Natural Killer (NK) Cells and Fertility

Introduction

Natural killer (NK) cells are a type of white blood cell (lymphocyte) that helps your body fight infection and cancer, and are an important part of your body's immune system. These cells can be found in a number of different places in your body including the blood stream and uterus (womb).

Some years ago research was undertaken based on a theory that the body's immune system and NK cells might play a role in causing unexplained infertility, recurrent miscarriage and recurrent IVF failure. However, research studies have not consistently confirmed a role for NK cells in these situations. The Royal College of Obstetricians and Gynaecologists (RCOG) have published a scientific advisory review regarding immunological testing for natural killer (NK) cells in reproduction, confirming that most of the medical and scientific community do not advocate testing or treatment for NK cells in the above situations. Their conclusion states;

"With the exception of anti-phospholipid testing among women with recurrent miscarriage, there is little evidence to support any particular test or immunomodulatory treatment in the investigation and treatment of couples with reproductive failure. These tests and treatments should be restricted only to women entered in formal research studies".

If you are being asked to take part in a research study, there will be a special consent form for you to complete that describes the study, outlines the risks and benefits of taking part in the study and indicates which ethics committee has reviewed and approved the study.

A range of treatments have been used, and all of them are designed to lower your body's immune response, and therefore are associated with an increased risk of infection. Furthermore, none of these treatments are licensed specifically for use in reproductive medicine in Australia, and there are potential serious side effects associated with such treatments. Therefore, if you are considering being tested for NK cells it is important to be aware of the current status of knowledge about NK cells and fertility.

Background Information

NK cells have been found in both the blood stream and the lining of the uterus. However, NK cells in the blood stream are of a different type from those found in the uterus. Uterine NK cells vary in number throughout the menstrual cycle, and their number is also influenced by such factors as whether a woman has recently had intercourse or if she is depressed.

In the early part of the menstrual cycle the number of NK cells is lower than in the latter part of the menstrual cycle or around the time of implantation. When the placenta is developing in early pregnancy there is normally a high number of NK cells and these are thought to play an important role in the formation of placental blood vessels. Far from finding a role for NK cells in causing miscarriage, many scientists think that NK cells play an important and beneficial role in establishing a good blood supply for the placenta. Research has shown that animals that lack uterine NK cells are more likely to have a miscarriage or a low birth weight baby, again suggesting that uterine NK cells have a beneficial impact on early pregnancy development.

Tests for NK Cells

The best available study to date found that the result of blood NK testing does not predict IVF success rates or the risk of miscarriage. This information is not surprising as we have previously indicated that blood NK cells are not the same type of cells as uterine NK cells, and blood NK cell numbers do not provide helpful information about numbers of uterine NK cells.

The other theoretical way of testing for NK cells is to obtain a sample of the lining of the uterus (endometrial biopsy) around the time in the cycle when implantation would occur. This kind of testing is a research tool and has not been proven as a helpful diagnostic test.

Summary

There is no current evidence to suggest that NK cells are implicated in causing infertility, IVF failure or recurrent miscarriage. Indeed NK cells are likely to play an important role in early pregnancy development.
Blood tests for NK cells do not provide useful information about NK cell levels in the uterus. Uterine testing for NK cells is a research tool.
There are significant potential side effects/risks associated with treatments designed to reduce NK cell numbers and potential treatments are not licensed for use in reproductive/fertility care in Australia.
The current consensus is that such tests and treatments should be limited to women who are participating in formal research studies.

If you would like further information or to obtain references to studies in this area you can email us at info@bubtree.com.au or call us at 1 300 853 546.