Step 5: effector phase. Cells implicated in the GVHD effector process are illustrated. In the gray box are the known mediators of tissue injury. IDO inhibits GVHD pathology by reducing the frequency of T-effector cells present in the colon. The red boxes below the mouse and the human recipient serve to highlight distinct features between these species. Question marks denote uncertain conclusions. NK indicates natural killer; TNF, tumor necrosis factor; and TNFa, tumor necrosis factor alpha.