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Competition for essential nutrients and/or release of toxic metabolites are mechanisms highly conserved in both bacteria and higher organisms as innate defense systems from external pathogens. One such mechanism is based on the activation of indoleamine 2,3-dioxygenase (IDO), an enzyme that catalyzes the catabolism of the essential amino acid tryptophan (rev. in 1). By depleting tryptophan, IDO limits the ability of many pathogens to synthesize proteins. In addition, activation of the enzyme results in the accumulation of tryptophan breakdown products, collectively known as kynurenines, some of which are toxic to bacteria and parasites.Consistent with the enzyme effects on the containment of pathogens' replication and invasion, constitutive IDO expression in higher organisms is found at mucosal interfaces, such as the mucosa of the intestine and the lung, which are barriers to external microorganisms (2). However, chronic inflammatory processes, particularly those sustained by interferon-driven immune responses, can induce high levels of IDO in many cell types including epithelial, endothelial, and myeloid cells (3–7). Intriguingly, under these conditions, active IDO appears to mute lymphocyte immune responses by “starving” actively proliferating leukocytes, thereby limiting the uncontrolled spread of inflammation. Both high levels of IDO as well as activation of the enzyme by redox active compounds present in the inflammatory microenvironment are required for the immunosuppressive effect (8).The ability of IDO to regulate lymphocyte functions has prompted many investigators to test the effect of IDO overexpression at transplantation sites as a mean to avert destructive allogeneic immune responses against heart, skin, lung, cornea, and bone marrow grafts (9 …

Pancreatic Islet Transplants and IDO: When Starving the Enemy Does You Good