Marilen Benner

CHAPTER 8 222 In Chapter 3 , we advance knowledge on Treg at the fetal-maternal interface. We describe, next to CD25Hi Treg, two additional functionally and phenotypically distinct Treg subsets of the decidua with regulatory capacities; PD1 Hi and TIGIT + T cells. All three Treg subsets express distinct transcriptional profiles compared to their phenotypic counterparts of peripheral blood. Both PD1 and TIGIT + Treg selectively inhibit CD4 + and not CD8 + Teff cells. The ability of decidual PD1 Hi to induce IL-10 expression in effector T cells suggests a positive feedback loop. This emphasizes the heightened capacity of decidual cells to produce IL-10, of which reduced presence is associated with pregnancy complications (23-26). Based on their lack of FOXP3, high PD1, IFN γ , and granzyme expression, and capacity to suppress CD4 + T cell proliferation through IL-10, PD1 Hi resemble Tr1 cells. However, they do not express CD49b and LAG3, supporting the notion that decidual Treg are a unique result of pregnancy-related antigen encounter. Accordingly, direct cell contact with extra villous trophoblast cells induced PD1 Hi cells. Also decidual macrophages were able to induce CD25 Hi and PD1 Hi Treg, upregulating FOXP3 and HELIOS expression. This fits with the timing of the observed differential expression, as Helios expression is associated with potent suppressive capacities. This was displayed by first trimester-derived decidual Treg, but only to a small extent by Treg isolated from term. In-depth mass cytometry-based phenotyping of decidual lymphocytes supports the presented findings of decidual Treg as heterogenous population with differential FOXP3 and HELIOS expression (27). Bright CD25 and Helios expression is associated with natural Treg (nTreg), which arise from the thymus to prevent anti-self-responses, in contrast to induced Treg (iTreg), that can dampen allo- recognition. It is tempting to speculate, based on our observed marker profiles, that the balance between nTreg to iTreg shifts towards more iTreg during gestation. However, the lack of clearly defining surface markers to specifically identify nTreg or iTreg leaves us questioning where (and to what purpose exactly) decidual Treg are induced. The functional and transcriptional differences between the selected Treg subsets of first trimester and term decidua underline that both timepoints deserve independent assessment. THE DECIDUA CONTAINS SPECIALIZED REGULATORY CELLS: IL-10 PRODUCING B CELLS The individual comparison of cells isolated from non-pregnant endometrium to decidua parietalis, and first trimester decidua to decidua parietalis, point out how focus on one time point will not suffice to understand the role of a particular cell at the fetal-maternal interface. We investigated lymphocytes isolated from menstrual blood alongside first trimester and term decidua parietalis by high-dimensional phenotyping (Chapter 4) . All studied time points contained uterine B cells, albeit at a low abundance (<2% of total lymphocytes). Systemically, B cells, together with T cells, are known as a major mediator of adaptive immunity. Next to their classical antibody-secreting function, B cells can act as APC, and modulate the local immune environment through cytokine