Chinese Hamster Ovary (CHO) cell lines are frequently used as hosts for the production of recombinant therapies, such as monoclonal antibodies, due to their ability to make correct post-translational modifications. A potential problem when using CHO cells for the production of therapeutic proteins is the selection of cell lines that do not retain stable protein expression during long-term culture (LTC). Expression instability affects process performance, effective use of time and money, and regulatory approval for the desired treatment.
In this study, we investigated an unstable GS-CHO cell line model over a continuous period of approximately 100 generations to determine markers for the mechanisms underlying the instability. In this cell line, expression stability was maintained for 40-50 generations, after which a 40% loss in antibody production was detected. The instability observed within the cell line was not due to a loss in the number of copies of the recombinant gene or a decrease in the expression of the mRNA encoding the H or L chain of the recombinant antibody but was associated with values of cell time lower cumulative levels and an apparent greater sensitivity to cellular stress (exemplified by increased mRNA expression of the stress-inducible gene GADD153).
Changes in cell metabolism were also observed during CTL (alterations in extracellular alanine accumulation and improved glucose and lactate utilization rates, during the exponential and decline phase of batch culture, respectively). Our data indicate the breadth of changes that can occur in recombinant CHO cells during CTL ranging from the instability of recombinant target production at a post-mRNA level to metabolic events. Defining the mechanisms, regulatory events, and linkages that underpin changes in the cellular phenotype requires a more detailed analysis at the molecular level.