Tryptophan catabolism and T-cell tolerance: immunosuppression by starvation?

Some macrophages inhibit microbial infections by producing indoleamine 2,3 dioxygenase (IDO), which catabolizes tryptophan. Here, Andrew Mellor and Da vid Munn discuss evidence that cells that synthesize IDO protect the mammal ian fetus from maternal T-cell attack and argue that this mechanism might h ave wider implications for the control of T-cell responses.