Figure 1.

Structure and function of the innate host defenses in conducting airways. Cartilaginous airways from the terminal bronchioles to the trachea are lined by a pseudostratified epithelium, whose surface is lined by ciliated and secretory cells, that together with submucosal glands, secrete mucins and other host-defense proteins into the periciliary fluids (a,b). Various transcription factors and associated proteins (b, bottom right) are selectively expressed in distinct subsets of epithelial cells lining the airways and submucosal glands. Secreted mucins (blue), such as MUC5AC and MUC5B, produced by goblet cells create a hydrated mucus gel (c,d) that binds particles and pathogens that are moved by the periciliary brush (b) up the airway for clearance from the lungs. Epithelial cells lining the airways and submucosal glands (b,d) create tight epithelial barriers and secrete a diversity of host-defense proteins that recognize microbial pathogens, which enhances the uptake and killing of those pathogens by professional cells of the immune system. The biophysical scaffolds created by the mucus gel, tight cell-cell junctions and communication among respiratory epithelial cells provide multiple barriers to infection. The secretion of fluid and mucus is coordinated with the directional beating of cilia (ultrastructure in electronmicrograph in b) mediated by Cnx43.