Provided herein is the use of donor T cells that express a continuously active form of STAT6, e.g., before or during BMT/HCT or after BMT/HCT, the use of autologous T cells genetically engineered to express a continuously active form of STAT6 to prevent, inhibit or treat immune diseases including autoimmune diseases and the use of T cells genetically engineered to express a continuously active form of STAT6 to enhance an immune response to a vaccine.