Background Beta-2 glycoprotein I (β2GPI) represents the major antigenic target for antiphospholipid antibodies (aPL), the hallmark of antiphospholipid syndrome (APS). β2GPI contains five homologous domains, with domain I (DI) being identified as the main antigenic epitope for pathogenic anti-β2GPI (aβ2GPI).
It has been reported that APS patients show an elevated amount of total β2GPI as compared with healthy donors and other autoimmune disease control groups. Moreover, in healthy individuals, β2GPI mainly exists in its biochemically reduced form whilst the post-translational oxidized structure (fish-hook linear configuration) is elevated in patients with APS.
Antibodies targeting β2GPI-DI (aDI) represent a key pathogenic sub-population of aPL, thus their detection may allow the identification of patients at highest clinical risk.
Objectives Our hypothesis is that the different redox state of β2GPI, from reduced (circular) to oxidised (linear), might be responsible for the exposure of DI, thus providing the antigenic drive for the aDI antibodies. However, no study has investigated the association between the circulating oxidized form of β2GPI and the presence of aDI in APS patients.
Methods Serum samples from 40 patients who fulfilled the classification criteria for APS were tested (34 primary and 6 secondary APS). A sandwich ELISA for quantifying total β2GPI levels within serum was performed for all samples. An in-house standard (consisting of pooled serum from 10 healthy controls) was used to obtain a standard curve in each ELISA. Biochemically reduced β2GPI was detected via labelling free thiols within serum proteins with a biotinylated free-thiol binding reagent and then, via coating on a streptavidin plate, detecting the presence of labelled β2GPI with an aβ2GPI antibody, based on published methods. Then samples were screened for IgG aDI, aβ2GPI and anti-cardiolipin (aCL).
Results The positivity for aDI, aβ2GPI and aCL IgG was 60%, 75% and 80% respectively. The relative proportion of reduced β2GPI was expressed as a percentage of that observed in the in house-standard after correction for the total amount of β2GPI. A significant negative correlation was observed between proportion of reduced β2GPI and aDI levels (Spearman coefficient r = -0.409, p=0.008) and aCL titre (r = -0.317, p=0.045). There was no significant correlation between proportion of reduced β2GPI and aβ2GPI (r = -0.059, p=0.713).
Conclusions The strong association between the oxidative state of β2GPI and aDI positivity supports the hypothesis that modulation of the redox state of β2GPI affects the production of antibodies against the DI epitope, which become exposed in the oxidised protein. However, it remains to be proven if the exposure of DI within the oxidised (hence linearized) form of β2GPI acts as the antigenic drive for development of aDI autoantibodies or whether the presence of these antibodies stabilizes the antigen-antibody complex maintaining the linear configuration of the molecule.
Disclosure of Interest None declared