Single-dose, virus-vectored vaccine protection against Yersinia pestis challenge: CD4+ cells are required at the time of challenge for optimal protection

Design of Replication-Competent VSV- and Ervebo-Vectored Vaccines Against SARS-CoV-2

Coronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is a global public health emergency. Several vaccine candidates have been developed in response to the COVID-19 pandemic. One approach is to construct live-recombinant viruses expressing the SARS-CoV-2 spike protein (S) as vaccine candidates. The vesicular stomatitis virus (VSV) vector is a mature vaccine platform which was successfully developed as a vaccine against Ebola virus (EBOV), leading to its licensure by the Food and Drug Administration (FDA) in December 2019. Based on this work, we developed two live, replication-competent VSV-vectored vaccines against SARS-CoV-2: (1) a VSV expressing the S protein of SARS-CoV-2 and (2) a bivalent VSV expressing the S protein of SARS-CoV-2 and the glycoprotein (GP) of EBOV. This protocol describes the methodologies for the design, cloning, rescue, and preparation of these recombinant VSV vaccines.

Rhabdoviruses as vectors for vaccines and therapeutics

Appropriate choice of vaccine vector is crucial for effective vaccine development. Rhabdoviral vectors, such as rabies virus and vesicular stomatitis virus, have been used in a variety of vaccine strategies. These viruses have small, easily manipulated genomes that can stably express foreign glycoproteins due to a well-established reverse genetics system for virus recovery. Both viruses have well-described safety profiles and have been demonstrated to be effective vaccine vectors. This review will describe how these Rhabdoviruses can be manipulated for use as vectors, their various applications as vaccines or therapeutics, and the advantages and disadvantages of their use.

Current State of the Problem of Vaccine Development for Specific Prophylaxis of Plague

Emergence of large-scale plague outbreaks in Africa and South America countries in the modern period, characterized by high frequency of pneumonic plague development (including with lethal outcome) keeps up the interest of scientists to the matters of development and testing of means for specific prophylaxis of this particularly dangerous infectious disease. WHO workshop that was held in 2018 identified the general principles of optimization of design and testing of new-generation vaccines effectively protecting the population from plague infection. Application of the achievements of biological and medical sciences for outlining rational strategy for construction of immunobiological preparations led to a certain progress in the creation of not only sub-unit vaccines based on recombinant antigens, but also live and vector preparations on the platform of safe bacterial strains and replicating and non-replicating viruses in recent years. The review comprehensively considers the relevant trends in vaccine construction for plague  prevention, describes advantages of the state-of-the art methodologies for their safety and efficiency enhancement.

Chikungunya, Influenza, Nipah, and Semliki Forest Chimeric Viruses with Vesicular Stomatitis Virus: Actions in the Brain

Recombinant vesicular stomatitis virus (VSV)-based chimeric viruses that include genes from other viruses show promise as vaccines and oncolytic viruses. However, the critical safety concern is the neurotropic nature conveyed by the VSV glycoprotein. VSVs that include the VSV glycoprotein (G) gene, even in most recombinant attenuated strains, can still show substantial adverse or lethal actions in the brain. Here, we test 4 chimeric viruses in the brain, including those in which glycoprotein genes from Nipah, chikungunya (CHIKV), and influenza H5N1 viruses were substituted for the VSV glycoprotein gene. We also test a virus-like vesicle (VLV) in which the VSV glycoprotein gene is expressed from a replicon encoding the nonstructural proteins of Semliki Forest virus. VSVΔG-CHIKV, VSVΔG-H5N1, and VLV were all safe in the adult mouse brain, as were VSVΔG viruses expressing either the Nipah F or G glycoprotein. In contrast, a complementing pair of VSVΔG viruses expressing Nipah G and F glycoproteins were lethal within the brain within a surprisingly short time frame of 2 days. Intranasal inoculation in postnatal day 14 mice with VSVΔG-CHIKV or VLV evoked no adverse response, whereas VSVΔG-H5N1 by this route was lethal in most mice. A key immune mechanism underlying the safety of VSVΔG-CHIKV, VSVΔG-H5N1, and VLV in the adult brain was the type I interferon response; all three viruses were lethal in the brains of adult mice lacking the interferon receptor, suggesting that the viruses can infect and replicate and spread in brain cells if not blocked by interferon-stimulated genes within the brain.IMPORTANCE Vesicular stomatitis virus (VSV) shows considerable promise both as a vaccine vector and as an oncolytic virus. The greatest limitation of VSV is that it is highly neurotropic and can be lethal within the brain. The neurotropism can be mostly attributed to the VSV G glycoprotein. Here, we test 4 chimeric viruses of VSV with glycoprotein genes from Nipah, chikungunya, and influenza viruses and nonstructural genes from Semliki Forest virus. Two of the four, VSVΔG-CHIKV and VLV, show substantially attenuated neurotropism and were safe in the healthy adult mouse brain. VSVΔG-H5N1 was safe in the adult brain but lethal in the younger brain. VSVΔG Nipah F+G was even more neurotropic than wild-type VSV, evoking a rapid lethal response in the adult brain. These results suggest that while chimeric VSVs show promise, each must be tested with both intranasal and intracranial administration to ensure the absence of lethal neurotropism.

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.

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.

A viable recombinant rhabdovirus lacking its glycoprotein gene and expressing influenza virus hemagglutinin and neuraminidase is a potent influenza vaccine

The emergence of novel influenza viruses that cause devastating human disease is an ongoing threat and serves as an impetus for the continued development of novel approaches to influenza vaccines. Influenza vaccine development has traditionally focused on producing humoral and/or cell-mediated immunity, often against the viral surface glycoproteins hemagglutinin (HA) and neuraminidase (NA). Here, we describe a new vaccine candidate that utilizes a replication-defective vesicular stomatitis virus (VSV) vector backbone that lacks the native G surface glycoprotein gene (VSVΔG). The expression of the H5 HA of an H5N1 highly pathogenic avian influenza virus (HPAIV), A/Vietnam/1203/04 (VN1203), and the NA of the mouse-adapted H1N1 influenza virus A/Puerto Rico/8/34 (PR8) in the VSVΔG vector restored the ability of the recombinant virus to replicate in cell culture, without the requirement for the addition of trypsin. We show here that this recombinant virus vaccine candidate was nonpathogenic in mice when given by either the intramuscular or intranasal route of immunization and that the in vivo replication of VSVΔG-H5N1 is profoundly attenuated. This recombinant virus also provided protection against lethal H5N1 infection after a single dose. This novel approach to vaccination against HPAIVs may be widely applicable to other emerging strains of influenza virus.

IMPORTANCE

Preparation for a potentially catastrophic influenza pandemic requires novel influenza vaccines that are safe, can be produced and administered quickly, and are effective, both soon after administration and for a long duration. We have created a new influenza vaccine that utilizes an attenuated vesicular stomatitis virus (VSV) vector, to deliver and express influenza virus proteins against which vaccinated animals develop potent antibody responses. The influenza virus hemagglutinin and neuraminidase proteins, expressed on the surface of VSV particles, allowed this vaccine to grow in cell culture and induced a potent antibody response in mice that was effective against infection with a lethal influenza virus. The mice showed no adverse reactions to the vaccine, and they were protected against an otherwise lethal influenza infection after only 14 days postvaccination and after as many as 140 days postvaccination. The ability to rapidly produce this safe and effective vaccine in cell culture is additionally advantageous.

RNA-based viral vectors

The advent of reverse genetic approaches to manipulate the genomes of both positive (+) and negative (-) sense RNA viruses allowed researchers to harness these genomes for basic research. Manipulation of positive sense RNA virus genomes occurred first largely because infectious RNA could be transcribed directly from cDNA versions of the RNA genomes. Manipulation of negative strand RNA virus genomes rapidly followed as more sophisticated approaches to provide RNA-dependent RNA polymerase complexes coupled with negative-strand RNA templates were developed. These advances have driven an explosion of RNA virus vaccine vector development. That is, development of approaches to exploit the basic replication and expression strategies of RNA viruses to produce vaccine antigens that have been engineered into their genomes. This study has led to significant preclinical testing of many RNA virus vectors against a wide range of pathogens as well as cancer targets. Multiple RNA virus vectors have advanced through preclinical testing to human clinical evaluation. This review will focus on RNA virus vectors designed to express heterologous genes that are packaged into viral particles and have progressed to clinical testing.

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.

A chimeric vesiculo/alphavirus is an effective alphavirus vaccine

While a large number of mosquito-transmitted alphaviruses are known to cause serious human diseases, there are no licensed vaccines that protect against alphavirus infections. The alphavirus chikungunya virus (CHIKV) has caused multiple recent outbreaks of chikungunya fever. This virus has the potential to cause a worldwide epidemic and has generated strong interest in development of a prophylactic CHIKV vaccine. We report here on the development of a potent experimental vaccine for CHIKV based on a chimeric vesicular stomatitis virus (VSV) expressing the entire CHIKV envelope polyprotein (E3-E2-6K-E1) in place of the VSV glycoprotein (G). These VSVΔG-CHIKV chimeras incorporated functional CHIKV glycoproteins into the viral envelope in place of VSV G. The chimeric viruses were attenuated for growth in tissue culture but could be propagated to high titers without VSV G complementation. They also generated robust neutralizing antibody and cellular immune responses to CHIKV in mice after a single dose and protected mice against CHIKV infection. VSVΔG-alphavirus chimeras could have general applicability as alphavirus vaccines.

Viral vectored granulocyte-macrophage colony stimulating factor inhibits vaccine protection in an SIV challenge model: protection correlates with neutralizing antibody

In a previous vaccine study, we reported significant and apparently sterilizing immunity to high-dose, mucosal, simian immunodeficiency virus (SIV) quasi-species challenge. The vaccine consisted of vectors based on vesicular stomatitis virus (VSV) expressing simian immunodeficiency virus (SIV) gag and env genes, a boost with propagating replicon particles expressing the same SIV genes, and a second boost with VSV-based vectors. Concurrent with that published study we had a parallel group of macaques given the same doses of vaccine vectors, but in addition, we included a third VSV vector expressing rhesus macaque GM-CSF in the priming immunization only. We report here that addition of the vector expressing GM-CSF did not enhance CD8 T cell or antibody responses to SIV antigens, and almost completely abolished the vaccine protection against high-dose mucosal challenge with SIV. Expression of GM-CSF may have limited vector replication excessively in the macaque model. Our results suggest caution in the use of GM-CSF as a vaccine adjuvant, especially when expressed by a viral vector. Combining vaccine group animals from this study and the previous study we found that there was a marginal but significant positive correlation between the neutralizing antibody to a neutralization resistant SIV Env and protection from infection.

The immune response to a vesicular stomatitis virus vaccine vector is independent of particulate antigen secretion and protein turnover rate

Vesicular stomatitis virus (VSV) is a highly cytopathic virus being developed as a vaccine vector due to its ability to induce strong protective T cell and antibody responses after a single dose. However, little is known regarding the mechanisms underlying the potent immune responses elicited by VSV. We previously generated a VSV vector expressing the hepatitis B virus middle envelope surface glycoprotein (MS) that induces strong MS-specific T cell and antibody responses in mice. After synthesis in the cytoplasm, the MS protein translocates to the endoplasmic reticulum, where it forms subviral particles that are secreted from the cell. To better understand the contributions of secreted and intracellular protein to the VSV-induced immune response, we produced a vector expressing a secretion-deficient MS mutant (MS(C69A)) and compared the immunogenicity of this vector to that of the wild-type VSV-MS vector in mice. As expected, the MS(C69A) protein was not secreted from VSV-infected cells and displayed enhanced proteasome-mediated degradation. Surprisingly, despite these differences in intracellular protein processing, the T cell and antibody responses generated to MS(C69A) were comparable to those elicited by virus expressing wild-type MS protein. Therefore, when it is expressed from VSV, the immune responses to MS are independent of particulate antigen secretion and the turnover rate of cytoplasmic protein. These results are consistent with a model in which the immune responses to VSV are strongly influenced by the replication cycle of the vector and demonstrate that characteristics of the vector have the capacity to affect vaccine efficacy more than do the properties of the antigen itself.

An encapsulated Yersinia pseudotuberculosis is a highly efficient vaccine against pneumonic plague

Background

Plague is still a public health problem in the world and is re-emerging, but no efficient vaccine is available. We previously reported that oral inoculation of a live attenuated Yersinia pseudotuberculosis, the recent ancestor of Yersinia pestis, provided protection against bubonic plague. However, the strain poorly protected against pneumonic plague, the most deadly and contagious form of the disease, and was not genetically defined.

Methodology and Principal Findings

The sequenced Y. pseudotuberculosis IP32953 has been irreversibly attenuated by deletion of genes encoding three essential virulence factors. An encapsulated Y. pseudotuberculosis was generated by cloning the Y. pestis F1-encoding caf operon and expressing it in the attenuated strain. The new V674pF1 strain produced the F1 capsule in vitro and in vivo. Oral inoculation of V674pF1 allowed the colonization of the gut without lesions to Peyer's patches and the spleen. Vaccination induced both humoral and cellular components of immunity, at the systemic (IgG and Th1 cells) and the mucosal levels (IgA and Th17 cells). A single oral dose conferred 100% protection against a lethal pneumonic plague challenge (33×LD₅₀ of the fully virulent Y. pestis CO92 strain) and 94% against a high challenge dose (3,300×LD₅₀). Both F1 and other Yersinia antigens were recognized and V674pF1 efficiently protected against a F1-negative Y. pestis.

Conclusions and Significance

The encapsulated Y. pseudotuberculosis V674pF1 is an efficient live oral vaccine against pneumonic plague, and could be developed for mass vaccination in tropical endemic areas to control pneumonic plague transmission and mortality.

Plague, among the most deadly infections of mankind's history, is present in Africa, Asia and America, and is currently re-emerging, recently causing cases in areas from where it had disappeared for decades. Pneumonic plague, its most deadly and contagious form, is responsible for human-to-human spreading of the infection. Vaccination would be an effective means to control the disease, but no efficient vaccine is currently available. Because live vaccines are potent inducers of protective immunity, our strategy was to use a Yersinia pseudotuberculosis, closely related to Y. pestis but genetically more stable, to make it suitable for use as live oral vaccine. We have developed a genetically defined Y. pseudotuberculosis strain strongly attenuated by deletion of virulence factors genes, which was also induced to produce the Y. pestis F1 pseudocapsule. A single oral dose was harmless and provided high- level protection against pneumonic plague. Such a candidate vaccine offers promising perspectives to control pneumonic plague mortality and transmission.

Developing live vaccines against plague

Three great plague pandemics caused by the gram-negative bacterium Yersinia pestis have killed nearly 200 million people and it has been linked to biowarfare in the past. Plague is endemic in many parts of the world. In addition, the risk of plague as a bioweapon has prompted increased research to develop plague vaccines against this disease. Injectable subunit vaccines are being developed in the United States and United Kingdom.  However, the live attenuated Y. pestis-EV NIIEG strain has been used as a vaccine for more than 70 years in the former Soviet Union and in some parts of Asia and provides a high degree of efficacy against plague.  This vaccine has not gained general acceptance because of safety concerns.  In recent years, modern molecular biological techniques have been applied to Y. pestis to construct strains with specific defined mutations designed to create safe, immunogenic vaccines with potential for use in humans and as bait vaccines to reduce the load of Y. pestis in the environment.  In addition, a number of live, vectored vaccines have been reported using attenuated viral vectors or attenuated Salmonella strains to deliver plague antigens. Here we summarize the progress of live attenuated vaccines against plagu.

Intranasal administration of an inactivated Yersinia pestis vaccine with interleukin-12 generates protective immunity against pneumonic plague

Inhalation of Yersinia pestis causes pneumonic plague, which rapidly progresses to death. A previously licensed killed whole-cell vaccine is presently unavailable due to its reactogenicity and inconclusive evidence of efficacy. The present study now shows that vaccination intranasally (i.n.) with inactivated Y. pestis CO92 (iYp) adjuvanted with interleukin-12 (IL-12) followed by an i.n. challenge with a lethal dose of Y. pestis CO92 prevented bacterial colonization and protected 100% of mice from pneumonic plague. Survival of the vaccinated mice correlated with levels of systemic and lung antibodies, reduced pulmonary pathology and proinflammatory cytokines, and the presence of lung lymphoid cell aggregates. Protection against pneumonic plague was partially dependent upon Fc receptors and could be transferred to naïve mice with immune mouse serum. On the other hand, protection was not dependent upon complement, and following vaccination, depletion of CD4 and/or CD8 T cells before challenge did not affect survival. In summary, the results demonstrate the safety, immunogenicity, and protective efficacy of i.n. administered iYp plus IL-12 in a mouse model of pneumonic plague.

Multi-Functional Characteristics of the Pseudomonas aeruginosa Type III Needle-Tip Protein, PcrV; Comparison to Orthologs in other Gram-negative Bacteria

Pseudomonas aeruginosa possesses a type III secretion system (T3SS) to intoxicate host cells and evade innate immunity. This virulence-related machinery consists of a molecular syringe and needle assembled on the bacterial surface, which allows delivery of T3 effector proteins into infected cells. To accomplish a one-step effector translocation, a tip protein is required at the top end of the T3 needle structure. Strains lacking expression of the functional tip protein fail to intoxicate host cells. P. aeruginosa encodes a T3S that is highly homologous to the proteins encoded by Yersinia spp. The needle-tip proteins of Yersinia, LcrV, and P. aeruginosa, PcrV, share 37% identity and 65% similarity. Other known tip proteins are AcrV (Aeromonas), IpaD (Shigella), SipD (Salmonella), BipD (Burkholderia), EspA (EPEC, EHEC), Bsp22 (Bordetella), with additional proteins identified from various Gram-negative species, such as Vibrio and Bordetella. The tip proteins can serve as a protective antigen or may be critical for sensing host cells and evading innate immune responses. Recognition of the host microenvironment transcriptionally activates synthesis of T3SS components. The machinery appears to be mechanically controlled by the assemblage of specific junctions within the apparatus. These junctions include the tip and base of the T3 apparatus, the needle proteins and components within the bacterial cytoplasm. The tip proteins likely have chaperone functions for translocon proteins, allowing the proper assembly of translocation channels in the host membrane and completing vectorial delivery of effector proteins into the host cytoplasm. Multi-functional features of the needle-tip proteins appear to be intricately controlled. In this review, we highlight the functional aspects and complex controls of T3 needle-tip proteins with particular emphasis on PcrV and LcrV.

Progress on plague vaccine development

Yersinia pestis (YP), the gram-negative plague bacterium, has shaped human history unlike any other pathogen known to mankind. YP (transmitted by the bite of an infected flea) diverged only recently from the related enteric pathogen Yersinia pseudotuberculosis but causes radically different diseases. Three forms of plague exist in humans: bubonic (swollen lymph nodes or bubos), septicemic (spread of YP through the lymphatics or bloodstream from the bubos to other organs), and contagious, pneumonic plague which can be communicated via YP-charged respiratory droplets resulting in person-person transmission and rapid death if left untreated (50-90% mortality). Despite the potential threat of weaponized YP being employed in bioterrorism and YP infections remaining prevalent in endemic regions of the world where rodent populations are high (including the four corner regions of the USA), an efficacious vaccine that confers immunoprotection has yet to be developed. This review article will describe the current vaccine candidates being evaluated in various model systems and provide an overall summary on the progress of this important endeavor.

Complementing defective viruses that express separate paramyxovirus glycoproteins provide a new vaccine vector approach

Replication-defective vaccine vectors based on vesicular stomatitis virus (VSV) lacking its envelope glycoprotein gene (G) are highly effective in animal models. However, such ΔG vectors are difficult to grow because they require complementation with the VSV G protein. In addition, the complementing G protein induces neutralizing antibodies in animals and thus limits multiple vector applications. In the process of generating an experimental Nipah virus (a paramyxovirus) vaccine, we generated two defective VSVΔG vectors, each expressing one of the two Nipah virus (NiV) glycoproteins (G and F) that are both required for virus entry to host cells. These replication-defective VSV vectors were effective at generating NiV neutralizing antibody in mice. Most interestingly, we found that these two defective viruses could be grown together and passaged in tissue culture cells in the absence of VSV G complementation. This mixture of complementing defective viruses was also highly effective at generating NiV neutralizing antibody in animals. This novel approach to growing and producing a vaccine from two defective viruses could be generally applicable to vaccine production for other paramyxoviruses or for other viruses where the expression of at least two different proteins is required for viral entry. Such an approach minimizes biosafety concerns that could apply to single, replication-competent VSV recombinants expressing all proteins required for infection.

TNFα and IFNγ contribute to F1/LcrV-targeted immune defense in mouse models of fully virulent pneumonic plague

Immunization with the Yersinia pestis F1 and LcrV proteins improves survival in mouse and non-human primate models of pneumonic plague. F1- and LcrV-specific antibodies contribute to protection, however, the mechanisms of antibody-mediated defense are incompletely understood and serum antibody titers do not suffice as quantitative correlates of protection. Previously we demonstrated roles for tumor necrosis factor-alpha (TNFα) and gamma-interferon (IFNγ) during defense against conditionally attenuated pigmentation (pgm) locus-negative Y. pestis. Here, using intranasal challenge with fully virulent pgm-positive Y. pestis strain CO92, we demonstrate that neutralizing TNFα and IFNγ interferes with the capacity of therapeutically administered F1- or LcrV-specific antibody to reduce bacterial burden and increase survival. Moreover, using Y. pestis strain CO92 in an aerosol challenge model, we demonstrate that neutralizing TNFα and IFNγ interferes with protection conferred by immunization with recombinant F1-LcrV fusion protein vaccine (p<0.0005). These findings establish that TNFα and IFNγ contribute to protection mediated by pneumonic plague countermeasures targeting F1 and LcrV, and suggest that an individual's capacity to produce these cytokines in response to Y. pestis challenge will be an important co-determinant of antibody-mediated defense against pneumonic plague.

Efficacy and safety of a modified vaccinia Ankara (MVA) vectored plague vaccine in mice

The efficacy and safety of plague vaccines based on the modified vaccinia Ankara (MVA) viral vector was evaluated. MVA recombinants were constructed expressing Yersinia pestis antigens under the translational control of the encephalomyocarditis virus (EMCV) internal ribosomal entry site (IRES) and/or fused to the tissue plasminogen activator (tPA) secretory signal. A MVA/Y. pestis recombinant that expressed a truncated version of the low-calcium response V antigen (MVA/IRES/tPA/V(307)), conferred significant protection (87.5-100%) against intranasal or intraperitoneal challenge with CO92 (encapsulated) or Java 9 (non-encapsulated) strains of Y. pestis, respectively. In contrast, a MVA/Y. pestis recombinant that expressed the full-length V antigen provided only 37.5% protection against challenge with CO92 or Java 9 strains, respectively. Interestingly, a MVA/Y. pestis recombinant that expressed the capsular protein (F1) did not elicit significant antibody titers but still conferred 50% and 25% protection against CO92 or Java 9 challenge, respectively. The MVA/Y. pestis recombinant viruses did not demonstrate any mortality or morbidity in SCID mice. Based on their safety and efficacy in mice, these MVA/Y. pestis recombinants are candidates for further development as biodefense and public health vaccines.

Protection against pneumonic plague following oral immunization with a non-replicating vaccine

Yersinia pestis is a dangerous bacterial pathogen that when inhaled can rapidly induce fatal pneumonic plague. Thus, there is a need for stable, safe, and easily administered mucosal vaccines capable of eliciting effective protection against pulmonary Y. pestis infections. Cationic liposome-nucleic acid complexes (CLDC) have been shown previously to be effective vaccine adjuvants for parenteral immunization, but have not been previously evaluated for use in oral immunization. Therefore, we investigated the ability of an orally administered CLDC adjuvanted vaccine to elicit protective immunity against lethal pneumonic plague. C57Bl/6 mice were vaccinated orally or subcutaneously using 10mug Y. pestis F1 antigen combined with CLDC and immune responses and protection from challenge was assessed. We found that oral immunization elicited high titers of anti-F1 antibodies, equivalent to those generated by parenteral immunization. Importantly, orally immunized mice were protected from lethal pulmonary challenge with virulent Y. pestis for up to 18 weeks following vaccination. Vaccine-induced protection following oral immunization was found to be dependent primarily on CD4+ T cells, with a partial contribution from CD8+ T cells. Thus, CLDC adjuvanted vaccines represent a new type of orally administered, non-replicating vaccine capable of generating effective protection against pulmonary infection with virulent Y. pestis.

Enhanced immunity against classical swine fever in pigs induced by prime-boost immunization using an alphavirus replicon-vectored DNA vaccine and a recombinant adenovirus

Classical swine fever (CSF) - caused by the classical swine fever virus (CSFV) - is a fatal disease of pigs that is responsible for extensive losses to the swine industry worldwide. We had demonstrated previously that a prime-boost vaccination strategy using an alphavirus (Semliki Forest virus, SFV) replicon-vectored DNA vaccine (pSFV1CS-E2) and a recombinant adenovirus (rAdV-E2) expressing the E2 glycoprotein of CSFV induced enhanced immune responses in a mouse model. In this study, we evaluated further the efficacy of the heterologous prime-boost immunization approach in pigs, the natural host of CSFV. The results showed that the pigs (n=5) receiving pSFV1CS-E2/rAdV-E2 heterologous prime-boost immunization developed significantly higher titers of CSFV-specific neutralizing antibodies and comparable CD4(+) and CD8(+) T-cell proliferation, compared to the pigs receiving double immunizations with rAdV-E2 alone. When challenged with virulent CSFV Shimen strain, the pigs of the heterologous prime-boost group did not show clinical symptoms or viremia, which were observed in one of the 5 pigs immunized with rAdV-E2 alone and all the 5 control pigs immunized with an empty adenovirus. The results demonstrate that the heterologous DNA prime and recombinant adenovirus boost strategy can induce solid protective immunity.

D27-pLpxL, an avirulent strain of Yersinia pestis, primes T cells that protect against pneumonic plague

Vaccinating with live, conditionally attenuated, pigmentation (Pgm)-deficient Yersinia pestis primes T cells that protect mice against pneumonic plague. However, Pgm-deficient strains are not considered safe for human use because they retain substantial virulence in animal models. Y. pestis strains engineered to express Escherichia coli LpxL are avirulent owing to constitutive production of lipopolysaccharide with increased Toll-like receptor 4-activating ability. We generated an LpxL-expressing Pgm-deficient strain (D27-pLpxL) and demonstrate here that this avirulent strain retains the capacity to prime protective T cells. Compared with unvaccinated controls, mice immunized intranasally with live D27-pLpxL exhibit a decreased bacterial burden and increased survival when challenged intranasally with virulent Y. pestis. T cells provide a substantial degree of this protection, as vaccine efficacy is maintained in B-cell-deficient muMT mice unless those animals are depleted of CD4 and CD8 T cells at the time of challenge. Upon challenge with Y. pestis, pulmonary T-cell numbers decline in naive mice, whereas immunized mice show increased numbers of CD44(high) CD43(high) effector T cells and T cells primed to produce tumor necrosis factor alpha and gamma interferon; neutralizing these cytokines at the time of challenge abrogates protection. Immunization does not prevent dissemination of Y. pestis from the lung but limits bacterial growth and pathology in visceral tissue, apparently by facilitating formation of granuloma-like structures. This study describes a new model for studying T-cell-mediated protection against pneumonic plague and demonstrates the capacity for live, highly attenuated, Y. pestis vaccine strains to prime protective memory T-cell responses safely.