The importance of the PD-1/PD-L1 pathway at the maternal-fetal interface

Background Our goal with this study was to investigate the contribution of PD-1/PD-L1 immune-checkpoint pathway to maternal immunotolerance mechanisms. Methods Thirteen healthy pregnant women and 10 non-pregnant controls were involved in this project. PBMCs and DICs were isolated from peripheral blood and from decidual tissues. After the characterization of different immune cell subsets, we used fluorochrome-conjugated monoclonal antibodies to measure the expression level of PD-1, PD-L1, NKG2D, and CD107a molecules by flow cytometry. Results We measured significant alternations in the proportion of decidual immune cell subsets compared to the periphery. Elevated PD-1 expression by decidual CD8+ T, CD4+ T, and NKT-like cells were also detected accompanied by the increased PD-L1 expression by decidual CD4+ T, Treg, NKT-like and CD56 + NK cell subsets compared to peripheral blood. The cytotoxic potential was significantly higher in PD-1- decidual immune cells compared to the periphery, however we measured a significantly lower cytotoxicity in the decidual PD-1+ CD8+ T cells compared with the peripheral subsets. An activation receptor NKG2D expression was decreased by the PD-1+ CD8+ T subsets in the first trimester compared to non-pregnant condition but the expression level of the decidual counterparts was significantly elevated compared to the periphery. The cytotoxic potential of decidual PD1/NKG2D double positive CD8+ T cells was significantly decreased compared to the peripheral subsets. Conclusions Based on our results we assume that PD-1/PD-L1 pathway might have a novel role in the maintaining of the local immunological environment. Accompanied by NKG2D activating receptor this checkpoint interaction could regulate decidual CD8 Tc cell subsets and may contribute maternal immunotolerance.