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

Development of MVA-d34 Tetravalent Dengue Vaccine: Design and Immunogenicity

Dengue fever, an infectious disease that affects more than 100 million people every year, is a global health problem. Vaccination may be the most effective prevention strategy for the disease. However, the development of vaccines against dengue fever is complicated by the high risk of developing an antibody-dependent increase in infection. This article describes the development of an MVA-d34 vaccine against the dengue virus based on a safe and effective MVA viral vector. The DIII domains of the envelope protein (E) of the dengue virus are used as vaccine antigens, as antibodies against these domains do not cause an enhancement of infection. The use of the DIII domains of each of the four dengue virus serotypes made it possible to generate a humoral response against all four dengue virus serotypes in immunized mice. We also showed that the sera of vaccinated mice present virus-neutralizing activity against dengue serotype 2. Thus, the developed MVA-d34 vaccine is a promising candidate vaccine against dengue fever.

Mice immunized with trimethyl chitosan nanoparticles containing DENV-2 envelope domain III elicit neutralizing antibodies with undetectable antibody-dependent enhancement activity

Dengue is a disease that poses a significant global public health concern. Although a tetravalent live-attenuated dengue vaccine has been licensed, its efficacy is still debated due to evidence of vaccine breakthrough infection. To avoid this issue, dengue vaccines should stimulate a high degree of serotype-specific response. Thus, envelope domain III (EDIII), which contains serotype-specific neutralizing epitopes, is an attractive target for dengue vaccine development. In this study, we investigated how EDIII encapsidated in N, N, N-trimethyl chitosan chloride nanoparticles (TMC NPs) stimulates a serotype-specific response and whether this response exerts a potential in vitro breakthrough infection. The immune response to DENV-2 elicited by EDIII TMC NP-immunized mice was monitored. We demonstrated that immunization with EDIII TMC NPs resulted in a high level of anti-EDIII antibody production. These antibodies included IgG, IgG1, and IgG2a subtypes. Importantly, antibodies from the immunized mice exerted efficient neutralizing activity with undetectable antibody dependent enhancement (ADE) activity. We also found that EDIII TMC NPs activated functional EDIII-specific CD4⁺ and CD8⁺ T cell responses. In conclusion, EDIII TMC NPs stimulated humoral immunity with a strong neutralizing antibody response, as well as a cellular immune response against DENV-2.

Dengue Virus and Vaccines: How Can DNA Immunization Contribute to This Challenge?

Dengue infections still have a tremendous impact on public health systems in most countries in tropical and subtropical regions. The disease is systemic and dynamic with broad range of manifestations, varying from mild symptoms to severe dengue (Dengue Hemorrhagic Fever and Dengue Shock Syndrome). The only licensed tetravalent dengue vaccine, Dengvaxia, is a chimeric yellow fever virus with prM and E genes from the different dengue serotypes. However, recent results indicated that seronegative individuals became more susceptible to develop severe dengue when infected after vaccination, and now WHO recommends vaccination only to dengue seropositive people. One possibility to explain these data is the lack of robust T-cell responses and antibody-dependent enhancement of virus replication in vaccinated people. On the other hand, DNA vaccines are excellent inducers of T-cell responses in experimental animals and it can also elicit antibody production. Clinical trials with DNA vaccines have improved and shown promising results regarding the use of this approach for human vaccination. Therefore, in this paper we review preclinical and clinical tests with DNA vaccines against the dengue virus. Most of the studies are based on the E protein since this antigen is the main target for neutralizing antibody production. Yet, there are other reports with DNA vaccines based on non-structural dengue proteins with protective results, as well. Combining structural and non-structural genes may be a solution for inducing immune responses aging in different infection moments. Furthermore, DNA immunizations are also a very good approach in combining strategies for vaccines against dengue, in heterologous prime/boost regimen or even administering different vaccines at the same time, in order to induce efficient humoral and cellular immune responses.

Molecular characterization of structural protein genes of dengue virus serotype 1 epidemic in Yunnan, Southwest China, in 2018

A dengue virus serotype 1 (DENV-1) epidemic occurred from October to December 2018 in Xishuangbanna, Yunnan, Southwest China, neighboring Myanmar, Laos, and Vietnam. In this study, we investigated the molecular characteristics, evolution, and potential source of DENV from Xishuangbanna. The C (capsid), prM (premembrane), and E (envelope) genes of DENV isolated from 87 serum samples obtained from local patients were amplified and sequenced, and the sequences were evaluated by identification of mutations, phylogenetic and homologous recombination analysis, and secondary structure prediction. Phylogenetic analysis showed that all of the epidemic DENV strains from Xishuangbanna could be grouped in a branch with DENV-1 isolates, and were most similar to the Fujian 2005 (China, DQ193572) and Singapore 2016 (MF314188) strains. When compared with DENV-1SS (the standard strain), there were 31 non-synonymous mutations, but no obvious homologous recombination signal was found. Secondary structure prediction showed that some changes had occurred in a helical region in proteins of the MN123849 and MN123854 strains, but there were few changes in the disordered region. This study reveals the molecular characteristics of the structural genes of the Xishuangbanna epidemic strains in 2018 and provides a reference for molecular epidemiology, infection, and pathogenicity research and vaccine development.

Immunogenicity and Efficacy of Zika Virus Envelope Domain III in DNA, Protein, and ChAdOx1 Adenoviral-Vectored Vaccines

The flavivirus envelope protein domain III (EDIII) was an effective immunogen against dengue virus (DENV) and other related flaviviruses. Whether this can be applied to the Zika virus (ZIKV) vaccinology remains an open question. Here, we tested the efficacy of ZIKV-EDIII against ZIKV infection, using several vaccine platforms that present the antigen in various ways. We provide data demonstrating that mice vaccinated with a ZIKV-EDIII as DNA or protein-based vaccines failed to raise fully neutralizing antibodies and did not control viremia, following a ZIKV challenge, despite eliciting robust antibody responses. Furthermore, we showed that ZIKV-EDIII encoded in replication-deficient Chimpanzee adenovirus (ChAdOx1-EDIII) elicited anti-ZIKV envelope antibodies in vaccinated mice but also provided limited protection against ZIKV in two physiologically different mouse challenge models. Taken together, our data indicate that contrary to what was shown for other flaviviruses like the dengue virus, which has close similarities with ZIKV-EDIII, this antigen might not be a suitable vaccine candidate for the correct induction of protective immune responses against ZIKV.

Adaptive Immunity to Dengue Virus: Slippery Slope or Solid Ground for Rational Vaccine Design?

The four serotypes of dengue virus are the most widespread causes of arboviral disease, currently placing half of the human population at risk of infection. Pre-existing immunity to one dengue virus serotype can predispose to severe disease following secondary infection with a different serotype. The phenomenon of immune enhancement has complicated vaccine development and likely explains the poor long-term safety profile of a recently licenced dengue vaccine. Therefore, alternative vaccine strategies should be considered. This review summarises studies dissecting the adaptive immune responses to dengue virus infection and (experimental) vaccination. In particular, we discuss the roles of (i) neutralising antibodies, (ii) antibodies to non-structural protein 1, and (iii) T cells in protection and pathogenesis. We also address how these findings could translate into next-generation vaccine approaches that mitigate the risk of enhanced dengue disease. Finally, we argue that the development of a safe and efficacious dengue vaccine is an attainable goal.

Immunodomination of Serotype-Specific CD4+ T-Cell Epitopes Contributed to the Biased Immune Responses Induced by a Tetravalent Measles-Vectored Dengue Vaccine

Dengue is an emerging mosquito-borne disease, and the use of prophylactic vaccines is still limited. We previously developed a tetravalent dengue vaccine (rMV-TDV) by a recombinant measles virus (MV) vector expressing envelope protein domain III (ED3). In this study, we used dengue-susceptible AG129 mice to evaluate the protective and/or pathogenic immune responses induced by rMV-TDV. Consistent with the previous study, rMV-TDV-immunized mice developed a significant neutralizing antibody response against all serotypes of DENV, as well as a significant IFN-γ response biased to DENV-3, compared to the vector controls. We further demonstrated that this DENV-3-specific IFN-γ response was dominated by one CD4⁺ T-cell epitope located in E_349-363. After DENV-2 challenge, rMV-TDV-immunized mice showed a significantly lower viremia and no inflammatory cytokine increase compared to the vector controls, which had an ~100 times higher viremia and a significant increase in IFN-γ and TNF-α. As a correlate of protection, a robust memory IFN-γ response specific to DENV-2 was boosted in rMV-TDV-immunized mice after challenge. This result suggested that pre-existing DENV-3-dominated T-cell responses did not cross-react, but a DENV-2-specific IFN-γ response, which was undetectable during immunization, was recalled. Interestingly, this recalled T-cell response recognized the epitope in the same position as the E_349-363 but in the DENV-2 serotype. This result suggested that immunodomination occurred in the CD4⁺ T-cell epitopes between dengue serotypes after rMV-TDV vaccination and resulted in a DENV-3-dominated CD4⁺ T-cell response. Although the significant increase in IgG against both DENV-2 and -3 suggested that cross-reactive antibody responses were boosted, the increased neutralizing antibodies and IgG avidity still remained DENV-2 specific, consistent with the serotype-specific T cell response post challenge. Our data reveal that immunodomination caused a biased T-cell response to one of the dengue serotypes after tetravalent dengue vaccination and highlight the roles of cross-reactive T cells in dengue protection.

The role of anti-flavivirus humoral immune response in protection and pathogenesis

Flavivirus infections are a public health threat in the world that requires the development of safe and effective vaccines. Therefore, the understanding of the anti-flavivirus humoral immune response is fundamental to future studies on flavivirus pathogenesis and the design of anti-flavivirus therapeutics. This review aims to provide an overview of the current understanding of the function and involvement of flavivirus proteins in the humoral immune response as well as the ability of the anti-envelope (anti-E) antibodies to interfere (neutralizing antibodies) or not (non-neutralizing antibodies) with viral infection, and how they can, in some circumstances enhance dengue virus infection on Fc gamma receptor (FcγR) bearing cells through a mechanism known as antibody-dependent enhancement (ADE). Thus, the dual role of the antibodies against E protein poses a formidable challenge for vaccine development. Also, we discuss the roles of antibody binding stoichiometry (the concentration, affinity, or epitope recognition) in the neutralization of flaviviruses and the "breathing" of flavivirus virions in the humoral immune response. Finally, the relevance of some specific antibodies in the design and improvement of effective vaccines is addressed.

Immunogenicity of a novel tetravalent dengue envelope protein domain III-based antigen in mice

Dengue virus is a mosquito-borne pathogen that causes dengue diseases. All four serotypes of dengue virus are infectious for humans. Therefore, an efficacious dengue vaccine should be tetravalent to provide protection against all types of virus. The goal of this study was to design a new tetravalent recombinant protein from envelope protein of dengue viruses to induce virus-neutralizing antibodies against all four serotypes in mice. A chimeric protein was designed from domain III of envelope protein of all serotypes of dengue virus. Four domain III fragments were linked together by alpha helix making linkers. The final sequence of the designed protein was analyzed in silico and the coding gene sequence was deduced by reverse translation. After cloning and expression of the recombinant protein (ED3-tetravalent protein), identity of the purified protein was confirmed using a pan-dengue specific monoclonal antibody in Western blotting. Then, the immunogenicity of the purified protein was studied in mice using antibody titration. The efficacy of induced antibodies in neutralization of the virus was studies by FRNT method. Furthermore, the induction of cellular immunity was studied by measurement of cytokines using ELISA method and measurement of lymphocyte proliferation using MTT assay. The ED3-tetravalent protein was able to enhance neutralizing immunogenic response against all four dengue serotypes; in similar way to that of tetravalent formulation of four individual domain III-based polypeptides. It is suggested that the ED3-tetravalent fusion protein can induce broadly neutralizing antibody responses against all four serotypes of dengue virus in mice.

Dengue viruses and promising envelope protein domain III-based vaccines

Dengue viruses are emerging mosquito-borne pathogens belonging to Flaviviridae family which are transmitted to humans via the bites of infected mosquitoes Aedes aegypti and Aedes albopictus. Because of the wide distribution of these mosquito vectors, more than 2.5 billion people are approximately at risk of dengue infection. Dengue viruses cause dengue fever and severe life-threatening illnesses as well as dengue hemorrhagic fever and dengue shock syndrome. All four serotypes of dengue virus can cause dengue diseases, but the manifestations are nearly different depending on type of the virus in consequent infections. Infection by any serotype creates life-long immunity against the corresponding serotype and temporary immunity to the others. This transient immunity declines after a while (6 months to 2 years) and is not protective against other serotypes, even may enhance the severity of a secondary heterotypic infection with a different serotype through a phenomenon known as antibody-depended enhancement (ADE). Although, it can be one of the possible explanations for more severe dengue diseases in individuals infected with a different serotype after primary infection. The envelope protein (E protein) of dengue virus is responsible for a wide range of biological activities, including binding to host cell receptors and fusion to and entry into host cells. The E protein, and especially its domain III (EDIII), stimulates host immunity responses by inducing protective and neutralizing antibodies. Therefore, the dengue E protein is an important antigen for vaccine development and diagnostic purposes. Here, we have provided a comprehensive review of dengue disease, vaccine design challenges, and various approaches in dengue vaccine development with emphasizing on newly developed envelope domain III-based dengue vaccine candidates.

Structural and Functional Characterization of a Cross-Reactive Dengue Virus Neutralizing Antibody that Recognizes a Cryptic Epitope

Understanding the molecular basis of the neutralizing antibody response to dengue virus (DENV) is an essential component in the design and development of effective vaccines and immunotherapeutics. Here we present the structure of a cross-reactive, neutralizing antibody, 3E31, in complex with domain III (DIII) of the DENV envelope (E) protein and reveal a conserved, temperature-sensitive, cryptic epitope on DIII that is not available in any of the known conformations of E on the dengue virion. We observed that 3E31 inhibits E-mediated membrane fusion, suggesting that the antibody is able to neutralize virus through binding an as-yet uncharacterized intermediate conformation of DENV E and sterically block trimer formation. Finally, we show that, unlike cross-reactive fusion peptide-specific antibodies, 3E31 does not promote antibody-dependent enhancement of infection at sub-neutralizing concentrations. Our results highlight the 3E31 epitope on the E protein DIII as a promising target for immunotherapeutics or vaccine design.

Neutralizing Antibody Response and Efficacy of Novel Recombinant Tetravalent Dengue DNA Vaccine Comprising Envelope Domain III in Mice

BACKGROUND

Dengue is a global arboviral threat to humans; causing 390 million infections per year. The availability of safe and effective tetravalent dengue vaccine is a global requirement to prevent epidemics, morbidity, and mortality associated with it.

METHODS

Five experimental groups (6 mice per group) each of 5-week-old BALB/c mice were immunized with vaccine and placebo (empty plasmid) (100 µg, i.m.) on days 0, 14 and 28. Among these, four groups (one group per serotype) of each were subsequently challenged 3 weeks after the last boost with dengue virus (DENV) serotypes 1-4 (100 LD₅₀, 20 µl intracerebrally) to determine vaccine efficacy. The fifth group of each was used as a control. The PBS immunized group was used as mock control. Serum samples were collected before and after subsequent immunizations. EDIII fusion protein expression was determined by Western blot. Total protein concentration was measured by Bradford assay. Neutralizing antibodies were assessed by TCID₅₀-CPE inhibition assay. Statistical analysis was performed using Stata/IC 10.1 software for Windows. One-way repeated measures ANOVA and Mann-Whitney test were used for neutralizing antibody analysis and vaccine efficacy, respectively.

RESULTS

The recombinant EDIII fusion protein was expressed adequately in transfected 293T cells. Total protein concentration was almost 3 times more than the control. Vaccine candidate induced neutralizing antibodies against all four DENV serotypes with a notable increase after subsequent boosters. Vaccine efficacy was 83.3% (DENV-1, -3, -4) and 50% (DENV-2).

CONCLUSION

Our results suggest that vaccine is immunogenic and protective; however, further studies are required to improve the immunogenicity particularly against DENV-2.

The successful induction of T-cell and antibody responses by a recombinant measles virus-vectored tetravalent dengue vaccine provides partial protection against dengue-2 infection

Dengue has a major impact on global public health, and the use of dengue vaccine is very limited. In this study, we evaluated the immunogenicity and protective efficacy of a dengue vaccine made from a recombinant measles virus (MV) that expresses envelope protein domain III (ED3) of dengue-1 to 4. Following immunization with the MV-vectored dengue vaccine, mice developed specific interferon-gamma and antibody responses against dengue virus and MV. Neutralizing antibodies against MV and dengue viruses were also induced, and protective levels of FRNT50 ≥ 10 to 4 serotypes of dengue viruses were detected in the MV-vectored dengue vaccine-immunized mice. In addition, specific interferon-gamma and antibody responses to dengue viruses were still induced by the MV-vectored dengue vaccine in mice that were pre-infected with MV. This finding suggests that the pre-existing immunity to MV did not block the initiation of immune responses. By contrast, mice that were pre-infected with dengue-3 exhibited no effect in terms of their antibody responses to MV and dengue viruses, but a dominant dengue-3-specific T-cell response was observed. After injection with dengue-2, a detectable but significantly lower viremia and a higher titer of anti-dengue-2 neutralizing antibodies were observed in MV-vectored dengue vaccine-immunized mice versus the vector control, suggesting that an anamnestic antibody response that provided partial protection against dengue-2 was elicited. Our results with regard to T-cell responses and the effect of pre-immunity to MV or dengue viruses provide clues for the future applications of an MV-vectored dengue vaccine.

The Immunodominance Change and Protection of CD4+ T-Cell Responses Elicited by an Envelope Protein Domain III-Based Tetravalent Dengue Vaccine in Mice

Dengue is the leading cause of mosquito-borne viral infections and no vaccine is available now. Envelope protein domain III (ED3) is the major target for the binding of dengue virus neutralizing antibodies; however, the ED3-specifc T-cell response is less well understood. To investigate the T-cell responses to four serotypes of dengue virus (DENV-1 to 4), we immunized mice using either a tetravalent ED3-based DNA or protein vaccine, or combined both as a DNA prime-protein boost strategy (prime-boost). A significant serotype-dependent IFN-γ or IL-4 response was observed in mice immunized with either the DNA or protein vaccine. The IFN-γ response was dominant to DENV-1 to 3, whereas the IL-4 response was dominant to DENV-4. Although the similar IgG titers for the four serotypes were observed in mice immunized with the tetravalent vaccines, the neutralizing antibody titers varied and followed the order of 2 = 3>1>4. Interestingly, the lower IFN-γ response to DENV-4 is attributable to the immunodominance change between two CD4⁺ T-cell epitopes; one T-cell epitope located at E_349-363 of DENV-1 to 3 was more immunogenic than the DENV-4 epitope E_313-327. Despite DENV-4 specific IFN-γ responses were suppressed by immunodominance change, either DENV-4-specific IFN-γ or neutralizing antibody responses were still recalled after DENV-4 challenge and contributed to virus clearance. Immunization with the prime-boost elicited both IFN-γ and neutralizing antibody responses and provided better protection than either DNA or protein immunization. Our findings shed light on how ED3-based tetravalent dengue vaccines sharpen host CD4 T-cell responses and contribute to protection against dengue virus.

Dengue E Protein Domain III-Based DNA Immunisation Induces Strong Antibody Responses to All Four Viral Serotypes

Dengue virus (DENV) infection is a major emerging disease widely distributed throughout the tropical and subtropical regions of the world affecting several millions of people. Despite constants efforts, no specific treatment or effective vaccine is yet available. Here we show a novel design of a DNA immunisation strategy that resulted in the induction of strong antibody responses with high neutralisation titres in mice against all four viral serotypes. The immunogenic molecule is an engineered version of the domain III (DIII) of the virus E protein fused to the dimerising CH3 domain of the IgG immunoglobulin H chain. The DIII sequences were also codon-optimised for expression in mammalian cells. While DIII alone is very poorly secreted, the codon-optimised fusion protein is rightly expressed, folded and secreted at high levels, thus inducing strong antibody responses. Mice were immunised using gene-gun technology, an efficient way of intradermal delivery of the plasmid DNA, and the vaccine was able to induce neutralising titres against all serotypes. Additionally, all sera showed reactivity to a recombinant DIII version and the recombinant E protein produced and secreted from mammalian cells in a mono-biotinylated form when tested in a conformational ELISA. Sera were also highly reactive to infective viral particles in a virus-capture ELISA and specific for each serotype as revealed by the low cross-reactive and cross-neutralising activities. The serotype specific sera did not induce antibody dependent enhancement of infection (ADE) in non-homologous virus serotypes. A tetravalent immunisation protocol in mice showed induction of neutralising antibodies against all four dengue serotypes as well.

Dengue disease is a mosquito-borne viral infection caused by Dengue virus (DENV), one of the most important human pathogens worldwide. DENV infection produces a systemic disease with a broad symptomatic spectrum ranging from mild febrile illness (Dengue Fever, DF) to severe haemorrhagic manifestations (Dengue Haemorrhagic fever and Dengue Shock Syndrome, DHF and DSS respectively). To date there is no vaccine available to prevent dengue disease. We show here a strategy of immunisation, tested in mice, that elicits a strong immune response against the four different DENV serotypes. The novelties presented in our work open the way to the development of an efficient vaccine accessible to developing countries.

Analysis of specific IgG titers against tick-borne encephalitis in patients with primary antibody deficiency under immunoglobulin substitution therapy: impact of plasma donor origin

Immunoglobulin (Ig) replacement therapy is effective in reducing infections in patients with primary antibody deficiency (PAD). Diversity of specific antibodies is achieved by pooling plasma from over 1000 donors usually of a given geographic region. However, there is no agreement with regard to an optimal vaccination schedule for plasma donors. Especially for tick-borne encephalitis (TBE), regional vaccination rates differ widely among populations due to the epidemiology of the disease. We analyzed specific antibody titers against TBE in comparison to total IgG levels in 162 serum samples collected from 110 PAD patients substituted with polyvalent intravenous IgG or subcutaneous IgG. Some patients received different IgG products over time leading to a total number of 122 different patient-IgG product combinations. Positive TBE-specific IgG levels were detected in 35 cases when measured by standard ELISA and could be confirmed by demonstration of neutralizing antibodies in 31 cases. The detection of specific antibody levels correlated with the geographic origin of the IgG preparations. No titers were detectable in patients substituted with IgG products from North-American donors, whereas variable degrees of anti-TBE titers were observed in patients receiving products from different European countries. We suggest considering the patients' personal risk for TBE when selecting an appropriate Ig preparation. These data support regional plasma donation in order to address the diverse local infection profile.

Neutralizing antibodies respond to a bivalent dengue DNA vaccine or/and a recombinant bivalent antigen

There is currently no effective vaccine to prevent dengue infection, despite the existence of multiple studies on potential methods of immunization. The aim of the present study was to explore the effect of DNA and/or recombinant protein on levels of neutralizing antibodies. For this purpose, envelope domain IIIs of dengue serotypes 1 and 2 (DEN-1/2)were spliced by a linker (Gly‑Gly‑Ser‑Gly‑Ser)3 and cloned into the prokaryotic expression plasmid pET30a (+) and eukaryotic vector pcDNA3.1 (+). The chimeric bivalent protein was expressed in Escherichia coli, and one‑step purification by high‑performance liquid chromatography was conducted. Protein expression levels of the DNA plasmid were tested in BHK‑21 cells by indirect immunofluorescent assay. In order to explore a more effective immunization strategy and to develop neutralizing antibodies against the two serotypes, mice were inoculated with recombinant bivalent protein, the DNA vaccine, or the two given simultaneously. Presence of the specific antibodies was tested by ELISA and the presence of the neutralizing antibodies was determined by plaque reduction neutralization test. Results of the analysis indicated that the use of a combination of DNA and protein induced significantly higher titers of neutralizing antibodies against either DEN‑1 or DEN‑2 (1:64.0 and 1:76.1, respectively) compared with the DNA (1:24.7 and 1:26.9, DEN‑1 and DEN‑2, respectively) or the recombinant protein (1:34.9 and 1:45.3 in DEN‑1 and DEN‑2, respectively). The present study demonstrated that the combination of recombinant protein and DNA as an immunization strategy may be an effective method for the development of a vaccine to prevent dengue virus infection.

A tetravalent dengue vaccine containing a mix of domain III-P64k and domain III-capsid proteins induces a protective response in mice

Recombinant fusion proteins containing domain III of the dengue virus envelope protein fused to the P64k protein from Neisseria meningitidis and domain III of dengue virus type 2 (D2) fused to the capsid protein of this serotype were immunogenic and conferred protection in mice against lethal challenge, as reported previously. Combining the domain III-P64k recombinant proteins of dengue virus types 1, 3 and 4 (D1, D3, and D4) with the domain III-capsid protein from D2, we obtained a novel tetravalent formulation containing different antigens. Here, the IgG and neutralizing antibody response, the cellular immune response, and the protective capacity against lethal challenge in mice immunized with this tetravalent formulation were evaluated. The neutralizing antibody response obtained against D1, D2 and D3, together with the high levels of IFNγ secretion induced after stimulation with the four dengue serotypes, supports the strategy of using a new tetravalent formulation containing domain III of the envelope protein fused to the capsid protein of each dengue virus serotype.

Induction of neutralizing antibodies against four serotypes of dengue viruses by MixBiEDIII, a tetravalent dengue vaccine

The worldwide expansion of four serotypes of dengue virus (DENV) poses great risk to global public health. Several vaccine candidates are under development. However, none is yet available for humans. In the present study, a novel strategy to produce tetravalent DENV vaccine based on envelope protein domain III (EDIII) was proposed. Tandem EDIIIs of two serotypes (type 1-2 and type 3-4) of DENV connected by a Gly-Ser linker ((Gly4Ser)3) were expressed in E. coli, respectively. Then, the two bivalent recombinant EDIIIs were equally mixed to form the tetravalent vaccine candidate MixBiEDIII, and used to immunize BALB/c mice. The results showed that specific IgG and neutralizing antibodies against all four serotypes of DENV were successfully induced in the MixBiEDIII employing Freund adjuvant immunized mice. Furthermore, in the suckling mouse model, sera from mice immunized with MixBiEDIII provided significant protection against four serotypes of DENV challenge. Our data demonstrated that MixBiEDIII, as a novel form of subunit vaccine candidates, might have the potential to be further developed as a tetravalent dengue vaccine in the near future.

Mapping and analysis of West Nile virus-specific monoclonal antibodies: prospects for vaccine development

Seasonal epidemics of West Nile virus (WNV) infection now occur throughout North America, causing clinical symptoms ranging from fever to encephalitis. There are no specific treatment options or licensed vaccines. Several classically developed vaccine candidates are being evaluated in clinical trials. However, questions of safety and/or immunogenicity may limit their usefulness. Mapping of human and murine antibody repertoires against the WNV envelope protein after WNV infection have revealed important insights into the protective immune response against the virus. This review will give an overview of vaccines under development and summarize current data on E-protein antigenicity that could aid in the design of next generation WNV vaccines.

A review of vaccine approaches for West Nile virus

The West Nile virus (WNC) first appeared in North America in 1999. The North American lineages of WNV were characterized by the presence of neuroinvasive and neurovirulent strains causing disease and death in humans, birds and horses. The 2012 WNV season in the United States saw a massive spike in the number of neuroinvasive cases and deaths similar to what was seen in the 2002–2003 season, according to the West Nile virus disease cases and deaths reported to the CDC by year and clinical presentation, 1999–2012, by ArboNET (Arboviral Diseases Branch, Centers for Disease Control and Prevention). In addition, the establishment and recent spread of lineage II WNV virus strains into Western Europe and the presence of neurovirulent and neuroinvasive strains among them is a cause of major concern. This review discusses the advances in the development of vaccines and biologicals to combat human and veterinary West Nile disease.

Optimization and validation of a plaque reduction neutralization test for the detection of neutralizing antibodies to four serotypes of dengue virus used in support of dengue vaccine development

A dengue plaque reduction neutralization test (PRNT) to measure dengue serotype–specific neutralizing antibodies for all four virus serotypes was developed, optimized, and validated in accordance with guidelines for validation of bioanalytical test methods using human serum samples from dengue-infected persons and persons receiving a dengue vaccine candidate. Production and characterization of dengue challenge viruses used in the assay was standardized. Once virus stocks were characterized, the dengue PRNT₅₀ for each of the four serotypes was optimized according to a factorial design of experiments approach for critical test parameters, including days of cell seeding before testing, percentage of overlay carboxymethylcellulose medium, and days of incubation post-infection to generate a robust assay. The PRNT₅₀ was then validated and demonstrated to be suitable to detect and measure dengue serotype-specific neutralizing antibodies in human serum samples with acceptable intra-assay and inter-assay precision, accuracy/dilutability, specificity, and with a lower limit of quantitation of 10.

A consensus envelope protein domain III can induce neutralizing antibody responses against serotype 2 of dengue virus in non-human primates

We have previously demonstrated that vaccination with a subunit dengue vaccine containing a consensus envelope domain III with aluminum phosphate elicits neutralizing antibodies against all four serotypes of dengue virus in mice. In this study, we evaluated the immunogenicity of the subunit dengue vaccine in non-human primates. After vaccination, monkeys that received the subunit vaccine with aluminum phosphate developed a significantly strong and long-lasting antibody response. A specific T cell response with cytokine production was also induced, and this correlated with the antibody response. Additionally, neutralizing antibodies against serotype 2 were detected in two of three monkeys. The increase in serotype-2-specific antibody titers and avidity observed in these two monkeys suggested that a serotype-2-biased antibody response occurs. These data provide evidence that a protective neutralizing antibody response was successfully elicited in non-human primates by the dengue subunit vaccine with aluminum phosphate adjuvant.

Expression and purification of an immunogenic dengue virus epitope using a synthetic consensus sequence of envelope domain III and Saccharomyces cerevisiae

A synthetic consensus gene was designed based on residues of the amino acid sequences of dengue envelope domain III (scEDIII) from all four serotypes, and codon optimization for expression was conducted using baker's yeast, Saccharomyces cerevisiae. The synthetic gene was cloned into a yeast episomal expression vector, pYEGPD-TER, which was designed to direct cloned gene expression using the glyceraldehyde-3-phosphate dehydrogenase (GPD) promoter, a functional signal peptide of the amylase 1A protein from rice, and the GAL7 terminator. PCR and back-transformation into Escherichia coli confirmed the presence of the scEDIII gene-containing plasmid in the transformants. Northern blot analysis showed the presence of the scEDIII-specific transcript. Western blot analysis indicated that expressed scEDIII, with mobility similar to purified EDIII from E. coli, was successfully secreted into the culture media. Quantitative ELISA revealed that the recombinant scEDIII comprised approximately 0.1-0.6% of cell-free extract. In addition, 0.1-0.6 mg of scEDIII protein per liter of culture filtrate was detected on day 1 and peaked on day 3 after cultivation. The secreted scEDIII protein can be purified to ≥90% purity with 85% recovery using a simple ion-exchange FPLC followed by molecular weight cut-off. Upon administration of the purified protein to mice, mouse sera contained antibodies that were specific to all four serotypes of dengue virus. Moreover, a balanced immune response against all four serotypes was observed, suggesting that it may be possible to develop an effective tetravalent dengue vaccine using S. cerevisiae.

Leptospirosis prevalence in patients with initial diagnosis of dengue

Objective. To determine the prevalence of leptospirosis in patients from Veracruz with initial diagnosis of dengue and its association with risk factors. Materials and Methods. Transversal study in patients who sought medical attention under the suspicion of dengue. Backgrounds were researched and blood samples were drawn to determine dengue (NS1, RT-PCR) and leptospirosis (IFI). Simple frequencies, central tendency and dispersion measures, and prevalence and trust intervals at 95% (IC95%) were obtained. Prevalence reasons (RP) and IC(95%) were obtained and a multivariate logistic model was applied, using SPSS V15. Results. 171 patients were included, 56% women (32 ± 14 years) and 44% men (32 ± 17 years). 48% of the cases (IC95% 40.5-55.4) was positive to dengue, with a cut point of 1 : 80, seroprevalence for leptospirosis was of 6% (IC(95%) 2.7-10); 12% (IC95% 7-16.5) was positive to both pathologies and 34% was negative to both tests. Although the largest number of isolations corresponded to serotype 2, the four dengue virus serotypes were identified. In the bivariate analysis, overcrowding RP = 1.33, (IC = 0.46-3.5), bathing in rivers (RP = 1.31, IC = 0.13-7.4), and walking barefoot (RP = 1.39, IC = 0.58-3.3) were the variables associated with leptospirosis, although the relation was not statistically significant. Conclusions. Leptospirosis prevalence in subjects under suspicion of dengue fever is high, as well as the coincidence of both infections. The results show the coexistence of overlapped outbreaks of several diseases sharing the side of transmission. It is necessary the intentional search of other pathologies, such as influenza, rickettsiosis, and brucella, among others.

A novel single-dose dengue subunit vaccine induces memory immune responses

To protect against dengue viral infection, a novel lipidated dengue subunit vaccine was rationally designed to contain the consensus amino acid sequences derived from four serotypes of dengue viruses. We found that the lipidated consensus dengue virus envelope protein domain III (LcED III) is capable of activating antigen-presenting cells and enhancing cellular and humoral immune responses. A single-dose of LcED III immunization in mice without extra adjuvant formulation is sufficient to elicit neutralizing antibodies against all four serotypes of dengue viruses. In addition, strong memory responses were elicited in mice immunized with a single-dose of LcED III. Quick, anamnestic neutralizing antibody responses to a live dengue virus challenge were elicited at week 28 post-immunization. These results demonstrate the promising possibility of a future successful tetravalent vaccine against dengue viral infections that utilizes one-dose vaccination with LcED III.

Next generation dengue vaccines: a review of candidates in preclinical development

Dengue represents a major public health problem of growing global importance. In the absence of specific dengue therapeutics, strategies for disease control have increasingly focused on the development of dengue vaccines. While a licensed dengue vaccine is not yet available, several vaccine candidates are currently being evaluated in clinical trials and are described in detail in accompanying articles. In addition, there are a large variety of candidates in preclinical development, which are based on diverse technologies, ensuring a continued influx of innovation into the development pipeline. Potentially, some of the current preclinical candidates may become next generation dengue vaccines with superior product profiles. This review provides an overview of the various technological approaches to dengue vaccine development and specifically focuses on candidates in preclinical development.

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.

Enhancement of anti-DIII antibodies by the C3d derivative P28 results in lower viral titers and augments protection in mice

Antibodies generated against West Nile virus (WNV) during infection are essential for controlling dissemination. Recent studies have demonstrated that epitopes in all three domains of the flavivirus envelope protein (E) are targets for neutralizing antibodies, with determinants in domain III (DIII) eliciting antibodies with strong inhibitory properties. In order to increase the magnitude and quality of the antibody response against the WNV E protein, DNA vaccines with derivatives of the WNV E gene (full length E, truncated E, or DIII region, some in the context of the pre-membrane [prM] gene) were conjugated to the molecular adjuvant P28. The P28 region of the complement protein C3d is the minimum CR2-binding domain necessary for the adjuvant activity of C3d. Delivery of DNA-based vaccines by gene gun and intramuscular routes stimulated production of IgG antibodies against the WNV DIII region of the E protein. With the exception of the vaccine expressing prM/E given intramuscularly, only mice that received DNA vaccines by gene gun produced protective neutralizing antibody titers (FRNT80 titer >1/40). Correspondingly, mice vaccinated by the gene gun route were protected to a greater level from lethal WNV challenge. In general, mice vaccinated with P28-adjuvated vaccines produced higher IgG titers than mice vaccinated with non-adjuvanted vaccines.

Domain III of the envelope protein as a dengue vaccine target

A dengue vaccine should induce long-lasting, simultaneous protection to the four dengue viruses while avoiding the immune enhancement of viral infection. Domain III of the dengue envelope protein has been implicated in receptor binding, and is also the target of specific neutralizing antibodies. Domain III has emerged as a promising region for a subunit vaccine candidate. Here, we review the current state of knowledge on vaccine candidates based on domain III. Due to the results obtained concerning the immune response and protection in mice and monkeys, particular attention is paid to the chimeric protein domain III fused to p64k of Neisseria meningitidis.

Passive protection assay of monoclonal antibodies against dengue virus in suckling mice

Dengue fever and dengue hemorrhagic fever/dengue shock syndrome are highly infectious diseases caused by dengue virus (DV). Specific monoclonal antibodies (mAbs) against DV are vital for diagnosis, pathological studies, and passive immune therapy. In this study, purified DV serotype 2 (DV2) was used as antigen and BALB/c mice were immunized to induce specific antibodies. We established five hybridoma cell lines, called 78#, 1E7, 7F7, 8F12, and 8H1, respectively, and evaluated them by enzyme-linked immunosorbent assay, indirect immunofluorescence assay, Western blot, plaque reduction neutralization test, and suckling mice protection assay. Lines 78#, 1E7, 7F7, and 8F12 showed a neutralizing effect, and lines 78#, 1E7, 8F12, and 8H1 recognized envelope glycoprotein of DV2. Among them, lines 78# and 8F12 had stronger neutralizing ability in vitro and could protect some suckling mice from virus challenge. Our results demonstrate that immunization with purified virion is efficient for the production of specific neutralizing mAbs against DV2, and these mAbs could be useful tools for studying or treating DV infection.

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.

A novel dengue vaccine candidate that induces cross-neutralizing antibodies and memory immunity

A novel dengue vaccine candidate comprised of a consensus dengue virus envelope protein domain III (cED III) was developed to fight against dengue virus infection. The amino acid sequence of this novel cED III was obtained by alignment of amino acid sequences from different isolates of the four serotypes of dengue viruses. A proof-of-concept study demonstrated that BALB/c mice immunized with the recombinant cED III developed neutralizing antibodies against all serotypes of dengue virus. Moreover, formulation of recombinant cED III with aluminum phosphate could induce long-lasting antibody responses and anamnestic neutralizing antibody responses following challenge with dengue virus at week 28 after priming. These results demonstrate the possibility of developing a single tetravalent vaccine against dengue viral infections.

Conservation and variability of dengue virus proteins: implications for vaccine design

Background

Genetic variation and rapid evolution are hallmarks of RNA viruses, the result of high mutation rates in RNA replication and selection of mutants that enhance viral adaptation, including the escape from host immune responses. Variability is uneven across the genome because mutations resulting in a deleterious effect on viral fitness are restricted. RNA viruses are thus marked by protein sites permissive to multiple mutations and sites critical to viral structure-function that are evolutionarily robust and highly conserved. Identification and characterization of the historical dynamics of the conserved sites have relevance to multiple applications, including potential targets for diagnosis, and prophylactic and therapeutic purposes.

Methodology/Principal Findings

We describe a large-scale identification and analysis of evolutionarily highly conserved amino acid sequences of the entire dengue virus (DENV) proteome, with a focus on sequences of 9 amino acids or more, and thus immune-relevant as potential T-cell determinants. DENV protein sequence data were collected from the NCBI Entrez protein database in 2005 (9,512 sequences) and again in 2007 (12,404 sequences). Forty-four (44) sequences (pan-DENV sequences), mainly those of nonstructural proteins and representing ∼15% of the DENV polyprotein length, were identical in 80% or more of all recorded DENV sequences. Of these 44 sequences, 34 (∼77%) were present in ≥95% of sequences of each DENV type, and 27 (∼61%) were conserved in other Flaviviruses. The frequencies of variants of the pan-DENV sequences were low (0 to ∼5%), as compared to variant frequencies of ∼60 to ∼85% in the non pan-DENV sequence regions. We further showed that the majority of the conserved sequences were immunologically relevant: 34 contained numerous predicted human leukocyte antigen (HLA) supertype-restricted peptide sequences, and 26 contained T-cell determinants identified by studies with HLA-transgenic mice and/or reported to be immunogenic in humans.

Conclusions/Significance

Forty-four (44) pan-DENV sequences of at least 9 amino acids were highly conserved and identical in 80% or more of all recorded DENV sequences, and the majority were found to be immune-relevant by their correspondence to known or putative HLA-restricted T-cell determinants. The conservation of these sequences through the entire recorded DENV genetic history supports their possible value for diagnosis, prophylactic and/or therapeutic applications. The combination of bioinformatics and experimental approaches applied herein provides a framework for large-scale and systematic analysis of conserved and variable sequences of other pathogens, in particular, for rapidly mutating viruses, such as influenza A virus and HIV.

Dengue viruses (DENVs) circulate in nature as a population of 4 distinct types, each with multiple genotypes and variants, and represent an increasing global public health issue with no prophylactic and therapeutic formulations currently available. Viral genomes contain sites that are evolutionarily stable and therefore highly conserved, presumably because changes in these sites have deleterious effects on viral fitness and survival. The identification and characterization of the historical dynamics of these sites in DENV have relevance to several applications such as diagnosis and drug and vaccine development. In this study, we have identified sequence fragments that were conserved across the majority of available DENV sequences, analyzed their historical dynamics, and evaluated their relevance as candidate vaccine targets, using various bioinformatics-based methods and immune assay in human leukocyte antigen (HLA) transgenic mice. This approach provides a framework for large-scale and systematic analysis of other human pathogens.

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.

Pediatric measles vaccine expressing a dengue antigen induces durable serotype-specific neutralizing antibodies to dengue virus

Dengue disease is an increasing global health problem that threatens one-third of the world's population. Despite decades of efforts, no licensed vaccine against dengue is available. With the aim to develop an affordable vaccine that could be used in young populations living in tropical areas, we evaluated a new strategy based on the expression of a minimal dengue antigen by a vector derived from pediatric live-attenuated Schwarz measles vaccine (MV). As a proof-of-concept, we inserted into the MV vector a sequence encoding a minimal combined dengue antigen composed of the envelope domain III (EDIII) fused to the ectodomain of the membrane protein (ectoM) from DV serotype-1. Immunization of mice susceptible to MV resulted in a long-term production of DV1 serotype-specific neutralizing antibodies. The presence of ectoM was critical to the immunogenicity of inserted EDIII. The adjuvant capacity of ectoM correlated with its ability to promote the maturation of dendritic cells and the secretion of proinflammatory and antiviral cytokines and chemokines involved in adaptive immunity. The protective efficacy of this vaccine should be studied in non-human primates. A combined measles–dengue vaccine might provide a one-shot approach to immunize children against both diseases where they co-exist.

Dengue is a tropical emerging disease that threatens one-third of the world's population, mainly children under the age of 15. The development of an affordable pediatric vaccine that could provide long-term protection against all four dengue serotypes remains a global public health priority. To address this challenge, we evaluated a strategy based on the expression of a minimal dengue antigen by live attenuated measles vaccine (MV), one of the most safe, stable, and effective human vaccines. As a proof-of-concept, we constructed a MV vector expressing a secreted dengue antigen composed of the domain III of the envelope glycoprotein (EDIII), which contains major serotype-specific neutralizing epitopes, fused to the ectodomain of the membrane protein (ectoM) from DV-1, as an adjuvant. This vector induced in mice durable serotype-specific virus-neutralizing antibodies against DV1. The remarkable adjuvant capacity of ectoM to EDIII immunogenicity was correlated to its capacity to mature dendritic cells, known to initiate immune response, and to activate the secretion of a panel of cytokines and chemokines determinant for the establishment of specific adaptive immunity. Such strategy might offer pediatric vaccines to immunize children simultaneously against measles and dengue in areas of the world where the diseases co-exist.

Immunization with West Nile virus envelope domain III protects mice against lethal infection with homologous and heterologous virus

The Japanese encephalitis virus (JEV) serocomplex-group consists of mosquito-borne flaviviruses, which include West Nile virus (WNV) and JEV, and both may cause severe encephalitis in humans. WNV has spread rapidly across the United States since its introduction in 1999 and its geographical distribution within the western hemisphere is expected to further expand, whereas, JEV is the most common cause of viral encephalitis in Southeast Asia, China and India. Currently, there is no registered human vaccine or specific therapy to prevent or treat WNV infection. Here we describe the efficacy of recombinant domain III (DIII) of WNV glycoprotein E in a mouse model. It induces high neutralizing antibody titers, as well as, protection against lethal WNV infection in C57BL/6 mice. This vaccine preparation also afforded partial protection against lethal JEV infection.

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.

High-level expression of recombinant dengue virus type 2 envelope domain III protein and induction of neutralizing antibodies in BALB/C mice

Dengue fever is a growing public health problem in many countries since so far no effective vaccines are available. In this study, the domain III of dengue virus type 2 envelope was expressed in Escherichia coli without fusion of any carrier protein. The recombinant protein was detected in the form of inclusion bodies, which were solubilized in 8M urea and could be purified subsequently by high-performance liquid chromatography (HPLC) on an ion exchange column. After refolding, the recombinant protein inhibited the DEN-2 plaque formation on C6/36 cells, demonstrated its function of receptor-interaction was retained. The recombinant protein was inoculated into BALB/c mice to test its immunogenicity and ability to induce neutralizing antibodies. The mice immunized with the purified protein developed high antibody titers. A neutralizing titer of 1:64 was also obtained by a cytopathogenic effect (CPE) inhibition assay in C6/36 cells. Mice challenged with lethal dose of DEN-2 in combination with sera from immunized mice were protected completely. The results suggested that these expression and purification strategies have the potential for development of an inexpensive vaccine.

An adenovirus prime/plasmid boost strategy for induction of equipotent immune responses to two dengue virus serotypes

Background

Dengue is a public health problem of global significance for which there is neither an effective antiviral therapy nor a preventive vaccine. It is a mosquito-borne viral disease, caused by dengue (DEN) viruses, which are members of the Flaviviridae family. There are four closely related serotypes, DEN-1, DEN-2, DEN-3 and DEN-4, each of which is capable of causing disease. As immunity to any one serotype can potentially sensitize an individual to severe disease during exposure to a heterologous serotype, the general consensus is that an effective vaccine should be tetravalent, that is, it must be capable of affording protection against all four serotypes. The current strategy of creating tetravalent vaccine formulations by mixing together four monovalent live attenuated vaccine viruses has revealed the phenomenon of viral interference leading to the manifestation of immune responses biased towards a single serotype.

Results

This work stems from the emergence of (i) the DEN virus envelope (E) domain III (EDIII) as the most important region of the molecule from a vaccine perspective and (ii) the adenovirus (Ad) as a promising vaccine vector platform. We describe the construction of a recombinant, replication-defective Ad (rAd) vector encoding a chimeric antigen made of in-frame linked EDIIIs of DEN virus serotypes 2 and 4. Using this rAd vector, in conjunction with a plasmid vector encoding the same chimeric bivalent antigen, in a prime-boost strategy, we show that it is possible to elicit equipotent neutralizing and T cell responses specific to both DEN serotypes 2 and 4.

Conclusion

Our data support the hypothesis that a DEN vaccine targeting more than one serotype may be based on a single DNA-based vector to circumvent viral interference. This work lays the foundation for developing a single Ad vector encoding EDIIIs of all four DEN serotypes to evoke a balanced immune response against each one of them. Thus, this work has implications for the development of safe and effective tetravalent dengue vaccines.