Inhibition of human platelet activation by polymorphonuclear leukocytes

1 The effect of unstimulated human polymorphonuclear leukocytes (PMNs) on platelet activation was examined. 2 Human platelet aggregation and adenosine 5′‐triphosphate (ATP) release induced by collagen (1–2 μg ml−1); thrombin (0.01–0.02 u ml−1) or arachidonic acid (AA) (0.1–0.2 mm) were markedly inhibited when conducted in the presence of unstimulated PMNs. 3 Platelet inhibition induced by PMNs was dependent on the number of PMNs and on the incubation time of the mixed cell suspension. 4 Platelet inhibition was not reversed in time when PMNs were depleted from the mixed‐cell suspension. 5 PMN‐mediated platelet‐inhibition was not mediated by AA metabolites, oxygen reactive intermediates, nitric oxide or proteases. 6 The factor(s) accounting for the platelet inhibition mediated by PMNs are not yet characterized.