Evaluation of the antigenic diversity of placenta-binding Plasmodium falciparum variants and the antibody repertoire among pregnant women

Reduced risk of placental parasitemia associated with complement fixation on Plasmodium falciparum by antibodies among pregnant women

BACKGROUND

The pathogenesis of malaria in pregnancy (MiP) involves accumulation of P. falciparum-infected red blood cells (pRBCs) in the placenta, contributing to poor pregnancy outcomes. Parasite accumulation is primarily mediated by P. falciparum erythrocyte membrane protein 1 (PfEMP1). Magnitude of IgG to pRBCs has been associated with reduced risk of MiP in some studies, but associations have been inconsistent. Further, antibody effector mechanisms are poorly understood, and the role of antibody complement interactions is unknown.

METHODS

Studying a longitudinal cohort of pregnant women (n=302) from a malaria-endemic province in Papua New Guinea (PNG), we measured the ability of antibodies to fix and activate complement using placental binding pRBCs and PfEMP1 recombinant domains. We determined antibody-mediated complement inhibition of pRBC binding to the placental receptor, chondroitin sulfate A (CSA), and associations with protection against placental parasitemia.

RESULTS

Some women acquired antibodies that effectively promoted complement fixation on placental-binding pRBCs. Complement fixation correlated with IgG1 and IgG3 antibodies, which dominated the response. There was, however, limited evidence for membrane attack complex activity or pRBC lysis or killing. Importantly, a higher magnitude of complement fixing antibodies was prospectively associated with reduced odds of placental infection at delivery. Using genetically modified P. falciparum and recombinant PfEMP1 domains, we found that complement-fixing antibodies primarily targeted a specific variant of PfEMP1 (known as VAR2CSA). Furthermore, complement enhanced the ability of antibodies to inhibit pRBC binding to CSA, which was primarily mediated by complement C1q protein.

CONCLUSIONS

These findings provide new insights into mechanisms mediating immunity to MiP and reveal potential new strategies for developing malaria vaccines that harness antibody-complement interactions.

High Antibodies to VAR2CSA in Response to Malaria Infection Are Associated With Improved Birthweight in a Longitudinal Study of Pregnant Women

Introduction

Pregnant women have an increased risk of P. falciparum infection, which is associated with low birth weight and preterm delivery. VAR2CSA, a variant surface antigen expressed on the parasitized erythrocyte surface, enables sequestration in the placenta. Few studies have prospectively examined relationships between antibody responses during pregnancy and subsequent adverse birth outcomes, and there are limited data outside Africa.

Methods

Levels of IgG against VAR2CSA domains (DBL3; DBL5) and a VAR2CSA-expressing placental-binding P. falciparum isolate (PfCS2-IE) were measured in 301 women enrolled at their first visit to antenatal care which occurred mid-pregnancy (median = 26 weeks, lower and upper quartiles = 22, 28). Associations between antibody levels at enrolment and placental infection, birthweight and estimated gestational age at delivery were assessed by linear and logistic regression with adjustment for confounders. For all outcomes, effect modification by gravidity and peripheral blood P. falciparum infection at enrolment was assessed.

Results

Among women who had acquired P. falciparum infection at enrolment, those with higher levels of VAR2CSA antibodies (75^th percentile) had infants with higher mean birthweight (estimates varied from +35g to +149g depending on antibody response) and reduced adjusted odds of placental infection (aOR estimates varied from 0.17 to 0.80), relative to women with lower levels (25^th percentile) of VAR2CSA antibodies. However, among women who had not acquired an infection at enrolment, higher VAR2CSA antibodies were associated with increased odds of placental infection (aOR estimates varied from 1.10 to 2.24).

Conclusions

When infected by mid-pregnancy, a better immune response to VAR2CSA-expressing parasites may contribute to protecting against adverse pregnancy outcomes.

The Development, Fine Specificity, and Importance of High-Avidity Antibodies to VAR2CSA in Pregnant Cameroonian Women Living in Yaoundé, an Urban City

Pregnant women infected with Plasmodium falciparum often produce antibodies (Abs) to VAR2CSA, a ligand that binds to placental chondroitin sulfate A causing placental malaria (PM). Antibodies to VAR2CSA are associated with improved pregnancy outcomes. Antibody avidity is a surrogate marker for the extent of maturation of the humoral immune response. Little is known about high avidity Abs to VAR2CSA for women living in urban African cities. Therefore, this study sought to determine: i) if high avidity Abs to full-length VAR2CSA (FV2) increase with gravidity in women in Yaoundé, Cameroon exposed to ~ 0.3-1.1 infectious mosquito bites per month, ii) if high avidity Abs to FV2 are directed against a specific region of VAR2CSA, and iii) if having high avidity Abs to FV2 improve pregnancy outcomes. Plasma samples collected at delivery from 695 women who had Abs to FV2 were evaluated. Ab levels and the Avidity Index (AI), defined as the percent Abs remaining bound to FV2 after incubation with 3M NH₄SCN, were determined. Similar Ab levels to FV2 were present in women of all gravidities (G1 through 6+; p=0.80), except significantly lower levels were detected in PM−negative (PM−) primigravidae (p <0.001). Median Ab avidities increased between gravidity 1 and 2 (p<0.001) and remained stable thereafter (G3-G6+: p=0.51). These results suggest that B cell clonal expansion began during the first pregnancy, with clonal selection primarily occurring during the second. However, the majority of women (84%) had AI <35, a level of high avidity Abs previously reported to be associated with improved pregnancy outcomes. When plasma from 107 Cameroonian women was tested against 8 different regions of FV2, high avidity Abs were predominately restricted to DBL5 with median AI of 50 compared to AI <25 for the other domains. The only significance influence of high avidity Abs on pregnancy outcome was that babies born to mothers with AI above the median were 104 g heavier than babies born to women with AI below the median (p=0.045). These results suggest that a vaccine that boosts maturation of the immune response to VAR2CSA may be beneficial for women residing in urban areas.

Naturally acquired immunity against immature Plasmodium falciparum gametocytes

The recent decline in global malaria burden has stimulated efforts toward Plasmodium falciparum elimination. Understanding the biology of malaria transmission stages may provide opportunities to reduce or prevent onward transmission to mosquitoes. Immature P. falciparum transmission stages, termed stages I to IV gametocytes, sequester in human bone marrow before release into the circulation as mature stage V gametocytes. This process likely involves interactions between host receptors and potentially immunogenic adhesins on the infected red blood cell (iRBC) surface. Here, we developed a flow cytometry assay to examine immune recognition of live gametocytes of different developmental stages by naturally exposed Malawians. We identified strong antibody recognition of the earliest immature gametocyte-iRBCs (giRBCs) but not mature stage V giRBCs. Candidate surface antigens (n = 30), most of them shared between asexual- and gametocyte-iRBCs, were identified by mass spectrometry and mouse immunizations, as well as correlations between responses by protein microarray and flow cytometry. Naturally acquired responses to a subset of candidate antigens were associated with reduced asexual and gametocyte density, and plasma samples from malaria-infected individuals were able to induce immune clearance of giRBCs in vitro. Infected RBC surface expression of select candidate antigens was validated using specific antibodies, and genetic analysis revealed a subset with minimal variation across strains. Our data demonstrate that humoral immune responses to immature giRBCs and shared iRBC antigens are naturally acquired after malaria exposure. These humoral immune responses may have consequences for malaria transmission potential by clearing developing gametocytes, which could be leveraged for malaria intervention.

Pregnancy-specific malarial immunity and risk of malaria in pregnancy and adverse birth outcomes: a systematic review

BACKGROUND

In endemic areas, pregnant women are highly susceptible to Plasmodium falciparum malaria characterized by the accumulation of parasitized red blood cells (pRBC) in the placenta. In subsequent pregnancies, women develop protective immunity to pregnancy-associated malaria and this has been hypothesized to be due to the acquisition of antibodies to the parasite variant surface antigen VAR2CSA. In this systematic review we provide the first synthesis of the association between antibodies to pregnancy-specific P. falciparum antigens and pregnancy and birth outcomes.

METHODS

We conducted a systematic review and meta-analysis of population-based studies (published up to 07 June 2019) of pregnant women living in P. falciparum endemic areas that examined antibody responses to pregnancy-specific P. falciparum antigens and outcomes including placental malaria, low birthweight, preterm birth, peripheral parasitaemia, maternal anaemia, and severe malaria.

RESULTS

We searched 6 databases and identified 33 studies (30 from Africa) that met predetermined inclusion and quality criteria: 16 studies contributed estimates in a format enabling inclusion in meta-analysis and 17 were included in narrative form only. Estimates were mostly from cross-sectional data (10 studies) and were heterogeneous in terms of magnitude and direction of effect. Included studies varied in terms of antigens tested, methodology used to measure antibody responses, and epidemiological setting. Antibody responses to pregnancy-specific pRBC and VAR2CSA antigens, measured at delivery, were associated with placental malaria (9 studies) and may therefore represent markers of infection, rather than correlates of protection. Antibody responses to pregnancy-specific pRBC, but not recombinant VAR2CSA antigens, were associated with trends towards protection from low birthweight (5 studies).

CONCLUSIONS

Whilst antibody responses to several antigens were positively associated with the presence of placental and peripheral infections, this review did not identify evidence that any specific antibody response is associated with protection from pregnancy-associated malaria across multiple populations. Further prospective cohort studies using standardized laboratory methods to examine responses to a broad range of antigens in different epidemiological settings and throughout the gestational period, will be necessary to identify and prioritize pregnancy-specific P. falciparum antigens to advance the development of vaccines and serosurveillance tools targeting pregnant women.

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.

Targets of complement-fixing antibodies in protective immunity against malaria in children

Antibodies against P. falciparum merozoites fix complement to inhibit blood-stage replication in naturally-acquired and vaccine-induced immunity; however, specific targets of these functional antibodies and their importance in protective immunity are unknown. Among malaria-exposed individuals, we show that complement-fixing antibodies to merozoites are more strongly correlated with protective immunity than antibodies that inhibit growth quantified using the current reference assay for merozoite vaccine evaluation. We identify merozoite targets of complement-fixing antibodies and identify antigen-specific complement-fixing antibodies that are strongly associated with protection from malaria in a longitudinal study of children. Using statistical modelling, combining three different antigens targeted by complement-fixing antibodies could increase the potential protective effect to over 95%, and we identify antigens that were common in the most protective combinations. Our findings support antibody-complement interactions against merozoite antigens as important anti-malaria immune mechanisms, and identify specific merozoite antigens for further evaluation as vaccine candidates.

Antibodies against Plasmodium falciparum merozoites that fix complement can inhibit blood-stage replication. Here, Reiling et al. show that complement-fixing antibodies strongly correlate with protective immunity in children, identify the merozoite targets, and predict antigen combinations that should result in strong protection.

Human antibodies activate complement against Plasmodium falciparum sporozoites, and are associated with protection against malaria in children

BACKGROUND

Antibodies targeting Plasmodium falciparum sporozoites play a key role in human immunity to malaria. However, antibody mechanisms that neutralize sporozoites are poorly understood. This has been a major constraint in developing highly efficacious vaccines, as we lack strong correlates of protective immunity.

METHODS

We quantified the ability of human antibodies from malaria-exposed populations to interact with human complement, examined the functional effects of complement activity against P. falciparum sporozoites in vitro, and identified targets of functional antibodies. In children and adults from malaria-endemic regions, we determined the acquisition of complement-fixing antibodies to sporozoites and their relationship with antibody isotypes and subclasses. We also investigated associations with protective immunity in a longitudinal cohort of children (n = 206) residing in a malaria-endemic region.

RESULTS

We found that antibodies to the major sporozoite surface antigen, circumsporozoite protein (CSP), were predominately IgG1, IgG3, and IgM, and could interact with complement through recruitment of C1q and activation of the classical pathway. The central repeat region of CSP, included in leading vaccines, was a key target of complement-fixing antibodies. We show that antibodies activate human complement on P. falciparum sporozoites, which consequently inhibited hepatocyte cell traversal that is essential for establishing liver-stage infection, and led to sporozoite death in vitro. The natural acquisition of complement-fixing antibodies in malaria-exposed populations was age-dependent, and was acquired more slowly to sporozoite antigens than to merozoite antigens. In a longitudinal cohort of children, high levels of complement-fixing antibodies were significantly associated with protection against clinical malaria.

CONCLUSIONS

These novel findings point to complement activation by antibodies as an important mechanism of anti-sporozoite human immunity, thereby enabling new strategies for developing highly efficacious malaria vaccines. We also present evidence that complement-fixing antibodies may be a valuable correlate of protective immunity in humans.

Evaluating antibody functional activity and strain-specificity of vaccine candidates for malaria in pregnancy using in vitro phagocytosis assays

BACKGROUND

Malaria in pregnancy is a major cause of poor maternal and infant health, and is associated with the sequestration of P. falciparum-infected erythrocytes (IE) in the placenta. The leading vaccine candidate for pregnancy malaria, VAR2CSA, has been shown to induce antibodies that inhibit IE adhesion to the placental receptor chondroitin sulfate A (CSA), potentially preventing placental infection. However, the ability of vaccination-induced antibodies to promote opsonic phagocytosis is not well defined, but likely to be an important component of protective immunity.

METHODS

We investigated the use of an opsonic phagocytosis assay to evaluate antibodies induced by pregnancy malaria vaccine candidate antigens based on VAR2CSA. Opsonic phagocytosis was measured by flow cytometry and visualized by electron microscopy. We measured vaccine-induced antibody reactivity to placental type IEs from different geographical origins, and the functional ability of antibodies raised in immunized rabbits to induce phagocytosis by a human monocyte cell line.

RESULTS

Immunization-induced antibodies showed a mixture of strain-specific and cross-reactive antibody recognition of different placental-binding parasite lines. Antibodies generated against the DBL5 and DBL3 domains of VAR2CSA effectively promoted the opsonic phagocytosis of IEs by human monocytes; however, these functional antibodies were largely allele-specific and not cross-reactive. This has significant implications for the development of vaccines aiming to achieve a broad coverage against diverse parasite strains. Using competition ELISAs, we found that acquired human antibodies among pregnant women targeted both cross-reactive and allele-specific epitopes, consistent with what we observed with vaccine-induced antibodies.

CONCLUSIONS

Vaccines based on domains of VAR2CSA induced opsonic phagocytosis of IEs in a strain-specific manner. Assays measuring this phagocytic activity have the potential to aid the development and evaluation of vaccines against malaria in pregnancy.

Quantitative Proteomic Profiling Reveals Novel Plasmodium falciparum Surface Antigens and Possible Vaccine Candidates

Despite recent efforts toward control and elimination, malaria remains a major public health problem worldwide. Plasmodium falciparum resistance against artemisinin, used in front line combination drugs, is on the rise, and the only approved vaccine shows limited efficacy. Combinations of novel and tailored drug and vaccine interventions are required to maintain the momentum of the current malaria elimination program. Current evidence suggests that strain-transcendent protection against malaria infection can be achieved using whole organism vaccination or with a polyvalent vaccine covering multiple antigens or epitopes. These approaches have been successfully applied to the human-infective sporozoite stage. Both systemic and tissue-specific pathology during infection with the human malaria parasite P. falciparum is caused by asexual blood stages. Tissue tropism and vascular sequestration are the result of specific binding interactions between antigens on the parasite-infected red blood cell (pRBC) surface and endothelial receptors. The major surface antigen and parasite ligand binding to endothelial receptors, PfEMP1 is encoded by about 60 variants per genome and shows high sequence diversity across strains. Apart from PfEMP1 and three additional variant surface antigen families RIFIN, STEVOR, and SURFIN, systematic analysis of the infected red blood cell surface is lacking. Here we present the most comprehensive proteomic investigation of the parasitized red blood cell surface so far. Apart from the known variant surface antigens, we identified a set of putative single copy surface antigens with low sequence diversity, several of which are validated in a series of complementary experiments. Further functional and immunological investigation is underway to test these novel P. falciparum blood stage proteins as possible vaccine candidates.

A new method for sequencing the hypervariable Plasmodium falciparum gene var2csa from clinical samples

Background

VAR2CSA, a member of the Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) family, mediates the binding of P. falciparum-infected erythrocytes to chondroitin sulfate A, a surface-associated molecule expressed in placental cells, and plays a central role in the pathogenesis of placental malaria. VAR2CSA is a target of naturally acquired immunity and, as such, is a leading vaccine candidate against placental malaria. This protein is very polymorphic and technically challenging to sequence. Published var2csa sequences, mostly limited to specific domains, have been generated through the sequencing of cloned PCR amplicons using capillary electrophoresis, a method that is both time consuming and costly, and that performs poorly when applied to clinical samples that are commonly polyclonal. A next-generation sequencing platform, Pacific Biosciences (PacBio), offers an alternative approach to overcome these issues.

Methods

PCR primers were designed that target a 5 kb segment in the 5′ end of var2csa and the resulting amplicons were sequenced using PacBio sequencing. The primers were optimized using two laboratory strains and were validated on DNA from 43 clinical samples, extracted from dried blood spots on filter paper or from cryopreserved P. falciparum-infected erythrocytes. Sequence reads were assembled using the SMRT-analysis ConsensusTools module.

Results

Here, a PacBio sequencing-based approach for recovering a segment encoding the majority of VAR2CSA’s extracellular region is described; this segment includes the totality of the first four domains in the 5′ end of var2csa (~5 kb), from clinical malaria samples. The feasibility of the method is demonstrated, showing a high success rate from cryopreserved samples and more limited success from dried blood spots stored at room temperature, and characterized the genetic variation of the var2csa locus.

Conclusions

This method will facilitate a detailed analysis of var2csa genetic variation and can be adapted to sequence other hypervariable P. falciparum genes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-017-1976-8) contains supplementary material, which is available to authorized users.

The exported chaperone Hsp70-x supports virulence functions for Plasmodium falciparum blood stage parasites

Malaria is caused by five different Plasmodium spp. in humans each of which modifies the host erythrocyte to survive and replicate. The two main causes of malaria, P. falciparum and P. vivax, differ in their ability to cause severe disease, mainly due to differences in the cytoadhesion of infected erythrocytes (IE) in the microvasculature. Cytoadhesion of P. falciparum in the brain leads to a large number of deaths each year and is a consequence of exported parasite proteins, some of which modify the erythrocyte cytoskeleton while others such as PfEMP1 project onto the erythrocyte surface where they bind to endothelial cells. Here we investigate the effects of knocking out an exported Hsp70-type chaperone termed Hsp70-x that is present in P. falciparum but not P. vivax. Although the growth of Δhsp70-x parasites was unaffected, the export of PfEMP1 cytoadherence proteins was delayed and Δhsp70-x IE had reduced adhesion. The Δhsp70-x IE were also more rigid than wild-type controls indicating changes in the way the parasites modified their host erythrocyte. To investigate the cause of this, transcriptional and translational changes in exported and chaperone proteins were monitored and some changes were observed. We propose that PfHsp70-x is not essential for survival in vitro, but may be required for the efficient export and functioning of some P. falciparum exported proteins.

Multiplexing detection of IgG against Plasmodium falciparum pregnancy-specific antigens

Background

Pregnant women exposed to Plasmodium falciparum generate antibodies against VAR2CSA, the parasite protein that mediates adhesion of infected erythrocytes to the placenta. There is a need of high-throughput tools to determine the fine specificity of these antibodies that can be used to identify immune correlates of protection and exposure. Here we aimed at developing a multiplex-immunoassay to detect antibodies against VAR2CSA antigens.

Methods and findings

We constructed two multiplex-bead arrays, one composed of 3 VAR2CSA recombinant-domains (DBL3X, DBL5Ɛ and DBL6Ɛ) and another composed of 46 new peptides covering VAR2CSA conserved and semi-conserved regions. IgG reactivity was similar in multiplexed and singleplexed determinations (Pearson correlation, protein array: R² = 0.99 and peptide array: R² = 0.87). IgG recognition of 25 out of 46 peptides and all recombinant-domains was higher in pregnant Mozambican women (n = 106) than in Mozambican men (n = 102) and Spanish individuals (n = 101; p<0.05). Agreement of IgG levels detected in cryopreserved plasma and in elutions from dried blood spots was good after exclusion of inappropriate filter papers. Under heterogeneous levels of exposure to malaria, similar seropositivity cutoffs were obtained using finite mixture models applied to antibodies measured on pregnant Mozambican women and average of antibodies measured on pregnant Spanish women never exposed to malaria. The application of the multiplex-bead array developed here, allowed the assessment of higher IgG levels and seroprevalences against VAR2CSA-derived antigens in women pregnant during 2003–2005 than during 2010–2012, in accordance with the levels of malaria transmission reported for these years in Mozambique.

Conclusions

The multiplex bead-based immunoassay to detect antibodies against selected 25 VAR2CSA new-peptides and recombinant-domains was successfully implemented. Analysis of field samples showed that responses were specific among pregnant women and dependent on the level of exposure to malaria. This platform provides a high-throughput approach to investigating correlates of protection and identifying serological markers of exposure for malaria in pregnancy.

Synergistic effect of IL-12 and IL-18 induces TIM3 regulation of γδ T cell function and decreases the risk of clinical malaria in children living in Papua New Guinea

BACKGROUND

γδ T cells are important for both protective immunity and immunopathogenesis during malaria infection. However, the immunological processes determining beneficial or detrimental effects on disease outcome remain elusive. The aim of this study was to examine expression and regulatory effect of the inhibitory receptor T-cell immunoglobulin domain and mucin domain 3 (TIM3) on γδ T cells. While TIM3 expression and function on conventional αβ T cells have been clearly defined, the equivalent characterization on γδ T cells and associations with disease outcomes is limited. This study investigated the functional capacity of TIM3+ γδ T cells and the underlying mechanisms contributing to TIM3 upregulation and established an association with malaria disease outcomes.

METHODS

We analyzed TIM3 expression on γδ T cells in 132 children aged 5-10 years living in malaria endemic areas of Papua New Guinea. TIM3 upregulation and effector functions of TIM3+ γδ T cells were assessed following in vitro stimulation with parasite-infected erythrocytes, phosphoantigen and/or cytokines. Associations between the proportion of TIM3-expressing cells and the molecular force of infection were tested using negative binomial regression and in a Cox proportional hazards model for time to first clinical episode. Multivariable analyses to determine the association of TIM3 and IL-18 levels were conducted using general linear models. Malaria infection mouse models were utilized to experimentally investigate the relationship between repeated exposure and TIM3 upregulation.

RESULTS

This study demonstrates that even in the absence of an active malaria infection, children of malaria endemic areas have an atypical population of TIM3-expressing γδ T cells (mean frequency TIM3+ of total γδ T cells 15.2% ± 12). Crucial factors required for γδ T cell TIM3 upregulation include IL-12/IL-18, and plasma IL-18 was associated with TIM3 expression (P = 0.002). Additionally, we show a relationship between TIM3 expression and infection with distinct parasite clones during repeated exposure. TIM3+ γδ T cells were functionally impaired and were associated with asymptomatic malaria infection (hazard ratio 0.54, P = 0.032).

CONCLUSIONS

Collectively our data demonstrate a novel role for IL-12/IL-18 in shaping the innate immune response and provide fundamental insight into aspects of γδ T cell immunoregulation. Furthermore, we show that TIM3 represents an important γδ T cell regulatory component involved in minimizing malaria symptoms.

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.

A single point in protein trafficking by Plasmodium falciparum determines the expression of major antigens on the surface of infected erythrocytes targeted by human antibodies

Antibodies to blood-stage antigens of Plasmodium falciparum play a pivotal role in human immunity to malaria. During parasite development, multiple proteins are trafficked from the intracellular parasite to the surface of P. falciparum-infected erythrocytes (IEs). However, the relative importance of different proteins as targets of acquired antibodies, and key pathways involved in trafficking major antigens remain to be clearly defined. We quantified antibodies to surface antigens among children, adults, and pregnant women from different malaria-exposed regions. We quantified the importance of antigens as antibody targets using genetically engineered P. falciparum with modified surface antigen expression. Genetic deletion of the trafficking protein skeleton-binding protein-1 (SBP1), which is involved in trafficking the surface antigen PfEMP1, led to a dramatic reduction in antibody recognition of IEs and the ability of human antibodies to promote opsonic phagocytosis of IEs, a key mechanism of parasite clearance. The great majority of antibody epitopes on the IE surface were SBP1-dependent. This was demonstrated using parasite isolates with different genetic or phenotypic backgrounds, and among antibodies from children, adults, and pregnant women in different populations. Comparisons of antibody reactivity to parasite isolates with SBP1 deletion or inhibited PfEMP1 expression suggest that PfEMP1 is the dominant target of acquired human antibodies, and that other P. falciparum IE surface proteins are minor targets. These results establish SBP1 as part of a critical pathway for the trafficking of major surface antigens targeted by human immunity, and have key implications for vaccine development, and quantifying immunity in populations.

Severe malaria: what's new on the pathogenesis front?

Plasmodium falciparum causes the most severe and fatal form of malaria in humans with over half a million deaths each year. Cerebral malaria, a complex neurological syndrome of severe falciparum malaria, is often fatal and represents a major public health burden. Despite vigorous efforts, the pathophysiology of cerebral malaria remains to be elucidated, thereby hindering the development of adjunctive therapies. In recent years, multidisciplinary and collaborative approaches have led to groundbreaking progress both in the laboratory and in the field. Here we review the latest breakthroughs in severe malaria pathogenesis, with a specific focus on new pathogenetic mechanisms leading to cerebral malaria. The most recent findings point towards specific parasite phenotypes targeting brain microvasculature, endothelial dysfunction and subsequent oedema-induced brain swelling.

Antibody responses to Plasmodium falciparum and Plasmodium vivax blood-stage and sporozoite antigens in the postpartum period

During pregnancy a variety of immunological changes occur to accommodate the fetus. It is unknown whether these changes continue to affect humoral immunity postpartum or how quickly they resolve. IgG levels were measured to P. falciparum and P. vivax antigens in 201 postpartum and 201 controls over 12 weeks. Linear mixed-effects models assessed antibody maintenance over time and the effect of microscopically confirmed Plasmodium spp. infection on antibody levels, and whether this was different in postpartum women compared with control women. Postpartum women had reduced Plasmodium spp. antibody levels compared to controls at baseline. Over 12 weeks, mean antibody levels in postpartum women increased to levels observed in control women. Microscopically confirmed P. falciparum and P. vivax infections during follow-up were associated with an increase in species-specific antibodies with similar magnitudes of boosting observed in postpartum and control women. Antibodies specific for pregnancy-associated, VAR2CSA-expressing parasites did not rapidly decline postpartum and did not boost in response to infection in either postpartum or control women. After pregnancy, levels of malaria-specific antibodies were reduced, but recovered to levels seen in control women. There was no evidence of an impaired ability to mount a boosting response in postpartum women.

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.

CD14(hi)CD16+ monocytes phagocytose antibody-opsonised Plasmodium falciparum infected erythrocytes more efficiently than other monocyte subsets, and require CD16 and complement to do so

BACKGROUND

With more than 600,000 deaths from malaria, mainly of children under five years old and caused by infection with Plasmodium falciparum, comes an urgent need for an effective anti-malaria vaccine. Limited details on the mechanisms of protective immunity are a barrier to vaccine development. Antibodies play an important role in immunity to malaria and monocytes are key effectors in antibody-mediated protection by phagocytosing antibody-opsonised infected erythrocytes (IE). Eliciting antibodies that enhance phagocytosis of IE is therefore an important potential component of an effective vaccine, requiring robust assays to determine the ability of elicited antibodies to stimulate this in vivo. The mechanisms by which monocytes ingest IE and the nature of the monocytes which do so are unknown.

METHODS

Purified trophozoite-stage P. falciparum IE were stained with ethidium bromide, opsonised with anti-erythrocyte antibodies and incubated with fresh whole blood. Phagocytosis of IE and TNF production by individual monocyte subsets was measured by flow cytometry. Ingestion of IE was confirmed by imaging flow cytometry.

RESULTS

CD14(hi)CD16+ monocytes phagocytosed antibody-opsonised IE and produced TNF more efficiently than CD14(hi)CD16- and CD14(lo)CD16+ monocytes. Blocking experiments showed that Fcγ receptor IIIa (CD16) but not Fcγ receptor IIa (CD32a) or Fcγ receptor I (CD64) was necessary for phagocytosis. CD14(hi)CD16+ monocytes ingested antibody-opsonised IE when peripheral blood mononuclear cells were reconstituted with autologous serum but not heat-inactivated autologous serum. Antibody-opsonised IE were rapidly opsonised with complement component C3 in serum (t1/2 = 2-3 minutes) and phagocytosis of antibody-opsonised IE was inhibited in a dose-dependent manner by an inhibitor of C3 activation, compstatin. Compared to other monocyte subsets, CD14(hi)CD16+ monocytes expressed the highest levels of complement receptor 4 (CD11c) and activated complement receptor 3 (CD11b) subunits.

CONCLUSIONS

We show a special role for CD14(hi)CD16+ monocytes in phagocytosing opsonised P. falciparum IE and production of TNF. While ingestion was mediated by Fcγ receptor IIIa, this receptor was not sufficient to allow phagocytosis; despite opsonisation with antibody, phagocytosis of IE also required complement opsonisation. Assays which measure the ability of vaccines to elicit a protective antibody response to P. falciparum should consider their ability to promote phagocytosis and fix complement.

An assay to probe Plasmodium falciparum growth, transmission stage formation and early gametocyte development

Conversion from asexual proliferation to sexual differentiation initiates the production of the gametocyte, which is the malaria parasite stage required for human-to-mosquito transmission. This protocol describes an assay designed to probe the effect of drugs or other perturbations on asexual replication, sexual conversion and early gametocyte development in the major human malaria parasite Plasmodium falciparum. Synchronized asexually replicating parasites are induced for gametocyte production by the addition of conditioned medium, and they are then exposed to the treatment of interest during sexual commitment or at any subsequent stage of early gametocyte development. Flow cytometry is used to measure asexual proliferation and gametocyte production via DNA dye staining and the gametocyte-specific expression of a fluorescent protein, respectively. This screening approach may be used to identify and evaluate potential transmission-blocking compounds and to further investigate the mechanism of sexual conversion in malaria parasites. The full protocol can be completed in 11 d.

Risk factors for malaria and adverse birth outcomes in a prospective cohort of pregnant women resident in a high malaria transmission area of Papua New Guinea

BACKGROUND

Low birth weight (LBW), anaemia and malaria are common in Papua New Guinean women.

METHODS

To identify risk factors for LBW, anaemia and preterm delivery (PTD), pregnant women recruited into a cohort study in Madang, Papua New Guinea, were followed to delivery.

RESULTS

Of 470 women enrolled, delivery data were available for 328 (69.7%). By microscopy, 34.4% (113/328) of women had malaria parasitaemia at enrolment and 12.5% (41/328) at delivery; at each time point, PCR detected sub-microscopic parasitaemia in substantially more. Most infections were with Plasmodium falciparum; the remainder being predominantly P. vivax. Anaemia and smoking were associated with lower birth weight, and LBW (16.7%; 51/305) and PTD (21.8%; 63/290) were common. Histopathologically diagnosed chronic placental malaria was associated with LBW (adjusted odds ratio [aOR] 3.3; p=0.048) and PTD (aOR 4.2; p=0.01). Lack of maternal education predisposed to PTD. Sub-microscopic parasitaemia at delivery appeared to increase the risk of LBW. Of the genetic polymorphisms, Southeast Asian ovalocytosis, α(+)-thalassaemia and complement receptor 1 (CR1) deficiency, a CR1 heterozygous genotype was associated with decreased risk of anaemia and substantial but non-significant effects were noted in other comparisons.

CONCLUSIONS

In coastal Papua New Guinea, malaria and anaemia are important causes of adverse pregnancy outcomes.

Limited antigenic diversity of Plasmodium falciparum apical membrane antigen 1 supports the development of effective multi-allele vaccines

BACKGROUND

Polymorphism in antigens is a common mechanism for immune evasion used by many important pathogens, and presents major challenges in vaccine development. In malaria, many key immune targets and vaccine candidates show substantial polymorphism. However, knowledge on antigenic diversity of key antigens, the impact of polymorphism on potential vaccine escape, and how sequence polymorphism relates to antigenic differences is very limited, yet crucial for vaccine development. Plasmodium falciparum apical membrane antigen 1 (AMA1) is an important target of naturally-acquired antibodies in malaria immunity and a leading vaccine candidate. However, AMA1 has extensive allelic diversity with more than 60 polymorphic amino acid residues and more than 200 haplotypes in a single population. Therefore, AMA1 serves as an excellent model to assess antigenic diversity in malaria vaccine antigens and the feasibility of multi-allele vaccine approaches. While most previous research has focused on sequence diversity and antibody responses in laboratory animals, little has been done on the cross-reactivity of human antibodies.

METHODS

We aimed to determine the extent of antigenic diversity of AMA1, defined by reactivity with human antibodies, and to aid the identification of specific alleles for potential inclusion in a multi-allele vaccine. We developed an approach using a multiple-antigen-competition enzyme-linked immunosorbent assay (ELISA) to examine cross-reactivity of naturally-acquired antibodies in Papua New Guinea and Kenya, and related this to differences in AMA1 sequence.

RESULTS

We found that adults had greater cross-reactivity of antibodies than children, although the patterns of cross-reactivity to alleles were the same. Patterns of antibody cross-reactivity were very similar between populations (Papua New Guinea and Kenya), and over time. Further, our results show that antigenic diversity of AMA1 alleles is surprisingly restricted, despite extensive sequence polymorphism. Our findings suggest that a combination of three different alleles, if selected appropriately, may be sufficient to cover the majority of antigenic diversity in polymorphic AMA1 antigens. Antigenic properties were not strongly related to existing haplotype groupings based on sequence analysis.

CONCLUSIONS

Antigenic diversity of AMA1 is limited and a vaccine including a small number of alleles might be sufficient for coverage against naturally-circulating strains, supporting a multi-allele approach for developing polymorphic antigens as malaria vaccines.

Strategies for designing and monitoring malaria vaccines targeting diverse antigens

After more than 50 years of intensive research and development, only one malaria vaccine candidate, “RTS,S,” has progressed to Phase 3 clinical trials. Despite only partial efficacy, this candidate is now forecast to become the first licensed malaria vaccine. Hence, more efficacious second-generation malaria vaccines that can significantly reduce transmission are urgently needed. This review will focus on a major obstacle hindering development of effective malaria vaccines: parasite antigenic diversity. Despite extensive genetic diversity in leading candidate antigens, vaccines have been and continue to be formulated using recombinant antigens representing only one or two strains. These vaccine strains represent only a small fraction of the diversity circulating in natural parasite populations, leading to escape of non-vaccine strains and challenging investigators’ abilities to measure strain-specific efficacy in vaccine trials. Novel strategies are needed to overcome antigenic diversity in order for vaccine development to succeed. Many studies have now cataloged the global diversity of leading Plasmodium falciparum and Plasmodium vivax vaccine antigens. In this review, we describe how population genetic approaches can be applied to this rich data source to predict the alleles that best represent antigenic diversity, polymorphisms that contribute to it, and to identify key polymorphisms associated with antigenic escape. We also suggest an approach to summarize the known global diversity of a given antigen to predict antigenic diversity, how to select variants that best represent the strains circulating in natural parasite populations and how to investigate the strain-specific efficacy of vaccine trials. Use of these strategies in the design and monitoring of vaccine trials will not only shed light on the contribution of genetic diversity to the antigenic diversity of malaria, but will also maximize the potential of future malaria vaccine candidates.

Genetic diversity of VAR2CSA ID1-DBL2Xb in worldwide Plasmodium falciparum populations: impact on vaccine design for placental malaria

In placental malaria (PM), sequestration of infected erythrocytes in the placenta is mediated by an interaction between VAR2CSA, a Plasmodium falciparum protein expressed on erythrocytes, and chondroitin sulfate A (CSA) on syncytiotrophoblasts. Recent works have identified ID1-DBL2Xb as the minimal CSA-binding region within VAR2CSA able to induce strong protective immunity, making it the leading candidate for the development of a vaccine against PM. Assessing the existence of population differences in the distribution of ID1-DBL2Xb polymorphisms is of paramount importance to determine whether geographic diversity must be considered when designing a candidate vaccine based on this fragment. In this study, we examined patterns of sequence variation of ID1-DBL2Xb in a large collection of P. falciparum field isolates (n=247) from different malaria-endemic areas, including Africa (Benin, Senegal, Cameroon and Madagascar), Asia (Cambodia), Oceania (Papua New Guinea), and Latin America (Peru). Detection of variants and estimation of their allele frequencies were performed using next-generation sequencing of DNA pools. A considerable amount of variation was detected along the whole gene segment, suggesting that several allelic variants may need to be included in a candidate vaccine to achieve broad population coverage. However, most sequence variants were common and extensively shared among worldwide parasite populations, demonstrating long term persistence of those polymorphisms, probably maintained through balancing selection. Therefore, a vaccine mixture including such stable antigen variants will be putatively applicable and efficacious in all world regions where malaria occurs. Despite similarity in ID1-DBL2Xb allele repertoire across geographic areas, several peaks of strong population differentiation were observed at specific polymorphic loci, pointing out putative targets of humoral immunity subject to positive immune selection.

Surface antigens of Plasmodium falciparum-infected erythrocytes as immune targets and malaria vaccine candidates

Understanding the targets and mechanisms of human immunity to malaria caused by Plasmodium falciparum is crucial for advancing effective vaccines and developing tools for measuring immunity and exposure in populations. Acquired immunity to malaria predominantly targets the blood stage of infection when merozoites of Plasmodium spp. infect erythrocytes and replicate within them. During the intra-erythrocytic development of P. falciparum, numerous parasite-derived antigens are expressed on the surface of infected erythrocytes (IEs). These antigens enable P. falciparum-IEs to adhere in the vasculature and accumulate in multiple organs, which is a key process in the pathogenesis of disease. IE surface antigens, often referred to as variant surface antigens, are important targets of acquired protective immunity and include PfEMP1, RIFIN, STEVOR and SURFIN. These antigens are highly polymorphic and encoded by multigene families, which generate substantial antigenic diversity to mediate immune evasion. The most important immune target appears to be PfEMP1, which is a major ligand for vascular adhesion and sequestration of IEs. Studies are beginning to identify specific variants of PfEMP1 linked to disease pathogenesis that may be suitable for vaccine development, but overcoming antigenic diversity in PfEMP1 remains a major challenge. Much less is known about other surface antigens, or antigens on the surface of gametocyte-IEs, the effector mechanisms that mediate immunity, and how immunity is acquired and maintained over time; these are important topics for future research.

The antibody response of pregnant Cameroonian women to VAR2CSA ID1-ID2a, a small recombinant protein containing the CSA-binding site

In pregnant women, Plasmodium falciparum-infected erythrocytes expressing the VAR2CSA antigen bind to chondroitin sulfate A in the placenta causing placental malaria. The binding site of VAR2CSA is present in the ID1-ID2a region. This study sought to determine if pregnant Cameroonian women naturally acquire antibodies to ID1-ID2a and if antibodies to ID1-ID2a correlate with absence of placental malaria at delivery. Antibody levels to full-length VAR2CSA and ID1-ID2a were measured in plasma samples from 745 pregnant Cameroonian women, 144 Cameroonian men, and 66 US subjects. IgM levels and IgG avidity to ID1-ID2a were also determined. As expected, antibodies to ID1-ID2a were absent in US controls. Although pregnant Cameroonian women developed increasing levels of antibodies to full-length VAR2CSA during pregnancy, no increase in either IgM or IgG to ID1-ID2a was observed. Surprisingly, no differences in antibody levels to ID1-ID2a were detected between Cameroonian men and pregnant women. For example, in rural settings only 8–9% of males had antibodies to full-length VAR2CSA, but 90–96% had antibodies to ID1-ID2a. In addition, no significant difference in the avidity of IgG to ID1-ID2a was found between pregnant women and Cameroonian men, and no correlation between antibody levels at delivery and absence of placental malaria was found. Thus, the response to ID1-ID2a was not pregnancy specific, but predominantly against cross-reactivity epitopes, which may have been induced by other PfEMP1 antigens, malarial antigens, or microbes. Currently, ID1-ID2a is a leading vaccine candidate, since it binds to the CSA with the same affinity as the full-length molecule and elicits binding-inhibitory antibodies in animals. Further studies are needed to determine if the presence of naturally acquired cross-reactive antibodies in women living in malaria endemic countries will alter the response to ID1-ID2a following vaccination with ID1-ID2a.

VAR2CSA signatures of high Plasmodium falciparum parasitemia in the placenta

Plasmodium falciparum infected erythrocytes (IE) accumulate in the placenta through the interaction between Duffy-binding like (DBL) domains of parasite-encoded ligand VAR2CSA and chondroitin sulphate-A (CSA) receptor. Polymorphisms in these domains, including DBL2X and DBL3X, may affect their antigenicity or CSA-binding affinity, eventually increasing parasitemia and its adverse effects on pregnancy outcomes. A total of 373 DBL2X and 328 DBL3X sequences were obtained from transcripts of 20 placental isolates infecting Mozambican women, resulting in 176 DBL2X and 191 DBL3X unique sequences at the protein level. Sequence alignments were divided in segments containing combinations of correlated polymorphisms and the association of segment sequences with placental parasite density was tested using Bonferroni corrected regression models, taking into consideration the weight of each sequence in the infection. Three DBL2X and three DBL3X segments contained signatures of high parasite density (P<0.003) that were highly prevalent in the parasite population (49-91%). Identified regions included a flexible loop that contributes to DBL3X-CSA interaction and two DBL3X motifs with evidence of positive natural selection. Limited antibody responses against signatures of high parasite density among malaria-exposed pregnant women could not explain the increased placental parasitemia. These results suggest that a higher binding efficiency to CSA rather than reduced antigenicity might provide a biological advantage to parasites with high parasite density signatures in VAR2CSA. Sequences contributing to high parasitemia may be critical for the functional characterization of VAR2CSA and the development of tools against placental malaria.

HIV and co-infections

Despite significant reductions in morbidity and mortality secondary to availability of effective combination anti-retroviral therapy (cART), human immunodeficiency virus (HIV) infection still accounts for 1.5 million deaths annually. The majority of deaths occur in sub-Saharan Africa where rates of opportunistic co-infections are disproportionately high. In this review, we discuss the immunopathogenesis of five common infections that cause significant morbidity in HIV-infected patients globally. These include co-infection with Mycobacterium tuberculosis, Cryptococcus neoformans, hepatitis B virus, hepatitis C virus, and Plasmodium falciparum. Specifically, we review the natural history of each co-infection in the setting of HIV, the specific immune defects induced by HIV, the effects of cART on the immune response to the co-infection, the pathogenesis of immune restoration disease (IRD) associated with each infection, and advances in the areas of prevention of each co-infection via vaccination. Finally, we discuss the opportunities and gaps in knowledge for future research.

Plasmodium falciparum variability and immune evasion proceed from antigenicity of consensus sequences from DBL6ε; generalization to all DBL from VAR2CSA

We studied all consensus sequences within the four least ‘variable blocks’ (VB) present in the DBL6ε domain of VAR2CSA, the protein involved in the adhesion of infected red blood cells by Plasmodium falciparum that causes the Pregnancy-Associated Malaria (PAM). Characterising consensus sequences with respect to recognition of antibodies and percentage of responders among pregnant women living in areas where P. falciparum is endemic allows the identification of the most antigenic sequences within each VB. When combining these consensus sequences among four serotypes from VB1 or VB5, the most often recognized ones are expected to induce pan-reactive antibodies recognizing VAR2CSA from all plasmodial strains. These sequences are of main interest in the design of an immunogenic molecule. Using a similar approach than for DBL6ε, we studied the five other DBL and the CIDRpam from VAR2CSA, and again identified VB segments with highly conserved consensus sequences. In addition, we identified consensus sequences in other var genes expressed by non-PAM parasites. This finding paves the way for vaccine design against other pathologies caused by P. falciparum.

Platelet factor 4 and Duffy antigen required for platelet killing of Plasmodium falciparum

Platelets restrict the growth of intraerythrocytic malaria parasites by binding to parasitized cells and killing the parasite within. Here, we show that the platelet molecule platelet factor 4 (PF4 or CXCL4) and the erythrocyte Duffy-antigen receptor (Fy) are necessary for platelet-mediated killing of Plasmodium falciparum parasites. PF4 is released by platelets on contact with parasitized red cells, and the protein directly kills intraerythrocytic parasites. This function for PF4 is critically dependent on Fy, which binds PF4. Genetic disruption of Fy expression inhibits binding of PF4 to parasitized cells and concomitantly prevents parasite killing by both human platelets and recombinant human PF4. The protective function afforded by platelets during a malarial infection may therefore be compromised in Duffy-negative individuals, who do not express Fy.

Defining the antigenic diversity of Plasmodium falciparum apical membrane antigen 1 and the requirements for a multi-allele vaccine against malaria

Apical Membrane Antigen 1 (AMA1) is a leading malaria vaccine candidate and a target of naturally-acquired human immunity. Plasmodium falciparum AMA1 is polymorphic and in vaccine trials it induces strain-specific protection. This antigenic diversity is a major roadblock to development of AMA1 as a malaria vaccine and understanding how to overcome it is essential. To assess how AMA1 antigenic diversity limits cross-strain growth inhibition, we assembled a panel of 18 different P. falciparum isolates which are broadly representative of global AMA1 sequence diversity. Antibodies raised against four well studied AMA1 alleles (W2Mef, 3D7, HB3 and FVO) were tested for growth inhibition of the 18 different P. falciparum isolates in growth inhibition assays (GIA). All antibodies demonstrated substantial cross-inhibitory activity against different isolates and a mixture of the four different AMA1 antibodies inhibited all 18 isolates tested, suggesting significant antigenic overlap between AMA1 alleles and limited antigenic diversity of AMA1. Cross-strain inhibition by antibodies was only moderately and inconsistently correlated with the level of sequence diversity between AMA1 alleles, suggesting that sequence differences are not a strong predictor of antigenic differences or the cross-inhibitory activity of anti-allele antibodies. The importance of the highly polymorphic C1-L region for inhibitory antibodies and potential vaccine escape was assessed by generating novel transgenic P. falciparum lines for testing in GIA. While the polymorphic C1-L epitope was identified as a significant target of some growth-inhibitory antibodies, these antibodies only constituted a minor proportion of the total inhibitory antibody repertoire, suggesting that the antigenic diversity of inhibitory epitopes is limited. Our findings support the concept that a multi-allele AMA1 vaccine would give broad coverage against the diversity of AMA1 alleles and establish new tools to define polymorphisms important for vaccine escape.

The Plasmodium falciparum erythrocyte invasion ligand Pfrh4 as a target of functional and protective human antibodies against malaria

Background

Acquired antibodies are important in human immunity to malaria, but key targets remain largely unknown. Plasmodium falciparum reticulocyte-binding-homologue-4 (PfRh4) is important for invasion of human erythrocytes and may therefore be a target of protective immunity.

Methods

IgG and IgG subclass-specific responses against different regions of PfRh4 were determined in a longitudinal cohort of 206 children in Papua New Guinea (PNG). Human PfRh4 antibodies were tested for functional invasion-inhibitory activity, and expression of PfRh4 by P. falciparum isolates and sequence polymorphisms were determined.

Results

Antibodies to PfRh4 were acquired by children exposed to P. falciparum malaria, were predominantly comprised of IgG1 and IgG3 subclasses, and were associated with increasing age and active parasitemia. High levels of antibodies, particularly IgG3, were strongly predictive of protection against clinical malaria and high-density parasitemia. Human affinity-purified antibodies to the binding region of PfRh4 effectively inhibited erythrocyte invasion by P. falciparum merozoites and antibody levels in protected children were at functionally-active concentrations. Although expression of PfRh4 can vary, PfRh4 protein was expressed by most isolates derived from the cohort and showed limited sequence polymorphism.

Conclusions

Evidence suggests that PfRh4 is a target of antibodies that contribute to protective immunity to malaria by inhibiting erythrocyte invasion and preventing high density parasitemia. These findings advance our understanding of the targets and mechanisms of human immunity and evaluating the potential of PfRh4 as a component of candidate malaria vaccines.

Targets of antibodies against Plasmodium falciparum-infected erythrocytes in malaria immunity

Plasmodium falciparum is the major cause of malaria globally and is transmitted by mosquitoes. During parasitic development, P. falciparum-infected erythrocytes (P. falciparum-IEs) express multiple polymorphic proteins known as variant surface antigens (VSAs), including the P. falciparum erythrocyte membrane protein 1 (PfEMP1). VSA-specific antibodies are associated with protection from symptomatic and severe malaria. However, the importance of the different VSA targets of immunity to malaria remains unclear, which has impeded an understanding of malaria immunity and vaccine development. In this study, we developed assays using transgenic P. falciparum with modified PfEMP1 expression to quantify serum antibodies to VSAs among individuals exposed to malaria. We found that the majority of the human antibody response to the IE targets PfEMP1. Furthermore, our longitudinal studies showed that individuals with PfEMP1-specific antibodies had a significantly reduced risk of developing symptomatic malaria, whereas antibodies to other surface antigens were not associated with protective immunity. Using assays that measure antibody-mediated phagocytosis of IEs, an important mechanism in parasite clearance, we identified PfEMP1 as the major target of these functional antibodies. Taken together, these data demonstrate that PfEMP1 is a key target of humoral immunity. These findings advance our understanding of the targets and mediators of human immunity to malaria and have major implications for malaria vaccine development.

Increased sensitivity for detecting malaria parasites in human umbilical cord blood using scaled-up DNA preparation

Background

All mothers donating umbilical cord blood units to the NHS cord blood bank undergo an assessment for the likelihood of prior exposure to malaria infection. Those deemed at risk due to a history of travel to, or residence in, malaria endemic regions are screened serologically to detect anti-malaria antibodies. A positive result excludes the use of the cord blood for transplant therapy unless a risk assessment can ensure that malaria transmission is extremely unlikely. This paper details the screening of cord blood units from malaria serology positive mothers to detect malaria parasite DNA using a highly sensitive nested PCR.

Methods

Uninfected blood from a healthy volunteer was spiked with known quantities of malaria parasites and 5 millilitre and 200 microlitre aliquots were subjected to DNA extraction using QIAamp DNA maxi and DNA mini kits respectively. Nested PCR, to detect malarial SSU rRNA sequences, was performed on the purified DNA samples to determine the limit of detection for this assay with both extraction methodologies. Following assay validation, 54 cord blood units donated by mothers who were positive for anti-malaria antibodies were screened by this approach.

Results

When DNA was purified from 5 millilitres of blood it was possible to routinely detect as few as 50 malaria parasites per millilitre using nested PCR. This equates to a significant increase in the sensitivity of the current gold standard nucleic acid amplification technique used to detect malaria parasites (routinely performed from > 200 microlitre volumes of blood). None of the 54 donated cord blood units from serology positive mothers tested positive for malaria parasites using this scaled up DNA preparation method.

Conclusion

Serological testing for malaria parasites may be overly conservative, leading to unnecessary rejection of cord blood donations that lack malaria parasites and which are, therefore, safe for use in stem cell therapy.

Sequence polymorphism, segmental recombination and toggling amino acid residues within the DBL3X domain of the VAR2CSA placental malaria antigen

Plasmodium falciparum malaria remains one of the world's foremost health problems, primarily in highly endemic regions such as Sub-Saharan Africa, where it is responsible for substantial morbidity, mortality and economic losses. Malaria is a significant cause of severe disease and death in pregnant women and newborns, with pathogenesis being associated with expression of a unique variant of the multidomain Plasmodium falciparum Erythrocyte Membrane Protein 1 (PfEMP1) called VAR2CSA. Here, we characterize the polymorphism of the DBL3X domain of VAR2CSA and identify regions under selective pressure among placental parasites from women living in endemic western Kenya. In addition to significant levels of polymorphism, our analysis reveals evidence for diversification through intra-segmental recombination and novel mutations that likely contributed to the high number of unique VAR2CSA sequence types identified in this study. Interestingly, we also identified a number of critical residues that may be implicated in immune evasion through switching (or toggling) to alternative amino acids, including an arginine residue within the predicted binding pocket in subdomain III, which was previously implicated in binding to placental CSA. Overall, these findings are important for understanding parasite diversity in pregnant women and will be useful for identifying epitopes and variants of DBL3X to be included in a vaccine against placental malaria.

The case for PfEMP1-based vaccines to protect pregnant women against Plasmodium falciparum malaria

Vaccines are very cost-effective tools in combating infectious disease mortality and morbidity. Unfortunately, vaccines efficiently protecting against infection with malaria parasites are not available and are not likely to appear in the near future. An alternative strategy would be vaccines protecting against the disease and its consequences rather than against infection per se, by accelerating the development of the protective immunity that is normally acquired after years of exposure to malaria parasites in areas of stable transmission. This latter strategy is being energetically pursued to develop a vaccine protecting pregnant women and their offspring against mortality and morbidity caused by the accumulation of Plasmodium falciparum-infected erythrocytes in the placenta. It is based on a detailed understanding of the parasite antigen and the host receptor involved in this accumulation, as well as knowledge regarding the protective immune response that is acquired in response to placental P. falciparum infection. Nevertheless, it remains controversial in some quarters whether such a vaccine would have the desired impact, or indeed whether the strategy is meaningful. This article critically examines the relevance of several perceived obstacles to development of a vaccine against placental malaria.

Prospects and Pitfalls of Pregnancy-Associated Malaria Vaccination Based on the Natural Immune Response to Plasmodium falciparum VAR2CSA-Expressing Parasites

Pregnancy-associated malaria, a manifestation of severe malaria, is the cause of up to 200,000 infant deaths a year, through the effects of placental insufficiency leading to growth restriction and preterm delivery. Development of a vaccine is one strategy for control. Plasmodium falciparum-infected red blood cells accumulate in the placenta through specific binding of pregnancy-associated parasite variants that express the VAR2CSA antigen to chondroitin sulphate A on the surface of syncytiotrophoblast cells. Parasite accumulation, accompanied by an inflammatory infiltrate, disrupts the cytokine balance of pregnancy with the potential to cause placental damage and compromise foetal growth. Multigravid women develop immunity towards VAR2CSA-expressing parasites in a gravidity-dependent manner which prevents unfavourable pregnancy outcomes. Although current vaccine design, targeting VAR2CSA antigens, has succeeded in inducing antibodies artificially, this candidate may not provide protection during the first trimester and may only protect those women living in areas endemic for malaria. It is concluded that while insufficient information about placental-parasite interactions is presently available to produce an effective vaccine, incremental progress is being made towards achieving this goal.

Relevant assay to study the adhesion of Plasmodium falciparum-infected erythrocytes to the placental epithelium

In placental malaria, Plasmodium falciparum-infected erythrocytes adhere to the apical plasma membrane of the placental epithelium, triggering an impairment of placental function detrimental to the fetus. The design of anti-adhesion intervention strategies requires a detailed understanding of the mechanisms involved. However, most adhesion assays lack in vivo relevance and are hardly quantitative. Here, we describe a flow cytometry-based adhesion assay that is fully relevant by using apical epithelial plasma membrane vesicles as the adhesion matrix, and being applicable to infected erythrocytes directly isolated from patients. Adhesion is measured both as the percentage of pathogens bound to epithelial membrane vesicles as well as the mean number of vesicles bound per infected erythrocytes. We show that adhesins alternative to those currently identified could be involved. This demonstrates the power of this assay to advance our understanding of epithelial adhesion of infected erythrocytes and in the design of intervention strategies.

Induction of strain-transcendent antibodies to placental-type isolates with VAR2CSA DBL3 or DBL5 recombinant proteins

Background

Pregnancy associated malaria is a severe clinical syndrome associated with sequestration of Plasmodium falciparum-infected erythrocytes in the placenta. Placental binding is mediated by VAR2CSA, which adheres to chondroitin sulphate A (CSA). VAR2CSA is a large and polymorphic protein that has six Duffy binding-like (DBL) domains. There is still limited understanding as to how effective individual VAR2CSA domains are at generating inhibitory antibodies or the number of domain variants needed for universal vaccine coverage.

Methods

To investigate the immunogenic properties of single domain VAR2CSA recombinant proteins, rats or rabbits were immunized with five of the six VAR2CSA domains produced in Pichia pastoris. Immune plasma was analysed against a geographically diverse panel of CSA-binding lab lines to assess antibody breadth and inhibitory activity.

Results

Of the five domains, DBL3, and to a lesser extent DBL5, induced antibodies that cross-reacted on five diverse CSA-binding parasite lines by flow cytometry. By comparison, anti-DBL6 antibodies were highly strain-specific and anti-DBL1 and anti-DBL4 antibodies were poorly reactive by flow cytometry. From this series of recombinant proteins, adhesion-blocking activity was restricted to a single rat immunized against a DBL4 recombinant protein.

Conclusions

Single domain VAR2CSA recombinant proteins produced in P. pastoris had limited efficacy in eliciting adhesion blocking antibody responses, but VAR2CSA DBL3 and DBL5 domains contain strain-transcendent epitopes that can be targeted by vaccination and may have application for vaccine development.

Antibodies to a full-length VAR2CSA immunogen are broadly strain-transcendent but do not cross-inhibit different placental-type parasite isolates

The high molecular weight, multidomain VAR2CSA protein mediating adhesion of Plasmodium falciparum-infected erythrocytes in the placenta is the leading candidate for a pregnancy malaria vaccine. However, it has been difficult so far to generate strong and consistent adhesion blocking antibody responses against most single-domain VAR2CSA immunogens. Recent advances in expression of the full-length recombinant protein showed it binds with much greater specificity and affinity to chondroitin sulphate A (CSA) than individual VAR2CSA domains. This raises the possibility that a specific CSA binding pocket(s) is formed in the full length antigen and could be an important target for vaccine development. In this study, we compared the immunogenicity of a full-length VAR2CSA recombinant protein containing all six Duffy binding-like (DBL) domains to that of a three-domain construct (DBL4-6) in mice and rabbits. Animals immunized with either immunogen acquired antibodies reacting with several VAR2CSA individual domains by ELISA, but antibody responses against the highly conserved DBL4 domain were weaker in animals immunized with full-length DBL1-6 recombinant protein compared to DBL4-6 recombinant protein. Both immunogens induced cross-reactive antibodies to several heterologous CSA-binding parasite lines expressing different VAR2CSA orthologues. However, antibodies that inhibited adhesion of parasites to CSA were only elicited in rabbits immunized with full-length immunogen and inhibition was restricted to the homologous CSA-binding parasite. These findings demonstrate that partial and full-length VAR2CSA immunogens induce cross-reactive antibodies, but inhibitory antibody responses to full-length immunogen were highly allele-specific and variable between animal species.

Allelic diversity of the Plasmodium falciparum erythrocyte membrane protein 1 entails variant-specific red cell surface epitopes

The clonally variant Plasmodium falciparum PfEMP1 adhesin is a virulence factor and a prime target of humoral immunity. It is encoded by a repertoire of functionally differentiated var genes, which display architectural diversity and allelic polymorphism. Their serological relationship is key to understanding the evolutionary constraints on this gene family and rational vaccine design. Here, we investigated the Palo Alto/VarO and IT4/R29 and 3D7/PF13_003 parasites lines. VarO and R29 form rosettes with uninfected erythrocytes, a phenotype associated with severe malaria. They express an allelic Cys2/group A NTS-DBL1α₁ PfEMP1 domain implicated in rosetting, whose 3D7 ortholog is encoded by PF13_0003. Using these three recombinant NTS-DBL1α₁ domains, we elicited antibodies in mice that were used to develop monovariant cultures by panning selection. The 3D7/PF13_0003 parasites formed rosettes, revealing a correlation between sequence identity and virulence phenotype. The antibodies cross-reacted with the allelic domains in ELISA but only minimally with the Cys4/group B/C PFL1955w NTS-DBL1α. By contrast, they were variant-specific in surface seroreactivity of the monovariant-infected red cells by FACS analysis and in rosette-disruption assays. Thus, while ELISA can differentiate serogroups, surface reactivity assays define the more restrictive serotypes. Irrespective of cumulated exposure to infection, antibodies acquired by humans living in a malaria-endemic area also displayed a variant-specific surface reactivity. Although seroprevalence exceeded 90% for each rosetting line, the kinetics of acquistion of surface-reactive antibodies differed in the younger age groups. These data indicate that humans acquire an antibody repertoire to non-overlapping serotypes within a serogroup, consistent with an antibody-driven diversification pressure at the population level. In addition, the data provide important information for vaccine design, as production of a vaccine targeting rosetting PfEMP1 adhesins will require engineering to induce variant-transcending responses or combining multiple serotypes to elicit a broad spectrum of immunity.

Evidence that the erythrocyte invasion ligand PfRh2 is a target of protective immunity against Plasmodium falciparum malaria

Abs targeting blood-stage Ags of Plasmodium falciparum are important in acquired immunity to malaria, but major targets remain unclear. The P. falciparum reticulocyte-binding homologs (PfRh) are key ligands used by merozoites during invasion of erythrocytes. PfRh2a and PfRh2b are functionally important members of this family and may be targets of protective immunity, but their potential role in human immunity has not been examined. We expressed eight recombinant proteins covering the entire PfRh2 common region, as well as PfRh2a- and PfRh2b-specific regions. Abs were measured among a cohort of 206 Papua New Guinean children who were followed prospectively for 6 mo for reinfection and malaria. At baseline, Abs were associated with increasing age and active infection. High levels of IgG to all PfRh2 protein constructs were strongly associated with protection from symptomatic malaria and high-density parasitemia. The predominant IgG subclasses were IgG1 and IgG3, with little IgG2 and IgG4 detected. To further understand the significance of PfRh2 as an immune target, we analyzed PfRh2 sequences and found that polymorphisms are concentrated in an N-terminal region of the protein and seem to be under diversifying selection, suggesting immune pressure. Cluster analysis arranged the sequences into two main groups, suggesting that many of the haplotypes identified may be antigenically similar. These findings provide evidence suggesting that PfRh2 is an important target of protective immunity in humans and that Abs act by controlling blood-stage parasitemia and support its potential for vaccine development.