Naked DNA vaccines expressing the prM and E genes of Russian spring summer encephalitis virus and Central European encephalitis virus protect mice from homologous and heterologous challenge

A recombinant Modified Vaccinia virus Ankara expressing prME of tick-borne encephalitis virus affords mice full protection against TBEV infection

Introduction

Tick-borne encephalitis virus (TBEV) is an important human pathogen that can cause a serious disease involving the central nervous system (tick-borne encephalitis, TBE). Although approved inactivated vaccines are available, the number of TBE cases is rising, and breakthrough infections in fully vaccinated subjects have been reported in recent years.

Methods

In the present study, we generated and characterized a recombinant Modified Vaccinia virus Ankara (MVA) for the delivery of the pre-membrane (prM) and envelope (E) proteins of TBEV (MVA-prME).

Results

MVA-prME was tested in mice in comparison with a licensed vaccine FSME-IMMUN® and proved to be highly immunogenic and afforded full protection against challenge infection with TBEV.

Discussion

Our data indicate that MVA-prME holds promise as an improved next-generation vaccine for the prevention of TBE.

A nucleic acid-based orthopoxvirus vaccine targeting the vaccinia virus L1, A27, B5 and A33 proteins protects rabbits against lethal rabbitpox virus aerosol challenge

In the age of COVID, nucleic acid vaccines have garnered much attention, at least in part, because of the simplicity of construction, production, and flexibility to adjust and adapt to an evolving outbreak. Orthopoxviruses remain a threat on multiple fronts, especially as emerging zoonosis. In response, we developed a DNA vaccine, termed 4pox, that protected nonhuman primates against monkeypox virus (MPXV) induced severe disease. Here, we examined the protective efficacy of the 4pox DNA vaccine delivered by intramuscular (i.m.) electroporation (EP) in rabbits challenged with aerosolized rabbitpox virus (RPXV), a model that recapitulates the respiratory route of exposure and low dose associated with natural smallpox exposure in humans. We found that 4pox vaccinated rabbits developed immunogen-specific antibodies, including neutralizing antibodies and did not develop any clinical disease, indicating protection against aerosolized RPXV. In contrast, unvaccinated animals developed significant signs of disease, including lesions, and were euthanized. These findings demonstrate that an unformulated, non-adjuvanted DNA vaccine delivered (i.m.) can protect against an aerosol exposure. Importance The eradication of smallpox and subsequent cessation of vaccination has left a majority of the population susceptible to variola virus or other emerging poxvirus. This is exemplified by human monkeypox, as evidenced by the increase in reported endemic and imported cases over the past decades. Therefore, a malleable vaccine technology that can be mass produced, and doesn't require complex conditions for distribution and storage is sought. Herein, we show that a DNA vaccine, in the absence of a specialized formulation or adjuvant, can protect against a lethal aerosol insult of rabbitpox virus.

On-Demand Patient-Specific Phenotype-to-Genotype Ebola Virus Characterization

Biosafety, biosecurity, logistical, political, and technical considerations can delay or prevent the wide dissemination of source material containing viable virus from the geographic origin of an outbreak to laboratories involved in developing medical countermeasures (MCMs). However, once virus genome sequence information is available from clinical samples, reverse-genetics systems can be used to generate virus stocks de novo to initiate MCM development. In this study, we developed a reverse-genetics system for natural isolates of Ebola virus (EBOV) variants Makona, Tumba, and Ituri, which have been challenging to obtain. These systems were generated starting solely with in silico genome sequence information and have been used successfully to produce recombinant stocks of each of the viruses for use in MCM testing. The antiviral activity of MCMs targeting viral entry varied depending on the recombinant virus isolate used. Collectively, selecting and synthetically engineering emerging EBOV variants and demonstrating their efficacy against available MCMs will be crucial for answering pressing public health and biosecurity concerns during Ebola disease (EBOD) outbreaks.

Distinct neutralizing antibody correlates of protection among related Zika virus vaccines identify a role for antibody quality

The emergence of Zika virus (ZIKV) in the Americas stimulated the development of multiple ZIKV vaccine candidates. We previously developed two related DNA vaccine candidates encoding ZIKV structural proteins that were immunogenic in animal models and humans. We sought to identify neutralizing antibody (NAb) properties induced by each vaccine that correlated with protection in nonhuman primates (NHPs). Despite eliciting equivalent NAb titers in NHPs, these vaccines were not equally protective. The transfer of equivalent titers of vaccine-elicited NAb into AG129 mice also revealed nonequivalent protection, indicating qualitative differences among antibodies (Abs) elicited by these vaccines. Both vaccines elicited Abs with similar binding titers against envelope protein monomers and those incorporated into virus-like particles, as well as a comparable capacity to orchestrate phagocytosis. Functional analysis of vaccine-elicited NAbs from NHPs and humans revealed a capacity to neutralize the structurally mature form of the ZIKV virion that varied in magnitude among vaccine candidates. Conversely, sensitivity to the virion maturation state was not a characteristic of NAbs induced by natural or experimental infection. Passive transfer experiments in mice revealed that neutralization of mature ZIKV virions more accurately predicts protection from ZIKV infection. These findings demonstrate that NAb correlates of protection may differ among vaccine antigens when assayed using standard neutralization platforms and suggest that measurements of Ab quality, including the capacity to neutralize mature virions, will be critical for defining correlates of ZIKV vaccine-induced immunity.

Tick-Borne Encephalitis Virus: A Quest for Better Vaccines against a Virus on the Rise

Tick-borne encephalitis virus (TBEV), a member of the family Flaviviridae, is one of the most important tick-transmitted viruses in Europe and Asia. Being a neurotropic virus, TBEV causes infection of the central nervous system, leading to various (permanent) neurological disorders summarized as tick-borne encephalitis (TBE). The incidence of TBE cases has increased due to the expansion of TBEV and its vectors. Since antiviral treatment is lacking, vaccination against TBEV is the most important protective measure. However, vaccination coverage is relatively low and immunogenicity of the currently available vaccines is limited, which may account for the vaccine failures that are observed. Understanding the TBEV-specific correlates of protection is of pivotal importance for developing novel and improved TBEV vaccines. For affording robust protection against infection and development of TBE, vaccines should induce both humoral and cellular immunity. In this review, the adaptive immunity induced upon TBEV infection and vaccination as well as novel approaches to produce improved TBEV vaccines are discussed.

A Phase 2a Randomized, Double-Blind, Dose-Optimizing Study to Evaluate the Immunogenicity and Safety of a Bivalent DNA Vaccine for Hemorrhagic Fever with Renal Syndrome Delivered by Intramuscular Electroporation

Hantaan virus (HTNV) and Puumala virus (PUUV) are pathogenic hantaviruses found in Asia and Europe, respectively. DNA vaccines targeting the envelope glycoproteins of these viruses have been constructed and found to elicit neutralizing antibodies when delivered to humans by various technologies including intramuscular electroporation. Here, we report findings from a Phase 2a clinical trial of a combined HTNV/PUUV DNA vaccine delivered at varying doses and administration schedules using the Ichor Medical Systems TriGrid intramuscular electroporation delivery technology. The study was designed to characterize the effects of DNA vaccine dose and number of administrations on the frequency and magnitude of immunological response. Subjects (n = 120) were divided into four cohorts. Cohorts 1 and 2 received a dose of 2 mg of DNA (1 mg per plasmid), and cohorts 3 and 4 received a dose of 1 mg of DNA (0.5 mg per plasmid) each vaccination. Each of the four cohorts received a series of four administrations (days 0, 28, 56 and 168). For cohorts 1 and 3, the DNA vaccine candidate was delivered at each of the four administrations. For cohorts 2 and 4, in order to maintain blinding, subjects received the DNA vaccine on days 0, 56 and 168, but on day 28 received only the phosphate buffered saline vehicle rather the DNA vaccine. Sera were collected on days 0, 28, 56, 84, 140, 168, 196, 252 and 365 and evaluated for the presence of neutralizing antibodies by PUUV and HTNV pseudovirion neutralization assays (PsVNAs). Day 84 was also evaluated by a plaque reduction neutralization test (PRNT). Overall the PsVNA50 geometric mean titers (GMTs) and seropositivity rates among cohorts were similar. Cohort 3 exhibited the highest frequency of subjects that became seropositive to both PUUV and HTNV after vaccination, the highest peak GMT against both viruses, and the highest median titers against both viruses.

Evaluation of immunogenicity and protective efficacy of a CpG-adjuvanted DNA vaccine against Tembusu virus

Tembusu virus (TMUV) is a contagious pathogen of waterfowl including ducks and geese, which causes symptoms of high fever, loss of appetite and reduced egg production. The development of an effective vaccine is important for the prevention and control of the disease. We evaluated a DNA vaccine based on a recombinant pre-membrane (prM) and envelope (E) protein, using CpG oligodeoxynucleotide (ODN) as an adjuvanted, and tested it for protection efficacy. BHK21 cells were transfected with the recombinant plasmid pVAX1-prM/E-CpG, and the antigenicity of the expressed protein was detected using an indirect immunofluorescence assay (IFA) and western blot assay. One-day-old ducklings were intramuscularly injected with 200 μg doses of pVAX1-prM/E-CpG or pVAX1-CpG, or PBS at ten day intervals. The neutralizing antibodies and cell-mediated immune responses elicited by the DNA vaccine were detected using serum neutralization tests (SNTs) and ELISAs. At 20 days old, the ducks were challenged with 10³EID50 doses of TMUV SD/02 strain and observed for 15 days post challenge. After the second DNA vaccination and during the monitoring period, the levels of TMUV neutralizing antibodies increased in the pVAX1-prM/E-CpG vaccinated ducks. Vaccination with pVAX1-prM/E-CpG resulted in 100.0% protection of the ducks, whereas approximately 40% of ducks vaccinated with pVAX-CpG or PBS manifested clinical symptoms. Expressions of IFN-γ and IL-6 in the pVAX1-prM/E-CpG group were significantly increased (p <  0.01) compared with the control groups during the entire experimental period. The results revealed that a vaccine co-expressing prM and E, and using a CpG-ODN motif as an adjuvant, could elicit effective neutralizing antibody titers and provide efficient protection to ducks against TMUV infection.

A Multiagent Alphavirus DNA Vaccine Delivered by Intramuscular Electroporation Elicits Robust and Durable Virus-Specific Immune Responses in Mice and Rabbits and Completely Protects Mice against Lethal Venezuelan, Western, and Eastern Equine Encephalitis Virus Aerosol Challenges

There remains a need for vaccines that can safely and effectively protect against the biological threat agents Venezuelan (VEEV), western (WEEV), and eastern (EEEV) equine encephalitis virus. Previously, we demonstrated that a VEEV DNA vaccine that was optimized for increased antigen expression and delivered by intramuscular (IM) electroporation (EP) elicited robust and durable virus-specific antibody responses in multiple animal species and provided complete protection against VEEV aerosol challenge in mice and nonhuman primates. Here, we performed a comparative evaluation of the immunogenicity and protective efficacy of individual optimized VEEV, WEEV, and EEEV DNA vaccines with that of a 1 : 1 : 1 mixture of these vaccines, which we have termed the 3-EEV DNA vaccine, when delivered by IM EP. The individual DNA vaccines and the 3-EEV DNA vaccine elicited robust and durable virus-specific antibody responses in mice and rabbits and completely protected mice from homologous VEEV, WEEV, and EEEV aerosol challenges. Taken together, the results from these studies demonstrate that the individual VEEV, WEEV, and EEEV DNA vaccines and the 3-EEV DNA vaccine delivered by IM EP provide an effective means of eliciting protection against lethal encephalitic alphavirus infections in a murine model and represent viable next-generation vaccine candidates that warrant further development.

A DNA vaccine delivered by dermal electroporation fully protects cynomolgus macaques against Lassa fever

Lassa virus (LASV) is an ambisense RNA virus in the Arenaviridae family and is the etiological agent of Lassa fever, a severe hemorrhagic disease endemic to West and Central Africa.^1,2 There are no US Food and Drug Administration (FDA)-licensed vaccines available to prevent Lassa fever.^1,2 in our previous studies, we developed a gene-optimized DNA vaccine that encodes the glycoprotein precursor gene of LASV (Josiah strain) and demonstrated that 3 vaccinations accompanied by dermal electroporation protected guinea pigs from LASV-associated illness and death. Here, we describe an initial efficacy experiment in cynomolgus macaque nonhuman primates (NHPs) in which we followed an identical 3-dose vaccine schedule that was successful in guinea pigs, and a follow-on experiment in which we used an accelerated vaccination strategy consisting of 2 administrations, spaced 4 weeks apart. In both studies, all of the LASV DNA-vaccinated NHPs survived challenge and none of them had measureable, sustained viremia or displayed weight loss or other disease signs post-exposure. Three of 10 mock-vaccinates survived exposure to LASV, but all of them became acutely ill post-exposure and remained chronically ill to the study end point (45 d post-exposure). Two of the 3 survivors experienced sensorineural hearing loss (described elsewhere). These results clearly demonstrate that the LASV DNA vaccine combined with dermal electroporation is a highly effective candidate for eventual use in humans.

Epitope mapping of Ebola virus dominant and subdominant glycoprotein epitopes facilitates construction of an epitope-based DNA vaccine able to focus the antibody response in mice

We performed epitope mapping studies on the major surface glycoprotein (GP) of Ebola virus (EBOV) using Chemically Linked Peptides on Scaffolds (CLIPS), which form linear and potential conformational epitopes. This method identified monoclonal antibody epitopes and predicted additional epitopes recognized by antibodies in polyclonal sera from animals experimentally vaccinated against or infected with EBOV. Using the information obtained along with structural modeling to predict epitope accessibility, we then constructed 2 DNA vaccines encoding immunodominant and subdominant epitopes predicted to be accessible on EBOV GP. Although a construct designed to produce a membrane-bound oligopeptide was poorly immunogenic, a construct generating a secreted oligopeptide elicited strong antibody responses in mice. When this construct was administered as a boost to a DNA vaccine expressing the complete EBOV GP gene, the resultant antibody response was focused largely toward the less immunodominant epitopes in the oligopeptide. Taken together, the results of this work suggest a utility for this method for immune focusing of antibody responses elicited by vaccination.

An immunoinformatics-derived DNA vaccine encoding human class II T cell epitopes of Ebola virus, Sudan virus, and Venezuelan equine encephalitis virus is immunogenic in HLA transgenic mice

Immunoinformatics tools were used to predict human leukocyte antigen (HLA) class II-restricted T cell epitopes within the envelope glycoproteins and nucleocapsid proteins of Ebola virus (EBOV) and Sudan virus (SUDV) and the structural proteins of Venezuelan equine encephalitis virus (VEEV). Selected epitopes were tested for binding to soluble HLA molecules representing 5 class II alleles (DRB1*0101, DRB1*0301, DRB1*0401, DRB1*0701, and DRB1*1501). All but one of the 25 tested peptides bound to at least one of the DRB1 alleles, and 4 of the peptides bound at least moderately or weakly to all 5 DRB1 alleles. Additional algorithms were used to design a single "string-of-beads" expression construct with 44 selected epitopes arranged to avoid creation of spurious junctional epitopes. Seventeen of these 44 predicted epitopes were conserved between the major histocompatibility complex (MHC) of humans and mice, allowing initial testing in mice. BALB/c mice vaccinated with the multi-epitope construct developed statistically significant cellular immune responses to EBOV, SUDV, and VEEV peptides as measured by interferon (IFN)-γ ELISpot assays. Significant levels of antibodies to VEEV, but not EBOV, were also detected in vaccinated BALB/c mice. To assess immunogenicity in the context of a human MHC, HLA-DR3 transgenic mice were vaccinated with the multi-epitope construct and boosted with a mixture of the 25 peptides used in the binding assays. The vaccinated HLA-DR3 mice developed significant cellular immune responses to 4 of the 25 (16%) tested individual class II peptides as measured by IFN-γ ELISpot assays. In addition, these mice developed antibodies against EBOV and VEEV as measured by ELISA. While a low but significant level of protection was observed in vaccinated transgenic mice after aerosol exposure to VEEV, no protection was observed after intraperitoneal challenge with mouse-adapted EBOV. These studies provide proof of concept for the use of an informatics approach to design a multi-agent, multi-epitope immunogen and provide a basis for further testing aimed at focusing immune responses toward desired protective T cell epitopes.

A Phase 1 clinical trial of a DNA vaccine for Venezuelan equine encephalitis delivered by intramuscular or intradermal electroporation

Venezuelan equine encephalitis virus (VEEV), a mosquito-borne alphavirus, causes periodic epizootics in equines and is a recognized biological defense threat for humans. There are currently no FDA-licensed vaccines against VEEV. We developed a candidate DNA vaccine expressing the E3-E2-6K-E1 genes of VEEV (pWRG/VEE) and performed a Phase 1 clinical study to assess the vaccine's safety, reactogenicity, tolerability, and immunogenicity when administered by intramuscular (IM) or intradermal (ID) electroporation (EP) using the Ichor Medical Systems TriGrid™ Delivery System. Subjects in IM-EP groups received 0.5mg (N=8) or 2.0mg (N=9) of pWRG/VEE or a saline placebo (N=4) in a 1.0ml injection. Subjects in ID-EP groups received 0.08mg (N=8) or 0.3mg (N=8) of DNA or a saline placebo (N=4) in a 0.15ml injection. Subjects were monitored for a total period of 360 days. No vaccine- or device-related serious adverse events were reported. Based on the results of a subject questionnaire, the IM- and ID-EP procedures were both considered to be generally acceptable for prophylactic vaccine administration, with the acute tolerability of ID EP delivery judged to be greater than that of IM-EP delivery. All subjects (100%) in the high and low dose IM-EP groups developed detectable VEEV-neutralizing antibodies after two or three administrations of pWRG/VEE, respectively. VEEV-neutralizing antibody responses were detected in seven of eight subjects (87.5%) in the high dose and five of eight subjects (62.5%) in the low dose ID-EP groups after three vaccine administrations. There was a correlation between the DNA dose and the magnitude of the resulting VEEV-neutralizing antibody responses for both IM and ID EP delivery. These results indicate that pWRG/VEE delivered by either IM- or ID-EP is safe, tolerable, and immunogenic in humans at the evaluated dose levels. Clinicaltrials.gov registry number NCT01984983.

Immunogenicity against Far Eastern and Siberian subtypes of tick-borne encephalitis (TBE) virus elicited by the currently available vaccines based on the European subtype: systematic review and meta-analysis

Tick-borne encephalitis (TBE) virus, which is usually divided into European, Far Eastern and Siberian subtypes, is a serious public health problem in several European and Asian countries. Vaccination is the most effective measure to prevent TBE; cross-subtype protection elicited by the TBE vaccines is biologically plausible since all TBE virus subtypes are closely related. This manuscript systematically explores available data on the cross-subtype immunogenicity elicited by the currently available Western vaccines based on the European subtype. Completed immunization course of 3 doses of both Western vaccines determined very high seroconversion/seropositivity rates against both Far Eastern and Siberian subtypes among previously flavivirus-naïve subjects. All but one study found no statistically significant difference in titers of neutralizing antibodies against strains belonging to homologous and heterologous subtypes. Pooled analysis of randomized controlled trials on head-to-head comparison of immunogenicity of Western and Russian TBE vaccines did not reveal differences in seroconversion rates against Far Eastern isolates in either hemagglutination inhibition (risk ratio = 0.98, p = 0.83) or enzyme-linked immunosorbent (risk ratio = 0.95, p = 0.44) assays after 2 vaccine doses. This suggests that, in regions where a heterogeneous TBE virus population circulates, vaccines based on the European subtype may be used alongside vaccines based on the Far Eastern subtype. Studies on the field effectiveness of TBE vaccines and investigation of vaccination failures, especially in countries where different subtypes co-circulate, will further elucidate TBE vaccination-induced cross-subtype protection.

Enhanced Efficacy of a Codon-Optimized DNA Vaccine Encoding the Glycoprotein Precursor Gene of Lassa Virus in a Guinea Pig Disease Model When Delivered by Dermal Electroporation

Lassa virus (LASV) causes a severe, often fatal, hemorrhagic fever endemic to West Africa. Presently, there are no FDA-licensed medical countermeasures for this disease. In a pilot study, we constructed a DNA vaccine (pLASV-GPC) that expressed the LASV glycoprotein precursor gene (GPC). This plasmid was used to vaccinate guinea pigs (GPs) using intramuscular electroporation as the delivery platform. Vaccinated GPs were protected from lethal infection (5/6) with LASV compared to the controls. However, vaccinated GPs experienced transient viremia after challenge, although lower than the mock-vaccinated controls. In a follow-on study, we developed a new device that allowed for both the vaccine and electroporation pulse to be delivered to the dermis. We also codon-optimized the GPC sequence of the vaccine to enhance expression in GPs. Together, these innovations resulted in enhanced efficacy of the vaccine. Unlike the pilot study where neutralizing titers were not detected until after virus challenge, modest neutralizing titers were detected in guinea pigs before challenge, with escalating titers detected after challenge. The vaccinated GPs were never ill and were not viremic at any timepoint. The combination of the codon-optimized vaccine and dermal electroporation delivery is a worthy candidate for further development.

Construction and nonclinical testing of a Puumala virus synthetic M gene-based DNA vaccine

Puumala virus (PUUV) is a causative agent of hemorrhagic fever with renal syndrome (HFRS). Although PUUV-associated HFRS does not result in high case-fatality rates, the social and economic impact is considerable. There is no licensed vaccine or specific therapeutic to prevent or treat HFRS. Here we report the synthesis of a codon-optimized, full-length M segment open reading frame and its cloning into a DNA vaccine vector to produce the plasmid pWRG/PUU-M(s2). pWRG/PUU-M(s2) delivered by gene gun produced high-titer neutralizing antibodies in hamsters and nonhuman primates. Vaccination with pWRG/PUU-M(s2) protected hamsters against infection with PUUV but not against infection by related HFRS-associated hantaviruses. Unexpectedly, vaccination protected hamsters in a lethal disease model of Andes virus (ANDV) in the absence of ANDV cross-neutralizing antibodies. This is the first evidence that an experimental DNA vaccine for HFRS can provide protection in a hantavirus lethal disease model.

Side-by-side comparison of gene-based smallpox vaccine with MVA in nonhuman primates

Orthopoxviruses remain a threat as biological weapons and zoonoses. The licensed live-virus vaccine is associated with serious health risks, making its general usage unacceptable. Attenuated vaccines are being developed as alternatives, the most advanced of which is modified-vaccinia virus Ankara (MVA). We previously developed a gene-based vaccine, termed 4pox, which targets four orthopoxvirus antigens, A33, B5, A27 and L1. This vaccine protects mice and non-human primates from lethal orthopoxvirus disease. Here, we investigated the capacity of the molecular adjuvants GM-CSF and Escherichia coli heat-labile enterotoxin (LT) to enhance the efficacy of the 4pox gene-based vaccine. Both adjuvants significantly increased protective antibody responses in mice. We directly compared the 4pox plus LT vaccine against MVA in a monkeypox virus (MPXV) nonhuman primate (NHP) challenge model. NHPs were vaccinated twice with MVA by intramuscular injection or the 4pox/LT vaccine delivered using a disposable gene gun device. As a positive control, one NHP was vaccinated with ACAM2000. NHPs vaccinated with each vaccine developed anti-orthopoxvirus antibody responses, including those against the 4pox antigens. After MPXV intravenous challenge, all control NHPs developed severe disease, while the ACAM2000 vaccinated animal was well protected. All NHPs vaccinated with MVA were protected from lethality, but three of five developed severe disease and all animals shed virus. All five NHPs vaccinated with 4pox/LT survived and only one developed severe disease. None of the 4pox/LT-vaccinated animals shed virus. Our findings show, for the first time, that a subunit orthopoxvirus vaccine delivered by the same schedule can provide a degree of protection at least as high as that of MVA.

A Phase 1 clinical trial of Hantaan virus and Puumala virus M-segment DNA vaccines for hemorrhagic fever with renal syndrome

Candidate DNA vaccines for hemorrhagic fever with renal syndrome expressing the envelope glycoprotein genes of Hantaan (HTNV) or Puumala (PUUV) viruses were evaluated in an open-label, single-center Phase 1 study consisting of three vaccination groups of nine volunteers. The volunteers were vaccinated by particle-mediated epidermal delivery (PMED) three times at four-week intervals with the HTNV DNA vaccine, the PUUV DNA vaccine or both vaccines. At each dosing, the volunteers received 8 μg DNA/4 mg gold. There were no study-related serious adverse events, and all injection site pain was graded as mild. The most commonly reported systemic adverse events were fatigue, headache, malaise, myalgia, and lymphadenopathy. Blood samples were collected on days 0, 28, 56, 84, 140, and 180, and assayed for the presence of neutralizing antibodies. In the single vaccine groups, neutralizing antibodies to HTNV or PUUV were detected in 30% or 44% of individuals, respectively. In the combined vaccine group, 56% of the volunteers developed neutralizing antibodies to one or both viruses. These results demonstrate that the HTNV and PUUV DNA vaccines are safe and can be immunogenic in humans when delivered by PMED.

Progress in recombinant DNA-derived vaccines for Lassa virus and filoviruses

Developing vaccines for highly pathogenic viruses such as those causing Lassa, Ebola, and Marburg hemorrhagic fevers is a daunting task due to both scientific and logistical constraints. Scientific hurdles to overcome include poorly defined relationships between pathogenicity and protective immune responses, genetic diversity of viruses, and safety in a target population that includes a large number of individuals with compromised immune systems. Logistical obstacles include the requirement for biosafety level-4 containment to study the authentic viruses, the poor public health infrastructure of the endemic disease areas, and the cost of developing these vaccines for use in non-lucrative markets. Recombinant DNA-based vaccine approaches offer promise of overcoming some of these issues. In this review, we consider the status of various recombinant DNA candidate vaccines against Lassa virus and filoviruses which have been tested in animals.

Tick-borne Encephalitis Vaccines

Tick-borne encephalitis (TBE) is a disease that is found from western Europe across Asia and into Japan. In recent years the incidence rate has been increasing as has the endemic range of the virus. Tick-borne encephalitis is caused by three genetically distinct sutypes of viruses within a single TBE virus (TBEV) serocomplex. These three subtypes consist of Far-eastern subtype TBEV (TBEV-FE), Siberian subtype (TBEV-Sib) and European subtype (TBEV-Eu). Each of these subtypes cause clinically distinct diseases with varying degrees of severity. Development of the first vaccines for TBEV began in the late 1930s shortly after the first isolation of TBEV-FE in Russia. In the 1970s Austria began large scale vaccine production and a nationalized vaccine campaign that significantly reduced the incidence rate of TBE. Currently there are four licensed TBE vaccines, two in Europe and two in Russia. These vaccines are all quite similar formalin-inactivated virus vaccines but the each use a different virus strain for production. Published studies have shown that European vaccines are cross-protective in rodent studies and elicit cross-reactive neutralizing antibody responses in human vaccines. European vaccines have been licensed for a rapid vaccine schedule that could be used in response to a significant outbreak and reasonable neutralizing antibody titers can be achieved after a single dose although a second dose provides nearly complete and long-lasting protection. This review focuses on the current status of licensed TBE vaccines and provides a brief summary of technology currently being developed for new vaccines.

Immunogenicity and protective efficacy of a DNA vaccine against Venezuelan equine encephalitis virus aerosol challenge in nonhuman primates

A study to evaluate the immunogenicity and protective efficacy of a Venezuelan equine encephalitis virus (VEEV) DNA vaccine in an aerosol model of nonhuman primate infection was performed. Cynomolgus macaques vaccinated with a plasmid expressing the 26S structural genes of VEEV subtype IAB by particle-mediated epidermal delivery (PMED) developed virus-neutralizing antibodies. No serum viremia was detected in two out of three macaques vaccinated with the VEEV DNA after aerosol challenge with homologous virus, while one displayed a low viremia on a single day postchallenge. In contrast, all three macaques vaccinated with empty vector DNA developed a high viremia that persisted for at least 3 days after challenge. In addition, macaques vaccinated with the VEEV DNA had reduced febrile reactions, lymphopenia, and clinical signs of disease postchallenge as compared to negative control macaques. Therefore, although the sample size was small in this pilot study, these results indicate that a VEEV DNA vaccine administered by PMED can at least partially protect nonhuman primates against an aerosol VEEV challenge.

Evaluating the orthopoxvirus type I interferon-binding molecule as a vaccine target in the vaccinia virus intranasal murine challenge model

The biological threat imposed by orthopoxviruses warrants the development of safe and effective vaccines. We developed a candidate orthopoxvirus DNA-based vaccine, termed 4pox, which targets four viral structural components, A33, B5, A27, and L1. While this vaccine protects mice and nonhuman primates from lethal infections, we are interested in further enhancing its potency. One approach to enhance potency is to include additional orthopoxvirus immunogens. Here, we investigated whether vaccination with the vaccinia virus (VACV) interferon (IFN)-binding molecule (IBM) could protect BALB/c mice against lethal VACV challenge. We found that vaccination with this molecule failed to significantly protect mice from VACV when delivered alone. IBM modestly augmented protection when delivered together with the 4pox vaccine. All animals receiving the 4pox vaccine plus IBM lived, whereas only 70% of those receiving a single dose of 4pox vaccine survived. Mapping studies using truncated mutants revealed that vaccine-generated antibodies spanned the immunoglobulin superfamily domains 1 and 2 and, to a lesser extent, 3 of the IBM. These antibodies inhibited IBM cell binding and IFN neutralization activity, indicating that they were functionally active. This study shows that DNA vaccination with the VACV IBM results in a robust immune response but that this response does not significantly enhance protection in a high-dose challenge model.

Directed molecular evolution improves the immunogenicity and protective efficacy of a Venezuelan equine encephalitis virus DNA vaccine

We employed directed molecular evolution to improve the cross-reactivity and immunogenicity of the Venezuelan equine encephalitis virus (VEEV) envelope glycoproteins. The DNA encoding the E1 and E2 proteins from VEEV subtypes IA/B and IE, Mucambo virus (MUCV), and eastern and western equine encephalitis viruses (EEEV and WEEV) were recombined in vitro to create libraries of chimeric genes expressing variant envelope proteins. ELISAs specific for all five parent viruses were used in high-throughput screening to identify those recombinant DNAs that demonstrated cross-reactivity to VEEV, MUCV, EEEV, and WEEV after administration as plasmid vaccines in mice. Selected variants were then used to vaccinate larger cohorts of mice and their sera were assayed by both ELISA and by plaque reduction neutralization test (PRNT). Representative variants from a library in which the E1 gene from VEEV IA/B was held constant and only the E2 genes of the five parent viruses were recombined elicited significantly increased neutralizing antibody titers to VEEV IA/B compared to the parent DNA vaccine and provided improved protection against aerosol VEEV IA/B challenge. Our results indicate that it is possible to improve the immunogenicity and protective efficacy of alphavirus DNA vaccines using directed molecular evolution.

Hantaan virus nucleocapsid protein binds to importin alpha proteins and inhibits tumor necrosis factor alpha-induced activation of nuclear factor kappa B

Hantaviruses such as Hantaan virus (HTNV) and Andes virus cause two human diseases, hemorrhagic fever with renal syndrome and hantavirus pulmonary syndrome, respectively. For both, disease pathogenesis is thought to be immunologically mediated and there have been numerous reports of patients with elevated levels of proinflammatory and inflammatory cytokines, including tumor necrosis factor alpha (TNF-alpha), in their sera. Multiple viruses have developed evasion strategies to circumvent the host cell inflammatory process, with one of the most prevalent being the disruption of nuclear factor kappa B (NF-kappaB) activation. We hypothesized that hantaviruses might also moderate host inflammation by interfering with this pathway. We report here that the nucleocapsid (N) protein of HTNV was able to inhibit TNF-alpha-induced activation of NF-kappaB, as measured by a reporter assay, and the activation of endogenous p65, an NF-kappaB subunit. Surprisingly, there was no defect in the degradation of the inhibitor of NF-kappaB (IkappaB) protein, nor was there any alteration in the level of p65 expression in HTNV N-expressing cells. However, immunofluorescence antibody staining demonstrated that cells expressing HTNV N protein and a green fluorescent protein-p65 fusion had limited p65 nuclear translocation. Furthermore, we were able to detect an interaction between HTNV N protein and importin alpha, a nuclear import molecule responsible for shuttling NF-kappaB to the nucleus. Collectively, our data suggest that HTNV N protein can sequester NF-kappaB in the cytoplasm, thus inhibiting NF-kappaB activity. These findings, which were obtained using cells transfected with cDNA representing the HTNV N gene, were confirmed using HTNV-infected cells.

Heterogeneity in the A33 protein impacts the cross-protective efficacy of a candidate smallpox DNA vaccine

We previously developed a gene-based vaccine, termed 4pox, which targets four orthopoxvirus proteins (A33, L1, B5, and A27). Because any subunit orthopoxvirus vaccine must protect against multiple species of orthopoxviruses, we are interested in understanding the cross-protective potential of our 4pox vaccine target immunogens. In our current studies, we focused on the A33 immunogen. We found one monoclonal antibody against A33, MAb-1G10, which could not bind the monkeypox virus A33 ortholog, A35. MAb-1G10 binding could be rescued if A35 amino acids 118 and 120 were substituted with those from A33. MAb-1G10 has been shown to protect mice from VACV challenge, thus our findings indicated a protective epitope differs among orthopoxviruses. Accordingly, we tested the cross-protective efficacy of a DNA vaccine consisting of A35R against VACV challenge and compared it to vaccination with A33R DNA. Mice vaccinated with A35R had greater mortality and more weight loss compared to those vaccinated with A33R. These findings demonstrate that despite high homology between A33R orthologs, amino acid differences can impact cross-protection. Furthermore, our results caution that adequate cross-protection by any pan-orthopoxvirus subunit vaccine will require not only careful evaluation of cross-protective immunity, but also of targeting of multiple orthopoxvirus immunogens.

Targeting the vaccinia virus L1 protein to the cell surface enhances production of neutralizing antibodies

The current live-orthopoxvirus vaccine is associated with minor to serious adverse affects, and is contraindicated for use in a significant portion of the population. As an alternative vaccine, we have previously shown that a DNA subunit vaccine (4pox) based on four orthopoxvirus immunogens (L1R, B5R, A27L and A33R) can produce protective immunity against lethal orthopoxvirus challenges in mice and nonhuman primates. Because antibodies are critical for protection against secondary orthopoxvirus infections, we are now interested in strategies that will enhance the humoral immune response against vaccine targets. Here, we tested the immunogenicity of an L1R construct to which a tissue plasminogen activator signal sequence was placed in frame with the full-length L1R gene. The tPA-L1R construct produced a more robust neutralizing antibody response in vaccinated mice when the DNA vaccine was administered by gene-gun as a prime/single boost. When the tPA-L1R construct was substituted for the unmodified L1R gene in the 4pox vaccine, given as a prime and single boost, animals were better protected from lethal challenge with vaccinia virus (VACV). These findings indicate that adding a tPA-leader sequence can enhance the immunogenicity of the L1R gene when given as a DNA vaccine. Furthermore, our results demonstrate that a DNA-based vaccine is capable of establishing protection from lethal orthopoxvirus challenges when administered as a prime and single boost without requiring adjuvant.

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.

Production of pseudoinfectious yellow fever virus with a two-component genome

Application of genetically modified, deficient-in-replication flaviviruses that are incapable of developing productive, spreading infection is a promising means of designing safe and effective vaccines. Here we describe a two-component genome yellow fever virus (YFV) replication system in which each of the genomes encodes complete sets of nonstructural proteins that form the replication complex but expresses either only capsid or prM/E instead of the entire structural polyprotein. Upon delivery to the same cell, these genomes produce together all of the viral structural proteins, and cells release a combination of virions with both types of genomes packaged into separate particles. In tissue culture, this modified YFV can be further passaged at an escalating scale by using a high multiplicity of infection (MOI). However, at a low MOI, only one of the genomes is delivered into the cells, and infection cannot spread. The replicating prM/E-encoding genome produces extracellular E protein in the form of secreted subviral particles that are known to be an effective immunogen. The presented strategy of developing viruses defective in replication might be applied to other flaviviruses, and these two-component genome viruses can be useful for diagnostic or vaccine applications, including the delivery and expression of heterologous genes. In addition, the achieved separation of the capsid-coding sequence and the cyclization signal in the YFV genome provides a new means for studying the mechanism of the flavivirus packaging process.

Influences of glycosylation on antigenicity, immunogenicity, and protective efficacy of ebola virus GP DNA vaccines

The Ebola virus (EBOV) envelope glycoprotein (GP) is the primary target of protective immunity. Mature GP consists of two disulfide-linked subunits, GP1 and membrane-bound GP2. GP is highly glycosylated with both N- and O-linked carbohydrates. We measured the influences of GP glycosylation on antigenicity, immunogenicity, and protection by testing DNA vaccines comprised of GP genes with deleted N-linked glycosylation sites or with deletions in the central hypervariable mucin region. We showed that mutation of one of the two N-linked GP2 glycosylation sites was highly detrimental to the antigenicity and immunogenicity of GP. Our data indicate that this is likely due to the inability of GP2 and GP1 to dimerize at the cell surface and suggest that glycosylation at this site is required for achieving the conformational integrity of GP2 and GP1. In contrast, mutation of two N-linked sites on GP1, which flank previously defined protective antibody epitopes on GP, may enhance immunogenicity, possibly by unmasking epitopes. We further showed that although deleting the mucin region apparently had no effect on antigenicity in vitro, it negatively impacted the elicitation of protective immunity in mice. In addition, we confirmed the presence of previously identified B-cell and T-cell epitopes in GP but show that when analyzed individually none of them were neither absolutely required nor sufficient for protective immunity to EBOV. Finally, we identified other potential regions of GP that may contain relevant antibody or T-cell epitopes.

A chimeric tetravalent dengue DNA vaccine elicits neutralizing antibody to all four virus serotypes in rhesus macaques

DNA shuffling and screening technologies were used to produce chimeric DNA constructs expressing antigens that shared epitopes from all four dengue serotypes. Three shuffled constructs (sA, sB and sC) were evaluated in the rhesus macaque model. Constructs sA and sC expressed pre-membrane and envelope genes, whereas construct sB expressed only the ectodomain of envelope protein. Five of six, and four of six animals vaccinated with sA and sC, respectively, developed antibodies that neutralized all 4 dengue serotypes in vitro. Four of six animals vaccinated with construct sB developed neutralizing antibodies against 3 serotypes (den-1, -2 and -3). When challenged with live dengue-1 or dengue-2 virus, partial protection against dengue-1 was observed. These results demonstrate the utility of DNA shuffling as an attractive tool to create tetravalent chimeric dengue DNA vaccine constructs, as well as a need to find ways to improve the immune responses elicited by DNA vaccines in general.

Production and characterization of vaccines based on flaviviruses defective in replication

To develop new vaccine candidates for flavivirus infections, we have engineered two flaviviruses, yellow fever virus (YFV) and West Nile virus (WNV), that are deficient in replication. These defective pseudoinfectious viruses (PIVs) lack a functional copy of the capsid (C) gene in their genomes and are incapable of causing spreading infection upon infection of cells both in vivo and in vitro. However, they produce extracellular E protein in form of secreted subviral particles (SVPs) that are known to be an effective immunogen. PIVs can be efficiently propagated in trans-complementing cell lines making high levels of C or all three viral structural proteins. PIVs derived from YFV and WNV, demonstrated very high safety and immunization produced high levels of neutralizing antibodies and protective immune response. Such defective flaviviruses can be produced in large scale under low biocontainment conditions and should be useful for diagnostic or vaccine applications.

Dengue tetravalent DNA vaccine inducing neutralizing antibody and anamnestic responses to four serotypes in mice

We developed a dengue tetravalent DNA vaccine consisting of plasmids expressing premembrane and envelope genes of each of four serotypes of dengue viruses. BALB/c mice immunized twice with the tetravalent vaccine at a dose of 100 microg (25 microg for each serotype) using a needle-free jet injector developed neutralizing antibodies against all serotypes. There was no interference among the four components included in this combination vaccine. Tetravalent vaccine-immunized mice showed anamnestic neutralizing antibody responses following challenge with each dengue serotype: responses to challenges from serotypes different to those used for neutralization tests were also induced.

Immunogenicity of combination DNA vaccines for Rift Valley fever virus, tick-borne encephalitis virus, Hantaan virus, and Crimean Congo hemorrhagic fever virus

DNA vaccines for Rift Valley fever virus (RVFV), Crimean Congo hemorrhagic fever virus (CCHFV), tick-borne encephalitis virus (TBEV), and Hantaan virus (HTNV), were tested in mice alone or in various combinations. The bunyavirus vaccines (RVFV, CCHFV, and HTNV) expressed Gn and Gc genes, and the flavivirus vaccine (TBEV) expressed the preM and E genes. All vaccines were delivered by gene gun. The TBEV DNA vaccine and the RVFV DNA vaccine elicited similar levels of antibodies and protected mice from challenge when delivered alone or in combination with other DNAs. Although in general, the HTNV and CCHFV DNA vaccines were not very immunogenic in mice, there were no major differences in performance when given alone or in combination with the other vaccines.

Comparison of live and inactivated tick-borne encephalitis virus vaccines for safety, immunogenicity and efficacy in rhesus monkeys

Three antigenic chimeric live attenuated tick-borne encephalitis virus (TBEV) vaccine candidates were compared for level of replication in murine and human neuroblastoma cells, for neurovirulence and neuroinvasiveness in mice, and for safety, immunogenicity and efficacy in rhesus monkeys. Two chimeric viruses were generated by replacing the membrane precursor and envelope protein genes of dengue type 4 virus (DEN4) with the corresponding genes of a Far Eastern TBEV, Sofjin strain, in the presence (TBEV/DEN4Delta30) or absence (TBEV/DEN4) of a 30 nucleotide deletion (Delta30) in the 3' noncoding region of the DEN4 part of the chimeric genome. A third chimeric TBEV vaccine candidate was based on the antigenically distant, but naturally attenuated Langat virus (LGT). Chimerization of LGT with DEN4 resulted in decreased neurovirulence and neuroinvasiveness in mice and highly restricted viremia in rhesus monkeys. Also, the LGT/DEN4 chimera was highly restricted in replication in both murine and human neuroblastoma cells. In contrast, TBEV/DEN4 and TBEV/DEN4Delta30 were neither attenuated for neurovirulence in the mice nor restricted in replication in the neuroblastoma cells. However, both were highly attenuated for neuroinvasiveness in mice. TBEV/DEN4 replicated to moderately high titer in rhesus monkeys (mean peak viremia=10(3.1)PFU/ml) indicating that the TBEV/DEN4 chimerization had only a modest, if any, attenuating effect in monkeys. However, the addition of the Delta30 mutation to TBEV/DEN4 greatly attenuated the chimeric virus for rhesus monkeys (mean peak viremia=10(0.7)PFU/ml) and induced a higher level of antibody against the TBEV than did LGT/DEN4. A single dose of either highly attenuated TBEV/DEN4Delta30 or LGT/DEN4 vaccine candidate or three doses of an inactivated TBEV vaccine were efficacious in monkeys against wild-type LGT challenge. These results indicate that both TBEV/DEN4Delta30 and LGT/DEN4 are safe and efficacious in rhesus monkeys and should be further evaluated as vaccine candidates for use in humans.

The role of particle-mediated DNA vaccines in biodefense preparedness

Particle-mediated epidermal delivery (PMED) of DNA vaccines is based on the acceleration of DNA-coated gold directly into the cytoplasm and nuclei of living cells of the epidermis, facilitating DNA delivery and gene expression. Professional antigen-presenting cells and keratinocytes in the skin are both targeted, resulting in antigen presentation via direct transfection and cross-priming mechanisms. Only a small number of cells need to be transfected to elicit humoral, cellular and memory responses, requiring only a low DNA dose. In recent years, data have accumulated on the utility of PMED for delivery of DNA vaccines against a number of viral pathogens, including filoviruses, flaviviruses, poxviruses, togaviruses and bunyaviruses. PMED DNA immunization of rodents and nonhuman primates results in the generation of neutralizing antibody, cellular immunity, and protective efficacy against a broad range of viruses of public health concern.

Epidermal delivery of protein and DNA vaccines

Targeting vaccines to the skin epidermis results in the activation of an immune inductive site that is rich in antigen-presenting cells. The superficial location of the skin makes it accessible to vaccine delivery. However, it is difficult to access the epidermis using needle and syringe delivery, and vaccine antigens are too large to be effectively delivered using standard topical formulations. Needle-free vaccine delivery systems have been developed for efficient delivery of particulate vaccines into the epidermal tissue. Particle-mediated epidermal delivery of DNA vaccines is based on the delivery of DNA-coated gold particles directly into the cytoplasm and nuclei of living cells of the epidermis, facilitating DNA delivery and gene expression. Alternatively, protein vaccines can be formulated into a dense powder, which can be propelled into the skin epidermis by epidermal powder immunisation using similar delivery devices and principles, but in this instance the protein is delivered to the extracellular space. Preclinical and clinical data will be reviewed, demonstrating applications of epidermal vaccine delivery to a wide range of experimental infectious disease vaccines.

Venezuelan equine encephalitis virus, southern Mexico

Equine epizootics of Venezuelan equine encephalitis (VEE) occurred in the southern Mexican states of Chiapas in 1993 and Oaxaca in 1996. To assess the impact of continuing circulation of VEE virus (VEEV) on human and animal populations, serologic and viral isolation studies were conducted in 2000 to 2001 in Chiapas State. Human serosurveys and risk analyses indicated that long-term endemic transmission of VEEV occurred among villages with seroprevalence levels of 18% to 75% and that medical personnel had a high risk for VEEV exposure. Seroprevalence in wild animals suggested cotton rats as possible reservoir hosts in the region. Virus isolations from sentinel animals and genetic characterizations of these strains indicated continuing circulation of a subtype IE genotype, which was isolated from equines during the recent VEE outbreaks. These data indicate long-term enzootic and endemic VEEV circulation in the region and continued risk for disease in equines and humans.

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.

Particle-mediated DNA vaccine delivery to the skin

Particle-mediated DNA vaccines employ a physical, intracellular delivery device to achieve the deposition of plasmid DNA-based expression vectors directly into the interior of cells of the skin. The resultant bolus of transient antigen expression in keratinocytes and trafficking dendritic cells results in the induction of humoral and cellular immune responses in various animal models and humans, mimicking characteristics of live or live-vectored vaccines. Ultimately, DNA vaccine success in the clinic will depend on both the successful intracellular delivery of a plasmid vector and an immunostimulator or adjuvant to maximise humoral and cellular immune responses to the encoded antigen(s). To this end, recent DNA vaccine clinical trials are confirming the importance of an intracellular delivery system, while preclinical studies in animal models are demonstrating the feasibility of augmenting responses through the use of DNA-encoded immunostimulators. Particle-mediated DNA vaccines represent a promising tool for developing candidate vaccines against some of the more difficult infectious, parasitic and oncologic disease targets.

Tick-borne encephalitis and the impact of vaccination

TBE virus is endemic in many parts of Europe and Northern Asia, and in these regions it causes more than 10,000 severe cases of central nervous system disease in humans each year. The virus is primarily transmitted to humans when infected ticks take a blood meal, but infections due to the consumption of unpasteurized milk, primarily from goats, occur in certain regions. Based on genetic analyses, three closely related subtypes can be distinguished and are designated European, Siberian, and Far Eastern subtype according to their primary geographic distribution. Consistent with their close antigenic relationships, immunization studies in animals have revealed a high degree of cross-protection between virus strains belonging to different subtypes. The commercially available vaccines in Europe consist of highly purified inactivated whole TBE virus. Austria is the country with the highest coverage of TBE vaccination (86% of the total population) and this has led to a dramatic reduction in the annual number of clinical cases and proves under field conditions that vaccination is an effective means for the prophylaxis of TBE.

Vaccines and animal models for arboviral encephalitides

Arthropod-borne viruses ("arboviruses") cause significant human illness ranging from mild, asymptomatic infection to fatal encephalitis or hemorrhagic fever. The most significant arboviruses causing human illness belong to genera in three viral families, Togaviridae, Flaviviridae, and Bunyaviridae. These viruses represent a significant public health threat to many parts of the world, and, as evidenced by the recent introduction of the West Nile virus (WNV) to the Western Hemisphere, they can no longer be considered specific to any one country or region of the world. Like most viral diseases, there are no specific therapies for the arboviral encephalitides; therefore, effective vaccines remain the front line of defense for these diseases. With this in mind, the development of new, more effective vaccines and the appropriate animal models in which to test them become paramount. In fact, for many important arboviruses (e.g. California serogroup and St. Louis encephalitis viruses), there are currently no approved vaccines available for human use. For others, such as the alphaviruses, human vaccines are available only as Investigational New Drugs, and thus are not in widespread use. On the other hand, safe and effective vaccines against tick-borne encephalitis virus (TBEV) and Japanese encephalitis virus (JEV) have been in use for decades. New challenges in vaccine development have been met with new technologies in vaccine research. Many of the newer vaccines are now being developed by recombinant DNA technology. For example, chimeric virus vaccines have been developed using infectious clone technology for many of the arboviruses including, WNV, JEV, and TBEV. Other successful approaches have involved the use of naked DNA encoding and subsequently expressing the desired protective epitopes. Naked DNA vaccines have been used for TBEV and JEV and are currently under development for use against WNV. The development of less expensive, more authentic animal models to evaluate new vaccines against arboviral diseases will become increasingly important as these new approaches in vaccine research are realized. This article reviews the current status of vaccines, both approved for use and those in developmental stages, against the major arboviral encephalitides causing human disease. In addition, research on animal models, both past and present, for these diseases are discussed.

Needle-free Biojector injection of a dengue virus type 1 DNA vaccine with human immunostimulatory sequences and the GM-CSF gene increases immunogenicity and protection from virus challenge in Aotus monkeys

A dengue-1 DNA vaccine containing sequences encoding premembrane and envelope proteins (DIME) was previously shown to elicit virus neutralizing antibodies in rhesus and Aotus monkeys, and the primates were partially protected from viremia upon challenge. To increase the neutralizing antibody levels and subsequent protection from virus challenge, four strategies were evaluated: (a) coimmunization with a plasmid expressing Aotus GM-CSF gene; (b) coimmunization with a plasmid containing human immunostimulatory sequences (ISS); (c) coimmunization with both the GM-CSF gene and ISS; and (d) delivery of vaccine using the needle-free Biojector system. Vaccination with the mixed formulation containing DIME, GM-CSF gene, and ISS, by either needle injection or Biojector, led to neutralizing antibody titers that were stable for up to 6 months after vaccination. Furthermore, 6 of 7 monkeys (85%), and 7 of 8 monkeys (87%) receiving this formulation were completely protected from viremia when challenged 1 and 6 months after vaccination, respectively. This is a significant improvement compared to our previous study in which one of three monkeys (33%) receiving just the DIME vaccine was completely protected from viremia at 6 months after immunization.

Comparison of individual and combination DNA vaccines for B. anthracis, Ebola virus, Marburg virus and Venezuelan equine encephalitis virus

Multiagent DNA vaccines for highly pathogenic organisms offer an attractive approach for preventing naturally occurring or deliberately introduced diseases. Few animal studies have compared the feasibility of combining unrelated gene vaccines. Here, we demonstrate that DNA vaccines to four dissimilar pathogens that are known biowarfare agents, Bacillus anthracis, Ebola (EBOV), Marburg (MARV), and Venezuelan equine encephalitis virus (VEEV), can elicit protective immunity in relevant animal models. In addition, a combination of all four vaccines is shown to be equally as effective as the individual vaccines for eliciting immune responses in a single animal species. These results demonstrate for the first time the potential of combined DNA vaccines for these agents and point to a possible method of rapid development of multiagent vaccines for disparate pathogens such as those that might be encountered in a biological attack.