OC 16.4 - Domain and residue mapping reveal unexpected similarities and differences between IgG, IgM, and IgA anti-B2GPI antibodies

Background: Anti-B2-glycoprotein I (B2GPI) antibodies are often found in Antiphospholipid Syndrome (APS) patients, the most commonly acquired thrombophilia. B2GPI is an elongated protein comprising five domains: DI, DII, DIII, DIV, and DV. Previous studies have documented that DI is the dominant epitope of IgG in thrombotic APS patients. However, there is less information about other isotypes and about the diversity of autoantibodies in relation to DI residue specificity.

Aims: To establish domain and residue specificity of IgG, IgM, and IgA anti-B2GPI antibodies and clinical correlates.

Methods: Levels of IgG, IgM, and IgA were measured in 32 APS patients and 8 healthy controls using in-house ELISAs with immobilized plasma purified and recombinant B2GPI. Domain and residue mapping studies were performed using five new deleted domain variants and nine DI variants.

Results: IgG is the most prevalent isotype, followed by IgA and IgM. The IgG and IgA isotypes exhibit a pronounced preference for DI, while the IgM isotype demonstrates lower specificity. While less frequent, we also observed a significant reduction in binding when DII and, especially, DV were removed, indicating specificity beyond DI and cross-reactivity between domains. Finally, we found that anti-DI antibodies are diverse, as they recognize conformational epitopes within DI that are closely related but physically distinct: one located north and one south of R43. We confirmed that anti-DI antibodies are prevalent in patients with thrombosis. We found no obvious clinical correlations with newly discovered epitopes. However, our cohort of APS patients is small. New studies should be designed for this purpose.

Conclusion(s): This study identifies striking similarities between IgG and IgA isotypes and establishes that IgM are less specific. This might help explain the dominant role of IgG in APS pathogenesis and suggests that IgA, when present, may play an important role, too.