Airway epithelial cell tolerance to Pseudomonas aeruginosa
© Wu et al. 2005
Received: 21 December 2004
Accepted: 01 April 2005
Published: 01 April 2005
The respiratory tract epithelium is a critical environmental interface that regulates inflammation. In chronic infectious airway diseases, pathogens may permanently colonize normally sterile luminal environments. Host-pathogen interactions determine the intensity of inflammation and thus, rates of tissue injury. Although many cells become refractory to stimulation by pathogen products, it is unknown whether the airway epithelium becomes either tolerant or hypersensitive in the setting of chronic infection. Our goals were to characterize the response of well-differentiated primary human tracheobronchial epithelial cells to Pseudomonas aeruginosa, to understand whether repeated exposure induced tolerance and, if so, to explore the mechanism(s).
The apical surface of well-differentiated primary human tracheobronchial epithelial cell cultures was repetitively challenged with Pseudomonas aeruginosa culture filtrates or the bacterial media control. Toxicity, cytokine production, signal transduction events and specific effects of dominant negative forms of signaling molecules were examined. Additional experiments included using IL-1β and TNFα as challenge agents, and performing comparative studies with a novel airway epithelial cell line.
An initial challenge of the apical surface of polarized human airway epithelial cells with Pseudomonas aeruginosa culture filtrates induced phosphorylation of IRAK1, JNK, p38, and ERK, caused degradation of IκBα, generation of NF-κB and AP-1 transcription factor activity, and resulted in IL-8 secretion, consistent with activation of the Toll-like receptor signal transduction pathway. These responses were strongly attenuated following a second Pseudomonas aeruginosa, or IL-1β, but not TNFα, challenge. Tolerance was associated with decreased IRAK1 protein content and kinase activity and dominant negative IRAK1 inhibited Pseudomonas aeruginosa -stimulated NF-κB transcriptional activity.
The airway epithelial cell response to Pseudomonas aeruginosa entails adaptation and tolerance likely mediated, in part, by down-regulation of IRAK1.
The innate immune system suppresses pathogen attachment, colonization, growth, and invasion and co-ordinates adaptive immunity . Innate immunity entails recognition of microbial signatures by the cellular repertoire of Toll-like receptors (TLRs [2, 3]). TLR agonists initiate receptor-specific downstream signaling pathways, ultimately enhancing production of anti-microbial molecules and inflammatory mediators . Much data has been derived from monocyte/macrophages and myelocytic cell lines, but the TLR pathway also functions in epithelial cells where receptor and co-receptor expression levels, and the activity of downstream signal transduction intermediates, likely determine cellular sensitivity to pathogen products [5–7]. In the human airway, polymorphisms in TLR4, the LPS receptor, modulated the response to inhaled LPS, and TLR4 was found on primary tracheobronchial epithelial (hTBE) cells . RNA for TLR1-6 was present in hTBE cells in vitro, and high doses of commercial LPS activated NF-κB and induced the neutrophil chemotactic cytokine IL-8 and the anti-microbial peptide beta defensin 2 (hBD-2) . More recent studies demonstrate TLR2-dependent IL-8 and hBD-2 production by hTBE cells . Culture filtrates of both Gram-positive and -negative bacteria and a TLR2 agonist enhanced IL-8 secretion by hTBE cells 3–5 fold . Hemophilus influenzae, an important respiratory tract pathogen, signals via TLR2 in epithelial cells . Recent studies indicate a role for TLR2 and TLR5 in stimulation of airway epithelial cells by flagellin or live Gram-positive and -negative bacteria [13, 14] and TLR2 was apparently recruited to lipid rafts at the apical epithelial cell surface . TLR1-10 expression and positive responses to several TLR agonists were recently reported in airway epithelial cells. Thus, the TLR signal transduction pathway is likely an important regulator of airway immunity and inflammation.
The regulation of TLR signaling is dynamic. Up-regulation of TLRs, for example by interferon  or virus , may enhance epithelial cell sensitivity to pathogen products. On the other hand, LPS exposure induces hypo-responsiveness to a second challenge, termed LPS tolerance (reviewed in ). Other molecules acting through the TLR pathway, including mycobacterial products  and lipoteichoic acid from Gram-positive bacteria , also induce tolerance. Tolerance is associated with decreased degradation of NF-κB inhibitory proteins, reduced MAP kinase phosphorylation, prevention of NF-κB and AP-1 activation, altered transcriptional responses and suppression of pro-inflammatory cytokine and chemokine production . Decreased cell surface TLR4 protein  was associated with tolerance, but tolerance could not be attributed solely to receptor loss, since cells that did not decrease TLR4 still became tolerant, and over-expression of CD14, TLR4 and MD-2 in HEK293 cells did not prevent LPS tolerance [20, 24]. However, a hypo-responsive state can be induced at the level of the plasma membrane by the expression of endogenous, functionally inactive members of the Toll-interleukin 1 receptor superfamily . Many substances acting though the TLR signal transduction pathway induce cross tolerance, including LPS and IL-1β , LPS and mycobacterial products , or LPS and lipoteichoic acid . Tolerance without down-regulation of surface receptors and cross-tolerance suggest negative regulation of common elements in the downstream signal transduction pathway. IRAK1 functions just distal to TLRs and their adaptor proteins , and tolerance is associated with decreased IRAK1 protein many cell types [26–31]. Alternatively, signaling through IRAK1 may be impaired due to decreased TLR4-MyD88 complex formation , lack of dissociation from the receptor complex , or increased function of inhibitory forms of IRAK such as IRAK-M . Hypo-responsiveness may also be due to events closer to activation of transcription factors, for example, abrogation of IκBα polyubiqitination , over-expression of unique IκB inhibitory proteins , or production of NF-κB p50 homo-dimers , or other factors that block NF-κB DNA binding . These diverse negative regulatory processes may be generally important to protect the host from overly exuberant, destructive inflammatory responses.
In cystic fibrosis (CF), the lack of functioning CFTR impairs mucociliary and cough clearance , forming a nidus for infection and allowing organisms such as Pseudomonas aeruginosa (Ps. a.) to evade host defenses. Inability to clear infected mucus results in continuous exposure of airway epithelial cells to bacteria and their products. Ongoing host-pathogen interactions determine the extent of the inflammatory response, and in turn, rates of tissue destruction and loss of pulmonary function. Strategically located between luminal bacterial masses and the host circulation, the airway epithelium is in a key position to regulate inflammation, but it remains unknown whether the airway epithelium becomes either hypersensitive or tolerant to the chronic presence of bacterial products. Our goals were to characterize the response of well-differentiated, primary hTBE cells to an apical surface challenge with Ps. a. products, to determine whether repeated exposure induced tolerance and, if so, to explore the mechanism(s). We show that Ps. a. products activate the TLR pathway and that hTBE cells become tolerant via a mechanism likely involving down-regulation of IRAK1.
Antibodies against phosphorylated or total c-jun NH2-terminal kinases (JNK), p38, extracellular signal-regulated kinases (ERK), and total IκBα were from Cell Signaling Technology (Beverly, MA). Antibody against IRAK1 was from Upstate Biotechnology, Inc. (Lake Placid, NY). Anti-NF-κB p50 and p65 subunits were from Santa Cruz Biotechnology (Santa Cruz, CA). An ELISA kit for IL-8 (DuoSet ELISA Development System) was from R&D Systems (Minneapolis, MN). An in vitro toxicology assay kit for lactate dehydrogenase (LDH) was from Sigma (St. Louis, MO), as were other standard reagents unless otherwise specified.
Preparation of Ps. a. filtrate
Ps. a. strain ATCC 27853 was grown in trypticase soy broth (TSB) for 72 hours at 37°C with shaking at 250 RPM. Following centrifugation at 5,500 × G (4°C) for 30 minutes, the supernatant was 0.45 μm filtered, aliquoted and stored at -20°C. TSB treated similarly was used as a control. When used in experiments with cell lines cultured on plastic, Ps. a. filtrates or TSB were boiled for 10 minutes to eliminate protease activity. Consistent with prior reports , we found IL-8 stimulatory activity in Ps. a. filtrates to be heat-resistant.
Under an Institutional Review Board-approved protocol, hTBE cell cultures were prepared as previously described. Briefly, epithelial cells were removed from the lower trachea and bronchi by protease XIV digestion and cells were plated in BEGM medium on collagen-coated dishes. Passage 2 cells were cultured on type VI collagen (Sigma) coated Millicell CM inserts (0.4 μM pore size, Millipore Corporation, Bedford, MA) in ALI medium. The cell seeding density for 10 mm and 30 mm diameter inserts was 0.15 × 106 and 1 × 106 cells per insert, respectively. Following confluence after 5–7 days, cultures were maintained with an air-liquid interface until well-differentiated and were used at 21 days. Endotoxin in ALI medium was less than 100 pg/ml (LAL assay, Bio-Whittaker, Walkersville, MD). A recently described immortalized cell line, referred to as AALEB, was derived from hTBE cells by infection with retroviruses expressing SV40 early region and telomerase reverse transcriptase . AALEB cells were grown on plastic dishes in BEGM medium under standard culture conditions.
Ps. a. filtrate challenge
Experiments with well-differentiated hTBE cells on 12 and 30 mm Millicell inserts were performed in 12 or 6 well plates, respectively. Before challenge, the apical culture surface was rinsed once with Dulbecco's PBS. Ps. a. filtrate in ALI medium supplemented with 10% human serum (Sigma #H4522) was added to the apical culture surface, using 100 μl for 12 mm inserts and 500 μl for 30 mm inserts. One or two ml of ALI medium was added to the basolateral side of the 12 or 6 well plates, respectively, and the cultures were incubated at 37°C in 5% CO2 for 24 hours. Following removal of basolateral medium for IL-8 or LDH assay, the apical and basolateral surfaces were washed twice with PBS. After incubation at 37°C for 1–2 hours with fresh basolateral ALI medium, the cultures were re-challenged apically with Ps. a. filtrate plus serum as described above. Challenges with IL-1β (10 ng/ml) or TNFα (25 ng/ml) were performed in ALI medium without serum. IL-8 and LDH assays were performed with commercial kits as specified previously  and are based on results from triplicate wells using cells from at least three different individuals, unless stated otherwise.
Western blot analysis
At specified times following challenge, cells were harvested from 30 mm inserts into ice-cold lysis buffer (100 mM TrisHCl pH 8.0, 100 mM NaCl, 5.0 mM NaF, 2 mM EDTA, 1% NP-40, 1 mM Na3VO4, 100 μM TPCK, 100 μM quercetin, 1 mM PMSF, 1 μg/ml leupeptin, and 1 μg/ml pepstatin) using a cell scraper, transferred to tubes and set on ice for 20 minutes. Following centrifugation, protein concentrations were determined using the BCA Protein Assay Reagent (Pierce, Rockford, IL). Samples were resolved by SDS-PAGE (4–20% tris-glycine gels, Invitrogen, San Diego, CA) and blotted onto Immobilon-P membranes (Millipore Corp., Bedford, MA). Blots were blocked in TBS with 0.05% Tween 20 and 5% dry milk powder, incubated with primary then secondary antibodies (Jackson ImmunoResearch Laboratories, Inc., West Grove, PA) followed by chemiluminescence detection of peroxidase (Pierce).
Cells were scraped from 30 mm inserts into 0.8 ml ice-cold PBS containing protease inhibitors (1 μM PMSF, 1 μg/ml leupeptin, 1 μg/ml pepstatin, and 0.5 μM DTT) and were centrifuged. The cell pellet was re-suspended in buffer (10 mM Hepes, pH 7.9, 10 mM KCl, 1.5 mM MgCl2, 0.2 mM EDTA, 0.5 mM DTT, 0.25% NP-40, 1 μM PMSF, 1 μg/ml leupeptin and 1 μg/ml pepstatin) on ice for 10 minutes and cells were lysed with a Dounce homogenizer (Kontes Scientific Glassware, Vineland, NJ). The nuclei were extracted with high salt buffer (20 mM Hepes, pH 7.9, 450 mM NaCl, 1.5 mM MgCl2, 0.2 mM EDTA, 0.5 mM DTT, 25% glycerol, 1 μM PMSF, 1 μg/ml leupeptin and 1 μg/ml pepstatin) on ice for 20 minutes with occasional vortexing. Supernatants were prepared by centrifugation and protein concentration was determined as above.
Electrophoretic mobility shift assays
NF-κB-specific consensus oligonucleotide (5' AGTTGAGGGGACTTTCCCAGGC3') and AP1-specific consensus oligonucleotide (5' CGCTTGATGAGTCAGCCGGAA3') were from Promega (Madison, WI). DNA probes were 32P end labeled with T4 polynucleotide kinase (Promega). Nuclear extracts (2.5 μg) were incubated with 40,000–60,000 cpm of 32P end labeled oligonucleotide probe in binding buffer (final volume of 10 μl) containing 1 μg poly dI-dC (Sigma), 10 mM TrisHCl, pH 7.9, 50 mM KCl, 1 mM DTT, 0.25 mg/ml BSA, 4% glycerol for 20 minutes at room temperature. For supershift analysis, nuclear extracts were preincubated with 0.5 μl of antisera against NF-κB p50 or p65 sub-units for 10 minutes in binding buffer. Unlabeled NF-κB, AP-1 or SP1 (5' ATTCGATCGGGGCGGGGCGAGC3') oligonucleotides were used as competitors. Complexes were separated on 5% non-denaturing polyacrylamide-urea gels, which were dried and exposed to a PhosphorImager screen (Amersham Pharmacia Biotech, Piscataway, NJ).
IRAK1 in vitro kinase assay
Cells were harvested from 30 mm inserts into 600 μl ice-cold lysis buffer (50 mM HEPES, pH 7.6, 150 mM NaCl, 1 mM EDTA, 1% NP-40, 1 mM Na3VO4, 20 mMβ-glycerophosphate, 1 mM NaF, 1 mM benzamidine, 5 mM para-nitrophenylphosphate, 1 mM DTT, 1 mM PMSF, 1 μg/ml leupeptin, 1 μg/ml aprotinin, and 1 μg/ml pepstatin) using a cell scraper. After brief vortexing and incubation on ice for 20 minutes, tubes were centrifuged and supernatant protein concentrations were determined as above. For immunoprecipitation, 1000 μg of protein extract was precleared with 2 μg of normal rabbit IgG and 20 μl of protein G-agarose slurry. Protein G beads were pelleted and the supernatant was incubated with 2 μg of rabbit IgG against IRAK1. Protein G-agarose slurry (20 μl) was added and incubated for 1 hour and the beads were washed 3 times with lysis buffer and twice with kinase buffer without 32P ATP (see below). Beads were suspended in 20 μl of kinase buffer (20 mM Tris-HCl, pH 7.6. 20 mM MgCl2, 20 mM β-glycerophosphate, 1 mM benzamidine, 20 mM para-nitrophenylphosphate, 0.4 mM PMSF; 1 mM sodium metabisulfite, 2 μM cold ATP and 10 μCi γ-32P ATP). Reactions were allowed to proceed at 30°C for 30 min and terminated with SDS sample buffer. Samples were then run on 4–20% polyacrylamide gels, dried, and exposed to a PhosphorImager screen as above.
NF-κB reporter assay and expression of dominant negative IRAK1
Adenoviral vectors constitutvely expressing the LacZ gene from the CMV promoter (Ad.CMV-lacZ) and NF-κB-responsive firefly luciferase (Ad.NF-κB-fLuc) have been described previously . We created an adenoviral vector expressing the DD domain of IRAK1 (NCBI accession # NM_001569, amino acids 1–80), which is reported to function as a dominant negative (dn) . A plasmid encoding human IRAK1 was kindly provided by Dr. X. Li (The Cleveland Clinic, Cleveland, OH) and was used as a template for PCR using primers (forward: 5'-CTC GAG GTG CCA GGC TGT GA-3', reverse: 5'-GCT AGC CGG CAG CCA TGG-3') adding 5' and 3' XhoI and NheI sites, respectively. The amplified fragment was cloned into the pCR2.1 vector (Invitrogen). The resulting plasmid was digested with XhoI /NheI and the fragment was ligated into XhoI /NheI-digested pShuttle-IRES-hrGFP-1 adenoviral expression vector plasmid (Stratagene, LaJolla, CA). This strategy placed the DD domain of IRAK1 in frame with the 3X Flag tag of the vector, and the final construct was verified by sequencing. Adenoviral vectors were created, plaque-purified and amplified using conventional methods. AALEB or hTBE cells were transfected with Ad.CMV-lacZ and Ad.NF-κB-fLuc and pShuttle-IRES-hrGFP-1 empty vector or pShuttle-dnIRAK1-IRES-hrGFP-1, using a 1:10 ratio of reporter and expression vectors, respectively. Cells were exposed to viruses for 2 hours, 48 hours prior to experimental challenge, using transient permeabilization  for hTBE cells. Eight hours after challenge, cells were lysed and fLuc and β-galactosidase activity measured as described previously [46, 47].
The initial response of hTBE cells to Ps. a. products
The response of hTBE cells to repeated Ps. a. challenge
IRAK-1 as a critical determinant of hTBE cell sensitivity to Ps. a.
To examine the effects of Ps. a. on NF-κB dependent gene transcription, we transfected hTBE and AALEB cells with adenoviruses expressing NF-κB-driven luciferase and constitutively expressed LacZ reporter genes. Transient permeabilization with caproic acid  was necessary for efficient gene transfer to hTBE cells. Luciferase activity normalized for β gal was measured at baseline and 8 and 24 hours following stimulation with the 4 possible combinations of Ps. a. or TSB. Ps. a. strongly simulated NF-κB-driven luciferase activity in naive or TSB pre-treated hTBE or AALEB cells, but the response was strongly attenuated in Ps. a. pretreated cells (compare the slopes of the 0–8 hour and 25–33 hour groups in Figure 7C and 7D). These results suggest a strong NF-κB driven transcriptional component of Ps. a. stimulation that was inhibited in tolerant airway epithelial cells.
Having established a reporter assay in a relevant cell type, it was now possible to examine whether IRAK1 down-regulation was correlative or causal in airway epithelial tolerance. AALEB cells were transfected with a 1:10 ratio of adenoviral particles expressing both reporter genes and dnIRAK1  or the empty vector control, respectively. In both hTBE and AALEB cells, NF-κB driven luciferase activity was approximately 5-fold greater in response to Ps. a. in cells infected with the control, empty vector. Luciferase activity was reduced significantly (52% in hTBE cells, n = 2 independent experiments; 55 ± 8 % in AALEB cells, n = 3 independent experiments), but not completely, by expression of dnIRAK1. Representative experiments are illustrated in Figure 7E and 7F, respectively. In AALEB cells, the dnIRAK1 construct caused an average 86 ± 1% decrease in IL-1β-stimulated NF-κB-driven luciferase activity but did not affect TNFα stimulated activity. These results confirm an important role for IRAK1 in the response to Ps. a. products, presumably via its integral function in the MyD88-dependent portion of the TLR pathway, and strongly suggest that loss or inhibition of IRAK1 is an important component of tolerance.
The host must eradicate pathogens while preventing tissue injury due to inflammation. Repeated exposure of airway epithelium to microbial products is a hallmark of chronic infectious lung diseases but the adaptation has not been previously studied in these key cells. Polarized, well-differentiated hTBE cell cultures used in the current studies recapitulate the morphology and mucus transport function found in vivo . We challenged the apical surface of these cultures with soluble products of Ps. a., an important airway pathogen. Prior studies focused on the direct interaction of live Ps. a. bacteria with epithelial cells (see  for review). However, bacteria in chronically infected CF lungs are present mostly as intra-luminal masses distal from the airway epithelial surface . Thus, we chose to model the interaction of the apical membrane of polarized hTBE cells with diffusible Ps. a. products, rather than live bacteria. We used late stationary phase TSB cultures of a common strain of Ps. a. instead of clinical isolates. Ps. a. in CF patients frequently become mucoid, produce modified forms of LPS  and also secrete quorum sensing molecules . Future studies will be necessary to determine whether modifications of Ps. a. typically found in chronic human infections alter epithelial responses. Although we used non-inoculated TSB processed in parallel as a negative control, we cannot completely exclude that a portion of the stimulation was due to TSB breakdown products induced by bacterial metabolism of the broth. Well-differentiated hTBE cells likely have greater physiologic relevance than cell lines grown directly on plastic. However, well-characterized cell lines facilitate mechanistic studies, so we also used a newly developed airway epithelial cell line (AALEB).
Generic IRAK was first recognized as a crucial component of the IL-1β signalling pathway and molecular cloning revealed homology to the Drosophila protein kinase Pelle . More recent studies have revealed 4 IRAK isoforms , a critical role for tandem action of IRAK1 and 4  and an inhibitory role for IRAK-M . Ligation of TIR domain-containing receptors induces IRAK1 phosphorylation both auto-catalytically and by unidentified kinases. Phosphorylated IRAK1 becomes degraded by proteasomes in cells stimulated by IL-1β but not TNFα, which correspondingly results in desensitization of the IL-1β, but not TNFα response . The precise mechanisms regulating IRAK1 localization and function during the propagation and termination of the TLR signal in hTBE cells are not fully understood, and require further study.
While the kinetics of the initial response of naive cells suggested a direct interaction of Ps. a. products to trigger the TLR pathway, airway epithelial cells at 24 hours were likely exposed to released autocrine/paracrine factors that may have also contributed to tolerance. However, this was probably not mediated by IL-1β, since this cytokine was consistently undetectable in hTBE cell media after Ps. a. stimulation (SHR and MWV unpublished observations). Moreover, such factors would have to selectively target the upstream TLR/IL-1β, pathway since TNFα responses were not affected.
Ps. a. is a complex and adaptable bacterium that secretes many factors known to damage host cells and/or induce inflammation such as pilin, flagellin, pyocyanin, hemolysins, autoinducer, LPS, proteases, and small unidentified heat-stable factors [9, 40, 50, 56–59]. There is a broad array of mechanisms by which these agents may act, including TLR activation, generalized oxidative stress due to redox cycling of pyocyanin, and proteolytic modification of target cells by Ps. a. elastase or alkaline protease. IRAK activation in a human bronchial cell line has been shown following neutrophil elastase . Rapid activation of IRAK1, in conjunction with desensitization of the IL-1β but not the TNFα response, suggests that Ps. a. products directly trigger an MyD88-dependent TLR pathway. Activation of airway epithelial cells by a variety of live bacteria and bacterial products is TLR2-dependent [10, 12–14]. Polymyxin B treatment is commonly used to block LPS effects , but it did not reduce Ps. a.-stimulated hTBE cell IL-8 secretion, and Ps. a. derived LPS is a poor inducer of IL-8 in these cells (SHR and MWV unpublished observations), suggesting that LPS stimulation of TLR4 was not predominant. Further studies are needed to chemically identify the most quantitatively important TLR pathway-stimulating substance(s) produced by Pseudomonas aeruginosa. In this regard, recent studies indicate a potential role for TLR2 activation by lipopeptides encoded by Ps. a. genes .
While controversial, several lines of evidence suggest that inflammatory responses in the CF airway may be intrinsically enhanced or protracted due to the absence of functioning CFTR (see  and references therein). Thus, examination of differences in the development of tolerance in CF cells is worthy of further scrutiny. Although hTBE cells in our in vitro model became tolerant to Ps. a., chronically infected airways in CF humans are typically severely inflamed. Whether tolerance exists in vivo, if it is helpful or harmful, different in CF, or if it is overwhelmed by the heavy bacterial burden are important, but unanswered questions. Multiple cell types, which are constantly turning over, contribute to in vivo responses. Bacterial products may penetrate beyond the epithelial barrier to trigger inflammation or systemic responses. We speculate that epithelial tolerance to Ps. a. occurs in vivo, and that inflammation would be even more severe in its absence. However, further clinical and/or animal model studies are needed to address these key questions.
We have shown that exposure of primary, well-differentiated hTBE cells to Ps. a. products causes selective tolerance. Ps. a. products elicit a response via the TLR signal transduction pathway that becomes down-regulated, likely due to decreased IRAK1 protein content and inhibition of IRAK1 phosphorylation. A greater understanding of the precise mechanisms decreasing airway epithelial cell generation of inflammatory mediators will suggest new avenues for anti-inflammatory therapies to minimize lung destruction typical of CF and other chronic infectious airway diseases.
The authors thank the staff of the UNC CF Center Tissue Procurement and Cell Culture Core for providing human airway epithelial cells, Kelly Plonk for technical assistance and Lisa Brown for help preparing the manuscript. We thank Dr. X. Li for IRAK plasmids and Dr. J. Engelhardt for the Ad.NF-κB-fLuc vector. The UNC Gene Therapy Center Vector Core assisted by producing adenovirus. Supported by grants from the Cystic Fibrosis Foundation and NIH to SHR.
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