A Study of Efficacy and Safety of Rosnilimab in Subjects With Moderate to Severe Ulcerative Colitis (ROSETTA)

Inclusion Criteria: * Male or female ≥18 * Participants with a clinical diagnosis of UC for prior to Day 1 * Subject has moderate to severe, active UC, defined as a mMS ≥ 5 with an endoscopy subscore ≥2 * Subject has had a surveillance colonoscopy that did not detect potential dysplasia or colon cancer performed within 1 year of Day 1. * Subject has a history of an inadequate response, loss of response, or intolerance to any combination of at least 2 UC therapy classes defined as, but not limited to, aminosalicylates, corticosteroids, immunomodulators, calcineurin inhibitors, or advanced UC therapies (e.g., biologics, JAK inhibitors, oral S1P receptor modulators, etc.) Exclusion Criteria: * Subject has a diagnosis of Crohn's disease or indeterminate colitis. * Subject has a diagnosis of fulminant colitis and/or toxic megacolon. * Subject has a history of an inadequate response, loss of response, or intolerance to any combination of 3 or more advanced UC therapy classes but not limited to, 1) anti-TNF antibodies (e.g., adalimumab, golimumab, infliximab), 2) other biologics (e.g., ustekinumab, vedolizumab), 3) oral JAK inhibitors (e.g., tofacitinib, upadacitinib), and 4) oral S1P receptor modulators (e.g., ozanimod). * Subject has disease limited to the rectum (ulcerative proctitis) * Subject has a history of colectomy (total or subtotal), ileoanal pouch, Kock pouch, or ileostomy or is planning bowel surgery. * The subject had prior exposure to a PD-1 or PD-L1 agonist, antagonist, or modulator.