For many years now the leading cell system for producing human proteins with therapeutic use has been Chinese Hamster Ovary (CHO) cells. Their use is widespread and popular and on an industrial scale (Walsh, 2018). A number of others reasons is because they’ve been used too in toxicity studies, evaluation of nutritional strategies and general genetics. They also offer the all-important post-translational protein modification capabilities such as glycosylation missing from more basic protein manufacturing systems.
These cells are derived from an epithelial cell line. Their use in medical research has been well established for over 100 years. The Chinese Hamster Ovary cell line was developed by Theodore Puck. They grow well in suspension cultures and produce high yields of protein.
The cells have a low chromosomal number for a mammal (2n = 22) which makes them suitable for general tissue culturing and advanced mammalian cell genetic studies.
The CHO cell lines of today are not able to synthesize the amino acid, proline nor can they express the epidermal grow factor receptor (EGFR).
Industrial Uses
Chinese Hamster ovary cells are used in industrial bioreactors for the expression of mammalian recombinant proteins. They can produce up to 10 grams of protein per litre which is extraordinarily high for a mammalian cell line.
One of the main benefits in general of mammalian cell lines is that human derived proteins are expressed that have then undergone post-translational modification. This makes the cell lines ideal for producing proteins that are suitable for human use. The CHO cell line is no different in this regard.
A number of variants have been developed for specific culturing purposes. One variant is unable to express dihydrofolate reductase (dhfr-) and this is often used in genetic manipulation studies. The approach in the selection of cells containing the gene of choice relies on firstly molecular cloning of the gene of interest along with the DHFR gene. This is into a single mammalian expression system such as a plasmid. The plasmid carrying both genes is transfected into CHO cells and these are grown in a medium lacking thymidine. The surviving cells are ones containing both gene of interest and the exogenous DHFR gene which are integrated into the CHO genome.. growth and rate and level of recombinant protein production for each cell line is highly variable.
CHO cells can also grow and produce protein when immobilized on porous microcarriers (Chen et al., 2001).
The types of human proteins produced include prothrombin which is a blood coagulation protein needed by people with blood-thinning or poor blood clotting disorders (Chen et al., 2001). CHO cells were used to produce erythropoietin which is a glycoprotein needed for the production of red blood cells (Davis et al., 1987; Adamikova et al., 2019).
Issues with CHO Cell Lines
Undoubtedly, CHO cells are extremely popular on an industrial scale however they suffer from some weaknesses. They have an inherent genomic plasticity which means both their genomic and phenotypic stability is highly questionable especially for long-term cultivation. It appears as with a number of other cell lines, there is a series of progressive changes occurring which negatively impact cell productivity and the production of proteins. Without the capability for sustained productivity and uncertainty on consistent product quality, the use development of a commercially viable process using these cells is markedly and commercially compromised.
References
Chen, Z., Lütkemeyer, D., Iding, K. et al. (2001) High-density culture of recombinant Chinese hamster ovary cells producing prothrombin in protein-free medium. Biotechnology Letters 23, pp. 767–770 (Article)
, & (2019). The fickle CHO: A review of the causes, implications, and potential alleviation of the CHO cell line instability problem. Current Opinion in Biotechnology, 60, pp. 128–137 (Article).
Davis, J. M., Arakawa, T., Strickland, T. W., & Yphantis, D. A. (1987). Characterization of recombinant human erythropoietin produced in Chinese hamster ovary cells. Biochemistry, 26(9), pp. 2633-2638
(2018). Biopharmaceutical benchmarks 2018. Nature Biotechnology, 36, pp. 1136–1145 (Article).
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