Antiphospholipid antibodies: specificity and pathophysiology

Antiphospholipid antibodies are autoantibodies that can be detected in plasma or serum with phospholipid-dependent coagulation tests or solid-phase immunoassays. The presence of these autoantibodies is strongly associated with an increased risk for arterial and venous thrombosis, recurrent fetal loss and thrombocytopenia. This paradoxical association of the in vitro prolongation of clotting assays and in vivo thrombosis has stimulated the search for the real antigen to which the autoantibodies are directed. A large number of potential pathological mechanisms have been proposed, and although disturbance of a certain metabolic pathway by the antibodies can explain a thrombotic tendency in one patient, no general pathological mechanism explaining thrombosis in the whole patient population has been found. This suggests that the antiphospholipid antibodies are a heterogeneous group of autoantibodies and is supported by the recent observations that antiphospholipid antibodies are not directed against phospholipids alone but against a combination of phospholipids and phospholipid-binding proteins. Both the phospholipid and the protein are part of the antigen. For the detection of antiphospholipids in an ELISA set-up, beta 2-glycoprotein I is the protein cofactor. In the coagulation tests, beta 2-glycoprotein, as well as prothrombin, can act as cofactor. However, the presence of these two proteins as a part of the epitope of the antiphospholipid antibodies does not explain the thrombotic tendency in the patient group. We have found that more physiologically relevant cofactors such as protein C and protein S, for which it is known that a partial deficiency is correlated with a thrombotic tendency, can also act as cofactors for the binding of antiphospholipid antibodies. It is concluded that antiphospholipid antibodies are a heterogeneous group of autoantibodies with varying affinity for different protein-phospholipid complexes and that inhibition of the biological activity of the protein part of the complex determines the pathological capacity of the antibodies.