IDO-mediated Inhibition of GVHD lethality occurs independently of the presence of donor Tregs. (A) Balb/c TCD BM with or without 3 × 10⁶ Balb/c CD25-depleted CD4⁺ and CD8⁺ T cells were infused into lethally irradiated B6 or IDO^−/− recipients. Survival plots are shown. ● indicates BM→B6; ■, CD25-depleted T cells→B6; ○, BM→IDO^−/−; □, CD25-depleted T cells→IDO^−/−. T cell groups of B6 vs IDO^−/− (P < .005). Data are from one experiment; n = 8 per group. (B) Balb/c TCD BM with or without 2 × 10⁶ Balb/c CD4⁺CD25⁻ T cells were infused into lethally irradiated B6 or IDO^−/− recipients. Survival plots of B6 vs IDO^−/− are shown. ● indicates BM→B6; ■, CD4⁺CD25⁻ T cells→B6; ○, BM→IDO^−/−; □, CD4⁺CD25⁻ T cells→IDO^−/−. T cell groups (P < .02). Data are from one experiment; n = 8 mice/group. (C) Balb/c TCD BM with or without 1 × 10⁶ Balb/c CD4⁺ T cells or CD4⁺CD25⁻ T cells were infused into IDO^−/− recipients. Survival plots of CD25-depleted versus nondepleted are shown. ○ indicates BM→IDO^−/−; □, CD4⁺ T cells→IDO^−/−; △, CD4⁺CD25⁻ T cells→IDO^−/−. T cell groups (P < .02). Data are from one experiment; n = 8 mice/group.