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

Calcium influx through the TRPV1 channel of endothelial cells (ECs) correlates with a stronger adhesion between monocytes and ECs

PURPOSE

Atherosclerosis is thought to be initiated by the transendothelial migration of monocytes. In the early stage of this process, the adhesion of monocytes to endothelial cells is supported by an increase in the intracellular concentration of calcium ion ([Ca(2+)]i) in endothelial cells. However, the main source of Ca(2+) has been unclear. In this study, the changes in ionic transmittance and [Ca(2+)]i due to the adhesion of monocytes were continuously measured by an electrophysiological technique and fluorescent imaging. Especially, we focused on transient receptor potential vanilloid channel 1 (TRPV1) as a Ca(2+) channel that could influence the adhesion of monocytes.

MATERIAL AND METHODS

Whole-cell current was continuously recorded in human umbilical vein endothelial cells (HUVECs) by a patch electrode.

RESULTS

The adhesion of monocytes (THP-1) induced a transient inward current in HUVECs, as well as an elevation of [Ca(2+)]i. This inward element was abolished by the application of 100 nM SB366,791, a selective antagonist of TRPV1 channel. Furthermore, SB366,791 significantly decreased the number of THP-1 cells that adhered to HUVECs (control: 231 ± 38, SB366,791: 96 ± 16 cells/mm2).

CONCLUSION

These results suggest that an inward calcium current via the TRPV1 channels of endothelial cells correlates with a stronger adhesion between monocytes and endothelial cells.

Live-cell visualization of the trans-cellular mode of monocyte transmigration across the vascular endothelium, and its relationship with endothelial PECAM-1

In response to atherogenic stimuli, blood monocytes transmigrate across the vascular endothelium not only through endothelial cell-cell junctions (para-cellular) but also through endothelial cells themselves (trans-cellular). The molecular mechanism of the latter is mostly unknown, because it rarely happens, especially in vitro. Although many reports have recognized trans-cellular migration from snapshot images of leukocytes halfway across the endothelium at non-junctional locations, it often produces a false-positive result, because some leukocytes that initiate trans-cellular migration withdraw and return to the apical endothelial surface. Thus, analyzing the entire process is essential. In this study, complete monocyte trans-cellular migration was successfully captured for live cells, with simultaneous visualization of endothelial PECAM-1. We suggest the possible existence of both PECAM-1-related migration at peri-junctional sites and PECAM-1-unrelated migration at sites remote from junctions. This is the first report to describe the entire process of monocyte trans-cellular migration for live cells and its relationship with endothelial PECAM-1.

Monocyte trans-endothelial migration augments subsequent transmigratory activity with increased PECAM-1 and decreased VE-cadherin at endothelial junctions

BACKGROUND

Although the importance of monocyte trans-endothelial migration in early atherogenesis is well recognized, it is unclear whether and how one transmigration event affects endothelium to facilitate subsequent ones. In this study, we tested the hypothesis that monocyte transmigration alters endothelial junctional organization to facilitate subsequent transmigration.

METHODS AND RESULTS

When human monocytes were added twice at intervals of ≈30 min to IL-1beta-prestimulated human umbilical vein endothelial cells in vitro, significant augmentation of transmigration was observed at the second addition (≈1.5-fold, analyzed from a total of 231 monocytes in 3 experiments). Endothelial surface expressions of two major junctional molecules, PECAM-1 and VE-cadherin, increased and decreased respectively, in response to monocyte addition, which could facilitate subsequent transmigration. To further investigate spatiotemporal dynamics of the increasing molecule, PECAM-1, we constructed a PECAM-1-GFP expression system and found that monocyte transmigration induced local accumulation of endothelial PECAM-1 around the transmigration spot, which was followed by transmigration of subsequent monocyte around the same location. Detailed analysis revealed that within the defined region around one transmigration event, 50% of later transmigrating monocytes used the same or similar location as the previous one (10 out of 20 transmigrating monocytes in 11 experiments).

CONCLUSIONS

These findings show that monocyte trans-endothelial migration alters endothelial junctional organization to a more monocyte-permeable state (increased PECAM-1 and decreased VE-cadherin), resulting in the augmented transmigratory activity at a later stage. This positive feedback mechanism is partially associated with monocyte transmigration-induced local accumulation of endothelial PECAM-1, which promotes transmigration of following monocytes at the same location.

Adhesion and transcellular migration of neutrophils and B lymphocytes on fibroblasts

During tissue inflammation, infiltrated leukocytes may have physical contacts with fibroblasts. We observed that neutrophils and B lymphocytes adhered in a larger proportion than T cells on cultured fibroblasts. Microscopy showed that adhesion was also characterized by leukocyte engulfment by the fibroblasts. In migration assays, only neutrophils and B lymphocytes were selectively able to migrate through a fibroblast barrier. Adhesion and migration were increased by stimulation with tumor necrosis factor-alpha (TNF-alpha) and phorbol-12-myristate-13-acetate (PMA). Antibodies against ICAM-1/beta2 integrin blocked the interaction of neutrophils to fibroblasts. For B lymphocytes the couple VCAM-1/alpha4 integrin was also involved in this interaction. Human skin fibroblasts presented similar adhesion characteristics as rat cardiac fibroblasts. By measuring the distance between the border of migration holes and cadherin-positive adherens junctions, more than 65% of the holes correspond to the transcellular route over the paracellular route. Furthermore, vimentin staining revealed that the migration holes were highly nested by intermediate filaments in accordance with the transcellular route. Our results demonstrated that engulfment of neutrophils and B lymphocytes by fibroblasts resulted in selective passage by a transcellular route.

Oxidized LDL specifically promotes the initiation of monocyte invasion during transendothelial migration with upregulated PECAM-1 and downregulated VE-cadherin on endothelial junctions

It is poorly understood how oxidized LDL (oxLDL) promotes monocyte dynamics in transendothelial migration (TEM) in atherogenesis. We developed an in vitro 3D-live-single cell TEM assay system with subendothelial oxLDL embedded in ultra-thin collagen gels, mimicking subendothelial oxLDL accumulation in vivo. With dividing monocyte dynamics into three stages (1: adhesion on endothelium, 2: invasion and 3: complete transmigration below endothelium), we analyzed the stage transition dynamics of individual living human monocytes. OxLDL did not enhance initial monocyte adhesion to endothelium (stage 1), but it specifically primed adherent monocytes to start invasion (stage 1-->2). Once invasion started, it had no effect thereafter on monocyte stage transition (stage 2-->3). OxLDL upregulated PECAM-1 and downregulated VE-cadherin on endothelial junctions without monocyte addition, both of which could promote monocyte entry by enhanced homophilic binding to monocyte PECAM-1, and by disrupted junctional barrier, respectively. Meanwhile, monocyte speed at neither locomotion on endothelium (stage 1) nor subendothelial migration (stage 3) was altered by oxLDL. These data indicate that before monocyte adhesion, endothelial junctions changed their conformation to more monocyte-acceptable state in response to oxLDL, resulting the stage-specific promotion of monocyte TEM (stage 1-->2; initiation of invasion) with no enhancement of its initial adhesion or migration speed.

Multi-cell agent-based simulation of the microvasculature to study the dynamics of circulating inflammatory cell trafficking

Leukocyte trafficking through the microcirculation and into tissues is central in angiogenesis, inflammation, and the immune response. Although the literature is rich with mechanistic detail describing molecular mediators of these processes, integration of signaling events and cell behaviors within a unified spatial and temporal framework at the multi-cell tissue-level is needed to achieve a fuller understanding. We have developed a novel computational framework that combines agent-based modeling (ABM) with a network flow analysis to study monocyte homing. A microvascular network architecture derived from mouse muscle was incorporated into the ABM. Each individual cell was represented by an individual agent in the simulation. The network flow model calculates hemodynamic parameters (blood flow rates, fluid shear stress, and hydrostatic pressures) throughout the simulated microvascular network. These are incorporated into the ABM to affect monocyte transit through the network and chemokine/cytokine concentrations. In turn, simulated monocytes respond to their local mechanical and biochemical environments and make behavioral decisions based on a rule set derived from independent literature. Simulated cell behaviors give rise to emergent leukocyte rolling, adhesion, and extravasation. Molecular knockout simulations were performed to validate the model, and predictions of monocyte adhesion, rolling, and extravasation show good agreement with the independently published corresponding mouse studies.

Calculation of arterial wall temperature in atherosclerotic arteries: effect of pulsatile flow, arterial geometry, and plaque structure

Background

This paper presents calculations of the temperature distribution in an atherosclerotic plaque experiencing an inflammatory process; it analyzes the presence of hot spots in the plaque region and their relationship to blood flow, arterial geometry, and inflammatory cell distribution. Determination of the plaque temperature has become an important topic because plaques showing a temperature inhomogeneity have a higher likelihood of rupture. As a result, monitoring plaque temperature and knowing the factors affecting it can help in the prevention of sudden rupture.

Methods

The transient temperature profile in inflamed atherosclerotic plaques is calculated by solving an energy equation and the Navier-Stokes equations in 2D idealized arterial models of a bending artery and an arterial bifurcation. For obtaining the numerical solution, the commercial package COMSOL 3.2 was used. The calculations correspond to a parametric study where arterial type and size, as well as plaque geometry and composition, are varied. These calculations are used to analyze the contribution of different factors affecting arterial wall temperature measurements. The main factors considered are the metabolic heat production of inflammatory cells, atherosclerotic plaque length l_p, inflammatory cell layer length l_mp, and inflammatory cell layer thickness d_mp.

Results

The calculations indicate that the best location to perform the temperature measurement is at the back region of the plaque (0.5 ≤ l/l_p ≤ 0.7). The location of the maximum temperature, or hot spot, at the plaque surface can move during the cardiac cycle depending on the arterial geometry and is a direct result of the blood flow pattern. For the bending artery, the hot spot moves 0.6 millimeters along the longitudinal direction; for the arterial bifurcation, the hot spot is concentrated at a single location due to the flow recirculation observed at both ends of the plaque. Focusing on the thermal history of different points selected at the plaque surface, it is seen that during the cardiac cycle the temperature at a point located at l/l_p = 0.7 can change between 0.5 and 0.1 degrees Celsius for the bending artery, while no significant variation is observed in the arterial bifurcation. Calculations performed for different values of inflammatory cell layer thickness d_mp indicate the same behavior reported experimentally; that corresponds to an increase in the maximum temperature observed, which for the bending artery ranges from 0.6 to 2.0 degrees Celsius, for d_mp = 25 and 100 micrometers, respectively.

Conclusion

The results indicate that direct temperature measurements should be taken (1) as close as possible to the plaque/lumen surface, as the calculations show a significant drop in temperature within 120 micrometers from the plaque surface; (2) in the presence of blood flow, temperature measurement should be performed in the downstream edge of the plaque, as it shows higher temperature independently of the arterial geometry; and (3) it is necessary to perform measurements at a sampling rate that is higher than the cardiac cycle; the measurement should be extended through several cardiac cycles, as variations of up to 0.7 degrees Celsius were observed at l/l_p = 0.7 for the bending artery.

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.