Intranasal immunization protects against Acinetobacter baumannii-associated pneumonia in mice

Development and Evaluation of an Immunoinformatics-Based Multi-Peptide Vaccine against Acinetobacter baumannii Infection

Multi-drug-resistant (MDR) Acinetobacter baumannii is an opportunistic pathogen associated with hospital-acquired infections. Due to its environmental persistence, virulence, and limited treatment options, this organism causes both increased patient mortality and incurred healthcare costs. Thus, prophylactic vaccination could be ideal for intervention against MDR Acinetobacter infection in susceptible populations. In this study, we employed immunoinformatics to identify peptides containing both putative B- and T-cell epitopes from proteins associated with A. baumannii pathogenesis. A novel Acinetobacter Multi-Epitope Vaccine (AMEV2) was constructed using an A. baumannii thioredoxin A (TrxA) leading protein sequence followed by five identified peptide antigens. Antisera from A. baumannii infected mice demonstrated reactivity to rAMEV2, and subcutaneous immunization of mice with rAMEV2 produced high antibody titer against the construct as well as peptide components. Immunization results in increased frequency of IL-4-secreting splenocytes indicative of a Th2 response. AMEV2-immunized mice were protected against intranasal challenge with a hypervirulent strain of A. baumannii and demonstrated reduced bacterial burden at 48 h. In contrast, all mock vaccinated mice succumbed to infection within 3 days. Results presented here provide insight into the effectiveness of immunoinformatic-based vaccine design and its potential as an effective strategy to combat the rise of MDR pathogens.

Enhanced immunoprotection against Acinetobacter baumannii infection: Synergistic effects of Bap and BauA in a murine model

BACKGROUND

The rise of multi-drug resistant Acinetobacter baumannii poses a grave threat to hospital settings, resulting in increased mortality rates and garnering global attention. The formation of biofilms facilitated by biofilm-associated protein (Bap) and the iron absorption capabilities mediated by Baumannii acinetobactin utilization A (BauA) contribute to the persistence and survival of multidrug-resistant strains. In this study, we aimed to investigate the potential of disrupting the function of BauA and Bap simultaneously as a strategy for controlling A. baumannii.

METHODS

Recombinant Bap and BauA were expressed, purified, and subcutaneously administered individually and in combination to BALB/c mice. Subsequently, mice were intraperitoneally challenged with A. baumannii, and the bacterial load and tissue damage in the spleen, lung, and liver were assessed. Serum samples were evaluated to determine antibody titers in surviving mice.

RESULTS

Specific IgG antibodies were significantly increased. A combination of the antigens resulted in enhanced titer of specific IgGs in comparison to either BauA or Bap alone. The antibodies remained stable over a seven-month period. The combination of Bap and BauA exhibited superior immunoprotection against A. baumannii infection compared to individual administration, resulting in a further reduction in bacterial load in the liver, spleen, and lungs. The histopathological analysis demonstrated successful protection of the tissues against A. baumannii-induced damage upon administration of the two immunogens.

CONCLUSIONS

The combination of Bap and BauA has the potential to target a broader range of A. baumannii strains, including those expressing either Bap or BauA, thereby increasing its efficacy against a diverse array of strains.

Epitope mapping of Acinetobacter baumannii outer membrane protein W (OmpW) and laboratory study of an OmpW-derivative peptide

Outer membrane protein W (OmpW) is a less-known A. baumannii antigen with potential immunogenic properties. The epitopes of this protein are not well-identified yet. Therefore, in the present study, B- and T-cell epitopes of A. baumannii OmpW were found using comprehensive in silico and partially in vitro studies. The T-cell (both class-I and class-II) and B-cell (both linear and conformational) epitopes were predicted and screened through many bioinformatics approaches including the prediction of IFN-γ production, immunogenicity, toxicity, allergenicity, human similarity, and clustering. A single 15-mer epitopic peptide containing a linear B-cell and both classes of T-cell epitopes were found and used for further assays. For in vitro assays, patient- and healthy control-derived peripheral blood mononuclear cells were stimulated with the 15-mer peptide, Phytohemagglutinin, or medium alone, and cell proliferation and IFN-γ production assays were performed. The bioinformatics studies led to mapping OmpW epitopes and introducing a 15-mer peptide. In vitro assays to some extent showed its potency in cell proliferation but not in IFN-γ induction, although the responses were not very expressive and faced some questions/limitations. In general, in the current study, we mapped the most immunogenic epitopes of OmpW that may be used for future studies and also assayed one of these epitopes in vitro, which was shown to have an immunogenicity potential. However, the induced immune responses were not strong which suggests that the present peptide needs a series of biotechnological manipulations to be used as a potential vaccine candidate. More studies in this field are recommended.

The Prevalence of Multidrug-Resistant Acinetobacter baumannii and Its Vaccination Status among Healthcare Providers

There is growing concern among healthcare providers worldwide regarding the prevalence of multidrug-resistant Acinetobacter baumannii (A. baumannii). Some of the worst hospital-acquired infections, often in intensive care units (ICUs), are caused by this bacterial pathogen. In recent years, the rise in multidrug-resistant A. baumannii has been linked to the overuse of antimicrobial drugs and the lack of adequate infection control measures. Infections caused by this bacterial pathogen are the result of prolonged hospitalization and ICU stays, and they are associated with increased morbidity and mortality. This review outlines the epidemiology, risk factors, and antimicrobial resistance associated with A. baumannii in various countries, with a special focus on the Kingdom of Saudi Arabia. In response to the growing concern regarding this drug-resistant bacteria, fundamental information about its pathology has been incorporated into the development of vaccines. Although these vaccines have been successful in animal models, their effectiveness in humans remains unproven. The review will discuss the development of A. baumannii vaccines, potential related obstacles, and efforts to find an effective strategy against this pathogen.

Using an Aluminum Hydroxide–Chitosan Matrix Increased the Vaccine Potential and Immune Response of Mice against Multi-Drug-Resistant Acinetobacter baumannii

Acinetobacter baumannii is a Gram-negative, immobile, aerobic nosocomial opportunistic coccobacillus that causes pneumonia, septicemia, and urinary tract infections in immunosuppressed patients. There are no commercially available alternative antimicrobials, and multi-drug resistance is an urgent concern that requires emergency measures and new therapeutic strategies. This study evaluated a multi-drug-resistant A. baumannii whole-cell vaccine, inactivated and adsorbed on an aluminum hydroxide–chitosan (mAhC) matrix, in an A. baumannii sepsis model in immunosuppressed mice by cyclophosphamide (CY). CY-treated mice were divided into immunized, non-immunized, and adjuvant-inoculated groups. Three vaccine doses were given at 0D, 14D, and 28D, followed by a lethal dose of 4.0 × 108 CFU/mL of A. baumannii. Immunized CY-treated mice underwent a significant humoral response, with the highest IgG levels and a higher survival rate (85%); this differed from the non-immunized CY-treated mice, none of whom survived (p < 0.001), and from the adjuvant group, with 45% survival (p < 0.05). Histological data revealed the evident expansion of white spleen pulp from immunized CY-treated mice, whereas, in non-immunized and adjuvanted CY-treated mice, there was more significant organ tissue damage. Our results confirmed the proof-of-concept of the immune response and vaccine protection in a sepsis model in CY-treated mice, contributing to the advancement of new alternatives for protection against A. baumannii infections.

Vaccine development for bacterial pathogens: Advances, challenges and prospects

The advent and use of antimicrobials have played a key role in treating potentially life-threatening infectious diseases, improving health, and saving the lives of millions of people worldwide. However, the emergence of multidrug resistant (MDR) pathogens has been a significant health challenge that has compromised the ability to prevent and treat a wide range of infectious diseases that were once treatable. Vaccines offer potential as a promising alternative to fight against antimicrobial resistance (AMR) infectious diseases. Vaccine technologies include reverse vaccinology, structural biology methods, nucleic acid (DNA and mRNA) vaccines, generalised modules for membrane antigens, bioconjugates/glycoconjugates, nanomaterials and several other emerging technological advances that are offering a potential breakthrough in the development of efficient vaccines against pathogens. This review covers the opportunities and advancements in vaccine discovery and development targeting bacterial pathogens. We reflect on the impact of the already-developed vaccines targeting bacterial pathogens and the potential of those currently under different stages of preclinical and clinical trials. More importantly, we critically and comprehensively analyse the challenges while highlighting the key indices for future vaccine prospects. Finally, the issues and concerns of AMR for low-income countries (sub-Saharan Africa) and the challenges with vaccine integration, discovery and development in this region are critically evaluated.

Non-antibiotic prevention and treatment against Acinetobacter baumannii infection: Are vaccines and adjuvants effective strategies?

Acinetobacter baumannii (A. baumannii) is a Gram-negative opportunistic pathogen widely attached to the surface of medical instruments, making it one of the most common pathogens of nosocomial infection, and often leading to cross-infection and co-infection. Due to the extensive antibiotic and pan-resistance, A. baumannii infection is facing fewer treatment options in the clinic. Therefore, the prevention and treatment of A. baumannii infection have become a tricky global problem. The requirement for research and development of the new strategy is urgent. Now, non-antibiotic treatment strategies are urgently needed. This review describes the research on A. baumannii vaccines and antibacterial adjuvants, discusses the advantages and disadvantages of different candidate vaccines tested in vitro and in vivo, especially subunit protein vaccines, and shows the antibacterial efficacy of adjuvant drugs in monotherapy.

Specific egg yolk antibody raised to biofilm associated protein (Bap) is protective against murine pneumonia caused by Acinetobacter baumannii

Acinetobacter baumannii easily turns into pan drug-resistant (PDR) with a high mortality rate. No effective commercial antibiotic or approved vaccine is available against drug-resistant strains of this pathogen. Egg yolk immunoglobulin (IgY) could be used as a simple and low-cost biotherapeutic against its infections. This study evaluates the prophylactic potential of IgY against A. baumannii in a murine pneumonia model. White Leghorn hens were immunized with intramuscular injection of the recombinant biofilm-associated protein (Bap) from A. baumannii on days 0, 21, 42, and 63. The reactivity and antibiofilm activity of specific IgYs raised against the Bap was evaluated by indirect ELISA and a microtiter plate assay for biofilm formation. The IgYs against Bap were able to decrease the biofilm formation ability of A. baumannii and protect the mice against the challenge of A. baumannii. IgYs antibody raised here shows a good antigen-specificity and protectivity which can be used in passive immunotherapy against A. baumannii. In conclusion, the IgY against biofilm-associated protein proves prophylactic in a murine pneumonia model.

Insights into Acinetobacter baumannii protective immunity

Acinetobacter baumannii is a nosocomic opportunistic Gram-negative bacteria known for its extensive drug-resistant phenotype. A. baumannii hospital-acquired infections are major contributors to increased costs and mortality observed during the COVID-19 pandemic. With few effective antimicrobials available for treatment of this pathogen, immune-based therapy becomes an attractive strategy to combat multi-drug resistant Acinetobacter infection. Immunotherapeutics is a field of growing interest with advances in vaccines and monoclonal antibodies providing insight into the protective immune response required to successfully combat this pathogen. This review focuses on current knowledge describing the adaptive immune response to A. baumannii, the importance of antibody-mediated protection, developments in cell-mediated protection, and their respective therapeutic application going forward. With A. baumannii’s increasing resistance to most current antimicrobials, elucidating an effective host adaptive immune response is paramount in the guidance of future immunotherapeutic development.

Whole-cell vaccine candidates induce a protective response against virulent Acinetobacter baumannii

Acinetobacter baumannii causes multi-system diseases in both nosocomial settings and a pre-disposed general population. The bacterium is not only desiccation-resistant but also notoriously resistant to multiple antibiotics and drugs of last resort including carbapenem, colistin, and sulbactam. The World Health Organization has categorized carbapenem-resistant A. baumannii at the top of its critical pathogen list in a bid to direct urgent countermeasure development. Several early-stage vaccines have shown a range of efficacies in healthy mice, but no vaccine candidates have advanced into clinical trials. Herein, we report our findings that both an ionizing γ-radiation-inactivated and a non-ionizing ultraviolet C-inactivated whole-cell vaccine candidate protects neutropenic mice from pulmonary challenge with virulent AB5075, a particularly pathogenic isolate. In addition, we demonstrate that a humoral response is sufficient for this protection via the passive immunization of neutropenic mice.

Immunity induced by valine-glycine repeat protein G imparts histoprotection of vital body organs against Acinetobacter baumannii

Background

Efforts toward the development of an effective vaccine against Acinetobacter baumannii, one of the most notorious nosocomial pathogens, are still ongoing. In this regard, virulence factors are interesting targets. Type VI secretion system (T6SS) participates in the pathogenicity of A. baumannii. VgrG is a crucial component of T6SS prevalent among A. baumannii strains. This study was conducted to evaluate the immunoprotectivity of recombinant VgrG (rVgrG) cloned and over-expressed in Escherichia coli BL21 (DE3). BALB/c mice were immunized with the purified rVgrG. Specific anti-VgrG IgG titers were assessed by ELISA. Actively and passively immunized mice were challenged with lethal doses of A. baumannii ATCC 19606. The survival rate, the bacterial burden, and histopathology of tissues in infected mice were examined.

Results

Anti-VgrG IgG (p < 0.0001) was significantly increased in immunized mice. No death was seen in actively immunized mice infected with the lethal dose (LD) of 1.9 × 10⁸ CFU of A. baumannii ATCC 19606 within 72 h. Challenge with 2.4 × 10⁸ CFU of the pathogen showed a 75% survival rate. All immunized mice infected with 3.2 × 10⁸ CFU of the pathogen died within 12 h. In passive immunization, no death was observed in mice that received LD of the bacteria incubated with the 1:250 dilution of the immune sera. An increased number of neutrophils around the peribronchial and perivascular areas were seen in unimmunized mouse lungs while passively immunized mice revealed moderate inflammation with infiltration of mixed mononuclear cells and neutrophils. The livers of the unimmunized mice showed inflammation and necrosis in contrast to the livers from immunized mice. Hyperplasia of the white pulp and higher neutrophils were evident in the spleen of unimmunized mice as against the normal histology of the immunized group.

Conclusions

VgrG is a protective antigen that could be topologically accessible to the host antibodies. Although VgrG is not sufficient to be assigned as a stand-alone antigen for conferring full protection, it could participate in multivalent vaccine developments for elevated efficacy.

Supplementary Information

The online version contains supplementary material available at 10.1186/s43141-022-00325-4.

Vaccine development to control the rising scourge of antibiotic-resistant Acinetobacter baumannii: a systematic review

Acinetobacter baumannii has emerged as one of major nosocomial pathogen and global emergence of multidrug-resistant strains has become a challenge for developing effective treatment options. A. baumannii has developed resistance to almost all the antibiotics viz. beta-lactams, carbapenems, tigecycline and now colistin, a last resort of antibiotics. The world is on the cusp of post antibiotic era and the evolution of multi-, extreme- and pan–drug-resistant A. baumannii strains is its obvious harbinger. Various combinations of antibiotics have been investigated but no successful treatment option is available. All these failed efforts have led researchers to develop and implement prophylactic vaccination for the prevention of infections caused by this pathogen. In this review, the advantages and disadvantages of active and passive immunization, the types of sub-unit and multi-component vaccine candidates investigated against A. baumannii viz. whole cell organism, outer membrane vesicles, outer membrane complexes, conjugate vaccines and sub-unit vaccines have been discussed. In addition, the benefits of Reverse vaccinology are emphasized here in which the potential vaccine candidates are predicted using bioinformatic online tools prior to in vivo validations.

Effective Strategies to Deal With Infection in Burn Patient

Treatment of bacterial infection is difficult. Treatment protocol of burned patient is hard. Furthermore, treatment in burned patients is accompanied with problems such as complexity in diagnosis of infection's agent, multiple infections, being painful, and involving with different organelles. There are different infections of Gram-positive and Gram-negative bacteria in burned patients. From important bacteria can be noted to Pseudomonas aeruginosa, Acinetobacter baumannii, and Staphylococcus aureus that have high range of morbidity and mortality. Treatment of those bacterial infections is extremely important. Hence, many studies about methods of treatment of bacterial infections have published. Herein, we have suggested practical methods for example ant virulence therapies, nanotechnology, vaccine, and photodynamic therapy in treatment of bacterial infections. Those methods have been done in many researches and had good effect.

Sequential Vaccination With Heterologous Acinetobacter baumannii Strains Induces Broadly Reactive Antibody Responses

Antibody therapy may be an alternative treatment option for infections caused by the multi-drug resistant (MDR) bacterium Acinetobacter baumannii. As A. baumannii has multiple capsular serotypes, a universal antibody therapy would need to target conserved protein antigens rather than the capsular polysaccharides. We have immunized mice with single or multiple A. baumannii strains to induce antibody responses to protein antigens, and then assessed whether these responses provide cross-protection against a collection of genetically diverse clinical A. baumannii isolates. Immunized mice developed antibody responses to multiple protein antigens. Flow cytometry IgG binding assays and immunoblots demonstrated improved recognition of both homologous and heterologous clinical strains in sera from mice immunized with multiple strains compared to a single strain. The capsule partially inhibited bacterial recognition by IgG and the promotion of phagocytosis by human neutrophils. However, after immunization with multiple strains, serum antibodies to protein antigens promoted neutrophil phagocytosis of heterologous A. baumannii strains. In an infection model, mice immunized with multiple strains had lower bacterial counts in the spleen and liver following challenge with a heterologous strain. These data demonstrate that antibodies targeting protein antigens can improve immune recognition and protection against diverse A. baumannii strains, providing support for their use as an antibody therapy.

A hypothetical adhesin protein induces anti-biofilm antibodies against multi-drug resistant Acinetobacter baumannii

The increase in multidrug-resistant (MDR) Acinetobacter baumannii strains in hospital environments has generated great concern around the world. Biofilm is one of the forms of bacterial adaptation that is increasingly leading to antimicrobial resistance and therapeutic failure. The search for alternative therapeutic strategies, especially non-antibiotic-based, is urgently needed. In this study, we produce polyclonal antibodies (pAbs) in murine models against recombinant CAM87009.1 antigen, an A. baumannii fimbriae protein. The pAbs produced were isotyped and anti-biofilm activity evaluated in the A. baumannii ATCC® 19606 standard strain and nine MDR clinical isolates. All clinical isolates were analyzed for the presence of the cam87009.1 gene using the PCR technique, and one of the isolates did not have the gene in its genome. After four intraperitoneal immunizations (days 0, 14, 21, and 28) of mice with rCAM87009.1 and Freund's adjuvant, a significant antibody titer was detected by indirect enzyme-linked immunosorbent assay (ELISA) since the first immunization (1:6400), and the level increased until the 4th immunization (1:819,200). IgM, IgA, IgG1, IgG2a, IgG2b, and IgG3 isotypes were identified in the serum of immunized mice (P < 0.001). The anti-rCAM87009.1 pAb was able to inhibit biofilm formation in 80 % of the strains evaluated in this study, including the ATCC® 19606 strain. The rCAM87009.1 proves to be a promising target in the development of alternative strategies to control biofilm-forming in A. baumannii MDR strains.

Mapping Global Prevalence of Acinetobacter baumannii and Recent Vaccine Development to Tackle It

Acinetobacter baumannii is a leading cause of nosocomial infections that severely threaten public health. The formidable adaptability and resistance of this opportunistic pathogen have hampered the development of antimicrobial therapies which consequently leads to very limited treatment options. We mapped the global prevalence of multidrug-resistant A. baumannii and showed that carbapenem-resistant A. baumannii is widespread throughout Asia and the Americas. Moreover, when antimicrobial resistance rates of Acinetobacter spp. exceed a threshold level, the proportion of A. baumannii isolates from clinical samples surges. Therefore, vaccines represent a realistic alternative strategy to tackle this pathogen. Research into anti-A. baumannii vaccines have enhanced in the past decade and multiple antigens have been investigated preclinically with varying results. This review summarises the current knowledge of virulence factors relating to A. baumannii–host interactions and its implication in vaccine design, with a view to understanding the current state of A. baumannii vaccine development and the direction of future efforts.

Mechanical ventilation enhances Acinetobacter baumannii-induced lung injury through JNK pathways

BACKGROUND

Patients in intensive care units (ICUs) often received broad-spectrum antibiotic treatment and Acinetobacter baumannii (A.b.) and Pseudomonas aeruginosa (P.a.) were the most common pathogens causing ventilator-associated pneumonia (VAP). This study aimed to examine the effects and mechanism of mechanical ventilation (MV) on A.b.-induced lung injury and the involvement of alveolar macrophages (AMs).

METHODS

C57BL/6 wild-type (WT) and c-Jun N-terminal kinase knockout (JNK1-/-) mice received MV for 3 h at 2 days after nasal instillation of A.b., P.a. (1 × 106 colony-forming unit, CFU), or normal saline.

RESULTS

Intranasal instillation of 106 CFU A.b. in C57BL/6 mice induced a significant increase in total cells and protein levels in the bronchoalveolar lavage fluid (BALF) and neutrophil infiltration in the lungs. MV after A.b. instillation increases neutrophil infiltration, interleukin (IL)-6 and vascular cell adhesion molecule (VCAM) mRNA expression in the lungs and total cells, IL-6 levels, and nitrite levels in the BALF. The killing activity of AMs against A.b. was lower than against P.a. The diminished killing activity was parallel with decreased tumor necrosis factor-α production by AMs compared with A.b. Inducible nitric oxide synthase inhibitor, S-methylisothiourea, decreased the total cell number in BALF on mice receiving A.b. instillation and ventilation. Moreover, MV decreased the A.b. and P.a. killing activity of AMs. MV after A.b. instillation induced less total cells in the BALF and nitrite production in the serum of JNK1-/- mice than those of WT mice.

CONCLUSION

A.b. is potent in inducing neutrophil infiltration in the lungs and total protein in the BALF. MV enhances A.b.-induced lung injury through an increase in the expression of VCAM and IL-6 levels in the BALF and a decrease in the bacteria-killing activity of AMs. A lower inflammation level in JNK1-/- mice indicates that A.b.-induced VAP causes lung injury through JNK signaling pathway in the lungs.

Specific egg yolk immunoglobulin as a promising non-antibiotic biotherapeutic product against Acinetobacter baumannii pneumonia infection

Acinetobacter baumannii is a serious health threat with a high mortality rate. We have already reported prophylactic effects of IgYs raised against OmpA and Omp34 as well as against inactivated whole-cell (IWC) of A. baumannii in a murine pneumonia model. However, the infection was exacerbated in the mice group that received IgYs raised against the combination of OmpA and Omp34. The current study was conducted to propose reasons for the observed antibody-dependent enhancement (ADE) in addition to the therapeutic effect of specific IgYs in the murine pneumonia model. This phenomenon was hypothetically attributed to topologically inaccessible similar epitopes of OmpA and Omp34 sharing similarity with peptides of mice proteins. In silico analyses revealed that some inaccessible peptides of OmpA shared similarity with peptides of Omp34 and Mus musculus. Specific anti-OmpA and anti-Omp34 IgYs cross-reacted with Omp34 and OmpA respectively. Specific IgYs showed different protectivity against A. baumannii AbI101 in the murine pneumonia model. IgYs triggered against OmpA or IWC of A. baumannii were the most protective antibodies. IgY triggered against Omp34 is ranked next after those against OmpA. The lowest protection was observed in mice received IgYs raised against the combination of rOmpA and rOmp34. In conclusion, specific IgYs against OmpA, Omp34, and IWC of A. baumannii could serve as novel biotherapeutics against A. baumannii pneumonia.

Recent Advances in the Pursuit of an Effective Acinetobacter baumannii Vaccine

Acinetobacter baumannii has been a major cause of nosocomial infections for decades. The absence of an available vaccine coupled with emerging multidrug resistance has prevented the medical community from effectively controlling this human pathogen. Furthermore, the ongoing pandemic caused by SARS-CoV-2 has increased the risk of hospitalized patients developing ventilator-associated pneumonia caused by bacterial opportunists including A. baumannii. The shortage of antibiotics in the development pipeline prompted the World Health Organization to designate A. baumannii a top priority for the development of new medical countermeasures, such as a vaccine. There are a number of important considerations associated with the development of an A. baumannii vaccine, including strain characteristics, diverse disease manifestations, and target population. In the past decade, research efforts have revealed a number of promising new immunization strategies that could culminate in a safe and protective vaccine against A. baumannii. In this review, we highlight the recent progress in the development of A. baumannii vaccines, discuss potential challenges, and propose future directions to achieve an effective intervention against this human pathogen.

Host Innate Immune Responses to Acinetobacter baumannii Infection

Acinetobacter baumannii has emerged as a major threat to global public health and is one of the key human pathogens in healthcare (nosocomial and community-acquired)-associated infections. Moreover, A. baumannii rapidly develops resistance to multiple antibiotics and is now globally regarded as a serious multidrug resistant pathogen. There is an urgent need to develop novel vaccines and immunotherapeutics as alternatives to antibiotics for clinical management of A. baumannii infection. However, our knowledge of host immune responses to A. baumannii infection and the identification of novel therapeutic targets are significantly lacking. This review highlights the recent advances and critical gaps in our understanding how A. baumannii interacts with the host innate pattern-recognition receptors, induces a cascade of inflammatory cytokine and chemokine responses, and recruits innate immune effectors (such as neutrophils and macrophages) to the site of infection for effective control of the infection. Such knowledge will facilitate the identification of new targets for the design and development of effective therapeutics and vaccines to fight this emerging threat.

Blp1 protein shows virulence-associated features and elicits protective immunity to Acinetobacter baumannii infection

BACKGROUND

Multidrug resistant Acinetobacter baumannii is one of the major infection agents causing nosocomial pneumonia. Therefore, new therapeutic approaches against this bacterium are needed. Surface-exposed proteins from bacterial pathogens are implicated in a variety of virulence-related traits and are considered as promising candidates for vaccine development.

RESULTS

We show in this study that a large Blp1 protein from opportunistic pathogen A. baumannii is encoded in all examined clinical strains of globally spread international clonal lineages I (IC I) and II (IC II). The two blp1 gene variants exhibit lineage-specific distribution profile. By characterization of blp1 deletion mutants and their complementation with blp1 alleles we show that blp1 gene is required for A. baumannii biofilm formation and adhesion to epithelial cells in IC I strain but not in the IC II strain. Nevertheless both alleles are functional in restoring the deficient phenotypes of IC I strain. Moreover, the blp1 gene is required for the establishing of A. baumannii virulence phenotype in nematode and murine infection models. Additionally, we demonstrate that C-terminal 711 amino acid fragment of Blp1 elicits an efficient protection to lethal A. baumannii infection in a murine model using active and passive immunization approaches. Antiserum obtained against Blp1-specific antigen provides opsonophagocytic killing of A. baumannii in vitro.

CONCLUSIONS

Lineage-specific variants of surface-exposed components of bacterial pathogens complicate the development of new therapeutic approaches. Though we demonstrated different impact of Blp1 variants on adherence of IC I and IC II strains, Blp1-specific antiserum neutralized A. baumannii strains of both clonal lineages. Together with the observed increased survival rate in vaccinated mice these results indicate that A. baumannii Blp1 protein could be considered as a new vaccine candidate.

Heme uptake and utilization by hypervirulent Acinetobacter baumannii LAC-4 is dependent on a canonical heme oxygenase (abHemO)

Acinetobacter baumannii is an opportunistic pathogen that causes serious infections in critically ill and immune compromised patients. The ability to acquire iron from the hosts iron and heme containing proteins is critical to their survival and virulence. The majority of A. baumannii hypervirulent strains encode a heme uptake system that includes a putative heme oxygenase (hemO). Despite reports indicating A. baumannii can grow on heme direct evidence of extracellular heme uptake and metabolism has not been shown. Through isotopic labeling (¹³C-heme) we show the hypervirulent A. baumannii LAC-4 metabolizes heme to biliverdin IXα (BVIXα), whereas ATC 17978 that lacks the hemO gene cluster cannot efficiently utilize heme. Expression and purification of the protein encoded by the A. baumannii LAC-4 hemO gene confirmed catalytic conversion of heme to BVIX. We further show inhibition of abHemO with previously characterized P. aeruginosa HemO inhibitors in a fluorescence based assay that couples HemO catalytic activity to the BVIXα binding phytochrome IFP1.4. Furthermore, the hemO gene cluster encodes genes with homology to heme-dependent extra cytoplasmic function (ECF) σ factor systems. The hemophore-dependent ECF system in Pseudomonas aeruginosa has been shown to play a critical role in heme sensing and virulence within the host. The prevalence of a hemO gene cluster in A. baumannii LAC4 and other hypervirulent strains suggests it is required within the host to adapt and utilize heme and is a major contributor to virulence.

The Mechanisms of Disease Caused by Acinetobacter baumannii

Acinetobacter baumannii is a Gram negative opportunistic pathogen that has demonstrated a significant insurgence in the prevalence of infections over recent decades. With only a limited number of “traditional” virulence factors, the mechanisms underlying the success of this pathogen remain of great interest. Major advances have been made in the tools, reagents, and models to study A. baumannii pathogenesis, and this has resulted in a substantial increase in knowledge. This article provides a comprehensive review of the bacterial virulence factors, the host immune responses, and animal models applicable for the study of this important human pathogen. Collating the most recent evidence characterizing bacterial virulence factors, their cellular targets and genetic regulation, we have encompassed numerous aspects important to the success of this pathogen, including membrane proteins and cell surface adaptations promoting immune evasion, mechanisms for nutrient acquisition and community interactions. The role of innate and adaptive immune responses is reviewed and areas of paucity in our understanding are highlighted. Finally, with the vast expansion of available animal models over recent years, we have evaluated those suitable for use in the study of Acinetobacter disease, discussing their advantages and limitations.

MF59 adjuvant enhances the immunogenicity and protective immunity of the OmpK/Omp22 fusion protein from Acineterbacter baumannii through intratracheal inoculation in mice

Acinetobacter baumannii (A baumannii) is an emerging nosocomial pathogenic bacterium which leads to hospital infections. The increase in drug-resistant A baumannii strains makes it difficult to control by using common antibiotics. The development of effective vaccines is an alternative means to avoid A baumannii infections. In the present study, Balb/c mice were inoculated intratracheally with 30 μg of OmpK/Omp22 fusion protein alone or OmpK/Omp22 formulated with MF59 adjuvant. After two times of boosting at day 14 and 21, the antigen-specific antibody levels and the protective immunity against A baumannii challenge were evaluated. The results showed that the OmpK/Omp22 formulated with MF59 immunized mice produced much higher level of antigen-specific antibodies compared to mice immunized with OmpK/Omp22 alone (P < 0.01). Mice immunized with 30 μg of OmpK/Omp22 formulated with MF59 also provided more potent protection post-challenge, which showed lower bacterial loads in the blood and lung tissue, lower level of blood inflammatory cytokines and higher survival rate (83.3%) than mice immunized with OmpK/Omp22 alone (P < 0.001). In conclusion, this study demonstrated that OmpK/Omp22 fusion protein adjuvanted with MF59 induced superior immune response and better protection than OmpK/Omp22 alone through intratracheal inoculation in mice.

Vaccines against major ICU pathogens: where do we stand?

PURPOSE OF REVIEW

Multidrug resistance of bacterial pathogens has confronted physicians around the world with the threat of inefficacy of the antibiotic regime, which is particularly important for patients with sepsis. Antibiotic resistance has revived search for alternative nonantibiotic strategies. Among them, prophylaxis by vaccination is an appealing concept.

RECENT FINDINGS

This review provides a compact overview on available vaccines against community-acquired pathogens such as pneumococci (in synergy with influenza) and meningococci and provides an overview on the ongoing developments of vaccines targeting typical nosocomial pathogens such as Clostridium difficile, Staphylococcus aureus, Acintetobacter baumannii, Klebsiella pneumonia, and Pseudomonas aeruginosa.

SUMMARY

The effects achieved by some conjugated vaccines (e.g. against Haemophilus influenzae B and Streptococcus pneumoniae) are encouraging. Their widespread use has resulted in a decrease or almost elimination of invasive diseases by the covered pneumococcal serotypes or Haemophilus influenzae B, respectively. These vaccines confer not only individual protection but also exploit herd protection effects. However, a multitude of failures reflects the obstacles on the way to effective and well tolerated bacterial vaccines. Regional differences in strain prevalence and variability of antigens that limit cross-protectivity remain major obstacles. However, promising candidates are in clinical development.

Vaccination with a live attenuated Acinetobacter baumannii deficient in thioredoxin provides protection against systemic Acinetobacter infection

Multi-drug resistant Acinetobacter baumannii (MDR-Ab), an opportunistic pathogen associated with nosocomial and combat related infections, has a high mortality due to its virulence and limited treatment options. Deletion of the thioredoxin gene (TrxA) from a clinical isolate of MDR-Ab resulted in a 100-fold increase in 50% lethal dose (LD₅₀) in a systemic challenge murine model. Thus, we investigated the potential use of this attenuated strain as a live vaccine against MDR-Ab. Mice were vaccinated by subcutaneous (s.c.) injection of 2×10⁵ CFU of the ΔtrxA mutant, boosted 14days later with an equivalent inoculum, and then challenged 30days post-vaccination by i.p. injection with 10 LD₅₀ of the wild type (WT) Ci79 strain. Efficacy of vaccination was evaluated by monitoring MDR-Ab specific antibody titers and cytokine production, observing pathology and organ burdens after WT challenge, and measuring levels of serum pentraxin-3, a molecular correlate of A. baumannii infection severity, before and after challenge. Mice vaccinated with ΔtrxA were fully protected against the lethal challenge of WT. However, minimal immunoglobulin class switching was observed with IgM predominating. Spleens harvested from vaccinated mice exhibited negligible levels of IL-4, IFN-γ and IL-17 production when stimulated with UV-inactivated WT Ci79. Importantly, tissues obtained from vaccinated mice displayed reduced pathology and organ burden compared to challenged non-vaccinated mice. Additionally, serum pentraxin-3 concentrations were not increased 24h after challenge in vaccinated mice, correlating with reduction of WT MDR-Ab infection in ΔtrxA immunized mice. Furthermore, passive immunization with ΔtrxA-immune sera provided protection against lethal systemic Ci79 challenge. Collectively, the defined live attenuated ΔtrxA strain is a vaccine candidate against emerging MDR Acinetobacter infection.

Current advances and challenges in the development of Acinetobacter vaccines

Acinetobacter baumannii is a major cause of healthcare-associated infections worldwide with high morbidity and mortality. The clinical treatment of A. baumannii infections has become increasingly difficult because of the rapid emerging of multidrug and extremely drug resistant strains. Thus, there is an urgent need for the development of novel intervention strategies to combat this multidrug-resistant pathogen. Vaccine is one of the most effective medical measures for infection control and is likely to overcome the development of multidrug resistance by A. baumannii. Here we discussed the recent advances and potential challenges in development of A. baumannii vaccines with a focus on the 3 most important steps in the preclinical vaccine development: antigen selection, immune correlates of protection, and animal models for efficacy evaluation.

Host resistance to intranasal Acinetobacter baumannii reinfection in mice

Acinetobacter baumannii is a major causative agent of healthcare-associated infection and develops multidrug resistance rapidly. However, little is known in the host defense mechanisms against this infection. In this study, we examined if mice recovered from a previous intranasal A. baumannii infection (recovered mice) are fully protected against a subsequent reinfection. We found that, despite the presence of specific serum IgG and mucosal IgA responses prior to the reinfection, the recovered mice were only marginally better protected against intranasal challenge with low doses of homologous or heterologous A. baumannii strains than the naïve mice. Post-challenge immune and inflammatory (cells and cytokines) responses were generally comparable between recovered and naïve mice although the recovered mice produced significantly higher amounts of IFN-γ and IL-17 and had higher percentages and numbers of resident lung CD44(hi)CD62L(-)CD4(+) and CD19(+) B lymphocytes. Taken together, our results suggest that mice recovered from a previous A. baumannii infection remain susceptible to reinfection, indicating the complexity of immune protection mechanism for this Gram-negative, multidrug-resistant emerging pathogen.

Hemagglutinin and neuraminidase containing virus-like particles produced in HEK-293 suspension culture: An effective influenza vaccine candidate

Virus-like particles (VLPs) constitute a promising alternative as influenza vaccine. They are non-replicative particles that mimic the morphology of native viruses which make them more immunogenic than classical subunit vaccines. In this study, we propose HEK-293 cells in suspension culture in serum-free medium as an efficient platform to produce large quantities of VLPs. For this purpose, a stable cell line expressing the main influenza viral antigens hemagglutinin (HA) and neuraminidase (NA) (subtype H1N1) under the regulation of a cumate inducible promoter was developed (293HA-NA cells). The production of VLPs was evaluated by transient transfection of plasmids encoding human immunodeficiency virus (HIV) Gag or M1 influenza matrix protein. To facilitate the monitoring of VLPs production, Gag was fused to the green fluorescence protein (GFP). The transient transfection of the gag containing plasmid in 293HA-NA cells increased the release of HA and NA seven times more than its counterpart transfected with the M1 encoding plasmid. Consequently, the production of HA-NA containing VLPs using Gag as scaffold was evaluated in a 3-L controlled stirred tank bioreactor. The VLPs secreted in the culture medium were recovered by ultracentrifugation on a sucrose cushion and ultrafiltered by tangential flow filtration. Transmission electron micrographs of final sample revealed the presence of particles with the average typical size (150-200nm) and morphology of HIV-1 immature particles. The concentration of the influenza glycoproteins on the Gag-VLPs was estimated by single radial immunodiffusion and hemagglutination assay for HA and by Dot-Blot for HA and NA. More significantly, intranasal immunization of mice with influenza Gag-VLPs induced strong antigen-specific mucosal and systemic antibody responses and provided full protection against a lethal intranasal challenge with the homologous virus strain. These data suggest that, with further optimization and characterization the process could support mass production of safer and better-controlled VLPs-based influenza vaccine candidate.

Immunization with a 22-kDa outer membrane protein elicits protective immunity to multidrug-resistant Acinetobacter baumannii

A. baumannii infections are becoming more and more serious health issues with rapid emerging of multidrug and extremely drug resistant strains, and therefore, there is an urgent need for the development of nonantibiotic-based intervention strategies. This study aimed at identifying whether an outer membrane protein with molecular weight of about 22 kDa (Omp22) holds the potentials to be an efficient vaccine candidate and combat A. baumannii infection. Omp22 which has a molecule length of 217 amino acids kept more than 95% conservation in totally 851 reported A. baumannii strains. Recombinant Omp22 efficiently elicited high titers of specific IgG in mice. Both active and passive immunizations of Omp22 increased the survival rates of mice, suppressed the bacterial burdens in the organs and peripheral blood, and reduced the levels of serum inflammatory cytokines and chemokines. Opsonophagocytosis assays showed in vitro that Omp22 antiserum had highly efficient bactericidal activities on clonally distinct clinical A. baumannii isolates, which were partly complements-dependent and opsonophagocytic killing effects. Additionally, administration with as high as 500 μg of Omp22 didn’t cause obvious pathological changes in mice. In conclusion, Omp22 is a novel conserved and probably safe antigen for developing effective vaccines or antisera to control A. baumannii infections.

Identification of novel vaccine candidates against Acinetobacter baumannii using reverse vaccinology

Acinetobacter baumannii (Ab) is a global emerging bacterium causing nosocomial infections such as pneumonia, meningitis, bacteremia and soft tissue infections especially in intensive care units. Since Ab is resistant to almost all conventional antibiotics, it is now one of the 6 top-priorities of the dangerous microorganisms listed by the Infectious Disease Society of America. The development of vaccine is one of the most promising and cost-effective strategies to prevent infections. In this study, we identified potential protective vaccine candidates using reverse vaccinology. We have analyzed 14 on-line available Ab genome sequences and found 2752 homologous core genes. Using information obtained from immuno-proteomic experiments, published proteomic information and the bioinformatics PSORTb v3.0 software to predict the location of extracellular and/or outer membrane proteins, 77 genes were identified and selected for further studies. After excluding those antigens have been used as vaccine candidates reported by the in silico search-engines of PubMed and Google Scholar, 13 proteins could potentially be vaccine candidates. We have selected and cloned the genes of 3 antigens that were further expressed and purified. These antigens were found to be highly immunogenic and conferred partial protection (60%) in a pneumonia animal model. The strategy described in the present study incorporates the advantages of reverse vaccinology, bioinformatics and immuno-proteomic platform technologies and is easy to perform to identify novel immunogens for multi-component vaccines development.

OmpW is a potential target for eliciting protective immunity against Acinetobacter baumannii infections

Acinetobacter baumannii (A. baumannii) is an important conditioned pathogen that causes nosocomial and community-associated infections. In this study, we sought to investigate whether outer membrane protein W (OmpW) is a potential target for eliciting protective immunity against A. baumannii infections. Mice immunized with the fusion protein thioredoxin-OmpW generated strong OmpW-specific IgG responses. In a sepsis model, both active and passive immunizations against OmpW effectively protected mice from A. baumannii infections. This protection was demonstrated by a significantly improved survival rate, reduced bacterial burdens within organs, and the suppressed accumulation of inflammatory cytokines and chemokines in sera. Opsonophagocytic assays with murine macrophage RAW264.7 cells indicated that the bactericidal effects of the antisera derived from the immunized mice are mediated synergistically by specific antibodies and complement components. The antisera presented significant opsonophagocytic activities against homologous strains and clonally distinct clinical isolates in vitro. Protein data analysis showed that the sequence of OmpW, which has a molecule length of 183 amino acids, is more than 91% conserved in reported A. baumannii strains. In conclusion, we identified OmpW as a highly immunogenic and conserved protein as a valuable antigen candidate for the development of an effective vaccine or the preparation of antisera to control A. baumannii infections.

Code blue: Acinetobacter baumannii, a nosocomial pathogen with a role in the oral cavity

Actinetobacter baumannii is an important nosocomial pathogen that can cause a wide range of serious conditions including pneumonia, meningitis, necrotizing fasciitis and sepsis. It is also a major cause of wound infections in military personnel injured during the conflicts in Afghanistan and Iraq, leading to its popular nickname of 'Iraqibacter'. Contributing to its success in clinical settings is resistance to environmental stresses such as desiccation and disinfectants. Moreover, in recent years there has been a dramatic increase in the number of A. baumannii strains with resistance to multiple antibiotic classes. Acinetobacter baumannii is an inhabitant of oral biofilms, which can act as a reservoir for pneumonia and chronic obstructive pulmonary disease. Subgingival colonization by A. baumannii increases the risk of refractory periodontitis. Pathogenesis of the organism involves adherence, biofilm formation and iron acquisition. In addition, A. baumannii can induce apoptotic cell death in epithelial cells and kill hyphal forms of Candida albicans. Virulence factors that have been identified include pili, the outer membrane protein OmpA, phospholipases and extracellular polysaccharide. Acinetobacter baumannii can sense blue light through a blue-light sensing using flavin (BLUF) domain protein, BlsA. The resulting conformational change in BlsA leads to changes in gene expression, including virulence genes.