KIR2DL4 (CD158d) genotype influences expression and function in NK cells

KIR2DL4 is an IL-2-regulated NK cell receptor that exhibits limited expression in humans but triggers strong IFN-gamma production

Killer cell Ig-like receptor (KIR)2DL4 (2DL4, CD158d) was previously described as the only KIR expressed by every human NK cell. It is also structurally atypical among KIRs because it possesses a basic transmembrane residue, which is characteristic of many activating receptors, but also contains a cytoplasmic immunoreceptor tyrosine-based inhibitory motif (ITIM). We expressed epitope-tagged 2DL4 in an NK-like cell line to study receptor function. Three distinct 2DL4 cDNA clones were analyzed: one encoding the "conventional" 2DL4 with the cytoplasmic ITIM (2DL4.1) and two encoding different cytoplasmic truncated forms lacking the ITIM (2DL4.2 and 2DL4(*)). Surprisingly, one truncated receptor (2DL4.2), which is the product of a prevalent human 2DL4 allele, was not expressed on the cell surface, indicating that some individuals may lack functional 2DL4 protein expression. Conversely, both 2DL4.1 and 2DL4(*) were expressed on the cell surface and up-regulated by IL-2. Analysis of primary NK cells with anti-2DL4 mAb confirmed the lack of surface expression in a donor with the 2DL4.2 genotype. Donors with the 2DL4.1 genotype occasionally expressed receptor only on CD56(high) NK cells, although their expression was up-regulated by IL-2. Interestingly, Ab engagement of epitope-tagged 2DL4 triggered rapid and robust IFN-gamma production, but weak redirected cytotoxicity in an NK-like cell line, which was the opposite pattern to that observed upon engagement of another NK cell activating receptor, NKp44. Importantly, both 2DL4.1 and 2DL4(*) exhibited similar activation potential, indicating that the ITIM does not influence 2DL4.1 activating function. The unique activation properties of 2DL4 suggest linkage to a distinct signaling pathway.

Human uterine leukocytes and pregnancy

In human pregnancy, the embryo implants into the specialized mucosal wall of the uterus (decidua) and the placenta starts to form. Cells from the placenta (trophoblasts) invade into the uterine mucosa in order to open up maternal uterine arteries to ensure an adequate supply of blood to the developing fetus. The trophoblasts have a unique immunological phenotype compared to most cells especially with regard to their expression of major histocompatibility complex (MHC) antigens. On the other side of the interaction, the uterine mucosa (endometrium) differentiates in preparation for implantation. One of the changes that takes place is the appearance in the endometrium of a large number of maternal leukocytes in the final part of the menstrual cycle. If pregnancy ensues, these leukocytes continue to increase in number and are found in close contact with trophoblasts. The composition of this population of maternal immune cells is unusual compared to that seen at other mucosal sites. A lot of research has focused on whether maternal T-cell responses are suppressed or modified during pregnancy. Research has also concentrated on the specialized uterine natural killer (NK) cells, which are found in the decidua in large numbers during early pregnancy. These uterine NK cells have been shown to express receptors for trophoblast MHC antigens, but their role in pregnancy is still mysterious. The purpose of this review is to give an overview of what is known about the immunology at the implantation site and also to provide an update of some of the most recent findings in this field.