Antiphospholipid syndrome: Complement activation, complement gene mutations, and therapeutic implications

Antiphospholipid syndrome (APS) is an acquired thromboinflammatory disorder characterized by the presence of antiphospholipid antibodies as well as an increased frequency of venous or arterial thrombosis and/or obstetrical morbidity. The spectrum of disease varies from asymptomatic to a severe form characterized by widespread thrombosis and multiorgan failure, termed catastrophic APS (CAPS). CAPS affects only about ∼1% of APS patients, often presents as a thrombotic microangiopathy and has a fulminant course with >40% mortality, despite the best available therapy. Animal models have implicated complement in the pathophysiology of thrombosis in APS, with more recent data from human studies confirming the interaction between the coagulation and complement pathways. Activation of the complement cascade via antiphospholipid antibodies can cause cellular injury and promote coagulation via multiple mechanisms. Finally, analogous to classic complement-mediated diseases such as atypical hemolytic uremic syndrome, a subset of patients with APS may be at increased risk for development of CAPS because of the presence of germline variants in genes crucial for complement regulation. Together, these data make complement inhibition an attractive and potentially lifesaving therapy to mitigate morbidity and mortality in severe thrombotic APS and CAPS.