Histones recruit fibrinogen and cause platelet aggregation through mechanisms dependent and independent of αIIbβ3 integrins. Fluorescence imaging of wild-type (A) or β3-integrin–deficient (B) platelets stimulated in the presence or absence of fibrinogen with ADP (50μM) or recombinant histone H4 (10 μg/mL). Histone H4 or ADP induce large platelet aggregates of wild-type but not β3-integrin–deficient platelets. Recombinant histone H4 but not ADP induces microaggregation of β3-integrin–deficient platelets. This microaggregation depended on the presence of fibrinogen. (C) Histones induce β3-integrin–independent fibrinogen binding to platelets. Washed platelets stimulated with the indicated concentrations of histones or ADP (50μM) were incubated with fluorescent fibrinogen and analyzed by flow cytometry. (D) Hypothesized interactions of fibrinogen with αIIbβ3 and histones in HiPA. Fibrinogen can bind to αIIbβ3 and histones on platelets. Fluorescence imaging of wild-type (green) or β3-integrin–deficient (red) platelets mixed with fibrinogen in response to ADP or histones in the presence (E) or absence (F) of CaCl2. Recombinant histone H4 but not ADP induced the formation of large platelet aggregates consisting of wild-type and β3-deficient platelets. This process depended on exogenous CaCl2. Scale bar = 100 μm. Data presented are representative of ≥ 3 independent experiments.
