A CTLL-2 cell proliferation assay was used to determine the biological activity of the rhIL-15 and IL-15/SuIL-15Rα-IgG4 Fc complexes. In brief, CTLL-2 cells in logarithmic growth phase were washed thrice with RPMI medium containing 10% FBS (basal medium) and then seeded into 96-well culture plates at 1 × 104 cells per well (60 μL). The rhIL-15 and IL-15/SuIL-15Rα-IgG4 Fc complexes were serially diluted with basal medium and added into the plates at 40 μL per well. After 48 h of incubation, the cell viability was measured using the MTT colorimetric assay. The results were analyzed by GraphPad Prism software, and EC50 values were calculated using the four-parameter fit logistic equation.
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