A Study to Evaluate the Efficacy and Safety of Belimumab Administered in Combination With Rituximab to Adult Subjects With Systemic Lupus Erythematosus (SLE) - BLISS-BELIEVE

Inclusion Criteria

• Subjects must be >=18 years of age at the time of signing the informed consent.
• Subjects who have clinical diagnosis of SLE based on 4 or more of the 11 American College of Rheumatology (ACR) criteria.
• Subjects who have a screening SLEDAI-2K score >=6 (This refers to the total score. Serological activity, i.e., anti-double stranded deoxyribonucleic acid [dsDNA]) positivity and/or hypocomplementemia is not required to be present in SLEDAI-2K assessment, but are scored if present).
• Subjects who have unequivocally positive autoantibody test results defined as an anti-nuclear (ANA) titer >=1:80 and/or a positive anti-dsDNA (>=30 International Units per milliliter [IU/mL]) serum antibody test from 2 independent time points as follows: Positive test results from 2 independent time points within the study screening period. Screening results must be based on the study's central laboratory results. Or, one positive historical test result and 1 positive test result during the screening period.
• Subjects who are on a stable SLE treatment regimen consisting of any of these medications (alone or in combination) for a period of at least 30 days prior to Day 1 (i.e. day of first dose of study treatment) with the exception that switching one agent for another of the same class for tolerability or availability reasons, which will be allowed within 30 days of Day 1: Corticosteroids (prednisone or prednisone equivalent); For those subjects on alternating daily doses of steroids, use the average of 2 daily doses to calculate the average daily steroid dose; Any immunosuppressant or immunomodulatory agents including methotrexate, azathioprine, leflunomide, mycophenolate (including mycophenolate mofetil, mycophenolate mofetil hydrochloride, and mycophenolate sodium), calcineurin inhibitors (example [e.g.] tacrolimus, cyclosporine), sirolimus, oral cyclophosphamide, 6-mercaptopurine, mizoribine, or thalidomide; Anti-malarials (e.g., hydroxychloroquine, chloroquine, quinacrine); Non steroidal anti-inflammatory drugs (NSAIDs).
• Male and/or female. A female subject is eligible to participate if she is not pregnant not breastfeeding, and at least one of the these conditions applies: Not a woman of childbearing potential (WOCBP) or A WOCBP who agrees to follow the contraceptive guidance during the treatment period and for at least 16 weeks after the last dose of belimumab, or at least 12 months after the last dose of rituximab or rituximab-placebo.
• Capable of giving signed informed consent which includes compliance with the requirements and restrictions listed in the informed consent form (ICF) and in this protocol.

Exclusion Criteria

• Symptomatic herpes zoster within 3 months prior to screening.
• Evidence of active or latent tuberculosis (TB). Documentation may include medical history and examination, chest X-rays (posterior, anterior, and lateral), and TB testing: either a positive tuberculin skin test (TST; defined as a skin induration ≥5 mm at 48 to 72 hours, regardless of Baccillus Calmette-Guerin (BCG) or other vaccination history) or a positive (not indeterminate) QuantiFERON-TB Gold Plus test.
• Significant allergies to humanized monoclonal antibodies.
• History of hypersensitivity to belimumab and/or rituximab or known to have titers of human anti-mouse antibody or history of hypersensitivity reactions when treated with other diagnostic or therapeutic monoclonal antibodies.
• Lymphoma, leukemia, or any malignancy within the past 5 years (yrs) except for basal cell or squamous epithelial carcinomas of the skin that have been resected with no evidence of metastatic disease for 3 yrs.
• Alanine transferase (ALT) greater than 2 times upper limit of normal (ULN).
• Bilirubin greater than 1.5 times ULN (isolated bilirubin greater than 1.5 times ULN is acceptable if bilirubin is fractionated and direct bilirubin less than 35%).
• Immunoglobulin A (IgA) deficiency (IgA level less than 10 milligram per deciliter [mg/d