Populations of NK Cells and Regulatory T Cells in the Endometrium of Cycling Mares—A Preliminary Study

SIMPLE SUMMARY: Endometrial immune cells, such as regulatory T cells and NK cells, are not well known in mares. Nevertheless, these cells play important roles both in physiological and pathological processes in equine reproduction. This work aimed to characterize populations of regulatory T cells and NK cells in the endometrium of cycling mares. With the use of specific antibodies, we showed that two different types of regulatory T cells and NK cells are present both during estrus and diestrus in mares. ABSTRACT: Endometrial immune cells are essential to support uterine functions across the estrous cycle and in preparation for pregnancy. It has been acknowledged that changes in phenotype and/or numbers of lymphocytes, such as regulatory T cells (Tregs) and NK cells, might result in lower fertility in women and mice. Little is known about equine endometrial immune cells across the estrous cycle. Here, we compared the populations of endometrial Tregs and NK cells in estrus and diestrus in mares. Endometrial biopsy and blood samples were taken in estrus and diestrus from 11 mares ages 4–12 years. Flow cytometry with anti-CD4, -CD25 and -FOXP3 and anti-NKp46 and -CD3 antibodies was used to determine the populations of Tregs and NK cells, respectively. The concentration of progesterone was measured with chemiluminescence immunoassay. The results were analyzed with paired Student t tests. The mean percentage of endometrial CD4(+)FOXP3(+) Tregs was 13.7 ± 6.2% in diestrus and 14.5 ± 5.9% in estrus, while the mean percentage of endometrial CD4(+)FOXP3(+)CD25(+) Tregs changed from 3.6 ± 2.1% in diestrus to 2 ± 2% in estrus (p = 0.0947). The mean proportion of CD3(−)NKp46(+) lymphocytes in the endometrium was not significantly different, with 6 ± 1% in estrus and 6.5 ± 1.4% in diestrus. There was a large variation in the percentage of NK cells between mares of 2.1–12.7%. This study showed, for the first time, the presence of CD4(+)FOXP3(+)CD25(+) Tregs and CD3(−)NKp46(+) NK cells in the endometrium of non-pregnant cycling mares. The percentage of Tregs, and to a greater extent NK cells, showed large fluctuations between mares. Both Tregs and NK cells might be important for the preparation of the endometrium for semen deposition and pregnancy; however, further research is required.