Subcutaneous breast tumor formation in IL-15 TG, IL-15 KO and C57BL/6 mice with MT cell line. (A) 1 × 106 MT cells were injected subcutaneously into IL-15 KO, C57BL/6 and IL-15 TG mice (n = 5/group). IL-15 TG mice were resistant to tumor formation in comparison to either the IL-15 KO or C57BL/6 mice. (B) Depletion of NK cells in C57BL/6 mice via NK1.1 antibody led to decreased survival (n = 5/group). (C) NK cells isolated from the spleen of C57BL/6 mice (E-Effector) efficiently kill MT tumor cells (T-Target) in a killing assay (n = 3). *p < 0.05, **p < 0.01.