Indoleamine 2,3-Dioxygenase in Endometriosis

Endometriosis (EMS) is a chronic inflammatory and estrogen-dependent gynecological disease characterized by the presence of endometrial tissue outside the uterine cavity. Although it is a benign disease, EMS is tumor-like in several aspects, which include unrestrained growth, decreased apoptosis, and aggressive invasion. EMS involves endocrine disorders and immunological factors. Indoleamine 2,3-dioxygenase (IDO) is an intracellular enzyme that catalyzes the initial and rate-limiting step of the metabolism of tryptophan. IDO is a potential candidate facilitating EMS development. Increased IDO expression in both eutopic and ectopic endometria of women with EMS is biologically important in aspects, which include regulation of endometrial stromal cell function and modulation of adjacent local immunocytes to generate a supportive microenvironment. In turn, the expression of IDO can be regulated by the complex endocrine-immune microenvironment networks in endometrial lesions. Here, we systematically review the roles of IDO in EMS to explore its pathological implications and treatment potential.