Trafficking of circulating pro-NK cells to the decidualizing uterus: Regulatory mechanisms in the mouse and human

Natural Killer (NK) lymphocytes, strongly expressing CD56, become abundant in the human uterus three to five days after the mid-menstrual cycle surge in pituitary-derived luteinizing hormone (LH). The primary functions of LH are to initiate final oocyte maturation/ovulation and to contribute to decidualization of the uterine stroma. Decidualization is the transformation of estrogen-primed uterine stromal fibroblasts into large hormone-producing cells under the influence of progesterone (P₄). Decidual CD56^bright (dNK) cells are a distinct, transient, tissue-specific NK cell subset that undergoes proliferation, terminal differentiation, and then death prior to menses. If pregnancy occurs, dNK cells increase during first trimester, then decline and are virtually absent in late pregnancy. In mouse models, pregnancy-associated uterine NK (uNK) cells appear coincident with onset of decidualization during embryonic implantation. Murine uNK cells traffic from the circulation to the antimesometrial side of the uterus and migrate to the mesometrial side of each implantation site. Here they proliferate and are implicated in regulation of midgestation structural changes to major arteries supplying the placenta, before dying in late gestation. Emerging data indicate that interactions between lymphocytes and endothelial cells within the uterine microenvironment are mediated by classical molecules associated with lymphocyte trafficking in immune surveillance and in response to inflammation. Here, we review factors influencing NK cell trafficking to decidualizing murine and human uteri and the differentiation and functions of these cells within the uterus.