8 The expression of inhibitory receptors includes NKG2A, KIR2DL4, KIR2DL1, KIR2DL2/L3, and ILT-242,45–47
which might function to inhibit the cytotoxic potential of dNK cells, as discussed below. Although dNK cells are in close contact with fetal-derived click here trophoblasts they do not exert cytolytic functions against trophoblast cells.48 Several studies have shown that the general cytotoxicity of dNK cells is reduced compared with peripheral blood NK cells,42,49 despite the fact that they express several activating receptors (as mentioned above), as well as high levels of perforin and granzyme A and B.27,42,50 The cytotoxic activity of dNK cells, although potentially low, is still preserved, as engagement of NKp46 (but not NKp30) in freshly isolated dNK cells induced intracalcium mobilization, perforin polarization, granule exocytosis and triggered apoptosis in target cells.45 Such existing killing potential of dNK cells might be important in case of uterine viral infection. Several different explanations for the lack of cytotoxicity toward trophoblast cells have been proposed. First, this phenomenon could be a result of inhibitory interactions Angiogenesis inhibitor between the non-classical class I MHC- molecules HLA-G and HLA-E and the inhibitory receptors expressed on dNK cells, e.g. ILT-2, KIR2DL4 , and CD94/NKG2A.45,51 However, ILT-2,
the most dominant HLA-G binding NK inhibitory receptor is only expressed on ∼20% of dNK cells, and whether KIR2DL4 could interact with HLA-G and inhibit NK cell activity is still controversial.52 Second, it has been suggested by EGFR inhibitor Kopcow et al.44 that dNK cells are unable to form mature activating synapses and to polarize perforin. This might also not be the only explanation, because as mentioned above, NKp46 is cytotoxic in dNK cells.45 Vacca et al.42 provided another possible explanation according to which, the cytotoxic activity of dNK cells is inhibited by the receptor 2B4, which delivers inhibitory signals that correlate with low or absent signaling lymphocyte activation molecule-associated protein (SAP) expression in dNK cells. Finally, it seems, of course, reasonable that
interactions of dNK cells with neighboring immune and non-immune cells at the decidua further inhibit their ability to damage the local tissue. The decidual microenvironment probably encourages dNK cells to exert their constructive functions. The landmark studies of Croy’s group demonstrated the novel concept of constructive functions for mouse dNK cells in vivo at the fetal-maternal interface and their involvement in tissue homeostasis.53 Their work demonstrated that depletion of dNK cells in the mouse decidua resulted in abnormal implantation sites and inadequate remodeling of the decidual spiral arteries. Furthermore, they showed that these abnormalities were a result of dNK-derived IFN-γ, which positively regulates the diameter of the lumen of the spiral arteries during decidualization.