Figure 1. An overview of SARS-CoV-2 infection via airway epithelial cells. ACE2 is expressed in various types of airway epithelial cells, such as C, G, CC, and type II pneumocytes (ATII). The infectivity and ACE2 expression are gradually decreased from the upper airway (red color) to lower airway (blue color). Cellular protease, TMPRSS2, is promoting the cellular entry of the virus with other co-factors. SARS-CoV-2 can activate transcription factors and can stimulate to produce proinflammatory cytokines, chemokines, and IFNs. ACE2 is upregulated by IFNs, so it acts as an ISG.

C, ciliated cells; G, goblet cells; CC, club cells; ATI, alveolar type I cell; ATII, alveolar type II cell; ADAM17, a disintegrin and metalloprotease domain metallopeptidase domain 17.