Recently, clinical studies have been completed or underway assessing Treg to improve outcomes after SOTx or with AlloHCT to prevent GVHD.5, 23–25 These clinical studies have also shown that large numbers of CD4+CD25+ Treg can be expanded in the presence of high doses of IL-2 ex vivo and infused safely.5, 26–34 The number of clinical trials utilizing Treg cell therapy in transplantation and other fields are expected to increase. Here, CD4 is linked to graft versus host disease.