Selection of Useful Cell Sub-populations

In addition to metabohc engineering, it is sometimes possible to isolate useful subpopulations of cell lines. 

A limitation in the use of CHO cell lines for producing biopharmaceutical proteins has been the long time it can take to adapt such cell lines to single cell suspension culture in serum- or protein-free media. A variant of the CHO-Kl cell line that grows spontaneously in protein-free suspension culture has been described for use with the GS system [59]. The isolation of natural variants has also been exploited to isolate an NSO clone which no longer requires cholesterol [60]. This nutrient is insoluble and its addition to protein-free media is not straightforward. 

In summary, a number of cell engineering approaches have been evaluated with GS cell lines to improve volumetric productiv- 

In recent years there has been a drive to remove serum, serum proteins and other animal-derived materials from cell culture media, motivated in large part by concerns regarding the potential introduction of adventitious agents. The removal of complex additions such as proteins offers other advantages particularly cost reduction and easier purification of product. In addition, chemical definition of the medium greatly assists process optimization. 

Serum and serum proteins have diverse functions which are now reasonably well understood for the industrially important cell lines, and which can generally be substituted by non-protein alternatives. Mammalian cells typically require a source of fatty acids, which were historically supplied by serum. To supply these, serum-free media usually contain plasma lipoprotein fractions, free fatty acids complexed to serum albumin or fatty acid/phospholipid microemulsions [61]. A high-density lipoprotein serum-fraction in medium containing bovine serum albumin was used by Seamans et al. [62] to replace serum in cultures of a recombinant antibody-producing GS-NSO cell line. Further, they found that the serum-fraction could be replaced with a commercially available non-protein-aceous lipid emulsion and a pluronic F-68/cholesterol emulsion. This gave equivalent growth and productivity (100 mg L ). 

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