Immunodeficiency abrogates the antitumor effect of RT. (A) WT C57BL/6 or nude mice (n = 10) were injected with 2 × 106 B16 melanoma cells and treated 7 days later with 20 Gy. The radiation group in WT but not in nude mice showed significantly smaller tumor size (**P = .002 at day 10 after RT). (B) WT or nude mice (n = 8-12) were injected with 2 × 105 B16-SIY and treated 10 days later with 25 Gy. The radiation group showed significantly smaller tumor size (***P < .001 on day 12 after RT). A similar trend of the inhibition was also detected with single 20 Gy. A total of 60% WT mice were cured, whereas 100% nude mice die with 20 Gy. (C) Tumor growth curve and (D) survival for WT mice injected with 105 B16 and treated on day 14 with 15 Gy given on days 0, 1, and 2 after RT. A total of 200 μg/mouse anti-CD8 antibody was administered on days 0, 4, and 8 after RT (n = 5-9 per group). After RT plus depletion of CD8, the size of tumor increased significantly from RT alone (**P = .007 at day 14). Survival increased after RT (***P < .001), but with CD8 depletion survival was significantly reduced: *P < .05; **P < .01; ***P < .001. Similar experiments were repeated 3 times (A-D).
