Further polymorphism of the RAET1E/ULBP4 gene in humans

Functional Characterisation and Analysis of the Soluble NKG2D Ligand Repertoire Detected in Umbilical Cord Blood Plasma

We previously reported that cord blood plasma (CBP) contains significantly more soluble NKG2D ligands (sNKG2DLs), such as sMICB and sULBP1, than healthy adult plasma. Viral infection or malignant transformation upregulates expression of NKG2D ligand on affected cells, leading to NK group 2, member D (NKG2D)-mediated natural killer (NK) cell lysis. Conversely, sNKG2DL engagement of NKG2D decreases NK cell cytotoxicity leading to viral or tumour immune escape. We hypothesised that sNKG2DLs detected in CBP may represent an additional fetal–maternal tolerance mechanism. To further understand the role of sNKG2DL in pregnancy and individual contributions of the various ligand types, we carried out functional analysis using 181 CBP samples. To test the ability of CBP to suppress the function of NK cells in vitro, we measured expression of NKG2D, CD107a, and IFN-γ in NK cells from control donors after exposure to 181 individual CBP samples and characterised the sMICA, sMICB, and sULBP1 content of each one. Furthermore, to detect possible allelic differences between samples that may also affect function, we carried out umbilical cord blood typing for MHC class I-related chain A (MICA) and MHC class I-related chain B (MICB) coding and promoter allelic types. Strongest functional correlations related to increasing concentration of exosomal sULBP1, which was present in all CBP samples tested. In addition, common MICB alleles, such as MICB*005:02, resulted in increased concentration of sMICB. Interestingly, MICB*005:02 uniquely associated with eight different promoter types. Among promoter polymorphisms, P2 resulted in the highest expression of sMICB and P9 the least and was confirmed using luciferase reporter assays. Higher levels of sMICB associated with lower IFN-γ production, indicating that sMICB also suppressed NK cell function. We also examined the MICA functional dimorphism encoding methionine (met) or valine (val) at residue 129 associated with strong or weak NKG2D binding, respectively. Most sMICA associated with val/val, some with met/val but none with met/met and, counter-intuitively, the presence of sMICA in CBP increased NK cell cytotoxicity. We propose a model for fetal–maternal tolerance, whereby NK cell activity is limited by sULBP1 and sMICB in CBP. The release of 129val sMICA with weak NKG2D signalling may reduce the overall net suppressive signal and break tolerance thus allowing fetal NK cells to overcome immunological threats in utero.

Cutting an NKG2D Ligand Short: Cellular Processing of the Peculiar Human NKG2D Ligand ULBP4

Stress-induced cell surface expression of MHC class I-related glycoproteins of the MIC and ULBP families allows for immune recognition of dangerous "self cells" by human cytotoxic lymphocytes via the NKG2D receptor. With two MIC molecules (MICA and MICB) and six ULBP molecules (ULBP1-6), there are a total of eight human NKG2D ligands (NKG2DL). Since the discovery of the NKG2D-NKG2DL system, the cause for both redundancy and diversity of NKG2DL has been a major and ongoing matter of debate. NKG2DL diversity has been attributed, among others, to the selective pressure by viral immunoevasins, to diverse regulation of expression, to differential tissue expression as well as to variations in receptor interactions. Here, we critically review the current state of knowledge on the poorly studied human NKG2DL ULBP4. Summarizing available facts and previous studies, we picture ULBP4 as a peculiar ULBP family member distinct from other ULBP family members by various aspects. In addition, we provide novel experimental evidence suggesting that cellular processing gives rise to mature ULBP4 glycoproteins different to previous reports. Finally, we report on the proteolytic release of soluble ULBP4 and discuss these results in the light of known mechanisms for generation of soluble NKG2DL.

Increasing polymorphism of the RAET1E/ULBP4 gene in humans

Description of three novel RAET1E/ULBP4 allele and promoter polymorphisms identified by sequence-based typing.

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.