Which Types of Regulatory T cells Play Important Roles in Implantation and Pregnancy Maintenance?

Regulatory T cells (Treg) play essential roles in implantation and allogeneic pregnancy maintenance in mice and humans. Recent data have shown the heterogeneity of Treg, such as thymic (naturally occurring) Treg, extrathymic (inducible or peripheral) Treg, naïve Treg, effector Treg, resting (non-proliferating) Treg and activated (proliferating) Treg. Importantly, Foxp3, which was believed to be a specific marker for Treg, is transiently expressed in T cells when conventional T cells are activated and proliferating in humans showing that Foxp3 is not a specific marker for Treg. Therefore, we should evaluate the true Treg level and clarify which types of Treg cells play important roles in implantation and pregnancy maintenance in mice and humans.