Anti-beta 2-glycoprotein I antibodies: a marker of antiphospholipid syndrome?

Anticardiolipin (aCL) and anti-beta2-glycoprotein I(antiβ2GPI) antibodies have been shown in animal models as not cross-reacting antibody populations. This observation prompted us to prove if anti-β2GPI exist in human sera by using a reliable method and then to investigate if these are independent from aCl antibodies. We have developed a new ELISA for the detection of anti-β2GPI antibodies employing the coating of the protein in carbonate buffer to irradiated microtitre plates and the filtration of serum samples, that makes irrelevant the binding to the uncoated wells. IgG F(ab)2fragments from IgG positive sera were shown bind β2GPI, providing that the binding was a specific antibody binding, mediated by the antigen binding site of the antibody molecule; moreover the antibodies were not able to differentiate native and delipidated β2GPI coated plates, making a possible role of a phospholipid contaminant unlikely. On the other hand, the phosphorus content of native as well as delipitated β2GPI was undetectable. IgG, but not IgM, anti-β2GPI antibodies were classically inhibited by the addition of soluble β2GPI, while cardiolipin liposomes appear to modify the reaction in a completely different way, possibly by the described interaction between cardiolipin and β2GPI. The application of the new ELISA to the study of patients has shown that: (1) the presence of anti-β2GPI is statistically associated with the presence of aCL antibodies (P < 0.0001), (2) anti-β2GPI antibodies are related to the classical features of antiphospholipid syndrome (thrombosis: P < 0.001; fetal loss: P < 0.001) while, in this series of patients, aCl antibodies are not (thrombosis: P < 0.126; fetal loss: P < 0.061).