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

Delivery of progeny virus from the infectious clone of cucumber green mottle mosaic virus and quantification of the viral load in different host plants

Cucumber green mottle mosaic virus (CGMMV, genus Tobamovirus) is a widely occurring tobamovirus in cucurbits. The genome of CGMMV has been used previously for the expression of foreign genes in the plant. High throughput delivery and high viral titer are important requirements of foreign protein expression in plant through virus genome-based vector, in this study, Agrobacterium containing infectious construct of CGMMV was infiltrated through syringe, vacuum and high-speed spray to N. benthamiana, cucumber and bottle gourd leaves. The success rate of systemic infection of CGMMV agro-construct through all three methods was higher (80-100%) in N. benthamiana compared to the cucurbits (40-73.3%). To determine the high-throughput delivery of CGMMV in the plant system, four delivery methods viz. rubbing, syringe infiltration, vacuum infiltration and high-speed spray using the progeny virus derived through CGMMV agro-construct were compared in the three different plant species. Based on the rate of systemic infection and time required to perform delivery by different methods, vacuum infiltration was found most efficient for the high-throughput delivery of CGMMV. The quantification of CGMMV through qPCR revealed that CGMMV load varied considerably in leaf and fruit tissues depending with the time of infection. Immediately after expression of symptoms, a high load of CGMMV (~ 1 µg/100 mg of tissues) was noticed in young leaves of N. benthamiana and cucumber. In bottle gourd leaves, the CGMMV load was far low compared to N. benthamiana and cucumber plants. In the fruit tissues of cucumber and bottle gourd higher virus load was observed in mature fruit but not in immature fruit. The findings of the present study will serve as an important base line information to produce foreign protein through CGMMV genome-vector.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-023-03630-y.

Build a Sustainable Vaccines Industry with Synthetic Biology

The vaccines industry has not changed appreciably in decades regarding technology, and has struggled to remain viable, with large companies withdrawing from production. Meanwhile, there has been no let-up in outbreaks of viral disease, at a time when the biopharmaceuticals industry is discussing downsizing. The distributed manufacturing model aligns well with this, and the advent of synthetic biology promises much in terms of vaccine design. Biofoundries separate design from manufacturing, a hallmark of modern engineering. Once designed in a biofoundry, digital code can be transferred to a small-scale manufacturing facility close to the point of care, rather than physically transferring cold-chain-dependent vaccine. Thus, biofoundries and distributed manufacturing have the potential to open up a new era of biomanufacturing, one based on digital biology and information systems. This seems a better model for tackling future outbreaks and pandemics.

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.

Heterologous Prime-Boost Enhances the Antitumor Immune Response Elicited by Plant-Virus-Based Cancer Vaccine

New cancer vaccine strategies are required to vanquish the self-tolerance and elicit robust immune responses against tumor-associated antigens and/or neoantigens. Contemporary approaches in nanomedicine center on the use of a single nanocarrier modified with multiple copies of multiple different functional domains, e.g., epitopes for vaccines. Therefore, we set out to develop a combinatorial approach toward the next-generation concept of epitope delivery: a prime-boost strategy in which the same epitope is delivered using different nanocarriers. We tested this concept in the setting of HER2⁺ breast cancer. We synthesized HER2-based cancer vaccines using three icosahedral plant viruses as carriers and evaluated the immune response as a result of repetitive, homologous immunization using BALB/c mice. Two of the vaccines induced a Th2-predominant response and the other a Th1-predominant response. To enhance the immunogenicity of the vaccines, we developed a heterologous prime-boost strategy with each of the vaccines administered only once, yielding higher titers of HER2-specific immunoglobulins and increasing the toxicity of the antisera toward cancer cells. The prime-boost also induced a Th1-predominant response. An in vivo tumor challenge showed that the prime-boost regimen reduced tumor growth and improved survival in mice. This novel strategy to elicit robust immune responses against weakly immunogenic antigens in principle could be broadly applicable to cancers and other diseases.

Development of a GFP expression vector for Cucurbit chlorotic yellows virus

Background

Cucurbit chlorotic yellows virus (CCYV), a bipartite crinivirus, causes chlorotic leaf spots and yellowing symptoms on cucurbit leaves. We previously developed an infectious clone of CCYV. Limited work has been conducted on the construction of a crinivirus green fluorescence protein (GFP) expression vector to date.

Finding

We constructed a CCYV GFP expression vector using the “add a gene” strategy based on CCYV RNA2 cDNA constrcut. Three resultant clones, pCCYVGFP_SGC, pCCYVGFP_CGC, and pCCYVGFP_CGS, were constructed with different promoters used to initiate GFP and CP expression. At 25 dpi GFP fluorescence was detectable not only in leaf veins but also in the surrounding cells. pCCYVGFP_CGC-infected cucumber leaves exhibited cell spread at 25 dpi, whereas pCCYVGFP_SGC and pCCYVGFP_CGS were mainly found in single cells. Further observation of pCCYVGFP_CGC GFP expression at 30 dpi, 40 dpi, and 50 dpi showed phloem-limited localization in the systemic leaves.

Conclusions

We developed of a CCYV GFP expression vector that will be useful for further study of CCYV movement in cucurbits.

In Planta Production of Fluorescent Filamentous Plant Virus-Based Nanoparticles

Viral nanoparticles are attractive platforms for biomedical applications and are frequently employed for optical imaging in tissue culture and preclinical animal models as fluorescent probes. Chemical modification with organic dyes remains the most common strategy to develop such fluorescent probes. Here we report a genetic engineering approach to incorporate fluorescent proteins in viral nanoparticles, which can be propagated in their plant host. The fluorescent viral nanoparticles so obtained obviate post-harvest modifications and thereby maximize yields. Our engineering approach transforms filamentous potato virus X (PVX) to display green fluorescent protein (GFP) or mCherry as N-terminal coat protein (CP) fusions at a 1:3 fusion protein to CP ratio through integration of the foot-and-mouth disease 2A sequence. The in planta propagation of recombinant GFP-PVX or mCherry-PVX thus produced in Nicotiana benthamiana can be easily documented using fluorescence imaging. Molecular farming protocols can be accordingly optimized by monitoring chimera stability over the course of the infection cycle. Moreover, we also demonstrate the utility of recombinant mCherry-PVX in optical imaging of human cancer cells and tumor tissue in preclinical mice model. Together, these features make genetically engineered fluorescent PVX particles ideally suited for molecular imaging applications.

Disease Prevention: An Opportunity to Expand Edible Plant-Based Vaccines?

The lethality of infectious diseases has decreased due to the implementation of crucial sanitary procedures such as vaccination. However, the resurgence of pathogenic diseases in different parts of the world has revealed the importance of identifying novel, rapid, and concrete solutions for control and prevention. Edible vaccines pose an interesting alternative that could overcome some of the constraints of traditional vaccines. The term “edible vaccine” refers to the use of edible parts of a plant that has been genetically modified to produce specific components of a particular pathogen to generate protection against a disease. The aim of this review is to present and critically examine “edible vaccines” as an option for global immunization against pathogenic diseases and their outbreaks and to discuss the necessary steps for their production and control and the list of plants that may already be used as edible vaccines. Additionally, this review discusses the required standards and ethical regulations as well as the advantages and disadvantages associated with this powerful biotechnology tool.

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.

Plant Molecular Farming: Much More than Medicines

Plants have emerged as commercially relevant production systems for pharmaceutical and nonpharmaceutical products. Currently, the commercially available nonpharmaceutical products outnumber the medical products of plant molecular farming, reflecting the shorter development times and lower regulatory burden of the former. Nonpharmaceutical products benefit more from the low costs and greater scalability of plant production systems without incurring the high costs associated with downstream processing and purification of pharmaceuticals. In this review, we explore the areas where plant-based manufacturing can make the greatest impact, focusing on commercialized products such as antibodies, enzymes, and growth factors that are used as research-grade or diagnostic reagents, cosmetic ingredients, and biosensors or biocatalysts. An outlook is provided on high-volume, low-margin proteins such as industrial enzymes that can be applied as crude extracts or unprocessed plant tissues in the feed, biofuel, and papermaking industries.

Identification of the subgenomic promoter of the coat protein gene of cucumber fruit mottle mosaic virus and development of a heterologous expression vector

Heterologous gene expression using plant virus vectors enables research on host-virus interactions and the production of useful proteins, but the host range of plant viruses limits the practical applications of such vectors. Here, we aimed to develop a viral vector based on cucumber fruit mottle mosaic virus (CFMMV), a member of the genus Tobamovirus, whose members infect cucurbits. The subgenomic promoter (SGP) in the coat protein (CP) gene, which was used to drive heterologous expression, was mapped by analyzing deletion mutants from a CaMV 35S promoter-driven infectious CFMMV clone. The region from nucleotides (nt) -55 to +160 relative to the start codon of the open reading frame (ORF) of CP was found to be a fully active promoter, and the region from nt -55 to +100 was identified as the active core promoter. Based on these SGPs, we constructed a cloning site in the CFMMV vector and successfully expressed enhanced green fluorescent protein (EGFP) in Nicotiana benthamiana and watermelon (Citrullus lanatus). Co-inoculation with the P19 suppressor increased EGFP expression and viral replication by blocking degradation of the viral genome. Our CFMMV vector will be useful as an expression vector in cucurbits.

Molecular pharming - VLPs made in plants

Plant-based expression offers a safe, inexpensive and potentially limitless way to produce therapeutics in a quick and flexible manner. Plants require only simple inorganic nutrients, water, carbon dioxide and sunlight for efficient growth. Virus-like particles (VLPs) are convincing look-alikes of viruses but without carrying infectious genomic material. However, they can still elicit a very potent immune response which makes them ideal vaccine candidates. In this review the different methods of plant expression are described together with the most recent developments in the field of transiently-expressed plant-made VLPs.

Mechanisms of assembly and genome packaging in an RNA virus revealed by high-resolution cryo-EM

Cowpea mosaic virus is a plant-infecting member of the Picornavirales and is of major interest in the development of biotechnology applications. Despite the availability of >100 crystal structures of Picornavirales capsids, relatively little is known about the mechanisms of capsid assembly and genome encapsidation. Here we have determined cryo-electron microscopy reconstructions for the wild-type virus and an empty virus-like particle, to 3.4 Å and 3.0 Å resolution, respectively, and built de novo atomic models of their capsids. These new structures reveal the C-terminal region of the small coat protein subunit, which is essential for virus assembly and which was missing from previously determined crystal structures, as well as residues that bind to the viral genome. These observations allow us to develop a new model for genome encapsidation and capsid assembly.

Little is known about how the plant-infecting cowpea mosaic virus (CPMV)—an invaluable tool in several biotechnology applications—packages its single-strand RNA genome into the capsid. Here the authors present two high-resolution cryo-EM structures of CPMV and a new model for RNA recognition and capsid assembly.

Current status of viral expression systems in plants and perspectives for oral vaccines development

During the last 25 years, the technology to produce recombinant vaccines in plant cells has evolved from modest proofs of the concept to viable technologies adopted by some companies due to significant improvements in the field. Viral-based expression strategies have importantly contributed to this success owing to high yields, short production time (which is in most cases free of tissue culture steps), and the implementation of confined processes for production under GMPs. Herein the distinct expression systems based on viral elements are analyzed. This review also presents the outlook on how these technologies have been successfully applied to the development of plant-based vaccines, some of them being in advanced stages of development. Perspectives on how viral expression systems could allow for the development of innovative oral vaccines constituted by minimally-processed plant biomass are discussed.

Plant–derived vaccines: a look back at the highlights and a view to the challenges on the road ahead

The sobering reality is that each year, 33 million children remain unvaccinated for vaccine-preventable diseases. Universal childhood vaccination would have profound effects on leveling the health inequities in many parts of the world. As an alternative to administration of vaccines by needle and syringe, oral vaccines offer significant logistical advantages, as the polio eradication campaign has demonstrated. Over the past decade, the expression of subunit vaccine antigens in plants has emerged as a convenient, safe and potentially economical platform technology, with the potential to provide a novel biotechnological solution to vaccine production and delivery. As this technology has come of age, many improvements have been made on several fronts, as a growing number of research groups worldwide have extensively investigated plants as factories for vaccine production. This review attempts to highlight some of the achievements over the past 15 years, identify some of the potential problems and discuss the promises that this technology could fulfill.

Plant molecular pharming for the treatment of chronic and infectious diseases

Plant molecular pharming has emerged as a niche technology for the manufacture of pharmaceutical products indicated for chronic and infectious diseases, particularly for products that do not fit into the current industry-favored model of fermenter-based production campaigns. In this review, we explore the areas where molecular pharming can make the greatest impact, including the production of pharmaceuticals that have novel glycan structures or that cannot be produced efficiently in microbes or mammalian cells because they are insoluble or toxic. We also explore the market dynamics that encourage the use of molecular pharming, particularly for pharmaceuticals that are required in small amounts (such as personalized medicines) or large amounts (on a multi-ton scale, such as blood products and microbicides) and those that are needed in response to emergency situations (pandemics and bioterrorism). The impact of molecular pharming will increase as the platforms become standardized and optimized through adoption of good manufacturing practice (GMP) standards for clinical development, offering a new opportunity to produce inexpensive medicines in regional markets that are typically excluded under current business models.

Exploiting plant virus-derived components to achieve in planta expression and for templates for synthetic biology applications

This review discusses the varying roles that have been played by many plant-viral regulatory sequences and proteins in the creation of plant-based expression systems and virus particles for use in nanotechnology. Essentially, there are two ways of expressing an exogenous protein: the creation of transgenic plants possessing a stably integrated gene construction, or the transient expression of the desired gene following the infiltration of the gene construct. Both depend on disarmed strains of Agrobacterium tumefaciens to deliver the created gene construction into cell nuclei, usually through the deployment of virus-derived components. The importance of efficient mRNA translation in the latter process is highlighted. Plant viruses replicate to sustain an infection to promote their survival. The major product of this, the virus particle, is finding increasing roles in the emerging field of bionanotechnology. One of the major products of plant-viral expression is the virus-like particle (VLP). These are increasingly playing a role in vaccine development. Similarly, many VLPs are suitable for the investigation of the many facets of the emerging field of synthetic biology, which encompasses the design and construction of new biological functions and systems not found in nature. Genetic and chemical modifications to plant-generated VLPs serve as ideal starter templates for many downstream synthetic biology applications.

Plant virus expression vectors set the stage as production platforms for biopharmaceutical proteins

Transgenic plants present enormous potential as a cost-effective and safe platform for large-scale production of vaccines and other therapeutic proteins. A number of different technologies are under development for the production of pharmaceutical proteins from plant tissues. One method used to express high levels of protein in plants involves the employment of plant virus expression vectors. Plant virus vectors have been designed to carry vaccine epitopes as well as full therapeutic proteins such as monoclonal antibodies in plant tissue both safely and effectively. Biopharmaceuticals such as these offer enormous potential on many levels, from providing relief to those who have little access to modern medicine, to playing an active role in the battle against cancer. This review describes the current design and status of plant virus expression vectors used as production platforms for biopharmaceutical proteins.

Plant viral vectors for delivery by Agrobacterium

Plant viral vectors delivered by Agrobacterium are the basis of several manufacturing processes that are currently in use for producing a wide range of proteins for multiple applications, including vaccine antigens, antibodies, protein nanoparticles such as virus-like particles (VLPs), and other protein and protein-RNA scaffolds. Viral vectors delivered by agrobacterial T-DNA transfer (magnifection) have also become important tools in research. In recent years, essential advances have been made both in the development of second-generation vectors designed using the 'deconstructed virus' approach, as well as in the development of upstream manufacturing processes that are robust and fully scalable. The strategy relies on Agrobacterium as a vector to deliver DNA copies of one or more viral RNA/DNA replicons; the bacteria are delivered into leaves by vacuum infiltration, and the viral machinery takes over from the point of T-DNA transfer to the plant cell nucleus, driving massive RNA and protein production and, if required, cell-to-cell spread of the replicons. Among the most often used viral backbones are those of the RNA viruses Tobacco mosaic virus (TMV), Potato virus X (PVX) and Cowpea mosaic virus (CPMV), and the DNA geminivirus Bean yellow dwarf virus. Prototypes of industrial processes that provide for high yield, rapid scale up and fast manufacturing cycles have been designed, and several GMP-compliant and GMP-certified manufacturing facilities are in place. These efforts have been successful as evidenced by the fact that several antibodies and vaccine antigens produced by magnifection are currently in clinical development.

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.

Development of expression vectors based on pepino mosaic virus

BACKGROUND

Plant viruses are useful expression vectors because they can mount systemic infections allowing large amounts of recombinant protein to be produced rapidly in differentiated plant tissues. Pepino mosaic virus (PepMV) (genus Potexvirus, family Flexiviridae), a widespread plant virus, is a promising candidate expression vector for plants because of its high level of accumulation in its hosts and the absence of severe infection symptoms. We report here the construction of a stable and efficient expression vector for plants based on PepMV.

RESULTS

Agroinfectious clones were produced from two different PepMV genotypes (European and Chilean), and these were able to initiate typical PepMV infections. We explored several strategies for vector development including coat protein (CP) replacement, duplication of the CP subgenomic promoter (SGP) and the creation of a fusion protein using the foot-and-mouth disease virus (FMDV) 2A catalytic peptide. We found that CP replacement vectors were unable to move systemically and that vectors with duplicated SGPs (even heterologous SGPs) suffered from significant transgene instability. The fusion protein incorporating the FMDV 2A catalytic peptide gave by far the best results, maintaining stability through serial passages and allowing the accumulation of GFP to 0.2-0.4 g per kg of leaf tissue. The possible use of PepMV as a virus-induced gene silencing vector to study gene function was also demonstrated. Protocols for the use of this vector are described.

CONCLUSIONS

A stable PepMV vector was generated by expressing the transgene as a CP fusion using the sequence encoding the foot-and-mouth disease virus (FMDV) 2A catalytic peptide to separate them. We have generated a novel tool for the expression of recombinant proteins in plants and for the functional analysis of virus and plant genes. Our experiments have also highlighted virus requirements for replication in single cells as well as intercellular and long-distance movement.

Plant-made vaccines for humans and animals

The concept of using plants to produce high-value pharmaceuticals such as vaccines is 20 years old this year and is only now on the brink of realisation as an established technology. The original reliance on transgenic plants has largely given way to transient expression; proofs of concept for human and animal vaccines and of efficacy for animal vaccines have been established; several plant-produced vaccines have been through Phase I clinical trials in humans and more are scheduled; regulatory requirements are more clear than ever, and more facilities exist for manufacture of clinic-grade materials. The original concept of cheap edible vaccines has given way to a realisation that formulated products are required, which may well be injectable. The technology has proven its worth as a means of cheap, easily scalable production of materials: it now needs to find its niche in competition with established technologies. The realised achievements in the field as well as promising new developments will be reviewed, such as rapid-response vaccines for emerging viruses with pandemic potential and bioterror agents.

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.

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.

Endothelial targeting of cowpea mosaic virus (CPMV) via surface vimentin

Cowpea mosaic virus (CPMV) is a plant comovirus in the picornavirus superfamily, and is used for a wide variety of biomedical and material science applications. Although its replication is restricted to plants, CPMV binds to and enters mammalian cells, including endothelial cells and particularly tumor neovascular endothelium in vivo. This natural capacity has lead to the use of CPMV as a sensor for intravital imaging of vascular development. Binding of CPMV to endothelial cells occurs via interaction with a 54 kD cell-surface protein, but this protein has not previously been identified. Here we identify the CPMV binding protein as a cell-surface form of the intermediate filament vimentin. The CPMV-vimentin interaction was established using proteomic screens and confirmed by direct interaction of CPMV with purified vimentin, as well as inhibition in a vimentin-knockout cell line. Vimentin and CPMV were also co-localized in vascular endothelium of mouse and rat in vivo. Together these studies indicate that surface vimentin mediates binding and may lead to internalization of CPMV in vivo, establishing surface vimentin as an important vascular endothelial ligand for nanoparticle targeting to tumors. These results also establish vimentin as a ligand for picornaviruses in both the plant and animal kingdoms of life. Since bacterial pathogens and several other classes of viruses also bind to surface vimentin, these studies suggest a common role for surface vimentin in pathogen transmission.

Cowpea mosaic virus-based systems for the expression of antigens and antibodies in plants

This chapter describes the use of Cowpea mosaic virus-based vectors for the production of foreign proteins such as antigens and antibodies in plants. The systems include vectors based on both full-length and deleted versions of RNA-2. In both cases, the modified RNA-2 is replicated by coinoculation with RNA-1. The constructs based on full-length RNA-2 retain the ability to spread systemically throughout an inoculated plant and the infection can be passaged. The vector based on a deleted version of RNA-2 can stably incorporate larger inserts but lacks the ability to move systemically. However, it has the added advantage of biocontainment. In both cases, vector constructs modified to contain a foreign gene of interest can be delivered by agroinfiltration to obtain transient expression of the foreign protein. If required, the same constructs can also be used for stable nuclear transformation. Both types of vector have proved effective for the production in plants of a diverse range of proteins including antigens and antibodies.

Cowpea Mosaic Virus

Cowpea mosaic virus (CPMV) is the type member of the genus Comovirus, which comprises one of three genera of the family Comoviridae. The genome of CPMV consists of two molecules of positive-strand RNA, which are separately encapsidated in isometric particles consisting of 60 copies each of two types of coat protein. Since its initial isolation, CPMV has been extensively studied from both a genetic and structural point of view and has become a paradigm for plant viruses that express their genome through the synthesis and processing of precursor polyproteins. In this regard, as well as in its particle structure, CPMV resembles other members of the family Picornaviridae. More recently, the virus has been used for a number of biotechnological applications.

Improving containment strategies in biopharming

This review examines the challenges of segregating biopharmed crops expressing pharmaceutical or veterinary agents from mainstream crops, particularly those destined for food or feed use. The strategy of using major food crops as production vehicles for the expression of pharmaceutical or veterinary agents is critically analysed in the light of several recent episodes of contamination of the human food chain by non-approved crop varieties. Commercially viable strategies to limit or avoid biopharming intrusion into the human food chain require the more rigorous segregation of food and non-food varieties of the same crop species via a range of either physical or biological methods. Even more secure segregation is possible by the use of non-food crops, non-crop plants or in vitro plant cultures as production platforms for biopharming. Such platforms already under development range from outdoor-grown Nicotiana spp. to glasshouse-grown Arabidopsis, lotus and moss. Amongst the more effective methods for biocontainment are the plastid expression of transgenes, inducible and transient expression systems, and physical containment of plants or cell cultures. In the current atmosphere of heightened concerns over food safety and biosecurity, the future of biopharming may be largely determined by the extent to which the sector is able to maintain public confidence via a more considered approach to containment and security of its plant production systems.

The Potential of Plant Virus Vectors for Vaccine Production

Plants viruses are versatile vectors that allow the rapid and convenient production of recombinant proteins in plants. Compared with production systems based on transgenic plants, viral vectors are easier to manipulate and recombinant proteins can be produced more quickly and in greater yields. Over the last few years, there has been much interest in the development of plant viruses as vectors for the production of vaccines, either as whole polypeptides or epitopes displayed on the surface of chimeric viral particles. Several viruses have been extensively developed for vaccine production, including tobacco mosaic virus, potato virus X and cowpea mosaic virus. Vaccine candidates have been produced against a range of human and animal diseases, and in many cases have shown immunogenic activity and protection in the face of disease challenge. In this review, we discuss the advantages of plant virus vectors, the development of different viruses as vector systems, and the immunological experiments that have demonstrated the principle of plant virus-derived vaccines.