Inhibition of endothelial cell activation caused by anti–annexin A2 mAb or β₂GPI and anti-β₂GPI antibodies by anti–annexin A2 mAb–derived Fab fragments. Endothelial cells were cultured in 96-well microplates and prepared as described in “Materials and methods.” In (A), cells were then incubated for 4 hours with medium alone, anti–annexin A2 mAb (600 nM), anti–annexin A2 mAb (600 nM) in the presence of Fab fragments derived from control murine IgG1 (▧, control Fab) or anti–annexin A2 mAb (▦) at the designated concentrations, LPS (1 μg/mL), or LPS (1 μg/mL) and 4000 nM anti–annexin A2 Fab fragments (anti–annexin A2 Fab). In (B), cells were incubated for 4 hours with medium, β₂GPI (100 nM) and rabbit anti-β₂GPI antibodies (600 nM), β₂GPI (100 nM) and rabbit anti-β₂GPI antibodies (600 nM) in the presence of Fab fragments derived from control murine IgG1 (▧, control Fab) or anti–annexin A2 mAb (▦) at the designated concentrations, LPS (1 μg/mL), or LPS (1 μg/mL) and 4000 nM anti–annexin A2 Fab fragments. Error bars represent the mean plus or minus SEM of quadruplicate points. *P < .05, **P < .001, ***P < .0001 versus anti–annexin A2 mAb (A) or β₂GPI and anti-β₂GPI antibodies alone (B). This experiment is representative of 3 so performed.