Umbilical cord blood plasma contains soluble NKG2D ligands that mediate loss of natural killer cell function and cytotoxicity

NK cells play a key role in innate elimination of virally infected or neoplastic cells but they can be circumvented by immunoevasive mechanisms enabling viral spread or tumor progression. Engagement of the NKG2D activating receptor with soluble forms of its ligand is one such mechanism of inducing NK cell hyporesponsiveness. Interestingly, this immunoevasive strategy among others is described at the maternal-fetal interface where tolerance of the semi-allogeneic fetus is required to allow successful human pregnancy. Understanding of maternal-fetal tolerance is increasing but mechanisms preventing alloreactivity of fetal immune cells against the maternal host are less well understood. The study of umbilical cord blood has enabled insight of the fetal immune system, which appears immature and inert. We have found that soluble NKG2D ligands (sNKG2DLs) are present in cord blood plasma (CBP) and associate with adult NK cell hyporesponsiveness demonstrated by reduced CD107a expression and secretion of IFN-γ upon stimulation. The capacity of NK cells to kill K562 cells or proliferate was also reduced by incubation with CBP; however, physical removal of sNKG2DL from CBP restored K562 lytic function and NKG2D expression. Therefore, our results strongly suggest sNKG2DLs are expressed in CBP as a mechanism of fetal-maternal tolerance in human pregnancy.