Reduced expression of junctional adhesion molecule and platelet/endothelial cell adhesion molecule-1 (CD31) at human vascular endothelial junctions by cytokines tumor necrosis factor-alpha plus interferon-gamma Does not reduce leukocyte transmigration under flow

The combination of tumor necrosis factor (TNF)-alpha plus interferon (IFN)-gamma has been shown previously to promote redistribution of platelet/endothelial cell adhesion molecule-1 (PECAM-1) (CD31), junctional adhesion molecule (JAM), and VE-cadherin away from lateral junctions of human umbilical vein endothelial cell monolayers. In parallel, neutrophil transmigration was significantly reduced. Because PECAM-1 and JAM have been implicated in leukocyte transmigration, the observed redistribution by cytokine activation was presumed to represent the mechanism causing decreased transmigration under static conditions. The current results confirm that culture of human umbilical vein endothelial cells with TNF-alpha plus IFN-gamma caused a decrease in surface-expressed and junctional-localized JAM and PECAM-1, but did not cause decreased leukocyte transmigration in an in vitro flow assay. Furthermore, blocking monoclonal antibody to PECAM-1 still significantly reduced monocyte transmigration, demonstrating that it retains a functional role even though its levels were reduced and redistributed away from junctions, whereas a panel of monoclonal antibodies to JAM failed to reduce leukocyte transmigration. Given the alterations in junction protein location, permeability function was assessed. IFN-gamma alone or TNF-alpha plus IFN-gamma significantly increased permeability, but TNF-alpha alone did not, suggesting lack of correlation between transmigration and loss of permeability. In conclusion, cytokine activation induced loss and redistribution of PECAM-1 and JAM away from lateral junctions, but per se does not negatively regulate either neutrophil or monocyte transmigration under flow.

IL-12, STAT4-dependent up-regulation of CD4(+) T cell core 2 beta-1,6-n-acetylglucosaminyltransferase, an enzyme essential for biosynthesis of P-selectin ligands

TCR activation of naive T cells in the presence of IL-12 drives polarization toward a Th1 phenotype and synthesis of P- and E-selectin ligands. Fucosyltransferase VII (Fuc-T VII) and core 2 beta-1,6-N-acetylglucosaminyltransferase (C2GnT) are critical for biosynthesis of selectin ligands. P-selectin glycoprotein ligand-1 is the best characterized ligand for P-selectin and also binds E-selectin. The contributions of TCR and cytokine signaling pathways to up-regulate Fuc-T VII and C2GnT during biosynthesis of E- and P-selectin ligands, such as P-selectin glycoprotein ligand 1, are unknown. IL-12 signals via the STAT4 pathway. Here, naive DO11.10 TCR transgenic and STAT4(-/-) TCR transgenic CD4(+) T cells were stimulated with Ag and IL-12 (Th1 condition), IL-4 (Th2), or neutralizing anti-IL-4 mAb only (Th0). The levels of Fuc-T VII and C2GnT mRNA in these cells were compared with their adhesive interactions with P- and E-selectin in vitro under flow. The data show IL-12/STAT4 signaling is necessary for induction of C2GnT, but not Fuc-TVII mRNA, and that STAT4(-/-) Th1 cells do not traffic normally to sites of inflammation in vivo, do not interact with P-selectin, and exhibit a partial reduction of E-selectin interactions under shear stress in vitro. Ag-specific TCR activation in CD4(+) T cells was sufficient to trigger induction of Fuc-TVII, but not C2GnT, mRNA and expression of E-selectin, but not P-selectin, ligands. Thus, Fuc-T VII and C2GnT are regulated by different signals during Th cell differentiation, and both cytokine and TCR signals are necessary for the expression of E- and P-selectin ligands.

Real-time imaging of vascular endothelial-cadherin during leukocyte transmigration across endothelium

Vascular endothelial-cadherin (VE-cadherin) is a component of the adherens junctions of endothelial cells whose role in endothelial transmigration of leukocytes has been controversial. Using a VE-cadherin/green fluorescent protein fusion construct (VEcadGFP) that mimics the native molecule, we visualized alterations in endothelial junctional structure in real time during transmigration of human neutrophils and monocytes in an in vitro flow model. We observed abundant transmigration occurring exclusively at the cell borders (paracellularly). Surprisingly, transmigration occurred both through de novo formation of transient gaps in VEcadGFP junctional distribution, and also through preexisting gaps. De novo gaps 4-6 microm in size were formed after a leukocyte arrived at a junction, whereas preexisting gaps were present even before the leukocyte had interacted with the endothelial cells contributing to a junction. Gaps rapidly resealed within 5 min after leukocyte transmigration. Migrating leukocytes appeared to push aside VEcadGFP in the plane of the junction, and this displaced material subsequently diffused back to refill the junction. To our knowledge, this is the first example where molecular events at the lateral junction have been tracked in real time during transmigration.

Fc gamma RIII mediates neutrophil recruitment to immune complexes. a mechanism for neutrophil accumulation in immune-mediated inflammation

Neutrophil accumulation is a hallmark of immune complex-mediated inflammatory disorders. Current models of neutrophil recruitment envision the capture of circulating neutrophils by activated endothelial cells. We now demonstrate that immobilized immune complexes alone support the rapid attachment of neutrophils, under physiologic flow conditions. Initial cell tethering requires the low-affinity Fc gamma receptor IIIB (Fc gamma RIIIB), and the beta(2) integrins are additionally required for the subsequent shear-resistant adhesion. The attachment function of Fc gamma RIIIB may be facilitated by its observed presentation on neutrophil microvilli. In vivo, in a model of acute antiglomerular basement membrane nephritis in which immune complexes are accessible to circulating neutrophils, Fc gamma RIII-deficient mice had a significant reduction in neutrophil recruitment. Thus, the interaction of immune complexes with Fc gamma RIII may mediate early neutrophil recruitment in immune complex-mediated inflammation.

Alpha4beta1-integrin activation is necessary for high-efficiency T-cell subset interactions with VCAM-1 under flow

OBJECTIVE

The purpose of this study was to examine the relationship between alpha4beta1-integrin state of activation on CD4+ T-cell subsets and their adhesive interaction to VCAM-1 under flow.

METHODS

Human CD4+ memory and naive T-cells were freshly isolated and effector-helper T-cell subsets. Th1 and Th2 cells, were differentiated in vitro from CD4+ naive T-cells. The expression of activation/ligand induced epitopes on beta1-integrins of each T-cell subset was assessed using mAb HUTS21 and mAb 15/7. T-cell subsets attachment and rolling on VCAM-1 was determined under defined flow conditions and the rates of attachment (ka), accumulation, and instantaneous rolling velocities were correlated to their beta1-integrin activation epitope expression.

RESULTS

A subset of memory T-cells constitutively express activation/ligand induced epitopes on beta1-integrins recognized by mAb HUTS21 and 15/7, whereas expression levels on naive T-cells is low or not detectable. Consistent with an activated phenotype, memory T-cells exhibit significantly higher rates of attachment and accumulation on VCAM-1 under flow as compared to naive T-cells. Interestingly, the expression of activation/ligand induced epitopes on beta1-integrins on Th2 cells and the ability of these cells to interact with VCAM-1 are comparable to memory T-cells. In contrast, Th1 cells did not interact as efficiently with VCAM-1, which correlated with lower expression of activation/ligand induced epitopes on these cells. VCAM-1 interactions are inhibited completely by pretreatment of the T-cells with blocking mAb to alpha4-integrins or beta1-integrins, indicating that alpha4beta1 is the predominant T-cell integrin involved.

CONCLUSIONS

Memory T-cells express constitutively active alpha4beta1-integrins, as compared to naive T-cells, which mediate high rates of initial attachment and sustained high-affinity adhesive interactions with VCAM-1 under flow conditions in vitro. Similarly, in vitro differentiated Th2 cells but not Th1 cells, which also express elevated levels of activated alpha4beta1-integrins, are capable of sustaining high-affinity adhesive interactions with VCAM-1. The differences observed in beta1-integrin activation on T-cell subsets may underlie selective recruitment patterns of T-cell subsets in vivo.

C-C and C-X-C chemokines trigger firm adhesion of monocytes to vascular endothelium under flow conditions

In summary, our findings indicate that specific chemokines that are elaborated by endothelial cells after cytokine or endotoxin activation can play an essential role in monocyte recruitment beyond their chemoattractant activities. We show that this action is to translate initial monocyte tethering into firm adhesion via rapid leukocyte integrin activation. The in vitro model presented here provides a sensitive system for investigating the modulating ability of chemokines and reveals an important biological effect that is not predicted by results in simpler in vitro assays, such as measurement of calcium transients or chemotaxis. The surprising finding that the C-X-C chemokine IL-8 can trigger monocyte firm adhesion to vascular endothelium suggests a potential role for this chemokine in monocyte recruitment and underscores the biological complexity of the chemokine family.

Integrin-mediated neutrophil adhesion and retinal leukostasis in diabetes

PURPOSE

A critical early event in the pathogenesis of diabetic retinopathy is leukocyte adhesion to the diabetic retinal vasculature. The process is mediated, in part, by intercellular adhesion molecule-1 (ICAM-1) and results in blood-retinal barrier breakdown and capillary nonperfusion. This study evaluated the expression and function of the corresponding ICAM-1-binding leukocyte beta2-integrins in experimental diabetes.

METHODS

Diabetes was induced in Long Evans rats with streptozotocin. The expression of the surface integrin subunits CD11a, CD11b, and CD18 on rat neutrophils isolated from peripheral blood was quantitated with flow cytometry. In vitro neutrophil adhesion was studied using quantitative endothelial cell-neutrophil adhesion assays. The adhesive role of the integrin subunits CD11a, CD11b, and CD18 was tested using specific neutralizing monoclonal antibodies. CD18 bioactivity was blocked in vivo with anti-CD18 F(ab')2 fragments, and the effect on retinal leukocyte adhesion was quantitated with acridine orange leukocyte fluorography.

RESULTS

Neutrophil CD11a, CD11b, and CD18 surface integrin levels were 62% (n = 5, P = 0.006), 54% (n = 5, P = 0.045), and 38% (n = 5, P = 0.009) greater in diabetic versus nondiabetic animals, respectively. Seventy-five percent more neutrophils from diabetic versus nondiabetic animals adhered to rat endothelial cell monolayers (n = 6, P = 0.02). Pretreatment of leukocytes with either anti-CD11b or anti-CD18 antibodies lowered the proportion of adherent diabetic neutrophils by 41% (n = 6, P = 0.01 for each treatment), whereas anti-CD11a antibodies had no significant effect (n = 6, P = 0.5). In vivo, systemic administration of anti-CD18 F(ab')2 fragments decreased diabetic retinal leukostasis by 62% (n = 5, P = 0.001).

CONCLUSIONS

Neutrophils from diabetic animals exhibit higher levels of surface integrin expression and integrin-mediated adhesion. In vivo, CD18 blockade significantly decreases leukostasis in the diabetic retinal microvasculature. Integrin adhesion molecules may serve as therapeutic targets for the treatment and/or prevention of early diabetic retinopathy.

Acquisition of selectin binding and peripheral homing properties by CD4(+) and CD8(+) T cells

Different T cell subsets exhibit distinct capacities to migrate into peripheral sites of inflammation, and this may in part reflect differential expression of homing receptors and chemokine receptors. Using an adoptive transfer approach, we examined the ability of functionally distinct subsets of T cells to home to a peripheral inflammatory site. The data directly demonstrate the inability of naive T cells and the ability of effector cells to home to inflamed peritoneum. Furthermore, interleukin (IL)-12 directs the differentiation of either CD4(+) or CD8(+) T cells into effector populations that expresses functional E- and P-selectin ligand and that are preferentially recruited into the inflamed peritoneum compared with T cells differentiated in the presence of IL-4. Recruitment can be blocked by anti-E- and -P-selectin antibodies. The presence of antigen in the peritoneum promotes local proliferation of recruited T cells, and significantly amplifies the Th1 polarization of the lymphocytic infiltrate. Preferential recruitment of Th1 cells into the peritoneum is also seen when cytokine response gene 2 (CRG-2)/interferon gamma-inducible protein 10 (IP-10) is used as the sole inflammatory stimulus. We have also found that P-selectin binds only to antigen-specific T cells in draining lymph nodes after immunization, implying that both antigen- and cytokine-mediated signals are required for expression of functional selectin-ligand.

Memory B lymphocytes from secondary lymphoid organs interact with E-selectin through a novel glycoprotein ligand

Recirculation of B lymphocytes through the secondary lymphoid organs is key for recognition and response to foreign antigen. B lymphocytes within secondary lymphoid organs comprise a heterogeneous population of cells at distinct differentiation stages. To ascribe a particular adhesive behavior to discrete B-cell subsets within secondary lymphoid organs, we investigated their functional interaction with endothelial selectins under flow. We describe herein the characterization of a subset of human tonsillar B cells that interact with E-selectin but not P-selectin. E-selectin-interacting B cells had a phenotype of non-germinal center (CD10(-), CD38(-), CD44(+)), memory (IgD-) cells. Furthermore, FucT-VII was expressed selectively in CD44(+) E-selectin-adherent B lymphocytes. B-cell rolling on E-selectin required sialic acid but was independent of previously described selectin ligands. A novel glycoprotein ligand of 240 kDa carrying N-linked glycans was isolated from B-cell membranes by an E-selectin immunoadhesin. Binding of this protein was strictly Ca2+ dependent, was inhibited by a cell adhesion-blocking mAb against E-selectin, and required the presence of sialic acid but not N-linked carbohydrates. Our results enable us to assign to resident memory B lymphocytes a novel adhesion function, the rolling on E-selectin, that provides insights on the adhesion pathways involved in homing of memory B cells to tertiary sites.

Cellular and molecular mechanisms of inflammation and thrombosis

In the last 20 years, the cellular and molecular mechanisms of inflammation and thrombosis have been characterised. These are essentially cell adhesion processes which are regulated by vascular endothelium. Many of the cell adhesion molecules and leucocyte chemoattractants expressed and generated at sites of inflammation have been sequenced and cloned. These inflammatory molecules work together in concert to mediate the adhesion between leucocytes, platelets and vascular endothelium which occurs during the occlusive, thromboembolic, reperfusion and septic complications of atherosclerotic and diabetic vascular diseases. This review aims to summarise our current understanding of the molecular basis of these disorders and the therapeutic implications.

MCP-1 and IL-8 trigger firm adhesion of monocytes to vascular endothelium under flow conditions

Monocytes contribute to the development of atherosclerotic lesions in mouse models. The chemoattractant proteins (chemokines), monocyte chemoattractant protein-1 (MCP-1) and interleukin-8 (IL-8), are found in human atheroma, and mice lacking receptors for these chemokines are less susceptible to atherosclerosis and have fewer monocytes in vascular lesions. Although MCP-1 has a powerful effect on monocytes, IL-8 is thought to act predominantly on neutrophils and it is unclear how it could recruit monocytes. Here we investigate the ability of chemokines to control the interaction of monocytes under flow conditions with vascular endothelium that has been transduced to express specific leukocyte-adherence receptors. We find that MCP-1 and IL-8 can each rapidly cause rolling monocytes to adhere firmly onto monolayers expressing E-selectin, whereas related chemokines do not. These effects do not correlate with either the induction of a calcium transient or chemotaxis. We conclude that chemokines are important modulators of monocyte-endothelial interactions under flow conditions. Moreover, our finding that IL-8 is a powerful trigger for firm adhesion of monocytes to vascular endothelium reveals an unexpected role for this chemokine in monocyte recruitment.

Expression of functional selectin ligands on Th cells is differentially regulated by IL-12 and IL-4

Immune responses may be qualitatively distinct depending on whether Th1 or Th2 cells predominate at the site of Ag exposure. T cell subset-specific expression of ligands for vascular selectins may underlie the distinct patterns of recruitment of Th1 or Th2 cells to peripheral inflammatory sites. Here we examine the regulation of selectin ligand expression during murine T helper cell differentiation. Large numbers of Th1 cells interacted with E- and P-selectin under defined flow conditions, while few Th2 and no naive T cells interacted. Th1 cells also expressed more fucosyltransferase VII mRNA than naive or Th2 cells. IL-12 induced expression of P-selectin ligands on Ag-activated naive T cells, even in the presence of IL-4, and on established Th2 cells restimulated in the presence of IL-12 and IFN-gamma. In contrast, Ag stimulation alone induced only E-selectin ligand. Interestingly, restimulation of established Th2 cells in the presence of IL-12 and IFN-gamma induced expression of P-selectin ligands but not E-selectin ligands; IFN-gamma alone did not enhance expression of either selectin ligand. In summary, functional P- and E-selectin ligands are expressed on most Th1 cells, few Th2 cells, but not naive T cells. Furthermore, selectin ligand expression is regulated by the cytokine milieu during T cell differentiation. IL-12 induces P-selectin ligand, while IL-4 plays a dominant role in down-regulating E-selectin ligand.

The cutaneous lymphocyte antigen is an essential component of the L-selectin ligand induced on human vascular endothelial cells

L-selectin mediates leukocyte rolling on vascular endothelium during inflammation. Although vascular endothelium can be activated with inflammatory cytokines to express functional L-selectin ligands, these ligands have not been well characterized. In this study, fucosyltransferase VII cDNA (Fuc-TVII) transfection of the EA.hy926 human vascular endothelial cell line (926-FtVII) induced functional L-selectin ligand expression and expression of sialyl Lewisx (sLex), as defined by HECA-452 (cutaneous lymphocyte antigen; CLA) and CSLEX-1 mAbs. Cytokine activation of human umbilical vein endothelial cells (HUVEC) also induced functional L-selectin ligand expression, with increased CLA expression and Fuc-TVII transcription. The majority of L-selectin-dependent lymphocyte attachment to activated HUVEC and 926-FtVII cells was blocked specifically by treating the endothelial cells with the HECA-452 mAb, but not the CSLEX-1 mAb. CLA-bearing ligands on vascular endothelium also required sulfation and appropriate molecular scaffolds for functional activity, but were distinct from the L-selectin ligands previously identified by the MECA-79 mAb. These findings demonstrate that the HECA-452- defined antigen, CLA, is an essential carbohydrate component of vascular L-selectin ligands.

Important contributions of P-selectin glycoprotein ligand-1-mediated secondary capture to human monocyte adhesion to P-selectin, E-selectin, and TNF-alpha-activated endothelium under flow in vitro

In this study, an in vitro flow model and a blocking mAb to P-selectin glycoprotein ligand-1 (PSGL-1) were used to define the role of PSGL-1 in monocyte attachment and rolling on E- and P-selectin and in attachment and accumulation on 6-h TNF-alpha-activated HUVEC. KPL1, an adhesion-blocking mAb directed against the tyrosine sulfate motif of PSGL-1, abolished monocyte-adhesive interactions with P-selectin, but only partially blocked monocyte interaction with E-selectin. Further analysis showed that on E-selectin, KPL1 blocked only secondary (i.e., monocyte/monocyte) interactions, but did not block primary (i.e., monocyte/E-selectin) interactions, with secondary adhesion accounting for 90% of the total adhesive interactions on either E- or P-selectin. On cytokine-activated HUVEC, monocytes initially attached and formed linear strings of adherent cells, which involved both primary and secondary adhesion. PSGL-1 or L-selectin mAb reduced string formation, and the combination of PSGL-1 and L-selectin mAb prevented monocyte strings and inhibited 86% of accumulation. Monocyte attachment and rolling on purified adherent monocytes were also critically dependent on PSGL-1 on the adherent monocytes. These studies document that secondary interactions between monocytes, mediated by PSGL-1, are crucial for monocyte initial attachment, rolling, and accumulation on activated endothelium under laminar shear flow.

Adhesion of monocytes to vascular cell adhesion molecule-1-transduced human endothelial cells: implications for atherogenesis

To study the role of vascular cell adhesion molecule-1 (VCAM-1) in monocyte recruitment and atherogenesis, we constructed a recombinant adenovirus, AdRSVrVCAM-1, carrying the rabbit VCAM-1 cDNA. We have previously shown that AdRSVrVCAM-1-transduced human umbilical vein endothelial cells (HUVECs) support the adhesion of CD4+ CD45RO+ memory T lymphocytes under laminar flow conditions. We now demonstrate that AdRSVrVCAM-1-transduced HUVECs support the adhesion of peripheral blood monocytes at a shear stress of < or = 1.5 dyne/cm2. Although VCAM-1 supported only firm adhesion of lymphocytes, it was able to mediate monocyte rolling, firm adhesion, and transmigration when expressed in the context of otherwise unactivated vascular endothelium. VCAM-1-transduced HUVECs supported the adhesion of as many as 4-fold more monocytes than T cells under laminar flow. The greater monocyte adhesion was explained at least in part by leukocyte-leukocyte interactions (secondary adhesions), which were not seen with T cells. These secondary monocyte interactions were specifically blocked by monoclonal antibodies to L-selectin and P-selectin glycoprotein ligand-1. These data demonstrate that VCAM-1 expressed in the context of unactivated vascular endothelium supports the adhesion of the leukocyte populations present in atherosclerotic plaque and may contribute to the predominance of monocytes over lymphocytes.

A novel P-selectin glycoprotein ligand-1 monoclonal antibody recognizes an epitope within the tyrosine sulfate motif of human PSGL-1 and blocks recognition of both P- and L-selectin

Interactions between P-selectin and P-selectin glycoprotein ligand-1 (PSGL-1) mediate the earliest "rolling" of leukocytes on the lumenal surface of endothelial cells at sites of inflammation. Previously, PSGL-1 has been shown to be the primary mediator of interactions between neutrophils and P-selectin, but studies on the ability of PSGL-1 to mediate interactions between P-selectin and other subsets of leukocytes have yielded variable and conflicting results. A novel IgG monoclonal antibody (MoAb) to human PSGL-1 was generated, and the specificity of this MoAb was confirmed by both flow cytometric analysis and Western blotting of cells transfected with human PSGL-1. This newly developed MoAb, KPL1, inhibited interactions between P-selectin expressing COS cells and either HL60 cells, neutrophils, or lymphocytes. Furthermore, KPL1 completely inhibited interactions between P-selectin and either purified CD4 T cells or neutrophils in a flow assay under physiological conditions, but had no effect on interactions of T cells or neutrophils with E-selectin. In addition, KPL1 blocked interactions between lymphoid cells transfected with L-selectin and COS cells expressing PSGL-1. The KPL1 epitope was mapped to a site within a consensus tyrosine sulfation motif of PSGL-1, previously shown to be essential for interaction with P-selectin and now shown to be essential for interaction with L-selectin, and to be distinct from the epitope identified by the PL1 function blocking anti-PSGL-1 MoAb. Two-color flow cytometry of normal leukocytes showed that while natural killer (NK) cells (CD16(+)), monocytes, CD4 and CD8 T cells, and alpha/beta and gamma/delta T cells were uniformly positive for PSGL-1, B cells expressed low levels of the KPL1 epitope. This low level of KPL1 staining was also observed immunohistologically in germinal centers, which had no detectable KPL1 staining, whereas T-cell areas (interfollicular region) were positive for KPL1. Interestingly, plasma cells in situ and interleukin-6-dependent myeloma cell lines were KPL1(+). Thus, PSGL-1 is expressed on essentially all blood neutrophils, NK cells, B cells, T cells, and monocytes. Variation in tyrosine sulfation during B-cell differentiation may affect the ability of B cells to interact with P- and L-selectin.

Endothelial-dependent mechanisms regulate leukocyte transmigration: a process involving the proteasome and disruption of the vascular endothelial-cadherin complex at endothelial cell-to-cell junctions

Although several adhesion molecules expressed on leukocytes (beta1 and beta2 integrins, platelet endothelial cell adhesion molecule 1 [PECAM-1], and CD47) and on endothelium (intercellular adhesion molecule 1, PECAM-1) have been implicated in leukocyte transendothelial migration, less is known about the role of endothelial lateral junctions during this process. We have shown previously (Read, M.A., A.S. Neish, F.W. Luscinskas, V.J. Palambella, T. Maniatis, and T. Collins. 1995. Immunity. 2:493-506) that inhibitors of the proteasome reduce lymphocyte and neutrophil adhesion and transmigration across TNF-alpha-activated human umbilical vein endothelial cell (EC) monolayers in an in vitro flow model. The current study examined EC lateral junction proteins, principally the vascular endothelial (VE)-cadherin complex and the effects of proteasome inhibitors (MG132 and lactacystin) on lateral junctions during leukocyte adhesion, to gain a better understanding of the role of EC junctions in leukocyte transmigration. Both biochemical and indirect immunofluorescence analyses of the adherens junction zone of EC monolayers revealed that neutrophil adhesion, not transmigration, induced disruption of the VE-cadherin complex and loss of its lateral junction localization. In contrast, PECAM-1, which is located at lateral junctions and is implicated in neutrophil transmigration, was not altered. These findings identify new and interrelated endothelial-dependent mechanisms for leukocyte transmigration that involve alterations in lateral junction structure and a proteasome-dependent event(s).

Reduced intracellular oxidative metabolism promotes firm adhesion of human polymorphonuclear leukocytes to vascular endothelium under flow conditions

The interaction of polymorphonuclear leukocytes (PMN) with the vascular endothelium and their subsequent extravasation to the tissues is a key step during different physiological and pathological processes. In certain of these pathologies the oxygen tension becomes very low, leading to reduced cellular oxidative status. To evaluate the effect of lowering the intracellular redox status in the interaction of PMN with the endothelium, exposure to hypoxic conditions as well as treatment with different antioxidant agents was carried out. PMN exposure to hypoxia enhanced beta2 integrin-dependent adhesion to intercellular adhesion molecule-1-coated surfaces, concomitant with a decrease in the intracellular redox status of the cell. As occurs with hypoxia, treatment with antioxidants produced a decrease in the oxidation state of PMN. These agents enhanced adhesion of PMN to human umbilical vein endothelial cells stimulated with tumor necrosis factor-alpha (TNF-alpha), and this effect was also mediated by beta2 integrins LFA-1 and Mac-1. Adhesion studies under defined laminar flow conditions showed that the antioxidant treatment induced an enhanced adhesion mediated by beta2 integrins with a decrease in the fraction of PMN rolling on TNF-alpha-activated endothelial cells. The up-regulated PMN adhesion was correlated to an increase in the expression and activation of integrin Mac-1, without loss of L-selectin surface expression. Altogether, these results demonstrate that a reduction in the intracellular oxidative state produces an enhanced beta2 integrin-dependent adhesion of PMN to stimulated endothelial cells under conditions of flow.

L-selectin shedding does not regulate human neutrophil attachment, rolling, or transmigration across human vascular endothelium in vitro

Current models of the multistep adhesion cascade for leukocyte-endothelial interactions predict loss of L-selectin from the leukocyte surface before transendothelial migration. We have tested this hypothesis using in vitro adhesion and transendothelial migration assays and a zinc-dependent metalloproteinase inhibitor, Ro 31-9790 (N-2-((2s)-[(hydroxycarbamoyl)methyl)-4-methylvaleryl]-N-1,3 -dimethyl-L-valinamide), which prevents chemoattractant-induced (e.g., IL-8, FMLP, C5a, platelet-activating factor) L-selectin endoproteolytic cleavage from isolated human neutrophils. Inhibitor and vehicle-treated neutrophils exhibited identical behavior during both adhesive interactions with 4- and 24-h TNF-alpha-activated HUVEC monolayers under flow, (including rate of initial attachment, rolling velocities, stable adhesion, and transmigration) and in static adhesion assays. Flow cytometric analysis of transmigrated neutrophils with mAb to L-selectin revealed that vehicle treated neutrophils had minimal detectable surface L-selectin, whereas inhibitor-treated neutrophils retained comparable levels of L-selectin on their surface as neutrophils maintained at 37 degrees C. In addition, mAb to L-selectin that induce rapid shape change and homotypic adhesion (LAM1-116) did not enhance the rate or extent of neutrophil transmigration under flow or static conditions. Neutrophils preincubated with LAM 1-116 displayed similar behavior to neutrophils preincubated with the control anti-L-selectin mAb, LAM1-101. In summary, these results demonstrate that there is no requirement for L-selectin to be shed from the surface of neutrophils before, or during, their migration across endothelial monolayers, and that prevention of surface L-selectin proteolytic cleavage does not enhance or inhibit neutrophil-endothelial cell adhesive interactions.

L-selectin shedding does not regulate human neutrophil attachment, rolling, or transmigration across human vascular endothelium in vitro

Current models of the multistep adhesion cascade for leukocyte-endothelial interactions predict loss of L-selectin from the leukocyte surface before transendothelial migration. We have tested this hypothesis using in vitro adhesion and transendothelial migration assays and a zinc-dependent metalloproteinase inhibitor, Ro 31-9790 (N-2-((2s)-[(hydroxycarbamoyl)methyl)-4-methylvaleryl]-N-1,3 -dimethyl-L-valinamide), which prevents chemoattractant-induced (e.g., IL-8, FMLP, C5a, platelet-activating factor) L-selectin endoproteolytic cleavage from isolated human neutrophils. Inhibitor and vehicle-treated neutrophils exhibited identical behavior during both adhesive interactions with 4- and 24-h TNF-alpha-activated HUVEC monolayers under flow, (including rate of initial attachment, rolling velocities, stable adhesion, and transmigration) and in static adhesion assays. Flow cytometric analysis of transmigrated neutrophils with mAb to L-selectin revealed that vehicle treated neutrophils had minimal detectable surface L-selectin, whereas inhibitor-treated neutrophils retained comparable levels of L-selectin on their surface as neutrophils maintained at 37 degrees C. In addition, mAb to L-selectin that induce rapid shape change and homotypic adhesion (LAM1-116) did not enhance the rate or extent of neutrophil transmigration under flow or static conditions. Neutrophils preincubated with LAM 1-116 displayed similar behavior to neutrophils preincubated with the control anti-L-selectin mAb, LAM1-101. In summary, these results demonstrate that there is no requirement for L-selectin to be shed from the surface of neutrophils before, or during, their migration across endothelial monolayers, and that prevention of surface L-selectin proteolytic cleavage does not enhance or inhibit neutrophil-endothelial cell adhesive interactions.

Isolated P-selectin glycoprotein ligand-1 dynamic adhesion to P- and E-selectin

Leukocyte adhesion to vascular endothelium under flow involves an adhesion cascade consisting of multiple receptor pairs that may function in an overlapping fashion. P-selectin glycoprotein ligand-1 (PSGL-1) and L-selectin have been implicated in neutrophil adhesion to P- and E-selectin under flow conditions. To study, in isolation, the interaction of PSGL-1 with P- and E-selectin under flow, we developed an in vitro model in which various recombinant regions of extracellular PSGL-1 were coupled to 10-microm-diameter microspheres. In a parallel plate chamber with well defined flow conditions, live time video microscopy analyses revealed that microspheres coated with PSGL-1 attached and rolled on 4-h tumor necrosis factor-alpha-activated endothelial cell monolayers, which express high levels of E-selectin, and CHO monolayers stably expressing E- or P-selectin. Further studies using CHO-E and -P monolayers demonstrate that the first 19 amino acids of PSGL-1 are sufficient for attachment and rolling on both E- and P-selectin and suggest that a sialyl Lewis x-containing glycan at Threonine-16 is critical for this sequence of amino acids to mediate attachment to E- and P-selectin. The data also demonstrate that a sulfated, anionic polypeptide segment within the amino terminus of PSGL-1 is necessary for PSGL-1-mediated attachment to P- but not to E-selectin. In addition, the results suggest that PSGL-1 has more than one binding site for E-selectin: one site located within the first 19 amino acids of PSGL-1 and one or more sites located between amino acids 19 through 148.

CD45RA-RO+ (memory) but not CD45RA+RO- (naive) T cells roll efficiently on E- and P-selectin and vascular cell adhesion molecule-1 under flow

This study examines the molecular mechanisms that underlie the observed preferential interactions of memory vs naive T cells with activated vascular endothelium. Many more CD4+ CD45RO+ (memory) cells adhered to 6-h TNF-alpha-activated human umbilical vein endothelium under flow than CD4+CD45RA+ (naive) cells. Adhesion studies were performed using Chinese hamster ovary (CHO) cell monolayers expressing human E- or P-selectin (CHO-E and CHO-P, respectively) or with soluble vascular cell adhesion molecule-1 (VCAM-1)-coated glass surfaces. Under flow at 1.8 dynes/cm2, RO+ T cells rolled extensively at low velocity on both CHO-P and CHO-E monolayers and VCAM-1, whereas very few RA+ T cells interacted with these surfaces. VCAM-1-dependent rolling was blocked completely by anti-very late Ag-4 (VLA-4) Abs. Purified CD4+RA+ T cells could be converted to RO+ cells by mitogen stimulation and 7-day culture in vitro, and this correlated with the acquisition of the ability to roll on E- or P-selectin, but not on VCAM-1 under flow. In summary, these data indicate that CD45RO+ cells interact with E- and P-selectins and VCAM-1 much more effectively than do CD45RA+ cells under flow conditions, and these adhesion pathways may contribute, either individually or in combination, to the preferential recruitment of memory T cells to peripheral sites of inflammation.

CD45RA-RO+ (memory) but not CD45RA+RO- (naive) T cells roll efficiently on E- and P-selectin and vascular cell adhesion molecule-1 under flow

This study examines the molecular mechanisms that underlie the observed preferential interactions of memory vs naive T cells with activated vascular endothelium. Many more CD4+ CD45RO+ (memory) cells adhered to 6-h TNF-alpha-activated human umbilical vein endothelium under flow than CD4+CD45RA+ (naive) cells. Adhesion studies were performed using Chinese hamster ovary (CHO) cell monolayers expressing human E- or P-selectin (CHO-E and CHO-P, respectively) or with soluble vascular cell adhesion molecule-1 (VCAM-1)-coated glass surfaces. Under flow at 1.8 dynes/cm2, RO+ T cells rolled extensively at low velocity on both CHO-P and CHO-E monolayers and VCAM-1, whereas very few RA+ T cells interacted with these surfaces. VCAM-1-dependent rolling was blocked completely by anti-very late Ag-4 (VLA-4) Abs. Purified CD4+RA+ T cells could be converted to RO+ cells by mitogen stimulation and 7-day culture in vitro, and this correlated with the acquisition of the ability to roll on E- or P-selectin, but not on VCAM-1 under flow. In summary, these data indicate that CD45RO+ cells interact with E- and P-selectins and VCAM-1 much more effectively than do CD45RA+ cells under flow conditions, and these adhesion pathways may contribute, either individually or in combination, to the preferential recruitment of memory T cells to peripheral sites of inflammation.

Adhesion of memory lymphocytes to vascular cell adhesion molecule-1-transduced human vascular endothelial cells under simulated physiological flow conditions in vitro

The accumulation of mononuclear leukocytes is an early and persistent finding in atherosclerotic plaques. These mononuclear leukocytes are mostly monocyte-derived, but up to 20% are lymphocytes, predominantly CD4+ CD45RO+ (memory) T cells. To evaluate the potential of adenovirus vectors for studies of mononuclear leukocyte recruitment in vitro, we studied the effects of adenovirus vectors per se on human umbilical vein endothelial cells (HUVECs), a well-characterized in vitro model of vascular endothelium. A recombinant adenovirus containing the seven-domain isoform of rabbit vascular cell adhesion molecule-1 (rVCAM-1) was constructed and used to study lymphocyte adhesion under defined laminar flow conditions in transduced HUVEC monolayers. No increase in basal HUVEC surface expression of the inducible endothelial adhesion molecules and markers of activation, E-selectin and VCAM-1, was noted across a broad range of multiplicity of infection. A modest dose-dependent increase in surface intercellular adhesion molecule-1 expression was detectable by flow cytometry at an MOI of > 30 plaque-forming units per cell. Under defined laminar flow from 1.5 to 0.5 dyne/cm2, the adenovirus vector carrying rVCAM-1 mediated stable adhesion of both a Jurkat T-cell line and primary human CD4+ CD45RO+ (memory) T cells. Monoclonal antibodies to alpha 4-integrin or rVCAM-1 abolished adhesion, whereas monoclonal antibodies to CD18 or P-selectin had no effect. We conclude that adenoviral gene transfer in useful for studies of VCAM-1-dependent leukocyte adhesion in vitro and that endothelial expression of VCAM-1 alone, in the absence of over endothelial cell activation, is sufficient under simulated physiological flow conditions to support adhesion of memory T cells, the predominant lymphocyte subset in atherosclerotic plaque.

A human colon carcinoma cell line exhibits adhesive interactions with P-selectin under fluid flow via a PSGL-1-independent mechanism

It has been postulated that endothelial cell adhesion molecules involved in leukocyte recruitment play a role in metastasis. Using an in vitro flow model, we studied the adhesion of the human colon carcinoma cell line KM12-L4 to P-selectin, an inducible endothelial-expressed adhesion molecule involved in leukocyte recruitment. Recombinant forms of P-selectin and Chinese hamster ovary cells stably expressing P-selectin supported attachment and rolling of KM12-L4 cells at 1 to 2 dynes/cm2. The adhesive interactions to P-selectin were abolished by pretreatment of the KM12-L4 cells with neuraminidase but were unaltered by pretreatment of the KM12-L4 cells with O-sialoglycoprotein endopeptidase, an enzyme that cleaves mucin type glycoproteins such as P-selectin glycoprotein ligand-1 (PSGL-1). PSGL-1 is the only counter-receptor for P-selectin known to mediate myeloid cell adhesion to P-selectin under flow. Flow cytometric and Northern blot analyses revealed that KM12-L4 cells did not express PSGL-1 and monoclonal antibody PL1, a function-blocking monoclonal antibody to PSGL-1, had no inhibitory effect on KM12-L4 adhesion to P-selectin under flow. Compared with HL-60 cells, which express PSGL-1, the KM12-L4 cells exhibited a slightly lower rate of attachment to P-selectin and rolled at a significantly higher velocity. In summary, KM12-L4 human colon carcinoma cells interact with P-selectin, under flow, through a PSGL-1-independent adhesion pathway.

Immunology of atherosclerosis: the promise of mouse models

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L- and P-selectins, but not CD49d (VLA-4) integrins, mediate monocyte initial attachment to TNF-alpha-activated vascular endothelium under flow in vitro

Monocyte adhesion to the vascular endothelium is a pivotal step during their egress to tissues at sites of inflammation and immune reactions, and during atherogenesis. In this study, an in vitro flow model and blocking mAb were used to define the role of adhesion molecules in monocyte interactions with activated HUVEC under flow conditions. By videomicroscopy, freely flowing monocytes abruptly halted (initial attachment) on 6-h TNF-alpha-activated HUVEC under flow via L- and P-selectin, whereas E-selectin was not involved. CD49d/CD29 integrin (VLA-4), which can mediate initial attachment of certain T cells to VCAM-1 under flow, did not support monocyte initial attachment. Once initially attached, a small number of monocytes began rolling at 9 microns/s through a mechanism involving L-selectin, as well as CD49d and CD11/CD18 integrins, while the remaining monocytes became firmly adherent, or released to the flow stream. Monocyte stable arrest and subsequent transendothelial migration occurred rapidly and efficiently through either CD49d or CD18 integrin adhesion pathways. Transendothelial passage was also dependent on PECAM-1 (CD31). These data reveal monocytes initially attach to activated endothelium via an L-selectin-dependent mechanism, with a smaller contribution from P-selectin and no contribution by CD49d. Subsequent monocyte rolling, arrest, and transmigration require overlapping functions between multiple members of the selectin, integrin, and Ig gene families.

L- and P-selectins, but not CD49d (VLA-4) integrins, mediate monocyte initial attachment to TNF-alpha-activated vascular endothelium under flow in vitro

Monocyte adhesion to the vascular endothelium is a pivotal step during their egress to tissues at sites of inflammation and immune reactions, and during atherogenesis. In this study, an in vitro flow model and blocking mAb were used to define the role of adhesion molecules in monocyte interactions with activated HUVEC under flow conditions. By videomicroscopy, freely flowing monocytes abruptly halted (initial attachment) on 6-h TNF-alpha-activated HUVEC under flow via L- and P-selectin, whereas E-selectin was not involved. CD49d/CD29 integrin (VLA-4), which can mediate initial attachment of certain T cells to VCAM-1 under flow, did not support monocyte initial attachment. Once initially attached, a small number of monocytes began rolling at 9 microns/s through a mechanism involving L-selectin, as well as CD49d and CD11/CD18 integrins, while the remaining monocytes became firmly adherent, or released to the flow stream. Monocyte stable arrest and subsequent transendothelial migration occurred rapidly and efficiently through either CD49d or CD18 integrin adhesion pathways. Transendothelial passage was also dependent on PECAM-1 (CD31). These data reveal monocytes initially attach to activated endothelium via an L-selectin-dependent mechanism, with a smaller contribution from P-selectin and no contribution by CD49d. Subsequent monocyte rolling, arrest, and transmigration require overlapping functions between multiple members of the selectin, integrin, and Ig gene families.

Salicylates inhibit I kappa B-alpha phosphorylation, endothelial-leukocyte adhesion molecule expression, and neutrophil transmigration

The expression of leukocyte adhesion molecules on endothelial cells is induced by TNF-alpha and other inflammatory cytokines. This induction of endothelial-leukocyte adhesion molecule-1, vascular cell adhesion molecule-1, and intercellular adhesion molecule-1 requires the transcription factor nuclear factor-kappa B (NF-kappa B). Recent work has suggested that some nonsteroidal anti-inflammatory agents, including sodium salicylate and aspirin, can inhibit NF-kappa B-dependent gene activation. We studied the effects of salicylates on expression of adhesion molecules in HUVECs. We found that sodium salicylate inhibited activation of NF-kappa B (p50/p65 and p65/p65) by preventing phosphorylation and subsequent degradation of the inhibitor 1 kappa B-alpha. Salicylate treatment had no effect on TNF-alpha-induced phosphorylation of the transcription factor ATF-2. Salicylate blocked the TNF-alpha-induced increase in mRNA levels of adhesion molecules and gave a dose-dependent inhibition of TNF-alpha-induced surface expression of vascular cell adhesion molecule-1, and intercellular adhesion molecule-1 with higher doses required to inhibit endothelial-leukocyte adhesion molecule-1 expression. Indomethacin, a nonsalicylate cyclooxygenase inhibitor, had no effect on surface expression of adhesion molecules, suggesting that the effects were not due to inhibition of cyclooxygenase. Treatment of endothelial cell monolayers with sodium salicylate inhibited transendothelial migration of neutrophils but had no significant effect on neutrophil adhesion under flow conditions. The clinical importance of high-dose salicylates in inflammation may be due, in part, to the ability to prevent expression of inducible adhesion molecules and recruitment of leukocytes.

Salicylates inhibit I kappa B-alpha phosphorylation, endothelial-leukocyte adhesion molecule expression, and neutrophil transmigration

The expression of leukocyte adhesion molecules on endothelial cells is induced by TNF-alpha and other inflammatory cytokines. This induction of endothelial-leukocyte adhesion molecule-1, vascular cell adhesion molecule-1, and intercellular adhesion molecule-1 requires the transcription factor nuclear factor-kappa B (NF-kappa B). Recent work has suggested that some nonsteroidal anti-inflammatory agents, including sodium salicylate and aspirin, can inhibit NF-kappa B-dependent gene activation. We studied the effects of salicylates on expression of adhesion molecules in HUVECs. We found that sodium salicylate inhibited activation of NF-kappa B (p50/p65 and p65/p65) by preventing phosphorylation and subsequent degradation of the inhibitor 1 kappa B-alpha. Salicylate treatment had no effect on TNF-alpha-induced phosphorylation of the transcription factor ATF-2. Salicylate blocked the TNF-alpha-induced increase in mRNA levels of adhesion molecules and gave a dose-dependent inhibition of TNF-alpha-induced surface expression of vascular cell adhesion molecule-1, and intercellular adhesion molecule-1 with higher doses required to inhibit endothelial-leukocyte adhesion molecule-1 expression. Indomethacin, a nonsalicylate cyclooxygenase inhibitor, had no effect on surface expression of adhesion molecules, suggesting that the effects were not due to inhibition of cyclooxygenase. Treatment of endothelial cell monolayers with sodium salicylate inhibited transendothelial migration of neutrophils but had no significant effect on neutrophil adhesion under flow conditions. The clinical importance of high-dose salicylates in inflammation may be due, in part, to the ability to prevent expression of inducible adhesion molecules and recruitment of leukocytes.

Endothelial-dependent mechanisms in chronic inflammatory leukocyte recruitment

Peripheral blood leukocytes interact with the vascular endothelium in a wide range of physiologic and pathophysiologic situations. A current working concept is that activation of vascular endothelium is an important event during the inflammatory response, conferring spatial and temporal localization and leukocyte-type selectivity to the recruitment process. This chapter highlights recent advances in our understanding of the endothelial-dependent molecular mechanisms that mediate recruitment of mononuclear leukocytes (lymphocytes and monocytes) and discusses these advances in the context of chronic inflammatory diseases and their potential therapeutic interventions.

Activation-dependent isolation and culture of murine pulmonary microvascular endothelium

OBJECTIVE

Establish a reproducible method for the isolation and cultivation of murine pulmonary microvascular endothelium. To this end, we exploited the localized pattern of microvascular endothelial activation induced in vivo by inflammatory stimuli to isolate a subpopulation of endothelium for in vitro study.

METHODS

Immunohistochemical analyses of the pulmonary vasculature of mice treated systemically with gram-negative bacterial endotoxin (LPS) demonstrated selective expression of VCAM-1 (CD106) in the endothelial lining of small collecting veins, venules, septal capillaries, and, infrequently, small arteries, which was not observed in control mice. Single cell suspensions prepared by enzymatic dissociation of peripheral lobular tissues dissected from the lungs of LPS-stimulated mice were incubated with a phycoerythrin-conjugated antimouse VCAM-1 monoclonal antibody (MK 1.91). Cells expressing this antigen were isolated by sterile fluorescence-activated cell sorting (FACS). Positive cell populations were collected and cultured for 1-2 weeks. When confluent, these primary cultures were further FACS enriched for endothelium, positively selecting for cells incorporating a fluorescent derivative of acetylated low density lipoprotein (Di-I-Ac-LDL).

RESULTS

The resulting population of cells (mouse lung endothelial cells, MLEC) were uniformly positive for the endothelial markers von Willebrand factor, thrombomodulin, and Dil-Ac-LDL uptake. MLEC readily formed tube-like structures when cultured on Matrigel and spontaneously demonstrated a sprouting phenotype on fibronectin or collagen matrices. MLEC retained responsiveness to cytokines (IL-1 alpha, IL-1 beta, TNF alpha, IFN gamma) up to at least eight passages from primary culture and demonstrated upregulation of E-selectin (CD62E) and P-selectin (CD62P) mRNA as early as 2 hr after LPS stimulation. Characteristic temporal expression patterns of cell surface E-selectin (maximal at 4 hr and declining toward baseline by 24 hr), VCAM-1 (maximal at 6-8 hr and remaining elevated for 24-48 hr), and ICAM-1 (maximal at 6-8 hr and maintained at 24 hr) were observed when cultured MLEC were treated with recombinant murine TNF alpha or recombinant human (rh) IL-1 alpha or rhIL-1 beta. The rolling, adhesion, and transmigration of human polymorphonuclear leukocytes was markedly increased on cytokine-activated MLEC monolayers under defined flow conditions.

CONCLUSION

The strategy of activation-dependent isolation allows for the reproducible selection of a specific subset of microvascular endothelial cells for in vitro study. This experimental approach should further facilitate study of the functional heterogeneity of endothelium and its pathophysiologic dysfunction.

Immunoselective targeting of an anti-thrombin agent to the surface of cytokine-activated vascular endothelial cells

An immunoconjugate was designed to target hirudin, a potent and specific inhibitor of thrombin, to the surface of activated endothelial cells. Hirudin was covalently cross-linked to the monoclonal antibody H18/7 that recognizes the extracellular domain of E-selectin (CD62E), an endothelium-leukocyte adhesion molecule that is expressed only on cytokine-activated endothelium. The hirudin-H18/7 immunoconjugate selectively bound to interleukin-1-activated but not to unactivated cultured human umbilical vein endothelial cells with a temporal profile similar to that of inducible cell-surface procoagulant activity. When bound to activated endothelial cells, the hirudin-H18/7 immunoconjugate significantly inhibited endogenous thrombin activity generated from coincubated human plasma and fibrin clot formation on the monolayer surface. Cellular responses that are mediated via the thrombin receptor, such as increases in cytoskeletal F-actin content, also were significantly downregulated, and monolayers were protected from thrombin-induced disruption by this treatment. The ability to selectively antagonize thrombin-dependent processes at the endothelium-blood interface may provide new insights into complex pathophysiological processes, such as thrombosis, inflammation, and atherogenesis. These studies also demonstrate the general feasibility of selective targeting of therapeutic agents to endothelial cells based on recognition of an activation-dependent surface phenotype.

MCP-1-stimulated monocytes preferentially utilize beta 2-integrins to migrate on laminin and fibronectin

Recruitment of monocytes to inflammatory sites involves a series of sequential attachments and detachments to extracellular matrix proteins in response to a chemoattractant gradient. In this study we compared the migration of human peripheral blood monocytes on different extracellular matrix proteins in response to monocyte chemoattractant protein-1 (MCP-1) and N-formylmethionyl-leucyl-phenylalanine. Monocytes migrated more effectively on laminin compared with other extracellular matrix proteins. In contrast, this preference was not observed with neutrophils, suggesting that the monocytes and neutrophils may have differences in their migration on extracellular matrix proteins. To study this further, function-blocking monoclonal antibodies were used to examine mechanistically whether beta 1- and beta 2-integrins were involved in monocyte migration on fibronectin or laminin in response to MCP-1. Monocyte migration on both laminin and fibronectin was blocked 100% (P < 0.05) by intact monoclonal antibody, F(ab') fragments, and F(ab')2 fragments to beta 2-integrins. We also determined that antibodies to beta 2-integrins block monocyte migration that has already been initiated. In contrast, antibody to the beta 1-integrins inhibited monocyte migration by approximately 40% (P < 0.05). Thus monocytes that express both beta 1- and beta 2-integrins require utilization of beta 2-integrins in migration on extracellular matrix proteins. The results also suggest that beta 1-integrins facilitate monocyte migration but that monocyte migration is not absolutely dependent on the interaction of beta 1-integrins with extracellular matrix proteins. In contrast, neutrophil migration is beta 2-integrin dependent and is not facilitated by beta 1-integrins.

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.

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.

MCP-1-stimulated monocyte attachment to laminin is mediated by beta 2-integrins

Migration of monocytes to sites of inflammation involves a series of attachments and detachments to extracellular matrix proteins. We examined the capacity of a chemokine, monocyte chemoattractant protein-1 (MCP-1), to regulate attachment of human monocytes to laminin, collagen I, collagen IV, or fibronectin. MCP-1 increased monocyte attachment to laminin in a dose- and time-dependent manner and stimulated a lesser increase to the other matrix proteins. Function-blocking monoclonal antibodies (MAbs) to the integrin beta 2-subunit (CD18), including Fab' fragments and alpha M (CD11b) blocked > 70% of attachment, whereas MAbs to the beta 1-integrin subunit reduced attachment by < 30%. This suggests that the CD11b/CD18 integrin is the predominant molecule involved in adhesion of MCP-1-stimulated monocytes to laminin. The association of CD11b with F-actin illustrated by confocal microscopy further supports this concept. In contrast, when monocytes were stimulated with the beta 1-stimulatory MAb TS2/16, monocyte adhesion to laminin occurred through beta 1-integrins. Thus MCP-1 can stimulate monocyte attachment to laminin, and this process is mediated through beta 2-integrins, principally CD11b/CD18.

Integrins as dynamic regulators of vascular function

The vascular endothelium lines the entire cardiovascular system and serves as a nonthrombogenic and selectively permeable boundary between the blood-stream and extravascular space. Endothelial cells are polar cells that are continuously subjected to fluid-generated forces on their luminal surface whereas their abluminal surface resides on basement membranes/extracellular matrix. The integrin family of cell-surface heterodimeric glycoproteins is located along both of these surfaces and participates in maintaining the normal endothelium and in the dynamic changes associated with the pathophysiology of the endothelium. Endothelial cell beta 1 and beta 3 integrins function together with other families of adhesion molecules during vasculogenesis, angiogenesis, inflammation, and wound healing. Leukocyte beta 1 and beta 2 integrins, in conjunction with members of the Ig and selectin gene families expressed on endothelium, mediate leukocyte recruitment to sites of inflammation. The structural and functional properties of integrins make them uniquely suited to mediate these essential and complex processes in the vasculature.

Monoclonal antibody blockade of L-selectin inhibits mononuclear leukocyte recruitment to inflammatory sites in vivo

L-selectin interacting with inducible endothelial counterreceptors mediates in part the initial adhesive interactions, termed rolling, between circulating blood leukocytes and vascular endothelium. While blockade of L-selectin function in in vivo models of inflammation reduces both neutrophil and lymphocyte influx at early times, little is known concerning the role of L-selectin in leukocyte recruitment at later times (> 24 hours). Using an in vivo murine model of experimentally induced inflammation of the peritoneum, the role of L-selectin in recruitment of mononuclear leukocytes to chronic sites of inflammation (48 hours) was investigated. Saturating levels of function blocking anti-L-selectin monoclonal antibody (MEL-14) or control rat IgG were maintained for 48 hours using surgically implanted mini-osmotic pumps; this treatment did not alter the circulating leukocyte cell count or differential. In animals receiving MEL-14 monoclonal antibody (MAb), macrophage and lymphocyte accumulation in response to thioglycollate was reduced by 60% (P < or = 0.0002) and > 90% (P < 0.001), respectively, at 48 hours as compared with animals implanted with pumps containing saline. Similarly, MEL-14 MAb dramatically inhibited granulocyte influx by 80% (P < 0.03) at 6 hours; recruitment at 24 and 48 hours was reduced by 50%. In contrast, the effects of purified rat IgG was not significantly different from saline. Our results suggest L-selectin, interacting with its inducible endothelial counterreceptor(s), plays an important role in circulating mononuclear leukocyte extravasation at sites of inflammation.

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.

Fluid flow modulates vascular endothelial cytosolic calcium responses to adenine nucleotides

OBJECTIVE

To determine whether fluid flow influences the action of soluble vasoactive agonists on vascular endothelium.

METHODS

Confluent monolayers of bovine aortic endothelial cells (BAEC) were cultured on glass coverslips, prelabeled with the Ca(2+)-sensitive dye fura-2, and placed in a parallel-plate flow chamber designed to generate defined laminar fluid flow. Cytosolic free Ca2+ concentration ([Ca2+]i) in individual BAEC was monitored during perfusion with medium containing adenine nucleotide under defined flow conditions.

RESULTS

Continuous perfusion with ATP (0.3-3.0 microM) or ADP (0.1-1.0 microM) evoked repetitive oscillations in [Ca2+]i in individual BAEC. The frequency of the [Ca2+]i oscillations was dependent on both nucleotide concentration and levels of applied shear stress; at constant bulk concentration of nucleotide, the frequency increased with shear stress. Stopping flow in the continuous presence of agonists immediately extinguished the oscillatory response. Elimination of extracellular Ca2+ did not inhibit the [Ca2+]i oscillations. In the presence of nonhydrolyzable nucleotide analog, ATP gamma S or ADP beta S, application of flow resulted in similar shear-dependent [Ca2+]i oscillations, suggesting that flow modulation of the [Ca2+]i response was not simply due to depletion of ATP or ADP in the vicinity of BAEC monolayers as a result of hydrolysis of nucleotides by ectonucleotidases.

CONCLUSIONS

These findings suggest that local hemodynamic conditions may modulate the action of vasoactive agents on the vascular endothelium in vivo.

Intravenous interleukin-8 inhibits granulocyte emigration from rabbit mesenteric venules without altering L-selectin expression or leukocyte rolling

Injection (i.v.) of the granulocyte chemoattractant/activator IL-8 has been shown to reduce neutrophil recruitment into dermal inflammatory sites in vivo. To further investigate the mechanism of this phenomenon, we examined the effect of i.v. [Ser-IL-8]72 (12-20 micrograms/kg) on leukocyte rolling and chemoattractant-induced emigration in mesenteric venules of New Zealand White rabbits and on expression of L-selectin (mAb LAM1-3) and CD18 (mAb 60.3) on circulating rabbit granulocytes. Within 1 min of IL-8 i.v., granulocytes virtually disappeared from carotid blood samples for approximately 5 min. Concomitantly, the flux of rolling leukocytes in mesenteric venules fell from 83 +/- 21 to 2 +/- 1 leukocytes/min. Both rolling leukocyte flux and systemic granulocyte count returned to or exceeded control values within less than 30 min. The chemoattractant/activator FMLP (0.15 microgram/kg i.v.) produced similar results. A second i.v. injection of IL-8 or FMLP, 90 min after the first challenge, had equipotent effects. Local extravascular application of IL-8 via micropipette close to a venule induced adhesion and emigration of 63 +/- 21 leukocytes per site before, but only 26 +/- 9 leukocytes per site 50 to 75 min after i.v. IL-8, when systemic granulocyte count and rolling leukocyte flux had reached or exceeded control values. This was not due to agonist-specific desensitization, because a similar reduction of leukocyte emigration was seen after FMLP i.v. Rabbit granulocytes circulating in vivo uniformly expressed near-control levels of L-selectin at all times between 3 and 360 min after IL-8 i.v. CD18 expression transiently increased after IL-8 i.v. and returned to base line by 90 min. These findings show that IL-8 i.v. reduces granulocyte recruitment to inflammatory sites by inhibiting function(s) necessary for transmigration that are independent of L-selectin and subsequent to rolling.

Intravenous interleukin-8 inhibits granulocyte emigration from rabbit mesenteric venules without altering L-selectin expression or leukocyte rolling

Injection (i.v.) of the granulocyte chemoattractant/activator IL-8 has been shown to reduce neutrophil recruitment into dermal inflammatory sites in vivo. To further investigate the mechanism of this phenomenon, we examined the effect of i.v. [Ser-IL-8]72 (12-20 micrograms/kg) on leukocyte rolling and chemoattractant-induced emigration in mesenteric venules of New Zealand White rabbits and on expression of L-selectin (mAb LAM1-3) and CD18 (mAb 60.3) on circulating rabbit granulocytes. Within 1 min of IL-8 i.v., granulocytes virtually disappeared from carotid blood samples for approximately 5 min. Concomitantly, the flux of rolling leukocytes in mesenteric venules fell from 83 +/- 21 to 2 +/- 1 leukocytes/min. Both rolling leukocyte flux and systemic granulocyte count returned to or exceeded control values within less than 30 min. The chemoattractant/activator FMLP (0.15 microgram/kg i.v.) produced similar results. A second i.v. injection of IL-8 or FMLP, 90 min after the first challenge, had equipotent effects. Local extravascular application of IL-8 via micropipette close to a venule induced adhesion and emigration of 63 +/- 21 leukocytes per site before, but only 26 +/- 9 leukocytes per site 50 to 75 min after i.v. IL-8, when systemic granulocyte count and rolling leukocyte flux had reached or exceeded control values. This was not due to agonist-specific desensitization, because a similar reduction of leukocyte emigration was seen after FMLP i.v. Rabbit granulocytes circulating in vivo uniformly expressed near-control levels of L-selectin at all times between 3 and 360 min after IL-8 i.v. CD18 expression transiently increased after IL-8 i.v. and returned to base line by 90 min. These findings show that IL-8 i.v. reduces granulocyte recruitment to inflammatory sites by inhibiting function(s) necessary for transmigration that are independent of L-selectin and subsequent to rolling.