Donor T cells up-regulate host IDO expression and worsen GVHD in IDO^−/− vs wt recipients. (A) Wt or IDO^−/− mice were lethally irradiated and infused with 3 × 10⁶ CD4⁺ and CD8⁺ T cells and killed on day 7. Cryosections of colon (left) and small intestine (right) were stained by immunohistochemistry for IDO. (i,v) Wild-type BM only. (ii,vi) Wild-type GVHD. (iii,vii) IDO^−/− BM only. (iv,viii) IDO^−/− GVHD. (B) Hematoxylin and eosin staining of wt and IDO^−/− colons (i-iv) and spleens (v-viii) 27 days after transplantation with 3 × 10⁶ CD4⁺ and CD8⁺ Balb/c T cells. (i,v) Wild-type BM only. (ii,vi) Wild-type GVHD. (iii,vii) IDO^−/− BM only. (iv,viii) IDO^−/− GVHD. (C) IHC staining of tissues in B for CD4. Cell numbers in colon are quantitated below (*P < .002). (D) IHC staining of tissues in B for CD8. Cell numbers in colon are shown below (*P < .02). Original magnification ×200, except spleens, which were ×100. Images are representative of 3 to 5 mice per group.