The present invention relates to compositions and methods for diagnosing, monitoring and/or treating an autoimmune or chronic inflammatory disease. In particular, the present invention provides methods for diagnosing, monitoring and treating an autoimmune disease (e.g., rheumatoid arthritis) or chronic inflammatory disease (e.g., systemic lupus erythematosus) based on detecting or altering (e.g., altering expression or methylation status of) autoimmune or chronic inflammatory disease proteins (e.g., CD70 and CD40L). The present invention also provides kits for detecting methylation status of autoimmune or chronic inflammatory disease proteins (e.g., CD70 and CD40L) and for diagnosing, monitoring and/or treating autoimmune or chronic inflammatory diseases.