Treatment arm 1: Top-down IFX treatment will consist of a total of 5 IFX infusions of 5 mg/kg (IFX induction at week 0, 2 and 6, followed by 2 maintenance infusions every 8 weeks) combined with oral azathioprine (AZA) 2-3 mg/kg once daily. AZA therapy will continue after the last IFX infusion to maintain remission.