Suppression of NF-kappaB-mediated beta-defensin gene expression in the mammalian airway by the Bordetella type III secretion system.

Expression of innate immune genes such as beta-defensins is induced in airway epithelium by bacterial components via activation of NF-kappaB. We show here that live Gram-negative bacteria can similarly stimulate this pathway, resulting in upregulation of the beta-defensin tracheal antimicrobial peptide (TAP) in primary cultures of bovine tracheal epithelial cells (TECs), by a Toll-like receptor 4 (TLR4)-mediated pathway. The Gram-negative airway pathogen Bordetella bronchiseptica possesses a type III secretion system previously suggested to inhibit the nuclear translocation of NF-kappaB in a cell line by immunohistochemistry. We therefore hypothesized that this pathogen might interfere in the innate immune response of the epithelium. Exposure of TECs to wild-type B. bronchiseptica suppressed the activation of NF-kappaB and the subsequent induction of TAP mRNA levels, whereas a type III secretion-defective strain did not. These results suggest a mechanism for bacterial evasion of the innate immune response in the airway, which could allow for the observed persistent colonization of this pathogen.