Plasmodium falciparum-infected erythrocytes adhere both in the intervillous space and on the villous surface of human placenta by binding to the low-sulfated chondroitin sulfate proteoglycan receptor

Acute response to pathogens in the early human placenta at single-cell resolution

The placenta is a selective maternal-fetal barrier that provides nourishment and protection from infections. However, certain pathogens can attach to and even cross the placenta, causing pregnancy complications with potential lifelong impacts on the child's health. Here, we profiled at the single-cell level the placental responses to three pathogens associated with intrauterine complications-Plasmodium falciparum, Listeria monocytogenes, and Toxoplasma gondii. We found that upon exposure to the pathogens, all placental lineages trigger inflammatory responses that may compromise placental function. Additionally, we characterized the responses of fetal macrophages known as Hofbauer cells (HBCs) to each pathogen and propose that they are the probable niche for T. gondii. Finally, we revealed how P. falciparum adapts to the placental microenvironment by modulating protein export into the host erythrocyte and nutrient uptake pathways. Altogether, we have defined the cellular networks and signaling pathways mediating acute placental inflammatory responses that could contribute to pregnancy complications.

A model of pregnancy-associated malaria for inducing adverse pregnancy outcomes in ICR mouse

BACKGROUND

Based on understanding of placental pathological features and safe medication in pregnancy-associated malaria (PAM), establishment of a stable pregnant mouse infection model with Plasmodium was urgently needed.

METHODS

ICR mice with vaginal plugs detected were randomly divided into post-pregnancy infection (Malaria+) and uninfected pregnancy (Malaria-) cohorts. Age-matched mice that had not been mated were infected as pre-pregnancy infection group (Virgin control), which were subsequently mated with ICR males. All mice were inoculated with 1 × 106Plasmodium berghei ANKA-infected RBCs by intraperitoneal injection, and the same amount of saline was given to Malaria- group. We recorded the incidence of adverse pregnancy outcomes and the amounts of offspring in each group.

RESULTS

The Virgin group mice were unable to conceive normally, and vaginal bleeding, abortion, or stillbirth appeared in the Malaria+ group. The incidence of adverse pregnancy outcomes was extremely high and statistically significant compared with the control (Malaria-) group (P < 0.05), of which placenta exhibited pathological features associated with human gestational malaria.

CONCLUSIONS

The intraperitoneal injection of 1 × 106Plasmodium berghei ANKA-infected RBCs could establish a model of pregnancy-associated malaria in ICR mouse.

A conserved epitope in VAR2CSA is targeted by a cross-reactive antibody originating from Plasmodium vivax Duffy binding protein

During Plasmodium falciparum infection in pregnancy, VAR2CSA is expressed on the surface of infected erythrocytes (IEs) and mediates their sequestration in the placenta. As a result, antibodies to VAR2CSA are largely restricted to women who were infected during pregnancy. However, we discovered that VAR2CSA antibodies can also be elicited by P. vivax Duffy binding protein (PvDBP). We proposed that infection with P. vivax in non-pregnant individuals can generate antibodies that cross-react with VAR2CSA. To better understand the specificity of these antibodies, we took advantage of a mouse monoclonal antibody (3D10) raised against PvDBP that cross-reacts with VAR2CSA and identified the epitopes targeted by this antibody. We screened two peptide arrays that span the ectodomain of VAR2CSA from the FCR3 and NF54 alleles. Based on the top epitope recognized by 3D10, we designed a 34-amino acid synthetic peptide, which we call CRP1, that maps to a highly conserved region in DBL3X. Specific lysine residues are critical for 3D10 recognition, and these same amino acids are within a previously defined chondroitin sulfate A (CSA) binding site in DBL3X. We showed by isothermal titration calorimetry that the CRP1 peptide can bind directly to CSA, and antibodies to CRP1 raised in rats significantly blocked the binding of IEs to CSA in vitro. In our Colombian cohorts of pregnant and non-pregnant individuals, at least 45% were seroreactive to CRP1. Antibody reactivities to CRP1 and the 3D10 natural epitope in PvDBP region II, subdomain 1 (SD1), were strongly correlated in both cohorts. These findings suggest that antibodies arising from PvDBP may cross-react with VAR2CSA through the epitope in CRP1 and that CRP1 could be a potential vaccine candidate to target a distinct CSA binding site in VAR2CSA.

Congenital Transmission of Apicomplexan Parasites: A Review

Apicomplexans are a group of pathogenic protists that cause various diseases in humans and animals that cause economic losses worldwide. These unicellular eukaryotes are characterized by having a complex life cycle and the ability to evade the immune system of their host organism. Infections caused by some of these parasites affect millions of pregnant women worldwide, leading to various adverse maternal and fetal/placental effects. Unfortunately, the exact pathogenesis of congenital apicomplexan diseases is far from being understood, including the mechanisms of how they cross the placental barrier. In this review, we highlight important aspects of the diseases caused by species of Plasmodium, Babesia, Toxoplasma, and Neospora, their infection during pregnancy, emphasizing the possible role played by the placenta in the host-pathogen interaction.

Promising nanomaterials in the fight against malaria

For more than one hundred years, several treatments against malaria have been proposed but they have systematically failed, mainly due to the occurrence of drug resistance in part resulting from the exposure of the parasite to low drug doses. Several factors are behind this problem, including (i) the formidable barrier imposed by the Plasmodium life cycle with intracellular localization of parasites in hepatocytes and red blood cells, (ii) the adverse fluidic conditions encountered in the blood circulation that affect the interaction of molecular components with target cells, and (iii) the unfavorable physicochemical characteristics of most antimalarial drugs, which have an amphiphilic character and can be widely distributed into body tissues after administration and rapidly metabolized in the liver. To surpass these drawbacks, rather than focusing all efforts on discovering new drugs whose efficacy is quickly decreased by the parasite's evolution of resistance, the development of effective drug delivery carriers is a promising strategy. Nanomaterials have been investigated for their capacity to effectively deliver antimalarial drugs at local doses sufficiently high to kill the parasites and avoid drug resistance evolution, while maintaining a low overall dose to prevent undesirable toxic side effects. In recent years, several nanostructured systems such as liposomes, polymeric nanoparticles or dendrimers have been shown to be capable of improving the efficacy of antimalarial therapies. In this respect, nanomaterials are a promising drug delivery vehicle and can be used in therapeutic strategies designed to fight the parasite both in humans and in the mosquito vector of the disease. The chemical analyses of these nanomaterials are essential for the proposal and development of effective anti-malaria therapies. This review is intended to analyze the application of nanomaterials to improve the drug efficacy on different stages of the malaria parasites in both the human and mosquito hosts.

A novel murine model for assessing fetal and birth outcomes following transgestational maternal malaria infection

Plasmodium falciparum infection during pregnancy is a major cause of severe maternal illness and neonatal mortality. Mouse models are important for the study of gestational malaria pathogenesis. When infected with Plasmodium chabaudi chabaudi AS in early gestation, several inbred mouse strains abort at midgestation. We report here that outbred Swiss Webster mice infected with P. chabaudi chabaudi AS in early gestation carry their pregnancies to term despite high parasite burden and malarial hemozoin accumulation in the placenta at midgestation, with the latter associated with induction of heme oxygenase 1 expression. Infection yields reduced fetal weight and viability at term and a reduction in pup number at weaning, but does not influence postnatal growth prior to weaning. This novel model allows for the exploration of malaria infection throughout pregnancy, modeling chronic infections observed in pregnant women prior to the birth of underweight infants and enabling the production of progeny exposed to malaria in utero, which is critical for understanding the postnatal repercussions of gestational malaria. The use of outbred mice allows for the exploration of gestational malaria in a genetically diverse model system, better recapitulating the diversity of infection responses observed in human populations.

Contribution of Murine Models to the Study of Malaria During Pregnancy

Annually, many pregnancies occur in areas of Plasmodium spp. transmission, particularly in underdeveloped countries with widespread poverty. Estimations have suggested that several million women are at risk of developing malaria during pregnancy. In particular cases, systemic infection caused by Plasmodium spp. may extend to the placenta, dysregulating local homeostasis and promoting the onset of placental malaria; these processes are often associated with increased maternal and fetal mortality, intrauterine growth restriction, preterm delivery, and reduced birth weight. The endeavor to understand and characterize the mechanisms underlying disease onset and placental pathology face several ethical and logistical obstacles due to explicit difficulties in assessing human gestation and biological material. Consequently, the advent of murine experimental models for the study of malaria during pregnancy has substantially contributed to our understanding of this complex pathology. Herein, we summarize research conducted during recent decades using murine models of malaria during pregnancy and highlight the most relevant findings, as well as discuss similarities to humans and the translational capacity of achieved results.

Composition of the gut microbiota transcends genetic determinants of malaria infection severity and influences pregnancy outcome

BACKGROUND

Malaria infection in pregnancy is a major cause of maternal and foetal morbidity and mortality worldwide. Mouse models for gestational malaria allow for the exploration of the mechanisms linking maternal malaria infection and poor pregnancy outcomes in a tractable model system. The composition of the gut microbiota has been shown to influence susceptibility to malaria infection in inbred virgin mice. In this study, we explore the ability of the gut microbiota to modulate malaria infection severity in pregnant outbred Swiss Webster mice.

METHODS

In Swiss Webster mice, the composition of the gut microbiota was altered by disrupting the native gut microbes through broad-spectrum antibiotic treatment, followed by the administration of a faecal microbiota transplant derived from mice possessing gut microbes reported previously to confer susceptibility or resistance to malaria. Female mice were infected with P. chabaudi chabaudi AS in early gestation, and the progression of infection and pregnancy were tracked throughout gestation. To assess the impact of maternal infection on foetal outcomes, dams were sacrificed at term to assess foetal size and viability. Alternatively, pups were delivered by caesarean section and fostered to assess neonatal survival and pre-weaning growth in the absence of maternal morbidity. A group of dams was also euthanized at mid-gestation to assess infection and pregnancy outcomes.

FINDINGS

Susceptibility to infection varied significantly as a function of source of transplanted gut microbes. Parasite burden was negatively correlated with the abundance of five specific OTUs, including Akkermansia muciniphila and OTUs classified as Allobaculum, Lactobacillus, and S24-7 species. Reduced parasite burden was associated with reduced maternal morbidity and improved pregnancy outcomes. Pups produced by dams with high parasite burdens displayed a significant reduction in survival in the first days of life relative to those from malaria-resistant dams when placed with foster dams. At midgestation, plasma cytokine levels were similar across all groups, but expression of IFNγ in the conceptus was elevated in infected dams, and IL-10 only in susceptible dams. In the latter, transcriptional and microscopic evidence of monocytic infiltration was observed with high density infection; likewise, accumulation of malaria haemozoin was enhanced in this group. These responses, combined with reduced vascularization of the placenta in this group, may contribute to poor pregnancy outcomes. Thus, high maternal parasite burden and associated maternal responses, potentially dictated by the gut microbial community, negatively impacts term foetal health and survival in the early postnatal period.

INTERPRETATION

The composition of the gut microbiota in Plasmodium chabaudi chabaudi AS-infected pregnant Swiss Webster mice transcends the outbred genetics of the Swiss Webster mouse stock as a determinant of malaria infection severity, subsequently influencing pregnancy outcomes in malaria-exposed progeny. FUND: Research reported in this manuscript was supported by the University of Florida College of Veterinary Medicine (JMM, MM, and MG), the National Institute of Allergy and Infectious Diseases, the National Institute of Diabetes and Digestive and Kidney Diseases, and the Eunice Kennedy Shriver National Institute of Child Health and Human Development of the National Institutes of Health under award numbers T32AI060546 (to CDMS), R01HD46860 and R21AI111242 (to JMM), and R01 DK109560 (to MM). MG was supported by Department of Infectious Diseases and Immunology and University of Florida graduate assistantships. AA was supported by the 2017-2019 Peach State LSAMP Bridge to the Doctorate Program at the University of Georgia (National Science Foundation, Award # 1702361). The content is solely the responsibility of the authors and does not necessarily represent official views of the Eunice Kennedy Shriver National Institute of Child Health and Human Development, the National Institute of Allergy and Infectious Diseases, the National Institute of Diabetes and Digestive and Kidney Diseases, or the National Institutes of Health.

Genesis of placental sequestration in malaria and possible targets for drugs for placental malaria

Malaria during pregnancy results in intrauterine growth restriction, fetal anemia, and infant mortality. Women are more susceptible to malaria during pregnancy due to malaria‐induced inflammation and the sequestration of infected red blood cells in the placenta, which bind to the chondroitin sulfate portion of syndecan‐1 on the syncytiotrophoblast and in the intervillous space. Syndecan‐1 is a dimeric proteoglycan with an extracellular ectodomain that is cleaved from the transmembrane domain (referred to as “shedding”) by matrix metalloproteinases (MMPs), likely the secreted MMP‐9. The ectodomain includes four binding sites for chondroitin sulfate, which are proximal to the transmembrane domain, and six distal binding sites primarily for heparan sulfate. This “shedding” of syndecan‐1 is inhibited by the presence of the heparan sulfate chains, which can be removed by heparanase. The intervillous space contains fibrin strands and syndecan‐1 ectodomains free of heparan sulfate. The following is proposed as the sequence of events that leads to and is primarily responsible for sequestration in the intervillous space of the placenta. Inflammation associated with malaria triggers increased heparanase activity that degrades the heparan sulfate on the membrane‐bound syndecan‐1. Inflammation also upregulates MMP‐9 and the removal of heparan sulfate gives MMP‐9 access to cleave syndecan‐1, thereby releasing dimeric syndecan‐1 ectodomains with at least four chondroitin sulfate chains attached. These multivalent ectodomains bind infected RBCs together leading to their aggregation and entrapment in intervillous fibrin. This mechanism suggests possible new targets for anti‐placental malaria drugs such as the inhibition of MMP‐9. Doxycycline is an antimalarial drug which inhibits MMP‐9.

Molecular Principles of Intrauterine Growth Restriction in Plasmodium Falciparum Infection

Malaria in pregnancy still constitutes a particular medical challenge in tropical and subtropical regions. Of the five Plasmodium species that are pathogenic to humans, infection with Plasmodium falciparum leads to fulminant progression of the disease with massive impact on pregnancy. Severe anemia of the mother, miscarriage, stillbirth, preterm delivery and intrauterine growth restriction (IUGR) with reduced birth weight are frequent complications that lead to more than 10,000 maternal and 200,000 perinatal deaths annually in sub-Saharan Africa alone. P. falciparum can adhere to the placenta via the expression of the surface antigen VAR2CSA, which leads to sequestration of infected erythrocytes in the intervillous space. This process induces a placental inflammation with involvement of immune cells and humoral factors. Especially, monocytes get activated and change the release of soluble mediators, including a variety of cytokines. This proinflammatory environment contributes to disorders of angiogenesis, blood flow, autophagy, and nutrient transport in the placenta and erythropoiesis. Collectively, they impair placental functions and, consequently, fetal growth. The discovery that women in endemic regions develop a certain immunity against VAR2CSA-expressing parasites with increasing number of pregnancies has redefined the understanding of malaria in pregnancy and offers strategies for the development of vaccines. The following review gives an overview of molecular processes in P. falciparum infection in pregnancy which may be involved in the development of IUGR.

Submicroscopic placental infection by non-falciparum Plasmodium spp

BACKGROUND

Among the Plasmodium species that infect humans, adverse effects of P. falciparum and P. vivax have been extensively studied and reported with respect to poor outcomes particularly in first time mothers and in pregnant women living in areas with unstable malaria transmission. Although, other non-falciparum malaria infections during pregnancy have sometimes been reported, little is known about the dynamics of these infections during pregnancy.

METHODS AND FINDINGS

Using a quantitative PCR approach, blood samples collected from Beninese pregnant women during the first antenatal visit (ANV) and at delivery including placental blood were screened for Plasmodium spp. Risk factors associated with Plasmodium spp. infection during pregnancy were assessed as well as the relationships with pregnancy outcomes. P. falciparum was the most prevalent Plasmodium species detected during pregnancy, irrespective either of parity, of age or of season during which the infection occurred. Although no P. vivax infections were detected in this cohort, P. malariae (9.2%) and P. ovale (5.8%) infections were observed in samples collected during the first ANV. These non-falciparum infections were also detected in maternal peripheral blood (1.3% for P. malariae and 1.2% for P. ovale) at delivery. Importantly, higher prevalence of P. malariae (5.5%) was observed in placental than peripheral blood while that of P. ovale was similar (1.8% in placental blood). Among the non-falciparum infected pregnant women with paired peripheral and placental samples, P. malariae infections in the placental blood was significantly higher than in the peripheral blood, suggesting a possible affinity of P. malariae for the placenta. However, no assoctiation of non-falciparum infections and the pregnancy outcomes was observed.

CONCLUSIONS

Overall this study provided insights into the molecular epidemiology of Plasmodium spp. infection during pregnancy, indicating placental infection by non-falciparum Plasmodium and the lack of association of these infections with adverse pregnancy outcomes.

Placental malaria and its effect on pregnancy outcomes in Sudanese women from Blue Nile State

BACKGROUND

Malaria infection during pregnancy can result in placental malaria and is associated with adverse pregnancy outcomes particularly among primigravidae. The aim of this study was to assess the prevalence and risk factors for placental malaria and its effect on pregnancy outcomes in Blue Nile state, Sudan.

METHODS

A cross-sectional hospital-based study was conducted consecutively during January 2012-December 2015 in three main hospitals in Blue Nile State, Sudan. At delivery, peripheral and placental blood samples were collected from consenting women. Finger prick blood was used for preparation of peripheral smears and for haemoglobin measurement. Smears were stained with Giemsa and examined microscopically for malaria parasites. Pregnancy outcomes in association to placental malaria were investigated.

RESULTS

A total of 1149 mothers and their newborns were recruited. The mean (SD) of the age was 23.3 (5.2) years. Detection of malaria parasites was confirmed in 37.8% of the peripheral blood films and 59.3% of the placental films with Plasmodium falciparum as the only species detected. In multivariate analysis, younger age ≤23.2 years old (AOR = 3.2, 95% CI 1.9-5.5; P < 0.001), primiparae (AOR = 3.9, CI 2.1-7.6; P < 0.001), secundiparae (AOR = 2.8, 95% CI 1.5-5.1; P < 0.001, no antenatal care (ANC) visits (AOR = 11.9, 95% CI 7.8-18.1; P < 0.001) and not using bed nets (AOR = 3.5, 95% CI 1.7-6.8; P < 0.001) were risk factors for placental malaria. Education and residence were not associated with placental malaria infection. Placental malaria was significantly associated with maternal anaemia (AOR = 41.6, 95% CI 23.3-74.4; P < 0.001) and low birth weight (LBW) (AOR = 25.2, 95% CI 15.1-41.3; P < 0.001).

CONCLUSION

During the study, there was a high prevalence of placental malaria in Blue Nile State-Sudan, as the enhanced control activities were not practiced, leading to adverse pregnancy outcomes, such as maternal anaemia and LBW.

Genetic analysis of ID1-DBL2X predicts its validity as a vaccine candidate in Colombia and supports at least two independently introduced Plasmodium falciparum populations in the region

Pregnancy-associated malaria (PAM) poses a threat to both the mother and fetus, increasing the risk of severe maternal anemia, fetal growth restriction and low birth weight infants. Two vaccines are currently in development to protect women from Plasmodium falciparum in pregnancy. Both vaccine constructs target the ID1-DBL2X domain of VAR2CSA, a protein expressed on the surface of infected erythrocytes (IEs) that mediates parasite sequestration in the placenta. Although development of an effective vaccine may be hampered by ID1-DBL2X polymorphisms expressed by field isolates, a recent study showed that genetic variation of this domain in South American parasite populations is much lower than in other geographical locations. This suggests that a recombinant vaccine designed to be efficacious in Africa and Asia is likely to be efficacious in South America. However, these studies did not include Colombian parasite populations in their analyses, which are known to be genetically distinct from other South American parasite populations due to their independent introduction from Africa. Therefore, we sought to determine the genetic variation of the ID1-DBL2X domain in Colombian parasites to assess the potential efficacy of the vaccine against PAM in this region. Through sequence analysis and population genetics, we show that there is a low degree of genetic variation amongst Colombian parasite populations and that a vaccine containing conserved antigen variants for worldwide populations is likely to be protective against PAM in Colombia. Our analysis also points towards an African origin for Colombian parasite populations, and suggests that their introduction into Colombia was a recurrent process encompassing multiple introduction events.

Pre-clinical and clinical development of the first placental malaria vaccine

INTRODUCTION

Malaria during pregnancy is a massive health problem in endemic areas. Placental malaria infections caused by Plasmodium falciparum are responsible for up to one million babies being born with a low birth weight every year. Significant efforts have been invested into preventing the condition. Areas covered: Pub Med was searched using the broad terms 'malaria parasite placenta' to identify studies of interactions between parasite and host, 'prevention of placental malaria' to identify current strategies to prevent placental malaria, and 'placental malaria vaccine' to identify pre-clinical vaccine development. However, all papers from these searches were not systematically included. Expert commentary: The first phase I clinical trials of vaccines are well underway. Trials testing efficacy are more complicated to carry out as only women that are exposed to parasites during pregnancy will contribute to endpoint measurements, further it may require extensive follow-up to establish protection. Future second generation vaccines may overcome the inherent challenges in making an effective placental malaria vaccine.

Comparison of functional assays used in the clinical development of a placental malaria vaccine

BACKGROUND

Malaria in pregnancy is associated with significant morbidity in pregnant women and their offspring. Plasmodium falciparum infected erythrocytes (IE) express VAR2CSA that mediates binding to chondroitin sulphate A (CSA) in the placenta. Two VAR2CSA-based vaccines for placental malaria are in clinical development. The purpose of this study was to evaluate the robustness and comparability of binding inhibition assays used in the clinical development of placental malaria vaccines.

METHODS

The ability of sera from animals immunised with different VAR2CSA constructs to inhibit IE binding to CSA was investigated in three in vitro assays using 96-well plates, petri dishes, capillary flow and an ex vivo placental perfusion assay.

RESULTS

The inter-assay variation was not uniform between assays and ranged from above ten-fold in the flow assay to two-fold in the perfusion assay. The intra-assay variation was highest in the petri dish assay. A positive correlation between IE binding avidity and the level of binding after antibody inhibition in the petri dish assay indicate that high avidity IE binding is more difficult to inhibit. The highest binding inhibition sensitivity was found in the 96-well and petri dish assays compared to the flow and perfusion assays where binding inhibition required higher antibody titers.

CONCLUSIONS

The inhibitory capacity of antibodies is not easily translated between assays and the high sensitivity of the 96-well and petri dish assays stresses the need for comparing serial dilutions of serum. Furthermore, IE binding avidity must be in the same range when comparing data from different days. There was an overall concordance in the capacity of antibody-mediated inhibition, when comparing the in vitro assays with the perfusion assay, which more closely represents in vivo conditions. Importantly the ID1-ID2a protein in a liposomal formulation, currently in a phase I trial, effectively induced antibodies that inhibited IE adhesion in placental tissue.

Antibody responses to the full-length VAR2CSA and its DBL domains in Cameroonian children and teenagers

BACKGROUND

Antigenic variation of Plasmodium falciparum erythrocyte membrane protein 1 is a key parasite mechanism for immune evasion and parasite survival. It is assumed that the number of parasites expressing the same var gene must reach high enough numbers before the host can produce detectable levels of antibodies (Ab) to the variant. VAR2CSA is a protein coded for by one of 60 var genes that is expressed on the surface of infected erythrocytes (IE) and mediates IE binding to the placenta. The idea that Ab to VAR2CSA are pregnancy-associated was challenged when VAR2CSA-specific Ab were reported in children and men. However, the frequency and conditions under which Ab to VAR2CSA are produced outside pregnancy is unclear. This study sought to determine frequency, specificity and level of Ab to VAR2CSA produced in children and whether children with hyperparasitaemia and severe malaria are more likely to produce Ab to VAR2CSA compared to healthy children.

METHODS

Antibody responses to a panel of recombinant proteins consisting of multiple VAR2CSA Duffy-binding-like domains (DBL) and full-length VAR2CSA (FV2) were characterized in 193 1-15 year old children from rural Cameroonian villages and 160 children with severe malaria from the city.

RESULTS

Low Ab levels to VAR2CSA were detected in children; however, Ab levels to FV2 in teenagers were rare. Children preferentially recognized DBL2 (56-70%) and DBL4 (50-60%), while multigravidae produced high levels of IgG to DBL3, DBL5 and FV2. Sixty-seven percent of teenage girls (n = 16/24) recognized ID1-ID2a region of VAR2CSA. Children with severe forms of malaria had significantly higher IgG to merozoite antigens (all p < 0.05), but not to VAR2CSA (all p > 0.05) when compared to the healthy children.

CONCLUSION

The study suggests that children, including teenage girls acquire Ab to VAR2CSA domains and FV2, but Ab levels are much lower than those needed to protect women from placental infections and repertoire of Ab responses to DBL domains is different from those in pregnant women. Interestingly, children with severe malaria did not have higher Ab levels to VAR2CSA compared to healthy children.

Knudsen L, Damm P, G. Theander T, R. Hansson S, A. Nielsen M, Salanti A. Placental Sequestration of Plasmodium falciparum Malaria Parasites Is Mediated by the Interaction Between VAR2CSA and Chondroitin Sulfate A on Syndecan-1

During placental malaria, Plasmodium falciparum infected erythrocytes sequester in the placenta, causing health problems for both the mother and fetus. The specific adherence is mediated by the VAR2CSA protein, which binds to placental chondroitin sulfate (CS) on chondroitin sulfate proteoglycans (CSPGs) in the placental syncytium. However, the identity of the CSPG core protein and the cellular impact of the interaction have remain elusive. In this study we identified the specific CSPG core protein to which the CS is attached, and characterized its exact placental location. VAR2CSA pull-down experiments using placental extracts from whole placenta or syncytiotrophoblast microvillous cell membranes showed three distinct CSPGs available for VAR2CSA adherence. Further examination of these three CSPGs by immunofluorescence and proximity ligation assays showed that syndecan-1 is the main receptor for VAR2CSA mediated placental adherence. We further show that the commonly used placental choriocarcinoma cell line, BeWo, express a different set of proteoglycans than those present on placental syncytiotrophoblast and may not be the most biologically relevant model to study placental malaria. Syncytial fusion of the BeWo cells, triggered by forskolin treatment, caused an increased expression of placental CS-modified syndecan-1. In line with this, we show that rVAR2 binding to placental CS impairs syndecan-1-related Src signaling in forskolin treated BeWo cells, but not in untreated cells.

Plasmodium falciparum is the most deadly malaria parasite, causing more than 500,000 deaths each year. The parasite infects the host’s red blood cells. In placental malaria infected red blood cells accumulate in placenta. The parasite protein VAR2CSA mediates this adherence, which causes complications for both mother and child. VAR2CSA binds a carbohydrate chain termed chondroitin sulfate (CS). CS is not a well-defined biochemical entity but constitute a family of oligosaccharides which each have unique sulfation patterns. The CS binding VAR2CSA is attached to proteoglycans expressed on the surface of placental cells. While much work has gone into understanding the nature of VAR2CSA and its interaction with placental CS, the protein to which the placental CS is attached is not known. To further the understanding of the molecular pathology of PM we characterized the CSPG receptor that the parasites adhere to by defining the exact proteoglycan that carries the placental CS. We further investigated the molecular and cellular consequences of VAR2CSA binding to the receptor. This work provides novel insights into the pathology of placental malaria and the nature of the parasite receptor. This may aid development of strategies to treat or prevent placental malaria.

Glycosaminoglycans and infection

Glycosaminoglycans (GAGs) are complex linear polysaccharides expressed in intracellular compartments, at the cell surface, and in the extracellular environment where they interact with various molecules to regulate many cellular processes implicated in health and disease. Subversion of GAGs is a pathogenic strategy shared by a wide variety of microbial pathogens, including viruses, bacteria, parasites, and fungi. Pathogens use GAGs at virtually every major portals of entry to promote their attachment and invasion of host cells, movement from one cell to another, and to protect themselves from immune attack. Pathogens co-opt fundamental activities of GAGs to accomplish these tasks. This ingenious strategy to subvert essential activities of GAGs likely prevented host organisms from deleting or inactivating these mechanisms during their evolution. The goal of this review is to provide a mechanistic overview of our current understanding of how microbes subvert GAGs at major steps of pathogenesis, using select GAG-pathogen interactions as representative examples.

Adhesion of Plasmodium falciparum infected erythrocytes in ex vivo perfused placental tissue: a novel model of placental malaria

Background

Placental malaria occurs when Plasmodium falciparum infected erythrocytes sequester in the placenta. Placental parasite isolates bind to chondroitin sulphate A (CSA) by expression of VAR2CSA on the surface of infected erythrocytes, but may sequester by other VAR2CSA mediated mechanisms, such as binding to immunoglobulins. Furthermore, other parasite antigens have been associated with placental malaria. These findings have important implications for placental malaria vaccine design. The objective of this study was to adapt and describe a biologically relevant model of parasite adhesion in intact placental tissue.

Results

The ex vivo placental perfusion model was modified to study adhesion of infected erythrocytes binding to CSA, endothelial protein C receptor (EPCR) or a transgenic parasite where P. falciparum erythrocyte membrane protein 1 expression had been shut down. Infected erythrocytes expressing VAR2CSA accumulated in perfused placental tissue whereas the EPCR binding and the transgenic parasite did not. Soluble CSA and antibodies specific against VAR2CSA inhibited binding of infected erythrocytes.

Conclusion

The ex vivo model provides a novel way of studying receptor-ligand interactions and antibody mediated inhibition of binding in placental malaria.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-016-1342-2) contains supplementary material, which is available to authorized users.

Murine Model for Preclinical Studies of Var2CSA-Mediated Pathology Associated with Malaria in Pregnancy

Plasmodium falciparum infection during pregnancy leads to abortions, stillbirth, low birth weight, and maternal mortality. Infected erythrocytes (IEs) accumulate in the placenta by adhering to chondroitin sulfate A (CSA) via var2CSA protein exposed on the P. falciparum IE membrane. Plasmodium berghei IE infection in pregnant BALB/c mice is a model for severe placental malaria (PM). Here, we describe a transgenic P. berghei parasite expressing the full-length var2CSA extracellular region (domains DBL1X to DBL6ε) fused to a P. berghei exported protein (EMAP1) and characterize a var2CSA-based mouse model of PM. BALB/c mice were infected at midgestation with different doses of P. berghei-var2CSA (P. berghei-VAR) or P. berghei wild-type IEs. Infection with 10(4) P. berghei-VAR IEs induced a higher incidence of stillbirth and lower fetal weight than P. berghei At doses of 10(5) and 10(6) IEs, P. berghei-VAR-infected mice showed increased maternal mortality during pregnancy and fetal loss, respectively. Parasite loads in infected placentas were similar between parasite lines despite differences in maternal outcomes. Fetal weight loss normalized for parasitemia was higher in P. berghei-VAR-infected mice than in P. berghei-infected mice. In vitro assays showed that higher numbers of P. berghei-VAR IEs than P. berghei IEs adhered to placental tissue. Immunization of mice with P. berghei-VAR elicited IgG antibodies reactive to DBL1-6 recombinant protein, indicating that the topology of immunogenic epitopes is maintained between DBL1-6-EMAP1 on P. berghei-VAR and recombinant DBL1-6 (recDBL1-6). Our data suggested that impairments in pregnancy caused by P. berghei-VAR infection were attributable to var2CSA expression. This model provides a tool for preclinical evaluation of protection against PM induced by approaches that target var2CSA.

VAR2CSA Domain-Specific Analysis of Naturally Acquired Functional Antibodies to Plasmodium falciparum Placental Malaria

BACKGROUND

Placental malaria is caused by Plasmodium falciparum-infected erythrocytes (IEs) that surface-express VAR2CSA and bind chondroitin sulfate A. The inflammatory response to placenta-sequestered parasites is associated with poor pregnancy outcomes, and protection may be mediated in part by VAR2CSA antibodies that block placental IE adhesion.

METHODS

In this study, we used a new approach to assess VAR2CSA domains for functional epitopes recognized by naturally acquired antibodies. Antigen-specific immunoglobulin (Ig) G targeting Duffy binding-like (DBL) domains from different alleles were sequentially purified from plasma pooled from multigravid women and then characterized using enzyme-linked immunosorbent assay, flow cytometry, and antiadhesion assays.

RESULTS

Different DBL domain-specific IgGs could react to homologous as well as heterologous antigens and parasites, suggesting that conserved epitopes are shared between allelic variants. Homologous blocking of IE binding was observed with ID1-DBL2-ID2a-, DBL4-, and DBL5-specific IgG (range, 42%-75%), whereas partial cross-inhibition activity was observed with purified IgG specific to ID1-DBL2-ID2a and DBL4 antigens. Plasma retained broadly neutralizing activity after complete depletion of these VAR2CSA specificities.

CONCLUSIONS

Broadly neutralizing antibodies of multigravidae are not depleted on VAR2CSA recombinant antigens, and hence development of VAR2CSA vaccines based on a single construct and variant might induce antibodies with limited broadly neutralizing activity.

Protective Antibodies against Placental Malaria and Poor Outcomes during Pregnancy, Benin

Immunity requires a vaccine that inhibits binding of infected erythrocytes to chondroitin sulfate.

Placental malaria is caused by Plasmodium falciparum–infected erythrocytes that bind to placental tissue. Binding is mediated by VAR2CSA, a parasite antigen coded by the var gene, which interacts with chondroitin sulfate A (CSA). Consequences include maternal anemia and fetal growth retardation. Antibody-mediated immunity to placental malaria is acquired during successive pregnancies, but the target of VAR2CSA-specific protective antibodies is unclear. We assessed VAR2CSA-specific antibodies in pregnant women and analyzed their relationships with protection against placental infection, preterm birth, and low birthweight. Antibody responses to the N-terminal region of VAR2CSA during early pregnancy were associated with reduced risks for infections and low birthweight. Among women infected during pregnancy, an increase in CSA binding inhibition was associated with reduced risks for placental infection, preterm birth, and low birthweight. These data suggest that antibodies against VAR2CSA N-terminal region mediate immunity to placental malaria and associated outcomes. Our results validate current vaccine development efforts with VAR2CSA N-terminal constructs.

Developing vaccines to prevent malaria in pregnant women

INTRODUCTION

Placental malaria (PM) is a major public health problem that constitutes a significant health concern for the mother, and especially for the developing fetus and offspring. Current means of prevention have limitations, including a restricted window of intervention that excludes the first trimester of pregnancy, and the fact that very few drugs can be used for this purpose. The identification of the VAR2CSA antigen, specific to PM parasites, offers an excellent opportunity to develop a vaccine against this disease. Proof of concept of a first-generation vaccine is nearing completion, and two clinical trials are underway.

AREAS COVERED

This review focuses on PM, which is mainly caused by Plasmodium falciparum. The review highlights recent advances and the key milestones that led to the identification of the optimal vaccine target within the large VAR2CSA protein. The paper also points out how future improvements can strengthen this process to achieve an effective vaccine in the field.

EXPERT OPINION

The approach taken to develop a P. falciparum erythrocyte membrane protein 1-based vaccine to protect pregnant women is very promising in view of the current difficulties of achieving a sterilizing vaccine against malaria parasite. This approach could help us to control the deleterious effect of malaria infections that characterize severe clinical forms.

Parasite accumulation in placenta of non-immune baboons during Plasmodium knowlesi infection

BACKGROUND

Placental malaria (PM) causes adverse pregnancy outcomes in the mother and her foetus. It is difficult to study PM directly in humans due to ethical challenges. This study set out to bridge this gap by determining the outcome of PM in non-immune baboons in order to develop a non-human primate model for the disease.

METHODS

Ten pregnant baboons were acquired late in their third trimester (day 150) and randomly grouped as seven infected and three non-infected. Another group of four nulligravidae (non-pregnant) infected was also included in the analysis of clinical outcome. Malaria infection was intravenously initiated by Plasmodium knowlesi blood-stage parasites through the femoral vein on 160(th) day of gestation (for pregnant baboons). Peripheral smear, placental smear, haematological samples, and histological samples were collected during the study period. Median values of clinical and haematological changes were analysed using Kruskal-Wallis and Dunn's Multiple Comparison Test. Parasitaemia profiles were analysed using Mann Whitney U test. A Spearman's rank correlation was run to determine the relationship between the different variables of severity scores. Probability values of P <0.05 were considered significant.

RESULTS

Levels of white blood cells increased significantly in pregnant infected (34%) than in nulligravidae infected baboons (8%). Placental parasitaemia levels was on average 19-fold higher than peripheral parasitaemia in the same animal. Infiltration of parasitized erythrocytes and inflammatory cells were also observed in baboon placenta. Malaria parasite score increased with increase in total placental damage score (rs = 0.7650, P <0.05) and inflammatory score (rs = 0.8590, P <0.05). Although the sample size was small, absence of parasitized erythrocytes in cord blood and foetal placental region suggested lack of congenital malaria in non-immune baboons.

CONCLUSION

This study has demonstrated accumulation of parasitized red blood cells and infiltration of inflammatory cells in the placental intravillous space (IVS) of baboons that are non-immune to malaria. This is a key feature of placental falciparum malaria in humans. This presents the baboon as a new model for the characterization of malaria during pregnancy.

Submicroscopic Plasmodium falciparum Infections Are Associated With Maternal Anemia, Premature Births, and Low Birth Weight

BACKGROUND

Molecular, as opposed to microscopic, detection measures the real prevalence of Plasmodium falciparum infections. Such occult infections are common during pregnancy but their impact on pregnancy outcomes is unclear. We performed a longitudinal study to describe that impact.

METHODS

In a cohort of 1037 Beninese pregnant women, we used ultrasound to accurately estimate gestational ages. Infection with P. falciparum, hemoglobin concentration, use of intermittent preventive treatment during pregnancy (IPTp) for malaria, and other parameters were recorded during pregnancy. Using multivariate analyses, we evaluated the impact of submicroscopic infections on maternal anemia, premature birth, and low birth weight.

RESULTS

At inclusion, polymerase chain reaction (PCR) and microscopy detected infection in 40% and 16% of women, respectively. The proportion infected declined markedly after 2 doses of IPTp but rebounded to 34% (by PCR) at delivery. Submicroscopic infections during pregnancy were associated with lower mean hemoglobin irrespective of gravidity, and with increased anemia risk in primigravidae (odds ratio [OR], 2.23; 95% confidence interval [CI], .98-5.07). Prospectively, submicroscopic infections at inclusion were associated with significantly increased risks of low birth weight in primigravidae (OR, 6.09; 95% CI, 1.16-31.95) and premature births in multigravidae (OR, 2.25; 95% CI, 1.13-4.46).

CONCLUSIONS

In this detailed longitudinal study, we document the deleterious impact of submicroscopic P. falciparum parasitemia during pregnancy on multiple pregnancy outcomes. Parasitemia occurs frequently during pregnancy, but routine microscopic and rapid diagnostic tests fail to detect the vast majority of episodes. Our findings imply caution in any revision of the current strategies for prevention of pregnancy-associated malaria.

Trying to remember: immunological B cell memory to malaria

In areas with stable transmission of Plasmodium falciparum parasites, even partially-protective immunity to malaria is acquired only after years of exposure and several infections. It has long been speculated that malaria parasites are directly able to undermine the establishment and maintenance of immunological memory, and that the often transient antibody responses to this parasite are evidence of such a dysfunction. We propose that long-lived antibody responses may not always be a prerequisite for protection, and that antibody longevity varies in an exposure- and age-dependent manner.

Macrophage migration inhibitory factor and placental malaria infection in an area characterized by unstable malaria transmission in central Sudan

Background: The pathogenesis of malaria during pregnancy is not fully understood. A proinflammatory cytokine, macrophage migration inhibitory factor (MIF) is suggested as a factor involved in the pathogenesis of malaria during pregnancy.

Methods: A cross-sectional study was conducted in Medani Hospital, Sudan to investigate MIF levels in placental malaria. Obstetrical and medical characteristics were gathered from each parturient woman using questionnaires. All women (151) were investigated for malaria using blood film and placental histology. MIF levels were measured using ELISA in paired maternal and cord blood samples.

Results: There were no P. falciparum-positive blood films obtained from maternal peripheral blood, placenta or cord samples. Out of 151 placentae, four (2.6%), one (0.7%), 32 (21.2%) showed acute, chronic and past infection on histopathology examinations respectively, while the rest (114; 75.5%) of them showed no signs of infection.There was no significant difference in the median (interquartile) of maternal [5.0 (3.7─8.8) vs 6.2(3.5─12.0) ng/ml, P=0.643] and cord [8.1(3.3─16.9) vs 8.3(4.2─16.9), ng/ml, P= 0.601] MIF levels between women with a positive result for placental malaria infection (n=37) and women with a negative result for placental malaria infection (n=114). In regression models placental malaria was not associated with maternal MIF, hemoglobin or birth weight. MIF was not associated with hemoglobin or birth weight .

Conclusion: There was no association between maternal and cord MIF levels, placental malaria, maternal hemoglobin and birth weight.

MyD88 signaling is directly involved in the development of murine placental malaria

Malaria is a widespread infectious disease caused by the parasite Plasmodium. During pregnancy, malaria infection leads to a range of complications that can affect both the mother and fetus, including stillbirth, infant mortality, and low birth weight. In this study, we utilized a mouse model of placental malaria (PM) infection to determine the importance of the protein MyD88 in the host immune response to Plasmodium during pregnancy. Initially, we demonstrated that Plasmodium berghei NK65GFP adhered to placental tissue via chondroitin sulfate A and induced PM in mice with a C57BL/6 genetic background. To evaluate the involvement of MyD88 in the pathology of PM, we performed a histopathological analysis of placentas obtained from MyD88(-/-) and wild-type (WT) mice following infection on the 19th gestational day. Our data demonstrated that the detrimental placental alterations observed in the infected mice were correlated with the expression of MyD88. Moreover, in the absence of this protein, production of interleukin 6 (IL-6) and tumor necrosis factor alpha (TNF-α) was significantly reduced in the infected mice. More importantly, in contrast to fetuses from infected WT mice, which exhibited a reduction in body weight, the fetuses from infected MyD88(-/-) mice did not display significant weight loss compared to their noninfected littermates. In addition, we observed a decrement of maternal care associated with malaria infection, which was attenuated in the MyD88-deficient mice. Collectively, the results of this study illustrate the pivotal importance of the MyD88 signaling pathway in the pathogenesis of placental malaria, thus presenting new possibilities for targeting MyD88 in therapeutic interventions.

Histopathologies, immunolocalization, and a glycan binding screen provide insights into Plasmodium falciparum interactions with the human placenta

During pregnancy, Plasmodium falciparum-infected erythrocytes cytoadhere to the placenta. Infection is likely initiated at two sites where placental trophoblasts contact maternal blood: 1) via syncytiotrophoblast (STB), a multicellular transporting and biosynthetic layer that forms the surface of chorionic villi and lines the intervillous space, and 2) through invasive cytotrophoblasts, which line uterine vessels that divert blood to the placenta. Here, we investigated mechanisms of infected erythrocyte sequestration in relationship to the microanatomy of the maternal-fetal interface. Histological analyses revealed STB denudation in placental malaria, which brought the stromal cores of villi in direct contact with maternal blood. STB denudation was associated with hemozoin deposition (P = 0.01) and leukocyte infiltration (P = 0.001) and appeared to be a feature of chronic placental malaria. Immunolocalization of infected red blood cell receptors (CD36, ICAM1/CD54, and chondroitin sulfate A) in placentas from uncomplicated pregnancies showed that STB did not stain, while the underlying villous stroma was immunopositive. Invasive cytotrophoblasts expressed ICAM1. In malaria, STB denudation exposed CD36 and chondroitin sulfate A in the villous cores to maternal blood, and STB expressed ICAM1. Finally, we investigated infected erythrocyte adherence to novel receptors by screening an array of 377 glycans. Infected erythrocytes bound Lewis antigens that immunolocalized to STB. Our results suggest that P. falciparum interactions with STB-associated Lewis antigens could initiate placental malaria. Subsequent pathologies, which expose CD36, ICAM1, and chondroitin sulfate A, might propagate the infection.

Reduced Arylsulfatase B activity in leukocytes from cystic fibrosis patients

The enzyme Arylsulfatase B (ARSB; N-acetylgalactosamine-4-sulfatase) removes 4-sulfate groups from chondroitin-4-sulfate and dermatan sulfate and is required for the degradation of these sulfated glycosaminoglycans (sGAGs). Since these GAGs accumulate in patients with Cystic Fibrosis (CF), we investigated the activity of ARSB in leukocytes of patients with CF, to consider if reduced activity of ARSB might contribute to the pathophysiology of CF. Previous cell-based experiments had demonstrated that when the deficiency of the cystic fibrosis transmembrane regulator (CFTR) was corrected in bronchial epithelial cells, the ARSB activity increased significantly. De-identified, citrated blood samples were collected from 16 children with CF and 31 control subjects, seen in the Pediatric Clinic at Rush University Medical Center. Polymorphonuclear leukocytes (PMN) and mononuclear cell (MC) populations were separated by density gradient, and blinded determinations of ARSB activity were performed using the exogenous substrate 4-methylumbilliferyl sulfate. Interleukin-6 was measured in the plasma samples by ELISA. ARSB activity was significantly less in the PMN and MC from the CF patients than controls (P < 0.0001, unpaired t-test, two-tailed). Interleukin-6 levels in plasma were significantly greater in the CF population (P < 0.001). Mean age, age range, and male:female ratio of CF patients and controls were similar, and no association of ARSB activity with age, gender, or CFTR genotype was evident. Since recombinant human ARSB is used successfully for replacement therapy in Mucopolysaccharidosis VI, it may be useful to restore ARSB activity to normal levels and increase degradation of sulfated GAGs in CF patients.

Intravital placenta imaging reveals microcirculatory dynamics impact on sequestration and phagocytosis of Plasmodium-infected erythrocytes

Malaria in pregnancy is exquisitely aggressive, causing a range of adverse maternal and fetal outcomes prominently linked to Plasmodium-infected erythrocyte cytoadherence to fetal trophoblast. To elucidate the physiopathology of infected erythrocytes (IE) sequestration in the placenta we devised an experimental system for intravital placental examination of P. berghei-infected mice. BALB/c females were mated to C57Bl/6 CFP+ male mice and infected with GFP+ P. berghei IE, and at gestational day 18, placentas were exposed for time-lapse imaging acquisition under two-photon microscopy. Real-time images and quantitative measurements revealed that trophoblast conformational changes transiently restrain blood flow in the mouse placental labyrinth. The complex dynamics of placental microcirculation promotes IE accumulation in maternal blood spaces with low blood flow and allows the establishment of stable IE-trophoblast contacts. Further, we show that the fate of sequestered IE includes engulfment by both macrophagic and trophoblastic fetal-derived cells. These findings reinforce the current paradigm that IE interact with the trophoblast and provide definitive evidence on two novel pathogenesis mechanisms: (1) trophoblast layer controls placental microcirculation promoting IE sequestration; and (2) fetal-derived placental cells engulf sequestered IE.

Malaria in pregnancy is exquisitely aggressive, causing a range of adverse effects impacting maternal and fetal health. Many of those effects are thought to derive from placental sequestration of red blood cells infected with the malaria parasite (Plasmodium falciparum) eliciting a placental inflammatory response that impairs maternal-fetal exchanges. We developed an experimental system for intravital microscopy to directly observe the course of placental infection in a mouse model of pregnancy-associated malaria. We found that microcirculation in infected placentas showed areas of low blood flow that promote sequestration of infected red blood cells. Furthermore, we observed that sequestered infected red blood cells are targeted and phagocytosed by fetal-derived cells in the materno-fetal interface. This work provides the first ever in vivo evidence that unique placental microcirculatory features promote infected red blood cell sequestration, implying a vascular component in placental malaria pathogenesis. Moreover, we reinforce the notion that fetal-derived cells contribute to the placental response against sequestered infected red blood cells.

Recent advances and perspectives on tropical diseases: Malaria

Malaria remains a major health problem in the world. It is a neglected disease because it occurs almost exclusively in poor developing countries, which offer negligible marketable and profitable opportunities. Malaria (together with Tuberculosis), is responsible for an unprecedented global health crisis with devastating effects in developing countries. The 2011 Word Malaria Report indicated that 106 countries showed endemic malaria. Malaria control depends mainly on drug treatment, which is increasingly difficult due to the spread of drug resistant parasites and requires expensive drug combinations. Part of the inability to combat this disease is attributed to an incomplete understanding of its pathogenesis and pathophysiology. Improving the knowledge of the underlying pathogenic mechanisms of malaria transmission and of the exclusive metabolic pathways of the parasites (protozoa of the genus Plasmodium), should promote efficient treatment of disease and help the identification of novel targets for potential therapeutic interventions. Moreover, the elucidation of determinants involved in the spread of malaria will provide important information for efficient planning of strategies for targeted control.

High avidity antibodies to full-length VAR2CSA correlate with absence of placental malaria

VAR2CSA mediates sequestration of Plasmodium falciparum-infected erythrocytes in the placenta, increasing the risk of poor pregnancy outcomes. Naturally acquired antibodies (Ab) to placental parasites at delivery have been associated with improved pregnancy outcomes, but Ab levels and how early in pregnancy Ab must be present in order to eliminate placental parasites before delivery remains unknown. Antibodies to individual Duffy-binding like domains of VAR2CSA have been studied, but the domains lack many of the conformational epitopes present in full-length VAR2CSA (FV2). Thus, the purpose of this study was to describe the acquisition of Ab to FV2 in women residing in high and low transmission areas and determine how Ab levels during pregnancy correlate with clearance of placental parasites. Plasma samples collected monthly throughout pregnancy from pregnant women living in high and low transmission areas in Cameroon were evaluated for Ab to FV2 and the proportion of high avidity Ab (i.e., Ab that remain bound in the presence of 3M NH₄SCN) was assessed. Ab levels and proportion of high avidity Ab were compared between women with placental malaria (PM⁺) and those without (PM⁻) at delivery. Results showed that PM⁻ women had significantly higher Ab levels (p = 0.0047) and proportion of high avidity Ab (p = 0.0009) than PM⁺ women throughout pregnancy. Specifically, women with moderate to high Ab levels (>5,000 MFI) and those with ≥35% high avidity Ab at 5–6 months were found to have 2.3 (95% CI, 1.0–4.9) and 7.6-fold (p = 0.0013, 95% CI: 1.2–50.0) reduced risk of placental malaria, respectively. These data show that high levels of Ab to FV2, particularly those with high avidity for FV2, produced by mid-pregnancy are important in clearing parasites from the placenta. Both high Ab levels and proportion of high avidity Ab to FV2 may serve as correlates of protection for assessing immunity against placental malaria.

Hypoxia reduces arylsulfatase B activity and silencing arylsulfatase B replicates and mediates the effects of hypoxia

This report presents evidence of 1) a role for arylsulfatase B (ARSB; N-acetylgalactosamine-4-sulfatase) in mediating intracellular oxygen signaling; 2) replication between the effects of ARSB silencing and hypoxia on sulfated glycosaminoglycan content, cellular redox status, and expression of hypoxia-associated genes; and 3) a mechanism whereby changes in chondroitin-4-sulfation that follow either hypoxia or ARSB silencing can induce transcriptional changes through galectin-3. ARSB removes 4-sulfate groups from the non-reducing end of chondroitin-4-sulfate and dermatan sulfate and is required for their degradation. For activity, ARSB requires modification of a critical cysteine residue by the formylglycine generating enzyme and by molecular oxygen. When primary human bronchial and human colonic epithelial cells were exposed to 10% O₂×1 h, ARSB activity declined by ∼41% and ∼30% from baseline, as nuclear hypoxia inducible factor (HIF)-1α increased by ∼53% and ∼37%. When ARSB was silenced, nuclear HIF-1α increased by ∼81% and ∼61% from baseline, and mRNA expression increased to 3.73 (±0.34) times baseline. Inversely, ARSB overexpression reduced nuclear HIF-1α by ∼37% and ∼54% from baseline in the epithelial cells. Hypoxia, like ARSB silencing, significantly increased the total cellular sulfated glycosaminoglycans and chondroitin-4-sulfate (C4S) content. Both hypoxia and ARSB silencing had similar effects on the cellular redox status and on mRNA expression of hypoxia-associated genes. Transcriptional effects of both ARSB silencing and hypoxia may be mediated by reduction in galectin-3 binding to more highly sulfated C4S, since the galectin-3 that co-immunoprecipitated with C4S declined and the nuclear galectin-3 increased following ARSB knockdown and hypoxia.

The Impact of HIV Coinfection on Cerebral Malaria Pathogenesis

HIV infection is widespread throughout the world and is especially prevalent in sub-Saharan Africa and Asia. Similarly, Plasmodium falciparum, the most common cause of severe malaria, affects large areas of sub-Saharan Africa, the Indian subcontinent, and Southeast Asia. Although initial studies suggested that HIV and malaria had independent impact upon patient outcomes, recent studies have indicated a more significant interaction. Clinical studies have shown that people infected with HIV have more frequent and severe episodes of malaria, and parameters of HIV disease progression worsen in individuals during acute malaria episodes. However, the effect of HIV on development of cerebral malaria, a manifestation of P. falciparum infection that is frequently fatal, has not been characterized. We review clinical and basic science studies pertaining to HIV and malaria coinfection and cerebral malaria in particular in order to highlight the likely role HIV plays in exacerbating cerebral malaria pathogenesis.

High levels of antibodies to multiple domains and strains of VAR2CSA correlate with the absence of placental malaria in Cameroonian women living in an area of high Plasmodium falciparum transmission

Placental malaria, caused by sequestration of Plasmodium falciparum-infected erythrocytes in the placenta, is associated with increased risk of maternal morbidity and poor birth outcomes. The parasite antigen VAR2CSA (variant surface antigen 2-chondroitin sulfate A) is expressed on infected erythrocytes and mediates binding to chondroitin sulfate A, initiating inflammation and disrupting homeostasis at the maternal-fetal interface. Although antibodies can prevent sequestration, it is unclear whether parasite clearance is due to antibodies to a single Duffy binding-like (DBL) domain or to an extensive repertoire of antibodies to multiple DBL domains and allelic variants. Accordingly, plasma samples collected longitudinally from pregnant women were screened for naturally acquired antibodies against an extensive panel of VAR2CSA proteins, including 2 to 3 allelic variants for each of 5 different DBL domains. Analyses were performed on plasma samples collected from 3 to 9 months of pregnancy from women living in areas in Cameroon with high and low malaria transmission. The results demonstrate that high antibody levels to multiple VAR2CSA domains, rather than a single domain, were associated with the absence of placental malaria when antibodies were present from early in the second trimester until term. Absence of placental malaria was associated with increasing antibody breadth to different DBL domains and allelic variants in multigravid women. Furthermore, the antibody responses of women in the lower-transmission site had both lower magnitude and lesser breadth than those in the high-transmission site. These data suggest that immunity to placental malaria results from high antibody levels to multiple VAR2CSA domains and allelic variants and that antibody breadth is influenced by malaria transmission intensity.

The effect of adjuvants on the immune response induced by a DBL4ɛ-ID4 VAR2CSA based Plasmodium falciparum vaccine against placental malaria

A vaccine protecting women against placental malaria could be based on the sub-domains of the VAR2CSA antigen, since antibodies against the DBL4ɛ-ID4 subunit of the VAR2CSA protein can inhibit parasite binding to the placental ligand chondroitin sulphate A (CSA). Here we tested the ability of DBL4ɛ-ID4 to induce binding-inhibitory antibodies when formulated with adjuvants approved for human use. We have characterized the immune response of DBL4ɛ-ID4 in combination with Freund's complete and incomplete adjuvant and with three adjuvants currently being used in clinical trials: Montanide(®) ISA 720, Alhydrogel(®) and CAF01. Antibodies induced against DBL4ɛ-ID4 in combination with these adjuvants inhibited parasite binding to CSA from 82% to 99%. Although, different epitope recognition patterns were obtained for the different formulations, all adjuvant combinations induced strong Th1 and Th2 type responses, resulting in IgG with similar binding strength, with to the DBL4ɛ-ID4 antigen. These results demonstrate that the DBL4ɛ-ID4 antigen is highly immunogenic and that binding inhibitory antibodies are induced when formulated with any of the tested adjuvants.

How specific is Plasmodium falciparum adherence to chondroitin 4-sulfate?

Plasmodium falciparum infection during pregnancy results in the sequestration of infected red blood cells (IRBCs) in the placenta, contributing to pregnancy associated malaria (PAM). IRBC adherence is mediated by the binding of a variant Plasmodium falciparum erythrocyte binding protein 1 named VAR2CSA to the low sulfated chondroitin 4-sulfate (C4S) proteoglycan (CSPG) present predominantly in the intervillous space of the placenta. IRBC binding is highly specific to the level and distribution of 4-sulfate groups in C4S. Given the strict specificity of IRBC-C4S interactions, it is better to use either placental CSPG or CSPGs bearing structurally similar C4S chains in defining VAR2CSA structural architecture that interact with C4S, evaluating VAR2CSA constructs for vaccine development or studying structure-based inhibitors as therapeutics for PAM.

Proteoglycans in host-pathogen interactions: molecular mechanisms and therapeutic implications

Many microbial pathogens subvert proteoglycans for their adhesion to host tissues, invasion of host cells, infection of neighbouring cells, dissemination into the systemic circulation, and evasion of host defence mechanisms. Where studied, specific virulence factors mediate these proteoglycan-pathogen interactions, which are thus thought to affect the onset, progression and outcome of infection. Proteoglycans are composites of glycosaminoglycan (GAG) chains attached covalently to specific core proteins. Proteoglycans are expressed ubiquitously on the cell surface, in intracellular compartments, and in the extracellular matrix. GAGs mediate the majority of ligand-binding activities of proteoglycans, and many microbial pathogens elaborate cell-surface and secreted factors that interact with GAGs. Some pathogens also modulate the expression and function of proteoglycans through known virulence factors. Several GAG-binding pathogens can no longer attach to and invade host cells whose GAG expression has been reduced by mutagenesis or enzymatic treatment. Furthermore, GAG antagonists have been shown to inhibit microbial attachment and host cell entry in vitro and reduce virulence in vivo. Together, these observations underscore the biological significance of proteoglycan-pathogen interactions in infectious diseases.

Chloroquine reduces arylsulphatase B activity and increases chondroitin-4-sulphate: implications for mechanisms of action and resistance

BACKGROUND

The receptors for adhesion of Plasmodium falciparum-infected red blood cells (RBC) in the placenta have been identified as chondroitin-4-sulphate (C4S) proteoglycans, and the more sulphate-rich chondroitin oligosaccharides have been reported to inhibit adhesion. Since the anti-malarial drug chloroquine accumulates in lysosomes and alters normal lysosomal processes, the effects of chloroquine on the lysosomal enzyme arylsulphatase B (ASB, N-acetylgalactosamine-4-sulphatase), which removes 4-sulphate groups from chondroitin-4-sulphate, were addressed. The underlying hypothesis derived from the recognized impairment of attachment of parasite-infected erythrocytes in the placenta, when chondroitin-4-sulphation was increased. If chloroquine reduced ASB activity, leading to increased chondroitin-4-sulphation, it was hypothesized that the anti-malarial mechanism of chloroquine might derive, at least in part, from suppression of ASB.

METHODS

Experimental methods involved cell culture of human placental, bronchial epithelial, and cerebrovascular cells, and the in vitro exposure of the cells to chloroquine at increasing concentrations and durations. Measurements of arylsulphatase B enzymatic activity, total sulphated glycosaminoglycans (sGAG), and chondroitin-4-sulphate (C4S) were performed using in vitro assays, following exposure to chloroquine and in untreated cell preparations. Fluorescent immunostaining of ASB was performed to determine the effect of chloroquine on cellular ASB content and localization. Mass spectrometry and high performance liquid chromatography were performed to document and to quantify the changes in chondroitin disaccharides following chloroquine exposure.

RESULTS

In the human placental, bronchial epithelial, and cerebrovascular cells, exposure to increasing concentrations of chloroquine was associated with reduced ASB activity and with increased concentrations of sGAG, largely attributable to increased C4S. The study data demonstrated: 1) decline in ASB activity following chloroquine exposure; 2) inverse correlation between ASB activity and C4S content; 3) increased content of chondroitin-4-sulphate disaccharides following chloroquine exposure; and 4) decline in extent of chloroquine-induced ASB reduction with lower baseline ASB activity. Confocal microscopy demonstrated the presence of ASB along the cell periphery, indicating extra-lysosomal localization.

CONCLUSIONS

The study data indicate that the therapeutic mechanism of chloroquine action may be attributable, at least in part, to reduction of ASB activity, leading to increased chondroitin-4-sulphation in human placental, bronchial epithelial, and cerebrovascular cells. In vivo, increased chondroitin-4-sulphation may reduce the attachment of P. falciparum-infected erythrocytes to human cells. Extra-lysosomal localization of ASB and reduced impact of chloroquine when baseline ASB activity is less suggest possible mechanisms of resistance to the effects of chloroquine.

Plasmodium falciparum: Assessment of parasite-infected red blood cell binding to placental chondroitin proteoglycan and bovine tracheal chondroitin sulfate A

In pregnant women infected with Plasmodium falciparum, the infected red blood cells (IRBCs) sequester in placenta by binding to the chondroitin 4-sulfate (C4S) chains of low sulfated chondroitin sulfate proteoglycan (CSPG). Placental CSPG, the natural receptor for IRBC adherence in the placenta, is the ideal molecule for studying structural interactions in IRBC adhesion to C4S, adhesion inhibitory antibody responses, and identification of parasite adhesive protein(s). However, because of difficulty involved in purifying placental CSPG, the commercially available bovine tracheal chondroitin sulfate A (bCSA), a copolymer having structural features of both C4S and C6S, has been widely used. To determine the validity of bCSA for C4S-IRBC interaction studies, we comparatively evaluated the characteristics of IRBC binding to placental CSPG and bCSA using three commonly used parasite strains. The results indicate that, in all three parasites studied, the characteristics of IRBC binding to placental CSPG and bCSA are qualitatively similar, but the binding capacity with respect to both the number of IRBCs bound per unit area of coated surface and binding strength is significantly higher for CSPG than bCSA regardless of whether parasites were selected on CSPG or bCSA. These results demonstrate that placental CSPG is best suited for studying interactions between parasite adhesive protein(s) and C4S, and have implications in understanding C4S-IRBC structural interactions.

Recrudescent Plasmodium berghei from pregnant mice displays enhanced binding to the placenta and induces protection in multigravida

Pregnancy-associated malaria (PAM) is associated with placenta pathology and poor pregnancy outcome but the mechanisms that control the malaria parasite expansion in pregnancy are still poorly understood and not amenable for study in human subjects. Here, we used a set of new tools to re-visit an experimental mouse model of pregnancy-induced malaria recrudescence, BALB/c with chronic Plasmodium berghei infection. During pregnancy 60% of the pre-exposed primiparous females showed pregnancy-induced malaria recrudescence and we demonstrated that the recrudescent P. berghei show an unexpected enhancement of the adherence to placenta tissue sections with a marked specificity for CSA. Furthermore, we showed that the intensity of parasitemia in primigravida was quantitatively correlated with the degree of thickening of the placental tissue and up-regulation of inflammation-related genes such as IL10. We also confirmed that the incidence of pregnancy-induced recrudescence, the intensity of the parasitemia peak and the impact on the pregnancy outcome decreased gradually from the first to the third pregnancy. Interestingly, placenta pathology and fetal impairment were also observed at low frequency among non-recrudescent females. Together, the data raise the hypothesis that recrudescent P. berghei displays selected specificity for the placenta tissue enabling on one hand, the triggering of the pathological process underlying PAM and on the other hand, the induction of PAM protection mechanisms that are revealed in subsequent pregnancies. Thus, by exploiting P. berghei pregnancy-induced recrudescence, this experimental system offers a mouse model to study the susceptibility to PAM and the mechanisms of disease protection in multigravida.

Structural interactions in chondroitin 4-sulfate mediated adherence of Plasmodium falciparum infected erythrocytes in human placenta during pregnancy-associated malaria

Infection with Plasmodium falciparum during pregnancy results in the adherence of infected red blood cells (IRBCs) in placenta, causing pregnancy-associated malaria with severe health complications in mothers and fetuses. The chondroitin 4-sulfate (C4S) chains of very low sulfated chondroitin sulfate proteoglycans (CSPGs) in placenta mediate the IRBC adherence. While it is known that partially sulfated but not fully sulfated C4S effectively binds IRBCs, structural interactions involved remain unclear and are incompletely understood. In this study, structurally defined C4S oligosaccharides of varying sulfate contents and sizes were evaluated for their ability to inhibit the binding of IRBCs from different P. falciparum strains to CSPG purified from placenta. The results clearly show that, with all parasite strains studied, dodecasaccharide is the minimal chain length required for the efficient adherence of IRBCs to CSPG and two 4-sulfated disaccharides within this minimal structural motif are sufficient for maximal binding. Together, these data demonstrate for the first time that the C4S structural requirement for IRBC adherence is parasite strain-independent. We also show that the carboxyl group on nonreducing end glucuronic acid in dodecasaccharide motif is important for IRBC binding. Thus, in oligosaccharides containing terminal 4,5-unsaturated glucuronic acid, the nonreducing end disaccharide moiety does not interact with IRBCs due to the altered spatial orientation of carboxyl group. In such C4S oligosaccharides, 14-mer but not 12-mer constitutes the minimal motif for inhibition of IRBC binding to placental CSPG. These data have important implications for the development and evaluation of therapeutics and vaccine for placental malaria.

Plasmodium falciparum during pregnancy: a puzzling parasite tissue adhesion tropism

P. falciparum malaria severely affects pregnant women and children. Despite immunity through lifelong exposure to malaria, pregnant women become susceptible to infections causing anaemia, abortions and low birth weight. They experience massive accumulation of infected erythrocytes (IEs) in the placenta. Adhesion of IEs to host endothelial receptors is mediated by members of a large diverse protein family called P. falciparum erythrocyte membrane protein 1 (PfEMP1). Pregnancy malaria is generally associated with the emergence of a distinct subset of parasites expressing a unique PfEMP1 that binds to the host-receptor chondroitin sulfate A (CSA). Resistance to pregnancy malaria is associated with the acquisition of antibodies that block IEs binding to placental CSA. The absence (or rare occurrence) of CSA-binding parasites in malaria patients (children, men and non-pregnant women) suggests that these parasites become virulent only during pregnancy. The molecular mechanisms used by P. falciparum to achieve the timely expression of the Pf-CSA ligand in pregnant women remain puzzling. In this review we will discuss two hypothetical mechanisms by which CSA-binding parasites may arise during pregnancy. The first, a selection process by the placenta of a distinct sub-population of P. falciparum expressing a particular PfEMP1. The second, an induction mechanism that facilitates the expression of a particular PfEMP1 protein by specific host factor(s) present only during pregnancy.

Pregnancy outcome and placenta pathology in Plasmodium berghei ANKA infected mice reproduce the pathogenesis of severe malaria in pregnant women

Pregnancy-associated malaria (PAM) is expressed in a range of clinical complications that include increased disease severity in pregnant women, decreased fetal viability, intra-uterine growth retardation, low birth weight and infant mortality. The physiopathology of malaria in pregnancy is difficult to scrutinize and attempts were made in the past to use animal models for pregnancy malaria studies. Here, we describe a comprehensive mouse experimental model that recapitulates many of the pathological and clinical features typical of human severe malaria in pregnancy. We used P. berghei ANKA-GFP infection during pregnancy to evoke a prominent inflammatory response in the placenta that entails CD11b mononuclear infiltration, up-regulation of MIP-1 alpha chemokine and is associated with marked reduction of placental vascular spaces. Placenta pathology was associated with decreased fetal viability, intra-uterine growth retardation, gross post-natal growth impairment and increased disease severity in pregnant females. Moreover, we provide evidence that CSA and HA, known to mediate P. falciparum adhesion to human placenta, are also involved in mouse placental malaria infection. We propose that reduction of maternal blood flow in the placenta is a key pathogenic factor in murine pregnancy malaria and we hypothesize that exacerbated innate inflammatory responses to Plasmodium infected red blood cells trigger severe placenta pathology. This experimental model provides an opportunity to identify cell and molecular components of severe PAM pathogenesis and to investigate the inflammatory response that leads to the observed fetal and placental blood circulation abnormalities.

Is human placenta proteoglycan remodeling involved in pre-eclampsia?

Impaired placento-fetal communication is a coherent symptom of exaggerated pre-eclampsia. The impact of the cellular expression of different glycosaminoglycans (GAGs) in this event on the placenta in pre-eclampsia is still obscure. This is the first study aimed at discovering the relationship between structural alterations of different sulfated GAGs at the molecular level and the development of pre-eclampsia in inflicted placenta. Sulfated GAGs were isolated and purified from control and pre-eclampsia placentas. The amount and the molecular weight of GAG in each tissue sample were measured. The polydispersity of the recovered GAG samples were determined by polyacrylamide gel electrophoresis. The disaccharide composition of chondroitin sulfate, dermatan sulfate and heparan sulfate were deduced by chondroitinase and heparinase depolymerization followed by liquid chromatography-mass spectrometry. The in vivo sulfo-modulation of GAGs in pre-eclampsia and control placenta were examined using RT-PCR to determine the transcription levels of different sulfotransferases involved in GAG biosynthesis. Marked differences in GAG sulfation patterns and mRNA level of encoding selected GAG O-sulfotransferases were observed in pre-eclampsia. These data suggest a linkage between pre-eclampsia and the observed alterations in placental GAGs and could provide new insights about the modulating role of GAGs in the development and the severity of placental pre-eclampsia.

New approaches to pathogenesis of malaria in pregnancy

Malaria infection during pregnancy is associated with poor maternal and foetal outcomes including low birth weight. In malaria-endemic areas, low birth weight is primarily a consequence of foetal growth restriction. Little is known on the pathogenesis of foetal growth restriction and our understanding of the relationship between epidemiological observations and the pathogenesis or consequences of disease is incomplete. In this review, we describe these gaps in our knowledge and also try to identify goals for future research into malaria in pregnancy. Foetal growth restriction results from a complex four-dimensional interaction between the foetus, the mother and the malaria parasite over gestation, and research into its pathogenesis may be advanced by combining longitudinal studies with techniques and approaches new to the field of malaria in pregnancy. Such approaches would greatly increase our knowledge on the pathogenesis of this disease and may provide new avenues for intervention strategies.

Plasmodiumin the placenta: parasites, parity, protection, prevention and possibly preeclampsia

The epidemiology of pregnancy malaria infection and disease is complex but reflects underlying interactions between thePlasmodium falciparumparasite, the mother, and the foetus. Parasites sequester in the human placenta by binding to chondroitin sulfate A (CSA), a novel receptor that does not commonly support binding of other parasites. Women become resistant toP. falciparummalaria over successive pregnancies as they acquire antibodies against the CSA-binding placental parasite forms. Due to acquired immunity, placental malaria is briefer and less inflammatory in multigravid women than primigravid women, and these parity differences may account for the different outcomes these women and their offspring experience. Commonly recognized sequelae of malaria-like maternal anaemia and low birth weight primarily occur in first and second pregnancies. Hypertension may result from maternal-foetal conflict over the inflammatory response to placental malaria, and occurs in young, first-time mothers. Placental malaria can either increase or decrease parasitaemia risk in the offspring, depending on the mother's parity. The burden of disease due to pregnancy malaria, and the benefits of an effective vaccine, may be much greater than is currently appreciated.