Donor T cell–produced IFN-γ is required for colon IDO induction and effects on T-cell proliferation and survival. (A) Wt (filled symbols) or IFN-γR^−/− (open symbols) B6 mice were lethally irradiated and infused with allogeneic BALB/c BM with (squares) or without (circles) 3 × 10⁶ whole T cells. n = 8 mice/group. Survival is shown. P < .001. (B) Mice from transplant in panel A were killed 7 days after transplantation, and colons were examined for IDO protein by IHC and IDO and CD4 mRNA by qPCR. For IDO in wt versus IFN-γR^−/− GVHD groups, n = 4 mice/group, P < .001. (C) Wt B6 mice were lethally irradiated and received BALB/c BM with or without 3 × 10⁶ wt or IFN-γ^−/− BALB/c T cells. At 7 days later, colons were harvested and assessed for IDO protein and mRNA. n = 4 mice/group, P = .03. (D) Wt (filled symbols) or IDO^−/− (open symbols) B6 mice were transplanted with BALB/c BM with (squares) or without (circles) 3 × 10⁶ wt (left) or IFN-γ^−/− (right) BALB/c T cells. Survival is shown. n = 8 mice/group. For wt donors, P = .04, for IFN-γ^−/− donors, P = .78. (E) Wt or IDO^−/− BALB/c mice were lethally irradiated and received B6 BM and 10⁶ wt or IFN-γ^−/− B6 T cells. Sixteen days after transplantation, T cells were isolated from colons and assessed by flow cytometry for annexin V and Ki-67. Data are pooled from 2 identical experiments with n = 3-4 mice/group; *P < .05. Only statistically significant differences are noted.