Jürgen Drossard

Plant cell cultures for the production of recombinant proteins

The use of whole plants for the synthesis of recombinant proteins has received a great deal of attention recently because of advantages in economy, scalability and safety compared with traditional microbial and mammalian production systems. However, production systems that use whole plants lack several of the intrinsic benefits of cultured cells, including the precise control over growth conditions, batch-to-batch product consistency, a high level of containment and the ability to produce recombinant proteins in compliance with good manufacturing practice. Plant cell cultures combine the merits of whole-plant systems with those of microbial and animal cell cultures, and already have an established track record for the production of valuable therapeutic secondary metabolites. Although no recombinant proteins have yet been produced commercially using plant cell cultures, there have been many proof-of-principle studies and several companies are investigating the commercial feasibility of such production systems.

Towards molecular farming in the future: transient protein expression in plants

Molecular farming in plants can be achieved by stable or transient expression of a recombinant protein. Transient expression of recombinant proteins in plants can rapidly provide large amounts of the proteins for detailed characterization. It is fast, flexible and can be carried out at field scale using viral vectors, but it lacks the increases in production volume that can be achieved easily with stable transgenic crops. This review article focuses on discussing the applications of transient expression using viral vectors, biolistic methods or agroinfiltration.

Rice cell culture as an alternative production system for functional diagnostic and therapeutic antibodies.

We investigated the suitability of transformed rice cell lines as a system for the production of therapeutic recombinant antibodies. Expression constructs encoding a single-chain Fv fragment (scFvT84.66, specific for CEA, the carcinoembryonic antigen present on many human tumours) were introduced into rice tissue by particle bombardment. We compared antibody production levels when antibodies were either secreted to the apoplast or retained in the endoplasmic reticulum (ER) using a KDEL retention signal. Production levels were up to 14 times higher when antibodies were retained in the ER. Additionally, we compared constructs encoding different leader peptides (plant codon optimised murine immunoglobulin heavy and light chain leader peptides) and carrying alternative 5′ untranslated regions (the petunia chalcone synthase gene 5′ UTR and the tobacco mosaic virus omega sequence). We observed no significant differences in antibody production levels among cell lines transformed with these constructs. The highest level of antibody production we measured was 3.8 μg g−1 callus (fresh weight). Immunological analysis of transgenic rice callus confirmed the presence of functional scFvT84.66. We discuss the potential merits of cell culture for the production of recombinant antibodies and other valuable macromolecules.

Affinity-purification of a TMV-specific recombinant full-size antibody from a transgenic tobacco suspension culture.

A TMV-specific full-size murine IgG-2b/K antibody (mAb24) was expressed in a Nicotiana tabacum cv. Petite Havana SR1 suspension culture (P9s), which was derived from a stably transformed transgenic plant (P9). The integration of an N-terminal murine leader peptide directed the assembled immunoglobulin for secretion. However, in suspension culture, the full-size recombinant antibody, rAb24, was retained by the plant cell wall and was not present in the culture medium. rAb24 expression reached a basal level of 15 microg per gram wet cell weight, corresponding to 0.3% of the total soluble plant cell protein. The level of rAb24 could be increased three-fold by amino acid supplementation of the culture medium. For purification of the recombinant antibody from batch-cultured tobacco suspension cells, the primary plant cell wall was partially digested by enzymatic treatment. This resulted in a total release of recombinant full-size rAb24 into the extraction buffer. A three-step procedure was used to purify the immunoglobulins, starting with cross-flow filtration (step 1) followed by protein A affinity chromatography (step 2) and gel filtration as a final purification step (step 3). This procedure gave a recovery of more than 80% of the expressed rAb24 from plant cell extracts. SDS-PAGE, IEF and immunoblot analyses demonstrated a high degree of homogeneity for the affinity-purified rAb24. An ELISA procedure demonstrated that the specificity and affinity of the protein A affinity purified antibody was indistinguishable from its murine counterpart, indicating the potential of plant cell suspension cultures as bio-reactors for the production of recombinant antibodies.

Towards molecular farming in the future: using plant-cell-suspension cultures as bioreactors

Plant‐suspension cells are an in vitro system that can be used for recombinant protein production under carefully controlled certified conditions. Plant‐suspension cells can be grown in shake flasks or fermenters to produce secondary metabolites, like vincristine and vinblastine, and to produce recombinant proteins after transformation. This review article focuses on discussing the generation of transformed suspension‐cell lines expressing recombinant proteins, like antibodies, and recombinant‐protein downstream processing and purification.

Towards molecular farming in the future: moving from diagnostic protein and antibody production in microbes to plants.

Molecular farming of pharmaceuticals in plants has the potential to provide almost unlimited amounts of recombinant proteins for use in disease diagnosis and therapy. Transgenic plants are attracting interest as bioreactors for the inexpensive production of large amounts of safe, functional, recombinant macromolecules, such as blood substitutes, vaccines and antibodies. In some cases, the function of expressed recombinant proteins can be rapidly analysed by expression in microbes or by transient expression in intact or virally infected plants. Protein production can be increased by upscaling production in fermenters, using yeast‐ or plant‐suspension cells or by using transient‐expression systems. Stable transgenic plants can be used to produce leaves or seeds rich in the recombinant protein for long‐term storage or direct processing. This demonstrates the promise for using plants as bioreactors for the molecular farming of recombinant therapeutics, diagnostics, blood substitutes and antibodies. We anticipate that this technology has the potential to greatly benefit human health by making safe recombinant pharmaceuticals widely available.

A carcinoembryonic antigen-specific diabody produced in tobacco

The feasibility of using tobacco for production of a recombinant antibody (T84.66/GS8 diabody) directed against the carcinoembryonic antigen (CEA) and used for tumor imaging was investigated. Two constructs were generated for targeting the protein either to the apoplast or to the endoplasmic reticulum. Expression of the diabody in tobacco leaves after vacuum‐assisted infiltration of engineered Agrobacteria (agro‐infiltration) and in regenerated transgenic tobacco plants was analyzed and compared. Results in terms of protein expression and accumulation between both systems showed a good correlation. His6‐tagged T84.66 diabody was readily purified from agro‐infiltrated tobacco leaves and from transgenic plants by immobilized metal ion affinity chromatography. The purified protein was analyzed by polyacrylamide gel electrophoresis, Western blot, gel filtration, electrospray mass spectrometry, direct and competition ELISA, electrophoretic mobility shift assay, and staining of CEA‐positive colon adenocarcinoma cell line LS174T. Our results demonstrate that tobacco is a competent production system for this clinically relevant diabody.

Regulatory approval and a first‐in‐human phase I clinical trial of a monoclonal antibody produced in transgenic tobacco plants

Although plant biotechnology has been widely investigated for the production of clinical-grade monoclonal antibodies, no antibody products derived from transgenic plants have yet been approved by pharmaceutical regulators for clinical testing. In the Pharma-Planta project, the HIV-neutralizing human monoclonal antibody 2G12 was expressed in transgenic tobacco (Nicotiana tabacum). The scientific, technical and regulatory demands of good manufacturing practice (GMP) were addressed by comprehensive molecular characterization of the transgene locus, confirmation of genetic and phenotypic stability over several generations of transgenic plants, and by establishing standard operating procedures for the creation of a master seed bank, plant cultivation, harvest, initial processing, downstream processing and purification. The project developed specifications for the plant-derived antibody (P2G12) as an active pharmaceutical ingredient (API) based on (i) the guidelines for the manufacture of monoclonal antibodies in cell culture systems; (ii) the draft European Medicines Agency Points to Consider document on quality requirements for APIs produced in transgenic plants; and (iii) de novo guidelines developed with European national regulators. From the resulting process, a GMP manufacturing authorization was issued by the competent authority in Germany for transgenic plant-derived monoclonal antibodies for use in a phase I clinical evaluation. Following preclinical evaluation and ethical approval, a clinical trial application was accepted by the UK national pharmaceutical regulator. A first-in-human, double-blind, placebo-controlled, randomized, dose-escalation phase I safety study of a single vaginal administration of P2G12 was carried out in healthy female subjects. The successful completion of the clinical trial marks a significant milestone in the commercial development of plant-derived pharmaceutical proteins.

Towards molecular farming in the future: Pichia pastoris‐based production of single‐chain antibody fragments

This review article focuses on the use of the methylotrophic yeast Pichia pastoris as a recombinant protein‐expression system. P. pastoris is a useful system for the expression of milligram‐to‐gram quantities of a protein, which can be scaled up to fermentation to meet greater demands. Compared with mammalian cells, Pichia do not require a complex growth medium or culture conditions, they are as easy to manipulate genetically as Escherichia coli and have a eukaryotic protein‐synthesis pathway. They seem suited to laboratory‐scale production of recombinant proteins for in‐house use or, in some cases, molecular farming of recombinant products. This review article focuses on the use of P. pastoris, describes a fermentation production run of a single‐chain antibody fragment and includes a discussion of fermentation as a production strategy.

Production of carcinoembryonic antigen (CEA) N‐A3 domain in Pichia pastoris by fermentation

Carcinoembryonic antigen (CEA) is a 180‐kDa glycoprotein found on the surface of normal colon and malignant human adenocarcinomas. Recently, a fusion protein containing two of the seven Ig‐like domains present in CEA (N and A3) has been constructed and expressed in Pichia pastoris [You, Hefta, Yazaki, Wu and Shively (1998) Anticancer Res. 18, 3193–3201]. Here, we report the generation and selection of a multi‐copy clone expressing this fusion protein, the optimization of the shake‐flask expression protocol and the upscaled production of CEA N‐A3 using fermentation technology. P. pastoris transformants secreting the CEA N‐A3 domain were generated by electrotransformation of the GS115 host strain with the pPIC9K vector containing the CEA N‐A3 cDNA [You, Hefta, Yazaki, Wu and Shively (1998) Anticancer Res. 18, 3193–3201] then screened for CEA N‐A3 expression and G418 resistance. The recombinant CEA N‐A3 domain was detected in the culture supernatant using the monoclonal anti‐CEA antibody T84.66. Optimization of methanol‐induction conditions resulted in a high‐methanol shake‐flask expression protocol yielding significantly increased CEA N‐A3 levels. Fermentation and culture conditions were optimized for 5‐l working‐volume fermentations and CEA N‐A3 was affinity purified using Ni‐IDA (imino di‐acetic acid) affinity chromatography from the clarified fermentation supernatant. Peptide N‐glycosidase F treatment revealed that the recombinant protein was heavily glycosylated but expressed as a single polypeptide of 28 kDa with no evidence of proteolytic degradation. Our results demonstrate that functional CEA N‐A3 domain can be produced in sufficient quantities in P. pastoris for structural analysis or diagnostic applications. To our knowledge, this article represents the first report on the production of a human tumour antigen through fermentation.