Design of Experiment in CHO and HEK transient transfection condition optimization

Transient gene expression (TGE) is a well-established enabling technology for rapid generation of recombinant proteins, with Human Embryonic Kidney (HEK) and Chinese Hamster Ovary (CHO) cell lines and polyethyleneimine (PEI) as the transfection reagent being its most popular components. However, despite considerable progress made in the field, volumetric titers can still be a limiting factor causing the manipulation of increasing quantities of culture media and DNA. Here, we report a systematic analysis of TGE conditions and their influence on yields and protein quality. Guided by Design of Experiments (DoE), we conclude that TGE yields with one test antibody can be maximized by a parallel increase of cell density - 2.4 to 3.0 × 10(6)cells/mL - and PEI concentration - 24 to 30 mg/L - while maintaining a 1:1 ratio of heavy chain and light chain encoding plasmids. Interestingly, we also show that in these conditions, DNA concentration can be maintained in the 1mg/L range, thereby limiting the need for large DNA preparations. Our optimized settings for PEI-mediated TGE in HEK and CHO cells evaluated on several proteins are generally applicable to recombinant antibodies and proteins.     

High-density transient gene expression in suspension-adapted 293 EBNA1 cells

Large-scale transient gene expression (TGE) in mammalian cells is an attractive method to rapidly produce recombinant proteins for pre-clinical studies, with some processes reported to reach 100 L. However, the yield remains low, hardly over 20 mg protein/L, mainly because the current TGEs have been performed at low cell density (approximately 5 x 10(5) cells/mL). In this study, the strategy to improve TGE focuses on facilitating transfection at high cell density. A high-density perfusion culture of 293 EBNA1 cells was established in 2-L bioreactor using Freestyle 293 expression medium (Invitrogen, Singapore) to grow the cells for transfection. Transfection was then carried out at 1 x 10(7) cells/mL using polyethylenimine (PEI) as DNA carrier, at the optimized conditions of 6 microg DNA/10(7) cells and 1:3 DNA to PEI mass ratio. During the post-transfection phase, 80.8 mg/L of the model protein, EPO was obtained at day 5.5 post-transfection (130 mg total EPO production) using a fed-batch culture mode. In comparison, perfusion cultures using an enriched SFM II medium resulted in a longer post-transfection production phase (8 days), and 227 mg of EPO was produced in 10.7 L medium, showing that high-density TGE enables the production of several hundreds of milligrams of protein in a 2 L bioreactor. In addition, a protocol for economical plasmid preparation based on anion exchange was also established to satisfy TGE's demand in terms of quality and quantity. To the best of our knowledge, this is the first report of transient transfections at a high cell density of up to 1 x 10(7) cells/mL.     

Bcl-2 overexpression in CHO cells improves polyethylenimine-mediated gene transfection

Transient gene expression (TGE) in Chinese hamster ovary (CHO) cells with polyethylenimine (PEI) as a transfection reagent has been considered as an attractive method to produce recombinant proteins rapidly for pre-clinical studies. A high level of transfection efficiency, which is required for high-level TGE in CHO cells, can be achieved by increasing the PEI concentration. However, PEI induces cytotoxicity in a dose-dependent manner. To overcome this problem, Bcl-2 protein, an anti-apoptotic protein, was overexpressed in CHO cells (DG44). At a ratio of PEI to DNA (an N/P ratio) of 10, there were no significant differences in transfection efficiency and cell viability between Bcl-2 overexpressing and non-overexpressing cells. The transfection efficiency and cell viability were 2–11% and 83–92%, respectively. However, there were significant differences (P < 0.05) in the transfection efficiency and cell viability between them at a higher N/P ratio. At an N/P ratio of 40, the transfection efficiency and cell viability of Bcl-2 non-overexpressing cells were 24–38% and 35–40%, respectively, while those of Bcl-2 overexpressing cells were 48–53% and 43–56%, respectively. Furthermore, compared with Bcl-2 non-overexpressing cells, more DNAs entered the Bcl-2 overexpressing cells, resulting in a higher rate of TGE per cell. PE-Annexin V apoptosis revealed that Bcl-2 overexpression suppressed PEI-induced apoptotic cell death at high N/P ratios. Taken together, Bcl-2 overexpression in CHO cells suppresses apoptotic cell death during PEI-mediated transient transfection, resulting in enhanced transfection efficiency and TGE.     

Nanoscale characterization coupled to multi-parametric optimization of Hi5 cell transient gene expression

Polyethylenimine (PEI)-based transient gene expression (TGE) is nowadays a well-established methodology for rapid protein production in mammalian cells, but it has been used to a much lower extent in insect cell lines. A fast and robust TGE methodology for suspension Hi5 (Trichoplusia ni) cells is presented. Significant differences in size and morphology of DNA:PEI polyplexes were observed in the different incubation solutions tested. Moreover, minimal complexing time (&lt; 1 min) between DNA and PEI in 150 mM NaCl solution provided the highest transfection efficiency. Nanoscopic characterization by means of cryo-EM revealed that DNA:PEI polyplexes up to 300–400 nm were the most efficient for transfection. TGE optimization was performed using eGFP as model protein by means of the combination of advanced statistical designs. A global optimal condition of 1.5 × 10⁶ cell/mL, 2.1 μg/mL of DNA, and 9.3 μg/mL PEI was achieved through weighted-based optimization of transfection, production, and viability responses. Under these conditions, a 60% transfection and 0.8 μg/10⁶ transfected cell·day specific productivity were achieved. The TGE protocol developed for Hi5 cells provides a promising baculovirus-free and worthwhile approach to produce a wide variety of recombinant proteins in a short period of time.

     

Establishment and characterization of conditions required for tumor colonization by intravenously delivered bacteria

Transient gene expression (TGE) provides a method for quickly delivering protein for research using mammalian cells. While high levels of recombinant proteins have been produced in TGE experiments in HEK 293 cells, TGE efforts in the commercially prominent CHO cell line still suffer from inadequate protein yields. Here, we describe a cell-engineering strategy to improve transient production of proteins using CHO cells. CHO-DG44 cells were engineered to overexpress the anti-apoptotic protein Bcl-x(L) and transiently transfected using polyethylenimine (PEI) in serum-free media. Pools and cell lines stably expressing Bcl-x(L) showed enhanced viable cell density and increased production of a glycosylated, therapeutic fusion protein in shake flask TGE studies. The improved cell lines showed fusion protein production levels ranging from 12.6 to 27.0 mg/L in the supernatant compared to the control cultures which produced 6.3-7.3 mg/L, representing a 70-270% increase in yield after 14 days of fed-batch culture. All Bcl-xL-expressing cell lines also exhibited an increase in specific productivity during the first 8 days of culture. In addition to increased production, Bcl-x(L) cell lines maintained viabilities above 90% and less apoptosis compared to the DG44 host which had viabilities below 60% after 14 days. Product quality was comparable between a Bcl-xL-engineered cell line and the CHO host. The work presented here provides the foundation for using anti-apoptosis engineered CHO cell lines for increased production of therapeutic proteins in TGE applications.     

Serum-free large-scale transient transfection of CHO cells

To date, methods for large-scale transient gene expression (TGE) in cultivated mammalian cells have focused on two transfection vehicles: polyethylenimine (PEI) and calcium phosphate (CaPi). Both have been shown to result in high transfection efficiencies at scales beyond 10 L. Unfortunately, both approaches yield higher levels of recombinant protein (r-protein) in the presence of serum than in its absence. Since serum is a major cost factor and an obstacle to protein purification, our goal was to develop a large-scale TGE process for Chinese hamster ovary (CHO) cells in the absence of serum. CHO-DG44 cells were cultivated and transfected in a chemically defined medium using linear 25 kDa PEI as a transfection vehicle. Parameters that were optimized included the DNA amount, the DNA-to-PEI ratio, the timing and solution conditions for complex formation, the transfection medium, and the cell density at the time of transfection. The highest levels of r-protein expression were observed when cultures at a density of 2.0 x 10(6) cells/ml were transfected with 2.5 microg/ml DNA in RPMI 1640 medium containing 25 mM HEPES at pH 7.1. The transfection complex was formed at a DNA:PEI ratio of 1:2 (w/w) in 150 mM NaCl with a 10-min incubation at room temperature prior to addition to the culture. The procedure was scaled up for a 20-L bioreactor, yielding expression levels of 10     

Optimization of a transient antibody expression platform towards high titer and efficiency

Transient gene expression (TGE) using mammalian cells is an extensively used technology for the production of antibodies and recombinant proteins and has been widely adopted by both academic and industrial labs. Chinese Hamster Ovary (CHO) cells have become one of the major workhorses for TGE of recombinant antibodies due to their attractive features: post-translational modifications, adaptation to high cell densities, and use of serum-free media. In this study, we describe the optimization of parameters for TGE for antibodies from CHO cells. Through a matrix evaluation of multiple factors including inoculum, transfection conditions, amount and type of DNA used, and post-transfection culture conditions, we arrived at an uniquely optimized process with higher titer and reduced costs and time, thus increasing the overall efficiency of early antibody material supply. We further investigated the amount of coding DNA used in TGE and the influence of kinetics and size of the transfection complex on the in vitro efficiency of the transfection. We present here the first report of an optimized TGE platform using Filler DNA in an early drug discovery setting for the screening and production of therapeutic mAbs.     

High cell density transient transfection of CHO cells for TGF‐β1 expression

High cell densities for transient transfection with polyethyleneimine (PEI) can be used for rapid and maximal production of recombinant proteins. High cell densities can be obtained by different cultivation systems, such as batch or perfusion systems. Herein, densities up to 18 million cells/mL were obtained by centrifugation for transfection evaluation. PEI transfection efficiency was easily determined by transfected enhanced green fluorescence protein (EGFP) reporter plasmid DNA (pDNA). A linear correlation between fluorescence intensity and transfection efficiency was improved. The transfection efficiency of PEI was highly dependent on the transfection conditions and directly related to the level of recombinant protein. Several factors were required to optimize the transient transfection process; these factors included the media type (which is compatible with low or high cell density transfection), the preculture CHO‐K1 suspension cell density, and the pDNA to PEI level. Based on design of experiment (DoE) analyses, the optimal transfection conditions for 10 × 10⁶ cells/mL in the CHOMACS CD medium achieved 73% transfection efficiency and a cell viability of over 80%. These results were confirmed for the production of transforming growth factor‐beta 1 (TGF‐β1) in a shake flask. The purified TGF‐β1 protein concentration from 60 mL supernatant was 27 µg/mL, and the protein was biologically active.     

Effects of the dilution rate on cell cycle distribution and PEI-mediated transient gene expression by CHO cells in continuous culture

In this study, a continuous culture system was applied to mammalian cells on large scale, and polyethyleneimine (PEI) mediated transient gene expression (TGE). PEI MAX 40,000 was chosen as a superior reagent from three types of PEI. The cell cycle distribution of cells in batch and continuous cultures was determined, in which the effects of cell cycle distribution on transfection efficiency, post-transfection proliferation and recombinant prothrombin expression were evaluated. Compared with cells from end-log and plateau phase in batch culture, cells from mid-log phase possessed a larger fraction of S and G2/M phase cells and a smaller fraction of G1 phase cells. In the continuous culture, the fraction of cells in the S and G2/M phases increased and the fraction of cells in the G1/G0 phase decreased with increasing dilution rates. Cells from the continuous culture run at highest dilution rate had excellent proliferation, transfection efficiency and protein expression. These results were confirmed by transfecting cells synchronized to different phases. The G2/M arrested cells exhibited a nearly 10-fold increase in recombinant human prothrombin production relative to that of non-dividing cells. The use of continuous culture for large scale transfection demonstrated a better cell physiological state for TGE process.     

Hyperosmolarity enhances transient recombinant protein yield in Chinese hamster ovary cells

The effect of hyperosmolarity on transient recombinant protein production in Chinese hamster ovary (CHO) cells was investigated. Addition of 90 mM NaCl to the production medium ProCHO5 increased the volumetric yield of recombinant antibody up to 4-fold relative to transfection in ProCHO5 alone. Volumetric yields up to 50 mg l⁻¹ were achieved in a 6 day batch culture of 3 l. In addition, hyperosmolarity reduced cell growth and increased cell size. The addition of salt to cultures of transiently transfected CHO cells is a simple and cost-effective method to increase TGE yields in this host.     

A CHO cell line engineered to express XBP1 and ERO1‐Lα has increased levels of transient protein expression

Transient gene expression (TGE) systems currently provide rapid and scalable (up to 100 L) methods for generating multigram quantities of recombinant heterologous proteins. Product titers of up to 1 g/L have been demonstrated in HEK293 cells 1 but reported yields from Chinese hamster ovary (CHO) cells are lower at ～300 mg/L. 2 We report on the establishment of an engineered CHOS cell line, which has been developed for TGE. This cell line has been engineered to express both X‐box binding protein (XBP‐1S) and endoplasmic reticulum oxidoreductase (ERO1‐Lα) and has been named CHOS‐XE. CHOS‐XE cells produced increased antibody (MAb) yields (5.3– 6.2 fold) in comparison to CHOS cells. Product quality was unchanged as assessed by size, charge, propensity to aggregate, major glycosylation species, and thermal stability. To further develop and test this TGE system, five commercial media were assessed, and one was shown to offer the greatest increase in antibody yields. With the addition of a commercial feed, MAb titers reached 875 mg/L. © 2013 American Institute of Chemical Engineers Biotechnol. Prog., 29:697–706, 2013     

Serum-free suspensin large-scale transient transfection of CHO cells in WAVE bioreactors

Here, we report the development of a large-scale transient expression platform utilizing Chinese hamster ovary (CHO) cells. The majority of recombinant proteins and antibodies that are produced for preclinical models and clinical trials are expressed in stably transfected CHO cells. A protocol for transient transfection of CHO cells that is rapid, reproducible, and cost-effective would therefore streamline the process from research to development and help avoid any potential host species induced variation in the molecule of interest. CHO cells were adapted to grow in serum-free suspension conditions in spinner flask cultures in a proprietary in-house developed growth medium. In developing this transient transfection protocol, the parameters optimized included the transfection reagent of choice, the cell density at the time of transfection, the plasmid DNA concentration, and the transfection reagent concentration. Using this optimized protocol, we have expressed recombinant proteins, including antibodies, at an expression level of up to 9.4 mg/L. We also report transient transfections from 500 mL working volume (w.v.) up to 20 L w.v. in a WAVE^TM bioreactor. Using this optimized protocol, it is possible to rapidly (within 10 d) produce up to 100 mg of recombinant protein for further study.     

Control of culture environment for improved polyethylenimine-mediated transient production of recombinant monoclonal antibodies by CHO cells

In this study we describe optimization of polyethylenimine (PEI)-mediated transient production of recombinant protein by CHO cells by facile manipulation of a chemically defined culture environment to limit accumulation of nonproductive cell biomass, increase the duration of recombinant protein production from transfected plasmid DNA, and increase cell-specific production. The optimal conditions for transient transfection of suspension-adapted CHO cells using branched, 25 kDa PEI as a gene delivery vehicle were experimentally determined by production of secreted alkaline phosphatase reporter in static cultures and recombinant IgG4 monoclonal antibody (Mab) production in agitated shake flask cultures to be a DNA concentration of 1.25 microg 10(6) cells(-1) mL(-1) at a PEI nitrogen:DNA phosphate ratio of 20:1. These conditions represented the optimal compromise between PEI cytotoxicity and product yield with most efficient recombinant DNA utilization. Separately, both addition of recombinant insulin-like growth factor (LR3-IGF) and a reduction in culture temperature to 32 degrees C were found to increase product titer 2- and 3-fold, respectively. However, mild hypothermia and LR3-IGF acted synergistically to increase product titer 11-fold. Although increased product titer in the presence of LR3-IGF alone was solely a consequence of increased culture duration, a reduction in culture temperature post-transfection increased both the integral of viable cell concentration (IVC) and cell-specific Mab production rate. For cultures maintained at 32 degrees C in the presence of LR3-IGF, IVC and qMab were increased 4- and 2.5-fold, respectively. To further increase product yield from transfected DNA, the duration of transgene expression in cell populations maintained at 32 degrees C in the presence of LR3-IGF was doubled by periodic resuspension of transfected cells in fresh media, leading to a 3-fold increase in accumulated Mab titer from approximately 13 to approximately 39 mg L(-1). Under these conditions, Mab glycosylation at Asn297 remained essentially constant and similar to that of the same Mab produced by stably transfected GS-CHO cells. From these data we suggest that the efficiency of transient production processes (protein output per rDNA input) can be significantly improved using a combination of mild hypothermia and growth factor(s) to yield an extended "activated hypothermic synthesis".     

Novel orbital shake bioreactors for transient production of CHO derived IgGs

Large-scale transient gene expression in mammalian cells is being developed for the rapid production of recombinant proteins for biochemical and preclinical studies. Here, the scalability of transient production of a recombinant human antibody in Chinese hamster ovary (CHO) cells was demonstrated in orbitally shaken disposable bioreactors at scales from 50 mL to 50 L. First, a small-scale multiparameter approach was developed to optimize the poly(ethylenimine)-mediated transfection in 50 mL shake tubes. This study confirmed the benefit, both in terms of extended cell culture viability and increased product yield, of mild hypothermic cultivation conditions for transient gene expression in CHO cells. Second, the scalability of the process was demonstrated in disposable shake bioreactors having nominal volumes of 5, 20, and 50 L with final antibody yields between 30 and 60 mg L(-1). Thus, the combination of transient gene expression with disposable shake bioreactors allows for rapid and cost-effective production of recombinant proteins in CHO cells.     

Transient gene expression: recombinant protein production with suspension-adapted HEK293-EBNA cells

Transient gene expression (TGE) in mammalian cells at the reactor scale is becoming increasingly important for the rapid production of recombinant proteins. We improved a process for transient calcium phosphate-based transfection of HEK293-EBNA cells in a 1-3 L bioreactor volume. Cells were adapted to suspension culture using a commercially available medium (BioWhittaker, Walkersville, MD). Process parameters were optimized using a plasmid reporter vector encoding the enhanced green fluorescent protein (EGFP/CLONTECH, Palo Alto, CA, USA). Using GFP as a marker-protein, we observed by microscopic examination transfection efficiencies between 70-100%. Three different recombinant proteins were synthesized within a timeframe of 7 days from time of transfection to harvest. The first, a human recombinant IgG(1)-type antibody, was secreted into the supernatant of the cell culture and achieved a final concentration of >20 mg/L. An E. coli-derived DNA-binding protein remained intracellular, as expected, but accumulated to such a concentration that the lysate of cells, taken up into the entire culture volume, gave a concentration of 18 mg/L. The third protein, a transmembrane receptor, was expressed at 3-6 x 10(6) molecules/cell.     

Gadd45-induced cell cycle G2/M arrest for improved transient gene expression in Chinese hamster ovary cells

Gadd45 is a p53-regulated protein and is involved in cell cycle arrest in the G2/M phase. In an effort to improve transient gene expression (TGE) in Chinese hamster ovary (CHO) cells, the effect of Gadd45-induced cell cycle arrest on TGE in CHO cells was investigated using the two different expression vectors encoding Fcfusion protein and recombinant antibody. To regulate the expression of Gadd45 in CHO cells, the CHO-TREx-gadd45 cell line was established using the T-REx system controlled by doxycycline. During the cultures for TGE, Gadd45 overexpression severely inhibited cell growth, but significantly enhanced TGE. Compared with the culture without Gadd45 overexpression, the TGE of Fc fusion protein and humanized antibody were increased by 111 and 93%, respectively. The enhanced TGE, despite the cell growth arrest induced by Gadd45 overexpression, was due to the significantly increased specific productivity, resulting from enhanced transfection efficiency, increased cell size, and active DNA demethylation. Taken together, the data obtained here demonstrate that Gadd45-induced cell cycle arrest in G2/M phase can significantly enhance TGE in CHO cells.     

Enhancement of DNA uptake in FUT8‐deleted CHO cells for transient production of afucosylated antibodies

Removal of the core α1,6 fucose from the glycans in the Fc region of IgG1 antibodies has been demonstrated to improve antibody‐dependent cellular cytotoxicity (ADCC) activity. In order to produce afucosylated antibodies using transient transfection, a FUT8 knockout (FUT8KO) cell line was generated in a CHO host cell line using the zinc finger nuclease technology. Transient transfection of DNA into mammalian cells using the cationic polymer, polyethylenimine (PEI), is commonly used for rapid generation of recombinant proteins. FUT8KO cells evaluated in PEI transfections yielded lower titers than parental CHO WT cells. FACS and HPLC analyses revealed that the FUT8KO cells had lower cell surface heparan sulfate (HS) levels than CHO WT. Removal of cell surface HS resulted in reduced uptake of PEI‐transfected DNA (PEI:DNA) and lower transfection titers suggesting that PEI:DNA relies on HS for binding and cellular entry. The absence of cell surface HS did not severely impact transfections performed with cationic liposomes. We undertook two approaches to improve transient production of afucosylated antibodies. First, we evaluated transfection of FUT8KO cells with cationic liposomes, which were observed to be less dependent on HS levels for uptake. Transfection of FUT8KO cells using the cationic liposome, DMRIE‐C, produced similar titers to CHO WT in both shake flask and large‐scale 10 L bioreactors. The second approach was to engineer a cell line overexpressing exostosin‐1 (EXT1), an enzyme responsible for HS chain elongation, to increase HS content. EXT1‐FUT8KO and CHO WT cells produced comparable levels of antibody from PEI transfections. Biotechnol. Bioeng. 2010;106: 751–763. © 2010 Wiley Periodicals, Inc.