LeA(H)Rning self-control

The aryl hydrocarbon receptor (AhR) is an important regulator of the immune response. A report by Puccetti and coworkers describes a regulatory pathway by which L-kynurenine (L-Kyn) produced by tryptophan 2,3-dioxygenase 2 (TDO2) activates AhR in cells of the innate immune system to limit endotoxin-triggered inflammation through a mechanism that involves the non-enzymatic anti-inflammatory activities of indoleamine 2,3-dioxygenase 1 (IDO1).