Cytokine networks in the uteroplacental unit: Macrophages as pivotal regulatory cells

Journal of Reproductive Immunology, 16 (1989) 1--17

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Elsevier Scientific Publishers Ireland Ltd.

JRI 00612

Review

Cytokine networks in the uteroplacental unit: Macrophages as pivotal regulatory cells Joan S. Hunt Department of Pathology, University of Kansas Medical Center, 39th Street and Rainbow Blvd., Kansas City, KS 66103 (U.S.A.) (Accepted for publication 12 June 1989)

Summary A rich array of potent regulatory molecules has been identified in the uteroplacental unit. Most recently uncovered are the cytokines, families of polypeptides that establish intercellular communications, a paracrine effect, and often bind to synthetic cells in autocrine regulatory loops. Nearly all of the disparate maternal and fetal cell types in the uteroplacental unit are integrated into the cytokine network. The highly versatile macrophage, abundant in uteroplacental tissues, has emerged as a potentially pivotal cell type because of its unique ability to send and receive cytokine signals. Elevated levels of cytokines, possibly secreted when uteroplacental macrophages are activated by either bacterial endotoxins or receptor-bound cytokines, may compromise pregnancy. In particular, cytokines have been implicated in the induction of pre-term labor associated with infections. Intensive research is required to delineate the temporal patterns of cytokine synthesis that characterize pregnancy, to evaluate the events leading to normal and premature pregnancy termination and to establish protocols for therapeutic interventions in cases of infection. Key words:

cytokines; macrophages; placenta; trophoblast cells; uteris.

Introduction " C y t o k i n e " is a term that was coined for ease o f reference to the newly recognized families of polypeptides that facilitate intercellular 0165-0378/89/$03.50 © 1989 Elsevier Scientific Publishers Ireland Ltd. Published and Printed in Ireland

communications. Originally identified and named by immunologists as "monokines" (monocyte/macrophages)or "lymphokines" (lymphocytes), it is now well established that synthesis of those potent regulatory molecules is not limited to cells of the immune system. Nor are macrophages and lymphocytes the only receptive cells; pleotrophic effects are characteristic of cytokines. Reproduction immunologists have become increasingly aware that cytokines, if present in uteroplacental tissues, could have a major impact on reproduction events. Recent experiments have been designed to identify cytokines in the uteroplacental unit, to determine which types of maternal and/or fetal cells synthesize and bind specific cytokines, and to evaluate the effects of cytokines on receptive cells. Although several uteroplacental cell types are clearly participating members in cytokine exchanges, the macrophage, a uniquely versatile cell with both synthetic and receptive features, has the potential to serve as a central cell in cytokine networks. The discussion below focuses on new reports of a few of the cytokines known to originate with macrophages, or to which macrophages are sensitive. Table 1 lists some of the cytokines identified in

TABLE 1 Cytokines and their receptors identified in uteroplacental tissues ~. Cytokine

Reference

Colony-stimulating factor-I

Pollard et al., 1987 (factor) Regenstreif and Rossant, 1989 (receptor)

Interleukin- 1

Flynn, Finke and Hilfiker, 1982 (factor) Main, Strizki and Schochet, 1987 (factor) Tamatani et al., 1988 (factor) Takacs et al., 1988 (factor)

Type I Interferons

Imakawa et al., 1987 (factor) Charpigny et al., 1988 (factor) Hansen et al., 1988 (factor) Godkin, Bazer and Roberts, 1984 (receptor)

Interleukin-2

Soubiran, Zapitelli and Schaffar, 1987 (factor) Boehm et al., 1989 (factor)

Transforming growth factor-beta

Frolik et al., 1983 (factor) Heine et al., 1987 (factor) Clark et al., 1988 (factor)

Tumor necrosis factor-alpha

Jaattela, Kuusela and Saksela, 1988 (factor) Eades, Cornelium and Pekala, 1988 (receptor)

•Limited to factors synthesized by macrophages, or for which macrophages have receptors.

Trophoblast cell lines (Hunt et al., 1989b) Macrophages (Wahl et al., 1987) Lymphocytes (Clark et al., 1988) Other ceils (see Sporn, 1987; Rizzino, 1988) Trophoblast cell lines (Berkowitz et al., 1988; Hunt et al., 1989b) Other cells (see Le and Vilcek, 1987)

Macrophages (Piacibello et al., 1985)

Placental macrophages (Flynn, Finke and Hilfiker, 1982) Trophoblast cells (Main et al., 1987) Uterine cells (Takacs et al., 1988) Other ceils (see Dinarello, 1988)

Uterine suppressor ceils (Clark et al., 1988) Macrophages (Assoian et al., 1987) Other cells (see Rizzino, 1988)

Macrophages (Beutler, 1985)

CSF-GM

IL-1

TGF-/3

TNF-a

aWith the exceptions noted above, uteroplacental macrophages have not been evaluated for either synthesis of cytokines or cytokine receptors. Citations are for either bone marrow derived macrophages or blood monocytes.

Trophoblast cell lines (Berkowitz et al., 1988; Hunt et al., 1989b) Macrophages (see Nathan, 1987) Other cells (see Dinarello, 1988)

Placental cells (Athananssakis et al., 1987)

Trophoblast cells (Regenstreif and Rossant, 1989; Pollard et al., 1989) Macrophages (Tushinski et al., 1982)

Luminal epithelium (Pollard et al., 1987) Macrophages (Rambaldi et al., 1987)

CSF- 1

Target cell (Reference)

Source (Reference)

Cytokinea

Sources of cytokines in the uteroplacental unit and potential target cells~.

TABLE 2

uteroplacental tissues. Maternal and extra-embryonic cells that are potential sources and/or target cells for a limited number of cytokines are given in Table 2. Distribution of macrophages in the uteroplacental unit Macrophages are a major cell type in both maternal and fetal compartments of the uteroplacental unit. In humans, macrophages are abundant in the decidua and in the fibrous tissues near the placenta (Bulmer and Johnson, 1984; Lessin et al., 1988). In rodents, macrophages cluster near the implantation site (Tachi et al., 1981). As pregnancy progresses, macrophages are prohibited from colonizing the decidua basalis, but are widely distributed throughout the myometrium, endometrial stroma and metrial gland of mice (Redline and Lu, 1988) and rats (Hunt and Atherton, 1989, unpublished data; Head, J.R., pers. commun.). In human tissues, macrophages can be identified in preparations of isolated placental cells (Moskalewski, Ptak, and Czarnik, 1975) and, by using immunohistology, in the mesenchymal stroma of the placenta (Wood et al., 1978; Hunt, King and Wood, 1984; Goldstein et al., 1988). In the extraplacental membranes, fetal macrophages populate the mesenchymal stroma between the amnion and chorion layers (Bulmer and Johnson, 1984; Lessin et al., 1988). Macrophages have been identified in association with the yolk sac membranes (Wood, 1980), are abundant in the fetal mesenchyme of the rodent chorioallantoic plate (Redline and Lu, 1988; Hunt and Atherton, 1989, unpublished data) and can be identified in both the mouse (Moskalewski, Ptak and Strzyzewska, 1974; Wood, 1980) and the rat placenta (Hunt and Atherton, 1989, unpublished data) late in gestation. Functions of uteroplacental macrophages High concentrations of macrophages in maternal and fetal tissues suggest that the cells perform specific pregnancy-associated tasks. One potential role for maternal macrophages is to assist in the tissue remodeling necessary to accommodate expansion of extraembryonic tissues. As their name implies, macrophages were first characterized by their phagocytic function (Metchnikoff, 1884). Macrophages in the uterus form a major line of defense against invading microorganisms, as shown recently by Redline and Lu (1988). The absolute numbers of macrophages in specific anatomic compartments may dictate local effects. For example, enhanced susceptibility to infections results from the paucity of macrophages in the murine decidua basalis (Redline and Lu, 1988). Maternal macrophages may also perform a service to the semi-allogeneic fetus by secreting prostaglandin E 2 and possibly other immu-

nosuppressive molecules (Hunt, Manning and Wood, 1984; Tawfik, Hunt and Wood, 1986a, 1986b; Clark et al., 1988; reviewed by Hunt, 1989) which diminish the likelihood that maternal lymphocytes, including those with potential anti-fetal reactivity, will proliferate in uterine tissues. Functions of placental macrophages are less certain. Wood and King (1982) have postulated that placental macrophages protect the fetus by removing maternal anti-fetal antibodies complexed to soluble fetal antigens. Expression of class II major histocompatibility antigens late in pregnancy (Lessin et al., 1988; Goldstein et al., 1988) may facilitate antigen presentation by placental macrophages. Macrophage functions, with the exception of phagocytosis, depend largely on their capacity to synthesize and secrete an impressive array of highly potent cytokines and other regulatory molecules (for a review, see Nathan, 1987). Macrophages produce substances that promote cellular growth and viability, and molecules that mediate cell death, which may be one and the same, dependent upon concentration. Levels of production rely on the receipt of signals that are either autocrine or paracrine in origin. In the complex ecosystem of the uteroplacental unit, macrophages may serve as message centers, receiving and relaying signals to other cells. Figure 1 illustrates the autocrine and paracrine loops of some macrophage molecules with potentially major effects on pregnancy events. ENDOTOXIN/INTERFERONS

/ ENDOTHELIAL CELLS FIBROBLASTS MYOCYTES LYMPHOCYTES TROPHOBLAST CELLS

\ LYMPHOCYTES TROPHOBLAST CELLS TUMOR CELLS

Fig. 1. Autocrine and paracrine regulation of macrophage synthesis of PGE 2 and the cytokines IL-1, TNF-a and TGF-/L Macrophages, activated by either endotoxin from gram negative bacteria or one of

the interferons, synthesize high levels of PGE2, IL-1, TNF-a and TGF-/~, which are biologically active on many types of cells (paracrine effects), and also have regulatory properties for macrophages (autocrine effects).

Growth-promotingcytokines Cytokine effects are dependent upon (1) the embryologic origin of the receptive cell, (2) whether the cell is normal or malignant and, critically (3) cytokine concentration. In many instances, cytokines have opposite results on different cell types (mesenchymal vs. epithelial cells for example), and opposite effects with high and low doses. Moreover, some cytokines have overlapping and/or synergistic biologic activities (reviewed by Le and Vilcek, 1987). Receptor density and both intracellular and extracellular second messenger pathways contribute to cytokine effects. One of the characteristic features of the cytokine genes is transient expression, allowing for finely-tuned alterations as required (for a review of cytokine gene regulation see Taniguchi, 1988).

Colony-stimulating factors Major candidates for growth promoters in normal pregnancy are the colony-stimulating factors (CSF) derived from macrophages and other types of cells in the uteroplacental unit. Athananssakis et al. (1987) have demonstrated that CSF for granulocytes and macrophages (CSF-GM) enhances murine placental cell proliferation and phagocytosis. Based on that observation, Wegmann (1987) has postulated that uterine cells, presumably macrophages, are stimulated to produce CSF-GM and other growth-promoting molecules by products of allogeneic cell-activated maternal lymphocytes. An alternative explanation may be that the usual stimuli for CSF-GM synthesis by uterine macrophages are phagocytosis of tissue debris and/or inflammatory mediators endogenous in the placental bed (Thorens et al., 1987). Uterine luminal epithelium is the major site of CSF-1 (colony stimulating factor for macrophages, M-CSF) synthesis in the murine uteroplacental unit, as shown by recent in situ hybridization studies (Pollard et al., 1987). Potential receptive cells, i.e., cells demonstrating the proto-oncogene c-finsderived CSF-1 receptor (Sherr et al., 1985), include both macrophages (Tushinski et al., 1982) and trophoblast cells of the placenta (Muller et al., 1983; Rettenmier et al., 1986; Pollard et al., 1989; Regenstreif and Rossant, 1989). A temporal relationship has been established between CSF-1 synthesis, trophoblast cell expression of the CSF-1 receptor and placental growth and development (Pollard et al., 1989; Regenstreif and Rossant, 1989). However, trophoblast cell and uteroplacental macrophage responses to CSF-1 remain to be fully explored. The chemotactic properties of CSF-1 for macrophages (Wang et al., 1988) may account for the increased density of uterine macrophages that characterizes the pregnant uterus (Hunt et al., 1985). However, CSF-1 may

diminish local macrophage proliferation by inducing Type I interferons (Moore et al., 1984). Interleukin-2 is synthesized by syncytiotrophoblast cells in the human placenta (Boehm et al., 1989) and is capable of co-operating with CSF-1 in the induction of macrophage natural killer-like activity (Li et al., 1989), illustrating the complexity of cytokine interplay that may operate in the uterus. Macrophages produce as well as respond to CSF-1 (Rambaldi, Young and Griffin, 1987; Becker, Devlin and Haskill, 1989). Synthesis and release of CSF-1 is stimulated by adherence (Haskill et al., 1988). Thus, resident uterine macrophages may contribute to the influx and differentiation of new inhabitants. Interleukin-1 A second potential growth-promoting cytokine originating with macrophages as well as other types of cells is interleukin-1 (IL-1) (reviewed by Dinarello, 1988). IL-1 is produced by human and murine placental macrophages (Flynn, Finke and Hilfiker, 1982) and human trophoblast cells (Main, Strizki and Schochet, 1987), and is found in normal amniotic fluid (Tamatani et al., 1988). In situ hybridization experiments demonstrate IL-1producing cells in the virgin mouse uterus (Takacs et al., 1988). One target cell for IL-1 may be trophoblast cells of the placenta. Both IL1-a and IL-1-/3 have a modest concentration-dependent ability to enhance DNA synthesis by rat trophoblast cell lines (Hunt et al., 1989b). Macrophages, endothelial cells, fibroblasts and myocytes also respond to ILl. Consequences of binding, of potential significance to reproduction events, are alteration of PGE 2 levels (endothelial cells, fibroblasts, myocytes) and synthesis of TNF-a (macrophages) (Nathan, 1987).

Inhibitory cytokines Two cytokines with growth inhibitory properties, TNF-a and TGF-/~, are discussed below. For a review of inhibitory polypeptides see Keski-Oji and Moses (1987). Tumor necrosis factor-alpha Cell death may also be a result of exposure to cytokines. Trophoblast cells bear a striking resemblance to tumor cells insofar as both trophoblast and tumor cells proliferate rapidly, invade normal tissues and use similar strategies for evading destruction by immune cells (Hunt and Fishback, manuscript in prep.). However, limits are clearly established for trophoblast cell migration and proliferation in the uterus, limits that could be related to trophoblast sensitivity to cytokines. Tumor necrosis factor-alpha (TNF-a), a

macrophage product (Beutler et al., 1985), may promote healthy pregnancy by preventing inappropriate trophoblast cell invasion of the uterus. The experimental observations leading to that conjecture are as follows. (i) TNF-a is present in the uteroplacental unit. TNF-a has been identified in human amniotic fluid and conditioned media from placental and decidual explants (Jaattela, Kuusela and Saksela, 1988). Preliminary experiments in our laboratory using highly sensitive "capture" enzyme immunoassays as well as bioassays demonstrate that low levels of TNF-a are synthesized by explants of rat uterus. (ii) Trophoblast cell DNA synthesis is limited by TNFa. Significantly, rat trophoblast cell lines, which have normal 2n, 4n DNA content (Hunt et al., 1989a), exhibit higher sensitivity to restriction of DNA synthesis by TNF-a (Hunt et al., 1989b) than trophoblast-derived choriocarcinoma cells (Berkowitz et al., 1988), which demonstrate aneuploidy. (iii) Intracellular second messenger pathways are operative in trophoblast cells. TNF-a sensitivity has been related to elevated intracellular levels of cAMP (Chun and Hoffman, 1987). Rat trophoblast cell lines are highly sensitive to down-regulation of DNA synthesis by analogues of cAMP (Soares et al., 1989). (iv) Trophoblast cells are likely to bind TNF-a. Receptors for TNF-a have been identified in homogenates of human placenta (Eades, Cornelium and Pekala, 1988). Taken together, those observations suggest that an encounter with TNF-a by migrating trophoblast cells could discourage DNA synthesis and consequent proliferation in the placental bed. In an amplification loop, TNF-a, which is chemotactic for monocytes (Ming et al., 1987), may recruit support from the blood monocyte pool. TNF-a may have other major effects on trophoblast ceils. Preliminary results in our laboratory indicate that TNF-a alters synthesis/release of both human chorionic gonadotrophin (hCG) and PGE 2 by trophoblast-derived choriocarcinoma cells (Hunt, unpublished results). The subtleties of factor interaction between extraembryonic and maternal cells, and the impact that those interrelationships may have on pregnancy are illustrated in the experiments of Bartocci, Pollard and Stanley (1986), who have demonstrated hormonal regulation of uterine CSF- 1. Although TNF-a was first identified as an inhibitory polypeptide, TNF-a may also encourage cellular proliferation (see Le and Vilcek, 1987). Thus, diverse effects on different types of cells in the uteroplacental unit could be expected of that potent molecule.

Transforming growthfactor-beta. TGF-/3 has growth-promoting effects on most cells of mesenchymal origin but inhibits growth by both cells of epithelial origin (reviewed by Keski-Oja and Moses, 1987), which include trophoblast cells, and lymphocytes (Wahl

et al., 1987). Like TNF-a, transforming growth factor-beta (TGF-/3), a product of both macrophages (Assoian et al., 1987) and other types of cells (reviewed by Sporn, 1987), is a highly effective concentration-dependent inhibitor of rat trophoblast cell DNA synthesis (Hunt et al., 1989b). Clark et al. (1988) have demonstrated production of a TGF-/~-like substance by murine uterine suppressor cells, and have postulated a role for that product in restricting maternal anti-fetal lymphocyte activities. Thus, TGF-/~ may exert triple effects, enhancing growth of mesenchymal cells, limiting inappropriate trophoblast invasion that would be detrimental to maternal health, and inhibiting lymphocyte proliferation that could compromise pregnancy. TGF-/J, which has been identified in human placentas (Frolik et al., 1983) and murine embryos (Heine et al., 1987), also has major effects on cell differentiation, which may be accomplished in part by the ability of that family of factors to regulate synthesis of extracellular matrix components (reviewed by Rizzino, 1988). Moreover, TGF-/3 could influence macrophage migration into the uteroplacental unit and production of cytokines following residency (Wahl et al., 1987). The potential for specific cytokine effects related to the embryologic origin of the receptive cell, to differential display of receptors, and to the cytokine concentration in the microenvironment, is particularly high for TGF-/L Special effects of cytokines: interferons Type I interferons (IFN-~, IFN-fl), molecules with multiple biologic activities (reviewed by Russell and Pace, 1987), are synthesized by macrophages and other leukocytes as well as fibroblasts in response to platelet-derived growth factor, CSF-1, and IL-1 (see Taniguchi, 1988). Type I interferons potentiate the effects of interferon-gamma on macrophages (Pace, 1988), and may therefore be major participants in the cytokine cascades that characterize infections of the uteroplacental unit (see below). Interferongamma, synthesized by activated lymphocytes, can overcome the negative regulatory effects that PGE 2 normally exerts on macrophage activation (Russell and Pace, 1984). Interferon-like molecules are also products of the trophectoderm. An ovine trophoblast protein (oTP-1) that binds to endometrium (Godkin, Bazer and Roberts, 1984), and promotes corpus luteum progesterone production via regulation of prostaglandin release or production, resembles IFN-a (Imakawa et al., 1987; Charpigny et al., 1988; Hansen et al., 1988). The specific endometrial cell type targeted by oTP-1 has not been identified. Regionalization of cytokines Synthesis of a limited group of cytokines with short-range and/or concentration-dependent effects by specific uteroplacental ceils suggests a

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requirement for regional specialization. In particular, compartmentalization of subpopulations of uteroplacental macrophages in different stages of maturation may dictate highly localized cytokine effects. In rats, phenotypically distinct macrophages inhabit specific anatomic compartments of maternal tissues both early (Head, J.R., pers. commun.) and late (Hunt and Atherton, 1989, unpublished data) in gestation; the distributional patterns for immature and mature maternal macrophages are different. Maternal macrophages differ from placental macrophages, as shown in recent studies in human tissues (Lessin et al., 1988; Goldstein et al., 1988; Bulmer, Morrison and Smith, 1988). Placental macrophages are immature, as demonstrated by the binding of stage-specific monoclonal antibodies in human (Goldstein et al., 1988) and rat (Hunt and Atherton, 1989, unpublished data) placentas. A potential for differential cytokine expression by uteroplacental macrophages in specific maturational stages is clearly present. The recent generation of anti-cytokine monoclonal antibodies and specific gene probes suitable for in situ hybridization studies will facilitate exploration of that question. Those tools may assist in resolving the same issue for extra-embryonic cells. Trophoblast cells in various stages of differentiation comprise the placenta. Receptors for the growth-promoting cytokine CSF-1 are differentially expressed by trophoblast cells, being highest in end-stage giant cells and trophoblast cells in the junctional zone (Pollard et al., 1989; Regenstreif and Rossant, 1989). Whether or not cytokine synthesis or the expression of other cytokine receptors by trophoblast cells will demonstrate a relationship to cell maturation in situ remains to be explored. Infections of the uteroplacental unit

In cases of infection, normal patterns of cytokine synthesis by macrophages and other cells in uteroplacental tissues may be drastically altered. Infections by organisms whose surface structures include the macrophage activator lipopolysaccharide (LPS, endotoxin) could enhance the levels of PGE 2 (Aderem et al., 1986), IL-1 (Dinarello, 1988), TNF-a (Beutler et al., 1985) and TGF-fl (Assoian et al., 1987), that are synthesized by uteroplacental macrophages. IL-1, TNF-a and TGF-fl all enhance PGE z synthesis by macrophages (Elias et al., 1987; Tashjian et al., 1985). Moreover, PGE 2 influences the production of IL-1 (Knudsen, Dinarello and Strom, 1986) and TNF-ot (Renz et al., 1988), thus completing an autocrine regulatory circuit. Amplification of endotoxin effects may be promoted by binding of one or more of the interferons, which facilitate macrophage

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activation (Pace, 1988). Some of the paracrine and autocrine effects of macrophage activation by endotoxin and/or interferons are shown in Fig. 1. Myometrium as target tissue PGE, is a critical intermediary molecule in the events subsequent to infection that lead to pre-term labor, and may be induced directly by bacterial products or indirectly by cytokines. Endotoxin has been identifed in the amniotic fluid of patients with pre-term labor (Romero, 1988), a leading cause of fetal mortality. Pre-term labor is associated with elevated levels of amniotic fluid PGE, (Lopez Bernal et al., 1987; Romero et al., 1987). Several types of cells, including decidual and chorion cells (Okazaki et al., 1981), uterine macrophages (Tawfik et al., 1986a, 1986b) and amnion cells (Okazaki et al., 1981; Lamont, Rose, and Elder, 1985; Lopez Bernal et al., 1987; Romero et al., 1987) produce PGE, and, as noted above, enhanced macrophage PGE, synthesis is a consequence of both LPS activation and encounter with cytokines. Because PGE, is a major inducer of myometrial contraction, pre-term labor can result from infection. Romero et al. (1988) have proposed a causal relationship between an unchecked cascade of endotoxin-stimulated cytokines and the induction of premature labor, with PGE, as an intermediary extracellular messenger. Trophoblast cells as targets Endotoxin in the uteroplacental unit may also have major effects on placental cells, either directly as a result of LPS binding, or indirectly by enhancing levels of macrophage products to which placental cells are sensitive. LPS receptors have been clearly identified on rat trophoblast cells (Hunt et al., 1989b). Although exposure to LPS does not have any effect on trophoblast cell DNA synthesis (Hunt et al., 1989b), other potential trophoblast cell responses have not been evaluated. In contrast, some (TNFa, TGF-fi) but not all (PGE,, IL-l) products of LPS-activated macrophages restrict DNA synthesis by rat trophoblast cell lines (Hunt et al., 1989b). Seemingly, trophoblast cells, as represented by the trophoblast cell lines, are not targets for proliferation control by the intermediary molecule PGE,. Those experimental results suggest that one major consequence of infection may be drastic reduction of trophoblast cell DNA synthesis by cytokines originating with endotoxin-activated macrophages in the uteroplacental unit. Unexplored territory

Interactions among cytokines and other potent molecules that are known products of macrophages, and are potential or described products of other

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uteroplacental ceils, remain to be fully explored. Those include fl-endorphin, bioactive lipids other than PGE 2, complement proteins, coagulation factors, fibroblast growth and activating factors, inhibitors of enzymes and cytokines, insulin-like factors (Fant et al., 1986; Taylor, Scott and Baxter, 1987), interleukin-2 (Soubiran, Zapitelli and Schaffar, 1987; Boehm et al., 1989), interleukin-6 (Tabibzadeh et al., 1989), platelet derived growth factor, reactive oxygen intermediates, transforming growth factor-alpha, and the newly described macrophage inflammatory protein-l, a prostaglandin-independent product of endotoxin-activated macrophages (Davatelis et al., 1989). Moreover, according to Adamson (1987), all of the proto-oncogenes that have been identified thus far are found in the placenta, where they undoubtedly serve major autocrine (Goustin et al., 1985) or paracrine functions. "With the exception of the elegant biologic story evolving with cfins and CSF-1, the potential interactions between uteroplacental cell oncogene expression and endogenous cytokines are uncharted.

Conclusions Cytokines are now recognized as major modulators of pregnancy events. The macrophage, a hitherto neglected cell type in reproduction, has emerged as a potentially pivotal cell type because of its ability to synthesize and respond to cytokines and other powerful regulatory molecules. PGE 2, a product of both macrophages and other types of uteroplacental cells, often acts as a critical intermediary effector molecule in the cytokine network. Moreover, other uterine cells as well as trophoblast cells of the placenta contribute to the complex cellular and molecular interplay required for successful pregnancy. A clear picture of cytokine events in temporal and spatial terms~through gestation has yet to emerge. However, exploration of macrophage functions and cytokine-mediated events in pregnancy are clearly promising avenues for future experimentation by reproduction immunologists. Recognition of temporal, synergistic and overlapping aspects of cytokine interactions is required; patterns revealed by the experiments performed thus far indicate that considerable complexity can be expected. The potential health benefits of cytokine research are high. In addition to clarifying normal events, experimentation may uncover effective therapeutic modalities for altering cytokine expression that would be useful when cytokines threaten pregnancy.

Acknowledgements These studies are supported by grants from the March of Dimes National

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Foundation for Birth Defects and the National Institutes of Health Biomedical Research Support Program (NIH BRSG SO7 RR05373). The author wishes to express her appreciation to Drs. H.C. Anderson, J.W. Pollard and M.J. Soares for helpful discussions and comments, and to Dr. G.W. Wood, whose interest in uterine macrophages stimulated her own. References Adamson, E.D. (1987) Review Article: Expression of proto-oncogenes in the placenta. Placenta 8,449-466. Aderem A.A., Cohen D.S., Wright S.D. and Cohn Z.A. (1986) Bacterial lipopolysaccharides prime macrophages for enhanced release of arachadonic acid metabolites. J. Exp. Med. 164, 165--179. Assoian, R.K., Fleurdelys, B.E., Stevenson, H.C., Miller, P.J., Madtes, D.K., Raines, E.W., Ross, R. and Sporn, M.B. (1987) Expression and secretion of type B transforming growth factor by activated human macrophages. Proc. Natl. Acad. Sci. USA 84, 6020--6024. Athanassakis, I., Bleackley, R.C., Paetkau, V., Guilbert, L., Barr, R.J., and Wegmann, T.G. (1987) The immunostimulatory effect of T cells and T cell lymphokines on murine fetally-derived placental cells. J. lmmunol. 138, 37--44. Bartocci, A., Pollard, J.W. and Stanley, E.R. (1986) Regulation of colony-stimulating factor 1 during pregnancy. J. Exp. Med. 164, 956--961. Becker, S., Devlin, R.B., and Haskill, J.S. (1989) Differential production of tumor necrosis factor, macrophage colony stimulating factor, and interleukin 1 by human alveolar macrophages. J. Leukocyte Biol. 45,353--361. Berkowitz, R.S., Hill, J.A., Kurtz, C.B. and Anderson D.J. (1988) Effects of products of activated leukocytes (lymphokines and monokines) on the growth of malignant trophoblast cells in vitro. Am. J. Obstet. Gynecol. 158, 199--203. Beutler, B., Mahoney, J., Trang, N.L., Pekala, P. and Cerami, A. (1985) Purification of cachectin, a lipoprotein lipase-suppressing hormone secreted by endotoxin-induced RAW 264.7 cells. J. Exp. Med. 161,984--995. Boehm, K.D., Kelley, M.F., Ilan, J. and Ilan, J. (1989) The interleukin 2 gene is expressed in the syncytiotrophoblast of the human placenta. Proc. Nat. Acad. Sci. USA 86, 656--660. Bulmer J.N. and Johnson, P.M. (1984) Macrophage populations in the human placenta and amniochorion. Clin. Exp. Immunol. 57,393--403. Bulmer, J.N., Morrison, L. and Smith, J.C. (1988) Expression of class II MHC gene products by macrophages in human uteroplacental tissue. Immunology 63,707--714. Charpigny, G., Reinaud, P., Huet, J-C., Guillomot, M., Charlier, M., Pernollet, J.-C. and Martal, J. (1988) High homology between trophoblastic protein (trophoblastin) isolated from ovine embryo and a-interferons. FEBS Lett. 228, 12--16. Chun, M. and Hoffmann, M.K. (1987) Intracellular cAMP regulates the eytotoxicity of recombinant tumor necrosis factor for L cells in vitro. Lymphokine Res. 6, 161--167. Clark, D.A., Falbo, M., Rowley, R.B., Banwatt, D. and Stedronska-Clark, J. (1988) Active suppression of host-vs-graft reaction in pregnant mice. IX. Soluble suppressor activity obtained from allopregnant mouse decidua that blocks the cytolytic effector response to IL-2 is related to transforming growth factor-B. J. Immunol. 141, 3833--3840. Davatelis, G., Wolpe, S.D., Sherry, B., Dayer, J-M., Chicheportiche, R. and Cerami, A. (1989) Macrophage inflammatory protein-1: a prostaglandin-independent endogenous pyrogen. Science 243, 1066--1068. Dinarello, C.A. (1988)Biology of interleukin-1. Fed. Am. Soc. Exp. Biol. J. 2, 108--115. Eades, D.K., Cornelium, P. and Pekala, P.H. (1988) Characterization of the tumour necrosis factor receptor in human placenta. Placenta 9, 247--251. Elias J.A., Gustilo, K., Baeder, W. and Freundlich, B. (1987) Synergistic stimulation of fibroblast prostaglandin production by recombinant interleukin 1 and tumor necrosis factor. J. Immunol. 138, 3812--3816.

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