Introduction
Recruitment of polymorphonuclear leukocytes (PMN) is a key event in the host defense against infectious and noxious stimuli, however pivotal also in the pathogenesis of inflammatory disorders. As a consequence of PMN activation and extravasation, the barrier function of the endothelial lining is adjusted, leading to increased vascular permeability for macromolecules and plasma exudation. Although known to be dependent on the function of leukocytic b2 integrins (CD11/CD18), the precise mechanisms by which activated PMN may impair endothelial cell (EC) barrier capacity remain unclear.
Methods
The kinetics of PMN-induced alterations in tight junctional permeability of cultured EC monolayers (transendothelial electrical resistance and albumin flux) have been investigated together with analysis of EC intracellular activation events (cytosolic free Ca2+ and filamentous actin assembly). PMN, added to the EC monolayer, were activated either by chemoattractant stimulation or through antibody cross-linking of CD11/CD18 (mimicking adhesion-dependent receptor engagement).
Results
Data derived from the model systems used indicate a signaling role for CD11/CD18 in the functional responsiveness of EC to PMN activation. Chemoattractant stimulation of PMN resting on EC results in both EC hyperpermeability and PMN transendothelial migration. Activation of PMN through CD18 cross-linking, bypassing chemoattractant receptor activation, provokes a similar increase in EC permeability that coincides with a rise in EC cytosolic free Ca2+ and rearrangement of actin filaments. These responses fully resemble those evoked by transendothelial chemotactic stimulation, in spite of absence of PMN adhesion and transmigration. Moreover, cell-free supernatant recovered from PMN in suspension activated via CD18 cross-linking triggers an EC response indistinguishable from that evoked by direct PMN activation. Consistent with these findings, data further reveal that ligation and clustering of b2 integrins leads to secretion of PMN proteins, seemingly responsible for mediating the PMN-evoked change in EC permeability.
Conclusions
Our data provide evidences for a causal connection between transmembrane signaling by the b2 integrins in PMN and the capacity of these cells to induce alterations in endothelial barrier function. They further indicate that, downstream to b2 integrin signaling, paracrine stimulation via PMN-derived proteins is involved in the cellular cross-talk between PMN and EC. A novel activation pathway with regard to PMN-evoked effects on vascular permeability in conjunction with leukocyte trafficking in inflammation is suggested by these findings.
References