Protection against fatal Pseudomonas aeruginosa pneumonia in mice after nasal immunization with a live, attenuated aroA deletion mutant

A case for immunization against nosocomial infections

Immunization is a highly effective public health measure that reduces the incidence of infectious diseases, yet there has been relatively little effort toward the development of vaccines for nosocomial infections. Many nosocomial infections originate on mucosal surfaces (e.g., respiratory or gastrointestinal mucosa). As patients who are hospitalized once are more likely to be hospitalized again, we propose a prime-boost immunization strategy, whereby a priming dose of vaccine for a nosocomial infection is administered mucosally. Upon readmission, a parenteral boost would elicit a rapid immune response locally and systemically. Such a strategy could reduce or ameliorate nosocomial infections and perhaps limit dissemination of nosocomial pathogens. Thus, a more aggressive effort to develop vaccines for nosocomial infections is warranted.

Pseudomonas aeruginosa lipopolysaccharide: a major virulence factor, initiator of inflammation and target for effective immunity

Pseudomonas aeruginosa is one of the most important bacterial pathogens encountered by immunocompromised hosts and patients with cystic fibrosis (CF), and the lipopolysaccharide (LPS) elaborated by this organism is a key factor in virulence as well as both innate and acquired host responses to infection. The molecule has a fair degree of heterogeneity in its lipid A and O-antigen structure, and elaborates two different outer-core glycoforms, of which only one is ligated to the O-antigen. A close relatedness between the chemical structures and genes encoding biosynthetic enzymes has been established, with 11 major O-antigen groups identified. The lipid A can be variably penta-, hexa- or hepta-acylated, and these isoforms have differing potencies when activating host innate immunity via binding to Toll-like receptor 4 (TLR4). The O-antigen is a major target for protective immunity as evidenced by numerous animal studies, but attempts, to date, to produce a human vaccine targeting these epitopes have not been successful. Newer strategies employing live attenuated P. aeruginosa, or heterologous attenuated bacteria expressing P. aeruginosa O-antigens are potential means to solve some of the existing problems related to making a P. aeruginosa LPS-specific vaccine. Overall, there is now a large amount of information available about the genes and enzymes needed to produce the P. aeruginosa LPS, detailed chemical structures have been determined for the major O-antigens, and significant biologic and immunologic studies have been conducted to define the role of this molecule in virulence and immunity to P. aeruginosa infection.

Application of vaccine technology to prevention of Pseudomonas aeruginosa infections

Development of an effective vaccine against the multiple presentations of Pseudomonas aeruginosa infection, including nosocomial pneumonia, bloodstream infections, chronic lung infections in cystic fibrosis patients and potentially sight-threatening keratitis in users of contact lenses, is a high priority. As with vaccine development for any pathogen, key information about the most effective immunologic effectors of immunity and target antigens needs to be established. For P. aeruginosa, although there is a role for cell-mediated immunity in animals following active vaccination, the bulk of the data indicate that opsonically-active antibodies provide the most effective mediators of acquired immunity. Major target antigens include the lipopolysaccharide O-polysaccharides, cell-surface alginate, flagella, components of the Type III secretion apparatus and outer membrane proteins with a potentially additive effect achieved by including immune effectors to toxins and proteases. A variety of active vaccination approaches have the potential for efficacy such as vaccination with purified or recombinant antigens incorporating multiple epitopes, conjugate vaccines incorporating proteins and carbohydrate antigens, and live attenuated vaccines, including heterologous antigen delivery systems expressing immunogenic P. aeruginosa antigens. A diverse range of passive immunotherapeutic approaches are also candidates for effective immunity, with a variety of human monoclonal antibodies described over the years with good preclinical efficacy and some early Phase I and II studies in humans. Finding an effective active and/or passive vaccination strategy for P. aeruginosa infections could be realized in the next 5 to 10 years, but will require that advances are made in the understanding of antigen expression and immune effectors that work in different human tissues and clinical settings, and also require a means to validate that clinical outcomes achieved in Phase III trials represent meaningful advances in management and treatment of P. aeruginosa infections.

Defect in early lung defence against Pseudomonas aeruginosa in DBA/2 mice is associated with acute inflammatory lung injury and reduced bactericidal activity in naive macrophages

Pseudomonas aeruginosa is an opportunistic pathogen that causes serious respiratory disease in the immune-compromised host. Using an aerosol infection model, 11 inbred mouse strains (129/Sv, A/J, BALB/c, C3H/HeN, C57BL/6, DBA/2, FVB, B10.D2/oSnJ, B10.D2/nSnJ, AKR/J and SWR/J) were tested for increased susceptibility to P. aeruginosa lung colonization. DBA/2 was the only mouse strain that had increased bacterial counts in the lung within 6 h post-infection. This deficiency incited a marked inflammatory response with reduced bacterial lung clearance and a mortality rate of 96.7 %. DBA/2 mice displayed progressive deterioration of lung pathology with extensive alveolar exudate and oedema formation at 48-72 h post-infection. The neutrophil-specific myeloperoxidase activity remained elevated throughout infection, suggesting that the increased leukocyte infiltration into alveoli caused acute inflammatory lung injury. DBA/2 mice lack the haemolytic complement; however, three additional mouse strains (AKR/J, SWR/J and A/J) with the same defect effectively cleared the infection, indicating that other host factors are involved in defence. Bone marrow-derived macrophages of DBA/2 showed an initial increase in phagocytosis, while their bactericidal activity was reduced compared to that of C57BL/6 macrophages. Comparison of pulmonary cytokine profiles of DBA/2 versus C57BL/6 or C3H/HeN indicated that DBA/2 had similar increases in tumour necrosis factor (TNF)-alpha, KC and interleukin (IL)-1a as C3H/HeN, but showed specific induction of IL-17, monocyte chemotactic protein (MCP)-1 and vascular endothelial growth factor (VEGF). Together, DBA/2 mice have a defect in the initial lung defence against P. aeruginosa colonization, which causes the host to produce a greater, but damaging, inflammatory response. Such a response may originate from the reduced antimicrobial activity of DBA/2 macrophages.

Development of signature-tagged mutagenesis in Burkholderia pseudomallei to identify genes important in survival and pathogenesis

Burkholderia pseudomallei, the causative agent of melioidosis, is an important human pathogen in Southeast Asia and northern Australia for which a vaccine is unavailable. A panel of 892 double signature-tagged mutants was screened for virulence using an intranasal BALB/c mouse model of infection. A novel DNA tag microarray identified 33 mutants as being attenuated in spleens, while 6 were attenuated in both lungs and spleens. The transposon insertion sites in spleen-attenuated mutants revealed genes involved in several stages of capsular polysaccharide biosynthesis and DNA replication and repair, a putative oxidoreductase, ABC transporters, and a lipoprotein that may be important in intercellular spreading. The six mutants identified as missing in both lungs and spleens were found to have insertions in recA involved in the SOS response and DNA repair; putative auxotrophs of leucine, threonine, p-aminobenzoic acid, and a mutant with an insertion in aroB causing auxotrophy for aromatic compounds were also found. Murine challenge studies revealed partial protection in BALB/c mice vaccinated with the aroB mutant. The refined signature-tagged mutagenesis approach developed in this study was used to efficiently identify attenuating mutants from this highly pathogenic species and could be applied to other organisms.

Biological safety concepts of genetically modified live bacterial vaccines

Live vaccines possess the advantage of having access to induce cell-mediated and antibody-mediated immunity; thus in certain cases they are able to prevent infection, and not only disease. Furthermore, live vaccines, particularly bacterial live vaccines, are relatively cheap to produce and easy to apply. Hence they are suitable to immunize large communities or herds. The induction of both cell-mediated immunity as well as antibody-mediated immunity, which is particularly beneficial in inducing mucosal immune responses, is obtained by the vaccine-strain's ability to colonize and multiply in the host without causing disease. For this reason, live vaccines require attenuation of virulence of the bacterium to which immunity must be induced. Traditionally attenuation was achieved simply by multiple passages of the microorganism on growth medium, in animals, eggs or cell cultures or by chemical or physical mutagenesis, which resulted in random mutations that lead to attenuation. In contrast, novel molecular methods enable the development of genetically modified organisms (GMOs) targeted to specific genes that are particularly suited to induce attenuation or to reduce undesirable effects in the tissue in which the vaccine strains can multiply and survive. Since live vaccine strains (attenuated by natural selection or genetic engineering) are potentially released into the environment by the vaccinees, safety issues concerning the medical as well as environmental aspects must be considered. These involve (i) changes in cell, tissue and host tropism, (ii) virulence of the carrier through the incorporation of foreign genes, (iii) reversion to virulence by acquisition of complementation genes, (iv) exchange of genetic information with other vaccine or wild-type strains of the carrier organism and (v) spread of undesired genes such as antibiotic resistance genes. Before live vaccines are applied, the safety issues must be thoroughly evaluated case-by-case. Safety assessment includes knowledge of the precise function and genetic location of the genes to be mutated, their genetic stability, potential reversion mechanisms, possible recombination events with dormant genes, gene transfer to other organisms as well as gene acquisition from other organisms by phage transduction, transposition or plasmid transfer and cis- or trans-complementation. For this, GMOs that are constructed with modern techniques of genetic engineering display a significant advantage over random mutagenesis derived live organisms. The selection of suitable GMO candidate strains can be made under in vitro conditions using basic knowledge on molecular mechanisms of pathogenicity of the corresponding bacterial species rather than by in vivo testing of large numbers of random mutants. This leads to a more targeted safety testing on volunteers and to a reduction in the use of animal experimentation.

Intratracheal immunization with pili protein protects against mortality associated with Pseudomonas aeruginosa pneumonia in mice

We examined the protective effect of intratracheal immunization with Pseudomonas aeruginosa pili protein against respiratory infection caused by P. aeruginosa. Mice were immunized intratracheally or subcutaneously with purified pili protein or bovine serum albumin as a control. Intratracheally but not subcutaneously pili protein-immunized mice showed significant improvement of survival after intratracheal challenge with the PAO1 strain. Furthermore, bacterial cell counts in pili protein-immunized murine lungs were significantly decreased compared to controls at 18 h after the challenge. Antipili protein antibody titers in bronchoalveolar lavage fluid of intratracheally pili protein-immunized mice were higher than in bovine serum albumin immunized mice. However, antipili antibody titers were not increased in bronchoalveolar lavage fluid of subcutaneously pili protein-immunized mice, despite the high serum antipili antibody titers. Inoculation of P. aeruginosa induced immediate increases in interleukin-12 and interferon-gamma in bronchoalveolar lavage fluid of pili protein-immunized mice, reflecting an adequate and rapid immune response against P. aeruginosa respiratory tract infection. Our findings suggest that intratracheal pili protein immunization is effective against respiratory tract infection caused by P. aeruginosa in mice.

Ablation of the complement C3a anaphylatoxin receptor causes enhanced killing ofPseudomonas aeruginosain a mouse model of pneumonia

The C3a anaphylatoxin is a 77-amino acid peptide that is generated by enzymatic cleavage of C3 during activation of the complement system. C3a mediates numerous biological functions on binding its receptor (C3aR), which is present on both myeloid and nonmyeloid cells. To investigate the biological impact of C3a-mediated effects during acute pneumonia caused by Pseudomonas aeruginosa, we subjected C3aR-deficient mice and matched wild-type (WT) mice to P. aeruginosa pulmonary infection. C3aR-deficient mice exhibited increased killing of P. aeruginosa in the lungs, less dissemination of bacteria into the bloodstream, and a decreased inflammatory response to P. aeruginosa pulmonary infection compared with WT mice. To examine whether the absence of C3aR would impact the humoral immune response to P. aeruginosa, we immunized WT and C3aR-deficient mice via intraperitoneal injection with live P. aeruginosa. Both groups of mice developed similar levels of antibody specific to P. aeruginosa. Immunized C3aR-deficient and WT mice were subjected to P. aeruginosa pulmonary infection, and C3aR-deficient mice again displayed increased killing of P. aeruginosa in the lungs, less dissemination of bacteria into the bloodstream, and a decreased inflammatory response in the lungs. Collectively, these data demonstrate that independently of antibody production, absence of C3aR causes enhanced killing of P. aeruginosa despite a diminished inflammatory response in a mouse model of pneumonia.

Intranasal immunisation with conjugate vaccine protects mice from systemic and respiratory tract infection with Pseudomonas aeruginosa

We tested intranasal application of anti-Pseudomonas conjugate vaccine in mice. Comparison of immunisation via the intra-muscular versus intranasal routes showed the induction of equivalent levels of specific serum IgG and IgG subclasses antibodies if cholera toxin was used as an adjuvant. In contrast, secretion of specific mucosal IgA antibodies in the upper respiratory tract was only observed after intranasal immunisation together with adjuvant. Systemic and mucosal immunity was also established via the intranasal route when CpG-containing oligonucleotides were used as adjuvant. The functionality of intranasally induced antibodies was proven in vitro by opsonophagocytosis and in vivo using the burn-wound sepsis and intra-tracheal lung infection models. These results demonstrate the feasibility of intranasal immunisation against P. aeruginosa with conjugate vaccine.

A live-attenuated Pseudomonas aeruginosa vaccine elicits outer membrane protein-specific active and passive protection against corneal infection

Pseudomonas aeruginosa can cause sight-threatening corneal infections in humans, particularly those who wear contact lenses. We have previously shown that a live-attenuated P. aeruginosa vaccine given intranasally protected mice against acute lethal pneumonia in a lipopolysaccharide (LPS) serogroup-specific manner. In the current study, we evaluated the protective and therapeutic efficacies, as well as the target antigens, of this vaccine in a murine corneal infection model. C3H/HeN mice were nasally immunized with the vaccine (an aroA deletion mutant of strain PAO1, designated PAO1DeltaaroA) or with Escherichia coli as a control and were challenged 3 weeks later by inoculating the scratch-injured cornea with P. aeruginosa. For passive prophylaxis and therapy, we utilized a serum raised in rabbits nasally immunized with PAO1DeltaaroA or E. coli. Outcome measures included corneal pathology scores and, in some experiments, reductions in total and internalized bacterial CFU. We found that both active and passive immunization reduced corneal pathology scores after challenge with a variety of P. aeruginosa strains, including several serogroup-heterologous strains. Even when given therapeutically starting as late as 24 h after infection, the rabbit antiserum to PAO1DeltaaroA was effective at reducing corneal pathology scores. Immunotherapy of established infections also reduced the numbers of total and internalized corneal P. aeruginosa bacteria. Experiments using absorbed sera showed that the protective antibodies are specific to outer membrane proteins. Thus, live-attenuated P. aeruginosa vaccines delivered nasally protect against corneal infections in mice and potentially can be used to prepare passive therapy reagents for the treatment of established P. aeruginosa corneal infections caused by diverse LPS serogroups.

Targeting mechanisms of Pseudomonas aeruginosa pathogenesis

Pseudomonas aeruginosa is an opportunistic pathogen responsible for ventilator-acquired pneumonia, acute lower respiratory tract infections in immunocompromised patients and chronic respiratory infections in cystic fibrosis patients. High incidence, infection severity and increasing resistance characterize P. aeruginosa infections, highlighting the need for new therapeutic options. One such option is to target the many pathogenic mechanisms conferred to P. aeruginosa by its large genome encoding many different virulence factors. This article reviews the pathogenic mechanisms and potential therapies targeting these mechanisms in P. aeruginosa respiratory infections.

Pseudomonas aeruginosa: the potential to immunise against infection

Pseudomonas aeruginosa remains a serious pathogen for specific cohorts of patients where chronic infection is a poor prognostic indicator, such as those with cystic fibrosis, burn wounds or those who are immunocompromised. Significant disease burden is associated with a diverse spectrum of both nosocomial and community-acquired infections. To date, vaccines against P. aeruginosa have shown limited and often conflicting efficacy data, especially against heterologous strains, which are increasingly identified as co-colonisers of biofilms. While few studies have gone beyond Phase II clinical trials, a particular concern is the ability of P. aeruginosa to evade the immune system while provoking an immune response that contributes to the destructive nature of infection. Therefore, vaccine development needs to focus on preventing attachment and colonisation, as well as preventing conversion to a mucoid phenotype that is characteristic of the chronic condition that promotes pathology.

Transcriptome analysis of quorum-sensing regulation and virulence factor expression in Pseudomonas aeruginosa

The opportunistic pathogen Pseudomonas aeruginosa possesses two well-studied quorum-sensing (QS) systems (las and rhl) that are important in the production of virulence factors, antibiotic sensitivity, and biofilm development. High-density oligonucleotide microarrays were used to further characterize the las QS system and to investigate the effect of environment (planktonic or biofilm mode of growth, absence or presence of oxygen) and nutritional conditions on detection of transcripts encoding QS-regulated virulence factors. Transcriptome results indicate that the QS system is far more complex than previously proposed. Interestingly, we found that many QS-regulated genes encoding virulence products were expressed in all conditions investigated.

Virulence of Pseudomonas aeruginosa in a murine model of gastrointestinal colonization and dissemination in neutropenia

Pseudomonas aeruginosa bacteremia in cancer patients develops from initial gastrointestinal (GI) colonization with translocation into the bloodstream in the setting of chemotherapy-induced neutropenia and GI mucosal damage. We established a reproducible mouse model of P. aeruginosa GI colonization and systemic spread during neutropenia. Mice received 2 mg of streptomycin/ml of drinking water and 1,500 U of penicillin G/ml for 4 days and then ingested 10(7) CFU of P. aeruginosa per ml of drinking water for 5 days. After GI colonization levels were determined, cyclophosphamide (Cy) was then injected intraperitoneally (i.p.) three times every other day or an antineutrophil monoclonal antibody, RB6-8C5, was injected i.p. once. Dissemination was defined by the presence of P. aeruginosa in spleens of moribund or dead mice. In this mouse model, P. aeruginosa colonizes the GI tract and then disseminates systemically once Cy or RB6-8C5 is administered. The duration and intensity of neutropenia, related to Cy dose, was found to be a means to compare the virulence of different P. aeruginosa strains, as exhibited by comparisons of strains lacking or producing the virulence-enhancing ExoU cytotoxin. The lipopolysaccharide outer core polysaccharide and O side chains were critical in establishing GI colonization, and P. aeruginosa mutants lacking the aroA gene (necessary for synthesizing aromatic amino acids) were able to establish GI colonization but unable to disseminate. Both the colonization and dissemination phases of P. aeruginosa pathogenesis can be studied in this model, which should prove useful for evaluating pathogenesis, therapies, and associated means to control P. aeruginosa nosocomial infections.

Oral vaccination of BALB/c mice with Salmonella enterica serovar Typhimurium expressing Pseudomonas aeruginosa O antigen promotes increased survival in an acute fatal pneumonia model

Pseudomonas aeruginosa is a leading cause of nosocomial pneumonia. We compared the efficacies of oral and intraperitoneal (i.p.) vaccinations of BALB/c mice with attenuated Salmonella enterica serovar Typhimurium SL3261 expressing P. aeruginosa serogroup O11 O antigen to protect against P. aeruginosa infection in an acute fatal pneumonia model. Oral and i.p. vaccines elicited O11-specific serum immunoglobulin G (IgG) antibodies, but IgA was observed only after oral immunization. Challenge of orally vaccinated mice with an O11 strain (9882-80) at 6 and 12 times the 50% lethal dose showed increased survival in mice that received the vaccine compared to phosphate-buffered saline (PBS)- and vector-treated controls; no difference in survival was seen with a heterologous strain, 6294 (serogroup O6). In addition, significant protection against 9882-80 was not observed in i.p. vaccinated animals. Bronchoalveolar lavage fluid taken from immunized mice harbored O11-specific IgA and IgG in orally immunized mice but only modest levels of IgG in i.p. vaccinated mice. To correlate protection, opsonophagocytosis assays were performed with pooled sera from orally immunized animals. Efficient killing of five O11 clinical isolates was observed, while no killing was noted with 6294, indicating that the recombinant SL3261 oral vaccine induces an O11-specific reaction. We next determined the ability of orally vaccinated animals to clear bacteria from their lungs. Following P. aeruginosa challenge, the numbers of viable bacteria were significantly fewer in orally vaccinated animals than in PBS- and vector-treated controls. Our results suggest that oral immunization with recombinant SL3261 is efficacious in protection against pneumonia caused by P. aeruginosa.

Human monoclonal antibodies to Pseudomonas aeruginosa alginate that protect against infection by both mucoid and nonmucoid strains

Two fully human mAbs specific for epitopes dependent on intact carboxylate groups on the C6 carbon of the mannuronic acid components of Pseudomonas aeruginosa alginate were found to promote phagocytic killing of both mucoid and nonmucoid strains as well as protection against both types of strains in a mouse model of acute pneumonia. The specificity of the mAbs for alginate was determined by ELISA and killing assays. Some strains of P. aeruginosa did not make detectable alginate in vitro, but in vivo protection against lethal pneumonia was obtained and shown to be due to rapid induction of expression of alginate in the murine lung. No protection against strains genetically unable to make alginate was achieved. These mAbs have potential to be passive therapeutic reagents for all strains of P. aeruginosa and the results document that alginate is a target for the proper type of protective Ab even when expressed at low levels on phenotypically nonmucoid strains.

Growth, virulence, and immunogenicity of Listeria monocytogenes aro mutants

Mutants of Listeria monocytogenes with deletions in genes of the common branch of the biosynthesis pathway leading to aromatic compounds were constructed as possible virulence-attenuated carrier strains for protein antigens or vaccine DNA. aroA, aroB, and in particular aroE mutants showed strongly reduced growth rates in epithelial cells and even in rich culture media. The metabolism of the aro mutants under these conditions was predominantly anaerobic. Aerobic metabolism and a wild-type growth rate were, however, regained upon the addition of vitamin K2, suggesting that the aro mutants are deficient in oxidative respiration due to the lack of menaquinone. Replication of the aro mutants in the host cell's cytosol and cell-to-cell spread were drastically slowed down, and all aro mutants showed high virulence attenuation in mice, i.e., the 50% lethal dose in BALB/c mice was increased at least 10(4)-fold for the aroA, aroB, and aroA/B mutants and >10(5)-fold for the aroE mutant compared to the parent strain. Nevertheless, mice preimmunized with aro mutant bacteria elicited good T-cell response and full protection against a subsequent challenge with the virulent wild-type strain. A total of 5 x 10(6) aroA, aroB, and aroA/B mutant bacteria were sufficient to obtain a protective T-cell response, while 5 x 10(8) aroE or aroA/E mutants were necessary to achieve comparable numbers of antigen-specific T cells. These numbers were well tolerated without causing any signs of disease, indicating that Listeria strains with deletions in genes of the basic branch of the aromatic amino acid pathway could be useful vaccine carriers for inducing T-cell immunity.

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.

Promises and pitfalls of Pseudomonas aeruginosa lipopolysaccharide as a vaccine antigen

Antibodies directed to the Pseudomonas aeruginosa lipopolysaccharide (LPS) O-antigens have clearly shown to mediate the most effective immunity to infection caused by LPS-smooth strains. Such strains are major causes of disease in immunocompromised hosts such as burn or cancer patients, individuals in intensive care units, and those who utilize extended-wear contact lenses. Yet producing an effective vaccine composed of non-toxic, immunogenic polysaccharides has been challenging. The chemical diversity among the different O-antigens representative of the 20 major serotypes, plus additional diversity among some O-antigens representing variant subtype antigens, translates into a large degree of serologic variability that increases the complexity of O-antigen specific vaccines. Further complications come from the poor immunogenicity of the major protective epitope expressed by some O-antigens, and a large degree of diversity in animal responses that preclude predicting the optimal vaccine formulation from such studies. Nonetheless human trials over the years of vaccines eliciting O-antigen immunity have been encouraging, though no vaccine has yet been fully evaluated and found to be clinically efficacious. Newer vaccine approaches such as using polysaccharide-protein conjugates and passive therapy with monoclonal or polyclonal immune sera offer some additional means to try and produce an effective immunotherapeutic reagent for this problematic pathogen.