Specific Aim #1: To conduct a multicenter pilot assessing the tolerability of abatacept (n=20). Patients will receive four doses (10 mg/kg IV on days -1, +5, +14 and +28), a schedule well tolerated by adolescents and adults with hematologic malignancies in a previous pilot. Abatacept will be combined with cyclosporine and mycophenolate mofetil.