In 1991, Kimumura et al showed that the human syncytiotrophoblasts produce this enzyme. Tthe indoleamine 2,3-dioxygenase activity was not detected in the placenta in the early stages of gestation. IDO was first detected in the placenta at around 14 weeks of gestation. Its amount increased rapidly thereafter and was maintained at high levels till near term. The indoleamine 2,3-dioxygenase activity was significantly lower in placenta with retarded intrauterine development.
T cells require tryptophan for their proliferation.
Exhaustion of the tryptophan leads to the inability of T cells to proliferate. For example, when macrophages produce IDO in response to the interferon gamma, the proliferation of T cells is inhibited. Treatment of the pregnant mice carrying allogeneic embryos led to the loss of the embryos. This loss was associated with degeneration of the embryos, an inflammatory response and surrounding areas of hemorrhage. In contrast, the syngeneic embryos were not affected by this treatment.