Indoleamine 2,3-dioxygenase activity in nonagenarians is markedly increased and predicts mortality.

Indoleamine 2,3-dioxygenase (IDO), an enzyme degrading tryptophan (trp) to kynurenine (kyn), suppresses T cell activity. Ageing of the immune system, immunosenescence, includes a decline in T cell function. We therefore sought to establish whether IDO activity is involved in immunosenescence and whether it predicts mortality in aged subjects. We measured kyn/trp, reflecting IDO activity, in 284 nonagenarians and 309 blood donor controls. IDO activity was significantly higher in nonagenarians compared with controls and IDO activity at study entry predicted subsequent mortality in nonagenarians. Thus, increased IDO activity might be a mechanism involved in the decline of T cell responses in immunosenescence.