Mer extracellular domain inhibits platelet aggregation induced by ADP and collagen and protects mice against collagen-epinephrine–induced thrombosis. (A-D) In vitro platelet aggregation was performed using human PRP and was analyzed on a BioData aggregometer. (A-B) Aggregation response of platelets in response to 2 μM ADP (A) or 4 μM ADP (B) following preincubation with different concentrations of Mer/Fc. (C) Platelet aggregation induced by 4 μM ADP after pretreatment with Ret/Fc. (D) Aggregation of platelets in response to 10 μg/mL collagen with and without preincubation with Mer/Fc. Squares on the x-axis represent 15-second intervals. Data shown are representative of 3 independent experiments. (E) Survival of mice pretreated with Mer/Fc (n = 10) or saline (PBS control, n = 8) after collagen-epinephrine injection. Protection from fatal thromboembolism by Mer/Fc was significant (P < .022, log-rank test). (F-G) Hematoxylin and eosin staining of lungs from control mouse (F) or Mer/Fc-pretreated mouse (G) after collagen-epinephrine injection. Extensive platelet thromboembolism is seen in control mice (arrows) compared to the Mer/Fc-pretreated mice. Scale bar represents 100 μm.