Identification of anti-prothrombin antibodies in the anti-phospholipid syndrome that display the prothrombinase activity

Objective. Prothrombin (PT) is one of the most important antigenic targets for aPL antibodies; however, the prothrombotic mechanism of anti-PT (aPT) antibodies in APS is not fully clarified. Considering that some autoantibodies possess the enzymatic activity, the aim of this study was to test the hypothesis that some aPT antibodies in APS may display prothrombinase activity. Methods. Six APS patient-derived PT-reactive monoclonal antibodies (mAbs) were analysed for prothrombinase activity on PT. One mAb with prothrombinase activity was examined for its proteolytic activity on PT. In addition, IgG was purified from plasma samples positive with IgG aPT antibodies, and their prothrombinase activity analysed. Results. Initial analysis of six mAbs revealed that, upon incubation with PT, IS6 mAb displayed prothrombinase activity and catalysed the proteolysis of PT to fragments. Analysis of plasma samples revealed that 9/21 (42.8%) APS patients had IgG antibodies against PT, based on a cut-off value equal to mean + 3 S.D. of the level in 21 normal controls. Importantly, of those samples positive for IgG aPT antibodies, two polyclonal IgG (P1 and P2) also displayed prothrombinase activity. Conclusions. In this study, we showed that some aPT antibodies displayed prothrombinase activity. Such catalytic aPT antibodies may contribute to thrombosis in APS.