Prevention of GvHD in Participants With Hematological Malignancies Undergoing Hematopoietic Stem Cell Transplant (HSCT)

Inclusion Criteria: 1. Subjects with one of the following hematologic malignancies: Acute Lymphoblastic Leukemia (B- or T-ALL), Acute Myeloid Leukemia (AML) and Myelodysplastic Syndrome (MDS), or Chronic myelomonocytic leukemia (CMML) 2. Males and Females Age ≥ 18 years. 3. Weight of ≥ 35 Kg. 4. Karnofsky performance status ≥ 70 %. 5. Available mismatched related (haploidentical) or unrelated donors for peripheral blood stem cell (PBSC) donation. 6. Subjects must otherwise fulfill institutional criteria for eligibility to undergo allogeneic stem cell transplantation. 7. Absence of uncontrolled bacterial, viral or fungal infection at time of enrollment. 8. Have adequate organ function. 9. Subjects \> 65-year-old receiving MAC conditioning will only be eligible if they have a HSCT-comorbidity index score \< 5. 10. Subjects must be able to understand and sign informed consent and be willing and able to complete all specified procedures and visits. Exclusion Criteria: 1. Prior allogeneic bone marrow, peripheral blood, or cord blood HSCT. 2. Any subject with a history of significant renal, hepatic, pulmonary, or cardiac dysfunction, or on treatment to support cardiac dysfunction. 3. HIV positive. 4. Positive hepatitis-B surface antigen. Subject may be included if they are HBV PCR negative. 5. Positive hepatitis-C antibody with positive Recombinant Immunoblot Assay (RIBA) or PCR unless the subject has received curative anti-viral treatment and confirmed negative viral load by PCR. 6. Received another investigational agent for treatment of disease understudy within 28 days (or 5 half-lives, whichever is shorter) of conditioning and/or have not recovered from treatment related toxicities. 7. Subjects with a previous history of Thrombotic Thrombocytopenic Purpura (TTP) or Hemolytic Uremic Syndrome (HUS) who are not good candidates for treatment with sirolimus. 8. Subjects that are pregnant, breast feeding or aim to become pregnant during the study period. (Subjects must agree to use a highly effective method of contraception). 9. Any serious illness, uncontrolled inter-current illness, psychiatric illness, active or uncontrolled infection, or other medical condition or history, including laboratory results.