Molecular interactions in the placenta during malaria infection

European Journal of Obstetrics & Gynecology and Reproductive Biology 152 (2010) 126–132

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European Journal of Obstetrics & Gynecology and Reproductive Biology journal homepage: www.elsevier.com/locate/ejogrb

Review

Molecular interactions in the placenta during malaria infection Petra F. Mens a,*, Edward C. Bojtor b, Henk D.F.H. Schallig a a b

Royal Tropical Institute, Department of Biomedical Research, Amsterdam, The Netherlands Vrije Universiteit, Faculty of Earth and Life Science Amsterdam, The Netherlands

A R T I C L E I N F O

A B S T R A C T

Article history: Received 17 December 2009 Received in revised form 12 March 2010 Accepted 24 May 2010

Placental malaria is the placental sequestration of Plasmodium falciparum infected erythrocytes that accumulate in the intervillous space, resulting in pathological alterations. The intervillous space, the main compartment for exchange of nutrients and delivery of oxygen to the fetus, is of utmost importance for fetal development. Events leading to adverse outcomes of placental malaria can be summarized in four steps: (1) accumulation of P. falciparum infected erythrocytes; (2) infiltration of monocytes and macrophages; (3) alteration of the placental cytokine balance and (4) pathogenesis of adverse pregnancy outcomes. These events are triggered by chemokines and cytokines leading to impaired materno–fetal exchange and damage to the placenta. This review describes the events during placental malaria infection at molecular level and presents a simplified model describing all crucial steps leading to adverse pregnancy outcomes based on a review of recent literature (August 2009). ß 2010 Elsevier Ireland Ltd. All rights reserved.

Keywords: Placental malaria Molecular interactions Malaria infection

Contents 1. 2. 3. 4. 5. 6. 7. 8.

The burden of malaria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Placental malaria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Molecular interactions between the placenta and infected erythrocytes Immunity induced processes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Placental cytokine balance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Placental malaria: pathology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Step by step model of the interactions and events in placental malaria Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. The burden of malaria Malaria is caused by protozoan parasites of the genus Plasmodium, which are transmitted by the bite of an infected female Anopheles mosquito. P. falciparum, the most wide spread species, is responsible for most severe disease and mortality [1]. Over two billion people are currently exposed to the threat of malaria, mainly in the sub-Saharan region of Africa [1] (Fig. 1). Following children below the age of 5, pregnant women are most vulnerable of acquiring malaria. Due to hormonal changes and reduced immunity during pregnancy, women might suffer

* Corresponding author at: Royal Tropical Institute, Biomedical Research, Parasitiolgy, Meibergdreef 39, Amsterdam 1105 AZ, The Netherlands. Tel.: +31 205665463; fax: +31 206971841. E-mail address: [email protected] (P.F. Mens). 0301-2115/$ – see front matter ß 2010 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.ejogrb.2010.05.013

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from an increased susceptibility to the disease [2]. The risk of attaining malaria is further increased if pregnant women are below the age of 20, primigravid, infected with human immunodeficiency virus [3] or if they have never been exposed to malaria like pregnant travellers from non-endemic countries. However, the main reason for attaining malaria during pregnancy lies in the molecular actions at the level of the placenta. Here accumulation is facilitated by the interaction of the parasites with the novel CSA receptor, which attracts and binds parasites to placental tissue and absence of specific immunity against the placenta specific isolates. In sub-Saharan Africa approximately 24 million women become pregnant each year [4]. In spite of measures to prevent disease, the prevalence of malaria in pregnant women is high (around 66%) [5]. Placental malaria has several adverse consequences. Up to half of the 3.5 million low birth weight (LBW) babies born in subSaharan Africa each year may be attributable to placental malaria [4]. LBW can be caused by fetal growth restriction and preterm

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Fig. 1. The malaria parasite life cycle in humans. During a blood meal, an infected Anopheles mosquito injects sporozoites into the human host (1). Sporozoites infect hepatocytes (2) and mature into schizonts (3), which rupture and release merozoites (4). (In P. vivax and P. ovale the dormant hypnozoites can persist in the liver for years before invading the bloodstream.) After replication in the liver (exo-erythrocytic phase (A)), the parasites undergo asexual multiplication in the erythrocytes (erythrocytic phase (B)). Merozoites infect erythrocytes (5) whereupon ring stage trophozoites mature into schizonts which rupture and release merozoites (6). Some parasites differentiate into sexual erythrocytic stages (gametocytes (7)). The gametocytes are ingested by an Anopheles mosquito during a blood meal (8). The parasites’ multiplication in the mosquito is known as the sporogonic cycle (not shown), which results in new sporozoites in the salivary glands of the mosquito ready to be injected in humans (1). Courtesy of Centers for Disease Control and Prevention, http://www.cdc.gov.

delivery which are both associated with placental malaria [6]. Spontaneous abortion is another adverse pregnancy outcome associated with placental malaria [7,8]. These fatal complications result globally into an estimated 75,000–200,000 deaths of infants each year [4]. Besides these pathological consequences for the child, placental malaria is also associated with maternal anemia [9]. 2. Placental malaria The placenta protects itself and the fetus from the maternal immune system. The placental trophoblast layer functions as a barrier preventing access to fetal cells by maternal immunity. In addition, a physiological placental barrier is created; several substances are continuously expressed by placental cells to suppress both the maternal adaptive and the innate immune system. This suppressed immunity may add to the increased susceptibility of malaria during pregnancy [2] and increased severity of disease [5]. Placental malaria is caused by accumulation of infected erythrocytes in the intervillous space of the placenta. In response,

several immunological events are triggered, which cause pathological alterations in the placenta, impairing materno–fetal interaction [10]. It is shown that the density of Plasmodium infected erythrocytes in the placenta is much higher compared to densities in peripheral blood [11]. Moreover, trophozoite and schizont stages of the parasite are found in the intervillous space while these are absent in peripheral blood. These findings suggest a selective accumulation of parasites in the intervillous space which could be mediated by surface ligands of infected erythrocytes that are able to bind to pregnancy specific placental adhesion receptors [12]. Next to accumulation of infected erythrocytes, histological observations show that malaria infected placentas are infiltrated by maternal monocytes and macrophages. These mononuclear phagocytes are found in the intervillous space with or without hemozoin pigment, which is a byproduct of parasite digestion of hemoglobin. In response to hemozoin, already present placental mononuclear phagocytes secrete chemokines which are attractants for additional monocytes and macrophages [13]. A malaria infected placenta is further characterized by a thickened trophoblastic basement membrane (TBM). This

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P.F. Mens et al. / European Journal of Obstetrics & Gynecology and Reproductive Biology 152 (2010) 126–132

Fig. 2. Placental tissue from uninfected women (A) and women infected with malaria (B). The red dotted lines indicate the outline of the trophoblast layer. The red arrows (B) indicate parasite infected erythrocytes in the intervillous space. The white asterisks (B) indicate stained monocytes and macrophages. Original magnification (oil immersion): 1000 adapted from [13]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

morphological alteration of the TBM is associated with the high density of infected erythrocytes and infiltrated maternal mononuclear cells in the placenta [14]. The TBM thickens as a protective measure in response to the enormous amount of secreted cytokines, which are damaging to placental host tissues. However, this trophoblastic alteration is also harmful for placental tissue as it damages the syncytiotrophoblastic surface of chorionic villi. The changes in the TBM can result in pathopysiological changes as well as mechanical blockage which impairs transport of maternal nutrients and oxygen across the placenta to the fetus. The exact role and importance of these changes is still the subject of ongoing research but it is evident that these events together impair materno–fetal exchange with pathological consequences as a result [8]. 3. Molecular interactions between the placenta and infected erythrocytes While the peripheral circulation of pregnant women might almost be free of Plasmodium infected erythrocytes, the placenta can experience high parasite densities indicating that placental malaria is caused by infected erythrocytes that are selected for receptors only present in the placenta [11,15]. Cluster of differentiation 36 (CD36) commonly supports adhesion of P. falciparum infected erythrocytes in non-pregnant hosts, thereby avoiding passage through the spleen and subsequent clearance by the spleen [16]. However, in pregnant women with malaria avoidance of passage through the spleen is mediated by chondroitin sulphate A (CSA), also known as chondroitin-4sulfate (C4S), which is the dominant adhesion receptor for infected erythrocytes in the placenta [12]. CSA, a sulfated glycosaminoglycan present on the syncytiotrophoblast in the intervillous space of the placenta, normally functions as a reversible immobilizer for cytokines, hormones and other molecules [17]. Sequestration of P. falciparum infected erythrocytes in the placenta is mediated by binding of specific antigens to adhesion receptor CSA. The binding to the placenta can be inhibited by the presence of antibodies that can block binding of infected erythrocytes to CSA. Primigravidae are exposed to these CSA-binding parasites for the first time and have not built up an effective immune response to it, resulting in placental malaria which can reach high levels of parasitaemia. However, the repeated exposure to CSA-binding parasites during successive pregnancies causes women to eventually acquire immunity to these parasites [18,19] (Fig. 2). Primigravid women lack or have delayed production of antibodies that can block binding of P. falciparum infected erythrocytes to the adhesion receptor CSA. In contrast, multigravid women already possess these antibodies against CSA-binding

parasites as they acquired them over successive pregnancies or acquire them earlier in pregnancy [18], which explains lower susceptibility to placental malaria compared to first pregnancies [18]. Moreover, women with high levels of antibodies against CSAbinding parasites are less likely to develop placental malaria [19]. High levels of antibodies against parasites that bind to CSA have been associated with an increased birth weight of the infant and reduced maternal anemia [20]. Furthermore, lack of adhesion to CD36 by CSA-binding parasites is the cause that these parasites are able to evade non-opsonic phagocytosis by monocytes [19]. Hyaluronic acid (HA) was until recently thought to be another adhesion receptor for P. falciparum infected erythrocytes [21] but convincing recent evidence has shown that parasites do not bind to this molecule [22,23]. Placental parasites express surface antigens that are different compared to those of parasites in non-pregnant hosts. Variant surface antigens (VSA) are selectively transcribed by all malaria parasites. Every parasite constantly changes the transcription of VSA antigens in order to prevent recognition by the hosts immune system. These antigens which are expresses on the surface of the red blood cell belong to the large and diverse P. falciparum erythrocyte membrane 1 (PfEMP1) protein family. PfEMP1 proteins are encoded by approximately 50–60 different var genes in the genome Placental isolates express distinct surface antigens belonging to the PfEMP1 family which has the expected characteristics for VSA in placental malaria. The malarial gene encoding the 350 kDa transmembrane protein VAR2CSA, var2csa, is selectively transcribed by malaria isolates involved in placental malaria, and its antigens are expressed on the surface of the red blood cell. When these VAR2CSA carrying red blood cells encounter the placenta they specifically recognize CSA [24,25]. As a result, VAR2CSA binds to CSA thereby retaining these parasites in the placenta. Various studies are supporting the relevance of VAR2CSA to placental malaria (Fig. 3). This concept is important for understanding immunity against placental malaria. Variant surface antigens are the most important targets for IgG which mediate the gradually developed immunity in reaction to repeated exposure to P. falciparum malaria in nonpregnant individuals in endemic regions [26]. However in primigravidae immunity is not yet developed against placental parasites expressing VAR2CSA. IgG from women exposed to P. falciparum malaria and who had multiple successive pregnancies is able to significantly inhibit adhesion of infected erythrocytes of primigravidae to CSA [27]. As expected, IgG from primigravidae and men from malaria endemic regions is not able to inhibit adhesion of these erythrocytes to CSA Other studies showed that IgG from malaria exposed pregnant women recognizes VSA expressed by infected erythrocytes and

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Fig. 3. The binding of variant surface antigen VAR2CSA to adhesion receptor chondroitin sulphate A. VAR2CSA binds to CSA which is present on the syncytiotrophoblast lining in the intervillous space. VAR2CSA consists of six binding domains (blue and yellow), a transmembrane segment and a cytoplasmic acid terminal segment (light blue) within the infected red blood cell. Binding domains of VAR2CSA that have been shown to bind CSA are yellow. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

inhibits parasite adhesion to CSA in a gravidity dependent manner: i.e. IgG from multigravidae inhibits adhesion of infected erythrocytes to CSA, whereas IgG from primigravidae does not [18]. Recent studies strengthened the theory of pregnancy specific VSA and pregnancy specific IgG. These specific antibodies are not present in primigravidae until week 20 of pregnancy, which might be caused by the immunosuppressive effect of the pregnancy hormone cortisol. The exact influence of cortisol in this matter is not yet understood [2]. Cortisol levels are higher in primigravidae compared to multigravid women and higher in primigravidae with placental malaria compared to uninfected primigravidae. This suggests that suppressed immunity by cortisol in pregnancy has an increasing effect on susceptibility to placental malaria, especially in primigravidae. However, it is also possible that increased cortisol levels reflect the stress response to malaria and are thus a consequence of the disease rather than a cause for the increased susceptibility to the disease [28]. Inactivation of the var2csa gene leads to an impaired ability of infected erythrocytes to adhere to CSA [29]. Importantly, levels of VAR2CSA-specific IgG antibodies in primigravidae, multigravidae and non-pregnant individuals are in line with the expected gravidity dependent and pregnancy specific pattern of VSA in placental malaria [30]. VAR2CSA is only present on the surface of placental infected erythrocytes and is absent on the surface of all other infected erythrocytes [30,31]. In addition, high serum levels of VAR2CSA-specific IgG are associated with protection against adverse pregnancy outcomes caused by placental malaria [30]. These findings suggest that VAR2CSA is the main antigen which mediates placental sequestration of P. falciparum infected erythrocytes. In non-pregnant women sequestration by rosette formation of infected erythrocytes can lead to microvascular obstruction in

organs. This phenomenon is uncommon during pregnancy and rarely found in the placenta [10]. This is probably because VAR2CSA proteins do not form rosettes in contrast to most other PfEMP proteins. The exact mechanism is still poorly understood. 4. Immunity induced processes Placental sequestration of infected erythrocytes stimulates maternal mononuclear cells to secrete chemokines, thereby initiating the inflammatory cascade. Macrophages secrete chemokines in response to malarial hemozoin pigment which is present in malaria infected placentas [13]. Several studies show significant elevated levels in the placenta of three b-chemokines and one achemokine during the infection all attracting mononuclear phagocytes (Table 1): monocyte chemoattractant protein 1 (MCP-1), macrophage-inflammatory protein 1a (MIP-1a), macrophage-inflammatory protein 1b (MIP-1b) and interferon-inducible protein 10 (IP-10) [13,32,33]. b-Chemokines MCP-1 MIP-1a and MIP-1b are secreted by maternal macrophages in response to hemozoin pigment. MIP-1a and MIP-1b recruit additional macrophages to the placenta and MCP-1 plays a significant role in recruiting monocytes [13,32]. Furthermore MCP-1 facilitates adhesion of monocytes to placental endothelial cells [13,32]. IP-10 is chemoattractant for, and expressed and secreted by, monocytes in response to macrophage activating cytokines [33]. b-Chemokine I-309 and the a-chemokine interleukin 8 (IL-8) which act as chemoattractant for monocytes and macrophages are also found in increased levels during placental malaria [32]. I-309 is secreted by T-lymphocytes and induces adhesion to fibrin which may play a role in placental monocyte accumulation and activation [32]. IL-8 is produced by macrophages in response to phagocytosis

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Table 1 Chemokines and cytokines with elevated levels during placental malaria and their function.

Chemokine MCP-1

MIP-1a MIP-1b IP-10 I-309 IL-8

Cytokine IFN-g MIF IL-10 TNF-a

Function during placental malaria

References

Recruitment of monocytes to the placenta Adhesion of monocytes to endothelial cells in the placenta Recruitment of macrophages to the placenta Recruitment of macrophages to the placenta Recruitment of monocytes to the placenta Induce adhesion to fibrin for placental monocyte recruitment and activation Recruitment of macrophages to the placenta Adhesion of monocytes to endothelial cells in the placenta

[13,32]

Enhance phagocytic activity of macrophages Inhibit random migration of macrophages Enhance phagocytic activity of macrophages Suppress Th1 cytokine dominance in the placenta Enhance phagocytic activity of macrophages Generate nitric oxide

[13,36,39] [33,37]

[13,32] [13,32] [33] [32] [32]

[13,36] [13,36,39]

of P. falciparum parasites. Neutrophil granulocytes are the primary target of IL-8, although macrophages can also be recruited. In addition IL-8 is thought to play a role in monocyte adhesion to the placental bed endothelium [32]. In response to secreted a- and b-chemokines, the intervillous space of the placenta is infiltrated by maternal mononuclear phagocytes. Macrophages play a critical role in combating P. falciparum infection by clearing malarial parasites in the placenta. Histological observations have shown that malaria infected placentas contain elevated numbers of macrophages loaded with malarial hemozoin pigment, indicating that these cells have phagocytized malarial parasites [14]. Monocytes also contribute to clearing the placenta of antigens by phagocytosis of malarial parasites and infected erythrocytes [34]. Placental infiltration of mononuclear phagocytes also has adverse consequences. Large numbers of placental monocytes and macrophages are associated with delivery of LBW babies [9]. Chemokines play a role in attracting and facilitating adhesion for monocytes and macrophages in the placenta. Once activated, these monocytes and macrophages produce inflammatory cytokines which aid in the elimination of parasites. However, if this production is left uncontrolled, the resulting large amount of cytokines can damage placental tissues and hamper materno–fetal exchange [9]. 5. Placental cytokine balance During normal pregnancy, the cytokine balance at the materno–fetal interface is dominated by T helper type 2 (Th2) cytokines which causes a down regulation of T helper type 1 (Th1) cytokine responses in the placenta. A placental Th1 cytokine dominance is harmful to both mother and fetus, causing maternal anemia, premature delivery and spontaneous abortion [35]. During

placental malaria, the Th2 cytokine dominance is neutralized by an increase of Th1 cytokines [13,36] that are produced by mononuclear phagocytes that are recruited into the placenta. Th1 cytokines are able to aid in clearing the placenta of malarial parasites and are therefore of parasitological importance. During placental malaria infections levels of pro-inflammatory Th1 cytokines tumor necrosis factor-a (TNF-a) and interferon-g (IFN-g) are elevated (Table 2) [9,13,36]. TNF-a is produced in response to phagocytosis of malarial parasites and the concentration correlates with the density of P. falciparum infected erythrocytes and monocyte/macrophage infiltration in the intervillous space of the placenta [9,13]. TNF-a concentrations are relatively low in peripheral blood compared to concentrations in placental blood with the highest concentrations found in teenage primigravidae. TNF-a aids in elimination of placental parasites by enhancing phagocytic activity of macrophages and by generating nitric oxide which is cytotoxic and cytostatic to blood stage malaria parasites. IFN-g is also found in significant increased levels in the intervillous space of women with placental malaria [13,36]. Besides the production of this cytokine by maternal lymphocytes IFN-g is additionally produced by fetal trophoblasts. The presence of malarial parasites in the placenta triggers trophoblasts to increase IFN-g production, which is known to have a protective function for the fetus against infectious microorganisms that are able to cross the placental barrier [13]. Increased levels of macrophage migration inhibitory factor (MIF) [33,37] are involved in activating macrophages to phagocytize parasites that have infected erythrocytes. The main function of MIF is however to inhibit the random migration of macrophages and thereby retain these phagocytes in the placenta. This function is in line with the characteristic placental accumulation of macrophages in the intervillous space [33,37]. Th2 cytokine interleukin 10 (IL-10) is also upregulated in malaria infected placentas: [13,36] and is also elevated in peripheral blood of women with placental malaria [38]. IL10 production is probably increased because of its immunoregulatory effect; IL-10 suppresses the Th1 dominance in the placenta [13]. Nevertheless, Th1 dominance still prevails during placental malaria. 6. Placental malaria: pathology Pathological changes of the placenta characterized by parasitized and loss of syncytial microvilli, intra-syncytial pigment accumulation and necrosis, thickening of the TBM, physiological changes of the placenta and accumulation of infected erythrocytes in the placenta, cause a reduction of the intervillous area exposed to maternal blood and can have negative effects on placental blood flow [14]. Consequently, materno–fetal exchanges are impaired and the nutritional ability of the placenta is diminished. On immunological level the pathological consequences of placental malaria are mainly caused by the increased placental concentrations of pro-inflammatory cytokines in response to the sequestration of malarial parasites in the placenta [35,36] (Table 2).

Table 2 Adverse pregnancy outcomes and the function of involved cytokines. Pregnancy outcome

Involved cytokine

Cytokine’s role leading to adverse pregnancy outcome

References

Fetal growth restriction Preterm delivery Spontaneous abortion

TNF-a IL-10 TNF-a IFN-g

[8,13,36,39] [40] [7,8]

Maternal anemia

TNF-a

Impair materno–fetal exchanges by damaging host tissues Contribute to anemia resulting in preterm delivery Cause necrosis of the implanted fetus Increase the risk of uterine contraction Induce and activate abortion involved natural killer cells Inflicts oxidative stress on erythrocyte membranes Suppression of erythropoiesis

[13,36,39]

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Elevated levels of TNF-a are associated with LBW and high concentrations of this cytokine damages host tissue and may cause fetal growth restriction [39]. Preterm delivery is associated with high levels of IL-10 in the placenta [40]. IL-10 is implicated in the pathogenesis of anemia, and is associated with protection of children against malaria, [41,42]. However, because anemia and preterm delivery are associated, high levels of IL-10 indirectly contribute to preterm delivery. Maternal anemia associated with placental malaria correlates with the accumulation of pigmented monocytes and macrophages in the placenta [9]. The oxidative stress that the resulting high levels of TNF-a inflicts are known to alter erythrocyte membranes, which leads to an increased destruction of these erythrocytes and consequently resulting in anaemia [43]. In addition TNF-a is associated with the suppression of erythropoiesis which in consequence also leads to increased anaemia [42]. Spontaneous abortion has also been associated with high levels of Th1 cytokines in the placenta infected with malaria. TNF-a causes fetal necrosis and increases the risk of uterine contraction resulting in fetal expulsion [39]. Furthermore, IFN-g, can induce and activate natural killer cells, that are found in significantly increased levels in peripheral blood of women who had repeated spontaneous abortion, suggesting that these cells, induced and activated by IFN-g, are involved in premature fetal expulsion [8]. 7. Step by step model of the interactions and events in placental malaria Events leading to adverse outcomes of placental malaria can be summarized in four steps: (1) accumulation of P. falciparum infected erythrocytes; (2) infiltration of monocytes and macrophages; (3) alteration of the placental cytokine balance; (4) pathogenesis of adverse pregnancy outcomes. Accumulation of P. falciparum infected erythrocytes 1. P. falciparum parasites in infected erythrocytes encounter the intervillous space of the placenta after their passage through the liver [14]. 2. In the presence of the placental adhesion receptor chondroitin sulfate A, the parasites that express the variant surface antigen VAR2CSA on the surface of the erythrocytes are retained in the placenta [30]. 3. A. IgG antibodies of malaria immune primigravidae are not able to recognize the VAR2CSA antigen, because this pregnancy specific antigen differs from malaria antigens expressed in non-pregnant hosts. Primigravidae are therefore more susceptible to placental malaria [27]. B. Elevated levels of immunosuppressive pregnancy hormone cortisol increase susceptibility in primigravidae [2]. 4. The expressed VAR2CSA on the surface of infected erythrocytes binds to CSA which is present on syncytiotrophoblasts of the placenta [12]. 5. Parasites multiply and infect new erythrocytes causing the placenta to be accumulated by infected erythrocytes [10]. Infiltration of monocytes and macrophages 6. In response to infected erythrocytes in the placenta, maternal mononuclear cells and lymphocytes secrete chemokines [14]. 7. Chemokine levels (MCP-1, IP-10, I-309 MIP-1a, MIP-1b and IL8) are raised in the placenta and attract monocytes and macrophages [13,32]. 8. In response to chemokines, maternal monocytes and macrophages infiltrate the placenta [14].

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Alteration of placental cytokine balance 9. Monocytes and macrophages in the placenta combat malaria parasites by phagocytosis and by secreting cytokines [14,34]. 10. A. Levels of pro-inflammatory Th1 cytokines TNF-a and IFN-g are raised aiding elimination of parasites by enhancing phagocytic activity of macrophages and by inflicting oxidative stress [13,36,39]. B. Cytokine levels of MIF are raised inhibiting random migration and enhancing phagocytic activity of macrophages [33,37]. 11. Elevated levels of Th1 cytokines cause a shift of the placental cytokine balance toward a Th1 profile [36]. 12. In response to placental Th1 dominance, levels of Th2 cytokine IL-10 are raised serving as a negative feedback mechanism to the placental Th1 cytokine dominance [13,36].

Pathogenesis of adverse pregnancy outcomes 13. A. Accumulation of P. falciparum infected erythrocytes, monocytes and macrophages in the placenta causes reduction of the intervillous area exposed to maternal blood harming placental blood flow and impairing materno–fetal exchanges [8]. B. Fetal growth restriction might be caused by elevated levels of TNF-a which impair materno–fetal exchanges by damaging placental host tissues [39]. C. Elevated levels of IL-10 contribute to anemia resulting in preterm delivery [40]. D. Elevated levels of TNF-a and IFN-g result in spontaneous abortion by causing fetal necrosis, influencing uterine contraction and inducing and activating abortion associated natural killer cells [7,8]. E. Elevated levels of TNF-a causes maternal anemia by inflicting oxidative stress on erythrocyte membranes, leading to an increased destruction of these erythrocytes [9,42,43] or by its inhibitory effects on erythropoiesis.

8. Discussion Fetal growth restriction, preterm delivery and spontaneous abortion are all associated with placental malaria [6–8]. Studies have shown that these adverse consequences for the child are associated with elevated cytokines levels in the placenta and blocking materno–fetal exchange. Although the step by step model sketches a fairly complete picture several unclear issues remain. Knowledge about the cytokines’ role in the pathogenesis of the adverse consequences is only hypothetical and should be studied further. The immunological nature of placental malaria offers the possibility of treatment with immunotherapy. For the development of this therapy, and other intervention methods to prevent pathological outcomes, it is important to have a greater understanding of the relation between placental immunopathology, the possible roles of associated cytokines and the pathogenesis of adverse pregnancy outcomes. Future research will not only complete the model but could ultimately lead to specific interventions reducing the adverse outcomes for mother and child. Acknowledgments We would like to thank Prof. Dr. Paul Klatser of the Royal Tropical Institute, Biomedical Research Amsterdam, The Netherlands for his guidance during the initial phase of the study.

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References [1] WHO. The African Malaria Report 2003. Geneva: WHO; 2004. pp. 17–23. [2] Bayoumi NK, Elhassan EM, Elbashir MI, Adam I. Cortisol, prolactin, cytokines and the susceptibility of pregnant Sudanese women to Plasmodium falciparum malaria. Ann Trop Med Parasitol 2009;103:111–7. [3] Mount AM, Mwapasa V, Elliott SR, et al. Impairment of humoral immunity to Plasmodium falciparum malaria in pregnancy by HIV infection. Lancet 2004;363:1860–7. [4] Steketee RW, Nahlen BL, Parise ME, Menendez C. The burden of malaria in pregnancy in malaria-endemic areas. Am J Trop Med Hyg 2001;64:28–35. [5] Brabin BJ. An analysis of malaria in pregnancy in Africa. Bull WHO 1983;61:1005–16. [6] Kalanda BF, Verhoeff FH, Chimsuku L, Harper G, Brabin BJ. Adverse birth outcomes in a malarious area. Epidemiol Infect 2006;134:659–66. [7] Calleja-Agius J, Brincat MP. Recurrent miscarriages: what is the role of cytokines? Gynecol Endocrinol 2008;24:663–8. [8] Okoko BJ, Enwere G, Ota MO. The epidemiology and consequences of maternal malaria: a review of immunological basis. Acta Tropica 2003;87:193–205. [9] Rogerson SJ, Pollina E, Getachew A, Tadesse E, Lema VM, Molyneux ME. Placental monocyte infiltrates in response to Plasmodium falciparum malaria infection and their association with adverse pregnancy outcomes. Am J Trop Med Hyg 2003;68:115–9. [10] Uneke CJ. Impact of placental Plasmodium falciparum malaria on pregnancy and perinatal outcome in sub-Saharan Africa: I: introduction to placental malaria. Yale J Biol Me 2007;80:39–50. [11] Beeson JG, Amin N, Kanjala M, Rogerson SJ. Selective accumulation of mature asexual stages of Plasmodium falciparum-infected erythrocytes in the placenta. Infect Immun 2002;70:5412–5. [12] Fried M, Duffy PE. Maternal malaria and parasite adhesion. J Mol Med 1998;76:162–71. [13] Suguitan Jr AL, Leke RG, Fouda G, et al. Changes in the levels of chemokines and cytokines in the placentas of women with Plasmodium falciparum malaria. J Infect Dis 2003;188:1074–82. [14] Ismail MR, Ordi J, Menendez C, et al. Placental pathology in malaria: a histological, immunohistochemical, and quantitative study. Hum Pathol 2000;31:85–93. [15] Gamain B, Trimnell AR, Scheidig C, Scherf A, Miller LH, Smith JD. Identification of multiple chondroitin sulfate A (CSA)-binding domains in the var2CSA gene transcribed in CSA-binding parasites. J Infect Dis 2005;191:1010–3. [16] McGilvray ID, Serghides L, Kapus A. Nonopsonic monocyte/macrophage phagocytosis of Plasmodium falciparum-parasitized erythrocytes: a role for CD36 in malarial clearance. Blood 2000;96:3231–40. [17] Rogerson S, Hviid L, Duffy P, Leke R, Taylor D. Malaria in pregnancy: pathogenesis and immunity. Lancet Infect Dis 2007;2:105–17. [18] O’Neil-Dunne I, Achur RN, Agbor-Enoh ST, et al. Gravidity-dependent production of antibodies that inhibit binding of Plasmodium falciparum-infected erythrocytes to placental chondroitin sulfate proteoglycan during pregnancy. Infect Immun 2001;69:7487–92. [19] Taylor DW, Zhou A, Marsillio LE, et al. Antibodies that inhibit binding of Plasmodium falciparum-infected erythrocytes to chondroitin sulfate A and to the C terminus of merozoite surface protein 1 correlate with reduced placental malaria in Cameroonian women. Infect Immun 2004;72:1603–7. [20] Staalsoe T, Shulman CE, Bulmer JN, Kawuondo K, Marsh K, Hviid L. Variant surface antigen-specific IgG and protection against clinical consequences of pregnancy-associated Plasmodium falciparum malaria. Lancet 2004;363: 283–9. [21] Beeson JG, Rogerson SJ, Cooke BM, et al. Adhesion of Plasmodium falciparuminfected erythrocytes to hyaluronic acid in placental malaria. Nat Med 2000;6:86–90. [22] Fried M, Domingo GJ, Gowda CD, et al. Plasmodium falciparum: chondroitin sulfate A is the major receptor for adhesion of parasitized erythrocytes in the placenta. Exp Parasitoy 2006;113:36–42.

[23] Muthusamy A, Achur RN, Valiyaveettil M, et al. Chondroitin sulfate proteoglycan but not hyaluronic acid is the receptor for the adherence of Plasmodium falciparum-infected erythrocytes in human placenta, and infected red blood cell adherence up-regulates the receptor expression. Am J Pathol 2007 Jun;170(6):1989–2000. [24] Tuikue Ndam NG, Salanti A, Bertin G, et al. High level of var2csa transcription by Plasmodium falciparum isolated from the placenta. J Infect Dis 2005;192:331–5. [25] Salanti A, Staalsoe T, Lavstsen T, et al. Selective upregulation of a single distinctly structured var gene in CSA-adhering Plasmodium falciparum involved in pregnancy-associated malaria. Mol Microbiol 2003;49: 179–91. [26] Nielsen MA, Staalsoe T, Kurtzhals JA, et al. Plasmodium falciparum variant surface antigen expression varies between isolates causing severe and nonsevere malaria and is modified by acquired immunity. J Immunol 2002; 168:3444–50. [27] Hviid L, Salanti A. VAR2CSA and protective immunity against pregnancyassociated Plasmodium falciparum malaria. Parasitol 2007;134:1871–6. [28] Bouyou-Akotet MK, Adegnika AA, Agnandji ST, et al. Cortisol and susceptibility to malaria during pregnancy. Microbes Infect 2005;7:1217–23. [29] Duffy MF, Maier AG, Byrne TJ, et al. VAR2CSA is the principal ligand for chondroitin sulfate A in two allogeneic isolates of Plasmodium falciparum. Mol Biochem Parasitol 2006;148:117–24. [30] Salanti A, Dahlba¨ck M, Turner L, et al. Evidence for the involvement of VAR2CSA in pregnancy-associated malaria. J Exp Med 2004;200:1197–203. [31] Magistrado P, Salanti A, Tuikue Ndam NG, et al. VAR2CSA expression on the surface of placenta-derived Plasmodium falciparum-infected erythrocytes. J Infect Dis 2008;198:1071–4. [32] Abrams ET, Brown H, Chensue SW, et al. Host response to malaria during pregnancy: placental monocyte recruitment is associated with elevated beta chemokine expression. J Immunol 2003;170(5):2759–64. [33] Chaisavaneeyakorn S, Moore JM, Othoro C, et al. Immunity to placental malaria. IV. Placental malaria is associated with up-regulation of macrophage migration inhibitory factor in intervillous blood. J Infect Dis 2002;186:1371–5. [34] Khusmith S, Druilhe P, Gentilini M. Enhanced Plasmodium falciparum merozoite phagocytosis by monocytes from immune individuals. Infect Immun 1982;35:874–9. [35] Kwak-Kim JY, Gilman-Sachs A, Kim CE. T helper 1 and 2 immune responses in relationship to pregnancy, nonpregnancy, recurrent spontaneous abortions and infertility of repeated implantation failures. Chem Immunol Allergy 2005;88:64–79. [36] Fievet N, Moussa M, Tami G, et al. Plasmodium falciparum induces a Th1/Th2 disequilibrium, favoring the Th1 type pathway, in the human placenta. J Infect Dis 2001;183:1530–4. [37] Chaisavaneeyakorn S, Lucchi N, Abramowsky C, et al. Immunohistological characterization of macrophage migration inhibitory factor expression in Plasmodium falciparum-infected placentas. Infect Immun 2005;73: 3287–3293. [38] Kabyemela ER, Muehlenbachs A, Fried M, et al. Maternal peripheral blood level of IL-10 as a marker for inflammatory placental malaria. Malaria J 2008;29:26. [39] Rogerson SJ, Brown HC, Pollina E, et al. Placental tumor necrosis factor alpha but not gamma interferon is associated with placental malaria and low birth weight in Malawian women. Infect Immun 2003;71267–70. [40] Suguitan AL, Cadigan TJ, Nguyen TA, et al. Malaria-associated cytokine changes in the placenta of women with pre-term deliveries in Yaounde, Cameroon. Am J Trop Med Hyg 2003;69:574–81. [41] Tilg H, Ulmer H, Kaser A, Weiss G. Role of IL-10 for induction of anemia during inflammation. J Immunol 2002;169:2204–9. [42] Mendez C, Flemming AF, Alonso PL. Malaria related anaemia. Parasitol Today 2006;16:469–76. [43] Becker K, Tilley L, Vennerstrom JL, et al. Oxidative stress in malaria parasiteinfected erythrocytes: host–parasite interactions. Int J Parasitol 2004;34: 163–89.