NETs induce persistent lung tissue damage via thromboinflammation without altering virus resolution in a mouse coronavirus model

Background: During Infection, neutrophil extracellular traps (NETs) are associated with severity of pulmonary diseases such as acute respiratory disease syndrome. NETs induce subsequent immune responses, are directly cytotoxic to pulmonary cells and highly procoagulant. Anticoagulation treatment was shown to reduce in-hospital mortality, indicating thromboinflammatory complications. However, little data is available on the involvement of NETs in secondary events after virus clearance, which can lead to persistent lung damage and post-acute sequelae with chronic fatigue and dyspnea.

Objectives: This study focuses on late phase events using a murine model of viral lung Infection with post-acute sequelae after virus resolution.

Methods: C57BL/6JRj mice were infected intranasally with the betacoronavirus Murine Coronavirus (MCoV, strain MHV-A95) and tissue samples were collected after two, four, and ten days. For NET modulation, mice were pretreated with OM-85 or GSK484 and DNase I were administered intraperitoneally between day 2-5 and day 4-7, respectively.

Results and conclusion: Rapid, platelet-attributed thrombus formation was followed by a second, late phase of thromboinflammation. This phase was characterized by negligible virus titers but pronounced tissue damage, Apoptosis, oxidative DNA damage, and presence of NETs. Inhibition of NETs during acute phase did not impact virus burden but decreased lung cell Apoptosis by 67% and oxidative stress by 94%. Prevention of neutrophil activation by immune training before virus Infection reduced damage by 75%, NETs by 31% and pulmonary thrombi by 93%. Neutrophil extracellular traps are detrimental inducers of tissue damage during respiratory virus Infection, but do not contribute to virus clearance.

Neutrophil Extracellular Traps; Oxidative Stress; Post-Acute COVID-19 Syndrome; Thromboinflammation; Thrombosis.