The effect of horse placental tissue extracts and equine chorionic gonadotrophin on the proliferation of horse lymphocytes stimulated in vitro

Placentation in Equids

This chapter focuses on the early stages of placental development in horses and their relatives in the genus Equus and highlights unique features of equid reproductive biology. The equine placenta is classified as a noninvasive, epitheliochorial type. However, equids have evolved a minor component of invasive trophoblast, the chorionic girdle and endometrial cups, which links the equine placenta with the highly invasive hemochorial placentae of rodents and, particularly, with the primate placenta. Two types of fetus-to-mother signaling in equine pregnancy are mediated by the invasive equine trophoblast cells. First, endocrinological signaling mediated by equine chorionic gonadotrophin (eCG) drives maternal progesterone production to support the equine conceptus between days 40 and 100 of gestation. Only in primates and equids does the placenta produce a gonadotrophin, but the evolutionary paths taken by these two groups of mammals to produce this placental signal were very different. Second, florid expression of paternal major histocompatibility complex (MHC) class I molecules by invading chorionic girdle cells stimulates strong maternal anti-fetal antibody responses that may play a role in the development of immunological tolerance that protects the conceptus from destruction by the maternal immune system. In humans, invasive extravillous trophoblasts also express MHC class I molecules, but the loci involved, and their likely function, are different from those of the horse. Comparison of the cellular and molecular events in these disparate species provides outstanding examples of convergent evolution and co-option in mammalian pregnancy and highlights how studies of the equine placenta have produced new insights into reproductive strategies.

Modulation of T-cell reactivity during equine pregnancy is antigen independent

PROBLEM

Pregnant mares demonstrate a reduction in cytotoxic T lymphocyte (CTL) reactivity against cells from the breeding stallion. We investigated whether this effect is limited to activity against paternal major histocompatibility complex (MHC) antigens, and whether it occurs during MHC-compatible pregnancy.

METHOD OF STUDY

Mares were mated to carry MHC-compatible or MHC-incompatible pregnancies. CTL activity of these mares when pregnant and non-pregnant was measured against cells from horses with MHC haplotypes unrelated to the mare or breeding stallion.

RESULTS

While carrying MHC-incompatible pregnancies, mares demonstrated reduced CTL activity against lymphocytes from third-party horses in addition to those from the breeding stallion. This effect was also observed in mares carrying MHC-compatible pregnancies.

CONCLUSIONS

The decrease in maternal T-cell reactivity characteristic of normal equine pregnancy is not restricted to paternal alloantigen, and occurs during MHC-matched matings. This suggests that antigen-independent mechanisms may be responsible for this reduction in cell-mediated immune activity.

Immunologic regulation in pregnancy: from mechanism to therapeutic strategy for immunomodulation

The immunologic interaction between the fetus and the mother is a paradoxical communication that is regulated by fetal antigen presentation and/or by recognition of and reaction to these antigens by the maternal immune system. There have been significant advances in understanding of abnormalities in the maternal-fetal immunologic relationship in the placental bed that can lead to pregnancy disorders. Moreover, immunologic recognition of pregnancy is vital for the maintenance of gestation, and inadequate recognition of fetal antigens may cause abortion. In this paper, we illustrate the complex immunologic aspects of human reproduction in terms of the role of human leukocyte antigen (HLA), immune cells, cytokines and chemokines, and the balance of immunity in pregnancy. In addition, we review the immunologic processes of human reproduction and the current immunologic therapeutic strategies for pathological disorders of pregnancy.

Maternal immune responses to trophoblast: the contribution of the horse to pregnancy immunology

The horse has proven to be a distinctively informative species in the study of pregnancy immunology for several reasons. First, unique aspects of the anatomy and physiology of the equine conceptus facilitate approaches that are not possible in other model organisms, such as non-surgical recovery of early stage embryos and conceptuses and isolation of pure trophoblast cell populations. Second, pregnant mares make strong cytotoxic antibody responses to paternal major histocompatibility complex class I antigens expressed by the chorionic girdle cells, permitting detailed evaluation of the antigenicity of these invasive trophoblasts and how they affect the maternal immune system. Third, there is abundant evidence for local maternal cellular immune responses to the invading trophoblasts in the pregnant mare. The survival of the equine fetus in the face of strong maternal immune responses highlights the complex immunoregulatory mechanisms that result in materno-fetal tolerance. Finally, the parallels between human and horse trophoblast cell types, their gene expression, and function make the study of equine pregnancy highly relevant to human health. Here, we review the most pertinent aspects of equine reproductive immunology and how studies of the pregnant mare have contributed to our understanding of maternal acceptance of the allogeneic fetus.

The equine immune response to endometrial cups

Out of all the areas of comparative immunological study in the horse, the field of reproductive immunology has proven to be one of the most fertile and exciting. Maternal immunological interactions with the fetus involve a set of events which prevent maternal rejection of trophoblastic tissue invading the uterus, and at the same time control this invasion to regulate growth and prevent damage to maternal tissues. Unique features of equine placentation make it exceptionally well-suited to studying these immunological interactions.

Susceptibility of equine chorionic girdle cells to lymphokine-activated killer cell activity

PROBLEM

Equine chorionic girdle cells are a subpopulation of highly invasive trophoblast cells that attach and invade the uterine epithelium on Day 35 (Day 0 = day of ovulation). These invading chorionic girdle cells form endometrial cups that are associated with a marked local maternal leukocytic response that may result in the demise of the cups at Day 120 of pregnancy. Once endometrial cups become established in the uterine wall they do not express MHC antigens, and therefore may only be susceptible to non-MHC restricted cytotoxic cells. The susceptibility of cultured chorionic girdle cells to LAK cell cytotoxicity was tested in order to evaluate the role of this type of cytotoxicity in the life-cycle of endometrial cup tissue.

METHODS

Chorionic girdle cells from ten Day 34 conceptuses were collected, cultured, and used as a target cell in a lymphokine activated killer assay to determine if these cells were susceptible to lymphokine activated killing.

RESULTS

Cultured chorionic girdle cells demonstrated similar in vitro morphological features and patterns of antigen expression to those seen in vivo in endometrial cups. Cultured chorionic girdle cells were susceptible to lymphokine activated killing.

CONCLUSIONS

Lymphokine activated killer cells may play a role in the degeneration of endometrial cup tissue. The potential mechanisms of regulation of uterine LAK cell activity in the horse are discussed.

Effects of the whole extract and the chromatographic fractions of the pig placenta on lymphocyte proliferation and humoral immune response

The immunoregulatory properties of pig fetal placenta extracts (PE) from 1 st, 2nd and 3rd month of pregnancy and five fractions (F1 to F5), isolated on Sephadex G-200 and additionally characterized by fast performance liquid chromatography, FPLC (Superose 12 HR) were studied in order to clarify the local immune regulation in diffuse epitheliochorial placentation. The obtained substances were added at 6.25 to 100 microg in cultures of Concanavalin A-stimulated mouse splenocytes and Phytohemagglutinin-stimulated pig and human PBL to monitor their influence on [(3)H]Thimidine uptake in proliferating lymphocytes. Their effects on the number of plaque-forming cells in spleen cell suspensions from mice treated ip simultaneously with sheep red blood cells and with 100 microg protein of PE, respectively, of each fraction were also investigated: PE and F1 had no effect while F4 and F5 suppressed the mitogen-induced lymphocyte proliferation in all studied species. F2 and F3 stimulated mouse and pig lymphocyte proliferation. The effects were dose-dependent and the suppression was not due to cytotoxic effects. The FPLC data allowed the suggestion that 110 kD protein(s) were involved in stimulation and 7 kD substance(s) - in suppression of cell proliferation. The PE from the 3 studied periods as well as the 5 fractions increased significantly the primary humoral immune response against T-dependent antigen. The results revealed that trophoblast of epitheliochorial placenta produces simultaneously immuno-stimulatory and -suppressive factors acting across the species barrier. Their presence at the feto-maternal interface may contribute to the regulation of local immune reactions and survival of the allogenic fetuses despite the morphological specificities of this type of placentation.

The equine endometrial cup reaction: a review

The function of eCG in equine pregnancy is far from clear but it has become evident that eCG has little or no FSH activity in the horse and is therefore probably not responsible for the secondary ovulations. eCG does have luteotrophic activity and it could play a role in the resurgence of the primary corpus luteum (1,7,44). Some evidence exists that the receptor population on the equine gonads is heterogenous in a way that makes it possible to distinguish eCG from eLH, resulting in different post-receptor effects (7). There is also evidence that eCG itself is heterogenous, both in glycosylation and in primary structure, not only between different individual animals but also within one animal during different stages of gestation. The differences could simply reflect the difference between stored and secreted hormone, but on the other hand the release of different eCG forms could be under endocrine control, allowing the mare to produce forms appropriate to specific biological needs (74). Thus some forms of eCG could play a role in immunological events taking place at the foeto-maternal interface. The role of cytotoxic antibodies in the equine pregnancy is not understood. The fact that they are not harmful to the pregnancy can be explained by the fact that their target, the paternal MHC molecules, are withdrawn from the endometrial cup tissue by the time the antibodies start appearing in the circulation. This unique way of regulation of MHC expression is also poorly understood.(ABSTRACT TRUNCATED AT 250 WORDS)

Human choriogonadotropin (hCG) and placental lactogen (hPL) inhibit interleukin-2 (IL-2) and increase interleukin-1 beta (IL-1 beta), -6 (IL-6) and tumor necrosis factor (TNF-alpha) expression in monocyte cell cultures

The effect of the human trophoblast hormones chorionic gonadotropin (hCG) and placental lactogen (hPL) on the expression of cytokines in peripheral blood mononuclear cell cultures was followed under a variety of culture conditions, (a) phytohemagglutinin stimulated cells (PHA-MSC), (b) allogenic mixed cells (AMC) and (c) spontaneously proliferating cells (SPC). A dose dependent enhanced release of IL-6, IL-1 beta and TNF-alpha by hCG and hPL was observed under all culture conditions. However, an inhibitory effect on the IL-2 release was seen in PHA-MSC by hPL and in AMC by hPL and hCG. The role of the suppression of IL-2 production/release on cytotoxicity towards trophoblast is discussed. These results suggest a sensitive, dose dependent hormonal control of the modulation of the immune response during pregnancy and strengthen the concept of a distinct regulation of monocytes and lymphocyte subpopulation by trophoblast hormones.

Immunosuppressive factor in cow follicular fluid

The activity of the immunosuppressive fraction and proteinase inhibitor isolated from cow follicular fluid was investigated. The immunosuppressive factor was separated from the accompanying proteinase inhibitor by Sepharose 6B and Sephacryl S-200 column chromatography. In vitro, the fraction significantly reduced mitogen-induced lymphocyte proliferation. In vivo, the fraction inhibited mouse plaque formation. However, the factor had no effect on the development of intact or zona free embryos. The active components have a molecular mass of about 110,000.