IDO1 and Arthritis: In previous studies, MSCs were found to secrete certain cytokines, such as IDO, TGF-β, NO, and PGE2, among which, PGE2 has been shown to be pivotal for the anti-inflammatory effect of MSCs in several inflammatory disease models, including wound disease, brain disease, arthritis, lung injury, periodontitis, and colitis [30–36].