All participants who completed the 12-month double-blind study period were eligible to continue in the open-label study. Participants received placebo, Abatacept 2 mg/kg, or Abatacept 10 mg/kg in the double-blind study. Participants receiving placebo in the double-blind study were switched 1:1 to continued treatment with placebo or Abatacept 2 mg/kg. Participants receiving Abatacept 2 mg/kg or Abatacept 10 mg/kg continued at the double-blind study dosage. After results from the double-blind period became available, all participants were switched to a weight-tiered 10 mg/kg dose of Abatacept.