Figure 2.  Mature NK cells also require very high concentrations of soluble IL2 or IL15 for growth
Mature NK cells were purified from adult spleen and cultured in medium containing serial
dilutions of IL2 or IL15.  Cell numbers were determined 4 days later
Conclusions
1.  Like immature NK cells, mature NK will only grow in very high concentrations of IL2.
2.  They will also grow in IL15, but again only when this is present at very high concentrations.