TLR1, (L)-arginine, I-kB, Beta-defensin 2, IKK-beta, Lipoteichoic acid (Gram positive bacteria), IFN-gamma, JAK1, iNOS, IFN-gamma receptor, 2 (L)-arginine + 3 NAD(P)H + 4 O(,2) = 4 H(,2)O + 3 NAD(P)('+) + 2 (S)-citrulline + 2 NO, NIK, TAK1, STAT1, IL-1 beta, PIAS1, IRAK4, TIRAP, LBP, LPS (Gram negative bacteria), Chloride ion extracellular region, CD14, MD-2, Nitric Oxide, <cytosol> chloride ion = <extracellular region> chloride ion, Chloride ion cytosol, Glycopeptide (PGN) (Gram positive bacteria), TLR5, IKK (cat), IRAK1/2, TAB1, TLR4, Flagellin, NF-kB, FasL, FasR, TAB2, IL-1RI, TRAF6, MyD88, JAK2, IL1RAP, IRF1, TLR2, IKK-gamma, IL-6, IKK-alpha, IL-8, CFTR
Bacterial infections in normal airways
The upper airways represent a primary site for the introduction of pathogenic microorganisms from inspired air. The ciliated epithelium features several powerful mechanisms for prevention of colonization by inhaled bacteria, thus the lower respiratory tract usually remains sterile. Toll-like receptors (TLRs) play a key role in facilitating the innate immune response to bacterial antigens .
Toll-like receptors (TLRs) belong to a family of transmembrane proteins that can recognize and discriminate a diverse array of microbial antigens. Following their activation by specific bacterial ligands, TLRs initiate intracellular signaling cascades that culminate in the activation of transcription factors and ultimately lead to activation of pro-inflammatory gene expression. Epithelial cells content of the airways provide both a physical barrier to infection and an active defense mechanism against invading microoranisms .
TLR2 is the predominant TLR expressed on the apical cell surface, with other TLRs (TLR3, 4 and 5) residing mainly intracellularly. However, in inflamed lung following stimulation with bacterial ligands TLR5 and TLR4 can be mobilized to the apical surface .
All TLRs, as well as Interleukin 1 receptor, type I ( IL-1RI ), induce the canonical pathway of nuclear factor kappa-B ( NF-kB ) activation which consists of Myeloid differentiation primary response gene 88 ( MyD88 )/ Interleukin-1 receptor-associated kinases 4, 1 and 2 ( IRAK4 and IRAK1/2 )/ TNF Receptor-associated factor 6 ( TRAF6 )/ Mitogen-activated protein kinase kinase kinase 7 interacting proteins 1 and 2 ( TAB1 and TAB2 )/ Mitogen-activated protein kinase kinase kinase 7 ( TAK1 )/ Mitogen-activated protein kinase kinase kinase 14 ( NIK )/ I-kB kinase complex ( IKK(cat) )/ Nuclear factor kappa-B inhibitor ( I-kB )/ NF-kB , , . TLR2 and TLR4 signaling pathways also require an additional adaptor Toll-Interleukin 1 receptor domain containing adaptor protein ( TIRAP ) , .
TLRs signaling and NF-kB activation are commonly involved in the up-regulation of chemotactic molecules and cytokines (such as Interleukins IL-1 beta, IL-6 and IL-8 ), production of mediators of innate immune response ( IFN-gamma, Nitric Oxide that is synthesized by inducible nitric oxide synthase ( iNOS )), enhanced expression of antimicrobial peptides (such as Beta-defensin 2 ). IL-1 beta signaling, in turn, regulates the levels of cystic fibrosis transmembrane conductance regulator ( CFTR ) , .
Of all the TLRs, TLR2 recognizes the broadest repertoire of ligands, such as, in conjunction with TLR1, lipoteichoid acid (LTA) and Glycopeptide (peptidoglycan, PGN) from gram positive bacteria followed by NF-kB activation and interleukin production , , . TLR5 is able to recognize Flagellin (from both gram positive and gram negative bacteria), and also stimulates the NF-kB signaling , , .
Pseudomonas aeruginosa has been shown to signal through TLR4/ MD-2/ CD14 complex with its LPS moiety , . Although TLR4 is expressed in airway epithelial cells, it does not appear to be prominently involved in signaling of P. aeruginosa presented at the apical surface of airway epithelial cells , , , . The low level of MD-2 expression is also proposed to limit the responses of human airway epithelia to endotoxin stimulation .
CFTR is a chloride channel that regulates chloride transport, fluid hydration and mucociliary clearance in the lung, thus preventing the bacterial growth in normal airways  , ,  . Normal CFTR promotes a rapid expression of Fas ligand, TNF superfamily, member 6 ( FasL ) and Fas, TNF receptor superfamily, member 6 ( FasR ), as well as an apoptotic response to P. aeruginosa infection. Mutant deltaF508 CFTR cells feature inhibited apoptosis and delayed FasL and FasR expression in response to infection .
Bacterial stimulation also leads to Interleukin 1 receptor, type I ( IL-1RI )-dependent NF-kB activation , . Rapid release of IL-1 beta (most probably NF-kB-dependent) is enhanced in the presence of functional CFTR, but not deltaF508 CFTR in respiratory epithelial cells .
The inducible form of nitric oxide (NO) synthase (( iNOS ) is expressed constitutively in normal human airway epithelium . Both NF-kB and IFN-gamma signaling components are necessary for normal iNOS expression. IFN-gamma activates Janus kinase 1 and 2 (JAK1 and JAK2)/ Signal transducer and activator of transcription 1 ( STAT1 ) signaling followed by Interferon regulatory factor 1 ( IRF1 ) and iNOS expression . Activation of Protein inhibitor of activated STAT1 ( PIAS1 ) results in reduced IRF1 and iNOS expression in CF, but not healthy, epithelial cells .