Therapeutic and prophylactic applications of alphavirus vectors

Advanced nanoscale delivery systems for mRNA-based vaccines

The effectiveness of messenger RNA (mRNA) vaccines, especially those designed for COVID-19, relies heavily on sophisticated delivery systems that ensure efficient delivery of mRNA to target cells. A variety of nanoscale vaccine delivery systems (VDSs) have been explored for this purpose, including lipid nanoparticles (LNPs), liposomes, and polymeric nanoparticles made from biocompatible polymers such as poly(lactic-co-glycolic acid), as well as viral vectors and lipid-polymer hybrid complexes. Among these, LNPs are particularly notable for their efficiency in encapsulating and protecting mRNA. These nanoscale VDSs can be engineered to enhance stability and facilitate uptake by cells. The choice of delivery system depends on factors like the specific mRNA vaccine, target cell types, stability requirements, and desired immune response. In this review, we shed light on recent advances in delivery mechanisms for self-amplifying RNA (saRNA) vaccines, emphasizing groundbreaking studies on nanoscale delivery systems aimed at improving the efficacy and safety of mRNA/saRNA vaccines.

New Generation Self-Replicating RNA Vaccines Derived from Pestivirus Genome

While mRNA vaccines have shown their worth, they have the same failing as inactivated vaccines, namely they have limited half-life, are non-replicating, and therefore limited to the size of the vaccine payload for the amount of material translated. New advances averting these problems are combining replicon RNA (RepRNA) technology with nanotechnology. RepRNA are large self-replicating RNA molecules (typically 12-15 kb) derived from viral genomes defective in at least one essential structural protein gene. They provide sustained antigen production, effectively increasing vaccine antigen payloads over time, without the risk of producing infectious progeny. The major limitations with RepRNA are RNase-sensitivity and inefficient uptake by dendritic cells (DCs), which need to be overcome for efficacious RNA-based vaccine design. We employed biodegradable delivery vehicles to protect the RepRNA and promote DC delivery. Condensing RepRNA with polyethylenimine (PEI) and encapsulating RepRNA into novel Coatsome-replicon vehicles are two approaches that have proven effective for delivery to DCs and induction of immune responses in vivo.

The mRNA Vaccine Revolution: COVID-19 Has Launched the Future of Vaccinology

During the COVID-19 pandemic, mRNA (mRNA) vaccines emerged as leading vaccine candidates in a record time. Nonreplicating mRNA (NRM) and self-amplifying mRNA (SAM) technologies have been developed into high-performing and clinically viable vaccines against a range of infectious agents, notably SARS-CoV-2. mRNA vaccines demonstrate efficient in vivo delivery, long-lasting stability, and nonexistent risk of infection. The stability and translational efficiency of in vitro transcription (IVT)-mRNA can be further increased by modulating its structural elements. In this review, we present a comprehensive overview of the recent advances, key applications, and future challenges in the field of mRNA-based vaccinology.

Advances in saRNA Vaccine Research against Emerging/Re-Emerging Viruses

Although conventional vaccine approaches have proven to be successful in preventing infectious diseases in past decades, for vaccine development against emerging/re-emerging viruses, one of the main challenges is rapid response in terms of design and manufacture. mRNA vaccines can be designed and produced within days, representing a powerful approach for developing vaccines. Furthermore, mRNA vaccines can be scaled up and may not have the risk of integration. mRNA vaccines are roughly divided into non-replicating mRNA vaccines and self-amplifying RNA (saRNA) vaccines. In this review, we provide an overview of saRNA vaccines, and discuss future directions and challenges in advancing this promising vaccine platform to combat emerging/re-emerging viruses.

Construction of a Chikungunya Virus, Replicon, and Helper Plasmids for Transfection of Mammalian Cells

The genome of Alphaviruses can be modified to produce self-replicating RNAs and virus-like particles, which are useful virological tools. In this work, we generated three plasmids for the transfection of mammalian cells: an infectious clone of Chikungunya virus (CHIKV), one that codes for the structural proteins (helper plasmid), and another one that codes nonstructural proteins (replicon plasmid). All of these plasmids contain a reporter gene (mKate2). The reporter gene in the replicon RNA and the infectious clone are synthesized from subgenomic RNA. Co-transfection with the helper and replicon plasmids has biotechnological/biomedical applications because they allow for the delivery of self-replicating RNA for the transient expression of one or more genes to the target cells.

Recent Advances in the Molecular Design and Delivery Technology of mRNA for Vaccination Against Infectious Diseases

Vaccines can prevent many millions of illnesses against infectious diseases and save numerous lives every year. However, traditional vaccines such as inactivated viral and live attenuated vaccines cannot adapt to emerging pandemics due to their time-consuming development. With the global outbreak of the COVID-19 epidemic, the virus continues to evolve and mutate, producing mutants with enhanced transmissibility and virulence; the rapid development of vaccines against such emerging global pandemics becomes more and more critical. In recent years, mRNA vaccines have been of significant interest in combating emerging infectious diseases due to their rapid development and large-scale production advantages. However, their development still suffers from many hurdles such as their safety, cellular delivery, uptake, and response to their manufacturing, logistics, and storage. More efforts are still required to optimize the molecular designs of mRNA molecules with increased protein expression and enhanced structural stability. In addition, a variety of delivery systems are also needed to achieve effective delivery of vaccines. In this review, we highlight the advances in mRNA vaccines against various infectious diseases and discuss the molecular design principles and delivery systems of associated mRNA vaccines. The current state of the clinical application of mRNA vaccine pipelines against various infectious diseases and the challenge, safety, and protective effect of associated vaccines are also discussed.

Packaging, Purification, and Titration of Replication-Deficient Semliki Forest Virus-Derived Particles as a Self-Amplifying mRNA Vaccine Vector

Background

Self-amplifying mRNA is the next-generation vaccine platform with the potential advantages in efficacy and speed of development against infectious diseases and cancer. The main aim was to present optimized and rapid methods for Semliki Forest virus (SFV)-PD self-amplifying mRNA (SAM) preparation, its packaging, and titer determination. These protocols are provided for producing and harvesting the high yields of virus replicon particle (VRP)-packaged SAM for vaccine studies.

Methods

pSFV-PD-EGFP plasmid was linearized and subjected to in vitro transcription. Different concentrations of SFV-PD SAM were first transfected into human embryonic kidney 293 cells (HEK-293) and baby hamster kidney cell line 21 (BHK-21) cell lines, and EGFP expression at different time points was evaluated by fluorescent microscopy. Replicon particle packaging was achieved by co-transfection of SFV-PD SAM and pSFV-Helper2 RNA into BHK-21 cells. The VRPs were concentrated using ultrafiltration with 100 kDa cut-off. The titers of replicon particles were determined by reverse transcription quantitative real-time PCR (RT-qPCR).

Results

In vitro transcribed SAM encoding EGFP was successfully transfected and expressed in HEK-293 and BHK-21 cell lines. Higher levels of EGFP expression was observed in BHK-21 compared to HEK-293 cells showing more stable protein overexpression and VRP packaging. Using ultrafiltration, the high yields of purified SFV-PD-EGFP particles were rapidly obtained with only minor loss of replicon particles. Accurate and rapid titer determination of replication-deficient particles was achieved by RT-qPCR.

Conclusion

Using optimized methods for SAM transfection, VRP packaging, and concentration, high yields of SFV-PD VRPs could be produced and purified. The RT-qPCR demonstrated to be an accurate and rapid method for titer determination of replication deficient VRPs.

Review: Development of SARS-CoV-2 immuno-enhanced COVID-19 vaccines with nano-platform

Vaccination is the most effective way to prevent coronavirus disease 2019 (COVID-19). Vaccine development approaches consist of viral vector vaccines, DNA vaccine, RNA vaccine, live attenuated virus, and recombinant proteins, which elicit a specific immune response. The use of nanoparticles displaying antigen is one of the alternative approaches to conventional vaccines. This is due to the fact that nano-based vaccines are stable, able to target, form images, and offer an opportunity to enhance the immune responses. The diameters of ultrafine nanoparticles are in the range of 1-100 nm. The application of nanotechnology on vaccine design provides precise fabrication of nanomaterials with desirable properties and ability to eliminate undesirable features. To be successful, nanomaterials must be uptaken into the cell, especially into the target and able to modulate cellular functions at the subcellular levels. The advantages of nano-based vaccines are the ability to protect a cargo such as RNA, DNA, protein, or synthesis substance and have enhanced stability in a broad range of pH, ambient temperatures, and humidity for long-term storage. Moreover, nano-based vaccines can be engineered to overcome biological barriers such as nonspecific distribution in order to elicit functions in antigen presenting cells. In this review, we will summarize on the developing COVID-19 vaccine strategies and how the nanotechnology can enhance antigen presentation and strong immunogenicity using advanced technology in nanocarrier to deliver antigens. The discussion about their safe, effective, and affordable vaccines to immunize against COVID-19 will be highlighted.

A Productive Expression Platform Derived from Host-Restricted Eilat Virus: Its Extensive Validation and Novel Strategy

Most alphaviruses are transmitted by mosquitoes and infect a wide range of insects and vertebrates. However, Eilat virus (EILV) is defective for infecting vertebrate cells at multiple levels of the viral life cycle. This host-restriction property renders EILV an attractive expression platform since it is not infectious for vertebrates and therefore provides a highly advantageous safety profile. Here, we investigated the feasibility of versatile EILV-based expression vectors. By replacing the structural genes of EILV with those of other alphaviruses, we generated seven different chimeras. These chimeras were readily rescued in the original mosquito cells and were able to reach high titers, suggesting that EILV is capable of packaging the structural proteins of different lineages. We also explored the ability of EILV to express authentic antigens via double subgenomic (SG) RNA vectors. Four foreign genetic materials of varied length were introduced into the EILV genome, and the expressed heterologous genetic materials were readily detected in the infected cells. By inserting an additional SG promoter into the chimera genome containing the structural genes of Chikungunya virus (CHIKV), we developed a bivalent vaccine candidate against CHIKV and Zika virus. These data demonstrate the outstanding compatibility of the EILV genome. The produced recombinants can be applied to vaccine and diagnostic tool development, but more investigations are required.

Perspectives on RNA Vaccine Candidates for COVID-19

With the current outbreak caused by SARS-CoV-2, vaccination is acclaimed as a public health care priority. Rapid genetic sequencing of SARS-CoV-2 has triggered the scientific community to search for effective vaccines. Collaborative approaches from research institutes and biotech companies have acknowledged the use of viral proteins as potential vaccine candidates against COVID-19. Nucleic acid (DNA or RNA) vaccines are considered the next generation vaccines as they can be rapidly designed to encode any desirable viral sequence including the highly conserved antigen sequences. RNA vaccines being less prone to host genome integration (cons of DNA vaccines) and anti-vector immunity (a compromising factor of viral vectors) offer great potential as front-runners for universal COVID-19 vaccine. The proof of concept for RNA-based vaccines has already been proven in humans, and the prospects for commercialization are very encouraging as well. With the emergence of COVID-19, mRNA-1273, an mRNA vaccine developed by Moderna, Inc. was the first to enter human trials, with the first volunteer receiving the dose within 10 weeks after SARS-CoV-2 genetic sequencing. The recent interest in mRNA vaccines has been fueled by the state of the art technologies that enhance mRNA stability and improve vaccine delivery. Interestingly, as per the "Draft landscape of COVID-19 candidate vaccines" published by the World Health Organization (WHO) on December 29, 2020, seven potential RNA based COVID-19 vaccines are in different stages of clinical trials; of them, two candidates already received emergency use authorization, and another 22 potential candidates are undergoing pre-clinical investigations. This review will shed light on the rationality of RNA as a platform for vaccine development against COVID-19, highlighting the possible pros and cons, lessons learned from the past, and the future prospects.

Nanomedicine-Based Approaches for mRNA Delivery

Messenger RNA (mRNA) has immense potential for developing a wide range of therapies, including immunotherapy and protein replacement. As mRNA presents no risk of integration into the host genome and does not require nuclear entry for transfection, which allows protein production even in nondividing cells, mRNA-based approaches can be envisioned as safe and practical therapeutic strategies. Nevertheless, mRNA presents unfavorable characteristics, such as large size, immunogenicity, limited cellular uptake, and sensitivity to enzymatic degradation, which hinder its use as a therapeutic agent. While mRNA stability and immunogenicity have been ameliorated by direct modifications on the mRNA structure, further improvements in mRNA delivery are still needed for promoting its activity in biological settings. In this regard, nanomedicine has shown the ability for spatiotemporally controlling the function of a myriad of bioactive agents in vivo. Direct engineering of nanomedicine structures for loading, protecting, and releasing mRNA and navigating in biological environments can then be applied for promoting mRNA translation toward the development of effective treatments. Here, we review recent approaches aimed at enhancing mRNA function and its delivery through nanomedicines, with particular emphasis on their applications and eventual clinical translation.

Contaminating viral sequences in high-throughput sequencing viromics: a linkage study of 700 sequencing libraries

OBJECTIVES

Sample preparation for high-throughput sequencing (HTS) includes treatment with various laboratory components, potentially carrying viral nucleic acids, the extent of which has not been thoroughly investigated. Our aim was to systematically examine a diverse repertoire of laboratory components used to prepare samples for HTS in order to identify contaminating viral sequences.

METHODS

A total of 322 samples of mainly human origin were analysed using eight protocols, applying a wide variety of laboratory components. Several samples (60% of human specimens) were processed using different protocols. In total, 712 sequencing libraries were investigated for viral sequence contamination.

RESULTS

Among sequences showing similarity to viruses, 493 were significantly associated with the use of laboratory components. Each of these viral sequences had sporadic appearance, only being identified in a subset of the samples treated with the linked laboratory component, and some were not identified in the non-template control samples. Remarkably, more than 65% of all viral sequences identified were within viral clusters linked to the use of laboratory components.

CONCLUSIONS

We show that high prevalence of contaminating viral sequences can be expected in HTS-based virome data and provide an extensive list of novel contaminating viral sequences that can be used for evaluation of viral findings in future virome and metagenome studies. Moreover, we show that detection can be problematic due to stochastic appearance and limited non-template controls. Although the exact origin of these viral sequences requires further research, our results support laboratory-component-linked viral sequence contamination of both biological and synthetic origin.

mRNA as a Transformative Technology for Vaccine Development to Control Infectious Diseases

In the last two decades, there has been growing interest in mRNA-based technology for the development of prophylactic vaccines against infectious diseases. Technological advancements in RNA biology, chemistry, stability, and delivery systems have accelerated the development of fully synthetic mRNA vaccines. Potent, long-lasting, and safe immune responses observed in animal models, as well as encouraging data from early human clinical trials, make mRNA-based vaccination an attractive alternative to conventional vaccine approaches. Thanks to these data, together with the potential for generic, low-cost manufacturing processes and the completely synthetic nature, the prospects for mRNA vaccines are very promising. In addition, mRNA vaccines have the potential to streamline vaccine discovery and development, and facilitate a rapid response to emerging infectious diseases. In this review, we overview the unique attributes of mRNA vaccine approaches, review the data of mRNA vaccines against infectious diseases, discuss the current challenges, and highlight perspectives about the future of this promising technology.

Self-Amplifying Replicon RNA Delivery to Dendritic Cells by Cationic Lipids

Advances in RNA technology during the past two decades have led to the construction of replication-competent RNA, termed replicons, RepRNA, or self-amplifying mRNA, with high potential for vaccine applications. Cytosolic delivery is essential for their translation and self-replication, without infectious progeny generation, providing high levels of antigen expression for inducing humoral and cellular immunity. Synthetic nanoparticle-based delivery vehicles can both protect the RNA molecules and facilitate targeting of dendritic cells—critical for immune defense development. Several cationic lipids were assessed, with RepRNA generated from classical swine fever virus encoding nucleoprotein genes of influenza A virus. The non-cytopathogenic nature of the RNA allowed targeting to dendritic cells without destroying the cells—important for prolonged antigen production and presentation. Certain lipids were more effective at delivery and at promoting translation of RepRNA than others. Selection of particular lipids provided delivery to dendritic cells that resulted in translation, demonstrating that delivery efficiency could not guarantee translation. The observed translation in vitro was reproduced in vivo by inducing immune responses against the encoded influenza virus antigens. Cationic lipid-mediated delivery shows potential for promoting RepRNA vaccine delivery to dendritic cells, particularly when combined with additional delivery elements.

Self-Replicating RNA Vaccine Delivery to Dendritic Cells

Most current vaccines are either inactivated pathogen-derived or protein/peptide-based, although attenuated and vector vaccines have also been developed. The former induce at best moderate protection, even as multimeric antigen, due to limitations in antigen loads and therefore capacity for inducing robust immune defense. While attenuated and vector vaccines offer advantages through their replicative nature, drawbacks and risks remain with potential reversion to virulence and interference from preexisting immunity. New advances averting these problems are combining self-amplifying replicon RNA (RepRNA) technology with nanotechnology. RepRNA are large self-replicating RNA molecules (12-15 kb) derived from viral genomes defective in at least one structural protein gene. They provide sustained antigen production, effectively increasing vaccine antigen payloads over time, without the risk of producing infectious progeny. The major limitation with RepRNA is RNase-sensitivity and inefficient uptake by dendritic cells (DCs)-absolute requirements for efficacious vaccine design. We employed biodegradable delivery vehicles to protect the RepRNA and promote DC delivery. Encapsulating RepRNA into chitosan nanoparticles, as well as condensing RepRNA with polyethylenimine (PEI), cationic lipids, or chitosans, has proven effective for delivery to DCs and induction of immune responses in vivo.

Enhanced Delivery of DNA or RNA Vaccines by Electroporation

Nucleic acid vaccines are a next-generation branch of vaccines which offer major benefits over their conventional protein, bacteria, or viral-based counterparts. However, to be effective in large mammals and humans, an enhancing delivery technology is required. Electroporation is a physical technique which results in improved delivery of large molecules through the cell membrane. In the case of plasmid DNA and mRNA, electroporation enhances both the uptake and expression of the delivered nucleic acids. The muscle is an attractive tissue for nucleic acid vaccination in a clinical setting due to the accessibility and abundance of the target tissue. Historical clinical studies of electroporation in the muscle have demonstrated the procedure to be generally well tolerated in patients. Previous studies have determined that optimized electroporation parameters (such as electrical field intensity, pulse length, pulse width and drug product formulation) majorly impact the efficiency of nucleic acid delivery. We provide an overview of DNA/RNA vaccination in the muscle of mice. Our results suggest that the technique is safe and effective and is highly applicable to a research setting as well as scalable to larger animals and humans.

Influenza Vaccine Research funded by the European Commission FP7-Health-2013-Innovation-1 project

Due to influenza viruses continuously displaying antigenic variation, current seasonal influenza vaccines must be updated annually to include the latest predicted strains. Despite all the efforts put into vaccine strain selection, vaccine production, testing, and administration, the protective efficacy of seasonal influenza vaccines is greatly reduced when predicted vaccine strains antigenically mismatch with the actual circulating strains. Moreover, preparing for a pandemic outbreak is a challenge, because it is unpredictable which strain will cause the next pandemic. The European Commission has funded five consortia on influenza vaccine development under the Seventh Framework Programme for Research and Technological Development (FP7) in 2013. The call of the EU aimed at developing broadly protective influenza vaccines. Here we review the scientific strategies used by the different consortia with respect to antigen selection, vaccine delivery system, and formulation. The issues related to the development of novel influenza vaccines are discussed.

Molecular determinants of alphavirus neuropathogenesis in mice

Alphaviruses are enveloped viruses with a positive-stranded RNA genome, of the family Togaviridae. In mammals and birds they are mosquito-transmitted and are of veterinary and medical importance. They cause primarily two types of disease: encephalitis and polyarthritis. Here we review attempts to understand the molecular basis of encephalitis and virulence for the central nervous system (CNS) in mouse models. Sindbis virus (SINV) was the first virus to be studied in this way. Other viruses analysed are Semliki Forest virus (SFV), Venezuelan equine encephalitis virus, Eastern equine encephalitis virus and Western equine encephalitis virus. Neurovirulence was found to be associated with damage to neurons in the CNS. It mapped mainly to the E2 region of the genome, and to the nsP3 gene. Also, avirulent natural isolates of both SINV and SFV have been found to have more rapid cleavage of nonstructural proteins due to mutations in the nsP1-nsP2 cleavage site. Immune-mediated demyelination for avirulent SFV has been shown to be associated with infection of oligodendrocytes. For Chikungunya virus, an emerging alphavirus that uncommonly causes encephalitis, analysis of the molecular basis of CNS pathogenicity is beginning. Experiments on SINV and SFV have indicated that virulence may be related to the resistance of virulent virus to interferon action. Although the E2 protein may be involved in tropism for neurons and passage across the blood-brain barrier, the role of the nsP3 protein during infection of neurons is unknown. More information in these areas may help to further explain the neurovirulence of alphaviruses.

Polyethylenimine-based polyplex delivery of self-replicating RNA vaccines

Self-amplifying replicon RNA (RepRNA) are large molecules (12-14 kb); their self-replication amplifies mRNA template numbers, affording several rounds of antigen production, effectively increasing vaccine antigen payloads. Their sensitivity to RNase-sensitivity and inefficient uptake by dendritic cells (DCs) - absolute requirements for vaccine design - were tackled by condensing RepRNA into synthetic, nanoparticulate, polyethylenimine (PEI)-polyplex delivery vehicles. Polyplex-delivery formulations for small RNA molecules cannot be transferred to RepRNA due to its greater size and complexity; the N:P charge ratio and impact of RepRNA folding would influence polyplex condensation, post-delivery decompaction and the cytosolic release essential for RepRNA translation. Polyplex-formulations proved successful for delivery of RepRNA encoding influenza virus hemagglutinin and nucleocapsid to DCs. Cytosolic translocation was facilitated, leading to RepRNA translation. This efficacy was confirmed in vivo, inducing both humoral and cellular immune responses. Accordingly, this paper describes the first PEI-polyplexes providing efficient delivery of the complex and large, self-amplifying RepRNA vaccines.

FROM THE CLINICAL EDITOR

The use of self-amplifying replicon RNA (RepRNA) to increase vaccine antigen payloads can potentially be useful in effective vaccine design. Nonetheless, its use is limited by the degradation during the uptake process. Here, the authors attempted to solve this problem by packaging RepRNA using polyethylenimine (PEI)-polyplex delivery vehicles. The efficacy was confirmed in vivo by the appropriate humoral and cellular immune responses. This novel delivery method may prove to be very useful for future vaccine design.

DNA-Launched Alphavirus Replicons Encoding a Fusion of Mycobacterial Antigens Acr and Ag85B Are Immunogenic and Protective in a Murine Model of TB Infection

There is an urgent need for effective prophylactic measures against Mycobacterium tuberculosis (Mtb) infection, particularly given the highly variable efficacy of Bacille Calmette-Guerin (BCG), the only licensed vaccine against tuberculosis (TB). Most studies indicate that cell-mediated immune responses involving both CD4+ and CD8+ T cells are necessary for effective immunity against Mtb. Genetic vaccination induces humoral and cellular immune responses, including CD4+ and CD8+ T-cell responses, against a variety of bacterial, viral, parasitic and tumor antigens, and this strategy may therefore hold promise for the development of more effective TB vaccines. Novel formulations and delivery strategies to improve the immunogenicity of DNA-based vaccines have recently been evaluated, and have shown varying degrees of success. In the present study, we evaluated DNA-launched Venezuelan equine encephalitis replicons (Vrep) encoding a novel fusion of the mycobacterial antigens α-crystallin (Acr) and antigen 85B (Ag85B), termed Vrep-Acr/Ag85B, for their immunogenicity and protective efficacy in a murine model of pulmonary TB. Vrep-Acr/Ag85B generated antigen-specific CD4+ and CD8+ T cell responses that persisted for at least 10 wk post-immunization. Interestingly, parenterally administered Vrep-Acr/Ag85B also induced T cell responses in the lung tissues, the primary site of infection, and inhibited bacterial growth in both the lungs and spleens following aerosol challenge with Mtb. DNA-launched Vrep may, therefore, represent an effective approach to the development of gene-based vaccines against TB, particularly as components of heterologous prime-boost strategies or as BCG boosters.

CD8 T-cell priming upon mRNA vaccination is restricted to bone-marrow-derived antigen-presenting cells and may involve antigen transfer from myocytes

Self-amplifying mRNAs (SAM(®) ) are a novel class of nucleic acid vaccines, delivered by a non-viral delivery system. They are effective at eliciting potent and protective immune responses and are being developed as a platform technology with potential to be used for a broad range of targets. However, their mechanism of action has not been fully elucidated. To date, no evidence of in vivo transduction of professional antigen-presenting cells (APCs) by SAM vector has been reported, while the antigen expression has been shown to occur mostly in the muscle fibres. Here we show that bone-marrow-derived APCs rather than muscle cells are responsible for induction of MHC class-I restricted CD8 T cells in vivo, but direct transfection of APCs by SAM vectors is not required. Based on all our in vivo and in vitro data we propose that upon SAM vaccination the antigen is expressed within muscle cells and then transferred to APCs, suggesting cross-priming as the prevalent mechanism for priming the CD8 T-cell response by SAM vaccines.

Transgenic DNA modules with pre-programmed self-destruction: Universal molecular devices to escape 'genetic litter' in gene and cell therapy

Gene delivery to human somatic cells is a well-established therapeutic strategy to treat a variety of diseases. In addition, gene transfer to human cells is required to generate human induced pluripotent cells and also to eliminate tumorigenic undifferentiated cells in many types of stem-cell derived transplantation material. The expression of transgenes in these medical technologies is often required only in some of the recipient cells and only in specific limited time-windows, with inappropriately located or untimely expressed transgenes presenting a risk of undesired collateral effects. Unfortunately, current gene transfer procedures commonly result in a number of cells in the patient's body containing fragments of transferred genetic material which are either not therapeutically necessary at all, are no longer necessary or are necessary but in some other cells. Such transgenic material in the patient, created as a by-product of the chosen therapeutic procedure, constitutes, in fact, 'genetic litter', that is, persisting potentially-hazardous foreign genetic material which is neither required therapeutically nor explicitly chosen by an informed and free-willing person as an artificial body element. Wider use and more frequent administration of gene and cell therapy in the future are likely to give greater prominence to the issue of misdelivered genetic medicines and of their unwanted remainders accumulating in human bodies. Thus, novel DNA templates, which, on the one hand, are capable of providing transgene expression over broad time-windows, and, on the other hand, do not leave unwanted permanent 'genetic traces', are required. I propose that the problem of 'genetic litter' in patients' bodies can be addressed through the employment of a new type of gene vectors delivering DNA-based transgenic modules with pre-programmed self-destruction. Such vectors could deliver therapeutic DNA cargo and then execute self-liquidation through pre-scheduled activation of co-delivered genome editing tools, such as CRISPR/Cas9 nucleases, specific for the DNA to be eliminated. In this model, all unnecessary transgenic DNA is edited away precisely at a desired time point. Activity of the gene correction apparatus for the specific and effective destruction of transgenic DNA could be turned on by well-timed external signals or could be triggered through intracellular sensors of particular epigenetic signatures. It is expected that the employment of the proposed DNA-based gene vectors equipped with a transgene self-destruct mechanism can extend the safe and ethical application of gene and cell therapy to a broader range of curative and lifestyle-choice medical treatments, e.g., full body prophylactic gene therapy of cancer.

Mutations conferring a noncytotoxic phenotype on chikungunya virus replicons compromise enzymatic properties of nonstructural protein 2

Chikungunya virus (CHIKV) (genus Alphavirus) has a positive-sense RNA genome. CHIKV nonstructural protein 2 (nsP2) proteolytically processes the viral nonstructural polyprotein, possesses nucleoside triphosphatase (NTPase), RNA triphosphatase, and RNA helicase activities, and induces cytopathic effects in vertebrate cells. Although alphaviral nsP2 mutations can result in a noncytotoxic phenotype, the effects of such mutations on nsP2 enzymatic activities are not well understood. In this study, we introduced a P718G (PG) mutation and selected for additional mutations in CHIKV nsP2 that resulted in a CHIKV replicon with a noncytotoxic phenotype in BHK-21 cells. Combinations of PG and either an E117K (EK) substitution or a GEEGS sequence insertion after residue T647 (5A) markedly reduced RNA synthesis; however, neither PG nor 5A prevented nsP2 nuclear translocation. Introducing PG into recombinant nsP2 inhibited proteolytic cleavage of nsP1/nsP2 and nsP3/nsP4 sites, reduced GTPase and RNA helicase activities, and abolished RNA stimulation of GTPase activity. 5A and EK modulated the effects of PG. However, only the RNA helicase activity of nsP2 was reduced by both of these mutations, suggesting that defects in this activity may be linked to a noncytotoxic phenotype. These results increase our understanding of the molecular basis for the cytotoxicity that accompanies alphaviral replication. Furthermore, adaptation of the CHIKV replicon containing both 5A and PG allowed the selection of a CHIKV replicon with adaptive mutations in nsP1 and nsP3 that enable persistence in human cell line. Such cell lines represent valuable experimental systems for discovering host factors and for screening inhibitors of CHIKV replication at lower biosafety levels.

IMPORTANCE

CHIKV is a medically important pathogen that causes febrile illness and can cause chronic arthritis. No approved vaccines or antivirals are available for CHIKV. The attenuation of CHIKV is critical to the establishment of experimental systems that can be used to conduct virus replication studies at a lower biosafety level. We applied a functional selection approach to develop, for the first time, a noncytotoxic CHIKV replicon capable of persisting in human cell lines. We anticipate that this safe and efficient research tool will be valuable for screening CHIKV replication inhibitors and for identifying and analyzing host factors involved in viral replication. We also analyzed, from virological and protein biochemistry perspectives, the functional defects caused by mutations conferring noncytotoxic phenotypes; we found that all known enzymatic activities of CHIKV nsP2, as well as its RNA-binding capability, were compromised by these mutations, which led to a reduced capacity for replication.

Self-Amplifying Replicon RNA Vaccine Delivery to Dendritic Cells by Synthetic Nanoparticles

Dendritic cells (DC) play essential roles determining efficacy of vaccine delivery with respect to immune defence development and regulation. This renders DCs important targets for vaccine delivery, particularly RNA vaccines. While delivery of interfering RNA oligonucleotides to the appropriate intracellular sites for RNA-interference has proven successful, the methodologies are identical for RNA vaccines, which require delivery to RNA translation sites. Delivery of mRNA has benefitted from application of cationic entities; these offer value following endocytosis of RNA, when cationic or amphipathic properties can promote endocytic vesicle membrane perturbation to facilitate cytosolic translocation. The present review presents how such advances are being applied to the delivery of a new form of RNA vaccine, replicons (RepRNA) carrying inserted foreign genes of interest encoding vaccine antigens. Approaches have been developed for delivery to DCs, leading to the translation of the RepRNA and encoded vaccine antigens both in vitro and in vivo. Potential mechanisms favouring efficient delivery leading to translation are discussed with respect to the DC endocytic machinery, showing the importance of cytosolic translocation from acidifying endocytic structures. The review relates the DC endocytic pathways to immune response induction, and the potential advantages for these self-replicating RNA vaccines in the near future.

A cationic nanoemulsion for the delivery of next-generation RNA vaccines

Nucleic acid-based vaccines such as viral vectors, plasmid DNA, and mRNA are being developed as a means to address a number of unmet medical needs that current vaccine technologies have been unable to address. Here, we describe a cationic nanoemulsion (CNE) delivery system developed to deliver a self-amplifying mRNA vaccine. This nonviral delivery system is based on Novartis's proprietary adjuvant MF59, which has an established clinical safety profile and is well tolerated in children, adults, and the elderly. We show that nonviral delivery of a 9 kb self-amplifying mRNA elicits potent immune responses in mice, rats, rabbits, and nonhuman primates comparable to a viral delivery technology, and demonstrate that, relatively low doses (75 µg) induce antibody and T-cell responses in primates. We also show the CNE-delivered self-amplifying mRNA enhances the local immune environment through recruitment of immune cells similar to an MF59 adjuvanted subunit vaccine. Lastly, we show that the site of protein expression within the muscle and magnitude of protein expression is similar to a viral vector. Given the demonstration that self-amplifying mRNA delivered using a CNE is well tolerated and immunogenic in a variety of animal models, we are optimistic about the prospects for this technology.

Self-replicating Replicon-RNA Delivery to Dendritic Cells by Chitosan-nanoparticles for Translation In Vitro and In Vivo

Self-amplifying replicon RNA (RepRNA) possesses high potential for increasing antigen load within dendritic cells (DCs). The major aim of the present work was to define how RepRNA delivered by biodegradable, chitosan-based nanoparticulate delivery vehicles (nanogel-alginate (NGA)) interacts with DCs, and whether this could lead to translation of the RepRNA in the DCs. Although studies employed virus replicon particles (VRPs), there are no reports on biodegradable, nanoparticulate vehicle delivery of RepRNA. VRP studies employed cytopathogenic agents, contrary to DC requirements—slow processing and antigen retention. We employed noncytopathogenic RepRNA with NGA, demonstrating for the first time the efficiency of RepRNA association with nanoparticles, NGA delivery to DCs, and RepRNA internalization by DCs. RepRNA accumulated in vesicular structures, with patterns typifying cytosolic release. This promoted RepRNA translation, in vitro and in vivo. Delivery and translation were RepRNA concentration-dependent, occurring in a kinetic manner. Including cationic lipids with chitosan during nanoparticle formation enhanced delivery and translation kinetics, but was not required for translation of immunogenic levels in vivo. This work describes for the first time the characteristics associated with chitosan-nanoparticle delivery of self-amplifying RepRNA to DCs, leading to translation of encoded foreign genes, namely influenza virus hemagglutinin and nucleoprotein.

Trial Watch: DNA vaccines for cancer therapy

During the past 2 decades, the possibility that preparations capable of eliciting tumor-specific immune responses would mediate robust therapeutic effects in cancer patients has received renovated interest. In this context, several approaches to vaccinate cancer patients against their own malignancies have been conceived, including the administration of DNA constructs coding for one or more tumor-associated antigens (TAAs). Such DNA-based vaccines conceptually differ from other types of gene therapy in that they are not devised to directly kill cancer cells or sensitize them to the cytotoxic activity of a drug, but rather to elicit a tumor-specific immune response. In spite of an intense wave of preclinical development, the introduction of this immunotherapeutic paradigm into the clinical practice is facing difficulties. Indeed, while most DNA-based anticancer vaccines are well tolerated by cancer patients, they often fail to generate therapeutically relevant clinical responses. In this Trial Watch, we discuss the latest advances on the use of DNA-based vaccines in cancer therapy, discussing the literature that has been produced around this topic during the last 13 months as well as clinical studies that have been launched in the same time frame to assess the actual therapeutic potential of this intervention.

Nucleic acid vaccines: prospects for non-viral delivery of mRNA vaccines

INTRODUCTION

Nucleic acid-based vaccines are being developed as a means to combine the positive attributes of both live-attenuated and subunit vaccines. Viral vectors and plasmid DNA vaccines have been extensively evaluated in human clinical trials and have been shown to be safe and immunogenic, although none have been licensed for human use. More recently, mRNA-based vaccine alternatives have emerged and might offer certain advantages over their DNA-based counterparts.

AREAS COVERED

This review describes the two main categories of mRNA vaccines: conventional non-amplifying and self-amplifying mRNA. It summarizes the initial clinical proof-of-concept studies and outlines the preclinical testing of the next wave of innovations for the technology. Finally, this review highlights the versatile functionality of the mRNA molecule and introduces opportunities for future improvements in vaccine design.

EXPERT OPINION

The prospects for mRNA vaccines are very promising. Like other types of nucleic acid vaccines, mRNA vaccines have the potential to combine the positive attributes of live attenuated vaccines while obviating many potential safety limitations. Although data from initial clinical trials appear encouraging, mRNA vaccines are far from a commercial product. These initial approaches have spurred innovations in vector design, non-viral delivery, large-scale production and purification of mRNA to quickly move the technology forward. Some improvements have already been tested in preclinical models for both prophylactic and therapeutic vaccine targets and have demonstrated their ability to elicit potent and broad immune responses, including functional antibodies, type 1 T helper cells-type T cell responses and cytotoxic T cells. Though the initial barriers for this nucleic acid vaccine approach seem to be overcome, in our opinion, the future and continued success of this approach lies in a more extensive evaluation of the many non-viral delivery systems described in the literature and gaining a better understanding of the mechanism of action to allow rational design of next generation technologies.

Genome-wide RNAi screen identifies SEC61A and VCP as conserved regulators of Sindbis virus entry

Alphaviruses are a large class of insect-borne human pathogens and little is known about the host-factor requirements for infection. To identify such factors, we performed a genome-wide RNAi screen using model Drosophila cells and validated 94 genes that impacted infection of Sindbis virus (SINV), the prototypical alphavirus. We identified a conserved role for SEC61A and valosin-containing protein (VCP) in facilitating SINV entry in insects and mammals. SEC61A and VCP selectively regulate trafficking of the entry receptor NRAMP2, and loss or pharmacological inhibition of these proteins leads to altered NRAMP2 trafficking to lysosomal compartments and proteolytic digestion within lysosomes. NRAMP2 is the major iron transporter in cells, and loss of NRAMP2 attenuates intracellular iron transport. Thus, this study reveals genes and pathways involved in both infection and iron homeostasis that may serve as targets for antiviral therapeutics or for iron-imbalance disorders.

A vaccinia virus recombinant transcribing an alphavirus replicon and expressing alphavirus structural proteins leads to packaging of alphavirus infectious single cycle particles

Poxviruses and Alphaviruses constitute two promising viral vectors that have been used extensively as expression systems, or as vehicles for vaccine purposes. Poxviruses, like vaccinia virus (VV) are well-established vaccine vectors having large insertion capacity, excellent stability, and ease of administration. In turn, replicons derived from Alphaviruses like Semliki Forest virus (SFV) are potent protein expression and immunization vectors but stocks are difficult to produce and maintain. In an attempt to demonstrate the use of a Poxvirus as a means for the delivery of small vaccine vectors, we have constructed and characterized VV/SFV hybrid vectors. A SFV replicon cDNA was inserted in the VV genome and placed under the control of a VV early promoter. The replicon, transcribed from the VV genome as an early transcript, was functional, and thus capable of initiating its own replication and transcription. Further, we constructed a VV recombinant additionally expressing the SFV structural proteins under the control of a vaccinia synthetic early/late promoter. Infection with this recombinant produced concurrent transcription of the replicon and expression of SFV structural proteins, and led to the generation of replicon-containing SFV particles that were released to the medium and were able to infect additional cells. This combined VV/SFV system in a single virus allows the use of VV as a SFV delivery vehicle in vivo. The combination of two vectors, and the possibility of generating in vivo single-cycle, replicon containing alphavirus particles, may open new strategies in vaccine development or in the design of oncolytic viruses.

Engineered alphavirus replicon vaccines based on known attenuated viral mutants show limited effects on immunogenicity

The immunogenicity of alphavirus replicon vaccines is determined by many factors including the level of antigen expression and induction of innate immune responses. Characterized attenuated alphavirus mutants contain changes to the genomic 5' UTR and mutations that result in altered non-structural protein cleavage timing leading to altered levels of antigen expression and interferon (IFN) induction. In an attempt to create more potent replicon vaccines, we engineered a panel of Venezuelan equine encephalitis-Sindbis virus chimeric replicons that contained these attenuating mutations. Modified replicons were ranked for antigen expression and IFN induction levels in cell culture and then evaluated in mice. The results of these studies showed that differences in antigen production and IFN induction in vitro did not correlate with large changes in immunogenicity in vivo. These findings indicate that the complex interactions between innate immune response and the replicon's ability to express antigen complicate rational design of more potent alphavirus replicons.

Enhanced Delivery and Potency of Self-Amplifying mRNA Vaccines by Electroporation in Situ

Nucleic acid-based vaccines such as viral vectors, plasmid DNA (pDNA), and mRNA are being developed as a means to address limitations of both live-attenuated and subunit vaccines. DNA vaccines have been shown to be potent in a wide variety of animal species and several products are now licensed for commercial veterinary but not human use. Electroporation delivery technologies have been shown to improve the generation of T and B cell responses from synthetic DNA vaccines in many animal species and now in humans. However, parallel RNA approaches have lagged due to potential issues of potency and production. Many of the obstacles to mRNA vaccine development have recently been addressed, resulting in a revival in the use of non-amplifying and self-amplifying mRNA for vaccine and gene therapy applications. In this paper, we explore the utility of EP for the in vivo delivery of large, self-amplifying mRNA, as measured by reporter gene expression and immunogenicity of genes encoding HIV envelope protein. These studies demonstrated that EP delivery of self-amplifying mRNA elicited strong and broad immune responses in mice, which were comparable to those induced by EP delivery of pDNA.

RNA: the new revolution in nucleic acid vaccines

Nucleic acid vaccines have the potential to address issues of safety and effectiveness sometimes associated with vaccines based on live attenuated viruses and recombinant viral vectors. In addition, methods to manufacture nucleic acid vaccines are suitable as generic platforms and for rapid response, both of which will be very important for addressing newly emerging pathogens in a timely fashion. Plasmid DNA is the more widely studied form of nucleic acid vaccine and proof of principle in humans has been demonstrated, although no licensed human products have yet emerged. The RNA vaccine approach, based on mRNA and engineered RNA replicons derived from certain RNA viruses, is gaining increased attention and several vaccines are under investigation for infectious diseases, cancer and allergy. Human clinical trials are underway and the prospects for success are bright.

Prime-boost approaches to tuberculosis vaccine development

Four individuals die from active TB disease each minute, while at least 2 billion are latently infected and at risk for disease reactivation. BCG, the only licensed TB vaccine, is effective in preventing childhood forms of TB; however its poor efficacy in adults, emerging drug-resistant TB strains and tedious chemotherapy regimes, warrant the development of novel prophylactic measures. Designing safe and effective vaccines against TB will require novel approaches on several levels, including the administration of rationally selected mycobacterial antigens in efficient delivery vehicles via optimal immunization routes. Given the primary site of disease manifestation in the lungs, development of mucosal immunization strategies to generate protective immune responses both locally, and in the circulation, may be important for effective TB prophylaxis. This review focuses on prime-boost immunization strategies currently under investigation and highlights the potential of mucosal delivery and rational vaccine design based on systems biology.

Functional RNA delivery targeted to dendritic cells by synthetic nanoparticles

Dendritic cells (DCs) are essential to many aspects of immune defense development and regulation. They provide important targets for prophylactic and therapeutic delivery. While protein delivery has had considerable success, RNA delivery is still expanding. Delivering RNA molecules for RNAi has shown particular success and there are reports on successful delivery of mRNA. Central, therein, is the application of cationic entities. Following endocytosis of the delivery vehicle for the RNA, cationic entities should promote vesicular membrane perturbation, facilitating cytosolic release. The present review explains the diversity of DC function in immune response development and control. Promotion of delivered RNA cytosolic release is discussed, relating to immunoprophylactic and therapeutic potential, and DC endocytic machinery is reviewed, showing how DC endocytic pathways influence the handling of internalized material. The potential advantages for application of replicating RNA are presented and discussed, in consideration of their value and development in the near future.

Nonviral delivery of self-amplifying RNA vaccines

Despite more than two decades of research and development on nucleic acid vaccines, there is still no commercial product for human use. Taking advantage of the recent innovations in systemic delivery of short interfering RNA (siRNA) using lipid nanoparticles (LNPs), we developed a self-amplifying RNA vaccine. Here we show that nonviral delivery of a 9-kb self-amplifying RNA encapsulated within an LNP substantially increased immunogenicity compared with delivery of unformulated RNA. This unique vaccine technology was found to elicit broad, potent, and protective immune responses, that were comparable to a viral delivery technology, but without the inherent limitations of viral vectors. Given the many positive attributes of nucleic acid vaccines, our results suggest that a comprehensive evaluation of nonviral technologies to deliver self-amplifying RNA vaccines is warranted.

Phoenix rising: gene therapy makes a comeback

Despite the first application of gene therapy in 1990, gene therapy has until recently failed to meet the huge expectations set forth by researchers, clinicians, and patients, thus dampening enthusiasm for an imminent cure for many life-threatening genetic diseases. Nonetheless, in recent years we have witnessed a strong comeback for gene therapy, with clinical successes in young and adult subjects suffering from inherited forms of blindness or from X-linked severe combined immunodeficiency disease. In this review, various gene therapy vectors progressing into clinical development and pivotal advances in gene therapy trials will be discussed.

The genetics of alphaviruses

Alphaviruses are emerging human pathogens that are transmitted by arthropod vectors. Their ability to infect a wide range of vertebrate hosts including humans, equines, birds and rodents has brought about a series of epidemic and epizootic outbreaks worldwide. Their potential to cause a pandemic has spurred the interest of researchers globally, leading to the rapid advancement on the characterization of genetic determinants of alphaviruses. In this review, the focal point is placed on the genetics of alphaviruses, whereby the genetic composition, clinical features, evolution and adaptation of alphaviruses, modulation of IFN response by alphavirus proteins and therapeutic aspects of alphaviruses will be discussed.

Rubella virus-like replicon particles: analysis of encapsidation determinants and non-structural roles of capsid protein in early post-entry replication

Rubella virus (RUBV) contains a plus-strand RNA genome with two ORFs, one encoding the non-structural replicase proteins (NS-ORF) and the second encoding the virion structural proteins (SP-ORF). This study describes development and use of a trans-encapsidation system for the assembly of infectious RUBV-like replicon particles (VRPs) containing RUBV replicons (self replicating genomes with the SP-ORF replaced with a reporter gene). First, this system was used to map signals within the RUBV genome that mediate packaging of viral RNA. Mutations within a proposed packaging signal did not significantly affect relative packaging efficiency. The insertion of various fragments derived from the RUBV genome into Sindbis virus replicons revealed that there are several regions within the RUBV genome capable of enhancing encapsidation of heterologous replicon RNAs. Secondly, the trans-encapsidation system was used to analyse the effect of alterations within the capsid protein (CP) on release of VRPs and subsequent initiation of replication in newly infected cells. Deletion of the N-terminal eight amino acids of the CP reduced VRP titre significantly, which could be partially complemented by native CP provided in trans, indicating that this mutation affected an entry or post-entry event in the replication cycle. To test this hypothesis, the trans-encapsidation system was used to demonstrate the rescue of a lethal deletion within P150, one of the virus replicase proteins, by CP contained within the virus particle. This novel finding substantiated the functional role of CP in early post-entry replication.

Progress in respiratory virus vaccine development

Viral respiratory infections cause significant morbidity and mortality in infants and young children as well as in at-risk adults and the elderly. Although many viral pathogens are capable of causing respiratory disease, vaccine development has to focus on a limited number of pathogens, such as those that commonly cause serious lower respiratory illness (LRI). Whereas influenza virus vaccines have been available for some time (see the review by Clark and Lynch in this issue), vaccines against other medically important viruses such as respiratory syncytial virus (RSV), the parainfluenza viruses (PIVs), and metapneumovirus (MPVs) are not available. This review aims to provide a brief update on investigational vaccines against RSV, the PIVs, and MPV that have been evaluated in clinical trials or are currently in clinical development.

A Semliki forest virus vector engineered to express IFNα induces efficient elimination of established tumors

Semliki Forest virus (SFV) represents a promising gene therapy vector for tumor treatment, because it produces high levels of recombinant therapeutic proteins while inducing apoptosis in infected cells. In this study, we constructed a SFV vector expressing murine interferon alpha (IFNα). IFNα displays antitumor activity mainly by enhancing an antitumor immune response, as well as by a direct antiproliferative effect. In spite of the antiviral activity of IFNα, SFV-IFN could be produced in BHK cells at high titers. This vector was able to infect TC-1 cells, a tumor cell line expressing E6 and E7 proteins of human papillomavirus, leading to high production of IFNα both in vitro and in vivo. When injected into subcutaneous TC-1 tumors implanted in mice, SFV-IFN was able to induce an E7-specific cytotoxic T lymphocyte response, and to modify tumor infiltrating immune cells, reducing the percentage of T regulatory cells and activating myeloid cells. As a consequence, SFV-IFN was able to eradicate 58% of established tumors treated 21 days after implantation with long-term tumor-free survival and very low toxicity. SFV-IFN was also able to induce significant antitumor responses in a subcutaneous tumor model of murine colon adenocarcimoma. These data suggest that local production of IFNα by intratumoral injection of recombinant SFV-IFN could represent a potent new strategy to treat tumors in patients.

Packaging of porcine reproductive and respiratory syndrome virus replicon RNA by a stable cell line expressing its nucleocapsid protein

Porcine reproductive and respiratory syndrome virus (PRRSV), a member of the Arteriviridae family, is one of the most common and economically important swine pathogens. Although both live-attenuated and killed-inactivated vaccines against the virus have been available for a decade, PRRSV is still a major problem in the swine industry worldwide. To explore the possibility of producing single-round infectious PRRSV replicon particles as a potential vaccine strategy, we have now generated two necessary components: 1) a stable cell line (BHK/Sinrepl9/PRRSV-N) that constitutively expresses the viral nucleocapsid (N) protein localized to the cytoplasm and the nucleolus and 2) a PRRSV replicon vector (pBAC/PRRSV/Replicon-AN) with a 177-nucleotide deletion, removing the 3′-half portion of ORF7 in the viral genome, from which the self-replicating propagation-defective replicon RNAs were synthesized in vitro by SP6 polymerase run-off transcription. Transfection of this replicon RNA into N protein-expressing BHK-21 cells led to the secretion of infectious particles that packaged the replicon RNA, albeit with a low production efficiency of 0.4 × 10² to 1.1 × 10² infectious units/ml; the produced particles had only single-round infectivity with no cell-to-cell spread. This trans-complementation system for PRRSV provides a useful platform for studies to define the packaging signals and motifs present within the viral genome and N protein, respectively, and to develop viral replicon-based antiviral vaccines that will stop the infection and spread of this pathogen.

Novel viral vectors utilizing intron splice-switching to activate genome rescue, expression and replication in targeted cells

BACKGROUND

The outcome of virus infection depends from the precise coordination of viral gene expression and genome replication. The ability to control and regulate these processes is therefore important for analysis of infection process. Viruses are also useful tools in bio- and gene technology; they can efficiently kill cancer cells and trigger immune responses to tumors. However, the methods for constructing tissue- or cell-type specific viruses typically suffer from low target-cell specificity and a high risk of reversion. Therefore novel and universal methods of regulation of viral infection are also important for therapeutic application of virus-based systems.

METHODS

Aberrantly spliced introns were introduced into crucial gene-expression units of adenovirus vector and alphavirus DNA/RNA layered vectors and their effects on the viral gene expression, replication and/or the release of infectious genomes were studied in cell culture. Transfection of the cells with splice-switching oligonucleotides was used to correct the introduced functional defect(s).

RESULTS

It was demonstrated that viral gene expression, replication and/or the release of infectious genomes can be blocked by the introduction of aberrantly spliced introns. The insertion of such an intron into an adenovirus vector reduced the expression of the targeted gene more than fifty-fold. A similar insertion into an alphavirus DNA/RNA layered vector had a less dramatic effect; here, only the release of the infectious transcript was suppressed but not the subsequent replication and spread of the virus. However the insertion of two aberrantly spliced introns resulted in an over one hundred-fold reduction in the infectivity of the DNA/RNA layered vector. Furthermore, in both systems the observed effects could be reverted by the delivery of splice-switching oligonucleotide(s), which corrected the splicing defects.

CONCLUSIONS

Splice-switch technology, originally developed for genetic disease therapy, can also be used to control gene expression of viral vectors. This approach represents a novel, universal and powerful method for controlling gene expression, replication, viral spread and, by extension, virus-induced cytotoxic effects and can be used both for basic studies of virus infection and in virus-based gene- and anti-cancer therapy.

Role of innate signalling pathways in the immunogenicity of alphaviral replicon-based vaccines

Background

Alphaviral replicon-based vectors induce potent immune responses both when given as viral particles (VREP) or as DNA (DREP). It has been suggested that the strong immune stimulatory effect induced by these types of vectors is mediated by induction of danger signals and activation of innate signalling pathways due to the replicase activity. To investigate the innate signalling pathways involved, mice deficient in either toll-like receptors or downstream innate signalling molecules were immunized with DREP or VREP.

Results

We show that the induction of a CD8⁺ T cell response did not require functional TLR3 or MyD88 signalling. However, IRF3, converging several innate signalling pathways and important for generation of pro-inflammatory cytokines and type I IFNs, was needed for obtaining a robust primary immune response. Interestingly, type I interferon (IFN), induced by most innate signalling pathways, had a suppressing effect on both the primary and memory T cell responses after DREP and VREP immunization.

Conclusions

We show that alphaviral replicon-based vectors activate multiple innate signalling pathways, which both activate and restrict the induced immune response. These results further show that there is a delicate balance in the strength of innate signalling and induction of adaptive immune responses that should be taken into consideration when innate signalling molecules, such as type I IFNs, are used as vaccine adjuvant.

A perspective on the 2009 A/H1N1 influenza pandemic in Mexico

In this article, we provide a chronological description of the 2009 H1N1 influenza pandemic in Mexico from the detection of severe respiratory disease among young adults in central Mexico and the identification of the novel swine-origin influenza virus to the response of Mexican public health authorities with the swift implementation of the National Preparedness and Response Plan for Pandemic Influenza. Furthermore, we review some features of the 2009 H1N1 influenza pandemic in Mexico in relation to the devastating 1918-1920 influenza pandemic and discuss opportunities for the application of mathematical modeling in the transmission dynamics of pandemic influenza. The value of historical data in increasing our understanding of past pandemic events is highlighted.

Biological challenges and technological opportunities for respiratory syncytial virus vaccine development

Respiratory syncytial virus (RSV) is an important cause of respiratory disease causing high rates of hospitalizations in infants, significant morbidity in children and adults, and excess mortality in the elderly. Major barriers to vaccine development include early age of RSV infection, capacity of RSV to evade innate immunity, failure of RSV-induced adaptive immunity to prevent reinfection, history of RSV vaccine-enhanced disease, and lack of an animal model fully permissive to human RSV infection. These biological challenges, safety concerns, and practical issues have significantly prolonged the RSV vaccine development process. One great advantage compared to other difficult viral vaccine targets is that passively administered neutralizing monoclonal antibody is known to protect infants from severe RSV disease. Therefore, the immunological goals for vaccine development are to induce effective neutralizing antibody to prevent infection and to avoid inducing T-cell response patterns associated with enhanced disease. Live-attenuated RSV and replication-competent chimeric viruses are in advanced clinical trials. Gene-based strategies, which can control the specificity and phenotypic properties of RSV-specific T-cell responses utilizing replication-defective vectors and which may improve on immunity from natural infection, are progressing through preclinical testing. Atomic level structural information on RSV envelope glycoproteins in complex with neutralizing antibodies is guiding design of new vaccine antigens that may be able to elicit RSV-specific antibody responses without induction of RSV-specific T-cell responses. These new technologies may allow development of vaccines that can protect against RSV-mediated disease in infants and establish a new immunological paradigm in the host to achieve more durable protection against reinfection.