Chinese Hamster Ovary (CHO) cells have emerged as the gold standard in the production of biologics. Their unique combination of beneficial quality characteristics make them the ideal cell line to produce many a protein in. Amongst other highly useful factors, the fact that they are easy to grow in large scale cell culture under defined conditions and allow human biosimilar post-translational modifications are most significant and render CHO cells a powerful bioprocess tool.
A Hamster History
The road that led to the modern-day cellular protein factory powerhouse that CHO cells are was far from straight forward. The history of the CHO cells we know today started in the beginning of the previous century and involves an American physician fleeing with a cage of hamsters from Mao Tse-Tung’s communist forces in the Chinese civil war, multiple severe mutagenic modifications and a career as the star of several different research fields.
Chinese hamsters (Cricetulus griseus) were gaining popularity in the 1920s as a medical research tool in China, as they were abundantly found in the fields and easily inoculated with a variety of maladies in research focus at that time, from pneumonia and tuberculosis to diphtheria and black fever. The animal soon became a popular disease model in European and North American research labs as well, albeit difficult to be bred in captivity. In the 1940s, Chinese hamsters started a career in genetics, when their low chromosome number (n=11 – although initially only 7 were detected) made them the ideal tool to study chromatic alterations in a time when DNA was studied by relatively poor resolution light microscopy of metaphase spreads.
In the 1950s it became increasingly obvious that the main roadblock for advances in human and animal genetics was the lack of mammalian cell lines. Initial first successes with human (the famous HeLa cells) and mouse cell lines made the Chinese hamster with its compact genome an ideal target to generate cell lines. In 1957, Theodore Puck, who previously successfully managed to isolate and propagate clonal HeLa cultures, obtained a single female Chinese hamster. He isolated the ovaries and grew extracted cells, later to be shown of fibroblastic lineage, in culture. The CHO cells were born.
Shortly after, Puck and his junior colleague Fa-Ten Kao, sub-cloned the hamster cells and generated the CHO-K1 cell line, which would become a standard of mammalian cell culture in the decades to come. Puck himself used to call the cells “the mammalian E. coli”.
Figure 1: The 22 chromosomes of the Chinese hamster and of the 21 chromosomes of CHO-K1 as identified by G-banding techniques. From F.M. Wurm (2013)
But the climax of the career of CHO cells was yet to come. After the initial success of insulin as the first recombinant protein for therapeutic use in the 70s, the initial euphoria surrounding protein therapeutics gradually wore off when it became clear that its success wasn’t easily reproduced. Many protein biologics rely on post-translational modifications that cannot be obtained by production in bacterial cell culture and initial attempts with mammalian cells had resulted in disappointingly low protein yields.
CHO cells were about to change that. A combination of discoveries and inventions led to CHO cells that would be the workhorse for decades for production of biologics. Chasin and Urlaub first generated dihydrofolate reductase (DHFR)-deficient CHO cells (CHO-DXB11 and -DG44) by mutagenesis and selective selection. DHFR- cells glycine, hypoxanthine, and thymidine to grow in culture. This allowed to select for transgene-positive cells after co-transfection of a gene of interest together with the DHFR-gene and transgenic CHO cell lines to express a target gene became a reality. The last step of developing the technology and adapting CHO cells to large-scale suspension cell culture in bioreactors was far from trivial, but lead the foundation for the most common cellular expression system today.
A wide variety of CHO cell lines have emerged since, often custom-tailored to the target biologic. Some adaptations are made to improve culture conditions, some for post-translational modifications (such as attempts to humanize glycosylation) and others for regulatory reasons (defined or even protein-free cell culture media). And many more are still to come – the versatility of the cell line is one of its major assets. CHO offer a wide variety of advantages for the production of biologics, and have become the dominant mammalian cellular expression system since the first approval of tissue plasminogen activator (tPA) as a CHO-derived protein in 1986. Their long-standing safety record, outstanding growth and productivity under controlled culture conditions and established selection systems will likely propel their widespread use even further for quite some time to come.
Figure 2: The family tree of prominent CHO cell lines. Sequenced lines are highlighted in blue. Where known, the name of those who isolated the strain and the year it was done are given in parentheses. From N.E. Lewis et al. (2013)
The Advantages of CHO Cells
Easy to Culture
Grow well in suspension and as adherent culture, rendering the cells ideal for GMP procedures. Their tolerance to variations in pH, oxygen levels, temperature or pressure make them the ideal cell for large-scale culture.
High recombinant protein yields and specific productivity. Thanks to genetic optimization, protein yields of 3-10 grams per liter of cell culture.
Defined Culture Conditions
Can be adapted for defined, serum-free culture conditions, as well as allowing for animal-free and protein-free production and better safety and stability profiles.
Various Selection Systems
Antibiotic and metabolic selection by DHFR- or glutamine synthase (GS)-deficiency to obtain stable clones of high productivity with ease.
Variety of post-translational modifications of the produced biologic, often allowing allow for a biosimilar, if not human-identical products with excellent pharmaceutical activity and biocompatibility.
Genetic Cell Engineering
Well-proven genetic tools are available to optimize CHO cells, from gene introduction to knock-out, knock-down and post-translational silencing.
Used for nearly 50 biotherapeutics already approved in the USA and EU.
Low Virus Susceptibility
Due to the hamster origin, the risk of propagation of human viruses is decreased, reducing production loss and increasing biosafety.
Whether you work in drug discovery, or upstream, or downstream bioprocessing, Enzo offers a range of products to help you maintain cell line viability, optimize and monitor product integrity, and maximize yield. For contamination monitoring, we offer numerous host cell protein ELISAs including the CHO host cell protein ELISA kit. This is a complete, colorimetric, immunometric immunoassay kit for the quantitative determination of CHO host cell protein contamination in bulk products expressed in CHO expression systems. Low intra-assay variability allows for streamlined testing within 3 hours.
Produced in CHO cells. aa 32-143, also known as Irisin, comprises the majority of the extracellular domain of FNDC5. It is fused at the N-terminus to a FLAG®-tag with an 8 aa linker between the FLAG®-tag and Irisin., ≥90% (SDS-PAGE), ELISA, WB, Activity assay, SDS-PAGE | Print as PDF
Produced in CHO cells. The fibrinogen-like domain (aa 284-498) of human Ang-1 (angiopoietin-1) is fused at the N-terminus to the coiled-coil domain of rat COMP (cartilage oligomeric matrix protein) and to a FLAG®-tag., ≥90% (SDS-PAGE) | Print as PDF
Produced in CHO cells. The sequence coding for the 4 extracellular Ig-like domains of human LAG-3 (D1-D4) is fused to the Fc portion of human IgG1., ≥99% (SDS-PAGE), ELISA, Flow Cytometry | Print as PDF
Produced in CHO cells. Secreted from the cells using a proprietary signal peptide. Full length extracellular human Nampt (visfatin/PBEF) (aa 1-491) is fused at the N-terminus to a FLAG®-tag., ≥95% (SDS-PAGE) | Print as PDF
Produced in CHO cells. The sequence coding for the 4 extracellular Ig-like domains of mouse LAG-3 (D1-D4) is fused to the Fc portion of mouse IgG2a (hinge-CH2-CH3)., ≥95% (SDS-PAGE), Flow Cytometry | Print as PDF