Endothelial Extracellular Vesicles Regulate Platelet Purinergic Signaling in Sickle Cell Disease

Introduction. Acute chest syndrome (ACS) is a type of acute lung injury and the leading cause of mortality among sickle cell disease (SCD) patients. ACS is often preceded by thrombocytopenia and involves massive thrombosis across pulmonary artery branches. Although, released during hemolysis, adenosine diphosphate (ADP) is known to activate platelets by stimulating their P2Y1 and P2Y12 purinergic receptors, antagonists of P2Y12 have not shown any benefit in ACS therapy, justifying the need for better understanding of purinergic signaling in SCD. CD39 maintains ADP homeostasis by degrading excessive ADP. Though CD39 inhibits ADP-dependent platelet activation, its role in the pathophysiology of SCD is still unidentified.

Methods. To evaluate SCD platelet response to ADP in vivo we used a state-of-the-art intravital lung microscopy and a novel in vivo model of ADP-triggered thrombocytopenia in transgenic humanized SCD mice. Additionally, both mouse and human SCD platelet ADP-dependent aggregation was examined using in vitro turbidimetric aggregation assay. Hemin, a major host-derived damage associated molecular pattern associated with SCD, was incubated with Human Lung Micro-Vascular Endothelial Cells (HMVEC-L). Extracellular vesicles (EVs) were obtained from mouse and human plasma samples, and cell culture medium samples using size exclusion chromatography. Obtained EVs were subjected to nanoparticle tracking analysis. EVs CD39 levels and activity were determined using ELISA, western blot and malachite green phosphate assays, respectively.

Results. Intravital lung microscopy and in vivo thrombocytopenia studies revealed that intravascular administration of ADP triggered acute pulmonary thrombosis in control but not in SCD mice. In vitro aggregation study confirmed our in vivo findings and demonstrated impaired SCD mouse and human platelet response to ADP, which was, further, significantly augmented by a specific CD39 inhibitor. Hemin triggered shedding of CD39-bearing EVs by HMVEC-L. Indeed, we discovered that isolated from mouse and human plasma endothelial cell-derived SCD EVs expressed higher CD39 levels and activity in comparison to control EVs.

Conclusions. Our findings suggest that CD39-bearing EVs prevent ADP-mediated platelet aggregation and pulmonary thrombosis in SCD. Current study explains why P2Y12 blockers are not effective in SCD therapy.

Jackson:Deleon Biosciences: Current equity holder in private company, Membership on an entity's Board of Directors or advisory committees, Other: Founder with equity interest; management position; intellectual property rights (e.g., royalties, patents, copyrights) assigned to the University of Pittsburgh and optioned to Deleon Biosciences, Inc., Patents & Royalties. Sundd:CSL Behring Inc: Research Funding; Novartis corporation: Research Funding; Bayer Hemophilia Award Program: Research Funding; IHP Therapeutics: Research Funding.