Donor IFN-γ is required for optimal GVL effects in sublethally irradiated allo-HCT recipients. B6D2F1 mice received 6 Gy TBI followed the next day by administration of 2 × 10⁷ splenocytes and 10⁷ BMCs from WT (▲) or GKO (♦) B6 donors (n = 8-12 per group). B6D2F1 mice that received sublethal TBI without HCT were used as controls (■; n = 7 per group). Leukemic recipients were additionally injected with 10⁴ (B) or 2 × 10⁴ (C,D) P815 tumor cells, either on day 0 (B,C) or day −2 (D) with respect to allo-HCT. Survivals of nonleukemic (A) and leukemic (B-D) recipients are shown.