Nitrite reductase from Pseudomonas aeruginosa induces inflammatory cytokines in cultured respiratory cells

Pseudomonas pyocyanin stimulates IL-8 expression through MAPK and NF-κB pathways in differentiated U937 cells

Background

Pyocyanin (PCN), an extracellular product of Pseudomonas aeruginosa and a blue redox active secondary metabolite, plays an important role in invasive pulmonary infection. However, the detailed inflammatory response triggered by PCN infection in inflammatory cells (particularly macrophages), if present, remains to be clarified. To investigate the effects of PCN on macrophages, the ability of PCN to induce inflammation reaction and the signaling pathway for IL-8 release in PCN-induced differentiated U937 cells were examined.

Results

It was found that PCN increased IL-8 release and mRNA expression in Phorbol 12-myristate 13-acetate (PMA) differentiated U937 cells in both a concentration- and time-dependent manner by reverse transcription-polymerase chain reaction (RT-PCR) and enzyme-linked immunosorbent assay (ELISA). P38 and ERK MAPKs were activated after 10 min of induction with PCN and their levels returned to baselines after 30 min by Western blotting. It was also found that within 10 min of PCN incubation, the level of p-I-κBα in the cytosol was increased, which returned to baseline level after 60 min. Meanwhile, the level of p-p65 was increased in the nuclear extract and cytosol, and maintained high in total cell lysates. The results were further confirmed by the observation that p38, ERK1/2 and NF-κB inhibitors inhibited PCN-induced NF-κB activation and attenuated PCN-induced IL-8 expression in U937 cells as a function of their concentrations. Moreover, it was shown that PCN induced oxidative stress in U937 cells and N-acetyl cysteine, an antioxidant, was able to inhibit PCN-induced IL-8 protein expression.

Conclusions

It is concluded that PCN induces IL-8 secretion and mRNA expression in PMA-differentiated U937 cells in a concentration- and time- dependent manner. Furthermore, p38 and ERK MAPKs and NF-κΒ signaling pathways may be involved in the expression of IL-8 in PCN-incubated PMA-differentiated U937 cells.

Airway epithelial cell tolerance to Pseudomonas aeruginosa

Background

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).

Methods

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.

Results

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.

Conclusion

The airway epithelial cell response to Pseudomonas aeruginosa entails adaptation and tolerance likely mediated, in part, by down-regulation of IRAK1.

Microarray analysis and functional characterization of the nitrosative stress response in nonmucoid and mucoid Pseudomonas aeruginosa

The type strain of Pseudomonas aeruginosa, PAO1, showed great upregulation of many nitrosative defense genes upon treatment with S-nitrosoglutathione, while the mucoid strain PAO578II showed no further upregulation above its constitutive upregulation of nor and fhp. NO* consumption however, showed that both strains mount functional, protein synthesis-dependent NO*-consumptive responses.

The Pseudomonas aeruginosa autoinducer N-3-oxododecanoyl homoserine lactone accelerates apoptosis in macrophages and neutrophils

Quorum-sensing systems are critical regulators of the expression of virulence factors of various organisms, including Pseudomonas aeruginosa. Las and Rhl are two major quorum-sensing components, and they are regulated by their corresponding autoinducers, N-3-oxododecanoyl homoserine lactone (3-oxo-C(12)-HSL) and N-butyryl-L-homoserine lactone (C(4)-HSL). Recent progress has demonstrated the potential of quorum-sensing molecules, especially 3-oxo-C(12)-HSL, for modulation of the host immune system. Here we show the specific ability of 3-oxo-C(12)-HSL to induce apoptosis in certain types of cells. When bone marrow-derived macrophages were incubated with synthetic 3-oxo-C(12)-HSL, but when they were incubated not C(4)-HSL, significant loss of viability was observed in a concentration (12 to 50 micro M)- and incubation time (1 to 24 h)-dependent manner. The cytotoxic activity of 3-oxo-C(12)-HSL was also observed in neutrophils and monocytic cell lines U-937 and P388D1 but not in epithelial cell lines CCL-185 and HEp-2. Cells treated with 3-oxo-C(12)-HSL revealed morphological alterations indicative of apoptosis. Acceleration of apoptosis in 3-oxo-C(12)-HSL-treated cells was confirmed by multiple criteria (caspases 3 and 8, histone-associated DNA fragments, phosphatidylserine expression). Structure-activity correlation experiments demonstrated that the fine structure of 3-oxo-C(12)-HSL, the HSL backbone, and side chain length are required for maximal activity. These data suggest that Pseudomonas 3-oxo-C(12)-HSL specifically promotes induction of apoptosis, which may be associated with 3-oxo-C(12)-HSL-induced cytotoxicity in macrophages and neutrophils. Our data suggest that the quorum-sensing molecule 3-oxo-C(12)-HSL has critical roles in the pathogenesis of P. aeruginosa infection, not only in the induction of bacterial virulence factors but also in the modulation of host responses.

Metalloproteases from Pseudomonas aeruginosa degrade human RANTES, MCP-1, and ENA-78

The gram-negative bacterium Pseudomonas aeruginosa is an opportunistic human pathogen associated with both an acute lung disease in patients with hospital-acquired pneumonia and a chronic, progressive lung disease in individuals with cystic fibrosis. A unique characteristic of this bacterium in its natural environment is the secretion of a wide variety of factors designed to ensure its growth and survival. Evidence suggests, however, that when present in the human host, these same factors may contribute to disease. In the course of studying the effect of P. aeruginosa secretory factors on airway epithelial cells, we observed that metalloproteases in bacterial-conditioned medium, as well as purified alkaline protease and elastase, degraded human RANTES, monocyte chemotactic protein-1 (MCP-1), and epithelial neutrophil-activating protein-78 (ENA-78). Under identical conditions, interleukin-8 (IL-8) was significantly more resistant to proteolysis. Degradation was accompanied by a loss of chemotactic activity. These data suggest that metalloproteases from P. aeruginosa could alter the relative amounts of critical immunomodulatory cytokines in the airway and, thus, could contribute to the pathophysiology observed in P. aeruginosa-associated lung disease.

Fourteen-member macrolides promote the phosphatidylserine receptor-dependent phagocytosis of apoptotic neutrophils by alveolar macrophages

An inflammation of the airway of patients with diffuse panbronchiolitis (DPB), is characterized by dense neutrophil infiltration. Resolution of the inflammation can be achieved by the removal of apoptotic neutrophils by human alveolar macrophages (AM) without liberating neutrophil proteases in the airway. To understand clinical efficacy for the treatment of DPB by 14- or 15-member macrolides, their effects on the phagocytosis of apoptotic neutrophils by AM were examined. Treatment of AM with erythromycin (ERY) or clarithromycin at clinically achievable levels significantly increased the levels of phagocytosis of apoptotic neutrophils. A serum factor was not essential for the enhancement by these 14-member macrolides. Of the antibiotics tested, these effects were specific for the 14-member macrolides and a 15-member macrolide, azithromycin, but not for the 16-member macrolides, clindamycin or beta-lactam antibiotics. The enhanced phagocytosis of apoptotic neutrophils by ERY had no effect on the levels of interleukin-8 or tumor necrosis factor alpha production by lipopolysaccharide-stimulated AM after phagocytosis of the apoptotic neutrophils. The increased phagocytosis of apoptotic neutrophils by ERY was also found to be phosphatidylserine receptor-dependent for AM. These data indicate a novel anti-inflammatory action of 14-member and 15-member macrolides, and suggest that such antibiotics achieve clinical efficacy for patients with DPB, in part, through enhancing the nonphlogistic phagocytosis of apoptotic neutrophils by AM.

Anaerobic metabolism and quorum sensing by Pseudomonas aeruginosa biofilms in chronically infected cystic fibrosis airways: rethinking antibiotic treatment strategies and drug targets

Recent evidence indicates that Pseudomonas aeruginosa residing as biofilms in airway mucus of cystic fibrosis (CF) patients is undergoing anaerobic metabolism, a form of growth requiring gene products that are not utilized during aerobic growth. The outer membrane protein, OprF, and the rhl quorum sensing circuit are two previously unrecognized cellular factors that are required for optimal anaerobic biofilm viability. Without OprF, bacteria grow extremely poorly because they lack nitrite reductase activity while lacking rhlR or rhlI forces bacteria to undergo metabolic suicide by overproduction of nitric oxide. Furthermore, anaerobic growth favors maintenance of the mucoid, alginate-overproducing phenotype. Thus, with increasing age of CF patients, mucoid populations predominate, indicating that anaerobic bacteria reside in the inspissated airway mucus. Because many frontline antibiotics used in the treatment of CF airway disease are either ineffective or show reduced efficacy during anaerobic conditions, we propose development of new drugs to combat anaerobic metabolism by P. aeruginosa for more effective treatment of chronic CF lung infections.

Crystal structure of the soluble domain of the major anaerobically induced outer membrane protein (AniA) from pathogenic Neisseria: a new class of copper-containing nitrite reductases

The major anaerobically induced outer membrane protein (AniA) from pathogenic Neisseria gonorrhoeae is essential for cell growth under oxygen limiting conditions in the presence of nitrite and is protective against killing by human sera. A phylogenic analysis indicates that AniA is a member of a new class of copper-containing nitrite reductases. Expression of the soluble domain of AniA yields a protein capable of reducing nitrite with specific activity of 160 units/mg, approximately 50 % of that measured for the nitrite reductase from the strong soil denitrifier Alcaligenes faecalis S-6. The crystal structure of the soluble domain of AniA was solved by molecular replacement and sixfold averaging to a resolution of 2.4 A. The nitrite soaked AniA crystal structure refined to 1.95 A reveals a bidentate mode of substrate binding to the type II copper. Despite low sequence identity (approximately 30 %), the core cupredoxin fold of AniA is similar to that found in copper-containing nitrite reductases from soil bacteria. The main structural differences are localized to two attenuated surface loops that map to deletions in the sequence alignment. In soil nitrite reductases, one of these surface loops is positioned near the type I copper site and contributes residues to the docking surface for proteaceous electron donors. In AniA, the attenuation of this loop results in a restructured hydrophobic binding surface that may be required to interact with a lipid anchored azurin. The second attenuated loop is positioned on the opposite side of AniA and may facilitate a more intimate interaction with the lipid membrane. A unique combination of structural effectors surrounding the type I copper site of sAnia contribute to a unusual visible absorption spectra with components observed previously in either green or blue type I copper sites.

Small molecular weight secretory factors from Pseudomonas aeruginosa have opposite effects on IL-8 and RANTES expression by human airway epithelial cells

Pseudomonas aeruginosa is an opportunistic human pathogen that causes both an acute lung disease in patients with hospital-acquired pneumonia and a chronic lung disease in individuals with cystic fibrosis. Many of the pathophysiologic effects of P. aeruginosa infection are due to factors secreted by the bacterium. Conditioned media from cultures of P. aeruginosa increased interleukin-8 expression and decreased regulated on activation, normal T cells expressed and secreted (RANTES) expression by human airway epithelial cells. Both of these activities were present in heat-treated, protease-treated, small molecular weight fractions. The activities were not inhibited by polymyxin B and were not extracted into ethyl acetate, suggesting that they were not due to endotoxin or autoinducer. Conversely, results from chloroform extractions and studies with a phenazine-minus mutant suggested that the blue pigment pyocyanin contributes to these activities when present. In addition to the effects of small molecular weight factors on cytokine expression, proteases in bacterial-conditioned media further decreased levels of RANTES. By altering expression, release, and/or activity of inflammatory cytokines, secretory factors from P. aeruginosa could disrupt the delicate balance that constitutes the immune response to bacterial infection and thus could contribute to the lung damage that occurs in P. aeruginosa-infected airways.

Role of cytokine-induced neutrophil chemoattractant-2 (CINC-2) alpha in a rat model of chronic bronchopulmonary infections with Pseudomonas aeruginosa

In order to investigate the role of the cytokine-induced neutrophil chemoattractant (CINC) in chronic bronchopulmonary infection, we developed a rat model of bronchopulmonary infection with Pseudomonas aeruginosa by using the agar bead method, and determined the kinetics of bacterial and cell number, as well as the concentrations of CINC-1, CINC-2, and CINC-3 in bronchoalveolar lavage (BAL) fluids in this model. The bacterial number in the lung rapidly increased from days 1 to 4, and declined 14 days after challenge. Neutrophil number in BAL fluid increased up to one day after challenge, and then slowly decreased during 14 days post-challenge. Among the CINCs, the local production of CINC-2 alpha sharply increased at day 1 and then decreased until day 4 post-challenge, while the local production of CINC-1 slightly increased at day 1 post-challenge. Neither CINC-2 beta nor CINC-3 were detected during the entire course of the infection. Increased CINC-2 mRNA expression in the lung tissue after challenge was associated with CINC-2 alpha production in BAL fluid. Moreover, an immunohistochemical study demonstrated the localization of CINC-1 and CINC-2 alpha primarily in alveolar macrophages and, to a much lesser extent, in bronchial epithelium of infected lung tissues, whereas CINC-2 beta and CINC-3 were not detected. When anti-CINC-1 or anti-CINC-2 alpha polyclonal antibodies were used for neutralizing neutrophil chemotactic activities in BAL fluids, the anti-CINC-2 alpha antibody inhibited 70% of the chemotactic activity in BAL fluids from infected rats at day 1 after challenge. No inhibition was observed by anti-CINC-1 antibody. These data indicate that CINC-2 alpha, which is produced by alveolar macrophages and bronchial epithelial cells, plays a pivotal role in neutrophil accumulation in the airway of a rat model of chronic bronchopulmonary infection with P. aeruginosa.

Induction of proinflammatory cytokines from human respiratory epithelial cells after stimulation by nontypeable Haemophilus influenzae

Nontypeable Haemophilus influenzae (NTHi) causes repeated respiratory infections in patients with chronic lung diseases. These infections are characterized by a brisk inflammatory response which results in the accumulation of polymorphonucleated cells in the lungs and is dependent on the expression and secretion of proinflammatory cytokines. We hypothesize that multiple NTHi molecules, including lipooligosaccharide (LOS), mediate cellular interactions with respiratory epithelial cells, leading to the production of proinflammatory cytokines. To address this hypothesis, we exposed 9HTEo- human tracheal epithelial cells to NTHi and compared the resulting profiles of cytokine gene expression and secretion using multiprobe RNase protection assays and enzyme-linked immunosorbent assays (ELISA), respectively. Dose-response experiments demonstrated a maximum stimulation of most cytokines tested, using a ratio of 100 NTHi bacterial cells to 1 9HTEo- tracheal epithelial cell. Compared with purified LOS, NTHi bacterial cells stimulated 3.6- and 4.5-fold increases in epithelial cell expression of interleukin-8 (IL-8) and IL-6 genes, respectively. Similar results were seen with epithelial cell macrophage chemotactic protein 1, IL-1alpha, IL-1beta, and tumor necrosis factor alpha expression. Polymyxin B completely inhibited LOS stimulation but only partially reduced NTHi whole cell stimulation. Taken together, these results suggest that multiple bacterial molecules including LOS contribute to the NTHi stimulation of respiratory epithelial cell cytokine production. Moreover, no correlation was seen between NTHi adherence to epithelial cells mediated by hemagglutinating pili, Hia, HMW1, HMW2, and Hap and epithelial cytokine secretion. These data suggest that bacterial molecules beyond previously described NTHi cell surface adhesins and LOS play a role in the induction of proinflammatory cytokines from respiratory epithelial cells.

CXC chemokine receptor CXCR2 is essential for protective innate host response in murine Pseudomonas aeruginosa pneumonia

Pulmonary infection due to Pseudomonas aeruginosa has emerged as a leading cause of mortality. A vigorous host response is required to effectively clear the organisms from the lungs. This host defense is dependent on the recruitment and activation of neutrophils and macrophages. A family of chemotactic cytokines (chemokines) has been shown to participate in this protective response. In this study, we assessed the role of the ELR(+) (glutamic acid-leucine-arginine motif positive) CXC chemokines and their CXC chemokine receptor (CXCR2) in lung antibacterial host defense. The intratracheal administration of Pseudomonas to mice resulted in the time-dependent influx of neutrophils to the lung, peaking at 12 to 24 h after inoculation. The influx of neutrophils was associated with a similar time-dependent expression of the ELR(+) CXC chemokines, KC, macrophage inflammatory protein 2 (MIP-2), and lipopolysaccharide-induced CXC chemokine (LIX). Selective neutralization of MIP-2 or KC resulted in modest changes in neutrophil influx but no change in bacterial clearance or survival. However, neutralization of CXCR2 resulted in a striking increase in mortality, which was associated with a marked decrease in neutrophil recruitment and bacterial clearance. Conversely, the site-specific transgenic expression of KC resulted in enhanced clearance of bacteria after Pseudomonas challenge. This study indicates that ELR(+) CXC chemokines are critical mediators of neutrophil-mediated host defense in Pseudomonas pneumonia.

Pro-inflammatory cytokine/chemokine gene expression in human corneal epithelial cells colonized by Pseudomonas aeruginosa

Pro-inflammatory cytokines interleukin-6 (IL-6), interleukin-1beta (IL- 1beta), tumour necrosis factor-alpha (TNF-alpha) and chemokine interleukin-8 (IL-8) may play a significant role in the regulation of bacterial corneal infection. The aim of this study was to investigate the gene expression of these four mediators in a human corneal epithelial cell line challenged with Pseudomonas aeruginosa within 12 h. Human corneal epithelial monolayers were colonized with P. aeruginosa strains Paerl, 6206 and 6294. Expression of IL-6, IL-8, IL-1beta and TNF-alpha was analysed using semi-quantitative reverse transcription-polymerase chain reaction. Results showed that both IL-6 and IL-8 mRNA were expressed very early (4 h) during bacterial colonization and remained at high levels until the end of the experiment. Expression of IL-1beta and TNF-alpha mRNA appeared at 8 h after bacterial stimulation. No expression of IL-8, IL-1beta and TNF-alpha mRNA was observed in unstimulated cells. Interleukin-6 mRNA was expressed at low levels in unstimulated cells. In conclusion, bacterial colonization of human corneal epithelial cells induced expression of IL-6 and IL-8 mRNA earlier and at higher levels than IL-1beta and TNF-alpha mRNA.

Induction of monocyte chemoattractant protein-1 (MCP-1) production by Pseudomonas nitrite reductase in human pulmonary type II epithelial-like cells

Monocyte chemoattractant protein-1 (MCP-1), a chemoattractant for monocytes, is presumed to play a pivotal role in the recruitment and accumulation of monocytes in various diseases including pulmonary infections. We examined here whether or not Pseudomonas nitrite reductase (PNR), a recently identified IL-8 inducer in various respiratory cells, could stimulate human pulmonary type II epithelial-like cells (A549) to induce MCP-1 production. A time- and dose-dependent induction of MCP-1 protein synthesis associated with an increase of MCP-1 mRNA expression by A549 cells was observed in response to PNR. New protein translation was not required for PNR-mediated MCP-1 mRNA expression in the same cells. When anti-human MCP-1 monoclonal antibody was used for neutralizing of monocyte chemotactic factor (MCF) activities in the culture supernatants of these cells stimulated with PNR, significant reductions of MCF activities (the mean reduction rate; 49-59%, P<0. 05) were observed. These data suggest that PNR may contribute to monocyte migration, through inducing pulmonary epithelial cell-derived MCP-1 production in the airway of patients with pneumonia due to P. aeruginosa.

Induction of interleukin 8 (IL-8) production by Pseudomonas nitrite reductase in human alveolar macrophages and epithelial cells

Interleukin-8 (IL-8) participates in the generation of dense neutrophil accumulations in bronchopulmonary infections caused by Pseudomonas aeruginosa (P. aeruginosa). We have recently reported that nitrite reductase, a bifunctional enzyme located in the periplasmic space of P. aeruginosa, induces IL-8 generation in bronchial epithelial cells (K. Oishi et al. Infect. Immun. 65: 2648-2655, 1997). We examined whether or not Pseudomonas nitrite reductase (PNR) could also stimulate human alveolar macrophages (AM) and pulmonary type II epithelial-like cells (A549) to induce IL-8 production and mRNA expression as well as the production of TNF alpha and IL-1beta. We demonstrated a time- and dose-dependent IL-8 protein synthesis and IL-8 mRNA expression, but no TNF alpha or IL-1beta production, by A549 cells in response to PNR. New protein translation was not required for PNR-mediated IL-8 mRNA expression in the same cells. Furthermore, simultaneous stimulation of PNR with serial doses of TNF alpha or IL-1beta resulted in additive IL-8 production in A549 cells. In adherent AM, PNR enhanced IL-8 protein synthesis and IL-8 mRNA expression in a time-dependent fashion. PNR similarly induced a time-dependent production of TNF alpha and IL-1beta by human adherent AM. Neutralization of TNF alpha or IL-1beta did not influence the levels of IL-8 production in adherent AM culture. We also evaluated whether the culture supernatants of the A549 cells or AM stimulated with PNR could similarly mediate neutrophil migration in vitro. When anti-human IL-8 immunoglobulin G was used for neutralizing neutrophil chemotactic factor (NCF) activities in the culture supernatants of these cells stimulated with 5 microg/ml of PNR, the mean percent reduction of NCF activities were 49-59% in A549 cells and 24-34% in AM. Our present data support that PNR directly stimulates AM and pulmonary epithelial cells to produce IL-8. PNR also mediates neutrophil migration, in part, through IL-8 production from AM and pulmonary epithelial cells. These data suggest the contribution of PNR to the pathogenesis of bronchopulmonary infections due to P. aeruginosa.

Essential role of transcription factor nuclear factor-kappaB in regulation of interleukin-8 gene expression by nitrite reductase from Pseudomonas aeruginosa in respiratory epithelial cells

Persistent infection with Pseudomonas aeruginosa increases interleukin-8 (IL-8) levels and causes dense neutrophil infiltrations in the airways of patients with chronic airway diseases. Recently, we have reported that nitrite reductase from P. aeruginosa induces the production of IL-8 in respiratory cells, including bronchial epithelial cells. To determine the molecular mechanism(s) of nitrite reductase-induced IL-8 expression in respiratory cells, A549 epithelial cells were transfected with plasmids containing serial deletions of the 5'-flanking region of the IL-8 gene and then exposed to nitrite reductase. Nitrite reductase significantly enhanced IL-8 gene promoter-driven reporter activity. This increased IL-8 gene expression was inhibited by mutating the nuclear factor-kappaB (NF-kappaB) binding element. Nitrite reductase enhanced nuclear localization of the NF-kappaB binding complex. Furthermore, nitrite reductase induced the degradation of IkappaBalpha, the major cytoplasmic inhibitor of NF-kappaB, and the expression of IkappaBalpha mRNA. These data support the critical role of the activation of NF-kappaB in nitrite reductase-induced IL-8 gene expression in airway epithelium.

Nitrite reductase from Pseudomonas aeruginosa released by antimicrobial agents and complement induces interleukin-8 production in bronchial epithelial cells

We have recently reported that nitrite reductase, a bifunctional enzyme located in the periplasmic space of Pseudomonas aeruginosa, could induce interleukin-8 (IL-8) generation in a variety of respiratory cells, including bronchial epithelial cells (K. Oishi et al. Infect. Immun. 65:2648-2655, 1997). In this report, we examined the mode of nitrite reductase (PNR) release from a serum-sensitive strain of live P. aeruginosa cells during in vitro treatment with four different antimicrobial agents or human complement. Bacterial killing of P. aeruginosa by antimicrobial agents induced PNR release and mediated IL-8 production in human bronchial epithelial (BET-1A) cells. Among these agents, imipenem demonstrated rapid killing of P. aeruginosa as well as rapid release of PNR and resulted in the highest IL-8 production. Complement-mediated killing of P. aeruginosa was also associated with PNR release and enhanced IL-8 production. The immunoprecipitates of the aliquots of bacterial culture containing imipenem or complement with anti-PNR immunoglobulin G (IgG) induced twofold-higher IL-8 production than did the immunoprecipitates of the aliquots of bacterial culture with a control IgG. These pieces of evidence confirmed that PNR released in the aliquots of bacterial culture was responsible for IL-8 production in the BET-1A cells. Furthermore, the culture supernatants of the BET-1A cells stimulated with aliquots of bacterial culture containing antimicrobial agents or complement similarly mediated neutrophil migration in vitro. These data support the possibility that a potent inducer of IL-8, PNR, could be released from P. aeruginosa after exposure to antimicrobial agents or complement and contributes to neutrophil migration in the airways during bronchopulmonary infections with P. aeruginosa.

Pseudomonas pyocyanin increases interleukin-8 expression by human airway epithelial cells

Pseudomonas aeruginosa, an opportunistic human pathogen, causes acute pneumonia in patients with hospital-acquired infections and is commonly associated with chronic lung disease in individuals with cystic fibrosis (CF). Evidence suggests that the pathophysiological effects of P. aeruginosa are mediated in part by virulence factors secreted by the bacterium. Among these factors is pyocyanin, a redox active compound that increases intracellular oxidant stress. We find that pyocyanin increases release of interleukin-8 (IL-8) by both normal and CF airway epithelial cell lines and by primary airway epithelial cells. Moreover, pyocyanin synergizes with the inflammatory cytokines tumor necrosis factor alpha and IL-1alpha. RNase protection assays indicate that increased IL-8 release is accompanied by increased levels of IL-8 mRNA. The antioxidant n-acetyl cysteine, general inhibitors of protein tyrosine kinases, and specific inhibitors of mitogen-activated protein kinases diminish pyocyanin-dependent increases in IL-8 release. Conversely, inhibitors of protein kinases C (PKC) and PKA have no effect. In contrast to its effects on IL-8 expression, pyocyanin inhibits cytokine-dependent expression of the monocyte/macrophage/T-cell chemokine RANTES. Increased release of IL-8, a potent neutrophil chemoattractant, in response to pyocyanin could contribute to the marked infiltration of neutrophils and subsequent neutrophil-mediated tissue damage that are observed in Pseudomonas-associated lung disease.

How bacteria initiate inflammation: aspects of the emerging story

Recent studies have shown that bacteria possess an array of proinflammatory molecules in addition to the extensively studied lipopolysaccharide and superantigens. These bacterial molecules include soluble and membrane-associated inducers of cytokine release, inducers of host cell apoptosis, and immunostimulatory DNA. There is therefore much greater diversity in the class of molecules and mechanisms by which bacteria engage the host immune system than previously appreciated.