β2‐Glycoprotein i reactivity of monoclonal anticardiolipin antibodies from patients with the antiphospholipid syndrome

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Abstract
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.