Antiphospholipid antibody mechanisms of thrombosis

Anti-phospholipid antibodies (aPL) are diagnostic and pathogenic antibodies. There is evidence from both in vitro and in vivo models that they can mediate thrombosis through several mechanisms. aPL may interact with beatta-2 glycoprotein I or with prothrombin and may interfere with fluid- phase coagulation steps: inhibition of protein C-S activity and fibrinolysis. More importantly beta-2 glycoprotein I –dependent aPL may recognize their own target on several cell types involved in the coagulation: endothelial cells, peripheral blood monocytes, and platelets. Once bound aPL may induce a pro-inflammatory and pro-coagulant endothelial phenotype, may induce tissue factor expression on monocytes, and can increase aggregation of platelets stimulated by another agonist. All these mechanisms may play a role in supporting the thrombophilic state of the anti-phospholipid syndrome. aPL are necessary for clotting but are not sufficient and require an additional stimulus (two- hit theory) and complement activation so explaining why thrombosis can occur only sometimes in spite of the persistent presence of the antibodies.