Development of new vaccines against dengue fever and Japanese encephalitis

Limited Transmission Potential of Takeda's Tetravalent Dengue Vaccine Candidate by Aedes albopictus

Recombinant live-attenuated chimeric tetravalent dengue vaccine viruses, TDV-1, -2, -3, and -4, contain the premembrane and envelope genes of dengue virus serotypes 1-4 in the replicative background of the attenuated dengue virus type-2 (DENV-2) PDK-53 vaccine strain. Previous results have shown that these recombinant vaccine viruses demonstrate limited infection and dissemination in Aedes aegypti and are unlikely to be transmitted by the primary mosquito vector of DENVs. In this report, we expand this analysis by assessing vector competence of all four serotypes of the TDV virus in Aedes albopictus, the secondary mosquito vector of DENVs. Our results indicate that these vaccine viruses demonstrate incompetence or defective infection and dissemination in these mosquitoes and will likely not be transmissible.

Dengue vaccines: Are they safe for travelers?

The four dengue viruses (DENV) circulate among nearly one-half of the world's population in tropical and semitropical countries imposing a huge morbidity burden on travelers. Sanofipasteur has developed a tetravalent live-attenuated vaccine, Dengvaxia, recently approved by the World Health Organization and licensed in four dengue-endemic countries. An additional two dengue vaccines, developed by the National Institute of Allergy and Infectious Diseases (NIAID), USA and Takeda, are entering phase III testing. Dengvaxia is composed of four yellow fever 17D-DENV chimeras, the NIAID vaccine contains three mutagenized DENV and one DENV2/4 chimera while the Takeda vaccine contains an attenuated DENV 2 and three DENV 2-DENV chimeras. Which of these vaccines might be useful in protecting travelers against dengue infections and disease? Dengvaxia requires three doses administered over the course of one year but in addition has safety signals suggesting that susceptible individuals should not be vaccinated. The NIAID vaccine is promising as a travel vaccine as a single dose fully protected susceptible adults against live dengue 2 virus challenge. The protective efficacy and safety of the Takeda vaccine remain to be demonstrated.

Cooperation between CD4+ T Cells and Humoral Immunity Is Critical for Protection against Dengue Using a DNA Vaccine Based on the NS1 Antigen

Dengue virus (DENV) is spread through most tropical and subtropical areas of the world and represents a serious public health problem. At present, the control of dengue disease is mainly hampered by the absence of antivirals or a vaccine, which results in an estimated half worldwide population at risk of infection. The immune response against DENV is not yet fully understood and a better knowledge of it is now recognized as one of the main challenge for vaccine development. In previous studies, we reported that a DNA vaccine containing the signal peptide sequence from the human tissue plasminogen activator (t-PA) fused to the DENV2 NS1 gene (pcTPANS1) induced protection against dengue in mice. In the present work, we aimed to elucidate the contribution of cellular and humoral responses elicited by this vaccine candidate for protective immunity. We observed that pcTPANS1 exerts a robust protection against dengue, inducing considerable levels of anti-NS1 antibodies and T cell responses. Passive immunization with anti-NS1 antibodies conferred partial protection in mice infected with low virus load (4 LD₅₀), which was abrogated with the increase of viral dose (40 LD₅₀). The pcTPANS1 also induced activation of CD4⁺ and CD8⁺ T cells. We detected production of IFN-γ and a cytotoxic activity by CD8⁺ T lymphocytes induced by this vaccine, although its contribution in the protection was not so evident when compared to CD4⁺ cells. Depletion of CD4⁺ cells in immunized mice completely abolished protection. Furthermore, transfer experiments revealed that animals receiving CD4⁺ T cells combined with anti-NS1 antiserum, both obtained from vaccinated mice, survived virus infection with survival rates not significantly different from pcTPANS1-immunized animals. Taken together, results showed that the protective immune response induced by the expression of NS1 antigen mediated by the pcTPANS1 requires a cooperation between CD4⁺ T cells and the humoral immunity.

Dengue is an emerging mosquito-borne disease present in an extensive area of the globe with an estimated risk exposure of half of the world’s population. Unfortunately, no specific treatment or vaccine is available to control this disease, which leads to approximately 20,000 casualties annually. The protective immune response against this pathogen consists of an important goal for the development of anti-dengue strategies. For years, the presence of neutralizing antibodies was believed to represent the major response for protection against dengue. However, a recent clinical trial showed that despite the induction of a balanced antibody response against all serotypes, vaccination had only a partial efficacy. In the present work, we aimed to elucidate the contribution of the cellular and humoral responses elicited by a DNA vaccine candidate encoding the non-structural 1 protein (NS1) from dengue virus. We observed that antibody as well as T cell responses are important for protection against dengue in a cooperative way. Our results demonstrated that an effective defense against virus was not achieved with antibodies or T cells alone, but rather with the combination of both responses. Therefore, we suggest that an ideal vaccine against dengue should induce both arms of the immune system.

An adjuvanted, tetravalent dengue virus purified inactivated vaccine candidate induces long-lasting and protective antibody responses against dengue challenge in rhesus macaques

The immunogenicity and protective efficacy of a candidate tetravalent dengue virus purified inactivated vaccine (TDENV PIV) formulated with alum or an Adjuvant System (AS01, AS03 tested at three different dose levels, or AS04) was evaluated in a 0, 1-month vaccination schedule in rhesus macaques. One month after dose 2, all adjuvanted formulations elicited robust and persisting neutralizing antibody titers against all four dengue virus serotypes. Most of the formulations tested prevented viremia after challenge, with the dengue serotype 1 and 2 virus strains administered at 40 and 32 weeks post-dose 2, respectively. This study shows that inactivated dengue vaccines, when formulated with alum or an Adjuvant System, are candidates for further development.

Dengue vaccine: priorities and progress

Dengue transmission has increased considerably in the past 20 years. Currently, it can only be reduced by mosquito control; however, the application of vector-control methods are labor intensive, require discipline and diligence, and are hard to sustain. In this context, a safe dengue vaccine that confers long-lasting protection against infection with the four dengue viruses is urgently required. This review will discuss the requirements of a dengue vaccine, problems, and advances that have been made. Finally, new targets for research will be presented.

Virus antibody dynamics in primary and secondary dengue infections

Dengue viral infections show unique infection patterns arising from its four serotypes, (DENV-1,2,3,4). Its effects range from simple fever in primary infections to potentially fatal secondary infections. We analytically and numerically analyse virus dynamics and humoral response in a host during primary and secondary dengue infection for long periods using micro-epidemic models. The models presented here incorporate time delays, antibody dependent enhancement, a dynamic switch and a correlation factor between different DENV serotypes. We find that the viral load goes down to undetectable levels within 7-14 days as is observed for dengue infection, in both cases. For primary infection, the stability analysis of steady states shows interesting dependence on the time delay involved in the production of antibodies from plasma cells. We demonstrate the existence of a critical value for the immune response parameter, beyond which the infection gets completely cured. For secondary infections with a different serotype, the homologous antibody production is enhanced due to the influence of heterologous antibodies. The antibody production is also controlled by the correlation factor, which is a measure of similarities between the different DENV serotypes involved. Our results agree with clinically observed humoral responses for primary and secondary infections.

Assessing the interest to participate in a dengue vaccine efficacy trial among residents of Puerto Rico

Dengue, endemic in Puerto Rico, is a major public health problem. Vaccines are thought the best means to prevent dengue because vector control alone has been largely ineffective. We implemented qualitative studies in 2006 and 2010 to determine the acceptability of conducting placebo-controlled dengue vaccine efficacy trials in Puerto Rican children. Key informant interviews and focus groups with parents and children were conducted in municipalities with high dengue incidence. We used structured open-ended questions to determine motivators and attitudes regarding vaccine trial participation. Knowledge about dengue risk and prevention, and knowledge, attitudes, and beliefs regarding vaccines and vaccine trials were assessed. Using grounded theory, we conducted content analysis and established categories and sub-categories of participant responses. All participants were knowledgeable about dengue prevention and perceived children as most affected age groups. Participants were aware of vaccines benefits and they thought a vaccine could prevent dengue. However, most would not allow their children to participate in a placebo-controlled vaccine trial. Barriers included lack of trust in new vaccines and vaccine trial procedures; fear of developing dengue or side effects from the vaccine and lack of information about candidate dengue vaccines. Participants thought information, including results of previous trials might overcome barriers to participation. Motivators for participation were altruism, protection from dengue, free medical attention, and compensation for transportation and participation. Parents would consider children participation if accurate vaccine trial information is provided.

Development of DENVax: a chimeric dengue-2 PDK-53-based tetravalent vaccine for protection against dengue fever

Dengue. virus infection is the leading arboviral cause of disease worldwide. A vaccine is being developed based on the attenuated DEN-2 virus, DEN-2 PDK-53. In this review, we summarize the characteristics of the parent DEN-2 PDK-53 strain as well as the chimeric viruses containing the prM and E genes of DEN-1, DEN-3 or DEN-4 virus in the genetic backbone of the DEN-2 PDK-53 virus (termed DENVax). Tetravalent DENVax formulations containing cloned, fully sequenced isolates of the DEN-2 PDK-53 virus and the three chimeras have been evaluated for safety and efficacy in preclinical animal models. Based on the safety, immunogenicity and efficacy in preclinical studies, Phase 1 clinical testing of DENVax has been initiated.

Efficacy of a tetravalent chimeric dengue vaccine (DENVax) in Cynomolgus macaques

Three tetravalent formulations of chimeric dengue (DENVax) viruses containing the pre-membrane and envelope genes of serotypes 1-4 expressed by the attenuated DENV-2 PDK-53 genome were tested for safety, immunogenicity, and efficacy in cynomolgus macaques (Macaca fascicularis). Subcutaneous injection of the DENVax formulations was well-tolerated. Low levels of viremia of only one of the four vaccine viruses were detected yet virus neutralizing antibody titers were induced against all four dengue virus serotypes after one or two administrations of vaccine. All animals immunized with the high-dose formulation were protected from viremia, and all immunized animals were completely protected from DENV-3 and DENV-4 challenge. A lower dose of DENVax formulation partially protected animals from DENV-1 or DENV-2 challenge. In contrast, all control animals developed high levels of viremia for multiple days after challenge with DENV 1-4. This study highlights the immunogenicity and efficacy of the tetravalent DENVax formulations in nonhuman primates.

DNA vaccines against dengue virus type 2 based on truncate envelope protein or its domain III

Two DNA vaccines were constructed encoding the ectodomain (domains I, II and III) of the DENV2 envelope protein (pE1D2) or only its domain III (pE2D2), fused to the human tissue plasminogen activator signal peptide (t-PA). The expression and secretion of recombinant proteins was confirmed in vitro in BHK cells transfected with the two plasmids, detected by immunofluorescence or immunoprecipitation of metabolically labeled gene products, using polyclonal and monoclonal antibodies against DENV2. Besides, results reveal that the ectodomain of the E protein can be efficiently expressed in vivo, in a mammalian system, without the prM protein that is hypothesized to act as a chaperonin during dengue infection. Balb/c mice were immunized with the DNA vaccines and challenged with a lethal dose of DENV2. All pE1D2-vaccinated mice survived challenge, while 45% of animals immunized with the pE2D2 died after infection. Furthermore, only 10% of pE1D2-immunized mice presented some clinical signs of infection after challenge, whereas most of animals inoculated with the pE2D2 showed effects of the disease with high morbidity degrees. Levels of neutralizing antibodies were significantly higher in pE1D2-vaccinated mice than in pE2D2-immunized animals, also suggesting that the pE1D2 vaccine was more protective than the pE2D2.

Reverse genetics and the study of dengue virus

Dengue virus infection causes the most important arthropod-borne disease of humans. Currently, there are no dengue vaccines or antiviral therapies in clinical use, although their development is a global health priority. Using a technique known as ‘reverse genetics’, the dengue virus RNA genome can be manipulated, either by the introduction of specific mutations or the deletion and/or substitution of entire genes. This has led to the production of novel recombinant viruses that have potential as vaccines and the production of noninfectious viral subgenomes (termed replicons) useful for drug screening. Reverse genetics is also an invaluable tool for studying the role of dengue virus RNA elements and proteins in replication and pathogenesis. This review describes the contribution of reverse genetics to dengue virus research to date, highlighting the potential use of this technology in the development of effective control measures against dengue in the future.

Dengue Vaccine: The Current Status

Dengue fever is a re-emerging public health problem with two-fifths of the world population being at risk of infection. Since there are no antiviral drugs available against the dengue virus, and vector control programmes have been largely unsuccessful in preventing outbreaks, vaccination seems to be the most viable option for preventing infection. An ideal dengue vaccine should provide long lasting immunity against all four serotypes of the virus. The envelope protein of the virus plays a key role in vaccine development. The present day candidate vaccines includes a live attenuated tetravalent vaccine, intertypic chimaeric vaccines based on live attenuated dengue virus vectors, chimaeric vaccines based on the live attenuated Yellow Fever 17D vector and recombinant vaccines which include vaccines based on flavivirus and non-flavivirus vectors. Tetravalent live attenuated vaccines, intertypic chimaeric vaccines and chimaeric vaccines are being tested in human trials. Recombinant DNA vaccines based on flavivirus and non-flavivirus vectors are being tested in animal trials. Recent studies have shown that the tetravalent formulations may elicit an unbalanced immune response. Research is continuing to find means of obtaining a balanced response to all antigens in the tetravalent formulations.

Multiple antigenic peptides as vaccine platform for the induction of humoral responses against dengue-2 virus

Dengue is an important agent of human disease for which no licensed vaccine is available to the public. We used multiple antigenic peptides (MAPs) as an antigen carrier for the development of subunit vaccines against dengue-2 virus (DEN-2). Commercially available software (MacVector 7.0) was used to identify potential antigenic B-cell epitopes of E-glycoprotein. A total of 60 BALB/c mice were immunized with 12 recombinant DEN-2-specific MAPs and the humoral immune response was assessed by anti-DEN-2 ELISA and PRNT50 assays. Anti-DEN-2 ELISA showed high levels of anti-DEN-2 antibodies and post-immune sera reduced viral infectivity and prevented infection of monkey kidney cells (LLC-MK2) with live DEN-2 virus. Seven neutralizing DEN-2 epitopes were identified that generated PRNT50 titers of up to 1:160. Our findings show that the MAP platform can be used as an antigen-presenting platform for dengue vaccine development.

Induction of neutralizing antibodies against dengue virus type 2 upon mucosal administration of a recombinant Lactococcus lactis strain expressing envelope domain III antigen

Mucosal vaccines present several advantages over conventional parenteral vaccines including their ease of administration and low cost, both criteria being priorities for developing countries plagued by infectious diseases. A recombinant Lactococcus lactis strain producing the envelope domain III (EDIII) antigen from dengue virus serotype 2 was engineered, and the ability of the live recombinant bacteria to trigger a systemic anti-EDIII IgG antibody response upon nasal or oral administration to BALB/c and C57BL/6 mice was investigated. Results showed that the antibody response depended on the route of administration and on the mouse strain inoculated. Out of six, two and three C57BL/6 mice orally and nasally inoculated with the recombinant bacteria, respectively, displayed anti-EDIII antibody responses higher than that obtained in the mouse group intraperitoneally (i.p.) immunized with heat-inactivated dengue 2 virus. The protective potential of the immune sera was measured using the plaque reduction neutralizing test (PRNT) and results indicated that high anti-EDIII antibody levels did not correlate directly with high neutralizing activities. Immune sera from orally inoculated mice were found the most potent to neutralize in vitro dengue infection with neutralizing antibody activities in some cases higher than that obtained with the immune sera from mice i.p. injected with heat-inactivated virus.

Induction of tetravalent protective immunity against four dengue serotypes by the tandem domain III of the envelope protein

In the present study, the domain IIIs of all four dengue virus (DENV) serotypes were connected sequentially to construct the tandem domain III. The resulting DNA fragment was then cloned into pBAD/Topo ThioFusion plasmid. After induction, the Escherichia coli expression protein was purified and used to immunize BALB/c mice by subcutaneous route. The sera from mice immunized with the purified protein were confirmed to contain specific high antibody titers against DEN1, DEN2, and DEN4, and moderate antibody titer against DEN3. In suckling mouse model, 70% of the mice challenged with DEN1, DEN2, and DEN4 in combination with sera from mice immunized with the purified protein were protected, and 18% of the mice challenged with DEN3 in combination with the same sera were protected. Our data suggest that the tandem domain III of the envelope protein can be used as a potential tetravalent dengue vaccine based on a single antigen.

DNA vaccines against dengue virus based on the ns1 gene: The influence of different signal sequences on the protein expression and its correlation to the immune response elicited in mice

We analyzed four DNA vaccines based on DENV-2 NS1: pcENS1, encoding the C-terminal from E protein plus the NS1 region; pcENS1ANC, similar to pcENS1 plus the N-terminal sequence from NS2a (ANC); pcTPANS1, coding the t-PA signal sequence fused to NS1; and pcTPANS1ANC, similar to pcTPANS1 plus the ANC sequence. The NS1 was detected in lysates and culture supernatants from pcTPANS1-, pcENS1- and pcENS1ANC-transfected cells and not in cells with pcTPANS1ANC. Only the pcENS1ANC leads the expression of NS1 in plasma membrane, confirming the importance of ANC sequence for targeting NS1 to cell surface. High levels of antibodies recognizing conformational epitopes of NS1 were induced in mice immunized with pcTPANS1 and pcENS1, while only few pcENS1ANC-inoculated animals presented detectable anti-NS1 IgG. Protection against DENV-2 was verified in pcTPANS1- and pcENS1-immunized mice, although the plasmid pcTPANS1 induced slight higher protective immunity. These plasmids seem to activate distinct patterns of the immune system.

Tetravalent neutralizing antibody response against four dengue serotypes by a single chimeric dengue envelope antigen

We employed DNA shuffling and screening technologies to develop a single recombinant dengue envelope (E) antigen capable of inducing neutralizing antibodies against all four antigenically distinct dengue serotypes. By DNA shuffling of codon-optimized dengue 1-4 E genes, we created a panel of novel chimeric clones expressing C-terminal truncated E antigens that combined epitopes from all four dengue serotypes. DNA vaccines encoding these novel chimeras induced multivalent T cell and neutralizing antibody responses against all four dengue serotypes in mice. By contrast, a mixture of four unshuffled, parental DNA vaccines failed to produce tetravalent neutralizing antibodies in mice. The neutralizing antibody titers for some of these antigens could be further improved by extending the sequences to express full-length pre-membrane and envelope proteins. The chimeric antigens also protected mice against a lethal dengue-2 virus challenge. These data demonstrate that DNA shuffling and associated screening can lead to the selection of multi-epitope antigens against closely related dengue virus serotypes and suggest a broad utility for these technologies in optimizing vaccine antigens.

New developments in flavivirus vaccines with special attention to yellow fever

PURPOSE OF REVIEW

Here we review recent epidemiological trends in flavivirus diseases, findings related to existing vaccines, and new directions in flavivirus vaccine research. We emphasize the need for stepped-up efforts to stop further spread and intensification of these infections worldwide.

RECENT FINDINGS

Although the incidence and geographic distribution of flavivirus diseases have increased in recent years, human vaccines are available only for yellow fever, Japanese encephalitis, tick-borne encephalitis and Kyasanur forest disease. Factors contributing to resurgence include insufficient supplies of available vaccines, incomplete vaccination coverage and relaxation in vector control. Research has been underway for 60 years to develop effective vaccines against dengue, and recent progress is encouraging. The development of vaccines against West Nile, virus recently introduced to North America, has been initiated. In addition, there is considerable interest in improving existing vaccines with respect to increasing safety (e.g. eliminating the newly recognized syndrome of yellow fever vaccine-associated viscerotropic adverse disease), and to reducing the cost and number of doses required for effective immunization.

SUMMARY

Traditional approaches to flavivirus vaccines are still employed, while recent advancements in biotechnology produced new approaches to vaccine design, such as recombinant live virus, subunit and DNA vaccines. Live chimeric vaccines against dengue, Japanese encephalitis and West Nile based on yellow fever 17D virus (ChimeriVax) are in phase I/II trials, with encouraging results. Other chimeric dengue, tick-borne encephalitis and West Nile virus candidates were developed based on attenuated dengue backbones. To further reduce the impact of flavivirus diseases, vaccination policies and vector control programs in affected countries require revision.

Chimeric dengue 2 PDK-53/West Nile NY99 viruses retain the phenotypic attenuation markers of the candidate PDK-53 vaccine virus and protect mice against lethal challenge with West Nile virus

Chimeric dengue serotype 2/West Nile (D2/WN) viruses expressing prM-E of WN NY99 virus in the genetic background of wild-type D2 16681 virus and two candidate D2 PDK-53 vaccine variants (PDK53-E and PDK53-V) were engineered. The viability of the D2/WN viruses required incorporation of the WN virus-specific signal sequence for prM. Introduction of two mutations at M-58 and E-191 in the chimeric cDNA clones further improved the viability of the chimeras constructed in all three D2 carriers. Two D2/WN chimeras (D2/WN-E2 and -V2) engineered in the backbone of the PDK53-E and -V viruses retained all of the PDK-53 vaccine characteristic phenotypic markers of attenuation and were immunogenic in mice and protected mice from a high-dose 10(7) PFU challenge with wild-type WN NY99 virus. This report further supports application of the genetic background of the D2 PDK-53 virus as a carrier for development of live-attenuated, chimeric flavivirus vaccines in general and the development of a chimeric D2/WN vaccine virus against WN disease in particular.

Induction of protective antibodies against dengue virus by tetravalent DNA immunization of mice with domain III of the envelope protein

Dengue fever is a growing public health concern around the world and despite vaccine development efforts, there are currently no effective dengue vaccines. In the present study we report the induction of protective antibodies against dengue virus by DNA immunization with domain III (DIII) region of the envelope protein (E) in a mouse model. The DIII region of all four dengue virus serotypes were cloned separately into pcDNA 3 plasmid. Protein expression was tested in COS-7 cells. Each plasmid, or a tetravalent combination, were used to immunize BALB/c mice by intramuscular route. Presence of specific antibodies was evaluated by ELISA, and neutralizing antibodies were tested using a cytopathogenic effect (CPE) inhibition assay in BHK-21 cells, as well as in newborn mice challenged intracranially with dengue 2 virus. Mice immunized with individual DIII constructs or the tetravalent formulation developed antibodies against each corresponding dengue serotype. Antibody titers by ELISA were similar for all serotypes and no significant differences were observed when boosters were administered, although antibody responses were dose-dependent. CPE inhibition assays using Den-2 virus showed neutralization titers of 1:10 in mice immunized with individual DIII plasmid or those immunized with the tetravalent formulations. 43% of newborn mice challenged with Den-2 in combination with sera from mice immunized with Den-2 DIII plasmid were protected, whereas sera from mice immunized with the tetravalent formulation conferred 87% protection. Our results suggest that DIII can be used as a tetravalent DNA formulation to induce neutralizing and protective antibodies against dengue virus.

Emerging flaviviruses: the spread and resurgence of Japanese encephalitis, West Nile and dengue viruses

Mosquito-borne flaviviruses provide some of the most important examples of emerging and resurging diseases of global significance. Here, we describe three of them: the resurgence of dengue in tropical and subtropical areas of the world, and the spread and establishment of Japanese encephalitis and West Nile viruses in new habitats and environments. These three examples also illustrate the complexity of the various factors that contribute to their emergence, resurgence and spread. Whereas some of these factors are natural, such as bird migration, most are due to human activities, such as changes in land use, water impoundments and transportation, which result in changed epidemiological patterns. The three examples also show the ease with which mosquito-borne viruses can spread to and colonize new areas, and the need for continued international surveillance and improved public health infrastructure to meet future emerging disease threats.

Recent advancement in flavivirus vaccine development

Lately, the magnitude of cumulative diseases burden caused by flaviviruses, such as dengue virus, Japanese encephalitis virus, tick-borne encephalitis virus, West Nile virus and yellow fever virus, has reached an unprecedented level with the sizes of human and animal populations at risk increasing sharply. These diseases present highly complex medical, economic and ecologic problems, some effecting primarily human and others affecting human, livestock and wildlife. The large body of recent publications on the development of vaccines taking advantage of new generations of bio-engineering techniques clearly reflects the profound interests and deep sense of urgency in the scientific and medical communities in combating those diseases. This review reveals a collection of remarkable progresses thus far made in flaviviral vaccine research not only employing a diverse range of new strategies but also re-tooling old techniques to improve the existing vaccines. The efficacy and safety of some of the new vaccine candidates have been evaluated and proven in human clinical trials. Besides the technical advancement in vaccine development, in this review, the importance of somewhat neglected and yet critical subjects, such as adequacy of animal model, vaccine safety, vaccine formulation and delivery, complication in serodiagostics and economic factor, was examined in-depth.

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.

Cost-effectiveness of a pediatric dengue vaccine

To ascertain the economic feasibility of a pediatric tetravalent dengue vaccine, we developed and calibrated a cost-effectiveness model of vaccinating children at 15 months in Southeast (SE) Asia using a societal perspective. We assumed that full immunization would require two doses at prices of US$ 0.50 and US$ 10 per dose in the public and private sectors, respectively. The gross cost per 1000 population (of all ages) of the vaccination program would be US$ 154. Due to projected savings in dengue treatment, the net cost per capita would be only US$ 17 (89% below the gross cost). The cost per disability adjusted life year (DALY) saved by a pediatric vaccine would be US$ 50, making the potential vaccine highly cost-effective. Eventually, vaccination may be able to replace environmental control as a strategy for dengue prevention and be cost saving.

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.

Dengue 2 PDK-53 virus as a chimeric carrier for tetravalent dengue vaccine development

Attenuation markers of the candidate dengue 2 (D2) PDK-53 vaccine virus are encoded by mutations that reside outside of the structural gene region of the genome. We engineered nine dengue virus chimeras containing the premembrane (prM) and envelope (E) genes of wild-type D1 16007, D3 16562, or D4 1036 virus within the genetic backgrounds of wild-type D2 16681 virus and the two genetic variants (PDK53-E and PDK53-V) of the D2 PDK-53 vaccine virus. Expression of the heterologous prM-E genes in the genetic backgrounds of the two D2 PDK-53 variants, but not that of wild-type D2 16681 virus, resulted in chimeric viruses that retained PDK-53 characteristic phenotypic markers of attenuation, including small plaque size and temperature sensitivity in LLC-MK(2) cells, limited replication in C6/36 cells, and lack of neurovirulence in newborn ICR mice. Chimeric D2/1, D2/3, and D2/4 viruses replicated efficiently in Vero cells and were immunogenic in AG129 mice. Chimeric D2/1 viruses protected adult AG129 mice against lethal D1 virus challenge. Two tetravalent virus formulations, comprised of either PDK53-E- or PDK53-V-vectored viruses, elicited neutralizing antibody titers in mice against all four dengue serotypes. These antibody titers were similar to the titers elicited by monovalent immunizations, suggesting that viral interference did not occur in recipients of the tetravalent formulations. The results of this study demonstrate that the unique attenuation loci of D2 PDK-53 virus make it an attractive vector for the development of live attenuated flavivirus vaccines.

Policymakers’ views on dengue fever/dengue haemorrhagic fever and the need for dengue vaccines in four southeast Asian countries

A survey of policymakers and other influential professionals in four southeast Asian countries (Cambodia, Indonesia, Philippines and Vietnam) was conducted to determine policymakers' views on the public health importance of dengue fever and dengue haemorrhagic fever (DHF), the need for a vaccine and the determinants influencing its potential introduction. The survey, which involved face-to-face interviews with policymakers, health programme managers, researchers, opinion leaders and other key informants, revealed an almost uniformly high level of concern about dengue fever/DHF and a high perceived need for a dengue vaccine. Several characteristics of the disease contribute to this high sense of priority, including its geographic spread, occurrence in outbreaks, the recurrent risk of infection each dengue season, its severity and the difficulty in diagnosis and management, its urban predominance, its burden on hospitals, and its economic toll on governments and families. Research felt to be key to future decision-making regarding dengue vaccine introduction include: disease surveillance studies, in-country vaccine trials or pilot projects, and studies on the economic burden of dengue and the cost-effectiveness of dengue vaccines. The results suggest favourable conditions for public and private sector markets for dengue vaccines and the need for creative financing strategies to ensure their accessibility to poor children in dengue-endemic countries.

Evidence for antigen production in muscles by dengue and Japanese encephalitis DNA vaccines and a relation to their immunogenicity in mice

This study demonstrated viral antigen production in muscle tissues following inoculation with DNA vaccines and examined its relation to antibody induction in mice using the flavivirus system. To achieve detectable levels of antigen production, we used a needle-free jet injector and examined 10% homogenate of quadriceps muscle for viral antigens in a sandwich enzyme-linked immunosorbent assay. We compared DNA vaccines against dengue type 1 (designated pcD1ME), dengue type 2 (pcD2ME) and Japanese encephalitis (pcJEME). The amounts of viral envelope (E) antigen contained in muscle homogenate 1, 2, 3 and 4 days following inoculation with 50 microg of pcJEME were 1.1, 1.0, 0.3 and <0.1 ng/ml, respectively. Muscles from pcD2ME- and pcD1ME-inoculated mice did not contain detectable levels of E antigen (<0.1 ng/ml) during 4 days following inoculation. The E amounts released from Vero cells transfected with DNAs were in the order pcJEME>pcD2ME>pcD1ME. Levels of neutralizing antibody induced by two immunizations with 100 microg of each DNA vaccine using needle-free or normal needle/syringe injection systems also were in the order pcJEME>pcD2ME>pcD1ME, 2-11 weeks after the first immunization. However, the difference in antibody levels among three DNA vaccines 14-18 weeks after immunization was smaller than that in the early phase of immunization. These results provide fundamental information useful for developing combination DNA vaccines, such as a dengue tetravalent DNA vaccine, which require adjustment of immunogenicity of each component.

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.

Bioterrorism and catastrophe response: a quick-reference guide to resources

BACKGROUND

Dentists' responses to catastrophe have been redefined by bioterrorism. Informed response requires accurate information about agents and diseases that have the potential to be used as weapons.

METHODS

The authors reviewed information about the most probable bioterrorist weapons (those from the Center for Disease Control and Prevention's Category A) from the World Wide Web and print journals and distilled it into a resource list that is current, relevant to dentistry and noncommercial. The Web sites cited include those sponsored by federal agencies, academic institutions and professional organizations. The articles cited include those published in English within the last six years in refereed journals that are available in most higher education institutions.

RESULTS

The authors present the information in a table that provides a quick-reference guide to resources describing agents and diseases with the greatest potential for use as weapons: anthrax, botulism, plague, smallpox, tularemia and viral hemorrhagic fevers. This article presents Web site and journal citations for background and patient-oriented information (fact sheets), signs and symptoms, and prophylactic measures and treatment for each of the agents and diseases. The table facilitates quick access to this information, especially in an emergency. This article also points out guidelines for response should a suspected attack occur.

CONCLUSIONS

Armed with information about biological weapons, dentists can provide faster diagnosis, inform their patients about risks, prophylaxis or treatment and rethink their own role in terrorism response.

CLINICAL IMPLICATIONS

Fast, accurate diagnosis limits the spread of exceptionally contagious diseases. Providing accurate information to patients minimizes misinformation and the associated public fear and panic that, unchecked, could overwhelm health care systems.

Vaccines for the prevention of neglected diseases--dengue fever

Dengue and dengue hemorrhagic fever have spread to all tropical areas of the developing world, but still remain largely neglected diseases. Several promising vaccine candidates in the form of live attenuated and chimeric vaccines have been developed and are currently in human clinical trials. However, significant practical, logistic, and scientific challenges remain before these vaccines can widely and safely be applied to vulnerable populations. Vector control, community education and public health measures must be pursued in parallel with vaccine development.

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.

Chimeric Flaviviruses: Novel Vaccines against Dengue Fever, Tick-borne Encephalitis, and Japanese Encephalitis

Many arthropod-borne flaviviruses are important human pathogens responsible for diverse illnesses, including YF, JE, TBE, and dengue. Live, attenuated vaccines have afforded the most effective and economical means of prevention and control, as illustrated by YF 17D and JE SA14-14-2 vaccines. Recent advances in recombinant DNA technology have made it possible to explore a novel approach for developing live attenuated flavivirus vaccines against other flaviviruses. Full-length cDNA clones allow construction of infectious virus bearing attenuating mutations or deletions incorporated in the viral genome. It is also possible to create chimeric flaviviruses in which the structural protein genes for the target antigens of a flavivirus are replaced by the corresponding genes of another flavivirus. By combining these molecular techniques, the DNA sequences of DEN4 strain 814669, DEN2 PDK-53 candidate vaccine and YF 17D vaccine have been used as the genetic backbone to construct chimeric flaviviruses with the required attenuation phenotype and expression of the target antigens. Encouraging results from preclinical and clinical studies have shown that several chimeric flavivirus vaccines have the safety profile and satisfactory immunogenicity and protective efficacy to warrant further evaluation in humans. The chimeric flavivirus strategy has led to the rapid development of novel live-attenuated vaccines against dengue, TBE, JE, and West Nile viruses.

Epidemic dengue/dengue hemorrhagic fever as a public health, social and economic problem in the 21st century

Dengue fever/dengue hemorrhagic fever is now one of the most important public health problems in tropical developing countries and also has major economic and societal consequences.