NK cells that differ in expression of NKp46 might play different roles in endometrium

Dual Role of Natural Killer Cells in Early Pregnancy: Immunopathological Implications and Therapeutic Potential in Recurrent Spontaneous Abortion and Recurrent Implantation Failure

Natural killer (NK) cells are critical regulators of immune processes during early pregnancy, playing a key role in maintaining maternal-foetal immune tolerance and supporting successful implantation. In particular, uterine NK cells, a specialised subset of NK cells, facilitate trophoblast invasion, spiral artery remodelling and placental establishment. Dysregulation of NK cell activity, however, has been implicated in pregnancy complications, notably recurrent spontaneous abortion (RSA) and recurrent implantation failure (RIF). Aberrant NK cell functions, such as heightened cytotoxicity or defective immune signalling, can disrupt the balance between immune tolerance and response, leading to impaired placental development, reduced trophoblast activity and compromised uteroplacental blood flow. This review examines the role of NK cells in early pregnancy, emphasising their contributions to immune modulation and placentation. It also investigates the mechanisms by which NK cell dysfunction contributes to RSA and RIF, and explores therapeutic strategies aimed at restoring NK cell balance to improve pregnancy outcomes. A deeper understanding of NK cell interactions during early pregnancy may provide critical insights into the pathogenesis of pregnancy failure and facilitate targeted immunotherapeutic approaches.

Expression of NKp46 and other activating inhibitory receptors on uterine endometrial NK cells in females with various reproductive failures: A review

Background

Uterine endometrial natural killer (uNK) cells represent major leukocytes in the mid-secretory phase of the cell cycle, and their number is further increased during early pregnancy. The activating and inhibitory receptors expressed on their surface mediate various functions of uNK cells, such as cytotoxicity, cytokine production, spiral artery remodeling, and self-recognition.

Methods

This study reviewed the most recent information (PubMed database, 175 articles included) regarding the activating and inhibitory receptors on uNK cells in human females with healthy pregnancies and the evidence indicating their significance in various reproductive failures.

Main Findings

Numerous studies have indicated that the natural cytotoxic receptors, killer cell immunoglobulin-like receptors, and C-type lectin receptors, particularly those expressed on uNK cells, play crucial roles in successful pregnancy.

Conclusion

As studies on human uNK cells are limited owing to the low availability of fertile samples, and the extrapolation of animal models has certain limitations, the in vivo role of uNK cells has not yet been fully elucidated. However, immunotherapies focusing on modulating uNK cell function have been controversial in terms of pregnancy outcomes. Further research is required to elucidate the role of uNK cells in reproduction.

Higher CD16-NKp46bright uterine endometrial natural killer cells may predict pregnancy success in women experiencing recurrent reproductive failure

RESEARCH QUESTION

Could CD16-NKp46bright uterine endometrial natural killer (uNK) cells serve as a predictor of pregnancy success in women with unknown recurrent reproductive failure (URRF), and what are the underlying mechanisms involved?

DESIGN

A prospective study involving 63 women with URRF, followed up for 2 years. After age adjustment, 17 women remained in both the pregnant and non-pregnant groups. The pregnant group was further divided into the live birth (n = 10) and miscarriage (n = 4) groups, with three women lost to follow-up. Surface antigens expressed and cytokine produced in uNK cells were analysed with multicolour flow cytometry.

RESULTS

Expression NKp46+ uNK (P = 0.034), NKp46bright uNK (P = 0.045), CD16-NKp46bright uNK (P = 0.026), NKp46brightNKG2D+ uNK (P = 0.004) and NKp46+NKG2D+ uNK (P = 0.037) cells was significantly lower in the non-pregnant group compared with the pregnant group. Also, the expression of CD16-NKp46bright uNK cells was significantly (P = 0.040) higher in the live birth group compared with the non-pregnant group. The threshold 44.9% of CD16-NKp46bright uNK cells showed the largest area under the curve. Women with decreased CD16-NKp46bright uNK cells (<44.9%), produced significantly higher TNF-α+IFN-γ+ in CD56+ uNK (P = 0.014) and in CD56bright uNK cells (P = 0.013) and significantly lower TNF-α-IFN-γ- in CD56+ uNK (P = 0.039) and in CD56bright uNK cells (P = 0.017), and had an elevated risk of failing to achieve live birth or pregnancy (OR 21.60, 95% CI 2.14 to 218.58; P = 0.004 (OR 11.20, 95% CI 2.20 to 56.93; P = 0.005).

CONCLUSIONS

CD16-NKp46bright uNK cells are a protective factor as well as an appropriate candidate for predicting pregnancy success in URRF.

Exploring Natural Killer Cell Testing in Embryo Implantation and Reproductive Failure: An Overview of Techniques and Controversies

The blastocyst nidation is the most crucial stage to a successful pregnancy, as the white cells work to promote a favorable endometrial microenvironment for this process. Intriguingly, this implantation window lasts, on average, 6 days in most regular women, and its quality is affected by many pathological conditions. Since the grounds of reproductive failure in healthy couples are still uncharted, studies have widely suggested a potential hostile role of the immune system in the equilibrium of the maternal-fetal interface. In recent years, natural killer cells have been the highlight as they represent the greatest lymphocyte in the uterus and have immune surveillance through cytotoxicity during the implantation window. This review explored the main techniques used for natural killer (NK) cell testing in the implantation window over the last 13 years on the PubMed® database. Of 2167 published articles potentially relevant for the review, only thirty-three were about cell evaluation in healthy women, met the inclusion criteria, and had their methodology critically analyzed. Here, we bring a summary from the study group and sample collection to evidence comments about their findings and correlations. Meanwhile, we also summarize the current relationship between NK cells and endometrial receptivity with reproductive failure to help enhance the possibilities for future research. In conclusion, our overview points out that restricted and unstandardized methods support the controversy between the NK population and unsuccessful embryo implantation, which is an obstacle to studying why healthy eggs do not thrive and finding a solution for one of the most controversial topics in human reproduction.

A Blinded Investigation: Accentuated NK Lymphocyte CD335 (NKp46) Expression Predicts Pregnancy Failures

AIM

NKp46 is an NK cell receptor uniquely expressed by NK cells and a small subset of innate lymphoid cells. In our previous studies, we suggested a tight connection between the activity of NK cells and the expression of NKp46 and supported the clinical significance of NKp46 expression in NK cells in women with reproductive failures. In this study, we investigated the expression of NKp46 in NK cells in the peripheral blood of women in early pregnancy and analyzed its association with pregnancy loss.

METHODS

In a blinded study, we examined blood samples and analyzed the subsequent pregnancy outcomes from 98 early pregnant women (5th-7th week of gestation-w.g.) and 66 women in the 11th-13th week of pregnancy who served as controls. We studied the expression of NKp46 and the levels of anti-cardiolipin antibodies (aCL). The results of aCL were shared with the clinic, while the expression of NKp46 was blinded and not analyzed until the end of the study.

RESULTS

A misbalance in the NKp46⁺NK cells subpopulations was associated with an unfavorable ongoing pregnancy. A decreased level of NKp46^high cells (<14%) was strongly associated with miscarriage. A decreased level of the double-bright subpopulation (NKp46^hightCD56⁺⁺) also was a negative prognostic factor for the pregnancy course, but its increased level (>4%) was strongly associated with a successful pregnancy course.

CONCLUSIONS

Our results showed that accentuated levels of NKp46⁺NK cells lead to a negative prognosis for early pregnancy courses in women.

Co-expression of activating and inhibitory receptors on peritoneal fluid NK cells in women with endometriosis

The detailed mechanism underlying endometriosis development remains unclear; few reports have suggested the involvement of immune and genetic factors. This study aims to investigate the role of NK cells in endometriosis by analyzing the co-expression of activating (NKp46, NKG2C, and NKG2D) and inhibitory receptors (NKG2A and CD158a) on NK cells and their subsequent cytokine production in the peritoneal fluid (PF). Sixty-two patients were enrolled for this study from Hyogo Medical University between February 2018 and April 2022. Results showed that the proportions of CD56⁺/NKp46⁺, CD56^dim/NKp46^＋, NKG2C^＋/NKp46^＋, and NKG2D^＋/NKp46^＋ NK cells were significantly lower in the endometriosis group than those in the control group. Meanwhile, within the peritoneal endometriosis (n = 21) and deep infiltrating endometriosis (n = 11) groups, the co-expression of NKG2D^＋/NKp46^＋ and CD16^＋/NKp46^＋. Additionally, the abundance of IFN-γ-producing NK cells was significantly increased in the endometriosis group compared to controls, and a significant negative correlation was noted between NKp46 expression on NK cells and type 1 cytokine (IFN-γ and TNF-α) production. Taken together, the findings of this study indicate that NK cell cytotoxicity in endometriosis is reduced due to changes in NKp46 expression, as well as activating receptors co-expressed with NKp46. Consequently, NK cells do not eliminate endometrial cells in the abdominal cavity, resulting in the production of TNF-α and IFN-γ.

Uterine Natural Killer Cells: A Rising Star in Human Pregnancy Regulation

Uterine natural killer (uNK) cells are an immune subset located in the uterus. uNK cells have distinct tissue-specific characteristics compared to their counterparts in peripheral blood and lymphoid organs. Based on their location and the pregnancy status of the host, uNK cells are classified as endometrial NK (eNK) cells or decidua NK (dNK) cells. uNK cells are important in protecting the host from pathogen invasion and contribute to a series of physiological processes that affect successful pregnancy, including uterine spiral artery remodeling, fetal development, and immunity tolerance. Abnormal alterations in uNK cell numbers and/or impaired function may cause pregnancy complications, such as recurrent miscarriage, preeclampsia, or even infertility. In this review, we introduce recent advances in human uNK cell research under normal physiological or pathological conditions, and summarize their unique influences on the process of pregnancy complications or uterine diseases. Finally, we propose the potential clinical use of uNK cells as a novel cellular immunotherapeutic approach for reproductive disorders.

Effect of Antiviral Therapy During Pregnancy on Natural Killer Cells in Pregnant Women With Chronic HBV Infection

OBJECTIVE

To study the effect of antiviral therapy during pregnancy on the frequency of natural killer (NK) cells in peripheral blood of women with HBV DNA positive chronic hepatitis B (CHB).

METHOD

In total 124 female subjects were divided into four groups: 11 healthy non-pregnant women (Normal group), 26 non-pregnant women in immune tolerance period of chronic hepatitis B virus (HBV) infection (CHB group), 41 pregnant CHB women without antiviral treatment during pregnancy (Untreated group), and 46 pregnant CHB women receiving antiviral treatment during pregnancy (Treated group). The frequency of NK cells in peripheral blood were detected by flow cytometry.

RESULT

The frequency of NK cells in healthy women [15.30 (12.80, 18.40)] was higher than that in women with HBV infection, but there was no significant statistical difference (p=0.436). The frequency of NK cells in CHB group [10.60 (6.00, 18.30)] was higher than those in pregnant CHB women [Untreated: 6.90 (4.89, 10.04), P=0.001; Treated: 9.42 (6.55, 14.10), P=0.047]. The frequency of NK cells in treated group was significantly higher than that in untreated group (P = 0.019). The frequencies of NK cells, CD56bright NK cells and NKp46dim NK cells at 12 and 24 weeks postpartum in the untreated group were increased significantly than those before delivery. In treated group, the frequencies of NK cells, CD56bright NK cells, NKp46+ NK cells and NKp46dim NK cells were significantly increased at 6 and 12 weeks than those before delivery. The frequencies of NK cells and CD56bright NK cells postpartum were increased significantly in treated group than those in untreated group. The frequencies of CD56dim NK cells decreased significantly after delivery in treated than those in untreated patients. Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) significantly increased after delivery than those before delivery. The results showed that the postpartum ALT level was weak positive correlated with NKp46high frequency (r=0.199) and was weak negative correlated with NKp46dim frequency (r= -0.199).

CONCLUSION

Antiviral treatment during pregnancy could significantly increase the frequency of NK cells postpartum. Postpartum hepatitis may be related to the immune injury caused by change of NK cell frequency and HBV infection.

Evaluation of NKp46 expression and cytokine production of decidual NK cells in women with recurrent pregnancy loss

Purpose

NKp46, a receptor on NK cells, is involved in cytotoxicity and cytokine production. The authors aimed to evaluate the effect of NKp46 on decidual NK (dNK) cells during pregnancy and whether it can be a marker for immunological abnormalities in women with recurrent pregnancy loss (RPL).

Methods

Flow-cytometric analysis was made to assess NKp46 expression and intracellular cytokine production of dNK cells. The proportion of NKp46+ dNK cells was analyzed among RPL patients who aborted karyotypically normal pregnancies and those who either aborted karyotypically abnormal pregnancies or without genetic studies, and controls who were going through the induced abortion.

Results

The %NKp46+ and %NKp46bright dNK cells were significantly lower in the RPL women who aborted karyotypically normal pregnancies than in the control group. The %NKp46bright dNK cells were significantly correlated with the NK1/NK2 ratio of dNK cells. The %NKp46+ dNK cell cutoff for RPL with immunological abnormalities was determined by the ROC curve analysis. In women with the low %NKp46+ dNK, NK1/NK2 ratios were significantly higher than those with the high.

Conclusion

RPL patients with an immunological abnormality have decreased NKp46 expression and NK1 shift in dNK cells. NKp46 expression could be a marker for RPL of immunological abnormalities.