This is a randomised double-blinded placebo-controlled multi centre study in 98 patients with ACPA positive arthralgia. The study is composed of 2 arms with a 1:1 randomisation at baseline: Treatment phase will be 6 months. Group 1: Abatacept s.c. 125 mg weekly for 6 months vs. Group 2: Placebo s.c. for 6 months. After 6 months both groups will run into the follow up period for another 12 months up to month 18. Patients developing arthritis will be treated according to local guidelines.

Inclusion Criteria:

• Females and males aged ≥ 18 years at time of consent
• ACPA (with or without RF)
• Joint pain (hand, feet, knee, shoulder or elbow) present for at least 6 weeks prior to inclusion or in past history
• Presence of synovitis (synovialitis or tenosynovitis) or osteitis in MRI of the dominant hand at baseline
• Women of childbearing potential or men capable of fathering children must be using effective contraception during treatment with abatacept and up to 14 weeks after the last dose of abatacept treatment.
• Must understand and voluntarily sign an informed consent form including written consent for data protection
• Must be able to adhere to the study visit schedule and other protocol requirements

Exclusion Criteria:

• Presence of arthritis in hand, feet, knee, shoulder or elbow joints defined as swelling
• Current treatment with glucocorticoids conventional or biologic DMARDs
• Previous treatment with abatacept
• Investigational study drug within 4 weeks (or 5 half lives, whichever is longer) prior to randomization
• Any condition, including the presence of laboratory abnormalities, which places the subject at unacceptable risk if he/she were to participate in the study or confounds the ability to interpret data from the study
• Any other autoimmune or inflammatory disease such as SLE, PSS, MCTD, SpA, Behcet disease, vasculitis or autoimmune hepatitis.
• Any malignancy in the last 5 years
• Chronic infection such as latent TB (TB not adequately treated according to guidelines) or hepatitis B or C infection
• Immunocompromised or HIV-positive patients
• Uncontrolled severe concomitant disease
• Patients who are younger than 18 years or are incapable to understand the aim, importance and consequences of the study and to give legal informed consent (according to § 40 Abs. 4 and § 41 Abs. 2 and Abs. 3 AMG).
• Pregnant or lactating females
• Patients who possibly are dependent on the Principal Investigator or Investigator (e.g. family members).