Vaccine Mediated Protection Against Zika Virus-Induced Congenital Disease

CpG-Recoding in Zika Virus Genome Causes Host-Age-Dependent Attenuation of Infection With Protection Against Lethal Heterologous Challenge in Mice

Experimental increase of CpG dinucleotides in an RNA virus genome impairs infection providing a promising approach for vaccine development. While CpG recoding is an emerging and promising vaccine approach, little is known about infection phenotypes caused by recoded viruses in vivo. For example, infection phenotypes, immunogenicity, and protective efficacy induced by CpG-recoded viruses in different age groups were not studied yet. This is important, because attenuation of infection phenotypes caused by recoded viruses may depend on the population-based expression of cellular components targeting viral CpG dinucleotides. In the present study, we generated several Zika virus (ZIKV) variants with the increasing CpG content and compared infection in neonatal and adult mice. Increasing the CpG content caused host-age-dependent attenuation of infection with considerable attenuation in neonates and high attenuation in adults. Expression of the zinc-finger antiviral protein (ZAP)—the host protein targeting viral CpG dinucleotides—was also age-dependent. Similar to the wild-type virus, ZIKV variants with the increased CpG content evoked robust cellular and humoral immune responses and protection against lethal challenge. Collectively, the host age should be accounted for in future studies on mechanisms targeting viral CpG dinucleotides, development of safe dinucleotide recoding strategies, and applications of CpG-recoded vaccines.

Cell-Mediated Immunity Generated in Response to a Purified Inactivated Vaccine for Dengue Virus Type 1

Dengue is a tropical disease transmitted by mosquitoes, and nearly half of the world’s population lives in areas where individuals are at risk of infection. Several vaccines for dengue are in development, including one which was recently licensed in several countries, although its utility is limited to people who have already been infected with one of the four dengue viruses. One major hurdle to understanding whether a dengue vaccine will work for everyone—before exposure—is the necessity of knowing which marker can be measured in the blood to signal that the individual has protective immunity. This report describes an approach measuring multiple different parts of immunity in order to characterize which signals one candidate vaccine imparted to a small number of human volunteers. This approach was designed to be able to be applied to any dengue vaccine study so that the data can be compared and used to inform future vaccine design and/or optimization strategies.

Dengue is the most prevalent arboviral disease afflicting humans, and a vaccine appears to be the most rational means of control. Dengue vaccine development is in a critical phase, with the first vaccine licensed in some countries where dengue is endemic but demonstrating insufficient efficacy in immunologically naive populations. Since virus-neutralizing antibodies do not invariably correlate with vaccine efficacy, other markers that may predict protection, including cell-mediated immunity, are urgently needed. Previously, the Walter Reed Army Institute of Research developed a monovalent purified inactivated virus (PIV) vaccine candidate against dengue virus serotype 1 (DENV-1) adjuvanted with alum. The PIV vaccine was safe and immunogenic in a phase I dose escalation trial in healthy, flavivirus-naive adults in the United States. From that trial, peripheral blood mononuclear cells obtained at various time points pre- and postvaccination were used to measure DENV-1-specific T cell responses. After vaccination, a predominant CD4⁺ T cell-mediated response to peptide pools covering the DENV-1 structural proteins was observed. Over half (13/20) of the subjects produced interleukin-2 (IL-2) in response to DENV peptides, and the majority (17/20) demonstrated peptide-specific CD4⁺ T cell proliferation. In addition, analysis of postvaccination cell culture supernatants demonstrated an increased rate of production of cytokines, including gamma interferon (IFN-γ), IL-5, and granulocyte-macrophage colony-stimulating factor (GM-CSF). Overall, the vaccine was found to have elicited DENV-specific CD4⁺ T cell responses as measured by enzyme-linked immunosorbent spot (ELISpot), intracellular cytokine staining (ICS), lymphocyte proliferation, and cytokine production assays. Thus, together with antibody readouts, the use of a multifaceted measurement of cell-mediated immune responses after vaccination is a useful strategy for more comprehensively characterizing immunity generated by dengue vaccines.

IMPORTANCE Dengue is a tropical disease transmitted by mosquitoes, and nearly half of the world’s population lives in areas where individuals are at risk of infection. Several vaccines for dengue are in development, including one which was recently licensed in several countries, although its utility is limited to people who have already been infected with one of the four dengue viruses. One major hurdle to understanding whether a dengue vaccine will work for everyone—before exposure—is the necessity of knowing which marker can be measured in the blood to signal that the individual has protective immunity. This report describes an approach measuring multiple different parts of immunity in order to characterize which signals one candidate vaccine imparted to a small number of human volunteers. This approach was designed to be able to be applied to any dengue vaccine study so that the data can be compared and used to inform future vaccine design and/or optimization strategies.

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.

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.

Adenovirus Vector-Based Vaccines Confer Maternal-Fetal Protection against Zika Virus Challenge in Pregnant IFN-αβR-/- Mice

Maternal infection with Zika virus (ZIKV) can lead to microcephaly and other congenital abnormalities of the fetus. Although ZIKV vaccines that prevent or reduce viremia in non-pregnant mice have been described, a maternal vaccine that provides complete fetal protection would be desirable. Here, we show that adenovirus (Ad) vector-based ZIKV vaccines induce potent neutralizing antibodies that confer robust maternal and fetal protection against ZIKV challenge in pregnant, highly susceptible IFN-αβR^-/- mice. Moreover, passive transfer of maternal antibodies from vaccinated dams protected pups against post-natal ZIKV challenge. These data suggest that Ad-based ZIKV vaccines may be able to provide protection in pregnant females against fetal ZIKV transmission in utero as well as in infants against ZIKV infection after birth.

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.

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.

Genetic stability of live-attenuated Zika vaccine candidates

Zika virus (ZIKV) has drawn global attention as the etiologic agent of Zika Congenital Syndrome in babies born to infected pregnant women. To prevent future ZIKV outbreaks and protect persons at risk for severe disease, we developed two live-attenuated vaccine (LAV) candidates containing 10- or 20-nucleotide deletions in the 3'UTR of the viral genome (Δ10 and Δ20). After a single-dose immunization, both Δ10 and Δ20 LAVs protected mice and non-human primates against ZIKV infection. Here, we characterized the stability, safety, and efficacy of the LAVs after continuously culturing them on manufacture Vero cells for ten rounds. Whole genome sequencing showed that passage 10 (P10) LAVs retained the engineered Δ10 and Δ20 deletions; one to four additional mutations emerged at different regions of the genome. In A129 mice, the P10 LAVs exhibited viremia higher than the un-passaged LAVs, but lower than wild-type ZIKV; unlike wild-type ZIKV-infected mice, none of the P10 LAV-infected mice developed disease or death, demonstrating that the P10 LAVs remained attenuated. Mice immunized with a single dose of the P10 LAVs developed robust neutralizing antibody titers (1/1,000 to 1/10,000) and were protected against epidemic ZIKV challenge. The P10 LAVs did not exhibit increased neurovirulence. Intracranial inoculation of one-day-old CD1 pups with 10³ focus-forming units of the P10 Δ10 and Δ20 LAVs resulted in 100% and ≥80% survival, respectively. Furthermore, the P10 LAVs remained incompetent in infecting Aedes aegypti mosquitoes after intrathoracic microinjection. Our results support the phenotypic stability and further development of these promising LAVs for ZIKV.

A Gorilla Adenovirus-Based Vaccine against Zika Virus Induces Durable Immunity and Confers Protection in Pregnancy

The teratogenic potential of Zika virus (ZIKV) has made the development of an effective vaccine a global health priority. Here, we generate two gorilla adenovirus-based ZIKV vaccines that encode for pre-membrane (prM) and envelope (E) proteins (GAd-Zvp) or prM and the ectodomain of E protein (GAd-Eecto). Both vaccines induce humoral and cell-mediated immune responses and prevent lethality after ZIKV challenge in mice. Protection is antibody dependent, CD8⁺ T cell independent, and for GAd-Eecto requires the complement component C1q. Immunization of GAd-Zvp induces antibodies against a key neutralizing epitope on domain III of E protein and confers durable protection as evidenced by memory B and long-lived plasma cell responses and challenge studies 9 months later. In two models of ZIKV infection during pregnancy, GAd-Zvp prevents maternal-to-fetal transmission. The gorilla adenovirus-based vaccine platform encoding full-length prM and E genes is a promising candidate for preventing congenital ZIKV syndrome and possibly infection by other flaviviruses.

Therapeutic Advances Against ZIKV: A Quick Response, a Long Way to Go

Zika virus (ZIKV) is a mosquito-borne flavivirus that spread throughout the American continent in 2015 causing considerable worldwide social and health alarm due to its association with ocular lesions and microcephaly in newborns, and Guillain-Barré syndrome (GBS) cases in adults. Nowadays, no licensed vaccines or antivirals are available against ZIKV, and thus, in this very short time, the scientific community has conducted enormous efforts to develop vaccines and antivirals. So that, different platforms (purified inactivated and live attenuated viruses, DNA and RNA nucleic acid based candidates, virus-like particles, subunit elements, and recombinant viruses) have been evaluated as vaccine candidates. Overall, these vaccines have shown the induction of vigorous humoral and cellular responses, the decrease of viremia and viral RNA levels in natural target organs, the prevention of vertical and sexual transmission, as well as that of ZIKV-associated malformations, and the protection of experimental animal models. Some of these vaccine candidates have already been assayed in clinical trials. Likewise, the search for antivirals have also been the focus of recent investigations, with dozens of compounds tested in cell culture and a few in animal models. Both direct acting antivirals (DAAs), directed to viral structural proteins and enzymes, and host acting antivirals (HAAs), directed to cellular factors affecting all steps of the viral life cycle (binding, entry, fusion, transcription, translation, replication, maturation, and egress), have been evaluated. It is expected that this huge collaborative effort will produce affordable and effective therapeutic and prophylactic tools to combat ZIKV and other related still unknown or nowadays neglected flaviviruses. Here, a comprehensive overview of the advances made in the development of therapeutic measures against ZIKV and the questions that still have to be faced are summarized.

Attenuation of Zika Virus by Passage in Human HeLa Cells

Zika virus (ZIKV) is a mosquito-borne Flavivirus. Previous studies have shown that mosquito-transmitted flaviviruses, including yellow fever, Japanese encephalitis, and West Nile viruses, could be attenuated by serial passaging in human HeLa cells. Therefore, it was hypothesized that wild-type ZIKV would also be attenuated after HeLa cell passaging. A human isolate from the recent ZIKV epidemic was subjected to serial HeLa cell passaging, resulting in attenuated in vitro replication in both Vero and A549 cells. Additionally, infection of AG129 mice with 10 plaque forming units (pfu) of wild-type ZIKV led to viremia and mortality at 12 days, whereas infection with 10³ pfu of HeLa-passage 6 (P6) ZIKV led to lower viremia, significant delay in mortality (median survival: 23 days), and increased cytokine and chemokine responses. Genomic sequencing of HeLa-passaged virus identified two amino acid substitutions as early as HeLa-P3: pre-membrane E87K and nonstructural protein 1 R103K. Furthermore, both substitutions were present in virus harvested from HeLa-P6-infected animal tissue. Together, these data show that, similarly to other mosquito-borne flaviviruses, ZIKV is attenuated following passaging in HeLa cells. This strategy can be used to improve understanding of substitutions that contribute to attenuation of ZIKV and be applied to vaccine development across multiple platforms.

Gottesfeld-Hohler Memorial Foundation Zika Virus Think Tank Summary

At a think tank bringing together experts on fetal neuroimaging, obstetric infectious diseases, and public health, we discussed trends in all of these areas for Zika virus. There is a wide variety of imaging findings in affected fetuses, influenced by timing of infection and probably host factors. The resources for diagnosis and interventions also vary by location with the hardest hit areas often having the fewest resources. We identified potential areas for both research and clinical collaboration as the Zika virus epidemic continues to evolve.

Human 5' UTR design and variant effect prediction from a massively parallel translation assay

The ability to predict the impact of cis-regulatory sequences on gene expression would facilitate discovery in fundamental and applied biology. Here we combine polysome profiling of a library of 280,000 randomized 5' untranslated regions (UTRs) with deep learning to build a predictive model that relates human 5' UTR sequence to translation. Together with a genetic algorithm, we use the model to engineer new 5' UTRs that accurately direct specified levels of ribosome loading, providing the ability to tune sequences for optimal protein expression. We show that the same approach can be extended to chemically modified RNA, an important feature for applications in mRNA therapeutics and synthetic biology. We test 35,212 truncated human 5' UTRs and 3,577 naturally occurring variants and show that the model predicts ribosome loading of these sequences. Finally, we provide evidence of 45 single-nucleotide variants (SNVs) associated with human diseases that substantially change ribosome loading and thus may represent a molecular basis for disease.

Human Monoclonal Antibodies Potently Neutralize Zika Virus and Select for Escape Mutations on the Lateral Ridge of the Envelope Protein

The mosquito-borne Zika virus (ZIKV) has been causing epidemic outbreaks on a global scale. Virus infection can result in severe disease in humans, including microcephaly in newborns and Guillain-Barré syndrome in adults. Here, we characterized monoclonal antibodies isolated from a patient with an active Zika virus infection that potently neutralized virus infection in Vero cells at the nanogram-per-milliliter range. In addition, these antibodies enhanced internalization of virions into human leukemia K562 cells in vitro, indicating their possible ability to cause antibody-dependent enhancement of disease. Escape variants of the ZIKV MR766 strain to a potently neutralizing antibody, AC10, exhibited an amino acid substitution at residue S368 in the lateral ridge region of the envelope protein. Analysis of publicly availably ZIKV sequences revealed the S368 site to be conserved among the vast majority (97.6%) of circulating strains. We validated the importance of this residue by engineering a recombinant virus with an S368R point mutation that was unable to be fully neutralized by AC10. Four out of the 12 monoclonal antibodies tested were also unable to neutralize the virus with the S368R mutation, suggesting this region to be an important immunogenic epitope during human infection. Last, a time-of-addition infection assay further validated the escape variant and showed that all monoclonal antibodies inhibited virus binding to the cell surface. Thus, the present study demonstrates that the lateral ridge region of the envelope protein is likely an immunodominant, neutralizing epitope.IMPORTANCE Zika virus (ZIKV) is a global health threat causing severe disease in humans, including microcephaly in newborns and Guillain-Barré syndrome in adults. Here, we analyzed the human monoclonal antibody response to acute ZIKV infection and found that neutralizing antibodies could not elicit Fc-mediated immune effector functions but could potentiate antibody-dependent enhancement of disease. We further identified critical epitopes involved with neutralization by generating and characterizing escape variants by whole-genome sequencing. We demonstrate that the lateral ridge region, particularly the S368 amino acid site, is critical for neutralization by domain III-specific antibodies.

Update on the Animal Models and Underlying Mechanisms for ZIKV-Induced Microcephaly

The circulation of Zika virus (ZIKV) in nearly 80 countries and territories poses a significant global threat to public health. ZIKV is causally linked to severe developmental defects in the brain, recognized as congenital Zika syndrome (CZS), which includes microcephaly and other serious congenital neurological complications. Since the World Health Organization declared the ZIKV outbreak a public health emergency of international concern, remarkable progress has been made in the generation of different ZIKV infection animal models to gain insight into cellular targets and pathogenesis and to explore the associated underlying mechanisms. Here we focus on summarizing our current understanding of the effects of ZIKV on mammalian brain development in different developmental stages and discuss the potential underlying mechanisms of ZIKV-induced CZS, as well as future perspectives.

Why Should We Advocate Maternal Immunization?

Maternal vaccination provides a method for protecting the pregnant woman, fetus and neonate during a period when there is increased susceptibility to infectious diseases. A dynamic state of immune tolerance during pregnancy and the need to develop adaptive memory to a new foreign antigen-rich environment lead to windows of vulnerability to infection for the mother and neonate, respectively. Passive transfer of humoral immunity through the placenta and breast milk from the mother can bridge the gap in immunity for the neonate. Studies on boosting this natural process of antibody transfer have led to the recommendation for administering inactivated influenza, diphtheria, tetanus toxoid and acellular pertussis vaccines during pregnancy. Several new maternal vaccine candidates are on the horizon.

Assessing the utility of antivirals for preventing maternal-fetal transmission of zika virus in pregnant mice

Zika virus (ZIKV) infection during pregnancy has been associated with adverse outcomes and birth defects such as microcephaly in newborn children. Congenital malformations associated with ZIKV are believed to occur via direct infection of the fetus. Unfortunately, there are no licensed therapeutic or preventative tools to block maternal-fetal transmission of ZIKV. In this study, we developed a mouse model of ZIKV infection that specifically establishes vertical maternal-fetal transmission of ZIKV in 40-60% of fetuses when the dams acquire ZIKV infection during pregnancy. This mouse model somewhat mirrors the experience in humans at the peak of the epidemic in the Americas. Using this model, we demonstrate that a well-documented directly acting antiviral (DAA) compound that targets flaviviral RNA synthesis can completely prevent fetal infection when the treatment is started at the time of infection. Notably, we show that the treatment commenced at the time of peak viremia is still able to reduce the risk of fetal infection concomitant with significant reduction in placental viral load. Our results show for the first time the potential for clinical development of antiviral drugs for preventing vertical maternal-fetal transmission of ZIKV.

A Comparison of Plasmid DNA and mRNA as Vaccine Technologies

This review provides a comparison of the theoretical issues and experimental findings for plasmid DNA and mRNA vaccine technologies. While both have been under development since the 1990s, in recent years, significant excitement has turned to mRNA despite the licensure of several veterinary DNA vaccines. Both have required efforts to increase their potency either via manipulating the plasmid DNA and the mRNA directly or through the addition of adjuvants or immunomodulators as well as delivery systems and formulations. The greater inherent inflammatory nature of the mRNA vaccines is discussed for both its potential immunological utility for vaccines and for the potential toxicity. The status of the clinical trials of mRNA vaccines is described along with a comparison to DNA vaccines, specifically the immunogenicity of both licensed veterinary DNA vaccines and select DNA vaccine candidates in human clinical trials.

A Biomimetic Nanodecoy Traps Zika Virus To Prevent Viral Infection and Fetal Microcephaly Development

Zika virus (ZIKV) has emerged as a global health threat due to its unexpected causal link to devastating neurological disorders such as fetal microcephaly; however, to date, no approved vaccine or specific treatment is available for ZIKV infection. Here we develop a biomimetic nanodecoy (ND) that can trap ZIKV, divert ZIKV away from its intended targets, and inhibit ZIKV infection. The ND, which is composed of a gelatin nanoparticle core camouflaged by mosquito medium host cell membranes, effectively adsorbs ZIKV and inhibits ZIKV replication in ZIKV-susceptible cells. Using a mouse model, we demonstrate that NDs significantly attenuate the ZIKV-induced inflammatory responses and degenerative changes and thus improve the survival rate of ZIKV-challenged mice. Moreover, by trapping ZIKV, NDs successfully prevent ZIKV from passing through physiologic barriers into the fetal brain and thereby mitigate ZIKV-induced fetal microcephaly in pregnant mice. We anticipate that this study will provide new insights into the development of safe and effective protection against ZIKV and various other viruses that threaten public health.

Nonhuman Primate Models of Zika Virus Infection, Immunity, and Therapeutic Development

Zika virus is a re-emerging flavivirus transmitted primarily by arthropod vectors. The recent devastating outbreak of Zika virus in Brazil was preceded by the slow global encroachment of this virus over many decades. To date, significant research efforts are underway to understand the spread and the unique pathogenesis of this virus; with the intent to rapidly develop vaccines and therapeutics. Several model systems have emerged to study Zika. This review will focus on the use of nonhuman primates to model Zika infection.

Pre-Clinical Pregnancy Models for Evaluating Zika Vaccines

Zika virus (ZIKV) infection during pregnancy can result in a variety of developmental abnormalities in the fetus, referred to as Congenital Zika Syndrome (CZS). The effects of CZS can range from the loss of the viable fetus to a variety of neurological defects in full-term infants, including microcephaly. The clinical importance of ZIKV-induced CZS has driven an intense effort to develop effective vaccines. Consequently, there are approximately 45 different ZIKV vaccine candidates at various stages of development with several undergoing phase I and II clinical trials. These vaccine candidates have been shown to effectively prevent infection in adult animal models, however, there has been less extensive testing for their ability to block vertical transmission to the fetus during pregnancy or prevent the development of CZS. In addition, it is becoming increasingly difficult to test vaccines in the field as the intensity of the ZIKV epidemic has declined precipitously, making clinical endpoint studies difficult. These ethical and practical challenges in determining efficacy of ZIKV vaccine candidates in preventing CZS have led to increased emphasis on pre-clinical testing in animal pregnancy models. Here we review the current status of pre-clinical pregnancy models for testing the ability of ZIKV vaccines to prevent CZS.

Systemic mRNA Therapy for the Treatment of Fabry Disease: Preclinical Studies in Wild-Type Mice, Fabry Mouse Model, and Wild-Type Non-human Primates

Fabry disease is an X-linked lysosomal storage disease caused by loss of alpha galactosidase A (α-Gal A) activity and is characterized by progressive accumulation of globotriaosylceramide and its analogs in all cells and tissues. Although enzyme replacement therapy (ERT) is considered standard of care, the long-term effects of ERT on renal and cardiac manifestations remain uncertain and thus novel therapies are desirable. We herein report preclinical studies evaluating systemic messenger RNA (mRNA) encoding human α-Gal A in wild-type (WT) mice, α-Gal A-deficient mice, and WT non-human primates (NHPs). The pharmacokinetics and distribution of h-α-Gal A mRNA encoded protein in WT mice demonstrated prolonged half-lives of α-Gal A in tissues and plasma. Single intravenous administration of h-α-Gal A mRNA to Gla-deficient mice showed dose-dependent protein activity and substrate reduction. Moreover, long duration (up to 6 weeks) of substrate reductions in tissues and plasma were observed after a single injection. Furthermore, repeat i.v. administration of h-α-Gal A mRNA showed a sustained pharmacodynamic response and efficacy in Fabry mice model. Lastly, multiple administrations to non-human primates confirmed safety and translatability. Taken together, these studies across species demonstrate preclinical proof-of-concept of systemic mRNA therapy for the treatment of Fabry disease and this approach may be useful for other lysosomal storage disorders.

Advances in Zika Virus⁻Host Cell Interaction: Current Knowledge and Future Perspectives

Emerging mosquito-transmitted RNA viruses, such as Zika virus (ZIKV) and Chikungunya represent human pathogens of an immense global health problem. In particular, ZIKV has emerged explosively since 2007 to cause a series of epidemics in the South Pacific and most recently in the Americas. Although typical ZIKV infections are asymptomatic, ZIKV infection during pregnancy is increasingly associated with microcephaly and other fetal developmental abnormalities. In the last few years, genomic and molecular investigations have established a remarkable progress on the pathogenic mechanisms of ZIKV infection using in vitro and in vivo models. Here, we highlight recent advances in ZIKV-host cell interaction studies, including cellular targets of ZIKV, ZIKV-mediated cell death mechanisms, host cell restriction factors that limit ZIKV replication, and immune evasion mechanisms utilized by ZIKV. Understanding of the mechanisms of ZIKV⁻host interaction at the cellular level will contribute crucial insights into the development of ZIKV therapeutics and vaccines.

How mRNA therapeutics are entering the monoclonal antibody field

In 1975, Milstein and Köhler revolutionized the medical world with the development of the hybridoma technique to produce monoclonal antibodies. Since then, monoclonal antibodies have entered almost every branch of biomedical research. Antibodies are now used as frontline therapeutics in highly divergent indications, ranging from autoimmune disease over allergic asthma to cancer. Wider accessibility and implementation of antibody-based therapeutics is however hindered by manufacturing challenges and high development costs inherent to protein-based drugs. For these reasons, alternative ways are being pursued to produce and deliver antibodies more cost-effectively without hampering safety. Over the past decade, messenger RNA (mRNA) based drugs have emerged as a highly appealing new class of biologics that can be used to encode any protein of interest directly in vivo. Whereas current clinical efforts to use mRNA as a drug are mainly situated at the level of prophylactic and therapeutic vaccination, three recent preclinical studies have addressed the feasibility of using mRNA to encode therapeutic antibodies directly in vivo. Here, we highlight the potential of mRNA-based approaches to solve several of the issues associated with antibodies produced and delivered in protein format. Nonetheless, we also identify key hurdles that mRNA-based approaches still need to take to fulfill this potential and ultimately replace the current protein antibody format.

mRNA as a Transformative Technology for Vaccine Development to Control Infectious Diseases

In the last two decades, there has been growing interest in mRNA-based technology for the development of prophylactic vaccines against infectious diseases. Technological advancements in RNA biology, chemistry, stability, and delivery systems have accelerated the development of fully synthetic mRNA vaccines. Potent, long-lasting, and safe immune responses observed in animal models, as well as encouraging data from early human clinical trials, make mRNA-based vaccination an attractive alternative to conventional vaccine approaches. Thanks to these data, together with the potential for generic, low-cost manufacturing processes and the completely synthetic nature, the prospects for mRNA vaccines are very promising. In addition, mRNA vaccines have the potential to streamline vaccine discovery and development, and facilitate a rapid response to emerging infectious diseases. In this review, we overview the unique attributes of mRNA vaccine approaches, review the data of mRNA vaccines against infectious diseases, discuss the current challenges, and highlight perspectives about the future of this promising technology.

Dengue and Zika Virus Cross-Reactive Human Monoclonal Antibodies Protect against Spondweni Virus Infection and Pathogenesis in Mice

Spondweni virus (SPOV) is the flavivirus that is most closely related to Zika virus (ZIKV). Although SPOV causes sporadic human infections in Africa, recently it was found in Culex mosquitoes in Haiti. To investigate the pathogenic spectrum of SPOV, we developed infection models in mice. Although two SPOV strains failed to cause disease in immunocompetent mice, each accumulated in the brain, spleen, eye, testis, and kidney when type I interferon signaling was blocked and unexpectedly caused infection, immune cell infiltration, and swelling in the ankle. In pregnant mice, SPOV replicated in the placenta and fetus but did not cause placental insufficiency or microcephaly. We identified human antibodies from ZIKV or DENV immune subjects that neutralized SPOV infection and protected against lethal challenge. Our experiments describe similarities and differences in clinical syndromes between SPOV and ZIKV and suggest that their serological relatedness has implications for antibody therapeutics and flavivirus vaccine development.

Demonstrating vaccine effectiveness during a waning epidemic: A WHO/NIH meeting report on approaches to development and licensure of Zika vaccine candidates

Since its peak in early 2016, the incidence of Zika virus (ZIKV) cases has declined to such low levels that Phase 3 field efficacy trials may be infeasible. While great progress was made to rapidly advance several vaccine candidates into Phase 1 and 2 clinical trials, in the absence of sustained viral transmission it may be difficult to evaluate the effectiveness of ZIKV vaccine candidates by conducting traditional clinical disease endpoint efficacy studies. However, ZIKV is still circulating at low levels in some areas and is likely to re-emerge in naïve populations or in sites of prior epidemics once population immunity wanes. Therefore, the public health need for a ZIKV vaccine remains. To facilitate continued ZIKV vaccine development efforts, the World Health Organization's Initiative for Vaccine Research and the National Institutes of Health's National Institute of Allergy and Infectious Diseases co-hosted a meeting of experts in March 2018 to identify strategies to demonstrate vaccine effectiveness in view of waning ZIKV disease incidence. This paper outlines points for consideration for developers, regulators, and other stakeholders working towards a licensed ZIKV vaccine. These deliberations may also be applicable to development of vaccines for other emerging infections where the size, unpredictability, and ephemeral nature of outbreaks makes clinical disease endpoint efficacy trials to demonstrate vaccine effectiveness infeasible.

Systemic Alanine Glyoxylate Aminotransferase mRNA Improves Glyoxylate Metabolism in a Mouse Model of Primary Hyperoxaluria Type 1

Primary Hyperoxaluria Type 1 (PH1) is an autosomal recessive disorder of glyoxylate metabolism. Loss of alanine glyoxylate aminotransferase (AGT) function to convert intermediate metabolite glyoxylate to glycine causes the accumulation and reduction of glyoxylate to glycolate, which eventually is oxidized to oxalate. Excess oxalate in PH1 patients leads to the formation and deposition of calcium oxalate crystals in the kidney and urinary tract. Oxalate crystal deposition causes a decline in renal function, systemic oxalosis, and eventually end-stage renal disease and premature death. mRNA-based therapies are a new class of drugs that work by replacing the missing enzyme. mRNA encoding AGT has the potential to restore normal glyoxylate to glycine metabolism, thus preventing the buildup of calcium oxalate in various organs. Panels of codon-optimized AGT mRNA constructs were screened in vitro and in wild-type mice for the production of a functional AGT enzyme. Two human constructs, wild-type and engineered AGT (RHEAM), were tested in Agxt^-/- mice. Repeat dosing in Agxt^-/- mice resulted in a 40% reduction in urinary oxalate, suggesting therapeutic benefit. These studies suggest that mRNA encoding AGT led to increased expression and activity of the AGT enzyme in liver that translated into decrease in urinary oxalate levels. Taken together, our data indicate that AGT mRNA may have the potential to be developed into a therapeutic for PH1.

A 'Furry-Tale' of Zika Virus Infection: What Have We Learned from Animal Models?

The worldwide attention that the Zika virus (ZIKV) attracted, following its declaration as a Public Health Emergency of International concern by WHO in 2016, has led to a large collective effort by the international scientific community to understand its biology. Despite the mild symptoms caused by ZIKV in most infected people, the virus displays a number of worrying features, such as its ability to cause transplacental infection, fetal abnormalities and vector independent transmission through body fluids. In addition, the virus has been associated with the induction of Guillain-Barre syndrome in a number of infected individuals. With travelling, the virus has spread outside the original ZIKV endemic areas making it imperative to find ways to control it. Thus far, the large number of animal models developed to study ZIKV pathogenesis have proven to be valuable tools in understanding how the virus replicates and manifests itself in the host, its tissue tropism and the type of immune responses it induces. Still, vital questions, such as the molecular mechanisms of ZIKV persistence and the long-term consequences of ZIKV infection in the developing brain, remain unanswered. Here, we reviewed and discussed the major and most recent findings coming from animal studies and their implications for a ZIKV vaccine design.

CD4+ T cells promote humoral immunity and viral control during Zika virus infection

Several Zika virus (ZIKV) vaccines designed to elicit protective antibody (Ab) responses are currently under rapid development, but the underlying mechanisms that control the magnitude and quality of the Ab response remain unclear. Here, we investigated the CD4+ T cell response to primary intravenous and intravaginal infection with ZIKV. Using the LysMCre+Ifnar1fl/fl (myeloid type I IFN receptor-deficient) C57BL/6 mouse models, we identified six I-Ab-restricted ZIKV epitopes that stimulated CD4+ T cells with a predominantly cytotoxic Th1 phenotype in mice primed with ZIKV. Intravenous and intravaginal infection with ZIKV effectively induced follicular helper and regulatory CD4+ T cells. Treatment of mice with a CD4+ T cell-depleting Ab reduced the plasma cell, germinal center B cell, and IgG responses to ZIKV without affecting the CD8+ T cell response. CD4+ T cells were required to protect mice from a lethal dose of ZIKV after infection intravaginally, but not intravenously. However, adoptive transfer and peptide immunization experiments showed a role for memory CD4+ T cells in ZIKV clearance in mice challenged intravenously. These results demonstrate that CD4+ T cells are required mainly for the generation of a ZIKV-specific humoral response but not for an efficient CD8+ T cell response. Thus, CD4+ T cells could be important mediators of protection against ZIKV, depending on the infection or vaccination context.

Poly(lactic acid) nanoparticles and cell-penetrating peptide potentiate mRNA-based vaccine expression in dendritic cells triggering their activation

Messenger RNA-based vaccines have the potential to trigger robust cytotoxic immune responses, which are essential for fighting cancer and infectious diseases like HIV. Dendritic Cells (DCs) are choice targets for mRNA-based vaccine strategies, as they link innate and adaptive immune responses and are major regulators of cytotoxic and humoral adaptive responses. However, efficient delivery of antigen-coding mRNAs into DC cytosol has been highly challenging. In this study, we developed an alternative to lipid-based mRNA delivery systems, using poly(lactic acid) nanoparticles (PLA-NPs) and cationic cell-penetrating peptides as mRNA condensing agent. The formulations are assembled in two steps: (1) formation of a polyplex between mRNAs and amphipathic cationic peptides (RALA, LAH4 or LAH4-L1), and (2) adsorption of polyplexes onto PLA-NPs. LAH4-L1/mRNA polyplexes and PLA-NP/LAH4-L1/mRNA nanocomplexes are taken up by DCs via phagocytosis and clathrin-dependent endocytosis, and induce strong protein expression in DCs in vitro. They modulate DC innate immune response by activating both endosome and cytosolic Pattern Recognition Receptors (PRRs), and induce markers of adaptive responses in primary human DCs in vitro, with prevalent Th1 signature. Thus, LAH4-L1/mRNA and PLA-NP/LAH4-L1/mRNA represent a promising platform for ex vivo treatment and mRNA vaccine development.

Efficacy of a T Cell-Biased Adenovirus Vector as a Zika Virus Vaccine

Zika virus (ZIKV) is a major public health concern due to the risk of congenital Zika syndrome in developing fetuses and Guillain-Barre syndrome in adults. Currently, there are no approved vaccines available to protect against infection. Adenoviruses are safe and highly immunogenic vaccine vectors capable of inducing lasting humoral and cellular immune responses. Here, we developed two Adenovirus (Ad) vectored Zika virus vaccines by inserting a ZIKV prM-E gene expression cassette into human Ad types 4 (Ad4-prM-E) and 5 (Ad5-prM-E). Immune correlates indicate that Ad5-prM-E vaccination induces both an anti-ZIKV antibody and T-cell responses whereas Ad4-prM-E vaccination only induces a T-cell response. In a highly lethal challenge in an interferon α/β receptor knockout mice, 80% of Ad5 vaccinated animals and 33% of Ad4 vaccinated animals survived a lethal ZIKV challenge, whereas no animals in the sham vaccinated group survived. In an infection model utilizing immunocompetent C57BL/6 mice that were immunized and then treated with a blocking anti-IFNAR-1 antibody immediately before ZIKV challenge, 100% of Ad4-prM-E and Ad5-prM-E vaccinated mice survived. This indicates that Ad4-prM-E vaccination is protective without the development of detectable anti-ZIKV antibodies. The protection seen in these highly lethal mouse models demonstrate the efficacy of Ad vectored vaccines for use against ZIKV.

Recent Advances in Zika Virus Vaccines

The recent outbreaks of Zika virus (ZIKV) infections and associated microcephaly in newborns has resulted in an unprecedented effort by researchers to target this virus. Significant advances have been made in developing vaccine candidates, treatment strategies and diagnostic assays in a relatively short period of time. Being a preventable disease, the first line of defense against ZIKV would be to vaccinate the highly susceptible target population, especially pregnant women. Along those lines, several vaccine candidates including purified inactivated virus (PIV), live attenuated virus (LAV), virus like particles (VLP), DNA, modified RNA, viral vectors and subunit vaccines have been in the pipeline with several advancing to clinical trials. As the primary objective of Zika vaccination is the prevention of vertical transmission of the virus to the unborn fetus, the safety and efficacy requirements for this vaccine remain unique when compared to other diseases. This review will discuss these recent advances in the field of Zika vaccine development.

Purified Inactivated Zika Vaccine Candidates Afford Protection against Lethal Challenge in Mice

In response to the 2016 global public health emergency of international concern announced by the World Health Organization surrounding Zika virus (ZIKV) outbreaks, we developed a purified inactivated Zika virus vaccine (PIZV) candidate from ZIKV strain PRVABC59, isolated during the outbreak in 2015. The virus isolate was plaque purified, creating six sub-isolated virus stocks, two of which were selected to generate PIZV candidates for preclinical immunogenicity and efficacy evaluation in mice. The alum-adjuvanted PIZV candidates were highly immunogenic in both CD-1 and AG129 mice after a 2-dose immunization. Further, AG129 mice receiving 2 doses of PIZV formulated with alum were fully protected against lethal ZIKV challenge and mouse immune sera elicited by the PIZV candidates were capable of neutralizing ZIKVs of both African and Asian genetic lineages in vitro. Additionally, passive immunization of naïve mice with ZIKV-immune serum showed strong positive correlation between neutralizing ZIKV antibody (NAb) titers and protection against lethal challenge. This study supported advancement of the PIZV candidate toward clinical development.

Zika Virus Vaccine Development: Progress in the Face of New Challenges

Zika virus (ZIKV) emerged at a global level when it spread to the Americas and began causing congenital malformations and microcephaly in 2015. A rapid response by academia, government, public health infrastructure, and industry has enabled the expedited development and testing of a suite of vaccine platforms aiming to control and eliminate ZIKV-induced disease. Analysis of key immunization and pathogenesis studies in multiple animal models, including during pregnancy, has begun to define immune correlates of protection. Nonetheless, the deployment of ZIKV vaccines, along with the confirmation of their safety and efficacy, still has major challenges, one of which is related to the waning of the epidemic. In this review, we discuss the measures that enabled rapid progress and highlight the path forward for successful deployment of ZIKV vaccines.

Zika Virus Liquid Biopsy: A Dendritic Ru(bpy)3 2+-Polymer-Amplified ECL Diagnosis Strategy Using a Drop of Blood

Zika virus (ZIKV) is a mosquito-borne flavivirus that leads to devastating consequences for fetal development. However, accurate diagnosis of ZIKV is made difficult by the fact that most infected patients are asymptomatic or present with symptoms similar to those of other febrile illnesses. Thus, the development of a simple, accurate, highly sensitive, and reliable method for the biomedical analysis and diagnosis of ZIKV is needed. Herein, a novel ZIKV liquid biopsy system was constructed via a dendritic Ru(bpy)₃ ²⁺-polymer-amplified electro-chemiluminescence (ECL) strategy. This system accomplished amplification-free analysis of ZIKV using a drop of blood, and simultaneously achieved a high sensitivity of 500 copies and superior specificity. This strategy adopted the humoral biomarker as the diagnostic index, which greatly simplified the analysis process, and established a nondestructive detection mode. Furthermore, the performance index for biomedical analysis of clinical ZIKV samples was investigated, and the results indicated that the dendritic Ru(bpy)₃ ²⁺-polymer-amplified ECL strategy reliably responded to ZIKV from the body fluid (blood, saliva, and urine). Hence, this system suitably met the strict clinical requirements for ZIKV detection and thus has the potential to serve as a new paradigm for the biomedical analysis and diagnosis of ZIKV.

Zika virus shedding in the stool and infection through the anorectal mucosa in mice

Zika virus (ZIKV) has elicited global concern due to its unique biological features, unusual transmission routes, and unexpected clinical outcomes. Although ZIKV transmission through anal intercourse has been reported in humans, it remains unclear if ZIKV is detectable in the stool, if it can infect the host through the anal canal mucosa, and what the pathogenesis of such a route of infection might be in the mouse model. Herein, we demonstrate that ZIKV RNA can be recovered from stools in multiple mouse models, as well as from the stool of a ZIKV patient. Remarkably, intra-anal (i.a.) inoculation with ZIKV leads to efficient infection in both Ifnar1-/- and immunocompetent mice, characterized by extensive viral replication in the blood and multiple organs, including the brain, small intestine, testes, and rectum, as well as robust humoral and innate immune responses. Moreover, i.a. inoculation of ZIKV in pregnant mice resulted in transplacental infection and delayed fetal development. Overall, our results identify the anorectal mucosa as a potential site of ZIKV infection in mice, reveal the associated pathogenesis of i.a. infection, and highlight the complexity of ZIKV transmission through anal intercourse.

A Single-Dose Live-Attenuated Zika Virus Vaccine with Controlled Infection Rounds that Protects against Vertical Transmission

Zika virus (ZIKV) infection of the mother during pregnancy causes devastating Zika congenital syndrome in the offspring. A ZIKV vaccine with optimal safety and immunogenicity for use in pregnant women is critically needed. Toward this goal, we have developed a single-dose live-attenuated vaccine candidate that infects cells with controlled, limited infection rounds. The vaccine contains a 9-amino-acid deletion in the viral capsid protein and replicates to titers of > 106 focus-forming units (FFU)/mL in cells expressing the full-length capsid protein. Immunization of A129 mice with one dose (105 FFU) did not produce viremia, but elicited protective immunity that completely prevented viremia, morbidity, and mortality after challenge with an epidemic ZIKV strain (106 PFU). A single-dose vaccination also fully prevented infection of pregnant mice and maternal-to-fetal transmission. Intracranial injection of the vaccine (104 FFU) to 1-day-old mice did not cause any disease or death, underscoring the safety of this vaccine candidate.

Rapid travel to a Zika vaccine: are we heading towards success or more questions?

INTRODUCTION

The emergence of the Zika virus (ZIKV) in Latin America in 2015-2016 led to an expeditious search for vaccine candidates, with a DNA-based candidate having progressed to Phase II. However, several features of ZIKV infection and epidemiology are not understood, which may be key to maximizing efficacy and ensuring safety of ZIKV vaccines.

AREAS COVERED

Conceivable problems related to vaccine development and policy include: (1) paucity of diagnostics to satisfactorily discriminate between past ZIKV and dengue virus (DENV) exposure; (2) insufficient knowledge of the mechanisms of ZIKV neurovirulence, amongst other unknowns in the biology of this infection, is particularly relevant from a vaccine safety perspective; and (3) the potential for disease enhancement, as observed with DENV infection and vaccine.

EXPERT OPINION

Vaccine candidates that entered phase I/II trials have demonstrated protection in naïve animal models, while ZIKV epidemics occurred in populations that had encountered DENV before. The resulting cross-reactive antibodies pose problems for reliable serologic diagnostic assays, and for the potential of disease enhancement. The alleged neurological complications also warrant further exploration in order to reassure regulators of the safety profile of these vaccines in target populations. These research aspects should be an integral part of the efforts to develop a vaccine.

Zika virus vaccines

The recent epidemic of Zika virus (ZIKV) in the Americas has revealed the devastating consequences of ZIKV infection, particularly in pregnant women. Congenital Zika syndrome, characterized by malformations and microcephaly in neonates as well as developmental challenges in children, highlights the need for the development of a safe and effective vaccine. Multiple vaccine candidates have been developed and have shown promising results in both animal models and phase I clinical trials. However, important challenges remain for the clinical development of these vaccines. In this Progress article, we discuss recent preclinical studies and lessons learned from first-in-human clinical trials with ZIKV vaccines.

Biomedical applications of mRNA nanomedicine

As an attractive alternative to plasmid DNA, messenger RNA (mRNA) has recently emerged as a promising class of nucleic acid therapeutics for biomedical applications. Advances in addressing the inherent shortcomings of mRNA and in the development of nanoparticle-based delivery systems have prompted the development and clinical translation of mRNA-based medicines. In this review, we discuss the chemical modification strategies of mRNA to improve its stability, minimize immune responses, and enhance translational efficacy. We also highlight recent progress in nanoparticle-based mRNA delivery. Considerable attention is given to the increasingly widespread applications of mRNA nanomedicine in the biomedical fields of vaccination, protein-replacement therapy, gene editing, and cellular reprogramming and engineering.

New Vaccine Technologies to Combat Outbreak Situations

Ever since the development of the first vaccine more than 200 years ago, vaccinations have greatly decreased the burden of infectious diseases worldwide, famously leading to the eradication of small pox and allowing the restriction of diseases such as polio, tetanus, diphtheria, and measles. A multitude of research efforts focuses on the improvement of established and the discovery of new vaccines such as the HPV (human papilloma virus) vaccine in 2006. However, radical changes in the density, age distribution and traveling habits of the population worldwide as well as the changing climate favor the emergence of old and new pathogens that bear the risk of becoming pandemic threats. In recent years, the rapid spread of severe infections such as HIV, SARS, Ebola, and Zika have highlighted the dire need for global preparedness for pandemics, which necessitates the extremely rapid development and comprehensive distribution of vaccines against potentially previously unknown pathogens. What is more, the emergence of antibiotic resistant bacteria calls for new approaches to prevent infections. Given these changes, established methods for the identification of new vaccine candidates are no longer sufficient to ensure global protection. Hence, new vaccine technologies able to achieve rapid development as well as large scale production are of pivotal importance. This review will discuss viral vector and nucleic acid-based vaccines (DNA and mRNA vaccines) as new approaches that might be able to tackle these challenges to global health.

Zika Virus Vaccines: Challenges and Perspectives

Zika virus is an arbovirus that has rapidly spread within the Americas since 2014, presenting a variety of clinical manifestations and neurological complications resulting in congenital malformation, microcephaly, and possibly, in male infertility. These significant clinical manifestations have led investigators to develop several candidate vaccines specific to Zika virus. In this review we describe relevant targets for the development of vaccines specific for Zika virus, the development status of various vaccine candidates and their different platforms, as well as their clinical progression.

A single-dose plasmid-launched live-attenuated Zika vaccine induces protective immunity

BACKGROUND

Vaccines are the most effective means to fight and eradicate infectious diseases. Live-attenuated vaccines (LAV) usually have the advantages of single dose, rapid onset of immunity, and durable protection. DNA vaccines have the advantages of chemical stability, ease of production, and no cold chain requirement. The ability to combine the strengths of LAV and DNA vaccines may transform future vaccine development by eliminating cold chain and cell culture with the potential for adventitious agents.

METHODS

A DNA-launched LAV was developed for ZIKV virus (ZIKV), a pathogen that recently caused a global public health emergency. The cDNA copy of a ZIKV LAV genome was engineered into a DNA plasmid. The DNA-LAV plasmid was delivered into mice using a clinically proven device TriGrid™ to launch the replication of LAV.

FINDINGS

A single-dose immunization as low as 0.5 μg of DNA-LAV plasmid conferred 100% seroconversion in A129 mice. All seroconverted mice developed sterilizing immunity, as indicated by no detectable infectious viruses and no increase of neutralizing antibody titers after ZIKV challenge. The immunization also elicited robust T cell responses. In pregnant mice, the DNA-LAV vaccination fully protected against ZIKV-induced disease and maternal-to-fetal transmission. High levels of neutralizing activities were detected in fetal serum, indicating maternal-to-fetal humoral transfer. In male mice, a single-dose vaccination completely prevented testis infection, injury, and oligospermia.

INTERPRETATION

The remarkable simplicity and potency of ZIKV DNA-LAV warrant further development of this vaccine candidate. The DNA-LAV approach may serve as a universal vaccine platform for other plus-sense RNA viruses. FUND: National Institute of Health, Kleberg Foundation, Centers for Disease Control and Prevention, University of Texas Medical Branch.

The emergence of Zika virus and its new clinical syndromes

Zika virus (ZIKV) is a mosquito-transmitted flavivirus that has emerged as a global health threat because of its potential to generate explosive epidemics and ability to cause congenital disease in the context of infection during pregnancy. Whereas much is known about the biology of related flaviviruses, the unique features of ZIKV pathogenesis, including infection of the fetus, persistence in immune-privileged sites and sexual transmission, have presented new challenges. The rapid development of cell culture and animal models has facilitated a new appreciation of ZIKV biology. This knowledge has created opportunities for the development of countermeasures, including multiple ZIKV vaccine candidates, which are advancing through clinical trials. Here we describe the recent advances that have led to a new understanding of the causes and consequences of the ZIKV epidemic.