What is the role of regulatory T cells in the success of implantation and early pregnancy?

Problem: The immune system is well controlled by the balance between immunostimulation and immunoregulation. CD4⁺CD25⁺ regulatory T (Treg) cells and an enzyme called indoleamine-2, 3-dioxygenase (IDO) mediate maternal tolerance of the allogeneic fetus. Treg cells, therefore, may prevent early pregnancy loss due to maternal 'rejection.' Methods: The latest understanding of tolerance during pregnancy is reviewed. Results and conclusions: Recent data show that CD4⁺CD25⁺ Treg cells play essential roles in the induction and maintenance of tolerance, and that they augment the IDO activity in dendritic cells and macrophages. Therefore, CD4⁺CD25⁺ Treg cells and IDO enzyme may cooperate in the induction of tolerance during pregnancy. Treg deficiency is associated with very early post-implantation loss and spontaneous abortion in animal models, and low Treg levels are associated with recurrent miscarriages in humans.