PAF mediates neutrophil adhesion to thrombin or TNF-stimulated endothelial cells under shear stress

Abstract

Platelet-activating factor (PAF) is known to modulate polymorphonuclear leukocyte (PMN) adhesion to endothelial cells cultured under static conditions and activated by thrombin. In contrast, there are no data on the role of PAF in PMN adhesion to cells exposed to flow conditions and activated by stimuli other than thrombin. Here we used the PAF receptor antagonist L-659,989 to evaluate PMN adhesion to human umbilical vein endothelial cells (HUVEC) in basal conditions or upon challenge with thrombin or tumor necrosis factor-alpha (TNF-alpha). Experiments were performed under dynamic flow using a parallel-plate flow chamber and a computer-based image analysis system. Rolling and adhesion of PMNs to endothelial cells significantly increased upon stimulation with thrombin. Thrombin-stimulated HUVEC also synthesized higher amounts of PAF than untreated cells. Pretreatment of PMNs with L-659,989 significantly reduced their rolling and adhesion to thrombin-activated HUVEC. Stimulation of HUVEC with TNF-alpha significantly increased the number of rolling and adherent PMNs as compared with untreated cells. Adhesion of PMNs to and migration across TNF-alpha-stimulated HUVEC were reduced by L-659,989, whereas cell rolling was unchanged. We conclude that PAF mediates leukocyte interaction under flow conditions with HUVEC activated by inflammatory stimuli.

Differential Ability of Exogenous Chemotactic Agents to Disrupt Transendothelial Migration of Flowing Neutrophils

“…Considering undisturbed migration through TNF-treated HUVEC first, others have implicated IL-8 and PAF as neutrophil activators (17,44,45). However, in our previous flow-based studies using TNF-stimulated HUVEC, treatment of neutrophils with PAF receptor antagonists or of HUVEC with neutralizing Ab against IL-8 had no effect on migration (27).…”

“…In the present studies exogenous PAF actually augmented diapedesis through HUVEC treated with 2 U/ml TNF, but had little effect with 100 U/ml TNF. If added PAF can itself induce transmigration, its failure to do so at 100 U/ml TNF may have because 1) a strong migratory stimulus was already present, and PAF could not add to this; 2) the endogenous activator actually desensitized the migratory ability of PAF; 3) PAF in the soluble phase activated neutrophils but could not induce transmigration (i.e., mode of presentation was crucial); or 4) PAF was already present, as suggested by some studies (44,45). The last possibility is not consistent with failure of the residual immobilized cells to migrate after Ab blockade of CXCR2 or of PAF receptor antagonist to affect migration in our hands.…”

Pathogenesis and Pathophysiology of Pneumococcal Meningitis

“…PAF is a protein produced by neutrophils and endothelial cells in response to inflammatory stimuli, and it facilitates adhesion of leukocytes to the vascular endothelium (43,308,460). In rabbits, PAF administered intrathecally induced blood-brain barrier permeability and cerebral edema at doses much lower than those at which it induced leukocytosis (82).…”

The influence of platelet activating factor on the effects of platelet agonists and antiplatelet agents in vitro

“…In addition, the effects of PAF may be an important contributor to apparent resistance to antiplatelet therapy. PAF is an important intercellular mediator of inflammation [8], secreted by many cell types including activated endothelial cells [24,25] and neutrophils [26][27][28]. Our results suggest that one mechanism by which inflammation may promote atherothrombotic complications is by increasing platelet reactivity, even in patients treated with antiplatelet therapy.…”