This is a Phase II, randomized, double-blind, placebo-controlled multicenter study to evaluate the efficacy and safety of rontalizumab compared with placebo in patients with moderately to severely active systemic lupus erythematosus (SLE).

The study will be conducted in 3 parts. Parts 1 and 2 of the study will include a double-blind treatment period of 24 weeks and a safety follow-up period of 48 weeks for participants who do not continue onto Part 3. Patients meeting the eligibility criteria for enrollment in Part 3, will enter Part 3 after completion of the Week 24 visit but prior to completion of the Week 72 visit. In Part 1, participants will be randomized in a 2:1 ratio (active drug:placebo) to receive either rontalizumab 750 mg or matching placebo intravenously every 4 weeks for 24 weeks. Part 2 was will be initiated upon the completion of recruitment for Part 1. In Part 2, participants will be randomized in a 2:1 ratio (active drug:placebo) to receive either rontalizumab 300 mg or matching placebo subcutaneously every 2 weeks for 24 weeks. After Week 24, patients will enter a 48-week safety follow-up period, or, after the open label extension became available via protocol amendment, will have the option of entering Part 3 of the study, if eligible. In Part 3, all participants will receive rontalizumab 750 mg intravenously every 4 weeks for 120 weeks (up to 144 weeks total).

Inclusion Criteria:

• Diagnosis of SLE.
• Active disease at the time of screening.
• Agreement to use an effective form of contraception for the duration of the study.

Exclusion Criteria:

• Acutely life- or organ-threatening manifestations of systemic lupus erythematosus (SLE) (eg, proliferative nephritis, unstable neuropsychiatric disease).
• Pregnancy or breastfeeding.
• History of severe allergic or anaphylactic reactions to monoclonal antibodies or intravenous (IV) immunoglobulin.
• Significant, uncontrolled medical disease in any organ system not related to SLE that in the investigator's opinion would preclude patient participation.
• Concomitant conditions that required systemic corticosteroid use within 1 year prior to screening. Use of topical, intraarticular, or inhaled corticosteroids is not exclusionary.
• History of cancer within 5 years of screening.
• Any current or recent (within 4 weeks of screening) signs or symptoms of infection, except for minor infections, fungal infections of the nail beds, or oral or vaginal candidiasis.
• History of severe systemic bacterial, fungal, viral, or parasitic infections (2 or more hospitalizations or 2 or more courses of IV antibiotics) within 6 months prior to screening