Indoleamine 2,3-dioxygenase (IDO) in inflammation and allergy to Aspergillus

Innate responses combine with adaptive immunity to generate the most effective form of resistance against Aspergillus fumigatus. A complex set of signaling networks initiate both innate and adaptive immunity in response to the different fungal morphotypes. In response, the fungus has developed or acquired sophisticated mechanisms to avoid, counteract and subvert sensors, signaling networks and a range of effector functions that constitute the host immune response. Host response and fungal countermeasures may contribute to the balance of pro-inflammatory and anti-inflammatory signaling that is eventually required to benefit both parties. Here we highlight the important contribution of the enzyme IDO (indoleamine 2,3-dioxygenase) and tryptophan catabolites to such a homeostatic condition in Aspergillus infection and allergy. By providing the host with immune defense mechanisms adequate for protection, without necessarily eliminating the fungus or causing an unacceptable level of tissue damage, IDO and tryptophan metabolites may prove to be potent regulators capable of taming innate and adaptive pathogenic inflammatory host responses.