IDO1 and rheumatoid arthritis: The expression of IDO by different cell types, as well as the capacity of IDO competent DCregs to induce Treg cells through high levels of IDO activity, could have broader immunological significance in tolerance and immunoregulation; for example, it has been suggested that defects in the immunoregulatory mechanism initiated by IDO are involved in the development of autoimmune conditions, such as multiple sclerosis, systemic lupus erythematosus, rheumatoid arthritis, and autoimmune diabetes [27–29].