The mammalian intestine harbors a beneficial microbiota numbering approximately 10(12) organisms per gram of colonic content. The host tolerates this tremendous bacterial load while maintaining the ability to efficiently respond to pathogenic organisms. In this study, we show that the Bacteroides use a mammalian-like pathway to decorate numerous surface capsular polysaccharides and glycoproteins with l-fucose, an abundant surface molecule of intestinal epithelial cells, resulting in the coordinated expression of this surface molecule by host and symbiont. A Bacteroides mutant deficient in the ability to cover its surface with L-fucose is defective in colonizing the mammalian intestine under competitive conditions.

Immune cells are activated during cellular responses to antigen by two described mechanisms: (i) direct uptake of antigen and (ii) extraction and internalization of membrane components from antigen-presenting cells. Although endocytosis of microbial antigens by pattern recognition molecules (PRM) also activates innate immunity, it is not known whether this involves extraction and internalization of microbial surface components. Epithelial cells on mucosal surfaces use a variety of receptors that are distinct from the classical endocytic PRM to bind and internalize intact microorganisms. Nonclassical receptor molecules theoretically could act as a type of endocytic PRM if these molecules could recognize, bind, extract, and internalize a pathogen-associated molecule and initiate cell signaling. We report here that the interaction between the cystic fibrosis transmembrane conductance regulator (CFTR) and the outer core oligosaccharide of the lipopolysaccharide (LPS) in the outer membrane of Pseudomonas aeruginosa satisfies all of these conditions. P. aeruginosa LPS was specifically recognized and bound by CFTR, extracted from the organism's surface, and endocytosed by epithelial cells, leading to a rapid (5-to 15-min) and dynamic translocation of nuclear transcription factor NF-B. Inhibition of epithelial cell internalization of P. aeruginosa LPS prevented NF-B activation. Cellular activation depended on expression of wild-type CFTR, because both cultured ⌬F508 CFTR human airway epithelial cells and lung epithelial cells of transgenic-CF mice failed to endocytose LPS and translocate NF-B. CFTR serves as a critical endocytic PRM in the lung epithelium, coordinating the effective innate immune response to P. aeruginosa infection.

Innate immunity is critical for clearing Pseudomonas aeruginosa from the lungs. In response to P. aeruginosa infection, a central transcriptional regulator of innate immunity-NF-B-is translocated within 15 min to the nuclei of respiratory epithelial cells expressing wild-type (WT) cystic fibrosis (CF) transmembrane conductance regulator (CFTR). P. aeruginosa clearance from lungs is impaired in CF, and rapid NF-B nuclear translocation is defective in cells with mutant or missing CFTR. We used WT and mutant P. aeruginosa and strains of transgenic mice lacking molecules involved in innate immunity to identify additional mediators required for P. aeruginosa-induced rapid NF-B nuclear translocation in lung epithelia. We found neither Toll-like receptor 2 (TLR2) nor TLR4 nor TLR5 were required for this response. However, both MyD88-deficient mice and interleukin-1 receptor (IL-1R)-deficient mice failed to rapidly translocate NF-B to the nuclei of respiratory epithelial cells in response to P. aeruginosa. Cultured human bronchial epithelial cells rapidly released IL-1␤ in response to P. aeruginosa; this process was maximized by expression of WT-CFTR and dramatically muted in cells with ⌬F508-CFTR. The IL-1R antagonist blocked P. aeruginosa-induced NF-B nuclear translocation. Oral inoculation via drinking water of IL-1R knockout mice resulted in higher rates of lung colonization and elevated P. aeruginosaspecific antibody titers in a manner analogous to that of CFTR-deficient mice. Overall, rapid IL-1 release and signaling through IL-1R represent key steps in the innate immune response to P. aeruginosa infection, and this process is deficient in cells lacking functional CFTR.