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Patterns of Epithelial Cell Invasion by Different Species of The INFECTION AND IMMUNITY, Aug. 2002, p. 4547–4555 Vol. 70, No. 8 0019-9567/02/$04.00ϩ0 DOI: 10.1128/IAI.70.8.4547–4555.2002 Copyright © 2002, American Society for Microbiology. All Rights Reserved. Patterns of Epithelial Cell Invasion by Different Species of the Burkholderia cepacia Complex in Well-Differentiated Human Airway Epithelia Ute Schwab,1* Margaret Leigh,1 Carla Ribeiro,1 James Yankaskas,1 Kimberly Burns,1 Peter Gilligan,1 Pamela Sokol,2 and Richard Boucher1 Cystic Fibrosis/Pulmonary Research and Treatment Center, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-7248,1 and Department of Microbiology and Infectious Diseases, University of Calgary Health Sciences Center, Calgary, Alberta, Canada T2N 4N12 Received 25 February 2002/Returned for modification 2 April 2002/Accepted 8 May 2002 Burkholderia cepacia has emerged as a serious respiratory pathogen in cystic fibrosis (CF) patients. The clinical course of B. cepacia infections is variable, but ϳ20% of patients eventually succumb to the cepacia syndrome, which is characterized as a fatal necrotizing pneumonia with bacteremia. The mechanisms that permit B. cepacia to cause bacteremia are not yet known but probably involve sequential penetration of airway barriers. This study evaluated the abilities of different species of the B. cepacia complex, including a strain from the ET12 lineage (BC-7, genomovar III, cblA؉), which is associated with most cepacia syndrome fatalities among CF populations, a genomovar IV strain (HI2258), and a genomovar II strain (J-1) to penetrate polarized, well-differentiated human airway epithelial cell cultures. As revealed by light and electron micros- copy, all three B. cepacia strains tested circumvented the mechanical barriers of mucus and ciliary transport to penetrate the airway epithelium but they used different routes. The BC-7 strain (genomovar III) formed biofilms in close proximity to the apical cell surface, followed by invasion and destruction of epithelial cells. This process involved disruption of the glycocalyx and rearrangements of the actin cytoskeleton. The HI2258 strain (genomovar IV) did not form biofilms, and the majority of bacteria that penetrated the epithelium were located between epithelial cells, suggesting paracytosis. Strain J-1 penetrated the epithelium both by cell destruction and paracytosis. These studies suggest that the distinct invasion pathways employed by B. cepacia may account for differences in virulence between B. cepacia genomovars. In cystic fibrosis (CF), chronic bacterial infection results in cepacia and are associated with most cases of cepacia syn- bronchiectasis and ultimately respiratory failure. The initial drome (18). infective organisms may include Haemophilus influenzae and The mechanism(s) that permits B. cepacia to cause chronic Staphylococcus aureus. Then, chronic persistent infections with versus invasive infections with bacteremia is not yet known. mucoid strains of Pseudomonas aeruginosa develop in the ma- One possibility is that, during chronic infection, B. cepacia jority of CF patients (5, 8, 12). Despite chronic infections with forms biofilms confined to the intraluminal mucus layer. This these organisms, necrotizing pneumonia and bacteremia are notion is supported in part by the observation that B. cepacia rare occurrences in CF. Recently, Burkholderia cepacia has is able to form biofilms in vitro (2, 6, 11, 22, 25), and an emerged as an important pathogen for CF patients (9). Al- outbreak of B. cepacia in hemodialysis patients has been re- though not as prevalent as P. aeruginosa, B. cepacia can be ported to result from biofilm-forming organisms in catheters isolated from as many as 13.3% of CF patients (Cystic Fibrosis (15). In addition Nielsen et al. (22) showed that both P. aerugi- Foundation Patient Registry, Annual data report, 2000: all nosa and B. cepacia grown in flow chambers formed coaggre- cystic fibrosis patients currently under care at 117 CF care gated or single-species biofilms depending on growth condi- centers in the United States, 2001), and highly transmissible tions, suggesting that P. aeruginosa and B. cepacia may coexist strains have been reported (32). Most patients harboring B. in CF airways. However, localization of B. cepacia to intralu- cepacia exhibit a chronic infection associated with periods of minal mucus in CF lungs has yet not been reported. ϳ acute exacerbations. Importantly, 20% of patients harboring In contrast, it appears likely that invasion and bacteremia B. cepacia eventually succumb to the cepacia syndrome, a rap- are associated with penetration of airway epithelia and trans- idly fatal clinical deterioration with acute necrotizing pneumo- mission via macrophages. For example, several investigators nia and bacteremia (13). Up to nine genomovars are known to have shown that B. cepacia can survive in A549 human epithe- occur in CF patients (4, 34). Of these, genomovar III strains lial cells and macrophages (3, 16, 19, 26). Recently, B. cepacia occur in approximately 50% of CF patients infected with B. has been localized by immunohistology in patients with cepacia syndrome in the exudates within bronchial lumens and be- tween airway epithelial cells (27). These CF patients were * Corresponding author. Mailing address: Cystic Fibrosis/Pulmo- infected by the transmissible clonal ET12 (Edinburgh-To- nary Treatment Center, 6013 Thurston-Bowles Bldg., CB no. 7248, ronto) lineage belonging to genomovar III. The isolates of this The University of North Carolina, Chapel Hill, NC 27599-7248. Phone: (919) 966-7031. Fax: (919) 966-7524. E-mail: [email protected] clone carry the cblA gene, which encodes the major subunit for .edu. surface cable pili, which have been shown to mediate binding 4547 4548 SCHWAB ET AL. INFECT.IMMUN. to respiratory mucins (28), and thus may facilitate invasion by Preparation of infected airway epithelial cell cultures for light, scanning, and this bacterium. It is not known whether the immunohistologi- electron microscopy. Infected and noninfected epithelial cell cultures were cal findings in the report of Sajjan et al. (27) represent features fixed with 1% OsO4-perfluorocarbon for 30 min, a fixation method which does not perturb the airway surface liquid layer (see Fig. 1A) (20). For light characteristic of this ET12 strain or of late-stage pulmonary microscopy (LM), the fixed cultures were embedded in epoxy and sectioned. infection with B. cepacia in general. Semithin sections from the center of the cell culture preparation were stained In the present study, we examined different species of the B. with Richardson’s stain. The following characteristics were determined in cepacia complex invasion pathways in well-differentiated hu- sections encompassing the overall length of airway epithelial cell cultures (12 man airway epithelial cell cultures. In this in vitro model sys- mm): presence of biofilms, single bacteria found between cells or within cells, tem, we tested whether different strains of B. cepacia devel- and damage of the airway epithelium, evaluated as mild, moderate, or severe. For transmission electron microscopy (TEM), ultrathin sections (90 nm) were oped different invasion pathways, i.e., biofilm formation cut from blocks and mounted on grids and stained with uranyl acetate and associated with invasion, single-cell entry, and/or paracytosis. lead citrate and areas of interest were examined with an electron microscope. In addition, we asked whether biofilm-mediated invasion in- For scanning electron microscopy (SEM), the cultures were fixed with 4% volved glycocalyx disruption and rearrangement of the actin paraformaldehyde for 10 min and postfixed in 1% OsO4 prior to dehydration filaments on the apical cell surface. through a graded ethanol series. After being coated with gold-palladium the specimens were viewed by SEM. Immunohistology. Infected and noninfected airway epithelial cell cultures MATERIALS AND METHODS were fixed in 4% paraformaldehyde for 15 min and washed in phosphate-buff- Human airway epithelial cell cultures. Well-differentiated human airway ep- ered saline (PBS). Paraffin-embedded sections were cut, deparaffinized by xylene ithelial cell cultures were grown on Costar (Cambridge, Mass.) 12-mm-diameter treatment, rehydrated in graded alcohols, washed in PBS, and then incubated Transwell-Col (T-Col) supports as previously described (1). Cells were removed with different dilutions of the rabbit antiserum specific for the B. cepacia complex by protease dissociation from portions of the main stem or lobar bronchi from (Heiner Neubauer, FAF Medical Academy, Institute of Microbiology, Munich, excess donor tissue obtained at the time of lung transplantation under the Germany) or normal rabbit serum (negative control). Afterwards, we added auspices of the University of North Carolina Institutional Committee on the sequentially the secondary antibody (biotinylated goat anti-rabbit immunoglob- Protection of The Rights of Human Subjects. Dissociated primary airway epi- ulin G; diluted 1:250), streptavidin-horseradish peroxidase conjugate reagent, thelial cells were grown on 100-mm-diameter tissue culture dishes in bronchial and the substrate, diaminobenzidine tetrahydrochloride. The sections were coun- epithelial culture (BEC) medium. Passage 2 cells were seeded on the T-Col terstained with light green and examined by LM. The strongest signal was membrane supports (Costar) under air-liquid interface conditions at a density of obtained by using the B. cepacia antiserum at a 1:500 dilution, and
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