Host IDO induction and IDO-mediated spleen and colon protection do not require donor Tregs. (A) IHC for IDO in colon 7 days after transplantation with Balb/c TCD BM with or without 2 × 10⁶ CD4⁺CD25⁻ T cells. (B) Hematoxylin and eosin staining of colon (i-iv) and spleen (v-viii) 7 days after transplantation with Balb/c TCD BM with or without 2 × 10⁶ CD4⁺CD25⁻ T cells. (i,v) Wild-type BM only. (ii,vi) Wild-type GVHD. (iii,vii) IDO^−/− BM only. (iv,viii) IDO^−/− GVHD. (C) Tissues from panel B stained for CD4 by IHC. Original magnification ×200 for colons, ×100 for spleens.