People with respiratory inflammation may be at higher health risk of bioaerosols: A comparative study of air-liquid interface and submerged exposure models

Bioaerosols are ubiquitous presence in ambient air, and inhalation exposure to bioaerosols may be associated with a series of respiratory diseases. However, the cytotoxic mechanism of bioaerosols remains poorly understood, emphasizing need for more physiologically relevant in vitro exposure models that accurately simulate respiratory microenvironment. In susceptible populations with respiratory inflammation, mucus hypersecretion and airway surface liquid (ASL) accumulation are common. Herein, two exposure models were designed: an air-liquid interface (ALI) model simulating normal respiratory epithelium and a novel submerged model simulating inflamed respiratory epithelium with pathological ASL accumulation. Cytotoxicity of Pseudomonas aeruginosa (P. aeruginosa) bioaerosol was explored with the two models. P. aeruginosa bioaerosol exposure (10⁵ and 10⁶ CFU/m³) increased apoptosis by 3.5 % and 8.8 % in submerged model, compared to 0.1 % and 2.4 % in ALI model. In submerged model, ASL accumulation resulted in mucin hypersecretion and upregulation of bacterial adhesins, enhancing P. aeruginosa adhesion, invasion, biofilm formation, and ultimately led to higher cytotoxicity. In ALI model, P. aeruginosa bioaerosol exposure caused greater increases in epithelial permeability despite exhibiting lower cytotoxicity. This disparity resulted from attenuated upregulation of intercellular junction proteins in ALI model (vs. submerged). Our study revealed distinct cytotoxic mechanisms of P. aeruginosa bioaerosols in two models, underscoring the heightened health risks of bioaerosol exposure in individuals with pre-existing respiratory inflammation.