β 2 ‐Glycoprotein i reactivity of monoclonal anticardiolipin antibodies from patients with the antiphospholipid syndrome

Objective . To elucidate the specificity of anticardiolipin antibodies (aCL) from patients with the antiphospholipid syndrome (APS) to various phospholipids (PLs), DNA, and β 2 ‐glycoprotein I (β 2 ‐GPI). Methods . Five monoclonal aCL were established from peripheral blood lymphocytes of 3 patients with the APS. The reactivity of monoclonal aCL with various PLs, with DNA, and with β 2 ‐GPI was examined by enzyme‐linked immunosorbent assay (ELISA). Results . All of the monoclonal aCL bound to anionic PLs, only in the presence of β 2 ‐GPI. Neither monoclonal aCL nor β 2 ‐GPI bound to DNA. Monoclonal aCL bound to solid‐phase β 2 ‐GPI on polystyrene ELISA plates that had carboxyl groups on their surface, but did not react with solid‐phase β 2 ‐GPI on ordinary polystyrene plates. A mixture of β 2 ‐GPI and CL inhibited the binding of monoclonal aCL to β 2 ‐GPI, but CL or β 2 ‐GPI alone did not. Conclusion . Monoclonal aCL may recognize a cryptic epitope, which appears as a result of β 2 ‐GPI binding to anionic PLs or to polystyrene with carboxyl groups.