Anticardiolipin antibodies (ACA) directed not to cardiolipin but to a plasma protein cofactor

Abstract

The binding of affinity-purified anticardiolipin antibodies (ACA) to liposomes that contained cardiolipin or phosphatidylserine was investigated. ACA bound to these liposomes only in the presence of plasma or serum, which indicated a requirement for a plasma component. This component--referred to as aca-cofactor--was purified; its activity to support ACA binding to liposomes that contained cardiolipin was not destroyed by heat (10 min at 90 degrees C), but was greatly diminished on incubation with trypsin. aca-cofactor bound liposomes that contained negatively charged phospholipid but had no affinity for liposomes that contained neutral phospholipid (eg, phosphatidylcholine); this binding was independent of calcium ions. aca-cofactor was essential for ACA to bind to liposomes that contained cardiolipin or phosphatidylserine and, when coated on a microtitre plate in the absence of any phospholipid, aca-cofactor was an apparent antigen for ACA in an enzyme-linked immunosorbent assay. aca-cofactor is a single chain polypeptide with an apparent molecular weight of 50 kD (non-reduced), which increases to 70 kD upon reduction, and its properties closely resemble those of beta 2-glycoprotein I (apolipoprotein H).

Specificities, properties, and clinical significance of antiprothrombin antibodies

“…␤ 2 GPI plays an important role in the binding of aPL to phospholipids. In 1990, three groups of investigators, Galli et al (5), McNeil et al (6), and Matsuura et al (7), independently reported that anticardiolipin antibodies (aCL), which were associated with the antiphospholipid syndrome, were not directed against CL alone but required a plasma protein (␤ 2 GPI) as a cofactor to bind CL in enzyme-linked immunosorbent assay (ELISA) plates. ␤ 2 GPI bears the epitopes for aCL binding that are exposed when ␤ 2 GPI binds to negatively charged phospholipids or irradiated plastic plates (13).…”

Antibodies to β2‐glycoprotein I and clinical manifestations in patients with systemic lupus erythematosus

“…More recent studies have shown that cardiolipin binding of IgG-class aCL found in patients with the antiphospholipid syndrome (APS) depends on the existence of the cofactor P,-glycoprotein I (P2GPI) (17)(18)(19)(20)(21). These findings have raised the question of the specificity of aPL.…”

Pathogenicity of human anti-platelet factor 4 (PF4)/heparin in vivo: generation of mouse anti-PF4/heparin and induction of thrombocytopenia by heparin

“…As irrelevant antigen, b 2 -glycoprotein I at 10 mg/ well (b 2 -GPI; kindly donated by Professor Angela Tincani, Brescia, Italy) was added in a similar manner. b 2 -GPI was chosen because it has been reported to serve as a bridge for binding of anti-cardiolipin antibodies to endothelial cells as well as to cardiolipin [36][37][38][39]. Mouse sera or purified IgG (1-100 mg/ml in Hanks'/ 0 .…”