Recent insights into endothelial control of leukocyte extravasation

Abstract

In the process of leukocyte migration from the circulation across the vascular wall, the crosstalk with endothelial cells that line the blood vessels is essential. It is now firmly established that in endothelial cells important signaling events are initiated upon leukocyte adhesion that impinge on the regulation of cell–cell contact and control the efficiency of transendothelial migration. In addition, several external factors such as shear force and vascular stiffness were recently identified as important regulators of endothelial signaling and, consequently, leukocyte transmigration. Here, I review recent insights into endothelial signaling events that are linked to leukocyte migration across the vessel wall. In this field, protein phosphorylation and Rho-mediated cytoskeletal dynamics are still widely studied using increasingly sophisticated mouse models. In addition, activation of tyrosine phosphatases, changes in endothelial cell stiffness as well as different vascular beds have all been established as important factors in endothelial signaling and leukocyte transmigration. Finally, I address less-well-studied but interesting components in the endothelium that also control transendothelial migration, such as the ephrins and their Eph receptors, that provide novel insights in the complexity associated with this process.

Box 1: The classical transmigration paradigm for acute inflammation

The swift response of our bodies’ defense mechanism to foreign invaders, such as bacteria on a sharp kitchen knife, requires that circulating leukocytes such as neutrophils deploy to the area of infestation a.s.a.p. While so doing, however, these cells (our ‘first line of defense’) encounter a number of problems to which an adequate response is mandatory. This response requires the leukocytes, following their recruitment to the inflamed site, to initiate a now well-established standard operating protocol for their approach (see schematic and text below).

The first phase of this multi-step process is the rolling along and adhesion to the inner wall of the blood vessel. This wall is paved with a monolayer of endothelial cells (EC) which can be locally activated by inflammatory cytokines. Upregulated by these cytokines, carbohydrate-conjugated endothelial cell surface proteins known as selectins (mainly E-selectin) orchestrate the ‘first contact’; their low-affinity interactions with cognate receptors such as PSGL-1 (P-selectin glycoprotein ligand-1) are key to the rolling behavior of leukocytes under flow. The flow in itself can induce leukocyte to initiate the TEM program, a response known as chemorheotaxis. Despite the continuous contact with large numbers of red blood cells, many rolling leukocytes remain associated with the vessel wall. Locally accumulated chemokines instruct the leukocytes to transform their circular shape into a spread-out, polarized and motile form, imitating directional motility (chemotaxis). This transformation is not possible without the rapid activation of special adhesion molecules, the β1 and β2 integrins, that allow the cells to further resist the external forces imposed by the blood plasma and its constituents. Activated integrins will lock on to their endothelial ligands ICAM-1 and VCAM-1, securing firm leukocyte adhesion. These ligands may also present as a gradient on the endothelial cells’ surface, driving haptotaxis, migration along a gradient of adhesion molecules.

The crawling along the vessel wall is next, directing the cells to optimal sites for TEM that are marked by largely unknown markers. Some of these may involve local differences in EC stiffness, leading to durotaxis. Next is the key process of TEM: the cells’ reversion to a round shape, possibly driven by regions of low stiffness, a phenomenon recently coined ‘tenertaxis’. Subsequently, the cells will protrude and cross the endothelial barrier. Once on the other side, the turbulence of the blood is no more but now the cells need to crawl in a novel way through dense, 3-dimensional networks of extracellular matrix (ECM) proteins and along other types of cell, such as pericytes and smooth muscle cells, in search for the inflammatory culprit.

The inflamed vascular endothelium, in the meantime, has been anything but oblivious to the spectacle and has welcomed the leukocytes not only by presenting sufficient adhesion sites in the shape of upregulated ICAM-1 and VCAM-1, but also by forming ‘docking structures’ or ‘transmigratory cups’ that promote the entrance into the inflamed tissue. These endothelial responses are driven by complicated intracellular signaling machinery, of which new components and interactions are still being discovered.

To conclude, be it by crossing the endothelial barrier through the seemingly obvious way of passage, i.e. through the intercellular junction, or by penetration of an endothelial cell’s body while aiming for an as yet unknown target, this incursion of inflamed tissue results in a rapid resolution of the original problem through the tracking down and killing of invader pathogens.