The purpose of this study is to assess whether co-administration of belimumab and a single cycle of rituximab will optimize treatment with belimumab, which will result in improvements of clinical status with a favorable safety profile, by comparing subjects randomized to belimumab plus rituximab versus belimumab plus rituximab-placebo. Approximately 292 subjects will be randomized in a 1:2:1 ratio to 1 of 3 treatment arms; belimumab plus rituximab-placebo (Arm A, control), belimumab plus rituximab (Arm B, combination), or belimumab plus standard therapy (Arm C, reference). Belimumab will be administered as subcutaneous (SC) and rituximab-placebo or rituximab will be administered by intravenous (IV) infusions. The total duration of the study is for 104 weeks.