Domain III of the envelope protein as a dengue vaccine target

Recombinant domains III of Tick-Borne Encephalitis Virus envelope protein in combination with dextran and CpGs induce immune response and partial protectiveness against TBE virus infection in mice

BACKGROUND

E protein of tick-borne encephalitis virus (TBEV) and other flaviviruses is located on the surface of the viral particle. Domain III of this protein seems to be a promising component of subunit vaccines for prophylaxis of TBE and kits for diagnostics of TBEV.

METHODS

Three variants of recombinant TBEV E protein domain III of European, Siberian and Far Eastern subtypes fused with dextran-binding domain of Leuconostoc citreum KM20 were expressed in E. coli and purified. The native structure of domain III was confirmed by ELISA antibody kit and sera of patients with tick-borne encephalitis. Immunogenic and protective properties of the preparation comprising these recombinant proteins immobilized on a dextran carrier with CpG oligonucleotides as an adjuvant were investigated on the mice model.

RESULTS

All 3 variants of recombinant proteins immobilized on dextran demonstrate specific interaction with antibodies from the sera of TBE patients. Thus, constructed recombinant proteins seem to be promising for TBE diagnostics. The formulation comprising the 3 variants of recombinant antigens immobilized on dextran and CpG oligonucleotides, induces the production of neutralizing antibodies against TBEV of different subtypes and demonstrates partial protectivity against TBEV infection.

CONCLUSIONS

Studied proteins interact with the sera of TBE patients, and, in combination with dextran and CPGs, demonstrate immunogenicity and limited protectivity on mice compared with reference "Tick-E-Vac" vaccine.

Responses of primary human nasal epithelial cells to EDIII-DENV stimulation: the first step to intranasal dengue vaccination

Background

About half of the world’s population are living in the endemic area of dengue viruses implying that a rapid-mass vaccination may be required. In addition, a major target of dengue vaccine are children, thus, a needle-free administration is more attractive. These problems may be overcome by the alternative route of vaccination such as topical, oral and intranasal vaccination. Here, we investigated the possibility to deliver a dengue immunogen intranasally, a painless route of vaccination. The tested immunogen was the domain III of dengue serotype-3 E protein (EDIII-D3) loaded into trimethyl chitosan nanoparticles (EDIII-D3 TMC NPs). The primary human nasal epithelial cells, HNEpCs, were used as an in vitro model for nasal responses.

Results

At tested concentrations, EDIII-D3 TMC NPs not only exerted no detectable toxicity toward HNEpC cultures but also efficiently delivered EDIII-D3 immunogens into HNEpCs. Moreover, HNEpCs quickly and strongly produced proinflammatory cytokines (IL-1β, IL-6, TNF-α), type-I IFN, the growth factors (GM-CSF, IL-7), the chemokines (MCP-1, MIP-1β, IL-8), Th1-related cytokines (IL-2, IL-12p70, IL-17, IFN-γ) and Th2-related cytokine (IL-4) in response to EDIII-D3 TMC NPs treatment.

Conclusions

A potential mucosal delivery system for dengue immunogens was revealed and found to stimulate a strong local innate antiviral response which possibly leading to a systemic adaptive immunity.

Dengue Fever: Causes, Complications, and Vaccine Strategies

Dengue is a highly endemic infectious disease of the tropical countries and is rapidly becoming a global burden. It is caused by any of the 4 serotypes of dengue virus and is transmitted within humans through female Aedes mosquitoes. Dengue disease varies from mild fever to severe conditions of dengue hemorrhagic fever and shock syndrome. Globalization, increased air travel, and unplanned urbanization have led to increase in the rate of infection and helped dengue to expand its geographic and demographic distribution. Dengue vaccine development has been a challenging task due to the existence of four antigenically distinct dengue virus serotypes, each capable of eliciting cross-reactive and disease-enhancing antibody response against the remaining three serotypes. Recently, Sanofi Pasteur's chimeric live-attenuated dengue vaccine candidate has been approved in Mexico, Brazil, and Philippines for usage in adults between 9 and 45 years of age. The impact of its limited application to the public health system needs to be evaluated. Simultaneously, the restricted application of this vaccine candidate warrants continued efforts in developing a dengue vaccine candidate which is additionally efficacious for infants and naïve individuals. In this context, alternative strategies of developing a designed vaccine candidate which does not allow production of enhancing antibodies should be explored, as it may expand the umbrella of efficacy to include infants and naïve individuals.

Production of dengue virus envelope protein domain III-based antigens in tobacco chloroplasts using inducible and constitutive expression systems

Dengue fever is a disease in many parts of the tropics and subtropics and about half the world's population is at risk of infection according to the World Health Organization. Dengue is caused by any of the four related dengue virus serotypes DEN-1, -2, -3 and -4, which are transmitted to people by Aedes aegypti mosquitoes. Currently there is only one vaccine (Dengvaxia(®)) available (limited to a few countries) on the market since 2015 after half a century's intensive efforts. Affordable and accessible vaccines against dengue are hence still urgently needed. The dengue envelop protein domain III (EDIII), which is capable of eliciting serotype-specific neutralizing antibodies, has become the focus for subunit vaccine development. To contribute to the development of an accessible and affordable dengue vaccine, in the current study we have used plant-based vaccine production systems to generate a dengue subunit vaccine candidate in tobacco. Chloroplast genome engineering was applied to express serotype-specific recombinant EDIII proteins in tobacco chloroplasts using both constitutive and ethanol-inducible expression systems. Expression of a tetravalent antigen fusion construct combining EDIII polypeptides from all four serotypes was also attempted. Transplastomic EDIII-expressing tobacco lines were obtained and homoplasmy was verified by Southern blot analysis. Northern blot analyses showed expression of EDIII antigen-encoding genes. EDIII protein accumulation levels varied for the different recombinant EDIII proteins and the different expression systems, and reached between 0.8 and 1.6 % of total cellular protein. Our study demonstrates the suitability of the chloroplast compartment as a production site for an EDIII-based vaccine candidate against dengue fever and presents a Gateway(®) plastid transformation vector for inducible transgene expression.

Evaluation of protection induced by a dengue virus serotype 2 envelope domain III protein scaffold/DNA vaccine in non-human primates

We describe the preclinical development of a dengue virus vaccine targeting the dengue virus serotype 2 (DENV2) envelope domain III (EDIII). This study provides proof-of-principle that a dengue EDIII protein scaffold/DNA vaccine can protect against dengue challenge. The dengue vaccine (EDIII-E2) is composed of both a protein particle and a DNA expression plasmid delivered simultaneously via intramuscular injection (protein) and gene gun (DNA) into rhesus macaques. The protein component can contain a maximum of 60 copies of EDIII presented on a multimeric scaffold of Geobacillus stearothermophilus E2 proteins. The DNA component is composed of the EDIII portion of the envelope gene cloned into an expression plasmid. The EDIII-E2 vaccine elicited robust antibody responses to DENV2, with neutralizing antibody responses detectable following the first boost and reaching titers of greater than 1:100,000 following the second and final boost. Vaccinated and naïve groups of macaques were challenged with DENV2. All vaccinated macaques were protected from detectable viremia by infectious assay, while naïve animals had detectable viremia for 2-7 days post-challenge. All naïve macaques had detectable viral RNA from day 2-10 post-challenge. In the EDIII-E2 group, three macaques were negative for viral RNA and three were found to have detectable viral RNA post challenge. Viremia onset was delayed and the duration was shortened relative to naïve controls. The presence of viral RNA post-challenge corresponded to a 10-30-fold boost in neutralization titers 28 days post challenge, whereas no boost was observed in the fully protected animals. Based on these results, we determine that pre-challenge 50% neutralization titers of >1:6000 correlated with sterilizing protection against DENV2 challenge in EDIII-E2 vaccinated macaques. Identification of the critical correlate of protection for the EDIII-E2 platform in the robust non-human primate model lays the groundwork for further development of a tetravalent EDIII-E2 dengue vaccine.

Recombinant lipidated dengue-3 envelope protein domain III stimulates broad immune responses in mice

The linkage of an immunogen with a toll-like receptor ligand has great potential to induce highly potent immune responses with the initial features of antigen-presenting cell activation. In the current study, we expressed recombinant dengue-3 envelope protein domain III (D3ED III) in lipidated form using an Escherichia coli-based system. The recombinant lipidated dengue-3 envelope protein domain III (LD3ED III) augments the expression levels of IL-12 family cytokines. LD3ED III-immunized mice enhance wide ranges of T cell responses as indicated by IFN-γ, IL-17, IL-21 production. Additionally, LD3ED III-immunized mice increase the frequencies of anti-D3ED III antibody producing cells. The boosted antibody titers cover various IgG isotypes, including IgG1, IgG2a, IgG2b, and IgG3. Importantly, LD3ED III-immunized mice induce neutralizing antibody capacity associated with a reduction of viremia levels after challenges. In contrast, mice that are immunized with D3ED III formulated with aluminum phosphate (D3ED III/Alum) only enhance Th2 responses and boost IgG1 antibody titers. Neither neutralizing antibody responses nor the inhibition of viremia levels after challenge is observed in mice that are immunized with D3ED III/Alum. These results suggest that LD3ED III can induce broad profiles of cellular and humoral immune responses.

Key concepts, strategies, and challenges in dengue vaccine development: an opportunity for sub-unit candidates?

Despite 70 years of research that has intensified in the past decade, a safe and efficacious dengue vaccine has yet to be available. In addition to the expected challenges such as identifying immune correlates of protection, the dengue vaccine field has faced additional hurdles including the necessity to design a tetravalent formulation and the risk of antibody-mediated disease enhancement. Nevertheless, tetravalent live attenuated vaccine candidates have reached efficacy trials and demonstrated some benefit, despite imbalanced immunogenicity and incomplete protection against the four serotypes. Meanwhile, the development of sub-unit dengue vaccines has gained momentum. As the target of most of the neutralizing antibodies so far reported, the virus envelope E protein has been the focus of much effort and represents the leading dengue sub-unit vaccine candidate. However, its notorious poor immunogenicity has prompted the development of innovative approaches to make E-derived constructs part of the second generation dengue vaccines portfolio.

Dengue Virus Infection of Aedes aegypti Requires a Putative Cysteine Rich Venom Protein

Dengue virus (DENV) is a mosquito-borne flavivirus that causes serious human disease and mortality worldwide. There is no specific antiviral therapy or vaccine for DENV infection. Alterations in gene expression during DENV infection of the mosquito and the impact of these changes on virus infection are important events to investigate in hopes of creating new treatments and vaccines. We previously identified 203 genes that were ≥5-fold differentially upregulated during flavivirus infection of the mosquito. Here, we examined the impact of silencing 100 of the most highly upregulated gene targets on DENV infection in its mosquito vector. We identified 20 genes that reduced DENV infection by at least 60% when silenced. We focused on one gene, a putative cysteine rich venom protein (SeqID AAEL000379; CRVP379), whose silencing significantly reduced DENV infection in Aedes aegypti cells. Here, we examine the requirement for CRVP379 during DENV infection of the mosquito and investigate the mechanisms surrounding this phenomenon. We also show that blocking CRVP379 protein with either RNAi or specific antisera inhibits DENV infection in Aedes aegypti. This work identifies a novel mosquito gene target for controlling DENV infection in mosquitoes that may also be used to develop broad preventative and therapeutic measures for multiple flaviviruses.

Dengue virus envelope domain III protein based on a tetravalent antigen secreted from insect cells: Potential use for serological diagnosis

In the present study, we developed a tetravalent protein by connecting the receptor-binding envelope domain III (EDIII) of the four dengue virus serotypes in the order of D1-D3-D4-D2. Using a baculovirus expression system, the protein was secreted into the supernatant of infected sf9 cells in a stable form with preserved native conformation. Using immobilized affinity chromatography, the recombinant EDIII (rEDIII) protein was purified with a yield of 300μg per 10(6) cells. The purity and reactivity of the protein were determined via SDS-PAGE and Western blot respectively. A MAC-ELISA method based on the secreted rEDIII protein was subsequently established and evaluated using a panel of pre-characterized dengue IgM-positive and -negative human sera. We obtained a specificity of 100% and sensitivity of 93% using this method. Our data collectively suggest that the secreted tetravalent rEDIII protein has potential utility in the diagnosis of dengue virus infections.

Immunogenicity and efficacy of flagellin-envelope fusion dengue vaccines in mice and monkeys

The envelope (E) protein of flaviviruses includes three domains, EI, EII, and EIII, and is the major protective antigen. Because EIII is rich in type-specific and subcomplex-specific neutralizing epitopes and is easy to express, it is particularly attractive as a recombinant vaccine antigen. VaxInnate has developed a vaccine platform that genetically links vaccine antigens to bacterial flagellin, a Toll-like receptor 5 ligand. Here we report that tetravalent dengue vaccines (TDVs) consisting of four constructs, each containing two copies of EIII fused to flagellin (R3.2x format), elicited robust and long-lived neutralizing antibodies (geometric mean titers of 200 to 3,000), as measured with a 50% focus reduction neutralization test (FRNT50). In an immunogenicity study, rhesus macaques (n = 2) immunized subcutaneously with 10 μg or 90 μg of TDV three or four times, at 4- to 6-week intervals, developed neutralizing antibodies to four dengue virus (DENV) serotypes (mean post-dose 3 FRNT50 titers of 102 to 601). In an efficacy study, rhesus macaques (n = 4) were immunized intramuscularly with 16 μg or 48 μg of TDV or a placebo control three times, at 1-month intervals. The animals that received 48-μg doses of TDV developed neutralizing antibodies against the four serotypes (geometric mean titers of 49 to 258) and exhibited reduced viremia after DENV-2 challenge, with a group mean viremia duration of 1.25 days and 2 of 4 animals being completely protected, compared to the placebo-treated animals, which all developed viremia, with a mean duration of 4 days. In conclusion, flagellin-EIII fusion vaccines are immunogenic and partially protective in a nonhuman primate model.

Understanding aggregation-based assays: nature of protein corona and number of epitopes on antigen matters

In this study, we systematically examine how the nature of the protein corona on NPs, formed from either antibody or antigen, and how the number of binding sites or epitopes on the antigen affect aggregation.

Direct and indirect interactions in the recognition between a cross-neutralizing antibody and the four serotypes of dengue virus

Dengue fever is the most important vector-borne viral disease. Four serotypes of dengue virus, DENV1 to DENV4, coexist. Secondary infection by a different serotype is a risk factor for severe dengue. Monoclonal antibody mAb4E11 neutralizes the four serotypes of DENV with varying efficacies by recognizing an epitope located within domain-III (ED3) of the viral envelope (E) protein. To better understand the cross-reactivities between mAb4E11 and the four serotypes of DENV, we constructed mutations in both Fab4E11 fragment and ED3, and we searched for indirect interactions in the crystal structures of the four complexes. According to the serotype, 7 to 12 interactions are mediated by one water molecule, 1 to 10 by two water molecules, and several of these interactions are conserved between serotypes. Most interfacial water molecules make hydrogen bonds with both antibody and antigen. Some residues or atomic groups are engaged in both direct and water-mediated interactions. The doubly-indirect interactions are more numerous in the complex of lowest affinity. The third complementarity determining region of the light chain (L-CDR3) of mAb4E11 does not contact ED3. The structures and double-mutant thermodynamic cycles showed that the effects of (hyper)-mutations in L-CDR3 on affinity were caused by conformational changes and indirect interactions with ED3 through other CDRs. Exchanges of residues between ED3 serotypes showed that their effects on affinity were context dependent. Thus, conformational changes, structural context, and indirect interactions should be included when studying cross-reactivity between antibodies and different serotypes of viral antigens for a better design of diagnostics, vaccine, and therapeutic tools against DENV and other Flaviviruses.

Receptor-activated human α2-macroglobulin interacts with the envelope protein of dengue virus and protects virions from temperature-induced inactivation through multivalent binding

Based on the hypothesis that interactions between virions and serum components may influence the outcome of dengue virus (DENV) infections, we decided to use affinity chromatography with domain III from the envelope (E) protein of DENV2 (DIIIE2) as a ligand to isolate virus-binding proteins from human plasma. This approach yielded serum amyloid P (SAP) and α2-macroglobulin (α2M) as novel viral interactors. After confirming the specific binding of both SAP and α2M to DIIIE2 by ELISA, the latter interaction was examined in greater detail. We obtain evidence suggesting that the binding species was actually the receptor-activated form of α2M (α2M*), that α2M* could bind monovalently to recombinant domain III from all four DENV serotypes with affinities in the micromolar range ranking as DENV4>DENV1~DENV2>DENV3 and that this interaction exhibited a strong avidity effect when multivalent binding was favoured (KD 8 × 10(-8) M for DIIIE2). We also showed that α2M* bound to DENV virions of the four serotypes, protecting the virus from temperature-induced inactivation in the absence of serum and enhancing infectivity. The latter effect exhibited an ED50 of 2.9 × 10(-8) M, also suggesting an avidity effect due to multivalent binding. These results will further contribute to the characterization of the virus-host factor interaction network during human DENV 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.

Evaluation of tetravalent and conserved synthetic peptides vaccines derived from Dengue virus Envelope domain I and II

Dengue is a major worldwide public health problem, especially in the tropical and subtropical regions of the world. Primary infection with a single Dengue virus (DENV) serotype causes a mild, self-limiting febrile illness called dengue fever. However, a subset of patients experiencing a secondary infection with a different serotype progress to the severe form of the disease, called dengue hemorrhagic fever. In this study, the vaccine potential of three tetravalent and conserved synthetic peptides derived from DENV envelope domain I (named Pep01) and II (named Pep02 and Pep03) was evaluated. Human dengue IgM/IgG positive serum (n=16) showed reactivity against Pep01, Pep02 and Pep03 in different degrees. Mice immunization experiments showed that these peptides were able to induce a humoral response characterized by antibodies with low neutralizing activity. The spleen cells derived from mice immunized with the peptides showed a significant cytotoxic activity (only for Pep02 and Pep03), a high expression of IL-10 (P<0.01) and a reduced expression of TNF-α and IFN-gamma (P<0.001) compared to DENV-1 infected splenocytes. Thus these peptides, and specially the Pep03, can induce a humoral response characterized by antibodies with low neutralizing activities and probably a T cell response that could be beneficial to induce an effective immune response against all DENV serotypes and do not contributed to the immunopathogenesis. However, further studies in peptide sequence will be required to induce the production of neutralizing antibodies against all four DENV serotypes and also to improve immunogenicity of these peptides.

Development of a multiplex bead-based assay to monitor dengue virus seroconversion

Dengue virus (DENV) envelope protein is responsible for viral attachment to host cells and as such is a target of neutralizing antibody responses. However, the presence of envelope-specific antibodies against a given serotype may contribute to enhanced disease during secondary infection with another serotype. There is a need therefore for a standardized, high-throughput low-volume assay which permits the simultaneous screening of reactivity to multiple DENV serotypes. Here, we describe a method of identifying DENV serotype-specific response in exposed individuals using a multiplexed bead-based immunoassay. The ED3 domain of a specific DENV serotype is cloned into pQEAM containing 6xHIS, TEV protease, and AviTag biotinylation sites. Biotinylated ED3 proteins are expressed in E. coli CVB101 and purified by sequential column fractionation followed by coupling onto fluorescent avidin-coated microspheres. Methods for determining the optimum amount of biotinylated ED3 protein coupled onto the microsphere are described. The assay demonstrates both a high degree of sensitivity and specificity using well-characterized patient plasma samples. The nature of the assay permits further development to include a variety of DENV serotypes and regionally important sub-serotypes.

Recombinant lipidated dengue-4 envelope protein domain III elicits protective immunity

The combination of recombinant protein antigens with an immunostimulator has the potential to greatly increase the immunogenicity of recombinant protein antigens. In the present study, we selected the dengue-4 envelope protein domain III as a dengue vaccine candidate and expressed the protein in lipidated form using an Escherichia coli-based system. The recombinant lipidated dengue-4 envelope protein domain III folded into the proper conformation and competed with the dengue-4 virus for cellular binding sites. Mice immunized with lipidated dengue-4 envelope protein domain III without exogenous adjuvant had higher frequencies of dengue-4 envelope protein domain III-specific B cells secreting antibodies than mice immunized with the nonlipidated form. Importantly, lipidated dengue-4 envelope protein domain III-immunized mice demonstrated a durable neutralizing antibody response and had reduced viremia levels after challenge. The study demonstrates that lipidated dengue-4 envelope protein domain III is immunogenic and may be a potential dengue vaccine candidate. Furthermore, the lipidation strategy can be applied to other serotypes of dengue virus.

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.

Lipidated dengue-2 envelope protein domain III independently stimulates long-lasting neutralizing antibodies and reduces the risk of antibody-dependent enhancement

BACKGROUND

Dengue virus is a mosquito-transmitted virus that can cause self-limiting dengue fever, severe life-threatening dengue hemorrhagic fever and dengue shock syndrome. The existence of four serotypes of dengue virus has complicated the development of an effective and safe dengue vaccine. Recently, a clinical phase 2b trial of Sanofi Pasteur's CYD tetravalent dengue vaccine revealed that the vaccine did not confer full protection against dengue-2 virus. New approaches to dengue vaccine development are urgently needed. Our approach represents a promising method of dengue vaccine development and may even complement the deficiencies of the CYD tetravalent dengue vaccine.

METHODOLOGY/PRINCIPAL FINDINGS

Two important components of a vaccine, the immunogen and immunopotentiator, were combined into a single construct to generate a new generation of vaccines. We selected dengue-2 envelope protein domain III (D2ED III) as the immunogen and expressed this protein in lipidated form in Escherichia coli, yielding an immunogen with intrinsic immunopotentiation activity. The formulation containing lipidated D2ED III (LD2ED III) in the absence of exogenous adjuvant elicited higher D2ED III-specific antibody responses than those obtained from its nonlipidated counterpart, D2ED III, and dengue-2 virus. In addition, the avidity and neutralizing capacity of the antibodies induced by LD2ED III were higher than those elicited by D2ED III and dengue-2 virus. Importantly, we showed that after lipidation, the subunit candidate LD2ED III exhibited increased immunogenicity while reducing the potential risk of antibody-dependent enhancement of infection in mice.

CONCLUSIONS/SIGNIFICANCE

Our study suggests that the lipidated subunit vaccine approach could be applied to other serotypes of dengue virus and other pathogens.

Understanding the dengue viruses and progress towards their control

Traditionally, the four dengue virus serotypes have been associated with fever, rash, and the more severe forms, haemorrhagic fever and shock syndrome. As our knowledge as well as understanding of these viruses increases, we now recognise not only that they are causing increasing numbers of human infections but also that they may cause neurological and other clinical complications, with sequelae or fatal consequences. In this review we attempt to highlight some of these features in the context of dengue virus pathogenesis. We also examine some of the efforts currently underway to control this "scourge" of the tropical and subtropical world.

Cross-reactivities between human IgMs and the four serotypes of dengue virus as probed with artificial homodimers of domain-III from the envelope proteins

Background

Dengue fever is the most important vector-borne viral disease. Four serotypes of dengue virus, DENV1 to DENV4, coexist. Infection by one serotype elicits long-lasting immunity to that serotype but not the other three. Subsequent infection by a different serotype is a risk factor for severe dengue. Domain III (ED3) of the viral envelope protein interacts with cell receptors and contains epitopes recognized by neutralizing antibodies. We determined the serotype specificity and cross-reactivity of human IgMs directed against ED3 by using a well-characterized collection of 90 DENV-infected and 89 DENV-uninfected human serums.

Methods

The recognitions between the four serotypes of ED3 and the serums were assayed with an IgM antibody-capture ELISA (MAC-ELISA) and artificial homodimeric antigens. The results were analyzed with Receiving Operator Characteristic (ROC) curves.

Results

The DENV-infected serums contained IgMs that reacted with one or several ED3 serotypes. The discrimination by ED3 between serums infected by the homotypic DENV and uninfected serums varied with the serotype in the decreasing order DENV1 > DENV2 > DENV3 > DENV4. The ED3 domain of DENV1 gave the highest discrimination between DENV-infected and DENV-uninfected serums, whatever the infecting serotype, and thus behaved like a universal ED3 domain for the detection of IgMs against DENV. Some ED3 serotypes discriminated between IgMs directed against the homotypic and heterotypic DENVs. The patterns of cross-reactivities and discriminations varied with the serotype.

Conclusions

The results should help better understand the IgM immune response and protection against DENV since ED3 is widely used as an antigen in diagnostic assays and an immunogen in vaccine candidates.

Induction of robust immunity by the emulsification of recombinant lipidated dengue-1 envelope protein domain III

Many attempts have focused on the use of either immunomodulators or antigen delivery systems to obtain an efficacious vaccine. Here, we report a novel approach that combined an immunomodulator and delivery system to enhance antigen association and induce robust immunity. We expressed a recombinant lipidated dengue-1 envelope protein domain III (LD1ED III) and its non-lipidated form, D1ED III, in an Escherichia coli system. The LD1ED III contains a bacterial lipid moiety, which is a potent immunomodulator. We demonstrated that LD1ED III possesses an inherent immunostimulation ability that can activate RAW 264.7 macrophage cells by up-regulating their expression of CD40, CD80, CD83, CD86 and MHC II, whereas D1ED III could not induce the up-regulation of these molecules. Moreover, combining LD1ED III with a multiphase emulsion system (called PELC) increased the antigen association more than either combining D1ED III with PELC or the antigen alone. Enhanced antigen association has been shown to correlate with stronger T cell responses, greater antibody avidity and improved neutralizing capacity. Our results demonstrate that combining recombinant lipoproteins with PELC improved both the intensity and the quality of the immune response. This approach is a promising strategy for the development of subunit vaccines that induce robust immunity.

The pathogenesis of dengue

PURPOSE OF REVIEW

Dengue is one of the most rapidly spreading vector-borne diseases in the world, with the incidence increasing 30-fold in the past 50 years. There are currently no licensed treatments or vaccines for dengue. This review covers the recent advances in our understanding of dengue pathogenesis, including host and viral determinants.

RECENT FINDINGS

The pathogenesis of severe dengue is thought to be immune-mediated due to the timing of the clinical manifestations and higher incidence in secondary infections with a heterologous serotype. Recent evidence has provided further information of neutralizing versus enhancing monoclonal antibodies and their target epitopes on the dengue virion, which has major implications for vaccine design. The role of T-cell immunopathology has also been advanced with recent evidence of cross-reactive high pro-inflammatory cytokine producing T cells predominating in severe dengue. Recent large genome-wide association studies have identified specific susceptibility loci associated with severe disease. Epidemiological studies have served to define certain at-risk groups and specific viral virulence factors have recently been described.

SUMMARY

The pathogenesis of dengue is likely to be a complex interplay of host immunity and genetic predisposition combined with certain viral virulence factors. Better understanding of the underlying mechanisms leading to severe dengue is crucial if we are to develop prognostic markers, novel diagnostics and therapeutics and ultimately a balanced and safe vaccine.

Highly efficient selection of epitope specific antibody through competitive yeast display library sorting

Combinatory antibody library display technologies have been invented and successfully implemented for the selection and engineering of therapeutic antibodies. Precise targeting of important epitopes on the protein of interest is essential for such isolated antibodies to serve as effective modulators of molecular interactions. We developed a strategy to efficiently isolate antibodies against a specific epitope on a target protein from a yeast display antibody library using dengue virus envelope protein domain III as a model target. A domain III mutant protein with a key mutation inside a cross-reactive neutralizing epitope was designed, expressed, and used in the competitive panning of a yeast display naïve antibody library. All the yeast display antibodies that bound to the wild type domain III but not to the mutant were selectively sorted and characterized. Two unique clones were identified and showed cross-reactive binding to envelope protein domain IIIs from different serotypes. Epitope mapping of one of the antibodies confirmed that its epitope overlapped with the intended neutralizing epitope. This novel approach has implications for many areas of research where the isolation of epitope-specific antibodies is desired, such as selecting antibodies against conserved epitope(s) of viral envelope proteins from a library containing high titer, high affinity non-neutralizing antibodies, and targeting unique epitopes on cancer-related proteins.

Targeting of envelope domain III protein of DENV type 2 to DEC-205 receptor elicits neutralizing antibodies in mice

Dengue virus (DENV) is the causal agent of severe disease and, in some cases, mortality in humans, but no licensed vaccines against dengue are available. An effective vaccine against dengue requires long-term humoral and cellular immune responses. Several viral proteins have been the subjects of intense research, especially the envelope (E) protein, aimed at developing a vaccine. Domain III of the envelope protein (EDIII) has been identified as a potential candidate because it is involved in binding to host cell receptors and contains epitopes that elicit virus neutralizing antibodies. However, this domain is not sufficiently antigenic when is expressed and administered as antigen to elicit a strong immune response. One alternative to enhance immunogenicity is to target the antigen to dendritic cells to induce T-cells for broad antibody responses. In this work, a single chain antibody fragment (scFv) raised against the DEC-205 receptor fused with the EDIII was successfully expressed in Nicotiana benthamiana. The recombinant protein was expressed and purified from the plant and evaluated in BALB/c mice to test its immunogenicity and ability to induce neutralizing antibodies against DENV. The mice immunized with the recombinant protein produced specific and strong humoral immune responses to DENV. Only two immunizations were required to generate a memory response to DENV without the presence of adjuvants. Also, recognition of the recombinant protein with sera from DENV-infected patients was observed. These findings suggest that this strategy has potential for development of an effective vaccine against DENV.

Thermodynamic stability of domain III from the envelope protein of flaviviruses and its improvement by molecular design

The Flavivirus genus includes widespread and severe human pathogens like the four serotypes of dengue virus (DENV1 to DENV4), yellow fever virus, Japanese encephalitis virus and West Nile virus. Domain III (ED3) of the viral envelope protein interacts with cell receptors and contains epitopes recognized by virus neutralizing antibodies. Its structural, antigenic and immunogenic properties have been thoroughly studied contrary to its physico-chemical properties. Here, the ED3 domains of the above pathogenic flaviviruses were produced in the periplasm of Escherichia coli. Their thermodynamic stabilities were measured and compared in experiments of unfolding equilibriums, induced with chemicals or heat and monitored through protein fluorescence. A designed ED3 domain, with the consensus sequence of DENV strains from all serotypes, was highly stable. The low stability of the ED3 domain from DENV3 was increased by three changes of residues in the protein core without affecting its reactivity towards DENV-infected human serums. Additional changes showed that the stability of ED3 varied with the DENV3 genotype. The T(m) of ED3 was higher than 69°C for all the tested viruses and reached 86°C for the consensus ED3. The latter, deprived of its disulfide bond by mutations, was predominantly unfolded at 20°C. These results will help better understand and design the properties of ED3 for its use as diagnostic, vaccine or therapeutic tools.

Virus-like particles displaying envelope domain III of dengue virus type 2 induce virus-specific antibody response in mice

OBJECTIVE

Currently, dengue represents one of the most significant arboviral disease worldwide, for which a vaccine is not yet available. Persistent challenges in live viral dengue vaccines have sparked a keen interest in exploring non-replicating dengue vaccines. We have examined the feasibility of using the methylotrophic yeast Pichia pastoris to develop a chimeric vaccine candidate displaying the dengue virus type-2 (DENV-2) envelope domain III (EDIII), implicated in host receptor binding and in the induction of virus-neutralizing antibodies, on the surface of non-infectious virus-like particles (VLP)-based on the Hepatitis B virus core antigen (HBcAg).

METHODS

We designed a fusion antigen by inserting DENV-2 EDIII into c/e1 loop of HBcAg. A codon-optimized gene encoding this fusion antigen was integrated into the genome of P. pastoris, under the control of the Alcohol Oxidase 1 promoter. The antigen was expressed by methanol induction and purified to near homogeneity by Ni(2+) affinity chromatography. The purified antigen was characterized physically and functionally to evaluate its ability to assemble into VLPs, and elicit DENV-2-specific antibodies in mice.

RESULTS

This fusion antigen was expressed successfully to high yields and purified to near homogeneity. Electron microscopy and competitive ELISA analyses showed that it formed VLPs in which the EDIII moiety was accessible to different EDIII-specific antibodies. These VLPs were immunogenic in mice, stimulating the production of antibodies that could specifically recognize DENV-2 and neutralize its infectivity. However, virus-neutralizing antibody titers were modest.

CONCLUSIONS

Our data show: (i) insertion of EDIII into the c/e1 loop of HBcAg does not compromise particle assembly; and (ii) the chimeric VLPs elicit a specific humoral response against DENV-2. The strategy of displaying dengue virus EDIII using a VLP platform will need further optimization before it may be developed into a viable alternative option.

Simplifying complex sequence information: a PCP-consensus protein binds antibodies against all four Dengue serotypes

Designing proteins that reflect the natural variability of a pathogen is essential for developing novel vaccines and drugs. Flaviviruses, including Dengue (DENV) and West Nile (WNV), evolve rapidly and can "escape" neutralizing monoclonal antibodies by mutation. Designing antigens that represent many distinct strains is important for DENV, where infection with a strain from one of the four serotypes may lead to severe hemorrhagic disease on subsequent infection with a strain from another serotype. Here, a DENV physicochemical property (PCP)-consensus sequence was derived from 671 unique sequences from the Flavitrack database. PCP-consensus proteins for domain 3 of the envelope protein (EdomIII) were expressed from synthetic genes in Escherichia coli. The ability of the purified consensus proteins to bind polyclonal antibodies generated in response to infection with strains from each of the four DENV serotypes was determined. The initial consensus protein bound antibodies from DENV-1-3 in ELISA and Western blot assays. This sequence was altered in 3 steps to incorporate regions of maximum variability, identified as significant changes in the PCPs, characteristic of DENV-4 strains. The final protein was recognized by antibodies against all four serotypes. Two amino acids essential for efficient binding to all DENV antibodies are part of a discontinuous epitope previously defined for a neutralizing monoclonal antibody. The PCP-consensus method can significantly reduce the number of experiments required to define a multivalent antigen, which is particularly important when dealing with pathogens that must be tested at higher biosafety levels.

Chimeric Hepatitis B core antigen virus-like particles displaying the envelope domain III of dengue virus type 2

Background

Dengue is a global public health problem for which no drug or vaccine is available. Currently, there is increasing interest in developing non-replicating dengue vaccines based on a discrete antigenic domain of the major structural protein of dengue viruses (DENVs), known as envelope domain III (EDIII). The use of bio-nanoparticles consisting of recombinant viral structural polypeptides, better known as virus-like particles (VLPs), has emerged as a potential platform technology for vaccine development. This work explores the feasibility of developing nanoparticles based on E. coli-expressed recombinant Hepatitis B virus core antigen (HBcAg) designed to display EDIII moiety of DENV on the surface.

Findings

We designed a synthetic gene construct encoding HBcAg containing an EDIII insert in its c/e1 loop. The fusion antigen HBcAg-EDIII-2 was expressed in E. coli, purified to near homogeneity using Ni⁺² affinity chromatography and demonstrated to assemble into discrete 35–40 nm VLPs by electron microscopy. Competitive ELISA analyses showed that the EDIII-2 moieties of the VLPs are accessible to anti-EDIII-2-specific monoclonal and polyclonal antibodies, suggesting that they are surface-displayed. The VLPs were highly immunogenic eliciting high titer anti-EDIII-2 antibodies that were able to recognize, bind and neutralize infectious DENV based on ELISA, immunofluorescence and virus-neutralization assays.

Conclusion

This work demonstrates that HBcAg-derived nanoparticles can serve as a useful platform for the display of DENV EDIII. The EDIII-displaying nanoparticles may have potential applications in diagnostics/vaccines for dengue.

Dengue-1 envelope protein domain III along with PELC and CpG oligodeoxynucleotides synergistically enhances immune responses

The major weaknesses of subunit vaccines are their low immunogenicity and poor efficacy. Adjuvants can help to overcome some of these inherent defects with subunit vaccines. Here, we evaluated the efficacy of the newly developed water-in-oil-in-water multiphase emulsion system, termed PELC, in potentiating the protective capacity of dengue-1 envelope protein domain III. Unlike aluminum phosphate, dengue-1 envelope protein domain III formulated with PELC plus CpG oligodeoxynucleotides induced neutralizing antibodies against dengue-1 virus and increased the splenocyte secretion of IFN-γ after in vitro re-stimulation. The induced antibodies contained both the IgG1 and IgG2a subclasses. A rapid anamnestic neutralizing antibody response against a live dengue virus challenge was elicited at week 26 after the first immunization. These results demonstrate that PELC plus CpG oligodeoxynucleotides broaden the dengue-1 envelope protein domain III-specific immune responses. PELC plus CpG oligodeoxynucleotides is a promising adjuvant for recombinant protein based vaccination against dengue virus.

Dengue is a mosquito-borne disease. Infection of dengue virus can cause clinical manifestations ranging from self-limiting dengue fever to potentially life-threatening dengue hemorrhagic fever or dengue shock syndrome. In recent years, dengue has spread to most tropical and subtropical areas, making it a global health concern. Specific approaches for dengue therapy do not exist; the development of a dengue vaccine would represent a major advance in the control of the disease. Currently, no licensed dengue vaccine is available. Subunit vaccines provide a great safety strategy for developing dengue vaccine. However, the major weaknesses of subunit vaccines are low immunogenicity and poor efficacy. Here we employed dengue-1 envelope protein domain III as a model vaccine candidate and described a newly developed water-in-oil-in water multiphase emulsion system to overcome the inherent defect of subunit vaccines. We showed that emulsification of dengue-1 envelope protein domain III and CpG oligodeoxynucleotides synergistically broadened immune responses and potentiated the protective capacity of dengue-1 envelope protein domain III. These results provide valuable information for development of recombinant protein based vaccination against dengue virus and future clinical studies.

Serotype specificity of recombinant fusion protein containing domain III and capsid protein of dengue virus 2

Recombinant fusion protein containing domain III of the dengue envelope protein fused to capsid protein from dengue 2 virus was immunogenic and conferred protection in mice against lethal challenge in previously report. Here, the antigenic specificity of this recombinant protein using anti-dengue antibodies from mice and humans and the cross-reactive humoral and cellular response induced in immunized mice were evaluated. The homologous anti-dengue antibodies showed a higher reactivity to the recombinant protein compared to the wide cross-reactivity observed for viral antigen as determined by ELISA. The IgG anti-dengue and functional antibodies, induced by the recombinant proteins in mice, were highly serotype specific by ELISA, hemaglutination inhibition and plaque reduction neutralizing tests. Accordingly, the cellular immune response determined by the IFNγ and TNFα secretion, was serotype specific. The specificity of serotype associated to this recombinant protein in addition to its high antigenicity, immunogenicity and protecting capacity suggest its advantage as a possible functional and safe vaccine candidate against dengue in a future tetravalent formulation.

Research advances in plant-made flavivirus antigens

Outbreaks of flaviviruses such as dengue (DV), yellow fever (YFV), Japanese encephalitis (JEV), tick-borne encephalitis (TBEV) and West Nile (WNV) affect numerous countries around the world. The fast spread of these viruses is the result of increases in the human population, rapid urbanisation and globalisation. While vector control is an important preventive measure against vector-borne diseases, it has failed to prevent the spread of these diseases, particularly in developing countries where the implementation of control measures is intermittent. As antiviral drugs against flaviviruses are not yet available, vaccination remains the most important tool for prevention. Although human vaccines for YFV, TBEV and JEV are available, on-going vaccination efforts are insufficient to prevent infection. No vaccines against DENV and WNV are available. Research advances have provided important tools for flavivirus vaccine development, such as the use of plants as a recombinant antigen production platform. This review summarises the research efforts in this area and highlights why a plant system is considered a necessary alternative production platform for high-tech subunit vaccines.

The serogroup A capsular polysaccharide from Neisseria meningitidis enhances the cell-mediated immunity and the protective capacity induced by a dengue fusion protein in mice

We previously tested in monkeys the P64k-DomIII fusion protein of DEN-2 (PD5), combined with the serogroup A capsular polysaccharide (CPS-A) from N. meningitidis as an immunopotentiator. The results revealed the induction of neutralizing antibodies and partial protection after DEN-2 challenge. Since one formulation of the CPS-A was only evaluated in monkeys, in the present study, we evaluated two CPS-A-based formulations in mice. Animals immunized with PD5 in alum with the highest dose of CPS-A produced the highest levels of INF-γ secretion upon viral stimulation, and accordingly, 100% protection. This is the first report that describes the dose effect of CPS-A and its capacity to potentiate the cell-mediated immunity induced by a heterologous antigen in mice.

Recombinant dengue type 2 viruses with altered e protein domain III epitopes are efficiently neutralized by human immune sera

Humans develop polyclonal, serotype-specific neutralizing antibody responses after dengue virus (DENV) infection. Many mouse antibodies that neutralize DENV bind to the lateral ridge or A strand epitopes on domain III of the viral envelope (EDIII) protein. It has been assumed that these epitopes are also the main target of human neutralizing antibodies. Using recombinant dengue serotype 2 viruses with altered EDIII epitopes, we demonstrate that EDIII epitopes are not the main target of human neutralizing antibody.

Recent emergence of dengue virus serotype 4 in French Polynesia results from multiple introductions from other South Pacific Islands

Background

Infection by dengue virus (DENV) is a major public health concern in hundreds of tropical and subtropical countries. French Polynesia (FP) regularly experiences epidemics that initiate, or are consecutive to, DENV circulation in other South Pacific Island Countries (SPICs). In January 2009, after a decade of serotype 1 (DENV-1) circulation, the first cases of DENV-4 infection were reported in FP. Two months later a new epidemic emerged, occurring about 20 years after the previous circulation of DENV-4 in FP. In this study, we investigated the epidemiological and molecular characteristics of the introduction, spread and genetic microevolution of DENV-4 in FP.

Methodology/Principal Findings

Epidemiological data suggested that recent transmission of DENV-4 in FP started in the Leeward Islands and this serotype quickly displaced DENV-1 throughout FP. Phylogenetic analyses of the nucleotide sequences of the envelope (E) gene of 64 DENV-4 strains collected in FP in the 1980s and in 2009–2010, and some additional strains from other SPICs showed that DENV-4 strains from the SPICs were distributed into genotypes IIa and IIb. Recent FP strains were distributed into two clusters, each comprising viruses from other but distinct SPICs, suggesting that emergence of DENV-4 in FP in 2009 resulted from multiple introductions. Otherwise, we observed that almost all strains collected in the SPICs in the 1980s exhibit an amino acid (aa) substitution V287I within domain I of the E protein, and all recent South Pacific strains exhibit a T365I substitution within domain III.

Conclusions/Significance

This study confirmed the cyclic re-emergence and displacement of DENV serotypes in FP. Otherwise, our results showed that specific aa substitutions on the E protein were present on all DENV-4 strains circulating in SPICs. These substitutions probably acquired and subsequently conserved could reflect a founder effect to be associated with epidemiological, geographical, eco-biological and social specificities in SPICs.

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.

Evaluation of envelope domain III-based single chimeric tetravalent antigen and monovalent antigen mixtures for the detection of anti-dengue antibodies in human sera

Background

Flavivirus cross-reactive antibodies in human sera interfere with the definitive identification of dengue virus (DENV) infections especially in areas with multiple co-circulating flaviviruses. Use of DENV envelope domain-III (EDIII) can partially resolve the problem. This study has examined the effect of (i) incorporating the EDIIIs of four DENV serotypes into a single chimeric antigen, and (ii) immobilizing the antigen through specific interaction on the sensitivity and specificity of anti-DENV antibody detection.

Methods

A sera panel (n = 164) was assembled and characterized using commercial kits for infection by DENV and a host of other pathogens. Anti-DENV antibodies of both IgM and IgG classes in this panel were detected in indirect ELISAs using a mixture of monovalent EDIIIs, a chimeric EDIII-based tetravalent antigen, EDIII-T, and a biotinylated version of the latter as coating antigens. The sensitivity and specificity of these assays were compared to those obtained using the PanBio Dengue IgG/IgM ELISAs.

Results

The performance of dengue IgG and IgM indirect ELISAs, using either a physical mixture of four EDIIIs or the single chimeric EDIII-T antigen, were comparable. Coating of a biotinylated version of the tetravalent antigen on streptavidin plates enhanced sensitivity without compromising specificity.

Conclusions

The incorporation of the EDIIIs of the four DENV serotypes into a single chimeric antigen did not adversely affect assay outcome in indirect ELISAs. Oriented, rather than random, immobilization of the tetravalent antigen enhanced sensitivity of detection of anti-DENV antibodies with retention of 100% specificity.

Immunodominance and functional activities of antibody responses to inactivated West Nile virus and recombinant subunit vaccines in mice

Factors controlling the dominance of antibody responses to specific sites in viruses and/or protein antigens are ill defined but can be of great importance for the induction of potent immune responses to vaccines. West Nile virus and other related important human-pathogenic flaviviruses display the major target of neutralizing antibodies, the E protein, in an icosahedral shell at the virion surface. Potent neutralizing antibodies were shown to react with the upper surface of domain III (DIII) of this protein. Using the West Nile virus system, we conducted a study on the immunodominance and functional quality of E-specific antibody responses after immunization of mice with soluble protein E (sE) and isolated DIII in comparison to those after immunization with inactivated whole virions. With both virion and sE, the neutralizing response was dominated by DIII-specific antibodies, but the functionality of these antibodies was almost four times higher after virion immunization. Antibodies induced by the isolated DIII had an at least 15-fold lower specific neutralizing activity than those induced by the virion, and only 50% of these antibodies were able to bind to virus particles. Our results suggest that immunization with the tightly packed E in virions focuses the DIII antibody response to the externally exposed sites of this domain which are the primary targets for virus neutralization, different from sE and isolated DIII, which also display protein surfaces that are cryptic in the virion. Despite its low potency for priming, DIII was an excellent boosting antigen, suggesting novel vaccination strategies that strengthen and focus the antibody response to critical neutralizing sites in DIII.

Recombinant dengue virus type 3 envelope domain III protein from Escherichia coli

Dengue is a public health problem of global significance for which there is neither an effective antiviral therapy nor a preventive vaccine. The envelope protein of dengue virus is the major antigen to elicit neutralizing antibody response and protective immunity in hosts. Optimization of culture media was carried out for enhanced production of recombinant dengue virus type 3 envelope domain III (rDen 3 EDIII) protein in E. coli. Further, batch and fed-batch cultivation process were also developed in optimized medium. After fed-batch cultivation, the dry cell weight was about 22.80 g/L of culture. The rDen 3 EDIII protein was purified using immobilized metal affinity chromatography. This process produced ∼649 mg of purified rDen 3 EDIII protein per liter of culture. The purity of the protein was determined by SDS-PAGE analysis and the reactivity was checked by Western blotting as well as ELISA. These results show that the purified protein may be used for the dengue diagnosis or further prophylactic studies for dengue infection.

Insights into dengue virus genome replication

Since many antiviral drugs are designed to interfere with viral genome replication, understanding this step in the viral replicative cycle has gained importance in recent years. Replication for many RNA viruses occurs in cellular compartments mainly originated from the production and reorganization of virus-induced membranes. Dengue virus translates, replicates and assembles new viral particles within virus-induced membranes from endoplasmic reticulum. In these compartments, all of the components required for replication are recruited, making the process efficient. In addition, membranes protect replication complexes from RNAases and proteases, and ultimately make them less visible to cellular defense sensors. Although several aspects in dengue virus replication are known, many others are yet to be understood. This article aims to summarize the advances in the understanding of dengue virus genome replication, highlighting the cis as well as trans elements that may have key roles in this process.

Pichia pastoris-expressed dengue virus type 2 envelope domain III elicits virus-neutralizing antibodies

A tetravalent dengue vaccine that can protect against all four serotypes of dengue viruses is a global priority. The host-receptor binding, multiple neutralizing epitope-containing carboxy-terminal region of the dengue envelope protein, known as domain III (EDIII), has emerged as a promising subunit vaccine antigen. One strategy to develop a tetravalent dengue subunit vaccine envisages mixing recombinant EDIIIs, corresponding to the four dengue virus serotypes. Towards this objective, a recombinant clone of the methylotrophic yeast Pichia pastoris, harboring the EDIII gene of dengue virus type 2 (EDIII-2) for its intracellular expression, was created. Recombinant EDIII-2 protein, expressed by this clone was purified to near homogeneity by affinity chromatography, with final yields of >50mg/l culture. Groups of Balb/c mice were immunized with this protein, separately formulated in two adjuvants, alum and montanide ISA 720. The EDIII-2 antigen, formulated in either adjuvant, elicited high levels of neutralizing antibodies to dengue virus type 2 in mice as analyzed by Plaque Reduction Neutralization Test (PRNT). This study demonstrates the feasibility of using P. pastoris to produce EDIII antigens capable of eliciting potent virus-neutralizing antibodies.