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To investigate the safety and efficacy of abatacept with steroid treatment in comparison to steroid treatment alone in up to a 28 week taper of steroid treatment to sustain remission of Giant Cell Arteritis in adults.

For more information regarding Bristol-Myers Squibb Clinical Trial participation, please visit www.BMSStudyConnect.com

Inclusion Criteria:

• New headache (new onset or new type of localized pain in the head)
• Elevated ESR (≥ 50 mm/h by the Westergren method) or CRP ≥ 1 mg/dL
• Temporal artery abnormality (i.e. temporal artery tenderness to palpation or decreased pulsation, unrelated to arteriosclerosis of cervical arteries)
• Temporal artery biopsy showing vasculitis characterized by a predominance of mononuclear cell infiltration or granulomatous inflammation, usually with multinucleated giant cells
• Large vessel biopsy showing vasculitis characterized by a predominance of mononuclear cell infiltration or granulomatous inflammation, usually with multinucleated giant cells or characteristic changes of large vessel stenosis or aneurysm secondary to GCA as seen by arteriography (Magnetic Resonance Imaging/ Magnetic Resonance Angiography), ultrasound (eg, halo sign on color duplex sonography), or CT scan
• Patients must be treated with prednisone or prednisolone of 20-60 mg/day (prednisone equivalent) and be on a dose between 20-60 mg/day for at least 2 weeks prior to enrollment into the study

Exclusion Criteria:

• Rheumatic disease other than GCA such as Takayasu's Arteritis, granulomatosis with polyangiitis (Wegener's), rheumatoid arthritis, systemic lupus erythematosus
• Patients with unilateral blindness (partial or complete) or who have unstable or recurrent visual symptoms attributable to GCA within 4 weeks of randomization
• Patients with a history of dissection of aorta
• Patients with a history of myocardial infarction, stroke or transient ischemic attack attributable to GCA within the 3 months of screening
• Patients who have been treated with intravenous ("pulse") doses of glucocorticoids defined as methylprednisolone > 1000 mg/day if given within 6 weeks of randomization
• Patients who will require oral or IV glucocorticoid treatment during the trial for conditions other than GCA
• Patients at risk of tuberculosis

Other protocol defined inclusion/exclusion criteria could apply