Summary
Plasmas from 16 patients that were found to be positive both for anticardiolipin antibodies
(ACA) and lupus anticoagulants (LA) were incubated with liposomes that contained anionic
phospholipids. In 11 of these plasmas, ACA could be cosedimented with the liposomes
in a dose-dependent manner, whereas LA activity of the remaining supernatant was unaffected.
LA activity of purified total IgG from 6 patients was measured in three different
coagulation tests, using normal plasmas from different species. Prolongation of the
aPTT, KCT and dRW clotting times was observed only with normal plasma from human origin,
not with bovine, rat or sheep plasma.
Highly purified coagulation factors Xa, Va and prothrombin, both of human and bovine
origin, were used to establish for two patient IgG's the effect of LA on the rate
of thrombin formation in the presence and absence of lipid vesicles composed of 20
mole% phosphatidylserine and 80 mole% phosphatidylcholine. A strong and dose dependent
inhibition by LA was observed only when human prothrombin was used as substrate in
the prothrombinase complex in the presence of lipids. No inhibition was found when
bovine prothrombin was used as substrate. The inhibitory effect observed in the presence
of human prothrombin was independent of the source of factors Xa and Va, and was not
found in the absence of lipid. Preliminary binding studies suggest that LA only associate
with a lipid surface, provided that human prothrombin and calcium ions are present.
These data indicate that LA are not directed to phospholipids alone, but presumably
recognize an epitope which becomes exposed upon Ca2+-mediated binding of human prothrombin to phospholipids.
