Alphaviruses as vectors for gene delivery

The amino-terminal domain of alphavirus capsid protein is dispensable for viral particle assembly but regulates RNA encapsidation through cooperative functions of its subdomains

Venezuelan equine encephalitis virus (VEEV) is a pathogenic alphavirus, which circulates in the Central, South, and North Americas, including the United States, and represents a significant public health threat. In recent years, strong progress has been made in understanding the structure of VEEV virions, but the mechanism of their formation has yet to be investigated. In this study, we analyzed the functions of different capsid-specific domains and its amino-terminal subdomains in viral particle formation. Our data demonstrate that VEEV particles can be efficiently formed directly at the plasma membrane without cytoplasmic nucleocapsid preassembly. The entire amino-terminal domain of VEEV capsid protein was found to be dispensable for particle formation. VEEV variants encoding only the capsid's protease domain efficiently produce genome-free VEEV virus-like particles (VLPs), which are very similar in structure to the wild-type virions. The amino-terminal domain of the VEEV capsid protein contains at least four structurally and functionally distinct subdomains, which mediate RNA packaging and the specificity of packaging in particular. The most positively charged subdomain is a negative regulator of the nucleocapsid assembly. The three other subdomains are not required for genome-free VLP formation but are important regulators of RNA packaging. Our data suggest that the positively charged surface of the VEEV capsid-specific protease domain and the very amino-terminal subdomain are also involved in interaction with viral RNA and play important roles in RNA encapsidation. Finally, we show that VEEV variants with mutated capsid acquire compensatory mutations in either capsid or nsP2 genes.

Generation of recombinant alphaviral vectors

The alphaviruses Semliki Forest virus and Sindbis virus have been used frequently as expression vectors in vitro and in vivo. Usually, these systems consist of replication-deficient vectors that require a helper vector for packaging of recombinant particles. Replication-proficient vectors have also been engineered. Alphaviral vectors can be used as nucleic-acid-based vectors (DNA and RNA) or infectious particles. High-titer viral production is achieved in <2 d. The broad host range of alphaviruses facilitates studies in mammalian and nonmammalian cell lines, primary cells in culture, and in vivo. The strong preference for expression in neuronal cells has made alphaviruses particularly useful in neurobiological studies. Unfortunately, their strong cytotoxic effect on host cells, relatively short-term transient expression patterns, and the reasonably high cost of viral production remain drawbacks. However, novel mutant alphaviruses have showed reduced cytotoxicity and prolonged expression. Membrane proteins (which are generally difficult to express at high levels in recombinant systems) have generated high yields and facilitate applications in structural biology. Alphaviruses have also been applied in vaccine development and gene therapy. This protocol describes the production of recombinant alphaviral vectors. The SFV- and SIN-based expression systems apply two vectors for recombinant particle production. In addition to the RNA-based vectors described here, DNA vectors with cytomegalovirus or other RNA polymerase type II promoters can be used for direct plasmid DNA transfections. Cotransfection of SFV-based pSCA expression and pSCA helper vectors generates recombinant viral particles.

Self Replicating Gene Vaccine Carrying P1-2A Gene of FMDV Serotype O and its Effects on the Immune Responses of Cattle

DNA vaccines are considered as alternatives to live attenuated ones for those diseases like foot-and-mouth disease (FMD) where the production and application of live vaccines have been found unsuccessful. However, stability of DNA and the quantity of antigen expressed are the major limitation with naked DNA vaccines. To address these issues self replicating gene vaccine construct was made for foot-and-mouth disease virus (FMDV) type 'O' and studied. The vector for vaccine construct, designated as pSinCMVVac carried CMV promoter and Poly(A) signal sequences at 5' and 3' end of Sindbis replicase gene respectively. Gene for structural protein precursor (P1-2A) of FMDV serotype 'O' was inserted into pSinCMVVac under subgenomic promoter. 5'UTR (untranslated region) of FMDV was introduced upstream of P1-2A to enhance the level of expression of cloned gene. Functionality of the vaccine construct was confirmed in vitro and in vivo. The self-replicating gene vaccine construct was tested in cattle in comparison with naked DNA vaccine carrying P1-2A and 3CD (pUP3CD). Humoral immune response by ELISA and SNT and cellular response by lymphoproliferation assay using MTT were studied. The default approach of using self replicating gene vaccine in high dose and multiple injection in cattle as followed in our studies might result in immunosuppression as this was observed in our subsequent experiments in guinea pigs. Hence based on dose response studies, vaccine strategy needs to be decided. However, the approach of using Sindbis polymerase gene and UTR in FMDV vaccine is the first report and shows future scope of developing such vaccines.

Apoptosis is essential for the increased efficacy of alphaviral replicase-based DNA vaccines

Alphaviral replicons can increase the efficacy and immunogenicity of naked nucleic acid vaccines. To study the impact of apoptosis on this increased effectiveness, we co-delivered an anti-apoptotic gene (Bcl-X(L)) with the melanocyte/melanoma differentiation antigen TRP-1. Although cells co-transfected with Bcl-X(L) lived longer, produced more antigen and elicited increased antibody production in vivo, co-delivery of pro-survival Bcl-X(L) with antigen significantly reduced the ability of the replicase-based vaccine to protect against an aggressive tumor challenge. These data show for the first time that the induction of apoptotic cell death of transfected cells in vivo is required for the increased effectiveness of replicase-based vaccines. Our findings also provide an explanation for the paradoxical observation that replicase-based DNA vaccines are much more immunogenic than conventional constructs despite reduced antigen production.

Semliki forest virus infection of laboratory mice: a model to study the pathogenesis of viral encephalitis

Semliki Forest virus (SFV) infection of the laboratory mouse provides an experimental system to study the pathogenesis of viral encephalitis. Following extraneural inoculation the virus is efficiently neuroinvasive and crosses the blood-brain barrier to initiate perivascular foci of infection in neurons and oligodendrocytes. The outcome of infection ranges from clinically unapparent mild encephalitis to fatal panencephalitis. SFV infections of the developing nervous system are always highly destructive and are generally fatal. In contrast, SFV infections of the mature nervous system can result in persistent infection with no apparent cell loss. This dramatic difference is attributable to developmental changes in the interactions between virus and CNS cells. Antibody responses clear the systemic infection and control the CNS infection. CD8+ T-cells are required to generate the lesions of inflammatory demyelination which can be a feature of the neuropathology. This article reviews the pathogenesis of SFV encephalitis, describing the neuropathology and the mechanisms which underlie it and which may be fundamental to many viral encephalitides.

AACR Special Conference in cancer research: ubiquitination in normal and cancer cells

The AACR Special Conference 2002 for Ubiquitination in Normal and Cancer Cells took place in Vancouver, BC, Canada. It was, indeed, special: a "special" location, a "special" audience and excellent talks that gave a detailed insight into the ubiquitination/proteasome-field. The following meeting highlights try to give a summary of some topics covered at the meeting, from basic research to successful applications of therapeutic agents, starting with cellular regulation, describing recently discovered structural features of enzymes involved in de-/ubiquitination, and, finally, presenting proteasome inhibition as a new approach in cancer chemotherapy.

Innovative cancer vaccine strategies based on the identification of tumour-associated antigens

The identification of tumour-associated antigens has opened up new approaches to cancer immunotherapy. While past research focused on CD8+ cytotoxic T-cell responses, accumulating evidence suggests that CD4+ T cells also play an important role in orchestrating the host immune response against cancer. In this article, we summarise new strategies for the identification of major histocompatibility complex (MHC) class II-associated tumour antigens and discuss the importance of engaging both CD4+ and CD8+ T cells in cancer immunotherapy. The cloning of MHC class I- or class II-associated antigens has made it possible to develop synthetic and recombinant cancer vaccines that express specific tumour antigens. There are three major types of synthetic and recombinant cancer vaccines: recombinant viral and bacterial vaccines; naked DNA or RNA vaccines; and recombinant protein and peptide vaccines. In this article, we also discuss a new generation of recombinant cancer vaccines, 'self-replicating' DNA and RNA vaccines. Studies on the mechanisms of 'self-replicating' nucleic acid vaccines revealed that the enhanced immunogenicity was not due to an enhanced antigen expression, suggesting that the quantitative difference may not be as important as the qualitative difference in antigen presentation. The presence of the RNA replicase in the 'self-replicating' nucleic acid vaccines mimics alphavirus infection, which triggers the innate antiviral pathways of the host cells. Studies on how viral and cellular modulators of the innate antiviral pathways affect vaccine function should provide molecular insights crucial to future vaccine design.

Recombinant Semliki Forest virus enhanced plasminogen activator inhibitor 1 expression and storage in the megakaryocytic cell line MEG-01

Platelet plasminogen activator inhibitor I (PAI-1), a trace alpha-granule protein, is a key physiological regulator of fibrinolysis. Because information on the packaging of PAI-1 into alpha-granules during megakaryocytopoiesis may reveal novel approaches for controlling hemostasis, this study investigated basal, plasmid-mediated, and alphavirus-mediated PAI-1 packaging into alpha-granules-like structures in the megakaryocytic cell line MEG-01. Differentiation of MEG-01 cells with phorbol myristate acetate (PMA) was observed to result in a four-fold increase in both secreted and cell-associated PAI-1 antigen over a four day period. Subcellular fractionation of PMA-treated MEG-01 cells on 45% self-forming Percoll gradients was employed to separate low density membrane and Golgi-rich fractions from a high density granule-containing region. A subsequent 30-60% pre-formed Percoll gradient was employed to remove contaminating lysosomes from the PAI-1/glycoprotein IIbIIIa-containing granules. Electron microscopy showed that these MEG-01 granules share a similar size distribution (350-600 nm) and morphology to platelet alpha-granules. PAI-1 (40 ng/mg protein) in isolated MEG-01 storage granules was approximately 10% of the levels present in isolated platelet alpha-granules. To elevate PAI-1 production/storage, two expression systems were investigated. Experiments with plasmids encoding PAI-1 and beta-galactosidase resulted in low transfection efficiency (0.001%). In contrast, Semliki Forest virus (SFV)-mediated gene transfer increased cellular PAI-1 by 31-fold (1,200 ng/10(6) cells at 10 MOI) in comparison to mock-infected cells. Pulse-chase experiments demonstrated that SFV/PAI-1 mediated gene expression could enhance PAI-1 storage 6-9-fold, reaching levels present within platelets. To document the ability of PAI-1 to be stored in a rapidly releasable form in MEG-01 cells, we isolated platelet-like particles from the media conditioned by the cells and examined secretagogue-induced release of PAI-1. Particles from SFV/PAI-1 infected cells display a 5-fold enhanced secretion of PAI-1 following treatment with ADP in comparison to particles incubated in the absence of secretagogue. These results suggest that SFV mediated gene expression in MEG-01 cells provides a useful framework for analyzing the production and storage of alpha-granule proteins.

Functions of alphavirus nonstructural proteins in RNA replication

Alphaviruses are enveloped positive-strand RNA viruses transmitted to vertebrate hosts by mosquitoes. Several alphaviruses are pathogenic to humans or domestic animals, causing serious central nervous system infections or milder infections, for example, arthritis, rash, and fever. The structure and replication of Semliki Forest virus (SFV) and Sindbis virus (SIN) have been studied extensively during the past 30 years. Alphaviruses have been important probes in cell biology to study the translation, glycosylation, folding, and transport of membrane glycoproteins, as well as endocytosis and membrane fusion mechanisms. A new organelle, the intermediate compartment, operating between the endoplasmic retieulum and the Golgi complex has been found by the aid of SFV. During the past 10 years, alphavirus replicons have been increasingly used as expression vectors for basic research, for the generation of vaccines, and for the production of recombinant proteins in industrial scale. The main approaches of laboratories in the recent years have been twofold. On one hand, they have discovered and characterized the enzymatic activities of the individual replicase proteins and on the other hand, they have studied the localization, membrane association, and other cell biological aspects of the replication complex.

Assessment of recombinants that arise from the use of a TMV-based transient expression vector

A potential use of virus-based transient expression vectors is the large-scale production of commercial specialty products, which would require the inoculation of many acres of plants with the viral vector. However, there are several concerns about the widespread use of virus-based vectors. Among these are the spread of the engineered virus to susceptible plants and the generation and persistence of recombinant viruses in the environment. Using a Tobacco mosaic virus (TMV)-based transient gene expression vector, 30B, which expresses the jellyfish green fluorescent protein (30B-GFP), we describe the predominant types of hybrid viruses that developed in plants. In general, the recombinants deleted the foreign gene and repeated sequences, retaining only those sequences required for optimal replication and movement. In pathogenicity studies and challenge experiments designed to make a comparative assessment of the competitiveness of the recombinants with the parent virus, the recombinants had reduced vigor and were less competitive and pathogenic than TMV, a virus which is already present in the areas where tobacco is grown.

Immunization of livestock with DNA vaccines: current studies and future prospects

Early studies using DNA immunization suggest the potential benefits of this form of immunization including: long-lived immunity, a broad spectrum of immune responses (both cell-mediated immunity and humoral responses) and the simultaneous induction of immunity to a variety of pathogens through the use of multivalent vaccines. Using marine and cow models, we studied methods to enhance and direct the immune response to polynucleotide vaccines. We demonstrated the ability to modulate the magnitude and direction of the immune response by co-administration of plasmid encoded cytokines and antigen. Also, we clearly demonstrated that the cellular components (cytosolic, membrane-anchored, or extracellular) to which the expressed antigen is delivered determines the types of immune responses induced. Since induction of immunity at mucosal surfaces (route of entry for many pathogens) is critical to prevent infection, various methods of delivering polynucleotide vaccines to animals including mucosal surfaces have been attempted and are described as future prospects for improving immune responses by DNA vaccination.

Improved Semliki Forest virus vectors for receptor research and gene therapy

We have modified Semliki Forest virus (SFV) vectors to broaden their application range. Here we describe a series of site-directed mutagenesis experiments on the SFV subgenomic 26S promoter to down-regulate the heterologous gene expression. Several mutants showed a dramatic effect on transgene expression levels in BHK cells. The luciferase activity was reduced to approximately 30%, 3%, and 1% compared to the wild type promoter. Similarly, a decrease in beta-galactosidase activity was observed in BHK cells and after injection into the striatum of male Wistar rats. Novel non-cytopathogenic and temperature-sensitive SFV vectors have recently been developed by introduction of point mutations in the viral nonstructural genes nsP2 and nsP4. These vectors do not show the typical shut down of host cell protein synthesis after SFV infections and therefore allow for a substantially prolonged survival of host cells. Both the mutant vectors demonstrating lower and more physiological expression levels and the non-cytopathogenic vectors should be valuable tools for various applications within receptor research. Furthermore, recent studies suggest that SFV vectors can be efficient gene delivery vehicles for gene therapy applications.

Nucleic acid vaccines: research tool or commercial reality

Polynucleotide immunization has captured the imagination of numerous researchers and commercial companies around the world as a novel approach for inducing immunity in animals. Clearly, the 'proof-of-principle' has been demonstrated both in rodents and various animal species. However, to date, no commercial veterinary vaccine has been developed, or to our knowledge, is in the licensing phase. The present review summarizes the types of pathogens and host species for which polynucleotide immunization has been tried. We have tried to identify possible barriers to commercialization of this technology and areas that need attention if this promising technology is ever to become a reality in the commercial arena.

A temperature-regulated replicon-based DNA expression system

We present a temperature-regulated, alphavirus replicon-based DNA expression system. The system is regulated by a viral temperature-sensitive RNA-dependent RNA replicase, creating a temperature-dependent RNA amplification loop. Because of this positive feedback, the system exhibits both low background and high inducibility. We observed 700-fold induction in transiently transfected cells, and over 104-fold induction in stably transfected cells. The high stringency of inducibility allowed the generation of stable cell lines expressing a highly toxic protein upon temperature shift. These data suggest that the present expression system could simplify bioprocess engineering strategies, especially in situations where the cloned protein has detrimental effects on host cell metabolism.

DNA and RNA-based vaccines: principles, progress and prospects

DNA vaccines were introduced less than a decade ago but have already been applied to a wide range of infectious and malignant diseases. Here we review the current understanding of the mechanisms underlying the activities of these new vaccines. We focus on recent strategies designed to enhance their function including the use of immunostimulatory (CpG) sequences, dendritic cells (DC), co-stimulatory molecules and cytokine- and chemokine-adjuvants. Although genetic vaccines have been significantly improved, they may not be sufficiently immunogenic for the therapeutic vaccination of patients with infectious diseases or cancer in clinical trials. One promising approach aimed at dramatically increasing the immunogenicity of genetic vaccines involves making them 'self-replicating'. This can be accomplished by using a gene encoding RNA replicase, a polyprotein derived from alphaviruses, such as Sindbis virus. Replicase-containing RNA vectors are significantly more immunogenic than conventional plasmids, immunizing mice at doses as low as 0.1 microg of nucleic acid injected once intramuscularly. Cells transfected with 'self-replicating' vectors briefly produce large amounts of antigen before undergoing apoptotic death. This death is a likely result of requisite double-stranded (ds) RNA intermediates, which also have been shown to super-activate DC. Thus, the enhanced immunogenicity of 'self-replicating' genetic vaccines may be a result of the production of pro-inflammatory dsRNA, which mimics an RNA-virus infection of host cells.

Production of recombinant subunit vaccines: protein immunogens, live delivery systems and nucleic acid vaccines

The first scientific attempts to control an infectious disease can be attributed to Edward Jenner, who, in 1796 inoculated an 8-year-old boy with cowpox (vaccinia), giving the boy protection against subsequent challenge with virulent smallpox. Thanks to the successful development of vaccines, many major diseases, such as diphtheria, poliomyelitis and measles, are nowadays kept under control, and in the case of smallpox, the dream of eradication has been fulfilled. Yet, there is a growing need for improvements of existing vaccines in terms of increased efficacy and improved safety, besides the development of completely new vaccines. Better technological possibilities, combined with increased knowledge in related fields, such as immunology and molecular biology, allow for new vaccination strategies. Besides the classical whole-cell vaccines, consisting of killed or attenuated pathogens, new vaccines based on the subunit principle, have been developed, e.g. the Hepatitis B surface protein vaccine and the Haemophilus influenzae type b vaccine. Recombinant techniques are now dominating in the strive for an ideal vaccine, being safe and cheap, heat-stable and easy to administer, preferably single-dose, and capable of inducing broad immune response with life-long memory both in adults and in infants. This review will describe different recombinant approaches used in the development of novel subunit vaccines, including design and production of protein immunogens, the development of live delivery systems and the state-of-the-art for nucleic acids vaccines.

Alphaviruses as tools in neurobiology and gene therapy

The broad host range and superior infectivity of alphaviruses have encouraged the development of efficient expression vectors for Semliki Forest virus (SFV) and Sindbis virus (SIN). The generation of high-titer recombinant alphavirus stocks has allowed high-level expression of a multitude of nuclear, cytoplasmic, membrane-associated and secreted proteins in a variety of different cell lines and primary cell cultures. Despite the viral cytopathogenic effects, functional assays on recombinant proteins are possible for a time-period of at least 24 hours post-infection. The high percentage (80-95%) of primary neurons infected with SFV has allowed localization and functional studies of recombinant proteins in these primary cell cultures. Through multiple infection studies the interaction of receptor and G protein subunits has become feasible. Establishment of efficient scale-up procedures has allowed production of large quantities of recombinant protein. Potential gene therapy applications of alphaviruses could be demonstrated by injection of recombinant SIN particles expressing beta-galactosidase into mouse brain. Tissue/cell specific infection has been achieved by introduction of an IgG-binding domain of protein A domain into one of the spike proteins of SIN. This enabled efficient targeting of infection to human lymphoblastoid cells.

G protein-coupled receptors: functional and mechanistic insights through altered gene expression

G protein-coupled receptors (GPCRs) comprise a large and diverse family of molecules that play essential roles in signal transduction. In addition to a constantly expanding pharmacological repertoire, recent advances in the ability to manipulate GPCR expression in vivo have provided another valuable approach in the study of GPCR function and mechanism of action. Current technologies now allow investigators to manipulate GPCR expression in a variety of ways. Graded reductions in GPCR expression can be achieved through antisense strategies or total gene ablation or replacement can be achieved through gene targeting strategies, and exogenous expression of wild-type or mutant GPCR isoforms can be accomplished with transgenic technologies. Both the techniques used to achieve these specific alterations and the consequences of altered expression patterns are reviewed here and discussed in the context of GPCR function and mechanism of action.

Outcome of immunization of cynomolgus monkeys with recombinant Semliki Forest virus encoding human immunodeficiency virus type 1 envelope protein and challenge with a high dose of SHIV-4 virus

Infection of macaques with chimeric simian-human immunodeficiency viruses (SHIVs) allows evaluation of HIV-1 envelope vaccines. SHIV-4 is based on SIVmac239 but carries the env, tat, and rev genes of HIV-1IIIB. In this study we used Semliki Forest virus (SFV) RNA vectors to express the envelope protein gp160 of HIV-1IIIB in cynomolgus macaques. Monkeys were immunized four times with recombinant suicide SFV. Whereas two of four monkeys showed T cell-proliferative responses, only one monkey had demonstrable levels of antibodies to HIV-1 gp41 and gp120 as shown by enzyme-linked immunosorbent assay (ELISA) and Western blot. The vaccinated monkeys and four control animals were challenged with 10,000 MID100 (100% minimum infectious doses) of cell-free monkey cell-grown SHIV-4 virus. As demonstrated by virus isolation, all macaques became infected after challenge. All vaccinated monkeys showed an HIV-1-specific anamnestic T cell-proliferative response. Three of four vaccines had developed HIV-1-Env-specific antibodies 2 weeks after challenge whereas none of the four controls showed any detectable immune response at this time point. Furthermore, three of four vaccinated monkeys had no demonstrable viral antigenemia and low viral load as opposed to one of the four naive control animals.

Alphaviruses as expression vectors

Alphavirus vectors have been used for efficient high-level expression of a variety of topologically different proteins, allowing studies of protein transport, localization and functional activity in a broad range of host cells. Complex transmembrane proteins have been produced in large quantities through the establishment of scale-up technology. Alphavirus vectors have also shown promising potential in vaccine production and preliminary gene therapy applications.

Alphavirus expression vectors and their use as recombinant vaccines: a minireview

Alphavirus vectors have become widely used in basic research to study the structure and function of proteins and for protein production purposes. Development of a variety of vectors has made it possible to deliver foreign sequences as naked RNA or DNA, or as suicide virus particles produced using helper vector strategies. Preliminary reports also suggest that these vectors may be useful for in vivo applications where transient, high-level protein expression is desired, such as recombinant vaccines. The initial studies have already shown that alphavirus vaccines can induce strong humoral and cellular immune responses with good immunological memory and protective effects.

Alphavirus-based expression vectors: strategies and applications

Alphaviruses are positive-strand RNA viruses that can mediate efficient cytoplasmic gene expression in insect and vertebrate cells. Through recombinant DNA technology, the alphavirus RNA replication machinery has been engineered for high-level expression of heterologous RNAs and proteins. Amplification of replication-competent alpha-virus RNAs (replicons) can be initiated by RNA or DNA transfection and a variety of packaging systems have been developed for producing high titers of infectious viral particles. Although normally cytocidal for vertebrate cells, variants with adaptive mutations allowing noncytopathic replication have been isolated from persistently infected cultures or selected using a dominant selectable marker. Such mutations have been mapped and used to create new alphavirus vectors for noncytopathic gene expression in mammalian cells. These vectors allow long-term expression at moderate levels and complement previous vectors designed for short-term high-level expression. Besides their use for a growing number of basic research applications, recombinant alphavirus RNA replicons may also facilitate genetic vaccination and transient gene therapy.

Vectors for cancer gene therapy

Many viral and non-viral vector systems have now been developed for gene therapy applications. In this article, the pros and cons of these vector systems are discussed in relation to the different cancer gene therapy strategies. The protocols used in cancer gene therapy can be broadly divided into six categories including gene transfer to explanted cells for use as cell-based cancer vaccines; gene transfer to a small number of tumour cells in situ to achieve a vaccine effect; gene transfer to vascular endothelial cells (VECs) lining the blood vessels of the tumour to interfere with tumour angiogenesis; gene transfer to T lymphocytes to enhance their antitumour effector capability; gene transfer to haemopoietic stem cells (HSCs) to enhance their resistance to cytotoxic drugs and gene transfer to a large number of tumour cells in situ to achieve nonimmune tumour reduction with or without bystander effect. Each of the six strategies makes unique demands on the vector system and these are discussed with reference to currently available vectors. Aspects of vector biology that are in need of further development are discussed in some detail. The final section points to the potential use of replicating viruses as delivery vehicles for efficient in vivo gene transfer to disseminated cancers.