Abstract & Commentary
Synopsis: In addition to its role as a chloride transport regulator, the normal cystic fibrosis transmembrane conductance regulator (CFTR) plays an important role in the innate immune response by internalizing endotoxin from the outer membrane of P aeruginosa with consequent activation of a host cell response.
Source: Schroeder TH, et al. CFTR is a pattern recognition molecule that extracts Pseudomonas aeruginosa LPS from the outer membrane into epithelial cells and activates NF-kB translocation. Proc Natl Acad Sci USA. 2002;99:6907-6912.
In work aimed at elucidating the reasons for the increased susceptibility of patients with cystic fibrosis to persisting pulmonary infection with Pseudomonas aeruginosa, Schroeder and colleagues examined the potential role of cystic fibrosis transmembrane conductance regulator (CFTR) in host defense against this organism. Specifically, they examined the hypothesis that CFTR is a pattern recognition molecule that extracts lipopolysaccharide (LPS) directly from the outer membrane of P aeruginosa and internalizes it, resulting in the cytoplasmic-nuclear translocation of NF-kappa B, which plays a key role in the regulation of, among other things, the immune response, apoptosis, inflammation, and cell-cycle progression.
In mice expressing wild-type CFTR, both intranasal inoculation of intact P aeruginosa and intratracheal instillation of P aeruginosa LPS resulted in rapid nuclear translocation of NF-kappa B in lung epithelial cells. In contrast, in mice expressing no or only mutant (DF508) CFTR, little or no NF-kappa B translocation resulted. Similar results were observed with coculture of wild type (wt) P aeruginosa and DF508 human epithelial cells after in vitro exposure. The NF-kappa B response by wt human cells was blocked by CFTR peptide 108-117 without prevention or binding of bacteria or LPS to the cells, indicating that their binding alone was insufficient to elicit NF-kappa B nuclear translocation. The unresponsiveness of DF508 CFTR expressing cells was overcome by exposure of the human cells to glycerol, which promotes trafficking of the mutant protein from intracellular compartments into the plasma membrane, indicating that the DF508 CFTR protein was capable of mediating NF-kappa B nuclear translocation when it was transported to this location. Other experiments demonstrated that wtCFTR cells rapidly internalize LPS from a bilayer, that this internalization is inhibited by prior incubation of LPS with CFTR peptide 108-117, and that LPS lacking the bacterial oligosaccharide ligand for CFTR was not internalized. Finally, these latter results were also obtained with intact cells of P aeruginosa, indicating that wtCFTR was required for extraction of LPS from the bacterial surface.
Comment by Stan Deresinski, MD, FACP
Patients with cystic fibrosis have a remarkable susceptibility to chronic pulmonary infection with P aeruginosa for reasons that remain incompletely understood.1 Cystic fibrosis is the result of mutation of the CFTR gene, the normal product of which is a phosphorylation-dependent epithelial chloride channel that is a central determinant of transepithelial salt transport, fluid flow, and ion concentration.2 CFTR is a member of the ABC-transporter family comprised of 2 membrane spanning, 2 nucleotide-binding, and one regulatory domain and is predominantly situated at the apical surface of epithelial cells. The most common mutation, DF508, results in retention of CFTR within the endoplasmic reticulum. CFTR knockout mice are unable to control experimental P aeruginosa infection.3
Previous work by Schroeder et al has demonstrated that CFTR binding to P aeruginosa results in the internalization of the bacteria into epithelial cells and their elimination by cellular desquamation.4 The current work demonstrates a role of CFTR in defense against this organism at a step prior to internalization in which it acts as a pattern recognition molecule (PRM) of the innate immune system. PRMs are receptors that recognize pathogen-associated molecular pattern (PAMP) molecules such as peptidoglycan, CpG-rich DNA, and LPS, resulting in activation of key components of the inflammatory response, such as NF-kappa B. PRMs are a key component of the evolutionarily ancient innate immune response; examples include mannose-binding lectin, CD14, and the toll-like receptors. The toll-like receptor, TL4, is the transducing subunit, together with CD14, of the soluble LPS-LPS binding protein receptor complex interaction with which activates NF-kappa B leading to the transcription of proinflammatory cytokines. Mutations in some of these previously known PRMs may be associated with altered response to infection.5 In contrast to the TL4 interaction with soluble LPS, which is normally present in low concentrations, CFTR apparently extracts LPS directly from the outer membrane of intact P aeruginosa, perhaps serving as an "early warning" and defense system.
The experiments reported by Schroeder et al demonstrate that wtCFTR binds, via a 10 amino acid portion of an extracellular loop, to the outer core LPS of P aeruginosa. Binding is followed by removal of the outer core LPS from the organism and its endocytosis by the host cell, resulting in activation and consequent nuclear translocation of NF-kappa B. NF-kappa B is normally sequestered in an inactive form in the cell cytoplasm, bound to members of the I-kappa B family of inhibitory proteins. Factors that lead to activation of NF-kappa B act by phosphorylating I-kappa B, with subsequent release of NF-kappa B and its translocation to the cell nucleus where it binds to various genes and activates their transcription. As indicated above, NF-kappa B plays a key role in the regulation of a variety of immune, inflammatory, and other functions.6
The findings described here, if they can be confirmed, provide a novel window into our understanding and approach to cystic fibrosis.
References
1. Ferkol TW, Look DC. Chinks in the armor of the airway. Pseudomonas infection in the cystic fibrosis lung. Am J Resp Crit Care Med. 2001;26:11-13.
2. Sheppard DN, Welsh MJ. Structure and function of the CFTR chloride channel. Physiol Rev. 1999; 79(Suppl 1): S23-S45.
3. Gosselin D, et al. Impaired ability of Cftr knockout mice to control lung infection with Pseudomonas aeruginosa. Am J Resp Crit Care Med. 1998;157:1253-1262.
4. Pier GB, et al. Cystic fibrosis transmembrane conductance regulator is an epithelial cell receptor for clearance of Pseudomonas aeruginosa from the lung. Proc Natl Acad Sci USA. 1997;94: 12088-12093.
5. Smirnova I, et al. Excess of rare amino acid polymorphisms in the Toll-like receptor 4 in humans. Genetics. 2001;158:1657-1664.
6. Caamano J, Hunter CA. NF-kappaB family of transcription factors: Central regulators of innate and adaptive immune functions. Clin Microbiol Rev. 2002;15:414-429.
Dr. Deresinski, Clinical Professor of Medicine, Stanford; Associate Chief of Infectious Diseases, Santa Clara Valley Medical Center, is Editor of Infectious Disease Alert.
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