T-cell proliferation and survival are increased in colons of IDO^−/− recipients. (A) B6 or IDO^−/− mice were lethally irradiated and infused with Balb/c TCD BM and 3 × 10⁶ CD4⁺ and CD8⁺ T cells. At 21 days after transplantation, T cells were isolated from colons and livers and counted; n = 4 mice/group (*P < .04). (B) T cells isolated from colon and liver were stained for annexin V; n = 4 mice/group (*P < .01). (C) Day 27 tissues from the transplant in Figure 2 were stained for CD4 in Cy5 (shown in red) and Ki-67 in FITC (shown in green), a nuclear marker of proliferation. Proliferating CD4⁺ cells are yellow. Original magnification ×400. Images are representative of 2 similar experiments, 2 to 5 fields/mouse, 3 to 5 mice/group.