Visualization of a neurotropic flavivirus infection in mouse reveals unique viscerotropism controlled by host type I interferon signaling

Breast milk transmission and involvement of mammary glands in tick-borne flavivirus infected mice

Tick-borne flaviviruses (TBFs) are transmitted to humans through milk and tick bites. Although a case of possible mother-to-child transmission of tick-borne encephalitis virus (TBEV) through breast milk has been reported, this route has not been confirmed in experimental models. Therefore, in this study, using type I interferon receptor-deficient A129 mice infected with Langat virus (LGTV), we aimed to demonstrate the presence of infectious virus in the milk and mammary glands of infected mice. Our results showed viral RNA of LGTV in the pup's stomach milk clots (SMCs) and blood, indicating that the virus can be transmitted from dam to pup through breast milk. In addition, we observed that LGTV infection causes tissue lesions in the mammary gland, and viral particles were present in mammary gland epithelial cells. Furthermore, we found that milk from infected mice could infect adult mice via the intragastric route, which has a milder infection process, longer infection time, and a lower rate of weight loss than other modes of infection. Specifically, we developed a nano-luciferase-LGTV reporter virus system to monitor the dynamics of different infection routes and observed dam-to-pup infection using in vivo bioluminescence imaging. This study provides comprehensive evidence to support breast milk transmission of TBF in mice and has helped provide useful data for studying TBF transmission routes.IMPORTANCETo date, no experimental models have confirmed mother-to-child transmission of tick-borne flavivirus (TBF) through breastfeeding. In this study, we used a mouse model to demonstrate the presence of infectious viruses in mouse breast milk and mammary gland epithelial cells. Our results showed that pups could become infected through the gastrointestinal route by suckling milk, and the infection dynamics could be monitored using a reporter virus system during breastfeeding in vivo. We believe our findings have provided substantial evidence to understand the underlying mechanism of breast milk transmission of TBF in mice, which has important implications for understanding and preventing TBF transmission in humans.

Novel antiviral discoveries for Japanese encephalitis virus infections through reporter virus-based high-throughput screening

Japanese encephalitis (JE) caused by JE virus (JEV), remains a global public health concern. Currently, there is no specific antiviral drug approved for the treatment of JE. While vaccines are available for prevention, they may not cover all at-risk populations. This underscores the urgent need for prophylaxis and potent anti-JEV drugs. In this context, a high-content JEV reporter system expressing Nanoluciferase (Nluc) was developed and utilized for a high-throughput screening (HTS) of a commercial antiviral library to identify potential JEV drug candidates. Remarkably, this screening process led to the discovery of five drugs with outstanding antiviral activity. Further mechanism of action analysis revealed that cepharanthine, an old clinically approved drug, directly inhibited virus replication by blocking GTP binding to the JEV RNA-dependent RNA polymerase. Additionally, treatment with cepharanthine in mice models alleviated JEV infection. These findings warrant further investigation into the potential anti-JEV activity of cepharanthine as a new therapeutic approach for the treatment of JEV infection. The HTS method employed here proves to be an accurate and convenient approach that facilitates the rapid development of antiviral drugs.

Validation of flavivirus infectious clones carrying fluorescent markers for antiviral drug screening and replication studies

Flaviviruses have emerged as major arthropod-transmitted pathogens and represent an increasing public health problem worldwide. High-throughput screening can be facilitated using viruses that easily express detectable marker proteins. Therefore, developing molecular tools, such as reporter-carrying versions of flaviviruses, for studying viral replication and screening antiviral compounds represents a top priority. However, the engineering of flaviviruses carrying either fluorescent or luminescent reporters remains challenging due to the genetic instability caused by marker insertion; therefore, new approaches to overcome these limitations are needed. Here, we describe reverse genetic methods that include the design and validation of infectious clones of Zika, Kunjin, and Dengue viruses harboring different reporter genes for infection, rescue, imaging, and morphology using super-resolution microscopy. It was observed that different flavivirus constructs with identical designs displayed strikingly different genetic stabilities, and corresponding virions resembled wild-type virus particles in shape and size. A successful strategy was assessed to increase the stability of rescued reporter virus and permit antiviral drug screening based on quantitative automated fluorescence microscopy and replication studies.

Rapid Generation of Recombinant Flaviviruses Using Circular Polymerase Extension Reaction

The genus Flavivirus is a group of arthropod-borne single-stranded RNA viruses, which includes important human and animal pathogens such as Japanese encephalitis virus (JEV), Zika virus (ZIKV), Dengue virus (DENV), yellow fever virus (YFV), West Nile virus (WNV), and Tick-borne encephalitis virus (TBEV). Reverse genetics has been a useful tool for understanding biological properties and the pathogenesis of flaviviruses. However, the conventional construction of full-length infectious clones for flavivirus is time-consuming and difficult due to the toxicity of the flavivirus genome to E. coli. Herein, we applied a simple, rapid, and bacterium-free circular polymerase extension reaction (CPER) method to synthesize recombinant flaviviruses in vertebrate cells as well as insect cells. We started with the de novo synthesis of the JEV vaccine strain SA-14-14-2 in Vero cells using CPER, and then modified the CPER method to recover insect-specific flaviviruses (ISFs) in mosquito C6/36 cells. Chimeric Zika virus (ChinZIKV) based on the Chaoyang virus (CYV) backbone and the Culex flavivirus reporter virus expressing green fluorescent protein (CxFV-GFP) were subsequently rescued in C6/36 cells. CPER is a simple method for the rapid generation of flaviviruses and other potential RNA viruses. A CPER-based recovery system for flaviviruses of different host ranges was established, which would facilitate the development of countermeasures against flavivirus outbreaks in the future.

Development of recombinant West Nile virus expressing mCherry reporter protein

West Nile virus (WNV) is transmitted to humans and animals by a mosquito and enters the central nervous system, leading to lethal encephalitis. Reporter viruses expressing fluorescent proteins enable detection of infected cells in vitro and in vivo, facilitating evaluation of the dynamics of viral infection, and the development of diagnostic or therapeutic methods. In this study, we developed a method for production of a recombinant replication-competent WNV expressing mCherry fluorescent protein. The expression of mCherry was observed in viral antigen-positive cells in vitro and in vivo, but the growth of the reporter WNV was reduced as compared to the parental WNV. The expression of mCherry was stable during 5 passages in reporter WNV-infected culture cells. Neurological symptoms were observed in mice inoculated intracranially with the reporter WNV. The reporter WNV expressing mCherry will facilitate research into WNV replication in mouse brains.

Japanese Encephalitis Virus-Infected Cells

RNA virus infections have been a leading cause of pandemics. Aided by global warming and increased connectivity, their threat is likely to increase over time. The flaviviruses are one such RNA virus family, and its prototypes such as the Japanese encephalitis virus (JEV), Dengue virus, Zika virus, West Nile virus, etc., pose a significant health burden on several endemic countries. All viruses start off their life cycle with an infected cell, wherein a series of events are set in motion as the virus and host battle for autonomy. With their remarkable capacity to hijack cellular systems and, subvert/escape defence pathways, viruses are able to establish infection and disseminate in the body, causing disease. Using this strategy, JEV replicates and spreads through several cell types such as epithelial cells, fibroblasts, monocytes and macrophages, and ultimately breaches the blood-brain barrier to infect neurons and microglia. The neurotropic nature of JEV, its high burden on the paediatric population, and its lack of any specific antivirals/treatment strategies emphasise the need for biomedical research-driven solutions. Here, we highlight the latest research developments on Japanese encephalitis virus-infected cells and discuss how these can aid in the development of future therapies.

Visualizing lymphocytic choriomeningitis virus infection in cells and living mice

Mammarenavirus are a large family of enveloped negative-strand RNA viruses that include several agents responsible for severe hemorrhagic fevers. Until now, no FDA-licensed drug has been admitted for treating an arenavirus infection, and only few effective anti-arenavirus drugs have been tested in vivo. In this work, we designed a recombinant reporter arenavirus lymphocytic choriomeningitis virus that stably expressed nanoluciferase (LCMV-Nluc). The LCMV-Nluc was proved to share similar biological properties with wild-type LCMV and the Nluc intensity reliably reflected viral replication both in vitro and in vivo. Replication of the Nluc-encoding virus in living mice can be visualized by real-time bioluminescent imaging, and bioluminescence can be detected in a variety of organs of infected mice. This work provides a novel approach that enables real-time study of the arenavirus infection and is a convenient and valuable tool for screening of compounds that are active against arenaviruses in vitro and in living mice.

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LCMV-Nluc was constructed and shared similar biological properties with LCMV-WT

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Replication of the LCMV-Nluc can be visualized by real-time bioluminescent imaging

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LCMV-Nluc is a valuable tool for screening antiviral compounds in vitro

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LCMV-Nluc is successfully applied for screening antiviral compounds in vivo

Flaviviruses: Innate Immunity, Inflammasome Activation, Inflammatory Cell Death, and Cytokines

The innate immune system is the host’s first line of defense against the invasion of pathogens including flavivirus. The programmed cell death controlled by genes plays an irreplaceable role in resisting pathogen invasion and preventing pathogen infection. However, the inflammatory cell death, which can trigger the overflow of a large number of pro-inflammatory cytokines and cell contents, will initiate a severe inflammatory response. In this review, we summarized the current understanding of the innate immune response, inflammatory cell death pathway and cytokine secretion regulation during Dengue virus, West Nile virus, Zika virus, Japanese encephalitis virus and other flavivirus infections. We also discussed the impact of these flavivirus and viral proteins on these biological processes. This not only provides a scientific basis for elucidating the pathogenesis of flavivirus, but also lays the foundation for the development of effective antiviral therapies.

Visualization of yellow fever virus infection in mice using a bioluminescent reporter virus

Yellow fever virus (YFV) is a re-emerging flavivirus, which can lead to severe clinical manifestations and high mortality, with no specific antiviral therapies available. The live-attenuated yellow fever vaccine 17D (YF17D) has been widely used for over eighty years. However, the emergence of yellow fever vaccine-associated viscerotropic disease (YFL-AVD) and yellow fever vaccine-associated neurotropic disease (YFL-AND) raised non-negligible concerns. Additionally, the attenuation mechanism of YF17D is still unclear. Thus, the development of convenient models is crucial to understand the mechanisms behind YF17D attenuation and its adverse effects. In this work, we generated a reporter YF17D expressing nano-luciferase (NLuc). In vitro and in vivo characterization demonstrated that the NLuc-YF17D shared similar biological properties with its parental strain and the NLuc activity can reflect viral infectivity reliably. Combined with in vivo bioluminescence imaging, a series of mice models of YF17D infection was established, which will be useful for the evaluation of antiviral medicines and novel vaccine candidates. Especially, we demonstrated that intraperitoneally (i.p.) infection of NLuc-YF17D in type I interferon receptor-deficient mice A129 resulted in outcomes resembling YEL-AVD and YEL-AND, evidenced by viral replication in multiple organs and invasion of the central neuronal system. Finally, in vitro and in vivo assays based on this reporter virus were established to evaluate the antiviral activities of validated antiviral agents. In conclusion, the bioluminescent reporter virus described herein provides a powerful platform to study YF17D attenuation and vaccine-associated diseases as well as to develop novel countermeasures against YFV.

Longitudinal neural connection detection using a ferritin-encoding adeno-associated virus vector and in vivo MRI method

The investigation of neural circuits is important for interpreting both healthy brain function and psychiatric disorders. Currently, the architecture of neural circuits is always investigated with fluorescent protein encoding neurotropic virus and ex vivo fluorescent imaging technology. However, it is difficult to obtain a whole‐brain neural circuit connection in living animals, due to the limited fluorescent imaging depth. Herein, the noninvasive, whole‐brain imaging technique of MRI and the hypotoxicity virus vector AAV (adeno‐associated virus) were combined to investigate the whole‐brain neural circuits in vivo. AAV2‐retro are an artificially‐evolved virus vector that permits access to the terminal of neurons and retrograde transport to their cell bodies. By expressing the ferritin protein which could accumulate iron ions and influence the MRI contrast, the neurotropic virus can cause MRI signal changes in the infected regions. For mice injected with the ferritin‐encoding virus vector (rAAV2‐retro‐CAG‐Ferritin) in the caudate putamen (CPu), several regions showed significant changes in MRI contrasts, such as PFC (prefrontal cortex), HIP (hippocampus), Ins (insular cortex) and BLA (basolateral amygdala). The expression of ferritin in those regions was also verified with ex vivo fluorescence imaging. In addition, we demonstrated that changes in T2 relaxation time could be used to identify the spread area of the virus in the brain over time. Thus, the neural connections could be longitudinally detected with the in vivo MRI method. This novel technique could be utilized to observe the viral infection process and detect the neural circuits in a living animal.

Development and characterization of recombinant tick-borne encephalitis virus expressing mCherry reporter protein: A new tool for high-throughput screening of antiviral compounds, and neutralizing antibody assays

The flavivirus, tick-borne encephalitis virus (TBEV) is transmitted by Ixodes spp. ticks and may cause severe and potentially lethal neurological tick-borne encephalitis (TBE) in humans. Studying TBEV requires the use of secondary methodologies to detect the virus in infected cells. To overcome this problem, we rationally designed and constructed a recombinant reporter TBEV that stably expressed the mCherry reporter protein. The resulting TBEV reporter virus (named mCherry-TBEV) and wild-type parental TBEV exhibited similar growth kinetics in cultured cells; however, the mCherry-TBEV virus produced smaller plaques. The magnitude of mCherry expression correlated well with progeny virus production but remained stable over <4 passages in cell culture. Using well-characterized antiviral compounds known to inhibit TBEV, 2'-C-methyladenosine and 2'-deoxy-2'-β-hydroxy-4'-azidocytidine (RO-9187), we demonstrated that mCherry-TBEV is suitable for high-throughput screening of antiviral drugs. Serum samples from a TBEV-vaccinated human and a TBEV-infected dog were used to evaluate the mCherry-based neutralization test. Collectively, recombinant mCherry-TBEV reporter virus described here provides a powerful tool to facilitate the identification of potential antiviral agents, and to measure levels of neutralizing antibodies in human and animal sera.

Identifying optimal capsid duplication length for the stability of reporter flaviviruses

ABSTRACT Mosquito-transmitted flaviviruses cause widespread disease across the world. To provide better molecular tools for drug screens and pathogenesis studies, we report a new approach to produce stable NanoLuc-tagged flaviviruses, including dengue virus serotypes 1-4, Japanese encephalitis virus, yellow fever virus, West Nile virus, and Zika virus. Since the reporter gene is often engineered at the capsid gene region, the capsid sequence must be duplicated to flank the reporter gene; such capsid duplication is essential for viral replication. The conventional approach for stabilizing reporter flaviviruses has been to shorten or modify the duplicated capsid sequence to minimize homologous recombination. No study has examined the effects of capsid duplication length on reporter virus stability. Here we report an optimal length to stabilize reporter flaviviruses. These viruses were stable after ten rounds of cell culture passaging, and in the case of stable NanoLuc-tagged Zika virus (ZIKV C38), the virus replicated to 107 FFU/ml in cell culture and produced robust luciferase signal after inoculation in mosquitoes. Mechanistically, the optimal length of capsid duplication may contain all the cis-acting RNA elements required for viral RNA replication, thus reducing the selection pressure for recombination. Together, these data describe an improved method of constructing optimal reporter flaviviruses.

A dengue type 2 reporter virus assay amenable to high-throughput screening

Dengue virus (DV) is an important mosquito-borne flavivirus threatening almost half of the world's population. Prophylaxis and potent anti-DV drugs are urgently needed. Here, we developed a high content imaging-based (HCI) assay with DV type 2 expressing the fluorescent protein mCherry (DV2/mCherry) to improve the efficiency and robustness of the drug discovery process. For the construction of the reporter virus, the mCherry gene followed by the ribosome-skipping 2A sequence of the Thosea asigna virus (T2A) was placed upstream of the full DV2 open reading frame. The biological characteristics including mCherry expression, virus replication rate, and plaque phenotype was examined and validated in BHK-21, Vero and C6/36 cells. A robust image-based antiviral assay combined with an automated robotic system was then developed, with a Z' factor of 0.73. To validate the image-based antiviral assay, a panel of reference compounds with different molecular mechanisms of anti-DV activity was assessed: (i) the glycosylation inhibitor, Celgosivir, (ii) two NS4b-targeting compounds: a 3-Acyl-indole derivative and NITD618, and (iii) two nucleoside viral polymerase inhibitors, 2'CMC and 7DMA. The inhibition profiles were quantified and obtained by means of HCI and RT-qPCR. Both methods resulted in very comparable inhibition profiles. In conclusion, a powerful and robust assay was developed with a fully automated data generation and processing pipeline. It makes the new reporter virus assay amenable to high-throughput screening of large libraries of small molecules.

Construction of Stable Reporter Flaviviruses and Their Applications

Flaviviruses are significant human pathogens that cause frequent emerging and reemerging epidemics around the world. Better molecular tools for studying, diagnosing, and treating these diseases are needed. Reporter viruses represent potent tools to fill this gap but have been hindered by genetic instability. Recent advances have overcome these hurdles, opening the way for increased use of stable reporter flaviviruses to diagnose infections, screen and study antiviral compounds, and serve as potential vaccine vectors.

Axl Deficiency Promotes the Neuroinvasion of Japanese Encephalitis Virus by Enhancing IL-1α Production from Pyroptotic Macrophages

Japanese encephalitis virus (JEV) is a mosquito-borne flavivirus that causes Japanese encephalitis (JE), the most commonly diagnosed viral encephalitis worldwide. The fatality rate of JE is 20%, and nearly half of the surviving patients develop neuropsychiatric sequelae. Axl is a receptor tyrosine kinase that plays multiple roles in flaviviral infections. Currently, the involvement of Axl in JEV infection remains enigmatic. In this study, we demonstrate that Axl impedes the pathogenesis of severe JE in mice by maintaining blood-brain-barrier (BBB) integrity and restricting viral neuroinvasion. Furthermore, serum IL-1α is a key mediator of this process and is primarily released by JEV-infected pyroptotic macrophages to elicit BBB breakdown, while an IL-1α antagonist can effectively reduce the incidence of severe JE. Our work uncovers the protective role of Axl in antagonizing severe JE and shows that the use of an IL-1α antagonist may be a promising tactic to prevent severe JE.

Japanese encephalitis virus (JEV) is a flavivirus that causes Japanese encephalitis (JE), which has an unclear pathogenesis. Despite vaccination, thousands of deaths attributed to JE are reported annually. In this study, we report that mice deficient for Axl, a receptor tyrosine kinase that plays multiple roles in flaviviral infection, displayed greater mortality upon JEV infection. The effect of Axl deficiency on JEV infection was mediated by markedly elevated serum interleukin-1α (IL-1α) levels, which devastated the blood-brain-barrier and promoted viral neuroinvasion within 24 h postinfection. Using an in situ infection model, we showed that dead macrophages were the primary source of observed increased serum IL-1α levels. Axl deficiency enhanced cell death and caused pyroptosis in 80% of JEV-infected macrophages by disrupting phosphatidylinositol 3-kinase (PI3K)-Akt signaling. Intriguingly, the primary effector released by pyroptotic macrophages in our model was IL-1α rather than IL-1β. Finally, we assessed the effect of an IL-1α antagonist and demonstrated that it effectively prevented the incidence of JE. Our results indicate that Axl plays a protective role in JEV infection, identify IL-1α released by pyroptotic macrophages as a crucial factor promoting JEV neuroinvasion, and suggest that an IL-1α antagonist may be a candidate for JE therapy.

IMPORTANCE Japanese encephalitis virus (JEV) is a mosquito-borne flavivirus that causes Japanese encephalitis (JE), the most commonly diagnosed viral encephalitis worldwide. The fatality rate of JE is 20%, and nearly half of the surviving patients develop neuropsychiatric sequelae. Axl is a receptor tyrosine kinase that plays multiple roles in flaviviral infections. Currently, the involvement of Axl in JEV infection remains enigmatic. In this study, we demonstrate that Axl impedes the pathogenesis of severe JE in mice by maintaining blood-brain-barrier (BBB) integrity and restricting viral neuroinvasion. Furthermore, serum IL-1α is a key mediator of this process and is primarily released by JEV-infected pyroptotic macrophages to elicit BBB breakdown, while an IL-1α antagonist can effectively reduce the incidence of severe JE. Our work uncovers the protective role of Axl in antagonizing severe JE and shows that the use of an IL-1α antagonist may be a promising tactic to prevent severe JE.

Generation and characterization of Japanese encephalitis virus expressing GFP reporter gene for high throughput drug screening

Japanese encephalitis virus (JEV), a major cause of Japanese encephalitisis, is an arbovirus that belongs to the genus Flavivirus of the family Flaviviridae. Currently, there is no effective drugs available for the treatment of JEV infection. Therefore, it is important to establish efficient antiviral screening system for the development of antiviral drugs. In this study, we constructed a full-length infectious clone of eGFP-JEV reporter virus by inserting the eGFP gene into the capsid-coding region of the viral genome. The reporter virus RNA transfected-BHK-21 cells generated robust eGFP fluorescence signals that were correlated well with viral replication. The reporter virus displayed growth kinetics similar to wild type (WT) virus although replicated a little slower. Using a known JEV inhibitor, NITD008, we demonstrated that the reporter virus could be used to identify inhibitors against JEV. Furthermore, an eGFP-JEV-based high throughput screening (HTS) assay was established in a 96-well format and used for screening of 1443 FDA-approved drugs. Sixteen hit drugs were identified to be active against JEV. Among them, five compounds which are lonafarnib, cetylpyridinium chlorid, cetrimonium bromide, nitroxoline and hexachlorophene, are newly discovered inhibitors of JEV, providing potential new therapies for treatment of JEV infection.

In vivo imaging of Zika virus reveals dynamics of viral invasion in immune-sheltered tissues and vertical propagation during pregnancy

Rationale: Zika virus (ZIKV) is a pathogenic virus known to cause a wide range of congenital abnormalities, including microcephaly, Guillain-Barre syndrome, meningoencephalitis, and other neurological complications, in humans. This study investigated the noninvasive detection of ZIKV infection in vivo, which is necessary for elucidating the virus's mechanisms of viral replication and pathogenesis, as well as to accelerate the development of anti-ZIKV therapeutic strategies.

Methods: In this study, a recombinant ZIKV harbouring Nluc gene (ZIKV-Nluc) was designed, recovered, and purified. The levels of bioluminescence were directly correlated with viral loads in vitro and in vivo. The dynamics of ZIKV infection in A129 (interferon (IFN)-α/β receptor deficient), AG6 (IFN-α/β and IFN-γ receptor deficient), and C57BL/6 mice were characterized. Pregnant dams were infected with ZIKV-Nluc at E10 via intra footpad injection. Then, the pooled immune sera (anti-ZIKV neutralizing antibodies) #22-1 in ZIKV-Nluc virus-infected mice were visualized.

Results: ZIKV-Nluc showed a high genetic stability and replicated well in cells with similar properties to the wild-type ZIKV (ZIKVwt). Striking bioluminescence signals were consistently observed in animal organs, including spleen, intestine, testis, uterus/ovary, and kidney. The ileocecal junction was found to be the crucial visceral target. Infection of pregnant dams with ZIKV-Nluc showed that ZIKV was capable of crossing the maternal-fetal barrier to infect the fetuses via vertical transmission. Furthermore, it was visualized that treatment with the pooled immune sera was found to greatly restrict the spread of the ZIKV-Nluc virus in mice.

Conclusions: This study is the first to report the real-time noninvasive tracking of the progression of ZIKV invading immune-sheltered tissues and propagating vertically during pregnancy. The results demonstrate that ZIKV-Nluc represents a powerful tool for the study of the replication, dissemination, pathogenesis, and treatment of ZIKV in vitro and in vivo.

A Chimeric Japanese Encephalitis Vaccine Protects against Lethal Yellow Fever Virus Infection without Inducing Neutralizing Antibodies

Efficient and safe vaccines against yellow fever (e.g., YFV-17D) that provide long-lasting protection by rapidly inducing neutralizing antibody responses exist. However, the vaccine supply cannot cope with an increasing demand posed by urban outbreaks in recent years. Here we report that JE-CVax/Imojev, a YFV-17D-based chimeric Japanese encephalitis vaccine, also efficiently protects against YFV infection in mice. In case of shortage of the YFV vaccine during yellow fever outbreaks, (off-label) use of JE-CVax/Imojev may be considered. Moreover, wider use of JE-CVax/Imojev in Asia may lower the risk of the much-feared YFV spillover to the continent. More generally, chimeric vaccines that combine surface antigens and replication machineries of two distinct flaviviruses may be considered dual vaccines for the latter pathogen without induction of surface-specific antibodies. Following this rationale, novel flavivirus vaccines that do not hold a risk for antibody-dependent enhancement (ADE) of infection (inherent to current dengue vaccines and dengue vaccine candidates) could be designed.

Recent outbreaks of yellow fever virus (YFV) in West Africa and Brazil resulted in rapid depletion of global vaccine emergency stockpiles and raised concerns about being unprepared against future YFV epidemics. Here we report that a live attenuated virus similar to the Japanese encephalitis virus (JEV) vaccine JE-CVax/Imojev that consists of YFV-17D vaccine from which the structural (prM/E) genes have been replaced with those of the JEV SA14-14-2 vaccine strain confers full protection in mice against lethal YFV challenge. In contrast to the YFV-17D-mediated protection against YFV, this protection is not mediated by neutralizing antibodies but correlates with YFV-specific nonneutralizing antibodies and T cell responses against cell-associated YFV NS1 and other YFV nonstructural (NS) proteins. Our findings reveal the potential of YFV NS proteins to mediate protection and demonstrate that chimeric flavivirus vaccines, such as Imojev, could confer protection against two flaviviruses. This dual protection may have implications for the possible off-label use of JE-CVax in case of emergency and vaccine shortage during YFV outbreaks. In addition, populations in Asia that have been vaccinated with Imojev may already be protected against YFV should outbreaks ever occur on that continent, as several countries/regions in the Asia-Pacific are vulnerable to international spread of the YFV.

Visualization of chikungunya virus infection in vitro and in vivo

Chikungunya virus (CHIKV), a mosquito-borne alphavirus, has become an important re-emerging pathogen with its rapid spread to many non-endemic areas. The lack of effective vaccines and antiviral agents is largely attributed to the elusive infection and dissemination dynamics in vivo. In this study, we designed and developed a novel, replication-competent, CHIKV reporter virus (CHIKV-iRFP) encoding a near infrared fluorescent protein (iRFP). In vitro and in vivo characterization demonstrated that CHIKV-iRFP retained similar replication and virulence phenotypes to its parental virus. Neonatal BABL/c mice and IFNAR^−/− A129 mice were highly susceptible to CHIKV-iRFP infection. Following intracranial (i.c.) inoculation, CHIKV-iRFP efficiently replicated and disseminated into whole body, resulting in rapid death in an age-dependent manner. Remarkably, upon footpad injection, CHIKV-iRFP readily disseminated from footpad to head and whole skeleton, with a specific tropism for bone marrow. Taken together, this novel reporter virus provides a powerful tool to track real time CHIKV replication and to test the in vivo efficacy of vaccines and antiviral therapeutics.

Different Degrees of 5'-to-3' DAR Interactions Modulate Zika Virus Genome Cyclization and Host-Specific Replication

Mosquito-borne flaviviruses, which include many important human pathogens, such as West Nile virus (WNV), dengue virus (DENV), and Zika virus (ZIKV), have caused numerous emerging epidemics in recent years. Details of the viral genome functions necessary for effective viral replication in mosquito and vertebrate hosts remain obscure. Here, using ZIKV as a model, we found that the conserved "downstream of AUG region" (DAR), which is known to be an essential element for genome cyclization, is involved in viral replication in a host-specific manner. Mutational analysis of the DAR element showed that a single-nucleotide mismatch between the 5' DAR and the 3' DAR had little effect on ZIKV replication in mammalian cells but dramatically impaired viral propagation in mosquito cells. The revertant viruses passaged in mosquito cells generated compensatory mutations restoring the base pairing of the DAR, further confirming the importance of the complementarity of the DAR in mosquito cells. We demonstrate that a single-nucleotide mutation in the DAR is sufficient to destroy long-range RNA interaction of the ZIKV genome and affects de novo RNA synthesis at 28°C instead of 37°C, resulting in the different replication efficiencies of the mutant viruses in mosquito and mammalian cells. Our results reveal a novel function of the circular form of the flavivirus genome in host-specific viral replication, providing new ideas to further explore the functions of the viral genome during host adaptation.IMPORTANCE Flaviviruses naturally cycle between the mosquito vector and vertebrate hosts. The disparate hosts provide selective pressures that drive virus genome evolution to maintain efficient replication during host alteration. Host adaptation may occur at different stages of the viral life cycle, since host-specific viral protein processing and virion conformations have been reported in the individual hosts. However, the viral determinants and the underlying mechanisms associated with host-specific functions remain obscure. In this study, using Zika virus, we found that the DAR-mediated genome cyclization regulates viral replication differently and is under different selection pressures in mammalian and mosquito cells. A more constrained complementarity of the DAR is required in mosquito cells than in mammalian cells. Since the DAR element is stably maintained among mosquito-borne flaviviruses, our findings could provide new information for understanding the role of flavivirus genome cyclization in viral adaptation and RNA evolution in the two hosts.

Novel Approach for Insertion of Heterologous Sequences into Full-Length ZIKV Genome Results in Superior Level of Gene Expression and Insert Stability

Zika virus (ZIKV) emerged in the Americas in 2015, presenting unique challenges to public health. Unlike other arboviruses of the Flaviviridae family, it is transmissible by sexual contact, which facilitates the spread of the virus into new geographic areas. Additionally, ZIKV can be transmitted from mother to fetus, causing microcephaly and other severe developmental abnormalities. Reliable and easy-to-work-with clones of ZIKV expressing heterologous genes will significantly facilitate studies aimed at understanding the virus pathogenesis and tissue tropism. Here, we developed and characterized two novel approaches for expression of heterologous genes of interest in the context of full-length ZIKV genome and compared them to two previously published strategies for ZIKV-mediated gene expression. We demonstrated that among the four tested viruses expressing nLuc gene, the virus constructed using a newly developed approach of partial capsid gene duplication (PCGD) attained the highest titer in Vero cells and resulted in the highest level of nLuc expression. Suitability of the PCGD approach for expression of different genes of interest was validated by replacing nLuc sequence with that of eGFP gene. The generated constructs were further characterized in cell culture. Potential applications of ZIKV clones stably expressing heterologous genes include development of detection assays, antivirals, therapeutics, live imaging systems, and vaccines.

Non-invasive bioluminescence imaging of HCoV-OC43 infection and therapy in the central nervous system of live mice

Human coronaviruses (HCoVs) are important pathogens that cause upper respiratory tract infections and have neuroinvasive abilities; however, little is known about the dynamic infection process of CoVs in vivo, and there are currently no specific antiviral drugs to prevent or treat HCoV infection. Here, we verified the replication ability and pathogenicity of a reporter HCoV-OC43 strain expressing Renilla luciferase (Rluc; rOC43-ns2DelRluc) in mice with different genetic backgrounds (C57BL/6 and BALB/c). Additionally, we monitored the spatial and temporal progression of HCoV-OC43 through the central nervous system (CNS) of live BALB/c mice after intranasal or intracerebral inoculation with rOC43-ns2DelRluc. We found that rOC43-ns2DelRluc was fatal to suckling mice after intranasal inoculation, and that viral titers and Rluc expression were detected in the brains and spinal cords of mice infected with rOC43-ns2DelRluc. Moreover, viral replication was initially observed in the brain by non-invasive bioluminescence imaging before the infection spread to the spinal cord of BALB/c mice, consistent with its tropism in the CNS. Furthermore, the Rluc readout correlated with the HCoV replication ability and protein expression, which allowed quantification of antiviral activity in live mice. Additionally, we validated that chloroquine strongly inhibited rOC43-ns2DelRluc replication in vivo. These results provide new insights into the temporal and spatial dissemination of HCoV-OC43 in the CNS, and our methods provide an extremely sensitive platform for evaluating the efficacy of antiviral therapies to treat neuroinvasive HCoVs in live mice.

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We verified the pathogenicity of a reporter HCoV-OC43 strain expressing Renilla luciferase (rOC43-ns2DelRluc) in mice.

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HCoV-OC43 spatio-temporal progression in CNS of mice was monitored by non-invasive bioluminescence imaging (BLI).

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Chloroquine was validated strongly inhibited rOC43-ns2DelRluc replication in in live mice.

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rOC43-ns2DelRluc-based BLI was reported as a promising platform for non-invasively screening antiviral compounds in vivo.

Review of Emerging Japanese Encephalitis Virus: New Aspects and Concepts about Entry into the Brain and Inter-Cellular Spreading

Japanese encephalitis virus (JEV) is an emerging flavivirus of the Asia-Pacific region. More than two billion people live in endemic or epidemic areas and are at risk of infection. Recently, the first autochthonous human case was recorded in Africa, and infected birds have been found in Europe. JEV may spread even further to other continents. The first section of this review covers established and new information about the epidemiology of JEV. The subsequent sections focus on the impact of JEV on humans, including the natural course and immunity. Furthermore, new concepts are discussed about JEV’s entry into the brain. Finally, interactions of JEV and host cells are covered, as well as how JEV may spread in the body through latently infected immune cells and cell-to-cell transmission of virions or via other infectious material, including JEV genomic RNA.

Trans Complementation of Replication-defective Omsk Hemorrhagic Fever Virus for Antiviral Study

Omsk hemorrhagic fever virus (OHFV) is a tick-borne flavivirus classified as a biosafety level-4 (BSL4) pathogen. Studies of OHFV are restricted to be conducted within BSL4 laboratories. Currently, no commercial vaccines or antiviral drugs are available against OHFV infection. In this study, we recovered a replication-deficient OHFV with an NS1 deletion (OHFV-ΔNS1) and reporter virus replacing NS1 with the Gaussia luciferase (Gluc) (OHFV-ΔNS1-Gluc). Both the defective OHFV-ΔNS1 and OHFV-ΔNS1-Gluc virus could only replicate efficiently in the BHK21 cell line expressing NS1 (BHK21_NS1) but not in naïve BHK21 cells. The Gluc reporter gene of OHFV-ΔNS1-Gluc virus was maintained stably after serial passaging of BHK21_NS1 cells and was used to surrogate the replication of OHFV. Using NITD008, OHFV-ΔNS1-Gluc virus was validated for antiviral screening, and high-throughput screening parameters were optimized in a 96-well plate format with a calculated Z' value above 0.5. The OHFV-ΔNS1-Gluc reporter virus is a powerful tool for antiviral screening as well as viral replication and pathogenesis studies in BSL2 laboratories.

A Simple and Robust Approach for Evaluation of Antivirals Using a Recombinant Influenza Virus Expressing Gaussia Luciferase

Influenza A virus (IAV) causes seasonal epidemics and occasional but devastating pandemics, which are major public health concerns. Because the effectiveness of seasonal vaccines is highly variable and the currently available drugs are limited in their efficacy because of the emergence of drug resistance, there is an urgent need to develop novel antivirals. In this study, we characterized a recombinant IAV-carrying Gaussia luciferase (Gluc) gene and determined its potential as a tool for evaluating therapeutics. We demonstrated that this recombinant IAV is replication-competent in tissue culture and pathogenic in mice, although it is slightly attenuated compared to the parental virus. Luciferase expression correlated well with virus propagation both in vitro and in vivo, providing a simple measure for viral replication in tissue culture and in mouse lungs. To demonstrate the utility of this virus, ribavirin and oseltamivir phosphate were used to treat the IAV-infected cells and mice, and we observed the dose-dependent inhibition of viral replication by a luciferase assay. Moreover, the decreased luciferase expression in the infected lungs could predict the protective efficacy of antiviral interventions as early as day 2 post virus challenge. In summary, this study provides a new and quantitative approach to evaluate antivirals against IAV.

North American domestic pigs are susceptible to experimental infection with Japanese encephalitis virus

Japanese encephalitis virus (JEV) is a mosquito-borne flavivirus that is capable of causing encephalitic diseases in children. While humans can succumb to severe disease, the transmission cycle is maintained by viremic birds and pigs in endemic regions. Although JEV is regarded as a significant threat to the United States (U.S.), the susceptibility of domestic swine to JEV infection has not been evaluated. In this study, domestic pigs from North America were intravenously challenged with JEV to characterize the pathological outcomes. Systemic infection followed by the development of neutralizing antibodies were observed in all challenged animals. While most clinical signs were limited to nonspecific symptoms, virus dissemination and neuroinvasion was observed at the acute phase of infection. Detection of infectious viruses in nasal secretions suggest infected animals are likely to promote the vector-free transmission of JEV. Viral RNA present in tonsils at 28 days post infection demonstrates the likelihood of persistent infection. In summary, our findings indicate that domestic pigs can potentially become amplification hosts in the event of an introduction of JEV into the U.S. Vector-free transmission to immunologically naïve vertebrate hosts is also likely through nasal shedding of infectious viruses.

In-vivo monitoring of infectious diseases in living animals using bioluminescence imaging

Traditional methods of localizing and quantifying the presence of pathogenic microorganisms in living experimental animal models of infections have mostly relied on sacrificing the animals, dissociating the tissue and counting the number of colony forming units. However, the discovery of several varieties of the light producing enzyme, luciferase, and the genetic engineering of bacteria, fungi, parasites and mice to make them emit light, either after administration of the luciferase substrate, or in the case of the bacterial lux operon without any exogenous substrate, has provided a new alternative. Dedicated bioluminescence imaging (BLI) cameras can record the light emitted from living animals in real time allowing non-invasive, longitudinal monitoring of the anatomical location and growth of infectious microorganisms as measured by strength of the BLI signal. BLI technology has been used to follow bacterial infections in traumatic skin wounds and burns, osteomyelitis, infections in intestines, Mycobacterial infections, otitis media, lung infections, biofilm and endodontic infections and meningitis. Fungi that have been engineered to be bioluminescent have been used to study infections caused by yeasts (Candida) and by filamentous fungi. Parasitic infections caused by malaria, Leishmania, trypanosomes and toxoplasma have all been monitored by BLI. Viruses such as vaccinia, herpes simplex, hepatitis B and C and influenza, have been studied using BLI. This rapidly growing technology is expected to continue to provide much useful information, while drastically reducing the numbers of animals needed in experimental studies.

Establishment of pseudovirus infection mouse models for in vivo pharmacodynamics evaluation of filovirus entry inhibitors

Filoviruses cause severe and fatal viral hemorrhagic fever in humans. Filovirus research has been extensive since the 2014 Ebola outbreak. Due to their high pathogenicity and mortality, live filoviruses require Biosafety Level-4 (BSL-4) facilities, which have restricted the development of anti-filovirus vaccines and drugs. An HIV-based pseudovirus cell infection assay is widely used for viral entry studies in BSL-2 conditions. Here, we successfully constructed nine in vitro pseudo-filovirus models covering all filovirus genera and three in vivo pseudo-filovirus-infection mouse models using Ebola virus, Marburg virus, and Lloviu virus as representative viruses. The pseudo-filovirus-infected mice showed visualizing bioluminescence in a dose-dependent manner. A bioluminescence peak in mice was reached on day 5 post-infection for Ebola virus and Marburg virus and on day 4 post-infection for Lloviu virus. Two known filovirus entry inhibitors, clomiphene and toremiphene, were used to validate the model. Collectively, our study shows that all genera of filoviruses can be well-pseudotyped and are infectious in vitro. The pseudo-filovirus-infection mouse models can be used for in vivo activity evaluation of anti-filovirus drugs. This sequential in vitro and in vivo evaluation system of filovirus entry inhibitors provides a secure and efficient platform for screening and assessing anti-filovirus agents in BSL-2 facilities.