Amino acid changes in the recombinant Dengue 3 Envelope domain III determine its antigenicity and immunogenicity in mice

Homotypic antibodies target novel E glycoprotein domains after natural DENV 3 infection/vaccination

The envelope (E) glycoprotein is the primary target of type-specific (TS) neutralizing antibodies (nAbs) after infection with any of the four distinct dengue virus serotypes (DENV1-4). nAbs can be elicited to distinct structural E domains (EDs) I, II, or III. However, the relative contribution of these domain-specific antibodies is unclear. To identify the primary DENV3 nAb targets in sera after natural infection or vaccination, chimeric DENV1 recombinant encoding DENV3 EDI, EDII, or EDIII were generated. DENV3 EDII is the principal target of TS polyclonal nAb responses and encodes two or more neutralizing epitopes. In contrast, some were individuals vaccinated with a DENV3 monovalent vaccine-elicited serum TS nAbs targeting each ED in a subject-dependent fashion, with an emphasis on EDI and EDIII. Vaccine responses were also sensitive to DENV3 genotypic variation. This DENV1/3 panel allows the measurement of serum ED TS nAbs, revealing differences in TS nAb immunity after natural infection or vaccination.

Engineered Dengue Virus Domain III Proteins Elicit Cross-Neutralizing Antibody Responses in Mice

Dengue virus is the most globally prevalent mosquito-transmitted virus. Primary infection with one of four cocirculating serotypes (DENV-1 to -4) causes a febrile illness, but secondary infection with a heterologous serotype can result in severe disease, due in part to antibody-dependent enhancement of infection (ADE). In ADE, cross-reactive but nonneutralizing antibodies, or subprotective levels of neutralizing antibodies, promote uptake of antibody-opsonized virus in Fc-γ receptor-positive cells. Thus, elicitation of broadly neutralizing antibodies (bNAbs), but not nonneutralizing antibodies, is desirable for dengue vaccine development. Domain III of the envelope glycoprotein (EDIII) is targeted by bNAbs and thus is an attractive immunogen. However, immunization with EDIII results in sera with limited neutralization breadth. We developed "resurfaced" EDIII immunogens (rsDIIIs) in which the A/G strand epitope that is targeted by bNAb 4E11 is maintained but less desirable epitopes are masked. RsDIIIs bound 4E11, but not serotype-specific or nonneutralizing antibodies. One rsDIII and, unexpectedly, wild-type (WT) DENV-2 EDIII elicited cross-neutralizing antibody responses against DENV-1 to -3 in mice. While these sera were cross-neutralizing, they were not sufficiently potent to protect AG129 immunocompromised mice at a dose of 200 μl (50% focus reduction neutralization titer [FRNT₅₀], ∼1:60 to 1:130) against mouse-adapted DENV-2. Our results provide insight into immunogen design strategies based on EDIII.IMPORTANCE Dengue virus causes approximately 390 million infections per year. Primary infection by one serotype causes a self-limiting febrile illness, but secondary infection by a heterologous serotype can result in severe dengue syndrome, which is characterized by hemorrhagic fever and shock syndrome. This severe disease is thought to arise because of cross-reactive, non- or poorly neutralizing antibodies from the primary infection that are present in serum at the time of secondary infection. These cross-reactive antibodies enhance the infection rather than controlling it. Therefore, induction of a broadly and potently neutralizing antibody response is desirable for dengue vaccine development. Here, we explore a novel strategy for developing immunogens based on domain III of the E glycoprotein, where undesirable epitopes (nonneutralizing or nonconserved) are masked by mutation. This work provides fundamental insight into the immune response to domain III that can be leveraged for future immunogen design.

Genotype-specific and cross-reactive neutralizing antibodies induced by dengue virus infection: detection of antibodies with different levels of neutralizing activities against homologous and heterologous genotypes of dengue virus type 2 in common marmosets (Callithrix jacchus)

BACKGROUND

A vaccine against all four dengue virus (DENV) serotypes includes the formulation of one genotype of each serotype. Although genetic similarities among genotypes within a serotype are higher as compared to those among serotypes, differences in the immunogenicity of the included genotypes would be a critical issue in maximizing successful dengue vaccine development. Thus, we determined the neutralizing antibody responses against three genotypes of dengue virus serotype 2 (DENV-2), namely Cosmopolitan, Asian I, and Asian/American, after primary and secondary inoculation with DENV-2 in a dengue animal model, the common marmoset (Callithrix jacchus).

METHODS

A total of fifty-four plasma samples were obtained from thirty-four marmosets that were inoculated with clinically-isolated DENV strains or DENV candidate vaccines, were used in this study. Plasma samples were obtained from marmosets after primary inoculation with DENV-2 infection, secondary inoculation with homologous or heterologous genotypes, and tertiary inoculation with heterologous DENV. Neutralizing antibody titers against DENV-2 (Cosmopolitan, Asian I, and Asian/American genotypes) and DENV-1 were determined using a conventional plaque reduction neutralization assay.

RESULTS

In marmosets that were inoculated with the Cosmopolitan genotype in primary infection, neutralizing antibody neutralized 3 genotypes, and the titers to Asian I genotype were significantly higher than those to homologous Cosmopolitan genotype. After secondary DENV-2 infection with heterologous genotype (Asian I in primary and Asian/American in secondary), neutralizing antibody titers to Asian/American genotype was significantly higher than those against Cosmopolitan and Asian I genotypes. Following tertiary infection with DENV-1 following DENV-2 Asian I and Cosmopolitan genotypes, neutralizing antibody titers to Asian/American were also significantly higher than those against Cosmopolitan and Asian I genotypes.

CONCLUSION

The present study demonstrated that different levels of neutralizing antibodies were induced against variable DENV-2 genotypes after primary, secondary and tertiary infections, and that neutralizing antibody titers to some heterologous genotypes were higher than those to homologous genotypes within a serotype. The results indicate that heterogeneity and homogeneity of infecting genotypes influence the levels and cross-reactivity of neutralizing antibodies induced in following infections. The results also suggest that certain genotypes may possess advantage in terms of breakthrough infections against vaccination.

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.

Integrated process for the purification and immobilization of the envelope protein domain III of dengue virus type 2 expressed in Rachiplusia nu larvae and its potential application in a diagnostic assay

Dengue incidence has grown dramatically in the last years, with about 40% of the world population at risk of infection. Recently, a vaccine developed by Sanofi Pasteur has been registered, but only in a few countries. Moreover, specific antiviral drugs are not available. Thus, an efficient and accurate diagnosis is important for disease management. To develop a low-cost immunoassay for dengue diagnosis, in the present study we expressed the envelope protein domain III of dengue virus type 2 in Rachiplusia nu larvae by infection with a recombinant baculovirus. The antigen was expressed as a fusion to hydrophobin I (DomIIIHFBI) to easily purify it by an aqueous two-phase system (ATPS) and to efficiently immobilize it in immunoassay plates. A high level of recombinant DomIIIHFBI was obtained in R. nu, where yields reached 4.5 mg per g of larva. Also, we were able to purify DomIIIHFBI by an ATPS with 2% of Triton X-114, reaching a yield of 73% and purity higher than 80% in a single purification step. The recombinant DomIIIHFBI was efficiently immobilized in hydrophobic surface plates. The immunoassay we developed with the immobilized antigen was able to detect IgG specific for dengue virus type 2 in serum samples and not for other serotypes.

Serologic and Virologic Studies of an Imported Dengue Case Occurring in 2014 in Okinawa, Japan

After returning from Bali, Indonesia, in February 2014, a 72-year-old man was hospitalized in Okinawa owing to a high fever and rash. Dengue was clinically suspected, and the patient tested positive for IgM against dengue using a commercial kit. Serologically, the patient showed secondary seroreactivity. Significant increases in neutralization titers (N-titers) against all 4 serotypes of dengue virus (DENV) and Japanese encephalitis virus (JEV) strains were recognized in convalescent-phase sera comparing to acute phase sera. The N-titer against DENV serotype 1 (DENV-1) was the highest among all DENV serotypes. Interestingly, the N-titers against JEV strains were significantly higher than those against all types of DENV comparing to acute phase sera. The virus was isolated from the acute-phase serum and identified as DENV-1 and designated RD14/Okinawa. The patient's symptoms were due to DENV-1 infection. Phylogenetic sequencing analysis indicated that the isolate RD14/Okinawa belonged to genotype I of DENV-1, which is closely related to the Southeast Asian strains and isolates found during the dengue outbreak in Japan in 2014. We should undertake control measures against dengue in Okinawa, which is a subtropical area with Aedes albopictus activity throughout year.

The relevance of dengue virus genotypes surveillance at country level before vaccine approval

Dengue is a major threat for public health in tropical and subtropical countries around the world. In the absence of a licensed vaccine and effective antiviral therapies, control measures have been based on education activities and vector elimination. Current efforts for developing a vaccine are both promising and troubling. At the advent of the introduction of a tetravalent dengue vaccine, molecular surveillance of the circulating genotypes in different geographical regions has gained considerable importance. A growing body of in vitro, preclinical, and clinical phase studies suggest that vaccine conferred protection in a geographical area could depends on the coincidence of the dengue virus genotypes included in the vaccine and those circulating. In this review we present the state-of-the-art in this field, highlighting the need of deeper knowledge on neutralizing immune response for making decisions about future vaccine approval and the potential need for different vaccine composition for regional administration.

Virus-like particle secretion and genotype-dependent immunogenicity of dengue virus serotype 2 DNA vaccine

Dengue virus (DENV), composed of four distinct serotypes, is the most important and rapidly emerging arthropod-borne pathogen and imposes substantial economic and public health burdens. We constructed candidate vaccines containing the DNA of five of the genotypes of dengue virus serotype 2 (DENV-2) and evaluated the immunogenicity, the neutralizing (Nt) activity of the elicited antibodies, and the protective efficacy elicited in mice immunized with the vaccine candidates. We observed a significant correlation between the level of in vitro virus-like particle secretion, the elicited antibody response, and the protective efficacy of the vaccines containing the DNA of the different DENV genotypes in immunized mice. However, higher total IgG antibody levels did not always translate into higher Nt antibodies against homologous and heterologous viruses. We also found that, in contrast to previous reports, more than 50% of total IgG targeted ectodomain III (EDIII) of the E protein, and a substantial fraction of this population was interdomain highly neutralizing flavivirus subgroup-cross-reactive antibodies, such as monoclonal antibody 1B7-5. In addition, the lack of a critical epitope(s) in the Sylvatic genotype virus recognized by interdomain antibodies could be the major cause of the poor protection of mice vaccinated with the Asian 1 genotype vaccine (pVD2-Asian 1) from lethal challenge with virus of the Sylvatic genotype. In conclusion, although the pVD2-Asian 1 vaccine was immunogenic, elicited sufficient titers of Nt antibodies against all DENV-2 genotypes, and provided 100% protection against challenge with virus of the homologous Asian 1 genotype and virus of the heterologous Cosmopolitan genotype, it is critical to monitor the potential emergence of Sylvatic genotype viruses, since vaccine candidates under development may not protect vaccinated humans from these viruses.

IMPORTANCE

Five genotype-specific dengue virus serotype 2 (DENV-2) DNA vaccine candidates were evaluated for their immunogenicity, homologous and heterologous neutralizing (Nt) antibody titers, and cross-genotype protection in a murine model. The immunity elicited by our prototype vaccine candidate (Asian 1 genotype strain 16681) in mice was protective against viruses of other genotypes but not against virus of the Sylvatic genotype, whose emergence and potential risk after introduction into the human population have previously been demonstrated. The underlying mechanism of a lack of protection elicited by the prototype vaccine may at least be contributed by the absence of a flavivirus subgroup-cross-reactive, highly neutralizing monoclonal antibody 1B7-5-like epitope in DENV-2 of the Sylvatic genotype. The DENV DNA vaccine directs the synthesis and assembly of virus-like particles (VLPs) and induces immune responses similar to those elicited by live-attenuated vaccines, and its flexibility permits the fast deployment of vaccine to combat emerging viruses, such as Sylvatic genotype viruses. The enhanced VLP secretion obtained by replacement of ectodomain I-II (EDI-II) of the Cosmopolitan genotype vaccine construct (VD2-Cosmopolitan) with the Asian 1 EDI-II elicited significantly higher total IgG and Nt antibody titers and suggests a novel approach to enhance the immunogenicity of the DNA vaccine. A DENV vaccine capable of eliciting protective immunity against viruses of existing and emerging genotypes should be the focus of future DENV vaccine development.

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.

Dengue viruses - an overview

Dengue viruses (DENVs) cause the most common arthropod-borne viral disease in man with 50-100 million infections per year. Because of the lack of a vaccine and antiviral drugs, the sole measure of control is limiting the Aedes mosquito vectors. DENV infection can be asymptomatic or a self-limited, acute febrile disease ranging in severity. The classical form of dengue fever (DF) is characterized by high fever, headache, stomach ache, rash, myalgia, and arthralgia. Severe dengue, dengue hemorrhagic fever (DHF), and dengue shock syndrome (DSS) are accompanied by thrombocytopenia, vascular leakage, and hypotension. DSS, which can be fatal, is characterized by systemic shock. Despite intensive research, the underlying mechanisms causing severe dengue is still not well understood partly due to the lack of appropriate animal models of infection and disease. However, even though it is clear that both viral and host factors play important roles in the course of infection, a fundamental knowledge gap still remains to be filled regarding host cell tropism, crucial host immune response mechanisms, and viral markers for virulence.

Emergence potential of sylvatic dengue virus type 4 in the urban transmission cycle is restrained by vaccination and homotypic immunity

Sylvatic dengue viruses (DENV) are both evolutionarily and ecologically distinct from human DENV and are maintained in an enzootic transmission cycle. Evidence of sylvatic human infections from West Africa and Southeast Asia suggests that sylvatic DENV come into regular contact with humans. Thus, this potential of emergence into the human transmission cycle could limit the potential for eradicating this cycle with vaccines currently in late stages of development. We assessed the likelihood of sylvatic DENV-4 emergence in the face of natural immunity to current human strains and vaccination with two DENV-4 vaccine candidates. Our data indicate homotypic neutralization of sylvatic and human DENV-4 strains by human primary convalescent and vaccinee sera but limited heterotypic immunity. These results suggest that emergence of sylvatic strains into the human cycle would be limited by homotypic immunity mediated by virus neutralizing antibodies produced by natural infection or vaccination.

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.

An alphavirus vector-based tetravalent dengue vaccine induces a rapid and protective immune response in macaques that differs qualitatively from immunity induced by live virus infection

Despite many years of research, a dengue vaccine is not available, and the more advanced live attenuated vaccine candidate in clinical trials requires multiple immunizations with long interdose periods and provides low protective efficacy. Here, we report important contributions to the development of a second-generation dengue vaccine. First, we demonstrate that a nonpropagating vaccine vector based on Venezuelan equine encephalitis virus replicon particles (VRP) expressing two configurations of dengue virus E antigen (subviral particles [prME] and soluble E dimers [E85]) successfully immunized and protected macaques against dengue virus, while antivector antibodies did not interfere with a booster immunization. Second, compared to prME-VRP, E85-VRP induced neutralizing antibodies faster, to higher titers, and with improved protective efficacy. Third, this study is the first to map antigenic domains and specificities targeted by vaccination versus natural infection, revealing that, unlike prME-VRP and live virus, E85-VRP induced only serotype-specific antibodies, which predominantly targeted EDIII, suggesting a protective mechanism different from that induced by live virus and possibly live attenuated vaccines. Fourth, a tetravalent E85-VRP dengue vaccine induced a simultaneous and protective response to all 4 serotypes after 2 doses given 6 weeks apart. Balanced responses and protection in macaques provided further support for exploring the immunogenicity and safety of this vaccine candidate in humans.

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.

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.

Development and characterization of a reverse genetic system for studying dengue virus serotype 3 strain variation and neutralization

Dengue viruses (DENV) are enveloped single-stranded positive-sense RNA viruses transmitted by Aedes spp. mosquitoes. There are four genetically distinct serotypes designated DENV-1 through DENV-4, each further subdivided into distinct genotypes. The dengue scientific community has long contended that infection with one serotype confers lifelong protection against subsequent infection with the same serotype, irrespective of virus genotype. However this hypothesis is under increased scrutiny and the role of DENV genotypic variation in protection from repeated infection is less certain. As dengue vaccine trials move increasingly into field-testing, there is an urgent need to develop tools to better define the role of genotypic variation in DENV infection and immunity. To better understand genotypic variation in DENV-3 neutralization and protection, we designed and constructed a panel of isogenic, recombinant DENV-3 infectious clones, each expressing an envelope glycoprotein from a different DENV-3 genotype; Philippines 1982 (genotype I), Thailand 1995 (genotype II), Sri Lanka 1989 and Cuba 2002 (genotype III) and Puerto Rico 1977 (genotype IV). We used the panel to explore how natural envelope variation influences DENV-polyclonal serum interactions. When the recombinant viruses were tested in neutralization assays using immune sera from primary DENV infections, neutralization titers varied by as much as ∼19-fold, depending on the expressed envelope glycoprotein. The observed variability in neutralization titers suggests that relatively few residue changes in the E glycoprotein may have significant effects on DENV specific humoral immunity and influence antibody mediated protection or disease enhancement in the setting of both natural infection and vaccination. These genotypic differences are also likely to be important in temporal and spatial microevolution of DENV-3 in the background of heterotypic neutralization. The recombinant and synthetic tools described here are valuable for testing hypotheses on genetic determinants of DENV-3 immunopathogenesis.

The human antibody response to dengue virus infection

Dengue viruses (DENV) are the causative agents of dengue fever (DF) and dengue hemorrhagic fever (DHF). Here we review the current state of knowledge about the human antibody response to dengue and identify important knowledge gaps. A large body of work has demonstrated that antibodies can neutralize or enhance DENV infection. Investigators have mainly used mouse monoclonal antibodies (MAbs) to study interactions between DENV and antibodies. These studies indicate that antibody neutralization of DENVs is a “multi-hit” phenomenon that requires the binding of multiple antibodies to neutralize a virion. The most potently neutralizing mouse MAbs bind to surface exposed epitopes on domain III of the dengue envelope (E) protein. One challenge facing the dengue field now is to extend these studies with mouse MAbs to better understand the human antibody response. The human antibody response is complex as it involves a polyclonal response to primary and secondary infections with 4 different DENV serotypes. Here we review studies conducted with immune sera and MAbs isolated from people exposed to dengue infections. Most dengue-specific antibodies in human immune sera are weakly neutralizing and bind to multiple DENV serotypes. The human antibodies that potently and type specifically neutralize DENV represent a small fraction of the total DENV-specific antibody response. Moreover, these neutralizing antibodies appear to bind to novel epitopes including complex, quaternary epitopes that are only preserved on the intact virion. These studies establish that human and mouse antibodies recognize distinct epitopes on the dengue virion. The leading theory proposed to explain the increased risk of severe disease in secondary cases is antibody dependent enhancement (ADE), which postulates that weakly neutralizing antibodies from the first infection bind to the second serotype and enhance infection of FcγR bearing myeloid cells such as monocytes and macrophages. Here we review results from human, animal and cell culture studies relevant to the ADE hypothesis. By understanding how human antibodies neutralize or enhance DENV, it will be possible to better evaluate existing vaccines and develop the next generation of novel vaccines.

Phenotypic characterization of patient dengue virus isolates in BALB/c mice differentiates dengue fever and dengue hemorrhagic fever from dengue shock syndrome

Background

Dengue virus (DENV) infection is the most common arthropod-borne viral disease in man and there are approximately 100 million infections annually. Despite the global burden of DENV infections many important questions regarding DENV pathogenesis remain unaddressed due to the lack of appropriate animal models of infection and disease. A major problem is the fact that no non-human species naturally develop disease similar to human dengue fever (DF) or dengue hemorrhagic fever (DHF) and dengue shock syndrome (DSS). Apart from other risk factors for severe dengue such as host genetics and secondary infection with a heterologous DENV, virus virulence is a risk factor that is not well characterized.

Results

Three clinical DENV-1 isolates from Cambodian patients experiencing the various forms of dengue disease (DF, DHF, and DSS) were inoculated in BALB/c mice at three different concentrations. The DENV-1 isolates had different organ and cell tropism and replication kinetics. The DENV-1 isolate from a DSS patient infected the largest number of mice and was primarily neurotropic. In contrast, the DENV-1 isolates from milder clinical dengue cases infected predominantly lungs and liver, and to a lesser extent brain. In addition, infection with the DENV isolate derived from a DSS patient persisted for more than two weeks in a majority of mice compared to the other DENV-1 isolates that peaked during the first week.

Conclusions

These results confirm the in vitro findings of the same DENV-1 isolates, that showed that the isolate derived from a DSS patient can be distinguished based on phenotypic characteristics that differ from the isolates derived from a DF and DHF case [1]. We observed in this study that the DSS virus isolate persist longer in vivo with extensive neuroinvasion in contrast to the other DENV-1 isolates originating in milder human cases. Genomic characterization of the three clinical isolates identified six amino acid substitutions unique for the DSS isolates that were located both in structural genes (M and E) and in non-structural genes (NS1, NS3, and NS5). The characterization of these clinically distinct DENV-1 isolates highlight that DENVs within the same genotype may have different in vivo phenotypes.

Highlights

• Clinical DENV-1 isolates have different organ tropism in BALB/c mice.

• The isolate from a DSS patient is primarily neurotropic compared to the other isolates.

• The DENV-1 isolates have different in vivo replication kinetics.

• The isolate from a DSS patient persists longer compared to the other isolates.

• These phenotypic differences confirm our earlier in vitro findings with the same DENV-1 isolates. Thus, DENVs within the same serotype and genotype may differ enough to affect clinical conditions in vivo.

The two component adjuvant IC31® potentiates the protective immunity induced by a dengue 2 recombinant fusion protein in mice

Here we evaluated the suitability of the synthetic adjuvant IC31® to potentiate the protective capacity of PD5 protein (domain III of the envelope protein of dengue 2 virus fused to the carrier protein P64k). Unlike Alum, PD5 mixed with IC31® induced complete protection against virus challenge in mice and increased IFN-γ secretion after in vitro re-stimulation. The induced antibody response was highly specific to the homologous serotype and showed both IgG1 and IgG2a subtypes. IC31® is a promising adjuvant for PD5 recombinant protein based vaccination against dengue. Future work should address the suitability of PD5/IC31® formulations in non-human primate models.

Pediatric measles vaccine expressing a dengue tetravalent antigen elicits neutralizing antibodies against all four dengue viruses

Dengue disease is an increasing global health problem that threatens one-third of the world's population. To control this emerging arbovirus, an efficient preventive vaccine is still needed. Because four serotypes of dengue virus (DV) coexist and antibody-dependent enhanced infection may occur, most strategies developed so far rely on the administration of tetravalent formulations of four live attenuated or chimeric viruses. Here, we evaluated a new strategy based on the expression of a single minimal tetravalent DV antigen by a single replicating viral vector derived from pediatric live-attenuated measles vaccine (MV). We generated a recombinant MV vector expressing a DV construct composed of the four envelope domain III (EDIII) from the four DV serotypes fused with the ectodomain of the membrane protein (ectoM). After two injections in mice susceptible to MV infection, the recombinant vector induced neutralizing antibodies against the four serotypes of dengue virus. When immunized mice were further inoculated with live DV from each serotype, a strong memory neutralizing response was raised against all four serotypes. A combined measles-dengue vaccine might be attractive to immunize infants against both diseases where they co-exist.

Nucleocapsid-like particles of dengue-2 virus enhance the immune response against a recombinant protein of dengue-4 virus

In this study, we evaluate in mice a novel formulation containing nucleocapsid-like particles of dengue-2 virus (recNLP) co-immunized with a chimeric protein composed of the dengue-4 envelope domain III fused twice within the meningococcal P64k protein of Neisseria meningitidis (PD24). The animals receiving the PD24-recNLP mixture showed the highest levels of antiviral antibodies. Similar results were obtained for IFNγ secretion levels, indicating a functional Th1 cellular response. Consistently, the percentage of mice surviving after viral challenge was significantly higher for those immunized with the mixture than for those inoculated with PD24 protein alone. In addition, in vivo depletion experiments demonstrated the decisive role of CD4(+) and CD8(+) cells in the protection conferred by immunization with PD24-recNLP. In conclusion, this report demonstrates for the first time the adjuvant capacity of dengue-2 virus recNLP. Additionally, the evidence presented highlights the potential of these particles for enhancing the immune response against heterologous recombinant proteins.

Identification of mutations in a candidate dengue 4 vaccine strain 341750 PDK20 and construction of a full-length cDNA clone of the PDK20 vaccine candidate

Dengue 4 virus strain 341750 serially passaged 20 times in primary dog kidney (PDK) cells was shown to have reduced infectivity for rhesus monkeys but was immunogenic and protected the monkeys from challenge with low passage parent dengue 4 virus. The dengue 4 PDK20 virus was also shown to be attenuated for human volunteers. We compared the genomic nucleotide sequences of low passage parent and PDK20 attenuated vaccine strains and identified 11 nucleotide (nt) substitutions in the PDK20 genome. Five mutations caused amino acid changes in viral proteins E (N366N/S), NS1 (E146Q), NS4B (S/L112L and A240V), and NS5 (F/L790L). Silent mutations occurred in genes encoding NS1 (nt 2609), NS3 (nt 6113, 6230 and 6239) and NS5 (nt 8081 and 8588). A full-length cDNA clone of the dengue 4 strain 341750 PDK20 was constructed and RNA transcripts of the clone were infectious in monkey kidney (LLC-MK(2)) and Aedes albopictus (C6/36) cells. The sequence analysis and availability of an infectious clone provide molecular tools to investigate the basis for the attenuation of dengue 4 virus.

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.

Natural strain variation and antibody neutralization of dengue serotype 3 viruses

Dengue viruses (DENVs) are emerging, mosquito-borne flaviviruses which cause dengue fever and dengue hemorrhagic fever. The DENV complex consists of 4 serotypes designated DENV1-DENV4. Following natural infection with DENV, individuals develop serotype specific, neutralizing antibody responses. Monoclonal antibodies (MAbs) have been used to map neutralizing epitopes on dengue and other flaviviruses. Most serotype-specific, neutralizing MAbs bind to the lateral ridge of domain III of E protein (EDIII). It has been widely assumed that the EDIII lateral ridge epitope is conserved within each DENV serotype and a good target for vaccines. Using phylogenetic methods, we compared the amino acid sequence of 175 E proteins representing the different genotypes of DENV3 and identified a panel of surface exposed amino acids, including residues in EDIII, that are highly variant across the four DENV3 genotypes. The variable amino acids include six residues at the lateral ridge of EDIII. We used a panel of DENV3 mouse MAbs to assess the functional significance of naturally occurring amino acid variation. From the panel of antibodies, we identified three neutralizing MAbs that bound to EDIII of DENV3. Recombinant proteins and naturally occurring variant viruses were used to map the binding sites of the three MAbs. The three MAbs bound to overlapping but distinct epitopes on EDIII. Our empirical studies clearly demonstrate that the antibody binding and neutralization capacity of two MAbs was strongly influenced by naturally occurring mutations in DENV3. Our data demonstrate that the lateral ridge “type specific” epitope is not conserved between strains of DENV3. This variability should be considered when designing and evaluating DENV vaccines, especially those targeting EDIII.

Dengue viruses are mosquito-borne flaviviruses and the agents of dengue fever and dengue hemorrhagic fever. It has been widely assumed that antibodies that neutralize dengue bind to regions on the viral envelope (E) protein that are conserved within each serotype. However, few studies have explored how natural variation influences dengue-antibody interactions. Mouse antibodies that strongly neutralize dengue bind to a region on domain III of E protein. This region has been the focus of much recent work because it might be the target of protective human antibodies as well. We compared a large number of E protein sequences and discovered that the region on E protein domain III targeted by neutralizing antibodies was highly variable between strains of dengue serotype 3. Using a panel of antibodies, we experimentally demonstrate that natural strain variation in dengue serotype 3 has a strong influence on antibody binding and neutralization. Our results challenge the dogma that neutralizing antibody binding regions are conserved within each serotype. The results of this study are relevant to the current global effort to develop and evaluate dengue vaccines.

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.

Antibodies induced by dengue virus type 1 and 2 envelope domain III recombinant proteins in monkeys neutralize strains with different genotypes

In the present work, we evaluated the neutralizing capacity of the antibodies induced by dengue virus type 1 and 2 envelope domain III recombinant proteins in monkeys against strains of different dengue virus type 1 and 2 genotypes. Here we demonstrated that dengue virus type 1 and 2 recombinant proteins induced high titers of neutralizing antibodies against different genotype strains.

Neutralizing antibody response variation against dengue 3 strains

To evaluate the neutralizing antibody activity of a human sera panel against seven strains of the homotypic virus. Sera were collected from DENV-3 immune individuals. Two DENV-3 genotypes and strains isolated at different time-points during the 2000 and 2001-2002 Havana epidemics were included. A panel of 20 late convalescent sera collected 16-18 months after acute illness from DF and DHF patients are studied. These individuals were infected during the 2001-2002 Havana DENV-3 epidemic. All but four sera collected from DF cases had a secondary DENV-1/DENV-3 infection. Sera neutralizing antibody titer against the seven DENV-3 strains were determined by plaque reduction neutralization technique. Sera samples were tested simultaneously. Studied sera showed higher levels of neutralizing antibodies to DENV-3 strains of genotype III compared to genotype V. Interesting, higher levels of neutralizing antibodies were detected to DENV-3 strain isolated at the end of the epidemic 2001-2002. An increased tendency of GMT of neutralizing antibodies according to epidemic evolution was observed for the 2001-2002 outbreak. In general, antibody levels in sera collected from DF cases were higher. Differences in the neutralization capacity of immune DENV-3 sera tested against two homologous genotypes including strains of the same genotype are demonstrated. Observed results suggest that virus changed in the course of the epidemic. The implications of this finding in terms of dengue pathogenesis and vaccine development need to be considered.

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.