Creation of a nonspreading Rift Valley fever virus

Genomic and phylogenetic profiling of RNA of tick-borne arboviruses in Hainan Island, China

Ticks act as vectors and hosts of numerous arboviruses. Examples of medically important arboviruses include the tick-borne encephalitis virus, Crimean Congo hemorrhagic fever, and severe fever with thrombocytopenia syndrome. Recently, some novel arboviruses have been identified in blood specimens of patients with unexplained fever and a history of tick bites in Inner Mongolia. Consequently, tick-borne viruses are a major focus of infectious disease research. However, the spectrum of tick-borne viruses in subtropical areas of China has yet to be sufficiently characterized. In this study, we collected 855 ticks from canine and bovine hosts in four locations in Hainan Province. The ticks were combined into 18 pools according to genus and location. Viral RNA-sequence libraries were subjected to transcriptome sequencing analysis. Molecular clues from metagenomic analyses were used to classify sequence reads into virus species, genera, or families. The diverse viral reads closely associated with mammals were assigned to 12 viral families and important tick-borne viruses, such as Jingmen, Beiji nairovirus, and Colorado tick fever. Our virome and phylogenetic analyses of the arbovirus strains provide basic data for preventing and controlling human infectious diseases caused by tick-borne viruses in the subtropical areas of China.

Quantifying Rift Valley fever virus transmission efficiency in a lamb-mosquito-lamb model

Rift Valley fever virus (RVFV) is a (re)emerging mosquito-borne pathogen impacting human and animal health. How RVFV spreads through a population depends on population-level and individual-level interactions between vector, host and pathogen. Here, we estimated the probability for RVFV to transmit to naive animals by experimentally exposing lambs to a bite of an infectious mosquito, and assessed if and how RVFV infection subsequently developed in the exposed animal. Aedes aegypti mosquitoes, previously infected via feeding on a viremic lamb, were used to expose naive lambs to the virus. Aedes aegypti colony mosquitoes were used as they are easy to maintain and readily feed in captivity. Other mosquito spp. could be examined with similar methodology. Lambs were exposed to either 1-3 (low exposure) or 7-9 (high exposure) infectious mosquitoes. All lambs in the high exposure group became viremic and showed characteristic signs of Rift Valley fever within 2-4 days post exposure. In contrast, 3 out of 12 lambs in the low exposure group developed viremia and disease, with similar peak-levels of viremia as the high exposure group but with some heterogeneity in the onset of viremia. These results suggest that the likelihood for successful infection of a ruminant host is affected by the number of infectious mosquitoes biting, but also highlights that a single bite of an infectious mosquito can result in disease. The per bite mosquito-to-host transmission efficiency was estimated at 28% (95% confidence interval: 15 - 47%). We subsequently combined this transmission efficiency with estimates for life traits of Aedes aegypti or related mosquitoes into a Ross-McDonald mathematical model to illustrate scenarios under which major RVFV outbreaks could occur in naïve populations (i.e., R0 >1). The model revealed that relatively high vector-to-host ratios as well as mosquitoes feeding preferably on competent hosts are required for R0 to exceed 1. Altogether, this study highlights the importance of experiments that mimic natural exposure to RVFV. The experiments facilitate a better understanding of the natural progression of disease and a direct way to obtain epidemiological parameters for mathematical models.

Advancements in Rift Valley fever vaccines: a historical overview and prospects for next generation candidates

Rift Valley fever (RVF) is a zoonotic viral disease transmitted by mosquitoes and causes abortion storms, fetal malformations, and newborn animal deaths in livestock ruminants. In humans, RVF can manifest as hemorrhagic fever, encephalitis, or retinitis. Outbreaks of RVF have been occurring in Africa since the early 20th century and continue to pose a threat to both humans and animals in various regions such as Africa, Madagascar, the Comoros, Saudi Arabia, and Yemen. The development of RVF vaccines is crucial in preventing mortality and morbidity and reducing the spread of the virus. While several veterinary vaccines have been licensed in endemic countries, there are currently no licensed RVF vaccines for human use. This review provides an overview of the existing RVF vaccines, as well as potential candidates for future studies on RVF vaccine development, including next-generation vaccines that show promise in combating the disease in both humans and animals.

Segmented, Negative-Sense RNA Viruses of Humans: Genetic Systems and Experimental Uses of Reporter Strains

Negative-stranded RNA viruses are a large group of viruses that encode their genomes in RNA across multiple segments in an orientation antisense to messenger RNA. Their members infect broad ranges of hosts, and there are a number of notable human pathogens. Here, we examine the development of reverse genetic systems as applied to these virus families, emphasizing conserved approaches illustrated by some of the prominent members that cause significant human disease. We also describe the utility of their genetic systems in the development of reporter strains of the viruses and some biological insights made possible by their use. To conclude the review, we highlight some possible future uses of reporter viruses that not only will increase our basic understanding of how these viruses replicate and cause disease but also could inform the development of new approaches to therapeutically intervene.

Viral Replicon Systems and Their Biosafety Aspects

Introduction

Viral RNA replicons are self-amplifying RNA molecules generated by deleting genetic information of one or multiple structural proteins of wild-type viruses. Remaining viral RNA is used as such (naked replicon) or packaged into a viral replicon particle (VRP), whereby missing genes or proteins are supplied via production cells. Since replicons mostly originate from pathogenic wild-type viruses, careful risk consideration is crucial.

Methods

A literature review was performed compiling information on potential biosafety risks of replicons originating from positive- and negative-sense single-stranded RNA viruses (except retroviruses).

Results

For naked replicons, risk considerations included genome integration, persistence in host cells, generation of virus-like vesicles, and off-target effects. For VRP, the main risk consideration was formation of primary replication competent virus (RCV) as a result of recombination or complementation. To limit the risks, mostly measures aiming at reducing the likelihood of RCV formation have been described. Also, modifying viral proteins in such a way that they do not exhibit hazardous characteristics in the unlikely event of RCV formation has been reported.

Discussion and Conclusion

Despite multiple approaches developed to reduce the likelihood of RCV formation, scientific uncertainty remains on the actual contribution of the measures and on limitations to test their effectiveness. In contrast, even though effectiveness of each individual measure is unclear, using multiple measures on different aspects of the system may create a solid barrier. Risk considerations identified in the current study can also be used to support risk group assignment of replicon constructs based on a purely synthetic design.

Pseudotyped Viruses for Phlebovirus

Rift Valley fever virus (RVFV) is a member of the Phlebovirus genus, one of the 20 genera in the Phenuiviridae family. RVFV causes disease in animals and humans and is transmitted by sandflies or ticks. However, research into RVFV is limited by the requirement for biosafety level 3 (BSL-3) containment. Pseudotyped virus overcomes this limitation as it can be handled in a BSL-2 environment. Pseudotyped RVFV possesses an identical envelope protein structure to that of the authentic virus, simulating the same process of receptor binding and membrane fusion to host cells. Pseudotyped phleboviruses are therefore useful tools to study the infection mechanism of these viruses and for the screening of inhibitory drugs and the development of therapeutic monoclonal antibodies.

Incomplete bunyavirus particles can cooperatively support virus infection and spread

Bunyaviruses lack a specific mechanism to ensure the incorporation of a complete set of genome segments into each virion, explaining the generation of incomplete virus particles lacking one or more genome segments. Such incomplete virus particles, which may represent the majority of particles produced, are generally considered to interfere with virus infection and spread. Using the three-segmented arthropod-borne Rift Valley fever virus as a model bunyavirus, we here show that two distinct incomplete virus particle populations unable to spread autonomously are able to efficiently complement each other in both mammalian and insect cells following co-infection. We further show that complementing incomplete virus particles can co-infect mosquitoes, resulting in the reconstitution of infectious virus that is able to disseminate to the mosquito salivary glands. Computational models of infection dynamics predict that incomplete virus particles can positively impact virus spread over a wide range of conditions, with the strongest effect at intermediate multiplicities of infection. Our findings suggest that incomplete particles may play a significant role in within-host spread and between-host transmission, reminiscent of the infection cycle of multipartite viruses.

Proteome-Wide and Protein-Specific Multi-Epitope Vaccine Constructs Against the Rift Valley Fever Virus Outbreak Using Integrated Omics Approaches

Rift Valley fever (RVF) is a viral disease caused by a member of the Bunyavirales family causing severe infections in humans. The RVF virus is an enveloped, negative-sense, single-stranded RNA virus that can infect both animals and humans. The symptoms associated with these infections span from minor (fever and headaches) to severe (meningoencephalitis and hemorrhagic fever syndrome) symptoms. Despite the outbreaks of the RVF virus being reported in different parts of the world, no effective therapy is available. Herein, the development of an efficient vaccine is critical for the control of infections associated with the RVF virus. Moreover, computational vaccine approaches are helpful in the design of specific, safe, and stable peptide-based designs when compared to the conventional methods of vaccine development. In this study, the whole proteome of the virus, comprising four proteins (NP, L, GP, and NSP), was screened to find putative vaccine epitope sequences (T cell, B cell, and HTL) specific for each protein. These shortlisted epitopes were then combined with flexible linkers to design protein-specific and proteome-wide immunogenic multi-epitope-based vaccine constructs. The results revealed that these multi-epitope vaccine constructs (MEVCs) are strongly antigenic and non-allergenic in nature. The efficacy of these constructs was further validated by docking with immune receptors, which revealed strong binding interactions with human TLR8. Using the MD simulation approach, the binding stability and residual flexibility of the best vaccine construct (proteome-wide) were confirmed, which revealed stable dynamic and favorable features. Furthermore, in-silico cloning and immune simulation analysis confirmed the expression and production of immune factors, that is, IgM, IgG, and IL-6, against the proposed vaccine designs. Additionally, 3D models of all the MEVC constructs have been developed and evaluated for potential immunization against the RVF virus. Finally, the proteome-wide vaccine candidate (MEVC-PW-RVFV) with the highest immune reinforcement potential provides new insights into the development of future vaccines against the emerging RVF virus.

Safety and immunogenicity of four-segmented Rift Valley fever virus in the common marmoset

Rift Valley fever virus (RVFV) is an emerging mosquito-borne bunyavirus that is highly pathogenic to wild and domesticated ruminants, camelids, and humans. While animals are exclusively infected via mosquito bites, humans can also be infected via contact with contaminated tissues or blood. No human vaccine is available and commercialized veterinary vaccines do not optimally combine efficacy with safety. We previously reported the development of two novel live-attenuated RVF vaccines, created by splitting the M genome segment and deleting the major virulence determinant NSs. The vaccine candidates, referred to as the veterinary vaccine vRVFV-4s and the human vaccine hRVFV-4s, were shown to induce protective immunity in multiple species after a single vaccination. Anticipating accidental exposure of humans to the veterinary vaccine and the application of hRVFV-4s to humans, the safety of each vaccine was evaluated in the most susceptible nonhuman primate model, the common marmoset (Callithrix jacchus). Marmosets were inoculated with high doses of each vaccine and were monitored for clinical signs as well as for vaccine virus dissemination, shedding, and spreading to the environment. To accurately assess the attenuation of both vaccine viruses, separate groups of marmosets were inoculated with the parent wild-type RVFV strains. Both wild-type strains induced high viremia and disseminated to primary target organs, associated with mild-to-severe morbidity. In contrast, both vaccines were well tolerated with no evidence of dissemination and shedding while inducing potent neutralizing antibody responses. The results of the studies support the unprecedented safety profile of both vaccines for animals and humans.

Identification of Single Amino Acid Changes in the Rift Valley Fever Virus Polymerase Core Domain Contributing to Virus Attenuation In Vivo

Rift Valley fever (RVF) is an arboviral zoonotic disease affecting many African countries with the potential to spread to other geographical areas. RVF affects sheep, goats, cattle and camels, causing a high rate of abortions and death of newborn lambs. Also, humans can be infected, developing a usually self-limiting disease that can turn into a more severe illness in a low percentage of cases. Although different veterinary vaccines are available in endemic areas in Africa, to date no human vaccine has been licensed. In previous works, we described the selection and characterization of a favipiravir-mutagenized RVFV variant, termed 40Fp8, with potential as a RVF vaccine candidate due to the strong attenuation shown in immunocompromised animal models. Compared to the parental South African 56/74 viral strain, 40Fp8 displayed 7 amino acid substitutions in the L-protein, three of them located in the central region corresponding to the catalytic core of the RNA-dependent RNA polymerase (RdRp). In this work, by means of a reverse genetics system, we have analyzed the effect on virulence of these amino acid changes, alone or combined, both in vitro and in vivo. We found that the simultaneous introduction of two changes (G924S and A1303T) in the heterologous ZH548-RVFV Egyptian strain conferred attenuated phenotypes to the rescued viruses as shown in infected mice without affecting virus immunogenicity. Our results suggest that both changes induce resistance to favipiravir likely associated to some fitness cost that could be the basis for the observed attenuation in vivo. Conversely, the third change, I1050V, appears to be a compensatory mutation increasing viral fitness. Altogether, these results provide relevant information for the safety improvement of novel live attenuated RVFV vaccines.

Immunogenicity of a DNA-Based Sindbis Replicon Expressing Crimean-Congo Hemorrhagic Fever Virus Nucleoprotein

Crimean–Congo hemorrhagic fever virus (CCHFV) infrequently causes hemorrhagic fever in humans with a case fatality rate of 30%. Currently, there is neither an internationally approved antiviral drug nor a vaccine against the virus. A replicon based on the Sindbis virus vector encoding the complete open reading frame of a CCHFV nucleoprotein from a South African isolate was prepared and investigated as a possible candidate vaccine. The transcription of CCHFV RNA and recombinant protein production by the replicon were characterized in transfected baby hamster kidney cells. A replicon encoding CCHFV nucleoprotein inserted in plasmid DNA, pSinCCHF-52S, directed transcription of CCHFV RNA in the transfected cells. NIH-III heterozygous mice immunized with pSinCCHF-52S generated CCHFV IgG specific antibodies with notably higher levels of IgG2a compared to IgG1. Splenocytes from mice immunized with pSinCCHF-52S secreted IFN-γ and IL-2, low levels of IL-6 or IL-10, and no IL-4. No specific cytokine production was registered in splenocytes of mock-immunized mice (p < 0.05). Thus, our study demonstrated the expression of CCHFV nucleoprotein by a Sindbis virus vector and its immunogenicity in mice. The spectrum of cytokine production and antibody profile indicated predominantly Th1-type of an anti-CCHFV immune response. Further studies in CCHFV-susceptible animals are necessary to determine whether the induced immune response is protective.

Vaccine Efficacy of Self-Assembled Multimeric Protein Scaffold Particles Displaying the Glycoprotein Gn Head Domain of Rift Valley Fever Virus

Compared to free antigens, antigens immobilized on scaffolds, such as nanoparticles, generally show improved immunogenicity. Conventionally, antigens are conjugated to scaffolds through genetic fusion or chemical conjugation, which may result in impaired assembly or heterogeneous binding and orientation of the antigens. By combining two emerging technologies-i.e., self-assembling multimeric protein scaffold particles (MPSPs) and bacterial superglue-these shortcomings can be overcome and antigens can be bound on particles in their native conformation. In the present work, we assessed whether this technology could improve the immunogenicity of a candidate subunit vaccine against the zoonotic Rift Valley fever virus (RVFV). For this, the head domain of glycoprotein Gn, a known target of neutralizing antibodies, was coupled on various MPSPs to further assess immunogenicity and efficacy in vivo. The results showed that the Gn head domain, when bound to the lumazine synthase-based MPSP, reduced mortality in a lethal mouse model and protected lambs, the most susceptible RVFV target animals, from viremia and clinical signs after immunization. Furthermore, the same subunit coupled to two other MPSPs (Geobacillus stearothermophilus E2 or a modified KDPG Aldolase) provided full protection in lambs as well.

Visualizing the ribonucleoprotein content of single bunyavirus virions reveals more efficient genome packaging in the arthropod host

Bunyaviruses have a genome that is divided over multiple segments. Genome segmentation complicates the generation of progeny virus, since each newly formed virus particle should preferably contain a full set of genome segments in order to disseminate efficiently within and between hosts. Here, we combine immunofluorescence and fluorescence in situ hybridization techniques to simultaneously visualize bunyavirus progeny virions and their genomic content at single-molecule resolution in the context of singly infected cells. Using Rift Valley fever virus and Schmallenberg virus as prototype tri-segmented bunyaviruses, we show that bunyavirus genome packaging is influenced by the intracellular viral genome content of individual cells, which results in greatly variable packaging efficiencies within a cell population. We further show that bunyavirus genome packaging is more efficient in insect cells compared to mammalian cells and provide new insights on the possibility that incomplete particles may contribute to bunyavirus spread as well.

Chinese tree shrew: a permissive model for in vitro and in vivo replication of human adenovirus species B

Human adenovirus (HAdV) species B can cause severe acute respiratory diseases. However, the researches to combat this infection have been hampered by the lack of an animal model permissive to the virus. Here, we report in vitro and in vivo HAdV species B infections of tree shrews, the closest relative of primates. HAdV-3, -7, -14, and -55 efficiently replicated in primary cell cultures. After intranasal inoculation of tree shrews with HAdV-55, the viral replication in the oropharyngeal region remained high until day 5 post-infection and was still detected until day 12. HAdV-55 in the lung or turbinate bone tissues reached the highest levels between days 3 and 5 post-infection, which indicated viral replication in the upper and lower respiratory tracts. HAdV-55 infection caused severe interstitial pneumonia in the animal. IL-8, IL-10, IL-17A, and IFN-γ expression in the peripheral blood mononuclear cells from infected animals was up-regulated. The pre-vaccination with HAdV-55 cleared the virus faster in the respiratory tract, mitigated lung pathological changes. Finally, HAdV-55 infection was propagated among tree shrews. Our study demonstrated that the tree shrew is a permissive animal model for HAdV species B infection and may serve as a valuable platform for testing multiple anti-viral treatments.

A single vaccination with four-segmented rift valley fever virus prevents vertical transmission of the wild-type virus in pregnant ewes

Rift Valley fever virus (RVFV) is a mosquito-transmitted bunyavirus that causes severe outbreaks among wild and domesticated ruminants, of which sheep are the most susceptible. Outbreaks are characterised by high mortality rates among new-born lambs and abortion storms, in which all pregnant ewes in a flock may abort their foetuses. In endemic areas, Rift Valley fever (RVF) can be controlled by vaccination with either inactivated or live-attenuated vaccines. Inactivated vaccines are safe for animals during all physiological stages, including pregnancy. However, optimal efficacy of these vaccines depends on multiple vaccinations and yearly re-vaccination. Live-attenuated vaccines are generally highly efficacious after a single vaccination, but currently available live-attenuated vaccines may transmit to the ovine foetus, resulting in stillbirths, congenital malformations or abortion. We have previously reported the development of a novel live-attenuated RVFV vaccine, named RVFV-4s. This vaccine virus was created by splitting the M genome segment and deleting the major virulence determinant NSs, and was shown to be safe even for the most susceptible species, including pregnant ewes. The demonstrated efficacy and safety profile suggests that RVFV-4s holds promise for veterinary and human application. The RVFV-4s vaccine for veterinary application, here referred to as vRVFV-4s, was shown to provide complete protection after a single vaccination of lambs, goats and cattle. In this work, we evaluated the efficacy of the vRVFV-4s vaccine in pregnant ewes. Anticipating on the extremely high susceptibility of pregnant ewes for RVFV, both a single vaccination and double vaccination were evaluated in two independent experiments. The combined results suggest that a single vaccination with vRVFV-4s is sufficient to protect pregnant ewes and to prevent transmission to the ovine foetus.

Metagenomic Profiling of Viruses Associated with Rhipicephalus microplus Ticks in Yunnan Province, China

Ticks are well known as vectors of many viruses which usually do great harm to human and animal health. Yunnan Province, widely covered by flourishing vegetation and mainly relying on farming husbandry, is abundant with Rhipicephalus microplus ticks. Therefore, it is of great significance to characterize the viral profile present in R. microplus parasitizing on cattle in Yunnan Province. In this study, a total of 7387 R. microplus ticks were collected from cattle and buffalo in the northwest and southeast areas of Yunnan Province from 2015 to 2017. We investigated the virome of R. microplus using next-generation sequencing (NGS) and the prevalence of important identified viruses among tick groups by RT-PCR. It revealed the presence of diverse virus concerning chu-, rhabdo-, phlebo-, flavi- and parvo- viruses in Yunnan. These viruses consist of single-stranded, circular and segmented sense RNAs, showing a greatly diversity in genomic organization. Furthermore, continuous epidemiological survey among ticks reveals broad prevalence of three viruses (Yunnan mivirus 1, Wuhan tick vrius 1 and YN tick-associated phlebovirus 1) and two possible prevalent viruses including a flavivirus-like segmented virus (Jingmen tick virus) and a bovine hokovirus 2 in Yunnan. Serological investigation among cattle indicates that these identified viruses may be infectious to cattle and can elicit corresponding antibody. Our findings on R. microplus-associated viral community will contribute to the prevention of viral disease and tracking the viral evolution. Further analysis is needed to better elucidate the pathogenicity and natural circulation of these viruses.

A Broad-Spectrum Antiviral Peptide Blocks Infection of Viruses by Binding to Phosphatidylserine in the Viral Envelope

The ongoing threat of viral infections and the emergence of antiviral drug resistance warrants a ceaseless search for new antiviral compounds. Broadly-inhibiting compounds that act on elements shared by many viruses are promising antiviral candidates. Here, we identify a peptide derived from the cowpox virus protein CPXV012 as a broad-spectrum antiviral peptide. We found that CPXV012 peptide hampers infection by a multitude of clinically and economically important enveloped viruses, including poxviruses, herpes simplex virus-1, hepatitis B virus, HIV-1, and Rift Valley fever virus. Infections with non-enveloped viruses such as Coxsackie B3 virus and adenovirus are not affected. The results furthermore suggest that viral particles are neutralized by direct interactions with CPXV012 peptide and that this cationic peptide may specifically bind to and disrupt membranes composed of the anionic phospholipid phosphatidylserine, an important component of many viral membranes. The combined results strongly suggest that CPXV012 peptide inhibits virus infections by direct interactions with phosphatidylserine in the viral envelope. These results reiterate the potential of cationic peptides as broadly-acting virus inhibitors.

Multimeric single-domain antibody complexes protect against bunyavirus infections

The World Health Organization has included three bunyaviruses posing an increasing threat to human health on the Blueprint list of viruses likely to cause major epidemics and for which no, or insufficient countermeasures exist. Here, we describe a broadly applicable strategy, based on llama-derived single-domain antibodies (VHHs), for the development of bunyavirus biotherapeutics. The method was validated using the zoonotic Rift Valley fever virus (RVFV) and Schmallenberg virus (SBV), an emerging pathogen of ruminants, as model pathogens. VHH building blocks were assembled into highly potent neutralizing complexes using bacterial superglue technology. The multimeric complexes were shown to reduce and prevent virus-induced morbidity and mortality in mice upon prophylactic administration. Bispecific molecules engineered to present two different VHHs fused to an Fc domain were further shown to be effective upon therapeutic administration. The presented VHH-based technology holds great promise for the development of bunyavirus antiviral therapies.

Viral replicons as valuable tools for drug discovery

RNA viruses can cause severe diseases such as dengue, Lassa, chikungunya and Ebola. Many of these viruses can only be propagated under high containment levels, necessitating the development of low containment surrogate systems such as subgenomic replicons and minigenome systems. Replicons are self-amplifying recombinant RNA molecules expressing proteins sufficient for their own replication but which do not produce infectious virions. Replicons can persist in cells and are passed on during cell division, enabling quick, efficient and high-throughput testing of drug candidates that act on viral transcription, translation and replication. This review will explore the history and potential for drug discovery of hepatitis C virus, dengue virus, respiratory syncytial virus, Ebola virus and norovirus replicon and minigenome systems.

Rift Valley Fever - epidemiological update and risk of introduction into Europe

Rift Valley fever (RVF) is a vector-borne disease transmitted by a broad spectrum of mosquito species, especially Aedes and Culex genus, to animals (domestic and wild ruminants and camels) and humans. Rift Valley fever is endemic in sub-Saharan Africa and in the Arabian Peninsula, with periodic epidemics characterised by 5-15 years of inter-epizootic periods. In the last two decades, RVF was notified in new African regions (e.g. Sahel), RVF epidemics occurred more frequently and low-level enzootic virus circulation has been demonstrated in livestock in various areas. Recent outbreaks in a French overseas department and some seropositive cases detected in Turkey, Tunisia and Libya raised the attention of the EU for a possible incursion into neighbouring countries. The movement of live animals is the most important pathway for RVF spread from the African endemic areas to North Africa and the Middle East. The movement of infected animals and infected vectors when shipped by flights, containers or road transport is considered as other plausible pathways of introduction into Europe. The overall risk of introduction of RVF into EU through the movement of infected animals is very low in all the EU regions and in all MSs (less than one epidemic every 500 years), given the strict EU animal import policy. The same level of risk of introduction in all the EU regions was estimated also considering the movement of infected vectors, with the highest level for Belgium, Greece, Malta, the Netherlands (one epidemic every 228-700 years), mainly linked to the number of connections by air and sea transports with African RVF infected countries. Although the EU territory does not seem to be directly exposed to an imminent risk of RVFV introduction, the risk of further spread into countries neighbouring the EU and the risks of possible introduction of infected vectors, suggest that EU authorities need to strengthen their surveillance and response capacities, as well as the collaboration with North African and Middle Eastern countries.

Immunogenicity Assessment of Rift Valley Fever Virus Virus-Like Particles in BALB/c Mice

Rift Valley fever (RVF) is an acute, febrile zoonotic disease that is caused by the RVF virus (RVFV) and is spread by arthropod vectors. Virus-like particle (VLP) vaccines, which have the advantages of strong immunogenicity and safety, play an important role in the prevention of this disease. VLPs for RVFV were successfully prepared by our research group using a baculovirus-insect cell expression system. To study the immunogenicity of these RVFV VLPs, a correct 3rd or 4th generation recombinant baculovirus, rBac-N-G, was identified and used to infect Sf9 cells, which were cultured in suspension at a large scale. Subsequently, cell debris was removed by centrifugation, and the VLPs were concentrated by ultracentrifugation and purified using a sucrose gradient, after which they were used to immunize BALB/c mice by intramuscular injection. The results showed that the RVFV VLPs prepared by our research group could effectively induce mice to produce RVFV neutralizing antibodies and that the prepared VLPs could stimulate mouse spleen cells to produce high levels of the cytokines IL-4 and IFN-γ. Moreover, the proportion of lymphocytes producing IL-4 and IFN-γ in the spleen of mice immunized with RVFV VLPs was significantly increased. Therefore, the RVFV VLPs prepared in this study had strong immunogenicity and could effectively activate humoral and cellular immunity in mice. This study lays a solid foundation for the development of RVFV VLP vaccine candidates and promotes the healthy development of animal husbandry and human public health.

Rift Valley fever virus targets the maternal-foetal interface in ovine and human placentas

Background

Rift Valley fever virus (RVFV) is an arbovirus of the order Bunyavirales that causes severe disease in ruminants and humans. Outbreaks in sheep herds are characterised by newborn fatalities and abortion storms. The association of RVFV infections with abortions of ovines and other ruminants is well recognized, whereas the pathology resulting in abortion has remained undescribed. Accumulating evidence suggests that RVFV is abortogenic in humans as well, warranting more research on the interaction of RVFV with the ruminant and human placenta.

Methodology/Principal findings

Pregnant ewes were inoculated with a highly virulent strain of RVFV and necropsied at different days post infection. Tissues were collected and analysed by PCR, virus isolation, and immunohistochemistry. The results show that RVFV replicates efficiently in maternal placental epithelial cells before the virus infects foetal trophoblasts. Moreover, the virus was shown to bypass the maternal epithelial cell layer by directly targeting foetal trophoblasts in the haemophagous zone, a region of the ovine placenta where maternal blood is in direct contact with foetal cells. Abortion was associated with widespread necrosis of placental tissues accompanied with severe haemorrhages. Experiments with human placental explants revealed that the same virus strain replicates efficiently in both cyto- and syncytiotrophoblasts.

Conclusions/Significance

This study demonstrates that RVFV targets the foetal-maternal interface in both ovine and human placentas. The virus was shown to cross the ovine placental barrier via two distinct routes, ultimately resulting in placental and foetal demise followed by abortion. Our finding that RVFV replicates efficiently in human trophoblasts underscores the risk of RVFV infection for human pregnancy.

Rift Valley fever virus (RVFV) is a mosquito-borne RNA virus that causes severe disease in ruminants, wildlife and humans in Africa and the Arabian Peninsula. Outbreaks are characterised by high mortality rates among newborn lambs and abortion storms in sheep herds. The severe outcome of RVFV infection during pregnancy in livestock is well documented, whereas the pathological changes that result in abortion have not yet been described. To investigate how RVFV crosses the placenta and how infection results in abortion, pregnant ewes were infected with RVFV and target cells in maternal and foetal tissues were identified at different time points after inoculation. We show that epithelial cells of the ovine placenta and foetal trophoblasts are primary target cells of RVFV and that placental demise is the primary cause of abortion. The same RVFV strain was shown to replicate efficiently in human placental explants, targeting both cyto- and syncytiotrophoblasts.

TRIM69 Inhibits Vesicular Stomatitis Indiana Virus

Vesicular stomatitis Indiana virus (VSIV) is a veterinary pathogen that is also used as a backbone for many oncolytic and vaccine strategies. In natural and therapeutic settings, viral infections like VSIV are sensed by the host, and as a result the host cells make proteins that can protect them from viruses. In the case of VSIV, these antiviral proteins constrain viral replication and protect most healthy tissues from virus infection. In order to understand how VSIV causes disease and how healthy tissues are protected from VSIV-based therapies, it is crucial that we identify the proteins that inhibit VSIV. Here, we show that TRIM69 is an antiviral defense that can potently and specifically block VSIV infection.

Vesicular stomatitis Indiana virus (VSIV), formerly known as vesicular stomatitis virus (VSV) Indiana (VSV_IND), is a model virus that is exceptionally sensitive to the inhibitory action of interferons (IFNs). Interferons induce an antiviral state by stimulating the expression of hundreds of interferon-stimulated genes (ISGs). These ISGs can constrain viral replication, limit tissue tropism, reduce pathogenicity, and inhibit viral transmission. Since VSIV is used as a backbone for multiple oncolytic and vaccine strategies, understanding how ISGs restrict VSIV not only helps in understanding VSIV-induced pathogenesis but also helps us evaluate and understand the safety and efficacy of VSIV-based therapies. Thus, there is a need to identify and characterize the ISGs that possess anti-VSIV activity. Using arrayed ISG expression screening, we identified TRIM69 as an ISG that potently inhibits VSIV. This inhibition was highly specific as multiple viruses, including influenza A virus, HIV-1, Rift Valley fever virus, and dengue virus, were unaffected by TRIM69. Indeed, just one amino acid substitution in VSIV can govern sensitivity/resistance to TRIM69. Furthermore, TRIM69 is highly divergent in human populations and exhibits signatures of positive selection that are consistent with this gene playing a key role in antiviral immunity. We propose that TRIM69 is an IFN-induced inhibitor of VSIV and speculate that TRIM69 could be important in limiting VSIV pathogenesis and might influence the specificity and/or efficacy of vesiculovirus-based therapies.

IMPORTANCE Vesicular stomatitis Indiana virus (VSIV) is a veterinary pathogen that is also used as a backbone for many oncolytic and vaccine strategies. In natural and therapeutic settings, viral infections like VSIV are sensed by the host, and as a result the host cells make proteins that can protect them from viruses. In the case of VSIV, these antiviral proteins constrain viral replication and protect most healthy tissues from virus infection. In order to understand how VSIV causes disease and how healthy tissues are protected from VSIV-based therapies, it is crucial that we identify the proteins that inhibit VSIV. Here, we show that TRIM69 is an antiviral defense that can potently and specifically block VSIV infection.

Use of a Scalable Replicon-Particle Vaccine to Protect Against Lethal Lassa Virus Infection in the Guinea Pig Model

Lassa fever is a viral zoonosis that can be transmitted from person to person, especially in the hospital setting. The disease is endemic to several countries in West Africa and can be a major contributor to morbidity and mortality in affected areas. There are no approved vaccines to prevent Lassa virus infection. In this work, we present a vaccine candidate that combines the scalability and efficacy benefits of a live vaccine with the safety benefits of single-cycle replication. The system consists of Lassa virus replicon particles devoid of the virus essential glycoprotein gene, and a cell line that expresses the glycoprotein products, enabling efficient vaccine propagation. Guinea pigs vaccinated with these particles showed no clinical reaction to the inoculum and were protected against fever, weight loss, and lethality after infection with Lassa virus.

The One Health Approach is Necessary for the Control of Rift Valley Fever Infections in Egypt: A Comprehensive Review

Rift Valley fever (RVF) is an emerging transboundary, mosquito-borne, zoonotic viral disease caused high morbidity and mortality in both human and ruminant populations. It is considered an important threat to both agriculture and public health in African and the Middle Eastern countries including Egypt. Five major RVF epidemics have been reported in Egypt (1977, 1993, 1994, 1997, and 2003). The virus is transmitted in Egypt by different mosquito’s genera such as Aedes, Culex, Anopheles, and Mansonia, leading to abortions in susceptible animal hosts especially sheep, goat, cattle, and buffaloes. Recurrent RVF outbreaks in Egypt have been attributed in part to the lack of routine surveillance for the virus. These periodic epizootics have resulted in severe economic losses. We posit that there is a critical need for new approaches to RVF control that will prevent or at least reduce future morbidity and economic stress. One Health is an integrated approach for the understanding and management of animal, human, and environmental determinants of complex problems such as RVF. Employing the One Health approach, one might engage local communities in surveillance and control of RVF efforts, rather than continuing their current status as passive victims of the periodic RVF incursions. This review focuses upon endemic and epidemic status of RVF in Egypt, the virus vectors and their ecology, transmission dynamics, risk factors, and the ecology of the RVF at the animal/human interface, prevention, and control measures, and the use of environmental and climate data in surveillance systems to predict disease outbreaks.

Genome packaging of the Bunyavirales

The order Bunyavirales comprises nine families of enveloped, negative-strand RNA viruses. Depending on the family and genus, bunyaviruses (i.e. now referring to all members of the Bunyavirales) contain genomes consisting of two to six segments. Each genome segment is encapsidated by multiple copies of the nucleocapsid (N) protein and one or a few molecules of the viral polymerase, forming so-called ribonucleoproteins (RNPs). Incorporation of RNPs into virions is mediated by the interaction of N with the cytoplasmic tails of the structural glycoproteins. Although some selectivity exists in the packaging of RNPs into virions, which seems to be driven by the 5' and 3'-untranslated regions of the genomic RNA segments, evidence is accumulating that bunyavirus genome packaging is a stochastic process.

Interferon-Stimulated Gene (ISG)-Expression Screening Reveals the Specific Antibunyaviral Activity of ISG20

Bunyaviruses pose a significant threat to human health, prosperity, and food security. In response to viral infections, interferons (IFNs) upregulate the expression of hundreds of interferon-stimulated genes (ISGs), whose cumulative action can potently inhibit the replication of bunyaviruses. We used a flow cytometry-based method to screen the ability of ∼500 unique ISGs from humans and rhesus macaques to inhibit the replication of Bunyamwera orthobunyavirus (BUNV), the prototype of both the Peribunyaviridae family and the Bunyavirales order. Candidates possessing antibunyaviral activity were further examined using a panel of divergent bunyaviruses. Interestingly, one candidate, ISG20, exhibited potent antibunyaviral activity against most viruses examined from the Peribunyaviridae, Hantaviridae, and Nairoviridae families, whereas phleboviruses (Phenuiviridae) largely escaped inhibition. Similar to the case against other viruses known to be targeted by ISG20, the antibunyaviral activity of ISG20 is dependent upon its functional RNase activity. Through use of an infectious virus-like particle (VLP) assay (based on the BUNV minigenome system), we confirmed that gene expression from all 3 viral segments is strongly inhibited by ISG20. Using in vitro evolution, we generated a substantially ISG20-resistant BUNV and mapped the determinants of ISG20 sensitivity/resistance. Taking all the data together, we report that ISG20 is a broad and potent antibunyaviral factor but that some bunyaviruses are remarkably ISG20 resistant. Thus, ISG20 sensitivity/resistance may influence the pathogenesis of bunyaviruses, many of which are emerging viruses of clinical or veterinary significance.

IMPORTANCE There are hundreds of bunyaviruses, many of which cause life-threatening acute diseases in humans and livestock. The interferon (IFN) system is a key component of innate immunity, and type I IFNs limit bunyaviral propagation both in vitro and in vivo. Type I IFN signaling results in the upregulation of hundreds of IFN-stimulated genes (ISGs), whose concerted action generates an “antiviral state.” Although IFNs are critical in limiting bunyaviral replication and pathogenesis, much is still unknown about which ISGs inhibit bunyaviruses. Using ISG-expression screening, we examined the ability of ∼500 unique ISGs to inhibit Bunyamwera orthobunyavirus (BUNV), the prototypical bunyavirus. Using this approach, we identified ISG20, an interferon-stimulated exonuclease, as a potent inhibitor of BUNV. Interestingly, ISG20 possesses highly selective antibunyaviral activity, with multiple bunyaviruses being potently inhibited while some largely escape inhibition. We speculate that the ability of some bunyaviruses to escape ISG20 may influence their pathogenesis.

Mutation of adjacent cysteine residues in the NSs protein of Rift Valley fever virus results in loss of virulence in mice

The NSs protein encoded by the S segment of Rift Valley fever virus (RVFV) is the major virulence factor, counteracting the host innate antiviral defence. It contains five highly conserved cysteine residues at positions 39, 40, 149, 178 and 194, which are thought to stabilize the tertiary and quaternary structure of the protein. Here, we report significant differences between clinical, virological, histopathological and host gene responses in BALB/c mice infected with wild-type RVFV (wtRVFV) or a genetic mutant having a double cysteine-to-serine substitution at residues 39 and 40 of the NSs protein (RVFV-C39S/C40S). Mice infected with the wtRVFV developed a fatal acute disease; characterized by high levels of viral replication, severe hepatocellular necrosis, and massive up-regulation of transcription of genes encoding type I and -II interferons (IFN) as well as pro-apoptotic and pro-inflammatory cytokines. The RVFV-C39S/C40S mutant did not cause clinical disease and its attenuated virulence was consistent with virological, histopathological and host gene expression findings in BALB/c mice. Clinical signs in mice infected with viruses containing cysteine-to-serine substitutions at positions 178 or 194 were similar to those occurring in mice infected with the wtRVFV, while a mutant containing a substitution at position 149 caused mild, non-fatal disease in mice. As mutant RVFV-C39S/C40S showed an attenuated phenotype in mice, the molecular mechanisms behind this attenuation were further investigated. The results show that two mechanisms are responsible for the attenuation; (1) loss of the IFN antagonistic propriety characteristic of the wtRVFV NSs and (2) the inability of the attenuated mutant to degrade Proteine Kinase R (PKR).

Transmission of Rift Valley fever virus from European-breed lambs to Culex pipiens mosquitoes

Background

Rift Valley fever virus (RVFV) is a mosquito-borne bunyavirus of the genus Phlebovirus that is highly pathogenic to ruminants and humans. The disease is currently confined to Africa and the Arabian Peninsula, but globalization and climate change may facilitate introductions of the virus into currently unaffected areas via infected animals or mosquitoes. The consequences of such an introduction will depend on environmental factors, the availability of susceptible ruminants and the capacity of local mosquitoes to transmit the virus. We have previously demonstrated that lambs native to the Netherlands are highly susceptible to RVFV and we here report the vector competence of Culex (Cx.) pipiens, the most abundant and widespread mosquito species in the country. Vector competence was first determined after artificial blood feeding of laboratory-reared mosquitoes using the attenuated Clone 13 strain. Subsequently, experiments with wild-type RVFV and mosquitoes hatched from field-collected eggs were performed. Finally, the transmission of RVFV from viremic lambs to mosquitoes was studied.

Principal findings

Artificial feeding experiments using Clone 13 demonstrated that indigenous, laboratory-reared Cx. pipiens mosquitoes are susceptible to RVFV and that the virus can be transmitted via their saliva. Experiments with wild-type RVFV and mosquitoes hatched from field-collected eggs confirmed the vector competence of Cx. pipiens mosquitoes from the Netherlands. To subsequently investigate transmission of the virus under more natural conditions, mosquitoes were allowed to feed on RVFV-infected lambs during the viremic period. We found that RVFV is efficiently transmitted from lambs to mosquitoes, although transmission was restricted to peak viremia. Interestingly, in the mosquito-exposed skin samples, replication of RVFV was detected in previously unrecognized target cells.

Significance

We here report the vector competence of Cx. pipiens mosquitoes from the Netherlands for RVFV. Both laboratory-reared mosquitoes and well as those hatched from field-collected eggs were found to be competent vectors. Moreover, RVFV was transmitted efficiently from indigenous lambs to mosquitoes, although the duration of host infectivity was found to be shorter than previously assumed. Interestingly, analysis of mosquito-exposed skin samples revealed previously unidentified target cells of the virus. Our findings underscore the value of including natural target species in vector competence experiments.

The consequences of first introductions of mosquito-borne viruses into previously unaffected areas depend on environmental factors, the availability of susceptible hosts and local vector populations. We have previously demonstrated that sheep breeds native to the Netherlands are highly susceptible to Rift Valley fever virus (RVFV), a mosquito-borne virus that causes severe outbreaks among domesticated ruminants and humans in Africa and the Arabian Peninsula. To gain further insight into the risk of a future RVFV introduction into the Netherlands, we have now investigated the vector competence of Cx. pipiens, the most abundant mosquito species in the country. Vector competence was first determined after artificial blood feeding and subsequently after feeding on viremic lambs. The results from artificial feeding experiments suggested that indigenous Cx. pipiens mosquitoes are competent vectors. The vector competence of Cx. pipiens was confirmed after feeding on viremic lambs. Transmission from lambs to mosquitoes was found to be very efficient, although largely confined to peak viremia. The localized inflammatory response resulting from mosquito bites was associated with enhanced virus replication in the skin.

Current Status of Rift Valley Fever Vaccine Development

Rift Valley Fever (RVF) is a mosquito-borne zoonotic disease that presents a substantial threat to human and public health. It is caused by Rift Valley fever phlebovirus (RVFV), which belongs to the genus Phlebovirus and the family Phenuiviridae within the order Bunyavirales. The wide distribution of competent vectors in non-endemic areas coupled with global climate change poses a significant threat of the transboundary spread of RVFV. In the last decade, an improved understanding of the molecular biology of RVFV has facilitated significant progress in the development of novel vaccines, including DIVA (differentiating infected from vaccinated animals) vaccines. Despite these advances, there is no fully licensed vaccine for veterinary or human use available in non-endemic countries, whereas in endemic countries, there is no clear policy or practice of routine/strategic livestock vaccinations as a preventive or mitigating strategy against potential RVF disease outbreaks. The purpose of this review was to provide an update on the status of RVF vaccine development and provide perspectives on the best strategies for disease control. Herein, we argue that the routine or strategic vaccination of livestock could be the best control approach for preventing the outbreak and spread of future disease.

Mapping of Transcription Termination within the S Segment of SFTS Phlebovirus Facilitated Generation of NSs Deletant Viruses

SFTS phlebovirus (SFTSV) is an emerging tick-borne bunyavirus that was first reported in China in 2009. Here we report the generation of a recombinant SFTSV (rHB29NSsKO) that cannot express the viral nonstructural protein (NSs) upon infection of cells in culture. We show that rHB29NSsKO replication kinetics are greater in interferon (IFN)-incompetent cells and that the virus is unable to suppress IFN induced in response to viral replication. The data confirm for the first time in the context of virus infection that NSs acts as a virally encoded IFN antagonist and that NSs is dispensable for virus replication. Using 3' rapid amplification of cDNA ends (RACE), we mapped the 3' end of the N and NSs mRNAs, showing that the mRNAs terminate within the coding region of the opposite open reading frame. We show that the 3' end of the N mRNA terminates upstream of a 5'-GCCAGCC-3' motif present in the viral genomic RNA. With this knowledge, and using virus-like particles, we could demonstrate that the last 36 nucleotides of the NSs open reading frame (ORF) were needed to ensure the efficient termination of the N mRNA and were required for recombinant virus rescue. We demonstrate that it is possible to recover viruses lacking NSs (expressing just a 12-amino-acid NSs peptide or encoding enhanced green fluorescent protein [eGFP]) or an NSs-eGFP fusion protein in the NSs locus. This opens the possibility for further studies of NSs and potentially the design of attenuated viruses for vaccination studies.IMPORTANCE SFTS phlebovirus (SFTSV) and related tick-borne viruses have emerged globally since 2009. SFTSV has been shown to cause severe disease in humans. For bunyaviruses, it has been well documented that the nonstructural protein (NSs) enables the virus to counteract the human innate antiviral defenses and that NSs is one of the major determinants of virulence in infection. Therefore, the use of reverse genetics systems to engineer viruses lacking NSs is an attractive strategy to rationally attenuate bunyaviruses. Here we report the generation of several recombinant SFTS viruses that cannot express the NSs protein or have the NSs open reading frame replaced with a reporter gene. These viruses cannot antagonize the mammalian interferon (IFN) response mounted to virus infection. The generation of NSs-lacking viruses was achieved by mapping the transcriptional termination of two S-segment-derived subgenomic mRNAs, which revealed that transcription termination occurs upstream of a 5'-GCCAGCC-3' motif present in the virus genomic S RNA.

Rift Valley fever vaccines: an overview of the safety and efficacy of the live-attenuated MP-12 vaccine candidate

INTRODUCTION

Rift Valley fever (RVF) is a mosquito-borne zoonotic viral disease endemic to Africa and the Arabian Peninsula. High rates of abortion among infected ruminants and hemorrhagic fever in infected humans are major public health concerns. Commercially available veterinary RVF vaccines are important for preventing the spread of the Rift Valley fever virus (RVFV) in endemic countries; however, RVFV outbreaks continue to occur frequently in endemic countries in the 21st century. In the U.S., the live-attenuated MP-12 vaccine has been developed for both animal and human vaccination. This vaccine strain is well attenuated, and a single dose induces neutralizing antibodies in both ruminants and humans. Areas covered: This review describes scientific evidences of MP-12 vaccine efficacy and safety, as well as MP-12 variants recently developed by reverse genetics, in comparison with other RVF vaccines. Expert commentary: The containment of active RVF outbreaks and long-term protection from RVF exposure to infected mosquitoes are important goals for RVF vaccination. MP-12 vaccine will allow immediate vaccination of susceptible animals in case of an unexpected RVF outbreak in the U.S., whereas MP-12 vaccine may be also useful for the RVF control in endemic regions.

Veterinary Replicon Vaccines

Vaccination is essential in livestock farming and in companion animal ownership. Nucleic acid vaccines based on DNA or RNA provide an elegant alternative to those classical veterinary vaccines that have performed suboptimally. Recent advances in terms of rational design, safety, and efficacy have strengthened the position of nucleic acid vaccines in veterinary vaccinology. The present review focuses on replicon vaccines designed for veterinary use. Replicon vaccines are self-amplifying viral RNA sequences that, in addition to the sequence encoding the antigen of interest, contain all elements necessary for RNA replication. Vaccination results in high levels of in situ antigen expression and induction of potent immune responses. Both positive- and negative-stranded viruses have been used to construct replicons, and they can be delivered as RNA, DNA, or viral replicon particles. An introduction to the biology and the construction of different viral replicon vectors is given, and examples of veterinary replicon vaccine applications are discussed.

A glycoprotein subunit vaccine elicits a strong Rift Valley fever virus neutralizing antibody response in sheep

Rift Valley fever virus (RVFV), a member of the Bunyaviridae family, is a mosquito-borne zoonotic pathogen that causes serious morbidity and mortality in livestock and humans. The recent spread of the virus beyond its traditional endemic boundaries in Africa to the Arabian Peninsula coupled with the presence of susceptible vectors in nonendemic countries has created increased interest in RVF vaccines. Subunit vaccines composed of specific virus proteins expressed in eukaryotic or prokaryotic expression systems are shown to elicit neutralizing antibodies in susceptible hosts. RVFV structural proteins, amino-terminus glycoprotein (Gn), and carboxyl-terminus glycoprotein (Gc), were expressed using a recombinant baculovirus expression system. The recombinant proteins were reconstituted as a GnGc subunit vaccine formulation and evaluated for immunogenicity in a target species, sheep. Six sheep were each immunized with a primary dose of 50 μg of each vaccine immunogen with the adjuvant montanide ISA25; at day 21, postvaccination, each animal received a second dose of the same vaccine. The vaccine induced a strong antibody response in all animals as determined by indirect enzyme-linked immunosorbent assay (ELISA). A plaque reduction neutralization test (PRNT80) showed the primary dose of the vaccine was sufficient to elicit potentially protective virus neutralizing antibody titers ranging from 40 to 160, and the second vaccine dose boosted the titer to more than 1280. Furthermore, all animals tested positive for neutralizing antibodies at day 328 postvaccination. ELISA analysis using the recombinant nucleocapsid protein as a negative marker antigen indicated that the vaccine candidate is DIVA (differentiating infected from vaccinated animals) compatible and represents a promising vaccine platform for RVFV infection in susceptible species.

Single-Molecule FISH Reveals Non-selective Packaging of Rift Valley Fever Virus Genome Segments

The bunyavirus genome comprises a small (S), medium (M), and large (L) RNA segment of negative polarity. Although genome segmentation confers evolutionary advantages by enabling genome reassortment events with related viruses, genome segmentation also complicates genome replication and packaging. Accumulating evidence suggests that genomes of viruses with eight or more genome segments are incorporated into virions by highly selective processes. Remarkably, little is known about the genome packaging process of the tri-segmented bunyaviruses. Here, we evaluated, by single-molecule RNA fluorescence in situ hybridization (FISH), the intracellular spatio-temporal distribution and replication kinetics of the Rift Valley fever virus (RVFV) genome and determined the segment composition of mature virions. The results reveal that the RVFV genome segments start to replicate near the site of infection before spreading and replicating throughout the cytoplasm followed by translocation to the virion assembly site at the Golgi network. Despite the average intracellular S, M and L genome segments approached a 1:1:1 ratio, major differences in genome segment ratios were observed among cells. We also observed a significant amount of cells lacking evidence of M-segment replication. Analysis of two-segmented replicons and four-segmented viruses subsequently confirmed the previous notion that Golgi recruitment is mediated by the Gn glycoprotein. The absence of colocalization of the different segments in the cytoplasm and the successful rescue of a tri-segmented variant with a codon shuffled M-segment suggested that inter-segment interactions are unlikely to drive the copackaging of the different segments into a single virion. The latter was confirmed by direct visualization of RNPs inside mature virions which showed that the majority of virions lack one or more genome segments. Altogether, this study suggests that RVFV genome packaging is a non-selective process.

The bunyavirus family is one of the largest virus families on Earth, of which several members cause severe disease in humans, animals or plants. Little is known about the mechanisms that facilitate the production of infectious bunyavirus virions, which should contain at least one copy of the small (S), medium (M) and large (L) genome segment. In this study, we investigated the genome packaging process of the Rift Valley fever virus (RVFV) by visualizing individual genome segments inside infected cells and virions. Experiments performed with wild-type virus, two- and four-segmented variants, and a variant with a codon-shuffled M segment showed that the production of infectious virions is a non-selective process and is unlikely to involve the formation of a supramolecular viral RNA complex. These observations have broad implications for understanding the bunyavirus replication cycle and may facilitate the development of new vaccines and the identification of novel antiviral targets.

RNA Encapsidation and Packaging in the Phleboviruses

The Bunyaviridae represents the largest family of segmented RNA viruses, which infect a staggering diversity of plants, animals, and insects. Within the family Bunyaviridae, the Phlebovirus genus includes several important human and animal pathogens, including Rift Valley fever virus (RVFV), severe fever with thrombocytopenia syndrome virus (SFTSV), Uukuniemi virus (UUKV), and the sandfly fever viruses. The phleboviruses have small tripartite RNA genomes that encode a repertoire of 5–7 proteins. These few proteins accomplish the daunting task of recognizing and specifically packaging a tri-segment complement of viral genomic RNA in the midst of an abundance of host components. The critical nucleation events that eventually lead to virion production begin early on in the host cytoplasm as the first strands of nascent viral RNA (vRNA) are synthesized. The interaction between the vRNA and the viral nucleocapsid (N) protein effectively protects and masks the RNA from the host, and also forms the ribonucleoprotein (RNP) architecture that mediates downstream interactions and drives virion formation. Although the mechanism by which all three genomic counterparts are selectively co-packaged is not completely understood, we are beginning to understand the hierarchy of interactions that begins with N-RNA packaging and culminates in RNP packaging into new virus particles. In this review we focus on recent progress that highlights the molecular basis of RNA genome packaging in the phleboviruses.

Emergency deployment of genetically engineered veterinary vaccines in Europe

On the 9th of November 2015, preceding the World Veterinary Vaccine Congress, a workshop was held to discuss how veterinary vaccines can be deployed more rapidly to appropriately respond to future epizootics in Europe. Considering their potential and unprecedented suitability for surge production, the workshop focussed on vaccines based on genetically engineered viruses and replicon particles. The workshop was attended by academics and representatives from leading pharmaceutical companies, regulatory experts, the European Medicines Agency and the European Commission. We here outline the present regulatory pathways for genetically engineered vaccines in Europe and describe the incentive for the organization of the pre-congress workshop. The participants agreed that existing European regulations on the deliberate release of genetically engineered vaccines into the environment should be updated to facilitate quick deployment of these vaccines in emergency situations.

A Recombinant Rift Valley Fever Virus Glycoprotein Subunit Vaccine Confers Full Protection against Rift Valley Fever Challenge in Sheep

Rift Valley fever virus (RVFV) is a mosquito-borne zoonotic pathogen causing disease outbreaks in Africa and the Arabian Peninsula. The virus has great potential for transboundary spread due to the presence of competent vectors in non-endemic areas. There is currently no fully licensed vaccine suitable for use in livestock or humans outside endemic areas. Here we report the evaluation of the efficacy of a recombinant subunit vaccine based on the RVFV Gn and Gc glycoproteins. In a previous study, the vaccine elicited strong virus neutralizing antibody responses in sheep and was DIVA (differentiating naturally infected from vaccinated animals) compatible. In the current efficacy study, a group of sheep (n = 5) was vaccinated subcutaneously with the glycoprotein-based subunit vaccine candidate and then subjected to heterologous challenge with the virulent Kenya-128B-15 RVFV strain. The vaccine elicited high virus neutralizing antibody titers and conferred complete protection in all vaccinated sheep, as evidenced by prevention of viremia, fever and absence of RVFV-associated histopathological lesions. We conclude that the subunit vaccine platform represents a promising strategy for the prevention and control of RVFV infections in susceptible hosts.

Making Bunyaviruses Talk: Interrogation Tactics to Identify Host Factors Required for Infection

The identification of host cellular genes that act as either proviral or antiviral factors has been aided by the development of an increasingly large number of high-throughput screening approaches. Here, we review recent advances in which these new technologies have been used to interrogate host genes for the ability to impact bunyavirus infection, both in terms of technical advances as well as a summary of biological insights gained from these studies.

Nonspreading Rift Valley Fever Virus Infection of Human Dendritic Cells Results in Downregulation of CD83 and Full Maturation of Bystander Cells

Vaccines based on nonspreading Rift Valley fever virus (NSR) induce strong humoral and robust cellular immune responses with pronounced Th1 polarisation. The present work was aimed to gain insight into the molecular basis of NSR-mediated immunity. Recent studies have demonstrated that wild-type Rift Valley fever virus efficiently targets and replicates in dendritic cells (DCs). We found that NSR infection of cultured human DCs results in maturation of DCs, characterized by surface upregulation of CD40, CD80, CD86, MHC-I and MHC-II and secretion of the proinflammatory cytokines IFN-β, IL-6 and TNF. Interestingly, expression of the most prominent marker of DC maturation, CD83, was consistently downregulated at 24 hours post infection. Remarkably, NSR infection also completely abrogated CD83 upregulation by LPS. Downregulation of CD83 was not associated with reduced mRNA levels or impaired CD83 mRNA transport from the nucleus and could not be prevented by inhibition of the proteasome or endocytic degradation pathways, suggesting that suppression occurs at the translational level. In contrast to infected cells, bystander DCs displayed full maturation as evidenced by upregulation of CD83. Our results indicate that bystander DCs play an important role in NSR-mediated immunity.

Serum levels of inflammatory cytokines in Rift Valley fever patients are indicative of severe disease

BACKGROUND

Rift Valley fever (RVF) is a mosquito-borne viral zoonosis affecting domestic and wild ruminants, camels and humans. Outbreaks of RVF are characterized by a sudden onset of abortions and high mortality amongst domestic ruminants. Humans develop disease ranging from a mild flu-like illness to more severe complications including hemorrhagic syndrome, ocular and neurological lesions and death. During the RVF outbreak in South Africa in 2010/11, a total of 278 human cases were laboratory confirmed, including 25 deaths. The role of the host inflammatory response to RVF pathogenesis is not completely understood.

METHODS

Virus load in serum from human fatal and non-fatal cases was determined by standard tissue culture infective dose 50 (TCID50) titration on Vero cells. Patient serum concentration of chemokines and cytokines involved in inflammatory responses (IL-8, RANTES, CXCL9, MCP-1, IP-10, IL-1β, IL-6, IL-10, TNF and IL-12p70) was determined using cytometric bead assays and flow cytometry.

RESULTS

Fatal cases had a 1-log10 higher TCID50/ml serum concentration of RVF virus (RVFV) than survivors (p < 0.05). There were no significant sequence differences between isolates recovered from fatal and non-fatal cases. Chemokines and pro- and anti-inflammatory cytokines were detected at significantly increased (IL-8, CXCL9, MCP-1, IP-10, IL-10) or decreased (RANTES) levels when comparing fatal cases to infected survivors and uninfected controls, or when comparing combined infected patients to uninfected controls.

CONCLUSIONS

The results suggest that regulation of the host inflammatory responses plays an important role in the outcome of RVFV infection in humans. Dysregulation of the inflammatory response contributes to a fatal outcome. The cytokines and chemokines identified in this study that correlate with fatal outcomes warrant further investigation as markers for disease severity.

Rift Valley fever virus: A review of diagnosis and vaccination, and implications for emergence in Europe

Rift Valley fever virus (RVFV) is a mosquito-borne virus, and is the causative agent of Rift Valley fever (RVF), a zoonotic disease characterised by an increased incidence of abortion or foetal malformation in ruminants. Infection in humans can also lead to clinical manifestations that in severe cases cause encephalitis or haemorrhagic fever. The virus is endemic throughout much of the African continent. However, the emergence of RVFV in the Middle East, northern Egypt and the Comoros Archipelago has highlighted that the geographical range of RVFV may be increasing, and has led to the concern that an incursion into Europe may occur. At present, there is a limited range of veterinary vaccines available for use in endemic areas, and there is no licensed human vaccine. In this review, the methods available for diagnosis of RVFV infection, the current status of vaccine development and possible implications for RVFV emergence in Europe, are discussed.

Reassortment and distinct evolutionary dynamics of Rift Valley Fever virus genomic segments

Rift Valley Fever virus (RVFV) is a member of Bunyaviridae family that causes a febrile disease affecting mainly ruminants and occasionally humans in Africa, with symptoms that range from mid to severe. RVFV has a tri-segmented ssRNA genome that permits reassortment and could generate more virulent strains. In this study, we reveal the importance of reassortment for RVFV evolution using viral gene genealogy inference and phylodynamics. We uncovered seven events of reassortment that originated RVFV lineages with discordant origins among segments. Moreover, we also found that despite similar selection regimens, the three segments have distinct evolutionary dynamics; the longer segment L evolves at a significant lower rate. Episodes of discordance between population size estimates per segment also coincided with reassortment dating. Our results show that RVFV segments are decoupled enough to have distinct demographic histories and to evolve under different molecular rates.

Preliminary Evaluation of a Bunyavirus Vector for Cancer Immunotherapy

Replicon particles of Rift Valley fever virus, referred to as nonspreading Rift Valley fever virus (NSR), are intrinsically safe and highly immunogenic. Here, we demonstrate that NSR-infected human dendritic cells can activate CD8(+) T cells in vitro and that prophylactic and therapeutic vaccinations of mice with NSR encoding a tumor-associated CD8 peptide can control the outgrowth of lymphoma cells in vivo. These results suggest that the NSR system holds promise for cancer immunotherapy.

Single-cycle replicable Rift Valley fever virus mutants as safe vaccine candidates

Rift Valley fever virus (RVFV) is an arbovirus circulating between ruminants and mosquitoes to maintain its enzootic cycle. Humans are infected with RVFV through mosquito bites or direct contact with materials of infected animals. The virus causes Rift Valley fever (RVF), which was first recognized in the Great Rift Valley of Kenya in 1931. RVF is characterized by a febrile illness resulting in a high rate of abortions in ruminants and an acute febrile illness, followed by fatal hemorrhagic fever and encephalitis in humans. Initially, the virus was restricted to the eastern region of Africa, but the disease has now spread to southern and western Africa, as well as outside of the African continent, e.g., Madagascar, Saudi Arabia and Yemen. There is a serious concern that the virus may spread to other areas, such as North America and Europe. As vaccination is an effective tool to control RVFV epidemics, formalin-inactivated vaccines and live-attenuated RVFV vaccines have been used in endemic areas. The formalin-inactivated vaccines require boosters for effective protection, whereas the live-attenuated vaccines enable the induction of protective immunity by a single vaccination. However, the use of live-attenuated RVFV vaccines for large human populations having a varied health status is of concern, because of these vaccines' residual neuro-invasiveness and neurovirulence. Recently, novel vaccine candidates have been developed using replication-defective RVFV that can undergo only a single round of replication in infected cells. The single-cycle replicable RVFV does not cause systemic infection in immunized hosts, but enables the conferring of protective immunity. This review summarizes the properties of various RVFV vaccines and recent progress on the development of the single-cycle replicable RVFV vaccines.

A virus-like particle system identifies the endonuclease domain of Crimean-Congo hemorrhagic fever virus

Crimean-Congo hemorrhagic fever virus (CCHFV; genus Nairovirus) is an extremely pathogenic member of the Bunyaviridae family. Since handling of the virus requires a biosafety level 4 (BSL-4) facility, little is known about pathomechanisms and host interactions. Here, we describe the establishment of a transcriptionally competent virus-like particle (tc-VLP) system for CCHFV. Recombinant polymerase (L), nucleocapsid protein (N) and a reporter minigenome expressed in human HuH-7 cells resulted in formation of transcriptionally active nucleocapsids that could be packaged by coexpressed CCHFV glycoproteins into tc-VLPs. The tc-VLPs resembled authentic virus particles in their protein composition and neutralization sensitivity to anti-CCHFV antibodies and could recapitulate all steps of the viral replication cycle. Particle attachment, entry, and primary transcription were modeled by infection of naive cells. The subsequent steps of genome replication, secondary transcription, and particle assembly and release can be obtained upon passaging the tc-VLPs on cells expressing CCHFV structural proteins. The utility of the VLP system was demonstrated by showing that the endonuclease domain of L is located around amino acid D693, as was predicted in silico by B. Morin et al. (PLoS Pathog 6:e1001038, 2010, http://dx.doi.org/10.1371/journal.ppat.1001038). The tc-VLP system will greatly facilitate studies and diagnostics of CCHFV under non-BSL-4 conditions.

IMPORTANCE

Crimean-Congo hemorrhagic fever virus (CCHFV) is an extremely virulent pathogen of humans. Since the virus can be handled only at the highest biosafety level, research is restricted to a few specialized laboratories. We developed a plasmid-based system to produce virus-like particles with the ability to infect cells and transcribe a reporter genome. Due to the absence of viral genes, the virus-like particles are unable to spread or cause disease, thus allowing study of aspects of CCHFV biology under relaxed biosafety conditions.

Four-segmented Rift Valley fever virus induces sterile immunity in sheep after a single vaccination

Rift Valley fever virus (RVFV), a mosquito-borne virus in the Bunyaviridae family, causes recurrent outbreaks with severe disease in ruminants and occasionally humans. The virus comprises a segmented genome consisting of a small (S), medium (M) and large (L) RNA segment of negative polarity. The M-segment encodes a glycoprotein precursor (GPC) protein that is co-translationally cleaved into Gn and Gc, which are required for virus entry and fusion. Recently we developed a four-segmented RVFV (RVFV-4s) by splitting the M-genome segment, and used this virus to study RVFV genome packaging. Here we evaluated the potential of a RVFV-4s variant lacking the NSs gene (4s-ΔNSs) to induce protective immunity in sheep. Groups of seven lambs were either mock-vaccinated or vaccinated with 10(5) or 10(6) tissue culture infective dose (TCID50) of 4s-ΔNSs via the intramuscular (IM) or subcutaneous (SC) route. Three weeks post-vaccination all lambs were challenged with wild-type RVFV. Mock-vaccinated lambs developed high fever and high viremia within 2 days post-challenge and three animals eventually succumbed to the infection. In contrast, none of the 4s-ΔNSs vaccinated animals developed clinical signs during the course of the experiment. Vaccination with 10(5) TCID50 via the IM route provided sterile immunity, whereas a 10(6) dose was required to induce sterile immunity via SC vaccination. Protection was strongly correlated with the presence of RVFV neutralizing antibodies. This study shows that 4s-ΔNSs is able to induce sterile immunity in the natural target species after a single vaccination, preferably administrated via the IM route.

The Rift Valley fever accessory proteins NSm and P78/NSm-GN are distinct determinants of virus propagation in vertebrate and invertebrate hosts

Rift Valley fever virus (RVFV) is an enzootic virus circulating in Africa that is transmitted to its vertebrate host by a mosquito vector and causes severe clinical manifestations in humans and ruminants. RVFV has a tripartite genome of negative or ambisense polarity. The M segment contains five in-frame AUG codons that are alternatively used for the synthesis of two major structural glycoproteins, G_N and G_C, and at least two accessory proteins, NSm, a 14-kDa cytosolic protein, and P78/NSm-G_N, a 78-kDa glycoprotein. To determine the relative contribution of P78 and NSm to RVFV infectivity, AUG codons were knocked out to generate mutant viruses expressing various sets of the M-encoded proteins. We found that, in the absence of the second AUG codon used to express NSm, a 13-kDa protein corresponding to an N-terminally truncated form of NSm, named NSm′, was synthesized from AUG 3. None of the individual accessory proteins had any significant impact on RVFV virulence in mice. However, a mutant virus lacking both NSm and NSm′ was strongly attenuated in mice and grew to reduced titers in murine macrophages, a major target cell type of RVFV. In contrast, P78 was not associated with reduced viral virulence in mice, yet it appeared as a major determinant of virus dissemination in mosquitoes. This study demonstrates how related accessory proteins differentially contribute to RVFV propagation in mammalian and arthropod hosts.

Paramyxovirus-based production of Rift Valley fever virus replicon particles

Replicon-particle-based vaccines combine the efficacy of live-attenuated vaccines with the safety of inactivated or subunit vaccines. Recently, we developed Rift Valley fever virus (RVFV) replicon particles, also known as nonspreading RVFV (NSR), and demonstrated that a single vaccination with these particles can confer sterile immunity in target animals. NSR particles can be produced by transfection of replicon cells, which stably maintain replicating RVFV S and L genome segments, with an expression plasmid encoding the RVFV glycoproteins, Gn and Gc, normally encoded by the M-genome segment. Here, we explored the possibility to produce NSR with the use of a helper virus. We show that replicon cells infected with a Newcastle disease virus expressing Gn and Gc (NDV-GnGc) were able to produce high levels of NSR particles. In addition, using reverse genetics and site-directed mutagenesis, we were able to create an NDV-GnGc variant that lacks the NDV fusion protein and contains two amino acid substitutions in, respectively, Gn and HN. The resulting virus uses a unique entry pathway that facilitates the efficient production of NSR in a one-component system. The novel system provides a promising alternative for transfection-based NSR production.