PACT is a non-randomized multicentre phase I/II study to evaluate the feasibility and safety of the prophylactic administration of donor derived TCM. Patients with Acute Myeloid Leukemia (AML) or Myelodysplastic Syndrome (MDS) who are planned to undergo a HLA -matched (9/10 or 10/10) allogeneic hematopoietic stem cell transplantation and who are either 50+ years old or have a high comorbidity score are included according to criteria as described below. TCM will be applied in escalating doses to a maximum of 30 patients who have received T cell depleted Human leukocyte antigen (HLA)-matched alloHSCT grafts and qualify for TCM transfer.