Distinctive phenotypes and functions of innate lymphoid cells in human decidua during early pregnancy.

During early pregnancy, decidual innate lymphoid cells (dILCs) interact with surrounding maternal cells and invading fetal extravillous trophoblasts (EVT). Here, using mass cytometry, we characterise five main dILC subsets: decidual NK cells (dNK)1-3, ILC3s and proliferating NK cells. Following stimulation, dNK2 and dNK3 produce more chemokines than dNK1 including XCL1 which can act on both maternal dendritic cells and fetal EVT.

Activating KIR2DS4 Is Expressed by Uterine NK Cells and Contributes to Successful Pregnancy.

Tissue-specific NK cells are abundant in the pregnant uterus and interact with invading placental trophoblast cells that transform the maternal arteries to increase the fetoplacental blood supply. Genetic case-control studies have implicated killer cell Ig-like receptor (KIR) genes and their HLA ligands in pregnancy disorders characterized by failure of trophoblast arterial transformation. Activating KIR2DS1 or KIR2DS5 (when located in the centromeric region as in Africans) lower the risk of disorders when there is a fetal HLA-C allele carrying a C2 epitope.

Tissue-Specific Education of Decidual NK Cells.

During human pregnancy, fetal trophoblast cells invade the decidua and remodel maternal spiral arteries to establish adequate nutrition during gestation. Tissue NK cells in the decidua (dNK) express inhibitory NK receptors (iNKR) that recognize allogeneic HLA-C molecules on trophoblast. Where this results in excessive dNK inhibition, the risk of pre-eclampsia or growth restriction is increased.

Maternal KIR in combination with paternal HLA-C2 regulate human birth weight.

Human birth weight is subject to stabilizing selection; babies born too small or too large are less likely to survive. Particular combinations of maternal/fetal immune system genes are associated with pregnancies where the babies are ≤ 5th birth weight centile, specifically an inhibitory maternal KIR AA genotype with a paternally derived fetal HLA-C2 ligand. We have now analyzed maternal KIR and fetal HLA-C combinations at the opposite end of the birth weight spectrum.

Maternal uterine NK cell-activating receptor KIR2DS1 enhances placentation.

Reduced trophoblast invasion and vascular conversion in decidua are thought to be the primary defect of common pregnancy disorders including preeclampsia and fetal growth restriction. Genetic studies suggest these conditions are linked to combinations of polymorphic killer cell Ig-like receptor (KIR) genes expressed by maternal decidual NK cells (dNK) and HLA-C genes expressed by fetal trophoblast. Inhibitory KIR2DL1 and activating KIR2DS1 both bind HLA-C2, but confer increased risk or protection from pregnancy disorders, respectively.

The effect of pregnancy on the uterine NK cell KIR repertoire.

The major leukocyte population in the decidua during the first trimester of pregnancy consists of NK cells that express receptors capable of recognizing MHC class I molecules expressed by placental trophoblast. These include members of the killer immunoglobulin-like receptor (KIR) family, the two-domain KIR (KIR2D), which recognize HLA-C. Interactions between decidual NK (dNK) cell KIR2D and placental HLA-C contribute to the success of pregnancy and dNK cells express KIR2D at higher frequency than peripheral NK (pNK) cells.

Maternal activating KIRs protect against human reproductive failure mediated by fetal HLA-C2.

Many common disorders of pregnancy are attributed to insufficient invasion of the uterine lining by trophoblast, fetal cells that are the major cell type of the placenta. Interactions between fetal trophoblast and maternal uterine NK (uNK) cells--specifically interactions between HLA-C molecules expressed by the fetal trophoblast cells and killer Ig-like receptors (KIRs) on the maternal uNK cells--influence placentation in human pregnancy. Consistent with this, pregnancies are at increased risk of preeclampsia in mothers homozygous for KIR haplotype A (KIR AA).