High Affinity Binding of β2-Glycoprotein I to Human Endothelial Cells Is Mediated by Annexin II*

β2-Glycoprotein I (β2GPI) is an abundant plasma phospholipid-binding protein and an autoantigen in the antiphospholipid antibody syndrome. Binding of β2GPI to endothelial cells targets them for activation by anti-β2GPI antibodies, which circulate and are associated with thrombosis in patients with the antiphospholipid antibody syndrome. However, the binding of β2GPI to endothelial cells has not been characterized and is assumed to result from association of β2GPI with membrane phospholipid. Here, we characterize the binding of β2GPI to endothelial cells and identify the β2GPI binding site.125I-β2GPI bound with high affinity (K d ∼18 nm) to human umbilical vein endothelial cells (HUVECs). Using affinity purification, we isolated β2GPI-binding proteins of ∼78 and ∼36 kDa from HUVECs and EAHY.926 cells. Amino acid sequences of tryptic peptides from each of these were identical to sequences within annexin II. A role for annexin II in binding of β2GPI to cells was confirmed by the observations that annexin II-transfected HEK 293 cells bound ∼10-fold more 125I-β2GPI than control cells and that anti-annexin II antibodies inhibited the binding of125I-β2GPI to HUVECs by ∼90%. Finally, surface plasmon resonance studies revealed high affinity binding between annexin II and β2GPI. These results demonstrate that annexin II mediates the binding of β2GPI to endothelial cells.