Mutations in the NS3 gene and 3'-NCR of Japanese encephalitis virus isolated from an unconventional ecosystem and implications for natural attenuation of the virus

Tick-borne encephalitis epidemic in Hungary 1951-2021: The story and lessons learned

The authors analysed epidemiological data of the Hungarian tick-borne encephalitis epidemic from the past seven decades. A total of 911 meningitis serosa cases were described from 1930-1950 s by local hospital physicians, indicating that the virus had been present in the country decades before its official identification in 1952. The virus spread freely in the 1950s-1960s, occupying almost all habitats where ticks occurred in large numbers. The increasing number of cases drove authorities to classify this illness as a notifiable disease in 1977 and to organize the first measures to stop the epidemic. Statistical analysis revealed that the large-scale vaccination launched from the 1990s was responsible for the sharp decrease in the number of human cases from 1997. A significant negative correlation was found between the number of vaccine doses sold and human cases 6 years later. The TBEV endemic area covers 16.57% of the territory and 16.65% of the population of the country. In the last 10 years, 186,000 vaccine doses/year in average were enough to keep the incidence of human TBEV infections between 0.45 and 0.06/100,000 persons. A 20-year-long study found evidence for easing clinical signs in TBEV-infected hospitalized patients. Statistics found a sharp decrease in the number of samples sent for TBEV diagnosis after 1989. Male dominance of patients was characteristic of the epidemics since the 1940s, but now analysis of detailed data from the 1981-2021 period (60.5%-87.5%) proved the statistical significance of this dominance. Obviously, the voluntary vaccination programme was the tool which broke the spread of the epidemic. Widespread public awareness of the disease and the tick vector, probable evolutionary spread of less pathogenic virus strains supplemented with the vaccination campaign led to a negligible level of human TBE cases in Hungary in the last years.

Pupal cannibalism by worker honey bees contributes to the spread of deformed wing virus

Transmission routes impact pathogen virulence and genetics, therefore comprehensive knowledge of these routes and their contribution to pathogen circulation is essential for understanding host–pathogen interactions and designing control strategies. Deformed wing virus (DWV), a principal viral pathogen of honey bees associated with increased honey bee mortality and colony losses, became highly virulent with the spread of its vector, the ectoparasitic mite Varroa destructor. Reproduction of Varroa mites occurs in capped brood cells and mite-infested pupae from these cells usually have high levels of DWV. The removal of mite-infested pupae by worker bees, Varroa Sensitive Hygiene (VSH), leads to cannibalization of pupae with high DWV loads, thereby offering an alternative route for virus transmission. We used genetically tagged DWV to investigate virus transmission to and between worker bees following pupal cannibalisation under experimental conditions. We demonstrated that cannibalization of DWV-infected pupae resulted in high levels of this virus in worker bees and that the acquired virus was then transmitted between bees via trophallaxis, allowing circulation of Varroa-vectored DWV variants without the mites. Despite the known benefits of hygienic behaviour, it is possible that higher levels of VSH activity may result in increased transmission of DWV via cannibalism and trophallaxis.

NS2B/NS3 mutations enhance the infectivity of genotype I Japanese encephalitis virus in amplifying hosts

Genotype I (GI) virus has replaced genotype III (GIII) virus as the dominant Japanese encephalitis virus (JEV) in the epidemic area of Asia. The mechanism underlying the genotype replacement remains unclear. Therefore, we focused our current study on investigating the roles of mosquito vector and amplifying host(s) in JEV genotype replacement by comparing the replication ability of GI and GIII viruses. GI and GIII viruses had similar infection rates and replicated to similar viral titers after blood meal feedings in Culex tritaeniorhynchus. However, GI virus yielded a higher viral titer in amplifying host-derived cells, especially at an elevated temperature, and produced an earlier and higher viremia in experimentally inoculated pigs, ducklings, and young chickens. Subsequently we identified the amplification advantage of viral genetic determinants from GI viruses by utilizing chimeric and recombinant JEVs (rJEVs). Compared to the recombinant GIII virus (rGIII virus), we observed that both the recombinant GI virus and the chimeric rJEVs encoding GI virus-derived NS1-3 genes supported higher replication ability in amplifying hosts. The replication advantage of the chimeric rJEVs was lost after introduction of a single substitution from a GIII viral mutation (NS2B-L99V, NS3-S78A, or NS3-D177E). In addition, the gain-of-function assay further elucidated that rGIII virus encoding GI virus NS2B-V99L/NS3-A78S/E177E substitutions re-gained the enhanced replication ability. Thus, we conclude that the replication advantage of GI virus in pigs and poultry is the result of three critical NS2B/NS3 substitutions. This may lead to more efficient transmission of GI virus than GIII virus in the amplifying host-mosquito cycle.

Flaviviral vertebrate amplifying host(s), invertebrate vector(s), genetics, and environmental factors shape the viral geographical distribution and epidemic disease pattern. Newly emerging dengue virus genotypes, West Nile virus clades, or Zika virus strains exhibited an enhancement in mosquito vector competence. However, hosts and viral determinants responsible for the occurrence of JEV genotype replacement remains unclear. Here, we demonstrated that emerging GI viruses with enhanced transmission potential in amplifying hosts such as pigs and avian species was encoded by three critical GI-specific mutations in NS2B/NS3 proteins. This discovery provides insight into the viral genetic mechanism underlying the GI virus advantage and adaptation in the pig/avian species-mosquito cycle. Our results also emphasize the importance of monitoring viral evolution in amplifying vertebrate hosts to clarify the role of avian species in local transmission of GI virus in JE endemic and epidemic countries.

An Ecological Survey of Mosquitoes and the Distribution of Japanese Encephalitis Virus in Ishikawa Prefecture, Japan, between 2010 and 2014

Japanese encephalitis virus (JEV) is a flavivirus, responsible for over 30,000 annual cases of encephalitis worldwide, with a mortality rate of approximately 30%. Therefore, it is important to examine the distribution of mosquitos carrying JEV in the fields, even though recently, the number of Japanese encephalitis cases has been approximately 5 per year in Japan. We report the seasonal dynamics of mosquitoes between 2010 and 2014 in Ishikawa Prefecture, Japan. We collected 39,308 female adult mosquitoes, 98.2% of which were classified as Culex tritaeniorhynchus Giles. We identified JEV genomic RNA belonging to genotype 1 from the homogenate of Cx. tritaeniorhynchus, collected during our study using reverse transcription-PCR and nucleotide sequencing techniques. Our results indicate that mosquito vectors for JEV are distributed not only in areas in Ishikawa, but also throughout Japan, and the results suggest that we must be careful regarding JEV outbreaks in Japan in the future.

Cell type-dependent RNA recombination frequency in the Japanese encephalitis virus

Japanese encephalitis virus (JEV) is one of approximately 70 flaviviruses, frequently causing symptoms involving the central nervous system. Mutations of its genomic RNA frequently occur during viral replication, which is believed to be a force contributing to viral evolution. Nevertheless, accumulating evidences show that some JEV strains may have actually arisen from RNA recombination between genetically different populations of the virus. We have demonstrated that RNA recombination in JEV occurs unequally in different cell types. In the present study, viral RNA fragments transfected into as well as viral RNAs synthesized in mosquito cells were shown not to be stable, especially in the early phase of infection possibly via cleavage by exoribonuclease. Such cleaved small RNA fragments may be further degraded through an RNA interference pathway triggered by viral double-stranded RNA during replication in mosquito cells, resulting in a lower frequency of RNA recombination in mosquito cells compared to that which occurs in mammalian cells. In fact, adjustment of viral RNA to an appropriately lower level in mosquito cells prevents overgrowth of the virus and is beneficial for cells to survive the infection. Our findings may also account for the slower evolution of arboviruses as reported previously.

The nature of replication of tick-borne encephalitis virus strains isolated from residents of the Russian Far East with inapparent and clinical forms of infection

We describe the biological properties and molecular characteristics of complete genomes of 33 tick-borne encephalitis virus (TBEV) strains that induced different forms of infection, from inapparent to severe focal ones resulting in fatal outcome. Hemagglutinating activity of Oshima-like strains was higher at pH 5.8, while activity of Sofjin- and Senhzang-like strains were higher at pH 6.2 and 6.8, respectively. We determined susceptibility of porcine kidney (PK) cell cultures to these TBEV strains by cytopathic effect (CPE), plaque formation, and size of plaques. The clinical TBEV strains had higher virus titers both in tissue culture infectious dose 50(TCID50) and in plaque-forming unit (PFU) titers and larger plaques than the inapparent strains. A comparison of virus multiplication kinetics by PFU in culture fluid with kinetics of ELISA antigen and hemagglutinin accumulation suggested a different mechanism of interaction between these virus strains and PK cells at the initial stage of cell infection.

Development of a live attenuated vaccine candidate against duck Tembusu viral disease

Duck Tembusu virus (DTMUV) is a newly emerging pathogenic flavivirus that is causing massive economic loss in the Chinese duck industry. To obtain a live vaccine candidate against the disease, the DTMUV isolate FX2010 was passaged serially in chicken embryo fibroblasts (CEFs). Characterization of FX2010-180P revealed that it was unable to replicate efficiently in chicken embryonated eggs, nor intranasally infect mice or shelducks at high doses of 5.5log10 tissue culture infectious doses (TCID50). FX2010-180P did not induce clinical symptoms, or pathological lesions in ducks at a dose of 5.5log10TCID50. The attenuation of FX2010-180P was due to 19 amino acid changes and 15 synonymous mutations. Importantly, FX2010-180P elicited good immune responses in ducks inoculated at low doses (3.5log10TCID50) and provided complete protection against challenge with a virulent strain. These results indicate that FX2010-180P is a promising candidate live vaccine for prevention of duck Tembusu viral disease.

West Nile virus drug discovery

The outbreak of West Nile virus (WNV) in 1999 in the USA, and its continued spread throughout the Americas, parts of Europe, the Middle East and Africa, underscored the need for WNV antiviral development. Here, we review the current status of WNV drug discovery. A number of approaches have been used to search for inhibitors of WNV, including viral infection-based screening, enzyme-based screening, structure-based virtual screening, structure-based rationale design, and antibody-based therapy. These efforts have yielded inhibitors of viral or cellular factors that are critical for viral replication. For small molecule inhibitors, no promising preclinical candidate has been developed; most of the inhibitors could not even be advanced to the stage of hit-to-lead optimization due to their poor drug-like properties. However, several inhibitors developed for related members of the family Flaviviridae, such as dengue virus and hepatitis C virus, exhibited cross-inhibition of WNV, suggesting the possibility to re-purpose these antivirals for WNV treatment. Most promisingly, therapeutic antibodies have shown excellent efficacy in mouse model; one of such antibodies has been advanced into clinical trial. The knowledge accumulated during the past fifteen years has provided better rationale for the ongoing WNV and other flavivirus antiviral development.

Comparison of genomic and amino acid sequences of eight Japanese encephalitis virus isolates from bats

We compared nucleotide and deduced amino acid sequences of eight Japanese encephalitis virus (JEV) isolates derived from bats in China. We also compared the bat JEV isolates with other JEV isolates available from GenBank to determine their genetic similarity. We found a high genetic homogeneity among the bat JEVs isolated in different geographical areas from various bat species at different time periods. All eight bat JEV isolates belonged to genotype III. The mean evolutionary rate of bat JEV isolates was lower than those of isolates of other origin, but this difference was not statistically significant. Based on these results, we presume that the bat JEV isolates might be evolutionarily conserved. The eight bat JEV isolates were phylogenetically similar to mosquito BN19 and human Liyujie isolates of JEV. These results indicate that bats might be involved in natural cycle of JEV.

First detection of the Africa/Caribbean/Latin American subtype of Culex flavivirus in Asian country, Taiwan

Culex flavivirus (CxFV), a member of the genus flavivirus, is a novel insect-specific flaviviruses that can be divided into two subtypes, the cytopathic Asia/U.S. and the noncytopathic Africa/Caribbean/Latin American subtypes. The CxFV circulates in several Asian countries, and here we conducted the first study investigating CxFV in Taiwan. A total of 14,016 mosquitoes were collected between 2010 and 2012 and 3.4% (6/179) of the pools were CxFV-positive. The phylogenetic analyses indicate that the Taiwan isolates are closely related to the Africa/Caribbean/Latin American subtype, but form an independent cluster. In the cytology experiments, the CxFV Taiwan isolate infected only mosquito cells and caused cell-cell fusion that might be associated with a unique glycine residue at position 117 within the envelope protein, which is shared with the cytopathic effect-causing Asia/US subtype. This study marks the first time the Africa/Caribbean/Latin American subtype of CxFV has been identified in an Asian country and grouped into a novel cluster.

Japanese encephalitis virus genotype replacement, Taiwan, 2009-2010

Genotype I of Japanese encephalitis virus first appeared in Taiwan in 2008. Phylogenetic analysis of 37 viruses from pig farms in 2009-2010 classified these viruses into 2 unique subclusters of genotype I viruses and suggested multiple introductions and swift replacement of genotype III by genotype I virus in Taiwan.

Autoimmunity-related demyelination in infection by Japanese encephalitis virus

Japanese encephalitis (JE) virus is the most common cause of epidemic viral encephalitis in the world. The virus mainly infects neuronal cells and causes an inflammatory response after invasion of the parenchyma of the brain. The death of neurons is frequently observed, in which demyelinated axons are commonly seen. The mechanism that accounts for the occurrence of demyelination is ambiguous thus far. With a mouse model, the present study showed that myelin-specific antibodies appeared in sera, particularly in those mice with evident symptoms. Meanwhile, specific T cells proliferating in response to stimulation by myelin basic protein (MBP) was also shown in these mice. Taken together, our results suggest that autoimmunity may play an important role in the destruction of components, e.g., MBP, of axon-surrounding myelin, resulting in demyelination in the mouse brain after infection with the JE virus.

The blood-brain barrier in the cerebrum is the initial site for the Japanese encephalitis virus entering the central nervous system

Japanese encephalitis (JE) virus is a member of the encephalitic flaviviruses and frequently causes neurological sequelae in a proportion of patients who survive the acute phase of the infection. In the present study, we molecularly identified viral infection in the brain of mice with rigidity of hindlimbs and/or abnormal gait, in which JE virus particles appeared within membrane-bound vacuoles of neurons throughout the central nervous system. Deformation of tight junctions (TJs) shown as dissociation of endothelial cells in capillaries, implying that the integrity of the blood-brain barrier (BBB) has been compromised by JE virus infection. BBB permeability evidently increased in the cerebrum, but not in the cerebellum, of JE virus-infected mice intravenously injected with the tracer of Evans blue dye. This suggests that the permeability of the BBB differentially changed in response to viral infection, leading to the entry of JE virions and/or putatively infected leukocytes from the periphery to the cerebrum as the initial site of infection in the central nervous system (CNS). Theoretically, the virus spread to the cerebellum soon after the cerebrum became infected.

Upregulation of a novel eukaryotic translation initiation factor 5A (eIF5A) in dengue 2 virus-infected mosquito cells

BACKGROUND

Dengue virus, a mosquito-borne flavivirus, is the etiological agent of dengue fever, dengue hemorrhagic fever, and dengue shock syndrome. It generally induces apoptosis in mammalian cells, but frequently results in persistent infection in mosquito cells. That mechanism remains to be explored. In turn, a genomic survey through subtractive hybridization (PCR-select cDNA subtraction) was conducted in order to find gene(s) that may play a role in interactions between the virus and its host cells.

RESULTS

Through this technique, we identified a novel eukaryotic translation initiation factor 5A (eIF5A) which is upregulated in Aedes albopictus-derived C6/36 cells infected by the type 2 dengue (Den-2) virus. The full-length of the identified eIF5A gene consisted of 1498 bp of nucleotides with a 41.39% G+C content, and it possessed a higher similarity and shorter evolutionary distance with insects than with other organisms. Upregulation of eIF5A in response to Den-2 virus infection was validated at both the RNA and protein levels. This phenomenon was also observed by confocal microscopy. In addition, cell death obviously occurred when eIF5A activity was inhibited in C6/36 cells even when they were infected by the virus. However, viral multiplication was not obviously affected in infected C6/36 cells when eIF5A activity was reduced.

CONCLUSIONS

Taken together, we postulated that eIF5A plays a role in preventing mosquito cells from death in response to Den-2 viral infection, thus facilitating continued viral growth and potential persistent infection in mosquito cells. It would be worthwhile to further investigate how its downstream factors or cofactors contribute to this phenomenon of dengue infection.

Experimental evidence that RNA recombination occurs in the Japanese encephalitis virus

Due to the lack of a proofreading function and error-repairing ability of genomic RNA, accumulated mutations are known to be a force driving viral evolution in the genus Flavivirus, including the Japanese encephalitis (JE) virus. Based on sequencing data, RNA recombination was recently postulated to be another factor associated with genomic variations in these viruses. We herein provide experimental evidence to demonstrate the occurrence of RNA recombination in the JE virus using two local pure clones (T1P1-S1 and CJN-S1) respectively derived from the local strains, T1P1 and CJN. Based on results from a restriction fragment length polymorphism (RFLP) assay on the C/preM junction comprising a fragment of 868 nucleotides (nt 10-877), the recombinant progeny virus was primarily formed in BHK-21 cells that had been co-infected with the two clones used in this study. Nine of 20 recombinant forms of the JE virus had a crossover in the nt 123-323 region. Sequencing data derived from these recombinants revealed that no nucleotide deletion or insertion occurred in this region favoring crossovers, indicating that precisely, not aberrantly, homologous recombination was involved. With site-directed mutagenesis, three stem-loop secondary structures were destabilized and re-stabilized in sequence, leading to changes in the frequency of recombination. This suggests that the conformation, not the free energy, of the secondary structure is important in modulating RNA recombination of the virus. It was concluded that because RNA recombination generates genetic diversity in the JE virus, this must be considered particularly in studies of viral evolution, epidemiology, and possible vaccine safety.

Japanese encephalitis virus NS2B-NS3 protease induces caspase 3 activation and mitochondria-mediated apoptosis in human medulloblastoma cells

Japanese encephalitis virus (JEV) causes severe neurological diseases with a high fatality rate. Clinical, neurophysiological and radiological features of Japanese encephalitis JE patients showed that JEV infection resulted in widespread involvement of the nervous system, including thalamus, basal ganglia, brainstem, cerebellum, cerebral cortex and spinal cord. In this study, we characterized the apoptotic effect of JEV infection and its viral proteins on the TE671 human medulloblastoma cells. JEV replicated in TE671 cells, inducing caspase 3-mediated apoptosis in MOI- and time-dependent manners. Of viral proteins, co-expression of JEV NS3 protease with NS2B cofactor significantly induced higher degrees of apoptosis and triggered higher caspase 3 activities than single expression of E, NS1, NS2B or NS3 protease in human medulloblastoma cells. Moreover, JEV NS2B-NS3 protease induced reduction of mitochondrial membrane potential and release of mitochondrial cytochrome C, which were responsible for the mitochondria-mediated apoptosis. In addition, the production of reactive oxygen species production and activation of ASK1-p38 MAPK signaling pathway might be associated with JEV NS2B-NS3 protease-induced mitochondria-mediated apoptosis. The results demonstrated that the JEV infection and the co-expression of JEV NS3 protease with NS2B cofactor induced caspase 3 activation and mitochondria-mediated apoptosis in human medulloblastoma cells, being valuable insight for cellular and molecular levels of JEV pathogenesis.

Identification of genetic determinants of a tick-borne flavivirus associated with host-specific adaptation and pathogenicity

Tick-borne flaviviruses are maintained in nature in an enzootic cycle involving a tick vector and a vertebrate host. Thus, the virus replicates in two disparate hosts, each providing selective pressures that can influence virus replication and pathogenicity. To identify viral determinants associated with replication in the individual hosts, plaque purified Langat virus (TP21pp) was adapted to growth in mouse or tick cell lines to generate two virus variants, MNBp20 and ISEp20, respectively. Virus adaptation to mouse cells resulted in four amino acid changes in MNBp20 relative to TP21pp, occurring in E, NS4A and NS4B. A comparison between TP21pp and ISEp20 revealed three amino acid modifications in M, NS3 and NS4A of ISEp20. ISEp20, but not MNBp20, was attenuated following intraperitoneal inoculation of mice. Following isolation from mice brains, additional mutations reproducibly emerged in E and NS3 of ISEp20 that were possibly compensatory for the initial adaptation to tick cells. Thus, our data implicate a role for E, M, NS3, NS4A and NS4B in host adaptation and pathogenicity of tick-borne flaviviruses.

Bovine lactoferrin inhibits Japanese encephalitis virus by binding to heparan sulfate and receptor for low density lipoprotein

Lactoferrin is a natural anti-microbial protein which affects Japanese encephalitis virus (JEV) activity. Binding of lactoferrin to cell surface expressed heparan sulfate (HS), one possible receptor for JEV, has been postulated to be the possible mechanism of anti-JEV antiviral activity. In this study, we evaluate the effects of bovine lactoferrin (bLF) against JEV infection in vitro, using both wild-type (WT) and laboratory-adapted strains. bLF inhibited the infectivity of all the JEV strains tested. In particular the infectivity of the HS-adapted JEV strains was strongly reduced, whereas the non HS-adapted JEV strains were inhibited to lesser extent. Using both HS-adapted CJN-S1 and non HS-adapted CJN-L1 viruses, the results showed that bLF inhibited the early events essential to initiate JEV infection, which includes blocking virus attachment to cellular membranes and reducing viral penetration. This anti-JEV activity was the highest using HS-adapted CJN-S1 strain on HS-expressed CHO-K1 cells. Also, binding of bLF to heparin-sepharose blocked JEV binding; and soluble HS attenuated the anti-JEV activity of bLF. The results support the premise that the interaction of bLF with cell surface expressed glycosaminoglycans, in particular the highly sulfated HS, plays an essential role in the antiviral activity of bLF. However, bLF was functional in inhibiting CJN-S1 entry into HS-deficient CHO-pgsA745 cells, and bLF-treated CHO-K1 and -pgsA745 cells also prevented non HS-adapted CJN-L1 virus entry, indicating that a non-HS pathway may be involved in bLF inhibition of JEV entry. The low-density lipoprotein receptor (LDLR), possibly involved in the entry of several RNA viruses, also binds to bLF. We found that both rLDLR and anti-LDLR antibodies reduced the effectiveness of bLF inhibition of JEV infection. This finding provided evidence to suggest that cell surface-expressed LDLR may play a role in JEV infection, especially for non HS-adapted strains.

Protective immunity of E. coli-synthesized NS1 protein of Japanese encephalitis virus

Immunogenicity and protective efficacy of recombinant Japanese encephalitis virus (JEV) NS1 proteins generated using DNA vaccines and recombinant viruses have been demonstrated to induce protection in mice against a challenge of JEV at a lethal dose. The West Nile virus NS1 region expressed in E. coli is recognized by these protective monoclonal antibodies and, in this study, we compare immunogenicity and protective immunity of the E. coli-synthesized NS1 protein with another protective immunogen, the envelope domain III (ED3). Pre-challenge, detectable titers of JEV-specific neutralizing antibody were detected in the immunized mice with E. coli-synthesized ED3 protein (PRNT50 = 1:28) and the attenuated JEV strain T1P1 (PRNT50 = 1:53), but neutralizing antibodies were undetectable in the immunized mice with E. coli-synthesized NS1 protein (PRNT50 < 1:10). However, the survival rate of the NS1-immunized mice against the JEV challenge was 87.5% (7/8), showing significantly higher levels of protection than the ED3-immunized mice, 62.5% (5/8) (P = 0.041). In addition, E. coli-synthesized NS1 protein induced a significant increase of anti-NS1 IgG1 antibodies, resulting in an ELISA titer of 100,1000 in the immunized sera before lethal JEV challenge. Surviving mice challenged with the virulent JEV strain Beijing-1 showed a ten-fold or greater rise in IgG1 and IgG2b titers of anti-NS1 antibodies, implying that the Th2 cell activation might be predominantly responsible for antibody responses and mice protection.

High antibody prevalence in an unconventional ecosystem is related to circulation of a low-virulent strain of Japanese encephalitis virus

Liu-Chiu islet, a relatively isolated ecosystem that is free of rice cultivation, has long considered free of Japanese encephalitis (JE). However, a new strain (T1P1) of JE virus was isolated from the mosquito, Armigeres subalbatus, and a rather broad distribution of neutralizing antibody has been reported on the islet, suggesting that the circulating virus could be an attenuated strain. In an assessment on 219 blood samples obtained from residents of Liu-Chiu islet, the positive rate of JEV-specific IgM antibodies decreased with age while that of neutralizing antibodies increased with age. Both antibodies were mainly responsive to the T1P1 strain since higher positive rates and titers of specific neutralizing antibodies are shown in this investigation. Importantly, the T1P1 strain is herein characterized as being broader in neutralizing virus strains, stable in genetic traits, and productive in Vero cells. Taken together, the JE virus strain endemically circulating on Liu-Chiu islet may have served as a natural form of a live-attenuated vaccine. As a result, it possibly can be utilized as a new and effective vaccine candidate in the future.

Parallel infection of Japanese encephalitis virus and Wolbachia within cells of mosquito salivary glands

The endosymbiont Wolbachia usually causes cytoplasmic incompatibility in dipteran hosts, including mosquitoes. However, some important arbovirus-transmitting mosquitoes such as Aedes aegypti (L.) are not heritably infected by Wolbachia. In Wolbachia-harboring mosquito Armigeres subalbatus Coquillett, colocalization of Wolbachia and inoculated Japanese encephalitis virus (family Flaviviridae, genus Flavivirus, JEV) in salivary gland (SG) cells was shown by electron microscopy. The infection rate of JEV in SGs, detected with either immunofluorescent antibody test or reverse transcription-polymerase chain reaction, did not show significant differences between Wolbachia-infected and -free colonies. It is suggested that Wolbachia did not mediate resistance of SG cells to superinfection by JEV, although both microorgamisms coexist in the same niche, i.e., the same SG cell. Therefore, a SG escape barrier may not be elevated due to Wolbachia infection, which presumably has no deleterious effects on vector competence in Wolbachia-harboring mosquitoes.

Japanese encephalitis virus NS2B-NS3 protease binding to phage-displayed human brain proteins with the domain of trypsin inhibitor and basic region leucine zipper

Flavivirus NS2B-NS3 proteases are associated with neurovirulence, becoming an important target for insight into the virus-induced pathogenesis. In this study, a phage-displayed human brain cDNA library was used to detect possible interaction between brain proteins and the Japanese encephalitis virus (JEV) NS2B-NS3 protease. After six rounds of biopanning, eight high-affinity NS2B-NS3 protease-interacting phages were identified. Identified NS2B-NS3 protease-interacting brain proteins contained several repeats of the consensus motifs E(R/K)(R/K)K and G(R/K)(R/K) with the dibasic residues, being similar to the conserved cleavage sites among flavivirus proteases. In addition, three identified brain proteins (phage-24, 34, and 44) were predicted as the domain of trypsin inhibitor and basic region leucine zipper (bZIP) using the SMART genome search. Immunoprecipitation and cleavage of two brain fusion proteins (phage-24 and phage-46) by the NS2B-NS3 protease confirmed the specific interaction between identified brain proteins and the JEV NS2B-NS3 protease. Fluorogenic peptide substrate assays revealed dose-manner inhibitory effects of these two brain fusion proteins on the trans-cleavage activity of NS2B-NS3 protease. Moreover, in vitro signaling pathway assay revealed that the JEV NS2B-NS3 protease significantly inhibited the signaling pathway of activator protein 1(AP1), a member of the bZIP family. Our results provide an insight into the protein interaction network of the JEV NS2B-NS3 protease in human brain.

A tick-borne Langat virus mutant that is temperature sensitive and host range restricted in neuroblastoma cells and lacks neuroinvasiveness for immunodeficient mice

Langat virus (LGT), the naturally attenuated member of the tick-borne encephalitis virus (TBEV) complex, was tested extensively in clinical trials as a live TBEV vaccine and was found to induce a protective, durable immune response; however, it retained a low residual neuroinvasiveness in mice and humans. In order to ablate or reduce this property, LGT mutants that produced a small plaque size or temperature-sensitive (ts) phenotype in Vero cells were generated using 5-fluorouracil. One of these ts mutants, clone E5-104, exhibited a more than 10(3)-fold reduction in replication at the permissive temperature in both mouse and human neuroblastoma cells and lacked detectable neuroinvasiveness for highly sensitive immunodeficient mice. The E5-104 mutant possessed five amino acid substitutions in the structural protein E and one change in each of the nonstructural proteins NS3 and NS5. Using reverse genetics, we demonstrated that a Lys(46)-->Glu substitution in NS3 as well as a single Lys(315)-->Glu change in E significantly impaired the growth of LGT in neuroblastoma cells and reduced its peripheral neurovirulence for SCID mice. This study and our previous experience with chimeric flaviviruses indicated that a decrease in viral replication in neuroblastoma cells might serve as a predictor of in vivo attenuation of the neurotropic flaviviruses. The combination of seven mutations identified in the nonneuroinvasive E5-104 mutant provided a useful foundation for further development of a live attenuated TBEV vaccine. An evaluation of the complete sequence of virus recovered from brain of SCID mice inoculated with LGT mutants identified sites in the LGT genome that promoted neurovirulence/neuroinvasiveness.

Fitness of Japanese encephalitis virus to Neuro-2a cells is determined by interactions of the viral envelope protein with highly sulfated glycosaminoglycans on the cell surface

Genetically different subpopulations were identified and purified from Japanese Encephalitis virus (JEV). Those with small plaques (SPs; <2 mm in diameter), derived from strains of T1P1, CJN, and CC27, were more competent than those with large plaques (LPs; >5 mm in diameter) when passaged in Neuro-2a cells. Differences in amino acids between SPs and LPs from each strain were shown in the viral envelope (E) protein. The amino acid at E-306 was Glu in LP but was substituted by Lys in SP in the T1P1 strain. A similar substitution occurred at E-138 in the CJN strain. However, the amino acid was Asp in LP but was substituted by Asn in SP at E-389 in the CC27 strain. All SPs were shown to have a higher affinity to the cellular membrane when compared to LPs, and this resulted in more-efficient infection of Neuro-2a cells, suggesting that the differential fitness of JEV variants to Neuro-2a cells appeared in the early phase of infection. In addition, glycosaminoglycans (GAGs) on the surface of many mammalian cells have been demonstrated to be critical for infection by JEV, especially SP variants. The present results suggest that T1P1-SP1 viruses infected Neuro-2a cells more efficiently in spite of the sparse distribution of cell surface GAGs. We conclude that highly sulfated forms of GAGs expressed by Neuro-2a cells play an important role in selecting JEV variants with specific mutations in the E glycoprotein.

5'- and 3'-noncoding regions in flavivirus RNA

The flavivirus genome is a capped, positive-sense RNA approximately 10.5 kb in length. It contains a single long open reading frame (ORF), flanked by a 5´ noncoding regions (NCR), which is about 100 nucleotides in length, and a 3´ NCR ranging in size from about 400 to 800 nucleotides in length. The conserved structural and nucleotide sequence elements of these NCRs and their function in RNA replication and translation are the subjects of this chapter. The 5´ and 3´ NCRs play a role in the initiation of negative-strand synthesis on virus RNA released from entering virions, switching from negative-strand synthesis to synthesis of progeny plus strand RNA at late times after infection, and possibly in the initiation of translation and in the packaging of virus plus strand RNA into particles. The presence of conserved and nonconserved complementary nucleotide sequences near the 5´ and 3´ termini of flavivirus genomes suggests that ‘‘panhandle’’ or circular RNA structures are formed transiently by hydrogen bonding at some stage during RNA replication.

E/NS1 modifications of dengue 2 virus after serial passages in mammalian and/or mosquito cells

OBJECTIVE

Dengue viruses are routinely maintained in nature by transmission cycles involving the passage of virus between humans and AEDES mosquitoes. The number of dengue virus lineages has been increasing over time. The aim of this study was to identify the genetic diversity of dengue 2 virus serially transferred in mammalian and/or mosquito cells.

METHODS

The E/NS1 gene of dengue 2 virus variants derived from serial passages in Vero or C6/36 cells, or alternately in both cell systems, was amplified and sequenced in order to observe gene modification after serial passages.

RESULTS

Three nucleotides (two in E and one in NS1) or two amino acids (one each in E and NS1) changed in the virus that was continuously cultured in Vero cells for 20 passages, whereas four nucleotides (two each in E and NS1) or three amino acids (one in E and two in NS1) changed in the virus cultured for 30 passages. The genome of dengue 2 virus remained stable even when the virus was serially transferred in C6/36 cells for 30 generations. However, there was one amino acid substitution (E46 I-->V) resulting from a single nucleotide change in the E region of dengue 2 virus alternately transferred in C6/36 and Vero cells for either 20 or 30 passages. In addition, dengue 2 virus obtained from serially cultured Vero cells usually replicated better when it reinfected Vero cells, reflecting its high adaptation fitness to the host cell.

CONCLUSIONS

It is concluded that genetic changes of dengue 2 virus are constrained in Vero (mammalian) cells, resulting in a variety of genome-related quasispecies populations. Some populations of the virus are subsequently selected by and genetically (at least in the E/NS1 portion of the viral genome) maintained in C6/36 (mosquito) cells during replicative competition.

Recent advances in Japanese encephalitis

Japanese encephalitis (JE), the most important cause of epidemic encephalitis worldwide, is confined to Asia, but its geographical area is spreading. West Nile virus, and other closely related flaviviruses, cause similar disease elsewhere. Recent cryoelectron microscopic studies have characterized the flavivirus envelope protein as a new class of viral fusion protein (class II), and examined its arrangement on the virion surface. Changes in the envelope protein's hinge region, or its putative receptor-binding domain, are associated with changes in neurovirulence in animal models of JE. Clinically, JE causes a wide range of presentations, including a poliolike flaccid paralysis. Seizures and raised intracranial pressure are associated with a poor outcome, and may be potentially treatable. A safe efficacious formalin-inactivated vaccine against JE has been available for many years, but is too expensive for use in most Asian countries. A newer live attenuated vaccine has been used in China, but its use elsewhere has been restricted by regulatory concerns. A chimeric vaccine in which JE structural proteins are inserted into the 17D yellow fever vaccine backbone is one of several vaccines in development. There are no established antiviral treatments against JE. Interferon alpha was the most promising drug in small open trials, but a recent double-blind placebo controlled trial showed that it did not affect the outcome in children with JE.