Donor T cells cause accelerated GVHD in IDO^−/− versus wt recipients. (A) B6 or IDO^−/− mice were lethally irradiated (9.0 Gy) and infused with 10⁷ MHC-mismatched Balb/c non–T cell–depleted BM with or without 3 × 10⁶ Balb/c CD4⁺ T cells. Survival plots of B6 versus IDO^−/− recipients are shown. ● indicates BM→B6; ■, CD4⁺ T cells→B6; ○, BM→IDO^−/−; □, CD4⁺ T cells→IDO^−/− (P < .03). Data are from one experiment; n = 8 mice/group. (B) B6 or IDO^−/− mice were lethally irradiated and given Balb/c TCD BM with or without 3 × 10⁶ donor CD4⁺ and CD8⁺ T cells. Survival plots of B6 vs IDO^−/− recipients are shown. ● indicates BM→B6; ■, CD4⁺ and CD8⁺ T cells→B6; ○, BM→IDO^−/−; □, CD4⁺ and CD8⁺ T cells→IDO^−/− (P < .04). Data are from one experiment; n = 8 mice/group. (C) Balb/c or IDO^−/− mice were lethally irradiated (7.0 Gy) and infused with 10⁷ B6 TCD BM with or without 2 × 10⁶ CD4⁺ and CD8⁺ T cells. Survival plots of Balb/c versus IDO^−/− recipients are shown. ● indicates BM→Balb/c; ■, CD4⁺ and CD8⁺ T cells→Balb/c; ○, BM→IDO^−/−; □, CD4⁺ and CD8⁺ T cells→IDO^−/−. Data are pooled from 2 experiments; n = 16 mice/group (P < .001).