Mechanisms sensitive to different drugs mediate a pro-adhesive and pro-thrombotic platelet phenotype in anti-phospholipid syndrome.

Antiphospholipid antibodies (aPL) mediate platelet- and leukocyte-interaction with damaged endothelium, contributing to antiphospholipid syndrome (APS) vasculopathy. This study aimed to understand the mechanisms sustaining the pro-adhesive/-thrombotic platelet phenotype and the in vitro effects of different drugs. We included thirty-four primary APS (PAPS) patients and twelve healthy subjects (HS). All patients had medium/high aPL levels with vascular/obstetric symptoms according to the 2023 ACR/EULAR classification. In vivo, we evaluated by flow-cytometry platelet activation markers [P-selectin, activated GPIIbIIIa (aGPIIbIII)], Tissue Factor (TF), ApoER2 and β2GPI expression and plateletmonocyte and-granulocyte aggregates (PMA and PGA)]. In vitro, the impact of antiplatelet and anti-inflammatory drugs on platelet activation induced by different aPL subpopulations was investigated. PAPS patients exhibited greater percentages of circulating ApoER2pos-, P-selectinpos-, aGPIIbIIIapos-, TFpos-platelets, and TFpos-platelet-leukocyte aggregates. In vitro, HS blood incubation with PAPS plasma fully reproduced the activation found in vivo. While anti-β2GPI-Domain(D)1, but not anti- D4,5, IgG upregulated platelet TF expression only, the addition of IL-6 also induced P-Selectin and aGPIIbIIIa upregulation. An IL-6 receptor-blocking monoclonal antibody prevented the proadhesive/- coagulant platelet phenotype and the formation of platelet-leukocyte aggregates mediated by PAPS plasma or by total IgG plus exogenous IL-6. While aspirin and P2Y12 inhibitor fully inhibited platelet activation, hydroxychloroquine (HCQ) did not blunt TF expression. PAPS patients exhibit circulating pro-adhesive/-coagulant (TF-positive) platelets and plateletleukocyte aggregates mediated by a2GPI-D1-dependent IgG and an inflammatory trigger. While Aspirin and P2Y12 inhibitor significantly inhibited the aPL-mediated P-Selectin and TF upregulation, HCQ affected the adhesion phenotype only, and might not be adequate to prevent plateletmediated thrombosis.