Influenza virus infection increases ACE2 expression and shedding in human small airway epithelial cells.

Patients with COVID-19 caused by severe acute respiratory syndrome coronavirus (SARS-Co-V)-2 demonstrate high rates of co-infection with respiratory viruses, including influenza A (IAV), suggesting pathogenic interactions. We investigated how IAV may increase the risk for COVID-19 lung disease, focusing on the receptor Angiotensin Convertase Enzyme 2 (ACE2) and the protease TMPRSS2, which cooperate to uptake SARS-CoV-2 intracellular. We found, using single cell RNA sequencing of distal human non-diseased lung homogenates, that at baseline, ACE2 is minimally expressed in basal, goblet, ciliated, and secretory epithelial cells populating small airways. We focused on human small airway epithelial cells (SAEC), central to the pathogenesis of lung injury following viral infections. Primary SAEC from non-diseased donor lungs apically infected (at air-liquid interface) with IAV (up to 3×105 pfu; ∼1 MOI) markedly (8-fold) boosted the expression of ACE2, paralleling that of STAT1, a transcription factor activated by viruses. IAV increased the apparent electrophoretic mobility of intrac¬ellular ACE2 and generated an ACE2 fragment (90 kDa) in apical secretions, suggesting cleavage of this receptor. IAV also increased the expression of two proteases known to cleave ACE2, sheddase ADAM17 (TACE) and TMPRSS2 and increased the TMPRSS2 zymogen and its mature fragments, implicating proteolytic autoactivation. These results indicate that IAV amplifies the expression of molecules necessary for SARS-CoV-2 infection of the distal lung. Further, posttranslational changes in ACE2 by IAV may increase the vulnerability to lung injury such as ARDS during viral co-infections. These findings support prevention and treatment efforts of influenza infections during the COVID-19 pandemic.