Genotype I of Japanese Encephalitis Virus Virus-like Particles Elicit Sterilizing Immunity against Genotype I and III Viral Challenge in Swine

Epitope(s) involving amino acids of the fusion loop of Japanese encephalitis virus envelope protein is(are) important to elicit protective immunity

Dengue vaccine candidates have been shown to improve vaccine safety and efficacy by altering the residues or accessibility of the fusion loop on the virus envelope protein domain II (DIIFL) in an ex vivo animal study. The current study aimed to comprehensively investigate the impact of DIIFL mutations on the antigenicity, immunogenicity, and protective efficacy of Japanese encephalitis virus (JEV) virus-like particles (VLPs) in mice. We found the DIIFL G106K/L107D (KD) and W101G/G106K/L107D (GKD) mutations altered the binding activity of JEV VLP to cross-reactive monoclonal antibodies but had no effect on their ability to elicit total IgG antibodies in mice. However, JEV VLPs with KD or GKD mutations induced significantly less neutralizing antibodies against JEV. Only 46% and 31% of the KD and GKD VLPs-immunized mice survived compared to 100% of the wild-type (WT) VLP-immunized mice after a lethal JEV challenge. In passive protection experiments, naïve mice that received sera from WT VLP-immunized mice exhibited a significantly higher survival rate of 46.7% compared to those receiving sera from KD VLP- and GKD VLP-immunized mice (6.7% and 0%, respectively). This study demonstrated that JEV DIIFL is crucial for eliciting potently neutralizing antibodies and protective immunity against JEV.

IMPORTANCE

Introduction of mutations into the fusion loop is one potential strategy for generating safe dengue and Zika vaccines by reducing the risk of severe dengue following subsequent infections, and for constructing live-attenuated vaccine candidates against newly emerging Japanese encephalitis virus (JEV) or Japanese encephalitis (JE) serocomplex virus. The monoclonal antibody studies indicated the fusion loop of JE serocomplex viruses primarily comprised non-neutralizing epitopes. However, the present study demonstrates that the JEV fusion loop plays a critical role in eliciting protective immunity in mice. Modifications to the fusion loop of JE serocomplex viruses might negatively affect vaccine efficacy compared to dengue and zika serocomplex viruses. Further studies are required to assess the impact of mutant fusion loop encoded by commonly used JEV vaccine strains on vaccine efficacy or safety after subsequent dengue virus infection.

Low-temperature culture enhances production of flavivirus virus-like particles in mammalian cells

Flavivirus virus-like particles (VLPs) exhibit a striking structural resemblance to viral particles, making them highly adaptable for various applications, including vaccines and diagnostics. Consequently, increasing VLPs production is important and can be achieved by optimizing expression plasmids and cell culture conditions. While attempting to express genotype III (GIII) Japanese encephalitis virus (JEV) VLPs containing the G104H mutation in the envelope (E) protein, we failed to generate VLPs in COS-1 cells. However, VLPs production was restored by cultivating plasmid-transfected cells at a lower temperature, specifically 28 °C. Furthermore, we observed that the enhancement in JEV VLPs production was independent of amino acid mutations in the E protein. The optimal condition for JEV VLPs production in plasmid-transfected COS-1 cells consisted of an initial culture at 37 °C for 6 h, followed by a shift to 28 °C (37/28 °C) for cultivation. Under 37/28 °C cultivation conditions, flavivirus VLPs production significantly increased in various mammalian cell lines regardless of whether its expression was transiently transfected or clonally selected cells. Remarkably, clonally selected cell lines expressing flavivirus VLPs consistently achieved yields exceeding 1 μg/ml. Binding affinity analyses using monoclonal antibodies revealed similar binding patterns for VLPs of genotype I (GI) JEV, GIII JEV, West Nile virus (WNV), and dengue virus serotype 2 (DENV-2) produced under both 37 °C or 37/28 °C cultivation conditions. In summary, our study demonstrated that the production of flavivirus VLPs can be significantly improved under 37/28 °C cultivation conditions without affecting the conformational structure of the E protein. KEYPOINTS: • Low-temperature culture (37/28 °C) enhances production of flavivirus VLPs. • Flavivirus VLPs consistently achieved yields exceeding 1 μg/ml. • 37/28 °C cultivation did not alter the structure of flavivirus VLPs.

Japanese encephalitis genotype I virus-like particles stably expressed in BHK-21 cells serves as potential antigen in JE IgM ELISA

Abstract

Japanese encephalitis virus (JEV) is one of the leading causes of epidemic encephalitis in South Asian countries. Due to the short-term viremia, detecting IgM antibodies by ELISA is treated as the front-line diagnostic assay. Co-circulation and multiple exposures to antigenically cross-reactive flaviviruses in India pose a challenge in serodiagnosis. Replacing the whole virus antigen currently used in the JE IgM detection kits (ELISA) may improve the specificity and sensitivity of the existing JE MAC ELISA kits. For this purpose, we developed a stably transfected cell clone, BHK-IE6, which expresses a high amount of VLPs up to 37 µg/ml and is consistent in expression up to 40 passages. For the expression of VLPs in the secretory form, we cloned the JEV G-I prM-E coding gene along with the C-terminal signal sequence of capsid protein in the BHK-21 cells using the pcDNA3.1 + mammalian expression vector. The immune assays performed demonstrated its immune reactivity equivalent to the parental JEV strain. Simultaneously performed ELISAs using the whole virus antigen and newly developed antigen gave comparable results for JE positive and negative samples, which established the utility of developed JEV E-VLP as an antigen. Reduced cross-reactivity and increased specificity were observed when tested with dual positive sera for anti-JEV and DENV antibodies. These findings confirm the efficiency and reliability of newly developed recombinant E-VLP antigen expressed by the BHK-IE6 cell clone as an antigen in serodiagnostic assays. The implementation and progress in developing cross-reactivity-reduced antigens would improve serodiagnosis and disease burden estimates of flavivirus infection.

Key points

• pcDNA3.1/JE-Sig-prM-E plasmid transfected BHK-21 cells stably express VLPs.

• Sodium butyrate induction enhanced the extracellular expression of VLPs.

• Application of JEV-E VLPs increases the specificity of JE IgM ELISA.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00253-022-11825-1.

Flavivirus vaccines: Virus-like particles and single-round infectious particles as promising alternatives

The genus flavivirus of the Flaviridae family includes several human pathogens, like dengue, Zika, Japanese encephalitis, and yellow fever virus. These viruses continue to be a significant threat to human health. Vaccination remains the most useful approach to reduce the impact of flavivirus fever. However, currently available vaccines can induce severe side effects or have low effectiveness. An alternative is the use of recombinant vaccines, of which virus-like particles (VLP) and single-round infectious particles (SRIP) are of especial interest. VLP consist of the virus structural proteins produced in a heterologous system that self-assemble in a structure almost identical to the native virus. They are highly immunogenic and have been effective vaccines for other viruses for over 30 years. SRIP are promising vaccine candidates, as they induce both cellular and humoral responses, as viral proteins are expressed. Here, the state of the art to produce both types of particles and their use as vaccines against flaviviruses are discussed. We summarize the different approaches used for the design and production of flavivirus VLP and SRIP, the evidence for their safety and efficacy, and the main challenges for their use as commercial vaccines.

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.

Duck Tembusu virus induces stronger cellular responses than Japanese encephalitis virus in primary duck neurons and fibroblasts

Duck Tembusu virus (DTMUV) and Japanese encephalitis virus (JEV) are mosquito-borne flaviviruses. These two viruses infect ducks; however, they show different neurological outcomes. The mechanism of DTMUV- and JEV-induced neuronal death has not been well investigated. In the present study, we examined the differences in the mechanisms involved in virus-induced cell death and innate immune responses between DTMUV KPS54A61 strain and JEV JaGAr-01 strain using primary duck neurons (DN) and duck fibroblasts (CCL-141). DN and CCL-141 were permissive for the infection and replication of these two viruses, which upregulated the expression of innate immunity genes. Both DTMUV and JEV induced cell death via a caspase-3-dependent manner; however, DTMUV triggered more cell death than JEV did in both CCL-141 and DN. These findings suggest that DTMUV infection causes apoptosis in duck neurons and fibroblasts more strongly than JEV. Levels of the mRNA expression of innate immunity-related genes after DTMUV infection were generally higher than levels after JEV infection, suggesting that DTMUV-induced immune response in duck cells may exhibit toxic effect rather than protective effects. This article is protected by copyright. All rights reserved.

A recombinant platform for flavivirus vaccines and diagnostics using chimeras of a new insect-specific virus

Flaviviruses such as dengue, yellow fever, Zika, West Nile, and Japanese encephalitis virus present substantial global health burdens. New vaccines are being sought to address safety and manufacturing issues associated with current live attenuated vaccines. Here, we describe a new insect-specific flavivirus, Binjari virus, which was found to be remarkably tolerant for exchange of its structural protein genes (prME) with those of the aforementioned pathogenic vertebrate-infecting flaviviruses (VIFs). Chimeric BinJ/VIF-prME viruses remained replication defective in vertebrate cells but replicated with high efficiency in mosquito cells. Cryo-electron microscopy and monoclonal antibody binding studies illustrated that the chimeric BinJ/VIF-prME virus particles were structurally and immunologically similar to their parental VIFs. Pilot manufacturing in C6/36 cells suggests that high yields can be reached up to 10^9.5 cell culture infectious dose/ml or ≈7 mg/liter. BinJ/VIF-prME viruses showed utility in diagnostic (microsphere immunoassays and ELISAs using panels of human and equine sera) and vaccine applications (illustrating protection against Zika virus challenge in murine IFNAR^-/- mouse models). BinJ/VIF-prME viruses thus represent a versatile, noninfectious (for vertebrate cells), high-yield technology for generating chimeric flavivirus particles with low biocontainment requirements.

Recent Advances in the Development of Virus-Like Particle-Based Flavivirus Vaccines

Flaviviruses include several medically important viruses, such as Zika virus (ZIKV), Dengue virus (DENV), West Nile virus (WNV) and Japanese encephalitis virus (JEV). They have expanded in geographic distribution and refocused international attention in recent years. Vaccination is one of the most effective public health strategies for combating flavivirus infections. In this review, we summarized virus-like particle (VLP)-based vaccines against the above four mentioned flaviviruses. Potential strategies to improve the efficacy of VLP-based flavivirus vaccines were also illustrated. The applications of flavivirus VLPs as tools for viral detection and antiviral drug screening were finally proposed.

Phenotypic and Genotypic Comparison of a Live-Attenuated Genotype I Japanese Encephalitis Virus SD12-F120 Strain with Its Virulent Parental SD12 Strain

The phenotypic and genotypic characteristics of a live-attenuated genotype I (GI) strain (SD12-F120) of Japanese encephalitis virus (JEV) were compared with its virulent parental SD12 strain to gain an insight into the genetic changes acquired during the attenuation process. SD12-F120 formed smaller plaque on BHK-21 cells and showed reduced replication in mouse brains compared with SD12. Mice inoculated with SD12-F120 via either intraperitoneal or intracerebral route showed no clinical symptoms, indicating a highly attenuated phenotype in terms of both neuroinvasiveness and neurovirulence. SD12-F120 harbored 29 nucleotide variations compared with SD12, of which 20 were considered silent nucleotide mutations, while nine resulted in eight amino acid substitutions. Comparison of the amino acid variations of SD12-F120 vs. SD12 pair with those from other four isogenic pairs of the attenuated and their virulent parental strains revealed that the variations at E¹³⁸ and E¹⁷⁶ positions of E protein were identified in four and three pairs, respectively, while the remaining amino acid variations were almost unique to their respective strain pairs. These observations suggest that the genetic changes acquired during the attenuation process were likely to be strain-specific and that the mechanisms associated with JEV attenuation/virulence are complicated.

Vaccines Against Dengue and West Nile Viruses in India: The Need of the Hour

The circulation of flaviviruses, dengue (DEN), Japanese encephalitis (JE) and West Nile (WN) viruses, and others, is generating a major concern in many countries. Both JE along with DEN have been endemic in large regions of India. WN virus infection, although circulating in southern regions for many years, in recent years, WN encephalitis patients have been demonstrated. While vaccines against JE have been developed and decrease outbreaks, in case of DEN and WN, vaccines are still in developing level, especially, it has been difficult to achieve the long-term protective immune response. The first licensed DEN vaccine, which is a live attenuated vaccine, was administered in countries where the virus is endemic, and has a potential to cause serious side effects, especially when administered to younger population as observed in the Philippines vaccination drive. In the case of WN, although the purified inactivated virion-based vaccine worked effectively as a veterinary vaccine for horses, no effective vaccine has yet been licensed for humans. The induction of CD4⁺ and CD8⁺ T cell responses is essential to complete protection by these viruses, as evidenced by responses to asymptomatic infections. Many studies have shown that neutralizing antibody (NAb) response is against surface structural proteins; CD4⁺ and CD8⁺ responses are mainly directed against nonstructural proteins rather than NAb response. New data suggest that encapsulating virus vaccines in nanoparticles (NPs) will direct antigen in cytoplasmic compartment by antigen-presenting cells, which will improve presentation to CD4⁺ and CD8⁺ T cells. Since tissue culture-derived, purified inactivated viruses are easier to manufacture and safer than developing live virus vaccines, inclusion of NP provides an attractive alternative for generating robust flaviviral vaccines that are affordable with long-lived protection.

Mosquito Cell-Derived Japanese Encephalitis Virus-Like Particles Induce Specific Humoral and Cellular Immune Responses in Mice

The Japanese encephalitis virus (JEV) is the major cause of an acute encephalitis syndrome in many Asian countries, despite the fact that an effective vaccine has been developed. Virus-like particles (VLPs) are self-assembled multi-subunit protein structures which possess specific epitope antigenicities related to corresponding native viruses. These properties mean that VLPs are considered safe antigens that can be used in clinical applications. In this study, we developed a novel baculovirus/mosquito (BacMos) expression system which potentially enables the scalable production of JEV genotype III (GIII) VLPs (which are secreted from mosquito cells). The mosquito-cell-derived JEV VLPs comprised 30-nm spherical particles as well as precursor membrane protein (prM) and envelope (E) proteins with densities that ranged from 30% to 55% across a sucrose gradient. We used IgM antibody-capture enzyme-linked immunosorbent assays to assess the resemblance between VLPs and authentic virions and thereby characterized the epitope specific antigenicity of VLPs. VLP immunization was found to elicit a specific immune response toward a balanced IgG2a/IgG1 ratio. This response effectively neutralized both JEV GI and GIII and elicited a mixed Th1/Th2 response in mice. This study supports the development of mosquito cell-derived JEV VLPs to serve as candidate vaccines against JEV.

Protection of swine by potent neutralizing anti-Japanese encephalitis virus monoclonal antibodies derived from vaccination

Japanese encephalitis virus (JEV) is a mosquito-borne flavivirus endemic in the Asia Pacific region. Despite use of several highly effective vaccines, it is estimated that up to 44,000 new cases of Japanese encephalitis (JE) occur every year including 14,000 deaths and 24,000 survivors with permanent sequelae. Humoral immunity induced by vaccination is critical for effective protection. Potently neutralizing antibodies reactive with the JEV envelope (E) protein are important since protective immune responses induced by both live-attenuated and inactivated JE vaccines target the E protein. Our understanding of how vaccine-induced humoral immunity protects vaccinees from morbidity and mortality is, however, limited and largely obtained from in vitro studies. With the exception of neurovirulence mouse models, very few platforms are available for evaluating the protective efficacy of neutralizing antibodies against JEV in vivo. Swine are a major amplifying host in the natural JEV transmission cycle and develop multiple pathological outcomes similar to humans infected with JEV. In this study, prophylactic passive immunization was performed in a miniature swine model, using two vaccination-induced monoclonal antibodies (mAb), JEV-31 and JEV-169. These were selected as representatives for antibodies reactive with the major antigenic structures in the E protein of JEV and related flaviviruses. JEV-31 recognizes the lateral ridge of E protein domain III (EDIII) whilst JEV-169 has a broad footprint of binding involving residues throughout domains I (EDI) and II (EDII) of the E protein. Detection of neutralizing antibodies in the serum of immunized animals mimics the presence of neutralizing antibodies in vaccinated individuals. Passive immunization with both mAbs significantly reduced the severity of diseases that resemble the symptoms of human JE including fever, viremia, viral shedding, systemic infection, and neuroinvasion. In contrast to the uniformed decrease of viral loads in lymphoid and central nervous systems, distinct kinetics in the onset of fever and viremia between animals receiving JEV-31 and JEV-169 suggest potential differences in immune protection mechanisms between anti-EDI and anti-EDIII neutralizing antibodies elicited by vaccination. Our data demonstrate the feasibility of using swine models in characterizing the protective humoral immunity against JEV and increase our understanding of how clonal populations of anti-E mAbs derived from JE vaccination protect against infection in vivo.

Nanoparticle-based vaccine development and evaluation against viral infections in pigs

Virus infections possess persistent health challenges in swine industry leading to severe economic losses worldwide. The economic burden caused by virus infections such as Porcine Reproductive and Respiratory Syndrome Virus, Swine influenza virus, Porcine Epidemic Diarrhea Virus, Porcine Circovirus 2, Foot and Mouth Disease Virus and many others are associated with severe morbidity, mortality, loss of production, trade restrictions and investments in control and prevention practices. Pigs can also have a role in zoonotic transmission of some viral infections to humans. Inactivated and modified-live virus vaccines are available against porcine viral infections with variable efficacy under field conditions. Thus, improvements over existing vaccines are necessary to: (1) Increase the breadth of protection against evolving viral strains and subtypes; (2) Control of emerging and re-emerging viruses; (3) Eradicate viruses localized in different geographic areas; and (4) Differentiate infected from vaccinated animals to improve disease control programs. Nanoparticles (NPs) generated from virus-like particles, biodegradable and biocompatible polymers and liposomes offer many advantages as vaccine delivery platform due to their unique physicochemical properties. NPs help in efficient antigen internalization and processing by antigen presenting cells and activate them to elicit innate and adaptive immunity. Some of the NPs-based vaccines could be delivered through both parenteral and mucosal routes to trigger efficient mucosal and systemic immune responses and could be used to target specific immune cells such as mucosal microfold (M) cells and dendritic cells (DCs). In conclusion, NPs-based vaccines can serve as novel candidate vaccines against several porcine viral infections with the potential to enhance the broader protective efficacy under field conditions. This review highlights the recent developments in NPs-based vaccines against porcine viral pathogens and how the NPs-based vaccine delivery system induces innate and adaptive immune responses resulting in varied level of protective efficacy.

Virus-Like Particle Systems for Vaccine Development against Viruses in the Flaviviridae Family

Viruses in the Flaviviridae family are important human and animal pathogens that impose serious threats to global public health. This family of viruses includes emerging and re-emerging viruses, most of which are transmitted by infected mosquito or tick bites. Currently, there is no protective vaccine or effective antiviral treatment against the majority of these viruses, and due to their growing spread, several strategies have been employed to manufacture prophylactic vaccines against these infectious agents including virus-like particle (VLP) subunit vaccines. VLPs are genomeless viral particles that resemble authentic viruses and contain critical repetitive conformational structures on their surface that can trigger the induction of both humoral and cellular responses, making them safe and ideal vaccine candidates against these viruses. In this review, we focus on the potential of the VLP platform in the current vaccine development against the medically important viruses in the Flaviviridae family.

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

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

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

Partial cross-protection between Japanese encephalitis virus genotype I and III in mice

Genotype III (GIII) Japanese encephalitis virus (JEV) predominance has gradually been replaced by genotype I (GI) over the last 20 years in many Asian countries. This genotype shift raises concerns about the protective efficacy of Japanese encephalitis (JE) vaccines, as all of the currently licensed JE vaccines are derived from GIII strains. In this study, we conducted vaccination-challenge protection assays to evaluate the cross-protective efficacy of GI- or GIII-derived vaccines against the challenge of a heterologous genotype using a mouse challenge model. Titration of the neutralizing antibodies elicited by SA14-14-2 live-attenuated JE vaccine (SA14-14-2 vaccine), a GIII-derived vaccine, indicated that the titer of neutralizing antibodies specific to heterologous genotype GI stain was significantly lower than that specific to homologous genotype GIII strain in both pigs and mice immunized with the SA14-14-2 vaccine. Vaccination of mice with SA14-14-2 vaccine or a GIII-inactivated vaccine at high and medium doses completely protected vaccinated mice against challenge with the homologous genotype GIII strains, but failed to provide the vaccinated mice complete protection against the challenge of heterologous genotype GI strains. The protection rates against GI strain challenge were 60%–80%, showing that these vaccines were partially protective against GI strain challenge. Additionally, vaccination of mice with a GI-inactivated vaccine conferred 100% protection against the challenge of homologous genotype GI strains, but 50%–90% protection against the challenge of heterologous genotype GIII strains, showing a reduced protective efficacy of a GI-derived vaccine against GIII strain challenge. Overall, these observations demonstrated a partial cross-protection between GI and GIII strains and suggested a potential need for new JE vaccine strategies, including options like a bivalent vaccine, to control both genotype infection.

Japanese encephalitis virus (JEV) is a mosquito-borne flavivirus that causes Japanese encephalitis (JE) in humans and reproductive disorders in pigs. JEV is phylogenetically classified into five genotypes. JEV genotype III (GIII) was historically dominant throughout most of Asia, but has been replaced by genotype I (GI) over the last 20 years in many Asian countries. Amino acid variations in JEV envelope protein play major roles in determination of antigenicity. Elicitation of cross-neutralizing antibodies for GI and GIII strains has been reported, showing an antigenic difference between the two genotypes. These amino acid differences in JEV envelope proteins raise a concern about the protective efficacy of JE vaccines against the emerged GI strain infection, because all currently licensed JE vaccines are derived from GIII strains. We evaluated the protective efficacy of JE vaccines against the heterologous genotype strain using a mouse challenge model and found a partial cross-protection between GI- or GIII-derived vaccines against the challenge of the heterologous genotype. This partial cross-protective efficacy suggested a potential need for a new JE vaccine, one solution may be a bivalent vaccine, to control infection with either genotype. However, more comprehensive studies should be conducted to address the partial cross-protective efficacy of JE vaccines against the heterologous genotype strains using JEV natural hosts such as pigs.