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Infection and Immunity, August 2005, p. 5262-5268, Vol. 73, No. 8
0019-9567/05/$08.00+0 doi:10.1128/IAI.73.8.5262-5268.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
Department of Genetic Medicine,1 Department of Pediatrics,2 Division of Pulmonary and Critical Care Medicine,3 Department of Microbiology and Immunology, Weill Medical College of Cornell University, New York, New York4
Received 20 December 2004/ Returned for modification 9 February 2005/ Accepted 31 March 2005
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Although the airway epithelial cells are the dominant cell type affected by CFTR dysfunction in the CF lung, the interaction of organisms with cells of the pulmonary host immune system may be important in determining the extent of the inflammatory responses to these pathogens and the lack of bacterial clearance from the airways that characterizes the disease (5, 14, 18, 20, 21, 30). Alveolar macrophages (AM), the resident mononuclear phagocytes in the respiratory tract, are the first line of cell-mediated defenses against inhaled organisms (4, 18, 25). Besides their scavenger function upon interaction with pathogens, AM release a variety of mediators that play a central role in the inflammatory response to organisms, such as the recruitment of polymorphonuclear phagocytes to the respiratory epithelium (4, 18, 25, 26). The phagocytic contribution of AM to the defense against P. aeruginosa is thought to be negligible (9), and studies with mice have shown that temporary elimination of AM did not affect the course of P. aeruginosa pulmonary or systemic infection; however, it affected cytokine and chemokine production (13). AM likely play a role in the control and maintenance of the pulmonary inflammatory response to P. aeruginosa (9, 20, 21, 23, 27, 41). As AM express only negligible amounts of the CFTR gene, mutations in CFTR associated with CF are unlikely to be associated with AM dysfunction per se (44). This has also been confirmed by studies with transgenic G551D mice expressing the human CFTR gene in lung epithelium or AM, with an increase in pulmonary clearance of P. aeruginosa and a decrease in inflammatory cytokines in epithelial cell-corrected, but not macrophage-corrected, mice (28). However, the epithelial abnormalities associated with CFTR mutations present the AM with the challenge of clearing a large, persistent burden of P. aeruginosa and, in some late cases, B. cepacia. AM isolated from patients with CF produce larger amounts of inflammatory cytokines compared to non-CF AM (21, 30). The inability of AM to efficiently clear organisms like P. aeruginosa and B. cepacia from the lung, in combination with an exacerbated inflammatory milieu, may be important factors in the interaction of these pathogens with the host defense system in CF (5, 7, 9, 14, 15, 20, 41). One clue to the inability of AM to efficiently clear P. aeruginosa is the observation that AM are more susceptible than epithelial cells to P. aeruginosa-induced apoptosis (11, 12, 17, 40, 42, 43).
Since the clinical outcome of infection with late strains of P. aeruginosa and with B. cepacia is more severe than with early isolates of P. aeruginosa, we hypothesized that these differences could be due to a differential response of the AM upon exposure to these different pathogens. To understand the interaction of human AM with P. aeruginosa and B. cepacia, we used an unbiased approach based on microarray technology to assess the early changes in human AM gene expression in response to short time points of exposure to two clinical strains of P. aeruginosa (one early and one late isolate) and a strain of B. cepacia.
Changes in gene expression in AM infected with P. aeruginosa strains and B. cepacia. The P. aeruginosa strains used included laboratory strain PAK (provided by A. Prince, Columbia University, New York, NY) and two clinical strains isolated from the sputum of an individual with CF, AD2A and AD15B (provided by J. Burns, University of Washington, Seattle). AD2A is an early clinical isolate, and AD15B is a late clinical isolate; both were derived from the same individual. The B. cepacia strain was J2315, genomovar 3 (kindly provided by John LiPuma, University of Michigan). Human AM were obtained by bronchoalveolar lavage of healthy, nonsmoking or smoking volunteers (36). AM represented >95% of the cells in all cases. Cell viability (always >90%) was determined by trypan blue exclusion. The yield of AM derived from individuals with a history of smoking was four to five times higher compared to nonsmokers. After 3 h of adherence, the cells were infected with the bacteria at a dose of 5 CFU/cell in RPMI 1640-25 mM HEPES, pH 7.4, for 4 h. The viability of the cells following 4 h of infection was >85% as assessed by trypan blue exclusion for all bacterial strains. All analyses were carried out with the Affymetrix HuGeneFL chip using the protocols from Affymetrix (Santa Clara, CA) and the GeneSpring software (Silicon Genetics, Redwood City, CA). As the yield of AM derived from nonsmoking individuals was not sufficient to allow assessment of more than two conditions (e.g., control and infection with PAK) and the preliminary analysis of gene expression changes in AM infected with PAK demonstrated that the results were independent of whether the AM were derived from smokers or nonsmokers (Fig. 1), all subsequent experiments were performed with AM derived from smokers, which by virtue of their increased numbers (2, 4, 38) allowed infection with PAK, AD2A, AD15B, and B. cepacia within the same experiment and sample.
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) (102- and 62-fold compared to controls, respectively). The second largest functional group consisted of genes coding for factors related to signal transduction and growth control (12 out of 42). The majority of these are known to be induced by cytokine signaling (e.g., TNF receptor-associated factor I
B). Some of the upregulated genes in this category, such as Jagged, have not been previously reported as associated with bacterial stimulation of macrophages. A smaller group of upregulated genes codes for nuclear proteins involved in transcriptional regulation, including the enhancer of zeste homolog 2 (EZH2), also not previously associated with bacterial activation of macrophages.
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As an important component of the innate immune system in the lung, AM are in close contact with pulmonary pathogens such as P. aeruginosa and B. cepacia and are found in abundance in the inflamed local milieu of the CF lung (20, 21). Studies with mice suggest that AM are not directly involved in the clearance of P. aeruginosa (21). However, since the AM response to the bacteria contributes to the composition of the inflammatory milieu on the airway epithelial surface, it is relevant to ask whether there is variability of the response of AM to different P. aeruginosa strains or B. cepacia. Interestingly, the present study demonstrates that the human AM response to short-term exposure of various P. aeruginosa isolates or B. cepacia is relatively uniform. Although mRNA levels for a variety of cytokines and chemokines were rapidly induced following exposure of the AM to the various Pseudomonas strains and B. cepacia, the short-term response of AM to P. aeruginosa isolates and B. cepacia was, at least in vitro, relatively homogeneous. These homogenous responses are dominated by the induction of inflammatory cytokines and chemokines, which are important in the host responses that lead to elimination of the bacteria in the healthy host, with TNF-
and IL-6 showing the highest levels of induction (3, 26). Similarly, signal transduction pathways known to be related to cytokine activation, especially those mediated by transcription factor NF-
B (1, 3, 26), were rapidly induced to similar extents following exposure to the different P. aeruginosa isolates and B. cepacia. Future experiments need to identify the specificity of the gene expression changes with those observed with other, not lung or CF-related, gram-negative pathogens.
Confirmation by TaqMan real-time reverse transcription (RT)-PCR of selected genes. Most of the genes up- or downregulated in AM exposed to the P. aeruginosa strains or to B. cepacia corresponded to genes coding for inflammatory mediators or other proteins involved in pathways previously described as part of the response of AM to pathogens (25, 26, 34, 35). The agreement between the microarray data presented here and previous studies using other techniques to measure mRNA levels is in itself confirmation of the validity of the approach (10). Therefore, we chose to focus the validation of the results obtained by microarray analysis on four genes that had not been previously described in association with AM activation or response to pathogens: Jagged, EZH2, Lyl-1, and DOK1. The gene expression changes observed by microarray analysis were independently confirmed by TaqMan real-time RT-PCR as follows: Jagged was upregulated (PAK, 21.1-fold ± 6.7-fold; B. cepacia, 21.3-fold ± 3.6-fold; Fig. 3A), EZH2 was upregulated (PAK, 4.3-fold ± 0.4-fold; B. cepacia, 5.1-fold ± 0.6-fold; Fig. 3B), DOK1 was downregulated (PAK, 5.3-fold ± 0.4-fold; B. cepacia, 3.4-fold ± 0.8-fold; Fig. 3C), and Lyl-1 was downregulated (PAK, 4.1-fold ± 1.3-fold; B. cepacia, 4.8-fold ± 0.7-fold; Fig. 3D). Compared to the gene expression changes observed by microarray analysis, the magnitude of the changes was higher for the upregulated Jagged and EZH2 genes and lower for the downregulated Lyl-1 and DOK1 genes. Two-way analysis of variance confirmed that for each of the four genes there was a statistically significant effect of infection with PAK (P < 0.001) or B. cepacia (P < 0.001), but not methodology (P > 0.9 for PAK, P > 0.6 for B. cepacia).
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Taken together, relevant to the pathogenesis of Pseudomonas and Burkholderia infection in CF, the observations in the present study are consistent with the concept that the AM play an important role in contributing to the inflammatory milieu of the respiratory epithelium. However, while Pseudomonas clinical isolates late in the course and superinfection with Burkholderia are both associated with progressive deterioration (14, 37), the responses of human AM to different strains of Pseudomonas, as well as Burkholderia, seem to be remarkably similar, suggesting that AM are not responsible for the worsening of CF in association with different strains of bacteria.
| ACKNOWLEDGMENTS |
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These studies were supported, in part, by P01 HL51746, P01 HL59312, U01 HL66952, M01RR00047, the Will Rogers Memorial Fund (Los Angeles, CA), and the Cystic Fibrosis Foundation (Bethesda, MD).
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