Neutralization and antibody-dependent enhancement of dengue viruses

Recent advances in understanding the adaptive immune response to Zika virus and the effect of previous flavivirus exposure

Zika virus (ZIKV) caused explosive epidemics across the Americas, starting in Brazil in 2015, and has been associated with severe manifestations such as microcephaly in babies born to infected mothers and Guillain-Barré syndrome in adults. As the underlying mechanisms of pathogenesis remain largely unknown, diverse investigations have focused on a potential role for flavivirus cross-reactive antibodies in enhancing ZIKV infection. Antibody-dependent enhancement is especially concerning due to structural similarities between ZIKV and other flaviviruses, especially dengue virus (DENV), that co-circulate in areas affected by ZIKV. Conversely, investigating cross-neutralizing antibodies is important for understanding protection among flaviviruses, including ZIKV. In this review, we discuss the latest findings regarding ZIKV-induced adaptive immunity, such as monoclonal and polyclonal antibody responses, structural immunology, and T cell-mediated responses. Much progress has been made in a short amount of time, but many questions remain. Fully understanding the specificity, magnitude, and kinetics of B cell/antibody and T cell responses in ZIKV-infected individuals with or without prior exposure to flaviviruses is of great relevance for diagnostics and vaccine development.

Zika virus vaccines: immune response, current status, and future challenges

Zika virus (ZIKV) is the most recent mosquito-transmitted virus to cause a global health crisis following its entrance into a naïve population in the Western Hemisphere. Once the ZIKV outbreak began investigators rapidly established small and large animal models of pathogenesis, developed a number candidate vaccines using different platforms, and defined mechanisms of protection. In this review, we characterize the adaptive immune response elicited by ZIKV infections and vaccines, the status of ongoing clinical trials in humans, and discuss future challenges within the field.

Profile of the 2016 dengue outbreak in Nepal

OBJECTIVE

The objective of this study was to obtain clinical, virological and demographic data detailing the 2016 dengue outbreak in Nepal.

RESULTS

Dengue disease was first reported in Nepal in 2004 and several major outbreaks have occurred since then, with a significant impact on public health. An outbreak of dengue fever occurred in Nepal during June to November 2016, with a peak number of cases reported in September. 1473 patients with laboratory confirmed DENV infections visited or were admitted to hospitals during this period. The most common clinical symptoms included fever, headache, joint pain and thrombocytopenia. Serotyping of 75 serum samples from patients having fever for less than 4 days was carried out with a dengue virus (DENV) serotype-specific RT-PCR strategy. Our results indicate that the dengue outbreak in Nepal during 2016 was caused predominantly, if not exclusively, by DENV-1, representing a shift in the prevailing serotype from DENV-2, the dominant serotype characterizing the 2013 dengue epidemic in Nepal. Hopefully, this report will assist Nepalese public health agencies in developing improved dengue-related programs including mosquito-vector control, DENV surveillance, and diagnosis and treatment of dengue fever patients, in order to reduce the impact of future dengue epidemics.

Dengue envelope-based 'four-in-one' virus-like particles produced using Pichia pastoris induce enhancement-lacking, domain III-directed tetravalent neutralising antibodies in mice

Dengue is a significant public health problem worldwide, caused by four antigenically distinct mosquito-borne dengue virus (DENV) serotypes. Antibodies to any given DENV serotype which can afford protection against that serotype tend to enhance infection by other DENV serotypes, by a phenomenon termed antibody-dependent enhancement (ADE). Antibodies to the viral pre-membrane (prM) protein have been implicated in ADE. We show that co-expression of the envelope protein of all four DENV serotypes, in the yeast Pichia pastoris, leads to their co-assembly, in the absence of prM, into tetravalent mosaic VLPs (T-mVLPs), which retain the serotype-specific antigenic integrity and immunogenicity of all four types of their monomeric precursors. Following a three-dose immunisation schedule, the T-mVLPs elicited EDIII-directed antibodies in mice which could neutralise all four DENV serotypes. Importantly, anti-T-mVLP antibodies did not augment sub-lethal DENV-2 infection of dengue-sensitive AG129 mice, based on multiple parameters. The 'four-in-one' tetravalent T-mVLPs possess multiple desirable features which may potentially contribute to safety (non-viral, prM-lacking and ADE potential-lacking), immunogenicity (induction of virus-neutralising antibodies), and low cost (single tetravalent immunogen produced using P. pastoris, an expression system known for its high productivity using simple inexpensive media). These results strongly warrant further exploration of this vaccine candidate.

Which Dengue Vaccine Approach Is the Most Promising, and Should We Be Concerned about Enhanced Disease after Vaccination? The Risks of Incomplete Immunity to Dengue Virus Revealed by Vaccination

Immune enhancement of dengue disease continues to be a concern for those with incomplete immunity in endemic areas. Advanced testing and follow-up of a newly available live attenuated dengue vaccine has recently shown the ability of vaccination to predispose some recipients for a severe outcome on subsequent infection. To improve safety, recommendations have been made to restrict use of the vaccine to those who are likely to have had prior exposure to dengue virus (DENV). Researchers continue to investigate dengue immunity and seek evidence that dengue vaccines can be safely administered to all populations needing protection.

Which Dengue Vaccine Approach Is the Most Promising, and Should We Be Concerned about Enhanced Disease after Vaccination? There Is Only One True Winner

The scientific community now possesses information obtained directly from human beings that makes it possible to understand why breakthrough-enhanced dengue virus (DENV) infections occurred in children receiving Sanofi Pasteur's Dengvaxia tetravalent live attenuated vaccine and to predict the possibility of breakthrough-enhanced DENV infections following immunization with two other tetravalent live attenuated vaccines now in phase III testing. Based upon recent research, Dengvaxia, lacking DENV nonstructural protein antigens, did not protect seronegatives because it failed to raise a competent T-cell response and/or antibodies to NS1. It is also possible that chimeric structure does not present the correct virion conformation permitting the development of protective neutralizing antibodies. A premonitory signal shared by the Sanofi Pasteur and the Takeda vaccines was the failure of fully immunized subhuman primates to prevent low-level viremia and/or anamnestic antibody responses to live DENV challenge. The vaccine developed by the National Institute of Allergy and Infectious Diseases (National Institutes of Health [NIH]) has met virtually all of the goals needed to demonstrate preclinical efficacy and safety for humans. Each monovalent vaccine was comprehensively studied for reactogenicity and immunogenicity in human volunteers. Protective immunity in subjects receiving tetravalent candidate vaccines was evidenced by the fact that when vaccinated subjects were given further doses of vaccine or different strains of DENV the result was "solid immunity," a nonviremic and nonanamnestic immune response.

Safety issues from a Phase 3 clinical trial of a live-attenuated chimeric yellow fever tetravalent dengue vaccine

A tetravalent live-attenuated 3-dose vaccine composed of chimeras of yellow fever 17D and the four dengue viruses (CYD, also called Dengvaxia) completed phase 3 clinical testing in over 35,000 children leading to a recommendation that vaccine be administered to >/ = 9 year-olds residing in highly dengue- endemic countries. When clinical trial results were assessed 2 years after the first dose, vaccine efficacy among seropositives was high, but among seronegatives efficacy was marginal. Breakthrough dengue hospitalizations of vaccinated children occurred continuously over a period of 4-5 years post 3rd dose in an age distribution suggesting these children had been vaccinated when seronegative. This surmise was validated recently when the manufacturer reported that dengue NS1 IgG antibodies were absent in sera from hospitalized vaccinated children, an observation consistent with their having received Dengvaxia when seronegative. Based upon published efficacy data and in compliance with initial published recommendations by the manufacturer and WHO the Philippine government undertook to vaccinate 800,000-plus 9 year-olds starting in April 2016. Eighteen months later, dengue hospitalizations and a deaths were reported among vaccinated children. The benefits of administering Dengvaxia predicted by the manufacturer, WHO and others derive from scoring dengue hospitalizations of vaccinated children as vaccine failures rather than as vaccine enhanced dengue disease. Recommended regimens for administration of Dengvaxia should have been structured to warn of and avoid serious adverse events.

Nucleobases and corresponding nucleosides display potent antiviral activities against dengue virus possibly through viral lethal mutagenesis

Dengue virus affects millions of people worldwide each year. To date, there is no drug for the treatment of dengue-associated disease. Nucleosides are effective antivirals and work by inhibiting the accurate replication of the viral genome. Nucleobases offer a cheaper alternative to nucleosides for broad antiviral applications. Metabolic activation of nucleobases involves condensation with 5-phosphoribosyl-1-pyrophosphate to give the corresponding nucleoside-5’-monophosphate. This could provide an alternative to phosphorylation of a nucleoside, a step that is often rate limiting and inefficient in activation of nucleosides. We evaluated more than 30 nucleobases and corresponding nucleosides for their antiviral activity against dengue virus. Five nucleobases and two nucleosides were found to induce potent antiviral effects not previously described. Our studies further revealed that nucleobases were usually more active with a better tissue culture therapeutic index than their corresponding nucleosides. The development of viral lethal mutagenesis, an antiviral approach that takes into account the quasispecies behavior of RNA viruses, represents an exciting prospect not yet studied in the context of dengue replication. Passage of the virus in the presence of the nucleobase 3a (T-1105) and corresponding nucleoside 3b (T-1106), favipiravir derivatives, induced an increase in apparent mutations, indicating lethal mutagenesis as a possible antiviral mechanism. A more concerted and widespread screening of nucleobase libraries is a very promising approach to identify dengue virus inhibitors including those that may act as viral mutagens.

Dengue virus is a world-wide public health menace estimated to infect hundreds of millions of people per year. Vaccines to prevent dengue virus infection have had limited success due in part to the requirement to elicit effective immune responses against the four dengue serotypes. There is an urgent unmet need for anti-dengue virus therapies. Nucleosides are effective antiviral small molecules which usually work by inhibiting the accurate replication of the viral genome. Typically, nucleosides must be converted within the cell to their triphosphate form to inhibit virus replication, thus inefficient phosphorylation often leads to suboptimal activity. We screened a small library of nucleobases that require an activation pathway different from nucleosides to achieve the same active form. We identified some known and previously undescribed dengue virus nucleobase inhibitors and their corresponding nucleosides. Our investigation of the mechanism of action of one nucleobase and its corresponding nucleoside found evidence for enhanced mutagenesis of the dengue virus genome in the presence of the compounds in cell culture. A wide screening of nucleobases libraries is a promising strategy to discover dengue virus inhibitors including potential viral mutagens.

Dengue virus NS2 and NS4: Minor proteins, mammoth roles

Despite the ever-increasing global incidence of dengue fever, there are no specific chemotherapy regimens for its treatment. Structural studies on dengue virus (DENV) proteins have revealed potential drug targets. Major DENV proteins such as the envelope protein and non-structural (NS) proteins 3 and 5 have been extensively investigated in antiviral studies, but with limited success in vitro. However, the minor NS proteins NS2 and NS4 have remained relatively underreported. Emerging evidence indicating their indispensable roles in virus propagation and host immunomodulation should encourage us to target these proteins for drug discovery. This review covers current knowledge on DENV NS2 and NS4 proteins from structural and functional perspectives and assesses their potential as targets for antiviral design. Antiviral targets in NS2A include surface-exposed transmembrane regions involved in pathogenesis, while those in NS2B include protease-binding sites in a conserved hydrophilic domain. Ideal drug targets in NS4A include helix α4 and the PEPEKQR sequence, which are essential for NS4A-2K cleavage and NS4A-NS4B association, respectively. In NS4B, the cytoplasmic loop connecting helices α5 and α7 is an attractive target for antiviral design owing to its role in dimerization and NS4B-NS3 interaction. Findings implicating NS2A, NS2B, and NS4A in membrane-modulation and viroporin-like activities indicate an opportunity to target these proteins by disrupting their association with membrane lipids. Despite the lack of 3D structural data, recent topological findings and progress in structure-prediction methods should be sufficient impetus for targeting NS2 and NS4 for drug design.

Development of Dengue Virus Serotype-Specific NS1 Capture Assays for the Rapid and Highly Sensitive Identification of the Infecting Serotype in Human Sera

Dengue fever can be caused by one of four distinct dengue virus (DENV) serotypes that cocirculate in many parts of the world. Point of care serotype-specific nonstructural protein-1 (NS1) capture assays for the rapid serotyping of DENV in human sera would greatly support epidemiological surveillance and potentially also prognosis in individual patients. To ensure both serotype specificity and broad coverage of variants within serotypes, we have applied an innovative approach for the generation and selection of serotype-specific anti-NS1 mAbs. To elicit mAbs against conformational epitopes, NMRI mice were immunized with living HEK 293 transfectants expressing the native target Ags in multiple display on the cell surface. For each serotype, three different NS1 sequence variants were sequentially used for immunization of mice, hybridoma selection, and capture assay development, respectively. Selection of optimal combinations of capturing and detecting mAbs yielded highly sensitive and specific NS1 serotyping ELISAs (st-ELISAs) for the four serotypes. st-ELISA testing of 41 dengue patient sera showed a 100% concordance with the serotype determined by serotype-specific reverse transcriptase real-time quantitative PCR. The respective NS1 variants could be detected for ∼10 d after the onset of illness. Ab-dependent enhancement of DENV infections may be associated with a specific range of pre-existing anti-DENV serological Ab titers. Testing of patient sera with the developed st-ELISAs will not only be useful for epidemiological studies and surveillance, but it may also help to develop and validate assays that can distinguish protective versus enhancing Ab responses for risk assessment for the development of severe dengue disease in individual patients.

The expression profile of human peripheral blood mononuclear cell miRNA is altered by antibody-dependent enhancement of infection with dengue virus serotype 3

Background

Antibody-dependent enhancement (ADE) of dengue virus (DENV) infection has been identified as the main risk factor for severe dengue disease, although the underlying mechanisms leading to severe dengue fever remain unclear. MicroRNAs (miRNAs) participate in numerous pathological and biological processes, including host responses to viral infections.

Method

Here, we aimed to investigate the differences in miRNA expression patterns in human peripheral blood mononuclear cells (PBMCs) infected with DENV-3 and DENV-3-ADE at various time points employing high-throughput sequencing.

Results

According to miRNAs high-throughput sequencing, a total of 50 known miRNAs exhibited significant differences. GO (Gene Ontology) and pathway analysis of the predicted targets showed enrichment in the regulation of transcription, including multicellular organismal development, DNA-dependent transcription, negative regulation of cell differentiation and transcription. Afterwards, regulatory networks of miRNA predicted targets, miRNA transcription factors, miRNA pathways and miRNA GOs were formulated to expose the complex regulatory mechanisms of miRNAs during the infection phase. Finally, we analyzed hierarchical GO categories of the predicted targets involved in the MAPK signaling pathway, the cGMP-PKG signaling pathway, the cAMP signaling pathway, the endocytosis effect, and our analyses indicated that innate and adaptive immunity following DENV-3 and DENV-3-ADE infections may be signally distinct.

Conclusion

Our results demonstrate a novel describing miRNA expression profiles in human PBMCs with DENV-3 and DENV-3-ADE infections using high-throughput sequencing. Our findings could provide a beneficial basis for further studies on the regulatory roles of miRNAs relevant to the different immune responses caused by DENV-3 and DENV-3-ADE infections.

Electronic supplementary material

The online version of this article (10.1186/s12985-018-0963-1) contains supplementary material, which is available to authorized users.

Immune Responses to Dengue and Zika Viruses-Guidance for T Cell Vaccine Development

Despite numerous efforts to identify the molecular and cellular effectors of the adaptive immunity that induce a long-lasting immunity against dengue or Zika virus infection, the specific mechanisms underlying such protective immunity remain largely unknown. One of the major challenges lies in the high level of dengue virus (DENV) seroprevalence in areas where Zika virus (ZIKV) is circulating. In the context of such a pre-existing DENV immunity that can exacerbate ZIKV infection and disease, and given the lack of appropriate treatment for ZIKV infection, there is an urgent need to develop an efficient vaccine against DENV and ZIKV. Notably, whereas several ZIKV vaccine candidates are currently in clinical trials, all these vaccine candidates have been designed to induce neutralizing antibodies as the primary mechanism of immune protection. Given the difficulty to elicit simultaneously high levels of neutralizing antibodies against the different DENV serotypes, and the potential impact of pre-existing subneutralizing antibodies induced upon DENV infection or vaccination on ZIKV infection and disease, additional or alternative strategies to enhance vaccine efficacy, through T cell immunity, are now being considered. In this review, we summarize recent discoveries about cross-reactive B and T cell responses against DENV and ZIKV and propose guidelines for the development of safe and efficient T cell vaccines targeting both viruses.

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.

Comparative immunogenicity of preparations of yeast-derived dengue oral vaccine candidate

BACKGROUND

Dengue is listed as a neglected tropical disease by the Center for Disease Control and Preservation, as there are insufficient integrated surveillance strategies, no effective treatment, and limited licensed vaccines. Consisting of four genetically distinct serotypes, dengue virus (DENV) causes serious life-threatening infections due to its complexity. Antibody-dependent enhancement by pre-existing cross-reactive as well as homotypic antibodies further worsens the clinical symptoms of dengue. Thus, a vaccine conferring simultaneous and durable immunity to each of the four DENV serotypes is essential to restrict its escalation. In deeply affected resource-limited countries, oral vaccination using food-grade organisms is considered to be a beneficial approach in terms of costs, patient comfort, and simple logistics for mass immunization. The current study used a mouse model to explore the immunogenicity of an oral dengue vaccine candidate prepared using whole recombinant yeast cells (WC) and cell-free extracts (CFE) from cells expressing recombinant Escherichia coli heat-labile toxin protein B-subunit (LTB) fused to the consensus dengue envelope domain III (scEDIII). Mice were treated orally with recombinant WC and CFE vaccines in 2-week intervals for 4 weeks and changes in systemic and mucosal immune responses were monitored.

RESULTS

Both WC and CFE dosage applications of LTB-scEDIII stimulated a systemic humoral immune response in the form of dengue-specific serum IgG as well as mucosal immune response in the form of secretory sIgA. Antigen-specific B cell responses in isolated lymphoid cells from the spleen and Peyer's patches further indicated an elevated mucosal immune response. Cellular immune response estimated through lymphocyte proliferation assay indicated higher levels in CFE than WC dosage. Furthermore, sera obtained after both oral administrations successfully neutralized DENV-1, whereas CFE formulation only neutralized DENV-2 serotype, two representative serotypes which cause severe dengue infection. Sera from mice that were fed CFE preparations demonstrated markedly higher neutralizing titers compared to those from WC-fed mice. However, WC feeding elicited strong immune responses, which were similar to the levels induced by CFE feeding after intraperitoneal booster with purified scEDIII antigen.

CONCLUSIONS

CFE preparations of LTB-scEDIII produced strong immunogenicity with low processing requirements, signifying that this fusion protein shows promise as a potent oral vaccine candidate against dengue viral infection.

Biologic Evidence Required for Zika Disease Enhancement by Dengue Antibodies

The sudden appearance of overt human Zika virus infections that cross the placenta to damage fetal tissues, target sexual organs, and are followed in some instances by Guillain-Barré syndrome raises questions regarding whether these outcomes are caused by genetic mutations or if prior infection by other flaviviruses affects disease outcome. Because dengue and Zika viruses co-circulate in the urban Aedes aegypti mosquito–human cycle, a logical question, as suggested by in vitro data, is whether dengue virus infections result in antibody-dependent enhancement of Zika virus infections. This review emphasizes the critical role for epidemiologic studies (retrospective and prospective) in combination with the studies to identify specific sites of Zika virus infection in humans that are needed to establish antibody-dependent enhancement as a possibility or a reality.

The bright and the dark side of human antibody responses to flaviviruses: lessons for vaccine design

Zika and dengue viruses belong to the Flavivirus genus, a close group of antigenically related viruses that cause significant arthropod-transmitted diseases throughout the globe. Although infection by a given flavivirus is thought to confer lifelong protection, some of the patient's antibodies cross-react with other flaviviruses without cross-neutralizing. The original antigenic sin phenomenon may amplify such antibodies upon subsequent heterologous flavivirus infection, potentially aggravating disease by antibody-dependent enhancement (ADE). The most striking example is provided by the four different dengue viruses, where infection by one serotype appears to predispose to more severe disease upon infection by a second one. A similar effect was postulated for sequential infections with Zika and dengue viruses. In this review, we analyze the molecular determinants of the dual antibody response to flavivirus infection or vaccination in humans. We highlight the role of conserved partially cryptic epitopes giving rise to cross-reacting and poorly neutralizing, ADE-prone antibodies. We end by proposing a strategy for developing an epitope-focused vaccine approach to avoid eliciting undesirable antibodies while focusing the immune system on producing protective antibodies only.

Dengue virus infection-enhancement activity in neutralizing antibodies of healthy adults before dengue season as determined by using FcγR-expressing cells

Background

Antibodies are critical responses to protect the host from dengue virus(DENV) infection. Antibodies target DENV by two pathologic mechanisms: virus neutralization and infection enhancement. In dengue patients, the absence of neutralizing activity in the presence of FcγR implies that infection-enhancing activity hampers the neutralizing activity of antibodies, which could potentially lead to symptomatic presentations and severe clinical outcomes.

Methods

A total of 100 pair serum samples from adult healthy volunteers were obtained during the dengue season in Ha Noi in 2015 for evaluation of neutralizing and infection-enhancing activity. Additionally, 20 serum samples from acute secondary DENV infection patients were also used as the patient group in this study. PRNT was performed on BHK cells and FcγR-expressing BHK cell lines for all serum samples.

Results

Out of 100 residents, positive neutralizing antibodies (N.A) were found in 44.23 and 76.92% for DENV-1; 38.46 and 75% for DENV-2; 19.23 and 15.38% for DENV-3; and 1.92 and 9.62% for DENV-4 for pre and post-dengue season respectively. The percentage of post-exposure residents having positive responses against single, two, or more than three DENV serotypes were 38.46, 44.23 and 15.38%, respectively. A total of 34 residents were DENV seropositive before the dengue season and these individuals demonstrated further elevation of IgG antibodies after the dengue season. At the end of the season, 18 residents were confirmed to be new asymptomatic DENV infection cases. In both groups, N.A titers determined on BHK cells were higher than that on FcγR-expressing BHK cells. In heterotypic N.A responses, N.A titers to the infecting serotype from the samples obtained from pre-exposure group were significantly higher than those of the patient group. However, fold enhancement to the infecting serotypes from the samples in the pre-exposure group was substantially lower as compared to that of the patient group.

Conclusion

Before and after the dengue season, serum samples from healthy volunteers demonstrated high levels of neutralizing antibodies and low or absence of infection-enhancement activity. The results suggest that while infection-enhancement activity hampers neutralizing activity of antibodies, high levels of DENV neutralizing antibodies set a critical threshold in facilitating the prevention of disease progression.

Electronic supplementary material

The online version of this article (10.1186/s12879-017-2894-7) contains supplementary material, which is available to authorized users.

Pichia pastoris-Expressed Bivalent Virus-Like Particulate Vaccine Induces Domain III-Focused Bivalent Neutralizing Antibodies without Antibody-Dependent Enhancement in Vivo

Dengue, a significant public health problem in several countries around the world, is caused by four different serotypes of mosquito-borne dengue viruses (DENV-1, -2, -3, and -4). Antibodies to any one DENV serotype which can protect against homotypic re-infection, do not offer heterotypic cross-protection. In fact, cross-reactive antibodies may augment heterotypic DENV infection through antibody-dependent enhancement (ADE). A recently launched live attenuated vaccine (LAV) for dengue, which consists of a mixture of four chimeric yellow-fever/dengue vaccine viruses, may be linked to the induction of disease-enhancing antibodies. This is likely related to viral interference among the replicating viral strains, resulting in an unbalanced immune response, as well as to the fact that the LAV encodes prM, a DENV protein documented to elicit ADE-mediating antibodies. This makes it imperative to explore the feasibility of alternate ADE risk-free vaccine candidates. Our quest for a non-replicating vaccine centered on the DENV envelope (E) protein which mediates virus entry into the host cell and serves as an important target of the immune response. Serotype-specific neutralizing epitopes and the host receptor recognition function map to E domain III (EDIII). Recently, we found that Pichia pastoris-expressed DENV E protein, of all four serotypes, self-assembled into virus-like particles (VLPs) in the absence of prM. Significantly, these VLPs displayed EDIII and elicited EDIII-focused DENV-neutralizing antibodies in mice. We now report the creation and characterization of a novel non-replicating recombinant particulate vaccine candidate, produced by co-expressing the E proteins of DENV-1 and DENV-2 in P. pastoris. The two E proteins co-assembled into bivalent mosaic VLPs (mVLPs) designated as mE1E2_bv VLPs. The mVLP, which preserved the serotype-specific antigenic integrity of its two component proteins, elicited predominantly EDIII-focused homotypic virus-neutralizing antibodies in BALB/c mice, demonstrating its efficacy. In an in vivo ADE model, mE1E2_bv VLP-induced antibodies lacked discernible ADE potential, compared to the cross-reactive monoclonal antibody 4G2, as evidenced by significant reduction in the levels of IL-6 and TNF-α, suggesting inherent safety. The results obtained with these bivalent mVLPs suggest the feasibility of incorporating the E proteins of DENV-3 and DENV-4 to create a tetravalent mVLP vaccine.

A tetravalent virus-like particle vaccine designed to display domain III of dengue envelope proteins induces multi-serotype neutralizing antibodies in mice and macaques which confer protection against antibody dependent enhancement in AG129 mice

Background

Dengue is one of the fastest spreading vector-borne diseases, caused by four antigenically distinct dengue viruses (DENVs). Antibodies against DENVs are responsible for both protection as well as pathogenesis. A vaccine that is safe for and efficacious in all people irrespective of their age and domicile is still an unmet need. It is becoming increasingly apparent that vaccine design must eliminate epitopes implicated in the induction of infection-enhancing antibodies.

Methodology/principal findings

We report a Pichia pastoris-expressed dengue immunogen, DSV4, based on DENV envelope protein domain III (EDIII), which contains well-characterized serotype-specific and cross-reactive epitopes. In natural infection, <10% of the total neutralizing antibody response is EDIII-directed. Yet, this is a functionally relevant domain which interacts with the host cell surface receptor. DSV4 was designed by in-frame fusion of EDIII of all four DENV serotypes and hepatitis B surface (S) antigen and co-expressed with unfused S antigen to form mosaic virus-like particles (VLPs). These VLPs displayed EDIIIs of all four DENV serotypes based on probing with a battery of serotype-specific anti-EDIII monoclonal antibodies. The DSV4 VLPs were highly immunogenic, inducing potent and durable neutralizing antibodies against all four DENV serotypes encompassing multiple genotypes, in mice and macaques. DSV4-induced murine antibodies suppressed viremia in AG129 mice and conferred protection against lethal DENV-4 virus challenge. Further, neither murine nor macaque anti-DSV4 antibodies promoted mortality or inflammatory cytokine production when passively transferred and tested in an in vivo dengue disease enhancement model of AG129 mice.

Conclusions/significance

Directing the immune response to a non-immunodominant but functionally relevant serotype-specific dengue epitope of the four DENV serotypes, displayed on a VLP platform, can help minimize the risk of inducing disease-enhancing antibodies while eliciting effective tetravalent seroconversion. DSV4 has a significant potential to emerge as a safe, efficacious and inexpensive subunit dengue vaccine candidate.

Dengue is mosquito-borne viral disease which is currently a global public health problem. It is caused by four different types of dengue viruses. Nearly a 100 million people a year suffer from overt sickness, which may range from mild fever to potentially fatal disease. A virus-based dengue vaccine was launched for the first time in late 2015. Unexpectedly, this vaccine mimics the dengue viruses in that it appears to elicit disease-enhancing antibodies. To reduce such risk, safer vaccines that eliminate viral proteins responsible for undesirable antibodies are needed. We focused our attention on a small domain of the dengue virus surface protein known as envelope domain III (EDIII). Humans make only a small amount of antibodies against EDIII, but these antibodies are effective in blocking dengue virus from entering cells. We used a yeast expression system to display EDIIIs of all four types of dengue viruses on the surface of non-infectious virus-like particles (VLPs). These VLPs elicited antibodies, in mice and monkeys, which blocked all four dengue virus types and their variants from entering cells in culture. Importantly, these antibodies did not enhance dengue infection in a mouse model.

Development of Antibody Therapeutics against Flaviviruses

Recent outbreaks of Zika virus (ZIKV) highlight the urgent need to develop efficacious interventions against flaviviruses, many of which cause devastating epidemics around the world. Monoclonal antibodies (mAb) have been at the forefront of treatment for cancer and a wide array of other diseases due to their specificity and potency. While mammalian cell-produced mAbs have shown promise as therapeutic candidates against several flaviviruses, their eventual approval for human application still faces several challenges including their potential risk of predisposing treated patients to more severe secondary infection by a heterologous flavivirus through antibody-dependent enhancement (ADE). The high cost associated with mAb production in mammalian cell cultures also poses a challenge for the feasible application of these drugs to the developing world where the majority of flavivirus infection occurs. Here, we review the current therapeutic mAb candidates against various flaviviruses including West Nile (WNV), Dengue virus (DENV), and ZIKV. The progress of using plants for developing safer and more economical mAb therapeutics against flaviviruses is discussed within the context of their expression, characterization, downstream processing, neutralization, and in vivo efficacy. The progress of using plant glycoengineering to address ADE, the major impediment of flavivirus therapeutic development, is highlighted. These advancements suggest that plant-based systems are excellent alternatives for addressing the remaining challenges of mAb therapeutic development against flavivirus and may facilitate the eventual commercialization of these drug candidates.

Challenges of Vaccine Development for Zika Virus

The emergence of outbreaks of Zika virus (ZIKV) in Brazil in 2015 was associated with devastating effects on fetal development and prompted a world health emergency and multiple efforts to generate an effective vaccine against infection. There are now more than 40 vaccine candidates in preclinical development and six in clinical trials. Despite similarities with other flaviviruses to which successful vaccines have been developed, such as yellow fever virus and Japanese Encephalitis virus, there are unique challenges to the development and clinical trials of a vaccine for ZIKV.

EDiP: the Epitope Dilution Phenomenon. Lessons learnt from a malaria vaccine antigen and its applicability to polymorphic antigens

INTRODUCTION

Polymorphism in vaccine antigens poses major challenges to vaccinologists. The Plasmodium falciparum Apical Membrane Antigen 1 (AMA1) poses such a challenge. We found that immunization with a mixture of three variants yielded functional antibody levels to all variants comparable to levels induced by monovalent immunization. The mechanism behind the observed broadening was shown to be an increase in the fraction of cross-reactive antibodies, most likely because strain-specific epitopes are present at lower frequency relative to conserved epitopes. Areas covered: We hereby introduce the Epitope Dilution Phenomenon (EDiP) as a practical strategy for the induction of broad, cross-variant antibody responses against polymorphic antigens and discuss the utility and applicability of this phenomenon for the development of vaccines against polymorphic antigens of pathogens like Influenza, HIV, Dengue and Plasmodium. Expert commentary: EDiP can be used to broaden antibody responses by immunizing with a mixture of at least 3 antigenic variants, where the variants included can differ, yet yield broadened responses.

Detection, phenotyping and quantification of dengue virus-specific B cells using fluorescent probes

Dengue viruses are some of the most important mosquito-borne pathogens worldwide. They cause illness in 50-100 million individuals per year and have a significant global health impact in low- and middle-income countries. It is important to improve our understanding of the humoral response to dengue virus, as antibodies (Abs) are associated with protection from or susceptibility to severe dengue disease. In recent years, significant advances have been made toward identifying Ab targets and evaluating the functional properties of Abs. However, much less is known about the cellular source of Abs, B cells, in part because the reagents to phenotype and characterize antigen-specific B cells have been challenging to develop. Here, we discuss our recent experience with developing and using fluorescent viruses to probe the B cell response to dengue virus. We present the strengths and weaknesses of flow cytometric analysis of antigen-specific B cells and discuss the use of these probes to phenotype and characterize specific B cells during and after natural infection and in ongoing dengue vaccine trials.

Potent Plasmablast-Derived Antibodies Elicited by the National Institutes of Health Dengue Vaccine

Exposure to dengue virus (DENV) is thought to elicit lifelong immunity, mediated by DENV-neutralizing antibodies (nAbs). However, Abs generated by primary infections confer serotype-specific protection, and immunity against other serotypes develops only after subsequent infections. Accordingly, the induction of these nAb responses acquired after serial DENV infections has been a long-sought-after goal for vaccination. Nonetheless, it is still unclear if tetravalent vaccines can elicit or recall nAbs. In this study, we have characterized the responses from a volunteer who had been previously exposed to DENV and was immunized with the live attenuated tetravalent vaccine Butantan-DV, developed by the NIH and Butantan Institute. Eleven days after vaccination, we observed an ∼70-fold expansion of the plasmablast population. We generated 21 monoclonal Abs (MAbs) from singly sorted plasmablasts. These MAbs were the result of clonal expansions and had significant levels of somatic hypermutation (SHM). Nineteen MAbs (90.5%) neutralized at least one DENV serotype at concentrations of 1 μg/ml or less; 6 of the 21 MAbs neutralized three or more serotypes. Despite the tetravalent composition of the vaccine, we observed a neutralization bias in the induced repertoire: DENV3 was targeted by 18 of the 19 neutralizing MAbs (nMAbs). Furthermore, the P3D05 nMAb neutralized DENV3 with extraordinary potency (concentration to achieve half-maximal neutralization [Neut₅₀] = 0.03 μg/ml). Thus, the Butantan-DV vaccine engendered a mature, antigen-selected B cell repertoire. Our results suggest that preexisting responses elicited by a previous DENV3 infection were recalled by immunization.IMPORTANCE The dengue epidemic presents a global public health challenge that causes widespread economic burden and remains largely unchecked by existing control strategies. Successful control of the dengue epidemic will require effective prophylactic and therapeutic interventions. Several vaccine clinical efficacy trials are approaching completion, and the chances that one or more live attenuated tetravalent vaccines (LATVs) will be introduced worldwide is higher than ever. While it is widely accepted that dengue virus (DENV)-neutralizing antibody (nAb) titers are associated with protection, the Ab repertoire induced by LATVs remain uncharacterized. Here, we describe the isolation of potent (Neut₅₀ < 0.1 μg/ml) nAbs from a DENV-seropositive volunteer immunized with the tetravalent vaccine Butantan-DV, which is currently in phase III trials.

Elaboration of tetravalent antibody responses against dengue viruses using a subunit vaccine comprised of a single consensus dengue envelope sequence

Dengue viruses (DENVs) are re-emerging pathogens transmitted by mosquitoes mainly in tropical and subtropical regions. Each year, they are estimated to infect 390 million people globally. The major challenge confronting dengue vaccine development is the need to induce balanced, long lasting tetravalent immune responses against four co-circulating virus serotypes (DENV-I, -II, -III, -IV), because primary infection by any one of which may predispose infected individuals to more severe diseases during a heterotypic secondary infection. Another difficulty is to select representative strains in vaccine design to provide cross-protection against most circulating virus strains. In this study, aimed at developing a tetravalent subunit vaccine with a representative single protein, we designed two vaccines (named cE80(D4) and cE80(max)) based on the consensus sequences of the ectodomain of envelope protein of 3127 DENV strains, and then expressed them in the baculovirus expression system. Both vaccines were capable of eliciting specific antibodies against all four DENV serotypes, and the predominant IgG subtype elicited by the two vaccines was IgG1. Moreover, these vaccines activated both type I and type II antigen-specific helper T cells that secreted IFN-γ and IL-4, respectively. This proof-of-concept study has set foundation for further optimization of a single protein-based tetravalent DENV vaccine.

Characterization and epitope mapping of Dengue virus type 1 specific monoclonal antibodies

BACKGROUND

Dengue virus (DV) infection causes a spectrum of clinical diseases ranging from dengue fever to a life-threatening dengue hemorrhagic fever. Four distinct serotypes (DV1-4), which have similar genome sequences and envelope protein (E protein) antigenic properties, were divided. Among these 4 serotypes, DV1 usually causes predominant infections and fast diagnosis and effective treatments are urgently required to prevent further hospitalization and casualties.

METHODS

To develop antibodies specifically targeting and neutralizing DV1, we immunized mice with UV-inactivated DV1 viral particles and recombinant DV1 E protein from residue 1 to 395 (E395), and then generated 12 anti-E monoclonal antibodies (mAbs) as the candidates for a series of characterized assays such as ELISA, dot blot, immunofluorescence assay, western blot, and foci forming analyses.

RESULTS

Among the mAbs, 10 out of 12 showed cross-reactivity to four DV serotypes as well as Japanese encephalitis virus (JEV) in different cross-reactivity patterns. Two particular mAbs, DV1-E1 and DV1-E2, exhibited strong binding specificity and neutralizing activity against DV1 and showed no cross-reactivity to DV2, DV3, DV4 or JEV-infected cells as characterized by ELISA, dot blot, immunofluorescence assay, western blot, and foci forming analyses. Using peptide coated indirect ELISA, we localized the neutralizing determinants of the strongly inhibitory mAbs to a sequence-unique epitope on the later-ridge of domain III of the DV1 E protein, centered near residues T346 and D360 (346TQNGRLITANPIVTD360). Interestingly, the amino acid sequence of the epitope region is highly conserved among different genotypes of DV1 but diverse from DV2, DV3, DV4 serotypes and other flaviviruses.

CONCLUSIONS

Our results showed two selected mAbs DV1-E1 and DV1-E2 can specifically target and significantly neutralize DV1. With further research these two mAbs might be applied in the development of DV1 specific serologic diagnosis and used as a feasible treatment option for DV1 infection. The identification of DV1 mAbs epitope with key residues can also provide vital information for vaccine design.

Germline bias dictates cross-serotype reactivity in a common dengue-virus-specific CD8+ T cell response

Adaptive immune responses protect against infection with dengue virus (DENV), yet cross-reactivity with distinct serotypes can precipitate life-threatening clinical disease. We found that clonotypes expressing the T cell antigen receptor (TCR) β-chain variable region 11 (TRBV11-2) were 'preferentially' activated and mobilized within immunodominant human-leukocyte-antigen-(HLA)-A*11:01-restricted CD8⁺ T cell populations specific for variants of the nonstructural protein epitope NS3₁₃₃ that characterize the serotypes DENV1, DENV3 and DENV4. In contrast, the NS3₁₃₃-DENV2-specific repertoire was largely devoid of such TCRs. Structural analysis of a representative TRBV11-2⁺ TCR demonstrated that cross-serotype reactivity was governed by unique interplay between the variable antigenic determinant and germline-encoded residues in the second β-chain complementarity-determining region (CDR2β). Extensive mutagenesis studies of three distinct TRBV11-2⁺ TCRs further confirmed that antigen recognition was dependent on key contacts between the serotype-defined peptide and discrete residues in the CDR2β loop. Collectively, these data reveal an innate-like mode of epitope recognition with potential implications for the outcome of sequential exposure to heterologous DENVs.

Zika virus activates de novo and cross-reactive memory B cell responses in dengue-experienced donors

Zika virus (ZIKV) shares a high degree of homology with dengue virus (DENV), suggesting that preexisting immunity to DENV could affect immune responses to ZIKV. We have tracked the evolution of ZIKV-induced B cell responses in three DENV-experienced donors. The acute antibody (plasmablast) responses were characterized by relatively high somatic hypermutation and a bias toward DENV binding and neutralization, implying the early activation of DENV clones. A DENV-naïve donor in contrast showed a classical primary plasmablast response. Five months after infection, the DENV-experienced donors developed potent type-specific ZIKV neutralizing antibody responses in addition to DENV cross-reactive responses. Because cross-reactive responses were poorly neutralizing and associated with enhanced ZIKV infection in vitro, preexisting DENV immunity could negatively affect protective antibody responses to ZIKV. The observed effects are epitope-dependent, suggesting that a ZIKV vaccine should be carefully designed for DENV-seropositive populations.

Use of a Recombinant Gamma-2 Herpesvirus Vaccine Vector against Dengue Virus in Rhesus Monkeys

Research on vaccine approaches that can provide long-term protection against dengue virus infection is needed. Here we describe the construction, immunogenicity, and preliminary information on the protective capacity of recombinant, replication-competent rhesus monkey rhadinovirus (RRV), a persisting herpesvirus. One RRV construct expressed nonstructural protein 5 (NS5), while a second recombinant expressed a soluble variant of the E protein (E85) of dengue virus 2 (DENV2). Four rhesus macaques received a single vaccination with a mixture of both recombinant RRVs and were subsequently challenged 19 weeks later with 1 × 10⁵ PFU of DENV2. During the vaccine phase, plasma of all vaccinated monkeys showed neutralizing activity against DENV2. Cellular immune responses against NS5 were also elicited, as evidenced by major histocompatibility complex class I (MHC-I) tetramer staining in the one vaccinated monkey that was Mamu-A*01 positive. Unlike two of two unvaccinated controls, two of the four vaccinated monkeys showed no detectable viral RNA sequences in plasma after challenge. One of these two monkeys also showed no anamnestic increases in antibody levels following challenge and thus appeared to be protected against the acquisition of DENV2 following high-dose challenge. Continued study will be needed to evaluate the performance of herpesviral and other persisting vectors for achieving long-term protection against dengue virus infection.IMPORTANCE Continuing studies of vaccine approaches against dengue virus (DENV) infection are warranted, particularly ones that may provide long-term immunity against all four serotypes. Here we investigated whether recombinant rhesus monkey rhadinovirus (RRV) could be used as a vaccine against DENV2 infection in rhesus monkeys. Upon vaccination, all animals generated antibodies capable of neutralizing DENV2. Two of four vaccinated monkeys showed no detectable viral RNA after subsequent high-dose DENV2 challenge at 19 weeks postvaccination. Furthermore, one of these vaccinated monkeys appeared to be protected against the acquisition of DENV2 infection on the basis of undetectable viral loads and the lack of an anamnestic antibody response. These findings underscore the potential utility of recombinant herpesviruses as vaccine vectors.

Cost-Effectiveness of Dengue Vaccination Programs in Brazil

The first approved dengue vaccine, CYD-TDV, a chimeric, live-attenuated, tetravalent dengue virus vaccine, was recently licensed in 13 countries, including Brazil. In light of recent vaccine approval, we modeled the cost-effectiveness of potential vaccination policies mathematically based on data from recent vaccine efficacy trials that indicated that vaccine efficacy was lower in seronegative individuals than in seropositive individuals. In our analysis, we investigated several vaccination programs, including routine vaccination, with various vaccine coverage levels and those with and without large catch-up campaigns. As it is unclear whether the vaccine protects against infection or just against disease, our model incorporated both direct and indirect effects of vaccination. We found that in the presence of vaccine-induced indirect protection, the cost-effectiveness of dengue vaccination decreased with increasing vaccine coverage levels because the marginal returns of herd immunity decreases with vaccine coverage. All routine dengue vaccination programs that we considered were cost-effective, reducing dengue incidence significantly. Specifically, a routine dengue vaccination of 9-year-olds would be cost-effective when the cost of vaccination per individual is less than $262. Furthermore, the combination of routine vaccination and large catch-up campaigns resulted in a greater reduction of dengue burden (by up to 93%) than routine vaccination alone, making it a cost-effective intervention as long as the cost per course of vaccination is $255 or less. Our results show that dengue vaccination would be cost-effective in Brazil even with a relatively low vaccine efficacy in seronegative individuals.

Complexity of Human Antibody Response to Dengue Virus: Implication for Vaccine Development

The four serotypes of dengue virus (DENV) are the leading cause of arboviral diseases in humans. Decades of efforts have made remarkable progress in dengue vaccine development. Despite the first dengue vaccine (dengvaxia from Sanofi Pasteur), a live-attenuated tetravalent chimeric yellow fever-dengue vaccine, has been licensed by several countries since 2016, its overall moderate efficacy (56.5–60.8%) in the presence of neutralizing antibodies during the Phase 2b and 3 trials, lower efficacy among dengue naïve compared with dengue experienced individuals, and increased risk of hospitalization among young children during the follow-up highlight the need for a better understanding of humoral responses after natural DENV infection. Recent studies of more than 300 human monoclonal antibodies (mAbs) against DENV have led to the discovery of several novel epitopes on the envelope protein recognized by potent neutralizing mAbs. This information together with in-depth studies on polyclonal sera and B-cells following natural DENV infection has tremendous implications for better immunogen design for a safe and effective dengue vaccine. This review outlines the progress in our understanding of mouse mAbs, human mAbs, and polyclonal sera against DENV envelope and precursor membrane proteins, two surface proteins involved in vaccine development, following natural infection; analyses of these discoveries have provided valuable insight into new strategies involving molecular technology to induce more potent neutralizing antibodies and less enhancing antibodies for next-generation dengue vaccine development.

Micro-anatomical changes in major blood vessel caused by dengue virus (serotype 2) infection

Dengue virus (DENV) has emerged as a major economic concern in developing countries, with 2.5 billion people believed to be at risk. Vascular endothelial cells (ECs) lining the circulatory system from heart to end vessels perform crucial functions in the human body, by aiding gas exchange in lungs, gaseous, nutritional and its waste exchange in all tissues, including the blood brain barrier, filtration of fluid in the glomeruli, neutrophil recruitment, hormone trafficking, as well as maintenance of blood vessel tone and hemostasis. These functions can be deregulated during DENV infection. In this study, BALB/c mice infected with DENV serotype 2 were analyzed histologically for changes in major blood vessels in response to DENV infection. In the uninfected mouse model, blood vessels showed normal architecture with intact endothelial monolayer, tunica media, and tunica adventitia. In the infected mouse model, DENV distorted the endothelium lining and disturbed the smooth muscle, elastic laminae and their supporting tissues causing vascular structural disarrangement. This may explain the severe pathological illness in DENV-infected individuals. The overall DENV-induced damages on the endothelial and it's supporting tissues and the dysregulated immune reactions initiated by the host were discussed.

Serotyping dengue virus with isothermal amplification and a portable sequencer

The recent development of a nanopore-type portable DNA sequencer has changed the way we think about DNA sequencing. We can perform sequencing directly in the field, where we collect the samples. Here, we report the development of a novel method to detect and genotype tropical disease pathogens, using dengue fever as a model. By combining the sequencer with isothermal amplification that only requires a water bath, we were able to amplify and sequence target viral genomes with ease. Starting from a serum sample, the entire procedure could be finished in a single day. The analysis of blood samples collected from 141 Indonesian patients demonstrated that this method enables the clinical identification and serotyping of the dengue virus with high sensitivity and specificity. The overall successful detection rate was 79%, and a total of 58 SNVs were detected. Similar analyses were conducted on 80 Vietnamese and 12 Thai samples with similar performance. Based on the obtained sequence information, we demonstrated that this approach is able to produce indispensable information for etiologically analyzing annual or regional diversifications of the pathogens.

Zika Virus: Recent Advances towards the Development of Vaccines and Therapeutics

Zika is a rapidly emerging public health threat. Although clinical infection is frequently mild, significant neurological manifestations have been demonstrated in infants born to Zika virus (ZIKV) infected mothers. Due to the substantial ramifications of intrauterine infection, effective counter-measures are urgently needed. In order to develop effective anti-ZIKV vaccines and therapeutics, improved animal models and a better understanding of immunological correlates of protection against ZIKV are required. This review will summarize what is currently known about ZIKV, the clinical manifestations and epidemiology of Zika as well as, the development of animal models to study ZIKV infection, host immune responses against ZIKV, and the current state of development of vaccines and therapeutics against ZIKV.

Covalently linked dengue virus envelope glycoprotein dimers reduce exposure of the immunodominant fusion loop epitope

A problem in the search for an efficient vaccine against dengue virus is the immunodominance of the fusion loop epitope (FLE), a segment of the envelope protein E that is buried at the interface of the E dimers coating mature viral particles. Anti-FLE antibodies are broadly cross-reactive but poorly neutralizing, displaying a strong infection enhancing potential. FLE exposure takes place via dynamic ‘breathing' of E dimers at the virion surface. In contrast, antibodies targeting the E dimer epitope (EDE), readily exposed at the E dimer interface over the region of the conserved fusion loop, are very potent and broadly neutralizing. We here engineer E dimers locked by inter-subunit disulfide bonds, and show by X-ray crystallography and by binding to a panel of human antibodies that these engineered dimers do not expose the FLE, while retaining the EDE exposure. These locked dimers are strong immunogen candidates for a next-generation vaccine.

The immunodominant epitope of dengue virus envelope protein (E) induces poorly neutralizing antibodies, which poses a problem for vaccine development. Here, the authors engineer covalently locked E dimers exposing an epitope that has been shown to induce potent and broadly neutralizing antibodies.

Humoral cross-reactivity between Zika and dengue viruses: implications for protection and pathology

Zika virus (ZIKV) is a re-emerging mosquito-borne flavivirus that has recently caused extensive outbreaks in Central and South America and the Caribbean. Given its association with Guillain–Barré syndrome in adults and neurological and ocular malformities in neonates, ZIKV has become a pathogen of significant public health concern worldwide. ZIKV shares a considerable degree of genetic identity and structural homology with other flaviviruses, including dengue virus (DENV). In particular, the surface glycoprotein envelope (E), which is involved in viral fusion and entry and is therefore a chief target for neutralizing antibody responses, contains regions that are highly conserved between the two viruses. This results in immunological cross-reactivity, which in the context of prior DENV exposure, may have significant implications for the generation of immune responses to ZIKV and affect disease outcomes. Here we address the issue of humoral cross-reactivity between DENV and ZIKV, reviewing the evidence for and discussing the potential impact of this cross-recognition on the functional quality of antibody responses against ZIKV. These considerations are both timely and relevant to future vaccine design efforts, in view of the existing overlap in the distribution of ZIKV and DENV and the likely spread of ZIKV to additional DENV-naive and experienced populations.

Role of Mast Cells in Dengue Virus Pathogenesis

Dengue is currently regarded as the most common arthropod-borne viral disease in tropical and subtropical areas, with an estimated 50-100 million infections occurring each year. Nearly all patients experience a self-limiting clinical course; however, the illness ranges from undifferentiated fever to severe hemorrhagic fever with or without shock syndrome complications. There are several immune cells associated with the pathogenesis of dengue virus (DENV) infection and systemic spread, including dendritic cells, macrophages, and mast cells (MCs). MCs are widely recognized for their immune functions and as cellular regulators of vascular integrity in human skin. Furthermore, these cells are able to detect DENV, which results in activation and degranulation of potent vasoactive mediators prestored in the granules. These mediators can act directly on vascular endothelium, increasing permeability and inducing vascular leakage. This review is designed to present an insight into the role of MCs during DENV infection and the dual functions in immune protection and contribution to the most severe forms of dengue.

Characterization of Zika virus binding and enhancement potential of a large panel of flavivirus murine monoclonal antibodies

Zika viruses (ZIKVs) are circulating in parts of the world endemic for other flavivirus infections. Some cross-reactive antibodies (Abs) elicited by prior flavivirus exposures can bind to ZIKV and enhance infection of Fc receptor-bearing cells. Here, we measured ZIKV binding of 54 murine monoclonal Abs (mAbs) elicited by exposure with Dengue virus and West Nile virus antigens. We found that 8 of 54 mAbs recognized the envelope protein of ZIKV in conventional binding assays. These 8 cross-reactive mAbs have different specificities; most recognize the DI/II region of the envelope protein but one mAb recognized the DIII lateral ridge of the envelope protein. Interestingly, only 3 of these cross-reactive mAbs were able to enhance ZIKV infection in vitro, and enhancing potential was not strictly correlated with relative binding ability. These data suggest that the ability of flavivirus Abs to enhance ZIKV is dependent on multiple factors.

The Role of Heterotypic DENV-specific CD8+T Lymphocytes in an Immunocompetent Mouse Model of Secondary Dengue Virus Infection

Dengue is the most prevalent arthropod-borne viral disease worldwide and is caused by the four dengue virus serotypes (DENV-1-4). Sequential heterologous DENV infections can be associated with severe disease manifestations. Here, we present an immunocompetent mouse model of secondary DENV infection using non mouse-adapted DENV strains to investigate the pathogenesis of severe dengue disease. C57BL/6 mice infected sequentially with DENV-1 (strain Puerto Rico/94) and DENV-2 (strain Tonga/74) developed low platelet counts, internal hemorrhages, and increase of liver enzymes. Cross-reactive CD8⁺ T lymphocytes were found to be necessary and sufficient for signs of severe disease by adoptively transferring of DENV-1-immune CD8⁺T lymphocytes before DENV-2 challenge. Disease signs were associated with production of tumor necrosis factor (TNF)-α and elevated cytotoxicity displayed by heterotypic anti-DENV-1 CD8⁺ T lymphocytes. These findings highlight the critical role of heterotypic anti-DENV CD8⁺ T lymphocytes in manifestations of severe dengue disease.

Cross-reactive dengue human monoclonal antibody prevents severe pathologies and death from Zika virus infections

Zika virus (ZIKV) infections have been linked with neurological complications and congenital Zika syndrome. Given the high level of homology between ZIKV and the related flavivirus dengue virus (DENV), we investigated the level of cross-reactivity with ZIKV using a panel of DENV human mAbs. A majority of the mAbs showed binding to ZIKV virions, with several exhibiting neutralizing capacities against ZIKV in vitro. Three of the best ZIKV-neutralizing mAbs were found to recognize diverse epitopes on the envelope (E) glycoprotein: the highly conserved fusion-loop peptide, a conformation-specific epitope on the E monomer, and a quaternary epitope on the virion surface. The most potent ZIKV-neutralizing mAb (SIgN-3C) was assessed in 2 type I interferon receptor–deficient (IFNAR^–/–) mouse models of ZIKV infection. Treatment of adult nonpregnant mice with SIgN-3C rescued mice from virus-induced weight loss and mortality. The SIgN-3C variant with Leu-to-Ala mutations in the Fc region (SIgN-3C-LALA) did not induce antibody-dependent enhancement (ADE) in vitro but provided similar levels of protection in vivo. In pregnant ZIKV-infected IFNAR^–/– mice, treatment with SIgN-3C or SIgN-3C-LALA significantly reduced viral load in the fetal organs and placenta and abrogated virus-induced fetal growth retardation. Therefore, SIgN-3C-LALA holds promise as a ZIKV prophylactic and therapeutic agent.

Monoclonal Antibody Against Premembrane Viral Protein of Avian Tembusu Virus

Premembrane (prM) is a viral protein of flavivirus, which is important for the generation of infectious virion and for virus infection to the host. However, the biological properties and function of the prM of Avian Tembusu virus (ATMUV) have scarcely been studied to date. Monoclonal antibodies (mAbs) are a powerful tool for functional analysis of viral protein. To produce a mAb against prM protein of ATMUV, the prM gene sequence was amplified by reverse transcription polymerase chain reaction (RT-PCR) and cloned into the prokaryotic expression vector pET-28a (+). The recombinant prM protein was successfully expressed in BL21 (DE3). Using the purified prM as immunogen in mice, three hybridoma cells secreting mAbs against prM protein were obtained. These mAbs showed a strong reaction with ATMUV-infected DF-1 cells and pEGFP-C3-prM transfected 293-T cells in both Western blotting analysis and immunofluorescence assay. The mAbs developed in this study will be useful tools for analysis of the prM protein functions on ATMUV infection and the interaction between prM and its host molecules.

Polytopic vaccination with a live-attenuated dengue vaccine enhances B-cell and T-cell activation, but not neutralizing antibodies

Dengue, caused by dengue viruses (DENVs), is the most common arboviral disease of humans. Several dengue vaccine candidates are at different stages of clinical development and one has been licensed. Inoculation with live-attenuated DENV constructs is an approach that has been used by vaccine developers. Unfortunately, the simultaneous injection of all four attenuated DENV serotypes (DENV1-4) into a single injection site (monotopic vaccination) has been postulated to result in interference in the replication of some serotypes in favor of others, an important obstacle in obtaining a balanced immune response against all serotypes. Here, we demonstrate the virus replicative and immunostimulatory effects of polytopic monovalent dengue vaccination (PV) in which, each of the four components of the tetravalent vaccine is simultaneously delivered to four different sites versus the more traditional monotopic tetravalent vaccination (MV) in a non-human primate (NHP) model. With the exception of DENV-2, there was no significant difference in detectable viral RNA levels between PV and MV inoculation. Interestingly, longer periods of detection and higher viral RNA levels were seen in the lymph nodes of NHPs inoculated PV compared to MV. Induction of lymph node dendritic cell maturation and of blood T- and B-cell activation showed different kinetics in PV inoculated NHPs compared to MV. The MV inoculated group showed earlier maturation of dendritic cells and activation of B and T cells compared to PV inoculated NHPs. A similar kinetic difference was also observed in the cytokine response: MV induced earlier cytokine responses compared to PV. However, similar levels of DENV neutralizing antibodies were observed in PV and MV NHPs. These findings indicate that cellular immune response after vaccination may be affected by the location of inoculation. Design of vaccine delivery may need to take into account the effects of locations of vaccine delivery of multiples serotype live viral vaccine on the induction of immune response.

Cost-effectiveness of dengue vaccination in Yucatán, Mexico using a dynamic dengue transmission model

Background

The incidence of dengue fever (DF) is steadily increasing in Mexico, burdening health systems with consequent morbidities and mortalities. On December 9^th, 2015, Mexico became the first country for which the dengue vaccine was approved for use. In anticipation of a vaccine rollout, analysis of the cost-effectiveness of the dengue vaccination program that quantifies the dynamics of disease transmission is essential.

Methods

We developed a dynamic transmission model of dengue in Yucatán, Mexico and its proposed vaccination program to incorporate herd immunity into our analysis of cost-effectiveness analysis. Our model also incorporates important characteristics of dengue epidemiology, such as clinical cross-immunity and susceptibility enhancement upon secondary infection. Using our model, we evaluated the cost-effectiveness and economic impact of an imperfect dengue vaccine in Yucatán, Mexico.

Conclusions

Our study indicates that a dengue vaccination program would prevent 90% of cases of symptomatic DF incidence as well as 90% of dengue hemorrhagic fever (DHF) incidence and dengue-related deaths annually. We conclude that a dengue vaccine program in Yucatán, Mexico would be very cost-effective as long as the vaccination cost per individual is less than $140 and $214 from health care and societal perspectives, respectively. Furthermore, at an exemplary vaccination cost of $250 USD per individual on average, dengue vaccination is likely to be cost-effective 43% and 88% of the time from health care and societal perspectives, respectively.