Anti-β2GP1 antibodies have variable effects on platelet aggregation

Aim: To investigate the effects of affinity-purified rabbit antib2GP1, and anti-b2GP1 purified from patients with systemic lupus erythematosus (SLE), on adenosine diphosphate(ADP)-induced aggregation. Methods: Whole blood was collected and processed to obtain platelet poor plasma PPP) from normal controls(n¼15) and SLE patients (n¼15). 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 using purified b2GP1 coupled to a HiTrap NHS-activated HP column. Results: The effect of rabbit and human derived antib2GP1(0–100mg/mL), on ADP(2.5, 5mM) induced platelet aggregation were investigated using light transmission aggregometry. Rabbit anti-b2GP1 significantly inhibited all parameters of 5mM ADP-induced platelet aggregation; %Max ( p¼0.028), %AUC ( p¼0.014) and slope ( p<0.001). In contrast, anti-b2GP1 purified from SLE patients significantly enhanced the %Max ( p¼0.031)and %AUC ( p¼0.007) in a concentration dependent manner,but inhibited the slope ( p<0.05) of 5mM ADP-induced platelet aggregation. Conclusion: Our data suggest anti-b2GP1 purified from different species have variable effects on in vitro platelet aggregation. The disparity between rabbit and human anti-b2GP1 may be due to the heterogeneous nature of anti-b2GP1,varying avidity or different antibody binding specificities between species.