Platelet activation and turnover in the primary antiphospholipid syndrome

Thromboembolism is a common occurrence in patients with the antiphospholipid syndrome (APS). The mechanism responsible for this phenomenon is unclear; there are several theories. One possibility is that a pathogenic interaction exists between antiphospholipid antibodies and platelets, leading to their activation. This study examined the expression of the platelet activation markers CD62 and CD63 by flow cytometry in 20 patients with the primary antiphospholipid syndrome (PAPS). Levels of soluble P-selectin were also assayed. Reticulated platelets were measured as an indicator of increased platelet production and/or turnover. Median CD63 expression was significantly increased in patients (14.3%) compared to a group of healthy controls (10.1%, P = 0.0008). There was no significant difference in median CD62 expression or percent reticulated platelets between the two groups. The median level of soluble P-selectin was significantly higher in PAPS patients (35.5 ng/ml) compared to controls (18.8 ng/ml, P = 0.0028). Patients receiving aspirin had lower median CD63 values (13.1%) when compared to those patients who were not (18.0%, P = 0.023). However, aspirin therapy did not prevent significant platelet activation occurring in some individual patients. Our data suggest that although not excessive, there is a degree of increased platelet activation in some PAPS patients, which is not always suppressed by antiplatelet therapy with aspirin.