A chimeric antibody with the human γ1 constant region as a putative standard for assays to detect IgG β₂-glycoprotein I-dependent anticardiolipin and anti-β₂-glycoprotein I antibodies

Objective. Thromboembolic manifestations or thrombocytopenia in association with anticardiolipin antibodies (aCL) or lupus anticoagulant are known as the antiphospholipid syndrome (APS). Efforts have been made to elucidate precise clinical features and adequate therapeutic options for treating patients with APS. However, the lack of a proper international standard for measurement of aCL makes it difficult to compare data derived from different laboratories. We attempted to design a chimeric antibody with human γ constant regions and variable regions of WBCAL-1, a monoclonal antibody established from an APS-prone mouse which has a specificity similar to that of aCL in sera from humans with APS. Methods. Variable-region genes of WBCAL-1, which were cloned using reverse transcription-polymerase chain reaction, were inserted into plasmids containing human γ1 and κ constant- region genes. The construct was transfected to a mouse myeloma cell line. Stable transfectants that secreted a chimeric antibody, HCAL, into the culture supernatant were obtained. The reactivity of HCAL to cardiolipin and to β₂-glycoprotein I (β₂GPI) was studied using a solid-phase enzyme immunoassay. The binding of HCAL was compared with the binding of standards for IgG aCL and anti-β₂GPI antibody assays done in 18 independent laboratories. Results. In the presence of β₂GPI, HCAL bound to the wells of cardiolipin-coated microtiter plates in a dose-dependent manner and reacted with β₂GPI on oxygenated polystyrene plates. The aCL activity of HCAL can be converted into GPL units (IgG phospholipid units), which is widely used to quantify IgG aCL activity, using the following formula: 1 GPL unit = 32.9 x (concentration of HCAL [in μg/ml])^0.503. The reactivity of HCAL to cardiolipin or β₂GPI was similar to the reactivity of standards for IgG aCL or anti-β₂GPI antibody assays done in collaborative laboratories. Conclusion. Because the reactivity of HCAL is similar to that of aCL in sera from humans with APS, HCAL will be useful as a standard for human IgG aCL and anti-β₂GPI antibody assays.