FOXO transcription factors regulate innate immune mechanisms in respiratory epithelial cells.

Bacterial pathogens are a leading cause of lung infections and contribute to acute exacerbations in patients with chronic respiratory diseases. The innate immune system of the respiratory tract controls and prevents colonization of the lung with bacterial pathogens. Forkhead box transcription factor family O (FOXO) transcription factors are key regulators of cellular metabolism, proliferation, and stress resistance. In this study, our aim was to investigate the role of FOXO transcription factors in innate immune functions of respiratory epithelial cells. We show that bacterial pathogens potently activate FOXO transcription factors in cultured human respiratory epithelial cells in vitro. Infection of mice with bacterial pathogens resulted in the activation of FOXO transcription factors in alveolar and bronchial epithelial cells in vivo. Active FOXO was also detectable in human bronchial tissue obtained from subjects with different infection-related lung diseases. Small interfering RNA-mediated knockdown of FOXO in bronchial epithelial cells resulted in reduced expression of factors of the innate immune system such as antimicrobial peptides and proinflammatory cytokines, both under basal conditions and upon infection. FOXO deficiency further affected internalization of Haemophilus influenzae in bronchial epithelial cells. Finally, we show that TLR3 activates innate immune responses in a FOXO-dependent manner. In conclusion, FOXO transcription factors are involved in the cellular responses to bacterial stimuli and act as central regulators of innate immune functions in respiratory epithelial cells.

IL-17C is a mediator of respiratory epithelial innate immune response.

The IL-17 family of cytokines consists of at least six members (IL-17A to -F). IL-17 directly activates epithelial cells leading to the expression of inflammatory mediators and antimicrobial factors. Recent studies showed that IL-17C is expressed by epithelial cells. It was the purpose of this study to examine the expression of IL-17 family members in respiratory epithelial cells during bacterial infection. We show that common bacterial pathogens, such as Pseudomonas aeruginosa and Haemophilus influenzae, and ligands of Toll-like receptors 3 and 5 (flagellin, polyI:C) induced the expression and release of IL-17C in cultured human bronchial epithelial cells (HBECs). The expression of IL-17A, -B, -D, or -E was not induced by bacterial stimuli in HBECs. IL-17C enhanced inflammatory responses of respiratory epithelial cells infected with P. aeruginosa. Furthermore, we demonstrate that cigarette smoke suppressed the expression of IL-17C in HBECs in response to bacterial infection and in vivo in the upper airways of mice colonized with H. influenzae. IL-17C could also be detected in bronchial tissue of subjects with infection-related lung diseases. These data show that IL-17C is involved in the innate immune response of respiratory epithelial cells and is suppressed by cigarette smoke.