Figure 1.  Immature NK cells require very high concentrations of soluble IL2 or IL15 for growth
Immature NK cells were obtained from E14 thymus and cultured initially for 2 days in IL4
+ PMA.  They were then washed, cultured in medium containing serial dilutions of IL2 or IL15,
and cell numbers were determined 4 days later by automated cell counting on a flow cytometer.
Conclusions
1.  In agreement with our previous studies, immature NK cells only grow efficiently in very high
concentrations of IL2 [20nM].
2.  The concentrations of IL2 needed are in vast excess over those that would be required to
saturate high affinity IL2 receptors, and those that are likely to be generated in vivo.
3.  One interpretation of this finding would be that IL2 is acting as an inefficient surrogate of some
other factor that normally drives the proliferation of NK cells in vivo, the most likely candidate being
IL15.
4.  However, IL15 is no more efficient than IL2 in driving the proliferation of immature NK cells.