2.6 pH measurements
In addition to the online pH measurements from the STR, offline pH readings were also taken daily from the microwell and STR cultures using the RAPIDLab 348EX blood gas analyser (Siemens Healthineers, Erlangen, Germany). For the STR cultures, the pH was recalibrated if the online and offline pH value deviated by more than 0.05 pH units.
2.7 Quantification of lentiviral vector infectious titre
To quantify the infectious titre in terms of transducing units per ml (TU/ml), HEK293T cells were transduced with serial LV dilutions in the presence of polybrene (8 µg/ml) (MilliporeSigma, Burlington, Massachusetts, United States). Polybrene is a polycation agent which has shown to enhance transduction by retroviral vectors (Davis, Morgan, & Yarmush, 2002). The transduced cells were adherent HEK293T cells which were derived from a GSK in-house research cell bank. Cell counts were performed at the time of transduction. Following this, cells were cultured for 3 days at 37°C in a 5% (v/v) CO2humidified incubator. The eGFP expression in the transduced cells was measured by flow cytometry using the Accuri c6 Plus system (BD Biosciences, Franklin Lakes, New Jersey, United States). The titre was determined using the formula: Transducing Units/mL = [(% of GFP positive cells) x (number of cells transduced) x (dilution factor/transduction volume)].