<i>Pseudomonas aeruginosa</i> Activates the NOD-like Receptor Signaling Pathway by Targeting Nasopharyngeal Cells throughout the Whole Respiratory Tract

Bioaerosols are commonly present in air, and the exposure of bacteria in bioaerosols may lead to allergic reactions and respiratory diseases. However, precise mechanisms underlying the impact of pathogenic bacteria on respiratory health remain poorly understood, and their primary target cells in respiratory tract have not been identified. We systematically explored cytotoxicity of <i>Pseudomonas aeruginosa</i> (<i>P. aeruginosa</i>) on the whole epithelial cell lines representing different segments of the respiratory tract, including HNEpC (nasal mucosa), NP69 (nasopharynx), 16HBE (bronchus), and Beas-2B (lung) cells. By tracing the entire process of exposure to <i>P. aeruginosa</i>, NP69 cells emerged as the primary target, with HNEpC cells showing the lowest susceptibility among the four respiratory cell types. <i>P. aeruginosa</i> was transmitted from extracellular space to intracellular region of NP69 cells, accompanied by significant up-regulation (1.5-26.5 fold) of <i>NOD2</i> expression, an RNA molecule that specifically binds to bacterial cell walls. Furthermore, cell proliferation activity decreased by 6.8%-46.1%, while levels of inflammatory cytokine interleukin IL-6 and IL-1β increased by 14.5%-181.0% and 50.2%-238.2%, respectively, following intracellular transition of bacteria in NP69 cells. The changes in epithelial-mesenchymal transition (EMT) marker expression at genetic and protein levels and enhancement of functional phenotypes (migratory capacity) confirmed the potential of <i>P. aeruginosa</i> to induce malignant transformation of epithelial cells by activating the NOD-like receptor signaling pathway. This comprehensive examination of comparative cytotoxicity of bacteria on cells throughout the human respiratory tract elucidates specific target cells and damage mechanisms, offering valuable insights into health risk assessment, risk prevention, and bacteria control.