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

Patients are screened and must sign informed consent at visit 1. At the 2nd visit, all patients receive a baseline infusion of Natalizumab, which is followed by an 8 week washout Phase. After the washout Phase all patients receive fingolimod for 32 weeks.