Early Alveolar Epithelial Cell Necrosis is a Potential Driver of ARDS with COVID-19

ABSTRACT

BackgroundAcute respiratory distress syndrome (ARDS) with COVID-19 is aggravated by hyperinflammatory responses even after the peak of viral load has passed; however, its underlying mechanisms remain unclear. Alveolar epithelial injury is reported to be a very early event in ARDS with COVID-19. Herein, we assessed whether necrosis of alveolar epithelial cells and subsequent releases of damage associated molecular patterns (DAMPs) at an early disease stage aggravates ARDS with COVID-19 MethodsWe analyzed the levels of cytokeratin18-M65, an epithelial total cell death marker; CK18-M30, an epithelial apoptosis-specific marker; and HMGB-1, one of the DAMPs released from necrotic cells, in patients with COVID-19 with and without ARDS and healthy adults, in addition to the circulating alveolar epithelial and endothelial injury markers, namely sRAGE, angiopoietin-2, and surfactant protein-D. Molecular mechanisms of alveolar epithelial cell death and effects of neutralization on alveolar tissue injury were assessed using a mouse model mimicking COVID-19-induced ARDS. ResultsCOVID-19-induced ARDS was characterized by the elevation of sRAGE, an epithelial injury marker, at a very early disease stage. Although both serum levels of CK18-M65 and CK18-M30 were elevated in COVID-19-induced ARDS, the median CK18-M30/M65 ratio, an indicator of the fraction of apoptosis among total epithelial cell death, was 31.5% in serum from COVID-19 patients with ARDS, a value significantly lower than that of non-ARDS patients or healthy subjects. Moreover, the median M30/M65 ratio in bronchoalveolar lavage fluid (BALF) in COVID-19-induced ARDS was 27.8%, indicating that alveolar epithelial cell death is mainly caused by necrosis. Serum levels of HMGB-1 were also significantly elevated in ARDS versus non-ARDS patients. In a mouse model mimicking COVID-19-induced ARDS, the ratio of CK18-M30 to a total epithelial cell death marker in BALF was also lower than that in control subjects. Moreover, the alveolar epithelial cell necrosis involved two forms of programmed necrosis necroptosis and pyroptosis. Finally, neutralization of HMGB-1 attenuated alveolar tissue injury in the mouse model. ConclusionsNecrosis, including necroptosis and pyroptosis, seems to be the primary form of alveolar epithelial cell death and subsequent release of DAMPs is a potential driver of COVID-19-induced ARDS.