Human platelets secrete chemotactic activity for eosinophils

Thrombin-stimulated platelets liberate factors that induce chemotaxis of eosinophils and raise their cytosolic Ca²⁺ content ([Ca²⁺](i)). The sources of this activity are the dense- and α-granules because inhibition of prostaglandin endoperoxide/thromboxane A₂ formation and the platelet- activating factor receptor-antagonist WEB 2086 have no effect. Platelets from patients with Storage-Pool Deficiency show about 60% of the normal chemotactic activity with little effect on [Ca²⁺](i), whereas completely degranulated platelets fail to affect eosinophils. In concentrations secreted by the platelets, adenosine diphosphate (ADP), and platelet factor 4 have no effect, whereas adenosine triphosphate (ATP) induces a strong chemotactic response and increases [Ca²⁺](i). However, apart from ATP other modulating factors must be involved as platelet releasates induce more chemotaxis than ATP alone. Thus, platelets secrete factors that activate eosinophils and may contribute to inflammatory and allergic processes.