Inclusion Criteria:

• - Men and women.

• - Aged between 18 and 50 years old, - With multiple sclerosis already diagnosed and fulfilling the modified Mc Donald criteria.

• - Stable (no relapse in the last 6 months and MRI less than a year old, with no new lesions) and with at least 1 year of treatment with Natalizumab, Ocrelizumab, Teriflunomide or Fumarate.

• - Subject medically fit to give a maximum of 50 ml of extra whole blood.

• - Patient having given their consent to participate in the study.

Exclusion Criteria:

• - HIV serology known or discovered during the present episode.

• - Demented patient.

• - Patient treated with corticosteroids in the last month.

• - Non-affiliated person or non-beneficiary of a social security scheme.

• - Uncooperative patient.

• - Patient under legal protection, guardianship or curatorship.

- Pregnant or breastfeeding women

In a previous translational study (5-HT SEP), the expression modulation of 5-HT7 receptor in function of immunological context was investigated, in different group of MS patients:

• - Group 1: MS patients with an acute relapse.

• - Group 2: MS stable patients treated with Natalizumab.

• - Group 3: Healthy people (from Etablissement Français du Sang) From blood cells of different groups of individuals, the investigators determined the percentage and intensity of expression of the receptor on circulating lymphocytes by flow cytometry.

The investigators demonstrated in this first study that the 5-HT7 receptor is mainly expressed on the surface of B lymphocytes compared to T lymphocytes. Interestingly, while the percentage of T lymphocytes (CD4+, CD8+) remained unchanged between the 3 groups, the investigators observed that the expression of R5-HT7 on the surface of the T lymphocyte subpopulations (Th1, Th2, Th17 and Treg), is significantly increased in the blood of MS patients treated with Natalizumab compared to healthy individuals and MS patients in relapse. Considering litterature data and our preliminary results showing the therapeutic potential of 5-HT7R ligand in animal models of MS as well as the upregulation of 5-HT7 expression in NTZ-treated patients (5-HTSEP study), the investigators wish to go further with these investigations. The main objective of this new translational study is to demonstrate the interest of the 5-HT7 serotonin receptor as a biomarker in the therapeutic monitoring of patients suffering from Multiple Sclerosis. The investigators first want to know if the increase in the expression of R5-HT7 on the surface of lymphocyte populations in stable MS patients is due specifically to Natalizumab treatment or if other treatments would also induce a similar effect. To answer this question, the investigators will study the expression of the receptor not only on the surface of lymphocyte cells but also on the surface of monocytes, NK cells and polymorphonuclear cells (cells not investigated in the first 5-HTSEP study) in three groups of individuals:

• - Group 1: Healthy individuals, so-called "control" donors whose blood samples will be collected from the EFS (French Blood Establishment) - Group 2: Stable MS patients without inflammatory activity of the disease treated with high efficacy treatment (Natalizumab or Ocrelizumab) - Group 3: Stable MS patients without inflammatory disease activity treated with moderately effective treatment (Teriflunomide or Fumarate) Group 1 (healthy volunteers) is a control group necessary for the smooth running of the study and the interpretation of the results.

For group 2, the Natalizumab treatment has already been investigated during the previous 5-HTSEP study. This new study will make it possible to 1) compare the expression/activity of the receptor in the three groups 2) check the reproducibility of the results compared to the previous study 3) evaluate any variations linked to the temporality of the treatment with 2 blood samples, a first before injection of Natalizumab, a second 14 days later. Indeed, unlike the 5-HTSEP study, where a single sample 1 month after injection of Natalizumab was analysed, the investigators wish to take these two samples, in order to carry out a longitudinal study making it possible to observe the evolution of the expression of the 5-HT7 receptor as a function of the temporality of the treatment. Group 3 will assess whether other treatments (teriflunomide or fumarate) can induce an effect similar to that observed after Natalizumab treatment which is associated with an increase of R5-HT7 expression on the surface of lymphocyte populations in stable MS patients.

Arms

: Healthy individuals

Group 1: Healthy individuals, so-called "control" donors whose blood samples will be collected from the EFS (French Blood Establishment)

: Stable MS patients treated with high efficacy treatment

Group 2: Stable MS patients without inflammatory activity of the disease treated with high efficacy treatment (Natalizumab or Ocrelizumab)

: Stable MS patients treated with moderately effective treatment

Group 3: Stable MS patients without inflammatory disease activity treated with moderately effective treatment (Teriflunomide or Fumarate)

Interventions

Biological: - Blood sample

Blood sampling will be done on three groups: Group 1: Healthy volunteers, so-called "control" donors whose blood samples will be ordered from the EFS (French Blood Establishment) Group 2: Stable MS patients without inflammatory activity of the disease treated with high efficacy treatment (Natalizumab or Ocrelizumab) Group 3: Stable MS patients without inflammatory disease activity treated with moderately effective treatment (Teriflunomide or Fumarate)