Shear stress-mediated changes in the expression of leukocyte adhesion receptors on human umbilical vein endothelial cells in vitro

Integration of substrate- and flow-derived stresses in endothelial cell mechanobiology

Endothelial cells (ECs) lining all blood vessels are subjected to large mechanical stresses that regulate their structure and function in health and disease. Here, we review EC responses to substrate-derived biophysical cues, namely topography, curvature, and stiffness, as well as to flow-derived stresses, notably shear stress, pressure, and tensile stresses. Because these mechanical cues in vivo are coupled and are exerted simultaneously on ECs, we also review the effects of multiple cues and describe burgeoning in vitro approaches for elucidating how ECs integrate and interpret various mechanical stimuli. We conclude by highlighting key open questions and upcoming challenges in the field of EC mechanobiology.

Lymphatic Valves Separate Lymph Flow Into a Central Stream and a Slow-Moving Peri-Valvular Milieu

The lymphatic system plays a pivotal role in the transport of fats, waste, and immune cells, while also serving as a metastatic route for select cancers. Using live imaging and particle tracking, we experimentally characterized the lymph flow field distal from the inguinal lymph node in the vicinity of normal bileaflet and malformed unileaflet intraluminal valves. Particle tracking experiments demonstrated that intraluminal lymphatic valves concentrate higher velocity lymph flow in the center of the vessel, while generating adjacent perivalvular recirculation zones. The recirculation zones are characterized by extended particle residence times and low wall shear stress (WSS) magnitudes in comparison to the rest of the lymphangion. A malformed unileaflet valve skewed lymph flow toward the endothelium on the vessel wall, generating a stagnation point and a much larger recirculation zone on the opposite wall. These studies define physical consequences of bileaflet and unileaflet intraluminal lymphatic valves that affect lymph transport and the generation of a heterogeneous flow field that affects the lymphatic endothelium nonuniformly. The characterized flow fields were recreated in vitro connecting different flow environments present in the lymphangion to a lymphatic endothelial cell (LEC) pro-inflammatory phenotype. Unique and detailed insight into lymphatic flow is provided, with potential applications to a variety of diseases that affect lymph transport and drug delivery.

Lipopolysaccharide-Induced Vascular Inflammation Model on Microfluidic Chip

Inflammation is the initiation of defense of our body against harmful stimuli. Lipopolysaccharide (LPS), originating from outer membrane of Gram-negative bacteria, causes inflammation in the animal’s body and can develop several diseases. In order to study the inflammatory response to LPS of blood vessels in vitro, 2D models have been mainly used previously. In this study, a microfluidic device was used to investigate independent inflammatory response of endothelial cells by LPS and interaction of inflamed blood vessel with monocytic THP-1 cells. Firstly, the diffusion of LPS across the collagen gel into blood vessel was simulated using COMSOL. Then, inflammatory response to LPS in engineered blood vessel was confirmed by the expression of Intercellular Adhesion Molecule 1 (ICAM-1) and VE-cadherin of blood vessel, and THP-1 cell adhesion and migration assay. Upregulation of ICAM-1 and downregulation of VE-cadherin in an LPS-treated condition was observed compared to normal condition. In the THP-1 cell adhesion and migration assay, the number of adhered and trans-endothelial migrated THP-1 cells were not different between conditions. However, migration distance of THP-1 was longer in the LPS treatment condition. In conclusion, we recapitulated the inflammatory response of blood vessels and the interaction of THP-1 cells with blood vessels due to the diffusion of LPS.

Low immunogenic endothelial cells endothelialize the Left Ventricular Assist Device

Low haemocompatibility of left ventricular assist devices (LVAD) surfaces necessitates anticoagulative therapy. Endothelial cell (EC) seeding can support haemocompatibility, however, the availability of autologous ECs is limited. In contrast, allogeneic ECs are readily available in sufficient quantity, but HLA disparities induce harmful immune responses causing EC loss. In this study, we investigated the feasibility of using allogeneic low immunogenic ECs to endothelialize LVAD sintered inflow cannulas (SIC). To reduce the immunogenicity of ECs, we applied an inducible lentiviral vector to deliver short-hairpins RNA to silence HLA class I expression. HLA class I expression on ECs was conditionally silenced by up to 70%. Sufficient and comparable endothelialization rates were achieved with HLA-expressing or HLA-silenced ECs. Cell proliferation was not impaired by cell-to-Sintered Inflow Cannulas (SIC) contact or by silencing HLA expression. The levels of endothelial phenotypic and thrombogenic markers or cytokine secretion profiles remained unaffected. HLA-silenced ECs-coated SIC exhibited reduced thrombogenicity. In contrast to native ECs, HLA-silenced ECs showed lower cell lysis rates when exposed to allogeneic T cells or specific anti-HLA antibodies. Allogeneic HLA-silenced ECs could potentially become a valuable source for LVAD endothelialization to reduce immunogenicity and correspondingly the need for anticoagulative therapy which can entail severe side effects.

Deciphering Endothelial Dysfunction in the HIV-Infected Population

Cardiovascular disease (CVD), as a possible consequence of endothelial dysfunction, is prevalent among HIV-infected patients despite successful administration of antiretroviral drugs. This warrants the routine clinical assessment of endothelial function in HIV-positive patients to circumvent potential CVD events. Several different non-invasive strategies have been employed to assess endothelial function in clinical research studies yielding inconsistencies among these reports. This review summarises the different techniques used for assessing endothelial function, with a focus on proposed blood-based biomarkers, such as endothelial leukocyte adhesion molecule-1 (E-selectin), soluble intercellular adhesion molecule-1 (sICAM-1), soluble vascular cell adhesion molecule-1 (sVCAM-1), von Willebrand factor (vWF), TNF-α, interleukin 6 (IL6) and soluble thrombomodulin (sTM). The identification of suitable blood-based biomarkers, especially those that can be measured using a point-of-care device, would be more applicable in under-resourced countries where the prevalence of HIV is high.

Engineering a multicellular vascular niche to model hematopoietic cell trafficking

BACKGROUND

The marrow microenvironment and vasculature plays a critical role in regulating hematopoietic cell recruitment, residence, and maturation. Extensive in vitro and in vivo studies have aimed to understand the marrow cell types that contribute to hematopoiesis and the stem cell environment. Nonetheless, in vitro models are limited by a lack of complex multicellular interactions, and cellular interactions are not easily manipulated in vivo. Here, we develop an engineered human vascular marrow niche to examine the three-dimensional cell interactions that direct hematopoietic cell trafficking.

METHODS

Using soft lithography and injection molding techniques, fully endothelialized vascular networks were fabricated in type I collagen matrix, and co-cultured under flow with embedded marrow fibroblast cells in the matrix. Marrow fibroblast (mesenchymal stem cells (MSCs), HS27a, or HS5) interactions with the endothelium were imaged via confocal microscopy and altered endothelial gene expression was analyzed with RT-PCR. Monocytes, hematopoietic progenitor cells, and leukemic cells were perfused through the network and their adhesion and migration was evaluated.

RESULTS

HS27a cells and MSCs interact directly with the vessel wall more than HS5 cells, which are not seen to make contact with the endothelial cells. In both HS27a and HS5 co-cultures, endothelial expression of junctional markers was reduced. HS27a co-cultures promote perfused monocytes to adhere and migrate within the vessel network. Hematopoietic progenitors rely on monocyte-fibroblast crosstalk to facilitate preferential recruitment within HS27a co-cultured vessels. In contrast, leukemic cells sense fibroblast differences and are recruited preferentially to HS5 and HS27a co-cultures, but monocytes are able to block this sensitivity.

CONCLUSIONS

We demonstrate the use of a microvascular platform that incorporates a tunable, multicellular composition to examine differences in hematopoietic cell trafficking. Differential recruitment of hematopoietic cell types to distinct fibroblast microenvironments highlights the complexity of cell-cell interactions within the marrow. This system allows for step-wise incorporation of cellular components to reveal the dynamic spatial and temporal interactions between endothelial cells, marrow-derived fibroblasts, and hematopoietic cells that comprise the marrow vascular niche. Furthermore, this platform has potential for use in testing therapeutics and personalized medicine in both normal and disease contexts.

A tunable microfluidic 3D stenosis model to study leukocyte-endothelial interactions in atherosclerosis

Atherosclerosis, a chronic inflammatory disorder characterized by endothelial dysfunction and blood vessel narrowing, is the leading cause of cardiovascular diseases including heart attack and stroke. Herein, we present a novel tunable microfluidic atherosclerosis model to study vascular inflammation and leukocyte-endothelial interactions in 3D vessel stenosis. Flow and shear stress profiles were characterized in pneumatic-controlled stenosis conditions (0%, 50% and 80% constriction) using fluid simulation and experimental beads perfusion. Due to non-uniform fluid flow at the 3D stenosis, distinct monocyte (THP-1) adhesion patterns on inflamed [tumor necrosis factor-α (TNF-α) treated] endothelium were observed, and there was a differential endothelial expression of intercellular adhesion molecule-1 (ICAM-1) at the constriction region. Whole blood perfusion studies also showed increased leukocyte interactions (cell rolling and adherence) at the stenosis of healthy and inflamed endothelium, clearly highlighting the importance of vascular inflammation, flow disturbance, and vessel geometry in recapitulating atherogenic microenvironment. To demonstrate inflammatory risk assessment using leukocytes as functional biomarkers, we perfused whole blood samples into the developed microdevices (80% constriction) and observed significant dose-dependent effects of leukocyte adhesion in healthy and inflamed (TNF-α treated) blood samples. Taken together, the 3D stenosis chip facilitates quantitative study of hemodynamics and leukocyte-endothelial interactions, and can be further developed into a point-of-care blood profiling device for atherosclerosis and other vascular diseases.

Exercise-induced upregulation of endothelial adhesion molecules in human skeletal muscle and number of circulating cells with remodeling properties

Multipotent cells have received great interest because of their potential capacity to repair and remodel peripheral tissues. We examined the effect of an acute exercise bout on the number of circulating cells with known remodeling properties and the level of factors in plasma and skeletal muscle tissue with potential to recruit these cells. Twenty healthy male subjects performed a 60-min cycling exercise. Blood samples for flow cytometry were drawn from 10 subjects (group 1) before and up to 2 h after exercise, and absolute cell counts of the classical (CD14⁺⁺CD16^-), intermediate (CD14⁺⁺CD16⁺), and nonclassical (CD14⁺CD16⁺⁺) monocyte (MO) subpopulations and of CD45^dimCD34⁺VEGFR2⁺ endothelial progenitor cells (EPCs) were measured by bead-based determination. Plasma samples and vastus lateralis muscle biopsies were obtained from the other 10 subjects (group 2). In group 1, all MO subsets were increased directly after exercise, with CD14⁺CD16⁺⁺ MOs showing the greatest fold increase. After 2 h, only CD14⁺⁺CD16^- MOs were increased compared with resting levels. The number of EPCs showed a trend toward increasing with exercise (P = 0.08). In group 2, the mRNA levels of the endothelial adhesion molecules ICAM-1, VCAM-1, and E-selectin increased in the skeletal muscle tissue. VEGF-A increased in exercised skeletal muscle and stimulated the expression of VCAM-1 and E-selectin in human umbilical vein endothelial cells. In conclusion, exercise increases MO subsets with different temporal patterns and enhances the capacity of skeletal muscle tissue to recruit circulating cells as shown by increased expression of endothelial adhesion molecules.NEW & NOTEWORTHY In the present study we showed for the first time that the adhesion molecules ICAM-1, VCAM-1, and E-selectin, known to be able to recruit circulating cells to the peripheral tissue, increased in exercised human skeletal muscle concurrently with increased circulating levels of cells shown to have importance for skeletal muscle remodeling. These findings support the concept of cell recruitment from the circulation playing a role in skeletal muscle adaptation to exercise.

Caveolae: A Role in Endothelial Inflammation and Mechanotransduction?

Vascular inflammation and disease progression, such as atherosclerosis, are in part a consequence of haemodynamic forces generated by changes in blood flow. The haemodynamic forces, such as shear stress or stretch, interact with vascular endothelial cells, which transduce the mechanical stimuli into biochemical signals via mechanosensors, which can induce an upregulation in pathways involved in inflammatory signaling. However, it is unclear how these mechanosensors respond to shear stress and most significantly what cellular mechanisms are involved in sensing the haemodynamic stimuli. This review explores the transition from shear forces, stretch and pressure to endothelial inflammation and the process of mechanotransduction, specifically highlighting evidence to suggest that caveolae play as a role as mechanosensors.

Endothelialization approaches for viable engineered tissues

One of the main limitation in obtaining thick, 3-dimensional viable engineered constructs is the inability to provide a sufficient and functional blood vessel system essential for the in vitro survival and the in vivo integration of the construct. Different strategies have been proposed to simulate the ingrowth of new blood vessels into engineered tissue, such as the use of growth factors, fabrication scaffold technologies, in vivo prevascularization and cell-based strategies, and it has been demonstrated that endothelial cells play a central role in the neovascularization process and in the control of blood vessel function. In particular, different "environmental" settings (origin, presence of supporting cells, biomaterial surface, presence of hemodynamic forces) strongly influence endothelial cell function, angiogenic potential and the in vivo formation of durable vessels. This review provides an overview of the different techniques developed so far for the vascularization of tissue-engineered constructs (with their advantages and pitfalls), focusing the attention on the recent development in the cell-based vascularization strategy and the in vivo applications.

Flow-dependent mass transfer may trigger endothelial signaling cascades

It is well known that fluid mechanical forces directly impact endothelial signaling pathways. But while this general observation is clear, less apparent are the underlying mechanisms that initiate these critical signaling processes. This is because fluid mechanical forces can offer a direct mechanical input to possible mechanotransducers as well as alter critical mass transport characteristics (i.e., concentration gradients) of a host of chemical stimuli present in the blood stream. However, it has recently been accepted that mechanotransduction (direct mechanical force input), and not mass transfer, is the fundamental mechanism for many hemodynamic force-modulated endothelial signaling pathways and their downstream gene products. This conclusion has been largely based, indirectly, on accepted criteria that correlate signaling behavior and shear rate and shear stress, relative to changes in viscosity. However, in this work, we investigate the negative control for these criteria. Here we computationally and experimentally subject mass-transfer limited systems, independent of mechanotransduction, to the purported criteria. The results showed that the negative control (mass-transfer limited system) produced the same trends that have been used to identify mechanotransduction-dominant systems. Thus, the widely used viscosity-related shear stress and shear rate criteria are insufficient in determining mechanotransduction-dominant systems. Thus, research should continue to consider the importance of mass transfer in triggering signaling cascades.

DIFFERENTIAL EFFECTS OF SHEAR STRESS ON THE EXPRESSIONS OF ICAM-1 AND VCAM-1 INDUCED BY TNF-α IN ENDOTHELIAL CELLS

Vascular endothelial cells (ECs) are subjected to shear stress and cytokine stimulation. We studied the interplay between shear stress and cytokine in modulating the expression of adhesion molecule genes and the adhesive function of ECs. Shear stress (20 dynes/cm2) was applied to ECs prior to or following the addition of tumor necrosis factor (TNF)-α. Shear stress increased the TNF-α-induced expression of intercellular adhesion molecule-1 (ICAM-1) at both mRNA and surface protein levels, but decreased the TNF-α-induced expression of vascular adhesion molecule-1 (VCAM-1). The TNF-α-induced increase in EC adhesiveness for monocytic THP-1 cells was reduced by shear stress. After 24-h pre-shearing followed by 1 h of static incubation, the effect of pre-shearing on TNF-α-induced ICAM-1 mRNA expression vanished. The recovery of the TNF-α-induced VCAM-1 mRNA expression following pre-shearing, however, required a static incubation time of >6 h (completely recovery at 24 h). Pre- and post-shearing caused a reduction in the nuclear factor (NF)-κB-DNA binding activity induced by TNF-α in the EC nucleus. Our findings suggest that shear stress plays differential roles in modulating the TNF-α-induced EC expressions of ICAM-1 and VCAM-1 genes, which serve similar functions in vascular biology.

Analysis of flow and wall shear stress in the peroneal veins under external compression based on real-time MR images

As a widely accepted prophylaxis for deep vein thrombosis, the underlying mechanism of compression stocking still remains unclear. In this study, computational fluid dynamics was applied to in vivo data to provide quantitative insight into the hemodynamic response of the deep venous system to static external compression. The geometry and flow information of deep veins before and after compression was acquired from ten healthy volunteers using magnetic resonance imaging. Our results indicated that application of the compression stocking led to a small reduction in blood flow rate but a significant reduction in cross-sectional area of the peroneal veins in the calf, resulting in an increase in wall shear stress (WSS), but the individual effects were highly variable. The mean volume reduction of the deep veins was 58%, while the time-averaged WSS showed an average increase of 398% after compression (median 98%). The analysis also showed a strong linear correlation between the time-averaged WSS and mean blood velocity, suggesting that flow in the deep veins under the level of compression examined here can be approximated by Poiseuille's law despite local geometric variations. It is hoped that quantitative analysis of WSS in the deep venous system will aid in the future design and optimisation of the compression stocking.

TGF-β1/ALK5-induced monocyte migration involves PI3K and p38 pathways and is not negatively affected by diabetes mellitus

AIMS

Monocytes contribute to arteriogenesis by infiltration to sites of collateral growth and subsequent production and release of growth factors. Transforming growth factor β1 (TGF-β1) mediates monocyte motility and stimulates arteriogenesis. TGF-β1 signalling mechanisms mediating monocyte motility are unknown so far. Moreover, the influence of cardiovascular risk factor diabetes on TGF-β1-induced monocyte migration remains to be elucidated.

METHODS AND RESULTS

Stimulation of primary human monocytes with TGF-β1 endorsed phosphorylation of v-Akt murine thymoma viral oncogene analogues protein (AKT), p38, and extracellular signal-related kinase 1/2 (ERK1/2), besides the activation of the SMA/MAD homologues protein (SMAD) pathway. Inhibition of the TGF-βtype 1 receptor, alias activin receptor-like kinase 5 (ALK5), hindered monocyte chemotaxis towards TGF-β1 and TGF-β1-activated downstream signalling cascades. Individual genetic knock-downs for receptor-regulated SMAD2 and SMAD3 did not affect monocyte migration to TGF-β1. Inhibition of phosphoinositide 3 kinase (PI3K) activity, but not AKT, diminished both basal and TGF-β1-mediated monocyte motility. TGF-β1-induced monocyte chemotaxis did not rely on ERK1/2, but rather on p38. Remarkably, TGF-β1 was able to stimulate chemotaxis of diabetic monocytes.

CONCLUSION

The current study provides novel insights into the molecular mechanisms of TGF-β1-induced monocyte migration, requiring ALK5 kinase activity and signalling via PI3K and p38. TGF-β1-driven monocyte motogenicity is fully functional in diabetic conditions, which is in sharp contrast to the impaired chemotactic responses to certain other arteriogenic cytokines. Therefore, TGF-β1 may be a promising candidate for endogenously and exogenously stimulating collateral growth in diabetic patients.

Combinations of hydrostatic pressure and shear stress influence morphology and adhesion molecules in cultured endothelial cells

Endothelial cells are exposed to mechanical stimuli from blood flow and blood pressure. However, it is not yet fully understood how their simultaneous exposure affects endothelial function. Firstly, in this study we investigated the effect of combined stress on morphology of cultured human aortic endothelial cells (HAECs). In the results, HAECs exposed to steady flow (a pressure of 100 mmHg, and a shear stress of 1.5 Pa) were more elongated than those exposed to a hydrostatic pressure of 100 mmHg and HAECs exposed to pulsatile flow (a pressure of 80/120 mmHg, and a shear stress of 1.2/1.8 Pa) were more elongated than those exposed to steady flow. Similarly, HAECs exposed to pulsatile flow were most oriented to the flow direction among these three stresses. Secondly, we investigated the effect of combined stress on gene expression of cell adhesion molecules in HAECs. After stress exposure to HAECs the mRNA of intercellular adhesion molecule-1 (ICAM-1), vascular cell adhesion molecule-1 (VCAM-1) and E-selectin were measured by real time RT-PCR. In the results, the exposure of steady flow increased the mRNA levels of ICAM-1 compared to the exposure of hydrostatic pressure; however, the exposure of pulsatile flow decreased the mRNA level of ICAM-1 compared to the exposure of steady flow. These findings suggest that gene expression of cell adhesion molecules induced by combined stress were different to the superposition of individual stress and that not only difference in the components of combined stress but also difference in the magnitude of the components of combined stress are important.

The response of human aortic endothelial cells in a stenotic hemodynamic environment: effect of duration, magnitude, and spatial gradients in wall shear stress

Inflammation plays a key role in the development and stability of coronary plaques. Endothelial cells alter their expression in response to wall shear stress (WSS). Straight/tubular and asymmetric stenosis models were designed to study the localized expression of atheroprone molecules and inflammatory markers due to the presence of the spatial wall shear stress gradients created by an eccentric plaque. The effects of steady wall shear stress duration (0-24 h) and magnitude (4.5-18 dynes/cm(2)) were analyzed in human abdominal aortic endothelial cells through quantitative real-time polymerase chain reaction (PCR) and immunofluorescence analysis in straight/tubular models. Regional expression was assessed by immunofluorescence and confocal microscopy in stenosis models. Under steady fully developed flow, endothelial cells exhibited a sustained increase in levels of atheroprotective genes with WSS duration and magnitude. The local response in the stenosis model showed that expression of endothelial nitric oxide synthase and Kruppel-like factor 2 is magnitude rather than gradient dependent. A WSS magnitude dependent transient increase in translocation of transcription factor nuclear factor kappaB was observed. Intercellular adhesion molecule 1, vascular cell adhesion molecule 1, and E-selectin exhibited a sustained increase in protein expression with time. The mRNA levels of these molecules were transiently upregulated and this was followed by a decrease in expression to levels lower than static controls. Regionally, increased inflammatory marker expression was observed in regions of WSS gradients both proximal and distal to the stenosis when compared with the uniform flow regions, whereas the atheroprotective markers were expressed to a greater extent in regions of elevated WSS magnitudes. The results from the straight/tubular model cannot explain the regional variation seen in the stenosis models. This may help explain the localization of inflammatory cells at the shoulders of plaques in vivo.

Hemodynamic forces in endothelial dysfunction and vascular aging

Aging is a key risk factor associated with the onset of cardiovascular disease. Notably, vascular aging and cardiovascular disease are both associated with endothelial dysfunction, or a marked decrease in production and bioavailability the vasodilator of nitric oxide (NO). As a result of decreased nitric oxide availability, aging vessels often exhibit endothelial cell senescence and increased oxidative stress. One of the most potent activators of NO production is fluid shear stress produced by blood flow. Interestingly, age-related decrease in NO production partially results from endothelial insensitivity to shear stress. While the endothelial cell response to fluid shear stress has been well characterized in recent years, the exact mechanisms of how the mechanical force of fluid shear stress is converted into intracellular biochemical signals are relatively unknown. Therefore, gaining a better knowledge of mechanosignaling events in endothelial cells may prove to be beneficial for developing potential therapies for cardiovascular diseases.

Ventilator-induced endothelial activation and inflammation in the lung and distal organs

Introduction

Results from clinical studies have provided evidence for the importance of leukocyte-endothelial interactions in the pathogenesis of pulmonary diseases such as acute lung injury (ALI) and acute respiratory distress syndrome (ARDS), as well as in systemic events like sepsis and multiple organ failure (MOF). The present study was designed to investigate whether alveolar stretch due to mechanical ventilation (MV) may evoke endothelial activation and inflammation in healthy mice, not only in the lung but also in organs distal to the lung.

Methods

Healthy male C3H/HeN mice were anesthetized, tracheotomized and mechanically ventilated for either 1, 2 or 4 hours. To study the effects of alveolar stretch in vivo, we applied a MV strategy that causes overstretch of pulmonary tissue i.e. 20 cmH₂O peak inspiratory pressure (PIP) and 0 cmH₂0 positive end expiratory pressure (PEEP). Non-ventilated, sham-operated animals served as a reference group (non-ventilated controls, NVC).

Results

Alveolar stretch imposed by MV did not only induce de novo synthesis of adhesion molecules in the lung but also in organs distal to the lung, like liver and kidney. No activation was observed in the brain. In addition, we demonstrated elevated cytokine and chemokine expression in pulmonary, hepatic and renal tissue after MV which was accompanied by enhanced recruitment of granulocytes to these organs.

Conclusions

Our data implicate that MV causes endothelial activation and inflammation in mice without pre-existing pulmonary injury, both in the lung and distal organs.

Vascular cell adhesion molecule-1 expression in endothelial cells exposed to physiological coronary wall shear stresses

Atherosclerosis is consistently found in bifurcations and curved segments of the circulatory system, indicating disturbed hemodynamics may participate in disease development. In vivo and in vitro studies have shown that endothelial cells (ECs) alter their gene expression in response to their hemodynamic environment, in a manner that is highly dependent on the exact nature of the applied forces. This research exposes cultured ECs to flow patterns present in the coronary arterial network, in order to determine the role of hemodynamic forces in plaque initiation. Vascular cell adhesion molecule-1 (VCAM-1) was examined as an indicator of plaque growth, as it participates in monocyte adhesion, which is one of the initial steps in the formation of fatty lesions. The hemodynamics of a healthy right and left coronary artery were determined by reconstructing 3D models from cineangiograms and employing computational fluid dynamic models to establish physiological coronary flow patterns. Wall shear stress (WSS) profiles selected from these studies were applied to ECs in a cone and plate bioreactor. The cone and plate system was specifically designed to be capable of reproducing the high frequency harmonics present in physiological waveforms. The shear stresses chosen represent those from regions prone to disease development and healthier arterial segments. The levels of the transcriptional and cell surface anchored VCAM-1 were quantified by flow cytometry and real time RT-PCR over a number of timepoints to obtain a complete picture of the relationship between this adhesion molecule and the applied shear stress. The WSS profiles from regions consistently displaying a higher incidence of plaques in vivo, induced greater levels of VCAM-1, particularly at the earlier timepoints. Conversely, the WSS profile from a straight section of vessel with undisturbed flow indicated no upregulation in VCAM-1 and a significant downregulation after 24 h, when compared with static controls. Low shear stress from the outer wall of a bifurcation induced four times the levels of VCAM-1 messenger ribonucleic acid (mRNA) after four hours when compared with levels of mRNA induced by WSS from a straight arterial section. This shear profile also induced prolonged expression of the surface protein of this molecule. The current study has provided insight into the possible influences of coronary hemodynamics on plaque localization, with VCAM-1 only significantly induced by the WSS from disease prone regions.

DNA microarray study on gene expression profiles in co-cultured endothelial and smooth muscle cells in response to 4- and 24-h shear stress

Shear stress, a major hemodynamic force acting on the vessel wall, plays an important role in physiological processes such as cell growth, differentiation, remodelling, metabolism, morphology, and gene expression. We investigated the effect of shear stress on gene expression profiles in co-cultured vascular endothelial cells (ECs) and smooth muscle cells (SMCs). Human aortic ECs were cultured as a confluent monolayer on top of confluent human aortic SMCs, and the EC side of the co-culture was exposed to a laminar shear stress of 12 dyn/cm(2) for 4 or 24 h. After shearing, the ECs and SMCs were separated and RNA was extracted from the cells. The RNA samples were labelled and hybridized with cDNA array slides that contained 8694 genes. Statistical analysis showed that shear stress caused the differential expression (p < or = 0.05) of a total of 1151 genes in ECs and SMCs. In the co-cultured ECs, shear stress caused the up-regulation of 403 genes and down-regulation of 470. In the co-cultured SMCs, shear stress caused the up-regulation of 152 genes and down-regulation of 126 genes. These results provide new information on the gene expression profile and its potential functional consequences in co-cultured ECs and SMCs exposed to a physiological level of laminar shear stress. Although the effects of shear stress on gene expression in monocultured and co-cultured EC are generally similar, the response of some genes to shear stress is opposite between these two types of culture (e.g., ICAM-1 is up-regulated in monoculture and down-regulated in co-culture), which strongly indicates that EC-SMC interactions affect EC responses to shear stress.

Mechanopathobiology of Atherogenesis: A Review

Cardiovascular disease is the number one cause of mortality in the United States. Atherosclerosis, the primary etiology of cardiovascular disease is hypothesized to be a time-dependent response to arterial injury. Although risk factors for atherosclerosis are systemic in nature, certain arteries (e.g., coronary arteries) are more susceptible to plaque formation than others. The heterogeneous distribution of atherosclerosis in the vasculature is thought to be related to biomechanical factors. A review of the relevant pathological features of atherogenesis and how physiologically-consistent mechanical stimuli can impact those processes supports this notion. However, specific investigations geared toward finding the mechanistic link between mechanical stimuli and early atherogenic processes are required to differentiate those stimuli that facilitate and those that inhibit atherogenesis. Such knowledge is required for intelligent direction in the search for potential targets for clinical intervention.

Mechanical strain regulates endothelial cell patterning in vitro

Blood vessels of the vertebrate circulatory system typically exhibit tissue-specific patterning. However, the cues that guide the development of these patterns remain unclear. We investigated the effect of cyclic uniaxial strain on vascular endothelial cell dynamics and sprout formation in vitro in two-dimensional (2D) and three-dimensional (3D) culture systems under the influence of growth factors. Cells preferentially aligned and moved in the direction perpendicular to the major strain axis in monolayer culture, and mechanical strain also regulated the spatial location of cell proliferation in 2D cell culture. Cells in 3D cell culture could be induced to form sprouts by exposure to appropriate growth factor combinations (vascular endothelial growth factor and hepatocyte growth factor), and the strain direction regulated the directionality of this process. Moreover, cyclic uniaxial strain inhibited branching of the structures formed by endothelial cells and increased their thickness. Taken together, these data support the importance of external mechanical stimulation in the regulation of endothelial cell migration, proliferation, and differentiation into primitive vessels.

Vascular bed origin dictates flow pattern regulation of endothelial adhesion molecule expression

Endothelial cell phenotypes markedly differ, depending upon function and vascular bed of origin. Differences might account for specific susceptibility to pathological conditions. As leukocyte adhesion to activated endothelium is the initiating event in a range of diseases, we compared the influence of vascular bed-specific flow patterns on adhesion molecule expression in human saphenous vein (HSVEC) and coronary artery endothelial cells (HCAEC). In vitro, immune cell attachment was increased 1.6-fold when tumor necrosis factor (TNF)-α-stimulated HSVEC were exposed to coronary artery flow in place of physiological venous flow and 1.9-fold higher compared with attachment to cytokine-stimulated HCAEC exposed to coronary artery flow. This was associated with increased concentrations of soluble E-selectin, VCAM-1, and ICAM-1 in supernatants of HSVEC exposed to coronary artery flow compared with HCAEC exposed to the same flow pattern. Venous and coronary artery flow both increased TNF-α-induced E-selectin and ICAM-1 expression on HSVEC, but only coronary artery flow increased VCAM-1 expression. In marked contrast to HSVEC, venous and coronary artery flow attenuated TNF-α-induced E-selectin and VCAM-1 expression on HCAEC, whereas coronary artery flow further induced ICAM-1 on cytokine-stimulated HCAEC. With the exception of cytokine-induced ICAM-1, adhesion molecule expression on HSVEC exposed to coronary artery flow exceeded expression on HCAEC. Thus ICAM-1 expression involves complex flow-dependent and -independent pathways with marked dissimilarities between the two endothelial cell types studied. Interestingly, Kruppel-like factor (KLF) 4 overexpression in HCAEC and HSVEC significantly reduced TNF-α-induced E-selectin and VCAM-1 expression in static conditions, while ICAM-1 expression remained constant. Furthermore, both flow patterns induced KLF2 and KLF4 expression in HCAEC and HSVEC. Venous and coronary artery flow differentially influence endothelial adhesion molecule and transcription factor expression, depending on the vascular bed of origin. Differences in adhesion molecule expression and subsequent immune cell adhesion between HSVEC and HCAEC may contribute to different susceptibility to pathological conditions.

The influence of biomaterials on endothelial cell thrombogenicity

Driven by tissue engineering and regenerative medicine, endothelial cells are being used in combination with biomaterials in a number of applications for the purpose of improving blood compatibility and host integration. Endothelialized vascular grafts are beginning to be used clinically with some success in some centers, while endothelial seeding is being explored as a means of creating a vasculature within engineered tissues. The underlying assumption of this strategy is that when cultured on artificial biomaterials, a confluent layer of endothelial cells maintain their non-thrombogenic phenotype. In this review the existing knowledge base of endothelial cell thrombogenicity cultured on a number of different biomaterials is summarized. The importance of selecting appropriate endpoint measures that are most reflective of overall surface thrombogenicity is the focus of this review. Endothelial cells inhibit thrombosis through three interconnected regulatory systems (1) the coagulation cascade, (2) the cellular components of the blood such as leukocytes and platelets and (3) the complement cascade, and also through effects on fibrinolysis and vascular tone, the latter which influences blood flow. Thus, in order to demonstrate the thrombogenic benefit of seeding a biomaterial with EC, the conditions under which EC surfaces are more likely to exhibit lower thrombogenicity than unseeded biomaterial surfaces need to be consistent with the experimental context. The endpoints selected should be appropriate for the dominant thrombotic process that occurs under the given experimental conditions.

Vascular shear stress and activation of inflammatory genes

Atherosclerotic lesions form preferentially at distinct sites in the arterial tree, especially at or near branch points, bifurcations, and curvatures where there is disturbed or oscillatory blood flow. In contrast, straight regions of the vasculature exhibit uniform laminar shear stress, which is atheroprotective. The ability of laminar flow to exert an anti-inflammatory effect on the endothelial cell lining of the blood vessel is revealed by preventing monocyte adhesion, proliferation, and apoptosis. Changes in endothelial cell gene expression in response to laminar shear stress reflect these changes in cell physiology with the demonstration that physiologic flow inhibits the expression of inflammatory genes. Thus, shear stress is critically important in regulating vascular physiology and pathobiology of the vessel wall via the modulation of endothelial cell gene expression.

Adequate design of customized cDNA macroarray for convenient multiple gene expression analysis

To establish a convenient, cost-effective, and reasonably reliable method for monitoring multiple gene expression using customized membrane-based macroarray, we constructed a cDNA macroarray with multiple probes for 13 human vascular endothelial genes and assessed the accuracy of the macroarray measurements. For each gene, two cDNA probes (450-550 bp) were designed from different regions (coding region and 3'-untranslated region [3'-UTR], respectively) on the basis of simple criteria concerning length and sequence specificity and spotted on the macroarray. In addition, unmodified oligonucleotide probes (80 mer) targeted to a unique sequence from the coding region of each gene were spotted on the same macroarray. Using this macroarray, shear stress-induced mRNA expression changes were analyzed in human coronary artery endothelial cells. Comparison of the expression ratios obtained with those measured using quantitative real-time polymerase chain reaction (PCR) as a reference method revealed that cDNA probes designed from a sequence within the coding region provided a highly accurate expression profile, whereas results obtained from oligonucleotide probes showed no correlation with real-time PCR data, which might be caused by inadequate immobilization of oligonucletotide probes on the nylon membrane. In addition, we observed that cDNA probes targeting different regions of a gene yielded different signal intensities. Most cDNA probes designed from a sequence within the coding region showed detectable signals, whereas few cDNA probes designed from 3'-UTR did.

Normal and shear stresses influence the spatial distribution of intracellular adhesion molecule-1 expression in human umbilical vein endothelial cells exposed to sudden expansion flow

Patterns in cell adhesion molecule expression by endothelial cells may play a role in atherogenesis. Previous studies have shown dependence of intracellular adhesion molecule-1 (ICAM-1) expression in human umbilical vein endothelial cells (HUVEC) on shear stress and have indirectly linked ICAM-1 expression to spatial gradients in shear stress. The spatial distribution of ICAM-1 in HUVEC pre-exposed to flow for 8h was determined using fluorescence microscopy and a sudden expansion flow chamber with a 2.66 expansion ratio to simulate gradients in wall shear stress found near arterial branches in vivo. When ICAM-1 expression in the disturbed flow region was compared to theoretical stress distributions obtained from a computational model of sudden expansion flow, a modest trend (R2 = 0.327, p < 0.01)was observed between ICAM-1 and shear stress but the correlation between ICAM-1 and shear stress gradient was insignificant. In contrast, a moderately strong trend (R2 = 0.873, p < 0.01) was evident between ICAM-1 expression and the component of normal stress induced by the expansion. Thus, in this in vitro model, normal stress arising from sudden expansion flow modulates the effect of shear stress on ICAM-1 expression.

Role of small GTPases in endothelial cytoskeletal dynamics and the shear stress response

Fluid shear stress caused by blood flow is a major determinant of vascular remodeling and arterial tone and can lead to development of atherosclerosis. The endothelial monolayer in vivo acts as a signal transduction interface for hemodynamic forces; these forces determine the shape, cytoskeletal organization, and function of endothelial cells, allowing the vessels to cope with physiological or pathological conditions. The Ras superfamily of GTPases have been revealed to be master regulators of many cellular activities. In particular, the GTPases RhoA, Rac1, and Cdc42 are known to regulate cell shape changes through effects on the cytoskeleton, but their ability to influence polarity, microtubule dynamics, and transcription factor activity is just as significant. Shear stress modulates the activity of small GTPases, which are critical for both cytoskeletal reorganization and changes in gene expression in response to shear stress. The goal of this article is to review what is known about Ras and more so about Rho GTPases in mechanotransduction and the responses of cells to fluid flow. Several distinct signaling pathways can be coordinately activated by flow, and small GTPases are strongly implicated in some of them; thus possible connections will be explored and a unifying hypothesis offered.

The effects of direct current cardioversion for persistent atrial fibrillation on indices of endothelial damage/dysfunction

BACKGROUND

Atrial fibrillation is associated with increased thromboembolic risk, and this risk may occur even following cardioversion. Atrial fibrillation has been hypothesised to cause alterations in endothelial cell function through the influences of altered flow dynamics, and resultant endothelial dysfunction may be contributory to the generation of a prothrombotic state. The aim of this study was therefore to assess endothelial function before and after electrical cardioversion.

METHODS

We studied 30 consecutive patients undergoing elective cardioversion for AF and compared them with 20 healthy controls. Plasma levels of endothelial damage/dysfunction [von Willebrand factor (vWF), E-selectin (E-sel), soluble thrombomodulin (sTM)] and Circulating Endothelial Cells (CECs, an index of endothelial damage) in whole blood were measured in all subjects and on the AF group at baseline (pre-cardioversion) and at 2 h and 4 weeks following cardioversion.

RESULTS

Plasma levels of vWf were significantly increased in persistent AF at baseline compared to healthy controls (p<0.001). With restoration of sinus rhythm, vWF levels were significantly decreased at 4 weeks (p=0.0001), whilst levels of CECs (p=0.01) and sTM (p=0.022), although not increased at baseline, were significantly increased following cardioversion.

CONCLUSION

Although plasma vWF levels decreased post-cardioversion, suggesting some improvement in vascular endothelial function, the increases in sTM and CECs at 4 weeks may indicate endothelial injury sustained peri-cardioversion. This (delayed) injury and shedding of endothelial cells post-cardioversion may contribute to late thromboembolic risk.

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.

Molecular basis of the effects of shear stress on vascular endothelial cells

Blood vessels are constantly exposed to hemodynamic forces in the form of cyclic stretch and shear stress due to the pulsatile nature of blood pressure and flow. Endothelial cells (ECs) are subjected to the shear stress resulting from blood flow and are able to convert mechanical stimuli into intracellular signals that affect cellular functions, e.g., proliferation, apoptosis, migration, permeability, and remodeling, as well as gene expression. The ECs use multiple sensing mechanisms to detect changes in mechanical forces, leading to the activation of signaling networks. The cytoskeleton provides a structural framework for the EC to transmit mechanical forces between its luminal, abluminal and junctional surfaces and its interior, including the cytoplasm, the nucleus, and focal adhesion sites. Endothelial cells also respond differently to different modes of shear forces, e.g., laminar, disturbed, or oscillatory flows. In vitro studies on cultured ECs in flow channels have been conducted to investigate the molecular mechanisms by which cells convert the mechanical input into biochemical events, which eventually lead to functional responses. The knowledge gained on mechano-transduction, with verifications under in vivo conditions, will advance our understanding of the physiological and pathological processes in vascular remodeling and adaptation in health and disease.

Blood Flow in Stented Arteries: A Parametric Comparison of Strut Design Patterns in Three Dimensions

Background: Restenosis after stent implantation varies with stent design. Alterations in secondary flow patterns and wall shear stress (WSS) can modulate intimal hyperplasia via their effects on platelet and inflammatory cell transport toward the wall, as well as direct effects on the endothelium. Method of Approach: Detailed flow characteristics were compared by estimating the WSS in the near-strut region of realistic stent designs using three-dimensional computational fluid dynamics (CFD), under pulsatile high and low flow conditions. The stent geometry employed was characterized by three geometric parameters (axial strut pitch, strut amplitude, and radius of curvature), and by the presence or lack of the longitudinal connector. Results: Stagnation regions were localized around stent struts. The regions of low WSS are larger distal to the strut. Under low flow conditions, the percentage restoration of mean axial WSS between struts was lower than that for the high flow by 10–12%. The largest mean transverse shear stresses were 30–50% of the largest mean axial shear stresses. The percentage restoration in WSS in the models without the longitudinal connector was as much as 11% larger than with the connector. The mean axial WSS restoration between the struts was larger for the stent model with larger interstrut spacing. Conclusion: The results indicate that stent design is crucial in determining the fluid mechanical environment in an artery. The sensitivity of flow characteristics to strut configuration could be partially responsible for the dependence of restenosis on stent design. From a fluid dynamics point of view, interstrut spacing should be larger in order to restore the disturbed flow; struts should be oriented to the flow direction in order to reduce the area of flow recirculation. Longitudinal connectors should be used only as necessary, and should be parallel to the axis. These results could guide future stent designs toward reducing restenosis.

Integrity of actin fibers and microtubules influences metastatic tumor cell adhesion

Tumor cell adhesion within host organ microvasculature, its stabilization and invasion into host organ parenchyma appear to be important steps during formation of distant metastasis. These interactions of circulating tumor cells with the host organs occur in the presence of fluid shear forces and soluble and cellular environmental conditions of the blood that can modulate their cellular responses and possibly their metastatic efficiency. Cytoskeletal components, such as actin filaments and microtubules, can regulate biophysical characteristics and cellular signaling of the circulating cells. Therefore, we investigated the role of these cytoskeletal structures for early steps during metastasis formation in vivo and in vitro. Using an intravital observation technique, tumor cell adhesion of colon carcinoma cells within the hepatic microcirculation of rats and their invasion into liver parenchyma was observed. Disruption of actin filaments increased cell adhesion, whereas tubulin disruption inhibited adhesive interactions in vivo. The impairment of the cytoskeleton modulated adhesion-mediated cell signaling via focal adhesion kinase (FAK) and paxillin under flow conditions in vitro. In the presence of fluid flow, focal adhesions were enlarged and hyperphosphorylated, whereas stress fibers were reduced compared to static cell adhesion. Disruption of microtubules, however, partially inhibited these effects. Combining the in vivo and in vitro results, our study suggested that changes in cell rigidity and avidity of cell adhesion molecules after disruption of cytoskeletal components appear to be more important for initial adhesive interactions in vivo than their interference with adhesion-mediated cellular signal transduction.

The impact of heparin compounds on cellular inflammatory responses: a construct for future investigation and pharmaceutical development

Atherosclerotic disease is recognized as a chronic inflammatory disorder with intermittent and widely variable phases of cellular proliferation and heightened thrombotic activity. The multi-tiered links between inflammation, atherogenesis and thrombogenesis provide a unique opportunity for research and development of pharmaceuticals which target one or more critical pathobiologic steps (Fig. 1). The purpose of the following review on heparin compounds is to comprehensively examine the multi-cellular, pleuripotential effects of a commonly used anticoagulant drug in the context of normal and disease-altered vascular responses and illustrate possible constructs for avenues of subsequent investigation in the field of atherothrombosis. The overview is divided into five integrated parts; antiinflammatory properties of the normal vessel wall, the relationship between glycosaminoglycans and inflammation, heparin-mediated effects on cellular inflammatory responses, association between molecular weight and antiinflammatory capabilities, and oral heparin compounds for achieving prolonged cell-based inhibition.

The local physicochemical environment conditions the proinflammatory response of endothelial cells and thus modulates leukocyte recruitment

The locations at which vascular endothelial cells recruit leukocytes during physiological or pathological inflammatory responses are influenced by direct effects of local haemodynamics on leukocyte adhesion. However, the expression of genes by endothelial cells, and their ability to respond to inflammatory cytokines also depend on the flow forces to which they are exposed. In addition, cells of the underlying stroma can modify the phenotype and responsiveness of endothelial cells, and hence their ability to recruit leukocytes. Thus, endothelial cells are plastic in their responses, and we hypothesise that the pattern of recruitment of leukocytes to tissues is critically dependent on the variable modulation of the endothelium by the local physicochemical microenvironment.

Shear stress-induced shedding of soluble intercellular adhesion molecule-1 from saphenous vein endothelium

Within 6 h, shear stress upregulated intercellular adhesion molecule-1 (ICAM-1) (two- to four-fold, P<0.001) and induced matrix metalloproteinase-2 (MMP-2) in cultured human saphenous vein endothelial cells. By 8 h endothelial ICAM-1 levels returned to baseline, with concomitant increase in soluble ICAM-1 (sICAM-1) (P<0.001) and MMP-9 had been induced. Inclusion of a hydroxamate metalloproteinase inhibitor partially reversed the effects on ICAM-1 and sICAM-1 at 8 h, whereas TIMP-1, -2 or -3 had no effect. MMP-9, but not MMP-2, co-immunoprecipitated with ICAM-1. sICAM-1 was processed distal to Arg441, indicating that MMP-9, docking to ICAM-1, contributes to sICAM-1 shedding and attenuation of the shear stress-induced upregulation of ICAM-1.

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.

Analysis of the effect of disturbed flow on monocytic adhesion to endothelial cells

The preferential adhesion of monocytes to vascular endothelial cells (ECs) at regions near branches and curvatures of the arterial tree, where flow is disturbed, suggests that hemodynamic conditions play significant roles in monocyte adhesion. The present study aims to elucidate the effects of disturbed flow on monocyte adhesion to ECs and the adhesive properties of ECs. We applied, for the first time, the micron-resolution particle image velocimetry (microPIV) technique to analyze the characteristics of the disturbed flow produced in our vertical-step flow (VSF) chamber. The results demonstrated the existence of a higher near-wall concentration and a longer residence time of the monocytic analog THP-1 cells near the step and the reattachment point. THP-1 cells showed prominent adhesion to ECs pretreated with TNFalpha in the regions near the step and the reattachment point, but they showed virtually no adhesion to un-stimulated ECs. Pre-incubation of the TNFalpha-treated ECs with antibodies against intercellular adhesion molecule-1 (ICAM-1), vascular adhesion molecule-1 (VCAM-1), and E-selectin inhibited the THP-1 adhesion; the maximal inhibition was observed with a combination of these antibodies. Pre-exposure of ECs to disturbed flow in VSF for 24 h led to significant increases in their surface expressions of ICAM-1 and E-selectin, but not VCAM-1, and in the adhesion of THP-1 cells. Our findings demonstrate the importance of complex flow environment in modulating the adhesive properties of vascular endothelium and consequently monocyte adhesion in regions of prevalence of atherosclerotic lesions.