IDO1 and colitis: Although the mechanism by which MSCs improve the pathogenesis of this model is not fully understood, they have been reported to improve the severity of colitis by reprogramming macrophages, modifying the Th-cell paradigm, and inducing regulatory immune cells via the secretion of indoleamine 2,3-dioxygenase (IDO), PGE2, transforming growth factor-β1, TSG-6, and EVs, among others (8).