ACE2 and COVID-19: Importantly, human airway cultures grown in an ALI system is a physiologically relevant model for investigating anti-COVID-19 interventions because the cells express angiotensin-converting enzyme 2 (ACE2), the host cell receptor of SARS-CoV-2, and the serine protease TMPRSS2 that is required for the viral glycoprotein (protein S) priming (14, 15).