Cell-autonomous control of interferon type I expression by indoleamine 2,3-dioxygenase in regulatory CD19+ dendritic cells.

Following CD80/86 (B7) and TLR9 ligation, small subsets of splenic dendritic cells expressing CD19 (CD19(+) DC) acquire potent T cell regulatory functions due to induced expression of the intracellular enzyme indoleamine 2,3-dioxygenase (IDO), which catabolizes tryptophan. In CD19(+) DC, IFN type I (IFN-alpha) is the obligate inducer of IDO. We now report that IFN-alpha production needed to stimulate high-level expression of IDO following B7 ligation is itself dependent on basal levels of IDO activity.