Anti–annexin A2 antibodies directly activate endothelial cells. Endothelial cells were cultured in 96-well plates, prepared as described in “Materials and methods,” and then incubated with medium alone, anti–annexin A2 mAb (600 nM, purified from EH7A hybridoma cells), control murine IgG1 (600 nM), or LPS (1 μg/mL). (A) Adhesion of CMFDA-labeled Mono Mac 6 cells, measured in arbitrary fluorescence units (AFUs). (B) Expression of endothelial cell surface E-selectin, measured by ELISA. Error bars represent the mean plus or minus SEM of quadruplicate points. ***P < .0001 versus medium alone. This experiment is representative of 4 so performed.