MS is a chronic autoimmune disease in which blood cells that are supposed to protect the body from infection mistakenly attack the body's own tissue. In MS, the target of this attack is a protein called myelin that coats nerves throughout the body. Damage to this protective layer can lead to loss of neurologic function.

There are a number of treatments available to MS patients. Interferon beta, Copaxone, and other drugs can delay the worsening of the disease in some patients. For other patients, more aggressive treatment with chemotherapy drugs such as Cytoxan or Imuran are needed. These drugs attempt to slow the disease by limiting the activity of the entire immune system. Because of this, they can often have serious side effects.

The ACCLAIM Study is testing a medication called "abatacept." Abatacept works different from most other MS treatments, as it is more specific in the immune cells that it targets. While abatacept is considered experimental for MS, it has been approved by the FDA to treat rheumatoid arthritis.

In the first phase of the study, all participants will receive 8 intravenous treatments over a period of 24 weeks. Then, if a participant remains eligible, they will enter the second phase of the study and will receive another 8 treatments over the following 24 weeks. Two-thirds (2 out of 3) of participants will receive the study drug abatacept in the first phase, and then an inactive form of the drug in the second phase. The remaining one-third (1 in 3) will get the placebo first, then the study drug in the second phase if they remain eligible. Therefore, all participants in the ACCLAIM trial will have the opportunity to receive the study drug abatacept if they remain healthy during the study. Participants will be asked to return for a follow-up visit 12 weeks after all treatments have been completed.

Regular appointments scheduled during the trial will be used to monitor participants' health and progress in the study. These appointments will include: physical and neurological exams, blood tests and motor function assessments. A total of 11 MRI procedures are scheduled during the study. The study medication and procedures related to the study will be provided at no expense to the participant.