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Background: Excessive inflammation is pathogenic in pneumonitis associated to severe COVID-19. Neutrophils are among the most abundantly present leukocytes in the inflammatory infiltrates and may form neutrophil extracellular traps (NETs) under the local influence of cytokines. NETs constitute a defence mechanism against bacteria but have also been shown to mediate tissue damage in a number of diseases. Methods: In this retrospective cohort study, sixteen immediate post-mortem lung biopsies were methodologically analysed as exploratory and validation cohorts. NETs were quantitatively analysed by multiplexed immunofluorescence and correlated with local levels of IL-8 mRNA expression and the density of CD8+ T-cell infiltration. SARS-CoV-2 presence in tissue was quantified by RT-PCR and immunohistochemistry.Findings: NETs were found in the lung interstitium and surrounding the bronchiolar epithelium with interindividual and spatial heterogeneity. NET density did not correlate with SARS-CoV-2 tissue viral load. NETs were associated with local IL-8 mRNA levels. NETs were also detected in pulmonary thrombi and in only one out of eight liver tissues in spatial fashion. NET focal presence negatively correlated with CD8+ T-cell infiltration in the lungs. Interpretation: Abundant neutrophils undergoing NETosis are found in the lungs of patients with fatal COVID-19, showing no correlation with viral loads. The strong association between NETs and IL-8 focal expression points to this chemokine as the potential causative factor. The function of cytotoxic T-lymphocytes in the immune responses against SARS-CoV-2 may be interfered by the presence of NETs.Funding Information: This study was supported by Banco Bilbao Vizcaya (BBVA) Foundation, “Ayudas a Equipos de Investigación Científica SARS-CoV-2 y COVID-19”. Declaration of Interests: I.M. reports receiving commercial research grants from BMS, Bioncotech, Alligator, Pfizer, Leadartis and Roche; has received speakers bureau honoraria from MSD; and is a consultant or advisory board member for BMS, Roche, Genmab, F-Star, Bioncotech, Bayer, Numab, Pieris, Alligator, and Merck Serono. C.E.A reports research grants from AstraZeneca. All other authors declare no competing interests.Ethics Approval Statement: This study was approved by the ethics committee of the University of Navarra, Spain (Approval 2020.192). Tissue collections were obtained with consent from a first-degree relative, following a protocol approved by the ethics committee of the University of Navarra (Protocol 2020.192p).

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