OC 68.4 - CD39-Bearing Extracellular Vesicles Constrain Platelet Purinergic Signaling-Dependent Pulmonary Thrombosis in Sickle Cell Disease

Background: 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. 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.

Aims: To assess CD-39-dependent platelet purinergic signaling in SCD.

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. Human Lung Microvascular Endothelial Cells (HMVEC-L) were treated with hemin. Extracellular vesicles (EVs) were obtained from plasma and cell culture samples using size exclusion chromatography and subjected to nanoparticle tracking analysis. 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 demonstrated impaired SCD mouse and human platelet response to ADP, which was significantly augmented by a CD39 inhibitor. Hemin triggered shedding of CD39-bearing EVs by HMVEC-L. Endothelial cell-derived SCD EVs, from mouse and human plasma, expressed higher CD39 levels and activity in comparison to control EVs.

Conclusion(s): 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.