Immunologic characterization and functional properties of murine antibodies raised against deleted mutants of human β₂-glycoprotein I

β₂-Glycoprotein I (β₂GPI) is a 50 kDa molecule proposed as a principal target of 'autoimmune' antiphospholipid antibodies (aPL). We have used deleted mutants (DM) representing different domains of β₂GPI (I-IV, IV-V and V) for immunization of naive mice and studied the characteristics of the respective murine IgG preparations in comparison with affinity-purified IgG from two patients with primary antiphospholipid syndrome. Immunization with β₂GPI and with the DM produced anti-β₂GPI antibodies, part of which reacted with negatively charged phospholipids (PL), whereas reactivity with cardiolipin was evident only in the IgG from mice immunized with β₂GPI. These results are consistent with the presumption that aPL are induced following the in vivo association of β₂GPI (used for immunization) with resident negatively charged PL. Accordingly, DM which either lack the PL binding site or aPL attachment locus did not elicit, upon immunization, antibodies reactive with PL. Further, murine anti-β₂GPI IgG and human 'autoimmune' aPL were similar, albeit not identical, in terms of DM requirement for PL binding and charge dependency. Murine antibodies and human aPL, regardless of their binding characteristics, were found to bind significantly to platelets upon their activation with thrombin and to promote platelet activation. The results of the current study emphasize the dissimilarities between human 'autoimmune' aPL and murine anti-β₂GPI. Thus, anti-β₂GPI antibodies to different DM as well as human aPL are capable of binding and activating human platelets provided β₂GPI is present.