Uterine NK cells underexpress KIR2DL1/S1 and LILRB1 in reproductive failure

A significant proportion of recurrent miscarriage, recurrent implantation failure and infertility are unexplained, and these conditions have been proposed to have an etiology of immunological dysfunction at the maternal-fetal interface. Uterine Natural Killer cells (uNK) comprise three subsets and are the most numerous immune cells found in the uterine mucosa at the time of implantation. They are thought to play an important role in successful pregnancy by regulation of extravillous trophoblast (EVT) invasion and spiral artery remodelling. Here, we examine the frequency, phenotype and function of uNK1-3 from the uterine mucosa of 16 women with unexplained reproductive failure compared to 11 controls with no reproductive problems, during the window of implantation. We report that KIR2DL1/S1 and LILRB1 expression is lower in the reproductive failure group for both uNK (total uNK, uNK 2 and 3) and pNK. We also show that degranulation activity is significantly reduced in total uNK, and that TNF-α production is lower in all uNK subsets in the reproductive failure group. Taken together, our findings suggest that reproductive failure is associated with global reduction in expression of uNK receptors important for interaction with HLA-C and HLA-G on EVT during early pregnancy, leading to reduced uNK activation. This is the first study to examine uNK subsets during the window of implantation in women with reproductive failure and will serve as a platform to focus on particular aspects of phenotype and function of uNK subsets in future studies. Further understanding of uNK dysregulation is important to establish potential diagnostic and therapeutic targets in the population of women with unexplained reproductive failure.

Dynamic Changes in Uterine NK Cell Subset Frequency and Function Over the Menstrual Cycle and Pregnancy

Uterine natural killer cells (uNK) play an important role in promoting successful pregnancy by regulating trophoblast invasion and spiral artery remodelling in the first trimester. Recently, single-cell RNA sequencing (scRNAseq) on first-trimester decidua showed that uNK can be divided into three subsets, which may have different roles in pregnancy. Here we present an integration of previously published scRNAseq datasets, together with novel flow cytometry data to interrogate the frequency, phenotype, and function of uNK1–3 in seven stages of the reproductive cycle (menstrual, proliferative, secretory phases of the menstrual cycle; first, second, and third trimester; and postpartum). We found that uNK1 and uNK2 peak in the first trimester, but by the third trimester, the majority of uNK are uNK3. All three subsets are most able to degranulate and produce cytokines during the secretory phase of the menstrual cycle and express KIR2D molecules, which allow them to interact with HLA-C expressed by placental extravillous trophoblast cells, at the highest frequency during the first trimester. Taken together, our findings suggest that uNK are particularly active and able to interact with placental cells at the time of implantation and that uNK1 and uNK2 may be particularly involved in these processes. Our findings are the first to establish how uNK frequency and function change dynamically across the healthy reproductive cycle. This serves as a platform from which the relationship between uNK function and impaired implantation and placentation can be investigated. This will have important implications for the study of subfertility, recurrent miscarriage, pre-eclampsia, and pre-term labour.

Tbet promotes CXCR6 expression in immature natural killer cells and natural killer cell egress from the bone marrow

Tbet-deficient mice have reduced natural killer (NK) cells in blood and spleen, but increased NK cells in bone marrow and lymph nodes, a phenotype that is thought to be the result of defective migration. Here, we revisit the role of Tbet in NK cell bone marrow egress. We definitively show that the accumulation of NK cells in the bone marrow of Tbet-deficient Tbx21^-/- animals occurs because of a migration defect and identify a module of genes, co-ordinated by Tbet, which affects the localization of NK cells in the bone marrow. Cxcr6 is approximately 125-fold underexpressed in Tbx21^-/- , compared with wild-type, immature NK cells. Immature NK cells accumulate in the bone marrow of CXCR6-deficient mice, and CXCR6-deficient progenitors are less able to reconstitute the peripheral NK cell compartment than their wild-type counterparts. However, the CXCR6 phenotype is largely confined to immature NK cells, whereas the Tbet phenotype is present in both immature and mature NK cells, suggesting that genes identified as being more differentially expressed in mature NK cells, such as S1pr5, Cx3cr1, Sell and Cd69, may be the major drivers of the phenotype.

The Obese Liver Environment Mediates Conversion of NK Cells to a Less Cytotoxic ILC1-Like Phenotype

The liver contains both NK cells and their less cytotoxic relatives, ILC1. Here, we investigate the role of NK cells and ILC1 in the obesity-associated condition, non-alcoholic fatty liver disease (NAFLD). In the livers of mice suffering from NAFLD, NK cells are less able to degranulate, express lower levels of perforin and are less able to kill cancerous target cells than those from healthy animals. This is associated with a decreased ability to kill cancer cells in vivo. On the other hand, we find that perforin-deficient mice suffer from less severe NAFLD, suggesting that this reduction in NK cell cytotoxicity may be protective in the obese liver, albeit at the cost of increased susceptibility to cancer. The decrease in cytotoxicity is associated with a shift toward a transcriptional profile characteristic of ILC1, increased expression of the ILC1-associated proteins CD200R1 and CD49a, and an altered metabolic profile mimicking that of ILC1. We show that the conversion of NK cells to this less cytotoxic phenotype is at least partially mediated by TGFβ, which is expressed at high levels in the obese liver. Finally, we show that reduced cytotoxicity is also a feature of NK cells in the livers of human NAFLD patients.

Conventional NK cells and ILC1 are partially ablated in the livers of Ncr1 iCreTbx21 fl/fl mice

Mouse liver contains both Eomes-dependent conventional natural killer (cNK) cells and Tbet-dependent liver-resident type I innate lymphoid cells (ILC1). In order to better understand the role of ILC1, we attempted to generate mice that would lack liver ILC1, while retaining cNK, by conditional deletion of Tbet in NKp46+ cells. Here we report that the Ncr1 ^iCreTbx21 ^fl/fl mouse has a roughly equivalent reduction in both the cNK and ILC1 compartments of the liver, limiting its utility for investigating the relative contributions of these two cell types in disease models. We also describe the phenotype of these mice with respect to NK cells, ILC1 and NKp46 ⁺ ILC3 in the spleen and small intestine lamina propria.

Conventional NK cells and ILC1 are partially ablated in the livers of Ncr1 iCreTbx21 fl/fl mice

Mouse liver contains both Eomes-dependent conventional natural killer (cNK) cells and Tbet-dependent liver-resident type I innate lymphoid cells (ILC1). In order to better understand the role of ILC1, we attempted to generate mice that would lack liver ILC1, while retaining cNK, by conditional deletion of Tbet in NKp46+ cells. Here we report that the Ncr1 iCreTbx21 fl/fl mouse has a roughly equivalent reduction in both the cNK and ILC1 compartments of the liver, limiting its utility for investigating the relative contributions of these two cell types in disease models. We also describe the phenotype of these mice with respect to NK cells, ILC1 and NKp46 + ILC3 in the spleen and small intestine lamina propria.

Eomeshi NK Cells in Human Liver Are Long-Lived and Do Not Recirculate but Can Be Replenished from the Circulation

Human liver contains an Eomes^hi population of NK cells that is not present in the blood. In this study, we show that these cells are characterized by a molecular signature that mediates their retention in the liver. By examining liver transplants where donors and recipients are HLA mismatched, we distinguish between donor liver–derived and recipient-derived leukocytes to show that Eomes^lo NK cells circulate freely whereas Eomes^hi NK cells are unable to leave the liver. Furthermore, Eomes^hi NK cells are retained in the liver for up to 13 y. Therefore, Eomes^hi NK cells are long-lived liver-resident cells. We go on to show that Eomes^hi NK cells can be recruited from the circulation during adult life and that circulating Eomes^lo NK cells are able to upregulate Eomes and molecules mediating liver retention under cytokine conditions similar to those in the liver. This suggests that circulating NK cells are a precursor of their liver-resident counterparts.