• Assess the proportion of subjects with new clinical flare of SLE (Adjudicated BILAG "A" or "B") [ Time Frame: during the 1 year double blind treatment period ] [ Designated as safety issue: No ]
• Assess the long term safety and tolerability of abatacept in subjects with SLE who have completed the initial 12 month double-blind treatment period on a background of tapering glucocorticosteroids [ Time Frame: Open label treatment period ] [ Designated as safety issue: No ]

• Assess the safety of chronic use of abatacept [ Time Frame: double blind treatment period ] [ Designated as safety issue: No ]
• Assess the Immunogenicity of abatacept during chronic use [ Time Frame: double-blind treatment period ] [ Designated as safety issue: No ]
• Assess the proportion of subjects with a new clinical flare of SLE [ Time Frame: open label treatment period ] [ Designated as safety issue: No ]
• Assess the total number of BILAG A flares [ Time Frame: open label treatment period ] [ Designated as safety issue: No ]
• Assess SLICC/ACR Damage Index at Year 2 compared with the baseline and Year 1 [ Time Frame: open label treatment period ] [ Designated as safety issue: No ]
• Assess the total exposure to glucocorticosteroids as measured by total prednisone or prednisone equivalent are under the curve (AUC) [ Time Frame: open label treatment period ] [ Designated as safety issue: No ]
• Assess the immunogenicity of abatacept during chronic use [ Time Frame: open label treatment period ] [ Designated as safety issue: No ]

The purpose of this clinical research study is to learn whether Abatacept can treat and prevent lupus flares; specifically, in patients with active lupus flares in at least one of three organ systems: skin (discoid lesions); inflammation of the lining of the heart (pericarditis), or inflammation of the lining of the lung (pleuritis/pleurisy); or inflammation of more than 4 joints (arthritis). All subjects will receive prednisone or prednisone-equivalent treatment in combination with study medication. The safety of this treatment will also be studied.

• Drug: Abatacept
• Drug: Placebo
• Drug: Prednisone

• Active Comparator: Double Blind Period
• Placebo Comparator: Double Blind Period
• Experimental: Open Label

Inclusion Criteria:

• Subjects must be diagnosed with SLE and be experiencing an active lupus flare in at least one of three organ systems: skin (discoid lesions), inflammation of the lining of the heart (pericarditis), or inflammation of the lining of the lung (pleuritis/pleurisy); or inflammation of more than 4 joints within 14 days of a screening visit (arthritis)
• Stable dose of prednisone (<30mg) for at least one month

Exclusion Criteria:

• Subjects experiencing an active lupus flare in the kidney or central nervous systems
• Treatment with a stable dose of azathioprine, mycophenolate mofetil, hydroxychloroquine, chloroquine, or methotrexate for less than three months prior to the study
• Subjects with active viral or bacterial infections
• Subjects with any other autoimmune disease as a main diagnosis
• Prior treatment with rituximab