Abstract
BackgroundThe respiratory tract epithelium is a critical environmental interface that regulates inflammation. In chronic infectious airway diseases, pathogens may permanently colonize normally sterile luminal environments. Host-pathogen interactions determine the intensity of inflammation and thus, rates of tissue injury. Although many cells become refractory to stimulation by pathogen products, it is unknown whether the airway epithelium becomes either tolerant or hypersensitive in the setting of chronic infection. Our goals were to characterize the response of well-differentiated primary human tracheobronchial epithelial cells to Pseudomonas aeruginosa, to understand whether repeated exposure induced tolerance and, if so, to explore the mechanism(s).MethodsThe apical surface of well-differentiated primary human tracheobronchial epithelial cell cultures was repetitively challenged with Pseudomonas aeruginosa culture filtrates or the bacterial media control. Toxicity, cytokine production, signal transduction events and specific effects of dominant negative forms of signaling molecules were examined. Additional experiments included using IL-1β and TNFα as challenge agents, and performing comparative studies with a novel airway epithelial cell line.ResultsAn initial challenge of the apical surface of polarized human airway epithelial cells with Pseudomonas aeruginosa culture filtrates induced phosphorylation of IRAK1, JNK, p38, and ERK, caused degradation of IκBα, generation of NF-κB and AP-1 transcription factor activity, and resulted in IL-8 secretion, consistent with activation of the Toll-like receptor signal transduction pathway. These responses were strongly attenuated following a second Pseudomonas aeruginosa, or IL-1β, but not TNFα, challenge. Tolerance was associated with decreased IRAK1 protein content and kinase activity and dominant negative IRAK1 inhibited Pseudomonas aeruginosa -stimulated NF-κB transcriptional activity.ConclusionThe airway epithelial cell response to Pseudomonas aeruginosa entails adaptation and tolerance likely mediated, in part, by down-regulation of IRAK1.
Highlights
The respiratory tract epithelium is a critical environmental interface that regulates inflammation
Recent studies indicate a role for TLR2 and TLR5 in stimulation of airway epithelial cells by flagellin or live Gram-positive and -negative bacteria [13,14] and TLR2 was apparently recruited to lipid rafts at the apical epithelial cell surface [15]
We show that Ps. a. products activate the TLR pathway and that hTBE cells become tolerant via a mechanism likely involving down-regulation of IRAK1
Summary
The respiratory tract epithelium is a critical environmental interface that regulates inflammation. Much data has been derived from monocyte/macrophages and myelocytic cell lines, but the TLR pathway functions in epithelial cells where receptor and co-receptor expression levels, and the activity of downstream signal transduction intermediates, likely determine cellular sensitivity to pathogen products [5,6,7]. More recent studies demonstrate TLR2-dependent IL-8 and hBD-2 production by hTBE cells [10]. Culture filtrates of both Gram-positive and -negative bacteria and a TLR2 agonist enhanced IL-8 secretion by hTBE cells 3–5 fold [11]. The TLR signal transduction pathway is likely an important regulator of airway immunity and inflammation
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