The generation of a reverse genetics system for Kyasanur Forest Disease Virus and the ability to antagonize the induction of the antiviral state in vitro

Development and characterization of infectious clones of two strains of Usutu virus

Usutu virus (USUV; genus Flavivirus; family Flaviviridae) is a mosquito-borne, positive-sense RNA virus that is currently causing significant die-offs in numerous bird species throughout Europe and has caused infections in humans. Currently, there are no molecular clones for USUV, hence, hindering studies on the pathogenesis and transmission of USUV. Here, we demonstrate the development and characterization of infectious clones for two modern strains of USUV isolated from Europe and Africa. We show that the infectious clone-derived viruses replicated similarly to the parental strains in mammalian and insect cells. Additionally, we observed similar levels of replication and disease in two mouse models. These clones will aid the study of USUV infection, transmission, diagnostics, and vaccines.

Kyasanur Forest disease virus non-mouse animal models: a pilot study

Objectives

Mouse models have delivered variable recapitulation of Kyasanur Forest disease (KFD) pathology and consistently demonstrated neurological involvement which may be a limited feature of human disease. With the purpose of more accurately modelling human disease progression we infected several small-mammalian models: guinea pigs, hamsters and ferrets with a titered infectious dose of Kyasanur Forest disease virus (KFDV). Clinical indicators of disease severity were observed for seventeen days, on day eighteen a visual post-mortem analysis of visceral organs was conducted. Viral load in selected tissues was measured to infer disease signs and the establishment of viral replication.

Data description

Daily monitoring did not reveal any observable signs of illness; weight loss was minimal across species and gross pathology did not indicate severe viral infection. Tissue specific tropism and establishment of viral infection was monitored by quantitative real-time polymerase chain reaction (qRT-PCR). No viral replication was detected in ferrets (n = 0/3), but was present in the spleen of guinea pigs (n = 3/3) and the brain of hamsters (n = 3/3). Low levels of viral RNA were detected in multiple hamster tissues (kidney, liver, lung and spleen) suggesting the possibility of viral tropism and possible adaptation to the host. No serological tests were performed.

Immune Evasion Strategies Used by Zika Virus to Infect the Fetal Eye and Brain

Zika virus (ZIKV) is a mosquito-transmitted flavivirus that caused a public health emergency in the Americas when an outbreak in Brazil became linked to congenital microcephaly. Understanding how ZIKV could evade the innate immune defenses of the mother, placenta, and fetus has become central to determining how the virus can traffic into the fetal brain. ZIKV, like other flaviviruses, evades host innate immune responses by leveraging viral proteins and other processes that occur during viral replication to allow spread to the placenta. Within the placenta, there are diverse cell types with coreceptors for ZIKV entry, creating an opportunity for the virus to establish a reservoir for replication and infect the fetus. The fetal brain is vulnerable to ZIKV, particularly during the first trimester, when it is beginning a dynamic process, to form highly complex and specialized regions orchestrated by neuroprogenitor cells. In this review, we provide a conceptual framework to understand the different routes for viral trafficking into the fetal brain and the eye, which are most likely to occur early and later in pregnancy. Based on the injury profile in human and nonhuman primates, ZIKV entry into the fetal brain likely occurs across both the blood/cerebrospinal fluid barrier in the choroid plexus and the blood/brain barrier. ZIKV can also enter the eye by trafficking across the blood/retinal barrier. Ultimately, the efficient escape of innate immune defenses by ZIKV is a key factor leading to viral infection. However, the host immune response against ZIKV can lead to injury and perturbations in developmental programs that drive cellular division, migration, and brain growth. The combined effect of innate immune evasion to facilitate viral propagation and the maternal/placental/fetal immune response to control the infection will determine the extent to which ZIKV can injure the fetal brain.

Tick-Borne Flaviviruses and the Type I Interferon Response

Flaviviruses are globally distributed pathogens causing millions of human infections every year. Flaviviruses are arthropod-borne viruses and are mainly transmitted by either ticks or mosquitoes. Mosquito-borne flaviviruses and their interactions with the innate immune response have been well-studied and reviewed extensively, thus this review will discuss tick-borne flaviviruses and their interactions with the host innate immune response.

Viral Determinants of Virulence in Tick-Borne Flaviviruses

Tick-borne flaviviruses have a global distribution and cause significant human disease, including encephalitis and hemorrhagic fever, and often result in neurologic sequelae. There are two distinct properties that determine the neuropathogenesis of a virus. The ability to invade the central nervous system (CNS) is referred to as the neuroinvasiveness of the agent, while the ability to infect and damage cells within the CNS is referred to as its neurovirulence. Examination of laboratory variants, cDNA clones, natural isolates with varying pathogenicity, and virally encoded immune evasion strategies have contributed extensively to our understanding of these properties. Here we will review the major viral determinants of virulence that contribute to pathogenesis and influence both neuroinvasiveness and neurovirulence properties of tick-borne flaviviruses, focusing particularly on the envelope protein (E), nonstructural protein 5 (NS5), and the 3′ untranslated region (UTR).

Epidemiology, Pathogenesis, and Control of a Tick-Borne Disease- Kyasanur Forest Disease: Current Status and Future Directions

In South Asia, Haemaphysalis spinigera tick transmits Kyasanur Forest Disease Virus (KFDV), a flavivirus that causes severe hemorrhagic fever with neurological manifestations such as mental disturbances, severe headache, tremors, and vision deficits in infected human beings with a fatality rate of 3-10%. The disease was first reported in March 1957 from Kyasanur forest of Karnataka (India) from sick and dying monkeys. Since then, between 400 and 500 humans cases per year have been recorded; monkeys and small mammals are common hosts of this virus. KFDV can cause epizootics with high fatality in primates and is a level-4 virus according to the international biosafety rules. The density of tick vectors in a given year correlates with the incidence of human disease. The virus is a positive strand RNA virus and its genome was discovered to code for one polyprotein that is cleaved post-translationally into 3 structural proteins (Capsid protein, Envelope Glycoprotein M and Envelope Glycoprotein E) and 7 non-structural proteins (NS1, NS2A, NS2B, NS3, NS4A, NS4B, and NS5). KFDV has a high degree of sequence homology with most members of the TBEV serocomplex. Alkhurma virus is a KFDV variant sharing a sequence similarity of 97%. KFDV is classified as a NIAID Category C priority pathogen due to its extreme pathogenicity and lack of US FDA approved vaccines and therapeutics; also, the infectious dose is currently unknown for KFD. In India, formalin-inactivated KFDV vaccine produced in chick embryo fibroblast is being used. Nevertheless, further efforts are required to enhance its long-term efficacy. KFDV remains an understudied virus and there remains a lack of insight into its pathogenesis; moreover, specific treatment to the disease is not available to date. Environmental and climatic factors involved in disseminating Kyasanur Forest Disease are required to be fully explored. There should be a mapping of endemic areas and cross-border veterinary surveillance needs to be developed in high-risk regions. The involvement of both animal and health sector is pivotal for circumscribing the spread of this disease to new areas.

Limited Effects of Type I Interferons on Kyasanur Forest Disease Virus in Cell Culture

BACKGROUND

The tick-borne flavivirus, Kyasanur Forest disease virus (KFDV) causes seasonal infections and periodic outbreaks in south-west India. The current vaccine offers poor protection with reported issues of coverage and immunogenicity. Since there are no approved prophylactic therapeutics for KFDV, type I IFN-α/β subtypes were assessed for antiviral potency against KFDV in cell culture.

METHODOLOGY/PRINCIPAL FINDINGS

The continued passage of KFDV-infected cells with re-administered IFN-α2a treatment did not eliminate KFDV and had little effect on infectious particle production whereas the IFN-sensitive, green fluorescent protein-expressing vesicular stomatitis virus (VSV-GFP) infection was controlled. Further evaluation of the other IFN-α/β subtypes versus KFDV infection indicated that single treatments of either IFN-αWA and IFN-αΚ appeared to be more effective than IFN-α2a at reducing KFDV titres. Concentration-dependent analysis of these IFN-α/β subtypes revealed that regardless of subtype, low concentrations of IFN were able to limit cytopathic effects (CPE), while significantly higher concentrations were needed for inhibition of virion release. Furthermore, expression of the KFDV NS5 in cell culture before IFN addition enabled VSV-GFP to overcome the effects of IFN-α/β signalling, producing a robust infection.

CONCLUSIONS/SIGNIFICANCE

Treatment of cell culture with IFN does not appear to be suitable for KFDV eradication and the assay used for such studies should be carefully considered. Further, it appears that the NS5 protein is sufficient to permit KFDV to bypass the antiviral properties of IFN. We suggest that other prophylactic therapeutics should be evaluated in place of IFN for treatment of individuals with KFDV disease.

Rapid one-step construction of a Middle East Respiratory Syndrome (MERS-CoV) infectious clone system by homologous recombination

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This work describes a minimalist approach to the construction and validation of a coronavirus reverse genetics system utilizing homologous recombination for vector construction and a variety of molecular techniques for virus detection.

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Homologous recombination in S. cerevisiae proves an efficient and fast method of constructing the large and complex genome of MERS-CoV, this strategy will be helpful for future construction of other virus genomes.

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Detection of rescued virus by molecular assay (i.e.: RT-PCR) is a strong alternative to the more traditional immunological detection assays.

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Rescued MERS-CoV was genomically and phenotypically similar to the original isolate MERS-CoV/EMC-2012.

Background

Viral Infectious clone systems serve as robust platforms to study viral gene or replicative function by reverse genetics, formulate vaccines and adapt a wild type-virus to an animal host. Since the development of the first viral infectious clone system for the poliovirus, novel strategies of viral genome construction have allowed for the assembly of viral genomes across the identified viral families. However, the molecular profiles of some viruses make their genome more difficult to construct than others. Two factors that affect the difficulty of infectious clone construction are genome length and genome complexity.

Results

This work examines the available strategies for overcoming the obstacles of assembling the long and complex RNA genomes of coronaviruses and reports one-step construction of an infectious clone system for the Middle East Respiratory Syndrome coronavirus (MERS-CoV) by homologous recombination in S. cerevisiae.

Conclusions

Future use of this methodology will shorten the time between emergence of a novel viral pathogen and construction of an infectious clone system. Completion of a viral infectious clone system facilitates further study of a virus’s biology, improvement of diagnostic tests, vaccine production and the screening of antiviral compounds.

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.