Kinetics of the different steps during neutrophil migration through cultured endothelial monolayers treated with tumour necrosis factor-alpha

In vitro and in vivo assessment of biocompatibility of AZ31 alloy as biliary stents: a preclinical approach

INTRODUCTION

Biomaterial technology due to its lack of or minimal side effects in tissues has great potential. Traditionally biomaterials used were cobalt-chromium, stainless steel and nitinol alloys. Biomaterials such as magnesium (Mg) and zinc (Zn) have good biocompatibility and consequently can be a potential material for medical implants. To date, the effects of AZ31 alloy stent on cell apoptosis are still unclear. The current investigation was designed to determine the effect of AZ31 alloy stent on necrosis and apoptosis of common bile duct (CBD) epithelial cells.

MATERIAL AND METHODS

We experimented with application of different concentrations of AZ31 alloy stent to primary mouse extrahepatic bile epithelial cells (MEBECs) and estimated the effect on apoptosis and necrotic cells. Apoptosis and pro-apoptosis expression were estimated through real-time PCR. For in vivo protocol, we used rabbits, implanted the AZ31 bile stent, and estimated its effect on the CBD. AZ31 (40%) concentration showed an effect on the apoptotic and necrotic cells.

RESULTS

Real-time PCR revealed that AZ31 (40%) concentration increased the apoptotic genes such as NF-κB, caspase-3, Bax and Bax/Bcl-2 ratio as compared to the control group. In the in vivo experiment, AZ31 alloy stents were implanted into the CBD and showed an effect on the alteration the hematological, hepatic and non-hepatic parameters.

CONCLUSIONS

To conclude, it can be stated that AZ31 induces apoptosis via alteration in genes including nuclear factor kappa-B (NF-κB), caspase-3, Bax and Bax/Bcl-2 ratio and improved the hematological, hepatic and non-hepatic parameters.

Experimental and mathematical modelling of magnetically labelled mesenchymal stromal cell delivery

A key challenge for stem cell therapies is the delivery of therapeutic cells to the repair site. Magnetic targeting has been proposed as a platform for defining clinical sites of delivery more effectively. In this paper, we use a combined in vitro experimental and mathematical modelling approach to explore the magnetic targeting of mesenchymal stromal cells (MSCs) labelled with magnetic nanoparticles using an external magnet. This study aims to (i) demonstrate the potential of magnetic tagging for MSC delivery, (ii) examine the effect of red blood cells (RBCs) on MSC capture efficacy and (iii) highlight how mathematical models can provide both insight into mechanics of therapy and predictions about cell targeting in vivo. In vitro MSCs are cultured with magnetic nanoparticles and circulated with RBCs over an external magnet. Cell capture efficacy is measured for varying magnetic field strengths and RBC percentages. We use a 2D continuum mathematical model to represent the flow of magnetically tagged MSCs with RBCs. Numerical simulations demonstrate qualitative agreement with experimental results showing better capture with stronger magnetic fields and lower levels of RBCs. We additionally exploit the mathematical model to make hypotheses about the role of extravasation and identify future in vitro experiments to quantify this effect.

Characterization of Proangiogenic Monocytes from Blood in Patients with Chronic Ischemic Diabetic Foot Ulcers and Controls

Many persons with diabetes mellitus have limb ischemia, which is a major clinical problem. A subset of human monocytes that expresses TIE-2 may enhance neovascularization. We performed 179 phlebotomies on 142 patients (or donors), including 61 patients/donors without diabetes or ischemia (controls), 39 diabetic nonischemic patients (controls), and 42 diabetic patients with severe limb ischemia requiring amputation. We compared these groups for the presence of TIE-2-positive proangiogenic monocytes. The proportion of proangiogenic monocytes in the venous blood (on hospital admission) was significantly increased in diabetic patients without ischemia (9.22% ± 1.19%), compared to controls (6.53% ± 0.58%) or ischemic diabetic patients (5.44% ± 0.56%) (P < 0.05). In this pilot evaluation, we succeeded in extracting potential proangiogenic TIE-2 monocytes from the blood of diabetic patients without ischemia, but less in patients with ischemia. The implications for therapeutic neoangiogenesis require further studies.

Angiogenic factor-driven inflammation promotes extravasation of human proangiogenic monocytes to tumours

Recruitment of circulating monocytes is critical for tumour angiogenesis. However, how human monocyte subpopulations extravasate to tumours is unclear. Here we show mechanisms of extravasation of human CD14^dimCD16⁺ patrolling and CD14⁺CD16⁺ intermediate proangiogenic monocytes (HPMo), using human tumour xenograft models and live imaging of transmigration. IFNγ promotes an increase of the chemokine CX3CL1 on vessel lumen, imposing continuous crawling to HPMo and making these monocytes insensitive to chemokines required for their extravasation. Expression of the angiogenic factor VEGF and the inflammatory cytokine TNF by tumour cells enables HPMo extravasation by inducing GATA3-mediated repression of CX3CL1 expression. Recruited HPMo boosts angiogenesis by secreting MMP9 leading to release of matrix-bound VEGF-A, which amplifies the entry of more HPMo into tumours. Uncovering the extravasation cascade of HPMo sets the stage for future tumour therapies.

Circulating myeloid cells can leave the vasculature to infiltrate tumours and are thought to contribute to tumour angiogenesis. Here the authors live image monocytes that migrate to xenograft tumours and map an extravasation cascade of human proangiogenic monocytes into the tumour.

Monocyte Subsets Coregulate Inflammatory Responses by Integrated Signaling through TNF and IL-6 at the Endothelial Cell Interface

Two major monocyte subsets, CD14⁺CD16⁻ (classical) and CD14^+/dimCD16⁺ (nonclassical/intermediate), have been described. Each has different functions ascribed in its interactions with vascular endothelial cells (EC), including migration and promoting inflammation. Although monocyte subpopulations have been studied in isolated systems, their influence on EC and on the course of inflammation has been ignored. In this study, using unstimulated or cytokine-activated EC, we observed significant differences in the recruitment, migration, and reverse migration of human monocyte subsets. Associated with this, and based on their patterns of cytokine secretion, there was a difference in their capacity to activate EC and support the secondary recruitment of flowing neutrophils. High levels of TNF were detected in cocultures with nonclassical/intermediate monocytes, the blockade of which significantly reduced neutrophil recruitment. In contrast, classical monocytes secreted high levels of IL-6, the blockade of which resulted in increased neutrophil recruitment. When cocultures contained both monocyte subsets, or when conditioned supernatant from classical monocytes cocultures (IL-6^hi) was added to nonclassical/intermediate monocyte cocultures (TNF^hi), the activating effects of TNF were dramatically reduced, implying that when present, the anti-inflammatory activities of IL-6 were dominant over the proinflammatory activities of TNF. These changes in neutrophil recruitment could be explained by regulation of E-selectin on the cocultured EC. This study suggests that recruited human monocyte subsets trigger a regulatory pathway of cytokine-mediated signaling at the EC interface, and we propose that this is a mechanism for limiting the phlogistic activity of newly recruited monocytes.

Underlying Mechanisms of Protection Involved in Immunocloak

BACKGROUND

We have previously reported on a novel organ-specific immunomodifying therapy that provides protection from early allograft rejection in the absence of systemic immunosuppressive drugs. This novel therapy is a nanobarrier membrane called ImmunoCloak, consisting of a matrix of laminin, proteoglycans, fibronectin, and collagens. The membrane "immunocloaks" the luminal surfaces within the renal vasculature by covering the point of contact between donor vascular endothelial cells and the recipient's immune cells, without adversely affecting renal function. The resulting nonthrombogenic and nonimmunogenic apical surface significantly delays the onset of rejection fivefold over untreated controls. Currently, our focus is to elucidate the mechanisms of protection provided by placement of the membrane.

METHODS

The mechanisms underlying the protective effect of the ImmunoCloak treatment was evaluated using human peripheral blood mononuclear cells and by testing for antigen presentation by cytokine/chemokine analysis using the Luminex platform, T cell allogeneic responses were measured by flow cytometry, and diapedesis was assessed using transwell plates.

RESULTS

We now report that ImmunoCloak interrupts antigen presentation thereby preventing early T cell activation and interferes with diapedesis. There was significant inhibition in the synthesis of proinflammatory cytokines with a concordant blockade of T cell-mediated responses. The placement of the ImmunoCloak also significantly reduced leukocyte migration through the endothelial cell layer by 93%.

CONCLUSIONS

Eliminating the need for nephrotoxic immunosuppressive drugs during the early posttransplant period could help to ameliorate the severity of delayed graft function and could provide a path to using more ischemically damaged renal allografts.

Endocrine Regulation of Lymphocyte Trafficking In Vitro

Lymphocyte recruitment in inflammation can be influenced by many molecules including cytokines, chemokines, and adipokines. In our lab, we have examined the effects of the adipokines leptin and adiponectin on lymphocyte migration, and observed modulation of this process. Lymphocyte behavior can be assessed in the lab under static conditions, or can be studied under flow, simulating in vivo conditions. In this chapter, in vitro methods for analyzing adhesion and migration of lymphocytes isolated from blood are described in detail. In static adhesion and migration assays, lymphocytes are allowed to settle on top of endothelial cell monolayers cultured in plates for a desired period of time. In the flow-based assay, lymphocytes are perfused over the endothelium at a continuous rate through microchannels which are commercially available. Depending on the choice of method employed, the efficiency of lymphocytes to adhere to and migrate across the endothelial cell monolayer under different conditions can be evaluated. Static assays are less complex and are of higher throughput. However, these assays provide less detailed information regarding lymphocyte behaviors. On the other hand, the flow-based assays are more difficult to perform, but are more physiologically relevant due to the presence of flow and yield more detailed information about lymphocyte activities such as capture, immobilization, and migration in real-time.

Divergent JAM-C Expression Accelerates Monocyte-Derived Cell Exit from Atherosclerotic Plaques

Atherosclerosis, caused in part by monocytes in plaques, continues to be a disease that afflicts the modern world. Whilst significant steps have been made in treating this chronic inflammatory disease, questions remain on how to prevent monocyte and macrophage accumulation in atherosclerotic plaques. Junctional Adhesion Molecule C (JAM-C) expressed by vascular endothelium directs monocyte transendothelial migration in a unidirectional manner leading to increased inflammation. Here we show that interfering with JAM-C allows reverse-transendothelial migration of monocyte-derived cells, opening the way back out of the inflamed environment. To study the role of JAM-C in plaque regression we used a mouse model of atherosclerosis, and tested the impact of vascular JAM-C expression levels on monocyte reverse transendothelial migration using human cells. Studies in-vitro under inflammatory conditions revealed that overexpression or gene silencing of JAM-C in human endothelium exposed to flow resulted in higher rates of monocyte reverse-transendothelial migration, similar to antibody blockade. We then transplanted atherosclerotic, plaque-containing aortic arches from hyperlipidemic ApoE^-/- mice into wild-type normolipidemic recipient mice. JAM-C blockade in the recipients induced greater emigration of monocyte-derived cells and further diminished the size of atherosclerotic plaques. Our findings have shown that JAM-C forms a one-way vascular barrier for leukocyte transendothelial migration only when present at homeostatic copy numbers. We have also shown that blocking JAM-C can reduce the number of atherogenic monocytes/macrophages in plaques by emigration, providing a novel therapeutic strategy for chronic inflammatory pathologies.

Static and Dynamic Assays of Cell Adhesion Relevant to the Vasculature

Methods are described for analyzing adhesion of isolated cells (such as leukocytes, tumor cells, or precursor cells) to purified adhesion receptors or cultured endothelial cells. "Static" assays (where cells are allowed to settle on the adhesive substrates) and flow-based assays (where cells are perfused over the substrates) are compared. Direct observations of the time course of adhesion and migration can be made when purified proteins or endothelial cells are cultured in plates, after cells are allowed to settle onto them for a desired period. In the flow-based assay, cells are perfused through coated glass capillaries, flow-channels incorporating coated plates, or commercially available preformed channels. Again, direct video-microscopic observations are made. In this assay various stages of capture, immobilization, and migration can be followed. In general, the static systems have higher throughput and greatest ease of use, but yield less detailed information, while the flow-based assay is most difficult to set up but is most physiologically relevant if one is interested in the dynamics of adhesion in the vasculature.

The roles of integrins in function of human neutrophils after their migration through endothelium into interstitial matrix

We investigated the changes in neutrophil phenotype and function after transendothelial migration, and the roles played by integrin receptors in their behaviour. Neutrophils were tracked microscopically as they migrated through endothelial cells into collagen gels, and were retrieved at desired times. When endothelial cells were treated with increasing doses of tumour necrosis factor-α, neutrophils not only migrated in greater number, but also to a greater depth in the gel. Apoptosis was barely detectable in neutrophils retrieved after 24h, and many remained viable and motile at 48h. Neutrophils retrieved after 1h had increased oxidative capacity and at 24h had similar capacity as freshly-isolated neutrophils. However, by then they had impaired ability to phagocytose bacteria. Compared to fresh neutrophils, total mRNA was halved by 24h, but while β2-integrin expression decreased, β1- and β3-integrin increased along with ICAM-1. Studies of integrin blockade indicated that while β2-integrins were needed to cross the endothelial barrier, no integrins were required for migration within the gel. β2-integrins also contributed to phagocytosis, but their binding was not required for prolonged survival. These results demonstrate a model for integrated analysis of neutrophil migration and function, and describe development of effector functions and the roles of integrins in human neutrophils for the first time.

Integrin-substrate interactions underlying shear-induced inhibition of the inflammatory response of endothelial cells

Conditioning of endothelial cells by shear stress suppresses their response to inflammatory cytokines. We questioned whether signalling through different integrin-matrix interactions, previously associated with the pathogenic effects of disturbed flow, supported the anti-inflammatory action of steady shear. Primary human endothelial cells were cultured on different substrates and exposed to shear stress (2.0Pa) for varying periods before stimulation with tumour necrosis factor-α (TNF). Shear-conditioning inhibited cytokine-induced recruitment of flowing neutrophils. However, the effect was similar for culture on collagen, laminin or fibronectin, even when seeding was reduced to 2 hours, and shear to 3 hours before TNF treatment (to minimise deposition of endothelial matrix). Nevertheless, in short- or longer-term cultures, reduction in expression of β(1)-integrin (but not β(3)-integrin) using siRNA essentially ablated the effect of shear-conditioning on neutrophil recruitment. Studies of focal adhesion kinase (FAK) phosphorylation, siRNA against FAK and a FAK-inhibitor (PF573228) indicated that FAK activity was an essential component downstream of β(1)-integrin. In addition, MAP-kinase p38 was phosphorylated downstream of FAK and also required for functional modification. Mechanotransduction through β(1)-integrins, FAK and p38 is required for anti-inflammatory effects of steady shear stress. Separation of the pathways which underlie pathological versus protective responses of different patterns of flow is required to enable therapeutic modification or mimicry, respectively.

Epoxyeicosatrienoic acids protect rat hearts against tumor necrosis factor-α-induced injury

Epoxyeicosatrienoic acids (EET), the primary arachidonic acid metabolites of cytochrome P450 2J (CYP2J) epoxygenases, possess potent vasodilatory, anti-inflammatory, antiapoptotic, and mitogenic effects. To date, little is known about the role of CYP2J2 and EETs in tumor necrosis factor (TNF)-α-induced cardiac injury. We utilized cell culture and in vivo models to examine the effects of exogenously applied EETs or CYP2J2 overexpression on TNF-α-induced cardiac apoptosis and cardiac dysfunction. In neonatal rat cardiomyocytes, TNF-α-induced apoptosis was markedly attenuated by EETs or CYP2J2 overexpression, leading to significantly improved cell survival. Further studies showed that TNF-α decreased expression of the antiapoptotic proteins Bcl-2 and Bcl-xL, decreased IκBα and PPARγ, and also inhibited PI3K-dependent Akt and EGFR signaling. Both EETs and CYP2J2 overexpression reversed the effects of TNF-α on these pathways. Furthermore, overexpression of CYP2J2 in rats prevented the decline in cardiac function that is normally observed in TNF-α-challenged animals. These results demonstrate that EETs or CYP2J2 overexpression can prevent TNF-α-induced cardiac cell injury and cardiac dysfunction by inhibiting apoptosis, reducing inflammation, and enhancing PPARγ expression. Targeting the CYP2J2 epoxygenase pathway may represent a novel approach to mitigate cardiac injury in diseases such as heart failure, where increased TNF-α levels are known to occur.

Mesenchymal stem cells exhibit firm adhesion, crawling, spreading and transmigration across aortic endothelial cells: effects of chemokines and shear

Mesenchymal stem cells (MSCs) have anti-inflammatory and immunosuppressive properties and may be useful in the therapy of diseases such as arteriosclerosis. MSCs have some ability to traffic into inflamed tissues, however to exploit this therapeutically their migratory mechanisms need to be elucidated. This study examines the interaction of murine MSCs (mMSCs) with, and their migration across, murine aortic endothelial cells (MAECs), and the effects of chemokines and shear stress. The interaction of mMSCs with MAECs was examined under physiological flow conditions. mMSCs showed lack of interaction with MAECs under continuous flow. However, when the flow was stopped (for 10min) and then started, mMSCs adhered and crawled on the endothelial surface, extending fine microvillous processes (filopodia). They then spread extending pseudopodia in multiple directions. CXCL9 significantly enhanced the percentage of mMSCs adhering, crawling and spreading and shear forces markedly stimulated crawling and spreading. CXCL9, CXCL16, CCL20 and CCL25 significantly enhanced transendothelial migration across MAECs. The transmigrated mMSCs had down-regulated receptors CXCR3, CXCR6, CCR6 and CCR9. This study furthers the knowledge of MSC transendothelial migration and the effects of chemokines and shear stress which is of relevance to inflammatory diseases such as arteriosclerosis.

Immunoglobulin subclass determines ability of immunoglobulin (Ig)G to capture and activate neutrophils presented as normal human IgG or disease-associated anti-neutrophil cytoplasm antibody (ANCA)-IgG

Immunoglobulin G (IgG) is a potent neutrophil stimulus, particularly when presented as anti-neutrophil cytoplasm antibody (ANCA) in ANCA-associated vasculitis. We assessed whether IgG subclasses had differential effects on neutrophil activation and whether differences were dependent on specific Fc-receptor engagement. Using a physiologically relevant flow model, we compared adhesion of neutrophils to different subclasses of normal IgG coated onto solid surfaces, with adhesion of neutrophils treated with different subclasses of soluble ANCA IgG to P-selectin surfaces or endothelial cells (EC). Normal IgG captured flowing neutrophils efficiently in the order IgG3 > IgG1 > IgG2 > IgG4. Fc-receptor blockade reduced capture, IgG3 being more dependent on CD16 and IgG1/2 on CD32. Blockade of the integrin CD18 reduced neutrophil spreading, while inhibition of calcium-dependent signalling reduced both capture and spreading, suggesting that both were active processes. Neutrophils treated with ANCA IgG subclasses 1, 3 and 4 showed stabilization of adhesion to P-selectin surfaces and EC. ANCA changed neutrophil behaviour from rolling to static adhesion and the potency of the subclasses followed the same pattern as above: IgG3 > IgG1 > IgG4. Blockade of Fc receptors resulted in neutrophils continuing to roll, i.e. they were not ANCA-activated; differential utilization of Fc receptor by particular IgG subclasses was not as apparent as during neutrophil capture by normal IgG. IgG3 is the most effective subclass for inducing neutrophil adhesion and altered behaviour, irrespective of whether the IgG is surface bound or docks onto neutrophil surface antigens prior to engaging Fc receptors. Engagement of Fc receptors underpins these responses; the dominant Fc receptor depends on IgG subclass.

Stromal cells differentially regulate neutrophil and lymphocyte recruitment through the endothelium

Stromal fibroblasts modify the initial recruitment of leucocytes by endothelial cells (EC), but their effects on subsequent transendothelial migration remain unclear. Here, EC and dermal or synovial fibroblasts were cultured on opposite surfaces of 3-μm pore filters and incorporated in static or flow-based migration assays. Fibroblasts had little effect on tumour necrosis factor-α-induced transendothelial migration of neutrophils, but tended to increase the efficiency of migration away from the endothelium. Surprisingly, similar close contact between EC and fibroblasts strongly reduced lymphocyte migration in static assays, and nearly abolished stable lymphocyte adhesion from flow. Fibroblasts did not alter endothelial surface expression of adhesion molecules or messenger RNA for chemokines. Inhibition of attachment did not occur when EC-fibroblast contact was restricted by using 0.4-μm pore filters, but under these conditions pre-treatment with heparinase partially inhibited adhesion. In the 3-μm pore co-cultures, inhibition of metalloproteinase activity partially recovered lymphocyte adhesion, but addition of CXCL12 (SDF-1α) to the endothelial surface did not. Hence, the ability of EC to present activating chemokines for lymphocytes may have been enzymatically inhibited by direct contact with fibroblasts. To avoid contact, we cultured EC and fibroblasts on separate 3-μm pore filters one above the other. Here, fibroblasts promoted the transendothelial migration of lymphocytes. Fibroblasts generate CXCL12, but blockade of CXCL12 receptor had no effect on lymphocyte migration. While stromal cells can provide signal(s) promoting leucocyte migration away from the sub-endothelial space, direct cell contact (which might occur in damaged tissue) may cause disruption of chemokine signalling, specifically inhibiting lymphocyte rather than neutrophil recruitment.

Delay of migrating leukocytes by the basement membrane deposited by endothelial cells in long-term culture

We investigated the migration of human leukocytes through endothelial cells (EC), and particularly their underlying basement membrane (BM). EC were cultured for 20 days on 3 μm-pore filters or collagen gels to form a distinct BM, and then treated with tumour necrosis factor-α, interleukin-1β or interferon-γ. Neutrophil migration through the cytokine-treated EC and BM was delayed for 20-day compared to 4-day cultures. The BM alone obstructed chemotaxis of neutrophils, and if fresh EC were briefly cultured on stripped BM, there was again a hold-up in migration. In studies with lymphocytes and monocytes, we could detect little hold-up of migration for 20-day versus 4-day cultures, in either the filter- or gel-based models. Direct microscopic observations showed that BM also held-up neutrophil migration under conditions of flow. Treatment of upper and/or lower compartments of filters with antibodies against integrins, showed that neutrophil migration through the endothelial monolayer was dependent on β₂-integrins, but not β1- or β₃-integrins. Migration from the subendothelial compartment was supported by β1- and β₂-integrins for all cultures, but blockade of β₃-integrin only inhibited migration effectively for 20-day cultures. Flow cytometry indicated that there was no net increase in expression of β1- or β3-integrins during neutrophil migration, and that their specific subendothelial function was likely dependent on turnover of integrins during migration. These studies show that BM is a distinct barrier to migration of human neutrophils, and that β₃-integrins are particularly important in crossing this barrier. The lesser effect of BM on lymphocytes and monocytes supports the concept that crossing the BM is a separate, leukocyte-specific, regulated step in migration.

LFA-1 and Mac-1 define characteristically different intralumenal crawling and emigration patterns for monocytes and neutrophils in situ

To exit blood vessels, most (∼80%) of the lumenally adhered monocytes and neutrophils crawl toward locations that support transmigration. Using intravital confocal microscopy of anesthetized mouse cremaster muscle, we separately examined the crawling and emigration patterns of monocytes and neutrophils in blood-perfused unstimulated or TNF-α-activated venules. Most of the interacting cells in microvessels are neutrophils; however, in unstimulated venules, a greater percentage of the total monocyte population is adherent compared with neutrophils (58.2 ± 6.1% versus 13.6 ± 0.9%, adhered/total interacting), and they crawl for significantly longer distances (147.3 ± 13.4 versus 61.8 ± 5.4 μm). Intriguingly, after TNF-α activation, monocytes crawled for significantly shorter distances (67.4 ± 9.6 μm), resembling neutrophil crawling. Using function-blocking Abs, we show that these different crawling patterns were due to CD11a/CD18 (LFA-1)- versus CD11b/CD18 (Mac-1)-mediated crawling. Blockade of either Mac-1 or LFA-1 revealed that both LFA-1 and Mac-1 contribute to monocyte crawling; however, the LFA-1-dependent crawling in unstimulated venules becomes Mac-1 dependent upon inflammation, likely due to increased expression of Mac-1. Mac-1 alone was responsible for neutrophil crawling in both unstimulated and TNF-α-activated venules. Consistent with the role of Mac-1 in crawling, Mac-1 block (compared with LFA-1) was also significantly more efficient in blocking TNF-α-induced extravasation of both monocytes and neutrophils in cremaster tissue and the peritoneal cavity. Thus, mechanisms underlying leukocyte crawling are important in regulating the inflammatory responses by regulating the numbers of leukocytes that transmigrate.

Intercellular adhesion molecule-1 enrichment near tricellular endothelial junctions is preferentially associated with leukocyte transmigration and signals for reorganization of these junctions to accommodate leukocyte passage

Leukocyte transmigration occurs at specific locations (portals) on the endothelium, but the nature of these portals is not clear. Using intravital confocal microscopy of anesthetized mouse cremaster muscle in combination with immunofluorescence labeling, we showed that in microvessels transmigration is mainly junctional and preferentially occurs at tricellular endothelial junctional regions. Our data suggest that enrichment of ICAM-1 near approximately 43% of these junctions makes these locations preferred for transmigration by signaling the location of a nearby portal, as well as preparing the endothelial cell (EC) junctions, to accommodate leukocyte passage. Blockade of the extracellular domain of the ICAM-1 significantly reduced transmigration (by 68.8 + or - 4.5%) by reducing the ability of leukocytes to get to these portals. In contrast, blockade of the cytoplasmic tail of ICAM-1 reduced transmigration (by 71.1 + or - 7.0%) by disabling VE-cadherin rearrangement. Importantly, venular convergences are optimally equipped to support leukocyte transmigration. Differences in EC morphology result in a significantly higher number of tricellular junctions in convergences compared with straight venular regions (20.7 + or - 1.2 versus 12.43 + or - 1.1/6000 microm(2), respectively). Consequently, leukocyte adhesion and transmigration are significantly higher in convergences compared with straight regions (1.6- and 2.6-fold, respectively). Taken together, these data identify an important role for EC morphology and expression patterns of ICAM-1 in leukocyte transmigration.

A role for the endothelial glycosaminoglycan hyaluronan in neutrophil recruitment by endothelial cells cultured for prolonged periods

Glycosaminoglycans (GAGs) presented on the surface of endothelial cells (ECs) are believed to influence leukocyte recruitment during inflammation, but their roles remain uncertain. Here we report an in vitro model of prolonged culture of human EC in which the contributions of heparan sulphate (HS) and hyaluronan (HA) to the process of neutrophil recruitment could be studied. Previously, we reported that increasing EC culture duration (up to 20 days) enhanced neutrophil recruitment in response to low dose (1 U/ml) but not high dose (100 U/ml) of tumour necrosis factor-α (TNF). Here we found that HS and HA were present at much higher levels on the surface of day 20 cultures than day 3 cultures. Neutrophil recruitment on both day 3 and day 20 ECs was mediated through CXCR chemokine receptors and interleukin-8 (IL-8). In addition, mRNA levels for TNF receptors, signalling pathway constituents, adhesion receptors, and chemokines involved in neutrophil recruitment were similar for day 3 and day 20 ECs. To test whether the enhanced neutrophil recruitment on day 20 EC was mediated by GAGs, they were removed enzymatically. Removal of HA (but not HS) inhibited neutrophil recruitment, as did antibody blockade of CD44, a counter-receptor for HA on neutrophils. Supernatants from hyaluronidase-treated day 20 ECs were more potent in activating neutrophils than supernatants from untreated EC. Thus, HA has a role in neutrophil recruitment that is revealed in long-term cultures where it increases potency of response to sub-optimal levels of TNF. This effect appears to occur through a dual mechanism involving chemokine presentation and interaction with CD44.

Fibroblasts from different sites may promote or inhibit recruitment of flowing lymphocytes by endothelial cells

We examined the hypothesis that stromal fibroblasts modulate the ability of endothelial cells (EC) to recruit lymphocytes in a site-specific manner. PBL were perfused over HUVEC that had been cultured with fibroblasts isolated from the inflamed synovium or the skin of patients with rheumatoid arthritis or osteoarthritis, or from normal synovium, with or without exposure to the inflammatory cytokines TNF-alpha+IFN-gamma. Fibroblasts from inflamed synovium, but no others, caused unstimulated HUVEC to bind flowing lymphocytes. This adhesion was supported by alpha(4)beta(1)-VCAM-1 interaction and stabilised by activation of PBL through CXCR4-CXCL12. Antibody neutralisation of IL-6 during co-culture effectively abolished the ability of EC to bind lymphocytes. Cytokine-stimulated EC supported high levels of lymphocyte adhesion, through the presentation of VCAM-1, E-selectin and chemokine(s) acting through CXCR3. Interestingly, co-culture with dermal fibroblasts caused a marked reduction in cytokine-induced adhesion, while synovial fibroblasts had variable effects depending on their source. In the dermal co-cultures, neutralisation of IL-6 or TGF-beta caused partial recovery of cytokine-induced lymphocyte adhesion; this was complete when both were neutralised. Exogenous IL-6 was also found to inhibit response to TNF-alpha+IFN-gamma. Normal stromal fibroblasts appear to regulate the cytokine-sensitivity of vascular endothelium, while fibroblasts associated with chronic inflammation bypass this and develop a directly inflammatory phenotype. Actions of IL-6 might be pro-inflammatory or anti-inflammatory, depending on the local milieu.

Static and dynamic assays of cell adhesion relevant to the vasculature

Methods are described for analysing adhesion of isolated cells (such as leucocytes, tumor cells, or precursor cells) to purified adhesion receptors or cultured endothelial cells. "Static" assays (in which cells are allowed to settle on the adhesive substrates) and flow-based assays (in which cells are perfused over the substrates) are compared. Direct observations of the time course of adhesion and migration can be made when purified proteins or endothelial cells are cultured in plates, after cells are allowed to settle onto them for a desired period. In the flow-based assay, cells are perfused through coated glass capillaries or flow channels incorporating coated plates. Again, direct video-microscopic observations are made. In this assay, various stages of capture, immobilisation and migration can be followed. In general, the static systems have higher throughput and greatest ease of use, but yield less-detailed information, while the flow-based assay is most difficult to set up but is most physiologically relevant if one is interested in the dynamics of adhesion in the vasculature.

Direct observations of the kinetics of migrating T cells suggest active retention by endothelial cells with continual bidirectional migration

The kinetics and regulatory mechanisms of T cell migration through the endothelium have not been fully defined. In experimental, filter-based assays in vitro, transmigration of lymphocytes takes hours, compared with minutes, in vivo. We cultured endothelial cell (EC) monolayers on filters, solid substrates, or collagen gels and treated them with TNF-α, IFN-γ, or both prior to analysis of lymphocyte migration in the presence or absence of flow. PBL, CD4+ cells, or CD8+ cells took many hours to migrate through EC-filter constructs for all cytokine treatments. However, direct microscopic observations of EC filters, which had been mounted in a flow chamber, showed that PBL crossed the endothelial monolayer in minutes and were highly motile in the subendothelial space. Migration through EC was also observed on clear plastic, with or without flow. After a brief settling without flow, PBL and isolated CD3+ or CD4+ cells crossed EC in minutes, but the numbers of migrated cells varied little with time. Close observation revealed that lymphocytes migrated back and forth continuously across endothelium. Under flow, migration kinetics and the proportions migrating back and forth were altered little. On collagen gels, PBL again crossed EC in minutes and migrated back and forth but showed little penetration of the gel over hours. In contrast, neutrophils migrated efficiently through EC and into gels. These observations suggest a novel model for lymphoid migration in which EC support migration but retain lymphocytes (as opposed to neutrophils), and additional signal(s) are required for onward migration.

The anti-tumor agent, ingenol-3-angelate (PEP005), promotes the recruitment of cytotoxic neutrophils by activation of vascular endothelial cells in a PKC-delta dependent manner

The modes of action of the novel anti-skin tumor agent ingenol-3-angelate (PEP005) are incompletely understood. Crucially, the cytotoxic functions of neutrophils recruited to the tumor in response to topical application of PEP005 are necessary for effective ablation of the treated lesion. Here, we investigated the hypothesis that the phorbol ester-like properties of PEP005 and its ability to activate PKC could directly activate endothelial cells (EC) so that they support the recruitment of neutrophils. Exposure of EC to PEP005 induced mRNA and/or protein for E-selectin, ICAM-1 and IL-8 in a dose dependent manner, while in a flow based adhesion assay, PEP005 treated EC supported the recruitment of neutrophils at levels comparable to EC stimulated with TNF-alpha. Neutrophil adhesion was inhibited by antibody against E-selectin but not P-selectin. Activation of EC was inhibited by the PKC inhibitor bisindolylmaleimide-1 and confocal immuno-fluorescent studies demonstrated translocation of PKC-delta from the cytosol to the peri-nuclear membrane in response to PEP005. Importantly, the knock down of PKC-delta using siRNA completely abolished neutrophil recruitment to EC subsequently treated with PEP005. Thus, we describe a novel route by which the anti-tumor agent PEP005 regulates the recruitment of cytotoxic leukocytes by directly activating EC in a PKC-delta dependent manner.

LPS differentially regulates adhesion and transendothelial migration of human monocytes under static and flow conditions

One of the key components of the innate immune response is the recognition of microbial products such as LPS by Toll-like receptors on monocytes and neutrophils. We show here that short-term stimulation of primary human monocytes with LPS led to an increase in adhesion of monocytes to endothelial cells and a dramatic decrease in transendothelial migration under static conditions. In contrast, under normal physiological flow, monocyte adhesion and migration across a human umbilical vein endothelial cell monolayer appeared to be unaffected by LPS treatment. LPS stimulation of monocytes activated beta(1) and beta(2) integrins, but did not increase their surface expression levels. During septic shock, reduction in blood flow as a result of vasodilation and vascular permeability leads to adhesion and accumulation of LPS-stimulated circulating monocytes onto the blood vessel walls. The different findings of monocyte migration under static and flow conditions in our study may offer one explanation for this phenomenon. The rapid engagement of LPS-activated monocytes preventing transendothelial migration could represent a novel mechanism of bacterial exclusion from the vasculature. This occurs during the early stages of sepsis, and in turn may modulate the severity of the pathophysiology.

Transendothelial flow inhibits neutrophil transmigration through a nitric oxide-dependent mechanism: potential role for cleft shear stress

Endothelial cells in vivo are well known to respond to parallel shear stress induced by luminal blood flow. In addition, fluid filtration across endothelium (transendothelial flow) may trigger nitric oxide (NO) production, presumably via shear stress within intercellular clefts. Since NO regulates neutrophil-endothelial interactions, we determined whether transendothelial flow regulates neutrophil transmigration. Interleukin-1β-treated human umbilical vein endothelial cell (HUVEC) monolayers cultured on a polycarbonate filter were placed in a custom chamber with or without a modest hydrostatic pressure gradient (ΔP, 10 cmH2O) to induce transendothelial flow. In other experiments, cells were studied in a parallel plate flow chamber at various transendothelial flows (ΔP = 0, 5, and 10 cmH2O) and luminal flows (shear stress of 0, 1, and 2 dyn/cm2). In the absence of luminal flow, transendothelial flow reduced transmigration of freshly isolated human neutrophils from 57% to 14% ( P < 0.05) and induced an increase in NO detected with a fluorescent assay (DAF-2DA). The NO synthase inhibitor l-NAME prevented the effects of transendothelial flow on neutrophil transmigration, while a NO donor (DETA/NO, 1 mM) inhibited neutrophil transmigration. Finally, in the presence of luminal flow (1 and 2 dyn/cm2), transendothelial flow also inhibited transmigration. On the basis of HUVEC morphometry and measured transendothelial volume flow, we estimated cleft shear stress to range from 49 to 198 dyn/cm2. These shear stress estimates, while substantial, are of similar magnitude to those reported by others with similar analyses. These data are consistent with the hypothesis that endothelial cleft shear stress inhibits neutrophil transmigration via a NO-dependent mechanism.

Comparison of the pro-inflammatory potential of monocytes from healthy adults and those with peripheral arterial disease using an in vitro culture model

We adapted a monocyte:endothelial cell co-culture model to investigate the pro-inflammatory potential of monocytes from patients with peripheral arterial disease (PAD). Isolated monocytes were cultured with human umbilical vein endothelial cells (HUVEC) for 24h, after which the ability of the HUVEC to recruit flowing neutrophils was tested. Development of a usable protocol required comparisons of primary HUVEC with cells that had been passaged and/or frozen and thawed, evaluation of optimal culture media and comparison of monocytes from freshly drawn and stored blood. We found, for instance, that expansion of HUVEC was assisted by inclusion of hydrocortisone, but this agent was withdrawn before the test phase because it reduced responses of HUVEC. Using the optimal practical protocol, we found great variation in the ability of monocytes from different donors to cause neutrophil adhesion. Slightly more ( approximately 20%) monocytes from patients with PAD adhered to HUVEC than monocytes from healthy controls, and the monocytes from PAD patients induced approximately 70% greater subsequent adhesion of neutrophils. Thus, we developed a functional model of inflammatory potential usable in clinically-related studies and found that patients with PAD had circulating monocytes with greater than normal ability to activate endothelial cells.

JAM-C regulates unidirectional monocyte transendothelial migration in inflammation

Monocyte recruitment from the vasculature involves sequential engagement of multiple receptors, culminating in transendothelial migration and extravasation. Junctional adhesion molecule-C (JAM-C) is localized at endothelial intercellular junctions and plays a role in monocyte transmigration. Here, we show that blockade of JAM-B/-C interaction reduced monocyte numbers in the extravascular compartment through increased reverse transmigration rather than by reduced transmigration. This was confirmed in vivo, showing that an anti-JAM-C antibody reduced the number of monocytes in inflammatory tissue and increased the number of monocytes with a reverse-transmigratory phenotype in the peripheral blood. All together, our results suggest a novel mechanism of reducing accumulation of monocytes at inflammation sites by disruption of JAM-C-mediated monocyte retention.

The functional state of neutrophils correlates with the severity of renal dysfunction in children with hemolytic uremic syndrome

Hemolytic Uremic Syndrome (HUS) is the main cause of acute renal failure in children. The high percentage of patients who develop long-term sequelae constitutes an important medical concern. The identification of parameters that correlate with the degree of renal failure may be useful to plan the best treatment soon after hospitalization. Here, we investigated the functional state of neutrophils (PMN) from HUS patients on admission, before dialysis and/or transfusion, in relation to the severity of renal impairment reached during the acute period (AP). We found that all PMN activation parameters measured in severe cases of HUS (HUS AP3) were statistically lower comparing to children with mild cases of HUS (HUS AP1). As HUS PMN phenotype and dysfunction is compatible with that of cells undergoing cell death, we also studied spontaneous apoptosis. Not only were HUS PMN not apoptotic, but HUS AP3 PMN showed an increased survival. Almost all phenotypic and functional parameters measured on PMN correlated with severity. Our results revealed a marked deactivation of PMN in severe cases of HUS, and suggest that studying the functional state of PMN could be of prognostic value.

Analysis of leukocyte migration through monolayers of cultured endothelial cells

In this chapter methods are described for analyzing the adhesion and migration of isolated leukocytes on endothelial cell monolayers that have been cultured on different substrates and treated with cytokines. When endothelial cells are grown on porous filters inserted in wells, the levels of leukocyte adhesion and migration are calculated from the number added and the numbers retrieved from the upper and lower chambers. Fluorescence microscopic examination of the fixed filters can be used to ascertain whether leukocytes are retained above or below the filter. Direct observations of the time course of migration can be made when endothelial cells are cultured in six-well plates after leukocytes are allowed to settle onto them for a short period. In a more specialized assay, leukocytes are perfused through glass capillaries coated with endothelial cells, and again, direct video-microscopic observations are made. In this assay all stages of capture, immobilization, and migration can be followed. In general, the filter-based assay has the highest throughput and greatest ease of use but yields less detailed information, whereas the flow-based assay is most difficult to set up but is most physiologically relevant.

Molecular events during leukocyte diapedesis

The recruitment of leukocytes from the circulation into tissues requires leukocyte migration through the vascular endothelium. The mechanisms by which leukocytes attach and firmly adhere to the endothelial cell surface have been studied in detail. However, much less is known about the last step in this process, the diapedesis of leukocytes through the vascular endothelium. This minireview focuses on the interactions between leukocyte and endothelial cell adhesion molecules that are important during leukocyte extravasation. In the past few years a series of endothelial cell surface and adhesion molecules have been identified that are located at endothelial cell contacts and found to participate in leukocyte diapedesis. These junctional cell adhesion molecules are believed to have an active role in controlling the opening and closure of endothelial cell contacts to allow the passage of leukocytes between adjacent endothelial cells. Alternatively, leukocytes can cross the endothelium at nonjunctional locations, with leukocytes migrating through a single endothelial cell. Further work is clearly needed to understand, in greater detail, the molecular mechanisms that allow leukocytes to cross the endothelium via either the paracellular or the transcellular pathway.

Differing mechanisms of leukocyte recruitment and sensitivity to conditioning by shear stress for endothelial cells treated with tumour necrosis factor-alpha or interleukin-1beta

The cytokines tumour necrosis factor-alpha (TNFalpha) and interleukin-1beta (IL-1B) induce endothelial cells to recruit leukocytes. However, the exact adhesion and activation mechanisms induced by each cytokine, and their relative sensitivities to modulation by endothelial exposure to shear stress remain unclear. We cultured human umbilical vein endothelial cells (HUVEC) in glass capillaries at various shear stresses, with TNFalpha or IL-1B added for the last 4 h. Subsequently, human neutrophils were perfused over the HUVEC, and adhesion and migration were recorded. Both cytokines induced dose-dependent capture of neutrophils. However, while conditioning of HUVEC by increasing shear stress for 24 h diminished their response to TNFalpha, the response of HUVEC to IL-1B was similar at all shear stresses. The differing sensitivities were evident at levels of adhesive function and mRNA for adhesion molecules and chemokines. Analysis of nuclear factor kappaB (NF-kappaB)/Rel family of transcription factors showed that their expression and activation were modified by exposure to shear stress, but did not obviously explain differential responses to TNFalpha and IL-1B. Antibodies against selectins were effective against capture of neutrophils on TNFalpha-treated but not IL-1B-treated HUVEC. Stable adhesion was supported by beta2-integrins in each case. Activation of neutrophils occurred dominantly through CXC-chemokine receptor 2 (CXCR2) for TNFalpha-treated HUVEC, while blockade of CXCR1, CXCR2 and of platelet-activating factor receptors caused additive inhibition of migration on IL-1B-treated HUVEC. The mechanisms which underlie neutrophil recruitment, and their modulation by the haemodynamic environment, differ between cytokines. Interventions aimed against leukocyte recruitment may not operate equally in different inflammatory milieu.

Prolonged culture of endothelial cells and deposition of basement membrane modify the recruitment of neutrophils

We tested whether endothelial cell conditioning during prolonged culture and deposition of basement membrane (BM) could modify neutrophil recruitment induced by the inflammatory cytokine, tumour necrosis factor-alpha (TNF). Confluent endothelial cells (EC) from human umbilical veins were cultured for 1 to 20 days and then stimulated with 1, 10 or 100 U/ml of TNF for 4 h. When isolated neutrophils were settled on EC stimulated with the lower doses of TNF, the levels of adhesion and the proportion of adherent cells that transmigrated increased markedly with time of culture. At 100 U/ml TNF, time of culture had little effect on recruitment, but the transmigrated neutrophils moved more slowly under the monolayer in longer-term cultures. The inhibitory effects of function-blocking antibodies against E-selectin and beta2-integrin, and studies in which neutrophils were perfused over short- or long-term cultures, suggested that increased adhesion and migration arose from increased efficiency of neutrophil activation by the EC. Prolonged culture was also associated with deposition of a distinct BM. When fresh EC were seeded on day 20 BM, transmigrated neutrophils moved more slowly under the EC than under control monolayers. Thus, EC change their pro-inflammatory phenotype during prolonged culture, and the deposited basement membrane influences neutrophil migration.

Mini-review: Transendothelial migration of leukocytes: through the front door or around the side of the house?

Leukocyte adhesion to the endothelial cells lining the vessel wall and the subsequent migration of the leukocytes into the underlying tissue are key elements of both innate and adaptive immunity. Leukocyte extravasation is generally believed to take place through small gaps at intercellular endothelial cell junctions -- the paracellular route. This view has, however, been repeatedly challenged by morphological studies demonstrating leukocyte migration through the endothelial cells themselves -- the transcellular pathway. On the basis of the current experimental evidence, we propose consideration that both pathways are equally possible for a leukocyte's journey from the apical surface of the endothelium to its basal side.

A transmigratory cup in leukocyte diapedesis both through individual vascular endothelial cells and between them

The basic route and mechanisms for leukocyte migration across the endothelium remain poorly defined. We provide definitive evidence for transcellular (i.e., through individual endothelial cells) diapedesis in vitro and demonstrate that virtually all, both para- and transcellular, diapedesis occurs in the context of a novel “cuplike” transmigratory structure. This endothelial structure was comprised of highly intercellular adhesion molecule-1– and vascular cell adhesion molecule-1–enriched vertical microvilli-like projections that surrounded transmigrating leukocytes and drove redistribution of their integrins into linear tracks oriented parallel to the direction of diapedesis. Disruption of projections was highly correlated with inhibition of transmigration. These findings suggest a novel mechanism, the “transmigratory cup”, by which the endothelium provides directional guidance to leukocytes for extravasation.

Methods for exposing multiple cultures of endothelial cells to different fluid shear stresses and to cytokines, for subsequent analysis of inflammatory function

Endothelial cells are conditioned by physicochemical environmental factors, including shear stress applied by flowing blood. However, the effects of shear conditioning on the functional responses of endothelial cells, such as ability to recruit leukocytes, remain uncertain. Here we describe a system for culturing multiple samples of endothelial cells under flow for prolonged periods, either at different shear stresses, or exposed concurrently to different concentrations of cytokines, for instance, tumour necrosis factor-alpha (TNF). The endothelial cells were cultured in glass capillaries (microslides) that could be conveniently transferred to a flow-based adhesion assay, to test the ability of the cultures to support adhesion and migration of flowing leukocytes. Paired control, 'static' samples were exposed to the identical medium and culture geometry. We found that the type of tubing used in the culture flow circuit and its maintenance at 37 degrees C were critical design factors, which could influence the response to TNF of the static controls which were exposed to recirculated medium. Endothelial cells conditioned by culture under flow showed a reduction in response to TNF, as judged by ability to induce the capture and migration of neutrophils. We found that the higher the shear stress, the weaker the ability to recruit neutrophils. This sensitivity to shear stress was greater if the cells were allowed to stabilise under static conditions for 24 h, compared to cells exposed to flow immediately after seeding. The inhibition of neutrophil recruitment was similar for cultures exposed to steady flow or flow with a pulsatile element (flow oscillation approximately 20% about the mean). Thus, we have developed a versatile culture system which allows investigations of functional modifications of endothelial cells and demonstrates the potential sensitivity of inflammatory responses to the local fluid environment.

Endothelial cells proactively form microvilli-like membrane projections upon intercellular adhesion molecule 1 engagement of leukocyte LFA-1

Specific leukocyte/endothelial interactions are critical for immunity and inflammation, yet the molecular details of this interaction interface remain poorly understood. Thus, we investigated, with confocal microscopy, the distribution dynamics of the central adhesion molecules ICAM-1 and LFA-1 in this context. Monolayers of activated HUVECs stained with fluorescent anti-ICAM-1 Fabs or Chinese hamster ovary-K1 cells expressing ICAM-1-green fluorescent protein were allowed to bind LFA-1-bearing monocytes, neutrophils, or K562 LFA-1 transfectants. ICAM-1 was rapidly relocalized to newly formed microvilli-like membrane projections in response to binding LFA-1 on leukocytes. These ICAM-1-enriched projections encircled the leukocytes extending up their sides and clustered LFA-1 underneath into linear tracks. Projections formed independently of VCAM-1/very late Ag 4 interactions, shear, and proactive contributions from the LFA-1-bearing cells. In the ICAM-1-bearing endothelial cells, projections were enriched in actin but not microtubules, required intracellular calcium, and intact microfilament and microtubule cytoskeletons and were independent of Rho/Rho kinase signaling. Disruption of these projections with cytochalasin D, colchicine, or BAPTA-AM had no affect on firm adhesion. These data show that in response to LFA-1 engagement the endothelium proactively forms an ICAM-1-enriched cup-like structure that surrounds adherent leukocytes but is not important for firm adhesion. This finding leaves open a possible role in leukocyte transendothelial migration, which would be consistent with the geometry and kinetics of formation of the cup-like structure.

CD31 regulates direction and rate of neutrophil migration over and under endothelial cells

Mechanisms guiding migration of neutrophils through endothelium are poorly understood. We showed previously that CD31-CD31 binding acted as an 'accelerator' for neutrophils migrating on platelets, while neutrophil alpha(v)beta3-integrin acted as a sensor to align migration with the direction of imposed flow. Here, we perfused neutrophils over human umbilical vein endothelial cells (HUVEC) treated with tumour necrosis factor-alpha, and characterised the kinetics of migration over, through and underneath the HUVEC. Before penetrating the monolayer, activated neutrophils migrated relatively slowly over the surface (approximately 6 microm/min), preferentially in the direction of flow. Once transmigrated, neutrophils moved more rapidly (approximately 14 microm/min) without preferred direction. Treatment of HUVEC and/or neutrophils with function-blocking antibodies against CD31 reduced directionality but not velocity of migration on top of HUVEC, and reduced velocity of migration underneath the monolayer. If neutrophils were pre-activated with formyl peptide, they did not migrate through the HUVEC, but migrated with increased velocity and directionality on top. Under these circumstances, both velocity and directionality were reduced by blocking CD31. alpha(v)beta3-integrin did not regulate migration under any conditions. We conclude that CD31-CD31 bonds act as robust sensors which can guide neutrophil migration, and also modify its velocity. Thus mechanical and adhesive signals can regulate neutrophil migration driven by locally-acting chemotactic agents.

Exposure to fluid shear stress modulates the ability of endothelial cells to recruit neutrophils in response to tumor necrosis factor-alpha: a basis for local variations in vascular sensitivity to inflammation

Vascular endothelial cells are able to sense changes in the forces acting on them and respond, for instance, by modifying expression of a range of genes. However, there is little information on how such responses are integrated to modify homeostatic functions. We hypothesized that different shear stresses experienced in different regions of the circulation might influence endothelial sensitivity to inflammatory stimuli. We cultured human endothelial cells in tubes and exposed them for varying periods to shear stresses ranging from those typically found in postcapillary venules to those in arteries. When tumor necrosis factor-alpha was included in the flow cultures, we found startling differential effects of shear stress on the ability of endothelial cells to induce adhesion and migration of flowing neutrophils. Compared with static cultures, endothelial cells cultured at low shear stress (0.3 Pa) captured similar numbers of neutrophils but failed to induce their transendothelial migration. After exposure of endothelial cells to high shear stress (1.0 or 2.0 Pa), capture of neutrophils was largely ablated. The modification in response was detectable after 4 hours of exposure to flow but was much greater after 24 hours. From analysis of gene expression, loss of capture or migration was attributable to reduction in tumor necrosis factor-induced expression of selectins or CXC-chemokines, respectively. Thus, conditioning of endothelial cells by different flow environments may underlie variations in susceptibility to inflammation between different tissues or parts of the vascular tree.

Patients with postoperative infections have sticky neutrophils before operation

Appropriate polymorphonuclear neutrophil (PMN) recruitment is essential for host defense against infection. We investigated the significance of the preoperative PMN adhesion-migration process, as assessed by the flow chamber method, on postoperative infectious complications. Thirty-one consecutive patients with gastrointestinal malignancies, 21 colorectal and 10 gastric, who were undergoing elective surgery were enrolled. PMNs, isolated preoperatively from each patient's venous blood, were perfused onto a tumor necrosis factor alpha-stimulated human umbilical vein endothelial cell (HUVEC) monolayer through the flow chamber. We evaluated the adherent PMN number, the migrated PMN number, and the stuck PMN number by directly inspecting PMN interactions with a HUVEC monolayer under continuous shear flow simulating postcapillary venules. The expression of adhesion molecules on circulating PMNs was also measured. Patients were grouped into an infectious and a noninfectious group according to the occurrence of postoperative infectious complications defined by the Centers for Disease Control criteria. Eleven patients developed postoperative infectious complications. Although the number of preoperative in vitro adherent PMNs in patients with postoperative infection was significantly higher than in those without postoperative infection (P = 0.01), migrated PMN number was similar in both groups. Stuck PMN number tended to be higher in the infectious group than in the noninfectious group. The migrated PMN number showed a significant positive correlation with the adherent PMN number in the noninfectious group but not in the infectious group. Preoperative CD31 expression on circulating PMNs was significantly lower in the infectious group than in the noninfectious group. Preoperative in vitro derangement of the PMN adhesion-migration process is closely associated with postoperative infectious complications.

Neutrophils induce sequential focal changes in endothelial adherens junction components: role of elastase

OBJECTIVE

In vitro studies have indicated that polymorphonuclear leukocytes (PMNs) traverse endothelial cell monolayers via the paracellular pathway (i.e., through endothelial cell-cell junctions. Herein, we assessed whether the adherens junctions (AJs) are disrupted during PMN transendothelial cell migration.

METHODS

Human umbilical vein endothelial cells (HUVECs) were grown to confluence on porous membranes and activated with interleukin-1beta, and PMN transendothelial migration was facilitated by formyl-methionyl-leucyl-phenylalanine. Using dual immunofluorescence staining and laser scanning confocal microscopy, we assessed the effects of PMN-endothelial cell adhesive interactions (i.e., adhesion to and emigration across monolayers) on the AJ components vascular endothelial (VE)-cadherin, beta-catenin, alpha-catenin, and gamma-catenin.

RESULTS

In the AJ immediately adjacent to the adherent PMN, there was a loss of staining for some of the AJ components. AJ components further away from HUVEC-PMN adhesive interactions were unaffected. An iterative approach indicated that the four components were sequentially lost from the AJ. beta-catenin was lost first, followed by VE-cadherin, alpha-catenin, and, finally, gamma-catenin. In the absence of PMNs, the cross-linking of VE-cadherin, but not platelet endothelial cell adhesion molecule-1 or intercellular adhesion molecule-1, increased the cytoplasmic accumulation of beta-catenin. During PMN transendothelial migration, all of the junctional components under study were lost at the immediate site of monolayer penetration. Again, at regions removed from the actual site of PMN penetration of the monolayers, the AJ components were unaffected. PMN-induced disorganization of the AJs was partially prevented by an elastase inhibitor.

CONCLUSIONS

These findings suggest that adherent PMNs induce a localized, sequential disassembly of AJs, which is partially mediated by PMN-derived elastase and involves the initial loss of an intracellular component of AJs (i.e., beta-catenin).

Effects of pentoxifylline on the different steps during adhesion and transendothelial migration of flowing neutrophils

We used a flow system to observe the stepwise adhesion and migration of neutrophils on cultured human umbilical vein endothelial cells (HUVEC) stimulated with tumour necrosis factor-alpha (TNF) for 4 h, and to evaluate the effects of pentoxifylline (PTX) at each step. When HUVEC had been stimulated with 100 U ml(-1) TNF, treatment of neutrophils with PTX did not reduce the number captured from flow but did cause nearly all adherent cells ( > 90%) to roll, whereas most untreated cells became immobilized and approximately 30% transmigrated within minutes. On washout of the PTX, many rolling cells halted and started to migrate. Treatment of the HUVEC with PTX at the same time as 100 U ml(-1) TNF did not affect the number of neutrophils adhering, but there was a significant increase in the percentage of cells rolling even though PTX was no longer present. Thus PTX reduced presentation of activating agents by HUVEC, as well as inhibiting the response by neutrophils to surface-presented activating agent(s). If HUVEC were stimulated with 10 U ml(-1) TNF with PTX, the adhesion of flowing neutrophils was greatly inhibited compared to TNF alone. Surface ELISA indicated that PTX reduced TNF-induced upregulation of E-selectin. This reduction was only sufficient to reduce capture of neutrophils at the low dose of TNF. Thus, by using a flow-based model, we have been able to separate the effects of a multipotent agent such as pentoxifylline, which acts on leucocytes and endothelial cells, at each stage of migration.

A novel system for investigating the ability of smooth muscle cells and fibroblasts to regulate adhesion of flowing leukocytes to endothelial cells

Stromal cells may contribute to the inflammatory processes which lead to the recruitment of circulating leukocytes. Here, we describe a multicellular model in which chosen cellular elements of tissue can be cocultured with endothelial cells (EC). Cocultures can be incorporated into a novel parallel plate flow chamber to determine if stromal cells influence the patterns of leukocyte adhesion to the EC. As an example relevant to the pathology of atherosclerosis, EC were cultured with arterial smooth muscle cells (SMC) of the 'secretory' phenotype. EC and secretory SMC were cultured on the opposite faces of commercially available porous polyethylene terepthalate (PET) culture inserts, which fitted into a parallel plate flow chamber. Binding of flowing purified lymphocytes, labelled with the fluorochrome calcein-AM, to cocultured EC was assessed by fluorescence microscopy. Lymphocyte adhesion was negligible on unstimulated EC cultured alone or cocultured with SMC. However, when tumour necrosis factor-alpha (TNF) was added to cocultures, the EC supported greatly increased levels of lymphocyte adhesion compared to TNF-treated EC cultured alone. Additionally, cocultured EC responded to TNF at concentrations far below those at which EC cultured alone responded. This priming was specific in that skin fibroblasts cocultured with EC did not modify lymphocyte adhesion induced by TNF. Thus, we have developed a coculture model to determine the ability of tissue stromal cells to modify leukocyte recruitment. This may have wide applications in the study of the cellular pathology of inflammation by allowing the contribution of the local microenvironment to be assessed.

Shear stress affects migration behavior of polymorphonuclear cells arrested on endothelium

Polymorphonuclear cell (PMN) transmigration across the TNF-alpha-stimulated endothelial cell (HUVEC) monolayer in the presence of shear flow was monitored with time-lapse videotapes. More than half of the PMN that arrested on HUVEC transmigrated through endothelial cell junctions within the following 15 min. The kinetics of transmigration was significantly faster than that of PMN placed under static conditions. Once PMN crept into the subendothelial space, they showed random migration beneath the HUVEC monolayer. PMN that did not transmigrate moved on the apical surface of HUVEC in the direction of flow downstream. Anti-beta1 integrin mAb (4B4) and RGD peptide inhibited the transmigration more effectively than anti-beta2 integrin mAb (TS1/18) and almost totally abrogated transmigration. When HUVEC were cultured on fibronectin or laminin, the transmigration was significantly inhibited by anti-alpha5 or alpha6 integrin mAbs, respectively. Our data clearly indicate that shear stress affects the migration behavior of PMN arrested on endothelium and suggest that binding to subendothelial extracellular matrix via beta1 integrins is another essential step in leukocyte extravasation.

Differential ability of exogenous chemotactic agents to disrupt transendothelial migration of flowing neutrophils

Neutrophils migrate through endothelium using an ordered sequence of adhesive interactions and activating signals. To investigate the consequences of disruption of this sequence, we characterized adhesion and migration of neutrophils perfused over HUVEC that had been treated with TNF-alpha for 4 h and evaluated changes caused by exogenously added chemotactic agents. When HUVEC were treated with 2 U/ml TNF, flowing neutrophils adhered, with the majority rolling and relatively few migrating through the monolayer. If fMLP, IL-8, zymosan-activated plasma (a source of activated complement factor C5a), epithelial cell-derived neutrophil-activating peptide (ENA-78), or growth-regulating oncogene, GRO-alpha, was perfused over these neutrophils, they stopped rolling and rapidly migrated over the monolayer, but did not penetrate it. When HUVEC were treated with 100 U/ml TNF, the majority of adherent neutrophils transmigrated. If neutrophils were treated with fMLP, IL-8, C5a, ENA-78, or GRO-alpha just before perfusion over this HUVEC, transmigration, but not adhesion, was abolished. However, when platelet-activating factor was used to activate neutrophils, migration through HUVEC treated with 100 U/ml TNF was not impaired, and migration through HUVEC treated with 2 U/ml TNF was actually increased. Transmigration required ligation of CXC chemokine receptor-2 on neutrophils, and differential desensitization of this receptor (e.g., by fMLP but not platelet-activating factor) may explain the pattern of disruption of migration. Thus, transmigration may require presentation of the correct activators in the correct sequence, and inappropriate activation (e.g., by systemic activators) could cause pathological accumulation of neutrophils in the vessel lumen.