A highly structured, nuclease-resistant, noncoding RNA produced by flaviviruses is required for pathogenicity

Viral noncoding RNAs have been shown to play an important role in virus-host interplay to facilitate virus replication. We report that members of the genus Flavivirus, a large group of medically important encephalitic RNA viruses, produce a unique and highly structured noncoding RNA of 0.3-0.5 kb derived from the 3' untranslated region of the viral genome. Using West Nile virus as a model, we show that this subgenomic RNA is a product of incomplete degradation of viral genomic RNA by cellular ribonucleases. Highly conserved RNA structures located at the beginning of the 3' untranslated region render this RNA resistant to nucleases, and the resulting subgenomic RNA product is essential for virus-induced cytopathicity and pathogenicity. Thus, flaviviruses evolved a unique strategy to generate a noncoding RNA product that allows them to kill the host more efficiently.

West Nile virus encodes a microRNA-like small RNA in the 3' untranslated region which up-regulates GATA4 mRNA and facilitates virus replication in mosquito cells

West Nile virus (WNV) belongs to a group of medically important single-stranded, positive-sense RNA viruses causing deadly disease outbreaks around the world. The 3' untranslated region (3'-UTR) of the flavivirus genome, in particular the terminal 3' stem-loop (3'SL) fulfils multiple functions in virus replication and virus-host interactions. Using the Kunjin strain of WNV (WNV(KUN)), we detected a virally encoded small RNA, named KUN-miR-1, derived from 3'SL. Transcription of WNV(KUN) pre-miRNA (3'SL) in mosquito cells either from plasmid or Semliki Forest virus (SFV) RNA replicon resulted in the production of mature KUN-miR-1. Silencing of Dicer-1 but not Dicer-2 led to a reduction in the miRNA levels. Further, when a synthetic inhibitor of KUN-miR-1 was transfected into mosquito cells, replication of viral RNA was significantly reduced. Using cloning and bioinformatics approaches, we identified the cellular GATA4 mRNA as a target for KUN-miR-1. KUN-miR-1 produced in mosquito cells during virus infection or from plasmid DNA, SFV RNA replicon or mature miRNA duplex increased accumulation of GATA4 mRNA. Depletion of GATA4 mRNA by RNA silencing led to a significant reduction in virus RNA replication while a KUN-miR-1 RNA mimic enhanced replication of a mutant WNV(KUN) virus producing reduced amounts of KUN-miR-1, suggesting that GATA4-induction via KUN-miR-1 plays an important role in virus replication.

Noncoding subgenomic flavivirus RNA: multiple functions in West Nile virus pathogenesis and modulation of host responses

Flaviviruses are a large group of positive strand RNA viruses transmitted by arthropods that include many human pathogens such as West Nile virus (WNV), Japanese encephalitis virus (JEV), yellow fever virus, dengue virus, and tick-borne encephalitis virus. All members in this genus tested so far are shown to produce a unique subgenomic flavivirus RNA (sfRNA) derived from the 3' untranslated region (UTR). sfRNA is a product of incomplete degradation of genomic RNA by the cell 5'–3' exoribonuclease XRN1 which stalls at highly ordered secondary RNA structures at the beginning of the 3'UTR. Generation of sfRNA results in inhibition of XRN1 activity leading to an increase in stability of many cellular mRNAs. Mutant WNV deficient in sfRNA generation was highly attenuated displaying a marked decrease in cytopathicity in cells and pathogenicity in mice. sfRNA has also been shown to inhibit the antiviral activity of IFN-α/β by yet unknown mechanism and of the RNAi pathway by likely serving as a decoy substrate for Dicer. Thus, sfRNA is involved in modulating multiple cellular pathways to facilitate viral pathogenicity; however the overlying mechanism linking all these multiple functions of sfRNA remains to be elucidated.

Identification of pif-2, a third conserved baculovirus gene required for per os infection of insects

Infection of cultured insect cells with Spodoptera exigua multicapsid nucleopolyhedrovirus (SeMNPV) resulted in the generation of mutants with major genomic deletions. Some of the mutants lacked the ability to infect S. exigua larvae per os. The gene(s) responsible for this phenotype in SeMNPV was mapped within a contiguous sequence encoding ORFs 29-35. In this paper we have shown that SeMNPV ORFs 15-35 (including genes encoding cathepsin, chitinase, GP37, PTPT-2, EGT, PKIP-1 and ARIF-1) are not essential for virus replication in cell culture or by in vivo intrahaemocoelic injection. By site-specific deletion mutagenesis of a full-length infectious clone of SeMNPV (bacmid) using ET recombination in E. coli, a series of SeMNPV bacmid mutants with increasing deletions in ORFs 15-35 was generated. Analyses of these mutants indicated that a deletion of SeMNPV ORF35 (Se35) resulted in loss of oral infectivity of polyhedral occlusion bodies. Reinsertion of ORF35 in SeMNPV bacmids lacking Se35 rescued oral infectivity. We propose the name pif-2 for Se35 and its baculovirus homologues (e.g. Autographa californica MNPV ORF22), by analogy to a different gene recently characterized in Spodoptera littoralis NPV, which was designated per os infectivity factor (pif). Similar to the p74 gene, which encodes an essential structural protein of the occlusion-derived virus envelope, pif and pif-2 belong to a group of 30 genes that are conserved among the Baculoviradae.

Effective chikungunya virus-like particle vaccine produced in insect cells

The emerging arthritogenic, mosquito-borne chikungunya virus (CHIKV) causes severe disease in humans and represents a serious public health threat in countries where Aedes spp mosquitoes are present. This study describes for the first time the successful production of CHIKV virus-like particles (VLPs) in insect cells using recombinant baculoviruses. This well-established expression system is rapidly scalable to volumes required for epidemic responses and proved well suited for processing of CHIKV glycoproteins and production of enveloped VLPs. Herein we show that a single immunization with 1 µg of non-adjuvanted CHIKV VLPs induced high titer neutralizing antibody responses and provided complete protection against viraemia and joint inflammation upon challenge with the Réunion Island CHIKV strain in an adult wild-type mouse model of CHIKV disease. CHIKV VLPs produced in insect cells using recombinant baculoviruses thus represents as a new, safe, non-replicating and effective vaccine candidate against CHIKV infections.

Mosquito co-infection with Zika and chikungunya virus allows simultaneous transmission without affecting vector competence of Aedes aegypti

BACKGROUND

Zika virus (ZIKV) and chikungunya virus (CHIKV) are highly pathogenic arthropod-borne viruses that are currently a serious health burden in the Americas, and elsewhere in the world. ZIKV and CHIKV co-circulate in the same geographical regions and are mainly transmitted by Aedes aegypti mosquitoes. There is a growing number of case reports of ZIKV and CHIKV co-infections in humans, but it is uncertain whether co-infection occurs via single or multiple mosquito bites. Here we investigate the potential of Ae. aegypti mosquitoes to transmit both ZIKV and CHIKV in one bite, and we assess the consequences of co-infection on vector competence.

METHODOLOGY/PRINCIPAL FINDINGS

First, growth curves indicated that co-infection with CHIKV negatively affects ZIKV production in mammalian, but not in mosquito cells. Next, Ae. aegypti mosquitoes were infected with ZIKV, CHIKV, or co-infected via an infectious blood meal or intrathoracic injections. Infection and transmission rates, as well as viral titers of positive mosquitoes, were determined at 14 days after blood meal or 7 days after injection. Saliva and bodies of (co-)infected mosquitoes were scored concurrently for the presence of ZIKV and/or CHIKV using a dual-colour immunofluorescence assay. The results show that orally exposed Ae. aegypti mosquitoes are highly competent, with transmission rates of up to 73% for ZIKV, 21% for CHIKV, and 12% of mosquitoes transmitting both viruses in one bite. However, simultaneous oral exposure to both viruses did not change infection and transmission rates compared to exposure to a single virus. Intrathoracic injections indicate that the selected strain of Ae. aegypti has a strong salivary gland barrier for CHIKV, but a less profound barrier for ZIKV.

CONCLUSIONS/SIGNIFICANCE

This study shows that Ae. aegypti can transmit both ZIKV and CHIKV via a single bite. Furthermore, co-infection of ZIKV and CHIKV does not influence the vector competence of Ae. aegypti.

Induction and suppression of tick cell antiviral RNAi responses by tick-borne flaviviruses

Arboviruses are transmitted by distantly related arthropod vectors such as mosquitoes (class Insecta) and ticks (class Arachnida). RNA interference (RNAi) is the major antiviral mechanism in arthropods against arboviruses. Unlike in mosquitoes, tick antiviral RNAi is not understood, although this information is important to compare arbovirus/host interactions in different classes of arbovirus vectos. Using an Ixodes scapularis-derived cell line, key Argonaute proteins involved in RNAi and the response against tick-borne Langat virus (Flaviviridae) replication were identified and phylogenetic relationships characterized. Analysis of small RNAs in infected cells showed the production of virus-derived small interfering RNAs (viRNAs), which are key molecules of the antiviral RNAi response. Importantly, viRNAs were longer (22 nucleotides) than those from other arbovirus vectors and mapped at highest frequency to the termini of the viral genome, as opposed to mosquito-borne flaviviruses. Moreover, tick-borne flaviviruses expressed subgenomic flavivirus RNAs that interfere with tick RNAi. Our results characterize the antiviral RNAi response in tick cells including phylogenetic analysis of genes encoding antiviral proteins, and viral interference with this pathway. This shows important differences in antiviral RNAi between the two major classes of arbovirus vectors, and our data broadens our understanding of arthropod antiviral RNAi.

Comparative Usutu and West Nile virus transmission potential by local Culex pipiens mosquitoes in north-western Europe

Originating from Africa, Usutu virus (USUV) first emerged in Europe in 2001. This mosquito-borne flavivirus caused high mortality rates in its bird reservoirs, which strongly resembled the introduction of West Nile virus (WNV) in 1999 in the United States. Mosquitoes infected with USUV incidentally transmit the virus to other vertebrates, including humans, which can result in neuroinvasive disease. USUV and WNV co-circulate in parts of southern Europe, but the distribution of USUV extends into central and northwestern Europe. In the field, both viruses have been detected in the northern house mosquito Culex pipiens, of which the potential for USUV transmission is unknown. To understand the transmission dynamics and assess the potential spread of USUV, we determined the vector competence of C. pipiens for USUV and compared it with the well characterized WNV. We show for the first time that northwestern European mosquitoes are highly effective vectors for USUV, with infection rates of 11% at 18 °C and 53% at 23 °C, which are comparable with values obtained for WNV. Interestingly, at a high temperature of 28 °C, mosquitoes became more effectively infected with USUV (90%) than with WNV (58%), which could be attributed to barriers in the mosquito midgut. Small RNA deep sequencing of infected mosquitoes showed for both viruses a strong bias for 21-nucleotide small interfering (si)RNAs, which map across the entire viral genome both on the sense and antisense strand. No evidence for viral PIWI-associated RNA (piRNA) was found, suggesting that the siRNA pathway is the major small RNA pathway that targets USUV and WNV infection in C. pipiens mosquitoes.

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Northwestern European mosquitoes are highly effective vectors for USUV.

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Culex pipiens is significantly more competent for USUV than for WNV at 28 °C.

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The siRNA but not the piRNA pathway targets USUV and WNV infections in C. pipiens.

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USUV may be a prelude to WNV transmission in northwestern Europe.

Alphavirus Infection: Host Cell Shut-Off and Inhibition of Antiviral Responses

Alphaviruses cause debilitating disease in humans and animals and are transmitted by blood-feeding arthropods, typically mosquitoes. With a traditional focus on two models, Sindbis virus and Semliki Forest virus, alphavirus research has significantly intensified in the last decade partly due to the re-emergence and dramatic expansion of chikungunya virus in Asia, Europe, and the Americas. As a consequence, alphavirus–host interactions are now understood in much more molecular detail, and important novel mechanisms have been elucidated. It has become clear that alphaviruses not only cause a general host shut-off in infected vertebrate cells, but also specifically suppress different host antiviral pathways using their viral nonstructural proteins, nsP2 and nsP3. Here we review the current state of the art of alphavirus host cell shut-off of viral transcription and translation, and describe recent insights in viral subversion of interferon induction and signaling, the unfolded protein response, and stress granule assembly.

Vector competence of European mosquitoes for West Nile virus

West Nile virus (WNV) is an arthropod-borne flavivirus of high medical and veterinary importance. The main vectors for WNV are mosquito species of the Culex genus that transmit WNV among birds, and occasionally to humans and horses, which are ‘dead-end’ hosts. Recently, several studies have been published that aimed to identify the mosquito species that serve as vectors for WNV in Europe. These studies provide insight in factors that can influence vector competence of European mosquito species for WNV. Here, we review the current knowledge on vector competence of European mosquitoes for WNV, and the molecular knowledge on physical barriers, anti-viral pathways and microbes that influence vector competence based on studies with other flaviviruses. By comparing the 12 available WNV vector competence studies with European mosquitoes we evaluate the effect of factors such as temperature, mosquito origin and mosquito biotype on vector competence. In addition, we propose a standardised methodology to allow for comparative studies across Europe. Finally, we identify knowledge gaps regarding vector competence that, once addressed, will provide important insights into WNV transmission and ultimately contribute to effective strategies to control WNV.

Vector competence of northern European Culex pipiens biotypes and hybrids for West Nile virus is differentially affected by temperature

BACKGROUND

Outbreaks of West Nile virus (WNV) have not occurred in northern Europe despite nearby circulation of WNV in the southern part of the continent. The main vector for WNV, the mosquito Culex (Cx.) pipiens, consists of two behaviorally distinct biotypes, pipiens and molestus, which can form hybrids. Although temperature has been shown to influence vector competence of Cx. pipiens for WNV and biotypes are differentially susceptible towards infection, the interaction between the two has not been elucidated.

METHODS

We determined vector competence of the Cx. pipiens biotypes and hybrids, after 14 days of incubation at 18, 23 and 28 °C. Mosquitoes were orally infected by providing an infectious blood meal or by injecting WNV directly in the thorax. Infection and transmission rates were determined by testing the bodies and saliva for WNV presence. In addition, titers of mosquitoes with WNV-positive bodies and saliva samples were determined.

RESULTS

Orally infected biotype pipiens and hybrids showed significantly increased transmission rates with higher temperatures, up to 32 and 14 %, respectively. In contrast, the molestus biotype had an overall transmission rate of 10 %, which did not increase with temperature. All mosquitoes that were infected via WNV injections had (close to) 100 % infection and transmission rates, suggesting an important role of the mosquito midgut barrier. We found no effect of increasing temperature on viral titers.

CONCLUSIONS

Temperature differentially affected vector competence of the Cx. pipiens biotypes. This shows the importance of accounting for biotype-by-temperature interactions, which influence the outcomes of vector competence studies. Vector competence studies with Cx. pipiens mosquitoes differentiated to the biotype level are essential for proper WNV risk assessments.

Pivotal role of the non-hr origin of DNA replication in the genesis of defective interfering baculoviruses

The generation of deletion mutants, including defective interfering viruses, upon serial passage of Spodoptera exigua multicapsid nucleopolyhedrovirus (SeMNPV) in insect cell culture has been studied. Sequences containing the non-homologous region origin of DNA replication (non-hr ori) became hypermolar in intracellular viral DNA within 10 passages in Se301 insect cells, concurrent with a dramatic drop in budded virus and polyhedron production. These predominant non-hr ori-containing sequences accumulated in larger concatenated forms and were generated de novo as demonstrated by their appearance and accumulation upon infection with a genetically homogeneous bacterial clone of SeMNPV (bacmid). Sequences were identified at the junctions of the non-hr ori units within the concatemers, which may be potentially involved in recombination events. Deletion of the SeMNPV non-hr ori using RecE/RecT-mediated homologous ET recombination in Escherichia coli resulted in a recombinant bacmid with strongly enhanced stability of virus and polyhedron production upon serial passage in insect cells. This suggests that the accumulation of non-hr oris upon passage is due to the replication advantage of these sequences. The non-hr ori deletion mutant SeMNPV bacmid can be exploited as a stable eukaryotic heterologous protein expression vector in insect cells.

Mosquito and Drosophila entomobirnaviruses suppress dsRNA- and siRNA-induced RNAi

RNA interference (RNAi) is a crucial antiviral defense mechanism in insects, including the major mosquito species that transmit important human viruses. To counteract the potent antiviral RNAi pathway, insect viruses encode RNAi suppressors. However, whether mosquito-specific viruses suppress RNAi remains unclear. We therefore set out to study RNAi suppression by Culex Y virus (CYV), a mosquito-specific virus of the Birnaviridae family that was recently isolated from Culex pipiens mosquitoes. We found that the Culex RNAi machinery processes CYV double-stranded RNA (dsRNA) into viral small interfering RNAs (vsiRNAs). Furthermore, we show that RNAi is suppressed in CYV-infected cells and that the viral VP3 protein is responsible for RNAi antagonism. We demonstrate that VP3 can functionally replace B2, the well-characterized RNAi suppressor of Flock House virus. VP3 was found to bind long dsRNA as well as siRNAs and interfered with Dicer-2-mediated cleavage of long dsRNA into siRNAs. Slicing of target RNAs by pre-assembled RNA-induced silencing complexes was not affected by VP3. Finally, we show that the RNAi-suppressive activity of VP3 is conserved in Drosophila X virus, a birnavirus that persistently infects Drosophila cell cultures. Together, our data indicate that mosquito-specific viruses may encode RNAi antagonists to suppress antiviral RNAi.

Functional processing and secretion of Chikungunya virus E1 and E2 glycoproteins in insect cells

Background

Chikungunya virus (CHIKV) is a mosquito-borne, arthrogenic Alphavirus that causes large epidemics in Africa, South-East Asia and India. Recently, CHIKV has been transmitted to humans in Southern Europe by invading and now established Asian tiger mosquitoes. To study the processing of envelope proteins E1 and E2 and to develop a CHIKV subunit vaccine, C-terminally his-tagged E1 and E2 envelope glycoproteins were produced at high levels in insect cells with baculovirus vectors using their native signal peptides located in CHIKV 6K and E3, respectively.

Results

Expression in the presence of either tunicamycin or furin inhibitor showed that a substantial portion of recombinant intracellular E1 and precursor E3E2 was glycosylated, but that a smaller fraction of E3E2 was processed by furin into mature E3 and E2. Deletion of the C-terminal transmembrane domains of E1 and E2 enabled secretion of furin-cleaved, fully processed E1 and E2 subunits, which could then be efficiently purified from cell culture fluid via metal affinity chromatography. Confocal laser scanning microscopy on living baculovirus-infected Sf21 cells revealed that full-length E1 and E2 translocated to the plasma membrane, suggesting similar posttranslational processing of E1 and E2, as in a natural CHIKV infection. Baculovirus-directed expression of E1 displayed fusogenic activity as concluded from syncytia formation. CHIKV-E2 was able to induce neutralizing antibodies in rabbits.

Conclusions

Chikungunya virus glycoproteins could be functionally expressed at high levels in insect cells and are properly glycosylated and cleaved by furin. The ability of purified, secreted CHIKV-E2 to induce neutralizing antibodies in rabbits underscores the potential use of E2 in a subunit vaccine to prevent CHIKV infections.

Spontaneous excision of BAC vector sequences from bacmid-derived baculovirus expression vectors upon passage in insect cells

Repeated baculovirus infections in cultured insect cells lead to the generation of defective interfering viruses (DIs), which accumulate at the expense of the intact helper virus and compromise heterologous protein expression. In particular, Autographa californica multicapsid nucleopolyhedovirus (AcMNPV) DIs are enriched in an origin of viral DNA replication (ori) not associated with the homologous regions (hrs). This non-hr ori is located within the coding sequence of the non-essential p94 gene. We investigated the effect of a deletion of the AcMNPV non-hr ori on the heterologous protein expression levels following serial passage in Sf21 insect cells. Using homologous ET recombination in E. coli, deletions within the p94 gene were made in a bacterial artificial chromosome (BAC) containing the entire AcMNPV genome (bacmid). All bacmids were equipped with an expression cassette containing the green fluorescent protein gene and a gene encoding the classical swine fever virus E2 glycoprotein (CSFV-E2). For the parental (intact) bacmid only, a strong accumulation of DIs with reiterated non-hr oris was observed. This was not observed for the mutants, indicating that removal of the non-hr ori enhanced the genetic stability of the viral genome upon passaging. However, for all passaged viruses it was found that the entire BAC vector including the expression cassette was spontaneously deleted from the viral genome, leading to a rapid decrease in GFP and CSFV-E2 production. The rationale for the (intrinsic) genetic instability of the BAC vector in insect cells and the implications with respect to large-scale production of proteins with bacmid-derived baculoviruses are discussed.

Kunjin virus replicons: an RNA-based, non-cytopathic viral vector system for protein production, vaccine and gene therapy applications

The application of viral vectors for gene expression and delivery is rapidly evolving, with several entering clinical trials. However, a number of issues, including safety, gene expression levels, cell selectivity and antivector immunity, are driving the search for new vector systems. A number of replicon-based vectors derived from positive-strand RNA viruses have recently been developed, and this paper reviews the current knowledge on the first flavivirus replicon system, which is based on the Australian flavivirus Kunjin (KUN). Like most replicon systems, KUN replicons can be delivered as DNA, RNA or virus-like particles, they replicate their RNA in the cytoplasm and direct prolonged high-level gene expression. However, unlike most alphavirus replicon systems, KUN replicons are non-cytopathic, with transfected cells able to divide, allowing the establishment of cell lines stably expressing replicon RNA and heterologous genes. As vaccine vectors KUN replicons can induce potent, long-lived, protective, immunogen-specific CD8+ T cell immunity, a feature potentially related to extended production of antigen and double-stranded RNA-induced 'danger signals'. The identification of KUN replicon mutants that induce increased levels of IFN-alpha/beta has also spawned investigation of KUN replicons for use in cancer gene therapy. The unique characteristics of KUN replicons may thus make them suitable for specific protein production, vaccine and gene therapy applications.

Autographa californica baculoviruses with large genomic deletions are rapidly generated in infected insect cells

Defective interfering baculoviruses (DIs) lack considerable portions of the genome, interfere with the replication of helper virus, and cause the so-called "passage-effect" during serial passaging in insect cells and in bioreactor configurations. We investigated their origin by (nested) PCR and demonstrated that DIs lacking approximately 43% (d43) of their DNA are present in low-passage Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV)-E2 virus stocks and in polyhedra, but not in the authentic AcMNPV isolate obtained prior to passage in cell culture. To investigate whether DIs are rapidly generated de novo in Sf21 insect cells, a genetically homogeneous AcMNPV bacmid was serially passaged, resulting in the generation of d43 DIs within two passages. AT-rich sequences of up to 66 nucleotides of partly unknown origin were found at the deletion junctions in the d43 DI genomes. These data suggest that the rapid generation of DIs is an intrinsic property of baculovirus infection in insect cell culture and involves several recombination steps.

Functional RNA during Zika virus infection

Zika virus (ZIKV; family Flaviviridae; genus Flavivirus) is a pathogenic mosquito-borne RNA virus that currently threatens human health in the Americas, large parts of Asia and occasionally elsewhere in the world. ZIKV infection is often asymptomatic but can cause severe symptoms including congenital microcephaly and Guillain-Barré syndrome. The positive single-stranded RNA genome of the mosquito-borne ZIKV requires effective replication in two evolutionary distinct hosts - mosquitoes and primates. In addition to some of the viral proteins, the ZIKV genomic RNA and functional RNAs produced thereof aid in the establishment of productive infection and the evasion of host cell antiviral responses. ZIKV has evolved to contain a nucleotide composition and RNA modifications, such as methylation and the formation of G-quadruplexes that allow effective replication in both hosts. Furthermore, a number of host factors interact with the viral genome to modulate RNA replication. Importantly, the ZIKV genome produces non-coding subgenomic flavivirus RNA (sfRNA) due to stalling of host 5'- 3' ribonucleases on viral RNA structures in the 3' untranslated region (UTR). This sfRNA (sfRNA) exerts important proviral functions such as antagonizing the innate interferon response and RNA interference. Here, we discuss the ZIKV genomic RNA and functional RNAs thereof to assess their significance during ZIKV infection. Understanding the details of the ZIKV infection cycle will aid in the development of effective antiviral strategies and safe vaccines.

Chikungunya virus non-structural protein 2-mediated host shut-off disables the unfolded protein response

The unfolded protein response (UPR) is a cellular defence mechanism against high concentrations of misfolded protein in the endoplasmic reticulum (ER). In the presence of misfolded proteins, ER-transmembrane proteins PERK and IRE1α become activated. PERK phosphorylates eIF2α leading to a general inhibition of cellular translation, whilst the expression of transcription factor ATF4 is upregulated. Active IRE1α splices out an intron from XBP1 mRNA, to produce a potent transcription factor. Activation of the UPR increases the production of several proteins involved in protein folding, degradation and apoptosis. Here, we demonstrated that transient expression of chikungunya virus (CHIKV) (family Togaviridae, genus Alphavirus) envelope glycoproteins induced the UPR and that CHIKV infection resulted in the phosphorylation of eIF2α and partial splicing of XBP1 mRNA. However, infection with CHIKV did not increase the expression of ATF4 and known UPR target genes (GRP78/BiP, GRP94 and CHOP). Moreover, nuclear XBP1 was not observed during CHIKV infection. Even upon stimulation with tunicamycin, the UPR was efficiently inhibited in CHIKV-infected cells. Individual expression of CHIKV non-structural proteins (nsPs) revealed that nsP2 alone was sufficient to inhibit the UPR. Mutations that rendered nsP2 unable to cause host-cell shut-off prevented nsP2-mediated inhibition of the UPR. This indicates that initial UPR induction takes place in the ER but that expression of functional UPR transcription factors and target genes is efficiently inhibited by CHIKV nsP2.

Mosquito Small RNA Responses to West Nile and Insect-Specific Virus Infections in Aedes and Culex Mosquito Cells

Small RNA mediated responses are essential for antiviral defence in mosquitoes, however, they appear to differ per virus-vector combination. To further investigate the diversity of small RNA responses against viruses in mosquitoes, we applied a small RNA deep sequencing approach on five mosquito cell lines: Culex tarsalis CT cells, Aedes albopictus U4.4 and C6/36 cells, Ae. aegypti Aag2 cells (cleared from cell fusing agent virus and Culex Y virus (CYV) by repetitive dsRNA transfections) and Ae. pseudoscutellaris AP-61 cells. De novo assembly of small RNAs revealed the presence of Phasi Charoen-like virus (PCLV), Calbertado virus, Flock House virus and a novel narnavirus in CT cells, CYV in U4.4 cells, and PCLV in Aag2 cells, whereas no insect-specific viruses (ISVs) were detected in C6/36 and AP-61 cells. Next, we investigated the small RNA responses to the identified ISVs and to acute infection with the arthropod-borne West Nile virus (WNV). We demonstrate that AP-61 and C6/36 cells do not produce siRNAs to WNV infection, suggesting that AP-61, like C6/36, are Dicer-2 deficient. CT cells produced a strong siRNA response to the persistent ISVs and acute WNV infection. Interestingly, CT cells also produced viral PIWI-interacting (pi)RNAs to PCLV, but not to WNV or any of the other ISVs. In contrast, in U4.4 and Aag2 cells, WNV siRNAs, and pi-like RNAs without typical ping-pong piRNA signature were observed, while this signature was present in PCLV piRNAs in Aag2 cells. Together, our results demonstrate that mosquito small RNA responses are strongly dependent on both the mosquito cell type and/or the mosquito species and family of the infecting virus.

Cloning of biologically active genomes from a Helicoverpa armigera single-nucleocapsid nucleopolyhedrovirus isolate by using a bacterial artificial chromosome

Purification of genotypes from baculovirus isolates provides understanding of the diversity of baculoviruses and may lead to the development of better pesticides. Here, we report the cloning of different genotypes from an isolate of Helicoverpa armigera single-nucleocapsid nucleopolyhedrovirus (HaSNPV) by using a bacterial artificial chromosome (BAC). A transfer vector (pHZB10) was constructed which contained an Escherichia coli mini-F replicon cassette within the upstream and downstream arms of HaSNPV polyhedrin gene. Hz2e5 cells were co-transfected with wild-type HaSNPV DNA and pHZB10 to generate recombinant viruses by homologous recombination. The DNA of budded viruses (BVs) was used to transform E. coli. One of the bacmid colonies, HaBacHZ8, has restriction enzyme digestion profiles similar to an in vivo cloned strain HaSNPV-G4, the genome of which has been completely sequenced. For testing the oral infectivity, the polyhedrin gene of HaSNPV was reintroduced into HaBacHZ8 to generate the recombinant bacmid HaBacDF6. The results of one-step growth curves, electron microscopic examination, protein expression analysis and bioassays indicated that HaBacDF6 replicated as well as HaSNPV-G4 in vitro and in vivo. The biologically functional HaSNPV bacmids obtained in this research will facilitate future studies on the function genomics and genetic modification of HaSNPV.

Subgenomic flavivirus RNA binds the mosquito DEAD/H-box helicase ME31B and determines Zika virus transmission by Aedes aegypti

Zika virus (ZIKV) is an arthropod-borne flavivirus predominantly transmitted by Aedes aegypti mosquitoes and poses a global human health threat. All flaviviruses, including those that exclusively replicate in mosquitoes, produce a highly abundant, noncoding subgenomic flavivirus RNA (sfRNA) in infected cells, which implies an important function of sfRNA during mosquito infection. Currently, the role of sfRNA in flavivirus transmission by mosquitoes is not well understood. Here, we demonstrate that an sfRNA-deficient ZIKV (ZIKVΔSF1) replicates similar to wild-type ZIKV in mosquito cell culture but is severely attenuated in transmission by Ae. aegypti after an infectious blood meal, with 5% saliva-positive mosquitoes for ZIKVΔSF1 vs. 31% for ZIKV. Furthermore, viral titers in the mosquito saliva were lower for ZIKVΔSF1 as compared to ZIKV. Comparison of mosquito infection via infectious blood meals and intrathoracic injections showed that sfRNA is important for ZIKV to overcome the mosquito midgut barrier and to promote virus accumulation in the saliva. Next-generation sequencing of infected mosquitoes showed that viral small-interfering RNAs were elevated upon ZIKVΔSF1 as compared to ZIKV infection. RNA-affinity purification followed by mass spectrometry analysis uncovered that sfRNA specifically interacts with a specific set of Ae. aegypti proteins that are normally associated with RNA turnover and protein translation. The DEAD/H-box helicase ME31B showed the highest affinity for sfRNA and displayed antiviral activity against ZIKV in Ae. aegypti cells. Based on these results, we present a mechanistic model in which sfRNA sequesters ME31B to promote flavivirus replication and virion production to facilitate transmission by mosquitoes.

Arbovirus vaccines; opportunities for the baculovirus-insect cell expression system

The baculovirus-insect cell expression system is a well-established technology for the production of heterologous viral (glyco)proteins in cultured cells, applicable for basic scientific research as well as for the development and production of vaccines and diagnostics. Arboviruses form an emerging group of medically important viral pathogens that are transmitted to humans and animals via arthropod vectors, mostly mosquitoes, ticks or midges. Few arboviral vaccines are currently available, but there is a growing need for safe and effective vaccines against some highly pathogenic arboviruses such as Chikungunya, dengue, West Nile, Rift Valley fever and Bluetongue viruses. This comprehensive review discusses the biology and current state of the art in vaccine development for arboviruses belonging to the families Togaviridae, Flaviviridae, Bunyaviridae and Reoviridae and the potential of the baculovirus-insect cell expression system for vaccine antigen production The members of three of these four arbovirus families have enveloped virions and display immunodominant glycoproteins with a complex structure at their surface. Baculovirus expression of viral antigens often leads to correctly folded and processed (glyco)proteins able to induce protective immunity in animal models and humans. As arboviruses occupy a unique position in the virosphere in that they also actively replicate in arthropod cells, the baculovirus-insect cell expression system is well suited to produce arboviral proteins with correct folding and post-translational processing. The opportunities for recombinant baculoviruses to aid in the development of safe and effective subunit and virus-like particle vaccines against arboviral diseases are discussed.