MUC1 and infection: During infection, injured airway epithelium releases biomarkers such as Krebs von den Lungen-6 (KL-6), surfactant protein A (SP-A), and short palate, lung, and nasal epithelium clone 1 (SPLUNC1), which reflect epithelial barrier disruption and impaired airway homeostasis (Keir et al., 2021; Li et al., 2025c).