Study to Evaluate the Pharmacodynamics and Safety of HGS1025 in Patients With Ulcerative Colitis

2 mg/kg of HGS1025 administered by IV infusion on Day 0, 14, 28 and 56, and then every 28 days thereafter if patient achieves clinical response

10 mg/kg of HGS1025 administered by IV infusion on Day 0, 14, 28 and 56, and then every 28 days thereafter if patient achieves clinical response

Placebo administered by IV infusion on Day 0, 14, 28 and 56. Subjects receiving placebo during the first 8 weeks of treatment will be given the option to receive HGS1025 10 mg/kg in the continuation phase of the study.