Standardization of Immunoassays for Antiphospholipid Antibodies with β2GPI and Role of Other Phospholipid Cofactors

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Abstract
Presence of β2 Glycoprotein I (β2GPI), in addition to phos-pholipids, is an absolute requirement for binding APA. This binding is frequently observed with β2GPI coated alone, however many APA react only with β2GPI complexed to phospholipids, but not with phospholipids alone. We demonstrate that a subgroup of rabbit polyclonal antibodies to human β2GPI binds to this protein only when it is coated on a solid surface, but not if it is in solution. In addition, β2GPI present in goat serum is strongly fixed by the coated phospholipids and the complexes formed bind as well APA as the rabbit antibodies to β2GPI. The diluent used for testing APA, has a strong incidence on APA’s reactivity as it can be a source of β2GPI. Antibody binding to β2GPI, Proth-rombin, Protein S, and Annexin V, coated in the presence or in the absence of phospholipids, was tested in 55 patients with the antiphospholipid syndrome. The strongest binding of antibodies was observed in 39 plasma to a mixture of phospholipids and purified human β2GPI, however 17 samples also presented a significant reactivity to β2GPI alone. Nine plasmas contained antibodies to Prothrombin, 4 to Protein S, 3 to Annexin V, and 1 to Protein C. We conclude that most of the APA are directed to a complex of β2GPI and phospholipids although in some patients antibodies to β2GPI alone or to other phosphoslipid binding proteins are present.

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