Characterization of the reactivity of prothrombin-dependent anti-phospholipid antibodies with apoptotic cells

Anti-phospholipid antibodies (aPL) occur in patients with the anti-phospholipid syndrome, and are directed against various combinations of phospholipids and phospholipid-binding proteins (e.g., beta2-glycoprotein I and prothrombin). Lupus anticoagulants (LA), a subset of aPL, exhibit anticoagulant properties in vitro, but are strikingly procoagulant in vivo. We have previously demonstrated that some aPL bind specifically to apoptotic, but not viable, thymocytes in the presence of beta2-glycoprotein 1. Here, we demonstrate that prothrombin binds selectively to the surface of apoptotic Jurkat cells, and supports the binding of LA-positive murine monoclonal antibodies (mAb) and patient-derived IgG to apoptotic cells. Despite similar LA activity and reactivity with apoptotic cells, the mAb differed in affinity and specificity. One mAb was highly reactive with prothrombin alone, while the other required anionic phospholipid for elevated binding. These results demonstrate that aPL recognize multiple epitopes on apoptotic cells, suggesting that apoptotic antigens contribute to the induction and/or perpetuation of aPL