Granulated lymphoid cells of the pregnant uterus: morphological and functional features

Life Cycle of Decidual Cells

The decisive events in the development of decidual cells (DC) are presented through examples of human and rodent decidua. Human decidua is formed by large decidual cells (LDC), endometrial granulated cells (eGC), and small decidual cells. The LDC form the main type of decidual membranes, which determine the morphological characteristics of the decidua as a tissue. Immediate precursor cells of LDC are located below the basement membrane of the uterine epithelium before and during implantation. At the next stage of differentiation, LDC acquire a spindle-like shape. Rodent LDC form an epithelium-like structure with gland properties at the terminal stage of differentiation. The single-cell structure of human decidua is a derivative of the epithelial organization of rodent decidua. Spindle-like rat LDC are characterized by a high level of protein, RNA, and DNA synthesis and by intensive proliferation. At the beginning of pregnancy, a cell proliferation predominates over cell loss. By Days 12-13 of rat pregnancy LDC loss reaches 80% per day. Terminally differentiated LDC (tLDC) disappear from decidua due to apoptosis. Apoptosis of tLDC and the exhaustion of their precursors account for the disappearance of LDC in the middle of rat pregnancy. Human term decidua is composed of living cells. Human LDC (hLDC) comprise the largest part of human decidual cells (hLDC). hLDC account for 60-90% of hDC but their relative amount can decrease to 35% in the case of significant cell loss under unfavorable conditions. A decrease of LDC is not accompanied by DC proliferation. The lack of ability of decidua to compensate for DC loss suggests DC is a growing type of cell population without cambial cells. LDC function largely by blebbing and budding. Human and rat endometrial granulated cells (eGC) are characterized by a low level of natural killer (NK) activity and a high level of natural suppressor (NS) activity. The combination of NK and NS properties is characteristic of the eGC immunoregulatory function. Other functions of decidua include control of inflammation and trophoblast growth and expansion in the uterus. The life span of decidual cells is limited by pregnancy.

Uterine natural killer cells are targets for a trophoblast cell-specific cytokine, prolactin-like protein A

PRL-like protein A (PLP-A) is a member of the PRL family expressed in trophoblast cells coincident with establishment of the chorioallantoic placenta. The purpose of this investigation was to identify targets for PLP-A. Using an alkaline phosphatase-tagging strategy, we show that PLP-A specifically interacts with a population of natural killer (NK) lymphocytes within the mesometrial compartment of decidua from pregnant and pseudopregnant rats. These observations are supported by the codistribution of PLP-A targets with cells expressing the rat NK cell surface marker, gp42, the absence of PLP-A binding in conceptuses from NK cell-deficient tg epsilon26 mice, and the specific interaction of PLP-A with a rat NK cell line, RNK-16. We have further demonstrated that PLP-A effectively suppresses RNK-16 cell cytolytic activities. Our results provide evidence for a new paradigm of embryonic-maternal communication involving a PLP-A signaling pathway between trophoblast cells and uterine NK lymphocytes.

Influence of the estrous cycle on the presence and distribution of immune cells in the rat reproductive tract

PROBLEM

Previous studies have shown that the uterus and vagina contain cells that can present antigen to ovalbumin-specific T-cells. The objective of the present study was to systematically characterize the immune cells [major histocompatibility complex (MHC) class-II+, macrophages, granulocytes, dendritic cells, and CD8+ cells] present in the uterus and vagina of the rat and to examine their distribution at various stages of the estrous cycle.

METHOD OF STUDY

Uterine and vaginal tissues from female rats were selected at various stages of the estrous cycle and were examined by immunohistochemical analysis. MHC class-II (Ia)-positive cells were detected using the OX-6 monoclonal antibody; macrophages, granulocytes, and dendritic cells were detected by OX-41 monoclonal antibody and CD8-positive T-cells were identified by OX-8 monoclonal antibody.

RESULTS

Immunohistochemical analysis showed cycle-dependent changes in the immune cell populations in the uterus and vagina. Ia+ cells, macrophages, granulocytes, and dendritic cells were present in large numbers in the stroma of the endometrium and around the glandular epithelium in the uterus at estrus, the stage of the reproductive cycle when estradiol levels are known to be high, relative to those seen at diestrus, when estrogen levels are low and progesterone is the predominant hormone. CD8+ cells were observed in the uterus interspersed between glandular epithelial cells at estrus. Immune cells were more numerous in the vagina, relative to the uterus. OX-6 and OX-41-positive cells were present in greater numbers in the subepithelial layers of the vagina at diestrus, in contrast to estrus.

CONCLUSION

This study demonstrates that a variety of immune cells are present in the reproductive tract and that their number and distribution vary in a tissue-specific manner with the stage of the estrous cycle.

The involvement of interleukin (IL)-15 in regulating the differentiation of granulated metrial gland cells in mouse pregnant uterus

Previous studies have suggested that granulated metrial gland (GMG) cells are bone marrow-derived lymphoid cells, which differentiate in situ in the mouse pregnant uterus into natural killer (NK)-like cells. Similar to NK cells, GMG cells express an abundant level of cytolytic mediators such as perforin. The factor(s) regulating the differentiation of GMG cells remain(s) to be identified, although cytokines previously implicated in the stimulation/activation of NK cells (e.g., IL-2, IL-6, IL-7, and IL-12) can be considered as potential candidates. Recently, IL-15, a novel cytokine, which displays biological activities similar to IL-2, has also been shown to be capable of activating NK cells. Using reverse transcription-polymerase chain reaction (RT-PCR) analysis, we have demonstrated in the present study that IL-15 and its cognate receptor, but not the other cytokines, are expressed in the mouse pregnant uterus, with a time course concomitant with those of cytolytic mediators in differentiated GMG cells. Moreover, IL-15, though not IL-2, is capable of inducing the expression of perforin and granzymes in pregnant uterine tissues explanted in vitro. Data obtained from in situ hybridization study have suggested that the macrophages present in the pregnant uterus may be responsible for the production of IL-15. These results suggest that IL-15 is involved in regulating the differentiation of GMG cells during mouse pregnancy.

Perforin: structure and function

Perforin is a cytolytic mediator produced by killer lymphocytes, and is stored in and released by cytoplasmic granules. The protein is partially homologous to the terminal components of the membrane attack complex of complement and produces pores of up to 20 nm in diameter on target membranes. Its genomic and protein structures have recently been unraveled, and its function elucidated through the availability of genetically engineered, perforin-deficient mice. Here Chau-Ching Liu, Craig M. Walsh and John Ding-E Young briefly outline certain biochemical and molecular features of perforin, and discuss the still-evolving issues concerning the relevance of perforin and Fas in cell killing.