Innate immune cells in reproduction.

Fetomaternal immune tolerance is required to prevent fetal rejection during pregnancy; simultaneously, the maternal immune system must also protect the fetus from harmful endogenous and exogenous antigens, including those associated with viral and bacterial infections. Therefore, a delicate immune balance is critical for the maintenance of a successful pregnancy, while disruption of this balance can induce complications such as implantation failure, miscarriage, preterm birth/labor, preeclampsia and fetal growth restriction. While the adaptive immune system is critical for the development of tolerance, this review summarizes evidence that modulation of the innate immunity is also essential for achieving this delicate balance between tolerance and protection. Canonical cells of the innate immune system, including dendritic cells, macrophages, natural killer cells and invariant natural killer T cells, contribute to the modulation of the immune balance at the fetomaternal interface. The newly identified myeloid-derived suppressor cells and innate lymphoid cells also appear important for successful reproduction. Moreover, it is possible that sterile inflammation facilitates complications of pregnancy via the innate immune system. In this review, the characteristic features and functions of innate immune cells in fetomaternal immunity and their contributions to pregnancy complications related to sterile inflammation are discussed. These insights on innate immune system function in reproduction may provide new perspectives for understanding the mechanisms of fetomaternal tolerance and the etiology of pregnancy complications.