New jobs for an old enzyme: the revival of IDO

Unresponsiveness towards autoantigens or potentially harmful pathogens can be thought of as an active process of developing tolerance. The induction of this activity relies on the interaction of antigen-presenting cells (APCs) and T cells, causing T-cell tolerance. Very recently, a new feature of regulatory APCs was observed. The expression of indoleamine-2,3-dioxygenase (IDO) activity by certain dendritic cells (DCs), monocytes and macrophages has immunomodulatory effects on T cells that are related to the peri-cellular degradation of the essential amino acid tryptophan [ 1 ]. In their recent review, Grohmann and co-authors [ 2 ] described the central role of IDO in the control of T-cell activity during infection, pregnancy, autoimmunity, transplantation and neoplasia.