Annexin A2: biology and relevance to the antiphospholipid syndrome

Antiphospholipid antibodies (aPL), the majority of which are directed against β₂-glycoprotein I (β₂GPI), are associated with an increased incidence of venous and arterial thrombosis. The pathogenesis of antiphospholipid/anti-β₂GPI-associated thrombosis has not been defined, and is likely multifactorial. However, accumulating evidence suggests an important role for endothelial cell activation with the acquisition of a procoagulant phenotype by the activated endothelial cell. Previous work demonstrated that endothelial activation by antiphospholipid/anti-β₂GPI antibodies is β₂GPI-dependent. We extended these observations by defining annexin A2 as an endothelial β₂GPI binding site. We also observed that annexin A2 plays a critical role in endothelial cell activation induced by anti-β₂GPI antibodies, and others have described direct endothelial activation by anti-annexin A2 antibodies in patients with aPL . Similar findings have been reported using human monocytes, which also express annexin A2. Because annexin A2 is not a transmembrane protein, how binding of β₂GPI/anti-β₂GPI antibodies, or anti-annexin A2 antibodies, to endothelial annexin A2 causes cellular activation is unknown. Recent studies, however, suggest an important role for the Toll-like receptor family, particularly TLR4. In this article, we review the role of these interactions in the activation of endothelial cells by aPL . The influence of these antibodies on the ability of annexin A2 to enhance t-PA-mediated plasminogen activation is also discussed.