Was CHO the Right Choice for Biotech?
It’s 1984 and you have just joined one of the handful of biotechnology companies as a member of the cell line development team. The industry is so young that only one product produced using recombinant DNA technology has been approved, recombinant human insulin (Humulin®), and it is manufactured using E. coli.
Your first task is to develop a recombinant mammalian cell line to produce your experimental product, a recombinant human protein that has cured cancer in many mice and that your company hopes will be useful in treating certain cancers in humans as well. Those pesky post-translational modifications that only mammalian cells can do are essential for the function of your product, so you have a decision to make: what mammalian cell system to use? There are some recombinant monoclonal antibodies being developed as potential therapeutics and most companies are producing them using hybridomas — fusions between mouse spleen cells and immortal myeloma cells. You could turn to one of these myeloma cells themselves – such as NSO or a non-producing hybridomas such as Sp2/0, which grow well in suspension culture and secrete antibodies and recombinant proteins at reasonable levels. Remember, it is still 1984, so “reasonable” means something on the order of hundreds of mg/L. Sounds promising.
But there’s another option available: people have started to use Chinese Hamster Ovary (CHO) cells, a cell line that was originally isolated in the 1950s and that has been used extensively in genetic research. It’s an epithelial cell line that grows well when attached to a matrix but that can be also be adapted to grow in suspension. CHO is gaining a foothold in the industry for many reasons, not the least of which is the availability of CHO lines, such as DG44, that lack the essential DHFR gene and therefore enable strong selective pressure if DHFR is included in your expression vector. There’s a small fee to pay to the inventors of the DG44 line, but overall, it’s an attractive option.
It is this scenario that started the widespread use of CHO cells in biopharmaceutical production. CHO’s current position as the dominant mammalian host used to produce recombinant proteins for pharmaceutical use is well established, and the industry continues to devote time and money to perpetuate it. We conduct research to better understand CHO metabolic pathways; develop media to support CHO growth based on this new information; engineer CHO cell lines using ever more sophisticated technologies such as CRISPR/Cas-9; and design expression vectors and selection strategies that improve clone selection, cell line productivity, and culture longevity in the bioreactor.
But as Alan Dickson noted recently, NSO cells (which are currently used to produce, among other proteins, the marketed monoclonal antibodies Soliris®, Synagis®, and Tysabri®) could also have been a good choice for production of your product back in 1984 (Dickson, Alan, “CHO Cells 10 Years from Now: Today’s Future”; Presented at KNect365 Cell Line Development and Engineering Conference, June 5–7, 2017, San Diego, CA). If the industry had turned to NSO in the 1980s and applied the same energy to improving growth, productivity, and product quality, we might not face some of the current bottlenecks in CHO cell culture. NSO was derived from lymphocyte B cells which have naturally evolved to secrete large quantities of antibody when the host faces an immune challenge.
Today, cell and process advances, industry infrastructure and regulatory acceptance surrounding CHO cell culture are far more advanced than for any other mammalian host systems. We expect the that these efforts will continue with further advances in CHO cell engineering and corresponding culture processes to become even more reliable as assembly lines for proteins with tailored product quality attributes. No one expects the industry to turn back the clock and embrace alternate mammalian host systems in the near term so CHO will continue to be a dominant player for now. However, we do expect to see advances in other host cell systems with the of improved NSO and human cells and associated processes gaining momentum as these viable alternatives to CHO have their own desirable characteristics. Who knows, in thirty years we may be talking about CHO as the “system of the past.”