Appropriation of GPlbα from platelet-derived extracellular vesicles supports monocyte recruitment in systemic inflammation

Interactions between platelets, leukocytes and the vessel wall provide alternative pathological routes of thrombo-inflammatory leukocyte recruitment. We found that when platelets were activated by a range of agonists in whole blood, they shed platelet-derived extracellular vesides which rapidly and preferentially bound to blood monocytes compared to other leukocytes. Platelet-derived extracellular veside binding to monocytes was initiated by P-selectin-dependent adhesion and was stabilised by binding of phosphatidylserine. These interactions resulted in the progressive transfer of the platelet adhesion receptor GPIb alpha to monocytes. GPIb alpha(+)-monocytes tethered and rolled on immobilised von Willebrand Factor or were recruited and activated on endothelial cells treated with TGF-beta 1 to induce the expression of von Willebrand Factor. In both models monocyte adhesion was ablated by a function-blocking antibody against GPIb alpha. Monocytes could also bind platelet-derived extracellular vesicle in mouse blood in vitro and in viva. Intratracheal instillations of diesel nanoparticles, to model chronic pulmonary inflammation, induced accumulation of GPIb alpha on circulating monocytes. In intravital experiments, GPIb alpha(+)-monocytes adhered to the microcirculation of the TGF-beta 1-stimulated cremaster muscle, while in the ApoE(-/-) model of atherosclerosis, GPIb alpha(+)-monocytes adhered to the carotid arteries. In trauma patients, monocytes bore platelet markers within 1 hour of injury, the levels of which correlated with severity of trauma and resulted in monocyte dearance from the circulation. Thus, we have defined a novel thrombo-inflammatory pathway in which platelet-derived extracellular vesides transfer a platelet adhesion receptor to monocytes, allowing their recruitment in large and small blood vessels, and which is likely to be pathogenic.