BM-derived host cells are more critical than epithelial cells in suppressing donor T cells via the IDO pathway. (A) WT or IDO^−/− B6 mice were lethally irradiated and reconstituted with wt or IDO^−/− B6 BM. After 3 months, the chimeras were reirradiated and infused with allogeneic BALB/c BM and 3 × 10⁶ T cells. Survival is shown. For wt→wt (■) versus IDO^−/−→IDO^−/− (□) chimeras, P = .02. For wt→IDO^−/− (▲) versus IDO^−/−→IDO^−/− chimeras, P = .04. For wt→wt versus IDO^−/−→wt (△), P = .07. Representative of 2 experiments with n = 8-10 mice/group. (B) Chimeras were generated and transplanted as above. At 18 to 20 days later, T cells were isolated from colons and assessed by flow cytometry for Ki-67, annexin V, and PD-1. Data are pooled from 2 similar experiments with n = 3-5 mice/group. *P < .05.