Proteolytic cleavage of β₂-glycoprotein I: Reduction of antigenicity and the structural relationship

Binding of β₂-glycoprotein I (β₂-GPI)-dependent anticardiolipin antibodies (aCL) derived from antiphospholipid syndrome (APS) is significantly reduced in aCL ELISA due to loss of the phospholipid (PL) binding property of β₂-GPI by plasmin treatment. In the present study, the treatment generated a nicked form of β₂-GPI and resulted in loss of antigenicity for the autoantibodies detected in ELISA, using an β₂-GPI directly adsorbed polyoxygenated carboxylated plate, the assay system of which was not related to PL binding. The nicked form bound to neither Cu²⁺-oxidized low-density lipoprotein (oxLDL) nor to β₂-GPI-specific lipid ligands isolated from oxLDL, the result being a complete loss of subsequent binding of anti-β₂-GPI autoantibodies. The conformational change in the nicked domain V was predicted from its intact structure determined by an X-ray analysis (implemented in Protein Data Bank: 1C1Z), molecular modeling and epitope mapping of a monoclonal anti-β₂-GPI antibody, i.e. Cof-18, which recognizes the related structure. The analysis revealed that novel hydrophobic and electrostatic interactions appeared in domain V after the cleavage, thereby affecting the PL binding of β₂-GPI. Such a conformational change may have important implications for exposure of cryptic epitopes located in the domains such as domain IV.