Cell culture
CHO DG44 cell line (A1100001; Thermo Fisher Scientific, Waltham, MA, USA) was maintained in Dulbecco’s modified Eagle’s medium containing 25 mM glucose (SH30243.01; Hyclone, Waltham, MA, USA) supplemented with 10% fetal bovine serum (SH30919.03; Hyclone), 10 mM sodium hypoxanthine and 1.6 mM thymidine (2068642; Gibco, Waltham, MA, USA) along with 100 U/ml penicillin and 100 μg/ml streptomycin (SV30079.01; Hyclone). Cells were cultured in ambient air (20% O2) supplemented with 5% CO2. For batch culture, cells were cultivated in a 250 ml Erlenmeyer flask (73250; SPL, Pocheon, Korea) with a working volume of 30 ml. Cells were incubated with 150 rpm agitation, and passaged every 3 d.