Substitution of wild-type yellow fever Asibi sequences for 17D vaccine sequences in ChimeriVax-dengue 4 does not enhance infection of Aedes aegypti mosquitoes

Biodistribution and environmental safety of a live-attenuated YF17D-vectored SARS-CoV-2 vaccine candidate

New platforms are needed for the design of novel prophylactic vaccines and advanced immune therapies. Live-attenuated yellow fever vaccine YF17D serves as a vector for several licensed vaccines and platform for novel candidates. On the basis of YF17D, we developed an exceptionally potent COVID-19 vaccine candidate called YF-S0. However, use of such live RNA viruses raises safety concerns, such as adverse events linked to original YF17D (yellow fever vaccine-associated neurotropic disease [YEL-AND] and yellow fever vaccine-associated viscerotropic disease [YEL-AVD]). In this study, we investigated the biodistribution and shedding of YF-S0 in hamsters. Likewise, we introduced hamsters deficient in signal transducer and activator of transcription 2 (STAT2) signaling as a new preclinical model of YEL-AND/AVD. Compared with YF17D, YF-S0 showed improved safety with limited dissemination to brain and visceral tissues, absent or low viremia, and no shedding of infectious virus. Considering that yellow fever virus is transmitted by Aedes mosquitoes, any inadvertent exposure to the live recombinant vector via mosquito bites is to be excluded. The transmission risk of YF-S0 was hence compared with readily transmitting YF-Asibi strain and non-transmitting YF17D vaccine, with no evidence for productive infection of mosquitoes. The overall favorable safety profile of YF-S0 is expected to translate to other vaccines based on the same YF17D platform.

Defeat Dengue and Zika Viruses With a One-Two Punch of Vaccine and Vector Blockade

Dengue virus (DENV) and Zika virus (ZIKV) are two mosquito-borne flaviviruses afflicting nearly half of the world population. Human infection by these viruses can either be asymptomatic or manifest as clinical diseases from mild to severe. Despite more cases are presented as self-limiting febrile illness, severe dengue disease can be manifested as hemorrhagic fever and hemorrhagic shock syndrome, and ZIKV infection has been linked to increased incidence of peripheral neuropathy Guillain-Barre syndrome and central neural disease such as microcephaly. The current prevention and treatment of these infectious diseases are either non-satisfactory or entirely lacking. Because DENV and ZIKV have much similarities in genomic and structural features, almost identical mode of mosquito-mediated transmission, and probably the same pattern of host innate and adaptive immunity toward them, it is reasonable and often desirable to investigate these two viruses side-by-side, and thereby devise common countermeasures against both. Here, we review the existing knowledge on DENV and ZIKV regarding epidemiology, molecular virology, protective immunity and vaccine development, discuss recent new discoveries on the functions of flavivirus NS1 protein in viral pathogenesis and transmission, and propose a one-two punch strategy using vaccine and vector blockade to overcome antibody-dependent enhancement and defeat Dengue and Zika viruses.

A review of Dengvaxia®: development to deployment

Dengue is the world's most prevalent and important arboviral disease. More than 50% of the world's population lives at daily risk of infection and it is estimated more than 95 million people a year seek medical care following infection. Severe disease can manifest as plasma leakage and potential for clinically significant hemorrhage, shock, and death. Treatment is supportive and there is currently no licensed anti-dengue virus prophylactic or therapeutic compound. A single dengue vaccine, Sanofi Pasteur's Dengvaxia®, has been licensed in 20 countries but uptake has been poor. A safety signal in dengue seronegative vaccine recipients stimulated an international re-look at the vaccine performance profile, new World Health Organization recommendations for use, and controversy in the Philippines involving the government, regulatory agencies, Sanofi Pasteur, clinicians responsible for testing and administering the vaccine, and the parents of vaccinated children. In this review, we provide an overview of Dengvaxia's® development and discuss what has been learned about product performance since its licensure.

Live-attenuated tetravalent dengue vaccines: The needs and challenges of post-licensure evaluation of vaccine safety and effectiveness

Since December 2015, the first dengue vaccine has been licensed in several Asian and Latin American countries for protection against disease from all four dengue virus serotypes. While the vaccine demonstrated an overall good safety and efficacy profile in clinical trials, some key research questions remain which make risk-benefit-assessment for some populations difficult. As for any new vaccine, several questions, such as very rare adverse events following immunization, duration of vaccine-induced protection and effectiveness when used in public health programs, will be addressed by post-licensure studies and by data from national surveillance systems after the vaccine has been introduced. However, the complexity of dengue epidemiology, pathogenesis and population immunity, as well as some characteristics of the currently licensed vaccine, and potentially also future, live-attenuated dengue vaccines, poses a challenge for evaluation through existing monitoring systems, especially in low and middle-income countries. Most notable are the different efficacies of the currently licensed vaccine by dengue serostatus at time of first vaccination and by dengue virus serotype, as well as the increased risk of dengue hospitalization among young vaccinated children observed three years after the start of vaccination in one of the trials. Currently, it is unknown if the last phenomenon is restricted to younger ages or could affect also seronegative individuals aged 9years and older, who are included in the group for whom the vaccine has been licensed. In this paper, we summarize scientific and methodological considerations for public health surveillance and targeted post-licensure studies to address some key research questions related to live-attenuated dengue vaccines. Countries intending to introduce a dengue vaccine should assess their capacities to monitor and evaluate the vaccine's effectiveness and safety and, where appropriate and possible, enhance their surveillance systems accordingly. Targeted studies are needed, especially to better understand the effects of vaccinating seronegative individuals.

Unique safety issues associated with virus-vectored vaccines: Potential for and theoretical consequences of recombination with wild type virus strains

In 2003 and 2013, the World Health Organization convened informal consultations on characterization and quality aspects of vaccines based on live virus vectors. In the resulting reports, one of several issues raised for future study was the potential for recombination of virus-vectored vaccines with wild type pathogenic virus strains. This paper presents an assessment of this issue formulated by the Brighton Collaboration.

To provide an appropriate context for understanding the potential for recombination of virus-vectored vaccines, we review briefly the current status of virus-vectored vaccines, mechanisms of recombination between viruses, experience with recombination involving live attenuated vaccines in the field, and concerns raised previously in the literature regarding recombination of virus-vectored vaccines with wild type virus strains. We then present a discussion of the major variables that could influence recombination between a virus-vectored vaccine and circulating wild type virus and the consequences of such recombination, including intrinsic recombination properties of the parent virus used as a vector; sequence relatedness of vector and wild virus; virus host range, pathogenesis and transmission; replication competency of vector in target host; mechanism of vector attenuation; additional factors potentially affecting virulence; and circulation of multiple recombinant vectors in the same target population. Finally, we present some guiding principles for vector design and testing intended to anticipate and mitigate the potential for and consequences of recombination of virus-vectored vaccines with wild type pathogenic virus strains.

Recovery of West Nile Virus Envelope Protein Domain III Chimeras with Altered Antigenicity and Mouse Virulence

Flaviviruses are positive-sense, single-stranded RNA viruses responsible for millions of human infections annually. The envelope (E) protein of flaviviruses comprises three structural domains, of which domain III (EIII) represents a discrete subunit. The EIII gene sequence typically encodes epitopes recognized by virus-specific, potently neutralizing antibodies, and EIII is believed to play a major role in receptor binding. In order to assess potential interactions between EIII and the remainder of the E protein and to assess the effects of EIII sequence substitutions on the antigenicity, growth, and virulence of a representative flavivirus, chimeric viruses were generated using the West Nile virus (WNV) infectious clone, into which EIIIs from nine flaviviruses with various levels of genetic diversity from WNV were substituted. Of the constructs tested, chimeras containing EIIIs from Koutango virus (KOUV), Japanese encephalitis virus (JEV), St. Louis encephalitis virus (SLEV), and Bagaza virus (BAGV) were successfully recovered. Characterization of the chimeras in vitro and in vivo revealed differences in growth and virulence between the viruses, within vivo pathogenesis often not being correlated within vitro growth. Taken together, the data demonstrate that substitutions of EIII can allow the generation of viable chimeric viruses with significantly altered antigenicity and virulence.

IMPORTANCE

The envelope (E) glycoprotein is the major protein present on the surface of flavivirus virions and is responsible for mediating virus binding and entry into target cells. Several viable West Nile virus (WNV) variants with chimeric E proteins in which the putative receptor-binding domain (EIII) sequences of other mosquito-borne flaviviruses were substituted in place of the WNV EIII were recovered, although the substitution of several more divergent EIII sequences was not tolerated. The differences in virulence and tissue tropism observed with the chimeric viruses indicate a significant role for this sequence in determining the pathogenesis of the virus within the mammalian host. Our studies demonstrate that these chimeras are viable and suggest that such recombinant viruses may be useful for investigation of domain-specific antibody responses and the more extensive definition of the contributions of EIII to the tropism and pathogenesis of WNV or other flaviviruses.

Dual miRNA targeting restricts host range and attenuates neurovirulence of flaviviruses

Mosquito-borne flaviviruses are among the most significant arboviral pathogens worldwide. Vaccinations and mosquito population control programs remain the most reliable means for flavivirus disease prevention, and live attenuated viruses remain one of the most attractive flavivirus vaccine platforms. Some live attenuated viruses are capable of infecting principle mosquito vectors, as demonstrated in the laboratory, which in combination with their intrinsic genetic instability could potentially lead to a vaccine virus reversion back to wild-type in nature, followed by introduction and dissemination of potentially dangerous viral strains into new geographic locations. To mitigate this risk we developed a microRNA-targeting approach that selectively restricts replication of flavivirus in the mosquito host. Introduction of sequences complementary to a mosquito-specific mir-184 and mir-275 miRNAs individually or in combination into the 3’NCR and/or ORF region resulted in selective restriction of dengue type 4 virus (DEN4) replication in mosquito cell lines and adult Aedes mosquitos. Moreover a combined targeting of DEN4 genome with mosquito-specific and vertebrate CNS-specific mir-124 miRNA can silence viral replication in two evolutionally distant biological systems: mosquitoes and mouse brains. Thus, this approach can reinforce the safety of newly developed or existing vaccines for use in humans and could provide an additional level of biosafety for laboratories using viruses with altered pathogenic or transmissibility characteristics.

Despite advances in developing flavivirus live attenuated vaccine (LAV) candidates, a concern exists that they might not be safe in the environment due to their intrinsic genetic instability leading to potential reversion back to wild-type that could be associated with possible dissemination of these mutated viruses by mosquitoes. Here, we describe a miRNA targeting approach that can be adapted to support the design of environmentally-safe LAV restricted in their ability to infect and be transmitted by competent vectors, thereby limiting the possibility of subsequent viral evolution and unpredictable consequences. A combined co-targeting of flavivirus genome with mosquito- and vertebrate brain- specific miRNAs resulted in simultaneous restriction of dengue virus infection and replication in mosquitoes and in brains of newborn mice indicating that the miRNA-mediated approach for virus attenuation represents an alternative to non-specific strategies for the control of viral tissue tropism and pathogenesis in the vertebrate host and replicative efficacy in permissive vectors.

Live virus vaccines based on a yellow fever vaccine backbone: standardized template with key considerations for a risk/benefit assessment

The Brighton Collaboration Viral Vector Vaccines Safety Working Group (V3SWG) was formed to evaluate the safety of live, recombinant viral vaccines incorporating genes from heterologous viruses inserted into the backbone of another virus (so-called "chimeric virus vaccines"). Many viral vector vaccines are in advanced clinical trials. The first such vaccine to be approved for marketing (to date in Australia, Thailand, Malaysia, and the Philippines) is a vaccine against the flavivirus, Japanese encephalitis (JE), which employs a licensed vaccine (yellow fever 17D) as a vector. In this vaccine, two envelope proteins (prM-E) of YF 17D virus were exchanged for the corresponding genes of JE virus, with additional attenuating mutations incorporated into the JE gene inserts. Similar vaccines have been constructed by inserting prM-E genes of dengue and West Nile into YF 17D virus and are in late stage clinical studies. The dengue vaccine is, however, more complex in that it requires a mixture of four live vectors each expressing one of the four dengue serotypes. This vaccine has been evaluated in multiple clinical trials. No significant safety concerns have been found. The Phase 3 trials met their endpoints in terms of overall reduction of confirmed dengue fever, and, most importantly a significant reduction in severe dengue and hospitalization due to dengue. However, based on results that have been published so far, efficacy in preventing serotype 2 infection is less than that for the other three serotypes. In the development of these chimeric vaccines, an important series of comparative studies of safety and efficacy were made using the parental YF 17D vaccine virus as a benchmark. In this paper, we use a standardized template describing the key characteristics of the novel flavivirus vaccine vectors, in comparison to the parental YF 17D vaccine. The template facilitates scientific discourse among key stakeholders by increasing the transparency and comparability of information. The Brighton Collaboration V3SWG template may also be useful as a guide to the evaluation of other recombinant viral vector vaccines.

Yellow fever virus: genetic and phenotypic diversity and implications for detection, prevention and therapy

Yellow fever virus (YFV) is the prototypical hemorrhagic fever virus, yet our understanding of its phenotypic diversity and any molecular basis for observed differences in disease severity and epidemiology is lacking, when compared to other arthropod-borne and haemorrhagic fever viruses. This is, in part, due to the availability of safe and effective vaccines resulting in basic YFV research taking a back seat to those viruses for which no effective vaccine occurs. However, regular outbreaks occur in endemic areas, and the spread of the virus to new, previously unaffected, areas is possible. Analysis of isolates from endemic areas reveals a strong geographic association for major genotypes, and recent epidemics have demonstrated the emergence of novel sequence variants. This review aims to outline the current understanding of YFV genetic and phenotypic diversity and its sources, as well as the available animal models for characterizing these differences in vivo. The consequences of genetic diversity for detection and diagnosis of yellow fever and development of new vaccines and therapeutics are discussed.

Latest developments and future directions in dengue vaccines

Dengue is a mosquito-borne disease which is currently an expanding global health problem. The disease is caused by four closely related viruses, the dengue virus. There are no specific dengue therapeutics and prevention is currently limited to vector control measures. Development of an effective tetravalent dengue vaccine would therefore represent a major advance in the control of the disease and is considered a high public health priority. While a licensed dengue vaccine is not yet available, the scope and intensity of dengue vaccine development has increased dramatically in the last decade. The uniqueness of the dengue viruses and the spectrum of disease resulting from infection have made dengue vaccine development difficult. Several vaccine candidates are currently being evaluated in clinical studies. The candidate currently at the most advanced clinical development stage, a live-attenuated tetravalent vaccine based on chimeric yellow fever dengue virus, has progressed to phase III efficacy studies. Several other live-attenuated vaccines, as well as subunit, DNA and purified inactivated vaccine candidates, are at earlier stages of clinical development. Additional technological approaches, such as virus-vectored and virus-like particle-based vaccines, are under evaluation in preclinical studies.

Preclinical and clinical development of a YFV 17 D-based chimeric vaccine against West Nile virus

Substantial success has been achieved in the development and implementation of West Nile (WN) vaccines for horses; however, no human WN vaccines are approved. This review focuses on the construction, pre-clinical and clinical characterization of ChimeriVax-WN02 for humans, a live chimeric vaccine composed of a yellow fever (YF) 17D virus in which the prM-E envelope protein genes are replaced with the corresponding genes of the WN NY99 virus. Pre-clinical studies demonstrated that ChimeriVax-WN02 was significantly less neurovirulent than YF 17D in mice and rhesus and cynomolgus monkeys. The vaccine elicited neutralizing antibody titers after inoculation in hamsters and monkeys and protected immunized animals from lethal challenge including intracerebral inoculation of high dose of WN NY99 virus. Safety, viremia and immunogenicity of ChimeriVax-WN02 were assessed in one phase I study and in two phase II clinical trials. No safety signals were detected in the three clinical trials with no remarkable differences in incidence of adverse events (AEs) between vaccine and placebo recipients. Viremia was transient and the mean viremia levels were low. The vaccine elicited strong and durable neutralizing antibody and cytotoxic T cell responses. WN epidemiology impedes a classical licensure pathway; therefore, innovative licensure strategies should be explored.

A chimeric dengue virus vaccine using Japanese encephalitis virus vaccine strain SA14-14-2 as backbone is immunogenic and protective against either parental virus in mice and nonhuman primates

The development of a safe and efficient dengue vaccine represents a global challenge in public health. Chimeric dengue viruses (DENV) based on an attenuated flavivirus have been well developed as vaccine candidates by using reverse genetics. In this study, based on the full-length infectious cDNA clone of the well-known Japanese encephalitis virus live vaccine strain SA14-14-2 as a backbone, a novel chimeric dengue virus (named ChinDENV) was rationally designed and constructed by replacement with the premembrane and envelope genes of dengue 2 virus. The recovered chimeric virus showed growth and plaque properties similar to those of the parental DENV in mammalian and mosquito cells. ChinDENV was highly attenuated in mice, and no viremia was induced in rhesus monkeys upon subcutaneous inoculation. ChinDENV retained its genetic stability and attenuation phenotype after serial 15 passages in cultured cells. A single immunization with various doses of ChinDENV elicited strong neutralizing antibodies in a dose-dependent manner. When vaccinated monkeys were challenged with wild-type DENV, all animals except one that received the lower dose were protected against the development of viremia. Furthermore, immunization with ChinDENV conferred efficient cross protection against lethal JEV challenge in mice in association with robust cellular immunity induced by the replicating nonstructural proteins. Taken together, the results of this preclinical study well demonstrate the great potential of ChinDENV for further development as a dengue vaccine candidate, and this kind of chimeric flavivirus based on JE vaccine virus represents a powerful tool to deliver foreign antigens.

Fever versus fever: the role of host and vector susceptibility and interspecific competition in shaping the current and future distributions of the sylvatic cycles of dengue virus and yellow fever virus

Two different species of flaviviruses, dengue virus (DENV) and yellow fever virus (YFV), that originated in sylvatic cycles maintained in non-human primates and forest-dwelling mosquitoes have emerged repeatedly into sustained human-to-human transmission by Aedes aegypti mosquitoes. Sylvatic cycles of both viruses remain active, and where the two viruses overlap in West Africa they utilize similar suites of monkeys and Aedes mosquitoes. These extensive similarities render the differences in the biogeography and epidemiology of the two viruses all the more striking. First, the sylvatic cycle of YFV originated in Africa and was introduced into the New World, probably as a result of the slave trade, but is absent in Asia; in contrast, sylvatic DENV likely originated in Asia and has spread to Africa but not to the New World. Second, while sylvatic YFV can emerge into extensive urban outbreaks in humans, these invariably die out, whereas four different types of DENV have established human transmission cycles that are ecologically and evolutionarily distinct from their sylvatic ancestors. Finally, transmission of YFV among humans has been documented only in Africa and the Americas, whereas DENV is transmitted among humans across most of the range of competent Aedes vectors, which in the last decade has included every continent save Antarctica. This review summarizes current understanding of sylvatic transmission cycles of YFV and DENV, considers possible explanations for their disjunct distributions, and speculates on the potential consequences of future establishment of a sylvatic cycle of DENV in the Americas.

Impact of Wolbachia on infection with chikungunya and yellow fever viruses in the mosquito vector Aedes aegypti

Incidence of disease due to dengue (DENV), chikungunya (CHIKV) and yellow fever (YFV) viruses is increasing in many parts of the world. The viruses are primarily transmitted by Aedes aegypti, a highly domesticated mosquito species that is notoriously difficult to control. When transinfected into Ae. aegypti, the intracellular bacterium Wolbachia has recently been shown to inhibit replication of DENVs, CHIKV, malaria parasites and filarial nematodes, providing a potentially powerful biocontrol strategy for human pathogens. Because the extent of pathogen reduction can be influenced by the strain of bacterium, we examined whether the wMel strain of Wolbachia influenced CHIKV and YFV infection in Ae. aegypti. Following exposure to viremic blood meals, CHIKV infection and dissemination rates were significantly reduced in mosquitoes with the wMel strain of Wolbachia compared to Wolbachia-uninfected controls. However, similar rates of infection and dissemination were observed in wMel infected and non-infected Ae. aegypti when intrathoracic inoculation was used to deliver virus. YFV infection, dissemination and replication were similar in wMel-infected and control mosquitoes following intrathoracic inoculations. In contrast, mosquitoes with the wMelPop strain of Wolbachia showed at least a 10⁴ times reduction in YFV RNA copies compared to controls. The extent of reduction in virus infection depended on Wolbachia strain, titer and strain of the virus, and mode of exposure. Although originally proposed for dengue biocontrol, our results indicate a Wolbachia-based strategy also holds considerable promise for YFV and CHIKV suppression.

Mosquito-transmitted viruses such as dengue, yellow fever and chikungunya, are responsible for significant morbidity and mortality throughout tropical and sub-tropical regions of the world. These viruses are primarily transmitted by Aedes aegypti, a mosquito that due to its close association with humans has historically been difficult to control. An innovative control strategy involving the release of mosquitoes infected with the intracellular bacterium Wolbachia is currently being developed. This approach is based on the recent discovery that Wolbachia reduces infection of mosquitoes with dengue virus, malaria parasites and filarial nematodes. In the current study, we demonstrated that Wolbachia also blocks infection of chikungunya and yellow fever viruses in Ae. aegypti. The degree of virus inhibition depended on the strain of Wolbachia, the route of virus exposure, the virus strain and the titer of virus that the mosquitoes were exposed to. The implementation of Wolbachia-based control strategies has the capacity to transform the way that mosquitotransmitted diseases are controlled in the future.

Vector competence of Australian mosquitoes for yellow fever virus

The vector competence of Australian mosquitoes for yellow fever virus (YFV) was evaluated. Infection and transmission rates in Cairns and Townsville populations of Aedes aegypti and a Brisbane strain of Ae. notoscriptus were not significantly different from a well-characterized YFV-susceptible strain of Ae. aegypti. After exposure to 10⁷·² tissue culture infectious dose (TCID₅₀)/mL of an African strain of YFV, > 70% of Ae. aegypti and Ae. notoscriptus became infected, and > 50% transmitted the virus. When exposed to 10⁶·⁷) TCID₅₀/mL of a South American strain of YFV, the highest infection (64%) and transmission (56%) rates were observed in Ae. notoscriptus. The infection and transmission rates in the Cairns Ae. aegypti were both 24%, and they were 36% and 28%, respectively, for the Townsville population. Because competent vectors are present, the limited number of travelers from endemic areas and strict vaccination requirements will influence whether YFV transmission occurs in Australia.

Stability of yellow fever virus under recombinatory pressure as compared with chikungunya virus

Recombination is a mechanism whereby positive sense single stranded RNA viruses exchange segments of genetic information. Recent phylogenetic analyses of naturally occurring recombinant flaviviruses have raised concerns regarding the potential for the emergence of virulent recombinants either post-vaccination or following co-infection with two distinct wild-type viruses. To characterize the conditions and sequences that favor RNA arthropod-borne virus recombination we constructed yellow fever virus (YFV) 17D recombinant crosses containing complementary deletions in the envelope protein coding sequence. These constructs were designed to strongly favor recombination, and the detection conditions were optimized to achieve high sensitivity recovery of putative recombinants. Full length recombinant YFV 17D virus was never detected under any of the experimental conditions examined, despite achieving estimated YFV replicon co-infection levels of ∼2.4×10⁶ in BHK-21 (vertebrate) cells and ∼1.05×10⁵ in C₇10 (arthropod) cells. Additionally YFV 17D superinfection resistance was observed in vertebrate and arthropod cells harboring a primary infection with wild-type YFV Asibi strain. Furthermore recombination potential was also evaluated using similarly designed chikungunya virus (CHIKV) replicons towards validation of this strategy for recombination detection. Non-homologus recombination was observed for CHIKV within the structural gene coding sequence resulting in an in-frame duplication of capsid and E3 gene. Based on these data, it is concluded that even in the unlikely event of a high level acute co-infection of two distinct YFV genomes in an arthropod or vertebrate host, the generation of viable flavivirus recombinants is extremely unlikely.

IMOJEV(®): a Yellow fever virus-based novel Japanese encephalitis vaccine

Japanese encephalitis (JE) is a disease of the CNS caused by Japanese encephalitis virus (JEV). The disease appears in the form of frequent outbreaks in most south- and southeast Asian countries and the virus has become endemic in several areas. There is no licensed therapy available and disease control by vaccination is considered to be most effective. Mouse brain-derived inactivated JE vaccines, although immunogenic, have several limitations in terms of safety, availability and requirement for multiple doses. Owing to these drawbacks, the WHO called for the development of novel, safe and more efficacious JE vaccines. Several candidate vaccines have been developed and at least three of them that demonstrated strong immunogenicity after one or two doses of the vaccine in animal models were subsequently tested in various clinical trials. One of these vaccines, IMOJEV(®) (JE-CV and previously known as ChimeriVax™-JE), is a novel recombinant chimeric virus vaccine, developed using the Yellow fever virus (YFV) vaccine vector YFV17D, by replacing the cDNA encoding the envelope proteins of YFV with that of an attenuated JEV strain SA14-14-2. IMOJEV was found to be safe, highly immunogenic and capable of inducing long-lasting immunity in both preclinical and clinical trials. Moreover, a single dose of IMOJEV was sufficient to induce protective immunity, which was similar to that induced in adults by three doses of JE-VAX(®), a mouse brain-derived inactivated JE vaccine. Recently, Phase III trials evaluating the immunogenicity and safety of the chimeric virus vaccine have been successfully completed in some JE-endemic countries and the vaccine manufacturers have filed an application for vaccine registration. IMOJEV may thus be licensed for use in humans as an improved alternative to the currently licensed JE vaccines.

Replicating viral vectors as HIV vaccines: summary report from the IAVI-sponsored satellite symposium at the AIDS vaccine 2009 conference

In October 2009, The International AIDS Vaccine Initiative (IAVI) convened a satellite symposium entitled 'Replicating Viral Vectors for use in AIDS Vaccines' at the AIDS Vaccine 2009 Conference in Paris. The purpose of the symposium was to gather together researchers, representatives from regulatory agencies, and vaccine developers to discuss issues related to advancement of replication-competent viral vector- based HIV vaccines into clinical trials. The meeting introduced the rationale for accelerating the development of replicating viral vectors for use as AIDS vaccines. It noted that the EMEA recently published draft guidelines that are an important first step in providing guidance for advancing live viral vectors into clinical development. Presentations included case studies and development challenges for viral vector-based vaccine candidates. These product development challenges included cell substrates used for vaccine manufacturing, the testing needed to assess vaccine safety, conducting clinical trials with live vectors, and assessment of vaccination risk versus benefit. More in depth discussion of risk and benefit highlighted the fact that AIDS vaccine efficacy trials must be conducted in the developing world where HIV incidence is greatest and how inequities in global health dramatically influence the political and social environment in developing countries.

Preclinical and clinical development of YFV 17D-based chimeric vaccines against dengue, West Nile and Japanese encephalitis viruses

Dengue viruses (DENV), West Nile virus (WNV) and Japanese encephalitis virus (JEV) are major global health and growing medical problems. While a live-attenuated vaccine exists since decades against the prototype flavivirus, yellow fever virus (YFV), there is an urgent need for vaccines against dengue or West Nile diseases, and for improved vaccines against Japanese encephalitis. Live-attenuated chimeric viruses were constructed by replacing the genes coding for Premembrane (prM) and Envelope (E) proteins from YFV 17D vaccine strain with those of heterologous flaviviruses (ChimeriVax technology). This technology has been used to produce vaccine candidates for humans, for construction of a horse vaccine for West Nile fever, and as diagnostic reagents for dengue, Japanese encephalitis, West Nile and St. Louis encephalitis infections. This review focuses on human vaccines and their characterization from the early stages of research through to clinical development. Phenotypic and genetic properties and stability were examined, preclinical evaluation through in vitro or animal models, and clinical testing were carried out. Theoretical environmental concerns linked to the live and genetically modified nature of these vaccines have been carefully addressed. Results of the extensive characterizations are in accordance with the immunogenicity and excellent safety profile of the ChimeriVax-based vaccine candidates, and support their development towards large-scale efficacy trials and registration.

Effects of the number of amino acid residues in the signal segment upstream or downstream of the NS2B-3 cleavage site on production and secretion of prM/M-E virus-like particles of West Nile virus

Expression of genes for precursor M (prM) and envelope (E) proteins of West Nile virus (WNV) leads to the production of small, capsidless, and non-infectious virus-like particles (VLPs) possessing the E antigen which is responsible for viral entry and immune protection. It has been reported that processing of the secretion signal affects viral release. We examined the secretion efficiency of VLPs into the culture medium from RK13 or 293T cells transfected with expression vectors for prM and E proteins of WNV which were constructed to comprise different lengths of signal peptides upstream of the prM-E domain. The number of amino acid residues present in the segment markedly affected the production, processing, and secretion of VLPs. Secreted VLPs possessed both the processed M protein and the glycosylated E protein. In addition, immunization with VLPs induced neutralizing antibodies in C3H/HeN mice. These results indicate that the number of amino acid residues comprising the N-terminus of the signal segment controls the efficiency of assembly, maturation, and release of VLPs in the absence of viral protease, which in turn indicates the potential of VLPs as a candidate for an effective WNV subunit vaccine.

Recombinant chimeric virus with wild-type dengue 4 virus premembrane and envelope and virulent yellow fever virus Asibi backbone sequences is dramatically attenuated in nonhuman primates

Candidate vaccine ChimeriVax viruses are attenuated, efficacious, safe, and highly unlikely to be transmitted by arthropod vectors. Nevertheless, concerns have been raised about the use of these vaccines because of the potential for recombination between vaccine and wild-type (WT) strains. To evaluate the vertebrate pathogenicity of such a worst-case recombinant, ChimeriVax-dengue (DEN) 4 virus was chimerized with the WT Asibi yellow fever virus. In this worst-case scenario, chimeric viruses remained fully attenuated in nonhuman primates. We therefore conclude that, even in the highly unlikely event of "virulent" backbone reversion, the safety of ChimeriVax-DEN vaccines would not be compromised.