Epithelial cells interact directly with bacteria in the environment and play a critical role in airway defense against microbial pathogens. In this study, we examined the response of respiratory epithelial cells to infection with nontypable Haemophilus influenzae. Using an in vitro cell culture model, we found that epithelial cell monolayers released significant quantities of IL-8 and expressed increased levels of ICAM-1 mRNA and surface protein in response to H. influenzae. In contrast, levels of IL-1beta, TNF-alpha, and MHC class I were not significantly affected, suggesting preferential activation of a specific subset of epithelial genes directed toward defense against bacteria. Induction of ICAM-1 required direct bacterial interaction with the epithelial cell surface and was not reproduced by purified H. influenzae lipooligosaccharide. Consistent with a functional role for this response, induction of ICAM-1 by H. influenzae mediated increased neutrophil adherence to the epithelial cell surface. Furthermore, in an in vivo murine model of airway infection with H. influenzae, increased epithelial cell ICAM-1 expression coincided with increased chemokine levels and neutrophil recruitment in the airway. These results indicate that ICAM-1 expression on human respiratory epithelial cells is induced by epithelial cell interaction with H. influenzae and suggest that an ICAM-1-dependent mechanism can mediate neutrophil adherence to these cells independent of inflammatory mediator release by other cell types. Direct induction of specific epithelial cell genes (such as ICAM-1 and IL-8) by bacterial infection may allow for rapid and efficient innate defense in the airway.
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