First-trimester Plasmodium falciparum infections display a typical "placental" phenotype

Parasitism in viviparous vertebrates: an overview

The reproductive mode of viviparity has independently evolved in various animal taxa. It refers to the condition in which the embryos or young develop inside the female's body during gestation, providing advantages such as protection, nutrition, and improved survival chances. However, parasites and diseases can be an evolutionary force that limit the host's resources, leading to physiological, morphological, and behavioral changes that impose additional costs on both the pregnant female and her offspring. This review integrates the primary literature published between 1980 and 2021 on the parasitism of viviparous hosts. We describe aspects such as reproductive investment in females, offspring sex ratios, lactation investment in mammals, alterations in birth intervals, current reproductive investment, variations between environments, immune system activity in response to immunological challenges, and other factors that can influence the interaction between viviparous females and parasites. Maintaining pregnancy incurs costs in managing the mother's resources and regulating the immune system's responses to the offspring, while simultaneously maintaining an adequate defense against parasites and pathogens. Parasites can significantly influence this reproductive mode: parasitized females adjust their investment in survival and reproduction based on their life history, environmental factors, and the diversity of encountered parasites.

Malaria Infections and Placental Blood Flow: A Doppler Ultrasound Study From a Preconception Cohort in Benin

Background

Malaria in pregnancy (MiP) has been associated with fetal growth restriction, the underlying pathogenic mechanisms of which remain poorly understood. Malaria in pregnancy is suspected to induce abnormalities in placental vascularization, leading to impaired placental development. Our study evaluated MIP's effect on uterine artery (UtA) and umbilical artery (UA) blood flow.

Methods

The analysis included 253 Beninese women followed throughout pregnancy and screened monthly for submicroscopic and microscopic malaria. Uterine artery Doppler measurement was performed once between 21 and 25 weeks' gestation (wg), and UA Doppler measurement was performed 1-3 times from 28 wg. Linear and logistic regression models were used to assess the effect of malaria infections on UtA Doppler indicators (pulsatility index and presence of a notch), whereas a logistic mixed model was used to assess the association between malaria infections and abnormal UA Doppler (defined as Z-score ≥2 standard deviation or absent/reversed UA end-diastolic flow).

Results

Primigravidae represented 7.5% of the study population; 42.3% of women had at least 1 microscopic infection during pregnancy, and 29.6% had at least 1 submicroscopic infection (and no microscopic infection). Both microscopic and submicroscopic infections before Doppler measurement were associated with the presence of a notch (adjusted odds ratio [aOR] 4.5, 95% confidence interval [CI] = 1.2-16.3 and aOR 3.3, 95% CI = .9-11.9, respectively). No associations were found between malaria before the Doppler measurement and abnormal UA Doppler.

Conclusions

Malaria infections in the first half of pregnancy impair placental blood flow. This highlights the need to prevent malaria from the very beginning of pregnancy.

Safety of Treating Malaria with Artemisinin-Based Combination Therapy in the First Trimester of Pregnancy

There have been recent calls for the use of artemisinin-based combination therapies (ACTs) for uncomplicated malaria in the first trimester of pregnancy. Nevertheless, the 2021 WHO Guidelines for Malaria reaffirmed their position that there is not adequate clinical safety data on artemisinins to support that usage. The WHO's position is consistent with several issues with the existing clinical data. First, first trimester safety results from multiple ACTs were lumped in a meta-analysis which does not demonstrate that each of the included ACTs is equally safe. Second, safety results from all periods of the first trimester were lumped in the meta-analysis which does not demonstrate the same level of safety for all subperiods, particularly gestational Weeks 6 to 8 which is likely to be the most sensitive period. Third, even if there is evidence of a lack of an effect on miscarriage for a particular ACT, it does not follow then there are no developmental effects for any ACT. In monkeys, artesunate caused marked embryonal anemia leading to embryo death but the long-term consequences of lower levels of embryonal anemia are not known. Fourth, there have been advances in the sensitivity and usage of rapid diagnostic tests that will lead to diagnoses of malaria earlier in gestation which is less well studied and more likely sensitive to artemisinins. Any clinical studies of the safety of ACTs in the first trimester need to evaluate the results of treatment with individual ACTs during different 1- to 2-week periods of the first trimester.

A novel murine model of post-implantation malaria-induced preterm birth

Despite major advances made in malaria treatment and control over recent decades, the development of new models for studying disease pathogenesis remains a vital part of malaria research efforts. The study of malaria infection during pregnancy is particularly reliant on mouse models, as a means of circumventing many challenges and costs associated with pregnancy studies in endemic human populations. Here, we introduce a novel murine model that will further our understanding of how malaria infection affects pregnancy outcome. When C57BL/6J (B6) mice are infected with Plasmodium chabaudi chabaudi AS on either embryonic day (E) 6.5, 8.5, or 10.5, preterm birth occurs in all animals by E16.5, E17.5, or E18.5 respectively, with no evidence of intrauterine growth restriction. Despite having the same outcome, we found that the time to delivery, placental inflammatory and antioxidant transcript upregulation, and the relationships between parasitemia and transcript expression prior to preterm birth differed based on the embryonic day of infection. On the day before preterm delivery, E6.5 infected mice did not experience significant upregulation of the inflammatory or antioxidant gene transcripts examined; however, peripheral and placental parasitemia correlated positively with Il1β, Cox1, Cat, and Hmox1 placental transcript abundance. E8.5 infected mice had elevated transcripts for Ifnγ, Tnf, Il10, Cox1, Cox2, Sod1, Sod2, Cat, and Nrf2, while Sod3 was the only transcript that correlated with parasitemia. Finally, E10.5 infected mice had elevated transcripts for Ifnγ only, with a tendency for Tnf transcripts to correlate with peripheral parasitemia. Tumor necrosis factor deficient (TNF-/-) and TNF receptor 1 deficient (TNFR1-/-) mice infected on E8.5 experienced preterm birth at the same time as B6 controls. Further characterization of this model is necessary to discover the mechanism(s) and/or trigger(s) responsible for malaria-driven preterm birth caused by maternal infection during early pregnancy.

Peripheral Plasmodium falciparum Infection in Early Pregnancy Is Associated With Increased Maternal Microchimerism in the Offspring

BACKGROUND

Placental malaria has been associated with increased cord blood maternal microchimerism (MMc), which in turn may affect susceptibility to malaria in the offspring. We sought to determine the impact of maternal peripheral Plasmodium falciparum parasitemia during pregnancy on MMc and to determine whether maternal cells expand during primary parasitemia in the offspring.

METHODS

We conducted a nested cohort study of maternal-infant pairs from a prior pregnancy malaria chemoprevention study. Maternal microchimerism was measured by quantitative polymerase chain reaction targeting a maternal-specific marker in genomic DNA from cord blood, first P falciparum parasitemia, and preparasitemia. Logistic and negative binomial regression were used to assess the impact of maternal peripheral parasitemia, symptomatic malaria, and placental malaria on cord blood MMc. Generalized estimating equations were used to assess predictors of MMc during infancy.

RESULTS

Early maternal parasitemia was associated with increased detection of cord blood MMc (adjusted odds ratio = 3.91, P = .03), whereas late parasitemia, symptomatic malaria, and placental malaria were not. The first parasitemia episode in the infant was not associated with increased MMc relative to preparasitemia.

CONCLUSIONS

Maternal parasitemia early in pregnancy may increase the amount of MMc acquired by the fetus. Future work should investigate the impact of this MMc on immune responses in the offspring.

A Genotyping Study in Benin Comparing the Carriage of Plasmodium falciparum Infections Before Pregnancy and in Early Pregnancy: Story of a Persistent Infection

Background

Malaria infections in the first trimester of pregnancy are frequent and deleterious for both mother and child health. To investigate if these early infections are newly acquired or already present in the host, we assessed whether parasites detected before pregnancy and those detected in early pregnancy are the same infection.

Methods

We used data from the preconceptional “RECIPAL” study (Benin, 2014–2017). Sixty-three pregnant women of 411 included who had a malaria infection detected by quantitative polymerase chain reaction both before pregnancy and at the first antenatal care (ANC) visit were selected for this study. Two highly polymorphic markers, msp-2 and glurp, and a fragment-analysis method were used to enumerate the Plasmodium falciparum genotypes and to quantify their proportions within isolates. An infection was considered as persistent when identical msp-2 and glurp genotypes were found in the corresponding prepregnancy and early-pregnancy samples.

Results

The median time between the 2 malaria screenings was 3 months. The median gestational age at the first ANC visit was 6.4 weeks. Most infections before pregnancy were submicroscopic infections. Based on both msp-2 and glurp genotyping, the infection was similar before and in early pregnancy in 46% (29/63) of cases.

Conclusions

Almost half of P. falciparum infections detected in the first trimester originate before pregnancy. Protecting young women from malaria infection before pregnancy might reduce the prevalence of malaria in early pregnancy and its related poor maternal and birth outcomes.

The role of ultra-sensitive molecular methods for detecting malaria - the broader perspective

Ultra-sensitive molecular diagnostics are lowering the limit of detection for malaria parasites in the blood and providing insights not captured by conventional tool such as microscopy and rapid antigen tests. Low-level malaria infections identified by molecular tools may influence clinical outcomes, transmission events, and elimination efforts. While many ultra-sensitive molecular methods require well-equipped laboratories, technologies such as loop-mediated isothermal amplification (LAMP) or recombinase polymerase amplification (RPA) provide more portable and analytically sensitive solutions. These tools may benefit asymptomatic patient screening, antenatal care, and elimination campaigns. We review the recent evidence, offer our perspective on the impact of these new tests and identify future research priorities.

Progress and new horizons toward a VAR2CSA-based placental malaria vaccine

Introduction: Several malaria vaccines are under various phases of development with some promising results. In placental malaria (PM) a deliberately anti-disease approach is considered as many studies have underlined the key role of VAR2CSA protein, which therefore represents the leading vaccine candidate. However, evidence indicates that VAR2CSA antigenic polymorphism remains an obstacle to overcome.Areas covered: This review analyzes the progress made thus far in developing a VAR2CSA-based vaccine, and addresses the current issues and challenges that must be overcome to develop an effective PM vaccine.Expert opinion: Phase I trials of PAMVAC and PRIMVAC VAR2CSA vaccines have shown more or less satisfactory results with regards to safety and immunogenicity. The second generation of VAR2CSA-based vaccines could benefit from optimization approaches to broaden the activity spectrum against various placenta-binding isolates through continued advances in the structural understanding of the interaction with CSA.

Teratogen update: Malaria in pregnancy and the use of antimalarial drugs in the first trimester

Malaria is a particular problem in pregnancy because of enhanced sensitivity, the possibility of placental malaria, and adverse effects on pregnancy outcome. Artemisinin-containing combination therapies (ACTs) are the most effective antimalarials known. WHO recommends 7-day quinine therapy for uncomplicated Plasmodium falciparum malaria in the first trimester despite the superior tolerability and efficacy of 3-day ACT regimens because artemisinins caused embryolethality and/or cardiovascular malformations at relatively low doses in rats, rabbits, and monkeys. The developmental toxicity of artesunate, artemether, and DHA were similar in rats but artesunate was embryotoxic at lower doses in rabbits (5 mg/kg/day) than artemether (no effect level = 25 mg/kg/day). In clinical studies in Africa, treatment with artemether-lumefantrine in the first trimester was observed to be highly efficacious and the miscarriage rate (≤3.1%) was similar to no antimalarial treatment (2.6%). When data from the first-trimester use of largely artesunate-based therapies in Thailand were pooled together, there was no difference in miscarriage rate compared to quinine. However, individually, artesunate-mefloquine was associated with a higher miscarriage rate (15/71 = 21%) compared to other artemisinin-based therapies including 7-day artesunate + clindamycin (2/50 = 4%) and quinine (92/842 = 11%). Thus, appropriate statistical comparisons of individual ACT groups are needed prior to assuming that they all have the same risk for developmental toxicity. Current limitations in the assessment of the safety of ACTs in the first trimester are a lack of exposures early in gestation (gestational weeks 6-7), limited postnatal evaluation for cardiovascular malformations, and the pooling of all ACTs for the assessment of risk.

VAR2CSA Antibodies in Non-Pregnant Populations

The Plasmodium falciparum protein VAR2CSA is a critical mediator of placental malaria, and VAR2CSA antibodies (IgGs) are important to protect pregnant women. Although infrequently detected outside pregnancy, VAR2CSA IgGs were reported in men and children from Colombia and Brazil and in select African populations. These findings raise questions about the specificity of VAR2CSA IgGs and the mechanisms by which they are acquired outside pregnancy. Here we review the data on VAR2CSA IgGs in men and children from different malaria-endemic regions. We discuss experimental factors that may affect interpretation of the serological data and consider the biological relevance of VAR2CSA IgGs in non-pregnant populations. We propose potential mechanisms for the acquisition of VARCSA IgGs outside of pregnancy. We identify knowledge gaps and research priorities.

Effects of Malaria in the First Trimester of Pregnancy on Poor Maternal and Birth Outcomes in Benin

BACKGROUND

In sub-Saharan Africa, malaria in the first half of pregnancy is harmful for both the mother and her fetus. However, malaria in the first trimester of pregnancy, when women are usually not protected against malaria, has been little investigated. For the first time, we assessed the effects of malaria in the first trimester on maternal and birth outcomes using a preconceptional study design.

METHODS

From June 2014 to March 2017, 1214 women of reproductive age were recruited and followed monthly until 411 became pregnant. The pregnant women were then followed from 5-6 weeks of gestation until delivery. Path analysis was used to assess the direct effect (ie, not mediated by malaria in the second or third trimester) of malaria in the first trimester on maternal anemia and poor birth outcomes. The cumulative effect of infections during pregnancy on the same outcomes was also evaluated.

RESULTS

The prevalence of malaria infections in the first trimester was 21.8%. Malaria in the first trimester was significantly associated with maternal anemia in the third trimester (adjusted odds ratio 2.25, 95% confidence interval 1.11-4.55). While we did not find evidence of any direct effect of first trimester malaria infections on birth outcomes, their association with infections later in pregnancy tended to increase the risk of low birth weights.

CONCLUSIONS

Malaria infections in the first trimester were highly prevalent and have deleterious effects on maternal anemia. They highlight the need for additional preventive measures, starting in early pregnancy or even before conception.

Active case detection of malaria in pregnancy using loop-mediated amplification (LAMP): a pilot outcomes study in South West Ethiopia

BACKGROUND

125 million women are pregnant each year in malaria endemic areas and are, therefore, at risk of Malaria in Pregnancy (MiP). MiP is the direct consequence of Plasmodium infection during pregnancy. The sequestration of Plasmodium falciparum parasites in the placenta adversely affects fetal development and impacts newborn birth weight. Importantly, women presenting with MiP commonly develop anaemia. In Ethiopia, the Ministry of Health recommends screening symptomatic women only at antenatal care visits with no formal intermittent preventive therapy. Since MiP can display low-level parasitaemia, current tests which include microscopy and RDT are challenged to detect these cases. Loop mediated isothermal Amplification (LAMP) technology is a highly sensitive technique for DNA detection and is field compatible. This study aims to evaluate the impact of active malaria case detection during pregnancy using LAMP technology in terms of birth outcomes.

METHODS

A longitudinal study was conducted in two health centres of the Kafa zone, South West Ethiopia. Both symptomatic and asymptomatic pregnant women were enrolled in the first or second trimester and allocated to either Standard of Care (SOC-microscopy and RDT) or LAMP (LAMP, microscopy and RDT). Women completed at least three visits prior to delivery, and the patient was referred for treatment if Plasmodium infection was detected by any of the testing methods. The primary outcome was to measure absolute birth weight, proportion of low birth weight, and maternal/neonatal haemoglobin in each arm. Secondary outcomes were to assess the performance of microscopy and RDT versus LAMP conducted in the field.

RESULTS

One hundred and ninety-nine women were included and assigned to either LAMP or SOC. Six were lost to follow up. In this cohort, 66.8% of women did not display any clinical symptoms and 70.9% were multi-parous. A reduced proportion of low birth weight newborns was observed in the LAMP group (0%) compared to standard of care (14%) (p <0.001). Improved neonatal haemoglobin was observed in the LAMP (13.1 g/dL) versus the SOC (12.8 g/dL) (p = 0.024) arm. RDT and microscopy had an analytical sensitivity of 66.7% and 55.6% compared to LAMP as a reference standard.

CONCLUSIONS

These results support the use of highly sensitive tools for rapid on-site active case detection of MiP which may improve birth outcomes in the absence of IPT. However, further large-scale studies are required to confirm this finding.

Use of a highly-sensitive rapid diagnostic test to screen for malaria in pregnancy in Indonesia

Background

The sensitivity of rapid diagnostic tests (RDTs) for malaria is inadequate for detecting low-density, often asymptomatic infections, such as those that can occur when screening pregnant women for malaria. The performance of the Alere™ Ultra-sensitive Malaria Ag Plasmodium falciparum RDT (uRDT) was assessed retrospectively in pregnant women in Indonesia.

Methods

The diagnostic performance of the uRDT and the CareStart™ Malaria HRP2/pLDH VOM (Plasmodium vivax, Plasmodium ovale and Plasmodium malariae) Combo RDT (csRDT) were assessed using 270 stored red blood cell pellets and plasma samples from asymptomatic pregnant women. These included 112 P. falciparum negative and 158 P. falciparum positive samples detected by a composite test (qPCR, LAMP, nPCR) as reference standard. Diagnostic indicators: sensitivity, specificity, negative predictive value (NPV), positive predictive value (PPV), diagnostic odds ratio (DOR) and the level of agreement (kappa) were calculated for comparison.

Results

Compared with the reference test, the uRDT had a sensitivity of 19.6% (95% CI 13.9–26.8) and specificity of 98.2% (93.1–99.7%). The csRDT was 22.8% (16.7–30.3) sensitive and 95.5% (89.4–98.3) specific for P. falciparum infections. Performance of the uRDT was non-significantly different to the csRDT (p = 0.169). RDT outcome was stratified by qPCR cycling threshold (Ct), and performance of the RDTs was found to be comparable across parasite loads.

Conclusion

The uRDT performed similarly to the currently used csRDTs in detecting P. falciparum infections in asymptomatic pregnant women. In these settings, molecular diagnostics are currently the most sensitive for malaria.

Prevalence and Associated Risk Factors of Malaria in the First Trimester of Pregnancy: A Preconceptional Cohort Study in Benin

Background

There is a lack of data on the burden of malaria in the first trimester of pregnancy in Africa, mainly because pregnant women generally attend the maternity clinic late. Bed nets are rarely provided to women before the second trimester of pregnancy and intermittent preventive treatment with sulfadoxine-pyrimethamine is not recommended before the second trimester, leaving women insufficiently or not protected in early pregnancy.

Methods

To assess the burden of first trimester malaria, 387 women were followed up monthly from preconception to delivery. They were screened for malaria monthly from early pregnancy until delivery. A logistic multilevel model was used to assess maternal factors associated with malaria during the first trimester.

Results

The proportion of women with at least 1 microscopic malaria infection during the first trimester of pregnancy was 20.8%. Women infected with malaria preconception were more likely to be infected during the first trimester (adjusted odds ratio: 2.68; 95% confidence interval, 1.24-5.78). Early gestational age was also positively correlated with malaria infection.

Conclusions

Using a preconceptional study design, we showed that malaria was highly prevalent in early pregnancy. This calls for the assessment of new strategies that could protect women as soon as the first trimester.

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.

Plasmodium falciparum Infection Early in Pregnancy has Profound Consequences for Fetal Growth

Malaria during pregnancy constitutes a large health problem in areas of endemicity. The World Health Organization recommends that interventions are initiated at the first antenatal visit, and these improve pregnancy outcomes. This study evaluated fetal growth by ultrasonography and birth outcomes in women who were infected prior to the first antenatal visit (gestational age, <120 days) and not later in pregnancy. Compared with uninfected controls, women with early Plasmodium falciparum exposure had retarded intrauterine growth between gestational ages of 212 and 253 days (difference between means, 107 g [95% confidence interval {CI}, 26-188]; P = .0099) and a shorter pregnancy duration (difference between means, 6.6 days [95% CI, 1.0-112.5]; P = .0087). The birth weight (difference between means, 221 g [95% CI, 6-436]; P = .044) and the placental weight (difference between means, 84 g [95% CI, 18-150]; P = .013) at term were also reduced. The study suggests that early exposure to P. falciparum, which is not targeted for prevention by current control strategies, has a profound impact on fetal growth, pregnancy duration, and placental weight at term.

Fetal-Derived MyD88 Signaling Contributes to Poor Pregnancy Outcomes During Gestational Malaria

Placental malaria (PM) remains a severe public health problem in areas of high malaria transmission. Despite the efforts to prevent infection poor outcomes in Plasmodium endemic areas, there is still a considerable number of preterm births and newborns with low birth weight resulting from PM. Although local inflammation triggered in response to malaria is considered crucial in inducing placental damage, little is known about the differential influence of maternal and fetal immune responses to the disease progression. Therefore, using a PM mouse model, we sought to determine the contribution of maternal and fetal innate immune responses to PM development. For this, we conducted a series of cross-breeding experiments between mice that had differential expression of the MyD88 adaptor protein to obtain mother and correspondent fetuses with distinct genetic backgrounds. By evaluating fetal weight and placental vascular spaces, we have shown that the expression of MyD88 in fetal tissue has a significant impact on PM outcomes. Our results highlighted the existence of a distinct contribution of maternal and fetal immune responses to PM onset. Thus, contributing to the understanding of how inflammatory processes lead to the dysregulation of placental homeostasis ultimately impairing fetal development.

Matched Placental and Circulating Plasmodium falciparum Parasites are Genetically Homologous at the var2csa ID1-DBL2X Locus by Deep Sequencing

In pregnancy-associated malaria, infected erythrocytes accumulate in the placenta. It is unclear if in polyclonal infections this results in distinct peripheral and placental parasite populations. We used long amplicon deep sequencing of Plasmodium falciparum var2csa ID1-DBL2X from 15 matched peripheral and placental samples collected at delivery from a high transmission area to determine genetic homology. Despite substantial sequence variation and detecting 23 haplotypes, the matched pairs mostly contained the same genetic variants, with 11 pairs sharing 100% of their variants, whereas others showed some heterogeneity. Thus, at delivery, peripheral and placental parasites appear to intermix and placental genotypes can be inferred through peripheral sampling.

Malaria in pregnancy: the relevance of animal models for vaccine development

Malaria during pregnancy due to Plasmodium falciparum or P. vivax is a major public health problem in endemic areas, with P. falciparum causing the greatest burden of disease. Increasing resistance of parasites and mosquitoes to existing tools, such as preventive antimalarial treatments and insecticide-treated bed nets respectively, is eroding the partial protection that they offer to pregnant women. Thus, development of effective vaccines against malaria during pregnancy is an urgent priority. Relevant animal models that recapitulate key features of the pathophysiology and immunology of malaria in pregnant women could be used to accelerate vaccine development. This review summarizes available rodent and nonhuman primate models of malaria in pregnancy, and discusses their suitability for studies of biologics intended to prevent or treat malaria in this vulnerable population.

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.

Burden, pathology, and costs of malaria in pregnancy: new developments for an old problem

Over the past 10 years, knowledge of the burden, economic costs, and consequences of malaria in pregnancy has improved, and the prevalence of malaria caused by Plasmodium falciparum has declined substantially in some geographical areas. In particular, studies outside of Africa have increased the evidence base of Plasmodium vivax in pregnancy. Rapid diagnostic tests have been poor at detecting malaria in pregnant women, while PCR has shown a high prevalence of low density infection, the clinical importance of which is unknown. Erythrocytes infected with P falciparum that express the surface protein VAR2CSA accumulate in the placenta, and VAR2CSA is an important target of protective immunity. Clinical trials for a VAR2CSA vaccine are ongoing, but sequence variation needs to be carefully studied. Health system and household costs still limit access to prevention and treatment services. Within the context of malaria elimination, pregnant women could be used to monitor malaria transmission. This Series paper summarises recent progress and highlights unresolved issues related to the burden of malaria in pregnancy.

Variations in the leukocyte and cytokine profiles between placental and maternal circulation in pregnancy-associated malaria

BACKGROUND

Activation of immune cells by malaria infection induces the secretion of cytokines and the synthesis of other inflammatory mediators. This study compared the cytokine levels and leukocyte count between malaria-infected peripheral and placental blood of pregnant women before delivery and postpartum. The cytokines assessed include interferon gamma (IFN-γ), tumor necrosis factor alpha (TNF-α), interleukin-4 (IL-4), interleukin-6 (IL-6) and interleukin-10 (IL-10).

MATERIALS AND METHODS

The subjects comprised 144 malaria-infected pregnant women and 60 malaria-infected women at post-partum stage (for placental blood collection). Others were 60 malaria-uninfected pregnant women and 40 malaria-uninfected women at postpartum stage (for placental blood collection). Forty malaria-infected and 40 malaria-uninfected nonpregnant women served as control subjects. The test groups were asymptomatic, and the control groups were apparently healthy subjects. All were aged between 17 and 44 years. Ethical approval for the study was obtained at Abia State University Teaching Hospital and Living Word Mission Hospital, Aba. Informed consent was obtained from the participants. Blood samples were aseptically collected initially from the maternal peripheral circulation and from the placenta on delivery, and tested for HIV and malaria using standard methods. IFN-γ, TNF-α, IL-4, IL-6 and IL-10 were measured by enzyme-linked immunosorbent assay technique. Kruskal-Wallis test was used for comparison of the groups.

RESULTS

IFN-γ was significantly higher in the peripheral than in placental blood (P=0.001). IL-4 and IL-10 were significantly lower in the peripheral than in placental blood (P=0.001 and P=0.004, respectively). The total leukocytes, neutrophils and lymphocyte counts were significantly higher in the placenta than in peripheral blood (P=0.001), and the mixed differential count was significantly higher in the placenta than in peripheral blood (P=0.012).

CONCLUSION

This study has shown that the cytokine levels and leukocyte counts may differ between the peripheral and placental blood of the same women. Therefore, measurement of parameters in the peripheral circulation may not always reflect the levels in the placental blood for the assessment of immune cellular response at the materno-fetal interface.

Plasmodium falciparum genotypes in matched peripheral, placental and umbilical cord blood in an area characterised by unstable malaria transmission in eastern Sudan

Background

There are few published studies on Plasmodium falciparum genotypes in peripheral, placental and umbilical cord blood in areas characterised by unstable malaria transmission.

Method

A cross-sectional study was conducted to investigate P. falciparum genotypes in matched peripheral, placental and umbilical cord blood in eastern Sudan. Thick blood smears and P. falciparum merozoite surface protein 1 (MSP1) and 2 (MSP2) genes as polymorphic markers in polymerase chain reactions were investigated in 3 kinds of samples of 153 pregnant women at delivery.

Results

There was no significant difference in the prevalence of blood film-detected P. falciparum in which 5 (3.3%), 7 (4.6%) and 3 (2.0%) (P = 0.437) of the 153 samples were determined to be P. falciparum-positive by microscopy for maternal peripheral, placental and cord blood samples, respectively.

Out of these 145 samples, 24 (16.6%), 39 (26.9%) and 24 (16.6%) (P = 0.039) of the peripheral, placental and cord samples, respectively, had submicroscopic parasitaemia (blood films were negative). There was no association between submicroscopic parasitaemia and age or parity.

RO33 and K1 (MSP1 alleles) were detected in 21/29 (72.4%), 42/46 (85.7%), 26/27 (92.2%) and 6/29 (20.6), 16/46 (32.6) and 0(0) (P < 0.001) of the maternal, placental and cord samples, respectively. MAD20 was not detected in any of the samples.

While the 3D7/IC1 allele was detected in 12 (41.3%), 30 (65.2%) and 4 (14.8%) (P < 0.001) of the peripheral, placental and cord samples, respectively, the FC (MSP2) allele was detected in only the 6 (20.6) placental samples. Multi-clonal infection was detected in 10 (34.4), 27 (58.6) and 3 (11.1) (P < 0.001) of the maternal placental and cord samples, respectively.

Conclusion

Compared with the peripheral and cord samples, placental samples had a higher prevalence of submicroscopic parasitaemia. MSP1 alleles were predominant in the cord, while MSP2 alleles were predominant in the placental samples, which had a significant higher multiplicity of the infection.

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.

Association between malaria immunity and pregnancy outcomes among Malawian pregnant women receiving nutrient supplementation

Background

Malaria antibody responses measured at delivery have been associated with protection from maternal anaemia and low birth weight deliveries. Whether malarial antibodies present in the first half of pregnancy may protect from these or other poor birth outcomes is unclear. To determine whether malaria antibodies in the first half of pregnancy predict pregnancy outcomes, antibodies were measured to a range of merozoite antigens and to antigens expressed on the surface of parasitized red blood cells (pRBCs) in plasma samples collected at 14–20 weeks of gestation from Malawian women. The latter antibodies were measured as total IgG to pRBCs, and antibodies promoting opsonic phagocytosis of pRBCs. Associations between antibodies and maternal haemoglobin in late pregnancy or newborn size were investigated, after adjusting for potential covariates.

Results

Antibodies to pRBC surface antigens were associated with higher haemoglobin concentration at 36 weeks. Total IgG to pRBCs was associated with 0.4 g/l [(95% confidence interval (0.04, 0.8)] increase in haemoglobin, and opsonizing antibody with 0.5 (0.05, 0.9) increase in haemoglobin for each 10% increase in antibody. These antibodies were not associated with birthweight, placental malaria, or newborn anthropometrics. Antibodies to merozoite antigens and non-placental-binding IEs were not associated with decreased risk of any of these outcomes. In some instances, they were negatively associated with outcomes of interest.

Conclusion

Antibodies to placental-binding infected erythrocytes may be associated with higher haemoglobin levels in pregnancy, whereas antibodies to other malaria antigens may instead be markers of malaria exposure.

Trial registration clinicaltrials.gov NCT01239693. Registered Nov 10, 2010.

Parasite dynamics in the peripheral blood and the placenta during pregnancy-associated malaria infection

Background

Malaria infections during pregnancy lead to sequestration of parasite infected red blood cells in the placenta. Placental infection can result in adverse outcomes for mothers and infants. Despite many studies, it remains unclear which peripheral blood infections during pregnancy lead to development of placental malaria. Understanding the timing of peripheral infections that lead to placental malaria and the ability of intermittent preventive treatment with sulfadoxine–pyrimethamine (SP-IPT) and artemisinin-based combination therapy to clear infections will enable the rational design of new interventions to decrease the burden of malaria in pregnancy.

Methods

Microsatellite markers were used to genotype peripheral and placental malaria infections in an observational cohort in Blantyre, Malawi. Genotypes were compared to determine the timing of infections that sequester in the placenta. The effects of SP-IPT and artemether–lumefantrine as curative treatment were also evaluated by assessing the occurrence of peripheral infections or matching genotypes between peripheral and placental parasites following treatment.

Results

Genotypes from 92 peripheral samples prior to delivery, 26 peripheral samples at delivery, and 29 placental samples were compared. Thirty percent of women with genotyped parasites in their placentas that had peripheral infections detected during pregnancy had matching peripheral-placental genotypes. Matching genotypes were not associated with gestational age and occurred from 13 to 39 weeks. Among women with more than one genotyped peripheral infection during pregnancy, 80 % had persistent infection with the same genotype while the remaining were new infections. Among infections treated with SP or artemether–lumefantrine, 28/84 (33 %) and 9/56 (16 %) had infection detected after treatment, respectively. Recrudescent infections were detected after both treatments and occurred up to 76 days after treatment. Women treated with SP-IPT and artemether–lumefantrine had genotypes matching treated infections detected in the placenta.

Conclusions

Placental malaria can occur at any time during pregnancy. In the context of late enrollment in antenatal care, interventions that protect all women of childbearing age and throughout pregnancy are needed. Currently used medications do not always clear peripheral or placental infections. The ability of anti-malarial drugs to prevent or clear placental infections should be considered in the development of future interventions.

Heterologous Infection of Pregnant Mice Induces Low Birth Weight and Modifies Offspring Susceptibility to Malaria

Pregnancy malaria (PM) is associated with poor pregnancy outcomes, and can arise due to relapse, recrudescence or a re-infection with heterologous parasites. We have used the Plasmodium chabaudi model of pregnancy malaria in C57BL/6 mice to examine recrudescence and heterologous infection using CB and AS parasite strains. After an initial course of patent parasitemia and first recrudescence, CB but not AS parasites were observed to recrudesce again in most animals that became pregnant. Pregnancy exacerbated heterologous CB infection of AS-experienced mice, leading to mortality and impaired post-natal growth of pups. Parasites were detected in placental blood without evidence of sequestration, unlike P. falciparum but similar to other malaria species that infect pregnant women. Inflammatory cytokine levels were elevated in pregnant females during malaria, and associated with intensity of infection and with poor outcomes. Pups born to dams during heterologous infection were more resistant to malaria infections at 6–7 weeks of age, compared to pups born to malaria-experienced but uninfected dams or to malaria-naïve dams. In summary, our mouse model reproduces several features of human PM, including recrudescences, heterologous infections, poor pregnancy outcomes associated with inflammatory cytokines, and modulation of offspring susceptibility to malaria. This model should be further studied to explore mechanisms underlying PM pathogenesis.

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.

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.

Identification of a Major Dimorphic Region in the Functionally Critical N-Terminal ID1 Domain of VAR2CSA

The VAR2CSA protein of Plasmodium falciparum is transported to and expressed on the infected erythrocyte surface where it plays a key role in placental malaria (PM). It is the current leading candidate for a vaccine to prevent PM. However, the antigenic polymorphism integral to VAR2CSA poses a challenge for vaccine development. Based on detailed analysis of polymorphisms in the sequence of its ligand-binding N-terminal region, currently the main focus for vaccine development, we assessed var2csa from parasite isolates infecting pregnant women. The results reveal for the first time the presence of a major dimorphic region in the functionally critical N-terminal ID1 domain. Parasite isolates expressing VAR2CSA with particular motifs present within this domain are associated with gravidity- and parasite density-related effects. These observations are of particular interest in guiding efforts with respect to optimization of the VAR2CSA-based vaccines currently under development.

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.

The impact of lipid-based nutrient supplementation on anti-malarial antibodies in pregnant women in a randomized controlled trial

Background

Malaria and undernutrition frequently coexist, especially in pregnant women and young children. Nutrient supplementation of these vulnerable groups might reduce their susceptibility to malaria by improving immunity.

Methods

Antibody immunity to antigens expressed by a placental-binding parasite isolate, a non-placental binding parasite isolate, merozoites and schizonts at enrolment (before 20 gestation weeks) and at 36 gestation weeks were measured in 1,009 Malawian pregnant women receiving a daily lipid-based nutrient supplement, multiple micronutrients or iron and folic acid, who were participants in a randomized clinical trial assessing the effects of nutrient supplementation on pregnancy outcomes and child development(registration ID: NCT01239693).

Results

Antibodies to placental-binding isolates significantly increased while antibodies to most merozoite antigens declined over pregnancy. Overall, after adjustment for covariates, the type of supplementation did not influence antibody levels at 36 gestation weeks or the rate of change in antibody levels from enrolment to 36 weeks. A negative association between maternal body mass index and opsonizing antibodies to placental-binding antigens (coefficient (95% CI) -1.04 (−1.84, −0.24), was observed. Similarly, women with higher socioeconomic status had significantly lower IgG and opsonizing antibodies to placental-binding antigens. Neither of these associations was significantly influenced by the supplementation type.

Conclusions

In the current cohort nutrient supplementation did not affect anti-malarial antibody responses, but poor and undernourished mothers should be a priority group in future trials.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-015-0707-2) contains supplementary material, which is available to authorized users.

Parity-dependent recognition of DBL1X-3X suggests an important role of the VAR2CSA high-affinity CSA-binding region in the development of the humoral response against placental malaria

Plasmodium falciparum multidomain protein VAR2CSA stands today as the leading vaccine candidate against pregnancy-associated malaria (PAM). Most of the studies aiming to decrypt how naturally acquired immunity develops have assessed the immune recognition of individual VAR2CSA Duffy-binding-like (DBL) domains, thus overlooking the presence of conformational epitopes resulting from the overall folding of the full-length protein. In order to characterize the development of humoral immunity toward VAR2CSA, we made use of a large cohort of 293 Senegalese pregnant women to assess the level of recognition by plasma IgG of the full-length VAR2CSA protein of the 3D7 parasite strain (3D7-VAR2CSA), the CSA-binding multidomains 3D7-DBL1X to -DBL3X (3D7-DBL1X-3X), and the CSA nonbinding multidomains 3D7-DBL4ε to -DBL6ε (3D7-DBL4ε-6ε), as well as individual 3D7-DBL domains. Our results revealed a parity-dependent recognition of the full-length 3D7-VAR2CSA and of the CSA-binding region, 3D7-DBL1X-3X. Indeed, multigravid women possess significantly higher levels of antibodies directed against these constructs than primigravidae. Our results suggest an important role of antibodies targeting the CSA-binding region in the development of immunity against PAM, therefore providing new insights on how natural protection might be acquired and further information for the design of VAR2CSA-based vaccines.

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.

Llama immunization with full-length VAR2CSA generates cross-reactive and inhibitory single-domain antibodies against the DBL1X domain

VAR2CSA stands today as the leading vaccine candidate aiming to protect future pregnant women living in malaria endemic areas against the severe clinical outcomes of pregnancy associated malaria (PAM). The rational design of an efficient VAR2CSA-based vaccine relies on a profound understanding of the molecular interactions associated with P. falciparum infected erythrocyte sequestration in the placenta. Following immunization of a llama with the full-length VAR2CSA recombinant protein, we have expressed and characterized a panel of 19 nanobodies able to recognize the recombinant VAR2CSA as well as the surface of erythrocytes infected with parasites originating from different parts of the world. Domain mapping revealed that a large majority of nanobodies targeted DBL1X whereas a few of them were directed towards DBL4ε, DBL5ε and DBL6ε. One nanobody targeting the DBL1X was able to recognize the native VAR2CSA protein of the three parasite lines tested. Furthermore, four nanobodies targeting DBL1X reproducibly inhibited CSA adhesion of erythrocytes infected with the homologous NF54-CSA parasite strain, providing evidences that DBL1X domain is part or close to the CSA binding site. These nanobodies could serve as useful tools to identify conserved epitopes shared between different variants and to characterize the interactions between VAR2CSA and CSA.

Burden of malaria in early pregnancy: a neglected problem?

According to the current World Health Organization guidelines, the drug prevention of malaria during pregnancy does not adequately cover the first trimester of gestation in high-transmission areas. Although the pathophysiological mechanisms of early infections are not completely understood yet, a review of the most recent studies on the topic suggests that their consequences are serious in terms of maternal anemia and low birth weight. Consequently, there is a need to focus on the awareness of women in a period hard to access, to develop safe drugs to be used in the first trimester, and to consider preconceptional interventions in teenage girls, such as a new malaria vaccine to be used in pregnancy.

Cytoadherence phenotype of Plasmodium falciparum-infected erythrocytes is associated with specific pfemp-1 expression in parasites from children with cerebral malaria

Background

Cytoadherence of Plasmodium falciparum-infected erythrocytes (IEs) in deep microvasculature endothelia plays a major role in the pathogenesis of cerebral malaria (CM). This biological process is thought to be mediated by P. falciparum erythrocyte membrane protein-1 (PfEMP-1) and human receptors such as CD36 and ICAM-1. The relationship between the expression of PfEMP-1 variants and cytoadherence phenotype in the pathology of malaria is not well established.

Methods

Cytoadherence phenotypes of IEs to CD36, ICAM-1, CSPG and the transcription patterns of A, B, var2csa, var3, var gene groups and domain cassettes DC8 and DC13 were assessed in parasites from children with CM and uncomplicated malaria (UM) to determine if cytoadherence is related to a specific transcription profile of pfemp-1 variants.

Results

Parasites from CM patients bind significantly more to CD36 than those from UM patients, but no difference was observed in their binding ability to ICAM-1 and CSPG. CM isolates highly transcribed groups A, B, var2csa, var3, DC8 and DC13 compared to UM parasites. The high transcription levels of var genes belonging to group B positively correlated with increased binding level to CD36.

Conclusion

CM isolates bind significantly more to CD36 than to ICAM-1, which was correlated with high transcription level of group B var genes, supporting their implication in malaria pathogenesis.

Estimated risk of placental infection and low birthweight attributable to Plasmodium falciparum malaria in Africa in 2010: a modelling study

BACKGROUND

Plasmodium falciparum infection during pregnancy leads to adverse outcomes including low birthweight; however, contemporary estimates of the potential burden of malaria in pregnancy in Africa (in the absence of interventions) are poor. We aimed to estimate the need to protect pregnant women from malaria across Africa.

METHODS

Using a mathematical model applied to estimates of the geographical distribution of P falciparum across Africa in 2010, we estimated the number of pregnant women who would have been exposed to infection that year in the absence of pregnancy-specific intervention. We then used estimates of the parity-dependent acquisition of immunity to placental infection and associated risk of low birthweight to estimate the number of women who would have been affected.

FINDINGS

We estimate that, without pregnancy-specific protection, 12·4 million pregnant women (44·9% of all 27·6 million livebirths in malaria endemic areas in Africa in 2010) would have been exposed to infection, with 11·4 million having placental infection (41·2% of all livebirths). This infection leads to an estimated 900,000 (95% credible interval [CrI] 530,000-1,240,000) low birthweight deliveries per year. Around the end of the first trimester, when the placenta becomes susceptible to infection, is a key period during which we estimate that 65·2% (95% CrI 60·9-70·0) of placental infections first occur.

INTERPRETATION

Our calculations are the only contemporary estimates of the geographical distribution of placental infection and associated low birthweight. The risk of placental infection across Africa in unprotected women is high. Prevention of malaria before conception or very early in pregnancy is predicted to greatly reduce incidence of low birthweight, especially in primigravidae. The underlying lifetime risk of low birthweight changes slowly with decreasing transmission, drawing attention to the need to maintain protection as transmission falls.

FUNDING

Malaria in Pregnancy Consortium and the Bill & Melinda Gates Foundation.

Dynamics in the cytoadherence phenotypes of Plasmodium falciparum infected erythrocytes isolated during pregnancy

Pregnant women become susceptible to malaria infection despite their acquired immunity to this disease from childhood. The placental sequestration of Plasmodium falciparum infected erythrocytes (IE) is the major feature of malaria during pregnancy, due to ability of these parasites to bind chondroitin sulfate A (CSA) in the placenta through the VAR2CSA protein that parasites express on the surface of IE. We collected parasites at different times of pregnancy and investigated the adhesion pattern of freshly collected isolates on the three well described host receptors (CSPG, CD36 and ICAM-1). Var genes transcription profile and VAR2CSA surface-expression were assessed in these isolates. Although adhesion of IE to CD36 and ICAM-1 was observed in some isolates, CSA-adhesion was the predominant binding feature in all isolates analyzed. Co-existence in the peripheral blood of several adhesion phenotypes in early pregnancy isolates was observed, a diversity that gradually tightens with gestational age in favour of the CSA-adhesion phenotype. Infections occurring in primigravidae were often by parasites that adhered more to CSA than those from multigravidae. Data from this study further emphasize the specificity of CSA adhesion and VAR2CSA expression by parasites responsible for pregnancy malaria, while drawing attention to the phenotypic complexity of infections occurring early in pregnancy as well as in multigravidae.

Kinetics of B cell responses to Plasmodium falciparum erythrocyte membrane protein 1 in Ghanaian women naturally exposed to malaria parasites

Naturally acquired protective immunity to Plasmodium falciparum malaria takes years to develop. It relies mainly on Abs, particularly IgG specific for Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) proteins on the infected erythrocyte surface. It is only partially understood why acquisition of clinical protection takes years to develop, but it probably involves a range of immune-evasive parasite features, not least of which are PfEMP1 polymorphism and clonal variation. Parasite-induced subversion of immunological memory and expansion of "atypical" memory B cells may also contribute. In this first, to our knowledge, longitudinal study of its kind, we measured B cell subset composition, as well as PfEMP1-specific Ab levels and memory B cell frequencies, in Ghanaian women followed from early pregnancy up to 1 y after delivery. Cell phenotypes and Ag-specific B cell function were assessed three times during and after pregnancy. Levels of IgG specific for pregnancy-restricted, VAR2CSA-type PfEMP1 increased markedly during pregnancy and declined after delivery, whereas IgG levels specific for two PfEMP1 proteins not restricted to pregnancy did not. Changes in VAR2CSA-specific memory B cell frequencies showed typical primary memory induction among primigravidae and recall expansion among multigravidae, followed by contraction postpartum in all. No systematic changes in the frequencies of memory B cells specific for the two other PfEMP1 proteins were identified. The B cell subset analysis confirmed earlier reports of high atypical memory B cell frequencies among residents of P. falciparum-endemic areas, and indicated an additional effect of pregnancy. Our study provides new knowledge regarding immunity to P. falciparum malaria and underpins efforts to develop PfEMP1-based vaccines against this disease.

Functional antibodies against VAR2CSA in nonpregnant populations from colombia exposed to Plasmodium falciparum and Plasmodium vivax

In pregnancy, parity-dependent immunity is observed in response to placental infection with Plasmodium falciparum. Antibodies recognize the surface antigen, VAR2CSA, expressed on infected red blood cells and inhibit cytoadherence to the placental tissue. In most settings of malaria endemicity, antibodies against VAR2CSA are predominantly observed in multigravid women and infrequently in men, children, and nulligravid women. However, in Colombia, we detected antibodies against multiple constructs of VAR2CSA among men and children with acute P. falciparum and Plasmodium vivax infection. The majority of men and children (>60%) had high levels of IgGs against three recombinant domains of VAR2CSA: DBL5ε, DBL3X, and ID1-ID2. Surprisingly, these antibodies were observed only in pregnant women, men, and children exposed either to P. falciparum or to P. vivax. Moreover, the anti-VAR2CSA antibodies are of high avidity and efficiently inhibit adherence of infected red blood cells to chondroitin sulfate A in vitro, suggesting that they are specific and functional. These unexpected results suggest that there may be genotypic or phenotypic differences in the parasites of this region or in the host response to either P. falciparum or P. vivax infection outside pregnancy. These findings may hold significant clinical relevance to the pathophysiology and outcome of malaria infections in this region.

Impact of pregnancy-associated malaria on infant malaria infection in southern Benin

BACKGROUND

Infants of mothers with placental Plasmodium falciparum infections at delivery are themselves more susceptible to malaria attacks or to infection in early life.

METHODOLOGY/ PRINCIPAL FINDINGS

To assess the impact of either the timing or the number of pregnancy-associated malaria (PAM) infections on the incidence of parasitemia or malaria attacks in infancy, we followed 218 mothers through pregnancy (monthly visits) up to delivery and their infants from birth to 12 months of age (fortnightly visits), collecting detailed clinical and parasitological data. After adjustment on location, mother's age, birth season, bed net use, and placental malaria, infants born to a mother with PAM during the third trimester of pregnancy had a significantly increased risk of infection (OR [95% CI]: 4.2 [1.6; 10.5], p = 0.003) or of malaria attack (4.6 [1.7; 12.5], p = 0.003). PAM during the first and second trimesters had no such impact. Similarly significant results were found for the effect of the overall number of PAM episodes on the time to first parasitemia and first malaria attack (HR [95% CI]: 2.95 [1.58; 5.50], p = 0.001 and 3.19 [1.59; 6.38], p = 0.001) respectively.

CONCLUSIONS/ SIGNIFICANCE

This study highlights the importance of protecting newborns by preventing repeated episodes of PAM in their mothers.

Differential adhesion-inhibitory patterns of antibodies raised against two major variants of the NTS-DBL2X region of VAR2CSA

BACKGROUND

VAR2CSA is a large polymorphic Plasmodium falciparum protein expressed on infected erythrocytes (IE) that allows their binding in the placenta, thus precipitating placental malaria (PM). The N-terminal part of VAR2CSA that contains the binding site to placental chondroitin sulfate A (CSA) is currently recognized as the most attractive region for vaccine development. An ultimate challenge is to define epitopes in this region that induce a broad cross-reactive adhesion inhibitory antibody response.

METHODS

Based on phylogenetic data that identified a dimorphic sequence motif in the VAR2CSA DBL2X, we raised antibodies against the NTS-DBL2X constructs containing one sequence or the other (3D7 and FCR3) and tested their functional properties on P. falciparum isolates from pregnant women and on laboratory-adapted strains.

RESULTS

The CSA binding inhibitory capacity of the antibodies induced varied from one parasite isolate to another (range, 10%–100%), but the combined analysis of individual activity highlighted a broader functionality that increased the total number of isolates inhibited. Interestingly, the differential inhibitory effect of the antibodies observed on field isolates resulted in significant inhibition of all field isolates tested, suggesting that optimal inhibitory spectrum on field isolates from pregnant women might be achieved with antibodies targeting limited variants of the N-terminal VAR2CSA.

CONCLUSIONS

Our findings indicate that the NTS-DBL2X region of VAR2CSA can elicit strain-transcending anti-adhesion antibodies and suggest that the combination of the two major variants used here could represent the basis for an effective bivalent VAR2CSA-based vaccine.

Cytoadherence of erythrocytes invaded by Plasmodium falciparum: quantitative contact-probing of a human malaria receptor

Cytoadherence of red blood cells (RBCs) invaded by Plasmodium falciparum parasites is an important contributor to the sequestration of RBCs, causing reduced microcirculatory flow associated with fatal malaria syndromes. The phenomenon involves a parasite-derived variant antigen, the P. falciparum erythrocyte membrane protein 1 (PfEMP1), and several human host receptors, such as chondroitin sulfate A (CSA), which has been explicitly implicated in placental malaria. Elucidating the molecular mechanisms of cytoadherence requires quantitative evaluation, under physiologically relevant conditions, of the specific receptor-ligand interactions associated with pathological states of cell-cell adhesion. Such quantitative studies have not been reported thus far for P. falciparum malaria under conditions of febrile temperatures that accompany malarial infections. In this study, single RBCs infected with P. falciparum parasites (CSA binding phenotype) in the trophozoite stage were engaged in mechanical contact with the surface of surrogate cells specifically expressing CSA, so as to quantify cytoadherence to human syncytiotrophoblasts in a controlled manner. From these measurements, a mean rupture force of 43pN was estimated for the CSA-PfEMP1 complex at 37°C. Experiments carried out at febrile temperature showed a noticeable decrease in CSA-PfEMP1 rupture force (by about 23% at 41°C and about 20% after a 40°C heat treatment), in association with an increased binding frequency. The decrease in rupture force points to a weakened receptor-ligand complex after exposure to febrile temperature, while the rise in binding frequency suggests an additional display of nonspecific binding molecules on the RBC surface. The present work establishes a robust experimental method for the quantitative assessment of cytoadherence of diseased cells with specific molecule-mediated binding.