Intradermal vaccination with a Pseudomonas aeruginosa vaccine adjuvanted with a mutant bacterial ADP-ribosylating enterotoxin protects against acute pneumonia

Development of acute Pseudomonas aeruginosa and Acinetobacter baumannii lung mono-challenge models in mice using oropharyngeal aspiration

Antimicrobial-resistant pathogens such as Pseudomonas aeruginosa and Acinetobacter baumannii can cause potentially fatal infections in susceptible individuals, with respiratory tract infections among the most common clinical presentations. The development of novel treatments or prophylactic interventions to combat these infections is urgently needed and requires robust, reliable animal models for their preclinical evaluation. In particular, the bacterial burden needs to be accurately determined before and after administration of the potential therapy under evaluation to quantify the effectiveness of the treatment. We provide two reliable, non-invasive murine acute lung challenge models with either P. aeruginosa or A. baumannii using an oropharyngeal aspiration technique, which has been widely overlooked in studies testing vaccines or treatments for these pathogens. Here, we show that this non-surgical technique to deliver suspensions into mouse lungs does not significantly impact animal welfare (based on welfare monitoring and weight) and allows uniform bilateral distribution of the bacterial dose, resulting in even bioburden in both lungs. The optimal timepoint for humane killing and organ harvest was 24 h after challenge for both pathogens, and at least 4×106 and 107 c.f.u. per mouse were needed to obtain a reproducible P. aeruginosa or A. baumannii bioburden, respectively. These mouse challenge models offer a valuable tool to assess therapeutic interventions against P. aeruginosa or A. baumannii infections.

Recent advances in enterotoxin vaccine adjuvants

Enterotoxin adjuvants have been researched for their ability to promote immunity to co-delivered antigens. Outside of cholera vaccines, however, these proteins have yet to be included in any currently licensed vaccines. They include molecules derived from the bacterial toxins of Vibrio cholerae, cholera toxin, or Escherichia coli, heat-labile toxin, such as detoxified mutants or subunits. This class of adjuvants is distinguished by their delivery possibilities, which include parenteral injection, skin applications, or direct mucosal administration by oral, sublingual, or nasal routes. In addition, inclusion of an enterotoxin adjuvant is associated with development of multifaceted cellular and humoral immune responses to vaccination. Here, we review exciting progress in the past few years in clinical trials for safety and efficacy, preclinical vaccines studies, and new mechanistic insights for enterotoxin adjuvants. This includes recent reports of their use in vaccines targeting microbial infections (bacterial, viral, parasitic) or substance abuse drugs.

Systems serology in cystic fibrosis: Anti-Pseudomonas IgG1 responses and reduced lung function

Nearly one-half of patients with cystic fibrosis (CF) carry the homozygous F508del mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) gene but exhibit variable lung function phenotypes. How adaptive immunity influences their lung function remains unclear, particularly the serological antibody responses to antigens from mucoid Pseudomonas in sera from patients with CF with varying lung function. Sera from patients with CF with reduced lung function show higher anti-outer membrane protein I (OprI) immunoglobulin G1 (IgG1) titers and greater antibody-mediated complement deposition. Induction of anti-OprI antibody isotypes with complement activity enhances lung inflammation in preclinical mouse models. This enhanced inflammation is absent in immunized Rag2-/- mice and is transferrable to unimmunized mice through sera. In a CF cohort undergoing treatment with elexacaftor-tezacaftor-ivacaftor, the declination in anti-OprI IgG1 titers is associated with lung function improvement and reduced hospitalizations. These findings suggest that antibody responses to specific Pseudomonas aeruginosa (PA) antigens worsen lung function in patients with CF.

Establishment of isotype-switched, antigen-specific B cells in multiple mucosal tissues using non-mucosal immunization

Although most pathogens infect the human body via mucosal surfaces, very few injectable vaccines can specifically target immune cells to these tissues where their effector functions would be most desirable. We have previously shown that certain adjuvants can program vaccine-specific helper T cells to migrate to the gut, even when the vaccine is delivered non-mucosally. It is not known whether this is true for antigen-specific B cell responses. Here we show that a single intradermal vaccination with the adjuvant double mutant heat-labile toxin (dmLT) induces a robust endogenous, vaccine-specific, isotype-switched B cell response. When the vaccine was intradermally boosted, we detected non-circulating vaccine-specific B cell responses in the lamina propria of the large intestines, Peyer's patches, and lungs. When compared to the TLR9 ligand adjuvant CpG, only dmLT was able to drive the establishment of isotype-switched resident B cells in these mucosal tissues, even when the dmLT-adjuvanted vaccine was administered non-mucosally. Further, we found that the transcription factor Batf3 was important for the full germinal center reaction, isotype switching, and Peyer's patch migration of these B cells. Collectively, these data indicate that specific adjuvants can promote mucosal homing and the establishment of activated, antigen-specific B cells in mucosal tissues, even when these adjuvants are delivered by a non-mucosal route. These findings could fundamentally change the way future vaccines are formulated and delivered.

Gearing up for battle: Harnessing adaptive T cell immunity against gram-negative pneumonia

Pneumonia is one of the leading causes of morbidity and mortality worldwide and Gram-negative bacteria are a major cause of severe pneumonia. Despite advances in diagnosis and treatment, the rise of multidrug-resistant organisms and hypervirulent strains demonstrates that there will continue to be challenges with traditional treatment strategies using antibiotics. Hence, an alternative approach is to focus on the disease tolerance components that mediate immune resistance and enhance tissue resilience. Adaptive immunity plays a pivotal role in modulating these processes, thus affecting the incidence and severity of pneumonia. In this review, we focus on the adaptive T cell responses to pneumonia induced by Klebsiella pneumoniae, Pseudomonas aeruginosa, and Acinetobacter baumannii. We highlight key factors in these responses that have potential for therapeutic targeting, as well as the gaps in current knowledge to be focused on in future work.

Pseudomonas aeruginosa: Recent Advances in Vaccine Development

Pseudomonas aeruginosa is an important opportunistic human pathogen. Using its arsenal of virulence factors and its intrinsic ability to adapt to new environments, P. aeruginosa causes a range of complicated acute and chronic infections in immunocompromised individuals. Of particular importance are burn wound infections, ventilator-associated pneumonia, and chronic infections in people with cystic fibrosis. Antibiotic resistance has rendered many of these infections challenging to treat and novel therapeutic strategies are limited. Multiple clinical studies using well-characterised virulence factors as vaccine antigens over the last 50 years have fallen short, resulting in no effective vaccination being available for clinical use. Nonetheless, progress has been made in preclinical research, namely, in the realms of antigen discovery, adjuvant use, and novel delivery systems. Herein, we briefly review the scope of P. aeruginosa clinical infections and its major important virulence factors.

Bacteriophages as drivers of bacterial virulence and their potential for biotechnological exploitation

Bacteria-infecting viruses (phages) and their hosts maintain an ancient and complex relationship. Bacterial predation by lytic phages drives an ongoing phage-host arms race, whereas temperate phages initiate mutualistic relationships with their hosts upon lysogenization as prophages. In human pathogens, these prophages impact bacterial virulence in distinct ways: by secretion of phage-encoded toxins, modulation of the bacterial envelope, mediation of bacterial infectivity and the control of bacterial cell regulation. This review builds the argument that virulence-influencing prophages hold extensive, unexplored potential for biotechnology. More specifically, it highlights the development potential of novel therapies against infectious diseases, to address the current antibiotic resistance crisis. First, designer bacteriophages may serve to deliver genes encoding cargo proteins which repress bacterial virulence. Secondly, one may develop small molecules mimicking phage-derived proteins targeting central regulators of bacterial virulence. Thirdly, bacteria equipped with phage-derived synthetic circuits which modulate key virulence factors could serve as vaccine candidates to prevent bacterial infections. The development and exploitation of such antibacterial strategies will depend on the discovery of other prophage-derived, virulence control mechanisms and, more generally, on the dissection of the mutualistic relationship between temperate phages and bacteria, as well as on continuing developments in the synthetic biology field.

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.

Development of a Broadly Protective, Self-Adjuvanting Subunit Vaccine to Prevent Infections by Pseudomonas aeruginosa

Infections caused by the opportunistic pathogen Pseudomonas aeruginosa can be difficult to treat due to innate and acquired antibiotic resistance and this is exacerbated by the emergence of multi-drug resistant strains. Unfortunately, no licensed vaccine yet exists to prevent Pseudomonas infections. Here we describe a novel subunit vaccine that targets the P. aeruginosa type III secretion system (T3SS). This vaccine is based on the novel antigen PaF (Pa Fusion), a fusion of the T3SS needle tip protein, PcrV, and the first of two translocator proteins, PopB. Additionally, PaF is made self-adjuvanting by the N-terminal fusion of the A1 subunit of the mucosal adjuvant double-mutant heat-labile enterotoxin (dmLT). Here we show that this triple fusion, designated L-PaF, can activate dendritic cells in vitro and elicits strong IgG and IgA titers in mice when administered intranasally. This self-adjuvanting vaccine expedites the clearance of P. aeruginosa from the lungs of challenged mice while stimulating host expression of IL-17A, which may be important for generating a protective immune response in humans. L-PaF's protective capacity was recapitulated in a rat pneumonia model, further supporting the efficacy of this novel fusion vaccine.

Protective Efficacy of the OprF/OprI/PcrV Recombinant Chimeric Protein Against Pseudomonas aeruginosa in the Burned BALB/c Mouse Model

Background

Pseudomonas aeruginosa infection is the major cause of death in burn patients. Thus, in this study, a chimeric vaccine harboring the OprF_185–350–OprI_22–83–PcrV was designed and expressed in Escherichia coli. The immunogenicity of the recombinant chimer, OprI, OprF, and PcrV was studied in a burned mouse model.

Methodology

Recombinant proteins including the proposed chimer, OprF, OprI, and PcrV were expressed in the E.coli. Mice were immunized with the purified recombinant proteins, and the antibody titre was estimated in the sera obtained from immunized mice. Immunized and control mice were challenged with 2, 5, and 10xLD₅₀ of the P. aeruginosa strains (PAO1, PAK, and R₅), and microbial counts were measured in the skin, liver, spleen, and kidney of the studied mice.

Results

Results showed that the antibody titre (total IgG) was significantly increased by injection of 10 μg of chimeric protein in the experimental groups compared to the control groups. The antibody survival titre was high until 235 days after administration of the second booster. The survival rate of the mice infected with 10xLD₅₀ was significantly increased and the number of bacteria was reduced, especially in the internal organs (kidney, spleen, and liver) compared to the mice immunized with any of the OprF, OprI, and PcrV proteins alone.

Conclusion

The findings of our study revealed that the chimeric protein is a promising vaccine candidate for control of the P. aeruginosa infection.

Mechanical Ventilation Impairs IL-17 Cytokine Family Expression in Ventilator-Associated Pneumonia

Mechanical ventilation (MV) is the primary risk factor for the development of ventilator-associated pneumonia (VAP). Besides inducing a pro-inflammatory T-helper (Th)-1 cytokine response, MV also induces an anti-inflammatory Th2 cytokine response, marked by increased IL-4 secretion and reduced bacterial phagocytic capacity of rodent lung macrophages. Since IL-4 is known to downregulate both Th1 and Th17 cytokines, the latter is important in mediating mucosal immunity and combating bacterial and fungal growth, we studied and showed here in a rat model of MV that Th17 cytokines (IL-17A, IL-17F, and IL-22) were significantly upregulated in the lung as a response to different MV strategies currently utilized in clinic. To study whether the increased IL-4 levels are associated with downregulation of the anti-bacterial Th17 cytokines, we subsequently challenged mechanically ventilated rats with an intratracheal inoculation of Pseudomonas aeruginosa (VAP model) and showed a dramatic downregulation of IL-17A, IL-17F, and IL-22, compared to animals receiving the same bacterial burden without MV. For the studied Th1 cytokines (IFNγ, TNFα, IL-6, and IL-1β), only IFNγ showed a significant decrease as a consequence of bacterial infection in mechanically ventilated rats. We further studied IL-17A, the most studied IL-17 family member, in intensive care unit (ICU) pneumonia patients and showed that VAP patients had significantly lower levels of IL-17A in the endotracheal aspirate compared to patients entering ICU with pre-existing pneumonia. These translational data, obtained both in animal models and in humans, suggest that a deficient anti-bacterial Th17 response in the lung during MV is associated with VAP development.

Immunological considerations in the development of Pseudomonas aeruginosa vaccines

Pseudomonas aeruginosa is an opportunistic human pathogen capable of causing a wide range of potentially life-threatening infections. With multidrug-resistant P. aeruginosa infections on the rise, the need for a rationally-designed vaccine against this pathogen is critical. A number of vaccine platforms have shown promising results in pre-clinical studies, but no vaccine has successfully advanced to licensure. Growing evidence suggests that an effective P. aeruginosa vaccine may require Th17-type CD4⁺ T cells to prevent infection. In this review, we summarize recent pre-clinical studies of P. aeruginosa vaccines, specifically focusing on those that induce Th17-type cellular immunity. We also highlight the importance of adjuvant selection and immunization route in vaccine design in order to target vaccine-induced immunity to infected tissues. Advances in cellular immunology and adjuvant biology may ultimately influence better P. aeruginosa vaccine platforms that can protect targeted human populations.