Natural Killer Cells in Pregnancy

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The implantation site during early human pregnancy is characterized by extensive remodeling of the vasculature, invasion of fetal trophoblast cells, and by an abundance of maternal NK cells. The remodeling of the maternal vasculature, which occurs over the first twelve weeks of pregnancy, is essential to establish sufficient blood supply to the fetus. How NK-trophoblast cell interactions influence vascular remodeling is unknown. Invading trophoblast cells from the fetus express the non-classical major histocompatibility class I molecule HLA-G. It was long thought that the role of HLA-G was to inhibit maternal NK cells, which would otherwise attack the fetus. However, our work has led to a radically different understanding of the function of NK cells in pregnancy. The endosomal innate receptor CD158d (KIR2DL4) induces cellular senescence in human NK cells in response to soluble ligand (HLA-G or agonist antibody). These senescent NK cells display a senescence-associated secretory phenotype (SASP) and their secretome promotes vascular remodeling and angiogenesis. To understand how CD158d initiates signaling for a senescence response, we mapped the region in its cytoplasmic tail that controls signaling. We identified a conserved TRAF6 binding motif, which was required for CD158d-induced NF-κB activation and IL-8 secretion, for TRAF6 association with CD158d, and for TRAF6 recruitment to CD158d+ endosomes in transfected cells. The adaptor TRAF6 is known to couple proximal signals from receptors such as endosomal TLRs and CD40 through the kinase TAK1 for NF-κB-dependent pro-inflammatory responses. siRNA-mediated silencing of TRAF6 and TAK1, and inhibition of TAK1 blocked CD158d-dependent IL-8 secretion. Stimulation of primary, resting NK cells with soluble Ab to CD158d induced TAK1 phosphorylation, and inhibition of TAK1 blocked the CD158d-dependent reprogramming of NK cells that produces the SASP signature. Our results reveal that a prototypic TLR and TNF-receptor signaling pathway is used by a killer cell immunoglobulin-like receptor that promotes secretion of pro-inflammatory and pro-angiogenic mediators as part of a unique senescence phenotype in NK cells