Anticardiolipin antibodies recognize β₂-glycoprotein I structure altered by interacting with an oxygen modified solid phase surface

Anticardiolipin antibodies (aCL) derived from the sera of individuals exhibiting the antiphospholipid syndrome (APS) directly bind to β₂- glycoprotein I (β₂-GPI), which is adsorbed to an oxidized polystyrene surface. Oxygen atoms were introduced on a polystyrene surface by irradiation with electron or γ-ray radiation. X-ray photoelectron spectroscopy revealed the irradiated surfaces were oxidized to generate C-O and C=O moieties. aCL derived from either APS patients or (NZW x BXSB)F₁ mice bound to β₂-GPI coated on the irradiated plates, depending on the radiation dose. Antibody binding to β₂-GPI on the irradiated plates was competitively inhibited by simultaneous addition of cardiolipin (CL)-coated latex beads mixed together with β₂-GPI but were unaffected by addition of excess β₂-GPI, CL micelles, or CL-coated latex beads alone. There was a high correlation between binding values of aCL in sera from 40 APS patients obtained by the anti-β₂-GPI enzyme-linked immunosorbent assay (ELISA) using the irradiated plates and those by the β₂-GPI-dependent aCL ELISA. Therefore, aCL have specificity for an epitope on β₂-GPI. This epitope is expressed by a conformational change occurring when β₂-GPI interacts with an oxygen- substituted solid phase surface.