Aggregation of washed platelets induced by various stimuli is inhibited by β₂ GPI. Aggregation of washed platelets was induced with ADP (50 μM)/epinephrine (50 μM; A) or collagen (2 μg/mL; B) in the absence ( ) or presence ( ) of β₂ GPI (2.2 μM). Relative levels of total aggregation (mean ± SD; n = 3; **P < .001) in the absence or presence of β₂ GPI (2.2 μM) are depicted in the insets of panels A and B. (C) Aggregation of washed platelets was induced with ristocetin-activated VWF (10 μg/mL VWF and 0.3 mg/mL ristocetin) in the absence (line I) or presence (line IV) of β₂ GPI (2.2 μM). β₂ GPI (2.2 μM) was also added in the presence of LAC-negative anti–β₂ GPI antibody 1F12 (2.2 μM; line III) or LAC-positive anti–β₂ GPI antibody 27G7 (2.2 μM; line II). A typical example of the aggregation curves is shown. All aggregation experiments were performed 3 times, using blood of different donors. (C inset) Ristocetin-independent platelet aggregation induced by VWF/R1306Q (10 μg/mL) in the absence () or presence () of β₂ GPI (2.2 μM). (D) Total aggregation of washed platelets induced with 2 μg/mL collagen was set at 100%. Relative aggregation (%) in the presence of β₂ GPI (2.2 μM), anti-VWF antibody ALX0081 recognizing the GpIbα-binding site (20 μg/mL), anti-GpIbα antibody 6D1 (20 μg/mL), or anti-VWF antibody MoAb 9 recognizing the α_IIbβ₃-binding site (20 μg/mL) was calculated. Data represent the mean (± SD) of 5 experiments. ***P < .001.