The human anti-HIV antibodies 2F5, 2G12, and PG9 differ in their susceptibility to proteolytic degradation: down-regulation of endogenous serine and cysteine proteinase activities could improve antibody production in plant-based expression platforms

Improving transient protein expression in agroinfiltrated Nicotiana benthamiana

Agroinfiltration of Nicotiana benthamiana is routinely used in plant science and molecular pharming to transiently express proteins of interest. Here, we discuss four phenomena that should be avoided to improve transient expression. Immune responses can be avoided by depleting immune receptors and employing pathogen-derived effectors; transcript degradation by using silencing inhibitors or RNA interference machinery mutants; endoplasmic reticulum stress by co-expressing chaperones; and protein degradation can be avoided with subcellular targeting, protease mutants and co-expressing protease inhibitors. We summarise the reported increased yields for various recombinant proteins achieved with these approaches and highlight remaining challenges to further improve the efficiency of this versatile protein expression platform.

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.

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.

Prevention of Shiga toxin 1-caused colon injury by plant-derived recombinant IgA

Immunoglobulin A (IgA) is a candidate antibody for oral passive immunization against mucosal pathogens like Shiga toxin-producing Escherichia coli (STEC). We previously established a mouse IgG monoclonal antibody (mAb) neutralizing Shiga toxin 1 (Stx1), a bacterial toxin secreted by STEC. We designed cDNA encoding an anti-Stx1 antibody, in which variable regions were from the IgG mAb and all domains of the heavy chain constant region from a mouse IgA mAb. Considering oral administration, we expressed the cDNA in a plant expression system aiming at the production of enough IgA at low cost. The recombinant-IgA expressed in Arabidopsis thaliana formed the dimeric IgA, bound to the B subunit of Stx1, and neutralized Stx1 toxicity to Vero cells. Colon injury was examined by exposing BALB/c mice to Stx1 via the intrarectal route. Epithelial cell death, loss of crypt and goblet cells from the distal colon were observed by electron microscopy. A loss of secretory granules containing MUC2 mucin and activation of caspase-3 were observed by immunohistochemical methods. Pretreatment of Stx1 with the plant-based recombinant IgA completely suppressed caspase-3 activation and loss of secretory granules. The results indicate that a plant-based recombinant IgA prevented colon damage caused by Stx1 in vivo.

Transient proteolysis reduction of Nicotiana benthamiana-produced CAP256 broadly neutralizing antibodies using CRISPR/Cas9

The hypersensitive response is elicited by Agrobacterium infiltration of Nicotiana benthamiana, including the induction and accumulation of pathogenesis-related proteins, such as proteases. This includes the induction of the expression of several cysteine proteases from the C1 (papain-like cysteine protease) and C13 (legumain-like cysteine protease) families. This study demonstrates the role of cysteine proteases: NbVPE-1a, NbVPE-1b, and NbCysP6 in the proteolytic degradation of Nicotiana benthamiana (glycosylation mutant ΔXTFT)-produced anti-human immunodeficiency virus broadly neutralizing antibody, CAP256-VRC26.25. Three putative cysteine protease cleavage sites were identified in the fragment crystallizable region. We further demonstrate the transient coexpression of CAP256-VRC26.25 with CRISPR/Cas9-mediated genome editing vectors targeting the NbVPE-1a, NbVPE-1b, and NbCysP6 genes which resulted in a decrease in CAP256-VRC26.25 degradation. No differences in structural features were observed between the human embryonic kidney 293 (HEK293)-produced and ΔXTFT broadly neutralizing antibodies produced with and without the coexpression of genome-editing vectors. Furthermore, despite the presence of proteolytically degraded fragments of plant-produced CAP256-VRC26.25 without the coexpression of genome editing vectors, no influence on the in vitro functional activity was detected. Collectively, we demonstrate an innovative in planta strategy for improving the quality of the CAP256 antibodies through the transient expression of the CRISPR/Cas9 vectors.

Structure-based discovery of small molecules improving stability of human broadly-neutralizing anti-HIV antibody 2F5 in plant suspension cells

The production of biopharmaceuticals in engineered plant-based systems is a promising technology that has proven its suitability for the production of various recombinant glyco-proteins that are currently undergoing clinical trials. However, compared to mammalian cell lines, the productivity of plant-based systems still requires further improvement. A major obstacle is the proteolytic degradation of recombinant target proteins by endogenous plant proteases mainly from the subtilisin family of serine proteases. In the present study, the authors screened for putative small molecule inhibitors for subtilases that are secreted from tobacco BY-2 suspension cells using an in silico approach. The effectiveness of the substances identified in this screen was subsequently tested in degradation assays using the human broadly-neutralizing anti-HIV monoclonal antibody 2F5 (mAb2F5) and spent BY-2 culture medium as a model system. Among 16 putative inhibitors identified by in silico studies, three naphthalene sulfonic acid derivatives showed inhibitory activity in in vitro degradation assays and are similar to or even more effective than phenylmethylsulfonyl fluoride (PMSF), a classical inhibitor of serine proteases, which served as positive control.

Reteplase Fc-fusions produced in N. benthamiana are able to dissolve blood clots ex vivo

Thrombolytic and fibrinolytic therapies are effective treatments to dissolve blood clots in stroke therapy. Thrombolytic drugs activate plasminogen to its cleaved form plasmin, a proteolytic enzyme that breaks the crosslinks between fibrin molecules. The FDA-approved human tissue plasminogen activator Reteplase (rPA) is a non-glycosylated protein produced in E. coli. rPA is a deletion mutant of the wild-type Alteplase that benefits from an extended plasma half-life, reduced fibrin specificity and the ability to better penetrate into blood clots. Different methods have been proposed to improve the production of rPA. Here we show for the first time the transient expression in Nicotiana benthamiana of rPA fused to the immunoglobulin fragment crystallizable (Fc) domain on an IgG1, a strategy commonly used to improve the stability of therapeutic proteins. Despite our success on the expression and purification of dimeric rPA-Fc fusions, protein instability results in high amounts of Fc-derived degradation products. We hypothesize that the "Y"- shape of dimeric Fc fusions cause steric hindrance between protein domains and leads to physical instability. Indeed, mutations of critical residues in the Fc dimerization interface allowed the expression of fully stable rPA monomeric Fc-fusions. The ability of rPA-Fc to convert plasminogen into plasmin was demonstrated by plasminogen zymography and clot lysis assay shows that rPA-Fc is able to dissolve blood clots ex vivo. Finally, we addressed concerns with the plant-specific glycosylation by modulating rPA-Fc glycosylation towards serum-like structures including α2,6-sialylated and α1,6-core fucosylated N-glycans completely devoid of plant core fucose and xylose residues.

Engineering Approaches in Plant Molecular Farming for Global Health

Since the demonstration of the first plant-produced proteins of medical interest, there has been significant growth and interest in the field of plant molecular farming, with plants now being considered a viable production platform for vaccines. Despite this interest and development by a few biopharmaceutical companies, plant molecular farming is yet to be embraced by ‘big pharma’. The plant system offers a faster alternative, which is a potentially more cost-effective and scalable platform for the mass production of highly complex protein vaccines, owing to the high degree of similarity between the plant and mammalian secretory pathway. Here, we identify and address bottlenecks in the use of plants for vaccine manufacturing and discuss engineering approaches that demonstrate both the utility and versatility of the plant production system as a viable biomanufacturing platform for global health. Strategies for improving the yields and quality of plant-produced vaccines, as well as the incorporation of authentic posttranslational modifications that are essential to the functionality of these highly complex protein vaccines, will also be discussed. Case-by-case examples are considered for improving the production of functional protein-based vaccines. The combination of all these strategies provides a basis for the use of cutting-edge genome editing technology to create a general plant chassis with reduced host cell proteins, which is optimised for high-level protein production of vaccines with the correct posttranslational modifications.

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.

Generation of enzymatically competent SARS-CoV-2 decoy receptor ACE2-Fc in glycoengineered Nicotiana benthamiana

Human angiotensin-converting enzyme 2 (ACE2) is the primary host cell receptor for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) binding and cell entry. Administration of high concentrations of soluble ACE2 can be utilized as a decoy to block the interaction of the virus with cellular ACE2 receptors and potentially be used as a strategy for treatment or prevention of coronavirus disease 2019. Human ACE2 is heavily glycosylated and its glycans impact on binding to the SARS-CoV-2 spike protein and virus infectivity. Here, we describe the production of a recombinant soluble ACE2-fragment crystallizable (Fc) variant in glycoengineered Nicotiana benthamiana. Our data reveal that the produced dimeric ACE2-Fc variant is glycosylated with mainly complex human-type N-glycans and functional with regard to enzyme activity, affinity to the SARS-CoV-2 receptor-binding domain, and wild-type virus neutralization.

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.

Identification of two subtilisin-like serine proteases engaged in the degradation of recombinant proteins in Nicotiana benthamiana

The tobacco variant Nicotiana benthamiana has recently emerged as a versatile host for the manufacturing of protein therapeutics, but the fidelity of many recombinant proteins generated in this system is compromised by inadvertent proteolysis. Previous studies have revealed that the anti-HIV-1 antibodies 2F5 and PG9 as well as the protease inhibitor α1 -antitrypsin (A1AT) are particularly susceptible to N. benthamiana proteases. Here, we identify two subtilisin-like serine proteases (NbSBT1 and NbSBT2) whose combined action is sufficient to account for all major cleavage events observed upon expression of 2F5, PG9 and A1AT in N. benthamiana. We propose that downregulation of NbSBT1 and NbSBT2 activities could constitute a powerful means to optimize the performance of this promising platform for the production of biopharmaceuticals. DATABASES: NbSBT sequence data are available in the DDBJ/EMBL/GenBank databases under the accession numbers MN534996 to MN535005.

The Instability of Dimeric Fc-Fusions Expressed in Plants Can Be Solved by Monomeric Fc Technology

The potential therapeutic value of many proteins is ultimately limited by their rapid in vivo clearance. One strategy to limit clearance by metabolism and excretion, and improving the stability of therapeutic proteins, is their fusion to the immunoglobulin fragment crystallizable region (Fc). The Fc region plays multiple roles in (i) dimerization for the formation of "Y"-shaped structure of Ig, (ii) Fc-mediated effector functions, (iii) extension of serum half-life, and (iv) a cost-effective purification tag. Plants and in particular Nicotiana benthamiana have proven to be suitable expression platforms for several recombinant therapeutic proteins. Despite the enormous success of their use for the production of full-length monoclonal antibodies, the expression of Fc-fused therapeutic proteins in plants has shown limitations. Many Fc-fusion proteins expressed in plants show different degrees of instability resulting in high amounts of Fc-derived degradation products. To address this issue, we used erythropoietin (EPO) as a reporter protein and evaluated the efforts to enhance the expression of full-length EPO-Fc targeted to the apoplast of N. benthamiana. Our results show that the instability of the fusion protein is independent from the Fc origin or IgG subclass and from the peptide sequence used to link the two domains. We also show that a similar instability occurs upon the expression of individual heavy chains of monoclonal antibodies and ScFv-Fc that mimic the "Y"-shape of antibodies but lack the light chain. We propose that in this configuration, steric hindrance between the protein domains leads to physical instability. Indeed, mutations of critical residues located on the Fc dimerization interface allowed the expression of fully stable EPO monomeric Fc-fusion proteins. We discuss the limitations of Fc-fusion technology in N. benthamiana transient expression systems and suggest strategies to optimize the Fc-based scaffolds on their folding and aggregation resistance in order to improve the stability.

A New Plant Expression System for Producing Pharmaceutical Proteins

In the past decade, interest in the production of recombinant pharmaceutical proteins in plants has tremendously progressed because plants do not harbor mammalian viruses, are economically competitive, easily scalable, and capable of carrying out complex post-translational modifications required for recombinant pharmaceutical proteins. Mucuna bracteata is an essential perennial cover crop species widely planted as an underground cover in oil palm and rubber plantations. As a legume, they have high biomass, thrive in its habitat, and can fix nitrogen. Thus, M. bracteata is a cost-efficient crop that shows ideal characteristics as a platform for mass production of recombinant protein. In this study, we established a new platform for the transient production of a recombinant protein in M. bracteata via vacuum-assisted agro-infiltration. Five-week-old M. bracteata plants were vacuum infiltrated with Agrobacterium tumefaciens harboring a plasmid that encodes for an anti-toxoplasma immunoglobulin (IgG) under different parameters, including trifoliate leaf positional effects, days to harvest post-infiltration, and the Agrobacterium strain used. Our results showed that vacuum infiltration of M. bracteata plant with A. tumefaciens strain GV3101 produced the highest concentration of heterologous protein in its bottom trifoliate leaf at 2 days post-infiltration. The purified anti-toxoplasma IgG was then analyzed using Western blot and ELISA. It was demonstrated that, while structural heterogeneity existed in the purified anti-toxoplasma IgG from M. bracteata, its transient expression level was two-fold higher than the model platform, Nicotiana benthamiana. This study has laid the foundation towards establishing M. bracteata as a potential platform for the production of recombinant pharmaceutical protein.

Steric Accessibility of the Cleavage Sites Dictates the Proteolytic Vulnerability of the Anti-HIV-1 Antibodies 2F5, 2G12, and PG9 in Plants

Broadly neutralizing antibodies (bNAbs) to human immunodeficiency virus type 1 (HIV-1) hold great promise for immunoprophylaxis and the suppression of viremia in HIV-positive individuals. Several studies have demonstrated that plants as Nicotiana benthamiana are suitable hosts for the generation of protective anti-HIV-1 antibodies. However, the production of the anti-HIV-1 bNAbs 2F5 and PG9 in N. benthamiana is associated with their processing by apoplastic proteases in the complementarity-determining-region (CDR) H3 loops of the heavy chains. Here, it is shown that apoplastic proteases can also cleave the CDR H3 loop of the bNAb 2G12 when the unusual domain exchange between its heavy chains is prevented by the replacement of Ile19 with Arg. It is demonstrated that CDR H3 proteolysis leads to a strong reduction of the antigen-binding potencies of 2F5, PG9, and 2G12-I19R. Inhibitor profiling experiments indicate that different subtilisin-like serine proteases account for bNAb fragmentation in the apoplast. Differential scanning calorimetry experiments corroborate that the antigen-binding domains of wild-type 2G12 and 4E10 are more compact than those of proteolysis-sensitive antibodies, thus shielding their CDR H3 regions from proteolytic attack. This suggests that the extent of proteolytic inactivation of bNAbs in plants is primarily dictated by the steric accessibility of their CDR H3 loops.

Pilot Production of SARS-CoV-2 Related Proteins in Plants: A Proof of Concept for Rapid Repurposing of Indoor Farms Into Biomanufacturing Facilities

The current CoVid-19 crisis is revealing the strengths and the weaknesses of the world's capacity to respond to a global health crisis. A critical weakness has resulted from the excessive centralization of the current biomanufacturing capacities, a matter of great concern, if not a source of nationalistic tensions. On the positive side, scientific data and information have been shared at an unprecedented speed fuelled by the preprint phenomena, and this has considerably strengthened our ability to develop new technology-based solutions. In this work, we explore how, in a context of rapid exchange of scientific information, plant biofactories can serve as a rapid and easily adaptable solution for local manufacturing of bioreagents, more specifically recombinant antibodies. For this purpose, we tested our ability to produce, in the framework of an academic lab and in a matter of weeks, milligram amounts of six different recombinant monoclonal antibodies against SARS-CoV-2 in Nicotiana benthamiana. For the design of the antibodies, we took advantage, among other data sources, of the DNA sequence information made rapidly available by other groups in preprint publications. mAbs were engineered as single-chain fragments fused to a human gamma Fc and transiently expressed using a viral vector. In parallel, we also produced the recombinant SARS-CoV-2 N protein and the receptor binding domain (RBD) of the Spike protein in planta and used them to test the binding specificity of the recombinant mAbs. Finally, for two of the antibodies, we assayed a simple scale-up production protocol based on the extraction of apoplastic fluid. Our results indicate that gram amounts of anti-SARS-CoV-2 antibodies could be easily produced in little more than 6 weeks in repurposed greenhouses with little infrastructure requirements using N. benthamiana as production platform. Similar procedures could be easily deployed to produce diagnostic reagents and, eventually, could be adapted for rapid therapeutic responses.

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.

Activity-based proteomics reveals nine target proteases for the recombinant protein-stabilizing inhibitor SlCYS8 in Nicotiana benthamiana

Co-expression of protease inhibitors like the tomato cystatin SlCYS8 is useful to increase recombinant protein production in plants, but key proteases involved in protein proteolysis are still unknown. Here, we performed activity-based protein profiling to identify proteases that are inhibited by SlCYS8 in agroinfiltrated Nicotiana benthamiana. We discovered that SlCYS8 selectively suppresses papain-like cysteine protease (PLCP) activity in both apoplastic fluids and total leaf extracts, while not affecting vacuolar-processing enzyme and serine hydrolase activity. A robust concentration-dependent inhibition of PLCPs occurred in vitro when purified SlCYS8 was added to leaf extracts, indicating direct cystatin-PLCP interactions. Activity-based proteomics revealed that nine different Cathepsin-L/-F-like PLCPs are strongly inhibited by SlCYS8 in leaves. By contrast, the activity of five other Cathepsin-B/-H-like PLCPs, as well as 87 Ser hydrolases, was unaffected by SlCYS8. SlCYS8 expression prevented protein degradation by inhibiting intermediate and mature isoforms of granulin-containing proteases from the Resistant-to-Desiccation-21 (RD21) PLCP subfamily. Our data underline the key role of endogenous PLCPs on recombinant protein degradation and reveal candidate proteases for depletion strategies.

Assessing Carnivorous Plants for the Production of Recombinant Proteins

The recovery of recombinant proteins from plant tissues is an expensive and time-consuming process involving plant harvesting, tissue extraction, and subsequent protein purification. The downstream process costs can represent up to 80% of the total cost of production. Secretion-based systems of carnivorous plants might help circumvent this problem. Drosera and Nepenthes can produce and excrete out of their tissues a digestive fluid containing up to 200 mg. L-1 of natural proteins. Based on the properties of these natural bioreactors, we have evaluated the possibility to use carnivorous plants for the production of recombinant proteins. In this context, we have set up original protocols of stable and transient genetic transformation for both Drosera and Nepenthes sp. The two major drawbacks concerning the proteases naturally present in the secretions and a polysaccharidic network composing the Drosera glue were overcome by modulating the pH of the plant secretions. At alkaline pH, digestive enzymes are inactive and the interactions between the polysaccharidic network and proteins in the case of Drosera are subdued allowing the release of the recombinant proteins. For D. capensis, a concentration of 25 μg of GFP/ml of secretion (2% of the total soluble proteins from the glue) was obtained for stable transformants. For N. alata, a concentration of 0.5 ng of GFP/ml secretions (0.5% of total soluble proteins from secretions) was reached, corresponding to 12 ng in one pitcher after 14 days for transiently transformed plants. This plant-based expression system shows the potentiality of biomimetic approaches leading to an original production of recombinant proteins, although the yields obtained here were low and did not allow to qualify these plants for an industrial platform project.

Comparative Antigenicity of Thiourea and Adipic Amide Linked Neoglycoconjugates Containing Modified Oligomannose Epitopes for the Carbohydrate-Specific anti-HIV Antibody 2G12

Novel neoglycoproteins containing oligomannosidic penta- and heptasaccharides as structural variants of oligomannose-type N-glycans found on human immunodeficiency virus type 1 gp120 have been prepared using different conjugation methods. Two series of synthetic ligands equipped with 3-aminopropyl spacer moieties and differing in the anomeric configuration of the reducing mannose residue were activated either as isothiocyanates or as adipic acid succinimidoyl esters and coupled to bovine serum albumin. Coupling efficiency for adipic acid connected neoglycoconjugates was better than for the thiourea-linked derivatives; the latter constructs, however, exhibited higher reactivity toward antibody 2G12, an HIV-neutralizing antibody with exquisite specificity for oligomannose-type glycans. 2G12 binding avidities for the conjugates, as determined by Bio-Layer Interferometry, were mostly higher for the β-linked ligands and, as expected, increased with the numbers of covalently linked glycans, leading to approximate K_D values of 10 to 34 nM for optimized ligand-to-BSA ratios. A similar correlation was observed by enzyme-linked immunosorbent assays. In addition, dendrimer-type ligands presenting trimeric oligomannose epitopes were generated by conversion of the amino-spacer group into a terminal azide, followed by triazole formation using "click chemistry". The severe steric bulk of the ligands, however, led to poor efficiency in the coupling step and no increased antibody binding by the resulting neoglycoconjugates, indicating that the low degree of substitution and the spatial orientation of the oligomannose epitopes within these trimeric ligands are not conducive to multivalent 2G12 binding.

Host Cell Proteome of Physcomitrella patens Harbors Proteases and Protease Inhibitors under Bioproduction Conditions

Host cell proteins are inevitable contaminants of biopharmaceuticals. Here, we performed detailed analyses of the host cell proteome of moss ( Physcomitrella patens) bioreactor supernatants using mass spectrometry and subsequent bioinformatics analysis. Distinguishing between the apparent secretome and intracellular contaminants, a complex extracellular proteolytic network including subtilisin-like proteases, metallo-proteases, and aspartic proteases was identified. Knockout of a subtilisin-like protease affected the overall extracellular proteolytic activity. Besides proteases, also secreted protease-inhibiting proteins such as serpins were identified. Further, we confirmed predicted cleavage sites of 40 endogenous signal peptides employing an N-terminomics approach. The present data provide novel aspects to optimize both product stability of recombinant biopharmaceuticals as well as their maturation along the secretory pathway. Data are available via ProteomeXchange with identifier PXD009517.

Three unrelated protease inhibitors enhance accumulation of pharmaceutical recombinant proteins in Nicotiana benthamiana

Agroinfiltrated Nicotiana benthamiana is a flexible and scalable platform for recombinant protein (RP) production, but its great potential is hampered by plant proteases that degrade RPs. Here, we tested 29 candidate protease inhibitors (PIs) in agroinfiltrated N. benthamiana leaves for enhancing accumulation of three unrelated RPs: glycoenzyme α-Galactosidase; glycohormone erythropoietin (EPO); and IgG antibody VRC01. Of the previously described PIs enhancing RP accumulation, we found only cystatin SlCYS8 to be effective. We identified three additional new, unrelated PIs that enhance RP accumulation: N. benthamiana NbPR4, NbPot1 and human HsTIMP, which have been reported to inhibit cysteine, serine and metalloproteases, respectively. Remarkably, accumulation of all three RPs is enhanced by each PI similarly, suggesting that the mechanism of degradation of unrelated RPs follows a common pathway. Inhibitory functions HsTIMP and SlCYS8 are required to enhance RP accumulation, suggesting that their target proteases may degrade RPs. Different PIs additively enhance RP accumulation, but the effect of each PI is dose-dependent. Activity-based protein profiling (ABPP) revealed that the activities of papain-like Cys proteases (PLCPs), Ser hydrolases (SHs) or vacuolar processing enzymes (VPEs) in leaves are unaffected upon expression of the new PIs, whereas SlCYS8 expression specifically suppresses PLCP activity only. Quantitative proteomics indicates that the three new PIs affect agroinfiltrated tissues similarly and that they all increase immune responses. NbPR4, NbPot1 and HsTIMP can be used to study plant proteases and improve RP accumulation in molecular farming.

The transcriptome, extracellular proteome and active secretome of agroinfiltrated Nicotiana benthamiana uncover a large, diverse protease repertoire

Infiltration of disarmed Agrobacterium tumefaciens into leaves of Nicotiana benthamiana (agroinfiltration) facilitates quick and safe production of antibodies, vaccines, enzymes and metabolites for industrial use (molecular farming). However, yield and purity of proteins produced by agroinfiltration are hampered by unintended proteolysis, restricting industrial viability of the agroinfiltration platform. Proteolysis may be linked to an immune response to agroinfiltration, but understanding of the response to agroinfiltration is limited. To identify the proteases, we studied the transcriptome, extracellular proteome and active secretome of agroinfiltrated leaves over a time course, with and without the P19 silencing inhibitor. Remarkably, the P19 expression had little effect on the leaf transcriptome and no effect on the extracellular proteome. 25% of the detected transcripts changed in abundance upon agroinfiltration, associated with a gradual up-regulation of immunity at the expense of photosynthesis. By contrast, 70% of the extracellular proteins increased in abundance, in many cases associated with increased efficiency of extracellular delivery. We detect a dynamic reprogramming of the proteolytic machinery upon agroinfiltration by detecting transcripts encoding for 975 different proteases and protease homologs. The extracellular proteome contains peptides derived from 196 proteases and protease homologs, and activity-based proteomics displayed 17 active extracellular Ser and Cys proteases in agroinfiltrated leaves. We discuss unique features of the N. benthamiana protease repertoire and highlight abundant extracellular proteases in agroinfiltrated leaves, being targets for reverse genetics. This data set increases our understanding of the plant response to agroinfiltration and indicates ways to improve a key expression platform for both plant science and molecular farming.

Low Protease Content in Medicago truncatula Cell Cultures Facilitates Recombinant Protein Production

Medicago truncatula is an established model for studying legume biology. More recently, it has also been exploited as a Molecular Farming platform for the production of recombinant proteins, with the successful expression of fungal and human proteins in plants and cell suspension cultures of this species. One of the challenges that now must be overcome is the degradation of final products during production and downstream processing stages. In the M. truncatula genome, there are more than 400 putative protease-encoding genes, but to date, the proteolytic content of Medicago cell cultures has not been studied. In this report, the proteolytic activities that can potentially hamper the successful production of recombinant proteins in this system are evaluated. The potential proteases responsible for the degradation of target proteins are identified. Interestingly, the number of proteases found in Medicago spent medium is considerably lower than that of the well-established tobacco bright yellow 2 (BY-2) system. Papain-like cysteine proteases are found to be the major contributors to recombinant protein degradation in Medicago. This knowledge is used to engineer a cell line with reduced endogenous protease activity by expressing a selective protease inhibitor, further improving this expression platform.

Cloning and plant-based production of antibody MC10E7 for a lateral flow immunoassay to detect [4-arginine]microcystin in freshwater

Antibody MC10E7 is one of a small number of monoclonal antibodies that bind specifically to [Arg4]-microcystins, and it can be used to survey natural water sources and food samples for algal toxin contamination. However, the development of sensitive immunoassays in different test formats, particularly user-friendly tests for on-site analysis, requires a sensitive but also cost-effective antibody. The original version of MC10E7 was derived from a murine hybridoma, but we determined the sequence of the variable regions using the peptide mass-assisted cloning strategy and expressed a scFv (single-chain variable fragment) format of this antibody in yeast and a chimeric full-size version in leaves of Nicotiana tabacum and Nicotiana benthamiana to facilitate inexpensive and scalable production. The specific antigen-binding activity of the purified antibody was verified by surface plasmon resonance spectroscopy and ELISA, confirming the same binding specificity as its hybridoma-derived counterpart. The plant-derived antibody was used to design a lateral flow immunoassay (dipstick) for the sensitive detection of [Arg4]-microcystins at concentrations of 100-300 ng/L in freshwater samples collected at different sites. Plant-based production will likely reduce the cost of the antibody, currently the most expensive component of the dipstick immunoassay, and will allow the development of further antibody-based analytical devices and water purification adsorbents for the efficient removal of toxic contaminants.

Vacuolar Targeting and Characterization of Recombinant Antibodies

Plant-based platforms are extensively use for the expression of recombinant proteins, including monoclonal antibodies (mAbs). Generally, immunoglobulins (Igs) are sorted to the apoplast, which is often afflicted with intense proteolysis. Here, we describe methods to transiently express mAbs sorted to central vacuole in Nicotiana benthamiana leaves and to characterize the obtained IgG. Central vacuole is an appropriate compartment for the efficient production of Abs, consequently vacuolar sorting should be considered as an alternative strategy to obtain high protein yields.

Protection of Human Colon Cells from Shiga Toxin by Plant-based Recombinant Secretory IgA

Shiga toxin is a major virulence factor of food-poisoning caused by Escherichia coli such as O157:H7. Secretory immunoglobulin (Ig) A (SIgA) is supposed to prevent infection of the mucosal surface and is a candidate agent for oral immunotherapy. We previously established a recombinant monoclonal antibody (mAb) consisting of variable regions from a mouse IgG mAb specific for the binding subunit of Shiga toxin 1 (Stx1) and the Fc region of mouse IgA. Here we produced a secretory form of the recombinant IgA (S-hyIgA) with transgenic Arabidopsis thaliana plant. All the S-hyIgA cDNAs (heavy, light, J chain and secretory component) were expressed under the control of a bidirectional promoter of a chlorophyll a/b-binding protein of A. thaliana without using a viral promoter. The plant-based S-hyIgA exhibited antigen binding, and was modified with plant-specific N-linked sugar chains. The Ig heavy chain and secretory components were observed in an intracellular protein body-like structure of the transgenic leaves on immuno-electron microscopy. An extract of the transgenic leaves neutralized the cytotoxicity of Stx1 toward butyrate-treated Caco-2 cells, a human colon carcinoma cell line. These results will contribute to the development of edible therapeutic antibodies such as those for the treatment of mucosal infection.

The papain-like cysteine proteinases NbCysP6 and NbCysP7 are highly processive enzymes with substrate specificities complementary to Nicotiana benthamiana cathepsin B

The tobacco-related plant Nicotiana benthamiana is gaining interest as a versatile host for the production of monoclonal antibodies and other protein therapeutics. However, the susceptibility of plant-derived recombinant proteins to endogenous proteolytic enzymes limits their use as biopharmaceuticals. We have now identified two previously uncharacterized N. benthamiana proteases with high antibody-degrading activity, the papain-like cysteine proteinases NbCysP6 and NbCysP7. Both enzymes are capable of hydrolysing a wide range of synthetic substrates, although only NbCysP6 tolerates basic amino acids in its specificity-determining S2 subsite. The overlapping substrate specificities of NbCysP6 and NbCysP7 are also documented by the closely related properties of their other subsites as deduced from the action of the enzymes on proteome-derived peptide libraries. Notable differences were observed to the substrate preferences of N. benthamiana cathepsin B, another antibody-degrading papain-like cysteine proteinase. The complementary activities of NbCysP6, NbCysP7 and N. benthamiana cathepsin B indicate synergistic roles of these proteases in the turnover of recombinant and endogenous proteins in planta, thus representing a paradigm for the shaping of plant proteomes by the combined action of papain-like cysteine proteinases.

Vacuolar targeting of recombinant antibodies in Nicotiana benthamiana

Plant-based platforms are extensively used for the expression of recombinant proteins, including monoclonal antibodies. However, to harness the approach effectively and leverage it to its full potential, a better understanding of intracellular processes that affect protein properties is required. In this work, we examined vacuolar (vac) targeting and deposition of the monoclonal antibody (Ab) 14D9 in Nicotiana benthamiana leaves. Two distinct vacuolar targeting signals (KISIA and NIFRGF) were C-terminal fused to the heavy chain of 14D9 (vac-Abs) and compared with secreted and ER-retained variants (sec-Ab, ER-Ab, respectively). Accumulation of ER- and vac-Abs was 10- to 15-fold higher than sec-Ab. N-glycan profiling revealed the predominant presence of plant typical complex fucosylated and xylosylated GnGnXF structures on sec-Ab while vac-Abs carried mainly oligomannosidic (Man 7-9) next to GnGnXF forms. Paucimannosidic glycans (commonly assigned as typical vacuolar) were not detected. Confocal microscopy analysis using RFP fusions showed that sec-Ab-RFP localized in the apoplast while vac-Abs-RFP were exclusively detected in the central vacuole. The data suggest that vac-Abs reached the vacuole by two different pathways: direct transport from the ER bypassing the Golgi (Ab molecules containing Man structures) and trafficking through the Golgi (for Ab molecules containing complex N-glycans). Importantly, vac-Abs were correctly assembled and functionally active. Collectively, we show that the central vacuole is an appropriate compartment for the efficient production of Abs with appropriate post-translational modifications, but also point to a reconsideration of current concepts in plant glycan processing.

An Accessory Protease Inhibitor to Increase the Yield and Quality of a Tumour-Targeting mAb in Nicotiana benthamiana Leaves

The overall quality of recombinant IgG antibodies in plants is dramatically compromised by host endogenous proteases. Different approaches have been developed to reduce the impact of endogenous proteolysis on IgGs, notably involving site-directed mutagenesis to eliminate protease-susceptible sites or the in situ mitigation of host protease activities to minimize antibody processing in the cell secretory pathway. We here characterized the degradation profile of H10, a human tumour-targeting monoclonal IgG, in leaves of Nicotiana benthamiana also expressing the human serine protease inhibitor α1-antichymotrypsin or the cysteine protease inhibitor tomato cystatin SlCYS8. Leaf extracts revealed consistent fragmentation patterns for the recombinant antibody regardless of leaf age and a strong protective effect of SlCYS8 in specific regions of the heavy chain domains. As shown using an antigen-binding ELISA and LC-MS/MS analysis of antibody fragments, SlCYS8 had positive effects on both the amount of fully-assembled antibody purified from leaf tissue and the stability of biologically active antibody fragments containing the heavy chain Fc domain. Our data confirm the potential of Cys protease inhibitors as convenient antibody-stabilizing expression partners to increase the quality of therapeutic antibodies in plant protein biofactories.

Agroinfiltration contributes to VP1 recombinant protein degradation

There is a growing interest in applying tobacco agroinfiltration for recombinant protein production in a plant based system. However, in such a system, the action of proteases might compromise recombinant protein production. Protease sensitivity of model recombinant foot-and-mouth disease (FMD) virus P1-polyprotein (P1) and VP1 (viral capsid protein 1) as well as E. coli glutathione reductase (GOR) were investigated. Recombinant VP1 was more severely degraded when treated with the serine protease trypsin than when treated with the cysteine protease papain. Cathepsin L- and B-like as well as legumain proteolytic activities were elevated in agroinfiltrated tobacco tissues and recombinant VP1 was degraded when incubated with such a protease-containing tobacco extract. In silico analysis revealed potential protease cleavage sites within the P1, VP1 and GOR sequences. The interaction modeling of the single VP1 protein with the proteases papain and trypsin showed greater proximity to proteolytic active sites compared to modeling with the entire P1-polyprotein fusion complex. Several plant transcripts with differential expression were detected 24 hr post-agroinfiltration when the RNA-seq technology was applied to identify changed protease transcripts using the recently available tobacco draft genome. Three candidate genes were identified coding for proteases which included the Responsive-to-Desiccation-21 (RD21) gene and genes for coding vacuolar processing enzymes 1a (NbVPE1a) and 1b (NbVPE1b). The data demonstrates that the tested recombinant proteins are sensitive to protease action and agroinfiltration induces the expression of potential proteases that can compromise recombinant protein production.

Production of a tumour-targeting antibody with a human-compatible glycosylation profile in N. benthamiana hairy root cultures

Hairy root (HR) cultures derived from Agrobacterium rhizogenes transformation of plant tissues are an advantageous biotechnological manufacturing platform due to the accumulation of recombinant proteins in an otherwise largely protein free culture medium. In this context, HRs descending from transgenic Nicotiana tabacum plants were successfully used for the production of several functional mAbs with plant-type glycans. Here, we expressed the tumor-targeting monoclonal antibody mAb H10 in HRs obtained either by infecting a transgenic N. tabacum line expressing H10 with A. rhizogenes or a glyco-engineered N. benthamiana line (ΔXTFT) with recombinant A. rhizogenes carrying mAb H10 heavy and light chain cDNAs. Selected HR clones derived from both plants accumulated mAb H10 in the culture medium with similar yields (2-3 mg/L). N-glycosylation profiles of antibodies purified from HR supernatant revealed the presence of plant-typical complex structures for N. tabacum-derived mAb H10 and of GnGn structures lacking xylose and fucose for the ΔXTFT-derived counterpart. Both antibody glyco-formats exhibited comparable antigen binding activities. Collectively, these data demonstrate that the co-infection of ΔXTFT Nicotiana benthamiana with recombinant A. rhizogenes is an efficient procedure for the generation of stable HR cultures expressing the tumor-targeting mAb H10 with a human-compatible glycosylation profile, thus representing an important step towards the exploitation of root cultures for the production of 'next generation' human therapeutic antibodies.

The death enzyme CP14 is a unique papain-like cysteine proteinase with a pronounced S2 subsite selectivity

The cysteine protease CP14 has been identified as a central component of a molecular module regulating programmed cell death in plant embryos. CP14 belongs to a distinct subfamily of papain-like cysteine proteinases of which no representative has been characterized thoroughly to date. However, it has been proposed that CP14 is a cathepsin H-like protease. We have now produced recombinant Nicotiana benthamiana CP14 (NbCP14) lacking the C-terminal granulin domain. As typical for papain-like cysteine proteinases, NbCP14 undergoes rapid autocatalytic activation when incubated at low pH. The mature protease is capable of hydrolysing several synthetic endopeptidase substrates, but cathepsin H-like aminopeptidase activity could not be detected. NbCP14 displays a strong preference for aliphatic over aromatic amino acids in the specificity-determining P2 position. This subsite selectivity was also observed upon digestion of proteome-derived peptide libraries. Notably, the specificity profile of NbCP14 differs from that of aleurain-like protease, the N. benthamiana orthologue of cathepsin H. We conclude that CP14 is a papain-like cysteine proteinase with unusual enzymatic properties which may prove of central importance for the execution of programmed cell death during plant development.

Site-targeted mutagenesis for stabilization of recombinant monoclonal antibody expressed in tobacco (Nicotiana tabacum) plants

This study examined the degradation pattern of a murine IgG1κ monoclonal antibody expressed in and extracted from transformedNicotiana tabacum Gel electrophoresis of leaf extracts revealed a consistent pattern of recombinant immunoglobulin bands, including intact and full-length antibody, as well as smaller antibody fragments. N-terminal sequencing revealed these smaller fragments to be proteolytic cleavage products and identified a limited number of protease-sensitive sites in the antibody light and heavy chain sequences. No strictly conserved target sequence was evident, although the peptide bonds that were susceptible to proteolysis were predominantly and consistently located within or near to the interdomain or solvent-exposed regions in the antibody structure. Amino acids surrounding identified cleavage sites were mutated in an attempt to increase resistance. Different Guy's 13 antibody heavy and light chain mutant combinations were expressed transiently inN. tabacumand demonstrated intensity shifts in the fragmentation pattern, resulting in alterations to the full-length antibody-to-fragment ratio. The work strengthens the understanding of proteolytic cleavage of antibodies expressed in plants and presents a novel approach to stabilize full-length antibody by site-directed mutagenesis.-Hehle, V. K., Paul, M. J., Roberts, V. A., van Dolleweerd, C. J., Ma, J. K.-C. Site-targeted mutagenesis for stabilization of recombinant monoclonal antibody expressed in tobacco (Nicotiana tabacum) plants.

Tackling Unwanted Proteolysis in Plant Production Hosts Used for Molecular Farming

Although the field of molecular farming has significantly matured over the last years, some obstacles still need to be resolved. A major limiting factor for a broader application of plant hosts for the production of valuable recombinant proteins is the low yield of intact recombinant proteins. These low yields are at least in part due to the action of endogenous plant proteases on the foreign recombinant proteins. This mini review will present the current knowledge of the proteolytic enzymes involved in the degradation of different target proteins and strategies that are applied to suppress undesirable proteolytic activities in order to safeguard recombinant proteins during the production process.

Nicotiana benthamiana cathepsin B displays distinct enzymatic features which differ from its human relative and aleurain-like protease

The tobacco-related plant species Nicotiana benthamiana has recently emerged as a versatile expression platform for the rapid generation of recombinant biopharmaceuticals, but product yield and quality frequently suffer from unintended proteolysis. Previous studies have highlighted that recombinant protein fragmentation in plants involves papain-like cysteine proteinases (PLCPs). For this reason, we have now characterized two major N. benthamiana PLCPs in detail: aleurain-like protease (NbALP) and cathepsin B (NbCathB). As typical for PLCPs, the precursor of NbCathB readily undergoes autocatalytic activation when incubated at low pH. On the contrary, maturation of NbALP requires the presence of a cathepsin L-like PLCP as processing enzyme. While the catalytic features of NbALP closely resemble those of its mammalian homologue cathepsin H, NbCathB displays remarkable differences to human cathepsin B. In particular, NbCathB appears to be a far less efficient peptidyldipeptidase (removing C-terminal dipeptides) than its human counterpart, suggesting that it functions primarily as an endopeptidase. Importantly, NbCathB was far more efficient than NbALP in processing the human anti-HIV-1 antibody 2F5 into fragments observed during its production in N. benthamiana. This suggests that targeted down-regulation of NbCathB could improve the performance of this plant-based expression platform.

Overcoming low yields of plant-made antibodies by a protein engineering approach

The commercial development of plant-based antibody production platforms is often limited by low and variable yields, but little is known about the factors that affect antibody accumulation during and after translation. Here, we present a strategy to identify yield-limiting regions in the transcript and protein. We exchanged variable heavy chain (VH) domain sequences between two human antibodies at structurally conserved positions, thus creating ten chimeric VH domains containing sequences from M12 (∼1000 μg/g leaf fresh weight [FW]) and 4E10 (∼100 μg/g FW). After transient expression in Nicotiana benthamiana leaves, we measured mRNA and protein levels by quantitative real-time PCR and surface plasmon resonance spectroscopy, respectively. Transcript levels were similar for all constructs, but antibody levels ranged from ∼250 μg/g to over 2000 μg/g FW. Analysis of the expression levels showed that: i) 4E10 yields were only marginally increased by suppression of post-transcriptional gene silencing; ii) the CDR3 of 4E10 contains a protease site; and iii) a bipartite, yield-limiting region exists in the CDR2/CDR3. Our findings highlight the strong impact of cotranslational and posttranslational events on antibody yields and show that protein engineering is a powerful tool that can be used to overcome the remaining limitations affecting antibody production in plants.

Putting the Spotlight Back on Plant Suspension Cultures

Plant cell suspension cultures have several advantages that make them suitable for the production of recombinant proteins. They can be cultivated under aseptic conditions using classical fermentation technology, they are easy to scale-up for manufacturing, and the regulatory requirements are similar to those established for well-characterized production systems based on microbial and mammalian cells. It is therefore no surprise that taliglucerase alfa (Elelyso®)-the first licensed recombinant pharmaceutical protein derived from plants-is produced in plant cell suspension cultures. But despite this breakthrough, plant cells are still largely neglected compared to transgenic plants and the more recent plant-based transient expression systems. Here, we revisit plant cell suspension cultures and highlight recent developments in the field that show how the rise of plant cells parallels that of Chinese hamster ovary cells, currently the most widespread and successful manufacturing platform for biologics. These developments include medium optimization, process engineering, statistical experimental designs, scale-up/scale-down models, and process analytical technologies. Significant yield increases for diverse target proteins will encourage a gold rush to adopt plant cells as a platform technology, and the first indications of this breakthrough are already on the horizon.

A scalable low-cost cGMP process for clinical grade production of the HIV inhibitor 5P12-RANTES in Pichia pastoris

In the continued absence of an effective anti-HIV vaccine, approximately 2 million new HIV infections occur every year, with over 95% of these in developing countries. Calls have been made for the development of anti-HIV drugs that can be formulated for topical use to prevent HIV transmission during sexual intercourse. Because these drugs are principally destined for use in low-resource regions, achieving production costs that are as low as possible is an absolute requirement. 5P12-RANTES, an analog of the human chemokine protein RANTES/CCL5, is a highly potent HIV entry inhibitor which acts by achieving potent blockade of the principal HIV coreceptor, CCR5. Here we describe the development and optimization of a scalable low-cost production process for 5P12-RANTES based on expression in Pichia pastoris. At pilot (150 L) scale, this cGMP compliant process yielded 30 g of clinical grade 5P12-RANTES. As well as providing sufficient material for the first stage of clinical development, this process represents an important step towards achieving production of 5P12-RANTES at a cost and scale appropriate to meet needs for topical HIV prevention worldwide.

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New medicines are required to prevent the spread of HIV/AIDS in low-resource regions.

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5P12-RANTES, a chemokine analog, is a promising new candidate drug.

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We describe a process for producing clinical grade cGMP 5P12-RANTES in Pichia pastoris.

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This is a key step to achieving production at cost and scale appropriate for use worldwide.

Glycan modulation and sulfoengineering of anti-HIV-1 monoclonal antibody PG9 in plants

Broadly neutralizing anti-HIV-1 monoclonal antibodies, such as PG9, and its derivative RSH hold great promise in AIDS therapy and prevention. An important feature related to the exceptional efficacy of PG9 and RSH is the presence of sulfated tyrosine residues in their antigen-binding regions. To maximize antibody functionalities, we have now produced glycan-optimized, fucose-free versions of PG9 and RSH in Nicotiana benthamiana. Both antibodies were efficiently sulfated in planta on coexpression of an engineered human tyrosylprotein sulfotransferase, resulting in antigen-binding and virus neutralization activities equivalent to PG9 synthesized by mammalian cells ((CHO)PG9). Based on the controlled production of both sulfated and nonsulfated variants in plants, we could unequivocally prove that tyrosine sulfation is critical for the potency of PG9 and RSH. Moreover, the fucose-free antibodies generated in N. benthamiana are capable of inducing antibody-dependent cellular cytotoxicity, an activity not observed for (CHO)PG9. Thus, tailoring of the antigen-binding site combined with glycan modulation and sulfoengineering yielded plant-produced anti-HIV-1 antibodies with effector functions superior to PG9 made in CHO cells.

Antibody proteolysis: a common picture emerging from plants

We have recently characterized the degradation profiles of 2 human IgG1 monoclonal antibodies, the tumor-targeting mAb H10 and the anti-HIV mAb 2G12. Both mAbs were produced in plants either as stable transgenics or using a transient expression system based on leaf agroinfiltration. The purified antibodies were separated by 1DE and protein bands were characterized by N-terminal sequencing. The proteolytic cleavage sites identified in the heavy chain (HC) of both antibodies were localized in 3 inter-domain regions, suggesting that the number of proteolytic cleavage events taking place in plants is limited. One of the cleavage sites, close to the hinge region, was common to both antibodies.

Editorial: biomolecular technology of proteins - BioToP

Current doctoral students who will be the driving force of science in the future. Christian Obinger, speaker of the BioToP Phd program "Biomolecular Technology of Proteins - BioToP", reviews the articles in this BioToP Special issue.

Suppression of IgM Proteolysis by Conformational Stabilization Through Excipients

Protease activity from host cell lines may cause product loss or affect the quality of recombinant proteins. In this study, we showed that excipients like glycine and sorbitol reduce the proteolysis of an immunoglobulin M (IgM) in the presence of added proteases like α-chymotrypsin, papain, and pepsin. The activity of the proteases in the IgM-protective environments was conserved or even enhanced as tested using low molecular weight substrates. Thus, a higher resistance against proteolytic degradation appears to be caused by the conformational stabilization of the IgM due to preferential exclusion of sorbitol and glycine.

Extracellular peptidase hunting for improvement of protein production in plant cells and roots

Plant-based recombinant protein production systems have gained an extensive interest over the past few years, because of their reduced cost and relative safety. Although the first products are now reaching the market, progress are still needed to improve plant hosts and strategies for biopharming. Targeting recombinant proteins toward the extracellular space offers several advantages in terms of protein folding and purification, but degradation events are observed, due to endogenous peptidases. This paper focuses on the analysis of extracellular proteolytic activities in two production systems: cell cultures and root-secretion (rhizosecretion), in Arabidopsis thaliana and Nicotiana tabacum. Proteolytic activities of extracellular proteomes (secretomes) were evaluated in vitro against two substrate proteins: bovine serum albumin (BSA) and human serum immunoglobulins G (hIgGs). Both targets were found to be degraded by the secretomes, BSA being more prone to proteolysis than hIgGs. The analysis of the proteolysis pH-dependence showed that target degradation was mainly dependent upon the production system: rhizosecretomes contained more peptidase activity than extracellular medium of cell suspensions, whereas variations due to plant species were smaller. Using class-specific peptidase inhibitors, serine, and metallopeptidases were found to be responsible for degradation of both substrates. An in-depth in silico analysis of genomic and transcriptomic data from Arabidopsis was then performed and led to the identification of a limited number of serine and metallo-peptidases that are consistently expressed in both production systems. These peptidases should be prime candidates for further improvement of plant hosts by targeted silencing.

Site-specific proteolytic degradation of IgG monoclonal antibodies expressed in tobacco plants

Plants are promising hosts for the production of monoclonal antibodies (mAbs). However, proteolytic degradation of antibodies produced both in stable transgenic plants and using transient expression systems is still a major issue for efficient high-yield recombinant protein accumulation. In this work, we have performed a detailed study of the degradation profiles of two human IgG1 mAbs produced in plants: an anti-HIV mAb 2G12 and a tumour-targeting mAb H10. Even though they use different light chains (κ and λ, respectively), the fragmentation pattern of both antibodies was similar. The majority of Ig fragments result from proteolytic degradation, but there are only a limited number of plant proteolytic cleavage events in the immunoglobulin light and heavy chains. All of the cleavage sites identified were in the proximity of interdomain regions and occurred at each interdomain site, with the exception of the VL /CL interface in mAb H10 λ light chain. Cleavage site sequences were analysed, and residue patterns characteristic of proteolytic enzymes substrates were identified. The results of this work help to define common degradation events in plant-produced mAbs and raise the possibility of predicting antibody degradation patterns 'a priori' and designing novel stabilization strategies by site-specific mutagenesis.

The increasing value of plant-made proteins

The production of high-value proteins in plants is maturing, as shown by the recent approval of innovative products and the latest studies that showcase plant-based production systems using technologies and approaches that are well established in other fields. These include host cell engineering, medium optimization, scalable unit operations for downstream processing (DSP), bioprocess optimization and detailed cost analysis. Product-specific benefits of plant-based systems have also been exploited, including bioencapsulation and the mucosal delivery of minimally processed topical and oral products with a lower entry barrier than pharmaceuticals for injection. Success stories spearheaded by the FDA approval of Elelyso developed by Protalix have revitalized the field and further interest has been fueled by the production of experimental Ebola treatments in plants.

Subcellular Targeting of Proteins Involved in Modification of Plant N- and O-Glycosylation

Plants are attractive expression hosts for the production of recombinant glycoprotein therapeutics. The quality and efficiency of these biopharmaceuticals are very often influenced by the glycosylation profile. Consequently, approaches are needed that enable the production of recombinant glycoproteins with customized and homogenous N- and O-glycan structures. Here, we describe convenient tools that allow targeting and retention of glycan-modifying enzymes in the early secretory pathway of plants. These protocols can be used to fine-tune the subcellular localization of glycosyltransferases and glycosidases in plants and consequently to increase the homogeneity of glycosylation on recombinant glycoproteins.