Recombinant vaccinia virus producing the prM and E proteins of yellow fever virus protects mice from lethal yellow fever encephalitis

The Present and Future of Yellow Fever Vaccines

The disease yellow fever (YF) is prevented by a live-attenuated vaccine, termed 17D, which has been in use since the 1930s. One dose of the vaccine is thought to give lifelong (35+ years) protective immunity, and neutralizing antibodies are the correlate of protection. Despite being a vaccine-preventable disease, YF remains a major public health burden, causing an estimated 109,000 severe infections and 51,000 deaths annually. There are issues of supply and demand for the vaccine, and outbreaks in 2016 and 2018 resulted in fractional dosing of the vaccine to meet demand. The World Health Organization (WHO) has established the “Eliminate Yellow Fever Epidemics” (EYE) initiative to reduce the burden of YF over the next 10 years. As with most vaccines, the WHO has recommendations to assure the quality, safety, and efficacy of the YF vaccine. These require the use of live 17D vaccine only produced in embryonated chicken eggs, and safety evaluated in non-human primates only. Thus, any second-generation vaccines would require modification of WHO recommendations if they were to be used in endemic countries. There are multiple second-generation YF vaccine candidates in various stages of development that must be shown to be non-inferior to the current 17D vaccine in terms of safety and immunogenicity to progress through clinical trials to potential licensing. The historic 17D vaccine continues to shape the global vaccine landscape in its use in the generation of multiple licensed recombinant chimeric live vaccines and vaccine candidates, in which its structural protein genes are replaced with those of other viruses, such as dengue and Japanese encephalitis. There is no doubt that the YF 17D live-attenuated vaccine will continue to play a role in the development of new vaccines for YF, as well as potentially for many other pathogens.

Development of a highly specific serodiagnostic ELISA for West Nile virus infection using subviral particles

West Nile virus (WNV), a member of the Japanese encephalitis virus (JEV) serocomplex group, causes lethal encephalitis in humans and horses. Because serodiagnosis of WNV and JEV is hampered by cross-reactivity, the development of a simple, secure, and WNV-specific serodiagnostic system is required. The coexpression of prM protein and E protein leads to the secretion of subviral particles (SPs). Deletion of the C-terminal region of E protein is reported to affect the production of SPs by some flaviviruses. However, the influence of such a deletion on the properties and antigenicity of WNV E protein is unclear. We analyzed the properties of full-length E protein and E proteins lacking the C-terminal region as novel serodiagnostics for WNV infection. Deletion of the C-terminal region of E protein suppressed the formation of SPs but did not affect the production of E protein. The sensitivity of an enzyme-linked immunosorbent assay (ELISA) using the full-length E protein was higher than that using the truncated E proteins. Furthermore, in the ELISA using full-length E protein, there was little cross-reactivity with anti-JEV antibodies, and the sensitivity was similar to that of the neutralization test.

Plant-made dengue virus-like particles produced by co-expression of structural and non-structural proteins induce a humoral immune response in mice

Dengue virus (DENV) is an emerging threat causing an estimated 390 million infections per year. Dengvaxia, the only licensed vaccine, may not be adequately safe in young and seronegative patients; hence, development of a safer, more effective vaccine is of great public health interest. Virus-like particles (VLPs) are a safe and very efficient vaccine strategy, and DENV VLPs have been produced in various expression systems. Here, we describe the production of DENV VLPs in Nicotiana benthamiana using transient expression. The co-expression of DENV structural proteins (SP) and a truncated version of the non-structural proteins (NSPs), lacking NS5 that contains the RNA-dependent RNA polymerase, led to the assembly of DENV VLPs in plants. These VLPs were comparable in appearance and size to VLPs produced in mammalian cells. Contrary to data from other expression systems, expression of the protein complex prM-E was not successful, and strategies used in other expression systems to improve the VLP yield did not result in increased yields in plants but, rather, increased purification difficulties. Immunogenicity assays in BALB/c mice revealed that plant-made DENV1-SP + NSP VLPs led to a higher antibody response in mice compared with DENV-E domain III displayed inside bluetongue virus core-like particles and a DENV-E domain III subunit. These results are consistent with the idea that VLPs could be the optimal approach to creating candidate vaccines against enveloped viruses.

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

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

Structure-Function of the Yellow Fever Virus Envelope Protein: Analysis of Antibody Epitopes

Yellow fever virus (YFV) is the prototype member of the genus Flavivirus, which contains more than 60 positive-sense, single-stranded RNA viruses, many of which are considered public health threats. YF disease is controlled by a live attenuated vaccine, 17D, which was generated empirically through serial passage of the wild-type (WT) strain Asibi in chicken tissue. The vaccine, which has been used for over 80 years, is considered to be one of the safest and most effective live attenuated vaccines. It has been shown that the humoral immune response is essential to a positive disease outcome during infection. As such, the neutralizing antibody response and its correlation to long-term protection are a critical measure of 17D efficacy. The primary target of these antibodies is the envelope (E) protein, which is the major component of the virion. Monoclonal antibodies can distinguish WT strain Asibi and vaccine strain 17D by many different measures, including physical binding, hemagglutination inhibition, neutralization, and passive protection. This makes the WT-vaccine pair ideal candidates to study the structure-function relationship of the E protein in the attenuation and immunogenicity of flaviviruses. In this study, we provide an overview of structure-function of YFV E protein and its involvement in protective immunity.

The Many Faces of a Dynamic Virion: Implications of Viral Breathing on Flavivirus Biology and Immunogenicity

Flaviviruses are arthropod-borne RNA viruses that are a significant threat to global health due to their widespread distribution, ability to cause severe disease in humans, and capacity for explosive spread following introduction into new regions. Members of this genus include dengue, tick-borne encephalitis, yellow fever, and Zika viruses. Vaccination has been a highly successful means to control flaviviruses, and neutralizing antibodies are an important component of a protective immune response. High-resolution structures of flavivirus structural proteins and virions, alone and in complex with antibodies, provide a detailed understanding of viral fusion mechanisms and virus-antibody interactions. However, mounting evidence suggests these structures provide only a snapshot of an otherwise structurally dynamic virus particle. The contribution of the structural ensemble arising from viral breathing to the biology, antigenicity, and immunity of flaviviruses is discussed, including implications for the development and evaluation of flavivirus vaccines.

Recent Advances in Zika Virus Vaccines

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

Using a Virion Assembly-Defective Dengue Virus as a Vaccine Approach

Dengue virus (DENV) is the most prevalent mosquito-transmitted viral pathogen in humans. The recently licensed dengue vaccine has major weaknesses. Therefore, there is an urgent need to develop improved dengue vaccines. Here, we report a virion assembly-defective DENV as a vaccine platform. DENV containing an amino acid deletion (K188) in nonstructural protein 2A (NS2A) is fully competent in viral RNA replication but is completely defective in virion assembly. When trans-complemented with wild-type NS2A protein, the virion assembly defect could be rescued, generating pseudoinfectious virus (PIV_NS2A) that could initiate single-round infection. The trans-complementation efficiency could be significantly improved through selection for adaptive mutations, leading to high-yield PIV_NS2A production, with titers of >10⁷ infectious-focus units (IFU)/ml. Mice immunized with a single dose of PIV_NS2A elicited strong T cell immune responses and neutralization antibodies and were protected from wild-type-virus challenge. Collectively, the results proved the concept of using assembly-defective virus as a vaccine approach. The study also solved the technical bottleneck in producing high yields of PIV_NS2A vaccine. The technology could be applicable to vaccine development for other viral pathogens.IMPORTANCE Many flaviviruses are significant human pathogens that pose global threats to public health. Although licensed vaccines are available for yellow fever, Japanese encephalitis, tick-borne encephalitis, and dengue viruses, new approaches are needed to develop improved vaccines. Using dengue virus as a model, we developed a vaccine platform using a virion assembly-defective virus. We show that such an assembly-defective virus could be rescued to higher titers and infect cells for a single round. Mice immunized with the assembly-defective virus were protected from wild-type-virus infection. This vaccine approach could be applicable to other viral pathogens.

Development of a serodiagnostic IgM-ELISA for tick-borne encephalitis virus using subviral particles with strep-tag

Tick-borne encephalitis virus (TBEV) is a zoonotic agent causing severe encephalitis in humans. IgM antibody detection is useful for the serological diagnosis of TBEV infection, because IgM has high specificity for each flavivirus and indicates a recent infection. Commercial IgM-ELISA kits are somewhat expensive and difficulties in their sensitivity have been suggested due to their format and formalin-inactivated antigens. Therefore, the development of an inexpensive IgM-ELISA with high specificity and sensitivity is needed. In this study, a μ-capture ELISA was developed to detect TBEV-specific IgM antibodies using subviral particles (SPs) with strep-tag (strep-SP-IgM-ELISA). The results of our strep-SP-IgM-ELISA were highly correlated with diagnoses made by the neutralization test (sensitivity: 94.1%), and our strep-SP-IgM-ELISA could detect anti-TBEV IgM antibodies in patients who could not be diagnosed with the neutralization test. Besides, 51 of 52 positive samples by a commercial IgM-ELISA were also diagnosed as positive by our strep-SP-IgM-ELISA (98.1%), and our strep-SP-IgM-ELISA could detect anti-TBEV IgM antibodies in all samples that were inconclusive based on the commercial IgM-ELISA. Our strep-SP-IgM-ELISA will be useful for diagnoses in TBE-endemic areas.

Development of Virus-Like-Particle Vaccine and Reporter Assay for Zika Virus

Recent worldwide outbreaks of Zika virus (ZIKV) infection and the lack of an approved vaccine raise serious concerns regarding preparedness to combat this emerging virus. We used a virus-like particle (VLP)-based approach to develop a vaccine and a microneutralization assay for ZIKV. A synthetic capsid-premembrane-envelope (C-prM-E) gene construct of ZIKV was used to generate reporter virus particles (RVPs) that package a green fluorescent protein (GFP) reporter-expressing West Nile virus (WNV) replicon. The assay was adapted to a 96-well format, similar to the plaque reduction neutralization test (PRNT), and showed high reproducibility with specific detection of ZIKV neutralizing antibodies. Furthermore, C-prM-E and prM-E VLPs were tested as vaccine candidates in mice and compared to DNA vaccination. While the ZIKV prM-E construct alone was sufficient for generating VLPs, efficient VLP production from the C-prM-E construct could be achieved in the presence of the WNV NS2B-3 protease, which cleaves C from prM, allowing virus release. Immunization studies in mice showed that VLPs generated higher neutralizing antibody titers than those with the DNA vaccines, with C-prM-E VLPs giving slightly higher titers than those with prM-E VLPs. The superiority of C-prM-E VLPs suggests that inclusion of capsid may have benefits for ZIKV and other flaviviral VLP vaccines. To facilitate the VLP platform, we generated a stable cell line expressing high levels of ZIKV prM-E proteins that constitutively produce VLPs as well as a cell line expressing ZIKV C-prM-E proteins for RVP production. While several vaccine platforms have been proposed for ZIKV, this study describes a safe, effective, and economical VLP-based vaccine against ZIKV.IMPORTANCE To address the growing Zika virus epidemic, we undertook this study with two objectives: first, to develop a safe, effective, and economical vaccine for ZIKV, and second, to develop a rapid and versatile assay to detect the anti-ZIKV immune response. We generated a cell line stably expressing ZIKV prM-E that produces large amounts of VLPs in the supernatant and a ZIKV C-prM-E cell line that produces reporter virus particles upon transfection with a GFP replicon plasmid. The prM-E VLPs induced a strong neutralizing antibody response in mice that was better when the capsid was included. VLP-based vaccines showed significantly better neutralizing antibody responses than those with their DNA counterparts. The RVP-based microneutralization assay worked similarly to the PRNT assay, with a rapid GFP readout in a 96-well format. Our VLP-based platform provides a source for a ZIKV vaccine and diagnosis that can rapidly be adapted to current outbreaks.

Current status and future prospects of yellow fever vaccines

Yellow fever 17D vaccine is one of the oldest live-attenuated vaccines in current use that is recognized historically for its immunogenic and safe properties. These unique properties of 17D are presently exploited in rationally designed recombinant vaccines targeting not only flaviviral antigens but also other pathogens of public health concern. Several candidate vaccines based on 17D have advanced to human trials, and a chimeric recombinant Japanese encephalitis vaccine utilizing the 17D backbone has been licensed. The mechanism(s) of attenuation for 17D are poorly understood; however, recent insights from large in silico studies have indicated particular host genetic determinants contributing to the immune response to the vaccine, which presumably influences the considerable durability of protection, now in many cases considered to be lifelong. The very rare occurrence of severe adverse events for 17D is discussed, including a recent fatal case of vaccine-associated viscerotropic disease.

A DNA vaccine against yellow fever virus: development and evaluation

Attenuated yellow fever (YF) virus 17D/17DD vaccines are the only available protection from YF infection, which remains a significant source of morbidity and mortality in the tropical areas of the world. The attenuated YF virus vaccine, which is used worldwide, generates both long-lasting neutralizing antibodies and strong T-cell responses. However, on rare occasions, this vaccine has toxic side effects that can be fatal. This study presents the design of two non-viral DNA-based antigen formulations and the characterization of their expression and immunological properties. The two antigen formulations consist of DNA encoding the full-length envelope protein (p/YFE) or the full-length envelope protein fused to the lysosomal-associated membrane protein signal, LAMP-1 (pL/YFE), aimed at diverting antigen processing/presentation through the major histocompatibility complex II precursor compartments. The immune responses triggered by these formulations were evaluated in H2b and H2d backgrounds, corresponding to the C57Bl/6 and BALB/c mice strains, respectively. Both DNA constructs were able to induce very strong T-cell responses of similar magnitude against almost all epitopes that are also generated by the YF 17DD vaccine. The pL/YFE formulation performed best overall. In addition to the T-cell response, it was also able to stimulate high titers of anti-YF neutralizing antibodies comparable to the levels elicited by the 17DD vaccine. More importantly, the pL/YFE vaccine conferred 100% protection against the YF virus in intracerebrally challenged mice. These results indicate that pL/YFE DNA is an excellent vaccine candidate and should be considered for further developmental studies.

Target selection for structural genomics of infectious diseases

This chapter describes the protocols used to identify, filter, and annotate potential protein targets from an organism associated with infectious diseases. Protocols often combine computational approaches for mining information in public databases or for checking whether the protein has already been targeted for structure determination, with manual strategies that examine the literature for information on the biological role of the protein or the experimental strategies that explore the effects of knocking out the protein. Publicly available computational tools have been cited as much as possible. Where these do not exist, the concepts underlying in-house tools developed for the Center for Structural Genomics of Infectious Diseases have been described.

Cross-reactivity of human monoclonal antibodies generated with peripheral blood lymphocytes from dengue patients with Japanese encephalitis virus

Background

Hybridomas that produce human monoclonal antibodies (HuMAbs) against Dengue virus (DV) had been prepared previously using peripheral blood lymphocytes from patients with DV during the acute and convalescent phases of a secondary infection. Anti-DV envelope glycoprotein (E) 99 clones, anti-DV premembrane protein (prM) 8 clones, and anti-DV nonstructural protein 1 (NS1) 4 clones were derived from four acute-phase patients, and anti-DV E 2 clones, anti-DV prM 2 clones, and anti-DV NS1 8 clones were derived from five convalescent-phase patients.

Methods and results

In the present study, we examined whether these clones cross-reacted with Japanese encephalitis virus (JEV), which belongs to the same virus family. Forty-six of the above-described 99 (46/99) anti-E, 0/8 anti-prM, and 2/4 anti-NS1 HuMAbs from acute-phase, and 0/2 anti-E, 0/2 anti-prM, and 5/8 anti-NS1 HuMAbs from convalescent-phase showed neutralizing activity against JEV. Thus, most of the anti-E and anti-NS1 (but not the anti-prM) antibodies cross-reacted with JEV and neutralized this virus. Interestingly, 3/46 anti-E HuMAbs derived from acute-phase patients and 3/5 anti-NS1 HuMAbs from convalescent-phase patients showed particularly high neutralizing activity against JEV. Consequently, the HuMAbs showing neutralization against JEV mostly consisted of two populations: one was HuMAbs recognizing DV E and showing neutralization activity against all four DV serotypes (complex-type) and the other was HuMAbs recognizing DV NS1 and showing subcomplex-type cross-reaction with DV.

Conclusion

Anti-DV E from acute phase (46/99) and anti-DV NS1 (7/12) indicate neutralizing activity against JEV. In particular, three of 46 anti-DV E clones from acute phase and three of five anti-NS1 clones from convalescent phase showed strong neutralizing activity against JEV.

Japanese Encephalitis Virus Generated Neurovirulence, Antigenicity, and Host Immune Responses

In response to a JE virus attack, infected body cells start secretion of different cytokines and activate innate immune response. Virus starts neuronal invasion by entering into nerve cells and inflecting the central nervous system. It avoids exposure of body’s natural immunity and generates neurotrophic effects. Virus causes acute susceptibility to CNS and establishes encephalitis syndrome that results in very high fatality in children. In survivors, JEV inhibits the growth and proliferation of NCPs and imposes permanent neuronal disorders like cognitive, motor, and behavioral impairments. However, body cells start TCR mediated interactions, to recognize viral antigens with class I MHC complex on specific target cells, and operate mass killing of virus infected cells by increased CTL activity. Thus, both cell mediated and antibody interactions plays a central role in protection against JEV. In the present review article virus generated neurovirulence, antigenicity, and host immune responses are described in detail. More emphasis is given on diagnosis, clinical care, and active immunization with well-designed potential antiflavivirus vaccines. Further, for achieving an elite success against JEV, global eradication strategies are to be needed for making vaccination program more responsible and effective in endemic areas.

Antibodies generated by immunization with the NS1 protein of West Nile virus confer partial protection against lethal Japanese encephalitis virus challenge

Japanese encephalitis virus (JEV) and West Nile virus (WNV) are two medically important flaviviruses that can cause severe hemorrhagic and encephalitic diseases in humans. Immune responses directed against the NS1 protein of flaviviruses can confer protection against lethal viral challenge. Previous studies have shown that the WNV NS1 protein harbors epitopes that elicit antibodies that cross react with JEV. Here we demonstrate that the WNV NS1 protein not only contains cross-reactive epitopes, but that the antibodies elicited by these cross-reactive epitopes provide partial protection against lethal JEV challenge in a mouse model. Mice immunized with WNV NS1 protein showed reduced morbidity and mortality following both intracerebral and intraperitoneal JEV challenge. WNV NS1 immunization attenuated the extent of lung pathology generated following JEV challenge, and delayed the appearance of other pathological findings including vascular cuffing. By screening and identifying the specific WNV NS1 protein-derived peptides recognized by serum antibodies elicited by immunization with WNV NS1 protein and by JEV challenge, we found after JEV challenge will induce several new epitopes, but which epitope primarily contribute to antibody-mediated cross protection need further evaluation. The knowledge and reagents generated in this study have potential applications in vaccine and subunit vaccine development for WNV and JEV.

Intradermal vaccination to protect against yellow fever and influenza

The viral infections yellow fever and influenza can lead to large epidemics, which may deplete limited vaccine supplies. The intradermal vaccination route of yellow fever and influenza vaccines has received renewed attention, because it allows dose reduction without loss of efficacy. In this chapter, we review these two vaccines, the history of vaccine development, correlates of protection, immune response to vaccination and current knowledge concerning intradermal vaccination, including the immunological background, both in healthy subjects and immunocompromized individuals.

Pre-clinical efficacy and safety of experimental vaccines based on non-replicating vaccinia vectors against yellow fever

BACKGROUND

Currently existing yellow fever (YF) vaccines are based on the live attenuated yellow fever virus 17D strain (YFV-17D). Although, a good safety profile was historically attributed to the 17D vaccine, serious adverse events have been reported, making the development of a safer, more modern vaccine desirable.

METHODOLOGY/PRINCIPAL FINDINGS

A gene encoding the precursor of the membrane and envelope (prME) protein of the YFV-17D strain was inserted into the non-replicating modified vaccinia virus Ankara and into the D4R-defective vaccinia virus. Candidate vaccines based on the recombinant vaccinia viruses were assessed for immunogenicity and protection in a mouse model and compared to the commercial YFV-17D vaccine. The recombinant live vaccines induced γ-interferon-secreting CD4- and functionally active CD8-T cells, and conferred full protection against lethal challenge already after a single low immunization dose of 10(5) TCID(50). Surprisingly, pre-existing immunity against wild-type vaccinia virus did not negatively influence protection. Unlike the classical 17D vaccine, the vaccinia virus-based vaccines did not cause mortality following intracerebral administration in mice, demonstrating better safety profiles.

CONCLUSIONS/SIGNIFICANCE

The non-replicating recombinant YF candidate live vaccines induced a broad immune response after single dose administration, were effective even in the presence of a pre-existing immunity against vaccinia virus and demonstrated an excellent safety profile in mice.

Flavivirus-induced antibody cross-reactivity

Dengue viruses (DENV) cause countless human deaths each year, whilst West Nile virus (WNV) has re-emerged as an important human pathogen. There are currently no WNV or DENV vaccines licensed for human use, yet vaccines exist against other flaviviruses. To investigate flavivirus cross-reactivity, sera from a human cohort with a history of vaccination against tick-borne encephalitis virus (TBEV), Japanese encephalitis virus (JEV) and yellow fever virus (YFV) were tested for antibodies by plaque reduction neutralization test. Neutralization of louping ill virus (LIV) occurred, but no significant neutralization of Murray Valley encephalitis virus was observed. Sera from some individuals vaccinated against TBEV and JEV neutralized WNV, which was enhanced by YFV vaccination in some recipients. Similarly, some individuals neutralized DENV-2, but this was not significantly influenced by YFV vaccination. Antigenic cartography techniques were used to generate a geometric illustration of the neutralization titres of selected sera against WNV, TBEV, JEV, LIV, YFV and DENV-2. This demonstrated the individual variation in antibody responses. Most sera had detectable titres against LIV and some had titres against WNV and DENV-2. Generally, LIV titres were similar to titres against TBEV, confirming the close antigenic relationship between TBEV and LIV. JEV was also antigenically closer to TBEV than WNV, using these sera. The use of sera from individuals vaccinated against multiple pathogens is unique relative to previous applications of antigenic cartography techniques. It is evident from these data that notable differences exist between amino acid sequence identity and mapped antigenic relationships within the family Flaviviridae.

A dynamic landscape for antibody binding modulates antibody-mediated neutralization of West Nile virus

Neutralizing antibodies are a significant component of the host's protective response against flavivirus infection. Neutralization of flaviviruses occurs when individual virions are engaged by antibodies with a stoichiometry that exceeds a required threshold. From this “multiple-hit” perspective, the neutralizing activity of antibodies is governed by the affinity with which it binds its epitope and the number of times this determinant is displayed on the surface of the virion. In this study, we investigated time-dependent changes in the fate of West Nile virus (WNV) decorated with antibody in solution. Experiments with the well-characterized neutralizing monoclonal antibody (MAb) E16 revealed a significant increase in neutralization activity over time that could not be explained by the kinetics of antibody binding, virion aggregation, or the action of complement. Additional kinetic experiments using the fusion-loop specific MAb E53, which has limited neutralizing activity because it recognizes a relatively inaccessible epitope on mature virions, identified a role of virus “breathing” in regulating neutralization activity. Remarkably, MAb E53 neutralized mature WNV in a time- and temperature-dependent manner. This phenomenon was confirmed in studies with a large panel of MAbs specific for epitopes in each domain of the WNV envelope protein, with sera from recipients of a live attenuated WNV vaccine, and in experiments with dengue virus. Given enough time, significant inhibition of infection was observed even for antibodies with very limited, or no neutralizing activity in standard neutralization assays. Together, our data suggests that the structural dynamics of flaviviruses impacts antibody-mediated neutralization via exposure of otherwise inaccessible epitopes, allowing for antibodies to dock on the virion with a stoichiometry sufficient for neutralization.

Neutralizing antibodies are a critical aspect of protection from flavivirus infection. The primary targets of neutralizing antibodies are the envelope (E) proteins incorporated into virions. The neutralizing activity of antibodies is determined by the affinity with which they interact with the virion, and the total number of sites available for binding. In this study, we investigate the impact of dynamic motion of the viral E proteins on antibody-mediated neutralization. Using panels of monoclonal antibodies and immune sera, we demonstrate that the dynamic motion of virions significantly impacts antibody-mediated neutralization of West Nile and dengue viruses by modulating epitope accessibility. Increasing the length of the interactions between antibody and virus resulted in increased neutralization reflecting engagement of epitopes that are not exposed on the surface of the virion in its average state, but instead become accessible through the dynamic motion of E proteins. While examples of the impact of structural dynamics on antibody binding have been described previously, our data suggests this phenomenon plays a role in neutralization by all antibodies that bind the array of E proteins on the virion. Our data identifies epitope accessibility as a critical, yet dynamic, factor that governs the neutralizing activity of anti-flavivirus antibodies.

Repeated in vivo stimulation of T and B cell responses in old mice generates protective immunity against lethal West Nile virus encephalitis

Older adults exhibit higher morbidity and mortality from infectious diseases compared with those of the general population. The introduction and rapid spread of West Nile virus (WNV) throughout the continental United States since 1999 has highlighted the challenge of protecting older adults against emerging pathogens: to this day there is no therapy or vaccine approved for human use against West Nile encephalitis. In this study, we describe the characterization of T and B cell responses in old mice after vaccination with RepliVAX WN, a novel West Nile encephalitis vaccine based on single-cycle flavivirus particles. In adult mice, RepliVAX WN induced robust and long-lasting CD4(+) and CD8(+) T cell and Ab (B cell) responses against natural WNV epitopes, similar to those elicited by primary WNV infection. Primary and memory T and B cell responses in old mice against RepliVAX WN vaccination were significantly lower than those seen in younger mice, similar to the response of old mice to infection with WNV. Surprisingly, both the quality and the quantity of the recall Ab and T cell responses in vaccinated old mice were improved to equal or exceed those in adult animals. Moreover, these responses together (but not individually) were sufficient to protect both old and adult mice from severe WNV disease upon challenge. Therefore, at least two cycles of in vivo restimulation are needed for selection and expansion of protective lymphocytes in older populations, and live, single-cycle virus vaccines that stimulate both cellular and humoral immunity can protect older individuals against severe viral disease.

Conjugation to nickel-chelating nanolipoprotein particles increases the potency and efficacy of subunit vaccines to prevent West Nile encephalitis

Subunit antigens are attractive candidates for vaccine development, as they are safe, cost-effective, and rapidly produced. Nevertheless, subunit antigens often need to be adjuvanted and/or formulated to produce products with acceptable potency and efficacy. Here, we describe a simple method for improving the potency and efficacy of a recombinant subunit antigen by its immobilization on nickel-chelating nanolipoprotein particles (NiNLPs). NiNLPs are membrane mimetic nanoparticles that provide a delivery and presentation platform amenable to binding any recombinant subunit immunogens featuring a polyhistidine tag. A His-tagged, soluble truncated form of the West Nile virus (WNV) envelope protein (trE-His) was immobilized on NiNLPs. Single inoculations of the NiNLP-trE-His produced superior anti-WNV immune responses and provided significantly improved protection against a live WNV challenge compared to mice inoculated with trE-His alone. These results have broad implications in vaccine development and optimization, as NiNLP technology is well-suited to many types of vaccines, providing a universal platform for enhancing the potency and efficacy of recombinant subunit immunogens.

Efficient assembly and secretion of recombinant subviral particles of the four dengue serotypes using native prM and E proteins

Background

Flavivirus infected cells produce infectious virions and subviral particles, both of which are formed by the assembly of prM and E envelope proteins and are believed to undergo the same maturation process. Dengue recombinant subviral particles have been produced in cell cultures with either modified or chimeric proteins but not using the native forms of prM and E.

Methodology/Principal Findings

We have used a codon optimization strategy to obtain an efficient expression of native viral proteins and production of recombinant subviral particles (RSPs) for all four dengue virus (DV) serotypes. A stable HeLa cell line expressing DV1 prME was established (HeLa-prME) and RSPs were analyzed by immunofluorescence and transmission electron microscopy. We found that E protein is mainly present in the endoplasmic reticulum (ER) where assembly of RSPs could be observed. Biochemical characterization of DV1 RSPs secretion revealed both prM protein cleavage and homodimerization of E proteins before their release into the supernatant, indicating that RSPs undergo a similar maturation process as dengue virus. Pulse chase experiment showed that 8 hours are required for the secretion of DV1 RSPs. We have used HeLa-prME to develop a semi-quantitative assay and screened a human siRNA library targeting genes involved in membrane trafficking. Knockdown of 23 genes resulted in a significant reduction in DV RSP secretion, whereas for 22 others we observed an increase of RSP levels in cell supernatant.

Conclusions/Significance

Our data describe the efficient production of RSPs containing native prM and E envelope proteins for all dengue serotypes. Dengue RSPs and corresponding producing cell lines are safe and novel tools that can be used in the study of viral egress as well as in the development of vaccine and drugs against dengue virus.

Effects of the number of amino acid residues in the signal segment upstream or downstream of the NS2B-3 cleavage site on production and secretion of prM/M-E virus-like particles of West Nile virus

Expression of genes for precursor M (prM) and envelope (E) proteins of West Nile virus (WNV) leads to the production of small, capsidless, and non-infectious virus-like particles (VLPs) possessing the E antigen which is responsible for viral entry and immune protection. It has been reported that processing of the secretion signal affects viral release. We examined the secretion efficiency of VLPs into the culture medium from RK13 or 293T cells transfected with expression vectors for prM and E proteins of WNV which were constructed to comprise different lengths of signal peptides upstream of the prM-E domain. The number of amino acid residues present in the segment markedly affected the production, processing, and secretion of VLPs. Secreted VLPs possessed both the processed M protein and the glycosylated E protein. In addition, immunization with VLPs induced neutralizing antibodies in C3H/HeN mice. These results indicate that the number of amino acid residues comprising the N-terminus of the signal segment controls the efficiency of assembly, maturation, and release of VLPs in the absence of viral protease, which in turn indicates the potential of VLPs as a candidate for an effective WNV subunit vaccine.

Progress on the development of therapeutics against West Nile virus

A decade has passed since the appearance of West Nile virus (WNV) in humans in the Western Hemisphere in New York City. During this interval, WNV spread inexorably throughout North and South America and caused millions of infections ranging from a sub-clinical illness, to a self-limiting febrile syndrome or lethal neuroinvasive disease. Its entry into the United States triggered intensive research into the basic biology of WNV and the elements that comprise a protective host immune response. Although no therapy is currently approved for use in humans, several strategies are being pursued to develop effective prophylaxis and treatments. This review describes the current state of knowledge on epidemiology, clinical presentation, pathogenesis, and immunobiology of WNV infection, and highlights progress toward an effective therapy.

The structural immunology of antibody protection against West Nile virus

SUMMARY

Recent investigations of the interaction between the West Nile virus (WNV) envelope protein (E) and monoclonal antibodies (mAbs) have elucidated fundamental insights into the molecular mechanisms of neutralization. Structural studies have defined an epitope on the lateral ridge of domain III (DIII-lr) of the WNV E protein that is recognized by antibodies with the strongest neutralizing activity in vitro and in vivo. Antibodies that bind this epitope are highly potent because they efficiently block at a post-entry step of viral infection with relatively low virion occupancy requirements. In this review, we discuss the structural, molecular, and immunologic basis for antibody-mediated protection against WNV, and its implications for novel therapeutic or vaccine strategies.

Yellow fever vaccine: past, present and future

BACKGROUND

Yellow fever is a re-emerging infectious disease, as vector control and routine immunisation have dwindled in endemic areas in the last few decades. There is a constant threat of outbreaks in the large susceptible non-immune population of the megacities in tropical countries with an ongoing virus life cycle in the rainforests. For this population and for travellers to endemic areas, vaccination is the only effective protective measure against the disease and the spread of the virus.

OBJECTIVE/METHODS

We discuss the history of yellow fever vaccine development, and focus on practical aspects of vaccine safety, contraindications for vaccination, and future vaccine developments.

RESULTS/CONCLUSIONS

Vaccination with the live attenuated yellow fever-17D vaccine (YF-17D) induces low-grade viraemia in half of the vaccinees and elicits protective neutralising antibody levels in 99%. Reports of serious adverse events in the elderly and immunocompromised, and the inability to produce large quantities of yellow fever vaccine at short notice in combination with limited vaccine stockpiles highlight the need for further study of this highly effective and safe vaccine.

Molecular mechanisms of antibody-mediated neutralisation of flavivirus infection

Flaviviruses are a group of positive-stranded RNA viruses that cause a spectrum of severe illnesses globally in more than 50 million individuals each year. While effective vaccines exist for three members of this group (yellow fever, Japanese encephalitis, and tick-borne encephalitis viruses), safe and effective vaccines for several other flaviviruses of clinical importance, including West Nile and dengue viruses, remain in development. An effective humoral immune response is critical for protection against flaviviruses and an essential goal of vaccine development. The effectiveness of virus-specific antibodies in vivo reflects their capacity to inhibit virus entry and spread through several mechanisms, including the direct neutralisation of virus infection. Recent advances in our understanding of the structural biology of flaviviruses, coupled with the use of small-animal models of flavivirus infection, have promoted significant advances in our appreciation of the factors that govern antibody recognition and inhibition of flaviviruses in vitro and in vivo. In this review, we discuss the properties that define the potency of neutralising antibodies and the molecular mechanisms by which they inhibit virus infection. How recent advances in this area have the potential to improve the development of safe and effective vaccines and immunotherapeutics is also addressed.

Maturation of West Nile virus modulates sensitivity to antibody-mediated neutralization

West Nile virions incorporate 180 envelope (E) proteins that orchestrate the process of virus entry and are the primary target of neutralizing antibodies. The E proteins of newly synthesized West Nile virus (WNV) are organized into trimeric spikes composed of pre-membrane (prM) and E protein heterodimers. During egress, immature virions undergo a protease-mediated cleavage of prM that results in a reorganization of E protein into the pseudo-icosahedral arrangement characteristic of mature virions. While cleavage of prM is a required step in the virus life cycle, complete maturation is not required for infectivity and infectious virions may be heterogeneous with respect to the extent of prM cleavage. In this study, we demonstrate that virion maturation impacts the sensitivity of WNV to antibody-mediated neutralization. Complete maturation results in a significant reduction in sensitivity to neutralization by antibodies specific for poorly accessible epitopes that comprise a major component of the human antibody response following WNV infection or vaccination. This reduction in neutralization sensitivity reflects a decrease in the accessibility of epitopes on virions to levels that fall below a threshold required for neutralization. Thus, in addition to a role in facilitating viral entry, changes in E protein arrangement associated with maturation modulate neutralization sensitivity and introduce an additional layer of complexity into humoral immunity against WNV.

West Nile virus (WNV) virions incorporate 180 envelope (E) proteins that are the primary target of neutralizing antibodies. As newly formed WNV virions are released from infected cells, the E proteins undergo a significant organizational change associated with maturation into an infectious virus. However, this process is not always efficient, as populations of infectious WNV include virions that did not complete the maturation process and may be heterogeneous with respect to the arrangement of E proteins on the virion. In this study, we found that neutralization by antibodies specific for epitopes commonly recognized in vivo is strongly impacted by the maturation state of WNV. Our studies suggest that maturation of WNV reduces the accessibility of some, but not all, epitopes on the virion for antibody binding. Virions that retain some immature character can be neutralized by monoclonal antibodies that fail to block infection of populations of WNV composed solely of mature virions. Similar results were found using polyclonal human serum obtained from volunteers of two clinical trials of candidate WNV vaccines. These studies identify unappreciated aspects of the antigenic complexity of WNV and highlight the importance of understanding the heterogenous forms of WNV that may be introduced into or replicating within the host.

The relative contribution of antibody and CD8+ T cells to vaccine immunity against West Nile encephalitis virus

West Nile virus (WNV) is a mosquito borne, neurotropic flavivirus that causes a severe central nervous system (CNS) infection in humans and animals. Although commercial vaccines are available for horses, none is currently approved for human use. In this study, we evaluated the efficacy and mechanism of immune protection of two candidate WNV vaccines in mice. A formalin-inactivated WNV vaccine induced higher levels of specific and neutralizing antibodies compared to a DNA plasmid vaccine that produces virus-like particles. Accordingly, partial and almost complete protection against a highly stringent lethal intracranial WNV challenge were observed in mice 60 days after single dose immunization with the DNA plasmid and inactivated virus vaccines, respectively. In mice immunized with a single dose of DNA plasmid or inactivated vaccine, antigen-specific CD8(+) T cells were induced and contributed to protective immunity as acquired or genetic deficiencies of CD8(+) T cells lowered the survival rates. In contrast, in boosted animals, WNV-specific antibody titers were higher, survival rates after challenge were greater, and an absence of CD8(+) T cells did not appreciably affect mortality. Overall, our experiments suggest that in mice, both inactivated WNV and DNA plasmid vaccines are protective after two doses, and the specific contribution of antibody and CD8(+) T cells to vaccine immunity against WNV is modulated by the prime-boost strategy.

Complement protein C1q inhibits antibody-dependent enhancement of flavivirus infection in an IgG subclass-specific manner

Severe dengue virus infection can occur in humans with pre-existing antibodies against the virus. This observation led to the hypothesis that a subneutralizing antibody level in vivo can increase viral burden and cause more severe disease. Indeed, antibody-dependent enhancement of infection (ADE) in vitro has been described for multiple viruses, including the flaviviruses dengue virus and West Nile virus. Here, we demonstrate that the complement component C1q restricts ADE by anti-flavivirus IgG antibodies in an IgG subclass-specific manner in cell culture and in mice. IgG subclasses that avidly bind C1q induced minimal ADE in the presence of C1q. These findings add a layer of complexity for the analysis of humoral immunity and flavivirus infection.

Yellow Fever virus NS3 plays an essential role in virus assembly independent of its known enzymatic functions

In flaviviruses it has been proposed that there is a coupling between genome replication and virion assembly and that nonstructural proteins are involved in this process. It was previously reported that mutations in yellow fever virus (YFV) nonstructural protein NS2A blocked production of infectious virus and that this block could be released by a suppressor mutation in NS3. Here, based on studies using a YFV replicon-based trans-packaging system as well as full-length YFV cDNA, we report that mutation of a conserved tryptophan at position 349 in the helicase domain of NS3 blocks production of infectious virus particles, revealing an as-yet-unknown role for NS3 in virus assembly. Mutation of tryptophan 349 to alanine (W349A) had no effect on viral replication, as demonstrated by wild-type levels of viral RNA amplification and protein expression in W349A-transfected cells. Although release of infectious virus was not detected, release of capsidless subviral particles was not blocked. The assembly defect in W349A could be trans-complemented inefficiently using BHK-REP cells (a cell line containing persistently replicating YFV replicon RNA). trans-complementation was also demonstrated by supplying wild-type NS2B-3 or NS3 protein alone as well as by supplying inactive NS2B-3 protein, indicating that this function of NS3 in virus assembly was independent of its known enzymatic functions.

The stoichiometry of antibody-mediated neutralization and enhancement of West Nile virus infection

Antibody binding to the icosahedral arrangement of envelope proteins on the surface of flaviviruses can result in neutralization or enhancement of infection. We evaluated how many antibodies must bind to a given epitope on West Nile virus (WNV) to achieve neutralization. The most potent monoclonal antibodies (mAbs) block infection at concentrations that result in low occupancy of accessible sites on the virion, with neutralization occurring when as few as 30 of 180 envelope proteins are bound. In contrast, weakly neutralizing mAbs recognize fewer sites on the virion and require almost complete occupancy to inhibit WNV infection. For all mAbs studied, enhancement of infection is possible in cells bearing activating Fc-gamma receptors when the number of mAbs docked to the virion is not sufficient for neutralization. Thus, neutralization is best described by a model requiring "multiple hits" with the cumulative functional outcome determined by interplay between antibody affinity and epitope accessibility.

Type- and subcomplex-specific neutralizing antibodies against domain III of dengue virus type 2 envelope protein recognize adjacent epitopes

Neutralization of flaviviruses in vivo correlates with the development of an antibody response against the viral envelope (E) protein. Previous studies demonstrated that monoclonal antibodies (MAbs) against an epitope on the lateral ridge of domain III (DIII) of the West Nile virus (WNV) E protein strongly protect against infection in animals. Based on X-ray crystallography and sequence analysis, an analogous type-specific neutralizing epitope for individual serotypes of the related flavivirus dengue virus (DENV) was hypothesized. Using yeast surface display of DIII variants, we defined contact residues of a panel of type-specific, subcomplex-specific, and cross-reactive MAbs that recognize DIII of DENV type 2 (DENV-2) and have different neutralizing potentials. Type-specific MAbs with neutralizing activity against DENV-2 localized to a sequence-unique epitope on the lateral ridge of DIII, centered at the FG loop near residues E383 and P384, analogous in position to that observed with WNV-specific strongly neutralizing MAbs. Subcomplex-specific MAbs that bound some but not all DENV serotypes and neutralized DENV-2 infection recognized an adjacent epitope centered on the connecting A strand of DIII at residues K305, K307, and K310. In contrast, several MAbs that had poor neutralizing activity against DENV-2 and cross-reacted with all DENV serotypes and other flaviviruses recognized an epitope with residues in the AB loop of DIII, a conserved region that is predicted to have limited accessibility on the mature virion. Overall, our experiments define adjacent and structurally distinct epitopes on DIII of DENV-2 which elicit type-specific, subcomplex-specific, and cross-reactive antibodies with different neutralizing potentials.

Complex adenovirus-mediated expression of West Nile virus C, PreM, E, and NS1 proteins induces both humoral and cellular immune responses

West Nile Virus (WNV), a member of the family Flaviviridae, was first identified in Africa in 1937. In recent years, it has spread into Europe and North America. The clinical manifestations of WNV infection range from mild febrile symptoms to fatal encephalitis. Two genetic lineages (lineages I and II) are recognized; lineage II is associated with mild disease, while lineage I has been associated with severe disease, including encephalitis. WNV has now spread across North America, significantly affecting both public and veterinary health. In the efforts to develop an effective vaccine against all genetic variants of WNV, we have studied the feasibility of inducing both neutralizing and cellular immune responses by de novo synthesis of WNV antigens using a complex adenoviral vaccine (CAdVax) vector. By expressing multiple WNV proteins from a single vaccine vector, we were able to induce both humoral and cellular immune responses in vaccinated mice. Neutralization assays demonstrated that the antibodies were broadly neutralizing against both lineages of WNV, with a significant preference for the homologous lineage II virus. The results from this study show that multiple antigens synthesized de novo from a CAdVax vector are capable of inducing both humoral and cellular immune responses against WNV and that a multiantigen approach may provide broad protection against multiple genetic variants of WNV.

Functional requirements of the yellow fever virus capsid protein

Although it is known that the flavivirus capsid protein is essential for genome packaging and formation of infectious particles, the minimal requirements of the dimeric capsid protein for virus assembly/disassembly have not been characterized. By use of a trans-packaging system that involved packaging a yellow fever virus (YFV) replicon into pseudo-infectious particles by supplying the YFV structural proteins using a Sindbis virus helper construct, the functional elements within the YFV capsid protein (YFC) were characterized. Various N- and C-terminal truncations, internal deletions, and point mutations of YFC were analyzed for their ability to package the YFV replicon. Consistent with previous reports on the tick-borne encephalitis virus capsid protein, YFC demonstrates remarkable functional flexibility. Nearly 40 residues of YFC could be removed from the N terminus while the ability to package replicon RNA was retained. Additionally, YFC containing a deletion of approximately 27 residues of the C terminus, including a complete deletion of C-terminal helix 4, was functional. Internal deletions encompassing the internal hydrophobic sequence in YFC were, in general, tolerated to a lesser extent. Site-directed mutagenesis of helix 4 residues predicted to be involved in intermonomeric interactions were also analyzed, and although single mutations did not affect packaging, a YFC with the double mutation of leucine 81 and valine 88 was nonfunctional. The effects of mutations in YFC on the viability of YFV infection were also analyzed, and these results were similar to those obtained using the replicon packaging system, thus underscoring the flexibility of YFC with respect to the requirements for its functioning.

Expression of particulate-form of Japanese encephalitis virus envelope protein in a stably transfected Drosophila cell line

Background

Japanese encephalitis virus (JEV), a member of the family Flaviviridae, is an important mosquito-borne human pathogen. Its envelope glycoprotein (E) is the major determinant of the pathogenicity and host immune responses. In the present study, we explored the feasibility of producing recombinant JEV E protein in the virus-free Drosophila expression system.

Results

The coding sequence for the signal sequence of premembrane and E protein was cloned into the Drosophila expression vector pAc5.1/V5-His. A Drosophila cell line S2 was cotransfected with this construct as well as a plasmid providing hygromycin B resistance. A cell line expressing the JEV E protein was selected by immunofluoresence, confocal microscopy, and western blot analysis using three different monoclonal antibodies directed against JEV E protein. This cell line was stable in the yield of JEV E protein during two months in vitro maintenance in the presence of hygromycin B. The results showed that the recombinant E protein had an expected molecular weight of about 50 kilodalton, was immunoreactive with all three monoclonal antibodies, and found in both the cytoplasm and culture supernatant. Sucrose gradient ultracentrifugation analysis revealed that the secreted E protein product was in a particulate form. It migrated to the sucrose fraction with a density of 1.13 g/ml. Balb/c mice immunised with the sucrose fraction containing the E protein particles developed specific antibodies. These data show that functioning JEV E protein was expressed in the stable S2 cell line.

Conclusion

The Drosophila expression system is a more convenient, cheaper and safer approach to the production of vaccine candidates and diagnostic reagents for JEV.

Characterization of the dengue virus envelope glycoprotein expressed in Pichia pastoris

The full-length and truncated forms of recombinant envelope (E) glycoprotein from Dengue virus type 1, Singapore strain S275/90 were expressed in the yeast, Pichia pastoris, using a secretory vector. A truncated form of the E protein in which the transmembrane domain was deleted was secreted successfully into the culture medium. The E protein was also co-expressed with C and prM proteins using a non-secretory yeast vector. The co-expression of C, prM and E proteins resulted in the spontaneous formation of virus-like particles (VLPs), which were confirmed by sucrose gradient analysis and transmission electron microscopy. Furthermore, the VLPs were used to immunise rabbits, and shown to be immunogenic by immunofluorescence staining of dengue virus-infected Vero cells. The yeast-expressed E protein was treated with PNGase F, which showed that although the protein was modified by the addition of N-linked glycans, the recombinant expressed E protein was not hyperglycosylated.

Structural analysis of viral nucleocapsids by subtraction of partial projections

The nucleocapsid of flavivirus particles does not have a recognizable capsid structure when using icosahedral averaging for cryo-electron microscopy structure determinations. The apparent absence of a definitive capsid structure could be due to a lack of synchronization of the symmetry elements of the external glycoprotein layer with those of the core or because the nucleocapsid does not have the same structure within each particle. A technique has been developed to determine the structure of the capsid, and possibly also of the genome, for icosahedral viruses, such as flaviviruses, using cryo-electron microscopy. The method is applicable not only to the analyses of viral cores, but also to the missing structure of multi-component complexes due to symmetry mismatches. The density contributed by external glycoprotein and membrane layers, derived from previously determined three-dimensional icosahedrally averaged reconstructions, was subtracted from the raw images of the virus particles. The resultant difference images were then used for a three-dimensional reconstruction. After appropriate test data sets were constructed and tested, the procedure was applied to examine the nucleocapsids of flaviviruses, which showed that there is no distinct protein density surrounding the genome. Furthermore, there was no evidence of any icosahedral symmetry within the nucleocapsid core.

Development of effective therapies against West Nile virus infection

Since its entry into North America in 1999, West Nile virus has spread throughout the USA and Canada, and now annually causes a clinical spectrum of human disease ranging from a self-limiting acute febrile illness to potentially lethal encephalitis. Although no therapy is currently approved for use in humans, several strategies are being pursued to develop effective prophylaxis and treatments. This review describes the epidemiology, clinical presentation and pathogenesis of West Nile virus infection, and highlights recent progress towards an effective therapy.