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

Antiphospholipid syndrome (APS) is characterized by thrombosis, recurrent miscarriages and the stable presence of antiphospholipid antibodies (aPL). Circulating aPL 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 pro-coagulant and pro-inflammatory extracellular microvesicles (EMV). This study analyzed the presence of EMV 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 more elevated in APS patients than in healthy donors. Moreover, we observed the presence of the main autoantigenic targets, β2-GPI and cardiolipin, on the surface of EMV from APS patients' plasma, by both biochemical and flow cytometry analysis. It also revealed significantly higher cardiolipin levels on EMV from patients with obstetrical APS compared to those without pregnancy morbidity, specifically in women with a history of fetal death. To analyze the effect of the EMV from APS patients on endothelial cell activation and signaling, we then incubated in vitro human microvascular endothelial cells with patient-derived EMV, demonstrating a significant phosphorylation of IRAK, NF-κB, as well as increased expression and release of tissue factor. These findings suggest that circulating EMV 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 EMV may reflect disease features, but also actively participate in the pathogenesis, by triggering intracellular signaling pathways that sustain vascular inflammation and thrombosis.