Hairy Platelet-Derived Extracellular Vesicles Promote Lung Vaso-Occlusion in Sickle Cell Disease

Background: Acute chest syndrome (ACS) is a type of acute lung injury and the leading cause of mortality in Sickle Cell Disease (SCD). Current treatments for ACS are primarily supportive, and there is a critical need for rescue therapies. ACS is often a sequela of acute systemic vaso-occlusive crisis and preceded by thrombocytopenia. However, the role of platelets in the pathogenesis of ACS remains largely unknown.

Methods: We used our validated model of vaso-occlusive crisis in transgenic, humanized SCD mice, which is triggered by intravenous challenge with nanogram levels of the TLR4 ligand, lipopolysaccharide (LPS). Platelet-neutrophil aggregates and blood flow in the lung microcirculation was visualized in real time in vivo, using multi-photon-excitation microscopy of intact lung in live SCD mice. SCD or control human blood was perfused through microfluidic channels in vitro and neutrophil-platelet aggregation was visualized using fluorescence microscopy. Platelet derived extracellular vesicles were characterized using nanoparticle tracking and biochemical approaches.

Results: We have made a novel finding that the arrest of blood flow and injury in the lung is secondary to blockade of pulmonary arterioles by platelet-neutrophil aggregates. Using in vitro microfluidic studies, we confirmed that platelet-neutrophil aggregation is higher in LPS-treated SCD patient blood compared with healthy controls, and this correlates with increased numbers of platelet-derived extracellular vesicles (EVs) that express IL-1β. Our studies also reveal that platelet-neutrophil aggregation in pulmonary arterioles of SCD mice is associated with an increase in peripheral blood levels of platelet-derived EVs containing IL-1β. Remarkably, inhibition of TLR4 or TLR4/NLRP3-inflammasome activated caspase-1, or inhibition of IL-1β signaling, attenuated release of platelet EVs and platelet-neutrophil aggregation in the lung arterioles of SCD mice in vivo and SCD human blood in vitro .

Conclusions: TLR4 and NLRP3-inflammasome-mediated caspase-1 activation in platelets during vaso-occlusive crisis leads to release of IL-1β-containing EVs into the circulation. These circulating platelet EVs promote platelet-neutrophil aggregation in pulmonary arterioles, which results in arrest of blood flow in the lung, leading to ACS. Therapeutic inhibition of TLR4/NLRP3-caspase-1 signaling in platelets or IL-1β signaling is a potential therapy for ACS in SCD patients.

Acknowledgments: This study was supported by 1R01HL128297-01 (P.S.) and VMI startup funds (P.S.).