Virulence determinants between New York 99 and Kunjin strains of West Nile virus

Evaluation of NS4A, NS4B, NS5 and 3'UTR Genetic Determinants of WNV Lineage 1 Virulence in Birds and Mammals

West Nile virus (WNV) is amplified in an enzootic cycle involving birds as amplifying hosts. Because they do not develop high levels of viremia, humans and horses are considered to be dead-end hosts. Mosquitoes, especially from the Culex genus, are vectors responsible for transmission between hosts. Consequently, understanding WNV epidemiology and infection requires comparative and integrated analyses in bird, mammalian, and insect hosts. So far, markers of WNV virulence have mainly been determined in mammalian model organisms (essentially mice), while data in avian models are still missing. WNV Israel 1998 (IS98) is a highly virulent strain that is closely genetically related to the strain introduced into North America in 1999, NY99 (genomic sequence homology > 99%). The latter probably entered the continent at New York City, generating the most impactful WNV outbreak ever documented in wild birds, horses, and humans. In contrast, the WNV Italy 2008 strain (IT08) induced only limited mortality in birds and mammals in Europe during the summer of 2008. To test whether genetic polymorphism between IS98 and IT08 could account for differences in disease spread and burden, we generated chimeric viruses between IS98 and IT08, focusing on the 3' end of the genome (NS4A, NS4B, NS5, and 3'UTR regions) where most of the non-synonymous mutations were detected. In vitro and in vivo comparative analyses of parental and chimeric viruses demonstrated a role for NS4A/NS4B/5'NS5 in the decreased virulence of IT08 in SPF chickens, possibly due to the NS4B-E249D mutation. Additionally, significant differences between the highly virulent strain IS98 and the other three viruses were observed in mice, implying the existence of additional molecular determinants of virulence in mammals, such as the amino acid changes NS5-V258A, NS5-N280K, NS5-A372V, and NS5-R422K. As previously shown, our work also suggests that genetic determinants of WNV virulence can be host-dependent.

The envelope protein of Usutu virus attenuates West Nile virus virulence in immunocompetent mice

West Nile virus (WNV) and Usutu virus (USUV) are the two most widespread mosquito-borne flaviviruses in Europe causing severe neuroinvasive disease in humans. Here, following standardization of the murine model with wild type (wt) viruses, we engineered WNV and USUV genome by reverse genetics. A recombinant virus carrying the 5' UTR of WNV within the USUV genome backbone (r-USUV_{5'-UTR WNV}) was rescued; when administered to mice this virus did not cause signs or disease as wt USUV suggesting that 5' UTR of a marked neurotropic parental WNV was not per se a virulence factor. Interestingly, a chimeric virus carrying the envelope (E) protein of USUV in the WNV genome backbone (r-WNV_E-USUV) showed an attenuated profile in mice compared to wt WNV but significantly more virulent than wt USUV. Moreover, except when tested against serum samples originating from a live WNV infection, r-WNV_E-USUV showed an identical antigenic profile to wt USUV confirming that E is also the major immunodominant protein of USUV.

Molecular Determinants of West Nile Virus Virulence and Pathogenesis in Vertebrate and Invertebrate Hosts

West Nile virus (WNV), like the dengue virus (DENV) and yellow fever virus (YFV), are major arboviruses belonging to the Flavivirus genus. WNV is emerging or endemic in many countries around the world, affecting humans and other vertebrates. Since 1999, it has been considered to be a major public and veterinary health problem, causing diverse pathologies, ranging from a mild febrile state to severe neurological damage and death. WNV is transmitted in a bird–mosquito–bird cycle, and can occasionally infect humans and horses, both highly susceptible to the virus but considered dead-end hosts. Many studies have investigated the molecular determinants of WNV virulence, mainly with the ultimate objective of guiding vaccine development. Several vaccines are used in horses in different parts of the world, but there are no licensed WNV vaccines for humans, suggesting the need for greater understanding of the molecular determinants of virulence and antigenicity in different hosts. Owing to technical and economic considerations, WNV virulence factors have essentially been studied in rodent models, and the results cannot always be transported to mosquito vectors or to avian hosts. In this review, the known molecular determinants of WNV virulence, according to invertebrate (mosquitoes) or vertebrate hosts (mammalian and avian), are presented and discussed. This overview will highlight the differences and similarities found between WNV hosts and models, to provide a foundation for the prediction and anticipation of WNV re-emergence and its risk of global spread.

Rift Valley fever: a review

Rift Valley fever (RVF) is a mosquito-borne viral disease, principally of ruminants, that is endemic to Africa. The causative Phlebovirus, Rift Valley fever virus (RVFV), has a broad host range and, as such, also infects humans to cause primarily a self-limiting febrile illness. A small number of human cases will also develop severe complications, including haemorrhagic fever, encephalitis and visual impairment. In parts of Africa, it is a major disease of domestic ruminants, causing epidemics of abortion and mortality. It infects and can be transmitted by a broad range of mosquitos, with those of the genus Aedes and Culex thought to be the major vectors. Therefore, the virus has the potential to become established beyond Africa, including in Australia, where competent vector hosts are endemic. Vaccines for humans have not yet been developed to the commercial stage. This review examines the threat of this virus, with particular reference to Australia, and assesses gaps in our knowledge that may benefit from research focus.

A Single Mutation at Position 156 in the Envelope Protein of Tembusu Virus Is Responsible for Virus Tissue Tropism and Transmissibility in Ducks

Duck Tembusu virus (TMUV), like other mosquito-borne flaviviruses, such as Japanese encephalitis virus, West Nile virus, and Bagaza virus, is able to transmit vector-independently. To date, why these flaviviruses can be transmitted without mosquito vectors remains poorly understood. To explore the key molecular basis of flavivirus transmissibility, we compared virus replication and transmissibility of an early and a recent TMUV in ducks. The recent TMUV strain FX2010 replicated systemically and transmitted efficiently in ducks, while the replication of early strain MM1775 was limited and did not transmit among ducks. The TMUV envelope protein and its domain I were responsible for tissue tropism and transmissibility. The mutation S156P in the domain I resulted in disruption of N-linked glycosylation at amino acid 154 of the E protein and changed the conformation of "150 loop" of the E protein, which reduced virus replication in lungs and abrogated transmission in ducks. These data indicate that the 156S in the envelope protein is critical for TMUV tissue tropism and transmissibility in ducks in the absence of mosquitos. Our findings provide novel insights on understanding TMUV transmission among ducks.IMPORTANCE Tembusu virus, similar to other mosquito-borne flaviviruses such as WNV, JEV, and BAGV, can be transmitted without the presence of mosquito vectors. We demonstrate that the envelope protein of TMUV and its amino acid (S) at position 156 is responsible for tissue tropism and transmission in ducks. The mutation S156P results in disruption of N-linked glycosylation at amino acid 154 of the E protein and changes the conformation of "150 loop" of the E protein, which induces limited virus replication in lungs and abrogates transmission between ducks. Our findings provide new knowledge about TMUV transmission among ducks.

Virulence and Evolution of West Nile Virus, Australia, 1960-2012

Despite the absence of disease in humans and animals, virulent virus strains have been circulating for >30 years.

Worldwide, West Nile virus (WNV) causes encephalitis in humans, horses, and birds. The Kunjin strain of WNV (WNV_KUN) is endemic to northern Australia, but infections are usually asymptomatic. In 2011, an unprecedented outbreak of equine encephalitis occurred in southeastern Australia; most of the ≈900 reported cases were attributed to a newly emerged WNV_KUN strain. To investigate the origins of this virus, we performed genetic analysis and in vitro and in vivo studies of 13 WNV_KUN isolates collected from different regions of Australia during 1960–2012. Although no disease was recorded for 1984, 2000, or 2012, isolates collected during those years (from Victoria, Queensland, and New South Wales, respectively) exhibited levels of virulence in mice similar to that of the 2011 outbreak strain. Thus, virulent strains of WNV_KUN have circulated in Australia for >30 years, and the first extensive outbreak of equine disease in Australia probably resulted from a combination of specific ecologic and epidemiologic conditions.

Fitness peaks of dengue virus populations

The role of intra-host genetic diversity in dengue viral populations remains a topic of debate, particularly the impact on transmission of changes in this diversity. Several approaches have been taken to increasing and decreasing the genetic diversity of populations of RNA viruses and have drawn what appear to be contradictory conclusions. A 2-6 fold increase in genetic diversity of a wild type population of dengue virus serotype 1(DENV1) and of an infectious clone population derived from the wild type population, produced by treatment with nucleotide analogue 5 fluorouracil (5FU), drove the populations to extinction. Removal of 5FU immediately prior to extinction, resulted in a return to pre-treatment levels of fitness and genetic diversity, albeit with novel single nucleotide polymorphisms. These observations support the concept that DENV populations exist on fitness peaks determined by their transmission requirements and either an increase or a decrease in genetic diversity may result in a loss of fitness.

Helicase Domain of West Nile Virus NS3 Protein Plays a Role in Inhibition of Type I Interferon Signalling

West Nile virus (WNV) is a neurotropic flavivirus that can cause encephalitis in mammalian and avian hosts. In America, the virulent WNV strain (NY99) is causing yearly outbreaks of encephalitis in humans and horses, while in Australia the less virulent Kunjin strain of WNV strain has not been associated with significant disease outbreaks until a recent 2011 large outbreak in horses (but not in humans) caused by NSW2011 strain. Using chimeric viruses between NY99 and NSW2011 strains we previously identified a role for the non-structural proteins of NY99 strain and especially the NS3 protein, in enhanced virus replication in type I interferon response-competent cells and increased virulence in mice. To further define the role of NY99 NS3 protein in inhibition of type I interferon response, we have generated and characterised additional chimeric viruses containing the protease or the helicase domains of NY99 NS3 on the background of the NSW2011 strain. The results identified the role for the helicase but not the protease domain of NS3 protein in the inhibition of type I interferon signalling and showed that helicase domain of the more virulent NY99 strain performs this function more efficiently than helicase domain of the less virulent NSW2011 strain. Further analysis with individual amino acid mutants identified two amino acid residues in the helicase domain primarily responsible for this difference. Using chimeric replicons, we also showed that the inhibition of type I interferon (IFN) signalling was independent of other known functions of NS3 in RNA replication and assembly of virus particles.

Infectious DNAs derived from insect-specific flavivirus genomes enable identification of pre- and post-entry host restrictions in vertebrate cells

Flaviviruses such as West Nile virus (WNV), dengue virus and Zika virus are mosquito-borne pathogens that cause significant human diseases. A novel group of insect-specific flaviviruses (ISFs), which only replicate in mosquitoes, have also been identified. However, little is known about the mechanisms of ISF host restriction. We report the generation of infectious cDNA from two Australian ISFs, Parramatta River virus (PaRV) and Palm Creek virus (PCV). Using circular polymerase extension cloning (CPEC) with a modified OpIE2 insect promoter, infectious cDNA was generated and transfected directly into mosquito cells to produce infectious virus indistinguishable from wild-type virus. When infectious PaRV cDNA under transcriptional control of a mammalian promoter was used to transfect mouse embryo fibroblasts, the virus failed to initiate replication even when cell entry steps were by-passed and the type I interferon response was lacking. We also used CPEC to generate viable chimeric viruses between PCV and WNV. Analysis of these hybrid viruses revealed that ISFs are also restricted from replication in vertebrate cells at the point of entry. The approaches described here to generate infectious ISF DNAs and chimeric viruses provide unique tools to further dissect the mechanisms of their host restriction.

The in vitro and in vivo antiviral properties of combined monoterpene alcohols against West Nile virus infection

West Nile Virus (WNV) is a mosquito-borne flavivirus that can cause neuroinvasive disease in humans and animals for which no therapies are currently available. We studied an established combination of monoterpene alcohols (CMA) derived from Melaleuca alternifolia, against WNV infection. The in vitro results show that CMA exhibits virucidal activity, as well as reduces the viral titres and percentage of infected cells. The antiviral mechanism of action of CMA was studied. We found that CMA did not alter the intracellular pH, neither induced apoptosis, but did induce cell cycle arrest in the G0/G1-phase although that was not the antiviral mechanism. Furthermore, we tested CMA in vivo using IRF 3(-)(/)(-)/7(-/-)mice and it was found that CMA treatment significantly delayed morbidity due to WNV infection, reduced the loss of body weight and reduced the viral titres in brain. These findings suggest that CMA could be a therapeutic agent against WNV infection.

Tissue-specific transcription profile of cytokine and chemokine genes associated with flavivirus control and non-lethal neuropathogenesis in rabbits

We previously showed that New Zealand White (NZWRs) and cottontail rabbits (CTRs) are a suitable model for studying immune mechanisms behind virus control and non-lethal neuropathogenesis associated with West Nile virus (WNV) and Murray Valley encephalitis virus (MVEV) infections. In the current study, we observed that MVEV infection induced high IFNα, TNFα, IL6, and CXCL10 transcript levels in the brains of weanling NZWRs, unlike infection with the less virulent WNVNSW2011. These transcript levels also correlated with encephalitis severity. Widespread STAT1 protein expression in brain with moderate neuropathology suggests that IFN-I signaling is crucial for limiting neural infection and mediating non-lethal neuropathogenesis. Unlike NZWRs, CTRs limit neuroinvasion without upregulation of many cytokine/chemokine transcripts, suggesting a species-dependent virus control mechanism. However, the common IFNγ, TNFα and IL6 transcript upregulation in specific lymphoid organs suggest some conserved elements in the response against flaviviruses, unique to all rabbits.

In Vitro and in Vivo Evaluation of Mutations in the NS Region of Lineage 2 West Nile Virus Associated with Neuroinvasiveness in a Mammalian Model

West Nile virus (WNV) strains may differ significantly in neuroinvasiveness in vertebrate hosts. In contrast to genetic lineage 1 WNVs, molecular determinants of pathogenic lineage 2 strains have not been experimentally confirmed so far. A full-length infectious clone of a neurovirulent WNV lineage 2 strain (578/10; Central Europe) was generated and amino acid substitutions that have been shown to attenuate lineage 1 WNVs were introduced into the nonstructural proteins (NS1 (P250L), NS2A (A30P), NS3 (P249H) NS4B (P38G, C102S, E249G)). The mouse neuroinvasive phenotype of each mutant virus was examined following intraperitoneal inoculation of C57BL/6 mice. Only the NS1-P250L mutation was associated with a significant attenuation of virulence in mice compared to the wild-type. Multiplication kinetics in cell culture revealed significantly lower infectious virus titres for the NS1 mutant compared to the wild-type, as well as significantly lower amounts of positive and negative stranded RNA.

The E glycoprotein plays an essential role in the high pathogenicity of European-Mediterranean IS98 strain of West Nile virus

West Nile virus (WNV) is the most widespread arbovirus in the world. Several recent outbreaks and epizootics have been reported in Europe and the Mediterranean basin with increased virulence. In contrast to the well-characterized American and Australian strains, little is known about the virulence determinants of the WNV European-Mediterranean strains. To investigate the viral factors involved in the virulence of these strains, we generated chimeras between the highly neuropathogenic Israel 1998 (IS-98-ST1, IS98) strain and the non-pathogenic Malaysian Kunjin virus (KJMP-502). In vivo analyses in a mouse model of WNV pathogenesis shows that chimeric virus where KJMP-502 E glycoprotein was replaced by that of IS98 is neuropathogenic, demonstrating that this protein is a major virulence determinant. Presence of the N-glycosylation site had limited impact on virus virulence and the 5'UTR does not seem to influence pathogenesis. Finally, mice inoculated with KJMP-502 virus were protected against lethal IS98 infection.

The challenge of West Nile virus in Europe: knowledge gaps and research priorities

West Nile virus (WNV) is continuously spreading across Europe, and other continents, i.e. North and South America and many other regions of the world. Despite the overall sporadic nature of outbreaks with cases of West Nile neuroinvasive disease (WNND) in Europe, the spillover events have increased and the virus has been introduced into new areas. The high genetic diversity of the virus, with remarkable phenotypic variation, and its endemic circulation in several countries, require an intensification of the integrated and multidisciplinary research efforts built under the 7th Framework Programme of the European Union (FP7). It is important to better clarify several aspects of WNV circulation in Europe, including its ecology, genomic diversity, pathogenicity, transmissibility, diagnosis and control options, under different environmental and socio-economic scenarios. Identifying WNV endemic as well as infection-free areas is becoming a need for the development of human vaccines and therapeutics and the application of blood and organs safety regulations. This review, produced as a joint initiative among European experts and based on analysis of 118 scientific papers published between 2004 and 2014, provides the state of knowledge on WNV and highlights the existing knowledge and research gaps that need to be addressed with high priority in Europe and neighbouring countries.

Clinical outcome and genetic differences within a monophyletic Dengue virus type 2 population

The exact mechanisms of interplay between host and viral factors leading to severe dengue are yet to be fully understood. Even though previous studies have implicated specific genetic differences of Dengue virus (DENV) in clinical severity and virus attenuation, similar studies with large-scale, whole genome screening of monophyletic virus populations are limited. Therefore, in the present study, we compared 89 whole genomes of DENV-2 cosmopolitan clade III isolates obtained from patients diagnosed with dengue fever (DF, n = 58), dengue hemorrhagic fever (DHF, n = 30) and dengue shock syndrome (DSS, n = 1) in Singapore between July 2010 and January 2013, in order to determine the correlation of observed viral genetic differences with clinical outcomes. Our findings showed no significant difference between the number of primary and secondary infections that progressed to DHF and DSS (p>0.05) in our study cohort. Despite being highly homogenous, study isolates possessed 39 amino acid substitutions of which 10 substitutions were fixed in three main groups of virus isolates. None of those substitutions were specifically associated with DHF and DSS. Notably, two evolutionarily unique virus groups possessing C-P43T+NS1-S103T+NS2A-V83I+NS3-R337K+ NS3-I600T+ NS5-P136S and NS2A-T119N mutations were exclusively found in patients with DF, the benign form of DENV infections. Those mutants were significantly associated with mild disease outcome. These observations indicated that disease progression into DHF and DSS within our patient population was more likely to be due to host than virus factors. We hypothesize that selection for potentially less virulent groups of DENV-2 in our study cohort may be an evolutionary adaptation of viral strains to extend their survival in the human-mosquito transmission cycle.

Systematic analysis of viral genes responsible for differential virulence between American and Australian West Nile virus strains

A variant Australian West Nile virus (WNV) strain, WNVNSW2011, emerged in 2011 causing an unprecedented outbreak of encephalitis in horses in south-eastern Australia. However, no human cases associated with this strain have yet been reported. Studies using mouse models for WNV pathogenesis showed that WNVNSW2011 was less virulent than the human-pathogenic American strain of WNV, New York 99 (WNVNY99). To identify viral genes and mutations responsible for the difference in virulence between WNVNSW2011 and WNVNY99 strains, we constructed chimeric viruses with substitution of large genomic regions coding for the structural genes, non-structural genes and untranslated regions, as well as seven individual non-structural gene chimeras, using a modified circular polymerase extension cloning method. Our results showed that the complete non-structural region of WNVNSW2011, when substituted with that of WNVNY99, significantly enhanced viral replication and the ability to suppress type I IFN response in cells, resulting in higher virulence in mice. Analysis of the individual non-structural gene chimeras showed a predominant contribution of WNVNY99 NS3 to increased virus replication and evasion of IFN response in cells, and to virulence in mice. Other WNVNY99 non-structural proteins (NS2A, NS4B and NS5) were shown to contribute to the modulation of IFN response. Thus a combination of non-structural proteins, likely NS2A, NS3, NS4B and NS5, is primarily responsible for the difference in virulence between WNVNSW2011 and WNVNY99 strains, and accumulative mutations within these proteins would likely be required for the Australian WNVNSW2011 strain to become significantly more virulent.

Last 20 aa of the West Nile virus NS1' protein are responsible for its retention in cells and the formation of unique heat-stable dimers

West Nile virus (WNV), a mosquito-borne flavivirus, is the major cause of arboviral encephalitis in the USA. As with other members of the Japanese encephalitis virus serogroup, WNV produces an additional non-structural protein, NS1', a C-terminal extended product of NS1 generated as the result of a -1 programmed ribosomal frameshift (PRF). We have previously shown that mutations abolishing the PRF, and consequently NS1', resulted in reduced neuroinvasiveness. However, whether this was caused by the PRF event itself or by the lack of a PRF product, NS1', or a combination of both, remains undetermined. Here, we showed that WNV NS1' formed a unique subpopulation of heat- and low-pH-stable dimers. C-terminal truncations and mutational analysis employing an NS1'-expressing plasmid showed that stability of NS1' dimers was linked to the penultimate 10 aa. To examine the role of NS1' heat-stable dimers in virus replication and pathogenicity, a stop codon mutation was introduced into NS1' to create a WNV producing a truncated version of NS1' lacking the last 20 aa, but not affecting the PRF. NS1' protein produced by this mutant virus was secreted more efficiently than WT NS1', indicating that the sequence of the last 20 aa of NS1' was responsible for its cellular retention. Further analysis of this mutant showed growth kinetics in cells and virulence in weanling mice after peripheral infection similar to the WT WNVKUN, suggesting that full-length NS1' was not essential for virus replication in vitro and for virulence in mice.

Role of enhanced vector transmission of a new West Nile virus strain in an outbreak of equine disease in Australia in 2011

Background

In 2011, a variant of West Nile virus Kunjin strain (WNV_KUN) caused an unprecedented epidemic of neurological disease in horses in southeast Australia, resulting in almost 1,000 cases and a 9% fatality rate. We investigated whether increased fitness of the virus in the primary vector, Culex annulirostris, and another potential vector, Culex australicus, contributed to the widespread nature of the outbreak.

Methods

Mosquitoes were exposed to infectious blood meals containing either the virus strain responsible for the outbreak, designated WNV_KUN2011, or WNV_KUN2009, a strain of low virulence that is typical of historical strains of this virus. WNV_KUN infection in mosquito samples was detected using a fixed cell culture enzyme immunoassay and a WNV_KUN- specific monoclonal antibody. Probit analysis was used to determine mosquito susceptibility to infection. Infection, dissemination and transmission rates for selected days post-exposure were compared using Fisher’s exact test. Virus titers in bodies and saliva expectorates were compared using t-tests.

Results

There were few significant differences between the two virus strains in the susceptibility of Cx. annulirostris to infection, the kinetics of virus replication and the ability of this mosquito species to transmit either strain. Both strains were transmitted by Cx. annulirostris for the first time on day 5 post-exposure. The highest transmission rates (proportion of mosquitoes with virus detected in saliva) observed were 68% for WNV_KUN2011 on day 12 and 72% for WNV_KUN2009 on day 14. On days 12 and 14 post-exposure, significantly more WNV_KUN2011 than WNV_KUN2009 was expectorated by infected mosquitoes. Infection, dissemination and transmission rates of the two strains were not significantly different in Culex australicus. However, transmission rates and the amount of virus expectorated were significantly lower in Cx. australicus than Cx. annulirostris.

Conclusions

The higher amount of WNV_KUN2011 expectorated by infected mosquitoes may be an indication that this virus strain is transmitted more efficiently by Cx. annulirostris compared to other WNV_KUN strains. Combined with other factors, such as a convergence of abundant mosquito and wading bird populations, and mammalian and avian feeding behaviour by Cx. annulirostris, this may have contributed to the scale of the 2011 equine epidemic.

Flavivirus reverse genetic systems, construction techniques and applications: a historical perspective

The study of flaviviruses, which cause some of the most important emerging tropical and sub-tropical human arbovirus diseases, has greatly benefited from the use of reverse genetic systems since its first development for yellow fever virus in 1989. Reverse genetics technology has completely revolutionized the study of these viruses, making it possible to manipulate their genomes and evaluate the direct effects of these changes on their biology and pathogenesis. The most commonly used reverse genetics system is the infectious clone technology. Whilst flavivirus infectious clones provide a powerful tool, their construction as full-length cDNA molecules in bacterial vectors can be problematic, laborious and time consuming, because they are often unstable, contain unwanted induced substitutions and may be toxic for bacteria due to viral protein expression. The incredible technological advances that have been made during the past 30years, such as the use of PCR or new sequencing methods, have allowed the development of new approaches to improve preexisting systems or elaborate new strategies that overcome these problems. This review summarizes the evolution and major technical breakthroughs in the development of flavivirus reverse genetics technologies and their application to the further understanding and control of these viruses and their diseases.

Programmed ribosomal frameshift alters expression of west nile virus genes and facilitates virus replication in birds and mosquitoes

West Nile virus (WNV) is a human pathogen of significant medical importance with close to 40,000 cases of encephalitis and more than 1,600 deaths reported in the US alone since its first emergence in New York in 1999. Previous studies identified a motif in the beginning of non-structural gene NS2A of encephalitic flaviviruses including WNV which induces programmed −1 ribosomal frameshift (PRF) resulting in production of an additional NS protein NS1′. We have previously demonstrated that mutant WNV with abolished PRF was attenuated in mice. Here we have extended our previous observations by showing that PRF does not appear to have a significant role in virus replication, virion formation, and viral spread in several cell lines in vitro. However, we have also shown that PRF induces an over production of structural proteins over non-structural proteins in virus-infected cells and that mutation abolishing PRF is present in ∼11% of the wild type virus population. In vivo experiments in house sparrows using wild type and PRF mutant of New York 99 strain of WNV viruses showed some attenuation for the PRF mutant virus. Moreover, PRF mutant of Kunjin strain of WNV showed significant decrease compared to wild type virus infection in dissemination of the virus from the midgut through the haemocoel, and ultimately the capacity of infected mosquitoes to transmit virus. Thus our results demonstrate an important role for PRF in regulating expression of viral genes and consequently virus replication in avian and mosquito hosts.

Programmed ribosomal frameshift (PRF) is a strategy used by some viruses to regulate expression of viral genes and/or generate additional gene products for the benefit of the virus. Encephalitic flaviruses from Japanese encephalitis virus serogroup encode PRF motif in the beginning of nonstructural gene NS2A that results in production of an additional nonstructural protein NS1′ which for West Nile virus (WNV) consists of NS1 protein with 52 amino acid addition at the C terminus. Our previous studies showed that abolishing PFR and NS1′ production attenuated WNV virulence in mice. Here we show by using wild type and PRF-deficient WNV mutant that PRF induces overproduction of structural proteins, which facilitates virus replication in birds and mosquitoes while having no advantage for virus replication in cell lines in vitro. Presence of PRF/NS1′ allowed more efficient virus dissemination in the body of mosquitoes after taking infected blood meal and subsequent accumulation of the virus in saliva to facilitate transmission. Combined with our previous data in mice, the results obtained in this study demonstrate that while having no advantage for WNV replication in vitro, PRF provides advantage for WNV replication in vivo in mammalian, avian and mosquito hosts most likely by overproducing viral structural proteins and generating NS1′.

Mechanism of West Nile virus neuroinvasion: a critical appraisal

West Nile virus (WNV) is an important emerging neurotropic virus, responsible for increasingly severe encephalitis outbreaks in humans and horses worldwide. However, the mechanism by which the virus gains entry to the brain (neuroinvasion) remains poorly understood. Hypotheses of hematogenous and transneural entry have been proposed for WNV neuroinvasion, which revolve mainly around the concepts of blood-brain barrier (BBB) disruption and retrograde axonal transport, respectively. However, an over‑representation of in vitro studies without adequate in vivo validation continues to obscure our understanding of the mechanism(s). Furthermore, WNV infection in the current rodent models does not generate a similar viremia and character of CNS infection, as seen in the common target hosts, humans and horses. These differences ultimately question the applicability of rodent models for pathogenesis investigations. Finally, the role of several barriers against CNS insults, such as the blood-cerebrospinal fluid (CSF), the CSF-brain and the blood-spinal cord barriers, remain largely unexplored, highlighting the infancy of this field. In this review, a systematic and critical appraisal of the current evidence relevant to the possible mechanism(s) of WNV neuroinvasion is conducted.

In vitro and in vivo characterization of a West Nile virus MAD78 infectious clone

The viral determinants governing the varied neuropathogenicity of different West Nile virus (WNV) strains are poorly understood. Here, we generated an infectious clone (WNV-MAD^IC) of the non-pathogenic strain WNV-MAD78 and compared its replication to that of parental WNV-MAD78 and a WNV-MAD78 infectious clone (WNV-MAD^TX-UTRs) containing the 5′ and 3′ untranslated regions (UTRs) of the pathogenic strain WNV-TX. All three viruses replicated at similar rates and caused similar lethality in mice. Thus, the infectious clone is indistinguishable from parental virus in replication and neurovirulence, and the UTRs alone do not account for the increased virulence of WNV-TX compared to WNV-MAD78.

A novel approach to propagate flavivirus infectious cDNA clones in bacteria by introducing tandem repeat sequences upstream of virus genome

Despite tremendous efforts to improve the methodology for constructing flavivirus infectious cDNAs, the manipulation of flavivirus cDNAs remains a difficult task in bacteria. Here, we successfully propagated DNA-launched type 2 dengue virus (DENV2) and Japanese encephalitis virus (JEV) infectious cDNAs by introducing seven repeats of the tetracycline-response element (7×TRE) and a minimal cytomegalovirus (CMVmin) promoter upstream of the viral genome. Insertion of the 7×TRE-CMVmin sequence upstream of the DENV2 or JEV genome decreased the cryptic E. coli promoter (ECP) activity of the viral genome in bacteria, as measured using fusion constructs containing DENV2 or JEV segments and the reporter gene Renilla luciferase in an empty vector. The growth kinetics of recombinant viruses derived from DNA-launched DENV2 and JEV infectious cDNAs were similar to those of parental viruses. Similarly, RNA-launched DENV2 infectious cDNAs were generated by inserting 7×TRE-CMVmin, five repeats of the GAL4 upstream activating sequence, or five repeats of BamHI linkers upstream of the DENV2 genome. All three tandem repeat sequences decreased the ECP activity of the DENV2 genome in bacteria. Notably, 7×TRE-CMVmin stabilized RNA-launched JEV infectious cDNAs and reduced the ECP activity of the JEV genome in bacteria. The growth kinetics of recombinant viruses derived from RNA-launched DENV2 and JEV infectious cDNAs displayed patterns similar to those of the parental viruses. These results support a novel methodology for constructing flavivirus infectious cDNAs, which will facilitate research in virology, viral pathogenesis and vaccine development of flaviviruses and other RNA viruses.

The West Nile virus-like flavivirus Koutango is highly virulent in mice due to delayed viral clearance and the induction of a poor neutralizing antibody response

The mosquito-borne West Nile virus (WNV) is responsible for outbreaks of viral encephalitis in humans, horses, and birds, with particularly virulent strains causing recent outbreaks of disease in eastern Europe, the Middle East, North America, and Australia. Previous studies have phylogenetically separated WNV strains into two main genetic lineages (I and II) containing virulent strains associated with neurological disease. Several WNV-like strains clustering outside these lineages have been identified and form an additional five proposed lineages. However, little is known about whether these strains have the potential to induce disease. In a comparative analysis with the highly virulent lineage I American strain (WNVNY99), the low-pathogenicity lineage II strain (B956), a benign Australian strain, Kunjin (WNVKUN), the African WNV-like Koutango virus (WNVKOU), and a WNV-like isolate from Sarawak, Malaysia (WNVSarawak), were assessed for neuroinvasive properties in a murine model and for their replication kinetics in vitro. While WNVNY99 replicated to the highest levels in vitro, in vivo mouse challenge revealed that WNVKOU was more virulent, with a shorter time to onset of neurological disease and higher morbidity. Histological analysis of WNVKOU- and WNVNY99-infected brain and spinal cords demonstrated more prominent meningoencephalitis and the presence of viral antigen in WNVKOU-infected mice. Enhanced virulence of WNVKOU also was associated with poor viral clearance in the periphery (sera and spleen), a skewed innate immune response, and poor neutralizing antibody development. These data demonstrate, for the first time, potent neuroinvasive and neurovirulent properties of a WNV-like virus outside lineages I and II.

IMPORTANCE

In this study, we characterized the in vitro and in vivo properties of previously uncharacterized West Nile virus strains and West Nile-like viruses. We identified a West Nile-like virus, Koutango virus (WNVKOU), that was more virulent than a known virulent lineage I virus, WNVNY99. The enhanced virulence of WNVKOU was associated with poor viral clearance and the induction of a poor neutralizing antibody response. These findings provide new insights into the pathogenesis of West Nile virus.

Expression of mosquito microRNA Aae-miR-2940-5p is downregulated in response to West Nile virus infection to restrict viral replication

West Nile virus (WNV) is an enveloped virus with a single-stranded positive-sense RNA genome from the Flaviviridae family. WNV is spread by mosquitoes and able to infect humans, causing encephalitis and meningitis that can be fatal; it therefore presents a significant risk for human health. In insects, innate response to RNA virus infection mostly relies on RNA interference and JAK/SAT pathways; however, some evidence indicates that it can also involve microRNAs (miRNAs). miRNAs are small noncoding RNAs that regulate gene expression at posttranscriptional level and play an important role in a number of processes, including immunity and antiviral response. In this study, we focus on the miRNA-mediated response to WNV in mosquito cells. We demonstrate that in response to WNV infection the expression of a mosquito-specific miRNA, aae-miR-2940, is selectively downregulated in Aedes albopictus cells. This miRNA is known to upregulate the metalloprotease m41 FtsH gene, which we have also shown to be required for efficient WNV replication. Correspondingly, downregulation of aae-miR-2940 reduced the metalloprotease level and restricted WNV replication. Thus, we have identified a novel miRNA-dependent mechanism of antiviral response to WNV in mosquitoes.

IMPORTANCE

A detailed understanding of vector-pathogen interactions is essential to address the problems posed by vector-borne diseases. Host and viral miRNAs play an important role in regulating expression of viral and host genes involved in endogenous processes, including antiviral response. There has been no evidence to date for the role of mosquito miRNAs in response to flaviviruses. In this study, we show that downregulation of aae-miR-2940 in mosquito cells acts as a potential antiviral mechanism in the mosquito host to inhibit WNV replication by repressing the expression of the metalloprotease m41 FtsH gene, which is required for efficient WNV replication. This is the first identification of an miRNA-dependent antiviral mechanism in mosquitoes, which inhibits replication of WNV. Our findings should facilitate identification of targets in the mosquito genome that can be utilized to suppress vector population and/or limit WNV replication.

Evaluation of a mouse model for the West Nile virus group for the purpose of determining viral pathotypes

West Nile virus (WNV; family Flaviviridae; genus Flavivirus) group members are an important cause of viral meningoencephalitis in some areas of the world. They exhibit marked variation in pathogenicity, with some viral lineages (such as those from North America) causing high prevalence of severe neurological disease, whilst others (such as Australian Kunjin virus) rarely cause disease. The aim of this study was to characterize WNV disease in a mouse model and to elucidate the pathogenetic features that distinguish disease variation. Tenfold dilutions of five WNV strains (New York 1999, MRM16 and three horse isolates of WNV-Kunjin: Boort and two isolates from the 2011 Australian outbreak) were inoculated into mice by the intraperitoneal route. All isolates induced meningoencephalitis in different proportions of infected mice. WNVNY99 was the most pathogenic, the three horse isolates were of intermediate pathogenicity and WNVKUNV-MRM16 was the least, causing mostly asymptomatic disease with seroconversion. Infectivity, but not pathogenicity, was related to challenge dose. Using cluster analysis of the recorded clinical signs, histopathological lesions and antigen distribution scores, the cases could be classified into groups corresponding to disease severity. Metrics that were important in determining pathotype included neurological signs (paralysis and seizures), meningoencephalitis, brain antigen scores and replication in extra-neural tissues. Whereas all mice infected with WNVNY99 had extra-neural antigen, those infected with the WNV-Kunjin viruses only occasionally had antigen outside the nervous system. We conclude that the mouse model could be a useful tool for the assessment of pathotype for WNVs.

Experimental infections of wild birds with West Nile virus

Avian models of West Nile virus (WNV) disease have become pivotal in the study of infection pathogenesis and transmission, despite the intrinsic constraints that represents this type of experimental research that needs to be conducted in biosecurity level 3 (BSL3) facilities. This review summarizes the main achievements of WNV experimental research carried out in wild birds, highlighting advantages and limitations of this model. Viral and host factors that determine the infection outcome are analyzed in detail, as well as recent discoveries about avian immunity, viral transmission, and persistence achieved through experimental research. Studies of laboratory infections in the natural host will help to understand variations in susceptibility and reservoir competence among bird species, as well as in the epidemiological patterns found in different affected areas.

Comparison of the neuropathology induced by two West Nile virus strains

Some strains of West Nile virus (WNV) are neuroinvasive and may induce fatal encephalitis/meningitis in a variety of animal species including humans. Whether, however, there is a strain-specific signature in the brain is as yet unknown. Here we investigated the neuropathogenesis induced by two phylogenetically distant WNV strains of lineage 1, WNV_IS98 and WNV_KUN35 911. While four-week old C57Bl/6J mice were susceptible to both strains and succumbed rapidly after intraperitoneal inoculation, differences were observed in virulence and clinical disease. WNV_KUN35 911, the less virulent strain as judged by determination of LD₅₀, induced typical signs of encephalitis. Such signs were not observed in WNV_IS98-infected mice, although they died more rapidly. Histological examination of brain sections also revealed differences, as the level of apoptosis and inflammation was higher in WNV_KUN35 911- than WNV_IS98-infected mice. Moreover, staining for cleaved caspase 3 showed that the two WNV strains induced apoptotic death through different molecular mechanisms in one particular brain area. Finally, the two strains showed similar tropism in cortex, striatum, brainstem, and cerebellum but a different one in hippocampus. In summary, our data show that, upon peripheral administration, WNV_IS98 and WNV_KUN35 911 strains induce partially distinct lesions and tissue tropism in the brain. They suggest that the virulence of a WNV strain is not necessarily correlated with the severity of apoptotic and inflammatory lesions in the brain.

Differential virulence and pathogenesis of West Nile viruses

West Nile virus (WNV) is a neurotropic flavivirus that cycles between mosquitoes and birds but that can also infect humans, horses, and other vertebrate animals. In most humans, WNV infection remains subclinical. However, 20%-40% of those infected may develop WNV disease, with symptoms ranging from fever to meningoencephalitis. A large variety of WNV strains have been described worldwide. Based on their genetic differences, they have been classified into eight lineages; the pathogenic strains belong to lineages 1 and 2. Ten years ago, Beasley et al. (2002) found that dramatic differences exist in the virulence and neuroinvasion properties of lineage 1 and lineage 2 WNV strains. Further insights on how WNV interacts with its hosts have recently been gained; the virus acts either at the periphery or on the central nervous system (CNS), and these observed differences could help explain the differential virulence and neurovirulence of WNV strains. This review aims to summarize the current state of knowledge on factors that trigger WNV dissemination and CNS invasion as well as on the inflammatory response and CNS damage induced by WNV. Moreover, we will discuss how WNV strains differentially interact with the innate immune system and CNS cells, thus influencing WNV pathogenesis.

Molecular epidemiology and evolution of West Nile virus in North America

West Nile virus (WNV) was introduced to New York in 1999 and rapidly spread throughout North America and into parts of Central and South America. Displacement of the original New York (NY99) genotype by the North America/West Nile 2002 (NA/WN02) genotype occurred in 2002 with subsequent identification of a novel genotype in 2003 in isolates collected from the southwestern Unites States region (SW/WN03 genotype). Both genotypes co-circulate to date. Subsequent WNV surveillance studies have confirmed additional genotypes in the United States that have become extinct due to lack of a selective advantage or stochastic effect; however, the dynamic emergence, displacement, and extinction of multiple WNV genotypes in the US from 1999–2012 indicates the continued evolution of WNV in North America.

Molecular characterization and phylogenetic analysis of Murray Valley encephalitis virus and West Nile virus (Kunjin subtype) from an arbovirus disease outbreak in horses in Victoria, Australia, in 2011

Virus was detected in the central nervous system (CNS) tissue of 11 horses from Victoria that died displaying neurological symptoms during an outbreak of disease in Australia in 2011. Five horses were identified as being infected with Murray Valley encephalitis virus (MVEV) and 6 as being infected with West Nile virus subtype Kunjin (WNV(KUN)). Analysis of partial sequence information from the NS5 and E genes indicated that the MVEVs within the samples were highly homogenous and all belonged to lineage I, which is enzootic to the tropical regions of northern Australia. Likewise, analysis of partial NS5 and E gene and full genome sequences indicated that the WNV(KUN) within the samples were also highly homogenous and clustered with WNV lineage 1, clade b, which is consistent with other WNV(KUN) isolates. Full genomes of 1 MVEV isolate and 2 WNV(KUN) isolates were sequenced and characterized. The genome sequences of Victorian WNV(KUN) are almost identical (3 amino acid differences) to that of the recently sequenced WNV isolate WNV(NSW2011). Metagenome sequencing directly from CNS tissue identified the presence of WNV(KUN) and MVEV within infected CNS tissue.

NS1' colocalizes with NS1 and can substitute for NS1 in West Nile virus replication

NS1' is a C-terminally extended form of the NS1 protein produced only by encephalitic flaviviruses from the Japanese encephalitis virus serogroup. Here we show that West Nile virus (WNV) NS1' and NS1 localize to the same cellular compartments when expressed from plasmid DNAs and also colocalize to viral RNA replication sites in infected cells. Using complementation analysis with NS1-deleted WNV cDNA, we demonstrated that NS1' is able to substitute for the crucial function of NS1 in virus replication.

Characterization of virulent West Nile virus Kunjin strain, Australia, 2011

To determine the cause of an unprecedented outbreak of encephalitis among horses in New South Wales, Australia, in 2011, we performed genomic sequencing of viruses isolated from affected horses and mosquitoes. Results showed that most of the cases were caused by a variant West Nile virus (WNV) strain, WNV(NSW2011), that is most closely related to WNV Kunjin (WNV(KUN)), the indigenous WNV strain in Australia. Studies in mouse models for WNV pathogenesis showed that WNV(NSW2011) is substantially more neuroinvasive than the prototype WNV(KUN) strain. In WNV(NSW2011), this apparent increase in virulence over that of the prototype strain correlated with at least 2 known markers of WNV virulence that are not found in WNV(KUN). Additional studies are needed to determine the relationship of the WNV(NSW2011) strain to currently and previously circulating WNV(KUN) strains and to confirm the cause of the increased virulence of this emerging WNV strain.

A novel bacterium-free method for generation of flavivirus infectious DNA by circular polymerase extension reaction allows accurate recapitulation of viral heterogeneity

A novel bacterium-free approach for rapid assembly of flavivirus infectious cDNAs using circular polymerase extension reaction was applied to generate infectious cDNA for the virulent New South Wales isolate of the Kunjin strain of West Nile virus (KUNV) that recently emerged in Australia. Recovered virus recapitulated the genetic heterogeneity present in the original isolate. The approach was utilized to generate viral mutants with designed phenotypic properties and to identify E protein glycosylation as one of the virulence determinants.

Effect of Wolbachia on replication of West Nile virus in a mosquito cell line and adult mosquitoes

Wolbachia as an endosymbiont is widespread in insects and other arthropods and is best known for reproductive manipulations of the host. Recently, it has been shown that wMelpop and wMel strains of Wolbachia inhibit the replication of several RNA viruses, including dengue virus, and other vector-borne pathogens (e.g., Plasmodium and filarial nematodes) in mosquitoes, providing an alternative approach to limit the transmission of vector-borne pathogens. In this study, we tested the effect of Wolbachia on the replication of West Nile Virus (WNV). Surprisingly, accumulation of the genomic RNA of WNV for all three strains of WNV tested (New York 99, Kunjin, and New South Wales) was enhanced in Wolbachia-infected Aedes aegypti cells (Aag2). However, the amount of secreted virus was significantly reduced in the presence of Wolbachia. Intrathoracic injections showed that replication of WNV in A. aegypti mosquitoes infected with wMel strain of Wolbachia was not inhibited, whereas wMelPop strain of Wolbachia significantly reduced the replication of WNV in mosquitoes. Further, when wMelPop mosquitoes were orally fed with WNV, virus infection, transmission, and dissemination rates were very low in Wolbachia-free mosquitoes and were completely inhibited in the presence of Wolbachia. The results suggest that (i) despite the enhancement of viral genomic RNA replication in the Wolbachia-infected cell line the production of secreted virus was significantly inhibited, (ii) the antiviral effect in intrathoracically infected mosquitoes depends on the strain of Wolbachia, and (iii) replication of the virus in orally fed mosquitoes was completely inhibited in wMelPop strain of Wolbachia.

IS-98-ST1 West Nile virus derived from an infectious cDNA clone retains neuroinvasiveness and neurovirulence properties of the original virus

Infectious clones of West Nile virus (WNV) have previously been generated and used to decipher the role of viral proteins in WNV virulence. The majority of molecular clones obtained to date have been derived from North American, Australian, or African isolates. Here, we describe the construction of an infectious cDNA clone of a Mediterranean WNV strain, IS-98-ST1. We characterized the biological properties of the recovered recombinant virus in cell culture and in mice. The growth kinetics of recombinant and parental WNV were similar in Vero cells. Moreover, the phenotype of recombinant and parental WNV was indistinguishable as regards viremia, viral load in the brain, and mortality in susceptible and resistant mice. Finally, the pathobiology of the infectious clone was examined in embryonated chicken eggs. The capacity of different WNV strains to replicate in embryonated chicken eggs closely paralleled their ability to replicate in mice, suggesting that inoculation of embryonated chicken eggs could provide a practical in vivo model for the study of WNV pathogenesis. In conclusion, the IS-98-ST1 infectious clone will allow assessment of the impact of selected mutations and novel genomic changes appearing in emerging European strains pathogenicity and endemic or epidemic potential. This will be invaluable in the context of an increasing number of outbreaks and enhanced severity of infections in the Mediterranean basin and Eastern Europe.

Identification of residues in West Nile virus pre-membrane protein that influence viral particle secretion and virulence

The pre-membrane protein (prM) of West Nile virus (WNV) functions as a chaperone for correct folding of the envelope (E) protein, and prevents premature fusion during virus egress. However, little is known about its role in virulence. To investigate this, we compared the amino acid sequences of prM between a highly virulent North American strain (WNV(NY99)) and a weakly virulent Australian subtype (WNV(KUN)). Five amino acid differences occur in WNV(NY99) compared with WNV(KUN) (I22V, H43Y, L72S, S105A and A156V). When expressed in mammalian cells, recombinant WNV(NY99) prM retained native antigenic structure, and was partially exported to the cell surface. In contrast, WNV(KUN) prM (in the absence of the E protein) failed to express a conserved conformational epitope and was mostly retained at the pre-Golgi stage. Substitutions in residues 22 (Ile to Val) and 72 (Leu to Ser) restored the antigenic structure and cell surface expression of WNV(KUN) prM to the same level as that of WNV(NY99), and enhanced the secretion of WNV(KUN) prME particles when expressed in the presence of E. Introduction of the prM substitutions into a WNV(KUN) infectious clone (FLSDX) enhanced the secretion of infectious particles in Vero cells, and enhanced virulence in mice. These findings highlight the role of prM in viral particle secretion and virulence, and suggest the involvement of the L72S and I22V substitutions in modulating these activities.