 The purpose of this clinical research study is to evaluate the safety and effectiveness of treatment with abatacept Abatacept 125mg injected subcutaneously under the skin weekly for 16 weeks versus placebo injectionsa substance with no active ingredients and therefore may have no treatment benefit in subjects with SLE and inflammatory polyarthritis. The effectiveness will be assessed primarily by the number of swollen, tender joints called a joint count at each of study visits. 