Bacterial infections in CF airways

Bacterial infections in CF 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 [1].

Defective mucociliary clearance is associated with the absence or dysfunction of the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) in airway epithelium. This defect plays the key role in the initial bacterial colonization. CFTR is a chloride channel. The genetic defects in CFTR (e.g. deltaF508, the most common mutation) cause reduced secretion of chloride and fluid hydration. Reduced mucociliary clearance, as well as damaged airway epithelium and excessive secretion of mucins, produce a biological matrix that facilitates the bacterial growth in biofilm. Mucus plastering against the airway epithelium flattens cilia and disrupts mucocociliary clearance [1], [2], [3].

Pseudomonas aeruginosa, a Gram negative bacterium, is an opportunistic pathogen that colonizes instrumented airways, immunocompromised hosts, and individuals with cystic fibrosis (CF). The idiosyncratic susceptibility in CF airways to respiratory infection with P. aeruginosa is a severe condition, Over 80% of individuals with CF suffer from considerable (>75%) morbidity due to chronic lung infection with this pathogen [1], [4], [5]. Mutations in CFTR are associated with severe lung diseases and are generally resulted in reduced CFTR protein expression and function in the apical plasma membranes of the airway epithelial cells that are first colonized with P. aeruginosa, followed by the progression to infection and severe inflammation [2]. It is common to see co-infections with other Gram negative bacteria (Stenotrophomonas maltophilia, Burkholderia cepacia, Haemophilus influenzae) and certain specific Gram positive bacteria (Staphylococcus aureus) [6], [7], [8], [9], [10]. However, the specific molecular and cellular mechanisms of hypersusceptibility of CF patients to P. aeruginosa infection are not fully elucidated.

P. aeruginosa antigens, such as lipopolysaccharide (LPS), virulence factor, exoenzyme S (ExoS (P.aeruginosa)), flagellin (Flagellin (P.aeruginosa)) and pilin (PilA (P.aeruginosa)) are recognized by the surface receptors asialo-ganglioside GA1 and Toll-like receptors (TLRs) [1], [5], [11]. Asialo-ganglioside GA1 and TLR2 receptors are increased in cells expressing mutant CFTR and in areas of regenerating epithelium that are likely present in the inflamed CF airway [5], [12], [13], [14]. Among the different TLRs, TLR2 and TLR5 play a major role in signaling epithelial responses to P. aeruginosa in the lung.

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 [8].

Of all the TLRs, TLR2 recognizes the broadest repertoire of ligands, such as ExoS (P.aeruginosa) [15], [16], or, in conjunction with TLR1, lipoteichoid acid (LTA) and Glycopeptide (peptidoglycan, PGN) of gram positive bacteria [8], [14], [17].

All TLRs induce the canonical pathway of Nuclear factor kappa-B (NF-kB) activating: 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 [8], [18], [19]. TLR2 and TLR4 signaling pathways require an additional Toll-Interleukin 1 receptor domain containing adaptor protein (TIRAP) [20], [21].

ExoS (P.aeruginosa) was shown to induce Tumor necrosis factor alpha (TNF-alpha) production through activation of both TLR2 and TLR4. The ability to activate cells expressing TLR2 was attributed to the C terminus of ExoS, whereas the ability to activate TLR4/ MD-2/ CD14 complex was attributed to the N terminus of ExoS [15], [16]. In addition, P. aeruginosa has been shown to signal through TLR4/ MD-2/ CD14 complex with its LPS moiety [22], [23]. 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 [1], [8], [16], [24]. The regulation of MD-2 expression under pathological conditions is also proposed to determine the airway epithelial responses to LPS [25].

Flagellin (P.aeruginosa) [26] and PilA (P. aeruginosa) [27] bind bacteria to the host cell glycolipid receptor, asialo-ganglioside GA1. TLR2 forms a receptor complex with asialo-ganglioside GA1 and activates NF-kB signaling and Interleukin-8 (IL-8) production [1], [13], [28]. TLR5 also recognizes Flagellin from P.aeruginosa and stimulates a similar signaling cascade [1], [29]. Expression of Interleukin-6 (IL-6) and IL-8 are increased in CF epithelial cells in response to stimulation by P. aeruginosa antigens, which may contribute to the excessive inflammatory response in CF [30], [31], [32].

TLR2 can also mediate Beta-defensin 2 expression via NF-kB in response to bacterial antigens in the human airway epithelia [33]. The antimicrobial activity of defensins is compromised by changes in airway surface liquid composition in lungs of CF patients, therefore contributing to the bacterial colonization in the lung. It has been demonstrated that Beta-defensin 2 is susceptible to degradation and inactivation by the host cysteine proteases Cathepsin B, Cathepsin L, and Cathepsin S [34]. Cathepsins are not present in the healthy lung. In chronic lung diseases, such as CF and emphysema, overexpression of cathepsins may lead to accelerated degradation of beta-defensins, thereby favoring bacterial infection and colonization [34].

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, whereas cells with deltaF508 CFTR show little apoptosis and delayed FasL and FasR expression [35].

Mucins are among the most abundant polymers in CF airways. P. aeruginosa has mucin-specific adhesins that mediate interactions between bacterial cells and mucins. Flagellin (P. aeruginosa) appears to play a prominent role in an interaction with Mucin1 (MUC1) [36]. Dehydrated mucus present in CF generates a unique environment in which bacteria are confined spatially. This increases the local concentration of autoinducers, leading to accelerated formation of biofilm [1], [4], [37]. Moreover, MUC1 is overexpressed in CF compare with a health airway [38], [39], [40], probably in a NF-kB-dependent manner [41]. By an unknown mechanism, MUC1 can suppress Flagellin (P.aeruginosa)-induced TLR5 signaling [42], [43].

P. aeruginosa infection causes Interleukin 1 receptor, type I (IL-1RI)-dependent NF-kB activation [8], [44]. Rapid release of IL-1 beta (most probably NF-kB-dependent) in respiratory epithelial cells in response to P. aeruginosa is enhanced in the presence of functional CFTR, but not deltaF508 CFTR [44]. In response to IL-1 beta CF airway epithelial cells induce NF-kB-dependent Chemokine (C-C) ligand 20 (CCL20) and IL-8 production [45], [46].

One well-studied component of bacterial killing, Nitric Oxide, is defective in CF [1]. Epithelial cells with abnormal CFTR activity have reduced inducible nitric oxide synthase (iNOS) expression [47]. Abnormalities in CF reduce both NF-kB and IFN-gamma signaling components that are necessary for complete iNOS expression. Active signal transducer and activator of transcription 1 (STAT1), necessary for both iNOS and Interferon regulatory factor 1 (IRF1) expression, was found to be bound to the Protein inhibitor of activated STAT1 (PIAS1), resulting in reduced IRF1 and iNOS expression in CF epithelial cells [48].

P. aeruginosa antigens can induce cytotoxicity and facilitate progress of infection in CF airway. Surfactant pulmonary-associated proteins A and D (SP-A and SP-D) are cleaved by zinc-metalloprotease elastase LasB (P. aeruginosa) [49]. Degradation of SP-A and SP-D occurs in the cystic fibrosis airway environment, and this degradation eliminates many normal immune functions of this proteins [49], [50].

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