25 cm2 t flasks

Adherent CHO-K1 cell lines were maintained in Dulbecco’s modified Eagle’s medium (DMEM, Gibco, Gaithersburg, MD) supplemented with 10% (v/v) fetal bovine serum (FBS, Hyclone, Logan, UT). Recombinant CHO-K1 cell lines expressing DHFR, EGFP, or dupilumab were maintained in DMEM supplemented with 10% dialyzed FBS (dFBS, Thermo Fisher Scientific). The cells were maintained in monolayer cultures in 25 cm² T-flasks (Nunc, Roskilde, Denmark) with a working volume of 5 ml and incubated at 37°C in a humidified 5% CO₂ atmosphere. The cell lines that were amplified by MTX were maintained in the maintenance media supplemented with 20, 50, 100, 200 nM, or 1 μM MTX (Sigma-Aldrich, St. Louis, MO). Viable cell density and viability were measured with the trypan blue dye exclusion method using an automated cell counter (Countess II FL; Invitrogen, Carlsbad, CA).

K562 (human chronic myeloid leukemia cell line) was procured from NCCS, Pune and grown in RPMI 1640 supplemented with 10% FBS, 100 U/ml of penicillin and 100 µg/ml of streptomycin in 25 cm² T-flasks (Nunc, Roskilde, Denmark) with vented caps and incubated at 37°C in a humidified atmosphere of 5% CO₂ in air.