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The Journal of Immunology, 2002, 168: 588-596.
Copyright © 2002 by The American Association of Immunologists

Migration of Human Hematopoietic Progenitor Cells Across Bone Marrow Endothelium Is Regulated by Vascular Endothelial Cadherin1

Jaap D. van Buul*, Carlijn Voermans*, Veronique van den Berg*, Eloise C. Anthony*, Frederik P. J. Mul*, Sandra van Wetering*, C. Ellen van der Schoot*,{dagger} and Peter L. Hordijk2,*

* Department of Experimental Immunohematology, CLB and Laboratory for Experimental and Clinical Immunology, and {dagger} Department of Hematology, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
The success of stem cell transplantation depends on the ability of i.v. infused stem cells to engraft the bone marrow, a process referred to as homing. Efficient homing requires migration of CD34+ cells across the bone marrow endothelium, most likely through the intercellular junctions. In this study, we show that loss of vascular endothelial (VE)-cadherin-mediated endothelial cell-cell adhesion increases the permeability of monolayers of human bone marrow endothelial cells (HBMECs) and stimulates the transendothelial migration of CD34+ cells in response to stromal cell-derived factor-1{alpha}. Stromal cell-derived factor-1{alpha}-induced migration was dependent on VCAM-1 and ICAM-1, even in the absence of VE-cadherin function. Cross-linking of ICAM-1 to mimic the leukocyte-endothelium interaction induced actin stress fiber formation but did not induce loss of endothelial integrity, whereas cross-linking of VCAM-1 increased the HBMEC permeability and induced gaps in the monolayer. In addition, VCAM-1-mediated gap formation in HBMEC was accompanied by and dependent on the production of reactive oxygen species. These data suggest that modulation of VE-cadherin function directly affects the efficiency of transendothelial migration of CD34+ cells and that activation of ICAM-1 and, in particular, VCAM-1 plays an important role in this process through reorganization of the endothelial actin cytoskeleton and by modulating the integrity of the bone marrow endothelium through the production of reactive oxygen species.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
Hematopoietic stem cell transplantation is applied to restore hematopoiesis in cancer patients after myelo-ablative chemotherapy and/or after irradiation. The success of the transplantation depends on the ability of the hematopoietic stem cells to engraft the bone marrow, a process referred to as homing (1). An important step in homing is the actual transmigration of reinfused stem cells across the bone marrow endothelium to the bone marrow stroma. Although much is known about the migration of granulocytes and T cells, the factors that control the transendothelial migration of hematopoietic stem cells are still poorly understood.

Recently, the first powerful chemoattractant for hematopoietic stem cells (CD34+ cells) has been described and identified as stromal cell-derived factor-1{alpha} (SDF-1{alpha}).3 SDF-1{alpha} is produced by several types of stromal cell, including those of the bone marrow (2, 3, 4, 5), and signals through a G protein-coupled receptor called Fusin, leukocyte-derived seven-transmembrane domain receptor, or CXCR-4 (6, 7, 8, 9). SDF-1{alpha}-driven homing of CD34+ cells has been suggested to be a multistep process similar to the extravasation process of leukocytes at inflammatory sites (10) and is mediated by adhesion molecules both on CD34+ cells (11) and on bone marrow endothelial cells. In the final stage of homing, CD34+ cells migrate across the bone marrow endothelium, presumably via the intercellular junctions. Therefore, endothelial cell-cell adhesion is most likely an important regulatory factor in the homing of CD34+ cells.

Endothelial cell-cell adhesion is largely dependent on the homotypic cell-cell adhesion molecule vascular endothelial (VE)-cadherin (cadherin-5, CD144). VE-cadherin is a transmembrane protein that, like other members of the cadherin family (12), associates via its cytoplasmic tail with various cytosolic proteins, including {alpha}-, {beta}-, and {gamma}-catenin (plakoglobin) and p120/p100. These proteins link VE-cadherin to the cortical actin cytoskeleton (13, 14, 15). The role of VE-cadherin in leukocyte transendothelial migration was first described by Gotsch et al. (16), who showed an accelerated extravasation of neutrophils in a mouse peritonitis model in vivo upon i.v. injection of a mAb against mouse VE-cadherin. Transfection experiments and gene inactivation studies have shown that VE-cadherin expression reduces monolayer permeability and promotes cell aggregation, motility, and growth, and that VE-cadherin is required for the organization of vascular-like structures in embryoid bodies (17, 18, 19). Moreover, VE-cadherin, together with {beta}-catenin, seems to be involved in cell survival (20). Regulation of VE-cadherin, and thereby of endothelial cell-cell adhesion, may occur through tyrosine or serine phosphorylation (21, 22, 23, 24), association with regulatory proteins (20), and modulation of the endothelial actin cytoskeleton (17, 25). In addition, several studies have proposed a role for leukocyte adhesion-induced signaling, e.g., through activation of myosin light chain kinase in the endothelial cells, which may indirectly regulate VE-cadherin function in the process of transendothelial migration (26, 27, 28, 29).

A role for specific adhesion molecules in endothelial cell signaling has been suggested by studies on VCAM-1 (CD106) and ICAM-1 (CD54). Lorenzon et al. (30) concluded that VCAM-1 and endothelial selectins, in addition to their role as adhesion receptors, mediate endothelial stimulation by adherent leukocytes. Moreover, activation of ICAM-1 on endothelial cells after binding of T cells has been reported to induce tyrosine phosphorylation of the actin-binding protein cortactin (31). In line with this observation, it was suggested that ICAM-1 mediates cell shape changes through coupling to the p21Rho GTPase and by inducing phosphorylation of cytoskeletal proteins and transcription factors (32).

In the present study, the role of VE-cadherin in the control of permeability of bone marrow endothelium and of the transmigration of primary CD34+ cells was investigated. For this purpose, we used immortalized human bone marrow endothelial cells (HBMECs) (33) and primary CD34+ cells isolated from cord blood (CB) or from peripheral blood (PB) of healthy untreated volunteers. In this study, we show that transendothelial migration of primary CD34+ cells is promoted by reduced VE-cadherin-mediated cell-cell adhesion and is accompanied by a focal loss of VE-cadherin at sites of transmigration. Cross-linking of VCAM-1, but not ICAM-1, was found to induce loss of endothelial cell-cell adhesion and increased permeability. VCAM-1-mediated gap formation was accompanied by and dependent on the generation of reactive oxygen species (ROS). These data indicate that activation of Ig-like adhesion molecules, in particular VCAM-1, regulate VE-cadherin function, which might facilitate SDF-1{alpha}-driven transendothelial migration of primary CD34+ cells across the bone marrow endothelium.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Materials

mAbs against VE-cadherin (cl75) and {beta}-catenin were from BD Transduction Laboratories (Amsterdam, The Netherlands). VE-cadherin Ab 7H1 was from BD PharMingen (San Diego, CA). Hybridoma supernatant TEA1.31 was a kind gift from Dr. E. Dejana (Instituto di Ricerche Farmacologiche Mario Negri, Milan, Italy). Polyclonal Ab against VE-cadherin (C19) was from SanverTech (Heerhugowaard, The Netherlands). Recombinant human IL-1{beta} was from PeproTech (Rocky Hill, NJ); calcein-acetoxymethyl, Texas-Red phalloidin, FITC-Dextran 3000, ALEXA 488-labeled goat anti-mouse Ig (G{alpha}M-Ig), and ALEXA 488-labeled goat anti-rabbit Ig secondary Abs were from Molecular Probes (Leiden, The Netherlands). PE-labeled secondary Abs and botulinum C3 toxin were from DAKO (Glostrup, Denmark). Pooled human serum, human serum albumin, fibronectin (FN), and control Abs IgG1 and IgG2a were obtained from the CLB (Amsterdam, The Netherlands). FCS was from Life Technologies (Paisley, U.K.). Basic fibroblast factor was from Boehringer Mannheim (Mannheim, Germany). CXCR-4 expression was quantitated with PE-labeled anti-human Fusin (12G5; BD PharMingen). mAbs against ICAM-1 (84H10) and VCAM-1 (1G11) were purchased from Immunotech (Marseille, France). Additional mAbs against ICAM-1 (15.2; CLB) and VCAM-1 (4B2; PeproTech) were also used to exclude epitope dependence of the effects. Cross-linking studies were performed with F(ab')2 of G{alpha}M-IgG from Jackson ImmunoResearch Laboratories (Baltimore, MD). Thrombin and N-acetyl-cysteine (N-AC) were from Sigma-Aldrich (St. Louis, MO).

Isolation of CD34+ hematopoietic progenitor cells

CB was collected after delivery, according to the guidelines of Eurocord, and PB from healthy volunteers was obtained from the local blood bank. These volunteers were normal donors that were not treated with G-CSF or with chemotherapy. Mononuclear leukocytes from PB (500 ml) and from CB were enriched by density gradient centrifugation over Ficoll-Paque (1.077 g/ml; Pharmacia Biotech, Uppsala, Sweden). Then the PB mononuclear fraction was purified from thrombocytes by elutriation and further processed, similar to CB CD34+ cell isolation, with the VarioMacs system (Miltenyi Biotec, Gladbach, Germany) as described (2). At least 95% of the cells from CB and >90% of the cells from PB expressed CD34 as determined by FACS analysis with a CD34 Ab (no. 581; Immunotech).

Cell cultures

The HBMEC line has been described previously (33). The cells were cultured in FN-coated culture flasks (Nunc, Roskilde, Denmark; Life Technologies) in Medium 199 (Life Technologies) supplemented with 10% (v/v) pooled, heat-inactivated human serum, 10% (v/v) heat-inactivated FCS, 1 ng/ml basic fibroblast factor, 5 U/ml heparin, 300 µg/ml glutamine, 100 U/ml penicillin, and 100 µg/ml streptomycin. After reaching confluency, the cells were passaged by treatment with trypsin/EDTA (Life Technologies). In all experiments, HBMEC monolayers were pretreated with IL-1{beta} for 4 h. HL-60 and KG-1a cell lines were obtained from the American Type Culture Collection (Manassas, VA) and were maintained in IMDM (BioWhittaker, Brussels, Belgium) containing L-glutamine, 100 U/ml penicillin, 100 µg/ml streptomycin, and 10% FCS. All cell lines were cultured at 37°C at 5% CO2.

Permeability assay

Permeability of HBMEC monolayers, cultured on 5-µm-pore, 6.5-mm Transwell filters (Costar, Cambridge, MA), was assayed using FITC-labeled 3000 Dextran as described (17). The permeability response to thrombin (1 U/ml) after 30 min was set to 100%. In some experiments, monolayers were pretreated with Abs (10 µg/ml) for 30 min. Unbound blocking Abs against VCAM-1 and ICAM-1 were washed away before the start of the permeability or transmigration assays. Cross-linking Abs and Abs to VE-cadherin were present during the permeability or transmigration assay. Botulinum C3 toxin was added to the HBMEC monolayers 6 h before the assay. In our previous publication (17), the incubation time for C3 was 18 h. However, we found that a 6-h incubation of HBMEC with C3 was already sufficient to inhibit actin polymerization. The 6-h incubation with C3 did not affect the VE-cadherin localization, whereas the 18-h incubation with C3 resulted in diffuse staining of VE-cadherin (17). Kinetics of VCAM-1-mediated increase in permeability was performed as described by Corada et al. (34). In brief, at indicated time points, 50 µl were taken from the lower compartment and fluorescence was measured. After the assay, filters were washed with ice-cold Ca2+- and Mg2+-containing PBS and then fixed with 2% paraformaldehyde and 1% Triton X-100-containing PBS and stained with Texas-Red phalloidin to inspect the HBMEC monolayer by confocal laser scanning microscopy.

Transendothelial migration assay

Migration assays were performed in Transwell plates of 6.5-mm diameter with 5-µm pore filters. Endothelial cells were plated at 20,000–30,000 cells/Transwell on FN-coated filters. Nonadherent cells were removed after 18 h. The adherent cells were cultured for 2–3 days to obtain confluent endothelial monolayers. Monolayers of endothelial cells were pretreated for 4 h with IL-1{beta}. Before adding CD34+ cells to the upper compartment, the endothelial monolayers were washed three times with assay medium (IMDM with 0.25% (w/v) BSA (fraction V; Sigma-Aldrich)). Freshly isolated CD34+ cells (50,000–100,000) were added to the upper compartment in 0.1 ml of assay medium, and 0.6 ml of assay medium with or without the indicated concentrations of recombinant human SDF-1{alpha} (Strathmann Biotech, Hannover, Germany) was added to the lower compartment. A 0.1-ml sample containing cells in assay medium was diluted in 0.5 ml of assay medium and was kept as input control for quantitation of the number of migrated cells. The Transwell plates were incubated at 37°C, 5% CO2, for 4 h. Preliminary experiments showed that after 4 h, a substantial fraction of the CD34+ cells had migrated. Cells that had migrated to the lower compartment were collected in a FACS tube to which a fixed number of control cell line cells (kG-1a) labeled with calcein-acetoxymethyl was added. FACS analysis was used to determine the ratio between labeled and unlabeled cells, with characteristic light scatter parameters, in the migrated fraction as described before (2). By comparison of this ratio to that of the input control, the number of migrated cells was quantitated. Using this method, we were able to determine reliably a minimum number of 200 migrated cells. In blocking experiments, HBMECs were preincubated for 30 min at 37°C with mAbs (10 µg/ml), followed by washing. As controls, IgG1 and IgG2a isotypes were used. Blocking Abs against VCAM-1 and ICAM-1 were not present during the transendothelial migration assay. However, blocking Abs against VE-cadherin (cl75) were present during the transendothelial migration assay. After the assay, the filters were fixed and stained with Texas-Red phalloidin to inspect HBMEC monolayers by confocal laser scanning microscopy.

Immunocytochemistry

HBMECs were cultured on FN-coated glass coverslips and were fixed and immunostained as described (17) with mAb against VE-cadherin (7H1, 10 µg/ml) or anti-{beta}-catenin (10 µg/ml) followed by staining with fluorescently labeled secondary Abs (10 µg/ml). Filamentous actin (F-actin) was visualized by Texas-Red phalloidin (1 U/ml). In some experiments, cells were pretreated with mAb against VE-cadherin (blocking cl75, 10 µg/ml; partially blocking TEA1.31; hybridoma supernatant 1/2 dilution; or the nonblocking 7H1, 10 µg/ml) for 30 min. In the analysis after the permeability experiments, a polyclonal Ab against VE-cadherin was used for immunostaining. Images were recorded with a confocal microscope with appropriate filter settings (LSM510; Zeiss, Oberkochen, Germany). Cross-talk between the green and red channels was avoided by use of sequential scanning.

Analysis of ROS production

To measure intracellular ROS production, HBMECs were cultured on FN-coated six-well plates and loaded with di-hydro-rhodamine-1,2,3 (DHR; 30 µM; Molecular Probes) for 60 min in the presence of catalase (60 µg/ml) and NaN3 (2 mM) at 37°C. Adhesion molecules were subsequently cross-linked at 37°C as described in Permeability assay. After 30 min, cells were incubated for 1 min with trypsin at 37°C, collected on ice, and washed with ice-cold Ca2+- and Mg2+-containing PBS, and DHR fluorescence was quantitated by FACS in the FL-2 channel ({lambda}EX, 488 nm; {lambda}EM, 585 nm) for 10,000 counted cells per incubation. For the kinetics of the production of ROS, the endothelial cells were cultured on FN-coated glass coverslips and loaded as described above. Then ROS production was recorded and quantitated by time-lapse confocal microscopy of DHR-loaded HBMECs. Intensity values are shown as the percentage of increase relative to the values at the start of the experiment. In some experiments, cells were pretreated for 18 h with 5 mM of the oxygen radical scavenger N-AC to prevent ROS-mediated signaling. FACS analysis showed that N-AC preincubation had no effect on the IL-1{beta}-mediated up-regulation of adhesion molecules such as VCAM-1 and ICAM-1.

Statistics

All results that were performed at least three times were expressed as the mean ± SD or SEM, as indicated in the legend. Differences were tested by using the Student t test. A value of p < 0.05 was considered significant.


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
Role of VE-cadherin in permeability and integrity of HBMEC monolayers

In the initial series of experiments, the role of VE-cadherin in the control of monolayer integrity of HBMECs was examined. Ab-mediated inhibition of VE-cadherin function using two independent blocking Abs resulted in increased permeability of HBMEC monolayers, whereas a nonblocking, isotype-matched VE-cadherin Ab and an irrelevant IgG1 did not have any effect on the permeability of HBMECs (Fig. 1Goa). Immunofluorescent staining of IL-1{beta}-prestimulated HBMECs after treatment with the nonblocking VE-cadherin Ab showed a jagged distribution of VE-cadherin (Fig. 1GobA) and normal F-actin stress fiber formation (Fig. 1GobB). At the ends of the F-actin stress fibers, colocalization of VE-cadherin with actin was observed (Fig. 1GobC), as previously described for primary HUVECs (17, 34, 35). Pretreatment of HBMEC monolayers with a blocking Ab against VE-cadherin (cl75) resulted in a redistribution of VE-cadherin over the cell surface (Fig. 1GobD) and a marked reorganization of the actin cytoskeleton (Fig. 1GobE). Although the partially blocking TEA1.31 Ab to VE-cadherin did increase the HBMEC permeability (Fig. 1Goa), it did not dramatically affect the localization of VE-cadherin (Fig. 1GobG) or the actin cytoskeleton of the HBMECs (Fig. 1GobH), in agreement with published results obtained with primary HUVECs (35). These findings underscore the essential role of VE-cadherin in the regulation of the integrity of HBMEC monolayers and show that its function (i.e., control of permeability) and localization can be modulated differentially by different blocking Abs.



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  		FIGURE 1. a, VE-cadherin-mediated permeability of HBMEC monolayers. Cells were grown to confluency on FN-coated Transwell filters and prestimulated with IL-1{beta}, followed by pretreatment for 30 min with Abs (10 µg/ml) or hybridoma supernatant TEA1.31 (tissue culture supernatant diluted 1/2 in medium). The Transwells were incubated for 3 h with FITC-Dextran 3000 in the upper compartment in the presence of the Abs. Next, fluorescence in the lower compartment was measured in a fluorometer ({lambda}EX, 485 nm; {lambda}EM, 525 nm) and expressed as described in Materials and Methods. The blocking c175 and the partially blocking TEA1.31 Abs to VE-cadherin, but not the isotype control Ab to VE-cadherin 7H1, increased monolayer permeability significantly. Thrombin-induced permeability was set to 100%, as described in Materials and Methods. Data are mean ± SEM of at least three independent experiments (aA, p < 0.001). b, Blocking Abs to VE-cadherin alter VE-cadherin distribution and induce cytoskeletal reorganization. HBMECs were grown to confluency on FN-coated glass coverslips, pretreated with IL-1{beta}, and incubated with the different anti-VE-cadherin Abs for 1 h. VE-cadherin and F-actin were visualized as described in Materials and Methods. The overlays show VE-cadherin in green (A, D, and G) and F-actin in red (B, E, and H); colocalization appears in yellow (C, F, and I). The nonblocking 7H1 Ab (10 µg/ml) did not affect VE-cadherin distribution, whereas the blocking cl75 Ab (10 µg/ml) caused loss of junctional localization of VE-cadherin (D) and a reorganization of the F-actin cytoskeleton as revealed by the loss of stress fibers (E). The TEA1.31 Ab induced a partial loss of cell-cell contacts (H) which, however, was not as prominent as with the blocking cl75 Ab. VE-cadherin remained localized to cell-cell junctions (G). Bar, 50 µm.

 
Role of VE-cadherin in transmigration of CD34+ cells across HBMECs

Purified primary CD34+ cells from CB and PB and the human leukemic cell line HL-60, which all express CXCR-4, were tested for their ability to migrate across FN or HBMECs to a gradient of SDF-1{alpha}. HL-60 cells migrated across FN with a similar efficiency as primary PB CD34+ cells to 30 ng/ml SDF-1{alpha}, whereas primary CB CD34+ cells showed a higher migration efficiency (Fig. 2Goa). However, CXCR-4 expression of PB and CB CD34+ was comparable (data not shown). Because of the limited supply of primary CD34+ cells, CXCR-4-expressing HL-60 cells were used as a model in dose response studies of SDF-1{alpha}-induced migration across IL-1{beta}-prestimulated HBMECs. The results show a bell-shaped response with optimal migration at 70 ng/ml SDF-1{alpha} (Fig. 2Gob). When VE-cadherin was blocked with the cl75 Ab on IL-1{beta}-prestimulated HBMECs, a shift of the optimal concentration for migration from 70 to 30 ng/ml SDF-1{alpha} and a significant increase in migration across HBMECs were observed (Fig. 2Gob). Moreover, migration of primary CB CD34+ cells to 30 ng/ml SDF-1{alpha} across HBMECs pretreated with either the partially blocking TEA1.31 or the blocking cl75 Ab resulted in a significant increase in transmigration efficiency (Fig. 2Goc). These findings show that VE-cadherin function is an important regulator of efficient migration of primary CD34+ cells across bone marrow endothelium.



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  		FIGURE 2. a, Migration of primary CD34+ cells across FN. SDF-1{alpha} (30 ng/ml)-induced migration of CB CD34+ cells (hatched bars) across FN-coated filters was significantly higher than the migration of HL-60 cells (filled bars) or PB CD34+ cells from healthy untreated volunteers (open bars). b, SDF-1{alpha}-induced transendothelial migration of HL-60 cells. Dose response of SDF-1{alpha}-induced migration of HL-60 cells across HBMECs showed a bell-shaped curve with an optimal migration at 70 ng/ml SDF-1{alpha} (filled bars). Open bars represent HBMECs that were pretreated for 30 min and subsequently incubated during the assay with blocking Ab cl75 against VE-cadherin (10 µg/ml), resulting in a significantly increased migration at 30 ng/ml SDF-1{alpha} and a shift of the dose for optimal migration from 70 to 30 ng/ml SDF-1{alpha}. c, Effect of Ab-mediated loss of VE-cadherin function on SDF-1{alpha}-induced transendothelial migration of primary CB CD34+ cells. Pretreatment and incubation of HBMECs with the blocking cl75 Ab (10 µg/ml) or the partially blocking TEA1.31 hybridoma supernatant to VE-cadherin showed a significant increase in transmigration to SDF-1{alpha}. Data are mean ± SD of at least three independent experiments. bA and cA, p < 0.05; cB, p < 0.01; aC, p < 0.001.

 
A more detailed analysis of VE-cadherin distribution revealed a focal loss of VE-cadherin immunostaining at sites of transmigration of CD34+ cells (Fig. 3Goa) where stress fibers seemed to converge at the periphery of the transmigrating CD34+ cell (Fig. 3Go, b and c). The cell is fixed during its passage through the filter and the endothelial junctions (Fig. 3God). Similarly, also {beta}-catenin was redistributed over the surface of HBMECs at such sites (data not shown). These observations suggest a coordinated interaction between the actin cytoskeleton and VE-cadherin at sites of transmigrating CD34+ cells.



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  		FIGURE 3. Transmigration of CB CD34+ cells induces focal loss of VE-cadherin. Primary CB CD34+ cells and HBMECs were stained and fixed after 3 h of transmigration. Loss of VE-cadherin localization (green) was observed at the site of migration of a CB CD34+ cell, indicated by the arrowhead (a). The migrating cell is visualized by F-actin staining in red (b, arrowhead). Yellow indicates colocalization of F-actin and VE-cadherin at the periphery of the migrating cell (c, arrowhead). The X-Z section shows the same migrating CD34+ cell in red, protruding into a filter pore (d). The dashed gray arrow in d corresponds to the gray arrow in the schematic. The drawing represents a cell (red) protruding into the endothelial cells (gray). Bar, 10 µm.

 
Role of p21Rho in VE-cadherin-mediated transmigration of CD34+ cells and in the permeability and integrity of HBMEC monolayers

Blocking VE-cadherin function by the cl75 Ab was accompanied by a reorganization of the actin cytoskeleton (Fig. 1Gob). Together with the results shown in Fig. 3Go, these findings indicate that the actin cytoskeleton, in concert with VE-cadherin, regulates barrier function in HBMECs. Regulation of cytoskeletal contractility in endothelial cells can be mediated by changes in cAMP levels (36) and by the small GTPase p21Rho (37, 38). p21Rho is required for actin stress fiber formation in response to extracellular stimuli in many cell types and has also been implicated in the organization of cadherin-based cell-cell adhesion in epithelial cells (39, 40). Moreover, inactivation of p21Rho by pretreatment of primary HUVECs with the botulinum C3 toxin causes cytoskeletal reorganization, mainly reflected in a loss of actin stress fibers (17). To investigate the role of p21Rho in the control of transendothelial migration of CD34+ cells and endothelial permeability, HBMECs were pretreated with the C3 toxin. As a result, a loss of actin stress fibers (Fig. 4Goa), but no loss of VE-cadherin localization (Fig. 4Gob), was observed. When, in addition to C3 pretreatment, the blocking cl75 Ab was used (Fig. 4God), VE-cadherin was diffusely distributed over the cell surface (Fig. 4Goe) and the cells had lost cell-cell contact as a consequence of reduced VE-cadherin function. This result also indicates that cellular contractility is not an absolute requirement for the Ab-mediated loss of VE-cadherin-mediated cell-cell adhesion. Inactivation of p21Rho by C3 did not significantly affect the migration of primary CD34+ cells across HBMECs (Fig. 4Gog), indicating a minor role for p21Rho in the control of transmigration of primary CD34+ cells. Moreover, inhibition of active p21Rho by C3 did not affect the permeability of HBMECs either (Fig. 4Goh).



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  		FIGURE 4. Role of p21Rho in the transmigration of CB CD34+ cells and in permeability and integrity of HBMEC monolayers. HBMECs were treated for 6 h with 10 µg/ml C3 toxin to inactivate p21Rho and were immunostained for F-actin and VE-cadherin. The treatment with C3 caused a loss of stress fibers (a), but staining of VE-cadherin remained localized to cell-cell junctions (b). Colocalization appears in yellow (c). In addition to the C3 treatment, the monolayer was pretreated for 30 min with the blocking c175 Ab (10 µg/ml), which caused a loss of HBMEC cell-cell adhesion (d) and diffuse staining of VE-cadherin over the surface (e). Colocalization appears in yellow (f). Inactivation of p21Rho had no effect on the SDF-1{alpha}-induced migration of CB CD34+ cells across HBMECs with or without treatment with the blocking c175 Ab (10 µg/ml) (g). Similarly, inactivation of p21Rho had no significant effect on the permeability of HBMECs to FITC-Dextran 3000 (h). The VE-cadherin Ab remained present during the assays. Data are mean ± SD of three independent experiments. {alpha}-VE, VE-cadherin Ab cl75. Bar, 50 µm.

 
Role of ICAM-1 and VCAM-1 in VE-cadherin-mediated transmigration of CD34+ cells

It was previously shown by our group and others that the {beta}1 integrins very late Ag (VLA)-4 and VLA-5 are required for efficient transendothelial migration of CD34+ cells (2, 11, 41, 42). Therefore, we investigated which adhesion molecules that are known ligands for VLA-4 and VLA-5 mediate the migration of CD34+ cells across HBMECs. ICAM-1 is highly expressed on IL-1{beta}-prestimulated HBMECs, whereas VCAM-1 is expressed at a lower level. Pretreatment of the HBMEC monolayers with blocking Abs to ICAM-1 inhibited the transmigration of PB and CB CD34+ cells across HBMECs. Blocking Abs against VCAM-1 also inhibited the transmigration of PB and CB CD34+ cells. The combination of Abs against ICAM-1 and VCAM-1 inhibited the transmigration of PB and CB CD34+ cells significantly (Fig. 5Go, a and b, respectively). After pretreatment of the HBMECs with the blocking cl75 Ab to VE-cadherin, the transmigration of untreated PB CD34+ (Fig. 5Goa) and CB CD34+ cells (Fig. 5Gob) remained dependent on ICAM-1 and VCAM-1. Other blocking mAbs to VCAM-1 and ICAM-1 showed similar results.



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  		FIGURE 5. Role for endothelial adhesion molecules in SDF-1{alpha}-induced transmigration of primary CD34+ cells across HBMECs. a, HBMECs were cultured on Transwell filters and prestimulated with IL-1{beta} as described in Materials and Methods. Before the addition of the CD34+ cells from PB of healthy untreated volunteers to the upper compartment, the monolayers on the filter were incubated for 30 min with blocking Abs against adhesion molecules, followed by washing. mAbs against ICAM-1 (10 µg/ml; {alpha}-ICAM-1) and to VCAM-1 (10 µg/ml; {alpha}-VCAM-1) inhibited the SDF-1{alpha}-induced migration of PB CD34+ cells (30 ng/ml SDF-1{alpha}). The combination of Abs against ICAM-1 and VCAM-1 ({alpha}-I-1 + {alpha}-V-1) showed a significant inhibition of the transmigration. Pretreatment of HBMECs with blocking Ab c175 against VE-cadherin (10 µg/ml, filled bars; VE-cadherin Ab present during the assay) increased basal migration of PB CD34+ cells. Under these conditions, ICAM-1 was still required for efficient migration. b, mAbs against ICAM-1 (10 µg/ml) and VCAM-1 (10 µg/ml) also inhibited the migration of CB CD34+ cells. Blocking Ab cl75 against VE-cadherin (10 µg/ml, filled bars) increased basal SDF-1{alpha}-induced migration of CB CD34+ cells, but mAbs against ICAM-1 and VCAM-1 were still able to inhibit. Data are mean ± SD of at least three independent experiments. bA, p < 0.05; aB and bB, p < 0.01; aC, p < 0.001.

 
Role of ICAM-1 and VCAM-1 in permeability and integrity of HBMEC monolayers

As was shown in Figs. 1Go and 2Go, loss of VE-cadherin function increased the permeability of HBMEC monolayers and promoted transendothelial migration of CD34+ cells. To establish a role for intracellular signaling in the control of VE-cadherin-mediated cell-cell adhesion, induced by specific adhesion molecules, permeability assays in combination with Ab-mediated cross-linking were performed. Cross-linking of ICAM-1 resulted in a significant decrease in the HBMEC permeability (Fig. 6Goa). In contrast, cross-linking of VCAM-1 induced a significant increase in the permeability of HBMEC monolayers (Fig. 6Goa). Cross-linking of ICAM-1 induced stress fiber formation (Fig. 6GobD), which was not accompanied, however, by altered VE-cadherin distribution at cell-cell junctions (Fig. 6GobE) or discernible gaps in the HBMEC monolayer (Fig. 6GobF). In contrast, cross-linking of VCAM-1 did induce loss of cell-cell contacts of endothelial cells (Figs. 6GobG and 7d) and concomitant loss of VE-cadherin localization at sites of gap formation (Fig. 6Go, bH and bI). Kinetics of VCAM-1-induced permeability showed maximal increase in permeability already after 90 min (Fig. 6Goa, inset).



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  		FIGURE 6. a, Effect of cross-linking of adhesion molecules on the permeability of HBMEC monolayers. HBMEC monolayer permeability was measured after pretreatment of the endothelial cells with mAbs against ICAM-1, VCAM-1, or IgG1 (30 min, 10 µg/ml). Together with the addition of FITC-Dextran 3000, the Abs were cross-linked with G{alpha}M-IgG, which remained present during the 3-h assay. Cross-linking of ICAM-1 decreased the permeability of HBMECs (p < 0.001, compared with control), whereas cross-linking of VCAM-1 showed an increase in permeability of HBMECs (p < 0.001, compared with control). The oxygen radical scavenger N-AC significantly inhibited the permeability induced by VCAM-1 cross-linking (V1 + N-AC) (p < 0.05, compared with cross-linked VCAM-1-induced permeability). Thrombin-induced permeability after 30 min was set to 100%. Data are mean ± SD of at least three independent experiments. Inset, Kinetics of VCAM-1-induced permeability, measured as described in Materials and Methods. {circ}, Control; {blacksquare}, VCAM-1 cross-linking. b, Effect of cross-linking of adhesion molecules on the morphology of HBMECs. HBMECs were grown to confluency on FN-coated glass coverslips, treated as indicated, fixed, and stained for VE-cadherin and F-actin. The images show F-actin in red (A, D, and G) and VE-cadherin (VE-cad) in green, immunostained with a polyclonal Ab (B, E, and H). Colocalization appears in yellow (C, F, and I). HBMEC monolayers were incubated for 30 min with mAbs against ICAM-1 (X-ICAM-1), followed by 30 min of cross-linking with G{alpha}M-IgG. F-actin staining showed induction of stress fibers, but this was not accompanied by the formation of gaps between the cells (D). VE-cadherin was localized normally to cell-cell junctions (E). Cross-linking of VCAM-1 (X-VCAM-1) induced stress fibers and gap formation in the HBMEC monolayer (G), with loss of discrete VE-cadherin localization at sites of absent cell-cell contacts (H). Staining of endothelial nuclei was due to nonspecific binding of the ALEXA 488-labeled goat anti-rabbit Ig secondary Ab. Bar, 20 µm.

 
Role of ROS in VCAM-1-mediated gap formation

Cross-linking of VCAM-1 on activated HUVECs has been reported to be accompanied by the production of low levels of ROS (43). Moreover, ROS have been described to mediate intercellular gap formation, cell shape change, and F-actin cytoskeletal reorganization in endothelial cells (44). In line with these data, we found that incubation with the oxygen radical scavenger N-AC significantly inhibited the VCAM-1-mediated increase in permeability (Fig. 6Goa). In addition, cross-linking of VCAM-1, but not ICAM-1, on HBMECs induced ROS production (Fig. 7Goa). Kinetic analysis, performed by time-lapse confocal microscopy, revealed a rapid increase in ROS production in HBMECs within 5–15 min after VCAM-1 cross-linking (Fig. 7Gob). Moreover, transmigration of primary CB CD34+ cells across HBMECs was significantly diminished when HBMECs were pretreated with N-AC, indicating a role for oxygen radicals during transendothelial migration (Fig. 7Goc). In addition, pretreatment of HBMECs with N-AC, which by itself did not affect the actin cytoskeleton or VE-cadherin distribution, prevented VCAM-1-induced gap formation (Fig. 7God).



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  		FIGURE 7. VCAM-1 activation induces generation of ROS in HBMECs. a, VCAM-1 and ICAM-1 were cross-linked as described in Materials and Methods on DHR-loaded and IL-1{beta}-pretreated HBMECs. After 30 min, the production of ROS was analyzed by FACS as described in Materials and Methods. Cross-linking of VCAM-1 showed a significant 20% increase in ROS production compared with control levels (A, p < 0.05), whereas ICAM-1 cross-linking had no effect. Ten millimolar H2O2 was used as a positive control and increased the DHR response by 44% (data not shown). Data are mean ± SD of three independent experiments. b, Kinetics of ROS production were measured by time-lapse confocal microscopy during 30 min of VCAM-1 cross-linking as described in Materials and Methods. Data are shown as percentage increase relative to the values at the start of the experiment. {blacksquare}, Control; {square}, VCAM-1 cross-linking. Data represent the mean of two independent experiments. c, Role for ROS in SDF-1{alpha}-induced transendothelial migration of primary CB CD34+ cells. Pretreatment and incubation of HBMECs with the oxygen radical scavenger N-AC (5 mM) showed a significant decrease in transmigration to SDF-1{alpha}. Data represent the mean ± SD of at least three independent experiments (cA, p < 0.05). d, Phase-contrast images show that gap formation, indicated by the arrowheads, occurred already after 15 min of VCAM-1 cross-linking. To prevent ROS-mediated signaling, HBMECs were pretreated for 18 h with 5 mM N-AC. This treatment prevented VCAM-1-mediated gap formation. Data are representative of three independent experiments. Bar, 100 µm.

 
Together, these results suggest that, upon adhesion of CD34+ cells, activation of VCAM-1, probably in conjunction with ICAM-1, induces signaling in the endothelial cells that leads to increased stress fiber formation and reduced endothelial cell-cell adhesion.


   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
The present study demonstrates that VE-cadherin is a crucial regulator of the permeability of HBMECs and that its adhesive properties control the efficiency of migration of hematopoietic progenitor cells (CD34+ cells) across HBMECs. In addition, endothelial adhesion molecules such as ICAM-1 and, in particular, VCAM-1 were found to be important for CD34+ cell transmigration and, upon activation, for the modulation of endothelial integrity.

Primary CD34+ cells induced a focal loss of VE-cadherin and of {beta}-catenin at sites of transmigration. This focal loss of VE-cadherin was reversible. The normal jagged pattern of VE-cadherin distribution was restored after the transmigration assays (data not shown). A similar phenomenon was also reported by Allport et al. (35), who studied monocyte migration across HUVECs under flow, albeit in the absence of a chemotactic gradient. One of the proposed mechanisms for the loss of VE-cadherin function during leukocyte passage is the trapdoor mechanism: the VE-cadherin-catenin complex is mechanically pushed aside by the migrating leukocyte and simply re-adheres after leukocyte passage (35). However, despite the fact that the VE-cadherin distribution was lost at sites of CD34+ cell transmigration, the migration across HBMECs remained dependent on VCAM-1- and ICAM-1-mediated adhesion in the absence of VE-cadherin function. It is important to note that the transendothelial migration assay is performed under static conditions. Although the VE-cadherin distribution during transmigration under static conditions was similar to what Allport and colleagues (35) found using shear stress, the effect of blocking Abs to VCAM-1 and ICAM-1 on the transendothelial migration of CD34+ cells under flow remains to be investigated. Recently, Cinamon et al. (45) showed that shear stress itself induced migration of lymphocytes across endothelium. Therefore, shear stress might also have its effect on stem cell homing and on the role of specific adhesion molecules in this process.

Conflicting results on the role of VCAM-1 and ICAM-1 in the transmigration of CD34+ cells have been published (46, 47). Therefore, we tested the hypothesis that leukocyte-endothelium interactions, mediated by VCAM-1 and ICAM-1, induce endothelial signaling that might control endothelial cell-cell adhesion. Cross-linking experiments supported a role for VCAM-1 in the regulation of endothelial cell-cell junctions. In contrast, ICAM-1 failed to induce changes in endothelial cell-cell junctions or formation of gaps between the cells after cross-linking, despite the induction of actin stress fibers. Similar findings have been reported by Del Maschio et al. (28). Moreover, activation of the small GTPase p21Rho by ICAM-1 cross-linking in HUVECs has been suggested (32). Our studies also indicate that in HBMECs, ICAM-1 may activate Rho, as deduced from the induction of stress fibers. However, this response was not sufficient to induce loss of HBMEC integrity. In line with this notion, inhibition of p21Rho by the C3 toxin did not affect the transmigration of CD34+ cells or the integrity of HBMEC monolayers, suggesting only a minor role for the p21Rho GTPase in the control of CD34+ cell transmigration. Although several reports suggest a signaling role for ICAM-1 during leukocyte transmigration, e.g., through activation of p21Rho (36, 48, 49), these findings indicate a differential, possibly cell-type-specific role for a p21Rho signaling pathway in the control of endothelial monolayer integrity during leukocyte transmigration. Moreover, in the absence of p21Rho activity, the endothelial cell-cell contacts are still reduced upon Ab-mediated loss of VE-cadherin function. This finding shows that gap formation, induced by anti-VE-cadherin Ab or cross-linking of VCAM-1, might occur independently of cellular contractility.

The increased HBMEC permeability after VCAM-1 cross-linking and the accompanying formation of endothelial gaps suggest that this Ig-like cell adhesion molecule can act as a bona fide signaling receptor. Lorenzon et al. (30) already described a prominent role for VCAM-1 in the induction of cytoskeletal changes in endothelial cells, although these authors did not analyze changes in endothelial permeability or VE-cadherin localization. Our present work adds new data to this issue in that the production of ROS appears to be a crucial signaling event in VCAM-1-induced gap formation. The fact that VCAM-1-induced loss of cell-cell adhesion is dependent on the production of ROS represents a new mechanism by which cell surface molecules can modulate cadherin-mediated cell-cell adhesion. However, activation of ICAM-1 did not increase significant generation of ROS or gap formation or changes of VE-cadherin localization in HBMECs. Therefore, ICAM-1 seems to function mainly as an endothelial adhesion molecule for CD34+ cells rather than as a signaling molecule in HBMECs. In contrast, VCAM-1 displays, besides its adhesive role in the interaction between the CD34+ cells and the bone marrow endothelium, a function as a signaling receptor that modulates HBMEC cell-cell contact, presumably through the production of ROS. Interestingly, ICAM-1-mediated production of ROS in HUVECs has recently been implicated in cytoskeletal remodeling (50). Although we did not observe ROS production upon ICAM-1 activation in HBMECs, this might very well represent a general signaling event, involved in adhesion-mediated signaling in endothelial cells. Decreased migration of CD34+ cells across HBMECs, which were pretreated with the oxygen radical scavenger N-AC, supports the role for intracellular endothelial ROS production during transmigration.

Obviously, there remain many unresolved issues with respect to this signaling pathway. First, the nature of the oxidase-generating system in endothelial cells is obscure, although it has been suggested that endothelial cells express a similar NADPH oxidase, as is also found in neutrophils (51). However, additional oxidase-like enzymes have recently been described that may be expressed in endothelial cells as well (52). Second, the pathway through which the endothelial oxidase is activated by cell surface receptors is unknown. Studies on the neutrophil NADPH oxidase suggest an extremely complex mechanism of activation involving Ser/Thr kinases, small GTPases, and correct intracellular targeting and assembly of the NADPH oxidase complex (53, 54, 55). Finally, although an important role for ROS has been described in the regulation of endothelial barrier function (44), the components of the pathway by which ROS lead to altered cadherin function are unclear.

An intriguing possibility is that ROS production plays a more general role in receptor-mediated modulation of endothelial cell-cell adhesion. Our own recent results, which showed the importance of ROS production for endothelial cell migration and for thrombin-mediated loss of cell-cell adhesion (J. D. van Buul, S. van Wetering, and P. L. Hordijk, unpublished observations), underscore the relevance for this signaling event in endothelial cell function.

Our present results suggest that the transendothelial migration of primary CD34+ cells requires VCAM-1 and ICAM-1 for firm adhesion to the bone marrow endothelium and that the binding to VCAM-1 may lead to the focal loss of VE-cadherin function through the production of ROS. Detailed analysis of VCAM-1-induced signaling to VE-cadherin-mediated cell-cell adhesion thus will be of relevance, not only for our understanding of the role of the endothelium in transmigration of hematopoietic stem cells, but also for our insight in endothelial cell function, which is important in a wide range of (patho)physiological conditions.


   Acknowledgments
 
We thank Dr. D. Roos for critically reading the manuscript and Dr. E. Dejana for the TEA1.31 Ab.


   Footnotes
 
1 This work was supported by Dutch Cancer Society Grants CLB99-2000 (to J.D.v.B. and E.C.A.) and CLB2001-2544 (to C.V.) and by Landsteiner Foundation for Blood Transfusion Research Grant 9910 (to F.P.J.M. and S.v.W.). Back

2 Address correspondence and reprint requests to Dr. Peter L. Hordijk, Department of Experimental Immunohematology, CLB, Academic Medical Center, University of Amsterdam, Plesmanlaan 125, 1066 CX, Amsterdam, The Netherlands. E-mail address: Hordijk{at}clb.nl Back

3 Abbreviations used in this paper: SDF-1{alpha}, stromal cell-derived factor-1{alpha}; VE, vascular endothelial; HBMEC, human bone marrow endothelial cell; CB, cord blood; PB, peripheral blood; ROS, reactive oxygen species; G{alpha}M-Ig, goat anti-mouse Ig; FN, fibronectin; N-AC, N-acetyl-cysteine; F-actin, filamentous actin; DHR, di-hydro-rhodamine-1,2,3; VLA, very late Ag. Back

Received for publication July 23, 2001. Accepted for publication November 16, 2001.


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