Efficient Transient Expression of Recombinant Proteins in Plants by the Novel pEff Vector Based on the Genome of Potato Virus X

Plant-Produced Chimeric Hepatitis E Virus-like Particles as Carriers for Antigen Presentation

A wide range of virus-like particles (VLPs) is extensively employed as carriers to display various antigens for vaccine development to fight against different infections. The plant-produced truncated variant of the hepatitis E virus (HEV) coat protein is capable of forming VLPs. In this study, we demonstrated that recombinant fusion proteins comprising truncated HEV coat protein with green fluorescent protein (GFP) or four tandem copies of the extracellular domain of matrix protein 2 (M2e) of influenza A virus inserted at the Tyr485 position could be efficiently expressed in Nicotiana benthamiana plants using self-replicating vector based on the potato virus X genome. The plant-produced fusion proteins in vivo formed VLPs displaying GFP and 4M2e. Therefore, HEV coat protein can be used as a VLP carrier platform for the presentation of relatively large antigens comprising dozens to hundreds of amino acids. Furthermore, plant-produced HEV particles could be useful research tools for the development of recombinant vaccines against influenza.

Green Biologics: Harnessing the Power of Plants to Produce Pharmaceuticals

Plants are increasingly used for the production of high-quality biological molecules for use as pharmaceuticals and biomaterials in industry. Plants have proved that they can produce life-saving therapeutic proteins (Elelyso™-Gaucher's disease treatment, ZMapp™-anti-Ebola monoclonal antibodies, seasonal flu vaccine, Covifenz™-SARS-CoV-2 virus-like particle vaccine); however, some of these therapeutic proteins are difficult to bring to market, which leads to serious difficulties for the manufacturing companies. The closure of one of the leading companies in the sector (the Canadian biotech company Medicago Inc., producer of Covifenz) as a result of the withdrawal of investments from the parent company has led to the serious question: What is hindering the exploitation of plant-made biologics to improve health outcomes? Exploring the vast potential of plants as biological factories, this review provides an updated perspective on plant-derived biologics (PDB). A key focus is placed on the advancements in plant-based expression systems and highlighting cutting-edge technologies that streamline the production of complex protein-based biologics. The versatility of plant-derived biologics across diverse fields, such as human and animal health, industry, and agriculture, is emphasized. This review also meticulously examines regulatory considerations specific to plant-derived biologics, shedding light on the disparities faced compared to biologics produced in other systems.

Molecular Farming of Pembrolizumab and Nivolumab

Immune checkpoint inhibitors (ICIs) are a class of immunotherapy agents capable of alleviating the immunosuppressive effects exerted by tumorigenic cells. The programmed cell death protein 1 (PD-1)/programmed death-ligand 1 (PD-L1) immune checkpoint is one of the most ubiquitous checkpoints utilized by tumorigenic cells for immune evasion by inducing apoptosis and inhibiting the proliferation and cytokine production of T lymphocytes. Currently, the most frequently used ICIs targeting the PD-1/PD-L1 checkpoint include monoclonal antibodies (mAbs) pembrolizumab and nivolumab that bind to PD-1 on T lymphocytes and inhibit interaction with PD-L1 on tumorigenic cells. However, pembrolizumab and nivolumab are costly, and thus their accessibility is limited in low- and middle-income countries (LMICs). Therefore, it is essential to develop novel biomanufacturing platforms capable of reducing the cost of these two therapies. Molecular farming is one such platform utilizing plants for mAb production, and it has been demonstrated to be a rapid, low-cost, and scalable platform that can be potentially implemented in LMICs to diminish the exorbitant prices, ultimately leading to a significant reduction in cancer-related mortalities within these countries.

Plant-Produced Nanoparticles Based on Artificial Self-Assembling Peptide Bearing the Influenza M2e Epitope

Despite advances in vaccine development, influenza remains a persistent global health threat and the search for a broad-spectrum recombinant vaccine against influenza continues. The extracellular domain of the transmembrane protein M2 (M2e) of the influenza A virus is highly conserved and can be used to develop a universal vaccine. M2e is a poor immunogen by itself, but it becomes highly immunogenic when linked to an appropriate carrier. Here, we report the transient expression of a recombinant protein comprising four tandem copies of M2e fused to an artificial self-assembling peptide (SAP) in plants. The hybrid protein was efficiently expressed in Nicotiana benthamiana using the self-replicating potato virus X-based vector pEff. The protein was purified using metal affinity chromatography under denaturing conditions. The hybrid protein was capable of self-assembly in vitro into spherical particles 15-30 nm in size. The subcutaneous immunization of mice with M2e-carrying nanoparticles induced high levels of M2e-specific IgG antibodies in serum and mucosal secretions. Immunization provided mice with protection against a lethal influenza A virus challenge. SAP-based nanoparticles displaying M2e peptides can be further used to develop a recombinant "universal" vaccine against influenza A produced 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.

CryoEM and stability analysis of virus-like particles of potyvirus and ipomovirus infecting a common host

Sweet potato feathery mottle virus (SPFMV) and Sweet potato mild mottle virus (SPMMV) are members of the genera Potyvirus and Ipomovirus, family Potyviridae, sharing Ipomoea batatas as common host, but transmitted, respectively, by aphids and whiteflies. Virions of family members consist of flexuous rods with multiple copies of a single coat protein (CP) surrounding the RNA genome. Here we report the generation of virus-like particles (VLPs) by transient expression of the CPs of SPFMV and SPMMV in the presence of a replicating RNA in Nicotiana benthamiana. Analysis of the purified VLPs by cryo-electron microscopy, gave structures with resolutions of 2.6 and 3.0 Å, respectively, showing a similar left-handed helical arrangement of 8.8 CP subunits per turn with the C-terminus at the inner surface and a binding pocket for the encapsidated ssRNA. Despite their similar architecture, thermal stability studies reveal that SPMMV VLPs are more stable than those of SPFMV.

Biotechnologically Engineered Plants

The development of recombinant DNA technology during the past thirty years has enabled scientists to isolate, characterize, and manipulate a myriad of different animal, bacterial, and plant genes. This has, in turn, led to the commercialization of hundreds of useful products that have significantly improved human health and well-being. Commercially, these products have been mostly produced in bacterial, fungal, or animal cells grown in culture. More recently, scientists have begun to develop a wide range of transgenic plants that produce numerous useful compounds. The perceived advantage of producing foreign compounds in plants is that compared to other methods of producing these compounds, plants seemingly provide a much less expensive means of production. A few plant-produced compounds are already commercially available; however, many more are in the production pipeline.

Expressing recombinant human lactoferrin with antibacterial activity in Nicotiana benthamiana

Lactoferrin is a non-hematic iron-binding 80-kDa protein that exhibits antimicrobial activity. Higher plants function as "green bioreactors" for large-scale recombinant protein production. In this study, we transiently expressed recombinant human lactoferrin (rhLF) in Nicotiana benthamiana at a yield of approximately 40 µg g-1 fresh mass (gFM) using the Tsukuba system. Additionally, the expression level of rhLF increased when it was fused with KDEL, an endoplasmic reticulum retention motif. Purified plant-derived rhLF possesses antibacterial activity that inhibits the growth of Escherichia coli. These results indicated that rhLF containing antimicrobial activity can be produced in N. benthamiana using the Tsukuba system.

Harnessing plant viruses in the metagenomics era: from the development of infectious clones to applications

Recent metagenomic studies which focused on virus characterization in the entire plant environment have revealed a remarkable viral diversity in plants. The exponential discovery of viruses also requires the concomitant implementation of high-throughput methods to perform their functional characterization. Despite several limitations, the development of viral infectious clones remains a method of choice to understand virus biology, their role in the phytobiome, and plant resilience. Here, we review the latest approaches for efficient characterization of plant viruses and technical advances built on high-throughput sequencing and synthetic biology to streamline assembly of viral infectious clones. We then discuss the applications of plant viral vectors in fundamental and applied plant research as well as their technical and regulatory limitations, and we propose strategies for their safer field applications.

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

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

A contemporary reassessment of the enhanced transient expression system based on the tombusviral silencing suppressor protein P19

Transient transgenic expression accelerates pharming and facilitates protein studies in plants. One embodiment of the approach involves leaf infiltration of Agrobacterium strains whose T-DNA is engineered with the gene(s) of interest. However, gene expression during 'agro-infiltration' is intrinsically and universally impeded by the onset of post-transcriptional gene silencing (PTGS). Nearly 20 years ago, a simple method was developed, whereby co-expression of the tombusvirus-encoded P19 protein suppresses PTGS and thus enhances transient gene expression. Yet, how PTGS is activated and suppressed by P19 during the process has remained unclear to date. Here, we address these intertwined questions in a manner also rationalizing how vastly increased protein yields are achieved using a minimal viral replicon as a transient gene expression vector. We also explore, in side-by-side analyses, why some proteins do not accumulate to the expected high levels in the assay, despite vastly increased mRNA levels. We validate that enhanced co-expression of multiple constructs is achieved within the same transformed cells, and illustrate how the P19 system allows rapid protein purification for optimized downstream in vitro applications. Finally, we assess the suitability of the P19 system for subcellular localization studies - an originally unanticipated, yet increasingly popular application - and uncover shortcomings of this specific implement. In revisiting the P19 system using contemporary knowledge, this study sheds light onto its hitherto poorly understood mechanisms while further illustrating its versatility but also some of its limits.

The Tobacco Mosaic Virus Origin of Assembly Sequence is Dispensable for Specific Viral RNA Encapsidation but Necessary for Initiating Assembly at a Single Site

We have investigated whether the presence of the origin of assembly sequence (OAS) of tobacco mosaic virus (TMV) is necessary for the specific encapsidation of replicating viral RNA. To this end TMV coat protein was expressed from replicating RNA constructs with or without the OAS in planta. In both cases the replicating RNA was specifically encapsidated to give nucleoprotein nanorods, though the yield in the absence of the OAS was reduced to about 60% of that in its presence. Moreover, the nanorods generated in the absence of the OAS were more heterogeneous in length and contained frequent structural discontinuities. These results strongly suggest that the function of the OAS is to provide a unique site for the initiation of viral assembly, leading to a one-start helix, rather than the selection of virus RNA for packaging.

High-Yield Production of Chimeric Hepatitis E Virus-Like Particles Bearing the M2e Influenza Epitope and Receptor Binding Domain of SARS-CoV-2 in Plants Using Viral Vectors

Capsid protein of Hepatitis E virus (HEV) is capable of self-assembly into virus-like particles (VLPs) when expressed in Nicotiana benthamiana plants. Such VLPs could be used as carriers of antigens for vaccine development. In this study, we obtained VLPs based on truncated coat protein of HEV bearing the M2e peptide of Influenza A virus or receptor-binding domain of SARS-CoV-2 spike glycoprotein (RBD). We optimized the immunogenic epitopes' presentation by inserting them into the protruding domain of HEV ORF2 at position Tyr485. The fusion proteins were expressed in Nicotiana benthamiana plants using self-replicating potato virus X (PVX)-based vector. The fusion protein HEV/M2, targeted to the cytosol, was expressed at the level of about 300-400 μg per gram of fresh leaf tissue and appeared to be soluble. The fusion protein was purified using metal affinity chromatography under native conditions with the final yield about 200 μg per gram of fresh leaf tissue. The fusion protein HEV/RBD, targeted to the endoplasmic reticulum, was expressed at about 80-100 μg per gram of fresh leaf tissue; the yield after purification was up to 20 μg per gram of fresh leaf tissue. The recombinant proteins HEV/M2 and HEV/RBD formed nanosized virus-like particles that could be recognized by antibodies against inserted epitopes. The ELISA assay showed that antibodies of COVID-19 patients can bind plant-produced HEV/RBD virus-like particles. This study shows that HEV capsid protein is a promising carrier for presentation of foreign antigen.

Optimising expression and extraction of recombinant proteins in plants

Recombinant proteins are of paramount importance for research, industrial and medical use. Numerous expression chassis are available for recombinant protein production, and while bacterial and mammalian cell cultures are the most widely used, recent developments have positioned transgenic plant chassis as viable and often preferential options. Plant chassis are easily maintained at low cost, are hugely scalable, and capable of producing large quantities of protein bearing complex post-translational modification. Several protein targets, including antibodies and vaccines against human disease, have been successfully produced in plants, highlighting the significant potential of plant chassis. The aim of this review is to act as a guide to producing recombinant protein in plants, discussing recent progress in the field and summarising the factors that must be considered when utilising plants as recombinant protein expression systems, with a focus on optimising recombinant protein expression at the genetic level, and the subsequent extraction and purification of target proteins, which can lead to substantial improvements in protein stability, yield and purity.

Rapid Transient Expression of Receptor-Binding Domain of SARS-CoV-2 and the Conserved M2e Peptide of Influenza A Virus Linked to Flagellin in Nicotiana benthamiana Plants Using Self-Replicating Viral Vector

The development of recombinant vaccines against SARS-CoV-2 and influenza A is an important task. The combination of the conserved influenza A antigen, the extracellular domain of the transmembrane protein M2 (M2e), and the receptor-binding domain of the SARS-CoV-2 spike glycoprotein (RBD) provides the opportunity to develop a bivalent vaccine against these infections. The fusion of antigens with bacterial flagellin, the ligand for Toll-like receptor 5 and potent mucosal adjuvant, may increase the immunogenicity of the candidate vaccines and enable intranasal immunization. In this study, we report the transient expression of RBD alone, RBD coupled with four copies of M2e, and fusions of RBD and RBD-4M2e with flagellin in Nicotiana benthamiana plants using the self-replicating potato virus X-based vector pEff. The yields of purified recombinant proteins per gram of fresh leaf tissue were about 20 µg for RBD, 50-60 µg for RBD-4M2e and the fusion of RBD with flagellin, and about 90 µg for RBD-4M2e fused to flagellin. Targeting to the endoplasmic reticulum enabled the production of glycosylated recombinant proteins comprising RBD. Our results show that plant-produced RBD and RBD-4M2e could be further used for the development of subunit vaccines against COVID-19 and a bivalent vaccine against COVID-19 and influenza A, while flagellin fusions could be used for the development of intranasal vaccines.

Plant-based expression platforms to produce high-value metabolites and proteins

Plant-based heterologous expression systems can be leveraged to produce high-value therapeutics, industrially important proteins, metabolites, and bioproducts. The production can be scaled up, free from pathogen contamination, and offer post-translational modifications to synthesize complex proteins. With advancements in molecular techniques, transgenics, CRISPR/Cas9 system, plant cell, tissue, and organ culture, significant progress has been made to increase the expression of recombinant proteins and important metabolites in plants. Methods are also available to stabilize RNA transcripts, optimize protein translation, engineer proteins for their stability, and target proteins to subcellular locations best suited for their accumulation. This mini-review focuses on recent advancements to enhance the production of high-value metabolites and proteins necessary for therapeutic applications using plants as bio-factories.

High-level transient production of a protease-resistant mutant form of human basic fibroblast growth factor in Nicotiana benthamiana leaves

The human basic fibroblast growth factor (bFGF) is a protein that plays a pivotal role in cellular processes like cell proliferation and development. As a result, it has become an important component in cell culture systems, with applications in biomedical engineering, cosmetics, and research. Alternative production techniques, such as transient production in plants, are becoming a feasible option as the demand continues to grow. High-level bFGF production was achieved in this study employing an optimized Agrobacterium-mediated transient expression system, which yielded about a 3-fold increase in production over a conventional system. This yield was further doubled at about 185 µg g^-1 FW using a mutant protease-resistant version that degraded/aggregated at a three-fold slower rate in leaf crude extracts. To achieve a pure product, a two-step purification technique was applied. The capacity of the pure protease-resistant bFGF (PRbFGF) to stimulate cell proliferation was tested and was found to be comparable to that of E. coli-produced bFGF in HepG2 and CHO-K1 cells. Overall, this study demonstrates a high-level transient production system of functional PRbFGF in N. benthamiana leaves as well as an efficient tag-less purification technique of leaf crude extracts.

[The analysis of immunoreactivity of individual B-cell epitopes of hepatitis C virus (Flaviviridae: Hepacivirus: Hepatitis С virus) NS4a antigen]

INTRODUCTION

Chronic viral hepatitis C (CHC) is a ubiquitous infectious disease, a significant limitation of which WHO attributes to the use of a new highly effective antiviral therapy. Previously, two B-cell epitopes were identified in NS4a antigen of the hepatitis C virus (HCV). It was shown that certain titers of antibodies (ABs) to the extended C-terminal epitope (1687-1718 a.a.) can predict a high probability of achieving a sustained virological response (SVR) to standard therapy with pegylated interferon-α and ribavirin.The aim of the work was to determine immunoreactivity of two B-cell epitopes (middle and C-terminal) of NS4a antigen, and to estimate a possible association of ABs to them with the achievement of SVR after standard interferon therapy and treatment with direct antiviral drugs (DAAs) daclatasvir and sofosbuvir (velpanat).

MATERIALS AND METHODS

Blood serum samples of patients with CHC (n = 113), of which 55 participants received standard interferon therapy, 50 received velpanate treatment, the remaining 8 received no therapy were examined. The middle B-cell epitope (positions 24-34 a.a.) of NS4a was synthesized by the solid-phase method, while the C-terminal epitope (34-54 a.a.) was obtained using genetically engineered techniques. Enzyme immunoassay (ELISA) testing of the sera collected before treatment was performed for the two selected epitopes according to the conventional methods.

RESULTS

The antibodies to the C-terminal epitope were detected significantly more frequently than those to the middle one (p = 0.01) when analyzing the blood sera of patients (n = 113). The presence of ABs to the C-terminal epitope in the serum samples of participants who completed standard interferon therapy was associated with the achievement of SVR (p = 0.0245). In the blood sera of participants who completed therapy with velpanate, an association of the presence of ABs to the C-terminal epitope with the achievement of SVR was also established (p < 0.0001). The presence of ABs to the middle B epitope was not associated with the achievement of SVR, regardless of the therapy used.

DISCUSSION

The observed difference in the immunoreactivity of the two B-cell determinants may be associated with the localization of the nearest Th-epitopes, the sensitivity of NS4a antigen to proteolytic enzymes, and the peculiarities of epitope presentation by antigen-presenting cells. However, it should be noted that the immunoreactivity of the middle B-epitope is poorly studied. Although the association of ABs to the C-terminal epitope with the achievement of SVR has been shown by several scientific teams, the detailed molecular mechanism of their influence on the effectiveness of therapy is unclear.

CONCLUSION

In CHC, ABs to the C-terminal epitope of NS4a are produced more frequently than those to the median epitope. The presence of ABs to the C-terminal epitope is a predictive marker of a high probability of achieving SVR, regardless of the type of therapy and antibody titer.

Current Status and Perspective on the Use of Viral-Based Vectors in Eukaryotic Microalgae

During the last two decades, microalgae have attracted increasing interest, both commercially and scientifically. Commercial potential involves utilizing valuable natural compounds, including carotenoids, polysaccharides, and polyunsaturated fatty acids, which are widely applicable in food, biofuel, and pharmaceutical industries. Conversely, scientific potential focuses on bioreactors for producing recombinant proteins and developing viable technologies to significantly increase the yield and harvest periods. Here, viral-based vectors and transient expression strategies have significantly contributed to improving plant biotechnology. We present an updated outlook covering microalgal biotechnology for pharmaceutical application, transformation techniques for generating recombinant proteins, and genetic engineering tactics for viral-based vector construction. Challenges in industrial application are also discussed.

Further Characterization of MUAS35SCP and FUAS35SCP Recombinant Promoters and Their Implication in Translational Research

Recombinant promoters are of high value in translational research. Earlier, we developed two recombinant promoters, namely MUAS35SCP and FUAS35SCP, and their transcriptional activities were found to be stronger than that of the most widely used CaMV35S promoter in dicot plants. Presently, we are reporting constitutive expression of both GUS and GFP reporters under the control of these promoters in several monocots, including rice, wheat, and pearl millet. We observed that these promoters could express the reporter genes constitutively, and their expression abilities were almost equal to that of the CaMV35S² promoter. Plant-derived enriched PaDef (Persea americana var. drymifolia defensin) and NsDef2 (Nigella sativa L. defensin 2) antimicrobial peptides expressed under the control of these promoters arrest the growth of devastating phytopathogens like Pseudomonas syringae, Rhodococcus fascians, and Alternaria alternata. We observed that plant-derived NsDef2 and PaDef under control of these promoters showed approximately 80-90% inhibitory activity against Pseudomonas syringae. Hence, these promoters were constitutive and universal, as they can drive the expression of transgenes in both dicot and monocot plants. Alongside, these promoters could become a valuable tool for raising genetically modified plants with in-built resistance toward phytopathogens.

A GoldenBraid-Compatible Virus-Based Vector System for Transient Expression of Heterologous Proteins in Plants

We have developed a Potato virus X (PVX)-based vector system compatible with the GoldenBraid 2.0 (GB) cloning strategy to transiently express heterologous proteins or peptides in plants for biotechnological purposes. This vector system consists of three domestication vectors carrying three GB parts-the cauliflower mosaic virus (CaMV) 35S promoter with PVX upstream of the second subgenomic promoter of the PVX coat protein (PVX CP SGP), nopaline synthase (NOS) terminator with PVX downstream of the first PVX CP SGP and the gene of interest (GOI). The full-length PVX clone carrying the sequence encoding a green fluorescent protein (GFP) as GOI was incorporated into the binary GB vector in a one-step reaction of three GB parts using the four-nucleotide GB standard syntax. We investigated whether the obtained vector named GFP/pGBX enables systemic PVX infection and expression of GFP in Nicotiana benthamiana plants. We show that this GB-compatible vector system can be used for simple and efficient assembly of PVX-based expression constructs and that it meets the current need for interchange of standard biological parts used in different expression systems.

The Use of a Replicating Virus Vector For in Planta Generation of Tobacco Mosaic Virus Nanorods Suitable For Metallization

The production of designer-length tobacco mosaic virus (TMV) nanorods in plants has been problematic in terms of yields, particularly when modified coat protein subunits are incorporated. To address this, we have investigated the use of a replicating potato virus X-based vector (pEff) to express defined length nanorods containing either wild-type or modified versions of the TMV coat protein. This system has previously been shown to be an efficient method for producing virus-like particles of filamentous plant viruses. The length of the resulting TMV nanorods can be controlled by varying the length of the encapsidated RNA. Nanorod lengths were analyzed with a custom-written Python computer script coupled with the Nanorod UI user interface script, thereby generating histograms of particle length. In addition, nanorod variants were produced by incorporating coat protein subunits presenting metal-binding peptides at their C-termini. We demonstrate the utility of this approach by generating nanorods that bind colloidal gold nanoparticles.

Genetic Manipulation and Bioreactor Culture of Plants as a Tool for Industry and Its Applications

In recent years, there has been a considerable increase in interest in the use of transgenic plants as sources of valuable secondary metabolites or recombinant proteins. This has been facilitated by the advent of genetic engineering technology with the possibility for direct modification of the expression of genes related to the biosynthesis of biologically active compounds. A wide range of research projects have yielded a number of efficient plant systems that produce specific secondary metabolites or recombinant proteins. Furthermore, the use of bioreactors allows production to be increased to industrial scales, which can quickly and cheaply deliver large amounts of material in a short time. The resulting plant production systems can function as small factories, and many of them that are targeted at a specific operation have been patented. This review paper summarizes the key research in the last ten years regarding the use of transgenic plants as small, green biofactories for the bioreactor-based production of secondary metabolites and recombinant proteins; it simultaneously examines the production of metabolites and recombinant proteins on an industrial scale and presents the current state of available patents in the field.

Plant-Derived Recombinant Vaccines against Zoonotic Viruses

Emerging and re-emerging zoonotic diseases cause serious illness with billions of cases, and millions of deaths. The most effective way to restrict the spread of zoonotic viruses among humans and animals and prevent disease is vaccination. Recombinant proteins produced in plants offer an alternative approach for the development of safe, effective, inexpensive candidate vaccines. Current strategies are focused on the production of highly immunogenic structural proteins, which mimic the organizations of the native virion but lack the viral genetic material. These include chimeric viral peptides, subunit virus proteins, and virus-like particles (VLPs). The latter, with their ability to self-assemble and thus resemble the form of virus particles, are gaining traction among plant-based candidate vaccines against many infectious diseases. In this review, we summarized the main zoonotic diseases and followed the progress in using plant expression systems for the production of recombinant proteins and VLPs used in the development of plant-based vaccines against zoonotic viruses.

Non-Integrating Lentiviral Vectors in Clinical Applications: A Glance Through

Lentiviral vectors (LVs) play an important role in gene therapy and have proven successful in clinical trials. LVs are capable of integrating specific genetic materials into the target cells and allow for long-term expression of the cDNA of interest. The use of non-integrating LVs (NILVs) reduces insertional mutagenesis and the risk of malignant cell transformation over integrating lentiviral vectors. NILVs enable transient expression or sustained episomal expression, especially in non-dividing cells. Important modifications have been made to the basic human immunodeficiency virus (HIV) structures to improve the safety and efficacy of LVs. NILV-aided transient expression has led to more pre-clinical studies on primary immunodeficiencies, cytotoxic cancer therapies, and hemoglobinopathies. Recently, the third generation of self-inactivating LVs was applied in clinical trials for recombinant protein production, vaccines, gene therapy, cell imaging, and induced pluripotent stem cell (iPSC) generation. This review discusses the basic lentiviral biology and the four systems used for generating NILV designs. Mutations or modifications in LVs and their safety are addressed with reference to pre-clinical studies. The detailed application of NILVs in promising pre-clinical studies is also discussed.

The B1 Domain of Streptococcal Protein G Serves as a Multi-Functional Tag for Recombinant Protein Production in Plants

Plants have long been considered a cost-effective platform for recombinant production. A recently recognized additional advantage includes the low risk of contamination of human pathogens, such as viruses and bacterial endotoxins. Indeed, a great advance has been made in developing plants as a "factory" to produce recombinant proteins to use for biopharmaceutical purposes. However, there is still a need to develop new tools for recombinant protein production in plants. In this study, we provide data showing that the B1 domain of Streptococcal protein G (GB1) can be a multi-functional domain of recombinant proteins in plants. N-terminal fusion of the GB1 domain increased the expression level of various target proteins ranging from 1.3- to 3.1-fold at the protein level depending on the target proteins. GB1 fusion led to the stabilization of the fusion proteins. Furthermore, the direct detection of GB1-fusion proteins by the secondary anti-IgG antibody eliminated the use of the primary antibody for western blot analysis. Based on these data, we propose that the small GB1 domain can be used as a versatile tag for recombinant protein production in plants.

The Amino-Proximal Region of the Coat Protein of Cucumber Vein Yellowing Virus (Family Potyviridae) Affects the Infection Process and Whitefly Transmission

Most plant viruses rely on vector transmission for their spread and specific interactions between vector and virus have evolved to regulate this relationship. The whitefly Bemisia tabaci- transmitted cucumber vein yellowing virus (CVYV; genus Ipomovirus, family Potyviridae) is endemic in the Mediterranean Basin, where it causes significant losses in cucurbit crops. In this study, the role of the coat protein (CP) of CVYV for B. tabaci transmission and plant infection was investigated using a cloned and infectious CVYV cDNA and a collection of point and deletion mutants derived from this clone. Whitefly transmission of CVYV was abolished in a deletion mutant lacking amino acids in position 93-105 of the CP. This deletion mutant caused more severe disease symptoms compared to the cDNA clone representing the wild-type (wt) virus and movement efficiency was likewise affected. Two virus mutants carrying a partially restored CP were transmissible and showed symptoms comparable to the wt virus. Collectively, our data demonstrate that the N-terminus of the CVYV CP is a determinant for transmission by the whitefly vector and is involved in plant infection and symptom expression.

High-Yield Production of Receptor Binding Domain of SARS-CoV-2 Linked to Bacterial Flagellin in Plants Using Self-Replicating Viral Vector pEff

The development of recombinant vaccines against SARS-CoV-2 is required to eliminate the COVID-19 pandemic. We reported the expression of a recombinant protein Flg-RBD comprising receptor binding domain of SARS-CoV-2 spike glycoprotein (RBD) fused to flagellin of Salmonella typhimurium (Flg), known as mucosal adjuvant, in Nicotiana benthamiana plants. The fusion protein, targeted to the cytosol, was transiently expressed using the self-replicating vector pEff based on potato virus X genome. The recombinant protein Flg-RBD was expressed at the level of about 110-140 μg per gram of fresh leaf tissue and was found to be insoluble. The fusion protein was purified using metal affinity chromatography under denaturing conditions. To increase the yield of Flg-RBD, the flow-through fraction obtained after loading of the protein sample on the Ni-NTA resin was re-loaded on the sorbent. The yield of Flg-RBD after purification reached about 100 μg per gram of fresh leaf tissue and the purified protein remained soluble after dialysis. The control flagellin was expressed in a soluble form and its yield after purification was about 300 μg per gram of fresh leaf biomass. Plant-produced Flg-RBD protein could be further used for the development of intranasal recombinant mucosal vaccines against COVID-19.

Robust Agrobacterium-Mediated Transient Expression in Two Duckweed Species (Lemnaceae) Directed by Non-replicating, Replicating, and Cell-to-Cell Spreading Vectors

Plant-based transient expression systems have recognized potential for use as rapid and cost-effective alternatives to expression systems based on bacteria, yeast, insect, or mammalian cells. The free-floating aquatic plants of the Lemnaceae family (duckweed) have compact architecture and can be vegetatively propagated on low-cost nutrient solutions in aseptic conditions. These features provide an economically feasible opportunity for duckweed-based production of high-value products via transient expression of recombinant products in fully contained, controlled, aseptic and bio-safe conditions in accordance with the requirements for pharmaceutical manufacturing and environmental biosafety. Here, we demonstrated Agrobacterium-mediated high-yield transient expression of a reporter green fluorescent protein using deconstructed vectors based on potato virus X and sweet potato leaf curl virus, as well as conventional binary vectors, in two representatives of the Lemnaceae (Spirodela polyrhiza and Landoltia punctata). Aseptically cultivated duckweed populations yielded reporter protein accumulation of >1 mg/g fresh biomass, when the protein was expressed from a deconstructed potato virus X-based vector, which is capable of replication and cell-to-cell movement of the replicons in duckweed. The expression efficiency demonstrated here places duckweed among the most efficient host organisms for plant-based transient expression systems, with the additional benefits of easy scale-up and full containment.

Production of Genistein in Amaranthus tricolor var. tristis and Spinacia oleracea by Expression of Glycine max Isoflavone Synthase

Isoflavonoids, the diverse group of secondary metabolites derived from the phenylpropanoid pathway, are distributed predominantly in leguminous plants. It has received considerable attention in recent days due to its health promoting benefits and is known to prevent certain diseases in humans. These isoflavonoids are synthesized from flavonoid intermediates of phenylpropanoid pathway by the enzyme isoflavone synthase. Metabolic engineering of isoflavonoid biosynthesis in non-legume crop plants could offer the health benefits of these compounds in diverse plant species further contributing for crop improvement. The transient expression of heterologous genes in the host is considered as an alternative to stable expression, that can provide a rapid way of studying the pathway engineering for metabolite production and could also act as a production platform for nutraceuticals and biopharmaceuticals. In this study, isoflavone genistein was produced in Amaranthus tricolor var. tristis and Spinacia oleracea by transiently expressing Glycine max isoflavone synthase (GmIFS). The GmIFS gene was cloned in plant expression vector pEarleyGate 102 HA and pEAQ-HT-DEST 3 and transformed into plants by agroinfiltration. The presence of transgene in the agroinfiltrated leaves was confirmed by semiquantitative reverse-transcription polymerase chain reaction. The flavonoid substrate naringenin and isoflavonoid genistein were quantified using high performance liquid chromatography in both wild-type and infiltrated leaf samples of both the plants. The naringenin content varied in the range of 65.5-338.5 nM/g fresh weight, while the accumulation of genistein was observed with varying concentrations from 113 to 182.6 nM/g fresh weight in the agroinfiltrated leaf samples of both A. tricolor var. tristis and S. oleracea. These results indicate that the transient expression of GmIFS gene has led to the synthesis of isoflavonoid genistein in A. tricolor var. tristis and S. oleracea providing an insight that stable expression of this gene could enrich the nutraceutical content in the crop plants. To the best of our knowledge, this is the first report on transient expression of GmIFS gene for the production of genistein in A. tricolor var. tristis and S. oleracea.

Long-Term Potato Virus X (PVX)-Based Transient Expression of Recombinant GFP Protein in Nicotiana benthamiana Culture In Vitro

Plant molecular farming has a great potential to produce valuable proteins. Transient expression technology provides high yields of recombinant proteins in greenhouse-grown plants, but every plant must be artificially agroinfiltrated, and open greenhouse systems are less controlled. Here, we propose to propagate agrobacteria-free plants with high-efficient long-term self-replicated transient gene expression in a well-controlled closed in vitro system. Nicotiana benthamiana plant tissue culture in vitro, with transient expression of recombinant GFP, was obtained through shoot induction from leaf explants infected by a PVX-based vector. The transient expression occurs in new tissues and regenerants due to the natural systemic distribution of viral RNA carrying the target gene. Gene silencing was delayed in plants grown in vitro, and GFP was detected in plants for five to six months. Agrobacteria-free, GFP-expressing plants can be micropropagated in vitro (avoiding an agroinfiltration step), "rejuvenated" through regeneration (maintaining culture for years), or transferred in soil. The mean GFP in the regenerants was 18% of the total soluble proteins (TSP) (0.52 mg/g of fresh leaf weight (FW). The highest value reached 47% TSP (2 mg/g FW). This study proposes a new method for recombinant protein production combining the advantages of transient expression technology and closed cultural systems.

Development and Optimization of an Enzyme Immunoassay to Detect Serum Antibodies against the Hepatitis E Virus in Pigs, Using Plant-Derived ORF2 Recombinant Protein

Hepatitis E is an emerging global disease, mainly transmitted via the fecal-oral route in developing countries, and in a zoonotic manner in the developed world. Pigs and wild boar constitute the primary Hepatitis E virus (HEV) zoonotic reservoir. Consumption of undercooked animal meat or direct contact with infected animals is the most common source of HEV infection in European countries. The purpose of this study is to develop an enzyme immunoassay (EIA) for the detection of anti-hepatitis E virus IgG in pig serum, using plant-produced recombinant HEV-3 ORF2 as an antigenic coating protein, and also to evaluate the sensitivity and specificity of this assay. A recombinant HEV-3 ORF2 110-610_6his capsid protein, transiently expressed by pEff vector in Nicotiana benthamiana plants was used to develop an in-house HEV EIA. The plant-derived HEV-3 ORF2 110-610_6his protein proved to be antigenically similar to the HEV ORF2 capsid protein and it can self-assemble into heterogeneous particulate structures. The optimal conditions for the in-house EIA (iEIA) were determined as follows: HEV-3 ORF2 110-610_6his antigen concentration (4 µg/mL), serum dilution (1:50), 3% BSA as a blocking agent, and secondary antibody dilution (1:20 000). The iEIA developed for this study showed a sensitivity of 97.1% (95% Cl: 89.9-99.65) and a specificity of 98.6% (95% Cl: 92.5-99.96) with a Youden index of 0.9571. A comparison between our iEIA and a commercial assay (PrioCHECK™ Porcine HEV Ab ELISA Kit, ThermoFisher Scientific, MA, USA) showed 97.8% agreement with a kappa index of 0.9399. The plant-based HEV-3 ORF2 iEIA assay was able to detect anti-HEV IgG in pig serum with a very good agreement compared to the commercially available kit.

Synthetic Promoters from Strawberry Vein Banding Virus (SVBV) and Dahlia Mosaic Virus (DaMV)

We have constructed two intra-molecularly shuffled promoters, namely S100 and D100. The S100 recombinant promoter (621 bp) was generated by ligation of 250 bp long upstream activation sequence (UAS) of Strawberry vein banding virus (SV10UAS; - 352 to - 102 relative to TSS) with its 371 bp long TATA containing core promoter domain (SV10CP; - 352 to + 19). Likewise, 726 bp long D100 promoter was constructed by fusion of 170 bp long UAS of Dahlia mosaic virus (DaMV14UAS; - 203 to - 33) with its 556 bp long core promoter domain (DaMV4CP; - 474 to + 82). S100 and D100 promoters showed 1.8 and 2.2 times stronger activities than that of the CaMV35S promoter. The activity of the promoters is comparable to that of the CaMV35S2 promoter. Transcript analysis employing qRT-PCR and histochemical assays supported the above findings. Abscisic acid and salicylic acid induce the activity of the D100 promoter. Leaf protein obtained from Nicotiana tabacum plant expressing NSD2 gene (Nigella sativa L. defensin 2) driven by the D100 promoter showed antifungal activity against Alternaria alternata and Phoma exigua var. exigua and antibacterial activity against Pseudomonas aeruginosa and Staphylococcus aureus. Strong S100 and D100 promoters have potential to become efficient candidates for plant metabolic engineering and molecular pharming.

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.

Transient expression of recombinant proteins in plants using potato virus X based vectors

Plants become a promising biofactory for the large-scale production of recombinant proteins due to low cost, scalability, and safety. Agroinfiltration of plant leaves with a plant viral vector carrying a gene of interest is a rapid and efficient method for protein production in plants. Currently this method is in use for producing a wide range of proteins for multiple applications, including vaccine antigens, antibodies, and protein nanoparticles such as virus-like particles. A number of pharmaceutical proteins produced by transient expression are currently in clinical development. Here, we describe potato virus X based vector pEff-GFP enabling fast and high-level expression of recombinant proteins in Nicotiana benthamiana plants. The pEff vector provides green fluorescent protein expression levels of up to 30% of total soluble protein (about 1mg per g of fresh leaf tissue) and was successfully applied for the production of the immunogens of potential clinical interest.

Fluorescence activated cell sorting-A selective tool for plant cell isolation and analysis

Instrumentation for flow cytometry and sorting is designed around the assumption that samples are single-cell suspensions. However, with few exceptions, higher plants comprise complex multicellular tissues and organs, in which the individual cells are held together by shared cell walls. Single-cell suspensions can be obtained through digestion of the cells walls and release of the so-called protoplasts (plants without their cell wall). Here we describe best practices for protoplast preparation, and for analysis through flow cytometry and cell sorting. Finally, the numerous downstream applications involving sorted protoplasts are discussed.

A Replicating Viral Vector Greatly Enhances Accumulation of Helical Virus-Like Particles in Plants

The production of plant helical virus-like particles (VLPs) via plant-based expression has been problematic with previous studies suggesting that an RNA scaffold may be necessary for their efficient production. To examine this, we compared the accumulation of VLPs from two potexviruses, papaya mosaic virus and alternanthera mosaic virus (AltMV), when the coat proteins were expressed from a replicating potato virus X- based vector (pEff) and a non-replicating vector (pEAQ-HT). Significantly greater quantities of VLPs could be purified when pEff was used. The pEff system was also very efficient at producing VLPs of helical viruses from different virus families. Examination of the RNA content of AltMV and tobacco mosaic virus VLPs produced from pEff revealed the presence of vector-derived RNA sequences, suggesting that the replicating RNA acts as a scaffold for VLP assembly. Cryo-EM analysis of the AltMV VLPs showed they had a structure very similar to that of authentic potexvirus particles. Thus, we conclude that vectors generating replicating forms of RNA, such as pEff, are very efficient for producing helical VLPs.

Development and optimization of a pepino mosaic virus-based vector for rapid expression of heterologous proteins in plants

Plant-virus-derived vectors are versatile tools with multiple applications in agricultural and medical biotechnology. In this study, we developed pepino mosaic virus (PepMV) (family Alphaflexiviridae; genus Potexvirus) into a vector for heterologous protein expression in plants. PepMV was initially cloned in a step-wise manner, fully sequenced and the full-length infectious clone was tested for infectivity in Nicotiana benthamiana. Initial infectious clones resulted in poor replication of PepMV and lack of systemic movement. Mutations in the viral sequence affected systemic infection. Two suspected mutations were altered to restore systemic infectivity. PepMV infection was apparent as early as 4 days post agroinfiltration (dpa) inoculation in N. benthamiana. A multiple cloning site was inserted into the PepMV genome for introduction and expression of foreign genes. Several modifications to the wild-type vector were made, such as a replacing the native subgenomic promoter (SGP) with a heterologous SGP, and introduction of translational enhancers and terminators, to improve heterologous expression of the foreign gene-of-interest. GFP was used as a reporter for monitoring virus infection and protein production. Strong GFP expression was observed as early as 4 dpa with a translational enhancer. The PepMV-based vector produces rapid expression of the foreign gene in comparison to two other potexvirus-based vectors. GFP production was monitored over time and optimal protein production was recorded between 5 and 7 dpa. GFP protein levels reached up to 4% and decreased to 0.5% total soluble protein at 7 and 14 dpa, respectively. Future studies will evaluate this virus-based vector for large-scale production of pharmaceutical compounds. KEY POINTS: • A pepino mosaic virus isolate was developed into a plant-based expression vector. • Expression levels of the heterologous protein were comparable or exceeded previously developed viral vectors. • Protein levels in plants were highest between 5 and 7 days and decreased gradually.

Optimization of the human colorectal carcinoma antigen GA733-2 production in tobacco plants

The colorectal carcinoma-associated protein GA733-2 is one of the representative candidate protein for the development of plant-derived colorectal cancer vaccine. Despite of its significant importance for colorectal vaccine development, low efficiency of GA733-2 production limits its wide applications. To improve productivity of GA733-2 in plants, we here tested multiple factors that affect expression of recombinant GA733-2 (rGA733-2) and rGA733 fused to fragment crystallizable (Fc) domain (rGA733-Fc) protein. The rGA733-2 and rGA733-Fc proteins were highly expressed when the pBINPLUS vector system was used for transient expression in tobacco plants. In addition, the length of interval between rGA733-2 and left border of T-DNA affected the expression of rGA733 protein. Transient expression analysis using various combinations of Agrobacterium tumefaciens strains (C58C1, LBA4404, and GV3101) and tobacco species (Nicotiana tabacum cv. Xanthi nc and Nicotiana benthamiana) revealed that higher accumulation of rGA733-2 and rGA733-Fc proteins were obtained by combination of A. tumefaciens LBA4404 and Nicotiana benthamiana. Transgenic plants generated by introduction of the rGA733-2 and rGA733-Fc expression cassettes also significantly accumulated corresponding recombinant proteins. Bioactivity and stability of the plant-derived rGA733 and rGA733-Fc were evaluated by further in vitro assay, western blot and N-glycosylation analysis. Collectively, we here suggest the optimal condition for efficient production of functional rGA733-2 protein in tobacco system.

Overexpression and Purification of Gracilariopsis chorda Carbonic Anhydrase (GcCAα3) in Nicotiana benthamiana, and Its Immobilization and Use in CO2 Hydration Reactions

Carbonic anhydrase (CA; EC 4.2.2.1) is a Zn-binding metalloenzyme that catalyzes the reversible hydration of CO2. Recently, CAs have gained a great deal of attention as biocatalysts for capturing CO2 from industrial flue gases owing to their extremely fast reaction rates and simple reaction mechanism. However, their general application for this purpose requires improvements to stability at high temperature and under in vitro conditions, and reductions in production and scale-up costs. In the present study, we developed a strategy for producing GcCAα3, a CA isoform from the red alga Gracilariopsis chorda, in Nicotiana benthamiana. To achieve high-level expression and facile purification of GcCAα3, we designed various constructs by incorporating various domains such as translation-enhancing M domain, SUMO domain and cellulose-binding domain CBM3. Of these constructs, MC-GcCAα3 that had the M and CBM3 domains was expressed at high levels in N. benthamiana via agroinfiltration with a yield of 1.0 g/kg fresh weight. The recombinant protein was targeted to the endoplasmic reticulum (ER) for high-level accumulation in plants. Specific and tight CBM3-mediated binding of recombinant GcCAα3 proteins to microcrystalline cellulose beads served as a means for both protein purification from total plant extracts and protein immobilization to a solid surface for increased stability, facilitating multiple rounds of use in CO2 hydration reactions.

A plant-based transient expression system for the rapid production of highly immunogenic Hepatitis E virus-like particles

OBJECTIVE

Hepatitis E virus (HEV) infection is a major cause of acute hepatitis worldwide. The aim of the study is the development of plant expression system for the production of virus-like particles formed by HEV capsid and the characterization of their immunogenicity.

RESULTS

Open reading frame (ORF) 2 encodes the viral capsid protein and possesses candidate for vaccine production. In this study, we used truncated genotype 3 HEV ORF 2 consisting of aa residues 110 to 610. The recombinant protein was expressed in Nicotiana benthamiana plants using the self-replicating potato virus X-based vector pEff up to 10% of the soluble protein fraction. The yield of HEV 110-610 after purification was 150-200 µg per 1 g of green leaf biomass. The recombinant protein formed nanosized virus-like particles. The immunization of mice with plant-produced HEV 110-610 protein induced high levels of HEV-specific serum antibodies.

CONCLUSIONS

HEV ORF 2 (110-610 aa) can be used as candidate for the development of a plant-produced vaccine against Hepatitis E.

Development of Tomato bushy stunt virus-based vectors for fusion and non-fusion expression of heterologous proteins in an alternative host Nicotiana excelsiana

Plant virus-based expression systems are an alternative expression platform for the production of clinically and industrially useful recombinant proteins. Nonetheless, due to a lack of viral vector with the commercial potentials, it is urgent to design and develop new, versatile, and efficient plant virus vectors. The genome of Tomato bushy stunt virus (TBSV) offers an attractive alternative to being modified as a vector for producing heterologous proteins in plants. Here, we developed a set of novel fusion and non-fusion TBSV-CP replacement vectors, which provide more flexible and efficient tools for expressing proteins of interest in plants. An alternative tobacco plant, Nicotiana excelsiana, was used in this study as a host for newly constructed TBSV vectors because the unwanted necrotic effects were reported on the commonly used Nicotiana benthamiana host associated with expression of TBSV-encoded P19 protein. The data showed that TBSV vectors caused a symptomless infection and overexpressed reporter gene in N. excelsiana leaves, demonstrating that N. excelsiana is an ideal host plant for TBSV-mediated heterologous gene expression. Moreover, a TBSV non-fusion vector, dAUG, shows the similar accumulation level of reporter proteins to that of TMV- and PVX-based vectors in side-by-side comparison and provides more flexible aspects than the previously developed TBSV vectors. Collectively, our newly developed TBSV expression system adds a new member to the family of plant viral expression vectors and meanwhile offers a flexible and highly effective approach for producing proteins of interest in plants. KEY POINTS: • The TBSV-based transient expression system has been significantly improved. • The necrotic effects caused by viral P19 protein were avoided by the usage of N. excelsiana as a host plant. • The expression level of the non-fusion vector was similar to the most effective virus vectors reported so far.

Transient protein expression in tobacco BY-2 plant cell packs using single and multi-cassette replicating vectors

KEY MESSAGE

This is the first evidence that replicating vectors can be successfully used for transient protein expression in BY-2 plant cell packs. Transient recombinant protein expression in plants and recently also plant cell cultures are of increasing interest due to the speed, safety and scalability of the process. Currently, studies are focussing on the design of plant virus-derived vectors to achieve higher amounts of transiently expressed proteins in these systems. Here we designed and tested replicating single and multi-cassette vectors that combine elements for enhanced replication and hypertranslation, and assessed their ability to express and particularly co-express proteins by Agrobacterium-mediated transient expression in tobacco BY-2 plant cell packs. Substantial yields of green and red fluorescent proteins of up to ~ 700 ng/g fresh mass were detected in the plant cells along with position-dependent expression. This is the first evidence of the ability of replicating vectors to transiently express proteins in BY-2 plant cell packs.

Heterologous expression of biologically active Mambalgin-1 peptide as a new potential anticancer, using a PVX-based viral vector in Nicotiana benthamiana

Mambalgin-1 is a peptide that acts as a potent analgesic through inhibiting acid-sensing ion channels (ASIC) in nerve cells. Research has shown that ASIC channels are involved in the proliferation and growth of cancer cells; therefore, Mambalgin-1 can be a potential anti-cancer by inhibiting these channels. In the present study, the Nicotiana benthamiana codon optimized Mambalgin-1 gene was synthesized and cloned in PVX (potato virus X) viral vector. The two cultures of Agrobacterium containing Mambalgin-1 and P19 silencing suppressor genes were co-agroinfiltrated into N. benthamiana leaves. Five days post infiltration, the production of recombinant Mambalgin-1 was determined by western blotting. For biological activity, MTT (3(4, 5-dimethylthiazole-2-yl)-2, 5-diphenyltetrazolium bromide) was analyzed for the cytotoxicity recombinant Mambalgin-1 from the transformed plants on nervous (SH-SY5Y) and breast (MCF7) cancer cells. The results showed that the plants expressing open reading frame of Mambalgin-1 showed recombinant 7.4 kDa proteins in the entire plant extract. In the MTT test, it was found that Mambalgin-1 had cytotoxic effects on SH-SY5Y cancer cells, yet no effects on MCF7 cancer cells were observed. According to the results, the expression of the biologically active recombinant Mambalgin-1 in the transformed plant leaves was confirmed and Mambalgin-1 can also have anti-cancer (inhibition of ASIC channels) potential along with its already known analgesic effect.

Plant-Produced Recombinant Influenza A Virus Candidate Vaccine Based on Flagellin Linked to Conservative Fragments of M2 Protein and Hemagglutintin

The development of recombinant influenza vaccines with broad spectrum protection is an important task. The combination of conservative viral antigens, such as M2e, the extracellular domain of the transmembrane protein M2, and conserved regions of the second subunit of hemagglutinin (HA), provides an opportunity for the development of universal influenza vaccines. Immunogenicity of the antigens could be enhanced by fusion to bacterial flagellin, the ligand for Toll-like receptor 5, acting as a powerful mucosal adjuvant. In this study, we report the transient expression in plants of a recombinant protein comprising flagellin of Salmonella typhimurium fused to the conserved region of the second subunit of HA (76–130 a.a.) of the first phylogenetic group of influenza A viruses and four tandem copies of the M2e peptide. The hybrid protein was expressed in Nicotiana benthamiana plants using the self-replicating potato virus X-based vector pEff up to 300 µg/g of fresh leaf tissue. The intranasal immunization of mice with purified fusion protein induced high levels of M2e-specific serum antibodies and provided protection against lethal challenge with influenza A virus strain A/Aichi/2/68(H3N2). Our results show that M2e and hemagglutinin-derived peptide can be used as important targets for the development of a plant-produced vaccine against influenza.

Virus-based pharmaceutical production in plants: an opportunity to reduce health problems in Africa

BACKGROUND

Developing African countries face health problems that they struggle to solve. The major causes of this situation are high therapeutic and logistical costs. Plant-made therapeutics are easy to produce due to the lack of the safety considerations associated with traditional fermenter-based expression platforms, such as mammalian cells. Plant biosystems are easy to scale up and inexpensive, and they do not require refrigeration or a sophisticated medical infrastructure. These advantages provide an opportunity for plant-made pharmaceuticals to counteract diseases for which medicines were previously inaccessible to people in countries with few resources.

MAIN BODY

The techniques needed for plant-based therapeutic production are currently available. Viral expression vectors based on plant viruses have greatly enhanced plant-made therapeutic production and have been exploited to produce a variety of proteins of industrial, pharmaceutical and agribusiness interest. Some neglected tropical diseases occurring exclusively in the developing world have found solutions through plant bioreactor technology. Plant viral expression vectors have been reported in the production of therapeutics against these diseases occurring exclusively in the third world, and some virus-derived antigens produced in plants exhibit appropriate antigenicity and immunogenicity. However, all advances in the use of plants as bioreactors have been made by companies in Europe and America. The developing world is still far from acquiring this technology, although plant viral expression vectors may provide crucial help to overcome neglected diseases.

CONCLUSION

Today, interest in these tools is rising, and viral amplicons made in and for Africa are in progress. This review describes the biotechnological advances in the field of plant bioreactors, highlights factors restricting access to this technology by those who need it most and proposes a solution to overcome these limitations.

Rapid High-Yield Transient Expression of Swine Hepatitis E ORF2 Capsid Proteins in Nicotiana benthamiana Plants and Production of Chimeric Hepatitis E Virus-Like Particles Bearing the M2e Influenza Epitope

The Hepatitis E virus (HEV) is a causative agent of acute hepatitis, mainly transmitted by the fecal-oral route or zoonotic. Open reading frame (ORF) 2 encodes the viral capsid protein, which is essential for virion assembly, host interaction, and inducing neutralizing antibodies. In this study, we investigated whether full-length and N- and C-terminally modified versions of the capsid protein transiently expressed in N. benthamiana plants could assemble into highly-immunogenic, virus-like particles (VLPs). We also assessed whether such VLPs can act as a carrier of foreign immunogenic epitopes, such as the highly-conserved M2e peptide from the Influenza virus. Plant codon-optimized HEV ORF2 capsid genes were constructed in which the nucleotides coding the N-terminal, the C-terminal, or both parts of the protein were deleted. The M2e peptide was inserted into the P2 loop after the residue Gly556 of HEV ORF2 protein by gene fusion, and three different chimeric constructs were designed. Plants expressed all versions of the HEV capsid protein up to 10% of total soluble protein (TSP), including the chimeras, but only the capsid protein consisting of aa residues 110 to 610 (HEV 110–610) and chimeric M2 HEV 110–610 spontaneously assembled in higher order structures. The chimeric VLPs assembled into particles with 22–36 nm in diameter and specifically reacted with the anti-M2e antibody.

Second generation of pepino mosaic virus vectors: improved stability in tomato and a wide range of reporter genes

BACKGROUND

Vectors based on plant viruses are important tools for functional genomics, cellular biology, plant genome engineering and molecular farming. We previously reported on the construction of PepGFP2a, a viral vector based on pepino mosaic virus (PepMV) which expressed GFP efficiently and stably in plants of its experimental host Nicotiana benthamiana, but not in its natural host tomato. We have prepared a new set of PepMV-based vectors with improved stability that are able to express a wide range of reporter genes, useful for both N. benthamiana and tomato.

RESULTS

We first tested PepGFPm1 and PepGFPm2, two variants of PepGFP2a in which we progressively reduced a duplication of nucleotides encoding the N-terminal region of the coat protein. The new vectors had improved GFP expression levels and stability in N. benthamiana but not in tomato plants. Next, we replaced GFP by DsRed or mCherry in the new vectors PepDsRed and PepmCherry, respectively; while PepmCherry behaved similarly to PepGFPm2, PepDsRed expressed the reporter gene efficiently also in tomato plants. We then used PepGFPm2 and PepDsRed to study the PepMV localization in both N. benthamiana and tomato cells. Using confocal laser scanning microscopy (CLSM), we observed characteristic fluorescent bodies in PepMV-infected cells; these bodies had a cytoplasmic localization and appeared in close proximity to the cell nucleus. Already at 3 days post-agroinoculation there were fluorescent bodies in almost every cell of agroinoculated tissues of both hosts, and always one body per cell. When markers for the endoplasmic reticulum or the Golgi apparatus were co-expressed with PepGFPm2 or PepDsRed, a reorganisation of these organelles was observed, with images suggesting that both are intimately related but not the main constituents of the PepMV bodies. Altogether, this set of data suggested that the PepMV bodies are similar to the potato virus X (PVX) "X-bodies", which have been described as the PVX viral replication complexes (VRCs). To complete the set of PepMV-based vectors, we constructed a vector expressing the BAR herbicide resistance gene, useful for massive susceptibility screenings.

CONCLUSIONS

We have significantly expanded the PepMV tool box by producing a set of new vectors with improved stability and efficiency in both N. benthamiana and tomato plants. By using two of these vectors, we have described characteristic cellular bodies induced by PepMV infection; these bodies are likely the PepMV VRCs.

Synthetic Salicylic acid inducible recombinant promoter for translational research

In the present study, we have developed an inter-molecularly shuffled caulimoviral promoter for protein over-expression by placing the Upstream Activation Sequence (UAS) of Figwort Mosaic Virus (FMV; -249 to -54) at the 5'-end of the Cassava Vein Mosaic Virus (CsVMV) promoter fragment 8 (CsVMV8; -215 to +166) to design a hybrid promoter; FUASCsV8CP. The FUASCsV8CP promoter exhibited approximately 2.1 and 2.0 times higher GUS-activities than that obtained from the CaMV35S promoter, in tobacco (Xanthi Brad) protoplasts and in Agroinfiltration assays respectively. Hereto, when FUASCsV8CP was assayed using transgenic tobacco plants (T2- generation), it showed 2.0 times stronger activity than CaMV35S promoter and almost equivalent activity to that of CaMV35S2 promoter. The promoter displayed Salicylic acid (SA) inducibility and hence can also be used for ensuring effective gene expression in plants under constitutive as well as specific inducible conditions. Furthermore, FUASCsV8CP was used to drive the expression of victoviral Vin gene (encoding Victoriocin) transiently in tobacco. The recombinant Victoriocin could be successfully detected by western blotting three days post infiltration. Also, the in vitro Agar-based killing zone assays employing plant-derived Victoriocin-His (obtained from transient expression of Vin) revealed enhanced antifungal activity of Victoriocin against hemi-biotrophic pathogen Phoma exigua Desm. var. exigua.

Combination of M2e peptide with stalk HA epitopes of influenza A virus enhances protective properties of recombinant vaccine

Background

Influenza infection could be more effectively controlled if a multi-purpose vaccine with the ability to induce responses against most, or all, influenza A subtypes could be generated. Conserved viral proteins are a promising basis for the creation of a broadly protective vaccine. In the present study, the immunogenicity and protective properties of three recombinant proteins (vaccine candidates), comprising conserved viral proteins fused with bacterial flagellin, were compared.

Methods

Balb/c mice were immunized intranasally with recombinant proteins comprising either one viral protein (the ectodomain of the M2 protein, ‘M2e’) or two viral proteins (M2e and the hemagglutinin second subunit ‘HA2’ epitope) genetically fused with flagellin. Further, two different consensus variants of HA2 were used. Therefore, three experimental positives were used in addition to the negative control (Flg-his). The mucosal, humoral, and T-cell immune responses to these constructs were evaluated.

Result

We have demonstrated that insertion of the HA2 consensus polypeptide (aa 76–130), derived from either the first (HA2-1) or second (HA2-2) virus phylogenetic group, into the recombinant Flg4M2e protein significantly enhanced its immunogenicity and protective properties. Intranasal administration of the vaccine candidates (Flg-HA2-2-4M2e or Flg-HA2-1-4M2e) induced considerable mucosal and systemic responses directed at both the M2e-protein and, in general, the influenza A virus. However, the immune response elicited by the Flg-HA2-1-4M2e protein was weaker than the one generated by Flg-HA2-2-4M2e. These recombinant proteins containing both viral peptides provide complete protection from lethal challenge with various influenza viruses: A/H3N2; A/H2N2; and A/H5N1.

Conclusion

This study demonstrates that the intranasal administration of Flg-HA2-2-4M2e recombinant protein induces a strong immune response which provides broad protection against various influenza viruses. This construct is therefore a strong candidate for development as a universal vaccine.