Survival curves of wt or NK-depleted C57BL/6 mice injected with parental and IL-15Rα-transfected MC38 cells. (A) Expression levels of IL-15Rα on MC38 transfectants. Flow cytometry data showing the expression levels of mouse IL-15Rα on MC38 transfectants. (Left) Parental MC38/isotype control staining of all MC38 clones; (middle) MC38-mouse IL-15Rα (moderate); (right) MC38-mouse IL-15Rα (high). (B) Survival of wt mice challenged by IL-15Rα–transfected MC38 cells. Two million parental or IL-15Rα (moderate or high expression)–transfected MC38cells were injected via the tail vain into C57BL/6 mice (n = 11 for parental MC38 injected mice, and n = 6 for mice injected with IL-15Rα⁺[moderate] or IL-15Rα⁺MC38 cells, n = 4 for NK-depleted wt mice injected with IL-15Rα⁺MC38), and their survival was monitored for 60 days. The survival of mice injected with IL-15Rα⁺ (high) MC38 cells was significantly longer (P < .001) than that of mice injected with the parental MC38 cells. The left panel shows the levels of IL-15Rα expression on these MC38 transfectants as assessed by flow cytometry. When the IL-15Rα⁺ MC 38 cells were injected into mice that had been depleted of either NK cells or CD8 T cells, only NK-depleted mice allowed the development of IL-15Rα⁺ (high) fatal MC38 cell tumors.