Characteristics and pathogenic role of anti-β2-glycoprotein I single-chain Fv domains: Induction of experimental antiphospholipid syndrome

Antiphospholipid syndrome is characterized by the presence of high titers of anti-β₂-glycoprotein I (β₂GPI) antibodies, lupus anticoagulant associated with thromboembolic phenomena, thrombocytopenia and recurrent fetal loss. Single-chain Fv (scFv) were prepared from four anti-β₂GPI mAb, CAM, CAL, CAR and 2C4C2, and one anti-ssDNA. All five scFv showed the same antigen binding properties as the original mAb. Replacement of the pathogenic CAM V(H) domain with the non-pathogenic CAL V(H) or anti-ssDNA V(H) decreased the binding affinity of the scFv to β₂GPI and completely abrogated the anticoagulant activity. Exchanging the CAM V(H) with anti-DNA V(H) resulted in a shift from anti-β₂GPI to anti-ssDNA binding of the scFv. Replacement of the CAM V(L) with CAL V(L) did not affect the binding and activity. BALB/c mice were immunized with the anti-β₂GPI scFv, and the scFv resulting from the substitution of the heavy (H) and light (L) chains. The mice which were immunized with CAM, 2C4C2 and CAR scFv developed clinical manifestations of experimental anti-phospholipid syndrome. Elevated titers of mouse anti-cardiolipin (aCL), anti-β₂GPI, associated with lupus anticoagulant activity, thrombocytopenia, prolonged activated partial thromboplastin time and a high percentage of fetal resorptions were detected, in the CAM scFv group and in the scFv composed of CAM V(H) groups. High titers of aCL, anti-β₂GPI, anti-ss/dsDNA and anti-histone associated with lupus findings were observed in the sera of the 2C4C2 scFv-immunized mice. Immunization with CAL scFv did not lead to any clinical findings. The current study shows that scFv of pathogenic antibodies are capable of inducing the same clinical manifestations as the whole antibody molecule upon active immunization. Replacement of H/L chains point to the importance of the V(H) domains in the pathogenic potential of anti-β₂GPI.