Impact of Natalizumab Versus Fingolimod in Relapsing-Remitting Multiple Sclerosis (RRMS) Participants

Key Inclusion Criteria for MS Patients: * Must have a documented diagnosis of relapsing MS (McDonald 2010 Criteria) at study screening with EDSS score from 0.0 to 5.5. * If the subject is on Betaseron, Rebif, Avonex, Copaxone, Extavia, Tecfidera, and Aubagio (BRACE-TA) at study screening: * He/she must have been on therapy for at least 6 months (unless experiencing highly active disease), have at least 9 T2-hyperintense lesions on a brain MRI scan, and have experienced ≥1 relapse within the last 6 months prior to study screening with ≥1 new T1-Gd+ lesion on a brain MRI scan performed ≤6 months prior to study screening or ≥2 new T2 lesions on a brain MRI scan performed ≤6 months prior to study screening, with comparison made to a T2 MRI scan performed up to 18 months before study screening * If the subject has highly active disease, regardless of whether they are disease-modifying therapy (DMT)-naïve or had previous exposure to Betaseron, Rebif, Avonex, Copaxone, Extavia, Tecfidera, and Aubagio (BRACE-TA), they must have had ≥2 disabling relapses in the 12 months prior to study screening and either ≥1 new T1-Gd+ lesion on a brain MRI scan performed ≤6 months prior to study screening or ≥2 new T2 lesions on a brain MRI scan performed ≤6 months prior to study screening, with comparison made to a T2 MRI scan performed up to 18 months before study screening Key Exclusion Criteria for MS Patients: * Diagnosis of Primary Progressive Multiple Sclerosis and/or Secondary Progressive Multiple Sclerosis. * History or positive test result at study screening for human immunodeficiency virus (HIV), hepatitis C virus (HCV) antibody or current hepatitis B infection (defined as positive for hepatitis B surface antigen \[HBsAg\] and/or hepatitis B core antibody \[HBcAb\]). * Prior treatment with natalizumab or fingolimod. * History of or known active malignant disease, including solid tumors and hematologic malignancies (subjects with cutaneous basal and squamous cell carcinoma that has been completely excised and considered cured prior to study screening remain eligible). * History of opportunistic infections or any clinically significant major disease, as determined by the Investigator. * A clinically significant infectious illness (e.g., pneumonia, septicemia) within the 1 month prior to study screening. * History of drug or alcohol abuse (as defined by the Investigator) within 1 year prior to study screening. * Prior history of immunosuppressant use (e.g., mitoxantrone, azathioprine, methotrexate, cyclophosphamide, mycophenolate, cladribine, rituximab), or exposure to intravenous immunoglobulin (IGIV), monoclonal antibodies, cytokines, growth factors, soluble receptors, other recombinant products, or fusion proteins in the last 12 months prior to study screening. * History of myocardial infarction, unstable angina, stroke, transient ischemic attack, decompensated heart failure in last 6 months. * Treatment with Class Ia (e.g., procainamide, quinidine, ajmaline, disopyramide) or Class III (amiodarone, bretylium, dofelitide, sotalol, ibulitide, azilimide) anti-arrhythmic drugs. * Concurrent therapy with drugs that slow heart rate (e.g., beta-blockers, heart-rate lowering calcium channel blockers such as diltiazem or verapamil, or digoxin). * Hypertension not controlled with prescribed medications. * History of severe respiratory disease, pulmonary fibrosis or class III or IV chronic obstructive pulmonary disease. * The use of live or live attenuated vaccination within 8 weeks of study screening. Key Inclusion Criteria for Healthy Volunteers: * Subjects who are generally healthy as demonstrated by physical examination and by medical history, with no history or evidence of major illnesses, diseases, or disorders. * Subjects of childbearing potential must practice effective contraception and be willing and able to continue contraception for duration of the study. * No history of drug or alcohol abuse (as defined by the Investigator) within 1 year prior to study screening. Key Exclusion Criteria for Healthy Volunteers: * Claustrophobia sufficient to interfere with generating reliable MRI scans. * History of other major illness including neurological disorders as determined by the Investigator. * Presence of a metal device affected by MRI (e.g., any type of electronic, mechanical or magnetic implant, cardiac pacemaker, aneurysm clips, implanted cardiac defibrillator) or potential ferromagnetic foreign body (metal slivers, metal shavings, other metal objects), which would be a contraindication for MRI. * Women who are currently pregnant or breastfeeding, or who have a positive pregnancy test result at screening. NOTE: Other protocol defined Inclusion/Exclusion criteria may apply