Activated Complement System's Impact in Antiphospholipid Syndrome Thrombosis: From Pathophysiology to Treatment

Antiphospholipid syndrome (APS) is the most common acquired form of thrombophilia and is associated with the presence of antiphospholipid antibodies (aPL) in the patient's serum. Until now, the "double-hit" hypothesis remains the prevailing theory for APS pathogenesis. According to this model, the presence of aPL (first hit) is insufficient to trigger thrombosis. A secondary event, such as an inflammatory trigger or vascular injury (second hit), is required to initiate immunothrombosis, which ultimately leads to thromboembolism. Although immunothrombosis has a critical role in several mechanisms, such as in defense against pathogens, chronic immune system activation by aPL appears to disrupt its protective function. In the last three decades, the role of the complement system has gained increasing recognition in the pathophysiology of APS. aPL are involved in the dysregulation of multiple components, such as platelets, β2-glycoprotein I, and complement factor H, resulting in excessive activation of the complement system. Thus, the complement system is a key driver of thrombosis in APS and stands as a promising target for the development of future therapeutic strategies. In the current review article, we aim to summarize the ongoing research regarding the role of complement system dysregulation in APS-associated thrombosis development, while recognizing potential therapeutic targets. In the era of precision medicine, more data concerning targeted therapeutics in the field of APS are essential.