Mechanisms regulating the expression of indoleamine 2,3-dioxygenase during decidualization of human endometrium

BACKGROUND: Expression of the tryptophan catabolizing enzyme, indoleamine 2,3‐dioxygenase, in the mouse placenta has been shown to be critical in preventing immunological rejection of the fetal allograft. To clarify the physiological importance of indoleamine 2,3‐dioxygenase in human pregnancy, we have studied how the expression of this enzyme changes during decidualization of human endometrium at both the cell and tissue level. METHODS and RESULTS: The level of indoleamine 2,3‐dioxygenase mRNA expression (determined by RT–PCR) was higher in decidual than in endometrial tissue. Uterine decidual tissue in ectopic pregnancy similarly showed increased mRNA expression. Immunohistochemistry demonstrated that indoleamine 2,3‐dioxygenase protein immunoreactivity was found in glandular epithelium and in stromal cells. The intensity of this immunoreactivity was increased in decidualized tissue. In a cell culture model, the level of indoleamine 2,3‐dioxygenase mRNA was suppressed specifically by progesterone‐induced decidualization of isolated endometrial stromal cells. Indoleamine 2,3‐dioxygenase protein abundance (determined by Western blot) was also decreased by progesterone‐induced decidualization. However interferon‐γ, a potent stimulator of indoleamine 2,3‐dioxygenase gene expression, increased the level of indoleamine 2,3‐dioxygenase mRNA and protein in both non‐decidualized and in decidualized cells. Indoleamine 2,3‐dioxygenase activity (determined by measuring the concentration of tryptophan and its indoleamine 2,3‐dioxygenase catabolite, kynurenine) was also decreased by progesterone‐induced decidualization but enhanced following interferon‐γ treatment. Expression of other interferon‐γ inducible genes (STAT1 and tryptophanyl‐tRNA synthetase) showed the same pattern as that of indoleamine 2,3‐dioxygenase in tissue samples, but was not changed by decidualization in the cell culture model. CONCLUSIONS: These data suggest that despite suppression by progesterone, indoleamine 2,3‐dioxygenase expression in endometrial stromal cells may increase during decidualization due to stimulation by interferon‐γ secreted by infiltrating leukocytes.