Treatment Interruption of Natalizumab

This is a randomized, rater blinded trial in patients who interrupt treatment with natalizumab with or without being treated with other immunomodulatory drugs, or continue treatment with natalizumab. The main purpose of this study is to find out the following, when participants stop taking natalizumab for 24 weeks: * when MS symptoms return, and * if other drugs for MS may help control MS symptoms during the natalizumab-interruption period. This study will also explore how quickly the effects of natalizumab return after resuming natalizumab dosing.