Addition of Belimumab to B-cell Depletion in Relapsing-remitting Multiple Sclerosis

Inclusion Criteria: * Diagnosis of RRMS based on McDonald criteria 2017 * Age \> 18 * A clinical relapse in the past 12 months OR an enhancing lesion on brain/ spinal cord MRI in the past 6 months OR a new T2/FLAIR lesion on a brain/spinal cord MRI obtained in the past 6 months (compared to a previous MRI obtained within one year from the latest MRI) * Pre-existing pneumococcal antibody titers (\>1.0 mg/mL) to =\<9 of 23 vaccine serotypes * Female Subjects: Not pregnant or nursing and at least one of the following conditions apply: a. Non- childbearing potential defined as pre-menopausal females with a documented tubal ligation or hysterectomy or postmenopausal defined as 12 months of spontaneous amenorrhea. b. Child-bearing potential and agrees to use one of the contraception methods as described by the investigator or designee, from Day 0 until 24 weeks after the last dose of study medications (See details below). * Liver function at the time of screening: alanine aminotransferase (ALT) \< 2x upper limit of normal (ULN); bilirubin \<= 1.5x ULN (isolated bilirubin \>1.5x ULN is acceptable if bilirubin is fractionated and direct bilirubin \<35%). Exclusion Criteria: * Prior therapy at any time: has ever received any of the following: a) B-cell targeted therapy (e.g., rituximab, ocrelizumab, other anti-cluster of differentiation (CD)20 agents, anti-CD22 \[epratuzumab\], anti-CD52 \[alemtuzumab\], B lymphocyte stimulator (BLyS)-receptor fusion protein \[BR3\], Transmembrane activator and calcium-modulating ligand (CAML) interaction (TACI) fragment, crystallizable (Fc), or belimumab) * Prior use of cladribine, mitoxantrone, cyclophosphamide, or hematopoietic stem cell transplantation (HSCT) * Lymphopenia: a lymphocyte count \<500/ millimeter (mm)\^3 * Neutrophils \<1.5x 10E9/L. * Drug sensitivity: a history of sensitivity to any of the study medications, or components thereof or a history of drug or other allergies including a previous anaphylactic reaction to parenteral administration contrast agents, human or murine proteins or monoclonal antibodies * Treatment with steroids in the last 30 days * Clinically unstable medical or psychiatric disorder * Have evidence of serious suicide risk including any history of suicidal behavior in the last 6 months and/or any suicidal ideation in the last 2 months or who in the investigator's judgment, poses a significant suicide risk * Have a history of an anaphylactic reaction to parenteral administration of contrast agents, human or murine proteins or monoclonal antibodies * Substance abuse: has evidence of current drug or alcohol abuse or dependence * 365 Day prior therapy: has received a biologic investigational agent other than B-cell targeted therapy \[e.g., abetimus sodium, anti CD40L antibody (e.g., BG9588/ IDEC-131; investigational agent applies to any drug not approved for sale in the country in which it is being used\] * 30 Day prior therapy: has received any of the following within 30 days before Day 0: a) Any other MS disease-modifying therapy, not mentioned above (including fumaric acid esters, sphingosine-1-phosphate (S1P) receptor modulators, teriflunomide, and natalizumab). Glatiramer acetate and interferons are permitted up to the day of starting the investigational medication. Intravenous, oral, and Inhaled steroids and new topical immunosuppressive agents (e.g., eye drops, topical creams) are allowed. * 30 Day prior therapy: has received a live virus vaccine or a non-biologic investigational agent. * Malignancy: has a history of malignancy in the past 5 years except for adequately treated cancers of the skin (basal or squamous cell) or carcinoma in situ of the uterine cervix. * Have a history of a primary immunodeficiency * Have a significant IgG deficiency (IgG level \< 400 mg/dL) * Have an IgA deficiency (IgA level \< 10 mg/dL) * Infection history: * Currently on any suppressive therapy for chronic infection (such as tuberculosis, pneumocystis, cytomegalovirus, herpes simplex virus, herpes zoster, and atypical mycobacteria) * Hospitalization for treatment of infection within 60 days of Day 0. * Use of parenteral (IV or IM) antibiotics (anti-bacterial, antiviral, anti-fungal, or anti-parasitic agents) within 60 days of Day 0 * Other disease/conditions: has any of the following: a) clinical evidence of significant unstable or uncontrolled acute or chronic diseases (i.e., cardiovascular, pulmonary, hematologic, gastrointestinal, hepatic, neurological, malignancy or infectious diseases) which, in the opinion of the investigator, could confound the results of the study or put the subject at undue risk; b) a surgical procedure planned in the 6 months after Day 0; c) a known history of any other medical disease (e.g., cardiopulmonary), laboratory abnormality, or condition (e.g., poor venous access) that, in the opinion of the investigator, makes the subject unsuitable for the study * Hepatitis status: * Serologic evidence of current or past Hepatitis B (HB) infection based on the results of testing for HBsAg and HBcAb as follows: Patients positive for HBsAg or HBcAb are excluded * A positive test for Hepatitis C antibody * HIV: known to have a historically positive HIV test or tests positive at screening for HIV. * Laboratory abnormalities: has an abnormal laboratory assessment, which is judged clinically significant by the investigator. * Drug Sensitivity: has a history of sensitivity to any of the study medications, or components thereof or a history of drug or other allergies including a previous anaphylactic reaction to parenteral administration contrast agents, human or murine proteins or monoclonal antibodies that, in the opinion of the investigator or Medical Monitor, contraindicates their participation. * Any contraindication to undergoing MRI