Essay: Therapeutic antibodies

Therapeutic antibodies are mostly manufactured in mammalian host cell lines including Chinese hamster ovary (CHO) cells, PER.C6 human cells and NS0 murine myeloma cells. The expression system of choice is usually determined by its ability to deliver high productivity with acceptable product qualities.
Murine NS0 cells are non-immunoglobulin secreting myeloma cells that require the presence of cholesterol in culture medium for growth. Cholesterol-independent NS0 cells also have been established. Furthermore, NS0 cells lack endogenous glutamine synthetase (GS) enzyme activity making them suitable for use with GS as a selectable marker for recombinant antibody expression. High antibody productivity has been reported from non-GS NS0 cell lines as well. Mouse-derived cell lines, including NS0, produce N-glycolylneuraminic acid (Neu5Gc), a sialic acid that cannot be synthesized by humans. Sialic acid was initially believed to present a potential immunogenicity concern in humans, but it was later shown that CHO cells could also express low levels of Neu5Gc and that humans apparently absorb Neu5Gc into proteins from dietary sources, which presented immunogenicity concerns. Even though NS0 cells have been used in industry to produce therapeutic antibodies, the potential immunogenicity aspects have likely limited use of these cells for therapeutic antibody production. Compared to NS0 and CHO cells, PER.C6® cells incorporate a relatively new technology. PER.C6 cells are derived from human embryonic retina cells that have been immortalized by transfecting the E1 genes from adenovirus 5 DNA. In like manner to NS0 and CHO cells, PER.C6 cells can proliferate indefinitely in suspension under serum-free conditions.
CHO cells are the predominant host used to produce therapeutic recombinant proteins, particularly monoclonal antibodies. Despite the availability of a number of other mammalian cell lines, including baby hamster kidney, mouse myeloma-derived NS0, and the human retina-derived PerC6, about 70% of all recombinant therapeutic proteins manufactured today are produced via Chinese hamster ovary (CHO) cells. The reason for the wide use of CHO cells can be attributed to the following reasons. To begin with, it is easier to obtain the approval to bring therapeutic proteins from various regulatory agencies since CHO cells have been regarded as safe hosts for the past two decades. In addition, low specific productivity (one of the drawbacks of protein production via mammalian cells) can be overcome by gene amplification of CHO cells. For CHO cells, robust gene amplification systems including but not limited to dihydrofolate reductase (DHFR) or glutamine synthetase (GS) gene amplification exist. Furthermore, CHO cells have the capacity for efficient post-translational modification, and they produce recombinant proteins with similar glycosylation patterns to humans. Last but not least, CHO cells can be easily adapted to grow in serum-free suspension conditions, which is a preferred characteristic for large-scale culture in bioreactors.