Development of neutralization tests using single-round infectious particles and cytopathic effect as an alternative method for measuring antibody titers against Japanese encephalitis virus in national epidemiological surveillance program of vaccine-preventable diseases in Japan

The "National Epidemiological Surveillance of Vaccine-Preventable Diseases" is conducted annually in Japan for various infectious diseases. A susceptibility survey for Japanese encephalitis was conducted by measuring antibody titers using a focus-forming assay. However, this method is complicated to operate and hard to count small focuses; therefore, the development of new measurement methods is required. In this study, we developed and evaluated a single-round infectious particles (SRIPs) assay method and a cytopathic effect (CPE) assay method. The SRIPs assay showed a strong correlation (R = 0.94) with the focus-forming assay. The SRIPs assay method is not only simple and can be expected to improve accuracy, but also has the advantages of extremely low pathogenicity and safety for the operator. The CPE assay also showed a strong correlation (R = 0.80) with the focus-forming assay. It has the advantage of requiring almost no reagents and is easy to interpret. Both SRIPs and CPE assay methods can be used as alternatives to the focus-forming assay method, but further characterization of each method is necessary before one is selected for routine use.

Importin-7-dependent nuclear translocation of the Flavivirus core protein is required for infectious virus production

Flaviviridae is a family of positive-stranded RNA viruses, including human pathogens, such as Japanese encephalitis virus (JEV), dengue virus (DENV), Zika virus (ZIKV), and West Nile virus (WNV). Nuclear localization of the viral core protein is conserved among Flaviviridae, and this feature may be targeted for developing broad-ranging anti-flavivirus drugs. However, the mechanism of core protein translocation to the nucleus and the importance of nuclear translocation in the viral life cycle remain unknown. We aimed to identify the molecular mechanism underlying core protein nuclear translocation. We identified importin-7 (IPO7), an importin-β family protein, as a nuclear carrier for Flaviviridae core proteins. Nuclear import assays revealed that core protein was transported into the nucleus via IPO7, whereas IPO7 deletion by CRISPR/Cas9 impaired their nuclear translocation. To understand the importance of core protein nuclear translocation, we evaluated the production of infectious virus or single-round-infectious-particles in wild-type or IPO7-deficient cells; both processes were significantly impaired in IPO7-deficient cells, whereas intracellular infectious virus levels were equivalent in wild-type and IPO7-deficient cells. These results suggest that IPO7-mediated nuclear translocation of core proteins is involved in the release of infectious virus particles of flaviviruses.

Identification and epidemiological study of an uncultured flavivirus from ticks using viral metagenomics and pseudoinfectious viral particles

During their blood-feeding process, ticks are known to transmit various viruses to vertebrates, including humans. Recent viral metagenomic analyses using next-generation sequencing (NGS) have revealed that blood-feeding arthropods like ticks harbor a large diversity of viruses. However, many of these viruses have not been isolated or cultured, and their basic characteristics remain unknown. This study aimed to present the identification of a difficult-to-culture virus in ticks using NGS and to understand its epidemic dynamics using molecular biology techniques. During routine tick-borne virus surveillance in Japan, an unknown flaviviral sequence was detected via virome analysis of host-questing ticks. Similar viral sequences have been detected in the sera of sika deer and wild boars in Japan, and this virus was tentatively named the Saruyama virus (SAYAV). Because SAYAV did not propagate in any cultured cells tested, single-round infectious virus particles (SRIP) were generated based on its structural protein gene sequence utilizing a yellow fever virus-based replicon system to understand its nationwide endemic status. Seroepidemiological studies using SRIP as antigens have demonstrated the presence of neutralizing antibodies against SAYAV in sika deer and wild boar captured at several locations in Japan, suggesting that SAYAV is endemic throughout Japan. Phylogenetic analyses have revealed that SAYAV forms a sister clade with the Orthoflavivirus genus, which includes important mosquito- and tick-borne pathogenic viruses. This shows that SAYAV evolved into a lineage independent of the known orthoflaviviruses. This study demonstrates a unique approach for understanding the epidemiology of uncultured viruses by combining viral metagenomics and pseudoinfectious viral particles.

Recapitulation of the Powassan virus life cycle in cell culture

Powassan virus (POWV) is a tick-borne flavivirus known for causing fatal neuroinvasive diseases in humans. Recently, there has been a noticeable increase in POWV infections, emphasizing the urgency of understanding viral replication, pathogenesis, and developing interventions. Notably, there are no approved vaccines or therapeutics for POWV, and its classification as a biosafety level-3 (BSL-3) agent hampers research. To overcome these obstacles, we developed a replicon system, a self-replicating RNA lacking structural proteins, making it safe to operate in a BSL-2 environment. We constructed a POWV replicon carrying the Gaussia luciferase (Gluc) reporter gene and blasticidin (BSD) selectable marker. Continuous BSD selection led to obtain a stable POWV replicon-carrying Huh7 cell lines. We identified cell culture adaptive mutations G4079A, G4944T and G6256A, resulting in NS2AR195K, NS3G122G, and NS3V560M, enhancing RNA replication. We demonstrated the utility of the POWV replicon system for high-throughput screening (HTS) assay to identify promising antivirals against POWV replication. We further explored the applications of the POWV replicon system, generating single-round infectious particles (SRIPs) by transfecting Huh7-POWV replicon cells with plasmids encoding viral capsid (C), premembrane (prM), and envelope (E) proteins, and revealed the distinct antigenic profiles of POWV with ZIKV. In summary, the POWV replicon and SRIP systems represent crucial platforms for genetic and functional analysis of the POWV life cycle and facilitating the discovery of antiviral drugs.IMPORTANCEIn light of the recent surge in human infections caused by POWV, a biosafety level-3 (BSL-3) classified virus, there is a pressing need to understand the viral life cycle and the development of effective countermeasures. To address this, we have pioneered the establishment of a POWV RNA replicon system and a replicon-based POWV SRIP system. Importantly, these systems are operable in BSL-2 laboratories, enabling comprehensive investigations into the viral life cycle and facilitating antiviral screening. In summary, these useful tools are poised to advance our understanding of the POWV life cycle and expedite the development of antiviral interventions.

Role of pre-existing immunity in driving the dengue virus serotype 2 genotype shift in the Philippines: A retrospective analysis of serological data

OBJECTIVE

The invasion of dengue virus (DENV)-2 Cosmopolitan genotype into the Philippines, where the Asian II genotype previously circulated challenges the principle of dengue serotype-specific immunity. Assessment of antibodies in this population may provide a mechanistic basis for how new genotypes emerge in dengue-endemic areas.

METHODS

We evaluated the neutralizing antibody (nAb) and antibody-dependent enhancement (ADE) responses against the two genotypes using archived serum samples collected from 333 patients with confirmed dengue in Metro Manila, Philippines, before, during, and after the introduction of the Cosmopolitan genotype. We quantified nAb titers in baby hamster kidney (BHK-21) cells with or without the Fcγ receptor IIA (FcγRIIA) to detect the capacity of virus-antibody complexes to neutralize or enhance DENV.

RESULTS

The nAb potency of the archived serum samples against the two genotypes was greatly affected by the presence of FcγRIIA. We found significant differences in nAb titers between the two genotypes in BHK-21 cells with FcγRIIA (P <0.0001). The archived serum samples were incapable of fully neutralizing the Cosmopolitan genotype, but instead strongly promoted its ADE compared to the Asian II genotype (P <0.0001).

CONCLUSION

These results reinforce the role of pre-existing immunity in driving genotype shifts. Our finding that specific genotypes exhibit differing susceptibilities to ADE by cross-reactive antibodies may have implications for dengue vaccine development.

The Dynamic Landscape of Capsid Proteins and Viral RNA Interactions in Flavivirus Genome Packaging and Virus Assembly

The Flavivirus genus of the Flaviviridae family of enveloped single-stranded RNA viruses encompasses more than 70 members, many of which cause significant disease in humans and livestock. Packaging and assembly of the flavivirus RNA genome is essential for the formation of virions, which requires intricate coordination of genomic RNA, viral structural, and nonstructural proteins in association with virus-induced, modified endoplasmic reticulum (ER) membrane structures. The capsid (C) protein, a small but versatile RNA-binding protein, and the positive single-stranded RNA genome are at the heart of the elusive flavivirus assembly process. The nucleocapsid core, consisting of the genomic RNA encapsidated by C proteins, buds through the ER membrane, which contains viral glycoproteins prM and E organized as trimeric spikes into the lumen, forming an immature virus. During the maturation process, which involves the low pH-mediated structural rearrangement of prM and E and furin cleavage of prM in the secretory pathway, the spiky immature virus with a partially ordered nucleocapsid core becomes a smooth, mature virus with no discernible nucleocapsid. This review focuses on the mechanisms of genome packaging and assembly by examining the structural and functional aspects of C protein and viral RNA. We review the current lexicon of critical C protein features and evaluate interactions between C and genomic RNA in the context of assembly and throughout the life cycle.

Rise of the RNA machines - self-amplification in mRNA vaccine design

mRNA vaccines have won the race for early COVID-19 vaccine approval, yet improvements are necessary to retain this leading role in combating infectious diseases. A next generation of self-amplifying mRNAs, also known as replicons, form an ideal vaccine platform. Replicons induce potent humoral and cellular responses with few adverse effects upon a minimal, single-dose immunization. Delivery of replicons is achieved with virus-like replicon particles (VRPs), or in nonviral vehicles such as liposomes or lipid nanoparticles. Here, we discuss innovative advances, including multivalent, mucosal, and therapeutic replicon vaccines, and highlight novelties in replicon design. As soon as essential safety evaluations have been resolved, this promising vaccine concept can transform into a widely applied clinical platform technology taking center stage in pandemic preparedness.

N-linked glycosylation of flavivirus E protein contributes to viral particle formation

In the case of the Japanese encephalitis virus (JEV), the envelope protein (E), a major component of viral particles, contains a highly conserved N-linked glycosylation site (E: N154). Glycosylation of the E protein is thought to play an important role in the ability of the virus to attach to target cells during transmission; however, its role in viral particle formation and release remains poorly understood. In this study, we investigated the role of N-glycosylation of flaviviral structural proteins in viral particle formation and secretion by introducing mutations in viral structural proteins or cellular factors involved in glycoprotein transport and processing. The number of secreted subviral particles (SVPs) was significantly reduced in N154A, a glycosylation-null mutant, but increased in D67N, a mutant containing additional glycosylation sites, indicating that the amount of E glycosylation regulates the release of SVPs. SVP secretion was reduced in cells deficient in galactose, sialic acid, and N-acetylglucosamine modifications in the Golgi apparatus; however, these reductions were not significant, suggesting that glycosylation mainly plays a role in pre-Golgi transport. Fluorescent labeling of SVPs using a split green fluorescent protein (GFP) system and time-lapse imaging by retention using selective hooks (RUSH) system revealed that the glycosylation-deficient mutant was arrested before endoplasmic reticulum (ER)- Golgi transport. However, the absence of ERGIC-53 and ERGIC-L, ER-Golgi transport cargo receptors that recognize sugar chains on cargo proteins, does not impair SVP secretion. In contrast, the solubility of the N154A mutant of E or the N15A/T17A mutant of prM in cells was markedly lower than that of the wild type, and proteasome-mediated rapid degradation of these mutants was observed, indicating the significance of glycosylation of both prM and E in proper protein folding and assembly of viral particles in the ER.

Flavivirus vaccines: Virus-like particles and single-round infectious particles as promising alternatives

The genus flavivirus of the Flaviridae family includes several human pathogens, like dengue, Zika, Japanese encephalitis, and yellow fever virus. These viruses continue to be a significant threat to human health. Vaccination remains the most useful approach to reduce the impact of flavivirus fever. However, currently available vaccines can induce severe side effects or have low effectiveness. An alternative is the use of recombinant vaccines, of which virus-like particles (VLP) and single-round infectious particles (SRIP) are of especial interest. VLP consist of the virus structural proteins produced in a heterologous system that self-assemble in a structure almost identical to the native virus. They are highly immunogenic and have been effective vaccines for other viruses for over 30 years. SRIP are promising vaccine candidates, as they induce both cellular and humoral responses, as viral proteins are expressed. Here, the state of the art to produce both types of particles and their use as vaccines against flaviviruses are discussed. We summarize the different approaches used for the design and production of flavivirus VLP and SRIP, the evidence for their safety and efficacy, and the main challenges for their use as commercial vaccines.

Seroprevalence of Flavivirus Neutralizing Antibodies in Thailand by High-Throughput Neutralization Assay: Endemic Circulation of Zika Virus before 2012

Thailand is a hyperendemic country for flavivirus infections in Southeast Asia. Although the reporting system for flavivirus surveillance in Thailand is well established, syndromic surveillance tends to underestimate the true epidemiological status of flaviviruses due to the majority of infections being asymptomatic. To accurately understand the prevalence of flaviviruses in endemic regions, we performed neutralization tests against multiple flaviviruses using 147 serum samples from healthy donors collected from four distinct regions in Thailand. Single-round infectious particles (SRIP) for six flaviviruses, dengue virus types 1 to 4 (DENV-1 to -4), Japanese encephalitis virus (JEV), and Zika virus (ZIKV), were used as antigens for developing a safe, high-throughput neutralization assay. Titers of neutralizing antibodies (NAbs) against the six flaviviruses revealed that DENV-1 and DENV-2, followed by ZIKV were the predominant circulating flaviviruses in a total of four regions, whereas the prevalence of NAbs against JEV varied among regions. Although the seroprevalence of ZIKV was low relative to that of DENV-1 and DENV-2, the findings strongly suggested that ZIKV has been circulating at a sustained level in Thailand since before 2012. These findings not only demonstrated the application of an SRIP-neutralization test in a serological study, but also elucidated the circulation and distribution trends of different flaviviruses in Thailand. IMPORTANCE Neutralization tests are the most reliable assay for flavivirus antibody detection; however, these assays are not suitable for high-throughput processing due to their time-consuming and labor-intensive nature. In this study, we developed single-round infectious particles (SRIPs) with a luciferase gene for dengue virus types 1 to 4, Japanese encephalitis virus, and Zika virus for use in a safe, high-throughput neutralization assay. We performed neutralization tests against multiple flaviviruses using 147 serum samples that were collected from healthy donors residing in four distinct regions of Thailand in 2011 to 2012. The assay was useful for surveys of flavivirus seroprevalence. The data revealed that dengue virus type 1 (DENV-1) and DENV-2 were the predominant circulating flaviviruses in Thailand and that Zika virus has been circulating at a sustained level in Thailand since before 2012.

Endoplasmic Reticulum-Associated Degradation Controls Virus Protein Homeostasis, Which Is Required for Flavivirus Propagation

Many positive-stranded RNA viruses encode polyproteins from which viral proteins are generated by processing the polyproteins. This system produces an equal amount of each viral protein, though the required amounts for each protein are not the same. In this study, we found the extra membrane-anchored nonstructural (NS) proteins of Japanese encephalitis virus and dengue virus are rapidly and selectively degraded by the endoplasmic reticulum-associated degradation (ERAD) pathway. Our gene targeting study revealed that ERAD involving Derlin2 and SEL1L, but not Derlin1, is required for the viral genome replication. Derlin2 is predominantly localized in the convoluted membrane (CM) of the viral replication organelle, and viral NS proteins are degraded in the CM. Hence, these results suggest that viral protein homeostasis is regulated by Derlin2-mediated ERAD in the CM, and this process is critical for the propagation of these viruses. IMPORTANCE The results of this study reveal the cellular ERAD system controls the amount of each viral protein in virus-infected cells and that this "viral protein homeostasis" is critical for viral propagation. Furthermore, we clarified that the "convoluted membrane (CM)," which was previously considered a structure with unknown function, serves as a kind of waste dump where viral protein degradation occurs. We also found that the Derlin2/SEL1L/HRD1-specific pathway is involved in this process, whereas the Derlin1-mediated pathway is not. This novel ERAD-mediated fine-tuning system for the stoichiometries of polyprotein-derived viral proteins may represent a common feature among polyprotein-encoding viruses.

Molecular Determinants of Flavivirus Virion Assembly

Virion assembly is an important step in the life cycle of all viruses. For viruses of the Flavivirus genus, a group of enveloped positive-sense RNA viruses, the assembly step represents one of the least understood processes in the viral life cycle. While assembly is primarily driven by the viral structural proteins, recent studies suggest that several nonstructural proteins also play key roles in coordinating the assembly and packaging of the viral genome. This review focuses on describing recent advances in our understanding of flavivirus virion assembly, including the intermolecular interactions between the viral structural (capsid) and nonstructural proteins (NS2A and NS2B-NS3), host factors, as well as features of the viral genomic RNA required for efficient flavivirus virion assembly.

Single-Round Infectious Particle Production by DNA-Launched Infectious Clones of Bungowannah Pestivirus

Reverse genetics systems are powerful tools for functional studies of viral genes or for vaccine development. Here, we established DNA-launched reverse genetics for the pestivirus Bungowannah virus (BuPV), where cDNA flanked by a hammerhead ribozyme sequence at the 5′ end and the hepatitis delta ribozyme at the 3′ end was placed under the control of the CMV RNA polymerase II promoter. Infectious recombinant BuPV could be rescued from pBuPV-DNA-transfected SK-6 cells and it had very similar growth characteristics to BuPV generated by conventional RNA-based reverse genetics and wild type BuPV. Subsequently, DNA-based E^RNS deleted BuPV split genomes (pBuPV∆E^RNS/E^RNS)—co-expressing the E^RNS protein from a separate synthetic CAG promoter—were constructed and characterized in vitro. Overall, DNA-launched BuPV genomes enable a rapid and cost-effective generation of recombinant BuPV and virus mutants, however, the protein expression efficiency of the DNA-launched systems after transfection is very low and needs further optimization in the future to allow the use e.g., as vaccine platform.

A Simple and High-Throughput ELISA-Based Neutralization Assay for the Determination of Anti-Flavivirus Neutralizing Antibodies

Mosquito-borne flavivirus infections, including dengue virus and Zika virus, are major public health threats globally. While the plaque reduction neutralization test (PRNT) is considered the gold standard for determining neutralizing antibody levels to flaviviruses, the assay is time-consuming and laborious. This study, therefore, aimed to develop an enzyme-linked immunosorbent assay (ELISA)-based microneutralization test (EMNT) for the detection of neutralizing antibodies to mosquito-borne flaviviruses. The inhibition of viral growth due to neutralizing antibodies was determined colorimetrically by using EMNT. Given the significance of Fcγ-receptors (FcγR) in antibody-mediated neutralization and antibody-dependent enhancement (ADE) of flavivirus infection, non-FcγR and FcγR-expressing cell lines were used in the EMNT to allow the detection of the sum of neutralizing and immune-enhancing antibody activity as the neutralizing titer. Using anti-flavivirus monoclonal antibodies and clinical samples, the utility of EMNT was evaluated by comparing the end-point titers of the EMNT and the PRNT. The correlation between EMNT and PRNT titers was strong, indicating that EMNT was robust and reproducible. The new EMNT assay combines the biological functional assessment of virus neutralization activity and the technical advantages of ELISA and, is simple, reliable, practical, and could be automated for high-throughput implementation in flavivirus surveillance studies and vaccine trials.

Reporter Replicons for Antiviral Drug Discovery against Positive Single-Stranded RNA Viruses

Single-stranded positive RNA ((+) ssRNA) viruses include several important human pathogens. Some members are responsible for large outbreaks, such as Zika virus, West Nile virus, SARS-CoV, and SARS-CoV-2, while others are endemic, causing an enormous global health burden. Since vaccines or specific treatments are not available for most viral infections, the discovery of direct-acting antivirals (DAA) is an urgent need. Still, the low-throughput nature of and biosafety concerns related to traditional antiviral assays hinders the discovery of new inhibitors. With the advances of reverse genetics, reporter replicon systems have become an alternative tool for the screening of DAAs. Herein, we review decades of the use of (+) ssRNA viruses replicon systems for the discovery of antiviral agents. We summarize different strategies used to develop those systems, as well as highlight some of the most promising inhibitors identified by the method. Despite the genetic alterations introduced, reporter replicons have been shown to be reliable systems for screening and identification of viral replication inhibitors and, therefore, an important tool for the discovery of new DAAs.

Split Nano Luciferase-based Assay to Measure Assembly of Japanese Encephalitis Virus

Cells infected with flavivirus release various forms of infectious and non-infectious particles as products and by-products. Comprehensive profiling of the released particles by density gradient centrifugation is informative for understanding viral particle assembly. However, it is difficult to detect low-abundance minor particles in such analyses. We developed a method for viral particle analysis that integrates a high-sensitivity split luciferase system and density gradient centrifugation. This protocol enables high-resolution profiling of particles produced by cells expressing Japanese encephalitis virus factors.

The Rescue and Characterization of Recombinant, Microcephaly-Associated Zika Viruses as Single-Round Infectious Particles

Zika virus (ZIKV) is transmitted by Aedes mosquitoes and exhibits genetic variation with African and Asian lineages. ZIKV Natal RGN strain, an Asian-lineage virus, has been identified in brain tissues from fetal autopsy cases with microcephaly and is suggested to be a neurotropic variant. However, ZIKV Natal RGN strain has not been isolated; its biological features are not yet illustrated. This study rescued and characterized recombinant, single-round infectious particles (SRIPs) of the ZIKV Natal RGN strain using reverse genetic and synthetic biology techniques. The DNA-launched replicon of ZIKV Natal RGN was constructed and contains the EGFP reporter, lacks prM-E genes, and replicates under CMV promoter control. The peak in the ZIKV Natal RGN SRIP titer reached 6.25 × 10⁶ TCID50/mL in the supernatant of prM-E-expressing packaging cells 72 h post-transfection with a ZIKV Natal RGN replicon. The infectivity of ZIKV Natal RGN SRIPs has been demonstrated to correlate with the green florescence intensity of the EGFP reporter, the SRIP-induced cytopathic effect, and ZIKV’s non-structural protein expression. Moreover, ZIKV Natal RGN SRIPs effectively self-replicated in rhabdomyosarcoma/muscle, glioblastoma/astrocytoma, and retinal pigmented epithelial cells, displaying unique cell susceptibility with differential attachment activity. Therefore, the recombinant ZIKV Natal RGN strain was rescued as SRIPs that could be used to elucidate the biological features of a neurotropic strain regarding cell tropism and pathogenic components, apply for antiviral agent screening, and develop vaccine candidates.

Functional Correlation between Subcellular Localizations of Japanese Encephalitis Virus Capsid Protein and Virus Production

The flavivirus capsid protein is considered to be essential for the formation of nucleocapsid complexes with viral genomic RNA at the viral replication organelle that appears on the endoplasmic reticulum (ER), as well as for incorporation into virus particles. However, this protein is also detected at the lipid droplet (LD) and nucleolus, and physiological roles of these off-site localizations are still unclear. In this study, we made a series of alanine substitution mutants of Japanese encephalitis virus (JEV) capsid protein that cover all polar and hydrophobic amino acid residues to identify the molecular surfaces required for virus particle formation and for localization at the LD and nucleolus. Five mutants exhibited a defect in the formation of infectious particles, and two of these mutants failed to be incorporated into the subviral particles (SVP). Three mutants lost the ability to localize to the nucleolus, and only a single mutant, with mutations at α2, was unable to localize to the LD. Unlike the cytoplasmic capsid protein, the nucleolar capsid protein was resistant to detergent treatment, and the α2 mutant was hypersensitive to detergent treatment. To scrutinize the relationship between these localizations and viral particle formation, we made eight additional alanine substitution mutants and found that all the mutants that did not localize at the LD or nucleolus failed to form normal viral particles. These results support the functional correlation between LD or nucleolus localization of the flaviviral capsid protein and the formation of infectious viral particles.IMPORTANCE This study is the first to report the comprehensive mutagenesis of a flavivirus capsid protein. We assessed the requirement of each molecular surface for infectious viral particle formation as well as for LD and nucleolar localization and found functional relationships between the subcellular localization of the virus capsid protein and infectious virus particle formation. We developed a system to independently assess the packaging of viral RNA and that of the capsid protein and found a molecular surface of the capsid protein that is crucial for packaging of viral RNA but not for packaging of the capsid protein itself. We also characterized the biochemical properties of capsid protein mutants and found that the capsid protein localizes at the nucleolus in a different manner than for its localization to the LD. Our comprehensive alanine-scanning mutagenesis study will aid in the development of antiflavivirus small molecules that can target the flavivirus capsid protein.

Activation of protein kinase R by hepatitis C virus RNA-dependent RNA polymerase

Hepatitis C virus (HCV) was shown to activate protein kinase R (PKR), which inhibits expression of interferon (IFN) and IFN-stimulated genes by controlling the translation of newly transcribed mRNAs. However, it is unknown exactly how HCV activates PKR. To address the molecular mechanism(s) of PKR activation mediated by HCV infection, we examined the effects of viral proteins on PKR activation. Here, we show that expression of HCV NS5B strongly induced PKR and eIF2α phosphorylation, and attenuated MHC class I expression. In contrast, expression of Japanese encephalitis virus RNA-dependent RNA polymerase did not induce phosphorylation of PKR. Co-immunoprecipitation analyses showed that HCV NS5B interacted with PKR. Furthermore, expression of NS5B with polymerase activity-deficient mutation failed to phosphorylate PKR, suggesting that RNA polymerase activity is required for PKR activation. These results suggest that HCV activates PKR by association with NS5B, resulting in translational suppression of MHC class I to establish chronic infection.

The Multiples Fates of the Flavivirus RNA Genome During Pathogenesis

The Flavivirus genus comprises many viruses (including dengue, Zika, West Nile and yellow fever viruses) which constitute important public health concerns worldwide. For several of these pathogens, neither antivirals nor vaccines are currently available. In addition to this unmet medical need, flaviviruses are of particular interest since they constitute an excellent model for the study of spatiotemporal regulation of RNA metabolism. Indeed, with no DNA intermediate or nuclear step, the flaviviral life cycle entirely relies on the cytoplasmic fate of a single RNA species, namely the genomic viral RNA (vRNA) which contains all the genetic information necessary for optimal viral replication. From a single open reading frame, the vRNA encodes a polyprotein which is processed to generate the mature viral proteins. In addition to coding for the viral polyprotein, the vRNA serves as a template for RNA synthesis and is also selectively packaged into newly assembled viral particles. Notably, vRNA translation, replication and encapsidation must be tightly coordinated in time and space via a fine-tuned equilibrium as these processes cannot occur simultaneously and hence, are mutually exclusive. As such, these dynamic processes involve several vRNA secondary and tertiary structures as well as RNA modifications. Finally, the vRNA can be detected as a foreign molecule by cytosolic sensors which trigger upon activation antiviral signaling pathways and the production of antiviral factors such as interferons and interferon-stimulated genes. However, to create an environment favorable to infection, flaviviruses have evolved mechanisms to dampen these antiviral processes, notably through the production of a specific vRNA degradation product termed subgenomic flavivirus RNA (sfRNA). In this review, we discuss the current understanding of the fates of flavivirus vRNA and how this is regulated at the molecular level to achieve an optimal replication within infected cells.

The Multiples Fates of the Flavivirus RNA Genome During Pathogenesis

The Flavivirus genus comprises many viruses (including dengue, Zika, West Nile and yellow fever viruses) which constitute important public health concerns worldwide. For several of these pathogens, neither antivirals nor vaccines are currently available. In addition to this unmet medical need, flaviviruses are of particular interest since they constitute an excellent model for the study of spatiotemporal regulation of RNA metabolism. Indeed, with no DNA intermediate or nuclear step, the flaviviral life cycle entirely relies on the cytoplasmic fate of a single RNA species, namely the genomic viral RNA (vRNA) which contains all the genetic information necessary for optimal viral replication. From a single open reading frame, the vRNA encodes a polyprotein which is processed to generate the mature viral proteins. In addition to coding for the viral polyprotein, the vRNA serves as a template for RNA synthesis and is also selectively packaged into newly assembled viral particles. Notably, vRNA translation, replication and encapsidation must be tightly coordinated in time and space via a fine-tuned equilibrium as these processes cannot occur simultaneously and hence, are mutually exclusive. As such, these dynamic processes involve several vRNA secondary and tertiary structures as well as RNA modifications. Finally, the vRNA can be detected as a foreign molecule by cytosolic sensors which trigger upon activation antiviral signaling pathways and the production of antiviral factors such as interferons and interferon-stimulated genes. However, to create an environment favorable to infection, flaviviruses have evolved mechanisms to dampen these antiviral processes, notably through the production of a specific vRNA degradation product termed subgenomic flavivirus RNA (sfRNA). In this review, we discuss the current understanding of the fates of flavivirus vRNA and how this is regulated at the molecular level to achieve an optimal replication within infected cells.

High-throughput neutralization assay for multiple flaviviruses based on single-round infectious particles using dengue virus type 1 reporter replicon

Diseases caused by the genus Flavivirus, including dengue virus (DENV) and Zika virus (ZIKV), have a serious impact on public health worldwide. Due to serological cross-reactivity among flaviviruses, current enzyme-linked immunosorbent assay (ELISA) for IgM/G cannot reliably distinguish between infection by different flaviviruses. In this study, we developed a reporter-based neutralization assay using single-round infectious particles (SRIPs) derived from representative flaviviruses. SRIPs were generated by transfection of human embryonic kidney 293 T cells with a plasmid encoding premembrane and envelope (prME) proteins from DENV1-4, ZIKV, Japanese encephalitis virus, West Nile virus, yellow fever virus, Usutu virus, and tick-borne encephalitis virus, along with a plasmid carrying DENV1 replicon containing the luciferase gene and plasmid for expression of DENV1 capsid. Luciferase activity of SRIPs-infected cells was well correlated with number of infected cells, and each reporter SRIP was specifically neutralized by sera from mice immunized with each flavivirus antigen. Our high-throughput reporter SRIP-based neutralization assay for multiple flaviviruses is a faster, safer, and less laborious diagnostic method than the conventional plaque reduction neutralization test to screen the cause of primary flavivirus infection. The assay may also contribute to the evaluation of vaccine efficacy and assist in routine surveillance and outbreak response to flaviviruses.

Infection with flaviviruses requires BCLXL for cell survival

BCL2 family proteins including pro-survival proteins, BH3-only proteins and BAX/BAK proteins control mitochondria-mediated apoptosis to maintain cell homeostasis via the removal of damaged cells and pathogen-infected cells. In this study, we examined the roles of BCL2 proteins in the induction of apoptosis in cells upon infection with flaviviruses, such as Japanese encephalitis virus, Dengue virus and Zika virus. We showed that survival of the infected cells depends on BCLX_L, a pro-survival BCL2 protein due to suppression of the expression of another pro-survival protein, MCL1. Treatment with BCLX_L inhibitors, as well as deficient BCLX_L gene expression, induced BAX/BAK-dependent apoptosis upon infection with flaviviruses. Flavivirus infection attenuates cellular protein synthesis, which confers reduction of short-half-life proteins like MCL1. Inhibition of BCLX_L increased phagocytosis of virus-infected cells by macrophages, thereby suppressing viral dissemination and chemokine production. Furthermore, we examined the roles of BCLX_L in the death of JEV-infected cells during in vivo infection. Haploinsufficiency of the BCLX_L gene, as well as administration of BH3 mimetic compounds, increased survival rate after challenge of JEV infection and suppressed inflammation. These results suggest that BCLX_L plays a crucial role in the survival of cells infected with flaviviruses, and that BCLX_L may provide a novel antiviral target to suppress propagation of the family of Flaviviridae viruses.

The genus Flavivirus including Japanese encephalitis virus, Dengue virus, and Zika virus all of which are mosquito-borne human pathogen and cause serious diseases in humans. Therefore, the development of effective vaccines and antivirals against several flaviviruses is still needed. BCL2 family proteins control mitochondria-mediated apoptosis to maintain cell homeostasis via the removal of damaged cells and pathogen-infected cells, deregulation of which leads to severe diseases including cancer and autoimmune diseases. Here, we showed that BCLX_L is a critical cell survival factor during infection with flaviviruses, and that inhibition of BCLX_L by treatment with BH3 mimetics restricts the production of infectious particles and the expression of chemokines in vitro and in vivo. Inhibition of BCLX_L induces apoptosis in cells infected with flaviviruses and these cells are quickly removed by engulfment of phagocytes, which leads to inhibition of virus dissemination without any inflammatory reaction. Based on these data, BCLX_L would appear to be a suitable target for the development of novel antivirals against a broad range of flavivirus infections.

Establishment and Application of Flavivirus Replicons

Dengue virus (DENV) and Zika virus (ZIKV) are enveloped, positive-strand RNA viruses belonging to the genus Flavivirus in the family Flaviviridae. The genome of ~11 kb length encodes one long open reading frame flanked by a 5' and a 3' untranslated region (UTR). The 5' end is capped and the 3' end lacks a poly(A) tail. The encoded single polyprotein is cleaved co-and posttranslationally by cellular and viral proteases. The first one-third of the genome encodes the structural proteins (C-prM-E), whereas the nonstructural (NS) proteins NS1-NS2A-NS3-NS4A-2K-NS4B-NS5 are encoded by the remaining two-thirds of the genome.Research on flaviviruses was driven forward by the ability to produce recombinant viruses using reverse genetics technology. It is known that the purified RNA of flaviviruses is per se infectious, which allows initiation of a complete viral life cycle by transfecting the genomic RNA into susceptible cells. In 1989, the first infectious flavivirus RNA was transcribed from full-length cDNA templates of yellow fever virus (YFV) facilitating molecular genetic analyses of this virus. In addition to the production of infectious recombinant viruses, reverse genetics can also be used to establish non-infectious replicons. Replicons contain an in-frame deletion in the structural protein genes but still encode all nonstructural proteins and contain the UTRs necessary to mediate efficient replication, a factor that enables their analyses under Biosafety Level (BSL) 1 conditions. This is particularly important since many flaviviruses are BSL3 agents.The review will cover strategies for generating flavivirus replicons, including the establishment of bacteriophage (T7 or SP6) promoter-driven constructs as well as cytomegalovirus (CMV) promoter-driven constructs. Furthermore, different reporter replicons or replicons expressing selectable marker proteins will be outlined using examples of their application to answer basic questions of the flavivirus replication cycle, to select and test antiviral compounds or to produce virus replicon particles. The establishment and application of flavivirus replicons will further be exemplified by my own data using an established YFV reporter replicon to study the role of YFV NS2A in the viral life cycle. In addition, we established a reporter replicon of a novel insect-specific flavivirus, namely Niénokoué virus (NIEV), to define the barrier(s) involved in host range restriction.

Investigating Tick-borne Flaviviral-like Particles as a Delivery System for Gene Therapy

Background

Research on the biogenesis of tick-borne encephalitis virus (TBEV) would benefit gene therapy. Due to specific arrangements of genes along the TBEV genome, its viral-like particles (VLPs) could be exploited as shuttles to deliver their replicon, which carries therapeutic genes, to immune system cells.

Objective

To develop a flaviviral vector for gene delivery as a part of gene therapy research that can be expressed in secretable VLP suicidal shuttles and provide abundant unique molecular and structural data supporting this gene therapy concept.

Method

TBEV structural gene constructs of a Swedish Torö strain were cloned into plasmids driven by the promoters CAG and CMV and then transfected into various cell lines, including COS-1 and BHK-21. Time-course sampling of the cells, culture fluid, cell lysate supernatant, and pellet specimens were performed. Western blotting and electron microscopy analyses of collected specimens were used to investigate molecular and structural processing of TBEV structural proteins.

Results

Western blotting analysis showed differences between promoters in directing the gene expression of the VLPs constructs. The premature flaviviral polypeptides as well as mature VLPs could be traced. Using electron microscopy, the premature and mature VLP accumulation in cellular compartments—and also endoplasmic reticulum proliferation as a virus factory platform—were observed in addition to secreted VLPs.

Conclusions

The abundant virologic and cellular findings in this study show the natural processing and safety of inserting flaviviral structural genes into suicidal VLP shuttles. Thus, we propose that these VLPs are a suitable gene delivering system model in gene therapy.

A highly pathogenic porcine reproductive and respiratory syndrome virus candidate vaccine based on Japanese encephalitis virus replicon system

In the swine industry, porcine reproductive and respiratory syndrome (PRRS) is a highly contagious disease which causes heavy economic losses worldwide. Effective prevention and disease control is an important issue. In this study, we described the construction of a Japanese encephalitis virus (JEV) DNA-based replicon with a cytomegalovirus (CMV) promoter based on the genome of Japanese encephalitis live vaccine virus SA14-14-2, which is capable of offering a potentially novel way to develop and produce vaccines against a major pathogen of global health. This JEV DNA-based replicon contains a large deletion in the structural genes (C-prM-E). A PRRSV GP5/M was inserted into the deletion position of JEV DNA-based replicons to develop a chimeric replicon vaccine candidate for PRRSV. The results showed that BALB/c mice models with the replicon vaccines pJEV-REP-G-2A-M-IRES and pJEV-REP-G-2A-M stimulated antibody responses and induced a cellular immune response. Analysis of ELSA data showed that vaccination with the replicon vaccine expressing GP5/M induced a better antibodies response than traditional DNA vaccines. Therefore, the results suggested that this ectopic expression system based on JEV DNA-based replicons may represent a useful molecular platform for various biological applications, and the JEV DNA-based replicons expressing GP5/M can be further developed into a novel, safe vaccine candidate for PRRS.

Development of hepatoma-derived, bidirectional oval-like cells as a model to study host interactions with hepatitis C virus during differentiation

Directed differentiation of human stem cells including induced pluripotent stem cells into hepatic cells potentially leads to acquired susceptibility to hepatitis C virus (HCV) infection. However, cellular determinants that change their expression during cell reprogramming or hepatic differentiation and are pivotal for supporting the HCV life cycle remain unclear. In this study, by introducing a set of reprogramming factors, we established HuH-7-derived oval-like cell lines, Hdo-17 and -23, which possess features of bipotential liver precursors. Upon induction of hepatocyte differentiation, expression of mature hepatocyte markers and hepatoblast markers in cells increased and decreased, respectively. In contrast, in response to cholangiocytic differentiation induction, gene expression of epithelium markers increased and cells formed round cysts with a central luminal space. Hdo cells lost their susceptibility to HCV infection and viral RNA replication. Hepatic differentiation of Hdo cells potentially led to recovery of permissiveness to HCV RNA replication. Gene expression profiling showed that most host-cell factors known to be involved in the HCV life cycle, except CD81, are expressed in Hdo cells comparable to HuH-7 cells. HCV pseudoparticle infectivity was significantly but partially recovered by ectopic expression of CD81, suggesting possible involvement of additional unidentified factors in HCV entry. In addition, we identified miR200a-3p, which is highly expressed in Hdo cells and stem cells but poorly expressed in differentiated cells and mature hepatocytes, as a novel negative regulator of HCV replication. In conclusion, our results showed that epigenetic reprogramming of human hepatoma cells potentially changes their permissivity to HCV.

Single-Round Infectious Particle Antiviral Screening Assays for the Japanese Encephalitis Virus

Japanese Encephalitis virus (JEV) is a mosquito-borne flavivirus with a positive-sense single-stranded RNA genome that contains a big open reading frame (ORF) flanked by 5′- and 3′- untranslated regions (UTRs). Nearly 30,000 JE cases with 10,000 deaths are still annually reported in East Asia. Although the JEV genotype III vaccine has been licensed, it elicits a lower protection against other genotypes. Moreover, no effective treatment for a JE case is developed. This study constructed a pBR322-based and cytomegaloviruses (CMV) promoter-driven JEV replicon for the production of JEV single-round infectious particles (SRIPs) in a packaging cell line expressing viral structural proteins. Genetic instability of JEV genome cDNA in the pBR322 plasmid was associated with the prokaryotic promoter at 5′ end of the JEV genome that triggers the expression of the structural proteins in E. coli. JEV structural proteins were toxic E. coli, thus the encoding region for structural proteins was replaced by a reporter gene (enhanced green fluorescent protein, EGFP) that was in-frame fused with the first eight amino acids of the C protein at N-terminus and the foot-and-mouth disease virus (FMDV) 2A peptide at C-terminus in a pBR322-based JEV-EGFP replicon. JEV-EGFP SRIPs generated from JEV-EGFP replicon-transfected packaging cells displayed the infectivity with cytopathic effect induction, self-replication of viral genomes, and the expression of EGFP and viral proteins. Moreover, the combination of JEV-EGFP SRIP plus flow cytometry was used to determine the half maximal inhibitory concentration (IC50) values of antiviral agents according to fluorescent intensity and positivity of SRIP-infected packaging cells post treatment. MJ-47, a quinazolinone derivative, significantly inhibited JEV-induced cytopathic effect, reducing the replication and expression of JEV-EGFP replicon in vitro. The IC50 value of 6.28 µM for MJ-47 against JEV was determined by the assay of JEV-EGFP SRIP infection in packaging cells plus flow cytometry that was more sensitive, effective, and efficient compared to the traditional plaque assay. Therefore, the system of JEV-EGFP SRIPs plus flow cytometry was a rapid and reliable platform for screening antiviral agents and evaluating antiviral potency.

Neutralizing and enhancing antibody responses to five genotypes of dengue virus type 1 (DENV-1) in DENV-1 patients

Dengue virus (DENV) has four distinct serotypes, DENV-1-4, with four to six genotypes in each serotype. The World Health Organization recommends tetravalent formulations including one genotype of each serotype as safe and effective dengue vaccines. Here, we investigated the impact of genotype on the neutralizing antibody responses to DENV-1 in humans. Convalescent sera collected from patients with primary infection of DENV-1 were examined for neutralizing antibody against single-round infectious particles of the five DENV-1 genotypes (GI-GV). In both GI- and GIV-infected patients, their neutralizing antibody titres against the five genotypes were similar, differing ≤4-fold from the homogenotypic responses. The enhancing activities against the five genotypes were also similar in these sera. Thus, the genotype strains of DENV-1 showed no significant antigenic differences in these patients, suggesting that GI- or GIV-derived vaccine antigens should induce equivalent levels of neutralizing antibodies against all DENV-1 genotypes.

Utility of Japanese encephalitis virus subgenomic replicon-based single-round infectious particles as antigens in neutralization tests for Zika virus and three other flaviviruses

The introduction of a foreign virus into an area may cause an outbreak, as with the Zika virus (ZIKV) outbreak in the Americas. Preparedness for handling a viral outbreak involves the development of tests for the serodiagnosis of foreign virus infections. We previously established a gene-based technology to generate some flaviviral antigens useful for functional antibody assays. The technology utilizes a Japanese encephalitis virus subgenomic replicon to generate single-round infectious particles (SRIPs) that possess designed surface antigens. In the present study, we successfully expanded the capacity of SRIPs to four human-pathogenic mosquito-borne flaviviruses that could potentially be introduced from endemic to non-endemic countries: ZIKV, Sepik virus, Wesselsbron virus, and Usutu virus. Flavivirus-crossreactive monoclonal antibodies dose-dependently neutralized these SRIPs. ZIKV-SRIPs also produced antibody-dose-dependent neutralization curves equivalent to those shown by authentic ZIKV particles using sera from a Zika fever patient. The faithful expression of designed surface antigens on SRIPs will allow their use in neutralization tests to diagnose foreign flaviviral infections.

Unique Requirement for ESCRT Factors in Flavivirus Particle Formation on the Endoplasmic Reticulum

Flavivirus infection induces endoplasmic reticulum (ER) membrane rearrangements to generate a compartment for replication of the viral genome and assembly of viral particles. Using quantitative mass spectrometry, we identified several ESCRT (endosomal sorting complex required for transport) proteins that are recruited to sites of virus replication on the ER. Systematic small interfering RNA (siRNA) screening revealed that release of both dengue virus and Japanese encephalitis virus was dramatically decreased by single depletion of TSG101 or co-depletion of specific combinations of ESCRT-III proteins, resulting in ≥1,000-fold titer reductions. By contrast, release was unaffected by depletion of some core ESCRTs, including VPS4. Reintroduction of ESCRT proteins to siRNA-depleted cells revealed interactions among ESCRT proteins that are crucial for flavivirus budding. Electron-microscopy studies revealed that the CHMP2 and CHMP4 proteins function directly in membrane deformation at the ER. Thus, a unique and specific subset of ESCRT contributes to ER membrane biogenesis during flavivirus infection.

Bivalent vaccine platform based on Japanese encephalitis virus (JEV) elicits neutralizing antibodies against JEV and hepatitis C virus

Directly acting antivirals recently have become available for the treatment of hepatitis C virus (HCV) infection, but there is no prophylactic vaccine for HCV. In the present study, we took advantage of the properties of Japanese encephalitis virus (JEV) to develop antigens for use in a HCV vaccine. Notably, the surface-exposed JEV envelope protein is tolerant of inserted foreign epitopes, permitting display of novel antigens. We identified 3 positions that permitted insertion of the HCV E2 neutralization epitope recognized by HCV1 antibody. JEV subviral particles (SVP) containing HCV-neutralization epitope (SVP-E2) were purified from culture supernatant by gel chromatography. Sera from mice immunized with SVP-E2 inhibited infection by JEV and by trans-complemented HCV particles (HCVtcp) derived from multi-genotypic viruses, whereas sera from mice immunized with synthetic E2 peptides did not show any neutralizing activity. Furthermore, sera from mice immunized with SVP-E2 neutralized HCVtcp with N415K escape mutation in E2. As with the SVP-E2 epitope-displaying particles, JEV SVPs with HCV E1 epitope also elicited neutralizing antibodies against HCV. Thus, this novel platform harboring foreign epitopes on the surface of the particle may facilitate the development of a bivalent vaccine against JEV and other pathogens.

Dengue virus infection-enhancing antibody activities against Indonesian strains in inhabitants of central Thailand

Dengue virus (DENV) infection-enhancing antibodies are a hypothetic factor to increase the dengue disease severity. In this study, we investigated the enhancing antibodies against Indonesian strains of DENV-1-4 in 50 healthy inhabitants of central Thailand (Bangkok and Uthai Thani). Indonesia and Thailand have seen the highest dengue incidence in Southeast Asia. The infection history of each subject was estimated by comparing his/her neutralizing antibody titers against prototype DENV-1-4 strains. To resolve the difficulty in obtaining foreign live viruses for use as assay antigens, we used a recombinant system to prepare single-round infectious dengue viral particles based on viral sequence information. Irrespective of the previously infecting serotype(s), most serum samples showed significantly higher enhancement titers against Indonesian DENV-2 strains than against Thai DENV-2 strains, whereas the opposite effect was observed for the DENV-3 strains. Equivalent enhancing activities were observed against both DENV-1 and DENV-4. These results suggest that the genotype has an impact on enhancing antibody activities against DENV-2 and DENV-3, because the predominant circulating genotypes of each serotype differ between Indonesia and Thailand.

Japanese encephalitis virus replicon-based vaccine expressing enterovirus-71 epitope confers dual protection from lethal challenges

BACKGROUND

To construct safer recombinant flavivirus vaccine, we exploited Japanese encephalitis virus (JEV) replicon-based platform to generate single-round infectious particles (SRIPs) that expressed heterologous neutralizing epitope SP70 derived from enterovirus-71 (EV71). Such pseudo-infectious virus particles, named SRIP-SP70, although are not genuine viable viruses, closely mimic live virus infection to elicit immune responses within one round of viral life cycle.

RESULTS

We found that, besides gaining of full protection to thwart JEV lethal challenge, female outbred ICR mice, when were immunized with SRIP-SP70 by prime-boost protocol, could not only induce SP70-specific and IgG2a predominant antibodies but also provide their newborns certain degree of protection against EV71 lethal challenge.

CONCLUSIONS

Our results therefore exemplify that this vaccination strategy could indeed confer an immunized host a dual protective immunity against subsequent lethal challenge from JEV or EV71.

Construction of an infectious molecular clone of Japanese encephalitis virus genotype V and its derivative subgenomic replicon capable of expressing a foreign gene

Japanese encephalitis virus (JEV) genotype V was originally isolated in Malaysia in 1952 and has long been restricted to the area. In 2009, sudden emergence of the genotype V in China and Korea was reported, suggesting expansion of its geographical distribution. Although studies on the genotype V are becoming more important, they have been limited partly due to lack of its infectious molecular clone. In this study, a plasmid carrying cDNA corresponding to the entire genome of JEV Muar strain, which belongs to genotype V, in the downstream of T7 promoter was constructed. Electroporation of viral RNA transcribed by T7 RNA polymerase (T7RNAP) in vitro from the plasmid led to production of progeny viruses both in mammalian and mosquito cells. Also, transfection of the infectious clone plasmid into mammalian cells expressing T7RNAP transiently or stably was demonstrated to generate infectious progenies. When the viral structural protein genes were partially deleted from the full-length cDNA, the subgenomic RNA transcribed in vitro from the modified plasmid was shown to replicate itself in mammalian cells as a replicon. The replicon carrying the firefly luciferase gene in place of the deleted structural protein genes was also shown to efficiently replicate itself and express luciferase in mammalian cells. Compared with the replicon derived from JEV genotype III (Nakayama strain), the genotype V-derived replicon appeared to be more tolerant to introduction of a foreign gene. The infectious clone and the replicons constructed in this study may serve as useful tools for characterizing JEV genotype V.

Development of a novel protocol for generating flavivirus reporter particles

Infection with West Nile virus (WNV), a mosquito-borne flavivirus, is a growing public and animal health concern worldwide. Prevention, diagnosis and treatment strategies for the infection are urgently required. Recently, viral reverse genetic systems have been developed and applied to clinical WNV virology. We developed a protocol for generating reporter virus particles (RVPs) of WNV with the aim of overcoming two major problems associated with conventional protocols, the difficulty in generating RVPs due to the specific skills required for handling RNAs, and the potential for environmental contamination by antibiotic-resistant genes encoded within the genome RNA of the RVPs. By using the proposed protocol, cells were established in which the RVP genome RNA is replicated constitutively and does not encode any antibiotic-resistant genes, and used as the cell supply for RVP genome RNA. Generation of the WNV RVPs requires only the simple transfection of the expression vectors for the viral structural proteins into the cells. Therefore, no RNA handling is required in this protocol. The WNV RVP yield obtained using this protocol was similar that obtained using the conventional protocol. According to these results, the newly developed protocol appears to be a good alternative for the generation of WNV RVPs, particularly for clinical applications.

[Reverse genetics system for flaviviruses]

Flaviviruses such as Japanese encephalitis virus, West Nile virus, yellow fever virus, dengue virus, and tick-borne encephalitis virus belong to a family Flaviviridae. These viruses are transmitted to vertebrates by infected mosquitoes or ticks, producing diseases, which have a serious impact on global public health. Reverse genetics is a powerful tool for studying the viruses. Although infectious full-length clones have been obtained for multiple flaviviruses, their early-stage development had the difficulty because of the instability problem of the viral cDNA in E. coli. Several strategies have been developed to circumvent the problem of infectious clone instability. The current knowledge accumulated on reverse genetics system of flaviviruses and its application are summarized in this review.