Pigs aerogenously immunized with genetically inactivated (ghosts) or irradiated Actinobacillus pleuropneumoniae are protected against a homologous aerosol challenge despite differing in pulmonary cellular and antibody responses

Bactofection, Bacterial-Mediated Vaccination, and Cancer Therapy: Current Applications and Future Perspectives

From the first report in 1891 by Dr. Coley of the effective treatment of tumors in 1000 patients with Streptococcus and the first successful use of bacterial vectors for transferring therapeutic genes in 1980 by Dr. Schnaffer, bactofection has been shown to be a promising strategy in the fields of vaccination, gene therapy, and cancer therapy. This review describes the general theory of bactofection and its advantages, disadvantages, challenges, and expectations, compiling the most notable advances in 14 vaccination studies, 27 cancer therapy studies, and 13 clinical trials. It also describes the current scope of bactofection and promising results. The extensive knowledge of Salmonella biology, as well as the multiple adequacies of the Ty21a vaccination platform, has allowed notable developments worldwide that have mainly been reflected in therapeutic efforts against cancer. In this regard, we strongly recommend the creation of a recombinant Ty21a model that constitutively expresses the GtgE protease from S. typhimurium, allowing this vector to be used in animal trials, thus enhancing the likelihood of favorable results that could quickly transition to clinical trials. From the current perspective, it is necessary to explore a greater diversity of bacterial vectors and find the best combination of implemented attenuations, generating personalized models that guarantee the maximum effectiveness in cancer therapy and vaccination.

Development of thermostable vaccine adjuvants

INTRODUCTION

The importance of vaccine thermostability has been discussed in the literature. Nevertheless, the challenge of developing thermostable vaccine adjuvants has sometimes not received appropriate emphasis. Adjuvants comprise an expansive range of particulate and molecular compositions, requiring innovative thermostable formulation and process development approaches.

AREAS COVERED

Reports on efforts to develop thermostable adjuvant-containing vaccines have increased in recent years, and substantial progress has been made in enhancing the stability of the major classes of adjuvants. This narrative review summarizes the current status of thermostable vaccine adjuvant development and looks forward to the next potential developments in the field.

EXPERT OPINION

As adjuvant-containing vaccines become more widely used, the unique challenges associated with developing thermostable adjuvant formulations merit increased attention. In particular, more focused efforts are needed to translate promising proof-of-concept technologies and formulations into clinical products.

The development of ghost vaccines trials

INTRODUCTION

Bacterial ghosts are intact bacterial cell envelopes that are emptied of their content by gentle biological or chemical poring methods. Ghost techniques increase the safety of the killed vaccines, while maintaining their antigenicity due to mild preparation procedures. Moreover, ghost-platforms may express and/or carry several antigens or plasmid-DNA encoding for protein epitopes.

AREAS COVERED

In this review, the development in ghost-vaccine production over the last 30 years is classified and discussed. The different applications of ghost-vaccines, how they trigger the immune system, their advantages and limitations are displayed. The phage-mediated lysis, molecular manipulation of the lysis-genes, and the biotechnological production of ghosts are described. The trials are classified according to the pattern of lysis and to the type of bacteria. Further subdivision includes chronological ordered application of the ghost as alternative-killed vaccine, recombinant antigen platform, plasmid DNA carrier, adjuvants, and dendritic cell inducer. Particular trials for specific pathogens or from distinct research schools are gathered.

EXPERT OPINION

Ghosts are highly qualified to act as immune-presenting platforms that express and/or carry several recombinant and DNA vaccines, as well as, being efficient alternative-killed vaccines. The coming years will show more molecular advances to develop ghost-production and to express more antigens.

Harnessing the Immunomodulatory Properties of Bacterial Ghosts to Boost the Anti-mycobacterial Protective Immunity

Tuberculosis (TB) pathogenesis is characterized by inadequate immune cell activation and delayed T cell response in the host. Recent immunotherapeutic efforts have been directed at stimulating innate immunity and enhancing interactions between antigen presenting cells and T cells subsets to improve the protective immunity against TB. In this study, we investigated the immunostimulatory properties of bacterial ghosts (BG) as a novel approach to potentiate the host immunity against mycobacterial infection. BG are intact cytoplasm-free Escherichia coli envelopes and have been developed as bacterial vaccines and adjuvant/delivery system in cancer immunotherapy. However, BG have yet to be exploited as immunopotentiators in the context of infectious diseases. Here, we showed that BG are potent inducers of dendritic cells (DC), which led to enhanced T cell proliferation and differentiation into effector cells. BG also induced macrophage activation, which was associated with enhanced nitric oxide production, a key anti-mycobacterial weapon. We further demonstrated that the immunostimulatory capability of BG far exceeds that of LPS and involves both TLR4-dependent and independent pathways. Consistently, BG treatment, but not LPS treatment, reduced the bacterial burden in infected mice, which correlated with increased influx of innate and adaptive effector immune cells and increased production of key cytokines in the lungs. Finally and importantly, enhanced bacilli killing was seen in mice co-administered with BG and second-line TB drugs bedaquiline and delamanid. Overall, this work paves the way for BG as potent immunostimulators that may be harnessed to improve mycobacteria killing at the site of infection.

Subcutaneous Immunization of Dogs With Bordetella bronchiseptica Bacterial Ghost Vaccine

The Bordetella species are Gram-negative bacterial pathogens that colonizes mammalian respiratory tract causing respiratory diseases in humans and animals. B. bronchiseptica causes clinical conditions in many mammals including immunocompromised humans. Using the dog model of respiratory infection, it has been shown in this study that a newly developed B. bronchiseptica Bacterial Ghost (BbBG) vaccine exhibited significant protection in the face of a severe pathogenic bacterial challenge in seronegative dogs. The protein E-specific lysis mechanism was used to produce BbBGs. Bacterial Ghosts (BGs) are the empty cell envelope of Gram-negative bacterium. They are genetically processed to form a microscopic hole in their membrane, through which all the cytoplasmic contents are expelled leaving behind intact empty bacterial shells. Due to the intact surface structures of BGs, they offer the safety of inactivated but efficacy of live attenuated vaccines. In this study, seronegative dogs were vaccinated subcutaneously (s/c) with two different doses of a newly developed BbBG vaccine [lower 10^∧5 (BbBG – 5) and higher 10^∧7 (BbBG – 7)] on day 0 and 21. The animals were challenged (by aerosol) with virulent live B. bronchiseptica strains 41 days after first vaccination. The dogs vaccinated s/c with BbBG – 7 vaccine had significantly lower spontaneous coughing scores (P = 0.0001) than dogs in negative control group. Furthermore, the tested BbBG – 7 vaccine was equivalent to the positive control vaccine Bronchicine CAe in terms of safety and efficacy. For the first time, we report the successful use of liquid formulated BGs vaccines in animal studies. Earlier reported studies using BGs vaccines were performed with resuspended freeze-dried BGs preparations.

Development of a dual-expression vector facilitated with selection-free PCR recombination cloning strategy

The conventional procedure for the construction of recombinant expression vector of a target gene includes PCR cloning and restriction enzyme mediated subcloning, which is time-consuming and sometimes troublesome because of the inefficiency of ligation. A variety of ligase-independent PCR cloning strategies have been developed, but they either involve complicated PCR procedures or need other DNA modifying enzymes. In this study, we report the design, and construction of an omnipotent expression vector pOmni, with which a target gene can be easily cloned through innovative selection-free PCR recombination cloning strategy with only one pair of primer and two times of PCR in one work day, without using any restriction enzymes, ligase and other DNA modifying enzymes. Furthermore, the target gene cloned in pOmni is ready to be high-efficiently expressed in either Escherichia coli cells or eukaryotic cells because of the elaborate design of compatible T7 promoter and CMV promoter expression elements in the vector. The cloning capability and reliability of selection-free PCR recombination cloning with pOmni were validated through cloning of 6 DNA fragments with length from 315 to 4557 bp, and the dual-expression function of the vector was verified through the cloning and expression of EGFP in E. coli BL21 and HeLa cells. pOmni developed in our study provides a powerful tool for gene cloning and expression, and is of special value for researches in which both prokaryotic and eukaryotic expression of a target gene are necessary.

Immunoprotective Efficacy of Six In vivo-Induced Antigens against Actinobacillus pleuropneumoniae as Potential Vaccine Candidates in Murine Model

Six in vivo-induced (IVI) antigens—RnhB, GalU, GalT, Apl_1061, Apl_1166, and HflX were selected for a vaccine trial in a mouse model. The results showed that the IgG levels in each immune group was significantly higher than that of the negative control (P < 0.001). Except rRnhB group, proliferation of splenocytes was observed in all immunized groups and a relatively higher proliferation activity was observed in rGalU and rGalT groups (P < 0.05). In the rGalT vaccinated group, the proportion of CD4+ T cells in spleen was significant higher than that of negative control (P < 0.05). Moreover, proportions of CD4+ T cells in other vaccinated groups were all up-regulated to varying degrees. Up-regulation of both Th1 (IFN-γ, IL-2) and Th2 (IL-4) cytokines were detected. A survival rate of 87.5, 62.5, and 62.5% were obtained among rGalT, rAPL_1166, and rHflX group, respectively while the remaining three groups was only 25%. Histopathological analyses of lungs indicated that surviving animals from the vaccinated groups showed relatively normal pulmonary structure alveoli. These findings confirm that IVI antigens used as vaccine candidates provide partial protection against Actinobacillus pleuropneumoniae infection in a mouse model, which could be used as potential vaccine candidates in piglets.

Delivery of a Chlamydial Adhesin N-PmpC Subunit Vaccine to the Ocular Mucosa Using Particulate Carriers

Trachoma, caused by the intracellular bacterium Chlamydia trachomatis (Ct), remains the world's leading preventable infectious cause of blindness. Recent attempts to develop effective vaccines rely on modified chlamydial antigen delivery platforms. As the mechanisms engaged in the pathology of the disease are not fully understood, designing a subunit vaccine specific to chlamydial antigens could improve safety for human use. We propose the delivery of chlamydia-specific antigens to the ocular mucosa using particulate carriers, bacterial ghosts (BGs). We therefore characterized humoral and cellular immune responses after conjunctival and subcutaneous immunization with a N-terminal portion (amino acid 1-893) of the chlamydial polymorphic membrane protein C (PmpC) of Ct serovar B, expressed in probiotic Escherichia coli Nissle 1917 bacterial ghosts (EcN BGs) in BALB/c mice. Three immunizations were performed at two-week intervals, and the immune responses were evaluated two weeks after the final immunization in mice. In a guinea pig model of ocular infection animals were immunized in the same manner as the mice, and protection against challenge was assessed two weeks after the last immunization. N-PmpC was successfully expressed within BGs and delivery to the ocular mucosa was well tolerated without signs of inflammation. N-PmpC-specific mucosal IgA levels in tears yielded significantly increased levels in the group immunized via the conjunctiva compared with the subcutaneously immunized mice. Immunization with N-PmpC EcN BGs via both immunization routes prompted the establishment of an N-PmpC-specific IFNγ immune response. Immunization via the conjunctiva resulted in a decrease in intensity of the transitional inflammatory reaction in conjunctiva of challenged guinea pigs compared with subcutaneously and non-immunized animals. The delivery of the chlamydial subunit vaccine to the ocular mucosa using a particulate carrier, such as BGs, induced both humoral and cellular immune responses. Further investigations are needed to improve the immunization scheme and dosage.

Irradiated wild-type and Spa mutant Staphylococcus aureus induce anti-S. aureus immune responses in mice which do not protect against subsequent intravenous challenge

Staphylococcus aureus remains an important human and animal pathogen. Its pathogenicity is determined in part by expression of the Spa-immune subversion protein, neutralising the activity of which provides partial protection in murine models, as does experimental infection with live S. aureus with Spa gene deletions followed by antibiotic-mediated cure in mice. Together, these data raise the question of whether Spa mutant S. aureus might represent a viable vaccine. Here, we find that gamma-irradiated S. aureus strains, both wild-type and null mutant of spa, are immunogenic in mice when administered intramuscularly, eliciting large amounts of anti-S. aureus antibodies, as judged by whole-cell immunoassay on fixed microorganisms. We used an intravenous challenge system to assess vaccine efficacy, the sensitivity of which was increased by studying renal bacterial concentrations in both kidneys. Despite this, protection from intravenous challenge was not observed (mean difference between vaccinated and unvaccinated mice 0.27 log(10) with 95% confidence interval -0.922 to 1.467). Surprisingly, antibody responses elicited against a panel of protective cell surface proteins were very low, indicating that most antibody induced is not protective. Additionally, these data suggest a limited role for irradiated wild-type or spa mutant S. aureus as vaccines.

Formulation and delivery of vaccines: Ongoing challenges for animal management

Development of a commercially successful animal vaccine is not only influenced by various immunological factors, such as type of antigen but also by formulation and delivery aspects. The latter includes the need for a vector or specific delivery system, the choice of route of administration and the nature of the target animal population and their habitat. This review describes the formulation and delivery aspects of various types of antigens such as killed microorganisms, proteins and nucleic acids for the development of efficacious and safe animal vaccines. It also focuses on the challenges associated with the different approaches that might be required for formulating and delivering species specific vaccines, particularly if their intended use is for improved animal management with respect to disease and/or reproductive control.

Immunoproteomic analysis of bacterial proteins of Actinobacillus pleuropneumoniae serotype 1

Background

Actinobacillus pleuropneumoniae (APP) is one of the most important swine pathogens worldwide. Identification and characterization of novel antigenic APP vaccine candidates are underway. In the present study, we use an immunoproteomic approach to identify APP protein antigens that may elicit an immune response in serotype 1 naturally infected swine and serotype 1 virulent strain S259-immunized rabbits.

Results

Proteins from total cell lysates of serotype 1 APP were separated by two-dimensional electrophoresis (2DE). Western blot analysis revealed 21 immunoreactive protein spots separated in the pH 4-7 range and 4 spots in the pH 7-11 range with the convalescent sera from swine; we found 5 immunoreactive protein spots that separated in the pH 4-7 range and 2 in the pH 7-11 range with hyperimmune sera from S259-immunized rabbits. The proteins included the known antigens ApxIIA, protective surface antigen D15, outer membrane proteins P5, subunit NqrA. The remaining antigens are being reported as immunoreactive proteins in APP for the first time, to our knowledge.

Conclusions

We identified a total of 42 immunoreactive proteins of the APP serotype 1 virulent strain S259 which represented 32 different proteins, including some novel immunoreactive factors which could be researched as vaccine candidates.

Bacterial Ghosts as antigen and drug delivery system for ocular surface diseases: Effective internalization of Bacterial Ghosts by human conjunctival epithelial cells

The purpose of the presented investigation was to examine the efficiency of the novel carrier system Bacterial Ghosts (BGs), which are empty bacterial cell envelopes of Gram-negative bacteria to target human conjunctival epithelial cells, as well as to test the endocytic capacity of conjunctival cells after co-incubation with BGs generated from different bacterial species, and to foreclose potential cytotoxic effects caused by BGs. The efficiency of conjunctival cells to internalize BGs was investigated using the Chang conjunctival epithelial cell line and primary human conjunctiva-derived epithelial cells (HCDECs) as in vitro model. A high capacity of HCDECs to functionally internalize BGs was detected with the level of internalization depending on the type of species used for BGs generation. Detailed analysis showed no cytotoxic effect of BGs on HCDECs independently of the used bacterial species. Moreover, co-incubation with BGs did not enhance expression of both MHC class I and class II molecules by HCDECs, but increased expression of ICAM-1. The high rates of BG's internalization by HCDECs with no BG-mediated cytotoxic impact designate this carrier system to be a promising candidate for an ocular surface drug delivery system. BGs could be useful for future therapeutic ocular surface applications and eye-specific disease vaccine development including DNA transfer.

Generation of Vibrio anguillarum ghost by coexpression of PhiX 174 lysis E gene and staphylococcal nuclease A gene

Vibrio anguillarum ghosts (VAG) were generated, for the first time, using a conjugation vector containing a ghost bacteria inducing cassette, pRK-lambdaP(R)-cI-Elysis, in which the expression of PhiX174 lysis gene E was controlled by the P ( R )/cI regulatory system of lambda phage. By scanning electron microscopy, holes ranging 80-200 nm in diameter were observed in the VAG. To avoid the presence of bacterial genomic DNA and an antibiotic resistance gene in the final VAG product, we constructed a new dual vector, pRK-lambdaP(R)-cI-E-SNA, containing the E-mediated lysis cassette and the staphylococcal nuclease A (SNA)-mediated DNA degradation cassette, and generated safety-enhanced VAG for use as a fish vaccine.

Selection of serotype-specific vaccine candidate genes in Actinobacillus pleuropneumoniae and heterologous immunization with Propionibacterium acnes

Actinobacillus pleuropneumoniae (A. pleuropneumoniae) is a highly contagious lethal causative agent of swine pleuropneumoniae. Vaccines for this disease are usually serotype specific. In order to identify immunogenic genes specific to serotypes, two differentially expressed gene cDNA libraries of A. pleuropneumoniae CCVC259 (serotype 1) and CCVC263 (serotype 5) had been constructed by using a cDNA representational difference analysis (cDNA-RDA). From the libraries, six potential vaccine candidate genes expressed only in serotype 1 and 13 genes in serotype 5 were identified by antibody screening after gene expression in vitro with a ribosome display system. Eight sequences out of these exhibited 77-100% identity to the corresponding genes in Propionibacterium acnes. The antisera raised against A. pleuropneumoniae serotypes 1 and 5 were reactive with P. acnes at a titer of 1:6400 and vice versa (ELISA titer, 1:3200). Mice immunized with P. acnes were protected against 10 x LD50 challenge with A. pleuropneumoniae serotypes 1 and 5, and the survival rates were 90% and 95%, respectively. Pigs vaccinated with the P. acnes strain could develop high level antibody cross-reacted with A. pleuropneumoniae and obtain noticeable protection from A. pleuropneumoniae infection. These data demonstrate that there were common antigens between A. pleuropneumoniae and P. acnes, and the cross protectivity highlights the possibility of using P. acnes vaccines for preventing infection by A. pleuropneumoniae.

Actinobacillus pleuropneumoniaevaccines: from bacterins to new insights into vaccination strategies

With the growing emergence of antibiotic resistance and rising consumer demands concerning food safety, vaccination to prevent bacterial infections is of increasing relevance.Actinobacillus pleuropneumoniaeis the etiological agent of porcine pleuropneumonia, a respiratory disease leading to severe economic losses in the swine industry. Despite all the research and trials that were performed withA. pleuropneumoniaevaccination in the past, a safe vaccine that offers complete protection against all serotypes has yet not reached the market. However, recent advances made in the identification of new potential vaccine candidates and in the targeting of specific immune responses, give encouraging vaccination perspectives. Here, we review past and current knowledge onA. pleuropneumoniaevaccines as well as the newly available genomic tools and vaccination strategies that could be useful in the design of an efficient vaccine againstA. pleuropneumoniaeinfection.

Bacterial ghosts as adjuvant particles

The development of more advanced and effective vaccines is of great interest in modern medicine. These new-generation vaccines, based on recombinant proteins or DNA, are often less reactogenic and immunogenic than traditional vaccines. Thus, there is an urgent need for the development of new and improved adjuvants. Besides many other immunostimulatory components, the bacterial ghost (BG) system is currently under investigation as a potent vaccine delivery system with intrinsic adjuvant properties. BGs are nonliving cell envelope preparations from Gram-negative cells, devoid of cytoplasmic contents, while their cellular morphology and native surface antigenic structures remain preserved. Owing to the particulate nature of BGs and the fact that they contain many well known immune-stimulating compounds, BGs have the potential to enhance immune responses against ghost-delivered target antigens.

Antigen discovery and delivery of subunit vaccines by nonliving bacterial ghost vectors

The bacterial ghost (BG) platform system is a novel vaccine delivery system endowed with intrinsic adjuvant properties. BGs are nonliving Gram-negative bacterial cell envelopes which are devoid of their cytoplasmic contents, yet maintain their cellular morphology and antigenic structures, including bioadhesive properties. The main advantages of BGs as carriers of subunit vaccines include their ability to stimulate a high immune response and to target the carrier itself to primary antigen-presenting cells. The intrinsic adjuvant properties of BGs enhance the immune response to target antigens, including T-cell activation and mucosal immunity. Since native and foreign antigens can be carried in the envelope complex of BGs, combination vaccines with multiple antigens of diverse origin can be presented to the immune system simultaneously. Beside the capacity of BGs to function as carriers of protein antigens, they also have a high loading capacity for DNA. Thus, loading BGs with recombinant DNA takes advantage of the excellent bioavailability for DNA-based vaccines and the high expression rates of the DNA-encoded antigens in target cell types such as macrophages and dendritic cells. There are many spaces within BGs including the inner and outer membranes, the periplasmic space and the internal lumen which can carry antigens, DNA or mediators of the immune response. All can be used for subunit antigen to design new vaccine candidates with particle presentation technology. In addition, the fact that BGs can also carry piggyback large-size foreign antigen particles, increases the technologic usefulness of BGs as combination vaccines against viral and bacterial pathogens. Furthermore, the BG antigen carriers can be stored as freeze-dried preparations at room temperature for extended periods without loss of efficacy. The potency, safety and relatively low production cost of BGs offer a significant technical advantage over currently utilized vaccine technologies.

Bacterial ghosts as antigen delivery vehicles

The bacterial ghost system is a novel vaccine delivery system unusual in that it combines excellent natural intrinsic adjuvant properties with versatile carrier functions for foreign antigens. The efficient tropism of bacterial ghosts (BG) for antigen presenting cells promotes the generation of both cellular and humoral responses to heterologous antigens and carrier envelope structures. The simplicity of both BG production and packaging of (multiple) target antigens makes them particularly suitable for use as combination vaccines. Further advantages of BG vaccines include a long shelf-life without the need of cold-chain storage due to their freeze-dried status, they are safe as they do not involve host DNA or live organisms, they exhibit improved potency with regard to target antigens compared to conventional approaches, they are versatile with regards to DNA or protein antigen choice and size, and as a delivery system they offer high bioavailability.

Construction of an Actinobacillus pleuropneumoniae serotype 2 prototype live negative-marker vaccine

Deletions were introduced into the ureC and apxIIA genes of an Actinobacillus pleuropneumoniae serotype 2 strain by homologous recombination and counterselection. The double-mutant contains no foreign DNA, is highly attenuated, protects pigs from homologous challenge upon a single aerosol application, and facilitates the serological discrimination of immunized and infected herds.

Extended recombinant bacterial ghost system

Controlled expression of cloned PhiX174 gene E in Gram-negative bacteria results in lysis of the bacteria by formation of an E-specific transmembrane tunnel structure built through the cell envelope complex. Bacterial ghosts from a variety of bacteria are used as non-living candidate vaccines. In the recombinant ghost system, foreign proteins are attached on the inside of the inner membrane as fusions with specific anchor sequences. Ghosts have a sealed periplasmic space and the export of proteins into this space vastly extends the capacity of ghosts or recombinant ghosts to function as carriers of foreign antigens. In addition, S-layer proteins forming shell-like self assembly structures can be expressed in candidate vaccine strains prior to E-mediated lysis. Such recombinant S-layer proteins carrying foreign epitopes further extend the possibilities of ghosts as carriers of foreign epitopes. As ghosts have inherent adjuvant properties, they can be used as adjuvants in combination with subunit vaccines. Subunits or other ligands can also be coupled to matrixes like dextran which are used to fill the internal lumen of ghosts. Oral, aerogenic or parenteral immunization of experimental animals with recombinant ghosts induced specific humoral and cellular immune responses against bacterial and target components including protective mucosal immunity. The most relevant advantage of recombinant bacterial ghosts as immunogens is that no inactivation procedures that denature relevant immunogenic determinants are employed in this production. This fact explains the superior quality of ghosts when compared to other inactivated vaccines. The endotoxic component of the outer membrane does not limit the use of ghosts as vaccine candidates but triggers the release of several potent immunoregulatory cytokines. As carriers, there is no limitation in the size of foreign antigens that can be inserted in the membrane and the capacity of all spaces including the membranes, peri-plasma and internal lumen of the ghosts can be fully utilized. This extended recombinant ghost system represents a new strategy for adjuvant free combination vaccines.