Effects of Intravascular Administration of Mesenchymal Stromal Cells Derived from Wharton's Jelly of the Umbilical Cord on Systemic Immunomodulation and Neuroinflammation After Traumatic Brain Injury.

20 healthy volunteers will be included for MRI normalization Volunteer eligibility criteria Inclusion criteria : * Age 18-50 years * ASA 1 classification (healthy patient) Exclusion criteria : * Lack of written consent * Neurological history likely to alter the image (epilepsy, transient ischaemic attack, meningitis, head trauma) * Vulnerable person according to article L1121-6 of the CSP * Protected adult person * No affiliation to a social security regime * Pregnancy * Contraindication for MRI and PET-MRI * patients with Pacemaker and defibrillator * MR-incompatible prosthetic heart valve * Metallic intraocular, intra cerebral or intra medullary foreign bodies * Implantable neurostimulation systems * Cochlear implants/ear implant * Metallic fragments such as bullets, shotgun pellets, and metal shrapnel * Cerebral artery aneurysm clips * Ventriculo peritoneal shunt with metallic component generating significant artefacts on the MR sequence * Catheters with metallic components (Swan-Ganz catheter) * Patient unable to remain supine and motionless during the duration of the examination 68 severe TBI patients with the following inclusion and exclusion criteria will be included" Patient Inclusion criteria * Age 18-50 years * Severe TBI defined by: * Glasgow score \<12 within the 48 first hours, * Brain traumatic lesion on CT scan, * Need for intracranial pressure monitoring * No other significant organ trauma (AIS \<2) * Unresponsive to verbal commands 5 days after sedation discontinuation, for whom, after usual clinical and paraclinical evaluation there has been no decision to interrupt active therapies within 10 days after sedation discontinuation * Written consent signed by the close relative Patient Exclusion criteria * History of disease or treatment impairing current or previous year immunity function ( hematologic disease (leukemia, myeloma), viral disease affecting immunity (like HIV), immunological treatment (corticoid, anti rejection medication, anti TNFα, chemotherapy) * History of severe neurological or psychiatric disease likely to alter neurological assessment * HTAP \> grade III OMS/WHO * Ongoing uncontrolled infection with organ failure (septic shock, ARDS) including those due to severe COVID-19 * Platelets \<100 G/L or \<100000/μL, Hb \<8 g/dL, lymphocytes count \<1.5 G/L or 1500 μL , neutrophils count \< 2.5G/L or \<2500/μL, , creatinin \> 100 μmol/L * Liver function abnormalities (bilirubin\> 2.5mg / dL or transaminases\> 5x the ULN). Patients with Gilbert's disease are eligible if liver tests are normal excluding bilirubinemia * Known HIV seropositivity * Neoplasia ongoing or treated in the 3 years before screening * Bone marrow transplant recipient * History of transfusion reaction or hypersensitivity * Pregnancy * Contraindication for MRI and PET-MRI: * Patient with Pacemaker and defibrillator * MR-incompatible prosthetic heart valve o Metallic intraocular, intra cerebral or intra medullary foreign bodies * Implantable neurostimulation systems o Cochlear implants/ ear implant * Metallic fragments such as bullets, shotgun pellets, and metal shrapnel * Cerebral artery aneurysm clips * Ventriculo peritoneal shunt with metallic component generating significant artefacts on the MR sequence * Catheters with metallic components (Swan-Ganz catheter) * Patient unable to remain supine and motionless during the duration of the examination * Participation in another interventional clinical trial of an investigational therapy within 30 days of consent * No affiliation to a social security regime * Vulnerable person according to article L1121-6 of the CSP * Protected adult person