Protection against experimental Pseudomonas aeruginosa infection in mice by active immunization with exotoxin A toxoids

Understanding Pseudomonas aeruginosa-Host Interactions: The Ongoing Quest for an Efficacious Vaccine

Pseudomonas aeruginosa is a leading cause of chronic respiratory infections in people with cystic fibrosis (CF), bronchiectasis or chronic obstructive pulmonary disease (COPD), and acute infections in immunocompromised individuals. The adaptability of this opportunistic pathogen has hampered the development of antimicrobial therapies, and consequently, it remains a major threat to public health. Due to its antimicrobial resistance, vaccines represent an alternative strategy to tackle the pathogen, yet despite over 50 years of research on anti-Pseudomonas vaccines, no vaccine has been licensed. Nevertheless, there have been many advances in this field, including a better understanding of the host immune response and the biology of P. aeruginosa. Multiple antigens and adjuvants have been investigated with varying results. Although the most effective protective response remains to be established, it is clear that a polarised Th2 response is sub-optimal, and a mixed Th1/Th2 or Th1/Th17 response appears beneficial. This comprehensive review collates the current understanding of the complexities of P. aeruginosa-host interactions and its implication in vaccine design, with a view to understanding the current state of Pseudomonal vaccine development and the direction of future efforts. It highlights the importance of the incorporation of appropriate adjuvants to the protective antigen to yield optimal protection.

A trivalent vaccine consisting of "flagellin A+B and pilin" protects against Pseudomonas aeruginosa infection in a murine burn model

Pseudomonas aeruginosa is a common nosocomial pathogen in burn patients, and rapidly achieves antibiotic resistance, and thus, developing an effective vaccine is critically important for combating P. aeruginosa infection. Flagella and pili play important roles in colonization of P. aeruginosa at the burn wound site and its subsequent dissemination to deeper tissue and organs. In the present study, we evaluated protective efficacy of a trivalent vaccine containing flagellins A and B (FlaA + FlaB) + pilin (PilA) in a murine burn model of infection. "FlaA + FlaB + PilA" induced greater protection in P. aeruginosa murine burn model than the single components alone, and it showed broad immune protection against P. aeruginosa strains. Immunization with "FlaA + FlaB + PilA" induced strong opsonophagocytic antibodies and resulted in reduced bacterial loads, systemic IL-12/IL-10 cytokine expression, and increased survival after challenge with three times lethal dose fifty (LD50) of P. eruginosa strains. Moreover, the protective efficacy of "FlaA + FlaB + PilA" vaccination was largely attributed to specific antibodies. Taken together, these data further confirm that the protective effects of "FlaA + FlaB + PilA" vaccine significantly enhance efficacy compared with antibodies against either mono or divalent antigen, and that the former broadens the coverage against P. eruginosa strains that express two of the three antigens.

Polysaccharide conjugate vaccine protein carriers as a "neglected valency" - Potential and limitations

The development of vaccines against polysaccharide-encapsulated pathogens (e.g. Haemophilus influenzae type b, pneumococci, meningococci) is challenging because polysaccharides do not elicit a strong and long-lasting immune response (i.e. T-cell independent). This can be overcome by conjugating the polysaccharide to a protein carrier (e.g. tetanus toxoid, cross-reacting material 197 [CRM]), which vastly improves the immune response and induces memory to the polysaccharide (T-cell dependent). Although it is well documented that protein carriers additionally induce an immune response against themselves, this potential "additional valency" has so far not been recognized. The only exception is for the protein D carrier (derived from non-typeable Haemophilus influenzae [NTHi]) used in a pneumococcal conjugate vaccine, which may have a beneficial impact on NTHi acute otitis media. In this review, we describe the immunogenicity of various protein carriers and discuss their potential dual function: as providers of T-cell helper epitopes and as protective antigens. If this "additional valency" could be proven to be protective, it may be possible to consider its potential effect on the number of required immunizations. We also describe the potential for positive or negative interference between conjugate vaccines using the same protein carriers, the resulting desire for novel carriers, and information on potential new carriers. The range of conjugate vaccines is ever expanding, with different carriers and methods of conjugation. We propose that new conjugate vaccine trials should assess immunogenicity to both the polysaccharide and carrier. Ultimately, this so-far "neglected valency" could be an exploitable characteristic of polysaccharide conjugate vaccines.

Review: Evidence against the hypothesis that antibodies to the inner core of lipopolysaccharides in antisera raised by immunization with enterobacterial deep-rough mutants confer broad-spectrum protection during Gram-negative bacterial sepsis

Antisera to rough enterobacterial mutants of chemotypes Ra, Rc, and Re have been reported to confer broad-spectrum protection against wild-type smooth strains. It has been hypothesized that binding and neutralization of lipopolysaccharides (LPS) by antibodies to common core epitopes underlies such protection. This review summarizes experiments by our laboratory and others that do not confirm this concept and proposes reasons for the divergent results. Studies indicating broad-spectrum protection by rough-mutant antisera often had defects in experimental design or methodology. These include the failure: (i) to use matched pre- and postimmune sera from the same donors to control for variable protective activity of normal sera; (ii) to exclude the role of natural and polyclonally stimulated antibodies with proven protective activity against the infecting bacterial strain (e.g. O-specific, capsular, Pseudomonas exotoxin A); (iii) to exclude protective effects of acute-phase serum factors; (iv) to exclude protective effects of endotoxin contamination after adsorption or fractionation of antibody preparations; (v) to use non-boiled bacteria and LPS not subjected to acid-hydrolysis or gel-fractionation, and to exclude nonspecific adsorption, to demonstrate physiologically meaningful binding of rough-mutant antibodies to smooth enterobacteria and their LPS.

Active immunization using exotoxin A confers protection against Pseudomonas aeruginosa infection in a mouse burn model

BACKGROUND

Pseudomonas aeruginosa is an important cause of nosocomial infection and may lead to septicemia and death. We evaluated the immunogenicity of semi-purified exotoxin A from the bacterium in a mouse burn model.

METHODS

The toxoid was prepared from exotoxin A taken from toxigenic strains of P. aeruginosa (PA 103). 50 mice were immunized with the toxoid, burned with hot metal and infected with 1 x 10(8) CFU of toxigenic strains of P. aeruginosa (experimental group); 25 non-immunized mice were also burned and infected (control group). The mortality rate and presence of any exotoxin and P. aeruginosa in the sera, liver and spleen were determined.

RESULTS

In the experimental group, 2 mice died before the burns were administered and were excluded from the study. The remainder (48 mice) were challenged with a lethal dose of P. aeruginosa and followed for 70 days. 3 of these mice died. Neither P. aeruginosa nor exotoxin A was not detected in the liver, spleen or sera of the surviving mice. The protective efficacy of toxoid vaccination was therefore 93.8%. In the control group, all mice died from bacteremia and septicemia, most (80%) within 6 days, and P. aeruginosa and exotoxin A were isolated from sera, spleen and liver.

CONCLUSION

Active immunization of mice using a semi-purified exotoxin A derived from P. aeruginosa was 93.8% effective at protecting mice from subsequent P. aeruginosa infections in a mouse burn model.

Intranasal immunization strategy to impede pilin-mediated binding of Pseudomonas aeruginosa to airway epithelial cells

Prevention of pulmonary Pseudomonas aeruginosa infections represents a critical unmet medical need for cystic fibrosis (CF) patients. We have examined the tenet that a mucosal immunization approach can reduce interactions of a piliated form of this opportunistic pathogen with respiratory epithelial cells. Vaccinations were performed using ntPEpilinPAK, a protein chimera composed of a nontoxic form of P. aeruginosa exotoxin A (ntPE), where the C-terminal loop amino acid sequence of the PAK strain pilin protein was inserted in place of the ntPE Ib domain. Intranasal (i.n.) immunization of BALB/c mice with ntPEpilinPAK generated both serum and saliva immune responses. A series of in vitro studies showed that diluted samples of saliva obtained from immunized mice reduced pilin-dependent P. aeruginosa binding to polarized human tracheal epithelial cells, protected human pulmonary epithelial cells from cytotoxic actions associated with bacterial challenge, and reduced exotoxin A toxicity. Overall, i.n. administration of ntPEpilinPAK induced mucosal and systemic immune responses that may be beneficial for blocking early stage adhesion and/or infection events of epithelial cell-P. aeruginosa interactions at oropharyngeal surfaces.

Pseudomonas immunotherapy: a historical overview

The historic development of vaccines to be used as immunotherapy for Pseudomonas aeruginosa infections, in various patient populations, is reviewed. Commentary is offered concerning the relevance of each approach in light of our current understanding of the pathological process of these infections.

Conformational integrity of a recombinant toxoid of Pseudomonas aeruginosa exotoxin A containing a deletion of glutamic acid-553

A mutant form of Pseudomonas aeruginosa exotoxin A (ETA) carrying a deletion of glutamic acid-553, an important active-site residue, was expressed in an ETA-negative strain of P. aeruginosa and shown to be exported from the cells as efficiently as wild-type ETA. The mutant protein, purified from the culture medium, was devoid of ADP-ribosyltransferase activity. Protein conformation was barely perturbed by the deletion, as determined by a number of measures, including affinity for substrate NAD, proteinase sensitivity, absorbance and fluorescence spectroscopy, and differential scanning calorimetry. The conformational integrity and stability of the mutant toxin are consistent with potential use of the protein in vaccines or as a carrier in preparing conjugate vaccines.

Production and characterization of human monoclonal antibody recognizing the N-terminal residues of Pseudomonas aeruginosa exotoxin A

Human cell lines producing monoclonal antibodies (MAbs) against Pseudomonas aeruginosa exotoxin A were established by EBV transformation followed by cell fusion. Monoclonal antibody FK-001, IgM (mu, kappa), was demonstrated to be specifically reactive with exotoxin A in ELISA and immunoblotting, by recognizing N-terminal 16 amino acid residues of exotoxin A as an epitope. This epitope region belongs to domain I which is required for the binding of exotoxin A to the receptor on target cells. FK-001 showed a partial neutralizing activity for cell toxicity caused by exotoxin A and appeared to be effective against exotoxin A-producing P. aeruginosa infection in mice. A line of evidence suggests that monoclonal antibody FK-001 neutralizes exotoxin A-induced cell toxicity by the interference of accessibility and/or binding of exotoxin A to animal cell receptors.

Generation and characterization of murine antiflagellum monoclonal antibodies that are protective against lethal challenge with Pseudomonas aeruginosa

Two murine monoclonal antibodies, IIG5 (IgG3) and IVE8 (IgG2a), that bind to Pseudomonas aeruginosa type a flagella and type b flagella, respectively, were prepared by conventional hybridoma methodology. Specificity of each monoclonal antibody for type a or type b flagella was demonstrated by enzyme-linked immunosorbent assay, indirect immunofluorescence, and immunoblotting. The percentage of P. aeruginosa isolates recognized by each monoclonal antibody was analyzed by enzyme-linked immunosorbent assay. Among a panel of 257 flagellated P. aeruginosa clinical isolates, IIG5 bound to 67.7% of the isolates and IVE8 bound to another 30.7%, for a combined coverage of 98.4%. Inhibition of motility of P. aeruginosa by the monoclonal antibodies was observed in vitro in a soft agar assay and was dose dependent. The protective efficacy of IIG5 and IVE8 was examined in a mouse burn wound sepsis model. The antiflagellum monoclonal antibodies provided specific and significant prophylactic and therapeutic protection against lethal challenge with P. aeruginosa strains.

Cloning and characterization of cDNAs coding for the heavy and light chains of a monoclonal antibody specific for Pseudomonas aeruginosa outer membrane protein I

A set of seven monoclonal antibodies (MAb) directed against outer membrane proteins of Pseudomonas aeruginosa has been examined by Western blot analysis, indirect immunofluorescence tests and subclass typing. The hybridoma cell line secreting MAb 6A4, which reacts with outer membrane protein I, belongs to the IgG2a subclass and crossreacts with the 17 P. aeruginosa serotypes as listed in the International Antigenic Typing System, was selected as source for the preparation of poly(A)+RNA which in turn was used as template for cDNA synthesis and cloning. Full length cDNA clones of the gamma heavy chain as well as the kappa light chain were obtained and characterized by nucleotide sequence analysis. The complete cDNA sequences coding for the heavy and light chains will be the prerequisite for the construction and heterologous expression of a chimeric human-mouse monoclonal antibody which might be used in therapy of P. aeruginosa infections.

Prospects for the prevention and control of gram-negative nosocomial infections

Nosocomial infections of bacterial origin continue to be a leading cause of morbidity and mortality among virtually all hospitalized patient populations. It is estimated that nearly 5% of all hospitalized individuals will acquire an infection during their stay. Approximately 3% of nosocomial infections will contribute to an eventual fatal outcome. Preeminent among nosocomial pathogens are the aerobic Gram-negative bacilli, with Escherichia coli, Pseudomonas aeruginosa and Klebsiella spp. being the causative agents for the majority of life-threatening infections. This review describes the current situation and future prospects for combatting these infections.

Role of exotoxin A and elastase in the pathogenicity of Pseudomonas aeruginosa strain PAO experimental mouse burn infection

We examined the virulence of Pseudomonas aeruginosa strain PAO and xcp (extracellular proteins deficient) and xch (extracellular proteins hyperproducing) mutants derived from strain PAO in an experimental mouse burn infection model. The results showed that xcp mutants, which produced little or no extracellular elastase and exotoxin A, were as virulent as their corresponding xcp+ strains. The xch mutants produced more elastase and exotoxin A than the wild type strain, however, they had significantly lower virulence, probably due to reduced ability of these strains to take up iron. Treatment of mice with ferric ammonium citrate had no effect on the wild type strain but enhanced mortality in mice challenged with xch mutants. Neither elastase nor exotoxin A seem to play any role in burn infections with P. aeruginosa strain PAO. However, ability for iron uptake is an important virulence factor.

Variation and adaptation of Pseudomonas aeruginosa toxicity to HeLa cells and fibroblasts

The toxic components of supernatants from Pseudomonas aeruginosa cultures directed against HeLa cells and Staphylococcus aureus were evaluated with the aim of discovering interactions. Supernatants of eight different strains of P. aeruginosa were assayed for cytotoxic activity. All were active against HeLa cells; seven were toxic for S. aureus. On repeated suspension of P. aeruginosa in 0.9% sodium chloride solution, a shift from HeLa cell toxicity to staphylococcal lytic activity occurred along with a change of toxic activity from a high (50,000 +/- 5,000) to a low (8,000 +/- 400) molecular weight (MW) range on gel filtration. Addition of protein to the minimal medium of cultures producing material toxic only for S. aureus reactivated the generation of HeLa cell-toxic material. Cultivation of P. aeruginosa in the presence of HeLa cells and a chloramphenicol supplement produced suppression of the generation of material toxic for S. aureus but facilitated that of HeLa-toxic material of high MW. Adaptation of toxicity against fibroblasts developed only on cocultivation of P. aeruginosa together with S. aureus and in the presence of fibroblasts. Under these conditions a strong lytic activity for S. aureus appeared, even in the presence of chloramphenicol. Chloramphenicol caused the material toxic for fibroblasts to elute at a low MW well separated from that toxic for HeLa cells. In contrast to the high-MW toxic substances, the low-MW material did not induce antibodies after injection into rabbits. This may explain failures of vaccination against P. aeruginosa infection and of serum therapy of homologous sepsis in humans.

Active and passive immunization with Pseudomonas aeruginosa ribosomal vaccines and antisera in the burned rat model

Pseudomonas aeruginosa ribosomal vaccines were tested for their ability to protect rats subjected to a 20% total body surface burn against the lethal effects of infection with homologous organisms. When administered prior to burning, the vaccines provided 100% protection. When administered postburning, the vaccine from one strain also provided 100% protection when the time interval between vaccination and infection was 3 days. When this time interval was reduced to 1 or 2 days, approximately 50% protection was obtained with the same vaccine. The vaccine from a second strain tested provided about 50% protection with a 3-day time interval. In addition, passive immunization using antiserum to a ribosomal vaccine was also demonstrated to be effective in protecting burned and infected rats, especially when multiple doses of antiserum were used. In this case, 80% protection was obtained (with no protection observed using multiple doses of normal serum). Finally, a comparison of ribosomal and lipopolysaccharide vaccines and antisera was also performed.

Prophylaxis of Pseudomonas aeruginosa infections in leukopenic mice by a combination of active and passive immunization

Mice rendered leukopenic with cyclophosphamide and then challenged with viable Pseudomonas aeruginosa were used to determine the protective efficacy of active immunization against exotoxin A and of passive immunization with human antiserum to Escherichia coli J5, a rough mutant of Escherichia coli O111:B4. Neither treatment alone provided a greater degree of protection than its respective control. However, the combination of these treatments produced a moderate, yet consistent, increase in the survival of infected immunosuppressed mice.

Reaction of antibody in sera from cystic fibrosis patients with non-toxic forms of Pseudomonas aeruginosa exotoxin A

Sera from 48 cystic fibrosis patients from two hospitals were screened for antibody against rods, non-toxic macromolecular structures which share antigenic determinants with Pseudomonas aeruginosa exotoxin A. A solid-phase radioimmunoassay employing (125I)-staphylococcal protein A was used to detect anti-rod IgG. Antibodies recognizing rods, exotoxin A, or both antigens, were demonstrated using a competitive radioimmunoassay in cystic fibrosis patient sera, and in sera from animals immunized with exotoxin A, rods, or infected with Pseudomonas aeruginosa. Anti-rod titers of cystic fibrosis patients (1.07 to 14 X control serum levels) inversely correlated with aggregate clinical evaluation scores, and in most instances, with X-ray scores. Since rods are non-toxic and cross-reactive with exotoxin A, they may represent therapeutically useful antigens for producing immunity to exotoxin A.

Production and characterization of monoclonal antibodies to exotoxin A from Pseudomonas aeruginosa

Hybridomas secreting monoclonal antibodies specific for exotoxin A from Pseudomonas aeruginosa strain PA103 were derived from the fusion of spleen cells from mice immunized with: (i) purified exotoxin A, (ii) Formalin-treated exotoxin A, (iii) exotoxin A covalently coupled to Sepharose 4B, or (iv) P. aeruginosa-infected mice. All hybridomas were screened and selected by using an enzyme-linked immuno-adsorbent assay. All antibody isotypes were represented (immunoglobulins G, A, and M) as determined by enzyme-linked immunoadsorbent assay. The most productive fusions resulted from immunization with antigens coupled to an insoluble matrix, such as Sepharose 4B, or by infection of mice. Several hybridomas were selected and cloned by limiting dilution. The specificity of the monoclonal antibodies for exotoxin A was demonstrated by indirect immunoprecipitation of 125I-labeled exotoxin A followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis and by the immunoblotting technique. The protective ability of certain monoclonal antibodies was demonstrated in vitro by toxin neutralization in tissue culture and in vivo by prolonged survival time in the burned mouse infection model, after passive immunization. One monoclonal antitoxin displayed specificity for PA103-derived exotoxin yet failed to react with exotoxin purified from PAO-PR1 or PAO1, suggesting that structural differences exist between these exotoxins.

Protection against fatal Pseudomonas aeruginosa burn wound sepsis by immunization with lipopolysaccharide and high-molecular-weight polysaccharide

A murine burn wound model was employed to evaluate the relative efficacy of purified Pseudomonas aeruginosa lipopolysaccharide (LPS) and high-molecular-weight polysaccharide as protective immunogens. LPS was found to be both highly immunogenic and protective. As little as three 0.001-microgram doses elicited good immunoglobulin M and G titers and increased the mean lethal dose more than 1,000-fold. The level of protection against a live challenge correlated with antibody titers and was found to be serotype specific. An immunizing regimen which evoked only an immunoglobulin M response was still found to offer substantial protection. Immunization with a high-molecular-weight polysaccharide was also found to be protective. However, approximately 1,000-fold more high-molecular-weight polysaccharide, as compared with LPS, was needed to protect mice to an equivalent degree. Immunization with LPS was found to promote bacterial clearance and prevent establishment of bacteremia. A multivalent LPS vaccine conferred high levels of protection (110- to 53,000-fold) against eight different challenge strains of various serotypes.

Locus of the Pseudomonas aeruginosa toxin A gene

The gene for Pseudomonas aeruginosa toxin A has been mapped in the late region of the chromosome of strain PAO. Strain PAO-PR1, which produces parental levels of toxin A antigen that is enzymatically inactive and nontoxic, was used as the donor for R68.45 plasmid-mediated genetic exchange. Strain PAO-PR1 (toxA1) was mated with toxin A-producing strains, and exconjugates for selected prototrophic markers were tested for the transfer of toxA1. The toxA1 gene was located between cnu-9001 and pur-67 at approximately 85 min on the PAO chromosome.

Protection against Pseudomonas aeruginosa infection in a murine burn wound sepsis model by passive transfer of antitoxin A, antielastase, and antilipopolysaccharide

The protective capacity of passively transferred immunoglobulin G (IgG) fractions from antitoxin (AT-IgG), antielastase (AE-IgG), and antilipopolysaccharide (ALPS-IgG) against Pseudomonas aeruginosa infection was evaluated in a murine burn wound sepsis model. Complete protection was afforded by homologous ALPS-IgG against intermediate challenge doses (10 50% lethal doses) of P. aeruginosa PA220, whereas AT-IgG and AE-IgG offered no significant protection (P less than 0.5). The simultaneous transfer of AT-IgG or AE-IgG with ALPS-IgG gave no additional protection above that seen with ALPS-IgG alone. The transfer of ALPS-IgG did not dramatically alter bacterial multiplication in the skin at the site of infection. However, bacteremia and infection of the liver were prevented. In parallel experiments, AT-IgG or AE-IgG did not significantly alter either the course of the infection or the number of bacteria seen in the blood, liver, or skin when compared with controls. ALPS-IgG administered 24 h before infection, at the time of infection, or 4 h postinfection provided complete protection. Even when ALPS-IgG was transferred at a time when the infection was well established locally in the skin (8 h postinfection), highly significant protection (P greater than 0.999) was obtained. Protection afforded by ALPS-IgG was serotype specific. These results indicate that antibody to lipopolysaccharide is of critical importance for protection against P. aeruginosa challenge in a relevant animal model.

Enhancement of Pseudomonas aeruginosa lung clearance after local immunization with a temperature-sensitive mutant

We investigated the capacity of the temperature-sensitive mutant strain A/10/25 of Pseudomonas aeruginosa (ts-Psa) to induce enhancement of lung defenses against wild type P. aeruginosa (wt-Psa). Mice of the DBA/2J inbred strain were immunized by aerosolization with a single dose of 2 x 10(5) to 4 x 10(5) CFU of ts-Psa and were challenged 7, 14, and 21 days later with wt-Psa. The uncleared bacteria ratio was determined 4 h after aerosol exposure; significant enhancement in lung clearance of wt-Psa (P less than 0.01) was evident as early as 7 days after immunization and detectable for at least 21 days. Aerosol immunization with Staphylococcus aureus did not enhance lung clearance of wt-Psa; however, slight but significant enhancement in S. aureus clearance was observed in mice immunized 7 days before with ts-Psa. No enhancement of S. aureus clearance was seen in ts-Psa immunized animals after 14 and 21 days. Analysis of the cell composition of lung lavage fluids revealed a transient cell response characterized by rapid increase in the absolute number of polymorphonuclear leukocytes, followed later by an increase in alveolar macrophages. The characteristics of lung lavages returned to base-line values 6 days after aerosol immunization, and a second exposure to a ts-Psa aerosol produced a response of similar magnitude and quality. We conclude that aerosol immunization with a temperature-sensitive mutant of P. aeruginosa enhances specific pulmonary defense mechanisms against the parental pathogen in mice.

Temperature-sensitive mutants of Pseudomonas aeruginosa: isolation and preliminary immunological evaluation

The immunogenicity of two temperature-sensitive (ts) mutants of Pseudomonas aeruginosa immunotype 1, isolated and characterized for the development of a safe, live vaccine strain, was evaluated in a mouse protection model. One mutant, A/10/25, had a limited "coasting" property (i.e., continued replication for two divisions) at the nonpermissive temperature (36 degrees C), whereas the other mutant, E/9/9, continued replication for five generations after transfer to 36 degrees C. Groups of 3- to 5-week-old ICR mice were immunized intraperitoneally with various doses of the two ts mutants; at various times thereafter, the mice were challenged intraperitoneally with lethal doses of the parental wild type. The more extensive coaster, E/9/9, induced 100% protection at immunizing doses lower than those required for A/10/25 to induce the same protection (1 x 10(8) to 2 x 10(8) and 6 x 10(8) colony-forming units, respectively). Both ts strains induced significant protection for up to 5 weeks after immunization. The results of these studies suggest that the use of P. aeruginosa ts mutants might provide a novel approach to the prevention of P. aeruginosa colonization of patients with cystic fibrosis.