Evaluation of Cell Membrane Expression of Annexin A2 on Monocytes by Flow Cytometry in Primary Antiphospholipid Syndrome

Annexin A2 (ANXA2), an endothelial cell receptor for plasminogen and tissue plasminogen activator, plays a pivotal role in regulation of fibrinolysis in vitro and in vivo and has been identified as a new autoantigen in antiphospholipid syndrome (APS). ANXA2 can exist as a monomer or a heterotetrameric complex with S100A10 protein. The aim of this study was to evaluate the cell membrane expression of ANXA2 on circulating monocytes in APS by flow cytometry. Several pathogenic mechanisms are involved in APS such as activation of endothelial cells, platelets and monocytes, inhibition of the natural anticoagulant protein C/protein S pathway, activation of the complement system and also impairment of fibrinolysis. Annexin A2 which hits binding partner S100A10, ANXA2 forms a cell surface complex that regulates generation of plasmin. ANXA2 is involved in the pathogenesis of APS-associated through several possible mechanisms. Human peripheral blood monocytes represent the major circulating ANXA2-expressing cell and ANXA2-mediated assembly of plasminogen and tissue activator of plasminogen (tPA) on monocyte/macrophages contributes to plasmin generation. Thus the investigators could suppose that decrease of cell membrane expression of ANXA2 on circulating monocytes represent a new pathogenic mechanism in APS.