stroma, and cytotoxic lymphocytes, play a critical role in determining the fate of small numbers of leukemic cells surviving after cytotoxic chemotherapy.

Chemokines and tissue injury

Accumulation of leukocytes at sites of inflammation is essential for host defense, yet secretory products of the white cells may augment injury by damaging surrounding healthy tissues. Members of the chemokine family of chemotactic cytokines play a fundamental role in this process by attracting and stimulating specific subsets of leukocytes. In vitro studies suggest that chemokines participate in at least three phases of leukocyte recruitment. First, they foster tight adhesion of circulating leukocytes to the vascular endothelium by activating leukocytic integrins. Second, because of their chemoattractant properties, chemokines guide leukocytes through the endothelial junctions and underlying tissue to the inflammatory focus. Finally, chemokines activate effector functions of leukocytes, including production of reactive oxygen intermediates and exocytosis of degradative enzymes. Animal studies in which antibodies are used to neutralize the activity of individual members of the chemokine family confirm that these mediators contribute to the development of both acute and chronic inflammatory conditions. A number of mechanisms may operate in vivo to limit the proinflammatory properties of chemokines. Therapies that target chemokines directly or enhance the body's mechanisms for controlling their activity may prove to be reasonable approaches for treatment of inflammatory diseases.

The proteasome pathway is required for cytokine-induced endothelial-leukocyte adhesion molecule expression

Multiple cell adhesion proteins are up-regulated in vascular endothelial cells in response to TNF alpha and other inflammatory cytokines. This increase in cell adhesion gene expression is thought to require the transcription factor NF-kappa B. Here, we show that peptide aldehyde inhibitors of the proteasome, a multicatalytic protease recently shown to be required for the activation of NF-kappa B, block TNF alpha induction of the leukocyte adhesion molecules E-selectin, VCAM-1, and ICAM-1. Striking functional consequences of this inhibition were observed in analyses of leukocyte-endothelial interactions under defined flow conditions. Lymphocyte attachment to TNF alpha-treated endothelial monolayers was totally blocked, while neutrophil attachment was partially reduced but transmigration was essentially prevented.

Transendothelial migration of neutrophils involves integrin-associated protein (CD47)

Inflammation is a primary pathological process. The development of an inflammatory reaction involves the movement of white blood cells through the endothelial lining of blood vessels into tissues. This process of transendothelial cell migration of neutrophils has been shown to involve neutrophil beta 2 integrins (CD18) and endothelial cell platelet-endothelium cell adhesion molecules (PECAM-1; CD31). We now show that F(ab')2 fragments of the monoclonal antibody B6H12 against integrin-associated protein (IAP) blocks the transendothelial migration of neutrophils stimulated by an exogenous gradient of the chemokine interleukin 8 (IL-8; 60% inhibition), by the chemotactic peptide N-formyl-methionylleucylphenylalanine (FMLP; 76% inhibition), or by the activation of the endothelium by the cytokine tumor necrosis factor alpha (98% inhibition). The antibody has two mechanisms of action: on neutrophils it prevents the chemotactic response to IL-8 and FMLP, and on endothelium it prevents an unknown but IL-8-independent process. Blocking antibodies to IAP do not alter the expression of adhesion proteins or production of IL-8 by endothelial cells, and thus the inhibition of neutrophil transendothelial migration is selective. These data implicate IAP as the third molecule essential for neutrophil migration through endothelium into sites of inflammation.

PECAM-1: its expression and function as a cell adhesion molecule on hemopoietic and endothelial cells

PECAM-1, the platelet-endothelial cell adhesion molecule, is expressed on a variety of mature hemopoietic cell types (including neutrophils, monocytes and T cell subsets) and is also present on endothelia. In such cases, this glycoprotein functions as either a homotypic or heterotypic adhesion molecule contributing to cell migration, inflammatory processes and wound healing. We have recently shown that PECAM-1 is expressed on a variety of hemopoietic progenitor cell types and on stromal macrophages from human bone marrow. In this review, we discuss the possible functional significance of this molecule for both hemopoietic cell differentiation and for mature cells.

P-selectin/ICAM-1 double mutant mice: acute emigration of neutrophils into the peritoneum is completely absent but is normal into pulmonary alveoli

Neutrophil emigration during an inflammatory response is mediated through interactions between adhesion molecules on endothelial cells and neutrophils. P-Selectin mediates rolling or slowing of neutrophils, while intercellular adhesion molecule-1 (ICAM-1) contributes to the firm adhesion and emigration of neutrophils. Removing the function of either molecule partially prevents neutrophil emigration. To analyze further the role of P-selectin and ICAM-1, we have generated a line of mice with mutations in both of these molecules. While mice with either mutation alone show a 60-70% reduction in acute neutrophil emigration into the peritoneum during Streptococcus pneumoniae-induced peritonitis, double mutant mice show a complete loss of neutrophil emigration. In contrast, neutrophil emigration into the alveolar spaces during acute S. pneumoniae-induced pneumonia is normal in double mutant mice. These data demonstrate organ-specific differences, since emigration into the peritoneum requires both adhesion molecules while emigration into the lung requires neither. In the peritoneum, P-selectin-independent and ICAM-1-independent adhesive mechanisms permit reduced emigration when one of these molecules is deficient, but P-selectin-independent mechanisms cannot lead to ICAM-1-independent firm adhesion and emigration.

Mapping the homotypic binding sites in CD31 and the role of CD31 adhesion in the formation of interendothelial cell contacts

CD31 is a member of the immunoglobulin superfamily consisting of six Ig-related domains. It is constitutively expressed by platelets, monocytes, and some lymphocytes, but at tenfold higher levels on vascular endothelial cells. CD31 has both homotypic and heterotypic adhesive properties. We have mapped the homotypic binding sites using a deletion series of CD31-Fc chimeras and a panel of anti-CD31 monoclonal antibodies. An extensive surface of CD31 is involved in homotypic binding with domains 2 and 3 and domains 5 and 6 playing key roles. A model consistent with the experimental data is that CD31 on one cell binds to CD31 on an apposing cell in an antiparallel interdigitating mode requiring full alignment of the six domains of each molecule. In addition to establishing intercellular homotypic contacts. CD31 binding leads to augmented adhesion via beta 1 integrins. The positive cooperation between CD31 and beta 1 integrins can occur in heterologous primate cells (COS cells). The interaction is specific to both CD31 and beta 1 integrins. Neither intercellular adhesion molecule-1 (ICAM-1)/leukocyte function-associated antigen-1 (LCAM-1) nor neural cell adhesion molecule (NCAM)/NCAM adhesion leads to recruitment of beta 1 integrin adhesion pathways. Establishment of CD31 contacts have effects on the growth and morphology of endothelial cells. CD31(D1-D6)Fc inhibits the growth of endothelial cells in culture. In addition, papain fragments of anti-CD31 antibodies (Fab fragments) disrupt interendothelial contact formation and monolayer integrity when intercellular contacts are being formed. The same reagents are without effect once these contacts have been established, suggesting that CD31-CD31 interactions are critically important only in the initial phases of intercellular adhesion.

P-selectin and vascular cell adhesion molecule 1 mediate rolling and arrest, respectively, of CD4+ T lymphocytes on tumor necrosis factor alpha-activated vascular endothelium under flow

This report examines the adhesive interactions of human CD4+ T lymphocytes with tumor necrosis factor alpha-activated human endothelial cell monolayers in an in vitro model that mimics microcirculatory flow conditions. Resting CD4+ T cell interactions with activated endothelium consisted of initial attachment followed by rolling, stable arrest, and then spreading and transendothelial migration. P-selectin, but not E-, or L-selectin, mediated most of this initial contact and rolling, whereas beta 1-integrins (alpha 4 beta 1), interacting with endothelial-expressed vascular cell adhesion molecule 1, participated in rolling and mediated stable arrest. In contrast, beta 2-integrins were primarily involved in spreading and transmigration. These findings highlight an important role for P-selectin and suggest discrete functions for beta 1- and beta 2-integrins during lymphocyte recruitment to sites of immune-mediated inflammation.

Localization of multiple functional domains on human PECAM-1 (CD31) by monoclonal antibody epitope mapping

PECAM-1, a cell adhesion molecule of the immunoglobulin gene (Ig) superfamily, has been implicated in white cell transmigration, integrin activation on lymphocytes, and cell-cell adhesion. The purpose of this investigation was to identify specific regions of the PECAM-1 extracellular domain mediating these functions by identifying the location of epitopes of bioactive anti-PECAM-1 monoclonal antibodies. The binding regions of mAbs important in PECAM-1-mediated leukocyte transmigration (Hec 7.2 and 3D2) were mapped to N-terminal Ig-like domains. The epitopes of monoclonal antibodies that activated integrin function on lymphocytes were dispersed over the entire extracellular region, but those that had the strongest activating effect were preferentially localized to the N-terminus of the molecule. The binding regions of mAbs that blocked PECAM-1-mediated heterophilic L-cell aggregation were located either in Ig-like domain 2 (NIH31.4) or Ig-like domain 6 (4G6 and 1.2). Site-directed mutagenesis further pinpointed the epitope of the 4G6 mAb to a hexapeptide, CAVNEG, within Ig-like domain 6. These results demonstrate that PECAM-1 contains multiple functional domains. Regions within N-terminal Ig-like domains appear to be required for transmigration. In contrast, two distinct regions were implicated in L-cell mediated heterophilic aggregation.

Murine macrophage scavenger receptor: in vivo expression and function as receptor for macrophage adhesion in lymphoid and non-lymphoid organs

Macrophage scavenger receptors are trimeric integral membrane glycoproteins which have been implicated in various macrophage functions including uptake of oxidized lipoprotein and the serum-dependent, divalent cation-independent adhesion of macrophages to tissue culture-treated plastic. In this study we have used a recently defined monoclonal antibody (2F8) which recognizes murine macrophage scavenger receptor, to explore its expression in lymphoid and non-lymphoid organs of the normal adult. Scavenger receptor was detected in the red pulp and marginal zone of normal adult mouse spleen, medulla of the thymus and subcapsular region of lymph nodes. Kupffer cells in the liver, alveolar macrophages in the lung and lamina propria macrophages in the gut all reacted with 2F8 monoclonal antibody. The antigen was not detected on any non-macrophage cells, with the exception of sinusoidal endothelial cells in the liver. In the spleen, lymph node and liver, scavenger receptor antigen expression was associated specifically with phagocytic cells which had taken up colloidal carbon. To examine macrophage adhesion in a context relevant to the interactions occurring within lymphoid and non-lymphoid organs, and the contribution of macrophage scavenger receptor to this adhesion, we designed an assay of macrophage adhesion to frozen tissue sections. Adhesion to most tissues was high and uniform in the absence of any chelating agents. The chelation of Ca2+ and Mg2+ revealed specific patterns of macrophage adhesion in lymphoid and non-lymphoid organs which was completely inhibited by 2F8. The ability of this antibody to block the EDTA-resistant adhesion correlated with tissue expression of the antigen in some tissues. Unlike adhesion to tissue culture-treated plastic, macrophage scavenger receptor-dependent adhesion of macrophages to frozen tissue sections did not exhibit an absolute requirement for exogenous fetal bovine serum indicating the presence of an endogenous ligand for scavenger receptor within the tissues. We propose that macrophage scavenger receptor is a candidate homing or retention molecule for macrophage localization within ligand-rich tissues.

Leukocytes contribute to hepatic ischemia/reperfusion injury via intercellular adhesion molecule-1-mediated venular adherence

BACKGROUND

Leukocytes are suggested to modulate ischemia/reperfusion injury via membrane receptor-controlled interaction with the microvascular endothelium.

METHODS

With the use of intravital fluorescence microscopy we investigated the role of the intercellular adhesion molecule-1 (ICAM-1) in a rat model of hepatic reperfusion injury with a neutralizing monoclonal antibody (anti-ICAM-1).

RESULTS

Sixty minutes of left lobar ischemia and reperfusion (isotype-matched immunoglobulin G1 control antibody) caused leukostasis in sinusoids (240 +/- 15 cells per liver lobule), leukocyte adherence in postsinusoidal venules (679 +/- 76 cells per mm2 endothelial surface of postsinusoidal venules), nutritive perfusion failure (15% +/- 2% nonperfused sinusoids), excretory dysfunction (bile flow, 1.2 +/- 0.3 microliters.min-1.gm-1), and loss of hepatocellular integrity (serum aspartate aminotransferase, 1353 +/- 317 units.L-1; serum alanine aminotransferase, 1055 +/- 265 units.L-1). Anti-ICAM-1 did not affect sinusoidal leukostasis; however, it effectively inhibited postischemic leukocyte adherence to the venular endothelial lining (217 +/- 38 cells/mm2, p < 0.01). Concomitantly, hepatic reperfusion injury, including sinusoidal perfusion (6% +/- 1% nonperfused sinusoids, p < 0.01), excretory function (bile flow, 1.8 +/- 0.1 microliters.min-1.gm-1, p < 0.05), and hepatocellular integrity (aspartate aminotransferase, 480 +/- 108 units.L-1; alanine aminotransferase, 447 +/- 80 units.L-1, p < 0.05), was significantly ameliorated by anti-ICAM-1.

CONCLUSIONS

These findings prove in vivo the pivotal role of ICAM-1 in leukocyte-dependent manifestation of postischemic liver damage.

Vascular adhesion molecule expression in viral chronic hepatitis: evidence of neoangiogenesis in portal tracts

BACKGROUND/AIMS

T cell-mediated immune reactions could be crucial for hepatocellular damage in viral chronic hepatitis. The aims of this study were to compare the expression of activation and cell adhesion molecules on peripheral blood and intrahepatic lymphocytes from chronic hepatitis C and to analyze the intrahepatic expression of vascular adhesion molecules in viral chronic hepatitis.

METHODS

Lymphocytes from patients with chronic hepatitis C were studied by flow cytometry. Intrahepatic expression of vascular adhesion molecules was assessed by immunohistochemistry.

RESULTS

Liver-derived T cells showed a high expression of activation and cell adhesion molecules. Interestingly, we observed that vascular cell adhesion molecule 1 was up-regulated on both sinusoidal endothelial and portal dendritic cells. A novel finding was the neoformation of microvessels in inflamed portal tracts. An enhanced expression of endoglin was located on sinusoidal endothelial cells and on portal tracts.

CONCLUSIONS

Activated cytotoxic T cells, which showed an up-regulated expression of cell adhesion molecules, composed the majority of intrahepatic lymphocytes in chronic hepatitis C. The expression of vascular cell adhesion molecule 1 on portal dendritic cells and the microvessels neoformation in portal tracts from viral chronic hepatitis could define the main pathway for the recruitment and priming of liver-infiltrating T cells.

The use of anti-PECAM reagents in the control of inflammation

Platelet/endothelial cell adhesion molecule 1 (PECAM-1/CD31) is expressed on the surfaces of neutrophils and monocytes and concentrated at the junctional surfaces of vascular endothelial cells. Monoclonal antibodies against PECAM-1 and soluble recombinant PECAM-1 selectively block the passage of these leukocytes across the endothelial monolayer without interfering with earlier adhesion events in the emigration pathway. This block is seen both in vitro and in several in vivo models of acute inflammation. Since PECAM-1 appears to be crucial for a distinct step in the emigration of leukocytes into a focus of inflammation, PECAM-1 appears to be a new and potentially important target for anti-inflammatory therapy.

Expression of ICAM-1 and VCAM-1 and monocyte adherence in arteries exposed to altered shear stress

Local shear stresses generated by blood flow exert direct mechanical effects on adhesion of circulating leukocytes to vascular endothelium, but their effects on expression of endothelial-leukocyte adhesion molecules have not been determined. Shear stress in rabbit carotid arteries was increased by 170% or decreased by 73% in 5 days by surgical manipulations. En face immunofluorescence staining with the monoclonal antibody Rb1/9 revealed that vascular cell adhesion molecule-1 (VCAM-1) expression was greatly increased under low shear stress, but the distribution of staining was patchy. Thus, 71.4 +/- 7.8% of fields were VCAM-1 positive versus 2.4 +/- 0.47% of fields in control arteries. Frequently, large regions showed consistent but heterogeneous staining. Occasionally, small islands of cells were labeled intensely. Monocytes, detected by use of the monocyte-specific antibody HAM 56, adhered to endothelium under low shear stress; 64.5 +/- 8.2% of the monocytes colocalized with detectable VCAM-1, although many (83.2 +/- 2.8%) VCAM-1-positive regions were devoid of monocytes. VCAM-1 expression also increased significantly but to a lesser extent when shear stress was approximately doubled. Thus, 8.7 +/- 1.5% of fields were VCAM-1 positive under high shear versus 2.5 +/- 0.87% under normal shear stress. No monocytes were detected at high shear stress. At normal shear stresses, intercellular adhesion molecule-1 (ICAM-1), detected by use of the monoclonal antibody Rb2/3, was extensively distributed; thus, 53.5 +/- 5.5% of fields contained ICAM-1-positive cells. The junctional regions of the cells were heavily stained.(ABSTRACT TRUNCATED AT 250 WORDS)

Murine M phi scavenger receptor: adhesion function and expression

In the absence of divalent cations M phi lose their spread morphology but remain adherent to tissue culture treated plastic. We have exploited this property of M phi adhesion in vitro, to isolate a rat monoclonal 2F8, which totally inhibits the divalent cation-independent adhesion of M phi to tissue culture treated plastic and is partly responsible for the trypsin-resistant adhesion of M phi to the same substratum. Immunoprecipitation from macrophages and stably transfected Chinese Hamster Ovary Cells revealed that the antigen recognised by monoclonal antibody 2F8 is identical to the murine macrophage scavenger receptor. Macrophage scavenger receptors are trimeric integral membrane glycoproteins which have been implicated in various macrophage functions including uptake of oxidised lipoprotein and the formation of foam cells in atherosclerotic lesions. We have used monoclonal antibody 2F8 to explore the expression of murine macrophage scavenger receptor in lymphoid and non-lymphoid organs of the normal adult mouse and to examine the contribution of macrophage scavenger receptor to macrophage adhesion within tissues.

Cell adhesion molecules in coronary artery disease

To date, six families of cell adhesion molecules are known. These are cell surface receptors that mediate adhesion of cells to each other or to components of the extracellular matrix and include integrins, selectins, the immunoglobulin superfamily, cadherins, proteoglycans and mucins. These cell adhesion molecules play a key role in cell-cell interaction (such as among endothelium, monocytes, smooth muscle cells and platelets) and cell-extracellular matrix interaction (such as between leukocytes, platelets or fibroblasts and the extracellular matrix). The importance of these interactions has recently been demonstrated in clinical trials with the use of an antibody fragment directed against the platelet alpha IIb beta IIIa integrin, with reduction of arterial thrombosis and restenosis after percutaneous coronary interventions. A fundamental role for cell adhesion molecules has been suggested for several other relevant disease processes, including atherosclerosis, acute coronary syndromes, reperfusion injury and allograft vasculopathy. This review focuses on providing the clinically relevant biology of these families of adhesion molecules, setting the foundation for delineation of their emerging role in cardiovascular therapeutics.

Changes in expression of the cell adhesion molecule PECAM-1 (CD31) during differentiation of human leukemic cell lines

PECAM-1, a member of the immunoglobulin gene superfamily, is widely distributed on cells of the vascular system, and mediates cellular interactions through both homotypic and heterotypic adhesive mechanisms. Previous studies have demonstrated that PECAM-1 is initially expressed at high levels on CD34+ multipotential progenitors in the bone marrow, but is subsequently downregulated in more committed precursors of all lineages. Interestingly, although PECAM-1 expression is high on circulating monocytes and neutrophils, little is known about the upregulation of PECAM-1 expression during terminal myelomonocytic differentiation. We have further characterized this process by examining PECAM-1 expression during chemically-induced differentiation of the U937, HL-60 and HEL cell lines. Quantitative Western blot analysis of cellular lysates indicated that PECAM-1 expression could be upregulated in U937 and HL-60 cells by phorbol esters or dimethyl sulfoxide. Northern blot analysis showed that PECAM-1 mRNA levels appeared to increase in parallel with that of PECAM-1 protein. We also observed a marked difference in the apparent molecular mass of PECAM-1 that was lineage-specific, both in differentiated leukemic cell lines and in their corresponding leukocyte population. Immunofluorescence localization indicated that the cellular distribution of PECAM-1 in U937 and HL-60 cells was similar to that of their normal circulating counterparts, and that the pattern of distribution again displayed lineage fidelity. The ability to induce the expression of PECAM-1 molecules having different glycosylation and surface expression patterns may prove useful for further elucidation of the role of PECAM-1 in hematopoiesis, as well as studies of the cell lineage-specific modulation of PECAM-1 function.

Molecular and functional aspects of PECAM-1/CD31

Among vascular cell adhesion molecules, platelet-endothelial cell adhesion molecule (PECAM-1/CD31) has the distinctive feature of being expressed on several of the major cell types associated with the vascular compartment. This makes it uniquely positioned to mediate multiple and important cell-cell interactions involving platelets, leukocytes and endothelial cells. Thus, PECAM-1 may represent a potential target for new therapeutic agents directed at a variety of pathological states.

A novel role for E- and P-selectins: shape control of endothelial cell monolayers

The migration of neutrophils from blood vessels to peripheral tissues is a key step of inflammation. This requires the formation of transient gaps between endothelial cells with concomitant leucocyte squeezing through these narrow apertures and immediate restoration of endothelium continuity. It is currently considered that the main role of selectins is to mediate the initial contact between flowing leucocytes and endothelial cells. We show here that the binding of E- or P-selectins by specific antibodies induces a marked ‘rounding up’ of interleukin-1- or thrombin-activated human endothelial cells, respectively. Also, anti-E-selectin antibodies trigger a transient increase in cytosolic calcium involving intracellular calcium stores. No such effect is observed when von Willebrand factor or intercellular adhesion molecule 1 are similarly bound. Thus, in addition to promoting the initial interaction between activated endothelium and moving leucocytes, selectins might play a role in the induction of subsequent endothelial deformation, which would facilitate leucocyte arrest and transmigration towards peripheral tissues, and enhance the diffusion of soluble molecules between intravascular and peripheral compartments. Our results are consistent with this hypothesis and demonstrate a new property of endothelial selectins.

Platelet endothelial cell adhesion molecule-1 (PECAM-1/CD31): alternatively spliced, functionally distinct isoforms expressed during mammalian cardiovascular development

The establishment of the cardiovascular system represents an early, critical event essential for normal embryonic development. An important component of vascular ontogeny is the differentiation and development of the endothelial and endocardial cell populations. This involves, at least in part, the expression and function of specific cell surface receptors required to mediate cell-cell and cell-matrix adhesion. Platelet endothelial cell adhesion molecule-1 (PECAM-1, CD31) may well serve such a function. It is a member of the immunoglobulin superfamily expressed by the entire vascular endothelium in the adult. It is capable of mediating adhesion by a heterophilic mechanism requiring glycosaminoglycans, as well as by a homophilic, glycosaminoglycan independent, mechanism. It has been shown to regulate the expression of other adhesion molecules on naive T cells. This report documents by RT-PCR and immunohistochemical analysis the expression of PECAM-1 during early post implantation mouse embryo development. PECAM-1 was expressed by early endothelial precursors first within the yolk sac and subsequently within the embryo itself. Interestingly, embryonic PECAM-1 was expressed as multiple isoforms in which one or more clusters of polypeptides were missing from the cytoplasmic domain. The sequence and location of the deleted polypeptides corresponded to exons found in the human PECAM-1 gene. The alternatively spliced isoforms were capable of mediating cell-cell adhesion when transfected into L-cells. The isoforms differed, however, in their sensitivity to a panel of anti-PECAM-1 monoclonal antibodies. These data suggest that changes in the cytoplasmic domain of PECAM-1 may affect its function during cardiovascular development, and are consistent with our earlier report that systematic truncation of the cytoplasmic domain of human PECAM-1 resulted in changes in its ligand specificity, divalent cation and glycosaminoglycan dependence, as well as its susceptibility to adhesion blocking monoclonal antibodies. This is the first report of naturally occurring alternatively spliced forms of PECAM-1 having possible functional implications.

C5a-induced expression of P-selectin in endothelial cells

Human umbilical vein endothelial cells have recently been shown to respond to C5a with increases in intracellular Ca2+, production of D-myo-inositol 1,4,5-triphosphate and superoxide anion generation. In the current studies, C5a had been found to cause in a time- and dose-dependent manner rapid expression of endothelial P-selectin, secretion of von Willebrand factor, and adhesiveness for human neutrophils. The effects of C5a in P-selectin expression and adhesiveness of neutrophils were similar to the effects of histamine and thrombin on endothelial cells. The adhesiveness of C5a-stimulated endothelium for neutrophils was blocked by anti-P-selectin, but not by antibodies to intercellular adhesion molecule 1, E-selectin, or CD18. A cell-based ELISA technique has confirmed upregulation of P-selectin in endothelial cells exposed to C5a. Binding of C5a to endothelial cells has been demonstrated, with molecules bound being approximately 10% of those binding to neutrophils. By a reverse transcriptase-PCR technique, endothelial cells have been shown to contain mRNA for the C5a receptor. These data suggest that C5a may be an important inflammatory mediator for the early adhesive interactions between neutrophils and endothelial cells in the acute inflammatory response.

The necrotic venom of the brown recluse spider induces dysregulated endothelial cell-dependent neutrophil activation. Differential induction of GM-CSF, IL-8, and E-selectin expression

Brown recluse spider (Loxosceles reclusa) venom induces severe dermonecrotic lesions. The mechanism for this is unknown but presents an interesting paradox: necrosis is completely dependent on the victim's neutrophils, yet neutrophils are not activated by the venom. We show Loxosceles venom is a potent, but disjointed, endothelial cell agonist. It weakly induced E-selectin expression, but not intercellular adhesion molecule-1 or IL-6 expression, yet significantly stimulated release of IL-8 and large amounts of GM-CSF by 4 h. In contrast, TNF strongly induced all of these, except for GM-CSF. PMN bound to E-selectin on venom-activated endothelial cells, apparently via counterreceptors different from those that bind E-selectin on TNF alpha-activated monolayers. Notably, PMN bound venom-activated monolayers only at intercellular junctions, did not polarize, and completely failed to migrate beneath the monolayer. Despite this, bound PMN demonstrated increased intracellular Ca2+ levels and secreted primary and secondary granule markers. The latter event was suppressed by sulfones used to treat envenomation. We have defined a new endothelial cell agonist, Loxosceles venom, that differentially stimulates the inflammatory response of endothelial cells. This, in turn, leads to a dysregulated PMN response where adhesion and degranulation are completely dissociated from shape change and transmigration.

Monocyte rolling, arrest and spreading on IL-4-activated vascular endothelium under flow is mediated via sequential action of L-selectin, beta 1-integrins, and beta 2-integrins

Leukocyte interactions with vascular endothelium at sites of inflammation can be dynamically regulated by activation-dependent adhesion molecules. Current models, primarily based on studies with polymorphonuclear leukocytes, suggest the involvement of multiple members of the selectin, integrin, and immunoglobulin gene families, sequentially, in the process of initial attachment (rolling), stable adhesion (arrest), spreading and ultimate diapedesis. In the current study, IL-4-activated human umbilical vein endothelium, which selectively expresses VCAM-1 and an L-selectin ligand but not E-selectin, and appropriate function blocking monoclonal antibodies, were used to study monocyte-endothelial interactions in an in vitro model that mimics microcirculatory flow conditions. In this system, L-selectin mediates monocyte rolling and also facilitates alpha 4 beta 1-integrin-dependent arrest, whereas beta 2-integrins are required for spreading of firmly attached monocytes on the endothelial cell surface but not their arrest. These findings provide the first in vitro evidence for human monocyte rolling on cytokine-activated endothelium, and suggest a sequential requirement for both beta 1- and beta 2-integrin-dependent adhesive mechanisms in monocyte-endothelial interactions.

Control of lymphocyte homing

The expression of immunity, both protective and pathologic, is critically dependent on the appropriate distribution of 'lymphoid resources' among the tissues of the body. The 'homing' mechanisms mediating this distribution have proven to have an astounding plasticity--directing, under strict microenvironmental control, the selective recruitment of specific lymphocyte subsets to the various secondary lymphoid tissues and extralymphoid immune effector sites. The past year has seen significant progress in our understanding in three areas: the molecular basis of lymphocyte interactions with endothelium, providing new insight into the complex multistep process of lymphocyte extravasation; the role of extravascular matrix and cells in retaining lymphocytes within tissues; and the mechanisms by which local microenvironments differentially regulate adhesion molecule expression and function so as to provide for site-selective lymphocyte homing.