Complement involvement in antiphospholipid syndrome

Antiphospholipid syndrome (APS) is an acquired thrombophilia characterized by thrombotic and non-thrombotic (or non-criteria) manifestations, in the context of persistent presence of autoantibodies targeting phospholipids and associated proteins. The complement system, which shares common actors with coagulation cascade, is nowadays well established to be implicated in APS pathophysiology in different ways. Animal models using knock out strains or complement blocking therapies have helped to decipher the different complement components implicated in the processes of thrombosis and fetal morbidity. In APS patients, complement activation may be assessed through the detection of activation fragments (C4d, C3a, C5a, sC5b9) in plasma and on blood cells surface (C4d, C3d and C5b9) or on APS-affected tissues such as cardiac valves, vessels walls, kidneys and placentae. APS patients are currently treated to avoid thrombosis recurrence by long-term treatment by vitamin K antagonists but various complement targeting molecules are tested in trials or now available and may be of major interest to treat APS patients. Several cases report described the use of eculizumab, an anti-C5 monoclonal antibody, to treat severe forms of APS (recurrent thrombosis, Catastrophic APS) but these studies are not sufficient and need to be more standardized. C4d measurement may be useful to assess classical and lectin pathways activation, C5a may allow evaluating the C5a/C5aR axis activity whereas, associated with sC5b9, it may also assess the terminal pathway activation but also the therapeutic efficacy of complement blocking molecules. Thus, assessment of good complement biomarkers and their kinetics needs to be done to determine personalized therapeutic options.