For women who have experienced miscarriages frequently or have a poor obstetric history, they may find natural killer cells as a possible cause for their inability to sustain pregnancy. While these cells are responsible for protecting the body against infections and illnesses such as cancer, they are also singled out with increasing frequency as a potential cause of infertility as well as recurring miscarriage.
As the name implies, natural killer cells search for and destroy cells that are harmful to the body. Once NK cells identify cells of foreign or infectious origin, they bind to them and produce a cytotoxic chemical known as tumor necrosis factor that results in their destruction. Used in chemotherapy drugs, tumor necrosis factor creates holes in the membrane of the target cell, ultimately killing it.
During early pregnancy, there is a dramatic increase in uterine natural killer cells. This occurs as a result of the close interaction between the endocrine system and the immune system, particularly during implantation and maintenance of pregnancy. At the endometrium, NK cells multiply at an increased rate.
Although the role of endometrial NK cells is not completely clear, theory suggests that they promote the growth of placental and trophoblastic cells. In some women, the natural activity of the natural killer cells is to view the embryo as a foreign invader. The NK cells thus increase in number in order to increase their efficacy. They then attach to the embryo and destroy it as they would any invading cell. For women experiencing multiple miscarriages, this may be the reason.
Diagnosing and treating autoimmune-related miscarriages due to NK cell activity is not easy. A complete blood count with differential count to measure the level of natural killer cells cannot differentiate uterine NK cells. There is no substantial evidence that suggests the number and activity of NK cells in the bloods is correlated to the number and activity of uterine NK cells. As far as treatment options are concerned, high-dose steroids, intravenous immunoglobulin (IVIg) and tumor necrosis factor-a (TNF-a) blocking agents are commonly prescribed. However, these treatment options require further study, as they are not approved for use in reproductive medicine. Furthermore, there is little supporting evidence that suggests these treatment options are effective or beneficial.
Tests for NK cells are currently very inconsistent and may be an unreliable method for measuring NK cells and their activity levels. If tests are done when the NK cells are inactive, it becomes increasingly difficult to measure and properly assess the levels of NK cells found in blood, making the test unreliable.
There are several explanations for increased levels of natural killer cells in women with recurring miscarriages. Stress is a common example, since some researchers speculate that a number of women tend to have an immune system that responds more readily to tension and anxiety.
Scientists still need to understand the effect of NK cells on pregnancy, as well as uncover any evidence that immunosuppressive therapy actually works to lower the incidence rates for miscarriage.