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Cytokine network at the feto-maternal interface
Journal of Reproductive Immunology 47 (2000) 87 – 103 www.elsevier.com/locate/jreprimm
Review
Cytokine network at the feto-maternal interface Shigeru Saito * Department of Obstetrics and Gynecology, Toyama Medical and Pharmaceutical Uni6ertsity, 2630 Sugitani Toyama-shi, Toyama 930 -0194, Japan Accepted 31 August 1999
Abstract There is much evidence that cytokines play a very important role in the maintenance of pregnancy by modulating immune and endocrine systems. Placental tissue produces cytokines and hormones that are essential to the regulation of the feto-maternal unit. Decidual lymphocytes express cell surface markers for activation, such as CD69 and HLA-DR, and these cells secrete many cytokines. Recent studies suggested that in pregnant women, cytokines produced by Th2 cells predominate over those produced by Th1 cells, resulting in the maintenance of pregnancy. This review article focuses on the unique cytokine network at the feto-maternal interface in humans. Recently, we demonstrated that Th2 cells were dominant within the decidua in early pregnancy in humans. The Th2-derived cytokines, IL-4 and IL-6, induce the release of hCG from trophoblasts, and the hCG stimulate progesterone production from corpus luteum in pregnancy. Progesterone stimulates the secretion of Th2 and reduces the secretion of Th1 cytokines. Thus, Th2 type cytokines appear to contribute to the maintenance of pregnancy by controlling the immune and endocrine systems and promoting the function of the trophoblasts at the implantation site. © 2000 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Cytokine; Pregnancy; Immune function; Endocrine; Trophoblast
* Tel.: + 81-764-342281; fax: + 81-764-345036. E-mail address: [email protected] (S. Saito) 0165-0378/00/$ - see front matter © 2000 Elsevier Science Ireland Ltd. All rights reserved. PII: S 0 1 6 5 - 0 3 7 8 ( 0 0 ) 0 0 0 6 0 - 7
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1. Introduction Cytokines are regulatory peptides or glycoproteins that can be produced by virtually every nucleated cell type in the body and have pleiotropic regulatory effects on hematopoietic, endocrine, neural and many other cell types. Unlike hormones, cytokines usually act as intercellular (paracrine) and/or intracellular (autocrine) signals in local tissues, only occasionally spilling over into the circulation to act as endocrine mediators (Miyajima et al., 1992; Kishimoto et al., 1994). Cytokines are closely involved in reproduction, i.e. ovarian follicular development, embryo implantation, endometrial development, and trophoblast growth and differentiation (Hunt and Roby, 1994; Savino and Dardenne, 1995; Simon et al., 1995; Petraglia et al., 1996). Although the roles of cytokines were considered to be quite complicated during the early stages of their identification, technological advances in recent years have contributed significantly to the clarification of their functions. CD4+-T cells play a central role in the immune system because they produce large amounts of cytokines and regulate a variety of immune functions. CD4+-T cells can be classified into Th1 and Th2 types by their pattern of cytokine production (Mosmann and Sad, 1996). The former type, which produces IL-2, TNF-b and IFN-g, is involved in the development of delayed type hypersensitivity and is associated with cytotoxic T cell functions. The latter type, which produces IL-4, IL-5, IL-6, IL-10 and IL-13, is involved in the antibody production response. The dominance of Th2 cells seems to be of prime importance in maintaining pregnancy in mice and human (Wegmann et al., 1993; Hill et al., 1995; Guilbert 1996; Krishnan et al., 1996; Marzi et al., 1996; Vince and Johnson, 1996; Ekerfelt et al., 1997; Haynes and Smith, 1997; Piccinni et al., 1998). These Th2 type cytokines, which are produced by trophoblasts and other types of cells, regulate maternal endocrine functions as well as trophoblastic functions (Nishino et al., 1990; De Moraes Pinto et al., 1996; Roth et al., 1996; Saito et al., 1997). In this study, the author discusses the characteristics of cytokines in the immune system at the human feto-maternal interface. Then the author mentions the existence of a specific cytokine cross-talk between the human trophoblast and decidual lymphocytes.
2. Characteristics of the lymphocytes in the decidua Endometrial granulated lymphocytes (EGLs) increase in number during the late secretory stage of the endometrium and the EGL level, at its peak, accounts for up to 70–80% of lymphocytes in the decidua during the first trimester of pregnancy then rapidly decreases during the period after the
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second trimester (Bulmer et al., 1988; Starkey et al., 1988; King and Loke, 1991). Although these lymphocytes clearly exhibit CD56, an NK-marker, they express no other NK-markers such as CD16 or CD57 (Nagler et al., 1989, 1990; Nishikawa et al., 1990, 1991; King et al., 1991) (Table 1). EGLs also contain the cytolytic molecules perforin, TIA 1, and granzyme A (Lin et al., 1991; King et al., 1993). They exhibited weak lytic activity against the NK cell-sensitive target K562, although they have been reported to have augmented NK activity after IL-2 stimulation (Nagler et al., 1990; Nishikawa et al., 1990; King and Loke, 1990; Nishikawa et al., 1991; Saito et al., 1993a,b,c,d,e). These characteristics suggest that EGLs are a type of NK cell. These cells are known by various names such as CD16−CD56bright NK cells, CD562 + large granular lymphocytes (LGLs) and CD562 + NK cells. Table 2 gives the results of decidual lymphocyte subsets obtained using flow cytometry conducted in our laboratory. The percentage of CD16−CD56bright NK cells in the peripheral blood is less than one percent. In the decidua, however, more than 80% of lymphocytes are CD16−CD56bright NK cells. Although CD16+ NK cells are regarded as the main component of NK cells in the peripheral blood, their content in the decidua is only 2% (Table 2). The characteristics of CD16−CD56bright NK cells can be summarized as follows: (1) they express c-kit receptor, which is expressed on bone marrow hematopoietic stem cells, indicating that CD16−CD56bright NK cells are an immature type of lymphocyte (Matos et al., 1993; Umekage et al., 1998); (2) they express both the early T cell markers, CD2 and CD7, and the NK cell marker, CD56, killer inhibitory receptor (KIR) and killer activatory receptor (KAR) (Nagler et al., 1989; King et al., 1991; Nishikawa et al., 1991; Miki et al., 1998); (3) they exist in fetal liver that contains hematopoietic stem cells and lymphoid progenitor cells (Phillips et al., 1992); (4) CD16− CD56bright NK cells express RAG-1 mRNA and RAG-2 mRNA that are Table 1 Characteristics of decidual CD16−CD56bright NK cells Hematopoietic cell NK marker Early T cell marker Late T cell marker Cell adhesion molecules Cytokine receptor Activation marker CD3 complexes RAG gene
CD45+ CD562+, CD16−, CD57−, KIR+, KAR+ CD2+, CD7+ CD4−, CD8+/−, CD3−, CD5− L-selectin(−), CD11a+, CD11b+/−, CD11c+, CD18+, ICAM-I+, CD44+, fibronectin receptor (a4b1+, a5b1+, a4b7+) IL-2Radim, IL-2Rb2+, IL2Rg+, IL-4R+, c-kit CD69+, HLA-DR+/−, CD71+/− CDo (intracellular)+, (cell surface)− CDj+ RAG1+/−, RAG2+/−
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Table 2 subsets of decidual and peripheral blood lymphocytes Subsets
decidua (n= 12) (%)
Peripheral blood (Pregnant women) (n= 12) (%)
CD3+-T CD4+-T CD8+-T ab+T/CD3+ gd+T/CD3+ CD45RO+CD4+/CD4+ CD3+CD56dim/CD3+ CD16+-NK CD16−CD56bright-NK CD20+-B
12.29 4.6 5.692.1 4.791.9 94.193.8 6.192.3 85.1910.1 29.5910.2 2.791.2 84.895.2 0.990.3
71.89 3.5 43.49 6.5 24.99 7.2 95.39 2.1 5.791.6 53.19 11.6 4.69 2.3 20.39 4.3 0.59 0.3 8.49 2.5
involved in gene recombination, despite the absence of T cell receptors (TCRs) and cell surface CD3 antigen (Phillips et al., 1992; Hayakawa et al., 1994; Gudelj et al., 1996). (5) Among CD3 molecules that are associated with TCRs, CD3o and CD3z exist in the cytoplasm and are not expressed on the cell surface (Phillips et al., 1992; King et al., 1998). (6) CD16− CD56bright NK cells appeared earlier than T cells or CD16+ NK cells when lymphocytes of donor origin were examined after bone marrow transplantation (Jacobs et al., 1992). Therefore CD16−CD56bright NK cells seem to be not only undifferentiated cells but also progenitor cells which are able to differentiate into T cells or NK cells. Regarding whether CD16−CD56bright NK cells express RAG-1 mRNA and RAG-2 mRNA, there is considerable divergence of opinion among researchers (Hayakawa et al., 1994; King et al., 1998). CD3+ T cells are the main population of peripheral blood lymphocytes. The CD3+-T cell count in the decidua, however, decreases to a level 1/9 of that in the peripheral blood. The specific recognition of antigen by T cells is mediated by the heterodimeric TCR. T cells can be classified as ab T or gdT by TCR. According to the results of immunohistological staining and flow cytometry, the number of gdT cells existing in the human decidua is limited and less than that in the mouse decidua (Dietl et al., 1990; Morii et al., 1993; Vassiliadou and Bulmer, 1998a; Arck et al., 1999). Our data showed that decidual gdT cells accounted for 5% of all decidual T cells and this indicates that there is no difference between the percentage of Tgd cells among T cells in the decidua and that in the peripheral blood. MinchevaNilsson et al. (1994) pointed out that gd epitopes easily disappeared after a fixation and freezing procedure. Therefore it is difficult to detect them by
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using conventional immunohistological methods. They used an electron microscope to observe gdT cells in the decidua and detected many gdT cells. In normal pregnancy, gdT cells in the decidua express mainly VgI mRNA which is known to respond not only to HSP-60 but also to mouse or human trophoblasts in a non-MHC-restricted manner (Divers et al., 1994; Heyborne et al., 1994; Hayakawa et al., 1996; Clark et al., 1998). More information is urgently needed on the mechanism by which gdT cells, which react to trophoblasts, contribute to the maintenance of pregnancy. CD3 expression and TCRab and TCRgd expression on decidual T cells were significantly down regulated compared to on peripheral blood T cells (Dietl et al., 1990; Morii et al., 1993). It has been reported that T cells which showed intermediate CD3 fluorescence intensity existed in mouse liver sinusoid and that these unique T cells, which express large amounts of IL-2Rb, were extrathymic T cells (Iiai et al., 1992; Watanabe et al., 1993). Various factors contribute to their proliferation, such as aging, tumor bearing state, pregnancy, infection, viral infection, and autoimmune disease (Ohteki et al., 1990; Kimura et al., 1995; Ohtsuka et al., 1995). Recently the existence of NKT cells, which expressed both NK 1.1 antigen, an NK cell marker and CD3, T cell marker, was confirmed. Watanabe et al. (1995) demonstrated that almost half of the extrathymic T cells in the liver were NKT cells with NK 1.1 antigen and CD3 antigen. T cells in human decidua possess down-regulated CD3 and increased levels of the IL-2Rb chain molecules like extrathymic T cells in mice (Saito et al., 1992). Furthermore CD56 positive T cells were frequently observed in decidua (Nishikawa et al., 1991) (Table 2). These findings suggest that decidual T cells are extrathymic T cells or NKT cells. In our recent study, it was found that some of the T cells in the decidua expressed Va24, an NKT cell marker (unpublished data). It remains to be clarified whether T cells in human decidua are extrathymic T cells or not.
3. Activation of the lymphocytes in the decidua and cytokine production by these lymphocytes Cytokines are produced by the lymphocytes after activated via CD3– TCR complexes or cytokine –cytokine receptor signals. The process of cytokine production by T cells is as follows; the native T cells of CD45RA+, CD45RO− and CD29−, which do not secrete any cytokines, are activated by some stimulation and become memory T cells which secrete a variety of cytokines (Fig. 1). Moreover memory T cells are differentiated into Th1 cells and Th2 cells (Fig. 1). The activated T cells and NK cells express CD69, an early activation marker, 2 or 3 h later and their expression
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continues for at least 24 h (Testi et al., 1989). Afterward they express HLA-DR antigen, a late activation marker. During the first trimester of pregnancy, 58.3% of CD16−CD56bright NK cells in the decidua express CD69 antigens (Table 3). Similar findings were reported by King et al. (1991), Nishikawa et al. (1991); Kodama et al. (1998); Maruyama et al. (1992). On the other hand, Vassiliadou and Bulmer (1998b) reported that EGL did not express CD69 using immunohistological staining. They speculated that CD16−CD56bright NK cells were activated during the separation of lymphocytes from decidual tissue. This method, however, has poor sensitivity compared to flow cytometry in detecting CD69 antigens. Kodama et al. (1998) reported that the intensity of CD69 expression on CD562 + LGL cells was the highest at the proliferative stage of the endometrium during the non-pregnant period, and that the antigen level showed a tendency to decrease early in the pregnancy. Nishikawa et al. (1991) reported that expression of CD69, HLA-DR and CD71 on the decidual CD16−CD56bright NK cells increased when a small amount of IL-2 was added to the medium in vitro. These results indicate that the intensity of CD69 antigen expression on the CD16−CD56bright NK cells is low during normal pregnancy, but the following findings demonstrate that CD16−CD56bright NK cells in the decidua had been activated. CD16−CD56bright NK cells in the peripheral blood which had not been activated have no granules in their cytoplasm. Once they are stimulated in
Fig. 1. T cell development from native T to memory T cells.
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Table 3 Expression of activation marker on decidual and peripheral blood T cells and CD16− CD56bright-NK cells in the first trimester of pregnancy Decidua (n= 12) (%)a
Peripheral (n =12) (%)
CD4 CD69 HLA-DR IL-2Ra IL2Rb
58.39 12.7* 67.79 14.8* 26.99 5.0** 8.694.6***
0.9 9 0.5 11.4 9 4.6 20.59 5.2 1.59 0.5
CD8 CD69 HLA-DR IL-2Ra IL-2Rb
73.3 914.3* 82.9913.5* 6.292.9**** 17.797.2**
3.590.7 18.69 8.0 2.59 1.5 10.69 3.4
CD16−CD56 bright CD69 HLA-DR
69.4913.4* 13.495.3****
7.792.2 77.7 920.2
a
Percentage of CD4, CD8 or CD16−CD56bright cells. * PB0.0001; ** PB0.005; *** PB0.001; **** PB0.01.
vitro by IL-2, granules are formed in the cells (Nagler et al., 1989, 1990) and there exists a morphological similarity between these cells and EGLs in the decidua. Next, these activated CD16−CD56bright NK cells in peripheral blood come to produce TNFa and IFNg (Nagler et al., 1989), and decidual CD16−CD56bright NK cells also express various types of cytokine mRNA such as TNFa, IFNg, M-CSF (CSF-1), G-CSF, GM-CSF and LIF mRNA (Saito et al., 1993a; Jokhi et al., 1994). The in vitro activation of CD45RA+ CD45RO− NK cells results in the acquisition of CD45RO expression and loss of CD45RA expression. CD45RA− CD45RO+ NK cells appear to be more functionally hyper-reactive and differentiated than CD45RA+ CD45RO− NK cells (Braakmann et al., 1991). Since most CD16−CD56bright NK cells in early pregnancy decidua are CD45RA− CD45RO+ (Kodama et al., 1998), these NK cells may have been functionally hyper-reactive and differentiated. These results strongly suggest that CD16−CD56bright NK cells have been activated by some stimulation. According to Kodama et al. (1998), the intensity of CD69 antigen expression on the CD562 + NK cells was highest at the proliferative phase and decreased gradually throughout the menstrual cycle. Therefore neither fertilized ova nor trophoblasts can be thought of as expressing antigens which stimulate NK cells. At present we
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cannot define the stimulation supposed to promote NK cell activation. Another problem we should clarify is the fact that although CD16− CD56bright NK cells express CD69, the intensity of HLA-DR expression in the decidua is lower than that in the peripheral blood (Table 2). The activation of CD16−CD56bright NK cells in the decidua is a subject of controversy. However, it is acknowledged that T cells in the decidua are activated. According to the results obtained in immunohistological staining and flow cytometry, T cells in the decidua express CD69 antigen (Saito et al., 1992; Chen et al., 1995; Vassiliadou and Bulmer, 1998b). As shown in Table 3, as a late activation marker, HLA-DR antigens are also expressed on decidual T cells. T cells can be classified into CD45RA+ CD45RO− native T cells and CD45RA− CD45RO+ memory T cells (Akbar et al., 1991). Activation of CD45RA+ CD45RO− native T cells result in the acquisition of CD45RO and loss of CD45RA expression, so CD45RA− CD45RO+ memory T cells are more functionally differentiated. As a characteristic of memory T cells, a small amount of activation marker such as IL-2Ra and IL-2Rb is expressed (Akbar et al., 1991). Thus, it can be considered that memory T cells have received continuous low-grade stimulation over a long period of time. As demonstrated in Table 3, most of the T cells in the decidua are CD45RA− CD45RO+ memory T cells and the intensity of IL-2Ra expression and of IL-2Rb expression are low. Therefore T cells in the decidua may constantly receive weak antigenic stimulation. Various types of cytokines are produced from native T cells and memory T cells. Native T cells produce only IL-2, while memory T cells produce a large variety of cytokines including IL-3, IL-4, IFNg and IL-6 (Akbar et al., 1991). Because most T cells in early pregnancy decidua are memory T cells (Saito et al., 1994a), they secrete locally large amounts of cytokines. The isolation of T cells is difficult in the decidua because their number is limited, so it is unclear which cytokines are produced by T cells in this tissue. Recently a new technique using flow cytometry has been developed to determine simultaneously both the cell surface markers and intracellular cytokines (Ferrick et al., 1995; Prussin and Metcalfe, 1995). This method has enabled us to examine the cytokine production at the single cell level and to analyze many cells in a short time. We used a system to detect the pan T marker, CD3 and intracellular cytokines (IFN-g, IL-4) simultaneously. We collected peripheral blood samples from women in the first trimester and made a comparison between the Th1 and Th2 cell ratios. These values showed no changes. On the other hand, the percentage of Th1 cells was lowest the decidua in early pregnancy and highest in endometrium during the proliferative phase followed by endometrium in the secretory phase and decidua in early pregnancy. The percentage of Th2 cells was highest in the decidua in early pregnancy and lowest in the endometrium
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during the proliferative phase. The percentage of Th0 cells was significantly increased in the endometrium during the proliferative and secretory stages compared to the decidua in early pregnancy (Saito et al., 1999). These findings suggested that Th0 cells differentiated into Th2 in the early pregnancy decidua. Piccinni et al. (1998) pointed out the predominance of Th2 cytokines in pregnancy because T cell clones established by the normal human decidua produced more Th2 cytokines; and Th2 producing clones decreased in cases of unknown recurrent miscarriage. It remains to be clarified how and where Th1 and Th2 cells differentiate from Th0 cells in pregnant subjects. Although more data are urgently needed, recent results suggested the involvement of intrathymic NKT cells (Nishizawa and Koyasu, 1997). These cells which secrete both IFNg and IL-4 seem to be involved in the differentiation of Th0 cells into Th1 or Th2 cells in the thymus. NKT cells in the decidua might regulate the Th1/Th2 balance in pregnancy. In mouse decidua, gdT cells which produce TNFa and IFNg and gdT cells which produce TGFb2 and IL-10 have been identified (Arck et al., 1999). These cells are probably involved in the differentiation of Th1 and Th2 cells in the decidua. Special attention should be directed to determining whether these NKT cells and gdT cells change in the endometrium in cases of recurrent miscarriage or implantation failure.
4. Cross talk mediated by cytokines in the maternal endocrine system, decidual immune system and trophoblast function Why are Th2 cells dominant in the decidua? IL-4 and IL-10 are known as the factors that induce Th2 cells to differentiate from Th0 cells (Mosmann and Sad, 1996). Furthermore trophoblasts produce IL-4 and IL-10 which induce Th2 cells (De Moraes Pinto et al., 1996; Roth et al., 1996) (Table 4). IL-10 mRNA is markedly recognizable in extravillous trophoblasts (Roth et al., 1996) and this could explain the larger population of Th2 cells in the decidua. G-CSF, which is abundant in the placenta and decidua (Saito et al., 1993b, 1994b), also induces Th2 cells (Pan et al., 1995). Progesterone is a hormone that accumulates during pregnancy and is indispensable to maintaining pregnancy. When progesterone is liberated by the corpus luteum after ovulation, it prepares the uterus for the reception and development of the fertilized ovum by differentiating the endometrium from the proliferative to the secretory stage. At this time, the Th1/Th2 cell ratio decreases significantly during the secretory stage in preparation for the implantation of the fertilized ovum (Saito et al., 1999). This may be due to progesterone, which has been reported to induce the conversion of Th0 cells into Th2 cells (Piccinni et al., 1995, 1998; Lim et al. 1998). However, we
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should not forget the involvement of prostaglandin and other factors existing in the placenta and fertilized ova in the process of the rapid induction to Th2 cells after pregnancy (Fortin et al., 1997; Kelly and Critchley, 1997; Kelemen et al., 1998; Miranda et al., 1998). Regarding other mechanisms, we must consider the cross talk mediated by cytokines in the maternal endocrine system, decidual immune system and trophoblast function. When a female becomes pregnant, hCG is secreted from trophoblasts and pregnancy is successfully maintained because hCG prevents apoptosis of the corpus luteum. Of special interest is that IL-4, IL-6 and LIF are Th2 cytokines which promote hCG release from trophoblasts (Nishino et al., 1990; Sawai et al., 1995; Saito et al., 1997). The released hCG could induce the production of progesterone from corpus luteum, resulting in Th2 cell induction. Estrogen and progesterone increase the M-CSF concentration in the mouse uterus (Pollard et al., 1987). M-CSF is produced by CD16−CD56bright NK cells, decidual stromal cells, and endometrial gland cells (Pollard et al., 1987; Saito et al., 1993a,d,e; Jokhi et al., 1994). M-CSF induces the differentiation and proliferation of trophoblasts and Table 4 Cytokine production in chorionic or placental tissue Cytokine
Trophoblast
IL-1a IL-1b IL-2 IL-3 IL-4 IL-5 IL-6 IL-7 IL-8 IL-10 G-CSF GM-CSF M-CSF LIF TNFa TGFb SCF HGF VEGF EGF TGFa IFNg
+ 9 − − + − + ? + + 9 − + + + + + − + + + −
(syncytiotrophoblast\cytotrophoblast)
(syncytiotrophoblastBcytotrophoblast) (syncytiotrophoblast\cytotrophoblast) (syncytiotrophoblast\cytotrophoblast) (syncytiotrophoblastBcytotrophoblast) (syncytiotrophoblast= cytotrophoblast) (syncytiotrophoblastBcytotrophoblast) (syncytiotrophoblast only) (syncytiotrophoblast\cytotrophoblast) (cytotrophoblast only) (syncytiotrophoblastBcytotrophoblast) 1st trimester only (syncytiotrophoblast\cytotrophoblast) (syncytiotrophoblastBcytotrophoblast)
Stromal cell 9 + − − + − + + + + + 9 + − + + + + + + + −
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Fig. 2. Cytokine and endocrine networks at the materno-fetal interface.
promotes hCG production (Arceci et al., 1989; Saito et al., 1991, 1993e; Garcia-Lloret et al., 1994; Saito et al., 1994c). As a result, hCG induces the production of progenterone by the corpus luteum so that Th2 cells can be stimulated to differentiate from Th0 cells by progesterone (Fig. 2). M-CSF seems to regulate macrophage functions in the decidua to support the maintenance of pregnancy (Pollard et al., 1991; Hunt and Robertson, 1996). Recently Munn et al. (1998) demonstrated that trophoblasts and macrophages had tryptophan catabolic enzymes and that these enzymes controlled the activation of maternal cytotoxic T cells to protect the placenta against attack by maternal cytotoxic T cells (Fig. 2). In this case, macrophages whose differentiation is mediated by M-CSF possess tryptophan catabolic enzymes that protect placenta from maternal T cell attack. Among the Th2 type cytokines, IL-4 inhibits the expression of IL-2Ra, IL-2Rb and IL-2Rg on CD16−CD56bright NK cells, thereby controling the augmentation of NK activity by IL-2 (Saito et al., 1996). Th2 type cytokines may control intrauterine NK cells to prevent them from attacking trophoblasts. Cytokines deliberately mediate the endocrine and immune systems of the maternal body, fetus and placenta. The cross talk between them could contribute to the maintenance of pregnancy. The disturbance of this delicate balance may result in abnormal pregnancy, i.e. implantation failure,
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recurrent abortion, preeclampsia etc. The clarification of various problems in the field of reproductive medicine, including the relationship between maternal and fetal immunity and endocrine systems, is awaited. The mammalian fetus has been perceived, paradoxically, as a successful allograft, successful tumor, and a successful parasite. The maternal immune and endocrine systems are involved in the success of pregnancy. Cytokines appear to contribute to the maintenance of pregnancy by controlling the immune and endocrine systems and promoting trophoblast function at the implantation site. Thus, the cytokine messengers of the immune, endocrine, and neuroendocrine systems interact within the intrauterine tissues during pregnancy to determine correct placental growth and differentiation and to ensure the feto-maternal well being.
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Review
Cytokine network at the feto-maternal interface Shigeru Saito * Department of Obstetrics and Gynecology, Toyama Medical and Pharmaceutical Uni6ertsity, 2630 Sugitani Toyama-shi, Toyama 930 -0194, Japan Accepted 31 August 1999
Abstract There is much evidence that cytokines play a very important role in the maintenance of pregnancy by modulating immune and endocrine systems. Placental tissue produces cytokines and hormones that are essential to the regulation of the feto-maternal unit. Decidual lymphocytes express cell surface markers for activation, such as CD69 and HLA-DR, and these cells secrete many cytokines. Recent studies suggested that in pregnant women, cytokines produced by Th2 cells predominate over those produced by Th1 cells, resulting in the maintenance of pregnancy. This review article focuses on the unique cytokine network at the feto-maternal interface in humans. Recently, we demonstrated that Th2 cells were dominant within the decidua in early pregnancy in humans. The Th2-derived cytokines, IL-4 and IL-6, induce the release of hCG from trophoblasts, and the hCG stimulate progesterone production from corpus luteum in pregnancy. Progesterone stimulates the secretion of Th2 and reduces the secretion of Th1 cytokines. Thus, Th2 type cytokines appear to contribute to the maintenance of pregnancy by controlling the immune and endocrine systems and promoting the function of the trophoblasts at the implantation site. © 2000 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Cytokine; Pregnancy; Immune function; Endocrine; Trophoblast
* Tel.: + 81-764-342281; fax: + 81-764-345036. E-mail address: [email protected] (S. Saito) 0165-0378/00/$ - see front matter © 2000 Elsevier Science Ireland Ltd. All rights reserved. PII: S 0 1 6 5 - 0 3 7 8 ( 0 0 ) 0 0 0 6 0 - 7
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1. Introduction Cytokines are regulatory peptides or glycoproteins that can be produced by virtually every nucleated cell type in the body and have pleiotropic regulatory effects on hematopoietic, endocrine, neural and many other cell types. Unlike hormones, cytokines usually act as intercellular (paracrine) and/or intracellular (autocrine) signals in local tissues, only occasionally spilling over into the circulation to act as endocrine mediators (Miyajima et al., 1992; Kishimoto et al., 1994). Cytokines are closely involved in reproduction, i.e. ovarian follicular development, embryo implantation, endometrial development, and trophoblast growth and differentiation (Hunt and Roby, 1994; Savino and Dardenne, 1995; Simon et al., 1995; Petraglia et al., 1996). Although the roles of cytokines were considered to be quite complicated during the early stages of their identification, technological advances in recent years have contributed significantly to the clarification of their functions. CD4+-T cells play a central role in the immune system because they produce large amounts of cytokines and regulate a variety of immune functions. CD4+-T cells can be classified into Th1 and Th2 types by their pattern of cytokine production (Mosmann and Sad, 1996). The former type, which produces IL-2, TNF-b and IFN-g, is involved in the development of delayed type hypersensitivity and is associated with cytotoxic T cell functions. The latter type, which produces IL-4, IL-5, IL-6, IL-10 and IL-13, is involved in the antibody production response. The dominance of Th2 cells seems to be of prime importance in maintaining pregnancy in mice and human (Wegmann et al., 1993; Hill et al., 1995; Guilbert 1996; Krishnan et al., 1996; Marzi et al., 1996; Vince and Johnson, 1996; Ekerfelt et al., 1997; Haynes and Smith, 1997; Piccinni et al., 1998). These Th2 type cytokines, which are produced by trophoblasts and other types of cells, regulate maternal endocrine functions as well as trophoblastic functions (Nishino et al., 1990; De Moraes Pinto et al., 1996; Roth et al., 1996; Saito et al., 1997). In this study, the author discusses the characteristics of cytokines in the immune system at the human feto-maternal interface. Then the author mentions the existence of a specific cytokine cross-talk between the human trophoblast and decidual lymphocytes.
2. Characteristics of the lymphocytes in the decidua Endometrial granulated lymphocytes (EGLs) increase in number during the late secretory stage of the endometrium and the EGL level, at its peak, accounts for up to 70–80% of lymphocytes in the decidua during the first trimester of pregnancy then rapidly decreases during the period after the
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second trimester (Bulmer et al., 1988; Starkey et al., 1988; King and Loke, 1991). Although these lymphocytes clearly exhibit CD56, an NK-marker, they express no other NK-markers such as CD16 or CD57 (Nagler et al., 1989, 1990; Nishikawa et al., 1990, 1991; King et al., 1991) (Table 1). EGLs also contain the cytolytic molecules perforin, TIA 1, and granzyme A (Lin et al., 1991; King et al., 1993). They exhibited weak lytic activity against the NK cell-sensitive target K562, although they have been reported to have augmented NK activity after IL-2 stimulation (Nagler et al., 1990; Nishikawa et al., 1990; King and Loke, 1990; Nishikawa et al., 1991; Saito et al., 1993a,b,c,d,e). These characteristics suggest that EGLs are a type of NK cell. These cells are known by various names such as CD16−CD56bright NK cells, CD562 + large granular lymphocytes (LGLs) and CD562 + NK cells. Table 2 gives the results of decidual lymphocyte subsets obtained using flow cytometry conducted in our laboratory. The percentage of CD16−CD56bright NK cells in the peripheral blood is less than one percent. In the decidua, however, more than 80% of lymphocytes are CD16−CD56bright NK cells. Although CD16+ NK cells are regarded as the main component of NK cells in the peripheral blood, their content in the decidua is only 2% (Table 2). The characteristics of CD16−CD56bright NK cells can be summarized as follows: (1) they express c-kit receptor, which is expressed on bone marrow hematopoietic stem cells, indicating that CD16−CD56bright NK cells are an immature type of lymphocyte (Matos et al., 1993; Umekage et al., 1998); (2) they express both the early T cell markers, CD2 and CD7, and the NK cell marker, CD56, killer inhibitory receptor (KIR) and killer activatory receptor (KAR) (Nagler et al., 1989; King et al., 1991; Nishikawa et al., 1991; Miki et al., 1998); (3) they exist in fetal liver that contains hematopoietic stem cells and lymphoid progenitor cells (Phillips et al., 1992); (4) CD16− CD56bright NK cells express RAG-1 mRNA and RAG-2 mRNA that are Table 1 Characteristics of decidual CD16−CD56bright NK cells Hematopoietic cell NK marker Early T cell marker Late T cell marker Cell adhesion molecules Cytokine receptor Activation marker CD3 complexes RAG gene
CD45+ CD562+, CD16−, CD57−, KIR+, KAR+ CD2+, CD7+ CD4−, CD8+/−, CD3−, CD5− L-selectin(−), CD11a+, CD11b+/−, CD11c+, CD18+, ICAM-I+, CD44+, fibronectin receptor (a4b1+, a5b1+, a4b7+) IL-2Radim, IL-2Rb2+, IL2Rg+, IL-4R+, c-kit CD69+, HLA-DR+/−, CD71+/− CDo (intracellular)+, (cell surface)− CDj+ RAG1+/−, RAG2+/−
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Table 2 subsets of decidual and peripheral blood lymphocytes Subsets
decidua (n= 12) (%)
Peripheral blood (Pregnant women) (n= 12) (%)
CD3+-T CD4+-T CD8+-T ab+T/CD3+ gd+T/CD3+ CD45RO+CD4+/CD4+ CD3+CD56dim/CD3+ CD16+-NK CD16−CD56bright-NK CD20+-B
12.29 4.6 5.692.1 4.791.9 94.193.8 6.192.3 85.1910.1 29.5910.2 2.791.2 84.895.2 0.990.3
71.89 3.5 43.49 6.5 24.99 7.2 95.39 2.1 5.791.6 53.19 11.6 4.69 2.3 20.39 4.3 0.59 0.3 8.49 2.5
involved in gene recombination, despite the absence of T cell receptors (TCRs) and cell surface CD3 antigen (Phillips et al., 1992; Hayakawa et al., 1994; Gudelj et al., 1996). (5) Among CD3 molecules that are associated with TCRs, CD3o and CD3z exist in the cytoplasm and are not expressed on the cell surface (Phillips et al., 1992; King et al., 1998). (6) CD16− CD56bright NK cells appeared earlier than T cells or CD16+ NK cells when lymphocytes of donor origin were examined after bone marrow transplantation (Jacobs et al., 1992). Therefore CD16−CD56bright NK cells seem to be not only undifferentiated cells but also progenitor cells which are able to differentiate into T cells or NK cells. Regarding whether CD16−CD56bright NK cells express RAG-1 mRNA and RAG-2 mRNA, there is considerable divergence of opinion among researchers (Hayakawa et al., 1994; King et al., 1998). CD3+ T cells are the main population of peripheral blood lymphocytes. The CD3+-T cell count in the decidua, however, decreases to a level 1/9 of that in the peripheral blood. The specific recognition of antigen by T cells is mediated by the heterodimeric TCR. T cells can be classified as ab T or gdT by TCR. According to the results of immunohistological staining and flow cytometry, the number of gdT cells existing in the human decidua is limited and less than that in the mouse decidua (Dietl et al., 1990; Morii et al., 1993; Vassiliadou and Bulmer, 1998a; Arck et al., 1999). Our data showed that decidual gdT cells accounted for 5% of all decidual T cells and this indicates that there is no difference between the percentage of Tgd cells among T cells in the decidua and that in the peripheral blood. MinchevaNilsson et al. (1994) pointed out that gd epitopes easily disappeared after a fixation and freezing procedure. Therefore it is difficult to detect them by
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using conventional immunohistological methods. They used an electron microscope to observe gdT cells in the decidua and detected many gdT cells. In normal pregnancy, gdT cells in the decidua express mainly VgI mRNA which is known to respond not only to HSP-60 but also to mouse or human trophoblasts in a non-MHC-restricted manner (Divers et al., 1994; Heyborne et al., 1994; Hayakawa et al., 1996; Clark et al., 1998). More information is urgently needed on the mechanism by which gdT cells, which react to trophoblasts, contribute to the maintenance of pregnancy. CD3 expression and TCRab and TCRgd expression on decidual T cells were significantly down regulated compared to on peripheral blood T cells (Dietl et al., 1990; Morii et al., 1993). It has been reported that T cells which showed intermediate CD3 fluorescence intensity existed in mouse liver sinusoid and that these unique T cells, which express large amounts of IL-2Rb, were extrathymic T cells (Iiai et al., 1992; Watanabe et al., 1993). Various factors contribute to their proliferation, such as aging, tumor bearing state, pregnancy, infection, viral infection, and autoimmune disease (Ohteki et al., 1990; Kimura et al., 1995; Ohtsuka et al., 1995). Recently the existence of NKT cells, which expressed both NK 1.1 antigen, an NK cell marker and CD3, T cell marker, was confirmed. Watanabe et al. (1995) demonstrated that almost half of the extrathymic T cells in the liver were NKT cells with NK 1.1 antigen and CD3 antigen. T cells in human decidua possess down-regulated CD3 and increased levels of the IL-2Rb chain molecules like extrathymic T cells in mice (Saito et al., 1992). Furthermore CD56 positive T cells were frequently observed in decidua (Nishikawa et al., 1991) (Table 2). These findings suggest that decidual T cells are extrathymic T cells or NKT cells. In our recent study, it was found that some of the T cells in the decidua expressed Va24, an NKT cell marker (unpublished data). It remains to be clarified whether T cells in human decidua are extrathymic T cells or not.
3. Activation of the lymphocytes in the decidua and cytokine production by these lymphocytes Cytokines are produced by the lymphocytes after activated via CD3– TCR complexes or cytokine –cytokine receptor signals. The process of cytokine production by T cells is as follows; the native T cells of CD45RA+, CD45RO− and CD29−, which do not secrete any cytokines, are activated by some stimulation and become memory T cells which secrete a variety of cytokines (Fig. 1). Moreover memory T cells are differentiated into Th1 cells and Th2 cells (Fig. 1). The activated T cells and NK cells express CD69, an early activation marker, 2 or 3 h later and their expression
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continues for at least 24 h (Testi et al., 1989). Afterward they express HLA-DR antigen, a late activation marker. During the first trimester of pregnancy, 58.3% of CD16−CD56bright NK cells in the decidua express CD69 antigens (Table 3). Similar findings were reported by King et al. (1991), Nishikawa et al. (1991); Kodama et al. (1998); Maruyama et al. (1992). On the other hand, Vassiliadou and Bulmer (1998b) reported that EGL did not express CD69 using immunohistological staining. They speculated that CD16−CD56bright NK cells were activated during the separation of lymphocytes from decidual tissue. This method, however, has poor sensitivity compared to flow cytometry in detecting CD69 antigens. Kodama et al. (1998) reported that the intensity of CD69 expression on CD562 + LGL cells was the highest at the proliferative stage of the endometrium during the non-pregnant period, and that the antigen level showed a tendency to decrease early in the pregnancy. Nishikawa et al. (1991) reported that expression of CD69, HLA-DR and CD71 on the decidual CD16−CD56bright NK cells increased when a small amount of IL-2 was added to the medium in vitro. These results indicate that the intensity of CD69 antigen expression on the CD16−CD56bright NK cells is low during normal pregnancy, but the following findings demonstrate that CD16−CD56bright NK cells in the decidua had been activated. CD16−CD56bright NK cells in the peripheral blood which had not been activated have no granules in their cytoplasm. Once they are stimulated in
Fig. 1. T cell development from native T to memory T cells.
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Table 3 Expression of activation marker on decidual and peripheral blood T cells and CD16− CD56bright-NK cells in the first trimester of pregnancy Decidua (n= 12) (%)a
Peripheral (n =12) (%)
CD4 CD69 HLA-DR IL-2Ra IL2Rb
58.39 12.7* 67.79 14.8* 26.99 5.0** 8.694.6***
0.9 9 0.5 11.4 9 4.6 20.59 5.2 1.59 0.5
CD8 CD69 HLA-DR IL-2Ra IL-2Rb
73.3 914.3* 82.9913.5* 6.292.9**** 17.797.2**
3.590.7 18.69 8.0 2.59 1.5 10.69 3.4
CD16−CD56 bright CD69 HLA-DR
69.4913.4* 13.495.3****
7.792.2 77.7 920.2
a
Percentage of CD4, CD8 or CD16−CD56bright cells. * PB0.0001; ** PB0.005; *** PB0.001; **** PB0.01.
vitro by IL-2, granules are formed in the cells (Nagler et al., 1989, 1990) and there exists a morphological similarity between these cells and EGLs in the decidua. Next, these activated CD16−CD56bright NK cells in peripheral blood come to produce TNFa and IFNg (Nagler et al., 1989), and decidual CD16−CD56bright NK cells also express various types of cytokine mRNA such as TNFa, IFNg, M-CSF (CSF-1), G-CSF, GM-CSF and LIF mRNA (Saito et al., 1993a; Jokhi et al., 1994). The in vitro activation of CD45RA+ CD45RO− NK cells results in the acquisition of CD45RO expression and loss of CD45RA expression. CD45RA− CD45RO+ NK cells appear to be more functionally hyper-reactive and differentiated than CD45RA+ CD45RO− NK cells (Braakmann et al., 1991). Since most CD16−CD56bright NK cells in early pregnancy decidua are CD45RA− CD45RO+ (Kodama et al., 1998), these NK cells may have been functionally hyper-reactive and differentiated. These results strongly suggest that CD16−CD56bright NK cells have been activated by some stimulation. According to Kodama et al. (1998), the intensity of CD69 antigen expression on the CD562 + NK cells was highest at the proliferative phase and decreased gradually throughout the menstrual cycle. Therefore neither fertilized ova nor trophoblasts can be thought of as expressing antigens which stimulate NK cells. At present we
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cannot define the stimulation supposed to promote NK cell activation. Another problem we should clarify is the fact that although CD16− CD56bright NK cells express CD69, the intensity of HLA-DR expression in the decidua is lower than that in the peripheral blood (Table 2). The activation of CD16−CD56bright NK cells in the decidua is a subject of controversy. However, it is acknowledged that T cells in the decidua are activated. According to the results obtained in immunohistological staining and flow cytometry, T cells in the decidua express CD69 antigen (Saito et al., 1992; Chen et al., 1995; Vassiliadou and Bulmer, 1998b). As shown in Table 3, as a late activation marker, HLA-DR antigens are also expressed on decidual T cells. T cells can be classified into CD45RA+ CD45RO− native T cells and CD45RA− CD45RO+ memory T cells (Akbar et al., 1991). Activation of CD45RA+ CD45RO− native T cells result in the acquisition of CD45RO and loss of CD45RA expression, so CD45RA− CD45RO+ memory T cells are more functionally differentiated. As a characteristic of memory T cells, a small amount of activation marker such as IL-2Ra and IL-2Rb is expressed (Akbar et al., 1991). Thus, it can be considered that memory T cells have received continuous low-grade stimulation over a long period of time. As demonstrated in Table 3, most of the T cells in the decidua are CD45RA− CD45RO+ memory T cells and the intensity of IL-2Ra expression and of IL-2Rb expression are low. Therefore T cells in the decidua may constantly receive weak antigenic stimulation. Various types of cytokines are produced from native T cells and memory T cells. Native T cells produce only IL-2, while memory T cells produce a large variety of cytokines including IL-3, IL-4, IFNg and IL-6 (Akbar et al., 1991). Because most T cells in early pregnancy decidua are memory T cells (Saito et al., 1994a), they secrete locally large amounts of cytokines. The isolation of T cells is difficult in the decidua because their number is limited, so it is unclear which cytokines are produced by T cells in this tissue. Recently a new technique using flow cytometry has been developed to determine simultaneously both the cell surface markers and intracellular cytokines (Ferrick et al., 1995; Prussin and Metcalfe, 1995). This method has enabled us to examine the cytokine production at the single cell level and to analyze many cells in a short time. We used a system to detect the pan T marker, CD3 and intracellular cytokines (IFN-g, IL-4) simultaneously. We collected peripheral blood samples from women in the first trimester and made a comparison between the Th1 and Th2 cell ratios. These values showed no changes. On the other hand, the percentage of Th1 cells was lowest the decidua in early pregnancy and highest in endometrium during the proliferative phase followed by endometrium in the secretory phase and decidua in early pregnancy. The percentage of Th2 cells was highest in the decidua in early pregnancy and lowest in the endometrium
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during the proliferative phase. The percentage of Th0 cells was significantly increased in the endometrium during the proliferative and secretory stages compared to the decidua in early pregnancy (Saito et al., 1999). These findings suggested that Th0 cells differentiated into Th2 in the early pregnancy decidua. Piccinni et al. (1998) pointed out the predominance of Th2 cytokines in pregnancy because T cell clones established by the normal human decidua produced more Th2 cytokines; and Th2 producing clones decreased in cases of unknown recurrent miscarriage. It remains to be clarified how and where Th1 and Th2 cells differentiate from Th0 cells in pregnant subjects. Although more data are urgently needed, recent results suggested the involvement of intrathymic NKT cells (Nishizawa and Koyasu, 1997). These cells which secrete both IFNg and IL-4 seem to be involved in the differentiation of Th0 cells into Th1 or Th2 cells in the thymus. NKT cells in the decidua might regulate the Th1/Th2 balance in pregnancy. In mouse decidua, gdT cells which produce TNFa and IFNg and gdT cells which produce TGFb2 and IL-10 have been identified (Arck et al., 1999). These cells are probably involved in the differentiation of Th1 and Th2 cells in the decidua. Special attention should be directed to determining whether these NKT cells and gdT cells change in the endometrium in cases of recurrent miscarriage or implantation failure.
4. Cross talk mediated by cytokines in the maternal endocrine system, decidual immune system and trophoblast function Why are Th2 cells dominant in the decidua? IL-4 and IL-10 are known as the factors that induce Th2 cells to differentiate from Th0 cells (Mosmann and Sad, 1996). Furthermore trophoblasts produce IL-4 and IL-10 which induce Th2 cells (De Moraes Pinto et al., 1996; Roth et al., 1996) (Table 4). IL-10 mRNA is markedly recognizable in extravillous trophoblasts (Roth et al., 1996) and this could explain the larger population of Th2 cells in the decidua. G-CSF, which is abundant in the placenta and decidua (Saito et al., 1993b, 1994b), also induces Th2 cells (Pan et al., 1995). Progesterone is a hormone that accumulates during pregnancy and is indispensable to maintaining pregnancy. When progesterone is liberated by the corpus luteum after ovulation, it prepares the uterus for the reception and development of the fertilized ovum by differentiating the endometrium from the proliferative to the secretory stage. At this time, the Th1/Th2 cell ratio decreases significantly during the secretory stage in preparation for the implantation of the fertilized ovum (Saito et al., 1999). This may be due to progesterone, which has been reported to induce the conversion of Th0 cells into Th2 cells (Piccinni et al., 1995, 1998; Lim et al. 1998). However, we
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should not forget the involvement of prostaglandin and other factors existing in the placenta and fertilized ova in the process of the rapid induction to Th2 cells after pregnancy (Fortin et al., 1997; Kelly and Critchley, 1997; Kelemen et al., 1998; Miranda et al., 1998). Regarding other mechanisms, we must consider the cross talk mediated by cytokines in the maternal endocrine system, decidual immune system and trophoblast function. When a female becomes pregnant, hCG is secreted from trophoblasts and pregnancy is successfully maintained because hCG prevents apoptosis of the corpus luteum. Of special interest is that IL-4, IL-6 and LIF are Th2 cytokines which promote hCG release from trophoblasts (Nishino et al., 1990; Sawai et al., 1995; Saito et al., 1997). The released hCG could induce the production of progesterone from corpus luteum, resulting in Th2 cell induction. Estrogen and progesterone increase the M-CSF concentration in the mouse uterus (Pollard et al., 1987). M-CSF is produced by CD16−CD56bright NK cells, decidual stromal cells, and endometrial gland cells (Pollard et al., 1987; Saito et al., 1993a,d,e; Jokhi et al., 1994). M-CSF induces the differentiation and proliferation of trophoblasts and Table 4 Cytokine production in chorionic or placental tissue Cytokine
Trophoblast
IL-1a IL-1b IL-2 IL-3 IL-4 IL-5 IL-6 IL-7 IL-8 IL-10 G-CSF GM-CSF M-CSF LIF TNFa TGFb SCF HGF VEGF EGF TGFa IFNg
+ 9 − − + − + ? + + 9 − + + + + + − + + + −
(syncytiotrophoblast\cytotrophoblast)
(syncytiotrophoblastBcytotrophoblast) (syncytiotrophoblast\cytotrophoblast) (syncytiotrophoblast\cytotrophoblast) (syncytiotrophoblastBcytotrophoblast) (syncytiotrophoblast= cytotrophoblast) (syncytiotrophoblastBcytotrophoblast) (syncytiotrophoblast only) (syncytiotrophoblast\cytotrophoblast) (cytotrophoblast only) (syncytiotrophoblastBcytotrophoblast) 1st trimester only (syncytiotrophoblast\cytotrophoblast) (syncytiotrophoblastBcytotrophoblast)
Stromal cell 9 + − − + − + + + + + 9 + − + + + + + + + −
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Fig. 2. Cytokine and endocrine networks at the materno-fetal interface.
promotes hCG production (Arceci et al., 1989; Saito et al., 1991, 1993e; Garcia-Lloret et al., 1994; Saito et al., 1994c). As a result, hCG induces the production of progenterone by the corpus luteum so that Th2 cells can be stimulated to differentiate from Th0 cells by progesterone (Fig. 2). M-CSF seems to regulate macrophage functions in the decidua to support the maintenance of pregnancy (Pollard et al., 1991; Hunt and Robertson, 1996). Recently Munn et al. (1998) demonstrated that trophoblasts and macrophages had tryptophan catabolic enzymes and that these enzymes controlled the activation of maternal cytotoxic T cells to protect the placenta against attack by maternal cytotoxic T cells (Fig. 2). In this case, macrophages whose differentiation is mediated by M-CSF possess tryptophan catabolic enzymes that protect placenta from maternal T cell attack. Among the Th2 type cytokines, IL-4 inhibits the expression of IL-2Ra, IL-2Rb and IL-2Rg on CD16−CD56bright NK cells, thereby controling the augmentation of NK activity by IL-2 (Saito et al., 1996). Th2 type cytokines may control intrauterine NK cells to prevent them from attacking trophoblasts. Cytokines deliberately mediate the endocrine and immune systems of the maternal body, fetus and placenta. The cross talk between them could contribute to the maintenance of pregnancy. The disturbance of this delicate balance may result in abnormal pregnancy, i.e. implantation failure,
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recurrent abortion, preeclampsia etc. The clarification of various problems in the field of reproductive medicine, including the relationship between maternal and fetal immunity and endocrine systems, is awaited. The mammalian fetus has been perceived, paradoxically, as a successful allograft, successful tumor, and a successful parasite. The maternal immune and endocrine systems are involved in the success of pregnancy. Cytokines appear to contribute to the maintenance of pregnancy by controlling the immune and endocrine systems and promoting trophoblast function at the implantation site. Thus, the cytokine messengers of the immune, endocrine, and neuroendocrine systems interact within the intrauterine tissues during pregnancy to determine correct placental growth and differentiation and to ensure the feto-maternal well being.
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