This study focuses on individuals with ulcerative colitis (UC) that has been assessed as being moderate to severe. The purpose of the study is to determine whether an investigational drug called PRA023 is safe and effective in treating UC. PRA023 is a monoclonal antibody (man-made antibodies that act like human antibodies in the immune system) that may reduce inflammation and fibrosis associated with inflammatory bowel disease, such as UC and Crohn’s disease. Participants will be randomly assigned to received either PRA023 or placebo (inactive substance).