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-gamma, 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-gamma treatment. Expression of other interferon-gamma 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-gamma secreted by infiltrating leukocytes.