Histones induce platelet aggregation, prothrombinase activity, PS exposure, and secretion. (A) Histones (0-80 μg/mL) were added to stirred gel-filtered human platelets, and aggregation was monitored for 10 minutes. Where indicated, gel-filtered platelets were pretreated with PGE₁ (4 μmol/L) before the addition of histones (40 μg/mL). Data are reported as means of the maximum percentage of aggregation induced by histones ± SD of 4 independent experiments with gel-filtered platelets from different donors. *Lowest concentration producing significant aggregation (P < .05); **P < .05 compared with histones (40 μg/mL). (B) Prothrombinase activity of gel-filtered platelets stimulated with histones (0-80 μg/mL) for 15 minutes was determined after the addition of human FXa and prothrombin; the amount of thrombin generated was evaluated after 90 seconds by cleavage of the chromogenic substrate Spectrozyme TH. Data are means ± SD, n = 3. *Lowest concentration producing a significant increase (P < .05). (C) Typical flow cytometric analysis of PS exposure (top panel), P-selectin expression (middle panel), and FV/Va expression (bottom panel). Gel-filtered platelets stimulated with histones (40 μg/mL) for 15 minutes were analyzed by flow cytometry as detailed in “Methods.” Region 2 indicating a positive population for the antigen of interest was arbitrarily chosen.