Introduction: There is growing evidence that polymorphic variants in genes other than CFTR play an important role in determining severity of CF lung disease. We recently demonstrated that the innate immune receptor TLR5 mediates much of the damaging inflammatory response generated by CF airway cells following exposure to P. aeruginosa. Furthermore, it has been suggested that TLR5 is involved in bacterial clearance from the airways in humans and mice. Together, this evidence identifies TLR5 as a biologically plausible candidate gene that may modify CF lung disease. Objective: To confirm the pro-inflammatory phenotype of CF airway epithelial cells and to determine if polymorphisms in the TLR5 gene modify the severity of pulmonary disease in patients with CF. Methods: CF and non-CF cell lines were stimulated with P. aeruginosa wt and a strain lacking flagellin (?fliC). We genotyped one functional non-synonymous SNP in the TLR5 gene (TLR5 R392X, rs5744168) in healthy individuals and stimulated PBMCs from carriers and non carriers with purified flagellin to asses the impact of the SNP on TLR5 signaling capacity. TLR5 genotypes of 2,441 CF patients were correlated with severity of CF lung disease. Outcome variables measured were a) lung function (i.e. cross-sectional measurement of FEV1 %predicted) and b) the annual decline of lung function (i.e. longitudinal measurement of FEV1 %predicted). Results and Conclusions: CF epithelial cell lines recognize P. aeruginosa through the flagellin-TLR5 interaction and produce an exaggerated pro-inflammatory cytokine response. The TLR5 R392X SNP was functionally active, significantly decreasing TLR5 responsiveness of PBMCs up to 75% (p < 0.0001). Our data identify TLR5 as an excellent biologically plausible candidate gene that may modify CF lung disease. As a follow up to this study, the TLR5 R392X SNP has been genotyped in 2,441 CF patients and ongoing statistical analysis of the impact of this SNP on lung function will examine whether genetic variation in TLR5 modifies CF lung disease.

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