Companion Protease Inhibitors for the In Situ Protection of Recombinant Proteins in Plants

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.

Co-expression With Replicating Vector Overcoming Competitive Effects Derived by a Companion Protease Inhibitor in Plants

Plant-based expression platforms are currently gaining acceptance as a viable alternative for the production of recombinant proteins (RPs), but the degradation of RPs by proteases in cells hinders their superb potentials. Co-expression of a protease inhibitor (PI) shows promise as a strategy to prevent RP from proteolytic degradation in plants. However, competitive effects behind the PI-RP co-expression system may mask or obfuscate the in situ protective effects of a companion PI. Here, we explored the competitive effects by co-expressing reteplase (rPA) with three unrelated PIs, namely NbPR4, HsTIMP, and SlCYS8, in Nicotiana benthamiana leaves. Remarkably, the accumulation of rPA was significantly repressed by each of the three PIs, suggesting that the competitive effects may be common among the PIs. The repression can be attenuated by reducing the PI inoculum dose in the co-inoculation mixtures, showing a negative correlation between the PI abundance of the PI-RP system and competitive effects. Interestingly, when a replicating vector was used to modulate the relative abundance of PI and RP in vivo, rPA was still boosted even at the maximal testing dose of PI, indicating that the competitive effects reduced to an ignorable level by this in vivo approach. Furthermore, a 7- to 12-fold increase of rPA was achieved, proving that it is a useful way for stimulating the potentials of a companion PI by overcoming competitive effects. And, this approach can be applied to molecular farming for improving the RP yields of plant expression systems.

Proteases of Nicotiana benthamiana: an emerging battle for molecular farming

Molecular farming increasingly uses the tobacco relative Nicotiana benthamiana for production of recombinant proteins through transient expression. Several proteins are produced efficiently with this expression platform, but yields for other proteins are often very low. These low yields are frequently due to endogenous proteases. The latest genome annotations indicate that N. benthamiana encodes for at least 1243 putative proteases that probably act redundantly and consecutively on substrates in different subcellular compartments. Here, we discuss the N. benthamiana protease repertoire that may affect recombinant protein production and recent advances in protease depletion strategies to increase recombinant protein production in N. benthamiana.

Production of Biopharmaceuticals in Nicotiana benthamiana-Axillary Stem Growth as a Key Determinant of Total Protein Yield

Data are scarce about the influence of basic cultural conditions on growth patterns and overall performance of plants used as heterologous production hosts for protein pharmaceuticals. Higher plants are complex organisms with young, mature, and senescing organs that show distinct metabolic backgrounds and differ in their ability to sustain foreign protein expression and accumulation. Here, we used the transient protein expression host Nicotiana benthamiana as a model to map the accumulation profile of influenza virus hemagglutinin H1, a clinically promising vaccine antigen, at the whole plant scale. Greenhouse-grown plants submitted to different light regimes, submitted to apical bud pruning, or treated with the axillary growth-promoting cytokinin 6-benzylaminopurine were vacuum-infiltrated with agrobacteria harboring a DNA sequence for H1 and allowed to express the viral antigen for 7 days in growth chamber under similar environmental conditions. Our data highlight the importance of young leaves on H1 yield per plant, unlike older leaves which account for a significant part of the plant biomass but contribute little to total antigen titer. Our data also highlight the key contribution of axillary stem leaves, which contribute more than 50% of total yield under certain conditions despite representing only one-third of the total biomass. These findings underline the relevance of both considering main stem leaves and axillary stem leaves while modeling heterologous protein production in N. benthamiana. They also demonstrate the potential of exogenously applied growth-promoting hormones to modulate host plant architecture for improvement of protein yields.

Recombinant cystatins in plants

Dozens of studies have assessed the practical value of plant cystatins as ectopic inhibitors of Cys proteases in biological systems. The potential of these proteins in crop protection to control herbivorous pests and pathogens has been documented extensively over the past 25 years. Their usefulness to regulate endogenous Cys proteases in planta has also been considered recently, notably to implement novel traits of agronomic relevance in crops or to generate protease activity-depleted environments in plants or plant cells used as bioreactors for recombinant proteins. After a brief update on the basic structural characteristics of plant cystatins, we summarize recent advances on the use of these proteins in plant biotechnology. Attention is also paid to the molecular improvement of their structural properties for the improvement of their protease inhibitory effects or the fine-tuning of their biological target range.

Recombinant protein susceptibility to proteolysis in the plant cell secretory pathway is pH-dependent

Cellular engineering approaches have been proposed to mitigate unintended proteolysis in plant protein biofactories, involving the design of protease activity-depleted environments by gene silencing or in situ inactivation with accessory protease inhibitors. Here, we assessed the impact of influenza virus M2 proton channel on host protease activities and recombinant protein processing in the cell secretory pathway of Nicotiana benthamiana leaves. Transient co-expression assays with M2 and GFP variant pHluorin were first conducted to illustrate the potential of proton export from the Golgi lumen to promote recombinant protein yield. A fusion protein-based system involving protease-sensitive peptide linkers to attach inactive variants of tomato cystatin SlCYS8 was then designed to relate the effects of M2 on protein levels with altered protease activities in situ. Secreted versions of the cystatin fusions transiently expressed in leaf tissue showed variable 'fusion to free cystatin' cleavage ratios, in line with the occurrence of protease forms differentially active against the peptide linkers in the secretory pathway. Variable ratios were also observed for the fusions co-expressed with M2, but the extent of fusion cleavage was changed for several fusions, positively or negatively, as a result of pH increase in the Golgi. These data indicating a remodelling of endogenous protease activities upon M2 expression confirm that the stability of recombinant proteins in the plant cell secretory pathway is pH-dependent. They suggest, in practice, the potential of M2 proton channel to modulate the stability of protease-susceptible secreted proteins in planta via a pH-related, indirect effect on host resident proteases.

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.

A Chimeric Affinity Tag for Efficient Expression and Chromatographic Purification of Heterologous Proteins from Plants

The use of plants as expression hosts for recombinant proteins is an increasingly attractive option for the production of complex and challenging biopharmaceuticals. Tools are needed at present to marry recent developments in high-yielding gene vectors for heterologous expression with routine protein purification techniques. In this study, we designed the Cysta-tag, a new purification tag for immobilized metal affinity chromatography (IMAC) of plant-made proteins based on the protein-stabilizing fusion partner SlCYS8. We show that the Cysta-tag may be used to readily purify proteins under native conditions, and then be removed enzymatically to isolate the protein of interest. We also show that commonly used protease recognition sites for linking purification tags are differentially stable in leaves of the commonly used expression host Nicotiana benthamiana, with those linkers susceptible to cysteine proteases being less stable then serine protease-cleavable linkers. As an example, we describe a Cysta-tag experimental scheme for the one-step purification of a clinically useful protein, human α1-antitrypsin, transiently expressed in N. benthamiana. With potential applicability to the variety of chromatography formats commercially available for IMAC-based protein purification, the Cysta-tag provides a convenient means for the efficient and cost-effective purification of recombinant proteins from plant tissues.