Primary objective: To assess the efficacy and safety of different dose regimens of ALX-0061 administered subcutaneously (s.c.) to subjects with moderate to severe active, seropositive systemic lupus erythematosus (SLE) compared to placebo. Secondary objectives: To assess the pharmacokinetics (PK), pharmacodynamics (PD), immunogenicity, flare rate, steroid reduction and health-related quality of life, with different dose regimens of ALX-0061.
Inclusion Criteria: 1. Man or woman ≥ 18 years and \< 65 years of age 2. Have a diagnosis of SLE for at least 6 months prior to screening and fulfill the 1997 American College of Rheumatology (ACR) or 2012 Systemic Lupus International Collaborating Clinics (SLICC) classification criteria 3. Have moderate to severe active SLE 4. Have seropositive disease at screening 5. Subject must be at least on one or more of the treatments for SLE as listed in the protocol 6. Others as defined in the protocol Exclusion Criteria: 1. Have an A score on the revised BILAG-2004 other than in the mucocutaneous and/or musculoskeletal system at screening and at baseline for the organ systems that can be clinically assessed 2. Have a systemic inflammatory disease other than SLE 3. Clinically significant infection treated or needing treatment 4. Any active or recurrent viral infection that based on the Investigator´s clinical assessment makes the subject unsuitable for the study 5. Have received prior therapy blocking the interleukin-6 (IL-6) pathway 6. Others as defined in the protocol