Platelet-induced granulocyte aggregation in vitro

Recent studies have shown that the response of granulocytes to stimulation is increased by the presence of platelets. Using an assay based on counting aggregates with a Coulter Counter, we found that heparinized or citrated platelet-rich plasma directly caused aggregation of isolated granulocytes. Platelets themselves were frequently present within the granulocyte aggregates. The degree of aggregation depended on the platelet:granulocyte ratio, and increased over a 2--5 min period of mixing, with a slower increase thereafter. Aggregation was calcium-dependent, increased by stimulation of the platelets with ADP or collagen but not serotonin, and only slightly reduced by a thrombin inhibitor (hirudin, at concentrations up to 0.9 U/ml). A similar adhesive interaction in vivo might be relevant to thrombotic and ischaemic pathology.