MHC-II expressed by tumor cells mediates antitumor immunity via cytotoxic CD4⁺T cells. (A) CD4⁺/CD8⁺ T-cell ratio in the TIL population. Tumor cells (5.0 × 10⁶) were inoculated subcutaneously. TILs were prepared from tumors 14 days after tumor cell inoculation and analyzed with flow cytometry (n = 6 per group). (B) Proportion of GrB⁺CD4⁺ T cells in the TIL population. The in vivo experiments were performed as described in panel A. Representative staining (left) and summary for the frequency (right) are shown. (C) Tumor growth of E.G7/B2MKO/CIITA in CD4⁺ T-cell– or CD8⁺ T-cell–deleted mice (left and right, respectively). Tumor cells (5.0 × 10⁶) were inoculated subcutaneously into C57BL/6 mice on day 0 (n = 6 per group). For CD4⁺ and CD8⁺ T-cell deletion, anti-CD4 mAbs and anti-CD8β mAbs, respectively, were administered intraperitoneally 1 day before tumor cell injection and every 7 days after tumor cell inoculation. Tumor growth was monitored every 3 days. All in vivo experiments were performed twice with similar results. *P < .05; **P < .01.