Optimized Blanching Reduces the Host Cell Protein Content and Substantially Enhances the Recovery and Stability of Two Plant-Derived Malaria Vaccine Candidates

Design, optimization, production and activity testing of recombinant immunotoxins expressed in plants and plant cells for the treatment of monocytic leukemia

Antibody-drug conjugates (ADCs) can improve therapeutic indices compared to plain monoclonal antibodies (mAbs). However, ADC synthesis is complex because the components are produced separately in CHO cells (mAb) and often by chemical synthesis (drug). They are individually purified, coupled, and then the ADC is purified, increasing production costs compared to regular mAbs. In contrast, it is easier to produce recombinant fusion proteins consisting of an antibody derivative, linker and proteinaceous toxin, i.e. a recombinant immunotoxin (RIT). Plants are capable of the post-translational modifications needed for functional antibodies and can also express active protein toxins such as the recombinant mistletoe lectin viscumin, which is not possible in prokaryotes and mammalian cells respectively. Here, we used Nicotiana benthamiana and N. tabacum plants as well as tobacco BY-2 cell-based plant cell packs (PCPs) to produce effective RITs targeting CD64 as required for the treatment of myelomonocytic leukemia. We compared RITs with different subcellular targeting signals, linkers, and proteinaceous toxins. The accumulation of selected candidates was improved to ~ 40 mg kg-1 wet biomass using a design of experiments approach, and corresponding proteins were isolated with a purity of ~ 80% using an optimized affinity chromatography method with an overall yield of ~ 84%. One anti-CD64 targeted viscumin-based drug candidate was characterized in terms of storage stability and cytotoxicity test in vitro using human myelomonocytic leukemia cell lines. We identified bottlenecks in the plant-based expression platform that require further improvement and assessed critical process parameters that should be considered during process development for plant-made RITs.

Product safety aspects of plant molecular farming

Plant molecular farming (PMF) has been promoted since the 1990s as a rapid, cost-effective and (most of all) safe alternative to the cultivation of bacteria or animal cells for the production of biopharmaceutical proteins. Numerous plant species have been investigated for the production of a broad range of protein-based drug candidates. The inherent safety of these products is frequently highlighted as an advantage of PMF because plant viruses do not replicate in humans and vice versa. However, a more nuanced analysis of this principle is required when considering other pathogens because toxic compounds pose a risk even in the absence of replication. Similarly, it is necessary to assess the risks associated with the host system (e.g., the presence of toxic secondary metabolites) and the production approach (e.g., transient expression based on bacterial infiltration substantially increases the endotoxin load). This review considers the most relevant host systems in terms of their toxicity profile, including the presence of secondary metabolites, and the risks arising from the persistence of these substances after downstream processing and product purification. Similarly, we discuss a range of plant pathogens and disease vectors that can influence product safety, for example, due to the release of toxins. The ability of downstream unit operations to remove contaminants and process-related toxic impurities such as endotoxins is also addressed. This overview of plant-based production, focusing on product safety aspects, provides recommendations that will allow stakeholders to choose the most appropriate strategies for process development.

Optimising expression and extraction of recombinant proteins in plants

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

The transient expression of recombinant proteins in plant cell packs facilitates stable isotope labelling for NMR spectroscopy

Nuclear magnetic resonance (NMR) spectroscopy can be used to determine the structure, dynamics and interactions of proteins. However, protein NMR requires stable isotope labelling for signal detection. The cells used for the production of recombinant proteins must therefore be grown in medium containing isotopically labelled substrates. Stable isotope labelling is well established in Escherichia coli, but bacteria are only suitable for the production of simple proteins without post-translational modifications. More complex proteins require eukaryotic production hosts, but their growth can be impaired by labelled media, thus reducing product yields and increasing costs. To address this limitation, we used media supplemented with isotope-labelled substrates to cultivate the tobacco-derived cell line BY-2, which was then cast into plant cell packs (PCPs) for the transient expression of a labelled version of the model protein GB1. Mass spectrometry confirmed the feasibility of isotope labelling with ¹⁵ N and ² H using this approach. The resulting NMR spectrum featured a signal dispersion comparable to recombinant GB1 produced in E. coli. PCPs therefore offer a rapid and cost-efficient alternative for the production of isotope-labelled proteins for NMR analysis, especially suitable for complex proteins that cannot be produced in microbial systems.

Enhanced HIV SOSIP Envelope yields in plants through transient co-expression of peptidyl-prolyl isomerase B and calreticulin chaperones and ER targeting

High yield production of recombinant HIV SOSIP envelope (Env) trimers has proven elusive as numerous disulfide bonds, proteolytic cleavage and extensive glycosylation pose high demands on the host cell machinery and stress imposed by accumulation of misfolded proteins may ultimately lead to cellular toxicity. The present study utilized the Nicotiana benthamiana/p19 (N.b./p19) transient plant system to assess co-expression of two ER master regulators and 5 chaperones, crucial in the folding process, to enhance yields of three Env SOSIPs, single chain BG505 SOSIP.664 gp140, CH505TF.6R.SOSIP.664.v4.1 and CH848-10.17-DT9. Phenotypic changes in leaves induced by SOSIP expression were employed to rapidly identify chaperone-assisted improvement in health and expression. Up to 15-fold increases were obtained by co-infiltration of peptidylprolvl isomerase (PPI) and calreticulin (CRT) which were further enhanced by addition of the ER-retrieval KDEL tags to the SOSIP genes; levels depending on individual SOSIP type, day of harvest and chaperone gene dosage. Results are consistent with reducing SOSIP misfolding and cellular stress due to increased exposure to the plant host cell's calnexin/calreticulin network and accelerating the rate-limiting cis-trans isomerization of Xaa-Pro peptide bonds respectively. Plant transient co-expression facilitates rapid identification of host cell factors and will be translatable to other complex glycoproteins and mammalian expression systems.

Strategies for Efficient and Sustainable Protein Extraction and Purification from Plant Tissues

Recombinant proteins produced in whole plants are usually extracted and purified from leaf or seed tissues. Here we describe a workflow that is suitable for most soluble pharmaceutical proteins produced in leafy biomass. We highlight options to streamline the process, reducing the total process time and costs, and eliminating certain unit operations if the product allows. The overall process is a combination of batch-wise or continuous extraction followed by a series of solid-liquid separation steps, which can be supported by chemically defined additives, and concludes with a sequence of optional membrane-based and regular chromatographic purification operations.

Improving Recombinant Protein Recovery from Plant Tissue Using Heat Precipitation

Plants are increasingly viewed as suitable expression hosts for the production of recombinant proteins, especially when oxidative folding and/or posttranslational modification is essential for protein stability and functionality. In contrast to traditional platforms such as yeast and mammalian cells, where the product is secreted into the culture medium, recombinant proteins expressed in plants are usually retained within the cells so additional effort is required during extraction and purification. Various extraction processes are used to release soluble proteins from plant tissues, followed by clarification to remove fibers and particulates before the target protein is purified. Fermentation media generally contain few proteins, making it easier to recover a secreted product, whereas the green juice extracted from plants usually contains a large number of host proteins that interfere with target isolation and purification. In this chapter, we describe the use of heat precipitation to remove a large portion of the host cell proteins, thus improving the efficiency of subsequent purification steps and the quality of the purified recombinant protein.

Expression of Biofilm-Degrading Enzymes in Plants and Automated High-Throughput Activity Screening Using Experimental Bacillus subtilis Biofilms

Biofilm-forming bacteria are sources of infections because they are often resistant to antibiotics and chemical removal. Recombinant biofilm-degrading enzymes have the potential to remove biofilms gently, but they can be toxic toward microbial hosts and are therefore difficult to produce in bacteria. Here, we investigated Nicotiana species for the production of such enzymes using the dispersin B-like enzyme Lysobacter gummosus glyco 2 (Lg2) as a model. We first optimized transient Lg2 expression in plant cell packs using different subcellular targeting methods. We found that expression levels were transferable to differentiated plants, facilitating the scale-up of production. Our process yielded 20 mg kg⁻¹ Lg2 in extracts but 0.3 mg kg⁻¹ after purification, limited by losses during depth filtration. Next, we established an experimental biofilm assay to screen enzymes for degrading activity using different Bacillus subtilis strains. We then tested complex and chemically defined growth media for reproducible biofilm formation before converting the assay to an automated high-throughput screening format. Finally, we quantified the biofilm-degrading activity of Lg2 in comparison with commercial enzymes against our experimental biofilms, indicating that crude extracts can be screened directly. This ability will allow us to combine high-throughput expression in plant cell packs with automated activity screening.

Nicotiana spp. for the Expression and Purification of Functional IgG3 Antibodies Directed Against the Staphylococcus aureus Alpha Toxin

Immunoglobulin subclass IgG1 is bound and neutralized effectively by Staphylococcus aureus protein A, allowing the bacterium to evade the host’s adaptive immune response. In contrast, the IgG3 subclass is not bound by protein A and can be used to treat S. aureus infections, including drug-resistant strains such as methicillin-resistant Staphylococcus aureus (MRSA). However, the yields of recombinant IgG3 are generally low because this subclass is prone to degradation, and recovery is hindered by the inability to use protein A as an affinity ligand for antibody purification. Here, we investigated plants (Nicotiana spp.) as an alternative to microbes and mammalian cell cultures for the production of an IgG3 antibody specific for the S. aureus alpha toxin. We targeted recombinant IgG3 to different subcellular compartments and tested different chromatography conditions to improve recovery and purification. Finally, we tested the antigen-binding capacity of the purified antibodies. The highest IgG3 levels in planta (>130 mg kg−1 wet biomass) were achieved by targeting the endoplasmic reticulum or apoplast. Although the purity of IgG3 exceeded 95% following protein G chromatography, product recovery requires further improvement. Importantly, the binding affinity of the purified antibodies was in the nanomolar range and thus comparable to previous studies using murine hybridoma cells as the production system.

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.

Ma J, McDonald KA, Murad A, Nandi S, O’Keef B, Parthiban S, Paul MJ, Ponndorf D, Rech E, Rodrigues JC, Ruf S, Schillberg S, Schwestka J, Shah PS, Singh R, Stoger E, Twyman RM, Varghese IP, Vianna GR, Webster G, Wilbers RHP, Christou P, Oksman‐Caldentey K, Capell T. Contributions of the international plant science community to the fight against infectious diseases in humans-part 2: Affordable drugs in edible plants for endemic and re-emerging diseases

The fight against infectious diseases often focuses on epidemics and pandemics, which demand urgent resources and command attention from the health authorities and media. However, the vast majority of deaths caused by infectious diseases occur in endemic zones, particularly in developing countries, placing a disproportionate burden on underfunded health systems and often requiring international interventions. The provision of vaccines and other biologics is hampered not only by the high cost and limited scalability of traditional manufacturing platforms based on microbial and animal cells, but also by challenges caused by distribution and storage, particularly in regions without a complete cold chain. In this review article, we consider the potential of molecular farming to address the challenges of endemic and re-emerging diseases, focusing on edible plants for the development of oral drugs. Key recent developments in this field include successful clinical trials based on orally delivered dried leaves of Artemisia annua against malarial parasite strains resistant to artemisinin combination therapy, the ability to produce clinical-grade protein drugs in leaves to treat infectious diseases and the long-term storage of protein drugs in dried leaves at ambient temperatures. Recent FDA approval of the first orally delivered protein drug encapsulated in plant cells to treat peanut allergy has opened the door for the development of affordable oral drugs that can be manufactured and distributed in remote areas without cold storage infrastructure and that eliminate the need for expensive purification steps and sterile delivery by injection.

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.

Precision analysis for the determination of steric mass action parameters using eight tobacco host cell proteins

Plants are advantageous as biopharmaceutical manufacturing platforms because they allow the economical and scalable upstream production of proteins, including those requiring post-translational modifications, but do not support the replication of human viruses. However, downstream processing can be more labor-intensive compared to fermenter-based systems because the product is often mixed with abundant host cell proteins (HCPs). Modeling chromatographic separation can minimize the number of process development experiments and thus reduce costs. An important part of such modeling is the sorption isotherm, such as the steric mass action (SMA) model, which describes the multicomponent protein-salt equilibria established in ion-exchange systems. Here we purified ten HCPs, including 2-Cys-peroxiredoxin, from tobacco (Nicotiana tabacum and N. benthamiana). For eight of these HCPs, we obtained sufficient quantities to determine the SMA binding parameters (KSMA and ν) under different production-relevant conditions. We studied the parameters for 2-Cys-peroxiredoxin on Q-Sepharose HP in detail, revealing that pH, resin batch and buffer batch had little influence on KSMA and ν, with coefficients of variation (COVs) less than 0.05 and 0.21, respectively. In contrast, the anion-exchange resins SuperQ-650S, Q-Sepharose FF and QAE-550C led to COVs of 0.69 for KSMA and 0.05 for ν, despite using the same quaternary amine functional group as Q-Sepharose HP. Plant cultivation in summer vs winter resulted in COVs of 0.09 for KSMA and 0.02 for ν, revealing a small impact compared to COVs of 17.15 for KSMA and 0.20 for ν when plants were grown in different settings (climate-controlled phytotron vs greenhouse). We conclude that plant cultivation can substantially affect protein properties and the resulting SMA parameters. Accordingly, plant growth but also protein purification and characterization for chromatography model building should be tightly controlled and well documented.

A combined pH and temperature precipitation step facilitates the purification of tobacco-derived recombinant proteins that are sensitive to extremes of either parameter

Incubation at pH 4.0 or blanching at ∼65°C facilitates the purification of biopharmaceutical proteins from plants by precipitating most of the host cell proteins (HCPs) before chromatography. However, both methods are compatible only with pH or thermostable target proteins whereas many target proteins may irreversibly denature, e.g., at pH < 4.0. Here, we developed a combined pH/temperature treatment for clarified tobacco extracts and intact leaves. The latter were subjected to a blanching procedure, i.e., the submersion into a hot buffer. Using a design of experiments approach we identified conditions that remove ∼70% of HCPs at ∼55°C, using the thermosensitive antibody 2G12 and the pH-sensitive DsRed as model proteins. We found that pH and temperature exerted a combined effect during the precipitation of HCPs in the pH range 5.0-7.0 at 35°C-60°C. For clarified extracts, the temperature required to achieve a DsRed purity threshold of 20% total soluble protein (TSP) increased from 54°C to 63°C when the pH was increased from 6.4 to 7.3. The pH-stable antibody 2G12 was less responsive to the combined treatment, but the purity of 1% TSP was achieved at 35°C instead of 44°C when the pH was reduced from 6.3 to 5.8. When blanching intact leaves, product losses were not exacerbated at pH 4.0. Indeed, the highest DsRed purity (58% TSP) was achieved at this pH, combined with a temperature of 60°C and an incubation time of 30 min. In contrast, the highest 2G12 purity (0.7% TSP) was achieved at pH 5.1 and 40°C with an incubation time of 20 min. Our data suggest that a combined pH/temperature regime can avoid extreme values of either parameter; therefore, broadening the applicability of these simple purification techniques to other recombinant proteins.

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

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

Plant-made immunotoxin building blocks: A roadmap for producing therapeutic antibody-toxin fusions

Molecular farming in plants is an emerging platform for the production of pharmaceutical proteins, and host species such as tobacco are now becoming competitive with commercially established production hosts based on bacteria and mammalian cell lines. The range of recombinant therapeutic proteins produced in plants includes replacement enzymes, vaccines and monoclonal antibodies (mAbs). But plants can also be used to manufacture toxins, such as the mistletoe lectin viscumin, providing an opportunity to express active antibody-toxin fusion proteins, so-called recombinant immunotoxins (RITs). Mammalian production systems are currently used to produce antibody-drug conjugates (ADCs), which require the separate expression and purification of each component followed by a complex and hazardous coupling procedure. In contrast, RITs made in plants are expressed in a single step and could therefore reduce production and purification costs. The costs can be reduced further if subcellular compartments that accumulate large quantities of the stable protein are identified and optimal plant growth conditions are selected. In this review, we first provide an overview of the current state of RIT production in plants before discussing the three key components of RITs in detail. The specificity-defining domain (often an antibody) binds cancer cells, including solid tumors and hematological malignancies. The toxin provides the means to kill target cells. Toxins from different species with different modes of action can be used for this purpose. Finally, the linker spaces the two other components to ensure they adopt a stable, functional conformation, and may also promote toxin release inside the cell. Given the diversity of these components, we extract broad principles that can be used as recommendations for the development of effective RITs. Future research should focus on such proteins to exploit the advantages of plants as efficient production platforms for targeted anti-cancer therapeutics.

Optimizing product quality in molecular farming

The production of biopharmaceuticals in plant-based systems had faced several challenges that hampered broader adoption of this technology. In recent years, various plant production hosts have been improved by genetic engineering approaches to overcome obstacles with regard to post-translational modifications and integrity of target proteins. Together with optimized extraction and purification processes, those advances have put plant molecular farming in a more competitive position compared to established production systems. Certain biopharmaceuticals can be derived from plant systems with unique desired properties, qualifying them as biobetters.

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.

A Combined Ultrafiltration/Diafiltration Step Facilitates the Purification of Cyanovirin-N From Transgenic Tobacco Extracts

The production of biopharmaceutical proteins in plants offers many advantages over traditional expression platforms, including improved safety, greater scalability and lower upstream production costs. However, most products are retained within plant cells or the apoplastic space instead of being secreted into a liquid medium, so downstream processing necessarily involves tissue and cell disruption followed by the removal of abundant particles and host cell proteins (HCPs). We investigated whether ultrafiltration/diafiltration (UF/DF) can simplify the purification of the model recombinant protein cyanovirin-N (CVN), an ~ 11 kDa HIV-neutralizing lectin, from tobacco extracts prior to chromatography. We compared different membrane types and process conditions, and found that at pH 8.0 and 50 mS cm^-1 an UF step using a 100 kDa regenerated cellulose membrane removed more than 80% of the ~ 0.75 mg mL^-1 total soluble protein present in the clarified plant extract. We recovered ~ 70% of the CVN and the product purity increased ~ 3-fold in the permeate. The underlying effects of tobacco HCP retention during the UF/DF step were investigated by measuring the zeta potential and particle size distribution in the 2-10,000 nm range. Combined with a subsequent 10 kDa DF step, this approach simultaneously reduced the process volume, conditioned the process intermediate, and facilitated early, chromatography-free purification. Due to the generic, size-based nature of the method, it is likely to be compatible with most products smaller than ~50 kDa.

Seasonal Weather Changes Affect the Yield and Quality of Recombinant Proteins Produced in Transgenic Tobacco Plants in a Greenhouse Setting

Transgenic plants have the potential to produce recombinant proteins on an agricultural scale, with yields of several tons per year. The cost-effectiveness of transgenic plants increases if simple cultivation facilities such as greenhouses can be used for production. In such a setting, we expressed a novel affinity ligand based on the fluorescent protein DsRed, which we used as a carrier for the linear epitope ELDKWA from the HIV-neutralizing antibody 2F5. The DsRed-2F5-epitope (DFE) fusion protein was produced in 12 consecutive batches of transgenic tobacco (Nicotiana tabacum) plants over the course of 2 years and was purified using a combination of blanching and immobilized metal-ion affinity chromatography (IMAC). The average purity after IMAC was 57 ± 26% (n = 24) in terms of total soluble protein, but the average yield of pure DFE (12 mg kg^-1) showed substantial variation (± 97 mg kg^-1, n = 24) which correlated with seasonal changes. Specifically, we found that temperature peaks (>28°C) and intense illuminance (>45 klx h^-1) were associated with lower DFE yields after purification, reflecting the loss of the epitope-containing C-terminus in up to 90% of the product. Whereas the weather factors were of limited use to predict product yields of individual harvests conducted for each batch (spaced by 1 week), the average batch yields were well approximated by simple linear regression models using two independent variables for prediction (illuminance and plant age). Interestingly, accumulation levels determined by fluorescence analysis were not affected by weather conditions but positively correlated with plant age, suggesting that the product was still expressed at high levels, but the extreme conditions affected its stability, albeit still preserving the fluorophore function. The efficient production of intact recombinant proteins in plants may therefore require adequate climate control and shading in greenhouses or even cultivation in fully controlled indoor farms.

Downstream processing of a plant-derived malaria transmission-blocking vaccine candidate

Plants as a platform for recombinant protein expression are now economically comparable to well-established systems, such as microbes and mammalian cells, thanks to advantages such as scalability and product safety. However, downstream processing accounts for the majority of the final product costs because plant extracts contain large quantities of host cell proteins (HCPs) that must be removed using elaborate purification strategies. Heat precipitation in planta (blanching) can remove ∼80% of HCPs and thus simplify further purification steps, but this is only possible if the target protein is thermostable. Here we describe a combination of blanching and chromatography to purify the thermostable transmission-blocking malaria vaccine candidate FQS, which was transiently expressed in Nicotiana benthamiana leaves. If the blanching temperature exceeded a critical threshold of ∼75 °C, FQS was no longer recognized by the malaria transmission-blocking monoclonal antibody 4B7. A design-of-experiments approach revealed that reducing the blanching temperature from 80 °C to 70 °C restored antibody binding while still precipitating most HCPs. We also found that blanching inhibited the degradation of FQS in plant extracts, probably due to the thermal inactivation of proteases. We screened hydrophobic interaction chromatography materials using miniature columns and a liquid-handling station. Octyl Sepharose achieved the highest FQS purity during the primary capture step and led to a final purity of ∼72% with 60% recovery via step elution. We found that 30-75% FQS was lost during ultrafiltration/diafiltration, giving a final yield of 9 mg kg-1 plant material after purification based on an initial yield of ∼49 mg kg-1 biomass after blanching.

Expression and purification of human phosphatase and actin regulator 1 (PHACTR1) in plant-based systems

Cardiovascular diseases are a prevalent cause of morbidity and mortality especially in industrialized countries. The human phosphatase and actin regulator 1 (PHACTR1) may be involved in such diseases, but its precise regulatory function remains unclear due to the large number of potential interaction partners. The same phenomenon makes this protein difficult to express in mammalian cells, but it is also an intrinsically disordered protein that likely aggregates when expressed in bacteria due to the absence of chaperones. We therefore used a design of experiments approach to test the suitability of three plant-based systems for the expression of satisfactory quantities of recombinant PHACTR1, namely transient expression in tobacco (Nicotiana tabacum) BY-2 plant cell packs (PCPs), whole N. benthamiana leaves and BY-2 cell lysate (BYL). The highest yield was achieved using the BYL: up to 120 mg product kg-1 biomass equivalent within 48 h of translation. This was 1.3-fold higher than transient expression in N. benthamiana together with the silencing inhibitor p19, and 6-fold higher than the PCP system. The presence of Triton X-100 in the extraction buffer increased the recovery of PHACTR1 by 2-200-fold depending on the conditions. PHACTR1 was incompatible with biomass blanching and was stable for less than 16 h in raw plant extracts. Purification using a DDK-tag proved inefficient whereas 15% purity was achieved by immobilized metal affinity chromatography.

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.

A Rapid Laser Probing Method Facilitates the Non-invasive and Contact-free Determination of Leaf Thermal Properties

Plants can produce valuable substances such as secondary metabolites and recombinant proteins. The purification of the latter from plant biomass can be streamlined by heat treatment (blanching). A blanching apparatus can be designed more precisely if the thermal properties of the leaves are known in detail, i.e., the specific heat capacity and thermal conductivity. The measurement of these properties is time consuming and labor intensive, and usually requires invasive methods that contact the sample directly. This can reduce the product yield and may be incompatible with containment requirements, e.g., in the context of good manufacturing practice. To address these issues, a non-invasive, contact-free method was developed that determines the specific heat capacity and thermal conductivity of an intact plant leaf in about one minute. The method involves the application of a short laser pulse of defined length and intensity to a small area of the leaf sample, causing a temperature increase that is measured using a near infrared sensor. The temperature increase is combined with known leaf properties (thickness and density) to determine the specific heat capacity. The thermal conductivity is then calculated based on the profile of the subsequent temperature decline, taking thermal radiation and convective heat transfer into account. The associated calculations and critical aspects of sample handling are discussed.

Comparison of Tobacco Host Cell Protein Removal Methods by Blanching Intact Plants or by Heat Treatment of Extracts

Plants not only provide food, feed and raw materials for humans, but have also been developed as an economical production system for biopharmaceutical proteins, such as antibodies, vaccine candidates and enzymes. These must be purified from the plant biomass but chromatography steps are hindered by the high concentrations of host cell proteins (HCPs) in plant extracts. However, most HCPs irreversibly aggregate at temperatures above 60 °C facilitating subsequent purification of the target protein. Here, three methods are presented to achieve the heat precipitation of tobacco HCPs in either intact leaves or extracts. The blanching of intact leaves can easily be incorporated into existing processes but may have a negative impact on subsequent filtration steps. The opposite is true for heat precipitation of leaf extracts in a stirred vessel, which can improve the performance of downstream operations albeit with major changes in process equipment design, such as homogenizer geometry. Finally, a heat exchanger setup is well characterized in terms of heat transfer conditions and easy to scale, but cleaning can be difficult and there may be a negative impact on filter capacity. The design-of-experiments approach can be used to identify the most relevant process parameters affecting HCP removal and product recovery. This facilitates the application of each method in other expression platforms and the identification of the most suitable method for a given purification strategy.