Antibodies to beta 2 glycoprotein I: standardized immunoassay and a reference interval for healthy controls.

Patients with antiphospholipid syndrome (APS) demonstrate an antibody reactivity with beta 2 glycoprotein I (beta 2 GPI) independent of the anionic phospholipids that previously had been believed to be the relevant autoantigens associated with this syndrome. Immunoassays for IgG anti-beta 2 GPI have, however, suffered from a lack of standardization. In this article, we describe an assay based on reference standards that we developed recently. The assay exhibits excellent linearity, with regard to both application of the standards and dilution of out-of-range specimens. Precision was assessed both within the between run and was judged to be satisfactory. A reference interval was developed on the basis of a control group consisting of 111 men and 93 women, yielding a range of 0-19 standard IgG anti-beta 2 GPI units (SGU) for the ninety-fifth percentile of values. These data suggest that this standardized anti-beta 2 GPI assay may be useful in the clinical diagnostic laboratory.

Binding of beta 2 glycoprotein I to activated polystyrene and its recognition by human IgG autoantibodies.

Beta 2 glycoprotein I (apolipoprotein H, beta 2GPI) is involved in the formation of epitope(s) recognized by clinically relevant autoantibodies from patients with antiphospholipid syndrome. We studied the binding of beta 2GPI to chemically activated polystyrene in a microtitre plate format. Adsorption isotherms (at 37 degrees C) were generated for beta 2GPI on activated polystyrene and on unactivated polystyrene, with both human serum antibodies and rabbit polyclonal IgG antibodies as probes, and horseradish peroxidase (HRP)-tagged anti-IgG to detect binding. Additionally, beta 2GPI was biotinylated and isotherms were developed by using HRP-streptavidin as the recognition sequence. Human serum autoantibodies, which did not precipitate beta 2GPI in solution, yielded a characteristic chemisorption isotherm on activated polystyrene but did not recognize beta 2GPI bound to untreated polystyrene. The rabbit IgG, which did precipitate beta 2GPI in solution, detected beta 2GPI bound to both activated polystyrene and, to a lesser extent, to untreated polystyrene. The binding of beta 2GPI to untreated polystyrene was confirmed by the use of biotinylated beta 2GPI. To assess the prevalence of IgG anti-beta 2GPI autoantibodies, we surveyed 113 sera submitted to our laboratory for anticardiolipin antibody (aCL) testing. Only nine (8%) had anti-beta 2GPI activity greater than two standard deviations above the mean for those sera in which aCL activity was within normal limits. We conclude that epitope presentation of beta 2GPI for human autoantibody binding is dependent on surface properties of the polystyrene, and that beta 2GPI autoantibodies are found only in a subpopulation of sera positive for aCL.