Engineering redox sensors into CHO cells enables near‐real‐time quantification of intracellular redox in bioprocesses

The production of biologics that treat complex diseases, such as cancer, autoimmune, and infectious disease, requires careful monitoring and control of cell cultures. While bioprocess optimizations have dramatically improved production yields, a lack of analytical tools has made it challenging to identify accompanying intracellular improvements. Intracellular redox can diminish the growth and productivity of biologics-producing cells and adversely impact product quality profiles yet characterizing redox is challenging due to its complex and highly transient nature. In this study, we integrated a fluorescent thiol-based redox biosensor to monitor intracellular redox in one bisAb- and two monoclonal antibody-producing clonal cell lines in a 14-day fed-batch bioreactor. We characterized biosensor functionality using three fluorescence measurement techniques and determined sensor oxidation correlates with the intracellular ratio of reduced (GSH) and oxidized glutathione (GSSG), an important cellular antioxidant. Our fed-batch bioreactor studies showed that sensor expression minimally affected bioprocess outcomes, including growth, productivity, product quality attributes, or intracellular redox attributes, including mitochondrial reactive oxygen species and total cellular GSH levels in all cell lines tested. Biosensor measurements taken throughout the culture revealed that the intracellular environment in these cell lines became more reduced throughout the culture, with the exception of a high pH condition which became more oxidized. Our results demonstrate the potential of using biosensors to monitor intracellular changes in near-real-time with minimal process effects, thus potentially improving future bioprocess optimizations.