Marilen Benner

B CELLS CONTRIBUTE TO DECIDUAL IMMUNITY 93 4 encounter with invading fetal trophoblast cells. So far, there is only scant availability of data regarding decidual IL-10 expression in vivo (51). Also, in our hands, using immunohistochemistry, in vivo IL-10 producing B cells were not readily detectable. Indeed, IL-10 staining in tissues poses a technical challenge; IL-10 has a short half-life, it is rapidly secreted, and in vivo secreted levels in the local micro-environment, although functionally relevant, may likely be too low to yield positive staining results (52). While unsupervised clustering of decidual B cells did not show significant differences in subsets between 1 st and 2 nd trimester, less IL-10 production upon in vitro stimulation of decidual immune cells was observed in 2 nd trimester. It may be possible that the potency of IL-10 production declines independent from the unchanged phenotype. CD20 is a well-known marker expressed by multiple B cell subsets, amongst which are B cells with regulatory function (53). Of note, unsupervised analysis showed that the most abundantly expressed B cell populations lacked CD20. Hasan and colleagues showed the diverse heterogeneity of marker expression by Breg, but unfortunately, they did not include CD20 in their study (24). While the biological function of CD20 remains poorly understood, future studies might benefit from considering CD20 as differentially expressed marker. We found that decidual B cells colocalized with T cells, similar to what has been described for non-pregnant endometrium (Yeaman et al., 1997). This colocalization supports the possibility of a functional interaction that may go both ways: T cell derived cytokines may facilitate the induction of Breg (Rosser et al., 2014), whereas in turn, through the production of IL-10, Breg may contribute to the induction of Foxp3 + Treg (34, 53, 54). In decidua, we detected Foxp3 + T cells in clusters of lymphocytes, closely located to B cells. In healthy individuals, peripheral blood Breg convert effector T cells to Treg through IL-10 production, and inhibit development of naive T cells towards Th1 or Th17 cells (34), thereby mediating suppression in case of a local inflammatory reaction (54). It is thus tempting to speculate that the IL-10 secreted by nearby B cells contributes to placental Treg induction and maintenance; these latter cells are well recognized for their contribution to successful pregnancy (55, 56). So, besides the idea that colocalization of decidual T and B cells is a phenomenon exclusive to pathologies (57), the interaction between Breg and T cells may just as well contribute to the required homeostasis and the tight regulation needed for healthy placentation, while preserving immune competence at the fetal-maternal interface. Unfortunately, the low amount of B cells that can be isolated per sample restricts experimental possibilities to assess their suppressive capacity on T cells. Even though their contribution in numbers within decidual lymphocytes is small, we propose that their effect might be amplified through close proximity and effect on T cells and possible Treg induction (Figure 4E). In conclusion, the current study highlights the complex dynamics of the human uterine immune landscape, including B cells with the potential to contribute to the immune-regulatory environment of the uterus during pregnancy.