A multiagent filovirus DNA vaccine delivered by intramuscular electroporation completely protects mice from ebola and Marburg virus challenge

DNA Vaccines for Epidemic Preparedness: SARS-CoV-2 and Beyond

We highlight the significant progress in developing DNA vaccines during the SARS-CoV-2 pandemic. Specifically, we provide a comprehensive review of the DNA vaccines that have progressed to Phase 2 testing or beyond, including those that have received authorization for use. DNA vaccines have significant advantages with regard to the rapidity of production, thermostability, safety profile, and cellular immune responses. Based on user needs and cost, we compare the three devices used in the SARS-CoV-2 clinical trials. Of the three devices, the GeneDerm suction device offers numerous benefits, particularly for international vaccination campaigns. As such, DNA vaccines represent a promising option for future pandemics.

Ebola Virus Disease Vaccines: Development, Current Perspectives & Challenges

The global outgoing outbreaks of Ebola virus disease (EVD) in different regions of Sudan, Uganda, and Western Africa have brought into focus the inadequacies and restrictions of pre-designed vaccines for use in the battle against EVD, which has affirmed the urgent need for the development of a systematic protocol to produce Ebola vaccines prior to an outbreak. There are several vaccines available being developed by preclinical trials and human-based clinical trials. The group of vaccines includes virus-like particle-based vaccines, DNA-based vaccines, whole virus recombinant vaccines, incompetent replication originated vaccines, and competent replication vaccines. The limitations and challenges faced in the development of Ebola vaccines are the selection of immunogenic, rapid-responsive, cross-protective immunity-based vaccinations with assurances of prolonged protection. Another issue for the manufacturing and distribution of vaccines involves post authorization, licensing, and surveillance to ensure a vaccine's efficacy towards combating the Ebola outbreak. The current review focuses on the development process, the current perspective on the development of an Ebola vaccine, and future challenges for combatting future emerging Ebola infectious disease.

A Historical Review of Military Medical Strategies for Fighting Infectious Diseases: From Battlefields to Global Health

The environmental conditions generated by war and characterized by poverty, undernutrition, stress, difficult access to safe water and food as well as lack of environmental and personal hygiene favor the spread of many infectious diseases. Epidemic typhus, plague, malaria, cholera, typhoid fever, hepatitis, tetanus, and smallpox have nearly constantly accompanied wars, frequently deeply conditioning the outcome of battles/wars more than weapons and military strategy. At the end of the nineteenth century, with the birth of bacteriology, military medical researchers in Germany, the United Kingdom, and France were active in discovering the etiological agents of some diseases and in developing preventive vaccines. Emil von Behring, Ronald Ross and Charles Laveran, who were or served as military physicians, won the first, the second, and the seventh Nobel Prize for Physiology or Medicine for discovering passive anti-diphtheria/tetanus immunotherapy and for identifying mosquito Anopheline as a malaria vector and plasmodium as its etiological agent, respectively. Meanwhile, Major Walter Reed in the United States of America discovered the mosquito vector of yellow fever, thus paving the way for its prevention by vector control. In this work, the military relevance of some vaccine-preventable and non-vaccine-preventable infectious diseases, as well as of biological weapons, and the military contributions to their control will be described. Currently, the civil-military medical collaboration is getting closer and becoming interdependent, from research and development for the prevention of infectious diseases to disasters and emergencies management, as recently demonstrated in Ebola and Zika outbreaks and the COVID-19 pandemic, even with the high biocontainment aeromedical evacuation, in a sort of global health diplomacy.

A Guide to Nucleic Acid Vaccines in the Prevention and Treatment of Infectious Diseases and Cancers: From Basic Principles to Current Applications

Faced with the challenges posed by infectious diseases and cancer, nucleic acid vaccines present excellent prospects in clinical applications. Compared with traditional vaccines, nucleic acid vaccines have the characteristics of high efficiency and low cost. Therefore, nucleic acid vaccines have potential advantages in disease prevention and treatment. However, the low immunogenicity and instability of nucleic acid vaccines have limited their development. Therefore, a large number of studies have been conducted to improve their immunogenicity and stability by improving delivery methods, thereby supporting progress and development for clinical applications. This article mainly reviews the advantages, disadvantages, mechanisms, delivery methods, and clinical applications of nucleic acid vaccines.

Cationic Polymers for the Delivery of the Ebola DNA Vaccine Encoding Artificial T-Cell Immunogen

Background: According to current data, an effective Ebola virus vaccine should induce both humoral and T-cell immunity. In this work, we focused our efforts on methods for delivering artificial T-cell immunogen in the form of a DNA vaccine, using generation 4 polyamidoamine dendrimers (PAMAM G4) and a polyglucin:spermidine conjugate (PG). Methods: Optimal conditions were selected for obtaining complexes of previously developed DNA vaccines with cationic polymers. The sizes, mobility and surface charge of the complexes with PG and PAMAM 4G have been determined. The immunogenicity of the obtained vaccine constructs was investigated in BALB/c mice. Results: It was shown that packaging of DNA vaccine constructs both in the PG envelope and the PAMAM 4G envelope results in an increase in their immunogenicity as compared with the group of mice immunized with the of vector plasmid pcDNA3.1 (a negative control). The highest T-cell responses were shown in mice immunized with complexes of DNA vaccines with PG and these responses significantly exceeded those in the groups of animals immunized with both the combination of naked DNAs and the combination DNAs coated with PAMAM 4G. In the group of animals immunized with complexes of the DNA vaccines with PAMAM 4G, no statistical differences were found in the ability to induce T-cell responses, as compared with the group of mice immunized with the combination of naked DNAs. Conclusions: The PG conjugate can be considered as a promising and safe means to deliver DNA-based vaccines. The use of PAMAM requires further optimization.

Innate Molecular and Cellular Signature in the Skin Preceding Long-Lasting T Cell Responses after Electroporated DNA Vaccination

DNA vaccines delivered with electroporation (EP) have shown promising results in preclinical models and are evaluated in clinical trials. In this study, we aim to characterize early mechanisms occurring in the skin after intradermal injection and EP of the auxoGTUmultiSIV DNA vaccine in nonhuman primates. First, we show that EP acts as an adjuvant by enhancing local inflammation, notably via granulocytes, monocytes/macrophages, and CD1a^int-expressing cell recruitment. EP also induced Langerhans cell maturation, illustrated by CD86, CD83, and HLA-DR upregulation and their migration out of the epidermis. Second, we demonstrate the crucial role of the DNA vaccine in soluble factors release, such as MCP-1 or IL-15. Transcriptomic analysis showed that EP played a major role in gene expression changes postvaccination. However, the DNA vaccine is required to strongly upregulate several genes involved in inflammatory responses (e.g., Saa4), cell migration (e.g., Ccl3, Ccl5, or Cxcl10), APC activation (e.g., Cd86), and IFN-inducible genes (e.g., Ifit3, Ifit5, Irf7, Isg15, orMx1), illustrating an antiviral response signature. Also, AIM-2, a cytosolic DNA sensor, appeared to be strongly upregulated only in the presence of the DNA vaccine and trends to positively correlate with several IFN-inducible genes, suggesting the potential role of AIM-2 in vaccine sensing and the subsequent innate response activation leading to strong adaptive T cell responses. Overall, these results demonstrate that a combined stimulation of the immune response, in which EP and the auxoGTUmultiSIV vaccine triggered different components of the innate immunity, led to strong and persistent cellular recall responses.

Nanoplasmid Vectors Co-expressing Innate Immune Agonists Enhance DNA Vaccines for Venezuelan Equine Encephalitis Virus and Ebola Virus

DNA vaccines expressing codon-optimized Venezuelan equine encephalitis virus (VEEV) and Ebola virus (EBOV) glycoprotein genes provide protective immunity to mice and nonhuman primates when delivered by intramuscular (IM) electroporation (EP). To achieve equivalent protective efficacy in the absence of EP, we evaluated VEEV and EBOV DNA vaccines constructed using minimalized Nanoplasmid expression vectors that are smaller than conventional plasmids used for DNA vaccination. These vectors may also be designed to co-express type I interferon inducing innate immune agonist genes that have an adjuvant effect. Nanoplasmid vaccinated mice had increased antibody responses as compared to those receiving our conventional pWRG7077-based vaccines when delivered by IM injection, and these responses were further enhanced by the inclusion of the innate immune agonist genes. The Nanoplasmid VEEV DNA vaccines also significantly increased protection against aerosol VEEV challenge as compared to the pWRG7077 VEEV DNA vaccine. Although all mice receiving the pWRG7077 and Nanoplasmid EBOV DNA vaccines at the dose tested survived EBOV challenge, only mice receiving the Nanoplasmid EBOV DNA vaccine that co-expresses the innate immune agonist genes failed to lose weight after challenge. Our results suggest that Nanoplasmid vectors can improve the immunogenicity and protective efficacy of alphavirus and filovirus DNA vaccines.

Vaccines against Ebola virus and Marburg virus: recent advances and promising candidates

The filoviruses Ebola virus and Marburg virus are among the most dangerous pathogens in the world. Both viruses cause viral hemorrhagic fever, with case fatality rates of up to 90%. Historically, filovirus outbreaks had been relatively small, with only a few hundred cases reported. However, the recent West African Ebola virus outbreak underscored the threat that filoviruses pose. The three year-long outbreak resulted in 28,646 Ebola virus infections and 11,323 deaths. The lack of Food and Drug Administration (FDA) licensed vaccines and antiviral drugs hindered early efforts to contain the outbreak. In response, the global scientific community has spurred the advanced development of many filovirus vaccine candidates. Novel vaccine platforms, such as viral vectors and DNA vaccines, have emerged, leading to the investigation of candidate vaccines that have demonstrated protective efficacy in small animal and nonhuman primate studies. Here, we will discuss several of these vaccine platforms with a particular focus on approaches that have advanced into clinical development.

The Genetic Adjuvants Interleukin-12 and Granulocyte-Macrophage Colony Stimulating Factor Enhance the Immunogenicity of an Ebola Virus Deoxyribonucleic Acid Vaccine in Mice

In previous studies, we showed that deoxyribonucleic acid (DNA) vaccines expressing codon-optimized filovirus envelope glycoprotein genes protect mice and nonhuman primates from viral challenge when delivered by intramuscular (IM) electroporation (EP). To determine whether we could achieve equivalent immunogenicity and protective efficacy by a simplified delivery method, we generated DNA vaccine plasmids expressing genetic adjuvants to potentiate immune responses. We tested the Th1-inducing cytokine interleukin-12 and the granulocyte growth factor granulocyte-macrophage colony stimulating factor, both of which have demonstrated significant adjuvant effect when included in clinical DNA vaccine formulations. In addition, because interferon (IFN)-αβ is required for DNA vaccine-induced immunity, we tested inclusion of a potent stimulator of the IFN-αβ pathway. Our data suggest that IM vaccination of mice with plasmid DNA encoding genetic adjuvants enhances vaccine immunogenicity, resulting in increased anti-Ebola virus (EBOV) immunoglobulin G and T-cell responses. Codelivery of genetic adjuvants also improved EBOV neutralizing capability compared with vaccine alone. Finally, IM vaccination with plasmid EBOV and genetic adjuvants provided complete protection against EBOV challenge. Overall, our data suggest that codelivery of genetic adjuvants with filovirus DNA vaccines using IM delivery can provide comparable efficacy to the same DNA vaccines when delivered using IM-EP devices.

In silico Designed Ebola Virus T-Cell Multi-Epitope DNA Vaccine Constructions Are Immunogenic in Mice

Background: The lack of effective vaccines against Ebola virus initiates a search for new approaches to overcoming this problem. The aim of the study was to design artificial polyepitope T-cell immunogens⁻⁻candidate DNA vaccines against Ebola virus and to evaluate their capacity to induce a specific immune response in a laboratory animal model. Method: Design of two artificial polyepitope T-cell immunogens, one of which (EV.CTL) includes cytotoxic and the other (EV.Th)⁻⁻T-helper epitopes of Ebola virus proteins was carried out using original TEpredict/PolyCTLDesigner software. Synthesized genes were cloned in pcDNA3.1 plasmid vector. Target gene expression was estimated by synthesis of specific mRNAs and proteins in cells transfected with recombinant plasmids. Immunogenicity of obtained DNA vaccine constructs was evaluated according to their capacity to induce T-cell response in BALB/c mice using IFNγ ELISpot and ICS. Results: We show that recombinant plasmids pEV.CTL and pEV.Th encoding artificial antigens provide synthesis of corresponding mRNAs and proteins in transfected cells, as well as induce specific responses both to CD4+ and CD8+ T-lymphocytes in immunized animals. Conclusions: The obtained recombinant plasmids can be regarded as promising DNA vaccine candidates in future studies of their capacity to induce cytotoxic and protective responses against Ebola virus.

A Recombinant Rabies Virus Expressing the Marburg Virus Glycoprotein Is Dependent upon Antibody-Mediated Cellular Cytotoxicity for Protection against Marburg Virus Disease in a Murine Model

Marburg virus (MARV) is a filovirus related to Ebola virus (EBOV) associated with human hemorrhagic disease. Outbreaks are sporadic and severe, with a reported case mortality rate of upward of 88%. There is currently no antiviral or vaccine available. Given the sporadic nature of outbreaks, vaccines provide the best approach for long-term control of MARV in regions of endemicity. We have developed an inactivated rabies virus-vectored MARV vaccine (FILORAB3) to protect against Marburg virus disease. Immunogenicity studies in our labs have shown that a Th1-biased seroconversion to both rabies virus and MARV glycoproteins (GPs) is beneficial for protection in a preclinical murine model. As such, we adjuvanted FILORAB3 with glucopyranosyl lipid adjuvant (GLA), a Toll-like receptor 4 agonist, in a squalene-in-water emulsion. Across two different BALB/c mouse challenge models, we achieved 92% protection against murine-adapted Marburg virus (ma-MARV). Although our vaccine elicited strong MARV GP antibodies, it did not strongly induce neutralizing antibodies. Through both in vitro and in vivo approaches, we elucidated a critical role for NK cell-dependent antibody-mediated cellular cytotoxicity (ADCC) in vaccine-induced protection. Overall, these findings demonstrate that FILORAB3 is a promising vaccine candidate for Marburg virus disease.IMPORTANCE Marburg virus (MARV) is a virus similar to Ebola virus and also causes a hemorrhagic disease which is highly lethal. In contrast to EBOV, only a few vaccines have been developed against MARV, and researchers do not understand what kind of immune responses are required to protect from MARV. Here we show that antibodies directed against MARV after application of our vaccine protect in an animal system but fail to neutralize the virus in a widely used virus neutralization assay against MARV. This newly discovered activity needs to be considered more when analyzing MARV vaccines or infections.

New Vaccine Technologies to Combat Outbreak Situations

Ever since the development of the first vaccine more than 200 years ago, vaccinations have greatly decreased the burden of infectious diseases worldwide, famously leading to the eradication of small pox and allowing the restriction of diseases such as polio, tetanus, diphtheria, and measles. A multitude of research efforts focuses on the improvement of established and the discovery of new vaccines such as the HPV (human papilloma virus) vaccine in 2006. However, radical changes in the density, age distribution and traveling habits of the population worldwide as well as the changing climate favor the emergence of old and new pathogens that bear the risk of becoming pandemic threats. In recent years, the rapid spread of severe infections such as HIV, SARS, Ebola, and Zika have highlighted the dire need for global preparedness for pandemics, which necessitates the extremely rapid development and comprehensive distribution of vaccines against potentially previously unknown pathogens. What is more, the emergence of antibiotic resistant bacteria calls for new approaches to prevent infections. Given these changes, established methods for the identification of new vaccine candidates are no longer sufficient to ensure global protection. Hence, new vaccine technologies able to achieve rapid development as well as large scale production are of pivotal importance. This review will discuss viral vector and nucleic acid-based vaccines (DNA and mRNA vaccines) as new approaches that might be able to tackle these challenges to global health.

Advances in Designing and Developing Vaccines, Drugs, and Therapies to Counter Ebola Virus

Ebola virus (EBOV), a member of the family Filoviridae, is responsible for causing Ebola virus disease (EVD) (formerly named Ebola hemorrhagic fever). This is a severe, often fatal illness with mortality rates varying from 50 to 90% in humans. Although the virus and associated disease has been recognized since 1976, it was only when the recent outbreak of EBOV in 2014-2016 highlighted the danger and global impact of this virus, necessitating the need for coming up with the effective vaccines and drugs to counter its pandemic threat. Albeit no commercial vaccine is available so far against EBOV, a few vaccine candidates are under evaluation and clinical trials to assess their prophylactic efficacy. These include recombinant viral vector (recombinant vesicular stomatitis virus vector, chimpanzee adenovirus type 3-vector, and modified vaccinia Ankara virus), Ebola virus-like particles, virus-like replicon particles, DNA, and plant-based vaccines. Due to improvement in the field of genomics and proteomics, epitope-targeted vaccines have gained top priority. Correspondingly, several therapies have also been developed, including immunoglobulins against specific viral structures small cell-penetrating antibody fragments that target intracellular EBOV proteins. Small interfering RNAs and oligomer-mediated inhibition have also been verified for EVD treatment. Other treatment options include viral entry inhibitors, transfusion of convalescent blood/serum, neutralizing antibodies, and gene expression inhibitors. Repurposed drugs, which have proven safety profiles, can be adapted after high-throughput screening for efficacy and potency for EVD treatment. Herbal and other natural products are also being explored for EVD treatment. Further studies to better understand the pathogenesis and antigenic structures of the virus can help in developing an effective vaccine and identifying appropriate antiviral targets. This review presents the recent advances in designing and developing vaccines, drugs, and therapies to counter the EBOV threat.

A prophylactic multivalent vaccine against different filovirus species is immunogenic and provides protection from lethal infections with Ebolavirus and Marburgvirus species in non-human primates

The search for a universal filovirus vaccine that provides protection against multiple filovirus species has been prompted by sporadic but highly lethal outbreaks of Ebolavirus and Marburgvirus infections. A good prophylactic vaccine should be able to provide protection to all known filovirus species and as an upside potentially protect from newly emerging virus strains. We investigated the immunogenicity and protection elicited by multivalent vaccines expressing glycoproteins (GP) from Ebola virus (EBOV), Sudan virus (SUDV), Taï Forest virus (TAFV) and Marburg virus (MARV). Immune responses against filovirus GP have been associated with protection from disease. The GP antigens were expressed by adenovirus serotypes 26 and 35 (Ad26 and Ad35) and modified Vaccinia virus Ankara (MVA) vectors, all selected for their strong immunogenicity and good safety profile. Using fully lethal NHP intramuscular challenge models, we assessed different vaccination regimens for immunogenicity and protection from filovirus disease. Heterologous multivalent Ad26-Ad35 prime-boost vaccination regimens could give full protection against MARV (range 75%-100% protection) and EBOV (range 50% to 100%) challenge, and partial protection (75%) against SUDV challenge. Heterologous multivalent Ad26-MVA prime-boost immunization gave full protection against EBOV challenge in a small cohort study. The use of such multivalent vaccines did not show overt immune interference in comparison with monovalent vaccines. Multivalent vaccines induced GP-specific antibody responses and cellular IFNγ responses to each GP expressed by the vaccine, and cross-reactivity to TAFV GP was detected in a trivalent vaccine expressing GP from EBOV, SUDV and MARV. In the EBOV challenge studies, higher humoral EBOV GP-specific immune responses (p = 0.0004) were associated with survival from EBOV challenge and less so for cellular immune responses (p = 0.0320). These results demonstrate that it is feasible to generate a multivalent filovirus vaccine that can protect against lethal infection by multiple members of the filovirus family.

Advancements in DNA vaccine vectors, non-mechanical delivery methods, and molecular adjuvants to increase immunogenicity

A major advantage of DNA vaccination is the ability to induce both humoral and cellular immune responses. DNA vaccines are currently used in veterinary medicine, but have not achieved widespread acceptance for use in humans due to their low immunogenicity in early clinical studies. However, recent clinical data have re-established the value of DNA vaccines, particularly in priming high-level antigen-specific antibody responses. Several approaches have been investigated for improving DNA vaccine efficacy, including advancements in DNA vaccine vector design, the inclusion of genetically engineered cytokine adjuvants, and novel non-mechanical delivery methods. These strategies have shown promise, resulting in augmented adaptive immune responses in not only mice, but also in large animal models. Here, we review advancements in each of these areas that show promise for increasing the immunogenicity of DNA vaccines.

DNA vaccines elicit durable protective immunity against individual or simultaneous infections with Lassa and Ebola viruses in guinea pigs

We previously developed optimized DNA vaccines against both Lassa fever and Ebola hemorrhagic fever viruses and demonstrated that they were protective individually in guinea pig and nonhuman primate models. In this study, we vaccinated groups of strain 13 guinea pigs two times, four weeks apart with 50 µg of each DNA vaccine or a mock vaccine at discrete sites by intradermal electroporation. Five weeks following the second vaccinations, guinea pigs were exposed to lethal doses of Lassa virus, Ebola virus, or a combination of both viruses simultaneously. None of the vaccinated guinea pigs, regardless of challenge virus and including the coinfected group, displayed weight loss, fever or other disease signs, and all survived to the study endpoint. All of the mock-vaccinated guinea pigs that were infected with Lassa virus, and all but one of the EBOV-infected mock-vaccinated guinea pigs succumbed. In order to determine if the dual-agent vaccination strategy could protect against both viruses if exposures were temporally separated, we held the surviving vaccinates in BSL-4 for approximately 120 days to perform a cross-challenge experiment in which guinea pigs originally infected with Lassa virus received a lethal dose of Ebola virus and those originally infected with Ebola virus were infected with a lethal dose of Lassa virus. All guinea pigs remained healthy and survived to the study endpoint. This study clearly demonstrates that DNA vaccines against Lassa and Ebola viruses can elicit protective immunity against both individual virus exposures as well as in a mixed-infection environment.

Combinatorial peptide-based epitope mapping from Ebola virus DNA vaccines and infections reveals residue-level determinants of antibody binding

Ebola virus (EBOV) infection is highly lethal and results in severe febrile bleeding disorders that affect humans and non-human primates. One of the therapeutic approaches for treating EBOV infection focus largely on cocktails of monoclonal antibodies (mAbs) that bind to specific regions of the EBOV glycoprotein (GP) and neutralize the virus. Recent structural studies using cryo-electron microscopy have identified key epitopes for several EBOV mAbs. While such information has yielded deep insights into antibody binding, limitations on resolution of these structures often preclude a residue-level analysis of EBOV epitopes. In this study, we performed combinatorial peptide-based epitope mapping of EBOV GP against a broad panel of mAbs and polyclonal sera derived from several animal species vaccinated with EBOV DNA and replicon vaccines and/or exposed to EBOV infection to identify residue-level determinants of antibody binding. The peptide-based epitope mapping obtained from a wide range of serum and mAb samples, combined with available cryo-EM structure reconstructions revealed fine details of antibody-virus interactions, allowing for a more precise and comprehensive mapping of antibody epitopes on EBOV GP. We show how these residue-level epitope definitions can be used to characterize antigenic variation across different filoviruses, and provide a theoretical basis for predicting immunity and cross-neutralization in potential future outbreaks.

Ebola and Marburg virus vaccines

The filoviruses, Ebola virus (EBOV), and Marburg virus (MARV), are among the most pathogenic viruses known to man and the causative agents of viral hemorrhagic fever outbreaks in Africa with case fatality rates of up to 90%. Nearly 30,000 infections were observed in the latest EBOV epidemic in West Africa; previous outbreaks were much smaller, typically only affecting less than a few hundred people. Compared to other diseases such as AIDS or Malaria with millions of cases annually, filovirus hemorrhagic fever (FHF) is one of the neglected infectious diseases. There are no licensed vaccines or therapeutics available to treat EBOV and MARV infections; therefore, these pathogens can only be handled in maximum containment laboratories and are classified as select agents. Under these limitations, a very few laboratories worldwide conducted basic research and countermeasure development for EBOV and MARV since their respective discoveries in 1967 (MARV) and 1976 (EBOV). In this review, we discuss several vaccine platforms against EBOV and MARV, which have been assessed for their protective efficacy in animal models of FHF. The focus is on the most promising approaches, which were accelerated in clinical development (phase I-III trials) during the EBOV epidemic in West Africa.

Assessing the safety and immunogenicity of recombinant vesicular stomatitis virus Ebola vaccine in healthy adults: a randomized clinical trial

BACKGROUND

The 2013-2016 Ebola virus outbreak in West Africa was the most widespread in history. In response, alive attenuated recombinant vesicular stomatitis virus (rVSV) vaccine expressing Zaire Ebolavirus glycoprotein (rVSVΔG-ZEBOV-GP) was evaluated in humans.

METHODS

In a phase 1, randomized, dose-ranging, observer-blind, placebo-controlled trial, healthy adults aged 18-65 years were randomized into 4 groups of 10 to receive one of 3 vaccine doses or placebo. Follow-up visits spanned 180 days postvaccination for safety monitoring, immunogenicity testing and any rVSV virus shedding.

RESULTS

Forty participants were injected with rVSVΔG-ZEBOV-GP vaccine (n = 30) or saline placebo (n = 10). No serious adverse events related to the vaccine or participant withdrawals were reported. Solicited adverse events during the 14-day follow-up period were mild to moderate and self-limited, with the exception of injection-site pain and headache. Viremia following vaccination was transient and no longer detectable after study day 3, with no virus shedding in saliva or urine. All vaccinated participants developed serum immunoglobulin G (IgG), as measured by Ebola virus envelope glycoprotein-based enzyme-linked immunosorbent assay (ELISA). Immunogenicity was comparable across all dose groups, and sustained IgG titers were detectable through to the last visit, at study day 180.

INTERPRETATION

In this phase 1 study, there were no safety concerns after a single dose of rVSVΔG-ZEBOV-GP vaccine. IgG ELISA showed persistent high titers at 180 days postimmunization. There was a period of reactogenicity, but in general, the vaccine was well tolerated. This study provides evidence of the safety and immunogenicity of rVSVΔG-ZEBOV-GP vaccine and importance of its further investigation. Trial registration: Clinical-Trials.gov no., NCT02374385.

Epitope mapping of Ebola virus dominant and subdominant glycoprotein epitopes facilitates construction of an epitope-based DNA vaccine able to focus the antibody response in mice

We performed epitope mapping studies on the major surface glycoprotein (GP) of Ebola virus (EBOV) using Chemically Linked Peptides on Scaffolds (CLIPS), which form linear and potential conformational epitopes. This method identified monoclonal antibody epitopes and predicted additional epitopes recognized by antibodies in polyclonal sera from animals experimentally vaccinated against or infected with EBOV. Using the information obtained along with structural modeling to predict epitope accessibility, we then constructed 2 DNA vaccines encoding immunodominant and subdominant epitopes predicted to be accessible on EBOV GP. Although a construct designed to produce a membrane-bound oligopeptide was poorly immunogenic, a construct generating a secreted oligopeptide elicited strong antibody responses in mice. When this construct was administered as a boost to a DNA vaccine expressing the complete EBOV GP gene, the resultant antibody response was focused largely toward the less immunodominant epitopes in the oligopeptide. Taken together, the results of this work suggest a utility for this method for immune focusing of antibody responses elicited by vaccination.

Advancements in DNA vaccine vectors, non-mechanical delivery methods, and molecular adjuvants to increase immunogenicity

A major advantage of DNA vaccination is the ability to induce both humoral and cellular immune responses. DNA vaccines are currently used in veterinary medicine, but have not achieved widespread acceptance for use in humans due to their low immunogenicity in early clinical studies. However, recent clinical data have re-established the value of DNA vaccines, particularly in priming high-level antigen-specific antibody responses. Several approaches have been investigated for improving DNA vaccine efficacy, including advancements in DNA vaccine vector design, the inclusion of genetically engineered cytokine adjuvants, and novel non-mechanical delivery methods. These strategies have shown promise, resulting in augmented adaptive immune responses in not only mice, but also in large animal models. Here, we review advancements in each of these areas that show promise for increasing the immunogenicity of DNA vaccines.

An immunoinformatics-derived DNA vaccine encoding human class II T cell epitopes of Ebola virus, Sudan virus, and Venezuelan equine encephalitis virus is immunogenic in HLA transgenic mice

Immunoinformatics tools were used to predict human leukocyte antigen (HLA) class II-restricted T cell epitopes within the envelope glycoproteins and nucleocapsid proteins of Ebola virus (EBOV) and Sudan virus (SUDV) and the structural proteins of Venezuelan equine encephalitis virus (VEEV). Selected epitopes were tested for binding to soluble HLA molecules representing 5 class II alleles (DRB1*0101, DRB1*0301, DRB1*0401, DRB1*0701, and DRB1*1501). All but one of the 25 tested peptides bound to at least one of the DRB1 alleles, and 4 of the peptides bound at least moderately or weakly to all 5 DRB1 alleles. Additional algorithms were used to design a single "string-of-beads" expression construct with 44 selected epitopes arranged to avoid creation of spurious junctional epitopes. Seventeen of these 44 predicted epitopes were conserved between the major histocompatibility complex (MHC) of humans and mice, allowing initial testing in mice. BALB/c mice vaccinated with the multi-epitope construct developed statistically significant cellular immune responses to EBOV, SUDV, and VEEV peptides as measured by interferon (IFN)-γ ELISpot assays. Significant levels of antibodies to VEEV, but not EBOV, were also detected in vaccinated BALB/c mice. To assess immunogenicity in the context of a human MHC, HLA-DR3 transgenic mice were vaccinated with the multi-epitope construct and boosted with a mixture of the 25 peptides used in the binding assays. The vaccinated HLA-DR3 mice developed significant cellular immune responses to 4 of the 25 (16%) tested individual class II peptides as measured by IFN-γ ELISpot assays. In addition, these mice developed antibodies against EBOV and VEEV as measured by ELISA. While a low but significant level of protection was observed in vaccinated transgenic mice after aerosol exposure to VEEV, no protection was observed after intraperitoneal challenge with mouse-adapted EBOV. These studies provide proof of concept for the use of an informatics approach to design a multi-agent, multi-epitope immunogen and provide a basis for further testing aimed at focusing immune responses toward desired protective T cell epitopes.

Ebola vaccines in clinical trial: The promising candidates

Ebola virus disease (EVD) has become a great threat to humans across the world in recent years. The 2014 Ebola epidemic in West Africa caused numerous deaths and attracted worldwide attentions. Since no specific drugs and treatments against EVD was available, vaccination was considered as the most promising and effective method of controlling this epidemic. So far, 7 vaccine candidates had been developed and evaluated through clinical trials. Among them, the recombinant vesicular stomatitis virus-based vaccine (rVSV-EBOV) is the most promising candidate, which demonstrated a significant protection against EVD in phase III clinical trial. However, several concerns were still associated with the Ebola vaccine candidates, including the safety profile in some particular populations, the immunization schedule for emergency vaccination, and the persistence of the protection. We retrospectively reviewed the current development of Ebola vaccines and discussed issues and challenges remaining to be investigated in the future.

Neglected filoviruses

Eight viruses are currently assigned to the family Filoviridae Marburg virus, Sudan virus and, in particular, Ebola virus have received the most attention both by researchers and the public from 1967 to 2013. During this period, natural human filovirus disease outbreaks occurred sporadically in Equatorial Africa and, despite high case-fatality rates, never included more than several dozen to a few hundred infections per outbreak. Research emphasis shifted almost exclusively to Ebola virus in 2014, when this virus was identified as the cause of an outbreak that has thus far involved more than 28 646 people and caused more than 11 323 deaths in Western Africa. Consequently, major efforts are currently underway to develop licensed medical countermeasures against Ebola virus infection. However, the ecology of and mechanisms behind Ebola virus emergence are as little understood as they are for all other filoviruses. Consequently, the possibility of the future occurrence of a large disease outbreak caused by other less characterized filoviruses (i.e. Bundibugyo virus, Lloviu virus, Ravn virus, Reston virus and Taï Forest virus) is impossible to rule out. Yet, for many of these viruses, not even rudimentary research tools are available, let alone medical countermeasures. This review summarizes the current knowledge on these less well-characterized filoviruses.

Codon-optimized filovirus DNA vaccines delivered by intramuscular electroporation protect cynomolgus macaques from lethal Ebola and Marburg virus challenges

Cynomolgus macaques were vaccinated by intramuscular electroporation with DNA plasmids expressing codon-optimized glycoprotein (GP) genes of Ebola virus (EBOV) or Marburg virus (MARV) or a combination of codon-optimized GP DNA vaccines for EBOV, MARV, Sudan virus and Ravn virus. When measured by ELISA, the individual vaccines elicited slightly higher IgG responses to EBOV or MARV than did the combination vaccines. No significant differences in immune responses of macaques given the individual or combination vaccines were measured by pseudovirion neutralization or IFN-γ ELISpot assays. Both the MARV and mixed vaccines were able to protect macaques from lethal MARV challenge (5/6 vs. 6/6). In contrast, a greater proportion of macaques vaccinated with the EBOV vaccine survived lethal EBOV challenge in comparison to those that received the mixed vaccine (5/6 vs. 1/6). EBOV challenge survivors had significantly higher pre-challenge neutralizing antibody titers than those that succumbed.

Preclinical Development of Inactivated Rabies Virus-Based Polyvalent Vaccine Against Rabies and Filoviruses

We previously described the generation of a novel Ebola virus (EBOV) vaccine based on inactivated rabies virus (RABV) containing EBOV glycoprotein (GP) incorporated in the RABV virion. Our results demonstrated safety, immunogenicity, and protective efficacy in mice and nonhuman primates (NHPs). Protection against viral challenge depended largely on the quality of the humoral immune response against EBOV GP.Here we present the extension and improvement of this vaccine by increasing the amount of GP incorporation into virions via GP codon-optimization as well as the addition of Sudan virus (SUDV) and Marburg virus (MARV) GP containing virions. Immunogenicity studies in mice indicate similar immune responses for both SUDV GP and MARV GP compared to EBOV GP. Immunizing mice with multiple antigens resulted in immune responses similar to immunization with a single antigen. Moreover, immunization of NHP with the new inactivated RABV EBOV vaccine resulted in high titer neutralizing antibody levels and 100% protection against lethal EBOV challenge when applied with adjuvant.Our results indicate that an inactivated polyvalent vaccine against RABV filoviruses is achievable. Finally, the novel vaccines are produced on approved VERO cells and a clinical grade RABV/EBOV vaccine for human trials has been produced.

Human Polyclonal Antibodies Produced through DNA Vaccination of Transchromosomal Cattle Provide Mice with Post-Exposure Protection against Lethal Zaire and Sudan Ebolaviruses

DNA vaccination of transchromosomal bovines (TcBs) with DNA vaccines expressing the codon-optimized (co) glycoprotein (GP) genes of Ebola virus (EBOV) and Sudan virus (SUDV) produce fully human polyclonal antibodies (pAbs) that recognize both viruses and demonstrate robust neutralizing activity. Each TcB was vaccinated by intramuscular electroporation (IM-EP) a total of four times and at each administration received 10 mg of the EBOV-GP_co DNA vaccine and 10 mg of the SUDV-GP_co DNA vaccine at two sites on the left and right sides, respectively. After two vaccinations, robust antibody responses (titers > 1000) were detected by ELISA against whole irradiated EBOV or SUDV and recombinant EBOV-GP or SUDV-GP (rGP) antigens, with higher titers observed for the rGP antigens. Strong, virus neutralizing antibody responses (titers >1000) were detected after three vaccinations when measured by vesicular stomatitis virus-based pseudovirion neutralization assay (PsVNA). Maximal neutralizing antibody responses were identified by traditional plaque reduction neutralization tests (PRNT) after four vaccinations. Neutralizing activity of human immunoglobulins (IgG) purified from TcB plasma collected after three vaccinations and injected intraperitoneally (IP) into mice at a 100 mg/kg dose was detected in the serum by PsVNA up to 14 days after administration. Passive transfer by IP injection of the purified IgG (100 mg/kg) to groups of BALB/c mice one day after IP challenge with mouse adapted (ma) EBOV resulted in 80% protection while all mice treated with non-specific pAbs succumbed. Similarly, interferon receptor 1 knockout (IFNAR ^-/-) mice receiving the purified IgG (100 mg/kg) by IP injection one day after IP challenge with wild type SUDV resulted in 89% survival. These results are the first to demonstrate that filovirus GP DNA vaccines administered to TcBs by IM-EP can elicit neutralizing antibodies that provide post-exposure protection. Additionally, these data describe production of fully human IgG in a large animal system, a system which is capable of producing large quantities of a clinical grade therapeutic product.

Cross-protection conferred by filovirus virus-like particles containing trimeric hybrid glycoprotein

Filoviruses are causative agents of hemorrhagic fever, and to date no effective vaccine or therapeutic has been approved to combat infection. Filovirus glycoprotein (GP) is the critical immunogenic component of filovirus vaccines, eliciting high levels of antibody after successful vaccination. Previous work has shown that protection against both Ebola virus (EBOV) and Marburg virus (MARV) can be achieved by vaccinating with a mixture of virus-like particles (VLPs) expressing either EBOV GP or MARV GP. In this study, the potential for eliciting effective immune responses against EBOV, Sudan virus, and MARV with a single GP construct was tested. Trimeric hybrid GPs were produced that expressed the sequence of Marburg GP2 in conjunction with a hybrid GP1 composed EBOV and Sudan virus GP sequences. VLPs expressing these constructs, along with EBOV VP40, provided comparable protection against MARV challenge, resulting in 75 or 100% protection. Protection from EBOV challenge differed depending upon the hybrid used, however, with one conferring 75% protection and one conferring no protection. By comparing the overall antibody titers and the neutralizing antibody titers specific for each virus, it is shown that higher antibody responses were elicited by the C terminal region of GP1 than by the N terminal region, and this correlated with protection. These data collectively suggest that GP2 and the C terminal region of GP1 are highly immunogenic, and they advance progress toward the development of a pan-filovirus vaccine.

Electroporation-enhanced delivery of nucleic acid vaccines

The naked delivery of nucleic acid vaccines is notoriously inefficient, and an enabling delivery technology is required to direct efficiently these constructs intracellularly. A delivery technology capable of enhancing nucleic acid uptake in both cells in tissues and in culture is electroporation (EP). EP is a physical delivery mechanism that increases the permeability of mammalian cell membranes and allows the trafficking of large macromolecules into the cell. EP has now been used extensively in the clinic and been shown to be an effective method to increase both the uptake of the construct and the breadth and magnitude of the resulting immune responses. Excitingly, 2014 saw the announcement of the first EP-enhanced DNA vaccine Phase II trial demonstrating clinical efficacy. This review seeks to introduce the reader to EP as a technology to enhance the delivery of DNA and RNA vaccines and highlight several published clinical trials using this delivery modality.

Ebola virus vaccines: an overview of current approaches

Ebola hemorrhagic fever is one of the most fatal viral diseases worldwide affecting humans and nonhuman primates. Although infections only occur frequently in Central Africa, the virus has the potential to spread globally and is classified as a category A pathogen that could be misused as a bioterrorism agent. As of today there is no vaccine or treatment licensed to counteract Ebola virus infections. DNA, subunit and several viral vector approaches, replicating and non-replicating, have been tested as potential vaccine platforms and their protective efficacy has been evaluated in nonhuman primate models for Ebola virus infections, which closely resemble disease progression in humans. Though these vaccine platforms seem to confer protection through different mechanisms, several of them are efficacious against lethal disease in nonhuman primates attesting that vaccination against Ebola virus infections is feasible.

Induction of broad cytotoxic T cells by protective DNA vaccination against Marburg and Ebola

Marburg and Ebola hemorrhagic fevers have been described as the most virulent viral diseases known to man due to associative lethality rates of up to 90%. Death can occur within days to weeks of exposure and there is currently no licensed vaccine or therapeutic. Recent evidence suggests an important role for antiviral T cells in conferring protection, but little detailed analysis of this response as driven by a protective vaccine has been reported. We developed a synthetic polyvalent-filovirus DNA vaccine against Marburg marburgvirus (MARV), Zaire ebolavirus (ZEBOV), and Sudan ebolavirus (SUDV). Preclinical efficacy studies were performed in guinea pigs and mice using rodent-adapted viruses, whereas murine T-cell responses were extensively analyzed using a novel modified assay described herein. Vaccination was highly potent, elicited robust neutralizing antibodies, and completely protected against MARV and ZEBOV challenge. Comprehensive T-cell analysis revealed cytotoxic T lymphocytes (CTLs) of great magnitude, epitopic breadth, and Th1-type marker expression. This model provides an important preclinical tool for studying protective immune correlates that could be applied to existing platforms. Data herein support further evaluation of this enhanced gene-based approach in nonhuman primate studies for in depth analyses of T-cell epitopes in understanding protective efficacy.

Forty-five years of Marburg virus research

In 1967, the first reported filovirus hemorrhagic fever outbreak took place in Germany and the former Yugoslavia. The causative agent that was identified during this outbreak, Marburg virus, is one of the most deadly human pathogens. This article provides a comprehensive overview of our current knowledge about Marburg virus disease ranging from ecology to pathogenesis and molecular biology.