Marilen Benner

CHAPTER 2 36 and stays over the course of pregnancy until term. Between term decidua and endometrium, the distribution of Th1, Th2, Th17, and Th1-like cells did not significantly differ. The similar Th cell distribution between endometrium and term decidua suggests that the pre-pregnancy endometrium might already be prepared for pregnancy. It can therefore be envisaged that women who experience recurrent miscarriages already have an imbalance in their endometrium affecting a successful pregnancy outcome. For instance, Shimada et al. (44) showed that in the endometrium of women with recurrent miscarriages less CD4 + IFN- γ + T cells were present. In-depth analysis of the Th1/Th2/Th17/Treg balance in the endometrium of women with fertility issues might give us more insight into the underlying pathogenesis of pregnancy complications and could potentially predict subsequent pregnancy outcome. Interestingly, when looking at the actual intrinsic capacity of CD4 + T cells to express IL-17 and IFN- γ intracellular, we found more expression of IFN- γ and IL-17 by decidual T cells compared to peripheral and endometrial CD4 + T cells. In first trimester decidua, the percentage of IL-17 + T cells was also found to be higher compared to peripheral blood (16, 45), while the expression of IFN- γ was lower (46). Although IFN- γ and IL-17 have been related to pregnancy complications, and an excess of inflammation was associated with a negative impact on pregnancy outcome (16, 18, 45, 47, 48), the presence of IFN- γ + and IL-17 + T cells in healthy term decidua suggests that these cytokines may play an important role during pregnancy. IFN- γ for instance, was shown to be essential for implantation, decidual integrity and placental growth in mice (47, 49). IL-17 plays an important role in host defense against pathogens (15), but it was also shown that IL-17 increases the production of progesterone by JEG-3 cells and supports the survival, proliferation and invasive capacity of trophoblast cells (50-52). The presence of normal, balanced levels of IFN- γ and IL-17 could play a role during placentation and/or prevention of intrauterine infection, but this is still far from understood. While we did not show a difference in the capacity of peripheral and endometrial CD4 + T cell to express IL-17, Hosseini et al. (53) showed more IL-17 + CD3 + T cells in menstrual blood compared to peripheral blood. This difference can be explained by a different gating approach since we looked at percentage of CD4 + T cells, while they looked at CD3 + T cells. When we would gate our data in a similar way, we found a similar, although not significant, difference between peripheral and menstrual blood. This can be explained by the higher percentages in our peripheral blood samples (average 4.5%) compared to theirs (average 0.9%). Several reasons could explain this discrepancy: duration of stimulation (4h vs 6h) or differences in stimulus and/ or concentration used (PMA (12.5 ng/ml)/Ionomycin (500 ng/ml)/Brefeldin A (5 µg/ml) by us versus PMA (25 ng/ml)/Ionomycin (500 ng/ml)/Monensin (1 µM/ml) by them). A limitation or our study is that we were not able to study decidual tissue at several time-points during pregnancy, to actually show the changes that occur with time. We could only infer this from the data collected from pre-pregnancy endometrium (implantation stage) and term decidua.