Our findings that IFN-γ+ Treg expanded after alloanergization were potently allosuppressive are consistent with recent studies that have shown that human CD4+ Treg with capacity to secrete IFN-γ are critical for controlling T-cell responses in proinflammatory environments19 and are required for effective suppression of GvHD in murine transplant models.17,29 However, although subpopulations of IFN-γ+ Treg expanded after alloanergization may play an important role in controlling alloresponses in proinflammatory conditions, these cells represent only a small proportion of the expanded Treg pool. Here, CD4 is linked to graft versus host disease.