5.1.1. Mucus production. An incoming virus first must find epithelial cells to invade the host.<br>Mucus layers can effectively trap the virus before it can enter the host cells (Figure 4). Mucus<br>secreted from the submucosal glands within the lamina propria serves as a mechanical barrier and<br>as a chemical barrier by its antimicrobial properties (74, 75). Components of the mucus are 93–<br>97% w/w water, 3–7% w/w solids, 1–3% w/w glycoproteins, 1% w/w proteins, 0.5–1% w/w<br>lipids, and 0.70–1.4% w/w minerals (76). The major glycoproteins in the airway mucus are<br>secretory mucin proteins MUC5AC and MUC5B (77). Cold environments have been linked to exacerbations of chronic obstructive pulmonary disease (COPD), which manifests in chronic<br>airflow obstruction, inflammation, and hypersecretion of respiratory mucus (78, 79). In the<br>normal human bronchial epithelial (NHBE) cells isolated from COPD patients, the expression<br>level of the transient receptor potential melastatin 8 (TRPM8), which is the cold receptor<br>activated by temperature under 27°C or cooling agents including menthol (80, 81), is upregulated<br>(82). Furthermore, cold (18°C) exposure or menthol treatment of cultured NHBE cells increases<br>MUC5AC secretion in a TRPM8-dependent manner (82, 83). Another study examined the effect<br>of temperature, humidity, and airflow mimicking respiration on mucin secretion from human<br>nasal epithelial cells using a climate chamber for cell culture (84). Mucin production increased<br>under 25°C, 40% RH compared to 37°C, 80% RH. Airflow increased mucin production under<br>25°C, 40% RH, but not at 37°C, 80% RH. These results suggest that impairment of MCC under<br>low temperature and low humidity includes hypersecretion of mucin.
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