Chimpanzee adenovirus vaccine protects against Zaire Ebola virus

History and impact of the mouse-adapted Ebola virus model

Ebola virus (EBOV) is a member of the filoviridae family, which are comprised of negative sense, enveloped RNA hemorrhagic fever viruses that can cause severe disease and high lethality rates. These viruses require BSL-4 containment laboratories for study. Early studies of EBOV pathogenesis relied heavily on the use of nonhuman primates, which are expensive and cumbersome to handle in large numbers. Guinea pig models were also developed, but even to this day limited reagents are available in this model. In 1998, Mike Bray and colleagues developed a mouse-adapted EBOV (maEBOV) that caused lethality in adult immunocompetent mice. This model had significant advantages, including being inexpensive, allowing for higher animal numbers for statistical analysis, availability of reagents for studying pathogenesis, and availability of a vast array of genetically modified strains. The model has been used to test vaccines, therapeutic drugs, EBOV mutants, and pathogenesis, and its importance is demonstrated by the hundreds of citations referencing the original publication. This review will cover the history of the maEBOV model and its use in filovirus research.

In vivo editing of the pan-endothelium by immunity evading simian adenoviral vector

Biological applications deriving from the clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9 site-specific nuclease system continue to impact and accelerate gene therapy strategies. Safe and effective in vivo co-delivery of the CRISPR/Cas9 system to target somatic cells is essential in the clinical therapeutic context. Both non-viral and viral vector systems have been applied for this delivery matter. Despite elegant proof-of-principle studies, available vector technologies still face challenges that restrict the application of CRISPR/Cas9-facilitated gene therapy. Of note, the mandated co-delivery of the gene-editing components must be accomplished in the potential presence of pre-formed anti-vector immunity. Additionally, methods must be sought to limit the potential of off-target editing. To this end, we have exploited the molecular promiscuities of adenovirus (Ad) to address the key requirements of CRISPR/Cas9-facilitated gene therapy. In this regard, we have endeavored capsid engineering of a simian (chimpanzee) adenovirus isolate 36 (SAd36) to achieve targeted modifications of vector tropism. The SAd36 vector with the myeloid cell-binding peptide (MBP) incorporated in the capsid has allowed selective in vivo modifications of the vascular endothelium. Importantly, vascular endothelium can serve as an effective non-hepatic cellular source of deficient serum factors relevant to several inherited genetic disorders. In addition to allowing for re-directed tropism, capsid engineering of nonhuman primate Ads provide the means to circumvent pre-formed vector immunity. Herein we have generated a SAd36. MBP vector that can serve as a single intravenously administered agent allowing effective and selective in vivo editing for endothelial target cells of the mouse spleen, brain and kidney. DATA AVAILABILITY: The data that support the findings of this study are available from the corresponding author upon reasonable request.

Adapting Simon's Two-Stage Design for Efficient Screening of Filovirus Vaccines in Non-Human Primates

The cynomolgus monkey (Macaca fascicularis) non-human primate (NHP) is widely used for filovirus vaccine testing. To use limited BSL-4 resources efficiently and minimize NHP usage, Simon's two-stage design was adapted to screen candidate Ebola virus (EBOV) vaccines in up to six NHPs with two (optimal), three, or four NHPs in Stage 1. Using the optimal design, two NHPs were tested in Stage 1. If neither survived, the candidate was rejected. Otherwise, it was eligible for Stage 2 testing in four NHPs. Candidates advanced if four or more NHPs were protected over both stages. An 80% efficacious candidate vaccine had 88.5% probability of advancing, and a 40% efficacious candidate vaccine had 83% probability of rejection. Simon's two-stage design was used to screen 27 EBOV vaccine candidates in 43 candidate regimens that varied in dose, adjuvant, formulation, or schedule. Of the 30 candidate regimens tested using two NHPs in Stage 1, 15 were rejected, nine were withdrawn, and six were tested in Stage 2. All six tested in Stage 2 qualified to advance in the product development pipeline. Multiple regimens for the EBOV vaccines approved by the European Medicines Agency (EMA) and the US Food and Drug Administration (FDA) in 2019 were tested in this program. This approach may also prove useful for screening Sudan virus (SUDV) and Marburg virus (MARV) vaccine candidates.

Review: Development of SARS-CoV-2 immuno-enhanced COVID-19 vaccines with nano-platform

Vaccination is the most effective way to prevent coronavirus disease 2019 (COVID-19). Vaccine development approaches consist of viral vector vaccines, DNA vaccine, RNA vaccine, live attenuated virus, and recombinant proteins, which elicit a specific immune response. The use of nanoparticles displaying antigen is one of the alternative approaches to conventional vaccines. This is due to the fact that nano-based vaccines are stable, able to target, form images, and offer an opportunity to enhance the immune responses. The diameters of ultrafine nanoparticles are in the range of 1-100 nm. The application of nanotechnology on vaccine design provides precise fabrication of nanomaterials with desirable properties and ability to eliminate undesirable features. To be successful, nanomaterials must be uptaken into the cell, especially into the target and able to modulate cellular functions at the subcellular levels. The advantages of nano-based vaccines are the ability to protect a cargo such as RNA, DNA, protein, or synthesis substance and have enhanced stability in a broad range of pH, ambient temperatures, and humidity for long-term storage. Moreover, nano-based vaccines can be engineered to overcome biological barriers such as nonspecific distribution in order to elicit functions in antigen presenting cells. In this review, we will summarize on the developing COVID-19 vaccine strategies and how the nanotechnology can enhance antigen presentation and strong immunogenicity using advanced technology in nanocarrier to deliver antigens. The discussion about their safe, effective, and affordable vaccines to immunize against COVID-19 will be highlighted.

Understanding Post Entry Sorting of Adenovirus Capsids; A Chance to Change Vaccine Vector Properties

Adenovirus vector-based genetic vaccines have emerged as a powerful strategy against the SARS-CoV-2 health crisis. This success is not unexpected because adenoviruses combine many desirable features of a genetic vaccine. They are highly immunogenic and have a low and well characterized pathogenic profile paired with technological approachability. Ongoing efforts to improve adenovirus-vaccine vectors include the use of rare serotypes and non-human adenoviruses. In this review, we focus on the viral capsid and how the choice of genotypes influences the uptake and subsequent subcellular sorting. We describe how understanding capsid properties, such as stability during the entry process, can change the fate of the entering particles and how this translates into differences in immunity outcomes. We discuss in detail how mutating the membrane lytic capsid protein VI affects species C viruses' post-entry sorting and briefly discuss if such approaches could have a wider implication in vaccine and/or vector development.

The Ebola virus glycoprotein and its immune responses across multiple vaccine platforms

Introduction: For over 40 years, ebolaviruses have been responsible for sporadic outbreaks of severe and often fatal hemorrhagic fever in humans and nonhuman primates across western and central Africa. In December 2013, an unprecedented Ebola virus (EBOV) epidemic began in West Africa and resulted in the largest outbreak to date. The past and current epidemics in West Africa and the Democratic Republic of the Congo has focused attention on the potential vaccine platforms developed over the past 20 years.Areas covered: This review summarizes the extraordinary progress using a variety of vaccination platforms including DNA, subunit, and several viral vector approaches, replicating and non-replicating, incorporating the primary antigen of EBOV, the glycoprotein. These vaccine constructs have shown varying degrees of protective efficacy in the 'gold-standard' nonhuman primate model for EBOV infections and were immunogenic in human clinical trials.Expert commentary: A number of these vaccine platforms have moved into phase III clinical trials over the past years and with the recent approval of the first EBOV vaccine in the European Union and the USA there is a strong potential to prevent future outbreaks/epidemics of EBOV infections on the scale of the West African epidemic.

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.

Hantavirus entry: Perspectives and recent advances

Hantaviruses are important zoonotic pathogens of public health importance that are found on all continents except Antarctica and are associated with hemorrhagic fever with renal syndrome (HFRS) in the Old World and hantavirus pulmonary syndrome (HPS) in the New World. Despite the significant disease burden they cause, no FDA-approved specific therapeutics or vaccines exist against these lethal viruses. The lack of available interventions is largely due to an incomplete understanding of hantavirus pathogenesis and molecular mechanisms of virus replication, including cellular entry. Hantavirus Gn/Gc glycoproteins are the only viral proteins exposed on the surface of virions and are necessary and sufficient to orchestrate virus attachment and entry. In vitro studies have implicated integrins (β_1-3), DAF/CD55, and gC1qR as candidate receptors that mediate viral attachment for both Old World and New World hantaviruses. Recently, protocadherin-1 (PCDH1) was demonstrated as a requirement for cellular attachment and entry of New World hantaviruses in vitro and lethal HPS in vivo, making it the first clade-specific host factor to be identified. Attachment of hantavirus particles to cellular receptors induces their internalization by clathrin-mediated, dynamin-independent, or macropinocytosis-like mechanisms, followed by particle trafficking to an endosomal compartment where the fusion of viral and endosomal membranes can occur. Following membrane fusion, which requires cholesterol and acid pH, viral nucleocapsids escape into the cytoplasm and launch genome replication. In this review, we discuss the current mechanistic understanding of hantavirus entry, highlight gaps in our existing knowledge, and suggest areas for future inquiry.

Chimpanzee adenoviral vector prime-boost regimen elicits potent immune responses against Ebola virus in mice and rhesus macaques

In the last few decades, Ebola virus (EBOV) has emerged periodically and infected people in Africa, resulting in an extremely high mortality rate. With no available prophylaxis or cure so far, a highly effective Ebola vaccine is urgently needed. In this study, we developed a novel chimpanzee adenovirus-based prime-boost vaccine by exploiting two recombinant replication-deficient chimpanzee adenoviral vectors, AdC7 and AdC68, which express glycoproteins (GP) of the EBOV strain identified in the 2014 outbreak. Our results indicated that a single immunization using AdC7 or AdC68 could stimulate potent EBOV-specific antibody responses, whereas the AdC7 prime-AdC68 boost regimen induced much stronger and sustained humoral and cellular immune responses in both mice and rhesus monkeys, compared with AdC7 or AdC68 single vaccination or the AdC68 prime-AdC7 boost regimen. This prime-boost vaccine could also protect mice from the simulated infection with EBOV-like particle (EBOVLP) in biosafety level 2 (BSL-2) laboratories, and antibodies from the prime-boost immunized rhesus macaques could passively provide protection against EBOVLP infection. Altogether, our results show that the AdC7 prime-AdC68 boost vaccine is a promising candidate for further development to combat EBOV infections.

A novel oncolytic adenovirus based on simian adenovirus serotype 24

Among the oncolytic virotherapy, an emerging treatment for tumor, adenoviruses are widely used at present in preclinical and clinical trials. Traditionally, oncolytic adenoviruses were developed based on the human adenovirus serotype 5 (AdHu5). However, AdHu5 has the drawbacks of preexisting anti-AdHu5 immunity in most populations, and extensive sequestration of Adhu5 by the liver through hexon, blood coagulation factor X (FX), and FX receptor interactions. To tackle these problems, we explored a novel oncolytic adenovirus AdC7-SP/E1A-ΔE3 for cancer treatment. AdC7-SP/E1A-ΔE3 was constructed by replacing the E1A promoter with tumor specific promoter survivin promoter and deleting E3 region using direct cloning methods based on simian adenovirus serotype 24 (namely AdC7). We showed that AdC7-SP/E1A-ΔE3 significantly killed tumor cell lines NCI-H508 and Huh7, and inhibited tumor growth in both NCI-H508 and Huh7 xenograft tumor models. Importantly, AdC7-SP/E1A-ΔE3 exhibited the antitumor efficacy via systemic administration. Mechanistically, infected cells were killed by AdC7-SP/E1A-ΔE3 via the p53-independent mitochondrial apoptosis pathway in which phosphorylation of BAD markedly declined and the expresses of Bik significantly went up. Therefore, AdC7-SP/E1A-ΔE3 is a promising candidate for liver and colon tumor treatment.

Recombinant Chimpanzee Adenovirus Vaccine AdC7-M/E Protects against Zika Virus Infection and Testis Damage

The recent outbreak of Zika virus (ZIKV) has emerged as a global health concern. ZIKV can persist in human semen and be transmitted by sexual contact, as well as by mosquitoes, as seen for classical arboviruses. We along with others have previously demonstrated that ZIKV infection leads to testis damage and infertility in mouse models. So far, no prophylactics or therapeutics are available; therefore, vaccine development is urgently demanded. Recombinant chimpanzee adenovirus has been explored as the preferred vaccine vector for many pathogens due to the low preexisting immunity against the vector among the human population. Here, we developed a ZIKV vaccine based on recombinant chimpanzee adenovirus type 7 (AdC7) expressing ZIKV M/E glycoproteins. A single vaccination of AdC7-M/E was sufficient to elicit potent neutralizing antibodies and protective immunity against ZIKV in both immunocompetent and immunodeficient mice. Moreover, vaccinated mice rapidly developed neutralizing antibody with high titers within 1 week postvaccination, and the elicited antiserum could cross-neutralize heterologous ZIKV strains. Additionally, ZIKV M- and E-specific T cell responses were robustly induced by AdC7-M/E. Moreover, one-dose inoculation of AdC7-M/E conferred mouse sterilizing immunity to eliminate viremia and viral burden in tissues against ZIKV challenge. Further investigations showed that vaccination with AdC7-M/E completely protected against ZIKV-induced testicular damage. These data demonstrate that AdC7-M/E is highly effective and represents a promising vaccine candidate for ZIKV control.IMPORTANCE Zika virus (ZIKV) is a pathogenic flavivirus that causes severe clinical consequences, including congenital malformations in fetuses and Guillain-Barré syndrome in adults. Vaccine development is a high priority for ZIKV control. In this study, to avoid preexisting anti-vector immunity in humans, a rare serotype chimpanzee adenovirus (AdC7) expressing the ZIKV M/E glycoproteins was used for ZIKV vaccine development. Impressively, AdC7-M/E exhibited exceptional performance as a ZIKV vaccine, as follows: (i) protective efficacy by a single vaccination, (ii) rapid development of a robust humoral response, (iii) durable immune responses, (iv) robust T cell responses, and (v) sterilizing immunity achieved by a single vaccination. These advantages of AdC7-M/E strongly support its potential application as a promising ZIKV vaccine in the clinic.

From bench to almost bedside: the long road to a licensed Ebola virus vaccine

INTRODUCTION

The Ebola virus (EBOV) disease epidemic during 2014-16 in West Africa has accelerated the clinical development of several vaccine candidates that have demonstrated efficacy in the gold standard nonhuman primate (NHP) model, namely cynomolgus macaques.

AREAS COVERED

This review discusses the pre-clinical research and if available, clinical evaluation of the currently available EBOV vaccine candidates, while emphasizing the translatability of pre-clinical data generated in the NHP model to clinical data in humans.

EXPERT OPINION

Despite the existence of many successful EBOV vaccine candidates in the pre-clinical stages, only two platforms became the focus of Phase 2/3 efficacy trials in Liberia, Sierra Leone, and Guinea near the peak of the epidemic: the Vesicular stomatitis virus (VSV)-vectored vaccine and the chimpanzee adenovirus type 3 (ChAd3)-vectored vaccine. The results of three distinct clinical trials involving these candidates may soon pave the way for a licensed, safe and efficacious EBOV vaccine to help combat future epidemics.

Protected to death: systematic exclusion of pregnant women from Ebola virus disease trials

BACKGROUND

For 30 years, women have sought equal opportunity to be included in trials so that drugs are equitably studied in women as well as men; regulatory guidelines have changed accordingly. Pregnant women, however, continue to be excluded from trials for non-obstetric conditions, though they have been included for trials of life-threatening diseases because prospects for maternal survival outweighed potential fetal risks. Ebola virus disease is a life-threatening infection without approved treatments or vaccines. Previous Ebola virus (EBOV) outbreak data showed 89-93% maternal and 100% fetal/neonatal mortality. Early in the 2013-2016 EBOV epidemic, an expert panel pointed to these high mortality rates and the need to prioritize and preferentially allocate unregistered interventions in favor of pregnant women (and children). Despite these recommendations and multiple ethics committee requests for their inclusion on grounds of justice, equity, and medical need, pregnant women were excluded from all drug and vaccine trials in the affected countries, either without justification or on grounds of potential fetal harm. An opportunity to offer pregnant women the same access to potentially life-saving interventions as others, and to obtain data to inform their future use, was lost. Once again, pregnant women were denied autonomy and their right to decide.

CONCLUSION

We recommend that, without clear justification for exclusion, pregnant women are included in clinical trials for EBOV and other life-threatening conditions, with lay language on risks and benefits in information documents, so that pregnant women can make their own decision to participate. Their automatic exclusion from trials for other conditions should be questioned.

Progress towards a vaccine against Ebola to meet emergency medical countermeasure needs

The Ebola virus epidemic in West Africa proved to be the largest in the history of filovirus outbreaks, causing the World Health Organization to declare a public health emergency of international concern in August of 2014. In collaboration with domestic and international partners, the Biomedical Advanced Research and Development Authority (BARDA) initiated several vaccine development projects in support of the overall response efforts. The urgency associated with the epidemic triggered the clinical evaluation of lead vaccine candidates starting in late 2014. Here we will discuss development of the lead vaccine candidates for Ebola virus, specifically Zaire ebolavirus.

Ebola virus disease: an update on post-exposure prophylaxis

The massive outbreak of Ebola virus disease in west Africa between 2013 and 2016 resulted in intense efforts to evaluate the efficacy of several specific countermeasures developed through years of preclinical work, including the first clinical trials for therapeutics and vaccines. In this Review, we discuss how the experience and data generated from that outbreak have helped to advance the understanding of the use of these countermeasures for post-exposure prophylaxis against Ebola virus infection. In future outbreaks, post-exposure prophylaxis could play an important part in reducing community transmission of Ebola virus by providing more immediate protection than does immunisation as well as providing additional protection for health-care workers who are inadvertently exposed over the course of their work. We propose provisional guidance for use of post-exposure prophylaxis in Ebola virus disease and identify the priorities for future preparedness and further research.

Chimpanzee adenoviral vectors as vaccines - challenges to move the technology into the fast lane

INTRODUCTION

In recent years, replication-defective chimpanzee-derived adenoviruses have been extensively evaluated as genetic vaccines. These vectors share desirable properties with human adenoviruses like the broad tissue tropism and the ease of large-scale manufacturing. Additionally, chimpanzee adenoviruses have the advantage to overcome the negative impact of pre-existing anti-human adenovirus immunity. Areas covered: Here the authors review current pre-clinical research and clinical trials that utilize chimpanzee-derived adenoviral vectors as vaccines. A wealth of studies are ongoing to evaluate different vector backbones and administration routes with the aim of improving immune responses. The challenges associated with the identification of an optimal chimpanzee vector and immunization strategies for different immunological outcomes will be discussed. Expert commentary: The demonstration that chimpanzee adenoviruses can be safely used in humans has paved the way to the use of a whole new array of vectors of different serotypes. However, so far no predictive signature of vector immunity in humans has been identified. The high magnitude of T cell responses elicited by chimpanzee adenoviruses has allowed dissecting the qualitative aspects that may be important for protective immunity. Ultimately, only the results from the most clinically advanced products will help establish the efficacy of the vaccine vector platform in the field of disease prevention.

Ethics review of studies during public health emergencies - the experience of the WHO ethics review committee during the Ebola virus disease epidemic

BACKGROUND

Between 2013 and 2016, West Africa experienced the largest ever outbreak of Ebola Virus Disease. In the absence of registered treatments or vaccines to control this lethal disease, the World Health Organization coordinated and supported research to expedite identification of interventions that could control the outbreak and improve future control efforts. Consequently, the World Health Organization Research Ethics Review Committee (WHO-ERC) was heavily involved in reviews and ethics discussions. It reviewed 24 new and 22 amended protocols for research studies including interventional (drug, vaccine) and observational studies.

WHO-ERC REVIEWS

WHO-ERC provided the reviews within on average 6 working days. The WHO-ERC often could not provide immediate approval of protocols for reasons which were not Ebola Virus Disease specific but related to protocol inconsistencies, missing information and complex informed consents. WHO-ERC considerations on Ebola Virus Disease specific issues (benefit-risk assessment, study design, exclusion of pregnant women and children from interventional studies, data and sample sharing, collaborative partnerships including international and local researchers and communities, community engagement and participant information) are presented.

CONCLUSIONS

To accelerate study approval in future public health emergencies, we recommend: (1) internally consistent and complete submissions with information documents in language participants are likely to understand, (2) close collaboration between local and international researchers from research inception, (3) generation of template agreements for data and sample sharing and use during the ongoing global consultations on bio-banks, (4) formation of Joint Scientific Advisory and Data Safety Review Committees for all studies linked to a particular intervention or group of interventions, (5) formation of a Joint Ethics Review Committee with representatives of the Ethics Committees of all institutions and countries involved to strengthen reviews through the different perspectives provided without the 'opportunity costs' for time to final approval of multiple, independent reviews, (6) direct information exchange between the chairs of advisory, safety review and ethics committees, (7) more Ethics Committee support for investigators than is standard and (8) a global consultation on criteria for inclusion of pregnant women and children in interventional studies for conditions which put them at particularly high risk of mortality or other irreversible adverse outcomes under standard-of-care.

A prime-boost immunization regimen based on a simian adenovirus 36 vectored multi-stage malaria vaccine induces protective immunity in mice

Malaria remains a considerable burden on public health. In 2015, the WHO estimates there were 212 million malaria cases causing nearly 429,000 deaths globally. A highly effective malaria vaccine is needed to reduce the burden of this disease. We have developed an experimental vaccine candidate (PyCMP) based on pre-erythrocytic (CSP) and erythrocytic (MSP1) stage antigens derived from the rodent malaria parasite P. yoelii. Our protein-based vaccine construct induces protective antibodies and CD4+ T cell responses. Based on evidence that viral vectors increase CD8+ T cell-mediated immunity, we also have tested heterologous prime-boost immunization regimens that included human adenovirus serotype 5 vector (Ad5), obtaining protective CD8+ T cell responses. While Ad5 is commonly used for vaccine studies, the high prevalence of pre-existing immunity to Ad5 severely compromises its utility. Here, we report the use of the novel simian adenovirus 36 (SAd36) as a candidate for a vectored malaria vaccine since this virus is not known to infect humans, and it is not neutralized by anti-Ad5 antibodies. Our study shows that the recombinant SAd36PyCMP can enhance specific CD8+ T cell response and elicit similar antibody titers when compared to an immunization regimen including the recombinant Ad5PyCMP. The robust immune responses induced by SAd36PyCMP are translated into a lower parasite load following P. yoelii infectious challenge when compared to mice immunized with Ad5PyCMP.

Ebola Vaccine: How Far are we?

Ebola viruses have been identified as an emerging threat as it causes severe haemorrhagic fever in human with mortality rates ranging from 50 to 90%. In addition to being a global health concern, the virus also is considered a potential biological threat agent. As for now, no licensed vaccine is available for pre or post exposure treatment. Recent epidemic of this disease in South Africa has led to concern towards development of an effective vaccine on a priority basis. This review is an attempt to look upon current progress in the development of Ebola virus vaccines and highlights strategies that have the greatest potential for commercial development.

Assessing Antiviral Countermeasures Using Mouse Models of Ebolavirus Infection

Mouse models of Ebola virus (EBOV) have demonstrated their utility as important tools for screening the efficacy of candidate therapeutics and vaccines. In this chapter we explain the various mouse models that utilize either wild-type or mouse-adapted EBOV variants.

Accelerating Vaccine Development During the 2013-2016 West African Ebola Virus Disease Outbreak

The Ebola virus disease outbreak that began in Western Africa in December 2013 was unprecedented in both scope and spread, and the global response was slower and less coherent than was optimal given the scale and pace of the epidemic. Past experience with limited localized outbreaks, lack of licensed medical countermeasures, reluctance by first responders to direct scarce resources to clinical research, community resistance to outside interventions, and lack of local infrastructure were among the factors delaying clinical research during the outbreak. Despite these hurdles, the global health community succeeded in accelerating Ebola virus vaccine development, in a 5-month interval initiating phase I trials in humans in September 2014 and initiating phase II/III trails in February 2015. Each of the three Ebola virus disease-affected countries, Sierra Leone, Guinea, and Liberia, conducted a phase II/III Ebola virus vaccine trial. Only one of these trials evaluating recombinant vesicular stomatitis virus expressing Ebola virus glycoprotein demonstrated vaccine efficacy using an innovative mobile ring vaccination trial design based on a ring vaccination strategy responsible for eradicating smallpox that reached areas of new outbreaks. Thoughtful and intensive community engagement in each country enabled the critical community partnership and acceptance of the phase II/III in each country. Due to the delayed clinical trial initiation, relative to the epidemiologic peak of the outbreak in the three countries, vaccine interventions may or may not have played a major role in bringing the epidemic under control. Having demonstrated that clinical trials can be performed during a large outbreak, the global research community can now build on the experience to implement trials more rapidly and efficiently in future outbreaks. Incorporating clinical research needs into planning for future health emergencies and understanding what kind of trial designs is needed for reliable results in an epidemic of limited duration should improve global response to future infectious disease outbreaks.

Safety, Immunogenicity and Efficacy of Prime-Boost Vaccination with ChAd63 and MVA Encoding ME-TRAP against Plasmodium falciparum Infection in Adults in Senegal

Malaria transmission is in decline in some parts of Africa, partly due to the scaling up of control measures. If the goal of elimination is to be achieved, additional control measures including an effective and durable vaccine will be required. Studies utilising the prime-boost approach to deliver viral vectors encoding the pre-erythrocytic antigen ME-TRAP (multiple epitope thrombospondin-related adhesion protein) have shown promising safety, immunogenicity and efficacy in sporozoite challenge studies. More recently, a study in Kenyan adults, similar to that reported here, showed substantial efficacy against P. falciparum infection. One hundred and twenty healthy male volunteers, living in a malaria endemic area of Senegal were randomised to receive either the Chimpanzee adenovirus (ChAd63) ME-TRAP as prime vaccination, followed eight weeks later by modified vaccinia Ankara (MVA) also encoding ME-TRAP as booster, or two doses of anti-rabies vaccine as a comparator. Prior to follow-up, antimalarials were administered to clear parasitaemia and then participants were monitored by PCR for malaria infection for eight weeks. The primary endpoint was time-to-infection with P. falciparum malaria, determined by two consecutive positive PCR results. Secondary endpoints included adverse event reporting, measures of cellular and humoral immunogenicity and a meta-analysis of combined vaccine efficacy with the parallel study in Kenyan adults.We show that this pre-erythrocytic malaria vaccine is safe and induces significant immunogenicity, with a peak T-cell response at seven days after boosting of 932 Spot Forming Cells (SFC)/10⁶ Peripheral Blood Mononuclear Cells(PBMC) compared to 57 SFC/ 10⁶ PBMCs in the control group. However, a vaccine efficacy was not observed: 12 of 57 ME-TRAP vaccinees became PCR positive during the intensive monitoring period as compared to 13 of the 58 controls (P = 0.80). This trial confirms that vaccine efficacy against malaria infection in adults may be rapidly assessed using this efficient and cost-effective clinical trial design. Further efficacy evaluation of this vectored candidate vaccine approach in other malaria transmission settings and age-de-escalation into the main target age groups for a malaria vaccine is in progress.

Ebola virus vaccines - reality or fiction?

For 40 years ebolaviruses have been responsible for sporadic outbreaks of severe and often fatal hemorrhagic fever in humans and nonhuman primates. In December 2013 an unprecedented Zaire ebolavirus epidemic began in West Africa. Although "patient zero" has finally been reached after 2 years, the virus is again causing disease in the region. Currently there are no licensed vaccines or therapeutic countermeasures against ebolaviruses; however, the epidemic in West Africa has focused attention on the potential vaccine platforms developed over the past 15 years. There has been remarkable progress using a variety of platforms including DNA, subunit, and several viral vector approaches, replicating and non-replicating, which have shown varying degrees of protective efficacy in the "gold-standard" nonhuman primate models for Ebolavirus infections. A number of these vaccine platforms have moved into clinical trials over the past year with the hope of finding an efficacious vaccine to prevent future outbreaks/epidemics of Ebola hemorrhagic fever on the scale of the West African epidemic.

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.

Simian adenoviruses as vaccine vectors

Replication incompetent human adenovirus serotype 5 (HAdV-C5) has been extensively used as a delivery vehicle for gene therapy proteins and infectious disease antigens. These vectors infect replicating and nonreplicating cells, have a broad tissue tropism, elicit high immune responses and are easily purified to high titers. However, the utility of HAdV-C5 vectors as potential vaccines is limited due to pre-existing immunity within the human population that significantly reduces the immunogenicity of HAdV-C5 vaccines. In recent years, adenovirus vaccine development has focused on simian-derived adenoviral vectors, which have the desirable vector characteristics of HAdV-C5 but with negligible seroprevalence in the human population. Here, we discuss recent advances in simian adenovirus vaccine vector development and evaluate current research specifically focusing on clinical trial data.

Xenogenic Adenoviral Vectors

Many nonhuman adenoviruses (AdVs) of simian, bovine, porcine, canine, ovine, murine, and fowl origin are being developed as gene delivery systems for recombinant vaccines and gene therapy applications. In addition to circumventing preexisting human AdV (HAdV) immunity, nonhuman AdV vectors utilize coxsackievirus-adenovirus receptor or other receptors for vector internalization, thereby expanding the range of cell types that can be targeted. Nonhuman AdV vectors also provide excellent platforms for veterinary vaccines. A specific nonhuman AdV vector when used in its species of origin could provide an excellent animal model for evaluating the vector efficacy and pathogenesis. These vectors are useful in prime–boost approaches with other AdV vectors or with other gene delivery systems including DNA immunization and viral or bacterial vectors. When multiple vector inoculations are required, nonhuman AdV vectors could supplement HAdV or other viral vectors.

Novel chimpanzee adenovirus-vectored respiratory mucosal tuberculosis vaccine: overcoming local anti-human adenovirus immunity for potent TB protection

Pulmonary tuberculosis (TB) remains to be a major global health problem despite many decades of parenteral use of Bacillus Calmette-Guérin (BCG) vaccine. Developing safe and effective respiratory mucosal TB vaccines represents a unique challenge. Over the past decade or so, the human serotype 5 adenovirus (AdHu5)-based TB vaccine has emerged as one of the most promising candidates based on a plethora of preclinical and early clinical studies. However, anti-AdHu5 immunity widely present in the lung of humans poses a serious gap and limitation to its real-world applications. In this study we have developed a novel chimpanzee adenovirus 68 (AdCh68)-vectored TB vaccine amenable to the respiratory route of vaccination. We have evaluated AdCh68-based TB vaccine for its safety, T-cell immunogenicity, and protective efficacy in relevant animal models of human pulmonary TB with or without parenteral BCG priming. We have also compared AdCh68-based TB vaccine with its AdHu5 counterpart in both naive animals and those with preexisting anti-AdHu5 immunity in the lung. We provide compelling evidence that AdCh68-based TB vaccine is not only safe when delivered to the respiratory tract but, importantly, is also superior to its AdHu5 counterpart in induction of T-cell responses and immune protection, and limiting lung immunopathology in the presence of preexisting anti-AdHu5 immunity in the lung. Our findings thus suggest AdCh68-based TB vaccine to be an ideal candidate for respiratory mucosal immunization, endorsing its further clinical development in humans.

Recent advances in the development of vaccines for Ebola virus disease

Ebola virus is one of the most dangerous microorganisms in the world causing hemorrhagic fevers in humans and non-human primates. Ebola virus (EBOV) is a zoonotic infection, which emerges and re-emerges in human populations. The 2014 outbreak was caused by the Zaire strain, which has a kill rate of up to 90%, though 40% was recorded in the current outbreak. The 2014 outbreak is larger than all 20 outbreaks that have occurred since 1976, when the virus was first discovered. It is the first time that the virus was sustained in urban centers and spread beyond Africa into Europe and USA. Thus far, over 22,000 cases have been reported with about 50% mortality in one year. There are currently no approved therapeutics and preventive vaccines against Ebola virus disease (EVD). Responding to the devastating effe1cts of the 2014 outbreak and the potential risk of global spread, has spurred research for the development of therapeutics and vaccines. This review is therefore aimed at presenting the progress of vaccine development. Results showed that conventional inactivated vaccines produced from EBOV by heat, formalin or gamma irradiation appear to be ineffective. However, novel vaccines production techniques have emerged leading to the production of candidate vaccines that have been demonstrated to be effective in preclinical trials using small animal and non-human primates (NHP) models. Some of the promising vaccines have undergone phase 1 clinical trials, which demonstrated their safety and immunogenicity. Many of the candidate vaccines are vector based such as Vesicular Stomatitis Virus (VSV), Rabies Virus (RABV), Adenovirus (Ad), Modified Vaccinia Ankara (MVA), Cytomegalovirus (CMV), human parainfluenza virus type 3 (HPIV3) and Venezuelan Equine Encephalitis Virus (VEEV). Other platforms include virus like particle (VLP), DNA and subunit vaccines.

Pre-existing immunity against Ad vectors: humoral, cellular, and innate response, what's important?

Pre-existing immunity against human adenovirus (HAd) serotype 5 derived vector in the human population is widespread, thus hampering its clinical use. Various components of the immune system, including neutralizing antibodies (nAbs), Ad specific T cells and type I IFN activated NK cells, contribute to dampening the efficacy of Ad vectors in individuals with pre-existing Ad immunity. In order to circumvent pre-existing immunity to adenovirus, numerous strategies, such as developing alternative Ad serotypes, varying immunization routes and utilizing prime-boost regimens, are under pre-clinical or clinical phases of development. However, these strategies mainly focus on one arm of pre-existing immunity. Selection of alternative serotypes has been largely driven by the absence in the human population of nAbs against them with little attention paid to cross-reactive Ad specific T cells. Conversely, varying the route of immunization appears to mainly rely on avoiding Ad specific tissue-resident T cells. Finally, prime-boost regimens do not actually circumvent pre-existing immunity but instead generate immune responses of sufficient magnitude to confer protection despite pre-existing immunity. Combining the above strategies and thus taking into account all components regulating pre-existing Ad immunity will help further improve the development of Ad vectors for animal and human use.

Clinical development of Ebola vaccines

The ongoing outbreak of Ebola virus disease in West Africa highlighted the lack of a licensed drug or vaccine to combat the disease and has renewed the urgency to develop a pipeline of Ebola vaccines. A number of different vaccine platforms are being developed by assessing preclinical efficacy in animal models and expediting clinical development. Over 15 different vaccines are in preclinical development and 8 vaccines are now in different stages of clinical evaluation. These vaccines include DNA vaccines, virus-like particles and viral vectors such as live replicating vesicular stomatitis virus (rVSV), human and chimpanzee adenovirus, and vaccinia virus. Recently, in preliminary results reported from the first phase III trial of an Ebola vaccine, the rVSV-vectored vaccine showed promising efficacy. This review charts this rapidly advancing area of research focusing on vaccines in clinical development and discusses the future opportunities and challenges faced in the licensure and deployment of Ebola vaccines.

Adenovirus-vectored Ebola vaccines

The 2014 outbreak of Ebola virus disease in West Africa has highlighted the need for the availability of effective vaccines against outbreak pathogens that are suitable for use in frontline workers who risk their own health in the course of caring for those with the disease, and also for members of the community in the affected area. Along with effective contact tracing and quarantine, use of a vaccine as soon as an outbreak is identified could greatly facilitate rapid control and prevent the outbreak from spreading. This review describes the progress that has been made in producing and testing adenovirus-based Ebola vaccines in both pre-clinical and clinical studies, and considers the likely future use of these vaccines.

Neutralizing antibody responses to enterovirus and adenovirus in healthy adults in China

Hand, foot and mouth disease (HFMD) is an important public health problem that has emerged over the past several years. HFMD predominantly infects children under seven years old and occasionally causes severe disease in adults. Among the enteroviruses, enterovirus 71 (EV71) and coxsackievirus 16 (CA16) are the major causative agents of HFMD. In addition, adenovirus cocirculates with enterovirus and has become a possible additional pathogenic factor for HFMD in some cases. Here, we have investigated the neutralizing antibody responses to both enterovirus and adenovirus in adults, with the aim of exploring the prevalence trends of these viruses and the nature of protective immunity in humans to these viral infections. Sera from 391 healthy adults from 21 provinces and cities in China were tested for the presence of antibodies against EV71, CA16, adenovirus human serotype 5 (AdHu5) and chimpanzee adenovirus pan7 (AdC7) using neutralization tests. High seroprevalence rates of EV71, CA16 and AdHu5 were found in the population (85.7%, 58.8% and 74.2%, respectively). The coseropositivity rate of these three viruses was 39.4% (154 of 391), with median neutralizing antibody titers of 80, 40 and 640, respectively, and the neutralizing antibody titer for EV71 was found to be correlated with those of CA16 and AdHu5. AdC7 was found to be a rare adenovirus serotype in the human population, with a seropositivity rate of 11.8%, suggesting that it could be a good choice for a vaccine carrier that could be used in vaccine development.

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.

Adenovirus-Based Vectors for the Development of Prophylactic and Therapeutic Vaccines

Emerging and reemerging infectious diseases as well as cancer pose great global health impacts on the society. Vaccines have emerged as effective treatments to prevent or reduce the burdens of already developed diseases. This is achieved by means of activating various components of the immune system to generate systemic inflammatory reactions targeting infectious agents or diseased cells for control/elimination. DNA virus-based genetic vaccines gained significant attention in the past decades owing to the development of DNA manipulation technologies, which allowed engineering of recombinant viral vectors encoding sequences for foreign antigens or their immunogenic epitopes as well as various immunomodulatory molecules. Despite tremendous progress in the past 50 years, many hurdles still remain for achieving the full clinical potential of viral-vectored vaccines. This chapter will present the evolution of vaccines from “live” or “attenuated” first-generation agents to recombinant DNA and viral-vectored vaccines. Particular emphasis will be given to human adenovirus (Ad) for the development of prophylactic and therapeutic vaccines. Ad biological properties related to vaccine development will be highlighted along with their advantages and potential hurdles to be overcome. In particular, we will discuss (1) genetic modifications in the Ad capsid protein to reduce the intrinsic viral immunogenicity, (2) antigen capsid incorporation for effective presentation of foreign antigens to the immune system, (3) modification of the hexon and fiber capsid proteins for Ad liver de-targeting and selective retargeting to cancer cells, (4) Ad-based vaccines carrying “arming” transgenes with immunostimulatory functions as immune adjuvants, and (5) oncolytic Ad vectors as a new therapeutic approach against cancer. Finally, the combination of adenoviral vectors with other non-adenoviral vector systems, the prime/boost strategy of immunization, clinical trials involving Ad-based vaccines, and the perspectives for the field development will be discussed.

Emerging targets and novel approaches to Ebola virus prophylaxis and treatment

Ebola is a highly virulent pathogen causing severe hemorrhagic fever with a high case fatality rate in humans and non-human primates (NHPs). Although safe and effective vaccines or other medicinal agents to block Ebola infection are currently unavailable, a significant effort has been put forth to identify several promising candidates for the treatment and prevention of Ebola hemorrhagic fever. Among these, recombinant adenovirus-based vectors have been identified as potent vaccine candidates, with some affording both pre- and post-exposure protection from the virus. Recently, Investigational New Drug (IND) applications have been approved by the US Food and Drug Administration (FDA) and phase I clinical trials have been initiated for two small-molecule therapeutics: anti-sense phosphorodiamidate morpholino oligomers (PMOs: AVI-6002, AVI-6003) and lipid nanoparticle/small interfering RNA (LNP/siRNA: TKM-Ebola). These potential alternatives to vector-based vaccines require multiple doses to achieve therapeutic efficacy, which is not ideal with regard to patient compliance and outbreak scenarios. These concerns have fueled a quest for even better vaccination and treatment strategies. Here, we summarize recent advances in vaccines or post-exposure therapeutics for prevention of Ebola hemorrhagic fever. The utility of novel pharmaceutical approaches to refine and overcome barriers associated with the most promising therapeutic platforms are also discussed.

Development of chimpanzee adenoviruses as vaccine vectors: challenges and successes emerging from clinical trials

Replication-defective chimpanzee adenovirus vectors are emerging as a promising new class of genetic vaccine carriers. Chimpanzee adenovirus vectors have now reached the clinical stage and appear to be endowed with all the properties needed for human vaccine development, including high quality and magnitude of the immune response induced against the encoded antigens, good safety and ease of manufacturing on a large-scale basis. Here the authors review the recent findings of this novel class of adenovirus vectors and compare their properties to other clinical stage vaccine vectors derived from poxvirus, alphavirus and human adenovirus.

Modeling pre-existing immunity to adenovirus in rodents: immunological requirements for successful development of a recombinant adenovirus serotype 5-based ebola vaccine

Pre-existing immunity (PEI) to human adenovirus serotype 5 (Ad5) worldwide is the primary limitation to routine clinical use of Ad5-based vectors in immunization platforms. Using systemic and mucosal PEI induction models in rodents (mice and guinea pigs), we assessed the influence of PEI on the type of adaptive immune response elicited by an Ad5-based vaccine for Ebola with respect to immunization route. Splenocytes isolated from vaccinated animals revealed that immunization by the same route in which PEI was induced significantly compromised Ebola Zaire glycoprotein (ZGP)-specific IFN-γ+ CD8+ T cells and ZGP-specific multifunctional CD8+ T cell populations. ZGP-specific IgG1 antibody levels were also significantly reduced and a sharp increase in serum anti-Ad5 neutralizing antibody (NAB) titers were noted following immunization. These immune parameters correlated with poor survival after lethal challenge with rodent-adapted Ebola Zaire virus (ZEBOV). Although the number of IFN-γ+ CD8+ T cells was reduced in animals given the vaccine by a different route from that used for PEI induction, the multifunctional CD8+ T cell response was not compromised. Survival rates in these groups were higher than when PEI was induced by the same route as immunization. These results suggest that antigen-specific multifunctional CD8(+) T cell and Th2 type antibody responses compromised by PEI to Ad5 are required for protection from Ebola. They also illustrate that methods for induction of PEI used in preclinical studies must be carefully evaluated for successful development of novel Ad5-based vaccines.

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.

Vaccination to conserved influenza antigens in mice using a novel Simian adenovirus vector, PanAd3, derived from the bonobo Pan paniscus

Among approximately 1000 adenoviruses from chimpanzees and bonobos studied recently, the Pan Adenovirus type 3 (PanAd3, isolated from a bonobo, Pan paniscus) has one of the best profiles for a vaccine vector, combining potent transgene immunogenicity with minimal pre-existing immunity in the human population. In this study, we inserted into a replication defective PanAd3 a transgene expressing a fusion protein of conserved influenza antigens nucleoprotein (NP) and matrix 1 (M1). We then studied antibody and T cell responses as well as protection from challenge infection in a mouse model. A single intranasal administration of PanAd3-NPM1 vaccine induced strong antibody and T cell responses, and protected against high dose lethal influenza virus challenge. Thus PanAd3 is a promising candidate vector for vaccines, including universal influenza vaccines.

Airway delivery of an adenovirus-based Ebola virus vaccine bypasses existing immunity to homologous adenovirus in nonhuman primates

Anti-adenovirus serotype 5 antibodies are capable of neutralizing adenovirus serotype 5-based vaccines. In mice and guinea pigs, intranasal delivery of adenovirus serotype 5-based vaccine bypasses induced adenovirus serotype 5 preexisting immunity, resulting in protection against species-adapted Ebola virus challenge. In this study, nonhuman primates were vaccinated with adenovirus serotype 5-based vaccine either intramuscularly or via the airway route (intranasally/intratracheally) in the presence or absence of adenovirus serotype 5 preexisting immunity. Immune responses were evaluated to determine the effect of both the vaccine delivery route and preexisting immunity before and after a lethal Ebola virus (Zaïre strain Kikwit 95) challenge. Intramuscular vaccination fully protected nonhuman primates in the absence of preexisting immunity, whereas the presence of preexisting immunity abrogated vaccine efficacy and resulted in complete mortality. In contrast, the presence of preexisting immunity to adenovirus serotype 5 did not alter the survival rate of nonhuman primates receiving the adenovirus serotype 5-based Ebola virus vaccine in the airway. This study shows that airway vaccination with adenovirus serotype 5-based Ebola virus vaccine can efficiently bypass preexisting immunity to adenovirus serotype 5 and induce protective immune responses, albeit at lower efficacy than that using an intramuscular vaccine delivery route.

Immune parameters correlate with protection against ebola virus infection in rodents and nonhuman primates

Ebola virus causes severe hemorrhagic fever in susceptible hosts. Currently, no licensed vaccines or treatments are available; however, several experimental vaccines have been successful in protecting rodents and nonhuman primates (NHPs) from the lethal Zaire ebolavirus (ZEBOV) infection. The objective of this study was to evaluate immune responses correlating with survival in these animals after lethal challenge with ZEBOV. Knockout mice with impaired ability to generate normal T and/or B cell responses were vaccinated and challenged with ZEBOV. Vaccine-induced protection in mice was mainly mediated by B cells and CD4(+) T cells. Vaccinated, outbred guinea pigs and NHPs demonstrated the highest correlation between survival and levels of total immunoglobulin G (IgG) specific to the ZEBOV glycoprotein (ZGP). These results highlight the relevance of total ZGP-specific IgG levels as a meaningful correlate of protection against ZEBOV exposure.

Mouse models for filovirus infections

The filoviruses marburg- and ebolaviruses can cause severe hemorrhagic fever (HF) in humans and nonhuman primates. Because many cases have occurred in geographical areas lacking a medical research infrastructure, most studies of the pathogenesis of filoviral HF, and all efforts to develop drugs and vaccines, have been carried out in biocontainment laboratories in non-endemic countries, using nonhuman primates (NHPs), guinea pigs and mice as animal models. NHPs appear to closely mirror filoviral HF in humans (based on limited clinical data), but only small numbers may be used in carefully regulated experiments; much research is therefore done in rodents. Because of their availability in large numbers and the existence of a wealth of reagents for biochemical and immunological testing, mice have become the preferred small animal model for filovirus research. Since the first experiments following the initial 1967 marburgvirus outbreak, wild-type or mouse-adapted viruses have been tested in immunocompetent or immunodeficient mice. In this paper, we review how these types of studies have been used to investigate the pathogenesis of filoviral disease, identify immune responses to infection and evaluate antiviral drugs and vaccines. We also discuss the strengths and weaknesses of murine models for filovirus research, and identify important questions for further study.

Delivery of antigens by viral vectors for vaccination

Viral vectors have been developed as vaccine platforms for a number of pathogens and tumors. In particular, adenovirus (Ad)-based vectors expressing genes coding for pathogen or tumor antigens have proven efficacious to induce protective immunity. Major challenges in the use of Ad vectors are the high prevalence of anti-Ad immunity and the recent observation during an Ad-based HIV vaccine trial that led to increased HIV-1 acquisition in the presence of circulating anti-Ad5 neutralizing antibodies. In this review we summarize strategies to address these challenges and focus on modifications of the Ad capsid to enhance the adjuvant effect of anti-Ad immunogenicity and to circumvent pre-existing immunity. In addition, we summarize the current status and potential of other viral vector vaccines based on adeno-associated viruses, lentiviruses and poxviruses.

Current ebola vaccines

INTRODUCTION

Ebolaviruses cause severe viral hemorrhagic fever in humans and non-human primates (NHPs), with case fatality rates of up to 90%. Currently, neither a specific treatment nor a vaccine licensed for use in humans is available. However, a number of vaccine candidates have been developed in the last decade that are highly protective in NHPs, the gold standard animal model for ebola hemorrhagic fever.

AREAS COVERED

This review analyzes a number of scenarios for the use of ebolavirus vaccines, discusses the requirements for ebolavirus vaccines in these scenarios and describes current ebolavirus vaccines. Among these vaccines are recombinant adenoviruses, recombinant vesicular stomatitis viruses (VSVs), recombinant human parainfluenza viruses and virus-like particles. Interestingly, one of these vaccine platforms, based on recombinant VSVs, has also demonstrated post-exposure protection in NHPs.

EXPERT OPINION

The most pressing remaining challenge is now to move these vaccine candidates forward into human trials and toward licensure. In order to achieve this, it will be necessary to establish the mechanisms and correlates of protection for these vaccines, and to continue to demonstrate their safety, particularly in potentially immunocompromised populations. However, already now there is sufficient evidence that, from a scientific perspective, a vaccine protective against ebolaviruses is possible.