Natural killer (NK) cells have been in the spotlight in recent years with numerous theories postulated about their potential role in miscarriage and infertility. Numerous fertility clinics are measuring levels of NK cells and their activity with a view to reduce these with immunosuppressant therapy.
While NK cells are known to play a role in viral infections, knowledge regarding their relevance in fertility issues is not as robust and the scientific basis on which these treatments are based on is conflicting.
What are NK cells?
NK cells are defined as large granular lymphocytes (white blood cells) and are an important component of the innate immune system. NK cells function as one of the first lines of defence, providing protection against viral and bacterial infections and helping detect and limit the development of cancerous growth.
This category of cells were initially given their name as they demonstrated the ability to “naturally” kill tumour cells without any priming or prior activation. This is an important action as other immune cells, like T lymphocytes, can only detect and destroy harmful cells that have markers, such as MHC I.
NK cells are known to mature and differentiate in different places in the body, including the bone marrow and lymphatic system, before entering circulation where they become known as peripheral natural killer (pNK) cells and display their immune activity as described above.
What are uterine NK cells?
A specific subset of NK cells, known as uterine natural killer (uNK) cells or CD56+ NK cells, are the most prominent immune cell found in the endometrium; yet a “healthy range” of uNK cells has not been established. Largely due to a lack of quality information, the actions and importance of uNK cells are also largely unknown.
However, what is known is that the levels of uNK cells fluctuate throughout the menstrual cycle and become particularly concentrated at the site of placentation, in close proximity to infiltrating trophoblast cells (specialised cells of the placenta that play a role in embryo implantation and interaction with the uterus). There is no known evidence that uNK have “killing” activity on an embryo. In fact, they are only in contact with placental trophoblast cells. They cannot impact on the trophoblast unless exposed to high doses of interleukin (IL)-2, a cytokine absent from the normal pregnant uterus.
The prevailing view is that uNK cells play an important role in building a healthy placenta, protecting the mother from the foetal tissue and support the foetus during its development, however this is not confirmed as yet.
How are NK cells measured?
pNK cells are measured in serum blood test. pNK cell numbers do not appear to relate to uNK numbers in the endometrium, which are measured by endometrial biopsy. The timing of this procedure is important as uNK cell numbers vary considerably during menstrual cycle.
While they can be measured, there is not a consensus on what constitutes a healthy range of uNK cells, nor is there a consistent time at which they are measured. Many researchers argue that until this is established, measuring uNK cell activity in relation to fertility does not serve a purpose. Until more is known, there is no clinically relevant information that can be gathered from performing a uNK cell test.
While pNK cell activity is not associated with uNK, it may offer clinical benefit. Elevated pNK levels indicate a heightened immune response and may indicate a body is under stress, which warrants further investigation.
What does the evidence say?
High levels of pNK cell activity were initially considered a risk factor for unexplained infertility, in vitro fertilisation (IVF) failure and recurrent miscarriage (RM). High levels of uNK cells have been hypothesised as playing a central role in cases of RM and infertility due to defective implantation, and multiple studies have been conducted to determine if they play a role of clinical significance.
A 1999 study found there were increased mean numbers of uNK cells in the endometrium of women with recurrent early miscarriage only. However, there was no correlation with uNK cells and the number of previous births, the number of live births or the time since the last miscarriage. The authors noted a considerable overlap in uNK cell numbers between women who miscarried and control subjects, indicating that no single pathology was underlying all cases of RM.
A more recent study conducted in 2007 also found significantly higher numbers of uNK cells in women with RM. However these numbers did not predict subsequent pregnancy outcome.
A link has been made between higher number of uNK cells and a decreased production of cortisol by decidualising cells (cells regulating initial changes to the pregnant endometrium) and suboptimal induction of enzymes involved in lipid production in pregnancy. This evidence suggests that uNK cells may be a result of low uterine cortisol, rather than uNK cell numbers causing the steroid deficiency.
Conversely, multiple meta-analyses and comprehensive reviews that evaluated uNK cells showed no significant difference in women with RM compared with controls, however pNK cell numbers and percentages were significantly higher in women with RM compared with controls.[5,12]
A small portion (11%) of women with repeated IVF failure (RIF) had significantly increased pNK cell activity. While the peripheral blood test for NK cells was able to define a subgroup of women, it is still unclear whether this is a clinically useful test to do for all cases of infertility.
Additional research suggests that an appropriate level of NK cell activation may be essential for distributing maternal and foetal resources, and ultimately for successful pregnancies. uNK cells express a range of receptors that respond to a variety of signals, which are either induced by the hormonal changes of pregnancy or expressed by trophoblast cells.
As discussed above, the current evidence is not conclusive on either side. Indeed, there is a lack of knowledge of detrimental effects of uNK cells and the prognostic value of measuring uNK or pNK cell parameters remain uncertain.
What can be done to improve fertility?
Due to the theory that elevated uNK and pNK levels indicate an overactive immune system, medical suppression of the immune system has been trialled in women with recurrent miscarriage and unexplained infertility. Immunotherapies have included white blood cells (leukocytes) from the woman’s partner/donor, products derived from early embryos (trophoblast membranes), antibodies derived from blood (immunoglobulins) and steroid treatment.
A Cochrane review on immunotherapy for recurrent miscarriage found no overall difference in live birth rates between immunotherapies and control. However preliminary trials utilising prednisolone to reduce uNK cell numbers report inconclusive increases in live births compared to placebo groups. Two systematic recent reviews of the literature also caution against the use of any therapy aimed at suppressing NK cells due to the extent of the conflicting and inconclusive evidence available.[12,17]
A holistic approach addresses the suspected cause of these raised levels of immune cells. NK cells are often raised due to an up-regulation of inflammation. Tackling inflammation with 2g of fish oil a day has been shown to reduce placental inflammation during pregnancy. Promising animal studies have also demonstrated that maternal dietary supplementation with fish oil enhances the placental capacity to resolve inflammation.
Nutrient deficiency should be investigated and addressed. Zinc deficiency before pregnancy can impair hormone production from the corpus luteum and pituitary, impair the maternal immune system, as well as adversely affect the physical and mental development of children. Vitamin D3 deficiency has also been linked to recurrent pregnancy losses and immune dysregulation.
Deficiency of vitamin D3 is also associated with increased risk of obstetrical complications during pregnancy.
By seeking to restore homeostasis, supporting normal hormone production (including cortisol) and modulating immune function, practitioners can support the body and prepare it for a healthy pregnancy.
- Leischner C, Burkard M, Pfeiffer M, et al. Nutritional immunology: function of natural killer cells and their modulation by resveratrol for cancer prevention and treatment. Nutr J 2015;15:47. [Full Text]
- Vidal S, Khakoo S, Biron C. Natural killer cell responses during viral infections: flexibility and conditioning of innate immunity by experience. Curr Opin Virol 2011;1;1(6):497-512. [Full Text]
- Clifford K, Flanagan AM, Regan L. Endometrial CD56+ natural killer cells in women with recurrent miscarriage: a histomorphometric study. Hum Reprod 1999;14(11):2727-2730. [Abstract]
- Rodrigues M, Favaron P, Dombrowski J, et al. Role of natural killer (NK) cells during pregnancy: A review. OJAS 2013;3:138-144. [Full Text]
- Moffett A, Shreeve N. First do no harm: uterine natural killer (NK) cells in assisted reproduction. Hum Reprod 2015;30(7):1519-1525. [Full Text]
- Zhang J, Dunk C, Croy A, et al. To serve and to protect: the role of decidual innate immune cells on human pregnancy. Cell Tissue Res 2016;363(1):249-265. [Abstract]
- Laird S, Mariee N, Wei L, et al. Measurements of CD56+ cells in peripheral blood and endometrium by flow cytometry and immunohistochemical staining in situ. Hum Reprod 2011;26(6):1331-1337. [Full Text]
- Wang Q, Li T, Wu Y. Reappraisal of peripheral NK cells in women with recurrent miscarriage. Reprod Biomed Online 2008;17(6):814-819. [Abstract]
- Laird GE, Li TC, Bulmer JN. The role of natural killer cells in human fertility (scientific impact paper no. 53). Royal College of Obstetricians and Gynaecologists 2016. [Link]
- Tuckerman E, Laird SM, Prakash A, et al. Prognostic value of the measurement of uterine natural killer cells in the endometrium of women with recurrent miscarriage. Hum Reprod 2007;22:2208-2213. [Full Text]
- Kuroda K, Venkatakrishnan R, James S, et al. Elevated periimplantation uterine natural killer cell density in human endometrium is associated with impaired corticosteroid signaling in decidualizing stromal cells. J Clin Endocrinol Metab 2013;98(11):4429-4437. [Full Text]
- Seshadri S, Sunkara SK. Natural killer cells in female infertility and recurrent miscarriage: a systematic review and meta-analysis. Hum Reprod Update 2014;20(3):429-438. [Full Text]
- Sacks G, Yang Y, Gowen E, et al. Detailed analysis of peripheral blood natural killer cells in women with repeated IVF failure. Am J Reprod Immunol 2012;67(5):434-442. [Full Text]
- Moffett A, Colucci F. Uterine NK cells: active regulators at the maternal-fetal interface. J Clin Invest 2014;124(5):1872-1879. [Full Text]
- Moffett A, Regan L, Braude P. Natural killer cells, miscarriage, and infertility. BMJ 2004;27;329(7477):1283-1285. [Full Text]
- Wong L, Porter T, Scott J. Immunotherapy for recurrent miscarriage. Cochrane Database Syst Rev 2014;21;(10):CD000112. [Full Text]
- Tang AW, Alfirevic Z, Quenby S. Natural killer cells and pregnancy outcomes in women with recurrent miscarriage and infertility: a systematic review. Hum Reprod 2011;26(8):1971-1980. [Full Text]
- Haghiac M, Yang X, Presley L, et al. Dietary omega-3 fatty acid supplementation reduces inflammation in obese pregnant women: a randomized double-blind controlled clinical trial. PLoS One 2015;10(9):e0137309. [Full Text]
- Jones M, Mark P, Keelan J, et al. Maternal dietary omega-3 fatty acid intake increases resolvin and protectin levels in the rat placenta. J Lipid Res 2013;54(8):2247-2254. [Full Text]
- Grant E. Nutritional supplements to prevent pregnancy complications - response to clinical review threatened miscarriage: evaluation and management. BMJ 2004;329:152. [Link]
- Kwak-Kim J, Skariah A, Wu L, et al. Humoral and cellular autoimmunity in women with recurrent pregnancy losses and repeated implantation failures: a possible role of vitamin D. Autoimmun Rev 2016;15(10):943-947. [Abstract]
- Ota K, Dambaeva S, Han AR, et al. Vitamin D deficiency may be a risk factor for recurrent pregnancy losses by increasing cellular immunity and autoimmunity. Hum Reprod 2014;29(2):208-219. [Full Text]