Marilen Benner

SUMMARY AND DISCUSSION 219 8 Human reproduction depends on complex adaptations of the uterus. Every month, hormones, tissue, and local immunity orchestrate the optimal conditions for implantation of a fertilized egg. Successful initiation of pregnancy depends on a balanced reciprocal interaction of maternal and fetal cells. A key challenge in this interaction is the natural antigenic mismatch between mother and child. How does the maternal immune system allow semi-allogeneic fetal cells to connect to the uterine circulation enabling formation of the placenta? In this thesis, we investigate this fetal-maternal interface to unravel a number of involved immunological adaptations. In Part I of this thesis, we investigate the role of uterine immune cells for healthy pregnancy. We illustrate how mucosal immunity of the pregnant uterus gain memory throughout gestation. Previously unassessed Treg and B cells, unique to the uterus, are part of the complex interaction of local immune cells. We show that a combination of immune cell subsets, isolated from the uterus and systemically, is altered in women suffering from recurrent pregnancy loss (RPL). In Part II , we examine if, and to what extent, microbiota could contribute to the modulation of the uterine immune environment. We conclude that microbiota and their compounds have a potential influence on receptivity. Maternal immunity is influenced through antibiotics, with possible impact on the developing child. This thesis highlights how an open approach is needed to discover new pieces in the puzzle that reproductive immunology still poses. LOCAL IMMUNITY EVOLVES THROUGHOUT PREGNANCY The endometrium, the mucosal lining of the uterus, forms the basis for implantation. Endometrial stromal fibroblasts differentiate in the mid-to-late luteal phase of the menstrual cycle to form the decidua. Formation of this specialized tissue allows for engulfment of villous cells in species such as humans, where trophoblast cells invade beyond the luminal epithelium (hemochorial placentation) (1). This transformation, termed decidualization, forms the basis for successful placentation. Existing data on uterine immunity mostly focus on specific cell types in either endometrium or decidua. We first investigated the immune signature of the non-pregnant uterine mucosa in parallel with term decidua to assess pregnancy-induced adaptations (Chapter 2) . Monthly, in absence of conception, decreasing progesterone levels initiate shedding of the superficial layer of the decidualized uterine mucosa to allow for renewal of the receptive surface (2). Discharge of the endometrial lining involves an efflux of immune cells that reside in the tissue. We asked women to collect menstrual blood using a menstrual cup, to allow for a non-invasive access to endometrial lymphocytes. We previously established that the subsequent isolation of immune cells yields cells similar to lymphocytes obtained through endometrial biopsies (3). We also made use of this now broadly accepted strategy in Chapter 4 and 5 . At term, placentae can be easily obtained, and the maternal side of the placental membranes (decidua parietalis) can be separated from the fetal layers, which allows for the retrieval of maternal uterine cells post pregnancy. Analyses of material sampled at either time point, non-pregnant and post-pregnancy,