Compositions and methods for the therapy and diagnosis of Inflammatory Bowel Disease (IBD), including Crohn's Disease and Ulcerative Colitis, are disclosed. Illustrative compositions comprise one or more bacterial polypeptides, immunogenic portions thereof, polynucleotides that encode such polypeptides, antigen presenting cell that expresses such polypeptides, and T cells that are specific for cells expressing such polypeptides. The disclosed compositions are useful, for example, in the diagnosis, prevention and/or treatment of IBD.