Platelet activation induced by histones, antihistone antibodies, and histone/antihistone immune complexes. PS exposure and CD62P expression of washed platelets incubated with high concentrations of histone octamers and antihistone antibodies alone (A-B); with various concentrations of histone octamers mixed with antihistone antibodies (B-C); and histone dimers H2A/H2B with a mix of the corresponding antihistone antibodies (D-E). PS exposure was assessed after annexin-V staining and CD62P expression in flow cytometry. Immune complexes were produced by incubating human histones and mouse antihistone antibodies dose dependently. High concentrations of histone octamers were able to activate platelets. This was independent of FcγRIIa because platelet activation could not be blocked by the monoclonal antibody IV.3 (which inhibits platelet activation via FcγRIIa) (A-B). In contrast, 10-fold lower concentrations of histone octamers were only able to activate platelets after incubation with the corresponding antihistone antibodies. This could be blocked by the monoclonal antibody IV.3 showing that activation by immune complexes was FcγRIIa dependent (B-C). The same was observed for histone dimers H2A/H2B incubated with the corresponding antihistone antibodies ∗∗P < .01, ∗∗∗P < .001, ∗∗∗∗P < .0001. MFI, mean fluorescence intensity; PBS, phosphate-buffered saline; TRAP/Conv, thrombin receptor activating peptide + convulxin.