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 β₂‐glycoprotein I (β₂‐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 β₂‐GPI was examined by enzyme‐linked immunosorbent assay (ELISA). Results. All of the monoclonal aCL bound to anionic PLs, only in the presence of β₂‐GPI. Neither monoclonal aCL nor β₂‐GPI bound to DNA. Monoclonal aCL bound to solid‐phase β₂‐GPI on polystyrene ELISA plates that had carboxyl groups on their surface, but did not react with solid‐phase β₂‐GPI on ordinary polystyrene plates. A mixture of β₂‐GPI and CL inhibited the binding of monoclonal aCL to β₂‐GPI, but CL or β₂‐GPI alone did not. Conclusion. Monoclonal aCL may recognize a cryptic epitope, which appears as a result of β₂‐GPI binding to anionic PLs or to polystyrene with carboxyl groups.