Radioresistant cells expressing TLR5 control the respiratory epithelium's innate immune responses to flagellin

Abstract

Bacterial products (such as endotoxins and flagellin) trigger innate immune responses through TLRs. Flagellin-induced signalling involves TLR5 and MyD88 and, according to some reports, TLR4. Whereas epithelial and dendritic cells are stimulated by flagellin in vitro, the cell contribution to the in vivo response is still unclear. Here, we studied the respective roles of radioresistant and radiosensitive cells in flagellin-induced airway inflammation in mice. We found that i.n. delivery of flagellin elicits a transient change in respiratory function and an acute, pro-inflammatory response in the lungs, characterized by TLR5-and MyD88-dependent chemokine secretion and neutrophil recruitment. In contrast, TLR4, CD14 and TRIF were not essential for flagellin-mediated responses, indicating that TLR4 does not cooperate with TLR5 in the lungs. Respiratory function, chemokine secretion and airway infiltration by neutrophils were dependent on radioresistant, TLR5-expressing cells. Furthermore, lung haematopoietic cells also responded to flagellin by activating TNF-a production. We suggest that the radioresistant lung epithelial cells are essential for initiating early, TLR5-dependent signalling in response to flagellin and thus triggering the lung's innate immune responses.Key words: Chemokine . Epithelium . Flagellin . Lung inflammation . TLR5 IntroductionUpon challenge with respiratory pathogens, the airways rapidly develop a pro-inflammatory response initiated by environmental agents including microbe-associated molecular patterns such as Gram-negative bacteria LPS or endotoxins. This proinflammatory reaction plays an important role in the innate defense against respiratory pathogens [1,2]. Physiological changes induced by endotoxin inhalation include pulmonary à These authors contributed equally to this work. Eur. J. Immunol. 2009. 39: 1587-1596 DOI 10.1002 Innate immunity 1587 function and bronchoconstriction, cytokine and chemokine production, PMN recruitment, increased permeability of the alveolar epithelium and the associated endothelium [3][4][5][6]. TLR4 [7], together with MD-2, LPS-binding protein and CD14 detect endotoxins and play a critical role in the initiation of the pulmonary response to systemic and mucosal endotoxin administration [5,[8][9][10]. The pulmonary inflammation to endotoxin involves TLR4 signalling in both the parenchyma cells like endothelial and epithelial cells and the haematopoietic cells, i.e. monocytes, dendritic cells and neutrophils [11]. In addition, TLR4 signalling contributes to the development and progression of chronic respiratory disease including asthma [12][13][14][15][16].Flagellin is the major constituent of flagella from Gramnegative and Gram-positive bacteria. TLR5 has been identified as the main pattern recognition receptor required for flagellinmediated immune signalling [17]. TLR5 signalling depends on MyD88 and results in activation of MAPK, nuclear translocation of NF-kB and production of various pro-inflammatory cytokines and chemokines [18]. Moreover, a previous ...

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