Rapid high-yield expression of a candidate influenza vaccine based on the ectodomain of M2 protein linked to flagellin in plants using viral vectors

Outlining recent updates on influenza therapeutics and vaccines: A comprehensive review

Influenza virus has presented a considerable healthcare challenge during the past years, particularly in vulnerable groups with compromised immune systems. Therapeutics and vaccination have always been in research annals since the spread of influenza. Efforts have been going on to develop an antiviral therapeutic approach that could assist in better disease management and reduce the overall disease complexity, resistance development, and fatality rates. On the other hand, vaccination presents a chance for effective, long-term, cost-benefit, and preventive response against the morbidity and mortality associated with the influenza. However, the issues of resistance development, strain mutation, antigenic variability, and inability to cure wide-spectrum and large-scale strains of the virus by available vaccines remain there. The article gathers the updated data for the therapeutics and available influenza vaccines, their mechanism of action, shortcomings, and trials under clinical experimentation. A methodological approach has been adopted to identify the prospective therapeutics and available vaccines approved and within the clinical trials against the influenza virus. Review contains influenza therapeutics, including traditional and novel antiviral drugs and inhibitor therapies against influenza virus as well as research trials based on newer drug combinations and latest technologies such as nanotechnology and organic and plant-based natural products. Most recent development of influenza vaccine has been discussed including some updates on traditional vaccination protocols and discussion on next-generation and upgraded novel technologies. This review will help the readers to understand the righteous approach for dealing with influenza virus infection and for deducing futuristic approaches for novel therapeutic and vaccine trials against Influenza.

Development of transgenic algae strain expressing CTB-M2e fusion gene an approach towards the development of a universal edible vaccine in algae

Avian Influenza, the most studied virus, is of high concern due to its zoonotic pandemic potential. In recent years, several influenza vaccines have been used with the broad goal of managing and in certain cases, eliminating the disease. The matrix 2 extracellular domain (M2e), is one of the key targets of the universal influenza vaccine, a liner peptide that is conserved throughout all influenza A subtypes virus. Many recombinant influenza proteins have been expressed in yeast and plants for vaccine development. A remarkable development has been made in the field of biotechnology to explore the potential of microalga as an expression host. In this study, we designed a fusion gene code for M2e peptide and CTB protein as M2e's natural form has a low level of immunogenicity. The fusion gene was cloned in the Chloroplast transformation vector pSRSapI and expressed in the TN72 mutant strain of Chlamydomonas reinhardii. The expression of the targeted protein was confirmed by ECL western blot analysis. A GM1-ELISA was carried out to detect the affinity of fusion protein for GM1 monosialoganglioside and the significant P-value is lower than 0.05. Immunogenicity assay on chicken detected the anti-M2e bodies in chicken serum. This study gives evidence of therapeutic protein production through algae chloroplast and a stable, selection free and low cost oral delivery for universal vaccine against influenza A virus.

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.

The race toward a universal influenza vaccine: Front runners and the future directions

Influenza virus is the pathogen of influenza (flu) and millions of people suffer from the infection worldwide, posing a significant health risk. The current influenza vaccines induce neutralizing antibodies against hemagglutinin (HA) to achieve strain-specific neutralization. The effectiveness of seasonal vaccines is usually low and unpredictable because of the antigenic variation and genetic plasticity of viruses, as well as the interference of preexisting immunity. A universal influenza vaccine is urgently needed to prevent a wide variety of influenza viruses. Nevertheless, reaching this difficult optimal goal requires a step-by-step approach. Innovative strategies and vaccine platforms are being developed in order to generate robust cross-protective immunity. In this review, we summarize candidate influenza vaccines that meet two criteria: first, they are designed to provide protection against multiple influenza viruses; second, they had passed regulatory evaluations and have entered various stages of clinical trials. We discuss these vaccine candidates based on the different vaccine-production platforms, with the focus on antigen selection, design, adjuvants, immunomodulators, and routes of vaccine delivery in the development of universal influenza vaccines.

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.

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-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.

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.

Oral Immunization With a Plant HSP90-SAG1 Fusion Protein Produced in Tobacco Elicits Strong Immune Responses and Reduces Cyst Number and Clinical Signs of Toxoplasmosis in Mice

Plant 90kDa heat shock protein (HSP90) is a potent adjuvant that increases both humoral and cellular immune responses to diverse proteins and peptides. In this study, we explored whether Arabidopsis thaliana HSP90 (AtHsp81.2) can improve the immune effects of a Toxoplasma gondii surface antigen 1 (SAG1). We designed two constructs containing the sequence of mature antigen (SAG1m), from aa77 to aa322, and B- and T-cell antigenic epitope-containing SAG1HC, from aa221 to aa319 fused to AtHsp81.2 sequence. When comparing the transient expression in Nicotiana tabacum X-27-8 leaves, which overexpress the suppressor helper component protease HC-Pro-tobacco etch virus (TEV), to that in N. benthamiana leaves, co-agroinfiltrated with the suppressor p19, optimal conditions included 6-week-old N. benthamiana plants, 7-day time to harvest, Agrobacterium tumefaciens cultures with an OD600nm of 0.6 for binary vectors and LED lights. While AtHsp81.2-SAG1m fusion protein was undetectable by Western blot in any of the evaluated conditions, AtHsp81.2-SAG1HC was expressed as intact fusion protein, yielding up to 90μg/g of fresh weight. Besides, the AtHsp81.2-SAG1HC mRNA was strongly expressed compared to the endogenous Nicotiana tabacum elongation factor-alpha (NtEFα) gene, whereas the AtHsp81.2-SAG1m mRNA was almost undetectable. Finally, mice were orally immunized with AtHsp81.2-SAG1HC-infiltrated fresh leaves (plAtHsp81.2-SAG1HC group), recombinant AtHsp81.2-SAG1HC purified from infiltrated leaves (rAtHsp81.2-SAG1HC group), non-infiltrated fresh leaves (control group), or phosphate-buffered saline (PBS group). Serum samples from plAtHsp81.2-SAG1HC-immunized mice had significantly higher levels of IgGt, IgG2a, and IgG2b anti-SAG1HC antibodies than serum from rAtHsp81.2-SAG1HC, control, and PBS groups. The number of cysts per brain in the plAtHsp81.2-SAG1HC-immunized mice was significantly reduced, and the parasite load in brain tissue was also lower in this group compared with the remaining groups. In an immunoblot assay, plant-expressed AtHsp81.2-SAG1HC was shown to react with antibodies present in sera from T. gondii-infected people. Therefore, the plant expression of a T. gondii antigen fused to the non-pathogenic adjuvant and carrier plant HSP90 as formulations against T. gondii can improve the vaccine efficacy, and plant extract can be directly used for vaccination without the need to purify the protein, making this platform a suitable and powerful biotechnological system for immunogenic antigen expression against toxoplasmosis.

Application of Plant Viruses in Biotechnology, Medicine, and Human Health

Plant-based nanotechnology programs using virus-like particles (VLPs) and virus nanoparticles (VNPs) are emerging platforms that are increasingly used for a variety of applications in biotechnology and medicine. Tobacco mosaic virus (TMV) and potato virus X (PVX), by virtue of having high aspect ratios, make ideal platforms for drug delivery. TMV and PVX both possess rod-shaped structures and single-stranded RNA genomes encapsidated by their respective capsid proteins and have shown great promise as drug delivery systems. Cowpea mosaic virus (CPMV) has an icosahedral structure, and thus brings unique benefits as a nanoparticle. The uses of these three plant viruses as either nanostructures or expression vectors for high value pharmaceutical proteins such as vaccines and antibodies are discussed extensively in the following review. In addition, the potential uses of geminiviruses in medical biotechnology are explored. The uses of these expression vectors in plant biotechnology applications are also discussed. Finally, in this review, we project future prospects for plant viruses in the fields of medicine, human health, prophylaxis, and therapy of human diseases.

Producing Vaccines against Enveloped Viruses in Plants: Making the Impossible, Difficult

The past 30 years have seen the growth of plant molecular farming as an approach to the production of recombinant proteins for pharmaceutical and biotechnological uses. Much of this effort has focused on producing vaccine candidates against viral diseases, including those caused by enveloped viruses. These represent a particular challenge given the difficulties associated with expressing and purifying membrane-bound proteins and achieving correct assembly. Despite this, there have been notable successes both from a biochemical and a clinical perspective, with a number of clinical trials showing great promise. This review will explore the history and current status of plant-produced vaccine candidates against enveloped viruses to date, with a particular focus on virus-like particles (VLPs), which mimic authentic virus structures but do not contain infectious genetic material.

Combating Human Viral Diseases: Will Plant-Based Vaccines Be the Answer?

Molecular pharming or the technology of application of plants and plant cell culture to manufacture high-value recombinant proteins has progressed a long way over the last three decades. Whether generated in transgenic plants by stable expression or in plant virus-based transient expression systems, biopharmaceuticals have been produced to combat several human viral diseases that have impacted the world in pandemic proportions. Plants have been variously employed in expressing a host of viral antigens as well as monoclonal antibodies. Many of these biopharmaceuticals have shown great promise in animal models and several of them have performed successfully in clinical trials. The current review elaborates the strategies and successes achieved in generating plant-derived vaccines to target several virus-induced health concerns including highly communicable infectious viral diseases. Importantly, plant-made biopharmaceuticals against hepatitis B virus (HBV), hepatitis C virus (HCV), the cancer-causing virus human papillomavirus (HPV), human immunodeficiency virus (HIV), influenza virus, zika virus, and the emerging respiratory virus, severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) have been discussed. The use of plant virus-derived nanoparticles (VNPs) and virus-like particles (VLPs) in generating plant-based vaccines are extensively addressed. The review closes with a critical look at the caveats of plant-based molecular pharming and future prospects towards further advancements in this technology. The use of biopharmed viral vaccines in human medicine and as part of emergency response vaccines and therapeutics in humans looks promising for the near future.

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.

Establishment of a novel virus-induced virulence effector assay for the identification of virulence effectors of plant pathogens using a PVX-based expression vector

Plant pathogens deliver virulence effectors into plant cells to modulate plant immunity and facilitate infection. Although species-specific virulence effector screening approaches have been developed for several pathogens, these assays do not apply to pathogens that cannot be cultured and/or transformed outside of their hosts. Here, we established a rapid and parallel screening assay, called the virus-induced virulence effector (VIVE) assay, to identify putative effectors in various plant pathogens, including unculturable pathogens, using a virus-based expression vector. The VIVE assay uses the potato virus X (PVX) vector to transiently express candidate effector genes of various bacterial and fungal pathogens into Nicotiana benthamiana leaves. Using the VIVE assay, we successfully identified Avh148 as a potential virulence effector of Phytophthora sojae. Plants infected with PVX carrying Avh148 showed strong viral symptoms and high-level Avh148 and viral RNA accumulation. Analysis of P. sojae Avh148 deletion mutants and soybean hairy roots overexpressing Avh148 revealed that Avh148 is required for full pathogen virulence. In addition, the VIVE assay was optimized in N. benthamiana plants at different developmental stages across a range of Agrobacterium cell densities. Overall, we identified six novel virulence effectors from seven pathogens, thus demonstrating the broad effectiveness of the VIVE assay in plant pathology research.

Highly Immunogenic Nanoparticles Based on a Fusion Protein Comprising the M2e of Influenza A Virus and a Lipopeptide

The highly conserved extracellular domain of the transmembrane protein M2 (M2e) of the influenza A virus is a promising target for the development of broad-spectrum vaccines. However, M2e is a poor immunogen by itself and must be linked to an appropriate carrier to induce an efficient immune response. In this study, we obtained recombinant mosaic proteins containing tandem copies of M2e fused to a lipopeptide from Neisseria meningitidis surface lipoprotein Ag473 and alpha-helical linkers and analyzed their immunogenicity. Six fusion proteins, comprising four or eight tandem copies of M2e flanked by alpha-helical linkers, lipopeptides, or a combination of both of these elements, were produced in Escherichia coli. The proteins, containing both alpha-helical linkers and lipopeptides at each side of M2e repeats, formed nanosized particles, but no particulate structures were observed in the absence of lipopeptides. Animal study results showed that proteins with lipopeptides induced strong M2e-specific antibody responses in the absence of external adjuvants compared to similar proteins without lipopeptides. Thus, the recombinant M2e-based proteins containing alpha-helical linkers and N. meningitidis lipopeptide sequences at the N- and C-termini of four or eight tandem copies of M2e peptide are promising vaccine candidates.

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.

Synthetic biology and healthcare

Through the application of the engineering paradigm of ‘design–build–test–learn’ allied to recent advances in DNA sequencing, bioinformatics and, critically, the falling cost of DNA synthesis, Synthetic Biology promises to make existing therapies more accessible and be at the centre of the development of new types of advanced therapies. As existing pharmaceutical companies integrate Synthetic Biology tools into their normal ways of working, existing products are being produced by cheaper and more sustainable methods. Vaccine design and production is becoming driven by the molecular design allied to rapidly scalable production methods to combat the threat of pandemics and the ability of pathogens to escape the immune system by mutation. Advanced therapies, such as chimeric antigen receptor T cell therapy, are able to capitalise on the tools of Synthetic Biology to design new proteins and molecular ‘kill switches’ as well as design scalable and effective vectors for cellular transduction. This review highlights how Synthetic Biology is having an impact across the various therapeutic modalities from existing products to new therapies.

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.

An Inactivated Influenza Virus Vaccine Approach to Targeting the Conserved Hemagglutinin Stalk and M2e Domains

Universal influenza virus vaccine candidates that focus on the conserved hemagglutinin (HA) stalk domain and the extracellular domain of the matrix protein 2 (M2e) have been developed to increase the breadth of protection against multiple strains. In this study, we report a novel inactivated influenza virus vaccine approach that combines these two strategies. We inserted a human consensus M2e epitope into the immunodominant antigenic site (Ca2 site) of three different chimeric HAs (cHAs). Sequential immunization with inactivated viruses containing these modified cHAs substantially enhanced M2e antibody responses while simultaneously boosting stalk antibody responses. The combination of additional M2e antibodies with HA stalk antibodies resulted in superior antibody-mediated protection in mice against challenge viruses expressing homologous or heterosubtypic hemagglutinin and neuraminidase compared to vaccination strategies that targeted the HA stalk or M2e epitopes in isolation.

Application of an immunoglobulin Y-alkaline phosphatase bioconjugate as a diagnostic tool for influenza A virus

The diagnosis of influenza A virus is essential since it can be confused with influenza A like illness and lead to inaccurate drug prescription. In this study, the M2e peptide, a strategic antigen that is conserved in all virus subtypes, was used as a diagnostic marker of influenza A. For the first time, M2e-specific IgY antibody was covalently conjugated to alkaline phosphatase (ALP) enzyme in the presence of glutaraldehyde. The antibody-enzyme bioconjugate was characterized by fluorescence and Fourier-transform infrared spectroscopy. Subsequently, the diagnostic value of this bioconjugate was evaluated by direct sandwich ELISA using nasopharyngeal swab samples positive/negative for H1N1 and H3N2, which were previously analyzed by rRT-PCR for influenza. In conclusion, the M2e-specific IgY-ALP bioconjugate demonstrated positive results for Influenza A in samples that were diagnosed as Influenza A via the RT-PCR method.

Production of Human IFNγ Protein in Nicotiana benthamiana Plant through an Enhanced Expression System Based on Bamboo mosaic Virus

Plant-based systems are safe alternatives to the current platforms for the production of biologically active therapeutic proteins. However, plant-based expression systems face certain major challenges, including the relatively low productivity and the generation of target proteins in biologically active forms. The use of plant virus-based expression systems has been shown to enhance yields, but further improvement is still required to lower the production cost. In this study, various strategies were employed to increase the yields of an important therapeutic protein, human interferon gamma (IFNγ), in Nicotiana benthamiana through modifications of expression vectors based on potexviruses. Among these, the vector based on a coat protein (CP)-deficient Bamboo mosaic virus (BaMV), pKB△CHis, was shown to exhibit the highest expression level for the unmodified IFNγ. Truncation of the N-terminal signal peptide of IFN (designated mIFNγ) resulted in a nearly seven-fold increase in yield. Co-expression of a silencing suppressor protein by replacing the coding sequence of BaMV movement protein with that of P19 led to a 40% increase in mIFNγ accumulation. The fusion of endoplasmic reticulum (ER) retention signal with mIFNγ significantly enhanced the accumulation ratio of biologically active dimeric mIFNγ to 87% relative to the non-active monomeric form. The construct pKB19mIFNγER, employing the combination of all the above enhancement strategies, gave the highest level of protein accumulation, up to 119 ± 0.8 μg/g fresh weight, accounting for 2.5% of total soluble protein (TSP) content. These findings advocate the application of the modified BaMV-based vector as a platform for high-level expression of therapeutic protein in N. benthamiana.

Prospects on the Use of Schizochytrium sp. to Develop Oral Vaccines

Although oral subunit vaccines are highly relevant in the fight against widespread diseases, their high cost, safety and proper immunogenicity are attributes that have yet to be addressed in many cases and thus these limitations should be considered in the development of new oral vaccines. Prominent examples of new platforms proposed to address these limitations are plant cells and microalgae. Schizochytrium sp. constitutes an attractive expression host for vaccine production because of its high biosynthetic capacity, fast growth in low cost culture media, and the availability of processes for industrial scale production. In addition, whole Schizochytrium sp. cells may serve as delivery vectors; especially for oral vaccines since Schizochytrium sp. is safe for oral consumption, produces immunomodulatory compounds, and may provide bioencapsulation to the antigen, thus increasing its bioavailability. Remarkably, Schizochytrium sp. was recently used for the production of a highly immunoprotective influenza vaccine. Moreover, an efficient method for transient expression of antigens based on viral vectors and Schizochytrium sp. as host has been recently developed. In this review, the potential of Schizochytrium sp. in vaccinology is placed in perspective, with emphasis on its use as an attractive oral vaccination vehicle.

Repurposing Plant Virus Nanoparticles

Plants have been explored for many years as inexpensive and versatile platforms for the generation of vaccines and other biopharmaceuticals. Plant viruses have also been engineered to either express subunit vaccines or act as epitope presentation systems. Both icosahedral and helical, filamentous-shaped plant viruses have been used for these purposes. More recently, plant viruses have been utilized as nanoparticles to transport drugs and active molecules into cancer cells. The following review describes the use of both icosahedral and helical plant viruses in a variety of new functions against cancer. The review illustrates the breadth of variation among different plant virus nanoparticles and how this impacts the immune response.

Hydroponic Treatment of Nicotiana benthamiana with Kifunensine Modifies the N-glycans of Recombinant Glycoprotein Antigens to Predominantly Man9 High-Mannose Type upon Transient Overexpression

Nicotiana benthamiana transient overexpression systems offer unique advantages for rapid and scalable biopharmaceuticals production, including high scalability and eukaryotic post-translational modifications such as N-glycosylation. High-mannose-type glycans (HMGs) of glycoprotein antigens have been implicated in the effectiveness of some subunit vaccines. In particular, Man₉GlcNAc₂ (Man9) has high binding affinity to mannose-specific C-type lectin receptors such as the mannose receptor and dendritic cell-specific intracellular adhesion molecule 3-grabbing non-integrin (DC-SIGN). Here, we investigated the effect of kifunensine, an α-mannosidase I inhibitor, supplemented in a hydroponic culture of N. benthamiana for the production of Man9-rich HMG glycoproteins, using N-glycosylated cholera toxin B subunit (gCTB) and human immunodeficiency virus gp120 that are tagged with a H/KDEL endoplasmic reticulum retention signal as model vaccine antigens. Biochemical analysis using anti-fucose and anti-xylose antibodies as well as Endo H and PNGase F digestion showed that kifunensine treatment effectively reduced plant-specific glycoforms while increasing HMGs in the N-glycan compositions of gCTB. Detailed glycan profiling revealed that plant-produced gp120 had a glycan profile bearing mostly HMGs regardless of kifunensine treatment. However, the gp120 produced under kifunensine-treatment conditions showed Man9 being the most prominent glycoform (64.5%), while the protein produced without kifunensine had a substantially lower Man9 composition (20.3%). Our results open up possibilities for efficient production of highly mannosylated recombinant vaccine antigens in plants.

Plant Virus Expression Vectors: A Powerhouse for Global Health

Plant-made biopharmaceuticals have long been considered a promising technology for providing inexpensive and efficacious medicines for developing countries, as well as for combating pandemic infectious diseases and for use in personalized medicine. Plant virus expression vectors produce high levels of pharmaceutical proteins within a very short time period. Recently, plant viruses have been employed as nanoparticles for novel forms of cancer treatment. This review provides a glimpse into the development of plant virus expression systems both for pharmaceutical production as well as for immunotherapy.

Synthetic Botany

Plants are attractive platforms for synthetic biology and metabolic engineering. Plants' modular and plastic body plans, capacity for photosynthesis, extensive secondary metabolism, and agronomic systems for large-scale production make them ideal targets for genetic reprogramming. However, efforts in this area have been constrained by slow growth, long life cycles, the requirement for specialized facilities, a paucity of efficient tools for genetic manipulation, and the complexity of multicellularity. There is a need for better experimental and theoretical frameworks to understand the way genetic networks, cellular populations, and tissue-wide physical processes interact at different scales. We highlight new approaches to the DNA-based manipulation of plants and the use of advanced quantitative imaging techniques in simple plant models such as Marchantia polymorpha. These offer the prospects of improved understanding of plant dynamics and new approaches to rational engineering of plant traits.

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

Agroinfiltration of plant leaves with binary vectors carrying a gene of interest within a plant viral vector is a rapid and efficient method for protein production in plants. Previously, we constructed a self-replicating vector, pA7248AMV, based on the genetic elements of potato virus X (PVX), and have shown that this vector can be used for the expression of recombinant proteins in Nicotiana benthamiana. However, this vector is almost 18 kb long and therefore not convenient for genetic manipulation. Furthermore, for efficient expression of the target protein it should be co-agroinfiltrated with an additional binary vector expressing a suppressor of post-transcriptional gene silencing. Here, we improved this expression system by creating the novel pEff vector. Its backbone is about 5 kb shorter than the original vector and it contains an expression cassette for the silencing suppressor, P24, from grapevine leafroll-associated virus-2 alongside PVX genetic elements, thus eliminating the need of co-agroinfiltration. The pEff vector provides green fluorescent protein expression levels of up to 30% of total soluble protein. The novel vector was used for expression of the influenza vaccine candidate, M2eHBc, consisting of an extracellular domain of influenza virus M2 protein (M2e) fused to hepatitis B core antigen. Using the pEff system, M2eHBc was expressed to 5-10% of total soluble protein, several times higher than with original pA7248AMV vector. Plant-produced M2eHBc formed virus-like particles in vivo, as required for its use as a vaccine. The new self-replicating pEff vector could be used for fast and efficient production of various recombinant proteins in plants.

Plant viral nanoparticles-based HER2 vaccine: Immune response influenced by differential transport, localization and cellular interactions of particulate carriers

Cancer vaccines are designed to elicit an endogenous adaptive immune response that can successfully recognize and eliminate residual or recurring tumors. Such approaches can potentially overcome shortcomings of passive immunotherapies by generating long-lived therapeutic effects and immune memory while limiting systemic toxicities. A critical determinant of vaccine efficacy is efficient transport and delivery of tumor-associated antigens to professional antigen presenting cells (APCs). Plant viral nanoparticles (VNPs) with natural tropism for APCs and a high payload carrying capacity may be particularly effective vaccine carriers. The applicability of VNP platform technologies is governed by stringent structure-function relationships. We compare two distinct VNP platforms: icosahedral cowpea mosaic virus (CPMV) and filamentous potato virus X (PVX). Specifically, we evaluate in vivo capabilities of engineered VNPs delivering human epidermal growth factor receptor 2 (HER2) epitopes for therapy and prophylaxis of HER2⁺ malignancies. Our results corroborate the structure-function relationship where icosahedral CPMV particles showed significantly enhanced lymph node transport and retention, and greater uptake by/activation of APCs compared to filamentous PVX particles. These enhanced immune cell interactions and transport properties resulted in elevated HER2-specific antibody titers raised by CPMV- vs. PVX-based peptide vaccine. The 'synthetic virology' field is rapidly expanding with numerous platforms undergoing development and preclinical testing; our studies highlight the need for systematic studies to define rules guiding the design and rational choice of platform, in the context of peptide-vaccine display technologies.

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.

Plant virus nanoparticles: new applications and new benefits

For over two decades now, plants have been explored for their potential to act as production platforms for biopharmaceuticals, such as vaccines and monoclonal antibodies. More recently, plant viruses have been designed as nontoxic nanoparticles that can target a variety of cancers and thus empower the immune system to slow or even reverse tumor progression. The following paper describes the employment of plant virus expression vectors for the treatment of some of the most challenging diseases known today. The paper concludes with a projection of the multiple avenues by which virus nanoparticles could impact developing countries.

Edible plants for oral delivery of biopharmaceuticals

Molecular farming is the use of plants for the production of high value recombinant proteins. Over the last 25 years, molecular farming has achieved the inexpensive, scalable and safe production of pharmaceutical proteins using a range of strategies. One of the most promising approaches is the use of edible plant organs expressing biopharmaceuticals for direct oral delivery. This approach has proven to be efficacious in several clinical vaccination and tolerance induction trials as well as multiple preclinical studies for disease prevention. The production of oral biopharmaceuticals in edible plant tissues could revolutionize the pharmaceutical industry by reducing the cost of production systems based on fermentation, and also eliminating expensive downstream purification, cold storage and transportation costs. This review considers the unique features that make plants ideal as platforms for the oral delivery of protein-based therapeutics and describes recent developments in the production of plant derived biopharmaceuticals for oral administration.

High immunogenicity of plant-produced candidate influenza vaccine based on the M2e peptide fused to flagellin

The ectodomain of the conserved influenza matrix protein M2 (M2e) is a promising target for the development of a universal influenza vaccines. Immunogenicity of M2e could be enhanced by its fusion to bacterial flagellin, the ligand for Toll-like receptor 5. Previously we reported the transient expression in plants of a recombinant protein Flg-4M comprising flagellin fused to 4 tandem copies of the M2e. The use of self-replicating recombinant vector based on the potato virus X allowed expression of Flg-4M in Nicotiana benthaminana leaves at a very high level, up to about 1 mg/g of fresh leaf tissue. Intranasal immunization of mice with Flg-4M induced M2e-specific serum antibodies and provided protection against lethal challenge with different strains of influenza A virus. Here we show that immunization with Flg-4M not only generates a strong immune response, but also redirects the response from the carrier flagellin toward the M2e epitopes. Significant IgG response to M2e was also developed in bronchoalveolar lavages of immunized mice. Protective activity of Flg-4M upon lethal influenza challenge correlated with a decrease of virus titers in lungs relative to the control. Overall these data show the potential for the development of a plant-produced M2e-flagellin universal influenza vaccine.

Supplemented vaccination with tandem repeat M2e virus-like particles enhances protection against homologous and heterologous HPAI H5 viruses in chickens

Highly pathogenic avian influenza (HPAI) H5 viruses derived from A/Goose/Guangdong/1/96 have been continuously circulating globally, severely affecting the public health and poultry industries. The matrix 2 protein ectodomain (M2e) is considered a promising candidate for a universal cross-protective influenza vaccine that provides more effective control over HPAI H5 viruses harboring variant hemagglutinin (HA)-antigens. Here, we evaluated the protective efficacy of a tandem repeat construct of heterologous M2e presented on virus-like particles (M2e5x VLPs) either alone or as a supplement against HPAI H5 viruses in a chicken model. Chickens immunized with M2e5x VLPs alone induced M2e-specific antibodies but were not protected against HPAI H5. The homo- and cross-protective efficacy of M2e5x VLP-supplemented vaccination of chickens was also examined. Importantly, supplementation with M2e5x VLPs induced significantly higher levels of antibodies specific for M2e and different viruses as well as provided improved protection against homologous and heterologous HPAI H5 viruses. Considering the limited efficacy of inactivated vaccines, supplement vaccination with M2e5x VLPs may be an effective measure for preventing outbreaks of HPAI viruses that have the ability to constantly change their antigenic properties in poultry.

Production of H5N1 Influenza Virus Matrix Protein 2 Ectodomain Protein Bodies in Tobacco Plants and in Insect Cells as a Candidate Universal Influenza Vaccine

The spread of influenza A viruses is partially controlled and prevented by vaccination. The matrix protein 2 ectodomain (M2e) is the most conserved sequence in influenza A viruses, and is therefore a good potential target for a vaccine to protect against multiple virus subtypes. We explored the feasibility of an M2e-based universal influenza A vaccine candidate based on the highly pathogenic avian influenza A virus, H5N1. A synthetic M2e gene was human- and plant-codon optimized and fused in-frame with a sequence encoding the N-terminal proline-rich domain (Zera(®)) of the γ-zein protein of maize. Zera(®)M2e was expressed transiently in Nicotiana benthamiana and Sf21 baculovirus/insect cell expression systems, and Zera(®)M2e protein bodies (PBs) were successfully produced in both expression systems. The plant-produced Zera(®)M2e PBs were purified and injected into Balb/c mice. Western blot analysis using insect cell-produced Zera(®)M2e PBs and multiple tandem M2e sequences (5xM2e) fused with the avian influenza H5N1 transmembrane and cytosolic tail (5xM2e_tHA) confirmed the presence of M2e-specific antibodies in immunized mice sera. The immunogenicity of the Zera(®)M2e indicates that our plant-produced protein has potential as an inexpensive universal influenza A vaccine.