Epitopes on β₂-GPI recognized by anticardiolipin antibodies

Anticardiolipin antibodies (aCL) found in sera from patients with antiphospholipid syndrome recognize a cryptic epitope that appears on the β₂-glycoprotein I (β₂-GPI) molecule when β₂-GPI interacts with a lipid membrane composed of negatively charged phospholipid or when β₂-GPI is adsorbed on a polyoxygenated polystyrene plate. A homology based model of β₂-GPI was constructed based on the NMR coordinates of sushi domains of human factor H. The conformation was like a cylinder consisting of five domains, its IV and V domains being glued by electrostatic interaction. We used phage-displayed random peptide libraries to search the epitopes of human aCL. Structures similar to consensus sequences selected by a biopanning method was found on domain IV of β₂-GPI.