Challenges in therapeutic glycoprotein production

A human embryonic kidney 293T cell line mutated at the Golgi alpha-mannosidase II locus

Disruption of Golgi alpha-mannosidase II activity can result in type II congenital dyserythropoietic anemia and induce lupus-like autoimmunity in mice. Here, we isolated a mutant human embryonic kidney (HEK) 293T cell line called Lec36, which displays sensitivity to ricin that lies between the parental HEK 293T cells, in which the secreted and membrane-expressed proteins are dominated by complex-type glycosylation, and 293S Lec1 cells, which produce only oligomannose-type N-linked glycans. Stem cell marker 19A was transiently expressed in the HEK 293T Lec36 cells and in parental HEK 293T cells with and without the potent Golgi alpha-mannosidase II inhibitor, swainsonine. Negative ion nano-electrospray ionization mass spectra of the 19A N-linked glycans from HEK 293T Lec36 and swainsonine-treated HEK 293T cells were qualitatively indistinguishable and, as shown by collision-induced dissociation spectra, were dominated by hybrid-type glycosylation. Nucleotide sequencing revealed mutations in each allele of MAN2A1, the gene encoding Golgi alpha-mannosidase II: a point mutation that mapped to the active site was found in one allele, and an in-frame deletion of 12 nucleotides was found in the other allele. Expression of the wild type but not the mutant MAN2A1 alleles in Lec36 cells restored processing of the 19A reporter glycoprotein to complex-type glycosylation. The Lec36 cell line will be useful for expressing therapeutic glycoproteins with hybrid-type glycans and as a sensitive host for detecting mutations in human MAN2A1 causing type II congenital dyserythropoietic anemia.

Plants as bioreactors: Recent developments and emerging opportunities

In recent years, the use of plants as bioreactors has emerged as an exciting area of research and significant advances have created new opportunities. The driving forces behind the rapid growth of plant bioreactors include low production cost, product safety and easy scale up. As the yield and concentration of a product is crucial for commercial viability, several strategies have been developed to boost up protein expression in transgenic plants. Augmenting tissue-specific transcription, elevating transcript stability, tissue-specific targeting, translation optimization and sub-cellular accumulation are some of the strategies employed. Various kinds of products that are currently being produced in plants include vaccine antigens, medical diagnostics proteins, industrial and pharmaceutical proteins, nutritional supplements like minerals, vitamins, carbohydrates and biopolymers. A large number of plant-derived recombinant proteins have reached advanced clinical trials. A few of these products have already been introduced in the market.

2-D DIGE to expedite downstream process development for human monoclonal antibody purification

Two-dimensional fluorescence difference gel electrophoresis (2-D DIGE) is an established method for assessing protein expression strategies, understanding pathogenesis mechanisms, characterizing biomarkers, and controlling therapeutic processes. We applied 2-D DIGE to facilitate the development of a purification process for a recombinant IgG1 antibody against Rhesus D antigen expressed by Chinese hamster ovary cells. The variability of two expression clones as well as the influence of cell viability on the host-cell protein pattern was assessed quantitatively. Up to 800 different spots were identified. 2-D DIGE showed that differences in cell viability had more influence on the protein expression pattern than did the expression clone itself. After purification of the IgG from different culture supernatants, the protein patterns on 2-D DIGE were identical, indicating the validity of purification scheme.

Emerging methods for the production of homogeneous human glycoproteins

Most circulating human proteins exist as heterogeneously glycosylated variants (glycoforms) of an otherwise homogeneous polypeptide. Though glycan heterogeneity is most likely important to glycoprotein function, the preparation of homogeneous glycoforms is important both for the study of the consequences of glycosylation and for therapeutic purposes. This review details selected approaches to the production of homogeneous human N- and O-linked glycoproteins with human-type glycans. Particular emphasis is placed on recent developments in the engineering of glycosylation pathways within yeast and bacteria for in vivo production, and on the in vitro remodeling of glycoproteins by enzymatic means. The future of this field is very exciting.

Discovery and characterization of a peptide motif that specifically recognizes a non-native conformation of human IgG induced by acidic pH conditions

In therapeutic antibody preparation, acidic pH conditions are generally used for elution from Protein A affinity column of IgG or for its viral inactivation. Exposing IgG to low pH conditions induces conformational changes, leading to its functional damage or loss, although the mechanisms have not been fully elucidated. In this study using random peptide T7 phage display libraries, we isolated a unique and novel peptide motif that specifically recognized the non-native conformer (acid conformer) of human IgG that was generated by the low pH treatment, but not the native conformer. We examined the generation conditions and biochemical properties of acid conformer using the peptide motif as an affinity ligand. The acid conformer was easily generated at acidic pH (<pH 3.0) and at moderate temperatures (20-40 degrees C). The conformer was present in a monomeric form functionally maintaining antigen or Fc receptor binding, but showed a tendency to aggregate with a long incubation time at neutral pH (>25 degrees C). The peptides isolated here could contribute to the elucidation of the mechanisms of antibody dysfunction or aggregation during acid exposure as well as storage of human IgG.

Evaluation of efficacy of mammalian and baculovirus expressed E2 subunit vaccine candidates to bovine viral diarrhoea virus

Bovine viral diarrhoea virus (BVDV) is a worldwide pathogen of cattle causing a wide spectrum of clinical disease. The major envelope glycoprotein of BVDV, E2, induces the production of neutralising antibodies. In this study we compared the protection afforded to cattle after BVDV challenge by two separate E2 vaccine candidates produced by different heterologous protein expression systems. E2 antigen was expressed using the baculovirus expression system (brE2) and a mammalian cell expression system (mrE2). In the first vaccination study the quantity of recombinant protein expressed by the two systems differed. Vaccination of cattle with a higher dose of brE2 or low dose mrE2 gave comparable protection from viral challenge. Immunised animals showed no pyrexia and reduced leucopaenia which contrasted to the unvaccinated controls. In addition virus shedding from the nasal mucosa was decreased in the vaccinated groups and strong humoral responses were evident post-challenge. However, the efficacy of the brE2 vaccine was greatly diminished when a reduced dose was tested, indicating the importance of assessing the type of expression system used in antigen production.

Targeting and post-translational processing of human alpha1-antichymotrypsin in BY-2 tobacco cultured cells

The post-translational processing of human alpha(1)-antichymotrypsin (AACT) in Bright Yellow-2 (BY-2) tobacco cells was assessed in relation to the cellular compartment targeted for accumulation. As determined by pulse-chase labelling experiments and immunofluorescence microscopy, AACT sent to the vacuole or the endoplasmic reticulum (ER) was found mainly in the culture medium, similar to a secreted form targeted to the apoplast. Unexpectedly, AACT expressed in the cytosol was found in the nucleus under a stable, non-glycosylated form, in contrast with secreted variants undergoing multiple post-translational modifications during their transit through the secretory pathway. All secreted forms of AACT were N-glycosylated, with the presence of complex glycans as observed naturally on human AACT. Proteolytic trimming was also observed for all secreted variants, both during their intracellular transit and after their secretion in the culture medium. Overall, the targeting of human AACT to different compartments of BY-2 tobacco cells led to the production of two protein products: (i) a stable, non-glycosylated protein accumulated in the nucleus; and (ii) a heterogeneous mixture of secreted variants resulting from post-translational N-glycosylation and proteolytic processing. Overall, these data suggest that AACT is sensitive to resident proteases in the ER, the Golgi and/or the apoplast, and that the production of intact AACT in the plant secretory pathway will require innovative approaches to protect its structural integrity in vivo. Studies are now needed to assess the activity of the different AACT variants, and to identify the molecular determinants for the nuclear localization of AACT expressed in the cytosol.

Nucleocytoplasmic transit of human alpha1-antichymotrypsin in tobacco leaf epidermal cells

Recently, we have observed a nuclear localization for human alpha(1)-antichymotrypsin (AACT) expressed in the cytosol of transgenic Bright Yellow-2 (BY-2) tobacco cultured cells (see accompanying paper: Benchabane, M., Saint-Jore-Dupas, C., Bardor, M., Faye, L., Michaud, D. and Gomord, V. (2008a) Targeting and post-translational processing of human alpha(1)-antichymotrypsin in BY-2 tobacco cultured cells. Plant Biotechnol. J. doi: 10.1111/j.1467-7652.2008.00382.x). In the present article, we assess whether the intrinsic DNA-binding activity of AACT can explain its nuclear localization, and whether this same activity has an impact on its protease inhibitory potency and stability in planta. An engineered form of AACT with no DNA-binding activity, rAACTDeltaK, was compared with the wild-type polypeptide, rAACT, in terms of chymotrypsin inhibitory potency, stability in planta and distribution in tobacco cells. In accordance with available data reporting distinct sites for protease inhibition and DNA binding, rAACT and rAACTDeltaK showed similar antichymotrypsin activity, similar to the activity of native AACT purified from human plasma. As observed for AACT in BY-2 tobacco cells, a green fluorescent protein (GFP)-AACT fusion transiently expressed in the cytosol of tobacco leaf epidermal cells was detected mainly in the nucleus by confocal laser microscopy. By contrast, rAACTDeltaK expressed as a GFP fusion showed a balanced distribution between the cytosol and the nucleus, similar to the distribution pattern of free GFP exhibiting no DNA-binding affinity. In line with immunodetection data showing higher accumulation levels for GFP-AACT in tobacco leaf cells, rAACTDeltaK was more susceptible than rAACT to tryptic digestion in the presence of DNA. Overall, these observations suggest the following: (i) a retention effect of DNA on AACT in the nucleus; and (ii) a stabilizing effect of the AACT-DNA interaction on rAACT challenged with non-target proteases, which, possibly, may be useful in protecting this protein in plant expression platforms.

Mass spectrometry of N-linked glycans

The analysis of the N-glycan portions of glycoproteins has become important for the detection of various diseases involving altered glycosylation patterns, including cancer and congenital disorders of glycosylation, and in the quality control of therapeutic recombinant glycoproteins. Here in this chapter we give detailed analysis procedures necessary for the complete structural elucidation of any N-glycan mixture found in naturally occurring and recombinant glycoproteins. The protocols include monosaccharide composition analysis on the intact glycoprotein, the enzymatic release of N-glycans from glycoprotein and their separation from O-glycopeptides and peptides, and the structural characterization of the glycans by exoglycosidase digestions, methylation ("linkage") analysis, and mass spectrometry/tandem mass spectrometry of permethylated glycans using matrix-assisted laser desorption and electrospray ionization methods.

Production of recombinant proteins in suspension-cultured plant cells

Plants have emerged in the past decade as a suitable alternative to the current production systems for recombinant pharmaceutical proteins and, today their potential for low-cost production of high quality, much safer and biologically active mammalian proteins is largely documented. Among various plant expression systems being explored, genetically modified suspension-cultured plant cells offer a promising system for production of biopharmaceuticals. Indeed, when compared to other plant-based production platforms that have been explored, suspension-cultured plant cells have the advantage of being totally devoid of problems associated with the vagaries of weather, pest, soil and gene flow in the environment. Because of short growth cycles, the timescale needed for the production of recombinant proteins in plant cell culture can be counted in days or weeks after transformation compared to months needed for the production in transgenic plants. Moreover, recovery and purification of recombinant proteins from plant biomass is an expensive and technically challenging business that may amount to 80-94% of the final product cost. One additional advantage of plant cell culture is that the recombinant protein fused with a signal sequence can be expressed and secreted into the culture medium, and therefore recovered and purified in the absence of large quantities of contaminating proteins. Consequently, the downstream processing of proteins extracted from plant cell culture medium is less expensive, which may/does balance the higher costs of fermentation. When needed for clinical use, recombinant proteins are easily produced in suspension-cultured plant cells under certified, controllable and sterile conditions that offer improved safety and provide advantages for good manufacturing practices and regulatory compliance. In this chapter, we present basic protocols for rapid generation of transgenic suspension-cultured cells of Nicotiana tabacum, Oriza sativa and Arabidopis thaliana. These systems are powerful tools for plant-made pharmaceuticals production in highly controlled conditions.

Challenges in monoclonal antibody-based therapies

Therapeutic monoclonal antibodies (mAbs) are the fastest growing class of new therapeutic molecules. They hold great promises for the treatment of a variety of diseases, including chronic inflammatory diseases and cancer. However, the current manufacturing and purification processes cause limitations in the production capacity of therapeutic antibodies, leading to an increase in cost. Genetic delivery of therapeutic monoclonal antibodies by in vivo production offers a new potential solution to these problems. Firstly, therapeutic efficacy can be improved by maintaining stable therapeutic, non-toxic levels within the blood circulation over a long period of time. Repeated high-dose bolus injections could be avoided, thereby reducing the possibility of side-effects. Secondly, the high cost of manufacturing and purification of the therapeutic antibodies could be reduced, making an in vivo/ex vivo mAb gene transfer an economically viable and attractive option. In general, three approaches can be used for the stable long-term expression and secretion of therapeutic antibodies in vivo: 1) direct in vivo administration of integrating vectors carrying a mAb gene, 2) grafting of ex vivo genetically modified autologous cells, and 3) implantation of an encapsulated antibody producing heterologous or autologous cells. This paper describes the key factors and problems associated with the current antibody-based immunotherapies and reviews prospects for genetic in vivo delivery of therapeutic antibodies.

Mass spectrometric quantification of neutral and sialylated N-glycans from a recombinant therapeutic glycoprotein produced in the two Chinese hamster ovary cell lines

Quality control and assurance of glycan profiles of a recombinant glycoprotein from lot to lot is a critical issue in the pharmaceutical industry. To develop an easy and simple quantitative and qualitative glycan profile method based on matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry (MALDI-TOF MS), the modification with Girard's reagent T (GT) was exploited. Because GT-derivatized quantification of oligosaccharides using MALDI-TOF MS is possible only with neutral glycans, sialylated glycans are not subjected to quantitative analysis with MALDI-TOF MS. To solve this problem, mild methyl esterification and subsequent GT derivatization were employed, enabling us to perform rapid qualitative and quantitative analysis of sialylated and neutral N-linked oligosaccharides using MALDI-TOF MS. This modified method was used in the comparative quantification of N-glycans from the recombinant therapeutic glycoprotein expressed in two different Chinese hamster ovary (CHO) cell lines. The percentages of sialylated N-glycans to total were 22.5 and 5.2% in CHO-I and CHO-II cells, respectively, resulting in a significant difference in the biological activity of the recombinant glycoprotein.

Properties, production, and applications of camelid single-domain antibody fragments

Camelids produce functional antibodies devoid of light chains of which the single N-terminal domain is fully capable of antigen binding. These single-domain antibody fragments (VHHs or Nanobodies®) have several advantages for biotechnological applications. They are well expressed in microorganisms and have a high stability and solubility. Furthermore, they are well suited for construction of larger molecules and selection systems such as phage, yeast, or ribosome display. This minireview offers an overview of (1) their properties as compared to conventional antibodies, (2) their production in microorganisms, with a focus on yeasts, and (3) their therapeutic applications.

Co-expression of human protein disulfide isomerase (hPDI) enhances secretion of bovine follicle-stimulating hormone (bFSH) in Pichia pastoris

Bovine follicle-stimulating hormone (bFSH) is a pituitary gonadotropin composed of two non-covalently associated polypeptide subunits, which must be glycosylated, folded, and assembled as a heterodimer to be biologically active. Low-level expression of the recombinant bFSH is the factor that limits its usefulness as a superovulation treatment for cows. To increase the production of recombinant bFSH, human protein disulfide isomerase (hPDI) was expressed simultaneously in engineered Pichia strains. The secretion characteristics of bFSH with or without hPDI were examined. The co-expression of bFSH and hPDI is increased to 1.56 mg/l of heterodimer in the culture medium, which is 6-fold higher when compared with the control strain carrying the bFSH gene only. These results may be generally applicable to increase the expression of other glycoprotein hormones in yeast.