This study will include a sub-study to evaluate the efficacy, safety and tolerability of tofacitinib 5 mg BID administered as monotherapy after methotrexate withdrawal compared to tofacitinib 5 mg BID continued in combination with methotrexate. The sub-study will be available to subjects who have completed at least 24 months of participation in the open-label extension study and meet eligibility criteria for the sub-study.