Abstract: Leukocytes and platelets must adhere to the wall of blood vessels to carry out their protective functions in inflammation and haemostasis. Recruitment is critically dependent on rheological variables (wall shear rate and stress, red cell aggregation and haematocrit) which affect delivery to the vessel wall as well as velocities and forces experienced there. Leukocyte recruitment is efficient only up to wall shear rates of about 300 s−1 and usually restricted to low-shear post-capillary venules in inflammation. Being smaller, platelets experience lower velocities and shear forces adjacent to the wall and can adhere at much higher shear rates for haemostasis in arteries. In addition, we found quite different effects of variations in haematocrit or red cell aggregation on attachment of neutrophils or platelets, which also assist their separate recruitment in venules or arteries. However, it has become increasingly evident that inflammatory and thrombotic responses may occur together, with platelets promoting the adhesion and activation of neutrophils and monocytes. Indeed, it is 30 years since we demonstrated that platelets could cause neutrophils to aggregate in suspension and, when attached to a surface, could support selectin-mediated rolling of all leukocytes. Thrombin-activated platelets could further induce neutrophil activation and immobilisation. In some conditions, platelets could bind to intact endothelial monolayers and capture neutrophils or monocytes. Subsequently, we found that extracellular vesicles released by activated platelets (PEV) fulfilled similar functions when deposited on surfaces or bound to endothelial cells. In murine models, platelets or PEV could act as bridges for monocytes in inflamed vessels. Thus, leukocytes and platelets are rheologically adapted for their separate functions, while novel thrombo-inflammatory pathways using platelets or PEV may underlie pathogenic leukocyte recruitment.
