Y.W. Loke

HLA-E is expressed on trophoblast and interacts with CD94/NKG2 receptors on decidual NK cells

Non‐classical MHC class I molecule HLA‐E is the ligand for CD94 / NKG2 NK cell receptors. Surface expression of HLA‐E requires binding of specific HLA class I leader sequences. The uterine mucosa in early pregnancy (decidua) is infiltrated by large numbers of NK cells, which are closely associated with placental trophoblast cells. In this study we demonstrate that trophoblast cells express HLA‐E on their cell surface in addition to the previously reported expression of HLA‐G and HLA‐C. Furthermore, we show that the vast majority of decidual NK cells bind to HLA‐E tetrameric complexes and this binding is inhibited by mAb to CD94. Thus, recognition of fetal HLA‐E by decidual NK cells may play a key role in regulation of placentation. The functional consequences of decidual NK cell interaction were investigated in cytotoxicity assays using polyclonal decidual NK cells. The overall effect of CD94 / NKG2 interaction with HLA‐E is inhibition of cytotoxicity by decidual NK cells. However, since decidual NK cells are unable to kill trophoblast even in the presence of mAb to MHC class I molecules and NK cell receptors, HLA‐E interaction with CD94 / NKG2 receptors may regulate other functions besides cytolysis during implantation.

Immunocytochemical characterization of the unusual large granular lymphocytes in human endometrium throughout the menstrual cycle.

Leukocyte populations were studied at all stages of the menstrual cycle using a panel of monoclonal antibodies to natural killer cells, T cells, B cells, and macrophages on frozen sections of endometrium. At the time of implantation (mid-secretory phase) the majority of leukocytes appear to be Leu19+, CD16-, Leu7-, CD2+, CD3-, CD5-, CD7-, and HLA DR- with the morphology of large granular lymphocytes. The numbers of these cells showed a marked increase in the mid-secretory phase. These cells exhibited similar phenotypic characteristics to those found in decidua. These findings, therefore, suggest that the recruitment of these large granular lymphocytes to the uterus is under hormonal control and is not a local response to the presence of invading trophoblast.

CD3- leukocytes present in the human uterus during early placentation: phenotypic and morphologic characterization of the CD56++ population.

In this study, the CD3- LGL/NK cells present in the pregnant human uterus have been characterized. Phenotypic and morphologic analyses of decidual LGL revealed many similarities to the minor CD56bright+, CD16- subset in peripheral blood, but there were some important differences. The relative surface density of CD56+ is greatly increased on decidual LGL to 22 x that found on the majority of CD56 peripheral blood NK cells. The CD56bright cells in decidua show LGL morphology, whereas in peripheral blood, they are .mainly agranular. Proliferation of CD56+ cells occurs predominantly during the nonpregnant secretory (luteal) phase, indicating these CD56+ uterine LGL do not migrate as terminally differentiated cells. The appearance of CD56 cells was examined at the ultrastructural level using immunoelectron microscopy. Cells with phenotypic characteristics of decidual LGL occur in a higher percentage (1.11%) in the peripheral blood of women of reproductive age than in men (0.66%). On the basis of these results, it is proposed that the CD56bright+ uterine leukocytes represent a distinctive, hormonally regulated subset possibly adapted to control human placentation.

Early human decidual cells exhibit NK activity against the K562 cell line but not against first trimester trophoblast

The susceptibility of cultured first trimester human trophoblast cells to lysis by NK cells has been studied. Trophoblast cells are resistant to lysis by NK cells from both peripheral blood and decidua. However, decidual cells extracted by enzymatic disaggregation do exhibit cytotoxicity against the NK-sensitive cell line K562. The relevance of these findings to successful implantation of the blastocyst is discussed.

Human trophoblast and JEG choriocarcinoma cells are sensitive to lysis by IL-2-stimulated decidual NK cells

Freshly isolated decidual large granular lymphocytes (LGL) show natural killer (NK) activity against K562 cells but not against normal human trophoblast. We now show that these decidual LGL proliferate in vitro in response to recombinant interleukin-2 (rIL-2) and that these rIL-2-stimulated cells acquire a broad cytolytic potential that is characteristic of lymphokine-activated killer (LAK) cells. Both fetal fibroblasts and JEG-3 choriocarcinoma cells are resistant to lysis by freshly isolated decidual effectors but are readily killed by IL-2-stimulated decidual LGL. The ability to kill these target cells is acquired after only 18 hr exposure to rIL-2. rIL-2-activated decidual LGL also kill cultured normal trophoblast cells but much lower levels of cytolysis were seen even after the effectors had been stimulated with rIL-2 for 4-6 days. The preferential killing of malignant over normal human trophoblast cells raises questions about the potential role of IL-2-activated decidual LGL in the control of unduly invasive or malignant trophoblast populations in vivo.

Expression of adhesion molecules by endovascular trophoblast and decidual endothelial cells: implications for vascular invasion during implantation.

Summary During the process of implantation, maternal spiral arteries within the decidua are invaded by trophoblast cells that adhere to and migrate along the luminal surface of the vascular endothelial cells. This phenomenon resembles the events that occur during the migration of neutrophils into an acute inflammatory site, therefore it is possible that similar mechanisms are involved. Indeed, previous observations have shown that endovascular trophoblast expresses the blood group-related antigen sialyl Le x . In this study, we show, by immunohistology, the expression of both E- and P-selectin by vascular endothelial cells only in the decidua basalis and not in decidua parietalis. In contrast, ICAM-1 is expressed by all vascular endothelium throughout the decidua. Expression of VCAM-1 is variable at the implantation site, and is not expressed by vascular endothelial cells in decidua parietalis. Interestingly, we demonstrate the strong expression of a polysialylated form of NCAM by endovascular trophoblast. Our data suggests that vascular invasion by trophoblast is regulated by the expression of appropriate adhesion molecules which permit interaction between endovascular trophoblast and decidual endothelial cells.

The immunological paradox of pregnancy: A reappraisal

The survival of the allogeneic conceptus has long been an immunological paradox. Medawar was the first to propose an evasive mechanism based on the concept of self/non-self recognition described in classical transplantation immunology. Since then, several newer models of self/non-self recognition have been proposed, such as the PAMP/PRR system, the Missing Self and the Danger Hypothesis. The present paper considers the fetal-maternal relationship in the context of all these models. The conclusion reached is that none of them is really appropriate because the interface between trophoblast cells of the fetal placenta and the leukocytes of the maternal decidua is unique. Pregnancy is not simply a case of acceptance or rejection like a transplant. The immunological mechanism must provide a balanced environment whereby the conceptus is nurtured by the mother and yet prevented from excessive invasion. Future identification of trophoblast ligands and their respective receptors on uterine Natural Killer cells and other leukocytes is likely to offer the best insight as to how this symbiotic state is achieved.

Uterine NK cells and trophoblast HLA class I molecules.

PROBLEM: To investigate the proposal that NK cells in decidua may control trophoblast migration during implantation of the human placenta.

Uterine large granular lymphocytes: a possible role in embryonic implantation?

Leukocytes in endometrium and decidua are phenotypically and morphologically similar to granulated lymphocytes at other mucosal surfaces. Although their exact functions are not known, we suggest they may have a role in the control of implantation and the transformation of the uterine vasculature by trophoblast on which the blood supply to the fetoplacental unit depends.

Human trophoblast cells cultured in modified medium and supported by extracellular matrix

This paper describes a culture procedure which consistently yields 80 to 90 per cent trophoblast from human first-trimester placentae. The trophoblast cells are selected and maintained but there is no increase in proliferation. The cultured cells are found to resemble extravillous rather than villous trophoblast in their immunocytochemical characteristics. This technique provides a means of obtaining human trophoblast cells with a sufficient degree of homogeneity and viability to be used for in vitro experiments.