Plant-based rapid production of recombinant subunit hemagglutinin vaccines targeting H1N1 and H5N1 influenza

Protein Expression Platforms and the Challenges of Viral Antigen Production

Several protein expression platforms exist for a wide variety of biopharmaceutical needs. A substantial proportion of research and development into protein expression platforms and their optimization since the mid-1900s is a result of the production of viral antigens for use in subunit vaccine research. This review discusses the seven most popular forms of expression systems used in the past decade-bacterial, insect, mammalian, yeast, algal, plant and cell-free systems-in terms of advantages, uses and limitations for viral antigen production in the context of subunit vaccine research. Post-translational modifications, immunogenicity, efficacy, complexity, scalability and the cost of production are major points discussed. Examples of licenced and experimental vaccines are included along with images which summarize the processes involved.

Design and Escherichia coli Expression of a Natively Folded Multi-Disulfide Bonded Influenza H1N1-PR8 Receptor-Binding Domain (RBD)

Refolding multi-disulfide bonded proteins expressed in E. coli into their native structure is challenging. Nevertheless, because of its cost-effectiveness, handiness, and versatility, the E. coli expression of viral envelope proteins, such as the RBD (Receptor-Binding Domain) of the influenza Hemagglutinin protein, could significantly advance research on viral infections. Here, we show that H1N1-PR8-RBD (27 kDa, containing four cysteines forming two disulfide bonds) expressed in E. coli and was purified with nickel affinity chromatography, and reversed-phase HPLC was successfully refolded into its native structure, as assessed with several biophysical and biochemical techniques. Analytical ultracentrifugation indicated that H1N1-PR8-RBD was monomeric with a hydrodynamic radius of 2.5 nm. Thermal denaturation, monitored with DSC and CD at a wavelength of 222 nm, was cooperative with a midpoint temperature around 55 °C, strongly indicating a natively folded protein. In addition, the 15N-HSQC NMR spectrum exhibited several 1H-15N resonances indicative of a beta-sheeted protein. Our results indicate that a significant amount (40 mg/L) of pure and native H1N1-PR8-RBD can be produced using an E. coli expression system with our refolding procedure, offering potential insights into the molecular characterization of influenza virus infection.

Multidimensional futuristic approaches to address the pandemics beyond COVID-19

Globally, the impact of the coronavirus disease 2019 (COVID-19) pandemic has been enormous and unrelenting with ∼6.9 million deaths and ∼765 million infections. This review mainly focuses on the recent advances and potentially novel molecular tools for viral diagnostics and therapeutics with far-reaching implications in managing the future pandemics. In addition to briefly highlighting the existing and recent methods of viral diagnostics, we propose a couple of potentially novel non-PCR-based methods for rapid, cost-effective, and single-step detection of nucleic acids of viruses using RNA mimics of green fluorescent protein (GFP) and nuclease-based approaches. We also highlight key innovations in miniaturized Lab-on-Chip (LoC) devices, which in combination with cyber-physical systems, could serve as ideal futuristic platforms for viral diagnosis and disease management. We also discuss underexplored and underutilized antiviral strategies, including ribozyme-mediated RNA-cleaving tools for targeting viral RNA, and recent advances in plant-based platforms for rapid, low-cost, and large-scale production and oral delivery of antiviral agents/vaccines. Lastly, we propose repurposing of the existing vaccines for newer applications with a major emphasis on Bacillus Calmette-Guérin (BCG)-based vaccine engineering.

Three Parts of the Plant Genome: On the Way to Success in the Production of Recombinant Proteins

Recombinant proteins are the most important product of current industrial biotechnology. They are indispensable in medicine (for diagnostics and treatment), food and chemical industries, and research. Plant cells combine advantages of the eukaryotic protein production system with simplicity and efficacy of the bacterial one. The use of plants for the production of recombinant proteins is an economically important and promising area that has emerged as an alternative to traditional approaches. This review discusses advantages of plant systems for the expression of recombinant proteins using nuclear, plastid, and mitochondrial genomes. Possibilities, problems, and prospects of modifications of the three parts of the genome in light of obtaining producer plants are examined. Examples of successful use of the nuclear expression platform for production of various biopharmaceuticals, veterinary drugs, and technologically important proteins are described, as are examples of a high yield of recombinant proteins upon modification of the chloroplast genome. Potential utility of plant mitochondria as an expression system for the production of recombinant proteins and its advantages over the nucleus and chloroplasts are substantiated. Although these opportunities have not yet been exploited, potential utility of plant mitochondria as an expression system for the production of recombinant proteins and its advantages over the nucleus and chloroplasts are substantiated.

Design and assembly of plant-based COVID-19 candidate vaccines: reсent development and future prospects

An outbreak of a new variant of the coronavirus infection, known as COVID-19, occurred at the end of 2019 in China, in the city of Wuhan. It was caused by the SARS-CoV-2 virus. This variant of the virus is characterized by a high degree of variability and, as the current situation with its spread across different regions of the globe shows, it can lead to a progressive spread of infection among the human population and become the cause of a pandemic. The world scientific community is making tremendous efforts to develop means of protection,prevention and treatment of this disease based on modern advances in molecular biology, immunology and vaccinology. This review provides information on the current state of research in the field of vaccine development against COVID-19 with an emphasis on the role of plants in solving this complex problem. Although plants have long been used by mankind as sources of various medicinal substances, in a pandemic, plant expression systems become attractive as biofactories or bioreactors for the production of artificially created protein molecules that include protective antigens against viral infection. The design and creation of such artificial molecules underlies the development of recombinant subunit vaccines aimed at a rapid response against the spread of infections with a high degree of variability. The review presents the state of research covering a period of just over two years, i. e. since the emergence of the new outbreak of coronavirus infection. The authors tried to emphasize the importance of rapid response of research groups from various scientific fields towards the use of existing developments to create means of protection against various pathogens. With two plant expression systems – stable and transient – as examples, the development of work on the creation of recombinant subunit vaccines against COVID-19 in various laboratories and commercial companies is shown. The authors emphasize that plant expression systems have promise for the development of not only protective means under conditions of rapid response (subunit vaccines), but also therapeutic agents in the form of monoclonal antibodies against COVID-19 synthesized in plant cells.

Needleless administration of advanced therapies into the skin via the appendages using a hypobaric patch

Needleless delivery into the skin would overcome a major barrier to efficient clinical utilization of advanced therapies such as nanomaterials and macromolecules. This study demonstrates that controlled skin stretching (in porcine, rat, and mouse models) using a patch comprising a hypobaric chamber, to open the skin appendages, can increase the permeability of the tissue and provide a means to enable direct delivery of advanced therapies directly into the skin without the use of a needle or injection system. This technology can facilitate the self-administration of therapeutics including vaccines, RNA, and antigens, thus improving the translation of these products into effective clinical use.

Advanced therapies are commonly administered via injection even when they act within the skin tissue, and this increases the chances of off-target effects. Here we report the use of a skin patch containing a hypobaric chamber that induces skin dome formation to enable needleless delivery of advanced therapies directly into porcine, rat, and mouse skin. Finite element method modeling showed that the hypobaric chamber in the patch opened the skin appendages by 32%, thinned the skin, and compressed the appendage wall epithelia. These changes allowed direct delivery of an H1N1 vaccine antigen and a diclofenac nanotherapeutic into the skin. Fluorescence imaging and infrared mapping of the skin showed needleless delivery via the appendages. The in vivo utility of the patch was demonstrated by a superior immunoglobulin G response to the vaccine antigen in mice compared to intramuscular injection and a 70% reduction in rat paw swelling in vivo over 5 h with diclofenac without skin histology changes.

Contributions of the international plant science community to the fight against human infectious diseases - part 1: epidemic and pandemic diseases

Infectious diseases, also known as transmissible or communicable diseases, are caused by pathogens or parasites that spread in communities by direct contact with infected individuals or contaminated materials, through droplets and aerosols, or via vectors such as insects. Such diseases cause ˜17% of all human deaths and their management and control places an immense burden on healthcare systems worldwide. Traditional approaches for the prevention and control of infectious diseases include vaccination programmes, hygiene measures and drugs that suppress the pathogen, treat the disease symptoms or attenuate aggressive reactions of the host immune system. The provision of vaccines and biologic drugs such as antibodies is hampered by the high cost and limited scalability of traditional manufacturing platforms based on microbial and animal cells, particularly in developing countries where infectious diseases are prevalent and poorly controlled. Molecular farming, which uses plants for protein expression, is a promising strategy to address the drawbacks of current manufacturing platforms. In this review article, we consider the potential of molecular farming to address healthcare demands for the most prevalent and important epidemic and pandemic diseases, focussing on recent outbreaks of high-mortality coronavirus infections and diseases that disproportionately affect the developing world.

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.

A Comprehensive Overview on the Production of Vaccines in Plant-Based Expression Systems and the Scope of Plant Biotechnology to Combat against SARS-CoV-2 Virus Pandemics

Many pathogenic viral pandemics have caused threats to global health; the COVID-19 pandemic is the latest. Its transmission is growing exponentially all around the globe, putting constraints on the health system worldwide. A novel coronavirus, severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), causes this pandemic. Many candidate vaccines are available at this time for COVID-19, and there is a massive international race underway to procure as many vaccines as possible for each country. However, due to heavy global demand, there are strains in global vaccine production. The use of a plant biotechnology-based expression system for vaccine production also represents one part of this international effort, which is to develop plant-based heterologous expression systems, virus-like particles (VLPs)-vaccines, antiviral drugs, and a rapid supply of antigen-antibodies for detecting kits and plant origin bioactive compounds that boost the immunity and provide tolerance to fight against the virus infection. This review will look at the plant biotechnology platform that can provide the best fight against this global pandemic.

Plant Platforms for Efficient Heterologous Protein Production

Production of recombinant proteins is primarily established in cultures of mammalian, insect and bacterial cells. Concurrently, concept of using plants to produce high-value pharmaceuticals such as vaccines, antibodies, and dietary proteins have received worldwide attention. Newer technologies for plant transformation such as plastid engineering, agroinfiltration, magnifection, and deconstructed viral vectors have been used to enhance the protein production in plants along with the inherent advantage of speed, scale, and cost of production in plant systems. Production of therapeutic proteins in plants has now a more pragmatic approach when several plant-produced vaccines and antibodies successfully completed Phase I clinical trials in humans and were further scheduled for regulatory approvals to manufacture clinical grade products on a large scale which are safe, efficacious, and meet the quality standards. The main thrust of this review is to summarize the data accumulated over the last two decades and recent development and achievements of the plant derived therapeutics. It also attempts to discuss different strategies employed to increase the production so as to make plants more competitive with the established production systems in this industry.

Plant Molecular Farming as a Strategy Against COVID-19 - The Italian Perspective

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has killed more than 37,000 people in Italy and has caused widespread socioeconomic disruption. Urgent measures are needed to contain and control the virus, particularly diagnostic kits for detection and surveillance, therapeutics to reduce mortality among the severely affected, and vaccines to protect the remaining population. Here we discuss the potential role of plant molecular farming in the rapid and scalable supply of protein antigens as reagents and vaccine candidates, antibodies for virus detection and passive immunotherapy, other therapeutic proteins, and virus-like particles as novel vaccine platforms. We calculate the amount of infrastructure and production capacity needed to deal with predictable subsequent waves of COVID-19 in Italy by pooling expertise in plant molecular farming, epidemiology and the Italian health system. We calculate the investment required in molecular farming infrastructure that would enable us to capitalize on this technology, and provide a roadmap for the development of diagnostic reagents and biopharmaceuticals using molecular farming in plants to complement production methods based on the cultivation of microbes and mammalian cells.

Potential Applications of Plant Biotechnology against SARS-CoV-2

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

Molecular farming - The slope of enlightenment

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

The Emergency Response Capacity of Plant-Based Biopharmaceutical Manufacturing-What It Is and What It Could Be

Several epidemic and pandemic diseases have emerged over the last 20 years with increasing reach and severity. The current COVID-19 pandemic has affected most of the world's population, causing millions of infections, hundreds of thousands of deaths, and economic disruption on a vast scale. The increasing number of casualties underlines an urgent need for the rapid delivery of therapeutics, prophylactics such as vaccines, and diagnostic reagents. Here, we review the potential of molecular farming in plants from a manufacturing perspective, focusing on the speed, capacity, safety, and potential costs of transient expression systems. We highlight current limitations in terms of the regulatory framework, as well as future opportunities to establish plant molecular farming as a global, de-centralized emergency response platform for the rapid production of biopharmaceuticals. The implications of public health emergencies on process design and costs, regulatory approval, and production speed and scale compared to conventional manufacturing platforms based on mammalian cell culture are discussed as a forward-looking strategy for future pandemic responses.

A nanoemulsion-adjuvanted intranasal H5N1 influenza vaccine protects ferrets against homologous and heterologous H5N1 lethal challenge

BACKGROUND

Flu vaccines administered intramuscularly (IM) have shown seasonally fluctuating efficacy, 20-60%, throughout the last 15 years. We formulated a recombinant H5 (rH5) in our Nanovax® (NE01) (rH5/NE01) adjuvant for intranasal vaccination in ferrets. We evaluated the regimen, one vs two immunization, and cross clade protection a ferret challenge model.

METHODS

Plant derived recombinant H5 (rH5) antigen was formulated with NE01 and administered intranasally to ferrets. Immunogenicity (IgG), hemagglutination inhibition (HI), and protection against lethal challenge, were measured following one or two immunizations. Protection against homologous (strain A/Indo) and heterologous (strain A/Vn) was evaluated in ferrets following two immunizations.

RESULTS

IN immunization with rH5/NE01 induced significant IgG levels after one and two immunizations. One vaccination did not induce any HI while low HI was measured after two immunizations. Homologous challenge with H5N1 A/ Indonesia showed 100% survival, with minimal weight loss in animals vaccinated twice compared to the unvaccinated controls. Analysis of nasal wash from these challenged ferrets vaccinated twice showed decreased viral shedding compared to unvaccinated controls. Interestingly, animals that received one vaccination showed 88% survival with moderate weight loss. Cross clade protection was evaluated using an increased antigen dose (45 µg rH5). Vaccinated animals demonstrated increased IgG and HAI antibody responses. Both homologous (A/Indo) and heterologous challenge (A/Vietnam) following two immunizations showed 100% survival with no loss of body weight. However viral clearance was more rapid against the homologous (day 3) compared to the heterologous (day 5) post challenge.

CONCLUSION

Intranasal administration of NE01 adjuvant-formulated rH5 vaccine elicited systemic and probably mucosal immunity that conferred protection against lethal challenge with homologous or heterologous viral strains. It also enhanced viral clearance with decreased shedding. These outcomes strongly suggest that intranasal immunization using NE01 against flu infections warrants clinical testing.

Soluble Recombinant Hemagglutinin Protein of H1N1pdm09 Influenza Virus Elicits Cross-Protection Against a Lethal H5N1 Challenge in Mice

Currently, influenza vaccines are produced using embryonated chicken eggs. Recently, recombinant influenza vaccines have been developed as a potential alternative to egg-grown vaccines. In this study, we evaluated the efficacy of soluble recombinant hemagglutinin (HA) protein produced in human cell culture (Expi293F cells) as an influenza vaccine against homosubtypic and heterosubtypic influenza virus challenges in mice. Mice were immunized intramuscularly with purified soluble HA protein of H1N1pdm09 virus and then challenged with a lethal dose of H1N1pdm09, seasonal H3N2, or highly pathogenic avian influenza (HPAI) H5N1 virus. Vaccinated mice showed better morbidity than mock-vaccinated mice following H1N1pdm09 challenge. By contrast, all mice died following H3N2 challenge. Interestingly, all vaccinated mice survived challenge with H5N1 virus, whereas all mock-vaccinated mice died. These results suggest that intramuscular immunization with recombinant HA proteins produced in Expi 293F cells could be of value in influenza vaccine strategies.

A linear epitope coupled to DsRed provides an affinity ligand for the capture of monoclonal antibodies

Monoclonal antibodies (mAbs) dominate the market for biopharmaceutical proteins because they provide active and passive immunotherapies for many different diseases. However, for most mAbs, two expensive manufacturing platforms are required. These are mammalian cell cultures for upstream production and Protein A chromatography for product capture during downstream processing. Here we describe a novel affinity ligand based on the fluorescent protein DsRed as a carrier for the linear epitope ELDKWA, which can capture the HIV-neutralizing antibody 2F5. We produced the DsRed-2F5-Epitope (DFE) in transgenic tobacco (Nicotiana tabacum) plants and purified it using a combination of heat treatment and immobilized metal-ion affinity chromatography, resulting in a yield of 24 mg kg-1 at 90% purity. Using a design-of-experiments approach, we coupled up to 15 mg DFE per mL Sepharose. The resulting affinity resin was able to capture 2F5 from the clarified extract of N. benthamiana plants, achieving a purity of 97%, a recovery of >95% and an initial dynamic binding capacity at 10% product breakthrough of 4 mg mL-1 after a contact time of 2 min. The resin capacity declined to 15% of the starting value within 25 cycles when 1.25 M magnesium chloride was used for elution. We confirmed the binding activity of the 2F5 product by surface plasmon resonance spectroscopy. DFE is not yet optimized, and a cost analysis revealed that boosting DFE expression and increasing its capacity by fourfold will make the resin cost-competitive with some Protein A counterparts. The affinity resin can also be exploited to purify idiotype-specific mAbs.

Production of complex viral glycoproteins in plants as vaccine immunogens

Plant molecular farming offers a cost-effective and scalable approach to the expression of recombinant proteins which has been proposed as an alternative to conventional production platforms for developing countries. In recent years, numerous proofs of concept have established that plants can produce biologically active recombinant proteins and immunologically relevant vaccine antigens that are comparable to those made in conventional expression systems. Driving many of these advances is the remarkable plasticity of the plant proteome which enables extensive engineering of the host cell, as well as the development of improved expression vectors facilitating higher levels of protein production. To date, the only plant-derived viral glycoprotein to be tested in humans is the influenza haemagglutinin which expresses at ~50 mg/kg. However, many other viral glycoproteins that have potential as vaccine immunogens only accumulate at low levels in planta. A critical consideration for the production of many of these proteins in heterologous expression systems is the complexity of post-translational modifications, such as control of folding, glycosylation and disulphide bridging, which is required to reproduce the native glycoprotein structure. In this review, we will address potential shortcomings of plant expression systems and discuss strategies to optimally exploit the technology for the production of immunologically relevant and structurally authentic glycoproteins for use as vaccine immunogens.

VaxArray for hemagglutinin and neuraminidase potency testing of influenza vaccines

Practical methods to measure the potency of influenza vaccines are needed as alternatives for the standard single radial immunodiffusion (SRID) assay. VaxArray assays for influenza hemagglutinin (HA) and neuraminidase (NA) have been developed to address this need. In this report, we evaluate the use of these assays to assess the potency of HA and NA of an A/H3N2 subunit vaccine by determining the correlation between the amounts measured by VaxArray and the immunogenicity in mice. The antibody response after one and two doses of five formulations of the vaccine ranging from 5 µg/mL to 80 µg/mL of HA, was measured by hemagglutination inhibition (HAI) and neuraminidase inhibition (NAI) assays. For hemagglutinin, vaccine potency determined by VaxArray was equivalent to potency measured SRID and these amounts were predictive of immunogenicity, with excellent correlation between potency measured by VaxArray and the HAI geometric mean titers (GMT). Likewise, the amount of NA measured by VaxArray was predictive of the NAI GMT. The VaxArray NA assay reported non-detectable levels of intact NA for a sample that had been heat degraded at 56 °C for 20 h, demonstrating that the assay measures the native, active form of NA. Similarly, the HA potency measured by VaxArray in this heat-treated sample was very low when a monoclonal antibody was used to detect the amount of antigen bound. Importantly, the force degraded sample induced low HAI titers and the NAI titers were not measurable, supporting the conclusion that the VaxArray HA and NA assays measure the immunogenic forms of these A/H3N2 antigens. This study indicates that VaxArray assays can be used to assess the potency of HA and NA components in influenza vaccines as a proxy for immunogenicity.

Plant-Produced Subunit Vaccine Candidates against Yellow Fever Induce Virus Neutralizing Antibodies and Confer Protection against Viral Challenge in Animal Models

Yellow fever (YF) is a viral disease transmitted by mosquitoes and endemic mostly in South America and Africa with 20-50% fatality. All current licensed YF vaccines, including YF-Vax^® (Sanofi-Pasteur, Lyon, France) and 17DD-YFV (Bio-Manguinhos, Rio de Janeiro, Brazil), are based on live attenuated virus produced in hens' eggs and have been widely used. The YF vaccines are considered safe and highly effective. However, a recent increase in demand for YF vaccines and reports of rare cases of YF vaccine-associated fatal adverse events have provoked interest in developing a safer YF vaccine that can be easily scaled up to meet this increased global demand. To this point, we have engineered the YF virus envelope protein (YFE) and transiently expressed it in Nicotiana benthamiana as a stand-alone protein (YFE) or as fusion to the bacterial enzyme lichenase (YFE-LicKM). Immunogenicity and challenge studies in mice demonstrated that both YFE and YFE-LicKM elicited virus neutralizing (VN) antibodies and protected over 70% of mice from lethal challenge infection. Furthermore, these two YFE-based vaccine candidates induced VN antibody responses with high serum avidity in nonhuman primates and these VN antibody responses were further enhanced after challenge infection with the 17DD strain of YF virus. These results demonstrate partial protective efficacy in mice of YFE-based subunit vaccines expressed in N. benthamiana. However, their efficacy is inferior to that of the live attenuated 17DD vaccine, indicating that formulation development, such as incorporating a more suitable adjuvant, may be required for product development.

Recombinant Plant Engineering for Immunotherapeutic Production

PURPOSE OF REVIEW

The requirement for large quantities of therapeutic proteins has fueled a great interest in the production of recombinant proteins in plant bioreactors. The vaccines and bio-therapeutic protein production in plants hold the promise of significantly lowering the cost of manufacturing life-saving drugs. This review will reflect the current status and challenges that the molecular farming platform faces becoming a strategic solution for the development of low-cost bio-therapeutics for developing countries.

RECENT FINDINGS

Different plant parts have been successfully identified as suitable expression systems for the commercial production of therapeutic proteins for some human and animal diseases ranging from common cold to AIDS. The processed therapeutics from such sources are devoid of any toxic components. The large-scale cultivation of these transgenic plants would be possible anywhere in the world including developing countries, which lack sophisticated drug manufacturing units. A couple of such commercially generated products have already hit the market with success. Newer methods using suitable plant viruses and recombinant gene expression systems have already been devised for producing therapeutic proteins and peptides.

SUMMARY

Plants are promising bio-factories for therapeutic protein production because of their several advantages over the other expression systems especially the advanced mechanisms for protein synthesis and post-translational modification which are very much similar to animal cells. Plant biotechnologists are much attracted to the bio-farming because of its flexibility, scalability, low manufacturing cost, as well as the lack of risk of toxic or pathogenic contamination. A number of projects on bio-farming are designed and are at various developmental stages but have not yet become available to the pharmaceutical industry. Therefore, we need further advancement in the optimization of lab protocols for up-scaling the production of such therapeutics at commercial level with a promise to offer their best clinical use.

Egg-Independent Influenza Vaccines and Vaccine Candidates

Vaccination remains the principal way to control seasonal infections and is the most effective method of reducing influenza-associated morbidity and mortality. Since the 1940s, the main method of producing influenza vaccines has been an egg-based production process. However, in the event of a pandemic, this method has a significant limitation, as the time lag from strain isolation to final dose formulation and validation is six months. Indeed, production in eggs is a relatively slow process and production yields are both unpredictable and highly variable from strain to strain. In particular, if the next influenza pandemic were to arise from an avian influenza virus, and thus reduce the egg-laying hen population, there would be a shortage of embryonated eggs available for vaccine manufacturing. Although the production of egg-derived vaccines will continue, new technological developments have generated a cell-culture-based influenza vaccine and other more recent platforms, such as synthetic influenza vaccines.

Plant-based vaccines against respiratory diseases: current status and future prospects

INTRODUCTION

Respiratory infections have an enormous, worldwide epidemiologic impact on humans and animals. Among the prophylactic measures, vaccination has the potential to neutralize this impact. New technologies for vaccine production and delivery are of importance in this field since they offer the potential to develop new immunization approaches overriding the current limitations that comprise high cost, safety issues, and limited efficacy. Areas covered: In the present review, the state of the art in developing plant-based vaccines against respiratory diseases is presented. The review was based on the analysis of current biomedical literature. Expert commentary: Preclinical and clinical evaluations of several vaccine candidates against influenza, tuberculosis, respiratory syncytial virus, pneumonia, anthrax and asthma are discussed and placed in perspective.

Intranasal vaccination with a plant-derived H5 HA vaccine protects mice and ferrets against highly pathogenic avian influenza virus challenge

Highly pathogenic avian influenza H5N1 infection remains a public health threat and vaccination is the best measure of limiting the impact of a potential pandemic. Mucosal vaccines have the advantage of eliciting immune responses at the site of viral entry, thereby preventing infection as well as further viral transmission. In this study, we assessed the protective efficacy of hemagglutinin (HA) from the A/Indonesia/05/05 (H5N1) strain of influenza virus that was produced by transient expression in plants. The plant-derived vaccine, in combination with the mucosal adjuvant (3′,5′)-cyclic dimeric guanylic acid (c-di-GMP) was used for intranasal immunization of mice and ferrets, before challenge with a lethal dose of the A/Indonesia/05/05 (H5N1) virus. Mice vaccinated with 15 μg or 5 μg of adjuvanted HA survived the viral challenge, while all control mice died within 10 d of challenge. Vaccinated animals elicited serum hemagglutination inhibition, IgG and IgA antibody titers. In the ferret challenge study, all animals vaccinated with the adjuvanted plant vaccine survived the lethal viral challenge, while 50% of the control animals died. In both the mouse and ferret models, the vaccinated animals were better protected from weight loss and body temperature changes associated with H5N1 infection compared with the non-vaccinated controls. Furthermore, the systemic spread of the virus was lower in the vaccinated animals compared with the controls. Results presented here suggest that the plant-produced HA-based influenza vaccine adjuvanted with c-di-GMP is a promising vaccine/adjuvant combination for the development of new mucosal influenza vaccines.

Engineering and expression of a RhoA peptide against respiratory syncytial virus infection in plants

MAIN CONCLUSION : A RhoA-derived peptide fused to carrier molecules from plants showed enhanced biological activity of in vitro assays against respiratory syncytial virus compared to the RhoA peptide alone or the synthetic RhoA peptide. A RhoA-derived peptide has been reported for over a decade as a potential inhibitor of respiratory syncytial virus (RSV) infection both in vitro and in vivo and is anticipated to be a promising alternative to monoclonal antibody-based therapy against RSV infection. However, there are several challenges to furthering development of this antiviral peptide, including improvement in the peptide’s bioavailability, development of an efficient delivery system and identification of a cost-effective production platform. In this study, we have engineered a RhoA peptide as a genetic fusion to two carrier molecules, either lichenase (LicKM) or the coat protein (CP) of Alfalfa mosaic virus. These constructs were introduced into Nicotiana benthamiana plants using a tobacco mosaic virus-based expression vector and targets purified. The results demonstrated that the RhoA peptide fusion proteins were efficiently expressed in N. benthamiana plants, and that two of the resulting fusion proteins, RhoA-LicKM and RhoA2-FL-d25CP, inhibited RSV growth in vitro by 50 and 80 %, respectively. These data indicate the feasibility of transient expression of this biologically active antiviral RhoA peptide in plants and the advantage of using a carrier molecule to enhance target expression and efficacy.

Potential of plants to produce recombinant protein products

Plants have great potential as photosynthetic factories to produce pharmaceutically important and commercially valuable biomedicines and industrial proteins at low cost. The U.S. Food and Drug Administration (U.S. FDA) has approved the drug Elelyso (taliglucerase alfa) produced by carrot cells for treatment of type 1 Gaucher's disease in 2012. The commercial potential of biomedicines produced by molecular farming has dramatically improved due to the success of an experimental drug called ZMapp, which has immunological activity in Ebola patients. A cocktail of three monoclonal antibodies was produced in tobacco (Nicotiana benthamiana) plants (Chen and Davis 2016). At present, very few drugs made by this technology have been approved by worldwide authorities such as the U.S. FDA. However, plants have been proposed as a novel paradigm for commercial production of proteins over the next decade. In recent years, leading researchers on molecular farming have given more priority to the area of animal-free therapeutic proteins such as parenteral and oral vaccines. Although plant-based platforms have considerable advantages over traditional systems such as bacterial and animal systems, there are several obstacles to commercial-scale production, especially with regards to improving the quality and quantity of plant-produced biologics and industrial materials. One of the biggest barriers to commercialization of this technology is the intense scrutiny of these new plant varieties by regulatory agencies and the public as well as the high costs associated with their regulatory approval.

Plant-based vaccines for animals and humans: recent advances in technology and clinical trials

It has been about 30 years since the first plant engineering technology was established. Although the concept of plant-based pharmaceuticals or vaccines motivates us to develop practicable commercial products using plant engineering, there are some difficulties in reaching the final goal: to manufacture an approved product. At present, the only plant-made vaccine approved by the United States Department of Agriculture is a Newcastle disease vaccine for poultry that is produced in suspension-cultured tobacco cells. The progress toward commercialization of plant-based vaccines takes much effort and time, but several candidate vaccines for use in humans and animals are in clinical trials. This review discusses plant engineering technologies and regulations relevant to the development of plant-based vaccines and provides an overview of human and animal vaccines currently under clinical trials.

Plant-made oral vaccines against human infectious diseases-Are we there yet?

Although the plant-made vaccine field started three decades ago with the promise of developing low-cost vaccines to prevent infectious disease outbreaks and epidemics around the globe, this goal has not yet been achieved. Plants offer several major advantages in vaccine generation, including low-cost production by eliminating expensive fermentation and purification systems, sterile delivery and cold storage/transportation. Most importantly, oral vaccination using plant-made antigens confers both mucosal (IgA) and systemic (IgG) immunity. Studies in the past 5 years have made significant progress in expressing vaccine antigens in edible leaves (especially lettuce), processing leaves or seeds through lyophilization and achieving antigen stability and efficacy after prolonged storage at ambient temperatures. Bioencapsulation of antigens in plant cells protects them from the digestive system; the fusion of antigens to transmucosal carriers enhances efficiency of their delivery to the immune system and facilitates successful development of plant vaccines as oral boosters. However, the lack of oral priming approaches diminishes these advantages because purified antigens, cold storage/transportation and limited shelf life are still major challenges for priming with adjuvants and for antigen delivery by injection. Yet another challenge is the risk of inducing tolerance without priming the host immune system. Therefore, mechanistic aspects of these two opposing processes (antibody production or suppression) are discussed in this review. In addition, we summarize recent progress made in oral delivery of vaccine antigens expressed in plant cells via the chloroplast or nuclear genomes and potential challenges in achieving immunity against infectious diseases using cold-chain-free vaccine delivery approaches.

Plant-produced candidate countermeasures against emerging and reemerging infections and bioterror agents

Despite progress in the prevention and treatment of infectious diseases, they continue to present a major threat to public health. The frequency of emerging and reemerging infections and the risk of bioterrorism warrant significant efforts towards the development of prophylactic and therapeutic countermeasures. Vaccines are the mainstay of infectious disease prophylaxis. Traditional vaccines, however, are failing to satisfy the global demand because of limited scalability of production systems, long production timelines and product safety concerns. Subunit vaccines are a highly promising alternative to traditional vaccines. Subunit vaccines, as well as monoclonal antibodies and other therapeutic proteins, can be produced in heterologous expression systems based on bacteria, yeast, insect cells or mammalian cells, in shorter times and at higher quantities, and are efficacious and safe. However, current recombinant systems have certain limitations associated with production capacity and cost. Plants are emerging as a promising platform for recombinant protein production due to time and cost efficiency, scalability, lack of harboured mammalian pathogens and possession of the machinery for eukaryotic post-translational protein modification. So far, a variety of subunit vaccines, monoclonal antibodies and therapeutic proteins (antivirals) have been produced in plants as candidate countermeasures against emerging, reemerging and bioterrorism-related infections. Many of these have been extensively evaluated in animal models and some have shown safety and immunogenicity in clinical trials. Here, we overview ongoing efforts to producing such plant-based countermeasures.

Biochemical composition of haemagglutinin-based influenza virus-like particle vaccine produced by transient expression in tobacco plants

Influenza virus-like particles (VLPs) are noninfectious particles resembling the influenza virus representing a promising vaccine alternative to inactivated influenza virions as antigens. Medicago inc. has developed a plant-based VLP manufacturing platform allowing the large-scale production of GMP-grade influenza VLPs. In this article, we report on the biochemical compositions of these plant-based influenza candidate vaccines, more particularly the characterization of the N-glycan profiles of the viral haemagglutinins H1 and H5 proteins as well as the tobacco-derived lipid content and residual impurities. Mass spectrometry analyses showed that all N-glycosylation sites of the extracellular domain of the recombinant haemagglutinins carry plant-specific complex-type N-glycans having core α(1,3)-fucose, core β(1,2)-xylose epitopes and Lewis(a) extensions. Previous phases I and II clinical studies have demonstrated that no hypersensibility nor induction of IgG or IgE directed against these glycans was observed. In addition, this article showed that the plant-made influenza vaccines are highly pure VLPs preparations while detecting no protein contaminants coming either from Agrobacterium or from the enzymes used for the enzyme-assisted extraction process. In contrast, VLPs contain few host cell proteins and glucosylceramides associated with plant lipid rafts. Identification of such raft markers, together with the type of host cell impurity identified, confirmed that the mechanism of VLP formation in planta is similar to the natural process of influenza virus assembly in mammals.

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

BACKGROUND

The extracellular domain of matrix protein 2 (M2e) of influenza A virus is a promising target for the development of a universal vaccine against influenza because M2e sequences are highly conserved among human influenza A strains. However, native M2e is poorly immunogenic, but its immunogenicity can be increased by delivery in combination with adjuvants or carrier particles. It was previously shown that fusion of M2e to bacterial flagellin, the ligand for Toll-like receptor (TLR) 5 and powerful mucosal adjuvant, significantly increases the immunogenicity and protective capacity of M2e.

RESULTS

In this study, we report for the first time the transient expression in plants of a recombinant protein Flg-4M comprising flagellin of Salmonella typhimurium fused to four tandem copies of the M2e peptide. The chimeric construct was expressed in Nicotiana benthamiana plants using either the self-replicating potato virus X (PVX) based vector, pA7248AMV-GFP, or the cowpea mosaic virus (CPMV)-derived expression vector, pEAQ-HT. The highest expression level up to 30% of total soluble protein (about 1 mg/g of fresh leaf tissue) was achieved with the PVX-based expression system. Intranasal immunization of mice with purified Flg-4M protein induced high levels of M2e-specific serum antibodies and provided protection against lethal challenge with influenza virus.

CONCLUSIONS

This study confirms the usefulness of flagellin as a carrier of M2e and its relevance for the production of M2e-based candidate influenza vaccines in plants.

Current challenges and future perspectives of plant and agricultural biotechnology

Advances in understanding plant biology, novel genetic resources, genome modification, and omics technologies generate new solutions for food security and novel biomaterials production under changing environmental conditions. New gene and germplasm candidates that are anticipated to lead to improved crop yields and other plant traits under stress have to pass long development phases based on trial and error using large-scale field evaluation. Therefore, quantitative, objective, and automated screening methods combined with decision-making algorithms are likely to have many advantages, enabling rapid screening of the most promising crop lines at an early stage followed by final mandatory field experiments. The combination of novel molecular tools, screening technologies, and economic evaluation should become the main goal of the plant biotechnological revolution in agriculture.

Vaccines against influenza A viruses in poultry and swine: Status and future developments

Influenza A viruses are important pathogens with a very broad host spectrum including domestic poultry and swine. For preventing clinical disease and controlling the spread, vaccination is one of the most efficient tools. Classical influenza vaccines for domestic poultry and swine are conventional inactivated preparations. However, a very broad range of novel vaccine types ranging from (i) nucleic acid-based vaccines, (ii) replicon particles, (iii) subunits and virus-like particles, (iv) vectored vaccines, or (v) live-attenuated vaccines has been described, and some of them are now also used in the field. The different novel approaches for vaccines against avian and swine influenza virus infections are reviewed, and additional features like universal vaccines, novel application approaches and the "differentiating infected from vaccinated animals" (DIVA)-strategy are summarized.

Recent developments in therapeutic protein expression technologies in plants

Infectious diseases and cancers are some of the commonest causes of deaths throughout the world. The previous two decades have witnessed a combined endeavor across various biological sciences to address this issue in novel ways. The advent of recombinant DNA technologies has provided the tools for producing recombinant proteins that can be used as therapeutic agents. A number of expression systems have been developed for the production of pharmaceutical products. Recently, advances have been made using plants as bioreactors to produce therapeutic proteins directed against infectious diseases and cancers. This review highlights the recent progress in therapeutic protein expression in plants (stable and transient), the factors affecting heterologous protein expression, vector systems and recent developments in existing technologies and steps towards the industrial production of plant-made vaccines, antibodies, and biopharmaceuticals.

Oral immunization of haemaggulutinin H5 expressed in plant endoplasmic reticulum with adjuvant saponin protects mice against highly pathogenic avian influenza A virus infection

Pandemics in poultry caused by the highly pathogenic avian influenza (HPAI) A virus occur too frequently globally, and there is growing concern about the HPAI A virus due to the possibility of a pandemic among humans. Thus, it is important to develop a vaccine against HPAI suitable for both humans and animals. Various approaches are underway to develop such vaccines. In particular, an edible vaccine would be a convenient way to vaccinate poultry because of the behaviour of the animals. However, an edible vaccine is still not available. In this study, we developed a strategy of effective vaccination of mice by the oral administration of transgenic Arabidopsis plants (HA-TG) expressing haemagglutinin (HA) in the endoplasmic reticulum (ER). Expression of HA in the ER resulted in its high-level accumulation, N-glycosylation, protection from proteolytic degradation and long-term stability. Oral administration of HA-TG with saponin elicited high levels of HA-specific systemic IgG and mucosal IgA responses in mice, which resulted in protection against a lethal influenza virus infection with attenuated inflammatory symptoms. Based on these results, we propose that oral administration of freeze-dried leaf powders from transgenic plants expressing HA in the ER together with saponin is an attractive strategy for vaccination against influenza A virus.

Hydrophobin fusion of an influenza virus hemagglutinin allows high transient expression in Nicotiana benthamiana, easy purification and immune response with neutralizing activity

The expression of recombinant hemagglutinin in plants is a promising alternative to the current egg-based production system for the influenza vaccines. Protein-stabilizing fusion partners have been developed to overcome the low production yields and the high downstream process costs associated with the plant expression system. In this context, we tested the fusion of hydrophobin I to the hemagglutinin ectodomain of the influenza A (H1N1)pdm09 virus controlled by the hybrid En2PMA4 transcriptional promoter to rapidly produce high levels of recombinant antigen by transient expression in agro-infiltrated Nicotiana benthamiana leaves. The fusion increased the expression level by a factor of ∼ 2.5 compared to the unfused protein allowing a high accumulation level of 8.6% of the total soluble proteins. Hemagglutinin was located in ER-derived protein bodies and was successfully purified by combining an aqueous-two phase partition system and a salting out step. Hydrophobin interactions allowed the formation of high molecular weight hemagglutinin structures, while unfused proteins were produced as monomers. Purified protein was shown to be biologically active and to induce neutralizing antibodies after mice immunization. Hydrophobin fusion to influenza hemagglutinin might therefore be a promising approach for rapid, easy, and low cost production of seasonal or pandemic influenza vaccines in plants.

Advances and challenges in the development and production of effective plant-based influenza vaccines

Influenza infections continue to present a major threat to public health. Traditional modes of influenza vaccine manufacturing are failing to satisfy the global demand because of limited scalability and long production timelines. In contrast, subunit vaccines (SUVs) can be produced in heterologous expression systems in shorter times and at higher quantities. Plants are emerging as a promising platform for SUV production due to time efficiency, scalability, lack of harbored mammalian pathogens and possession of the machinery for eukaryotic post-translational protein modifications. So far, several organizations have utilized plant-based transient expression systems to produce SUVs against influenza, including vaccines based on virus-like particles. Plant-produced influenza SUV candidates have been extensively evaluated in animal models and some have shown safety and immunogenicity in clinical trials. Here, the authors review ongoing efforts and challenges to producing influenza SUV candidates in plants and discuss the likelihood of bringing these products to the market.

Plant-based vaccines against viruses

Plant-made or "biofarmed" viral vaccines are some of the earliest products of the technology of plant molecular farming, and remain some of the brightest prospects for the success of this field. Proofs of principle and of efficacy exist for many candidate viral veterinary vaccines; the use of plant-made viral antigens and of monoclonal antibodies for therapy of animal and even human viral disease is also well established. This review explores some of the more prominent recent advances in the biofarming of viral vaccines and therapies, including the recent use of ZMapp for Ebolavirus infection, and explores some possible future applications of the technology.

A potential protein-based vaccine for influenza H5N1 from the recombinant HA1 domain of avian influenza A/H5N1 expressed in Pichia pastoris

ABSTRACT 

The HA1 genes from influenza A strains A/Puerto Rico/8/1934 H1N1 (A/PR/8/34) and A/Hatay/2004 H5N1 were each cloned in Pichia pastoris vectors in the correct reading frame with the yeast α-factor secretion signal and the C-terminus His-tag, resulting in simple, fast purification of expressed H1HA1 and H5HA1 protein from the culture medium. Mice vaccinated with the purified proteins showed robust T cell, anti-HA1 IgG responses and developed a high antibody response for hemagglutination inhibition (HI) at titer 7.6 log2. Chickens vaccinated with a dose of 200 µg of H5HA1 mixed with either Montanide or Freund's adjuvants gave HI values of up to 7 log2 at the third week comparable with a licensed inactivated H5N1 vaccine.

Immunogenicity of H1N1 influenza virus-like particles produced in Nicotiana benthamiana

The H1N1 influenza pandemic of 2009 stimulated interest in developing safe and effective subunit influenza vaccines using rapid and cost-effective recombinant technologies that can avoid dependence on hens' eggs supply and live viruses for production. Among alternative approaches to subunit vaccine development, virus-like particles (VLPs) represent an attractive strategy due to their safety and immunogenicity. Previously, we have produced a recombinant monomeric hemagglutinin (HA) protein derived from the A/California/04/09 (H1N1) strain of influenza virus in a plant-based transient expression system and demonstrated immunogenicity and safety of this monomeric HA in animal models and human volunteers. In an effort to produce higher potency influenza vaccine in plants, we have designed and generated enveloped VLPs using the ectodomain of HA from the A/California/04/09 strain and heterologous sequences. The resulting H1 HA VLPs (HAC-VLPs) elicited robust hemagglutination inhibition antibody responses in mice at doses lower than 1 µg in the presence or absence of Alhydrogel adjuvant. These results suggest enhanced immunogenicity of recombinant HA in the form of an enveloped VLP over soluble antigen.

A novel plant-produced Pfs25 fusion subunit vaccine induces long-lasting transmission blocking antibody responses

Malaria transmission blocking vaccines (TBV) directed against proteins expressed on sexual stages of Plasmodium falciparum in the mosquito midgut are considered an effective means to reduce malaria transmission. Antibodies induced by TBV block sporogonic development in the mosquito, and thus transmission to the next human host. The Pfs25 protein, expressed on the surface of gametes, zygotes and ookinetes, is one of the primary targets for TBV development. Using a plant virus-based transient expression system, we have successfully produced Pfs25 fused to a modified lichenase (LicKM) carrier in Nicotiana benthamiana, purified and characterized the protein (Pfs25-FhCMB), and evaluated this vaccine candidate in animal models for the induction of transmission blocking antibodies. Soluble Pfs25-FhCMB was expressed in plants at a high level, and induced transmission blocking antibodies that persisted for up to 6 months post immunization in mice and rabbits. These data demonstrate the potential of the new malaria vaccine candidate and also support feasibility of expressing Plasmodium antigens in a plant-based system.

Transgenic plants: a 5-year update on oral antipathogen vaccine development

The progressive interest in transgenic plants as advantageous platforms for the production and oral delivery of vaccines has led to extensive research and improvements in this technology over recent years. In this paper, the authors examine the most significant advances in this area, including novel approaches for higher yields and better containment, and the continued evaluation of new vaccine prototypes against several infectious diseases. The use of plants to deliver vaccine candidates against viruses, bacteria, and eukaryotic parasites within the last 5 years is discussed, focusing on innovative expression strategies and the immunogenic potential of new vaccines. A brief section on the state of the art in mucosal immunity is also included.

Safety and immunogenicity of a plant-produced recombinant monomer hemagglutinin-based influenza vaccine derived from influenza A (H1N1)pdm09 virus: a Phase 1 dose-escalation study in healthy adults

BACKGROUND

Novel influenza viruses continue to pose a potential pandemic threat worldwide. In recent years, plants have been used to produce recombinant proteins, including subunit vaccines. A subunit influenza vaccine, HAC1, based on recombinant hemagglutinin from the 2009 pandemic A/California/04/2009 (H1N1) strain of influenza virus, has been manufactured using a plant virus-based transient expression technology in Nicotiana benthamiana plants and demonstrated to be immunogenic and safe in pre-clinical studies (Shoji et al., 2011).

METHODS

A first-in-human, Phase 1, single-center, randomized, placebo-controlled, single-blind, dose escalation study was conducted to investigate safety, reactogenicity and immunogenicity of an HAC1 formulation at three escalating dose levels (15 μg, 45 μg and 90 μg) with and without Alhydrogel(®), in healthy adults 18-50 years of age (inclusive). Eighty participants were randomized into six study vaccine groups, a saline placebo group and an approved monovalent H1N1 vaccine group. Recipients received two doses of vaccine or placebo (except for the monovalent H1N1 vaccine cohort, which received a single dose of vaccine, later followed by a dose of placebo).

RESULTS

The experimental vaccine was safe and well tolerated, and comparable to placebo and the approved monovalent H1N1 vaccine. Pain and tenderness at the injection site were the only local solicited reactions reported following vaccinations. Nearly all adverse events were mild to moderate in severity. The HAC1 vaccine was also immunogenic, with the highest seroconversion rates, based on serum hemagglutination-inhibition and virus microneutralization antibody titers, in the 90 μg non-adjuvanted HAC1 vaccine group after the second vaccine dose (78% and 100%, respectively).

CONCLUSIONS

This is the first study demonstrating the safety and immunogenicity of a plant-produced subunit H1N1 influenza vaccine in healthy adults. The results support further clinical investigation of the HAC1 vaccine as well as demonstrate the feasibility of the plant-based technology for vaccine antigen production.