Marilen Benner

CHAPTER 8 220 confirmed that we successfully isolated immune cells of the uterine mucosa; the observed NK cell profile consisting of almost exclusively NKbright cells displays the phenotype associated with uterine cells (4). Additionally, the presence of CD103 + , an integrin subunit mostly observed on tissue resident cells in combination with the absence of CD62L, a marker associated with circulating leucocytes, further confirmed that our observed results reflect uterine immunity rather than peripheral blood contamination. Our results again reveal the general consensus of NK and T cell fluctuations throughout gestation: NK cells are abundant in non-pregnant endometrium but decrease in term decidua parietalis (5-7). The opposite is seen for T cells, accumulating throughout gestation (8-10). Of note, all samples of uterine mucosa contained, albeit small, a CD19 + B cell fraction. Using high dimensional flowcytometry, we showed a gain in immune cell memory from pre- pregnancy mucosal immunology up to term decidua. Less naïve, and more effector and central memory CD4 + and CD8 + T cells were observed in decidua parietalis (Chapter 2) . Likewise, an increase in B cell memory was observed in the study described in Chapter 4 . Especially in contrast to lymphocytes of the non-pregnant mucosa, this suggests that cells become activated and differentiate over the course of pregnancy. Indeed, in vitro , endometrial cells were able to differentiate and display a similar switch in phenotype upon T cell activation. The T cell signature we observed in term decidua is in accordance with earlier data by Tilburgs and colleagues that highlights the distinct functional profiles of these highly differentiated decidual T cells, compared to their counterparts in peripheral blood (11). Building up on this, it was later shown that decidual T cells are able to recognize fetal antigen as previously hypothesized (12). Frequencies of HY- specific T cells, in response to male fetuses, were 10-fold higher in decidua (0,6% of CD8 + T cells) compared to peripheral blood highlighting the local character of the occurring recognition. In an in vitro mixed lymphocyte reaction, decidual CD4 + and CD8 + T cells were shown to proliferate more potently than peripheral blood T cells in response to irradiated fetal cord blood cells. Powell and colleagues additionally observed how Treg are essential to control this proliferation (12). These and other studies highlight the importance of local Treg to contain inflammation of the decidua (8). To establish an overview of the evolving immune environment throughout pregnancy, we also investigated Th subsets in Chapter 2 . The paradigm that a Th2-type immunity, marked by IL-4, IL-5, and IL-13 secretion, dominates a Th1-type, characterized by an IL-2, and IFN γ secretion profile, dominated the field in the 1980s to 1990s (13). This Th2 profile was thought to dampen a harmful Th1-type response of the maternal immune system towards fetal antigens. It has now been established that there are more aspects to consider, such as Treg and IL- 17 secreting Th17 cells (13, 14). Likewise, asides from Treg, we investigated a number of Th subsets. In the flow cytometric approach used in this study, we assessed Th frequencies using