Comparing Cyclophosphamide and Abatacept With Standard of Care Treatment Following Stem Cell Transplantation

Day 3 and day 4 following transplant. Dosing will be based on patients' actual weight up to 120% of ideal body weight, above which it will be based on adjusted ideal body weight (ideal weight plus 50% of the difference between ideal and actual weight).

Abatacept at a dose of 10mg/kg will be administered on days +5, +14 and +28, +56, +84, +112, +140, +168

standard of care Methotrexate 5 mg/m2 on Days 1, 3, 6 and 11

Tacrolimus per institutional guidelines