Abstract
Pregnancy is a unique type of immunological process. Healthy pregnancy is associated with a series of inflammatory events: implantation (inflammation), gestation (anti-inflammation), and parturition (inflammation). As the most abundant leukocytes during pregnancy, natural killer (NK) cells are recruited and activated by ovarian hormones and have pivotal roles throughout pregnancy. During the first trimester, NK cells represent up to 50–70% of decidua lymphocytes. Differently from peripheral-blood NK cells, decidual natural killer (dNK) cells are poorly cytolytic, and they release cytokines/chemokines that induce trophoblast invasion, tissue remodeling, embryonic development, and placentation. NK cells can also shift to a cytotoxic identity and carry out immune defense if infected in utero by pathogens. At late gestation, premature activation of NK cells can lead to a breakdown of tolerance of the maternal–fetal interface and, subsequently, can result in preterm birth. This review is focused on the role of dNK cells in normal pregnancy and pathological pregnancy, including preeclampsia, recurrent spontaneous abortion, endometriosis, and recurrent implantation failure. dNK cells could be targets for the treatment of pregnancy complications.
Highlights
By comparison with pNK cells and first-trimester decidual natural killer (dNK) cells, they suggested that the proportion of term-pregnancy dNK cells among CD45+ cells were significantly lower and dNK cells in a term pregnancy had an increased degranulation response, but lower capacity to respond to human CMV-infected cells
This review emphasizes the important role of dNK cells throughout pregnancy (Figure 1)
In the early stages of pregnancy, dNK cells do not present a cytotoxic response against the semi-allogeneic embryo. dNK cells interact with HLA ligands expressed on extravillous trophoblast (EVT) to depress the cytotoxic capability of dNK cells and mediate immune tolerance at the maternal–fetal interface
Summary
Successful pregnancy in humans is reliant on a series of critical events: embryo implantation, decidualization, placentation, and parturition. By comparison with pNK cells and first-trimester dNK cells, they suggested that the proportion of term-pregnancy dNK cells among CD45+ cells were significantly lower and dNK cells in a term pregnancy had an increased degranulation response, but lower capacity to respond to human CMV-infected cells They identified that expression of a set of NK cell receptors, and found that term pregnancy dNK had fewer HLA-C receptors (KIR2DL1, KIR2DL2/3, and KIR2DS1) but more HLA-E receptor NKG2D compared with first trimester dNK. They show increased expression of NKG2C and ILT2 and enhanced production of IFN-g and VEGFa, and display a special type of innate memory: these cells are termed “pregnancy-trained dNK cells” [25]. For dNK cells, ~60% are CD11b−CD27− NK cells and >20% are CD27+ NK cells [32]
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