The maximum length of the study is 35 weeks, which includes the 2-week screening period, 1-week baseline period, 24-week open label treatment, and 8-week follow-up period. Eligible subjects are to continue their current MTX treatment regimen, a stable dose of at least 15mg/week, during the entire length of the study (24 weeks). At Day 0, patients who meet inclusion criteria, will be dosed from Day 0 to Week 24 with Abatacept 125 mg SC injection.