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

CT584 Is Not a Protective Vaccine Antigen against Respiratory Chlamydial Challenge in Mice

Background:Chlamydia trachomatis is the most prevalent bacterial sexually transmitted pathogen in humans worldwide. Since chlamydial infection is largely asymptomatic with the potential for serious complications, a preventative vaccine is likely the most viable long-term answer to this public health threat. Cell-free protein synthesis (CFPS) utilizes the cellular protein manufacturing machinery decoupled from the requirement for maintaining cellular viability, offering the potential for flexible, rapid, and decentralized production of recombinant protein vaccine antigens. Methods: Here, we use CFPS to produce the full-length putative chlamydial type three secretion system (T3SS) needle-tip protein, CT584, for evaluation as a vaccine antigen in mouse models. High-speed atomic force microscopy (HS-AFM) (RIBM, Tsukuba, Japan) imaging and computer simulations confirm that CFPS-produced CT584 retains a native-like structure prior to immunization. Female mice were primed with CT584 adjuvanted with CpG-1826 intranasally (i.n.) or CpG-1826 + Montanide ISA 720 intramuscularly (i.m.), followed four weeks later by an i.m. boost before respiratory challenge with 104 inclusion forming units (IFU) of Chlamydia muridarum. Results: Immunization with CT584 generated robust antibody responses but weak cell-mediated immunity and failed to protect against i.n. challenge as demonstrated by body weight loss, increased lung weights, and the presence of high numbers of IFUs in the lungs. Conclusion: While CT584 was not a protective vaccine candidate, the speed and flexibility with which CFPS can be used to produce other potential chlamydial antigens make it an attractive technique for antigen production.

Progress on the research and development of plague vaccines with a call to action

There is a compelling demand for approved plague vaccines due to the endemicity of Yersinia pestis and its potential for pandemic spread. Whilst substantial progress has been made, we recommend that the global funding and health security systems should work urgently to translate some of the efficacious vaccines reviewed herein to expedite clinical development and to prevent future disastrous plague outbreaks, particularly caused by antimicrobial resistant Y. pestis strains.Content includes material subject to Crown Copyright © 2024.This is an open access article under the Open Government License ( http://www.nationalarchives.gov.uk/doc/open-government-licence/version/3/ ).

Evaluation in mice of cell-free produced CT584 as a Chlamydia vaccine antigen

Chlamydia trachomatis is the most prevalent bacterial sexually transmitted pathogen worldwide. Since chlamydial infection is largely asymptomatic with the potential for serious complications, a preventative vaccine is likely the most viable long-term answer to this public health threat. Cell-free protein synthesis (CFPS) utilizes the cellular protein manufacturing machinery decoupled from the requirement for maintaining cellular viability, offering the potential for flexible, rapid, and de-centralized production of recombinant protein vaccine antigens. Here, we use CFPS to produce the putative chlamydial type three secretion system (T3SS) needle-tip protein, CT584, for use as a vaccine antigen in mouse models. High-speed atomic force microscopy (HS-AFM) imaging and computer simulations confirm that CFPS-produced CT584 retains a native-like structure prior to immunization. Female mice were primed with CT584 adjuvanted with CpG-1826 intranasally (i.n.) or CpG-1826 + Montanide ISA 720 intramuscularly (i.m.), followed four-weeks later by an i.m. boost before respiratory challenge with 104 inclusion forming units (IFU) of Chlamydia muridarum. Immunization with CT584 generated robust antibody responses but weak cell mediated immunity and failed to protect against i.n. challenge as demonstrated by body weight loss, increased lungs' weights and the presence of high numbers of IFUs in the lungs. While CT584 alone may not be the ideal vaccine candidate, the speed and flexibility with which CFPS can be used to produce other potential chlamydial antigens makes it an attractive technique for antigen production.

Plague Gives Surprises in the Second Decade of the Twenty-First Century

From 2010 through 2019, the six leading countries by numbers of human plague cases reported to the WHO were, in order from highest to lowest, Madagascar, Congo, Uganda, Peru, Tanzania, and the United States. From these countries, there was a total of 4,547 cases, of whom 786 (17%) died. Top plague events were four outbreaks of primary pneumonic plague in Madagascar that affected 1,936 persons, including index cases, of whom 137 died. One of the outbreaks was caused by a streptomycin-resistant strain of Yersinia pestis. Person-to-person transmission occurred in a taxi, in households with family caregivers, at burial ceremonies and wakes for victims, and at a hospital where cases were treated. Unique clinical presentations in the United States included a dog owner who acquired pneumonic plague from his sick dog, a boy with septicemic plague who developed complications of osteomyelitis and arthritis that required surgery for bone removal and bone grafting, and a prairie dog handler who acquired bubonic plague from a bite by a sick prairie dog. Efficacy of antibiotics in a model of pneumonic plague in African green monkeys for use in bioterrorism revealed the most effective drugs to be gentamicin, ciprofloxacin, and levofloxacin. A recombinant vaccine containing Fraction 1 antigen and V antigen of Y. pestis designed for first responders during a bioterrorism attack and military personnel was tested for safety and immunogenicity but was not licensed for use by the end of the decade.

Predictors of Survival after Vaccination in a Pneumonic Plague Model

Background: The need for an updated plague vaccine is highlighted by outbreaks in endemic regions together with the pandemic potential of this disease. There is no easily available, approved vaccine. Methods: Here we have used a murine model of pneumonic plague to examine the factors that maximise immunogenicity and contribute to survival following vaccination. We varied vaccine type, as either a genetic fusion of the F1 and V protein antigens or a mixture of these two recombinant antigens, as well as antigen dose-level and formulation in order to correlate immune response to survival. Results: Whilst there was interaction between each of the variables of vaccine type, dose level and formulation and these all contributed to survival, vaccine formulation in protein-coated microcrystals (PCMCs) was the key contributor in inducing antibody titres. From these data, we propose a cut-off in total serum antibody titre to the F1 and V proteins of 100 µg/mL and 200 µg/mL, respectively. At these thresholds, survival is predicted in this murine pneumonic model to be >90%. Within the total titre of antibody to the V antigen, the neutralising antibody component correlated with dose level and was enhanced when the V antigen in free form was formulated in PCMCs. Antibody titre to F1 was limited by fusion to V, but this was compensated for by PCMC formulation. Conclusions: These data will enable clinical assessment of this and other candidate plague vaccines that utilise the same vaccine antigens by identifying a target antibody titre from murine models, which will guide the evaluation of clinical titres as serological surrogate markers of efficacy.

The African Green Monkey Model of Pneumonic Plague and US Food and Drug Administration Approval of Antimicrobials Under the Animal Rule

BACKGROUND

Additional treatment options for pneumonic plague, the most severe form of infection by Yersinia pestis, are needed, as past US Food and Drug Administration (FDA) approvals were not based on clinical trials that meet today's standards, and multiple drugs are sought to counter resistance or use in special populations. Due to the sporadic nature of outbreaks and the low number of pneumonic cases of disease, we sought FDA approval of antimicrobials for treatment under the Animal Efficacy Rule, where efficacy can be demonstrated in 1 or more well-characterized animal models that sufficiently represent human disease.

METHODS

A model was developed in African green monkeys (AGMs) after challenge with a lethal dose of Y. pestis delivered as an aerosol, in 4 independent studies in 3 laboratories. The primary data points were bacteremia (daily), body temperature and heart rate (continuously monitored by telemetry), and survival. In antimicrobial efficacy studies, human-equivalent doses of gentamicin, ciprofloxacin, levofloxacin, and doxycycline were administered upon fever onset for 10 days.

RESULTS

Disease in AGMs was similar to case reports of human disease. Fever was determined to be a reliable sign of disease and selected as a treatment trigger. Gentamicin was 60%-80% effective depending on the dose given to animals. Ciprofloxacin and levofloxacin were found to be >90% efficacious. These data were submitted to FDA and plague indications were approved. Doxycycline was less effective.

CONCLUSIONS

The AGM model of pneumonic plague is reproducible, well-characterized, and mimics human disease. It has been used to support plague indications for fluoroquinolones and to test the efficacy of additional antimicrobials.

A new generation needle- and adjuvant-free trivalent plague vaccine utilizing adenovirus-5 nanoparticle platform

A plague vaccine with a fusion cassette of YscF, F1, and LcrV encoding genes in an adenovirus-5 vector (rAd5-YFV) is evaluated for efficacy and immune responses in mice. Two doses of the vaccine provides 100% protection when administered intranasally against challenge with Yersinia pestis CO92 or its isogenic F1 mutant in short- or long- term immunization in pneumonic/bubonic plague models. The corresponding protection rates drop in rAd5-LcrV monovalent vaccinated mice in plague models. The rAd5-YFV vaccine induces superior humoral, mucosal and cell-mediated immunity, with clearance of the pathogen. Immunization of mice with rAd5-YFV followed by CO92 infection dampens proinflammatory cytokines and neutrophil chemoattractant production, while increasing Th1- and Th2-cytokine responses as well as macrophage/monocyte chemo-attractants when compared to the challenge control animals. This is a first study showing complete protection of mice from pneumonic/bubonic plague with a viral vector-based vaccine without the use of needles and the adjuvant.

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.

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.

Humoral and cellular immune correlates of protection against bubonic plague by a live Yersinia pseudotuberculosis vaccine

Immunization with the live-attenuated Yersinia pseudotuberculosis VTnF1 strain producing a Yersinia pestis F1 pseudocapsule efficiently protects mice against bubonic and pneumonic plague. In clinical trials, demonstration of a plague vaccine's efficacy in humans will not be feasible, and correlates of protection will be needed to bridge the immune response of protected animals to that of vaccinated humans. Using serum transfer and vaccination of antibody-deficient µMT mice, we established that both humoral and cellular responses elicited by VTnF1 independently conferred protection against bubonic plague. Thus, correlates were searched for in both responses, using blood only. Mice were vaccinated with increasing doses of VTnF1 to provide a range of immune responses and survival outcomes. The cellular response was evaluated using an in vitro IFNγ release assay, and IFNγ levels were significantly associated with protection, although some survivors were negative for IFNγ, so that IFNγ release is not a fully satisfactory correlate. Abundant serum IgG against the F1 capsule, Yop injectable toxins, and also non-F1 Y.pestis antigens were found, but none against the LcrV antigen. All readouts correlated to survival and to each other, confirming that vaccination triggered multiple protective mechanisms developing in parallel. Anti-F1 IgG was the most stringent correlate of protection, in both inbred BALB/c mice and outbred OF1 mice. This indicates that antibodies (Ab) to F1 play a dominant role for protection even in the presence of Ab to many other targets. Easy to measure, the anti-F1 IgG titer will be useful to evaluate the immune response in other animal species and in clinical trials.

Immunisation of two rodent species with new live-attenuated mutants of Yersinia pestis CO92 induces protective long-term humoral- and cell-mediated immunity against pneumonic plague

We showed recently that the live-attenuated Δlpp ΔmsbB Δail and Δlpp ΔmsbB::ailL2 mutants of Yersinia pestis CO92 provided short-term protection to mice against developing subsequent lethal pneumonic plague. These mutants were either deleted for genes encoding Braun lipoprotein (Lpp), an acetyltransferase (MsbB) and the attachment invasion locus (Ail) (Δlpp ΔmsbB Δail) or contained a modified version of the ail gene with diminished virulence (Δlpp ΔmsbB::ailL2). Here, long-term immune responses were first examined after intramuscular immunisation of mice with the above-mentioned mutants, as well as the newly constructed Δlpp ΔmsbB Δpla mutant, deleted for the plasminogen-activator protease (pla) gene instead of ail. Y. pestis-specific IgG levels peaked between day 35 and 56 in the mutant-immunised mice and were sustained until the last tested day 112. Splenic memory B cells peaked earlier (day 42) before declining in the Δlpp ΔmsbB::ailL2 mutant-immunised mice while being sustained for 63 days in the Δlpp ΔmsbB Δail and Δlpp ΔmsbB Δpla mutant-immunised mice. Splenic CD4⁺ T cells increased in all immunised mice by day 42 with differential cytokine production among the immunised groups. On day 120, immunised mice were exposed intranasally to wild-type (WT) CO92, and 80–100% survived pneumonic challenge. Mice immunised with the above-mentioned three mutants had increased innate as well as CD4⁺ responses immediately after WT CO92 exposure, and coupled with sustained antibody production, indicated the role of both arms of the immune response in protection. Likewise, rats vaccinated with either Δlpp ΔmsbB Δail or the Δlpp ΔmsbB Δpla mutant also developed long-term humoral and cell-mediated immune responses to provide 100% protection against developing pneumonic plague. On the basis of the attenuated phenotype, the Δlpp ΔmsbB Δail mutant was recently excluded from the Centers for Disease Control and Prevention select agent list.

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

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

Intramuscular Immunization of Mice with a Live-Attenuated Triple Mutant of Yersinia pestis CO92 Induces Robust Humoral and Cell-Mediated Immunity To Completely Protect Animals against Pneumonic Plague

Earlier, we showed that the Δlpp ΔmsbB Δail triple mutant of Yersinia pestis CO92 with deleted genes encoding Braun lipoprotein (Lpp), an acyltransferase (MsbB), and the attachment invasion locus (Ail), respectively, was avirulent in a mouse model of pneumonic plague. In this study, we further evaluated the immunogenic potential of the Δlpp ΔmsbB Δail triple mutant and its derivative by different routes of vaccination. Mice were immunized via the subcutaneous (s.c.) or the intramuscular (i.m.) route with two doses (2 × 10(6) CFU/dose) of the above-mentioned triple mutant with 100% survivability of the animals. Upon subsequent pneumonic challenge with 70 to 92 50% lethal doses (LD(50)) of wild-type (WT) strain CO92, all of the mice survived when immunization occurred by the i.m. route. Since Ail has virulence and immunogenic potential, a mutated version of Ail devoid of its virulence properties was created, and the genetically modified ail replaced the native ail gene on the chromosome of the Δlpp ΔmsbB double mutant, creating a Δlpp ΔmsbB::ailL2 vaccine strain. This newly generated mutant was attenuated similarly to the Δlpp ΔmsbB Δail triple mutant when administered by the i.m. route and provided 100% protection to animals against subsequent pneumonic challenge. Not only were the two above-mentioned mutants cleared rapidly from the initial i.m. site of injection in animals with no histopathological lesions, the immunized mice did not exhibit any disease symptoms during immunization or after subsequent exposure to WT CO92. These two mutants triggered balanced Th1- and Th2-based antibody responses and cell-mediated immunity. A substantial increase in interleukin-17 (IL-17) from the T cells of vaccinated mice, a cytokine of the Th17 cells, further augmented their vaccine potential. Thus, the Δlpp ΔmsbB Δail and Δlpp ΔmsbB::ailL2 mutants represent excellent vaccine candidates for plague, with the latter mutant still retaining Ail immunogenicity but with a much diminished virulence potential.