IDO and tolerance to tumors

Tumors create a pathological state of tolerance towards tumor-associated antigens. The mechanisms by which this occurs are still largely unknown. Recent studies suggest that one mechanism contributing to this phenomenon could be tryptophan catabolism carried out by the immunoregulatory enzyme indoleamine 2,3-dioxygenase (IDO). IDO can be expressed by tumor cells themselves, and we propose that the recruitment of antigen-presenting cells expressing IDO to tumor-draining lymph nodes might constitute another potent mechanism for inducing tolerance to tumor-derived antigens. Therapeutic strategies based on blocking this pathway might represent a novel class of adjuvants for tumor immunotherapy.