Comparison of CD20 Binding Affinities of Rituximab Produced in Nicotiana benthamiana Leaves and Arabidopsis thaliana Callus

Plants are promising drug-production platforms with high economic efficiency, stability, and convenience in mass production. However, studies comparing the equivalency between the original antibodies and those produced in plants are limited. Amino acid sequences that constitute the Fab region of an antibody are diverse, and the post-transcriptional modifications that occur according to these sequences in animals and plants are also highly variable. In this study, rituximab, a blockbuster antibody drug used in the treatment of non-Hodgkin's lymphoma, was produced in Nicotiana benthamiana leaves and Arabidopsis thaliana callus, and was compared to the original rituximab produced in CHO cells. Interestingly, the epitope recognition and antigen-binding abilities of rituximab from N. benthamiana leaves were almost lost. In the case of rituximab produced in A. thaliana callus, the specific binding ability and CD20 capping activity were maintained, but the binding affinity was less than 50% of that of original rituximab from CHO cells. These results suggest that different plant species exhibit different binding affinities. Accordingly, in addition to the differences in PTMs between mammals and plants, the differences between the species must also be considered in the process of producing antibodies in plants.

Purity by design: Reducing impurities in bioproduction by stimulus-controlled global translational downregulation of non-product proteins

Capitalizing on the ability of mammalian cells to conduct complex post-translational modifications, most protein therapeutics are currently produced in cell culture systems. Addition of a signal peptide to the product protein enables its accumulation in the cell culture supernatant, but separation of the product from endogenously secreted proteins remains costly and labor-intensive. We considered that global downregulation of translation of non-product proteins would be an efficient strategy to minimize downstream processing requirements. Therefore, taking advantage of the ability of mammalian protein kinase R (PKR) to switch off most cellular translation processes in response to infection by viruses, we fused a caffeine-inducible dimerization domain to the catalytic domain of PKR. Addition of caffeine to this construct results in homodimerization and activation of PKR, effectively rewiring rapid global translational downregulation to the addition of the stimulus in a dose-dependent manner. Then, to protect translation of the target therapeutic, we screened viral and cellular internal ribosomal entry sites (IRESes) known or suspected to be resistant to PKR-induced translational stress. After choosing the best-in-class Seneca valley virus (SVV) IRES, we additionally screened for IRES transactivation factors (ITAFs) as well as for supplementary small molecules to further boost the production titer of the product protein under conditions of global translational downregulation. Importantly, the residual global translation activity of roughly 10% under maximal downregulation is sufficient to maintain cellular viability during a production timeframe of at least five days. Standard industrially used adherent as well as suspension-adapted cell lines transfected with this synthetic biology-inspired Protein Kinase R-Enhanced Protein Production (PREPP) system could produce several medicinally relevant protein therapeutics, such as the blockbuster drug rituximab, in substantial quantities and with significantly higher purity than previous culture technologies. We believe incorporation of such purity-by-design technology in the production process will alleviate downstream processing bottlenecks in future biopharmaceutical manufacturing.

Multiplexed, targeted gene editing in Nicotiana benthamiana for glyco-engineering and monoclonal antibody production

Biopharmaceutical glycoproteins produced in plants carry N-glycans with plant-specific residues core α(1,3)-fucose and β(1,2)-xylose, which can significantly impact the activity, stability and immunogenicity of biopharmaceuticals. In this study, we have employed sequence-specific transcription activator-like effector nucleases (TALENs) to knock out two α(1,3)-fucosyltransferase (FucT) and the two β(1,2)-xylosyltransferase (XylT) genes within Nicotiana benthamiana to generate plants with improved capacity to produce glycoproteins devoid of plant-specific residues. Among plants regenerated from N. benthamiana protoplasts transformed with TALENs targeting either the FucT or XylT genes, 50% (80 of 160) and 73% (94 of 129) had mutations in at least one FucT or XylT allele, respectively. Among plants regenerated from protoplasts transformed with both TALEN pairs, 17% (18 of 105) had mutations in all four gene targets, and 3% (3 of 105) plants had mutations in all eight alleles comprising both gene families; these mutations were transmitted to the next generation. Endogenous proteins expressed in the complete knockout line had N-glycans that lacked β(1,2)-xylose and had a significant reduction in core α(1,3)-fucose levels (40% of wild type). A similar phenotype was observed in the N-glycans of a recombinant rituximab antibody transiently expressed in the homozygous mutant plants. More importantly, the most desirable glycoform, one lacking both core α(1,3)-fucose and β(1,2)-xylose residues, increased in the antibody from 2% when produced in the wild-type line to 55% in the mutant line. These results demonstrate the power of TALENs for multiplexed gene editing. Furthermore, the mutant N. benthamiana lines provide a valuable platform for producing highly potent biopharmaceutical products.

Expression of bioactive anti-CD20 antibody fragments and induction of ER stress response in Arabidopsis seeds

Seed-based expression system is an attractive platform for the production of recombinant proteins in molecular farming. Despite the many advantages of molecular farming, little is known about the effect of the different subcellular accumulation of recombinant proteins on the endoplasmic reticulum (ER) quality control system in host plants. In this study, we analyzed the expression of anti-CD20 antibody fragments in seeds of Arabidopsis thaliana (ecotype Columbia) and corresponding glycosylation mutants, and evaluated the influence of three different signal sequences on the expression levels of scFv-Fc of C2B8. The highest protein accumulation level, with a maximum of 6.12 % total soluble proteins, was observed upon fusing proteins to the signal peptide of Arabidopsis seed storage albumin 2. The ER stress responses in developing seeds at 13 days post-anthesis were also compared across different transgenic lines under normal and heat shock conditions. Based on the gene expression profiles of ER stress transducers, our results suggest that accumulation of antibody fragments in the ER exerts more stress on ER homeostasis. In addition, quantitative PCR results also implicate enhanced activation of ER-associated degradation in transgenic lines. Last but not the least, we also demonstrate the anti-tumor potency of plant-derived proteins by showing the anti-tumor activity of purified scFv-Fc proteins against Daudi cells. Together, our data implies that better understanding of the interaction between exogenous protein production and the cellular quality control system of the host plant is necessary for the development of an optimal expression strategy that will be especially beneficial to commercial protein manufacturing.