Accumulative scFv-Fc antibody gene integration into the hprt chromosomal locus of Chinese hamster ovary cells

The new frontier in CHO cell line development: From random to targeted transgene integration technologies

Cell line development represents a crucial step in the development process of a therapeutic glycoprotein. Chinese hamster ovary (CHO) cells are the most frequently employed mammalian host cell system for the industrial manufacturing of biologics. The predominant application of CHO cells for heterologous recombinant protein expression lies in the relative simplicity of stably introducing ectopic DNA into the CHO host cell genome. Since CHO cells were first used as expression host for the industrial production of biologics in the late 1980s, stable genomic transgene integration has been achieved almost exclusively by random integration. Since then, random transgene integration had become the gold standard for generating stable CHO production cell lines due to a lack of viable alternatives. However, it was eventually demonstrated that this approach poses significant challenges on the cell line development process such as an increased risk of inducing cell line instability. In recent years, significant discoveries of new and highly potent (semi)-targeted transgene integration systems have paved the way for a technological revolution in the cell line development sector. These advanced methodologies comprise the application of transposase-, recombinase- or Cas9 nuclease-mediated site-specific genomic integration techniques, which enable a scarless transfer of the transgene expression cassette into transcriptionally active loci within the host cell genome. This review summarizes recent advancements in the field of transgene integration technologies for CHO cell line development and compare them to the established random integration approach. Moreover, advantages and limitations of (semi)-targeted integration techniques are discussed, and benefits and opportunities for the biopharmaceutical industry are outlined.

Inducible transgene expression in CHO cells using an artificial transcriptional activator with estrogen-binding domain

Biopharmaceuticals, including therapeutic antibodies, are rapidly growing products in the pharmaceutical market. Mammalian cells, such as Chinese hamster ovary (CHO) cells, are widely used as production hosts because recombinant antibodies require complex three-dimensional structures modified with sugar chains. Recombinant protein production using mammalian cells is generally performed with cell growth. In this study, we developed a technology that controls cell growth and recombinant protein production to induce recombinant protein production with predetermined timing. Expression of green fluorescent protein (GFP) gene and a single-chain antibody fused with the Fc-region of the human IgG1 (scFv-Fc) gene can be induced and mediated by the estrogen receptor-based artificial transcription factor Gal4-ERT2-VP16 and corresponding inducer drugs. We generated CHO cells using an artificial gene expression system. The addition of various concentrations of inducer drugs to the culture medium allowed control of proliferation and transgene expression of the engineered CHO cells. Use of 4-hydroxytamoxifen, an antagonist of estrogen, as an inducing agent yielded high gene expression at a concentration more than 10-fold lower than that of β-estradiol. When scFv-Fc was produced under inducing conditions, continuous production was possible for more than 2 weeks while maintaining high specific productivity (57 pg cell-1 day-1 ). This artificial gene expression control system that utilizes the estrogen response of estrogen receptors can be an effective method for inducible production of biopharmaceuticals.

CRISPR-Cas9 Mediated Stable Expression of Exogenous Proteins in the CHO Cell Line through Site-Specific Integration

Chinese hamster ovary (CHO) cells are a popular choice in biopharmaceuticals because of their beneficial traits, including high-density suspension culture, safety, and exogenously produced proteins that closely resemble natural proteins. Nevertheless, a decline in the expression of exogenous proteins is noted as culture time progresses. This is a consequence of foreign gene recombination into chromosomes by random integration. The current investigation employs CRISPR-Cas9 technology to integrate foreign genes into a particular chromosomal location for sustained expression. Results demonstrate the successful integration of enhanced green fluorescent protein (EGFP) and human serum albumin (HSA) near base 434814407 on chromosome NC_048595.1 of CHO-K1 cells. Over 60 successive passages, monoclonal cell lines were produced that consistently expressed all relevant external proteins without discernible variation in expression levels. In conclusion, the CHO-K1 cell locus, NC_048595.1, proves an advantageous locus for stable exogenous protein expression. This study provides a viable approach to establishing a CHO cell line capable of enduring reliable exogenous protein expression.

High-Level Production of scFv-Fc Antibody Using an Artificial Promoter System with Transcriptional Positive Feedback Loop of Transactivator in CHO Cells

With the increasing demand for therapeutic antibodies, CHO cells have become the de facto standard as producer host cells for biopharmaceutical production. High production yields are required for antibody production, and developing a high-titer production system is increasingly crucial. This study was established to develop a high-production system using a synthetic biology approach by designing a gene expression system based on an artificial transcription factor that can strongly induce the high expression of target genes in CHO cells. To demonstrate the functionality of this artificial gene expression system and its ability to induce the high expression of target genes in CHO cells, a model antibody (scFv-Fc) was produced using this system. Excellent results were obtained with the plate scale, and when attempting continuous production in semi-continuous cultures using bioreactor tubes with high-cell-density suspension culture using a serum-free medium, high-titer antibody production at the gram-per-liter level was achieved. Shifting the culture temperature to a low temperature of 33 °C achieved scFv-Fc concentrations of up to 5.5 g/L with a specific production rate of 262 pg/(cell∙day). This artificial gene expression system should be a powerful tool for CHO cell engineering aimed at constructing high-yield production systems.

CRISPR-interceded CHO cell line development approaches

For industrial production of recombinant protein biopharmaceuticals, Chinese hamster ovary (CHO) cells represent the most widely adopted host cell system, owing to their capacity to produce high-quality biologics with human-like posttranslational modifications. As opposed to random integration, targeted genome editing in genomic safe harbor sites has offered CHO cell line engineering a new perspective, ensuring production consistency in long-term culture and high biotherapeutic expression levels. Corresponding the remarkable advancements in knowledge of CRISPR-Cas systems, the use of CRISPR-Cas technology along with the donor design strategies has been pushed into increasing novel scenarios in cell line engineering, allowing scientists to modify mammalian genomes such as CHO cell line quickly, readily, and efficiently. Depending on the strategies and production requirements, the gene of interest can also be incorporated at single or multiple loci. This review will give a gist of all the most fundamental recent advancements in CHO cell line development, such as different cell line engineering approaches along with donor design strategies for targeted integration of the desired construct into genomic hot spots, which could ultimately lead to the fast-track product development process with consistent, improved product yield and quality.

Cell Engineering and Cultivation of Chinese Hamster Ovary Cells for the Development of Orthogonal Eukaryotic Cell-free Translation Systems

The investigation of protein structures, functions and interactions often requires modifications to adapt protein properties to the specific application. Among many possible methods to equip proteins with new chemical groups, the utilization of orthogonal aminoacyl-tRNA synthetase/tRNA pairs enables the site-specific incorporation of non-canonical amino acids at defined positions in the protein. The open nature of cell-free protein synthesis reactions provides an optimal environment, as the orthogonal components do not need to be transported across the cell membrane and the impact on cell viability is negligible. In the present work, it was shown that the expression of orthogonal aminoacyl-tRNA synthetases in CHO cells prior to cell disruption enhanced the modification of the pharmaceutically relevant adenosine A2a receptor. For this purpose, in complement to transient transfection of CHO cells, an approach based on CRISPR/Cas9 technology was selected to generate a translationally active cell lysate harboring endogenous orthogonal aminoacyl-tRNA synthetase.

Generation of a Host Cell line containing a MAR-rich landing pad for site-specific integration and expression of transgenes

In recent years, targeted gene integration (TI) has been introduced as a strategy for the generation of recombinant mammalian cell lines for the production of biotherapeutics. Besides reducing the immense heterogeneity within a pool of recombinant transfectants, TI also aims at shortening the duration of the current cell line development process. Here we describe the generation of a host cell line carrying Matrix‐Attachment Region (MAR)‐rich landing pads (LPs), which allow for the simultaneous and site‐specific integration of multiple genes of interest (GOIs). We show that several copies of each chicken lysozyme 5'MAR‐based LP containing either BxB1 wild type or mutated recombination sites, integrated at one random chromosomal locus of the host cell genome. We further demonstrate that these LP‐containing host cell lines can be used for the site‐specific integration of several GOIs and thus, generation of transgene‐expressing stable recombinant clones. Transgene expression was shown by site‐specific integration of heavy and light chain genes coding for a monospecific antibody (msAb) as well as for a bi‐specific antibody (bsAb). The genetic stability of the herein described LP‐based recombinant clones expressing msAb or bsAb was demonstrated by cultivating the recombinant clones and measuring antibody titers over 85 generations. We conclude that the host cell containing multiple copies of MAR‐rich landing pads can be successfully used for stable expression of one or several GOIs.

How to train your cell - Towards controlling phenotypes by harnessing the epigenome of Chinese hamster ovary production cell lines

Recent advances in omics technologies and the broad availability of big datasets have revolutionized our understanding of Chinese hamster ovary cells in their role as the most prevalent host for production of complex biopharmaceuticals. In consequence, our perception of this "workhorse of the biopharmaceutical industry" has successively shifted from that of a nicely working, but unknown recombinant protein producing black box to a biological system governed by multiple complex regulatory layers that might possibly be harnessed and manipulated at will. Despite the tremendous progress that has been made to characterize CHO cells on various omics levels, our understanding is still far from complete. The well-known inherent genetic plasticity of any immortalized and rapidly dividing cell line also characterizes CHO cells and can lead to problematic instability of recombinant protein production. While the high mutational frequency has been a focus of CHO cell research for decades, the impact of epigenetics and its role in differential gene expression has only recently been addressed. In this review we provide an overview about the current understanding of epigenetic regulation in CHO cells and discuss its significance for shaping the cell's phenotype. We also look into current state-of-the-art technology that can be applied to harness and manipulate the epigenetic network so as to nudge CHO cells towards a specific phenotype. Here, we revise current strategies on site-directed integration and random as well as targeted epigenome modifications. Finally, we address open questions that need to be investigated to exploit the full repertoire of fine-tuned control of multiplexed gene expression using epigenetic and systems biology tools.

Novel transgenic Chlamydomonas reinhardtii strain with retargetable genomic transgene integration using Cre-loxP system

The use of Chlamydomonas for biofuel and biopharmaceutical production has been anticipated. However, the genetic engineering technology for Chlamydomonas is not as advanced as that for other organisms. Here, we established transgenic Chlamydomonas strains capable of high and stable transgene expression. The established cells exhibited stable reporter gene expression at a high level throughout long-term culture (∼60 days), even in the absence of drug pressure. The transgene insertion sites in the cell genome that may be suitable for exogenous gene expression were identified. Because the transgene contains a loxP site, the cells can be used as founders for retargeting other transgenes using the Cre-loxP system to generate transgenic Chlamydomonas producing useful substances. As a model biopharmaceutical gene, an interferon expression cassette was integrated into the genomic locus of the cells using Cre recombinase. The transgenic cells stably produced interferon protein in medium for 12 passages under non-selective conditions. These results indicate that the Chlamydomonas cells established in this study can serve as valuable and powerful tools not only for basic research on microalgae but also for the rapid establishment of cell lines expressing exogenous genes.

LINE-1 vectors mediate recombinant antibody gene transfer by retrotransposition in Chinese hamster ovary cells

Retrotransposons, such as long interspersed element-1 (LINE-1), can copy themselves to other genomic loci via a transposition event (termed retrotransposition). Retrotransposons, therefore, have potential use as an efficient gene delivery tool to integrate multiple copies of a target gene into a host genome. Here, we developed a retrotransposon vector based on LINE-1 that achieves target gene integration of multiple transgene copies. The retrotransposon vector contains a neomycin resistance gene split by an intron as a marker gene, and a gene encoding an antibody single-chain variable fragment (Fv) fused with the constant antibody region (Fc) (scFv-Fc) as a model target gene. G418-resistant Chinese hamster ovary cells were generated using this retrotransposon vector, and scFv-Fc was produced in the culture medium. To regulate retrotransposition, we developed a retrotransposon vector system that separately expressed the two open reading frames (ORF1 and ORF2) of LINE-1. Genomic PCR analysis detected the transgene sequence in almost all tested clones. Compared with clones established using the intact LINE-1 vector, clones generated with the split ORF1 and ORF2 system showed similar specific scFv-Fc productivity and retrotransposition efficiency. This approach of using a retrotransposon-based vector system has the potential to provide a new gene delivery tool for mammalian cells.

Development of a targeted integration Chinese hamster ovary host directly targeting either one or two vectors simultaneously to a single locus using the Cre/Lox recombinase-mediated cassette exchange system

Cell line development (CLD) by random integration (RI) can be labor intensive, inconsistent, and unpredictable due to uncontrolled gene integration after transfection. Unlike RI, targeted integration (TI) based CLD introduces the antibody-expressing cassette to a predetermined site by recombinase-mediated cassette exchange (RMCE). The key to success for the development of a TI host for therapeutic antibody production is to identify a transcriptionally active hotspot that enables highly efficient RMCE and antibody expression with good stability. In this study, a genome wide search for hotspots in the Chinese hamster ovary (CHO)-K1-M genome by either RI or PiggyBac (PB) transposase-based integration has been described. Two CHO-K1-M derived TI host cells were established with the Cre/Lox RMCE system and are described here. Both TI hosts contain a GFP-expressing landing pad flanked by two incompatible LoxP recombination sites (L3 and 2L). In addition, a third incompatible LoxP site (LoxFAS) is inserted in the GFP landing pad to enable an innovative two-plasmid based RMCE strategy, in which two separate vectors can be targeted to a single locus simultaneously. Cell lines generated by the TI system exhibit comparable or higher productivity, better stability and fewer sequence variant (SV) occurrences than the RI cell lines.

Multicopy Targeted Integration for Accelerated Development of High-Producing Chinese Hamster Ovary Cells

The ever-growing biopharmaceutical industry relies on the production of recombinant therapeutic proteins in Chinese hamster ovary (CHO) cells. The traditional timelines of CHO cell line development can be significantly shortened by the use of targeted gene integration (TI). However, broad use of TI has been limited due to the low specific productivity (q_P) of TI-generated clones. Here, we show a 10-fold increase in the q_P of therapeutic glycoproteins in CHO cells through the development and optimization of a multicopy TI method. We used a recombinase-mediated cassette exchange (RMCE) platform to investigate the effect of gene copy number, 5' and 3' gene regulatory elements, and landing pad features on q_P. We evaluated the limitations of multicopy expression from a single genomic site as well as multiple genomic sites and found that a transcriptional bottleneck can appear with an increase in gene dosage. We created a dual-RMCE system for simultaneous multicopy TI in two genomic sites and generated isogenic high-producing clones with q_P of 12-14 pg/cell/day and product titer close to 1 g/L in fed-batch. Our study provides an extensive characterization of the multicopy TI method and elucidates the relationship between gene copy number and protein expression in mammalian cells. Moreover, it demonstrates that TI-generated CHO cells are capable of producing therapeutic proteins at levels that can support their industrial manufacture.

Secretion analysis of intracellular "difficult-to-express" immunoglobulin G (IgG) in Chinese hamster ovary (CHO) cells

The Chinese hamster ovary (CHO) cell line is the most widely used host cell for therapeutic antibody production. Although its productivity has been improved by various strategies to satisfy the growing global demand, some difficult-to-express (DTE) antibodies remain at low secretion levels. To improve the production of various therapeutic antibodies, it is necessary to determine possible rate-limiting steps in DTE antibody secretion in comparison with other high IgG producers. Here, we analyzed the protein secretion process in CHO cells producing the DTE immunoglobulin G (IgG) infliximab. The results from chase assays using a translation inhibitor revealed that infliximab secretion could be nearly completed within 2 h, at which time the cells still retained about 40% of heavy chains and 65% of light chains. Using fluorescent microscopy, we observed that these IgG chains remained in the endoplasmic reticulum and Golgi apparatus. The cells inefficiently form fully assembled heterodimer IgG by making LC aggregates, which may be the most serious bottleneck in the production of DTE infliximab compared with other IgG high producers. Our study could contribute to establish the common strategy for constructing DTE high-producer cells on the basis of rate-limiting step analysis.

Site-specific Integration Ushers in a New Era of Precise CHO Cell Line Engineering

Chinese hamster ovary (CHO) cells are widely used for the production of therapeutic proteins. Customarily, CHO production cell lines are established through random integration, which requires laborious screening of many clones to isolate suitable producers. In contrast, site-specific integration (SSI) accelerates cell line development by targeting integration of transgenes to pre-validated genomic loci capable of supporting high and stable expression. To date, a relatively small number of these so called 'hot spots' have been identified, mainly through empirical methods. Nevertheless, nuclease-mediated and recombinase-mediated SSI have revolutionized cell line engineering by enabling rational and reproducible transgene targeting.

Rapid development of stable transgene CHO cell lines by CRISPR/Cas9-mediated site-specific integration into C12orf35

Chinese hamster ovary (CHO) cells are the most widely used mammalian hosts for recombinant protein production. However, by conventional random integration strategy, development of a high-expressing and stable recombinant CHO cell line has always been a difficult task due to the heterogenic insertion and its caused requirement of multiple rounds of selection. Site-specific integration of transgenes into CHO hot spots is an ideal strategy to overcome these challenges since it can generate isogenic cell lines with consistent productivity and stability. In this study, we investigated three sites with potential high transcriptional activities: C12orf35, HPRT, and GRIK1, to determine the possible transcriptional hot spots in CHO cells, and further construct a reliable site-specific integration strategy to develop recombinant cell lines efficiently. Genes encoding representative proteins mCherry and anti-PD1 monoclonal antibody were targeted into these three loci respectively through CRISPR/Cas9 technology. Stable cell lines were generated successfully after a single round of selection. In comparison with a random integration control, all the targeted integration cell lines showed higher productivity, among which C12orf35 locus was the most advantageous in both productivity and cell line stability. Binding affinity and N-glycan analysis of the antibody revealed that all batches of product were of similar quality independent on integrated sites. Deep sequencing demonstrated that there was low level of off-target mutations caused by CRISPR/Cas9, but none of them contributed to the development process of transgene cell lines. Our results demonstrated the feasibility of C12orf35 as the target site for exogenous gene integration, and strongly suggested that C12orf35 targeted integration mediated by CRISPR/Cas9 is a reliable strategy for the rapid development of recombinant CHO cell lines.

Targeted knock-in of an scFv-Fc antibody gene into the hprt locus of Chinese hamster ovary cells using CRISPR/Cas9 and CRIS-PITCh systems

Chinese hamster ovary (CHO) cells have been used as host cells for the production of pharmaceutical proteins. For the high and stable production of target proteins, the transgene should be integrated into a suitable genomic locus of host cells. Here, we generated knock-in CHO cells, in which transgene cassettes without a vector backbone sequence were integrated into the hprt locus of the CHO genome using clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 and CRISPR-mediated precise integration into target chromosome (CRIS-PITCh) systems. We investigated the efficiency of targeted knock-in of transgenes using these systems. As a practical example, we generated knock-in CHO cells producing an scFv-Fc antibody using the CRIS-PITCh system mediated by microhomology sequences for targeting. We found that the CRIS-PITCh system can facilitate targeted knock-in for CHO cell engineering.