Endothelial cell-derived plasminogen activator inhibitor-1 potentiates thrombosis in antiphospholipid syndrome

Plasminogen activator inhibitor-1 (PAI-1) plays a key role in modulating fibrinolysis. However, its role in the thrombotic autoimmune disease known as antiphospholipid syndrome (APS) remains understudied. We characterized PAI-1 expression by APS endothelial cells through single-cell RNA sequencing (scRNA-seq) of skin biopsies from APS patients with active livedo racemosa and age-matched controls. Total and active PAI-1 were quantified in plasma samples from 167 patients with APS, 37 antiphospholipid antibody-positive carriers without clinical APS, 18 patients with a history of non-APS thrombosis, and 48 healthy controls. In vitro, human microvascular endothelial cells (MVECs) were cultured with IgG isolated from patients with APS, and PAI-1 mRNA and protein levels were evaluated by quantitative PCR and ELISA, respectively. Finally, a mouse model of APS IgG-accelerated venous thrombosis induced by electrolytic injury of the inferior vena cava was used to determine the impact of the PAI-1 inhibitor MDI-2268 on thrombosis. scRNA-seq revealed significantly increased SERPINE1 (PAI-1) expression in endothelial cells from patients with APS. Compared with healthy controls, active PAI-1 was detected at significantly higher levels in the plasma of patients with thrombotic APS, where it showed a positive correlation with absolute neutrophil count, erythrocyte sedimentation rate, and C-reactive protein. APS patients with increased antibody burden, as determined by a higher adjusted global APS score (aGAPSS ≥7 versus <7), had more active PAI-1 in their plasma. In vitro, APS IgG stimulation of MVECs increased PAI-1 transcription and secretion, which were dependent on NF-κB signaling. In mice, the administration of APS IgG significantly increased thrombus weights, which was mitigated by MDI-2268. Together, these findings suggest that endothelial cells are a likely source of increased active PAI-1 in APS. Furthermore, blocking PAI-1 activity effectively reduced APS IgG-accelerated thrombosis in mice, suggesting that PAI-1 inhibition has therapeutic potential in APS. • The active form of PAI-1 is associated with smoldering inflammation in APS. • Patient profiling suggests endothelial cells are a key source of PAI-1 in APS. • Patient IgG induces PAI-1 expression in endothelial cells via NF-κB signaling. • Inhibiting PAI-1 alleviates thrombosis in a model of APS-accelerated thrombosis.