Monocyte microvesicles transfer TF and PSGL-1 to activated platelets via PSGL-1. (A) Collagen-activated or unstimulated washed platelets were incubated with either monocyte microvesicles or control buffer. Platelets were sedimented and unbound microvesicles were removed by washing 5 times. The platelets were pelleted, lysed in SDS sample buffer, and analyzed by SDS-PAGE and Western blotting for PSGL-1 and TF. (B) Flow cytometric analysis of platelets incubated with either microvesicles or buffer, handled as described in (A). Samples using resting platelets plus microvesicles, activated platelets alone, and samples probed with control fluorescent IgG served as the negative controls in these experiments. (C) Treatment with anti–PSGL-1 (KPL-1) or an anti–P-selectin antibody, but not a nonspecific mouse IgG, blocked the transfer of TF and PSGL-1.