Rapid development of a DNA vaccine for Zika virus

Live vaccine infection burden elicits adaptive humoral and cellular immunity required to prevent Zika virus infection

BACKGROUND

The emergence of Zika virus (ZIKV) as an important cause of congenital and childhood developmental disorders presents another challenge to global health. Efforts to develop a Zika vaccine have begun although vaccine development against flaviviruses, of which ZIKV belongs to, has proven to be time-consuming and challenging. Defining the vaccine attributes that elicit adaptive immune response necessary for preventing ZIKV infection could provide an evidence-based guide to Zika vaccine development.

METHODS

We used a previously described attenuated ZIKV DN-2 strain in a type-I interferon receptor deficient mouse model and tested the hypothesis that duration of vaccine burden rather than peak level of infection, is a determinant of immunogenicity. We quantified both humoral and cellular responses against ZIKV using plaque reduction neutralisation test and flow cytometry with ELISPOT assays, respectively. Vaccinated mice were challenged with wild-type ZIKV (H/PF/2013 strain) to determine the level of protection against infection.

FINDINGS

We found that the overall vaccine burden is directly correlated with neutralising antibody titres. Reduced duration of vaccine burden lowered neutralising antibody titres that resulted in subclinical infection, despite unchanged peak vaccine viraemia levels. We also found that sterilising immunity is dependant on both neutralising antibody and CD8⁺T cell responses; depletion of CD8⁺T cells in vaccinated animals led to wild-type ZIKV infection, especially in the male reproductive tract.

INTERPRETATION

Our findings indicate that duration of attenuated virus vaccine burden is a determinant of humoral and cellular immunity and also suggest that vaccines that elicit both arms of the adaptive immune response are needed to fully prevent ZIKV transmission.

FUNDING

This study was supported by the National Medical Research Council through the Clinician-Scientist Award (Senior Investigator) to E.E.O. Salary support for S.W. was from a Competitive Research Programme grant awarded by the National Research Foundation of Singapore.

Zika vaccine pre-clinical and clinical data review with perspectives on the future development

Zika is an arboviral illness caused by infection with the Zika flavivirus. Transmission most commonly occurs during a feeding event involving an infected Aedes mosquito or vertical transmission between an infected mother to her fetus. Infection outcomes range from asymptomatic to devastating neurologic injuries in children infected in utero. The recognition of Congenital Zika Syndrome prompted the declaration of an international health emergency and a call to rapidly develop medical countermeasures such as vaccines and therapeutics. A flurry of research and development activity in industry, government, non-governmental organizations, and academia during the most recent Zika epidemic (2015) stimulated the development of a number of vaccine candidate prototypes, generation of pre-clinical data, and the conduct of early phase human trials. The safety and immunogenicity of different vaccine platforms were demonstrated and mouse and non-human primate passive transfer studies hinted at the potential for clinical benefit in humans and defining an immune correlate of protection. A rapid decline in regional transmission, however, prevented the conduct a clinical endpoint efficacy trial. The pathway to licensure of a Zika vaccine remains unclear.

SARS-CoV-2 mRNA vaccine design enabled by prototype pathogen preparedness

A vaccine for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is needed to control the coronavirus disease 2019 (COVID-19) global pandemic. Structural studies have led to the development of mutations that stabilize Betacoronavirus spike proteins in the prefusion state, improving their expression and increasing immunogenicity1. This principle has been applied to design mRNA-1273, an mRNA vaccine that encodes a SARS-CoV-2 spike protein that is stabilized in the prefusion conformation. Here we show that mRNA-1273 induces potent neutralizing antibody responses to both wild-type (D614) and D614G mutant2 SARS-CoV-2 as well as CD8+ T cell responses, and protects against SARS-CoV-2 infection in the lungs and noses of mice without evidence of immunopathology. mRNA-1273 is currently in a phase III trial to evaluate its efficacy.

Reverse genetic approaches for the development of Zika vaccines and therapeutics

In 2015-2016, the little known Zika virus (ZIKV) caused an epidemic, in which it became recognized as a unique human pathogen associated with a range of devastating congenital abnormalities collectively categorized as congenital Zika syndrome (CZS). In adults, the virus can trigger the autoimmune disorder Guillain-Barré syndrome (GBS), characterized by ascending paralysis. In February 2016, the World Health Organization (WHO) declared ZIKV to be a Public Health Emergency of International Concern. The global public health problem prompted academia, industry, and governments worldwide to initiate development of an effective vaccine to prevent another ZIKV epidemic that would put millions at risk. The development of reverse genetic systems for the study and manipulation of RNA viral genomes has revolutionized the field of virology, providing platforms for vaccine and antiviral development. In this review, we discuss the impact of reverse genetic systems on the rapid progress of ZIKV vaccines and antiviral therapeutics.

Analytical characterization of liposomes and other lipid nanoparticles for drug delivery

Lipid nanoparticles, especially liposomes and lipid/nucleic acid complexed nanoparticles have shown great success in the pharmaceutical industry. Their success is attributed to stable drug loading, extended pharmacokinetics, reduced off-target side effects, and enhanced delivery efficiency to disease targets with formidable blood-brain or plasma membrane barriers. Therefore, they offer promising formulation options for drugs limited by low therapeutic indexes in traditional dosage forms and current "undruggable" targets. Recent development of siRNA, antisense oligonucleotide, or the CRISPR complex-loaded lipid nanoparticles and liposomal vaccines also shed light on their potential in enabling versatile formulation platforms for new pharmaceutical modalities. Analytical characterization of these nanoparticles is critical to drug design, formulation development, understanding in vivo performance, as well as quality control. The multi-lipid excipients, unique core-bilayer structure, and nanoscale size all underscore their complicated critical quality attributes, including lipid species, drug encapsulation efficiency, nanoparticle characteristics, product stability, and drug release. To address these challenges and facilitate future applications of lipid nanoparticles in drug development, we summarize available analytical approaches for physicochemical characterizations of lipid nanoparticle-based pharmaceutical modalities. Furthermore, we compare advantages and challenges of different techniques, and highlight the promise of new strategies for automated high-throughput screening and future development.

Vesicular Stomatitis Virus and DNA Vaccines Expressing Zika Virus Nonstructural Protein 1 Induce Substantial but Not Sterilizing Protection against Zika Virus Infection

The nonstructural protein 1 (NS1) of several flaviviruses, including West Nile, dengue, and yellow fever viruses, is capable of inducing variable degrees of protection against flavivirus infection in animal models. However, the immunogenicity of NS1 protein of Zika virus (ZIKV) is less understood. Here, we determined the efficacy of ZIKV NS1-based vaccine candidates using two delivery platforms, methyltransferase-defective recombinant vesicular stomatitis virus (mtdVSV) and a DNA vaccine. We first show that expression of ZIKV NS1 could be significantly enhanced by optimizing the signal peptide. A single dose of mtdVSV-NS1-based vaccine or two doses of DNA vaccine induced high levels of NS1-specfic antibody and T cell immune responses but provided only partial protection against ZIKV viremia in BALB/c mice. In Ifnar1^-/- mice, neither NS1-based vaccine provided protection against a lethal high dose (10⁵ PFU) ZIKV challenge, but mtdVSV-NS1-based vaccine prevented deaths from a low dose (10³ PFU) challenge, though they experienced viremia and body weight loss. We conclude that ZIKV NS1 alone conferred substantial, but not complete, protection against ZIKV infection. Nevertheless, these results highlight the value of ZIKV NS1 for vaccine development.IMPORTANCE Most Zika virus (ZIKV) vaccine research has focused on the E or prM-E proteins and the induction of high levels of neutralizing antibodies. However, these ZIKV neutralizing antibodies cross-react with other flaviviruses, which may aggravate the disease via an antibody-dependent enhancement (ADE) mechanism. ZIKV NS1 protein may be an alternative antigen for vaccine development, since antibodies to NS1 do not bind to the virion, thereby eliminating the risk of ADE. Here, we show that recombinant VSV and DNA vaccines expressing NS1, alone, confer partial protection against ZIKV infection in both immunocompetent and immunodeficient mice, highlighting the value of NS1 as a potential vaccine candidate.

A Single Dose of NILV-Based Vaccine Provides Rapid and Durable Protection against Zika Virus

Zika virus, a member of the Flaviviridae family, is primarily transmitted by infected Aedes species mosquitoes. In 2016, Zika infection emerged as a global health emergency for its explosive spread and the remarkable neurological defects in the developing fetus. Development of a safe and effective Zika vaccine remains a high priority owing to the risk of re-emergence and limited understanding of Zika virus epidemiology. We engineered a non-integrating lentiviralvector(NILV)-based Zika vaccine encoding the consensus pre-membrane and envelope glycoprotein of circulating Zika virus strains. We further evaluated the immunogenicity and protective efficacy of this vaccine in both immunocompromised and immunocompetent mouse models. A single immunization in both mouse models elicited a robust neutralizing antibody titer and afforded full protection against Zika challenge as early as 7 days post-immunization. This NILV-based vaccine also induced a long-lasting immunity when immunized mice were challenged 6 months after immunization. Altogether, our NILV Zika vaccine provides a rapid yet durable protection through a single dose of immunization without extra adjuvant formulation. Our data suggest a promising Zika vaccine candidate for an emergency situation, and demonstrate the capacity of lentiviral vector as an efficient vaccine delivery platform.

Development of a potent Zika virus vaccine using self-amplifying messenger RNA

Zika virus (ZIKV) is the cause of a pandemic associated with microcephaly in newborns and Guillain-Barre syndrome in adults. Currently, there are no available treatments or vaccines for ZIKV, and the development of a safe and effective vaccine is a high priority for many global health organizations. We describe the development of ZIKV vaccine candidates using the self-amplifying messenger RNA (SAM) platform technology delivered by cationic nanoemulsion (CNE) that allows bedside mixing and is particularly useful for rapid responses to pandemic outbreaks. Two immunizations of either of the two lead SAM (CNE) vaccine candidates elicited potent neutralizing antibody responses to ZIKV in mice and nonhuman primates. Both SAM (CNE) vaccines protected these animals from ZIKV challenge, with one candidate providing complete protection against ZIKV infection in nonhuman primates. The data provide a preclinical proof of concept that a SAM (CNE) vaccine candidate can rapidly elicit protective immunity against ZIKV.

Enhanced effect of modified Zika virus E antigen on the immunogenicity of DNA vaccine

It has been reported worldwide that the Zika virus (ZIKV) could be transmitted through placentas and sexual contact. ZIKV can also cause Guillain-Barre syndrome, microcephaly and neurological abnormalities. However, there are no approved vaccines available. We constructed six DNA vaccine candidates and tested the immunogenicity. Tandem repeated envelope domain Ⅲ (ED Ⅲ × 3) induced highly total IgG and neutralization antibody, as well as CD8⁺ T cell responses. Also, stem region-removed envelope (E ΔSTEM) elicited a robust production of IFN-γ in mice. To examine in vivo protection, we used mice treated with an IFNAR1 blocking antibody before and after the challenge. Vaccination with the two candidates led to a decline in the level of viral RNAs in organs. Moreover, the sera from the vaccinated mice did not enhance the infection of Dengue virus in K562 cells. These findings suggest the potential for the development of a novel ZIKV DNA vaccine.

Immunopathology of Zika virus infection

Zika virus (ZIKV) is a mosquito-borne virus of the flavivirus genus in the Flaviviridae family. Flaviviruses are single-stranded, positive-sense RNA viruses that have been responsible for numerous human epidemics. Notable flaviviruses include mosquito-borne viruses such as yellow fever virus (YFV), Dengue virus (DENV), West Nile virus (WNV), Japanese encephalitis virus (JEV), as well as tick-borne viruses including Powassan virus (POWV) and tick-borne encephalitis virus (TBEV). Despite having been relatively obscure until the past decade, ZIKV has become a major global health concern, and is a topic of active research following multiple outbreaks across the globe. Here, we discuss ZIKV pathogenesis and the associated immunopathology, as well as advances in research, therapies, and vaccines developed using models of ZIKV pathogenesis.

Identification of SARS-CoV-2 Vaccine Epitopes Predicted to Induce Long-Term Population-Scale Immunity

Here we propose a SARS-CoV-2 vaccine design concept based on identification of highly conserved regions of the viral genome and newly acquired adaptations, both predicted to generate epitopes presented on major histocompatibility complex (MHC) class I and II across the vast majority of the population. We further prioritize genomic regions that generate highly dissimilar peptides from the human proteome and are also predicted to produce B cell epitopes. We propose sixty-five 33-mer peptide sequences, a subset of which can be tested using DNA or mRNA delivery strategies. These include peptides that are contained within evolutionarily divergent regions of the spike protein reported to increase infectivity through increased binding to the ACE2 receptor and within a newly evolved furin cleavage site thought to increase membrane fusion. Validation and implementation of this vaccine concept could specifically target specific vulnerabilities of SARS-CoV-2 and should engage a robust adaptive immune response in the vast majority of the population.

Role of Lipid-Based and Polymer-Based Non-Viral Vectors in Nucleic Acid Delivery for Next-Generation Gene Therapy

The field of gene therapy has experienced an insurgence of attention for its widespread ability to regulate gene expression by targeting genomic DNA, messenger RNA, microRNA, and short-interfering RNA for treating malignant and non-malignant disorders. Numerous nucleic acid analogs have been developed to target coding or non-coding sequences of the human genome for gene regulation. However, broader clinical applications of nucleic acid analogs have been limited due to their poor cell or organ-specific delivery. To resolve these issues, non-viral vectors based on nanoparticles, liposomes, and polyplexes have been developed to date. This review is centered on non-viral vectors mainly comprising of cationic lipids and polymers for nucleic acid-based delivery for numerous gene therapy-based applications.

WHO informal consultation on the guidelines for evaluation of the quality, safety, and efficacy of DNA vaccines, Geneva, Switzerland, December 2019

Consultations have been held to promote the revision of the WHO guidelines for assuring the quality and nonclinical safety evaluation of DNA vaccines adopted by the Expert Committee on Biological Standardization (ECBS) in 2005. The drivers for this revision are described, including the need for regulatory convergence highlighted by the WHO R&D Blueprint. These consultations have driven the revision to its current form, where a new guideline that includes quality, nonclinical, and clinical evaluation of plasmid DNA vaccines is being prepared for public consultation with a view to present to an upcoming ECBS. Major changes to the guidelines include streamlining the existing quality (part A) and nonclinical (part B) sections to reflect the two decades of experience, with manufacturing and control, nonclinical evaluation, and clinical testing of plasmid DNA vaccines, as a platform technology. The urgency for gaining regulatory convergence on this topic is that development of such a platform technology as DNA vaccines for routine use immunizations will prepare manufacturers and regulators across the globe in dealing with rapid development of medical countermeasures against emerging infectious diseases even in the face of an emergency setting. Two examples are described of Zika candidate vaccines that have rapidly advanced in development based on preexisting nonclinical and clinical data that precluded the need to repeat nonclinical toxicology. This report describes the progress stemming from the most recent consultation on the guidelines, including topics discussed and consensus reached.

Bat-borne virus diversity, spillover and emergence

Most viral pathogens in humans have animal origins and arose through cross-species transmission. Over the past 50 years, several viruses, including Ebola virus, Marburg virus, Nipah virus, Hendra virus, severe acute respiratory syndrome coronavirus (SARS-CoV), Middle East respiratory coronavirus (MERS-CoV) and SARS-CoV-2, have been linked back to various bat species. Despite decades of research into bats and the pathogens they carry, the fields of bat virus ecology and molecular biology are still nascent, with many questions largely unexplored, thus hindering our ability to anticipate and prepare for the next viral outbreak. In this Review, we discuss the latest advancements and understanding of bat-borne viruses, reflecting on current knowledge gaps and outlining the potential routes for future research as well as for outbreak response and prevention efforts.

SARS-CoV-2 mRNA Vaccine Development Enabled by Prototype Pathogen Preparedness

A SARS-CoV-2 vaccine is needed to control the global COVID-19 public health crisis. Atomic-level structures directed the application of prefusion-stabilizing mutations that improved expression and immunogenicity of betacoronavirus spike proteins. Using this established immunogen design, the release of SARS-CoV-2 sequences triggered immediate rapid manufacturing of an mRNA vaccine expressing the prefusion-stabilized SARS-CoV-2 spike trimer (mRNA-1273). Here, we show that mRNA-1273 induces both potent neutralizing antibody and CD8 T cell responses and protects against SARS-CoV-2 infection in lungs and noses of mice without evidence of immunopathology. mRNA-1273 is currently in a Phase 2 clinical trial with a trajectory towards Phase 3 efficacy evaluation.

Modeling mosquito-borne and sexual transmission of Zika virus in an enzootic host, the African green monkey

Mosquito-borne and sexual transmission of Zika virus (ZIKV), a TORCH pathogen, recently initiated a series of large epidemics throughout the Tropics. Animal models are necessary to determine transmission risk and study pathogenesis, as well screen antivirals and vaccine candidates. In this study, we modeled mosquito and sexual transmission of ZIKV in the African green monkey (AGM). Following subcutaneous, intravaginal or intrarectal inoculation of AGMs with ZIKV, we determined the transmission potential and infection dynamics of the virus. AGMs inoculated by all three transmission routes exhibited viremia and viral shedding followed by strong virus neutralizing antibody responses, in the absence of clinical illness. All four of the subcutaneously inoculated AGMs became infected (mean peak viremia: 2.9 log10 PFU/mL, mean duration: 4.3 days) and vRNA was detected in their oral swabs, with infectious virus being detected in a subset of these specimens. Although all four of the intravaginally inoculated AGMs developed virus neutralizing antibody responses, only three had detectable viremia (mean peak viremia: 4.0 log10 PFU/mL, mean duration: 3.0 days). These three AGMs also had vRNA and infectious virus detected in both oral and vaginal swabs. Two of the four intrarectally inoculated AGMs became infected (mean peak viremia: 3.8 log10 PFU/mL, mean duration: 3.5 days). vRNA was detected in oral swabs collected from both of these infected AGMs, and infectious virus was detected in an oral swab from one of these AGMs. Notably, vRNA and infectious virus were detected in vaginal swabs collected from the infected female AGM (peak viral load: 7.5 log10 copies/mL, peak titer: 3.8 log10 PFU/mL, range of detection: 5-21 days post infection). Abnormal clinical chemistry and hematology results were detected and acute lymphadenopathy was observed in some AGMs. Infection dynamics in all three AGM ZIKV models are similar to those reported in the majority of human ZIKV infections. Our results indicate that the AGM can be used as a surrogate to model mosquito or sexual ZIKV transmission and infection. Furthermore, our results suggest that AGMs are likely involved in the enzootic maintenance and amplification cycle of ZIKV.

Current Status of Zika Virus Vaccines: Successes and Challenges

The recently emerged Zika virus (ZIKV) spread to the Americas, causing a spectrum of congenital diseases including microcephaly in newborn and Guillain-Barré syndrome (GBS) in adults. The unprecedented nature of the epidemic and serious diseases associated with the viral infections prompted the global research community to understand the immunopathogenic mechanisms of the virus and rapidly develop safe and efficacious vaccines. This has led to a number of ZIKV vaccine candidates that have shown significant promise in human clinical trials. These candidates include nucleic acid vaccines, inactivated vaccines, viral-vectored vaccines, and attenuated vaccines. Additionally, a number of vaccine candidates have been shown to protect animals in preclinical studies. However, as the epidemic has waned in the last three years, further development of the most promising vaccine candidates faces challenges in clinical efficacy trials, which is needed before a vaccine is brought to licensure. It is important that a coalition of government funding agencies and private sector companies is established to move forward with a safe and effective vaccine ready for deployment when the next ZIKV epidemic occurs.

Lipid Nanoparticle Formulation Increases Efficiency of DNA-Vectored Vaccines/Immunoprophylaxis in Animals Including Transchromosomic Bovines

The use of nucleic acid as a drug substance for vaccines and other gene-based medicines continues to evolve. Here, we have used a technology originally developed for mRNA in vivo delivery to enhance the immunogenicity of DNA vaccines. We demonstrate that neutralizing antibodies produced in rabbits and nonhuman primates injected with lipid nanoparticle (LNP)-formulated Andes virus or Zika virus DNA vaccines are elevated over unformulated vaccine. Using a plasmid encoding an anti-poxvirus monoclonal antibody (as a reporter of protein expression), we showed that improved immunogenicity is likely due to increased in vivo DNA delivery, resulting in more target protein. Specifically, after four days, up to 30 ng/mL of functional monoclonal antibody were detected in the serum of rabbits injected with the LNP-formulated DNA. We pragmatically applied the technology to the production of human neutralizing antibodies in a transchromosomic (Tc) bovine for use as a passive immunoprophylactic. Production of neutralizing antibody was increased by >10-fold while utilizing 10 times less DNA in the Tc bovine. This work provides a proof-of-concept that LNP formulation of DNA vaccines can be used to produce more potent active vaccines, passive countermeasures (e.g., Tc bovine), and as a means to produce more potent DNA-launched immunotherapies.

Immune-profiling of ZIKV-infected patients identifies a distinct function of plasmacytoid dendritic cells for immune cross-regulation

Zika virus (ZIKV) is a mosquito-borne pathogen with increasing public health significance. To characterize immune responses to ZIKV, here we examine transcriptional signatures of CD4 T, CD8 T, B, and NK cells, monocytes, myeloid dendritic cells (mDCs), and plasmacytoid dendritic cells (pDCs) from three individuals with ZIKV infection. While gene expression patterns from most cell subsets display signs of impaired antiviral immune activity, pDCs from infected host have distinct transcriptional response associated with activation of innate immune recognition and type I interferon signaling pathways, but downregulation of key host factors known to support ZIKV replication steps; meanwhile, pDCs exhibit a unique expression pattern of gene modules that are correlated with alternative cell populations, suggesting collaborative interactions between pDCs and other immune cells, particularly B cells. Together, these results point towards a discrete but integrative function of pDCs in the human immune responses to ZIKV infection.

Heterologous prime-boost: an important candidate immunization strategy against Tembusu virus

BACKGROUND

Tembusu virus (TMUV), a newly emerging pathogenic flavivirus, spreads rapidly between ducks, causing massive economic losses in the Chinese duck industry. Vaccination is the most effective method to prevent TMUV. Therefore, it is urgent to look for an effective vaccine strategy against TMUV. Heterologous prime-boost regimens priming with vaccines and boosting with recombinant adenovirus vaccines have been proven to be successful strategies for protecting against viruses in experimental animal models.

METHODS

In this study, heterologous and homologous prime-boost strategies using an attenuated salmonella vaccine and a recombinant adenovirus vaccine expressing prM-E or the E gene of TMUV were evaluated to protect ducks against TMUV infection for the first time, including priming and boosting with the attenuated salmonella vaccine, priming and boosting with the recombinant adenovirus vaccine, and priming with the attenuated salmonella vaccine and boosting with the recombinant adenovirus vaccine. Humoral and cellular immune responses were detected and evaluated. We then challenged the ducks with TMUV at 12 days after boosting to assay for clinical symptoms, mortality, viral loads and histopathological lesions after these different strategies.

RESULTS

Compared with the homologous prime-boost strategies, the heterologous prime-boost regimen produced higher levels of neutralizing antibodies and IgG antibodies against TMUV. Additionally, it could induce higher levels of IFN-γ than homologous prime-boost strategies in the later stage. Interestingly, the heterologous prime-boost strategy induced higher levels of IL-4 in the early stage, but the IL-4 levels gradually decreased and were even lower than those induced by the homologous prime-boost strategy in the later stage. Moreover, the heterologous prime-boost strategy could efficiently protect ducks, with low viral titres, no clinical symptoms and histopathological lesions in this experiment after challenge with TMUV, while slight clinical symptoms and histopathological lesions were observed with the homologous prime-boost strategies.

CONCLUSIONS

Our results indicated that the heterologous prime-boost strategy induced higher levels of humoral and cellular immune responses and better protection against TMUV infection in ducks than the homologous prime-boost strategies, suggesting that the heterologous prime-boost strategy is an important candidate for the design of a novel vaccine strategy against TMUV.

Preparedness of public health-care system for Zika virus outbreak: An Indian perspective

Zika virus is a mosquito-borne flavivirus that has emerged recently and affected in many countries. Since its discovery in Uganda in 1947, two major outbreaks were reported from Yap Islands in 2007 and French Polynesia in 2013. In 2015, the first case of ZIKV infection was confirmed from Brazil followed by a report of cases from American and Caribbean countries. In February 2016, the World Health Organization declared ZIKV infection a Public Health Emergency of International Concern. India reported the first Zika case in 2017. Subsequently, 157 laboratory-confirmed cases of ZIKV including 63 pregnant women were reported from Rajasthan, India in 2018. Since 2014, many countries took initiatives to boost their public health system to combat ZIKV. However, there is still scope for the improvement. This review describes ZIKV outbreaks, diagnostic challenges, surveillance and control measures in India and the future perspective to deal with the ZIKV outbreak in India.

A SARS-CoV-2 Vaccination Strategy Focused on Population-Scale Immunity

Here we propose a vaccination strategy for SARS-CoV-2 based on identification of both highly conserved regions of the virus and newly acquired adaptations that are presented by MHC class I and II across the vast majority of the population, are highly dissimilar from the human proteome, and are predicted B cell epitopes. We present 65 peptide sequences that we expect to result in a safe and effective vaccine which can be rapidly tested in DNA, mRNA, or synthetic peptide constructs. These include epitopes that are contained within evolutionarily divergent regions of the spike protein reported to increase infectivity through increased binding to the ACE2 receptor, and within a novel furin cleavage site thought to increase membrane fusion. This vaccination strategy specifically targets unique vulnerabilities of SARS-CoV-2 and should engage a robust adaptive immune response in the vast majority of the human population.

Current Efforts in the Development of Vaccines for the Prevention of Zika and Chikungunya Virus Infections

Arboviruses represent major challenges to public health, particularly in tropical, and subtropical regions, and a substantial risk to other parts of the world as respective vectors extend their habitats. In recent years, two viruses transmitted by Aedes mosquitoes, Chikungunya and Zika virus, have gathered increased interest. After decades of regionally constrained outbreaks, both viruses have recently caused explosive outbreaks on an unprecedented scale, causing immense suffering and massive economic burdens in affected regions. Chikungunya virus causes an acute febrile illness that often transitions into a chronic manifestation characterized by debilitating arthralgia and/or arthritis in a substantial subset of infected individuals. Zika infection frequently presents as a mild influenza-like illness, often subclinical, but can cause severe complications such as congenital malformations in pregnancy and neurological disorders, including Guillain-Barré syndrome. With no specific treatments or vaccines available, vector control remains the most effective measure to manage spread of these diseases. Given that both viruses cause antibody responses that confer long-term, possibly lifelong protection and that such responses are cross-protective against the various circulating genetic lineages, the development of Zika and Chikungunya vaccines represents a promising route for disease control. In this review we provide a brief overview on Zika and Chikungunya viruses, the etiology and epidemiology of the illnesses they cause and the host immune response against them, before summarizing past and current efforts to develop vaccines to alleviate the burden caused by these emerging diseases. The development of the urgently needed vaccines is hampered by several factors including the unpredictable epidemiology, feasibility of rapid clinical trial implementation during outbreaks and regulatory pathways. We will give an overview of the current developments.

A SARS-CoV-2 Vaccination Strategy Focused on Population-Scale Immunity

Here we propose a vaccination strategy for SARS-CoV-2 based on identification of both highly conserved regions of the virus and newly acquired adaptations that are presented by MHC class I and II across the vast majority of the population, are highly dissimilar from the human proteome, and are predicted B cell epitopes. We present 65 peptide sequences that we expect to result in a safe and effective vaccine which can be rapidly tested in DNA, mRNA, or synthetic peptide constructs. These include epitopes that are contained within evolutionarily divergent regions of the spike protein reported to increase infectivity through increased binding to the ACE2 receptor, and within a novel furin cleavage site thought to increase membrane fusion. This vaccination strategy specifically targets unique vulnerabilities of SARS-CoV-2 and should engage a robust adaptive immune response in the vast majority of the human population.

Homologous prime-boost with Zika virus envelope protein and poly (I:C) induces robust specific humoral and cellular immune responses

The recent outbreaks of Zika virus (ZIKV) infection and the potential association with Guillain-Barré syndrome in adults and with congenital abnormalities have highlighted the urgency for an effective vaccine. The ZIKV Envelope glycoprotein (E_ZIKV) is the most abundant protein on the virus surface, and has been evaluated together with the pre-membrane protein (prM) of the viral coat as a vaccine candidate in clinical trials. In this study, we performed a head-to-head comparison of the immune response induced by different E_ZIKV-based vaccine candidates in mice. We compared different platforms (DNA, recombinant protein), adjuvants (poly (I:C), CpG ODN 1826) and immunization strategies (homologous, heterologous). The hierarchy of adjuvant potency showed that poly (I:C) was a superior adjuvant than CpG ODN. While poly (I:C) assisted immunization reached a plateau in antibody titers after two doses, the CpG ODN group required an extra immunization dose. Besides, the administration of poly (I:C) induced higher E_ZIKV-specific cellular immune responses than CpG ODN. We also show that immunization with homologous prime-boost E_ZIKV protein + poly (I:C) regimen induced a more robust humoral response than homologous DNA (pVAX-E_ZIKV) or heterologous regimens (DNA/protein or protein/DNA). A detailed analysis of cellular immune responses revealed that homologous (E_ZIKV + poly (I:C)) and heterologous (pVAX-E_ZIKV/E_ZIKV + poly (I:C)) prime-boost regimens induced the highest magnitude of IFN-γ secreting cells and cytokine-producing CD4⁺ T cells. Overall, our data demonstrate that homologous E_ZIKV + poly (I:C) prime-boost immunization is sufficient to induce more robust specific-E_ZIKV humoral and cellular immune responses than the other strategies that contemplate homologous DNA (pVAX-E_ZIKV) or heterologous (pVAX-E_ZIKV/E_ZIKV + poly (I:C), and vice-versa) immunizations.

In Vivo Assembly of Nanoparticles Achieved through Synergy of Structure-Based Protein Engineering and Synthetic DNA Generates Enhanced Adaptive Immunity

Nanotechnologies are considered to be of growing importance to the vaccine field. Through decoration of immunogens on multivalent nanoparticles, designed nanovaccines can elicit improved humoral immunity. However, significant practical and monetary challenges in large-scale production of nanovaccines have impeded their widespread clinical translation. Here, an alternative approach is illustrated integrating computational protein modeling and adaptive electroporation-mediated synthetic DNA delivery, thus enabling direct in vivo production of nanovaccines. DNA-launched nanoparticles are demonstrated displaying an HIV immunogen spontaneously self-assembled in vivo. DNA-launched nanovaccines induce stronger humoral responses than their monomeric counterparts in both mice and guinea pigs, and uniquely elicit CD8+ effector T-cell immunity as compared to recombinant protein nanovaccines. Improvements in vaccine responses recapitulate when DNA-launched nanovaccines with alternative scaffolds and decorated antigen are designed and evaluated. Finally, evaluation of functional immune responses induced by DLnanovaccines demonstrates that, in comparison to control mice or mice immunized with DNA-encoded hemagglutinin monomer, mice immunized with a DNA-launched hemagglutinin nanoparticle vaccine fully survive a lethal influenza challenge, and have substantially lower viral load, weight loss, and influenza-induced lung pathology. Additional study of these next-generation in vivo-produced nanovaccines may offer advantages for immunization against multiple disease targets.

Two Is Better Than One: Evidence for T-Cell Cross-Protection Between Dengue and Zika and Implications on Vaccine Design

Dengue virus (DENV, family Flaviviridae, genus Flavivirus) exists as four distinct serotypes. Generally, immunity after infection with one serotype is protective and lifelong, though exceptions have been described. However, secondary infection with a different serotype can result in more severe disease for a minority of patients. Host responses to the first DENV infection involve the development of both cross-reactive antibody and T cell responses, which, depending upon their precise balance, may mediate protection or enhance disease upon secondary infection with a different serotype. Abundant evidence now exists that responses elicited by DENV infection can cross-react with other members of the genus Flavivirus, particularly Zika virus (ZIKV). Cohort studies have shown that prior DENV immunity is associated with protection against Zika. Cross-reactive antibody responses may enhance infection with flaviviruses, which likely accounts for the cases of severe disease seen during secondary DENV infections. Data for T cell responses are contradictory, and even though cross-reactive T cell responses exist, their clinical significance is uncertain. Recent mouse experiments, however, show that cross-reactive T cells are capable of mediating protection against ZIKV. In this review, we summarize and discuss the evidence that T cell responses may, at least in part, explain the cross-protection seen against ZIKV from DENV infection, and that T cell antigens should therefore be included in putative Zika vaccines.

A combination of two human monoclonal antibodies limits fetal damage by Zika virus in macaques

Zika virus (ZIKV) infection during pregnancy can cause fetal abnormalities. Vaccines against ZIKV are under development, but because of potential safety concerns due to disease-enhancing antibodies, and the time required by active immunization to induce protective antibodies, there is a need to explore alternative strategies. Recombinant monoclonal antibodies can be modified to prevent enhancement of infection, and thus could be an efficacious and safe alternative to vaccines to confer rapid protection. We show that prophylactic administration of two engineered antibodies, Z004 and Z021, to pregnant macaques partially protects against fetal neurologic damage and limits vertical transmission of ZIKV.

Human infection by Zika virus (ZIKV) during pregnancy can lead to vertical transmission and fetal aberrations, including microcephaly. Prophylactic administration of antibodies can diminish or prevent ZIKV infection in animal models, but whether passive immunization can protect nonhuman primates and their fetuses during pregnancy has not been determined. Z004 and Z021 are neutralizing monoclonal antibodies to domain III of the envelope (EDIII) of ZIKV. Together the two antibodies protect nonpregnant macaques against infection even after Fc modifications to prevent antibody-dependent enhancement (ADE) in vitro and extend their half-lives. Here we report on prophylactic coadministration of the Fc-modified antibodies to pregnant rhesus macaques challenged three times with ZIKV during first and second trimester. The two antibodies did not entirely eliminate maternal viremia but limited vertical transmission, protecting the fetus from neurologic damage. Thus, maternal passive immunization with two antibodies to EDIII can shield primate fetuses from the harmful effects of ZIKV.

An optimized purified inactivated Zika vaccine provides sustained immunogenicity and protection in cynomolgus macaques

The recent spread of Zika virus (ZIKV) through the Americas and Caribbean and its devastating consequences for pregnant women and their babies have driven the search for a safe and efficacious ZIKV vaccine. Among the vaccine candidates, a first-generation ZIKV purified inactivated vaccine (ZPIV), adjuvanted with aluminum hydroxide, developed by the Walter Reed Army Institute of Research (WRAIR), has elicited high seroconversion rates in participants in three phase-I clinical trials. In collaboration with the WRAIR, Sanofi Pasteur (SP) optimized the production scale, culture and purification conditions, and increased the regulatory compliance, both of which are critical for clinical development and licensure of this vaccine. Using a clinical batch of the first-generation ZPIV as a benchmark, we report that different doses of the optimized vaccine (ZPIV-SP) elicited sustained neutralizing antibodies, specific T- and memory B-cells, and provided complete protection against a ZIKV challenge in cynomolgus macaques. These data provide evidence that the ZPIV-SP vaccine performs at least as well as the ZPIV vaccine, and provide support for continued development in the event of future ZIKV outbreaks.

Active Immunoprophylaxis and Vaccine Augmentations Mediated by a Novel Plasmid DNA Formulation

Plasmid DNA (pDNA) gene delivery is a highly versatile technology that has the potential to address a multitude of unmet medical needs. Advances in pDNA delivery to host tissue with the employment of in vivo electroporation (EP) have led to significantly enhanced gene expression and the recent demonstration of clinical efficacy with the platform. Building upon this platform, this study reports that enzyme-mediated modification of the muscle tissue extracellular matrix structure at the site of pDNA delivery operates in a synergistic manner with EP to enhance both local and systemic gene expression further. Specifically, administration of chondroitinase ABC (Cho ABC) to the site of intramuscular delivery of pDNA led to transient disruption of chondroitin sulfate scaffolding barrier, permitting enhanced gene distribution and expression across the tissue. The employment of Cho ABC in combination with CELLECTRA^® intramuscular EP resulted in increased gene expression by 5.5-fold in mice and 17.98-fold in rabbits. The study demonstrates how this protocol can be universally applied to an active prophylaxis platform to increase the in vivo production of functional immunoglobulin G, and to DNA vaccine protocols to permit drug dose sparing. The data indicate the Cho ABC formulation to be of significant value upon combination with EP to drive enhanced gene expression levels in pDNA delivery protocols.

Characterization of a Species E Adenovirus Vector as a Zika virus vaccine

The development of a safe and efficacious Zika virus (ZIKV) vaccine remains a global health priority. In our previous work, we developed an Adenovirus vectored ZIKV vaccine using a low-seroprevalent human Adenovirus type 4 (Ad4-prM-E) and compared it to an Ad5 vector (Ad5-prM-E). We found that vaccination with Ad4-prM-E leads to the development of a strong anti-ZIKV T-cell response without eliciting significant anti-ZIKV antibodies, while vaccination with Ad5-prM-E leads to the development of both anti-ZIKV antibody and T-cell responses in C57BL/6 mice. However, both vectors conferred protection against ZIKV infection in a lethal challenge model. Here we continued to characterize the T-cell biased immune response observed in Ad4 immunized mice. Vaccination of BALB/c mice resulted in immune correlates similar to C57BL/6 mice, confirming that this response is not mouse strain-specific. Vaccination with an Ad4 expressing an influenza hemagglutinin (HA) protein resulted in anti-HA T-cell responses without the development of significant anti-HA antibodies, indicating this unique response is specific to the Ad4 serotype rather than the transgene expressed. Co-administration of a UV inactivated Ad4 vector with the Ad5-prM-E vaccine led to a significant reduction in anti-ZIKV antibody development suggesting that this serotype-specific immune profile is capsid-dependent. These results highlight the serotype-specific immune profiles elicited by different Adenovirus vector types and emphasize the importance of continued characterization of these alternative Ad serotypes.

Complete Protection in Macaques Conferred by Purified Inactivated Zika Vaccine: Defining a Correlate of Protection

A critical global health need exists for a Zika vaccine capable of mitigating the effects of future Zika epidemics. In this study we evaluated the antibody responses and efficacy of an aluminum hydroxide adjuvanted purified inactivated Zika vaccine (PIZV) against challenge with Zika virus (ZIKV) strain PRVABC59. Indian rhesus macaques received two doses of PIZV at varying concentrations ranging from 0.016 µg - 10 µg and were subsequently challenged with ZIKV six weeks or one year following the second immunization. PIZV induced a dose-dependent immune response that was boosted by a second immunization. Complete protection against ZIKV infection was achieved with the higher PIZV doses of 0.4 µg, 2 µg, and 10 µg at 6 weeks and  with 10 ug PIZV at  1 year following vaccination. Partial protection was achieved with the lower PIZV doses of 0.016 µg and 0.08 µg. Based on these data, a neutralizing antibody response above 3.02 log10 EC50 was determined as a correlate of protection in macaques. PIZV elicited a dose-dependent neutralizing antibody response which is protective for at least 1 year following vaccination.

A high infectious simian adenovirus type 23 vector based vaccine efficiently protects common marmosets against Zika virus infection

Zika virus (ZIKV) has spread in many countries or territories causing severe neurologic complications with potential fatal outcomes. The small primate common marmosets are susceptible to ZIKV, mimicking key features of human infection. Here, a novel simian adenovirus type 23 vector-based vaccine expressing ZIKV pre-membrane-envelope proteins (Sad23L-prM-E) was produced in high infectious titer. Due to determination of immunogenicity in mice, a single-dose of 3×10⁸ PFU Sad23L-prM-E vaccine was intramuscularly inoculated to marmosets. This vaccine raised antibody titers of 10^4.07 E-specific and 10^3.13 neutralizing antibody (NAb), as well as robust specific IFN-γ secreting T-cell response (1,219 SFCs/10⁶ cells) to E peptides. The vaccinated marmosets, upon challenge with a high dose of ZIKV (10⁵ PFU) six weeks post prime immunization, reduced viremia by more than 100 folds, and the low level of detectable viral RNA (<10³ copies/ml) in blood, saliva, urine and feces was promptly eliminated when the secondary NAb (titer >10^3.66) and T-cell response (>726 SFCs/10⁶ PBMCs) were acquired 1–2 weeks post exposure to ZIKV, while non-vaccinated control marmosets developed long-term high titer of ZIKV (10^5.73 copies/ml) (P<0.05). No significant pathological lesions were observed in marmoset tissues. Sad23L-prM-E vaccine was detectable in spleen, liver and PBMCs at least 4 months post challenge. In conclusion, a prime immunization with Sad23L-prM-E vaccine was able to protect marmosets against ZIKV infection when exposed to a high dose of ZIKV. This Sad23L-prM-E vaccine is a promising vaccine candidate for prevention of ZIKV infection in humans.

Zika virus (ZIKV) is a member of the Flaviviridaefamily) and causes severe neurologic diseases. The development of safe and effective vaccine is urgent need. In this study, we constructed a novel simian adenovirus type 23 vector-based vaccine expressing ZIKV pre-membrane-envelope proteins (Sad23L-prM-E). By vaccinating the common marmosets with prime immunization of vaccine, and upon challenge with a high dose of ZIKV to the vaccinated marmosets, the immune response and protective efficacy of vaccine were extensively evaluated. The data suggested that Sad23L-prM-E vaccine could protect marmosets against a high dose of ZIKV challenge, which provided a promising vaccine for preventing ZIKV infection in humans.

DNA vaccination before conception protects Zika virus-exposed pregnant macaques against prolonged viremia and improves fetal outcomes

Zika virus (ZIKV) infection of pregnant women is associated with congenital Zika syndrome (CZS) and no vaccine is available, although several are being tested in clinical trials. We tested the efficacy of ZIKV DNA vaccine VRC5283 in a rhesus macaque model of congenital ZIKV infection. Most animal vaccine experiments have a set pathogen exposure several weeks or months after vaccination. In the real world, people encounter pathogens years or decades after vaccination, or may be repeatedly exposed if the virus is endemic. To more accurately mimic how this vaccine would be used, we immunized macaques before conception and then exposed them repeatedly to ZIKV during early and mid-gestation. In comparison to unimmunized animals, vaccinated animals had a significant reduction in peak magnitude and duration of maternal viremia, early fetal loss, fetal infection, and placental and fetal brain pathology. Vaccine-induced neutralizing antibody titers on the day of first ZIKV exposure were negatively associated with the magnitude of maternal viremia, and the absence of prolonged viremia was associated with better fetal outcomes. These data support further clinical development of ZIKV vaccine strategies to protect against negative fetal outcomes.

Simian Immunodeficiency Virus Infection of Rhesus Macaques Results in Delayed Zika Virus Clearance

Flaviviruses are controlled by adaptive immune responses but are exquisitely sensitive to interferon-stimulated genes (ISGs). How coinfections, particularly simian immunodeficiency viruses (SIVs), that induce robust ISG signatures influence flavivirus clearance and pathogenesis is unclear. Here, we studied how Zika virus (ZIKV) infection is modulated in SIV-infected nonhuman primates. We measured ZIKV replication, cellular ZIKV RNA levels, and immune responses in non-SIV-infected and SIV-infected rhesus macaques (RMs), which we infected with ZIKV. Coinfected animals had a 1- to 2-day delay in peak ZIKV viremia, which was 30% of that in non-SIV-infected animals. However, ZIKV viremia was significantly prolonged in SIV-positive (SIV⁺) RMs. ISG levels at the time of ZIKV infection were predictive for lower ZIKV viremia in the SIV⁺ RMs, while prolonged ZIKV viremia was associated with muted and delayed adaptive responses in SIV⁺ RMs.IMPORTANCE Immunocompromised individuals often become symptomatic with infections which are normally fairly asymptomatic in healthy individuals. The particular mechanisms that underlie susceptibility to coinfections in human immunodeficiency virus (HIV)-infected individuals are multifaceted. ZIKV and other flaviviruses are sensitive to neutralizing antibodies, whose production can be limited in HIV-infected individuals but are also sensitive to type I interferons, which are expressed at high levels in HIV-infected individuals. Data in this study highlight how individual components of the innate and adaptive immune responses which become perturbed in HIV-infected individuals influence ZIKV infection.

An mRNA Vaccine Protects Mice against Multiple Tick-Transmitted Flavivirus Infections

Powassan virus (POWV) is an emerging tick-transmitted flavivirus that circulates in North America and Russia. Up to 5% of deer ticks now test positive for POWV in certain regions of the northern United States. Although POWV infections cause life-threatening encephalitis, there is no vaccine or countermeasure available for prevention or treatment. Here, we developed a lipid nanoparticle (LNP)-encapsulated modified mRNA vaccine encoding the POWV prM and E genes and demonstrated its immunogenicity and efficacy in mice following immunization with one or two doses. The POWV mRNA vaccine induced high titers of neutralizing antibody and sterilizing immunity against lethal challenge with different POWV strains. The mRNA vaccine also induced cross-neutralizing antibodies against multiple other tick-borne flaviviruses and protected mice against the distantly related Langat virus. These data demonstrate the utility of the LNP-mRNA vaccine platform for the development of vaccines with protective activity against multiple flaviviruses.

Zika Vaccine Development: Current Status

Zika virus outbreaks have been explosive and unpredictable and have led to significant adverse health effects-as well as considerable public anxiety. Significant scientific work has resulted in multiple candidate vaccines that are now undergoing further clinical development, with several vaccines now in phase 2 clinical trials. In this review, we survey current vaccine efforts, preclinical and clinical results, and ethical and other concerns that directly bear on vaccine development. It is clear that the world needs safe and effective vaccines to protect against Zika virus infection. Whether such vaccines can be developed through to licensure and public availability absent significant financial investment by countries, and other barriers discussed within this article, remains uncertain.

An attenuated Zika virus NS4B protein mutant is a potent inducer of antiviral immune responses

Live attenuated vaccines (LAVs) are one of the most important strategies to control flavivirus diseases. The flavivirus nonstructural (NS) 4B proteins are a critical component of both the virus replication complex and evasion of host innate immunity. Here we have used site-directed mutagenesis of residues in the highly conserved N-terminal and central hydrophobic regions of Zika virus (ZIKV) NS4B protein to identify candidate attenuating mutations. Three single-site mutants were generated, of which the NS4B-C100S mutant was more attenuated than the other two mutants (NS4B-C100A and NS4B-P36A) in two immunocompromised mouse models of fatal ZIKV disease. The ZIKV NS4B-C100S mutant triggered stronger type 1 interferons and interleukin-6 production, and higher ZIKV-specific CD4+ and CD8+ T-cell responses, but induced similar titers of neutralization antibodies compared with the parent wild-type ZIKV strain and a previously reported candidate ZIKV LAV with a 10-nucleotide deletion in 3'-UTR (ZIKV-3'UTR-Δ10). Vaccination with ZIKV NS4B-C100S protected mice from subsequent WT ZIKV challenge. Furthermore, either passive immunization with ZIKV NS4B-C100S immune sera or active immunization with ZIKV NS4B-C100S followed by the depletion of T cells affords full protection from lethal WT ZIKV challenge. In summary, our results suggest that the ZIKV NS4B-C100S mutant may serve as a candidate ZIKV LAV due to its attenuated phenotype and high immunogenicity.

Current Challenges in the Development of Vaccines and Drugs Against Emerging Vector-borne Diseases

Vectors are living organisms that transmit infectious diseases from an infected animal to humans or another animal. Biological vectors such as mosquitoes, ticks, and sand flies carry pathogens that multiply within their bodies prior to delivery to a new host. The increased prevalence of Vector-Borne Diseases (VBDs) such as Aedes-borne dengue, Chikungunya (CHIKV), Zika (ZIKV), malaria, Tick-Borne Disease (TBD), and scrub typhus has a huge impact on the health of both humans and livestock worldwide. In particular, zoonotic diseases transmitted by mosquitoes and ticks place a considerable burden on public health. Vaccines, drugs, and vector control methods have been developed to prevent and treat VBDs and have prevented millions of deaths. However, development of such strategies is falling behind the rapid emergence of VBDs. Therefore, a comprehensive approach to fighting VBDs must be considered immediately. In this review, I focus on the challenges posed by emerging outbreaks of VBDs and discuss available drugs and vaccines designed to overcome this burden. Research into promising drugs needs to be upgraded and fast-tracked, and novel drugs or vaccines being tested in in vitro and in vivo models need to be moved into human clinical trials. Active preventive tactics, as well as new and upgraded diagnostics, surveillance, treatments, and vaccination strategies, need to be monitored constantly if we are to manage VBDs of medical importance.

Capsid containing virus like particle vaccine against Zika virus made from a stable cell line

Zika virus infection during pregnancy is associated with severe birth defects including microcephaly in the new born. The lack of specific treatment calls for the development of a safe and effective vaccine for use in pregnant women. We recently tested the efficacy of a Virus Like Particle (VLP) vaccine for Zika virus in mice and found that Capsid-preMembrane-Env (CprME) VLPs generated a better neutralizing antibody response than preMembrane-Env (prME) VLPs. The superiority of CprME VLPs suggested that inclusion of capsid in the vaccine may enhance the immune response. However, production of CprME VLPs requires co-expression of NS2B-3 protease, which creates a major hurdle for generation of stable cell lines. To overcome this limitation, we generated a bicistronic vector that expresses CprME and NS2B-3 using an IRES sequence. This bicistronic expression cassette, in a lentiviral vector, was used to create a stable cell line that constitutively secretes CprME VLPs. The expression of NS2B-3, presence of capsid in the secreted VLPs, efficiency of VLP release, and stability of the cell line was extensively tested. Antigen sparing studies in mice using prME and CprME VLPs, both derived from stable cell lines, confirmed the superiority of CprME VLPs in generation of neutralizing antibody response. Capsid specific antibodies were detected in CprME VLP immunized mice providing mechanistic insights into the superiority of these VLPs. Challenge of CprME VLP immunized mice with Zika PRVABC59 showed complete protection against day 3 viremia further validating the efficacy of the vaccine. Our study is the first to generate a stable cell line secreting Zika CprME VLPs via natural NS2B-3 cleavage, demonstrate incorporation of capsid in CprME VLPs and complete protection in challenge studies. This is a major advancement for the Zika vaccine platform that is safe for use in pregnant women and readily scalable for use in developing countries.

Protective Efficacy of Nucleic Acid Vaccines Against Transmission of Zika Virus During Pregnancy in Mice

Zika virus (ZIKV) caused an epidemic of congenital malformations in 2015-2016. Although many vaccine candidates have been generated, few have demonstrated efficacy against congenital ZIKV infection. Here, we evaluated lipid-encapsulated messenger RNA (mRNA) vaccines and a DNA plasmid vaccine encoding the prM-E genes of ZIKV in mouse models of congenital infection. Although the DNA vaccine provided comparable efficacy against vertical transmission of ZIKV, the mRNA vaccines, including one that minimizes antibody-dependent enhancement of infection, elicited higher levels of antigen-specific long-lived plasma cells and memory B cells. Despite the induction of robust neutralizing antibody titers by all vaccines, breakthrough seeding of the placenta and fetal head was observed in a small subset of type I interferon signaling-deficient immunocompromised dams. In comparison, evaluation of one of the mRNA vaccines in a human STAT2-knockin transgenic immunocompetent mouse showed complete protection against congenital ZIKV transmission. These data will inform ongoing human ZIKV vaccine development efforts and enhance our understanding of the correlates of vaccine-induced protection.

A review of Zika virus: hurdles toward vaccine development and the way forward

The Zika virus (ZIKV) epidemic has recently emerged as a public health threat due to its teratogenic nature and association with the serious neurological condition Guillain-Barré syndrome (GBS). To date, no approved antiviral therapeutics to treat, nor vaccines to prevent, ZIKV infection are available. In order to develop effective anti-ZIKV vaccines, improved animal models and a better understanding of immunological correlates of protection against ZIKV are required. In this paper, we discuss the recent progress in developing vaccines against ZIKV and the hurdles to overcome in making efficacious anti-ZIKV vaccines. Here, we propose strategies to make efficacious and safe vaccines against ZIKV by using novel approaches including molecular attenuation of viruses and TLR-based nanoparticle vaccines. The question of exacerbating dengue virus infection or causing GBS through the production of cross-reactive immunity targeting viral or host proteins have been addressed in this paper. Challenges in implementing immunogenic and protective ZIKV vaccine trials in immunodepressed target populations (for example, pregnant women) have also been discussed.

Immunization With a Novel Human Type 5 Adenovirus-Vectored Vaccine Expressing the Premembrane and Envelope Proteins of Zika Virus Provides Consistent and Sterilizing Protection in Multiple Immunocompetent and Immunocompromised Animal Models

Background

Zika virus (ZIKV) infection may be associated with severe complications and disseminated via both vector-borne and nonvector-borne routes. Adenovirus-vectored vaccines represent a favorable controlling measure for the ZIKV epidemic because they have been shown to be safe, immunogenic, and rapidly generable for other emerging viral infections. Evaluations of 2 previously reported adenovirus-vectored ZIKV vaccines were performed using nonlethal animal models and/or nonepidemic ZIKV strain.

Methods

We constructed 2 novel human adenovirus 5 (Ad5)-vectored vaccines containing the ZIKV premembrane-envelope (Ad5-Sig-prM-Env) and envelope (Ad5-Env) proteins, respectively, and evaluated them in multiple nonlethal and lethal animal models using epidemic ZIKV strains.

Results

Both vaccines elicited robust humoral and cellular immune responses in immunocompetent BALB/c mice. Dexamethasone-immunosuppressed mice vaccinated with either vaccine demonstrated robust and durable antibody responses and significantly lower blood and tissue viral loads than controls (P < .05). Similar findings were also observed in interferon-α/β receptor-deficient A129 mice. In both of these immunocompromised animal models, Ad5-Sig-prM-Env-vaccinated mice had significantly (P < .05) higher titers of anti-ZIKV-specific neutralizing antibody titers and lower (undetectable) viral loads than Ad5-Env-vaccinated mice. The close correlation between the neutralizing antibody titer and viral load helped to explain the better protective effect of Ad5-Sig-prM-Env than Ad5-Env. Anamnestic response was absent in Ad5-Sig-prM-Env-vaccinated A129 mice.

Conclusions

Ad5-Sig-prM-Env provided sterilizing protection against ZIKV infection in mice.

An Attenuated Zika Virus Encoding Non-Glycosylated Envelope (E) and Non-Structural Protein 1 (NS1) Confers Complete Protection against Lethal Challenge in a Mouse Model

Zika virus (ZIKV), a mosquito-transmitted flavivirus, emerged in the last decade causing serious human diseases, including congenital microcephaly in newborns and Guillain-Barré syndrome in adults. Although many vaccine platforms are at various stages of development, no licensed vaccines are currently available. Previously, we described a mutant MR766 ZIKV (m2MR) bearing an E protein mutation (N154A) that prevented its glycosylation, resulting in attenuation and defective neuroinvasion. To further attenuate m2MR for its potential use as a live viral vaccine, we incorporated additional mutations into m2MR by substituting the asparagine residues in the glycosylation sites (N130 and N207) of NS1 with alanine residues. Examination of pathogenic properties revealed that the virus (m5MR) carrying mutations in E (N154A) and NS1 (N130A and N207A) was fully attenuated with no disease signs in infected mice, inducing high levels of humoral and cell-mediated immune responses, and protecting mice from subsequent lethal virus challenge. Furthermore, passive transfer of sera from m5MR-infected mice into naïve animals resulted in complete protection from lethal challenge. The immune sera from m5MR-infected animals neutralized both African and Asian lineage viruses equally well, suggesting that m5MR virus could be developed as a potentially broad live virus vaccine candidate.