Cowpea mosaic virus-based chimaeras. Effects of inserted peptides on the phenotype, host range, and transmissibility of the modified viruses

The Plant Viruses and Molecular Farming: How Beneficial They Might Be for Human and Animal Health?

Plant viruses have traditionally been studied as pathogens in the context of understanding the molecular and cellular mechanisms of a particular disease affecting crops. In recent years, viruses have emerged as a new alternative for producing biological nanomaterials and chimeric vaccines. Plant viruses were also used to generate highly efficient expression vectors, revolutionizing plant molecular farming (PMF). Several biological products, including recombinant vaccines, monoclonal antibodies, diagnostic reagents, and other pharmaceutical products produced in plants, have passed their clinical trials and are in their market implementation stage. PMF offers opportunities for fast, adaptive, and low-cost technology to meet ever-growing and critical global health needs. In this review, we summarized the advancements in the virus-like particles-based (VLPs-based) nanotechnologies and the role they played in the production of advanced vaccines, drugs, diagnostic bio-nanomaterials, and other bioactive cargos. We also highlighted various applications and advantages plant-produced vaccines have and their relevance for treating human and animal illnesses. Furthermore, we summarized the plant-based biologics that have passed through clinical trials, the unique challenges they faced, and the challenges they will face to qualify, become available, and succeed on the market.

Isolation of Cowpea Mosaic Virus-Binding Peptides

The plant virus cowpea mosaic virus (CPMV) is a natural nanocarrier that has been developed as a platform technology for the delivery of various payloads including peptide epitopes for vaccines, contrast agents for imaging, and drugs for therapy. Genetic fusion and chemical conjugations are the mainstay approaches to load the active ingredient to the exterior and/or interior of CPMV. However, these methods have limitations; genetic engineering is limited to biologics, and chemical alteration often requires multistep reactions with modification of both CPMV and the active ingredient. Either method can also result in particle instability. Therefore, to provide an alternate path toward CPMV functionalization, we report the isolation of peptides that specifically bind to CPMV, termed CPMV-binding peptides (CBP). We used a commercial M13 phage display 7-mer peptide library to pan for and select peptides that selectively bind to CPMV. Biopanning and characterization of lead candidates resulted in isolation of the motif "GWRVSEF/L" as the CPMV-specific motif with phenylalanine (F) at the seventh position being stronger than leucine (L). Specificity to CPMV was demonstrated, and cross-reactivity toward other plant viruses was not observed. To demonstrate cargo loading, GWRVSEF was tagged with biotin, fluorescein isothiocyanate (FITC), and a human epidermal growth factor receptor 2 (HER2)-specific targeting peptide ligand. Display of the active ingredient was confirmed, and utility of tagged and targeted CPMV in cell binding assays was demonstrated. The CBP functionalization strategy offers a new avenue for CPMV nanoparticle functionalization and should offer a versatile tool to add active ingredients that otherwise may be difficult to conjugate or display.

Stable Display of Artificially Long Foreign Antigens on Chimeric Bamboo mosaic virus Particles

Plant viruses can be genetically modified to generate chimeric virus particles (CVPs) carrying heterologous peptides fused on the surface of coat protein (CP) subunits as vaccine candidates. However, some factors may be especially significant in determining the properties of chimeras. In this study, peptides from various sources and of various lengths were inserted into the Bamboo mosaic virus-based (BaMV) vector CP N-terminus to examine the chimeras infecting and accumulating in plants. Interestingly, it was found that the two different strains Foot-and-mouth disease virus (FMDV) VP1 antigens with flexible linker peptides (77 or 82 amino acids) were directly expressed on the BaMV CP, and the chimeric particles self-assembled and continued to express FMDV antigens. The chimeric CP, when directly fused with a large foreign protein (117 amino acids), can self-fold into incomplete virus particles or disks. The physicochemical properties of heterologus peptides N-terminus, complex strand structures of heterologus peptides C-terminus and different flexible linker peptides, can affect the chimera accumulation. Based on these findings, using plant virus-based chimeras to express foreign proteins can increase their length limitations, and engineered plant-made CVP-based vaccines have increasing potential for further development as novel vaccines.

vB_EcoS_NBD2 bacteriophage-originated polytubes as a carrier for the presentation of foreign sequences

Virus-based nanoparticles constitute a promising platform for the creation of efficient vaccines and nanomaterials. Previously we demonstrated, that the recombinant tail tube protein gp39 of vB_EcoS_NBD2 bacteriophage self-assembles into extremely long (from 0.1 to >3.95 μm), flexible, and stable polytubes when produced in Saccharomyces cerevisiae. To develop a tubular platform for multivalent display of foreign antigens, yeast-derived recombinant tail tube protein gp39 was chosen as a scaffold. The carboxy-terminal fusions of gp39 with various antigens up to 238 amino acids in length resulted in different synthesis efficiency and self-assembly capacity. Recombinant gp39 fused with green fluorescent protein (eGFP) comprising 238 amino acid residues was capable to self-assemble into short fluorescent polytubes with retained eGFP functional activity. By demonstrating the display of active foreign antigens on the exterior surface of polytubes, these structures may provide a promising tool for diverse applications in nanotechnology.

Turnip Mosaic Virus Coat Protein Deletion Mutants Allow Defining Dispensable Protein Domains for 'in Planta' eVLP Formation

The involvement of different structural domains of the coat protein (CP) of turnip mosaic virus, a potyvirus, in establishing and/or maintaining particle assembly was analyzed through deletion mutants of the protein. In order to identify exclusively those domains involved in protein-protein interactions within the particle, the analysis was performed by agroinfiltration "in planta", followed by the assessment of CP accumulation in leaves and the assembly of virus-like particles lacking nucleic acids, also known as empty virus-like particles (eVLP). Thus, the interactions involving viral RNA could be excluded. It was found that deletions precluding eVLP assembly did not allow for protein accumulation either, probably indicating that non-assembled CP protein was degraded in the plant leaves. Deletions involving the CP structural core were incompatible with particle assembly. On the N-terminal domain, only the deletion avoiding the subdomain involved in interactions with other CP subunits was incorporated into eVLPs. The C-terminal domain was shown to be more permissive to deletions. Assembled eVLPs were found for mutants, eliminating the whole domain. The C-terminal domain mutants were unusually long, suggesting some role of the domain in the regulation of particle length. The identification of the CP domains responsible for eVLP formation will allow for new approaches to protein stretch replacement with peptides or proteins of nanobiotechnological interest. Finally, specific cases of application are considered.

Prevention and treatment of autoimmune diseases with plant virus nanoparticles

Plant viruses are natural, self-assembling nanostructures with versatile and genetically programmable shells, making them useful in diverse applications ranging from the development of new materials to diagnostics and therapeutics. Here, we describe the design and synthesis of plant virus nanoparticles displaying peptides associated with two different autoimmune diseases. Using animal models, we show that the recombinant nanoparticles can prevent autoimmune diabetes and ameliorate rheumatoid arthritis. In both cases, this effect is based on a strictly peptide-related mechanism in which the virus nanoparticle acts both as a peptide scaffold and as an adjuvant, showing an overlapping mechanism of action. This successful preclinical testing could pave the way for the development of plant viruses for the clinical treatment of human autoimmune diseases.

Plant Virus Nanoparticles for Vaccine Applications

In the rapidly evolving field of nanotechnology, plant virus nanoparticles (pVNPs) are emerging as powerful tools in diverse applications ranging from biomedicine to materials science. The proteinaceous structure of plant viruses allows the capsid structure to be modified by genetic engineering and/or chemical conjugation with nanoscale precision. This means that pVNPs can be engineered to display peptides and proteins on their external surface, including immunodominant peptides derived from pathogens allowing pVNPs to be used for active immunization. In this context, pVNPs are safer than VNPs derived from mammalian viruses because there is no risk of infection or reversion to pathogenicity. Furthermore, pVNPs can be produced rapidly and inexpensively in natural host plants or heterologous production platforms. In this review, we discuss the use of pVNPs for the delivery of peptide antigens to the host immune in pre-clinical studies with the final aim of promoting systemic immunity against the corresponding pathogens. Furthermore, we described the versatility of plant viruses, with innate immunostimulatory properties, in providing a huge natural resource of carriers that can be used to develop the next generation of sustainable vaccines.

Amino acids at the exposed C-terminus of the S coat protein of cowpea mosaic virus play different roles in particle formation and viral systemic movement

The icosahedral capsid of cowpea mosaic virus is formed by 60 copies of the large (L) and small (S) coat protein subunits. The 24-amino-acid C-terminal peptide of the S coat protein can undergo proteolytic cleavage without affecting particle stability or infectivity. Mutagenic studies have shown that this sequence is involved in particle assembly, virus movement, RNA encapsidation and suppression of gene silencing. However, it is unclear how these processes are related, and which part(s) of the sequence are involved in each process. Here, we have analysed the effect of mutations in the C-terminal region of the S protein on the assembly of empty virus-like particles and on the systemic movement of infectious virus. The results confirmed the importance of positively charged amino acids adjacent to the cleavage site for particle assembly and revealed that the C-terminal 11 amino acids are important for efficient systemic movement of the virus.

Protein Nanotubes: From Bionanotech towards Medical Applications

Nanobiotechnology involves the study of structures found in nature to construct nanodevices for biological and medical applications with the ultimate goal of commercialization. Within a cell most biochemical processes are driven by proteins and associated macromolecular complexes. Evolution has optimized these protein-based nanosystems within living organisms over millions of years. Among these are flagellin and pilin-based systems from bacteria, viral-based capsids, and eukaryotic microtubules and amyloids. While carbon nanotubes (CNTs), and protein/peptide-CNT composites, remain one of the most researched nanosystems due to their electrical and mechanical properties, there are many concerns regarding CNT toxicity and biodegradability. Therefore, proteins have emerged as useful biotemplates for nanomaterials due to their assembly under physiologically relevant conditions and ease of manipulation via protein engineering. This review aims to highlight some of the current research employing protein nanotubes (PNTs) for the development of molecular imaging biosensors, conducting wires for microelectronics, fuel cells, and drug delivery systems. The translational potential of PNTs is highlighted.

Vaccine and antibody production in plants: developments and computational tools

Plants as bioreactors have been widely used to express efficient vaccine antigens against viral, bacterial and protozoan infections. To date, many different plant-based expression systems have been analyzed, with a growing preference for transient expression systems. Antibody expression in diverse plant species for therapeutic applications is well known, and this review provides an overview of various aspects of plant-based biopharmaceutical production. Here, we highlight conventional and gene expression technologies in plants along with some illustrative examples. In addition, the portfolio of products that are being produced and how they relate to the success of this field are discussed. Stable and transient gene expression in plants, agrofiltration and virus infection vectors are also reviewed. Further, the present report draws attention to antibody epitope prediction using computational tools, one of the crucial steps of vaccine design. Finally, regulatory issues, biosafety and public perception of this technology are also discussed.

Use of plant viruses and virus-like particles for the creation of novel vaccines

In recent decades, the development of plant virology and genetic engineering techniques has resulted in the construction of plant virus-based vaccines for protection against different infectious agents, cancers and autoimmune diseases in both humans and animals. Interaction studies between plant viruses and mammalian organisms have suggested that plant viruses and virus-like particles (VLPs) are safe and noninfectious to humans and animals. Plant viruses with introduced antigens are powerful vaccine components due to their strongly organized, repetitive spatial structure; they can elicit strong immune responses similar to those observed with infectious mammalian viruses. The analysis of published data demonstrated that at least 73 experimental vaccines, including 61 prophylactic and 12 therapeutic vaccines, have been constructed using plant viruses as a carrier structure for presentation of different antigens. This information clearly demonstrates that noninfectious viruses are also applicable as vaccine carriers. Moreover, several plant viruses have been used for platform development, and corresponding vaccines are currently being tested in human and veterinary clinical trials. This review therefore discusses the main principles of plant VLP vaccine construction, emphasizing the physical, chemical, genetic and immunological aspects. Results of the latest studies suggest that several plant virus-based vaccines will join the list of approved human and animal vaccines in the near future.

Cowpea mosaic virus nanoparticles for cancer imaging and therapy

Nanoparticle platforms are particularly attractive for theranostic applications due to their capacity for multifunctionality and multivalency. Some of the most promising nano-scale scaffold systems have been co-opted from nature including plant viruses such as cowpea mosaic virus (CPMV). The use of plant viruses like CPMV as viral nanoparticles is advantageous for many reasons; they are non-infectious and nontoxic to humans and safe for use in intravital imaging and drug delivery. The CPMV capsid icosahedral shape allows for enhanced multifunctional group display and the ability to carry specific cargoes. The native tropism of CPMV for cell-surface displayed vimentin and the enhanced permeability and retention effect allow them to preferentially extravasate from tumor neovasculature and efficiently penetrate tumors. Furthermore, CPMVs can be engineered via several straightforward chemistries to display targeting and imaging moieties on external, addressable residues and they can be loaded internally with therapeutic drug cargoes. These qualities make them highly effective as biocompatible platforms for tumor targeting, intravital imaging and cancer therapy.

A protein nanocontainer targeting epithelial cancers: rational engineering, biochemical characterization, drug loading and cell delivery

The development of drug delivery and imaging tools is a major challenge in human health, in particular in cancer pathologies. This work describes the optimization of a protein nanocontainer, belonging to the lectin protein family, for its use in epithelial cancer diagnosis and treatment. Indeed, it specifically targets a glycosidic marker, the T antigen, which is known to be characteristic of epithelial cancers. Its quaternary structure reveals a large hydrated inner cavity able to transport small therapeutic molecules. Optimization of the nanocontainer by site directed mutagenesis allowed controlling loading and release of confined drugs. Doxorubicin confinement was followed, both theoretically and experimentally, and provided a proof of concept for the use of this nanocontainer as a vectorization system. In OVCAR-3 cells, a human ovarian adenocarcinoma cell line that expresses the T antigen, the drug was observed to be delivered inside late endosomes/lysosomes. These results show that this new type of vectorization and imaging device opens new exciting perspectives in nano-theranostic approaches.

Systemic Infection of Nicotiana benthamiana with Potato virus X Nanoparticles Presenting a Fluorescent iLOV Polypeptide Fused Directly to the Coat Protein

Plant virus-based nanoparticles can be produced in plants on a large scale and are easily modified to introduce new functions, making them suitable for applications such as vaccination and drug delivery, tissue engineering, and in vivo imaging. The latter is often achieved using green fluorescent protein and its derivatives, but the monovalent fluorescent protein iLOV is smaller and more robust. Here, we fused the iLOV polypeptide to the N-terminus of the Potato virus X (PVX) coat protein, directly or via the Foot-and-mouth disease virus 2A sequence, for expression in Nicotiana benthamiana. Direct fusion of the iLOV polypeptide did not prevent the assembly or systemic spread of the virus and we verified the presence of fusion proteins and iLOV hybrid virus particles in leaf extracts. Compared to wild-type PVX virions, the PVX particles displaying the iLOV peptide showed an atypical, intertwined morphology. Our results confirm that a direct fusion of the iLOV fluorescent protein to filamentous PVX nanoparticles offers a promising tool for imaging applications.

Bioinspired Silica Mineralization on Viral Templates

Plant virus capsids are attractive entities for nanotechnological applications because of their variation in shape and natural assembly ability. This chapter describes the production and modification of three differently shaped plant virus capsids for silica mineralization purposes. The chosen plant viruses exhibit either an icosahedral (cowpea mosaic virus, CPMV), or a flexuous rod-like structure (potato virus X, PVX), or a rigid rod-like shape (tobacco mosaic virus, TMV), and are well-known and frequently used plant viruses for biotechnological applications. We describe the production (including genetic or chemical modification) and purification of the plant viruses or of empty virus-like particles in the case of CPMV, as well as the characterization of these harvested templates. The mineralization procedures and differences in the protocols specific to the distinct viruses are described, and the analyses of the mineralization results are explained.

Engineering Potato Virus X Particles for a Covalent Protein Based Attachment of Enzymes

Plant virus nanoparticles are often used to display functional amino acids or small peptides, thus serving as building blocks in application areas as diverse as nanoelectronics, bioimaging, vaccination, drug delivery, and bone differentiation. This is most easily achieved by expressing coat protein fusions, but the assembly of the corresponding virus particles can be hampered by factors such as the fusion protein size, amino acid composition, and post-translational modifications. Size constraints can be overcome by using the Foot and mouth disease virus 2A sequence, but the compositional limitations cannot be avoided without the introduction of time-consuming chemical modifications. SpyTag/SpyCatcher technology is used in the present study to covalently attach the Trichoderma reesei endoglucanase Cel12A to Potato virus X (PVX) nanoparticles. The formation of PVX particles is confirmed by western blot, and the ability of the particles to display Cel12A is demonstrated by enzyme-linked immunosorbent assays and transmission electron microscopy. Enzymatic assays show optimal reaction conditions of 50 °C and pH 6.5, and an increased substrate conversion rate compared to free enzymes. It is concluded that PVX displaying the SpyTag can serve as new scaffold for protein display, most notably for proteins with post-translational modifications.

Combining high-resolution cryo-electron microscopy and mutagenesis to develop cowpea mosaic virus for bionanotechnology

Particles of cowpea mosaic virus (CPMV) have enjoyed considerable success as nanoparticles. The development of a system for producing empty virus-like particles (eVLPs) of the virus, which are non-infectious and have the potential to be loaded with heterologous material, has increased the number of possible applications for CPMV-based particles. However, for this potential to be realised, it was essential to demonstrate that eVLPs were accurate surrogates for natural virus particles, and this information was provided by high-resolution cryo-EM studies of eVLPs. This demonstration has enabled the approaches developed for the production of modified particles developed with natural CPMV particles to be applied to eVLPs. Furthermore, a combination of cryo-EM and mutagenic studies allowed the development of particles which are permeable but which could still assemble efficiently. These particles were shown to be loadable with cobalt, indicating that they can, indeed, be used as nano-containers.

Icosahedral plant viral nanoparticles - bioinspired synthesis of nanomaterials/nanostructures

Viral nanotechnology utilizes virus nanoparticles (VNPs) and virus-like nanoparticles (VLPs) of plant viruses as highly versatile platforms for materials synthesis and molecular entrapment that can be used in the nanotechnological fields, such as in next-generation nanoelectronics, nanocatalysis, biosensing and optics, and biomedical applications, such as for targeting, therapeutic delivery, and non-invasive in vivo imaging with high specificity and selectivity. In particular, plant virus capsids provide biotemplates for the production of novel nanostructured materials with organic/inorganic moieties incorporated in a very precise and controlled manner. Interestingly, capsid proteins of spherical plant viruses can self-assemble into well-organized icosahedral three-dimensional (3D) nanoscale multivalent architectures with high monodispersity and structural symmetry. Using viral genetic and protein engineering of icosahedral viruses with a variety of sizes, the interior, exterior and the interfaces between coat protein (CP) subunits can be manipulated to fabricate materials with a wide range of desirable properties allowing for biomineralization, encapsulation, infusion, controlled self-assembly, and multivalent ligand display of nanoparticles or molecules for varied applications. In this review, we discuss the various functional nanomaterials/nanostructures developed using the VNPs and VLPs of different icosahedral plant viruses and their nano(bio)technological and nanomedical applications.

Synthetic plant virology for nanobiotechnology and nanomedicine

Nanotechnology is a rapidly expanding field seeking to utilize nano-scale structures for a wide range of applications. Biologically derived nanostructures, such as viruses and virus-like particles (VLPs), provide excellent platforms for functionalization due to their physical and chemical properties. Plant viruses, and VLPs derived from them, have been used extensively in biotechnology. They have been characterized in detail over several decades and have desirable properties including high yields, robustness, and ease of purification. Through modifications to viral surfaces, either interior or exterior, plant-virus-derived nanoparticles have been shown to support a range of functions of potential interest to medicine and nano-technology. In this review we highlight recent and influential achievements in the use of plant virus particles as vehicles for diverse functions: from delivery of anticancer compounds, to targeted bioimaging, vaccine production to nanowire formation. WIREs Nanomed Nanobiotechnol 2017, 9:e1447. doi: 10.1002/wnan.1447 For further resources related to this article, please visit the WIREs website.

Production of Japanese Encephalitis Virus Antigens in Plants Using Bamboo Mosaic Virus-Based Vector

Japanese encephalitis virus (JEV) is among the major threats to public health in Asia. For disease control and prevention, the efficient production of safe and effective vaccines against JEV is in urgent need. In this study, we produced a plant-made JEV vaccine candidate using a chimeric virus particle (CVP) strategy based on bamboo mosaic virus (BaMV) for epitope presentation. The chimeric virus, designated BJ2A, was constructed by fusing JEV envelope protein domain III (EDIII) at the N-terminus of BaMV coat protein, with an insertion of the foot-and-mouth disease virus 2A peptide to facilitate the production of both unfused and epitope-presenting for efficient assembly of the CVP vaccine candidate. The strategy allowed stable maintenance of the fusion construct over long-term serial passages in plants. Immuno-electron microscopy examination and immunization assays revealed that BJ2A is able to present the EDIII epitope on the surface of the CVPs, which stimulated effective neutralizing antibodies against JEV infection in mice. This study demonstrates the efficient production of an effective CVP vaccine candidate against JEV in plants by the BaMV-based epitope presentation system.

Display of whole proteins on inner and outer surfaces of grapevine fanleaf virus-like particles

Virus-like particles (VLPs) derived from nonenveloped viruses result from the self-assembly of capsid proteins (CPs). They generally show similar structural features to viral particles but are noninfectious and their inner cavity and outer surface can potentially be adapted to serve as nanocarriers of great biotechnological interest. While a VLP outer surface is generally amenable to chemical or genetic modifications, encaging a cargo within particles can be more complex and is often limited to small molecules or peptides. Examples where both inner cavity and outer surface have been used to simultaneously encapsulate and expose entire proteins remain scarce. Here, we describe the production of spherical VLPs exposing fluorescent proteins at either their outer surface or inner cavity as a result of the self-assembly of a single genetically modified viral structural protein, the CP of grapevine fanleaf virus (GFLV). We found that the N- and C-terminal ends of the GFLV CP allow the genetic fusion of proteins as large as 27 kDa and the plant-based production of nucleic acid-free VLPs. Remarkably, expression of N- or C-terminal CP fusions resulted in the production of VLPs with recombinant proteins exposed to either the inner cavity or the outer surface, respectively, while coexpression of both fusion proteins led to the formation hybrid VLP, although rather inefficiently. Such properties are rather unique for a single viral structural protein and open new potential avenues for the design of safe and versatile nanocarriers, particularly for the targeted delivery of bioactive molecules.

Bioengineered protein-based nanocage for drug delivery

Nature, in its wonders, presents and assembles the most intricate and delicate protein structures and this remarkable phenomenon occurs in all kingdom and phyla of life. Of these proteins, cage-like multimeric proteins provide spatial control to biological processes and also compartmentalizes compounds that may be toxic or unstable and avoids their contact with the environment. Protein-based nanocages are of particular interest because of their potential applicability as drug delivery carriers and their perfect and complex symmetry and ideal physical properties, which have stimulated researchers to engineer, modify or mimic these qualities. This article reviews various existing types of protein-based nanocages that are used for therapeutic purposes, and outlines their drug-loading mechanisms and bioengineering strategies via genetic and chemical functionalization. Through a critical evaluation of recent advances in protein nanocage-based drug delivery in vitro and in vivo, an outlook for de novo and in silico nanocage design, and also protein-based nanocage preclinical and future clinical applications will be presented.

Nodavirus-based biological container for targeted delivery system

Biological containers such as virus-like particles (VLPs) have gained increasing interest in the fields of gene therapy and vaccine development. Several virus-based materials have been studied, but the toxicity, biodistribution, and immunology of these systems still require extensive investigation. The specific goal of this review is to provide information about nodaviruses, which are causative infectious agents of insects and aquatic animals, but not humans. By understanding the structure and biophysical properties of such viruses, further chemical or genetic modification for novel nanocarriers could be developed. Therefore, their application for therapeutic purposes, particularly in humans, is of great interest.

Coupling Peptide Antigens to Virus-Like Particles or to Protein Carriers Influences the Th1/Th2 Polarity of the Resulting Immune Response

We have conjugated the S9 peptide, a mimic of the group B streptococcal type III capsular polysaccharide, to different carriers in an effort to elicit an optimal immune response. As carriers, we utilized the soluble protein keyhole limpet hemocyanin and virus-like particles (VLPs) from two plant viruses, Cowpea Chlorotic Mottle Virus and Cowpea Mosaic Virus. We have found that coupling the peptide to the soluble protein elicits a Th2 immune response, as evidenced by the production of the peptide-specific IgG1 antibody and IL-4/IL-10 production in response to antigen stimulation, whereas the peptide conjugated to VLPs elicited a Th1 response (IgG2a, IFN-γ). Because the VLPs used as carriers package RNA during the assembly process, we hypothesize that this effect may result from the presence of nucleic acid in the immunogen, which affects the Th1/Th2 polarity of the response.

The evolution of plant virus transmission pathways

The evolution of plant virus transmission pathways is studied through transmission via seed, pollen, or a vector. We address the questions: under what circumstances does vector transmission make pollen transmission redundant? Can evolution lead to the coexistence of multiple virus transmission pathways? We restrict the analysis to an annual plant population in which reproduction through seed is obligatory. A semi-discrete model with pollen, seed, and vector transmission is formulated to investigate these questions. We assume vector and pollen transmission rates are frequency-dependent and density-dependent, respectively. An ecological stability analysis is performed for the semi-discrete model and used to inform an evolutionary study of trade-offs between pollen and seed versus vector transmission. Evolutionary dynamics critically depend on the shape of the trade-off functions. Assuming a trade-off between pollen and vector transmission, evolution either leads to an evolutionarily stable mix of pollen and vector transmission (concave trade-off) or there is evolutionary bi-stability (convex trade-off); the presence of pollen transmission may prevent evolution of vector transmission. Considering a trade-off between seed and vector transmission, evolutionary branching and the subsequent coexistence of pollen-borne and vector-borne strains is possible. This study contributes to the theory behind the diversity of plant-virus transmission patterns observed in nature.

A transgenic plant cell-suspension system for expression of epitopes on chimeric Bamboo mosaic virus particles

We describe a novel strategy to produce vaccine antigens using a plant cell-suspension culture system in lieu of the conventional bacterial or animal cell-culture systems. We generated transgenic cell-suspension cultures from Nicotiana benthamiana leaves carrying wild-type or chimeric Bamboo mosaic virus (BaMV) expression constructs encoding the viral protein 1 (VP1) epitope of foot-and-mouth disease virus (FMDV). Antigens accumulated to high levels in BdT38 and BdT19 transgenic cell lines co-expressing silencing suppressor protein P38 or P19. BaMV chimeric virus particles (CVPs) were subsequently purified from the respective cell lines (1.5 and 2.1 mg CVPs/20 g fresh weight of suspended biomass, respectively), and the resulting CVPs displayed VP1 epitope on the surfaces. Guinea pigs vaccinated with purified CVPs produced humoral antibodies. This study represents an important advance in the large-scale production of immunopeptide vaccines in a cost-effective manner using a plant cell-suspension culture system.

When plant virology met Agrobacterium: the rise of the deconstructed clones

In the early days of molecular farming, Agrobacterium-mediated stable genetic transformation and the use of plant virus-based vectors were considered separate and competing technologies with complementary strengths and weaknesses. The demonstration that 'agroinfection' was the most efficient way of delivering virus-based vectors to their target plants blurred the distinction between the two technologies and permitted the development of 'deconstructed' vectors based on a number of plant viruses. The tobamoviruses, potexviruses, tobraviruses, geminiviruses and comoviruses have all been shown to be particularly well suited to the development of such vectors in dicotyledonous plants, while the development of equivalent vectors for use in monocotyledonous plants has lagged behind. Deconstructed viral vectors have proved extremely effective at the rapid, high-level production of a number of pharmaceutical proteins, some of which are currently undergoing clinical evaluation.

Feasibility of Cowpea chlorotic mottle virus-like particles as scaffold for epitope presentations

Background & Methods

Within the last decade Virus-Like Particles (VLPs) have increasingly received attention from scientists for their use as a carrier of (peptide) molecules or as scaffold to present epitopes for use in subunit vaccines. To test the feasibility of Cowpea chlorotic mottle virus (CCMV) particles as a scaffold for epitope presentation and identify sites for epitope fusion or insertion that would not interfere with virus-like-particle formation, chimeric CCMV coat protein (CP) gene constructs were engineered, followed by expression in E. coli and assessment of VLP formation. Various constructs were made encoding a 6x-His-tag, or selected epitopes from Influenza A virus [IAV] (M2e, HA) or Foot and Mouth Disease Virus [FMDV] (VP1 and 2C). The epitopes were either inserted 1) in predicted exposed loop structures of the CCMV CP protein, 2) fused to the amino- (N) or carboxyl-terminal (C) ends, or 3) to a N-terminal 24 amino acid (aa) deletion mutant (N∆24-CP) of the CP protein.

Results

High levels of insoluble protein expression, relative to proteins from the entire cell lysate, were obtained for CCMV CP and all chimeric derivatives. A straightforward protocol was used that, without the use of purification columns, successfully enabled CCMV CP protein solubilization, reassembly and subsequent collection of CCMV CP VLPs. While insertions of His-tag or M2e (7-23 aa) into the predicted external loop structures did abolish VLP formation, high yields of VLPs were obtained with all fusions of His-tag or various epitopes (13- 27 aa) from IAV and FMDV at the N- or C-terminal ends of CCMV CP or N∆24-CP. VLPs derived from CCMV CP still encapsulated RNA, while those from CCMV CP-chimera containing a negatively charged N-terminal domain had lost this ability. The usefulness and rapid ease of exploitation of CCMV VLPs for the production of potential subunit vaccines was demonstrated with the synthesis of chimeric CCMV VLPs containing selected sequences from the G_N and G_C glycoproteins of the recently emerged Schmallenberg orthobunyavirus at both termini of the CP protein.

Conclusions

CCMV VLPs can be successfully exploited as scaffold for epitope fusions up to 31 aa at the N- and C-terminus, and at a N-terminal 24 amino acid (aa) deletion mutant (N∆24-CP) of the CP protein.

Plant Viruses as Nanoparticle-Based Vaccines and Adjuvants

Vaccines are considered one of the greatest medical achievements in the battle against infectious diseases. However, the intractability of various diseases such as hepatitis C, HIV/AIDS, malaria, tuberculosis, and cancer poses persistent hurdles given that traditional vaccine-development methods have proven to be ineffective; as such, these challenges have driven the emergence of novel vaccine design approaches. In this regard, much effort has been put into the development of new safe adjuvants and vaccine platforms. Of particular interest, the utilization of plant virus-like nanoparticles and recombinant plant viruses has gained increasing significance as an effective tool in the development of novel vaccines against infectious diseases and cancer. The present review summarizes recent advances in the use of plant viruses as nanoparticle-based vaccines and adjuvants and their mechanism of action. Harnessing plant-virus immunogenic properties will enable the design of novel, safe, and efficacious prophylactic and therapeutic vaccines against disease.

Presentation of epitope sequences from foreign viruses on the surface of apple latent spherical virus particles

Apple latent spherical virus (ALSV) has small isometric particles that are comprised of two single-stranded RNA species (RNA1 and RNA2) and three capsid proteins (Vp25, Vp20, and Vp24). We constructed ALSV vectors for presenting foreign peptides on the surface of virus particles. In these vectors, peptides can be fused to either of two C-terminal regions of Vp20 (amino acid positions between G171 and P172 or between P172 and L173) or the C-terminus (T192) of Vp24. An ALSV vector presenting the epitope sequences of the coat protein (CP) of zucchini yellow mosaic virus (ZYMV) could systemically infect host plants and was specifically recognized by antiserum against ZYMV by ELISA, immunoelectron microscopy, and immunoblotting. RT-PCR showed that the epitope sequences up to 20 amino acids were stably maintained in the chimeric ALSV for more than 10 serial passages and at least six months. Purified chimeric ALSV particles induced an immune response and the production of antibodies against ZYMV-CP in rabbits. The ALSV vector was also used for expression of an epitope from VP1 of foot-and-mouth disease virus.

Development of cowpea mosaic virus-based vectors for the production of vaccines in plants

Plant viruses are emerging as an attractive alternative to stable genetic transformation for the expression of foreign proteins in plants. The main advantages of using this strategy are that viral genomes are small and easy to manipulate, infection of plants with modified viruses is simpler and quicker than the regeneration of stably transformed plants and the sequence inserted into a virus vector will be highly amplified. One use of these virus expression systems is for vaccine production. Among plant viruses, cowpea mosaic virus makes an ideal candidate for the production of such vaccines because it grows extremely well in host plants, is very stable, and the purification of virus particles, if required, is straightforward. In this article, the authors review the progress made in the development of cowpea mosaic virus-based vectors for vaccine production, making use of two main approaches: epitope presentation and polypeptide expression.

Genetic engineering and characterization of Cowpea mosaic virus empty virus-like particles

The development of methods for the production of empty Cowpea mosaic virus (CPMV) virus-like particles (VLPs) that are devoid of RNA, eVLPs, has renewed promise in CPMV capsid technologies. The recombinant nature of CPMV eVLP production means that the extent and variety of genetic modifications that may be incorporated into the particles is theoretically much greater than those that can be made to infectious CPMV virions due to restrictions on viral propagation of the latter. Free of the infectious agent, the genomic RNA, these particles are now finding potential uses in vaccine development, in vivo imaging, drug delivery, and other nanotechnology applications that make use of internal loading of the empty particles. Here we describe methods for the genetic modification and production of CPMV eVLPs and describe techniques useful for their characterization.

Structure-based design and experimental engineering of a plant virus nanoparticle for the presentation of immunogenic epitopes and as a drug carrier

Biomaterials research for the discovery of new generation nanoparticles is one of the most active areas of nanotechnology. In the search of nature-made nanometer-sized objects, plant virus particles appear as symmetrically defined entities that can be formed by protein self-assembly. In particular, in the field of plant virology, there is plenty of literature available describing the exploitation of plant viral cages to produce safe vaccine vehicles and nanoparticles for drug delivery. In this context, we have investigated on the use of the artichoke mottled crinkle virus (AMCV) capsid both as a carrier of immunogenic epitopes and for the delivery of anticancer molecules. A dual approach that combines both in silico tools and experimental virology was applied for the rational design of immunologically active chimeric virus-like particles (VLPs) carrying immunogenic peptides. The atomic structures of wild type (wt) and chimeric VLPs were obtained by homology modeling. The effects of insertion of the HIV-1 2F5 neutralizing epitope on the structural stability of chimeric VLPs were predicted and assessed by detailed inspection of the nanoparticle intersubunit interactions at atomic level. Wt and chimeric VLPs, exposing on their surface the 2F5 epitope, were successfully produced in plants. In addition, we demonstrated that AMCV capsids could also function as drug delivery vehicles able to load the chemotherapeutic drug doxorubicin. To our knowledge, this is the first systematic predictive and empirical research addressing the question of how this icosahedral virus can be used for the production of both VLPs and viral nanoparticles for biomedical applications.

Protein nanotubes, channels and cages

Proteins are the work-horses of life and excute the essential processes involved in the growth and repair of cells. These roles include all aspects of cell signalling, metabolism and repair that allow living things to exist. They are not only chemical catalysts and machine components, they are also structural components of the cell or organism, capable of self-organisation into strong supramolecular cages, fibres and meshes. How proteins are encoded genetically and how they are sythesised in vivo is now well understood, and for an increasing number of proteins, the relationship between structure and function is known in exquisite detail. The next challenge in bionanoscience is to adapt useful protein systems to build new functional structures. Well-defined natural structures with potential useful shapes are a good starting point. With this in mind, in this chapter we discuss the properties of natural and artificial protein channels, nanotubes and cages with regard to recent progress and potential future applications. Chemistries for attaching together different proteins to form superstructures are considered as well as the difficulties associated with designing complex protein structures ab initio.

Virus-based nanocarriers for drug delivery

New nanocarrier platforms based on natural biological building blocks offer great promises in revolutionalizing medicine. The usage of specific protein cage structures: virus-like particles (VLPs) for drug packaging and targetted delivery is summarized here. Versatile chemical and genetic modifications on the outer surfaces and inner cavities of VLPs facilitate the preparation of new materials that could meet the biocompatibility, solubility and high uptake efficiency requirements for drug delivery. A full evaluation on the toxicity, bio-distribution and immunology of these materials are envisaged to boost their application potentials.

Induction of protective immunity in chickens immunized with plant-made chimeric Bamboo mosaic virus particles expressing very virulent Infectious bursal disease virus antigen

Very virulent Infectious bursal disease virus (vvIBDV) causes a highly contagious disease in young chickens and leads to significant economic loss in the poultry industry. Effective new vaccines are urgently needed. Autonomously replicating plant virus-based vector provides attractive means for producing chimeric virus particles (CVPs) in plants that can be developed into vaccines. In this study, we demonstrate the potential for vaccine development of Bamboo mosaic virus (BaMV) epitope-presentation system, where the antigen from vvIBDV VP2 was fused to the N-terminus of BaMV coat protein. Accordingly, an infections plasmid, pBIBD2, was constructed. Inoculation of the recombinant BaMV clone pBIBD2 enabled the generation of chimeric virus, BIBD2, and stable expression of IBDV VP2 antigen on its coat protein. After intramuscular immunization with BIBD2 CVPs, chickens produced antibodies against IBDV and were protected from vvIBDV (V263/TW strain) challenges. These results corroborate the feasibility of BaMV-based CVP platform in plants for the development and production of vaccines against IBDV.

Maize rayado fino virus virus-like particles expressed in tobacco plants: A new platform for cysteine selective bioconjugation peptide display

Maize rayado fino virus (MRFV) virus-like-particles (VLPs) produced in tobacco plants were examined for their ability to serve as a novel platform to which a variety of peptides can be covalently displayed when expressed through a Potato virus X (PVX)-based vector. To provide an anchor for chemical modifications, three Cys-MRFV-VLPs mutants were created by substituting several of the amino acids present on the shell of the wild-type MRFV-VLPs with cysteine residues. The mutant designated Cys 2-VLPs exhibited, under native conditions, cysteine thiol reactivity in bioconjugation reactions with a fluorescent dye. In addition, this Cys 2-VLPs was cross-linked by NHS-PEG4-Maleimide to 17 (F) and 8 (HN) amino acid long peptides, corresponding to neutralizing epitopes of Newcastle disease virus (NDV). The resulting Cys 2-VLPs-F and Cys 2-VLPs-HN were recognized in Western blots by antibodies to MRFV as well as to F and HN. The results demonstrated that plant-produced MRFV-VLPs have the ability to function as a novel platform for the multivalent display of surface ligands.

Applications of plant viruses in bionanotechnology

The capsids of most plant viruses are simple and robust structures consisting of multiple copies of one or a few types of protein subunit arranged with either icosahedral or helical symmetry. In many cases, capsids can be produced in large quantities either by the infection of plants or by the expression of the subunit(s) in a variety of heterologous systems. In view of their relative simplicity, stability and ease of production, plant virus particles or virus-like particles (VLPs) have attracted attention as potential reagents for applications in bionanotechnology. As a result, plant virus particles have been subjected to both genetic and chemical modification, have been used to encapsulate foreign material and have, themselves, been incorporated into supramolecular structures.

Immunogenic properties of chimeric potato virus X particles displaying the hepatitis C virus hypervariable region I peptide R9

The immunogenic properties of chimeric potato virus X (PVX) particles engineered to display the synthetic R9 peptide have been evaluated. The R9 peptide is a consensus sequence derived from diverse variants of the hypervariable region 1 from the hepatitis C virus (HCV) envelope protein E2. Two different constructs were designed, with the R9 peptide expressed either as an indirect fusion via the ribosomal skip 2A (PVX(R9-2A)CP) sequence or as a direct PVX coat protein fusion (PVX(R9)CP). Systemic infection of Nicotiana benthamiana plants was only achieved with PVX(R9-2A)CP constructs, and the presence of the R9 peptide was detected in extracts from these plants by ELISA, Western blot and electron microscopy using specific anti-R9 antibodies. The virus particles were recovered at yields of up to 125mg/kg from leaf material. BALB/c mice immunized with purified PVX(R9-2A)CP particles developed specific anti-R9 IgG titers of up to 1:50,000. Monoclonal anti-R9 antibodies were obtained from the spleen of a mouse immunized with PVX(R9-2A)CP particles and characterized by Western blot and electron microscopy. Sera from patients infected chronically with HCV were found to react specifically with PVX(R9-2A)CP particles in 35% of cases.

Development of Tobacco necrosis virus A as a vector for efficient and stable expression of FMDV VP1 peptides

Plant virus-based expression systems provide attractive alternatives for production of animal virus-originated antigenic peptides. In the present study, an infectious cDNA clone of Tobacco necrosis virus A Chinese isolate (TNV-A(C)) was used for expression of different peptides derived from Foot and mouth disease virus (FMDV) serotype O VP1 fused downstream of the coat protein (CP) open reading frame (ORF). Chenopodium amaranticolor inoculated with in vitro transcripts of the chimaeras developed symptoms similar to those caused by wild-type TNV-A(C). Western blot and RT-PCR detection of the infected leaves demonstrated that the chimaeras were infective, and a large number of self-assembled virions could be purified and observed under electron microscopy. Immunogold labelling revealed that highly expressed FMDV VP1 peptides could be displayed on the surfaces of virus particles. Additional immunoblotting and DNA sequence analyses showed that most of the chimaeras contained unmodified foreign peptides even after six successive passages in C. amaranticolor and three passages in Nicotiana benthamiana. Our results also suggest that the amino acid sequence and peptide length have a substantial influence on viral morphogenesis and systemic infections. Finally, animal experiments showed that purified chimaeric virus particles (CVPs) could induce a strong immune response against FMDV structural protein VP1 via an intramuscular route. And when inoculated nasally, CVPs could induce systemic and mucosal immune responses in mice.

PEGylated viral nanoparticles for biomedicine: the impact of PEG chain length on VNP cell interactions in vitro and ex vivo

PEGylation is an effective strategy for reducing biospecific interactions for pharmaceuticals. The plant virus Cowpea mosaic virus (CPMV) has been studied for potential nanobiomedical applications by virtue of its natural interactions with mammalian endothelial cells. To investigate the degree of PEGylation required to retarget CPMV-based formulations to other destinations, two CPMV-PEG formulations, CPMV-PEG1000 (P1) and CPMV-PEG2000 (P2) were tested. Modeling suggested that the PEG chains were displayed as flattened mushrooms on the particle with an estimated surface grafting area of 0.53% for P1 and 0.83% for P2. Only the P2 formulation effectively shielded the particles from interacting with cells or tissues, suggesting that either key interacting regions on the particle surface were blocked or that a sufficient hydration shell had been generated to inhibit cellular interactions. The large CPMV surface area available after PEGylation allows further attachment of imaging and therapeutic molecules to the particle to generate multifunctionality.

Cowpea mosaic virus: the plant virus-based biotechnology workhorse

In the 50 years since it was first described, Cowpea mosaic virus (CPMV) has become one of the most intensely studied plant viruses. Research in the past 15 to 20 years has shifted from studying the underlying genetics and structure of the virus to focusing on ways in which it can be exploited in biotechnology. This work led first to the use of virus particles to present peptides, then to the creation of a variety of replicating virus vectors and finally to the development of a highly efficient protein expression system that does not require viral replication. The circle has been completed by the use of the latter system to create empty particles for peptide presentation and other novel uses. The history of CPMV in biotechnology can be likened to an Ouroborus, an ancient symbol depicting a snake or dragon swallowing its own tail, thus forming a circle.

Interaction of Cowpea mosaic virus (CPMV) nanoparticles with antigen presenting cells in vitro and in vivo

Background

Plant viruses such as Cowpea mosaic virus (CPMV) are increasingly being developed for applications in nanobiotechnology including vaccine development because of their potential for producing large quantities of antigenic material in plant hosts. In order to improve efficacy of viral nanoparticles in these types of roles, an investigation of the individual cell types that interact with the particles is critical. In particular, it is important to understand the interactions of a potential vaccine with antigen presenting cells (APCs) of the immune system. CPMV was previously shown to interact with vimentin displayed on cell surfaces to mediate cell entry, but the expression of surface vimentin on APCs has not been characterized.

Methodology

The binding and internalization of CPMV by several populations of APCs was investigated both in vitro and in vivo by flow cytometry and fluorescence confocal microscopy. The association of the particles with mouse gastrointestinal epithelium and Peyer's patches was also examined by confocal microscopy. The expression of surface vimentin on APCs was also measured.

Conclusions

We found that CPMV is bound and internalized by subsets of several populations of APCs both in vitro and in vivo following intravenous, intraperitoneal, and oral administration, and also by cells isolated from the Peyer's patch following gastrointestinal delivery. Surface vimentin was also expressed on APC populations that could internalize CPMV. These experiments demonstrate that APCs capture CPMV particles in vivo, and that further tuning the interaction with surface vimentin may facilitate increased uptake by APCs and priming of antibody responses. These studies also indicate that CPMV particles likely access the systemic circulation following oral delivery via the Peyer's patch.

Plant production of veterinary vaccines and therapeutics

Plant-derived biologicals for use in animal health are becoming an increasingly important target for research into alternative, improved methods for disease control. Although there are no commercial products on the market yet, the development and testing of oral, plant-based vaccines is now beyond the proof-of-principle stage. Vaccines, such as those developed for porcine transmissible gastroenteritis virus, have the potential to stimulate both mucosal and systemic, as well as, lactogenic immunity as has already been seen in target animal trials. Plants are a promising production system, but they must compete with existing vaccines and protein production platforms. In addition, regulatory hurdles will need to be overcome, and industry and public acceptance of the technology are important in establishing successful products.

Monoclonal antibodies specific to Cucumber mosaic virus coat protein possess DNA-hydrolyzing activity

Monoclonal antibodies (mAbs) specific to Cucumber mosaic virus coat protein (CMV-CP) were designed from cDNA and deduced amino acid sequences of the light chain genes of 10 out of 14 different hybridoma cell lines. Ten of these mAbs revealed a very restricted germline family VkappaII, within which gene bd2 has identical amino acid sequences with VIPase and an i41SL 1-2 catalytic antibody light chain, both of which possess peptidase activity. Four out of the 14 mAbs illustrated another germline family VkappaIA, within which gene bb1.1 had high homology with BV04-01 light chain mAb, which hydrolyses ssDNA. Interestingly, our mAbs showed DNA-hydrolytic activity at an optimum pH of 4-5, which is a typical pattern of autoimmune diseases in which autoantibodies hydrolyze supercoiled plasmid DNA. This is the first evidence ever that CMV-CP could stimulate catalytic antibodies, which have an identical sequence homology with autoantibodies. Furthermore, the CMV-CP-specific mAbs will be important for isolating antibodies specific to the CPs of bacteria, viruses, cancer cells, etc. that could be used for medical therapy.