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.