The live attenuated chimeric vaccine rWN/DEN4Δ30 is well-tolerated and immunogenic in healthy flavivirus-naïve adult volunteers

Subgenomic flavivirus RNA as key target for live-attenuated vaccine development

Live-attenuated flavivirus vaccines confer long-term protection against disease, but the design of attenuated flaviviruses does not follow a general approach. The non-coding, subgenomic flavivirus RNA (sfRNA) is produced by all flaviviruses and is an essential factor in viral pathogenesis and transmission. We argue that modulating sfRNA expression is a promising, universal strategy to finetune flavivirus attenuation for developing effective flavivirus vaccines of the future.

Vaccination and Control Methods of West Nile Virus Infection in Equids and Humans

West Nile virus (WNV) is capable of causing severe neurologic disease in both humans and equines, making it a disease of importance in both human medicine and veterinary medicine. No targeted treatments exist for WNV infection in either humans or equines. Infection is treated symptomatically through management of symptoms like fever and seizures. As treatment for WNV is purely supportive, the response to WNV has focused primarily on methods of disease prevention. To this end, research efforts have yielded several effective vaccines for equine use as well as numerous conventional mosquito control techniques. Even with the implementation of these techniques, disease caused by WNV remains a concern since no human vaccine exists. Due to the lack of a human vaccine, novel preventative strategies are under active research and development. Of these strategies, some of the most conceptually promising are techniques using genetically modified mosquitoes, addressing the disease at the vector level with minimal ecological side effects. Taken together, the use of combined, synergistic methods, such as physical barriers, transgenic mosquitoes, and immunological targets, will be the best way to prevent WNV disease.

Development of Vaccines against Emerging Mosquito-Vectored Arbovirus Infections

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

Evaluation of NS4A, NS4B, NS5 and 3'UTR Genetic Determinants of WNV Lineage 1 Virulence in Birds and Mammals

West Nile virus (WNV) is amplified in an enzootic cycle involving birds as amplifying hosts. Because they do not develop high levels of viremia, humans and horses are considered to be dead-end hosts. Mosquitoes, especially from the Culex genus, are vectors responsible for transmission between hosts. Consequently, understanding WNV epidemiology and infection requires comparative and integrated analyses in bird, mammalian, and insect hosts. So far, markers of WNV virulence have mainly been determined in mammalian model organisms (essentially mice), while data in avian models are still missing. WNV Israel 1998 (IS98) is a highly virulent strain that is closely genetically related to the strain introduced into North America in 1999, NY99 (genomic sequence homology > 99%). The latter probably entered the continent at New York City, generating the most impactful WNV outbreak ever documented in wild birds, horses, and humans. In contrast, the WNV Italy 2008 strain (IT08) induced only limited mortality in birds and mammals in Europe during the summer of 2008. To test whether genetic polymorphism between IS98 and IT08 could account for differences in disease spread and burden, we generated chimeric viruses between IS98 and IT08, focusing on the 3' end of the genome (NS4A, NS4B, NS5, and 3'UTR regions) where most of the non-synonymous mutations were detected. In vitro and in vivo comparative analyses of parental and chimeric viruses demonstrated a role for NS4A/NS4B/5'NS5 in the decreased virulence of IT08 in SPF chickens, possibly due to the NS4B-E249D mutation. Additionally, significant differences between the highly virulent strain IS98 and the other three viruses were observed in mice, implying the existence of additional molecular determinants of virulence in mammals, such as the amino acid changes NS5-V258A, NS5-N280K, NS5-A372V, and NS5-R422K. As previously shown, our work also suggests that genetic determinants of WNV virulence can be host-dependent.

Recent Advancements in Mosquito-Borne Flavivirus Vaccine Development

Lately, the global incidence of flavivirus infection has been increasing dramatically and presents formidable challenges for public health systems around the world. Most clinically significant flaviviruses are mosquito-borne, such as the four serotypes of dengue virus, Zika virus, West Nile virus, Japanese encephalitis virus and yellow fever virus. Until now, no effective antiflaviviral drugs are available to fight flaviviral infection; thus, a highly immunogenic vaccine would be the most effective weapon to control the diseases. In recent years, flavivirus vaccine research has made major breakthroughs with several vaccine candidates showing encouraging results in preclinical and clinical trials. This review summarizes the current advancement, safety, efficacy, advantages and disadvantages of vaccines against mosquito-borne flaviviruses posing significant threats to human health.

Characteristics of Chimeric West Nile Virus Based on the Japanese Encephalitis Virus SA14-14-2 Backbone

West Nile virus disease (WND) is an arthropod-borne zoonosis responsible for nonspecific fever or severe encephalitis. The pathogen is West Nile virus belonging to the genus Flavivirus, family Flaviviridae. Every year, thousands of cases were reported, which poses significant public health risk. Here, we constructed a West Nile virus chimera, ChiVax-WN01, by replacing the prMΔE gene of JEV SA14-14-2 with that of the West Nile virus NY99. The ChiVax-WN01 chimera showed clear, different characters compared with that of JEV SA14-14-2 and WNV NY99 strain. An animal study indicated that the ChiVax-WN01 chimera presented moderate safety and immunogenicity for 4-week female BALB/c mice.

Immunoinformatics and molecular dynamics approaches: Next generation vaccine design against West Nile virus

West Nile Virus (WNV) is a life threatening flavivirus that causes significant morbidity and mortality worldwide. No preventive therapeutics including vaccines against WNV are available for human use. In this study, immunoinformatics approach was performed to design a multi epitope-based subunit vaccine against this deadly pathogen. Human (HLA) and Mice (H-2) allele specific potential T-cell and B-cell epitopes were shortlisted through a stringent procedure. Molecular docking showed selected epitopes that have stronger binding affinity with human TLR-4. Molecular dynamics simulation confirmed the stable nature of the docked complex. Furthermore, in silico cloning analysis ensures efficient expression of desired gene in the microbial system. Interestingly, previous studies showed that two of our selected epitopes have strong immune response against WNV. Therefore, selected epitopes could be strong vaccine candidates to prevent WNV infections in human. However, further in vitro and in vivo investigations could be strengthening the validation of the vaccine candidate against WNV.

Cost effectiveness and impact of a targeted age- and incidence-based West Nile virus vaccine strategy

BACKGROUND

West Nile virus (WNV) is the leading cause of arboviral disease in the United States and is associated with significant morbidity and mortality. A previous analysis found that a vaccination program targeting persons aged ≥60 years was more cost effective than universal vaccination, but costs remained high.

METHODS

We used a mathematical Markov model to evaluate cost-effectiveness of an age- and incidence-based WNV vaccination program. We grouped states and large counties (≥100,000 persons aged ≥60 years) by median annual WNV incidence rates from 2004 to 2017 for persons aged ≥60 years. We defined WNV incidence thresholds, in increments of 0.5 cases per 100,000 persons ≥60 years. We calculated potential cost per WNV vaccine-prevented case and per quality adjusted life years (QALYs) saved.

RESULTS

Vaccinating persons aged ≥60 years in states with an annual incidence of WNV neuroinvasive disease of ≥0.5 per 100,000 resulted in approximately half the cost per health outcome averted compared to vaccinating persons aged ≥60 years in all the contiguous United States. This approach could potentially prevent 37% of all neuroinvasive disease cases and 63% of WNV-related deaths nationally. Employing such a threshold at a county-level further improved cost-effectiveness ratios while preventing 19% and 30% of WNV-related neuroinvasive disease cases and deaths, respectively.

CONCLUSIONS

An age- and incidence-based WNV vaccination program could be a more cost-effective strategy than an age-based program while still having a substantial impact on lowering WNV-related morbidity and mortality.

A 20-year historical review of West Nile virus since its initial emergence in North America: Has West Nile virus become a neglected tropical disease?

After the unexpected arrival of West Nile virus (WNV) in the United States in 1999, the mosquito-borne virus quickly spread throughout North America. Over the past 20 years, WNV has become endemic, with sporadic epizootics. Concerns about the economic impact of infection in horses lead to the licensure of an equine vaccine as early as 2005, but few advances regarding human vaccines or treatments have since been made. There is a high level of virus transmission in hot/humid, subtropical climates, and high morbidity that may disproportionately affect vulnerable populations including the homeless, elderly, and those with underlying health conditions. Although WNV continues to cause significant morbidity and mortality at great cost, funding and research have declined in recent years. These factors, combined with neglect by policy makers and amenability of control measures, indicate that WNV has become a neglected tropical disease.

Pathogenicity and virulence of West Nile virus revisited eight decades after its first isolation

West Nile virus (WNV) is a flavivirus which transmission cycle is maintained between mosquitoes and birds, although it occasionally causes sporadic outbreaks in horses and humans that can result in serious diseases and even death. Since its first isolation in Africa in 1937, WNV had been considered a neglected pathogen until its recent spread throughout Europe and the colonization of America, regions where it continues to cause outbreaks with severe neurological consequences in humans and horses. Although our knowledge about the characteristics and consequences of the virus has increased enormously lately, many questions remain to be resolved. Here, we thoroughly update our knowledge of different aspects of the WNV life cycle: virology and molecular classification, host cell interactions, transmission dynamics, host range, epidemiology and surveillance, immune response, clinical presentations, pathogenesis, diagnosis, prophylaxis (antivirals and vaccines), and prevention, and we highlight those aspects that are still unknown and that undoubtedly require further investigation.

Management of West Nile Encephalitis: An Uncommon Complication of West Nile Virus

West Nile virus disease (WNVD) is a mosquito-borne disease that affects the meninges and central nervous system, causing West Nile virus (WNV) encephalitis, a debilitating, life-threatening condition, especially in the elderly. While there is a lot of research discussing different aspects of the disease, the treatment is mainly unknown. We conducted a literature review to explore the wide variety of treatment options that consolidate the knowledge about the most recent management of WNV encephalitis. We did a combined advanced search and Medical Subject Headings (MeSH) search on PubMed. Inclusion criteria included papers written in the English language and human subjects research for the past 25 years. We initially gather 110 papers, and after applying the inclusion/exclusion criteria, we end up with 30 articles for the paper's discussion. This review aims to provide clinicians with an overview of the latest approach in treating and managing hospitalized WNVD patients. It discusses case reports and the outcome of different treatment regimens done in vitro and in vivo. The study discusses all the advancements in treatment and prophylaxis and compares their effectiveness. However, more research is warranted to gain further insight to develop a single guideline for the management of this disease. This review discusses the following treatment modalities: ribavirin, interferon-alpha, intravenous immunoglobulin (IVIG), and other less-used drugs. More studies about ribavirin are needed to know if the drug is useful for WNV encephalitis. Interferon-alpha has been shown to have both protective and disease limiting properties. At the moment, there are no guidelines for the treatment of WNV encephalitis, nor is there a single Food and Drug Administration (FDA)-approved drug. For the time being, IVIG offers the best results in treating WNV encephalitis

Animal and Human Vaccines against West Nile Virus

West Nile virus (WNV) is a widely distributed enveloped flavivirus transmitted by mosquitoes, which main hosts are birds. The virus sporadically infects equids and humans with serious economic and health consequences, as infected individuals can develop a severe neuroinvasive disease that can even lead to death. Nowadays, no WNV-specific therapy is available and vaccines are only licensed for use in horses but not for humans. While several methodologies for WNV vaccine development have been successfully applied and have contributed to significantly reducing its incidence in horses in the US, none have progressed to phase III clinical trials in humans. This review addresses the status of WNV vaccines for horses, birds, and humans, summarizing and discussing the challenges they face for their clinical advance and their introduction to the market.

The Role of Microglia during West Nile Virus Infection of the Central Nervous System

Encephalitis resulting from viral infections is a major cause of hospitalization and death worldwide. West Nile Virus (WNV) is a substantial health concern as it is one of the leading causes of viral encephalitis in the United States today. WNV infiltrates the central nervous system (CNS), where it directly infects neurons and induces neuronal cell death, in part, via activation of caspase 3-mediated apoptosis. WNV infection also induces neuroinflammation characterized by activation of innate immune cells, including microglia and astrocytes, production of inflammatory cytokines, breakdown of the blood-brain barrier, and infiltration of peripheral leukocytes. Microglia are the resident immune cells of the brain and monitor the CNS for signs of injury or pathogens. Following infection with WNV, microglia exhibit a change in morphology consistent with activation and are associated with increased expression of proinflammatory cytokines. Recent research has focused on deciphering the role of microglia during WNV encephalitis. Microglia play a protective role during infections by limiting viral growth and reducing mortality in mice. However, it also appears that activated microglia are triggered by T cells to mediate synaptic elimination at late times during infection, which may contribute to long-term neurological deficits following a neuroinvasive WNV infection. This review will discuss the important role of microglia in the pathogenesis of a neuroinvasive WNV infection. Knowledge of the precise role of microglia during a WNV infection may lead to a greater ability to treat and manage WNV encephalitis.

Twenty Years of Progress Toward West Nile Virus Vaccine Development

Although West Nile virus (WNV) has been a prominent mosquito-transmitted infection in North America for twenty years, no human vaccine has been licensed. With a cumulative number of 24,714 neurological disease cases and 2314 deaths in the U.S. since 1999, plus a large outbreak in Europe in 2018 involving over 2000 human cases in 15 countries, a vaccine is essential to prevent continued morbidity, mortality, and economic burden. Currently, four veterinary vaccines are licensed, and six vaccines have progressed into clinical trials in humans. All four veterinary vaccines require multiple primary doses and annual boosters, but for a human vaccine to be protective and cost effective in the most vulnerable older age population, it is ideal that the vaccine be strongly immunogenic with only a single dose and without subsequent annual boosters. Of six human vaccine candidates, the two live, attenuated vaccines were the only ones that elicited strong immunity after a single dose. As none of these candidates have yet progressed beyond phase II clinical trials, development of new candidate vaccines and improvement of vaccination strategies remains an important area of research.

West Nile virus vaccines - current situation and future directions

West Nile virus (WNV) is a widely spread human pathogenic arthropod-borne virus. It can lead to severe, sometimes fatal, neurological disease. Over the last two decades, several vaccine candidates for the protection of humans from WNV have been developed. Some technologies were transferred into clinical testing, but these approaches have not yet led to a licensed product. This review summarizes the current status of a human WNV vaccine and discusses reasons for the lack of clinically advanced product candidates. It also discusses the problem of immunological cross-reactivity between flaviviruses and how it can be addressed during vaccine development.

Role of NS1 and TLR3 in Pathogenesis and Immunity of WNV

West Nile Virus (WNV) is a mosquito-transmitted flavivirus which causes encephalitis especially in elderly and immunocompromised individuals. Previous studies have suggested the protective role of the Toll-like receptor 3 (TLR3) pathway against WNV entry into the brain, while the WNV non-structural protein 1 (NS1) interferes with the TLR3 signaling pathway, besides being a component of viral genome replication machinery. In this study, we investigated whether immunization with NS1 could protect against WNV neuroinvasion in the context of TLR3 deficiency. We immunized mice with either an intact or deleted TLR3 system (TLR3KO) with WNV envelope glycoprotein (gE) protein, NS1, or a combination of gE and NS1. Immunization with gE or gE/NS1, but not with NS1 alone, induced WNV neutralizing antibodies and protected against WNV brain invasion and inflammation. The presence of intact TLR3 signaling had no apparent effect on WNV brain invasion. However, mock-immunized TLR3KO mice had higher inflammatory cell invasion upon WNV brain infection than NS1-immunized TLR3KO mice and wild type mice. Thus, immunization against NS1 may reduce brain inflammation in a context of TLR3 signaling deficiency.

An observer blinded, randomized, placebo-controlled, phase I dose escalation trial to evaluate the safety and immunogenicity of an inactivated West Nile virus Vaccine, HydroVax-001, in healthy adults

Background

West Nile virus (WNV) is the most common mosquito-borne infection in the United States. HydroVax-001 WNV is a hydrogen peroxide inactivated, whole virion (WNV-Kunjin strain) vaccine adjuvanted with aluminum hydroxide.

Methods

We performed a phase 1, randomized, placebo-controlled, double-blind (within dosing group), dose escalation clinical trial of the HydroVax-001 WNV vaccine administered via intramuscular injection. This trial evaluated 1 mcg and 4 mcg dosages of HydroVax-001 WNV vaccine given intramuscularly on day 1 and day 29 in healthy adults. The two dosing groups of HydroVax-001 were enrolled sequentially and each group consisted of 20 individuals who received HydroVax-001 and 5 who received placebo. Safety was assessed at all study days (days 1, 2, 4 and 15 post dose 1, and days 1, 2, 4, 15, 29, 57, 180 and 365 post dose 2), and reactogenicity was assessed for 14 days after administration of each dose. Immunogenicity was measured by WNV-specific plaque reduction neutralization tests (PRNT₅₀) in the presence or absence of added complement or by WNV-specific enzyme-linked immunosorbent assays (ELISA).

Results

HydroVax-001 was safe and well-tolerated as there were no serious adverse events or concerning safety signals. At the 1 mcg dose, HydroVax-001 was not immunogenic by PRNT₅₀ but elicited up to 41% seroconversion by WNV-specific ELISA in the per-protocol population (PP) after the second dose. At the 4 mcg dose, HydroVax-001 elicited neutralizing antibody responses in 31% of the PP following the second dose. In the presence of added complement, PRNT₅₀ seroconversion rates increased to 50%, and 75% seroconversion was observed by WNV-specific ELISA.

Conclusions

The HydroVax-001 WNV vaccine was found to be modestly immunogenic and welltolerated at all dose levels.

The Immune Responses of the Animal Hosts of West Nile Virus: A Comparison of Insects, Birds, and Mammals

Vector-borne diseases, including arboviruses, pose a serious threat to public health worldwide. Arboviruses of the flavivirus genus, such as Zika virus (ZIKV), dengue virus, yellow fever virus (YFV), and West Nile virus (WNV), are transmitted to humans from insect vectors and can cause serious disease. In 2017, over 2,000 reported cases of WNV virus infection occurred in the United States, with two-thirds of cases classified as neuroinvasive. WNV transmission cycles through two different animal populations: birds and mosquitoes. Mammals, particularly humans and horses, can become infected through mosquito bites and represent dead-end hosts of WNV infection. Because WNV can infect diverse species, research on this arbovirus has investigated the host response in mosquitoes, birds, humans, and horses. With the growing geographical range of the WNV mosquito vector and increased human exposure, improved surveillance and treatment of the infection will enhance public health in areas where WNV is endemic. In this review, we survey the bionomics of mosquito species involved in Nearctic WNV transmission. Subsequently, we describe the known immune response pathways that counter WNV infection in insects, birds, and mammals, as well as the mechanisms known to curb viral infection. Moreover, we discuss the bacterium Wolbachia and its involvement in reducing flavivirus titer in insects. Finally, we highlight the similarities of the known immune pathways and identify potential targets for future studies aimed at improving antiviral therapeutic and vaccination design.

An Overview of Current Approaches Toward the Treatment and Prevention of West Nile Virus Infection

The persistence of West Nile virus (WNV) infections throughout the USA since its inception in 1999 and its continuous spread throughout the globe calls for an urgent need of effective treatments and prevention measures. Although the licensing of several WNV vaccines for veterinary use provides a proof of concept, similar efforts on the development of an effective vaccine for humans remain still unsuccessful. Increased understanding of biology and pathogenesis of WNV together with recent technological advancements have raised hope that an effective WNV vaccine may be available in the near future. In addition, rapid progress in the structural and functional characterization of WNV and other flaviviral proteins have provided a solid base for the design and development of several classes of inhibitors as potential WNV therapeutics. Moreover, the therapeutic monoclonal antibodies demonstrate an excellent efficacy against WNV in animal models and represent a promising class of WNV therapeutics. However, there are some challenges as to the design and development of a safe and efficient WNV vaccine or therapeutic. In this chapter, we discuss the current approaches, progress, and challenges toward the development of WNV vaccines, therapeutic antibodies, and antiviral drugs.

Are we prepared for emerging flaviviruses in Europe? Challenges for vaccination

Tick-borne encephalitis and West Nile fever are endemic flavivirus diseases in Europe. Climate change, virus evolution, and social factors may increase the risk of these flavivirus infections and may lead to the emergence of other flaviviruses in Europe that are endemic in (sub)tropical regions of the world. Control of the spread of flaviviruses is very difficult considering the cycling of flaviviruses between arthropod vectors and animal reservoir hosts. The increasing threat of flavivirus infections emphasizes the necessity of a sustainable vector surveillance system, an active animal health surveillance system and an adequate human surveillance system for early detection of flavivirus infections. Vaccination is the most important approach to prevent flavivirus infections. Effective inactivated whole virus vaccines against tick-borne encephalitis (TBE) infection are available. Implementation of TBE vaccination based on favorable cost-effectiveness estimates per region and per target group can reduce the disease burden of TBE infection. At present, several West Nile virus (WNV) vaccine candidates are in various stages of clinical development. A major challenge for WNV vaccine candidates is to demonstrate efficacy, because of the sporadic nature of unpredictable WNV outbreaks. Universal WNV vaccination is unlikely to be cost-effective, vaccination of high-risk groups will be most appropriate to protect against WNV infections.

Cost effectiveness of a targeted age-based West Nile virus vaccination program

BACKGROUND

West Nile virus (WNV) is the leading cause of domestically-acquired arboviral disease in the United States. Several WNV vaccines are in various stages of development. We estimate the cost-effectiveness of WNV vaccination programs targeting groups at increased risk for severe WNV disease.

METHODS

We used a mathematical model to estimate costs and health outcomes of vaccination with WNV vaccine compared to no vaccination among seven cohorts, spaced at 10year intervals from ages 10 to 70years, each followed until 90-years-old. U.S. surveillance data were used to estimate WNV neuroinvasive disease incidence. Data for WNV seroprevalence, acute and long-term care costs of WNV disease patients, quality-adjusted life-years (QALYs), and vaccine characteristics were obtained from published reports. We assumed vaccine efficacy to either last lifelong or for 10years with booster doses given every 10years.

RESULTS

There was a statistically significant difference in cost-effectiveness ratios across cohorts in both models and all outcomes assessed (Kruskal-Wallis test p<0.0001). The 60-year-cohort had a mean cost per neuroinvasive disease case prevented of $664,000 and disability averted of $1,421,000 in lifelong model and $882,000 and $1,887,000, respectively in 10-year immunity model; these costs were statistically significantly lower than costs for other cohorts (p<0.0001). Vaccinating 70-year-olds had the lowest cost per death averted in both models at around $4.7 million (95%CI $2-$8 million). Cost per disease case averted was lowest among 40- and 50-year-old cohorts and cost per QALY saved lowest among 60-year cohorts in lifelong immunity model. The models were most sensitive to disease incidence, vaccine cost, and proportion of persons developing disease among infected.

CONCLUSIONS

Age-based WNV vaccination program targeting those at higher risk for severe disease is more cost-effective than universal vaccination. Annual variation in WNV disease incidence, QALY weights, and vaccine costs impact the cost effectiveness ratios.

A Live Attenuated Chimeric West Nile Virus Vaccine, rWN/DEN4Δ30, Is Well Tolerated and Immunogenic in Flavivirus-Naive Older Adult Volunteers

West Nile virus (WNV) is a major cause of mosquito-borne illness in the United States. Human disease ranges from mild febrile illness to severe fatal neurologic infection. Adults aged >60 years are more susceptible to neuroinvasive disease accompanied by a high mortality rate or long-lasting neurologic sequelae. A chimeric live attenuated West Nile virus vaccine, rWN/DEN4Δ30, was shown to be safe and immunogenic in healthy adults aged 18-50 years. This study evaluated rWN/DEN4Δ30 in flavivirus-naive adults aged 50-65 years and found it to be safe and immunogenic. Outbreaks of WNV infection tend to be unpredictable, and a safe and effective vaccine will be an important public health tool.

Neutropenia as an Adverse Event following Vaccination: Results from Randomized Clinical Trials in Healthy Adults and Systematic Review

BACKGROUND

In the context of early vaccine trials aimed at evaluating the safety profile of novel vaccines, abnormal haematological values, such as neutropenia, are often reported. It is therefore important to evaluate how these trials should be planned not to miss potentially important safety signals, but also to understand the implications and the clinical relevance.

METHODOLOGY

We report and discuss the results from five clinical trials (two with a new Shigella vaccine in the early stage of clinical development and three with licensed vaccines) where the absolute neutrophil counts (ANC) were evaluated before and after vaccination. Additionally, we have performed a systematic review of the literature on cases of neutropenia reported during vaccine trials to discuss our results in a more general context.

PRINCIPAL FINDINGS

Both in our clinical trials and in the literature review, several cases of neutropenia have been reported, in the first two weeks after vaccination. However, neutropenia was generally transient and had a benign clinical outcome, after vaccination with either multiple novel candidates or well-known licensed vaccines. Additionally, the vaccine recipients with neutropenia frequently had lower baseline ANC than non-neutropenic vaccinees. In many instances neutropenia occurred in subjects of African descent, known to have lower ANC compared to western populations.

CONCLUSIONS

It is important to include ANC and other haematological tests in early vaccine trials to identify potential safety signals. Post-vaccination neutropenia is not uncommon, generally transient and clinically benign, but many vaccine trials do not have a sampling schedule that allows its detection. Given ethnic variability in the level of circulating neutrophils, normal ranges taking into account ethnicity should be used for determination of trial inclusion/exclusion criteria and classification of neutropenia related adverse events.

TRIAL REGISTRATION

ClinicalTrials.gov NCT02017899, NCT02034500, NCT01771367, NCT01765413, NCT02523287.

A Novel Synthetic TLR-4 Agonist Adjuvant Increases the Protective Response to a Clinical-Stage West Nile Virus Vaccine Antigen in Multiple Formulations

West Nile virus (WNV) is a mosquito-transmitted member of the Flaviviridae family that has emerged in recent years to become a serious public health threat. Given the sporadic nature of WNV epidemics both temporally and geographically, there is an urgent need for a vaccine that can rapidly provide effective immunity. Protection from WNV infection is correlated with antibodies to the viral envelope (E) protein, which encodes receptor binding and fusion functions. Despite many promising E-protein vaccine candidates, there are currently none licensed for use in humans. This study investigates the ability to improve the immunogenicity and protective capacity of a promising clinical-stage WNV recombinant E-protein vaccine (WN-80E) by combining it with a novel synthetic TLR-4 agonist adjuvant. Using the murine model of WNV disease, we find that inclusion of a TLR-4 agonist in either a stable oil-in-water emulsion (SE) or aluminum hydroxide (Alum) formulation provides both dose and dosage sparing functions, whereby protection can be induced after a single immunization containing only 100 ng of WN-80E. Additionally, we find that inclusion of adjuvant with a single immunization reduced viral titers in sera to levels undetectable by viral plaque assay. The enhanced protection provided by adjuvanted immunization correlated with induction of a Th1 T-cell response and the resultant shaping of the IgG response. These findings suggest that inclusion of a next generation adjuvant may greatly enhance the protective capacity of WNV recombinant subunit vaccines, and establish a baseline for future development.

Development of a human live attenuated West Nile infectious DNA vaccine: Suitability of attenuating mutations found in SA14-14-2 for WN vaccine design

Direct attenuation of West Nile (WN) virus strain NY99 for the purpose of vaccine development is not feasible due to its high virulence and pathogenicity. Instead, we created highly attenuated chimeric virus W1806 with the serological identity of NY99. To further attenuate W1806, we investigated effects of mutations found in Japanese encephalitis virus vaccine SA14-14-2. WN viruses carrying all attenuating mutations lost infectivity in mammalian, but not in mosquito cells. No single reversion restored infectivity in mammalian cells, although increased infectivity in mosquito cells was observed. To identify a subset of mutations suitable for further attenuation of W1806, we analyzed effects of E138K and K279M changes on virulence, growth properties, and immunogenicity of derivatized W956, from which chimeric W1806 inherited its biological properties and attenuation profile. Despite strong dominant attenuating effect, introduction of only two mutations was not sufficient for attenuating W1806 to the safety level acceptable for human use.

Technologies for the development of West Nile virus vaccines

West Nile virus (WNV), an emerging mosquito-borne and zoonotic flavivirus, continues to spread worldwide and represents a major problem for human and veterinary medicine. In recent years, severe outbreaks were observed in the USA and Europe with neighboring countries, and the virus is considered to be endemic in an increasing number of areas. Although most infections remain asymptomatic, WNV can cause severe, even fatal, neurological disease, which affects mostly the elderly and immunocompromised individuals. Several vaccines have been licensed in the veterinary sector, but no human vaccine is available today. This review summarizes recent strategies that are being followed to develop WNV vaccines with emphasis on technologies suitable for the use in humans.

Development of a human live attenuated West Nile infectious DNA vaccine: conceptual design of the vaccine candidate

West Nile virus has become an important epidemiological problem attracting significant attention of health authorities, mass media, and the public. Although there are promising advancements toward addressing the vaccine need, the perspectives of the commercial availability of the vaccine remain uncertain. To a large extent this is due to lack of a sustained interest for further commercial development of the vaccines already undergoing the preclinical and clinical development, and a predicted insignificant cost effectiveness of mass vaccination. There is a need for a safe, efficacious and cost effective vaccine, which can improve the feasibility of a targeted vaccination program. In the present report, we summarize the background, the rationale, and the choice of the development pathway that we selected for the design of a live attenuated human West Nile vaccine in a novel infectious DNA format.

Plant-made vaccines against West Nile virus are potent, safe, and economically feasible

The threat of West Nile virus (WNV) epidemics with increasingly severe neuroinvasive infections demands the development and licensing of effective vaccines. To date, vaccine candidates based on inactivated, live-attenuated, or chimeric virus, and viral DNA and WNV protein subunits have been developed. Some have been approved for veterinary use or are under clinical investigation, yet no vaccine has been licensed for human use. Reaching the milestone of a commercialized human vaccine, however, may largely depend on the economics of vaccine production. Analysis suggests that currently only novel low-cost production technologies would allow vaccination to outcompete the cost of surveillance and clinical treatment. Here, we review progress using plants to address the economic challenges of WNV vaccine production. The advantages of plants as hosts for vaccine production in cost, speed and scalability, especially those of viral vector-based transient expression systems, are discussed. The progress in developing WNV subunit vaccines in plants is reviewed within the context of their expression, characterization, downstream processing, and immunogenicity in animal models. The development of vaccines based on enveloped and non-enveloped virus-like particles is also discussed. These advancements suggest that plants may provide a production platform that offers potent, safe and affordable human vaccines against WNV.

Correlation between serum levels of anti-endothelial cell autoantigen and anti-dengue virus nonstructural protein 1 antibodies in dengue patients

We have previously shown that anti-dengue virus nonstructural protein 1 (anti-DENV NS1) antibodies cross-react with endothelial cells, and several autoantigens have been identified. This study shows that the antibody levels against these self-proteins are higher in sera from patients with dengue hemorrhagic fever (DHF) than those in control sera. Anti-protein disulfide isomerase (PDI) and anti-heat shock protein 60 (anti-HSP60) IgM levels correlated with both anti-endothelial cells and anti-DENV NS1 IgM titers. A cross-reactive epitope on the NS1 amino acid residues 311-330 (P311-330) had been predicted. We further found that there were higher IgM and IgG levels against P311-330 in DHF patients' sera than those in the control sera. In addition, correlations were observed between anti-PDI with anti-P311-330 IgM and IgG levels, respectively. Therefore, our results indicate that DENV NS1 P311-330 is a major epitope for cross-reactive antibodies to PDI on the endothelial cell surface, which may play an important role in DENV infection-induced autoimmunity.

RNA-based viral vectors

The advent of reverse genetic approaches to manipulate the genomes of both positive (+) and negative (-) sense RNA viruses allowed researchers to harness these genomes for basic research. Manipulation of positive sense RNA virus genomes occurred first largely because infectious RNA could be transcribed directly from cDNA versions of the RNA genomes. Manipulation of negative strand RNA virus genomes rapidly followed as more sophisticated approaches to provide RNA-dependent RNA polymerase complexes coupled with negative-strand RNA templates were developed. These advances have driven an explosion of RNA virus vaccine vector development. That is, development of approaches to exploit the basic replication and expression strategies of RNA viruses to produce vaccine antigens that have been engineered into their genomes. This study has led to significant preclinical testing of many RNA virus vectors against a wide range of pathogens as well as cancer targets. Multiple RNA virus vectors have advanced through preclinical testing to human clinical evaluation. This review will focus on RNA virus vectors designed to express heterologous genes that are packaged into viral particles and have progressed to clinical testing.

Substitution of the premembrane and envelope protein genes of Modoc virus with the homologous sequences of West Nile virus generates a chimeric virus that replicates in vertebrate but not mosquito cells

Background

Most known flaviviruses, including West Nile virus (WNV), are maintained in natural transmission cycles between hematophagous arthropods and vertebrate hosts. Other flaviviruses such as Modoc virus (MODV) and Culex flavivirus (CxFV) have host ranges restricted to vertebrates and insects, respectively. The genetic elements that modulate the differential host ranges and transmission cycles of these viruses have not been identified.

Methods

Fusion polymerase chain reaction (PCR) was used to replace the capsid (C), premembrane (prM) and envelope (E) genes and the prM-E genes of a full-length MODV infectious cDNA clone with the corresponding regions of WNV and CxFV. Fusion products were directly transfected into baby hamster kidney-derived cells that stably express T7 RNA polymerase. At 4 days post-transfection, aliquots of each supernatant were inoculated onto vertebrate (BHK-21 and Vero) and mosquito (C6/36) cells which were then assayed for evidence of viral infection by reverse transcription-PCR, Western blot and plaque assay.

Results

Chimeric virus was recovered in cells transfected with the fusion product containing the prM-E genes of WNV. The virus could infect vertebrate but not mosquito cells. The in vitro replication kinetics and yields of the chimeric virus were similar to MODV but the chimeric virus produced larger plaques. Chimeric virus was not recovered in cells transfected with any of the other fusion products.

Conclusions

Our data indicate that genetic elements outside of the prM-E gene region of MODV condition its vertebrate-specific phenotype.

Current developments in understanding of West Nile virus central nervous system disease

PURPOSE OF REVIEW

West Nile virus (WNV) is the most important cause of epidemic encephalitis in the United States. We review articles published in the last 18 months related to the epidemiology, immunology, clinical features, and treatment of this disease.

RECENT FINDINGS

There was a resurgence in WNV disease in the United States in 2012. The WNV strain now predominant in the United States (NA/WN02) differs from the initial emergent isolate in 1999 (NY99). However, differences in the genetics of currently circulating United States WNV strains do not explain variations in epidemic magnitude or disease severity. Innate and acquired immunity are critical in control of WNV, and in some cases pathways are central nervous system specific. The clinical features of infection are now well understood, although nonconfirmed observations of chronic viral excretion in urine remain controversial. There is no specific antiviral therapy for WNV, but studies of antivirals specific for other flaviviruses may identify agents with promise against WNV. Phase I and II human WNV vaccine clinical trials have established that well tolerated and immunogenic WNV vaccines can be developed.

SUMMARY

WNV remains an important public health problem. Although recent studies have significantly increased our understanding of host immune and genetic factors involved in control of WNV infection, no specific therapy is yet available. Development of a well tolerated, immunogenic, and effective vaccine against WNV is almost certainly feasible, but economic factors and the lack of predictability of the magnitude and location of outbreaks are problematic for designing phase III trials and ultimate licensure.

Vaccine-mediated immunity against dengue and the potential for long-term protection against disease

It is estimated that over 2.5 billion people are at risk for contracting dengue, a virus responsible for 50–390 million infections in addition to thousands of hospitalizations and deaths each year. There are no licensed vaccines available to combat this pathogen but substantial efforts are underway to develop live-attenuated, inactivated, and subunit vaccines that will protect against each of the four serotypes of dengue. Unfortunately, the results of a recent Phase IIb efficacy trial involving a tetravalent live-attenuated chimeric dengue virus vaccine have raised questions with regard to our current understanding of vaccine-mediated immunity to this important flavivirus. Here, we will briefly summarize these vaccination efforts and discuss the importance of informative in vivo models for determining vaccine efficacy and the need to establish a quantitative correlate of immunity in order to predict the duration of vaccine-induced antiviral protection.

Assurance of neuroattenuation of a live vaccine against West Nile virus: a comprehensive study of neuropathogenesis after infection with chimeric WN/DEN4Δ30 vaccine in comparison to two parental viruses and a surrogate flavivirus reference vaccine

The upsurge of West Nile virus (WNV) human infections in 2012 suggests that the US can expect periodic WNV outbreaks in the future. Availability of safe and effective vaccines against WNV in endemic areas, particularly for aging populations that are at high risk of West Nile neuroinvasive disease (WNND), could be beneficial. WN/DEN4Δ30 is a live, attenuated chimeric vaccine against WNV produced by replacement of the genes encoding the pre-membrane and envelope protein genes of the vaccine virus against dengue virus type 4 (DEN4Δ30) with corresponding sequences derived from a wild type WNV. Following intrathalamic inoculation of nonhuman primates (NHPs), a comprehensive neuropathogenesis study was performed and neurovirulence of WN/DEN4Δ30 vaccine candidate was compared to that of two parental viruses (i.e., WNV and DEN4Δ30), as well as to that of an attenuated flavivirus surrogate reference (i.e., yellow fever YF 17D). Clinical and virological data, as well as results of a semi-quantitative histopathological analysis, demonstrated that WN/DEN4Δ30 vaccine is highly attenuated for the central nervous system (CNS) of NHPs in comparison to a wild type WNV. Importantly, based on the virus replicative ability in the CNS of NHPs and the degree of induced histopathological changes, the level of neuroattenuation of WN/DEN4Δ30 vaccine was similar to that of YF 17D, and therefore within an acceptable range. In addition, we show that the DEN4Δ30 vaccine tested in this study also has a low neurovirulence profile. In summary, our results demonstrate a high level of neuroattenuation of two vaccine candidates, WN/DEN4Δ30 and DEN4Δ30. We also show here a remarkable sensitivity of our WNV-NY99 NHP model, as well as striking resemblance of the observed neuropathology to that seen in human WNND. These results support the use of this NHP model for translational studies of WNV neuropathogenesis and/or testing the effectiveness of vaccines and therapeutic approaches.

A review of the epidemiological and clinical aspects of West Nile virus

The resurgence of West Nile virus (WNV) in North America and Europe in recent years has raised the concerns of local authorities and highlighted that mosquito-borne disease is not restricted to tropical regions of the world. WNV is maintained in enzootic cycles involving, primarily, Culex spp. mosquitoes and avian hosts, with epizootic spread to mammals, including horses and humans. Human infection results in symptomatic illness in approximately one-fifth of cases and neuroinvasive disease in less than 1% of infected persons. The most consistently recognized risk factor for neuroinvasive disease is older age, although diabetes mellitus, alcohol excess, and a history of cancer may also increase risk. Despite the increasing public health concern, the current WNV treatments are inadequate. Current evidence supporting the use of ribavirin, interferon α, and WNV-specific immunoglobulin are reviewed. Nucleic acid detection has been an important diagnostic development, which is particularly important for the protection of the donated blood supply. While effective WNV vaccines are widely available for horses, no human vaccine has been registered. Uncertainty surrounds the magnitude of future risk posed by WNV, and predictive models are limited by the heterogeneity of environmental, vector, and host factors, even in neighboring regions. However, recent history has demonstrated that for regions where suitable mosquito vectors and reservoir hosts are present, there will be a risk of major epidemics. Given the potential for these outbreaks to include severe neuroinvasive disease, strategies should be implemented to monitor for, and respond to, outbreak risk. While broadscale mosquito control programs will assist in reducing the abundance of mosquito populations and subsequently reduce the risks of disease, for many individuals, the use of topical insect repellents and other personal protective strategies will remain the first line of defense against infection.

Current trends in West Nile virus vaccine development

West Nile virus (WNV) is a mosquito-borne flavivirus that has become endemic in the United States. From 1999-2012, there have been 37088 reported cases of WNV and 1549 deaths, resulting in a 4.2% case-fatality rate. Despite development of effective WNV vaccines for horses, there is no vaccine to prevent human WNV infection. Several vaccines have been tested in preclinical studies and to date there have been eight clinical trials, with promising results in terms of safety and induction of antiviral immunity. Although mass vaccination is unlikely to be cost effective, implementation of a targeted vaccine program may be feasible if a safe and effective vaccine can be brought to market. Further evaluation of new and advanced vaccine candidates is strongly encouraged.

A review of successful flavivirus vaccines and the problems with those flaviviruses for which vaccines are not yet available

Genus flavivirus comprises many important human pathogens causing public health problems worldwide. Some flavivirus infections are characterized by a relatively high mortality rate and/or high sequelae rate in survivors. Because most flavivirus life cycles are maintained between arthropod vectors and amplifying/reservoir hosts in the absence of humans, eradication of flaviviruses might be extremely difficult. Flavivirus vaccine development is considered a reasonable method to prevent flavivirus infections. Some vaccines have been successfully developed, but others have not, regardless of much effort. This review article describes currently available flavivirus vaccines against yellow fever, Japanese encephalitis, and tick-borne encephalitis. In addition, the current status of dengue and West Nile virus vaccine development is reviewed and problems regarding their development are discussed.