Summary
The antiphospholipid antibodies (aPL) present in autoimmune disorders are associated
with thromboembolic episodes, and their binding to phospholipids (PL) is mediated
by a plasma cofactor, β2-glycoprotein I (β2GPI). Both PL and β2GPI seem necessary
for binding, thus indicating that the two components comprise the epitope against
which aPL are directed. Using an anti-β2GPI antibody ELISA with the antigen adsorbed
onto polyvinylchloride (PVC) plates, we detected high antibody titres in 12 out of
12 plasmas from patients with the antiphospholipid syndrome. No or very low positivity
was obtained when the same ELISA was carried out in polystyrene (PST) plates, while
an increasing positivity was found when processed (i.e. more hydrophilic) or COOH-surface
PST plates were used. When (β2GPI dependent IgG-aPL were purified using agarose-immobilized
cardio- lipin, 4 out of 4 preparations were highly positive in anti-β2GPI antibody
ELISA using PVC plates, while β2GPI was not fully recognized by aPL-IgG when adsorbed
onto PST plates. These findings demonstrate that aPL are, in fact, anti-β2GPI antibodies
directed against a cryptic epitope which is expressed when β2GPI is bound to anionic
phospholipid, or another suitable surface.
