The Role of Corticotropin-Releasing Hormone (CRH) on Implantation and Immunotolerance of the Fetus

Kalantaridou SN, Makrigiannakis A, Zoumakis E, et al.

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Introduction

The hypothalamic neuropeptide corticotropin-releasing hormone (CRH), as well as its receptors, have been identified in several reproductive organs, including the endometrial glands, the decidualized endometrial stroma and the placental trophoblast, synctiotrophoblast and decidua.1-9 “Reproductive” CRH is a form of “tissue” CRH (CRH found in peripheral tissues), analogous to the “immune” CRH detected in peripheral inflammatory sites.10 “Immune” CRH possesses potent proinflammatory properties, influencing both innate and acquired immune processes. Intrauterine CRH may participate in local immune phenomena associated with embryo implantation (Table 1).

Table 1. Intrauterine functions of corticotropin-releasing hormone.

Table 1

Intrauterine functions of corticotropin-releasing hormone.

During blastocyst implantation, the maternal endometrial response to the invading embryo has characteristics of an acute, aseptic inflammatory response; yet, once implanted, the embryo suppresses this response and prevents rejection. Simultaneously, the immune system of the mother prevents a graft versus host reaction deriving from the immune system of the fetus.

The Fas receptor and its ligand (FasL) play an important role in the regulation of immune tolerance. The major function of the Fas-FasL interaction is the induction of apoptosis in activated cells carrying Fas.11

We have reported that embryonic trophoblast and maternal decidua cells, i.e., cells located in the interface between the fetal placenta and the maternal endometrium, produce CRH and express FasL.12 Trophoblast cells express a cytoplasmic form of FasL, which is secreted via the release of microvesicles.13 CRH may play a crucial role in the implantation and the anti-rejection process that protects the fetus from the maternal immune system, primarily by killing activated T cells through the Fas-FasL interaction.12

Intrauterine CRH

Human and rat uterus express the CRH gene.2,3 Epithelial cells of both species are the main source of endometrial CRH, while stroma does not express it, unless it differentiates to decidua.2,3,14,15 In addition, CRH receptors type 1 are present in both epithelial and stroma cells of human endometrium16 and in human myometrium,17 indicating a local effect of endometrial CRH. Inducers of CRH, such as 8-bromo-cAMP, forskolin and epidermal growth factor, stimulate the activity of the CRH promoter.18 Estrogens and glucocorticoids inhibit and prostaglandin E2 (PGE2) stimulates the promoter of human CRH gene in transfected human endometrial cells, suggesting that the endometrial CRH gene is under the control of these hormones.19 The inhibitory effect of glucocorticoids on endometrial CRH is in agreement with that found in the hypothalamus and opposite to that observed in the human placenta, indicating that the regulation of the transcription of the CRH gene is tissue-specific. The cytokines interleukin-1 (IL-1) and IL-6 stimulate the activity of the CRH promoter, an effect possibly mediated by prostaglandins, as has been described in both the hypothalamus and the placenta.19

In the human endometrium, a phenomenon with characteristics of an “aseptic” inflammatory reaction takes place during the differentiation of endometrial stroma to decidua. It has been shown that CRH induces the decidualization of endometrial stroma15,20 and that it potentiates the decidualizing effect of progesterone. Progestins stimulate the expression of endometrial CRH in a cAMP-dependent manner.21 Indeed, in stromal cells, CRH may mediate, via the CRH-R1 receptor, the cAMP-dependent part of the decidualizing effect of progesterone, an effect blocked by the cAMP inhibitor Rp-Br-cAMP. In addition to progesterone, several locally produced pro-inflammatory immune factors also exert a decidualizing effect. Thus, prostaglandins and interleukins are important members of this category of modulators. Endometrial stroma produces several inflammatory factors, including PGE2, interleukin 1 (IL-1) and IL-6. In humans, PGE2 enhances, while IL-1 inhibits, the decidualizing effect of progesterone.

It has been shown that CRH inhibits the production of PGE2 by human endometrial stromal cells.20 Therefore, endometrial CRH, in addition to its direct decidualizing effect, may also alter the decidualizing action of progesterone via its influence on the locally produced modulators, including PGE2. In addition, CRH stimulates the production of both IL-1 and IL-6 in human endometrial stromal cells.20 Of note, IL-1 is a principal modulator of the decidualization process, blocking the differentiation of human endometrial stromal cells induced by ovarian steroids or cAMP.22 The stimulatory effect of CRH on stromal IL-1 suggests that the former may exert its decidualizing effect either as a principal regulator or as a modulator of progesterone, the classical decidualizing effector. Thus, a close interaction takes place within the human endometrium involving CRH, prostanoids and cytokines.

CRH Promotes Blastocyst Implantation and Early Maternal Tolerance

Early in pregnancy, the implantation sites in rat endometrium contain 3.5-fold higher concentrations of CRH compared to the interimplantation regions,14 indicating that CRH might participate in implantation. We have shown that CRH induces the expression of apoptotic FasL on invasive extravillous trophoblast and maternal decidual cells at the fetal-maternal interface. 12 Furthermore, CRH increases the apoptosis of activated T lymphocytes through FasL induction, participating in the processes of both implantation and early pregnancy tolerance (fig. 1). This effect of CRH is specifically mediated through CRH-R1.12

Figure 1. Role of CRH on implantation and immunotolerance of the fetus: CRH, produced locally by decidual and extravillous trophoblast cells, acts in an autocrine/paracrine fashion, through CRH-R1, to stimulate FasL expression and induce apoptosis of activated maternal T lymphocytes expressing Fas.

Figure 1

Role of CRH on implantation and immunotolerance of the fetus: CRH, produced locally by decidual and extravillous trophoblast cells, acts in an autocrine/paracrine fashion, through CRH-R1, to stimulate FasL expression and induce apoptosis of activated (more...)

During implantation, the invading blastocyst secretes several inflammatory mediators, including CRH,23 IL-1,24 IL-6, leukemia inhibitory factor (LIF)25 and PGE2. Blastocyst-deriving IL-1 plays an essential role on implantation and, in mice, blockade of its effect by the specific antagonist IL-1ra inhibits implantation.24 The effects of LIF are equally important.25

Preliminary data suggested that implantation could be blocked in mice by the administration of a polyclonal rabbit antiserum generated against rat or human CRH.26 This observation is further supported by experiments in rats using antalarmin, a CRH-R1 specific antagonist. Administration of antalarmin to early pregnant rats (days 1-6 of pregnancy) results in a 70% reduction in the number of implantation sites.12 In the rat, nidation occurs on days 4-5 of pregnancy, about 12 h after the embryo enters the uterine cavity.27 Thus, blocking of CRH has an anti-nidation effect when it occurs at a very early stage of pregnancy. It is evident that both methods of blocking the effects of uterine CRH (antibodies or antalarmin) do not completely abolish nidation, suggesting the presence of other, redundant mechanisms supporting the implanted embryo. This is also in agreement with the fact that CRH- and CRH-R1-knock-out mice are hypofertile, but not entirely sterile.28,29

We have shown that CRH participates in the nidation of the fertilized egg by inhibiting local maternal immune response to the implanted embryo.12 Our data are in agreement with previously published reports suggesting that expression of FasL by fetal extravillous trophoblast cells can induce apoptosis of activated T lymphocytes expressing the Fas receptor.13,30,31 It should be noted here that mice with missense or inactivating mutations of FasL gene (gld) can reproduce, suggesting that trophoblast FasL expression is not obligatory for maternal immunotolerance. Thus, in the absence of a functional Fas-FasL system, other mechanisms supporting maternal immunotolerance are sufficient to prevent total pregnancy failure.

It has been suggested that maternal and fetal FasL limits the migration of fetal cytotrophoblast cells into maternal tissue and vice versa.11 Our data strengthen this hypothesis, suggesting that locally produced CRH at the fetal-maternal interface regulates FasL production and affects the invasion process through a local auto/paracrine regulatory loop of cytotrophoblast cells.

If CRH-R1 blockade by antalarmin and other compounds prevents implantation, by reducing the inflammatory-like reaction of the endometrium to the invading blastocyst, they might represent a new class of nonsteroidal inhibitors of pregnancy at its very early stages. Given the promising future of CRH antagonists in the therapy of depression and anxiety disorders, 1 their ability to cause hypofertility or early miscarriages should be seriously considered. Neverthelss, in rats, administration of antalarmin after gestation day 5, and until the end of pregnancy, did not affect the embryos, suggesting that other than CRH-mediated FasL expression mechanisms occur in mid- and late-gestation.12 Therefore, the lack of an abortifacient or fetotoxic effect in mid- and late gestation suggests that CRH antagonists could be used to protect the fetus from maternal stress and/or to prevent premature labor, another potential use of this class of compounds.1,32

Conclusions

CRH, the principal regulator of the hypothalamic-pituitary-adrenal axis, as well as its receptors, have been identified in female reproductive organs, including the endometrial glands, decidualized endometrial stroma, and placental trophoblast, synctiotrophoblast and decidua.1-9 We have shown that uterine CRH participates in local immune phenomena associated with early pregnancy, such as differentiation of endometrial stroma to decidua and protection of the fetus from the maternal immune system.12 CRH induces the expression of proapoptotic FasL on invasive extravillous trophoblast and maternal decidual cells at the fetal-maternal interface.12 Therefore, CRH induces the apoptosis of activated maternal T lymphocytes through Fas-FasL interaction, thus, participating in the processes of implantation and early fetal immunotolerance.12

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