Mucins require L-selectin, PSGL-1, and platelet P-selectin to stimulate secretion of cathepsin G from neutrophils. (A-B) Leukocytes from mice of the indicated genotype were incubated with mucins in the presence or absence of washed platelets from WT mice. After pelleting the cells, the concentration of cathepsin G was measured using an enzymatic assay. Activity in control buffer-treated samples was subtracted as background. (C) Leukocytes and platelets of the indicated genotype were incubated with mucin in the presence or absence of control rat IgG or anti–L-selectin mAb Mel-14. After pelleting the cells, the concentration of cathepsin G was measured using an enzymatic assay. Activity in control buffer-treated samples was subtracted as background. The data represent the mean ± SEM from 3 experiments. *P < .01.