Rhamnolipids Are Virulence Factors That Promote Early Infiltration of Primary Human Airway Epithelia by Pseudomonas aeruginosa
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感染与免疫杂志 2006年第6期
Department of Cell Physiology and Metabolism
Department of Microbiology and Molecular Medicine, Medical Center, University of Geneva
Department of Oto-Rhino-Laryngology, University Hospital of Geneva,Geneva, Switzerland
ABSTRACT
The opportunistic bacterium Pseudomonas aeruginosa causes chronic respiratory infections in cystic fibrosis and immunocompromised individuals. Bacterial adherence to the basolateral domain of the host cells and internalization are thought to participate in P. aeruginosa pathogenicity. However, the mechanism by which the pathogen initially modulates the paracellular permeability of polarized respiratory epithelia remains to be understood. To investigate this mechanism, we have searched for virulence factors secreted by P. aeruginosa that affect the structure of human airway epithelium in the early stages of infection. We have found that only bacterial strains secreting rhamnolipids were efficient in modulating the barrier function of an in vitro-reconstituted human respiratory epithelium, irrespective of their release of elastase and lipopolysaccharide. In contrast to previous reports, we document that P. aeruginosa was not internalized by epithelial cells. We further report that purified rhamnolipids, applied on the surfaces of the epithelia, were sufficient to functionally disrupt the epithelia and to promote the paracellular invasion of rhamnolipid-deficient P. aeruginosa. The mechanism involves the incorporation of rhamnolipids within the host cell membrane, leading to tight-junction alterations. The study provides direct evidence for a hitherto unknown mechanism whereby the junction-dependent barrier of the respiratory epithelium is selectively altered by rhamnolipids.
INTRODUCTION
The airway mucosa is an efficient barrier to protect the host from infection by pathogens. An essential component of this barrier is the polarity of surface epithelial cells, which allows them to segregate the receptors that trigger the adherence and internalization of several pathogens within the basolateral domain of the membrane (51). Normally, this domain is not accessible to microbial organisms, due to the presence of tight junctions (TJ) that separate the apical from the basolateral membrane components and seal the paracellular space (37). To overcome this protective barrier, several microorganisms have developed strategies to alter either the apical membrane of epithelial cells (39) or the TJ barrier (53). Understanding the mechanism underlying these alterations is a prerequisite for development of novel therapeutic strategies targeted to specific molecular and cellular events. This is particularly necessary in the context of infections, such as those caused by Pseudomonas aeruginosa, which is difficult to eradicate by conventional antibiotic treatments (45).
P. aeruginosa is an opportunistic gram negative bacterium that does not invade normal mucosae but causes serious nosocomial infections in immunocompromised individuals and in cystic fibrosis patients (7). The pathogenic mechanism accounting for these infections is not fully clarified and has been variously attributed to the production of different cell-associated and secreted virulence factors (41). The finding that P. aeruginosa is internalized more readily by dispersed and migrating epithelial cells than by fully polarized cells has been taken as an indication that the bacteria need to access the basolateral membrane to interact with the receptors that mediate their internalization (44). This implies that the early steps of P. aeruginosa infection should involve some alterations in the paracellular route of the epithelium. Consistent with this view, several factors produced by P. aeruginosa, including lipopolysaccharide and elastase, have been reported to decrease the transepithelial resistance (TER) of various epithelia and to decrease the expression of TJ-associated proteins (2, 3, 5, 57). As yet, however, the specificity of these alterations remains to be determined. Indeed, in addition to the factors mentioned above, the virulence of P. aeruginosa may be also attributed to the type III secretion system (TTSS), which controls the production of cytotoxic proteins delivered to the host cells (41), as well as to factors regulated by the quorum-sensing (QS) systems (54). In P. aeruginosa, two QS systems, called Las and Rhl, control the expression of more than 100 genes in a cell-density-dependent manner (54). Once a sufficient amount of autoinducer molecules has accumulated, these signaling molecules bind to their cognate transcriptional activators LasR and RhlR. LasR regulates the transcription of several virulence genes, including lasA, lasB, and toxA, whereas RhlR enhances the transcription of lasB and the rhamnolipid synthesis genes rhlAB (for a review, see reference 48). Furthermore, previous studies have not shown whether the paracellular route was directly affected by Pseudomonas or tested the relevance of the tight junction changes for the invasion of a human respiratory epithelium.
To identify the mechanism whereby P. aeruginosa invades human epithelia, we have exposed an epithelium reconstituted with primary human respiratory cells to strains of P. aeruginosa featuring selective alterations in the expression of virulence factors. We have observed that only bacteria that efficiently secrete rhamnolipids infiltrate a respiratory epithelium, while strains expressing all other QS-regulated factors do not. We also document that, once applied to the apical surface of epithelia, purified rhamnolipids rapidly altered the transepithelial resistance and the paracellular permeability of the reconstituted epithelia. These changes were associated with alterations in the architecture of TJ that lead to rapid infiltration of P. aeruginosa via the paracellular route.
The data show that the initial steps of P. aeruginosa infiltration involve a rhamnolipid-dependent alteration of the epithelial barrier.
MATERIALS AND METHODS
Airway epithelia. Biopsies of noninvolved nasal mucosa were performed on 35 male and 18 female patients (age range, 21 to 60 years) according to the guidelines of the Ethical Committee for Clinical Studies of the Geneva State Hospital (authorization no. 04/019). Informed consent was obtained from all patients. Epithelial cells were dispersed from the biopsy specimens (29) and plated at a density of 5 x 105 cells/cm2 onto 0.6-cm2 collagen-coated filters (Millipore, Molsheim, France). Cells were cultured in Dulbecco's modified Eagle medium (DMEM)-F12 (Invitrogen, Basel, Switzerland) supplemented with 2% Ultroser G (Biosepra, Ciphergen Biosystems, Cergy-Saint-Christophe, France), 100 U/ml penicillin, and 100 mg/ml streptomycin. After 1 day, filters were taken at the air-liquid interface for 2 to 3 weeks. Twenty-four hours prior to the infection assay, penicillin and streptomycin were removed from the culture medium.
Pseudomonas aeruginosa. The P. aeruginosa strains, listed in Table 1, were transformed with plasmid pIAX2 to express the gene coding for green fluorescent protein (GFP) (a gift from I. Attree, CEA-Grenoble, France) (12). Bacteria were inoculated in Luria-Bertani (LB) medium overnight at 37°C, diluted in LB, and grown to an optical density at 600 nm of 0.5, under which conditions all strains grew similarly. Supernatants of an overnight culture of wild-type or mutated P. aeruginosa strains grown in DMEM-F12-HEPES were centrifuged, filtered on a 0.22-μm-pore-size filter, and adjusted to a pH of 7.5.
The rhlA promoter was fused to the GFP gene by replacing the X2 promoter of the pIAX2 plasmid with the rhlA promoter of plasmid pECP60 (42), between the SmaI and BamHI sites of pIAX2, to generate the pZC1 vector. Strains listed in Table 2 were electroporated with pZC1. Promoter activity was further analyzed in LB and DMEM-F12-HEPES culture media by assessing the fluorescence of GFP.
The rhlA mutation in the PT712 strains was complemented as follows. The HindIII-EcoRI fragment of plasmid pJPP6, containing the rhlABRI operon and the pheC gene (Jim Pearson, unpublished data), was ligated into the HindIII/EcoRI sites of pBluescript II SK(+) to generate plasmid pAKRHL. The EcoRI fragment of this plasmid, containing the rhlABRI operon, was ligated into the EcoRI site of the pIAX2 plasmid (12). The resulting pZC6 plasmid, which allows for expression of the rhl operon under the control of its own promoter, constitutively expresses the GFP gene under the control of the X2 promoter. Transformation of strain PT712 with pAKRHL and pZC6 generated strains PT1323 and MZ10, respectively. The promoter activity for rhlABRI was assessed on SW blue plates as described below, while expression of GFP was observed by fluorescence.
Elastase and rhamnolipid production. Elastase production was measured by an elastin Congo red assay, as previously described (52). Rhamnolipid production was measured on SW blue plates by inoculating strains in M9-based agar plates (36) supplemented with 0.2% (vol/vol) glycerol, 2 mM MgSO4, 5 mM KNO3 (instead of NH4Cl), 0.0005% (vol/vol) methylene blue, and 0.02% (vol/vol) cetyltrimethylammonium bromide (46). Plates were incubated at 37°C for 24 h and then kept for at least 48 h at room temperature until a blue halo appeared around colonies. For quantitative assays, rhamnolipids were extracted from supernatants of PAO1 cultures grown in M9 minimal medium supplemented with 2% glycerol, 2 mM MgSO4, 0.05% glutamate (instead of NH4Cl), and 0.05% Casamino Acids. After ether extraction, rhamnolipids were quantified by the orcinol procedure (42). Purified rhamnolipids JBR515 were also obtained from the Jeneil Company (Saukville, Wis.) and diluted in DMEM-F12-HEPES immediately before use.
Measurement of the epithelial barrier. The TER of the reconstituted epithelia was assessed using a Millicel ERS Volt-ohm meter (World Precision Instruments, New Haven, CT). Paracellular permeability was monitored after apical addition of 1 μCi/ml [3H]inulin in the presence or absence of 150 μg/ml rhamnolipids. At the indicated times, aliquots of the apical and basolateral media were sampled and counted.
Antibodies. Rabbit polyclonal antibodies to claudin-1, occludin, and ZO-1 were purchased from Zymed Laboratories (San Francisco, Calif.), rabbit polyclonal antibodies to ezrin from Upstate (Lake Placid, N.Y.), mouse monoclonal antibodies to mucin 5AC 1 from NeoMarker (Fremont, Calif.), mouse monoclonal anti-human cystic fibrosis transmembrane conductance regulator (CFTR) from R&D Systems, Inc. (Minneapolis, Minn.), horseradish peroxidase-conjugated anti-rabbit or anti-mouse antibodies from Bio-Rad (Reinach, Switzerland), and fluorescein isothiocyanate (FITC)-, Texas Red-, and Cy5-conjugated anti-mouse or anti-rabbit antibodies and Alexa Fluor 488, Texas Red, and Cy5 phalloidin from Molecular Probes (Leiden, The Netherlands). The mouse monoclonal antibody to JAM1 was given by M. Aurrand-Lions,Geneva, Switzerland. GM1 ganglioside was detected with horseradish peroxidase-conjugated cholera toxin B (Sigma). Fluorescent L-rhamnose and rhamnolipids were generated by coupling the diol group of the sugar moiety with 5-(4,6-dichlorotriazinyl) aminofluorescein (Molecular Probes) and purifying (Eurogentec, Seraing, Belgium).
Experimental treatments. The apical surfaces of reconstituted epithelia, featuring similar transepithelial resistances, were exposed for 10 min to 16 h to one of the following conditions: (i) no treatment, (ii) P. aeruginosa bacteria washed off the culture medium, (iii) supernatants of P. aeruginosa cultures, (iv) 15 to 150 μg/ml purified rhamnolipids, (v) purified rhamnolipids followed by Pseudomonas washed off the culture medium, (vi) purified rhamnolipids (unlabeled or FITC labeled) followed by 0.5 μm carboxylate-modified fluorospheres (Molecular Probes, Leiden, The Netherlands), or (vii) unlabeled or FITC-labeled L-rhamnose (Fluka). In each set of experiments, epithelia showing comparable TERs were exposed in parallel to several of these conditions. The experiments were stopped by extensive washing of the epithelia in DMEM-F12-HEPES, and metabolically active cells were evaluated using an MTT[1-4,5-dimethylthiazol-2-yl)-3,5-diphenylformazan] assay (Sigma). Cytotoxicity and viability were also assessed using a LIVE/DEAD viability/cytotoxicity assay kit (Molecular Probes, Leiden, The Netherlands). To this end, epithelia were stained with 4 μM calcein and 2 μM ethidium for 40 min at 37°C. Filters were then cut off and mounted for live confocal microscopy analysis.
Immunostaining. The reconstituted epithelia were fixed in 4% paraformaldehyde, permeabilized in 0.1% saponin, and incubated for 1 h with one of the primary antibodies listed above diluted in phosphate-buffered saline containing 1% bovine serum albumin. After a wash, the tissues were incubated again for 30 min with an appropriate secondary antibody as per standard protocols (58). Filters were cut off from the culture inserts, mounted in Vectorshield-DAPI (4',6'-diamidino-2-phenylindole) (Vector Laboratories) between glass coverslips, and observed with an LSM 510 confocal microscope (Zeiss). For convenience and consistency of image representations, some of the immunostaining was captured with the green (488 nm) and red (543 nm) channels and appears red and green, respectively, in some of the figures.
For live imaging, filters of control epithelia and of epithelia exposed to FITC-labeled rhamnolipids were cut off and mounted in culture media containing 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene-p-toluenesulfate (TMA-DPH; Molecular Probes, Leiden, The Netherlands), a cationic linear polyene that is readily incorporated into the plasma membranes of host cells. Time lapse video microscopy was performed using a Hamamatsu high-resolution black/white charge-coupled device camera coupled to Openlab software.
Electron microscopy. Tissues were fixed in 2.5% glutaraldehyde in 0.1 M phosphate buffer (pH 7.4) and processed for either conventional or freeze fracture electron microscopy as described previously (8). Sections and replicas were photographed with a Philips CM10 microscope (Eindhoven, The Netherlands). Quantitative analysis of TJ was carried out on photographs of 60 to 100 cells per condition. The area, length, and width encompassed by TJ fibrils were measured at a magnification of x34,000, using an Acecad graphic tablet connected to Quantimet 500 software (Leica). Numbers of strands and loose ends of fibrils were also scored.
Statistics. Values were expressed as means ± standard errors of the means (SEMs) and were compared by analysis of variance and t tests for independent variables using SPSS software (SPSS Inc., Chicago, Ill.).
RESULTS
Differentiated respiratory epithelia are reconstituted with cells of human nasal mucosa. After a 2- to 3-week culture at the air-liquid interface, cells of nasal mucosa formed epithelia showing a TER of 1,000 to 2,500 · cm2 and resembling a native respiratory epithelium, i.e., comprising basal, goblet, and ciliated cells (Fig. 1A). These cells were immunostained for both ezrin and CFTR (Fig. 1B). The polarization of goblet and ciliated cells correlated with the presence of continuous TJ belts, which freeze fracture electron microscopy revealed between the basolateral and apical domains of the cell membranes (Fig. 1C). Immunofluorescence showed that these belts were made of claudin-1, occludin, ZO-1, and JAM-1 and that they uninterruptedly surrounded each cell (Fig. 1D).
P. aeruginosa expressing the Rhl quorum-sensing system alters the epithelial barrier. When the apical surface of reconstituted epithelia was exposed to PAO1 in the exponential-growth phase that had been washed off its supernatant and added at a ratio of 1 to 20 bacteria per host cell, we observed no invasion or infiltration of the tissue for up to 8 h (Fig. 2A). In contrast, when the infection was prolonged to 16 to 24 h, we observed that GFP-PAO1 efficiently infiltrated the epithelium (Fig. 2A). When the epithelial surface was exposed to a filtered (i.e., devoid of bacteria) supernatant of an overnight PAO1 culture, a decrease in TER was observed which was sustained throughout the duration of the experiment (Fig. 2B). During the first 4 h, this reduction in TER was not paralleled by a decrease in the viability of epithelial cells, as evaluated by the MTT assay (data not shown). These results suggest that diffusible toxins released by a high density of bacteria, not the TTSS (10), are responsible for the alteration of the epithelial barrier.
To identify the relevant factors under the control of the quorum-sensing systems, the epithelia were exposed to strain PT531, which was derived from PAO1 and is deficient in both the las and rhl quorum-sensing systems (Table 1). After 16 h, strain PT531 multiplied to high titers (approximately 109 bacteria per ml) in the apical medium without inducing cell damage or infiltrating the epithelia (Fig. 2A). Similar observations were made with the rhlR mutant PT462 (Table 1). In contrast, invasion was detected with the lasR mutant PT498 (Fig. 2A).
Since these results showed that mutations affecting the rhl QS system impaired the infiltration of airway epithelia by P. aeruginosa, we next tested whether rhamnolipids, the synthesis of which depends mainly on the rhl QS system, were involved in bacterial invasion. We observed that after an overnight period, the reconstituted epithelium was not susceptible to infection by the rhlA mutant PT712, which is specifically impaired in rhamnolipid synthesis (Fig. 2A; Table 1). Since these results suggested that the activity of the rhl quorum-sensing system and the production of rhamnolipids were necessary and sufficient to promote infiltration by P. aeruginosa, we compared the effects on TER of filtered supernatants from strains deficient in both the las and rhl QS systems (PT531) or only in the rhlA gene (PT712). Even though a slight decrease in TER with time was observed for PT531 and PT712, neither strain induced the large TER drop observed after the epithelia were exposed to the supernatant from wild-type strain PAO1 (Fig. 2B). However, addition to the PT712 supernatant of purified rhamnolipids isolated from the PAO1 supernatant induced a drop in TER comparable to that induced by the PAO1 supernatant alone (Fig. 2B). Furthermore, when strain PT712 was transformed with plasmid pAKRHL, carrying the entire rhlABRI operon, and plasmid pZC6, carrying GFP under the control of the X2 promoter, we observed that the defective rhamnolipid expression was complemented, as evaluated on a blue SW plate (Fig. 3A), and that the complemented strains were able to infiltrate the epithelium (Table 2). These results suggest that rhamnolipids cause P. aeruginosa to invade respiratory epithelium by modulating the permeability of the tissue.
Elastase, another factor encoded by las and rhl, has been previously suggested to promote opening of the paracellular route (4, 2). To test its implications in our model, we measured the elastolytic activity in the supernatant of strain PT712. We found that this rhamnolipid-deficient strain, which did not invade the reconstituted epithelium, secreted elastase at even higher levels than the invasive strain PAO1 (Fig. 3A; Table 2). We further observed that the cytotoxic Pseudomonas aeruginosa isolate CHA from a cystic fibrosis patient (13) produced elastase levels lower than those produced by both strains PAO1 and PT712 (Fig. 3A). However, we observed that both CHA and its isogenic mutant CHAexsA, which lacks the type III secretion system (12, 25), infiltrated the epithelia (Fig. 2A; Table 2). In view of previous reports, we further analyzed the behavior of strain PAK (31, 20) and observed that it did not invade the epithelium (Fig. 2A), consistent with the finding that it also did not produce rhamnolipids under our culture conditions (Fig. 3A). The results show that invasion of P. aeruginosa does not correlate with elastolytic activity and type III secretion, at least during the early stages of infection.
To test whether rhamnolipids were actually produced under the conditions we used for the invasion assay, the promoter of the rhlA gene, which codes for rhamnosyltransferase, was fused to GFP to generate plasmid pZC1, which was used to transform PAO1 and PT712, yielding strains MZ2 and MZ6, respectively. We found that the expression of the rhlA promoter was activated in both strains as soon as the bacteria reached a density of >109/ml, indicating that the rhl quorum-sensing system and the rhlA pathways were properly activated, irrespective of the culture medium tested (data not shown). However, when reconstituted epithelia were exposed overnight to exponentially grown Pseudomonas strains MZ2 and MZ6, we observed that only the rhamnolipid-secreting strain MZ2 infiltrated the epithelium (Fig. 3B). Taken together, the results show that the actual release of rhamnolipids is essential for epithelial infiltration (Table 2).
Purified rhamnolipids alter the epithelial barrier without affecting cell viability. To determine whether exogenous rhamnolipids could reproduce the effects of P. aeruginosa or supernatants of high-density bacterial cultures, we applied purified rhamnolipids to the apical surface of the epithelia. We observed that rhamnolipids resulted in a rapid reduction in TER, which was dose and time dependent. Thus, whereas 15 μg/ml rhamnolipids did not significantly alter the values of TER after 3 h (846 ± 6 · cm2 [n = 4]) compared to those observed for untreated controls (996 ± 48 · cm2), 50 μg/ml rhamnolipids decreased the TER within 30 min, reaching less than 10% of the control value (P < 0.001) after 360 min (Fig. 4A). In the presence of 150 μg/ml rhamnolipids, such a drop in TER was observed within 10 min (163 ± 83 · cm2 [n = 5]) (Fig. 4A).
To determine whether this drop was paralleled by a change in epithelial permeability, we next studied the flux of [3H]inulin. We observed that, compared to the control value of 1.2%, the transepithelial passage of this extracellular marker was increased to 4.5% and 25% after 1-h and 4-h exposures of the epithelium to 150 μg/ml rhamnolipids, respectively (Fig. 4B), in spite of unchanged viability of the epithelial cells, as assessed by the MTT (Fig. 4C) and LIVE/DEAD cytotoxicity/viability (Fig. 4D) tests. Under these conditions, the proportion of dead cells (0.5%) was like that observed in untreated epithelia, showing that cell death could not account for the large alterations in permeability of rhamnolipid-treated epithelia.
Consistent with these alterations, we observed that within 30 min, GFP-PAO1 infiltrated epithelia that had previously been exposed for 60 min to 150 μg/ml rhamnolipids (Fig. 4E). Under these conditions, an average of 6 bacteria contacted 9% of the cells, compared to control values of 1.2 bacteria on 1.7% of the cells (31 filters were scored from 19 independent experiments). The number of bacteria reaching the basal surface of the epithelia was evaluated by plating the bacteria onto agar plates after extensive washing of the epithelia followed by hypotonic lysis. We observed that 7.4% of the wild-type bacteria that were applied on top of the epithelia had infiltrated (data not shown). Similar infiltration of rhamnolipid-exposed epithelia was also observed with Pseudomonas strains PAK, PT531, and PT712, which otherwise did not infiltrate our control epithelia even when applied at high densities (Table 2), as well as with inert carboxylate-modified microspheres (data not shown). These data indicate that once the paracellular pathway was made accessible, no further active process was required for epithelial invasion.
To identify the type of cells affected by rhamnolipids, we monitored the passage of GFP-P. aeruginosa through epithelia that had been exposed either to purified rhamnolipids or to supernatants of overnight cultures of PAO1. In both cases, we observed that the fluorescent bacteria infiltrated the epithelia at sites where the immunolabeling of ezrin was displaced from the apical (control group) to the basolateral (rhamnolipid-exposed group) membrane (Fig. 5A). Double immunolabeling for ezrin and MUC5AC showed that most of these cells did not express mucins and had a ciliated phenotype (Fig. 5B). These findings, together with the loss of cilia, which was observed after rhamnolipid exposure (Fig. 5A, C, and D), indicated that PAO1 infiltrated the epithelium close to cells featuring an altered polarity. To determine whether this infiltration occurred through the transcellular or the paracellular pathway, epithelia exposed to Pseudomonas for 16 to 24 h were examined by electron microscopy. Irrespective of the invading strain (PAO1 or CHA), the bacteria were seen exclusively within intercellular spaces throughout the duration of the experiment (Fig. 5C). However, after several hours of infection, a few necrotic cell profiles were observed, over which numerous P. aeruginosa bacteria were concentrated (Fig. 5C). The finding of similar profiles in epithelia exposed to rhamnolipids, to permit the infiltration of the PT712 and PT531 bacteria, suggests that this limited cell necrosis was not caused by factors secreted under the control of the quorum-sensing systems. Comparable observations were made in epithelia exposed to rhamnolipids and then to PAO1 for as long as 5 h (Fig. 5D).
Rhamnolipids localize within the lateral membranes of epithelial cells. To assess how rhamnolipids altered the epithelial barrier, we coupled fluorescein to their carbohydrate moiety and applied either the labeled molecules or fluorescein-tagged L-rhamnose to the apical surface of the epithelia. Labeled rhamnolipids drastically reduced TER, as did the unlabeled molecules (Fig. 6A). This effect, which was not observed with either unlabeled or FITC-labeled rhamnose, was reversible after washout (Fig. 6B). At early time points, fluorescent rhamnolipids were uniformly distributed within the apical membranes of the ezrin-positive cells (Fig. 6C). After 60 to 120 min, the rhamnolipids featured a sparser and patchier distribution in the apical membranes and became concentrated in the basolateral membranes (Fig. 6C). The incorporation of FITC-labeled rhamnolipids into the host membranes was confirmed by live imaging of treated epithelial cells stained with TMA-DPH (data not shown). This incorporation appears to depend on the alkyl chains of rhamnolipids, inasmuch as no labeled L-rhamnose bounded or was internalized by epithelial cells (data not shown).
In several systems, rhamnose promotes the binding of polysaccharides to specific receptors (49). To assess whether rhamnose promoted the insertion of rhamnolipids within the membranes of epithelial cells, we tried to compete for this insertion by using 0.3 to 2 mM L-rhamnose for 1 to 16 h. We found that L-rhamnose prevented neither the drastic drop in TER (Fig. 7A) nor the bacterial infiltration (Fig. 7B) induced by rhamnolipids.
Rhamnolipids alter tight-junction architecture. The opening of the paracellular route caused by rhamnolipids implies alterations of TJ. Consistent with this view, significant alterations in TJ morphology were seen, which increased with time after rhamnolipid treatment (Fig. 8A). Thus, whereas TJ of control cells formed regular and uninterrupted belts, made on average of five interconnected strands of fibrils (Fig. 8A), morphometric analysis revealed that the number of strands and the area of TJ belts were significantly reduced after a 30- to 60-min treatment with 150 μg/ml rhamnolipids (Fig. 8). After 4 h, TJ belts comprised fibrils that were often interrupted and either partially or fully disconnected from the TJ belt, resulting in an increase in the membrane area occupied by TJ fibrils, in spite of a reduced number of strands (Fig. 8).
DISCUSSION
The establishment and maintenance of cell polarity is essential for the integrity and function of epithelia, which is particularly critical for the prevention of invasion by pathogens. Previous studies have demonstrated that defects in the paracellular permeability of respiratory epithelia are a prerequisite for P. aeruginosa invasion (18, 44) and have suggested that bacterial toxins may induce these defects by decreasing levels of TJ-associated proteins (3, 57). However, the specificity of these findings and the mechanisms leading to TJ alterations have not been investigated. To address this question, we have developed a model of epithelia reconstituted with human cells, grown at the air-liquid interface.
Like the native normal epithelium, the reconstituted tissue was not susceptible to infection by P. aeruginosa for up to 8 h. However, after prolonged exposure to bacteria (16 to 24 h), which allows for expression and secretion of bacterial toxins, the reconstituted epithelium became selectively infiltrated by bacteria featuring a normal Rhl quorum-sensing system. Among the virulence factors regulated by this system, we found that rhamnolipids are necessary and sufficient to affect the epithelial barrier. Thus, purified rhamnolipids reproduced the drop in transepithelial resistance, the increase in epithelial permeability to inulin, and the disorganization of TJ belts which were induced either by high densities of wild-type P. aeruginosa or by bacteria-free supernatants of these pathogens but not by strains of P. aeruginosa that featured a global defect in the Rhl quorum-sensing system (PT531 and PT462) or were selectively deficient in rhamnolipid production (PT712 and PAK).
Previous studies have suggested that the production of the elastolytic metalloproteinase LasB (40), which is a consistent feature of pathogenic P. aeruginosa (35, 56), decreases the levels of TJ-associated proteins, thus altering the paracellular barrier function of epithelia (3). It has also been reported that bacterial invasion inversely correlates with the levels of ExoS, a protein of the type III secretion system (11), which is a substrate for P. aeruginosa elastases (11) and which accounts for P. aeruginosa cytotoxicity. Here we report that a P. aeruginosa strain that produces elastase but not rhamnolipids (PT712) cannot infiltrate the reconstituted epithelium, whereas the same PT712 strain complemented for the rhlA mutation (PT1323 and MZ10), as well as a P. aeruginosa strain that produces rhamnolipids but no elastase (CHA), can. We also report that a strain producing rhamnolipids but featuring a defective type III secretion system (CHAexsA) also infiltrated the epithelium, ruling out the type III mechanism as the trigger of this infiltration. Hence, our data show that rhamnolipids are necessary and sufficient to initiate the alterations of the paracellular pathway that allow for bacterial invasion. This conclusion does not exclude the possibility that elastase and the type III secretion system might also contribute to virulence at later stages of the infection process (11, 34).
Our study provides a first insight into the mode of action of rhamnolipids. Like other lipid molecules (1, 50), rhamnolipids are titrated as a function of their partition into the membranes of the host cells. Using FITC-labeled molecules, we show that rhamnolipids are initially incorporated within the apical membranes of epithelial cells and later are found within their basolateral membranes. Together with the obvious loss of cilia, the displacement of ezrin, and the alterations of the TJ, these findings indicate that rhamnolipids, whether chemically purified or produced by P. aeruginosa, cause a loss of cell polarity. As a result, TER was markedly decreased and the permeability of epithelia to extracellular markers and bacteria increased, in the absence of obvious cell death. It remains to be shown whether the loss of cell polarity is due to a direct effect of the rhamnolipids on TJ or, as suggested by the distribution of these molecules over large domains of the apical (initially) and basolateral (at later time points) membranes, to their effects on the lipid environment of the junctions, which conceivably could alter their fence and barrier functions.
At any rate, once TJ are opened, a variety of P. aeruginosa strains, including some that do not secrete rhamnolipids, enter the paracellular pathway. Our data show that this step does not involve an active mechanism, inasmuch as it is mimicked by inert particles with a size comparable to that of the bacteria. Electron microscopy revealed that, within the 24-h time frame of our experiments, the infiltrating P. aeruginosa remained in the intercellular spaces and was not internalized by the epithelial cells. This finding was not anticipated, in view of previous reports that had suggested that P. aeruginosa is internalized at advanced stages of pulmonary infection, possibly via a receptor located in the basolateral membrane (16). While this putative receptor remains to be identified, a possible role for CFTR has been suggested (22, 26, 43), even though this chloride channel is normally located within the apical membranes of epithelial cells (32, 55). In this situation, access to the basolateral membrane is not needed for P. aeruginosa to interact with CFTR, in contrast to the finding of such an early access documented in this and previous studies (19). Also, epithelia reconstituted with cells from cystic fibrosis patients carrying the homozygous F508 mutation behaved like control epithelia (data not shown). Thus, polarized epithelia lacking CFTR were infiltrated by PAO1 only when the bacteria had reached a high cell density and, throughout the time course of experiments, failed to show sizable internalization of P. aeruginosa (data not shown). Together, these data do not support a central role for CFTR (14, 43) in the early steps of P. aeruginosa invasion. Furthermore, comparison of invasive (PAO1), cytotoxic (CHA), and noncytotoxic (CHAexsA) strains showed that the early steps of P. aeruginosa infiltration also were not dependent on the type III secretion system. Rather, our findings document the requirement for access to the paracellular route, as previously suggested for cultures of cell lines (9, 24, 30). In these cases, P. aeruginosa internalization appeared to be dependent on the cell phenotype, inasmuch as bacteria were not incorporated by polarized epithelial cells (24, 30). Differences in the types of cells studied, in the multiplicity of infection, and in the presence or absence of antibiotic treatment may further account for the different observations made in this and previous studies (21). It is, however, not excluded that under environmental conditions not investigated here, such as antibiotic treatment, P. aeruginosa uses airway epithelial cells as a reservoir for persistence (21).
Our study is the first to investigate a 3-dimensional epithelium of primary human cells under conditions leading to differentiation of ciliated and goblet cells. Under such conditions, the early steps of invasion by P. aeruginosa require the opening of the paracellular route and do not involve incorporation of the bacteria by the cells. Here we have shown that this infiltration is dependent on the production of rhamnolipids, encoded by the Rhl quorum-sensing system, which open the paracellular route. The molecular mechanism whereby rhamnolipids alter the structures and mechanisms ensuring cell polarity remains to be determined. Rhamnolipids are found in the sputum (33) and lung secretions (23) of chronically infected patients at concentrations close to the concentration we tested experimentally (thus sufficiently high to promote P. aeruginosa infiltration). Rhamnolipids have also been reported to have deleterious effects on mucociliary clearance (28) and phagocytosis by macrophages (38) and are involved in fluid-channel formation in and dispersion of biofilms (6, 15, 17, 27, 47). Hence, these molecules are candidate targets for future therapeutic strategies aimed at specific modulation of the mucosal barrier.
ACKNOWLEDGMENTS
We thank C. van Delden, P.-A. Ruttiman for graphic help, J.-L. Dumas for the construction of plasmid pAKRHL, and the Jeneil Company for the gift of purified rhamnolipids.
L.Z. is supported by the Ernst and Lucie Schmidheiny Foundation, the Sir Jules Thorn Charitable Overseas Trust, and the Association Vaincre la Mucoviscidose. J.-S.L. is supported by a grant from the Swiss National Foundation (3100A0-100621-1). Work of the Meda team is supported by grants from the Swiss National Foundation (3100-00-109402), the Juvenile Diabetes Research Foundation International (1-2005-46), the European Union (QLRT-2001-01777), and the National Institute of Health (DK 63443-01).
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Department of Microbiology and Molecular Medicine, Medical Center, University of Geneva
Department of Oto-Rhino-Laryngology, University Hospital of Geneva,Geneva, Switzerland
ABSTRACT
The opportunistic bacterium Pseudomonas aeruginosa causes chronic respiratory infections in cystic fibrosis and immunocompromised individuals. Bacterial adherence to the basolateral domain of the host cells and internalization are thought to participate in P. aeruginosa pathogenicity. However, the mechanism by which the pathogen initially modulates the paracellular permeability of polarized respiratory epithelia remains to be understood. To investigate this mechanism, we have searched for virulence factors secreted by P. aeruginosa that affect the structure of human airway epithelium in the early stages of infection. We have found that only bacterial strains secreting rhamnolipids were efficient in modulating the barrier function of an in vitro-reconstituted human respiratory epithelium, irrespective of their release of elastase and lipopolysaccharide. In contrast to previous reports, we document that P. aeruginosa was not internalized by epithelial cells. We further report that purified rhamnolipids, applied on the surfaces of the epithelia, were sufficient to functionally disrupt the epithelia and to promote the paracellular invasion of rhamnolipid-deficient P. aeruginosa. The mechanism involves the incorporation of rhamnolipids within the host cell membrane, leading to tight-junction alterations. The study provides direct evidence for a hitherto unknown mechanism whereby the junction-dependent barrier of the respiratory epithelium is selectively altered by rhamnolipids.
INTRODUCTION
The airway mucosa is an efficient barrier to protect the host from infection by pathogens. An essential component of this barrier is the polarity of surface epithelial cells, which allows them to segregate the receptors that trigger the adherence and internalization of several pathogens within the basolateral domain of the membrane (51). Normally, this domain is not accessible to microbial organisms, due to the presence of tight junctions (TJ) that separate the apical from the basolateral membrane components and seal the paracellular space (37). To overcome this protective barrier, several microorganisms have developed strategies to alter either the apical membrane of epithelial cells (39) or the TJ barrier (53). Understanding the mechanism underlying these alterations is a prerequisite for development of novel therapeutic strategies targeted to specific molecular and cellular events. This is particularly necessary in the context of infections, such as those caused by Pseudomonas aeruginosa, which is difficult to eradicate by conventional antibiotic treatments (45).
P. aeruginosa is an opportunistic gram negative bacterium that does not invade normal mucosae but causes serious nosocomial infections in immunocompromised individuals and in cystic fibrosis patients (7). The pathogenic mechanism accounting for these infections is not fully clarified and has been variously attributed to the production of different cell-associated and secreted virulence factors (41). The finding that P. aeruginosa is internalized more readily by dispersed and migrating epithelial cells than by fully polarized cells has been taken as an indication that the bacteria need to access the basolateral membrane to interact with the receptors that mediate their internalization (44). This implies that the early steps of P. aeruginosa infection should involve some alterations in the paracellular route of the epithelium. Consistent with this view, several factors produced by P. aeruginosa, including lipopolysaccharide and elastase, have been reported to decrease the transepithelial resistance (TER) of various epithelia and to decrease the expression of TJ-associated proteins (2, 3, 5, 57). As yet, however, the specificity of these alterations remains to be determined. Indeed, in addition to the factors mentioned above, the virulence of P. aeruginosa may be also attributed to the type III secretion system (TTSS), which controls the production of cytotoxic proteins delivered to the host cells (41), as well as to factors regulated by the quorum-sensing (QS) systems (54). In P. aeruginosa, two QS systems, called Las and Rhl, control the expression of more than 100 genes in a cell-density-dependent manner (54). Once a sufficient amount of autoinducer molecules has accumulated, these signaling molecules bind to their cognate transcriptional activators LasR and RhlR. LasR regulates the transcription of several virulence genes, including lasA, lasB, and toxA, whereas RhlR enhances the transcription of lasB and the rhamnolipid synthesis genes rhlAB (for a review, see reference 48). Furthermore, previous studies have not shown whether the paracellular route was directly affected by Pseudomonas or tested the relevance of the tight junction changes for the invasion of a human respiratory epithelium.
To identify the mechanism whereby P. aeruginosa invades human epithelia, we have exposed an epithelium reconstituted with primary human respiratory cells to strains of P. aeruginosa featuring selective alterations in the expression of virulence factors. We have observed that only bacteria that efficiently secrete rhamnolipids infiltrate a respiratory epithelium, while strains expressing all other QS-regulated factors do not. We also document that, once applied to the apical surface of epithelia, purified rhamnolipids rapidly altered the transepithelial resistance and the paracellular permeability of the reconstituted epithelia. These changes were associated with alterations in the architecture of TJ that lead to rapid infiltration of P. aeruginosa via the paracellular route.
The data show that the initial steps of P. aeruginosa infiltration involve a rhamnolipid-dependent alteration of the epithelial barrier.
MATERIALS AND METHODS
Airway epithelia. Biopsies of noninvolved nasal mucosa were performed on 35 male and 18 female patients (age range, 21 to 60 years) according to the guidelines of the Ethical Committee for Clinical Studies of the Geneva State Hospital (authorization no. 04/019). Informed consent was obtained from all patients. Epithelial cells were dispersed from the biopsy specimens (29) and plated at a density of 5 x 105 cells/cm2 onto 0.6-cm2 collagen-coated filters (Millipore, Molsheim, France). Cells were cultured in Dulbecco's modified Eagle medium (DMEM)-F12 (Invitrogen, Basel, Switzerland) supplemented with 2% Ultroser G (Biosepra, Ciphergen Biosystems, Cergy-Saint-Christophe, France), 100 U/ml penicillin, and 100 mg/ml streptomycin. After 1 day, filters were taken at the air-liquid interface for 2 to 3 weeks. Twenty-four hours prior to the infection assay, penicillin and streptomycin were removed from the culture medium.
Pseudomonas aeruginosa. The P. aeruginosa strains, listed in Table 1, were transformed with plasmid pIAX2 to express the gene coding for green fluorescent protein (GFP) (a gift from I. Attree, CEA-Grenoble, France) (12). Bacteria were inoculated in Luria-Bertani (LB) medium overnight at 37°C, diluted in LB, and grown to an optical density at 600 nm of 0.5, under which conditions all strains grew similarly. Supernatants of an overnight culture of wild-type or mutated P. aeruginosa strains grown in DMEM-F12-HEPES were centrifuged, filtered on a 0.22-μm-pore-size filter, and adjusted to a pH of 7.5.
The rhlA promoter was fused to the GFP gene by replacing the X2 promoter of the pIAX2 plasmid with the rhlA promoter of plasmid pECP60 (42), between the SmaI and BamHI sites of pIAX2, to generate the pZC1 vector. Strains listed in Table 2 were electroporated with pZC1. Promoter activity was further analyzed in LB and DMEM-F12-HEPES culture media by assessing the fluorescence of GFP.
The rhlA mutation in the PT712 strains was complemented as follows. The HindIII-EcoRI fragment of plasmid pJPP6, containing the rhlABRI operon and the pheC gene (Jim Pearson, unpublished data), was ligated into the HindIII/EcoRI sites of pBluescript II SK(+) to generate plasmid pAKRHL. The EcoRI fragment of this plasmid, containing the rhlABRI operon, was ligated into the EcoRI site of the pIAX2 plasmid (12). The resulting pZC6 plasmid, which allows for expression of the rhl operon under the control of its own promoter, constitutively expresses the GFP gene under the control of the X2 promoter. Transformation of strain PT712 with pAKRHL and pZC6 generated strains PT1323 and MZ10, respectively. The promoter activity for rhlABRI was assessed on SW blue plates as described below, while expression of GFP was observed by fluorescence.
Elastase and rhamnolipid production. Elastase production was measured by an elastin Congo red assay, as previously described (52). Rhamnolipid production was measured on SW blue plates by inoculating strains in M9-based agar plates (36) supplemented with 0.2% (vol/vol) glycerol, 2 mM MgSO4, 5 mM KNO3 (instead of NH4Cl), 0.0005% (vol/vol) methylene blue, and 0.02% (vol/vol) cetyltrimethylammonium bromide (46). Plates were incubated at 37°C for 24 h and then kept for at least 48 h at room temperature until a blue halo appeared around colonies. For quantitative assays, rhamnolipids were extracted from supernatants of PAO1 cultures grown in M9 minimal medium supplemented with 2% glycerol, 2 mM MgSO4, 0.05% glutamate (instead of NH4Cl), and 0.05% Casamino Acids. After ether extraction, rhamnolipids were quantified by the orcinol procedure (42). Purified rhamnolipids JBR515 were also obtained from the Jeneil Company (Saukville, Wis.) and diluted in DMEM-F12-HEPES immediately before use.
Measurement of the epithelial barrier. The TER of the reconstituted epithelia was assessed using a Millicel ERS Volt-ohm meter (World Precision Instruments, New Haven, CT). Paracellular permeability was monitored after apical addition of 1 μCi/ml [3H]inulin in the presence or absence of 150 μg/ml rhamnolipids. At the indicated times, aliquots of the apical and basolateral media were sampled and counted.
Antibodies. Rabbit polyclonal antibodies to claudin-1, occludin, and ZO-1 were purchased from Zymed Laboratories (San Francisco, Calif.), rabbit polyclonal antibodies to ezrin from Upstate (Lake Placid, N.Y.), mouse monoclonal antibodies to mucin 5AC 1 from NeoMarker (Fremont, Calif.), mouse monoclonal anti-human cystic fibrosis transmembrane conductance regulator (CFTR) from R&D Systems, Inc. (Minneapolis, Minn.), horseradish peroxidase-conjugated anti-rabbit or anti-mouse antibodies from Bio-Rad (Reinach, Switzerland), and fluorescein isothiocyanate (FITC)-, Texas Red-, and Cy5-conjugated anti-mouse or anti-rabbit antibodies and Alexa Fluor 488, Texas Red, and Cy5 phalloidin from Molecular Probes (Leiden, The Netherlands). The mouse monoclonal antibody to JAM1 was given by M. Aurrand-Lions,Geneva, Switzerland. GM1 ganglioside was detected with horseradish peroxidase-conjugated cholera toxin B (Sigma). Fluorescent L-rhamnose and rhamnolipids were generated by coupling the diol group of the sugar moiety with 5-(4,6-dichlorotriazinyl) aminofluorescein (Molecular Probes) and purifying (Eurogentec, Seraing, Belgium).
Experimental treatments. The apical surfaces of reconstituted epithelia, featuring similar transepithelial resistances, were exposed for 10 min to 16 h to one of the following conditions: (i) no treatment, (ii) P. aeruginosa bacteria washed off the culture medium, (iii) supernatants of P. aeruginosa cultures, (iv) 15 to 150 μg/ml purified rhamnolipids, (v) purified rhamnolipids followed by Pseudomonas washed off the culture medium, (vi) purified rhamnolipids (unlabeled or FITC labeled) followed by 0.5 μm carboxylate-modified fluorospheres (Molecular Probes, Leiden, The Netherlands), or (vii) unlabeled or FITC-labeled L-rhamnose (Fluka). In each set of experiments, epithelia showing comparable TERs were exposed in parallel to several of these conditions. The experiments were stopped by extensive washing of the epithelia in DMEM-F12-HEPES, and metabolically active cells were evaluated using an MTT[1-4,5-dimethylthiazol-2-yl)-3,5-diphenylformazan] assay (Sigma). Cytotoxicity and viability were also assessed using a LIVE/DEAD viability/cytotoxicity assay kit (Molecular Probes, Leiden, The Netherlands). To this end, epithelia were stained with 4 μM calcein and 2 μM ethidium for 40 min at 37°C. Filters were then cut off and mounted for live confocal microscopy analysis.
Immunostaining. The reconstituted epithelia were fixed in 4% paraformaldehyde, permeabilized in 0.1% saponin, and incubated for 1 h with one of the primary antibodies listed above diluted in phosphate-buffered saline containing 1% bovine serum albumin. After a wash, the tissues were incubated again for 30 min with an appropriate secondary antibody as per standard protocols (58). Filters were cut off from the culture inserts, mounted in Vectorshield-DAPI (4',6'-diamidino-2-phenylindole) (Vector Laboratories) between glass coverslips, and observed with an LSM 510 confocal microscope (Zeiss). For convenience and consistency of image representations, some of the immunostaining was captured with the green (488 nm) and red (543 nm) channels and appears red and green, respectively, in some of the figures.
For live imaging, filters of control epithelia and of epithelia exposed to FITC-labeled rhamnolipids were cut off and mounted in culture media containing 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene-p-toluenesulfate (TMA-DPH; Molecular Probes, Leiden, The Netherlands), a cationic linear polyene that is readily incorporated into the plasma membranes of host cells. Time lapse video microscopy was performed using a Hamamatsu high-resolution black/white charge-coupled device camera coupled to Openlab software.
Electron microscopy. Tissues were fixed in 2.5% glutaraldehyde in 0.1 M phosphate buffer (pH 7.4) and processed for either conventional or freeze fracture electron microscopy as described previously (8). Sections and replicas were photographed with a Philips CM10 microscope (Eindhoven, The Netherlands). Quantitative analysis of TJ was carried out on photographs of 60 to 100 cells per condition. The area, length, and width encompassed by TJ fibrils were measured at a magnification of x34,000, using an Acecad graphic tablet connected to Quantimet 500 software (Leica). Numbers of strands and loose ends of fibrils were also scored.
Statistics. Values were expressed as means ± standard errors of the means (SEMs) and were compared by analysis of variance and t tests for independent variables using SPSS software (SPSS Inc., Chicago, Ill.).
RESULTS
Differentiated respiratory epithelia are reconstituted with cells of human nasal mucosa. After a 2- to 3-week culture at the air-liquid interface, cells of nasal mucosa formed epithelia showing a TER of 1,000 to 2,500 · cm2 and resembling a native respiratory epithelium, i.e., comprising basal, goblet, and ciliated cells (Fig. 1A). These cells were immunostained for both ezrin and CFTR (Fig. 1B). The polarization of goblet and ciliated cells correlated with the presence of continuous TJ belts, which freeze fracture electron microscopy revealed between the basolateral and apical domains of the cell membranes (Fig. 1C). Immunofluorescence showed that these belts were made of claudin-1, occludin, ZO-1, and JAM-1 and that they uninterruptedly surrounded each cell (Fig. 1D).
P. aeruginosa expressing the Rhl quorum-sensing system alters the epithelial barrier. When the apical surface of reconstituted epithelia was exposed to PAO1 in the exponential-growth phase that had been washed off its supernatant and added at a ratio of 1 to 20 bacteria per host cell, we observed no invasion or infiltration of the tissue for up to 8 h (Fig. 2A). In contrast, when the infection was prolonged to 16 to 24 h, we observed that GFP-PAO1 efficiently infiltrated the epithelium (Fig. 2A). When the epithelial surface was exposed to a filtered (i.e., devoid of bacteria) supernatant of an overnight PAO1 culture, a decrease in TER was observed which was sustained throughout the duration of the experiment (Fig. 2B). During the first 4 h, this reduction in TER was not paralleled by a decrease in the viability of epithelial cells, as evaluated by the MTT assay (data not shown). These results suggest that diffusible toxins released by a high density of bacteria, not the TTSS (10), are responsible for the alteration of the epithelial barrier.
To identify the relevant factors under the control of the quorum-sensing systems, the epithelia were exposed to strain PT531, which was derived from PAO1 and is deficient in both the las and rhl quorum-sensing systems (Table 1). After 16 h, strain PT531 multiplied to high titers (approximately 109 bacteria per ml) in the apical medium without inducing cell damage or infiltrating the epithelia (Fig. 2A). Similar observations were made with the rhlR mutant PT462 (Table 1). In contrast, invasion was detected with the lasR mutant PT498 (Fig. 2A).
Since these results showed that mutations affecting the rhl QS system impaired the infiltration of airway epithelia by P. aeruginosa, we next tested whether rhamnolipids, the synthesis of which depends mainly on the rhl QS system, were involved in bacterial invasion. We observed that after an overnight period, the reconstituted epithelium was not susceptible to infection by the rhlA mutant PT712, which is specifically impaired in rhamnolipid synthesis (Fig. 2A; Table 1). Since these results suggested that the activity of the rhl quorum-sensing system and the production of rhamnolipids were necessary and sufficient to promote infiltration by P. aeruginosa, we compared the effects on TER of filtered supernatants from strains deficient in both the las and rhl QS systems (PT531) or only in the rhlA gene (PT712). Even though a slight decrease in TER with time was observed for PT531 and PT712, neither strain induced the large TER drop observed after the epithelia were exposed to the supernatant from wild-type strain PAO1 (Fig. 2B). However, addition to the PT712 supernatant of purified rhamnolipids isolated from the PAO1 supernatant induced a drop in TER comparable to that induced by the PAO1 supernatant alone (Fig. 2B). Furthermore, when strain PT712 was transformed with plasmid pAKRHL, carrying the entire rhlABRI operon, and plasmid pZC6, carrying GFP under the control of the X2 promoter, we observed that the defective rhamnolipid expression was complemented, as evaluated on a blue SW plate (Fig. 3A), and that the complemented strains were able to infiltrate the epithelium (Table 2). These results suggest that rhamnolipids cause P. aeruginosa to invade respiratory epithelium by modulating the permeability of the tissue.
Elastase, another factor encoded by las and rhl, has been previously suggested to promote opening of the paracellular route (4, 2). To test its implications in our model, we measured the elastolytic activity in the supernatant of strain PT712. We found that this rhamnolipid-deficient strain, which did not invade the reconstituted epithelium, secreted elastase at even higher levels than the invasive strain PAO1 (Fig. 3A; Table 2). We further observed that the cytotoxic Pseudomonas aeruginosa isolate CHA from a cystic fibrosis patient (13) produced elastase levels lower than those produced by both strains PAO1 and PT712 (Fig. 3A). However, we observed that both CHA and its isogenic mutant CHAexsA, which lacks the type III secretion system (12, 25), infiltrated the epithelia (Fig. 2A; Table 2). In view of previous reports, we further analyzed the behavior of strain PAK (31, 20) and observed that it did not invade the epithelium (Fig. 2A), consistent with the finding that it also did not produce rhamnolipids under our culture conditions (Fig. 3A). The results show that invasion of P. aeruginosa does not correlate with elastolytic activity and type III secretion, at least during the early stages of infection.
To test whether rhamnolipids were actually produced under the conditions we used for the invasion assay, the promoter of the rhlA gene, which codes for rhamnosyltransferase, was fused to GFP to generate plasmid pZC1, which was used to transform PAO1 and PT712, yielding strains MZ2 and MZ6, respectively. We found that the expression of the rhlA promoter was activated in both strains as soon as the bacteria reached a density of >109/ml, indicating that the rhl quorum-sensing system and the rhlA pathways were properly activated, irrespective of the culture medium tested (data not shown). However, when reconstituted epithelia were exposed overnight to exponentially grown Pseudomonas strains MZ2 and MZ6, we observed that only the rhamnolipid-secreting strain MZ2 infiltrated the epithelium (Fig. 3B). Taken together, the results show that the actual release of rhamnolipids is essential for epithelial infiltration (Table 2).
Purified rhamnolipids alter the epithelial barrier without affecting cell viability. To determine whether exogenous rhamnolipids could reproduce the effects of P. aeruginosa or supernatants of high-density bacterial cultures, we applied purified rhamnolipids to the apical surface of the epithelia. We observed that rhamnolipids resulted in a rapid reduction in TER, which was dose and time dependent. Thus, whereas 15 μg/ml rhamnolipids did not significantly alter the values of TER after 3 h (846 ± 6 · cm2 [n = 4]) compared to those observed for untreated controls (996 ± 48 · cm2), 50 μg/ml rhamnolipids decreased the TER within 30 min, reaching less than 10% of the control value (P < 0.001) after 360 min (Fig. 4A). In the presence of 150 μg/ml rhamnolipids, such a drop in TER was observed within 10 min (163 ± 83 · cm2 [n = 5]) (Fig. 4A).
To determine whether this drop was paralleled by a change in epithelial permeability, we next studied the flux of [3H]inulin. We observed that, compared to the control value of 1.2%, the transepithelial passage of this extracellular marker was increased to 4.5% and 25% after 1-h and 4-h exposures of the epithelium to 150 μg/ml rhamnolipids, respectively (Fig. 4B), in spite of unchanged viability of the epithelial cells, as assessed by the MTT (Fig. 4C) and LIVE/DEAD cytotoxicity/viability (Fig. 4D) tests. Under these conditions, the proportion of dead cells (0.5%) was like that observed in untreated epithelia, showing that cell death could not account for the large alterations in permeability of rhamnolipid-treated epithelia.
Consistent with these alterations, we observed that within 30 min, GFP-PAO1 infiltrated epithelia that had previously been exposed for 60 min to 150 μg/ml rhamnolipids (Fig. 4E). Under these conditions, an average of 6 bacteria contacted 9% of the cells, compared to control values of 1.2 bacteria on 1.7% of the cells (31 filters were scored from 19 independent experiments). The number of bacteria reaching the basal surface of the epithelia was evaluated by plating the bacteria onto agar plates after extensive washing of the epithelia followed by hypotonic lysis. We observed that 7.4% of the wild-type bacteria that were applied on top of the epithelia had infiltrated (data not shown). Similar infiltration of rhamnolipid-exposed epithelia was also observed with Pseudomonas strains PAK, PT531, and PT712, which otherwise did not infiltrate our control epithelia even when applied at high densities (Table 2), as well as with inert carboxylate-modified microspheres (data not shown). These data indicate that once the paracellular pathway was made accessible, no further active process was required for epithelial invasion.
To identify the type of cells affected by rhamnolipids, we monitored the passage of GFP-P. aeruginosa through epithelia that had been exposed either to purified rhamnolipids or to supernatants of overnight cultures of PAO1. In both cases, we observed that the fluorescent bacteria infiltrated the epithelia at sites where the immunolabeling of ezrin was displaced from the apical (control group) to the basolateral (rhamnolipid-exposed group) membrane (Fig. 5A). Double immunolabeling for ezrin and MUC5AC showed that most of these cells did not express mucins and had a ciliated phenotype (Fig. 5B). These findings, together with the loss of cilia, which was observed after rhamnolipid exposure (Fig. 5A, C, and D), indicated that PAO1 infiltrated the epithelium close to cells featuring an altered polarity. To determine whether this infiltration occurred through the transcellular or the paracellular pathway, epithelia exposed to Pseudomonas for 16 to 24 h were examined by electron microscopy. Irrespective of the invading strain (PAO1 or CHA), the bacteria were seen exclusively within intercellular spaces throughout the duration of the experiment (Fig. 5C). However, after several hours of infection, a few necrotic cell profiles were observed, over which numerous P. aeruginosa bacteria were concentrated (Fig. 5C). The finding of similar profiles in epithelia exposed to rhamnolipids, to permit the infiltration of the PT712 and PT531 bacteria, suggests that this limited cell necrosis was not caused by factors secreted under the control of the quorum-sensing systems. Comparable observations were made in epithelia exposed to rhamnolipids and then to PAO1 for as long as 5 h (Fig. 5D).
Rhamnolipids localize within the lateral membranes of epithelial cells. To assess how rhamnolipids altered the epithelial barrier, we coupled fluorescein to their carbohydrate moiety and applied either the labeled molecules or fluorescein-tagged L-rhamnose to the apical surface of the epithelia. Labeled rhamnolipids drastically reduced TER, as did the unlabeled molecules (Fig. 6A). This effect, which was not observed with either unlabeled or FITC-labeled rhamnose, was reversible after washout (Fig. 6B). At early time points, fluorescent rhamnolipids were uniformly distributed within the apical membranes of the ezrin-positive cells (Fig. 6C). After 60 to 120 min, the rhamnolipids featured a sparser and patchier distribution in the apical membranes and became concentrated in the basolateral membranes (Fig. 6C). The incorporation of FITC-labeled rhamnolipids into the host membranes was confirmed by live imaging of treated epithelial cells stained with TMA-DPH (data not shown). This incorporation appears to depend on the alkyl chains of rhamnolipids, inasmuch as no labeled L-rhamnose bounded or was internalized by epithelial cells (data not shown).
In several systems, rhamnose promotes the binding of polysaccharides to specific receptors (49). To assess whether rhamnose promoted the insertion of rhamnolipids within the membranes of epithelial cells, we tried to compete for this insertion by using 0.3 to 2 mM L-rhamnose for 1 to 16 h. We found that L-rhamnose prevented neither the drastic drop in TER (Fig. 7A) nor the bacterial infiltration (Fig. 7B) induced by rhamnolipids.
Rhamnolipids alter tight-junction architecture. The opening of the paracellular route caused by rhamnolipids implies alterations of TJ. Consistent with this view, significant alterations in TJ morphology were seen, which increased with time after rhamnolipid treatment (Fig. 8A). Thus, whereas TJ of control cells formed regular and uninterrupted belts, made on average of five interconnected strands of fibrils (Fig. 8A), morphometric analysis revealed that the number of strands and the area of TJ belts were significantly reduced after a 30- to 60-min treatment with 150 μg/ml rhamnolipids (Fig. 8). After 4 h, TJ belts comprised fibrils that were often interrupted and either partially or fully disconnected from the TJ belt, resulting in an increase in the membrane area occupied by TJ fibrils, in spite of a reduced number of strands (Fig. 8).
DISCUSSION
The establishment and maintenance of cell polarity is essential for the integrity and function of epithelia, which is particularly critical for the prevention of invasion by pathogens. Previous studies have demonstrated that defects in the paracellular permeability of respiratory epithelia are a prerequisite for P. aeruginosa invasion (18, 44) and have suggested that bacterial toxins may induce these defects by decreasing levels of TJ-associated proteins (3, 57). However, the specificity of these findings and the mechanisms leading to TJ alterations have not been investigated. To address this question, we have developed a model of epithelia reconstituted with human cells, grown at the air-liquid interface.
Like the native normal epithelium, the reconstituted tissue was not susceptible to infection by P. aeruginosa for up to 8 h. However, after prolonged exposure to bacteria (16 to 24 h), which allows for expression and secretion of bacterial toxins, the reconstituted epithelium became selectively infiltrated by bacteria featuring a normal Rhl quorum-sensing system. Among the virulence factors regulated by this system, we found that rhamnolipids are necessary and sufficient to affect the epithelial barrier. Thus, purified rhamnolipids reproduced the drop in transepithelial resistance, the increase in epithelial permeability to inulin, and the disorganization of TJ belts which were induced either by high densities of wild-type P. aeruginosa or by bacteria-free supernatants of these pathogens but not by strains of P. aeruginosa that featured a global defect in the Rhl quorum-sensing system (PT531 and PT462) or were selectively deficient in rhamnolipid production (PT712 and PAK).
Previous studies have suggested that the production of the elastolytic metalloproteinase LasB (40), which is a consistent feature of pathogenic P. aeruginosa (35, 56), decreases the levels of TJ-associated proteins, thus altering the paracellular barrier function of epithelia (3). It has also been reported that bacterial invasion inversely correlates with the levels of ExoS, a protein of the type III secretion system (11), which is a substrate for P. aeruginosa elastases (11) and which accounts for P. aeruginosa cytotoxicity. Here we report that a P. aeruginosa strain that produces elastase but not rhamnolipids (PT712) cannot infiltrate the reconstituted epithelium, whereas the same PT712 strain complemented for the rhlA mutation (PT1323 and MZ10), as well as a P. aeruginosa strain that produces rhamnolipids but no elastase (CHA), can. We also report that a strain producing rhamnolipids but featuring a defective type III secretion system (CHAexsA) also infiltrated the epithelium, ruling out the type III mechanism as the trigger of this infiltration. Hence, our data show that rhamnolipids are necessary and sufficient to initiate the alterations of the paracellular pathway that allow for bacterial invasion. This conclusion does not exclude the possibility that elastase and the type III secretion system might also contribute to virulence at later stages of the infection process (11, 34).
Our study provides a first insight into the mode of action of rhamnolipids. Like other lipid molecules (1, 50), rhamnolipids are titrated as a function of their partition into the membranes of the host cells. Using FITC-labeled molecules, we show that rhamnolipids are initially incorporated within the apical membranes of epithelial cells and later are found within their basolateral membranes. Together with the obvious loss of cilia, the displacement of ezrin, and the alterations of the TJ, these findings indicate that rhamnolipids, whether chemically purified or produced by P. aeruginosa, cause a loss of cell polarity. As a result, TER was markedly decreased and the permeability of epithelia to extracellular markers and bacteria increased, in the absence of obvious cell death. It remains to be shown whether the loss of cell polarity is due to a direct effect of the rhamnolipids on TJ or, as suggested by the distribution of these molecules over large domains of the apical (initially) and basolateral (at later time points) membranes, to their effects on the lipid environment of the junctions, which conceivably could alter their fence and barrier functions.
At any rate, once TJ are opened, a variety of P. aeruginosa strains, including some that do not secrete rhamnolipids, enter the paracellular pathway. Our data show that this step does not involve an active mechanism, inasmuch as it is mimicked by inert particles with a size comparable to that of the bacteria. Electron microscopy revealed that, within the 24-h time frame of our experiments, the infiltrating P. aeruginosa remained in the intercellular spaces and was not internalized by the epithelial cells. This finding was not anticipated, in view of previous reports that had suggested that P. aeruginosa is internalized at advanced stages of pulmonary infection, possibly via a receptor located in the basolateral membrane (16). While this putative receptor remains to be identified, a possible role for CFTR has been suggested (22, 26, 43), even though this chloride channel is normally located within the apical membranes of epithelial cells (32, 55). In this situation, access to the basolateral membrane is not needed for P. aeruginosa to interact with CFTR, in contrast to the finding of such an early access documented in this and previous studies (19). Also, epithelia reconstituted with cells from cystic fibrosis patients carrying the homozygous F508 mutation behaved like control epithelia (data not shown). Thus, polarized epithelia lacking CFTR were infiltrated by PAO1 only when the bacteria had reached a high cell density and, throughout the time course of experiments, failed to show sizable internalization of P. aeruginosa (data not shown). Together, these data do not support a central role for CFTR (14, 43) in the early steps of P. aeruginosa invasion. Furthermore, comparison of invasive (PAO1), cytotoxic (CHA), and noncytotoxic (CHAexsA) strains showed that the early steps of P. aeruginosa infiltration also were not dependent on the type III secretion system. Rather, our findings document the requirement for access to the paracellular route, as previously suggested for cultures of cell lines (9, 24, 30). In these cases, P. aeruginosa internalization appeared to be dependent on the cell phenotype, inasmuch as bacteria were not incorporated by polarized epithelial cells (24, 30). Differences in the types of cells studied, in the multiplicity of infection, and in the presence or absence of antibiotic treatment may further account for the different observations made in this and previous studies (21). It is, however, not excluded that under environmental conditions not investigated here, such as antibiotic treatment, P. aeruginosa uses airway epithelial cells as a reservoir for persistence (21).
Our study is the first to investigate a 3-dimensional epithelium of primary human cells under conditions leading to differentiation of ciliated and goblet cells. Under such conditions, the early steps of invasion by P. aeruginosa require the opening of the paracellular route and do not involve incorporation of the bacteria by the cells. Here we have shown that this infiltration is dependent on the production of rhamnolipids, encoded by the Rhl quorum-sensing system, which open the paracellular route. The molecular mechanism whereby rhamnolipids alter the structures and mechanisms ensuring cell polarity remains to be determined. Rhamnolipids are found in the sputum (33) and lung secretions (23) of chronically infected patients at concentrations close to the concentration we tested experimentally (thus sufficiently high to promote P. aeruginosa infiltration). Rhamnolipids have also been reported to have deleterious effects on mucociliary clearance (28) and phagocytosis by macrophages (38) and are involved in fluid-channel formation in and dispersion of biofilms (6, 15, 17, 27, 47). Hence, these molecules are candidate targets for future therapeutic strategies aimed at specific modulation of the mucosal barrier.
ACKNOWLEDGMENTS
We thank C. van Delden, P.-A. Ruttiman for graphic help, J.-L. Dumas for the construction of plasmid pAKRHL, and the Jeneil Company for the gift of purified rhamnolipids.
L.Z. is supported by the Ernst and Lucie Schmidheiny Foundation, the Sir Jules Thorn Charitable Overseas Trust, and the Association Vaincre la Mucoviscidose. J.-S.L. is supported by a grant from the Swiss National Foundation (3100A0-100621-1). Work of the Meda team is supported by grants from the Swiss National Foundation (3100-00-109402), the Juvenile Diabetes Research Foundation International (1-2005-46), the European Union (QLRT-2001-01777), and the National Institute of Health (DK 63443-01).
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