IDO: A double-edged sword for TH1/TH2 regulation

Indoleamine 2,3-dioxygenase (IDO) has been well defined as one of the important immunosuppressive properties for T(H)1 cell-mediated immune responses, but its function in T(H)2 dominant system is poorly understood. Recently, an appreciable number of publications Suggest that the role of IDO in T(H)2 cell regulation may be different from that of T(H)1 immune responses. Here we review the evidence on the regulatory function of IDO and tryptophan metabolites in T(H)1/T(H)2 differentiation. We propose that IDO-kynurenine pathway can serve as a negative feedback loop for TIII cells but it may play a distinct role in up-regulating T(H)2 dominant immune responses. (C) 2008 Elsevier B.V. All rights reserved.