Phenotypically similar but functionally distinct NK cell populations within the human maternal-fetal interface

Natural killer (NK) cell function within tissues extends beyond exerting cytotoxicity, encompassing a range of functions that are just starting to become fully elucidated. In the context of human placentation, NK cells play a key role in enabling initial placentation, which is associated with the acquisition of tolerance-like properties. If and to which extent NK cells maintain these tolerance-like properties over the course of human pregnancy is still poorly understood. We asked if NK cells isolated from the decidual-placental interface of full-term human pregnancies are able to exert effector function. We observed a significant and striking lack in the ability of NK cells isolated from the decidual-placental interface (DPI) to produce interferon-g (IFN-γ) in response to the activating cytokines interleukin (IL)-12, IL-15, and IL-18. In contrast, NK cells from the decidua retained their responsiveness to cytokine-mediated activation. Notably, CD103+CD69+ tissue-resident NK cells were present in both DPI and decidua, yet exhibited distinct effector function from one another. Using high-parameter flow cytometry and single-cell sequencing, we found that this functional discrepancy was not directly predictable based on their cell surface phenotype or cell transcript. Together, our findings reveal the presence of distinct functional resident NK cell populations in 2 anatomically adjacent tissues at healthy full-term pregnancies.