Depression is one of the most common symptoms of multiple sclerosis (MS) with a life-time prevalence of major depressive disorder (MDD) of up to 50%. Depression occurs more frequently in MS than in other chronic diseases including other neurological and inflammatory disorders and may contribute to lower quality of life, cognitive problems, difficulties at work, and poorer long term health outcomes. Despite its clinical relevance, the biological mechanisms which may be responsible for the high risk for MS patients to develop depression are unknown.

In this observational study, investigators explore the molecular mechanisms responsible for the impaired regulation of immune cells in relapsing-remitting (RR) MS patients with depression. Investigators will compare the molecular and phenotypical profile of immune cells obtained from RRMS patients with clinical depression (n=50), matched MS patients who do not suffer from depression (n=50) as well as matched healthy controls (n=50) and matched patients with depression but without a comorbid neurological disorder (n=50).

Inclusion Criteria:

• Relapsing remitting MS by McDonald Criteria (for RRMS and RRMS+MDD groups).
• Currently meeting diagnostic criteria for Major Depressive Disorder (for MDD and RRMS+MDD groups)
• Age 18-55.
• On stable treatment regime, either untreated or on disease modifying therapy (DMT) for > 6 months

Exclusion Criteria:

• Secondary-progressive or primary progressive MS.
• Substance abuse for more than 6 months.
• Pregnancy
• Use of steroids or vaccination or infections in the previous 3 months.
• Signs of serious psychiatric pathology other than depression including schizophrenia, bipolar disorder or developmental and learning disorders including disorders on the autism spectrum
• Antidepressant medication.

Patients will be recruited at neurological outpatient clinics and neurologcial clinics of the Charité and neurologists' medical practices.