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.