Both kinetic data and epitope mapping provide clues for understanding the anti-coagulant effect of five murine monoclonal antibodies to human β₂- glycoprotein I

The interaction between five murine monoclonal antibodies (mAb) and β₂-glycoprotein I (β₂GPI) in the absence of phospholipids was studied using surface plasmon resonance-based biosensor technology. Two separate epitope regions were confirmed for the five mAb but epitopes of two mAb were shown to be overlapping but not identical. The characteristics of binding on both immobilized β₂GPI, using different chemistries of coupling to a dextran matrix and antibody surfaces prepared by two strategies of immobilization, were compared. Binding was strongly influenced by the orientation of the immobilized partner, and the five mAb showed heterogeneity in their binding to immobilized and soluble β₂GPI. The observed stoichiometries of mAb-β₂GPI complexes and the detailed analysis of the kinetics of the association and dissociation phases of the interactions with soluble and immobilized β₂GPI revealed differences in the dissociation rate constants, resulting in a 10-fold higher affinity for immobilized β₂GPI compared to soluble β₂GPI for four out of five mAb. This suggests bivalent binding of these mAb to immobilized β₂GPI. In addition, the kinetic data helped explain the differing anti-coagulant properties of these mAb.