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 Molecular Determinant of West Nile Virus Secretion and Morphology as a Target for Viral Attenuation.

West Nile virus (WNV), a member of the Flavivirus genus and currently one of the most common arboviruses worldwide, is associated with severe neurological disease in humans. Its high potential to reemerge and rapidly disseminate makes it a bona fide global public health problem. The surface membrane glycoprotein (M) has been associated with Flavivirus-induced pathogenesis. Here, we identified a key amino acid residue at position 36 of the M protein whose mutation impacts WNV secretion and promotes viral attenuation. We also identified a compensatory site at position M-43 whose mutation stabilizes M-36 substitution both in vitro and in vivo Moreover, we found that introduction of the two mutations together confers a full attenuation phenotype and protection against wild-type WNV lethal challenge, eliciting potent neutralizing-antibody production in mice. Our study thus establishes the M protein as a new viral target for rational design of attenuated WNV strains.IMPORTANCE West Nile virus (WNV) is a worldwide (re)emerging mosquito-transmitted Flavivirus causing fatal neurological diseases in humans. However, no human vaccine has been yet approved. One of the most effective live-attenuated vaccines was empirically obtained by serial passaging of wild-type yellow fever Flavivirus However, such an approach is not acceptable nowadays, and the development of a rationally designed vaccine is necessary. Generating molecular infectious clones and mutating specific residues known to be involved in Flavivirus virulence constitute a powerful tool to promote viral attenuation. WNV membrane glycoprotein is thought to carry such essential determinants. Here, we identified two residues of this protein whose substitutions are key to the full and stable attenuation of WNV in vivo, most likely through inhibition of secretion and possible alteration of morphology. Applied to other flaviviruses, this approach should help in designing new vaccines against these viruses, which are an increasing threat to global human health.

Synthesis of 4'-Substituted-2'-Deoxy-2'-α-Fluoro Nucleoside Analogs as Potential Antiviral Agents.

Nucleoside analogs are widely used for the treatment of viral diseases (Hepatitis B/C, herpes and human immunodeficiency virus, HIV) and various malignancies. ALS-8176, a prodrug of the 4'-chloromethyl-2'-deoxy-2'-fluoro nucleoside ALS-8112, was evaluated in hospitalized infants for the treatment of respiratory syncytial virus (RSV), but was abandoned for unclear reasons. Based on the structure of ALS-8112, a series of novel 4'-modified-2'-deoxy-2'-fluoro nucleosides were synthesized. Newly prepared compounds were evaluated against RSV, but also against a panel of RNA viruses, including Dengue, West Nile, Chikungunya, and Zika viruses. Unfortunately, none of the compounds showed marked antiviral activity against these viruses.

West Nile Virus fidelity modulates the capacity for host cycling and adaptation.

The fidelity of flaviviruses is thought to be tightly regulated for optimal fitness within and between hosts. West Nile virus (WNV) high-fidelity (HiFi) mutations V793I and G806R within the RNA-dependent RNA polymerase, and low-fidelity (LoFi) mutation T248I within the methyltransferase, were previously shown to attenuate infectivity and replicative fitness in Culex mosquitoes and Culex tarsalis (CXT) cells but not in mammalian cells. We hypothesized that fidelity alterations would modify adaptation and maintenance in a host-specific manner. To test this hypothesis, wild-type (WT), HiFi (V793I/G806R) and LoFi (T248I) variants were sequentially passaged eight times in avian (PDE) or mosquito cells, or alternately between the two. Initial characterization confirmed that fidelity mutants are attenuated in mosquito, but not avian, cells. Deep sequencing revealed mutations unique to both cell lines and fidelity mutants, including ENV G1378A, a mutation associated with avian cell adaptation. To characterize maintenance and adaptation, viral outputs were monitored throughout passaging and viral fitness was assessed. The results indicate that fidelity mutants can at times recover fitness during mosquito cell passage, but remain attenuated relative to WT. Despite similar initial fitness, LoFi mutants were impaired during sequential passage in avian cells. Conversely, HiFi mutants passaged in avian cells showed increased adaptation, suggesting that increased fidelity may be advantageous in avian hosts. Although some adaptation occurred with individual mutants, the output titres of fidelity mutants were on average lower and were often lost during host switching. These data confirm that arbovirus fidelity is likely fine-tuned to maximize survival in disparate hosts.

On the Fly: Interactions Between Birds, Mosquitoes, and Environment That Have Molded West Nile Virus Genomic Structure Over Two Decades.

West Nile virus (WNV) was first identified in North America almost 20 yr ago. In that time, WNV has crossed the continent and established enzootic transmission cycles, resulting in intermittent outbreaks of human disease that have largely been linked with climatic variables and waning avian seroprevalence. During the transcontinental dissemination of WNV, the original genotype has been displaced by two principal extant genotypes which contain an envelope mutation that has been associated with enhanced vector competence by Culex pipiens L. (Diptera: Culicidae) and Culex tarsalis Coquillett vectors. Analyses of retrospective avian host competence data generated using the founding NY99 genotype strain have demonstrated a steady reduction in viremias of house sparrows over time. Reciprocally, the current genotype strains WN02 and SW03 have demonstrated an inverse correlation between house sparrow viremia magnitude and the time since isolation. These data collectively indicate that WNV has evolved for increased avian viremia while house sparrows have evolved resistance to the virus such that the relative host competence has remained constant. Intrahost analyses of WNV evolution demonstrate that selection pressures are avian species-specific and purifying selection is greater in individual birds compared with individual mosquitoes, suggesting that the avian adaptive and/or innate immune response may impose a selection pressure on WNV. Phylogenomic, experimental evolutionary systems, and models that link viral evolution with climate, host, and vector competence studies will be needed to identify the relative effect of different selective and stochastic mechanisms on viral phenotypes and the capacity of newly evolved WNV genotypes for transmission in continuously changing landscapes.

West Nile Virus Capsid Protein Interacts With Biologically Relevant Host Lipid Systems.

West Nile and dengue viruses are closely related flaviviruses, originating mosquito-borne viral infections for which there are no effective and specific treatments. Their capsid proteins sequence and structure are particularly similar, forming highly superimposable α-helical homodimers. Measuring protein-ligand interactions at the single-molecule level yields detailed information of biological and biomedical relevance. In this work, such an approach was successfully applied on the characterization of the West Nile virus capsid protein interaction with host lipid systems, namely intracellular lipid droplets (an essential step for dengue virus replication) and blood plasma lipoproteins. Dynamic light scattering measurements show that West Nile virus capsid protein binds very low-density lipoproteins, but not low-density lipoproteins, and this interaction is dependent of potassium ions. Zeta potential experiments show that the interaction with lipid droplets is also dependent of potassium ions as well as surface proteins. The forces involved on the binding of the capsid protein with lipid droplets and lipoproteins were determined using atomic force microscopy-based force spectroscopy, proving that these interactions are K+-dependent rather than a general dependence of ionic strength. The capsid protein interaction with host lipid systems may be targeted in future therapeutic strategies against different flaviviruses. The biophysical and nanotechnology approaches employed in this study may be applied to characterize the interactions of other important proteins from different viruses, in order to understand their life cycles, as well as to find new strategies to inhibit them.

Seroprevalence, risk factors and spatial distribution of West Nile virus in Jordan.

Background: This is the first countrywide study of the seroprevalence, risk factors and spatial distribution of West Nile virus (WNV) in Jordan. Methods: A cross-sectional sample of 801 participants was administered a comprehensive questionnaire survey and tested for WNV immunoglobulin G antibodies. Results: The point seroprevalence rate for WNV infection was 8.61% (95% confidence interval 6.8 to 10.8). Multivariate regression analysis demonstrated that sex, age, climate, income and drinking water source were significantly associated with seropositivity (p≤0.05). Males had 1.73 greater odds of infection compared with females. Compared with 15 to 29-year-olds, adults 30-49 y old and adults ≥50 y old had 2.0 and 3.1 greater odds of infection, respectively. Individuals living in the Jordan Valley and Badia had 22.2 and 7.2 times greater odds of infection, respectively, compared with individuals living in the highlands. Households with an income of

Innate Immune Response of Primary Human Keratinocytes to West Nile Virus Infection and Its Modulation by Mosquito Saliva.

West Nile Virus (WNV) is a flavivirus involved in many human infections worldwide. This arthropod-borne virus is directly co-inoculated with mosquito saliva through the epidermis and the dermis during blood meal. WNV starts replicating in the skin before migrating to the draining lymph node, leading to widespread viremia and in some cases to neurological symptoms. Skin is a complex organ composed of different cell types that together perform essential functions such as pathogen sensing, barrier maintenance and immunity. Keratinocytes, which represent 90% of the cells of the epidermis, are the organism's first line of defense, initiating innate immune response by recognizing pathogens through their pattern recognition receptors. Although WNV was previously known to replicate in human primary keratinocytes, the induced inflammatory response remains unknown. The aim of this study was first to characterize the inflammatory response of human primary keratinocytes to WNV infection and then, to assess the potential role of co-inoculated mosquito saliva on the keratinocyte immune response and viral replication. A type I and III interferon inflammatory response associated with an increase of IRF7 but not IRF3 mRNA expression, and dependent on infectious dose, was observed during keratinocyte infection with WNV. Expression of several interferon-stimulated gene mRNA was also increased at 24 h post-infection (p.i.); they included CXCL10 and interferon-induced proteins with tetratricopeptide repeats (IFIT)-2 sustained up until 48 h p.i. Moreover, WNV infection of keratinocyte resulted in a significant increase of pro-inflammatory cytokines (TNFα, IL-6) and various chemokines (CXCL1, CXCL2, CXCL8 and CCL20) expression. The addition of Aedes aegypti or Culex quinquefasciatus mosquito saliva, two vectors of WNV infection, to infected keratinocytes led to a decrease of inflammatory response at 24 h p.i. However, only Ae. Aegypti saliva adjunction induced modulation of viral replication. In conclusion, this work describes for the first time the inflammatory response of human primary keratinocytes to WNV infection and its modulation in presence of vector mosquito saliva. The effects of mosquito saliva assessed in this work could be involved in the early steps of WNV replication in skin promoting viral spread through the body.

Insect-specific viruses: from discovery to potential translational applications.

Over the past decade the scientific community has experienced a new age of virus discovery in arthropods in general, and in insects in particular. Next generation sequencing and advanced bioinformatics tools have provided new insights about insect viromes and viral evolution. In this review, we discuss some high-throughput sequencing technologies used to discover viruses in insects and the challenges raised in data interpretations. Additionally, the discovery of these novel viruses that are considered as insect-specific viruses (ISVs) has gained increasing attention in their potential use as biological agents. As example, we show how the ISV Nhumirim virus was used to reduce West Nile virus transmission when co-infecting the mosquito vector. We also discuss new translational opportunities of using ISVs to limit insect vector competence by using them to interfere with pathogen acquisition, to directly target the insect vector or to confer pathogen resistance by the insect vector.

Experimental evaluation of infection, dissemination, and transmission rates for two West Nile virus strains in European Aedes japonicus under a fluctuating temperature regime.

West Nile virus (WNV) is continuously spreading in Eastern and Southern Europe. However, the extent of vector competence of Aedes japonicus (Theobald, 1901) is controversial. In this work, we elucidated the dynamics of virus growth in this invasive mosquito species. Females of Ae. japonicus were reared from eggs collected in the field in Switzerland and fed on bovine blood spiked with two WNV lineage 1 strains (FIN, Italy; NY99, USA). Fully engorged females were incubated for 14 days under a fluctuating temperature regime of 24 ± 7 °C (average 24 °C), 45-90% relative humidity, which is realistic for a Central European mid-summer day. Infection, dissemination, and transmission rates were assessed from individual mosquitoes by analyzing the abdomen, legs and wings, and saliva for the presence of viral RNA. Saliva was also investigated for the presence of infectious virus particles. Overall, 302 females were exposed to WNV strain FIN and 293 to strain NY99. A higher infection rate was observed for NY99 (57.4%) compared to FIN (30.4%) (p = 0.003). There was no statistical evidence that the dissemination rate (viral RNA in legs and wings) was different between females infected with FIN (57.1%) compared to NY99 (35.5%) (p = 0.16). Viral RNA load of FIN compared to NY99 was significantly higher in the hemocoel (p = 0.031) of exposed females but not at other sites (legs and wings, saliva). This is the first study describing the vector competence parameters for two WNV strains in a European population of Ae. japonicus. The high dissemination and transmission rates for WNV under a realistic temperature regime in Ae. japonicus together with recent findings on its opportunistic feeding behavior (mammals and birds) indicate its potential role in WNV transmission in Central Europe where it is highly abundant.

A Small-Molecule Oligosaccharyltransferase Inhibitor with Pan-flaviviral Activity.

The mosquito-borne flaviviruses include important human pathogens such as dengue, Zika, West Nile, and yellow fever viruses, which pose a serious threat for global health. Recent genetic screens identified endoplasmic reticulum (ER)-membrane multiprotein complexes, including the oligosaccharyltransferase (OST) complex, as critical flavivirus host factors. Here, we show that a chemical modulator of the OST complex termed NGI-1 has promising antiviral activity against flavivirus infections. We demonstrate that NGI-1 blocks viral RNA replication and that antiviral activity does not depend on inhibition of the N-glycosylation function of the OST. Viral mutants adapted to replicate in cells deficient of the OST complex showed resistance to NGI-1 treatment, reinforcing the on-target activity of NGI-1. Lastly, we show that NGI-1 also has strong antiviral activity in primary and disease-relevant cell types. This study provides an example for advancing from the identification of genetic determinants of infection to a host-directed antiviral compound with broad activity against flaviviruses.

El virus de nilo occidental y sus vectores: reseña histórica, biología y ecología de aedes albopictus, culex pipiens y cx. quinquefasciatus / Western Nile virus and its vectors: a historic review, biology and ecology of Aedes albopictus, Culex pipiens and Cx. Quinquefasciatus

Resumen Este trabajo obedece a la primera iniciativa para compilar el conocimiento de los mosquitos vectores del Virus del Nilo Occidental en Costa Rica, como parte del primer Congreso para el estudio de la epidemiología de este arbovirus. Se incluye la revisión de las bases de datos de mayor prestigio y mayor cantidad de artículos para completar la información referente a lo que se ha informado acerca de las especies de mosquitos en Costa Rica, acerca de los cuales la información es fragmentaria y de difícil acceso en muchos casos. Se recomienda incentivar la investigación en este importante grupo de insectos, además de la necesidad de estudios de la ecología de estas especies en nuestro país. Se incluye una clave para los principales géneros de mosquitos hallados en Costa Rica en los cuales hay especies informadas como posibles vectores en otras latitudes. Se hace una sinopsis de la necesidad de incluir factores psicosociales en el diseño de las campañas educativas.

Abstract This work is the first initiative to compile the knowledge of mosquito vectors of West Nile Virus in Costa Rica, as part of the first Congress to study the epidemiology of this arbovirus. It includes a review of the most prestigious databases and a greater number of articles to complete the information regarding what has been reported about mosquito species in Costa Rica, about which the information is fragmentary and difficult to access in many cases. It is recommended to encourage research in this important group of insects, in addition to the need for studies of the ecology of these species in our country. A key is included for the main genera of mosquitoes found in Costa Rica in which there are species reported as possible vectors in other latitudes. A synopsis is made of the need to include psychosocial factors in the design of educational campaigns.

Large-Scale Removal of Invasive Honeysuckle Decreases Mosquito and Avian Host Abundance.

Invasive species rank second only to habitat destruction as a threat to native biodiversity. One consequence of biological invasions is altered risk of exposure to infectious diseases in human and animal populations. The distribution and prevalence of mosquito-borne diseases depend on the complex interactions between the vector, the pathogen, and the human or wildlife reservoir host. These interactions are highly susceptible to disturbance by invasive species, including terrestrial plants. We conducted a 2-year field experiment using a Before-After/Control-Impact design to examine how removal of invasive Amur honeysuckle (Lonicera maackii) in a forest fragment embedded within a residential neighborhood affects the abundance of mosquitoes, including two of the most important vectors of West Nile virus, Culex pipiens and Cx. restuans. We also assessed any potential changes in avian communities and local microclimate associated with Amur honeysuckle removal. We found that (1) removal of Amur honeysuckle reduces the abundance of both vector and non-vector mosquito species that commonly feed on human hosts, (2) the abundance and composition of avian hosts is altered by honeysuckle removal, and (3) areas invaded with honeysuckle support local microclimates that are favorable to mosquito survival. Collectively, our investigations demonstrate the role of a highly invasive understory shrub in determining the abundance and distribution of mosquitoes and suggest potential mechanisms underlying this pattern. Our results also give rise to additional questions regarding the general impact of invasive plants on vector-borne diseases and the spatial scale at which removal of invasive plants may be utilized to effect disease control.

Mosquito species involved in the circulation of West Nile and Usutu viruses in Italy.

Usutu (USUV) and West Nile (WNV) are mosquito-borne Flavivirus emerged in Italy in 1996 and 1998, respectively, and reappeared 10 years later. The aim of this work is to review the Italian mosquito species found positive for WNV and USUV between 2008 and 2014. Moreover, the role of mosquitoes in promoting the overwintering of these viruses is discussed, as a result of the mosquito collections performed in Molise region between September 2010 and April 2011. Overall 99,000 mosquitoes were collected: 337 and 457 mosquito pools tested positive by real time reverse transcriptase polymerase chain reaction (real time RT-PCR) for WNV and USUV, respectively. West Nile virus was detected in pools of Culex pipiens s.l. (329), Ochlerotatus caspius (4), Culex modestus (2), and Culex spp. (2). Positive USUV pools were from Cx. pipiens s.l. (435), Aedes albopictus (12), Oc. caspius (5), Culex spp. (2), Anopheles maculipennis s.l. (1), Culiseta annulata (1), and Ochlerotatus detritus (1). In Molise region, 1,694 mosquitoes were collected, and USUV was identi ed in Cx. pipiens s.l., Cs. annulata, and Oc. detritus pools. This paper shows that Cx. pipiens s.l. is the mosquito species most involved in the WNV and USUV circulation in Italy, although other species would also support the spread of both the viruses during Winter.

Antiviral Activity of Nordihydroguaiaretic Acid and Its Derivative Tetra-O-Methyl Nordihydroguaiaretic Acid against West Nile Virus and Zika Virus.

Flaviviruses are positive-strand RNA viruses distributed all over the world that infect millions of people every year and for which no specific antiviral agents have been approved. These viruses include the mosquito-borne West Nile virus (WNV), which is responsible for outbreaks of meningitis and encephalitis. Considering that nordihydroguaiaretic acid (NDGA) has been previously shown to inhibit the multiplication of the related dengue virus and hepatitis C virus, we have evaluated the effect of NDGA, and its methylated derivative tetra-O-methyl nordihydroguaiaretic acid (M4N), on the infection of WNV. Both compounds inhibited the infection of WNV, likely by impairing viral replication. Since flavivirus multiplication is highly dependent on host cell lipid metabolism, the antiviral effect of NDGA has been previously related to its ability to disturb the lipid metabolism, probably by interfering with the sterol regulatory element-binding proteins (SREBP) pathway. Remarkably, we observed that other structurally unrelated inhibitors of the SREBP pathway, such as PF-429242 and fatostatin, also reduced WNV multiplication, supporting that the SREBP pathway may constitute a druggable target suitable for antiviral intervention against flavivirus infection. Moreover, treatment with NDGA, M4N, PF-429242, and fatostatin also inhibited the multiplication of the mosquito-borne flavivirus Zika virus (ZIKV), which has been recently associated with birth defects (microcephaly) and neurological disorders. Our results point to SREBP inhibitors, such as NDGA and M4N, as potential candidates for further antiviral development against medically relevant flaviviruses.

Effects of NS2B-NS3 protease and furin inhibition on West Nile and Dengue virus replication.

West Nile virus (WNV) and Dengue virus (DENV) replication depends on the viral NS2B-NS3 protease and the host enzyme furin, which emerged as potential drug targets. Modification of our previously described WNV protease inhibitors by basic phenylalanine analogs provided compounds with reduced potency against the WNV and DENV protease. In a second series, their decarboxylated P1-trans-(4-guanidino)cyclohexylamide was replaced by an arginyl-amide moiety. Compound 4-(guanidinomethyl)-phenylacetyl-Lys-Lys-Arg-NH2 inhibits the NS2B-NS3 protease of WNV with an inhibition constant of 0.11 µM. Due to the similarity in substrate specificity, we have also tested the potency of our previously described multibasic furin inhibitors. Their further modification provided chimeric inhibitors with additional potency against the WNV and DENV proteases. A strong inhibition of WNV and DENV replication in cell culture was observed for the specific furin inhibitors, which reduced virus titers up to 10,000-fold. These studies reveal that potent inhibitors of furin can block the replication of DENV and WNV.

The natural killer cell response to West Nile virus in young and old individuals with or without a prior history of infection.

West Nile virus (WNV) typically leads to asymptomatic infection but can cause severe neuroinvasive disease or death, particularly in the elderly. Innate NK cells play a critical role in antiviral defenses, yet their role in human WNV infection is poorly defined. Here we demonstrate that NK cells mount a robust, polyfunctional response to WNV characterized by cytolytic activity, cytokine and chemokine secretion. This is associated with downregulation of activating NK cell receptors and upregulation of NK cell activating ligands for NKG2D. The NK cell response did not differ between young and old WNV-naïve subjects, but a history of symptomatic infection is associated with more IFN-γ producing NK cell subsets and a significant decline in a specific NK cell subset. This NK repertoire skewing could either contribute to or follow heightened immune pathogenesis from WNV infection, and suggests that NK cells could play an important role in WNV infection in humans.

Mosquito cell-derived West Nile virus replicon particles mimic arbovirus inoculum and have reduced spread in mice.

Half of the human population is at risk of infection by an arthropod-borne virus. Many of these arboviruses, such as West Nile, dengue, and Zika viruses, infect humans by way of a bite from an infected mosquito. This infectious inoculum is insect cell-derived giving the virus particles distinct qualities not present in secondary infectious virus particles produced by infected vertebrate host cells. The insect cell-derived particles differ in the glycosylation of virus structural proteins and the lipid content of the envelope, as well as their induction of cytokines. Thus, in order to accurately mimic the inoculum delivered by arthropods, arboviruses should be derived from arthropod cells. Previous studies have packaged replicon genome in mammalian cells to produce replicon particles, which undergo only one round of infection, but no studies exist packaging replicon particles in mosquito cells. Here we optimized the packaging of West Nile virus replicon genome in mosquito cells and produced replicon particles at high concentration, allowing us to mimic mosquito cell-derived viral inoculum. These particles were mature with similar genome equivalents-to-infectious units as full-length West Nile virus. We then compared the mosquito cell-derived particles to mammalian cell-derived particles in mice. Both replicon particles infected skin at the inoculation site and the draining lymph node by 3 hours post-inoculation. The mammalian cell-derived replicon particles spread from the site of inoculation to the spleen and contralateral lymph nodes significantly more than the particles derived from mosquito cells. This in vivo difference in spread of West Nile replicons in the inoculum demonstrates the importance of using arthropod cell-derived particles to model early events in arboviral infection and highlights the value of these novel arthropod cell-derived replicon particles for studying the earliest virus-host interactions for arboviruses.

Interleukin-17A Promotes CD8+ T Cell Cytotoxicity To Facilitate West Nile Virus Clearance.

CD8+ T cells are crucial components of immunity and play a vital role in recovery from West Nile virus (WNV) infection. Here, we identify a previously unrecognized function of interleukin-17A (IL-17A) in inducing cytotoxic-mediator gene expression and promoting CD8+ T cell cytotoxicity against WNV infection in mice. We find that IL-17A-deficient (Il17a-/-) mice are more susceptible to WNV infection and develop a higher viral burden than wild-type (WT) mice. Interestingly, the CD8+ T cells isolated from Il17a-/- mice are less cytotoxic and express lower levels of cytotoxic-mediator genes, which can be restored by supplying recombinant IL-17A in vitro and in vivo Importantly, treatment of WNV-infected mice with recombinant IL-17A, as late as day 6 postinfection, significantly reduces the viral burden and increases survival, suggesting a therapeutic potential for IL-17A. In conclusion, we report a novel function of IL-17A in promoting CD8+ T cell cytotoxicity, which may have broad implications in other microbial infections and cancers. IMPORTANCE: Interleukin-17A (IL-17A) and CD8+ T cells regulate diverse immune functions in microbial infections, malignancies, and autoimmune diseases. IL-17A is a proinflammatory cytokine produced by diverse cell types, while CD8+ T cells (known as cytotoxic T cells) are major cells that provide immunity against intracellular pathogens. Previous studies have demonstrated a crucial role of CD8+ T cells in recovery from West Nile virus (WNV) infection. However, the role of IL-17A during WNV infection remains unclear. Here, we demonstrate that IL-17A protects mice from lethal WNV infection by promoting CD8+ T cell-mediated clearance of WNV. In addition, treatment of WNV-infected mice with recombinant IL-17A reduces the viral burden and increases survival of mice, suggesting a potential therapeutic. This novel IL-17A-CD8+ T cell axis may also have broad implications for immunity to other microbial infections and cancers, where CD8+ T cell functions are crucial.

A novel reverse genetics system for production of infectious West Nile virus using homologous recombination in mammalian cells.

Reverse genetics systems facilitate investigation of many aspects of the life cycle and pathogenesis of viruses. However, genetic instability in Escherichia coli has hampered development of a reverse genetics system for West Nile virus (WNV). In this study, we developed a novel reverse genetics system for WNV based on homologous recombination in mammalian cells. Introduction of the DNA fragment coding for the WNV structural protein together with a DNA-based replicon resulted in the release of infectious WNV. The growth rate and plaque size of the recombinant virus were almost identical to those of the parent WNV. Furthermore, chimeric WNV was produced by introducing the DNA fragment coding for the structural protein and replicon plasmid derived from various strains. Here, we report development of a novel system that will facilitate research into WNV infection.