Indoleamine 2,3-dioxygenase as a Modifier of Pathogenic Inflammation in Cancer and other Inflammation-Associated Diseases

Chronic inflammation underlies the basis for development and progression of cancers and a variety of other disorders, but what specifically defines its pathogenic nature remains largely undefined. Recent genetic and pharmacological studies in the mouse suggest that the immune modulatory enzyme indoleamine 2,3-dioxygenase (IDO), identified as an important mediator of immune escape in cancer, can also contribute to the development of pathology in the context of chronic inflammatory models of arthritis and allergic airway disease. IDO-deficient mice do not display spontaneous disorders of classical inflammation and small molecule inhibitors of IDO do not elicit generalized inflammatory reactions. Rather, in the context of a classical model of skin cancer that is promoted by chronic inflammation, or in models of inflammation-associated arthritis and allergic airway disease, IDO impairment can alleviate disease severity. Here we offer a survey of preclinical literature suggesting that IDO functions as a modifier of inflammatory states rather than simply as a suppressor of immune function. We propose that IDO induction in a chronically inflamed tissue may shape the inflammatory state to support, or in some cases retard, pathogenesis and disease severity.