Mucins require PSGL-1 to generate platelet-rich microthrombi in vivo and to activate platelets in whole blood. (A) Representative images of lung sections from WT or PSGL-1^−/− mice 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 = 4-7 mice for each group. *P < .001. (C) Hirudin-anticoagulated whole blood from WT or PSGL-1^−/− mice was incubated with mucins in the presence or absence of control rat IgG, nonblocking anti–PSGL-1 mAb 4RB12 or blocking anti–PSGL-1 mAb 4RA10. 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.