Thrombotic vs obstetric antiphospholipid syndrome: a role for constitutive endothelial dysfunction

Background Antiphospholipid syndrome (APS) is a thrombotic autoimmune disease defined by the onset of thrombotic (T-APS) or obstetric (O-APS) manifestations and the presence of antiphospholipid antibodies (aPLs). Endothelial dysfunction (ED), resulting from the interaction between aPLs and endothelial cells, contributes to the clinical manifestations of APS. However, it is not yet known whether constitutive ED may further predispose the host to disease onset. In this context, patient-specific endothelial colony-forming cells (ECFCs) are a valuable tool to investigate ED in APS patients. Aim To assess constitutive ED in primary T-APS and O-APS patients and identify the pathways involved. Methods ECFCs were isolated from T-APS and O-APS patients and healthy donors (HD). The presence of ED was investigated by assessing: ECFC senescence rate; ECFC expression of pro- and anticoagulant molecules by flow-cytometry; ECFC ability to promote thrombosis by thrombin generation assay (TGA). The study was approved by the medical ethics committee; patient informed consent was obtained (ICH424/22, VA_110/2022). Results A reduction in the frequency of subjects giving rise to ECFC colonies was observed in O-APS compared to HD. ECFC early senescence rates were comparable in all groups. The presence of constitutive ED was observed in both T-APS and O-APS ECFCs, as assessed as higher expression of the pro-coagulant molecule tissue factor than HD-ECFCs. T-APS ECFCs also showed lower levels of the anti-coagulant molecules thrombomodulin and endothelial cell protein C receptor (EPCR). Moreover, T-APS ECFCs promoted faster thrombin generation compared to both O-APS and HD. Conclusions Our results suggest that both T-APS and O-APS patients exhibit constitutive ED. Further analysis are underway to elucidate the differences in the underlying mechanisms that may contribute to the different pathological manifestations observed in T-APS and O-APS patients. Acknowledgements This research was funded by Fondazione Cariplo (2020-3606); R.C. received a fellowship from the PhD programme in Experimental Medicine (University of Milan).