Study participants receive standard therapy for SLE in addition to receiving the study drug, either placebo (no active medicine) or belimumab. The controlled period of the study is 52 weeks. The random assignment in this study is "1 to 1" which means that participants have an equal chance of receiving belimumab or placebo. After completion of the 52-week study period, participants will be contacted by phone annually for 4 more years to assess health status.