Neutrophil Control of Immune Cell Recruitment and Acute Lung Injury in SARS-CoV-2-infected Mice

Neutrophils are postulated to contribute to the severity of acute lung injury induced by SARS-CoV-2 infection. Neutrophils play both protective and damaging roles in bacterial and viral pneumonia. The present study examined the hypotheses that neutrophils play multiple roles in the pathophysiology of SARS-CoV-2 infection and that granule exocytosis is a contributor to immune cell recruitment and lung injury. Neutrophils were depleted beginning one day prior to, or one day after, infection of 8-10 month old C57BL/6 mice with a mouse-adapted SARS-CoV-2. In separate experiments neutrophil exocytosis was inhibited by administration of TAT-SNAP-23 beginning 1 to 3 days after infection. Lung tissue, BALF, and blood were obtained on day 5. SARS-CoV-2 pneumonia was associated with recruitment of mature and immature neutrophils, activated macrophages, and activated dendritic cells, but reduced anti-inflammatory macrophages. Mature neutrophil depletion prior to infection did not prevent lung injury or alter the changes in lung macrophages and dendritic cells. Depletion starting one day after infection significantly reduced lung injury and recruitment of macrophages and dendritic cells. A significant increase in immature neutrophils occurred with mature neutrophil depletion. Inhibition of neutrophil exocytosis significantly reduced lung injury and impaired recruitment of mature and immature neutrophils without altering macrophage or dendritic cell recruitment. We conclude that the presence or absence of mature neutrophils during the initial post-infection period altered myeloid cell recruitment and determined the pathway of acute lung injury. Neutrophil exocytosis is an important contributor to acute lung injury induced by SARS-CoV-2 and is a potential therapeutic target.