Evaluation of endometrial natural killer cell expression of CD4, CD103, and CD16 cells in women with unexplained infertility

Highlights

• Successful pregnancy requires coordination between a receptive endometrium and the implanting blastocyst.

• It is reasonable to assume that some of the women with unexplained infertility have an endometrial factor that is contributing to their infertility.

• High numbers of endometrial natural killer cells may cause immune and vascular abnormalities in the endometrium of women with unexplained infertility.

Abstract

Objective

To determine whether natural killer cell (NK) concentrations in the mid-secretory endometrial tissue of women with unexplained infertility differ from those in the mid-secretory endometrial tissue of fertile women.

Methods

This study was conducted with 22 patients with unexplained infertility caused by unsuccessful ovulation induced with gonadotrophins and 12 healthy fertile women, who formed a control group. Mid-secretory endometrial tissue samples were obtained with a Pipelle catheter, and the endometrial NK cell phenotypes were determined by flow cytometry.

Results

The median number of CD4+ cells in the endometrium was significantly lower in women with unexplained infertility compared with in the fertile controls (3.31 vs. 5.81; p = 0.02). The CD4⁺/CD8⁺ cell ratio in the unexplained-infertility group was significantly lower than that in the control group (0.42 vs. 0.76, p = 0.01). The median number of CD103⁺ cells in the endometrium of the infertile group was significantly lower than that in the endometrium of the control group (4.40 vs. 6.73, p = 0.02). The median number of CD16⁺ cells was significantly higher in infertile women than in control women (0.12 vs. 0.001, p = 0.01).

Conclusions

Significant reductions in the numbers of CD4⁺ and CD103⁺ cells as well as in the CD4⁺/CD8⁺ ratio but an increased number of CD16⁺ cells in the endometrium of infertile women suggest that disordered NK cell activity may be a contributor to the etiology of unexplained infertility.

Introduction

A successful pregnancy requires coordination between a receptive endometrium and the implanting blastocyst, which is dependent on successful placentation and the immune cells present in the decidua. Natural killer (NK) cells comprise 5–15% of peripheral blood lymphocytes. The origins of NK cells are undefined, but several sites have been proposed, including the bone marrow for CD34⁺ hematopoietic progenitor cells (Colucci et al., 2003). NK cells develop in lymphoid tissues and the thymus (Freud and Caligiuri, 2006, Vosshenrich et al., 2006) and have been classified into CD56^dim and CD16⁺ NK cells using flow cytometry; these include 90% of the NK cell population and are considered more cytotoxic than CD56^bright CD16⁻ NK cells, which comprise only 10% of peripheral blood NK cells (Cooper et al., 2001, Nagler et al., 1989). Decidual NK cells predominantly consist of CD56^bright CD16⁻ NK cells (Hanna et al., 2003). NK cells specialize in killing tumor- and virus-infected cells and secrete a variety of cytokines (Lieberman and Mandelboim, 2000, Zingoni et al., 2005).

In addition to lymphoid and non-lymphoid tissues, NK cells also populate the uterus (Hanna and Mandelboim, 2007). Uterine NK (uNK) cells differ from NK cells in other sites, as they contain numerous cytoplasmic granules and are scattered throughout the endometrium and spiral arteries (Croy et al., 2003, Veljkovic et al., 2013). Mice deficient in uNK cells have defective spiral arteries (van den Heuvel et al., 2005). T lymphocytes are the main immune cells in the human endometrium and are not CD56^brightNK cells. Indeed, only one-third of endometrial lymphocytes are NK cells (Manaster et al., 2008).

It is reasonable to assume that some women with unexplained infertility have an endometrial component contributing to their infertility. However, defining possible endometrial defects in a tissue that changes during each menstrual cycle is very difficult. Human uNK cells are found during every post-pubertal menstrual cycle (Moffett-King, 2002), when they provide immune protection for uterine mucosal surfaces and play crucial roles in endometrial remodeling, decidualization, and embryonic implantation (Bulmer et al., 2010). The presence of NK cells at the implantation site indicates that human uNK cells have an important role in fertility. It has been suggested that uNK cells play an important role during implantation and that an increase in cytotoxic uNK cells can affect implantation (Fukui et al., 1999). A literature review revealed 10 studies that have compared the numbers of NK cells in infertile women vs. those in controls (Seshadri and Sunkara, 2014). We found only two studies evaluating uNK cells in infertile women (McGrath et al., 2009, Parkin et al., 2011). Recent studies have shown that numbers of NK cells >12% above normal are associated with poor reproductive performance (Michou et al., 2003, Thum et al., 2004). King et al. demonstrated that NK cells >18% above normal are strongly associated with implantation failure (King et al., 2010). The objective of the present study was to determine whether the concentrations of NK cell lymphocyte subpopulations differed in the mid-secretory endometrium of women with unexplained infertility compared with those in normal fertile women.