FIG. 2.

IgG-coated targets activate platelets. (A) Platelets were exposed to IgG-coated 1.5-μm polystyrene beads, and CD42b-expressing platelets were assessed for activation by measuring CD62P surface expression (gray lines) or isotype-matched control IgG (black line) on platelets not in contact with beads (region R1) or in contact with beads (region R2). (B) Quantitative analysis of CD62P expression of CD42b-expressing platelets incubated with 0.5- or 1.5-μm polystyrene beads coated with BSA or IgG. Platelets incubated with 2 U/ml thrombin served as a positive control for platelet activation. Some samples were pretreated with either 5 μg/ml anti-FcγRIIA MAb IV.3 or 5 μg/ml isotype-matched control IgG to assess the dependence on FcγRIIA. Other samples were allowed to bind beads and then incubated with 20 μM cytochalasin D (Cyto D) to determine the role of actin rearrangement on activation. Data are means ± SD representative of one experiment performed in triplicate. Experiments were repeated on 3 different days with platelets from three healthy volunteers with similar results. MFI, mean fluorescence intensity.