Molecular farming for new drugs and vaccines. Current perspectives on the production of pharmaceuticals in transgenic plants

Expression strategies for the efficient synthesis of antimicrobial peptides in plastids

Antimicrobial peptides (AMPs) kill microbes or inhibit their growth and are promising next-generation antibiotics. Harnessing their full potential as antimicrobial agents will require methods for cost-effective large-scale production and purification. Here, we explore the possibility to exploit the high protein synthesis capacity of the chloroplast to produce AMPs in plants. Generating a large series of 29 sets of transplastomic tobacco plants expressing nine different AMPs as fusion proteins, we show that high-level constitutive AMP expression results in deleterious plant phenotypes. However, by utilizing inducible expression and fusions to the cleavable carrier protein SUMO, the cytotoxic effects of AMPs and fused AMPs are alleviated and plants with wild-type-like phenotypes are obtained. Importantly, purified AMP fusion proteins display antimicrobial activity independently of proteolytic removal of the carrier. Our work provides expression strategies for the synthesis of toxic polypeptides in chloroplasts, and establishes transplastomic plants as efficient production platform for antimicrobial peptides.

Genetic Containment for Molecular Farming

Plant molecular farming can provide humans with a wide variety of plant-based products including vaccines, therapeutics, polymers, industrial enzymes, and more. Some of these products, such as Taxol, are produced by endogenous plant genes, while many others require addition of genes by artificial gene transfer. Thus, some molecular farming plants are transgenic (or cisgenic), while others are not. Both the transgenic nature of many molecular farming plants and the fact that the products generated are of high-value and specific in purpose mean it is essential to prevent accidental cross-over of molecular farming plants and products into food or feed. Such mingling could occur either by gene flow during plant growth and harvest or by human errors in material handling. One simple approach to mitigate possible transfer would be to use only non-food non-feed species for molecular farming purposes. However, given the extent of molecular farming products in development, testing, or approval that do utilize food or feed crops, a ban on use of these species would be challenging to implement. Therefore, other approaches will need to be considered for mitigation of cross-flow between molecular farming and non-molecular-farming plants. This review summarized some of the production systems available for molecular farming purposes and options to implement or improve plant containment.

Plant-derived VLP: a worthy platform to produce vaccine against SARS-CoV-2

After its emergence in late 2019 SARS-CoV-2 was declared a pandemic by the World Health Organization on 11 March 2020 and has claimed more than 2.8 million lives. There has been a massive global effort to develop vaccines against SARS-CoV-2 and the rapid and low cost production of large quantities of vaccine is urgently needed to ensure adequate supply to both developed and developing countries. Virus-like particles (VLPs) are composed of viral antigens that self-assemble into structures that mimic the structure of native viruses but lack the viral genome. Thus they are not only a safer alternative to attenuated or inactivated vaccines but are also able to induce potent cellular and humoral immune responses and can be manufactured recombinantly in expression systems that do not require viral replication. VLPs have successfully been produced in bacteria, yeast, insect and mammalian cell cultures, each production platform with its own advantages and limitations. Plants offer a number of advantages in one production platform, including proper eukaryotic protein modification and assembly, increased safety, low cost, high scalability as well as rapid production speed, a critical factor needed to control outbreaks of potential pandemics. Plant-based VLP-based viral vaccines currently in clinical trials include, amongst others, Hepatitis B virus, Influenza virus and SARS-CoV-2 vaccines. Here we discuss the importance of plants as a next generation expression system for the fast, scalable and low cost production of VLP-based vaccines.

Plastid Transformation in Tomato: A Vegetable Crop and Model Species

Tomato (Solanum lycopersicum L.), a member of the nightshade family (Solanaceae), is one of the most important vegetable crops and has long been an important model species in plant biology. Plastid biology in tomato is especially interesting due to the chloroplast-to-chromoplast conversion occurring during fruit ripening. Moreover, as tomato represents a major food crop with a fleshy fruit that can be eaten raw, the development of a plastid transformation protocol for tomato was of particular interest to plant biotechnologists. Recent methodological improvements have made tomato plastid transformation more efficient, and facilitated applications in metabolic engineering and molecular farming. This chapter describes the basic methods involved in the generation and analysis of tomato plants with transgenic chloroplast genomes and summarizes recent applications of tomato plastid transformation in plant biotechnology.

Biotechnological Insights on the Expression and Production of Antimicrobial Peptides in Plants

The emergence of drug-resistant pathogens poses a serious critical threat to global public health and requires immediate action. Antimicrobial peptides (AMPs) are a class of short peptides ubiquitously found in all living forms, including plants, insects, mammals, microorganisms and play a significant role in host innate immune system. These peptides are considered as promising candidates to treat microbial infections due to its distinct advantages over conventional antibiotics. Given their potent broad spectrum of antimicrobial action, several AMPs are currently being evaluated in preclinical/clinical trials. However, large quantities of highly purified AMPs are vital for basic research and clinical settings which is still a major bottleneck hindering its application. This can be overcome by genetic engineering approaches to produce sufficient amount of diverse peptides in heterologous host systems. Recently plants are considered as potential alternatives to conventional protein production systems such as microbial and mammalian platforms due to their unique advantages such as rapidity, scalability and safety. In addition, AMPs can also be utilized for development of novel approaches for plant protection thereby increasing the crop yield. Hence, in order to provide a spotlight for the expression of AMP in plants for both clinical or agricultural use, the present review presents the importance of AMPs and efforts aimed at producing recombinant AMPs in plants for molecular farming and plant protection so far.

Efficient gene targeting in Nicotiana tabacum using CRISPR/SaCas9 and temperature tolerant LbCas12a

Nicotiana tabacum is a non-food herb that has the potential to be utilized as bio-factory for generating medicines, vaccines or valuable small metabolites. To achieve these goals, the improvement of genetic tools for pre-designed genome modifications is indispensable. The development of CRISPR/Cas nucleases allows the induction of site-specific double-strand breaks to enhance homologous recombination-mediated gene targeting (GT). However, the efficiency of GT is still a challenging obstacle for many crops including tobacco. Recently, studies in several plant species indicated that by replacing SpCas9 with other CRISPR/Cas-based nucleases, GT efficiencies might be enhanced considerably. Therefore, we tested SaCas9 as well as a temperature-insensitive version of LbCas12a (ttLbCas12a) for targeting the tobacco SuRB gene. At the same time, we also optimized the protocol for Agrobacterium-mediated tobacco transformation and tissue culture. In this way, we could improve GT efficiencies to up to a third of the inoculated cotyledons when using ttLbCas12a, which outperformed SaCas9 considerably. In addition, we could show that the conversion tract length of the GT reaction can be up to 606 bp long and in the majority of cases, it is longer than 250 bp. We obtained multiple heritable GT events, mostly heterozygous, but also biallelic GT events and some without T-DNA integration. Thus, we were not only able to obtain CRISPR/Cas-based heritable GT events in allotetraploid Nicotiana tabacum for the first time, but our results also indicate that ttLbCas12a might be a superior alternative for gene editing and GT in tobacco as well as in other crops.

Plant-made dengue virus-like particles produced by co-expression of structural and non-structural proteins induce a humoral immune response in mice

Dengue virus (DENV) is an emerging threat causing an estimated 390 million infections per year. Dengvaxia, the only licensed vaccine, may not be adequately safe in young and seronegative patients; hence, development of a safer, more effective vaccine is of great public health interest. Virus-like particles (VLPs) are a safe and very efficient vaccine strategy, and DENV VLPs have been produced in various expression systems. Here, we describe the production of DENV VLPs in Nicotiana benthamiana using transient expression. The co-expression of DENV structural proteins (SP) and a truncated version of the non-structural proteins (NSPs), lacking NS5 that contains the RNA-dependent RNA polymerase, led to the assembly of DENV VLPs in plants. These VLPs were comparable in appearance and size to VLPs produced in mammalian cells. Contrary to data from other expression systems, expression of the protein complex prM-E was not successful, and strategies used in other expression systems to improve the VLP yield did not result in increased yields in plants but, rather, increased purification difficulties. Immunogenicity assays in BALB/c mice revealed that plant-made DENV1-SP + NSP VLPs led to a higher antibody response in mice compared with DENV-E domain III displayed inside bluetongue virus core-like particles and a DENV-E domain III subunit. These results are consistent with the idea that VLPs could be the optimal approach to creating candidate vaccines against enveloped viruses.

Hepatitis B core-based virus-like particles: A platform for vaccine development in plants

Virus-like particles (VLPs) are a class of structures formed by the self-assembly of viral capsid protein subunits and contain no infective viral genetic material. The Hepatitis B core (HBc) antigen is capable of assembling into VLPs that can elicit strong immune responses and has been licensed as a commercial vaccine against Hepatitis B. The HBc VLPs have also been employed as a platform for the presentation of foreign epitopes to the immune system and have been used to develop vaccines against, for example, influenza A and Foot-and-mouth disease. Plant expression systems are rapid, scalable and safe, and are capable of providing correct post-translational modifications and reducing upstream production costs. The production of HBc-based virus-like particles in plants would thus greatly increase the efficiency of vaccine production. This review investigates the application of plant-based HBc VLP as a platform for vaccine production.

High-throughput evolutionary optimization of the induction medium towards recombinant protein production in BY-2 tobacco

Bright yellow (BY-2) tobacco cells combined with the XVE chemically inducible system are one of the most promising plant-based platforms for recombinant protein production. This offers a range of benefits, including the separation of the cell growth and heterologous gene expression, lack of risk of infecting the end product with prions and human viruses or appropriate protein glycosylation and folding. However, low protein productivity remains a major obstacle that limits the extensive commercialization of bioproduction in plants. A number of molecular, cell culture and down processing approaches have been made to overcome this problem. Media development for the specific nutritional and hormonal requirements of transgenic plant cells is one of the most efficient cell-culture approaches. We optimized the induction medium towards recombinant protein production in BY-2 and demonstrated the usefulness of evolutionary medium optimization for high-yield protein production in liquid plant cultures. A reliable XVE/GFP model, parallel conducting experiments in a microscale on 96-well plates, and dedicated Gene Game evolutionary optimization software allowed for an effective search of 7611 possible solutions of 11-component media. Within the 4608 formulations tested, the Induct X medium was found with a significant 107.14% increase in protein expression in relation to the standard BY-2 medium.

Plant-based vaccines and antibodies to combat COVID-19: current status and prospects

Globally, researchers are undertaking significant efforts to design and develop effective vaccines, therapeutics, and antiviral drugs to curb the spread of coronavirus disease 2019 (COVID-19). Plants have been used for the production of vaccines, monoclonal antibodies, immunomodulatory proteins, drugs, and pharmaceuticals via molecular farming/transient expression system and are considered as bioreactors or factories for their bulk production. These biological products are stable, safe, effective, easily available, and affordable. Plant molecular farming could facilitate rapid production of biologics on an industrial scale, and has the potential to fulfill emergency demands, such as in the present situation of the COVID-19 pandemic. This article aims to describe the methodology and basics of plant biopharming, in addition to its prospective applications for developing effective vaccines and antibodies to counter COVID-19.

Potential Applications of Plant Biotechnology against SARS-CoV-2

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a novel coronavirus responsible for an ongoing human pandemic (COVID-19). There is a massive international effort underway to develop diagnostic reagents, vaccines, and antiviral drugs in a bid to slow down the spread of the disease and save lives. One part of that international effort involves the research community working with plants, bringing researchers from all over the world together with commercial enterprises to achieve the rapid supply of protein antigens and antibodies for diagnostic kits, and scalable production systems for the emergency manufacturing of vaccines and antiviral drugs. Here, we look at some of the ways in which plants can and are being used in the fight against COVID-19.

New Hope for a "Cursed" Crop? Understanding Stakeholder Attitudes to Plant Molecular Farming With Modified Tobacco in Europe

Plant molecular farming (PMF) with tobacco could provide a sustainable and cheap platform for the production of high-value proteins for medical use. It could also offer European tobacco farmers an alternative, healthful end use for their crop. New plant breeding techniques (NPBTs) offer a means of quickly and precisely optimizing molecular farming platforms for this purpose. However, there has been little empirical research focussing on the barriers and facilitators of these technologies in the agricultural sphere. Here, we explore key stakeholder perceptions toward this combination of technologies, exploring their understanding of risk and opportunity. We interviewed N = 24 key stakeholders - tobacco farmers, agronomists, policymakers, and researchers - in three tobacco-growing areas of Spain and Italy. Our findings demonstrate these stakeholders have a favorable attitude toward PMF with tobacco due to its beneficial medical purpose and the opportunity it provides farmers to continue growing tobacco in a declining European market. Tobacco producers also reported favorable views toward NPBTs, though for some this was contingent on their use for non-food crops like tobacco. Most stakeholders' concerns are economic in nature, such as potential profitability and demands for new agronomic practices or infrastructure. Tobacco producer associations were thought to be important facilitators for future PMF scale-up. The attitude toward these technologies by smoking tobacco companies is, however, unknown and constitutes a potential risk to the development of PMF.

Shotguns vs Lasers: Identifying barriers and facilitators to scaling-up plant molecular farming for high-value health products

Plant molecular farming (PMF) is a convenient and cost-effective way to produce high-value recombinant proteins that can be used in the production of a range of health products, from pharmaceutical therapeutics to cosmetic products. New plant breeding techniques (NPBTs) provide a means to enhance PMF systems more quickly and with greater precision than ever before. However, the feasibility, regulatory standing and social acceptability of both PMF and NPBTs are in question. This paper explores the perceptions of key stakeholders on two European Union (EU) Horizon 2020 programmes-Pharma-Factory and Newcotiana-towards the barriers and facilitators of PMF and NPBTs in Europe. One-on-one qualitative interviews were undertaken with N = 20 individuals involved in one or both of the two projects at 16 institutions in seven countries (Belgium, France, Germany, Italy, Israel, Spain and the UK). The findings indicate that the current EU regulatory environment and the perception of the public towards biotechnology are seen as the main barriers to scaling-up PMF and NPBTs. Competition from existing systems and the lack of plant-specific regulations likewise present challenges for PMF developing beyond its current niche. However, respondents felt that the communication of the benefits and purpose of NPBT PMF could provide a platform for improving the social acceptance of genetic modification. The importance of the media in this process was highlighted. This article also uses the multi-level perspective to explore the ways in which NPBTs are being legitimated by interested parties and the systemic factors that have shaped and are continuing to shape the development of PMF in Europe.

Molecular farming - The slope of enlightenment

Molecular farming can be defined as the use of plants to produce recombinant protein products. The technology is now >30 years old. The early promise of molecular farming was based on three perceived advantages: the low costs of growing plants, the immense scalability of agricultural production, and the inherent safety of plants as hosts for the production of pharmaceuticals. This resulted in a glut of research publications in which diverse proteins were expressed in equally diverse plant-based systems, and numerous companies were founded hoping to commercialize the new technology. There was a moderate degree of success for companies producing non-pharmaceutical proteins, but in the pharmaceutical sector the anticipation raised by promising early research was soon met by the cold hard reality of industrial pragmatism. Plants did not have a track record of success in pharmaceutical protein manufacturing, lacked a regulatory framework, and did not perform as well as established industry platforms. Negative attitudes towards genetically modified plants added to the mix. By the early 2000s, major industry players started to lose interest and pharmaceutical molecular farming fell from a peak of expectation into a trough of disillusionment, just as predicted by the Gartner hype cycle. But many of the pioneers of molecular farming have refocused their activities and have worked to address the limitations that hampered the first generation of technologies. The field has now consolidated around a smaller number of better-characterized platforms and has started to develop standardized methods and best practices, mirroring the evolution of more mature industry sectors. Likewise, attention has turned from proof-of-principle studies to realistic techno-economic modeling to capture significant niche markets, replicating the success of the industrial molecular farming sector. Here we argue that these recent developments signify that pharmaceutical molecular farming is now climbing the slope of enlightenment and will soon emerge as a mature technology.

Molecular Pharming for low and middle income countries

Interest in applications and benefits that Molecular Pharming might offer to Low and Middle Income Countries has always been a potent driver for the research discipline, and a major reason why many scientists entered the field. Although enthusiasm remains high, the reality is that such a game-changing innovation would always take longer than traditional uptake of new technology in developed countries, and be complicated by external factors beyond technical feasibility. Excitingly, signs of increasing interest by LMICS in Molecular Pharming are now emerging. Here, three case studies from Thailand, South Africa and Brazil are used to identify some of the key issues when a new investment into Molecular Pharming manufacturing capacity is under consideration. At present, academic research is not necessarily addressing these issues. Only by understanding the concerns, can members of the academic community contribute to helping the development of Molecular Pharming for LMICs by focusing their research efforts appropriately.

Expression of the Biologically Active Insulin Analog SCI-57 in Nicotiana Benthamiana

Diabetes mellitus is a growing problem worldwide; however, only 23% of low-income countries have access to insulin, and ironically it costs higher in such countries than high-income ones. Therefore, new strategies for insulin and insulin analogs production are urgently required to improve low-cost access to therapeutic products, so as to contain the diabetes epidemic. SCI-57 is an insulin analog with a greater affinity for the insulin receptor and lower thermal degradation than native insulin. It also shows native mitogenicity and insulin-like biological activity. In this work, SCI-57 was transiently expressed in the Nicotiana benthamiana (Nb) plant, and we also evaluated some of its relevant biological effects. An expression plasmid was engineered to translate an N-terminal ubiquitin and C-terminal endoplasmic reticulum-targeting signal KDEL, in order to increase protein expression and stability. Likewise, the effect of co-expression of influenza M2 ion channel (M2) on the expression of insulin analog SCI-57 (SCI-57/M2) was evaluated. Although using M2 increases yield, it tends to alter the SCI-57 amino acid sequence, possibly promoting the formation of oligomers. Purification of SCI-57 was achieved by FPLC cation exchange and ultrafiltration of N. benthamiana leaf extract (NLE). SCI-57 exerts its anti-diabetic properties by stimulating glucose uptake in adipocytes, without affecting the lipid accumulation process. Expression of the insulin analog in agroinfiltrated plants was confirmed by SDS-PAGE, RP-HPLC, and MS. Proteome changes related to the expression of heterologous proteins on N. benthamiana were not observed; up-regulated proteins were related to the agroinfiltration process. Our results demonstrate the potential for producing a biologically active insulin analog, SCI-57, by transient expression in Nb.

Stable Co-Cultivation of the Moss Physcomitrella patens with Human Cells in vitro as a New Approach to Support Metabolism of Diseased Alzheimer Cells

Alzheimer's disease (AD) is a devastating slowly progressive neurodegenerative disorder with no cure. While there are many hypotheses, the exact mechanism causing this pathology is still unknown. Among many other features, AD is characterized by brain hypometabolism and decreased sugar availability, to which neurons eventually succumb. In light of this aspect of the disease, we hypothesized that boosting fuel supply to neurons may help them survive or at least alleviate some of the symptoms. Here we demonstrate that live moss Physcomitrella patens cells can be safely co-cultured with human fibroblasts in vitro and thus have a potential for providing human cells with energy and other vital biomolecules. These data may form the foundation for the development of novel approaches to metabolic bioengineering and treatment of diseased cells based on live plants. In addition, by providing alternative energy sources to human tissues, the biotechnological potential of this interkingdom setup could also serve as a springboard to foster innovative dietary processes addressing current challenges of mankind such as famine or supporting long-haul space flight.

"Therapeutic applications of the 'NPGP' family of viral 2As"

Oligopeptide "2A" and "2A-like" sequences ("2As"; 18-25aa) are found in a range of RNA virus genomes controlling protein biogenesis through "recoding" of the host-cell translational apparatus. Insertion of multiple 2As within a single open reading frame (ORF) produces multiple proteins; hence, 2As have been used in a very wide range of biotechnological and biomedical applications. During translation, these 2A peptide sequences mediate a eukaryote-specific, self-"cleaving" event, termed "ribosome skipping" with very high efficiency. A particular advantage of using 2As is the ability to simultaneously translate a number of proteins at an equal level in all eukaryotic systems although, naturally, final steady-state levels depend upon other factors-notably protein stability. By contrast, the use of internal ribosome entry site elements for co-expression results in an unbalanced expression due to the relative inefficiency of internal initiation. For example, a 1:1 ratio is of particular importance for the biosynthesis of the heavy-chain and light-chain components of antibodies: highly valuable as therapeutic proteins. Furthermore, each component of these "artificial polyprotein" systems can be independently targeted to different sub-cellular sites. The potential of this system was vividly demonstrated by concatenating multiple gene sequences, linked via 2A sequences, into a single, long, ORF-a polycistronic construct. Here, ORFs comprising the biosynthetic pathways for violacein (five gene sequences) and β-carotene (four gene sequences) were concatenated into a single cistron such that all components were co-expressed in the yeast Pichia pastoris. In this review, we provide useful information on 2As to serve as a guide for future utilities of this co-expression technology in basic research, biotechnology, and clinical applications.

High-level expression of the HIV entry inhibitor griffithsin from the plastid genome and retention of biological activity in dried tobacco leaves

KEY MESSAGE

The potent anti-HIV microbicide griffithsin was expressed to high levels in tobacco chloroplasts, enabling efficient purification from both fresh and dried biomass, thus providing storable material for inexpensive production and scale-up on demand. The global HIV epidemic continues to grow, with 1.8 million new infections occurring per year. In the absence of a cure and an AIDS vaccine, there is a pressing need to prevent new infections in order to curb the disease. Topical microbicides that block viral entry into human cells can potentially prevent HIV infection. The antiviral lectin griffithsin has been identified as a highly potent inhibitor of HIV entry into human cells. Here we have explored the possibility to use transplastomic plants as an inexpensive production platform for griffithsin. We show that griffithsin accumulates in stably transformed tobacco chloroplasts to up to 5% of the total soluble protein of the plant. Griffithsin can be easily purified from leaf material and shows similarly high virus neutralization activity as griffithsin protein recombinantly expressed in bacteria. We also show that dried tobacco provides a storable source material for griffithsin purification, thus enabling quick scale-up of production on demand.

Molecular Farming in Barley: Development of a Novel Production Platform to Produce Human Antimicrobial Peptide LL-37

The peptide LL-37, a component of the human innate immune system, represents a promising drug candidate. In particular, the development of low-cost production platform technology is a critical bottleneck in its use in medicine. In the present study, a viable approach for the LL-37 production in transgenic barley is developed. First, comparative analyses of the effects of different fused peptide epitope tags applicable for accumulation and purification on LL-37 production yield are performed using transient expression in tobacco leaves. Following the selection of the most yielding fusion peptide strategies, eight different constructs for the expression of codon optimized chimeric LL-37 genes in transgenic barley plants are created. The expression of individual constructs is driven either by an endosperm-specific promoter of the barley B1 hordein gene or by the maize ubiquitin promoter. The transgenes are stably integrated into the barley genome and inherited in the subsequent generation. All transgenic lines show normal phenotypes and are fertile. LL-37 accumulated in the barley seeds up to 0.55 mg per 1 kg of grain. The fused epitope tags are cleaved off by the use of enterokinase. Furthermore, in planta produced LL-37 including the fused versions is biologically active.

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.

Combination of Plant Virus Nanoparticle-Based in Situ Vaccination with Chemotherapy Potentiates Antitumor Response

Immunotherapeutics are gaining more traction in the armamentarium used to combat cancer. Specifically, in situ vaccination strategies have gained interest because of their ability to alter the tumor microenvironment to an antitumor state. Herein, we investigate whether flexuous plant virus-based nanoparticles formed by the potato virus X (PVX) can be used as an immunotherapeutic for in situ vaccine monotherapy. We further developed dual chemo-immunotherapeutics by incorporating doxorubicin (DOX) into PVX yielding a dual-functional nanoparticle (PVX-DOX) or by coadministration of the two therapeutic regimes, PVX immunotherapy and DOX chemotherapy (PVX+DOX). In the context of B16F10 melanoma, PVX was able to elicit delayed tumor progression when administered as an intratumoral in situ vaccine. Furthermore, the coadministration of DOX via PVX+DOX enhanced the response of the PVX monotherapy through increased survival, which was also represented in the enhanced antitumor cytokine/chemokine profile stimulated by PVX+DOX when compared to PVX or DOX alone. Importantly, coadministered PVX+DOX was better for in situ vaccination than PVX loaded with DOX (PVX-DOX). Whereas the nanomedicine field strives to design multifunctional nanoparticles that integrate several functions and therapeutic regimens into a single nanoparticle, our data suggest a paradigm shift; some therapeutics may need to be administered separately to synergize and achieve the most potent therapeutic outcome. Altogether, our studies show that development of plant viral nanoparticles for in situ vaccines for treatment is a possibility, and dual mechanistic therapeutics can increase efficacy. Nonetheless, combining immunotherapeutics with cytolytic chemotherapy requires detailed investigation to inform optimal integration of cytolytic and immunotherapies and maximize synergy and efficacy.

Exploiting virus-like particles as innovative vaccines against emerging viral infections

Emerging viruses pose a major threat to humans and livestock with global public health and economic burdens. Vaccination remains an effective tool to reduce this threat, and yet, the conventional cell culture often fails to produce sufficient vaccine dose. As an alternative to cell-culture based vaccine, virus-like particles (VLPs) are considered as a highpriority vaccine strategy against emerging viruses. VLPs represent highly ordered repetitive structures via macromolecular assemblies of viral proteins. The particulate nature allows efficient uptake into antigen presenting cells stimulating both innate and adaptive immune responses towards enhanced vaccine efficacy. Increasing research activity and translation opportunity necessitate the advances in the design of VLPs and new bioprocessing modalities for efficient and cost-effective production. Herein, we describe major achievements and challenges in this endeavor, with respect to designing strategies to harnessing the immunogenic potential, production platforms, downstream processes, and some exemplary cases in developing VLP-based vaccines.

Efficient expression of nuclear transgenes in the green alga Chlamydomonas: synthesis of an HIV antigen and development of a new selectable marker

The unicellular green alga Chlamydomonas reinhardtii has become an invaluable model system in plant biology. There is also considerable interest in developing this microalga into an efficient production platform for biofuels, pharmaceuticals, green chemicals and industrial enzymes. However, the production of foreign proteins in the nucleocytosolic compartment of Chlamydomonas is greatly hampered by the inefficiency of transgene expression from the nuclear genome. We have recently addressed this limitation by isolating mutant algal strains that permit high-level transgene expression and by determining the contributions of GC content and codon usage to gene expression efficiency. Here we have applied these new tools and explored the potential of Chlamydomonas to produce a recombinant biopharmaceutical, the HIV antigen P24. We show that a codon-optimized P24 gene variant introduced into our algal expression strains give rise to recombinant protein accumulation levels of up to 0.25% of the total cellular protein. Moreover, in combination with an expression strain, a resynthesized nptII gene becomes a highly efficient selectable marker gene that facilitates the selection of transgenic algal clones at high frequency. By establishing simple principles of successful transgene expression, our data open up new possibilities for biotechnological research in Chlamydomonas.

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.

Expression of rabies glycoprotein and ricin toxin B chain (RGP-RTB) fusion protein in tomato hairy roots: a step towards oral vaccination for rabies

Transgenic hairy roots of Solanum lycopersicum were engineered to express a recombinant protein containing a fusion of rabies glycoprotein and ricin toxin B chain (rgp-rtxB) antigen under the control of constitutive CaMV35S promoter. Asialofetuin-mediated direct ELISA of transgenic hairy root extracts was performed using polyclonal anti-rabies antibodies (Ab1) and epitope-specific peptidal anti-RGP (Ab2) antibodies which confirmed the expression of functionally viable RGP-RTB fusion protein. Direct ELISA based on asialofetuin-binding activity was used to screen crude protein extracts from five transgenic hairy root lines. Expressions of RGP-RTB fusion protein in different tomato hairy root lines varied between 1.4 and 8 µg in per gram of tissue. Immunoblotting assay of RGP-RTB fusion protein from these lines showed a protein band on monomeric size of ~84 kDa after denaturation. Tomato hairy root line H03 showed highest level of RGP-RTB protein expression (1.14 %) and was used further in bench-top bioreactor for the optimization of scale-up process to produce large quantity of recombinant protein. Partially purified RGP-RTB fusion protein was able to induce the immune response in BALB/c mice after intra-mucosal immunization. In the present investigation, we have not only successfully scaled up the hairy root culture but also established the utility of this system to produce vaccine antigen which subsequently will reduce the total production cost for implementing rabies vaccination programs in developing nations. This study in a way aims to provide consolidated base for low-cost preparation of improved oral vaccine against rabies.

Production of Hybrid Chimeric PVX Particles Using a Combination of TMV and PVX-Based Expression Vectors

We have generated hybrid chimeric potato virus X (PVX) particles by coexpression of different PVX coat protein fusions utilizing tobacco mosaic virus (TMV) and PVX-based expression vectors. Coinfection was achieved with a modified PVX overcoat vector displaying a fluorescent protein and a TMV vector expressing another PVX fluorescent overcoat fusion protein. Coexpression of the PVX-CP fusions in the same cells was confirmed by epifluorescence microscopy. Labeling with specific antibodies and transmission electron microscopy revealed chimeric particles displaying green fluorescent protein and mCherry on the surface. These data were corroborated by bimolecular fluorescence complementation. We used split-mCherry fragments as PVX coat fusions and confirmed an interaction between the split-mCherry fragments in coinfected cells. The presence of assembled split-mCherry on the surface confirmed the hybrid character of the chimeric particles.

Regulatory approval and a first-in-human phase I clinical trial of a monoclonal antibody produced in transgenic tobacco plants

Although plant biotechnology has been widely investigated for the production of clinical-grade monoclonal antibodies, no antibody products derived from transgenic plants have yet been approved by pharmaceutical regulators for clinical testing. In the Pharma-Planta project, the HIV-neutralizing human monoclonal antibody 2G12 was expressed in transgenic tobacco (Nicotiana tabacum). The scientific, technical and regulatory demands of good manufacturing practice (GMP) were addressed by comprehensive molecular characterization of the transgene locus, confirmation of genetic and phenotypic stability over several generations of transgenic plants, and by establishing standard operating procedures for the creation of a master seed bank, plant cultivation, harvest, initial processing, downstream processing and purification. The project developed specifications for the plant-derived antibody (P2G12) as an active pharmaceutical ingredient (API) based on (i) the guidelines for the manufacture of monoclonal antibodies in cell culture systems; (ii) the draft European Medicines Agency Points to Consider document on quality requirements for APIs produced in transgenic plants; and (iii) de novo guidelines developed with European national regulators. From the resulting process, a GMP manufacturing authorization was issued by the competent authority in Germany for transgenic plant-derived monoclonal antibodies for use in a phase I clinical evaluation. Following preclinical evaluation and ethical approval, a clinical trial application was accepted by the UK national pharmaceutical regulator. A first-in-human, double-blind, placebo-controlled, randomized, dose-escalation phase I safety study of a single vaginal administration of P2G12 was carried out in healthy female subjects. The successful completion of the clinical trial marks a significant milestone in the commercial development of plant-derived pharmaceutical proteins.

Farming of Plant-Based Veterinary Vaccines and Their Applications for Disease Prevention in Animals

Plants have been studied for the production of pharmaceutical compounds for more than two decades now. Ever since the plant-made poultry vaccine against Newcastle disease virus made a breakthrough and went all the way to obtain regulatory approval, research to use plants for expression and delivery of vaccine proteins for animals was intensified. Indeed, in view of the high production costs of veterinary vaccines, plants represent attractive biofactories and offer many promising advantages in the production of recombinant vaccine proteins. Furthermore, the possibility of conducting immunogenicity and challenge studies in target animals has greatly exaggerated the progress. Although there are no edible plant-produced animal vaccines in the market, plant-based vaccine technology has great potentials. In this review, development, uses, and advantages of plant-based recombinant protein production in various expression platforms are discussed. In addition, examples of plant-based veterinary vaccines showing strong indication in terms of efficacy in animal disease prevention are also described.

Recent advances on host plants and expression cassettes' structure and function in plant molecular pharming

Plant molecular pharming is a promising system to produce important recombinant proteins such as therapeutic antibodies, pharmaceuticals, enzymes, growth factors, and vaccines. The system provides an interesting alternative method to the direct extraction of proteins from inappropriate source material while offering the possibility to overcome problems related to product safety and source availability. Multiple factors including plant hosts, genes of interest, expression vector cassettes, and extraction and purification techniques play important roles in the plant molecular pharming. Plant species, as a biosynthesis platform, are a crucial factor in achieving high yields of recombinant protein in plant. The choice of recombinant gene and its expression strategy is also of great importance in ensuring a high amount of the recombinant proteins. Many studies have been conducted to improve expression, accumulation, and purification of the recombinant protein from molecular pharming systems. Re-engineered vectors and expression cassettes are also pivotal tools in enhancing gene expression at the transcription and translation level, and increasing protein accumulation, stability, retention and targeting of specific organelles. In this review, we report recent advances and strategies of plant molecular pharming while focusing on the choice of plant hosts and the role of some molecular pharming elements and approaches: promoters, codon optimization, signal sequences, and peptides used for upstream design, purification and downstream processing.

Comparative whole-genome analyses of selection marker-free rice-based cholera toxin B-subunit vaccine lines and wild-type lines

Background

We have developed a rice-based oral cholera vaccine named MucoRice-CTB (Cholera Toxin B-subunit) by using an Agrobacterium tumefaciens–mediated co-transformation system. To assess the genome-wide effects of this system on the rice genome, we compared the genomes of three selection marker–free MucoRice-CTB lines with those of two wild-type rice lines (Oryza sativa L. cv. Nipponbare). Mutation profiles of the transgenic and wild-type genomes were examined by next-generation sequencing (NGS).

Results

Using paired-end short-read sequencing, a total of more than 300 million reads for each line were obtained and mapped onto the rice reference genome. The number and distribution of variants were similar in all five lines: the numbers of line-specific variants ranged from 524 to 842 and corresponding mutation rates ranged from 1.41 × 10⁻⁶ per site to 2.28 × 10⁻⁶ per site. The frequency of guanine-to-thymine and cytosine-to-adenine transversions was higher in MucoRice-CTB lines than in WT lines. The transition-to-transversion ratio was 1.12 in MucoRice-CTB lines and 1.65 in WT lines. Analysis of variant-sharing profiles showed that the variants common to all five lines were the most abundant, and the numbers of line-specific variant for all lines were similar. The numbers of non-synonymous amino acid substitutions in MucoRice-CTB lines (15 to 21) were slightly higher than those in WT lines (7 or 8), whereas the numbers of frame shifts were similar in all five lines.

Conclusions

We conclude that MucoRice-CTB and WT are almost identical at the genomic level and that genome-wide effects caused by the Agrobacterium-mediated transformation system for marker-free MucoRice-CTB lines were slight. The comparative whole-genome analyses between MucoRice-CTB and WT lines using NGS provides a reliable estimate of genome-wide differences. A similar approach may be applicable to other transgenic rice plants generated by using this Agrobacterium-mediated transformation system.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1285-y) contains supplementary material, which is available to authorized users.

Plant-derived SAC domain of PAR-4 (Prostate Apoptosis Response 4) exhibits growth inhibitory effects in prostate cancer cells

The gene Par-4 (Prostate Apoptosis Response 4) was originally identified in prostate cancer cells undergoing apoptosis and its product Par-4 showed cancer specific pro-apoptotic activity. Particularly, the SAC domain of Par-4 (SAC-Par-4) selectively kills cancer cells leaving normal cells unaffected. The therapeutic significance of bioactive SAC-Par-4 is enormous in cancer biology; however, its large scale production is still a matter of concern. Here we report the production of SAC-Par-4-GFP fusion protein coupled to translational enhancer sequence (5' AMV) and apoplast signal peptide (aTP) in transgenic Nicotiana tabacum cv. Samsun NN plants under the control of a unique recombinant promoter M24. Transgene integration was confirmed by genomic DNA PCR, Southern and Northern blotting, Real-time PCR, and Nuclear run-on assays. Results of Western blot analysis and ELISA confirmed expression of recombinant SAC-Par-4-GFP protein and it was as high as 0.15% of total soluble protein. In addition, we found that targeting of plant recombinant SAC-Par-4-GFP to the apoplast and endoplasmic reticulum (ER) was essential for the stability of plant recombinant protein in comparison to the bacterial derived SAC-Par-4. Deglycosylation analysis demonstrated that ER-targeted SAC-Par-4-GFP-SEKDEL undergoes O-linked glycosylation unlike apoplast-targeted SAC-Par-4-GFP. Furthermore, various in vitro studies like mammalian cells proliferation assay (MTT), apoptosis induction assays, and NF-κB suppression suggested the cytotoxic and apoptotic properties of plant-derived SAC-Par-4-GFP against multiple prostate cancer cell lines. Additionally, pre-treatment of MAT-LyLu prostate cancer cells with purified SAC-Par-4-GFP significantly delayed the onset of tumor in a syngeneic rat prostate cancer model. Taken altogether, we proclaim that plant made SAC-Par-4 may become a useful alternate therapy for effectively alleviating cancer in the new era.

A double built-in containment strategy for production of recombinant proteins in transgenic rice

Using transgenic rice as a bioreactor for mass production of pharmaceutical proteins could potentially reduce the cost of production significantly. However, a major concern over the bioreactor transgenic rice is the risk of its unintended spreading into environment and into food or feed supplies. Here we report a mitigating method to prevent unwanted transgenic rice spreading by a double built-in containment strategy, which sets a selectively termination method and a visual tag technology in the T-DNA for transformation. We created transgenic rice with an inserted T-DNA that harbors a human proinsulin gene fused with the far-red fluorescent protein gene mKate_S158A, an RNAi cassette suppressing the expression of the rice bentazon detoxification enzyme CYP81A6, and an EPSPS gene as the selection marker for transformation. Herbicide spray tests indicated that such transgenic rice plants can be killed selectively by a spray of bentazon at regular field application dosage for rice weed control. Moreover, the transgenic rice seeds were bright red in color due to the fused far-red fluorescent protein, and could be easily visualized under daylight by naked eyes. Thus, the transgenic rice plants reported in this study could be selectively killed by a commonly used herbicide during their growth stage, and their seeds may be detected visually during processing and consumption after harvest. This double built-in containment strategy may greatly enhance the confinement of the transgenic rice.

Generation of protective immune response against anthrax by oral immunization with protective antigen plant-based vaccine

In concern with frequent recurrence of anthrax in endemic areas and inadvertent use of its spores as biological weapon, the development of an effective anthrax vaccine suitable for both human and veterinary needs is highly desirable. A simple oral delivery through expression in plant system could offer promising alternative to the current methods that rely on injectable vaccines extracted from bacterial sources. In the present study, we have expressed protective antigen (PA) gene in Indian mustard by Agrobacterium-mediated transformation and in tobacco by plastid transformation. Putative transgenic lines were verified for the presence of transgene and its expression by molecular analysis. PA expressed in transgenic lines was biologically active as evidenced by macrophage lysis assay. Intraperitoneal (i.p.) and oral immunization with plant PA in murine model indicated high serum PA specific IgG and IgA antibody titers. PA specific mucosal immune response was noted in orally immunized groups. Further, antibodies indicated lethal toxin neutralizing potential in-vitro and conferred protection against in-vivo toxin challenge. Oral immunization experiments demonstrated generation of immunoprotective response in mice. Thus, our study examines the feasibility of oral PA vaccine expressed in an edible plant system against anthrax.

Can plant biotechnology help break the HIV-malaria link?

The population of sub-Saharan Africa is at risk from multiple, poverty-related endemic diseases. HIV and malaria are the most prevalent, but they disproportionately affect different groups of people, i.e. HIV predominantly affects sexually-active adults whereas malaria has a greater impact on children and pregnant women. Nevertheless, there is a significant geographical and epidemiological overlap which results in bidirectional and synergistic interactions with important consequences for public health. The immunosuppressive effects of HIV increase the risk of infection when individuals are exposed to malaria parasites and also the severity of malaria symptoms. Similarly, acute malaria can induce a temporary increase in the HIV viral load. HIV is associated with a wide range of opportunistic infections that can be misdiagnosed as malaria, resulting in the wasteful misuse of antimalarial drugs and a failure to address the genuine cause of the disease. There is also a cumulative risk of toxicity when antiretroviral and antimalarial drugs are given to the same patients. Synergistic approaches involving the control of malaria as a strategy to fight HIV/AIDS and vice versa are therefore needed in co-endemic areas. Plant biotechnology has emerged as a promising approach to tackle poverty-related diseases because plant-derived drugs and vaccines can be produced inexpensively in developing countries and may be distributed using agricultural infrastructure without the need for a cold chain. Here we explore some of the potential contributions of plant biotechnology and its integration into broader multidisciplinary public health programs to combat the two diseases in developing countries.

Molecular farming in tobacco hairy roots by triggering the secretion of a pharmaceutical antibody

Recombinant pharmaceutical proteins expressed in hairy root cultures can be secreted into the medium to improve product homogeneity and to facilitate purification, although this may result in significant degradation if the protein is inherently unstable or particularly susceptible to proteases. To address these challenges, we used a design of experiments approach to develop an optimized induction protocol for the cultivation of tobacco hairy roots secreting the full-size monoclonal antibody M12. The antibody yield was enhanced 30-fold by the addition of 14 g/L KNO3 , 19 mg/L 1-naphthaleneacetic acid and 1.5 g/L of the stabilizing agent polyvinylpyrrolidone. Analysis of hairy root cross sections revealed that the optimized medium induced lateral root formation and morphological changes in the inner cortex and pericycle cells, indicating that the improved productivity was at least partially based on the enhanced efficiency of antibody secretion. We found that 57% of the antibody was secreted, yielding 5.9 mg of product per liter of induction medium. Both the secreted and intracellular forms of the antibody could be isolated by protein A affinity chromatography and their functionality was confirmed using vitronectin-binding assays. Glycan analysis revealed three major plant complex-type glycans on both forms of the antibody, although the secreted form was more homogeneous due to the predominance of a specific glycoform. Tobacco hairy root cultures therefore offer a practical solution for the production of homogeneous pharmaceutical antibodies in containment.

Transglutamination allows production and characterization of native-sized ELPylated spider silk proteins from transgenic plants

In the last two decades it was shown that plants have a great potential for production of specific heterologous proteins. But high cost and inefficient downstream processing are a main technical bottleneck for the broader use of plant-based production technology especially for protein-based products, for technical use as fibres or biodegradable plastics and also for medical applications. High-performance fibres from recombinant spider silks are, therefore, a prominent example. Spiders developed rather different silk materials that are based on proteins. These spider silks show excellent properties in terms of elasticity and toughness. Natural spider silk proteins have a very high molecular weight, and it is precisely this property which is thought to give them their strength. Transgenic plants were generated to produce ELPylated recombinant spider silk derivatives. These fusion proteins were purified by Inverse Transition Cycling (ITC) and enzymatically multimerized with transglutaminase in vitro. Layers produced by casting monomers and multimers were characterized using atomic force microscopy (AFM) and AFM-based nanoindentation. The layered multimers formed by mixing lysine- and glutamine-tagged monomers were associated with the highest elastic penetration modulus.

Proteolysis of recombinant proteins in bioengineered plant cells

Plants are increasingly used as alternative expression hosts for the production of recombinant proteins offering many advantages including higher biomass and the ability to perform post-translational modifications on complex proteins. Key challenges for optimized accumulation of recombinant proteins in a plant system still remain, including endogenous plant proteolytic activity, which may severely compromise recombinant protein stability. Several strategies have recently been applied to improve protein stability by limiting protease action such as recombinant protein production in various sub-cellular compartments or application of protease inhibitors to limit protease action. A short update on the current strategies applied is provided here, with particular focus on sub-cellular sites previously selected for recombinant protein production and the co-expression of protease inhibitors to limit protease activity.

Plastid transformation in tomato

Tomato (Solanum lycopersicum) is one of the most important vegetable crops and has long been an important model species in plant biology. Plastid biology in tomato is especially interesting due to the chloroplast-to-chromoplast conversion occurring during fruit ripening. Moreover, as tomato represents a major food crop with an edible fruit that can be eaten raw, the development of a plastid transformation protocol for tomato was of particular interest to plant biotechnology. Recent methodological improvements have made tomato plastid transformation more efficient and facilitated applications in metabolic engineering and molecular farming. This article describes the basic methods involved in the generation and analysis of tomato plants with transgenic chloroplast genomes and summarizes current applications of tomato plastid transformation.

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.