A Randomized, Multicenter Open Label Study Comparing Early Administration of Azathioprine Plus IFX to Steroids Plus Azathioprine for Acute Severe Colitis

Inclusion Criteria: * Age \> 18 years. * Diagnosis of UC according to Lennard-Jones criteria (Appendix 1). * Endoscopically demonstrated colorectal lesions localized above the anal margin and extending at least up to 15cm proximally (Endoscopic Mayo subscore ≥ 2). * Acute flare requiring hospitalization * Severe acute flare of UC with a Lichtiger Index score \> 10 at Day -3 * Adequate contraception for male or female subjects of childbearing potential, which will be continued throughout the study and at least 3 months after study termination. 3.2 EXCLUSION CRITERIA * Pregnant or breast-feeding woman. * Previous treatment with infliximab. * Treatment with adalimumab or golimumab within 8 weeks before randomization * Treatment with vedolizumab within 4 weeks before randomization * Azathioprine or 6-mercaptopurine treatment initiated more than 4 weeks before screening. * Ongoing intravenous steroids for more than 96 hours at time of the screening * Contraindication for anti-TNF therapy * Indication for immediate surgery. * History of colorectal dysplasia. * Diagnosis of Crohn's disease or indeterminate colitis * Positive stool tests for amoebiasis and/or positive bacteriological culture for Salmonella, Shigella, Yersinia and Campylobacter and/or presence of Clostridium difficile B toxin in the stools. * Renal failure (creatininemia \> upper limit of normal laboratory value). * Uncontrolled high blood pressure. * HIV, HBV viral infection (except the presence of positive anti-H…