Platelet Binding to Monocytes Induces Changes in Integrin Functionality Promoting Monocyte Adhesion and Transendothelial Migration. Do Platelets Migrate Along?.

Monocyte adhesion to and transmigration across the endothelium are essential steps in atherogenesis. We have shown that the adhesive interactions between monocytes and the activated endothelium are increased when platelets bind to the monocyte surface and form platelet-monocyte complexes (PMC). PMC formation is dependent on interactions between platelet-displayed P-selectin and PSGL-1 on the monocyte surface. To better understand the effect of platelet binding on the capacity of monocytes to adhere to activated endothelium the P-selectin-PSGL-1 interaction - induced changes in integrin functionality were studied. The binding of platelets to monocytes via P-selectin-PSGL-1 interactions was shown to increase expression and activity of α₄ beta;₁ - and α_M β₂ - integrins which, resulted in increased monocyte adhesion to ICAM-1, VCAM-1, fibronectin and subsequently to stimulated endothelial cells. Platelet binding also induced monocyte migration (up to a 3-fold increase), compared to monocytes without platelets on their surface. To investigate the role of platelets in this process we determined the fate of platelets (within the PMC) during monocyte transendothelial migration. After forming PMC by mixing freshly isolated and fluorescently labeled platelets and monocytes, PMC were seeded on endothelial cells cultured on top of a fibrin gel. The cells were allowed to migrate across the endothelial layer into the gel where afterwards the position of platelets and monocytes was analyzed by confocal laser scanning microscopy. We found that the platelets were retained at the endothelium, suggesting that they detach from the PMC and are left behind on the endothelial layer upon monocyte transendothelial migration. In line with this we observed that the monocytes that were in the fibrin gel, underneath the endothelial layer, did not carry any platelets. This was confirmed by PMC migration over endothelial layered transwell filters since almost only monocytes were found in the lower compartment after migration. After testing different migration barriers, platelets seem to be shed from the monocyte surface upon monocyte migration by mechanical stress rather than only endothelial interaction. Our data suggest that monocytes bound to platelets are in a higher state of activation and have an increased atherogenic capacity. Furthermore, platelets seem to mainly play a role in the monocyte recruitment to the endothelium because once the monocytes cross the endothelial layer, platelets detach from their surface. Altogether, our findings, by showing that PMC have a strong atherogenic capacity, might be helpful in finding new therapeutic ways to prevent atherosclerosis and inflammation.