Japanese encephalitis vaccines: current vaccines and future prospects

The roles of XJ13 and XJ44-specific mutations within the Candid #1 GPC in Junin virus attenuation

Junin virus (JUNV) is a member of the Arenaviridae family of viruses and is the pathogen responsible for causing Argentine hemorrhagic fever, a potentially lethal disease endemic to Argentina. A live attenuated vaccine for human use, called Candid#1, is approved only in Argentina. Candid#1 vaccine strain of Junin virus was obtained through serial passage in mouse brain tissues followed by passage in Fetal Rhesus macaque lung fibroblast (FRhL) cells. Previously, the mutations responsible for attenuation of this virus in Guinea pigs were mapped in the gene encoding for glycoprotein precursor (GPC) protein. The resulting Candid#1 glycoprotein complex has been shown to cause endoplasmic reticulum (ER) stress in vitro resulting in the degradation of the GPC. To evaluate the attenuating properties of specific mutations within GPC, we created recombinant viruses expressing GPC mutations specific to key Candid#1 passages and evaluated their pathogenicity in our outbred Hartley guinea pig model of Argentine hemorrhagic fever. Here, we provide evidence that early mutations in GPC obtained through serial passaging attenuate the visceral disease and increase immunogenicity in guinea pigs. Specific mutations acquired prior to the 13th mouse brain passage (XJ13) are responsible for attenuation of the visceral disease while having no impact on the neurovirulence of Junin virus. Additionally, our findings demonstrate that the mutation within an N-linked glycosylation motif, acquired prior to the 44th mouse brain passage (XJ44), is unstable but necessary for complete attenuation and enhanced immunogenicity of Candid#1 vaccine strain. The highly conserved N-linked glycosylation profiles of arenavirus glycoproteins could therefore be viable targets for designing attenuating viruses for vaccine development against other arenavirus-associated illnesses.

Epidemiological investigation of swine Japanese encephalitis virus based on RT-RAA detection method

JEV is one of the zoonotic pathogens that cause serious diseases in humans. JEV infection can cause abortion, mummified foetus and stillbirth in sows, orchitis and semen quality decline in boars, causing huge economic losses to pig industry. In order to investigate the epidemiology of JEV in pigs in Sichuan province, a rapid and efficient fluorescent Reverse transcription recombinase-aided amplification (RT-RAA) detection method was established. Aborted fetuses and testicular swollen boar samples were detected by RT-RAA in pigs in the mountain areas around Sichuan Basin, and the detection rate of JEV was 6.49%. The positive samples were identified as JEV GI strain and GIIIstrain by sequencing analysis. We analyzed the whole gene sequence of a positive sample for the GI virus. The Envelope Protein (E protein) phylogenetic tree analysis was far related to the Chinese vaccine strain SA14-14-2, and was most closely related to the JEV GI strains SH17M-07 and SD0810 isolated from China. The results showed that we established an efficient, accurate and sensitive method for clinical detection of JEV, and JEV GI strains were prevalent in Sichuan area. It provides reference for the prevention and control of JEV in Sichuan.

Re-Examining the Importance of Pigs in the Transmission of Japanese Encephalitis Virus

Japanese encephalitis virus (JEV), a mosquito-borne flavivirus, is the leading cause of pediatric encephalitis in Southeast Asia. The enzootic transmission of JEV involves two types of amplifying hosts, swine and avian species. The involvement of pigs in the transmission cycle makes JEV a unique pathogen because human Japanese encephalitis cases are frequently linked to the epizootic spillover from pigs, which can not only develop viremia to sustain transmission but also signs of neurotropic and reproductive disease. The existing knowledge of the epidemiology of JEV largely suggests that viremic pigs are a source of infectious viruses for competent mosquito species, especially Culex tritaeniorhynchus in the endemic regions. However, several recently published studies that applied molecular detection techniques to the characterization of JEV pathogenesis in pigs described the shedding of JEV through multiple routes and persistent infection, both of which have not been reported in the past. These findings warrant a re-examination of the role that pigs are playing in the transmission and maintenance of JEV. In this review, we summarize discoveries on the shedding of JEV during the course of infection and analyze the available published evidence to discuss the possible role of the vector-free JEV transmission route among pigs in viral maintenance.

Effectiveness of Live-Attenuated Genotype III Japanese Encephalitis Viral Vaccine against Circulating Genotype I Viruses in Swine

Expansion of genotype I (GI) Japanese encephalitis viruses (JEV) has resulted in the replacement of the dominant genotype III (GIII) viruses, raising serious public health concerns for using GIII virus-derived vaccines to effectively control JEV epidemics. Therefore, this study used swine as the model to estimate the effectiveness of GIII live-attenuated vaccine against GI virus infection by comparing the incidence of stillbirth/abortion in gilts from vaccinated and non-vaccinated pig farms during the GI-circulation period. In total, 389 and 213 litters of gilts were recorded from four vaccinated and two non-vaccinated pig farms, respectively. All viruses detected in the aborted fetuses and mosquitoes belonged to the GI genotype during the study period. We thus estimated that the vaccine effectiveness of GIII live-attenuated vaccine against GI viruses in naive gilts based on the overall incidence of stillbirth/abortion and incidence of JEV-confirmed stillbirth/abortion was 65.5% (50.8–75.7%) and 74.7% (34.5–90.2%), respectively. In contrast to previous estimates, the GIII live-attenuated vaccine had an efficacy of 95.6% (68.3–99.4%) to prevent the incidence of stillbirth/abortion during the GIII-circulating period. These results indicate that the vaccine effectiveness of GIII live-attenuated JEV vaccine to prevent stillbirth/abortion caused by GI viruses is lower than that against GIII viruses.

Immune responses and protective effects against Japanese encephalitis induced by a DNA vaccine encoding the prM/E proteins of the attenuated SA14-14-2 strain

Japanese encephalitis virus (JEV) is the causal pathogen of Japanese encephalitis (JE), which has become a severe public health problem and is one of the most rapidly spreading mosquito-borne diseases worldwide. Currently, there is no specific treatment for JEV. A vaccine would be an effective measure for reducing morbidity and mortality. Although the live attenuated vaccine SA14-14-2 has been approved in some countries, it is still necessary to develop safer, more effective, and less costly vaccines. In this study, a DNA vaccine candidate, pV-SA14ME, expressing the prM/E proteins of SA14-14-2 was inoculated into BALB/c mice via intramuscular electroporation, and the immunogenicity and degree of protection were evaluated. We found that administration of 50 μg pV-SA14ME via electroporation via three immunizations could induce persistent humoral and cellular immune responses and effectively protect mice against lethal JEV challenge. This study provides a basis for the subsequent promotion and use of the vaccine and lays the foundation for its further use in swine and humans.

Recent Advances in Zika Virus Vaccines

The recent outbreaks of Zika virus (ZIKV) infections and associated microcephaly in newborns has resulted in an unprecedented effort by researchers to target this virus. Significant advances have been made in developing vaccine candidates, treatment strategies and diagnostic assays in a relatively short period of time. Being a preventable disease, the first line of defense against ZIKV would be to vaccinate the highly susceptible target population, especially pregnant women. Along those lines, several vaccine candidates including purified inactivated virus (PIV), live attenuated virus (LAV), virus like particles (VLP), DNA, modified RNA, viral vectors and subunit vaccines have been in the pipeline with several advancing to clinical trials. As the primary objective of Zika vaccination is the prevention of vertical transmission of the virus to the unborn fetus, the safety and efficacy requirements for this vaccine remain unique when compared to other diseases. This review will discuss these recent advances in the field of Zika vaccine development.

North American domestic pigs are susceptible to experimental infection with Japanese encephalitis virus

Japanese encephalitis virus (JEV) is a mosquito-borne flavivirus that is capable of causing encephalitic diseases in children. While humans can succumb to severe disease, the transmission cycle is maintained by viremic birds and pigs in endemic regions. Although JEV is regarded as a significant threat to the United States (U.S.), the susceptibility of domestic swine to JEV infection has not been evaluated. In this study, domestic pigs from North America were intravenously challenged with JEV to characterize the pathological outcomes. Systemic infection followed by the development of neutralizing antibodies were observed in all challenged animals. While most clinical signs were limited to nonspecific symptoms, virus dissemination and neuroinvasion was observed at the acute phase of infection. Detection of infectious viruses in nasal secretions suggest infected animals are likely to promote the vector-free transmission of JEV. Viral RNA present in tonsils at 28 days post infection demonstrates the likelihood of persistent infection. In summary, our findings indicate that domestic pigs can potentially become amplification hosts in the event of an introduction of JEV into the U.S. Vector-free transmission to immunologically naïve vertebrate hosts is also likely through nasal shedding of infectious viruses.

Synthesis of p-methoxyphenyl sulfated β-GalNAc derivatives with inhibitory activity against Japanese encephalitis virus

The N-acetylgalactosamine (GalNAc) residue is one of the units of chondroitin sulfate E (CS-E) which has been reported to have inhibitory activity against Japanese encephalitis virus (JEV). Herein, we describe the synthesis of a series of p-methoxyphenyl β-GalNAc derivatives with a sulfate group at 3-, 4-, and/or 6-positions using an efficient route through a common synthetic intermediate. By measuring the inhibition activity of these compounds that bear different numbers and positions of sulfate groups, the effect of position specificity for interaction with the virus was determined. From these results, GalNAc6S and GalNAc4S6S derivatives inhibited JEV infections well; we suggest the 6-O-sulfate group is necessary for selective recognition by the virus.

Formalin Inactivation of Japanese Encephalitis Virus Vaccine Alters the Antigenicity and Immunogenicity of a Neutralization Epitope in Envelope Protein Domain III

Formalin-inactivated Japanese encephalitis virus (JEV) vaccines are widely available, but the effects of formalin inactivation on the antigenic structure of JEV and the profile of antibodies elicited after vaccination are not well understood. We used a panel of monoclonal antibodies (MAbs) to map the antigenic structure of live JEV virus, untreated control virus (UCV), formalin-inactivated commercial vaccine (FICV), and formalin-inactivated virus (FIV). The binding activity of T16 MAb against Nakayama-derived FICV and several strains of FIV was significantly lower compared to live virus and UCV. T16 MAb, a weakly neutralizing JEV serocomplex antibody, was found to inhibit JEV infection at the post-attachment step. The T16 epitope was mapped to amino acids 329, 331, and 389 within domain III (EDIII) of the envelope (E) glycoprotein. When we explored the effect of formalin inactivation on the immunogenicity of JEV, we found that Nakayama-derived FICV, FIV, and UCV all exhibited similar immunogenicity in a mouse model, inducing anti-JEV and anti-EDII 101/106/107 epitope-specific antibodies. However, the EDIII 329/331/389 epitope-specific IgG antibody and neutralizing antibody titers were significantly lower for FICV-immunized and FIV-immunized mouse serum than for UCV-immunized. Formalin inactivation seems to alter the antigenic structure of the E protein, which may reduce the potency of commercially available JEV vaccines. Virus inactivation by H₂O₂, but not by UV or by short-duration and higher temperature formalin treatment, is able to maintain the antigenic structure of the JEV E protein. Thus, an alternative inactivation method, such as H₂O₂, which is able to maintain the integrity of the E protein may be essential to improving the potency of inactivated JEV vaccines.

We demonstrated that formalin inactivation of Japanese encephalitis virus (JEV) alters the antigenic structure of the JEV envelope glycoprotein (E), in particular an epitope in domain III, and that this reduces the ability of the inactivated vaccine to elicit protective neutralizing antibodies. Ours and others’ previous studies have highlighted the importance of improving the immunogenicity of genotype III (GIII)-derived JEV vaccine in order to provide cross-protection against genotype I (GI) viruses, which are emerging and replacing GIII viruses in many JEV-endemic regions. Encouraging the wide use of live-attenuated or chimeric vaccines, such as SA14-14-2 or yellow-fever 17D/JEV vaccines, respectively, developing GI virus-derived inactivated or premembrane/E–containing, noninfectious virus-like particle (VLP) vaccines are two other possible ways to address this potential problem. In this exploratory study, we highlight an alternative inactivation method, such as H₂O₂ treatment, which may improve the antigenic stability and immunogenicity of JEV.

The Potential Use of Wolbachia-Based Mosquito Biocontrol Strategies for Japanese Encephalitis

Japanese encephalitis virus (JEV) is a zoonotic pathogen transmitted by the infectious bite of Culex mosquitoes. The virus causes the development of the disease Japanese encephalitis (JE) in a small proportion of those infected, predominantly affecting children in eastern and southern Asia. Annual JE incidence estimates range from 50,000–175,000, with 25%–30% of cases resulting in mortality. It is estimated that 3 billion people live in countries in which JEV is endemic. The virus exists in an enzootic transmission cycle, with mosquitoes transmitting JEV between birds as reservoir hosts and pigs as amplifying hosts. Zoonotic infection occurs as a result of spillover events from the main transmission cycle. The reservoir avian hosts include cattle egrets, pond herons, and other species of water birds belonging to the family Ardeidae. Irrigated rice fields provide an ideal breeding ground for mosquitoes and attract migratory birds, maintaining the transmission of JEV. Although multiple vaccines have been developed for JEV, they are expensive and require multiple doses to maintain efficacy and immunity. As humans are a “dead-end” host for the virus, vaccination of the human population is unlikely to result in eradication. Therefore, vector control of the principal mosquito vector, Culex tritaeniorhynchus, represents a more promising strategy for reducing transmission. Current vector control strategies include intermittent irrigation of rice fields and space spraying of insecticides during outbreaks. However, Cx. Tritaeniorhynchus is subject to heavy exposure to pesticides in rice fields, and as a result, insecticide resistance has developed. In recent years, significant advancements have been made in the potential use of the bacterial endosymbiont Wolbachia for mosquito biocontrol. The successful transinfection of Wolbachia strains from Drosophila flies to Aedes (Stegomyia) mosquitoes has resulted in the generation of “dengue-refractory” mosquito lines. The successful establishment of Wolbachia in wild Aedes aegypti populations has recently been demonstrated, and open releases in dengue-endemic countries are ongoing. This review outlines the current control methods for JEV in addition to highlighting the potential use of Wolbachia-based biocontrol strategies to impact transmission. JEV and dengue virus are both members of the Flavivirus genus, and the successful establishment of Drosophila Wolbachia strains in Cx. Tritaeniorhynchus, as the principal vector of JEV, is predicted to significantly impact JEV transmission.

Profiling of viral proteins expressed from the genomic RNA of Japanese encephalitis virus using a panel of 15 region-specific polyclonal rabbit antisera: implications for viral gene expression

Japanese encephalitis virus (JEV), a mosquito-borne flavivirus, is closely related to West Nile (WN), yellow fever (YF), and dengue (DEN) viruses. Its plus-strand genomic RNA carries a single open reading frame encoding a polyprotein that is cleaved into three structural (C, prM/M, and E) and at least seven nonstructural (NS1/NS1', NS2A, NS2B, NS3, NS4A, NS4B, and NS5) proteins, based on previous work with WNV, YFV, and DENV. Here, we aimed to profile experimentally all the viral proteins found in JEV-infected cells. We generated a collection of 15 JEV-specific polyclonal antisera covering all parts of the viral protein-coding regions, by immunizing rabbits with 14 bacterially expressed glutathione-S-transferase fusion proteins (for all nine viral proteins except NS2B) or with a chemically synthesized oligopeptide (for NS2B). In total lysates of JEV-infected BHK-21 cells, immunoblotting with these antisera revealed: (i) three mature structural proteins (~12-kDa C, ~8-kDa M, and ~53-kDa E), a precursor of M (~24-kDa prM) and three other M-related proteins (~10-14 kDa); (ii) the predicted ~45-kDa NS1 and its frameshift product, ~58-kDa NS1', with no evidence of the predicted ~25-kDa NS2A; (iii) the predicted but hardly detectable ~14-kDa NS2B and an unexpected but predominant ~12-kDa NS2B-related protein; (iv) the predicted ~69-kDa NS3 plus two major cleavage products (~34-kDa NS3N-term and ~35-kDa NS3C-term), together with at least nine minor proteins of ~16-52 kDa; (v) the predicted ~14-kDa NS4A; (vi) two NS4B-related proteins (~27-kDa NS4B and ~25-kDa NS4B'); and (vii) the predicted ~103-kDa NS5 plus at least three other NS5-related proteins (~15 kDa, ~27 kDa, and ~90 kDa). Combining these data with confocal microscopic imaging of the proteins' intracellular localization, our study is the first to provide a solid foundation for the study of JEV gene expression, which is crucial for elucidating the regulatory mechanisms of JEV genome replication and pathobiology.

Safety and immunogenicity of a delta inulin-adjuvanted inactivated Japanese encephalitis virus vaccine in pregnant mares and foals

In 2011, following severe flooding in Eastern Australia, an unprecedented epidemic of equine encephalitis occurred in South-Eastern Australia, caused by Murray Valley encephalitis virus (MVEV) and a new variant strain of Kunjin virus, a subtype of West Nile virus (WNV_KUN). This prompted us to assess whether a delta inulin-adjuvanted, inactivated cell culture-derived Japanese encephalitis virus (JEV) vaccine (JE-ADVAX™) could be used in horses, including pregnant mares and foals, to not only induce immunity to JEV, but also elicit cross-protective antibodies against MVEV and WNV_KUN. Foals, 74–152 days old, received two injections of JE-ADVAX™. The vaccine was safe and well-tolerated and induced a strong JEV-neutralizing antibody response in all foals. MVEV and WNV_KUN antibody cross-reactivity was seen in 33% and 42% of the immunized foals, respectively. JE-ADVAX™ was also safe and well-tolerated in pregnant mares and induced high JEV-neutralizing titers. The neutralizing activity was passively transferred to their foals via colostrum. Foals that acquired passive immunity to JEV via maternal antibodies then were immunized with JE-ADVAX™ at 36–83 days of age, showed evidence of maternal antibody interference with low peak antibody titers post-immunization when compared to immunized foals of JEV-naïve dams. Nevertheless, when given a single JE-ADVAX™ booster immunization as yearlings, these animals developed a rapid and robust JEV-neutralizing antibody response, indicating that they were successfully primed to JEV when immunized as foals, despite the presence of maternal antibodies. Overall, JE-ADVAX™ appears safe and well-tolerated in pregnant mares and young foals and induces protective levels of JEV neutralizing antibodies with partial cross-neutralization of MVEV and WNV_KUN.

Comparison of genotypes I and III in Japanese encephalitis virus reveals distinct differences in their genetic and host diversity

Japanese encephalitis (JE) is an arthropod-borne disease associated with the majority of viral encephalitis cases in the Asia-Pacific region. The causative agent, Japanese encephalitis virus (JEV), has been phylogenetically divided into five genotypes. Recent surveillance data indicate that genotype I (GI) is gradually replacing genotype III (GIII) as the dominant genotype. To investigate the mechanism behind the genotype shift and the potential consequences in terms of vaccine efficacy, human cases, and virus dissemination, we collected (i) all full-length and partial JEV molecular sequences and (ii) associated genotype and host information comprising a data set of 873 sequences. We then examined differences between the two genotypes at the genetic and epidemiological level by investigating amino acid mutations, positive selection, and host range. We found that although GI is dominant, it has fewer sites predicted to be under positive selection, a narrower host range, and significantly fewer human isolates. For the E protein, the sites under positive selection define a haplotype set for each genotype that shows striking differences in their composition and diversity, with GIII showing significantly more variety than GI. Our results suggest that GI has displaced GIII by achieving a replication cycle that is more efficient but is also more restricted in its host range.

IMPORTANCE

Japanese encephalitis is an arthropod-borne disease associated with the majority of viral encephalitis cases in the Asia-Pacific region. The causative agent, Japanese encephalitis virus (JEV), has been divided into five genotypes based on sequence similarity. Recent data indicate that genotype I (GI) is gradually replacing genotype III (GIII) as the dominant genotype. Understanding the reasons behind this shift and the potential consequences in terms of vaccine efficacy, human cases, and virus dissemination is important for controlling the spread of the virus and reducing human fatalities. We collected all available full-length and partial JEV molecular sequences and associated genotype and host information. We then examined differences between the two genotypes at the genetic and epidemiological levels by investigating amino acid mutations, positive selection, and host range. Our results suggest that GI has displaced GIII by achieving a replication cycle that is more efficient but more restricted in host range.

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.

A comprehensive analysis of predicted HLA binding peptides of JE viral proteins specific to north Indian isolates

Japanese encephalitis (JE), a viral disease has significantly increased worldwide especially, in the developing region due to challenges in immunization, vector control and lack of appropriate treatment methods. An effective, yet an expensive heat-killed vaccine is available for the disease. Therefore, the design and development of short peptide vaccine candidate is promising. We used immune-informatics methods to perform a comprehensive analysis of the entire JEV proteome of north Indian isolate to identify the conserved peptides binding known specific HLA alleles among the documented JEV genotypes 1, 2, 3, 4 and 5. The prediction analysis identified 102 class I (using propred I) and 118 class II (using propred) binding peptides at 4% threshold value. These predicted HLA allele binding peptides were further analyzed for potential conserved region using IEDB (an immune epitope database and analysis resource). This analysis shows that 78.81% of class II (in genotype 2) and 76.47% of HLA I (in genotype 3) bound peptides are conserved. The peptides IPIVSVASL, KGAQRLAAL, LAVFLICVL and FRTLFGGMS, VFLICVLTV, are top ranking with potential super antigenic property by binding to all HLA allele members of B7 and DR4 super-types, respectively. This data finds application in the design and development of short peptide vaccine candidates and diagnostic agents for JE following adequate validation and verification.

Japanese encephalitis: the virus and vaccines

Japanese encephalitis (JE) is an infectious disease of the central nervous system caused by Japanese encephalitis virus (JEV), a zoonotic mosquito-borne flavivirus. JEV is prevalent in much of Asia and the Western Pacific, with over 4 billion people living at risk of infection. In the absence of antiviral intervention, vaccination is the only strategy to develop long-term sustainable protection against JEV infection. Over the past half-century, a mouse brain-derived inactivated vaccine has been used internationally for active immunization. To date, however, JEV is still a clinically important, emerging, and re-emerging human pathogen of global significance. In recent years, production of the mouse brain-derived vaccine has been discontinued, but 3 new cell culture-derived vaccines are available in various parts of the world. Here we review current aspects of JEV biology, summarize the 4 types of JEV vaccine, and discuss the potential of an infectious JEV cDNA technology for future vaccine development.

An inactivated cell culture Japanese encephalitis vaccine (JE-ADVAX) formulated with delta inulin adjuvant provides robust heterologous protection against West Nile encephalitis via cross-protective memory B cells and neutralizing antibody

West Nile virus (WNV), currently the cause of a serious U.S. epidemic, is a mosquito-borne flavivirus and member of the Japanese encephalitis (JE) serocomplex. There is currently no approved human WNV vaccine, and treatment options remain limited, resulting in significant mortality and morbidity from human infection. Given the availability of approved human JE vaccines, this study asked whether the JE-ADVAX vaccine, which contains an inactivated cell culture JE virus antigen formulated with Advax delta inulin adjuvant, could provide heterologous protection against WNV infection in wild-type and β2-microglobulin-deficient (β2m(-/-)) murine models. Mice immunized twice or even once with JE-ADVAX were protected against lethal WNV challenge even when mice had low or absent serum cross-neutralizing WNV titers prior to challenge. Similarly, β2m(-/-) mice immunized with JE-ADVAX were protected against lethal WNV challenge in the absence of CD8(+) T cells and prechallenge WNV antibody titers. Protection against WNV could be adoptively transferred to naive mice by memory B cells from JE-ADVAX-immunized animals. Hence, in addition to increasing serum cross-neutralizing antibody titers, JE-ADVAX induced a memory B-cell population able to provide heterologous protection against WNV challenge. Heterologous protection was reduced when JE vaccine antigen was administered alone without Advax, confirming the importance of the adjuvant to induction of cross-protective immunity. In the absence of an approved human WNV vaccine, JE-ADVAX could provide an alternative approach for control of a major human WNV epidemic.

JE-ADVAX vaccine protection against Japanese encephalitis virus mediated by memory B cells in the absence of CD8(+) T cells and pre-exposure neutralizing antibody

JE-ADVAX is a new, delta inulin-adjuvanted, Japanese encephalitis (JE) candidate vaccine with a strong safety profile and potent immunogenicity that confers efficient immune protection not only against JE virus but also against related neurotropic flaviviruses such as West Nile virus. In this study, we investigated the immunological mechanism of protection by JE-ADVAX vaccine using knockout mice deficient in B cells or CD8(+) T cells and poor persistence of neutralizing antibody or by adoptive transfer of immune splenocyte subpopulations. We show that memory B cells induced by JE-ADVAX provide long-lived protection against JE even in the absence of detectable pre-exposure serum neutralizing antibodies and without the requirement of CD8(+) T cells. Upon virus encounter, these vaccine-induced memory B cells were rapidly triggered to produce neutralizing antibodies that then protected immunized mice from morbidity and mortality. The findings suggest that the extent of the B-cell memory compartment might be a better immunological correlate for clinical efficacy of JE vaccines than the currently recommended measure of serum neutralizing antibody. This may explain the paradox where JE protection is observed in some subjects even in the absence of detectable serum neutralizing antibody. Our investigation also established the suitability of a novel flavivirus challenge model (β(2)-microglobulin-knockout mice) for studies of the role of B-cell memory responses in vaccine protection.

Partially neutralizing potency against emerging genotype I virus among children received formalin-inactivated Japanese encephalitis virus vaccine

Background

Genotype I (GI) Japanese encephalitis virus (JEV) that replaced GIII virus has become the dominant circulating virus in Asia. Currently, all registered live and inactivated JEV vaccines are derived from genotype III viruses. In Taiwan, the compulsory JEV vaccination policy recommends that children receives four doses of formalin-inactivated Nakayama (GIII) JEV vaccine.

Methodology/Principal Findings

To evaluate the influence of genotype replacement on the post-vaccination viral neutralizing ability by GIII and GI viruses, the small panel of vaccinated-children serum specimens was assembled, and the reciprocal 50% plaque-reduction neutralizing antibody titers (PRNT₅₀) were measured against Nakayama vaccine strain, CJN GIII human brain isolate and TC2009-1 GI mosquito isolate. The seropositivity rate (PRNT₅₀≥1∶10) and geometric mean titers (GMT) against the TC2009-1 virus were the lowest among the three viruses. The protective threshold against the CJN and TC2009-1 viruses could only be achieved when the GMT against Nakayama virus was ≥1∶20 or ≥1∶80, respectively. Using undiluted vaccinees' sera, the enhancement of JEV infection in K562 cells was observed in some low or non-neutralizing serum specimens.

Conclusions/Significance

Our preliminary study has shown that neutralizing antibodies, elicited by the mouse brain-derived and formalin-inactivated JEV Nakayama vaccine among a limited number of vaccinees, have reduced neutralizing capacity against circulating GI virus, but more detailed studies are needed to address the potential impact on the future vaccine policy.

Genotype I (GI) Japanese encephalitis virus (JEV) that replaced GIII virus has become the dominant circulating virus in Asia; however, all available JEV vaccines are derived from genotype III viruses, and no study has been conducted on the cross-neutralization and protection elicited by GIII JEV vaccines against GI viruses using vaccinated children’s serum specimens collected from the general population. Genotype I virus was first detected in Taiwan in 2008, and became the dominant circulating JEV, and was island-wide within a year. In the present study, the small panel of GIII virus vaccinated-children serum specimens were not only showed lower strain-specific neutralization against GI virus as compared to the GIII vaccine and human isolates but also observed the enhancement of GI virus infection in K562 cells in some low or non-neutralizing serum specimens. These preliminary results indicated the reduced neutralization potency due to genotype replacement should be closely monitored in the JE epidemic/endemic regions in the future.

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.

Feasibility of cross-protective vaccination against flaviviruses of the Japanese encephalitis serocomplex

Serological cross-reactivity providing cross-protective immunity between antigenically related viruses is a cornerstone of vaccination. It was the immunological basis for the first human vaccine against smallpox introduced more than 200 years ago, and continues to underpin modern vaccine development as has recently been shown for human papillomavirus vaccines, which confer cross-protection against other oncogenic papillomavirus types not present in the vaccine. Here, we review the feasibility of cross-protective vaccination against an antigenic group of clinically important viruses belonging to the Japanese encephalitis serocomplex in the Flaviviridae family. We will discuss evidence suggesting that 'new generation' flavivirus vaccines may provide effective cross-protective immunity against heterologous Japanese encephalitis serocomplex viruses, and appraise potential risks associated with cross-reactive vaccine immunity. The review will also focus on the structural and mechanistic basis for cross-protective immunity among this group of flaviviruses, which is predominantly mediated by antibodies against a single viral surface protein.

Japanese Encephalitis Vaccines

Japanese encephalitis (JE) is a significant human health concern in Asia, Indonesia and parts of Australia with more than 3 billion people potentially at risk of infection with Japanese encephalitis virus (JEV), the causative agent of JE. Given the risk to human health and the theoretical potential for JEV use as a bioweapon, the development of safe and effective vaccines to prevent JEV infection is vital for preserving human health. The development of vaccines for JE began in the 1940s with formalin-inactivated mouse brain-derived vaccines. These vaccines have been shown to induce a protective immune response and to be very effective. Mouse brain-derived vaccines were still in use until May 2011 when the last lots of the BIKEN(®) JE-VAX(®) expired. Development of modern JE vaccines utilizes cell culture-derived viruses and improvements in manufacturing processes as well as removal of potential allergens or toxins have significantly improved vaccine safety. China has developed a live-attenuated vaccine that has proven to induce protective immunity following a single inoculation. In addition, a chimeric vaccine virus incorporating the prM and E structural proteins derived from the live-attenuated JE vaccine into the live-attenuated yellow fever 17D vaccine virus backbone is currently in clinical trials. In this article, we provide a summary of JE vaccine development and on-going clinical trials. We also discuss the potential risk of JEV as a bioweapon with a focus on virus sustainability if used as a weapon.

Pathogenesis of Japanese encephalitis virus infection in a golden hamster model and evaluation of flavivirus cross-protective immunity

Japanese encephalitis virus (JEV) is a mosquito-borne flavivirus endemic to Southeast Asia and surrounding Pacific Islands, and it has most recently emerged in northern Australia. JEV is closely related to West Nile virus (WNV) and St. Louis encephalitis virus (SLEV), both endemic to the United States. In the event that JEV is introduced into the Americas, it will be important to determine whether immunity to WNV or SLEV might afford protection from infection and development of viremia in susceptible hosts. We investigated a hamster model of JEV infection and showed that a large fraction of animals infected with either a genotype I or III isolate of virus developed viremia and encephalitic lesions without clinical signs of disease. Using this model, we showed that prior infection with WNV or SLEV, vaccination using a chimeric WNV vaccine, and passive immunization with anti-JEV immune sera prevented viremia in hamsters challenged with JEV.

IMOJEV(®): a Yellow fever virus-based novel Japanese encephalitis vaccine

Japanese encephalitis (JE) is a disease of the CNS caused by Japanese encephalitis virus (JEV). The disease appears in the form of frequent outbreaks in most south- and southeast Asian countries and the virus has become endemic in several areas. There is no licensed therapy available and disease control by vaccination is considered to be most effective. Mouse brain-derived inactivated JE vaccines, although immunogenic, have several limitations in terms of safety, availability and requirement for multiple doses. Owing to these drawbacks, the WHO called for the development of novel, safe and more efficacious JE vaccines. Several candidate vaccines have been developed and at least three of them that demonstrated strong immunogenicity after one or two doses of the vaccine in animal models were subsequently tested in various clinical trials. One of these vaccines, IMOJEV(®) (JE-CV and previously known as ChimeriVax™-JE), is a novel recombinant chimeric virus vaccine, developed using the Yellow fever virus (YFV) vaccine vector YFV17D, by replacing the cDNA encoding the envelope proteins of YFV with that of an attenuated JEV strain SA14-14-2. IMOJEV was found to be safe, highly immunogenic and capable of inducing long-lasting immunity in both preclinical and clinical trials. Moreover, a single dose of IMOJEV was sufficient to induce protective immunity, which was similar to that induced in adults by three doses of JE-VAX(®), a mouse brain-derived inactivated JE vaccine. Recently, Phase III trials evaluating the immunogenicity and safety of the chimeric virus vaccine have been successfully completed in some JE-endemic countries and the vaccine manufacturers have filed an application for vaccine registration. IMOJEV may thus be licensed for use in humans as an improved alternative to the currently licensed JE vaccines.

The effect of insecticide-treated mosquito nets (ITMNs) on Japanese encephalitis virus seroconversion in pigs and humans

The effect of insecticide-treated mosquito nets (ITMNs) on Japanese Encephalitis (JE) virus seroconversion in pigs and humans was studied in Assam, Northeast India. A sharp reduction of seroconversion rate in human and pig was found in treated localities after intervention. A marked reduction was achieved in humans (risk ratio [RR] = 0.28, 95% confidence interval [CI] = 0.16-0.49) and pigs (RR = 0.21, CI = 0.11-0.40) in the Kollolua locality where ITMNs were used on both humans and pigs compared with the other two area, Athabari and Rajmai, where ITMNs were covering only either humans or pigs. Monitoring of the mosquito population in and around cattle sheds during dusk revealed no significant decline (P > 0.05) of vector density during the post-intervention period in study localities. In spite of the high preponderance of potential JE vector outdoors during the post-intervention period, an encouraging line of defense against circulation of JE virus through the use of ITMNs can be achieved in endemic areas.

Role of amphotericin B upon enhancement of protective immunity elicited by oral administration with liposome-encapsulated-Japanese encephalitis virus nonstructural protein 1 (NS1) in mice

Amphotericin B (AmB) is an antifungal antibiotic the activity of which has been associated with modulation of pro-inflammatory cytokines expression in cultured cells. Herein we reveal that co-administration with AmB enhances the immunogenicity of oral Lip-JENS1 vaccine which derived from liposomes functionalized with DSPC (distearoylphosphatidylcholine) and cholesterol (2:1, molar ratio)-bearing JE virus NS1 protein (600 microg ml(-1)). Oral single dose of Lip-JENS1 elicited a detectable serum NS1-specific IgG antibody response from a mouse model. Remarkably, the addition of AmB (125 microg per mouse), particularly, 2 h prior to, but not simultaneously with, the administration of Lip-JENS1 significantly enhanced the systemic antigen-specific antibody response, providing superior protection against lethal JEV challenges. Further, we observed AmB-induced the transcription of cytokine expression and translocation of transcriptional factor NF-kappaB from the cytoplasm to the nucleus for the murine macrophage J774A.1. Moreover, Peyer's-patch lymphocytes (PPL) from AmB-treated mice produced high levels of IL-1beta, IL-6 and TNF-alpha expression compared to the corresponding control of cells from non-treated mice. Taken together, the results suggest that AmB exerts a profound influence upon mucosal vaccination with Lip-JENS1, possibly playing an adjuvant-augmented role to "fine-tune" humoral as well as cellular immune response, thus conferring enhanced protective immunity for immunising individuals against JE infection.

An inactivated Vero cell-grown Japanese encephalitis vaccine formulated with Advax, a novel inulin-based adjuvant, induces protective neutralizing antibody against homologous and heterologous flaviviruses

Advax is a polysaccharide-based adjuvant that potently stimulates vaccine immunogenicity without the increased reactogenicity seen with other adjuvants. This study investigated the immunogenicity of a novel Advax-adjuvanted Vero cell culture candidate vaccine against Japanese encephalitis virus (JEV) in mice and horses. The results showed that, in mice, a two-immunization, low-dose (50 ng JEV antigen) regimen with adjuvanted vaccine produced solid neutralizing immunity comparable to that elicited with live ChimeriVax-JE immunization and superior to that elicited with tenfold higher doses of a traditional non-adjuvanted JEV vaccine (JE-VAX; Biken Institute) or a newly approved alum-adjuvanted vaccine (Jespect; Novartis). Mice vaccinated with the Advax-adjuvanted, but not the unadjuvanted vaccine, were protected against live JEV challenge. Equine immunizations against JEV with Advax-formulated vaccine similarly showed enhanced vaccine immunogenicity, confirming that the adjuvant effects of Advax are not restricted to rodent models. Advax-adjuvanted JEV vaccine elicited a balanced T-helper 1 (Th1)/Th2 immune response against JEV with protective levels of cross-neutralizing antibody against other viruses belonging to the JEV serocomplex, including Murray Valley encephalitis virus (MVEV). The adjuvanted JEV vaccine was well tolerated with minimal reactogenicity and no systemic toxicity in immunized animals. The cessation of manufacture of traditional mouse brain-derived unadjuvanted JEV vaccine in Japan has resulted in a JEV vaccine shortage internationally. There is also an ongoing lack of human vaccines against other JEV serocomplex flaviviruses, such as MVEV, making this adjuvanted, cell culture-grown JEV vaccine a promising candidate to address both needs with one vaccine.

Preclinical and clinical development of YFV 17D-based chimeric vaccines against dengue, West Nile and Japanese encephalitis viruses

Dengue viruses (DENV), West Nile virus (WNV) and Japanese encephalitis virus (JEV) are major global health and growing medical problems. While a live-attenuated vaccine exists since decades against the prototype flavivirus, yellow fever virus (YFV), there is an urgent need for vaccines against dengue or West Nile diseases, and for improved vaccines against Japanese encephalitis. Live-attenuated chimeric viruses were constructed by replacing the genes coding for Premembrane (prM) and Envelope (E) proteins from YFV 17D vaccine strain with those of heterologous flaviviruses (ChimeriVax technology). This technology has been used to produce vaccine candidates for humans, for construction of a horse vaccine for West Nile fever, and as diagnostic reagents for dengue, Japanese encephalitis, West Nile and St. Louis encephalitis infections. This review focuses on human vaccines and their characterization from the early stages of research through to clinical development. Phenotypic and genetic properties and stability were examined, preclinical evaluation through in vitro or animal models, and clinical testing were carried out. Theoretical environmental concerns linked to the live and genetically modified nature of these vaccines have been carefully addressed. Results of the extensive characterizations are in accordance with the immunogenicity and excellent safety profile of the ChimeriVax-based vaccine candidates, and support their development towards large-scale efficacy trials and registration.

Evaluation of chimeric Japanese encephalitis and dengue viruses for use in diagnostic plaque reduction neutralization tests

The plaque reduction neutralization test (PRNT) is a specific serological test used to identify and confirm arbovirus infection in diagnostic laboratories and monitor immunological protection in vaccine recipients. Wild-type (wt) viruses used in the PRNT may be difficult to grow and plaque titrate, such as the dengue viruses (DENV), and/or may require biosafety level 3 (BSL3) containment, such as West Nile virus (WNV), St. Louis encephalitis virus (SLEV), and Japanese encephalitis virus (JEV). These requirements preclude their use in diagnostic laboratories with only BSL2 capacity. In addition, wt JEV falls under the jurisdiction of the select-agent program and can be used only in approved laboratories. The chimeric vaccine viruses ChimeriVax-WNV and -SLEV have previously been shown to elicit antibody reactivity comparable to that of parental wt WNV and SLEV. ChimeriVax viruses provide advantages for PRNT, as follows: they grow more rapidly than most wt flaviviruses, produce large plaques, require BSL2 conditions, and are not under select-agent restrictions. We evaluated the ChimeriVax-DENV serotype 1 (DENV1), -DENV2, -DENV3, -DENV4, and -JEV for use in PRNT on sera from DENV- and JEV-infected patients and from JEV vaccine recipients. Serostatus agreement was 100% between the ChimeriVax-DENV serotypes and wt prototype DENV and 97% overall with ChimeriVax-JEV compared to prototype Nakayama JEV, 92% in a subgroup of JEV vaccine recipients, and 100% in serum from encephalitis patients naturally infected with JEV. ChimeriVax-DENV and -JEV plaque phenotype and BSL2 requirements, combined with sensitive and specific reactivity, make them good substitutes for wt DENV and JEV in PRNT in public health diagnostic laboratories.

Live chimeric and inactivated Japanese encephalitis virus vaccines differ in their cross-protective values against Murray Valley encephalitis virus

The Japanese encephalitis virus (JEV) serocomplex, which also includes Murray Valley encephalitis virus (MVEV), is a group of antigenically closely related, mosquito-borne flaviviruses that are responsible for severe encephalitic disease in humans. While vaccines against the prominent members of this serocomplex are available or under development, it is unlikely that they will be produced specifically against those viruses which cause less-frequent disease, such as MVEV. Here we have evaluated the cross-protective values of an inactivated JEV vaccine (JE-VAX) and a live chimeric JEV vaccine (ChimeriVax-JE) against MVEV in two mouse models of flaviviral encephalitis. We show that (i) a three-dose vaccination schedule with JE-VAX provides cross-protective immunity, albeit only partial in the more severe challenge model; (ii) a single dose of ChimeriVax-JE gives complete protection in both challenge models; (iii) the cross-protective immunity elicited with ChimeriVax-JE is durable (>or=5 months) and broad (also giving protection against West Nile virus); (iv) humoral and cellular immunities elicited with ChimeriVax-JE contribute to protection against lethal challenge with MVEV; (v) ChimeriVax-JE remains fully attenuated in immunodeficient mice lacking type I and type II interferon responses; and (vi) immunization with JE-VAX, but not ChimeriVax-JE, can prime heterologous infection enhancement in recipients of vaccination on a low-dose schedule, designed to mimic vaccine failure or waning of vaccine-induced immunity. Our results suggest that the live chimeric JEV vaccine will protect against other viruses belonging to the JEV serocomplex, consistent with the observation of cross-protection following live virus infections.

Evaluation of Japanese encephalitis virus DNA vaccine candidates in rhesus monkeys [Macaca mulatta]

We have previously described DNA vaccine candidates against Japanese encephalitis virus (JEV) that were immunogenic in mice. Present study was conducted to evaluate their immunogenicity in rhesus monkeys (Macaca mulatta) and compare it with the commercial mouse brain-derived, formalin-inactivated vaccine. Groups of four monkeys were immunized with either pMEa (expressing the anchored form of the envelope protein along with the pre-membrane protein of JEV) or pMEs (expressing the secretory form of the envelope protein along with pre-membrane protein of JEV) by intra-muscular (IM, using needle) or intra-dermal (ID, using gene gun) routes. Following primary immunization with 1mg plasmid DNA given IM, or 5 microg plasmid DNA given ID, the monkeys were boosted after 1 and 2 months with 0.5mg DNA given IM or 5 microg DNA given ID, and observed for a period of 6 months. After the second booster, most of the monkeys sero-converted and developed JEV neutralizing antibodies, albeit of low titer. Importantly however, following a sham challenge with the mouse brain-derived inactivated JEV vaccine given 6 months after immunization, the neutralizing antibody titers rose rapidly indicating a vigorous anamnestic response. Based on the JEV neutralizing antibody response following the vaccination and the extent of anamnestic response generated in the immunized monkeys, plasmid pMEa was superior to pMEs. This study indicates that the JEV candidate DNA vaccine is capable of generating protective levels of JEV neutralizing antibodies in rhesus monkeys and prime the immune system effectively against a subsequent exposure to JEV.

Humanized monoclonal antibodies derived from chimpanzee Fabs protect against Japanese encephalitis virus in vitro and in vivo

Japanese encephalitis virus (JEV)-specific Fab antibodies were recovered by repertoire cloning from chimpanzees initially immunized with inactivated JE-VAX and then boosted with attenuated JEV SA14-14-2. From a panel of 11 Fabs recovered by different panning strategies, three highly potent neutralizing antibodies, termed Fabs A3, B2, and E3, which recognized spatially separated regions on the virion, were identified. These antibodies reacted with epitopes in different domains: the major determinant for Fab A3 was Lys(179) (domain I), that for Fab B2 was Ile(126) (domain II), and that for Fab E3 was Gly(302) (domain III) in the envelope protein, suggesting that these antibodies neutralize the virus by different mechanisms. Potent neutralizing antibodies reacted with a low number of binding sites available on the virion. These three Fabs and derived humanized monoclonal antibodies (MAbs) exhibited high neutralizing activities against a broad spectrum of JEV genotype strains. Demonstration of antibody-mediated protection of JEV infection in vivo is provided using the mouse encephalitis model. MAb B2 was most potent, with a 50% protective dose (ED(50)) of 0.84 microg, followed by MAb A3 (ED(50) of 5.8 microg) and then MAb E3 (ED(50) of 24.7 microg) for a 4-week-old mouse. Administration of 200 microg/mouse of MAb B2 1 day after otherwise lethal JEV infection protected 50% of mice and significantly prolonged the average survival time compared to that of mice in the unprotected group, suggesting a therapeutic potential for use of MAb B2 in humans.

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.

Vaccines for preventing Japanese encephalitis

BACKGROUND

Vaccination is recognized as the only practical measure for preventing Japanese encephalitis. Production shortage, costs, and issues of licensure impair vaccination programmes in many affected countries. Concerns over vaccine effectiveness and safety also have a negative impact on acceptance and uptake.

OBJECTIVES

To evaluate vaccines for preventing Japanese encephalitis in terms of effectiveness, adverse events, and immunogenicity.

SEARCH STRATEGY

In March 2007, we searched the Cochrane Infectious Diseases Group Specialized Register, CENTRAL (The Cochrane Library 2007, Issue 1), MEDLINE, EMBASE, LILACS, BIOSIS, and reference lists. We also attempted to contact corresponding authors and vaccine companies.

SELECTION CRITERIA

Randomized controlled trials (RCTs), including cluster-RCTs, comparing Japanese encephalitis vaccines with placebo (inert agent or unrelated vaccine), no intervention, or alternative Japanese encephalitis vaccine.

DATA COLLECTION AND ANALYSIS

Authors independently extracted data and assessed methodological quality. Dichotomous data were compared with relative risks and a 95% confidence interval (CI), and converted into percentage vaccine efficacy.

MAIN RESULTS

Eight RCTs involving 358,750 participants were included. These trials investigated two available and three pre-licensure vaccines. Two RCTs assessing efficacy of the commercially available inactivated Nakayama vaccine were identified. A two-dose schedule of the licensed vaccine provided significant protection of 95% (95% CI 10% to 100%) for one year only, while two doses of an unpurified precursor vaccine protected children by 81% (95% CI 45% to 94%) in year one and by 59% (95% CI 2% to 83%) in year two. Serious adverse events were not observed. Mild and moderate episodes of injection site soreness, fever, headache, and nausea were reported in less than 6% of children receiving inactivated vaccine compared to 0.6% of unvaccinated controls. One cluster-RCT compared the live-attenuated SA14-14-2 vaccine (widely used in China) with no intervention measuring adverse events. Fever was reported in 2.7% of vaccinees compared to 3.1% of controls, while 0.1% of both groups suffered diarrhoea or seizures. Four small pre-licensure RCTs assessing a genetically engineered vaccine and two cell culture-derived inactivated vaccines revealed high immunogenicity and relative safety.

AUTHORS' CONCLUSIONS

Only one of the three currently used vaccines has been assessed for efficacy in a RCT. Other RCTs have assessed their safety, however, and they appear to cause only occasional mild or moderate adverse events. Further trials of effectiveness and safety are needed for the currently used vaccines, especially concerning dose levels and schedules. Trials investigating several new vaccines are planned or in progress.

A hospital-based surveillance for Japanese encephalitis in Bali, Indonesia

BACKGROUND

Japanese encephalitis (JE) is presumed to be endemic throughout Asia, yet only a few cases have been reported in tropical Asian countries such as Indonesia, Malaysia and the Philippines. To estimate the true disease burden due to JE in this region, we conducted a prospective, hospital-based surveillance with a catchment population of 599,120 children less than 12 years of age in Bali, Indonesia, from July 2001 through December 2003.

METHODS

Balinese children presenting to any health care facility with acute viral encephalitis or aseptic meningitis were enrolled. A "confirmed" diagnosis of JE required the detection of JE virus (JEV)-specific IgM in cerebrospinal fluid, whereas a diagnosis of "probable JE" was assigned to those cases in which JEV-specific IgM was detected only in serum.

RESULTS

In all, 86 confirmed and 4 probable JE cases were identified. The annualized JE incidence rate was 7.1 and adjusted to 8.2 per 100,000 for children less than 10 years of age over the 2.5 consecutive years of study. Only one JE case was found among 96,920 children 10-11 years old (0.4 per 100,000). Nine children (10%) died and 33 (37%) of the survivors had neurological sequelae at discharge. JEV was transmitted in Bali year-round with 70% of cases in the rainy season.

CONCLUSION

JE incidence and case-fatality rates in Bali were comparable to those of other JE-endemic countries of Asia. Our findings contradict the common wisdom that JE is rare in tropical Asia. Hence, the geographical range of endemic JE is broader than previously described. The results of the study support the need to introduce JE vaccination into Bali.

NEUROLOGICAL INFECTIONS IN THE RETURNING INTERNATIONAL TRAVELER

Clinicians may encounter international travelers returning with exotic infections, emerging infectious diseases, or resurgent old-world infections. Many of these infectious diseases can affect the nervous system directly or indirectly. The contemporary neurologist should therefore be cognizant of the clinical manifestations, potential complications, and appropriate management of common travel-related infections. This chapter focuses on five important infections that affect the central nervous system and that may be encountered in returning travelers: Japanese encephalitis, malaria, rabies, dengue, and neurocysticercosis. The clinical manifestations, suggested evaluation, and treatment are discussed for each infection.

West Nile virus vaccines

West Nile virus (WNV) is a mosquito-borne flavivirus that is emerging as a global pathogen. In the last decade, virulent strains of the virus have been associated with significant outbreaks of human and animal disease in Europe, the Middle East and North America. Efforts to develop human and veterinary vaccines have taken both traditional and novel approaches. A formalin-inactivated whole virus vaccine has been approved for use in horses. DNA vaccines coding for the structural WNV proteins have also been assessed for veterinary use and have been found to be protective in mice, horses and birds. Live attenuated yellow fever WNV chimeric vaccines have also been successful in animals and are currently undergoing human trials. Additional studies have shown that immunisation with a relatively benign Australian variant of WNV, the Kunjin virus, also provides protective immunity against the virulent North American strain. Levels of efficacy and safety, as well as logistical, economic and environmental issues, must all be carefully considered before vaccine candidates are approved and selected for large-scale manufacture and distribution.

Identification of mimotopes of the Japanese encephalitis virus envelope protein using phage-displayed combinatorial peptide library

The phage-displayed combinatorial peptide library is a revolutionary method for discovering epitopes, in particular conformational epitopes. In this study, we characterized a Japanese encephalitis virus (JEV) conformational epitope by biopanning of phage-displayed random peptide libraries with a JEV envelope (E) protein-specific monoclonal antibody (mAb) 2H2. Eleven identified phage clones with high affinity to mAb 2H2 were identified using direct and inhibitory binding ELISA. Sequence alignment, structure modeling and mutational analysis revealed that the identified mimotopes for mAb 2H2 possess a conserved motif X(1)(D/E)(Y/T/S)X(2), fitting into a region at the domain III lateral surface of the E protein. The results of our study could provide useful information on the development of effective mimotope-based vaccines and diagnostic kits for the JEV infection.

Allergic reactions to Japanese encephalitis vaccine

The JEV widely is used in Asian countries each year and is an important vaccine for travelers to the East from other parts of the world. JE virus is a zoonotic disease with natural reservoirs and cannot be eliminated. Although a declining incidence of JE has been observed in Asia because of reduced transmission by agricultural approaches and vaccination, the most important control measure now, and in the future, is vaccination of humans against JE. The inactivated vaccine, produced from infected mouse-brain-derived tissue, is the only commercially available vaccine. There are several concerns with the use of this vaccine. It is expensive, requires two or three doses to achieve protective efficacy, and, in practice, requires further booster doses to maintain immunity. The apparent increase in allergic reactions in the first part of the 1990s has set focus on the safety of the JEV. A cheap, live attenuated SA 14-14-2 vaccine is used almost exclusively in China and parts of Korea, but there have been no trials of SA 14-14-2 vaccine outside JE endemic countries. The vaccine seems to be highly efficient, and few adverse events have been observed; however, PHK cells are used for the production of this vaccine, and these cells are not approved by the WHO. A satisfactory cell substrate is needed. A committee under the WHO has proposed that for the live JEV, there should be validity of the assays for retrovirus when applied to PHK cell substrate and validity of the mouse assays for neurovirulence. Further information should be reviewed on the long-term follow-up of recipients of the vaccine. Several new types of vaccines have reached the phase of clinical trials; however, studies remain to be completed. Until a new vaccine is available, the priority of surveillance of adverse events and the continuous reporting of such events to the users of the vaccines must be of importance. This fact is highlighted by the possibility of the varying frequency of adverse events with different batches over the years. The WHO offers information and recommendations for vaccines in the EPI and issues a series of updated papers on other vaccines that are of international public health importance (eg, JEV). The development of alternative efficient, safe, and appropriately priced JEVs is recommended, as is intensified surveillance of adverse events. Prospective vaccine studies of safety may be limited because of sample size and because rare adverse events may not be detected. Several new initiatives have been taken to improve surveillance of adverse events to vaccines within the past 10 years. In Japan, there is an increasing awareness of the importance of efforts taken to improve vaccine safety, and surveillance of adverse events and possibilities of compensation for vaccine-related injuries are in place. In Vietnam, a database to detect adverse events after vaccination has been established; the project involves active visits to data collectors at the vaccination sites. Comparative studies of adverse events, such as one recent study from Japan and the United States, are important for the evaluation of the reporting systems. The reporting rate for JEV adverse events from Japan was approximately one order of magnitude lower than that in the United States. Japan had strict predefined reporting criteria and time limits for observations. If time limits for the observation are too strict (eg, defining a possible neurologic reaction to occur within 1 week after vaccination), later reactions will not be included (eg, if ADEM is elicited by a vaccine, the symptoms cannot be expected to occur until weeks after the vaccination). The passive surveillance systems have limitations with an underreporting of adverse events, depending on clinical seriousness, temporal proximity to vaccination, awareness of healthcare workers, and tradition of reporting particular events. In developed countries, surveillance of adverse events is formalized, although not necessarily optimal. An increase in reporting would be expected when the reporting of adverse events is mandatory. Reports have been sent to VAERS, the Vaccine Safety Datalink Project, and the European Union Pharmacovigilance System. A Brighton collaboration has been implemented to enhance comparability of vaccine safety data. Public health authorities in specific countries, such as the CDC in the United States and the National Advisory Committee in Canada, regularly have published information on the JE situation in Asia and the preventive measures to be taken, including information on the vaccines and adverse reactions. The conventional recommendation is that travelers should be vaccinated if they will spend more than 1 month in a JE endemic area or in areas with epidemic transmission with even shorter periods. Although the risk for JE for short-term travelers is considered small (1 case per 1 million travelers per year), sporadic cases, including deaths, have been reported among tourists traveling to endemic areas. Risk for travelers in rural districts in the season of risk is considerably higher (range, 1 case per 5000 travelers to 1 case per 20,000 travelers per week). Doctors who advise travelers should be updated on the latest JE occurrences in Asia. Updates on the JE situation can be found on bulletins at http://www.promedmail.org or are available from the WHO or CDC. The allergic reactions primarily described after vaccination with the inactivated mouse-brain-derived JEV have been observed in several countries during the 1900s. Allergic reactions, including the mucocutaneous and neurologic reactions reported after JE vaccination, may vary in frequency, and these reactions should be evaluated meticulously yearly. This step enables recommendations, including information on possible side effects, to be given in an optimal way.

Current immunizations for travel

PURPOSE OF REVIEW

International travelers may be at risk from a variety of potentially severe and life-threatening infections. Some of these diseases are preventable, and vaccination remains a cornerstone of travel medicine. Vaccines that are important for international travel are reviewed, in a succinct update based on the most recent literature.

RECENT FINDINGS

Discussed are vaccines for enteric infections (polio, cholera, hepatitis A, and typhoid), as well as those for hepatitis B, Japanese encephalitis, yellow fever, and meningococcal vaccines. The controversial end to the polio eradication campaign and the recognition of vaccine-derived polioviruses are discussed. New monovalent cholera vaccines, including the live attenuated Peru-15 and CVD 103-HgR and the oral killed whole cell B subunit vaccine are reviewed, as well as a new oral bivalent vaccine that may offer protection against Vibrio cholerae 0139. Advances in typhoid vaccination include promising preclinical and clinical trial results of recombinant ZH9 and CVD 908-htrA vaccines, which, in addition to providing protection against typhoid fever, may be useful vectors for heterologous antigens. A growing recognition of rare adverse reactions to the 17D yellow fever vaccine, especially postvaccinal encephalitis, has led to a reassessment of its risks and benefits. Development of a novel chimeric vaccine may improve the safety and efficacy of the current Japanese encephalitis vaccine. Vaccination for meningococcal disease is characterized by the need for polyvalent, conjugate vaccines as well as a product that affords protection against serotype B.

SUMMARY

This travel vaccination review highlights progress in new travel-related vaccine development and updates the reader on issues surrounding licensed products. It will be useful for generalists, infectious disease physicians, and travel medicine specialists.

Stable high-producer cell clone expressing virus-like particles of the Japanese encephalitis virus e protein for a second-generation subunit vaccine

We produced and characterized a cell clone (J12#26 cells) that stably expresses Japanese encephalitis virus (JEV) cDNA, J12, which encodes the viral signal peptide, premembrane (prM), and envelope (E) proteins (amino acid positions 105 to 794). Rabbit kidney-derived RK13 cells were transfected with a J12 expression plasmid, selected by resistance to marker antibiotics, and cloned by two cycles of a limiting-dilution method in the presence of antibiotics, a procedure that prevents the successful generation of E-producing cell clones. J12#26 cells secreted virus-like particles containing the authentic E antigen (E-VLP) into the culture medium in a huge enzyme-linked immunosorbent assay-equivalent amount (2.5 micro g per 10(4) cells) to the internationally licensed JE vaccine JE-VAX. E-VLP production was stable after multiple cell passages and persisted over 1 year with 100% expressing cells without detectable cell fusion, apoptosis, or cell death, but was suspended when the cells grew to 100% confluency and contact inhibition occurred. Mice immunized with the purified J12#26 E-antigen without adjuvant developed high titers of neutralizing antibodies for at least 7 months and 100% protection against intraperitoneal challenge with 5 x 10(6) PFU of JEV when examined according to the JE vaccine standardization protocol. These results suggest that the recombinant E-VLP antigen produced by the J12#26 cell clone is an effective, safe, and low-cost second-generation subunit JE vaccine.

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.