Comparison of nucleotide and deduced amino acid sequence of the 5' non-coding region and structural protein genes of the wild-type Japanese encephalitis virus strain SA14 and its attenuated vaccine derivatives

Key virulence factors responsible for differences in pathogenicity between clinically proven live-attenuated Japanese encephalitis vaccine SA14-14-2 and its pre-attenuated highly virulent parent SA14

Japanese encephalitis virus (JEV), a neuroinvasive and neurovirulent orthoflavivirus, can be prevented in humans with the SA14-14-2 vaccine, a live-attenuated version derived from the wild-type SA14 strain. To determine the viral factors responsible for the differences in pathogenicity between SA14 and SA14-14-2, we initially established a reverse genetics system that includes a pair of full-length infectious cDNAs for both strains. Using this cDNA pair, we then systematically exchanged genomic regions between SA14 and SA14-14-2 to generate 20 chimeric viruses and evaluated their replication capability in cell culture and their pathogenic potential in mice. Our findings revealed the following: (i) The single envelope (E) protein of SA14-14-2, which contains nine mutations (eight in the ectodomain and one in the stem region), is both necessary and sufficient to render SA14 non-neuroinvasive and non-neurovirulent. (ii) Conversely, the E protein of SA14 alone is necessary for SA14-14-2 to become highly neurovirulent, but it is not sufficient to make it highly neuroinvasive. (iii) The limited neuroinvasiveness of an SA14-14-2 derivative that contains the E gene of SA14 significantly increases (approaching that of the wild-type strain) when two viral nonstructural proteins are replaced by their counterparts from SA14: (a) NS1/1', which has four mutations on the external surface of the core β-ladder domain; and (b) NS2A, which has two mutations in the N-terminal region, including two non-transmembrane α-helices. In line with their roles in viral pathogenicity, the E, NS1/1', and NS2A genes all contribute to the enhanced spread of the virus in cell culture. Collectively, our data reveal for the first time that the E protein of JEV has a dual function: It is the master regulator of viral neurovirulence and also the primary initiator of viral neuroinvasion. After the initial E-mediated neuroinvasion, the NS1/1' and NS2A proteins act as secondary promoters, further amplifying viral neuroinvasiveness.

Shift in dominant genotypes of Japanese encephalitis virus and its impact on current vaccination strategies

Japanese encephalitis (JE) is a zoonotic ailment from the Japanese encephalitis virus (JEV). JEV belongs to the flavivirus genus and is categorized into a solitary serotype consisting of five genetically diverse genotypes (I, II, III, IV, and V). The JEV genotype III (GIII) was the prevailing strain responsible for multiple outbreaks in countries endemic to JEV until 1990. In recent years, significant improvements have occurred in the epidemiology of JE, encompassing the geographical expansion of the epidemic zone and the displacement of prevailing genotypes. The dominant genotype of the JEV has undergone a progressive shift from GIII to GI due to variations in its adaptability within avian populations. From 2021 to 2022, Australia encountered an epidemic of viral encephalitis resulting from infection with the GIV JEV pathogen. The current human viral encephalitis caused by GIV JEV is the initial outbreak since its initial discovery in Indonesia during the late 1970s. Furthermore, following a time frame of 50 years, the detection and isolation of GV JEV have been reported in Culex mosquitoes across China and South Korea. Evidence suggests that the prevalence of GIV and GV JEV epidemic regions may be on the rise, posing a significant threat to public safety and the sustainable growth of animal husbandry. The global approach to preventing and managing JE predominantly revolves around utilizing the GIII strain vaccine for vaccination purposes. Nevertheless, research has demonstrated that the antibodies generated by the GIII strain vaccine exhibit limited capacity to neutralize the GI and GV strains. Consequently, these antibodies cannot protect against JEV challenge caused by animal GI and GV strains. The limited cross-protective and neutralizing effects observed between various genotypes may be attributed to the low homology of the E protein with other genotypes. In addition, due to the GIV JEV outbreak in Australia, further experiments are needed to evaluate the protective efficiency of the current GIII based JE vaccine against GIV JEV. The alteration of the prevailing genotype of JEV and the subsequent enlargement of the geographical extent of the epidemic have presented novel obstacles in JE prevention and control. This paper examines the emerging features of the JE epidemic in recent years and the associated problems concerning prevention and control.

Viral intra-host evolutionary dynamics revealed via serial passage of Japanese encephalitis virus in vitro

Analyses of viral inter- and intra-host mutations could better guide the prevention and control of infectious diseases. For a long time, studies on viral evolution have focused on viral inter-host variations. Next-generation sequencing has accelerated the investigations of viral intra-host diversity. However, the theoretical basis and dynamic characteristics of viral intra-host mutations remain unknown. Here, using serial passages of the SA14-14-2 vaccine strain of Japanese encephalitis virus (JEV) as the in vitro model, the distribution characteristics of 1,788 detected intra-host single-nucleotide variations (iSNVs) and their mutated frequencies from 477 deep-sequenced samples were analyzed. Our results revealed that in adaptive (baby hamster kidney (BHK)) cells, JEV is under a nearly neutral selection pressure, and both non-synonymous and synonymous mutations represent an S-shaped growth trend over time. A higher positive selection pressure was observed in the nonadaptive (C6/36) cells, and logarithmic growth in non-synonymous iSNVs and linear growth in synonymous iSNVs were observed over time. Moreover, the mutation rates of the NS4B protein and the untranslated region (UTR) of the JEV are significantly different between BHK and C6/36 cells, suggesting that viral selection pressure is regulated by different cellular environments. In addition, no significant difference was detected in the distribution of mutated frequencies of iSNVs between BHK and C6/36 cells.

Rhinolophus sinicus virome revealed multiple novel mosquito-borne zoonotic viruses

To exploit the Rhinolophus sinicus–specific virome, 29 Rhinolophus sinicus were gathered in Lincang, China. Enriched viral sequences of 22 virus families were acquired by metavirome techniques. Hereby, the part of virome in Rhinolophus sinicus, including Chikungunya virus (CHIKV), Getah virus, and Japanese encephalitis virus (JEV) were validated by PCR. Five CHIKV viral sequences were amplified, among which CHIKV-China/B2016C-1 shared the highest homology to CHIKV isolated from Italy in 2007, with the genotype as African ECS. Eight JEV viral sequences were amplified, of which JEV-China/B2016E-1 shared the highest homology with at least 91.3% nt identity with the JEV sequence found in South Korea in 1988 and was classified as genotype III. Notably, JEV was isolated for the first time in Rhinolophus sinicus. The newly isolated JEV-China/B2016-1 could increase infectivity while passaging in Vero cells from BHK-21 cells. Overall, the research sheds insight into the diversity and viral susceptibility dynamics of the virome in Rhinolophus sinicus and reveals new light on the ecology of other important viral hosts.

Interventions for the Prevention and Treatment of Japanese Encephalitis

Purpose of Review

Japanese encephalitis (JE), a clinical indication of JE virus–induced brain inflammation, is the most prevalent cause of viral encephalitis in the world. This review gives a comprehensive update on the epidemiology, clinical features, therapeutic trials and approaches for preventing the spread of JE. It also outlines the different JE vaccines used in various countries and recommendations for administration of JE vaccines.

Recent Findings

According to the WHO, annual incidence of JE is estimated to be approximately 68,000 cases worldwide. It is widespread across Asia–Pacific, with a potential for worldwide transmission. In endemic locations, JE is believed to affect children below 6 years of age, but in newly affected areas, both adults and children are at risk due to a lack of protective antibodies. Various vaccines have been developed for the prevention of JE and are being administered in endemic countries.

Summary

JE is a neuroinvasive disease that causes symptoms ranging from simple fever to severe encephalitis and death. Despite a vast number of clinical trials on various drugs, there is still no complete cure available, and it can only be prevented by adequate vaccination. Various nanotechnological approaches for the prevention and treatment of JE are outlined in this review.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11908-022-00786-1.

Molecular pathogenesis of Japanese encephalitis and possible therapeutic strategies

Japanese encephalitis virus (JEV), a single-stranded, enveloped RNA virus, is a health concern across Asian countries, associated with severe neurological disorders, especially in children. Primarily, pigs, bats, and birds are the natural hosts for JEV, but humans are infected incidentally. JEV requires a few host proteins for its entry and replication inside the mammalian host cell. The endoplasmic reticulum (ER) plays a significant role in JEV genome replication and assembly. During this process, the ER undergoes stress due to its remodelling and accumulation of viral particles and unfolded proteins, leading to an unfolded protein response (UPR). Here, we review the overall strategy used by JEV to infect the host cell and various cytopathic effects caused by JEV infection. We also highlight the role of JEV structural proteins (SPs) and non-structural proteins (NSPs) at various stages of the JEV life cycle that are involved in up- and downregulation of different host proteins and are potentially relevant for developing efficient therapeutic drugs.

The 5' and 3' Untranslated Regions of the Japanese Encephalitis Virus (JEV): Molecular Genetics and Higher Order Structures

The untranslated region (UTRs) of viral genome are important for viral replication and immune modulation. Japanese encephalitis virus (JEV) is the most significant cause of epidemic encephalitis worldwide. However, little is known regarding the characterization of the JEV UTRs. Here, systematic analyses of the UTRs of JEVs isolated from a variety of hosts worldwide spanning about 80 years were made. All the important cis-acting elements and structures were compared with other mosquito-borne Flaviviruses [West Nile virus (WNV), Yellow fever virus (YFV), Zika virus (ZIKV), Dengue virus (DENV)] and annotated in detail in the UTRs of different JEV genotypes. Our findings identified the JEV-specific structure and the sequence motif with unique JEV feature. (i) The 3’ dbsHP was identified as a small hairpin located in the DB region in the 3′ UTR of JEV, with the structure highly conserved among the JEV genotypes. (ii) The spacer sequence UARs of JEV consist of four discrete spacer sequences, whereas the UARs of other mosquito-borne Flaviviruses are continuous sequences. In addition, repetitive elements have been discovered in the UTRs of mosquito-borne Flaviviruses. The lengths, locations, and numbers of the repetitive elements of JEV also differed from other Flaviviruses (WNV, YFV, ZIKV, DENV). A 300 nt-length region located at the beginning of the 3′ UTR exhibited significant genotypic specificity. This study lays the basis for future research on the relationships between the JEV specific structures and elements in the UTRs, and their important biological function.

Japanese encephalitis virus live attenuated vaccine strains display altered immunogenicity, virulence and genetic diversity

Japanese encephalitis virus (JEV) is the etiological agent of Japanese encephalitis (JE). The most commonly used vaccine used to prevent JE is the live-attenuated strain SA14-14-2, which was generated by serial passage of the wild-type (WT) JEV strain SA14. Two other vaccine candidates, SA14-5-3 and SA14-2-8 were derived from SA14. Both were shown to be attenuated but lacked sufficient immunogenicity to be considered effective vaccines. To better contrast the SA14-14-2 vaccine with its less-immunogenic counterparts, genetic diversity, ribavirin sensitivity, mouse virulence and mouse immunogenicity of the three vaccines were investigated. Next generation sequencing demonstrated that SA14-14-2 was significantly more diverse than both SA14-5-3 and SA14-2-8, and was slightly less diverse than WT SA14. Notably, WT SA14 had unpredictable levels of diversity across its genome whereas SA14-14-2 is highly diverse, but genetic diversity is not random, rather the virus only tolerates variability at certain residues. Using Ribavirin sensitivity in vitro, it was found that SA14-14-2 has a lower fidelity replication complex compared to SA14-5-3 and SA14-2-8. Mouse virulence studies showed that SA14-2-8 was the most virulent of the three vaccine strains while SA14-14-2 had the most favorable combination of safety (virulence) and immunogenicity for all vaccines tested. SA14-14-2 contains genetic diversity and sensitivity to the antiviral Ribavirin similar to WT parent SA14, and this genetic diversity likely explains the (1) differences in genomic sequences reported for SA14-14-2 and (2) the encoding of major attenuation determinants by the viral E protein.

The pre membrane and envelope protein is the crucial virulence determinant of Japanese encephalitis virus

After sequence comparison, it was found that there are multiple amino acid mutations in pre-M and envelope (E) protein of Japanese encephalitis virus vaccine strain comparison with wild type (WT) strain SA14. It is generally acknowledged it is the mutations that have caused the virulence attenuation of vaccine strain, but lack of sufficient experimental evidences. For a better understanding of the mechanism of attenuation of Japanese encephalitis virus (JEV), in this study, we assessed whether prM/E is critical neurovirulence determinants of JEV with infectious cDNA clones technique. Substitutions prM/E of vaccine strain with that of WT SA14 did significantly increase the virulence of JEV to the similar level of wild type SA14, and simultaneously, replacement prM/E of JEV WT strain SA14 with that of vaccine strain SA14-14-2 decreased the virulence of JEV significantly to the similar level of vaccine stain. The results indicate that the prM/E protein is the crucial virulence determinant of Japanese encephalitis virus, although other proteins take part in the process to some extent.

A novel amino acid site closely associated with the neurovirulence of live, attenuated Japanese encephalitis vaccine (SA14-14-2 strain)

Japanese encephalitis (JE) poses a serious threat to the world's public health yet without a cure, the only way to prevent Japanese encephalitis virus (JEV) infection is vaccination. Live attenuated vaccine (SA14-14-2 strain) is the most widely used JE vaccine, and clinical data have confirmed its safety and effectiveness. Eight sitesassociated with virulence in the Envelope (E) protein are often the focus of quality control of JE vaccine. However, sequences retrieved from NCBI, as well as our previous results showed that the wild strain SA14 may harbor two different amino acids at amino acid residue 244 of the E glycoprotein (E244), and it may be related to virulence. In this study, we introduced a single mutation at nt1708 (G → A) in the full-length cDNA clone of SA14-14-2, replacing a Gly with Glu at amino acid residue 244 of the E glycoprotein, and successfully constructed the mutant virus (JEV E244). JEV E244 exhibited a similar plaque morphology and growth characteristics to JEV SA14-14-2 in cell culture. However, it had lethal neurovirulence in mice and could enter the brain following intraperitoneal inoculation. Moreover, the virulence of JEV E244 in the context of vaccine in mice is significantly different from that of the JEV E244 alone. These results suggested that E244 site should be included in the assessment of the genetic stability of the attenuated JE vaccine. The detection of minor mutations in vaccine population and influence on the safety of vaccine is discussed.

Japanese encephalitis vaccine-specific envelope protein E138K mutation does not attenuate virulence of West Nile virus

West Nile (WNV) and Japanese encephalitis viruses (JEV) are closely related, mosquito-borne neurotropic flaviviruses. Although there are no licensed human vaccines for WNV, JEV has multiple human vaccines, including the live, attenuated vaccine SA14-14-2. Investigations into determinants of attenuation of JE SA14-14-2 demonstrated that envelope (E) protein mutation E138K was crucial to the attenuation of mouse virulence. As WNV is closely related to JEV, we investigated whether or not the E-E138K mutation would be beneficial to be included in a candidate live attenuated WNV vaccine. Rather than conferring a mouse attenuated phenotype, the WNV E-E138K mutant reverted and retained a wild-type mouse virulence phenotype. Next-generation sequencing analysis demonstrated that, although the consensus sequence of the mutant had the E-E138K mutation, there was increased variation in the E protein, including a single-nucleotide variant (SNV) revertant to the wild-type glutamic acid residue. Modeling of the E protein and analysis of SNVs showed that reversion was likely due to the inability of critical E-protein residues to be compatible electrostatically. Therefore, this mutation may not be reliable for inclusion in candidate live attenuated vaccines in related flaviviruses, such as WNV, and care must be taken in translation of attenuating mutations from one virus to another virus, even if they are closely related.

Japanese encephalitis virus (JEV) and West Nile virus (WNV) are closely related neurotropic viruses—a live attenuated vaccine exists for JEV but not for WNV. A team led by Alan D.T. Barrett at the University of Texas investigated whether a key E-protein mutation (E138K) in the live attenuated JEV vaccine can also attenuate a candidate live WNV vaccine. The mutant WNV vaccine shows essentially identical behavior to the virulent parental strain in vitro but unexpectedly also has unimpaired lethality and neurotropism when mice are challenged intraperitoneally. Sequencing of the mutant WNV vaccine demonstrated increased frequencies of single-nucleotide variants clustered around residue 138—including reversion to the parental strain glutamic acid. E138K mutation is therefore unlikely to be a reliable means to attenuate candidate WNV vaccines.

Genotypic and phenotypic characterization of West Nile virus NS5 methyltransferase mutants

Although West Nile virus (WNV) causes annual cases of neurological disease and deaths in humans, a vaccine has not been licensed for human use. Several WNV genes have been targeted for mutagenesis in attempts to generate live attenuated vaccine candidates, including the non-structural protein NS5. Specifically, mutation of WNV NS5-K61A or NS5-E218A in the catalytic tetrad of the methyltransferase decreases enzyme activity of the NS5 protein and correspondingly attenuates the virus in mice. In this report, NS5-K61A, NS5-E218A, and a double mutant encoding both mutations (NS5-K61A/E218A) were compared both in vitro and in vivo. Each single mutant was strongly attenuated in highly susceptible outbred mice, whereas the double mutant unexpectedly was not attenuated. Sequencing analysis demonstrated that the double mutant was capable of reversion at both residues NS5-61 and NS5-218, whereas the genotype of the single mutants did not show evidence of reversion. Overall, either NS5-K61A or NS5-E218A methyltransferase mutations could be potential mutations to include in a candidate live WNV vaccine; however, multiple mutations in the catalytic tetrad of the methyltransferase are not tolerated.

Chimeric Japanese Encephalitis Virus SA14/SA14-14-2 Was Virulence Attenuated and Protected the Challenge of Wild-Type Strain SA14

The attenuated Japanese encephalitis virus (JEV) live vaccine SA14-14-2 prepared from wild-type (WT) strain SA14 was licensed to prevent Japanese encephalitis (JE) in 1989 in China. Many studies showed that the premembrane (prM) and envelope (E) protein were the crucial determinant of virulence and immunogenicity of JEV. So we are interested in whether the substitution of prM/E of JEV WT SA14 with those of vaccine strain SA14-14-2 could decrease neurovirulence and prevent the challenge of JEV WT SA14. Molecular clone technique was used to replace the prM/E gene of JEV WT strain SA14 with those of vaccine strain SA14-14-2 to construct the infectious clone of chimeric virus (designated JEV SA14/SA14-14-2), the chimeric virus recovered from BHK21 cells upon electrotransfection of RNA into BHK21 cells. The results showed that the recovered chimeric virus was highly attenuated in mice, and a single immunization elicited strong protective immunity in a dose-dependent manner. This study increases our understanding of the molecular mechanisms of neurovirulence attenuation and immunogenicity of JEV.

Acidity/Alkalinity of Japanese Encephalitis Virus E Protein Residue 138 Alters Neurovirulence in Mice

The Japanese encephalitis virus (JEV) envelope (E) protein, as one of mediators of virus entry into host cells, plays a critical role in determining virulence. The Glu-to-Lys mutation of residue 138 in E protein (E138) plays an important role in attenuating JEV vaccine strain SA14-14-2. However, it is not clear how E138 attenuates JEV. Here, we demonstrate that the Glu-to-Arg mutation of E138 also determines the attenuation of JEV strain 10S3. Likewise, for its parent strain (HEN0701), a virulence strain, the mutations of E138 are responsible for virulence alteration. Furthermore, we demonstrated that mutations of alkaline residues in E138 contributed to the attenuation of neurovirulence; in contrast, mutations of acidic residues enhanced the neurovirulence of the strains. Moreover, acidity in residue E47 had a similar effect on neurovirulence. Furthermore, the alkaline E138 residue enhanced susceptibility to heparin inhibition in vitro and limited JEV diffusion in mouse brain. These results suggest that the acidity/alkalinity of the E138 residue plays an important role in neurovirulence determination.IMPORTANCE The E protein is the only glycoprotein in mature JEV, and it plays an important role in viral neurovirulence. E protein mutations attenuate JEV neurovirulence through unclear mechanisms. Here, we discovered that E138 is a predominant determinant of JEV neurovirulence. We demonstrated that the alkalinity/acidity of E138 determines JEV neurovirulence. These data contribute to the characterization of the E protein and the rational development of novel JEV vaccines.

Metagenomic Analysis of Flaviviridae in Mosquito Viromes Isolated From Yunnan Province in China Reveals Genes From Dengue and Zika Viruses

More than 6,000 mosquitoes of six species from six sites were collected and tested for their virome using metagenomics sequencing and bioinformatic analysis. The identified viral sequences belonged to more than 50 viral families. The results were verified by PCR of selected viruses in all mosquitoes, followed by phylogenetic analysis. In the present study, we identified the partial dengue virus (DENV), Zika virus (ZIKV), and Japanese encephalitis virus (JEV) sequences in mosquitoes. Metagenomic analysis and the PCR amplification revealed three DENV sequences, one of which encodes a partial envelope protein. Two ZIKV sequences both encoding partial nonstructural protein 3 and one JEV sequence encoding the complete envelope protein were identified. There was variability in the viral titers of the newly isolated virus JEV-China/YN2016-1 of different passage viruses. The newly identified Zika virus gene from ZIKV-China/YN2016-1 was an Asian genotype and shared the highest nucleotide sequence identity (97.1%) with a ZIKV sequence from Thailand isolated in 2004. Phylogenetic analysis of ZIKV-China/YN2016-1 and ZIKV-China/YN2016-2 with known Flavivirus genes indicated that ZIKV has propagated in Yunnan province, China.

Metagenomic Sequencing From Mosquitoes in China Reveals a Variety of Insect and Human Viruses

We collected 8,700 mosquitoes in three sites in China, which belonged to seven species. Their viromes were tested using metagenomic sequencing and bioinformatic analysis. The abundant viral sequences were detected and annotated belonging to more than 50 viral taxonomic families. The results were verified by PCR, followed by phylogenetic analysis. In the present study, we identified partial viral genes of dengue virus (DENV), a novel circovirus (CCV), densovirus (DNV), Japanese encephalitis virus (JEV), and Wuhan mosquito virus (WMV) in mosquitoes. Metagenomic analysis and PCR amplification revealed three DENV sequences, which were as homologous to the NS3 gene of DENV from Singapore isolated in 2005, with at least 91% nucleotide (nt) identity. Seven fragments of JEV encoding structural proteins were identified belonging to genotype I. They all shared high homology with structural protein genes of JEV isolated from Laos in 2009. The production of infectious virus particles of the newly isolated virus YunnanJEV2017-4 increased after passage from the BHK-21 cell line to the Vero cell line. Novel circovirus-related genes were identified and as being related to an unnamed gene of a mosquito circovirus (MCCV) sequence from the USA isolated in 2011, with at least 41% nt identity: this distant relationship suggests that the parent virus might belong to a novel circovirus genus. Additionally, numerous known viruses and some unknown viruses were also detected in mosquitoes from Yunnan province, China, which will be tested for propagation.

A Simple Mechanism Based on Amino Acid Substitutions is not a Critical Determinant of High Mortality of Japanese Encephalitis Virus Infection in Mice

For the development of effective treatment strategies for Japanese encephalitis (JE), it is important to identify the viral factors causing severe disease during JE virus (JEV) infection. In this study, we assessed whether amino acid substitutions are critical factors for higher mortality of JaTH160 compared with JaOArS982 in mice using the technique of infectious cDNA clones. We raised the possibility that two amino acids of C₁₂₄ and NS3₄₈₂ of JaTH160 may contribute to increased mortality in mice. However, simultaneous substitutions of these amino acids did not significantly increase the virulence of JaOArS982, suggesting that high mortality due to JaTH160 viral infection cannot be simply attributed to the specific amino acids. Multiple and complex, but not simple, mechanisms may induce the high mortality of JaTH160 infection in mice.

Japanese encephalitis vaccines: Immunogenicity, protective efficacy, effectiveness, and impact on the burden of disease

Japanese encephalitis (JE) is a serious public health concern in most of Asia. The disease is caused by JE virus (JEV), a flavivirus transmitted by Culex mosquitoes. Several vaccines have been developed to control JE in endemic areas as well as to protect travelers and military personnel who visit or are commissioned from non-endemic to endemic areas. The vaccines include inactivated vaccines produced in mouse brain or cell cultures, live attenuated vaccines, and a chimeric vaccine based on the live attenuated yellow fever virus 17D vaccine strain. All the marketed vaccines belong to the JEV genotype III, but have been shown to be efficacious against other genotypes and strains, with varying degrees of cross-neutralization, albeit at levels deemed to be protective. The protective responses have been shown to last three or more years, depending on the type of vaccine and the number of doses. This review presents a brief account of the different JE vaccines, their immunogenicity and protective ability, and the impact of JE vaccines in reducing the burden of disease in endemic countries.

Envelope Protein Mutations L107F and E138K Are Important for Neurovirulence Attenuation for Japanese Encephalitis Virus SA14-14-2 Strain

The attenuated Japanese encephalitis virus (JEV) strain SA14-14-2 has been successfully utilized to prevent JEV infection; however, the attenuation determinants have not been fully elucidated. The envelope (E) protein of the attenuated JEV SA14-14-2 strain differs from that of the virulent parental SA14 strain at eight amino acid positions (E107, E138, E176, E177, E264, E279, E315, and E439). Here, we investigated the SA14-14-2-attenuation determinants by mutating E107, E138, E176, E177, and E279 in SA14-14-2 to their status in the parental virulent strain and tested the replication capacity, neurovirulence, neuroinvasiveness, and mortality associated with the mutated viruses in mice, as compared with those of JEV SA14-14-2 and SA14. Our findings indicated that revertant mutations at the E138 or E107 position significantly increased SA14-14-2 virulence, whereas other revertant mutations exhibited significant increases in neurovirulence only when combined with E138, E107, and other mutations. Revertant mutations at all eight positions in the E protein resulted in the highest degree of SA14-14-2 virulence, although this was still lower than that observed in SA14. These results demonstrated the critical role of the viral E protein in controlling JEV virulence and identified the amino acids at the E107 and E138 positions as the key determinants of SA14-14-2 neurovirulence.

Comparison of the live-attenuated Japanese encephalitis vaccine SA14-14-2 strain with its pre-attenuated virulent parent SA14 strain: similarities and differences in vitro and in vivo

Japanese encephalitis virus (JEV) is the main cause of acute viral encephalitis, primarily affecting children and young adults in the Asia-Pacific region. JEV is a vaccine-preventable pathogen, with four types of JE vaccine licensed in different regions of the world. To date, the most common JEV strain used in vaccine development and production is SA14-14-2, an attenuated strain derived from its wild-type parental strain SA14. In this study, we directly compared the phenotypic and genotypic characteristics of SA14 and SA14-14-2 to determine the biological and genetic properties associated with their differential virulence. In susceptible BHK-21 cells, SA14-14-2 grew slightly more slowly and formed smaller plaques than SA14, but unlike SA14, it showed almost no expression of the viral protein NS1', the product of a conserved predicted RNA pseudoknot-mediated ribosomal frameshift. In weanling ICR mice, SA14-14-2 was highly attenuated in terms of both neuroinvasiveness and neurovirulence, with its median lethal doses invariably over five logs higher than those of SA14 when inoculated intramuscularly and intracerebrally. Interestingly, the neurovirulence of SA14-14-2 was dependent on mouse age, with the 1- to 7-day-old mice being highly susceptible and the 14- to 21-day-old mice becoming resistant to intracerebral inoculation. At the genome level, SA14-14-2 differed from SA14 by 57 nucleotides, including one silent G-to-A substitution at position 3599 within the predicted RNA pseudoknot for NS1' synthesis; of the 57 differences, 25 resulted in amino acid substitutions. Our data pave the way for the development of new genetically modified JE vaccines.

The A66G back mutation in NS2A of JEV SA14-14-2 strain contributes to production of NS1' protein and the secreted NS1' can be used for diagnostic biomarker for virulent virus infection

Japanese encephalitis virus (JEV) is the most common cause of the prevalent encephalitis in Asia-Pacific region and poses a serious risk to public health. Here, we developed a reliable reverse genetics system based on the JEV SA14-14-2 strain to further explore the mechanism for the synthesis of NS1' protein and to investigate the function of NS1' protein during virus infection. NS1' is an additional form of NS1 protein with 52 amino acid carboxy-terminal extension and is expressed by the members of the Japanese encephalitis (JE) serogroup due to the translation frameshift. A66G substitution in NS2A gene of JEV SA14-14-2 strain contributed to recover the GC-rich pseudoknot and resulted in the formation of the NS1'. The NS1' protein has no significant effect on the virus replication properties in BHK-21 cells. Animal experiments demonstrated that the NS1' protein had a rather minor effect on neurovirulence of JEV SA14-14-2 strain. But the NS1'-expressing virus (rA66G) could induce a higher humoral immune response than the NS1'-non-expressing virus (rSA14-14-2). NS1' protein can be detected in the serum of JEV rA66G infected animal and in the cultural media of that infected mammalian cells. Interesting, only the dimer of NS1' can be detected in the cultural media of the infected BHK-21 cells and the amount of the secreted NS1' was in agreement with that of the secreted virion. In comparison with the live-attenuated JE vaccine strain which is incapable of formation of NS1', most of the virulent JEV strains produce the NS1' protein. And the secreted NS1' may serve as an early surrogate biomarker for viremia to distinguish the field infection from the vaccine inoculation. In total, in the present study, we identified the nt 66 in the viral NS2A gene as one of the critical site for the -1 programmed ribosomal frameshift to produce the NS1' protein and demonstrated the secreted NS1' could be used for diagnostic biomarker during JEV infection.

Development of a human live attenuated West Nile infectious DNA vaccine: conceptual design of the vaccine candidate

West Nile virus has become an important epidemiological problem attracting significant attention of health authorities, mass media, and the public. Although there are promising advancements toward addressing the vaccine need, the perspectives of the commercial availability of the vaccine remain uncertain. To a large extent this is due to lack of a sustained interest for further commercial development of the vaccines already undergoing the preclinical and clinical development, and a predicted insignificant cost effectiveness of mass vaccination. There is a need for a safe, efficacious and cost effective vaccine, which can improve the feasibility of a targeted vaccination program. In the present report, we summarize the background, the rationale, and the choice of the development pathway that we selected for the design of a live attenuated human West Nile vaccine in a novel infectious DNA format.

Genetic Determinants of Japanese Encephalitis Virus Vaccine Strain SA14-14-2 That Govern Attenuation of Virulence in Mice

The safety and efficacy of the live-attenuated Japanese encephalitis virus (JEV) SA14-14-2 vaccine are attributed to mutations that accumulated in the viral genome during its derivation. However, little is known about the contribution that is made by most of these mutations to virulence attenuation and vaccine immunogenicity. Here, we generated recombinant JEV (rJEV) strains containing JEV SA14-14-2 vaccine-specific mutations that are located in the untranslated regions (UTRs) and seven protein genes or are introduced from PCR-amplified regions of the JEV SA14-14-2 genome. The resulting mutant viruses were evaluated in tissue culture and in mice. The authentic JEV SA14-14-2 (E) protein, with amino acid substitutions L107F, E138K, I176V, T177A, E244G, Q264H, K279M, A315V, S366A, and K439R relative to the wild-type rJEV clone, was essential and sufficient for complete attenuation of neurovirulence. Individually, the nucleotide substitution T39A in the 5' UTR (5'-UTR-T39A), the capsid (C) protein amino acid substitution L66S (C-L66S), and the complete NS1/2A genome region containing 10 mutations each significantly reduced virus neuroinvasion but not neurovirulence. The levels of peripheral virulence attenuation imposed by the 5'-UTR-T39A and C-L66S mutations, individually, were somewhat mitigated in combination with other vaccine strain-specific mutations, which might be compensatory, and together did not affect immunogenicity. However, a marked reduction in immunogenicity was observed with the addition of the NS1/2A and NS5 vaccine virus genome regions. These results suggest that a second-generation recombinant vaccine can be rationally engineered to maximize levels of immunogenicity without compromising safety.

IMPORTANCE

The live-attenuated JEV SA14-14-2 vaccine has been vital for controlling the incidence of disease caused by JEV, particularly in rural areas of Asia where it is endemic. The vaccine was developed >25 years ago by passaging wild-type JEV strain SA14 in tissue cultures and rodents, with intermittent tissue culture plaque purifications, to produce a virus clone that had adequate levels of attenuation and immunogenicity. The vaccine and parent virus sequences were later compared, and mutations were identified throughout the vaccine virus genome, but their contributions to attenuation were never fully elucidated. Here, using reverse genetics, we comprehensively defined the impact of JEV SA14-14-2 mutations on attenuation of virulence and immunogenicity in mice. These results are relevant for quality control of new lots of the current live-attenuated vaccine and provide insight for the rational design of second-generation, live-attenuated, recombinant JEV vaccine candidates.

Genetic and phenotypic properties of vero cell-adapted Japanese encephalitis virus SA14-14-2 vaccine strain variants and a recombinant clone, which demonstrates attenuation and immunogenicity in mice

The live-attenuated Japanese encephalitis virus (JEV) SA14-14-2 vaccine, produced in primary hamster kidney cells, is safe and effective. Past attempts to adapt this virus to replicate in cells that are more favorable for vaccine production resulted in mutations that significantly reduced immunogenicity. In this study, 10 genetically distinct Vero cell-adapted JEV SA14-14-2 variants were isolated and a recombinant wild-type JEV clone, modified to contain the JEV SA14-14-2 polyprotein amino acid sequence, was recovered in Vero cells. A single capsid protein mutation (S66L) was important for Vero cell-adaptation. Mutations were also identified that modulated virus sensitivity to type I interferon-stimulation in Vero cells. A subset of JEV SA14-14-2 variants and the recombinant clone were evaluated in vivo and exhibited levels of attenuation that varied significantly in suckling mice, but were avirulent and highly immunogenic in weanling mice and are promising candidates for the development of a second-generation, recombinant vaccine.

A molecularly cloned, live-attenuated japanese encephalitis vaccine SA14-14-2 virus: a conserved single amino acid in the ij Hairpin of the Viral E glycoprotein determines neurovirulence in mice

Japanese encephalitis virus (JEV), a mosquito-borne flavivirus that causes fatal neurological disease in humans, is one of the most important emerging pathogens of public health significance. JEV represents the JE serogroup, which also includes West Nile, Murray Valley encephalitis, and St. Louis encephalitis viruses. Within this serogroup, JEV is a vaccine-preventable pathogen, but the molecular basis of its neurovirulence remains unknown. Here, we constructed an infectious cDNA of the most widely used live-attenuated JE vaccine, SA14-14-2, and rescued from the cDNA a molecularly cloned virus, SA14-14-2MCV, which displayed in vitro growth properties and in vivo attenuation phenotypes identical to those of its parent, SA14-14-2. To elucidate the molecular mechanism of neurovirulence, we selected three independent, highly neurovirulent variants (LD50, <1.5 PFU) from SA14-14-2MCV (LD50, >1.5×105 PFU) by serial intracerebral passage in mice. Complete genome sequence comparison revealed a total of eight point mutations, with a common single G1708→A substitution replacing a Gly with Glu at position 244 of the viral E glycoprotein. Using our infectious SA14-14-2 cDNA technology, we showed that this single Gly-to-Glu change at E-244 is sufficient to confer lethal neurovirulence in mice, including rapid development of viral spread and tissue inflammation in the central nervous system. Comprehensive site-directed mutagenesis of E-244, coupled with homology-based structure modeling, demonstrated a novel essential regulatory role in JEV neurovirulence for E-244, within the ij hairpin of the E dimerization domain. In both mouse and human neuronal cells, we further showed that the E-244 mutation altered JEV infectivity in vitro, in direct correlation with the level of neurovirulence in vivo, but had no significant impact on viral RNA replication. Our results provide a crucial step toward developing novel therapeutic and preventive strategies against JEV and possibly other encephalitic flaviviruses.

IXIARO updated: overview of clinical trials and developments with the inactivated vaccine against Japanese encephalitis

The mosquito-borne Japanese encephalitis virus causes an estimated 50,000 cases in Asia, accounting for at least 10,000 deaths and 15,000 cases of neuropsychiatric sequelae. IXIARO (Intercell AG, Vienna, Austria), an inactivated, Vero cell-derived vaccine against Japanese encephalitis was introduced in 2009. The vaccine is highly immunogenic, showing significantly higher geometric mean antibody titers compared with previous, mouse brain-derived vaccines. Postmarketing studies have confirmed the excellent safety profile. Studies on children aged 2 months to 18 years have been published. Based on these data, positive opinion from the EMA for vaccination of children has recently been given. Since a safe and effective vaccine against Japanese encephalitis is now available, outdated guidelines and recommendations have to be revised: travelers to rural areas of Asia should generally be recommended vaccination.

Matrix and backstage: cellular substrates for viral vaccines

Vaccines are complex products that are manufactured in highly dynamic processes. Cellular substrates are one critical component that can have an enormous impact on reactogenicity of the final preparation, level of attenuation of a live virus, yield of infectious units or antigens, and cost per vaccine dose. Such parameters contribute to feasibility and affordability of vaccine programs both in industrialized countries and developing regions. This review summarizes the diversity of cellular substrates for propagation of viral vaccines from primary tissue explants and embryonated chicken eggs to designed continuous cell lines of human and avian origin.

Recovery of a chemically synthesized Japanese encephalitis virus reveals two critical adaptive mutations in NS2B and NS4A

A full-length genome infectious clone is a powerful tool for functional assays in virology. In this study, using a chemical synthesized complete genome of Japanese encephalitis virus (JEV) strain SA14 (GenBank accession no. U14163), we constructed a full-length genomic cDNA clone of JEV. The recovered virus from the cDNA clone replicated poorly in baby hamster kidney (BHK-21) cells and in suckling mice brain. Following serial passage in BHK-21 cells, adaptive mutations within the NS2B and NS4A proteins were recovered in the passaged viruses leading to viruses with a large-plaque phenotype. Mutagenesis analysis, using a genome-length RNA and a replicon of JEV, demonstrated that the adaptive mutations restored replication to different degrees, and the restoration efficiencies were in the order: NS2B-T102M<NS4A-R79K<NS2B-T102M+NS4A-R79K. An in vivo virulence assay in mice showed that the recombinant virus containing double mutations showed similar virulence to the WT SA14 (GenBank accession no. M55506). This study reports the first chemically synthesized JEV. A reverse genetics assay demonstrated that substitutions of NS2B-T102M and NS4A-R79K altered JEV replication.

Japanese Encephalitis Virus Generated Neurovirulence, Antigenicity, and Host Immune Responses

In response to a JE virus attack, infected body cells start secretion of different cytokines and activate innate immune response. Virus starts neuronal invasion by entering into nerve cells and inflecting the central nervous system. It avoids exposure of body’s natural immunity and generates neurotrophic effects. Virus causes acute susceptibility to CNS and establishes encephalitis syndrome that results in very high fatality in children. In survivors, JEV inhibits the growth and proliferation of NCPs and imposes permanent neuronal disorders like cognitive, motor, and behavioral impairments. However, body cells start TCR mediated interactions, to recognize viral antigens with class I MHC complex on specific target cells, and operate mass killing of virus infected cells by increased CTL activity. Thus, both cell mediated and antibody interactions plays a central role in protection against JEV. In the present review article virus generated neurovirulence, antigenicity, and host immune responses are described in detail. More emphasis is given on diagnosis, clinical care, and active immunization with well-designed potential antiflavivirus vaccines. Further, for achieving an elite success against JEV, global eradication strategies are to be needed for making vaccination program more responsible and effective in endemic areas.

IXIARO® updated: overview of clinical trials and developments with the inactivated vaccine against Japanese encephalitis

The mosquito-borne Japanese encephalitis virus causes an estimated 50,000 cases in Asia, accounting for at least 10,000 deaths and 15,000 cases of neuropsychiatric sequelae. IXIARO® (Intercell AG, Vienna, Austria), an inactivated, Vero cell-derived vaccine against Japanese encephalitis was introduced in 2009. The vaccine is highly immunogenic, showing significantly higher geometric mean antibody titers compared with previous, mouse brain-derived vaccines. Postmarketing studies have confirmed the excellent safety profile. Studies on children aged 2 months to 18 years have been published. Based on these data, positive opinion from the EMA for vaccination of children has recently been given. Since a safe and effective vaccine against Japanese encephalitis is now available, outdated guidelines and recommendations have to be revised: travelers to rural areas of Asia should generally be recommended vaccination.

Japanese encephalitis: a review of the Indian perspective

Japanese encephalitis virus (JEV) causes Japanese encephalitis, which is a leading form of viral encephalitis in Asia, with around 50,000 cases and 10,000 deaths per year in children below 15 years of age. The JEV has shown a tendency to extend to other geographic regions. Case fatality averages 30% and a high percentage of the survivors are left with permanent neuropsychiatric sequelae. Currently, there is no cure for JEV, and treatment is mainly supportive. Patients are not infectious, but should avoid further mosquito bites. A number of antiviral agents have been investigated; however, none of these have convincingly been shown to improve the outcome of JEV. In this review, the current knowledge of the epidemiology and the pathogenesis of this deadly disease have been summarized.

Biological and genetic properties of SA₁₄-14-2, a live-attenuated Japanese encephalitis vaccine that is currently available for humans

Japanese encephalitis virus (JEV), a mosquito-borne flavivirus, is a major cause of acute encephalitis, a disease of significance for global public health. In the absence of antiviral therapy to treat JEV infection, vaccination is the most effective method of preventing the disease. In JE-endemic areas, the most widely used vaccine to date is SA(14)-14-2, a live-attenuated virus derived from its virulent parent SA(14). In this study, we describe the biological properties of SA(14)-14-2, both in vitro and in vivo, and report the genetic characteristics of its genomic RNA. In BHK-21 (hamster kidney) cells, SA(14)-14-2 displayed a slight delay in plaque formation and growth kinetics when compared to a virulent JEV strain, CNU/LP2, with no decrease in maximum virus production. The delay in viral growth was also observed in two other cell lines, SH-SY5Y (human neuroblastoma) and C6/36 (mosquito larva), which are potentially relevant to JEV pathogenesis and transmission. In 3-week-old ICR mice, SA(14)-14-2 did not cause any symptoms or death after either intracerebral or peripheral inoculation with a maximum dose of up to 1.5×10(3) plaque-forming units (PFU) per mouse. The SA(14)-14-2 genome consisted of 10977 nucleotides, one nucleotide longer than all the previously reported genomes of SA(14)-14-2, SA(14) and two other SA(14)-derived attenuated viruses. This difference was due to an insertion of one G nucleotide at position 10701 in the 3 noncoding region. Also, we noted a significant number of nucleotide and/or amino acid substitutions throughout the genome of SA(14)-14-2, except for the prM protein-coding region, that differed from SA(14) and/or the other two attenuated viruses. Our results, together with others', provide a foundation not only for the study of JEV virulence but also for the development of new and improved vaccines for JEV.

A single nucleotide mutation in NS2A of Japanese encephalitis-live vaccine virus (SA14-14-2) ablates NS1' formation and contributes to attenuation

Japanese encephalitis (JE) remains the leading cause of viral encephalitis in the Asia-Pacific region, and the live vaccine SA14-14-2 is currently recommended by WHO and widely used in Asian countries with a good safety and efficacy profile. In this study, we demonstrated that SA14-14-2 failed to produce NS1', the larger NS1-related protein, compared with its parental strain SA14 in various cells. Sequence analysis and secondary structure prediction identified a single silent mutation G66A in the NS2A-coding region of SA14-14-2 destabilized the conserved pseudoknot structure, which was associated with a -1 ribosomal frame shift event. Using reverse genetic technology and animal study, we provided solid evidence that this single silent mutation G66A in the NS2A gene abolished the production of NS1' in vitro and reduced neurovirulence and neuroinvasiveness in mice. These findings provide critical information in understanding the molecular mechanism of JE vaccine attenuation and is critical for JE vaccine quality control.

Phenotypic and genotypic characteristics of Japanese encephalitis attenuated live vaccine virus SA14-14-2 and their stabilities

A novel Japanese encephalitis (JE) attenuated live vaccine virus SA14-14-2 was licensed for commercial application in China in 1989. Since then this vaccine has been widely used in China and other countries in Asia, and no vaccine associated encephalitis case was reported. The neurovirulence of the SA14-14-2 was tested in JE susceptible laboratory animals, such as mice, monkeys, hamsters and athymic nude mice. The results showed that the attenuated virus strain was avirulent to these animals by intracerebral inoculation (i.c.) or intraperitoneal inoculation (i.p.). Studies on the neuroattenuation stability revealed that no reversion after 17 times tissue culture passages or one i.c. sucking mice passage. Mosquito infection studies indicated that after one mosquito intrathoracical passage, the progeny viruses in the infected mosquitoes were unable to cause sucking mice or weanling mice disease. Molecules characteristics' studies of the SA14-14-2 virus strain showed that there are 57-61 nucleotide changes and 24-31 amino acid substitutions, eight substitutions in the E protein gene are the critical amino acid related to the virus attenuation. The E gene sequence studies have showed that the 8 critical amino acids were not changed after 22 passages in tissue cultures or one passage in mosquitoes. Comparison of the full-length sequence to the parental SA14 virus has revealed that after 22 passages in the tissue cultures, only 8 nucleotides changed leading to 4 amino acid substitutions. However they were not the reverse mutation and none of the 8 critical residues changed. The homology of the nucleotide and amino acid between the virus of passage 22 and the primary seed virus in Genbank was 99.93% and 99.88% respectively. The above results demonstrated that the SA14-14-2 virus is highly attenuated for the various JE susceptible animals. The attenuated phenotypes and the genetic characteristics of the SA14-14-2 strain are highly stable after multiple in vitro passages or mosquitoes infection. Therefore the safety of the live JE vaccine is due to a high degree of neuroattenuation and a number of stable phenotypically and genetically characteristics, suggesting that reversion to neurovirulence of the vaccine strain would be highly unlikely.

Preventive strategies for frequent outbreaks of Japanese encephalitis in Northern India

Japanese encephalitis (JE) remains the most important cause of acute viral encephalitis and continues to spread to hitherto unaffected regions like Indonesia, Pakistan and Australia. Approximately 60% of the world population inhabits JE endemic areas. Despite its restricted range mostly in the developing countries,a high annual incidence of 50,000 cases and about 10,000 deaths has been reported. Disease can be fatal in 25% ases. Magnitude of the problem is even more alarming since the survivors are left with serious long-term neuropsychiatric sequelae. Almost every two years,epidemics of JE occur in Indian subcontinent with a high mortality. JE virus infection results in different disease manifestations in host from mild subclinical febrile illness to clinical infections leading to encephalitis. No antiviral treatment is so far available for JE. The prevention of JE can be achieved by controlling the vector or by immunization regime. The vector control in the rural areas,which are the worst affected ones,is practically almost impossible. Three vaccines that have been implicated against JE include inactivated mouse brain derived, inactivated cell culture derived and cell culture derived live attenuated JE vaccine. But each has its own limitation. Currently,attempts to synthesize recombinant DNA vaccine are being made. New therapeutics are on the way of development like use of minocycline, short interfering RNA, arctigenin, rosmarinic acid, DNAzymes etc. However,the immune mechanisms that lead to JE are complex and need to be elucidated further for the development of therapeutics as well as safe and efficacious JE vaccines.

Current use and development of vaccines for Japanese encephalitis

BACKGROUND

Japanese encephalitis (JE) is a significant cause of human morbidity and mortality throughout Asia. Vaccines for JE have been available for many years and their use has been effective in reducing the incidence of JE disease in several countries but, as disease incidence has decreased, concerns regarding adverse events following immunisation have increased.

OBJECTIVE

To review existing JE vaccines and new candidates in advanced preclinical or clinical evaluation.

METHODS

The review primarily covers published and some unpublished literature from the past decade describing current use of approved JE vaccines in various parts of the world, and advanced development and clinical testing of alternative vaccine candidates.

RESULTS/CONCLUSION

There is a clear need for additional licensing of existing or new JE vaccines. Several promising candidates are currently in use or completing clinical trials.

Sultam thiourea inhibition of West Nile virus

We have identified sultam thioureas as novel inhibitors of West Nile virus (WNV) replication. One such compound inhibited WNV, with a 50% effective concentration of 0.7 microM, and reduced reporter expression from cells that harbored a WNV-based replicon. Our results demonstrate that sultam thioureas can block a postentry, preassembly step of WNV replication.

JE Nakayama/JE SA14-14-2 virus structural region intertypic viruses: biological properties in the mouse model of neuroinvasive disease

A molecular clone of Japanese encephalitis (JE) virus Nakayama strain was used to create intertypic viruses containing either the 5'-C-prM-E or the prM-E region of the attenuated JE SA14-14-2 virus in the JE Nakayama background. These two intertypic JE viruses, JE-X/5'CprME(S) and JE-X/prME(S), respectively, generally resembled the parental JE virus in cell culture properties. Similar to virus derived from the JE Nakayama molecular clone (JE-XJN), JE-X/prME(S) was highly neuroinvasive and neurovirulent for young adult mice, whereas JE-X/5'CprME(S) was attenuated for neuroinvasiveness and only partially attenuated for neurovirulence. Immunization of young mice with JE-X/5'CprME(S) virus elicited neutralizing antibodies against JE Nakayama virus and conferred protection against encephalitis following challenge with JE Nakayama virus. The sequence of the JE-X/5'CprME(S) virus differed from that of JE-X/prME(S) virus at two nucleotides in the 5' UTR, 3 amino acid positions in the capsid protein, 4 positions in the prM protein and 1 in the envelope protein. For JE-X/prME(S) virus, the 4 differences in prM and the single substitution in the envelope represented reversions to the sequence of JE Nakayama virus. Overall, this study reveals that molecular determinants associated with the prM-E region of the attenuated JE SA14-14-2 virus are insufficient by themselves to confer an attenuation phenotype upon JE Nakayama virus. This suggests a role for determinants in the 5' UTR and/or the capsid protein of the JE SA 14-14-2 virus genome in influencing the virulence properties of the JE Nakayama virus in the mouse model.

Chimeric Japanese encephalitis virus/dengue 2 virus infectious clone: biological properties, immunogenicity and protection against dengue encephalitis in mice

A molecular clone of Japanese encephalitis virus (JE virus) was derived from the JE virus Nakayama strain and used to produce infectious JE virus in cell culture. The engineered JE virus resembled the parental JE virus in cell-culture properties and was related closely to other JE virus strains based on nucleotide sequence analysis. The JE virus clone was used as a genetic background for construction of a chimeric virus containing the structural proteins prM and E of Dengue virus, serotype 2. The chimeric JE/dengue 2 virus generated authentic dengue 2 structural proteins as assessed by immunoassays for the dengue E protein. It exhibited a small plaque size and less efficient growth in various cell lines than the parental JE virus. JE/dengue 2 virus was non-neuroinvasive for young adult mice, but displayed partial neurovirulence at doses up to 4 log p.f.u. given intracerebrally. Immunization of 3-week-old mice with JE/dengue 2 virus yielded neutralizing-antibody titres against dengue 2 virus and conferred protection against dengue encephalitis caused by neuroadapted dengue 2 virus. A rise in post-challenge neutralizing-antibody titres against dengue 2 virus in surviving mice suggests that immunization is associated with establishment of a memory antibody response in this model. This study demonstrates the capacity of JE virus to serve as a vector for expression of heterologous flavivirus structural proteins. Similar to previous studies with other chimeric flaviviruses, this approach may be useful as a genetic system for engineering experimental vaccines against Dengue virus and other medically important flaviviruses.

Sensitive and specific detection of strains of Japanese encephalitis virus using a one-step TaqMan RT-PCR technique

A rapid, sensitive, and accurate laboratory diagnostic test is needed for distinguishing Japanese encephalitis virus (JEV) from other diseases featuring similar clinical symptoms and also for preventing potential outbreaks. In this study, a TaqMan reverse transcription (RT)-polymerase chain reaction (PCR) assay was developed for rapid detection and quantification of the viral RNA of various JEV strains. A consensus JEV NS3 region was chosen to design the primers and the TaqMan probe. The JEV TaqMan assay used the EZ-rTtH RT-PCR system featuring advantages such as a one-step, high-temperature RT reaction modality and preventing carry-over contamination. The sensitivity of the JEV TaqMan assay for detecting in vitro-transcribed JEV NS3 RNA was estimated to be one to five copies of RNA per reaction. For cultured JE virions, less than 40 plaque forming unit (PFU)/ml of virus load (corresponding to 0.07 PFU/test) could be detected. In addition, the JEV TaqMan assay could detect all seven strains of JEV tested, but provided negative results for nine other flaviviruses and encephalitis viruses tested. The JEV TaqMan assay demonstrated greater sensitivity and specificity than traditional RT-PCR methods as has been previously reported. The application of the JEV TaqMan assay herein has been shown to the sensitive detection of the JEV from both mosquito pools and also JEV-spiking human blood. The assay should be of use in diagnostic laboratory conduct and could be used to replace or complement time-consuming viral-culture methods, thus achieving more rapid, sensitive, and highly specific identification of JEV infection.

Phenotypic and genotypic characterization of the neurovirulence and neuroinvasiveness of a large-plaque attenuated Japanese encephalitis virus isolate

The virulent phenotypes of Japanese encephalitis virus (JEV) can be divided into neuroinvasiveness (NI) and neurovirulence (NV). In this study, two JEV antigenic variants, CH2195LA (large-plaque, attenuated) and CH2195SA (small-plaque, non-attenuated), were passaged in suckling mice by intracerebral inoculation. Viruses at passage two and four were characterized in terms of NV and NI in weaning mice, as well as their in vitro growth characteristics in six cell lines. Following two brain-brain passages in mice, the attenuated variant CH2195LA was found to significantly restore the NV and NI by approximately 90% and 20-40%, respectively. The increased titers in THP-1 monocytic cells but not IMR-32 and Neuro-2A neuroblastoma cells were more correlated with the phenotypic changes of NI and NV in mice. Entire genomic sequencing was further performed to demonstrate that 14 nucleotides were altered in the attenuated variant CH2195LA following four brain-brain passages in mice, giving 12 amino acid changes, in prM-73, prM-80, E-161, E-170, E-276, NS2A-136, NS2A-215, NS3-346, NS4A-128, NS4B-196, NS4B-197, NS4B-198. This study indicated a cluster of amino acids which is involved in NV and NI of the JEV for mice and, perhaps, for humans. Elucidating the molecular basis of virulence of flaviviruses can provide valuable information for live-attenuated vaccine development.

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.

Origin and evolution of Japanese encephalitis virus in southeast Asia

Since it emerged in Japan in the 1870s, Japanese encephalitis has spread across Asia and has become the most important cause of epidemic encephalitis worldwide. Four genotypes of Japanese encephalitis virus (JEV) are presently recognized (representatives of genotypes I to III have been fully sequenced), but its origin is not known. We have determined the complete nucleotide and amino acid sequence of a genotype IV Indonesian isolate (JKT6468) which represents the oldest lineage, compared it with other fully sequenced genomes, and examined the geographical distribution of all known isolates. JKT6468 was the least similar, with nucleotide divergence ranging from 17.4 to 19.6% and amino acid divergence ranging from 4.7 to 6.5%. It included an unusual series of amino acids at the carboxy terminus of the core protein unlike that seen in other JEV strains. Three signature amino acids in the envelope protein (including E327 Leu-->Thr/Ser on the exposed lateral surface of the putative receptor binding domain) distinguished genotype IV strains from more recent genotypes. Analysis of all 290 JEV isolates for which sequence data are available showed that the Indonesia-Malaysia region has all genotypes of JEV circulating, whereas only more recent genotypes circulate in other areas (P < 0.0001). These results suggest that JEV originated from its ancestral virus in the Indonesia-Malaysia region and evolved there into the different genotypes which then spread across Asia. Our data, together with recent evidence on the origins of other emerging viruses, including dengue virus and Nipah virus, imply that tropical southeast Asia may be an important zone for emerging pathogens.

Japanese encephalitis vaccines: current vaccines and future prospects

Vaccination against JE ideally should be practiced in all areas of Asia where the virus is responsible for human disease. The WHO has placed a high priority on the development of a new vaccine for prevention of JE. Some countries in Asia (Japan, South Korea, North Korea, Taiwan, Vietnam, Thailand, and the PRC) manufacture JE vaccines and practice childhood immunization, while other countries suffering endemic or epidemic disease (India, Nepal, Laos, Cambodia, Bangladesh, Myanmar, Malaysia, Indonesia and the Philippines) have no JE vaccine manufacturing or policy for use. With the exception of the PRC, all countries practicing JE vaccination use formalin inactivated mouse brain vaccines, which are relatively expensive and are associated with rare but clinically significant allergic and neurological adverse events. New inactivated JE vaccines manufactured in Vero cells are in advanced preclinical or early clinical development in Japan, South Korea, Taiwan, and the PRC. An empirically derived, live attenuated vaccine (SA14-14-2) is widely used in the PRC. Trials in the PRC have shown SA14-14-2 to be safe and effective when administered in a two-dose regimen, but regulatory concerns over manufacturing and control have restricted international distribution. The genetic basis of attenuation of SA14-14-2 has been partially defined. A new live attenuated vaccine (ChimeriVax-JE) that uses a reliable flavivirus vaccine--yellow fever 17D--as a live vector for the envelope genes of SA14-14-2 virus is in early clinical trials and appears to be well tolerated and immunogenic after a single dose. Vaccinia and avipox vectored vaccines have also been tested clinically, but are no longer being pursued due to restricted effectiveness mediated by anti-vector immunity. Other approaches to JE vaccines--including naked DNA, oral vaccination, and recombinant subunit vaccines--have been reviewed.

Single mutation in the flavivirus envelope protein hinge region increases neurovirulence for mice and monkeys but decreases viscerotropism for monkeys: relevance to development and safety testing of live, attenuated vaccines

A chimeric yellow fever (YF) virus/Japanese encephalitis (JE) virus vaccine (ChimeriVax-JE) was constructed by insertion of the prM-E genes from the attenuated JE virus SA14-14-2 vaccine strain into a full-length cDNA clone of YF 17D virus. Passage in fetal rhesus lung (FRhL) cells led to the emergence of a small-plaque virus containing a single Met-->Lys amino acid mutation at E279, reverting this residue from the SA14-14-2 to the wild-type amino acid. A similar virus was also constructed by site-directed mutagenesis (J. Arroyo, F. Guirakhoo, S. Fenner, Z.-X. Zhang, T. P. Monath, and T. J. Chambers, J. Virol. 75:934-942, 2001). The E279 mutation is located in a beta-sheet in the hinge region of the E protein that is responsible for a pH-dependent conformational change during virus penetration from the endosome into the cytoplasm of the infected cell. In independent transfection-passage studies with FRhL or Vero cells, mutations appeared most frequently in hinge 4 (bounded by amino acids E266 to E284), reflecting genomic instability in this functionally important region. The E279 reversion caused a significant increase in neurovirulence as determined by the 50% lethal dose and survival distribution in suckling mice and by histopathology in rhesus monkeys. Based on sensitivity and comparability of results with those for monkeys, the suckling mouse is an appropriate host for safety testing of flavivirus vaccine candidates for neurotropism. After intracerebral inoculation, the E279 Lys virus was restricted with respect to extraneural replication in monkeys, as viremia and antibody levels (markers of viscerotropism) were significantly reduced compared to those for the E279 Met virus. These results are consistent with the observation that empirically derived vaccines developed by mouse brain passage of dengue and YF viruses have increased neurovirulence for mice but reduced viscerotropism for humans.

Amplification and cloning of the full-length genome of Japanese encephalitis virus by a novel long RT-PCR protocol in a cosmid vector

A novel and rapid full-length long RT-PCR technique was established to produce genome-length cDNA from Japanese encephalitis virus. In vitro positive strand RNA transcripts from the full-length RT-PCR amplicon including T7 promoter sequences at the 5' end were proved to be infectious upon transfection. The full-length amplicon without the T7 promoter was cloned into a cosmid vector under the SP6 promoter. This stable clone, designated as pJEV-1, was characterised further and used as a genetic resource for generation of infectious RNA transcripts, gene manipulation and expression. The 'run-off' transcript from pJEV-1 with vector sequences at the either end of the insert was not infectious, but transcripts of the full-length PCR amplicon from pJEV-1 produced infectious virus upon transfection. A transcript with an engineered Xho I site from two ligated PCR fragments amplified from pJEV-1 was also infectious. Furthermore, the coding region for premembrane and envelope proteins (preM-E) from pJEV-1 was subcloned and expressed in the Drosophila Expression System. The expressed protein showed correct molecular size and was immunoreactive with a Japanese encephalitis virus E protein-specific antibody. The derivation of genome-size cDNA from Japanese encephalitis virus and the stable clone will facilitate investigation of this virus and elucidation of its pathogenesis at the molecular level.