Non–bone marrow–derived SR-BI deficiency leads to platelet hyperreactivity and increased platelet cholesterol content. (A) WT platelets in PRP either from WT or SR-BI^−/− recipients were stimulated with ADP, PAR4-AP, or convulxin (CVX), and platelet integrin α_IIbβ₃ activation and P-selectin expression were determined. (B-C) Platelet aggregation in PRP isolated from chimeric WT or SR-BI^−/− mice with WT bone marrow was induced by 120μM PAR4-AP and was optically monitored. (B) Quantification of the aggregation data. Data are expressed as maximal amplitude of aggregation within 5 minutes after adding the agonist. (C) Representative aggregation curves are shown. (D-E) Platelets in PRP from chimeric WT or SR-BI^−/− mice with WT bone marrow (BM) in panel D as well as from WT and SR-BI^−/− mice in panel E were stained with 50 μg/mL filipin to label unesterified cholesterol and analyzed by flow cytometry. All above data are presented as mean ± SEM of at least 3 independent experiments. (F) WT platelets were isolated by gel-filtration from pooled blood of WT chimeras and loaded with cholesterol in vitro by incubation with 75μM cholesterol-chelated MβCD. Incubation with alpha-cyclodextrin (αCD) was used as control. After cholesterol loading, platelets were stimulated with ADP, thrombin, or CVX, and platelet integrin α_IIbβ₃ activation was determined by FACS analysis. Data are presented as mean ± SEM of measurements after 3 separate cholesterol loading, which were repeated twice. *P < .05, **P < .01, ***P < .001.