Immune regulation at the foetal-maternal interface; implications for healthy and complicated pregnancies

Abstract: For a successful pregnancy, the maternal immune system must acquire tolerance towards the paternal antigens present in the semi-allogeneic foetus. This tolerance is mainly established locally at the foetal-maternal interface, where foetally-derived trophoblasts invade the maternal endometrium (called decidua during pregnancy) and come in close proximity to maternal immune cells. The decidua is populated by maternal immune cells of a unique composition, characterised by their suppressive phenotypes that are essential for maintaining tissue homeostasis. Accordingly, failure of immune tolerance can lead to pregnancy complications. Macrophages and regulatory T-cells are enriched in the decidua and are believed to play important roles in the establishment of tolerance. However, there is limited information regarding the factors that regulate their functions and if their function is compromised in pregnancy complications.The aim of this thesis was to further elucidate which factors are responsible for induction of the regulatory phenotypes of macrophages and T-cells found in the decidua, how tissue resident cells in the decidua contribute to this and if this system is compromised during pregnancy complications, such as preeclampsia and recurrent pregnancy loss.Decidual stromal cells (DSCs) constitute the largest population of tissue resident cells in the decidua. In an in vitro system of macrophage differentiation, we found that Isolated peripheral blood monocytes cultured in conditioned medium (CM) from DSCs acquired a high expression of the regulatory M2 markers CD163, CD209 and CD14, and a low expression of CD86, characteristics of decidual macrophages. This induction was in part mediated by macrophage-colony stimulating factor (M-CSF), as neutralising its effects reduced the expression of CD163. However, since only a partial reduction was reached, other factors are involved. Another likely candidate for this polarisation is interleukin (IL)-34, a second ligand for the M-CSF receptor. We showed that IL-34 is expressed by both DSCs and the foetal placenta. Further, in vitro, IL-34 was able to induce macrophages with similar properties as that of M-CSF-induced macrophages, with high expression of CD163, CD209 and CD14. This was also coupled to a cytokine secretion profile similar to M-CSF-induced macrophages, with high production of IL-10, low production of tumour necrosis factor (TNF) and no production of IL-12. We found no evidence of IL-34 being aberrantly expressed in placentas from preeclamptic women.In addition to promoting induction of macrophages with a regulatory phenotype, CM from DSCs promoted expansion of Foxp3+CD25bright regulatory T (Treg) cells in an in vitro polarisation system, in a SMAD3 dependent manner. Protein profiling of DSCs revealed limited production of the Th2 related IL-13, IL-4, IL-33 and thymic stromal lymphopoietin (TSLP), as well as no production of the Th17 related IL-17A and chemokine (C-C motif) ligand (CCL) 20. Instead we found that DSCs were more prone to production of regulatory factors, such as M-CSF, leukaemia inhibitory factor (LIF) and transforming growth factor (TGF)-β, albeit with addition of the more pro-inflammatory IL-6, chemokine (C-X-C motif) ligand (CXCL) 8 and the Th1-related CXCL10.We also investigated if the placenta´s ability to induce Treg cells and regulatory M2 macrophages is compromised in women with a history of unexplained recurrent pregnancy loss (uRPL) and if the placental secreted protein profile is skewed to a pro-inflammatory response in uRPL. Using surplus materials from chorionic villous sampling (CVS), we generated CM from placental tissue taken from healthy and uRPL pregnancies and used this to polarise macrophages and T-cells in vitro. We found no difference in the ability to induce Treg cells and regulatory M2 macrophages between the healthy group and the uRPL group. Likewise, no differences in the protein profile was observed between the two groups.Taken together, our findings imply that DSCs produce a variety of factors promoting foetal tolerance by induction of Treg cells and regulatory M2 macrophages. Furthermore, we also showed that the placenta retained its ability to induce Treg cells and regulatory M2 macrophages in women with a history of uRPL.