Study to Assess Immune Function and MRI Disease Activity in RRMS Patients When Switching From Natalizumab to Gilenya

A trial in patients with relapsing remitting multiple sclerosis (RRMS) Main objectives: * To evaluate changes in the reconstitution of immune surveillance over time upon switching from natalizumab to fingolimod assessed by a change in the expression of CD49d. * To evaluate changes in the migratory capacity of immune cells/peripheral blood mononuclear cells (PBMCs) upon switching from natalizumab to fingolimod in an in-vitro model of the blood-brain-barrier (BBB). * To evaluate changes in paraclinical disease activity over time upon switching from natalizumab to fingolimod assessed by MRI (changes in Gd+, T2w lesions and DTI). * To evaluate changes in T1w / FLAIR lesions upon switching from natalizumab to fingolimod.