Localization of indoleamine 2,3-dioxygenase in human female reproductive organs and the placenta

Indoleamine 2,3-dioxygenase (IDO) has been implicated in regulation of feto-maternal tolerance and protection against intracellular and extracellular pathogens. We have studied the expression of IDO in the human female reproductive tract and the placenta by immunohistochemistry. Endometrial glandular and surface epithelial cells showed increasing IDO expression during the course of the menstrual cycle. In term placenta, IDO was irregularly localized to the mesenchymal core and found in isolated areas of the syncytiotrophoblast. In first trimester pregnancy, IDO was not present in placental villi, but was present in glandular epithelium of the decidua, and there were distinctly positive cells scattered in the connective tissue, sometimes in conjunction with lymphoid aggregates. The endothelium of spiral arteries and of capillaries showed some, albeit no generalized, reactivity. IDO was also present in the epithelium of cervical glands and of Fallopian tubes. Specificity of antibody binding was confirmed by Western blot analysis. IDO mRNA was detected in first trimester decidua as determined by RT–PCR. IDO is secreted, as determined by analysis of cervical mucus by high pressure liquid chromatography for the presence of the tryptophan metabolite L-kynurenine, indicating IDO activity. Our results support the concept of IDO providing a mechanism of innate immunity protecting against ascending infections in the female reproductive tract.