What are NK cells, and what role do they play in pregnancy?

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NK cells are a type of white blood cell (immune cell) in the innate immune system that primarily fights against infection.  

NK cells were first discovered due to their ability to target abnormal cells and kill them, particularly tumor cells, cancer cells, and other cells infected with a pathogen, such as a bacteria or virus.ⁱ NK cells found in the blood throughout the circulatory system are called peripheral blood NK cells. Peripheral NK cells can also be found in secondary lymphoid organs (i.e., spleen, tonsils), lymph nodes, and bone marrow. NK cells found in the uterus are called uterine natural killer cells (uNK cells). These uNK cells have different structures and functions compared to peripheral NK cells.

The term “killer cells” is fitting for a peripheral NK cell, but is somewhat of a misnomer for uNK cells, in that uNK cell activity is an important part of the implantation process. These cells modulate the immune system, immune cells, and immune responses by producing local factors that promote endometrial and placental growth. Some types of uNK cells have immune memory. The belief is that this memory may help improve placental development for any subsequent pregnancies that follow the first pregnancy.ⁱⁱ

While uNK cells promote endometrial and placental growth, they can also cause excess inflammation when they defend the uterus against pathogens (i.e., bacteria or viruses).ⁱⁱⁱ In some cases, an abnormal NK cell count and NK cell cytotoxicity (activity that is toxic to living cells) are thought to potentially be associated with recurrent implantation failure (RIF), recurrent pregnancy losses (RPL), or infertility.^iv

In addition to being found in the female reproductive tract, NK cells are also found in the male reproductive tract, particularly in the testis and prostate. While infection and inflammation of the male reproductive tract are widely accepted to be potential causes of male infertility, the role of NK cells in male infertility is poorly understood.^v

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How do NK cells impact fertility?

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Overall, there is a lack of consensus as to whether elevated NK cell levels or activity are associated with infertility and/or poor IVF outcomes. According to several studies,^vi,vii,viii higher levels of peripheral or uterine NK cells are associated with poor reproductive outcomes, including RPL (RPL is defined as more than two consecutive pregnancy losses before 20-week gestation).  

One study published in Human Reproduction (2019) compared peripheral blood NK cell levels in 104 women with RPL compared to 33 controls subjects (no RPL). They noted that a higher percentage of patients with RPL had elevated NK cell levels compared to controls (13 percent vs 3 percent).^ix  

Given that peripheral blood NK cells and uterine NK cells have different structure and function, a 2017 study compared uterine NK cell levels in 58 women with unexplained RPL to 17 fertile women.^x They noted higher concentrations of uterine NK cells in the group with unexplained RPL. Although this was a small study, results suggested that uNK cells may play a role in the pathophysiology of miscarriages.^xi

In contrast, other studies have not found evidence that elevated NK cells contribute to infertility outcome. One meta-analysis of 22 studies published in Human Reproduction Update (2013) reported no significant differences in the percentages of peripheral NK cells and uterine NK cells between fertile and infertile women. They also reported no significant difference in IVF outcomes between women with and without elevated NK cell levels.^xii

A 2019 meta-analysis observed that in some studies, elevated percentages of NK cells were reported in women with RPL or RIF; however, the higher levels did not predict poor IVF outcomes.^xiii The researchers’ own case-controlled study that included 115 women reported no differences in the percentage of total blood NK cells between women with RPL (9.4 percent), RIF (9.4 percent), and healthy controls (10.6 percent).^xiv

Given the conflicting evidence, it remains unclear whether NK cell levels are a useful marker for the management of infertility, including recurrent pregnancy loss and recurrent implantation failure.

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What causes high NK cells?

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Uterine NK cell levels vary throughout the menstrual cycle. Levels of NK cells and NK cell function are highly regulated by immune molecules (cytokines) secreted by the endometrium. Inflammation in the surrounding uterine environment or an underlying problem with the maternal hormonal and immune systems can lead to abnormal cytokine production. As a result, elevated NK cell levels or activity can occur.^xv

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How are NK cell levels tested?

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A uterine biopsy can be performed to test uNK cell levels. In this procedure, a physician will take a small sample of the uterine lining. The tissue sample is then analyzed in a laboratory under a high-resolution microscope to count the number of uNK cells present in the tissue. This process is called immunohistochemistry. Some researchers have suggested that a “normal range” should be defined as 40-300 uNK cells/mm² and a count above 300 uNK/mm² is considered “elevated”.^xvi

Because biopsy and testing for uNK cells is somewhat invasive and can be expensive, bloodwork has been proposed as an alternative to determine peripheral NK cell levels instead. However, it is a matter of debate as to whether testing peripheral NK cells provides an accurate representation of the levels and type of NK cells in the uterus, given that these two cell populations have separate functions.^xvii Peripheral NK cell levels are tested following a blood draw and no biopsy is required. The blood is analyzed using immunotherapy blood testing. This involves a specialized laboratory technique called flow cytometry, that examines the surface proteins expressed on the white blood cells to identify and quantify the NK cell population.^xviii

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What is the treatment for high levels of natural killer cells?

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Some clinics offer immunotherapy to patients who have unexplained recurrent pregnancy loss (RPL) or recurrent implantation failure (RIF), and who also have abnormal or elevated levels of NK cells. Several immunotherapy treatments have been proposed in an attempt to modify the uterine immune environment or to suppress the activity of NK cells. These treatments may include intravenous immunoglobulin (IVIg), intralipid infusions,^xix andcorticosteroids.^xx

IVIg Treatment: Intravenous immunoglobulin (IVIg) therapy is a product containing a mix of antibodies (immunoglobulin G, IgG) derived from thousands of donors. It is administered directly into a vein (intravenously) and is traditionally used to treat immune deficiencies (i.e., due to cancer treatment), as well as autoimmune and inflammatory disorders.  It has been proposed that IVIg may help women with elevated NK cell activity undergoing fertility treatments by shifting the immune environment towards a more anti-inflammatory and immuno-regulatory state.^xxi,xxii

In a 2012 study published in the American Journal of Reproductive Immunology, researchers investigated the effect of IVIg treatment for 40 IVF patients with RIF and/or RPL and elevated peripheral blood NK cell levels.^xxiii According to their findings, those who received IVIg therapy had a clinical pregnancy rate of 92.5 percent and live birth rate to 82.5 percent compared to 25 percent clinical pregnancy rate and 12.5 percent live birth rate among women who did not receive IVIg.^xxiv A study of 202 IVF cycles showed similar results for women with elevated peripheral NK cells; they observed an improvement in clinical pregnancy and live birth rates for those receiving IVIg treatment.^xxv In both studies, peripheral blood NK cell levels, not uterine NK cell levels were tested.  

It should be noted that not the published evidence supporting IVIg treatment in individuals undergoing fertility treatment is conflicting. Other studies examining IVIg immunotherapy have not observed an improvement in IVF outcomes.^xxvi,xxvii

Intralipid Infusions: Elevated levels of uNK cells can be treated with an intravenous infusion of intralipids during assisted reproductive technology (ART), such as IVF. This infusion is a 20 percent fat emulsion diluted in 250 mL of saline, and the intention is to suppress the activity of NK cells.^xxviii One possible intralipid treatment protocol involves scheduling the first infusion on the day of oocyte retrieval or the day of embryo transfer (for frozen transfer cycles). A second infusion occurs after the positive pregnancy test, and the following infusions take place every two weeks until the 12^th week of pregnancy.^xxix

Evidence supporting the effectiveness of intralipid IV infusions in improving fertility outcomes is inconsistent. A 2021 meta-analysis of several studies did not find a significant positive impact of intralipids on improving pregnancy rates in IVF cycles.^xxx

Corticosteroids: Corticosteroids, such as prednisone (pms-PREDNISOLONE®), work to suppress the immune system and decrease inflammation, and have been shown to decrease uterine NK cell activity.^xxxi They can be prescribed to RIF and RPL patients with altered immune cell activity.^xxxii

A study published in the Journal of Reproductive Immunology (2018) reported that not all RIF patients responded to prednisone treatment, with only 54.5 percent showing an improvement with a reduction in uNK cell activity after immunotherapy.^xxxiii The researchers concluded that only a portion of RIF patients may benefit from corticosteroid treatment. Importantly, the authors did not assess pregnancy outcomes following treatment.  

Corticosteroids given in combination with other treatments for RPL have been associated with improved clinical outcomes compared to when they are given alone. According to a double-blind placebo randomized controlled trial published in Archives of Gynecology and Obstetrics (2014) that included 160 patients with RPL, 70.3 percent of women who received prednisone in combination with low-dose aspirin and heparin had an ongoing pregnancy beyond 20 weeks' gestation, along with suppressed peripheral NK cell activity. Only 9.2 percent of the control group (heparin and aspirin with placebo) experienced this outcome.^xxxiv It is worth noting, however, that heparin and aspirin are not routine treatments for unexplained RPL. These therapies are usually reserved for women with a blood clotting abnormality (thrombophilia). Additionally, patients enrolled in this study had unexplained RPL, not elevated peripheral or uterine NK cell levels.

A study published in the Journal of Reproductive Immunology (2021) evaluated whether immunomodulatory (prednisone alone or prednisone+IVIg and anticoagulation (Lovenox® and low dose aspirin)) treatment improved IVF outcomes for 197 RPL and/or RIF patients with known immune cell abnormalities and thrombophilia.^xxxv The researchers observed that among all patients who received immunomodulatory treatment, there was a significant improvement in the pregnancy rate (48 percent) and live birth rate (40 percent) compared to historical controls (33 and 2 percent, respectively).^xxxvi Investigators concluded that immunomodulatory and anticoagulation treatment significantly improved reproductive outcomes of IVF cycles for women with immune-related RPL and/or RIF and thrombophilia.^xxxvii

Given the paucity of published evidence (small number of clinical trials) for corticosteroid use in IVF,^xxxviii and considering potential side effects/risks, the balance between risk and benefit should be carefully weighed by the physician and patient.^xxxix,xl

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What are the other risks or complications of elevated NK cells?

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Elevated NK cells can present additional potential problems. The activity of peripheral NK cells has been linked to several autoimmune diseases such as type I diabetes and lupus.^xli However, the presence of elevated NK cell levels can be protective or pathogenic (disease-causing). This depends on the types of NK cells present, as well as the tissue environment and stage or type of autoimmune disease.^xlii

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Is it possible to get pregnant with excess NK cells?

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According to a meta-analysis of 22 studies published in Human Reproduction Update (2013), significantly higher peripheral NK cell numbers existed in women with RPL. However, there was no significant difference in live birth rates between women with or without elevated peripheral NK cells or activity.^xliii

Researchers concluded that there is a lack of consistent evidence to determine whether elevated NK cell levels or activity will negatively impact reproductive outcomes. They recommended further research be conducted before NK cell measurements become a prognostic tool for infertility.^xliv

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Conclusion

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Knowing the potential role peripheral and uterine NK cells can play in miscarriage and infertility can help potential parents understand and navigate fertility treatments. It is important to recognize that while NK cells target diseased or abnormal cells, they are also an important part of menstrual cycle regulation and the process of implantation. More research is needed to determine how NK cells relate to fertility and if treatment to alter NK cell levels is beneficial.