OxHDL inhibits human platelet aggregation via SR-BI. (A) Human platelets isolated by gel filtration were preincubated with or without anti–SR-BI blocking antibody or nonimmune isotype control antibody for 5 minutes, followed by incubation with 200 μg/mL OxHDL for 5 minutes at 37°C and stimulation with 0.1 U/mL thrombin. (B) Data from panel A are presented as means plus or minus SD of 5 independent experiments. (C) Platelet aggregation was performed as in panel A, but platelets were stimulated with 5 μg/mL collagen type I. (D) Platelet aggregation was performed as in panel A, but platelets were stimulated with 10 μM ADP in the presence of 300 nM human fibrinogen. (E) Human platelets isolated by gel filtration were incubated with 5 μg/mL [¹²⁵I] lipoproteins for 30 minutes at room temperature in the presence or absence of indicated competitors, and platelet-bound radioactivity was quantified. Final concentrations of competitors were 200 μg/mL for lipoproteins, 40 μg/mL for phospholipids, and 20 μg/mL for antibodies. Data shown are means plus or minus SD from 3 independent experiments. (F) Human platelets isolated by gel filtration were activated by thrombin as described in “Materials and methods.” Activated and resting platelets were fixed with 2% formaldehyde, and SR-BI expression was analyzed by flow cytometry (left panel). Quantification of the data (means ± SD) from 3 independent experiments is shown in right panel. NA indicates no addition; α-SR-BI, anti–SR-BI blocking antibody; NI, nonimmune antibody; PAPC, unilamellar vesicles containing unoxidized phospholipids; and OxPAPC, unilamellar vesicles containing oxidized phospholipids. *P < .005; **P < .001.