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

In vivo electroporation of skeletal muscles increases the efficacy of Japanese encephalitis virus DNA vaccine

DNA vaccines can induce protective immunity against subsequent viral challenge. However, for some DNA vaccines to be effective when administered intramuscularly, cardiotoxin pretreatment is necessary. In this study, we used the technique of in vivo electroporation to facilitate DNA delivery and elicit an immune response without the use of cardiotoxin. Intramuscular delivery of DNA (pE) encoding the Japanese encephalitis virus (JEV) envelope protein-induced anti-E antibodies only when the injected muscles were pretreated with cardiotoxin. In vivo electrotransfer of pE eliminated the need for cardiotoxin pretreatment and produced higher antibody titer than that induced by conventional intramuscular injection. Moreover, the induced immunity also conferred protection against lethal viral challenge. Interestingly, like intramuscular immunization, in vivo electroporation immunization with plasmid pE generated anti-envelope antibodies that were predominantly of the immunoglobulin G2a (IgG2a) isotype. These results suggest that in vivo electroporation can be used as an efficient gene delivery system for DNA vaccines to provide efficient protection against viral infection.

Japanese encephalitis subunit vaccine composed of virus-like envelope antigen particles purified from serum-free medium of a high-producer J12#26 cell clone

A stable cell clone, J12#26, which continuously secretes large amounts of the envelope (E) antigen of Japanese encephalitis (JE) virus (J. Virol. 77 (2003) 8745) was adapted to serum-free medium. The J12#26 antigen possessed hemagglutinating activity, as well as the viral E and M proteins. More than 10 and 1mg of the antigen quantified with the licensed JE vaccine (JE-VAX) as a standard by E-ELISA and protein determination, respectively, were recovered from 500 ml of serum-free medium by membrane ultrafiltration, Sephacryl S-300 chromatography, sucrose gradient centrifugation and Sephadex G-25 chromatography. SDS-PAGE and Western blot analyses confirmed the high yield and purity of the J12#26 E antigen, which was comprised of small spherical virus-like particles (VLP) of approximately 25 nm in diameter. This antigen induced in mice without adjuvant neutralizing antibody (NT Ab) titers, as high as or higher than the licensed JE vaccine, and complete protection against challenge with wild-type virus. These results suggest that the J12#26 antigen is a promising second-generation JE subunit vaccine.

Construction of recombinant pseudorabies virus expressing NS1 protein of Japanese encephalitis (SA14-14-2) virus and its safety and immunogenicity

The bivalent genetic engineering vaccine of Japanese encephalitis (JE) and Aujeszkj disease (AD) was developed to provide a novel approach to prevent and control these two diseases. NS1 gene of Japanese encephalitis virus (JEV) SA14-14-2 strain was produced by reverse transcriptase-mediated PCR (RT-PCR) and was cloned into vector pUSK to form recombinant plasmid (designed as pUSK-NS1). A co-transfection experiment was performed in porcine kidney (PK-15) cells with pUSK-NS1 and the genome of the vector virus (PRV TK(-)/gG(-)/LacZ(+) mutant). By plaque purification, PCR detection and southern hybridization, recombinant pseudorabies virus (PRV) expressing NS1 protein of JEV was acquired and named TK(-)/gG(-)/NS1(+). Western blot analysis and ELISA demonstrated the NS1 protein expression. To evaluate the recombinant virus's potential application, we characterized the safety and immune responses in Balb/c mice and swine. The safety test indicated that, when receiving the recombinant virus at a concentration of 10(6.0)pfu, no virulence of the recombinant virus to the mice, piglets and pregnant sows was observed. The vaccinated animals could acquire protective immunity against lethal challenge of the virulent PRV Ea strain and develop a good humoral and cellular immune response against JEV. The above results revealed that the recombinant virus could be a suitable candidate vaccine strain for developing a novel genetic vaccine to combat pseudorabies and Japanese encephalitis in the pig industry.

Cell-mediated immune responses in healthy children with a history of subclinical infection with Japanese encephalitis virus: analysis of CD4+ and CD8+ T cell target specificities by intracellular delivery of viral proteins using the human immunodeficiency virus Tat protein transduction domain

Japanese encephalitis virus (JEV), a single-stranded positive-sense RNA virus of the family Flaviviridae, is the major cause of paediatric encephalitis in Asia. The high incidence of subclinical infections in Japanese encephalitis-endemic areas and subsequent evasion of encephalitis points to the development of immune responses against JEV. Humoral responses play a central role in protection against JEV; however, cell-mediated immune responses contributing to this end are not fully understood. The structural envelope (E) protein, the major inducer of neutralizing antibodies, is a poor target for T cells in natural JEV infections. The extent to which JEV non-structural proteins are targeted by T cells in subclinically infected healthy children would help to elucidate the role of cell-mediated immunity in protection against JEV as well as other flaviviral infections. The property of the Tat peptide of Human immunodeficiency virus to transduce proteins across cell membranes, facilitating intracellular protein delivery following exogenous addition to cultured cells, prompted us to express the four largest proteins of JEV, comprising 71 % of the JEV genome coding sequence, as Tat fusions for enumerating the frequencies of virus-specific CD4+ and CD8+ T cells in JEV-immune donors. At least two epitopes recognized by distinct HLA alleles were found on each of the non-structural proteins, with dominant antiviral Th1 T cell responses to the NS3 protein in nearly 96 % of the cohort. The data presented here show that non-structural proteins are frequently targeted by T cells in natural JEV infections and may be efficacious supplements for the predominantly antibody-eliciting E-based JEV vaccines.

Sub-fragments of the envelope gene are highly protective against the Japanese encephalitis virus lethal infection in DNA priming—protein boosting immunization strategies

The envelope (E) gene of Japanese encephalitis virus (JEV) plays a major protective role against JEV infection. In order to locate the part of E gene that is responsible for this protection, an N-terminal fragment EA (nucleotide number 933-1877 bp of JEV genome) and a C-terminal fragment EB (nucleotide number 1851-2330 bp of JEV genome) from E gene were prepared. Both of these fragments were used in the form of recombinant proteins (rEA and rEB) and plasmid DNA (pEA, pM15EA and pEB) for immunizations. Recombinant EA protein (rEA) was previously found to be non-protective because it was expressed in an insoluble form. Plasmid EA (pEA) was also found to be non-protective unless it is preceded by a 15 mer signal peptide derived from the very C-terminal of the membrane gene (M) of JEV to form pM15EA plasmid indicating the importance of the signal peptide in the expression of EA immunogenicity. Although pM15EA and pEB are both immunogenic and protective against JEV lethal infection, the protection by both fragments however is not optimal. Even when pM15EA and pEB were used together for immunization, maximum protection as those induced by control vaccine was not achieved. However, if individual fragments (EA or EB) were used in a DNA priming-protein boosting or protein priming-DNA boosting strategy, high levels of protection were achieved by both fragments. This was especially true for EA fragment where the level of protection against JEV lethal infection was equal to that induced by commercially available vaccine alone. The protection correlated very well with the neutralizing antibody titers and the T helper cell involved in this process in mainly the Th1 type.

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.

Enhancing effect of vaxfectin on the ability of a Japanese encephalitis DNA vaccine to induce neutralizing antibody in mice

Vaxfectin, a recently developed adjuvant, was evaluated for its enhancing effect on immunogenicity of a Japanese encephalitis (JE) DNA vaccine plasmid encoding the JE virus premembrane (prM) and envelope (E) genes (designated pcJEME), using BALB/c and ICR mice. Formulation of pcJEME with Vaxfectin provided > or =8-fold higher neutralizing antibody titers than those induced by pcJEME alone and reduced the amount of pcJEME to one-tenth to induce comparable levels of neutralizing antibody. Use of Vaxfectin did not alter a Th1 type IgG isotype immune response (IgG1 < IgG2a) induced by pcJEME in mice. These results indicate that Vaxfectin has an ability to enhance immunogenicity of pcJEME and is considered as a useful adjuvant for DNA vaccines in murine experimental models.

Induction of cross-protection against two wild-type Taiwanese isolates of Japanese encephalitis virus using Beijing-1 strain DNA vaccine

Japanese encephalitis virus (JEV), a mosquito-borne flavivirus, is an important human pathogen. Mouse brain-derived, inactivated JEV vaccines have contributed greatly to the reduction in numbers of JE patients in several countries, including Taiwan. However, mice immunized with the Nakayama strain inactivated vaccine show lower protection against a lethal strain of Beijing-1 JEV than those immunized with the homologous vaccine. DNA vaccine encoding the envelope (E) protein gene appears to provide protection against JEV in the mouse model, but it is not known whether such vaccines would confer cross-protection for mice against different strains of JEV. In this study, we evaluated the ability of pE, a plasmid DNA vaccine encoding the Beijing-1 envelope protein to elicit cross-protective immunity against infection with the homologous Beijing-1 strain and two lethal Taiwanese isolates of JEV, CH2195 and CEN. Our results showed that mice immunized with pE were protected against lethal challenge with Beijing-1 JEV as well as two Taiwanese isolates. In addition, nai;ve mice were cross-protected by passive transfer of sera from immunized animals, indicating the crucial role of humoral immunity in protection. These results demonstrate that JEV DNA vaccine can provide effective protection against infection not only with homologous virus, but also with heterologous virus.

Comparison of protective efficacies of plasmid DNAs encoding Japanese encephalitis virus proteins that induce neutralizing antibody or cytotoxic T lymphocytes in mice

Mice immunized with a plasmid DNA encoding the premembrane (prM) and envelope (E) proteins of Japanese encephalitis (JE) virus (designated pcJEME) produce neutralizing antibodies and are protected from JE. To determine the role of the immune response to other viral proteins in protection, we constructed plasmid DNAs encoding other JE virus proteins and made a direct comparison among these plasmids using a mouse model. Cytotoxic T lymphocytes (CTLs) were induced by plasmids encoding capsid (C) or nonstructural proteins, NS1, NS2A, NS2B, NS3 or NS5. However, these plasmids provided only a partial protection against intraperitoneal challenge with a lethal dose of JE virus, whereas mice immunized with pcJEME were fully protected. In mice inoculated with CTL-inducing plasmids, high virus titers were detected in plasma immediately (1h) following challenge and in brain on day 4 post-challenge, but no virus infectivity was detected in plasma and brain of pcJEME-immunized mice during the 5 days following challenge. These results indicate that protection provided by the prM/E-encoding DNA consists of neutralizing antibody that prevents virus dissemination from the peripheral site to the brain, and that this antibody-mediated mechanism of protection is more efficient than the immunity induced by plasmids that generate CTL responses capable of killing JE virus-infected cells.

Development of a recombinant vaccine against Japanese encephalitis

Japanese encephalitis (JE) is the major form of viral encephalitis in much of the South-East Asia, India, and China. The disease is caused by a mosquito-borne virus known as Japanese encephalitis virus (JEV). The virus spreads in the form of epidemics, although several endemic areas for JEV activity are known. In recent years, JEV has spread to newer geographic locations such as Australia and Pakistan, and thus has become an important emerging virus infection in these areas. A mouse brain-derived, formalin-inactivated vaccine is available for immunization against JE. Because the formalin-inactivated JEV vaccine has limitations in terms of safety, availability, and cost, attempts are being made to develop improved vaccine using the recombinant DNA technology. This article reviews various attempts in this direction and summarizes the latest developments such as the recombinant yellow fever virus- or the plasmid DNA-based JEV vaccine.

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.

Immunogenicity of a Japanese encephalitis DNA vaccine candidate in cynomolgus monkeys

A Japanese encephalitis (JE) vaccine candidate encoding JE virus premembrane (prM) and envelope (E) genes, designated pNJEME, was evaluated for safety and immunogenicity in non-human primate, cynomolgus monkeys. pNJEME was constructed using a vector (pNGVL4a) designed to address some of the safety concerns of DNA vaccine. In two different experiments, two immunizations with 300 microg of pNJEME by intramuscular (i.m.) injection, and 3 microg of pNJEME using a gene gun, and three immunizations by i.m. injection with 500 microg of pNJEME were performed. All the three protocols induced low to high levels of neutralizing antibody, indicating an ability of pNJEME to induce neutralizing antibody in monkeys with a wide individual variation in response to pNJEME. In one experiment designed to compare the DNA vaccine with a commercial inactivated JE vaccine, three immunizations by i.m. inoculation with 300 microg of pNJEME or by gene gun administration with 3 microg of pNJEME induced similar levels of neutralizing antibody to those induced by three immunizations with a human dose of the inactivated vaccine in most monkeys. After intranasal challenge with the Beijing P3 or JaTH160 strain of JE virus, pNJEME-immunized monkeys showed anamnestic neutralizing antibody responses, indicating that pNJEME induced memory B cells which were responsive to infection with JE virus. No systemic and local reactions were observed in any monkeys after i.m. or gene gun inoculations with plasmid DNAs.

T helper responses to Japanese encephalitis virus infection are dependent on the route of inoculation and the strain of mouse used

T helper cytokine and IgG subtype responses were studied in three strains of mice (C57BL/6J, Swiss albino, BALB/c; n=90 per strain) immunized with live Japanese encephalitis virus (JEV) by intraperitoneal (IP), subcutaneous (SC) and peroral (PO) routes. Lymphocytes obtained from the spleens of immunized and control mice were stimulated in vitro with JEV for 48 h and the supernatants were assayed for the presence of the cytokines IL-4 and IFN-gamma. JEV-specific IgG isotypes were also measured in the sera of immunized mice. T helper cytokine responses in mice immunized with JEV were found to be strain- and route-specific in the three species tested. Moreover, they were also dependent on the type of immunogen used (live vs killed virus), as well as the number of doses administered. C57BL/6J and BALB/c mice were more uniform in their T helper responses compared with the outbred Swiss albino mice and induced a good Th1 response (P<0.001). Among the three routes evaluated, the IP and SC routes consistently elicited a Th1 response compared with the PO route (P<0.001), where an initial Th2-type response reverted to a Th1 response after repeated immunization. Live JEV induced a Th1 response while the commercial killed vaccine induced a predominant Th2 profile.

Traditional and novel approaches to flavivirus vaccines

Yellow fever, dengue, Japanese encephalitis and tick-borne encephalitis viruses are the medically most important members of the Flavivirus genus composed primarily of arboviruses. In this paper, we review the commercially available traditional flavivirus vaccines against yellow fever, Japanese encephalitis, and tick-borne encephalitis, as well as modern approaches to flavivirus vaccines. Formalin inactivation technology has been employed to produce killed vaccines. Flaviviruses have been attenuated by multiple passages in animal tissues and cell cultures to produce empirical live attenuated vaccines. The use of traditional methods is being pursued to develop vaccines against other flavivirus diseases, such as dengue, and to improve existing vaccines, such as for Japanese encephalitis. With the recent development of infectious clones, rational approaches to attenuated flavivirus vaccines have employed the introduction of specific mutations into wild type viruses and chimerisation between different viruses. Novel methods for delivery of live vaccines, such as inoculation of infectious DNA or RNA, have been described. Other approaches, such as the construction of protein subunit, expression vector-based and naked DNA vaccines, have been proposed to create alternate vaccine candidates.

Inoculation of plasmids encoding Japanese encephalitis virus PrM-E proteins with colloidal gold elicits a protective immune response in BALB/c mice

We established a simple and effective method for DNA immunization against Japanese encephalitis virus (JEV) infection with plasmids encoding the viral PrM and E proteins and colloidal gold. Inoculation of plasmids mixed with colloidal gold induced the production of specific anti-JEV antibodies and a protective response against JEV challenge in BALB/c mice. When we compared the efficacy of different inoculation routes, the intravenous and intradermal inoculation routes were found to elicit stronger and more sustained neutralizing immune responses than intramuscular or intraperitoneal injection. After being inoculated twice, mice were found to resist challenge with 100,000 times the 50% lethal dose (LD(50)) of JEV (Beijing-1 strain) even when immunized with a relatively small dose of 0.5 micro g of plasmid DNA. Protective passive immunity was also observed in SCID mice following transfer of splenocytes or serum from plasmid DNA- and colloidal gold-immunized BALB/c mice. The SCID mice resisted challenge with 100 times the LD(50) of JEV. Analysis of histological sections detected expression of proteins encoded by plasmid DNA in the tissues of intravenously, intradermally, and intramuscularly inoculated mice 3 days after inoculation. DNA immunization with colloidal gold elicited encoded protein expression in splenocytes and might enhance immune responses in intravenously inoculated mice. This approach could be exploited to develop a novel DNA vaccine.

Inactivation of classical swine fever virus: association of hydrostatic pressure and ultraviolet irradiation

Reversible pressure-induced disassembly of several viruses has suggested the idea of using hydrostatic pressure to suppress virus infectivity. In this study, the effects of high hydrostatic pressure and ultraviolet (UV) irradiation were investigated on classical swine fever virus (CSFV) in an attempt to eliminate residual infectivity. The structural modifications were followed by intrinsic fluorescence and biological activity assays. The kinetics of CSFV inactivation showed that pressure-induced inactivation was not enough to eliminate viral infectivity. However, when pressure was applied in association with UV irradiation no infectious focus was observed. The application of these two methods against CSFV can be an attractive inactivation strategy for the development of a vaccine.

Tick-borne encephalitis

Tick-borne encephalitis (TBE) is one of the most dangerous human infections occurring in Europe and many parts of Asia. The etiological agent Tick-borne encephalitis virus (TBEV), is a member of the virus genus Flavivirus, of the family Flaviviridae. TBEV is believed to cause at least 11,000 human cases of encephalitis in Russia and about 3000 cases in the rest of Europe annually. Related viruses within the same group, Louping ill virus (LIV), Langat virus (LGTV) and Powassan virus (POWV), also cause human encephalitis but rarely on an epidemic scale. Three other viruses within the same group, Omsk hemorrhagic fever virus (OHFV), Kyasanur Forest disease virus (KFDV) and Alkhurma virus (ALKV), are closely related to the TBEV complex viruses and tend to cause fatal hemorrhagic fevers rather than encephalitis. This review describes the clinical manifestations associated with TBEV infections, the main molecular-biological properties of these viruses, and the different factors that define the incidence and severity of disease. The role of ticks and their local hosts in the emergence of new virus variants with different pathogenic characteristics is also discussed. This review also contains a brief history of vaccination against TBE including trials with live attenuated vaccine and modern tendencies in developing of vaccine virus strains.

Pathogenesis of flavivirus encephalitis

Within the flavivirus family, viruses that cause natural infections of the central nervous system (CNS) principally include members of the Japanese encephalitis virus (JEV) serogroup and the tick-borne encephalitis virus (TBEV) serocomplex. The pathogenesis of diseases involves complex interactions of viruses, which differ in neurovirulence potential, and a number of host factors, which govern susceptibility to infection and the capacity to mount effective antiviral immune responses both in the periphery and within the CNS. This chapter summarizes progress in the field of flavivirus neuropathogenesis. Mosquito-borne and tickborne viruses are considered together. Flavivirus neuropathogenesis involves both neuroinvasiveness (capacity to enter the CNS) and neurovirulence (replication within the CNS), both of which can be manipulated experimentally. Neuronal injury as a result of bystander effects may be a factor during flavivirus neuropathogenesis given that microglial activation and elaboration of inflammatory mediators, including IL-1β and TNF-α, occur in the CNS during these infections and may accompany the production of nitric oxide and peroxynitrite, which can cause neurotoxicity.

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 efficacy of a plasmid DNA encoding Japanese encephalitis virus envelope protein fused to tissue plasminogen activator signal sequences: studies in a murine intracerebral virus challenge model

We report the construction of chimeric DNA vaccine vectors in which secretory signal sequence derived from tissue plasminogen activator (TPA) was fused to the full length (pCMVTE) or 398 amino terminal amino acids (pCMVTdeltaE) of Japanese encephalitis virus (JEV) envelope (E) protein. Transfection studies indicate that E protein expressed from pCMVTdeltaE-transfected cells but not pCMVTE-transfected cells is secreted into the culture medium. Analysis of the potency of various DNA vaccine constructs in a murine intracerebral (i.c.) JEV challenge model indicates that pCMVTdeltaE confers the highest level (71%) of protection. Immunization with pCMVTdeltaE induces a mixed Th1 and Th2 T helper cell response while immunization with plasmids encoding nonsecretory forms of E protein induces a Th1 T helper response. Only low levels (<1:20) of virus neutralizing antibody titres were observed in DNA vaccinated mice which did not increase further after i.c. JEV challenge. Thus, immunization with a plasmid encoding secretory E protein results in an altered cytokine response and better protection against i.c. JEV challenge than that conferred by immunization with plasmids encoding nonsecretory forms of E protein. We also demonstrate that unlike peripheral JEV challenge, i.c. JEV challenge does not result in an increase in anamnestic antibody response suggesting that other components of immune system such as cytotoxic T cells and T helper cells contribute to protection against i.c. JEV challenge of DNA vaccinated mice.

Protection against lethal enterovirus 71 infection in newborn mice by passive immunization with subunit VP1 vaccines and inactivated virus

Enterovirus 71 (EV71), the newest member of Enteroviridae, is notable for its etiological role in epidemics of severe neurological diseases in children. Developing effective vaccines is considered a top choice among all control measures. We compared the inactivated virus vaccine (10 microg protein/mouse) with subunit vaccines--VP1 DNA vaccine (100 microg/mouse) or recombinant VP1 protein (10 microg/mouse)--in its ability to elicit maternal antibody and to provide protection against lethal infection of EV71 in suckling mice. Prior to gestation, all three groups of vaccinated dams possessed similar levels of neutralizing antibody. With a challenge dose of 2300 LD(50) virus/mouse, suckling mice born to dams immunized with inactivated virus showed 80% survival. The subunit vaccines provided protection only at a lower challenge dosage of 230 LD(50) per mouse, with 40% survival for DNA vaccine and 80% survival for VP1 protein. The cytokine profile produced by splenocytes showed a high level of IL-4 in the inactivated virus group, high levels of IFN-gamma and IL-12 in the DNA vaccine group, and high levels of IL-10 and IFN-gamma in the VP1 protein group. Overall, the inactivated virus elicited a much greater magnitude of immune response than the subunit vaccines, including total IgG, all four IgG subtypes, and T-helper-cell responses; these antibodies were shown to be protective against lethal infection when passively transferred to susceptible newborn mice. Our data indicated that inactivated virus is the choice of vaccine preparation capable of fulfilling the demand for effective control, and that VP1 subunit vaccines remain promising vaccine strategies that require further refinement.

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.

DNA vaccination with the Hantaan virus M gene protects Hamsters against three of four HFRS hantaviruses and elicits a high-titer neutralizing antibody response in Rhesus monkeys

Four hantaviruses-Hantaan virus (HTNV), Seoul virus (SEOV), Dobrava virus (DOBV) and Puumala virus-are known to cause hemorrhagic fever with renal syndrome (HFRS) in Europe and Asia. HTNV causes the most severe form of HFRS (5 to 15% case-fatality rate) and afflicts tens of thousands of people annually. Previously, we demonstrated that DNA vaccination with a plasmid expressing the SEOV M gene elicited neutralizing antibodies and protected hamsters against infection with SEOV and HTNV. Here, we report the construction and evaluation of a DNA vaccine that expresses the HTNV M gene products, G1 and G2. DNA vaccination of hamsters with the HTNV M gene conferred sterile protection against infection with HTNV, SEOV, and DOBV. DNA vaccination of rhesus monkeys with either the SEOV or HTNV M gene elicited high levels of neutralizing antibodies. These are the first immunogenicity data for hantavirus DNA vaccines in nonhuman primates. Because a neutralizing antibody response is considered a surrogate marker for protective immunity in humans, our protection data in hamsters combined with the immunogenicity data in monkeys suggest that hantavirus M gene-based DNA vaccines could protect humans against the most severe forms of HFRS.

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.

Suppression of immune response and protective immunity to a Japanese encephalitis virus DNA vaccine by coadministration of an IL-12-expressing plasmid

IL-12 plays a central role in both innate and acquired immunity and has been demonstrated to potentiate the protective immunity in several experimental vaccines. However, in this study, we show that IL-12 can be detrimental to the immune responses elicited by a plasmid DNA vaccine. Coadministration of the IL-12-expressing plasmid (pIL-12) significantly suppressed the protective immunity elicited by a plasmid DNA vaccine (pE) encoding the envelope protein of Japanese encephalitis virus. This suppressive effect was associated with marked reduction of specific T cell proliferation and Ab responses. A single dose of pIL-12 treatment with plasmid pE in initial priming resulted in significant immune suppression to subsequent pE booster immunization. The pIL-12-mediated immune suppression was dose dependent and evident only when the IL-12 gene was injected either before or coincident with the pE DNA vaccine. Finally, using IFN-gamma gene-disrupted mice, we showed that the suppressive activity of the IL-12 plasmid was dependent upon endogenous production of IFN-gamma. These results demonstrate that coexpression of the IL-12 gene can sometimes produce untoward effects to immune responses, and thus its application as a vaccine adjuvant should be carefully evaluated.