Abatacept for Rheumatoid Arthritis

Inclusion Criteria: M or F \>=18 y.o. with diagnosed RA. Disease related inclusion criteria: 1\. Patients being treated with MTX with or without csDMARDS at stable doses for at least 4 weeks, who are either: (a) about to start ABA, OR, (b) who are likely to remain on stable DMARDs and who might also be using a TNFi but in whom all therapies will have been stable for 4 or more weeks. 2.) meet diagnostic criteria for RA, based on 2010 ACR Criteria or 1987 RA criteria, OR, are one point short of meeting the criteria but are being treated for RA, OR, have previously identified RA typical erosions. 3\) have never received ABA or rituximab, and, if previously used a JAKi, will have stopped this for over a month, OR, if previously used an IL-6 inhibitor, will have stopped for at least 3 months. 4\) RA subject must not be taking prednisone at doses over 10 mg daily, and will not have received injectable Depomedrol or equivalent within 4 weeks of baseline or prior to the 3 or 6 month study assessment. Healthy control patients cannot be taking prednisone. Any subject can use oral or nasal inhalers that include glucocorticoids 5) have evidence of recent or currently active disease depending on treatment arm. Subjects starting ABA (Arm 1) are expected to have at least moderate disease activity, OR if CDAI is between 2.81 and 10, there should be two or more swollen and tender joints. For subjects being included in Arm 2, the stable treatment arm, CDAI \<13 and one swollen joint observed by a rheumatologist in the prior 6 months. Subjects recruited to the healthy control arm (Arm 3) will be free of any autoimmune disease or systemic form of an inflammatory arthritis. 6\) WOCBP must be using acceptable forms of contraception to avoid pregnancy throughout the study, i.e. oral contraceptives, other hormonal contraceptives. Specific Exclusion Criteria: 1. Have severe complications of RA that might require imminent escalation of therapy, e.g. pericarditis, active vasculitis of a major organ system. 2. Have an autoimmune disease or systemic inflammatory rheumatic disease (e.g., lupus erythematosus) that could confound T and B cell subset results 3. Have a concurrent serious medical disease (e.g., terminal malignancy) 4. Have a BMI indicating poor health (\<18 or \> 40) 5. Have received the following Prohibited Treatments and/or Therapies 1. treatment with rituximab 2. exposed to ABA or CTLA-4Ig 3. exposed to any investigational drug within 28 days. 4. received any live vaccines within 2 weeks prior to study start. Subjects cannot receive a live vaccine at any time during the study. 6. WOCBP with a positive pregnancy test on enrollment or prior to study start, OR who are unwilling or unable to use an acceptable method to avoid pregnancy despite continuing MTX. 7. part of a vulnerable population