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Mutations in the gene encoding the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR), a cAMP-regulated chloride channel found in apical membranes of epithelial cells lining secretory organs, lead to the clinical syndrome of the lethal hereditary disease Cystic Fibrosis (CF). This disease is characterized by neutrophil dominated lung inflammation with chronic bacterial infection, especially with Pseudomonas aeruginosa. Mutation and loss of function of CFTR led to the assumption that CF might be associated with an intrinsic dysregulated inflammatory response. In vivo as well as in vitro studies have yielded contradictory results as regards the expression of components of the innate immune system, such as Toll-like receptors (TLRs) and proinflammatory cytokines. Therefore, it is still unknown whether immune responses in CF are compromised or on the contrary excessively activated in early stages of the disease.
Our studies on CF bronchial epithelial cell line CFBE and its CFTR corrected counterpart showed that the TLR-4 mediated inflammatory response was compromised in CF cells after stimulation with P. aeruginosa and LPS. CFTR corrected cells were capable of inducing secretion of IL-8, IL-6 and IP-10 by a factor of 3 in response to stimuli of P. aeruginosa, whereas CF cells were only able to induce cytokine secretion by a factor of 2. The compromised inflammatory response in CF cells was due to a reduced surface expression of TLR-4, resulting in reduced binding of bacterial components and diminished activation of the transcription factor NF-κB. The reduced surface expression of TLR-4 was not associated with decreased mRNA expression. In fact, CF cells showed a strong intracellular localization of the receptor or its precursors. This could be a result of alterations due to the loss of CFTR function, which leads to an increase of intracellular pH. A defective acidification in CF cells might compromise the activity and efficiency of posttranslational modifications, such as glycosylation of membrane proteins. Therefore, a reduced availability of mature TLR-4 or required co-receptors could lead to a decreased integration into the cell membrane.
In sum, the results of the study support the hypothesis that the pathogenesis of CF lung disease is a result of the loss of CFTR function. Alterations of the TLR-4 signalling pathway could strongly influence and compromise effective clearing of pathogens via innate and adaptive immune defense mechanisms. This might predispose a CF lung to early infection and bacterial colonization.