MTOR and graft versus host disease: Thus, a number of ongoing clinical trials are repurposing approved drugs, such as the Treg growth factor IL-2 (aldesleukin) or Treg-stabilizing factors, e.g., the mTOR-inhibitor rapamycin to enhance Treg function as a therapeutic approach to control a variety of autoimmune diseases, GvHD, or organ transplant rejection (Abbas et al., 2018).