The reduced number of regulatory T cells in active IBD (20) could be influenced by an altered thymic development and peripheral homeostasis through factors like CD28, IL-2, or TGF-ß (63, 64) as Tregs are IL-2 and CD28-dependent for their development, activation and function (64–66) and TGF-ß is necessary for maintenance in the periphery (67, 68). Here, CD28 is linked to inflammatory bowel disease.