Human Umbilical Vein Endothelial Cells foster conversion of CD4+CD25-Foxp3- T cells into CD4+Foxp3+ Regulatory T Cells via Transforming Growth Factor-β.

Trans-placental cell trafficking is a naturally occurring process during pregnancy that results in the direct recognition of foreign maternal antigens by fetal tissue and vice versa. Immigration of potentially harmful allo-reactive maternal T cells into fetal circulation may provoke anti-fetal immune responses. However, the contact with fetal tissue may favor differentiation of maternal immune cells into cells with a regulatory phenotype. Human Umbilical Vein Endothelial Cells (HUVECs) possess immune-regulating properties and are one of the first fetal cells to get in contact with foreign maternal immune cells. Therefore, here we studied whether HUVECs induce the conversion of maternal T cells into regulatory T (Treg) cells. Moreover, we assessed whether this response is changing according to the sex of the HUVECs. Both female and male HUVECs induced the conversion of maternal T cells into Treg cells which is partially mediated via TGF-β. Female HUVECs showed a stronger capacity to induce Treg cells compared to male HUVECs. Our findings propose that HUVECs contribute to fetal-maternal tolerance by the increase of the Treg cell population. Sex-specific differences in Treg cell induction may partly account for the disparities on the incidence of infectious and autoimmune diseases between both sexes during early childhood.