A recombinant Semliki Forest virus particle vaccine encoding the prME and NS1 proteins of louping ill virus is effective in a sheep challenge model

Immunogenicity of virus-like Semliki Forest virus replicon particles expressing Indian HIV-1C gag, env and polRT genes

Development of a vaccine targeting human immunodeficiency virus-1 subtype C (HIV-1C) is an important public health priority in regions with a high prevalence of the clade C virus. The present study demonstrates the immunogenicity of recombinant Semliki Forest virus (SFV)-based virus-like replicon particles (VRPs) expressing Indian HIV-1C env/gag/polRT genes. Immunization of mice with recombinant VRPs in a homologous prime-boost protocol, either individually or in combination, elicited significant antigen-specific IFN-γ T cell responses as detected by the ELISPOT assay. Additionally, Gag-specific TNF-α secreting CD8⁺ and CD4⁺ T cells and Env-specific IL-2 secreting T cells were also elicited by mice immunized with Gag and Env constructs, respectively, as estimated by intracellular cytokine staining assay. Moreover, an HIV Pol-specific TNF-α response was elicited in mice immunized with a combination of the three VRP constructs. Furthermore, HIV-1C Gag and Env-specific binding antibodies were elicited as verified by gp120 ELISA and p24 Gag ELISA, respectively. The immunogenicity of VRPs was found to be higher as compared to that of RNA replicons and VRPs may therefore be promising preventive and therapeutic candidate vaccines for the control and management of HIV/AIDS.

Veterinary Replicon Vaccines

Vaccination is essential in livestock farming and in companion animal ownership. Nucleic acid vaccines based on DNA or RNA provide an elegant alternative to those classical veterinary vaccines that have performed suboptimally. Recent advances in terms of rational design, safety, and efficacy have strengthened the position of nucleic acid vaccines in veterinary vaccinology. The present review focuses on replicon vaccines designed for veterinary use. Replicon vaccines are self-amplifying viral RNA sequences that, in addition to the sequence encoding the antigen of interest, contain all elements necessary for RNA replication. Vaccination results in high levels of in situ antigen expression and induction of potent immune responses. Both positive- and negative-stranded viruses have been used to construct replicons, and they can be delivered as RNA, DNA, or viral replicon particles. An introduction to the biology and the construction of different viral replicon vectors is given, and examples of veterinary replicon vaccine applications are discussed.

Self-replicating alphavirus RNA vaccines

Recombinant nucleic acids are considered as promising next-generation vaccines. These vaccines express the native antigen upon delivery into tissue, thus mimicking live attenuated vaccines without having the risk of reversion to pathogenicity. They also stimulate the innate immune system, thus potentiating responses. Nucleic acid vaccines are easy to produce at reasonable cost and are stable. During the past years, focus has been on the use of plasmid DNA for vaccination. Now mRNA and replicon vaccines have come into focus as promising technology platforms for vaccine development. This review discusses self-replicating RNA vaccines developed from alphavirus expression vectors. These replicon vaccines can be delivered as RNA, DNA or as recombinant virus particles. All three platforms have been pre-clinically evaluated as vaccines against a number of infectious diseases and cancer. Results have been very encouraging and propelled the first human clinical trials, the results of which have been promising.

A sindbis virus replicon-based DNA vaccine encoding the rabies virus glycoprotein elicits immune responses and complete protection in mice from lethal challenge

A sindbis virus replicon-based DNA vaccine encoding rabies virus glycoprotein (G) was developed by subcloning rabies G gene into a sindbis virus replicon-based vaccine vector (pAlpha). The self-amplification of RNA transcripts and translation efficiency of rabies G was analyzed in pAlpha-Rab-G-transfected mammalian cells using RT-PCR, SDS-PAGE and Western blot analysis. The transfected cells also showed induction of apoptosis which is an important event in the enhancement of immune responses. Further, immune responses induced with replicon-based rabies DNA vaccine (pAlpha-Rab-G) was compared with conventional rabies DNA vaccine and commercial cell culture vaccine (Rabipur) in intramuscularly injected mice. The mice immunized with replicon-based rabies DNA vaccine induced humoral and cell mediated immune responses better than conventional rabies DNA vaccine however, comparable to Rabipur vaccine. On challenge with rabies virus CVS strain, replicon-based rabies DNA vaccine conferred complete protection similar to Rabipur. These results demonstrate that replicon-based rabies DNA vaccine is effective in inducing both humoral and cellular immune responses and can be considered as effective vaccine against rabies.

Human immunodeficiency virus type 1 env trimer immunization of macaques and impact of priming with viral vector or stabilized core protein

Currently there is limited information about the quality of immune responses elicited by candidate human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein (Env)-based immunogens in primates. Here we describe a comprehensive analysis of neutralizing antibody and T-cell responses obtained in cynomolgus macaques by three selected immunization regimens. We used the previously described YU2-based gp140 protein trimers administered in an adjuvant, preceded by two distinct priming strategies: either alphavirus replicon particles expressing matched gp140 trimers or gp120 core proteins stabilized in the CD4-bound conformation. The rationale for priming with replicon particles was to evaluate the impact of the expression platform on trimer immunogenicity. The stable core proteins were chosen in an attempt to expand selectively lymphocytes recognizing common determinants between the core and trimers to broaden the immune response. The results presented here demonstrate that the platform by which Env trimers were delivered in the priming (either protein or replicon vector) had little impact on the overall immune response. In contrast, priming with stable core proteins followed by a trimer boost strikingly focused the T-cell response on the core sequences of HIV-1 Env. The specificity of the T-cell response was distinctly different from that of the responses obtained in animals immunized with trimers alone and was shown to be mediated by CD4(+) T cells. However, this regimen showed limited or no improvement in the neutralizing antibody responses, suggesting that further immunogen design efforts are required to successfully focus the B-cell response on conserved neutralizing determinants of HIV-1 Env.

Therapeutic and prophylactic applications of alphavirus vectors

Alphavirus vectors are high-level, transient expression vectors for therapeutic and prophylactic use. These positive-stranded RNA vectors, derived from Semliki Forest virus, Sindbis virus and Venezuelan equine encephalitis virus, multiply and are expressed in the cytoplasm of most vertebrate cells, including human cells. Part of the genome encoding the structural protein genes, which is amplified during a normal infection, is replaced by a transgene. Three types of vector have been developed: virus-like particles, layered DNA-RNA vectors and replication-competent vectors. Virus-like particles contain replicon RNA that is defective since it contains a cloned gene in place of the structural protein genes, and thus are able to undergo only one cycle of expression. They are produced by transfection of vector RNA, and helper RNAs encoding the structural proteins. Layered DNA-RNA vectors express the Semliki Forest virus replicon from a cDNA copy via a cytomegalovirus promoter. Replication-competent vectors contain a transgene in addition to the structural protein genes. Alphavirus vectors are used for three main applications: vaccine construction, therapy of central nervous system disease, and cancer therapy.

Semliki Forest virus vectors expressing the H and HN genes of measles and mumps viruses reduce immunity induced by the envelope protein genes of rubella virus

A Semliki Forest virus (SFV) recombinant particle vaccine vector was constructed expressing the viral E1 and E2 envelope proteins of the RA27/3 vaccine strain of rubella virus. This vector induced high titres of antibody after intramuscular administration to Balb/C mice, both following initial vaccination and a boost 4 weeks later. This occurred for antibody as measured by ELISA and as measured by a latex agglutination test. However, co-administration of similar particles expressing the measles virus H protein and the mumps virus HN protein with the rubella protein expressing vector resulted in reduction of the anti-rubella immune response.

Particulate delivery systems for animal vaccines

The requirements for veterinary vaccines are different to those of human vaccines. Indeed, while more side effects can be tolerated in animals than in humans; there are stricter requirements in terms of cost, ease of delivery (including to wildlife), and a need to develop vaccines in species for which relatively little is known in terms of molecular immunology. By their nature particulate vaccine delivery systems are well suited to address these challenges. Here, we review particulate vaccine delivery systems, ranging from cm-sized long-distance ballistic devices to nano-bead technology for veterinary species and wildlife.

Mucosal delivery of vaccines in domestic animals

Mucosal vaccination is proving to be one of the greatest challenges in modern vaccine development. Although highly beneficial for achieving protective immunity, the induction of mucosal immunity, especially in the gastro-intestinal tract, still remains a difficult task. As a result, only very few mucosal vaccines are commercially available for domestic animals. Here, we critically review various strategies for mucosal delivery of vaccines in domestic animals. This includes live bacterial and viral vectors, particulate delivery-systems such as polymers, alginate, polyphosphazenes, immune stimulating complex and liposomes, and receptor mediated-targeting strategies to the mucosal tissues. The most commonly used routes of immunization, strategies for delivering the antigen to the mucosal surfaces, and future prospects in the development of mucosal vaccines are discussed.

Biochemical and immunogenic characterization of soluble human immunodeficiency virus type 1 envelope glycoprotein trimers expressed by semliki forest virus

The current lack of envelope glycoprotein immunogens that elicit broadly neutralizing antibody responses remains a major challenge for human immunodeficiency virus type 1 (HIV-1) vaccine development. However, the recent design and construction of stable soluble gp140 trimers have shown that some neutralization breadth can be achieved by using immunogens that better mimic the functional viral spike complex. The use of genetic delivery systems to drive the in vivo expression of such immunogens for the stimulation of neutralizing antibodies against HIV-1 may offer advantages by maintaining the quaternary structure of the trimeric envelope glycoproteins. Here, we describe the biochemical and immunogenic properties of soluble HIV-1 envelope glycoprotein trimers expressed by recombinant Semliki Forest virus (rSFV). The results presented here demonstrate that rSFV supports the expression of stable soluble gp140 trimers that retain recognition by conformationally sensitive antibodies. Further, we show that rSFV particle immunizations efficiently primed immune responses as measured after a single boost with purified trimeric gp140 protein, resulting in a Th1-biased antibody response. This differed from the Th2-biased antibody response obtained after repeated immunizations with purified gp140 protein trimers. Despite this difference, both regimens stimulated neutralizing antibody responses of similar potency. This suggests that rSFV may be a useful component of a viral vector prime-protein boost regimen aimed at stimulating both cell-mediated immune responses and neutralizing antibodies against HIV-1.

Efficient expansion of HIV-1-specific T cell responses by homologous immunization with recombinant Semliki Forest virus particles

Vaccines based on recombinant viruses represent a promising strategy for the development of a prophylactic vaccine against HIV-1. However, despite a proven capacity to stimulate potent HIV-1-specific immune responses, viral systems have limited utility in homologous prime-boost regimens due to the generation of anti-vector immune responses. It is therefore important to develop a diverse set of vaccine candidates that can be combined in different heterologous prime-boost regimens and/or to identify a vaccine candidate that is less sensitive to anti-vector mediated immunity. In this report, we describe the design and pre-clinical immunogenicity of a Semliki Forest virus-based vaccine, VREP-C, encoding Indian origin HIV-1 clade C antigens. We show that a single immunization with VREP-C stimulates HIV-1-specific IFNgamma ELISPOT responses, which were efficiently boosted by a second and a third homologous VREP-C immunization resulting in highly potent cytotoxic T cell responses. These results suggest that VREP-C may be a valuable component of a future prophylactic vaccine against HIV-1.

Enhanced immunogenicity using an alphavirus replicon DNA vaccine against human immunodeficiency virus type 1

With the human immunodeficiency virus type 1 (HIV-1) epidemic expanding at increasing speed, development of a safe and effective vaccine remains a high priority. One of the most central vaccine platforms considered is plasmid DNA. However, high doses of DNA and several immunizations are typically needed to achieve detectable T-cell responses. In this study, a Semliki Forest virus replicon DNA vaccine designed for human clinical trials, DREP.HIVA, encoding an antigen that is currently being used in human trials in the context of a conventional DNA plasmid, pTHr.HIVA, was generated. It was shown that a single immunization of DREP.HIVA stimulated HIV-1-specific T-cell responses in mice, suggesting that the poor immunogenicity of conventional DNA vaccines may be enhanced by using viral replicon-based plasmid systems. The results presented here support the evaluation of Semliki Forest virus replicon DNA vaccines in non-human primates and in clinical studies.

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.

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.

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.

Kunjin virus replicon vaccine vectors induce protective CD8+ T-cell immunity

The ability of self-replicating RNA (replicon) vaccine vectors derived from the Australian flavivirus Kunjin (KUN) to induce protective alphabeta CD8+ T-cell responses was examined. KUN replicons encoding a model immunogen were delivered by three different vaccine modalities: (i) as naked RNA transcribed in vitro, (ii) as plasmid DNA constructed to allow in vivo transcription of replicon RNA by cellular RNA polymerase II (DNA based), and (iii) as replicon RNA encapsidated into virus-like particles. A single immunization with any of these KUN replicon vaccines induced CD8+ T-cell responses at levels comparable to those induced by recombinant vaccinia virus encoding the same immunogen. Immunization with only 0.1 microg of DNA-based KUN replicons elicited CD8+ T-cell responses similar to those seen after immunization with 100 microg of a conventional DNA vaccine. Naked RNA immunization with KUN replicons also protected mice against challenges with recombinant vaccinia virus and B16 tumor cells. These results demonstrate the value of KUN replicon vectors for inducing protective antiviral and anticancer CD8+ T-cell responses.

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

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

The pathogenicity of louping ill virus for mice and lambs

Mice and lambs were infected with the LI/I, LI/31 or MA54 strain of louping ill virus (LIV) to provide information relevant to testing the efficacy and biosafety of a new generation of flavivirus vaccines based on a Semliki Forest virus (SFV) vector. Whereas clinical signs and neuropathological lesions were consistently severe in mice, the majority of lambs showed lesions of moderate severity and only lambs with severe lesions were clinically affected. For both species, dispersal of viral antigen occurred along neuronal cell processes, and neuronal degeneration and death were confirmed as central events after infection with LIV. In contrast to lambs, in which most lesions remained localized, mice showed widely dispersed lesions which were associated with less intense leucocytic infiltrates. Among the infiltrating cells, histiocytes predominated and apoptotic forms were prominent in severely affected animals. The intranasal route of infection provided an efficient avenue for entry of LIV into the brain and resulted in lesions which were more severe than those produced by subcutaneous or intraperitoneal inoculation.