Effects of Antiphospholipid Antibodies on In Vitro Platelet Aggregation

Antiphospholipid antibodies contribute to the development of thrombosis, although precise mechanisms remain to be elucidated. We determined the effects of affinity-purified anti-beta(2)-glycoprotein 1 (anti-beta(2)GP1) and anti-prothrombin (anti-PT) antibodies on in vitro platelet aggregation. Adenosine diphosphate (ADP) and collagen-induced platelet aggregation were performed using platelet-rich plasma ([PRP] 250 x 10(9)/L). Antiphospholipid antibodies (1.25-10 mu g/mL) were preincubated with PRP for 10 minutes at 37 degrees C prior to the addition of agonist. Anti-beta(2)GP1 antibodies significantly reduced platelet aggregation (percentage area under the curve; %AUC) in a concentration-dependent manner using both 5 mu mol/L (P < .001) and 2.5 mu mol/L (P = .038) ADP but did not significantly affect the rate of aggregation. Anti-PT antibodies significantly enhanced 5 A mu g/mL collagen-induced platelet aggregation (%AUC; P = .034) but did not affect ADP-induced platelet aggregation. These results suggest (1) interactions and effects of antiphospholipid antibodies on platelets are agonist and concentration dependent and (2) anti-beta(2)GP1 antibodies may inhibit dense granule release and/or inhibition of the arachidonic acid pathway.