Participation in this study may last up to 4 years. Participants will receive abatacept intravenously on specified days during Months 1, 2, and 3. They will also receive daily prednisone, which will be started at a dose of 40 to 60mg, then tapered to 20mg by Month 3, and finally further tapered until discontinuation is reached. At Month 3, participants who have achieved remission will be randomly assigned under double-blind conditions to either continue abatacept or be switched to placebo infusions. Both treatments will be given once a month at study visits. Blood samples will also be collected at the monthly study visits to conduct laboratory-based studies. Participants who remain in remission will continue to receive abatacept or placebo monthly until the common closing date, defined as 12 months after enrollment of the 33rd participant for each disease.