The purpose of this study will be to demonstrate improvement of the T regulatory cells (Treg) response, under add on low-dose Interleukin 2 (ld-IL2) in patients with bipolar disorders experiencing a depressive relapse

A hypothesis still under study concerning the pathophysiological mechanisms of bipolar disorder concerns an immunological disorder that could possibly be involved in this disease and during a depressive episode. In particular, that the multiplication of a certain category of white blood cells called "regulators" that block the toxic effect of more aggressive white blood cells could improve these disorders.

The aim of this research is therefore to check the effectiveness and safety of injections of Interleukin 2 (IL-2) to induce this multiplication of regulatory white blood cells.

IL-2 is a natural protein released by white blood cells that is used for the proper functioning of the immune defence system. The favourable effects of injecting small doses of IL-2 are known and assessed in other diseases, including autoimmune diseases.

The secondary objective of this study is to assess the effect of these injections on patient's mood, which could be improved by this treatment.

Inclusion Criteria:

• A depressive episode according to DSM-V criteria in the course of a bipolar disorder
• MADRS score > 17
• Already on a mood stabilizer and/or antidepressant
• Patient with a normal or controlled thyroid function
• Male or female both using effective methods of contraception during treatment if sexually active.

Exclusion Criteria:

• - Contraindication to IL-101 therapy:
• Hypersensitivity to active substance or excipient;
• Active infection requiring antibiotics therapy;
• Organ failure (e.g., liver, kidney, lung and heart);
• Immunosuppressed patient
• Hepatotoxic, nephrotoxic, myelotoxic or cardiotoxic drugs
• Other chronic diseases
• Signs of active infection requiring treatment
• Previous history of organ transplantation
• Leukocytes < 4000 / mm3, platelets < 100 000 / mm3, Hemoglobin < 10.0 g/dL or 6.2 mmol/L, red cell blood < 3.5 T/L.
• Anti-TPO or anti-TG or anti-TRACKS positive at inclusion.
• Use of anti-inflammatory medication on a regular basis for a chronic inflammatory/autoimmune Disorder (NSAD, immunosuppressant IV-Ig based treatment);
• Ongoing fever < 38
• uncontrolled diabetes type I or II;
• Existing cancer or history of cancer in the last 5 years (except skin epidermoid cancer or in-situ cervix cancer);
• Existing or planned pregnancy or lactation;
• Person under legal protection (1121-8 of CSP, Public Health Code
• Pregnant and parturient and Breast feeding women (1121-5 of CSP)
• legally detained person (1121-6 of CSP)
• hospitalisation without consent
• under the age of majority (1121-7of CSP)
• Immediate risk for suicidal behaviour (MADRS-item 10 >2 or columbia > 2 for suicide idea);
• Known HIV infection or clinically manifest Acquired Immune Deficiency Syndrome (AIDS), Parkinson's or Alzheimer's disease, or any other serious condition likely to interfere with the conduct of the trial;
• Participation to an interventional study concomitantly or within 30 days prior to this study, except in the cohorts studies aiming at the analysis of immuno-inflammatory biomarkers and/or brain imaging studies.
• Patients thought to be unreliable or incapable of complying with the requirements of the protocol;
• Patient is relative of, or staff directly reporting to the investigator;
• Patient is employee of the sponsor.