Fas expression on donor T cells is required for IL-18-mediated GVHD protection in MCH class II-disparate BMT, but it does not have a role in MHC class I-dependent GVHD induced by IL-18. (A) bm12 animals underwent irradiation and transplantation with TCD BM from B6 and CD4⁺ T cells from allogeneic Fas-deficient lpr (H-2b) donors and were injected with IL-18 (▪, n = 6) or the diluent (□, n = 5) as in Figure 1. IL-18 had no effect on the mortality (▪ vs □, P = .69). (B) bm1 (B; •, n = 11; ○, n = 11) recipients were given 13 Gy TBI and underwent transplantation as in Figure 1, with TCD BM from B6 donor plus CD8⁺ T cells from Fas-deficient lpr donors. For syngeneic BMT, bm1 (▴, n = 6) recipients received TCD BM plus CD8⁺ cells from bm1 donors. Mice that underwent transplantation received rmIL-18 (• and ▴) or the diluent (○) from day -2 to +2, as in “Materials and methods.” Percentage survival after BMT. • vs ○; P = .0005.