mAb-RC3 inhibits platelet procoagulant activity during ex vivo human whole blood flow. (A) Representative images and (B-E) quantitation of human thrombi formed after 3 minutes of a whole blood thrombosis assay using donors subsequently genotyped as homozygous at rs773902 for Thr120 or Ala120, or as heterozygous. (A) Shown are thrombin (green, thrombin probe), fibrin (purple, anti-fibrin), and brightfield image of thrombi. Note that the direct thrombin inhibitor, hirudin (800 U/mL), abolished thrombin activity and fibrin accumulation despite continued platelet deposition. Scale bars, 20 μm. No differences in (B) thrombin activity or (C) fibrin volume were observed between thrombi formed in individuals expressing the indicated PAR4 genotypes. Pretreatment with mAb-RC3 (100 μg/mL) significantly inhibited both (D) thrombin activity and (E) fibrin formation. Note that all blood donors were pooled in panels D and E with the different genotypes highlighted by the colors as indicated. Data are mean ± standard error of the mean of N = 3 to 8 per genotype and was analyzed by 1-way ANOVA (B-C) or unpaired Student t test (D-E). *P < .05.