Anti-β2GP1 antibodies have variable effects on platelet aggregation

Aim: To investigate the effects of affinity-purified rabbit anti-beta 2GP1, and anti-beta 2GP1 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 (n = 15) and SLE patients (n = 15). Using PPP, anti-beta 2GP1 titres were determined using an ELISA and IgG fractions isolated using a HiTrap protein G column. Anti-beta 2GP1 was purified from two SLE patients using purified beta 2GP1 coupled to a HiTrap NHS-activated HP column. Results: The effect of rabbit and human derived anti-beta 2GP1 (0-100 mu g/mL), on ADP (2.5, 5 mu M) induced platelet aggregation were investigated using light transmission aggregometry. Rabbit anti-beta 2GP1 significantly inhibited all parameters of 5 mu M ADP-induced platelet aggregation; %Max (p=0.028), % AUC (p=0.014) and slope (p<0.001). In contrast, anti-beta 2GP1 purified from SLE patients significantly enhanced the %Max (p=0.031) and % AUC (p=0.007) in a concentration dependent manner, but inhibited the slope (p<0.05) of 5 mu M ADP-induced platelet aggregation. Conclusion: Our data suggest anti-beta 2GP1 purified from different species have variable effects on in vitro platelet aggregation. The disparity between rabbit and human anti-beta 2GP1 may be due to the heterogeneous nature of anti-beta 2GP1, varying avidity or different antibody binding specificities between species.