Immunization with recombinant V10 protects cynomolgus macaques from lethal pneumonic plague

Antimicrobial therapies administrated during the Third Plague Pandemic in Europe

Plague raged in Europe for over 1400 years and was responsible for three major pandemics. Today, plague still poses a serious threat to global public health and surveillance is imperative. Plague is still present in natural reservoirs on several continents, including Africa, Asia and the Americas, and sometimes causes local cases and epidemics. The Third Plague Pandemic caused millions of deaths worldwide, including in Europe. Plague arrived in Europe in the autumn of 1896 mostly through maritime trade routes, where it spread with several epidemic events until 1945, when, in the port city of Taranto, the last known outbreak was recorded. In this paper, we present an overview of the natural history and pathogenicity of Yersinia pestis, the bacterium responsible for plague, its spread from Asia to Europe during the Third Pandemic, and the therapies used to treat and prevent the disease in Europe, with particular focus on the case of Taranto. In Taranto, the Pasteur Institute's antiserum antimicrobial therapy, and vaccination were used to treat and stop the advance of the bacterium, with mixed results.

A novel sORF gene mutant strain of Yersinia pestis vaccine EV76 offers enhanced safety and improved protection against plague

We recently identified two virulence-associated small open reading frames (sORF) of Yersinia pestis, named yp1 and yp2, and null mutants of each individual genes were highly attenuated in virulence. Plague vaccine strain EV76 is known for strong reactogenicity, making it not suitable for use in humans. To improve the immune safety of EV76, three mutant strains of EV76, Δyp1, Δyp2, and Δyp1&yp2 were constructed and their virulence attenuation, immunogenicity, and protective efficacy in mice were evaluated. All mutant strains were attenuated by the subcutaneous (s.c.) route and exhibited more rapid clearance in tissues than the parental strain EV76. Under iron overload conditions, only the mice infected with EV76Δyp1 survived, accompanied by less draining lymph nodes damage than those infected by EV76. Analysis of cytokines secreted by splenocytes of immunized mice found that EV76Δyp2 induced higher secretion of multiple cytokines including TNF-α, IL-2, and IL-12p70 than EV76. On day 42, EV76Δyp2 or EV76Δyp1&yp2 immunized mice exhibited similar protective efficacy as EV76 when exposed to Y. pestis 201, both via s.c. or intranasal (i.n.) routes of administration. Moreover, when exposed to 200-400 LD50 Y. pestis strain 201Δcaf1 (non-encapsulated Y. pestis), EV76Δyp2 or EV76Δyp1&yp2 are able to afford about 50% protection to i.n. challenges, significantly better than the protection afforded by EV76. On 120 day, mice immunized with EV76Δyp2 or EV76Δyp1&yp2 cleared the i.n. challenge of Y. pestis 201-lux as quickly as those immunized with EV76, demonstrating 90-100% protection. Our results demonstrated that deletion of the yp2 gene is an effective strategy to attenuate virulence of Y. pestis EV76 while improving immunogenicity. Furthermore, EV76Δyp2 is a promising candidate for conferring protection against the pneumonic and bubonic forms of plague.

Overview of Yersinia pestis Metallophores: Yersiniabactin and Yersinopine

The pathogenic anaerobic bacteria Yersinia pestis (Y. pestis), which is well known as the plague causative agent, has the ability to escape or inhibit innate immune system responses, which can result in host death even before the activation of adaptive responses. Bites from infected fleas in nature transmit Y. pestis between mammalian hosts causing bubonic plague. It was recognized that a host's ability to retain iron is essential in fighting invading pathogens. To proliferate during infection, Y. pestis, like most bacteria, has various iron transporters that enable it to acquire iron from its hosts. The siderophore-dependent iron transport system was found to be crucial for the pathogenesis of this bacterium. Siderophores are low-molecular-weight metabolites with a high affinity for Fe3+. These compounds are produced in the surrounding environment to chelate iron. The siderophore secreted by Y. pestis is yersiniabactin (Ybt). Another metallophore produced by this bacterium, yersinopine, is of the opine type and shows similarities with both staphylopine and pseudopaline produced by Staphylococcus aureus and Pseudomonas aeruginosa, respectively. This paper sheds light on the most important aspects of the two Y. pestis metallophores as well as aerobactin a siderophore no longer secreted by this bacterium due to frameshift mutation in its genome.

Yersinia pestis and Plague: some knowns and unknowns

Since its first identification in 1894 during the third pandemic in Hong Kong, there has been significant progress of understanding the lifestyle of Yersinia pestis, the pathogen that is responsible for plague. Although we now have some understanding of the pathogen's physiology, genetics, genomics, evolution, gene regulation, pathogenesis and immunity, there are many unknown aspects of the pathogen and its disease development. Here, we focus on some of the knowns and unknowns relating to Y. pestis and plague. We notably focus on some key Y. pestis physiological and virulence traits that are important for its mammal-flea-mammal life cycle but also its emergence from the enteropathogen Yersinia pseudotuberculosis. Some aspects of the genetic diversity of Y. pestis, the distribution and ecology of plague as well as the medical countermeasures to protect our population are also provided. Lastly, we present some biosafety and biosecurity information related to Y. pestis and plague.

Non-human primate models of human respiratory infections

Respiratory pathogens represent a great burden for humanity and a potential source of new pandemics, as illustrated by the recent emergence of coronavirus disease 2019 (COVID-19). In recent decades, biotechnological advances have led to the development of numerous innovative therapeutic molecules and vaccine immunogens. However, we still lack effective treatments and vaccines against many respiratory pathogens. More than ever, there is a need for a fast, predictive, preclinical pipeline, to keep pace with emerging diseases. Animal models are key for the preclinical development of disease management strategies. The predictive value of these models depends on their ability to reproduce the features of the human disease, the mode of transmission of the infectious agent and the availability of technologies for monitoring infection. This review focuses on the use of non-human primates as relevant preclinical models for the development of prevention and treatment for human respiratory infections.

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.

Binding Sites of Anti-Lcr V Monoclonal Antibodies Are More Critical than the Avidities and Affinities for Passive Protection against Yersinia pestis Infection in a Bubonic Plague Model

Plague is a zoonotic disease that is caused by Yersinia pestis. Monoclonal antibodies (mAbs) that bind to the V-antigen, a virulence factor that is produced by Y. pestis, can passively protect mice from plague. An analysis of protective mAbs that bind to V-antigen was made to assess binding sites, avidities, and affinities. Anti-V mAbs were screened for their efficacy in a murine model of plague. Antigen-binding sites of protective V mAbs were determined with a linear peptide library, V-antigen fragment, competitive binding, and surface plasmon resonance. The avidities to the V-antigen was determined by ELISA, and affinities of the mAbs to the V-antigen were determined by surface plasmon resonance. The most protective mAb 7.3 bound to a unique conformational site on the V-antigen, while a less protective mAb bound to a different conformational site located on the same V-antigen fragment as mAb 7.3. The avidity of mAb 7.3 for the V-antigen was neither the strongest overall nor did it have the highest affinity for the V-antigen. The binding site of the most protective mAb was critical in its ability to protect against a lethal plague challenge.

Yersinia pestis and plague: an updated view on evolution, virulence determinants, immune subversion, vaccination, and diagnostics

Plague is a vector-borne disease caused by Yersinia pestis. Transmitted by fleas from rodent reservoirs, Y. pestis emerged <6000 years ago from an enteric bacterial ancestor through events of gene gain and genome reduction. It is a highly remarkable model for the understanding of pathogenic bacteria evolution, and a major concern for public health as highlighted by recent human outbreaks. A complex set of virulence determinants, including the Yersinia outer-membrane proteins (Yops), the broad-range protease Pla, pathogen-associated molecular patterns (PAMPs), and iron capture systems play critical roles in the molecular strategies that Y. pestis employs to subvert the human immune system, allowing unrestricted bacterial replication in lymph nodes (bubonic plague) and in lungs (pneumonic plague). Some of these immunogenic proteins as well as the capsular antigen F1 are exploited for diagnostic purposes, which are critical in the context of the rapid onset of death in the absence of antibiotic treatment (less than a week for bubonic plague and <48 h for pneumonic plague). Here, we review recent research advances on Y. pestis evolution, virulence factor function, bacterial strategies to subvert mammalian innate immune responses, vaccination, and problems associated with pneumonic plague diagnosis.

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 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.

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.

Modern Advances against Plague

Plague has been a scourge of humanity, responsible for the deaths of millions. The etiological agent, Yersinia pestis, has evolved relatively recently from an enteropathogen, Yersinia pseudotuberculosis. The evolution of the plague pathogen has involved a complex series of genetic acquisitions, deletions, and rearrangements in its transition from an enteric niche to becoming a systemic, flea-vectored pathogen. With the advent of modern molecular biology techniques, we are starting to understand how the organism adapts to the diverse niches it encounters and how to combat the threat it poses.

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.

Characterization of a Cynomolgus Macaque Model of Pneumonic Plague for Evaluation of Vaccine Efficacy

The efficacy of a recombinant plague vaccine (rF1V) was evaluated in cynomolgus macaques (CMs) to establish the relationship among vaccine doses, antibody titers, and survival following an aerosol challenge with a lethal dose of Yersinia pestis strain Colorado 92. CMs were vaccinated with a range of rF1V doses on a three-dose schedule (days 0, 56, and 121) to provide a range of survival outcomes. The humoral immune response following vaccination was evaluated with anti-rF1, anti-rV, and anti-rF1V bridge enzyme-linked immunosorbent assays (ELISAs). Animals were challenged via aerosol exposure on day 149. Vaccine doses and antibody responses were each significantly associated with the probability of CM survival (P < 0.0001). Vaccination also decreased signs of pneumonic plague in a dose-dependent manner. There were statistically significant correlations between the vaccine dose and the time to onset of fever (P < 0.0001), the time from onset of fever to death (P < 0.0001), the time to onset of elevated respiratory rate (P = 0.0003), and the time to onset of decreased activity (P = 0.0251) postinfection in animals exhibiting these clinical signs. Delays in the onset of these clinical signs of disease were associated with larger doses of rF1V. Immunization with ≥ 12 μg of rF1V resulted in 100% CM survival. Since both the vaccine dose and anti-rF1V antibody titers correlate with survival, rF1V bridge ELISA titers can be used as a correlate of protection.

Quantitative proteomic analysis of Burkholderia pseudomallei Bsa type III secretion system effectors using hypersecreting mutants

Burkholderia pseudomallei is an intracellular pathogen and the causative agent of melioidosis, a severe disease of humans and animals. One of the virulence factors critical for early stages of infection is the Burkholderia secretion apparatus (Bsa) Type 3 Secretion System (T3SS), a molecular syringe that injects bacterial proteins, called effectors, into eukaryotic cells where they subvert cellular functions to the benefit of the bacteria. Although the Bsa T3SS itself is known to be important for invasion, intracellular replication, and virulence, only a few genuine effector proteins have been identified and the complete repertoire of proteins secreted by the system has not yet been fully characterized. We constructed a mutant lacking bsaP, a homolog of the T3SS "gatekeeper" family of proteins that exert control over the timing and magnitude of effector protein secretion. Mutants lacking BsaP, or the T3SS translocon protein BipD, were observed to hypersecrete the known Bsa effector protein BopE, providing evidence of their role in post-translational control of the Bsa T3SS and representing key reagents for the identification of its secreted substrates. Isobaric Tags for Relative and Absolute Quantification (iTRAQ), a gel-free quantitative proteomics technique, was used to compare the secreted protein profiles of the Bsa T3SS hypersecreting mutants of B. pseudomallei with the isogenic parent strain and a bsaZ mutant incapable of effector protein secretion. Our study provides one of the most comprehensive core secretomes of B. pseudomallei described to date and identified 26 putative Bsa-dependent secreted proteins that may be considered candidate effectors. Two of these proteins, BprD and BapA, were validated as novel effector proteins secreted by the Bsa T3SS of B. pseudomallei.

Multiple roles of Myd88 in the immune response to the plague F1-V vaccine and in protection against an aerosol challenge of Yersinia pestis CO92 in mice

The current candidate vaccine against Yersinia pestis infection consists of two subunit proteins: the capsule protein or F1 protein and the low calcium response V protein or V-antigen. Little is known of the recognition of the vaccine by the host's innate immune system and how it affects the acquired immune response to the vaccine. Thus, we vaccinated Toll-like receptor (Tlr) 2, 4, and 2/4-double deficient, as well as signal adaptor protein Myd88-deficient mice. We found that Tlr4 and Myd88 appeared to be required for an optimal immune response to the F1-V vaccine but not Tlr2 when compared to wild-type mice. However, there was a difference between the requirement for Tlr4 and MyD88 in vaccinated animals. When F1-V vaccinated Tlr4 mutant (lipopolysaccharide tolerant) and Myd88-deficient mice were challenged by aerosol with Y. pestis CO92, all but one Tlr4 mutant mice survived the challenge, but no vaccinated Myd88-deficient mice survived the challenge. Spleens from these latter nonsurviving mice showed that Y. pestis was not cleared from the infected mice. Our results suggest that MyD88 appears to be important for both an optimal immune response to F1-V and in protection against a lethal challenge of Y. pestis CO92 in F1-V vaccinated mice.

Remote monitoring of the progression of primary pneumonic plague in Brown Norway rats in high-capacity, high-containment housing

Development of new vaccines, diagnostics, and therapeutics for biodefense or other relatively rare infectious diseases is hindered by the lack of naturally occurring human disease on which to conduct clinical trials of efficacy. To overcome this experimental gap, the U.S. Food and Drug Administration established the Animal Rule, in which efficacy testing in two well-characterized animal models that closely resemble human disease may be accepted in lieu of large-scale clinical trials for diseases with limited natural human incidence. In this report, we evaluated the Brown Norway rat as a model for pneumonic plague and describe the natural history of clinical disease following inhalation exposure to Yersinia pestis. In high-capacity, high-containment housing, we monitored temperature, activity, heart rate, and rhythm by capturing electronic impulses transmitted from abdominal telemeter implants. Using this system, we show that reduced activity and development of fever are sensitive indications of disease progression. Furthermore, we identified heart arrhythmias as contributing factors to the rapid progression to lethality following the fever response. Together, these data validate the Brown Norway rat as an experimental model for human pneumonic plague and provide new insight that may ultimately lead to novel approaches in postexposure treatment of this devastating infection.

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.

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 novel post-exposure medical countermeasure L-97-1 improves survival and acute lung injury following intratracheal infection with Yersinia pestis

Yersinia pestis, a Gram-negative bacillus causing plague and Centers for Disease Control and Prevention (CDC) classified Category A pathogen, has high potential as a bioweapon. Lipopolysaccharide, a virulence factor for Y. pestis, binds to and activates A(1) adenosine receptor (AR)s and, in animals, A(1)AR antagonists block induced acute lung injury (ALI) and increase survival following cecal ligation and perforation. In this study, rats were infected intratracheally with viable Y. pestis [CO99 (pCD1( + )/Δpgm) 1 × 10( 8 ) CFU/animal] and treated daily for 3 d with ciprofloxacin (cipro), the A(1)AR antagonist L-97-1, or cipro plus L-97-1 starting at 0, 6, 24, 48, or 72 h post-Y. pestis. At 72 h post-Y. pestis, cipro plus L-97-1 significantly improved 6-d survival to 60-70% vs 28% for cipro plus H(2)O and 33% for untreated Y. pestis controls (P = 0.02, logrank test). Lung edema, hemorrhage and leukocyte infiltration index (LII) were evaluated histologically to produce ALI scores. Cipro plus L-97-1 significantly reduced lung edema, as well as aggregate lung injury scores vs controls or cipro plus H(2)O, and LII vs controls (P < 0.05, Student's unpaired t test). These results support efficacy for L-97-1 as a post-exposure medical countermeasure, adjunctive therapy to antibiotics for Y. pestis.

Establishment of a Swiss Webster mouse model of pneumonic plague to meet essential data elements under the animal rule

A recombinant vaccine (rF1V) is being developed for protection against pneumonic plague. This study was performed to address essential data elements to establish a well-characterized Swiss Webster mouse model for licensing the rF1V vaccine using the FDA's Animal Rule. These elements include the documentation of challenge material characteristics, aerosol exposure parameters, details of the onset and severity of clinical signs, pathophysiological response to disease, and relevance to human disease. Prior to animal exposures, an evaluation of the aerosol system was performed to determine and understand the variability of the aerosol exposure system. Standardized procedures for the preparation of Yersinia pestis challenge material also were developed. The 50% lethal dose (LD(50)) was estimated to be 1,966 CFU using Probit analysis. Following the LD(50) determination, pathology was evaluated by exposing mice to a target LD(99) (42,890 CFU). Mice were euthanized at 12, 24, 36, 48, 60, and 72 h postexposure. At each time point, samples were collected for clinical pathology, detection of bacteria in blood and tissues, and pathology evaluations. A general increase in incidence and severity of microscopic findings was observed in the lung, lymph nodes, spleen, and liver from 36 to 72 h postchallenge. Similarly, the incidence and severity of pneumonia increased throughout the study; however, some mice died in the absence of pneumonia, suggesting that disease progression does not require the development of pneumonia. Disease pathology in the Swiss Webster mouse is similar to that observed in humans, demonstrating the utility of this pneumonic plague model that can be used by researchers investigating plague countermeasures.

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.

Polymorphisms in the lcrV gene of Yersinia enterocolitica and their effect on plague protective immunity

Current efforts to develop plague vaccines focus on LcrV, a polypeptide that resides at the tip of type III secretion needles. LcrV-specific antibodies block Yersinia pestis type III injection of Yop effectors into host immune cells, thereby enabling phagocytes to kill the invading pathogen. Earlier work reported that antibodies against Y. pestis LcrV cannot block type III injection by Yersinia enterocolitica strains and suggested that lcrV polymorphisms may provide for escape from LcrV-mediated plague immunity. We show here that polyclonal or monoclonal antibodies raised against Y. pestis KIM D27 LcrV (LcrV(D27)) bind LcrV from Y. enterocolitica O:9 strain W22703 (LcrV(W22703)) or O:8 strain WA-314 (LcrV(WA-314)) but are otherwise unable to block type III injection by Y. enterocolitica strains. Replacing the lcrV gene on the pCD1 virulence plasmid of Y. pestis KIM D27 with either lcrV(W22703) or lcrV(WA-314) does not affect the ability of plague bacteria to secrete proteins via the type III pathway, to inject Yops into macrophages, or to cause lethal plague infections in mice. LcrV(D27)-specific antibodies blocked type III injection by Y. pestis expressing lcrV(W22703) or lcrV(WA-314) and protected mice against intravenous lethal plague challenge with these strains. Thus, although antibodies raised against LcrV(D27) are unable to block the type III injection of Y. enterocolitica strains, expression of lcrV(W22703) or lcrV(WA-314) in Y. pestis did not allow these strains to escape LcrV-mediated plague protective immunity in the intravenous challenge model.

Enhanced humoral and mucosal immune responses after intranasal immunization with chimeric multiple antigen peptide of LcrV antigen epitopes of Yersinia pestis coupled to palmitate in mice

Yersinia pestis is the causative agent of the most deadly disease plague. F1 and V antigens are the major vaccine candidates. Six protective epitopes of V antigen of varying length (15-25aa) were assembled on a lysine backbone as multiple antigen peptide (MAP) using standard Fmoc chemistry. Palmitate was coupled at amino terminus end. Amino acid analysis, SDS-PAGE, immunoblot and immunoreactivity proved the authenticity of MAP. MAP was immunized intranasally encapsulated in PLGA (polylactide-co-glycolide) microspheres and with/without/adjuvants murabutide and CpG ODN 1826 (CpG), in three strains of mice. Humoral and mucosal immune responses were studied till day 120 and memory response was checked after immunization with native V antigen on day 120. Epitope specific serum and mucosal washes IgG, IgA, IgG subclasses and specific activity were measured by indirect ELISA and sandwich ELISA, respectively. IgG and IgA peak antibody titers of all the MAP construct formulations in sera were ranging from 71,944 to 360,578 and 4493 to 28,644, respectively. MAP with CpG showed significantly high (p<0.0001) antibody titers ranging from 101,690 to 360,578 for IgG and 28,644 for IgA. Mucosal peak IgG and IgA titers were ranging from 1425 to 8072 and 1425 to 7183, respectively in intestinal washes and 799-4528 and 566-4027, respectively in lung washes. MAP with CpG showed significantly high (p<0.001) SIgA titers of 8000 in lung and 16,000 in intestinal washes. IgG isotyping revealed IgG2a/IgG1 ratio>1 with CpG. Serum and mucosal antipeptide IgG and IgA specific activities correlated well with antibody titers. All the constituent peptides contributed towards immune response. Structural analysis of MAP revealed little or no interaction between the peptides. Present study showed MAP to be highly immunogenic with high and long lasting antibody titers in serum and mucosal washes with good recall response with/without CpG as an adjuvant which can be used for vaccine development for plague.

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.

Prevention of pneumonic plague in mice, rats, guinea pigs and non-human primates with clinical grade rV10, rV10-2 or F1-V vaccines

Yersinia pestis causes plague, a disease with high mortality in humans that can be transmitted by fleabite or aerosol. A US Food and Drug Administration (FDA)-licensed plague vaccine is currently not available. Vaccine developers have focused on two subunits of Y. pestis: LcrV, a protein at the tip of type III secretion needles, and F1, the fraction 1 pilus antigen. F1-V, a hybrid generated via translational fusion of both antigens, is being developed for licensure as a plague vaccine. The rV10 vaccine is a non-toxigenic variant of LcrV lacking residues 271-300. Here we developed Current Good Manufacturing Practice (cGMP) protocols for rV10. Comparison of clinical grade rV10 with F1-V did not reveal significant differences in plague protection in mice, guinea pigs or cynomolgus macaques. We also developed cGMP protocols for rV10-2, a variant of rV10 with an altered affinity tag. Immunization with rV10-2 adsorbed to aluminum hydroxide elicited antibodies against LcrV and conferred pneumonic plague protection in mice, rats, guinea pigs, cynomolgus macaques and African Green monkeys. The data support further development of rV10-2 for FDA Investigational New Drug (IND) authorization review and clinical testing.

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.

Plague: Infections of Companion Animals and Opportunities for Intervention

Plague is a zoonotic disease, normally circulating in rodent populations, transmitted to humans most commonly through the bite of an infected flea vector. Secondary infection of the lungs results in generation of infectious aerosols, which pose a significant hazard to close contacts. In enzootic areas, plague infections have been reported in owners and veterinarians who come into contact with infected pets. Dogs are relatively resistant, but can import infected fleas into the home. Cats are acutely susceptible, and can present a direct hazard to health. Reducing roaming and hunting behaviours, combined with flea control measures go some way to reducing the risk to humans. Various vaccine formulations have been developed which may be suitable to protect companion animals from contracting plague, and thus preventing onward transmission to man. Since transmission has resulted in a number of fatal cases of plague, the vaccination of domestic animals such as cats would seem a low cost strategy for reducing the risk of infection by this serious disease in enzootic regions.

Plague: Infections of Companion Animals and Opportunities for Intervention

Plague is a zoonotic disease, normally circulating in rodent populations, transmitted to humans most commonly through the bite of an infected flea vector. Secondary infection of the lungs results in generation of infectious aerosols, which pose a significant hazard to close contacts. In enzootic areas, plague infections have been reported in owners and veterinarians who come into contact with infected pets. Dogs are relatively resistant, but can import infected fleas into the home. Cats are acutely susceptible, and can present a direct hazard to health. Reducing roaming and hunting behaviours, combined with flea control measures go some way to reducing the risk to humans. Various vaccine formulations have been developed which may be suitable to protect companion animals from contracting plague, and thus preventing onward transmission to man. Since transmission has resulted in a number of fatal cases of plague, the vaccination of domestic animals such as cats would seem a low cost strategy for reducing the risk of infection by this serious disease in enzootic regions.

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.

Recombinant (F1+V) vaccine protects cynomolgus macaques against pneumonic plague

Cynomolgus macaques, immunised at the 80 μg dose level with an rF1+rV vaccine (two doses, three weeks apart), were fully protected against pneumonic plague following inhalational exposure to a clinical isolate of Yersinia pestis (strain CO92) at week 8 of the schedule. At this time, all the immunised animals had developed specific IgG titres to rF1 and rV with geometric mean titres of 96.83±20.93 μg/ml and 78.59±12.07 μg/ml, respectively, for the 40 μg dose group; by comparison, the 80 μg dose group had developed titres of 114.4±22.1 and 90.8±15.8 μg/ml to rF1 and rV, respectively, by week 8. For all the immunised animals, sera drawn at week 8 competed with the neutralising and protective Mab7.3 for binding to rV antigen in a competitive ELISA, indicating that a functional antibody response to rV had been induced. All but one of the group immunised at the lower 40 μg dose-level were protected against infection; the single animal which succumbed had significantly reduced antibody responses to both the rF1 and rV antigens. Although a functional titre to rV antigen was detected for this animal, this was insufficient for protection, indicating that there may have been a deficiency in the functional titre to rF1 and underlining the need for immunity to both vaccine antigens to achieve protective efficacy against plague. This candidate vaccine, which has been evaluated as safe and immunogenic in clinical studies, has now been demonstrated to protect cynomolgus macaques, immunised in the clinical regimen, against pneumonic plague.

Histopathological observation of immunized rhesus macaques with plague vaccines after subcutaneous infection of Yersinia pestis

In our previous study, complete protection was observed in Chinese-origin rhesus macaques immunized with SV1 (20 µg F1 and 10 µg rV270) and SV2 (200 µg F1 and 100 µg rV270) subunit vaccines and with EV76 live attenuated vaccine against subcutaneous challenge with 6×10(6) CFU of Y. pestis. In the present study, we investigated whether the vaccines can effectively protect immunized animals from any pathologic changes using histological and immunohistochemical techniques. In addition, the glomerular basement membranes (GBMs) of the immunized animals and control animals were checked by electron microscopy. The results show no signs of histopathological lesions in the lungs, livers, kidneys, lymph nodes, spleens and hearts of the immunized animals at Day 14 after the challenge, whereas pathological alterations were seen in the corresponding tissues of the control animals. Giemsa staining, ultrastructural examination, and immunohistochemical staining revealed bacteria in some of the organs of the control animals, whereas no bacterium was observed among the immunized animals. Ultrastructural observation revealed that no glomerular immune deposits on the GBM. These observations suggest that the vaccines can effectively protect animals from any pathologic changes and eliminate Y. pestis from the immunized animals. The control animals died from multi-organ lesions specifically caused by the Y. pestis infection. We also found that subcutaneous infection of animals with Y. pestis results in bubonic plague, followed by pneumonic and septicemic plagues. The histopathologic features of plague in rhesus macaques closely resemble those of rodent and human plagues. Thus, Chinese-origin rhesus macaques serve as useful models in studying Y. pestis pathogenesis, host response and the efficacy of new medical countermeasures against plague.

Plague in Guinea pigs and its prevention by subunit vaccines

Human pneumonic plague is a devastating and transmissible disease for which a Food and Drug Administration-approved vaccine is not available. Suitable animal models may be adopted as a surrogate for human plague to fulfill regulatory requirements for vaccine efficacy testing. To develop an alternative to pneumonic plague in nonhuman primates, we explored guinea pigs as a model system. On intranasal instillation of a fully virulent strain, Yersinia pestis CO92, guinea pigs developed lethal lung infections with hemorrhagic necrosis, massive bacterial replication in the respiratory system, and blood-borne dissemination to other organ systems. Expression of the Y. pestis F1 capsule was not required for the development of pulmonary infection; however, the capsule seemed to be important for the establishment of bubonic plague. The mean lethal dose (MLD) for pneumonic plague in guinea pigs was estimated to be 1000 colony-forming units. Immunization of guinea pigs with the recombinant forms of LcrV, a protein that resides at the tip of Yersinia type III secretion needles, or F1 capsule generated robust humoral immune responses. Whereas LcrV immunization resulted in partial protection against pneumonic plague challenge with 250 MLD Y. pestis CO92, immunization with recombinant F1 did not. rV10, a vaccine variant lacking LcrV residues 271-300, elicited protection against pneumonic plague, which seemed to be based on conformational antibodies directed against LcrV.

Levofloxacin cures experimental pneumonic plague in African green monkeys

BACKGROUND

Yersinia pestis, the agent of plague, is considered a potential bioweapon due to rapid lethality when delivered as an aerosol. Levofloxacin was tested for primary pneumonic plague treatment in a nonhuman primate model mimicking human disease.

METHODS AND RESULTS

Twenty-four African Green monkeys (AGMs, Chlorocebus aethiops) were challenged via head-only aerosol inhalation with 3-145 (mean = 65) 50% lethal (LD(50)) doses of Y. pestis strain CO92. Telemetered body temperature >39 °C initiated intravenous infusions to seven 5% dextrose controls or 17 levofloxacin treated animals. Levofloxacin was administered as a "humanized" dose regimen of alternating 8 mg/kg and 2 mg/kg 30-min infusions every 24-h, continuing until animal death or 20 total infusions, followed by 14 days of observation. Fever appeared at 53-165 h and radiographs found multilobar pneumonia in all exposed animals. All control animals died of severe pneumonic plague within five days of aerosol exposure. All 16 animals infused with levofloxacin for 10 days survived. Levofloxacin treatment abolished bacteremia within 24 h in animals with confirmed pre-infusion bacteremia, and reduced tachypnea and leukocytosis but not fever during the first 2 days of infusions.

CONCLUSION

Levofloxacin cures established pneumonic plague when treatment is initiated after the onset of fever in the lethal aerosol-challenged AGM nonhuman primate model, and can be considered for treatment of other forms of plague. Levofloxacin may also be considered for primary presumptive-use, multi-agent antibiotic in bioterrorism events prior to identification of the pathogen.

Principles of antidote pharmacology: an update on prophylaxis, post-exposure treatment recommendations and research initiatives for biological agents

The use of biological agents has generally been confined to military-led conflicts. However, there has been an increase in non-state-based terrorism, including the use of asymmetric warfare, such as biological agents in the past few decades. Thus, it is becoming increasingly important to consider strategies for preventing and preparing for attacks by insurgents, such as the development of pre- and post-exposure medical countermeasures. There are a wide range of prophylactics and treatments being investigated to combat the effects of biological agents. These include antibiotics (for both conventional and unconventional use), antibodies, anti-virals, immunomodulators, nucleic acids (analogues, antisense, ribozymes and DNAzymes), bacteriophage therapy and micro-encapsulation. While vaccines are commercially available for the prevention of anthrax, cholera, plague, Q fever and smallpox, there are no licensed vaccines available for use in the case of botulinum toxins, viral encephalitis, melioidosis or ricin. Antibiotics are still recommended as the mainstay treatment following exposure to anthrax, plague, Q fever and melioidosis. Anti-toxin therapy and anti-virals may be used in the case of botulinum toxins or smallpox respectively. However, supportive care is the only, or mainstay, post-exposure treatment for cholera, viral encephalitis and ricin - a recommendation that has not changed in decades. Indeed, with the difficulty that antibiotic resistance poses, the development and further evaluation of techniques and atypical pharmaceuticals are fundamental to the development of prophylaxis and post-exposure treatment options. The aim of this review is to present an update on prophylaxis and post-exposure treatment recommendations and research initiatives for biological agents in the open literature from 2007 to 2009.

Cynomolgus macaque model for pneumonic plague

A recombinant vaccine (rF1V) is currently being developed for protection against pneumonic plague. An essential component in evaluating efficacy of the rF1V vaccine is the development of a well-understood animal model that shows similarity to human disease. The objective of this study was to determine the inhaled median lethal dose (LD₅₀), evaluate the pathophysiology of disease and identify appropriate study endpoints in a cynomolgus macaque (CM) model of pneumonic plague. Eighteen CMs were challenged by head-only aerosol exposure with seven dosages of Yersinia pestis CO92. An LD₅₀ of 24 colony forming units was estimated using Probit analysis. Disease pathology was evaluated by blood culture, clinical pathology, histopathology and telemetry. CMs that died became febrile following challenge and died 34-92 h after onset of fever. Bacteremia, increased respiration and heart rate, decreased blood pressure and loss of diurnal rhythm were also observed in conjunction with onset of fever. Histopathological examinations revealed significant findings in the lungs (intra-alveolar neutrophils and fibrinous pleuritis) consistent with pneumonic plague. These data indicate that the disease pathology observed in CMs following aerosol exposure to Y. pestis CO92 is similar to that of pneumonic plague in humans. Thus, the CM is an appropriate model to evaluate efficacy of a recombinant F1V vaccine candidate.

Protective immunity against a lethal respiratory Yersinia pestis challenge induced by V antigen or the F1 capsular antigen incorporated into adenovirus capsid

The aerosol form of the bacterium Yersinia pestis causes pneumonic plague, a rapidly fatal disease that is a biothreat if deliberately released. At present, no plague vaccines are available for use in the United States, but subunit vaccines based on the Y. pestis V antigen and F1 capsular protein show promise when administered with adjuvants. In the context that adenovirus (Ad) gene transfer vectors have a strong adjuvant potential related to the ability to directly infect dendritic cells, we hypothesized that modification of the Ad5 capsid to display either the Y. pestis V antigen or the F1 capsular antigen on the virion surface would elicit high V antigen- or F1-specific antibody titers, permit boosting with the same Ad serotype, and provide better protection against a lethal Y. pestis challenge than immunization with equivalent amounts of V or F1 recombinant protein plus conventional adjuvant. We constructed AdYFP-pIX/V and AdLacZ-pIX/F1, E1(-), E3(-) serotype 5 Ad gene transfer vectors containing a fusion of the sequence for either the Y. pestis V antigen or the F1 capsular antigen to the carboxy-terminal sequence of pIX, a capsid protein that can accommodate the entire V antigen (37 kDa) or F1 protein (15 kDa) without disturbing Ad function. Immunization with AdYFP-pIX/V followed by a single repeat administration of the same vector at the same dose resulted in significantly better protection of immunized animals compared with immunization with a molar equivalent amount of purified recombinant V antigen plus Alhydrogel adjuvant. Similarly, immunization with AdLacZ-pIX/F1 in a prime-boost regimen resulted in significantly enhanced protection of immunized animals compared with immunization with a molar-equivalent amount of purified recombinant F1 protein plus adjuvant. These observations demonstrate that Ad vaccine vectors containing pathogen-specific antigens fused to the pIX capsid protein have strong adjuvant properties and stimulate more robust protective immune responses than equivalent recombinant protein-based subunit vaccines administered with conventional adjuvant, suggesting that F1-and/or V-modified capsid Ad-based recombinant vaccines should be considered for development as anti-plague vaccines.

Primary pneumonic plague in the African Green monkey as a model for treatment efficacy evaluation

BACKGROUND

Primary pneumonic plague is rare among humans, but treatment efficacy may be tested in appropriate animal models under the FDA 'Animal Rule'.

METHODS

Ten African Green monkeys (AGMs) inhaled 44-255 LD(50) doses of aerosolized Yersinia pestis strain CO92. Continuous telemetry, arterial blood gases, chest radiography, blood culture, and clinical pathology monitored disease progression.

RESULTS

Onset of fever, >39°C detected by continuous telemetry, 52-80 hours post-exposure was the first sign of systemic disease and provides a distinct signal for treatment initiation. Secondary endpoints of disease severity include tachypnea measured by telemetry, bacteremia, extent of pneumonia imaged by chest x-ray, and serum lactate dehydrogenase enzyme levels.

CONCLUSIONS

Inhaled Y. pestis in the AGM results in a rapidly progressive and uniformly fatal disease with fever and multifocal pneumonia, serving as a rigorous test model for antibiotic efficacy studies.

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.

Milestones in progression of primary pneumonic plague in cynomolgus macaques

Vaccines against primary pneumonic plague, a potential bioweapon, must be tested for efficacy in well-characterized nonhuman primate models. Telemetered cynomolgus macaques (Macaca fascicularis) were challenged by the aerosol route with doses equivalent to approximately 100 50% effective doses of Yersinia pestis strain CO92 and necropsied at 24-h intervals postexposure (p.e.). Data for telemetered heart rates, respiratory rates, and increases in the temperature greater than the diurnal baseline values identified the onset of the systemic response at 55 to 60 h p.e. in all animals observed for at least 70 h p.e. Bacteremia was detected at 72 h p.e. by a Yersinia 16S rRNA-specific quantitative reverse transcription-PCR and was detected later by the culture method at the time of moribund necropsy. By 72 h p.e. multilobar pneumonia with diffuse septal inflammation consistent with early bacteremia was established, and all lung tissues had a high bacterial burden. The levels of cytokines or chemokines in serum were not significantly elevated at any time, and only the interleukin-1beta, CCL2, and CCL3 levels were elevated in lung tissue. Inhalational plague in the cynomolgus macaque inoculated by the aerosol route produces most clinical features of the human disease, and in addition the disease progression mimics the disease progression from the anti-inflammatory phase to the proinflammatory phase described for the murine model. Defined milestones of disease progression, particularly the onset of fever, tachypnea, and bacteremia, should be useful for evaluating the efficacy of candidate vaccines.

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.

Prevention of bubonic and pneumonic plague using plant-derived vaccines

Yersinia pestis, the causative agent of bubonic and pneumonic plague, is an extremely virulent bacterium but there are currently no approved vaccines for protection against this organism. Plants represent an economical and safer alternative to fermentation-based expression systems for the production of therapeutic proteins. The recombinant plague vaccine candidates produced in plants are based on the two most immunogenic antigens of Y. pestis: the fraction-1 capsular antigen (F1) and the low calcium response virulent antigen (V) either in combination or as a fusion protein (F1-V). These antigens have been expressed in plants using all three known possible strategies: nuclear transformation, chloroplast transformation and plant-virus-based expression vectors. These plant-derived plague vaccine candidates were successfully tested in animal models using parenteral, oral, or prime/boost immunization regimens. This review focuses on the recent research accomplishments towards the development of safe and effective pneumonic and bubonic plague vaccines using plants as bioreactors.

Yersinia pestis with regulated delayed attenuation as a vaccine candidate to induce protective immunity against plague

Two mutant strains of Yersinia pestis KIM5+, a Deltacrp mutant and a mutant with arabinose-dependent regulated delayed-shutoff crp expression (araC P(BAD) crp), were constructed, characterized in vitro, and evaluated for virulence, immunogenicity, and protective efficacy in mice. Both strains were highly attenuated by the subcutaneous (s.c.) route. The 50% lethal doses (LD(50)s) of the Deltacrp and araC P(BAD) crp mutants were approximately 1,000,000-fold and 10,000-fold higher than those of Y. pestis KIM5+, respectively, indicating that both strains were highly attenuated. Mice vaccinated s.c. with 3.8 x 10(7) CFU of the Deltacrp mutant developed high anti-Y. pestis and anti-LcrV serum IgG titers, both with a strong Th2 bias, and induced protective immunity against subcutaneous challenge with virulent Y. pestis (80% survival) but no protection against pulmonary challenge. Mice vaccinated with 3.0 x 10(4) CFU of the araC P(BAD) crp mutant also developed high anti-Y. pestis and anti-LcrV serum IgG titers but with a more balanced Th1/Th2 response. This strain induced complete protection against s.c. challenge and partial protection (70% survival) against pulmonary challenge. Our results demonstrate that arabinose-dependent regulated crp expression is an effective strategy to attenuate Y. pestis while retaining strong immunogenicity, leading to protection against the pneumonic and bubonic forms of plague.

Amino acid residues 196-225 of LcrV represent a plague protective epitope

LcrV, a protein that resides at the tip of the type III secretion needles of Yersinia pestis, is the single most important plague protective antigen. Earlier work reported monoclonal antibody MAb 7.3, which binds a conformational epitope of LcrV and protects experimental animals against lethal plague challenge. By screening monoclonal antibodies directed against LcrV for their ability to protect immunized mice against bubonic plague challenge, we examined here the possibility of additional protective epitopes. MAb BA5 protected animals against plague, neutralized the Y. pestis type III secretion pathway and promoted opsonophagocytic clearance of bacteria in blood. LcrV residues 196-225 were necessary and sufficient for MAb BA5 binding. Compared to full-length LcrV, a variant lacking its residues 196-225 retained the ability of eliciting plague protection. These results identify LcrV residues 196-225 as a linear epitope that is recognized by the murine immune system to confer plague protection.

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.