To compare the efficacy of MaaT033 with its placebo on OS at 12 months after alloHCT

To evaluate MaaT033 efficacy in gut microbiota diversity restoration using alpha-diversity (Richness index) To evaluate the cumulative incidence of grade 2-4 acute GvHD within 6 months after alloHCT To evaluate the cumulative incidence of non-relapse mortality within 12 months after alloHCT

Hämatologische Erkrankung, bei der eine allogene Stammzelltransplantation indiziert ist

Age ≥ 50 years old Presence of a hematologic malignancy for which an alloHCT is indicated with a reduced toxicity or reduced intensity conditioning regimen Patients with polynuclear neutrophils > 0.5 G/L Patients having received wide spectrum antibiotics within the last 90 days prior to inclusion Karnofsky index ≥ 70%

Patients planned to receive a non-myeloablative conditioning regimen (2 Gray total body irradiation (TBI) +/- purine analog, fludarabine + cyclophosphamide or equivalent) Patients planned to receive a conventional myeloablative conditioning regimen (e.g. high dose cyclophosphamide and high dose TBI (≥10Gy); high dose busulfan (12.8 mg/kg IV) + high dose cyclophosphamide) Patients receiving a manipulated graft (in-vitro T-cell depletion) Patients planned to receive a conditioning regimen with alemtuzumab Patients planned to receive alloHCT with cord blood cells Patients planned to receive alloHCT from unrelated donor with >= 3/10 HLA-mismatches Patients receiving a large spectrum antibiotic at time of randomization