The objective of the ORION study is to explore the changes of gut microbiota composition following MaaT013 administration and its impact on the immune system in GVHD patients.

The french regulatory authority ANSM approved in July 2019 the use of MaaT013, a pooled microbiome-based enema formulation, under a formalized named-patient use program in France called "Autorisation Temporaire d'Utilisation - ATU nominative protocolisée - ATUn" or Early Access.

The ATUn is indicated to provide a benefit in the treatment of patients with grade III-IV aGVHD:

• In case of initial resistance to CS alone or in association or in case of failure to other treaments
• In first-line therapy in association with CS in case of steroid-dependance (inability to taper CS dose <0.5 mg/kg/d)
• In case of digestive aGvHD with overlap syndrome The objective of the ORION study is to explore the changes of gut microbiota composition following MaaT013 administration and its impact on the immune system in GVHD patients.

Blood and stool samples will be collected at each visit which explained why the study is thus categorized as research involving the human person with low risks and constraints clinical trial (RIPH2).

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.