CD25⁺ donor cells are indispensable for the efficacy of SA-FasL in preventing aGVHD. Lethally irradiated F1 mice received bone marrow cells admixed with nonengineered or SA-FasL–engineered splenocytes or SA-FasL–engineered CD25-deplete splenocytes. Animals were followed for development of lethal acute GVHD and survival. For survival curve comparison, log-rank (Mantel-Cox) test was used. ∗P < .05; ∗∗P < .01