Non beta 2-glycoprotein I cofactors for antiphospholipid antibodies

The Antiphospholipid Syndrome is defined by the association between peculiar clinical manifestations, namely arterial and/or venous thrombosis, recurrent abortions and thrombocytopenia, and the antiphospholipid antibodies. These antibodies are directed to plasma proteins bound to anionic phospholipids or other anionic surfaces: so far, beta 2-glycoprotein I is the best known and characterized antiphospholipid 'cofactor' (this issue is specifically treated in other parts of this journal). In recent years, such a role has been reported also for prothrombin, activated Protein C, Protein S, Annexin V, Thrombomodulin, high- and low-molecular weight kininogens. Anti-prothrombin antibodies are detected in approximately 50% of the antiphospholipid-positive patients; conversely, limited data are available regarding the prevalence the other antibodies. 'Cofactors' are necessary for the expression of both the immunological and the functional properties of their respective antiphospholipid antibodies. In particular, the recognition of the calcium-mediated prothrombin/lipid complex by anti-prothrombin antibodies hampers prothrombin activation, thus causing the prolongation of the phospholipid-dependent coagulation reactions. The interaction between antiphospholipid antibodies and natural inhibitors of coagulation such as activated Protein C, its non-enzymatic accessory protein Protein S or Thrombomodulin might increase the risk to develop thromboembolic events. Similarly, the presence of antibodies to surface-bound Annexin V has been hypothesized to play a role in recurrent abortions and fetal deaths. However, to clearly establish whether and which antiphospholipid antibodies represent risk factors for the thromboembolic events of the antiphospholipid syndrome, further studies of their behaviour and properties as well as the identification and characterization of (possibly) other antibodies are required.