Mucins require the neutrophil proteinase, cathepsin G, to generate platelet-rich microthrombi in vivo, and require cathepsin G and PAR4 to activate platelets in whole blood. (A) Representative images of lung sections from mice of the indicated genotype prepared 5 minutes after intravenous injection of mucins or saline. The sections were labeled with antibody to the platelet marker, CD41. (B) Quantification of CD41-positive pixels in lung sections. The data represent the mean ± SEM, n = 7 for each group. *P < .001. (C,D) Hirudin-anticoagulated whole blood from mice of the indicated genotype was incubated with mucins. The percentage of P-selectin–positive platelets was measured by flow cytometry. P-selectin expression in control buffer-treated samples was subtracted as background. The data represent the mean ± SEM from 5 experiments. (E) Hirudin-anticoagulated whole blood from mice of the indicated genotype was incubated with mucins or the PAR4 receptor agonist AYPGKF peptide at the indicated concentration. The percentage of P-selectin–positive platelets was measured by flow cytometry. P-selectin expression in control buffer-treated samples was subtracted as background. The data represent the mean ± SEM from 5 experiments. *P < .01; **P < .05.