Development and application of West Nile virus subgenomic replicon RNA expressing secreted alkaline phosphatase

High-Throughput Platform for Detection of Neutralizing Antibodies Using Flavivirus Reporter Replicon Particles

Flavivirus outbreaks require fast and reliable diagnostics that can be easily adapted to newly emerging and re-emerging flaviviruses. Due to the serological cross-reactivity among flavivirus antibodies, neutralization tests (NT) are considered the gold standard for sero-diagnostics. Here, we first established wild-type single-round infectious virus replicon particles (VRPs) by packaging a yellow fever virus (YFV) replicon expressing Gaussia luciferase (Gluc) with YFV structural proteins in trans using a double subgenomic Sindbis virus (SINV) replicon. The latter expressed the YFV envelope proteins prME via the first SINV subgenomic promoter and the capsid protein via a second subgenomic SINV promoter. VRPs were produced upon co-electroporation of replicon and packaging RNA. Introduction of single restriction enzyme sites in the packaging construct flanking the prME sequence easily allowed to exchange the prME moiety resulting in chimeric VRPs that have the surface proteins of other flaviviruses including dengue virus 1-4, Zika virus, West Nile virus, and tick-borne encephalitis virus. Besides comparing the YF-VRP based NT assay to a YF reporter virus NT assay, we analyzed the neutralization efficiencies of different human anti-flavivirus sera or a monoclonal antibody against all established VRPs. The assays were performed in a 96-well high-throughput format setting with Gluc as readout in comparison to classical plaque reduction NTs indicating that the VRP-based NT assays are suitable for high-throughput analyses of neutralizing flavivirus antibodies.

The role of mouse 2',5'-oligoadenylate synthetase 1 paralogs

The interferon-induced oligoadenylate synthetase (OAS) family is one of the most important immune response proteins to the viral infection. The OAS protein binds dsRNA and is activated to produce 2',5'-oligoadenylates, which lead to the activation of latent form of RNase L, resulting in degradation of cellular and viral RNA and inhibition of viral replication. In mice, the Oas gene family locates on chromosome 5. The mouse Oas gene locus undergoes a recent series of duplication event, leading to the presence of eight paralogs of Oas1 genes (Oas1a through Oas1h) that forms Oas gene cluster with the Oas2, Oas3 and two OasL (OasL1 and OasL2) genes. Previous studies demonstrated that the mouse Oas1b gene conferred resistance to the flavivirus infection in mice; however, the antiviral activity of other mouse Oas1 gene family is still unknown. Therefore, in the present study, we have evaluated the mouse Oas1 paralogs regarding the enzymatic activity and antiviral activity against the two neurotropic flaviviruses, West Nile virus and tick-borne encephalitis virus. The mouse Oas1 genes were cloned from C57BL/6J (B6) as well as the Oas1b derived from feral mouse strain, MSM. The obtained results demonstrated that only Oas1a and Oas1g showed enzymatic activity. Although MSM-derived Oas1b showed antiviral activity to both viruses, all B6-derived OAS paralogs did not show antiviral activity. These results suggest that Oas1a and Oas1g play a role in potentiating viral RNA-induced interferon response in the cell, whereas the Oas1b works as a specific anti-flavivirus factor unless it is mutated. However, the role of other paralogs is unknown and should wait for further investigation.

Development of a subgenomic clone system for Kyasanur Forest disease virus

Emerging tropical viruses pose an increasing threat to public health because social, economic and environmental factors such as global trade and deforestation allow for their migration into previously unexposed populations and ecological niches. Among such viruses, Kyasanur Forest disease virus (KFDV) deserves particular recognition because it causes hemorrhagic fever. This work describes the completion of an antiviral testing platform (subgenomic system) for KFDV that could be used to quickly and safely screen compounds capable of inhibiting KFDV replication without the requirement for high containment, as the structural genes have been replaced with a luciferase reporter gene precluding the generation of infectious particles. The coordination of KFDV kinetics with the replication characteristics of the subgenomic system has provided additional insight into the timing of flavivirus replication events, as the genetically engineered KFDV genome began replication as early as 2h post cellular entry. Possession of such antiviral testing platforms by public health agencies should accelerate the testing of antiviral drugs against emerging or recently emerged viruses mitigating the effects of their disease and transmission.

Live Cell Reporter Systems for Positive-Sense Single Strand RNA Viruses

Cell-based reporter systems have facilitated studies of viral replication and pathogenesis, virus detection, and drug susceptibility testing. There are three types of cell-based reporter systems that express certain reporter protein for positive-sense single strand RNA virus infections. The first type is classical reporter system, which relies on recombinant virus, reporter virus particle, or subgenomic replicon. During infection with the recombinant virus or reporter virus particle, the reporter protein is expressed and can be detected in real time in a dose-dependent manner. Using subgenomic replicon, which are genetically engineered viral RNA molecules that are capable of replication but incapable of producing virions, the translation and replication of the replicon could be tracked by the accumulation of reporter protein. The second type of reporter system involves genetically engineered cells bearing virus-specific protease cleavage sequences, which can sense the incoming viral protease. The third type is based on viral replicase, which can report the specific virus infection via detection of the incoming viral replicase. This review specifically focuses on the major technical breakthroughs in the design of cell-based reporter systems and the application of these systems to the further understanding and control of viruses over the past few decades.

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.

Generation of West Nile virus infectious clones containing amino acid insertions between capsid and capsid anchor

West Nile virus (WNV) is a positive-sense RNA arbovirus responsible for recent outbreaks of severe neurological disease within the US and Europe. Large-scale analyses of antiviral compounds that inhibit virus replication have been limited due to the lack of an adequate WN reporter virus. Previous attempts to insert a reporter into the 3' untranslated region of WNV generated unstable viruses, suggesting that this region does not accommodate additional nucleotides. Here, we engineered two WNV infectious clones containing insertions at the Capsid (C)/Capsid Anchor (CA) junction of the viral polyprotein. Recombinant viruses containing a TAT(1-67) or Gaussia Luciferase (GLuc) gene at this location were successfully recovered. However, rapid loss of most, if not all, of the reporter sequence occurred for both viruses, indicating that the reporter viruses were not stable. While the GLuc viruses predominantly reverted back to wild-type WNV length, the TAT viruses retained up to 75 additional nucleotides of the reporter sequence. These additional nucleotides were stable over at least five passages and did not significantly alter WNV fitness. Thus, the C/CA junction of WNV can tolerate additional nucleotides, though insertions are subject to certain constraints.

Development and characterization of the replicon system of Japanese encephalitis live vaccine virus SA14-14-2

Background

Viral self-replicating sub-genomic replicons represent a powerful tool for studying viral genome replication, antiviral screening and chimeric vaccine development. Many kinds of flavivirus replicons have been developed with broad applications.

Findings

The replicon system of JEV live vaccine strain SA14-14-2 was successfully developed in this study. Two kinds of replicons that express enhanced green fluorescent protein (EGFP) and Renilla luciferase (R.luc) were constructed under the control of SP6 promoter, respectively. Robust EGFP and R.luc signals could be detected in the replicon-transfected BHK-21 cells. Furthermore, the potential effects of selected amino acids in the C-terminal of envelope protein on replication were characterized using the replicon system.

Conclusions

Our results provide a useful platform not only for the study of JEV replication, but also for antiviral screening and chimeric vaccine development.