Antiphospholipid antibody: functional specificity for inhibition of prothrombin activation by the prothrombinase complex

Abstract

Summary. The antiphospholipid syndrome (APS) is associated with production of autoantibodies with lupus anticoagulant (LA) activity. These antibodies cause prolongation of in vitro clotting tests by inhibition of the conversion of prothrombin to thrombin in the presence of anionic phospholipid (PL). The extent to which this inhibition reflects antibody binding to, or functional inhibition of, phospholipids alone, prothrombin alone, or a prothrombinphospholipid complex is pertinent to our understanding of the pathophysiology of this syndrome. Immunoglobulin fractions (IgG) from 18 patients with LA activity were tested for inhibitory activity against prothrombin activation by either factor Xa, in a purified prothrombinase system, or by purified fractions of snake venoms (E. carinatus, E. multisquamatus) which cleave prothrombin at the same initial site as the prothrombinase complex but do not require anionic phospholipid as a cofactor. Parallel testing of the same IgG samples for prothrombin binding by immunoassay was performed. Although all IgG samples inhibited the prothrombinase reaction, only three exhibited any inhibition of venom protease prothrombin activation in either the presence or absence of PL. Only one sample exhibited prothrombin binding by Western blot. These results suggest that lupus anticoagulant antibodies are heterogenous and that many, if not most, of the autoantibody populations responsible for LA activity impair prothrombin activation by interaction either with phospholipid alone or with a restricted range of prothrombin-phospholipid epitopes expressed by prothrombin only as part of the intact prothrombin-prothrombinase complex.

Specificities, properties, and clinical significance of antiprothrombin antibodies

“…Pierangeli et al (39) have suggested that APTs cause prolongation of in vitro clotting by inhibiting the conversion of prothrombin into thrombin. However, it seems unlikely that APTs prolong the clotting times by hampering the activation of prothrombin through binding near the activation sites in the molecule, since such a mechanism would not explain the neutralizing effects of high phospholipid concentrations.…”

Immune responses to oxidative neoepitopes on LDL and phospholipids modulate the development of atherosclerosis

“…A characteristic feature of the APS is the occurrence of both arterial and venous thrombosis. This raises the intriguing question as to whether antibodies to oxidized phospholipid–protein adducts enhance thrombosis [73, 74] and, if so, whether some of the autoantibodies formed during atherogenesis may also influence coagulation. Studies assessing this in vivo are currently underway in our laboratories, using a laser‐induced thrombosis model.…”

Das Antiphospholipidsyndrom Pathogenese, molekulare Grundlagen und klinische Aspekte

“…Die Hypothese, dass Antiphospholipidantikörper ein Neoepitop auf dem ␤ 2 -GPI erkennen, welches erst nach Bindung an ein anionisches Phospholipid präsentiert wird, wird weiter kontrovers diskutiert [94]. Man findet bei Patienten mit Antiphospholipidsyndrom Antikörper, die gegen ␤ 2 -GPI gerichtet sind und dieses in einem ELISA, in dem ␤ 2 -GPI in Abwesenheit von Cardiolipin oder anderen Phospholipiden auf eine Platte aufgebracht ist, binden.…”

“…Man findet bei Patienten mit Antiphospholipidsyndrom Antikörper, die gegen ␤ 2 -GPI gerichtet sind und dieses in einem ELISA, in dem ␤ 2 -GPI in Abwesenheit von Cardiolipin oder anderen Phospholipiden auf eine Platte aufgebracht ist, binden. Die in vivo postulierte Konformationsänderung, verbunden mit der Prä-sentation von Neoepitope(n) im ␤ 2 -GPI aufgrund der Bindung an Phospholipide, wird in diesem ELISA dadurch simuliert, dass ␤ 2 -GPI an bestrahlte Platten gebunden wird, die eine negative Oberflächenladung tragen [79,80,94]. Dieses wird auch durch die Beobachtung unterstützt, dass das Ausmaß der Bindung mit der Bestrahlung ansteigt.…”