Inhibitory effect of surface ERp57 on α_IIbβ₃integrin activation, P-selectin exposure, aggregation, and ATP secretion of PDI-null platelets. Mouse platelets were pretreated with nonimmune sheep immunoglobulin G (IgG) or blocking anti-ERp57 antibodies (50 μg/mL) and stimulated with 0.05 U/mL thrombin (thr). (A,B) α_IIbβ₃ integrin activation and P-selectin exposure were analyzed by flow cytometry as described in “Materials and methods.” Binding of anti-activated α_IIbβ₃ (JON/A) and anti–P-selectin antibodies to platelets was calculated by the ratio of the geometric mean intensity value of antibodies to that of control IgG. Then, antibody binding to resting WT platelets treated with nonimmune sheep IgG was normalized as 1 (white bar in the left panel). Data represent mean ± SD (n = 3). **P < .01 vs control IgG; ##P < .01 vs WT platelets after Student t test. (C,D) Platelet aggregation and ATP secretion were measured as described in Figure 2 (mean ± SD, n = 3). **P < .01 vs control IgG; ##P < .01 vs WT platelets after Student t test.