Allergic asthma is a complex disease characterized by eosinophilic pulmonary inflammation, mucus production and reversible airway obstruction1. Exposure to indoor allergens is a risk factor for asthma, but this disease is also associated with high household levels of total and particularly Gram-negative bacteria2. The ability of bacterial products to act as adjuvants3 suggests they might promote asthma by priming allergic sensitization to inhaled allergens. In support of this idea, house dust extracts (HDEs) can activate antigen-presenting dendritic cells (DCs) in vitro and promote allergic sensitization to inhaled innocuous proteins in vivo4. It is unknown which microbial products provide most of the adjuvant activity in HDEs. A screen for adjuvant activity of microbial products revealed that the bacterial protein flagellin (FLA) stimulated strong allergic airway responses to an innocuous inhaled protein, ovalbumin (OVA). Moreover, Toll-like receptor 5 (TLR5), the mammalian receptor for FLA5,6, was required for priming strong allergic responses to natural indoor allergens present in HDEs. In addition, individuals with asthma have higher serum levels of FLA-specific antibodies as compared to nonasthmatic individuals. Together, these findings suggest that household FLA promotes the development of allergic asthma by TLR5-dependent priming of allergic responses to indoor allergens.
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This work was supported by the Intramural Research Program of the US National Institutes of Health NIEHS. We thank the human subjects for their blood donations and B. Yingling, J. Marshburn and A. Rice of the NIEHS Clinical Research Unit and D. Beaver of Duke University for assistance with patient recruitment and serum isolation. J. Hollingsworth (Duke University) with permission from R. Medzhitov (Yale University) provided CD11c-Myd88 transgenic mice on a Myd88−/− background. S. Akira (Osaka University) provided Tlr2−/−, Tlr4−/− and Myd88−/− mice and J. Ting (University of North Carolina) provided Nlrc4−/− mice. D. Wozniak (Ohio State University) provided both strains of P. aeruginosa, F.-S. Shu (Wayne State University) provided purified FLA from P. aeruginosa and A. Gewirtz (Emory University) provided FLA from Escherichia coli. We also thank NIEHS members K. Nakano for genotyping, L. Perrow for mouse colony management, N. Flagler for histology, J. Aloor for measuring endotoxin levels in HDEs, M. Sifre and C. Bortner for help with FACS-based cell sorting, J. Ciencewicki for advice on airway epithelial cell culture and M. Fessler and S. London for suggestions on the manuscript.
The authors declare no competing financial interests.
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Wilson, R., Maruoka, S., Whitehead, G. et al. The Toll-like receptor 5 ligand flagellin promotes asthma by priming allergic responses to indoor allergens. Nat Med 18, 1705–1710 (2012). https://doi.org/10.1038/nm.2920
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