A Roadmap for the Molecular Farming of Viral Glycoprotein Vaccines: Engineering Glycosylation and Glycosylation-Directed Folding

Harnessing Plant Sugar Metabolism for Glycoengineering

Plants possess an innate ability to generate vast amounts of sugar and produce a range of sugar-derived compounds that can be utilized for applications in industry, health, and agriculture. Nucleotide sugars lie at the unique intersection of primary and specialized metabolism, enabling the biosynthesis of numerous molecules ranging from small glycosides to complex polysaccharides. Plants are tolerant to perturbations to their balance of nucleotide sugars, allowing for the overproduction of endogenous nucleotide sugars to push flux towards a particular product without necessitating the re-engineering of upstream pathways. Pathways to produce even non-native nucleotide sugars may be introduced to synthesize entirely novel products. Heterologously expressed glycosyltransferases capable of unique sugar chemistries can further widen the synthetic repertoire of a plant, and transporters can increase the amount of nucleotide sugars available to glycosyltransferases. In this opinion piece, we examine recent successes and potential future uses of engineered nucleotide sugar biosynthetic, transport, and utilization pathways to improve the production of target compounds. Additionally, we highlight current efforts to engineer glycosyltransferases. Ultimately, the robust nature of plant sugar biochemistry renders plants a powerful chassis for the production of target glycoconjugates and glycans.

Plant-produced RBD and cocktail-based vaccine candidates are highly effective against SARS-CoV-2, independently of its emerging variants

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a novel and highly pathogenic coronavirus that caused an outbreak in Wuhan City, China, in 2019 and then spread rapidly throughout the world. Although several coronavirus disease 2019 (COVID-19) vaccines are currently available for mass immunization, they are less effective against emerging SARS-CoV-2 variants, especially the Omicron (B.1.1.529). Recently, we successfully produced receptor-binding domain (RBD) variants of the spike (S) protein of SARS-CoV-2 and an antigen cocktail in Nicotiana benthamiana, which are highly produced in plants and elicited high-titer antibodies with potent neutralizing activity against SARS-CoV-2. In this study, based on neutralization ability, we demonstrate that plant-produced RBD and cocktail-based vaccine candidates are highly effective against SARS-CoV-2, independently of its emerging variants. These data demonstrate that plant-produced RBD and cocktail-based proteins are the most promising vaccine candidates and may protect against Delta and Omicron-mediated COVID-19. This is the first report describing vaccines against SARS-CoV-2, which demonstrate significant activities against Delta and Omicron variants.

Plant-made vaccines against viral diseases in humans and farm animals

Plants provide not only food and feed, but also herbal medicines and various raw materials for industry. Moreover, plants can be green factories producing high value bioproducts such as biopharmaceuticals and vaccines. Advantages of plant-based production platforms include easy scale-up, cost effectiveness, and high safety as plants are not hosts for human and animal pathogens. Plant cells perform many post-translational modifications that are present in humans and animals and can be essential for biological activity of produced recombinant proteins. Stimulated by progress in plant transformation technologies, substantial efforts have been made in both the public and the private sectors to develop plant-based vaccine production platforms. Recent promising examples include plant-made vaccines against COVID-19 and Ebola. The COVIFENZ^® COVID-19 vaccine produced in Nicotiana benthamiana has been approved in Canada, and several plant-made influenza vaccines have undergone clinical trials. In this review, we discuss the status of vaccine production in plants and the state of the art in downstream processing according to good manufacturing practice (GMP). We discuss different production approaches, including stable transgenic plants and transient expression technologies, and review selected applications in the area of human and veterinary vaccines. We also highlight specific challenges associated with viral vaccine production for different target organisms, including lower vertebrates (e.g., farmed fish), and discuss future perspectives for the field.

A plant-produced SARS-CoV-2 spike protein elicits heterologous immunity in hamsters

Molecular farming of vaccines has been heralded as a cheap, safe and scalable production platform. In reality, however, differences in the plant biosynthetic machinery, compared to mammalian cells, can complicate the production of viral glycoproteins. Remodelling the secretory pathway presents an opportunity to support key post-translational modifications, and to tailor aspects of glycosylation and glycosylation-directed folding. In this study, we applied an integrated host and glyco-engineering approach, NXS/T Generation™, to produce a SARS-CoV-2 prefusion spike trimer in Nicotiana benthamiana as a model antigen from an emerging virus. The size exclusion-purified protein exhibited a characteristic prefusion structure when viewed by transmission electron microscopy, and this was indistinguishable from the equivalent mammalian cell-produced antigen. The plant-produced protein was decorated with under-processed oligomannose N-glycans and exhibited a site occupancy that was comparable to the equivalent protein produced in mammalian cell culture. Complex-type glycans were almost entirely absent from the plant-derived material, which contrasted against the predominantly mature, complex glycans that were observed on the mammalian cell culture-derived protein. The plant-derived antigen elicited neutralizing antibodies against both the matched Wuhan and heterologous Delta SARS-CoV-2 variants in immunized hamsters, although titres were lower than those induced by the comparator mammalian antigen. Animals vaccinated with the plant-derived antigen exhibited reduced viral loads following challenge, as well as significant protection from SARS-CoV-2 disease as evidenced by reduced lung pathology, lower viral loads and protection from weight loss. Nonetheless, animals immunized with the mammalian cell-culture-derived protein were better protected in this challenge model suggesting that more faithfully reproducing the native glycoprotein structure and associated glycosylation of the antigen may be desirable.

Generation of multi-layered protein bodies in N. benthamiana for the encapsulation of vaccine antigens

The ability of plants to assemble particulate structures such as virus-like particles and protein storage organelles allows the direct bioencapsulation of recombinant proteins during the manufacturing process, which holds promise for the development of new drug delivery vehicles. Storage organelles found in plants such as protein bodies (PBs) have been successfully used as tools for accumulation and encapsulation of recombinant proteins. The fusion of sequences derived from 27-kDa-γ-zein, a major storage protein of maize, with a protein of interest leads to the incorporation of the chimeric protein into the stable and protected environment inside newly induced PBs. While this procedure has proven successful for several, but not all recombinant proteins, the aim of this study was to refine the technology by using a combination of PB-forming proteins, thereby generating multi-layered protein assemblies in N. benthamiana. We used fluorescent proteins to demonstrate that up to three proteinaceous components can be incorporated into different layers. In addition to 27-kDa-γ-zein, which is essential for PB initiation, 16-kDa-γ-zein was identified as a key element to promote the incorporation of a third zein-component into the core of the PBs. We show that a vaccine antigen could be incorporated into the matrix of multi-layered PBs, and the protein microparticles were characterized by confocal and electron microscopy as well as flow cytometry. In future, this approach will enable the generation of designer PBs that serve as drug carriers and integrate multiple components that can be functionalized in different ways.

In planta Production and Validation of Neuraminidase Derived from Genotype 4 Reassortant Eurasian Avian-like H1N1 Virus as a Vaccine Candidate

Influenza viruses are a major public health threat that causes repetitive outbreaks. In recent years, genotype 4 (G4) reassortant Eurasian avian-like (EA) H1N1 (G4 EA H1N1) has garnered attention as a potential novel pandemic strain. The necessity of developing vaccines against G4 EA H1N1 is growing because of the increasing cases of human infection and the low cross-reactivity of the strain with current immunity. In this study, we produced a G4 EA H1N1-derived neuraminidase (G4NA) as a vaccine candidate in Nicotiana benthamiana. The expressed G4NA was designed to be accumulated in the endoplasmic reticulum (ER). The M-domain of the human receptor-type tyrosine-protein phosphatase C was incorporated into the expression cassette to enhance the translation of G4NA. In addition, the family 3 cellulose-binding module and Brachypodium distachyon small ubiquitin-like modifier sequences were used to enable the cost-effective purification and removal of unnecessary domains after purification, respectively. The G4NA produced in plants displayed high solubility and assembled as a tetramer, which is required for the efficacy of an NA-based vaccine. In a mouse immunization model, the G4NA produced in plants could induce significant humoral immune responses. The plant-produced G4NA also stimulated antigen-specific CD4 T cell activation. These G4NA vaccine-induced immune responses were intensified by the administration of the antigen with a vaccine adjuvant. These results suggest that G4NA produced in plants has great potential as a vaccine candidate against G4 EA H1N1.

Augmenting glycosylation-directed folding pathways enhances the fidelity of HIV Env immunogen production in plants

Heterologous glycoprotein production relies on host glycosylation-dependent folding. When the biosynthetic machinery differs from the usual expression host, there is scope to remodel the assembly pathway to enhance glycoprotein production. Here we explore the integration of chaperone coexpression with glyco-engineering to improve the production of a model HIV-1 envelope antigen. Calreticulin was coexpressed to support protein folding together with Leishmania major STT3D oligosaccharyltransferase, to improve glycan occupancy, RNA interference to suppress the formation of truncated glycans, and Nicotiana benthamiana plants lacking α1,3-fucosyltransferase and β1,2-xylosyltransferase was used as an expression host to prevent plant-specific complex N-glycans forming. This approach reduced the formation of undesired aggregates, which predominated in the absence of glyco-engineering. The resulting antigen also exhibited increased glycan occupancy, albeit to a slightly lower level than the equivalent mammalian cell-produced protein. The antigen was decorated almost exclusively with oligomannose glycans, which were less processed compared with the mammalian protein. Immunized rabbits developed comparable immune responses to the plant-produced and mammalian cell-derived antigens, including the induction of autologous neutralizing antibodies when the proteins were used to boost DNA and modified vaccinia Ankara virus-vectored vaccines. This study demonstrates that engineering glycosylation-directed folding offers a promising route to enhance the production of complex viral glycoproteins in plants.

Virus-like Particles: Fundamentals and Biomedical Applications

Nanotechnology is a fast-evolving field focused on fabricating nanoscale objects for industrial, cosmetic, and therapeutic applications. Virus-like particles (VLPs) are self-assembled nanoparticles whose intrinsic properties, such as heterogeneity, and highly ordered structural organization are exploited to prepare vaccines; imaging agents; construct nanobioreactors; cancer treatment approaches; or deliver drugs, genes, and enzymes. However, depending upon the intrinsic features of the native virus from which they are produced, the therapeutic performance of VLPs can vary. This review compiles the recent scientific literature about the fundamentals of VLPs with biomedical applications. We consulted different databases to present a general scenario about viruses and how VLPs are produced in eukaryotic and prokaryotic cell lines to entrap therapeutic cargo. Moreover, the structural classification, morphology, and methods to functionalize the surface of VLPs are discussed. Finally, different characterization techniques required to examine the size, charge, aggregation, and composition of VLPs are described.

Plant-Derived Human Vaccines: Recent Developments

The idea of producing vaccines in plants originated in the late 1980s. Initially, it was contemplated that this notion could facilitate the concept of edible vaccines, making them more cost effective and easily accessible. Initial studies on edible vaccines focussed on the use of a variety of different transgenic plant host species for the production of vaccine antigens. However, adequate expression levels of antigens, the difficulties predicted with administration of consistent doses, and regulatory rules required for growth of transgenic plants gave way to the development of vaccine candidates that could be purified and administered parenterally. The field has subsequently advanced with improved expression techniques including a shift from using transgenic to transient expression of antigens, refinement of purification protocols, a deeper understanding of the biological processes and a wealth of evidence of immunogenicity and efficacy of plant-produced vaccine candidates, all contributing to the successful practice of what is now known as biopharming or plant molecular farming. The establishment of this technology has resulted in the development of many different types of vaccine candidates including subunit vaccines and various different types of nanoparticle vaccines targeting a wide variety of bacterial and viral diseases. This has brought further acceptance of plants as a suitable platform for vaccine production and in this review, we discuss the most recent advances in the production of vaccines in plants for human use.

Development of Plant-Based Vaccines for Prevention of Avian Influenza and Newcastle Disease in Poultry

Viral diseases, including avian influenza (AI) and Newcastle disease (ND), are an important cause of morbidity and mortality in poultry, resulting in significant economic losses. Despite the availability of commercial vaccines for the major viral diseases of poultry, these diseases continue to pose a significant risk to global food security. There are multiple factors for this: vaccine costs may be prohibitive, cold chain storage for attenuated live-virus vaccines may not be achievable, and commercial vaccines may protect poorly against local emerging strains. The development of transient gene expression systems in plants provides a versatile and robust tool to generate a high yield of recombinant proteins with superior speed while managing to achieve cost-efficient production. Plant-derived vaccines offer good stability and safety these include both subunit and virus-like particle (VLP) vaccines. VLPs offer potential benefits compared to currently available traditional vaccines, including significant reductions in virus shedding and the ability to differentiate between infected and vaccinated birds (DIVA). This review discusses the current state of plant-based vaccines for prevention of the AI and ND in poultry, challenges in their development, and potential for expanding their use in low- and middle-income countries.

Improving Protein Quantity and Quality—The Next Level of Plant Molecular Farming

Plants offer several unique advantages in the production of recombinant pharmaceuticals for humans and animals. Although numerous recombinant proteins have been expressed in plants, only a small fraction have been successfully put into use. The hugely distinct expression systems between plant and animal cells frequently cause insufficient yield of the recombinant proteins with poor or undesired activity. To overcome the issues that greatly constrain the development of plant-produced pharmaceuticals, great efforts have been made to improve expression systems and develop alternative strategies to increase both the quantity and quality of the recombinant proteins. Recent technological revolutions, such as targeted genome editing, deconstructed vectors, virus-like particles, and humanized glycosylation, have led to great advances in plant molecular farming to meet the industrial manufacturing and clinical application standards. In this review, we discuss the technological advances made in various plant expression platforms, with special focus on the upstream designs and milestone achievements in improving the yield and glycosylation of the plant-produced pharmaceutical proteins.

Production of Recombinant Proteins by Agrobacterium-Mediated Transient Expression

The agroinfiltration of plant tissue is a robust method that allows the rapid and transient expression of recombinant proteins. Using wild-type plants as biomass, agroinfiltration exploits the ability of plants to synthesize even complex multimeric proteins that require oxidative folding and/or post-translational modifications, while avoiding the expensive and time-consuming creation of stably transformed plant lines. Here we describe a generic method for the transient expression of recombinant proteins in Nicotiana benthamiana at the small to medium laboratory scale, including appropriate binary vectors, the design and cloning of expression constructs, the transformation, selection, and cultivation of recombinant Agrobacterium tumefaciens, the infiltration of plants using a syringe or vacuum device, and finally the extraction of recombinant proteins from plant tissues.

Plant-made vaccines against parasites: bioinspired perspectives to fight against Chagas disease

Introduction: Three decades of evidence have demonstrated that plants are an affordable platform for biopharmaceutical production and delivery. For instance, several plant-made recombinant proteins have been approved for commercialization under good manufacturing practice (GMP). Thus far, plant-based vaccine prototypes have been evaluated at pre- and clinical levels. Particularly, plant-made vaccines against parasitic diseases, such as malaria, cysticercosis, and toxoplasmosis have been successfully produced and orally delivered with promising outcomes in terms of immunogenicity and protection. The experience on several approaches and technical strategies over 30 years accounts for their potential low-cost, high scalability, and easy administration.Areas covered: This platform is an open technology to fight against Chagas disease, one of the most important neglected tropical diseases worldwide.Expert opinion: This review provides a perspective for the potential use of plants as a production platform and delivery system of Trypanosoma cruzi recombinant antigens, analyzing the advantages and limitations with respect to plant-made vaccines produced for other parasitic diseases. Plant-made vaccines are envisioned to fight against Chagas disease and other neglected tropical diseases in those countries suffering endemic prevalence.

Production of Gloeophyllum trabeum Endoglucanase Cel12A in Nicotiana benthamiana for Cellulose Degradation

Lignocellulosic biomass from plants has been used as a biofuel source and the potent acidic endoglucanase GtCel12A has been isolated from Gloeophyllum trabeum, a filamentous fungus. In this study, we established a plant-based platform for the production of active GtCel12A fused to family 3 cellulose-binding module (CBM3). We used the signal sequence of binding immunoglobulin protein (BiP) and the endoplasmic reticulum (ER) retention signal for the accumulation of the produced GtCel12A in the ER. To achieve enhanced enzyme expression, we incorporated the M-domain of the human receptor-type tyrosine-protein phosphatase C into the construct. In addition, to enable the removal of N-terminal domains that are not necessary after protein expression, we further incorporated the cleavage site of Brachypodium distachyon small ubiquitin-like modifier. The GtCel12A-CBM3 fusion protein produced in the leaves of Nicotiana benthamiana exhibited not only high solubility but also efficient endoglucanase activity on the carboxymethyl cellulose substrate as determined by 3,5-dinitrosalicylic acid assay. The endoglucanase activity of GtCel12A-CBM3 was maintained even when immobilized on microcrystalline cellulose beads. Taken together, these results indicate that GtCel12A endoglucanase produced in plants might be used to provide monomeric sugars from lignocellulosic biomass for bioethanol production.

Site-Specific Glycosylation of Recombinant Viral Glycoproteins Produced in Nicotiana benthamiana

There is an urgent need to establish large scale biopharmaceutical manufacturing capacity in Africa where the infrastructure for biologics production is severely limited. Molecular farming, whereby pharmaceuticals are produced in plants, offers a cheaper alternative to mainstream expression platforms, and is amenable to rapid large-scale production. However, there are several differences along the plant protein secretory pathway compared to mammalian systems, which constrain the production of complex pharmaceuticals. Viral envelope glycoproteins are important targets for immunization, yet in some cases they accumulate poorly in plants and may not be properly processed. Whilst the co-expression of human chaperones and furin proteases has shown promise, it is presently unclear how plant-specific differences in glycosylation impact the production of these proteins. In many cases it may be necessary to reproduce features of their native glycosylation to produce immunologically relevant vaccines, given that glycosylation is central to the folding and immunogenicity of these antigens. Building on previous work, we transiently expressed model glycoproteins from HIV and Marburg virus in Nicotiana benthamiana and mammalian cells. The proteins were purified and their site-specific glycosylation was determined by mass-spectrometry. Both glycoproteins yielded increased amounts of protein aggregates when produced in plants compared to the equivalent mammalian cell-derived proteins. The glycosylation profiles of the plant-produced glycoproteins were distinct from the mammalian cell produced proteins: they displayed lower levels of glycan occupancy, reduced complex glycans and large amounts of paucimannosidic structures. The elucidation of the site-specific glycosylation of viral glycoproteins produced in N. benthamiana is an important step toward producing heterologous viral glycoproteins in plants with authentic human-like glycosylation.

Investigating Constraints Along the Plant Secretory Pathway to Improve Production of a SARS-CoV-2 Spike Vaccine Candidate

Given the complex maturation requirements of viral glycoproteins and the challenge they often pose for expression in plants, the identification of host constraints precluding their efficient production is a priority for the molecular farming of vaccines. Building on previous work to improve viral glycoprotein production in plants, we investigated the production of a soluble SARS-CoV-2 spike comprising the ectopic portion of the glycoprotein. This was successfully transiently expressed in N. benthamiana by co-expressing the human lectin-binding chaperone calreticulin, which substantially increased the accumulation of the glycoprotein. The spike was mostly unprocessed unless the protease furin was co-expressed which resulted in highly efficient processing of the glycoprotein. Co-expression of several broad-spectrum protease inhibitors did not improve accumulation of the protein any further. The protein was successfully purified by affinity chromatography and gel filtration, although the purified product was heterogenous and the yields were low. Immunogenicity of the antigen was tested in BALB/c mice, and cellular and antibody responses were elicited after low dose inoculation with the adjuvanted protein. This work constitutes an important proof-of-concept for host plant engineering in the context of rapid vaccine development for SARS-CoV-2 and other emerging viruses.