Anti-B2GP1 Antibodies have Variable Effects on Platelet Aggregation

Aim: Anti-B2GP1 antibodies are strongly associated with thrombosis in autoimmune conditions such as SLE and APS. The aims of this study were to compare the effects on platelet aggregation of 1) IgG fractions from normal controls and SLE patients, and 2) rabbit anti-B2GP1 and anti-B2GP1 purified from patients with SLE. Methods: Whole blood was collected and processed to obtain platelet poor plasma (PPP) from normal controls (n=15) and SLE patients (n=15). Using PPP, anti-B2GP1 titres were determined using an ELISA and IgG fractions isolated using a HiTrap protein G column. Anti-B2GP1 was purified from two SLE patients with high titres using purified B2GP1 coupled to a HiTrap NHS-activated HP column. The effect of IgG fractions (0-100ug/mL) from controls and patients, as well as rabbit and human derived anti-B2GP1 (0-100ug/mL), on ADP (2.5 and 5uM) induced platelet aggregation were investigated using light transmission aggregometry. Results: IgG fractions from SLE patients significantly increased 2.5uM, but not 5uM, ADP-induced platelet aggregation [%Max (p=0.006), %AUC (p=0.015), slope (p=0.048)] in a dose dependent manner. IgG fractions from normal controls did not affect platelet aggregation. Rabbit anti-B2GP1 significantly inhibited 5uM ADP-induced platelet aggregation [%Max (p=0.028), %AUC (p=0.014), slope (p<0.001)]. In contrast, anti-B2GP1 purified from SLE patients significantly enhanced the %AUC (p=0.004), but inhibited the slope (p<0.05) of 5uM ADP-induced platelet aggregation. Conclusion: IgG fractions and anti-B2GP1 from SLE patients enhanced ADP-induced platelet aggregation, reflecting increased secondary aggregation. In contrast, rabbit anti-B2GP1 inhibited ADP-induced platelet aggregation, reflecting decreased primary and secondary aggregation. The disparity between rabbit and human anti-B2GP1 may be due to the heterogeneous nature of these antibodies, avidity or different antibody binding specificities between species. Alternatively two populations of anti-B2GP1 may exist.