Arenavirus reverse genetics for vaccine development

A novel BSL-2 Lassa virus reverse genetics system modelling the complete viral life cycle

Lassa virus (LASV), a risk-group 4 pathogen, must be handled in biosafety level-4 (BSL-4) conditions, thereby limiting its research and antiviral development. Here, we developed a novel LASV reverse genetics system which, to our knowledge, is the first to study the complete LASV life cycle under BSL-2 conditions. Viral particles can be produced efficiently when LASV minigenomic RNA harbouring minimal viral cis-elements and reporter genes is transfected into a helper cell line stably expressing viral NP, GP, Z and L proteins. The resulting defective virions, named LASVmg, can propagate only in the helper cell line, providing a BSL-2 model to study the complete LASV life cycle. Using this model, we found that a previously reported cellular receptor α-dystroglycan is dispensable for LASVmg infection. Furthermore, we showed that ribavirin can inhibit LASVmg infection by inducing viral mutations. This new BSL-2 system should facilitate studying the LASV life cycle and screening antivirals.

Plasmid-Based Lassa Virus Reverse Genetics

Several mammarenaviruses cause hemorrhagic fever (HF) disease in humans and pose a significant public health problem in their endemic regions. The Old World (OW) mammarenavirus Lassa virus (LASV) is estimated to infect several hundred thousand people yearly in West Africa, resulting in high numbers of Lassa fever (LF) cases, a disease associated with high morbidity and mortality. No licensed vaccines are available to combat LASV infection, and anti-LASV drug therapy is limited to the off-label use of ribavirin whose efficacy remains controversial. The development of reverse genetics approaches has provided investigators with a powerful approach for the investigation of the molecular, cell biology and pathogenesis of mammarenaviruses. The use of cell-based minigenome systems has allowed examining the cis- and trans-acting factors involved in viral genome replication and gene transcription, assembly, and budding, which has facilitated the identification of several anti-mammarenavirus candidate drugs. Likewise, it is possible now to rescue infectious recombinant mammarenaviruses from cloned cDNAs containing predetermined mutations in their genomes to investigate virus-host interactions and mechanisms of viral pathogenesis. Reverse genetics have also allowed the generation of mammarenaviruses expressing foreign genes to facilitate virus detection, to identify antiviral drugs, and to generate live-attenuated vaccine (LAV) candidates. Likewise, reverse genetics techniques have allowed the generation of single-cycle infectious, reporter-expressing mammarenaviruses to study some aspects of the biology of HF-causing human mammarenavirus without the need of high security biocontainment laboratories. In this chapter, we describe the experimental procedures to generate recombinant (r)LASV using state-of-the-art plasmid-based reverse genetics.

S RNA Intergenic Deletions Drive Viral Interference during Arenavirus Infections

Arenaviruses, a family of negative-sense RNA viruses spread by rodents, are a leading cause of severe hemorrhagic fever in humans. Due to a paucity of antivirals and vaccines for arenaviruses, there is a need to identify new mechanisms for interfering with arenavirus replication. In several negative-sense RNA viruses, natural viral interference results from the production of non-standard viral genomes (nsVGs) that activate the innate immune system and/or compete for essential viral products. Although it is well established that arenaviruses produce strong interfering activities, it is unknown if they produce interfering nsVGs. Here we show that arenaviruses produce deletions within the intergenic region of their Small (S) RNA genome, which prevents the production of viral mRNA and protein. These deletions are more abundant when arenaviruses are grown in high-interfering conditions and are associated with inhibited viral replication. Overall, we found that arenaviruses produce internal deletions within the S RNA intergenic region that are produced by arenaviruses and can block viral replication. These natural arenavirus interfering molecules provide a new target for the generation of antivirals as well as an alternative strategy for producing attenuated arenaviruses for vaccines.

Development of reverse genetic tools to study Chapare and Machupo viruses

Arenaviruses are highly pathogenic viruses that pose a serious public health threat. Chapare virus (CHAV) and Machupo virus (MACV), two New World arenaviruses, cause hemorrhagic fevers with case fatality rates of up to 45%. Research on therapeutic drug targets and vaccines for these viruses is limited because biosafety level 4 containment is required for handling them. In this study, we developed reverse genetics systems, including minigenomes and recombinant viruses, that will facilitate the study of these pathogens. The minigenome system is based on the S segment of CHAV or MACV genomes expressing the fluorescent reporter gene ZsGreen (ZsG). We also generated recombinant CHAV and MACV with and without the ZsG reporter gene. As a proof-of-concept study, we used both minigenomes and recombinant viruses to test the inhibitory effects of previously reported antiviral compounds. The new reverse genetics system described here will facilitate future therapeutic studies for these two life-threatening arenaviruses.

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.

Establishment of Recombinant Trisegmented Mopeia Virus Expressing Two Reporter Genes for Screening of Mammarenavirus Inhibitors

Highly pathogenic Arenaviruses, like the Lassa Virus (LASV), pose a serious public health threat in affected countries. Research and development of vaccines and therapeutics are urgently needed but hampered by the necessity to handle these pathogens under biosafety level 4 conditions. These containment restrictions make large-scale screens of antiviral compounds difficult. Therefore, the Mopeia virus (MOPV), closely related to LASV, is often used as an apathogenic surrogate virus. We established for the first time trisegmented MOPVs (r3MOPV) with duplicated S segments, in which one of the viral genes was replaced by the reporter genes ZsGreen (ZsG) or Renilla Luciferase (Rluc), respectively. In vitro characterization of the two trisegmented viruses (r3MOPV ZsG/Rluc and r3MOPV Rluc/ZsG), showed comparable growth behavior to the wild type virus and the expression of the reporter genes correlated well with viral titer. We used the reporter viruses in a proof-of-principle in vitro study to evaluate the antiviral activity of two well characterized drugs. IC₅₀ values obtained by Rluc measurement were similar to those obtained by virus titers. ZsG expression was also suitable to evaluate antiviral effects. The trisegmented MOPVs described here provide a versatile and valuable basis for rapid high throughput screening of broadly reactive antiviral compounds against arenaviruses under BSL-2 conditions.

Identification of potent inhibitors of arenavirus and SARS-CoV-2 exoribonucleases by fluorescence polarization assay

Viral exoribonucleases are uncommon in the world of RNA viruses. To date, they have only been identified in the Arenaviridae and the Coronaviridae families. The exoribonucleases of these viruses play a crucial role in the pathogenicity and interplay with host innate immune response. Moreover, coronaviruses exoribonuclease is also involved in a proofreading mechanism ensuring the genetic stability of the viral genome. Because of their key roles in virus life cycle, they constitute attractive target for drug design. Here we developed a sensitive, robust and reliable fluorescence polarization assay to measure the exoribonuclease activity and its inhibition in vitro. The effectiveness of the method was validated on three different viral exoribonucleases, including SARS-CoV-2, Lymphocytic Choriomeningitis and Machupo viruses. We performed a screening of a focused library consisting of 113 metal chelators. Hit compounds were recovered with an IC₅₀ at micromolar level. We confirmed 3 hits in SARS-CoV-2 infected Vero-E6 cells.

Generation of Reporter-Expressing New World Arenaviruses: A Systematic Comparison

Replication-competent reporter-expressing viruses are crucial tools in molecular virology with applications that range from antiviral screening to live-cell imaging of protein spatiotemporal dynamics. However, there is currently little information available regarding viable strategies to develop reporter-expressing arenaviruses. To address this, we used Tacaribe virus (TCRV), an apathogenic BSL2 arenavirus, to assess the feasibility of different reporter expression approaches. We first generated trisegmented TCRV viruses with either the glycoprotein (GP) or nucleoprotein (NP) replaced by a reporter (GFP, mCherry, or nanoluciferase). These viruses were all viable, but showed marked differences in brightness and attenuation. Next, we generated terminal fusions with each of the TCRV proteins (i.e., NP, GP, polymerase (L), matrix protein (Z)) either with or without a T2A self-cleavage site. We tested both the function of the reporter-fused proteins alone, and the viability of corresponding recombinant TCRVs. We successfully rescued viruses with both direct and cleavable reporter fusions at the C-terminus of Z, as well as cleavable N-terminal fusions with NP. These viruses all displayed detectable reporter activity, but were also moderately attenuated. Finally, reporter proteins were inserted into a flexible hinge region within L. These viruses were also viable and showed moderate attenuation; however, reporter expression was only detectable for the luminescent virus. These strategies provide an exciting range of new tools for research into the molecular biology of TCRV that can likely also be adapted to other arenaviruses.

The Virus–Host Interplay in Junín Mammarenavirus Infection

Junín virus (JUNV) belongs to the Arenaviridae family and is the causative agent of Argentine hemorrhagic fever (AHF), a severe human disease endemic to agricultural areas in Argentina. At this moment, there are no effective antiviral therapeutics to battle pathogenic arenaviruses. Cumulative reports from recent years have widely provided information on cellular factors playing key roles during JUNV infection. In this review, we summarize research on host molecular determinants that intervene in the different stages of the viral life cycle: viral entry, replication, assembly and budding. Alongside, we describe JUNV tight interplay with the innate immune system. We also review the development of different reverse genetics systems and their use as tools to study JUNV biology and its close teamwork with the host. Elucidating relevant interactions of the virus with the host cell machinery is highly necessary to better understand the mechanistic basis beyond virus multiplication, disease pathogenesis and viral subversion of the immune response. Altogether, this knowledge becomes essential for identifying potential targets for the rational design of novel antiviral treatments to combat JUNV as well as other pathogenic arenaviruses.

Generation and Characterization of Single-Cycle Infectious Canine Influenza A Virus (sciCIV) and Its Use as Vaccine Platform

Influenza A viruses (IAVs) infect a broad range of hosts, including multiple avian and mammalian species. The frequent emergence of novel IAV strains in different hosts, including in humans, results in the need for vigilance and ongoing development of new approaches to fighting or prevent those infections. Canine influenza is a contagious respiratory disease in dogs caused by two subtypes of IAV, the equine-origin H3N8 canine influenza virus (CIV), and the avian-origin H3N2 CIV. A novel approach to influenza vaccination involves single-cycle infectious influenza A viruses (sciIAVs), which are defective for an essential viral gene. They are propagated in complementing cell lines which provide the missing gene in trans. As sciIAV cannot complete their replication cycle in regular cells they are limited to a single round of viral replication. Because of their safety profile and ability to express foreign antigens inside infected cells, sciIAVs have served both as live-attenuated vaccines and as vaccine vectors for the expression of heterologous antigens. Here, we describe experimental procedures for the generation of a single-cycle infectious CIV (sciCIV), where the viral hemagglutinin (HA) gene was exchanged for the gene for green fluorescent protein (GFP). Complementation of the viral HA protein is provided in trans by stable HA-expressing cell lines. Methods for the in vitro characterization of HA deficient but GFP-expressing sciCIV (sciCIV ΔHA/GFP) are described, as well as its use as a potential vaccine.

Generation of Bi-Reporter-Expressing Tri-Segmented Arenavirus

Reverse genetics systems provide a powerful tool to generate recombinant arenavirus expressing reporters to facilitate the investigation of the arenavirus life cycle and also for the discovery of antiviral countermeasures. The plasmid-encoded viral ribonucleoprotein components initiate the transcription and replication of a plasmid-driven full-length viral genome, resulting in infectious virus. Thereby, this approach is ideal for the generation of recombinant arenaviruses expressing reporter genes that can be used as valid surrogates for virus replication. By splitting the small viral segment (S) into two viral segments (S1 and S2), each of them encoding a reporter gene, recombinant tri-segmented arenavirus can be rescued. Bi-reporter-expressing recombinant tri-segmented arenaviruses represent an excellent tool to study the biology of arenaviruses, including the identification and characterization of both prophylactic and therapeutic countermeasures for the treatment of arenaviral infections. In this chapter, we describe a detailed protocol on the generation and in vitro characterization of recombinant arenaviruses containing a tri-segment genome expressing two reporter genes based on the prototype member in the family, lymphocytic choriomeningitis virus (LCMV). Similar experimental approaches can be used for the generation of bi-reporter-expressing tri-segment recombinant viruses for other members in the arenavirus family.

Advances in Development and Application of Influenza Vaccines

Influenza A virus is one of the most important zoonotic pathogens that can cause severe symptoms and has the potential to cause high number of deaths and great economic loss. Vaccination is still the best option to prevent influenza virus infection. Different types of influenza vaccines, including live attenuated virus vaccines, inactivated whole virus vaccines, virosome vaccines, split-virion vaccines and subunit vaccines have been developed. However, they have several limitations, such as the relatively high manufacturing cost and long production time, moderate efficacy of some of the vaccines in certain populations, and lack of cross-reactivity. These are some of the problems that need to be solved. Here, we summarized recent advances in the development and application of different types of influenza vaccines, including the recent development of viral vectored influenza vaccines. We also described the construction of other vaccines that are based on recombinant influenza viruses as viral vectors. Information provided in this review article might lead to the development of safe and highly effective novel influenza vaccines.

Development of a Simian RNA Polymerase I Promoter-Driven Reverse Genetics for the Rescue of Recombinant Rift Valley Fever Virus from Vero Cells

Rift Valley fever (RVF), which has been designated as a priority disease by the World Health Organization (WHO), is one of the most pathogenic zoonotic diseases endemic to Africa and the Arabian Peninsula. Human vaccine preparation requires the use of appropriate cell substrates to support efficient production of seed vaccine with minimum concerns of tumorigenicity, oncogenicity, or adventitious agents. Vero cells, which were derived from the African green monkey kidney, represent one of the few mammalian cell lines that are used for vaccine manufacturing. This study demonstrated the rescue of RVFV MP-12 infectious clones in Vero cells using plasmids encoding the Macaca mulatta RNA polymerase I promoter. Although Vero cells demonstrated an approximately 20% transfection efficiency, only 0.5% of transfected cells showed the replication of viral genomic RNA, supported by the co-expression of RVFV N and L helper proteins. RVFV Infectious clones were detectable in the culture supernatants approximately 4 to 9 days posttransfection reaching maximum titers during the following 5 days. The re-amplification of rescued recombinant MP-12 (rMP-12) in Vero cells led to an increase in the genetic subpopulations, affecting the viral phenotype via amino acid substitutions in the NSs gene, whereas the rMP-12 re-amplified in human diploid MRC-5 cells did not increase viral sub-populations with NSs gene mutations. The strategy in which RVFV infectious clones are rescued in Vero cells and then subsequently amplified in MRC-5 cells will support the vaccine seed lot systems of live-attenuated recombinant RVFV vaccines for human use.IMPORTANCE RVF is a mosquito-transmitted, viral, zoonotic disease endemic to Africa and the Arabian Peninsula, and its spread outside of the endemic area will potentially cause devastating economic damages and serious public health problems. Different from classical live-attenuated vaccines, live-attenuated recombinant vaccines allow rational improvement of vaccine production efficiency, protective efficacy, and vaccine safety via the genetic engineering. This study demonstrated the generation of infectious Rift Valley fever (RVF) virus from cloned cDNA using Vero cells, which are one of a few mammalian cell lines used for vaccine manufacturing. Subsequent re-amplification of virus clones in Vero cells unexpectedly increased viral subpopulations encoding unfavorable mutations, whereas viral re-amplification in human diploid MRC-5 cells could minimize the emergence of such mutants. Rescue of recombinant RVFV from Vero cells and re-amplification in MRC-5 cells will support the vaccine seed lot systems of live-attenuated recombinant RVFV vaccines for human use.

Development of a Recombinant Pichinde Virus-Vectored Vaccine against Turkey Arthritis Reovirus and Its Immunological Response Characterization in Vaccinated Animals

Vaccination may be an effective way to reduce turkey arthritis reovirus (TARV)-induced lameness in turkey flocks. However, there are currently no commercial vaccines available against TARV infection. Here, we describe the use of reverse genetics technology to generate a recombinant Pichinde virus (PICV) that expresses the Sigma C and/or Sigma B proteins of TARV as antigens. Nine recombinant PICV-based TARV vaccines were developed carrying the wild-type S1 (Sigma C) and/or S3 (Sigma B) genes from three different TARV strains. In addition, three recombinant PICV-based TARV vaccines were produced carrying codon-optimized S1 and/or S3 genes of a TARV strain. The S1 and S3 genes and antigens were found to be expressed in virus-infected cells via reverse transcriptase polymerase chain reaction (RT-PCR) and the direct fluorescent antibody (DFA) technique, respectively. Turkey poults inoculated with the recombinant PICV-based TARV vaccine expressing the bivalent TARV S1 and S3 antigens developed high anti-TARV antibody titers, indicating the immunogenicity (and safety) of this vaccine. Future in vivo challenge studies using a turkey reovirus infection model will determine the optimum dose and protective efficacy of this recombinant virus-vectored candidate vaccine.

Reverse genetics approaches for the development of mammarenavirus live-attenuated vaccines

Several mammarenaviruses can cause severe hemorrhagic fever disease with a very high case fatality rate, representing important threats to human health within the viruses' endemic regions. To date, there are no United States (US) Food and Drug Administration (FDA)-licensed vaccines available to combat mammarenavirus infections in humans, and current anti-mammarenavirus therapy is limited to off-label use of the guanosine analog ribavirin, which has limited efficacy and has been associated with significant side effects. Vaccination is one of the most effective ways to prevent viral diseases, and live-attenuated vaccines (LAVs) have been shown to often provide long-term protection against a subsequent natural infection by the corresponding virulent form of the virus. The development of mammarenavirus reverse genetics systems has provided investigators with a powerful approach for the investigation of the molecular and cell biology of mammarenaviruses and also for the generation of recombinant viruses containing predetermined mutations in their genome for their implementation as LAVs for the treatment of mammarenavirus infections. In this review, we summarize the current knowledge on the mammarenavirus molecular and cell biology, and the use of reverse genetic approaches for the generation of recombinant mammarenaviruses. Moreover, we briefly discus some novel LAV approaches for the treatment of mammarenavirus infections based on the use of reverse genetics approaches.

Development of Reverse Genetics for the Prototype New World Mammarenavirus Tacaribe Virus

The New World mammarenavirus Tacaribe virus (TCRV) has been isolated from fruit bats, mosquitoes, and ticks, whereas all other known New World mammarenaviruses are maintained in rodents. TCRV has not been linked to human disease, but it has been shown to protect against Argentine hemorrhagic fever-like disease in marmosets infected with the New World mammarenavirus Junín virus (JUNV), indicating the potential of TCRV as a live-attenuated vaccine for the treatment of Argentine hemorrhagic fever. Implementation of TCRV as a live-attenuated vaccine or a vaccine vector would be facilitated by the establishment of reverse genetics systems for the genetic manipulation of the TCRV genome. In this study, we developed, for the first time, reverse genetics approaches for the generation of recombinant TCRV (rTCRV). We successfully rescued a wild-type (WT) rTCRV (a trisegmented form of TCRV expressing two reporter genes [r3TCRV]) and a bisegmented TCRV expressing a single reporter gene from a bicistronic viral mRNA (rTCRV/GFP). These reverse genetics approaches represent an excellent tool to investigate the biology of TCRV and to explore its potential use as a live-attenuated vaccine or a vaccine vector for the treatment of other viral infections. Notably, we identified a 39-nucleotide (nt) deletion (Δ39) in the noncoding intergenic region (IGR) of the viral large (L) segment that is required for optimal virus multiplication. Accordingly, an rTCRV containing this 39-nt deletion in the L-IGR (rTCRV/Δ39) exhibited decreased viral fitness in cultured cells, suggesting the feasibility of using this deletion in the L-IGR as an approach to attenuate TCRV, and potentially other mammarenaviruses, for their implementation as live-attenuated vaccines or vaccine vectors.IMPORTANCE To date, no Food and Drug Administration (FDA)-approved vaccines are available to combat hemorrhagic fever caused by mammarenavirus infections in humans. Treatment of mammarenavirus infections is limited to the off-label use of ribavirin, which is partially effective and associated with significant side effects. Tacaribe virus (TCRV), the prototype member of the New World mammarenaviruses, is nonpathogenic in humans but able to provide protection against Junín virus (JUNV), the causative agent of Argentine hemorrhagic fever, demonstrating the feasibility of using TCRV as a live-attenuated vaccine vector for the treatment of JUNV and potentially other viral infections. Here, we describe for the first time the feasibility of generating recombinant TCRV (rTCRV) using reverse genetics approaches, which paves the way to study the biology of TCRV and also its potential use as a live-attenuated vaccine or a vaccine vector for the treatment of mammarenavirus and/or other viral infections in humans.

Reverse genetics approaches for the development of bunyavirus vaccines

The Bunyavirales order is the largest group of RNA viruses, which includes important human and animal pathogens, that cause serious diseases. Licensed vaccines are often not available for many of these pathogens. The establishment of bunyavirus reverse genetics systems has facilitated the generation of recombinant infectious viruses, which have been employed as powerful tools for understanding bunyavirus biology and identifying important virulence factors. Technological advances in this area have enabled the development of novel strategies, including codon-deoptimization, viral genome rearrangement and single-cycle replicable viruses, for the generation of live-attenuated vaccine candidates. In this review, we have summarized the current knowledge of the bunyavirus reverse genetics approaches for the generation of live-attenuated vaccine candidates and their evaluation in animal models.

Rapid Generation of Attenuated Infectious Bursal Disease Virus from Dual-Promoter Plasmids by Reduction of Viral Ribonucleoprotein Activity

Infectious bursal disease virus (IBDV) of the Birnaviridae family leads to immunosuppression of young chickens by destroying B cells in the bursa of Fabricius (BFs). Given the increasing number of variant IBDV strains, we urgently require a method to produce attenuated virus for vaccine development. To accomplish this goal, the dual-promoter plasmids in which the RNA polymerase II and RNA polymerase I (Pol I) promoters were placed upstream of the IBDV genomic sequence, which was followed by mouse Pol I terminator and a synthetic polyadenylation signal, were developed for rapid generation of IBDV. This approach did not require trans-supplementation of plasmids for the expression of VP1 and VP3, the main components of IBDV ribonucleoprotein (RNP). Based on the finding in this study that the IBDV RNP activity was partially retained by VP1-FLAG, we successfully rescued the replication-competent IBDV/1FLAG expressing VP1-FLAG. Compared with its parental counterpart, IBDV/1FLAG formed smaller size plaques in cultured cells and induced the same 100% immune protection in vivo However, neither retarded development nor severe BFs lesion was observed in the IBDV/1FLAG-inoculated chickens. Collectively, this is the first report that viral RNP activity was affected by the addition of an epitope tag on the componential viral proteins. Furthermore, this work demonstrates the rapid generation of attenuated IBDV from dual-promoter plasmids via reducing viral RNP activity by a fused FLAG tag on the C terminus of VP1. This would be a convenient strategy to attenuate epidemic variant IBDV strains for rapid and efficient vaccine development.IMPORTANCE Immunosuppression in chickens as a result of infectious bursal disease virus (IBDV) infection leads to significant economic losses in the poultry industry worldwide every year. Currently, vaccination is still the best way to prevent the prevalence of IBDV. However, with the occurrence of increasing numbers of variant IBDV strains, it is challenging to develop antigen-matched live attenuated vaccine. Here, we first developed a dual-promoter reverse-genetic system for the rapid generation of IBDV. Using this system, the attenuated IBDV/1FLAG expressing VP1-FLAG, which displays the decreased viral RNP activity, was rescued. Moreover, IBDV/1FLAG inoculation induced a similar level of neutralizing antibodies to that of its parental counterpart, protecting chickens against lethal challenge. Our study, for the first time, describes a dual-promoter reverse-genetic approach for the rapid generation of attenuated IBDV while maintaining entire parental antigenicity, suggesting a potential new method to attenuate epidemic variant IBDV strains for vaccine development.

In vivo rescue of recombinant Zika virus from an infectious cDNA clone and its implications in vaccine development

Zika virus (ZIKV) is a mosquito-borne member of the Flaviviridae family that has been known to circulate for decades causing mild febrile illness. The more recent ZIKV outbreaks in the Americas and the Caribbean associated with congenital malformations and Guillain-Barré syndrome in adults have placed public health officials in high alert and highlight the significant impact of ZIKV on human health. New technologies to study the biology of ZIKV and to develop more effective prevention options are highly desired. In this study we demonstrate that direct delivery in mice of an infectious ZIKV cDNA clone allows the rescue of recombinant (r)ZIKV in vivo. A bacterial artificial chromosome containing the sequence of ZIKV strain Paraiba/2015 under the control of the cytomegalovirus promoter was complexed with a commercial transfection reagent and administrated using different routes in type-I interferon receptor deficient A129 mice. Clinical signs and death associated with ZIKV viremia were observed in mice. The rZIKV recovered from these mice remained fully virulent in a second passage in mice. Interestingly, infectious rZIKV was also recovered after intraperitoneal inoculation of the rZIKV cDNA in the absence of transfection reagent. Further expanding these studies, we demonstrate that a single intraperitoneal inoculation of a cDNA clone encoding an attenuated rZIKV was safe, highly immunogenic, and provided full protection against lethal ZIKV challenge. This novel in vivo reverse genetics method is a potentially suitable delivery platform for the study of wild-type and live-attenuated ZIKV devoid of confounding factors typical associated with in vitro systems. Moreover, our results open the possibility of employing similar in vivo reverse genetic approaches for the generation of other viruses and, therefore, change the way we will use reverse genetics in the future.

Novel Functions of IFI44L as a Feedback Regulator of Host Antiviral Responses

We describe a novel function for the interferon (IFN)-induced protein 44-like (IFI44L) gene in negatively modulating innate immune responses induced after virus infections. Furthermore, we show that decreasing IFI44L expression impairs virus production and that IFI44L expression negatively modulates the antiviral state induced by an analog of double-stranded RNA (dsRNA) or by IFN treatment. The mechanism likely involves the interaction of IFI44L with cellular FK506-binding protein 5 (FKBP5), which in turn interacts with kinases essential for type I and III IFN responses, such as inhibitor of nuclear factor kappa B (IκB) kinase alpha (IKKα), IKKβ, and IKKε. Consequently, binding of IFI44L to FKBP5 decreased interferon regulatory factor 3 (IRF-3)-mediated and nuclear factor kappa-B (NF-κB) inhibitor (IκBα)-mediated phosphorylation by IKKε and IKKβ, respectively. According to these results, IFI44L is a good target for treatment of diseases associated with excessive IFN levels and/or proinflammatory responses and for reduction of viral replication.IMPORTANCE Excessive innate immune responses can be deleterious for the host, and therefore, negative feedback is needed. Here, we describe a completely novel function for IFI44L in negatively modulating innate immune responses induced after virus infections. In addition, we show that decreasing IFI44L expression impairs virus production and that IFI44L expression negatively modulates the antiviral state induced by an analog of dsRNA or by IFN treatment. IFI44L binds to the cellular protein FKBP5, which in turn interacts with kinases essential for type I and III IFN induction and signaling, such as the kinases IKKα, IKKβ, and IKKε. IFI44L binding to FKBP5 decreased the phosphorylation of IRF-3 and IκBα mediated by IKKε and IKKβ, respectively, providing an explanation for the function of IFI44L in negatively modulating IFN responses. Therefore, IFI44L is a candidate target for reducing virus replication.

Interferon-Induced Protein 44 Interacts with Cellular FK506-Binding Protein 5, Negatively Regulates Host Antiviral Responses, and Supports Virus Replication

Using multiple viral systems, and performing silencing approaches, overexpression approaches, and experiments in knockout cells, we report, for the first time, that interferon (IFN)-induced protein 44 (IFI44) positively affects virus production and negatively modulates innate immune responses induced after viral infections. Moreover, IFI44 is able to rescue poly(I·C)- and IFN-mediated inhibition of virus growth. Furthermore, we report a novel interaction of IFI44 with the cellular factor FK506-binding protein 5 (FKBP5), which binds to cellular kinases such as the inhibitor of nuclear factor kappa B (IκB) kinases (IKKα, IKKβ, and IKKε). Importantly, in the presence of FKBP5, IFI44 decreases the ability of IKKβ to phosphorylate IκBα and the ability of IKKε to phosphorylate interferon regulatory factor 3 (IRF-3), providing a novel mechanism for the function of IFI44 in negatively modulating IFN responses. Remarkably, these new IFI44 functions may have implications for diseases associated with excessive immune signaling and for controlling virus infections mediated by IFN responses.IMPORTANCE Innate immune responses mediated by IFN and inflammatory cytokines are critical for controlling virus replication. Nevertheless, exacerbated innate immune responses could be detrimental for the host and feedback mechanisms are needed to maintain the cellular homeostasis. In this work, we describe a completely novel function for IFI44 in negatively modulating the innate immune responses induced after viral infections. We show that decreasing IFI44 expression by using small interfering RNAs (siRNAs) or by generating knockout (KO) cells impairs virus production and increases the levels of IFN responses. Moreover, we report a novel interaction of IFI44 with the cellular protein FKBP5, which in turn interacts with kinases essential for type I and III IFN induction and signaling, such as the inhibitor of nuclear factor kappa B (IκB) kinases IKKα, IKKβ, and IKKε. Our data indicate that binding of IFI44 to FKBP5 decreased the phosphorylation of IRF-3 and IκBα mediated by IKKε and IKKβ, respectively, providing a likely explanation for the function of IFI44 in negatively modulating IFN responses. These results provide new insights into the induction of innate immune responses and suggest that IFI44 is a new potential antiviral target for reducing virus replication.

Animal models for Lassa virus infection

In humans, Lassa virus infection can result in disease with hemorrhagic manifestations and high fatality rates. There are no approved treatments or vaccines available and the inherent danger of studying Lassa virus means it can only be studied in high containment labs (BSL4). Under these conditions, mouse models are becoming an important instrument in the study of Lassa virus infection, disease and host responses. While guinea pigs and non-human primates are the critical components in assessing treatments and vaccines and have recently been used with great affect in this capacity.

Reverse genetics systems as tools to overcome the genetic diversity of Lassa virus

Lassa virus is endemic in a large area of sub-Saharan Africa, and exhibits a large amount of genetic diversity. Of the four currently recognized lineages, lineages I-III circulate in Nigeria, and lineage IV circulates in Sierra Leone, Guinea, and Liberia. However, several newly detected lineages have been proposed. LASV genetic diversity may result in differences in pathogenicity or response to medical countermeasures, necessitating the testing of multiple lineages during the development of countermeasures and diagnostics. Logistical and biosafety concerns can make it difficult to obtain representative collections of divergent LASV clades for comparison studies. For example, lack of a cold chain in remote areas, or shipping restrictions on live viruses can prevent the dissemination of natural virus isolates to researchers. Reverse genetics systems that have been developed for LASV can facilitate acquisition of hard-to-obtain LASV strains and enable comprehensive development of medical countermeasures.

High-throughput screening for negative-stranded hemorrhagic fever viruses using reverse genetics

Viral hemorrhagic fevers (VHFs) cause thousands of fatalities every year, but the treatment options for their management remain very limited. In particular, the development of therapeutic interventions is restricted by the lack of commercial viability of drugs targeting individual VHF agents. This makes approaches like drug repurposing and/or the identification of broad range therapies (i.e. those directed at host responses or common proviral factors) highly attractive. However, the identification of candidates for such antiviral repurposing or of host factors/pathways important for the virus life cycle is reliant on high-throughput screening (HTS). Recently, such screening work has been increasingly facilitated by the availability of reverse genetics-based approaches, including tools such as full-length clone (FLC) systems to generate reporter-expressing viruses or various life cycle modelling (LCM) systems, many of which have been developed and/or greatly improved during the last years. In particular, since LCM systems are capable of modelling specific steps in the life cycle, they are a valuable tool for both targeted screening (i.e. for inhibitors of a specific pathway) and mechanism of action studies. This review seeks to summarize the currently available reverse genetics systems for negative-sense VHF causing viruses (i.e. arenaviruses, bunyaviruses and filoviruses), and to highlight the recent advancements made in applying these systems for HTS to identify either antivirals or new virus-host interactions that might hold promise for the development of future treatments for the infections caused by these deadly but neglected virus groups.

Hemorrhagic Fever-Causing Arenaviruses: Lethal Pathogens and Potent Immune Suppressors

Hemorrhagic fevers (HF) resulting from pathogenic arenaviral infections have traditionally been neglected as tropical diseases primarily affecting African and South American regions. There are currently no FDA-approved vaccines for arenaviruses, and treatments have been limited to supportive therapy and use of non-specific nucleoside analogs, such as Ribavirin. Outbreaks of arenaviral infections have been limited to certain geographic areas that are endemic but known cases of exportation of arenaviruses from endemic regions and socioeconomic challenges for local control of rodent reservoirs raise serious concerns about the potential for larger outbreaks in the future. This review synthesizes current knowledge about arenaviral evolution, ecology, transmission patterns, life cycle, modulation of host immunity, disease pathogenesis, as well as discusses recent development of preventative and therapeutic pursuits against this group of deadly viral pathogens.

DDX3 suppresses type I interferons and favors viral replication during Arenavirus infection

Several arenaviruses cause hemorrhagic fever (HF) diseases that are associated with high morbidity and mortality in humans. Accordingly, HF arenaviruses have been listed as top-priority emerging diseases for which countermeasures are urgently needed. Because arenavirus nucleoprotein (NP) plays critical roles in both virus multiplication and immune-evasion, we used an unbiased proteomic approach to identify NP-interacting proteins in human cells. DDX3, a DEAD-box ATP-dependent-RNA-helicase, interacted with NP in both NP-transfected and virus-infected cells. Importantly, DDX3 deficiency compromised the propagation of both Old and New World arenaviruses, including the HF arenaviruses Lassa and Junin viruses. The DDX3 role in promoting arenavirus multiplication associated with both a previously un-recognized DDX3 inhibitory role in type I interferon production in arenavirus infected cells and a positive DDX3 effect on arenavirus RNA synthesis that was dependent on its ATPase and Helicase activities. Our results uncover novel mechanisms used by arenaviruses to exploit the host machinery and subvert immunity, singling out DDX3 as a potential host target for developing new therapies against highly pathogenic arenaviruses.

Recombinant LCMV Vectors Induce Protective Immunity following Homologous and Heterologous Vaccinations

Successful vaccination against cancer and infectious diseases relies on the induction of adaptive immune responses that induce high-titer antibodies or potent cytoxic T cell responses. In contrast to humoral vaccines, the amplification of cellular immune responses is often hampered by anti-vector immunity that either pre-exists or develops after repeated homologous vaccination. Replication-defective lymphocytic choriomeningitis virus (LCMV) vectors represent a novel generation of vaccination vectors that induce potent immune responses while escaping recognition by neutralizing antibodies. Here, we characterize the CD8 T cell immune response induced by replication-defective recombinant LCMV (rLCMV) vectors with regard to expansion kinetics, trafficking, phenotype, and function and we perform head-to-head comparisons of the novel rLCMV vectors with established vectors derived from adenovirus, vaccinia virus, or Listeria monocytogenes. Our results demonstrate that replication-deficient rLCMV vectors are safe and ideally suited for both homologous and heterologous vaccination regimens to achieve optimal amplification of CD8 T cell immune responses in vivo.

Transcription and replication mechanisms of Bunyaviridae and Arenaviridae L proteins

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Bunyavirus and arenavirus are important public health threats.

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Bunyavirus and arenavirus molecular biology, common and differential features.

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Implications of LACV L protein structure for understanding viral RNA synthesis.

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Current state and future perspectives on bunya- and arenavirus antivirals.

Bunyaviridae and Arenaviridae virus families include an important number of highly pathogenic viruses for humans. They are enveloped viruses with negative stranded RNA genomes divided into three (bunyaviruses) or two (arenaviruses) segments. Each genome segment is coated by the viral nucleoproteins (NPs) and the polymerase (L protein) to form a functional ribonucleoprotein (RNP) complex. The viral RNP provides the necessary context on which the L protein carries out the biosynthetic processes of RNA replication and gene transcription. Decades of research have provided a good understanding of the molecular processes underlying RNA synthesis, both RNA replication and gene transcription, for these two families of viruses. In this review we will provide a global view of the common features, as well as differences, of the molecular biology of Bunyaviridae and Arenaviridae. We will also describe structures of protein and protein-RNA complexes so far determined for these viral families, mainly focusing on the L protein, and discuss their implications for understanding the mechanisms of viral RNA replication and gene transcription within the architecture of viral RNPs, also taking into account the cellular context in which these processes occur. Finally, we will discuss the implications of these structural findings for the development of antiviral drugs to treat human diseases caused by members of the Bunyaviridae and Arenaviridae families.

Reverse Genetics of Influenza B Viruses

Annual influenza epidemics are caused not only by influenza A viruses but also by influenza B viruses. Initially established for the generation of recombinant influenza A viruses, plasmid-based reverse genetics techniques have allowed researchers the generation of wild type and mutant viruses from full-length cDNA copies of the influenza viral genome. These reverse genetics approaches have allowed researchers to answer important questions on the biology of influenza viruses by genetically engineering infectious recombinant viruses. This has resulted in a better understanding of the molecular biology of influenza viruses, including both viral and host factors required for genome replication and transcription. With the ability to generate recombinant viruses containing specific mutations in the viral genome, these reverse genetics tools have also allowed the identification of viral and host factors involved in influenza pathogenesis, transmissibility, host-range interactions and restrictions, and virulence. Likewise, reverse genetics techniques have been used for the implementation of inactivated or live-attenuated influenza vaccines and the identification of anti-influenza drugs and their mechanism of antiviral activity. In 2002, these reverse genetics approaches allowed also the recovery of recombinant influenza B viruses entirely from plasmid DNA. In this chapter we describe the cloning of influenza B/Brisbane/60/2008 viral RNAs into the ambisense pDP-2002 plasmid and the experimental procedures for the successful generation of recombinant influenza B viruses.

Lassa Virus Reverse Genetics

The Old World (OW) arenavirus Lassa (LASV ) is estimated to infect several hundred thousand people yearly in West Africa, resulting in high numbers of Lassa fever (LF), a viral hemorrhagic fever (HF) disease associated with high morbidity and mortality. To date, no licensed vaccines are available to LASV infections, and anti-LASV drug therapy is limited to an off-label use of ribavirin (Rib) that is only partially effective. The development of reverse genetics has provided investigators with a novel and powerful approach for the investigation of the molecular, cell biology, and pathogenesis of LASV. The use of cell-based LASV minigenome (MG) systems has allowed examining the cis- and trans-acting factors involved in genome replication and gene transcription and the identification of novel drugable LASV targets. Likewise, it is now feasible to rescue infectious recombinant (r)LASV entirely from cloned cDNAs containing predetermined mutations in their genomes to investigate virus-host interactions and mechanisms of pathogenesis, as well as to facilitate screens to identify antiviral drugs against LASV and the implementation of novel strategies to develop live-attenuated vaccines (LAV). In this chapter we will summarize the state-of-the-art experimental procedures for implementation of LASV reverse genetics. In addition, we will briefly discuss some significant translational research developments that have been made possible upon the development of LASV reverse genetics.

Recombinant Tri-Segmented Pichinde Virus as a Novel Live Viral Vaccine Platform

Pichinde virus (PICV) is a nonpathogenic arenavirus with a bi-segmented RNA genome (L and S segments) that encodes four viral genes. We have developed a reverse genetics system to generate recombinant tri-segmented PICV (rP18tri) that packages three RNA segments (L, S1, and S2) and can encode up to two foreign genes. Using influenza virus HA and NP as model antigens, we show that the rP18tri vector can induce strong humoral and cell-mediated immunity, which further increases upon a booster dose. We propose that this novel rP18tri vector can be developed into a useful vaccine platform for other antigens, particularly when strong cellular immunity and prime-boost vaccination strategy are desired.

Development of Recombinant Arenavirus-Based Vaccines

The development of arenavirus reverse genetics has provided investigators with a novel and powerful approach for the investigation of the arenavirus molecular and cell biology. The use of cell-based minigenome systems has allowed examining the cis- and trans-acting factors involved in arenavirus replication and transcription, and the identification of novel anti-arenaviral drug targets without requiring the use of live forms of arenaviruses. Likewise, it is now feasible to rescue infectious arenaviruses entirely from cloned cDNAs containing predetermined mutations in their genomes to investigate virus-host interactions and mechanisms of pathogenesis. These advances in arenavirus genetics have also facilitated screens to identify anti-arenaviral drugs and the pursuit of novel strategies to generate live-attenuated arenavirus vaccine candidates. Moreover, the generation of tri-segmented (r3) arenaviruses expressing foreign genes of interest (GOI) has opened the possibility of implementing live-attenuated arenaviruses-based vaccine vector approaches. In this chapter, we will summarize the implementation of plasmid-based reverse genetics techniques for the development of r3 arenaviruses expressing foreign GOI for their implementation as vaccine vectors.

Reverse Genetics Approaches for the Development of Influenza Vaccines

Influenza viruses cause annual seasonal epidemics and occasional pandemics of human respiratory disease. Influenza virus infections represent a serious public health and economic problem, which are most effectively prevented through vaccination. However, influenza viruses undergo continual antigenic variation, which requires either the annual reformulation of seasonal influenza vaccines or the rapid generation of vaccines against potential pandemic virus strains. The segmented nature of influenza virus allows for the reassortment between two or more viruses within a co-infected cell, and this characteristic has also been harnessed in the laboratory to generate reassortant viruses for their use as either inactivated or live-attenuated influenza vaccines. With the implementation of plasmid-based reverse genetics techniques, it is now possible to engineer recombinant influenza viruses entirely from full-length complementary DNA copies of the viral genome by transfection of susceptible cells. These reverse genetics systems have provided investigators with novel and powerful approaches to answer important questions about the biology of influenza viruses, including the function of viral proteins, their interaction with cellular host factors and the mechanisms of influenza virus transmission and pathogenesis. In addition, reverse genetics techniques have allowed the generation of recombinant influenza viruses, providing a powerful technology to develop both inactivated and live-attenuated influenza vaccines. In this review, we will summarize the current knowledge of state-of-the-art, plasmid-based, influenza reverse genetics approaches and their implementation to provide rapid, convenient, safe and more effective influenza inactivated or live-attenuated vaccines.

Development of live-attenuated arenavirus vaccines based on codon deoptimization of the viral glycoprotein

Several arenaviruses, chiefly Lassa (LASV) in West Africa, cause hemorrhagic fever (HF) disease in humans and pose important public health problems in their endemic regions. To date, there are no FDA-approved arenavirus vaccines and current anti-arenaviral therapy is limited to the use of ribavirin that has very limited efficacy. In this work we document that a recombinant prototypic arenavirus lymphocytic choriomeningitis virus (LCMV) with a codon deoptimized (CD) surface glycoprotein (GP), rLCMV/CD, exhibited wild type (WT)-like growth properties in cultured cells despite barely detectable GP expression levels in rLCMV/CD-infected cells. Importantly, rLCMV/CD was highly attenuated in vivo but able to induce complete protection against a subsequent lethal challenge with rLCMV/WT. Our findings support the feasibility of implementing an arenavirus GP CD-based approach for the development of safe and effective live-attenuated vaccines (LAVs) to combat diseases caused by human pathogenic arenaviruses.

Reporter-Expressing, Replicating-Competent Recombinant Arenaviruses

Several arenaviruses cause hemorrhagic fever (HF) disease in humans and pose an important public health problem in their endemic regions. To date, no Food and Drug Administration (FDA)-licensed vaccines are available to combat human arenavirus infections, and current anti-arenaviral drug therapy is limited to an off-label use of ribavirin that is only partially effective. The development of arenavirus reverse genetic approaches has provided investigators with a novel and powerful approach for the study of arenavirus biology including virus–host interactions underlying arenavirus induced disease. The use of cell-based minigenome systems has allowed examining the cis- and trans-acting factors involved in arenavirus replication and transcription, as well as particle assembly and budding. Likewise, it is now feasible to rescue infectious arenaviruses containing predetermined mutations in their genomes to investigate virus-host interactions and mechanisms of pathogenesis. The use of reverse genetics approaches has also allowed the generation of recombinant arenaviruses expressing additional genes of interest. These advances in arenavirus molecular genetics have also facilitated the implementation of novel screens to identify anti-arenaviral drugs, and the development of novel strategies for the generation of arenavirus live-attenuated vaccines. In this review, we will summarize the current knowledge on reporter-expressing, replicating-competent arenaviruses harboring reporter genes in different locations of the viral genome and their use for studying and understanding arenavirus biology and the identification of anti-arenaviral drugs to combat these important human pathogens.

Replication-competent fluorescent-expressing influenza B virus

Influenza B viruses (IBVs) cause annual outbreaks of respiratory illness in humans and are increasingly recognized as a major cause of influenza-associated morbidity and mortality. Studying influenza viruses requires the use of secondary methodologies to identify virus-infected cells. To this end, replication-competent influenza A viruses (IAVs) expressing easily traceable fluorescent proteins have been recently developed. In contrast, similar approaches for IBV are mostly lacking. In this report, we describe the generation and characterization of replication-competent influenza B/Brisbane/60/2008 viruses expressing fluorescent mCherry or GFP fused to the C-terminal of the viral non-structural 1 (NS1) protein. Fluorescent-expressing IBVs display similar growth kinetics and plaque phenotype to wild-type IBV, while fluorescent protein expression allows for the easy identification of virus-infected cells. Without the need of secondary approaches to monitor viral infection, fluorescent-expressing IBVs represent an ideal approach to study the biology of IBV and an excellent platform for the rapid identification and characterization of antiviral therapeutics or neutralizing antibodies using high-throughput screening approaches. Lastly, fluorescent-expressing IBVs can be combined with the recently described reporter-expressing IAVs for the identification of novel therapeutics to combat these two important human respiratory pathogens.

Reverse Genetics Approaches to Control Arenavirus

Several arenavirus cause hemorrhagic fever disease in humans and pose a significant public health problem in their endemic regions. To date, no licensed vaccines are available to combat human arenavirus infections, and anti-arenaviral drug therapy is limited to an off-label use of ribavirin that is only partially effective. The development of arenavirus reverse genetics approaches provides investigators with a novel and powerful approach for the investigation of the arenavirus molecular and cell biology. The use of cell-based minigenome systems has allowed examining the cis- and trans-acting factors involved in arenavirus replication and transcription and the identification of novel anti-arenaviral drug targets without requiring the use of live forms of arenaviruses. Likewise, it is now feasible to rescue infectious arenaviruses entirely from cloned cDNAs containing predetermined mutations in their genomes to investigate virus-host interactions and mechanisms of pathogenesis, as well as to facilitate screens to identify anti-arenaviral drugs and development of novel live-attenuated arenavirus vaccines. Recently, reverse genetics have also allowed the generation of tri-segmented arenaviruses expressing foreign genes, facilitating virus detection and opening the possibility of implementing live-attenuated arenavirus-based vaccine vector approaches. Likewise, the development of single-cycle infectious, reporter-expressing, arenaviruses has provided a new experimental method to study some aspects of the biology of highly pathogenic arenaviruses without the requirement of high-security biocontainment required to study HF-causing arenaviruses. In this chapter we summarize the current knowledge on arenavirus reverse genetics and the implementation of plasmid-based reverse genetics techniques for the development of arenavirus vaccines and vaccine vectors.

Generation of Lymphocytic Choriomeningitis Virus Based Vaccine Vectors

Vaccination with a recombinant LCMV based vector expressing tumor-associated or viral antigens is a safe and versatile method to induce an immune response against tumors or viral infections. Here, we describe the generation of recombinant LCMV vectors in which the gene encoding the viral LCMV-GP was substituted with a gene of interest (vaccine antigen). This renders the vaccine vector propagation-incompetent while it preserves the property of eliciting a strong cytotoxic T cell response.

A Novel Live Pichinde Virus-Based Vaccine Vector Induces Enhanced Humoral and Cellular Immunity after a Booster Dose

Pichinde virus (PICV) is a bisegmented enveloped RNA virus that targets macrophages and dendritic cells (DCs) early in infection and induces strong innate and adaptive immunity in mice. We have developed a reverse genetics system to produce live recombinant PICV (strain P18) with a trisegmented RNA genome (rP18tri), which encodes all four PICV gene products and as many as two foreign genes. We have engineered the vector to express the green fluorescent protein (GFP) reporter gene (abbreviated as G in virus designations) and either the hemagglutination (HA [H]) or the nucleoprotein (NP [P]) gene of the influenza A/PR8 virus. The trisegmented viruses rP18tri-G/H and rP18tri-G/P showed slightly reduced growth in vitro and expressed HA and NP, respectively. Mice immunized with rP18tri-G/H were completely protected against lethal influenza virus challenge even 120 days after immunization. These rP18tri-based vectors could efficiently induce both neutralizing antibodies and antigen-specific T cell responses via different immunization routes. Interestingly, the immune responses were significantly increased upon a booster dose and remained at high levels even after three booster doses. In summary, we have developed a novel PICV-based live vaccine vector that can express foreign antigens to induce strong humoral and cell-mediated immunity and is ideal for a prime-and-boost vaccination strategy.

IMPORTANCE

We have developed a novel Pichinde virus (PICV)-based live viral vector, rP18tri, that packages three RNA segments and encodes as many as two foreign genes. Using the influenza virus HA and NP genes as model antigens, we show that this rP18tri vector can induce strong humoral and cellular immunity via different immunization routes and can lead to protection in mice. Interestingly, a booster dose further enhances the immune responses, a feature that distinguishes this from other known live viral vectors. In summary, our study demonstrates a unique feature of this live rP18tri vector to be used as a novel vaccine platform for a prime-and-boost vaccination strategy.

Arenavirus Genome Rearrangement for the Development of Live Attenuated Vaccines

Several members of the Arenaviridae family cause hemorrhagic fever disease in humans and pose serious public health problems in their geographic regions of endemicity as well as a credible biodefense threat. To date, there have been no FDA-approved arenavirus vaccines, and current antiarenaviral therapy is limited to an off-label use of ribavirin that is only partially effective. Arenaviruses are enveloped viruses with a bisegmented negative-stranded RNA genome. Each genome segment uses an ambisense coding strategy to direct the synthesis of two viral polypeptides in opposite orientations, separated by a noncoding intergenic region. Here we have used minigenome-based approaches to evaluate expression levels of reporter genes from the nucleoprotein (NP) and glycoprotein precursor (GPC) loci within the S segment of the prototypic arenavirus lymphocytic choriomeningitis virus (LCMV). We found that reporter genes are expressed to higher levels from the NP than from the GPC locus. Differences in reporter gene expression levels from the NP and GPC loci were confirmed with recombinant trisegmented LCM viruses. We then used reverse genetics to rescue a recombinant LCMV (rLCMV) containing a translocated viral S segment (rLCMV/TransS), where the viral NP and GPC open reading frames replaced one another. The rLCMV/TransS showed slower growth kinetics in cultured cells and was highly attenuated in vivo in a mouse model of lethal LCMV infection, but immunization with rLCMV/TransS conferred complete protection against a lethal challenge with wild-type LCMV. Attenuation of rLCMV/TransS was associated with reduced NP expression levels. These results open a new avenue for the development of arenavirus live attenuated vaccines based on rearrangement of their viral genome.

IMPORTANCE

Several arenaviruses cause severe hemorrhagic fever in humans and also pose a credible bioterrorism threat. Currently, no FDA-licensed vaccines are available to combat arenavirus infections and antiarenaviral therapy is limited to the off-label use of ribavirin, which is only partially effective and associated with side effects. Here we describe, for the first time, the generation of a recombinant LCMV where the viral protein products encoded by the S RNA segment (NP and GPC) were swapped to generate rLCMV/TransS. rLCMV/TransS exhibited reduced viral multiplication in cultured cells and was highly attenuated in vivo while conferring protection, upon a single immunization dose, against a lethal challenge with wild-type LCMV. Our studies provide a proof of concept for the rational development of safe and protective live attenuated vaccine candidates based on genome reorganization for the treatment of pathogenic arenavirus infections in humans.

Development of live-attenuated arenavirus vaccines based on codon deoptimization

Arenaviruses have a significant impact on public health and pose a credible biodefense threat, but the development of safe and effective arenavirus vaccines has remained elusive, and currently, no Food and Drug Administration (FDA)-licensed arenavirus vaccines are available. Here, we explored the use of a codon deoptimization (CD)-based approach as a novel strategy to develop live-attenuated arenavirus vaccines. We recoded the nucleoprotein (NP) of the prototypic arenavirus lymphocytic choriomeningitis virus (LCMV) with the least frequently used codons in mammalian cells, which caused lower LCMV NP expression levels in transfected cells that correlated with decreased NP activity in cell-based functional assays. We used reverse-genetics approaches to rescue a battery of recombinant LCMVs (rLCMVs) encoding CD NPs (rLCMV/NP(CD)) that showed attenuated growth kinetics in vitro. Moreover, experiments using the well-characterized mouse model of LCMV infection revealed that rLCMV/NP(CD1) and rLCMV/NP(CD2) were highly attenuated in vivo but, upon a single immunization, conferred complete protection against a subsequent lethal challenge with wild-type (WT) recombinant LCMV (rLCMV/WT). Both rLCMV/NP(CD1) and rLCMV/NP(CD2) were genetically and phenotypically stable during serial passages in FDA vaccine-approved Vero cells. These results provide proof of concept of the safety, efficacy, and stability of a CD-based approach for developing live-attenuated vaccine candidates against human-pathogenic arenaviruses.

IMPORTANCE

Several arenaviruses cause severe hemorrhagic fever in humans and pose a credible bioterrorism threat. Currently, no FDA-licensed vaccines are available to combat arenavirus infections, while antiarenaviral therapy is limited to the off-label use of ribavirin, which is only partially effective and is associated with side effects. Here, we describe the generation of recombinant versions of the prototypic arenavirus LCMV encoding codon-deoptimized viral nucleoproteins (rLCMV/NP(CD)). We identified rLCMV/NP(CD1) and rLCMV/NP(CD2) to be highly attenuated in vivo but able to confer protection against a subsequent lethal challenge with wild-type LCMV. These viruses displayed an attenuated phenotype during serial amplification passages in cultured cells. Our findings support the use of this approach for the development of safe, stable, and protective live-attenuated arenavirus vaccines.

Inhibition of arenavirus by A3, a pyrimidine biosynthesis inhibitor

Arenaviruses merit significant interest as important human pathogens, since several of them cause severe hemorrhagic fever disease that is associated with high morbidity and significant mortality. Currently, there are no FDA-licensed arenavirus vaccines available, and current antiarenaviral therapy is limited to an off-labeled use of the nucleoside analog ribavirin, which has limited prophylactic efficacy. The pyrimidine biosynthesis inhibitor A3, which was identified in a high-throughput screen for compounds that blocked influenza virus replication, exhibits a broad-spectrum antiviral activity against negative- and positive-sense RNA viruses, retroviruses, and DNA viruses. In this study, we evaluated the antiviral activity of A3 against representative Old World (lymphocytic choriomeningitis virus) and New World (Junin virus) arenaviruses in rodent, monkey, and human cell lines. We show that A3 is significantly more efficient than ribavirin in controlling arenavirus multiplication and that the A3 inhibitory effect is in part due to its ability to interfere with viral RNA replication and transcription. We document an additive antiarenavirus effect of A3 and ribavirin, supporting the potential combination therapy of ribavirin and pyrimidine biosynthesis inhibitors for the treatment of arenavirus infections.