Protective efficacy of recombinant Yersinia outer proteins against bubonic plague caused by encapsulated and nonencapsulated Yersinia pestis

Polyclonal Antibodies Derived from Transchromosomic Bovines Vaccinated with the Recombinant F1-V Vaccine Increase Bacterial Opsonization In Vitro and Protect Mice from Pneumonic Plague

Plague is an ancient disease that continues to be of concern to both the public health and biodefense research communities. Pneumonic plague is caused by hematogenous spread of Yersinia pestis bacteria from a ruptured bubo to the lungs or by directly inhaling aerosolized bacteria. The fatality rate associated with pneumonic plague is significant unless effective antibiotic therapy is initiated soon after an early and accurate diagnosis is made. As with all bacterial pathogens, drug resistance is a primary concern when developing strategies to combat these Yersinia pestis infections in the future. While there has been significant progress in vaccine development, no FDA-approved vaccine strategy exists; thus, other medical countermeasures are needed. Antibody treatment has been shown to be effective in animal models of plague. We produced fully human polyclonal antibodies in transchromosomic bovines vaccinated with the recombinant F1-V plague vaccine. The resulting human antibodies opsonized Y. pestis bacteria in the presence of RAW264.7 cells and afforded significant protection to BALB/c mice after exposure to aerosolized Y. pestis. These data demonstrate the utility of this technology to produce large quantities of non-immunogenic anti-plague human antibodies to prevent or possibly treat pneumonic plague in human.

Plague Prevention and Therapy: Perspectives on Current and Future Strategies

Plague, caused by the bacterial pathogen Yersinia pestis, is a vector-borne disease that has caused millions of human deaths over several centuries. Presently, human plague infections continue throughout the world. Transmission from one host to another relies mainly on infected flea bites, which can cause enlarged lymph nodes called buboes, followed by septicemic dissemination of the pathogen. Additionally, droplet inhalation after close contact with infected mammals can result in primary pneumonic plague. Here, we review research advances in the areas of vaccines and therapeutics for plague in context of Y. pestis virulence factors and disease pathogenesis. Plague continues to be both a public health threat and a biodefense concern and we highlight research that is important for infection mitigation and disease treatment.

Protection Elicited by Attenuated Live Yersinia pestis Vaccine Strains against Lethal Infection with Virulent Y. pestis

The etiologic agent of plague, Yersinia pestis, is a globally distributed pathogen which poses both a natural and adversarial threat. Due largely to the rapid course and high mortality of pneumonic plague, vaccines are greatly needed. Two-component protein vaccines have been unreliable and potentially vulnerable to vaccine resistance. We evaluated the safety and efficacy of eight live Y. pestis strains derived from virulent strains CO92 or KIM6+ and mutated in one or more virulence-associated gene(s) or cured of plasmid pPst. Stringent, single-dose vaccination allowed down-selection of the two safest and most protective vaccine candidates, CO92 mutants pgm- pPst- and ΔyscN. Both completely protected BALB/c mice against subcutaneous and aerosol challenge with Y. pestis. Strain CD-1 outbred mice were more resistant to bubonic (but not pneumonic) plague than BALB/c mice, but the vaccines elicited partial protection of CD-1 mice against aerosol challenge, while providing full protection against subcutaneous challenge. A ΔyscN mutant of the nonencapsulated C12 strain was expected to display antigens previously concealed by the capsule. C12 ΔyscN elicited negligible titers to F1 but comparable antibody levels to whole killed bacteria, as did CO92 ΔyscN. Although one dose of C12 ΔyscN was not protective, vaccination with two doses of either CO92 ΔyscN, or a combination of the ΔyscN mutants of C12 and CO92, protected optimally against lethal bubonic or pneumonic plague. Protection against encapsulated Y. pestis required inclusion of F1 in the vaccine and was associated with high anti-F1 titers.

Yersinia Outer Membrane Vesicles as Potential Vaccine Candidates in Protecting against Plague

Despite the relatively low incidence of plague, its etiological agent, Yersinia pestis, is an exceptional epidemic danger due to the high infectivity and mortality of this infectious disease. Reports on the isolation of drug-resistant Y. pestis strains indicate the advisability of using asymmetric responses, such as phage therapy and vaccine prophylaxis in the fight against this problem. The current relatively effective live plague vaccine is not approved for use in most countries because of its ability to cause heavy local and system reactions and even a generalized infectious process in people with a repressed immune status or metabolic disorders, as well as lethal infection in some species of nonhuman primates. Therefore, developing alternative vaccines is of high priority and importance. However, until now, work on the development of plague vaccines has mainly focused on screening for the potential immunogens. Several investigators have identified the protective potency of bacterial outer membrane vesicles (OMVs) as a promising basis for bacterial vaccine candidates. This review is aimed at presenting these candidates of plague vaccine and the results of their analysis in animal models.

Yersinia pestis Antigen F1 but Not LcrV Induced Humoral and Cellular Immune Responses in Humans Immunized with Live Plague Vaccine-Comparison of Immunoinformatic and Immunological Approaches

The recent progress in immunoinformatics provided the basis for an accelerated development of target-specific peptide vaccines as an alternative to the traditional vaccine concept. However, there is still limited information on whether the in silico predicted immunoreactive epitopes correspond to those obtained from the actual experiments. Here, humoral and cellular immune responses to two major Yersinia pestis protective antigens, F1 and LcrV, were studied in human donors immunized with the live plague vaccine (LPV) based on the attenuated Y. pestis strain EV line NIIEG. The F1 antigen provided modest specific cellular (mixed T helper 1 (Th1)/Th2 type) and humoral immune responses in vaccinees irrespective of the amount of annual vaccinations and duration of the post-vaccination period. The probing of the F1 overlapping peptide library with the F1-positive sera revealed the presence of seven linear B cell epitopes, which were all also predicted by in silico assay. The immunoinformatics study evaluated their antigenicity, toxicity, and allergenic properties. The epitope TSQDGNNH was mostly recognized by the sera from recently vaccinated donors rather than antibodies from those immunized decades ago, suggesting the usefulness of this peptide for differentiation between recent and long-term vaccinations. The in silico analysis predicted nine linear LcrV-specific B-cell epitopes; however, weak antibody and cellular immune responses prevented their experimental evaluation, indicating that LcrV is a poor marker of successful vaccination. No specific Th17 immune response to either F1 or LcrV was detected, and there were no detectable serum levels of F1-specific immunoglobulin A (IgA) in vaccinees. Overall, the general approach validated in the LPV model could be valuable for the rational design of vaccines against other neglected and novel emerging infections with high pandemic potency.

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.

Plague vaccine: recent progress and prospects

Three great plague pandemics, resulting in nearly 200 million deaths in human history and usage as a biowarfare agent, have made Yersinia pestis as one of the most virulent human pathogens. In late 2017, a large plague outbreak raged in Madagascar attracted extensive attention and caused regional panics. The evolution of local outbreaks into a pandemic is a concern of the Centers for Disease Control and Prevention (CDC) in plague endemic regions. Until now, no licensed plague vaccine is available. Prophylactic vaccination counteracting this disease is certainly a primary choice for its long-term prevention. In this review, we summarize the latest advances in research and development of plague vaccines.

A Replication-Defective Human Type 5 Adenovirus-Based Trivalent Vaccine Confers Complete Protection against Plague in Mice and Nonhuman Primates

Currently, no plague vaccine exists in the United States for human use. The capsular antigen (Caf1 or F1) and two type 3 secretion system (T3SS) components, the low-calcium-response V antigen (LcrV) and the needle protein YscF, represent protective antigens of Yersinia pestis We used a replication-defective human type 5 adenovirus (Ad5) vector and constructed recombinant monovalent and trivalent vaccines (rAd5-LcrV and rAd5-YFV) that expressed either the codon-optimized lcrV or the fusion gene designated YFV (consisting of ycsF, caf1, and lcrV). Immunization of mice with the trivalent rAd5-YFV vaccine by either the intramuscular (i.m.) or the intranasal (i.n.) route provided protection superior to that with the monovalent rAd5-LcrV vaccine against bubonic and pneumonic plague when animals were challenged with Y. pestis CO92. Preexisting adenoviral immunity did not diminish the protective response, and the protection was always higher when mice were administered one i.n. dose of the trivalent vaccine (priming) followed by a single i.m. booster dose of the purified YFV antigen. Immunization of cynomolgus macaques with the trivalent rAd5-YFV vaccine by the prime-boost strategy provided 100% protection against a stringent aerosol challenge dose of CO92 to animals that had preexisting adenoviral immunity. The vaccinated and challenged macaques had no signs of disease, and the invading pathogen rapidly cleared with no histopathological lesions. This is the first report showing the efficacy of an adenovirus-vectored trivalent vaccine against pneumonic plague in mouse and nonhuman primate (NHP) models.

Multiple antigens of Yersinia pestis delivered by live recombinant attenuated Salmonella vaccine strains elicit protective immunity against plague

Based on our improved novel Salmonella vaccine delivery platform, we optimized the recombinant attenuated Salmonella typhimurium vaccine (RASV) χ12094 to deliver multiple Yersinia pestis antigens. These included LcrV196 (amino acids, 131-326), Psn encoded on pYA5383 and F1 encoded in the chromosome, their synthesis did not cause adverse effects on bacterial growth. Oral immunization with χ12094(pYA5383) simultaneously stimulated high antibody titers to LcrV, Psn and F1 in mice and presented complete protection against both subcutaneous (s.c.) and intranasal (i.n.) challenges with high lethal doses of Y. pestis CO92. Moreover, no deaths or other disease symptoms were observed in SCID mice orally immunized with χ12094(pYA5383) over a 60-day period. Therefore, the trivalent S. typhimurium-based live vaccine shows promise for a next-generation plague vaccine.

Expression and Association of the Yersinia pestis Translocon Proteins, YopB and YopD, Are Facilitated by Nanolipoprotein Particles

Yersinia pestis enters host cells and evades host defenses, in part, through interactions between Yersinia pestis proteins and host membranes. One such interaction is through the type III secretion system, which uses a highly conserved and ordered complex for Yersinia pestis outer membrane effector protein translocation called the injectisome. The portion of the injectisome that interacts directly with host cell membranes is referred to as the translocon. The translocon is believed to form a pore allowing effector molecules to enter host cells. To facilitate mechanistic studies of the translocon, we have developed a cell-free approach for expressing translocon pore proteins as a complex supported in a bilayer membrane mimetic nano-scaffold known as a nanolipoprotein particle (NLP) Initial results show cell-free expression of Yersinia pestis outer membrane proteins YopB and YopD was enhanced in the presence of liposomes. However, these complexes tended to aggregate and precipitate. With the addition of co-expressed (NLP) forming components, the YopB and/or YopD complex was rendered soluble, increasing the yield of protein for biophysical studies. Biophysical methods such as Atomic Force Microscopy and Fluorescence Correlation Spectroscopy were used to confirm that the soluble YopB/D complex was associated with NLPs. An interaction between the YopB/D complex and NLP was validated by immunoprecipitation. The YopB/D translocon complex embedded in a NLP provides a platform for protein interaction studies between pathogen and host proteins. These studies will help elucidate the poorly understood mechanism which enables this pathogen to inject effector proteins into host cells, thus evading host defenses.

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.

Prophylaxis and therapy of plague

Plague has been a scourge of mankind for centuries, and outbreaks continue to the present day. The virulence mechanisms employed by the etiological agent Yersinia pestis are reviewed in the context of the available prophylactic and therapeutic strategies for plague. Although antibiotics are available, resistance is emerging in this dangerous pathogen. Therapeutics used in the clinic are discussed and innovative approaches to the design and development of new therapeutic compounds are reviewed. Currently there is no licensed vaccine available for prevention of plague in the USA or western Europe, although both live attenuated strains and killed whole-cell extracts have been used historically. Live strains are still approved for human use in some parts of the world, such as the former Soviet Union, but poor safety profiles render them unacceptable to many countries. The development of safe, effective next-generation vaccines, including the recombinant subunit vaccine currently used in clinical trials is discussed.

Protecting against plague: towards a next-generation vaccine

The causative organism of plague is the bacterium Yersinia pestis. Advances in understanding the complex pathogenesis of plague infection have led to the identification of the F1- and V-antigens as key components of a next-generation vaccine for plague, which have the potential to be effective against all forms of the disease. Here we review the roles of F1- and V-antigens in the context of the range of virulence mechanisms deployed by Y. pestis, in order to develop a greater understanding of the protective immune responses required to protect against plague.

Surface organelles assembled by secretion systems of Gram-negative bacteria: diversity in structure and function

Gram-negative bacteria express a wide variety of organelles on their cell surface. These surface structures may be the end products of secretion systems, such as the hair-like fibers assembled by the chaperone/usher (CU) and type IV pilus pathways, which generally function in adhesion to surfaces and bacterial-bacterial and bacterial-host interactions. Alternatively, the surface organelles may be integral components of the secretion machinery itself, such as the needle complex and pilus extensions formed by the type III and type IV secretion systems, which function in the delivery of bacterial effectors inside host cells. Bacterial surface structures perform functions critical for pathogenesis and have evolved to withstand forces exerted by the external environment and cope with defenses mounted by the host immune system. Given their essential roles in pathogenesis and exposed nature, bacterial surface structures also make attractive targets for therapeutic intervention. This review will describe the structure and function of surface organelles assembled by four different Gram-negative bacterial secretion systems: the CU pathway, the type IV pilus pathway, and the type III and type IV secretion systems.

Detection and Isolation of Yersinia pestis Without Fraction 1 Antigen by Monoclonal Antibody in Foods and Water

Most available immunoassays for Yersinia pestis are based on the detection of fraction 1 antigen (F1) when yersiniae are grown at 37°C. A monoclonal antibody (MAb) was developed based on the detection of surface antigens that are not F1. F1-deficient Y. pestis cells were induced and used to immunize BALB/c mice from which MAb (immunoglobulin G1), which specifically recognizes Y. pestis, with or without F1, was obtained. This MAb (6B5) did not cross-react with enteric bacteria, including Yersinia enterocolitica. Enzyme-linked immunosorbent assay results revealed that MAb 6B5 is specific for Y. pestis, with the exception of a minor cross-reaction with Yersinia pseudotuberculosis. Western immunoblot analysis revealed that MAb 6B5 recognizes a Y. pestis outer membrane protein of ca. 30 kDa. Magnetic beads that were coated with MAb 6B5 were compared with beads coated with polyclonal antibody (PAb; rabbit) against Y. pestis for the isolation of Y. pestis in food and water samples by using a PATHATRIX cell concentration apparatus. Enrichment cultures of Y. pestis in different foods by using two different times (6 and 24 h) in brain heart infusion broth at 37°C were evaluated. Results revealed MAb 6B5–coated magnetic beads were equivalent to magnetic beads coated with PAb against Y. pestis A1122 whole cells in concentrating Y. pestis for isolation, especially when samples were enriched for 6 h. However, the selectivity for Y. pestis of the magnetic beads coated with MAb 6B5 was greater than that coated with PAb.

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.

Humoral and cellular immune responses to Yersinia pestis infection in long-term recovered plague patients

Plague is one of the most dangerous diseases and is caused by Yersinia pestis. Effective vaccine development requires understanding of immune protective mechanisms against the bacterium in humans. In this study, the humoral and memory cellular immune responses in plague patients (n = 65) recovered from Y. pestis infection during the past 16 years were investigated using a protein microarray and an enzyme-linked immunosorbent spot assay (ELISpot). The seroprevalence to the F1 antigen in all recovered patients is 78.5%. In patients infected more than a decade ago, the antibody-positive rate still remains 69.5%. There is no difference in the antibody presence between gender, age, and infected years, but it seems to be associated with the F1 antibody titers during infection (r = 0.821; P < 0.05). Except F1 antibody, the antibodies against LcrV and YopD were detected in most of the patients, suggesting they could be the potential diagnostic markers for detecting the infection of F1-negative strains. Regarding cellular immunity, the cell number producing gamma interferon (IFN-γ), stimulated by F1 and LcrV, respectively, in vitro to the peripheral blood mononuclear cells of 7 plague patients and 4 negative controls, showed no significant difference, indicating F1 and LcrV are not dominant T cell antigens against plague for a longer time in humans. Our findings have direct implications for the future design and development of effective vaccines against Y. pestis infection and the development of new target-based diagnostics.

[Immunobiological properties of Yersinia pestis antigens]

The present review contains information concerning immunobiological properties of plague microbe antigens. All of the identified antigens are evaluated in relation to pathogenicity of Yersinia pestis namely a resistance to phagocytosis, toxicity, adhesiveness etc. as well as persistence ability and adaptation to variable environment. In addition, the role of antigens in immunogenicity of living plague microbe for experimental animals is considered. The data concerning mechanisms of antigenic contribution to the development of adaptive immunity are presented.

The role of immune correlates and surrogate markers in the development of vaccines and immunotherapies for plague

One of the difficulties in developing countermeasures to biothreat agents is the challenge inherent in demonstrating their efficacy in man. Since the first publication of the Animal Rule by the FDA, there has been increased discussion of potential correlates of protection in animal models and their use to establish surrogate markers of efficacy in man. The latter need to be relatively easy to measure in assays that are at least qualified, if not validated, in order to derive a quantitative assessment of the clinical benefit conferred. The demonstration of safety and clinical benefit is essential to achieve regulatory approval for countermeasures for which clinical efficacy cannot be tested directly, as is the case for example, for biodefence vaccines. 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 potential immune correlates of protection for plague, from which it may be possible to derive surrogate markers of efficacy, in order to predict the clinical efficacy of candidate prophylaxes and therapies.

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.

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.

EseD, a putative T3SS translocon component of Edwardsiella tarda, contributes to virulence in fish and is a candidate for vaccine development

Edwardsiella tarda has a type III secretion system (T3SS) essential for pathogenesis. EseD, together with EseB and EseC, has been suggested to form a putative T3SS translocon complex, although its further function is unclear. To investigate the physiological role of EseD, a mutant strain of E. tarda was constructed with an in-frame deletion of the entire eseD gene. One finding was that the ∆eseD mutant decreased the secretion levels of EseC and EseB proteins. Additionally, the ∆eseD mutant showed attenuated swarming and contact-hemolysis abilities. However, the ∆eseD mutant showed increased biofilm formation. Complementation of the mutant strain with eseD restored these phenotypes to those similar to the wild-type strain. Furthermore, infection experiments in fish showed that the ∆eseD mutant exhibited slower proliferation and a tenfold decrease in virulence in fish. These results indicate a specific role of EseD in the pathogenesis of E. tarda. Finally, recombinant EseD protein elicited high antibody titers in immunized fish and various levels of protection against lethal challenge with the wild-type strain. These results indicate that EseD protein may be a candidate antigen for development of a subunit vaccine against Edwardsiellosis.

Development of a vaccinia virus based reservoir-targeted vaccine against Yersinia pestis

Yersinia pestis, the causative organism of plague, is a zoonotic organism with a worldwide distribution. Although the last plague epidemic occurred in early 1900s, human cases continue to occur due to contact with infected wild animals. In this study, we have developed a reservoir-targeted vaccine against Y. pestis, to interrupt transmission of disease in wild animals as a potential strategy for decreasing human disease. A vaccinia virus delivery system was used to express the F1 capsular protein and the LcrV type III secretion component of Y. pestis as a fusion protein. Here we show that a single dose of this vaccine administered orally, generates a dose-dependent antibody response in mice. Antibody titers peak by 3 weeks after administration and remain elevated for a minimum of 45 weeks. Vaccination provided up to 100% protection against challenge with Y. pestis administered by intranasal challenge at 10 times the lethal dose with protection lasting a minimum of 45 weeks. An orally available, vaccinia virus expressed vaccine against Y. pestis may be a suitable vaccine for a reservoir targeted strategy for the prevention of enzootic 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.

Biosafety level 2 model of pneumonic plague and protection studies with F1 and Psa

Attenuated Yersinia pestis pgm strains, such as KIM5, lack the siderophore yersiniabactin. Strain KIM5 does not induce significant pneumonia when delivered intranasally. In this study, mice were found to develop pneumonia after intranasal challenge with strain KIM5 when they were injected intraperitoneally with iron dextran, though not with iron sulfate. KIM5-infected mice treated daily with 4 mg iron dextran died in 3 days with severe pneumonia. Pneumonia was less severe if 4 mg iron dextran was administered only once before infection. The best-studied experimental vaccine against plague currently consists of the Yersinia pestis capsular antigen F1 and the type 3 secreted protein LcrV. The F1 antigen was shown to be protective against KIM5 infections in mice administered iron dextran doses leading to light or severe pneumonia, supporting the use of an iron dextran-treated model of pneumonic plague. Since F1 has been reported to be incompletely protective in some primates, and bacterial isolates lacking F1 are still virulent, there has been considerable interest in identifying additional protective subunit immunogens. Here we showed that the highly conserved Psa fimbriae of Y. pestis (also called pH 6 antigen) are expressed in murine organs after infection through the respiratory tract. Studies with iron dextran-treated mice showed that vaccination with the Psa fimbrial protein together with an adjuvant afforded incomplete but significant protection in the mouse model described. Therefore, further investigations to fully characterize the protective properties of the Psa fimbriae are warranted.

Prospects for new plague vaccines

The potential application of Yersinia pestis for bioterrorism emphasizes the urgent need to develop more effective vaccines against airborne infection. The current status of plague vaccines has been reviewed. The present emphasis is on subunit vaccines based on the F1 and LcrV antigens. These provide good protection in animal models but may not protect against F1 strains with modifications to the type III secretion system. The duration of protection against pneumonic infection is also uncertain. Other strategies under investigation include defined live-attenuated vaccines, DNA vaccines, mucosal delivery systems and heterologous immunization. The live-attenuated strain Y. pestis EV NIIEG protects against aerosol challenge in animal models and, with further modification to reduce residual virulence and to optimize respiratory protection, it could provide a shortcut to improved vaccines. The regulatory problems inherent in licensing vaccines for which efficacy data are unavailable and their possible solutions are discussed herein.

Identification of in vivo-induced conserved sequences from Yersinia pestis during experimental plague infection in the rabbit

In an effort to identify the novel virulence determinants of Yersinia pestis, we applied the gene "discovery" methodology, in vivo-induced (IVI) antigen technology, to detect genes upregulated during infection in a laboratory rabbit model for bubonic plague. After screening over 70,000 Escherichia coli clones of Y. pestis DNA expression libraries, products from 25 loci were identified as being seroreactive to reductively adsorbed, pooled immune serum. Upon sequence analysis of the predicted IVI gene products, more frequently encountered conserved protein functional categories have emerged, to include type-V autotransporters and components of more complex secretion systems including types III and VI. The recombinant products from eight selected clones were subsequently immunoblotted against pooled immune serum from two naturally infected host species: the prairie dog, and a species refractory to lethal disease, the coyote. Immune prairie dog serum recognized 2-3 of the rabbit-reactive antigens, suggesting at least some overlap in the pathogen's in vivo survival mechanisms between these two hosts. Although the coyote serum failed to recognize most of the IVI antigens, LepA was universally reactive with all three host sera. Collectively, the profiles/patterns of IVI conserved sequences (IVICS) may represent immune "signatures" among different host species, possessing the potential for use as a diagnostic tool for plague. Further, the antigenic nature of IVICS makes them ideal for further evaluation as novel subunit vaccine candidates. The gathering of additional data and analysis of the intact IVI genes and the expressed IVICS products should provide insight into the unique biologic processes of Y. pestis during infection and reveal the genetic patterns of the pathogen's survival strategy in different hosts.

Plague

Killed whole cell vaccines for plague were first produced as long ago as the late 1890s and modified versions of these are still used, with evidence that they are efficacious against bubonic plague. Renewed efforts with modern technology have yielded new candidate vaccines which are less reactogenic, can be produced in a conventional pharmaceutical manufacturing plant and are protective against the life-threatening pneumonic form of the disease. This paper reviews the progress towards an improved vaccine for plague and assesses the likely impact of a prophylactic vaccine for bubonic and pneumonic plague.

Dual-function antibodies to Yersinia pestis LcrV required for pulmonary clearance of plague

Yersinia pestis causes pneumonic plague, a necrotic pneumonia that rapidly progresses to death without early treatment. Antibodies to the protective antigen LcrV are thought to neutralize its essential function in the type III secretion system (TTSS) and by themselves are capable of inducing immunity to plague in mouse models. To develop multivalent LcrV antibodies as a therapeutic treatment option, we screened for monoclonal antibodies (MAbs) to LcrV that could prevent its function in the TTSS. Although we were able to identify single and combination MAbs that provided the high-level inhibition of the TTSS, these did not promote phagocytosis in vitro and were only weakly protective in a mouse pneumonic plague model. Only one MAb, BA5, was able to protect mice from pneumonic plague. In vitro, MAb BA5 blocked the TTSS with efficiency equal to or even less than that of other MAbs as single agents or as combinations, but its activity led to increased phagocytic uptake. Polyclonal anti-LcrV was superior to BA5 in promoting phagocytosis and also was more efficient in protecting mice from pneumonic plague. Taken together, the data support a hypothesis whereby the pulmonary clearance of Y. pestis by antibodies requires both the neutralization of the TTSS and the simultaneous stimulation of innate signaling pathways used by phagocytic cells to destroy pathogens.

High-throughput identification of new protective antigens from a Yersinia pestis live vaccine by enzyme-linked immunospot assay

Yersinia pestis, the plague pathogen, is a facultative intracellular bacterium. Cellular immunity plays important roles in defense against infections. The identification of T-cell targets is critical for the development of effective vaccines against intracellular bacteria; however, the function of cellular immunity in protection from plague was not clearly understood. In this study, 261 genes from Y. pestis were selected on the basis of bioinformatics analysis and previous research results for expression in Escherichia coli BL21(DE3). After purification, 101 proteins were qualified for examination of their abilities to induce the production of gamma interferon in mice immunized with live vaccine EV76 by enzyme-linked immunospot assay. Thirty-four proteins were found to stimulate strong T-cell responses. The protective efficiencies for 24 of them were preliminarily evaluated using a mouse plague model. In addition to LcrV, nine proteins (YPO0606, YPO1914, YPO0612, YPO3119, YPO3047, YPO1377, YPCD1.05c, YPO0420, and YPO3720) may provide partial protection against challenge with a low dose (20 times the 50% lethal dose [20x LD(50)]) of Y. pestis, but only YPO0606 could partially protect mice from infection with Y. pestis at a higher challenge dosage (200x LD(50)). These proteins would be the potential components for Y. pestis vaccine development.

Immune defense against pneumonic plague

SUMMARY

Yersinia pestis is one of the world's most virulent human pathogens. Inhalation of this Gram-negative bacterium causes pneumonic plague, a rapidly progressing and usually fatal disease. Extensively antibiotic-resistant strains of Y. pestis exist and have significant potential for exploitation as agents of terrorism and biowarfare. Subunit vaccines comprised of the Y. pestis F1 and LcrV proteins are well-tolerated and immunogenic in humans but cannot be tested for efficacy, because pneumonic plague outbreaks are uncommon and intentional infection of humans is unethical. In animal models, F1/LcrV-based vaccines protect mice and cynomolgus macaques but have failed, thus far, to adequately protect African green monkeys. We lack an explanation for this inconsistent efficacy. We also lack reliable correlate assays for protective immunity. These deficiencies are hampering efforts to improve vaccine efficacy. Here, I review the immunology of pneumonic plague, focusing on evidence that humoral and cellular defense mechanisms collaborate to defend against pulmonary Y. pestis infection.

Vaccination of mice with a Yop translocon complex elicits antibodies that are protective against infection with F1- Yersinia pestis

Yersinia pestis, the bacterial agent of plague, secretes several proteins important for pathogenesis or host protection. The F1 protein forms a capsule on the bacterial cell surface and is a well-characterized protective antigen but is not essential for virulence. A type III secretion system that is essential for virulence exports Yop proteins, which function as antiphagocytic or anti-inflammatory factors. Yop effectors (e.g., YopE) are delivered across the host cell plasma membrane by a translocon, composed of YopB and YopD. Complexes of YopB, YopD, and YopE (BDE) secreted by Yersinia pseudotuberculosis were purified by affinity chromatography and used as immunogens to determine if antibodies to the translocon could provide protection against Y. pestis in mice. Mice vaccinated with BDE generated high-titer immunoglobulin G antibodies specific for BDE, as shown by enzyme-linked immunosorbent assay and immunoblotting, and were protected against lethal intravenous challenge with F1(-) but not F1(+) Y. pestis. Mice passively immunized with anti-BDE serum were protected from lethal challenge with F1(-) Y. pestis. The YopB protein or a complex of YopB and YopD (BD) was purified and determined by vaccination to be immunogenic in mice. Mice actively vaccinated with BD or passively vaccinated with anti-BD serum were protected against lethal challenge with F1(-) Y. pestis. These results indicate that anti-translocon antibodies can be used as immunotherapy to treat infections by F1(-) Y. pestis.

Current challenges in the development of vaccines for pneumonic plague

Inhalation of Yersinia pestis bacilli causes pneumonic plague, a rapidly progressing and exceptionally virulent disease. Extensively antibiotic-resistant Y. pestis strains exist and we currently lack a safe and effective pneumonic plague vaccine. These facts raise concern that Y. pestis may be exploited as a bioweapon. Here, I review the history and status of plague vaccine research and advocate that pneumonic plague vaccines should strive to prime both humoral and cellular immunity.

Neutralization of Yersinia pestis-mediated macrophage cytotoxicity by anti-LcrV antibodies and its correlation with protective immunity in a mouse model of bubonic plague

Plague is a life-threatening disease caused by Yersinia pestis, for which effective-licensed vaccines and reliable predictors of in vivo immunity are lacking. V antigen (LcrV) is a major Y. pestis virulence factor that mediates translocation of the cytotoxic Yersinia protein effectors (Yops). It is a well-established protective antigen and a part of currently tested plague subunit vaccines. We have developed a highly sensitive in vitro macrophage cytotoxicity neutralization assay which is mediated by anti-LcrV antibodies; and studied the potential use of these neutralizing antibodies as an in vitro correlate of plague immunity in mice. The assay is based on a Y. pestis strain with enhanced cytotoxicity to macrophages in which endogenous yopJ was replaced by the more effectively translocated yopP of Y. enterocolitica O:8. Mice passively immunized with rabbit anti-LcrV IgG or actively immunized with recombinant LcrV were protected against lethal doses of a virulent Y. pestis strain, in a mouse model of bubonic plague. This protection significantly correlated with the in vitro neutralizing activity of the antisera but not with their corresponding ELISA titers. In actively immunized mice, a cutoff value for serum neutralizing activity, above which survival was assured with high degree of confidence, could be established for different vaccination regimes. The impact of overall findings on the potential use of serum neutralizing activity as a correlate of protective immunity is discussed.

Relative immunogenicity and protection potential of candidate Yersinia Pestis antigens against lethal mucosal plague challenge in Balb/C mice

Yersinia Pestis outer proteins, plasminogen activator protease and Yop secretion protein F are necessary for the full virulence of Yesinia pestis and have been proposed as potential protective antigens for vaccines against plague. In the current study, we used DNA immunization as a tool to study the relative protective immunity of these proteins with a standardized intranasal challenge system in mice. While the natural full-length gene sequences for most of these Y. pestis proteins did not display a good level of protein expression in vitro when delivered by a DNA vaccine vector, the overall immunogenicity of these wild type gene DNA vaccines was low in eliciting antigen-specific antibody responses and gene sequence modifications improved both of these parameters. However, even modified YopD, YopO and YscF antigens were only able to partially protect immunized mice at various levels against lethal challenge with Y. pestis KIM 1001 strain while no protection was observed with either the YopB or Pla antigens. These results demonstrate that DNA immunization is effective in screening, optimizing and comparing optimal antigen designs and immunogenicity of candidate antigens for the development of a subunit-based plague vaccine.

Concomitant administration of Yersinia pestis specific monoclonal antibodies with plague vaccine has a detrimental effect on vaccine mediated immunity

Antibodies can be used to confer rapid immunity against infectious agents for short periods of time. By comparison, vaccine induced immunity is more protective, but takes a relatively long time to develop. Concomitant administration of antibody and vaccine by different routes was evaluated as a means of providing both rapid and long-term protection against plague. BALB/c mice were treated intraperitoneally with monoclonal antibodies, with specificities for Yersinia pestis LcrV and F1 antigens. A cohort of these mice was simultaneously vaccinated with rF1 and rLcrV by the intramuscular route. Antibody co-administration with vaccine reduced the level of vaccine mediated protection afforded against a high level Y. pestis challenge. Conversely, antibody-mediated protection was unaffected by vaccine co-administration and lasted for at least 8 weeks post administration. We also evaluated the effect of administering vaccine intradermally and antibody intratracheally and observed that, irrespective of administration route, concomitant administration of antibody reduced the effectiveness of vaccine mediated immunity. The results of passive transfer experiments supported the thesis that the development of protective antibody responses following vaccination is impaired by the presence of circulating monoclonal antibodies with specificities for important B-cell epitopes in the vaccine. We also noted that intradermal injection of LcrV antigen and cholera toxin adjuvant afforded good levels of protection against systemic and aerosol challenge with Y. pestis: intradermal injection might therefore be considered as a potential minimally invasive method of plague vaccine administration. These data have implications for the design of therapeutic strategies against plague infection.

Oral vaccination with salmonella simultaneously expressing Yersinia pestis F1 and V antigens protects against bubonic and pneumonic plague

The gut provides a large area for immunization enabling the development of mucosal and systemic Ab responses. To test whether the protective Ags to Yersinia pestis can be orally delivered, the Y. pestis caf1 operon, encoding the F1-Ag and virulence Ag (V-Ag) were cloned into attenuated Salmonella vaccine vectors. F1-Ag expression was controlled under a promoter from the caf1 operon; two different promoters (P), PtetA in pV3, PphoP in pV4, as well as a chimera of the two in pV55 were tested. F1-Ag was amply expressed; the chimera in the pV55 showed the best V-Ag expression. Oral immunization with Salmonella-F1 elicited elevated secretory (S)-IgA and serum IgG titers, and Salmonella-V-Ag(pV55) elicited much greater S-IgA and serum IgG Ab titers than Salmonella-V-Ag(pV3) or Salmonella-V-Ag(pV4). Hence, a new Salmonella vaccine, Salmonella-(F1+V)Ags, made with a single plasmid containing the caf1 operon and the chimeric promoter for V-Ag allowed the simultaneous expression of F1 capsule and V-Ag. Salmonella-(F1+V)Ags elicited elevated Ab titers similar to their monotypic derivatives. For bubonic plague, mice dosed with Salmonella-(F1+V)Ags and Salmonella-F1-Ag showed similar efficacy (>83% survival) against approximately 1000 LD(50) Y. pestis. For pneumonic plague, immunized mice required immunity to both F1- and V-Ags because the mice vaccinated with Salmonella-(F1+V)Ags protected against 100 LD(50) Y. pestis. These results show that a single Salmonella vaccine can deliver both F1- and V-Ags to effect both systemic and mucosal immune protection against Y. pestis.

Protective immunity against plague

Plague, an infectious disease that reached catastrophic proportions during three pandemics, continues to be a legitimate public health concern worldwide. Although antibiotic therapy for the causative agent Yersinia pestis is available, pharmaceutical supply limitations, multi-drug resistance from natural selection as well as malicious bioengineering are a reality. Consequently, plague vaccinology is a priority for biodefense research. Development of a multi-subunit vaccine with Fraction 1 and LcrV as protective antigens seems to be receiving the most attention. However, LcrV has been shown to cause immune suppression and Y. pestis mutants lacking F1 expression are thought to be fully virulent in nature and in animal experiments. The LcrV variant, rV10, retains the well documented protective antigenic properties of LcrV but with diminished inhibitory effects on the immune system. More research is required to examine the molecular mechanisms of vaccine protection afforded by surface protein antigens and to decipher the host mechanisms responsible for vaccine success.

Treatment of plague: promising alternatives to antibiotics

Plague still poses a significant threat to human health, and interest has been renewed recently in the possible use of Yersinia pestis as a biological weapon by terrorists. The septicaemic and pneumonic forms are always lethal if untreated. Attempts to treat this deadly disease date back to the era of global pandemics, when various methods were explored. The successful isolation of the plague pathogen led to the beginning of more scientific approaches to the treatment and cure of plague. This subsequently led to specific antibiotic prophylaxis and therapy for Y. pestis. The use of antibiotics such as tetracycline and streptomycin for the treatment of plague has been embraced by the World Health Organization Expert Committee on Plague as the 'gold standard' treatment. However, concerns regarding the development of antibiotic-resistant Y. pestis strains have led to the exploration of alternatives to antibiotics. Several investigators have looked into the use of alternatives, such as immunotherapy, non-pathogen-specific immunomodulatory therapy, phage therapy, bacteriocin therapy, and treatment with inhibitors of virulence factors. The alternative therapies reported in this review should be further investigated by comprehensive studies of their clinical application for the treatment of plague.

Vaccination with live Yersinia pestis primes CD4 and CD8 T cells that synergistically protect against lethal pulmonary Y. pestis infection

Vaccination with live attenuated Yersinia pestis confers protection against pneumonic plague but is not considered safe for general use. Subunit plague vaccines containing the Y. pestis F1 and LcrV proteins prime robust antibody responses but may not provide sufficient protection. To aid the development of a safe and effective plague vaccine, we are investigating roles for T cells during defense against Y. pestis infection. Here we demonstrate that vaccination of mice with live Y. pestis primes specific CD4 and CD8 T cells that, upon purification and direct transfer to naïve mice, synergistically protect against lethal intranasal Y. pestis challenge. While not preventing extrapulmonary dissemination, the coadministered T cells promote bacterial clearance and reduce bacteremia. These observations strongly suggest that development of pneumonic plague vaccines should strive to prime both CD4 and CD8 T cells. Finally, we demonstrate that vaccination with live Y. pestis primes CD4 and CD8 T cells that respond to Y. pestis strains lacking the capacity to express F1, LcrV, and all pCD1/pPCP-encoded proteins, suggesting that protective T cells likely recognize antigens distinct from those previously defined as targets for humoral immunity.

Oral inoculation with Type III secretion mutants of Yersinia pseudotuberculosis provides protection from oral, intraperitoneal, or intranasal challenge with virulent Yersinia

The enteric pathogen Yersinia pseudotuberculosis (Yptb) causes gastroenteritis, mesenteric lymphadenitis, and systemic infections in humans, livestock, and wild animals. Yptb Type III secretion system (pTTSS) mutants efficiently colonize lymphoid tissues, but not the gastrointestinal tract, spleen, or liver. Here, we show that a single oral inoculation of pTTSS mutants prevents morbidity in almost 100% of mice challenged intragastrically with virulent Yptb. In addition, a single oral inoculation of a pTTSS mutant protected 50% of mice challenged intraperitoneally or intranasally with virulent Yptb. In addition, the intranasally challenged mice that succumbed to infection lived significantly longer than non-immunized mice. Thus, pTTSS mutants can function as live attenuated vaccine when delivered orally. Potential uses for these attenuated strains include use as a livestock vaccine, a rodent plague control reagent in endemic areas around the world, and a vector for delivery of other antigens to the mesenteric lymph nodes.

Protective immunity against respiratory tract challenge with Yersinia pestis in mice immunized with an adenovirus-based vaccine vector expressing V antigen

The aerosol form of the bacterium Yersinia pestis causes the pneumonic plague, a rapidly fatal disease. At present, no plague vaccines are available for use in the United States. One candidate for the development of a subunit vaccine is the Y. pestis virulence (V) antigen, a protein that mediates the function of the Yersinia outer protein virulence factors and suppresses inflammatory responses in the host. On the basis of the knowledge that adenovirus (Ad) gene-transfer vectors act as adjuvants in eliciting host immunity against the transgene they carry, we tested the hypothesis that a single administration of a replication-defective Ad gene-transfer vector encoding the Y. pestis V antigen (AdsecV) could stimulate strong protective immune responses without a requirement for repeat administration. AdsecV elicited specific T cell responses and high IgG titers in serum within 2 weeks after a single intramuscular immunization. Importantly, the mice were protected from a lethal intranasal challenge of Y. pestis CO92 from 4 weeks up to 6 months after immunization with a single intramuscular dose of AdsecV. These observations suggest that an Ad gene-transfer vector expressing V antigen is a candidate for development of an effective anti-plague vaccine

Yersinia pestis outer membrane type III secretion protein YscC: expression, purification, characterization, and induction of specific antiserum

The type III secretion system (YscC) protein of Yersinia pestis plays an essential role in the translocation of Yersinia outer proteins (Yops) into eukaryotic target cells through a type III secretion mechanism. To assess the immunogenicity and potential protective efficacy of YscC against lethal plague challenge, we cloned, overexpressed, and purified YscC using two different bacterial expression and purification systems. The resulting expression plasmids for YscC, pETBlue-2-YscC and pTYB11-YscC, were regulated by robust T7 promoters that were induced with isopropyl-beta-D-thiogalactopyranoside. The intein-fusion pTYB11-YscC system and the six-histidine-tagging pETBlue-2-YscC system were both successful for producing and purifying YscC. The intein-mediated purification system produced about 1mg of soluble YscC per liter of bacterial culture while the YscC-His(6)-tag method resulted in 16mg of insoluble YscC per liter of bacterial culture. Protein identity for purified YscC-His(6) was confirmed by ion trap mass spectrometry. Antisera were produced against both YscC and YscC-His(6). The specific immune response generated in YscC-vaccinated mice was relative to the particular purified protein, YscC or YscC-His(6), which was used for vaccination as determined by Western blot analysis and ELISA. Regardless of the purification method, either form of the YscC protein failed to elicit a protective immune response against lethal plague challenge with either F1 capsule forming Y. pestis CO92 or the isogenic F1(-)Y. pestis C12.

Immunization of mice with YscF provides protection from Yersinia pestis infections

BACKGROUND

Yersinia pestis, the causative agent of plague, is a pathogen with a tremendous ability to cause harm and panic in populations. Due to the severity of plague and its potential for use as a bioweapon, better preventatives and therapeutics for plague are desirable. Subunit vaccines directed against the F1 capsular antigen and the V antigen (also known as LcrV) of Y. pestis are under development. However, these new vaccine formulations have some possible limitations. The F1 antigen is not required for full virulence of Y. pestis and LcrV has a demonstrated immunosuppressive effect. These limitations could damper the ability of F1/LcrV based vaccines to protect against F1-minus Y. pestis strains and could lead to a high rate of undesired side effects in vaccinated populations. For these reasons, the use of other antigens in a plague vaccine formulation may be advantageous.

RESULTS

Desired features in vaccine candidates would be antigens that are conserved, essential for virulence and accessible to circulating antibody. Several of the proteins required for the construction or function of the type III secretion system (TTSS) complex could be ideal contenders to meet the desired features of a vaccine candidate. Accordingly, the TTSS needle complex protein, YscF, was selected to investigate its potential as a protective antigen. In this study we describe the overexpression, purification and use of YscF as a protective antigen. YscF immunization triggers a robust antibody response to YscF and that antibody response is able to afford significant protection to immunized mice following challenge with Y. pestis. Additionally, evidence is presented that suggests antibody to YscF is likely not protective by blocking the activity of the TTSS.

CONCLUSION

In this study we investigated YscF, a surface-expressed protein of the Yersinia pestis type III secretion complex, as a protective antigen against experimental plague infection. Immunization of mice with YscF resulted in a high anti-YscF titer and provided protection against i.v. challenge with Y. pestis. This is the first report to our knowledge utilizing a conserved protein from the type III secretion complex of a gram-negative pathogen as a candidate for vaccine development.

Protein microarray for profiling antibody responses to Yersinia pestis live vaccine

A protein microarray representing 149 Yersinia pestis proteins was developed to profile antibody responses in EV76-immunized rabbits. Antibodies to 50 proteins were detected. There are 11 proteins besides F1 and V antigens to which the predominant antibody response occurred, and these proteins show promise for further evaluation as candidates for subunit vaccines and/or diagnostic antigens.

Defining the genome content of live plague vaccines by use of whole-genome DNA microarray

Yersinia pestis whole-genome DNA microarrays were developed to perform genomic comparison of a collection of live plague vaccines. By using the genomic DNA to probe the DNA microarrays, we detected dozens of deletions and amplifications of the genomic regions in the 19 vaccine strains analyzed. The revealed genomic differences within the vaccine strains of different origins provide us an unprecedented opportunity to understand the molecular background of the variability of the immunogenic and protective potency of plague live vaccine. The whole-genome DNA microarray also provides an ideal tool to perform the pre-evaluation of a vaccine strain for its high throughput to determine the genomic features essential or unallowable for the live vaccines.