Natural killer cells, miscarriage, and infertility

Uterine natural killer cells, implantation failure and recurrent miscarriage

Uterine natural killer (uNK) cells are the most abundant leukocytes in preimplantation endometrium and early pregnancy decidua. Maternal uNK cells are adjacent to, and have the ability to interact directly with, fetal trophoblasts. uNK cells can secrete an array of cytokines that are important in angiogenesis and thus placental development and the establishment of pregnancy. Increased numbers of uNK cells have been associated with reproductive failure. The number of preimplantation uNK cells has been reduced with prednisolone. However, despite these exciting advances in understanding of the uNK cells, considerably more work needs to be done to establish a specific role for uNK cells and to use uNK cells as a test of malfunctioning endometrium and the basis for future treatment for reproductive failure.

NK cells in autoimmune disease

The role of NK cells in autoimmunity has not been extensively studied. Speaking for a disease-promoting role for NK cells in autoimmune diseases are recent results suggesting that IFN-gamma production by NK cells may help adaptive immune responses diverge in the direction of a Th1 response. NK cells may also be involved in direct killing of tissue cells, which could lead to acceleration of autoimmunity. However, NK cells have also been shown to protect from some autoimmune diseases. A possible reason for this discrepancy may lie in the capacity of NK cells also to produce Th2 cytokines, which could downregulate the Th1 responses that are common in autoimmune disorders. Thus there is at present no coherent view on the role of NK cells in autoimmunity, and more work is needed to clarify why NK cells in some cases aggravate disease and in some cases protect from disease.

Trophoblast Cells as Immune Regulators

Medawar, in the early 1950s, recognized for the first time, the unique immunology of the maternal-fetal interface and its potential relevance for transplantation. In his original work, he described the “fetal allograft analogy” whereby the fetus may be viewed as a semi-allogeneic conceptus that has evaded rejection by the maternal immune system. Although numerous hypotheses have been proposed to prove this observation, none have demonstrated that the maternal immune system is antagonist to the invading trophoblast. In the present manuscript we have reviewed recent studies demonstrating the expression and function of TLRs in trophoblast cells and based on this data we propose an alternative view for maternal-fetal immune interactions.

Serious foetal growth restriction is associated with reduced proportions of natural killer cells in decidua basalis

Extravillous trophoblasts are major participants in placental development and remodelling of spiral arteries. Trophoblast invasion is regulated by maternal immune cells, and abnormal leucocyte subpopulation composition has been reported in implantation failure. In pre-eclampsia (PE), with or without foetal growth restriction (FGR), superficial trophoblast invasion and insufficient remodelling of spiral arteries are common findings. In the present study, we have compared spiral artery remodelling and leucocyte composition in decidual tissue from 30 cases (PE=8, FGR=5, PE + FGR=17) and 31 controls. Six histological remodelling criteria were established, and each pregnancy obtained a remodelling score. Numbers of natural killer (NK) cells (CD56+), T cells (CD3+) and activated (CD25+ or CD69+) leucocytes were determined and related to total leucocyte (CD45+) numbers in serial sections. Cases demonstrated significantly impaired spiral artery remodelling, inappropriate placental growth and reduced NK cell proportions, as compared to controls (P=0.02, P<0.001 and P=0.01, respectively). Reduced NK cell proportion was primarily found in pregnancies complicated by FGR, with or without PE, and a significant positive correlation was observed between NK cell proportion, trophoblast infiltration and placental growth. Our in vivo observations support the hypothesized association between NK cells, impaired placental development and pathogenesis of PE/FGR.

Prednisolone reduces preconceptual endometrial natural killer cells in women with recurrent miscarriage

OBJECTIVE

To test the hypothesis that high numbers of uterine natural killer (uNK) cells in the endometrium of women with recurrent miscarriage (RM) could be reduced with prednisolone.

DESIGN

A before and after study.

SETTING

A tertiary referral teaching hospital.

PATIENT(S)

Eighty-five women with idiopathic RM recruited from all over the UK and 18 women attending for sterilization (controls).

INTERVENTION(S)

An endometrial sample was taken on day 21 +/- 2 of the menstrual cycle. Immunohistochemistry was used to identify uNK (CD56+, CD16-, CD3-). Twenty-nine women with RM and >5% uNK agreed to take 20 mg oral prednisolone daily from day 1 to 21 of their menstrual cycle, when a second biopsy was obtained and analyzed.

MAIN OUTCOME MEASURE(S)

The percentage of stromal cells that were uNK. The normal range was defined using control samples as <5%.

RESULT(S)

Women with RM had significantly more uNK than the controls (P=.008). Prednisolone treatment significantly reduced the number of CD56 cells in the endometrium, from a median of 14% (before) to 9% (after) (P=.0004).

CONCLUSION(S)

We have demonstrated that high numbers of uNK in preimplantation endometrium of women with recurrent miscarriage can be reduced with administration of prednisolone.

Natural killer cells and reproductive failure

PURPOSE OF REVIEW

The purpose of this review is to provide an update of what is known about the role of natural killer cells in reproduction.

RECENT FINDINGS

In humans, natural killer cells are present in abundance in the uterus and appear to play an important role in early pregnancy. Women with reproductive failure have been shown to have higher levels of natural killer cell numbers and activity in peripheral blood. The evidence relating to uterine natural killer cells, however, is contradictory. While earlier studies suggested an increase of uterine natural killer cells in women with recurrent pregnancy loss, more recent investigations have not confirmed this trend. Uterine natural killer cell number or activity appear to be higher in spontaneous abortions with normal chromosomes compared with chromosomally abnormal pregnancies. Furthermore, a rise of the cytotoxic CD56(dim)CD16+ natural killer cell subset is seen in the follicular fluid of patients with idiopathic infertility and it is postulated that this increase may decrease fertilization rates in this group.

SUMMARY

Natural killer cell number and activity appear to be altered in women with reproductive failure although it is unclear if the difference is a cause or effect of reproductive failure. Whether natural killer cells are increased or decreased depends on whether peripheral blood, endometrium or first-trimester decidua is analyzed. Until more is known about the role of natural killer cells in normal pregnancy, there does not appear to be any benefit in offering natural killer cell testing to women with reproductive failure outside of research protocols.