Phagocytosis of mucoid and nonmucoid strains of Pseudomonas aeruginosa

Immunization against Pseudomonas aeruginosa using Alg-PLGA nano-vaccine

OBJECTIVES

Pseudomonas aeruginosa is the bacterium that causes of pulmonary infection among chronically hospitalized patients. Alginate is a common surface antigen of P. aeruginosa with a constant structure that which makes it an appropriate target for vaccines. In this study, P. aeruginosa alginate was conjugated with to PLGA nanoparticles, and its immunogenicity was characterized as a vaccine.

MATERIALS AND METHODS

Alginate was isolated from a mucoid strain of P. aeruginosa and conjugated with to PLGA with˝ N-(3-Dimethylaminopropyl)-N'-ethylcarbodiimide hydrochloride ˝= ˝EDAC˝ and N-Hydroxysuccinimide (NHS). Chemical characterization of prepared nano-vaccine was performed using FTIR Spectroscopy, Zetasizer, and Atomic Force Microscopy (AFM). The immunogenicity of this nano-vaccine was evaluated through intramuscular injection into BALB/c mice. Four groups of mice were subjected to the injection of alginate-PLGA, and two weeks after the last administration step, opsonophagocytosis assay, IgG detection, challenge, and cytokine determination via ELISA were carried out.

RESULTS

Alginate-PLGA conjugation was corroborated by FTIR, Zetasizer, and AFM. The ELISA consequence showed that alginate was prospering in the instigation of the humoral immunity.The immunogenicity enhanced against the alginate-PLGA. Remarkably diminished bacterial titer in the spleen of the immunized mice posterior to challenge with PAO1 strain in comparison with the alginate alone and control groups.

CONCLUSION

The bacterial burden in the spleen significantly decreased after the challenge (P<0.05). The opsonic activity was significantly increased in the alginate- PLGA group (P<0.05).

Microenvironmental characteristics and physiology of biofilms in chronic infections of CF patients are strongly affected by the host immune response

In vitro studies of Pseudomonas aeruginosa and other pathogenic bacteria in biofilm aggregates have yielded detailed insight into their potential growth modes and metabolic flexibility under exposure to gradients of substrate and electron acceptor. However, the growth pattern of P. aeruginosa in chronic lung infections of cystic fibrosis (CF) patients is very different from what is observed in vitro, for example, in biofilms grown in flow chambers. Dense in vitro biofilms of P. aeruginosa exhibit rapid O₂ depletion within <50-100 μm due to their own aerobic metabolism. In contrast, in vivo investigations show that P. aeruginosa persists in the chronically infected CF lung as relatively small cell aggregates that are surrounded by numerous PMNs, where the activity of PMNs is the major cause of O₂ depletion rendering the P. aeruginosa aggregates anoxic. High levels of nitrate and nitrite enable P. aeruginosa to persist fueled by denitrification in the PMN-surrounded biofilm aggregates. This configuration creates a potentially long-term stable ecological niche for P. aeruginosa in the CF lung, which is largely governed by slow growth and anaerobic metabolism and enables persistence and resilience of this pathogen even under the recurring aggressive antimicrobial treatments of CF patients. As similar slow growth of other CF pathogens has recently been observed in endobronchial secretions, there is now a clear need for better in vitro models that simulate such in vivo growth patterns and anoxic microenvironments in order to help unravel the efficiency of existing or new antimicrobials targeting anaerobic metabolism in P. aeruginosa and other CF pathogens. We also advocate that host immune responses such as PMN-driven O₂ depletion play a central role in the formation of anoxic microniches governing bacterial persistence in other chronic infections such as chronic wounds.

A Putative ABC Transporter Permease Is Necessary for Resistance to Acidified Nitrite and EDTA in Pseudomonas aeruginosa under Aerobic and Anaerobic Planktonic and Biofilm Conditions

Pseudomonas aeruginosa (PA) is an important airway pathogen of cystic fibrosis and chronic obstructive disease patients. Multiply drug resistant PA is becoming increasing prevalent and new strategies are needed to combat such insidious organisms. We have previously shown that a mucoid, mucA22 mutant PA is exquisitely sensitive to acidified nitrite (A-NO2−, pH 6.5) at concentrations that are well tolerated in humans. Here, we used a transposon mutagenesis approach to identify PA mutants that are hypersensitive to A-NO2−. Among greater than 10,000 mutants screened, we focused on PA4455, in which the transposon was found to disrupt the production of a putative cytoplasmic membrane-spanning ABC transporter permease. The PA4455 mutant was not only highly sensitive to A-NO2−, but also the membrane perturbing agent, EDTA and the antibiotics doxycycline, tigecycline, colistin, and chloramphenicol, respectively. Treatment of bacteria with A-NO2− plus EDTA, however, had the most dramatic and synergistic effect, with virtually all bacteria killed by 10 mM A-NO2−, and EDTA (1 mM, aerobic, anaerobic). Most importantly, the PA4455 mutant was also sensitive to A-NO2− in biofilms. A-NO2− sensitivity and an anaerobic growth defect was also noted in two mutants (rmlC and wbpM) that are defective in B-band LPS synthesis, potentially indicating a membrane defect in the PA4455 mutant. Finally, this study describes a gene, PA4455, that when mutated, allows for dramatic sensitivity to the potential therapeutic agent, A-NO2− as well as EDTA. Furthermore, the synergy between the two compounds could offer future benefits against antibiotic resistant PA strains.

Chronic Pseudomonas aeruginosa infection in cystic fibrosis airway disease: metabolic changes that unravel novel drug targets

The cystic fibrosis (CF) airways have an incompletely characterized defect in innate defense that eventually leads to bacterial infection and airway inflammation. Persistent Pseudomonas aerugionsa infection resulting from defective innate immunity and a bacterial phenotypic switch to a more intractable mucoid form inside the airway are now well established as important components of CF pathogenesis. Broad-based factors leading to chronic infection will be discussed with respect to: bacterial virulence in the context of biofilm formation, quorum sensing machinery and alginate overproduction, and failure of innate lung immunity in CF airways. In addition, a controversial question as to whether inflammation or infection comes first during CF airway disease will be addressed. Finally, a new hypothesis, that P. aeruginosa thrives as biofilms within the thickened anaerobic mucus layers, will be developed.

Novel concepts in evaluating antimicrobial therapy for bacterial lung infections in patients with cystic fibrosis

Cystic fibrosis (CF) patients suffer typically from bacterial infections of their airways. Whilst current antibiotic-based treatment of these infections has brought much benefit to patients, it has been difficult to make either direct or indirect assessments of the in vivo efficacy of any specific treatment used. Traditional culture-based assessment has for example been rarely used to determine the direct impact of therapy on the bacteria in the airways. Instead, the "success" of a treatment is most often gauged through measures of respiratory and general health. New culture-independent approaches though are emerging that offer much promise here however in allowing a more comprehensive evaluation of antimicrobial efficacy. These new methods offer an opportunity to examine bacterial outcomes rather than host outcomes alone. Application of these novel techniques in a systematic way will lead to the rationalisation and, likely greater still individualisation, of therapy for CF patients. This review discusses host and microbiological factors that may influence antibiotic efficacy. Moreover, the degree to which the inherent complexity of CF respiratory infections complicates the process of determining treatment impact and the need to identify more robust microbiological outcome measures will also be reviewed.

Mucoid Pseudomonas aeruginosa isolates maintain the biofilm formation capacity and the gene expression profiles during the chronic lung infection of CF patients

Phenotypic and genotypic diversifications of Pseudomonas aeruginosa in the airways of patients with cystic fibrosis (CF) promote long-term survival of bacteria during chronic lung infection. Twelve clonally related, sequential mucoid and non-mucoid paired P. aeruginosa isolates obtained from three Danish CF patients were investigated. The in vitro biofilm formation capacity was studied under static and flow through conditions and the global gene expression profiles were investigated by Affymetrix GeneChip. Regulatory genes of alginate production and quorum sensing (QS) system were sequenced and measurements of the alginate production and the detection of the QS signal molecules were performed. Comparisons of mucoid and non-mucoid isolates from early and late stages of the infection showed that the mucoid phenotype maintained over a decade the capacity to form in vitro biofilm and showed an unaltered transcriptional profile, whereas substantial alterations in the transcriptional profiles and loss of the capacity to form in vitro biofilms were observed in corresponding isolates of the non-mucoid phenotype. The conserved gene expression pattern in the mucoid isolates vs the diversity of changes in non-mucoid isolates observed in this particular P. aeruginosa clone reflects different adaptation strategies used by these two phenotypes in the different niches of the CF lung environment.

Pseudomonas aeruginosa alginate promotes Burkholderia cenocepacia persistence in cystic fibrosis transmembrane conductance regulator knockout mice

Pseudomonas aeruginosa, a major respiratory pathogen in cystic fibrosis (CF) patients, facilitates infection by other opportunistic pathogens. Burkholderia cenocepacia, which normally infects adolescent patients, encounters alginate elaborated by mucoid P. aeruginosa. To determine whether P. aeruginosa alginate facilitates B. cenocepacia infection in mice, cystic fibrosis transmembrane conductance regulator knockout mice were infected with B. cenocepacia strain BC7 suspended in either phosphate-buffered saline (BC7/PBS) or P. aeruginosa alginate (BC7/alginate), and the pulmonary bacterial load and inflammation were monitored. Mice infected with BC7/PBS cleared all of the bacteria within 3 days, and inflammation was resolved by day 5. In contrast, mice infected with BC7/alginate showed persistence of bacteria and increased cytokine levels for up to 7 days. Histological examination of the lungs indicated that there was moderate to severe inflammation and pneumonic consolidation in isolated areas at 5 and 7 days postinfection in the BC7/alginate group. Further, alginate decreased phagocytosis of B. cenocepacia by professional phagocytes both in vivo and in vitro. P. aeruginosa alginate also reduced the proinflammatory responses of CF airway epithelial cells and alveolar macrophages to B. cenocepacia infection. The observed effects are specific to P. aeruginosa alginate, because enzymatically degraded alginate or other polyuronic acids did not facilitate bacterial persistence. These observations suggest that P. aeruginosa alginate may facilitate B. cenocepacia infection by interfering with host innate defense mechanisms.

The exopolysaccharide alginate protects Pseudomonas aeruginosa biofilm bacteria from IFN-gamma-mediated macrophage killing

The ability of Pseudomonas aeruginosa to form biofilms and cause chronic infections in the lungs of cystic fibrosis patients is well documented. Numerous studies have revealed that P. aeruginosa biofilms are highly refractory to antibiotics. However, dramatically fewer studies have addressed P. aeruginosa biofilm resistance to the host's immune system. In planktonic, unattached (nonbiofilm) P. aeruginosa, the exopolysaccharide alginate provides protection against a variety of host factors yet the role of alginate in protection of biofilm bacteria is unclear. To address this issue, we tested wild-type strains PAO1, PA14, the mucoid cystic fibrosis isolate, FRD1 (mucA22+), and the respective isogenic mutants which lacked the ability to produce alginate, for their susceptibility to human leukocytes in the presence and absence of IFN-gamma. Human leukocytes, in the presence of recombinant human IFN-gamma, killed biofilm bacteria lacking alginate after a 4-h challenge at 37 degrees C. Bacterial killing was dependent on the presence of IFN-gamma. Killing of the alginate-negative biofilm bacteria was mediated through mononuclear cell phagocytosis since treatment with cytochalasin B, which prevents actin polymerization, inhibited leukocyte-specific bacterial killing. By direct microscopic observation, phagocytosis of alginate-negative biofilm bacteria was significantly increased in the presence of IFN-gamma vs all other treatments. Addition of exogenous, purified alginate to the alginate-negative biofilms restored resistance to human leukocyte killing. Our results suggest that although alginate may not play a significant role in bacterial attachment, biofilm development, and formation, it may play an important role in protecting mucoid P. aeruginosa biofilm bacteria from the human immune system.

Pseudomonas aeruginosa alginate is refractory to Th1 immune response and impedes host immune clearance in a mouse model of acute lung infection

Pseudomonas aeruginosa is an opportunistic respiratory pathogen that accounts for most of the morbidity and mortality in cystic fibrosis (CF) patients. In CF-affected lungs, the bacteria undergo conversion from a non-mucoid to a non-tractable mucoid phenotype, due to overproduction of alginate. The effect of alginate production on pathogenicity was investigated by using an acute lung infection mouse model that compared a non-mucoid P. aeruginosa strain, PAO1, to its constitutive alginate-overproducing derivative, Alg(+) PAOmucA22, and an alginate-defective strain, Alg(-) PAOalgD. Bacterial suspensions were instilled into the left bronchus and examined 24 and 48 h post-infection. The highest bacterial loads and the most severe lung pathology were observed with strain Alg(-) PAOalgD at 24 h post-infection, which may have been due to an increase in expression of bacterial elastase by the mutant. Significantly lower lung and spleen bacterial loads were found in the two non-mucoid (PAO1 and Alg(-) PAOalgD) groups, compared to the mucoid Alg(+) PAOmucA22 group, between 24 and 48 h post-infection. The positive correlation between lung bacteriology and lung macroscopic pathology in the Alg(+) PAOmucA22 group suggests that alginate production not only impedes pulmonary clearing, but also results in severe lung damage. Positive correlations between IL12 levels and lung macroscopic pathology, and between IL12 and IFN-gamma levels in the Alg(+) PAOmucA22 group, suggested a possible contribution of these pro-inflammatory cytokines to tissue damage. No significant differences were found between the three groups in lung cytokine responses at 24 or 48 h post-infection. However, on comparison within each group at 24 and 48 h post-infection, a significant increase in the pro-inflammatory cytokine IFN-gamma was observed. Higher ratios of IFN-gamma/IL4 and IFN-gamma/IL10, but lower IL10 levels, were also found in all three groups. These results indicate a Th1-predominated immune response in these animals. Such cytokine responses could have aided the clearance of non-mucoid P. aeruginosa, but were not sufficient to alleviate infection by the mucoid variants. Alginate production may promote survival and persistence of this pathogenic micro-organism in the lung.

Lung infections associated with cystic fibrosis

While originally characterized as a collection of related syndromes, cystic fibrosis (CF) is now recognized as a single disease whose diverse symptoms stem from the wide tissue distribution of the gene product that is defective in CF, the ion channel and regulator, cystic fibrosis transmembrane conductance regulator (CFTR). Defective CFTR protein impacts the function of the pancreas and alters the consistency of mucosal secretions. The latter of these effects probably plays an important role in the defective resistance of CF patients to many pathogens. As the modalities of CF research have changed over the decades from empirical histological studies to include biophysical measurements of CFTR function, the clinical management of this disease has similarly evolved to effectively address the ever-changing spectrum of CF-related infectious diseases. These factors have led to the successful management of many CF-related infections with the notable exception of chronic lung infection with the gram-negative bacterium Pseudomonas aeruginosa. The virulence of P. aeruginosa stems from multiple bacterial attributes, including antibiotic resistance, the ability to utilize quorum-sensing signals to form biofilms, the destructive potential of a multitude of its microbial toxins, and the ability to acquire a mucoid phenotype, which renders this microbe resistant to both the innate and acquired immunologic defenses of the host.

Implication of neutral polysaccharides associated to alginate in inhibition of murine macrophage response to Pseudomonas aeruginosa

There is evidence that exopolysaccharides (EPS) contribute to the persistence of Pseudomonas aeruginosa in cystic fibrosis lung. However, the relationship between the chemical composition of EPS and the modulation of phagocytic cells is poorly understood. In order to evaluate the role of the chemical composition of EPS in macrophage behavior changes, we pretreated macrophages with characterized EPS and assessed P. aeruginosa phagocytosis and reactive oxygen intermediate (ROI) production. The results showed that alginate and neutral polysaccharides are involved in phagocytic impairment of P. aeruginosa. Moreover, alginates were able to prime macrophages for increased P. aeruginosa-induced macrophage oxidative burst as determined by chemiluminescence. In contrast, neutral polysaccharides are responsible for the decrease of ROI by a scavenging effect evaluated by the xanthine-xanthine oxidase system. This study showed that the content of P. aeruginosa EPS in alginate, but also in neutral polysaccharides, influences the behavior of strains towards phagocytosis and macrophage oxidative burst.

Microbial pathogenesis in cystic fibrosis: mucoid Pseudomonas aeruginosa and Burkholderia cepacia

Respiratory infections with Pseudomonas aeruginosa and Burkholderia cepacia play a major role in the pathogenesis of cystic fibrosis (CF). This review summarizes the latest advances in understanding host-pathogen interactions in CF with an emphasis on the role and control of conversion to mucoidy in P. aeruginosa, a phenomenon epitomizing the adaptation of this opportunistic pathogen to the chronic chourse of infection in CF, and on the innate resistance to antibiotics of B. cepacia, person-to-person spread, and sometimes rapidly fatal disease caused by this organism. While understanding the mechanism of conversion to mucoidy in P. aeruginosa has progressed to the point where this phenomenon has evolved into a model system for studying bacterial stress response in microbial pathogenesis, the more recent challenge with B. cepacia, which has emerged as a potent bona fide CF pathogen, is discussed in the context of clinical issues, taxonomy, transmission, and potential modes of pathogenicity.

New perspectives in understanding and management of the respiratory disease in cystic fibrosis

In the past 40 years, the mean survival of patients with cystic fibrosis (CF) has increased from less than 1 year to 30 years. The identification of the gene mutated in CF in 1989 has already been followed by the first phase of somatic gene therapy in 1993. The target organ of somatic gene therapy is the respiratory epithelium, which is progressively damaged by the chronic infection and inflammation characteristic of the disease. Since in the future, more patients may benefit from somatic gene therapy, the understanding of the mechanisms leading to chronic infection and inflammation becomes increasingly important. In the future, current therapeutic measures to protect the respiratory epithelium from damage, such as intravenous antimicrobial treatment, will be improved by the additional delivery of new drugs to the bronchial tree by aerosol. Amiloride and recombinant human DNAse administered by this route have the potential to improve mucociliary clearance. Antibiotics as well as protease inhibitors delivered by aerosol should contribute to prevent damage by infection and inflammation in order to increase the probability of successful somatic gene therapy in this disease.

Inhibition of adherence of mucoid Pseudomonas aeruginosa by alginase, specific monoclonal antibodies, and antibiotics

The adherence of pseudomonal species was investigated by using a newly developed radiometric dacron fiber microcolumn assay. Pseudomonas aeruginosa, P. stutzeri, and Xanthomonas maltophilia were more adherent (approximately 20%) than P. pseudomallei, P. fluorescens, and P. cepacia (approximately 10%). Mucoid strains of P. aeruginosa were consistently more adherent than nonmucoid strains (30% versus 20%). Alginase was shown to inhibit the adherence of mucoid but not nonmucoid P. aeruginosa. Monoclonal antibodies to alginate were also shown to inhibit the adherence of mucoid but not nonmucoid P. aeruginosa. In addition, antibiotics active against P. aeruginosa were shown to inhibit the adherence of both mucoid and nonmucoid strains. Furthermore, synergism between dyadic combinations of monoclonal antibodies and antibiotic (ciprofloxacin), as well as alginase and antibiotic, was also observed. These results indicate that bacterial alginate has an intrinsic role in the adherence of mucoid P. aeruginosa and may have evolved not only for protection against dehydration in the water and soil ecosystem of this bacterium, but also as a means of attaching to soil substrates in the same ecosystem to enhance survival. They also suggest that synergistic combinations of antibiotics with alginase or monoclonal antibodies to alginate may be of value in the therapy of some pseudomonal infections.

Suppression of lymphocyte and neutrophil functions by Pseudomonas aeruginosa mucoid exopolysaccharide (alginate): reversal by physicochemical, alginase, and specific monoclonal antibody treatments

The mucoid exopolysaccharide (MEP or alginate) of Pseudomonas aeruginosa is thought to be a virulence factor for this organism by virtue of its ability to suppress local host defense mechanisms. We purified MEP from clinical isolates of mucoid P. aeruginosa, subjected it to degradation by ultrasonication, heat, alkali, and alginase, and reacted it with monoclonal antibodies specific for MEP epitopes. Partial reversal or complete abrogation of the inhibitory effects of alginate on human neutrophil random migration, chemotaxis, and hexose monophosphate shunt activity and lymphocyte transformation were observed following most of these treatments. Physicochemical analysis of degraded MEP revealed a positive correlation between changes in molecular size and viscosity and loss of biological properties. The biological properties of MEP were also shown to be dependent on the structural integrity of the O-acetyl groups substituted for the mannuronic acid residues. The results show that the capacity of MEP to suppress neutrophil and lymphocyte functions is dependent on its acetyl content and the physical properties of large size and viscosity and may provide part of the explanation for the propensity of mucoid P. aeruginosa to persist in the airways of patients with cystic fibrosis. These findings highlight the important role of MEP as one of the virulence factors in the pathogenesis of inflammatory damage and subsequent pulmonary destruction in cystic fibrosis.

Cystic fibrosis. Infection and immunity to Pseudomonas

Chronic pulmonary infection with P. aeruginosa in CF may result from: 1. An initial failure of clearance mechanisms (increased adherence) leading to the development of a highly compartmentalized inflammatory reaction; 2. Inhibition of clearing mechanisms for bacteria present in the bronchial lumen; and 3. A largely ineffective, and possibly damaging, hyperactivity of inflammatory cells in the lumen and bronchial wall. The special relationship between the CF host and P. aeruginos, always long-term, and frequently subtle in its complexity, needs further understanding in order to develop new strategies for the treatment of chronic lung infections with this organism.

Nonopsonic antibodies in cystic fibrosis. Pseudomonas aeruginosa lipopolysaccharide-specific immunoglobulin G antibodies from infected patient sera inhibit neutrophil oxidative responses

Antibody opsonins from cystic fibrosis (CF) patients were investigated using nonmucoid and mucoid lipopolysaccharide (LPS) immunotype 1 Pseudomonas aeruginosa as bacterial ligands and PMN phagocytes. CF sera were compared to normal sera, polyvalent PA LPS hyperimmune globulin, and isotype switch variant monoclonal antibodies (MAbs) specific for type 1 PA LPS. Sera from PA-infected CF patients (CF PA+) had elevated levels of PA LPS and alginate IgG antibodies and promoted significantly greater antibody-dependent PMN chemiluminescence responses than sera from uninfected CF patients (CF PA-) or normal human sera (NHS). After adjustment for autologous IgG PA LPS antibody content, however, CF PA+ sera had less antibody-dependent opsonic activity than sera from CF PA- patients (P less than 0.025) or NHS (P less than 0.0025), suggesting qualitative opsonic defects of IgG PA LPS antibodies in CF PA+ sera. Antigen-specific immunoprecipitation of PA LPS antibodies enhanced opsonization by 40% of CF PA+ sera while uniformly reducing that from CF PA- sera (P less than 0.01), indicating LPS-specific nonopsonic antibodies in some CF PA+ sera. Alginate antibodies were not critical opsonins in most uninfected CF patient sera. PA LPS IgG antibodies isolated by immunoaffinity chromatography from NHS, hyperimmune globulin, and CF PA- sources were opsonic and had greater activity at equal antigen-binding concentration than identical antibodies isolated from infected CF patients (P less than 0.01-0.05); the majority of isolates from CF PA+ sera did not promote PMN oxidative responses above nonopsonic baseline. A potential isotypic basis for these findings was supported by differences in PMN responses to PA opsonized with MAbs of identical specificity but differing isotypes. PA LPS-specific IgG antibodies inhibiting PMN oxidative responses in infected patient sera demonstrate antigen-specific immunomodulation of host responses by chronic bacterial parasitism in CF, which may play a role in the pathophysiology of lung disease.

Effect of Pseudomonas aeruginosa proteases on human leukocyte phagocytosis and bactericidal activity

The effect of Pseudomonas aeruginosa alkaline protease (AP) and elastase (Ela) on neutrophil phagocytosis and bactericidal activity was examined. It was found that both proteases reduced the phagocytic activity of the leukocytes against P. aeruginosa, whereas they had little effect on the phagocytosis of S. aureus. AP and Ela at concentration of up to 250 micrograms per ml (much higher than the levels detectable under in vivo conditions) did not interfere with the bactericidal activity of the leukocytes against both test organisms. Inhibition of phagocytosis by AP and Ela without effect on the bactericidal activity suggests that the P. aeruginosa proteases most probably exert their effect on the cell surface perhaps by proteolytic cleavage of the cell receptors which are necessary for phagocytosis.