Bacterial ghosts as an oral vaccine: a single dose of Escherichia coli O157:H7 bacterial ghosts protects mice against lethal challenge

A Novel Vaccine for Bovine Diarrhea Complex Utilizing Recombinant Enterotoxigenic Escherichia coli and Salmonella Expressing Surface-Displayed Chimeric Antigens from Enterohemorrhagic Escherichia coli O157:H7

BACKGROUND/OBJECTIVES

Enterohemorrhagic Escherichia coli (EHEC) O157:H7, a zoonotic pathogen primarily found in cattle, causes Hemolytic Uremic Syndrome (HUS) in humans, often through contaminated food. Its Type Three Secretion System (T3SS) facilitates gut colonization. In contrast, neonatal calf diarrhea (NCD) is mainly caused by pathogens like enterotoxigenic Escherichia coli (ETEC), Salmonella spp., Bovine Coronavirus (BCoV), and Bovine Rotavirus type A (BRoVA). This study engineered a chimeric protein combining EspB and Int280γ, two T3SS components, expressed in the membranes of Salmonella Dublin and ETEC.

METHODS

Immune responses in vaccinated mice and guinea pigs were assessed through ELISA assays.

RESULTS

Successful membrane anchorage and stability of the chimera were confirmed. Immune evaluations showed no enhancement from combining recombinant bacteria, indicating either bacterium suffices in a single formulation. Chimeric expression yielded immunogenicity equivalent to 10 µg of recombinant protein, with similar antibody titers. IgG1/IgG2a levels and Th1, Th2, and Th17 markers indicated a mixed immune response, providing broad humoral and cellular protection. Responses to BCoV, BRoVA, ETEC, and Salmonella antigens remained strong and did not interfere with chimera-specific responses, potentially boosting NCD vaccine efficacy.

CONCLUSIONS

The chimera demonstrated robust immunogenicity, supporting its potential as a viable vaccine candidate against EHEC O157:H7. This approach could enhance NCD vaccine valency by offering broader protection against calf diarrhea while reducing HUS transmission risks to humans.

In Vivo Validation of Novel Synthetic tbp1 Peptide-Based Vaccine Candidates against Haemophilus influenzae Strains in BALB/c Mice

Haemophilus influenzae is a Gram-negative bacterium characterized as a small, nonmotile, facultative anaerobic coccobacillus. It is a common cause of a variety of invasive and non-invasive infections. Among six serotypes (a-f), H. influenzae type b (Hib) is the most familiar and predominant mostly in children and immunocompromised individuals. Following Hib vaccination, infections due to other serotypes have increased in number, and currently, there is no suitable effective vaccine to induce cross-strain protective antibody responses. The current study was aimed to validate the capability of two 20-mer highly conserved synthetic tbp1 (transferrin-binding protein 1) peptide-based vaccine candidates (tbp1-E1 and tbp1-E2) predicted using in silico approaches to induce immune responses against H. influenzae strains. Cytokine induction ability, immune simulations, and molecular dynamics (MD) simulations were performed to confirm the candidacy of epitopic docked complexes. Synthetic peptide vaccine formulations in combination with two different adjuvants, BGs (Bacterial Ghosts) and CFA/IFA (complete/incomplete Freund's adjuvant), were used in BALB/c mouse groups in three booster shots at two-week intervals. An indirect ELISA was performed to determine endpoint antibody titers using the Student's t-distribution method. The results revealed that the synergistic use of both peptides in combination with BG adjuvants produced better results. Significant differences in absorbance values were observed in comparison to the rest of the peptide-adjuvant combinations. The findings of this study indicate that these tbp1 peptide-based vaccine candidates may present a preliminary set of peptides for the development of an effective cross-strain vaccine against H. influenzae in the future due to their highly conserved nature.

Cooperation between physiological defenses and immune resistance produces asymptomatic carriage of a lethal bacterial pathogen

Animals evolved two defense strategies to survive infections. Antagonistic strategies include immune resistance mechanisms that operate to kill invading pathogens. Cooperative or physiological defenses mediate host adaptation to the infected state, limiting physiological damage and disease, without killing the pathogen, and have been shown to cause asymptomatic carriage and transmission of lethal pathogens. Here, we demonstrate that physiological defenses cooperate with the adaptive immune response to generate long-term asymptomatic carriage of the lethal enteric murine pathogen, Citrobacter rodentium. Asymptomatic carriage of genetically virulent C. rodentium provided immune resistance against subsequent infections. Immune protection was dependent on systemic antibody responses and pathogen virulence behavior rather than the recognition of specific virulent antigens. Last, we demonstrate that an avirulent strain of C. rodentium in the field has background mutations in genes that are important for LPS structure. Our work reveals insight into how asymptomatic infections can arise mechanistically with immune resistance, mediating exclusion of phenotypically virulent enteric pathogen to promote asymptomatic carriage.

Cooperation between physiological defenses and immune resistance produces asymptomatic carriage of a lethal bacterial pathogen

Animals have evolved two defense strategies to survive infections. Antagonistic strategies include mechanisms of immune resistance that operate to sense and kill invading pathogens. Cooperative or physiological defenses mediate host adaptation to the infected state, limiting physiological damage and disease, without killing the pathogen, and have been shown to cause asymptomatic carriage and transmission of lethal pathogens. Here we demonstrate that physiological defenses cooperate with the adaptive immune response to generate long-term asymptomatic carriage of the lethal enteric murine pathogen, Citrobacter rodentium. Asymptomatic carriage of genetically virulent C. rodentium provided immune resistance against subsequent infections. Host immune protection was dependent on systemic antibody responses and pathogen virulence behavior, rather than the recognition of specific virulent factor antigens. Finally, we demonstrate that an avirulent strain of C. rodentium in the field has background mutations in two genes that are important for LPS structure. Our work reveals novel insight into how asymptomatic infections can arise mechanistically with immune resistance, mediating exclusion of phenotypically virulent enteric pathogen to promote asymptomatic carriage.

Oral immunocontraceptive vaccines: A novel approach for fertility control in wildlife

The overabundant populations of wildlife have caused many negative impacts, such as human-wildlife conflicts and ecological degradation. The existing approaches like injectable immunocontraceptive vaccines and lethal methods have limitations in many aspects, which has prompted the advancement of oral immunocontraceptive vaccine. There is growing interest in oral immunocontraceptive vaccines for reasons including high immunization coverage, easier administration, frequent boosting, the ability to induce systemic and mucosal immune responses, and cost-effectiveness. Delivery systems have been developed to protect oral antigens and enhance the immunogenicity, including live vectors, microparticles and nanoparticles, bacterial ghosts, and mucosal adjuvants. However, currently, no effective oral immunocontraceptive vaccine is available for field trials because of the enormous development challenges, including biological and physicochemical barriers of the gastrointestinal tract, mucosal tolerance, pre-existing immunity, antigen residence time in the small intestine, species specificity and other safety issues. To overcome these challenges, this article summarizes achievements in delivery systems and contraceptive antigens in oral immunocontraceptive vaccines and explores the potential barriers for future vaccine design and application.

Bacterial Ghosts of Pseudomonas aeruginosa as a Promising Candidate Vaccine and Its Application in Diabetic Rats

Infections with Pseudomonas aeruginosa (PA) pose a major clinical threat worldwide especially to immunocompromised patients. As a novel vaccine network for many kinds of bacteria, bacterial ghosts (BGs) have recently been introduced. In the present research, using Sponge-Like Reduced Protocol, P. aeruginosa ghosts (PAGs) were prepared to maintain surface antigens and immunogenicity. This is the first study, to our knowledge, on the production of chemically induced well-structured bacterial ghosts for PA using concentrations of different chemicals. The research was carried out using diabetic rats who were orally immunized at two-week intervals with three doses of PAGs. Rats were subsequently challenged either by the oral route or by the model of ulcer infection with PA. In challenged rats, in addition to other immunological parameters, organ bioburden and wound healing were determined, respectively. Examination of the scanning and transmission electron microscope (EM) proved that PAGs with a proper three-dimensional structure were obtained. In contrast to control groups, oral PAGs promoted the generation of agglutinating antibodies, the development of IFN-γ, and the increase in phagocytic activity in vaccinated groups. Antibodies of the elicited PAGs were reactive to PA proteins and lipopolysaccharides. The defense against the PA challenge was observed in PAGs-immunized diabetic rats. The resulting PAGs in orally vaccinated diabetic rats were able to evoke unique humoral and cell-mediated immune responses and to defend them from the threat of skin wound infection. These results have positive implications for future studies on the PA vaccine.

Engineered microbial systems for advanced drug delivery

The human body is a natural habitat for a multitude of microorganisms, with bacteria being the major constituent of the microbiota. These bacteria colonize discrete anatomical locations that provide suitable conditions for their survival. Many bacterial species, both symbiotic and pathogenic, interact with the host via biochemical signaling. Based on these attributes, commensal and attenuated pathogenic bacteria have been engineered to deliver therapeutic molecules to target specific diseases. Recent advances in synthetic biology have enabled us to perform complex genetic modifications in live bacteria and bacteria-derived particles, which simulate micron or submicron lipid-based vectors, for the targeted delivery of therapeutic agents. In this review, we highlight various examples of engineered bacteria or bacteria-derived particles that encapsulate, secrete, or surface-display therapeutic molecules for the treatment or prevention of various diseases. The review highlights recent studies on (i) the production of therapeutics by microbial cell factories, (ii) disease-triggered release of therapeutics by sense and respond systems, (iii) bacteria targeting tumor hypoxia, and (iv) bacteria-derived particles as chassis for drug delivery. In addition, we discuss the potential of such drug delivery systems to be translated into clinical therapies.

Proteomic and Antibody Profiles Reveal Antigenic Composition and Signatures of Bacterial Ghost Vaccine of Brucella abortus A19

Brucellosis is an important zoonotic disease that causes great economic losses. Vaccine immunisation is the main strategy for the prevention and control of brucellosis. Although live attenuated vaccines play important roles in the prevention of this disease, they also have several limitations, such as residual virulence and difficulty in the differentiation of immunisation and infection. We developed and evaluated a new bacterial ghost vaccine of Brucella abortus A19 by a new double inactivation method. The results showed that the bacterial ghost vaccine of Brucella represents a more safe and efficient vaccine for brucellosis. We further characterised the antigenic components and signatures of the vaccine candidate A19BG. Here, we utilised a mass spectrometry-based label-free relative quantitative proteomics approach to investigate the global proteomics changes in A19BGs compared to its parental A19. The proteomic analysis identified 2014 proteins, 1116 of which were differentially expressed compared with those in A19. The common immunological proteins of OMPs (Bcsp31, Omp25, Omp10, Omp19, Omp28, and Omp2a), HSPs (DnaK, GroS, and GroL), and SodC were enriched in the proteome of A19BG. By protein micro array-based antibody profiling, significant differences were observed between A19BG and A19 immune response, and a number of signature immunogenic proteins were identified. Two of these proteins, the BMEII0032 and BMEI0892 proteins were significantly different (P < 0.01) in distinguishing between A19 and A19BG immune sera and were identified as differential diagnostic antigens for the A19BG vaccine candidate. In conclusion, using comparative proteomics and antibody profiling, protein components and signature antigens were identified for the ghost vaccine candidate A19BG, which are valuable for further developing the vaccine and its monitoring assays.

Immunological characterization of the chemically prepared ghosts of Salmonella Typhimurium as a vaccine candidate

BACKGROUND

Bacterial ghosts are the evacuated bacterial cellular membranes from most of the genetic and protein contents which preserved their surface characters. Recently, bacterial ghosts exploited for different biomedical applications, for instance, vaccination. The purpose of this study is to measure the immunogenic protective response of bacterial ghosts of Salmonella Typhimurium in animals and to allow future testing this response in humans. The immunologic response was qualitatively, quantitatively, and functionally measured. We have measured the humoral and cellular immune responses, such as immunoglobulins elevation (IgG), increased granulocytes, serum antibacterial activity, clearance of virulence in feces and liver, and the survival rate.

RESULTS

The bacterial ghosts' vaccine was able to protect 100% of subcutaneously vaccinated rats and 75% of adjuvant subcutaneously vaccinated rats. The lowest survival rate was in the orally vaccinated group (25%). The maximum level of serum IgG titers, as well as serum and feces bactericidal activity (100% eradication), was exhibited in the subcutaneously vaccinated group with adjuvant vaccines followed by the subcutaneously vaccinated one. Additionally, the highest granulocytes' number was observed in the adjuvant vaccine subcutaneously immunized group. The bacterial load in liver homogenate was eliminated in the subcutaneously vaccinated rats after the virulence challenge.

CONCLUSIONS

The bacterial ghosts of Salmonella enterica serovar Typhimurium that prepared by Tween 80 Protocol showed an effective vaccine candidate that protected animals, eliminated the virulence in feces and liver. These findings show that chemically induced bacterial ghosts of Salmonella Typhimurium can be a promising vaccine.

Protective Immunity against Listeria monocytogenes in Rats, Provided by HCl- and NaOH-Induced Listeria monocytogenes Bacterial Ghosts (LMGs) as Vaccine Candidates

Listeria monocytogenes (Lm) bacterial ghosts (LMGs) were produced by the minimum inhibitory concentration (MIC) of HCl, H₂SO₄, and NaOH. Acid and alkali effects on the LMGs were compared by in vitro and in vivo analyses. Scanning electron microscope showed that all chemicals form lysis pores on the Lm cell envelopes. Real-time qPCR revealed a complete absence of genomic DNA in HCl- and H₂SO₄-induced LMGs but not in NaOH-induced LMGs. HCl-, H₂SO₄- and NaOH-induced LMGs showed weaker or missing protein bands on SDS-PAGE gel when compared to wild-type Lm. Murine macrophages exposed to the HCl-induced LMGs showed higher cell viability than those exposed to NaOH-induced LMGs or wild-type Lm. The maximum level of cytokine expression (TNF-α, iNOS, IFN-γ, and IL-10 mRNA) was observed in the macrophages exposed to NaOH-induced LMGs, while that of IL-1β mRNA was observed in the macrophages exposed to HCl-induced LMGs. To investigate LMGs as a vaccine candidate, mice were divided into PBS buffer-injected, HCl- and NaOH-induced LMGs immunized groups. Mice vaccinated with HCl- and NOH-induced LMGs, respectively, significantly increased in specific IgG antibodies, bactericidal activities of serum, and CD4⁺ and CD8⁺ T-cell population. Antigenic Lm proteins reacted with antisera against HCl- and NOH-induced LMGs, respectively. Bacterial loads in HCl- and NaOH-induced LMGs immunized mice were significantly lower than PBS-injected mice after virulent Lm challenges. It suggested that vaccination with LMGs induces both humoral and cell-mediated immune responses and protects against virulent challenges.

Immunization of chickens with Salmonella gallinarium ghosts expressing Salmonella Enteritidis NFliC-FimAC and CD40LC fusion antigen enhances cell-mediated immune responses and protects against wild-type challenges with both species

This study describes the construction and immunological characterization of a novel Salmonella gallinarium ghost vaccine to protect against S. gallinarium (SG) and S. Enteritidis (SE) serotypes. The SG ghost was designed to express N-terminus FliC (D0-D1 domain) and FimA retrieved from the SE genome, and the receptor-binding domain (RBD) of CD40L from the chicken as a single fusion construct. The construct was built in pJHL184, a phage lysis gene E-mediated ghost plasmid and the expression was confirmed by western blot resulting in an 85-kDa band. Chicken immunization was conducted by intramuscular route with SG ghost FliC-FimA-CD40L, vector control, or PBS alone in a prime-boost schedule. Antibody responses, cell-mediated immune responses (CMI), and cytokine induction was assessed in chicken demonstrating significantly high levels of IgY, CMI, cytokine responses in ghost immunized group delivering partial protection against SG wild type challenge and near complete protection against SE challenge wild type challenge.

Bacteria from Infectious Particles to Cell Based Anticancer Targeted Drug Delivery Systems

Bacterial ghosts (BGs) are empty cell envelopes of nonliving evacuated bacterial cells. They are free from their cytoplasmic contents; however, they sustain their cellular 3D morphology and antigenic structures, counting on bioadhesive properties. Lately, they have been tested as an advanced drug delivery system (DDS) for different materials like DNA, peptides, or drugs, either single components or combinations. Different studies have revealed that, BG DDS were paid the greatest attention in recent years. The current review explores the impact of BGs on the field of drug delivery and drug targeting. BGs have a varied area of applications, including vaccine and tumor therapy. Moreover, the use of BGs, their synthesis, their uniqueness as a delivery system and application principles in cancer are discussed. Furthermore, the safety issues of BGs and stability aspects of using ghost bacteria as delivery systems are discussed. Future perspective efforts that must be followed for this important system to continue to grow are important and promising.

Design, development, and evaluation of the efficacy of a nucleic acid-free version of a bacterial ghost candidate vaccine against avian pathogenic E. coli (APEC) O78:K80 serotype

One of the major bacterial infectious diseases in the poultry industry is avian pathogenic Escherichia coli (APEC), which causes colibacillosis in chickens. To develop a novel nucleic acid-free bacterial ghost (BG) vaccine against the O78:K80 serotype of APEC, in this study we constructed a plasmid that harbored E-lysis and S nuclease (SNUC). Following the expression, the O78:K80 bacteria lost all of their cytoplasmic content and nucleic acids by enzymatic digestion. The functionality of these two proteins in the production procedure of bacterial ghosts was confirmed by monitoring the number of colonies, scanning electron microscopy imaging, gel electrophoresis of genomic DNA, and qPCR on the plasmid content of bacterial ghosts. The protective efficacy of the ghost vaccine generated from O78:K80 serotype of APEC was tested in chickens by injection and inhalation routes and compared with that in chickens that received the injection of a killed vaccine. The O78:K80 BG vaccine candidate, used as injection and inhalation, in comparison with the killed vaccine, triggered higher proinflammatory cytokine expression including IL-6, IL-1β, and TNFSF15; a higher level of antibody-dependent humoral (IgY and IgA) and cellular immune responses (IFNγ and lymphocyte proliferation); and lower lesion scores. According to the results of this study, we suggest that the bacterial ghost technology has the potential to be applied for the development of novel vaccines against avian colibacillosis. This technology provides an effective and reliable approach to make multivalent vaccines for more prevalent APEC strains involved in the establishment of this infectious disease in the poultry industry.

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.

Parenteral immunization of Salmonella Typhimurium ghosts with surface-displayed Escherichia coli flagellin enhancesTLR-5 mediated activation of immune responses that protect the chicken against Salmonella infection

The present study investigates the enhancement of immunogenicity and protection efficacy of Salmonella Typhimurium ghosts surface-displayed with FliC against chicken salmonellosis. The membrane-anchored FliC is a potential TLR-5 agonist, delivers an essential adjuvant effect for the ghost vaccine candidate. The present ghost plasmid pJHL184 construct carries a convergent dual promoter system that has the temperature-dependent induction of the phage lysis gene E and the target antigen FliC at the same time. Under permissible conditions of temperatures, less than 30 °C at the presence of 20 mM l-arabinose effectively suppresses expression of the lysis gene. Once the temperature is up-lifted to 42 °C without arabinose, cause the generation of ST ghosts expelling the cytoplasmic content. The addition of FliC adjuvant significantly enhanced the IgY response, cell-mediated immune responses, regulatory cytokine induction and subsequently enhanced protection against Salmonella challenge. Further, intramuscular immunization with ST ghosts displaying FliC induced particularly high CD8⁺ response demarcating its proficiency to elicit Type I immune responses. Further, ST ghosts displaying FliC caused an increase in both CD4⁺ and CD8⁺ response compared to the PBS control suggesting its capability to engage both cell-mediated and humoral immune responses essential for the elimination of Salmonella. Upon the virulent challenge, we could observe a significant reduction in challenged bacterial load on spleen, liver and cecum tissues in the ST ghosts surface-displaying FliC adjuvant. Our study suggests the biological incorporation of FliC on ST ghosts enhances vaccine immunogenic potency and acts as a safe and effective prevention strategy against chicken salmonellosis.

Treatment Strategies for Infections With Shiga Toxin-Producing Escherichia coli

Infections with Shiga toxin-producing Escherichia coli (STEC) cause outbreaks of severe diarrheal disease in children and the elderly around the world. The severe complications associated with toxin production and release range from bloody diarrhea and hemorrhagic colitis to hemolytic-uremic syndrome, kidney failure, and neurological issues. As the use of antibiotics for treatment of the infection has long been controversial due to reports that antibiotics may increase the production of Shiga toxin, the recommended therapy today is mainly supportive. In recent years, a variety of alternative treatment approaches such as monoclonal antibodies or antisera directed against Shiga toxin, toxin receptor analogs, and several vaccination strategies have been developed and evaluated in vitro and in animal models. A few strategies have progressed to the clinical trial phase. Here, we review the current understanding of and the progress made in the development of treatment options against STEC infections and discuss their potential.

Salmonella enterica serovar Enteritidis ghosts displaying a surface FliC adjuvant elicit a robust immune response and effective protection against virulent challenge

Salmonella Enteritidis (SE) is one of the most common culprits of foodborne disease in humans due to its horizontal transmission from infected animals to humans. The development of a safe vaccine against Salmonella would be important for both farm animals and humans concerning disease containment. The SE ghosts carrying FliC were genetically constructed using a special ghost plasmid pJHL184 that co-expressed FliC and the phage lysis gene E. These SE ghosts were characterized by ghost generation efficacy by increasing the culture temperature to "42 °C" in the absence of L-arabinose. This temperature change led to an ghost generation with almost complete lysis of the SE host strain in 48 hs. The expression of FliC was confirmed by Western blot analysis. Also, indirect ELISA was used to prove FliC specific antibody generation in immunized mice. The parenteral adjuvant effect of the FliC antigen was demonstrated by immunizing mice with pJHL184::flC, pJHL184 alone, or PBS alone. The mice were intramuscularly immunized at six weeks of age (n = 8) and boosted after three weeks of primary inoculation. A total of 32 mice were equally divided into four groups. Each group was treated with pJHL-ghosts alone, ghost surface displaying FliC adjuvant, and compared to the PBS and naïve control groups. The immunized mice demonstrated greater IgG and IgA antibody responses than did the PBS control group. Furthermore, the addition of the ghosts to the FliC led to a significant increase in both the humoral and cell-mediated immune responses compared to those in the ghost alone group. Besides, the in vitro antigen uptake and presentation studies revealed efficient antigen presentation on the mouse macrophage cell surfaces. This finding further corroborated the potential efficacy of immune stimulation by SE ghosts. After the virulent challenge, we observed a significant reduction in the bacterial load in the spleen and liver tissues in SE ghosts surface, displaying FliC adjuvant. Our results demonstrate a safe and effective strategy to prevent salmonellosis. They also suggest that the surface expression of flagellin (FliC) significantly enhances antigen-specific humoral and cell-mediated immune responses. This FliC expression can also enhance the protective efficacy of the bacterial ghosts-based vaccine against virulent challenge.

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.

Immunogenic potential and protective efficacy of a sptP deletion mutant of Salmonella Enteritidis as a live vaccine for chickens against a lethal challenge

Salmonella Enteritidis (SE) is a highly adapted pathogen causing severe economic losses in the poultry industry worldwide. Chickens infected by SE are a major source of human food poisoning. Vaccination is an effective approach to control SE infections. This study evaluated the immunogenicity and protective efficacy of a SE sptP deletion mutant (C50336ΔsptP) as a live attenuated vaccine (LAV) candidate in chickens. 14 day-old specific pathogen-free (SPF) chickens were intramuscularly immunized with various doses of C50336ΔsptP. Several groups of chickens were challenged with the virulent wild-type SE strain Z-11 via the same route at 14 days post vaccination. Compared to the control group, the groups vaccinated with 1 × 10⁶, 1 × 10⁷ and 1 × 10⁸ colony-forming units (CFU) of C50336ΔsptP exhibited no clinical symptoms after immunization. Only slight pathological changes occurred in the organs of the 1 × 10⁹ CFU vaccinated group. C50336ΔsptP bacteria were cleared from the organs of immunized chickens within 14 days after vaccination. Lymphocyte proliferation and serum cytokine analyses indicated that significant cellular immune responses were induced after the vaccination of C50336ΔsptP. Compared to the control group, specific IgG antibody levels increased significantly in vaccinated chickens, and the levels increased markedly after the challenge. The 1 × 10⁷, 1 × 10⁸, and 1 × 10⁹ CFU vaccinated chickens groups showed no clinical symptoms or pathological changes, and no death after the lethal challenge. Whereas severe clinical signs of disease and pathological changes were observed in the control group chickens after the challenge. These results suggest that a single dose of C50336ΔsptP could be an effective LAV candidate to against SE infection in chickens.

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.

Use of a modified bacterial ghost lysis system for the construction of an inactivated avian pathogenic Escherichia coli vaccine candidate

Vaccination is an effective strategy to prevent avian colibacillosis. Bacterial ghosts (BGs) are prepared by the controlled expression of the phiX174 gene E, which mediates the lysis of Gram-negative bacteria. Staphylococcal nuclease A may be used to produce BGs for further inactivation of host bacteria and elimination of residual genetic material. In this study, the double promoter lysis plasmid (pUC19-ΔcI857-E-rrnB-pL-SN) was successfully constructed and BGs were prepared at 37 °C. The cleavage efficiency of Escherichia coli BGs was 99.9%. Furthermore, to evaluate the immunological effects of the BG vaccines in chickens, a BG vaccine was prepared using the serotype O2 avian pathogenic Escherichia coli deletion strain (DE17ΔluxSΔaroA). The results showed that the BG vaccine was able to achieve over 90% immune protection against virulent challenge using the same serotype O2 strain (DE17 or CE35), while it showed poor cross-protection against serotypes O1 and O78 (data not shown). The enzyme-linked immunosorbent assay results showed that the antibody levels in the immunized groups were higher than in the control group (p < 0.05), with the BG group being the highest. The cytokine tests showed that the levels of interferon-γ in the BG immune group were higher than in the phosphate-buffered saline (PBS) control group (non-immune) (p < 0.01) and the formalin-inactivated vaccine immune group (p < 0.05), and the levels of tumor necrosis factor-α in the BG group were higher than in the formalin-inactivated vaccine (p > 0.05) and the PBS control groups (p < 0.05). In addition, pathological analysis revealed that the PBS control group showed typical fibrinous pericarditis and perihepatitis, whereas the immune group showed no obvious pathological changes. In summary, our findings provide a new strategy for the prevention and control of avian colibacillosis.

Preparation of porcine enterotoxigenic Escherichia coli (ETEC) ghosts and immunogenic analysis in a mouse model

Enterotoxignenic Escherichia coli (ETEC)-associated colibacillosis causes high levels of morbidity and mortality in neonatal piglets. Vaccination is among effective strategy to fight against ETEC-related diseases. Bacterial ghosts (BGs) are empty bacterial envelopes, which substain subtle antigenic comformation in bacterial outer membrane. In this study, a BG vaccine was generated using porcine ETEC isolated strain DQ061 and evaluated its safety and immunogenicity in a mouse model. The recombinant bacteria were constructed by transformation of lysis plasmid pHH43 and generation of BGs was conducted in a lysis rate of 99.93% by incubation of the recombinant bacteria at 42 °C for 2 h. Mice were immunized subcutaneously twice in 2-week intervals with BGs, BGs emulsified with ISA 206 adjuvant, or formalin-inactivated ETEC vaccine after safety test. Mice with either of two BG vaccines developed higher titer of antibodies, secreted higher titer of interleukin 4, gamma interferon and alpha tumor necrosis factor after 2 doses than those with formalin-inactivated ETEC vaccine or those with adjuvant placebo (P < 0.01). The quantity of CD4⁺ and CD8⁺ T lymphocyte in spleen was higher in both BG groups than that in the inactivated vaccine group or adjuvant group 2 weeks post boost immunization (P < 0.05). The vaccinated mice were challenged intraperitoneally with 10 × LD₅₀ dose of DQ061. Mice with the BGs plus adjuvant were completely protected against challenge, compared to 60% protection of mice with the inactivated vaccine. Mice exhibited decreased tissue lesion and reduced bacterial loads in the BGs groups by comparison with those with the inactivated vaccine or adjuvant only. Our results validated that the ETEC BGs bear high safety and immunogenicity in a mouse model, suggesting a potential of further evaluation in a pig model.

Generation of a novel Streptococcus agalactiae ghost vaccine and examination of its immunogenicity against virulent challenge in tilapia

Streptococcus agalactiae (S. agalactiae) is a gram-positive pathogen that causes a wide range of infections in fish and other animals including humans. Bacterial ghosts (BGs) are nonliving, empty cell envelopes and are well represented as novel vaccine candidates. In this study, we examined the immunogenicity and protective efficacy of S. agalactiae ghosts (SAG) against a virulent challenge in tilapia. Nonliving SAG was generated by a culture with Penicillin and Streptolysin, and then treated with the MIC of sodium hydroxide. The formation of a transmembrane lysis tunnel structure in SAG was visualized by electron microscopy. To investigate the SAG as a vaccine candidate, fish were divided into three groups, A (SAG immunized), B [Formalin-inactivated S. agalactiae (FSA) immunized] and C (phosphate-buffered saline, PBS-immunized control). The IgM antibody responses were significantly stronger in the SAG-immunized group than in FSA-immunized group, which was higher than in the non-immunized control group (P < 0.05). Moreover, phagocytic activity (percent phagocytes, PP) was significantly higher (p < 0.05) in the SAG-immunized group than in FSA-immunized group, which was higher than in the non-immunized control group (P < 0.05). In addition, non-specific immune immunity, such as lysozyme and superoxide dismutase activities, in the SAG-immunized fish showed significantly higher activities than FSA-immunized fish and the control group fish (P < 0.05). Also, fish immunized with SAG and FSA showed significantly higher (p < 0.05) gene expression of IL-1β, TNF-α, IFN-γ and TGF-β in the head kidney and spleen than fish treated with PBS during the whole observed period. In addition, fish immunized with SAG showed significantly higher gene expression of L-1β, TNF-α, and TGF-β in the spleen than in the FSA-immunized fish. Although there was no significant (P > 0.05) difference of survival rate (SR) or relative percent survival (RPS) between SAG and FSA immunized groups, they were all significantly more protected against the S. agalactiae challenge (SR: 86.67%, RPS: 76.395) and (SR: 80.00%, RPS: 67.50%) respectively, compared to the PBS-treated group (SR: 33.33%). These results suggest that immunization with SAG induces immune responses and provides protection against a virulent S. agalactiae challenge.

Construction of Lactobacillus casei ghosts by Holin-mediated inactivation and the potential as a safe and effective vehicle for the delivery of DNA vaccines

BACKGROUND

Bacterial ghosts (BGs) are empty bacterial cell envelopes generated by releasing the cellular contents. In this study, a phage infecting Lactobacillus casei ATCC 393 (L. casei 393) was isolated and designated Lcb. We aimed at using L. casei 393 as an antigen delivery system to express phage-derived holin for development of BGs.

RESULTS

A gene fragment encoding holin of Lcb (hocb) was amplified by polymerase chain reaction (PCR). We used L. casei 393 as an antigen delivery system to construct the recombinant strain pPG-2-hocb/L. casei 393. Then the recombinants were induced to express hocb. The immunoreactive band corresponding to hocb was observed by western-blotting, demonstrating the efficiency and specificity of hocb expression in recombinants. The measurements of optical density at 600 nm (OD₆₀₀₎ after induction showed that expression of hocb can be used to convert L. casei cells into BGs. TEM showed that the cytomembrane and cell walls of hocb expressing cells were partially disrupted, accompanied by the loss of cellular contents, whereas control cells did not show any morphological changes. SEM showed that lysis pores were distributed in the middle or at the poles of the cells. To examine where the plasmid DNA was associated, we analyzed the L. casei ghosts loading SYBR Green I labeled pCI-EGFP by confocal microscopy. The result demonstrated that the DNA interacted with the inside rather than with the outside surface of the BGs. To further analyze where the DNA were loaded, we stained BGs with MitoTracker Green FM and the loaded plasmids were detected using EGFP-specific Cy-3-labeled probes. Z-scan sections through the BGs revealed that pCI-EGFP (red) was located within the BGs (green), but not on the outside. Flow cytometry and qPCR showed that the DNA was loaded onto BGs effectively and stably.

CONCLUSIONS

Our study constructed L. casei BGs by a novel method, which may be a promising technology for promoting the further application of DNA vaccine, providing experimental data to aid the development of other Gram-positive BGs.

Tumor lysate-loaded Bacterial Ghosts as a tool for optimized production of therapeutic dendritic cell-based cancer vaccines

Cancer immunotherapy with dendritic cell (DC)-based vaccines has been used to treat various malignancies for more than two decades, however generally showed a limited clinical success. Among various factors responsible for their modest clinical activity is the lack of universally applied, standardized protocols for the generation of clinical-grade DC vaccines, capable of inducing effective anti-tumor immune responses. We investigated Bacterial Ghosts (BGs) - empty envelopes of Gram-negative bacteria - as a tool for optimized production of DC vaccines. BGs possess various intact cell surface structures, exhibiting strong adjuvant properties required for the induction of DC maturation, whereas their empty internal space can be easily filled with a source tumor antigens, e.g. tumor lysate. Hence BGs emerge as an excellent platform for both the induction of immunogenic DC maturation and loading with tumor antigens in a single-step procedure. We compared the phenotype, cytokine secretion profile, functional activity and ability to induce immunogenic T-cell responses in vitro of human monocyte-derived DCs generated using BG platform and DCs matured with widely used lipopolysaccharide (LPS) plus interferon-γ cocktail and loaded with tumor lysate. Both approaches induced DC maturation, however BG-based protocol was superior to LPS-based protocol in terms of the ability to induce DCs with a lower tolerogenic potential, resulting in a more robust CD8⁺ T cell activation and their functional activity as well as significantly lower induction of regulatory T cells. These superior parameters are attributed, at least in part, to the ability of BG-matured DCs to resist potential immunosuppressive and pro-tolerogenic activity of various tumor cell lysates, including melanoma, renal carcinoma and glioblastoma.

Intestinal Pathogenic Escherichia coli: Insights for Vaccine Development

Diarrheal diseases are one of the major causes of mortality among children under five years old and intestinal pathogenic Escherichia coli (InPEC) plays a role as one of the large causative groups of these infections worldwide. InPECs contribute significantly to the burden of intestinal diseases, which are a critical issue in low- and middle-income countries (Asia, Africa and Latin America). Intestinal pathotypes such as enteropathogenic E. coli (EPEC) and enterotoxigenic E. coli (ETEC) are mainly endemic in developing countries, while ETEC strains are the major cause of diarrhea in travelers to these countries. On the other hand, enterohemorrhagic E. coli (EHEC) are the cause of large outbreaks around the world, mainly affecting developed countries and responsible for not only diarrheal disease but also severe clinical complications like hemorrhagic colitis and hemolytic uremic syndrome (HUS). Overall, the emergence of antibiotic resistant strains, the annual cost increase in the health care system, the high incidence of traveler diarrhea and the increased number of HUS episodes have raised the need for effective preventive treatments. Although the use of antibiotics is still important in treating such infections, non-antibiotic strategies are either a crucial option to limit the increase in antibiotic resistant strains or absolutely necessary for diseases such as those caused by EHEC infections, for which antibiotic therapies are not recommended. Among non-antibiotic therapies, vaccine development is a strategy of choice but, to date, there is no effective licensed vaccine against InPEC infections. For several years, there has been a sustained effort to identify efficacious vaccine candidates able to reduce the burden of diarrheal disease. The aim of this review is to summarize recent milestones and insights in vaccine development against InPECs.

Identification of fish source Vibrio alginolyticus and evaluation of its bacterial ghosts vaccine immune effects

Vibrio alginolyticus (V. alginolyticus) is a common pathogen for humans and marine aquatic animals. Vibriosis of marine aquatic animals, caused by V. alginolyticus, has become more prevalent globally in recent years. Hence, a safe and effective vaccine is urgently needed for the control of this disease. Here, the strain 16‐3 isolated from the large yellow croaker (Larimichthys crocea) suffered from canker was identified as V. alginolyticus based on morphological, biochemical, and 16S rDNA sequencing analysis. Then, recombinant temperature‐controlled lysis plasmid pBV220‐lysisE was electroporated into the strain 16‐3 to generate V. alginolyticus bacterial ghosts (VaBGs) by inducing lysis gene E expression, and the safety and immune effects of VaBGs were further investigated in mice and large yellow croaker. The results showed that VaBGs were as safe as formalin‐killed V. alginolyticus cells (FKC) to mice and fish. Compared with FKC and PBS groups, significant elevations of the serum agglutinating antibody titer, serum bactericidal activity, lymphocyte proliferative responses, and levels of four different cytokines (Th1 type: IL‐2, TNF‐α; Th2 type: IL‐4 and IL‐6) in serum were detected in the VaBGs group, indicating that a Th1/Th2‐mediated mixed immune response was elicited by the VaBGs. More importantly, after challenged with the parent strain 16‐3, VaBGs‐vaccinated mice and fish showed higher protection than FKC‐vaccinated mice, the relative percent of survival (RPS) being 60%, 66.7% and 40%, respectively. Taken together, this is the first demonstration that the newly constructed V. alginolyticus ghosts may be developed as a safe and effective vaccine against V. alginolyticus infection in aquaculture.

Bacterial ghosts as adjuvants: mechanisms and potential

Bacterial ghosts (BG) are empty cell envelopes derived from Gram-negative bacteria. They contain many innate immunostimulatory agonists, and are potent activators of a broad range of cell types involved in innate and adaptive immunity. Several considerable studies have demonstrated the effectiveness of BG as adjuvants as well as their ability to induce proinflammatory cytokine production by a range of immune and non-immune cell types. These proinflammatory cytokines trigger a generalized recruitment of T and B lymphocytes to lymph nodes that maximize the chances of encounter with their cognate antigen, and subsequent elicitation of potent immune responses. The plasticity of BG has allowed for the generation of envelope-bound foreign antigens in immunologically active forms that have proven to be effective vaccines in animal models. Besides their adjuvant property, BG also effectively deliver DNA-encoded antigens to dendritic cells, thereby leading to high transfection efficiencies, which subsequently result in higher gene expressions and improved immunogenicity of DNA-based vaccines. In this review, we summarize our understanding of BG interactions with the host immune system, their exploitation as an adjuvant and a delivery system, and address important areas of future research interest.

Protective immunity of grass carp induced by DNA vaccine encoding capsid protein gene (vp7) of grass carp reovirus using bacterial ghost as delivery vehicles

Grass carp reovirus (GCRV) is one of the most pathogenic aquareovirus and can cause lethal hemorrhagic disease in grass carp (Ctenopharyngodon idella). However, management of GCRV infection remains a challenge. Therefore, it is necessary to find effective means for the control of its infection. The uses of bacterial ghost (BG, non-living bacteria) as carriers for DNA delivery have received considerable attentions in veterinary and human vaccines studies. Nevertheless, there is still no report about intramuscular administration of bacterial ghost-based DNA vaccines in fish. In the current study, a novel vaccine based on Escherichia coli DH5α bacterial ghost (DH5α-BG), delivering a major capsid protein gene (vp7) of grass carp reovirus encoded DNA vaccine was developed to enhance the efficacy of a vp7 DNA vaccine against GCRV in grass carp. The grass carp was injected intramuscularly by different treatments -i) naked pcDNA-vp7 (containing plasmid 1, 2.5 and 5 μg, respectively), ii) DH5α-BG/pcDNA-vp7 (containing plasmid 1, 2.5 and 5 μg, respectively) and iii) naked pcDNA, DH5α-BG or phosphate buffered saline. The immune responses and disease resistance of grass carp were assessed in different groups, and results indicated that the antibody levels, serum total antioxidant capacity (T-AOC), superoxide dismutase (SOD) activity, acid phosphatase (ACP) activity and alkaline phosphatase (AKP) activity and immune-related genes were significantly enhanced in fish immunized with DH5α-BG/pcDNA-vp7 vaccine (DNA dose ranged from 2.5 to 5 μg). In addition, the relative percentage survival were significantly enhanced in fish immunized with DH5α-BG/pcDNA-vp7 vaccine and the relative percentage survival reached to 90% in DH5α-BG/pcDNA-vp7 group than that of naked pcDNA-vp7 (42.22%) at the highest DNA dose (5 μg) after 14 days of post infection. Moreover, the level of pcDNA-vp7 plasmid was higher in DH5α-BG/pcDNA-vp7 groups than naked pcDNA-vp7 groups in muscle and kidneys tissues after 21 days. Overall, those results suggested that DH5α bacterial ghost based DNA vaccine might be used as a promising vaccine for aquatic animals to fight against GCRV infection.

A safe and molecular-tagged Brucella canis ghosts confers protection against virulent challenge in mice

Canine brucellosis, caused by Brucella canis, is a persistent infectious reproductive disease in dogs. The absence of effective treatment to the intracellular pathogen and the irreversible consequence of infection makes the need of a specific vaccine urgent. Bacterial ghosts (BGs) are the empty envelopes of bacteria with no genome content inside, which emerge as a proper vaccine candidate due to its intact outer antigen. It is generally derived from a genetically engineered strain, through the expression of Bacteriophage phiX174 lysis E gene upon induction. In this study, we combined the homologous recombination (HR) and bacterial ghost technologies, generating a genetically stable B. canis ghost strain which bears no drug resistance gene. When the ghost strain grows to OD₆₀₀ of 0.6, 100% inactivation can be achieved under 42°C in 60h. The resultant BGs showed guaranteed safety and comparable immunogenicity to a live vaccine. The bacterial B0419 protein was depleted during HR process, which is subsequently proved to work as a molecular tag in distinguishing natural infection and BGs immunization through ELISA. Additionally, the BGs also conferred protection against B. canis RM6/66 and B. melitensis 16M. Therefore, the application of current BGs as a vaccine candidate and the corresponding serological diagnostic approach may provide better B. canis prevention strategy.

Oral Immunization with Recombinant Lactobacillus acidophilus Expressing espA-Tir-M Confers Protection against Enterohemorrhagic Escherichia coli O157:H7 Challenge in Mice

Enterohemorrhagic Escherichia coli O157:H7 (EHEC O157:H7) causes hemorrhagic colitis and the formation of characteristic attaching and effacing (A/E) lesions in humans. Given the severe sequelae of EHEC O157:H7 infection, it is critical to develop effective vaccines for human use. However, for achieving this goal many hurdles need to be addressed, such as the type or subset of antigens, adjuvant, and the delivery route. We developed a candidate vaccine by inserting the bivalent antigen espA-Tir-M composed of espA and the Tir central domain into Lactobacillus acidophilus. The recombinant L. acidophilus (LA-ET) was safe in a cell model and excluded EHEC O157:H7 from LoVo cells at rates of nearly 94 and 60% in exclusion and competition assays, respectively. LA-ET inhibited the induction of A/E lesions by EHEC O157:H7 cells in vitro. Oral immunization with LA-ET induced higher levels of specific mucosal and systemic antibody responses in mice. Moreover, LA-ET enhanced interferon-γ and interleukin-4 and -10 production, which was associated with mixed helper T (Th1/Th2) cell responses, and protected against EHEC O157:H7 colonization and infection in mice at a rate of 80%. Histopathological analyses revealed that orally administered LA-ET reduced or inhibited A/E lesions and toxin-induced systemic injury. These findings demonstrate that LA-ET induces both humoral and cellular immune responses in mice and is therefore a promising vaccine against EHEC O157:H7 infection.

Exosome-Based Cancer Therapy: Implication for Targeting Cancer Stem Cells

Drug resistance, difficulty in specific targeting and self-renewal properties of cancer stem cells (CSCs) all contribute to cancer treatment failure and relapse. CSCs have been suggested as both the seeds of the primary cancer, and the roots of chemo- and radio-therapy resistance. The ability to precisely deliver drugs to target CSCs is an urgent need for cancer therapy, with nanotechnology-based drug delivery system being one of the most promising tools to achieve this in the clinic. Exosomes are cell-derived natural nanometric vesicles that are widely distributed in body fluids and involved in multiple disease processes, including tumorigenesis. Exosome-based nanometric vehicles have a number of advantages: they are non-toxic, non-immunogenic, and can be engineered to have robust delivery capacity and targeting specificity. This enables exosomes as a powerful nanocarrier to deliver anti-cancer drugs and genes for CSC targeting therapy. Here, we will introduce the current explorations of exosome-based delivery system in cancer therapy, with particular focus on several exosomal engineering approaches that have improved their efficiency and specificity for CSC targeting.

A review on strategies for decreasing E. coli O157:H7 risk in animals

Enterohemorrhagic Escherichia coli (EHEC) serotype O157:H7 is a food-borne pathogen that younger children are most prone to this microorganism. Hemolytic Uremic Syndrome (HUS) caused by EHEC, leads to the destruction of red blood cells and kidney failure. The virulence of E.coli O157:H7 is attributed to fimbriae, that facilitate colonization of bacteria within the colon and verotoxins (VT) or Shiga toxins (Stx) that are released into the blood. Although, in most cases, the infection is self-limitedin young children and aged population, it may cause HUS. Therefore, several investigations are performed in order to offer effective therapies and vaccines, which can prevent and treat the infection in appropriate time. As the pathogenesis of this infection is complicated, a multi-targeted strategy is required. Since cattle are the most important reservoir of EHEC and the root of contamination, reducing E. coli O157:H7 at the farm level should decrease the risk of human illness. Several vaccine approaches have been employed with different proper outcomes in animal models, including recombinant proteins (virulence factors such as; Stx1/2, intimin, EspA, fusion proteins of A and B Stx subunits), avirulent ghost cells of EHEC O157:H7, live attenuated bacteria expressing recombinant proteins, recombinant fimbrial proteins. In addition to protein-based vaccines, DNA vaccines have provided proper prevention in the laboratory animal model. This review paper summarizes the previous studies, current status and future perspective of different immunization strategies for eradicating Enterohemorrhagic Escherichia coli O157:H7.

Effectiveness of F18+ Fimbrial Antigens Released by a Novel Autolyzed Salmonella Expression System as a Vaccine Candidate against Lethal F18+ STEC Infection

Porcine edema disease (ED) caused by Shiga toxin 2e producing Escherichia coli expressing F18ab⁺ fimbriae (F18ab⁺STEC) frequently occurs in post-weaned piglets, resulting in a significant economic loss in swine industries worldwide. In the present study, we proposed an efficient prevention scheme against ED in which the attenuated Salmonella Typhimurium inactivated by the E-mediated cell lysis to deliver target antigens, FedF and FedA, which function in fimbrial-mediated adhesion and as a major subunit of F18ab+fimbriae, respectively. The co-expression of FedA and FedF protein with outer membrane protein A signal peptide was confirmed in the resultant strains JOL1460 and JOL1464 by immunoblot analysis. Immunization with the candidate strains in mice led to the significant generation of immunoglobulin (Ig) G, specific to both antigens and secretory IgA specific to FedF (P < 0.05). The titers of IgG isotypes, IgG1 and IgG2a, used as markers for T-helpers (Th)-2 and Th-1lymphocytes, respectively, also significantly increased in the immunized group (P < 0.05). The increase in CD3⁺CD4⁺ T lymphocyte subpopulation and in vitro proliferative activity was observed in in vivo stimulated splenocytes, which indicated the immunostimulatory effect of the candidate strains. Moreover, the immunized mice were completely protected from a lethal challenge against wild-type F18⁺STEC whereas 28% of mice died in the non-immunized group. This study demonstrated that the inactivated Salmonella system could efficiently release FedF and FedA and induce robust immune responses specific to the target antigens, which is sufficient to protect the mice from the lethal challenge.

Characterization of Chemically-Induced Bacterial Ghosts (BGs) Using Sodium Hydroxide-Induced Vibrio parahaemolyticus Ghosts (VPGs)

Acellular bacterial ghosts (BGs) are empty non-living bacterial cell envelopes, commonly generated by controlled expression of the cloned lysis gene E of bacteriophage PhiX174. In this study, Vibrio parahaemolyticus ghosts (VPGs) were generated by chemically-induced lysis and the method is based on minimum inhibitory concentration (MIC) of sodium hydroxide (NaOH), acetic acid, boric acid, citric acid, maleic acid, hydrochloric acid, and sulfuric acid. The MIC values of the respective chemicals were 3.125, 6.25, <50.0, 25.0, 6.25, 1.56, and 0.781 mg/mL. Except for boric acid, the lysis efficiency reached more than 99.99% at 5 min after treatment of all chemicals. Among those chemicals, NaOH-induced VPGs appeared completely DNA-free, which was confirmed by quantitative real-time PCR. Besides, lipopolysaccharides (LPS) extracted from the NaOH-induced VPGs showed no distinctive band on SDS-PAGE gel after silver staining. On the other hand, LPS extracted from wild-type bacterial cells, as well as the organic acids-induced VPGs showed triple major bands and LPS extracted from the inorganic acids-induced VPGs showed double bands. It suggests that some surface structures in LPS of the NaOH-induced VPGs may be lost, weakened, or modified by the MIC of NaOH. Nevertheless, Limulus amoebocyte lysate assay revealed that there is no significant difference in endotoxic activity between the NaOH-induced VPGs and wild-type bacterial cells. Macrophages exposed to the NaOH-induced VPGs at 0.5 × 10⁶ CFU/mL showed cell viability of 97.9%, however, the MIC of NaOH did not reduce the cytotoxic effect of wild-type bacterial cells. Like Escherichia coli LPS, the NaOH-induced VPGs are an excellent activator of pro-inflammatory cytokines (IL-1β and iNOS), anti-inflammatory cytokine (IL-10), and dual activities (IL-6) in the stimulated macrophage cells. On the other hand, the induction of TNF-α mRNA was remarkable in the macrophages exposed with wild-type cells. Scanning electron microscopy showed the formation of trans-membrane lysis tunnel structures in the NaOH-induced VPGs. SDS-PAGE and agarose gel electrophoresis also confirmed that cytoplasmic proteins and genomic DNA released from the VPGs to culture medium through the lysis tunnel structures. Taken together, all these data indicate that the NaOH-induced VPGs show the potency of a safe, economical, and effective inactivated bacterial vaccine candidate.

Intramammary immunization with ultraviolet-killed Escherichia coli shows partial protection against late gestation intramammary challenge with a homologous strain

The objective of this study was to evaluate the efficacy of intramammary immunization with UV-killed Escherichia coli ECC-Z on prevention of intramammary colonization after a challenge with a dose of the homologous E. coli ECC-Z live bacteria. A total of 10 cows were included in a study to evaluate the efficacy of intramammary immunization. All 10 cows received an intramammary immunization of 100 cfu of UV-killed E. coli ECC-Z bacteria into one hind quarter at the time of dry off. Approximately 2wk before the anticipated calving date, both hind quarters of all cows were challenged with 100 cfu of live E. coli ECC-Z bacteria. Five of the cows were vaccinated parenterally with a commercial J5 bacterin, and 5 cows served as controls with no parenteral vaccination. The cows were then followed over time and infection risk, clinical scores, somatic cell count, and milk production were observed over time. The results of these 10 cows showed partial protection of intramammary immunization on the outcome of a subsequent homologous intramammary challenge. Immunization resulted in a lower probability of infection, a lower bacteria count, lower somatic cell counts and milk conductivity, a lower clinical mastitis score, and increased milk production compared with unimmunized control quarters. Once the analysis was corrected for immunization, parenteral J5 vaccination had no significant effect on any of the measured parameters. These results provide the first evidence that intramammary immunization may improve the outcome of an intramammary E. coli infection in late gestation and onset of mastitis immediately following parturition. Unlike systemic vaccination, which generally does not reduce the intramammary infection risk, the intramammary immunization did show a 5-times reduced odds of an established intramammary infection after challenge. Cytokine profiles indicated a local return of proinflammatory response after challenge as the data showed a more pronounced increase in in IFN-γ with a subsequent negative feedback due to a spike in the level of IL-10 in immunized quarters relative to nonimmunized quarters. Although these results are preliminary and obtained on only 10 cows, the results provide insight into the biological benefits of triggering mucosal immunity in the mammary gland.

Protective efficacy and immune responses by homologous prime-booster immunizations of a novel inactivated Salmonella Gallinarum vaccine candidate

Purpose

Salmonella enterica serovar Gallinarum (SG) ghost vaccine candidate was recently constructed. In this study, we evaluated various prime-boost vaccination strategies using the candidate strain to optimize immunity and protection efficacy against fowl typhoid.

Materials and Methods

The chickens were divided into five groups designated as group A (non-immunized control), group B (orally primed and boosted), group C (primed orally and boosted intramuscularly), group D (primed and boosted intramuscularly), and group E (primed intramuscularly and boosted orally). The chickens were primed with the SG ghost at 7 days of age and were subsequently boosted at the fifth week of age. Post-immunization, the plasma IgG and intestinal secretory IgA (sIgA) levels, and the SG antigen-specific lymphocyte stimulation were monitored at weekly interval and the birds were subsequently challenged with a virulent SG strain at the third week post-second immunization.

Results

Chickens in group D showed an optimized protection with significantly increased plasma IgG, sIgA, and lymphocyte stimulation response compared to all groups. The presence of CD4⁺ and CD8⁺ T cells and monocyte/macrophage (M/M) in the spleen, and splenic expression of cytokines such as interferon γ (IFN-γ) and interleukin 6 (IL-6) in the immunized chickens were investigated. The prime immunization induced significantly higher splenic M/M population and mRNA levels of IFN-γ whereas the booster showed increases of splenic CD4⁺ and CD8⁺ T-cell population and IL-6 cytokine in mRNA levels.

Conclusion

Our results indicate that the prime immunization with the SG ghost vaccine induced Th1 type immune response and the booster elicited both Th1- and Th2-related immune responses.

Protective efficacy by various doses of Salmonella ghost vaccine candidate carrying enterotoxigenic Escherichia coli fimbrial antigen against neonatal piglet colibacillosis

Humoral immune responses and protective efficacy by various doses of Salmonella ghost cells carrying enterotoxigenic Escherichia coli (ETEC) fimbrial antigens for protection against piglet colibacillosis were studied. All groups were orally primed and boosted at 11 and 14 wk of pregnancy, respectively. Group A sows were inoculated with phosphate-buffered saline (PBS), and groups B, C, and D sows were immunized with 2 × 10(9), 2 × 10(10), and 2 × 10(11) ghost cells, respectively. Serum immunoglobulin (Ig) G, and colostrum IgG and IgA levels of groups C and D sows were significantly higher than those of group A sows. In addition, serum IgG and IgA levels in group C and D piglets were significantly increased compared to those of group A piglets. After challenge with wild-type ETEC, diarrhea and mortality were not observed in group C and D piglets, while diarrhea was observed in 88.9% and 58.8% of groups A and B piglets, respectively, and 16.7% mortality was observed in group A piglets. These findings indicate that oral immunization of sows with 2 × 10(10) or 10(11) ghost cells can effectively protect their offspring from colibacillosis.

Evaluation of protective efficacy of a novel inactivated Salmonella Pullorum ghost vaccine against virulent challenge in chickens

Salmonella Gallinarum biovar Pullorum is the causative agent of pullorum disease in poultry, an acute systemic disease that results in a high mortality rate in young chickens. Vaccines have been considered in many developing countries where levels of infection are high and eradication is not a realistic option. An attenuated strain combined with protein E-mediated cell lysis was used to generate a safety enhanced Salmonella Pullorum ghost vaccine. Immune responses and protection induced by ghost vaccine in chickens were investigated following a prime-boost immunization administered via intramuscular and oral routes. Chickens from vaccinated groups showed significant increases in antigen-specific IgG, especially after booster immunization. Lymphocyte proliferation responses were also significantly increased in all immunized groups at 2-weeks post-final vaccination. The Salmonella Pullorum ghost vaccine provided satisfactory protection against virulent Salmonella Pullorum infection, as shown by the robust stimulation of both humoral and cell-mediated immune responses as well as the reduction in the number of bacterial recovered post-challenge. Moreover, the immune effects and survival rates indicated intramuscular injection is more efficient than oral vaccination. In conclusion, our results suggest that Salmonella Pullorum ghosts may be used as a safe and effective novel inactivated vaccine candidate to protect against virulent Salmonella Pullorum infection.

Novel fusion antigen displayed-bacterial ghosts vaccine candidate against infection of Escherichia coli O157:H7

Infection with Escherichia coli O157:H7 may develop into hemorrhagic colitis, or hemolytic uremic syndrome (HUS), which usually causes kidney failure or even death. The adhesion and toxins are the important virulent factors. In this study, a novel vaccine candidate rSOBGs was constructed based on the bacterial ghost (BG). rSOBGs maintained the integrity of cellular morphology and displayed the linear Stx2Am-Stx1B antigen on the surface of outer membrane. rSOBGs induced Stxs-specific IgA/IgG antibodies and stronger intimin-specific IgA/IgG antibodies effectively in sera in this study. In vivo, the rSOBGs provided the higher protection rate (52%) than native bacterial ghost-OBGs (12%) when challenged intragastricly with high dose (500 LD50) viable E. coli O157:H7. Meanwhile, the rSOBGs provided higher protection rate (73.33%) than OBGs when challenged with 2 LD50 even to 5 LD50 lysed E. coli O157:H7. In vitro, the rSOBGs-immunized sera possessed neutralizing activity to lysed pathogenic bacteria. Furthermore, the results of histopathology also displayed that the administration of rSOBGs have the ability to reduce or inhibit the adhesion lesions and toxins damages of organs. The novel vaccine candidate rSOBGs induced both anti-toxin and anti-adhesion immune protection, suggesting the possibility to prevent the infectious diseases caused by Escherichia coli O157:H7.

Multi-parameter flow cytometry as a process analytical technology (PAT) approach for the assessment of bacterial ghost production

Flow cytometry (FCM) is a tool for the analysis of single-cell properties in a cell suspension. In this contribution, we present an improved FCM method for the assessment of E-lysis in Enterobacteriaceae. The result of the E-lysis process is empty bacterial envelopes-called bacterial ghosts (BGs)-that constitute potential products in the pharmaceutical field. BGs have reduced light scattering properties when compared with intact cells. In combination with viability information obtained from staining samples with the membrane potential-sensitive fluorescent dye bis-(1,3-dibutylarbituric acid) trimethine oxonol (DiBAC4(3)), the presented method allows to differentiate between populations of viable cells, dead cells, and BGs. Using a second fluorescent dye RH414 as a membrane marker, non-cellular background was excluded from the data which greatly improved the quality of the results. Using true volumetric absolute counting, the FCM data correlated well with cell count data obtained from colony-forming units (CFU) for viable populations. Applicability of the method to several Enterobacteriaceae (different Escherichia coli strains, Salmonella typhimurium, Shigella flexneri 2a) could be shown. The method was validated as a resilient process analytical technology (PAT) tool for the assessment of E-lysis and for particle counting during 20-l batch processes for the production of Escherichia coli Nissle 1917 BGs.

Salmonella ghosts expressing enterotoxigenic Escherichia coli k88ab, k88ac, k99, and fasa fimbrial antigens induce robust immune responses in a mouse model

BACKGROUND

Bacterial ghosts can be developed as safe and effective vaccines against bacterial infectious disease such as enterotoxigenic Escherichia coli (ETEC)-induced diarrhea in neonatal piglets.

OBJECTIVE

Immune responses against a Salmonella ghost expressing ETEC K88ab, K88ac, K99, and FasA antigens with various adjuvants and inoculation routes were evaluated in mice.

ANIMALS AND METHODS

A ghost cell expressing K88ab, K88ac, K99, and FasA fimbrial antigens of ETEC on the envelope of △asd Salmonella typhimurium was constructed as a candidate vaccine against ETEC infection. To optimize the immunization strategy, 6-week-old female BALB/c mice were inoculated with the ghost and various adjuvants, and the immune responses against the individual fimbrial antigens were measured. Blood samples from caudal vein to evaluate serum IgG concentrations and fecal samples to evaluate mucosal IgA concentrations were collected up to 14 weeks post-prime immunization.

RESULTS

All groups with single, double, and triple inoculations of the ghost showed higher humoral and mucosal immune responses than the control group. In particular, the groups with intramuscular double and triple inoculations showed significantly higher immune responses. In addition, oral inoculation with a combination of the ghost and MONTANIDE IMS 1113 (MI1113) resulted in high and prolonged induction of intestinal IgA levels.

CONCLUSION

These results indicated that both systemic and mucosal immunity against ETEC fimbrial antigens expressed by the ghost are induced by intramuscular booster inoculation with the ghost, and that addition of M1113 to the ghost was found to result in prominent induction of mucosal immunity through oral inoculation.