Step 3: regulation of acute GVHD by T-cell subpopulations. T-cell subsets that have been implicated in GVHD generation include naive and effector T cells, and Th1, Th2, and Th17 cells. More uncertain is the role of memory T cells. Inhibitory T-cell populations include CD4⁺CD25⁺ regulator T cells and natural killer T cells. In rodents, a T-cell population with stem cell properties has been implicated in acute GVHD generation. The red boxes below the mouse and the human recipient serve to highlight distinct features between these species. Question marks denote uncertain conclusions. Treg indicates T regulatory cell; and NKT, natural killer T cell.