Inclusion Criteria:
* Age 12 - 35 years
* Acceptation of myelogram (bone marrow aspiration)
* Diagnosis of HbSS by Hb electrophoresis and genetic analysis to analyse the alpha locus
* Clinical history or ongoing evidence of severe sickle cell anemia with one OR more of the following clinical complications demonstrating disease severity:
* At least 3 vaso-occlusive crises requiring hospitalization, under hydroxyurea or transfusion, within 2 years prior to enrolment
* One severe acute chest syndrome (ACS) hospitalized in the intensive care unit
* At least 2 episodes of ACS, including one under HU.
* Acute priapism (at least 2 episodes \>3h in the preceding year or in the year prior to the start of a regular transfusion program), OR stuttering priapism ≥ 1 by week under sickle cell treatment (HU, transfusion or phlebotomy).
* Tricuspid regurgitation velocity \>2.8m/s on cardiac echocardiograph without pulmonary hypertension confirmed by right heart catheterization (mPAP\>\<25mmHg)
* Failed hydroxyurea (HU) therapy, OR Inadequate clinical response to HU, defined as any one of the following outcomes, while on HU for at least 3 months: 2 or more acute sickle pain crisis requiring hospitalization, requirement of transfusion to maintain Hb \>6.0g/dL, an episode of ACS despite adequate supportive care measures
* Karnovsky/Lansky performance score ≥ 60%
* Sexually active patients must be willing to use an acceptable method of double-barrier contraception for at least 12 months post-infusion (beyond 12 months at the discretion of the investigator)
* Procedure for obtaining consent (adults, dependent minors, to give their consent)
* Affiliation to social security
Exclusion Criteria:
* Existence of a matched sibling donor
* Based on myelogram, the presence of chromosomal (detected by karyotyping) or molecular abnormalities (detected by NGS) and retained dangerous by the Hemato-Oncology referent and validated during a specific multidisciplinary concerted meeting
* Hematologic evaluation: Leukopenia (WBC \<3,000/µL) or neutropenia (ANC \<1,000/µL) or thrombocytopenia (platelet count \<100,000/µL) within 90 days prior to mobilization or harvest (not due to an erythrapheresis procedure or possible acute viral infection)
* PT/INR or PTT \>1.5 times the upper limit of normal (ULN) or clinically significant bleeding disorder
* Two alpha deletions (risk of alpha-thalassemia after gene therapy)
* Hypersensitivity to the active substances of the administered drugs (plerixafor, busulfan, anti-inflammatory therapy) or to any of their excipients
* Patients who have already been treated with gene therapy
Evaluations within 6 months prior to screening visit:
* ALT or AST \>3 times ULN
* Severe liver iron overload evaluated by MRI (\>15mg Fe/g dry weight or \>270umol Fe/g dry weight) or liver cirrhosis suspicion on echography or elastometry or CT scan or MRI AND confirmed by histology
* Measured GFR \<60ml/min/1.73 m²
* Cardiac evaluation: LVEF \<40% by cardiac echocardiogram or by MUGA scan or clinically significant ECG abnormalities
* Stroke with significant CNS sequelae i.e., Rankin \>2
* Specific sickle cell disease cerebral vasculopathy confirmed by MRA (magnetic resonance angiography) OR transcranial doppler ultrasound with or without Moya-moya WITH an indication of chronic transfusion program (target HbS\<30%)
* Lung interstitial infiltrate AND Forced Vital Capacity less than 70% AND DLCO less than 60% at steady state
* Confirmed pulmonary hypertension defined by a right heart catheterization (PAPm \>25 mmHg). Right heart catheterization is required if tricuspid regurgitation velocity \>2.8m/s on cardiac echocardiograph OR \>2.5m/s with an abnormal Brain Natriuretic Peptide dosage or an important decrease in transcutaneous Hb O2 saturation during the 6 minutes' walk test.
* Seropositivity for HIV (Human Immunodeficiency Virus), HCV (Hepatitis C Virus), HTLV-1 (Human T-Lymphotropic Virus), or active Hepatitis B Virus, or active infection by CMV or parvovirus B19, based on positive blood PCR.
* Pregnancy or breastfeeding in a postpartum female
* Any current cancer or prior history of a malignant disease, with the exception of curatively treated non-melanoma skin cancer
* Immediate family member with an established or suspected Familial Cancer Syndrome
* Diagnosis of significant psychiatric disorder of the subject that could seriously impede the ability to participate in the study
* Patients who failed previous HSCT
* Any clinically significant active infection
* Participation in another clinical study with an investigational drug within 30 days of screening
* Any condition, based on perspective of the medical monitor and treating investigator, which may lead to increased safety risk or inability to comply with the protocol
