Uterine tissue in early human pregnancy is characterized by extensive vascular remodeling, invasion of fetal trophoblast cells, and by an abundance of maternal natural killer (NK) cells. Trophoblast cells express membrane-bound and soluble isoforms of the non-classical major histocompatibility class I molecule HLA-G. How NK-trophoblast cell interactions influence vascular remodeling is unknown. Signaling from endosomes is emerging as a mechanism by which selected receptors provide sustained signals distinct from those generated at the plasma membrane. The activity of natural killer (NK) cells, which are important effectors of innate immunity and regulators of adaptive immunity, is controlled primarily by receptors that are at the cell surface. Here we show that cytokine secretion by resting human NK cells is induced by soluble, but not solid-phase antibodies to the killer cell immunoglobulin-like receptor (KIR) 2DL4, a receptor for HLA-G. KIR2DL4 was constitutively internalized into Rab5-positive compartments by a dynamin-dependent process. Soluble HLA-G was endocytosed into KIR2DL4-containing compartments in NK cells and in 293T cells transfected with KIR2DL4. Chemokine secretion induced by KIR2DL4 transfection into 293T cells occurred only with recombinant forms of KIR2DL4 that trafficked to endosomes. The profile of genes upregulated by KIR2DL4 engagement on resting NK cells revealed a pro-inflammatory/pro-angiogenic response. Soluble HLA-G induced secretion of a similar set of cytokines and chemokines. This unique stimulation of resting NK cells by soluble HLA-G, which is endocytosed by KIR2DL4, implies that NK cells may provide useful functions at sites of HLA-G expression, such as promotion of vascularization in maternal decidua during early pregnancy.
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