Oxidation of beta₂-glycoprotein I (beta₂GPI) by the hydroxyl radical alters phospholipid binding and modulates recognition by anti-beta₂GPI autoantibodies

We investigated whether β₂GPI, the key antigen in the antiphospholipid syndrome, is susceptible to oxidative modifications by the hydroxyl radical (°OH) that may influence its lipid-binding and antigenic properties. We compared the effects on human and bovine β₂GPI of °OH free radicals generated by γ -radiolysis of water with ¹³⁷Cs and by the Fenton system composed of Fe-EDTA, ascorbate and H₂O₂. Radiolytic °OH caused a dose-dependent loss of tryptophan, production of dityrosine and carbonyl groups, dimerization and/or extensive aggregation of β₂GPI. It ensued a reduction in affinity binding to cardiolipin liposomes and loss of β₂GPI-dependent autoantibody binding to immobilized cardiolipin. Patient anti-β₂GPI antibodies segregated into two groups based on the effect in the β₂GPI-ELISA of β₂GPI pretreatment with radiolytic °OH : enhancement or suppression of IgG binding in groups A (common type) and B (rare subset), respectively. The avidities of group A antibodies for fluid-phase β₂GPI were low but increased in a dose-dependent manner upon β₂GPI irradiation, in relation to protein crosslinking. Distinguishing features of group B antibodies included higher avidities for fluid-phase β₂GPI that was no longer recognized after °OH treatment, and negative anticardiolipin tests suggesting epitope location near the phospholipid binding site. The °OH scavengers thiourea and mannitol efficiently protected against all above changes. In contrast, the Fenton system induced no major alteration in the structure and functions of β₂GPI.

Thus, oxidative modifications of β₂GPI via °OH attack of susceptible amino acids alters phospholipid binding, and modulates recognition by autoantibodies depending on their epitope specificities. These findings may be of clinical relevance for the generation and/or reactivity of anti-β₂GPI antibodies.