Indoleamine 2,3-dioxygenase: distribution and function in the developing human placenta

Studies in mice have suggested that the placenta is protected from immune rejection by maternal T cells by means of localised indoleamine 2,3-dioxygenase dependent depletion of tryptophan. To determine whether such mechanisms might operate in the human placenta, we have studied the physiological importance of human placental indoleamine 2,3-dioxygenase immunohistochemically and functionally. Indoleamine 2,3-dioxygenase is detectable immunohistochemically from day 6 human blastocysts and thereafter throughout pregnancy in syncytiotrophoblasts, extravillous cytotrophoblasts and macrophages in the villous stroma and in the fetal membranes. Interferon-γ added to villous explants markedly stimulates indoleamine 2,3-dioxygenase protein expression in macrophages. Indoleamine 2,3-dioxygenase-mediated tryptophan degradation in the first trimester villous and decidual tissue explants is stimulated by interferon-γ and inhibited by 1-methyl-tryptophan (an inhibitor of indoleamine 2,3-dioxygenase). Peripheral blood mononuclear cell proliferation is controlled by indoleamine 2,3-dioxygenase-mediated tryptophan degradation. These results suggest the cellular basis of a mechanism present at the human maternal–fetal interface involved in regulating the maternal immune response to conceptus.