IL15 and neoplasm: Next, because IL-15−/− mice lack both pre-mNK cells46 and NK cells, and because specific depletion of NK cells has little effect on tumor rejection in our system (Fig. 4), we reasoned that pre-mNK cells (NK1.1+B220+CD11cloGR-1− cells) had a potential role in suppressing antitumor immunity.