Reducing recombinant protein expression during CHO pool selection enhances frequency of high-producing cells

CHOBRI/rcTA cell pools were generated by transfecting cells with the expression vectors as described above. The day after transfection, the cells were centrifuged for 5 min at 250 rpm and seeded at density of 0.5 x 10e6 cells/mL in selection medium (PowerCHO2 medium supplemented with 25 uM of methionine sulfoximline).

Chinese hamster ovary (CHO) cells are the most widely used mammalian host for industrial-scale production of monoclonal antibodies (mAbs) and other protein biologics. Isolation of rare high-producing CHO cell lines from heterogeneous populations of stable transfectants is a daunting task and delays the process of manufacturing of novel biologics. A variety of factors that contribute to the low frequency of high-producing clones have been described; however, the impact of metabolic burden and other stresses (eg. ER stress) associated with sustained high-level expression of recombinant protein (r-protein) during selection of stable transfectants has not been fully appreciated. CHO cell line development has not traditionally received much optimization in this area because the vast majority of platforms use constitutive expression systems to produce biologics. Previously, we developed a cell line (CHO(BRI/rcTA)) containing a robust inducible expression system, based on the cumate gene switch, that allows r-protein expression to be down-regulated during selection. Using this switch, we generated inducible CHO(BRI/rcTA) pools expressing an Fc-fusion protein within two weeks of transfection with volumetric productivity of up to 1.1 g/L at 17 days post-induction in a fed-batch culture process. Herein, we show that the ability to regulate r-protein expression during pool generation confers a substantial advantage for selecting high-producing stable clones. Reducing expression levels ("off-state") during pool selection dramatically enhances high-producer frequency compared to a pool in which expression was maintained at a high level during selection ("on-state", mimicking a constitutive expression system). Overexpression of the r-protein during the pool selection process negatively affects pool recovery and is associated with subtle but significant increases in BiP expression and cell death compared to pool selection in the "off-state". Our data shows that the cumate gene switch is a valuable platform for stable clone generation and supports the wider application of inducible systems for scalable production of biologics in CHO cells.