FIG 1.

Mucosal defense in the respiratory tract during SARS-CoV-2 infection. (A) Human upper and lower respiratory tracts. (B) Respiratory epithelium with ciliated cells, goblet cells, and a submucosal gland. The soluble mucin MUC5AC is secreted by goblet cells, and the soluble mucin MUC5B is secreted by mucosal cells in the submucosal gland. (C) Ciliated epithelial cells express transmembrane mucins MUC1 (red), MUC4 (blue), and MUC16 (yellow) and the SARS-CoV-2 entry receptor ACE2. (D) Domain structure of transmembrane mucins MUC1, MUC4, and MUC16. Mucin O-glycan structures and, specifically, terminal sialic acids play an important role in virus-mucin interactions.