Plague Vaccine Development: Current Research and Future Trends

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.

Decrypting the multi-genome data for chimeric vaccine designing against the antibiotic resistant Yersinia pestis

Yersinia pestis, the causative agent of plague, is a gram-negative bacterium that can be fatal if not treated properly. Three types of plague are currently known: bubonic, septicemic, and pneumonic plague, among which the fatality rate of septicemic and pneumonic plague is very high. Bubonic plague can be treated, but only if antibiotics are used at the initial stage of the infection. But unfortunately, Y. pestis has also shown resistance to certain antibiotics such as kanamycin, minocycline, tetracycline, streptomycin, sulfonamides, spectinomycin, and chloramphenicol. Despite tremendous progress in vaccine development against Y. pestis, there is no proper FDA-approved vaccine available to protect people from its infections. Therefore, effective broad-spectrum vaccine development against Y. pestis is indispensable. In this study, vaccinomics-assisted immunoinformatics techniques were used to find possible vaccine candidates by utilizing the core proteome prepared from 58 complete genomes of Y. pestis. Human non-homologous, pathogen-essential, virulent, and extracellular and membrane proteins are potential vaccine targets. Two antigenic proteins were prioritized for the prediction of lead epitopes by utilizing reverse vaccinology approaches. Four vaccine designs were formulated using the selected B- and T-cell epitopes coupled with appropriate linkers and adjuvant sequences capable of inducing potent immune responses. The HLA allele population coverage of the T-cell epitopes selected for vaccine construction was also analyzed. The V2 constructs were top-ranked and selected for further analysis on the basis of immunological, physicochemical, and immune-receptor docking interactions and scores. Docking and molecular dynamic simulations confirmed the stability of construct V2 interactions with the host immune receptors. Immune simulation analysis anticipated the strong immune profile of the prioritized construct. In silico restriction cloning ensured the feasible cloning ability of the V2 construct in the expression system of E. coli strain K12. It is anticipated that the designed vaccine construct may be safe, effective, and able to elicit strong immune responses against Y. pestis infections and may, therefore, merit investigation using in vitro and in vivo assays.

Co-formulation of the rF1V plague vaccine with depot-formulated cytokines enhances immunogenicity and efficacy to elicit protective responses against aerosol challenge in mice

This study evaluated a depot-formulated cytokine-based adjuvant to improve the efficacy of the recombinant F1V (rF1V) plague vaccine and examined the protective response following aerosol challenge in a murine model. The results of this study showed that co-formulation of the Alhydrogel-adsorbed rF1V plague fusion vaccine with the depot-formulated cytokines recombinant human interleukin 2 (rhuIL-2) and/or recombinant murine granulocyte macrophage colony-stimulating factor (rmGM-CSF) significantly enhances immunogenicity and significant protection at lower antigen doses against a lethal aerosol challenge. These results provide additional support for the co-application of the depot-formulated IL-2 and/or GM-CSF cytokines to enhance vaccine efficacy.

New Bacteriophages with Podoviridal Morphotypes Active against Yersinia pestis: Characterization and Application Potential

Phages of highly pathogenic bacteria represent an area of growing interest for bacterial detection and identification and subspecies typing, as well as for phage therapy and environmental decontamination. Eight new phages-YpEc56, YpEc56D, YpEc57, YpEe58, YpEc1, YpEc2, YpEc11, and YpYeO9-expressing lytic activity towards Yersinia pestis revealed a virion morphology consistent with the Podoviridae morphotype. These phages lyse all 68 strains from 2 different sets of Y. pestis isolates, thus limiting their potential application for subtyping of Y. pestis strains but making them rather promising in terms of infection control. Two phages-YpYeO9 and YpEc11-were selected for detailed studies based on their source of isolation and lytic cross activity towards other Enterobacteriaceae. The full genome sequencing demonstrated the virulent nature of new phages. Phage YpYeO9 was identified as a member of the Teseptimavirus genus and YpEc11 was identified as a member of the Helsettvirus genus, thereby representing new species. A bacterial challenge assay in liquid microcosm with a YpYeO9/YpEc11 phage mixture showed elimination of Y. pestis EV76 during 4 h at a P/B ratio of 1000:1. These results, in combination with high lysis stability results of phages in liquid culture, the low frequency of formation of phage resistant mutants, and their viability under different physical-chemical factors indicate their potential for their practical use as an antibacterial mean.

Chitosan particle stabilized Pickering emulsion/interleukin-12 adjuvant system for Pgp3 subunit vaccine elicits immune protection against genital chlamydial infection in mice

Considering the shortcomings in current chlamydia infection control strategies, a major challenge in curtailing infection is the implementation of an effective vaccine. The immune response induced by C. trachomatis plasmid encoded Pgp3 was insufficient against C. trachomatis infection, which requires adjuvant applications to achieve the robust immune response induced by Pgp3. There is increasing promising in developing adjuvant systems relying on the delivery potential of Pickering emulsions and the immunomodulatory effects of interleukin (IL)-12. Here, owing to the polycationic nature, chitosan particles tended to absorb on the oil/water interphase to prepare the optimized chitosan particle-stabilized Pickering emulsion (CSPE), which was designed as a delivery system for Pgp3 protein and IL-12. Our results showed that the average droplets size of CSPE was 789.47 ± 44.26 nm after a series of optimizations and about 90% antigens may be absorbed by CSPE owing to the positively charged surface (33.2 ± 3mV), and CSPE promoted FITC-BSA proteins uptake by macrophages. Furthermore, as demonstrated by Pgp3-specific antibody production and cytokine secretion, CSPE/IL-12 system enhanced significantly higher levels of Pgp3-specific IgG, IgG1, IgG2a, sIgA and significant cytokines secretion of IFN-γ, IL-2, TNF-α, IL-4. Similarly, vaginal chlamydial shedding and hydrosalpinx pathologies were markedly reduced in mice immunized with Pgp3/CSPE/IL-12. Collectively, vaccination with Pgp3/CSPE/IL-12 regimen elicited robust cellular and humoral immune response in mice resulting in an obvious reduction of live chlamydia load in the vaginal and inflammatory pathologies in the oviduct, which further propells the development of vaccines against C. trachomatis infection.

The magnitude of the germinal center B cell and T follicular helper cell response predicts long-lasting antibody titers to plague vaccination

The development of a safe and effective vaccine against Yersinia pestis, the causative organism for plague disease, remains an important global health priority. Studies have demonstrated effective immune-based protection against plague challenge that is induced by plague antigen subunit vaccination in an aqueous alhydrogel formulation; however, whether these candidate vaccines in this formulation and presentation, induce long-lasting immunological memory in the form of durable cellular and antibody recall responses has not been fully demonstrated. In this study, we analyzed germinal center T follicular helper and germinal center B cell responses following F1V and F1 + V plague subunit immunization of mice with vaccines formulated in various adjuvants. Our data demonstrate that recombinant plague protein immunization formulated with IL-2/GM-CSF cytokines bound to alhydrogel adjuvant drive an increase in the magnitude of the germinal center T follicular helper and germinal center B cell responses following primary immunization, compared to vaccines formulated with Alhydrogel adjuvant alone. In contrast, plague protein subunit immunization combined with CpG ODN bound to alhydrogel increased the magnitude and duration of the germinal center Tfh and B cell responses following booster immunization. Importantly, enhanced germinal center Tfh and B cell responses correlated with long-lasting and high F1V-specific antibody titers and more robust antibody recall responses to F1V re-exposure. These findings indicate that vaccine formulations that drive enhancement of the germinal center Tfh and B cell responses are critical for inducing durable plague-specific humoral immunity.

Complete Protection Against Yersinia pestis in BALB/c Mouse Model Elicited by Immunization With Inhalable Formulations of rF1-V10 Fusion Protein via Aerosolized Intratracheal Inoculation

Pneumonic plague, caused by Yersinia pestis, is an infectious disease with high mortality rates unless treated early with antibiotics. Currently, no FDA-approved vaccine against plague is available for human use. The capsular antigen F1, the low-calcium-response V antigen (LcrV), and the recombinant fusion protein (rF1-LcrV) of Y. pestis are leading subunit vaccine candidates under intense investigation; however, the inability of recombinant antigens to provide complete protection against pneumonic plague in animal models remains a significant concern. In this study, we compared immunoprotection against pneumonic plague provided by rF1, rV10 (a truncation of LcrV), and rF1-V10, and vaccinations delivered via aerosolized intratracheal (i.t.) inoculation or subcutaneous (s.c.) injection. We further considered three vaccine formulations: conventional liquid, dry powder produced by spray freeze drying, or dry powder reconstituted in PBS. The main findings are: (i) rF1-V10 immunization with any formulation via i.t. or s.c. routes conferred 100% protection against Y. pestis i.t. infection; (ii) rF1 or rV10 immunization using i.t. delivery provided significantly stronger protection than rF1 or rV10 immunization via s.c. delivery; and (iii) powder formulations of subunit vaccines induced immune responses and provided protection equivalent to those elicited by unprocessed liquid formulations of vaccines. Our data indicate that immunization with a powder formulation of rF1-V10 vaccines via an i.t. route may be a promising vaccination strategy for providing protective immunity against pneumonic plague.

Plague vaccines: new developments in an ongoing search

As the reality of pandemic threats challenges humanity, exemplified during the ongoing SARS-CoV-2 infections, the development of vaccines targeting these etiological agents of disease has become increasingly critical. Of paramount concern are novel and reemerging pathogens that could trigger such events, including the plague bacterium Yersinia pestis. Y. pestis is responsible for more human deaths than any other known pathogen and exists globally in endemic regions of the world, including the four corners region and Northern California in the USA. Recent cases have been scattered throughout the world, including China and the USA, with serious outbreaks in Madagascar during 2008, 2013-2014, and, most recently, 2017-2018. This review will focus on recent advances in plague vaccine development, a seemingly necessary endeavor, as there is no Food and Drug Administration-licensed vaccine available for human distribution in western nations, and that antibiotic-resistant strains are recovered clinically or intentionally developed. Progress and recent development involving subunit, live-attenuated, and nucleic acid-based plague vaccine candidates will be discussed in this review. KEY POINTS: • Plague vaccine development remains elusive yet critical. • DNA, animal, and live-attenuated vaccine candidates gain traction.

Broadly reactive human CD4+ T cells against Enterobacteriaceae are found in the naïve repertoire and are clonally expanded in the memory repertoire

Enterobacteriaceae are a large family of Gram-negative bacteria that includes both commensals and opportunistic pathogens. The latter can cause severe nosocomial infections, with outbreaks of multi-antibiotics resistant strains, thus being a major public health threat. In this study, we report that Enterobacteriaceae-reactive memory Th cells were highly enriched in a CCR6+ CXCR3+ Th1*/17 cell subset and produced IFN-γ, IL-17A, and IL-22. This T cell subset was severely reduced in septic patients with K. pneumoniae bloodstream infection who also selectively lacked circulating K. pneumonie-reactive T cells. By combining heterologous antigenic stimulation, single cell cloning and TCR Vβ sequencing, we demonstrate that a large fraction of memory Th cell clones was broadly cross-reactive to several Enterobacteriaceae species. These cross-reactive Th cell clones were expanded in vivo and a large fraction of them recognized the conserved outer membrane protein A antigen. Interestingly, Enterobacteriaceae broadly cross-reactive T cells were also prominent among in vitro primed naïve T cells. Collectively, these data point to the existence of immunodominant T cell epitopes shared among different Enterobacteriaceae species and targeted by cross-reactive T cells that are readily found in the pre-immune repertoire and are clonally expanded in the memory repertoire.

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

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

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

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

Human plague: An old scourge that needs new answers

Yersinia pestis, the bacterial causative agent of plague, remains an important threat to human health. Plague is a rodent-borne disease that has historically shown an outstanding ability to colonize and persist across different species, habitats, and environments while provoking sporadic cases, outbreaks, and deadly global epidemics among humans. Between September and November 2017, an outbreak of urban pneumonic plague was declared in Madagascar, which refocused the attention of the scientific community on this ancient human scourge. Given recent trends and plague's resilience to control in the wild, its high fatality rate in humans without early treatment, and its capacity to disrupt social and healthcare systems, human plague should be considered as a neglected threat. A workshop was held in Paris in July 2018 to review current knowledge about plague and to identify the scientific research priorities to eradicate plague as a human threat. It was concluded that an urgent commitment is needed to develop and fund a strong research agenda aiming to fill the current knowledge gaps structured around 4 main axes: (i) an improved understanding of the ecological interactions among the reservoir, vector, pathogen, and environment; (ii) human and societal responses; (iii) improved diagnostic tools and case management; and (iv) vaccine development. These axes should be cross-cutting, translational, and focused on delivering context-specific strategies. Results of this research should feed a global control and prevention strategy within a "One Health" approach.

A forensic evaluation of plague - a re-emerging infectious disease with biowarfare potential

Plague is an acute infectious disease caused by the gram-negative cocco-bacillus Yersinia pestis. It has been responsible for 200 million deaths throughout history with three major pandemics. There are three forms: bubonic, septicaemic and pneumonic, each carrying a significant mortality rate. The usual transmission is from fleas carried by rodents. Recently, it has been listed as one of the reemerging infectious diseases globally, with a potential use in bioterrorism. At autopsy there may be lymphadenopathy, fulminant pneumonia or diffuse interstitial pneumonitis. However any organ may be affected with myocarditis, meningitis, pharyngitis and hepatic and splenic necrosis. The lethality of plague with the resurgence in numbers of cases, development of antibiotic resistance, recent occurrence in urban areas and the lack of a vaccine make it a disease not to be missed in the mortuary.

Vaccine Potential of a Recombinant Bivalent Fusion Protein LcrV-HSP70 Against Plague and Yersiniosis

To counteract the deadly pathogens, i.e., Y. pestis, Y. enetrocolitica, and Y. pseudotuberculosis, we prepared a recombinant DNA construct lcrV-hsp70 encoding the bivalent fusion protein LcrV-HSP70. The lcrV gene of Y. pestis and hsp70 domain II DNA fragment of M. tuberculosis were amplified by PCR. The lcrV amplicon was first ligated in the pET vector using NcoI and BamHI restriction sites. Just downstream to the lcrV gene, the hsp70 domain II was ligated using BamHI and Hind III restriction sites. The in-frame and the orientation of cloned lcrV-hsp70 were checked by restriction analysis and nucleotide sequencing. The recombinant bivalent fusion protein LcrV-HSP70 was expressed in E. coli and purified by affinity chromatography. The vaccine potential of LcrV-HSP70 fusion protein was evaluated in formulation with alum. BALB/c mice were vaccinated, and the humoral and cellular immune responses were studied. The fusion protein LcrV-HSP70 induced a strong and significant humoral immune response in comparison to control animals. We also observed a significant difference in the expression levels of IFN-γ and TNF-α in LcrV–HSP70-immunized mice in comparison to control, HSP70, and LcrV groups. To test the protective efficacy of the LcrV–HSP70 fusion protein against plague and Yersiniosis, the vaccinated mice were challenged with Y. pestis, Y. enterocolitica, and Y. pseudotuberculosis separately. The bivalent fusion protein LcrV–HSP70 imparted 100% protection against the plague. In the case of Yersiniosis, on day 2 post challenge, there was a significant reduction in the number of CFU of Y. enterocolitica and Y. pseudotuberculosis in the blood (CFU/ml) and the spleen (CFU/g) of vaccinated animals in comparison to the LcrV, HSP70, and control group animals.

Optimization and One-Step Purification of Recombinant V Antigen Production from Yersinia pestis

The purpose of this study was to develop an efficient and inexpensive method for the useful production of recombinant protein V antigen, an important virulence factor for Yersinia pestis. To this end, the synthetic gene encoding the V antigen was subcloned into the downstream of the intein (INT) and chitin-binding domain (CBD) from the pTXB1 vector using specific primers. In the following, the produced new plasmid, pTX-V, was transformed into E. coli ER₂₅₆₆ strain, and the expression accuracy was confirmed using electrophoresis and Western blotting. In addition, the effects of medium, inducer, and temperature on the enhancement of protein production were studied using the Taguchi method. Finally, the V antigen was purified by a chitin affinity column using INT and CBD tag. The expression was induced by 0.05 mM IPTG at 25 °C under optimal conditions including TB medium. It was observed that the expression of the V-INT–CBD fusion protein was successfully increased to more than 40% of the total protein. The purity of V antigen was as high as 90%. This result indicates that V antigen can be produced at low cost and subjected to one-step purification using a self-cleaving INT tag.

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

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

Promises and Challenges of the Type Three Secretion System Injectisome as an Antivirulence Target

Antibiotic resistance is a major public health threat that has stimulated the scientific community to search for nontraditional therapeutic targets. Because virulence, but not the growth, of many Gram-negative bacterial pathogens depends on the multicomponent type three secretion system injectisome (T3SSi), the T3SSi has been an attractive target for identifying small molecules, peptides, and monoclonal antibodies that inhibit its function to render the pathogen avirulent. While many small-molecule lead compounds have been identified in whole-cell-based high-throughput screens (HTSs), only a few protein targets of these compounds are known; such knowledge is an important step to developing more potent and specific inhibitors. Evaluation of the efficacy of compounds in animal studies is ongoing. Some efforts involving the development of antibodies and vaccines that target the T3SSi are further along and include an antibody that is currently in phase II clinical trials. Continued research into these antivirulence therapies, used alone or in combination with traditional antibiotics, requires combined efforts from both pharmaceutical companies and academic labs.

Yersinia pestis Pla Protein Thwarts T Cell Defense against Plague

Plague is a rapidly lethal human disease caused by the bacterium Yersinia pestis This study demonstrated that the Y. pestis plasminogen activator Pla, a protease that promotes fibrin degradation, thwarts T cell-mediated defense against fully virulent Y. pestis Introducing a single point mutation into the active site of Pla suffices to render fully virulent Y. pestis susceptible to primed T cells. Mechanistic studies revealed essential roles for fibrin during T cell-mediated defense against Pla-mutant Y. pestis Moreover, the efficacy of T cell-mediated protection against various Y. pestis strains displayed an inverse relationship with their levels of Pla activity. Together, these data indicate that Pla functions to thwart fibrin-dependent T cell-mediated defense against plague. Other important human bacterial pathogens, including staphylococci, streptococci, and borrelia, likewise produce virulence factors that promote fibrin degradation. The discovery that Y. pestis thwarts T cell defense by promoting fibrinolysis suggests novel therapeutic approaches to amplifying T cell responses against human pathogens.

Vaccines for emerging pathogens: prospects for licensure

Globally, there are a number of emerging pathogens. For most, there are no licensed vaccines available for human use, although there is ongoing research and development. However, given the extensive and increasing list of emerging pathogens and the investment required to bring vaccines into clinical use, the task is huge. Overlaid on this task is the risk of anti‐microbial resistance (AMR) acquisition by micro‐organisms which can endow a relatively harmless organism with pathogenic potential. Furthermore, climate change also introduces a challenge by causing some of the insect vectors and environmental conditions prevalent in tropical regions to begin to spread out from these traditional areas, thus increasing the risk of migration of zoonotic disease.

Vaccination provides a defence against these emerging pathogens. However, vaccines for pathogens which cause severe, but occasional, disease outbreaks in endemic pockets have suffered from a lack of commercial incentive for development to a clinical standard, encompassing Phase III clinical trials for efficacy. An alternative is to develop such vaccines to request US Emergency Use Authorization (EUA), or equivalent status in the United States, Canada and the European Union, making use of a considerable number of regulatory mechanisms that are available prior to licensing. This review covers the status of vaccine development for some of the emerging pathogens, the hurdles that need to be overcome to achieve EUA or an equivalent regional or national status and how these considerations may impact vaccine development for the future, such that a more comprehensive stockpile of promising vaccines can be achieved.

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.

The responses of lungs and adjacent lymph nodes in responding to Yersinia pestis infection: A transcriptomic study using a non-human primate model

Initiation of treatment during the pre-symptomatic phase of Yersinia pestis (Y. pestis) infection is particularly critical. The rapid proliferation of Y. pestis typically couples with the manifestation of common flu-like early symptoms that often misguides the medical intervention. Our study used African green monkeys (AGM) that did not exhibit clear clinical symptoms for nearly two days after intranasal challenge with Y. pestis and succumbed within a day after showing the first signs of clinical symptoms. The lung, and mediastinal and submandibular lymph nodes (LN) accumulated significant Y. pestis colonization immediately after the intranasal challenge. Hence, organ-specific molecular investigations are deemed to be the key to elucidating mechanisms of the initial host response. Our previous study focused on the whole blood of AGM, and we found early perturbations in the ubiquitin-microtubule-mediated host defense. Altered expression of the genes present in ubiquitin and microtubule networks indicated an early suppression of these networks in the submandibular lymph nodes. In concert, the upstream toll-like receptor signaling and downstream NFκB signaling were inhibited at the multi-omics level. The inflammatory response was suppressed in the lungs, submandibular lymph nodes and mediastinal lymph nodes. We posited a causal chain of molecular mechanisms that indicated Y. pestis was probably able to impair host-mediated proteolysis activities and evade autophagosome capture by dysregulating both ubiquitin and microtubule networks in submandibular lymph nodes. Targeting these networks in a submandibular LN-specific and time-resolved fashion could be essential for development of the next generation therapeutics for pneumonic plague.

Plague vaccine: recent progress and prospects

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

The Tip Complex: From Host Cell Sensing to Translocon Formation

Type III secretion systems are used by some Gram-negative bacteria to inject effector proteins into targeted eukaryotic cells for the benefit of the bacterium. The type III secretion injectisome is a complex nanomachine comprised of four main substructures including a cytoplasmic sorting platform, an envelope-spanning basal body, an extracellular needle and an exposed needle tip complex. Upon contact with a host cell, secretion is induced, resulting in the formation of a translocon pore in the host membrane. Translocon formation completes the conduit needed for effector secretion into the host cell. Control of type III secretion occurs in response to environmental signals, with the final signal being host cell contact. Secretion control occurs primarily at two sites-the cytoplasmic sorting platform, which determines secretion hierarchy, and the needle tip complex, which is critical for sensing and responding to environmental signals. The best-characterized injectisomes are those from Yersinia, Shigella and Salmonella species where there is a wealth of information on the tip complex and the two translocator proteins. Of these systems, the best characterized from a secretion regulation standpoint is Shigella. In the Shigella system, the tip complex and the first secreted translocon both contribute to secretion control and, thus, both are considered components of the tip complex. In this review, all three of these type III secretion systems are described with discussion focused on the structure and formation of the injectisome tip complex and what is known of the transition from nascent tip complex to assembled translocon pore.

Metabolomic analyses reveal lipid abnormalities and hepatic dysfunction in non-human primate model for Yersinia pestis

INTRODUCTION

Pneumonic plague is caused by the aerosolized form of Yersinia pestis and is a highly virulent infection with complex clinical consequences, and without treatment, the fatality rate approaches 100%. The exact mechanisms of disease progression are unclear, with limited work done using metabolite profiling to study disease progression.

OBJECTIVE

The aim of this pilot study was to profile the plasma metabolomics in an animal model of Y. pestis infection.

METHODS

In this study, African Green monkeys were challenged with the highly virulent, aerosolized Y. pestis strain CO92, and untargeted metabolomics profiling of plasma was performed using liquid and gas chromatography with mass spectrometry.

RESULTS

At early time points post-exposure, we found significant increases in polyunsaturated, long chain fatty acid metabolites with p values ranging from as low as 0.000001 (ratio = 1.94) for the metabolite eicosapentaenoate to 0.04 (ratio = 1.36) for the metabolite adrenate when compared to time-matched controls. Multiple acyl carnitines metabolites were increased at earlier time points and could be a result of fatty acid oxidation defects with p values ranging from as low as 0.00001 (ratio = 2.95) for the metabolite octanoylcarnitine to 0.04 (ratio = 1.33) for metabolite deoxycarnitine when compared to time-matched controls. Dicarboxylic acids are important metabolic products of fatty acids oxidation, and when compared to time matched controls, were higher at earlier time points where metabolite tetradecanedioate has a ratio of 4.09 with significant p value of 0.000002 and adipate with a ratio of 1.12 and p value of 0.004. The metabolites from lysolipids (with significant p values ranging from 0.00006 for 1-oleoylglycerophosphoethanolamine to 0.04 for 1-stearoylglycerophosphoethanolamine and a ratio of 0.47 and 0.78, respectively) and bile acid metabolism (with significant p values ranging from 0.02 for cholate to 0.04 for deoxycholate and a ratio of 0.39 and 0.66, respectively) pathways were significantly lower compared to their time-matched controls during the entire course of infection. Metabolite levels from amino acid pathways were disrupted, and a few from the leucine, isoleucine and valine pathway were significantly higher (p values ranging from 0.002 to 0.04 and ratios ranging from 1.3 to 1.5, respectively), whereas metabolites from the urea cycle, arginine and proline pathways were significantly lower (p values ranging from 0.00008 to 0.02 and ratios ranging from 0.5 to 0.7, respectively) during the course of infection.

CONCLUSIONS

The involvement of several lipid pathways post-infection suggested activation of pathways linked to inflammation and oxidative stress. Metabolite data further showed increased energy demand, and multiple metabolites indicated potential hepatic dysfunction. Integration of blood metabolomics and transcriptomics data identified linoleate as a core metabolite with cross-talk with multiple genes from various time points. Collectively, the data from this study provided new insights into the mechanisms of Y. pestis pathogenesis that may aid in development of therapeutics.

Escherichia coli expressed flagellin C (FliC) of Salmonella Typhi improved the protective efficacy of YopE against plague infection

In the current antibiotic resistance scenario, vaccines may provide best defense against lethal bacterial diseases. So far, there is no idealvaccine available against plague. Despite providing complete protection in small animal models, F1/LcrV based vaccine failed to provide ideal protection in non human primates. Here, we cloned, expressed and purified YopE of Yersinia pestis and flagellin C (FliC) of Salmonella Typhi. However the best possible protection needs the significant induction of IFN-γ and TNF-α. To determine the protective potential of the recombinant YopE alone or in formulation with FliC, Balb/C mice were immunized subcutaneously. The formulations were prepared with alum, a human compatible adjuvant. In our studies, the combination of YopE + FliC induced significantly strong humoral and cellular immune responses. A combination of YopE + FliC provided 83% protection whereas YopE alone provided only 50% against 100LD₅₀ of Y. pestis in a mouse model.

Targeting of the Yersinia pestis F1 capsular antigen by innate-like B1b cells mediates a rapid protective response against bubonic plague

The generation of adaptive immunity by vaccination is usually a prolonged process that requires multiple dosing over several months. Hence, vaccines are administered for disease prevention a relatively long time prior to possible infection as opposed to post-exposure prophylaxis, which typically requires rapid intervention such as antibiotic therapy. The emergence of pathogens resistant to common antibiotic treatments has prompted the search for alternative therapeutic strategies. We previously demonstrated that vaccination of mice with the F1 capsular antigen of Yersinia pestis elicits specific and effective yet, unexpectedly, rapid anti-plague immunity. Here, we show by applying genetic and immunological approaches that the F1 antigen is targeted by peritoneal innate-like B1b cells that generate a prompt T-independent (TI) anti-F1 humoral response. The rapid F1-mediated defense response was diminished in Xid (Btk^m) mice in which B1 cell numbers and activity are limited. Binding of fluorophore-labeled F1 to peritoneal B1b cells was detected as soon as 6 h post vaccination, emphasizing the high speed of this process. By assessing the ability to achieve rapid immunity with monomerized F1, we show that the natural polymeric structure of F1 is essential for (i) rapid association with peritoneal B1b cells, (ii) early induction of anti-F1 titers and (iii) rapid TI immunity in the mouse model of bubonic plague. These observations shed new light on the potential of novel as well as well-known protective antigens in generating rapid immunity and could be implemented in the rational design of future vaccines.

LFchimera protects HeLa cells from invasion by Yersinia spp. in vitro

Yersinia pestis is the causative agent of plague. As adequate antibiotic treatment falls short and currently no effective vaccine is available, alternative therapeutic strategies are needed. In order to contribute to solving this problem we investigated the therapeutic potential of the peptide construct LFchimera against the safer-to-handle Y. pestis simulants Yersinia enterocolitica and Yersinia pseudotuberculosis in vitro. LFchimera is a heterodimeric peptide construct mimicking two antimicrobial domains of bovine lactoferrin, i.e. lactoferrampin and lactoferricin. LFchimera has been shown to be a potent antimicrobial peptide against a variety of bacteria in vitro and in vivo. Also Y. enterocolitica and Y. pseudotuberculosis have been shown to be susceptible for LFchimera in vitro. As Yersiniae spp. adhere to and invade host cells upon infection, we here investigated the effects of LFchimera on these processes. It was found that LFchimera has the capacity to inhibit host-cell invasion by Yersiniae spp. in vitro. This effect appeared to be host-cell mediated, not bacteria-mediated. Furthermore it was found that exposure of human HeLa epithelial cells to both LFchimera and the bacterial strains evoked a pro-inflammatory cytokine release from the cells in vitro.

Dual route vaccination for plague with emergency use applications

Here, we report a dual-route vaccination approach for plague, able to induce a rapid response involving systemic and mucosal immunity, whilst also providing ease of use in those resource-poor settings most vulnerable to disease outbreaks. This novel vaccine (VypVaxDuo) comprises the recombinant F1 and V proteins in free association. VypVaxDuo has been designed for administration via a sub-cutaneous priming dose followed by a single oral booster dose and has been demonstrated to induce early onset immunity 14 days after the primary immunisation; full protective efficacy against live organism challenge was achieved in Balb/c mice exposed to 2 × 10⁴ median lethal doses of Yersinia pestis Co92, by the sub-cutaneous route at 25 days after the oral booster immunisation. This dual-route vaccination effectively induced serum IgG and serum and faecal IgA, specific for F1 and V, which constitute two key virulence factors in Y. pestis, and is therefore suitable for further development to prevent bubonic plague and for evaluation in models of pneumonic plague. This is an essential requirement for control of disease outbreaks in areas of the world endemic for plague and is supported further by the observed exceptional stability of the primary vaccine formulation in vialled form under thermostressed conditions (40 °C for 29 weeks, and 40 °C with 75% relative humidity for 6 weeks), meaning no cold chain for storage or distribution is needed. In clinical use, the injected priming dose would be administered on simple rehydration of the dry powder by means of a dual barrel syringe, with the subsequent single booster dose being provided in an enteric-coated capsule suitable for oral self-administration.

[The plague: An overview and hot topics]

Plague is a bacterial zoonosis caused by Yersinia pestis, usually found in fleas and small rodents that constitute the reservoir of the disease. It is transmitted to humans by flea bite, contact with rodents or inhalation of infected droplets. There are three clinical forms: bubonic plague, pulmonary plague and septicemic plague. The usual presentation is a flu-like syndrome possibly accompanied by an inflammatory lymphadenopathy which appears after 1 to 7days of incubation. Bubonic plague has a case fatality rate of about 50% while other forms of plague are almost always fatal without treatment. Diagnosis can be confirmed by usual bacteriological techniques (Gram examination, culture) but also by serological examination, use of rapid diagnostic tests or PCR. Although aminoglycosides are traditionally regarded as the most effective treatment, fluoroquinolones or cyclins are currently recommended in France. Plague is one of the re-emerging diseases according to the WHO and Madagascar suffered in 2017 the most important plague epidemic of the 21st century with more than 2000 cases and 200 deaths. Peru and the Democratic Republic of Congo are also considered endemic areas. Public health measures and a relentless fight against poverty are the cornerstone of the control of the disease. Vaccine improvement in endemic areas may also play an important role.

A safety and immunogenicity study of a novel subunit plague vaccine in cynomolgus macaques

Plague has led to millions of deaths in history and outbreaks continue to the present day. The efficacy limitations and safety concerns of the existing killed whole cell and live-attenuated vaccines call for the development of new vaccines. In this study, we evaluated the immunogenicity and safety of a novel subunit plague vaccine, comprising native F1 antigen and recombinant V antigen. The cynomolgus macaques in low- and high-dose vaccine groups were vaccinated at weeks 0, 2, 4 and 6, at dose levels of 15 μg F1 + 15 μg rV and 30 μg F1 + 30 μg rV respectively. Specific antibodies and interferon-γ and interleukin-2 expression in lymphocytes were measured. For safety, except for the general toxicity and local irritation, we made a systematic immunotoxicity study on the vaccine including immunostimulation, autoimmunity and anaphylactic reaction. The vaccine induced high levels of serum anti-F1 and anti-rV antibodies, and caused small increases of interferon-γ and interleukin-2 in monkeys. The vaccination led to a reversible increase in the number of peripheral blood eosinophils, the increases in serum IgE level in a few animals and histopathological change of granulomas at injection sites. The vaccine had no impact on general conditions, most clinical pathology parameters, percentages of T-cell subsets, organ weights and gross pathology of treated monkeys and had passable local tolerance. The F1 + rV subunit plague vaccine can induce very strong humoral immunity and low level of cellular immunity in cynomolgus macaques and has a good safety profile.

Host Iron Nutritional Immunity Induced by a Live Yersinia pestis Vaccine Strain Is Associated with Immediate Protection against Plague

Prompt and effective elicitation of protective immunity is highly relevant for cases of rapidly deteriorating fatal diseases, such as plague, which is caused by Yersinia pestis. Here, we assessed the potential of a live vaccine to induce rapid protection against this infection. We demonstrated that the Y. pestis EV76 live vaccine protected mice against an immediate lethal challenge, limiting the multiplication of the virulent pathogen and its dissemination into circulation. Ex vivo analysis of Y. pestis growth in serum derived from EV76-immunized mice revealed that an antibacterial activity was produced rapidly. This activity was mediated by the host heme- and iron-binding proteins hemopexin and transferrin, and it occurred in strong correlation with the kinetics of hemopexin induction in vivo. We suggest a new concept in which a live vaccine is capable of rapidly inducing iron nutritional immunity, thus limiting the propagation of pathogens. This concept could be exploited to design novel therapeutic interventions.