Yellow fever vector live-virus vaccines: West Nile virus vaccine development

Vaccine Preventable Zoonotic Diseases: Challenges and Opportunities for Public Health Progress

Zoonotic diseases represent a heavy global burden, causing important economic losses, impacting animal health and production, and costing millions of human lives. The vaccination of animals and humans to prevent inter-species zoonotic disease transmission is an important intervention. However, efforts to develop and implement vaccine interventions to reduce zoonotic disease impacts are often limited to the veterinary and agricultural sectors and do not reflect the shared burden of disease. Multisectoral collaboration, including co-development opportunities for human and animal vaccines, expanding vaccine use to include animal reservoirs such as wildlife, and strategically using vaccines to interrupt complex transmission cycles is needed. Addressing zoonoses requires a multi-faceted One Health approach, wherein vaccinating people and animals plays a critical role.

Highlights in Mesoporous Silica Nanoparticles as a Multifunctional Controlled Drug Delivery Nanoplatform for Infectious Diseases Treatment

Infectious diseases are a major global concern being responsible for high morbidity and mortality mainly due to the development and enhancement of multidrug-resistant microorganisms exposing the fragility of medicines and vaccines commonly used to these treatments. Taking into account the scarcity of effective formulation to treat infectious diseases, nanotechnology offers a vast possibility of ground-breaking platforms to design new treatment through smart nanostructures for drug delivery purposes. Among the available nanosystems, mesoporous silica nanoparticles (MSNs) stand out due their multifunctionality, biocompatibility and tunable properties make them emerging and actual nanocarriers for specific and controlled drug release. Considering the high demand for diseases prevention and treatment, this review exploits the MSNs fabrication and their behavior in biological media besides highlighting the most of strategies to explore the wide MSNs functionality as engineered, smart and effective controlled drug release nanovehicles for infectious diseases treatment. Graphical Abstract Schematic representation of multifunctional MSNs-based nanoplatforms for infectious diseases treatment.

West Nile Virus: Veterinary Health and Vaccine Development

West Nile virus (WNV) (Flaviviridae: Flavivirus) was discovered in Africa more than 80 yr ago and became recognized as an avian pathogen and a cause of neurologic disease in horses largely during periodic incursions into Europe. Introduction of WNV into North America stimulated great anxiety, particularly in the equine industry, but also for pet owners and livestock producers concerned about the effect of WNV on other domestic animals. Numerous subsequent studies of naturally occurring and experimentally induced disease greatly expanded our understanding of the host range and clinical consequences of WNV infection in diverse species and led to rapid development and deployment of efficacious vaccines for horses. In addition to humans, horses are clearly the animals most frequently affected by serious, sometimes lethal disease following infection with WNV, but are dead-end hosts due to the low-magnitude viremia they develop. Dogs, cats, and livestock species including chickens are readily infected with WNV, but only occasionally develop clinical disease and are considered dead-end hosts for the virus.

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.

West Nile Virus

Several infectious agents have emerged over the past two decades as a result of population migration and enhanced world travel. The introduction of arthropodborne infections to formerly unaffected geographic areas has occurred in the western hemisphere at an alarming rate. In 1999, the West Nile virus (WNV) first appeared in North America in Queens, New York and spread rapidly to infect bird and mosquito populations along the Atlantic and Gulf coasts. Significant human morbidity and mortality has been associated with the virus, with several patients deaths from encephalitis. Specific antiviral therapy is currently unavailable, but recommendations for a national plan to control and prevent the spread of this vector-borne disease have been made by the Centers for Disease Control and Prevention. Educating the public about how WNV is transmitted, how to best protect one's self, and what signs and symptoms are consistent with this infection are extremely important. Pharmacists, who have more patient interactions per unit time than any other health care provider group, are in an excellent position to play a pivotal role in this educational effort.

Vector-based genetically modified vaccines: Exploiting Jenner's legacy

The global vaccine market is diverse while facing a plethora of novel developments. Genetic modification (GM) techniques facilitate the design of ’smarter’ vaccines. For many of the major infectious diseases of humans, like AIDS and malaria, but also for most human neoplastic disorders, still no vaccines are available. It may be speculated that novel GM technologies will significantly contribute to their development. While a promising number of studies is conducted on GM vaccines and GM vaccine technologies, the contribution of GM technology to newly introduced vaccines on the market is disappointingly limited.

In this study, the field of vector-based GM vaccines is explored. Data on currently available, actually applied, and newly developed vectors is retrieved from various sources, synthesised and analysed, in order to provide an overview on the use of vector-based technology in the field of GM vaccine development. While still there are only two vector-based vaccines on the human vaccine market, there is ample activity in the fields of patenting, preclinical research, and different stages of clinical research. Results of this study revealed that vector-based vaccines comprise a significant part of all GM vaccines in the pipeline. This study further highlights that poxviruses and adenoviruses are among the most prominent vectors in GM vaccine development.

After the approval of the first vectored human vaccine, based on a flavivirus vector, vaccine vector technology, especially based on poxviruses and adenoviruses, holds great promise for future vaccine development. It may lead to cheaper methods for the production of safe vaccines against diseases for which no or less perfect vaccines exist today, thus catering for an unmet medical need. After the introduction of Jenner’s vaccinia virus as the first vaccine more than two centuries ago, which eventually led to the recent eradication of smallpox, this and other viruses may now be the basis for constructing vectors that may help us control other major scourges of mankind.

Recovery of West Nile Virus Envelope Protein Domain III Chimeras with Altered Antigenicity and Mouse Virulence

Flaviviruses are positive-sense, single-stranded RNA viruses responsible for millions of human infections annually. The envelope (E) protein of flaviviruses comprises three structural domains, of which domain III (EIII) represents a discrete subunit. The EIII gene sequence typically encodes epitopes recognized by virus-specific, potently neutralizing antibodies, and EIII is believed to play a major role in receptor binding. In order to assess potential interactions between EIII and the remainder of the E protein and to assess the effects of EIII sequence substitutions on the antigenicity, growth, and virulence of a representative flavivirus, chimeric viruses were generated using the West Nile virus (WNV) infectious clone, into which EIIIs from nine flaviviruses with various levels of genetic diversity from WNV were substituted. Of the constructs tested, chimeras containing EIIIs from Koutango virus (KOUV), Japanese encephalitis virus (JEV), St. Louis encephalitis virus (SLEV), and Bagaza virus (BAGV) were successfully recovered. Characterization of the chimeras in vitro and in vivo revealed differences in growth and virulence between the viruses, within vivo pathogenesis often not being correlated within vitro growth. Taken together, the data demonstrate that substitutions of EIII can allow the generation of viable chimeric viruses with significantly altered antigenicity and virulence.

IMPORTANCE

The envelope (E) glycoprotein is the major protein present on the surface of flavivirus virions and is responsible for mediating virus binding and entry into target cells. Several viable West Nile virus (WNV) variants with chimeric E proteins in which the putative receptor-binding domain (EIII) sequences of other mosquito-borne flaviviruses were substituted in place of the WNV EIII were recovered, although the substitution of several more divergent EIII sequences was not tolerated. The differences in virulence and tissue tropism observed with the chimeric viruses indicate a significant role for this sequence in determining the pathogenesis of the virus within the mammalian host. Our studies demonstrate that these chimeras are viable and suggest that such recombinant viruses may be useful for investigation of domain-specific antibody responses and the more extensive definition of the contributions of EIII to the tropism and pathogenesis of WNV or other flaviviruses.

Substitution of NS5 N-terminal domain of dengue virus type 2 RNA with type 4 domain caused impaired replication and emergence of adaptive mutants with enhanced fitness

Flavivirus NS3 and NS5 are required in viral replication and 5'-capping. NS3 has NS2B-dependent protease, RNA helicase, and 5'-RNA triphosphatase activities. NS5 has 5'-RNA methyltransferase (MT)/guanylyltransferase (GT) activities within the N-terminal 270 amino acids and the RNA-dependent RNA polymerase (POL) activity within amino acids 271-900. A chimeric NS5 containing the D4MT/D4GT and the D2POL domains in the context of wild-type (WT) D2 RNA was constructed. RNAs synthesized in vitro were transfected into baby hamster kidney cells. The viral replication was analyzed by an indirect immunofluorescence assay to monitor NS1 expression and by quantitative real-time PCR. WT D2 RNA-transfected cells were NS1- positive by day 5, whereas the chimeric RNA-transfected cells became NS1-positive ∼30 days post-transfection in three independent experiments. Sequence analysis covering the entire genome revealed the appearance of a single K74I mutation within the D4MT domain ∼16 days post-transfection in two experiments. In the third, D290N mutation in the conserved NS3 Walker B motif appeared ≥16 days post-transfection. A time course study of serial passages revealed that the 30-day supernatant had gradually evolved to gain replication fitness. Trans-complementation by co-expression of WT D2 NS5 accelerated viral replication of chimeric RNA without changing the K74I mutation. However, the MT and POL activities of NS5 WT D2 and the chimeric NS5 proteins with or without the K74I mutation are similar. Taken together, our results suggest that evolution of the functional interactions involving the chimeric NS5 protein encoded by the viral genome species is essential for gain of viral replication fitness.

Preclinical and clinical development of a YFV 17 D-based chimeric vaccine against West Nile virus

Substantial success has been achieved in the development and implementation of West Nile (WN) vaccines for horses; however, no human WN vaccines are approved. This review focuses on the construction, pre-clinical and clinical characterization of ChimeriVax-WN02 for humans, a live chimeric vaccine composed of a yellow fever (YF) 17D virus in which the prM-E envelope protein genes are replaced with the corresponding genes of the WN NY99 virus. Pre-clinical studies demonstrated that ChimeriVax-WN02 was significantly less neurovirulent than YF 17D in mice and rhesus and cynomolgus monkeys. The vaccine elicited neutralizing antibody titers after inoculation in hamsters and monkeys and protected immunized animals from lethal challenge including intracerebral inoculation of high dose of WN NY99 virus. Safety, viremia and immunogenicity of ChimeriVax-WN02 were assessed in one phase I study and in two phase II clinical trials. No safety signals were detected in the three clinical trials with no remarkable differences in incidence of adverse events (AEs) between vaccine and placebo recipients. Viremia was transient and the mean viremia levels were low. The vaccine elicited strong and durable neutralizing antibody and cytotoxic T cell responses. WN epidemiology impedes a classical licensure pathway; therefore, innovative licensure strategies should be explored.

Development of chimaeric West Nile virus attenuated vaccine candidate based on the Japanese encephalitis vaccine strain SA14-14-2

Mosquito-borne flaviviruses include a large group of important human medical pathogens. Several chimaeric flaviviruses have been constructed, and show potential for vaccine development. Although Japanese encephalitis virus (JEV) live vaccine SA14-14-2 has been widely used with ideal safety and efficacy profiles, no chimaeric flavivirus based on the JEV vaccine has been described to date. Based on the reverse genetic system of the JEV vaccine SA14-14-2, a novel live chimaeric flavivirus carrying the protective antigens of West Nile virus (WNV) was constructed and recovered in this study. The resulting chimaera (ChinWNV) replicated efficiently in both mammalian and mosquito cells and possessed genetic stability after in vitro serial passaging. ChinWNV exhibited a small-plaque phenotype, and its replication was significantly restricted in mouse peripheral blood and brain compared with parental WNV. Importantly, ChinWNV was highly attenuated with regard to both neurovirulence and neuroinvasiveness in mice. Furthermore, a single ChinWNV immunization stimulated robust WNV-specific adaptive immune responses in mice, conferring significant protection against lethal WNV infection. Our results demonstrate that chimaeric flaviviruses based on the JEV vaccine can serve as a powerful platform for vaccine development, and that ChinWNV represents a potential WNV vaccine candidate that merits further development.

Vaccines against diseases transmitted from animals to humans: a one health paradigm

This review focuses on the immunization of animals as a means of preventing human diseases (zoonoses). Three frameworks for the use of vaccines in this context are described, and examples are provided of successes and failures. Framework I vaccines are used for protection of humans and economically valuable animals, where neither plays a role in the transmission cycle. The benefit of collaborations between animal health and human health industries and regulators in developing such products is discussed, and one example (West Nile vaccine) of a single product developed for use in animals and humans is described. Framework II vaccines are indicated for domesticated animals as a means of preventing disease in both animals and humans. The agents of concern are transmitted directly or indirectly (e.g. via arthropod vectors) from animals to humans. A number of examples of the use of Framework II vaccines are provided, e.g. against brucellosis, Escherichia coli O157, rabies, Rift Valley fever, Venezuelan equine encephalitis, and Hendra virus. Framework III vaccines are used to immunize wild animals as a means of preventing transmission of disease agents to humans and domesticated animals. Examples are reservoir-targeted, oral bait rabies, Mycobacterium bovis and Lyme disease vaccines. Given the speed and lost cost of veterinary vaccine development, some interventions based on the immunization of animals could lead to rapid and relatively inexpensive advances in public health. Opportunities for vaccine-based approaches to preventing zoonotic and emerging diseases that integrate veterinary and human medicine (the One Health paradigm) are emphasized.

Current progress with serological assays for exotic emerging/re-emerging viruses

Recent decades have witnessed an unprecedented rise in the outbreak occurrence of infectious and primarily zoonotic viruses. Contributing factors to this phenomenon include heightened global connectivity via air travel and international trade links, as well as man-made environmental alterations, such as deforestation and climate change, which all serve to bring humans into closer contact with animal reservoirs and alter the habitat of vectors, thus facilitating the transmission of viruses between species. Serological assays are integral to tracking the epidemiological spread of a virus and evaluating mass vaccination programs by quantifying neutralizing antibody responses raised against antigenic epitopes on the viral surface. However, conventional serological tests are somewhat marred by equipment and reagent costs, the necessity for high-containment laboratories for studying many emerging viruses, and interlaboratory variability, among other issues. This review details ‘next-generation’ assays aimed at addressing some of the persistent problems with viral serology, focusing on how manipulating the genomes of RNA viruses can produce attenuated or chimeric viruses that can be exploited as surrogate viruses in neutralization assays. Despite the undoubted promise of such novel serological platforms, it must be remembered that these assays have to withstand rigorous validation and standardization measures before they can play an integral role in curtailing the severity of future emerging virus outbreaks.

Evaluation of chimeric Japanese encephalitis and dengue viruses for use in diagnostic plaque reduction neutralization tests

The plaque reduction neutralization test (PRNT) is a specific serological test used to identify and confirm arbovirus infection in diagnostic laboratories and monitor immunological protection in vaccine recipients. Wild-type (wt) viruses used in the PRNT may be difficult to grow and plaque titrate, such as the dengue viruses (DENV), and/or may require biosafety level 3 (BSL3) containment, such as West Nile virus (WNV), St. Louis encephalitis virus (SLEV), and Japanese encephalitis virus (JEV). These requirements preclude their use in diagnostic laboratories with only BSL2 capacity. In addition, wt JEV falls under the jurisdiction of the select-agent program and can be used only in approved laboratories. The chimeric vaccine viruses ChimeriVax-WNV and -SLEV have previously been shown to elicit antibody reactivity comparable to that of parental wt WNV and SLEV. ChimeriVax viruses provide advantages for PRNT, as follows: they grow more rapidly than most wt flaviviruses, produce large plaques, require BSL2 conditions, and are not under select-agent restrictions. We evaluated the ChimeriVax-DENV serotype 1 (DENV1), -DENV2, -DENV3, -DENV4, and -JEV for use in PRNT on sera from DENV- and JEV-infected patients and from JEV vaccine recipients. Serostatus agreement was 100% between the ChimeriVax-DENV serotypes and wt prototype DENV and 97% overall with ChimeriVax-JEV compared to prototype Nakayama JEV, 92% in a subgroup of JEV vaccine recipients, and 100% in serum from encephalitis patients naturally infected with JEV. ChimeriVax-DENV and -JEV plaque phenotype and BSL2 requirements, combined with sensitive and specific reactivity, make them good substitutes for wt DENV and JEV in PRNT in public health diagnostic laboratories.

Use of a surrogate chimeric virus to detect West Nile virus-neutralizing antibodies in avian and equine sera

A chimeric yellow fever virus/West Nile virus (WNV) was compared to WNV alone as a biosafety level 2 reagent in the plaque reduction neutralization test for determining WNV infection histories. Concordance was 96.3% among 188 avian and equine serum samples. Neutralizing antibody titers were frequently more than twofold lower with the chimera.

Safety of an attenuated West Nile virus vaccine, live Flavivirus chimera in horses

REASON FOR PERFORMING STUDY

West Nile virus (WNV) infection is endemic and able to cause disease in naive hosts. It is necessary therefore to evaluate the safety of new vaccines.

OBJECTIVES

To establish: 1) the safety of a modified live Flavivirus/West Nile virus (WN-FV) chimera by administration of an overdose and testing for shed of vaccine virus and spread to uninoculated sentinel horses; 2) that this vaccine did not become pathogenic once passaged in horses; and 3) vaccine safety under field conditions.

METHODS

There were 3 protocols: 1) In the overdose/shed and spread study, horses were vaccinated with a 100x immunogenicity overdose of WN-FV chimera vaccine and housed with sentinel horses. 2) A reversion to virulence study, where horses were vaccinated with a 20x immunogenicity overdose of WN-FV chimera vaccine. Horses in both studies were evaluated for abnormal health conditions and samples obtained to detect virus, seroconversion and dissemination into tissues. 3) In a field safety test 919 healthy horses of various ages, breeds and sex were used.

RESULTS

Vaccination did not result in site or systemic reactions in either experimental or field-injected horses. There was no shed of vaccine virus, no detection of vaccine virus into tissue and no reversion to virulence with passage.

CONCLUSIONS

WN-FV chimera vaccine is safe to use in horses with no evidence of ill effects from very high doses of vaccine. There was no evidence of reversion to virulence. In addition, administration of this vaccine to several hundred horses that may have been previously exposed to WNV or WNV vaccine resulted in no untoward reactions.

POTENTIAL RELEVANCE

These studies establish that this live attenuated Flavivirus chimera is safe to use for immunoprophylaxis against WNV disease in horses.

Human outbreak of St. Louis encephalitis detected in Argentina, 2005

BACKGROUND

An outbreak of flavivirus encephalitis occurred in 2005 in Córdoba province, Argentina.

OBJECTIVES

To characterize the epidemiologic and clinical features of that outbreak and provide the serologic results that identified St. Louis encephalitis virus (SLEV) as the etiologic agent.

STUDY DESIGN

From January to May 2005, patients with symptoms of encephalitis, meningitis, or fever with severe headache were evaluated and an etiologic diagnosis achieved by detection of flavivirus-specific antibody sera and cerebrospinal fluid.

RESULTS

The epidemic curve of 47 cases showed an explosive outbreak starting in January 2005 with one peak in mid-February and a second peak in mid-March; the epidemic ended in May. Cases occurred predominantly among persons 60 years and older. Nine deaths were reported. SLEV antibodies, when detected in 47 patients studied, had a pattern characteristic of a primary SLEV infection.

CONCLUSIONS

Even though isolated cases of St. Louis encephalitis have been reported in Argentina, this is the first description of a large SLEV encephalitis outbreak in Argentina.

The molecular basis of antibody protection against West Nile virus

West Nile virus (WNV) infection of mosquitoes, birds, and vertebrates continues to spread in the Western Hemisphere. In humans, WNV infects the central nervous system and causes severe disease, primarily in the immunocompromised and elderly. In this review we discuss the mechanisms by which antibody controls WNV infection. Recent virologic, immunologic, and structural experiments have enhanced our understanding on how antibodies neutralize WNV and protect against disease. These advances have significant implications for the development of novel antibody-based therapies and targeted vaccines.

Chimeric dengue 2 PDK-53/West Nile NY99 viruses retain the phenotypic attenuation markers of the candidate PDK-53 vaccine virus and protect mice against lethal challenge with West Nile virus

Chimeric dengue serotype 2/West Nile (D2/WN) viruses expressing prM-E of WN NY99 virus in the genetic background of wild-type D2 16681 virus and two candidate D2 PDK-53 vaccine variants (PDK53-E and PDK53-V) were engineered. The viability of the D2/WN viruses required incorporation of the WN virus-specific signal sequence for prM. Introduction of two mutations at M-58 and E-191 in the chimeric cDNA clones further improved the viability of the chimeras constructed in all three D2 carriers. Two D2/WN chimeras (D2/WN-E2 and -V2) engineered in the backbone of the PDK53-E and -V viruses retained all of the PDK-53 vaccine characteristic phenotypic markers of attenuation and were immunogenic in mice and protected mice from a high-dose 10(7) PFU challenge with wild-type WN NY99 virus. This report further supports application of the genetic background of the D2 PDK-53 virus as a carrier for development of live-attenuated, chimeric flavivirus vaccines in general and the development of a chimeric D2/WN vaccine virus against WN disease in particular.

West Nile virus. Primer for family physicians

OBJECTIVE

To provide primary care physicians with an understanding of West Nile virus in North America. This article focuses on epidemiology, clinical features, diagnosis, and prevention of infection.

QUALITY OF EVIDENCE

MEDLINE and EMBASE searches revealed epidemiologic, surveillance, cohort, and outcome studies providing level II evidence. There were no randomized controlled trials of treatment. Recommended prevention and treatment strategies are based on level II and III evidence.

MAIN MESSAGE

The mosquito-borne virus that first appeared on this continent in 1999 is now prevalent throughout North America. Most infections are asymptomatic. Fewer than 1% of those infected develop severe illness; 3% to 15% of those with severe illness die. While methods for controlling the mosquito population are available, we lack evidence that they reduce infection in the general human population. Family physicians have an important role in advising their patients on ways to prevent infection and in identifying patients who might be infected with West Nile virus.

CONCLUSION

The general population is at low risk of West Nile virus infection. Prevention of infection rests on controlling the mosquito population and educating people on how to protect themselves against mosquito bites.

West Nile premembrane-envelope genetic vaccine encoded as a chimera containing the transmembrane and cytoplasmic domains of a lysosome-associated membrane protein: increased cellular concentration of the transgene product, targeting to the MHC II compartment, and enhanced neutralizing antibody response

A genetic vaccine for West Nile virus (WN) has been synthesized with the WN premembrane-envelope (WN preM-E) gene sequences encoded as a chimera with the transmembrane and carboxyl terminal domains of the lysosome-associated membrane protein (LAMP). The LAMP sequences are used to direct the antigen protein to the major histocompatibility class II (MHC II) vesicular compartment of transfected professional antigen-presenting cells (APCs). Vaccine constructs encoding the native WN preM-E and WN preM-E/LAMP chimera were synthesized in pVAX1 and pITR plasmid backbones. Extracts of human fibroblast 293 and monkey kidney COS-7 cells transfected with the WN preM-E/LAMP chimera constructs contained much greater amounts of E than did the cells transfected with constructs encoding the native WN preM-E. This difference in the concentration of native E and the E/LAMP chimera in transfected cells is attributed to the secretion of native E. The amount of preM protein in cell extracts, in contrast to the E protein, and the levels of DNA and RNA transcripts, did not differ between WN preM-E- and WN preM-E/LAMP-transfected cells. Additionally, confocal and immunoelectron microscopic analyses of transfected B cells showed localization of the WN preM-E/LAMP chimera in vesicular compartments containing endogenous LAMP, MHC II, and H2-M, whereas native viral preM-E lacking the LAMP sequences was distributed within the cellular vesicular network with little LAMP or MHC II association. Mice immunized with a DNA construct expressing the WN preM-E/LAMP antigen induced significant antibody and long-term neutralization titers in contrast to the minimal and short-lived neutralization titer of mice vaccinated with a plasmid expressing the untargeted antigen. These results underscore the utility of LAMP targeting of the WN envelope to the MHC II compartments in the design of a genetic WN vaccine.

A recombinant envelope protein vaccine against West Nile virus

West Nile (WN) virus is a flavivirus that first appeared in North America in 1999. Since then, more than 600 human deaths and 22,000 equine infections have been attributed to the virus in the United States. We expressed a truncated form of WN virus envelope (E) protein in Drosophila S2 cells. This soluble recombinant E protein was recognized by antibodies from naturally infected horses, indicating that it contains native epitopes. Mice and horses produced high-titer antibodies when immunized with recombinant E protein combined with aluminum hydroxide. Immunized mice were resistant to challenge with a lethal viral dose. Sera from immunized horses, administered to naive mice, conferred resistance against a lethal WN viral challenge. In addition, sera of immunized horses neutralized West Nile virus in vitro, as demonstrated by plaque reduction assays. This recombinant form of E protein, combined with aluminum hydroxide, is a candidate vaccine that may protect humans and horses against WN virus infections.

An attenuated West Nile prototype virus is highly immunogenic and protects against the deadly NY99 strain: a candidate for live WN vaccine development

In a short time, West Nile virus has developed into a nationwide health and veterinary problem. The high virulence of the circulating virus and related lineage 1 WN strains hinders development of an attenuated live vaccine. We describe an attenuated WN isolate, WN1415, which is a molecularly cloned descendant of the WN prototype B956 strain. The parent virus belongs to lineage 2, members of which have not been associated with epidemic or epizootic outbreaks. A set of non-conservative mutations, mostly in non-structural protein genes, distinguishes the WN1415 isolate from the parent B956 prototype strain. Immunization with WN1415 (55-550,000 pfu) established a potent immunity, which protected the majority of mice against lethal challenge with WN NY99. The attenuated nature of the isolate and its excellent growth characteristics combined with the availability of a highly stable infectious clone make the isolate an attractive candidate for live WN vaccine development.

Recent advancement in flavivirus vaccine development

Lately, the magnitude of cumulative diseases burden caused by flaviviruses, such as dengue virus, Japanese encephalitis virus, tick-borne encephalitis virus, West Nile virus and yellow fever virus, has reached an unprecedented level with the sizes of human and animal populations at risk increasing sharply. These diseases present highly complex medical, economic and ecologic problems, some effecting primarily human and others affecting human, livestock and wildlife. The large body of recent publications on the development of vaccines taking advantage of new generations of bio-engineering techniques clearly reflects the profound interests and deep sense of urgency in the scientific and medical communities in combating those diseases. This review reveals a collection of remarkable progresses thus far made in flaviviral vaccine research not only employing a diverse range of new strategies but also re-tooling old techniques to improve the existing vaccines. The efficacy and safety of some of the new vaccine candidates have been evaluated and proven in human clinical trials. Besides the technical advancement in vaccine development, in this review, the importance of somewhat neglected and yet critical subjects, such as adequacy of animal model, vaccine safety, vaccine formulation and delivery, complication in serodiagostics and economic factor, was examined in-depth.

ChimeriVax-West Nile virus live-attenuated vaccine: preclinical evaluation of safety, immunogenicity, and efficacy

The availability of ChimeriVax vaccine technology for delivery of flavivirus protective antigens at the time West Nile (WN) virus was first detected in North America in 1999 contributed to the rapid development of the vaccine candidate against WN virus described here. ChimeriVax-Japanese encephalitis (JE), the first live- attenuated vaccine developed with this technology has successfully undergone phase I and II clinical trials. The ChimeriVax technology utilizes yellow fever virus (YF) 17D vaccine strain capsid and nonstructural genes to deliver the envelope gene of other flaviviruses as live-attenuated chimeric viruses. Amino acid sequence homology between the envelope protein (E) of JE and WN viruses facilitated targeting attenuating mutation sites to develop the WN vaccine. Here we discuss preclinical studies with the ChimeriVax-WN virus in mice and macaques. ChimeriVax-WN virus vaccine is less neurovirulent than the commercial YF 17D vaccine in mice and nonhuman primates. Attenuation of the virus is determined by the chimeric nature of the construct containing attenuating mutations in the YF 17D virus backbone and three point mutations introduced to alter residues 107, 316, and 440 in the WN virus E protein gene. The safety, immunogenicity, and efficacy of the ChimeriVax-WN(02) vaccine in the macaque model indicate the vaccine candidate is expected to be safe and immunogenic for humans.

Safety and efficacy of chimeric yellow Fever-dengue virus tetravalent vaccine formulations in nonhuman primates

To construct chimeric YF/DEN viruses (ChimeriVax-DEN), the premembrane (prM) and envelope (E) genes of yellow fever (YF) 17D virus were replaced with those of each wild-type (WT) dengue (DEN) virus representing serotypes 1 to 4. ChimeriVax-DEN1-4 vaccine viruses were prepared by electroporation of Vero cells with RNA transcripts prepared from viral cDNA (F. Guirakhoo, J. Arroyo, K. V. Pugachev, C. Miller, Z.-X. Zhang, R. Weltzin, K. Georgakopoulos, J. Catalan, S. Ocran, K. Soike, M. Ratteree, and T. P. Monath, J. Virol. 75:7290-7304, 2001; F. Guirakhoo, K. Pugachev, J. Arroyo, C. Miller, Z.-X. Zhang, R. Weltzin, K. Georgakopoulos, J. Catalan, S. Ocran, K. Draper, and T. P. Monath, Virology 298:146-159, 2002). Progeny viruses were subjected to three rounds of plaque purifications to produce the Pre-Master Seed viruses at passage 7 (P7). Three further passages were carried out using U.S. current Good Manufacturing Practices (cGMP) to produce the Vaccine Lot (P10) viruses. Preclinical studies demonstrated that the vaccine candidates are replication competent and genetically stable and do not become more neurovirulent upon 20 passages in Vero cells. The safety of a tetravalent vaccine was determined and compared to that of YF-VAX in a formal monkey neurovirulence test. Brain lesions produced by the tetravalent ChimeriVax-DEN vaccine were significantly less severe than those observed with YF-VAX. The immunogenicity and protective efficacy of four different tetravalent formulations were evaluated in cynomolgus monkeys following a single-dose subcutaneous vaccination followed by a virulent virus challenge 6 months later. All monkeys developed low levels of viremia postimmunization, and all the monkeys that had received equal concentrations of either a high-dose (5,5,5,5) or a low-dose (3,3,3,3) formulation seroconverted against all four DEN virus serotypes. Twenty-two (92%) of 24 monkeys were protected as determined by lack of viremia post-challenge. This report is the first to demonstrate the safety of a recombinant DEN virus tetravalent vaccine in a formal neurovirulence test, as well as its protective efficacy in a monkey challenge model.

West Nile virus: a growing concern?

West Nile virus was first detected in North America in 1999 and has subsequently spread throughout the United States and Canada and into Mexico and the Caribbean. This review describes the epidemiology and ecology of West Nile virus in North America and the prospects for effective treatments and vaccines.

Innate and adaptive immune responses determine protection against disseminated infection by West Nile encephalitis virus

WNV continues to spread throughout the Western Hemisphere as virus activity in insects and animals has been reported in the United States, Canada, Mexico, and the Caribbean islands. West Nile virus (WNV) infects the central nervous system and causes severe disease primarily in humans who are immunocompromised or elderly. In this review, we discuss the mechanisms by which the immune system limits dissemination of WNV infection. Recent experimental studies in animals suggest important roles for both the innate and the adaptive immune responses in controlling WNV infection. Interferons, antibody, complement components and CD8+ T cells coordinate protection against severe infection and disease. These findings are analyzed in the context of recent approaches to vaccine development and immunotherapy against WNV.

Molecularly engineered live-attenuated chimeric West Nile/dengue virus vaccines protect rhesus monkeys from West Nile virus

Two molecularly engineered, live-attenuated West Nile virus (WN) vaccine candidates were highly attenuated and protective in rhesus monkeys. The vaccine candidates are chimeric viruses (designated WN/DEN4) bearing the membrane precursor and envelope protein genes of WN on a backbone of dengue 4 virus (DEN4) with or without a deletion of 30 nucleotides (Delta 30) in the 3' noncoding region of DEN4. Viremia in WN/DEN4- infected monkeys was reduced 100-fold compared to that in WN- or DEN4-infected monkeys. WN/DEN4-3'Delta 30 did not cause detectable viremia, indicating that it is even more attenuated for monkeys. These findings indicate that chimerization itself and the presence of the Delta 30 mutation independently contribute to the attenuation phenotype for nonhuman primates. Despite their high level of attenuation in monkeys, the chimeras induced a moderate-to-high titer of neutralizing antibodies and prevented viremia in monkeys challenged with WN. The more attenuated vaccine candidate, WN/DEN4-3'Delta 30, will be evaluated first in our initial clinical studies.

Growth characteristics of the veterinary vaccine candidate ChimeriVax-West Nile (WN) virus in Aedes and Culex mosquitoes

In 1999 West Nile (WN) virus was introduced to North America where this flavivirus has spread rapidly among wildlife (especially birds) transmitted by various species of mosquitoes (Diptera: Culicidae). Increasing numbers of cases and deaths among humans, horses and other domestic animals require development of effective vaccines. 'ChimeriVax-West Nile(vet)' is being developed for use as a veterinary vaccine to protect against WN infection. This chimeric virus contains the pre-membrane (prM) and envelope (E) genes from the wild-type WN NY99 virus (isolated from a flamingo in New York zoo during the 1999 WN epidemic) in the backbone of yellow fever (YF) 17D vaccine virus. Replication kinetics of ChimeriVax-WN(vet) virus were evaluated in mosquito cell culture (Aedes albopictus C6/36), in WN vector mosquitoes [Culex tritaeniorhynchus Giles, Cx. nigripalpus Theobald and Cx. quinquefasciatus Say (Diptera: Culicidae)] and in YF vectors [Aedes aegypti (L) and Ae. albopictus (Skuse)], to determine whether these mosquitoes become infected through feeding on a viraemic vaccine, and their potential infectivity to transmit the virus. Growth of ChimeriVax-WN(vet) virus was found to be restricted in mosquitoes, compared to WN virus in Ae. albopictus C6/36 cells. When inoculated intrathoracically, ChimeriVax-WN(vet) and YF 17D viruses did not replicate in Cx. tritaeniorhynchus or Cx. nigripalpus; replication was very restricted compared to the wild-type WN virus in Cx. quinquefasciatus, Ae. aegypti and Ae. albopictus. When fed on hanging drops with ChimeriVax-WN(vet) virus (7.7 log10 PFU/mL), none of the Culex mosquitoes became infected; one Ae. albopictus and 10% of the Ae. aegypti became infected, but the titre was very low and virus did not disseminate to head tissue. ChimeriVax-WN(vet) virus had a replication profile similar to that of the attenuated vaccine virus YF 17D, which is not transmitted by mosquitoes. These results suggest that the natural mosquito vectors of WN and YF viruses, which may incidentally take a bloodmeal from a vaccinated host, will not become infected with ChimeriVax-WN(vet) virus.

Interaction of flaviviruses with cells of the vertebrate host and decoy of the immune response

Flaviviruses cause endemic and epidemic disease with significant morbidity and mortality throughout the world. In contrast to viruses that avoid the host immune response by down-regulating cell surface major histocompatibility complex expression, infection by members of the neurotropic Japanese encephalitis serogroup induce virus-directed functional increases in expression of class I and II major histocompatibility complex and various adhesion molecules, resulting in increased susceptibility to both virus- and major histocompatibility complex-specific cytotoxic T lymphocyte lysis. These changes are comodulated by T1 and T2 cytokines, as well as by cell cycle position and adherence status at infection. Infected skin dendritic (Langerhans) cells also show increased costimulatory molecule expression and local interleukin-1beta production causes accelerated migration of Langerhans cells to local draining lymph nodes, where initiation of antiviral immune responses occur. The exact mechanism(s) of up-regulation is unclear, but changes are associated with NF-kappaB activation and increased MHC and ICAM-1 gene transcription, independently of interferon or other pro-inflammatory cytokines. We hypothesize that these viruses may decoy the adaptive immune system into generating low-affinity, self-reactive T cells which clear virus poorly, as part of their survival strategy. This may enable viral growth and immune escape in cycling cells, which do not significantly up-regulate cell surface molecules. A possible side-effect of this might be immunopathology, caused by 'autoimmune' cross-reactive damage of uninfected high major histocompatibility complex and adhesion molecule-expressing cells, with consequent exacerbation of encephalitic disease. Results from a murine model of flavivirus encephalitis developed in this laboratory further suggest that interferon-gamma plays a crucial role in fatal immunopathology.

The molecular biology of West Nile Virus: a new invader of the western hemisphere

West Nile virus (WNV) is a mosquito-borne flavivirus that primarily infects birds but occasionally also infects humans and horses. In recent years, the frequency of WNV outbreaks in humans has increased, and these outbreaks have been associated with a higher incidence of severe disease. In 1999, the geographical distribution of WNV expanded to the Western hemisphere. WNV has a positive strand RNA genome of about 11 kb that encodes a single polyprotein. WNV replicates in the cytoplasm of infected cells. Although there are still many questions to be answered, a large body of data on the molecular biology of WNV and other flaviviruses has already been obtained. Aspects of virion structure, the viral replication cycle, viral protein function, genome structure, conserved viral elements, host factors, virus-host interactions, and vaccines are discussed in this review.

Chimeric Flaviviruses: Novel Vaccines against Dengue Fever, Tick-borne Encephalitis, and Japanese Encephalitis

Many arthropod-borne flaviviruses are important human pathogens responsible for diverse illnesses, including YF, JE, TBE, and dengue. Live, attenuated vaccines have afforded the most effective and economical means of prevention and control, as illustrated by YF 17D and JE SA14-14-2 vaccines. Recent advances in recombinant DNA technology have made it possible to explore a novel approach for developing live attenuated flavivirus vaccines against other flaviviruses. Full-length cDNA clones allow construction of infectious virus bearing attenuating mutations or deletions incorporated in the viral genome. It is also possible to create chimeric flaviviruses in which the structural protein genes for the target antigens of a flavivirus are replaced by the corresponding genes of another flavivirus. By combining these molecular techniques, the DNA sequences of DEN4 strain 814669, DEN2 PDK-53 candidate vaccine and YF 17D vaccine have been used as the genetic backbone to construct chimeric flaviviruses with the required attenuation phenotype and expression of the target antigens. Encouraging results from preclinical and clinical studies have shown that several chimeric flavivirus vaccines have the safety profile and satisfactory immunogenicity and protective efficacy to warrant further evaluation in humans. The chimeric flavivirus strategy has led to the rapid development of novel live-attenuated vaccines against dengue, TBE, JE, and West Nile viruses.

West Nile virus activity in the United States, 2001

West Nile virus (WNV) first appeared in the naive environment of the Western Hemisphere in 1999 in New York. Genetic analysis determined that the virus was introduced into the United States from the Mediterranean Basin. This review discusses the spread of the virus in 2001 from the initial focus in Queens, New York, to widespread activity in the eastern and midwestern United States. It concentrates on viral ecology, epizootiology, pathology, prediction, and prevention. Research questions to further our understanding of the transmission cycle of WNV are discussed, including host-preference studies, molecular confirmation of implicated mosquito vectors, and survival of WNV in the temperate environment of the United States. Comparisons are drawn with two other arboviruses enzootic in the United States, eastern equine encephalitis, and St. Louis encephalitis viruses. Although not recently introduced, these two viruses also demonstrated increased activity in the United States in 2001.

Immunization with heterologous flaviviruses protective against fatal West Nile encephalitis

Prior immunization of hamsters with three heterologous flaviviruses (Japanese encephalitis virus [JEV] SA14-2-8 vaccine, wild-type St. Louis encephalitis virus [SLEV], and Yellow fever virus [YFV] 17D vaccine) reduces the severity of subsequent West Nile virus (WNV) infection. Groups of adult hamsters were immunized with each of the heterologous flaviviruses; approximately 30 days later, the animals were injected intraperitoneally with a virulent New York strain of WNV. Subsequent levels of viremia, antibody response, and deaths were compared with those in nonimmune (control) hamsters. Immunity to JEV and SLEV was protective against clinical encephalitis and death after challenge with WNV. The antibody response in the sequentially infected hamsters also illustrates the difficulty in making a serologic diagnosis of WNV infection in animals (or humans) with preexisting Flavivirus immunity.

Single mutation in the flavivirus envelope protein hinge region increases neurovirulence for mice and monkeys but decreases viscerotropism for monkeys: relevance to development and safety testing of live, attenuated vaccines

A chimeric yellow fever (YF) virus/Japanese encephalitis (JE) virus vaccine (ChimeriVax-JE) was constructed by insertion of the prM-E genes from the attenuated JE virus SA14-14-2 vaccine strain into a full-length cDNA clone of YF 17D virus. Passage in fetal rhesus lung (FRhL) cells led to the emergence of a small-plaque virus containing a single Met-->Lys amino acid mutation at E279, reverting this residue from the SA14-14-2 to the wild-type amino acid. A similar virus was also constructed by site-directed mutagenesis (J. Arroyo, F. Guirakhoo, S. Fenner, Z.-X. Zhang, T. P. Monath, and T. J. Chambers, J. Virol. 75:934-942, 2001). The E279 mutation is located in a beta-sheet in the hinge region of the E protein that is responsible for a pH-dependent conformational change during virus penetration from the endosome into the cytoplasm of the infected cell. In independent transfection-passage studies with FRhL or Vero cells, mutations appeared most frequently in hinge 4 (bounded by amino acids E266 to E284), reflecting genomic instability in this functionally important region. The E279 reversion caused a significant increase in neurovirulence as determined by the 50% lethal dose and survival distribution in suckling mice and by histopathology in rhesus monkeys. Based on sensitivity and comparability of results with those for monkeys, the suckling mouse is an appropriate host for safety testing of flavivirus vaccine candidates for neurotropism. After intracerebral inoculation, the E279 Lys virus was restricted with respect to extraneural replication in monkeys, as viremia and antibody levels (markers of viscerotropism) were significantly reduced compared to those for the E279 Met virus. These results are consistent with the observation that empirically derived vaccines developed by mouse brain passage of dengue and YF viruses have increased neurovirulence for mice but reduced viscerotropism for humans.