Scale-down cultivation in mammalian cell bioreactors—The effect of bioreactor mixing time on the response of CHO cells to dissolved oxygen gradients

Abstract

Scale-down simulators have been widely applied to investigate the impact of various concentration gradients on the efficiency of industrial-scale fed-batch bioprocesses involving microbial cell factories. However, due to the general fragility and shear sensitivity of mammalian cells, these simulators, especially those with multiple compartments, have not been widely adopted to investigate the effects of similar concentration gradients on cell culture processes. In this work, we developed a multi-compartment scale-down simulator comprising a stirred tank bioreactor and a plug-flow reactor (PFR). CHO cells expressing a recombinant protein were recycled through the PFR whilst being subjected to dissolved oxygen gradients at varying residence times from 70 s to 120 s. The results show a clear cut switch in the metabolism of CHO cells, such that until a residence time of 90s was exceeded, the cells did not respond to the presence of dissolved oxygen gradients in the PFR. Beyond this residence time threshold, there was a marked accumulation of lactate and up to 15 % decline in viable cell density in the stationary phase, compared to the reference fed-batch process. Interestingly, the recombinant protein productivity was not affected by the dissolved oxygen gradients, although the purity of the product, measured as the concentration of host cell proteins in the supernatant was adversely affected. The response time threshold in the metabolism of the cells was used to propose a scale-up technique based on similar physiological indices between bioreactor scales. The demonstrated method is a possible tool for assessing the suitability of a process in a given large scale bioreactor with known mixing times.