Shear stress affects migration behavior of polymorphonuclear cells arrested on endothelium

Mechanosensation by endothelial PIEZO1 is required for leukocyte diapedesis

The extravasation of leukocytes is a critical step during inflammation which requires the localized opening of the endothelial barrier. This process is initiated by the close interaction of leukocytes with various adhesion molecules such as intercellular adhesion molecule-1 (ICAM-1) on the surface of endothelial cells. Here we reveal that mechanical forces generated by leukocyte-induced clustering of ICAM-1 synergistically with fluid shear stress exerted by the flowing blood increase endothelial plasma membrane tension to activate the mechanosensitive cation channel PIEZO1. This leads to increases in [Ca2+]i and activation of downstream signaling events including phosphorylation of SRC, PYK2 and myosin light chain resulting in opening of the endothelial barrier. Mice with endothelium-specific Piezo1 deficiency show decreased leukocyte extravasation in different inflammation models. Thus, leukocytes and the hemodynamic microenvironment synergize to mechanically activate endothelial PIEZO1 and subsequent downstream signaling to initiate leukocyte diapedesis.

Transendothelial migration induces differential migration dynamics of leukocytes in tissue matrix

Leukocyte extravasation into inflamed tissue is a complex process that is difficult to capture as a whole in vitro. We employed a blood-vessel-on-a-chip model in which human endothelial cells were cultured in a tube-like lumen in a collagen-1 matrix. The vessels are leak tight, creating a barrier for molecules and leukocytes. Addition of inflammatory cytokine TNF-α (also known as TNF) caused vasoconstriction, actin remodelling and upregulation of ICAM-1. Introducing leukocytes into the vessels allowed real-time visualization of all different steps of the leukocyte transmigration cascade, including migration into the extracellular matrix. Individual cell tracking over time distinguished striking differences in migratory behaviour between T-cells and neutrophils. Neutrophils cross the endothelial layer more efficiently than T-cells, but, upon entering the matrix, neutrophils display high speed but low persistence, whereas T-cells migrate with low speed and rather linear migration. In conclusion, 3D imaging in real time of leukocyte extravasation in a vessel-on-a-chip enables detailed qualitative and quantitative analysis of different stages of the full leukocyte extravasation process in a single assay. This article has an associated First Person interview with the first authors of the paper.

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.

A Flow Chamber Assay for Studying MAIT Cell Trafficking

Human MAIT cells show little expression of the selectin CD62L and the chemokine receptor CCR7, which are important for entering lymph nodes, and high expression of selectin ligands and chemokine receptors that mediate trafficking into inflamed tissue. Extravasation of leukocytes into tissue requires sequential steps including rolling, firm arrest, crawling, and transendothelial migration, and can be modeled using endothelial cell monolayers in flow chambers that approximate the sheer stress found in post-capillary venules. Using MAIT cells purified from elutriated lymphocytes by fluorescence-activated cell sorting, we have used flow chambers to demonstrate roles for individual chemokine receptors in specific steps required for extravasation. These methods provide a general way to study the molecular mechanisms underlying MAIT cell trafficking from blood into tissue.

Effect of ex vivo extracorporeal membrane oxygenation flow dynamics on immune response

BACKGROUND

Extracorporeal membrane oxygenation is a life-saving support for heart and/or lung failure patients. Despite technological advancement, abnormal physiology persists and has been associated with subsequent adverse events. These include thrombosis, bleeding, systemic inflammatory response syndrome and infection. However, the underlying mechanisms are yet to be elucidated. We aimed to investigate whether the different flow dynamics of extracorporeal membrane oxygenation would alter immune responses, specifically the overall inflammatory response, leukocyte numbers and activation/adhesion surface antigen expression.

METHODS

An ex vivo model was used with human whole blood circulating at 37°C for 6 hours at high (4 L/minute) or low (1.5 L/minute) flow dynamics, with serial blood samples taken for analysis.

RESULTS

During high flow, production of interleukin-1β (p < 0.0001), interleukin-6 (p = 0.0075), tumour necrosis factor-α (p = 0.0013), myeloperoxidase (p < 0.0001) and neutrophil elastase (p < 0.0001) were significantly elevated over time compared to low flow, in particular at 6 hours. While the remaining assessments exhibited minute changes between flow dynamics, a consistent trend of modulation in leukocyte subset numbers and phenotype was observed at 6 hours.

CONCLUSION

We conclude that prolonged circulation at high flow triggers a prominent pro-inflammatory cytokine response and activates neutrophil granule release, but further research is needed to better characterize the effect of flow during extracorporeal membrane oxygenation.

Development of an in vitro media perfusion model of Leishmania major macrophage infection

Background

In vitro assays are widely used in studies on pathogen infectivity, immune responses, drug and vaccine discovery. However, most in vitro assays display significant differences to the in vivo situation and limited predictive properties. We applied medium perfusion methods to mimic interstitial fluid flow to establish a novel infection model of Leishmania parasites.

Methods

Leishmania major infection of mouse peritoneal macrophages was studied within the Quasi Vivo QV900 macro-perfusion system. Under a constant flow of culture media at a rate of 360μl/min, L. major infected macrophages were cultured either at the base of a perfusion chamber or raised on 9mm high inserts. Mathematical and computational modelling was conducted to estimate medium flow speed, shear stress and oxygen concentration. The effects of medium flow on infection rate, intracellular amastigote division, macrophage phagocytosis and macropinocytosis were measured.

Results

Mean fluid speeds at the macrophage cell surface were estimated to be 1.45 x 10⁻⁹ m/s and 1.23 x 10⁻⁷ m/s for cells at the base of the chamber and cells on an insert, respectively. L. major macrophage infection was significantly reduced under both media perfusion conditions compared to cells maintained under static conditions; a 85±3% infection rate of macrophages at 72 hours in static cultures compared to 62±5% for cultures under slow medium flow and 55±3% under fast medium flow. Media perfusion also decreased amastigote replication and both macrophage phagocytosis (by 44±4% under slow flow and 57±5% under fast flow compared with the static condition) and macropinocytosis (by 40±4% under slow flow and 62±5% under fast flow compared with the static condition) as measured by uptake of latex beads and pHrodo Red dextran.

Conclusions

Perfusion of culture medium in an in vitro L. major macrophage infection model (simulating in vivo lymphatic flow) reduced the infection rate of macrophages, the replication of the intracellular parasite, macrophage phagocytosis and macropinocytosis with greater reductions achieved under faster flow speeds.

Go with the flow: GEF-H1 mediated shear stress mechanotransduction in neutrophils

Neutrophils in circulation experience significant shear forces due to blood flow when they tether to the vascular endothelium. Biochemical and biophysical responses of neutrophils to the physical force of flowing blood modulate their behavior and promote tissue recruitment under pro-inflammatory conditions. Neutrophil mechanotransduction responses occur through mechanisms that are not yet fully understood. In our recent work, we showed that GEF-H1, a RhoA specific guanine nucleotide exchange factor (GEF), is required to maintain neutrophil motility and migration in response to shear stress. GEF-H1 re-localizes to flottilin-rich uropods in neutrophils in response to fluid shear stress and promotes spreading and crawling on activated endothelial cells. GEF-H1 drives cellular contractility through myosin light chain (MLC) phosphorylation downstream of the Rho-ROCK signaling axis. We propose that GEF-H1-dependent cell spreading and crawling in shear stress-dependent neutrophil recruitment from the vasculature are due to the specific localization of Rho-induced contractility in the uropod.

Role of the Cytoskeleton in Myeloid Cell Function

During an innate immune response, myeloid cells undergo complex morphological adaptations in response to inflammatory cues, which allow them to exit the vasculature, enter the tissues, and destroy invading pathogens. The actin and microtubule cytoskeletons are central to many of the most essential cellular functions including cell division, cell morphology, migration, intracellular trafficking, and signaling. Cytoskeletal structure and regulation are crucial for many myeloid cell functions, which require rapid and dynamic responses to extracellular signals. In this chapter, we review the roles of the actin and microtubule cytoskeletons in myeloid cells, focusing primarily on their roles in chemotaxis and phagocytosis. The role of myeloid cell cytoskeletal defects in hematological disorders is highlighted throughout.

GEF-H1 is necessary for neutrophil shear stress-induced migration during inflammation

In their work, Fine et al. demonstrate that GEF-H1 is required for the spreading and crawling of neutrophils in response to intravascular blood flow. They uncover a novel mechanism that couples shear stress with Rho-dependent migratory behavior of neutrophils during inflammation.

Leukocyte crawling and transendothelial migration (TEM) are potentiated by shear stress caused by blood flow. The mechanism that couples shear stress to migration has not been fully elucidated. We found that mice lacking GEF-H1 (GEF-H1^−/−), a RhoA-specific guanine nucleotide exchange factor (GEF), displayed limited migration and recruitment of neutrophils into inflamed tissues. GEF-H1^−/− leukocytes were deficient in in vivo crawling and TEM in the postcapillary venules. We demonstrated that although GEF-H1 deficiency had little impact on the migratory properties of neutrophils under static conditions, shear stress triggered GEF-H1–dependent spreading and crawling of neutrophils and relocalization of GEF-H1 to flotillin-2–rich uropods. Our results identify GEF-H1 as a component of the shear stress response machinery in neutrophils required for a fully competent immune response to bacterial infection.

Leukocyte transendothelial migration: A local affair

Inflammation is part of the complex biological response of body tissues to harmful stimuli, such as pathogens. It serves as a protective response that involves leukocytes, blood vessels and molecular mediators with the purpose to eliminate the initial cause of cell injury and to initiate tissue repair. Inflammation is tightly regulated by the body and is associated with transient crossing of leukocytes through the blood vessel wall, a process called transendothelial migration (TEM) or diapedesis. TEM is a close collaboration between leukocytes on one hand and the endothelium on the other. Limiting vascular leakage during TEM but also when the leukocyte has crossed the endothelium is essential for maintaining vascular homeostasis. Although many details have been uncovered during the recent years, the molecular mechanisms from the vascular part that drive TEM still shows significant gaps in our understanding. This review will focus on the local signals that are induced in the endothelium that regulate leukocyte TEM and simultaneous preservation of endothelial barrier function.

Focused ultrasound delivers targeted immune cells to metastatic brain tumors

Natural killer (NK) cells are cytotoxic lymphocytes involved in innate immunity. NK-92, a human NK cell line, may be targeted to tumor-associated antigens in solid malignancies where it exhibits antitumor efficacy, but its clinical utility for treating brain tumors is limited by an inability to cross the blood-brain barrier (BBB). We investigated the potential for focused ultrasound (FUS) to deliver targeted NK-92 cells to the brain using a model of metastatic breast cancer. HER2-expressing human breast tumor cells were implanted into the brain of nude rats. The NK-92-scFv(FRP5)-zeta cell line expressing a chimeric HER2 antigen receptor was transfected with superparamagnetic iron oxide nanoparticles before intravenous injection, before and following BBB disruption using focused ultrasound (551.5 kHz focused transducer, 0.33 MPa average peak rarefaction pressure) in the presence of a microbubble contrast agent. Baseline and posttreatment 1.5T and 7T MR imaging was done, and histology used to identify NK-92 cells post-mortem. Contrast-enhanced MRI showed reproducible and consistent BBB disruption. 7T MR images obtained at 16 hours posttreatment revealed a significant reduction in signal indicating the presence of iron-loaded NK-92 cells at the tumor site. The average ratio of NK-92 to tumor cells was 1:100 when NK cells were present in the vasculature at the time of sonication, versus 2:1,000 and 1:1,000 when delivered after sonication and without BBB disruption, respectively. Our results offer a preclinical proof-of-concept that FUS can improve the targeting of immune cell therapy of brain metastases.

Microfluidics for in vitro biomimetic shear stress-dependent leukocyte adhesion assays

Recruitment of leukocytes from blood to tissues is a multi-step process playing a major role in the activation of inflammatory responses. Tethering and rolling of leukocytes along the vessel wall, followed by arrest and transmigration through the endothelium result from chemoattractant-dependent signals, inducing adhesive and migratory events. Shear forces exerted by the blood flow on leukocytes induce rolling via selectin-mediated interactions with endothelial cells and increase the probability of leukocytes to engage their chemokine receptors, facilitating integrin activation and consequent arrest. Flow-derived shear forces generate mechanical stimuli concurring with biochemical signals in the modulation of leukocyte-endothelial cell interactions. In the last few years, a host of in vitro studies have clarified the biochemical adhesion cascade and the role of shear stress in leukocyte extravasation. The limitation of the static environment in Boyden devices has been overcome both by the use of parallel-plate flow chambers and by custom models mimicking the in vivo conditions, along with widespread microfluidic approaches to in vitro modeling. These devices create an in vitro biomimetic environment where the multi-step transmigration process can be imaged and quantified under mechanical and biochemical controlled conditions, including fluid dynamic settings, channel design, materials and surface coatings. This paper reviews the technological solutions recently proposed to model, observe and quantify leukocyte adhesion behavior under shear flow, with a final survey of high-throughput solutions featuring multiple parallel assays as well as thorough and time-saving statistical interpretation of the experimental results.

Migrational guidance of neutrophils is mechanotransduced via high-affinity LFA-1 and calcium flux

Acute inflammation triggers the innate immune response of neutrophils that efficiently traffic from the bloodstream to concentrate at high numbers at the site of tissue infection or wounding. A gatekeeper in this process is activation of β(2) integrins, which form bond clusters with ICAM-1 on the endothelial surface. These bond clusters serve dual functions of providing adhesive strength to anchor neutrophils under the shear forces of blood flow and directional guidance for cell polarization and subsequent transmigration on inflamed endothelium. We hypothesized that shear forces transmitted through high-affinity LFA-1 facilitates the cooperation with the calcium release-activated channel Orai1 in directing localized cytoskeletal activation and directed migration. By using vascular mimetic microfluidic channels, we observed neutrophil arrest on a substrate of either ICAM-1 or allosteric Abs that stabilize a high- or low-affinity conformation of LFA-1. Neutrophils captured via low-affinity LFA-1 did not exhibit intracellular calcium flux, F-actin polymerization, cell polarization, or directional migration under shear flow. In contrast, high-affinity LFA-1 provided orientation along a uropod-pseudopod axis that required calcium flux through Orai1. We demonstrate how the shear stress of blood flow can transduce distinct outside-in signals at focal sites of high-affinity LFA-1 that provide contact-mediated guidance for neutrophil emigration.

Leukocyte transmigration requires kinesin-mediated microtubule-dependent membrane trafficking from the lateral border recycling compartment

Diapedesis of leukocytes across endothelial cells is a crucial step in both the innate and adaptive immune responses. Surface molecules on leukocytes and endothelial cells critical for diapedesis have been identified, but the mechanisms underlying this process are not understood. Homophilic interaction between platelet/endothelial cell adhesion molecule (PECAM) on leukocytes and PECAM at the endothelial border triggers targeted recycling of membrane from a reticulum localized close to the endothelial cell lateral border. This membrane surrounds the transmigrating leukocyte (Mamdouh, Z., X. Chen, L.M. Pierini, F.R. Maxfield, and W.A. Muller. 2003. Nature. 421:748-753). How this process occurs and whether it is required for diapedesis independent of PECAM are not known. We now report that targeted recycling from this lateral border recycling compartment (LBRC) is required for diapedesis, is mediated by kinesin family molecular motors, and requires normally functioning endothelial microtubules. Selective disruption of microtubules or inhibition of kinesin motor domain blocked targeted recycling and diapedesis of monocytes. Furthermore, targeted recycling of membrane from the LBRC was required for transmigration of lymphocytes, which migrate independently of PECAM. Thus, trafficking of membrane from the LBRC to surround leukocytes may be a general requirement for migration of leukocytes across the endothelial cell border. Furthermore, these data provide the first demonstration of a role for endothelial microtubules and kinesins in promoting diapedesis, and a mechanism to explain targeted recycling.

Assays of transendothelial migration in vitro

The inflammatory response is critical for our ability to heal wounds and fight off foreign microorganisms. Uncontrolled inflammation is also at the root of most pathologic conditions. Recruitment of leukocytes to the site of inflammation plays a defining role in the inflammatory response, and migration of leukocytes across endothelium is arguably the point of no return of the inflammatory response. Assays to study the transmigration of leukocytes have and will continue to shed light on the regulation of this vital response. Assays of transendothelial migration in vitro allow the controlled observation of this phenomenon, as well as experiments to study its regulation. In this chapter, we describe in vitro assays of transendothelial migration that have been used successfully in the authors' laboratories for decades and have proven to be reproducible, reliable, and predictive of the behavior of leukocytes and endothelial cells in models of inflammation in vivo.

Recoil and stiffening by adherent leukocytes in response to fluid shear

Prolonged exposure to fluid shear stress alters leukocyte functions associated with the immune response. We examined the initial response of freshly isolated human leukocytes to fluid shear stress under high magnification. Adherent leukocytes exhibit a rapid biomechanical response to physiological levels of fluid shear stress. After passive displacement in the direction of a constant fluid shear stress, adherent leukocytes actively recoil back in the opposite direction of the fluid flow. Recoil is observed within seconds of the applied fluid shear stress. Simultaneously, fluid shear stress induces a stiffening of the cell. The immediate cell displacement in response to a step increase in fluid shear stress is greatly attenuated in subsequent steps compared to the initial fluid shear stress step. Recoil is not mediated by actin polymerization-dependent mechanisms, as cytochalasin D had no effect on this early response. However, stiffening was determined in part by an intact actin cytoskeleton. Inhibiting myosin force generation with ML-7 abolished the recoil and stiffening responses, implicating force generation by myosin as an important contributor to the early leukocyte response to fluid shear stress. This initial shear stress response may be particularly important in facilitating leukocyte attachment under sustained fluid shear stress by the flowing blood in the microcirculation.

CD99 Is a Key Mediator of the Transendothelial Migration of Neutrophils

Transendothelial migration of leukocytes is a critical event for inflammation, but the molecular regulation of this event is only beginning to be understood. PECAM (CD31) is a major mediator of monocyte and neutrophil transmigration, and CD99 was recently defined as a second mediator of the transmigration of monocytes. Expression of CD99 on the surface of circulating polymorphonuclear cells (PMN) is low compared with expression of CD99 on monocytes or expression of PECAM on PMN. We demonstrate here that, despite low expression of CD99, Fab of Abs against CD99 blocked over 80% of human neutrophils from transmigrating across HUVEC monolayers in an in vitro model of inflammation. Blocking CD99 on either the neutrophil or endothelial cell side resulted in a quantitatively equivalent block, suggesting a homophilic interaction between CD99 on the neutrophil and CD99 on the endothelial cell. Blocking CD99 and PECAM together resulted in additive effects, suggesting the two molecules work at distinct steps. Confocal microscopy confirmed that CD99-blocked neutrophils lodged in endothelial cell junctions at locations distal to PECAM-blocked neutrophils. The CD99-blocked PMN exhibited dynamic lateral movement within endothelial cell junctions, indicating that only the diapedesis step was blocked by interference with CD99. Anti-CD99 mAb also blocked PMN transmigration in a second in vitro model that incorporated shear stress. Taken together, the evidence demonstrates that PECAM and CD99 regulate distinct, sequential steps in the transendothelial migration of neutrophils during inflammation.

CD157 plays a pivotal role in neutrophil transendothelial migration

Paracellular diapedesis, a key step in leukocyte recruitment to the site of inflammation, occurs at endothelial junctions and is regulated by highly coordinated interactions between leukocytes and endothelium. We found that CD157, a glycosylphosphatidylinositol-anchored ectoenzyme belonging to the NADase/ADP-ribosyl cyclase family, plays a crucial role for neutrophil diapedesis, because its ligation with specific monoclonal antibodies (both on neutrophils or endothelial cells) results in altered neutrophil movement on the apical surface of endothelium and, ultimately, in loss of diapedesis. Real-time microscopy revealed that CD157 behaves as a sort of compass during the interaction between neutrophils and endothelial cells; indeed, following CD157 ligation, neutrophils appear disoriented, meandering toward junctions where they eventually stop without transmigrating. These findings are relevant in vivo because CD157-deficient neutrophils obtained from patients with paroxysmal nocturnal hemoglobinuria are characterized by a severely impaired diapedesis.

Morphological analysis of tumor cell/endothelial cell interactions under shear flow

In the process of hematogenous cancer metastasis, tumor cells (TCs) must shed into the blood stream, survive in the blood circulation, migrate through the vascular endothelium (extravasation) and proliferate in the target organs. However, the precise mechanisms by which TCs penetrate the endothelial cell (EC) junctions remain one of the least understood aspects of TC extravasation. This question has generally been addressed under static conditions, despite the important role of flow induced mechanical stress on the circulating cell-endothelium interactions. Moreover, flow studies were generally focused on transient or firm adhesion steps of TC-EC interactions and did not consider TCs spreading or extravasation. In this paper, we used a parallel-plate flow chamber to investigate TC-EC interactions under flow conditions. An EC monolayer was cultured on the lower plate of the flow chamber to model the endothelial barrier. Circulating TCs were introduced into the flow channel under a well-defined flow field and TC cell shape changes on the EC monolayer were followed in vitro with live phase contrast and fluorescence microscopy. Two spreading patterns were observed: radial spreading which corresponds to TC extravasation, and axial spreading where TCs formed a mosaic TC-EC monolayer. By investigating the changes in area and minor/major aspect ratio, we have established a simple quantitative basis for comparing spreading modes under various shear stresses. Contrary to radial spreading, the extent of axial spreading was increased by shear stress.

A new antihistamine levocetirizine inhibits eosinophil adhesion to vascular cell adhesion molecule-1 under flow conditions

BACKGROUND

We previously demonstrated that low concentrations of a new antihistamine levocetirizine inhibited eosinophil transmigration through human microvascular endothelial cells.

OBJECTIVE

Here, the inhibitory effect of levocetirizine on eosinophil adhesion to recombinant human vascular cell adhesion molecule-1 (rhVCAM)-1 was examined under conditions of shear stress using an in vitro model of the post-capillary venules.

METHODS

Eosinophils isolated from normal subjects were pre-incubated with a concentration range of levocetirizine (10(-6)-10(-10) m) or negative dilution control. Resting or granulocyte macrophage-colony stimulating factor (GM-CSF)-stimulated cells were pumped through rhVCAM-1 (10 microg/mL) coated capillary tubes using a microfluidic syringe pump at a precise and constant flow rate (1 dyn/cm(2)). Images of rolling and firmly adherent eosinophils were captured using real-time video microscopy.

RESULTS

Levocetirizine significantly inhibited resting eosinophil adhesion to rhVCAM-1 with maximal effect at 10(-8) M with an EC(50) of 10(-9) m. Levocetirizine almost abolished resting eosinophil adhesion by the 15 min time-point. GM-CSF significantly enhanced eosinophil adhesion and their ability to flatten on rhVCAM-1. Both phenomena were inhibited by levocetirizine in a dose-dependent manner, at both 5 and 15 min (optimal concentration of 10(-8) m with an EC(50) of 10(-9) m). Real-time imaging revealed that the effect of levocetirizine on post-adhesion behaviour (detachment, flatness) contributed to its inhibitory action on eosinophil adhesion to rhVCAM-1. In contrast, very late antigen (VLA)-4 mAb inhibited eosinophil adhesion to rhVCAM-1 from the earliest time-points.

CONCLUSION

Physiologically relevant concentrations of levocetirizine inhibit resting and GM-CSF-stimulated firm eosinophil adhesion to rhVCAM-1 under flow conditions.

Neutrophils, lymphocytes, and monocytes exhibit diverse behaviors in transendothelial and subendothelial migrations under coculture with smooth muscle cells in disturbed flow

Atherosclerosis develops at regions of the arterial tree exposed to disturbed flow. The early stage of atherogenesis involves the adhesion of leukocytes (white blood cells [WBCs]) to and their transmigration across endothelial cells (ECs), which are located in close proximity to smooth muscle cells (SMCs). We investigated the effects of EC/SMC coculture and disturbed flow on the adhesion and transmigration of 3 types of WBCs (neutrophils, peripheral blood lymphocytes [PBLs], and monocytes) using our vertical-step flow (VSF) chamber, in which ECs were cocultured with SMCs in collagen gels. Such coculture significantly increased the adhesion and transmigration of neutrophils, PBLs, and monocytes under VSF, particularly in the reattachment area, where the rolling velocity of WBCs and their transmigration time were decreased, as compared with the other areas. Neutrophils, PBLs, and monocytes showed different subendothelial migration patterns under VSF. Their movements were more random and shorter in distance in the reattachment area. Coculture of ECs and SMCs induced their expressions of adhesion molecules and chemokines, which contributed to the increased WBC adhesion and transmigration. Our findings provide insights into the mechanisms of WBC interaction with the vessel wall (composed of ECs and SMCs) under the complex flow environments found in regions of prevalence for atherogenesis.

Eosinophil adhesion under flow conditions activates mechanosensitive signaling pathways in human endothelial cells

Leukocyte transmigration can be affected by shear stress; however, the mechanisms by which shear stress modulates transmigration are unknown. We found that adhesion of eosinophils or an eosinophilic cell line to intereukin 4–stimulated endothelial cells led to a shear-dependent increase in endothelial cell intracellular calcium and increased phosphorylation of extracellular signal-regulated kinase (ERK) 2, but not c-Jun NH₂-terminal kinase or p38 mitogen-activated protein kinase. Latex beads coated with antibodies were used to characterize the role of specific endothelial cell surface molecules in initiating signaling under shear conditions. We found that ligation of either vascular cell adhesion molecule–1 or E-selectin, but not major histocompatibility complex class I, induced a shear-dependent increase in ERK2 phosphorylation in cytokine-stimulated endothelial cells. Disassembly of the actin cytoskeleton with latrunculin A prevented ERK2 phosphorylation after adhesion under flow conditions, supporting a role for the cytoskeleton in mechanosensing. Rapid phosphorylation of focal adhesion kinase and paxillin occurred under identical conditions, suggesting that focal adhesions were also involved in mechanotransduction. Finally, we found that Rho-associated protein kinase and calpain were both critical in the subsequent transendothelial migration of eosinophils under flow conditions. These data suggest that ligation of leukocyte adhesion molecules under flow conditions leads to mechanotransduction in endothelial cells, which can regulate subsequent leukocyte trafficking.

De-Activation of Neutrophils in Suspension by Fluid Shear Stress: A Requirement for Erythrocytes

Leukocyte de-activation in response to a mechanical stimulus may be an important mechanism to reduce inflammation in the circulation and cardiovascular complications. We examine here a specific form of leukocyte activation in the form of pseudopod projection, a process that is important during cell spreading and migration, but if it occurs in circulating leukocytes, may also lead to their entrapment in the microvascular network. Fresh neutrophils were activated with fMLP, suspended without adhesion to endothelium, and sheared in a cone-and-plate device while both shear stress and shear rate were measured. A fraction of the activated neutrophils retracted their pseudopods under the influence of fluid shear and returned to round shape. Pseudopod retraction was observed only in the presence of erythrocytes (at shear stresses up to approximately 25 dyn/cm(2)). At a constant hematocrit and increasing plasma viscosities with addition of macromolecules, the number of de-activated neutrophils scaled with shear stress and less so with shear rate. We examined a biochemical and rheological role of erythrocytes during shear de-activation of neutrophils. Addition of superoxide dismutase (SOD) in phosphate buffer served to enhance neutrophil de-activation by fluid shear. Replacement of erythrocytes by solid microspheres (5.4 mum) to simulate the particle properties of the erythrocytes, did not serve to enhance neutrophil de-activation unless in the presence of SOD. At higher shear stresses without erythrocytes (38-77 dyn/cm(2)), we also observed neutrophil de-activation but only in the presence of SOD. These results suggest that erythrocytes play an important role in neutrophil de-activation by reducing the superoxide level in plasma. Shear stress, rather than shear rate, is the key determinant that regulates neutrophil de-activation.

Experimental approaches to lymphocyte migration in dermatology in vitro and in vivo

Lymphocyte trafficking through the dermal compartment is part of the physiological surveillance process of the adaptive immune system. On the other hand, persistent or recurrent lymphocyte infiltrates are hallmarks of both types of chronic inflammatory skin diseases, Th1-type such as psoriasis or Th2/allergic-type like atopic dermatitis. A better understanding of the mechanisms underlying lymphocyte movements is one of the key prerequisites for developing more effective therapies. In this review, we introduce a range of simple-to-sophisticated experimental in vitro and in vivo approaches to analyze lymphocyte migration. These methods start from static in vitro adhesion and chemotaxis assays, include dynamic endothelial flow chamber, intravital dual photon, and transcutaneous live-video microscopy, and finally encompass specific genetically deficient or engineered animal models. Discussing pros and cons of these assay systems hopefully generates both state-of-the-art knowledge about the factors involved in most common chronic skin diseases as well as an improved understanding of the limitations and chances of new biologic pharmaceuticals that are currently introduced into clinical practice.

Cytoskeletal remodeling and cellular activation during deformation of neutrophils into narrow channels

Neutrophils are subjected to mechanical stimulation as they deform into the narrow capillary segments of the pulmonary microcirculation. The present study seeks to understand the changes in the cytoskeletal structure and the extent of biological activation as a result of this process. Neutrophils were passed through narrow polycarbonate filter pores under physiological driving pressures, fixed, and stained downstream to visualize the F-actin content and distribution. Below a threshold capillary size, the cell remodeled its cytoskeleton through initial F-actin depolymerization, followed by recovery and increase in F-actin content associated with formation of pseudopods. This rapid depolymerization and subsequent recovery of F-actin was consistent with our previous observation of an immediate reduction in moduli with eventual recovery when the cells were subjected to deformation. Results also show that neutrophils must be retained in their elongated shape for an extended period of time for pseudopod formation, suggesting that a combination of low driving pressures and small capillary diameters promotes cellular activation. These observations show that mechanical deformation of neutrophils into narrow pulmonary capillaries have the ability to influence cytoskeletal structure, the degree of cellular activation, and migrational tendencies of the cells.

Vascular progenitor cells: origin and mechanisms of mobilization, differentiation, integration, and vasculogenesis

The recent discovery of progenitor cells in peripheral blood that can differentiate into endothelial or vascular smooth muscle cells has led to the re-evaluation of many traditionally held beliefs about vascular biology. Most notably, concepts of vascular regeneration and repair, previously considered limited to the proliferation of existing differentiated cells within vascular tissue, have been expanded to include the potential for postnatal vasculogenesis. These cells have since been identified in the bone marrow, heart, skeletal muscle, and other peripheral tissues, including the vasculature itself. The significance of these cells lies not only in developing our understanding of normal vascular biology, but also in the insights they may provide into vascular diseases such as atherosclerosis. In addition, a potential role in therapeutics has already been explored in early clinical trials in humans. The mechanisms underlying the mobilization, target tissue integration, differentiation, and the observed therapeutic benefits of these cells are now being elucidated. It is these mechanisms, and the current understanding of the lineage of these cells, that constitutes the focus of this review.

PECAM-1, alpha6 integrins and neutrophil elastase cooperate in mediating neutrophil transmigration

The heterogeneous nature of the perivascular basement membrane (composed primarily of laminin and collagen type IV) suggests the existence of an elaborate array of adhesive interactions and possibly proteolytic events in leukocyte migration through this barrier. In this context, blockade of alpha6 integrins (laminin receptors), neutrophil elastase (NE) or both inhibited neutrophil migration through interleukin-1beta (IL-1beta)-stimulated mouse cremasteric venules, as observed by intravital microscopy. Furthermore, analysis of tissues by confocal microscopy indicated a synergistic role for alpha6 integrins and NE in mediating neutrophil migration through the perivascular basement membrane. Using a combined in vitro and in vivo experimental approach, the findings of this study also suggest that alpha6 integrins and NE are mobilized from intracellular stores to the cell surface of transmigrating mouse neutrophils, although these events occur via mechanisms dependent on and independent of platelet/endothelial-cell adhesion molecule 1 (PECAM-1, CD31), respectively. Despite different regulatory mechanisms, blockade of alpha6 integrins or NE inhibited migration of murine neutrophils through laminin-coated filters in vitro. Collectively, the findings suggest that, whereas regulation of the expression of alpha6 integrins and NE occur via different adhesive mechanisms, these molecules might act in a cooperative manner in mediating neutrophil migration through venular walls, in particular the perivascular basement membrane.

Direct observation and quantitative analysis of spatiotemporal dynamics of individual living monocytes during transendothelial migration

OBJECTIVE

To visualize and quantitatively analyze spatiotemporal dynamics of individual living monocytes during transendothelial migration (TEM).

METHODS AND RESULTS

We developed an in vitro new experimental system using confocal laser scanning microscope with following two improvements: (1) ultra thin collagen gel layer (30-50 microm thick) constructed under human umbilical vein endothelial cell layer for three-dimensional observation with high magnification; (2) appropriate fluorescent labeling of living monocytes and endothelial cells to keep highest cell activity. Individual monocytes behaved quite diversely. Approximately 70% of adhered monocytes directionally crawled to intercellular junction, and started invasion. Time from adhesion to start of invasion was 8.6 +/- 5.4 min (mean +/- S.D., n=61 monocytes). Approximately 80% of such invading monocytes completed TEM, but remaining 20% of once invading monocytes hesitated transmigration, and returned onto the endothelial surface. Time from start to finish of invasion was 6.3 +/- 3.2 min (mean +/- S.D., n=53 monocytes).

CONCLUSIONS

Using our collagen gel-based newly-developed system, we visualized and quantitatively analyzed detailed spatiotemporal, three-dimensional dynamics of individual living monocytes during TEM. We revealed that monocytes encountered at least two hurdles, at starting invasion, and leaving endothelium, to achieve complete TEM. Approximately 56% (80% of 70% of adhered monocytes) passed both hurdles.

Pseudopod projection and cell spreading of passive leukocytes in response to fluid shear stress

Recent evidence suggests that circulating leukocytes respond to physiological levels of fluid shear stress. This study was designed to examine the shear stress response of individual leukocytes adhering passively to a glass surface. Human leukocytes were exposed to a step fluid shear stress with amplitude between 0.2 and 4 dyn/cm(2) and duration between 1 and 20 min. The response of the cells was determined in the form of projected cell area measurements by high-resolution observation before, during, and after fluid shear application. All cells selected initially had a round morphology. After application of fluid shear many cells projected pseudopodia and spread on the glass surface. The number of leukocytes responding with pseudopod projection and the extent of cell spreading increased with increasing amplitude and duration of fluid shear stress. Pseudopod projection after exposure to a step fluid shear occurs following a delay that is insensitive to the shear stress amplitude and duration. Leukocytes that did not project pseudopodia and spread in response to low shear stress could be shown to respond to a second shear step of higher amplitude. The spreading response requires an intact actin network and activated myosin molecules. Depleting the cell glycocalyx with protease treatment enhances the spreading response in sheared leukocytes. These results indicate that passive leukocytes respond to fluid shear stress with active pseudopod projection and cell spreading. This behavior may contribute to cell spreading on endothelium and other cells as well as to transendothelial migration of leukocytes in the microcirculation.

Chemoattractant signals and beta 2 integrin occupancy at apical endothelial contacts combine with shear stress signals to promote transendothelial neutrophil migration

Lymphocyte transendothelial migration (TEM) is promoted by fluid shear signals and apical endothelial chemokines. Studying the role of these signals in neutrophil migration across differently activated HUVEC in a flow chamber apparatus, we gained new insights into how neutrophils integrate multiple endothelial signals to promote TEM. Neutrophils crossed highly activated HUVEC in a beta(2) integrin-dependent manner but independently of shear. In contrast, neutrophil migration across resting or moderately activated endothelium with low-level beta(2) integrin ligand activity was dramatically augmented by endothelial-presented chemoattractants, conditional to application of physiological shear stresses and intact beta(2) integrins. Shear stress signals were found to stimulate extensive neutrophil invaginations into the apical endothelial interface both before and during TEM. A subset of invaginating neutrophils completed transcellular diapedesis through individual endothelial cells within <1 min. Our results suggest that low-level occupancy of beta(2) integrins by adherent neutrophils can mediate TEM only if properly coupled to stimulatory shear stress and chemoattractant signals transduced at the apical neutrophil-endothelial interface.

Mechanical deformation of neutrophils into narrow channels induces pseudopod projection and changes in biomechanical properties

Neutrophils traversing the pulmonary microcirculation are subjected to mechanical stimulation during their deformation into narrow capillaries. To better understand the time-dependant changes caused by this mechanical stimulus, neutrophils were caused to flow into a microchannel, which allowed simultaneous visualization of cell morphology and passive rheological measurement by tracking the Brownian motion of endogenous granules. Above a threshold stimulus, mechanical deformation resulted in neutrophil activation with pseudopod projection. The activation time was inversely correlated to the rate of mechanical deformation experienced by the neutrophils. A reduction in shear moduli was observed within seconds after the onset of the mechanical stimulus, suggesting a sudden disruption of the neutrophil cytoskeleton when subjected to mechanical deformation. However, the magnitude of the reduction in moduli was independent of the degree of deformation. Recovery to nearly the initial values of viscoelastic moduli occurred within 1 min. These observations confirm that mechanical deformation of neutrophils, similar to conditions encountered in the pulmonary capillaries, is not a passive event; rather, it is capable of activating the neutrophils and enhancing their migratory tendencies.

Cell migration/invasion assays and their application in cancer drug discovery

Invasive capacity is the single most important trait that distinguishes benign from malignant lesions. Tumour cells, during intravasation and extravasation of blood and lymphatic channels and when establishing colonies at secondary sites, must move through tissue boundaries that normal adult cells (other than, for example activated leukocytes) do not cross. Similar mechanisms are also utilised by activated endothelial cells during the generation of new blood vessels that enable the sustained growth and dissemination of tumours. It is now increasingly recognised that these processes--cell motility and invasion--might provide a rich source of novel targets for cancer therapy and that appropriate inhibitors may restrain both metastasis and neoangiogenesis. This new paradigm demands screening assays that can rapidly and quantitatively measure cell movement and the ability to traverse physiological barriers. We also need to consider whether simple reductionist in vitro approaches can reliably model the complexity of in vivo tumour invasion/neoangiogenesis. There are both opportunities and challenges ahead in developing a balanced portfolio of assays that will be able to evaluate accurately and finally deliver novel anti-invasive agents with therapeutic potential for clinical use.

Blockade of α6integrin inhibits IL-1β- but not TNF-α-induced neutrophil transmigration in vivo

In vitro and in vivo evidence supports a functional role for the integrin alpha6beta1 in neutrophil migration through the perivascular basement membrane, a response that in vivo appears to be associated with platelet/endothelial cell adhesion molecule-1 (PECAM-1)-mediated up-regulation of alpha6beta1 on the cell surface of transmigrating leukocytes. As the involvement of PECAM-1 in leukocyte migration is cytokine-specific, the aim of the present study was to investigate whether alpha6beta1 exhibited a similar profile of stimulus specificity in this context. The cytokines interleukin-1beta (IL-1beta) and tumor necrosis factor alpha (TNF-alpha) were used to elicit neutrophil migration in two murine models of inflammation, migration through cremasteric venules, as observed by intravital microscopy, and migration into the peritoneal cavity. The role of alpha6beta1 was investigated using an alpha6 integrin-blocking monoclonal antibody GoH3. In both models, GoH3 significantly inhibited neutrophil transmigration induced by IL-1beta but not TNF-alpha. This cytokine-specific role of alpha6 integrin was associated with enhanced cell-surface expression of alpha6beta1 on transmigrated neutrophils (as compared with blood cells) in response to IL-1beta but not TNF-alpha. Using lipopolysaccharide as an inflammatory stimulus in the cremaster muscle model, the study also provides evidence for the involvement of alpha6 integrin in leukocyte transmigration as mediated by endogenously generated IL-1beta. Collectively, the findings demonstrate that alpha6beta1 blockade inhibits neutrophil migration induced by exogenous and endogenous IL-1beta but not TNF-alpha, observations that are associated with increased expression of the integrin on transmigrated leukocytes.

Locomotion of monocytes on endothelium is a critical step during extravasation

Monocytes, like all leukocytes, undergo a series of sequential steps during extravasation from blood into tissues: tethering, rolling, adhesion and diapedesis. We have discovered an essential step, which we call locomotion, in which the monocyte moves from a site of firm adhesion to the nearest junction to begin diapedesis. Blocking CD11a-CD18 and CD11b-CD18 on human monocytes or adhesion molecules ICAM-1 and ICAM-2 on endothelial cells prevented the monocytes from reaching junctions. The blocked monocytes spun in circles as if they were unable to direct their movement despite being able to adhere and polarize normally. This step fills a gap in the paradigm of extravasation as a multistep process.

Tools for anti-inflammatory drug design:In vitro models of leukocyte migration

Inhibiting leukocyte recruitment is now a major focus in the design of novel anti-inflammatory drugs. Following the identification of lead compounds from conventional high-throughput screens using appropriate receptors or enzymes, it is important to validate the action of the compounds in a suitable in vitro model of leukocyte migration. Here, we review a range of different experimental approaches to modelling leukocyte migration, and identify the multi-well filter migration assay as the best compromise between the amount of resources required to screen multiple compounds and the amount of information gained about the effects of the compounds on cell movement behavior. However, there are pitfalls in the interpretation of data obtained using the multi-well filter migration assay, which arise from the imperfect correlation between the number of cells undergoing migration and the inhibitory activity of the test substances. We examine a number of such pitfalls and provide practical approaches to mitigate these problems as far as possible. We recommend a general strategy for screening inhibitors of cell migration using in vitro functional assays. While being more resource intensive than surrogate measures such as calcium flux, functional approaches nevertheless provide superior correlations with anti-inflammatory activity in vivo.

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.

The Fluid Shear Stress Distribution on the Membrane of Leukocytes in the Microcirculation

Recent in-vivo and in-vitro evidence indicates that fluid shear stress on the membrane of leukocytes has a powerful control over several aspects of their cell function. This evidence raises a question about the magnitude of the fluid shear stress on leukocytes in the circulation. The flow of plasma on the surface of a leukocyte at a very low Reynolds number is governed by the Stokes equation for the motion of a Newtonian fluid. We numerically estimated the distribution of fluid shear stress on a leukocyte membrane in a microvessel for the cases when the leukocyte is freely suspended, as well as rolling along or attached to a microvessel wall. The results indicate that the fluid shear stress distribution on the leukocyte membrane is nonuniform with a sharp increase when the leukocyte makes membrane attachment to the microvessel wall. In a microvessel (10 μm diameter), the fluid shear stress on the membrane of a freely suspended leukocyte (8 μm diameter) is estimated to be several times larger than the wall shear stress exerted by the undisturbed Poiseuille flow, and increases on an adherent leukocyte up to ten times. High temporal stress gradients are present in freely suspended leukocytes in shear flow due to cell rotation, which are proportional to the local shear rate. In comparison, the temporal stress gradients are reduced on the membrane of leukocytes that are rolling or firmly adhered to the endothelium. High temporal gradients of shear stress are also present on the endothelial wall. At a plasma viscosity of 1 cPoise, the peak shear stresses for suspended and adherent leukocytes are of the order of 10 dyn/cm2 and 100 dyn/cm2, respectively.

Monocyte recruitment to endothelial cells in response to oscillatory shear stress

Leukocyte recruitment to endothelial cells is a critical event in inflammatory responses. The spatial, temporal gradients of shear stress, topology, and outcome of cellular interactions that underlie these responses have so far been inferred from static imaging of tissue sections or studies of statically cultured cells. In this report, we developed micro-electromechanical systems (MEMS) sensors, comparable to a single endothelial cell (EC) in size, to link real-time shear stress with monocyte/EC binding kinetics in a complex flow environment, simulating the moving and unsteady separation point at the arterial bifurcation with high spatial and temporal resolution. In response to oscillatory shear stress (tau) at +/- 2.6 dyn/cm2 at a time-averaged shear stress (tau(ave))=0 and 0.5 Hz, individual monocytes displayed unique to-and-fro trajectories undergoing rolling, binding, and dissociation with other monocyte, followed by solid adhesion on EC. Our study quantified individual monocyte/EC binding kinetics in terms of displacement and velocity profiles. Oscillatory flow induces up-regulation of adhesion molecules and cytokines to mediate monocyte/EC interactions over a dynamic range of shear stress +/- 2.6 dyn/cm2 (P=0.50, n=10).

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.

Syndecans in inflammation

Cell surface heparan sulfate (HS) influences a multitude of molecules, cell types, and processes relevant to inflammation. HS binds to cell surface and matrix proteins, cytokines, and chemokines. These interactions modulate inflammatory cell maturation and activation, leukocyte rolling, and tight adhesion to endothelium, as well as extravasation and chemotaxis. The syndecan family of transmembrane proteoglycans is the major source of cell surface HS on all cell types. Recent in vitro and in vivo data suggest the involvement of syndecans in the modulation of leukocyte-endothelial interactions and extravasation, the formation of chemokine and kininogen gradients, participation in chemokine and growth factor signaling, as well as repair processes. Thus, the complex role of HS in inflammation is reflected by multiple functions of its physiological carriers, the syndecans. Individual and common functions of the four mammalian syndecan family members can be distinguished. Recently generated transgenic and knockout mouse models will facilitate analysis of the individual processes that each syndecan is involved in.

A real time in vitro assay for studying leukocyte transendothelial migration under physiological flow conditions

The mechanisms underlying leukocyte migration across endothelial barriers are largely elusive. Most of the current knowledge on transendothelial migration (TEM) of leukocytes has been derived from in vitro modified Boyden chamber transfilter migration assays. In these assays, leukocyte migration towards chemokine gradients constructed across the endothelial barrier is measured under shear-free conditions. These assays do not incorporate the contribution of shear flow to leukocyte adherence and migration across the endothelial barrier. Furthermore, transfilter assays do not reconstitute the physiological distribution of endothelial chemokines shown to be displayed in vivo at high levels on vessel walls. To overcome these two drawbacks, we have recently developed a novel in vitro assay to follow real time leukocyte migration across endothelial barriers under physiological flow conditions. Using this assay, we have found that apically displayed endothelial chemokines could trigger robust lymphocyte TEM through signaling to lymphocyte-expressed G-protein coupled receptors. This migration required continuous exposure of lymphocytes, adherent to the endothelial barrier, to fluid shear, but did not require a chemotactic gradient across the barrier. In the present review, we describe this new flow-based migration assay and discuss future applications for investigating TEM processes of different types of leukocytes across distinct endothelial barriers.

PECAM-1 (CD31) homophilic interaction up-regulates alpha6beta1 on transmigrated neutrophils in vivo and plays a functional role in the ability of alpha6 integrins to mediate leukocyte migration through the perivascular basement membrane

Platelet-endothelial cell adhesion molecule (PECAM)-1 has been implicated in leukocyte migration through the perivascular basement membrane (PBM) though the mechanisms involved are unclear. The present results demonstrate that the ability of alpha(6) integrins to mediate neutrophil migration through the PBM is PECAM-1 dependent, a response associated with PECAM-1-mediated increased expression of alpha(6)beta(1) on transmigrating neutrophils in vivo. An anti-alpha(6) integrins mAb (GoH3) inhibited (78%, P < 0.001) neutrophil migration through interleukin (IL)-1beta-stimulated cremasteric venules, primarily at the level of the PBM, as analyzed by intravital and electron microscopy. In PECAM-1-deficient mice (KO), a reduced level of neutrophil transmigration elicited by IL-1beta (4-h reaction) was observed in both the cremaster muscle (55% inhibition, P < 0.05) and in the peritoneum (57% inhibition, P < 0.01) but GoH3 had no additional inhibitory effect on these responses. FACS((R)) analysis of neutrophils demonstrated increased expression of alpha(6)beta(1) on transmigrated peritoneal neutrophils, as compared with blood neutrophils, in wild-type but not KO mice even though neutrophils from both strains of mice exhibited comparable levels of intracellular expression of alpha(6) as observed by immunofluorescent staining and confocal microscopy. Furthermore, mice deficient in either leukocyte or endothelial cell PECAM-1, as developed by bone marrow transplantation, demonstrated a similar level of reduced neutrophil transmigration and expression of alpha(6)beta(1) on transmigrated neutrophils as that detected in KO mice. The results demonstrate a role for PECAM-1 homophilic interaction in neutrophil transmigration and increased expression of alpha(6)beta(1) on the cell surface of transmigrated neutrophils in vivo, a response that could contribute to the mechanism of PECAM-1-mediated neutrophil migration through the PBM.

CD99 plays a major role in the migration of monocytes through endothelial junctions

CD99 is a heavily O-glycosylated 32-kD type I transmembrane protein that is expressed on most hematopoietic cells. We show here that CD99 is expressed on endothelial cells and is concentrated at the borders between confluent cells. We found that a monoclonal antibody to CD99, hec2, selectively inhibited diapedesis of monocytes across endothelial cells by >90%. Diapedesis involved the homophilic interaction of CD99 on monocytes with CD99 on endothelial junctions. CD99 functioned distally to the point at which platelet-endothelial cell adhesion molecule 1 (PECAM-1, also known as CD31), another adhesion molecule involved in transmigration, played its critical role. Confocal microscopy showed that anti-PECAM-1 arrested leukocytes on the apical surface of endothelium, whereas blocking CD99 arrested monocytes at a point where they were partially through the junction. Therefore, diapedesis, the forward migration of leukocytes through endothelial junctions, is regulated sequentially by two distinct molecules, PECAM-1 and CD99.

Shear-dependent eosinophil transmigration on interleukin 4-stimulated endothelial cells: a role for endothelium-associated eotaxin-3

Leukocyte infiltration into inflammatory sites is regulated by the expression of adhesion and activation proteins, yet the role of these proteins in shear-dependent transmigration is poorly understood. We examined eosinophil recruitment on cytokine-stimulated human umbilical vein endothelial cells (HUVECs) under laminar flow conditions. Eosinophils rapidly transmigrated on interleukin (IL)-4-, but not TNF-stimulated HUVECs. Transmigration was shear dependent, with up to 90% of eosinophils transmigrating in the presence of shear and less than 25% of cells transmigrating under static conditions. Eosinophils express CC chemokine receptor CCR3 and are responsive to various CC chemokines. The effects of chemokines are mediated primarily through G(alpha)i, which is pertussis toxin sensitive. Greater than 65% of shear-dependent eosinophil transmigration on IL-4-stimulated HUVECs was blocked by either pertussis toxin or by an anti-CCR3 monoclonal antibody. Using reverse transcription polymerase chain reaction (RT-PCR) and Western blots, we found that IL-4-stimulated HUVECs produce both mRNA and protein for eotaxin-3. Eotaxin-3 was both released by HUVECs and expressed on the endothelial cell surface. Pretreatment of HUVECs with an anti-eotaxin-3 antibody blocked eosinophil transmigration to the same extent as an anti-CCR3 antibody. These results indicate that IL-4-stimulated HUVECs support shear-dependent eosinophil transmigration by upregulating eotaxin-3, and that surface association is critical for the role of eotaxin-3 in transmigration.