Chinese hamsters had been used in research since 1919, where they were used in place of mice for typing pneumococci. They were subsequently found to be excellent vectors for transmission of kala-azar, facilitating Leishmania research. In 1948, the Chinese hamster was first used in the United States for breeding in research laboratories. In 1957, Theodore T. Puck obtained a female Chinese hamster from Dr. George Yerganian's laboratory at the Boston Cancer Research Foundation and used it to derive the original Chinese hamster ovary cell line. Since then, CHO cells have been a cell line of choice because of their rapid growth in suspension culture and high protein production. Having a very low chromosome number for a mammal, the Chinese hamster is also a good model for radiation cytogenetics and tissue culture.
Since the original CHO cell line was described in 1956, many variants of the cell line have been developed for various purposes. In 1957, CHO-K1 was generated from a single clone of CHO cells, CHO-K1 was mutagenized with ethyl methanesulfonate to generate a cell line lacking dihydrofolate reductase activity, referred to as CHO-DXB11. However, these cells, when mutagenized, could revert to DHFR activity, making their utility for research somewhat limited. Subsequently, CHO cells were mutagenized with gamma radiation to yield a cell line in which both alleles of the DHFR locus were completely eliminated, termed CHO-DG44 These DHFR-deficient strains require glycine, hypoxanthine, and thymidine for growth. Cell lines with mutated DHFR are useful for genetic manipulation as cells transfected with a gene of interest along with a functional copy of the DHFR gene can easily be screened for in thymidine-lacking media. Due to this, CHO cells lacking DHFR are the most widely used CHO cells for industrial protein production.
Genetic manipulation
Much of the genetic manipulation done in CHO cells is done in cells lacking DHFR enzyme. This genetic selection scheme remains one of the standard methods to establish transfected CHO cell lines for the production of recombinant therapeutic proteins. The process begins with the molecular cloning of the gene of interest and the DHFR gene into a single mammalian expression system. The plasmid DNA carrying the two genes is then transfected into cells, and the cells are grown under selective conditions in a thymidine-lacking medium. Surviving cells will have the exogenousDHFR gene along with the gene of interest integrated in its genome. The growth rate and the level of recombinant protein production of each cell line varies widely. To obtain a few stably transfected cell lines with the desired phenotypic characteristics, evaluating several hundred candidate cell lines may be necessary. The CHO and CHO-K1 cell lines can be obtained from a number of biological resource centres such as the European Collection of Cell Cultures, which is part of the Health Protection AgencyCulture Collections. These organizations also maintain data, such as growth curves, timelapse videos of growth, images, and subculture routine information.
Industrial use
CHO cells are the most common mammalian cell line used for mass production of therapeutic proteins. They can produce recombinant protein on the scale of 3-10 grams per liter of culture. CHO cells are also suitable for human applications, as they allow post-translational modifications to recombinant proteins which can function in humans.
