IDO1 (Indoleamine 2,3-Dioxygenase 1)

IDO1 is the host enzyme that catalyzes the first and rate-limiting step of the kynurenine pathway, converting tryptophan to N-formylkynurenine en route to kynurenine. Because it is expressed by host cells rather than by gut bacteria, IDO1 is one route by which microbial signals can raise systemic kynurenine indirectly, through host rather than bacterial metabolism.

IDO1-driven tryptophan depletion and kynurenine production are long-standing mechanisms of immune tolerance in tumors and in mucosal tissue such as the colon.