Role of PD1/PDL1 pathway, and TH17 and treg cells in maternal tolerance to the fetus.

Tolerance of the fetus by the maternal immune system is regulated through various mechanisms involving the different immune cells, both in the periphery and locally at the feto-maternal interface. The maternal T lymphocytes are aware of the paternal fetal antigens and a state of dynamic T cell homeostasis is maintained in the uterus during gestation, which involves increase in antigen-specific regulatory T cell (Treg) proliferation, increase in apoptosis of antigen-specific effector T cells, and inhibition of excessive inflammation post successful implantation to ensure tolerance to the fetus. The Tregs play an important role in the maintenance of tolerance during gestation. Recently, the inflammatory T helper type 17 (Th17) cells are reported to have a role in loss of tolerance to the fetus. The interaction between costimulatory molecule programmed death 1 (PD1) and its ligand PDL1 is known to play a role in regulating both the Tregs and Th17 cells. Here we discuss how the PD1/PDL1 pathway affects these two T cell populations and its role in feto-maternal tolerance.