Control of mitochondrial functions by Pseudomonas aeruginosa in cystic fibrosis

Cystic fibrosis (CF) is a genetic disease characterized by mutations of cystic fibrosis transmembrane conductance regulator (CFTR) gene, which lead to a dysfunctional chloride and bicarbonate channel. Abnormal mucus viscosity, persistent infections and hyperinflammation that preferentially affect the airways, referred to the pathogenesis of CF lung disease. It has largely demonstrated that Pseudomonas aeruginosa (P. aeruginosa) represents the most important pathogen that affect CF patients, leading to worsen inflammation by stimulating pro-inflammatory mediators release and tissue destruction. The conversion to mucoid phenotype and formation of biofilms, together with the increased frequency of mutations, are only few changes that characterize the P. aeruginosa's evolution during CF lung chronic infection. Recently, mitochondria received increasing attention due to their involvement in inflammatory-related diseases, including in CF. Alteration of mitochondrial homeostasis is sufficient to stimulate immune response. Exogenous or endogenous stimuli that perturb mitochondrial activity are used by cells, which, through the mitochondrial stress, potentiate immunity programs. Studies show the relationship between mitochondria and CF, supporting the idea that mitochondrial dysfunction endorses the exacerbation of inflammatory responses in CF lung. In particular, evidences suggest that mitochondria in CF airway cells are more susceptible to P. aeruginosa infection, with consequent detrimental effects that lead to amplify the inflammatory signals. This review discusses the evolution of P. aeruginosa in relationship with the pathogenesis of CF, a fundamental step to establish chronic infection in CF lung disease. Specifically, we focus on the role of P. aeruginosa in the exacerbation of inflammatory response, by triggering mitochondria in CF.

CFTR Modulators Restore Acidification of Autophago-Lysosomes and Bacterial Clearance in Cystic Fibrosis Macrophages

Cystic fibrosis (CF) human and mouse macrophages are defective in their ability to clear bacteria such as Burkholderia cenocepacia. The autophagy process in CF (F508del) macrophages is halted, and the underlying mechanism remains unclear. Furthermore, the role of CFTR in maintaining the acidification of endosomal and lysosomal compartments in CF cells has been a subject of debate. Using 3D reconstruction of z-stack confocal images, we show that CFTR is recruited to LC3-labeled autophagosomes harboring B. cenocepacia. Using several complementary approaches, we report that CF macrophages display defective lysosomal acidification and degradative function for cargos destined to autophagosomes, whereas non-autophagosomal cargos are effectively degraded within acidic compartments. Notably, treatment of CF macrophages with CFTR modulators (tezacaftor/ivacaftor) improved the autophagy flux, lysosomal acidification and function, and bacterial clearance. In addition, CFTR modulators improved CFTR function as demonstrated by patch-clamp. In conclusion, CFTR regulates the acidification of a specific subset of lysosomes that specifically fuse with autophagosomes. Therefore, our study describes a new biological location and function for CFTR in autophago-lysosomes and clarifies the long-standing discrepancies in the field.

Revisiting the Role of Leukocytes in Cystic Fibrosis

Cystic fibrosis in characterized by pulmonary bacterial colonization and hyperinflammation. Lymphocytes, monocytes/macrophages, neutrophils, and dendritic cells of patients with CF express functional CFTR and are directly affected by altered CFTR expression/function, impairing their ability to resolve infections and inflammation. However, the mechanism behind and the contribution of leukocytes in the pathogenesis of CF are still poorly characterized. The recent clinical introduction of specific CFTR modulators added an important tool not only for the clinical management of the disease but also to the investigation of the pathophysiological mechanisms related to CFTR dysfunction and dysregulated immunity. These drugs treat the basic defect in cystic fibrosis (CF) by increasing CFTR function with improvement of lung function and quality of life, and may improve clinical outcomes also by correcting the dysregulated immune function that characterizes CF. Measure of CFTR function, protein expression profiling and several omics methods were used to identify molecular changes in freshly isolated leukocytes of CF patients, highlighting two roles of leukocytes in CF: one more generally related to the mechanism(s) causing immune dysregulation in CF and unresolved inflammation, and another more applicative role, which identifies in myeloid cells, an important tool predictive of the therapeutic response of CF patients. In this review we will summarize available data on CFTR expression and function in leukocyte populations and will discuss potential clinical applications based on available data.

Bacterial Subversion of Autophagy in Cystic Fibrosis

Cystic fibrosis (CF) is a genetic disease affecting more than 70,000 people worldwide. It is caused by a mutation in the cftr gene, a chloride ion transporter localized in the plasma membrane of lung epithelial cells and other organs. The loss of CFTR function alters chloride, bicarbonate, and water transport through the plasma membrane, promoting the production of a thick and sticky mucus in which bacteria including Pseudomonas aeruginosa and Burkholderia cenocepacia can produce chronic infections that eventually decrease the lung function and increase the risk of mortality. Autophagy is a well-conserved lysosomal degradation pathway that mediates pathogen clearance and plays an important role in the control of bacterial infections. In this mini-review, we describe the principal strategies used by P. aeruginosa and B. cenocepacia to survive and avoid microbicidal mechanisms within the autophagic pathway leading to the establishment of chronic inflammatory immune responses that gradually compromise the lung function and the life of CF patients.

The Distribution and Role of the CFTR Protein in the Intracellular Compartments

Cystic fibrosis is a hereditary disease that mainly affects secretory organs in humans. It is caused by mutations in the gene encoding CFTR with the most common phenylalanine deletion at position 508. CFTR is an anion channel mainly conducting Cl⁻ across the apical membranes of many different epithelial cells, the impairment of which causes dysregulation of epithelial fluid secretion and thickening of the mucus. This, in turn, leads to the dysfunction of organs such as the lungs, pancreas, kidney and liver. The CFTR protein is mainly localized in the plasma membrane; however, there is a growing body of evidence that it is also present in the intracellular organelles such as the endosomes, lysosomes, phagosomes and mitochondria. Dysfunction of the CFTR protein affects not only the ion transport across the epithelial tissues, but also has an impact on the proper functioning of the intracellular compartments. The review aims to provide a summary of the present state of knowledge regarding CFTR localization and function in intracellular compartments, the physiological role of this localization and the consequences of protein dysfunction at cellular, epithelial and organ levels. An in-depth understanding of intracellular processes involved in CFTR impairment may reveal novel opportunities in pharmacological agents of cystic fibrosis.

Type I IFNs facilitate innate immune control of the opportunistic bacteria Burkholderia cenocepacia in the macrophage cytosol

The mammalian immune system is constantly challenged by signals from both pathogenic and non-pathogenic microbes. Many of these non-pathogenic microbes have pathogenic potential if the immune system is compromised. The importance of type I interferons (IFNs) in orchestrating innate immune responses to pathogenic microbes has become clear in recent years. However, the control of opportunistic pathogens-and especially intracellular bacteria-by type I IFNs remains less appreciated. In this study, we use the opportunistic, Gram-negative bacterial pathogen Burkholderia cenocepacia (Bc) to show that type I IFNs are capable of limiting bacterial replication in macrophages, preventing illness in immunocompetent mice. Sustained type I IFN signaling through cytosolic receptors allows for increased expression of autophagy and linear ubiquitination mediators, which slows bacterial replication. Transcriptomic analyses and in vivo studies also show that LPS stimulation does not replicate the conditions of intracellular Gram-negative bacterial infection as it pertains to type I IFN stimulation or signaling. This study highlights the importance of type I IFNs in protection against opportunistic pathogens through innate immunity, without the need for damaging inflammatory responses.

Mitochondrial Stress Responses and "Mito-Inflammation" in Cystic Fibrosis

Cystic fibrosis (CF) is a genetic disease associated to mutations in the cystic fibrosis transmembrane conductance regulator gene, which results in the alteration of biological fluid and electrolyte homeostasis. The characteristic pathological manifestation is represented by exaggerated proinflammatory response in lung of CF patients, driven by recurrent infections and worsen by hypersecretion of proinflammatory mediators and progressive tissue destruction. Treating inflammation remains a priority in CF. However, current anti-inflammatory treatments, including non-steroidal agents, are poorly effective and present dramatic side effects in CF patients. Different studies suggest an intimate relationship between mitochondria and CF lung disease, supporting the hypothesis that a decline in mitochondrial function endorses the development of the hyperinflammatory phenotype observed in CF lung. This allowed the implementation of a new concept: the "mito-inflammation," a compartmentalization of inflammatory process, related to the role of mitochondria in engage and sustain the inflammatory responses, resulting a druggable target to counteract the amplification of inflammatory signals in CF. Here, we will offer an overview of the contribution of mitochondria in the pathogenesis of CF lung disease, delving into mitochondrial quality control responses, which concur significantly to exacerbation of CF lung inflammatory responses. Finally, we will discuss the new therapeutic avenues that aim to target the mito-inflammation, an alternative therapeutic advantage for mitochondrial quality control that improves CF patient's inflammatory state.

Role of Cystic Fibrosis Bronchial Epithelium in Neutrophil Chemotaxis

A hallmark of cystic fibrosis (CF) chronic respiratory disease is an extensive neutrophil infiltrate in the mucosa filling the bronchial lumen, starting early in life for CF infants. The genetic defect of the CF Transmembrane conductance Regulator (CFTR) ion channel promotes dehydration of the airway surface liquid, alters mucus properties, and decreases mucociliary clearance, favoring the onset of recurrent and, ultimately, chronic bacterial infection. Neutrophil infiltrates are unable to clear bacterial infection and, as an adverse effect, contribute to mucosal tissue damage by releasing proteases and reactive oxygen species. Moreover, the rapid cellular turnover of lumenal neutrophils releases nucleic acids that further alter the mucus viscosity. A prominent role in the recruitment of neutrophil in bronchial mucosa is played by CF bronchial epithelial cells carrying the defective CFTR protein and are exposed to whole bacteria and bacterial products, making pharmacological approaches to regulate the exaggerated neutrophil chemotaxis in CF a relevant therapeutic target. Here we revise: (a) the major receptors, kinases, and transcription factors leading to the expression, and release of neutrophil chemokines in bronchial epithelial cells; (b) the role of intracellular calcium homeostasis and, in particular, the calcium crosstalk between endoplasmic reticulum and mitochondria; (c) the epigenetic regulation of the key chemokines; (d) the role of mutant CFTR protein as a co-regulator of chemokines together with the host-pathogen interactions; and (e) different pharmacological strategies to regulate the expression of chemokines in CF bronchial epithelial cells through novel drug discovery and drug repurposing.

Macrophage dysfunction in cystic fibrosis: Nature or nurture?

Mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) affect the homeostasis of chloride flux by epithelial cells. This has deleterious consequences, especially in respiratory epithelia, where the defect results in mucus accumulation distinctive of cystic fibrosis. CFTR is, however, also expressed in phagocytic cells, like macrophages. Immune cells are highly sensitive to conditioning by their environment; thus, CFTR dysfunction in epithelia influences macrophages by affecting the lung milieu, but the mutations also appear to be directly consequential for intrinsic macrophage functions. Particular mutations can alter CFTR's folding, traffic of the protein to the membrane and function. As such, understanding the intrinsic effects of CFTR mutation requires distinguishing the secondary effects of misfolded CFTR on cell stress pathways from the primary defect of CFTR dysfunction/absence. Investigations into CFTR's role in macrophages have exploited various models, each with their own advantages and limitations. This review summarizes these methodologic approaches, discussing their physiological correspondence and highlighting key findings. The controversy surrounding CFTR-dependent acidification is used as a case study to highlight difficulties in commensurability across model systems. Recent work in macrophage biology, including polarization and host-pathogen interaction studies, brought into the context of CFTR research, offers potential explanations for observed discrepancies between studies. Moreover, the rapid advancement of novel gene editing technologies and new macrophage model systems makes this assessment of the field's models and methodologies timely.

Cystic Fibrosis and Pseudomonas aeruginosa: the Host-Microbe Interface

In human pathophysiology, the clash between microbial infection and host immunity contributes to multiple diseases. Cystic fibrosis (CF) is a classical example of this phenomenon, wherein a dysfunctional, hyperinflammatory immune response combined with chronic pulmonary infections wreak havoc upon the airway, leading to a disease course of substantial morbidity and shortened life span. Pseudomonas aeruginosa is an opportunistic pathogen that commonly infects the CF lung, promoting an accelerated decline of pulmonary function. Importantly, P. aeruginosa exhibits significant resistance to innate immune effectors and to antibiotics, in part, by expressing specific virulence factors (e.g., antioxidants and exopolysaccharides) and by acquiring adaptive mutations during chronic infection. In an effort to review our current understanding of the host-pathogen interface driving CF pulmonary disease, we discuss (i) the progression of disease within the primitive CF lung, specifically focusing on the role of host versus bacterial factors; (ii) critical, neutrophil-derived innate immune effectors that are implicated in CF pulmonary disease, including reactive oxygen species (ROS) and antimicrobial peptides (e.g., LL-37); (iii) P. aeruginosa virulence factors and adaptive mutations that enable evasion of the host response; and (iv) ongoing work examining the distribution and colocalization of host and bacterial factors within distinct anatomical niches of the CF lung.

Inflammation in cystic fibrosis: An update

Inflammation plays a critical role in cystic fibrosis (CF) lung pathology and disease progression making it an active area of research and important therapeutic target. In this review, we explore the most recent research on the major contributors to the exuberant inflammatory response seen in CF as well as potential therapeutics to combat this response. Absence of functional cystic fibrosis transmembrane conductance regulator (CFTR) alters anion transport across CF airway epithelial cells and ultimately results in dehydration of the airway surface liquid. The dehydrated airway surface liquid in combination with abnormal mucin secretion contributes to airway obstruction and subsequent infection that may serve as a trigger point for inflammation. There is also evidence to suggest that airway inflammation may be excessive and sustained relative to the infectious stimuli. Studies have shown dysregulation of both pro-inflammatory mediators such as IL-17 and pro-resolution mediators including metabolites of the eicosanoid pathway. Recently, CFTR potentiators and correctors have garnered much attention in the CF community. Although these modulators address the underlying defect in CF, their impact on downstream consequences such as inflammation are not known. Here, we review pre-clinical and clinical data on the impact of CFTR modulators on inflammation. In addition, we examine other cell types including neutrophils, macrophages, and T-lymphocytes that express CFTR and contribute to the CF inflammatory response. Finally, we address challenges in developing anti-inflammatory therapies and highlight some of the most promising anti-inflammatory drugs under development for CF.

A putative lateral flagella of the cystic fibrosis pathogen Burkholderia dolosa regulates swimming motility and host cytokine production

Burkholderia dolosa caused an outbreak in the cystic fibrosis clinic at Boston Children's Hospital and was associated with high mortality in these patients. This species is part of a larger complex of opportunistic pathogens known as the Burkholderia cepacia complex (Bcc). Compared to other species in the Bcc, B. dolosa is highly transmissible; thus understanding its virulence mechanisms is important for preventing future outbreaks. The genome of one of the outbreak strains, AU0158, revealed a homolog of the lafA gene encoding a putative lateral flagellin, which, in other non-Bcc species, is used for movement on solid surfaces, attachment to host cells, or movement inside host cells. Here, we analyzed the conservation of the lafA gene and protein sequences, which are distinct from those of the polar flagella, and found lafA homologs to be present in numerous β-proteobacteria but notably absent from most other Bcc species. A lafA deletion mutant in B. dolosa showed a greater swimming motility than wild-type due to an increase in the number of polar flagella, but did not appear to contribute to biofilm formation, host cell invasion, or murine lung colonization or persistence over time. However, the lafA gene was important for cytokine production in human peripheral blood mononuclear cells, suggesting it may have a role in recognition by the human immune response.

Iron Acquisition Mechanisms and Their Role in the Virulence of Burkholderia Species

Burkholderia is a genus within the β-Proteobacteriaceae that contains at least 90 validly named species which can be found in a diverse range of environments. A number of pathogenic species occur within the genus. These include Burkholderia cenocepacia and Burkholderia multivorans, opportunistic pathogens that can infect the lungs of patients with cystic fibrosis, and are members of the Burkholderia cepacia complex (Bcc). Burkholderia pseudomallei is also an opportunistic pathogen, but in contrast to Bcc species it causes the tropical human disease melioidosis, while its close relative Burkholderia mallei is the causative agent of glanders in horses. For these pathogens to survive within a host and cause disease they must be able to acquire iron. This chemical element is essential for nearly all living organisms due to its important role in many enzymes and metabolic processes. In the mammalian host, the amount of accessible free iron is negligible due to the low solubility of the metal ion in its higher oxidation state and the tight binding of this element by host proteins such as ferritin and lactoferrin. As with other pathogenic bacteria, Burkholderia species have evolved an array of iron acquisition mechanisms with which to capture iron from the host environment. These mechanisms include the production and utilization of siderophores and the possession of a haem uptake system. Here, we summarize the known mechanisms of iron acquisition in pathogenic Burkholderia species and discuss the evidence for their importance in the context of virulence and the establishment of infection in the host. We have also carried out an extensive bioinformatic analysis to identify which siderophores are produced by each Burkholderia species that is pathogenic to humans.

Macrophages, but not neutrophils, are critical for proliferation of Burkholderia cenocepacia and ensuing host-damaging inflammation

Bacteria of the Burkholderia cepacia complex (Bcc) can cause devastating pulmonary infections in cystic fibrosis (CF) patients, yet the precise mechanisms underlying inflammation, recurrent exacerbations and transition from chronic stages to acute infection and septicemia are not known. Bcc bacteria are generally believed to have a predominant extracellular biofilm life style in infected CF lungs, similar to Pseudomonas aeruginosa, but this has been challenged by clinical observations which show Bcc bacteria predominantly in macrophages. More recently, Bcc bacteria have emerged in nosocomial infections of patients hospitalized for reasons unrelated to CF. Research has abundantly shown that Bcc bacteria can survive and replicate in mammalian cells in vitro, yet the importance of an intracellular life style during infection in humans is unknown. Here we studied the contribution of innate immune cell types to fatal pro-inflammatory infection caused by B. cenocepacia using zebrafish larvae. In strong contrast to the usual protective role for macrophages against microbes, our results show that these phagocytes significantly worsen disease outcome. We provide new insight that macrophages are critical for multiplication of B. cenocepacia in the host and for development of a fatal, pro-inflammatory response that partially depends on Il1-signalling. In contrast, neutrophils did not significantly contribute to disease outcome. In subcutaneous infections that are dominated by neutrophil-driven phagocytosis, the absence of a functional NADPH oxidase complex resulted in a small but measurably higher increase in bacterial growth suggesting the oxidative burst helps limit bacterial multiplication; however, neutrophils were unable to clear the bacteria. We suggest that paradigm-changing approaches are needed for development of novel antimicrobials to efficiently disarm intracellular bacteria of this group of highly persistent, opportunistic pathogens.

Iron chelation as novel treatment for lung inflammation in cystic fibrosis

Cystic fibrosis (CF) is an autosomal recessive genetic disorder that results in defective cystic fibrosis transmembrane conductance regulator (CFTR) protein expression and function in various tissues. The leading cause of CF mortality and morbidity is the progressive destruction of the lungs due to recurrent infections and chronic inflammation. CFTR defect also affects immune cells, including neutrophils, resulting in ineffective, severe and persistent inflammatory response. Since unopposed recruitment of neutrophils significantly contributes to lung tissue damage through the generation of reactive oxygen species (ROS), we hypothesize that the administration of iron chelators could serve as a novel treatment to attenuate chronic inflammation in CF lungs since iron is significantly involved in ROS production by neutrophils. Ideally, the iron chelator should sequester host iron effectively, prevent bacterial access to chelator-bound iron and penetrates lung tissues efficiently, e.g. by inhalational route of administration.

Immune Recognition of the Epidemic Cystic Fibrosis Pathogen Burkholderia dolosa

Burkholderia dolosa caused an outbreak in the cystic fibrosis (CF) clinic at Boston Children's Hospital from 1998 to 2005 and led to the infection of over 40 patients, many of whom died due to complications from infection by this organism. To assess whether B. dolosa significantly contributes to disease or is recognized by the host immune response, mice were infected with a sequenced outbreak B. dolosa strain, AU0158, and responses were compared to those to the well-studied CF pathogen Pseudomonas aeruginosa In parallel, mice were also infected with a polar flagellin mutant of B. dolosa to examine the role of flagella in B. dolosa lung colonization. The results showed a higher persistence in the host by B. dolosa strains, and yet, neutrophil recruitment and cytokine production were lower than those with P. aeruginosa The ability of host immune cells to recognize B. dolosa was then assessed, B. dolosa induced a robust cytokine response in cultured cells, and this effect was dependent on the flagella only when bacteria were dead. Together, these results suggest that B. dolosa can be recognized by host cells in vitro but may avoid or suppress the host immune response in vivo through unknown mechanisms. B. dolosa was then compared to other Burkholderia species and found to induce similar levels of cytokine production despite being internalized by macrophages more than Burkholderia cenocepacia strains. These data suggest that B. dolosa AU0158 may act differently with host cells and is recognized differently by immune systems than are other Burkholderia strains or species.

Evaluation of combination therapy for Burkholderia cenocepacia lung infection in different in vitro and in vivo models

Burkholderia cenocepacia is an opportunistic pathogen responsible for life-threatening infections in cystic fibrosis patients. B. cenocepacia is extremely resistant towards antibiotics and therapy is complicated by its ability to form biofilms. We investigated the efficacy of an alternative antimicrobial strategy for B. cenocepacia lung infections using in vitro and in vivo models. A screening of the NIH Clinical Collection 1&2 was performed against B. cenocepacia biofilms formed in 96-well microtiter plates in the presence of tobramycin to identify repurposing candidates with potentiator activity. The efficacy of selected hits was evaluated in a three-dimensional (3D) organotypic human lung epithelial cell culture model. The in vivo effect was evaluated in the invertebrate Galleria mellonella and in a murine B. cenocepacia lung infection model. The screening resulted in 60 hits that potentiated the activity of tobramycin against B. cenocepacia biofilms, including four imidazoles of which econazole and miconazole were selected for further investigation. However, a potentiator effect was not observed in the 3D organotypic human lung epithelial cell culture model. Combination treatment was also not able to increase survival of infected G. mellonella. Also in mice, there was no added value for the combination treatment. Although potentiators of tobramycin with activity against biofilms of B. cenocepacia were identified in a repurposing screen, the in vitro activity could not be confirmed nor in a more sophisticated in vitro model, neither in vivo. This stresses the importance of validating hits resulting from in vitro studies in physiologically relevant model systems.

A High Level of Soluble CD40L Is Associated with P. aeruginosa Infection in Patients with Cystic Fibrosis

Objective

The aim of this study was to evaluate whether the concentration of sCD40L, a product of platelet activation, correlates with the presence of Pseudomonas aeruginosa in the airway of patients with cystic fibrosis (CF) and to determine its possible clinical association.

Methods

Sixty patients with CF, ranging in age from 2 months to 36 years, were studied. The demographics, cystic fibrosis transmembrane conductance regulator (CFTR) genotype, spirometry measurements, radiographic and tomographic scans, platelet count in peripheral blood, sCD40L, IL-6, TNF-α and ICAM1 data were collected. Infection-colonization of the airway was evaluated using sputum and throat swab cultures; the levels of anti-Pseudomonas aeruginosa antibodies (Anti-PaAb) were evaluated.

Results

Patients with CF and chronic colonization had anti-PaAb values of 11.6 ± 2.1 ELISA units (EU) and sCD40L in plasma of 1530.9 ±1162.3 pg/mL; those with intermittent infection had 5.7 ± 2.7 EU and 2243.6 ± 1475.9 pg/mL; and those who were never infected had 3.46 ± 1.8 EU (p≤0.001) and 1008.1 ± 746.8 pg/mL (p≤0.01), respectively. The cutoff value of sCD40L of 1255 pg/mL was associated with an area under the ROC (receiver operating characteristic curve) of 0.84 (95% CI, 0.71 to 0.97), reflecting P. aeruginosa infection with a sensitivity of 73% and a specificity of 89%. Lung damage was determined using the Brasfield Score, the Bhalla Score, and spirometry (FVC%, FEV1%) and found to be significantly different among the groups (p≤0.001).

Conclusion

Circulating sCD40L levels are increased in patients with cystic fibrosis and P. aeruginosa infection. Soluble CD40L appears to reflect infection and provides a tool for monitoring the evolution of lung deterioration.

Cystic Fibrosis Lung Immunity: The Role of the Macrophage

Cystic fibrosis (CF) pathophysiology is hallmarked by excessive inflammation and the inability to efficiently resolve lung infections, contributing to major morbidity and eventually the mortality of patients with this disease. Macrophages (MΦs) are major players in lung homeostasis through their diverse contributions to both the innate and adaptive immune networks. The setting of MΦ function and activity in CF is multifaceted, encompassing the response to the unique environmental cues in the CF lung as well as the intrinsic changes resulting from CFTR dysfunction. The complexity is further enhanced with the identification of modifier genes, which modulate the CFTR contribution to disease, resulting in epigenetic and transcriptional shifts in MΦ phenotype. This review focuses on the contribution of MΦ to lung homeostasis, providing an overview of the diverse literature and various perspectives on the role of these immune guardians in CF.

Modulating Innate and Adaptive Immunity by (R)-Roscovitine: Potential Therapeutic Opportunity in Cystic Fibrosis

(R)-Roscovitine, a pharmacological inhibitor of kinases, is currently in phase II clinical trial as a drug candidate for the treatment of cancers, Cushing's disease and rheumatoid arthritis. We here review the data that support the investigation of (R)-roscovitine as a potential therapeutic agent for the treatment of cystic fibrosis (CF). (R)-Roscovitine displays four independent properties that may favorably combine against CF: (1) it partially protects F508del-CFTR from proteolytic degradation and favors its trafficking to the plasma membrane; (2) by increasing membrane targeting of the TRPC6 ion channel, it rescues acidification in phagolysosomes of CF alveolar macrophages (which show abnormally high pH) and consequently restores their bactericidal activity; (3) its effects on neutrophils (induction of apoptosis), eosinophils (inhibition of degranulation/induction of apoptosis) and lymphocytes (modification of the Th17/Treg balance in favor of the differentiation of anti-inflammatory lymphocytes and reduced production of various interleukins, notably IL-17A) contribute to the resolution of inflammation and restoration of innate immunity, and (4) roscovitine displays analgesic properties in animal pain models. The fact that (R)-roscovitine has undergone extensive preclinical safety/pharmacology studies, and phase I and II clinical trials in cancer patients, encourages its repurposing as a CF drug candidate.

Intracellular survival ofBurkholderia cepaciacomplex in phagocytic cells

Burkholderia cepacia complex (Bcc) species are a group of Gram-negative opportunistic pathogens that infect the airways of cystic fibrosis patients, and occasionally they infect other immunocompromised patients. Bcc bacteria display high-level multidrug resistance and chronically persist in the infected host while eliciting robust inflammatory responses. Studies using macrophages, neutrophils, and dendritic cells, combined with advances in the genetic manipulation of these bacteria, have increased our understanding of the molecular mechanisms of virulence in these pathogens and the molecular details of cell-host responses triggering inflammation. This article discusses our current view of the intracellular survival of Burkholderia cenocepacia within macrophages.

Inflammation in cystic fibrosis lung disease: Pathogenesis and therapy

Lung disease is the major cause of morbidity and mortality in patients with cystic fibrosis (CF). Although CF lung disease is primarily an infectious disorder, the associated inflammation is both intense and ineffective at clearing pathogens. Persistent high-intensity inflammation leads to permanent structural damage of the CF airways and impaired lung function that eventually results in respiratory failure and death. Several defective inflammatory responses have been linked to cystic fibrosis transmembrane conductance regulator (CFTR) deficiency including innate and acquired immunity dysregulation, cell membrane lipid abnormalities, various transcription factor signaling defects, as well as altered kinase and toll-like receptor responses. The inflammation of the CF lung is dominated by neutrophils that release oxidants and proteases, particularly elastase. Neutrophil elastase in the CF airway secretions precedes the appearance of bronchiectasis, and correlates with lung function deterioration and respiratory exacerbations. Anti-inflammatory therapies are therefore of particular interest for CF lung disease but must be carefully studied to avoid suppressing critical elements of the inflammatory response and thus worsening infection. This review examines the role of inflammation in the pathogenesis of CF lung disease, summarizes the results of past clinical trials and explores promising new anti-inflammatory options.

A Burkholderia cenocepacia gene encoding a non-functional tyrosine phosphatase is required for the delayed maturation of the bacteria-containing vacuoles in macrophages

Burkholderia cenocepacia infects patients with cystic fibrosis. We have previously shown that B. cenocepacia can survive in macrophages within membrane vacuoles [B. cenocepacia-containing vacuoles (BcCVs)] that preclude fusion with the lysosome. The bacterial factors involved in B. cenocepacia intracellular survival are not fully elucidated. We report here that deletion of BCAM0628, encoding a predicted low molecular weight protein tyrosine phosphatase (LMW-PTP) that is restricted to B. cenocepacia strains of the transmissible ET-12 clone, accelerates the maturation of the BcCVs. Compared to the parental strain and deletion mutants in other LMW-PTPs that are widely conserved in Burkholderia species, a greater proportion of BcCVs containing the ΔBCAM0628 mutant were targeted to the lysosome. Accelerated BcCV maturation was not due to reduced intracellular viability since ΔBCAM0628 survived and replicated in macrophages similarly to the parental strain. Therefore, BCAM0628 was referred to as dpm (delayed phagosome maturation). We provide evidence that the Dpm protein is secreted during growth in vitro and upon macrophage infection. Dpm secretion requires an N-terminal signal peptide. Heterologous expression of Dpm in Burkholderia multivorans confers to this bacterium a similar phagosomal maturation delay to that found with B. cenocepacia. We demonstrate that Dpm is an inactive phosphatase, suggesting that its contribution to phagosomal maturation arrest must be unrelated to tyrosine phosphatase activity.

IFN-γ stimulates autophagy-mediated clearance of Burkholderia cenocepacia in human cystic fibrosis macrophages

Burkholderia cenocepacia is a virulent pathogen that causes significant morbidity and mortality in patients with cystic fibrosis (CF), survives intracellularly in macrophages, and uniquely causes systemic infections in CF. Autophagy is a physiologic process that involves engulfing non-functional organelles and proteins and delivering them for lysosomal degradation, but also plays a role in eliminating intracellular pathogens, including B. cenocepacia. Autophagy is defective in CF but can be stimulated in murine CF models leading to increased clearance of B. cenocepacia, but little is known about autophagy stimulation in human CF macrophages. IFN-γ activates macrophages and increases antigen presentation while also inducing autophagy in macrophages. We therefore, hypothesized that treatment with IFN-γ would increase autophagy and macrophage activation in patients with CF. Peripheral blood monocyte derived macrophages (MDMs) were obtained from CF and non-CF donors and subsequently infected with B. cenocepacia. Basal serum levels of IFN-γ were similar between CF and non-CF patients, however after B. cenocepacia infection there is deficient IFN-γ production in CF MDMs. IFN-γ treated CF MDMs demonstrate increased co-localization with the autophagy molecule p62, increased autophagosome formation, and increased trafficking to lysosomes compared to untreated CF MDMs. Electron microscopy confirmed IFN-γ promotes double membrane vacuole formation around bacteria in CF MDMs, while only single membrane vacuoles form in untreated CF cells. Bacterial burden is significantly reduced in autophagy stimulated CF MDMs, comparable to non-CF levels. IL-1β production is decreased in CF MDMs after IFN-γ treatment. Together, these results demonstrate that IFN-γ promotes autophagy-mediated clearance of B. cenocepacia in human CF macrophages.

Burkholderia cenocepacia J2315 escapes to the cytosol and actively subverts autophagy in human macrophages

Selective autophagy functions to specifically degrade cellular cargo tagged by ubiquitination, including bacteria. Strains of the Burkholderia cepacia complex (Bcc) are opportunistic pathogens that cause life-threatening infections in patients with cystic fibrosis (CF) and chronic granulomatous disease (CGD). While there is evidence that defective macrophage autophagy in a mouse model of CF can influence B. cenocepacia susceptibility, there have been no comprehensive studies on how this bacterium is sensed and targeted by the host autophagy response in human macrophages. Here, we describe the intracellular life cycle of B. cenocepacia J2315 and its interaction with the autophagy pathway in human cells. Electron and confocal microscopy analyses demonstrate that the invading bacteria interact transiently with the endocytic pathway before escaping to the cytosol. This escape triggers theselective autophagy pathway, and the recruitment of ubiquitin, the ubiquitin-binding adaptors p62 and NDP52 and the autophagosome membrane-associated protein LC3B, to the bacterial vicinity. However, despite recruitment of these key autophagy pathway effectors, B. cenocepacia blocks autophagosome completion and replicates in the host cytosol. We find that a pre-infection increase in cellular autophagy flux can significantly inhibit B. cenocepacia replication and that lower autophagy flux in macrophages from immunocompromised CGD patients could contribute to increased B. cenocepacia susceptibility, identifying autophagy manipulation as a potential therapeutic approach to reduce bacterial burden in B. cenocepacia infections.

Burkholderia multivorans survival and trafficking within macrophages

Cystic fibrosis (CF) patients are at great risk of opportunistic lung infection, particularly by members of the Burkholderia cepacia complex (Bcc). This group of bacteria can cause damage to the lung tissue of infected patients and are difficult to eradicate due to their high levels of antibiotic resistance. Although the highly virulent Burkholderia cenocepacia has been the focus of virulence research for the past decade, Burkholderia multivorans is emerging as the most prevalent Bcc species infecting CF patients in North America. Despite several studies detailing the intramacrophage trafficking and survival of B. cenocepacia, no such data exist for B. multivorans. The results of this study demonstrated that the clinical CF isolates C5568 and C0514 and an environmental B. multivorans isolate, ATCC 17616, were able to replicate and survive within murine macrophages in a manner similar to that of B. cenocepacia strain K56-2. These strains were also able to survive but were unable to replicate within human THP-1 macrophages. Differences in macrophage uptake were observed among all three B. multivorans strains; these variances were attributed to major differences in O-antigen production. Unlike B. cenocepacia-containing vacuoles, which delay phagosomal maturation in murine macrophages by 6 h, all B. multivorans-containing vacuoles co-localized with lysosome-associated membrane protein-1, a late endosome/lysosomal marker, and the lysosomal marker dextran within 2 h of uptake. Together, these results indicated that, whilst both Bcc species were able to survive and replicate within macrophages, they utilized different intramacrophage survival strategies. To observe differences in virulence, the strains were compared using the Galleria mellonella (wax worm) model. When compared with the B. multivorans strains tested, B. cenocepacia K56-2 was highly virulent in this model and killed all worms within 24 h when injected at 10(7) c.f.u. B. multivorans clinical isolates C5568 and C0514 were significantly more virulent than the soil isolate ATCC 17616, which was avirulent even when worms were injected with 10(7) c.f.u. These results suggest strain differences in the virulence of B. multivorans isolates.

Macrophage-mediated inflammation and disease: a focus on the lung

The lung is exposed to a vast array of inhaled antigens, particulate matter, and pollution. Cells present in the airways must therefore be maintained in a generally suppressive phenotype so that excessive responses to nonserious irritants do not occur; these result in bystander damage to lung architecture, influx of immune cells to the airways, and consequent impairment of gas exchange. To this end, the resident cells of the lung, which are predominantly macrophages, are kept in a dampened state. However, on occasion the suppression fails and these macrophages overreact to antigenic challenge, resulting in release of inflammatory mediators, induction of death of lung epithelial cells, deposition of extracellular matrix, and development of immunopathology. In this paper, we discuss the mechanisms behind this macrophage-mediated pathology, in the context of a number of inflammatory pulmonary disorders.

Exaggerated inflammatory responses mediated by Burkholderia cenocepacia in human macrophages derived from Cystic fibrosis patients

Cystic fibrosis (CF) is accompanied with heightened inflammation worsened by drug resistant Burkholderia cenocepacia. Human CF macrophage responses to B. cenocepacia are poorly characterized and variable in the literature. Therefore, we examined human macrophage responses to the epidemic B. cenocepacia J2315 strain in order to identify novel anti-inflammatory targets. Peripheral blood monocyte derived macrophages were obtained from 23 CF and 27 non-CF donors. Macrophages were infected with B. cenocepacia J2315 and analyzed for cytokines, cytotoxicity, and microscopy. CF macrophages demonstrated significant increases in IL-1β, IL-10, MCP-1, and IFN-γ production in comparison to non-CF controls. CF patients on prednisone exhibited globally diminished cytokines compared to controls and other CF patients. CF macrophages also displayed increased bacterial burden and cell death. In conclusion, CF macrophages demonstrate exaggerated IL-1β, IL-10, MCP-1, and IFN-γ production and cell death during B. cenocepacia infection. Treatment with corticosteroids acutely suppressed cytokine responses.

Cystic fibrosis transmembrane conductance regulator contributes to reacidification of alkalinized lysosomes in RPE cells

The role of the cystic fibrosis transmembrane conductance regulator (CFTR) in lysosomal acidification has been difficult to determine. We demonstrate here that CFTR contributes more to the reacidification of lysosomes from an elevated pH than to baseline pH maintenance. Lysosomal alkalinization is increasingly recognized as a factor in diseases of accumulation, and we previously showed that cAMP reacidified alkalinized lysosomes in retinal pigmented epithelial (RPE) cells. As the influx of anions to electrically balance proton accumulation may enhance lysosomal acidification, the contribution of the cAMP-activated anion channel CFTR to lysosomal reacidification was probed. The antagonist CFTR(inh)-172 had little effect on baseline levels of lysosomal pH in cultured human RPE cells but substantially reduced the reacidification of compromised lysosomes by cAMP. Likewise, CFTR activators had a bigger impact on cells whose lysosomes had been alkalinized. Knockdown of CFTR with small interfering RNA had a larger effect on alkalinized lysosomes than on baseline levels. Inhibition of CFTR in isolated lysosomes altered pH. While CFTR and Lamp1 were colocalized, treatment with cAMP did not increase targeting of CFTR to the lysosome. The inhibition of CFTR slowed lysosomal degradation of photoreceptor outer segments while activation of CFTR enhanced their clearance from compromised lysosomes. Activation of CFTR acidified RPE lysosomes from the ABCA4(-/-) mouse model of recessive Stargardt's disease, whose lysosomes are considerably alkalinized. In summary, CFTR contributes more to reducing lysosomal pH from alkalinized levels than to maintaining baseline pH. Treatment to activate CFTR may thus be of benefit in disorders of accumulation associated with lysosomal alkalinization.

Activation of the pyrin inflammasome by intracellular Burkholderia cenocepacia

Burkholderia cenocepacia is an opportunistic pathogen that causes chronic infection and induces progressive respiratory inflammation in cystic fibrosis patients. Recognition of bacteria by mononuclear cells generally results in the activation of caspase-1 and processing of IL-1β, a major proinflammatory cytokine. In this study, we report that human pyrin is required to detect intracellular B. cenocepacia leading to IL-1β processing and release. This inflammatory response involves the host adapter molecule ASC and the bacterial type VI secretion system (T6SS). Human monocytes and THP-1 cells stably expressing either small interfering RNA against pyrin or YFP-pyrin and ASC (YFP-ASC) were infected with B. cenocepacia and analyzed for inflammasome activation. B. cenocepacia efficiently activates the inflammasome and IL-1β release in monocytes and THP-1. Suppression of pyrin levels in monocytes and THP-1 cells reduced caspase-1 activation and IL-1β release in response to B. cenocepacia challenge. In contrast, overexpression of pyrin or ASC induced a robust IL-1β response to B. cenocepacia, which correlated with enhanced host cell death. Inflammasome activation was significantly reduced in cells infected with T6SS-defective mutants of B. cenocepacia, suggesting that the inflammatory reaction is likely induced by an as yet uncharacterized effector(s) of the T6SS. Together, we show for the first time, to our knowledge, that in human mononuclear cells infected with B. cenocepacia, pyrin associates with caspase-1 and ASC forming an inflammasome that upregulates mononuclear cell IL-1β processing and release.

The CovS/CovR acid response regulator is required for intracellular survival of group B Streptococcus in macrophages

Group B Streptococcus (GBS) is a leading cause of neonatal meningitis and septicemia. The ability of this organism to survive inside phagocytic cells is poorly understood but thought to be an important step for the establishment of disease in the host. Here, we demonstrate that GBS shows prolonged survival within J774 macrophages and that the capacity to survive is not significantly changed across a diverse range of strains representing different serotypes, multilocus sequence types (MLST), and sites of clinical isolation. Using staining for the lysosome-associated membrane protein (LAMP) and by pharmacological inhibition of phagosome acidification, we demonstrate that streptococci reside in a phagosome and that acidification of the phagosome is required for GBS to survive intracellularly. Moreover, we show that the GBS two-component system CovS/CovR, which is the major acid response regulator in this organism, is required for survival inside the phagosome.

The Type VI secretion system of Burkholderia cenocepacia affects multiple Rho family GTPases disrupting the actin cytoskeleton and the assembly of NADPH oxidase complex in macrophages

Burkholderia cenocepacia is a Gram-negative opportunistic pathogen of patients with cystic fibrosis and chronic granulomatous disease. The bacterium survives intracellularly in macrophages within a membrane-bound vacuole (BcCV) that precludes the fusion with lysosomes. The underlying cellular mechanisms and bacterial molecules mediating these phenotypes are unknown. Here, we show that intracellular B. cenocepacia expressing a type VI secretion system (T6SS) affects the activation of the Rac1 and Cdc42 RhoGTPase by reducing the cellular pool of GTP-bound Rac1 and Cdc42. The T6SS also increases the cellular pool of GTP-bound RhoA and decreases cofilin activity. These effects lead to abnormal actin polymerization causing collapse of lamellipodia and failure to retract the uropod. The T6SS also prevents the recruitment of soluble subunits of the NADPH oxidase complex including Rac1 to the BcCV membrane, but is not involved in the BcCV maturation arrest. Therefore, T6SS-mediated deregulation of Rho family GTPases is a common mechanism linking disruption of the actin cytoskeleton and delayed NADPH oxidase activation in macrophages infected with B. cenocepacia.

Autophagy stimulation by rapamycin suppresses lung inflammation and infection by Burkholderia cenocepacia in a model of cystic fibrosis

Cystic fibrosis (CF) is the most common inherited lethal disease of Caucasians which results in multi organ dysfunction. However, 85% of the deaths are due to pulmonary infections. Infection by Burkholderia cenocepacia (B. cepacia) is a particularly lethal threat to CF patients because it causes severe and persistent lung inflammation and is resistant to nearly all available antibiotics. In CFTR ΔF508 mouse macrophages, B. cepacia persists in vacuoles that do not fuse with the lysosomes and mediates increased production of IL-1β. It is believed that intracellular bacterial survival contributes to the persistence of the bacterium. Here we show for the first time that in wild-type macrophages but not in ΔF508 macrophages, many B. cepacia reside in autophagosomes that fuse with lysosomes at later stages of infection. Accordingly, association and intracellular survival of B. cepacia are higher in CFTR-ΔF508 (ΔF508) macrophages than in WT macrophages. An autophagosome is a compartment that engulfs non-functional organelles and parts of the cytoplasm then delivers them to the lysosome for degradation to produce nutrients during periods of starvation or stress. Furthermore, we show that B. cepacia downregulates autophagy genes in WT and ΔF508 macrophages. However, autophagy dysfunction is more pronounced in ΔF508 macrophages since they already have compromised autophagy activity. We demonstrate that the autophagy-stimulating agent, rapamycin markedly decreases B. cepacia infection in vitro by enhancing the clearance of B. cepacia via induced autophagy. In vivo, Rapamycin decreases bacterial burden in the lungs of CF mice and drastically reduces signs of lung inflammation. Together, our studies reveal that if efficiently activated, autophagy can control B. cepacia infection and ameliorate the associated inflammation. Therefore, autophagy is a novel target for new drug development for CF patients to control B. cepacia infection and accompanying inflammation.

Akt-mediated proinflammatory response of mononuclear phagocytes infected with Burkholderia cenocepacia occurs by a novel GSK3β-dependent, IκB kinase-independent mechanism

The environmental bacterium Burkholderia cenocepacia causes opportunistic lung infections in immunocompromised individuals, particularly in patients with cystic fibrosis. Infections in these patients are associated with exacerbated inflammation leading to rapid decay of lung function, and in some cases resulting in cepacia syndrome, which is characterized by a fatal acute necrotizing pneumonia and sepsis. B. cenocepacia can survive intracellularly in macrophages by altering the maturation of the phagosome, but very little is known on macrophage responses to the intracellular infection. In this study, we have examined the role of the PI3K/Akt signaling pathway in B. cenocepacia-infected monocytes and macrophages. We show that PI3K/Akt activity was required for NF-κB activity and the secretion of proinflammatory cytokines during infection with B. cenocepacia. In contrast to previous observations in epithelial cells infected with other Gram-negative bacteria, Akt did not enhance IκB kinase or NF-κB p65 phosphorylation, but rather inhibited GSK3β, a negative regulator of NF-κB transcriptional activity. This novel mechanism of modulation of NF-κB activity may provide a unique therapeutic target for controlling excessive inflammation upon B. cenocepacia infection.

A decade of Burkholderia cenocepacia virulence determinant research

The Burkholderia cepacia complex (Bcc) is a group of genetically related environmental bacteria that can cause chronic opportunistic infections in patients with cystic fibrosis (CF) and other underlying diseases. These infections are difficult to treat due to the inherent resistance of the bacteria to antibiotics. Bacteria can spread between CF patients through social contact and sometimes cause cepacia syndrome, a fatal pneumonia accompanied by septicemia. Burkholderia cenocepacia has been the focus of attention because initially it was the most common Bcc species isolated from patients with CF in North America and Europe. Today, B. cenocepacia, along with Burkholderia multivorans, is the most prevalent Bcc species in patients with CF. Given the progress that has been made in our understanding of B. cenocepacia over the past decade, we thought that it was an appropriate time to review our knowledge of the pathogenesis of B. cenocepacia, paying particular attention to the characterization of virulence determinants and the new tools that have been developed to study them. A common theme emerging from these studies is that B. cenocepacia establishes chronic infections in immunocompromised patients, which depend more on determinants mediating host niche adaptation than those involved directly in host cells and tissue damage.

A cation counterflux supports lysosomal acidification

The profound luminal acidification essential for the degradative function of lysosomes requires a counter-ion flux to dissipate an opposing voltage that would prohibit proton accumulation. It has generally been assumed that a parallel anion influx is the main or only counter-ion transport that enables acidification. Indeed, defective anion conductance has been suggested as the mechanism underlying attenuated lysosome acidification in cells deficient in CFTR or ClC-7. To assess the individual contribution of counter-ions to acidification, we devised means of reversibly and separately permeabilizing the plasma and lysosomal membranes to dialyze the cytosol and lysosome lumen in intact cells, while ratiometrically monitoring lysosomal pH. Replacement of cytosolic Cl(-) with impermeant anions did not significantly alter proton pumping, while the presence of permeant cations in the lysosomal lumen supported acidification. Accordingly, the lysosomes were found to acidify to the same pH in both CFTR- and ClC-7-deficient cells. We conclude that cations, in addition to chloride, can support lysosomal acidification and defects in lysosomal anion conductance cannot explain the impaired microbicidal capacity of CF phagocytes.

Burkholderia cenocepacia creates an intramacrophage replication niche in zebrafish embryos, followed by bacterial dissemination and establishment of systemic infection

Bacteria belonging to the "Burkholderia cepacia complex" (Bcc) often cause fatal pulmonary infections in cystic fibrosis patients, yet little is know about the underlying molecular mechanisms. These Gram-negative bacteria can adopt an intracellular lifestyle, although their ability to replicate intracellularly has been difficult to demonstrate. Here we show that Bcc bacteria survive and multiply in macrophages of zebrafish embryos. Local dissemination by nonlytic release from infected cells was followed by bacteremia and extracellular replication. Burkholderia cenocepacia isolates belonging to the epidemic electrophoretic type 12 (ET12) lineage were highly virulent for the embryos; intravenous injection of <10 bacteria of strain K56-2 killed embryos within 3 days. However, small but significant differences between the clonal ET12 isolates K56-2, J2315, and BC7 were evident. In addition, the innate immune response in young embryos was sufficiently developed to control infection with other less virulent Bcc strains, such as Burkholderia vietnamiensis FC441 and Burkholderia stabilis LMG14294. A K56-2 cepR quorum-sensing regulator mutant was highly attenuated, and its ability to replicate and spread to neighboring cells was greatly reduced. Our data indicate that the zebrafish embryo is an excellent vertebrate model to dissect the molecular basis of intracellular replication and the early innate immune responses in this intricate host-pathogen interaction.

Disease-causing mutations in the cystic fibrosis transmembrane conductance regulator determine the functional responses of alveolar macrophages

Alveolar macrophages (AMs) play a major role in host defense against microbial infections in the lung. To perform this function, these cells must ingest and destroy pathogens, generally in phagosomes, as well as secrete a number of products that signal other immune cells to respond. Recently, we demonstrated that murine alveolar macrophages employ the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel as a determinant in lysosomal acidification (Di, A., Brown, M. E., Deriy, L. V., Li, C., Szeto, F. L., Chen, Y., Huang, P., Tong, J., Naren, A. P., Bindokas, V., Palfrey, H. C., and Nelson, D. J. (2006) Nat. Cell Biol. 8, 933-944). Lysosomes and phagosomes in murine cftr(-/-) AMs failed to acidify, and the cells were deficient in bacterial killing compared with wild type controls. Cystic fibrosis is caused by mutations in CFTR and is characterized by chronic lung infections. The information about relationships between the CFTR genotype and the disease phenotype is scarce both on the organismal and cellular level. The most common disease-causing mutation, DeltaF508, is found in 70% of patients with cystic fibrosis. The mutant protein fails to fold properly and is targeted for proteosomal degradation. G551D, the second most common mutation, causes loss of function of the protein at the plasma membrane. In this study, we have investigated the impact of CFTR DeltaF508 and G551D on a set of core intracellular functions, including organellar acidification, granule secretion, and microbicidal activity in the AM. Utilizing primary AMs from wild type, cftr(-/-), as well as mutant mice, we show a tight correlation between CFTR genotype and levels of lysosomal acidification, bacterial killing, and agonist-induced secretory responses, all of which would be expected to contribute to a significant impact on microbial clearance in the lung.

Interactions of Burkholderia cenocepacia and other Burkholderia cepacia complex bacteria with epithelial and phagocytic cells

Burkholderia cenocepacia is a member of the B. cepacia complex (Bcc), a group of opportunistic bacteria that infect the airways of patients with cystic fibrosis (CF) and are extraordinarily resistant to almost all clinically useful antibiotics. Infections in CF patients with Bcc bacteria generally lead to a more rapid decline in lung function, and in some cases to the ‘cepacia syndrome’, a virtually deadly exacerbation of the lung infection with systemic manifestations. These characteristics of Bcc bacteria contribute to higher morbidity and mortality in infected CF patients. In the last 10 years considerable progress has been made in understanding the interactions between Bcc bacteria and mammalian host cells. Bcc isolates can survive either intracellularly within eukaryotic cells or extracellularly in host tissues. They survive within phagocytes and respiratory epithelial cells, and they have the ability to breach the respiratory epithelium layer. Survival and persistence of Bcc bacteria within host cells and tissues are believed to play a key role in pulmonary infection and to contribute to the persistent inflammation observed in patients with CF. This review summarizes recent findings concerning the interaction between Bcc bacteria and epithelial and phagocytic cells.

Defective organellar acidification as a cause of cystic fibrosis lung disease: reexamination of a recurring hypothesis

The cellular mechanisms by which loss-of-function mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel produce cystic fibrosis (CF) lung disease remain uncertain. Defective organellar function has been proposed as an important determinant in the pathogenesis of CF lung disease. According to one hypothesis, reduced CFTR chloride conductance in organelles in CF impairs their acidification by preventing chloride entry into the organelle lumen, which is needed to balance the positive charge produced by proton entry. According to a different hypothesis, CFTR mutation hyperacidifies organelles by an indirect mechanism involving unregulated sodium efflux through epithelial sodium channels. There are reports of defective Golgi, endosomal and lysosomal acidification in CF epithelial cells, defective phagolysosomal acidification in CF alveolar macrophages, and organellar hyperacidification in CF respiratory epithelial cells. The common theme relating too high or low organellar pH to cellular dysfunction and CF pathogenesis is impaired functioning of organellar enzymes, such as those involved in ceramide metabolism and protein processing in epithelial cells and antimicrobial activity in alveolar macrophages. We review here the evidence for defective organellar acidification in CF. Significant technical and conceptual concerns are discussed regarding the validity of data showing too high/low organellar pH in CF cells, and rigorous measurements of organellar pH in CF cells are reviewed that fail to support defective organellar acidification in CF. Indeed, there is an expanding body of evidence supporting the involvement of non-CFTR chloride channels in organellar acidification. We conclude that biologically significant involvement of CFTR in organellar acidification is unlikely.

Delayed association of the NADPH oxidase complex with macrophage vacuoles containing the opportunistic pathogen Burkholderia cenocepacia

Burkholderia cenocepacia causes chronic lung infections in patients suffering from cystic fibrosis and chronic granulomatous disease. We have previously shown that B. cenocepacia survives intracellularly in macrophages within a membrane vacuole (BcCV) that delays acidification. Here, we report that after macrophage infection with live B. cenocepacia there is a approximately 6 h delay in the association of NADPH oxidase with BcCVs, while heat-inactivated bacteria are normally trafficked into NADPH oxidase-positive vacuoles. BcCVs in macrophages treated with a functional inhibitor of the cystic fibrosis transmembrane conductance regulator exhibited a further delay in the assembly of the NADPH oxidase complex at the BcCV membrane, but the inhibitor did not affect NADPH oxidase complex assembly onto vacuoles containing heat-inactivated B. cenocepacia or live Escherichia coli. Macrophages produced less superoxide following B. cenocepacia infection as compared to heat-inactivated B. cenocepacia and E. coli controls. Reduced superoxide production was associated with delayed deposition of cerium perhydroxide precipitates around BcCVs of macrophages infected with live B. cenocepacia, as visualized by transmission electron microscopy. Together, our results demonstrate that intracellular B. cenocepacia resides in macrophage vacuoles displaying an altered recruitment of the NADPH oxidase complex at the phagosomal membrane. This phenomenon may contribute to preventing the efficient clearance of this opportunistic pathogen from the infected airways of susceptible patients.

Revisiting the role of cystic fibrosis transmembrane conductance regulator and counterion permeability in the pH regulation of endocytic organelles

Organellar acidification by the electrogenic vacuolar proton-ATPase is coupled to anion uptake and cation efflux to preserve electroneutrality. The defective organellar pH regulation, caused by impaired counterion conductance of the mutant cystic fibrosis transmembrane conductance regulator (CFTR), remains highly controversial in epithelia and macrophages. Restricting the pH-sensitive probe to CFTR-containing vesicles, the counterion and proton permeability, and the luminal pH of endosomes were measured in various cells, including genetically matched CF and non-CF human respiratory epithelia, as well as cftr(+/+) and cftr(-/-) mouse alveolar macrophages. Passive proton and relative counterion permeabilities, determinants of endosomal, lysosomal, and phagosomal pH-regulation, were probed with FITC-conjugated transferrin, dextran, and Pseudomonas aeruginosa, respectively. Although CFTR function could be documented in recycling endosomes and immature phagosomes, neither channel activation nor inhibition influenced the pH in any of these organelles. CFTR heterologous overexpression also failed to alter endocytic organellar pH. We propose that the relatively large CFTR-independent counterion and small passive proton permeability ensure efficient shunting of the proton-ATPase-generated membrane potential. These results have implications in the regulation of organelle acidification in general and demonstrate that perturbations of the endolysosomal organelles pH homeostasis cannot be linked to the etiology of the CF lung disease.

Unimpaired lysosomal acidification in respiratory epithelial cells in cystic fibrosis

The mechanisms remain uncertain by which mutations in CFTR cause lung disease in cystic fibrosis (CF). Teichgräber et al. recently reported increased ceramide in CF lungs, which was proposed to result from defective lysosomal acidification in airway epithelial cells and consequent impairment of pH-dependent ceramide-metabolizing enzymes (Teichgräber, V., Ulrich, M., Endlich, N., Reithmüller, J., Wilker, B., Conceição Ce Olivereira-Munding, C., van Heeckeren, A. M., Barr, M. L., von Kürthy, G., Schmid, K. W., Weller, M., Tümmler, B., Lang, F., Grassme, H., Döring, G., and Gulbins, E. (2008) Nat. Med. 14, 382-391). Here, we measured lysosomal pH in several CFTR-expressing and -deficient cell lines, freshly isolated airway epithelial cells from non-CF and CF mice and humans, and well-differentiated primary cultures of human non-CF and CF airway epithelial cells. Lysosomal pH was measured by ratio imaging using a fluorescent pH indicator consisting of 40-kDa dextran conjugated to Oregon Green 488 and tetramethylrhodamine. In all cell types, lysosomal pH was approximately 4.5, unaffected by the thiazolidinone CFTR inhibitor CFTR(inh)-172, and increased to approximately 6.5 following bafilomycin inhibition of the vacuolar proton pump. Lysosomal pH did not differ significantly in airway epithelial cells from non-CF versus CF humans or mice. Our results provide direct evidence against the conclusions of Teichgräber et al. that lysosomal acidification is CFTR-dependent, impaired in CF, or responsible for ceramide accumulation. As such, alternative mechanisms are needed to explain increased ceramide in CF airways. Non-CFTR mechanisms, such as ClC-type chloride channels, are likely involved in maintaining electroneutrality during organellar acidification.