Extracellular microvesicles from patients with antiphospholipid syndrome carry antigenic targets and promote endothelial cell activation <i>in vitro</i>

Antiphospholipid syndrome (APS) is characterised by thrombosis, recurrent miscarriages and the stable presence of antiphospholipid antibodies (aPLs). Circulating aPLs can activate molecular mechanisms that may promote the activation of various cell types, particularly endothelial cells. Recently, endothelial activation has been associated with the release of procoagulant and proinflammatory extracellular microvesicles (EMVs). This study analysed the presence of EMVs in the plasma of APS patients, their correlation with clinical manifestations, and their role in carrying antigenic targets and promoting endothelial cell activation. Nanosight analysis revealed that EMV concentrations were significantly elevated in APS patients than in healthy donors. Moreover, we observed the presence of the main autoantigenic targets, β2-GPI and CL, on the surface of EMVs from APS patients' plasma, by both biochemical and flow cytometry analysis. It also revealed significantly higher CL levels on EMVs from patients with obstetrical APS compared to those without pregnancy morbidity, specifically in women with a history of foetal death. To analyze the effect of the EMVs from APS patients on endothelial cell activation and signaling, we then incubated in vitro human microvascular endothelial cells (HMEC-1) with patient-derived EMVs, demonstrating a significant phosphorylation of IRAK, NF-κB, as well as increased expression and release of TF. These findings suggest that circulating EMVs may act as platforms for aPL binding and propagation of pathogenic immune complexes, introducing a new task to explain the immunoreactivity of the main antigenic targets of APS patients. Moreover, the presence of EMVs may reflect disease features, but also actively participate in the pathogenesis, by triggering intracellular signaling pathways that sustain vascular inflammation and thrombosis.