EL4 cells expressing ULBPs are partially rejected in C57BL/6 SCID mice; IL-15 enhances the rejection. (A) C57BL/6 SCID mice were challenged subcutaneously with 3 × 10⁵ EL4 cells expressing vector alone (▪), ULBP1 (•), ULBP2 (▴), or ULBP3 (○). The number of tumor-bearing animals at the end of the experiment was 10 of 10 for EL4 mock, 8 of 10 for ULBP1⁺ EL4, 8 of 10 for ULBP2⁺ EL4, and 9 of 10 for ULBP3⁺ EL4. The EL4 mock mice were humanely killed on day 19 due to tumor burden. (B) C57BL/6 SCID mice were challenged as in panel A with EL4 mock cells (▪), EL4 mock–expressing IL-15 (□), EL4-expressing ULBP1 (•), or EL4-coexpressing ULBP1 and IL-15 (○). Mean tumor size (± SEM) was calculated from all of the mice. The number of tumor-bearing animals at the end of the experiment was 8 of 9 for EL4 mock, 7 of 7 for EL4 mock/IL-15⁺, 7 of 8 for ULBP1⁺, and 3 of 7 for ULBP1⁺/IL-15⁺. The EL4 mock and EL4 mock/IL-15⁺ mice were humanely killed on day 21 due to tumor burden. Data are representative of 3 independent experiments.