Mechanisms sensitive to different drugs mediate a proadhesive and pro-thrombotic platelet phenotype in...

Anti-phospholipid antibodies (aPL) mediate platelet- and leukocyte-interaction with damaged endothelium, contributing to anti-phospholipid 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 34 primary APS (PAPS) patients and 12 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 flowcytometry platelet activation markers (P-selectin, activated GPIIbIIIa [aGPIIbIII], tissue factor [TF], ApoER2 and β2GPI expression and platelet-monocyte 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, immunoglobulin (Ig)G upregulated platelet TF expression only, the addition of interleukin (IL)-6 also induced P-selectin and aGPIIbIIIa upregulation. An IL-6 receptor-blocking monoclonal antibody prevented the pro-adhesive/-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 platelet-leukocyte aggregates mediated by β2GPI-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 platelet-mediated thrombosis.