Developing a dengue vaccine: progress and future challenges

Influence of microbiota-driven natural antibodies on dengue transmission

Dengue has had a significant global health impact, with a dramatic increase in incidence over the past 50 years, affecting more than 100 countries. The absence of a specific treatment or widely applicable vaccine emphasizes the urgent need for innovative strategies. This perspective reevaluates current evidence supporting the concept of dual protection against the dengue virus (DENV) through natural antibodies (NAbs), particularly anti-α-Gal antibodies induced by the host's gut microbiome (GM). These anti-α-Gal antibodies serve a dual purpose. Firstly, they can directly identify DENV, as mosquito-derived viral particles have been observed to carry α-Gal, thereby providing a safeguard against human infections. Secondly, they possess the potential to impede virus development in the vector by interacting with the vector's microbiome and triggering infection-refractory states. The intricate interplay between human GM and NAbs on one side and DENV and vector microbiome on the other suggests a novel approach, using NAbs to directly target DENV and simultaneously disrupt vector microbiome to decrease pathogen transmission and vector competence, thereby blocking DENV transmission cycles.

Clinical Safety Experience of TAK-003 for Dengue Fever: A New Tetravalent Live Attenuated Vaccine Candidate

BACKGROUND

An unmet medical need remains for an effective dengue tetravalent vaccine that can be administered irrespective of previous dengue exposure. TAK-003, a dengue tetravalent vaccine, has demonstrated efficacy in an ongoing phase 3 trial in children and adolescents living in dengue-endemic areas, with an acceptable safety profile in both dengue-naive and dengue-exposed individuals.

METHODS

Safety findings are presented herein from an integrated analysis of data for healthy 4-60-year-olds from two phase 2 and three phase 3 double-blind, placebo-controlled clinical trials of TAK-003 (TAK-003, n = 14 627; placebo, n = 7167). Safety evaluation included analyses of postinjection reactogenicity, unsolicited adverse events (AEs), serious AEs (SAEs), and deaths. Subgroup analyses were performed by age group, baseline serostatus, and gender.

RESULTS

The most common local and systemic AEs were injection site pain (43% for TAK-003 and 26% for placebo) and headache (34% and 30%, respectively). Injection site AEs were mostly mild and resolved within 1-3 days. Unsolicited AEs and AEs leading to discontinuation occurred with similar frequency across both groups, while SAEs were fewer for TAK-003 recipients (6% vs 8% for placebo). Four of the 5 vaccine-related SAEs (which included hypersensitivity, dengue fever, and dengue hemorrhagic fever) occurred in the placebo group. No deaths were considered vaccine-related. Subgroup analyses showed no differences in safety by baseline serostatus or by gender, albeit analysis by age indicated greater local reactogenicity rates for adolescents (46% for TAK-003 and 28% for placebo) and adults (56% and 19%, respectively) than for children (37% and 25%, respectively).

CONCLUSIONS

No important safety risks were identified, and TAK-003 was well tolerated irrespective of age, gender, or baseline dengue serostatus in recipients aged 4-60 years.

A review of Dengvaxia®: development to deployment

Dengue is the world's most prevalent and important arboviral disease. More than 50% of the world's population lives at daily risk of infection and it is estimated more than 95 million people a year seek medical care following infection. Severe disease can manifest as plasma leakage and potential for clinically significant hemorrhage, shock, and death. Treatment is supportive and there is currently no licensed anti-dengue virus prophylactic or therapeutic compound. A single dengue vaccine, Sanofi Pasteur's Dengvaxia®, has been licensed in 20 countries but uptake has been poor. A safety signal in dengue seronegative vaccine recipients stimulated an international re-look at the vaccine performance profile, new World Health Organization recommendations for use, and controversy in the Philippines involving the government, regulatory agencies, Sanofi Pasteur, clinicians responsible for testing and administering the vaccine, and the parents of vaccinated children. In this review, we provide an overview of Dengvaxia's® development and discuss what has been learned about product performance since its licensure.

The Development of Peptide-based Antimicrobial Agents against Dengue Virus

Dengue fever has become an imminent threat to international public health because of global warming and climate change. The World Health Organization proclaimed that more than 50% of the world’s population is at risk of dengue virus (DENV) infection. Therefore, developing a clinically ap-proved vaccine and effective therapeutic remedy for treating dengue fever is imperative. Peptide drug de-velopment has become a novel pharmaceutical research field. This article reviews various peptides-based antimicrobial agents targeting three pathways involved in the DENV lifecycle. Specifically, they are peptide vaccines from immunomodulation, peptide drugs that inhibit virus entry, and peptide drugs that interfere with viral replication. Many antiviral peptide studies against DENV have been conducted in animal model trials, and progression to clinical trials for these promising peptide drugs is anticipated.

Clinical development and regulatory points for consideration for second-generation live attenuated dengue vaccines

Licensing and decisions on public health use of a vaccine rely on a robust clinical development program that permits a risk-benefit assessment of the product in the target population. Studies undertaken early in clinical development, as well as well-designed pivotal trials, allow for this robust characterization. In 2012, WHO published guidelines on the quality, safety and efficacy of live attenuated dengue tetravalent vaccines. Subsequently, efficacy and longer-term follow-up data have become available from two Phase 3 trials of a dengue vaccine, conducted in parallel, and the vaccine was licensed in December 2015. The findings and interpretation of the results from these trials released both before and after licensure have highlighted key complexities for tetravalent dengue vaccines, including concerns vaccination could increase the incidence of dengue disease in certain subpopulations. This report summarizes clinical and regulatory points for consideration that may guide vaccine developers on some aspects of trial design and facilitate regulatory review to enable broader public health recommendations for second-generation dengue vaccines.

Safety and Immunogenicity of a Tetravalent Dengue DNA Vaccine Administered with a Cationic Lipid-Based Adjuvant in a Phase 1 Clinical Trial

We conducted an open label, dose escalation Phase 1 clinical trial of a tetravalent dengue DNA vaccine (TVDV) formulated in Vaxfectin^® to assess safety and immunogenicity. A total of 40 dengue- and flavivirus-naive volunteers received either low-dose (1 mg) TVDV alone (N = 10, group 1), low-dose TVDV (1 mg) formulated in Vaxfectin (N = 10, group 2), or high-dose TVDV (2 mg, group 3) formulated in Vaxfectin^® (N = 20). Subjects were immunized intramuscularly with three doses on a 0-, 30-, 90-day schedule and monitored. Blood samples were obtained after each immunization and various time points thereafter to assess anti-dengue antibody and interferon gamma (IFNγ) T-cell immune responses. The most common adverse events (AEs) across all groups included mild to moderate pain and tenderness at the injection site, which typically resolved within 7 days. Common solicited signs and symptoms included fatigue (42.5%), headache (45%), and myalgias (47.5%). There were no serious AEs related to the vaccine or study procedures. No anti-dengue antibody responses were detected in group 1 subjects who received all three immunizations. There were minimal enzyme-linked immunosorbent assay and neutralizing antibody responses among groups 2 and 3 subjects who completed the immunization schedule. By contrast, IFNγ T-cell responses, regardless of serotype specificity, occurred in 70%, 50%, and 79% of subjects in groups 1, 2, and 3, respectively. The largest IFNγ T-cell responses were among group 3 subjects. We conclude that TVDV was safe and well-tolerated and elicited predominately anti-dengue T-cell IFNγ responses in a dose-related fashion.

Development of Peptide Vaccines in Dengue

Dengue is one of the most important arboviral infections worldwide, infecting up to 390 million people and causing 25,000 deaths annually. Although a licensed dengue vaccine is available, it is not efficacious against dengue serotypes that infect people living in South East Asia, where dengue is an endemic disease. Hence, there is an urgent need to develop an efficient dengue vaccine for this region. Data from different clinical trials indicate that a successful dengue vaccine must elicit both neutralizing antibodies and cell mediated immunity. This can be achieved by designing a multi-epitope peptide vaccine comprising B, CD8+ and CD4+ T cell epitopes. As recognition of T cell epitopes are restricted by human leukocyte antigens (HLA), T cell epitopes which are able to recognize several major HLAs will be preferentially included in the vaccine design. While peptide vaccines are safe, biocompatible and cost-effective, it is poorly immunogenic. Strategies to improve its immunogenicity by the use of long peptides, adjuvants and nanoparticle delivery mechanisms are discussed.

Working towards dengue as a vaccine-preventable disease: challenges and opportunities

INTRODUCTION

Dengue is an emerging viral disease that affects the human population around the globe. Recent advancements in dengue virus research have opened new avenues for the development of vaccines against dengue. The development of a vaccine against dengue is a challenging task because any of the four serotypes of dengue viruses can cause disease. The development of a dengue vaccine aims to provide balanced protection against all the serotypes. Several dengue vaccine candidates are in the developmental stages such as inactivated, live attenuated, recombinant subunit, and plasmid DNA vaccines. Area covered: The authors provide an overview of the progress made in the development of much needed dengue vaccines. The authors include their expert opinion and their perspectives for future developments. Expert opinion: Human trials of a live attenuated tetravalent chimeric vaccine have clearly demonstrated its potential as a dengue vaccine. Other vaccine candidate molecules such as DENVax, a recombinant chimeric vaccine andTetraVax, are at different stages of development at this time. The authors believe that the novel strategies for testing and improving the immune response of vaccine candidates in humans will eventually lead to the development of a successful dengue vaccine in future.

Dissecting Antibodies Induced by a Chimeric Yellow Fever-Dengue, Live-Attenuated, Tetravalent Dengue Vaccine (CYD-TDV) in Naive and Dengue-Exposed Individuals

Sanofi Pasteur has developed a chimeric yellow fever-dengue, live-attenuated, tetravalent dengue vaccine (CYD-TDV) that is currently approved for use in several countries. In clinical trials, CYD-TDV was efficacious at reducing laboratory-confirmed cases of dengue disease. Efficacy varied by dengue virus (DENV) serotype and prevaccination dengue immune status. We compared the properties of antibodies in naive and DENV-exposed individuals who received CYD-TDV. We depleted specific populations of DENV-reactive antibodies from immune serum samples to estimate the contribution of serotype-cross-reactive and type-specific antibodies to neutralization. Subjects with no preexisting immunity to DENV developed neutralizing antibodies to all 4 serotypes of DENV. Further analysis demonstrated that DENV4 was mainly neutralized by type-specific antibodies whereas DENV1, DENV2, and DENV3 were mainly neutralized by serotype cross-reactive antibodies. When subjects with preexisting immunity to DENV were vaccinated, they developed higher levels of neutralizing antibodies than naive subjects who were vaccinated. In preimmune subjects, CYD-TDV boosted cross-reactive neutralizing antibodies while maintaining type-specific neutralizing antibodies acquired before vaccination. Our results demonstrate that the quality of neutralizing antibodies induced by CYD-TDV varies depending on DENV serotype and previous immune status. We discuss the implications of these results for understanding vaccine efficacy.

Historical Perspectives on Flavivirus Research

The flaviviruses are small single-stranded RNA viruses that are typically transmitted by mosquito or tick vectors. These "arboviruses" are found around the world and account for a significant number of cases of human disease. The flaviviruses cause diseases ranging from mild or sub-clinical infections to lethal hemorrhagic fever or encephalitis. In many cases, survivors of neurologic flavivirus infections suffer long-term debilitating sequelae. Much like the emergence of West Nile virus in the United States in 1999, the recent emergence of Zika virus in the Americas has significantly increased the awareness of mosquito-borne viruses. The diseases caused by several flaviviruses have been recognized for decades, if not centuries. However, there is still a lot that is unknown about the flaviviruses as the recent experience with Zika virus has taught us. The objective of this review is to provide a general overview and some historical perspective on several flaviviruses that cause significant human disease. In addition, available medical countermeasures and significant gaps in our understanding of flavivirus biology are also discussed.

Vaccines licensed and in clinical trials for the prevention of dengue

Dengue has become a major global public health threat with almost half of the world's population living in at-risk areas. Vaccination would likely represent an effective strategy for the management of dengue disease in endemic regions, however to date there is only one licensed preventative vaccine for dengue infection. The development of a vaccine against dengue virus (DENV) has been hampered by an incomplete understanding of protective immune responses against DENV. The most clinically advanced dengue vaccine is the chimeric yellow fever-dengue vaccine (CYD) that employs the yellow fever virus 17D strain as the replication backbone (Chimerivax-DEN; CYD-TDV). This vaccine had an overall pooled protective efficacy of 65.6% but was substantially more effective against severe dengue and dengue hemorrhagic fever. Several other vaccine approaches have been developed including live attenuated chimeric dengue vaccines (DENVax and LAV Delta 30), DEN protein subunit V180 vaccine (DEN1-80E) and DENV DNA vaccines. These vaccines have been shown to be immunogenic in animals and also safe and immunogenic in humans. However, these vaccines are yet to progress to phase III trials to determine their protective efficacy against dengue. This review will summarize the details of vaccines that have progressed to clinical trials in humans.

Trials and Tribulations on the Path to Developing a Dengue Vaccine

Dengue is a rapidly expanding global health problem. Development of a safe and efficacious tetravalent vaccine along with strategic application of vector control activities represents a promising approach to reducing the global disease burden. Although many vaccine development challenges exist, numerous candidates are in clinical development and one has been tested in three clinical endpoint studies. The results of these studies have raised numerous questions about how we measure vaccine immunogenicity and how these readouts are associated with clinical outcomes in vaccine recipients who experience natural infection. In this review the authors discuss the dengue vaccine pipeline, development challenges, the dengue vaccine-immunologic profiling intersection, and research gaps.

Laboratory-confirmed Dengue in Children in Three Regional Hospitals in the Philippines in 2009-2010

BACKGROUND

The burden of dengue is high in the Philippines but the prevalence of confirmed cases is unknown, and the disease is subject to underreporting because surveillance of suspected cases is passive. We conducted a prospective epidemiological study to estimate the proportion of laboratory-confirmed dengue among clinically suspected hospitalized cases in the pediatric wards of 3 regional hospitals in the Philippines and to describe the clinical and laboratory features, age distributions, case fatality rates and serotype distributions of these hospitalized cases.

METHODS

Patients ≤18 years and hospitalized for suspected dengue were included if they had an axillary temperature ≥38°C for 2-7 days and 2 or more dengue-associated symptoms. Dengue infection was confirmed in acute blood samples by serotype-specific reverse transcription-polymerase chain reaction and IgM immunoassay.

RESULTS

We confirmed dengue infection in 1809 (86.1%) cases of 2103 suspected cases between November 2009 and November 2010. The 6- to 10-year-old age group had the highest proportion of cases overall (36.7%). Fever, anorexia, myalgia, abdominal pain and headache were the most common symptoms at admission. Hemorrhagic manifestations, signs of plasma leakage, thrombocytopenia and leucopenia were all significantly more common in confirmed than in nonconfirmed cases. Most cases (76.5%) developed dengue hemorrhagic fever or dengue shock syndrome, and the overall case fatality rate was 0.94%. Distributions of all 4 virus serotypes varied at each hospital.

CONCLUSIONS

The clinical burden of pediatric dengue continues to be substantial in the Philippines. Most hospitalized cases of suspected pediatric dengue can be laboratory confirmed and most develop severe disease.

Animal models for studying dengue pathogenesis and therapy

Development of a suitable animal model for dengue virus disease is critical for understanding pathogenesis and for preclinical testing of antiviral drugs and vaccines. Many laboratory animal models of dengue virus infection have been investigated, but the challenges of recapitulating the complete disease still remain. In this review, we provide a comprehensive coverage of existing models, from man to mouse, with a specific focus on recent advances in mouse models for addressing the mechanistic aspects of severe dengue in humans. This article forms part of a symposium in Antiviral Research on flavivirus drug discovery.

Characterization of N-Glycan Structures on the Surface of Mature Dengue 2 Virus Derived from Insect Cells

DENV envelope glycoprotein (E) is responsible for interacting with host cell receptors and is the main target for the development of a dengue vaccine based on an induction of neutralizing antibodies. It is well known that DENV E glycoprotein has two potential N-linked glycosylation sites at Asn67 and Asn153. The N-glycans of E glycoprotein have been shown to influence the proper folding of the protein, its cellular localization, its interactions with receptors and its immunogenicity. However, the precise structures of the N-glycans that are attached to E glycoprotein remain elusive, although the crystal structure of DENV E has been determined. This study characterized the structures of envelope protein N-linked glycans on mature DENV-2 particles derived from insect cells via an integrated method that used both lectin microarray and MALDI-TOF-MS. By combining these methods, a high heterogeneity of DENV N-glycans was found. Five types of N-glycan were identified on DENV-2, including mannose, GalNAc, GlcNAc, fucose and sialic acid; high mannose-type N-linked oligosaccharides and the galactosylation of N-glycans were the major structures that were found. Furthermore, a complex between a glycan on DENV and the carbohydrate recognition domain (CRD) of DC-SIGN was mimicked with computational docking experiments. For the first time, this study provides a comprehensive understanding of the N-linked glycan profile of whole DENV-2 particles derived from insect cells.

Safety and Immunogenicity of a Dengue Virus Serotype-1 Purified-Inactivated Vaccine: Results of a Phase 1 Clinical Trial

We describe the results from a human clinical trial of a dengue virus serotype-1, purified-inactivated vaccine (DENV-1 PIV) adjuvanted with aluminum hydroxide. This first-in-man, Phase 1, open-label clinical trial consisted of two groups of flavivirus-naïve healthy adult volunteers that received two intramuscular vaccine doses of either 2.5 μg or 5 μg of DENV-1 PIV administered on days 0 and 28. Following vaccination, both vaccine doses exhibited an acceptable safety profile with minimal injection site and systemic reactions. By study day 42, 2 weeks following the second vaccine dose, all volunteers in both vaccine groups developed serum-neutralizing antibodies against DENV-1. Additional testing using an enzyme-linked immunosorbent assay demonstrated induction of a humoral immune response following both vaccine doses. The DENV-1 PIV was safe and immunogenic in a small number of volunteers supporting development and further testing of a tetravalent DENV PIV formulation.

Dengue Virus NS Proteins Inhibit RIG-I/MAVS Signaling by Blocking TBK1/IRF3 Phosphorylation: Dengue Virus Serotype 1 NS4A Is a Unique Interferon-Regulating Virulence Determinant

Dengue virus (DENV) replication is inhibited by the prior addition of type I interferon or by RIG-I agonists that elicit RIG-I/MAVS/TBK1/IRF3-dependent protective responses. DENV infection of primary human endothelial cells (ECs) results in a rapid increase in viral titer, which suggests that DENV inhibits replication-restrictive RIG-I/interferon beta (IFN-β) induction pathways within ECs. Our findings demonstrate that DENV serotype 4 (DENV4) nonstructural (NS) proteins NS2A and NS4B inhibited RIG-I-, MDA5-, MAVS-, and TBK1/IKKε-directed IFN-β transcription (>80%) but failed to inhibit IFN-β induction directed by STING or constitutively active IRF3-5D. Expression of NS2A and NS4B dose dependently inhibited the phosphorylation of TBK1 and IRF3, which suggests that they function at the level of TBK1 complex activation. NS2A and NS4B from DENV1/2/4, as well as the West Nile virus NS4B protein, commonly inhibited TBK1 phosphorylation and IFN-β induction. A comparative analysis of NS4A proteins across DENVs demonstrated that DENV1, but not DENV2 or DENV4, NS4A proteins uniquely inhibited TBK1. These findings indicate that DENVs contain conserved (NS2A/NS4B) and DENV1-specific (NS4A) mechanisms for inhibiting RIG-I/TBK1-directed IFN responses. Collectively, our results define DENV NS proteins that restrict IRF3 and IFN responses and thereby facilitate DENV replication and virulence. Unique DENV1-specific NS4A regulation of IFN induction has the potential to be a virulence determinant that contributes to the increased severity of DENV1 infections and the immunodominance of DENV1 responses during tetravalent DENV1-4 vaccination.

Our findings demonstrate that NS2A and NS4B proteins from dengue virus serotypes 1, 2, and 4 are inhibitors of RIG-I/MDA5-directed interferon beta (IFN-β) induction and that they accomplish this by blocking TBK1 activation. We determined that IFN inhibition is functionally conserved across NS4B proteins from West Nile virus and DENV1, -2, and -4 viruses. In contrast, DENV1 uniquely encodes an extra IFN regulating protein, NS4A, that inhibits TBK1-directed IFN induction. DENV1 is associated with an increase in severe patient disease, and added IFN regulation by the DENV1 NS4A protein may contribute to increased DENV1 replication, immunodominance, and virulence. The regulation of IFN induction by nonstructural (NS) proteins suggests their potential roles in enhancing viral replication and spread and as potential protein targets for viral attenuation. DENV1-specific IFN regulation needs to be considered in vaccine strategies where enhanced DENV1 replication may interfere with DENV2-4 seroconversion within coadministered tetravalent DENV1-4 vaccines.

Reviewing dengue: still a neglected tropical disease?

Dengue is currently listed as a “neglected tropical disease” (NTD). But is dengue still an NTD or not? Classifying dengue as an NTD may carry advantages, but is it justified? This review considers the criteria for the definition of an NTD, the current diverse lists of NTDs by different stakeholders, and the commonalities and differences of dengue with other NTDs. We also review the current research gaps and research activities and the adequacy of funding for dengue research and development (R&D) (2003–2013). NTD definitions have been developed to a higher precision since the early 2000s, with the following main features: NTDs are characterised as a) poverty related, b) endemic to the tropics and subtropics, c) lacking public health attention, d) having poor research funding and shortcomings in R&D, e) usually associated with high morbidity but low mortality, and f) often having no specific treatment available. Dengue meets most of these criteria, but not all. Although dengue predominantly affects resource-limited countries, it does not necessarily only target the poor and marginalised in those countries. Dengue increasingly attracts public health attention, and in some affected countries it is now a high profile disease. Research funding for dengue has increased exponentially in the past two decades, in particular in the area of dengue vaccine development. However, despite advances in dengue research, dengue epidemics are increasing in frequency and magnitude, and dengue is expanding to new areas. Specific treatment and a highly effective vaccine remain elusive. Major research gaps exist in the area of integrated surveillance and vector control. Hence, although dengue differs from many of the NTDs, it still meets important criteria commonly used for NTDs. The current need for increased R&D spending, shared by dengue and other NTDs, is perhaps the key reason why dengue should continue to be considered an NTD.