Defective interfering RNAs of Japanese encephalitis virus found in mosquito cells and correlation with persistent infection

Characterization of Viral Interference in Aedes albopictus C6/36 Cells Persistently Infected with Dengue Virus 2

Arboviruses are an important group of pathogens that cause diseases of medical and veterinary concern worldwide. The interactions of these viruses with their host cells are complex, and frequently, the coexistence of two different viruses in the same cell results in the inhibition of replication in one of the viruses, which is a phenomenon called viral interference. This phenomenon can be exploited to develop antiviral strategies. Insect cell lines persistently infected with arboviruses are useful models with which to study viral interference. In this work, a model of C6/36-HT cells (from Aedes albopictus mosquitoes) persistently infected with Dengue virus, serotype 2, was used. Viral interference was evaluated via plaque and flow cytometry assays. The presence of heterotypic interference against the other serotypes of the same virus and homologous interference against yellow fever virus was determined; however, this cell line did not display heterologous viral interference against Sindbis virus. The mechanisms responsible for viral interference have not been fully elucidated, but small RNAs could be involved. However, the silencing of Ago3, a key protein in the genome-derived P-element-induced wimpy testis pathway, did not alter the viral interference process, suggesting that viral interference occurs independent of this pathway.

Differential miRNA Expression Profiling Reveals Correlation of miR125b-5p with Persistent Infection of Japanese Encephalitis Virus

MicroRNAs (miRNAs) play versatile roles in multiple biological processes. However, little is known about miRNA’s involvement in flavivirus persistent infection. Here, we used an miRNA array analysis of Japanese encephalitis virus (JEV)-infected cells to search for persistent infection-associated miRNAs in comparison to acute infection. Among all differentially expressed miRNAs, the miR-125b-5p is the most significantly increased one. The high level of miR-125b-5p in persistently JEV-infected cells was confirmed by Northern analysis and real-time quantitative polymerase chain reaction. As soon as the cells established a persistent infection, a significantly high expression of miR-125b-5p was readily observed. Transfecting excess quantities of a miR-125b-5p mimic into acutely infected cells reduced genome replication and virus titers. Host targets of miR125b-5p were analyzed by target prediction algorithms, and six candidates were confirmed by a dual-luciferase reporter assay. These genes were upregulated in the acutely infected cells and sharply declined in the persistently infected cells. The transfection of the miR125b-5p mimic reduced the expression levels of Stat3, Map2k7, and Triap1. Our studies indicated that miR-125b-5p targets both viral and host sequences, suggesting its role in coordinating viral replication and host antiviral responses. This is the first report to characterize the potential roles of miR-125b-5p in persistent JEV infections.

Defective viral genomes as therapeutic interfering particles against flavivirus infection in mammalian and mosquito hosts

Arthropod-borne viruses pose a major threat to global public health. Thus, innovative strategies for their control and prevention are urgently needed. Here, we exploit the natural capacity of viruses to generate defective viral genomes (DVGs) to their detriment. While DVGs have been described for most viruses, identifying which, if any, can be used as therapeutic agents remains a challenge. We present a combined experimental evolution and computational approach to triage DVG sequence space and pinpoint the fittest deletions, using Zika virus as an arbovirus model. This approach identifies fit DVGs that optimally interfere with wild-type virus infection. We show that the most fit DVGs conserve the open reading frame to maintain the translation of the remaining non-structural proteins, a characteristic that is fundamental across the flavivirus genus. Finally, we demonstrate that the high fitness DVG is antiviral in vivo both in the mammalian host and the mosquito vector, reducing transmission in the latter by up to 90%. Our approach establishes the method to interrogate the DVG fitness landscape, and enables the systematic identification of DVGs that show promise as human therapeutics and vector control strategies to mitigate arbovirus transmission and disease.

Mosquito cells persistently infected with dengue virus produce viral particles with host-dependent replication

Dengue viruses (DENV) are important arboviruses that can establish a persistent infection in its mosquito vector Aedes. Mosquitoes have a short lifetime in nature which makes trying to study the processes that take place during persistent viral infections in vivo. Therefore, C6/36 cells have been used to study this type of infection. C6/36 cells persistently infected with DENV 2 produce virions that cannot infect BHK -21 cells. We hypothesized that the following passages in mosquito cells have a deleterious impact on DENV fitness in vertebrate cells. Here, we demonstrated that the viral particles released from persistently infected cells were infectious to mosquito but not to vertebrate cells. This host restriction occurs at the replication level and is associated with several mutations in the DENV genome. In summary, our findings provide new information about viral replication fitness in a host-dependent manner.

The defective component of viral populations

Particles containing degenerate forms of the viral genome which interfere with virus replication and are non-replicative per se are known as defective interfering particles (DIPs). DIPs are likely to be produced upon infection by any virus in vitro and in nature. Until recently, roles of these non-viable particles as members of a multi-component viral system have been overlooked. In this review, we cover the most recent studies that shed light on critical roles of DIPs during the course of infection, including: the modulation of virus replication, innate immune responses, disease outcome and virus persistence, as well as the evolution of the viral population. Together, these reports allow us to conceive a more complete picture of the virion population, and highlight the fact that DIPs are not a negligible subset of this population but instead can greatly influence the fate of infection.

The conserved stem-loop II structure at the 3' untranslated region of Japanese encephalitis virus genome is required for the formation of subgenomic flaviviral RNA

Flaviviruses accumulate abundant subgenomic RNA (sfRNA) in infected cells. It has been reported that sfRNA results from stalling of host 5’-to-3’ exoribonuclease XRN1 at the highly structured RNA of the 3’ untranslated region (UTR). Although XRN1 digestion of a 3’-terminal 800-nt RNA could stall at a position to generate the sfRNA in vitro, we found that knocking out XRN1 had no effect on the accumulation of sfRNA in Japanese encephalitis virus (JEV) infected cells. Mutagenesis studies revealed that the stemloop II (SLII) at the 3’ UTR is required for the accumulation of sfRNA. According to the results of an in vitro RNA-dependent RNA polymerase (RdRp) assay, the (-)10431-10566 RNA fragment, containing the putative promoter on the antigenome for the sfRNA transcription, binds to RdRp protein and exhibits a strong promoter activity. Taken together, our results indicate that the JEV sfRNA could be transcribed initially and then be trimmed by XRN1 or other unidentified exoribonucleases.

New reverse genetics and transfection methods to rescue arboviruses in mosquito cells

Reverse genetics is a critical tool to decrypt the biological properties of arboviruses. However, whilst reverse genetics methods have been usually applied to vertebrate cells, their use in insect cells remains uncommon due to the conjunction of laborious molecular biology techniques and of specific difficulties surrounding the transfection of such cells. To leverage reverse genetics studies in both vertebrate and mosquito cells, we designed an improved DNA transfection protocol for insect cells and then demonstrated that the simple and flexible ISA (Infectious Subgenomic Amplicons) reverse-genetics method can be efficiently applied to both mammalian and mosquito cells to generate in days recombinant infectious positive-stranded RNA viruses belonging to genera Flavivirus (Japanese encephalitis, Yellow fever, West Nile and Zika viruses) and Alphavirus (Chikungunya virus). This method represents an effective option to potentially overcome technological issues related to the study of arboviruses.

Patchy DNA forms of the Zika virus RNA genome are generated following infection in mosquito cell cultures and in mosquitoes

Zika virus (ZIKV) is a mosquito-borne flavivirus and has historically been reported to cause mild symptomatic diseases during human infections. More recently, the explosion of microcephaly among infants born to ZIKV-infected women has made ZIKV a global public health concern. While ZIKV causes acute human diseases, infections of vector mosquitoes are basically non-pathogenic, allowing persistent infections and conferring lifelong ability to transmit the virus. Recent studies have revealed that DNA forms of arboviral RNA genomes play a significant role in viral persistence in mosquitoes. We have initiated experiments to determine whether ZIKV generates viral DNA (vDNA) forms following infection in mosquitoes. Here we show that vDNAs are generated following ZIKV infection both in mosquito cell cultures and in its primary vector Aedes aegypti. vDNA formation is more extensive in RNA interference (RNAi)-deficient Aedes albopictus-derived C6/36 cells compared to RNAi-proficient mosquito cells. In addition, vDNAs are generated via multiple template-switching events.

Participation of 14-3-3ε and 14-3-3ζ proteins in the phagocytosis, component of cellular immune response, in Aedes mosquito cell lines

BACKGROUND

Better knowledge of the innate immune system of insects will improve our understanding of mosquitoes as potential vectors of diverse pathogens. The ubiquitously expressed 14-3-3 protein family is evolutionarily conserved from yeast to mammals, and at least two isoforms of 14-3-3, the ε and ζ, have been identified in insects. These proteins have been shown to participate in both humoral and cellular immune responses in Drosophila. As mosquitoes of the genus Aedes are the primary vectors for arboviruses, causing several diseases such as dengue fever, yellow fever, Zika and chikungunya fevers, cell lines derived from these mosquitoes, Aag-2 from Aedes aegypti and C6/36 HT from Aedes albopictus, are currently used to study the insect immune system. Here, we investigated the role of 14-3-3 proteins (ε and ζ isoform) in phagocytosis, the main cellular immune responses executed by the insects, using Aedes spp. cell lines.

RESULTS

We evaluated the mRNA and protein expression of 14-3-3ε and 14-3-3ζ in C6/36 HT and Aag-2 cells, and demonstrated that both proteins were localised in the cytoplasm. Further, in C6/36 HT cells treated with a 14-3-3 specific inhibitor we observed a notable modification of cell morphology with filopodia-like structure caused through cytoskeleton reorganisation (co-localization of 14-3-3 proteins with F-actin), more importantly the decrease in Salmonella typhimurium, Staphylococcus aureus and E. coli phagocytosis and reduction in phagolysosome formation. Additionally, silencing of 14-3-3ε and 14-3-3ζ expression by mean of specific DsiRNA confirmed the decreased phagocytosis and phagolysosome formation of pHrodo labelled E. coli and S. aureus bacteria by Aag-2 cells.

CONCLUSION

The 14-3-3ε and 14-3-3ζ proteins modulate cytoskeletal remodelling, and are essential for phagocytosis of Gram-positive and Gram-negative bacteria in Aedes spp. cell lines.

Analysis of the miRNA profile in C6/36 cells persistently infected with dengue virus type 2

Dengue virus (DENV) is the most important arbovirus in the world; DENV is transmitted by the Aedes genus of mosquitoes and can establish a life-long persistent infection in mosquitoes. However, the exact mechanism by which persistent infection is established remains unknown. In this study the differential expression of miRNAs was analysed by deep sequencing and RT-qPCR using a previously established C6/36-HT cell line persistently infected with DENV 2 (C6-L) as a model. miR-927, miR-87, miR-210, miR-2a-3p, miR-190 and miR-970 were up-regulated, whereas miR-252, miR-263a-3p, miR-92b, miR-10-5p miR-9a-5p, miR-9a-1, miR-124, miR-286a and miR-286b were down-regulated in C6-L cells compared with C6/36 cells acutely infected with the same virus or mock-infected cells. Deep sequencing results were validated by RT-qPCR for the highly differentially expressed miR-927 and miR-9a-5p, which were up- and down-regulated, respectively, compared with both acutely and mock-infected C6/36 cells. The putative targets of these miRNAs include components of the ubiquitin conjugation pathway, vesicle-mediated transport, autophagy, and the JAK-STAT cascade as well as proteins with endopeptidase activity. Other putative targets include members of the Toll signalling pathway and proteins with kinase, ATPase, protease, scavenger receptor or Lectin C-type activity or that participate in fatty acid biosynthesis or oxidative stress. Our results suggest that several specific miRNAs help regulate the cellular functions that maintain equilibrium between viral replication and the antiviral response during persistent infection of mosquito cells. This study is the first report of a global miRNA profile in a mosquito cell line persistently infected with DENV.

DNA forms of arboviral RNA genomes are generated following infection in mosquito cell cultures

Although infections of vertebrate hosts by arthropod-borne viruses may lead to pathogenic outcomes, infections of vector mosquitoes result in persistent infections, where the virus replicates in the host without causing apparent pathological effects. It is unclear how persistent infections are established and maintained in mosquitoes. Several reports revealed the presence of flavivirus-like DNA sequences in the mosquito genome, and recent studies have shown that DNA forms of RNA viruses restrict virus replication in Drosophila, suggesting that DNA forms may have a role in developing persistent infections. Here, we sought to investigate whether arboviruses generate DNA forms following infection in mosquitoes. Our results with West Nile, Dengue, and La Crosse viruses demonstrate that DNA forms of the viral RNA genome are generated in mosquito cells; however, not the entire viral genome, but patches of viral RNA in DNA forms can be detected 24h post infection.

Transcriptome Analysis Reveals a Signature Profile for Tick-Borne Flavivirus Persistence in HEK 293T Cells

Tick-borne flaviviruses (TBFVs) cause febrile illnesses, which may progress to severe encephalitis and/or death in humans globally. Most people who recover from severe acute disease suffer from debilitating neurological sequelae, which may be due to viral persistence, infection-induced neurological cell damage, host response, or some combination of these. Acute TBFV infection of human embryonic kidney (HEK) 293T cells in vitro results in the death of >95% of infected cells by day 5. However, replacing cell growth medium allows surviving cells to repopulate and become persistently infected for extended periods of time. The mechanisms responsible for initiation and maintenance of viral persistence remain vague. We subjected the HEK 293T cell transcriptome to deep sequencing to identify genes differentially expressed during acute infection and persistent infection. A total of 451 genes showed unique significant differential expression levels in persistently infected cells relative to the acute phase of infection. Ingenuity Pathway Analysis results suggested that the expression of prosurvival oncogenes AKT2 and ERBB2 was upregulated in persistently infected cells, whereas proapoptotic genes, such as Bad and the beta interferon 1 (IFN-β1) gene, were downregulated. Genes encoding antiviral cytokines such as the CCL5, tumor necrosis factor alpha (TNF-α), and CXCL10 genes were upregulated during the acute phase, but the same genes were relatively quiescent in persistently infected cells. Exogenous induction of apoptosis demonstrated that persistently infected cells were resistant to apoptosis in a dose-dependent manner. In summary, the differential transcriptome profiles of acute-phase compared to persistently infected HEK 293T cells demonstrated an evasion of apoptosis, which may be critical for a chronic TBFV infection state. These results provide a basis for further study of the mechanisms of TBFV persistence.

IMPORTANCE

Tick-borne flaviviruses (TBFVs) cause life-threatening encephalitic disease in humans worldwide. Some people who recover from severe disease may suffer prolonged neurological symptoms due to either virus- or host response-induced cell damage or a combination of the two that are linked to viral persistence. By examining the genes that are significantly differentially expressed in acute TBFV infection versus persistent TBFV infection, we may be able to find the molecular basis of viral persistence. Here we used deep sequencing of the host cell transcriptome to discover that the expression levels of prosurvival genes were upregulated in persistently infected cells relative to acute TBFV infections whereas the expression levels of genes that promote programmed cell death were downregulated. In addition, persistently infected cells were also resistant to exogenous chemical induction of cell death, in a dose-dependent manner, compared to uninfected cells. Our results pave the way for further studies aimed at understanding the precise mechanisms of TBFV persistence.

Viral Interference and Persistence in Mosquito-Borne Flaviviruses

Mosquito-borne flaviviruses are important pathogens for humans, and the detection of two or more flaviviruses cocirculating in the same geographic area has often been reported. However, the epidemiological impact remains to be determined. Mosquito-borne flaviviruses are primarily transmitted through Aedes and Culex mosquitoes; these viruses establish a life-long or persistent infection without apparent pathological effects. This establishment requires a balance between virus replication and the antiviral host response. Viral interference is a phenomenon whereby one virus inhibits the replication of other viruses, and this condition is frequently associated with persistent infections. Viral interference and persistent infection are determined by several factors, such as defective interfering particles, competition for cellular factors required for translation/replication, and the host antiviral response. The interaction between two flaviviruses typically results in viral interference, indicating that these viruses share common features during the replicative cycle in the vector. The potential mechanisms involved in these processes are reviewed here.

Constant up-regulation of BiP/GRP78 expression prevents virus-induced apoptosis in BHK-21 cells with Japanese encephalitis virus persistent infection

Background

Persistent infection of the Japanese Encephalitis Virus (JEV) has been reported in clinical cases, experimental animals, and various cell culture systems. We previously reported the establishment of spontaneous JEV persistent infection, assisted by defective interfering particle accumulation and/or attenuated helper viruses, in BHK-21 cells devoid of virus-induced apoptosis, cBS6-2 and cBS6-3. However, cell-specific factors may play important roles in controlling JEV replication and have never been assessed for this specific phenomenon. Recent evidence suggests that viruses have evolved various mechanisms to cope with endoplasmic reticulum stress signaling pathways for their efficient amplification and transmission, including the unfolded protein response (UPR).

Results

To identify the host cell factors that affect JEV persistence, we investigated the expression of essential UPR factors in cBS6-2 and cBS6-3 cells. Of the selected UPR factors tested, the most noticeable deviations from those of the normal BHK-21 cells with JEV acute infection were as follows: the suppression of C/EBP homologous binding protein (CHOP) and the constant up-regulation of immunoglobulin binding protein (BiP) expression in cBS6-2 and cBS6-3 cells. In JEV acute infection on normal BHK-21 cells, silencing CHOP expression through specific siRNA blocked cell death almost completely. Meanwhile, depletion of BiP by specific siRNA unlocked CHOP expression in cBS6-2 and cBS6-3 cells, resulting in massive cell death. Fulminant apoptotic cell death for both cell clones on tunicamycin treatment revealed that the JEV persistently infected cells still contained functional arms for cell fate decisions.

Conclusions

BHK-21 cells with JEV persistent infection strive against virus-induced apoptosis through constant up-regulation of BiP expression, resulting in the complete depletion of CHOP even with apparent virus amplification in the cells. Accordingly, the attenuation of virus replication as well as the modifications to cell metabolism could be additional factors contributing to the development of JEV persistent infection in mammalian cells.

Electronic supplementary material

The online version of this article (doi:10.1186/s12985-015-0269-5) contains supplementary material, which is available to authorized users.

Integrative effect of defective interfering RNA accumulation and helper virus attenuation is responsible for the persistent infection of Japanese encephalitis virus in BHK-21 cells

Persistence of RNA viruses is often, but not always, associated with the production of defective interfering (DI) particles. To investigate possible roles of DI particles and helper viruses in RNA virus persistence, persistent infection with Japanese encephalitis virus (JEV) was established in baby hamster kidney (BHK-21) cells. At the 6th and 7th serial undiluted passages of JEV on BHK-21 cells, viral persistence was established spontaneously with DI RNA generation. Seven cell clones exhibiting persistent infection were obtained from the initial BHK-21 cell batches exhibiting JEV persistence, and maintained for over 400 days. Most cell clones produced infectious particles (10(1) -10(5)  PFU/ml) continuously, expressed viral proteins, and resisted homologous superinfection. Two helper viruses, chvBS6-3 and chvBS7-1, were isolated from two of the seven cell clones, and characterized to investigate their roles in JEV persistence. While chvBS6-3 was restored to its full cytopathicity in the absence of DI RNA, chvBS7-1 exhibited almost no cytopathicity, regardless of DI RNA co-replication. Attenuation of chvBS7-1 did not appear to be due to inadequate adsorption or genome replication, but due to inefficient egress of the assembled progeny virions, suggesting altered helper virus emergence during JEV persistence in BHK-21 cells. These observations suggest that at least two mechanisms are involved in JEV persistence; a DI RNA-dependent mechanism, where DI RNA co-replication nullifies the helper virus's cytopathicity, or a DI RNA-independent mechanism, where the helper virus is self-attenuated. This study provides a useful in vitro tool for understanding the mechanisms underlying RNA virus persistent infections.

Natural infection of Atlantic salmon (Salmo salar L.) with salmonid alphavirus 3 generates numerous viral deletion mutants

Salmon pancreas disease virus (SPDV) also referred to as salmonid alphavirus (SAV) is a virus causing pancreas disease in Atlantic salmon (Salmo salar L.) and rainbow trout (Oncorhynchus mykiss). Although the virus causes an economically important disease, relatively few full-length genome sequences of SAV strains are currently available. Here, we report full-length genome sequences of nine SAV3 strains from sites farming Atlantic salmon geographically spread along the Norwegian coastline. The virus genomes were sequenced directly from infected heart tissue, to avoid culture selection bias. Sequence analysis confirmed a high level of sequence identity within SAV3 strains, with a mean nucleotide diversity of 0.11 %. Sequence divergence was highest in 6K and E2, while lowest in the capsid protein and the non-structural proteins (nsP4 and nsP2). This study reports for the first time that numerous defective viruses containing genome deletions are generated during natural infection with SAV. Deletions occurred in all virus strains and were not distributed randomly throughout the genome but instead tended to aggregate in certain areas. We suggest imprecise homologous recombination as an explanation for generation of defective viruses with genome deletions. The presence of such viruses, provides a possible explanation for the difficulties in isolating SAV in cell culture. Primary virus isolation was successfully achieved for only two of eight strains, despite extensive attempts using three different cell lines. Both SAV isolates were easily propagated further and concomitant viral deletion mutants present in clinically infected heart tissue were maintained following serial passage in CHH-1 cells.

Japanese encephalitis virus non-coding RNA inhibits activation of interferon by blocking nuclear translocation of interferon regulatory factor 3

Noncoding RNA (ncRNA) plays a critical role in modulating a broad range of diseases. All arthropod-borne flaviviruses produce short fragment ncRNA (sfRNA) collinear with highly conserved regions of the 3'-untranslated region (UTR) in the viral genome. We show that the molar ratio of sfRNA to genomic RNA in Japanese encephalitis virus (JEV) persistently infected cells is greater than that in acutely infected cells, indicating an sfRNA role in establishing persistent infection. Transfecting excess quantities of sfRNA into JEV-infected cells reduced interferon-β (IFN-β) promoter activity by 57% and IFN-β mRNA levels by 52%, compared to mock-transfected cells. Transfection of sfRNA into JEV-infected cells also reduced phosphorylation of interferon regulatory factor-3 (IRF-3), the IFN-β upstream regulator, and blocked roughly 30% of IRF-3 nuclear localization. Furthermore, JEV-infected sfRNA transfected cells produced 23% less IFN-β-stimulated apoptosis than mock-transfected groups did. Taken together, these results suggest that sfRNA plays a role against host-cell antiviral responses, prevents cells from undergoing apoptosis, and thus contributes to viral persistence.

Detection and sequencing of defective viral genomes in C6/36 cells persistently infected with dengue virus 2

Dengue virus is the most important arbovirus that affects humans, and it can establish persistent infections, especially in insect-derived cell cultures. Defective viral genomes have been implicated in the establishment and maintenance of persistent infections with several flaviviruses; however, there exists almost no information concerning defective dengue virus genomes. Here, we report the detection of defective dengue 2 virus genomes in persistently infected mosquito C6/36 cells. The defective viral genomes were detected at a low ratio compared with the wild-type genome. Deletions of approximately 147 residues (222-368) were found in the E protein, and these mainly affected domain III (73 %) of the protein; deletions of approximately 153 residues (4-156) and 228 residues (597-825) were found in the methyltransferase and polymerase domains, respectively, of the NS5 protein. The truncated versions of NS5 could be detected by western blot only in the protein extracts derived from persistently infected cells.

Internally deleted WNV genomes isolated from exotic birds in New Mexico: function in cells, mosquitoes, and mice

Most RNA viruses exist in their hosts as a heterogeneous population of related variants. Due to error prone replication, mutants are constantly generated which may differ in individual fitness from the population as a whole. Here we characterize three WNV isolates that contain, along with full-length genomes, mutants with large internal deletions to structural and nonstructural protein-coding regions. The isolates were all obtained from lorikeets that died from WNV at the Rio Grande Zoo in Albuquerque, NM between 2005 and 2007. The deletions are approximately 2kb, in frame, and result in the elimination of the complete envelope, and portions of the prM and NS-1 proteins. In Vero cell culture, these internally deleted WNV genomes function as defective interfering particles, reducing the production of full-length virus when introduced at high multiplicities of infection. In mosquitoes, the shortened WNV genomes reduced infection and dissemination rates, and virus titers overall, and were not detected in legs or salivary secretions at 14 or 21 days post-infection. In mice, inoculation with internally deleted genomes did not attenuate pathogenesis relative to full-length or infectious clone derived virus, and shortened genomes were not detected in mice at the time of death. These observations provide evidence that large deletions may occur within flavivirus populations more frequently than has generally been appreciated and suggest that they impact population phenotype minimally. Additionally, our findings suggest that highly similar mutants may frequently occur in particular vertebrate hosts.

DnaJ homolog Hdj2 facilitates Japanese encephalitis virus replication

Background

Japanese encephalitis virus (JEV) is a member of the mosquito-borne Flaviviridae family of viruses that causes human encephalitis. Upon infection of a new host, replication of viral RNA involves not only the viral RNA-dependent RNA polymerase (RdRp), but also host proteins. Host factors involved in JEV replication are not well characterized.

Results

We identified Hdj2, a heat-shock protein 40 (Hsp40)/DnaJ homolog, from a mouse brain cDNA library interacting with JEV nonstructural protein 5 (NS5) encoding viral RdRp using yeast two-hybrid system. Specific interaction of Hdj2 with NS5 was confirmed by coimmunoprecipitation and colocalization in JEV-infected cells. Overexpression of Hdj2 in JEV-infected cells led to an increase of RNA synthesis, and the virus titer was elevated approximately 4.5- to 10-fold. Knocking down of Hdj2 by siRNA reduced the virus production significantly.

Conclusions

We conclude that Hdj2 directly associates with JEV NS5 and facilitates viral replication. This study is the first to demonstrate Hdj2 involved in JEV replication, providing insight into a potential therapeutic target and cell-based vaccine development of JEV infection.

West Nile virus genetic diversity is maintained during transmission by Culex pipiens quinquefasciatus mosquitoes

Due to error-prone replication, RNA viruses exist within hosts as a heterogeneous population of non-identical, but related viral variants. These populations may undergo bottlenecks during transmission that stochastically reduce variability leading to fitness declines. Such bottlenecks have been documented for several single-host RNA viruses, but their role in the population biology of obligate two-host viruses such as arthropod-borne viruses (arboviruses) in vivo is unclear, but of central importance in understanding arbovirus persistence and emergence. Therefore, we tracked the composition of West Nile virus (WNV; Flaviviridae, Flavivirus) populations during infection of the vector mosquito, Culex pipiens quinquefasciatus to determine whether WNV populations undergo bottlenecks during transmission by this host. Quantitative, qualitative and phylogenetic analyses of WNV sequences in mosquito midguts, hemolymph and saliva failed to document reductions in genetic diversity during mosquito infection. Further, migration analysis of individual viral variants revealed that while there was some evidence of compartmentalization, anatomical barriers do not impose genetic bottlenecks on WNV populations. Together, these data suggest that the complexity of WNV populations are not significantly diminished during the extrinsic incubation period of mosquitoes.

Japanese encephalitis virus co-opts the ER-stress response protein GRP78 for viral infectivity

The serum-free medium from Japanese encephalitis virus (JEV) infected Baby Hamster Kidney-21 (BHK-21) cell cultures was analyzed by liquid chromatography tandem mass spectrometry (LC-MS) to identify host proteins that were secreted upon viral infection. Five proteins were identified, including the molecular chaperones Hsp90, GRP78, and Hsp70. The functional role of GRP78 in the JEV life cycle was then investigated. Co-migration of GRP78 with JEV particles in sucrose density gradients was observed and co-localization of viral E protein with GRP78 was detected by immunofluorescence analysis in vivo. Knockdown of GRP78 expression by siRNA did not effect viral RNA replication, but did impair mature viral production. Mature viruses that do not co-fractionate with GPR78 displayed a significant decrease in viral infectivity. Our results support the hypothesis that JEV co-opts host cell GPR78 for use in viral maturation and in subsequent cellular infections.

Environmental and biological factors influencing Culex pipiens quinquefasciatus (Diptera: Culicidae) vector competence for West Nile Virus

Interactions between environmental and biological factors affect the vector competence of Culex pipiens quinquefasciatus for West Nile virus. Three age cohorts from two Cx. p. quinquefasciatus colonies were fed blood containing a low- or high-virus dose, and each group was held at two different extrinsic incubation temperatures (EIT) for 13 days. The colonies differed in the way that they responded to the effects of the environment on vector competence. The effects of mosquito age on aspects of vector competence were dependent on the EIT and dose, and they changed depending on the colony. Complex interactions must be considered in laboratory studies of vector competence, because the extent of the genetic and environmental variation controlling vector competence in nature is largely unknown. Differences in the environmental (EIT and dose) and biological (mosquito age and colony) effects from previous studies of Cx. p. quinquefasciatus vector competence for St. Louis encephalitis virus are discussed.

Virus susceptibility of Chinese hamster ovary (CHO) cells and detection of viral contaminations by adventitious agent testing

Biopharmaceuticals are of increasing importance in the treatment of a variety of diseases. A remaining concern associated with their production is the potential introduction of adventitious agents into their manufacturing process, which may compromise the pathogen safety of a product and potentially cause stock-out situations for important medical supplies. To ensure the safety of biological therapeutics, regulatory guidance requires adventitious agent testing (AAT) of the bulk harvest. AAT is a deliberately promiscuous assay procedure which has been developed to indicate, ideally, the presence of any viral contaminant. One of the most important cell lines used in the production of biopharmaceuticals is Chinese hamster ovary (CHO) cells and while viral infections of CHO cells have occurred, a systematic screen of their virus susceptibility has never been published. We investigated the susceptibility of CHO cells to infection by 14 different viruses, including members of 12 families and representatives or the very species that were implicated in previously reported production cell infections. Based on our results, four different infection outcomes were distinguished, based on the possible combinations of the two factors (i) the induction, or not, of a cytopathic effect and (ii) the ability, or not, to replicate in CHO cells. Our results demonstrate that the current AAT is effective for the detection of viruses which are able to replicate in CHO cells. Due to the restricted virus susceptibility of CHO cells and the routine AAT of bulk harvests, our results provide re-assurance for the very high safety margins of CHO cell-derived biopharmaceuticals.

Persistent, triple-virus co-infections in mosquito cells

Background

It is known that insects and crustaceans can carry simultaneous, active infections of two or more viruses without showing signs of disease, but it was not clear whether co-infecting viruses occupied the same cells or different cells in common target tissues. Our previous work showed that successive challenge of mosquito cell cultures followed by serial, split-passage resulted in stabilized cultures with 100% of the cells co-infected with Dengue virus (DEN) and an insect parvovirus (densovirus) (DNV). By addition of Japanese encephalitis virus (JE), we tested our hypothesis that stable, persistent, triple-virus co-infections could be obtained by the same process.

Results

Using immunocytochemistry by confocal microscopy, we found that JE super-challenge of cells dually infected with DEN and DNV resulted in stable cultures without signs of cytopathology, and with 99% of the cells producing antigens of the 3 viruses. Location of antigens for all 3 viruses in the triple co-infections was dominant in the cell nuclei. Except for DNV, this differed from the distribution in cells persistently infected with the individual viruses or co-infected with DNV and DEN. The dependence of viral antigen distribution on single infection or co-infection status suggested that host cells underwent an adaptive process to accommodate 2 or more viruses.

Conclusions

Individual mosquito cells can accommodate at least 3 viruses simultaneously in an adaptive manner. The phenomenon provides an opportunity for genetic exchange between diverse viruses and it may have important medical and veterinary implications for arboviruses.

Low genetic diversities of rabies virus populations within different hosts in Brazil

The low rates of nonsynonymous evolution observed in natural rabies virus (RABV) isolates are suggested to have arisen in association with the structural and functional constraints operating on the virus protein and the infection strategies employed by RABV within infected hosts to avoid strong selection by the immune response. In order to investigate the relationship between the genetic characteristics of RABV populations within hosts and the virus evolution, the present study examined the genetic heterogeneities of RABV populations within naturally infected dogs and foxes in Brazil, as well as those of bat RABV populations that were passaged once in suckling mice. Sequence analyses of complete RABV glycoprotein (G) genes showed that RABV populations within infected hosts were genetically highly homogeneous whether they were infected naturally or experimentally (nucleotide diversities of 0-0.95x10(-3)). In addition, amino acid mutations were randomly distributed over the entire region of the G protein, and the nonsynonymous/synonymous rate ratios (d(N)/d(S)) for the G protein gene were less than 1. These findings suggest that the low genetic diversities of RABV populations within hosts reflect the stabilizing selection operating on the virus, the infection strategies of the virus, and eventually, the evolutionary patterns of the virus.

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) interaction with 3' ends of Japanese encephalitis virus RNA and colocalization with the viral NS5 protein

Replication of the Japanese encephalitis virus (JEV) genome depends on host factors for successfully completing their life cycles; to do this, host factors have been recruited and/or relocated to the site of viral replication. Glyceraldehyde-3-phosphate dehydrogenase (GAPDH), a cellular metabolic protein, was found to colocalize with viral RNA-dependent RNA polymerase (NS5) in JEV-infected cells. Subcellular fractionation further indicated that GAPDH remained relatively constant in the cytosol, while increasing at 12 to 24 hours postinfection (hpi) and decreasing at 36 hpi in the nuclear fraction of infected cells. In contrast, the redistribution patterns of GAPDH were not observed in the uninfected cells. Co-immunoprecipitation of GAPDH and JEV NS5 protein revealed no direct protein-protein interaction; instead, GAPDH binds to the 3' termini of plus- and minus-strand RNAs of JEV by electrophoretic mobility shift assays. Accordingly, GAPDH binds to the minus strand more efficiently than to the plus strand of JEV RNAs. This study highlights the findings that infection of JEV changes subcellular localization of GAPDH suggesting that this metabolic enzyme may play a role in JEV replication.