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

Inclusion Criteria: * High risk hematologic malignancy justifying the need for an allogeneic hematopoetic stem cell transplantation: AML, ALL, CML in accelerated or blast phase, MDS/MPN, NHL, Hodgkin lymphoma, and multiple myeloma * Creatinine clearance \> 40 * Adequate hepatic function * Normal cardiac function (EF \> 50%) Exclusion Criteria: * Patients with hematologic malignancies for which transplant is not the only curative option, such as AML with good or intermediate cytogenetics or molecular markers in CR1 or CML in chronic phase * Inability to identify an 10/10 HLA-Matched Donor (related or unrelated) or a haploidentical donor * Active malignant disease relapse * Active, uncontrolled infection, uncontrolled cardiac angina, symptomatic congestive heart failure * Life expectancy \<3 months * Pregnancy or lactation * Patients may not be receiving any other investigational agents in the last 28 days * Patients with chronic myeloid leukemia in first chronic phase