Marilen Benner

THREE TYPES OF TREG CONTROL PLACENTAL INFLAMMATION 65 3 certain aspects of Treg-suppressive function, differentiation, and survival (51). However, the lack of HELIOS expression does not exclusively identify human iTregs (54). Here, we demonstrate that human decidual CD25 HI FOXP3 + Tregs have a high expression of HELIOS, which reduces in term pregnancy decidua as well as in cases of miscarriage, as was shown previously (14). Furthermore, we demonstrate that co- culture of CD4 + T cells with EVTs or decidual macrophages both significantly increased the expression of FOXP3 and HELIOS, advocating for either a local expansion of FOXP3 + and HELIOS + nTregs or a possible de novo induction of FOXP3 + and HELIOS + iTregs. The increased suppressive capacity of first-trimester CD25 HI Tregs compared to term pregnancy CD25 HI cells aligns with the observation that HELIOS expression increases and/ or stabilizes Treg function. Clonally expanded CD4 + CD25 HI CD127 – CD45RA – Treg populations were observed in term pregnancy decidua (15), and preferential recruitment of fetus-specific CD25 HI FOXP3 + Tregs from the maternal peripheral blood to the maternal-fetal interface has been suggested to occur in human pregnancy (2). Furthermore, CD4 + CD25 HI Tregs had increased suppressive function when they were isolated from decidual tissue with a maternal-fetal HLA-C mismatch compared to an HLA-C match (3). In neither of these studies was HELIOS expression investigated, but an explanation to align these observations may be that CD25 HI FOXP3 + Tregs are a mixed population of self-specific nTregs and fetus-specific iTregs. Another explanation may suggest that an increased induction of other iTreg types (e.g., PD1 HI Tregs) induced in response to fetal allo-antigens may indirectly increase or enhance CD25 HI FOXP3 + Tregs stability and/or function through secretion of immune-suppressive IL-10 and/or other factors. Because of the intracellular nature of HELIOS and the lack of other markers to distinguish CD25 HI FOXP3 + nTregs from iTregs, it is technically not possible to investigate functional differences between these cells from human tissues. A further focus on the mechanisms by which both CD25 HI FOXP3 + and PD1 HI IL-10 Tregs are induced by EVTs and decidual macrophages at the maternal-fetal interface as well as the effects of their mutual interactions will contribute to answering burning questions on their origin and antigen specificity. In contrast to our expectations, infection of EVTs with HCMVs did not diminish the ability of EVTs to increase CD25 HI FOXP3 + and PD1 HI Tregs, suggesting that HCMV infection does not alter the capacity of EVTs to promote immune tolerance. This is in line with a previous observation that decidual natural killer (NK) cells degranulated in response to HCMV-infected maternal decidual stromal cells, but not in response to HCMV-infected EVTs, suggesting that immune tolerance is maintained at the expense of efficient clearance of HCMV infection (40). Relevance of the distinct Treg populations should be addressed by studying their presence and functionality in cases of pregnancy complications. Decreased proportions of decidual CD4 + CD25 HI FOXP3 + and HELIOS + Tregs were associated with spontaneous recurrent miscarriage (13, 14) and preeclampsia (11, 12, 55). The detailed phenotypic characterization, combined with gene expression and extensive functional analysis of multiple Treg populations as presented here,