A1-A1 inhibits thrombus size in BALB/c mice in the presence of anti-β2GPI antibodies and does not affect thrombus size in the absence of anti-β2GPI antibodies. (A-B) Anti-β2GPI–positive APS IgG (2 mg per mouse) and A1-A1 (4 μg/g mouse). (A) Median-integrated platelet fluorescence studied in 4 mice in the absence (black curve, 33 thrombi) and in the presence (red curve, 39 thrombi) of A1-A1. In the presence of A1-A1, thrombus size decreased to 13% of the thrombus size in the absence of A1-A1 (P = .001). (B) Area under the curve of individual thrombi collected in the absence (black circles) and in the presence of A1-A1 (red circles). (C-D) Anti-β2GPI–positive APS IgG (2 mg per mouse) and LA6 (4 μg/g mouse). (C) Median-integrated platelet fluorescence studied in 7 mice in the absence (black curve, 37 thrombi) and in the presence (red curve, 30 thrombi) of LA6. (D) Area under the curve of individual thrombi collected in the absence (black circles) and in the presence of LA6 (red circles). The difference between 2 data sets is not statistically significant. (E-F) Purified patient anti-β2GPI IgG (16 μg per mouse) and A1-A1 (4 μg/g mouse). (E) Median-integrated platelet fluorescence studied in 4 mice in the absence (black curve, 29 thrombi) and in the presence (red curve, 26 thrombi) of A1-A1. In the presence of A1-A1, thrombus size decreased to 18% of the thrombus size in the absence of A1-A1 (P = .007). (F) Area under the curve of individual thrombi collected in the absence (black circles) and in the presence of A1-A1 (red circles). (G-H) PBS and A1-A1 (4 μg/g mouse). (G) Median-integrated platelet fluorescence studied in 4 mice in the absence (black curve, 28 thrombi) and in the presence (red curve, 29 thrombi) of A1-A1. (H) Area under the curve of individual thrombi collected in the absence (black circles) and in the presence of A1-A1 (red circles). The difference between 2 data sets is not statistically significant. Lines on panels B, D, F, and H correspond to areas under the curves of the median-integrated fluorescence calculated for all thrombi in a data set.