Marburg virus vaccines based upon alphavirus replicons protect guinea pigs and nonhuman primates

Venezuelan Equine Encephalitis Replicon Immunization Overcomes Intrinsic Tolerance and Elicits Effective Anti-tumor Immunity to the ‘Self’ tumor-associated antigen, neu in a Rat Mammary Tumor Model

Many tumor-associated antigens (TAAs) represent 'self' antigens and as such, are subject to the constraints of immunologic tolerance. There are significant barriers to eliciting anti-tumor immune responses of sufficient magnitude. We have taken advantage of a Venezuelan equine encephalitis-derived alphavirus replicon vector system with documented in vivo tropism for immune system dendritic cells. We have overcome the intrinsic tolerance to the 'self' TAA rat neu and elicited an effective anti-tumor immune response using this alphavirus replicon vector system and a designed target antigen in a rigorous rat mammary tumor model. We have demonstrated the capacity to generate 50% protection in tumor challenge experiments (p = 0.004) and we have confirmed the establishment of immunologic memory by both second tumor challenge and Winn Assay (p = 0.009). Minor antibody responses were identified and supported the establishment of T helper type 1 (Th1) anti-tumor immune responses by isotype. Animals surviving in excess of 300 days with established effective anti-tumor immunity showed no signs of autoimmune phenomena. Together these experiments support the establishment of T lymphocyte dependent, Th1-biased anti-tumor immune responses to a non-mutated 'self' TAA in an aggressive tumor model. Importantly, this tumor model is subject to the constraints of immunologic tolerance present in animals with normal developmental, temporal, and anatomical expression of a non-mutated TAA. These data support the continued development and potential clinical application of this alphaviral replicon vector system and the use of appropriately designed target antigen sequences for anti-tumor immunotherapy.

An alphavirus replicon particle chimera derived from venezuelan equine encephalitis and sindbis viruses is a potent gene-based vaccine delivery vector

Alphavirus replicon particle-based vaccine vectors derived from Sindbis virus (SIN), Semliki Forest virus, and Venezuelan equine encephalitis virus (VEE) have been shown to induce robust antigen-specific cellular, humoral, and mucosal immune responses in many animal models of infectious disease and cancer. However, since little is known about the relative potencies among these different vectors, we compared the immunogenicity of replicon particle vectors derived from two very different parental alphaviruses, VEE and SIN, expressing a human immunodeficiency virus type 1 p55(Gag) antigen. Moreover, to explore the potential benefits of combining elements from different alphaviruses, we generated replicon particle chimeras of SIN and VEE. Two distinct strategies were used to produce particles with VEE-p55(gag) replicon RNA packaged within SIN envelope glycoproteins and SIN-p55(gag) replicon RNA within VEE envelope glycoproteins. Each replicon particle configuration induced Gag-specific CD8(+) T-cell responses in murine models when administered alone or after priming with DNA. However, Gag-specific responses varied dramatically, with the strongest responses to this particular antigen correlating with the VEE replicon RNA, irrespective of the source of envelope glycoproteins. Comparing the replicons with respect to heterologous gene expression levels and sensitivity to alpha/beta interferon in cultured cells indicated that each might contribute to potency differences. This work shows that combining desirable elements from VEE and SIN into a replicon particle chimera may be a valuable approach toward the goal of developing vaccine vectors with optimal in vivo potency, ease of production, and safety.

Evaluation of Venezuelan Equine Encephalitis (VEE) replicon-based Outer surface protein A (OspA) vaccines in a tick challenge mouse model of Lyme disease

Venezuelan Equine Encephalitis (VEE) virus replicon particles (VRPs) encoding Borrelia burgdorferi Outer surface protein A (OspA) were evaluated for their ability to induce an immune response and provide protection from tick-borne spirochetes. VRPs expressing ospA that accumulated intracellularly (VRP OspA) or that was secreted from host cells (VRP tPA-OspA) were tested. Both VRP OspA and VRP tPA-OspA expressed ospA in immunized mice. Mice vaccinated with VRPs expressing secreted OspA produced significant amounts of anti-OspA antibodies, whereas VRPs expressing intracellular OspA were less immunogenic. The VRP method of delivery induced a Th1 type immune response unlike the recombinant OspA protein in Freund's adjuvant, which induced a mixed (Th1 and Th2) immune response. The VRP tPA-OspA construct induced an immune response that reduced the bacterial load in feeding Ixodes scapularis and blocked transmission to the host. These results indicate that VRPs are capable of providing protection against tick-borne B. burgdorferi, and potentially can be used for developing improved vaccines against Lyme disease.

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.

Characterization of monoclonal antibodies to Marburg virus (strain Musoke) glycoprotein and identification of two protective epitopes

Monoclonal antibodies (MAbs) reactive with Marburg virus (strain Musoke) were evaluated for both biological activity and specificity. Several of the Marburg virus- (MBGV) specific MAbs reduced the size and/or number of MBGV plaques in vitro. The ability of the MAbs to affect plaque formation in vitro was demonstrated to be specific for the glycoprotein (GP) of the strain of MBGV used for vaccination. Using deletion analysis and peptide mapping, the binding epitopes of several of these neutralizing MAbs were identified. Not unexpectedly, the epitopes were shown to lie in the most hypervariable and highly glycosylated region of MBGV GP. An analysis of the in vivo activity of several MAbs revealed that some antibodies provided substantial but incomplete protection of naive guinea pigs by passive transfer. These data suggest that neutralizing epitopes exist within MBGV GP but that induction of antibodies to these neutralizing epitopes may not be sufficient for protection from lethal infection.

Ebola virus: from discovery to vaccine

Ebola virus, being highly pathogenic for humans and non-human primates and the subject of former weapons programmes, is now one of the most feared pathogens worldwide. In addition, the lack of pre- and post-exposure interventions makes the development of rapid diagnostics, new antiviral agents and protective vaccines a priority for many nations. Further insight into the ecology, immunology and pathogenesis of Ebola virus will promote the delivery of these urgently required tools.

Transfection-independent production of alphavirus replicon particles based on poxvirus expression vectors

This report describes a transfection-independent system for packaging alphavirus replicon vectors using modified vaccinia virus Ankara (MVA) vectors to express all of the RNA components necessary for the production of Venezuelan equine encephalitis (VEE) virus replicon particles (VRP). Infection of mammalian cells with these recombinant MVA vectors resulted in robust expression of VEE structural genes, replication of the alphavirus vector and high titers of VRP. In addition, VRP packaging was achieved in a cell type (fetal rhesus lung) that has been approved for the manufacturing of vaccines destined for human use.

Alphavirus vectors for vaccine production and gene therapy

Alphavirus vectors demonstrate high expression of heterologous proteins in a broad range of host cells. Replication-deficient as well as replication-competent variants exist. Systemic delivery of many viral antigens has elicited strong antibody responses in immunized mice and primates, and protection against challenges with lethal viruses was obtained. Similarly, prophylactic vaccination was established against tumor challenges. Attention has been paid to the engineering of improved targeting to immunologically active cells, such as dendritic cells. In the area of gene therapy, intratumoral injections of alphavirus vectors have resulted in potentially promising tumor rejection. Moreover, encapsulation of alphavirus particles into liposomes demonstrated efficient tumor targeting in mice with severe combined immunodeficiency, which permitted the initiation of clinical trials for patients with advanced kidney carcinoma and melanoma.

A method of alphavirus replicon particle titration based on expression of functional replicase/transcriptase

Alphavirus replicon particles are being exploited for a variety of purposes both in vitro as gene expression vectors, and in vivo as vaccines or gene therapy vectors. There is a need for a simple and universal method of titration of replicon particles that is independent of expression of the foreign protein. We devised a method that uses modified vaccinia virus Ankara (MVA) as an indicator virus, to deliver a Venezuelan equine encephalitis virus (VEE) defective helper RNA encoding green fluorescent protein (GFP). Co-infection of cells with the MVA-based indicator and Venezuelan equine encephalitis virus replicon particles (VRP) results in expression of the GFP gene. VRP titer is readily determined by counting fluorescent cells.

Vaccine research efforts for filoviruses

Ebola and Marburg viruses belong to the family Filoviridae, and cause acute, frequently fatal, haemorrhagic fever in humans and non-human primates. No vaccines are available for human use. This review describes the status of research efforts to develop vaccines for these viruses and to identify the immune mechanisms of protection. The vaccine approaches discussed include DNA-based vaccines, and subunit vaccines vectored by adenovirus, alphavirus replicons, and vaccinia virus.

Mapping of two dominant sites of VP35 of Marburg virus

Five types of anti-VP35 monoclonal antibodies (MAbs), four immune sera against Marburg virus (MBGV), and 11 overlapping recombinant VP35 fragments were used to map the epitopes for VP35 of MBGV. The purified full-size recombinant VP35 was highly immunogenic and retained the B-cell epitopes that were identical to those of the viral VP35. Two major sites on VP35 and a set of truncated VP35 fragments were found by use of an enzyme immunoassay and immunoblot. Site I was located in a region between amino acids 1 and 174 of the VP35 sequence, and only polyclonal antibodies (PAbs) against MBGV recognized epitopes at this site. Site II was mapped by use of anti-VP35 MAbs to the region between amino acid residues 167 and 278 of VP35. Amino acids 252-278 of VP35 might be involved in the formation of the epitopes for MAbs. B-cell epitopes were not found on the C-terminus of VP35 by use of PAbs or MAbs.

Ebola virus: immune mechanisms of protection and vaccine development

Vaccination is one of our most powerful antiviral strategies. Despite the emergence of deadly viruses such as Ebola virus, vaccination efforts have focused mainly on childhood communicable diseases. Although Ebola virus was once believed to be limited to isolated outbreaks in distant lands, forces of globalization potentiate outbreaks anywhere in the world through incidental transmission. Moreover, since this virus has already been transformed into weapon-grade material, the potential exists for it to be used as a biological weapon with catastrophic consequences for any population vulnerable to attack. Ebola hemorrhagic fever (EHF) is a syndrome that can rapidly lead to death within days of symptom onset. The disease directly affects the immune system and vascular bed, with correspondingly high mortality rates. Patients with severe disease produce dangerously high levels of inflammatory cytokines, which destroy normal tissue and microcirculation, leading to profound capillary leakage, renal failure, and disseminated intravascular coagulation. Vaccine development has been fraught with obstacles, primarily of a biosafety nature. Case reports of acutely ill patients with EHF showing improvement with the transfusion of convalescent plasma are at odds with animal studies demonstrating further viral replication with the same treatment. Using mRNA extracted from bone marrow of Ebola survivors, human monoclonal antibodies against Ebola virus surface protein have been experimentally produced and now raise the hope for the development of a safe vaccine.

Eradication of established HPV16-transformed tumours after immunisation with recombinant Semliki Forest virus expressing a fusion protein of E6 and E7

Previously, we described the efficacy of immunisation with recombinant Semliki Forest virus (SFV), expressing the human papillomavirus 16 (HPV) oncoproteins E6 and E7, in inducing HPV-specific CTLs and anti-tumour responses. Recently, we developed a novel recombinant SFV construct encoding a relatively stable fusion protein of HPV16 E6 and E7 under control of a translational enhancer derived from the SFV capsid protein. In the present study we demonstrate that immunisation of tumour-bearing mice with this improved vector results in the regression and complete elimination of established tumours. We furthermore demonstrate that a long-term high level of CTL activity, up to 340 days, accompanies the anti-tumour response. Thus, immunisation with recombinant SFV particles encoding increased levels of a fusion protein of HPV16 E6 and E7 efficiently induces CTL activity and CTL memory resulting in a potent therapeutic anti-tumour effect.

Induction of primary virus-cross-reactive human immunodeficiency virus type 1-neutralizing antibodies in small animals by using an alphavirus-derived in vivo expression system

We have studied the induction of neutralizing antibodies by in vivo expression of the human immunodeficiency virus type 1 (HIV-1) envelope by using a Venezuelan equine encephalitis virus (VEE) replicon system with mice and rabbits. The HIV-1 envelope, clone R2, has broad sensitivity to cross-reactive neutralization and was obtained from a donor with broadly cross-reactive, primary virus-neutralizing antibodies (donor of reference serum, HIV-1-neutralizing serum 2 [HNS2]). It was expressed as gp160, as secreted gp140, and as gp160deltaCT with the cytoplasmic tail deleted. gp140 was expressed in vitro at a high level and was predominantly uncleaved oligomer. gp160deltaCT was released by cells in the form of membrane-bound vesicles. gp160deltaCT induced stronger neutralizing responses than the other forms. Use of a helper plasmid for replicon particle packaging, in which the VEE envelope gene comprised a wild-type rather than a host range-adapted sequence, also enhanced immunogenicity. Neutralizing activity fractionated with immunoglobulin G. This activity was cross-reactive among a panel of five nonhomologous primary clade B strains and a Chinese clade C strain and minimally reactive against a Chinese clade E (circulating recombinant form 1) strain. The comparative neutralization of these strains by immune mouse sera was similar to the relative neutralizing effects of HNS2, and responses induced in rabbits were similar to those induced in mice. Together, these results demonstrate that neutralizing antibody responses can be induced in mice within 2 to 3 months that are similar in potency and cross-reactivity to those found in the chronically infected, long-term nonprogressive donor of HNS2. These findings support the expectation that induction of highly cross-reactive HIV-1 primary virus-neutralizing activity by vaccination may be realized.

Venezuelan equine encephalitis virus-vectored vaccines protect mice against anthrax spore challenge

Anthrax, a disease usually associated with herbivores, is caused by the bacterium Bacillus anthracis. The current vaccine licensed for human use requires a six-dose primary series and yearly boosters and causes reactogenicity in up to 30% of vaccine recipients. A minimally reactogenic vaccine requiring fewer inoculations is warranted. Venezuelan equine encephalitis (VEE) virus has been configured for use as a vaccine vector for a wide variety of immunogens. The VEE vaccine vector is composed of a self-replicating RNA (replicon) containing all of the VEE virus nonstructural genes and a multiple-cloning site in place of the VEE structural genes. Four different anthrax vaccines were constructed by cloning the protective antigen (PA) gene from B. anthracis into the VEE vaccine vector. The anthrax vaccines were produced by assembling the vectors into propagation-deficient VEE replicon particles in vitro. A/J mice inoculated subcutaneously with three doses of the mature 83-kDa PA vaccine were completely protected from challenge with the Sterne strain of B. anthracis. Similar results were obtained with vaccines composed of the PA gene fused to either the B. anthracis secretory sequence or to a tissue plasminogen activator secretory sequence in three additional mouse strains. Mice were unprotected from challenge after inoculation with the carboxy-terminal 63-kDa PA vaccine. These results suggest that these VEE-vectored vaccines may be suitable as candidate vaccines against anthrax.

Defense against filoviruses used as biological weapons

The filoviruses, Marburg and Ebola, are classified as Category A biowarfare agents by the Centers for Disease Control. Most known human infections with these viruses have been fatal, and no vaccines or effective therapies are currently available. Filoviruses are highly infectious by the airborne route in the laboratory, but investigations of African outbreaks have shown that person-to-person spread requires direct contact with virus-containing material. In consequence, filovirus epidemics can be halted by isolating patients and instituting standard infection control and barrier nursing procedures. The filovirus disease syndrome resembles that caused by other hemorrhagic fever viruses, necessitating studies in a biocontainment laboratory to confirm the diagnosis. Some progress has been made in developing vaccines and antiviral drugs, but efforts are hindered by the limited number of maximum containment laboratories. Terrorists might have great difficulty acquiring a filovirus for use as a weapon, but my attempt to do so because of the agents' ability to inspire fear. Accurate information is the best tool to prevent panic in the event of an attack.

Alphavirus vectors and vaccination

Alphaviruses are positive-stranded RNA viruses that have a broad host range and therefore are capable of replicating in many vertebrate and invertebrate cells. The single-stranded alphavirus genome is divided into two ORFs. The first ORF encodes the nonstructural proteins that are translated upon entry of the virus into the cytoplasm and are responsible for transcription and replication of viral RNA. The second ORF is under the control of a subgenomic promoter and normally encodes the structural proteins, which are responsible for encapsidation of viral RNA and final assembly into enveloped particles. Expression vectors have been engineered from at least three alphaviruses in which the structural protein gene region has been replaced by heterologous genes and have been shown to express high levels of the heterologous protein in cultured cells. These RNA vectors, known as replicons, are capable of replicating on their own but are not packaged into virus-like particles unless the structural proteins are provided in trans. Thus, replicons are single cycle vectors incapable of spreading from infected to noninfected cells. Because of these features, alphavirus replicon vectors are being developed as a platform vaccine technology for numerous viral, bacterial, protozoan and tumour antigens where they have been shown to be efficient inducers of both humoral and T cell responses. In addition, as the alphavirus structural proteins are not expressed in vaccine recipients, antivector immune responses are generally minimal, allowing for multiple effective immunisations of the same individual.

Virus-based vectors for human vaccine applications

Vaccinology has experienced a dramatic resurgence recently, as traditional methodologies of using attenuated live pathogens or inactivated whole pathogens have been either ineffective or are not an acceptable risk for several disease targets, including HIV and Hepatitis C. Gene-based vaccines can stimulate potent humoral and cellular immune responses, and viral vectors might be an efficient strategy for both delivery of antigen-encoding genes, as well as facilitating and enhancing antigen presentation. Vectors derived from diverse viruses with distinct tropism and gene expression strategies have been developed, and are being evaluated in preclinical and clinical vaccine studies. Virus-based vaccines represent a promising approach for vaccines against infectious and malignant disease.

Evaluation in nonhuman primates of vaccines against Ebola virus

Ebola virus (EBOV) causes acute hemorrhagic fever that is fatal in up to 90% of cases in both humans and nonhuman primates. No vaccines or treatments are available for human use. We evaluated the effects in nonhuman primates of vaccine strategies that had protected mice or guinea pigs from lethal EBOV infection. The following immunogens were used: RNA replicon particles derived from an attenuated strain of Venezuelan equine encephalitis virus (VEEV) expressing EBOV glycoprotein and nucleoprotein; recombinant Vaccinia virus expressing EBOV glycoprotein; liposomes containing lipid A and inactivated EBOV; and a concentrated, inactivated whole-virion preparation. None of these strategies successfully protected nonhuman primates from robust challenge with EBOV. The disease observed in primates differed from that in rodents, suggesting that rodent models of EBOV may not predict the efficacy of candidate vaccines in primates and that protection of primates may require different mechanisms.

Immunologic control of HIV-1

By destroying CD4+ T cells, human immunodeficiency virus-1 (HIV-1) infection results in immunodeficiency and the inability of the immune system to contain the virus in most individuals. Although treatment of HIV-1 infection with potent antiretroviral medications has resulted in enormous clinical benefit, there is a growing recognition of the limitations of this therapy. As a result, novel approaches to treating HIV-1 infection are being considered. One such strategy is immunotherapy, which seeks to boost immune responses against HIV-1 and control the virus. This approach is based on studies of other viruses in which a coordinated immune response contains the chronic infection. Recent studies show that CD4+ helper responses, CD8+ T cell activity, and antibodies may contribute to control of the virus without antiretroviral therapy in some HIV-positive individuals. Based on this understanding of the immunologic correlates of control of HIV-1, exciting new immunotherapeutic strategies for HIV-1 infection are being designed and tested.

Lipid raft microdomains: a gateway for compartmentalized trafficking of Ebola and Marburg viruses

Spatiotemporal aspects of filovirus entry and release are poorly understood. Lipid rafts act as functional platforms for multiple cellular signaling and trafficking processes. Here, we report the compartmentalization of Ebola and Marburg viral proteins within lipid rafts during viral assembly and budding. Filoviruses released from infected cells incorporated raft-associated molecules, suggesting that viral exit occurs at the rafts. Ectopic expression of Ebola matrix protein and glycoprotein supported raft-dependent release of filamentous, virus-like particles (VLPs), strikingly similar to live virus as revealed by electron microscopy. Our findings also revealed that the entry of filoviruses requires functional rafts, identifying rafts as the site of virus attack. The identification of rafts as the gateway for the entry and exit of filoviruses and raft-dependent generation of VLPs have important implications for development of therapeutics and vaccination strategies against infections with Ebola and Marburg viruses.

Alphavirus replicon particles expressing the two major envelope proteins of equine arteritis virus induce high level protection against challenge with virulent virus in vaccinated horses

Replicon particles derived from a vaccine strain of Venezuelan equine encephalitis (VEE) virus were used as vectors for expression in vivo of the major envelope proteins (G(L) and M) of equine arteritis virus (EAV), both individually and in heterodimer form (G(L)/M). The immunogenicity of the different replicons was evaluated in horses, as was their ability to protectively immunize horses against intranasal and intrauterine challenge with a virulent strain of EAV (EAV KY84). Horses immunized with replicons that express both the G(L) and M proteins in heterodimer form developed neutralizing antibodies to EAV, shed little or no virus, and developed only mild or inapparent signs of equine viral arteritis (EVA) after challenge with EAV KY84. In contrast, unvaccinated horses and those immunized with replicons expressing individual EAV envelope proteins (M or G(L)) shed virus for 6-10 days in their nasal secretions and developed severe signs of EVA after challenge. These data confirm that replicons that co-express the G(L) and M envelope proteins effectively, induce EAV neutralizing antibodies and protective immunity in horses.

Heterologous gene expression from transmissible gastroenteritis virus replicon particles

We have recently isolated a transmissible gastroenteritis virus (TGEV) infectious construct designated TGEV 1000 (B. Yount, K. M. Curtis, and R. S. Baric, J. Virol. 74:10600-10611, 2000). Using this construct, a recombinant TGEV was constructed that replaced open reading frame (ORF) 3A with a heterologous gene encoding green fluorescent protein (GFP). Following transfection of baby hamster kidney (BHK) cells, a recombinant TGEV (TGEV-GFP2) was isolated that replicated efficiently and expressed GFP. Replicon constructs were constructed that lacked either the ORF 3B and E genes or the ORF 3B, E, and M genes [TGEV-Rep(AvrII) and TGEV-Rep(EcoNI), respectively]. As the E and M proteins are essential for TGEV virion budding, these replicon RNAs should replicate but not result in the production of infectious virus. Following cotransfection of BHK cells with the replicon RNAs carrying gfp, GFP expression was evident by fluorescent microscopy and leader-containing transcripts carrying gfp were detected by reverse transcription-PCR (RT-PCR). Subsequent passage of cell culture supernatants onto permissive swine testicular (ST) cells did not result in the virus, GFP expression, or the presence of leader-containing subgenomic transcripts, demonstrating the single-hit nature of the TGEV replicon RNAs. To prepare a packaging system to assemble TGEV replicon particles (TGEV VRP), the TGEV E gene was cloned into a Venezuelan equine encephalitis (VEE) replicon expression vector and VEE replicon particles encoding the TGEV E protein were isolated [VEE-TGEV(E)]. BHK cells were either cotransfected with TGEV-Rep(AvrII) (E gene deletion) and VEE-TGEV(E) RNA transcripts or transfected with TGEV-Rep(AvrII) RNA transcripts and subsequently infected with VEE VRPs carrying the TGEV E gene. In both cases, GFP expression and leader-containing GFP transcripts were detected in transfected cells. Cell culture supernatants, collected approximately 36 h posttransfection, were passed onto fresh ST cells where GFP expression was evident approximately 18 h postinfection. Leader-containing GFP transcripts containing the ORF 3B and E gene deletions were detected by RT-PCR. Recombinant TGEV was not released from these cultures. Under identical conditions, TGEV-GFP2 spread throughout ST cell cultures, expressed GFP, and formed viral plaques. The development of infectious TGEV replicon particles should assist studies of TGEV replication and assembly as well as facilitate the production of novel swine candidate vaccines.

Alphavirus vectors as tools in cancer gene therapy

Alphavirus vectors, particularly those based on the replicon of Semliki Forest virus, have shown great potential as gene delivery vehicles for various applications in cancer gene therapy. The rapid production of high-titer recombinant SFV particles, which show impressive transduction rates in various mammalian cell lines, primary cultures and in vivo, results in high levels of transgene expression. Additionally, SFV vectors induce apoptosis in transduced host cells, which can further increase their efficiency in tumor therapy. Because of the broad host range some attempts to target the gene delivery have been engineered for Sindbis virus vectors, where IgG binding domains of protein A have been introduced into the envelope structure of the recombinant particles to allow attachment of virus to host cells through the interaction of protein A with monoclonal antibodies. SFV vectors have also been employed for the production of retrovirus-like particles for establishment of long-term gene expression. Tumor vaccine approaches have been taken by injection of SFV vectors as naked RNA molecules, DNA plasmids or recombinant particles to achieve both therapeutic and prophylactic efficacy. The continuous improvement of alphavirus vectors will further expand the application range in the future.

Systemic, mucosal, and heterotypic immune induction in mice inoculated with Venezuelan equine encephalitis replicons expressing Norwalk virus-like particles

Norwalk-like viruses (NLVs) are a diverse group of single-stranded, nonenveloped, positive-polarity RNA viruses and are the leading cause of epidemic acute gastroenteritis in the United States. In this study, the major capsid gene of Norwalk virus, the prototype NLV, has been cloned and expressed in mammalian cells using a Venezuelan equine encephalitis (VEE) replicon expression system. Upon infection of baby hamster kidney (BHK) cells with VEE replicon particles (VRPs), the Norwalk virus capsid proteins self-assemble to generate high titers of Norwalk virus-like particles (VLPs) that are morphologically and antigenically analogous to wild-type Norwalk virus. Mice inoculated subcutaneously with VRPs expressing the Norwalk virus capsid protein (VRP-NV1) developed systemic and mucosal immune responses to Norwalk VLPs, as well as heterotypic antibody responses to the major capsid protein from another genogroup I NLV strain (NCFL) isolated from a recent outbreak. A second Norwalk virus capsid clone (NV2) containing three amino acid codon mutations from the NV1 clone was also expressed using VEE replicons (VRP-NV2), but upon infection of BHK cells failed to confer VLP self-assembly. Mice inoculated with VRP-NV2 elicited reduced systemic and mucosal immune responses to Norwalk VLPs, demonstrating the importance and potential utility of endogenous VLP presentation for maximum immune induction. Inoculation with either VRP-NV1 or VRP-NV2 resulted in serum antibody responses far superior to the induction in mice dosed orally with VLPs that were prepared using the VEE-NV1 replicon construct, a regimen similar to current models for NLV vaccination. Expression of NLV VLPs in mammalian cells offers a powerful approach for the design of novel NLV vaccines, either alone or in combination with current vaccination models.

Reverse genetics demonstrates that proteolytic processing of the Ebola virus glycoprotein is not essential for replication in cell culture

Ebola virus, a prime example of an emerging pathogen, causes fatal hemorrhagic fever in humans and in nonhuman primates. Identification of major determinants of Ebola virus pathogenicity has been hampered by the lack of effective strategies for experimental mutagenesis. Here we exploit a reverse genetics system that allows the generation of Ebola virus from cloned cDNA to engineer a mutant Ebola virus with an altered furin recognition motif in the glycoprotein (GP). When expressed in cells, the GP of the wild type, but not of the mutant, virus was cleaved into GP1 and GP2. Although posttranslational furin-mediated cleavage of GP was thought to be an essential step in Ebola virus infection, generation of a viable mutant Ebola virus lacking a furin recognition motif in the GP cleavage site demonstrates that GP cleavage is not essential for replication of Ebola virus in cell culture.

Individual and bivalent vaccines based on alphavirus replicons protect guinea pigs against infection with Lassa and Ebola viruses

Lassa and Ebola viruses cause acute, often fatal, hemorrhagic fever diseases, for which no effective vaccines are currently available. Although lethal human disease outbreaks have been confined so far to sub-Saharan Africa, they also pose significant epidemiological concern worldwide as demonstrated by several instances of accidental importation of the viruses into North America and Europe. In the present study, we developed experimental individual vaccines for Lassa virus and bivalent vaccines for Lassa and Ebola viruses that are based on an RNA replicon vector derived from an attenuated strain of Venezuelan equine encephalitis virus. The Lassa and Ebola virus genes were expressed from recombinant replicon RNAs that also encoded the replicase function and were capable of efficient intracellular self-amplification. For vaccinations, the recombinant replicons were incorporated into virus-like replicon particles. Guinea pigs vaccinated with particles expressing Lassa virus nucleoprotein or glycoprotein genes were protected from lethal challenge with Lassa virus. Vaccination with particles expressing Ebola virus glycoprotein gene also protected the animals from lethal challenge with Ebola virus. In order to evaluate a single vaccine protecting against both Lassa and Ebola viruses, we developed dual-expression particles that expressed glycoprotein genes of both Ebola and Lassa viruses. Vaccination of guinea pigs with either dual-expression particles or with a mixture of particles expressing Ebola and Lassa virus glycoprotein genes protected the animals against challenges with Ebola and Lassa viruses. The results showed that immune responses can be induced against multiple vaccine antigens coexpressed from an alphavirus replicon and suggested the possibility of engineering multivalent vaccines based upon alphavirus vectors for arenaviruses, filoviruses, and possibly other emerging pathogens.

Marburg virus vaccines: comparing classical and new approaches

An effort to develop a safe and effective vaccine for Marburg virus (MBGV), one of the filoviruses known to cause high mortality rates in humans, led us to compare directly some of the merits of modern versus classical vaccine approaches for this agent. Prior work had established the MBGV-glycoprotein (GP), the only known virion surface antigen, as a candidate for inclusion in a vaccine. In this study, we vaccinated groups of Hartley guinea pigs with killed MBGV, live attenuated MBGV, soluble MBGV-GP expressed by baculovirus recombinants, MBGV-GP delivered as a DNA vaccine, or MBGV-GP delivered via an alphavirus RNA replicon. Serological responses were evaluated, and animals were challenged with a lethal dose of MBGV given either subcutaneously or via aerosol. Killed MBGV and replicon-delivered MBGV-GP were notably immunogenic and protective against MBGV, but results did not exclude any approach and suggested a role for DNA vaccines in immunological priming.

Candidate vaccine against botulinum neurotoxin serotype A derived from a Venezuelan equine encephalitis virus vector system

A candidate vaccine against botulinum neurotoxin serotype A (BoNT/A) was developed by using a Venezuelan equine encephalitis (VEE) virus replicon vector. This vaccine vector is composed of a self-replicating RNA containing all of the VEE nonstructural genes and cis-acting elements and also a heterologous immunogen gene placed downstream of the subgenomic 26S promoter in place of the viral structural genes. In this study, the nontoxic 50-kDa carboxy-terminal fragment (H(C)) of the BoNT/A heavy chain was cloned into the replicon vector (H(C)-replicon). Cotransfection of BHK cells in vitro with the H(C)-replicon and two helper RNA molecules, the latter encoding all of the VEE structural proteins, resulted in the assembly and release of propagation-deficient, H(C) VEE replicon particles (H(C)-VRP). Cells infected with H(C)-VRP efficiently expressed this protein when analyzed by either immunofluorescence or by Western blot. To evaluate the immunogenicity of H(C)-VRP, mice were vaccinated with various doses of H(C)-VRP at different intervals. Mice inoculated subcutaneously with H(C)-VRP were protected from an intraperitoneal challenge of up to 100,000 50% lethal dose units of BoNT/A. Protection correlated directly with serum enzyme-linked immunosorbent assay titers to BoNT/A. The duration of the immunity achieved was tested at 6 months and at 1 year postvaccination, and mice challenged at these times remained refractory to challenge with BoNT/A.

Phosphorylation of Marburg virus VP30 at serines 40 and 42 is critical for its interaction with NP inclusions

The Marburg virus (MBGV) nucleocapsid complex is composed of four viral proteins (NP, L, VP35, and VP30) and the negative-strand nonsegmented genomic RNA. NP, L, and VP35 are functionally conserved among the order Mononegavirales, whereas VP30, a phosphoprotein, represents a filovirus-specific nucleocapsid protein. In the present paper, we have characterized the localization and function of VP30 phosphorylation. The main phosphorylation sites are represented by seven serine residues in the region of amino acid 40 to 51 of VP30. Additionally, trace amounts of phosphothreonine were detected. Substitution of serine residues 40 and 42 by alanine abolished the interaction of VP30 with NP-induced inclusion bodies, which contain nucleocapsid-like structures formed by NP. Substitution of the other phosphoserine residues had little effect on this interaction. Replacement of the introduced alanine residues 40 and 42 by aspartate restored the interaction between VP30 and the NP inclusions pointing to the importance of negative charges at these particular positions.

Alphavirus vectors: development and potential therapeutic applications

Alphaviruses are RNA enveloped viruses that are proving their value as expression vectors. They are particularly well-suited for this role as they are easily and quickly engineered and can be used to produce high levels of proteins of interest. A promising and important use is as vaccines against disease-causing agents such as HIV. The three alphaviruses now serving as vectors are Sindbis virus, Semliki Forest virus (SFV) and Venezuelan equine encephalitis (VEE) virus. Sindbis virus and SFV are well-known models for studies in molecular and cell biology; VEE virus is a human pathogen and had received some previous notoriety as a potential biological weapon. It is now becoming a potentially valuable vaccine vector. All three viruses are being tested as vaccines but, at present, only Sindbis virus and SFV have been considered for other uses. Sindbis virus vectors have been developed to screen libraries for the identification of new proteins and to devise sensitive assays to detect viruses more difficult to grow in culture. Both Sindbis virus and SFV vectors are serving as tools for fundamental studies in biology, examples include development in insects and analysis of protein functions in neuronal cells. In this article the replication strategy of alphaviruses and the different ways they can be engineered to serve as expression vectors is described. This provides an introduction to the ways these vectors have been used and illustrates the promise these vectors offer.

A PPxY motif within the VP40 protein of Ebola virus interacts physically and functionally with a ubiquitin ligase: implications for filovirus budding

VP40, the putative matrix protein of both Ebola and Marburg viruses, possesses a conserved proline-rich motif (PY motif) at its N terminus. We demonstrate that the VP40 protein can mediate its own release from mammalian cells, and that the PY motif is important for this self-exocytosis (budding) function. In addition, we used Western-ligand blotting to demonstrate that the PY motif of VP40 can mediate interactions with specific cellular proteins that have type I WW-domains, including the mammalian ubiquitin ligase, Nedd4. Single point mutations that disrupted the PY motif of VP40 abolished the PY/WW-domain interactions. Significantly, the full-length VP40 protein was shown to interact both physically and functionally with full-length Rsp5, a ubiquitin ligase of yeast and homolog of Nedd4. The VP40 protein was multiubiquitinated by Rsp5 in a PY-dependent manner in an in vitro ubiquitination assay. These data demonstrate that the VP40 protein of Ebola virus possesses a PY motif that is functionally similar to those described previously for Gag and M proteins of specific retroviruses and rhabdoviruses, respectively. Last, these studies imply that VP40 likely plays an important role in filovirus budding, and that budding of retroviruses, rhabdoviruses, and filoviruses may proceed via analogous mechanisms.

Infection of human dendritic cells by a sindbis virus replicon vector is determined by a single amino acid substitution in the E2 glycoprotein

The ability to target antigen-presenting cells with vectors encoding desired antigens holds the promise of potent prophylactic and therapeutic vaccines for infectious diseases and cancer. Toward this goal, we derived variants of the prototype alphavirus, Sindbis virus (SIN), with differential abilities to infect human dendritic cells. Cloning and sequencing of the SIN variant genomes revealed that the genetic determinant for human dendritic cell (DC) tropism mapped to a single amino acid substitution at residue 160 of the envelope glycoprotein E2. Packaging of SIN replicon vectors with the E2 glycoprotein from a DC-tropic variant conferred a similar ability to efficiently infect immature human DC, whereupon those DC were observed to undergo rapid activation and maturation. The SIN replicon particles infected skin-resident mouse DC in vivo, which subsequently migrated to the draining lymph nodes and upregulated cell surface expression of major histocompatibility complex and costimulatory molecules. Furthermore, SIN replicon particles encoding human immunodeficiency virus type 1 p55(Gag) elicited robust Gag-specific T-cell responses in vitro and in vivo, demonstrating that infected DC maintained their ability to process and present replicon-encoded antigen. Interestingly, human and mouse DC were differentially infected by selected SIN variants, suggesting differences in receptor expression between human and murine DC. Taken together, these data illustrate the tremendous potential of using a directed approach in generating alphavirus vaccine vectors that target and activate antigen-presenting cells, resulting in robust antigen-specific immune responses.

Expression of the two major envelope proteins of equine arteritis virus as a heterodimer is necessary for induction of neutralizing antibodies in mice immunized with recombinant Venezuelan equine encephalitis virus replicon particles

RNA replicon particles derived from a vaccine strain of Venezuelan equine encephalitis virus (VEE) were used as a vector for expression of the major envelope proteins (G(L) and M) of equine arteritis virus (EAV), both individually and in heterodimer form (G(L)/M). Open reading frame 5 (ORF5) encodes the G(L) protein, which expresses the known neutralizing determinants of EAV (U. B. R. Balasuriya, J. F. Patton, P. V. Rossitto, P. J. Timoney, W. H. McCollum, and N. J. MacLachlan, Virology 232:114-128, 1997). ORF5 and ORF6 (which encodes the M protein) of EAV were cloned into two different VEE replicon vectors that contained either one or two 26S subgenomic mRNA promoters. These replicon RNAs were packaged into VEE replicon particles by VEE capsid protein and glycoproteins supplied in trans in cells that were coelectroporated with replicon and helper RNAs. The immunogenicity of individual replicon particle preparations (pVR21-G(L), pVR21-M, and pVR100-G(L)/M) in BALB/c mice was determined. All mice developed antibodies against the recombinant proteins with which they were immunized, but only the mice inoculated with replicon particles expressing the G(L)/M heterodimer developed antibodies that neutralize EAV. The data further confirmed that authentic posttranslational modification and conformational maturation of the recombinant G(L) protein occur only in the presence of the M protein and that this interaction is necessary for induction of neutralizing antibodies.

Recombinant RNA replicons derived from attenuated Venezuelan equine encephalitis virus protect guinea pigs and mice from Ebola hemorrhagic fever virus

RNA replicons derived from an attenuated strain of Venezuelan equine encephalitis virus (VEE), an alphavirus, were configured as candidate vaccines for Ebola hemorrhagic fever. The Ebola nucleoprotein (NP) or glycoprotein (GP) genes were introduced into the VEE RNA downstream from the VEE 26S promoter in place of the VEE structural protein genes. The resulting recombinant replicons, expressing the NP or GP genes, were packaged into VEE replicon particles (NP-VRP and GP-VRP, respectively) using a bipartite helper system that provided the VEE structural proteins in trans and prevented the regeneration of replication-competent VEE during packaging. The immunogenicity of NP-VRP and GP-VRP and their ability to protect against lethal Ebola infection were evaluated in BALB/c mice and in two strains of guinea pigs. The GP-VRP alone, or in combination with NP-VRP, protected both strains of guinea pigs and BALB/c mice, while immunization with NP-VRP alone protected BALB/c mice, but neither strain of guinea pig. Passive transfer of sera from VRP-immunized animals did not confer protection against lethal challenge. However, the complete protection achieved with active immunization with VRP, as well as the unique characteristics of the VEE replicon vector, warrant further testing of the safety and efficacy of NP-VRP and GP-VRP in primates as candidate vaccines against Ebola hemorrhagic fever.

Immunization against potential biological warfare agents

The intentional release of biological agents by belligerents or terrorists is a possibility that has recently attracted increased attention. Law enforcement agencies, military planners, public health officials, and clinicians are gaining an increasing awareness of this potential threat. From a military perspective, an important component of the protective pre-exposure armamentarium against this threat is immunization. In addition, certain vaccines are an accepted component of postexposure prophylaxis against potential bioterrorist threat agents. These vaccines might, therefore, be used to respond to a terrorist attack against civilians. We review the development of vaccines against 10 of the most credible biological threats.

Vaccination of macaques against pathogenic simian immunodeficiency virus with Venezuelan equine encephalitis virus replicon particles

Vaccine vectors derived from Venezuelan equine encephalitis virus (VEE) that expressed simian immunodeficiency virus (SIV) immunogens were tested in rhesus macaques as part of the effort to design a safe and effective vaccine for human immunodeficiency virus. Immunization with VEE replicon particles induced both humoral and cellular immune responses. Four of four vaccinated animals were protected against disease for at least 16 months following intravenous challenge with a pathogenic SIV swarm, while two of four controls required euthanasia at 10 and 11 weeks. Vaccination reduced the mean peak viral load 100-fold. The plasma viral load was reduced to below the limit of detection (1,500 genome copies/ml) in one vaccinated animal between 6 and 16 weeks postchallenge and in another from week 6 through the last sampling time (40 weeks postchallenge). The extent of reduction in challenge virus replication was directly correlated with the strength of the immune response induced by the vectors, which suggests that vaccination was effective.

Role of dendritic cell targeting in Venezuelan equine encephalitis virus pathogenesis

The initial steps of Venezuelan equine encephalitis virus (VEE) spread from inoculation in the skin to the draining lymph node have been characterized. By using green fluorescent protein and immunocytochemistry, dendritic cells in the draining lymph node were determined to be the primary target of VEE infection in the first 48 h following inoculation. VEE viral replicon particles, which can undergo only one round of infection, identified Langerhans cells to be the initial set of cells infected by VEE directly following inoculation. These cells are resident dendritic cells in the skin, which migrate to the draining lymph node following activation. A point mutation in the E2 glycoprotein gene of VEE that renders the virus avirulent and compromises its ability to spread beyond the draining lymph blocked the appearance of virally infected dendritic cells in the lymph node in vivo. A second-site suppressor mutation that restores viral spread to lymphoid tissues and partially restore virulence likewise restored the ability of VEE to infect dendritic cells in vivo.

Comparison of the protective efficacy of naked DNA, DNA-based Sindbis replicon, and packaged Sindbis replicon vectors expressing Hantavirus structural genes in hamsters

Seoul virus (SEOV) is a member of the Hantavirus genus (family Bunyaviridae) and an etiological agent of hemorrhagic fever with renal syndrome. The medium (M) and small (S) gene segments of SEOV encode the viral envelope glycoproteins and nucleocapsid protein, respectively. We compared the immunogenicity and protective efficacy of naked DNA (pWRG7077), DNA-based Sindbis replicon (pSIN2.5), and packaged Sindbis replicon vectors (pSINrep5), containing either the M or S gene segment of SEOV in Syrian hamsters. All of the vectors elicited an anti-SEOV immune response to the expressed SEOV gene products. Vaccinated hamsters were challenged with SEOV and monitored for evidence of infection. Protection from infection was strongly associated with M-gene vaccination. A small number of S-gene-vaccinated animals also were protected. Hamsters vaccinated with the pWRG7077 vector expressing the M gene demonstrated the most consistent protection from SEOV infection and also were protected from heterologous hantavirus (Hantaan virus) infection.

Alphavirus vectors for gene expression and vaccines

Alphavirus expression vectors are finding novel uses in research. They are showing increasing promise as vaccines and are being developed for diagnostic assays of other viruses. Some highlights over the past couple of years include improvements in packaging of replicons, targeting of Sindbis virus replicons, stable cell lines that can be induced to produce replicons, and the isolation of noncytopathic variants of Sindbis virus replicons. Reports that alphavirus vectors can efficiently infect neurons in rat hippocampal slices should increase their use in neurobiological studies.