- Email: [email protected]
Pre-eclampsia and the inflammatory response
European Journal of Obstetrics & Gynecology and Reproductive Biology 95 (2001) 213±217
Pre-eclampsia and the in¯ammatory response M.M. Faasa,*, G.A. Schuilingb a
Reproductive Immunology, Medical Biology Branch, Department of Pathology and Laboratory Medicine, University of Groningen, P.O. Box 30.001, 9700 Groningen, The Netherlands b Department of Obstetrics and Gynaecology, University of Groningen, Groningen, The Netherlands
Keywords: Pre-eclampsia; In¯ammatory response; Leukocytes; Blastocyst; Implantation; Pregnancy
1. Introduction Pre-eclampsia, a human disease of pregnancy, is the leading cause of maternal morbidity in the western world. Despite the ubiquity of the disease and the plethora of studies concerning its aetiology and pathogenesis no comprehensive theory concerning its aetiology and pathogenesis have been put forward until now and so far there are no adequate therapies other than bed rest and, if necessary, early delivery. We recently developed an animal model for preeclampsia [1,2], whereby activation of the in¯ammatory response by low dose endotoxin resulted in pregnant rats, and not in non-pregnant rats, in a pre-eclampsia-like syndrome. Analysis of the in¯ammatory reaction induced by low dose endotoxin in pregnant Ð and non-pregnant rats revealed that this reaction was much more persistent and intense in pregnant rats as compared with non-pregnant rats; this is in line with the vast literature showing that pregnant individuals are much more sensitive to endotoxin than nonpregnant individuals [3±5]. Based on the animal model, we have thus put forward the hypothesis that also human preeclampsia results from activation of the in¯ammatory system [6,7]. Here, we raise and try to answer, from a biological point of view, various questions such as ``why is pregnancy a pro-in¯ammatory condition?'', ``what triggers the in¯ammatory response leading to human pre-eclampsia?'' and ``why is pre-eclampsia so relatively common in humans?''. 2. Pre-eclampsia In our society, pre-eclampsia is the most common and serious antenatal complication of pregnancy, characterised by hypertension, proteinuria and sometimes abnormal ¯uid retention. It affects about 2±3% of all pregnancies [8]. The disease may also be associated with abnormalities of the liver and the central nervous system [9], as well as with * Corresponding author. Tel.: 31-50-3613045; fax: 31-50-3611694. E-mail address: [email protected] (M.M. Faas).
disseminated intravascular coagulation [10]. Because the placenta of patients with pre-eclampsia is often underdeveloped, there generally is a poor placental circulation [11] and accordingly a poor exchange of nutrients and gasses. As a result, the foetus may suffer from severe intra-uterine growth retardation [12]. Pre-eclampsia appears to be a typical human disease, although it has been observed in two other primates, i.e. the patas monkey [13] and the lowland gorrila [14]. 3. Pathophysiology of pre-eclampsia Unfortunately, the pathophysiology of pre-eclampsia is relatively unknown. The current, generally accepted concept regarding the pathophysiology of pre-eclampsia is that it is a disease due to endothelial cell dysfunction [15,16]. This endothelial cell dysfunction is apparent from both morphological parameters, endotheliosis in the glomeruli of the kidney and ultrastructural changes in the placentabed [17,18], as well as from biochemical parameters, such as aberrations in endothelin-1 and elastase [19] and increased levels of factor VIII [20]. This, however, raises the question of how the endothelial cell dysfunction is brought about. Some authors claim that sera of pre-eclamptic patients contain factors cytotoxic to endothelial cells [15,16], while others suggest that pre-eclampsia is a maternal response to a pathogenic factor of trophoblastic origin [21,22]. More recently, we and others have suggested that pre-eclampsia is the result of an activated maternal in¯ammatory response, including activation of in¯ammatory cells, such as monocytes and granulocytes, as well as activation of endothelial cells, which are indeed part of the in¯ammatory system [2,7,22]. 4. Activation of the maternal inflammatory response in pregnant rats results in a pre-eclampsia-like syndrome We recently demonstrated that activation of the in¯ammatory response by infusion of an ultra low dose of
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M.M. Faas, G.A. Schuiling / European Journal of Obstetrics & Gynecology and Reproductive Biology 95 (2001) 213±217
endotoxin into pregnant rats induces a pre-eclampsia-like syndrome (experimental pre-eclampsia) [1]. This syndrome is characterised by hypertension, proteinuria and disseminated intravascular coagulation [1]. Like in human preeclampsia, experimental pre-eclampsia is very speci®c for pregnancy, since it can not be induced in non-pregnant rats [1]. Rats with experimental pre-eclampsia showed a pregnancy-speci®c glomerular in¯ammatory reaction [2]: as compared with non-pregnant (cyclic) rats, they showed a more intense and persistent in®ltration of (activated) granulocytes and monocytes into the glomeruli of the kidney [2]. There was also more persistent expression of adhesion molecules (intercellular adhesion molecule-1 and vascular cell adhesion molecule 1) on the endothelium and of their respective ligands (CD11a and CD49d) on the in®ltrated leukocytes [2]. Moreover, circulating leukocytes, especially monocytes and granulocytes, showed an activated phenotype, as characterised by decreased expression of CD14 (on monocytes and granulocytes) and CD62L (on granulocytes) and increased expression of CD11a and CD49d (on granulocytes) [23]. These data are consistent with a generalised activation of the in¯ammatory response in experimental preeclampsia, including activation of in¯ammatory cells and activation of endothelial cells. Treatment of rats with experimental pre-eclampsia with low dose aspirin [24] or superoxide dismutase (an oxygen radical scavenger) [25] attenuated not only the clinical signs of the disease but also the in¯ammatory response in the glomeruli of the kidney. Together, these data suggest a causal relationship between the endotoxin-induced pregnancy-speci®c persistent in¯ammatory response and the clinical signs of the pre-eclampsialike disease. These data also show that during pregnancy, a relatively weak pro-in¯ammatory stimulus, which does not induce an in¯ammatory response in the non-pregnant condition, may during pregnancy elicit a relatively strong, persistent in¯ammatory response. This thus suggests that pregnancy is a pro-in¯ammatory condition. Moreover, on the basis of the above mentioned experiments, we proposed that like in the endotoxin-induced pre-eclampsia-like syndrome in the pregnant rat, human pre-eclampsia should also be regarded as the result of an in¯ammatory response [2,6]. This hypothesis is consistent with ®ndings of Sacks et al., who showed that circulating leukocytes of pre-eclamptic patients exhibited an activated phenotype, as characterised by decreased expression of CD62L and an increased production of reactive oxygen species compared with normal pregnancy [26]. 5. Questions The above mentioned suggestions and hypothesis raise various questions, such as: ``why are pregnant individuals so
extremely sensitive to pro-in¯ammatory stimuli (including endotoxin)?''. It seems likely that this phenomenon, since it is potentially dangerous for both mother and child, serves a biological aim. ``What induces the in¯ammatory reaction leading to the signs of pre-eclampsia in humans?''. Although experimental pre-eclampsia is induced by endotoxin, there is no evidence for a role of endotoxin in the cause of preeclampsia. Or ``why is pre-eclampsia restricted to humans, since as far as we know, it does not affect pregnant females of other orders (except for a few primates)?'' Next, we will try and answer these questions from a biological point of view. 5.1. Why are pregnant individuals so extremely sensitive to pro-inflammatory stimuli? The reproductive strategy of humans is aimed at producing a limited number of offspring, in which much is invested [27]. Raising of human offspring takes many years and in fact a next child is usually born before the previous one in independent of parental care. The human reproductive potential, however, seems to con¯ict with this reproductive strategy: humans are very fertile (females ovulate every month and both males and females are continuously willing to mate). Humans, therefore, are in need of mechanisms reducing fertility. In the course of the evolution such mechanisms seem to have evolved: ®rst of all, subfertility is induced by breast feeding and women may practise fertility control by prolonged breast feeding [28]. Secondly, decreased fertility from the age of 35 onward and the de®nitive end of fertility around the age of 50 may contribute to the limitation of the number of offspring [29,30]. One might also explain the production of a high percentage of abnormal fertilised eggs [31,32] in the light of reducing female fertility; it appears that more than 50% of the fertilised eggs are abnormal [31,32]. Although the production of abnormal fertilised eggs can be seen as a strategy to reduce fertility, it can only be effective together with a very sensitive mechanism for selection and elimination of the zygotes which are abnormal. Indeed, in humans such a mechanism must to be operative, since although more then 50% of the zygotes are abnormal [31,32], the vast majority of children born are normal in every respect. This appears to be the result of a postimplantation loss rate, which has been estimated to be at least 43% [33], while others suggest it to be even 78% [34]. This reproductive strategy is unique to humans (although some primates may follow the same strategy). Human fecundability, ranging from 21 to 28% between the age of 20 and 30 years [35], is very low as compared with domestic animals and laboratory animals. Most domestic and laboratory animals have fertilisation rates between 90 and 100%. However, not all fertilised eggs develop to full-term foetuses: for instance, in sheep, 20±30% of the fertilised eggs fail to develop into full-term lambs [36]; in rabbits about
M.M. Faas, G.A. Schuiling / European Journal of Obstetrics & Gynecology and Reproductive Biology 95 (2001) 213±217
25% of the fertilised eggs are lost [37]; in laboratory rats, about 10% of the fertilised eggs are lost [38]. However, failures of fertilised eggs to develop to full-term in these animals is most likely due to adverse environmental circumstances, rather than to embryo abnormalities, since, in contrast to human's, embryo, abnormalities appear in a relatively low incidence [39]. Since the blastocyst is semiallogen, implantation of the blastocyst induces a local in¯ammatory response, which is characterised by in®ltration of (activated) immune cells [40± 42]. These immune cells, but also other cells at the maternofetal interface, may produce a variety of cytokines [43,44]. It has been demonstrated that at the materno-fetal interface, cytokines are produced by the zygote that promote implantation (for instance leukemia inhibitory factor, interleukin(IL)-1, IL-10 [45±47]). Cytokines that promote foetal loss are produced by maternal cells, i.e. by the in®ltrated immune cells and but also by decidual cells (for instance IFN-g and TNFa [48,49]). For implantation to occur, the zygote thus needs to regulate the in¯ammatory response in such a way that there is a balance between production of implantationpromoting and implantation-inhibiting cytokines. In our view, this in¯ammatory response aroused at the implantation site may serve as the selection mechanism for zygotes. It may be suggested that abnormal zygotes are not able to regulate the in¯ammatory reaction in such a way that they can implant, and they will thus be lost. This may for instance be the case for zygotes with evident chromosomal abnormalities. Only ``healthy'' zygotes are able to regulate the in¯ammatory reaction to their bene®t, so that they can implant. In order to make the selection process as critical as possible, the in¯ammatory response of the mother is set to a pro-in¯ammatory condition in pregnancy [2,26]; a condition in which even a mild pro-in¯ammatory stimulus, which does not induce an in¯ammatory reaction in nonpregnant individuals, may evoke an intense, persistent in¯ammatory reaction. Moreover, this setting of the in¯ammatory system may serve another biological aim, since the conceptus is semiallogen and, therefore, the speci®c immune response is suggested to be downregulated (notably the type 1 immune response [50]) and the pro-in¯ammatory state may, therefore, also be necessary to protect the mother from infection. 5.2. What induces the inflammatory reaction leading to the signs of pre-eclampsia in humans? In instances in which the zygote is not able to perfectly regulate the in¯ammatory response at the implantation site, but can still implant, the in¯ammatory response may impair implantation. Implantation will thus be defective. Moreover, since the zygote is not able to regulate the in¯ammatory response, this in¯ammatory response will not be downregulated. Since at this point the mother is pregnant, a mild pro-in¯ammatory stimulus (in this case induced by implan-
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tation) may induce a persistent generalised in¯ammatory response (see above). This in¯ammatory response may ultimately lead to the signs of pre-eclampsia and will not stop, unless pregnancy is terminated. A zygote may not be able to regulate the in¯ammatory response in case of a minor abnormality in the zygote; trisomy 13, for instance, appears to be associated with an increased incidence of pre-eclamspia [51,52]. On the other hand, a zygote may be perfectly normal, but the in¯ammatory response of the mother may be too intense for the zygote to be able to interfere. This may for instance be due to environmental circumstances arousing the in¯ammatory response; it has indeed been suggested that in crowded urban areas the incidence of pre-eclampsia is increased [53]. Also the genetic make-up of the mother may be responsible for a more intense reaction to a pro-in¯ammatory stimulus in general, and, therefore, also to implantation; familial factors are indeed implicated in the aetiology of preeclampsia [54]. In the case of oocyte donation, in which preeclampsia appears to be more frequent [55], the zygote is allogen, instead of semiallogen, and may, therefore, arouse a more intense in¯ammatory reaction, which it is not able to perfectly regulate. A similar problem may present itself in molar pregnancies, a pregnancy with a ``zygote'' of total paternal origin, in which pre-eclampsia is more frequent [56]. 5.3. Why is pre-eclampsia restricted to humans? According to the above mentioned hypothesis that preeclampsia is the result of an unsuccessful attack of the maternal in¯ammatory response at the implantation site on the implanting zygote, the fact that pre-eclampsia is mostly restricted to humans may be the result of their reproductive strategy. Since humans aim at the production of very few offspring, while on the other hand they are extremely fertile, they have developed strategies during the course of the evolution to reduce fertility. One of those strategies may be the production of a very high percentage of abnormal zygotes; in this we, humans, are unique. To be able to select only ``healthy'' zygotes for implantation, humans must have an extremely ®ne tuned mechanism, the in¯ammatory system (which setpoint is set so that it reacts to very mild pro-in¯ammatory stimuli), to detect and eliminate abnormal zygotes. This mechanism, however, may also be the cause of pre-eclampsia: a zygote may, for whatever reason, not be able to regulate the in¯ammatory response properly, but can still implant. 6. Conclusions We propose that the restriction of pre-eclampsia to humans (or primates) and the pro-in¯ammatory condition of pregnancy is a result of the unusual human reproductive strategy, in which many abnormal zygotes are produced. In
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this humans are unique. Therefore, they need a tightly controlled selection mechanism, the in¯ammatory system. To be able to carry out its task in an ef®cient way, the in¯ammatory response is set to pro-in¯ammatory state, in which it can respond to a mild pro-in¯ammatory stimulus. We also propose that pre-eclampsia may result from an unsuccessful attack of the maternal in¯ammatory response at the zygote at implantation. Since the zygote is not able to successfully downregulate the in¯ammatory response, this in¯ammatory response may hamper implantation, with as a result defective implantation and placentation. In the condition of pregnancy, the response may also become generalised and persistent, resulting in the signs of pre-eclampsia.
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[17] Roberts JM, Taylor RN, Friedman SA, Goldfien A. New developments in pre-eclampsia. Fetal Med Rev 1990;2:125±41. [18] Shanklin DR, Sibai BM. Ultrastructural aspects of pre-eclampsia. I. Placental bed and uterine boundary vessels. Am J Obstet Gynecol 1989;161:735±41. [19] Greer IA, Leask R, Hodson BA, Dawes J, Kilpatrich DC, Liston WA. Endothelin, elastase and endothelial dysfunction in pregnancyinduced hypertension. Lancet 1991;337:558. [20] Fournie A, Monrozies M, Pontonnier G, Boneu B, Bierme R. Factor VIII complex in normal pregnancy, pre-eclampsia and fetal growth retardation. Br J Obstet Gynaecol 1981;88:250±4. [21] Smarason AK, Sargent IL, Starkey PM, Redman CWG. The effect of placental syncytiotrophoblast microvillous membranes from normal and pre-eclamptic women on the growth of endothelial cells in vitro. Br J Obstet Gynaecol 1993;100:943±9. [22] Redman CWG, Sacks GP, Sargent IL. Preeclampsia: an excessive maternal inflammatory response to pregnancy. Am J Obstet Gynecol 1999;180:499±506. [23] Faas MM, Schuiling GA, Linton EA, Sargent IL, Redman CWG. Activation of peripheral leukocytes in rat pregnancy and experimental pre-eclampsia. Am J Obstet Gynecol 2000;182:351±7. [24] Faas MM, Schuiling GA, Baller JFW, Valkhof N, Bakker WW. Aspirin treatment of the low-dose-endotoxin-treated pregnant rat: pathophysiologic and immunohistologic aspects. J Lab Clin Med 1997;130:496±508. [25] Faas MM, Schuiling GA, Valkhof N, Baller JFW, Bakker WW. Superoxide-mediated glomerulopathy in the endotoxin-treated pregnant rat. Kidney and Blood Press Res 1998;21:432±7. [26] Sacks GP, Studena K, Sargent IL, Redman CWG. Normal pregnancy and preeclampsia both produce inflammatory changes in peripheral blood leukocytes akin to those of sepsis. Am J Obstet Gynecol 1998;179:80±6. [27] Stearns SC. Life history tactics: a review of the ideas. Quart Rev Biol 1976;51:3±47. [28] Howie PW, McNeilly AS. Effect of breast-feeding patterns on human birth intervals. J Reprod Fertil 1982;65:545±57. [29] Meldrum DR. Female reproductive aging-ovarian and uterine factors. Fertil Steril 2000;59:1±5. [30] Rogers A. Why menopause. Evolut Ecol 1993;7:406±20. [31] Hertig AT, Rock J, Adams EC, Menkin MC. Thirty-four fertilized human ova, good, bad and indifferent, recovered from 210 women of unknown fertility. Paediatrics 1952;23:202±11. [32] Edwards RG. Causes of early embryonic loss in human pregnancy. Hum Reprod 1986;1:185±98. [33] Miller JF, Williamson E, Glue J, Gordon YB, Grudzinskas JG, Sykes A. Fetal loss after implantation. A prospective study. Lancet 1980;2:554±6. [34] Roberts CJ, Lowe CR. Where have all the conceptions gone? Lancet 1975;1:498±9. [35] Short, RV. Species differences in reproductive mechanisms. Austin CR, Short RV, editors. Reproduction in mammals. Cambridge: Cambridge University Press, 1984. p. 24±61. [36] Quinlivan TD, Martin CA, Taylor WB, Cairney IM. Estimates of preand perinatal mortality in the New Zealand Romney Marsh ewe. I. Pre- and perinatal mortality in those ewes that conceived to one service. J Reprod Fertil 1966;11:379±90. [37] Palmer AK, Bottomley AM, Worden AN, Frohberg H, Bauer A. Effect of lindane on pregnancy in the rabbit and rat. Toxicology 1978;9:239±47. [38] Austin CR, Short RV. Reproduction in mammals. Cambridge: Cambridge University Press, 1972. [39] Wilmut I, Sales DI, Ashworth CJ. Maternal and embryonic factors associated with prenatal loss in mammals. J Reprod Fertil 1986;76:851±64. [40] Robertson SA, Brannstrom M, Seamark RF. Cytokines in rodent reproduction and the cytokine-endocrine interaction. Curr Opinion Immunol 1992;4:585±90.
M.M. Faas, G.A. Schuiling / European Journal of Obstetrics & Gynecology and Reproductive Biology 95 (2001) 213±217 [41] Seamark RF, Hadjisavas M, Robertson SA. Influence of the immune system on reproductive function. Anim Reprod Sci 1992;28:171±8. [42] McMaster MT, Dey SK, Andrews GK. Association of monocytes and neutrophils with early events of blastocyts implantation in mice. J Reprod Fertil 1993;99:561±9. [43] Cybulsky MI, Chan MKW, Movat HZ. Biology of disease: acute inflammation and microthrombosis induced by endotoxin, interleukin-1, and tumor necrosis factor and their implication in gram negative infection. Lab Invest 1988;58:365±78. [44] Chard T. Cytokines in implantation. Hum Reprod Update 1995;1:385±96. [45] Chaouat G, Menu E, Delage G, Moreau JF, Khrishnan L, Hui L, Meliani AA, Martal J, Raghupathy R, Leladier C. Immune-endocrine interactions in early pregnancy. Hum Reprod 1995;10(Suppl 2): 55±9. [46] Simon C, Pellicer A, Polan ML. Interleukin-1 system crosstalk between embryo and endometrium in implantation. Hum Reprod 1995;10(Suppl 2):43±54. [47] Khrishnan L, Guilbert LJ, Wegmann TG, Belosevic M, Mosmann TR. T helper-1 response against Leishmania in pregnant C57BL/6 mice increases implantation failure and fetal resorptions. Correlation with increased IFN-gamma and TNF and reduced IL-10 production by placental cells. J Immunol 1996;156:653±62. [48] Philips MR, Buyon JP, Winchester R, Weismann G, Abramson SB. Upregulation of the iC3b receptor (CR3) is neither necessary nor
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Pre-eclampsia and the in¯ammatory response M.M. Faasa,*, G.A. Schuilingb a
Reproductive Immunology, Medical Biology Branch, Department of Pathology and Laboratory Medicine, University of Groningen, P.O. Box 30.001, 9700 Groningen, The Netherlands b Department of Obstetrics and Gynaecology, University of Groningen, Groningen, The Netherlands
Keywords: Pre-eclampsia; In¯ammatory response; Leukocytes; Blastocyst; Implantation; Pregnancy
1. Introduction Pre-eclampsia, a human disease of pregnancy, is the leading cause of maternal morbidity in the western world. Despite the ubiquity of the disease and the plethora of studies concerning its aetiology and pathogenesis no comprehensive theory concerning its aetiology and pathogenesis have been put forward until now and so far there are no adequate therapies other than bed rest and, if necessary, early delivery. We recently developed an animal model for preeclampsia [1,2], whereby activation of the in¯ammatory response by low dose endotoxin resulted in pregnant rats, and not in non-pregnant rats, in a pre-eclampsia-like syndrome. Analysis of the in¯ammatory reaction induced by low dose endotoxin in pregnant Ð and non-pregnant rats revealed that this reaction was much more persistent and intense in pregnant rats as compared with non-pregnant rats; this is in line with the vast literature showing that pregnant individuals are much more sensitive to endotoxin than nonpregnant individuals [3±5]. Based on the animal model, we have thus put forward the hypothesis that also human preeclampsia results from activation of the in¯ammatory system [6,7]. Here, we raise and try to answer, from a biological point of view, various questions such as ``why is pregnancy a pro-in¯ammatory condition?'', ``what triggers the in¯ammatory response leading to human pre-eclampsia?'' and ``why is pre-eclampsia so relatively common in humans?''. 2. Pre-eclampsia In our society, pre-eclampsia is the most common and serious antenatal complication of pregnancy, characterised by hypertension, proteinuria and sometimes abnormal ¯uid retention. It affects about 2±3% of all pregnancies [8]. The disease may also be associated with abnormalities of the liver and the central nervous system [9], as well as with * Corresponding author. Tel.: 31-50-3613045; fax: 31-50-3611694. E-mail address: [email protected] (M.M. Faas).
disseminated intravascular coagulation [10]. Because the placenta of patients with pre-eclampsia is often underdeveloped, there generally is a poor placental circulation [11] and accordingly a poor exchange of nutrients and gasses. As a result, the foetus may suffer from severe intra-uterine growth retardation [12]. Pre-eclampsia appears to be a typical human disease, although it has been observed in two other primates, i.e. the patas monkey [13] and the lowland gorrila [14]. 3. Pathophysiology of pre-eclampsia Unfortunately, the pathophysiology of pre-eclampsia is relatively unknown. The current, generally accepted concept regarding the pathophysiology of pre-eclampsia is that it is a disease due to endothelial cell dysfunction [15,16]. This endothelial cell dysfunction is apparent from both morphological parameters, endotheliosis in the glomeruli of the kidney and ultrastructural changes in the placentabed [17,18], as well as from biochemical parameters, such as aberrations in endothelin-1 and elastase [19] and increased levels of factor VIII [20]. This, however, raises the question of how the endothelial cell dysfunction is brought about. Some authors claim that sera of pre-eclamptic patients contain factors cytotoxic to endothelial cells [15,16], while others suggest that pre-eclampsia is a maternal response to a pathogenic factor of trophoblastic origin [21,22]. More recently, we and others have suggested that pre-eclampsia is the result of an activated maternal in¯ammatory response, including activation of in¯ammatory cells, such as monocytes and granulocytes, as well as activation of endothelial cells, which are indeed part of the in¯ammatory system [2,7,22]. 4. Activation of the maternal inflammatory response in pregnant rats results in a pre-eclampsia-like syndrome We recently demonstrated that activation of the in¯ammatory response by infusion of an ultra low dose of
0301-2115/01/$ ± see front matter # 2001 Elsevier Science Ireland Ltd. All rights reserved. PII: S 0 3 0 1 - 2 1 1 5 ( 0 0 ) 0 0 4 9 3 - 0
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endotoxin into pregnant rats induces a pre-eclampsia-like syndrome (experimental pre-eclampsia) [1]. This syndrome is characterised by hypertension, proteinuria and disseminated intravascular coagulation [1]. Like in human preeclampsia, experimental pre-eclampsia is very speci®c for pregnancy, since it can not be induced in non-pregnant rats [1]. Rats with experimental pre-eclampsia showed a pregnancy-speci®c glomerular in¯ammatory reaction [2]: as compared with non-pregnant (cyclic) rats, they showed a more intense and persistent in®ltration of (activated) granulocytes and monocytes into the glomeruli of the kidney [2]. There was also more persistent expression of adhesion molecules (intercellular adhesion molecule-1 and vascular cell adhesion molecule 1) on the endothelium and of their respective ligands (CD11a and CD49d) on the in®ltrated leukocytes [2]. Moreover, circulating leukocytes, especially monocytes and granulocytes, showed an activated phenotype, as characterised by decreased expression of CD14 (on monocytes and granulocytes) and CD62L (on granulocytes) and increased expression of CD11a and CD49d (on granulocytes) [23]. These data are consistent with a generalised activation of the in¯ammatory response in experimental preeclampsia, including activation of in¯ammatory cells and activation of endothelial cells. Treatment of rats with experimental pre-eclampsia with low dose aspirin [24] or superoxide dismutase (an oxygen radical scavenger) [25] attenuated not only the clinical signs of the disease but also the in¯ammatory response in the glomeruli of the kidney. Together, these data suggest a causal relationship between the endotoxin-induced pregnancy-speci®c persistent in¯ammatory response and the clinical signs of the pre-eclampsialike disease. These data also show that during pregnancy, a relatively weak pro-in¯ammatory stimulus, which does not induce an in¯ammatory response in the non-pregnant condition, may during pregnancy elicit a relatively strong, persistent in¯ammatory response. This thus suggests that pregnancy is a pro-in¯ammatory condition. Moreover, on the basis of the above mentioned experiments, we proposed that like in the endotoxin-induced pre-eclampsia-like syndrome in the pregnant rat, human pre-eclampsia should also be regarded as the result of an in¯ammatory response [2,6]. This hypothesis is consistent with ®ndings of Sacks et al., who showed that circulating leukocytes of pre-eclamptic patients exhibited an activated phenotype, as characterised by decreased expression of CD62L and an increased production of reactive oxygen species compared with normal pregnancy [26]. 5. Questions The above mentioned suggestions and hypothesis raise various questions, such as: ``why are pregnant individuals so
extremely sensitive to pro-in¯ammatory stimuli (including endotoxin)?''. It seems likely that this phenomenon, since it is potentially dangerous for both mother and child, serves a biological aim. ``What induces the in¯ammatory reaction leading to the signs of pre-eclampsia in humans?''. Although experimental pre-eclampsia is induced by endotoxin, there is no evidence for a role of endotoxin in the cause of preeclampsia. Or ``why is pre-eclampsia restricted to humans, since as far as we know, it does not affect pregnant females of other orders (except for a few primates)?'' Next, we will try and answer these questions from a biological point of view. 5.1. Why are pregnant individuals so extremely sensitive to pro-inflammatory stimuli? The reproductive strategy of humans is aimed at producing a limited number of offspring, in which much is invested [27]. Raising of human offspring takes many years and in fact a next child is usually born before the previous one in independent of parental care. The human reproductive potential, however, seems to con¯ict with this reproductive strategy: humans are very fertile (females ovulate every month and both males and females are continuously willing to mate). Humans, therefore, are in need of mechanisms reducing fertility. In the course of the evolution such mechanisms seem to have evolved: ®rst of all, subfertility is induced by breast feeding and women may practise fertility control by prolonged breast feeding [28]. Secondly, decreased fertility from the age of 35 onward and the de®nitive end of fertility around the age of 50 may contribute to the limitation of the number of offspring [29,30]. One might also explain the production of a high percentage of abnormal fertilised eggs [31,32] in the light of reducing female fertility; it appears that more than 50% of the fertilised eggs are abnormal [31,32]. Although the production of abnormal fertilised eggs can be seen as a strategy to reduce fertility, it can only be effective together with a very sensitive mechanism for selection and elimination of the zygotes which are abnormal. Indeed, in humans such a mechanism must to be operative, since although more then 50% of the zygotes are abnormal [31,32], the vast majority of children born are normal in every respect. This appears to be the result of a postimplantation loss rate, which has been estimated to be at least 43% [33], while others suggest it to be even 78% [34]. This reproductive strategy is unique to humans (although some primates may follow the same strategy). Human fecundability, ranging from 21 to 28% between the age of 20 and 30 years [35], is very low as compared with domestic animals and laboratory animals. Most domestic and laboratory animals have fertilisation rates between 90 and 100%. However, not all fertilised eggs develop to full-term foetuses: for instance, in sheep, 20±30% of the fertilised eggs fail to develop into full-term lambs [36]; in rabbits about
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25% of the fertilised eggs are lost [37]; in laboratory rats, about 10% of the fertilised eggs are lost [38]. However, failures of fertilised eggs to develop to full-term in these animals is most likely due to adverse environmental circumstances, rather than to embryo abnormalities, since, in contrast to human's, embryo, abnormalities appear in a relatively low incidence [39]. Since the blastocyst is semiallogen, implantation of the blastocyst induces a local in¯ammatory response, which is characterised by in®ltration of (activated) immune cells [40± 42]. These immune cells, but also other cells at the maternofetal interface, may produce a variety of cytokines [43,44]. It has been demonstrated that at the materno-fetal interface, cytokines are produced by the zygote that promote implantation (for instance leukemia inhibitory factor, interleukin(IL)-1, IL-10 [45±47]). Cytokines that promote foetal loss are produced by maternal cells, i.e. by the in®ltrated immune cells and but also by decidual cells (for instance IFN-g and TNFa [48,49]). For implantation to occur, the zygote thus needs to regulate the in¯ammatory response in such a way that there is a balance between production of implantationpromoting and implantation-inhibiting cytokines. In our view, this in¯ammatory response aroused at the implantation site may serve as the selection mechanism for zygotes. It may be suggested that abnormal zygotes are not able to regulate the in¯ammatory reaction in such a way that they can implant, and they will thus be lost. This may for instance be the case for zygotes with evident chromosomal abnormalities. Only ``healthy'' zygotes are able to regulate the in¯ammatory reaction to their bene®t, so that they can implant. In order to make the selection process as critical as possible, the in¯ammatory response of the mother is set to a pro-in¯ammatory condition in pregnancy [2,26]; a condition in which even a mild pro-in¯ammatory stimulus, which does not induce an in¯ammatory reaction in nonpregnant individuals, may evoke an intense, persistent in¯ammatory reaction. Moreover, this setting of the in¯ammatory system may serve another biological aim, since the conceptus is semiallogen and, therefore, the speci®c immune response is suggested to be downregulated (notably the type 1 immune response [50]) and the pro-in¯ammatory state may, therefore, also be necessary to protect the mother from infection. 5.2. What induces the inflammatory reaction leading to the signs of pre-eclampsia in humans? In instances in which the zygote is not able to perfectly regulate the in¯ammatory response at the implantation site, but can still implant, the in¯ammatory response may impair implantation. Implantation will thus be defective. Moreover, since the zygote is not able to regulate the in¯ammatory response, this in¯ammatory response will not be downregulated. Since at this point the mother is pregnant, a mild pro-in¯ammatory stimulus (in this case induced by implan-
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tation) may induce a persistent generalised in¯ammatory response (see above). This in¯ammatory response may ultimately lead to the signs of pre-eclampsia and will not stop, unless pregnancy is terminated. A zygote may not be able to regulate the in¯ammatory response in case of a minor abnormality in the zygote; trisomy 13, for instance, appears to be associated with an increased incidence of pre-eclamspia [51,52]. On the other hand, a zygote may be perfectly normal, but the in¯ammatory response of the mother may be too intense for the zygote to be able to interfere. This may for instance be due to environmental circumstances arousing the in¯ammatory response; it has indeed been suggested that in crowded urban areas the incidence of pre-eclampsia is increased [53]. Also the genetic make-up of the mother may be responsible for a more intense reaction to a pro-in¯ammatory stimulus in general, and, therefore, also to implantation; familial factors are indeed implicated in the aetiology of preeclampsia [54]. In the case of oocyte donation, in which preeclampsia appears to be more frequent [55], the zygote is allogen, instead of semiallogen, and may, therefore, arouse a more intense in¯ammatory reaction, which it is not able to perfectly regulate. A similar problem may present itself in molar pregnancies, a pregnancy with a ``zygote'' of total paternal origin, in which pre-eclampsia is more frequent [56]. 5.3. Why is pre-eclampsia restricted to humans? According to the above mentioned hypothesis that preeclampsia is the result of an unsuccessful attack of the maternal in¯ammatory response at the implantation site on the implanting zygote, the fact that pre-eclampsia is mostly restricted to humans may be the result of their reproductive strategy. Since humans aim at the production of very few offspring, while on the other hand they are extremely fertile, they have developed strategies during the course of the evolution to reduce fertility. One of those strategies may be the production of a very high percentage of abnormal zygotes; in this we, humans, are unique. To be able to select only ``healthy'' zygotes for implantation, humans must have an extremely ®ne tuned mechanism, the in¯ammatory system (which setpoint is set so that it reacts to very mild pro-in¯ammatory stimuli), to detect and eliminate abnormal zygotes. This mechanism, however, may also be the cause of pre-eclampsia: a zygote may, for whatever reason, not be able to regulate the in¯ammatory response properly, but can still implant. 6. Conclusions We propose that the restriction of pre-eclampsia to humans (or primates) and the pro-in¯ammatory condition of pregnancy is a result of the unusual human reproductive strategy, in which many abnormal zygotes are produced. In
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this humans are unique. Therefore, they need a tightly controlled selection mechanism, the in¯ammatory system. To be able to carry out its task in an ef®cient way, the in¯ammatory response is set to pro-in¯ammatory state, in which it can respond to a mild pro-in¯ammatory stimulus. We also propose that pre-eclampsia may result from an unsuccessful attack of the maternal in¯ammatory response at the zygote at implantation. Since the zygote is not able to successfully downregulate the in¯ammatory response, this in¯ammatory response may hamper implantation, with as a result defective implantation and placentation. In the condition of pregnancy, the response may also become generalised and persistent, resulting in the signs of pre-eclampsia.
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