Particulate vaccine candidate for Japanese encephalitis induces long-lasting virus-specific memory T lymphocytes in mice

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.

Japanese encephalitis: development of new candidate vaccines

Japanese encephalitis (JE) is the most common form of viral encephalitis that appears in the form of frequent epidemics of brain fever throughout Southeast Asia, China and India. The disease is caused by a Flavivirus named Japanese encephalitis virus that is spread to humans by mosquitoes. An internationally approved mouse brain-derived inactivated vaccine has been available that is relatively expensive, gives immunity of uncertain duration and is not completely safe. Cell culture-derived inactivated and attenuated JE vaccines are in use in China, but these are not produced as per the norms acceptable in most countries. Several new promising JE vaccine candidates have been developed, some of which are under different stages of clinical evaluation. These new candidate JE vaccines have the potential to generate long-lasting immunity at low cost.

[Reverse genetics system for flaviviruses]

Flaviviruses such as Japanese encephalitis virus, West Nile virus, yellow fever virus, dengue virus, and tick-borne encephalitis virus belong to a family Flaviviridae. These viruses are transmitted to vertebrates by infected mosquitoes or ticks, producing diseases, which have a serious impact on global public health. Reverse genetics is a powerful tool for studying the viruses. Although infectious full-length clones have been obtained for multiple flaviviruses, their early-stage development had the difficulty because of the instability problem of the viral cDNA in E. coli. Several strategies have been developed to circumvent the problem of infectious clone instability. The current knowledge accumulated on reverse genetics system of flaviviruses and its application are summarized in this review.

Recombinant dengue virus-like particles from Pichia pastoris: efficient production and immunological properties

The envelope glycoprotein (E) of flavivirus is the major structural protein on the surface of the mature virions. The complexes of premembrane (prM) and E play important roles in virus assembly and fusion modulation and in potential immunity-inducing vaccines. In the present study, the cDNA encoding prM and E proteins of dengue virus type 2 (DENV-2) was subcloned into the pGAPZalphaA vector and further integrated into the genome of Pichia pastoris under the control of the glyceraldehyde-3-phosphate dehydrogenase (GAP) constitutive promoter. The high-level constitutive expression of recombinant E antigen was achieved in P. pastoris. Both the cell lysate and the culture supernatant, examined by electron microscopy, were found to contain DENV-2 virus-like particles (VLPs) with diameters of about 30 nm. After immunization of BALB/c mice, the VLPs exhibited similar efficacies as inactivated virus in terms of antibody induction and neutralization titer. These results suggest that recombinant DENV VLPs can be efficiently produced in the GAP promoter-based P. pastoris expression system. This system may be useful for the development of effective and economic dengue subunit vaccine.

Third-generation flavivirus vaccines based on single-cycle, encapsidation-defective viruses

Flaviviruses are arthropod-borne pathogens that cause significant disease on all continents of the world except Antarctica. Flavivirus diseases are particularly important in tropical regions where arthropod vectors are abundant. Live-attenuated virus vaccines (LAVs) and inactivated virus vaccines (INVs) exist for some of these diseases. LAVs are economical to produce and potent, but are not suitable for use in the immunocompromised. INVs are safer, but are more expensive to produce and less potent. Despite the success of both classes of these first-generation flavivirus vaccines, problems associated with their use indicate a need for improved products. Furthermore, there are no suitable vaccines available for important emerging flavivirus diseases, notably dengue and West Nile encephalitis (WNE). To address these needs, new products, including LAVs, INVs, viral-vectored, genetically engineered LAVs, naked DNA, and subunit vaccines are in various stages of development. Here we describe the current state of these first- and second-generation vaccine candidates, and compare these products to our recently described single-cycle, encapsidation defective flavivirus vaccine: RepliVAX. RepliVAX can be propagated in C-expressing cells (or as a unique two-component virus) using methods similar to those used to produce today's economical and potent LAVs. However, due to deletion of most of the gene for the C protein, RepliVAX cannot spread between normal cells, and is unable to cause disease in vaccinated animals. Nevertheless, RepliVAX is potent and efficacious in animal models for WNE and Japanese encephalitis, demonstrating its utility as a third-generation flavivirus vaccine that should be potent, economical to produce, and safe in the immunocompromised.

Construction and characterization of a second-generation pseudoinfectious West Nile virus vaccine propagated using a new cultivation system

Safer vaccines are needed to prevent flavivirus diseases. To help develop these products we have produced a pseudoinfectious West Nile virus (WNV) lacking a functional C gene which we have named RepliVAX WN. Here we demonstrate that RepliVAX WN can be safely propagated at high titer in BHK cells and vaccine-certified Vero cells engineered to stably express the C protein needed to trans-complement RepliVAX WN growth. Using these BHK cells we selected a better growing mutant RepliVAX WN population and used this to generate a second-generation RepliVAX WN (RepliVAX WN.2). RepliVAX WN.2 grown in these C-expressing cell lines safely elicit strong protective immunity against WNV disease in mice and hamsters. Taken together, these results indicate the clinical utility of RepliVAX WN.2 as a vaccine candidate against West Nile encephalitis.

Expression of particulate-form of Japanese encephalitis virus envelope protein in a stably transfected Drosophila cell line

Background

Japanese encephalitis virus (JEV), a member of the family Flaviviridae, is an important mosquito-borne human pathogen. Its envelope glycoprotein (E) is the major determinant of the pathogenicity and host immune responses. In the present study, we explored the feasibility of producing recombinant JEV E protein in the virus-free Drosophila expression system.

Results

The coding sequence for the signal sequence of premembrane and E protein was cloned into the Drosophila expression vector pAc5.1/V5-His. A Drosophila cell line S2 was cotransfected with this construct as well as a plasmid providing hygromycin B resistance. A cell line expressing the JEV E protein was selected by immunofluoresence, confocal microscopy, and western blot analysis using three different monoclonal antibodies directed against JEV E protein. This cell line was stable in the yield of JEV E protein during two months in vitro maintenance in the presence of hygromycin B. The results showed that the recombinant E protein had an expected molecular weight of about 50 kilodalton, was immunoreactive with all three monoclonal antibodies, and found in both the cytoplasm and culture supernatant. Sucrose gradient ultracentrifugation analysis revealed that the secreted E protein product was in a particulate form. It migrated to the sucrose fraction with a density of 1.13 g/ml. Balb/c mice immunised with the sucrose fraction containing the E protein particles developed specific antibodies. These data show that functioning JEV E protein was expressed in the stable S2 cell line.

Conclusion

The Drosophila expression system is a more convenient, cheaper and safer approach to the production of vaccine candidates and diagnostic reagents for JEV.

Formalin-inactivated whole virus and recombinant subunit flavivirus vaccines

The Flaviviridae is a family of arthropod-borne, enveloped, RNA viruses that contain important human pathogens such as yellow fever (YF), Japanese encephalitis (JE), tick-borne encephalitis (TBE), West Nile (WN), and the dengue (DEN) viruses. Vaccination is the most effective means of disease prevention for these viral infections. A live-attenuated vaccine for YF, and inactivated vaccines for JE and TBE have significantly reduced the incidence of disease for these viruses, while licensed vaccines for DEN and WN are still lacking despite a significant disease burden associated with these infections. This review focuses on inactivated and recombinant subunit vaccines (non-replicating protein vaccines) in various stages of laboratory development and human testing. A purified, inactivated vaccine (PIV) candidate for DEN will soon be evaluated in a phase 1 clinical trial, and a second-generation JE PIV produced using similar technology has advanced to phase 2/3 trials. The inactivated TBE vaccine used successfully in Europe for almost 30 years continues to be improved by additional purification, new stabilizers, an adjuvant, and better immunization schedules. The recent development of an inactivated WN vaccine for domestic animals demonstrates the possibility of producing a similar vaccine for human use. Advances in flavivirus gene expression technology have led to the production of several recombinant subunit antigen vaccine candidates in a variety of expression systems. Some of these vaccines have shown sufficient promise in animal models to be considered as candidates for evaluation in clinical trials. Feasibility of non-replicating flavivirus vaccines has been clearly demonstrated and further development is now warranted.

Vaccines and animal models for arboviral encephalitides

Arthropod-borne viruses ("arboviruses") cause significant human illness ranging from mild, asymptomatic infection to fatal encephalitis or hemorrhagic fever. The most significant arboviruses causing human illness belong to genera in three viral families, Togaviridae, Flaviviridae, and Bunyaviridae. These viruses represent a significant public health threat to many parts of the world, and, as evidenced by the recent introduction of the West Nile virus (WNV) to the Western Hemisphere, they can no longer be considered specific to any one country or region of the world. Like most viral diseases, there are no specific therapies for the arboviral encephalitides; therefore, effective vaccines remain the front line of defense for these diseases. With this in mind, the development of new, more effective vaccines and the appropriate animal models in which to test them become paramount. In fact, for many important arboviruses (e.g. California serogroup and St. Louis encephalitis viruses), there are currently no approved vaccines available for human use. For others, such as the alphaviruses, human vaccines are available only as Investigational New Drugs, and thus are not in widespread use. On the other hand, safe and effective vaccines against tick-borne encephalitis virus (TBEV) and Japanese encephalitis virus (JEV) have been in use for decades. New challenges in vaccine development have been met with new technologies in vaccine research. Many of the newer vaccines are now being developed by recombinant DNA technology. For example, chimeric virus vaccines have been developed using infectious clone technology for many of the arboviruses including, WNV, JEV, and TBEV. Other successful approaches have involved the use of naked DNA encoding and subsequently expressing the desired protective epitopes. Naked DNA vaccines have been used for TBEV and JEV and are currently under development for use against WNV. The development of less expensive, more authentic animal models to evaluate new vaccines against arboviral diseases will become increasingly important as these new approaches in vaccine research are realized. This article reviews the current status of vaccines, both approved for use and those in developmental stages, against the major arboviral encephalitides causing human disease. In addition, research on animal models, both past and present, for these diseases are discussed.

Changing perspectives in Japanese encephalitis in India

Japanese encephalitis is a serious public health problem with significant mortality in children and old people. It occurs throughout much of Asia (43 000 cases worldwide per year). In recent years it has caused many epidemics in different parts of the country. In view of the high mortality and severe sequelae which often leaves behind highly dependent and disabled survivors, the disease is assuming great importance. A review of the pathogenesis, epidemiology, management and prevention together with changing perspectives in all these areas is presented.

Enhancing biosynthesis and secretion of premembrane and envelope proteins by the chimeric plasmid of dengue virus type 2 and Japanese encephalitis virus

We have constructed a series of plasmids encoding premembrane (prM) and envelope (E) protein genes of dengue virus type 2 (DEN-2). These plasmids included an authentic DEN-2 prM-E construct (pCBD2-14-6), and two chimeric constructs, 90% DEN-2 E-10% Japanese encephalitis (JE) virus E (pCB9D2-1J-4-3) and 80% DEN-2 E-20% JE E (pCB8D2-2J-2-9-1). Monoclonal antibody (MAb) reactivity indicated that all three plasmids expressed authentic DEN-2 virus E protein epitopes representative of flavivirus domains 1, 2, and 3. However, only the pCB8D2-2J-2-9-1 construct secreted high levels of prM, M (membrane), and E proteins into the culture fluid of plasmid-transformed COS-1 cells. The major portion of the prM and E proteins expressed by COS-1 cells transformed by pCBD2-14-6 or pCB9D2-4-3 plasmids remained membrane-bound. The results supported the notion that an unidentified membrane retention sequence is located between E-397 and E-436 of DEN-2 virus E protein. Replacing the carboxyl-terminal 20% of DEN-2 E (397-450) with the corresponding JE sequence had no effect on anti-DEN-2 MAb reactivity, indicating that this region is antigenically inert, although it is required for antigen secretion. Plasmid pCBD2-2J-2-9-1, which expressed secreted forms of prM/M and E that have the potential to form subviral particles, was superior to other constructs in stimulating an antibody response. Ninety percent neutralization titers ranging from 1:40 to >1:1000 were observed in seven of nine serum specimens from pCB8D2-2J-2-9-1-immunized mice. Eleven of twelve 2-day-old neonatal mice, derived from a pCB8D2-2J-2-9-1 immunized female mouse, survived intraperitoneal challenge of DEN-2 New Guinea C virus.

Japanese encephalitis vaccines: current vaccines and future prospects

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

Protective mechanisms induced by a Japanese encephalitis virus DNA vaccine: requirement for antibody but not CD8(+) cytotoxic T-cell responses

We have previously shown that a plasmid (pE) encoding the Japanese encephalitis virus (JEV) envelope (E) protein conferred a high level of protection against a lethal viral challenge. In the present study, we used adoptive transfer experiments and gene knockout mice to demonstrate that the DNA-induced E-specific antibody alone can confer protection in the absence of cytotoxic T-lymphocyte (CTL) functions. Plasmid pE administered by either intramuscular or gene gun injection produced significant E-specific antibodies, helper T (Th)-cell proliferative responses, and CTL activities. Animals receiving suboptimal DNA vaccination produced low titers of anti-E antibodies and were only partially or not protected from viral challenge, indicating a strong correlation between anti-E antibodies and the protective capacity. This observation was confirmed by adoptive transfer experiments. Intravenous transfer of E-specific antisera but not crude or T-cell-enriched immune splenocytes to sublethally irradiated hosts conferred protection against a lethal JEV challenge. Furthermore, experiments with gene knockout mice showed that DNA vaccination did not induce anti-E titers and protective immunity in Igmu(-/-) and I-Abeta(-/-) mice, whereas in CD8alpha(-/-) mice the pE-induced antibody titers and protective rate were comparable to those produced in the wild-type mice. Taken together, these results demonstrate that the anti-E antibody is the most critical protective component in this JEV challenge model and that production of anti-E antibody by pE DNA vaccine is dependent on the presence of CD4(+) T cells but independent of CD8(+) T cells.

Immunization of mice against West Nile virus with recombinant envelope protein

West Nile (WN) virus is a mosquito-borne flavivirus that emerged in the United States in 1999 and can cause fatal encephalitis. Envelope (E) protein cDNA from a WN virus isolate recovered from Culex pipiens in Connecticut was expressed in Escherichia coli. The recombinant E protein was purified and used as Ag in immunoblot assays and immunization experiments. Patients with WN virus infection had Abs that recognized the recombinant E protein. C3H/HeN mice immunized with E protein developed E protein Abs and were protected from infection with WN virus. Passive administration of E protein antisera was also sufficient to afford immunity. E protein is a candidate vaccine to prevent WN virus infection.

A recombinant particulate antigen of Japanese encephalitis virus produced in stably-transformed cells is an effective noninfectious antigen and subunit immunogen

A COS-1 cell line, stably transformed by a plasmid encoding the premembrane and envelope glycoproteins of Japanese encephalitis virus, produced a noninfectious recombinant antigen expressed as extracellular particles. Extracellular particles purified by equilibrium density centrifugation in sucrose gradients followed by electron microscopy were characterized as spherical particles with an average diameter of approximately 30 nm and a buoyant density of 1.15 g/cc. Purified extracellular particles were shown by western blot to contain premembrane, membrane and envelope proteins. The gradient-purified particles exhibited hemagglutination activity at the same pH optimum (6.6) as Japanese encephalitis virus. Recombinant antigen from cell culture fluid was concentrated by precipitation with polyethylene glycol and evaluated for immunogenicity in 8-10-week-old ICR mice. Groups of five mice received only one immunization of recombinant antigen with or without Freund's incomplete adjuvant. Mice immunized with recombinant antigen plus Freund's incomplete adjuvant elicited the highest anti-viral titers as determined by both enzyme-linked immunosorbent assay (ELISA) and plaque-reduction neutralization tests. The polyethylene glycol-concentrated recombinant antigen was also evaluated for use in IgM antibody-capture ELISA and indirect IgG ELISA. The IgM-capture ELISA results using recombinant antigen correlated well with the results of a similar test using Japanese encephalitis virus-infected mouse brain antigen for the analysis of serum samples from patients with symptoms of acute encephalitis. Similar IgG titers were observed in an indirect ELISA comparing recombinant antigen and purified Japanese encephalitis virus as plate-bound antigens. Based on these studies, this entirely safe, easily produced antigen that expresses authentic Japanese encephalitis virus envelope glycoprotein would provide an excellent alternative to standard viral antigens used in various ELISA formats.

Fragment of Japanese encephalitis virus envelope protein produced in Escherichia coli protects mice from virus challenge

A fragment from the N-terminal part (E(A)) and a fragment from the C-terminal part (E(B)) of the envelope (E) protein of Japanese encephalitis virus (JEV) was synthesized in Escherichia coli. These two fragments were overlapping with each other by nine amino acids, however, they were not cross-reacting with each other at the antisera level. Both E(A)and E(B)are antigenic by themselves when injected into mice, but when tested against sera from mice, rabbit, swine and human that had been immunized or naturally infected with JEV, E(B)acted as a better antigen than E(A)by ELISA assays. E(B)also proved to be a better immunogen in protection against lethal JEV infection than E(A). The protection appears to be correlated with the neutralizing titres of the anti-JEV sera. The response elicited by E(B)is a Th1 response and the antibody produced contained higher neutralizing titre than E(A)fragment. The major difference between E(A)and E(B)fragments is the solubility during expression in E. coli, while E(B)fragment is soluble, E(A)was isolated from the insoluble inclusion bodies. Therefore the antigenicity and immunogenicity expressed by the E(B)fragment may probably be due to its proper folding to assume a correctly assembled form during expression in E. coli, a quality that is important for a protein to qualify as a good vaccine candidate.

Generation and characterization of a mammalian cell line continuously expressing Japanese encephalitis virus subviral particles

We have generated a cell line (F cells) producing a secreted form of Japanese encephalitis virus (JEV) subviral particle (extracellular particles [EPs]) that contains the JEV envelope glycoprotein (E) and a precursor (prM) of the virion membrane protein (M). The F cells were engineered to synthesize these JEV products from a cDNA encoding a mutated (furin proteinase resistant) form of prM, since stable cell lines expressing E and the authentic form of prM could not be obtained, due (in part) to the cell-fusing ability of EPs containing E and M. Our biochemical alteration of the prM protein was critical for the successful production of EP-producing cell lines. EPs produced by F cells share the biochemical properties of empty viral particles produced by JEV-infected cells, except that the F-cell EPs lack hemagglutinating activity and M. F-cell EPs were recognized by a panel of monoclonal antibodies to E, and EPs were shown to be useful as vaccine candidates in mice and as diagnostic reagents in evaluating human immune responses to JE vaccination. The amounts of E antigen released into the culture fluid of F cells were similar to those found in virion fractions of JEV-infected cell culture fluids or JEV-infected weanling mouse brains (the current source of antigen used to produce human vaccines for JE). Thus, the F-cell line would appear to be a useful source of antigen for JE vaccines and diagnostics.

A single intramuscular injection of recombinant plasmid DNA induces protective immunity and prevents Japanese encephalitis in mice

Plasmid vectors containing Japanese encephalitis virus (JEV) premembrane (prM) and envelope (E) genes were constructed that expressed prM and E proteins under the control of a cytomegalovirus immediate-early gene promoter. COS-1 cells transformed with this plasmid vector (JE-4B clone) secreted JEV-specific extracellular particles (EPs) into the culture media. Groups of outbred ICR mice were given one or two doses of recombinant plasmid DNA or two doses of the commercial vaccine JEVAX. All mice that received one or two doses of DNA vaccine maintained JEV-specific antibodies 18 months after initial immunization. JEVAX induced 100% seroconversion in 3-week-old mice; however, none of the 3-day-old mice had enzyme-linked immunosorbent assay titers higher than 1:400. Female mice immunized with this DNA vaccine developed plaque reduction neutralization antibody titers of between 1:20 and 1:160 and provided 45 to 100% passive protection to their progeny following intraperitoneal challenge with 5,000 PFU of virulent JEV strain SA14. Seven-week-old adult mice that had received a single dose of JEV DNA vaccine when 3 days of age were completely protected from a 50, 000-PFU JEV intraperitoneal challenge. These results demonstrate that a recombinant plasmid DNA which produced JEV EPs in vitro is an effective vaccine.

Structures and mechanisms in flavivirus fusion

This chapter focuses on the work carried out with tick-borne encephalitis (TBE) virus, the structurally best characterized of the flaviviruses. The data is related to those obtained with other flaviviruses, which are assumed to have a conserved structural organization, and compare the characteristics of flavivirus fusion to those of other enveloped viruses. Fusion proteins from several different virus families, including Orthomyxoviridae , Paramyxoviridae , Retroviridae , and Filoviridae have been shown to exhibit striking structural similarities; they all use a common mechanism for inducing membrane fusion, and the same general model applies to all of these cases. The flavivirus genome is a positive-stranded RNA molecule consisting of a single, long open reading frame of more than 10,000 nucleotides flanked by noncoding regions at the 5′ and 3′ ends. The fusion properties of flaviviruses have been investigated using several different assay systems, including virus-induced cell–cell fusion and virus–liposome fusion. All of these studies indicate that flaviviruses require an acidic pH for fusion, consistent with their proposed mode of entry.

Screening of protective antigens of Japanese encephalitis virus by DNA immunization: a comparative study with conventional viral vaccines

In this study, we evaluated the relative role of the structural and nonstructural proteins of the Japanese encephalitis virus (JEV) in inducing protective immunities and compared the results with those induced by the inactivated JEV vaccine. Several inbred and outbred mouse strains immunized with a plasmid (pE) encoding the JEV envelope protein elicited a high level of protection against a lethal JEV challenge similar to that achieved by the inactivated vaccine, whereas all the other genes tested, including those encoding the capsid protein and the nonstructural proteins NS1-2A, NS3, and NS5, were ineffective. Moreover, plasmid pE delivered by intramuscular or gene gun injections produced much stronger and longer-lasting JEV envelope-specific antibody responses than immunization of mice with the inactivated JEV vaccine did. Interestingly, intramuscular immunization of plasmid pE generated high-avidity antienvelope antibodies predominated by the immunoglobulin G2a (IgG2a) isotype similar to a sublethal live virus immunization, while gene gun DNA immunization and inactivated JEV vaccination produced antienvelope antibodies of significantly lower avidity accompanied by a higher IgG1-to-IgG2a ratio. Taken together, these results demonstrate that the JEV envelope protein represents the most critical antigen in providing protective immunity.

Immunogenicity, genetic stability, and protective efficacy of a recombinant, chimeric yellow fever-Japanese encephalitis virus (ChimeriVax-JE) as a live, attenuated vaccine candidate against Japanese encephalitis

Yellow fever (YF) 17D vaccine virus, having a 60-year history of safe and effective use, is an ideal vector to deliver heterologous genes from other medically important flaviviruses. A chimeric YF/Japanese encephalitis (JE) virus (ChimeriVax-JE virus) was constructed by insertion of the premembrane and envelope (prME) genes of an attenuated human vaccine strain (SA14-14-2) of Japanese encephalitis (JE) virus between core and nonstructural (NS) genes of a YF 17D infectious clone. The virus grew to high titers in cell cultures and was not neurovirulent for 3- to 4-week-old mice at doses </=6 log10 plaque forming units (pfu) inoculated by the intracerebral (IC) route. In contrast, commercial YF 17D vaccine was highly neurovirulent for weanling mice by the same route. Mice inoculated subcutaneously with one dose of >/=10(3) pfu of ChimeriVax-JE virus were solidly protected against intraperitoneal challenge with a virulent JE virus. Genetic stability of the chimera was assessed by sequential passages in cell cultures or in mouse brain. All attenuating residues and the avirulent phenotype were preserved after 18 passages in cell cultures or 6 passages in mouse brains.