Application of a Scalable Plant Transient Gene Expression Platform for Malaria Vaccine Development

Overview of Recombinant Tick Vaccines and Perspectives on the Use of Plant-Made Vaccines to Control Ticks of Veterinary Importance

Ticks are obligate hematophagous ectoparasites that affect animals, and some of them transmit a wide range of pathogens including viruses, bacteria, and protozoa to both animals and humans. Several vaccines have shown immunogenicity and protective efficacy against ticks in animal models and definitive hosts. After several decades on anti-tick vaccine research, only a commercial vaccine based on a recombinant antigen is currently available. In this context, plants offer three decades of research and development on recombinant vaccine production to immunize hosts and as a delivery vehicle platform. Despite the experimental advances in plant-made vaccines to control several parasitosis and infectious diseases, no vaccine prototype has been developed against ticks. This review examines a panorama of ticks of veterinary importance, recombinant vaccine experimental developments, plant-made vaccine platforms, and perspectives on using this technology as well as the opportunities and limitations in the field of tick vaccine research.

Plants as Sources of Natural and Recombinant Antimalaria Agents

Malaria is one of the severe infectious diseases that has victimized about half a civilization billion people each year worldwide. The application of long-lasting insecticides is the main strategy to control malaria; however, a surge in antimalarial drug development is also taking a leading role to break off the infections. Although, recurring drug resistance can compromise the efficiency of both conventional and novel antimalarial medicines. The eradication of malaria is significantly contingent on discovering novel potent agents that are low cost and easy to administer. In this context, plant metabolites inhibit malaria infection progression and might potentially be utilized as an alternative treatment for malaria, such as artemisinin. Advances in genetic engineering technology, especially the advent of molecular farming, have made plants more versatile in producing protein drugs (PDs) to treat infectious diseases, including malaria. These recent developments in genetic modifications have enabled the production of native pharmaceutically active compounds and the accumulation of diverse heterologous proteins such as human antibodies, booster vaccines, and many PDs to treat infectious diseases and genetic disorders. This review will discuss the pivotal role of a plant-based production system that expresses natural antimalarial agents or host protein drugs to cure malaria infections. The potential of these natural and induced compounds will support modern healthcare systems in treating malaria infections, especially in developing countries to mitigate human fatalities.

Recombinant Protein Production in Plants: A Brief Overview of Strengths and Challenges

The first recombinant proteins were produced in microbes and animal cells cultivated in bioreactors. These systems have become the standard for industrial-scale recombinant protein manufacturing. Later, the production of recombinant proteins was demonstrated in whole plants, which differ morphologically from cell-based systems and require completely different cultivation conditions. Over time, additional plant-based production platforms were established, including hairy roots and cell suspension cultures, which are more similar to conventional cell-based systems in terms of morphology, procedures, and equipment requirements. In this brief overview of the field, we explain why plant-based systems are becoming increasingly attractive for the production of valuable proteins with scientific and commercial applications, but also highlight the challenges that these systems must overcome to achieve more widespread industrial utilization. We discuss various laboratory protocols and approaches for the production of recombinant proteins in plants, as well as strategies to optimize yields, and the regulatory and legal framework.

Natural Bioactives: Back to the Future in the Fight against Human Papillomavirus? A Narrative Review

Human papillomavirus (HPV) still represents an important threat to health worldwide. Better therapy in terms of further improvement of outcomes and attenuation of related side-effects is desirable. The pharmaceutical industry has always targeted natural substances-phytochemicals in particular-to identify lead compounds to be clinically validated and industrially produced as antiviral and anticancer drugs. In the field of HPV, numerous naturally occurring bioactives and dietary phytochemicals have been investigated as potentially valuable in vitro and in vivo. Interference with several pathways and improvement of the efficacy of chemotherapeutic agents have been demonstrated. Notably, some clinical trials have been conducted. Despite being endowed with general safety, these natural substances are in urgent need of further assessment to foresee their clinical exploitation. This review summarizes the basic research efforts conducted so far in the study of anti-HPV properties of bio-actives with insights into their mechanisms of action and highlights the variety of their natural origin in order to provide comprehensive mapping throughout the different sources. The clinical studies available are reported, as well, to highlight the need of uniformity and consistency of studies in the future to select those natural compounds that may be suited to clinical application.

Sinigrin in combination with artesunate provides protection against lethal murine malaria via falcipain-3 inhibition and immune modulation

Plant-derived antimalarials are indispensable for malaria treatment and a platform for new drugs. The present study explores sinigrin, for malaria using in vitro, in silico and in vivo strategies and the immune response generated after administration. The compound exhibited promising activity against chloroquine (CQ)-resistant (RKL-9) IC₅₀ 5.14 μg/mL and CQ-sensitive (3D7) IC₅₀ 5.47 μg/mL strains of P. falciparum and was safe in both in vitro (CC₅₀ > 640 μg/mL) and in vivo (LD₅₀ > 2 g/kg) toxicity studies. In addition, virtual screening showed hydrogen bonding, hydrophobic and van der Waals interactions with amino acid residues of 3BPM (falcipain-3). In vivo studies revealed promising antimalarial activity of sinigrin (200 mg/kg) with 87.44% chemo-suppression on day 5 and significantly (p < 0.0001) enhanced the mean survival time (21 ± 4.74 days) in contrast to the infected control (5.4 ± 1.14 days). In combination therapy, sinigrin (100 mg/kg and 200 mg/kg) augmented the efficacy of artesunate (AS 50 mg/kg) with 100% survival and no recrudescence. These observations are further corresponded and supported by DLC, NO production, cytokine analysis, biochemical and histopathological studies. Treatment with the combination resulted in a regulated interplay of immune cells and cytokines aiding in parasite clearance in addition to its specific inhibitory activity. We report the antimalarial activity of sinigrin first time with best D-score against falcipain-3. These findings highlight sinigrin as a HIT molecule, which may potentially be used in drug and vaccine development approaches.

How can we develop an effective subunit vaccine to achieve successful malaria eradication?

Malaria, a mosquito-borne infection, is the most widespread parasitic disease. Despite numerous efforts to eradicate malaria, this disease is still a health concern worldwide. Owing to insecticide-resistant vectors and drug-resistant parasites, available controlling measures are insufficient to achieve a malaria-free world. Thus, there is an urgent need for new intervention tools such as efficient malaria vaccines. Subunit vaccines are the most promising malaria vaccines under development. However, one of the major drawbacks of subunit vaccines is the lack of efficient and durable immune responses including antigen-specific antibody, CD4⁺, and CD8⁺ T-cell responses, long-lived plasma cells, memory cells, and functional antibodies for parasite neutralization or inhibition of parasite invasion. These types of responses could be induced by whole organism vaccines, but eliciting these responses with subunit vaccines has been proven to be more challenging. Consequently, subunit vaccines require several policies to overcome these challenges. In this review, we address common approaches that can improve the efficacy of subunit vaccines against malaria.

Efficient Plant Production of Recombinant NS1 Protein for Diagnosis of Dengue

Dengue fever is endemic in more than 120 countries, which account for 3.9 billion people at risk of infection worldwide. The absence of a vaccine with effective protection against the four serotypes of this virus makes differential molecular diagnosis the key step for the correct treatment of the disease. Rapid and efficient diagnosis prevents progression to a more severe stage of this disease. Currently, the limiting factor in the manufacture of dengue (DENV) diagnostic kits is the lack of large-scale production of the non-structural 1 (NS1) protein (antigen) to be used in the capture of antibodies from the blood serum of infected patients. In this work, we use plant biotechnology and genetic engineering as tools for the study of protein production for research and commercial purposes. Gene transfer, integration and expression in plants is a valid strategy for obtaining large-scale and low-cost heterologous protein production. The authors produced NS1 protein of the dengue virus serotype 2 (NS1DENV2) in the Arabidopsis thaliana plant. Transgenic plants obtained by genetic transformation expressed the recombinant protein that was purified and characterized for diagnostic use. The yield was 203 μg of the recombinant protein per gram of fresh leaf. By in situ immunolocalization, transgenic protein was observed within the plant tissue, located in aggregates bodies. These antigens showed high sensitivity and specificity to both IgM (84.29% and 91.43%, respectively) and IgG (83.08% and 87.69%, respectively). The study goes a step further to validate the use of plants as a strategy for obtaining large-scale and efficient protein production to be used in dengue virus diagnostic tests.

Ready-to-Use Stocks of Agrobacterium tumefaciens Can Simplify Process Development for the Production of Recombinant Proteins by Transient Expression in Plants

Large-scale automated transient protein expression in plants requires the synchronization of cultivation and bacterial fermentation, especially if more than one bacterial strain. Therefore, a ready-to-use approach that decouples bacterial fermentation and infiltration is developed. It is found that bacterial cultures can easily be reconstituted in infiltration medium at a user-defined time, optical density, and quantity. This allows the process flow to be staggered, avoiding bottlenecks in process capacity and labor. Using the red fluorescent protein, DsRed, as a model product, the ready-to-use preparations achieved the same yields in infiltrated plant biomass as Agrobacterium tumefaciens derived from regular fermentations. It is possible to store the ready-to-use stocks at -20 °C and -80 °C for more than two months without loss of activity. Using a consolidated cost model for the current fermentation process, it is found that the ready-to-use strategy can reduce operational costs by 20-95% and investment costs by up to 75%, which would otherwise offset the economic advantages of plants over mammalian expression systems during upstream production. Furthermore, the staggered cultivation of plants and bacteria reduces the likelihood of batch failure and thus increases the robustness and flexibility of transient expression for the production of recombinant proteins in plants.

CRISPR/Cas9-mediated knockout of six glycosyltransferase genes in Nicotiana benthamiana for the production of recombinant proteins lacking β-1,2-xylose and core α-1,3-fucose

Plants offer fast, flexible and easily scalable alternative platforms for the production of pharmaceutical proteins, but differences between plant and mammalian N-linked glycans, including the presence of β-1,2-xylose and core α-1,3-fucose residues in plants, can affect the activity, potency and immunogenicity of plant-derived proteins. Nicotiana benthamiana is widely used for the transient expression of recombinant proteins so it is desirable to modify the endogenous N-glycosylation machinery to allow the synthesis of complex N-glycans lacking β-1,2-xylose and core α-1,3-fucose. Here, we used multiplex CRISPR/Cas9 genome editing to generate N. benthamiana production lines deficient in plant-specific α-1,3-fucosyltransferase and β-1,2-xylosyltransferase activity, reflecting the mutation of six different genes. We confirmed the functional gene knockouts by Sanger sequencing and mass spectrometry-based N-glycan analysis of endogenous proteins and the recombinant monoclonal antibody 2G12. Furthermore, we compared the CD64-binding affinity of 2G12 glycovariants produced in wild-type N. benthamiana, the newly generated FX-KO line, and Chinese hamster ovary (CHO) cells, confirming that the glyco-engineered antibody performed as well as its CHO-produced counterpart.

Critical Analysis of the Commercial Potential of Plants for the Production of Recombinant Proteins

Over the last three decades, the expression of recombinant proteins in plants and plant cells has been promoted as an alternative cost-effective production platform. However, the market is still dominated by prokaryotic and mammalian expression systems, the former offering high production capacity at a low cost, and the latter favored for the production of complex biopharmaceutical products. Although plant systems are now gaining widespread acceptance as a platform for the larger-scale production of recombinant proteins, there is still resistance to commercial uptake. This partly reflects the relatively low yields achieved in plants, as well as inconsistent product quality and difficulties with larger-scale downstream processing. Furthermore, there are only a few cases in which plants have demonstrated economic advantages compared to established and approved commercial processes, so industry is reluctant to switch to plant-based production. Nevertheless, some plant-derived proteins for research or cosmetic/pharmaceutical applications have reached the market, showing that plants can excel as a competitive production platform in some niche areas. Here, we discuss the strengths of plant expression systems for specific applications, but mainly address the bottlenecks that must be overcome before plants can compete with conventional systems, enabling the future commercial utilization of plants for the production of valuable proteins.

Platforms for Plant-Based Protein Production

Plant molecular farming depends on a diversity of plant systems for production of useful recombinant proteins. These proteins include protein biopolymers, industrial proteins and enzymes, and therapeutic proteins. Plant production systems include microalgae, cells, hairy roots, moss, and whole plants with both stable and transient expression. Production processes involve a narrowing diversity of bioreactors for cell, hairy root, microalgae, and moss cultivation. For whole plants, both field and automated greenhouse cultivation methods are used with products expressed and produced either in leaves or seeds. Many successful expression systems now exist for a variety of different products with a list of increasingly successful commercialized products. This chapter provides an overview and examples of the current state of plant-based production systems for different types of recombinant proteins.

Growth kinetics and scale-up of Agrobacterium tumefaciens

Production of recombinant proteins in plants through Agrobacterium-mediated transient expression is a promising method of producing human therapeutic proteins, vaccines, and commercial enzymes. This process has been shown to be viable at a large scale and involves growing large quantities of wild-type plants and infiltrating the leaf tissue with a suspension of Agrobacterium tumefaciens bearing the genes of interest. This study examined one of the steps in this process that had not yet been optimized: the scale-up of Agrobacterium production to sufficient volumes for large-scale plant infiltration. Production of Agrobacterium strain C58C1 pTFS40 was scaled up from shake flasks (50-100 mL) to benchtop (5 L) scale with three types of media: Lysogeny broth (LB), yeast extract peptone (YEP) media, and a sucrose-based defined media. The maximum specific growth rate (μ max) of the strain in the three types of media was 0.46 ± 0.04 h-1 in LB media, 0.43 ± 0.03 h-1 in YEP media, and 0.27 ± 0.01 h-1 in defined media. The maximum biomass concentration reached at this scale was 2.0 ± 0.1, 2.8 ± 0.1, and 2.6 ± 0.1 g dry cell weight (DCW)/L for the three media types. Production was successfully scaled up to a 100-L working volume reactor with YEP media, using k L a as the scale-up parameter.

Media development for large scale Agrobacterium tumefaciens culture

A chemically defined media was developed for growing Agrobacterium tumefaciens at large scale for commercial production of recombinant proteins by transient expression in plants. Design of experiments was used to identify major and secondary effects of ten media components: sucrose, ammonium sulfate ((NH₄ )₂ SO₄ ), magnesium sulfate heptahydrate (MgSO₄ *7H₂ O), calcium chloride dihydrate (CaCl₂ *2H₂ O), iron (II) sulfate heptahydrate (FeSO₄ *7H₂ O), manganese (II) sulfate monohydrate (MnSO₄ *H₂ O), zinc sulfate heptahydrate (ZnSO₄ *7H₂ O), sodium chloride (NaCl), potassium chloride (KCl) and a sodium/potassium phosphate buffer (Na₂ HPO₄ /KH₂ PO₄ ). Calcium and zinc were found to have no detectable impact on biomass concentration or transient expression level, and concentrations of the other components that maximized final biomass concentration were determined. The maximum specific growth rate of Agrobacterium strain C58C1 pTFS40 in this media was 0.33 ± 0.01 h^-1 and the final biomass concentration after 26 h of batch growth in shake flasks was 2.6 g dry cell weight/L. Transient expression levels of the reporter protein GUS following infiltration of a recombinant Agrobacterium strain C58C1 into N. benthamiana were comparable when the strain was grown in the defined media, Lysogeny Broth (LB) media, or yeast extract-peptone (YEP) media. In LB and YEP media, free amino acid concentration was measured at three points over the course of batch growth of Agrobacterium strain C58C1 pTFS40; results indicated that l-serine and l-asparagine were depleted from the media first, followed by l-alanine and l-glutamic acid. © 2017 American Institute of Chemical Engineers Biotechnol. Prog., 33:1218-1225, 2017.

The Last Ten Years of Advancements in Plant-Derived Recombinant Vaccines against Hepatitis B

Disease prevention through vaccination is considered to be the greatest contribution to public health over the past century. Every year more than 100 million children are vaccinated with the standard World Health Organization (WHO)-recommended vaccines including hepatitis B (HepB). HepB is the most serious type of liver infection caused by the hepatitis B virus (HBV), however, it can be prevented by currently available recombinant vaccine, which has an excellent record of safety and effectiveness. To date, recombinant vaccines are produced in many systems of bacteria, yeast, insect, and mammalian and plant cells. Among these platforms, the use of plant cells has received considerable attention in terms of intrinsic safety, scalability, and appropriate modification of target proteins. Research groups worldwide have attempted to develop more efficacious plant-derived vaccines for over 30 diseases, most frequently HepB and influenza. More inspiring, approximately 12 plant-made antigens have already been tested in clinical trials, with successful outcomes. In this study, the latest information from the last 10 years on plant-derived antigens, especially hepatitis B surface antigen, approaches are reviewed and breakthroughs regarding the weak points are also discussed.