An adjuvanted, tetravalent dengue virus purified inactivated vaccine candidate induces long-lasting and protective antibody responses against dengue challenge in rhesus macaques

Opportunities and challenges of mRNA technologies in development of dengue virus vaccine

Dengue virus (DENV) is a mosquito-borne virus with a significant human health concern. With 390 million infections annually and 96 million showing clinical symptoms, severe dengue can lead to life-threatening conditions like dengue hemorrhagic fever (DHF) and dengue shock syndrome (DSS). The only FDA-approved vaccine, Dengvaxia, has limitations due to antibody-dependent enhancement (ADE), necessitating careful administration. The recent pre-approval of TAK-003 by WHO in 2024 highlights ongoing efforts to improve vaccine options. This review explores recent advancements in dengue vaccine development, emphasizing potential utility of mRNA-based vaccines. By examining current clinical trial data and innovations, we aim to identify promising strategies to address the limitations of existing vaccines and enhance global dengue prevention efforts.

Unlocking Hope: Paving the Way for a Cutting-Edge Multi-Epitope Dengue Virus Vaccine

Dengue fever is a significant health issue in Pakistan, demanding a vaccine effective against all the viral strains. This study employs reverse vaccinology to develop potential dengue vaccine candidates (DVAX I-III). The study thoroughly examined conserved areas of dengue virus serotypes 1-4's structural and non-structural proteins. Key viral proteins were analyzed to find antigenic peptides, which were incorporated into vaccine candidates and potentiated with adjuvants. Computational methods predicted peptide structures and evaluated their binding to immune receptors TLR 2, TLR 4, HLA *A1101, and DRB*401. A molecular dynamics simulation lasting 100 ns of the DVAX II-TLR4 complex at different time intervals clearly indicated that the ligand is attached to the receptor. Normal mode analysis assessed the stability and flexibility of these interactions. Encouragingly, all three vaccine candidates demonstrated favorable interactions with these immune receptors and the potential to induce a robust immune response. These findings suggest their safety and warrant further in vivo studies to evaluate their efficacy for clinical development.

Enhancing Dengue Virus Production and Immunogenicity with Celcradle™ Bioreactor: A Comparative Study with Traditional Cell Culture Methods

The dengue virus, the primary cause of dengue fever, dengue hemorrhagic fever, and dengue shock syndrome, is the most widespread mosquito-borne virus worldwide. In recent decades, the prevalence of dengue fever has increased markedly, presenting substantial public health challenges. Consequently, the development of an efficacious vaccine against dengue remains a critical goal for mitigating its spread. Our research utilized Celcradle™, an innovative tidal bioreactor optimized for high-density cell cultures, to grow Vero cells for dengue virus production. By maintaining optimal pH levels (7.0 to 7.4) and glucose concentrations (1.5 g/L to 3.5 g/L) during the proliferation of cells and viruses, we achieved a peak Vero cell count of approximately 2.46 × 109, nearly ten times the initial count. The use of Celcradle™ substantially decreased the time required for cell yield and virus production compared to conventional Petri dish methods. Moreover, our evaluation of the immunogenicity of the Celcradle™-produced inactivated DENV4 through immunization of mice revealed that sera from these mice demonstrated cross-reactivity with DENV4 cultured in Petri dishes and showed elevated antibody titers compared to those from mice immunized with virus from Petri dishes. These results indicate that the dengue virus cultivated using the Celcradle™ system exhibited enhanced immunogenicity relative to that produced in traditional methods. In conclusion, our study highlights the potential of the Celcradle™ bioreactor for large-scale production of inactivated dengue virus vaccines, offering significant promise for reducing the global impact of dengue virus infections and accelerating the development of effective vaccination strategies.

Development of Vaccines against Emerging Mosquito-Vectored Arbovirus Infections

Among emergent climate-sensitive infectious diseases, some mosquito-vectored arbovirus infections have epidemiological, social, and economic effects. Dengue virus (DENV), West Nile virus (WNV), and Chikungunya virus (CHIKV) disease, previously common only in the tropics, currently pose a major risk to global health and are expected to expand dramatically in the near future if adequate containment measures are not implemented. The lack of safe and effective vaccines is critical as it seems likely that emerging mosquito-vectored arbovirus infections will be con-trolled only when effective and safe vaccines against each of these infections become available. This paper discusses the clinical characteristics of DENV, WNV, and CHIKV infections and the state of development of vaccines against these viruses. An ideal vaccine should be able to evoke with a single administration a prompt activation of B and T cells, adequate concentrations of protecting/neutralizing antibodies, and the creation of a strong immune memory capable of triggering an effective secondary antibody response after new infection with a wild-type and/or mutated infectious agent. Moreover, the vaccine should be well tolerated, safe, easily administrated, cost-effective, and widely available throughout the world. However, the development of vaccines against emerging mosquito-vectored arbovirus diseases is far from being satisfactory, and it seems likely that it will take many years before effective and safe vaccines for all these infections are made available worldwide.

Dengue Vaccination: Towards a New Dawn of Curbing Dengue Infection

Dengue is an infectious disease caused by dengue virus (DENV) and is a serious global burden. Antibody-dependent enhancement and the ability of DENV to infect immune cells, along with other factors, lead to fatal Dengue Haemorrhagic Fever and Dengue Shock Syndrome. This necessitates the development of a robust and efficient vaccine but vaccine development faces a number of hurdles. In this review, we look at the epidemiology, genome structure and cellular targets of DENV and elaborate upon the immune responses generated by human immune system against DENV infection. The review further sheds light on various challenges in development of a potent vaccine against DENV which is followed by presenting a current account of different vaccines which are being developed or have been licensed.

Immunogenicity of an AS01-adjuvanted respiratory syncytial virus prefusion F (RSVPreF3) vaccine in animal models

Respiratory syncytial virus (RSV) causes a high disease burden in older adults. An effective vaccine for this RSV-primed population may need to boost/elicit robust RSV-neutralizing antibody responses and recall/induce RSV-specific T cell responses. To inform the selection of the vaccine formulation for older adults, RSVPreF3 (RSV fusion glycoprotein engineered to maintain the prefusion conformation) with/without AS01 adjuvant was evaluated in mice and bovine RSV infection-primed cattle. In mice, RSVPreF3/AS01 elicited robust RSV-A/B-specific neutralization titers and RSV F-specific polyfunctional CD4+ T cell responses exceeding those induced by non-adjuvanted RSVPreF3. In primed bovines, RSVPreF3/AS01 tended to induce higher pre-/post-vaccination fold-increases in RSV-A/B-specific neutralization titers relative to non-adjuvanted and Alum-adjuvanted RSVPreF3 formulations, and elicited higher RSV F-specific CD4+ T cell frequencies relative to the non-adjuvanted vaccine. Though AS01 adjuvanticity varied by animal species and priming status, RSVPreF3/AS01 elicited/boosted RSV-A/B-specific neutralization titers and RSV F-specific CD4+ T cell responses in both animal models, which supported its further clinical evaluation as prophylactic candidate vaccine for older adults.

A Perspective on Current Flavivirus Vaccine Development: A Brief Review

The flavivirus genus contains several clinically important pathogens that account for tremendous global suffering. Primarily transmitted by mosquitos or ticks, these viruses can cause severe and potentially fatal diseases ranging from hemorrhagic fevers to encephalitis. The extensive global burden is predominantly caused by six flaviviruses: dengue, Zika, West Nile, yellow fever, Japanese encephalitis and tick-borne encephalitis. Several vaccines have been developed, and many more are currently being tested in clinical trials. However, flavivirus vaccine development is still confronted with many shortcomings and challenges. With the use of the existing literature, we have studied these hurdles as well as the signs of progress made in flavivirus vaccinology in the context of future development strategies. Moreover, all current licensed and phase-trial flavivirus vaccines have been gathered and discussed based on their vaccine type. Furthermore, potentially relevant vaccine types without any candidates in clinical testing are explored in this review as well. Over the past decades, several modern vaccine types have expanded the field of vaccinology, potentially providing alternative solutions for flavivirus vaccines. These vaccine types offer different development strategies as opposed to traditional vaccines. The included vaccine types were live-attenuated, inactivated, subunit, VLPs, viral vector-based, epitope-based, DNA and mRNA vaccines. Each vaccine type offers different advantages, some more suitable for flaviviruses than others. Additional studies are needed to overcome the barriers currently faced by flavivirus vaccine development, but many potential solutions are currently being explored.

Challenges on the development of a dengue vaccine: a comprehensive review of the state of the art

Dengue virus (DENV) is the mosquito-borne virus of greatest human health concern. There are four serotypes of DENV (1-4) that co-circulate in endemic areas. Each serotype of DENV is individually capable of causing the full spectrum of disease, ranging from self-resolving dengue fever to the more severe dengue haemorrhagic fever (DHF) or dengue shock syndrome (DSS). Based on data published by the CDC, one in four people who become infected with dengue will become ill. Of those that do develop symptomology, the symptoms can range from mild to severe. Symptoms can vary from rash, ocular aches and pains to more intense symptoms in the manifestation of severe dengue. Roughly, 1 in 20 people who become ill will develop severe dengue, which can result in shock, internal bleeding and death. There is currently no specific treatment for dengue and only one licensed vaccine (Dengvaxia) for children 9 through 16 years of age in just a few countries. Despite its licensure for clinical use, Dengvaxia has performed with low efficacy in children and dengue naïve individuals and critically has resulted in increased risk of developing severe dengue in young, vaccinated recipients. Currently, there are various novel strategies for the development of a dengue vaccine. In this review we have conducted a detailed overview of the DENV vaccine landscape, focusing on nine vaccines in the pipeline to provide a comprehensive overview of the most state-of-the-art developments in strategies for vaccines against DENV.

Preparedness for the Dengue Epidemic: Vaccine as a Viable Approach

Dengue fever is one of the significant fatal mosquito-borne viral diseases and is considered to be a worldwide problem. Aedes mosquito is responsible for transmitting various serotypes of dengue viruses to humans. Dengue incidence has developed prominently throughout the world in the last ten years. The exact number of dengue cases is underestimated, whereas plenty of cases are misdiagnosed as alternative febrile sicknesses. There is an estimation that about 390 million dengue cases occur annually. Dengue fever encompasses a wide range of clinical presentations, usually with undefinable clinical progression and outcome. The diagnosis of dengue depends on serology tests, molecular diagnostic methods, and antigen detection tests. The therapeutic approach relies completely on supplemental drugs, which is far from the real approach. Vaccines for dengue disease are in various stages of development. The commercial formulation Dengvaxia (CYD-TDV) is accessible and developed by Sanofi Pasteur. The vaccine candidate Dengvaxia was inefficient in liberating a stabilized immune reaction toward different serotypes (1-4) of dengue fever. Numerous promising vaccine candidates are now being developed in preclinical and clinical stages even though different serotypes of DENV exist that worsen the situation for a vaccine to be equally effective for all serotypes. Thus, the development of an efficient dengue fever vaccine candidate requires time. Effective dengue fever management can be a multidisciplinary challenge, involving international cooperation from diverse perspectives and expertise to resolve this global concern.

Characterization of B-cell and T-cell responses to a tetravalent dengue purified inactivated vaccine in healthy adults

The increasing global impact of dengue underscores the need for a dengue virus (DENV) vaccine. We assessed B-cell and T-cell responses following vaccination with four formulations of a tetravalent dengue purified inactivated vaccine (DPIV) in dengue-primed and dengue-naive adults from two studies (NCT01666652, NCT01702857). Frequencies of DPIV-induced memory B cells specific to each DENV serotype remained high up to 12 months post-vaccination, and were higher in the dengue-primed than dengue-naive adults. A subsequent DPIV booster dose induced strong anamnestic B-cell responses. Multifunctional CD4+ T cells (predominantly expressing IL-2) were induced by DPIV, with higher frequencies in dengue-primed adults. DPIV-induced CD4+ T cells also demonstrated in vitro proliferative capacity and antigen-specific production of GM-CSF, IFN-γ, and IL-13. CD8+ T-cell responses were undetectable in dengue-naive adults and low in dengue-primed individuals. B- and T-cell responses persisted up to 12 months post-vaccination in both dengue-primed and dengue-naive adults.

Dengue Vaccines: An Update

Dengue is one of the most prevalent mosquito-borne diseases in the world, affecting an estimated 390 million people each year, according to models. For the last two decades, efforts to develop safe and effective vaccines to prevent dengue virus (DENV) infections have faced several challenges, mostly related to the complexity of conducting long-term studies to evaluate vaccine efficacy and safety to rule out the risk of vaccine-induced DHS/DSS, particularly in children. At least seven DENV vaccines have undergone different phases of clinical trials; however, only three of them (Dengvaxia^®, TV003, and TAK-003) have showed promising results, and are addressed in detail in this review in terms of their molecular design, efficacy, and immunogenicity. Safety-related challenges during DENV vaccine development are also discussed.

Enhanced Immunogenicity of Inactivated Dengue Vaccines by Novel Polysaccharide-Based Adjuvants in Mice

Dengue fever, caused by any of four dengue viruses (DENV1-4), is a major global burden. Currently, there is no effective vaccine that prevents infection in dengue naïve populations. We tested the ability of two novel adjuvants (Advax-PEI and Advax-2), using aluminum hydroxide (alum) as control, to enhance the immunogenicity of formalin- or psoralen-inactivated (PIV or PsIV) DENV2 vaccines in mice. Mice were vaccinated on days 0 and 30, and serum samples were collected on days 30, 60, 90, and 101. Neutralizing antibodies were determined by microneutralization (MN) assays, and the geometric mean 50% MN (MN50) titers were calculated. For the PIV groups, after one dose MN50 titers were higher in the novel adjuvant groups compared to the alum control, while MN50 titers were comparable between the adjuvant groups after the second dose. For the PsIV groups, both novel adjuvants induced higher MN50 titers than the alum control after the second dose. Spleen cells were collected on days 45 and 101 for enzyme-linked immunospot (ELISPOT) for IFNγ and IL4. Both PIV and PsIV groups elicited different degrees of IFNγ and IL4 responses. Overall, Advax-2 gave the best responses just ahead of Advax-PEI. Given Advax-2’s extensive human experience in other vaccine applications, it will be pursued for further development.

Challenges for Vaccinologists in the First Half of the Twenty-First Century

The COVID-19 pandemic of 2020-2021 has highlighted the importance of vaccines and vaccination in human health. The pandemic has resulted in social distancing, travel restrictions, decreased trade, high unemployment, commodity price decline, and financial stress that has impacted the global economy. Since December 2020, a massive vaccination campaign is undergoing in every country on the planet to protect against SARS-CoV-2. Vaccination is the cheapest health-care interventions that can save more lives than any other drugs or therapies. Some of the common diseases of the twentieth century including smallpox and polio are seldom reported due to intense vaccination programs that eradicated it. Smallpox is completely eradicated globally; whereas, polio is confined to only a couple of countries. Vaccination has not only improved the health of man but also improved food security by preventing diseases in farm animals and aquacultured fish. Awareness of the principles of immunology and novel vaccines has led to effective vaccination strategies. Climate change could lead to generation of new strains of infectious microorganisms that would require development of novel vaccines. Recent years have seen the increase in incidence of brain-eating amoeba and flesh-eating bacteria (necrotizing fasciitis). There are no vaccines for these diseases. Though vaccination programs have eradicated several diseases and increased the quality of life, there are several diseases that have no effective vaccines. Currently there are no vaccines for cancer, neurodegenerative diseases, autoimmune diseases, as well as infectious diseases like tuberculosis, AIDS, and parasitic diseases including malaria. Spontaneous evolution of pathogenic microorganisms may lead to pandemics that impact the health of not only humanity but also other animals. Hence, the challenge to vaccinologists is the development of novel vaccines and vaccination strategies within limited time period and using minimum resources. In addition, the vaccine developed should be administered globally within a short duration so as to prevent generation of pathogenic variants more lethal than the parent strain.

Potential Role of Flavivirus NS2B-NS3 Proteases in Viral Pathogenesis and Anti-flavivirus Drug Discovery Employing Animal Cells and Models: A Review

Flaviviruses are known to cause a variety of diseases in humans in different parts of the world. There are very limited numbers of antivirals to combat flavivirus infection, and therefore new drug targets must be explored. The flavivirus NS2B-NS3 proteases are responsible for the cleavage of the flavivirus polyprotein, which is necessary for productive viral infection and for causing clinical infections; therefore, they are a promising drug target for devising novel drugs against different flaviviruses. This review highlights the structural details of the NS2B-NS3 proteases of different flaviviruses, and also describes potential antiviral drugs that can interfere with the viral protease activity, as determined by various studies. Moreover, optimized in vitro reaction conditions for studying the NS2B-NS3 proteases of different flaviviruses may vary and have been incorporated in this review. The increasing availability of the in silico and crystallographic/structural details of flavivirus NS2B-NS3 proteases in free and drug-bound states can pave the path for the development of promising antiflavivirus drugs to be used in clinics. However, there is a paucity of information available on using animal cells and models for studying flavivirus NS2B-NS3 proteases, as well as on the testing of the antiviral drug efficacy against NS2B-NS3 proteases. Therefore, on the basis of recent studies, an effort has also been made to propose potential cellular and animal models for the study of flavivirus NS2B-NS3 proteases for the purposes of exploring flavivirus pathogenesis and for testing the efficacy of possible drugs targets, in vitro and in vivo.

T cell and memory B cell responses in tetravalent DNA, tetravalent inactivated and tetravalent live-attenuated prime-boost dengue vaccines in rhesus macaques

We previously reported the efficacy of prime-boost vaccination using three tetravalent (T) dengue vaccines, DNA (TDNA), purified inactivated vaccine (TPIV), and live attenuated vaccine (TLAV). We demonstrated that the TPIV/TLAV prime-boost vaccination yielded the highest and most durable neutralizing antibodies and 100% protection to all 4 serotypes of dengue virus in rhesus macaques. This study compares gene transcription, T and B cell responses elicited by these prime-boost combinations in rhesus macaques. This study shows that the TLAV vaccine increased the expression of the innate immune genes, DDX58 and TLR7, IL1A, IL1B, TNF, CXCL8, CXCL10, IRF1, IRF7, and IFNB, more robustly as compared to TDNA and TPIV vaccines. Overall, two doses of TDNA and one dose of TLAV efficiently elicited a T cell IFNγ response to PrM/E with a comparable magnitude. Compared to TDNA vaccine, the TLAV vaccine elicited additional IFNγ response to C, NS1, NS3, and NS5. The TPIV vaccine alone produced poor IFNγ response; however, the TLAV significantly boosted its IFNγ response. The T cell response repertoire associated with TPIV/TLAV prime-boost was to both the structural C/PrM/E and NS proteins, and the T cells were multifunctional as the CD4⁺ T cells produced IFNγ, TNF α, and IL2 and the CD8⁺ cells produced TNF α and IFNγ. Opposite to the pattern of CMI, the TPIV vaccine alone elicited the highest B_Mem compared to the other two vaccines, which continuously remained as the highest after boosting. In summary, the TDNA and TLAV vaccines elicited a strong T cell response whereas the TPIV vaccine elicited a superior B_Mem. The T cell response of the TPIV vaccine was significantly boosted by the TLAV vaccine. The elevated T cell response may have provided T cell help for a sustained antibody response for TPIV/TLAV vaccines, which is required for a protective immunity against a live virus challenge.

Vaccine-Associated Enhanced Viral Disease: Implications for Viral Vaccine Development

Vaccine-associated enhanced disease (VAED) is a serious barrier to attaining successful virus vaccines in human and veterinary medicine. VAED occurs as two different immunopathologies, antibody-dependent enhancement (ADE) and vaccine-associated hypersensitivity (VAH). ADE contributes to the pathology of disease caused by four dengue viruses (DENV) through control of the intensity of cellular infection. Products of virus-infected cells are toxic. A partially protective yellow fever chimeric tetravalent DENV vaccine sensitized seronegative children to ADE breakthrough infections. A live-attenuated tetravalent whole virus vaccine in phase III testing appears to avoid ADE by providing durable protection against the four DENV. VAH sensitization by viral vaccines occurred historically. Children given formalin-inactivated measles or respiratory syncytial virus (RSV) vaccines experienced severe disease during breakthrough infections. Tissue responses demonstrated that VAH not ADE caused these vaccine safety problems. Subsequently, measles was successfully and safely contained by a live-attenuated virus vaccine. The difficulty in formulating a safe and effective RSV vaccine is troublesome evidence that avoiding VAH is a major research challenge. VAH-like tissue responses were observed during breakthrough homologous virus infections in monkeys given severe acute respiratory syndrome (SARS) or Middle East respiratory syndrome (MERS) vaccines.

Impact of prior Dengue immunity on Zika vaccine protection in rhesus macaques and mice

Pre-existing immunity to flaviviruses can influence the outcome of subsequent flavivirus infections. Therefore, it is critical to determine whether baseline DENV immunity may influence subsequent ZIKV infection and the protective efficacy of ZIKV vaccines. In this study, we investigated the impact of pre-existing DENV immunity induced by vaccination on ZIKV infection and the protective efficacy of an inactivated ZIKV vaccine. Rhesus macaques and mice inoculated with a live attenuated DENV vaccine developed neutralizing antibodies (NAbs) to multiple DENV serotypes but no cross-reactive NAbs responses to ZIKV. Animals with baseline DENV NAbs did not exhibit enhanced ZIKV infection and showed no overall reduction in ZIKV vaccine protection. Moreover, passive transfer of purified DENV-specific IgG from convalescent human donors did not augment ZIKV infection in STAT2 -/- and BALB/c mice. In summary, these results suggest that baseline DENV immunity induced by vaccination does not significantly enhance ZIKV infection or impair the protective efficacy of candidate ZIKV vaccines in these models. These data can help inform immunization strategies in regions of the world with multiple circulating pathogenic flaviviruses.

Enhanced dengue vaccine virus replication and neutralizing antibody responses in immune primed rhesus macaques

Antibody-dependent enhancement (ADE) is suspected to influence dengue virus (DENV) infection, but the role ADE plays in vaccination strategies incorporating live attenuated virus components is less clear. Using a heterologous prime-boost strategy in rhesus macaques, we examine the effect of priming with DENV purified inactivated vaccines (PIVs) on a tetravalent live attenuated vaccine (LAV). Sera exhibited low-level neutralizing antibodies (NAb) post PIV priming, yet moderate to high in vitro ADE activity. Following LAV administration, the PIV primed groups exhibited DENV-2 LAV peak viremias up to 1,176-fold higher than the mock primed group, and peak viremia correlated with in vitro ADE. Furthermore, PIV primed groups had more balanced and higher DENV-1–4 NAb seroconversion and titers than the mock primed group following LAV administration. These results have implications for the development of effective DENV vaccine prime-boost strategies and for our understanding of the role played by ADE in modulating DENV replication.

Whole inactivated dengue virus-loaded trimethyl chitosan nanoparticle-based vaccine: immunogenic properties in ex vivo and in vivo models

Dengue virus (DENV) is a mosquito-borne virus that poses an incomparable public health problem, particularly in tropical and subtropical areas. Vaccination remains the most rational measure for controlling DENV infection. In this study, an ultraviolet irradiation (UV)-inactivated DENV-2 carried by N,N,N-trimethyl chitosan nanoparticles (UV-inactivated DENV2 TMC NPs) was investigated as a potential non-replicating dengue vaccine candidate. Using a human ex vivo model, the human monocyte-derived dendritic cells (MoDCs), we showed that TMC served as both a vaccine vehicle and a potent adjuvant. TMC NPs not only efficiently enhanced UV-inactivated DENV2 internalization into MoDCs but also greatly increased the breadth of UV-inactivated DENV2 immunogenicity to drive the maturation of MoDCs. Moreover, UV-inactivated DENV2 TMC NPs were highly immunogenic in mice, inducing greater levels of antibodies (total IgG, IgG1, IgG2a and neutralizing antibodies) and T cells (activated CD4⁺ and CD8⁺ T cells) against DENV-2 compared to soluble DENV-2 immunogens. Notably, the neutralizing activity of sera from mice immunized with UV-inactivated DENV2 TMC NPs was significantly augmented in the presence of complement activation, leading to the strong elimination of both DENV-2 particles and infected cells. We further showed that the immunogenicity of an inactivated dengue-based vaccine was significantly improved in a concentration-dependent manner. These positive results warrant further investigations of this platform of vaccine delivery for tetravalent vaccines or monovalent vaccines in sequential immunizations.

Landscape of Monoclonal Antibodies Targeting Zika and Dengue: Therapeutic Solutions and Critical Insights for Vaccine Development

The unprecedented 2015-2016 Zika outbreak in the Americas sparked global concern and drove the rapid deployment of vaccine and therapeutic countermeasures against this re-emerging pathogen. Alongside vaccine development, a number of potent neutralizing antibodies against Zika and related flaviviruses have been identified in recent years. High-throughput antibody isolation approaches have contributed to a better understanding of the B cell responses elicited following infection and/or vaccination. Structure-based approaches have illuminated species-specific and cross-protective epitopes of therapeutic value. This review will highlight previously described monoclonal antibodies with the best therapeutic potential against ZIKV and related flaviviruses, and discuss their implications for the rational design of better vaccine strategies.

Immunogenicity of a Live-Attenuated Dengue Vaccine Using a Heterologous Prime-Boost Strategy in a Phase 1 Randomized Clinical Trial

BACKGROUND

Dengue is a global health problem and the development of a tetravalent dengue vaccine with durable protection is a high priority. A heterologous prime-boost strategy has the advantage of eliciting immune responses through different mechanisms and therefore may be superior to homologous prime-boost strategies for generating durable tetravalent immunity.

METHODS

In this phase 1 first-in-human heterologous prime-boost study, 80 volunteers were assigned to 4 groups and received a tetravalent dengue virus (DENV-1-4) purified inactivated vaccine (TDENV-PIV) with alum adjuvant and a tetravalent dengue virus (DENV-1-4) live attenuated vaccine (TDENV-LAV) in different orders and dosing schedules (28 or 180 days apart).

RESULTS

All vaccination regimens had acceptable safety profiles and there were no vaccine-related serious adverse events. TDEN-PIV followed by TDEN-LAV induced higher neutralizing antibody titers and a higher rate of tetravalent seroconversions compared to TDEN-LAV followed by TDEN-PIV. Both TDEN-PIV followed by TDEN-LAV groups demonstrated 100% tetravalent seroconversion 28 days following the booster dose, which was maintained for most of these subjects through the day 180 measurement.

CONCLUSIONS

A heterologous prime-boost vaccination strategy for dengue merits additional evaluation for safety, immunogenicity, and potential for clinical benefit.

CLINICAL TRIALS REGISTRATION

NCT02239614.

Anti-dengue Vaccines: From Development to Clinical Trials

Dengue Virus (DENV) is an arbovirus (arthropod-borne virus). Four serotypes of DENV are responsible for the infectious disease called dengue that annually affects nearly 400 million people worldwide. Although there is only one vaccine formulation licensed for use in humans, there are other vaccine formulations under development that apply different strategies. In this review, we present information about anti-dengue vaccine formulations regarding development, pre-clinical tests, and clinical trials. The improvement in vaccine development against dengue is much needed, but it should be considered that the correlate of protection is still uncertain. Neutralizing antibodies have been proposed as a correlate of protection, but this ignores the key role of T-cell mediated immunity in controlling DENV infection. It is important to confirm the accurate correlate of protection against DENV infection, and also to have other anti-dengue vaccine formulations licensed for use.

Challenges in Dengue Vaccines Development: Pre-existing Infections and Cross-Reactivity

Dengue is one of the most frequently transmitted mosquito-borne diseases in the world, which creates a significant public health concern globally, especially in tropical and subtropical countries. It is estimated that more than 390 million people are infected with dengue virus each year and around 96 million develop clinical pathologies. Dengue infections are not only a health problem but also a substantial economic burden. To date, there are no effective antiviral therapies and there is only one licensed dengue vaccine that only demonstrated protection in the seropositive (Immune), naturally infected with dengue, but not dengue seronegative (Naïve) vaccines. In this review, we address several immune components and their interplay with the dengue virus. Additionally, we summarize the literature pertaining to current dengue vaccine development and advances. Moreover, we review some of the factors affecting vaccine responses, such as the pre-vaccination environment, and provide an overview of the significant challenges that face the development of an efficient/protective dengue vaccine including the presence of multiple serotypes, antibody-dependent enhancement (ADE), as well as cross-reactivity with other flaviviruses. Finally, we discuss targeting T follicular helper cells (Tfh), a significant cell population that is essential for the production of high-affinity antibodies, which might be one of the elements needed to be specifically targeted to enhance vaccine precision to dengue regardless of dengue serostatus.

Vaccines Against Dengue and West Nile Viruses in India: The Need of the Hour

The circulation of flaviviruses, dengue (DEN), Japanese encephalitis (JE) and West Nile (WN) viruses, and others, is generating a major concern in many countries. Both JE along with DEN have been endemic in large regions of India. WN virus infection, although circulating in southern regions for many years, in recent years, WN encephalitis patients have been demonstrated. While vaccines against JE have been developed and decrease outbreaks, in case of DEN and WN, vaccines are still in developing level, especially, it has been difficult to achieve the long-term protective immune response. The first licensed DEN vaccine, which is a live attenuated vaccine, was administered in countries where the virus is endemic, and has a potential to cause serious side effects, especially when administered to younger population as observed in the Philippines vaccination drive. In the case of WN, although the purified inactivated virion-based vaccine worked effectively as a veterinary vaccine for horses, no effective vaccine has yet been licensed for humans. The induction of CD4⁺ and CD8⁺ T cell responses is essential to complete protection by these viruses, as evidenced by responses to asymptomatic infections. Many studies have shown that neutralizing antibody (NAb) response is against surface structural proteins; CD4⁺ and CD8⁺ responses are mainly directed against nonstructural proteins rather than NAb response. New data suggest that encapsulating virus vaccines in nanoparticles (NPs) will direct antigen in cytoplasmic compartment by antigen-presenting cells, which will improve presentation to CD4⁺ and CD8⁺ T cells. Since tissue culture-derived, purified inactivated viruses are easier to manufacture and safer than developing live virus vaccines, inclusion of NP provides an attractive alternative for generating robust flaviviral vaccines that are affordable with long-lived protection.

Route of inoculation and mosquito vector exposure modulate dengue virus replication kinetics and immune responses in rhesus macaques

Dengue virus (DENV) is transmitted by infectious mosquitoes during blood-feeding via saliva containing biologically-active proteins. Here, we examined the effect of varying DENV infection modality in rhesus macaques in order to improve the DENV nonhuman primate (NHP) challenge model. NHPs were exposed to DENV-1 via subcutaneous or intradermal inoculation of virus only, intradermal inoculation of virus and salivary gland extract, or infectious mosquito feeding. The infectious mosquito feeding group exhibited delayed onset of viremia, greater viral loads, and altered clinical and immune responses compared to other groups. After 15 months, NHPs in the subcutaneous and infectious mosquito feeding groups were re-exposed to either DENV-1 or DENV-2. Viral replication and neutralizing antibody following homologous challenge were suggestive of sterilizing immunity, whereas heterologous challenge resulted in productive, yet reduced, DENV-2 replication and boosted neutralizing antibody. These results show that a more transmission-relevant exposure modality resulted in viral replication closer to that observed in humans.

Dengue virus is transmitted into the skin of humans by mosquitoes as they take a blood meal. In contrast, many animal models are infected in the laboratory using a syringe to inject below the skin. Here, we looked at how different routes and methods of infection altered dengue infection in rhesus macaques. We found that infection via mosquito feeding resulted in a number of changes compared to other routes and methods, including a delay in the time to detection of dengue virus and overall greater quantities of dengue virus in the blood, and changes in the amounts of various components of blood that have been associated with dengue disease in humans. After 15 months, we exposed the macaques again to either the same or a different type of dengue virus. We found that animals exposed to the same type of dengue virus were protected from infection, whereas those animals exposed to a different type were only partially protected. Overall, our results show that dengue virus delivery using the natural transmission vector, mosquitoes, results in infection that is closer to what is observed in humans and may influence the interpretation of future studies of candidate vaccines.

Comparison of purified psoralen-inactivated and formalin-inactivated dengue vaccines in mice and nonhuman primates

Dengue fever, caused by dengue viruses (DENV 1-4) is a leading cause of illness and death in the tropics and subtropics. Therefore, an effective vaccine is urgently needed. Currently, the only available licensed dengue vaccine is a chimeric live attenuated vaccine that shows varying efficacy depending on serotype, age and baseline DENV serostatus. Accordingly, a dengue vaccine that is effective in seronegative adults, children of all ages and in immunocompromised individuals is still needed. We are currently researching the use of psoralen to develop an inactivated tetravalent dengue vaccine. Unlike traditional formalin inactivation, psoralen inactivates pathogens at the nucleic acid level, potentially preserving envelope protein epitopes important for protective anti-dengue immune responses. We prepared highly purified monovalent vaccine lots of formalin- and psoralen-inactivated DENV 1-4, using Capto DeVirS and Capto Core 700 resin based column chromatography. Tetravalent psoralen-inactivated vaccines (PsIV) and formalin-inactivated vaccines (FIV) were prepared by combining the four monovalent vaccines. Mice were immunized with either a low or high dose of PsIV or FIV to evaluate the immunogenicity of monovalent as well as tetravalent formulations of each inactivation method. In general, the monovalent and tetravalent PsIVs elicited equivalent or higher titers of neutralizing antibodies to DENV than the FIV dengue vaccines and this response was dose dependent. The immunogenicity of tetravalent dengue PsIVs and FIVs were also evaluated in nonhuman primates (NHPs). Consistent with what was observed in mice, significantly higher neutralizing antibody titers for each dengue serotype were observed in the NHPs vaccinated with the tetravalent dengue PsIV compared to those vaccinated with the tetravalent dengue FIV, indicative of the importance of envelope protein epitope preservation during psoralen inactivation of DENV.

Highly conserved epitopes of DENV structural and non-structural proteins: Candidates for universal vaccine targets

Dengue is a severe emerging arthropod borne viral disease occurring globally. Around two fifths of the world's population, or up to 3.9 billion people, are at a risk of dengue infection. Infection induces a life-long protective immunity to the homologous serotype but confers only partial and transient protection against subsequent infection caused by other serotypes. Thus, there is a need for a vaccine which is capable of providing a life- long protection against all the serotypes of dengue virus. In our study, comparative genomics of Dengue virus (DENV) was conducted to explore potential candidates for novel vaccine targets. From our analysis we successfully found 100% conserved epitopes in Envelope protein (RCPTQGE); NS3 (SAAQRRGR, PGTSGSPI); NS4A (QRTPQDNQL); NS4B (LQAKATREAQKRA) and NS5 proteins (QRGSGQV) in all DENV serotypes. Some serotype specific conserved motifs were also found in NS1, NS5, Capsid, PrM and Envelope proteins. Using comparative genomics and immunoinformatics approach, we could find conserved epitopes which can be explored as peptide vaccine candidates to combat dengue worldwide. Serotype specific epitopes can also be exploited for rapid diagnostics. All ten proteins are explored to find the conserved epitopes in DENV serotypes, thus making it the most extensively studied viral genome so far.

Role of Multivalency and Antigenic Threshold in Generating Protective Antibody Responses

Vaccines play a vital role in protecting our communities against infectious disease. Unfortunately, some vaccines provide only partial protection or in some cases vaccine-mediated immunity may wane rapidly, resulting in either increased susceptibility to that disease or a requirement for more booster vaccinations in order to maintain immunity above a protective level. The durability of antibody responses after infection or vaccination appears to be intrinsically determined by the structural biology of the antigen, with multivalent protein antigens often providing more long-lived immunity than monovalent antigens. This forms the basis for the Imprinted Lifespan model describing the differential survival of long-lived plasma cell populations. There are, however, exceptions to this rule with examples of highly attenuated live virus vaccines that are rapidly cleared and elicit only short-lived immunity despite the expression of multivalent surface epitopes. These exceptions have led to the concept that multivalency alone may not reliably determine the duration of protective humoral immune responses unless a minimum number of long-lived plasma cells are generated by reaching an appropriate antigenic threshold of B cell stimulation. Examples of long-term and in some cases, potentially lifelong antibody responses following immunization against human papilloma virus (HPV), Japanese encephalitis virus (JEV), Hepatitis B virus (HBV), and Hepatitis A virus (HAV) provide several lessons in understanding durable serological memory in human subjects. Moreover, studies involving influenza vaccination provide the unique opportunity to compare the durability of hemagglutinin (HA)-specific antibody titers mounted in response to antigenically repetitive whole virus (i.e., multivalent HA), or detergent-disrupted “split” virus, in comparison to the long-term immune responses induced by natural influenza infection. Here, we discuss the underlying mechanisms that may be associated with the induction of protective immunity by long-lived plasma cells and their importance in future vaccine design.

Detection of post-vaccination enhanced dengue virus infection in macaques: An improved model for early assessment of dengue vaccines

The need for improved dengue vaccines remains since the only licensed vaccine, Dengvaxia, shows variable efficacy depending on the infecting dengue virus (DENV) type, and increases the risk of hospitalization for severe dengue in children not exposed to DENV before vaccination. Here, we developed a tetravalent dengue purified and inactivated vaccine (DPIV) candidate and characterized, in rhesus macaques, its immunogenicity and efficacy to control DENV infection by analyzing, after challenge, both viral replication and changes in biological markers associated with dengue in humans. Although DPIV elicited cross-type and long-lasting DENV-neutralizing antibody responses, it failed to control DENV infection. Increased levels of viremia/RNAemia (correlating with serum capacity at enhancing DENV infection in vitro), AST, IL-10, IL-18 and IFN-γ, and decreased levels of IL-12 were detected in some vaccinated compared to non-vaccinated monkeys, indicating the vaccination may have triggered antibody-dependent enhancement of DENV infection. The dengue macaque model has been considered imperfect due to the lack of DENV-associated clinical signs. However, here we show that post-vaccination enhanced DENV infection can be detected in this model when integrating several parameters, including characterization of DENV-enhancing antibodies, viremia/RNAemia, and biomarkers relevant to dengue in humans. This improved dengue macaque model may be crucial for early assessment of efficacy and safety of future dengue vaccines.

Dengue virus (DENV) is responsible for the most widespread arboviral disease affecting humans. A pre-existing suboptimal immunity to DENV is accepted as being the major risk factor for severe dengue. Thus, if vaccination does not elicit optimal DENV-specific immunity, a vaccine might, instead, increase the risk of severe dengue in vaccinated individuals, as seen with the only licensed vaccine (Dengvaxia) in children naïve to DENV at vaccination. It is thus crucial to assess dengue vaccine safety at the earliest development stages, ideally in the preclinical stage. The dengue macaque model has been used to assess preclinical efficacy of dengue vaccines, with post-challenge DENV replication as the sole efficacy endpoint. However, this model had not predicted the Dengvaxia-associated safety signals. Here we characterized, in macaques, a dengue purified and inactivated vaccine (DPIV) candidate for its immunogenicity and efficacy/safety. Using a multiparameter approach, including characterization of viral replication and biomarkers relevant to dengue/severe dengue in humans, we were able to detect vaccine-associated safety signals in this model. While these results enabled us to discontinue at an early stage the DPIV development, this improved dengue macaque model may also be instrumental for early assessment of efficacy/safety of future dengue vaccines.

Aiming at the heart: the capsid protein of dengue virus as a vaccine candidate

INTRODUCTION

Dengue fever remains as a health problem worldwide. Although Dengvaxia®, was registered in several countries, the results after the immunization of people suggest an increase of risk in non-immune persons and children younger than 9 years old. No other vaccine is registered so far, thus the development of a safe and effective vaccine continues to be a priority for the WHO and the scientific community.

AREAS COVERED

This work reviews the structural and antigenic properties of the capsid protein of Dengue virus, along with results of studies performed to assess the immunogenicity and protective capacity in animals of vaccine candidates based on this protein.

EXPERT OPINION

The generation of a memory cellular immune response alone, after vaccination against Dengue virus, could be advantageous, as there would not be risk of increasing viral infectivity through sub-neutralizing antibodies. However, it is improbable to achieving sterilizing immunity. In this scenario, an infection could stablished but without the appearance of the severe disease. The cell-mediated immunity should keep the virus at bay. The capsid protein induces a protective immune response in animals without the induction of virus-binding antibodies. Vaccine candidates based on this protein could be an attractive strategy to induce protection against the severe Dengue disease.

Characterization of recent and minimally passaged Brazilian dengue viruses inducing robust infection in rhesus macaques

The macaque is widely accepted as a suitable model for preclinical characterization of dengue vaccine candidates. However, the only vaccine for which both preclinical and clinical efficacy results were reported so far showed efficacy levels that were substantially different between macaques and humans. We hypothesized that this model’s predictive capacity may be improved using recent and minimally passaged dengue virus isolates, and by assessing vaccine efficacy by characterizing not only the post-dengue virus challenge viremia/RNAemia but also the associated-cytokine profile. Ten recent and minimally passaged Brazilian clinical isolates from the four dengue virus serotypes were tested for their infectivity in rhesus macaques. For the strains showing robust replication capacity, the associated-changes in soluble mediator levels, and the elicited dengue virus-neutralizing antibody responses, were also characterized. Three isolates from dengue virus serotypes 1, 2 and 4 induced viremia of high magnitude and longer duration relative to previously reported viremia kinetics in this model, and robust dengue virus-neutralizing antibody responses. Consistent with observations in humans, increased MCP-1, IFN-γ and VEGF-A levels, and transiently decreased IL-8 levels were detected after infection with the selected isolates. These results may contribute to establishing a dengue macaque model showing a higher predictability for vaccine efficacy in humans.

EDIII-DENV3 nanospheres drive immature dendritic cells into a mature phenotype in an in vitro model

Domain III of E protein of dengue virus (DENV) is a target for vaccine development. Unfortunately, this protein based platform has low general immunogenicity. To circumvent this problem, the use of an adjuvant-nanoparticle delivery system to facilitate immunogenicity of soluble DENV-EDIII protein was investigated. One of the key features of this delivery system is its ability to simultaneously deliver antigens and exert adjuvanticity on specialized immune cells. In this study, N-trimethyl chitosan (TMC) nanoparticles (NPs) were generated to be used as adjuvant and carrier for soluble E-domain III of dengue virus serotype 3 (sEDIII-D3). Using ionotropic gelation, purified sEDIII-D3 was encapsulated into TMC NPs to form EDIII-D3 TMC NPs. After optimization, EDIII-D3 TMC particles exhibited a loading efficiency of 81% and a loading capacity of 41%. The immunogenicity of EDIII-D3 TMC NPs was tested using monocyte-derived dendritic cells (MoDCs). It was found that EDIII-D3 TMC NPs were well taken up by MoDCs. In addition, EDIII-D3 TMC NP treated MoDCs significantly upregulated maturation markers (CD80, CD83, CD86 and HLA-DR) and induced secretion of various cytokines and chemokines (IFN-α, IL-1β, IL-6, IL-2, IL-12p70, IFN-γ, IL-4, IL-10, IL-8, MCP-1, macrophage inflammatory protein-1β, granulocyte-colony stimulating factor, granulocyte-macrophage colony-stimulating factor and IL-7). These results indicate that EDIII-D3 TMC NPs are potent immunogens, at least in vitro, with the ability to induce maturation of DCs and highlight the potential use of TMC NPs for enhancing immunogenicity of a non-replicating dengue vaccine.

Phase I Randomized Study of a Tetravalent Dengue Purified Inactivated Vaccine in Healthy Adults from Puerto Rico

The safety and immunogenicity of four adjuvanted formulations of an investigational tetravalent dengue purified inactivated vaccine (DPIV) were evaluated in a predominantly dengue-primed population in Puerto Rico. In this placebo-controlled, randomized, observer-blind, phase I trial, 100 healthy adults were randomized 1:1:1:1:1 to receive DPIV at Day (D)0 and D28 (1 μg per dengue virus [DENV] type 1–4 adjuvanted with either alum, AS01_E or AS03_B, or 4 μg per DENV type adjuvanted with alum) or saline placebo. Functional antibody responses were assessed using a microneutralization assay at D56, Month (M)7, and M13. All DPIV formulations were well tolerated and no safety signals were identified through M13. The M13 according-to-protocol (ATP) immunogenicity cohort included 83 participants. The ATP analysis of immunogenicity was performed only on the 78 subjects seropositive for ≥ 1 DENV type at baseline: 69 tetravalent, three trivalent, two bivalent, and four monovalent. In all DPIV groups, geometric mean antibody titers (GMTs) increased from D0 to D56 and waned modestly through M13, while remaining well above prevaccination levels. The 4 μg + alum and the AS01_E- and AS03_B-adjuvanted formulations were highly immunogenic, with M13-neutralizing antibody GMTs against all four DENV types above 1,000. M13/D0 GMT ratios were highest in the 1 μg + AS03_B group (ranging 3.2–3.7 depending on the DENV type). These results encourage continued clinical development of DPIV (ClinicalTrials.gov: NCT01702857).

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.

Animal Models for Dengue and Zika Vaccine Development

The current status of animal models in the study of dengue and Zika are covered in this review. Mouse models deficient in IFN signaling are used to overcome the natural resistance of mice to non-encephalitic flaviviruses. Conditional IFNAR mice and non-human primates (NHP) are useful immuno-competent models. Sterile immunity after dengue vaccination is not observed in NHPs. Placental and fetal development in NHPs is similar to humans, facilitating studies on infection-mediated fetal impairment.

Marmosets (Callithrix jacchus) as a non-human primate model for evaluation of candidate dengue vaccines: induction and maintenance of specific protective immunity against challenges with clinical isolates

Dengue virus (DENV) is one of the major infectious diseases in tropical regions and approximately half of the world population is at risk of infection. Vaccines would offer an effective control measure against this disease. We previously reported on the utility of marmosets as an animal model for studying primary and secondary dengue infections. Infected marmosets consistently develop viraemia and antibody kinetics that reflect those of patients with dengue. Thus, it is important to determine the utility of marmosets as an animal model for demonstrating vaccine efficacy. In this study, marmosets were inoculated with candidate vaccine and parent strains and challenged with a clinical DENV strain. The viraemia and antibody kinetics in these marmosets were determined. Marmosets consistently develop lower viraemia with an attenuated vaccine strain. During secondary challenge, the IgM response was delayed, whereas the IgG levels rose rapidly, indicating a secondary antibody response. The neutralizing activities against the homotypic serotype were high; all marmosets were protected against viraemia following secondary inoculation. The viraemia markers and antibody responses were consistent with those of human DENV infection and vaccinees. These results demonstrate the utility of marmosets as an animal model for the study of vaccine efficacy.

Immune correlates of protection for dengue: State of the art and research agenda

Dengue viruses (DENV1-4) are mosquito-borne flaviviruses estimated to cause up to ∼400 million infections and ∼100 million dengue cases each year. Factors that contribute to protection from and risk of dengue and severe dengue disease have been studied extensively but are still not fully understood. Results from Phase 3 vaccine efficacy trials have recently become available for one vaccine candidate, now licensed for use in several countries, and more Phase 2 and 3 studies of additional vaccine candidates are ongoing, making these issues all the more urgent and timely. At the "Summit on Dengue Immune Correlates of Protection", held in Annecy, France, on March 8-9, 2016, dengue experts from diverse fields came together to discuss the current understanding of the immune response to and protection from DENV infection and disease, identify key unanswered questions, discuss data on immune correlates and plans for comparison of results across assays/consortia, and propose a research agenda for investigation of dengue immune correlates, all in the context of both natural infection studies and vaccine trials.

Phase 1 Randomized Study of a Tetravalent Dengue Purified Inactivated Vaccine in Healthy Adults in the United States

The safety and immunogenicity of four formulations of an investigational tetravalent dengue purified inactivated vaccine (DPIV), formulated at 1 or 4 μg with aluminum hydroxide (alum) or at 1 μg with an adjuvant system (AS01_E or AS03_B), were evaluated in a first-time-in-human, placebo-controlled, randomized, observer-blind, phase 1 trial in the continental United States. Two doses of vaccine or placebo were administered intramuscularly 4 weeks apart to 100 healthy adults 18–39 years of age, randomized 1:1:1:1:1 to receive one of four DPIV formulations or saline placebo. The response to a third dose was evaluated in a subset of nine participants remote from primary vaccination. Humoral immunogenicity was assessed using a 50% microneutralization assay. All DPIV formulations were well tolerated. No vaccine-related serious adverse events were observed through 12 months after the second vaccine dose. In all DPIV groups, geometric mean antibody titers peaked at Day 56, waned through 6 months after the second vaccine dose, and then stabilized. In the nine subjects where boosting was evaluated, a strong anamnestic response was observed. These results support continuation of the clinical development of this dengue vaccine candidate (clinicaltrials.gov: NCT01666652).

Prevention and control of dengue-the light at the end of the tunnel

Advances in the development of new dengue control tools, including vaccines and vector control, herald a new era of desperately needed dengue prevention and control. The burden of dengue has expanded for decades, and now affects more than 120 countries. Complex, large-scale global forces have and will continue to contribute to the expansion of dengue, including population growth, unplanned urbanisation, and suboptimal mosquito control in urban centres. Although no so-called magic bullets are available, there is new optimism following the first licensure of a dengue vaccine and other promising vaccine candidates, and the development of novel vector control interventions to help control dengue and other expanding mosquito-borne diseases such as Zika virus. Implementation of effective and sustainable immunisation programmes to complement existing methods will add complexity to the health systems of affected countries, which have varying levels of robustness and maturity. Long-term high prioritisation and adequate resources are needed. The way forward is full commitment to addressing a complex disease with a set of solutions integrating vaccination and vector control methods. A whole systems approach is thus needed to integrate these various approaches and strategies for controlling dengue within the goal of universal health coverage. The ultimate objective of these interventions will be to reduce the disease burden in a sustainable and equitable manner.

Potential for treatment and a Zika virus vaccine

PURPOSE OF REVIEW

Zika virus (ZIKV) has only recently been exposed as a significant public health threat, and much of our limited knowledge of its pathogenesis and triggered immune responses were discovered in only the last few years. There are currently no ZIKV-specific therapeutics or vaccines available. This review seeks to bring the reader up-to-date with the latest developments in finding a way to combat this emerging infectious disease.

RECENT FINDINGS

Current strategies used for developing ZIKV vaccines or treatments follow proven methods used against other flaviviruses. Unfortunately, ZIKV carries many unique challenges, such as the need to target drugs and vaccines towards immunocompromised populations (pregnant mothers and fetuses), the risk of stimulating harmful immune responses (either autoimmune or antibody-dependent enhancement of infection in those with previous flavivirus exposure), frequently silent infection that may delay treatment and increase risk of transmission to others, and multiple routes of transmission (arthropod vector, sexual, bloodborne, and potentially other body fluids).

SUMMARY

Current medical recommendations are directed towards resolving symptoms and not the actual infection; however, ZIKV treatments and vaccines are in development. Vector control and travel restrictions to endemic areas may remain our only available interventions for some time.

Protective efficacy of multiple vaccine platforms against Zika virus challenge in rhesus monkeys

Zika virus (ZIKV) is responsible for a major ongoing epidemic in the Americas and has been causally associated with fetal microcephaly. The development of a safe and effective ZIKV vaccine is therefore an urgent global health priority. Here we demonstrate that three different vaccine platforms protect against ZIKV challenge in rhesus monkeys. A purified inactivated virus vaccine induced ZIKV-specific neutralizing antibodies and completely protected monkeys against ZIKV strains from both Brazil and Puerto Rico. Purified immunoglobulin from vaccinated monkeys also conferred passive protection in adoptive transfer studies. A plasmid DNA vaccine and a single-shot recombinant rhesus adenovirus serotype 52 vector vaccine, both expressing ZIKV premembrane and envelope, also elicited neutralizing antibodies and completely protected monkeys against ZIKV challenge. These data support the rapid clinical development of ZIKV vaccines for humans.

Zanotto PM, Iampietro MJ, Badamchi-Zadeh A, Boyd M, Ng’ang’a D, Kirilova M, Nityanandam R, Mercado NB, Li Z, Moseley ET, Bricault CA, Borducchi EN, Giglio PB, Jetton D, Neubauer G, Nkolola JP, Maxfield LF, De La Barrera RA, Jarman RG, Eckels KH, Michael NL, Thomas SJ, Barouch DH. Vaccine protection against Zika virus from Brazil

Zika virus (ZIKV) is a flavivirus that is responsible for the current epidemic in Brazil and the Americas. ZIKV has been causally associated with fetal microcephaly, intrauterine growth restriction, and other birth defects in both humans and mice. The rapid development of a safe and effective ZIKV vaccine is a global health priority, but very little is currently known about ZIKV immunology and mechanisms of immune protection. Here we show that a single immunization with a plasmid DNA vaccine or a purified inactivated virus vaccine provides complete protection in susceptible mice against challenge with a strain of ZIKV involved in the outbreak in northeast Brazil. This ZIKV strain has recently been shown to cross the placenta and to induce fetal microcephaly and other congenital malformations in mice. We produced DNA vaccines expressing ZIKV pre-membrane and envelope (prM-Env), as well as a series of deletion mutants. The prM-Env DNA vaccine, but not the deletion mutants, afforded complete protection against ZIKV, as measured by absence of detectable viraemia following challenge, and protective efficacy correlated with Env-specific antibody titers. Adoptive transfer of purified IgG from vaccinated mice conferred passive protection, and depletion of CD4 and CD8 T lymphocytes in vaccinated mice did not abrogate this protection. These data demonstrate that protection against ZIKV challenge can be achieved by single-shot subunit and inactivated virus vaccines in mice and that Env-specific antibody titers represent key immunologic correlates of protection. Our findings suggest that the development of a ZIKV vaccine for humans is likely to be achievable.

Synthetic B-Cell Epitopes Eliciting Cross-Neutralizing Antibodies: Strategies for Future Dengue Vaccine

Dengue virus (DENV) is a major public health threat worldwide. A key element in protection from dengue fever is the neutralising antibody response. Anti-dengue IgG purified from DENV-2 infected human sera showed reactivity against several peptides when evaluated by ELISA and epitope extraction techniques. A multi-step computational approach predicted six antigenic regions within the E protein of DENV-2 that concur with the 6 epitopes identified by the combined ELISA and epitope extraction approach. The selected peptides representing B-cell epitopes were attached to a known dengue T-helper epitope and evaluated for their vaccine potency. Immunization of mice revealed two novel synthetic vaccine constructs that elicited good humoral immune responses and produced cross-reactive neutralising antibodies against DENV-1, 2 and 3. The findings indicate new directions for epitope mapping and contribute towards the future development of multi-epitope based synthetic peptide vaccine.

Recombinant Dengue 2 Virus NS3 Helicase Protein Enhances Antibody and T-Cell Response of Purified Inactivated Vaccine

Dengue virus purified inactivated vaccines (PIV) are highly immunogenic and protective over the short term, but may be poor at inducing cell-mediated immune responses and long-term protection. The dengue nonstructural protein 3 (NS3) is considered the main target for T-cell responses during viral infection. The amino (N)-terminal protease and the carboxy (C)-terminal helicase domains of DENV-2 NS3 were expressed in E. coli and analyzed for their immune-potentiating capacity. Mice were immunized with DENV-2 PIV with and without recombinant NS3 protease or NS3 helicase proteins, and NS3 proteins alone on days 0, 14 and 28. The NS3 helicase but not the NS3 protease was effective in inducing T-cell responses quantified by IFN-γ ELISPOT. In addition, markedly increased total IgG antibody titer against virus antigen was seen in mice immunized with the PIV/NS3 helicase combination in the ELISA, as well as increased neutralizing antibody titer measured by the plaque reduction neutralization test. These results indicate the potential immunogenic properties of the NS3 helicase protein and its use in a dengue vaccine formulation.

Recent advances in understanding dengue

Dengue is an emerging threat to billions of people worldwide. In the last 20 years, the incidence has increased four-fold and this trend appears to be continuing. Caused by one of four viral serotypes, dengue can present as a wide range of clinical phenotypes with the severe end of the spectrum being defined by a syndrome of capillary leak, coagulopathy, and organ impairment. The pathogenesis of severe disease is thought to be in part immune mediated, but the exact mechanisms remain to be defined. The current treatment of dengue relies on supportive measures with no licensed therapeutics available to date. There have been recent advances in our understanding of a number of areas of dengue research, of which the following will be discussed in this review: the drivers behind the global dengue pandemic, viral structure and epitope binding, risk factors for severe disease and its pathogenesis, as well as the findings of recent clinical trials including therapeutics and vaccines. We conclude with current and future dengue control measures and key areas for future research.

Dengue vaccine: hypotheses to understand CYD-TDV-induced protection

Dengue virus (DENV) is a human pathogen with a large impact on public health. Although no vaccine against DENV is currently licensed, a recombinant vaccine - chimeric yellow fever virus-DENV tetravalent dengue vaccine (CYD-TDV) - has shown efficacy against symptomatic dengue disease in two recent Phase III clinical trials. Safety observations were also recently reported for these trials. In this Opinion article, we review the data from recent vaccine clinical trials and discuss the putative mechanisms behind the observed efficacy of the vaccine against different forms of the disease, focusing on the interactions between the infecting virus, pre-existing host immunity and vaccine-induced immune responses.

Next generation dengue vaccines: A review of the preclinical development pipeline

Dengue represents a significant and growing public health problem across the globe, with approximately half of the world's population at risk. The increasing and expanding burden of dengue has highlighted the need for new tools to prevent dengue, including development of dengue vaccines. Recently, the first dengue vaccine candidate was evaluated in Phase 3 clinical trials, and other vaccine candidates are under clinical evaluation. There are also a number of candidates in preclinical development, based on diverse technologies, with promising results in animal models and likely to move into clinical trials and could eventually be next-generation dengue vaccines. This review provides an overview of the various technological approaches to dengue vaccine development with specific focus on candidates in preclinical development and with evaluation in non-human primates.