Involvement of CD8+ T cell-mediated immune responses in LcrV DNA vaccine induced protection against lethal Yersinia pestis challenge

Antioxidant anthocyanin synergistic immune enhancer nanopeptide C-I20 remarkably enhances the protective effect of largemouth bass against largemouth bass ranavirus

Largemouth bass ranavirus (LMBV) infection results in huge economic losses in largemouth bass (Micropterus salmoides) industry. Nanopeptide C-I20 and anthocyanins have a positive effect on promoting immune responses and antioxidant mechanisms in several aquatic organisms, and are therefore used to inhibit LMBV infection. In this study, we developed an LMBV immersion challenge model using three different viral concentrations (1 × 104 copies/mL, 1 × 105 copies/mL, and 1 × 106 copies/mL) to infect largemouth bass, and LMBV-MCP mRNA expression was detected in infected fish. Following infection, the fish exhibited severe external ulceration, redness swelling, and darkening of the skin. Histopathological examination revealed significant necrosis and inflammation in muscle tissue, epithelial cell shedding in renal tubules, macrophage aggregation centers and cellular vacuolization in spleen and head kidney, and cellular hypertrophy in liver. To mitigate LMBV infection, we explored the protective effects of a combined treatment strategy involving C-I20 and anthocyanin. Overall, the combination of anthocyanin and C-I20 demonstrated the highest protective efficacy, significantly reducing viral loads in muscle, liver, spleen, and head kidney. Moreover, this treatment regimen enhanced antioxidant enzyme activities (T-AOC, TSOD, GSH-Px, CAT) and modulated important immune genes (IL-1, IL-8, TNF-α, IL-10, Mx, and IgM) expression. In conclusion, the synergistic application of anthocyanin and C-I20 demonstrates significant efficacy in mitigating LMBV infection. This research introduces a novel and promising approach to managing infectious diseases in aquaculture settings.

Depletion of CD8+ T cells from vaccinated goats does not affect protection from challenge with wild-type peste des petits ruminants virus

Peste des petits ruminants (PPR) is a severe disease of goats and sheep that is widespread in Africa, the Middle East and Asia. The disease is caused by peste des petits ruminants virus (PPRV); cell culture-attenuated strains of PPRV have been shown, both experimentally and by extensive use in the field, to be effective vaccines and are widely used. We have previously demonstrated that these vaccines elicit both serological (PPRV-specific antibody) and cell-based (PPRV-specific CD4⁺ and CD8⁺ T cells) immune responses. However, it is not known which of these responses are required for protection from PPRV, information that would be useful in the evaluation of new vaccines that are being developed to provide the capability to differentiate infected and vaccinated animals (DIVA capability). To begin to address this issue, we have used a complement-fixing monoclonal antibody recognizing caprine CD8 to deplete >99.9% of circulating CD8⁺ T cells from vaccinated goats. Animals were then infected with wild-type PPRV. Despite the absence of the CD8⁺ T-cell component of the vaccine-induced immune response, the vaccinated animals were almost fully protected, showing no pyrexia or viraemia, and almost no clinical signs. These data suggest that a virus-specific CD8⁺ T-cell response is not critical for protection against PPRV and that virus-specific antibody and/or CD4⁺ T cells are the main mediators of protection. We have also shown that the leucopenia caused by infection with wild-type PPRV affects all major classes of circulating leucocytes.

Development of a multiple-antigen protein fusion vaccine candidate that confers protection against Bacillus anthracis and Yersinia pestis

Bacillus anthracis and Yersinia pestis are zoonotic bacteria capable of causing severe and sometimes fatal infections in animals and humans. Although considered as diseases of antiquity in industrialized countries due to animal and public health improvements, they remain endemic in vast regions of the world disproportionally affecting the poor. These pathogens also remain a serious threat if deployed in biological warfare. A single vaccine capable of stimulating rapid protection against both pathogens would be an extremely advantageous public health tool. We produced multiple-antigen fusion proteins (MaF1 and MaF2) containing protective regions from B. anthracis protective antigen (PA) and lethal factor (LF), and from Y. pestis V antigen (LcrV) and fraction 1 (F1) capsule. The MaF2 sequence was also expressed from a plasmid construct (pDNA-MaF2). Immunogenicity and protective efficacy were investigated in mice following homologous and heterologous prime-boost immunization. Antibody responses were determined by ELISA and anthrax toxin neutralization assay. Vaccine efficacy was determined against lethal challenge with either anthrax toxin or Y. pestis. Both constructs elicited LcrV and LF-specific serum IgG, and MaF2 elicited toxin-neutralizing antibodies. Immunizations with MaF2 conferred 100% and 88% protection against Y. pestis and anthrax toxin, respectively. In contrast, pDNA-MaF2 conferred only 63% protection against Y. pestis and no protection against anthrax toxin challenge. pDNA-MaF2-prime MaF2-boost induced 75% protection against Y. pestis and 25% protection against anthrax toxin. Protection was increased by the molecular adjuvant CARDif. In conclusion, MaF2 is a promising multi-antigen vaccine candidate against anthrax and plague that warrants further investigation.

Anthrax and plague are ancient infectious diseases that continue to affect people living in poor, endemic regions and to threaten industrialized nations due to their potential use in biowarfare. Candidate vaccines need improvement to minimize non-desirable effects and increase their efficacy. The purpose of this work was to develop and evaluate a single subunit vaccine capable of conferring protection against Bacillus anthracis and Yersinia pestis. To this end, specific regions from their genome or key protective protein sequences from both microorganisms were combined to obtain either recombinant plasmids or recombinant proteins and tested as vaccine candidates in mice. The recombinant protein MaF2 induced specific antibody responses and afforded full and partial protection against Y. pestis and B. anthracis, respectively. Meanwhile, the DNA vaccine equivalent to MaF2 conferred only partial protection against Y. pestis, which increased when combined with an MaF2 protein boost. MaF2 emerged as a promising dual pathogen recombinant vaccine that warrants further investigation.

Robust Th1 cellular and humoral responses generated by the Yersinia pestis rF1-V subunit vaccine formulated to contain an agonist of the CD137 pathway do not translate into increased protection against pneumonic plague

Yersinia pestis is the causative agent of plague and is a re-emerging pathogen that also has the potential as a biological weapon, necessitating the development of a preventive vaccine. Despite intense efforts for the last several decades, there is currently not a vaccine approved by the FDA. The rF1-V vaccine adjuvanted with Alhydrogel is a lead candidate subunit vaccine for plague and generates a strong Th2-mediate humoral response with a modest Th1 cellular response. As immune protection against Y. pestis requires both humoral and Th1 cellular responses, modifying the rF1-V subunit vaccine formulation to include a robust inducer of Th1 responses may improve efficacy. Thus, we reformulated the subunit vaccine to include SA-4-1BBL, an agonist of the CD137 costimulatory pathway and a potent inducer of Th1 response, and assessed its protective efficacy against pneumonic plague. We herein show for the first time a sex bias in the prophylactic efficacy of the Alhydrogel adjuvanted rF1-V vaccine, with female mice showing better protection against pneumonic plague than male. The sex bias for protection was irrespective of the generation of comparable levels of rF1-V-specific antibody titers and Th1 cellular responses in both sexes. The subunit vaccine reformulated with SA-4-1BBL generated robust Th1 cellular and humoral responses. A prime-boost vaccination scheme involving prime with rF1-V + Alhydrogel and boost with the rF1-V + SA-4-1BBL provided protection in male mice against pneumonic plague. In marked contrast, prime and boost with rF1-V reformulated with both adjuvants resulted in the loss of protection against pneumonic plague, despite generating high levels of humoral and Th1 cellular responses. While unexpected, these findings demonstrate the complexity of immune mechanisms required for protection. Elucidating mechanisms responsible for these differences in protection will help to guide the development of better prophylactic subunit vaccines effective against pneumonic plague.

The rise of pneumonic plague in Madagascar: current plague outbreak breaks usual seasonal mould

Madagascar has just emerged from the grip of an acute urban pneumonic plague outbreak, which began in August 2017, before the usual plague season of October-April and outside the traditional plague foci in the northern and central highlands. The World Health Organization reported a total of 2417 confirmed, probable and suspected cases, including 209 deaths between 1 August and 26 November 2017. The severity and scope of this outbreak, which has affected those in higher socioeconomic groups as well as those living in poverty, along with factors including the potential for use of multi-drug-resistant strains of plague in bioterrorism, highlights the ongoing threat posed by this ancient disease. Factors likely to have contributed to transmission include human behaviour, including burial practices and movement of people, poor urban planning leading to overcrowding and ready transmission by airborne droplets, climatic factors and genomic subtypes. The outbreak demonstrates the importance of identifying targeted pneumonic plague therapies and of developing vaccines that can be administered in planned programmes in developing countries such as Madagascar where plague is endemic. The dominance of pneumonic plague in this outbreak suggests that we need to focus more urgently on the danger of person-to-person transmission, as well as the problem of transmission of plague from zoonotic sources.

A Bivalent Anthrax-Plague Vaccine That Can Protect against Two Tier-1 Bioterror Pathogens, Bacillus anthracis and Yersinia pestis

Bioterrorism remains as one of the biggest challenges to global security and public health. Since the deadly anthrax attacks of 2001 in the United States, Bacillus anthracis and Yersinia pestis, the causative agents of anthrax and plague, respectively, gained notoriety and were listed by the CDC as Tier-1 biothreat agents. Currently, there is no Food and Drug Administration-approved vaccine against either of these threats for mass vaccination to protect general public, let alone a bivalent vaccine. Here, we report the development of a single recombinant vaccine, a triple antigen consisting of all three target antigens, F1 and V from Y. pestis and PA from B. anthracis, in a structurally stable context. Properly folded and soluble, the triple antigen retained the functional and immunogenicity properties of all three antigens. Remarkably, two doses of this immunogen adjuvanted with Alhydrogel^® elicited robust antibody responses in mice, rats, and rabbits and conferred complete protection against inhalational anthrax and pneumonic plague. No significant antigenic interference was observed. Furthermore, we report, for the first time, complete protection of animals against simultaneous challenge with Y. pestis and the lethal toxin of B. anthracis, demonstrating that a single biodefense vaccine can protect against a bioterror attack with weaponized B. anthracis and/or Y. pestis. This bivalent anthrax–plague vaccine is, therefore, a strong candidate for stockpiling, after demonstration of its safety and immunogenicity in human clinical trials, as part of national preparedness against two of the deadliest bioterror threats.

Novel CTL epitopes identified through a Y. pestis proteome-wide analysis in the search for vaccine candidates against plague

The causative agent of Plague, Yersinia pestis, is a highly virulent pathogen and a potential bioweapon. Depending on the route of infection, two prevalent occurrences of the disease are known, bubonic and pneumonic. The latter has a high fatality rate. In the absence of a licensed vaccine, intense efforts to develop a safe and efficacious vaccine have been conducted, and humoral-driven subunit vaccines containing the F1 and LcrV antigens are currently under clinical trials. It is well known that a cellular immune response might have an essential additive value to immunity and protection against Y. pestis infection. Nevertheless, very few documented epitopes eliciting a protective T-cell response have been reported. Here, we present a combined high throughput computational and experimental effort towards identification of CD8 T-cell epitopes. All 4067 proteins of Y. pestis were analyzed with state-of-the-art recently developed prediction algorithms aimed at mapping potential MHC class I binders. A compilation of the results obtained from several prediction methods revealed a total of 238,000 peptide candidates, which necessitated downstream filtering criteria. Our previously established and proven approach for enrichment of true positive CTL epitopes, which relies on mapping clusters rich in tandem or overlapping predicted MHC binders ("hotspots"), was applied, as well as considerations of predicted binding affinity. A total of 1532 peptides were tested for their ability to elicit a specific T-cell response by following the production of IFNγ from splenocytes isolated from vaccinated mice. Altogether, the screen resulted in 178 positive responders (11.8%), all novel Y. pestis CTL epitopes. These epitopes span 113 Y. pestis proteins. Substantial enrichment of membrane-associated proteins was detected for epitopes selected from hotspots of predicted MHC binders. These results considerably expand the repertoire of known CTL epitopes in Y. pestis and pave the way to attest their protective potential, and hence their contribution to a future potent subunit vaccine.

YopE specific CD8+ T cells provide protection against systemic and mucosal Yersinia pseudotuberculosis infection

Prior studies indicated that CD8+ T cells responding to a surrogate single antigen expressed by Y. pseudotuberculosis, ovalbumin, were insufficient to protect against yersiniosis. Herein we tested the hypothesis that CD8+ T cells reactive to the natural Yersinia antigen YopE would be more effective at providing mucosal protection. We first confirmed that immunization with the attenuated ksgA- strain of Y. pseudotuberculosis generated YopE-specific CD8+ T cells. These T cells were protective against challenge with virulent Listeria monocytogenes expressing secreted YopE. Mice immunized with an attenuated L. monocytogenes YopE+ strain generated large numbers of functional YopE-specific CD8+ T cells, and initially controlled a systemic challenge with virulent Y. pseudotuberculosis, yet eventually succumbed to yersiniosis. Mice vaccinated with a YopE peptide and cholera toxin vaccine generated robust T cell responses, providing protection to 60% of the mice challenged mucosally but failed to show complete protection against systemic infection with virulent Y. pseudotuberculosis. These studies demonstrate that vaccination with recombinant YopE vaccines can generate YopE-specific CD8+ T cells, that can provide significant mucosal protection but these cells are insufficient to provide sterilizing immunity against systemic Y. pseudotuberculosis infection. Our studies have implications for Yersinia vaccine development studies.

Plague Vaccine Development: Current Research and Future Trends

Plague is one of the world’s most lethal human diseases caused by Yersinia pestis, a Gram-negative bacterium. Despite overwhelming studies for many years worldwide, there is no safe and effective vaccine against this fatal disease. Inhalation of Y. pestis bacilli causes pneumonic plague, a fast growing and deadly dangerous disease. F1/LcrV-based vaccines failed to provide adequate protection in African green monkey model in spite of providing protection in mice and cynomolgus macaques. There is still no explanation for this inconsistent efficacy, and scientists leg behind to search reliable correlate assays for immune protection. These paucities are the main barriers to improve the effectiveness of plague vaccine. In the present scenario, one has to pay special attention to elicit strong cellular immune response in developing a next-generation vaccine against plague. Here, we review the scientific contributions and existing progress in developing subunit vaccines, the role of molecular adjuvants; DNA vaccines; live delivery platforms; and attenuated vaccines developed to counteract virulent strains of Y. pestis.

Characterization of the protective immune response to Yersinia pseudotuberculosis infection in mice vaccinated with an LcrV-secreting strain of Lactococcus lactis

BACKGROUND

Pseudotuberculosis is an infection caused by the bacterial enteropathogen Yersinia pseudotuberculosis and is considered to be a significant problem in veterinary medicine. We previously found that intranasal administration of a recombinant Lactococcus lactis strain that secretes the low-calcium response V (LcrV) antigen from Y. pseudotuberculosis (Ll-LcrV) confers protection against a lethal Y. pseudotuberculosis infection. Here, we aimed at characterizing the immunological basis of this LcrV-elicited protective response and at determining the duration of vaccine-induced immunity.

METHODS

Splenocytes from BALB/c mice intranasally immunized with Ll-LcrV or Ll as control were immunostained then analyzed by flow cytometry. Protection against a lethal intravenous injection of Y. pseudotuberculosis was also determined (i) in immunized BALB/c mice depleted or not of CD4⁺, CD8⁺ or CD25⁺ cells and (ii) in naïve BALB/c mice receiving serum from immunized mice by counting the number of bacteria in liver and spleen. Lastly, survival rate of immunized BALB/c mice following a lethal intravenous injection of Y. pseudotuberculosis was followed up to 9-months.

RESULTS

We found that T and B lymphocytes but not non-conventional lymphoid cells were affected by Ll-LcrV immunization. We also observed that depletion of CD4⁺ and CD25⁺ but not CD8⁺ cells in immunized mice eradicated protection against a lethal systemic Y. pseudotuberculosis infection, suggesting that activated CD4⁺ T lymphocytes are required for vaccine-induced protection. Adoptive transfer of LcrV-specific antibodies from Ll-LcrV-immunized animals significantly reduced the bacterial counts in the liver compared to non-vaccinated mice. Lastly, the protective immunity conferred by Ll-LcrV decreased slightly over time; nevertheless almost 60% of the mice survived a lethal bacterial challenge at 9months post-vaccination.

CONCLUSION

Mucosal vaccination of mice with Ll-LcrV induced cell- and antibody-mediated protective immunity against Y. pseudotuberculosis infection in the mouse and the protection is long-lasting.

Plague Vaccines: Status and Future

Three major plague pandemics caused by the gram-negative bacterium Yersinia pestis have killed nearly 200 million people in human history. Due to its extreme virulence and the ease of its transmission, Y. pestis has been used purposefully for biowarfare in the past. Currently, plague epidemics are still breaking out sporadically in most of parts of the world, including the United States. Approximately 2000 cases of plague are reported each year to the World Health Organization. However, the potential use of the bacteria in modern times as an agent of bioterrorism and the emergence of a Y. pestis strain resistant to eight antibiotics bring out severe public health concerns. Therefore, prophylactic vaccination against this disease holds the brightest prospect for its long-term prevention. Here, we summarize the progress of the current vaccine development for counteracting plague.

Oral administration of a recombinant attenuated Yersinia pseudotuberculosis strain elicits protective immunity against plague

A Yersinia pseudotuberculosis PB1+ (Yptb PB1+) mutant strain combined with chromosome insertion of the caf1R-caf1A-caf1M-caf1 operon and deletions of yopJ and yopK, χ10068 [pYV-ω2 (ΔyopJ315 ΔyopK108) ΔlacZ044::caf1R-caf1M-caf1A-caf1] was constructed. Results indicated that gene insertion and deletion did not affect the growth rate of χ10068 compared to wild-type Yptb cultured at 26 °C. In addition, the F1 antigen in χ10068 was synthesized and secreted on the surface of bacteria at 37 °C (mammalian body temperature), not at ambient culture temperature (26 °C). Immunization with χ10068 primed antibody responses and specific T-cell responses to F1 and YpL (Y. pestis whole cell lysate). Oral immunization with a single dose of χ10068 provided 70% protection against a subcutaneous (s.c.) challenge with ∼ 2.6 × 10(5) LD50 of Y. pestis KIM6+ (pCD1Ap) (KIM6+Ap) and 90% protection against an intranasal (i.n.) challenge with ∼ 500 LD50 of KIM6+Ap in mice. Our results suggest that χ10068 can be used as an effective precursor to make a safe vaccine to prevent plague in humans and to eliminate plague circulation among humans and animals.

LcrV delivered via type III secretion system of live attenuated Yersinia pseudotuberculosis enhances immunogenicity against pneumonic plague

Here, we constructed a Yersinia pseudotuberculosis mutant strain with arabinose-dependent regulated and delayed shutoff of crp expression (araC P(BAD) crp) and replacement of the msbB gene with the Escherichia coli msbB gene to attenuate it. Then, we inserted the asd mutation into this construction to form χ10057 [Δasd-206 ΔmsbB868::P(msbB) msbB(EC) ΔP(crp21)::TT araC P(BAD) crp] for use with a balanced-lethal Asd-positive (Asd(+)) plasmid to facilitate antigen synthesis. A hybrid protein composed of YopE (amino acids [aa]1 to 138) fused with full-length LcrV (YopE(Nt138)-LcrV) was synthesized in χ10057 harboring an Asd(+) plasmid (pYA5199, yopE(Nt138)-lcrV) and could be secreted through a type III secretion system (T3SS) in vitro and in vivo. Animal studies indicated that mice orally immunized with χ10057(pYA5199) developed titers of IgG response to whole-cell lysates of Y. pestis (YpL) and subunit LcrV similar to those seen with χ10057(pYA3332) (χ10057 plus an empty plasmid). However, only immunization of mice with χ10057(pYA5199) resulted in a significant secretory IgA response to LcrV. χ10057(pYA5199) induced a higher level of protection (80% survival) against intranasal (i.n.) challenge with ~240 median lethal doses (LD50) (2.4 × 10(4) CFU) of Y. pestis KIM6+(pCD1Ap) than χ10057(pYA3332) (40% survival). Splenocytes from mice vaccinated with χ10057(pYA5199) produced significant levels of gamma interferon (IFN-γ), tumor necrosis factor alpha (TNF-α), and interleukin-17 (IL-17) after restimulation with LcrV and YpL antigens. Our results suggest that it is possible to use an attenuated Y. pseudotuberculosis strain delivering the LcrV antigen via the T3SS as a potential vaccine candidate against pneumonic plague.

Improving the Th1 cellular efficacy of the lead Yersinia pestis rF1-V subunit vaccine using SA-4-1BBL as a novel adjuvant

The lead candidate plague subunit vaccine is the recombinant fusion protein rF1-V adjuvanted with alum. While alum generates Th2 regulated robust humoral responses, immune protection against Yersinia pestis has been shown to also involve Th1 driven cellular responses. Therefore, the rF1-V-based subunit vaccine may benefit from an adjuvant system that generates a mixed Th1 and humoral immune response. We herein assessed the efficacy of a novel SA-4-1BBL costimulatory molecule as a Th1 adjuvant to improve cellular responses generated by the rF1-V vaccine. SA-4-1BBL as a single adjuvant had better efficacy than alum in generating CD4(+) and CD8(+) T cells producing TNFα and IFNγ, signature cytokines for Th1 responses. The combination of SA-4-1BBL with alum further increased this Th1 response as compared with the individual adjuvants. Analysis of the humoral response revealed that SA-4-1BBL as a single adjuvant did not generate a significant Ab response against rF1-V, and SA-4-1BBL in combination with alum did not improve Ab titers. However, the combined adjuvants significantly increased the ratio of Th1 regulated IgG2c in C57BL/6 mice to the Th2 regulated IgG1. Finally, a single vaccination with rF1-V adjuvanted with SA-4-1BBL+alum had better protective efficacy than vaccines containing individual adjuvants. Taken together, these results demonstrate that SA-4-1BBL improves the protective efficacy of the alum adjuvanted lead rF1-V subunit vaccine by generating a more balanced Th1 cellular and humoral immune response. As such, this adjuvant platform may prove efficacious not only for the rF1-V vaccine but also against other infections that require both cellular and humoral immune responses for protection.

Pilot Study on the Use of DNA Priming Immunization to Enhance Y. pestis LcrV-Specific B Cell Responses Elicited by a Recombinant LcrV Protein Vaccine

Recent studies indicate that DNA immunization is powerful in eliciting antigen-specific antibody responses in both animal and human studies. However, there is limited information on the mechanism of this effect. In particular, it is not known whether DNA immunization can also enhance the development of antigen-specific B cell development. In this report, a pilot study was conducted using plague LcrV immunogen as a model system to determine whether DNA immunization is able to enhance LcrV-specific B cell development in mice. Plague is an acute and often fatal infectious disease caused by Yersinia pestis (Y. pestis). Humoral immune responses provide critical protective immunity against plague. Previously, we demonstrated that a DNA vaccine expressing LcrV antigen can protect mice from lethal mucosal challenge. In the current study, we further evaluated whether the use of a DNA priming immunization is able to enhance the immunogenicity of a recombinant LcrV protein vaccine, and in particular, the development of LcrV-specific B cells. Our data indicate that DNA immunization was able to elicit high-level LcrV antibody responses when used alone or as part of a prime-boost immunization approach. Most significantly, DNA immunization was also able to increase the levels of LcrV-specific B cell development. The finding that DNA immunization can enhance antigen-specific B cell responses is highly significant and will help guide similar studies in other model antigen systems.

DNA immunization

DNA immunization was discovered in early 1990s, and its use has been expanded from vaccine studies to a broader range of biomedical research areas, such as the generation of high-quality polyclonal and monoclonal antibodies as research reagents. In this unit, three common DNA immunization methods are described: needle injection, electroporation, and gene gun. In addition, several common considerations related to DNA immunization are discussed.

Post-translational intracellular trafficking determines the type of immune response elicited by DNA vaccines expressing Gag antigen of Human Immunodeficiency Virus Type 1 (HIV-1)

In the current study, immune responses induced by Gag DNA vaccines with different designs were evaluated in Balb/C mice. The results demonstrated that the DNA vaccine with the full length wild type gag gene (Wt-Gag) mainly produced Gag antigens intracellularly and induced a higher level of cell-mediated immune (CMI) responses, as measured by IFN-gamma ELISPOT, intracellular cytokine staining (ICS), and cytotoxic T lymphocytes (CTL) assays against a dominant CD8(+) T cell epitope (AMQMLKETI). In contrast, the addition of a tissue plasminogen activator (tPA) leader sequence significantly improved overall Gag protein expression/secretion and Gag-specific antibody responses; however, Gag-specific CMI responses were decreased. The mutation of zinc-finger motif changed Gag protein expression patterns and reduced the ability to generate both CMI and antibody responses against Gag. These findings indicate that the structure and post-translational processing of antigens expressed by DNA vaccines play a critical role in eliciting optimal antibody or CMI responses.

Mutated and bacteriophage T4 nanoparticle arrayed F1-V immunogens from Yersinia pestis as next generation plague vaccines

Pneumonic plague is a highly virulent infectious disease with 100% mortality rate, and its causative organism Yersinia pestis poses a serious threat for deliberate use as a bioterror agent. Currently, there is no FDA approved vaccine against plague. The polymeric bacterial capsular protein F1, a key component of the currently tested bivalent subunit vaccine consisting, in addition, of low calcium response V antigen, has high propensity to aggregate, thus affecting its purification and vaccine efficacy. We used two basic approaches, structure-based immunogen design and phage T4 nanoparticle delivery, to construct new plague vaccines that provided complete protection against pneumonic plague. The NH₂-terminal β-strand of F1 was transplanted to the COOH-terminus and the sequence flanking the β-strand was duplicated to eliminate polymerization but to retain the T cell epitopes. The mutated F1 was fused to the V antigen, a key virulence factor that forms the tip of the type three secretion system (T3SS). The F1mut-V protein showed a dramatic switch in solubility, producing a completely soluble monomer. The F1mut-V was then arrayed on phage T4 nanoparticle via the small outer capsid protein, Soc. The F1mut-V monomer was robustly immunogenic and the T4-decorated F1mut-V without any adjuvant induced balanced T_H1 and T_H2 responses in mice. Inclusion of an oligomerization-deficient YscF, another component of the T3SS, showed a slight enhancement in the potency of F1-V vaccine, while deletion of the putative immunomodulatory sequence of the V antigen did not improve the vaccine efficacy. Both the soluble (purified F1mut-V mixed with alhydrogel) and T4 decorated F1mut-V (no adjuvant) provided 100% protection to mice and rats against pneumonic plague evoked by high doses of Y. pestis CO92. These novel platforms might lead to efficacious and easily manufacturable next generation plague vaccines.

Plague caused by Yersinia pestis is a deadly disease that wiped out one-third of Europe's population in the 14th century. The organism is listed by the CDC as Tier-1 biothreat agent, and currently, there is no FDA-approved vaccine against this pathogen. Stockpiling of an efficacious plague vaccine that could protect people against a potential bioterror attack has been a national priority. The current vaccines based on the capsular antigen (F1) and the low calcium response V antigen, are promising against both bubonic and pneumonic plague. However, the polymeric nature of F1 with its propensity to aggregate affects vaccine efficacy and generates varied immune responses in humans. We have addressed a series of concerns and generated mutants of F1 and V, which are completely soluble and produced in high yields. We then engineered the vaccine into a novel delivery platform using the bacteriophage T4 nanoparticle. The nanoparticle vaccines induced robust immunogenicity and provided 100% protection to mice and rats against pneumonic plague. These highly efficacious new generation plague vaccines are easily manufactured, and the potent T4 platform which can simultaneously incorporate antigens from other biothreat or emerging infectious agents provides a convenient way for mass vaccination of humans against multiple pathogens.

Multiple antigen peptide containing B and T cell epitopes of F1 antigen of Yersinia pestis showed enhanced Th1 immune response in murine model

Yersinia pestis is a facultative bacterium that can survive and proliferate inside host macrophages and cause bubonic, pneumonic and systemic infection. Apart from humoral response, cell-mediated protection plays a major role in combating the disease. Fraction 1 capsular antigen (F1-Ag) of Y. pestis has long been exploited as a vaccine candidate. In this study, F1-multiple antigenic peptide (F1-MAP or MAP)-specific cell-mediated and cytokine responses were studied in murine model. MAP consisting of three B and one T cell epitopes of F1-antigen with one palmitoyl residue was synthesized using Fmoc chemistry. Mice were immunized with different formulations of MAP in poly DL-lactide-co-glycolide (PLGA) microspheres. F1-MAP with CpG oligodeoxynucleotide (CpG-ODN) as an adjuvant showed enhanced in vitro T cell proliferation and Th1 (IL-2, IFN-γ and TNF-α) and Th17 (IL-17A) cytokine secretion. Similar formulation also showed significantly higher numbers of cytokine (IL-2, IFN-γ)-secreting cells. Moreover, F1-MAP with CpG formulation showed significantly high (P < 0.001) percentage of CD4(+) IFN-γ(+) cells as compared to CD8(+) IFN-γ(+) cells, and also more (CD4- IFN-γ)(+) cells secrete perforin and granzyme as compared to (CD8- IFN-γ)(+) showing Th1 response. Thus, the study highlights the importance of Th1 cytokine and existence of CD4(+) and CD8(+) immune response. This study proposes a new perspective for the development of vaccination strategies for Y. pestis that trigger T cell immune response.

Intranasal prophylaxis with CpG oligodeoxynucleotide can protect against Yersinia pestis infection

Immunomodulatory agents potentially represent a new class of broad-spectrum antimicrobials. Here, we demonstrate that prophylaxis with immunomodulatory cytosine-phosphate-guanidine (CpG) oligodeoxynucleotide (ODN), a toll-like receptor 9 (TLR9) agonist, confers protection against Yersinia pestis, the etiologic agent of plague. The data establish that intranasal administration of CpG ODN 1 day prior to lethal pulmonary exposure to Y. pestis strain KIM D27 significantly improves survival of C57BL/6 mice and reduces bacterial growth in hepatic tissue, despite paradoxically increasing bacterial growth in the lung. All of these CpG ODN-mediated impacts, including the increased pulmonary burden, are TLR9 dependent, as they are not observed in TLR9-deficient mice. The capacity of prophylactic intranasal CpG ODN to enhance survival does not require adaptive immunity, as it is evident in mice lacking B and/or T cells; however, the presence of T cells improves long-term survival. The prophylactic regimen also improves survival and reduces hepatic bacterial burden in mice challenged intraperitoneally with KIM D27, indicating that intranasal delivery of CpG ODN has systemic impacts. Indeed, intranasal prophylaxis with CpG ODN provides significant protection against subcutaneous challenge with Y. pestis strain CO92 even though it fails to protect mice from intranasal challenge with that fully virulent strain.

Plague vaccines: current developments and future perspectives

Despite many decades of intensive studies of Yersinia pestis, the causative agent of plague, there is no safe and efficient vaccine against this devastating disease. A recently developed F1/V subunit vaccine candidate, which relies mainly on humoral immunity, showed promising results in animal studies; however, its efficacy in humans still has to be carefully evaluated. In addition, those developing next-generation plague vaccines need to pay particular attention to the importance of eliciting cell-mediated immunity. In this review, we analyzed the current progress in developing subunit, DNA and live carrier platforms of delivery by bacterial and viral vectors, as well as approaches for controlled attenuation of virulent strains of Y. pestis.

Cell-mediated immune response to epitopic MAP (multiple antigen peptide) construct of LcrV antigen of Yersinia pestis in murine model

Yersinia pestis is the causative agent of plague. Cellular immunity seems to play an important role in defense against this disease. The subunit vaccine based on V (Lcr V) antigen has been proved to be immunogenic in animals and in humans. The multiple antigen peptide (MAP), incorporating all the relevant B and T cell epitopes is highly immunogenic in mice through intranasal route of immunization in PLGA particles containing CpG-ODN as an immunoadjuvant inducing humoral and mucosal immune response. In the present study, cell-mediated immune response using same MAP was studied in murine model. Primary and memory T cell responses were studied in outbred and inbred mice immunized intranasally with MAP in the presence of two immunoadjuvants (Murabutide and CpG-ODN). All the three compartments (Spleen, Lamina propria and Peyer's patches) of the lymphoid system showed increased lymphoproliferative response. Highest lymphoproliferative response was observed especially with CpG-ODN. Cytokine profile in the culture supernatant showed highest Th(1) and Th(17) levels. FACS analysis showed expansion of both CD4(+) and CD8(+) T cells producing gamma-interferon, perforin and granzyme-B with major contribution from CD4(+) T cells.

The natural history and incidence of Yersinia pestis and prospects for vaccination

Plague is an ancient, serious, infectious disease which is still endemic in regions of the modern world and is a potential biothreat agent. This paper discusses the natural history of the bacterium and its evolution into a flea-vectored bacterium able to transmit bubonic plague. It reviews the incidence of plague in the modern world and charts the history of vaccines which have been used to protect against the flea-vectored disease, which erupts as bubonic plague. Current approaches to vaccine development to protect against pneumonic, as well as bubonic, plague are also reviewed. The considerable challenges in achieving a vaccine which is licensed for human use and which will comprehensively protect against this serious human pathogen are assessed.

Yersinia pestis YopE contains a dominant CD8 T cell epitope that confers protection in a mouse model of pneumonic plague

Septic bacterial pneumonias are a major cause of death worldwide. Several of the highest priority bioterror concerns, including anthrax, tularemia, and plague, are caused by bacteria that acutely infect the lung. Bacterial resistance to multiple antibiotics is increasingly common. Although vaccines may be our best defense against antibiotic-resistant bacteria, there has been little progress in the development of safe and effective vaccines for pulmonary bacterial pathogens. The Gram-negative bacterium Yersinia pestis causes pneumonic plague, an acutely lethal septic pneumonia. Historic pandemics of plague caused millions of deaths, and the plague bacilli's potential for weaponization sustains an ongoing quest for effective countermeasures. Subunit vaccines have failed, to date, to fully protect nonhuman primates. In mice, they induce the production of Abs that act in concert with type 1 cytokines to deliver high-level protection; however, the Y. pestis Ags recognized by cytokine-producing T cells have yet to be defined. In this study, we report that Y. pestis YopE is a dominant Ag recognized by CD8 T cells in C57BL/6 mice. After vaccinating with live attenuated Y. pestis and challenging intranasally with virulent plague, nearly 20% of pulmonary CD8 T cells recognize this single, highly conserved Ag. Moreover, immunizing mice with a single peptide, YopE(69-77), suffices to confer significant protection from lethal pulmonary challenge. These findings suggest YopE could be a valuable addition to subunit plague vaccines and provide a new animal model in which sensitive, pathogen-specific assays can be used to study CD8 T cell-mediated defense against acutely lethal bacterial infections of the lung.