Expression of an F1/V fusion protein in attenuated Salmonella typhimurium and protection of mice against plague

CD4+ T-Cell Epitope Prediction by Combined Analysis of Antigen Conformational Flexibility and Peptide-MHCII Binding Affinity

Antigen processing in the class II MHC pathway depends on conventional proteolytic enzymes, potentially acting on antigens in native-like conformational states. CD4+ epitope dominance arises from a competition among antigen folding, proteolysis, and MHCII binding. Protease-sensitive sites, linear antibody epitopes, and CD4+ T-cell epitopes were mapped in plague vaccine candidate F1-V to evaluate the various contributions to CD4+ epitope dominance. Using X-ray crystal structures, antigen processing likelihood (APL) predicts CD4+ epitopes with significant accuracy for F1-V without considering peptide-MHCII binding affinity. We also show that APL achieves excellent performance over two benchmark antigen sets. The profiles of conformational flexibility derived from the X-ray crystal structures of the F1-V proteins, Caf1 and LcrV, were similar to the biochemical profiles of linear antibody epitope reactivity and protease sensitivity, suggesting that the role of structure in proteolysis was captured by the analysis of the crystal structures. The patterns of CD4+ T-cell epitope dominance in C57BL/6, CBA, and BALB/c mice were compared to epitope predictions based on APL, MHCII binding, or both. For a sample of 13 diverse antigens, the accuracy of epitope prediction by the combination of APL and I-A^b-MHCII-peptide affinity reached 36%. When MHCII allele specificity was also diverse, such as in human immunity, prediction of dominant epitopes by APL alone reached 42% when using a stringent scoring threshold. Because dominant CD4+ epitopes tend to occur in conformationally stable antigen domains, crystal structures typically are available for analysis by APL, and thus, the requirement for a crystal structure is not a severe limitation.

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.

Single vector platform vaccine protects against lethal respiratory challenge with Tier 1 select agents of anthrax, plague, and tularemia

Bacillus anthracis, Yersinia pestis, and Francisella tularensis are the causative agents of Tier 1 Select Agents anthrax, plague, and tularemia, respectively. Currently, there are no licensed vaccines against plague and tularemia and the licensed anthrax vaccine is suboptimal. Here we report F. tularensis LVS ΔcapB (Live Vaccine Strain with a deletion in capB)- and attenuated multi-deletional Listeria monocytogenes (Lm)-vectored vaccines against all three aforementioned pathogens. We show that LVS ΔcapB- and Lm-vectored vaccines express recombinant B. anthracis, Y. pestis, and F. tularensis immunoprotective antigens in broth and in macrophage-like cells and are non-toxic in mice. Homologous priming-boosting with the LVS ΔcapB-vectored vaccines induces potent antigen-specific humoral and T-cell-mediated immune responses and potent protective immunity against lethal respiratory challenge with all three pathogens. Protection against anthrax was far superior to that obtained with the licensed AVA vaccine and protection against tularemia was comparable to or greater than that obtained with the toxic and unlicensed LVS vaccine. Heterologous priming-boosting with LVS ΔcapB- and Lm-vectored B. anthracis and Y. pestis vaccines also induced potent protective immunity against lethal respiratory challenge with B. anthracis and Y. pestis. The single vaccine platform, especially the LVS ΔcapB-vectored vaccine platform, can be extended readily to other pathogens.

A phase I safety and immunogenicity dose escalation trial of plague vaccine, Flagellin/F1/V, in healthy adult volunteers (DMID 08-0066)

INTRODUCTION

Intentional aerosolization of Yersinia pestis may result in pneumonic plague which is highly fatal if not treated early.

METHODS

We conducted a phase 1 randomized, double blind (within each group), placebo controlled, dose escalation trial to evaluate a plague vaccine, Flagellin/F1/V, in healthy adults aged 8 through 45years. Vaccine was administered intramuscularly on Days 0 and 28 at a dose of 1, 3, 6 or 10mcg. Subjects were observed for 4h after vaccination for cytokine release syndrome. Reactogenicity and adverse events (AE) were collected for 14 and 28days, respectively, after each vaccination. Serious AE were collected for the entire study. ELISA antibody and cytokines were measured at multiple time points. Subject's participation lasted 13months.

RESULTS

Sixty healthy subjects were enrolled; 52% males, 100% non-Hispanic, 91.7% white and mean age 30.8years. No severe reactogenicity events occurred; most AE were mild. No serious AE related to vaccine occurred. A dose response effect was observed to F1, V and flagellin. The peak ELISA IgG antibody titers (95% CI) after two 10mcg doses of vaccine were 260.0 (102.6-659.0) and 983.6 (317.3-3048.8), respectively, against F1 and V antigens. The 6mcg dose group provided similar titers. Titers were low for the placebo, 1mcg and 3mcg recipients. A positive antibody dose response was observed to F1, V and flagellin. Vaccine antigen specific serum IgE was not detected. There were no significant rises in serum or cellular cytokine responses and no significant IgG increase to flagellin after the second dose.

CONCLUSION

The Flagellin/F1/V vaccine exhibited a dose dependent increase in immunogenicity and was well tolerated at all doses. Antibody specific responses to F1, V and flagellin increased as dose increased. Given the results from this trial, testing higher doses of the vaccine may be merited.

Immunology of Yersinia pestis Infection

As a pathogen of plague, Yersinia pestis caused three massive pandemics in history that killed hundreds of millions of people. Yersinia pestis is highly invasive, causing severe septicemia which, if untreated, is usually fatal to its host. To survive in the host and maintain a persistent infection, Yersinia pestis uses several stratagems to evade the innate and the adaptive immune responses. For example, infections with this organism are biphasic, involving an initial "noninflammatory" phase where bacterial replication occurs initially with little inflammation and following by extensive phagocyte influx, inflammatory cytokine production, and considerable tissue destruction, which is called "proinflammatory" phase. In contrast, the host also utilizes its immune system to eliminate the invading bacteria. Neutrophil and macrophage are the first defense against Yersinia pestis invading through phagocytosis and killing. Other innate immune cells also play different roles, such as dendritic cells which help to generate more T helper cells. After several days post infection, the adaptive immune response begins to provide organism-specific protection and has a long-lasting immunological memory. Thus, with the cooperation and collaboration of innate and acquired immunity, the bacterium may be eliminated from the host. The research of Yersinia pestis and host immune systems provides an important topic to understand pathogen-host interaction and consequently develop effective countermeasures.

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.

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.

Yersinia pestis biovar Microtus strain 201, an avirulent strain to humans, provides protection against bubonic plague in rhesus macaques

Yersinia pestis biovar Microtus is considered to be a virulent to larger mammals, including guinea pigs, rabbits and humans. It may be used as live attenuated plague vaccine candidates in terms of its low virulence. However, the Microtus strain's protection against plague has yet to be demonstrated in larger mammals. In this study, we evaluated the protective efficacy of the Microtus strain 201 as a live attenuated plague vaccine candidate. Our results show that this strain is highly attenuated by subcutaneous route, elicits an F1-specific antibody titer similar to the EV and provides a protective efficacy similar to the EV against bubonic plague in Chinese-origin rhesus macaques. The Microtus strain 201 could induce elevated secretion of both Th1-associated cytokines (IFN-γ, IL-2 and TNF-α) and Th2-associated cytokines (IL-4, IL-5, and IL-6), as well as chemokines MCP-1 and IL-8. However, the protected animals developed skin ulcer at challenge site with different severity in most of the immunized and some of the EV-immunized monkeys. Generally, the Microtus strain 201 represented a good plague vaccine candidate based on its ability to generate strong humoral and cell-mediated immune responses as well as its good protection against high dose of subcutaneous virulent Y. pestis challenge.

Evaluation of YadC protein delivered by live attenuated Salmonella as a vaccine against plague

Yersinia pestis YadB and YadC are two new outer membrane proteins related to its pathogenicity. Here, codon-optimized yadC, yadC810 (aa 32-551), or yadBC antigen genes delivered by live attenuated Salmonella strains are evaluated in mice for induction of protective immune responses against Y. pestis CO92 through subcutaneous or intranasal challenge. Our findings indicate that mice immunized with Salmonella synthesizing YadC, YadC810, or YadBC develop significant serum IgG responses to purified recombinant YadC protein. For subcutaneous challenge (approximately 230 LD50 of Y. pestis CO92), mice immunized with Salmonella synthesizing YadC or YadC810 are afforded 50% protection, but no protection by immunization with the Salmonella strain synthesizing YadBC. None of these antigens provided protection against intranasal challenge (approximately 31 LD50 of Y. pestis CO92). In addition, subcutaneous immunization with purified YadC810 protein emulsified with alum adjuvant does not elicit a protective response against Y. pestis administered by either challenge route.

Acquisition of maternal antibodies both from the placenta and by lactation protects mouse offspring from Yersinia pestis challenge

Artificially passive immunization has been demonstrated to be effective against Yersinia pestis infection in animals. However, maternal antibodies' protective efficacy against plague has not yet been demonstrated. Here, we evaluated the kinetics, protective efficacy, and transmission modes of maternal antibodies, using mice immunized with plague subunit vaccine SV1 (20 μg of F1 and 10 μg of rV270). The results showed that the rV270- and F1-specific antibodies could be detected in the sera of newborn mice (NM) until 10 and 14 weeks of age, respectively. There was no antibody titer difference between the parturient mice immunized with SV1 (PM-S) and the caesarean-section newborns (CSN) from the PM-S or between the lactating mice immunized by SV1 (LM-S) and the cross-fostered mice (CFM) during 3 weeks of lactation. The NM had a 72% protection against 4,800 CFU Y. pestis strain 141 challenge at 6 weeks of age, whereas at 14 weeks of age, NM all succumbed to 5,700 CFU of Y. pestis challenge. After 7 weeks of age, CFM had an 84% protection against 5,000 CFU of Y. pestis challenge. These results indicated that maternal antibodies induced by the plague subunit vaccine in mother mice can be transferred to NM by both placenta and lactation. Passive antibodies from the immunized mothers could persist for 3 months and provide early protection for NM. The degree of early protection is dependent on levels of the passively acquired antibody. The results indicate that passive immunization should be an effective countermeasure against plague during its epidemics.

Construction and screening of attenuated ΔphoP/Q Salmonella typhimurium vectored plague vaccine candidates

Preclinical studies evaluating plague vaccine candidates have demonstrated that the F1 and V protein antigens of Yersinia pestis confer protection against challenge from virulent strains. Live-attenuated ΔphoP/Q Salmonella typhimurium recombinants were constructed expressing either F1, V antigens, F1 and V antigens, or a F1-V fusion from Asd (+) balanced-lethal plasmids. To improve antigen delivery, genes encoding plague antigens were modified in order to localize antigens to specific bacterial cellular compartments which include cytoplasmic, outer membrane, or secreted. Candidate vaccine strains were evaluated for growth characteristics, full-length lipopolysaccharide (LPS), plasmid stability, and antigen expression in vitro. Plague vaccine candidate strains with favorable in vitro profiles were evaluated in murine or rabbit preclinical oral immunogenicity studies. Attenuated S. typhimurium strains expressing cytoplasmically localized F1-V and V antigen antigens were more immunogenic than strains that secreted or localized plague antigens to the outer membrane. In particular, S. typhimurium M020 and M023, which express Asd(+)-plasmid derived soluble F1-V and soluble V antigen, respectively, at high levels in the bacterial cell cytoplasm were found to induce the highest levels of plague-specific serum antibodies. To further evaluate balanced-lethal plasmid retention capacity, ΔphoP/Q S. typhimurium PurB(+) and GlnA(+) balanced-lethal plasmid systems harboring F1-V were compared with M020 in vitro and in BALB/c mice in a immunogenicity study. Although there was no detectable difference in plague antigen expression in vitro, S. typhimurium M020 was the most immunogenic plague antigen vector strain evaluated, inducing high-titer serum IgG antibodies specific against F1, V and F1-V.

Chimeric flagellin expressed by Salmonella typhimurium induces an ESAT-6-specific Th1-type immune response and CTL effects following intranasal immunization

The flagellin component FliC of Salmonella typhimurium is capable of activating the innate immune system via specific interactions with TLR5 and can also act as a carrier of foreign antigen to elicit antigen-specific immune responses. Thus, we constructed an attenuated Salmonella strain SL5928(fliC/esat) expressing chimeric flagellin that contained the ESAT-6 antigen coding sequence of Mycobacterium tuberculosis inserted into the highly variable region of the Salmonella flagellin coding gene fliC(i). The chimeric flagellin functioned normally, as demonstrated using a flagella swarming assay and electron microscopy. To analyze the effects of chimeric flagellin, the cell-mediated immune response and cytotoxic T lymphocyte (CTL) effects specific for ESAT-6 antigen were tested after intranasal immunization of mice with flagellated Salmonella SL5928(fliC/esat). The results showed that SL5928(fliC/esat) intranasal immunization can strongly elicit an ESAT-6-specific T helper (Th) 1-type immune response in mucosal lymphoid tissues, such as nasopharynx-associated lymph nodes, lung and Peyer's patches, and a Th1/Th2 response was elicited in spleen and mesenteric lymph nodes. Furthermore, intranasal immunization of SL5928(fliC/esat) produced efficient CTL effects, as demonstrated using a 5- and 6-carboxyfluorescein diacetate succinimidyl ester (CFSE) assay. Thus, our study revealed that Salmonella flagellin acts as a carrier for foreign antigen and triggers strong Th1 and CTL responses during intranasal immunization. Chimeric flagellin is potentially an effective strategy for the development of novel vaccines against tuberculosis in humans and animals.

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.

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.

Enhancement of immune response to an antigen delivered by vaccinia virus by displaying the antigen on the surface of intracellular mature virion

Vaccinia virus (VACV) is the vaccine for smallpox and a widely used vaccine vector for infectious diseases and cancers. The majority of the antibodies elicited by live VACV vaccination respond to virion structural proteins, including many integral membrane proteins on the intracellular mature virion (MV). Here, we showed that antibody response to an exogenous antigen delivered by VACV was greatly enhanced by incorporating the antigen as an integral membrane protein of MV. We constructed recombinant VACV expressing a Yersinia pestis protective antigen, LcrV, unmodified or fused with either a signal peptide or with the transmembrane domain of VACV D8 protein (LcrV-TM). Electron microscopy showed that LcrV-TM was displayed on the surface of MV. Importantly, VACV expressing LcrV-TM elicited a significantly higher titer of anti-LcrV antibody in mice than viruses expressing other forms of LcrV. Only mice immunized with LcrV-TM-expressing VACV were protected from lethal Y. pestis and VACV WR challenges. Antigen engineering through fusion with D8 transmembrane domain may be broadly applicable for enhancing the immune response to antigens delivered by a VACV vector. The recombinant virus described here could also serve as the basis for developing a vaccine against both smallpox and plague.

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.

Purification and characterization of a recombinant Yersinia pestis V-F1 "Reversed" fusion protein for use as a new subunit vaccine against plague

We previously developed a unique recombinant protein vaccine against plague composed of a fusion between the Fraction 1 capsular antigen (F1) and the V antigen. To determine if overall expression, solubility, and recovery of the F1-V fusion protein could be enhanced, we modified the original fusion. Standard recombinant DNA techniques were used to reverse the gene order such that the V antigen coding sequence was fused at its C-terminus to the N-terminus of F1. The F1 secretion signal sequence (F1S) was subsequently fused to the N-terminus of V. This new fusion protein, designated F1S-V-F1, was then co-expressed with the Y. pestis Caf1M periplasmic chaperone protein in BL21-Star Escherichia coli. Recombinant strains expressing F1-V, F1S-F1-V, or F1S-V-F1 were compared by cell fractionation, SDS-PAGE, Western blotting, and suspension immunolabelling. F1S-V-F1 exhibited enhanced solubility and secretion when co-expressed with Caf1M resulting in a recombinant protein that is processed in a similar manner to the native F1 protein. Purification of F1S-V-F1 was accomplished by anion-exchange and hydrophobic interaction chromatography. The purification method produced greater than 1mg of purified soluble protein per liter of induced culture. F1S-V-F1 polymerization characteristics were comparable to the native F1. The purified F1S-V-F1 protein appeared equivalent to F1-V in its ability to be recognized by neutralizing antibodies.

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

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

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

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

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.

Long-term observation of subunit vaccine F1-rV270 against Yersinia pestis in mice

Long-term protection and antibody response for the subunit vaccine F1-rV270 were determined by using the mouse model. Antibodies to F1 and rV270 were still detectable over a period of 518 days. The complete protection against lethal challenge of Yersinia pestis could be achieved up to day 518 after primary immunization.

Technical transformation of biodefense vaccines

Biodefense vaccines are developed against a diverse group of pathogens. Vaccines were developed for some of these pathogens a long time ago but they are facing new challenges to move beyond the old manufacturing technologies. New vaccines to be developed against other pathogens have to determine whether to follow traditional vaccination strategies or to seek new approaches. Advances in basic immunology and recombinant DNA technology have fundamentally transformed the process of formulating a vaccine concept, optimizing protective antigens, and selecting the most effective vaccine delivery approach for candidate biodefense vaccines.

Mucosally delivered Salmonella typhi expressing the Yersinia pestis F1 antigen elicits mucosal and systemic immunity early in life and primes the neonatal immune system for a vigorous anamnestic response to parenteral F1 boost

Neonates respond poorly to conventional vaccines. This has been attributed, in part, to the immaturity of neonatal dendritic cells that lack full capacity for Ag presentation and T cell stimulation. We engineered an attenuated Salmonella Typhi strain to express and export the F1 Ag of Y. pestis (S. Typhi(F1)) and investigated its immunogenicity early in life using a heterologous prime-boost regimen. Newborn mice primed intranasally with a single dose of S. Typhi(F1) elicited mucosal Ab- and IFN-gamma-secreting cells 1 wk after immunization. They also developed a potent and fast anamnestic response to a subsequent parenteral boost with F1-alum, which surpassed those of mice primed and boosted with S. Typhi(F1) or F1-alum. Neonatal priming with S. Typhi(F1), as opposed to priming with F1-alum, resulted in a more balanced IgG2a/IgG1 profile, enhanced avidity maturation and stimulation of B memory cells, and strong Th1-type cell-mediated immunity. S. Typhi(F1) enhanced the activation and maturation of neonatal CD11c+ dendritic cells, shown by increased expression of CD80, CD86, CD40, and MHC-II cell surface markers and production of proinflammatory cytokines IL-12, TNF-alpha, IL-6, and MCP-1. S. Typhi(F1)-stimulated neonatal DC had improved capacity for Ag presentation and T cell stimulation in vitro and induced F1-specific CD4+ and CD8+ T cell responses when adoptively transferred to newborn mice. Mucosal immunization with S. Typhi expressing a foreign Ag effectively primes the neonatal immune system for potent, fast, and broader responses to a parenteral Ag boost. Such a strategy can prevent infectious diseases, including those considered biowarfare threats, early in life.

Flagellin-F1-V fusion protein is an effective plague vaccine in mice and two species of nonhuman primates

A number of studies have clearly demonstrated that flagellin is a potent adjuvant that promotes robust immune responses when it is given with a protein antigen. In view of the potential biological and practical benefits of a recombinant protein vaccine composed of a single fusion protein containing flagellin and antigen, we have evaluated the efficacy of a fusion protein composed of flagellin and two protective antigens of Yersinia pestis (F1 and V) in eliciting protection against respiratory challenge with Y. pestis. Flagellin-F1-V was produced and purified in high yield under good manufacturing practices conditions. The fusion protein retains full Toll-like receptor 5-stimulating activity in vitro. Using a prime-boost immunization protocol, we found that flagellin-F1-V elicits robust antigen-specific humoral immunity in mice and two species of nonhuman primates. Immune mice were fully protected against intranasal challenge with 150 mean tolerated doses of Y. pestis CO92. In immune mice, the bacteria were completely cleared within 3 days after challenge. Flagellin-F1-V exhibited full stability for at least 297 days at 4 degrees C and at least 168 days at 25 degrees C. At between 29 and 84 days at 37 degrees C, the protein exhibited a loss of biological activity that appeared to be associated with a substantial change in protein diameter, possibly due to oligomerization. On the basis of our results, we believe that flagellin-F1-V is an outstanding candidate for evaluation in studies with humans.

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.

Induction of innate immunity by lipid A mimetics increases survival from pneumonic plague

This study analysed the effect of priming the innate immune system using synthetic lipid A mimetics in a Yersinia pestis murine pulmonary infection model. Two aminoalkyl glucosaminide 4-phosphate (AGP) Toll-like receptor 4 (TLR4) ligands, delivered intranasally, extended time to death or protected against a lethal Y. pestis CO92 challenge. The level of protection was dependent upon the challenge dose of Y. pestis and the timing of AGP therapy. Protection correlated with cytokine induction and a decreased bacterial burden in lung tissue. AGP protection was TLR4-dependent and was not evidenced in transgenic TLR4-deficient mice. AGP therapy augmented with subtherapeutic doses of gentamicin produced dramatically enhanced survival. Combined, these results indicated that AGPs may be useful in protection of immunologically naive individuals against plague and potentially other infectious agents, and that AGP therapy may be used synergistically with other therapies.

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.

Yersinia pestis YadC: a novel vaccine candidate against plague

Current subunit vaccines provide partial protection against pneumonic plague if the infecting Y. pestis strain is encapsulated (F1+). Here we describe YadC, a novel Y. pestis outer membrane protein that provides partial protection against a F1(-) Y. pestis strain. Swiss-Webster mice were immunized subcutaneously with glutathione S-transferase (GST) or His6-tagged (HT) purified fusion proteins (GST-YadC137-409 or HT-LcrV) or buffer emulsified with Alhydrogel. Intravenous challenge with 1 x 10(4) F1(-) Deltapgm Y. pestis CO99-3015 revealed no protection for those mice immunized with GST-Alhydrogel alone, full protection for HT-LcrV-immunized mice, and partial protection for GST-YadC137-409-immunized mice. Similarly, C57BL/6 mice were immunized with GST-YadC137-409, HT-LcrV, or GST all with Alhydrogel adjuvant. After intranasal challenge with 3 x 10(3) F1(-) Y. pestis CO99-3015, 87% of GST-YadC137-409-immunized mice survived pneumonic plague. This is compared to the GST control group (0 surviving mice) and the LcrV-immunized group where 50% survived the challenge. This protection was correlated with a predominantly IgG1 response in LcrV-immunized mice and an IgG1/IgG3 antibody response in YadC-immunized mice. Additionally, we report the cytokine response from HT-LcrV- and GST-YadC137-409-stimulated peripherally derived macrophages. YadC-stimulated cells demonstrated a predominant pro-inflammatory cytokine production. This mixed Thl/Th2 response suggests that YadC's protection may involve a different adaptive immune response than the LcrV protein that currently is part of plague vaccines.

A comparison of immunogenicity and protective immunity against experimental plague by intranasal and/or combined with oral immunization of mice with attenuated Salmonella serovar Typhimurium expressing secreted Yersinia pestis F1 and V antigen

We investigated the relative immunogenicity and protective efficacy of recombinant X85MF1 and X85V strains of DeltacyaDeltacrpDeltaasd-attenuated Salmonella Typhimurium expressing, respectively, secreted Yersinia pestis F1 and V antigens, following intranasal (i.n.) or i.n. combined with oral immunization for a mouse model. A single i.n. dose of 10(8) CFU of X85MF1 or X85V induced appreciable serum F1- or V-specific IgG titres, although oral immunization did not. Mice i.n. immunized three times (i.n. x 3) with Salmonella achieved the most substantial F1/V-specific IgG titres, as compared with corresponding titres for an oral-primed, i.n.-boosted (twice; oral-i.n. x 2) immunization regimen. The level of V-specific IgG was significantly greater than that of F1-specific IgG (P<0.001). Analysis of the IgG antibodies subclasses revealed comparable levels of V-specific Th-2-type IgG1 and Th-1-type IgG2a, and a predominance of F1-specific Th-1-type IgG2a antibodies. In mice immunized intranasally, X85V stimulated a greater IL-10-secreting-cell response in the lungs than did X85MF1, but impaired the induction of gamma-interferon-secreting cells. A program of i.n. x 3 and/or oral-i.n. x 2 immunization with X85V provided levels of protection against a subsequent lethal challenge with Y. pestis, of, respectively, 60% and 20%, whereas 80% protection was provided following the same immunization but with X85MF1.

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.

Purification and protective efficacy of monomeric and modified Yersinia pestis capsular F1-V antigen fusion proteins for vaccination against plague

The F1-V vaccine antigen, protective against Yersinia pestis, exhibits a strong tendency to multimerize that affects larger-scale manufacture and characterization. In this work, the sole F1-V cysteine was replaced with serine by site-directed mutagenesis for characterization of F1-V non-covalent multimer interactions and protective potency without participation by disulfide-linkages. F1-V and F1-V(C424S) proteins were overexpressed in Escherichia coli, recovered using mechanical lysis/pH-modulation and purified from urea-solubilized soft inclusion bodies, using successive ion-exchange, ceramic hydroxyapatite, and size-exclusion chromatography. This purification method resulted in up to 2mg/g of cell paste of 95% pure, mono-disperse protein having < or =0.5 endotoxin units per mg by a kinetic chromogenic limulus amoebocyte lysate reactivity assay. Both F1-V and F1-V(C424S) were monomeric at pH 10.0 and progressively self-associated as pH conditions decreased to pH 6.0. Solution additives were screened for their ability to inhibit F1-V self-association at pH 6.5. An L-arginine buffer provided the greatest stabilizing effect. Conversion to >500-kDa multimers occurred between pH 6.0 and 5.0. Conditions for efficient F1-V adsorption to the cGMP-compatible alhydrogel adjuvant were optimized. Side-by-side evaluation for protective potency against subcutaneous plague infection in mice was conducted for F1-V(C424S) monomer; cysteine-capped F1-V monomer; cysteine-capped F1-V multimer; and a F1-V standard reported previously. After a two-dose vaccination with 2 x 20 microg of F1-V, respectively, 100%, 80%, 80%, and 70% of injected mice survived a subcutaneous lethal plague challenge with 10(8) LD(50)Y. pestis CO92. Thus, vaccination with F1-V monomer and multimeric forms resulted in significant, and essentially equivalent, protection.

Mechanisms of major histocompatibility complex class II-restricted processing and presentation of the V antigen of Yersinia pestis

We mapped mouse CD4 T-cell epitopes located in three structurally distinct regions of the V antigen of Yersinia pestis. T-cell hybridomas specific for epitopes from each region were generated to study the mechanisms of processing and presentation of V antigen by bone-marrow-derived macrophages. All three epitopes required uptake and/or processing from V antigen as well as presentation to T cells by newly synthesized major histocompatibility complex (MHC) class II molecules over a time period of 3-4 hr. Sensitivity to inhibitors showed a dependence on low pH and cysteine, serine and metalloproteinase, but not aspartic proteinase, activity. The data indicate that immunodominant epitopes from all three structural regions of V antigen were presented preferentially by the classical MHC class II-restricted presentation pathway. The requirement for processing by the co-ordinated activity of several enzyme families is consistent with the buried location of the epitopes in each region of V antigen. Understanding the structure-function relationship of multiple immunodominant epitopes of candidate subunit vaccines is necessary to inform choice of adjuvants for vaccine delivery. In the case of V antigen, adjuvants designed to target it to lysosomes are likely to induce optimal responses to multiple protective T-cell epitopes.

Sequential proteolytic processing of the capsular Caf1 antigen of Yersinia pestis for major histocompatibility complex class II-restricted presentation to T lymphocytes

We studied the mechanisms of antigen presentation of CD4 T cell epitopes of the capsular Caf1 antigen of Yersinia pestis using murine bone marrow macrophages as antigen presenting cells and T cell hybridomas specific for major histocompatibility complex (MHC) class II-restricted epitopes distributed throughout the Caf1 sequence. The data revealed diversity in the pathways used and the degrees of antigen processing required depending on the structural context of epitopes within the Caf1 molecule. Two epitopes in the carboxyl-terminal globular domain were presented by newly synthesized MHC class II after low pH-dependent lysosomal processing, whereas an epitope located in a flexible amino-terminal strand was presented by mature MHC class II independent of low pH and with no detectable requirement for proteolytic processing. A fourth epitope located between the two regions of Caf1 showed intermediate behavior. The data are consistent with progressive unfolding and cleavage of rCaf1 from the amino terminus as it traverses the endosomal pathway, the availability of epitopes determining which pool of MHC class II is preferentially loaded. The Caf1 capsular protein is a component of second generation plague vaccines and an understanding of the mechanisms and pathways of MHC class II-restricted presentation of multiple epitopes from this candidate vaccine antigen should inform the choice of delivery systems and adjuvants that target vaccines successfully to appropriate intracellular locations to induce protective immune responses against as wide a T cell repertoire as possible.

Flagellin is an effective adjuvant for immunization against lethal respiratory challenge with Yersinia pestis

Gram-negative flagellin, a Toll-like receptor 5 (TLR5) agonist, is a potent inducer of innate immune effectors such as cytokines and nitric oxide. In the lung, flagellin induces a localized and transient innate immune response characterized by neutrophil infiltration and the production of cytokines and chemokines. In view of the extraordinary potency of flagellin as an inducer of innate immunity and the contribution of innate responses to the development of adaptive immunity, we evaluated the efficacy of recombinant Salmonella flagellin as an adjuvant in an acellular plague vaccine. Mice immunized intranasally or intratracheally with the F1 antigen of Yersinia pestis and flagellin exhibited dramatic increases in anti-F1 plasma immunoglobulin G (IgG) titers that remained stable over time. In contrast, control mice had low or undetectable antibody responses. The IgG1/IgG2a ratio of antibody titers against F1 in immunized mice is consistent with a Th2 bias. However, no significant antigen-specific IgE production was detected. Interferons, tumor necrosis factor alpha, and interleukin-6 were not essential for the adjuvant effects of flagellin. Preexisting antiflagellin antibodies had no significant effect on the adjuvant activity of flagellin. Importantly, intranasal immunization with flagellin and the F1 antigen was protective against intranasal challenge with virulent Y. pestis CO92, with 93 to 100% survival of immunized mice. Lastly, vaccination of cynomolgus monkeys with flagellin and a fusion of the F1 and V antigens of Y. pestis induced a robust antigen-specific IgG antibody response.

Oral immunization with a live recombinant attenuated Salmonella typhimurium protects mice against Toxoplasma gondii

The natural site of infection for T. gondii is the mucosal surface of the intestine, so the protective immunity obtained after natural infection with T. gondii points to the importance of developing a vaccine that stimulates mucosal defences. In this study, an aroA- and aroD- attenuated strain of Salmonella typhimurium (BRD509) has been used to deliver the recombinant eukaryotic plasmid pSAG(1-2)/CTA2/B expressing a multi-antigenic gene encoding SAG1 and SAG2 of T. gondii linked to A2/B subunits of cholera toxin as a candidate oral T. gondii vaccine. Immunoblot analysis showed compound gene expression in HeLa cells in vitro and intragastric immunization of mice with the recombinant salmonella resulted in the induction of humoral and Th1 type cellular immune responses and afforded protection against RH strain T. gondii challenge. Anti-T. gondii IgG values increased markedly in the BRD509/pSAG(1-2)-CTA2/B immunized group; these values were significantly higher than those in the negative controls (P = 0.008). With CTA2/B genetic adjuvant, the T. gondii-specific response was predominantly Th1, indicating that the CTA(2)/B genetic adjuvant was able to overcome the strong Th2-bias of the antigen (IgG2a >> IgG1). Antigen-specific T cell proliferative responses and CTL activity were significantly enhanced when cholera toxin CTA2/B genetic adjuvant was used (P = 0.009; P = 0.006). Culture supernatants from antigen-stimulated splenocytes from mice in these groups were also examined by ELISA for Th1- and Th2-type cytokines; mean IFN-gamma levels produced after oral immunization with BRD509/pSAG(1-2)-CTA2/B were about nine-fold higher than after immunization with BRD509/pSAG(1-2) (P = 0.007). On the other hand, the levels of IL-4 were low for all groups and no increase was seen in the presence of CTA2/B genetic adjuvant. When the immunized mice were intraperitoneally challenged with 10(3) tachyzoites of the highly virulent RH strain, the survival time of the mice immunized with BRD509/pSAG(1-2)-CTA2/B was markedly longer than other groups (P = 0.003) and a 40% survival rate was achieved. This is the first report that demonstrates that an oral attenuated salmonella DNA vaccine can induce protective immunity against the acute phase of T. gondii infection.

Human immune response to a plague vaccine comprising recombinant F1 and V antigens

The human immune response to a new recombinant plague vaccine, comprising recombinant F1 (rF1) and rV antigens, has been assessed during a phase 1 safety and immunogenicity trial in healthy volunteers. All the subjects produced specific immunoglobulin G (IgG) in serum after the priming dose, which peaked in value after the booster dose (day 21), with the exception of one individual in the lowest dose level group, who responded to rF1 only. Three subjects, found to have an anti-rV titer at screening, were excluded from the overall analysis. Human antibody functionality has been assessed by quantification of antibody competing for binding to rV in vitro and also by the transfer of protective immunity in human serum into the naive mouse. Human and macaque IgG competed for binding to rV in vitro with a mouse monoclonal antibody, previously shown to protect mice against challenge with plague, suggesting that this protective B-cell epitope on rV is conserved between these three species. Total IgG to rV in individuals and the titer of IgG competing for binding to rV correlated significantly at days 21 (r = 0.72; P < 0.001) and 28 (r = 0.82; P < 0.001). Passive transfer of protective immunity into mice also correlated significantly with total IgG titer to rF1 plus rV at days 21 (r(2) = 98.6%; P < 0.001) and 28 (r(2) = 76.8%; P < 0.03). However, no significant vaccination-related change in activation of peripheral blood mononuclear cells was detected at any time. Potential serological immune correlates of protection have been investigated, but no trends specific to vaccination could be detected in cellular markers.

A Salmonella enterica serovar Typhi vaccine expressing Yersinia pestis F1 antigen on its surface provides protection against plague in mice

A recombinant strain of attenuated Salmonella enterica serovar Typhi surface-expressing Yersinia pestis F1 antigen was generated by transforming strain BRD1116 (aroA aroC htrA) with plasmid pAH34L encoding the Y. pestis caf operon. BRD1116/pAH34L was stable in vitro and in vivo. An immunisation regimen of two intranasal doses of 1 x 10(8) cfu of BRD1116/pAH34L given intranasally to mice 7 days apart induced the strongest immune response compared to other regimens and protected 13 out of 20 mice from lethal challenge with Y. pestis. Intranasal immunisation of mice constitutes a model for oral immunisation with Salmonella vaccines in humans. Thus, the results demonstrate that attenuated strains of S. enterica serovar Typhi which express Y. pestis F1 antigen may be developed to provide an oral vaccine against plague suitable for use in humans.

Yersinia pestis (plague) vaccines

Live attenuated and killed whole cell vaccines against disease caused by Yersinia pestis have been available since the early part of the last century. Although these vaccines indicate the feasibility of protecting against disease, they have a number of shortcomings. The live attenuated vaccine is highly reactogenic and is not licensed for use in humans. The killed whole cell vaccine, also reactogenic, provides poor protection against pneumonic plague and immunisation requires multiple doses of the vaccine. Against this background, a range of candidate vaccines, including rationally attenuated mutants, subunit vaccines and naked DNA vaccines have been described. Of these, an injected subunit vaccine is likely to offer the best near-term solution to the provision of a vaccine that protects against both bubonic and pneumonic plague.

Oral immunisation with live aroA attenuated Salmonella enterica serovar Typhimurium expressing the Yersinia pestis V antigen protects mice against plague

Bubonic and pneumonic plague are caused by the bacterium Yersinia pestis. The V antigen of Y. pestis is a protective antigen against plague. In this study, an aroA attenuated strain of Salmonella enterica serovar Typhimurium (SL3261) has been used to deliver the Y. pestis V antigen as a candidate oral plague vaccine. SL3261 was transformed with the expression plasmid pTrc-LcrV, containing the lcrV gene encoding V antigen. Immunoblot analysis showed V antigen expression in SL3261 in vitro and intragastric immunisation of mice with the recombinant Salmonella resulted in the induction of V antigen-specific serum antibody responses and afforded protection against Y. pestis challenge. However, the antibody responses induced by the recombinant Salmonella did not correlate with the protection afforded, indicating that immune responses other than antibody may play a role in the protection afforded against plague by this candidate vaccine.

Advances in the development of bacterial vector technology

The demand for new and improved vaccines against human diseases has continued unabated over the past century. While the need continues for traditional vaccines in areas such as infectious diseases, there is an increasing demand for new therapies in nontraditional areas, such as cancer treatment, bioterrorism and food safety. Prompted by these changes, there has been a renewed interest in the application and development of live, attenuated bacteria expressing foreign antigens as vaccines. The application of bacterial vector vaccines to human maladies has been studied most extensively in attenuted strains of Salmonella. Live, attenuated strains of Shigella, Listeria monocytogenes, Mycobacterium bovis-BCG and Vibrio cholerae provide unique alternatives in terms of antigen delivery and immune presentation, however and also show promise as potentially useful bacterial vectors.

Vaccines for Category A bioterrorism diseases

Vaccination programmes are very successful as a preventive strategy against many infectious diseases which have had a major impact on human morbidity and mortality. One of these diseases, smallpox, has been eliminated as a natural infection. The recent concern about biological attacks has turned attention to the use of an immunisation programme to prevent infection with what are considered the most significant potentially harmful biowarfare pathogens. This review puts into perspective the available information on current immunisation and newer vaccine options for anthrax, smallpox, tularaemia, plague and botulism.

Protection against plague following immunisation with microencapsulated V antigen is reduced by co-encapsulation with IFN-gamma or IL-4, but not IL-6

We have investigated intranasal delivery of novel vaccines for plague, based on poly-L-lactide (PLLA) microencapsulated recombinant V antigen (rV) of Yersinia pestis. Microspheres containing rV alone or co-encapsulated with the cytokines IFN-gamma, IL-4 or IL-6 were administered in a two-dose regimen and antibody responses and protective efficacy were monitored. All treatment groups stimulated high rV-specific antibody titres in serum, predominantly of the IgG1 isotype, which were maintained over several months. There was evidence of both IgG and IgA responses in lung samples from all groups. Formulations based on rV antigen alone or rV co-encapsulated with IL-6 provided complete protection against systemic challenge with Y. pestis strain GB; however protective efficacy was impaired by co-encapsulating either IFN-gamma or IL-4 with rV.