In vivo inhibition of antiphospholipid antibody-induced pathogenicity utilizing the antigenic target peptide domain I of β₂-glycoprotein I: Proof of concept

Objectives: In the antiphospholipid syndrome (APS), the immunodominant epitope for the majority of circulating pathogenic antiphospholipid antibodies (aPLs) is the N-terminal domain I (DI) of β₂- glycoprotein I. We have previously shown that recombinant DI inhibits the binding of aPLs in fluid phase to immobilized native antigen, and that this inhibition is greater with the DI(D8S/D9G) mutant and absent with the DI(R39S) mutant. Hence, we hypothesized that DI and DI(D8S/D9G) would inhibit aPL-induced pathogenicity in vivo. Methods: C57BL/6 mice (n = 5, each group) were injected with purified IgG derived from APS patients (IgG-APS, 500 μg) or IgG from normal healthy serum (IgG-NHS) and either recombinant DI, DI(R39S), DI(D8S/ D9G), or an irrelevant control peptide (at 10-40 μg). Outcome variables measured were femoral vein thrombus dynamics in treated and control groups following standardized vessel injury, expression of vascular cell adhesion molecule-1 (VCAM-1) on the aortic endothelial surface, and tissue factor (TF) activity in murine macrophages. Results: IgG-APS significantly increased thrombus size as compared with IgG-NHS. The IgG-APS thrombus enhancement effect was abolished in mice pretreated with recombinant DI (P ≤ 0.0001) and DI(D8S/D9G) (P ≤ 0.0001), but not in those treated with DI(R39S) or control peptide. This inhibitory effect by DI was dose-dependent, and at lower doses DI(D8S/ D9G) was a more potent inhibitor of thrombosis than wild-type DI (P ≤ 0.01). DI also inhibited IgG-APS induction of VCAM-1 on the aortic endothelial surface and TF production by murine macrophages. Conclusion: Our findings in this proof-of-concept study support the development of recombinant DI or the novel variant DI(D8S/D9G) as a potential future therapeutic agent for APS.