Inclusion Criteria:  Patient ≥18 years old with Grade III-IV aGVHD with gut involvement undergoing treatment with MaaT013 through named-patient use program ATUn / Early Access Signature of informed and written consent by the subject or by the subject's legally acceptable representative for patients under guardianship or trusteeship. Affiliated or recipient from a social security scheme  Exclusion Criteria:  Pregnancy and breastfeeding Vulnerable patients such as: minors, persons deprived of liberty, persons in Intensive Care Unit unable to provide informed consent prior to the intervention.