On the Optimal Ratio of Heavy to Light Chain Genes for Efficient Recombinant Antibody Production by CHO Cells

Proteins improving recombinant antibody production in mammalian cells

Mammalian cells have been successfully used for the industrial manufacture of antibodies due to their ability to synthesize antibodies correctly. Nascent polypeptides must be subjected to protein folding and assembly in the ER and the Golgi to be secreted as mature proteins. If these reactions do not proceed appropriately, unfolded or misfolded proteins are degraded by the ER-associated degradation (ERAD) pathway. The accumulation of unfolded proteins or intracellular antibody crystals accompanied by this failure triggers the unfolded protein response (UPR), which can considerably attenuate the levels of translation, folding, assembly, and secretion, resulting in reduction of antibody productivity. Accumulating studies by omics-based analysis of recombinant mammalian cells suggest that not only protein secretion processes including protein folding and assembly but also translation are likely to be the rate-limiting factors for increasing antibody production. Here, this review describes the mechanism of antibody folding and assembly and recent advantages which could improve recombinant antibody production in mammalian cells by utilizing proteins such as ER chaperones or UPR-related proteins.

An internal ribosome entry site (IRES) mutant library for tuning expression level of multiple genes in mammalian cells

A set of mutated Encephalomyocarditis virus (EMCV) internal ribosome entry site (IRES) elements with varying strengths is generated by mutating the translation initiation codons of 10^th, 11^th, and 12^th AUG to non-AUG triplets. They are able to control the relative expression of multiple genes over a wide range in mammalian cells in both transient and stable transfections. The relative strength of each IRES mutant remains similar in different mammalian cell lines and is not gene specific. The expressed proteins have correct molecular weights. Optimization of light chain over heavy chain expression by these IRES mutants enhances monoclonal antibody expression level and quality in stable transfections. Uses of this set of IRES mutants can be extended to other applications such as synthetic biology, investigating interactions between proteins and its complexes, cell engineering, multi-subunit protein production, gene therapy, and reprogramming of somatic cells into stem cells.

Using molecular markers to characterize productivity in Chinese hamster ovary cell lines

Selection of high producing cell lines to produce maximum product concentration is a challenging and time consuming task for the biopharmaceutical industry. The identification of early markers to predict high productivity will significantly reduce the time required for new cell line development. This study identifies candidate determinants of high productivity by profiling the molecular and morphological characteristics of a panel of six Chinese Hamster Ovary (CHO) stable cell lines with varying recombinant monoclonal antibody productivity levels ranging between 2 and 50 pg/cell/day. We examined the correlation between molecular parameters and specific productivity (q_p) throughout the growth phase of batch cultures. Results were statistically analyzed using Pearson correlation coefficient. Our study revealed that, overall, heavy chain (HC) mRNA had the strongest association with q_p followed by light chain (LC) mRNA, HC intracellular polypeptides, and intracellular antibodies. A significant correlation was also obtained between q_p and the following molecular markers: growth rate, biomass, endoplasmic reticulum, and LC polypeptides. However, in these cases, the correlation was not observed at all-time points throughout the growth phase. The repeated sampling throughout culture duration had enabled more accurate predictions of productivity in comparison to performing a single-point measurement. Since the correlation varied from day to day during batch cultivation, single-point measurement was of limited use in making a reliable prediction.

Polyethyleneimine-based transient gene expression processes for suspension-adapted HEK-293E and CHO-DG44 cells

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A brief overview of principles of TGE using mammalian cells.

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Description of TGE processes for HEK293 and CHO cells.

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Description of orbitally shaken bioreactors for suspension cell cultivation.

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Description of polyethylenime-based transfection processes.

Transient gene expression (TGE) from mammalian cells is an increasingly important tool for the rapid production of recombinant proteins for research applications in biochemistry, structural biology, and biomedicine. Here we review methods for the transfection of human embryo kidney (HEK-293) and Chinese hamster ovary (CHO) cells in suspension culture using the cationic polymer polyethylenimine (PEI) for gene delivery.

Comparison of internal ribosome entry site (IRES) and Furin-2A (F2A) for monoclonal antibody expression level and quality in CHO cells

Four versions of tricistronic vectors expressing IgG1 light chain (LC), IgG1 heavy chain (HC), and dihydrofolate reductase (DHFR) in one transcript were designed to compare internal ribosome entry site (IRES) and furin-2A (F2A) for their influence on monoclonal antibody (mAb) expression level and quality in CHO DG44 cells. LC and HC genes are arranged as either the first or the second cistron. When using mAb quantification methods based on the detection antibodies against HC Fc region, F2A-mediated tricistronic vectors appeared to express mAb at higher levels than the IRES-mediated tricistronic vectors in both transient and stable transfections. Further analysis revealed that more than 40% of products detected in stably transfected pools generated using the two F2A-mediated tricistronic vectors were aggregates. LC and HC from the F2A stably transfected pools were not properly processed, giving rise to LC+F2A+HC or HC+F2A+LC fusion proteins, LC and HC polypeptides with F2A remnants, and incorrectly cleaved signal peptides. Both IRES-mediated tricistronic vectors express mAb with correct sizes and signal peptide cleavage. Arrangement of LC as the first cistron in the IRES-mediated tricistronic vectors exhibits increased mAb expression level, better growth, and minimized product aggregation, while arrangement of HC as first cistron results in low expression, slower growth, and high aggregation. The results obtained will be beneficial for designing vectors that enhance mAb expression level and quality in mammalian cells.

Mammalian stable expression of biotherapeutics

Many therapeutically relevant proteins, like IgG antibodies, are highly complex, multimeric glycoproteins that are difficult to express in microbial systems and thus usually produced in mammalian host cells. During the past two decades, stable mammalian expression technologies have made huge progress resulting in highly increased speed of cell line development and yield of manufacturing processes. Here, we give an overview of technologies that are applied at different stages of state-of-the-art cell line development processes for biomanufacturing.

Large-scale transfection of mammalian cells

The large-scale transfection of mammalian cells allows moderate (milligram to gram) amounts of recombinant proteins (r-proteins) to be obtained for fundamental or clinical research. In this article, we describe a one-liter transfection using polyethyleneimine (PEI) for DNA delivery into human embryonic kidney (HEK-293) cells cultivated in serum-free suspension to produce a recombinant human monoclonal antibody that yields up to about 1 g/L in a 10-day process. The method is based on a DNA delivery step performed at high cell density (20×10(6) cells/mL) by direct addition of DNA and PEI to the culture. Subsequently, the cells are diluted 20-fold for the 10-day production phase in the presence of valproic acid (VPA), a histone deacetylase inhibitor. The methods for plasmid purification, antibody quantification by enzyme-linked immunosorbent assay (ELISA), and affinity purification with protein A are also described.

Using matrix attachment regions to improve recombinant protein production

Chinese hamster ovary (CHO) cells are the system of choice for the production of complex molecules, such as monoclonal antibodies. Despite significant progress in improving the yield from these cells, the process to the selection, identification, and maintenance of high-producing cell lines remains cumbersome, time consuming, and often of uncertain outcome. Matrix attachment regions (MARs) are DNA sequences that help generate and maintain an open chromatin domain that is favourable to transcription and may also facilitate the integration of several copies of the transgene. By incorporating MARs into expression vectors, an increase in the proportion of high-producer cells as well as an increase in protein production are seen, thereby reducing the number of clones to be screened and time to production by as much as 9 months. In this chapter, we describe how MARs can be used to increase transgene expression and provide protocols for the transfection of CHO cells in suspension and detection of high-producing antibody cell clones.

Transient expression technologies: past, present, and future

The first protocols describing transient gene expression in mammalian cells for the rapid generation of recombinant proteins emerged more than 10 years ago as an alternative to the establishment of stable, often amplified clonal cell lines, and relieved somewhat the bias against mammalian cell systems as being too complicated, labor intensive, and tedious to serve as a source for tool proteins in industrial research and academia. Over the past decade, these attempts have been refined and optimized, giving rise to expression protocols applicable in every lab in dependence on available tools, equipment, and envisaged outcome. This chapter summarizes the development of transient expression technologies over the past decade up to its current status and provides an outlook into what may be the future of transient technology development.

Efficient expression from one CMV enhancer controlling two core promoters

The human CMV promoter/enhancer is one of the strongest promoters for recombinant protein expression in mammalian cells, making the promoter very popular for production of recombinant antibodies. We used an antibody vector design where the antibody heavy and light chain genes were transcribed from a promoter complex consisting of two promoters arranged divergently with the 5' ends of the promoters in close proximity. However, when two identical CMV promoters constituted this promoter complex, the antibody expression observed was lower than expected based on the strength of the individual promoters. To optimize expression we prepared truncated promoter complexes where only one CMV enhancer controlled the initiation of transcription from two divergent minimal CMV core promoters. Antibody expression from the truncated promoter complexes was analyzed both when transiently transfected and upon stable site-specific integration into a CHO DG44 derived cell line. The data showed that it was possible for one enhancer to drive the expression of two core promoters. However, efficient expression from both divergent core promoters was seen only when the unique region upstream of the CMV enhancer was removed. Notably, a 12-fold increase in expression was found from the best of the truncated promoter complexes after stable site-specific integration when compared to the full-length double CMV promoter complex.

Impact of gene vector design on the control of recombinant monoclonal antibody production by Chinese hamster ovary cells

In this study, we systematically compare two vector design strategies for recombinant monoclonal antibody (Mab) synthesis by Chinese hamster ovary (CHO) cells; a dual open reading frame (ORF) expression vector utilizing separate cytomegalovirus (CMV) promoters to drive heavy chain (HC) and light chain (LC) expression independently, and a single ORF vector design employing a single CMV promoter to drive HC and LC polypeptide expression joined by a foot and mouth disease virus F2A polypeptide self-cleaving linker sequence. Initial analysis of stable transfectants showed that transfectants utilizing the single ORF vector designs exhibited significantly reduced Mab production. We employed an empirical modeling strategy to quantitatively describe the cellular constraints on recombinant Mab synthesis in all stable transfectants. In all transfectants, an intracellular molar excess of LC polypeptide over HC polypeptide was observed. For CHO cells transfected with the single ORF vectors, model-predicted, and empirical intracellular intermediate levels could only be reconciled by inclusion of nascent HC polypeptide degradation. Whilst a local sensitivity analysis showed that qMab of all transfectants was primarily constrained by recombinant mRNA translation rate, our data indicated that all single ORF transfectants exhibited a reduced level of recombinant gene transcription and that Mab folding and assembly reactions generically exerted greater control over qMab. We infer that the productivity of single ORF transfectants is limited by ER processing/degradation "capacity" which sets a limit on transcriptional input. We conclude that gene vector design for oligomeric recombinant proteins should be based on an understanding of protein-specific synthetic kinetics rather than polypeptide stoichiometry.

Stable expression and purification of a functional processed Fab' fragment from a single nascent polypeptide in CHO cells expressing the mCAT-1 retroviral receptor

Monoclonal antibodies and derivative formats such as Fab' fragments are used in a broad range of therapeutic, diagnostic and research applications. New systems and methodologies that can improve the production of these proteins are consequently of much interest. Here we present a novel approach for the rapid production of processed Fab' fragments in a CHO cell line that has been engineered to express the mouse cationic amino acid transporter receptor 1 (mCAT-1). This facilitated the introduction of the target antibody gene through retroviral transfection, rapidly producing stable expression. Using this system, we designed a single retroviral vector construct for the expression of a target Fab' fragment as a single polypeptide with a furin cleavage site and a FMDV 2A self-cleaving peptide introduced to bridge the light and truncated heavy chain regions. The introduction of these cleavage motifs ensured equimolar expression and processing of the heavy and light domains as exemplified by the production of an active chimeric Fab' fragment against the Fas receptor, routinely expressed in 1-2mg/L yield in spinner-flask cell cultures. These results demonstrate that this method could have application in the facile production of bioactive Fab' fragments.

A mechanistic understanding of production instability in CHO cell lines expressing recombinant monoclonal antibodies

One of the most significant problems in industrial bioprocessing of recombinant proteins using engineered mammalian cells is the phenomenon of cell line instability, where a production cell line suffers a loss of specific productivity (qP). This phenomenon occurs with unpredictable kinetics and has been widely observed in Chinese hamster ovary (CHO) cell lines and with all commonly used gene expression systems. The underlying causes (both genetic and physiological) and the precise molecular mechanisms underpinning cell line instability have yet to be fully elucidated, although recombinant gene silencing and loss of recombinant gene copies have been shown to cause qP loss. In this work we have investigated the molecular mechanisms underpinning qP instability over long-term sub-culture in CHO cell lines producing recombinant IgG1 and IgG2 monoclonal antibodies (Mab's). We demonstrate that production instability derives from two primary mechanisms: (i) epigenetic--methylation-induced transcriptional silencing of the CMV promoter driving Mab gene transcription and (ii) genetic--progressive loss of recombinant Mab gene copies in a proliferating CHO cell population. We suggest that qP decline resulting from loss of recombinant genes is a consequence of the inherent genetic instability of recombinant CHO cell lines.

An empirical modeling platform to evaluate the relative control discrete CHO cell synthetic processes exert over recombinant monoclonal antibody production process titer

In this study we have combined empirically derived mathematical models of intracellular Mab synthesis to quantitatively compare the degree to which individual cellular processes limit recombinant IgG(4) monoclonal antibody production by GS-CHO cells throughout a state-of-the-art industrial fed-batch culture process. Based on the calculation of a production process control coefficient for each stage of the intracellular Mab synthesis and secretion pathway, we identified the major cellular restrictions on Mab production throughout the entire culture process to be recombinant heavy chain gene transcription and heavy chain mRNA translation. Surprisingly, despite a substantial decline in the rate of cellular biomass synthesis during culture, with a concomitant decline in the calculated rate constants for energy-intensive Mab synthetic processes (Mab folding/assembly and secretion), these did not exert significant control of Mab synthesis at any stage of production. Instead, cell-specific Mab production was maintained by increased Mab gene transcription which offset the decline in cellular biosynthetic rates. Importantly, this study shows that application of this whole-process predictive modeling strategy should rationally precede and inform cell engineering approaches to increase production of a recombinant protein by a mammalian host cell--where control of productivity is inherently protein product and cell line specific.

Bispecific antibodies engage T cells for antitumor immunotherapy

INTRODUCTION

Although considerable evidence supports the hypothesis that T cells play a critical role in the immune response against cancer, the ability to mount and sustain tumor-specific cellular responses in vivo remains a challenge. A strategy that harnesses the cytotoxic advantage of T cell therapy is the use of bispecific antibodies designed to engage and activate endogenous polyclonal T cell populations via the CD3 complex, but only in the presence of a tumor antigen. While antibody constructs with dual specificity were first described as anticancer therapeutics over 25 years ago, it was not until recently that one subclass of bispecific single-chain antibody, the bispecific T cell engager (BiTE), emerged as superior to previous iterations in achieving efficacy in animal models and early clinical trials.

AREAS COVERED

The evolution of bispecific antibodies in antitumor immunotherapy is reviewed and the greatest hurdles impeding their clinical translation are discussed, specifically in the context of immunoprivileged sites as is the case for intracerebral malignancy.

EXPERT OPINION

The BiTE platform has great potential in the treatment of malignant disease. Despite burgeoning interest in bispecific antibodies and permutations thereof, the issues of stability and cost-effective production persist as obstacles.

Towards the molecular characterization of the stable producer phenotype of recombinant antibody-producing NS0 myeloma cells

The loss of heterologous protein expression is one of the major problems faced by industrial cell line developers and has been reported by several authors. Therefore, the understanding of the mechanisms involved in the generation of stable and high producer cell lines is a critical issue, especially for those processes based on long term continuous cultures. We characterized two recombinant NS0 myeloma cell lines expressing Nimotuzumab, a humanized anti-human epidermal growth factor receptor (EGFR) antibody. The hR3/H7 clone is a stable producer obtained from the unstable hR3/t16 clone. The unstable clone was characterized by a bimodal distribution of intracellular immunoglobulin staining using flow cytometry. Loss of antibody production was due to the emergence of a non-producer cell subpopulation that increased with cell generation number. Immunoglobulin heavy chain (HC) and light chain (LC) ratio (HC/LC) was lower for the unstable phenotype. Proteomic maps using two dimensional gel electrophoresis (2DE) were obtained for both clones, at initial cell culture time and after 40 generations. Fifteen proteins potentially associated with the phenomenon of production stability were identified. The hR3/H7 stable clone showed an up-regulated expression pattern for most of these proteins. The regulation of recombinant antibody production by the host NS0 myeloma cell line most likely involves simultaneously cellular processes such as DNA transcription, mRNA processing, protein synthesis and folding, vesicular transport, glycolysis and energy production, according to the proteins identified in the present proteomic study.

Conditional expression of full-length humanized anti-prion protein antibodies in Chinese hamster ovary cells

Because of their high antigen specificity and metabolic stability, genetically engineered human monoclonal antibodies are on the way to becoming one of the most promising medical diagnostics and therapeutics. In order to establish an in vitro system capable of producing such biosimilar antibodies, we used human constant chain sequences to design the novel human antibody expressing vector cassette pMAB-ABX. A bidirectional tetracycline (tet)-controllable promotor was used for harmonized expression of immunoglobulin type G (IgG) heavy and light chains. As an example we used anti-prion protein (anti-PrP) IgGs. Therefore, the variable heavy (V(H)) and light chain (V(L)) sequences of anti-PrP antibodies, previously generated in our laboratory by DNA immunization of prion protein knock-out mice, were isolated from murine hybridoma cell lines and inserted into pMAB-ABX vector. After transfection of Chinese hamster ovary (CHO) cells, a number of stable antibody producing cell clones were selected. One cell line (pMAB-ABX-13F10/3B5) stably expressing the recombinant humanized antibody (rechuAb) 13F10/3B5 was selected for detailed characterization by Western blot, immunofluorescence, and flow cytometric analyses. The full-length recombinant humanized IgG antibody showed a high level of expression in the cytoplasm. In conclusion, the new cell system described here is a suitable tool to produce functional intact full-length humanized IgG antibodies.

Anti-TNFα domain antibody construct CEP-37247: Full antibody functionality at half the size

We report preclinical data for CEP-37247, the first human framework domain antibody construct to enter the clinic. At approximately 11 - 13kDa, domain antibodies or dAbs are the smallest antibody domain able to demonstrate the antigen-recognition function of an antibody, e.g. high selectivity and affinity for target antigen. CEP-37247 is a bivalent anti-tumor necrosis factor (TNF)α domain antibody protein construct combining the antigen-recognition function of a dAb with the pharmacological advantages of an antibody Fc region. As a homodimer, with each chain comprising VL dAb, truncated CH1, hinge, CH2 and CH3 domains, CEP-37247 has a molecular mass of approximately 78kDa, which is about half the size of a conventional IgG molecule. Surface plasmon resonance data demonstrate that CEP-37247 possesses high selectivity and affinity for TNFα. CEP-37247 is a potent neutralizer of TNFα activity in vitro in the L929 TNF-mediated cytotoxicity assay. In a human TNFα-over-expressing mouse model of polyarthritis, CEP-37247 prevents development of disease, and is at least as effective as the marketed product etanercept. Fc functionality is intact - CEP-37247 is capable of mediating antibody-dependent cell-mediated cytotoxicity and has a circulating half-life of approximately 4.5 days in cynomolgus macaques. Given the favorable properties outlined above, and its high expression levels (approaching 7 g/L) in a CHOK1 based-expression system, CEP-37247 is progressing into the clinic, where other potential advantages such as enhanced efficacy due to improved tissue distribution, and beneficial immunogenicity profile, will be evaluated.

Evaluation of the influenza A replicon for transient expression of recombinant proteins in mammalian cells

Recombinant protein expression in mammalian cells has become a very important technique over the last twenty years. It is mainly used for production of complex proteins for biopharmaceutical applications. Transient recombinant protein expression is a possible strategy to produce high quality material for preclinical trials within days. Viral replicon based expression systems have been established over the years and are ideal for transient protein expression. In this study we describe the evaluation of an influenza A replicon for the expression of recombinant proteins. We investigated transfection and expression levels in HEK-293 cells with EGFP and firefly luciferase as reporter proteins. Furthermore, we studied the influence of different influenza non-coding regions and temperature optima for protein expression as well. Additionally, we exploited the viral replication machinery for the expression of an antiviral protein, the human monoclonal anti-HIV-gp41 antibody 3D6. Finally we could demonstrate that the expression of a single secreted protein, an antibody light chain, by the influenza replicon, resulted in fivefold higher expression levels compared to the usually used CMV promoter based expression. We emphasize that the influenza A replicon system is feasible for high level expression of complex proteins in mammalian cells.

Production of protein complexes via co-expression

Multi-protein complexes are involved in essentially all cellular processes. A protein's function is defined by a combination of its own properties, its interacting partners, and the stoichiometry of each. Depending on binding partners, a transcription factor can function as an activator in one instance and a repressor in another. The study of protein function or malfunction is best performed in the relevant context. While many protein complexes can be reconstituted from individual component proteins after being produced individually, many others require co-expression of their native partners in the host cells for proper folding, stability, and activity. Protein co-expression has led to the production of a variety of biological active complexes in sufficient quantities for biochemical, biophysical, structural studies, and high throughput screens. This article summarizes examples of such cases and discusses critical considerations in selecting co-expression partners, and strategies to achieve successful production of protein complexes.

Generation of full-length functional antibody against preS2 of hepatitis B virus in hepatic cells in vitro from bicistrons mediated by gutless adenovirus

BACKGROUND

Monoclonal antibodies (mAbs) have been developed as effective therapeutics for a wide variety of diseases. Delivery of mAbs by gene transfer provides an option for overcoming the difficulties in mAb production and manufacturing processes. However, for the polymeric structure of full-length mAbs, it is important to design an optimal gene transfer system for mAb generation.

METHODS

Gutless adenovirus and liver-specific promoter transthyretin (TTR) were combined to deliver bicistronic mAb genes in human hepatic cell lines. In order to optimize the bicistrons for mAb generation, four bicistrons were designed and compared, and the most efficient one was selected. ELISA and Western blot were conducted to evaluate mAb products in the supernatants.

RESULTS

Our data showed that all of four gutless adenoviruses elicited liver-specific mAb production in HepG2 and Hep3B hepatic cell lines. It was observed that the L2AH bicistron construct (comprising an immunoglobulin light-chain cDNA situated 5' of a heavy-chain cDNA, with a foot-and-mouth disease virus 2A cleavage site in the middle, subcloned into the helper-dependent adenovirus plasmid pGL) could induce the highest level expression of mAb (about 5.0 microg/mL in Hep3B) among these four constructs. Importantly, the mAb products by gene transfer methods retained specific antigen-binding activity.

CONCLUSION

Our studies gave further evidence that it was feasible to produce active full-length mAb in human hepatic cell lines in vitro by a special gene delivery system. Moreover, we developed an optimized bicistron gene transfer system for future gene therapy research, which may also be of use in industrial mAb production.

Expression, engineering and characterization of the tumor-targeting heterodimeric immunocytokine F8-IL12

Proinflammatory cytokines have been used for several years in patients with advanced cancer but their administration is typically associated with severe toxicity hampering their application to therapeutically active regimens. This problem can be overcome by using immunocytokines (cytokines fused to antibody or antibody fragments) which selectively deliver the active cytokine to the tumor environment. Preclinical and recent clinical results confirmed that this approach is a very promising avenue to go. We designed an immunocytokine consisting of the scFv(F8) specific to extra-domain A of fibronectin and the very potent human cytokine interleukin-12 (IL12). The heterodimeric nature of IL12 allows the engineering of various immunocytokine formats, based on different combinations of the two subunits (p35 and p40) together with the scFv. In comparison to monomeric or homodimeric cytokines, the construction of a heterodimeric immunocytokine poses many challenges, e.g. gene dosing, stable high-yield expression as well as good manufacture practice (GMP) purification and characterization. In this paper, we describe the successful construction, characterization and production of the heterodimeric immunocytokine F8-IL12. The positive outcome of this feasibility study leads now to GMP production of F8-IL12, which will soon enter clinical trials.

Combination of the 2A/furin technology with an animal component free cell line development platform process

The recently described 2A/furin technology combines both chains of the antibody in a single open reading frame. Upon translation and secretion, the peptide is processed by the cell to generate native fully functional IgG antibodies. Here, we describe the results of an evaluation study of this technology for an industrial CHO cell line development process. The 2A/furin expression cassette setup was combined with a Novartis vector system. A transfection, selection, and cloning procedure in chemically defined media was established at Novartis and applied for a monoclonal test antibody. The productivity of 2A/furin-vector-derived clones in non-optimized generic shake flask fed-batch models was in a comparable range with clones derived from the reference control vector. Higher clonal production stability was seen for the majority of clones generated with the 2A/furin technology compared to the clones generated with the reference control vector. Product quality was analyzed by SDS-PAGE and no significant difference was detected between the two systems. Thus, it was shown that the 2A/furin technology can be successfully combined with a Novartis CHO expression system and platform. Due to the single ORF setup, the 2A/furin technology may therefore offer a suitable approach to reduce vector size and complexity.

Triple light chain antibodies: factors that influence its formation in cell culture

THIOMABs are recombinant antibodies engineered with reactive cysteines, which can be covalently conjugated to drugs of interest to generate targeted therapeutics. During the analysis of THIOMABs secreted by stably transfected Chinese Hamster Ovary (CHO) cells, we discovered the existence of a new species--Triple Light Chain Antibody (3LC). This 3LC species is the product of a disulfide bond formed between an extra light chain and one of the engineered cysteines on the THIOMAB. We characterized the 3LC by size exclusion chromatography, mass spectrometry, and microchip electrophoresis. We also investigated the potential causes of 3LC formation during cell culture, focusing on the effects of free light chain (LC) polypeptide concentration, THIOMAB amino acid sequence, and glutathione (GSH) production. In studies covering 12 THIOMABs produced by 66 stable cell lines, increased free LC polypeptide expression--evaluated as the ratio of mRNA encoding for LC to the mRNA encoding for heavy chain (HC)--correlated with increased 3LC levels. The amino acid sequence of the THIOMAB molecule also impacted its susceptibility to 3LC formation: hydrophilic LC polypeptides showed elevated 3LC levels. Finally, increased GSH production--evaluated as the ratio of the cell-specific production rate of GSH (q(GSH)) to the cell-specific production rate of THIOMAB (q(p))--corresponded to decreased 3LC levels. In time-lapse studies, changes in extracellular 3LC levels during cell culture corresponded to changes in mRNA LC/HC ratio and q(GSH)/q(p) ratio. In summary, we found that cell lines with low mRNA LC/HC ratio and high q(GSH)/q(p) ratio yielded the lowest levels of 3LC. These findings provide us with factors to consider in selecting a cell line to produce THIOMABs with minimal levels of the 3LC impurity.

Intraclonal protein expression heterogeneity in recombinant CHO cells

Therapeutic glycoproteins have played a major role in the commercial success of biotechnology in the post-genomic era. But isolating recombinant mammalian cell lines for large-scale production remains costly and time-consuming, due to substantial variation and unpredictable stability of expression amongst transfected cells, requiring extensive clone screening to identify suitable high producers. Streamlining this process is of considerable interest to industry yet the underlying phenomena are still not well understood. Here we examine an antibody-expressing Chinese hamster ovary (CHO) clone at single-cell resolution using flow cytometry and vectors, which couple light and heavy chain transcription to fluorescent markers. Expression variation has traditionally been attributed to genetic heterogeneity arising from random genomic integration of vector DNA. It follows that single cell cloning should yield a homogeneous cell population. We show, in fact, that expression in a clone can be surprisingly heterogeneous (standard deviation 50 to 70% of the mean), approaching the level of variation in mixed transfectant pools, and each antibody chain varies in tandem. Phenotypic variation is fully developed within just 18 days of cloning, yet is not entirely explained by measurement noise, cell size, or the cell cycle. By monitoring the dynamic response of subpopulations and subclones, we show that cells also undergo slow stochastic fluctuations in expression (half-life 2 to 11 generations). Non-genetic diversity may therefore play a greater role in clonal variation than previously thought. This also has unexpected implications for expression stability. Stochastic gene expression noise and selection bias lead to perturbations from steady state at the time of cloning. The resulting transient response as clones reestablish their expression distribution is not ordinarily accounted for but can contribute to declines in median expression over timescales of up to 50 days. Noise minimization may therefore be a novel strategy to reduce apparent expression instability and simplify cell line selection.

Transcriptome and proteome analysis of Chinese hamster ovary cells under low temperature and butyrate treatment

Recombinant Chinese hamster ovary (CHO) cells selected for high productivity are capable of secreting immunoglobulin G (IgG) molecules at a level that rivals plasma cells in vivo. Following butyrate treatment at 33 degrees C, further increases in productivity are observed. To better understand the mechanisms by which this increased productivity is incurred, the transcriptional response of an antibody-producing cell line undergoing these treatments was investigated using oligo-DNA microarrays. Using distance calculations, more than 900 genes were identified as kinetically differentially expressed between the butyrate-treated 33 degrees C culture and the untreated culture. Furthermore, transcript levels of the heavy and light chain IgG genes increased following treatment. Using stable isotope labeling (SILAC), the secretion rate of IgG was investigated by tracking the decay of the isotope label upon switching to unlabeled medium. Both treated and untreated cultures exhibited very similar IgG secretion kinetics. In contrast, the intracellular IgG content was found to be elevated following treatment. This result suggests that increased productivity under treatment is attributable to elevated cellular secretory capacity, rather than shorter holding times in the secretory pathway. This hypothesis is further supported by the results of gene set enrichment analysis (GSEA), which revealed that elements of the secretory pathway, including Golgi apparatus, cytoskeleton protein binding and small GTPase-mediated signal transduction are enriched and thus may play a role in the increased recombinant protein production observed under butyrate treatment at 33 degrees C.

Alternative-splicing-based bicistronic vectors for ratio-controlled protein expression and application to recombinant antibody production

In the last decade polycistronic vectors have become essential tools for both basic science and gene therapy applications. In order to co-express heterologous polypeptides, different systems have been developed from Internal Ribosome Entry Site (IRES) based vectors to the use of the 2A peptide. Unfortunately, these methods are not fully suitable for the efficient and reproducible modulation of the ratio between the proteins of interest. Here we describe a novel bicistronic vector type based on the use of alternative splicing. By modifying the consensus sequence that governs splicing, we demonstrate that the ratio between the synthesized proteins could easily vary from 1 : 10 to 10 : 1. We have established this system with luciferase genes and we extended its application to the production of recombinant monoclonal antibodies. We have shown that these vectors could be used in several typical cell lines with similar efficiencies. We also present an adaptation of these vectors to hybrid alternative splicing/IRES constructs that allow a ratio-controlled expression of proteins of interest in stably transfected cell lines.

Mutated polyadenylation signals for controlling expression levels of multiple genes in mammalian cells

A set of mutated SV40 early polyadenylation signals (SV40pA) with varying strengths is generated by mutating the AATAAA sequence in the wild-type SV40pA. They are shown to control the expression level of a gene over a 10-fold range using luciferase reporter genes in transient transfection assays. The relative strength of these SV40pA variants remains similar under three commonly used mammalian promoters and in five mammalian cell lines. Application of SV40pA variants for controlling expression level of multiple genes is demonstrated in a study of monoclonal antibody (mAb) synthesis in mammalian cells. By using SV40pA variants of different strengths, the expression of light chain (LC) and heavy chain (HC) genes encoded in a single vector is independently altered which results in different ratios of LC to HC expression spanning a range from 0.24 to 16.42. The changes in gene expression are determined by measuring mRNA levels and intracellular LC and HC polypeptides. It is found that a substantial decrease of HC expression, which increases the LC/HC mRNA ratio, only slightly reduces mAb production. However, reducing the LC expression by a similar magnitude, which decreases the LC/HC mRNA ratio results in a sharp decline of mAb production to trace amounts. This set of SV40pA variants offers a new tool for accurate control of the relative expression levels of multiple genes. It will have wide-ranging applications in fields related to the study of biosynthesis of multi-subunit proteins, proteomic research on protein interactions, and multi-gene metabolic engineering.

A clone screening method using mRNA levels to determine specific productivity and product quality for monoclonal antibodies

To meet increasing demands for efficient and streamlined production processes of therapeutic antibodies, improved methods of screening clones are required. In this article, we examined the potential of using antibody transcript levels as criteria for clone screening. We evaluated the QuantiGene Plex, a commercially available, high-throughput assay for simultaneously measuring multiple transcripts from cell lysate. Using the development of stable Chinese hamster ovary cell lines as examples, we investigated the relationship between transcript and antibody levels through several rounds of screening. First, we observed that measured heavy chain transcript levels are generally correlated with specific productivity, enabling the identification of high-producing clones from mRNA. Second, we observed that low ratios (< 1.5) of light to heavy chain transcript levels may be indicative of high antibody aggregation levels, allowing for the rapid identification and elimination of clones of questionable product quality. Therefore, an efficient process of identifying high-producing clones of desirable product quality is possible by using QuantiGene Plex assay to measure antibody transcript levels.