Broad analysis reveals a consistent pattern of var gene transcription in Plasmodium falciparum repeatedly selected for a defined adhesion phenotype

The P. falciparum alternative histones Pf H2A.Z and Pf H2B.Z are dynamically acetylated and antagonized by PfSir2 histone deacetylases at heterochromatin boundaries

IMPORTANCE

The malaria parasite Plasmodium falciparum relies on variant expression of members of multi-gene families as a strategy for environmental adaptation to promote parasite survival and pathogenesis. These genes are located in transcriptionally silenced DNA regions. A limited number of these genes escape gene silencing, and switching between them confers variant fitness on parasite progeny. Here, we show that PfSir2 histone deacetylases antagonize DNA-interacting acetylated alternative histones at the boundaries between active and silent DNA. This finding implicates acetylated alternative histones in the mechanism regulating P. falciparum variant gene silencing and thus malaria pathogenesis. This work also revealed that acetylation of alternative histones at promoters is dynamically associated with promoter activity across the genome, implicating acetylation of alternative histones in gene regulation genome wide. Understanding mechanisms of gene regulation in P. falciparum may aid in the development of new therapeutic strategies for malaria, which killed 619,000 people in 2021.

Malaria-specific Type 1 regulatory T cells are more abundant in first pregnancies and associated with placental malaria

BACKGROUND

Malaria in pregnancy (MIP) causes higher morbidity in primigravid compared to multigravid women; however, the correlates and mechanisms underlying this gravidity-dependent protection remain incompletely understood. We aimed to compare the cellular immune response between primigravid and multigravid women living in a malaria-endemic region and assess for correlates of protection against MIP.

METHODS

We characterised the second trimester cellular immune response among 203 primigravid and multigravid pregnant women enrolled in two clinical trials of chemoprevention in eastern Uganda, utilizing RNA sequencing, flow cytometry, and functional assays. We compared responses across gravidity and determined associations with parasitaemia during pregnancy and placental malaria.

FINDINGS

Using whole blood RNA sequencing, no significant differentially expressed genes were identified between primigravid (n = 12) and multigravid (n = 11) women overall (log 2(FC) > 2, FDR < 0.1). However, primigravid (n = 49) women had higher percentages of malaria-specific, non-naïve CD4+ T cells that co-expressed IL-10 and IFNγ compared with multigravid (n = 85) women (p = 0.000023), and higher percentages of these CD4+ T cells were associated with greater risks of parasitaemia in pregnancy (Rs = 0.49, p = 0.001) and placental malaria (p = 0.0073). These IL-10 and IFNγ co-producing CD4+ T cells had a genomic signature of Tr1 cells, including expression of transcription factors cMAF and BATF and cell surface makers CTLA4 and LAG-3.

INTERPRETATION

Malaria-specific Tr1 cells were highly prevalent in primigravid Ugandan women, and their presence correlated with a higher risk of malaria in pregnancy. Understanding whether suppression of Tr1 cells plays a role in naturally acquired gravidity-dependent immunity may aid the development of new vaccines or treatments for MIP.

FUNDING

This work was funded by NIH (PO1 HD059454, U01 AI141308, U19 AI089674, U01 AI155325, U01 AI150741), the March of Dimes (Basil O'Connor award), and the Bill and Melinda Gates Foundation (OPP 1113682).

The exception that proves the rule: Virulence gene expression at the onset of Plasmodium falciparum blood stage infections

Controlled human malaria infections (CHMI) are a valuable tool to study parasite gene expression in vivo under defined conditions. In previous studies, virulence gene expression was analyzed in samples from volunteers infected with the Plasmodium falciparum (Pf) NF54 isolate, which is of African origin. Here, we provide an in-depth investigation of parasite virulence gene expression in malaria-naïve European volunteers undergoing CHMI with the genetically distinct Pf 7G8 clone, originating in Brazil. Differential expression of var genes, encoding major virulence factors of Pf, PfEMP1s, was assessed in ex vivo parasite samples as well as in parasites from the in vitro cell bank culture that was used to generate the sporozoites (SPZ) for CHMI (Sanaria PfSPZ Challenge (7G8)). We report broad activation of mainly B-type subtelomeric located var genes at the onset of a 7G8 blood stage infection in naïve volunteers, mirroring the NF54 expression study and suggesting that the expression of virulence-associated genes is generally reset during transmission from the mosquito to the human host. However, in 7G8 parasites, we additionally detected a continuously expressed single C-type variant, Pf7G8_040025600, that was most highly expressed in both pre-mosquito cell bank and volunteer samples, suggesting that 7G8, unlike NF54, maintains expression of some previously expressed var variants during transmission. This suggests that in a new host, the parasite may preferentially express the variants that previously allowed successful infection and transmission. Trial registration: ClinicalTrials.gov - NCT02704533; 2018-004523-36.

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

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

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.

Progress and Insights Toward an Effective Placental Malaria Vaccine

In areas where Plasmodium falciparum transmission is endemic, clinical immunity against malaria is progressively acquired during childhood and adults are usually protected against the severe clinical consequences of the disease. Nevertheless, pregnant women, notably during their first pregnancies, are susceptible to placental malaria and the associated serious clinical outcomes. Placental malaria is characterized by the massive accumulation of P. falciparum infected erythrocytes and monocytes in the placental intervillous spaces leading to maternal anaemia, hypertension, stillbirth and low birth weight due to premature delivery, and foetal growth retardation. Remarkably, the prevalence of placental malaria sharply decreases with successive pregnancies. This protection is associated with the development of antibodies directed towards the surface of P. falciparum-infected erythrocytes from placental origin. Placental sequestration is mediated by the interaction between VAR2CSA, a member of the P. falciparum erythrocyte membrane protein 1 family expressed on the infected erythrocytes surface, and the placental receptor chondroitin sulfate A. VAR2CSA stands today as the leading candidate for a placental malaria vaccine. We recently reported the safety and immunogenicity of two VAR2CSA-derived placental malaria vaccines (PRIMVAC and PAMVAC), spanning the chondroitin sulfate A-binding region of VAR2CSA, in both malaria-naïve and P. falciparum-exposed non-pregnant women in two distinct Phase I clinical trials (ClinicalTrials.gov, NCT02658253 and NCT02647489). This review discusses recent advances in placental malaria vaccine development, with a focus on the recent clinical data, and discusses the next clinical steps to undertake in order to better comprehend vaccine-induced immunity and accelerate vaccine development.

VAR2CSA-Mediated Host Defense Evasion of Plasmodium falciparum Infected Erythrocytes in Placental Malaria

Over 30 million women living in P. falciparum endemic areas are at risk of developing malaria during pregnancy every year. Placental malaria is characterized by massive accumulation of infected erythrocytes in the intervillous space of the placenta, accompanied by infiltration of immune cells, particularly monocytes. The consequent local inflammation and the obstruction of the maternofetal exchanges can lead to severe clinical outcomes for both mother and child. Even if protection against the disease can gradually be acquired following successive pregnancies, the malaria parasite has developed a large panel of evasion mechanisms to escape from host defense mechanisms and manipulate the immune system to its advantage. Infected erythrocytes isolated from placentas of women suffering from placental malaria present a unique phenotype and express the pregnancy-specific variant VAR2CSA of the Plasmodium falciparum Erythrocyte Membrane Protein (PfEMP1) family at their surface. The polymorphic VAR2CSA protein is able to mediate the interaction of infected erythrocytes with a variety of host cells including placental syncytiotrophoblasts and leukocytes but also with components of the immune system such as non-specific IgM. This review summarizes the described VAR2CSA-mediated host defense evasion mechanisms employed by the parasite during placental malaria to ensure its survival and persistence.

Fetal and Maternal Innate Immunity Receptors Have Opposing Effects on the Severity of Experimental Malaria in Pregnancy: Beneficial Roles for Fetus-Derived Toll-Like Receptor 4 and Type I Interferon Receptor 1

Malaria in pregnancy (MiP) is a distinctive clinical form of Plasmodium infection and is a cause of placental insufficiency leading to poor pregnancy outcomes. Maternal innate immunity responses play a decisive role in the development of placental inflammation, but the action of fetus-derived factors in MiP outcomes has been overlooked. We investigated the role of the Tlr4 and Ifnar1 genes, taking advantage of heterogenic mating strategies to dissect the effects mediated by maternally and fetally derived Toll-like receptor 4 (TLR4) or type I interferon receptor 1 (IFNAR1). Using a mouse infection system displaying severe MiP outcomes, we found that the expressions of TLR4 and IFNAR1 in the maternal compartment take part in deleterious MiP outcomes, but their fetal counterparts patently counteract these effects. We uncovered that fetal TLR4 contributes to the in vitro uptake of infected erythrocytes by trophoblasts and to the innate immune response in the placenta, offering robust protection of fetus viability, but had no sensible impact on the placental parasite burden. In contrast, we observed that the expression of IFNAR1 in the fetal compartment was associated with a reduced placental parasite burden but had little beneficial effect on fetus outcomes. Furthermore, the downregulation of Ifnar1 expression in infected placentas and in trophoblasts exposed to infected erythrocytes indicated that the interferon-IFNAR1 pathway is involved in the trophoblast response to infection. This work unravels that maternal and fetal counterparts of innate immune pathways drive opposing responses in murine placental malaria and implicates the activation of innate receptors in fetal trophoblast cells in the control of placental infection and in the protection of the fetus.

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.

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.

Antigenic Variation and the Genetics and Epigenetics of the PfEMP1 Erythrocyte Surface Antigens in Plasmodium falciparum Malaria

How immunity to malaria develops remains one of the great unresolved issues in bio-medicine and resolution of its various paradoxes is likely to be the key to developing effective malaria vaccines. The basic epidemiological observations are; under conditions of intense natural transmission, humans do become immune to P. falciparum malaria, but this is a slow process requiring multiple disease episodes which many, particularly young children, do not survive. Adult survivors are immune to the symptoms of malaria, and unless pregnant, can control the growth of most or all new inoculations. Sterile immunity is not achieved and chronic parasitization of apparently healthy adults is the norm. In this article, we analyse the best understood malaria "antigenic variation" system, that based on Plasmodium falciparum's PfEMP1-type cytoadhesion antigens, and critically review recent literature on the function and control of this multi-gene family of parasite variable surface antigens.

Impact of Malaria in Pregnancy as Latin America Approaches Elimination

In Latin America, four million pregnancies are at risk of malaria annually, but malaria in pregnancy is largely overlooked. As countries progress toward malaria elimination, targeting reservoirs of transmission is a priority. Pregnant women are an important risk group because they harbor asymptomatic infections and dormant liver stages of Plasmodium vivax that cause relapses. Of significant concern is the discovery that most infections in pregnant women fail to be detected by routine diagnostics. We review here recent findings on malaria in pregnancy within Latin America. We focus on the Amazon basin and Northwest Colombia, areas that harbor the greatest burden of malaria, and propose that more sensitive diagnostics and active surveillance at antenatal clinics will be necessary to eliminate malaria from these final frontiers.

Large screen approaches to identify novel malaria vaccine candidates

Until recently, malaria vaccine development efforts have focused almost exclusively on a handful of well characterized Plasmodium falciparum antigens. Despite dedicated work by many researchers on different continents spanning more than half a century, a successful malaria vaccine remains elusive. Sequencing of the P. falciparum genome has revealed more than five thousand genes, providing the foundation for systematic approaches to discover candidate vaccine antigens. We are taking advantage of this wealth of information to discover new antigens that may be more effective vaccine targets. Herein, we describe different approaches to large-scale screening of the P. falciparum genome to identify targets of either antibody responses or T cell responses using human specimens collected in Controlled Human Malaria Infections (CHMI) or under conditions of natural exposure in the field. These genome, proteome and transcriptome based approaches offer enormous potential for the development of an efficacious malaria vaccine.

Risk factors for placental malaria and associated adverse pregnancy outcomes in Rufiji, Tanzania: a hospital based cross sectional study

BACKGROUND

Prevention and treatment of malaria during pregnancy is crucial for reduction of malaria in pregnancy and its adverse outcomes. The spread of parasite resistance to Sulphadoxine-Pyrimethamine (SP) used for Intermittent Preventive Treatment for malaria in pregnancy (IPTp), particularly in East Africa has raised concerns about the usefulness and the reliability of the IPTp regimen. We aimed to assess the effectiveness of two doses of SP in treating and preventing occurrence of adverse pregnancy outcomes.

METHODOLOGY

The study was an analytical cross sectional study which enrolled 350 pregnant women from Kibiti Health Centre, South Eastern Tanzania. Structured questionnaires were used to obtain previous obstetrics and medical history of participants and verified by reviewing antenatal clinic cards. Maternal placental blood samples for microscopic examination of malaria parasites were collected after delivery. Data was analyzed for associations between SP dosage, risk for PM and pregnancy outcome. Sample size was estimated based on precision.

RESULTS

Prevalence of placental maternal (PM) was 8% among pregnant women (95%CI, 4.4-13.1%). Factors associated with increased risk of PM were primigravidity (P<0.001) and history of fever during pregnancy (P= 0.02). Use of at least 2 doses of SP for IPTp during pregnancy was insignificantly associated with reducing the risk PM (P=0.08), low birth weight (P=0.73) and maternal anemia (P=0.71) but associated significantly with reducing the risk of preterm birth (P<0.001).

CONCLUSION

Two doses of SP for IPTp regime are ineffective in preventing and treating PM and adverse pregnancy outcome. Hence a review to the current IPTp regimen should be considered with possibility of integrating it with other malaria control strategies.

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.

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.

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.

Treatment and prevention of malaria in pregnancy: opportunities and challenges

Control of malaria in pregnancy through prevention or treatment may save lives of mothers and babies. Selection of drugs for treatment of infected pregnant women, or for prevention in exposed populations is problematic owing to resistance to established drugs and lack of pregnancy-specific safety and pharmacological data for new drugs. Encouragingly, a number of new drugs and combinations of drugs hold promise for effective treatment, but adequate data on their safety in pregnancy is currently lacking. Our principal challenges are to decide which drugs to develop for use in malaria treatment and prevention in pregnancy and to develop mechanisms to rapidly and comprehensively evaluate their safety. Prevention of pregnancy malaria by vaccination may also become possible, but targets must be closely defined, and strategies developed to test candidates against meaningful end points.

Malarial parasite diversity in chimpanzees: the value of comparative approaches to ascertain the evolution of Plasmodium falciparum antigens

BACKGROUND

Plasmodium falciparum shares its most recent common ancestor with parasites found in African apes; these species constitute the so-called Laverania clade. In this investigation, the evolutionary history of Plasmodium lineages found in chimpanzees (Pan troglodytes) was explored.

METHODS

Here, the remainders of 74 blood samples collected as part of the chimpanzees' routine health examinations were studied. For all positive samples with parasite lineages belonging to the Laverania clade, the complete mitochondrial genome (mtDNA), the gene encoding dihydrofolate reductase-thymidylate synthase (dhfr-ts), the chloroquine resistance transporter (Pfcrt), the circumsporozoite protein (csp), merozoite surface protein 2 (msp2), and the DBL-1 domain from var2CSA were amplified, cloned, and sequenced. Other Plasmodium species were included in the mtDNA, dhfr-ts, and csp analyses. Phylogenetic and evolutionary genetic analyses were performed, including molecular clock analyses on the mtDNA.

RESULTS/CONCLUSIONS

Nine chimpanzees were malaria positive (12.2%); four of those infections were identified as P. falciparum, two as a Plasmodium reichenowi-like parasite or Plasmodium sp., one as Plasmodium gaboni, and two as Plasmodium malariae. All P. falciparum isolates were resistant to chloroquine indicating that the chimpanzees acquired such infections from humans in recent times. Such findings, however, are not sufficient for implicating chimpanzees as an animal reservoir for P. falciparum.Timing estimates support that the Laverania clade has co-existed with hominids for a long-period of time. The proposed species P. gaboni, Plasmodium billbrayi, and Plasmodium billcollinsi are monophyletic groups supporting that they are indeed different species.An expanded CSP phylogeny is presented, including all the Laverania species and other malarial parasites. Contrasting with other Plasmodium, the Laverania csp exhibits great conservation at the central tandem repeat region. Msp2 and var2CSA, however, show extended recent polymorphism in P. falciparum that likely originated after the P. reichenowi-P. falciparum split. The accumulation of such diversity may indicate adaptation to the human host. These examples support the notion that comparative approaches among P. falciparum and its related species will be of great value in understanding the evolution of proteins that are important in parasite invasion of the human red blood cell, as well as those involved in malaria pathogenesis.

Nuclear repositioning precedes promoter accessibility and is linked to the switching frequency of a Plasmodium falciparum invasion gene

Variation of surface adhesins, such as the Plasmodium falciparum erythrocyte invasion ligand PfRh4, is critical for virulence and immune evasion in many microbes. While phenotypic switching is linked to transcriptional changes and chromatin function, the determinants of switching frequency remain poorly defined. By expressing a prokaryotic DNA methylase in P. falciparum, we directly assayed accessibility of transcriptionally active and silent chromatin at the PfRh4 locus. Parasites selected for in vivo PfRh4 activation show a reversible increase in promoter accessibility and exhibit perinuclear repositioning of the locus from a silent to a conserved activation domain. Forced activation of a proximal gene results in a similar repositioning of the PfRh4 locus, with a concomitant increase in PfRh4 activation in a subpopulation of parasites and promoter accessibility correlating with actively transcribed loci. Thus, nuclear repositioning is associated with increased P. falciparum switching frequency, while promoter accessibility is tightly linked to clonally active PfRh4 promoters.

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.

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.

Expression and strain variation of the novel "small open reading frame" (smorf) multigene family in Babesia bovis

Small open reading frame (smorf) genes comprise the second largest Babesia bovis multigene family. All known 44 variant smorf genes are located in close chromosomal proximity to ves1 genes, which encode proteins that mediate cytoadhesion and contribute to immune evasion. In this study, we characterised the general topology of smorf genes and investigated the gene repertoire, transcriptional profile and SMORF expression in two distinct strains, T2Bo and Mo7. Sequence analysis using degenerate primers identified additional smorf genes in each strain and demonstrated that the smorf gene repertoire varies between strains, with conserved and unique genes in both. Smorf genes have multiple semi-conserved and variable blocks, and a large hypervariable insertion in 20 of the 44 genes defines two major branches of the family, termed smorf A and smorf B. A total of 32 smorf genes are simultaneously transcribed in T2Bo strain B. bovis merozoites obtained from deep brain tissue of an acutely infected animal. SMORF peptide-specific antiserum bound in immunoblots to multiple proteins with a range of sizes predicted by smorf genes, confirming translation of smorf gene products from these transcripts. These results indicate that the smorf multigene family is larger than previously described and demonstrate that smorf genes are expressed and are undergoing variation, both within strains and in a lineage-specific pattern independent of strain specificity. The function of these novel proteins is unknown.

Highly co-ordinated var gene expression and switching in clinical Plasmodium falciparum isolates from non-immune malaria patients

Antigenic variation to fool the immune system is one of the molecular tricks Plasmodium uses to maintain infection in its human host. The exclusive expression of the surface-exposed PfEMP1 molecules, encoded by var genes, is the best example for this. Central questions regarding the dynamics of antigenic variation, namely the rate of switching and the regulation of var gene expression in Plasmodium falciparum, are yet unanswered. To elucidate the in vivo situation, we studied var gene switching by analysing the var transcripts from parasites isolated from 20 non-immune malaria patients as well as during subsequent in vitro generations. Parasites were found to be highly co-ordinated as the whole population isolated from individual patients usually expressed only one dominant - preferentially group A -var gene. While some isolates have very low switching rates, others switched their var gene expression in every generation. However, during extended cultivation the co-ordinated expression and switching is lost resulting in random expression of all var gene groups. Switching as observed on the RNA level was also supported on the protein level using PfEMP1-specific antibodies. The results suggest that var genes switch in an ordered, hierarchical manner at much higher rates than previously described.

Var2CSA minimal CSA binding region is located within the N-terminal region

Var2CSA, a key molecule linked with pregnancy-associated malaria (PAM), causes sequestration of Plasmodium falciparum infected erythrocytes (PEs) in the placenta by adhesion to chondroitin sulfate A (CSA). Var2CSA possesses a 300 kDa extracellular region composed of six Duffy-binding like (DBL) domains and a cysteine-rich interdomain region (CIDRpam) module. Although initial studies implicated several individual var2CSA DBL domains as important for adhesion of PEs to CSA, new studies revealed that these individual domains lack both the affinity and specificity displayed by the full-length extracellular region. Indeed, recent evidence suggests the presence of a single CSA-binding site formed by a higher-order domain organization rather than several independent binding sites located on the different domains. Here, we search for the minimal binding region within var2CSA that maintains high affinity and specificity for CSA binding, a characteristic feature of the full-length extracellular region. Accordingly, truncated recombinant var2CSA proteins comprising different domain combinations were expressed and their binding characteristics assessed against different sulfated glycosaminoglycans (GAGs). Our results indicate that the smallest region within var2CSA with similar binding properties to those of the full-length var2CSA is DBL1X-3X. We also demonstrate that inhibitory antibodies raised in rabbit against the full-length DBL1X-6ε target principally DBL3X and, to a lesser extent, DBL5ε. Taken together, our results indicate that efforts should focus on the DBL1X-3X region for developing vaccine and therapeutic strategies aimed at combating PAM.

Expression of P. falciparum var genes involves exchange of the histone variant H2A.Z at the promoter

Plasmodium falciparum employs antigenic variation to evade the human immune response by switching the expression of different variant surface antigens encoded by the var gene family. Epigenetic mechanisms including histone modifications and sub-nuclear compartmentalization contribute to transcriptional regulation in the malaria parasite, in particular to control antigenic variation. Another mechanism of epigenetic control is the exchange of canonical histones with alternative variants to generate functionally specialized chromatin domains. Here we demonstrate that the alternative histone PfH2A.Z is associated with the epigenetic regulation of var genes. In many eukaryotic organisms the histone variant H2A.Z mediates an open chromatin structure at promoters and facilitates diverse levels of regulation, including transcriptional activation. Throughout the asexual, intraerythrocytic lifecycle of P. falciparum we found that the P. falciparum ortholog of H2A.Z (PfH2A.Z) colocalizes with histone modifications that are characteristic of transcriptionally-permissive euchromatin, but not with markers of heterochromatin. Consistent with this finding, antibodies to PfH2A.Z co-precipitate the permissive modification H3K4me3. By chromatin-immunoprecipitation we show that PfH2A.Z is enriched in nucleosomes around the transcription start site (TSS) in both transcriptionally active and silent stage-specific genes. In var genes, however, PfH2A.Z is enriched at the TSS only during active transcription in ring stage parasites. Thus, in contrast to other genes, temporal var gene regulation involves histone variant exchange at promoter nucleosomes. Sir2 histone deacetylases are important for var gene silencing and their yeast ortholog antagonises H2A.Z function in subtelomeric yeast genes. In immature P. falciparum parasites lacking Sir2A or Sir2B high var transcription levels correlate with enrichment of PfH2A.Z at the TSS. As Sir2A knock out parasites mature the var genes are silenced, but PfH2A.Z remains enriched at the TSS of var genes; in contrast, PfH2A.Z is lost from the TSS of de-repressed var genes in mature Sir2B knock out parasites. This result indicates that PfH2A.Z occupancy at the active var promoter is antagonized by PfSir2A during the intraerythrocytic life cycle. We conclude that PfH2A.Z contributes to the nucleosome architecture at promoters and is regulated dynamically in active var genes.

Plasmodium falciparum is a protist parasite that causes malaria and kills more than 800,000 people per year. The parasite escapes from the human immune response by antigenic variation through switching between expression of different var genes. These encode different variant antigens that are expressed on the surface of the infected erythrocyte and mediate pathogenic adhesion of the infected erythrocytes to host receptors. Understanding how this process is regulated may lead to the identification of factors that are essential for immune evasion and that could represent novel drug targets. Here, we have identified the parasite's histone variant PfH2A.Z as a novel contributor to the transcriptional regulation of antigenic variation. PfH2A.Z is enriched in the promoter of many genes, but enrichment correlates with gene expression only in var genes. Furthermore we show that PfH2A.Z enrichment in var promoters is antagonised by the var gene silencing factor PfSir2A. These findings further extend our knowledge of the complex mechanisms regulating gene expression in P. falciparum.

Molecular aspects of Plasmodium falciparum Infection during pregnancy

Cytoadherence of Plasmodium-falciparum-parasitized red blood cells (PRBCs) to host receptors is the key phenomenon in the pathological process of the malaria disease. Some of these interactions can originate poor outcomes responsible for 1 to 3 million annual deaths mostly occurring among children in sub-Saharan Africa. Pregnancy-associated malaria (PAM) represents an important exception of the disease occurring at adulthood in malaria endemic settings. Consequences of this are shared between the mother (maternal anemia) and the baby (low birth weight and infant mortality). Demonstrating that parasites causing PAM express specific variant surface antigens (VSA(PAM)), including the P. falciparum erythrocyte membrane protein 1 (P f EMP1) variant VAR2CSA, that are targets for protective immunity has strengthened the possibility for the development of PAM-specific vaccine. In this paper, we review the molecular basis of malaria pathogenesis attributable to the erythrocyte stages of the parasites, and findings supporting potential anti-PAM vaccine components evidenced in PAM.

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

Pregnant women are infected by specific variants of Plasmodium falciparum that adhere and accumulate in the placenta. Using serological and molecular approaches, we assessed the global antigenic diversity of surface antigens expressed by placenta-binding isolates to better understand immunity to malaria in pregnancy and evolution of polymorphisms and to inform vaccine development. We found that placenta-binding isolates originating from all major regions where malaria occurs were commonly recognized by antibodies in different populations of pregnant women. There was substantial antigenic overlap and sharing of epitopes between isolates, including isolates from distant geographic locations, suggesting that there are limitations to antigenic diversity; however, differences between populations and isolates were also seen. Many women had cross-reactive antibodies and/or a broad repertoire of antibodies to different isolates. Studying VAR2CSA as the major antigen expressed by placenta-binding isolates, we identified antibody epitopes encoded by variable sequence blocks in the DBL3 domain. Analysis of global var2csa DBL3 sequences demonstrated that there was extensive sharing of variable blocks between Africa, Asia, Papua New Guinea, and Latin America, which likely contributes to the high level of antigenic overlap between different isolates. However, there was also evidence of geographic clustering of sequences and differences in VAR2CSA sequences between populations. The results indicate that there is limited antigenic diversity in placenta-binding isolates and may explain why immunity to malaria in pregnancy can be achieved after exposure during one pregnancy. Inclusion of a limited number of variants in a candidate vaccine may be sufficient for broad population coverage, but geographic considerations may also have to be included in vaccine design.

Differential recognition of P. falciparum VAR2CSA domains by naturally acquired antibodies in pregnant women from a malaria endemic area

Background

Plasmodium falciparum infected red blood cells (iRBC) express variant surface antigens (VSA) of which VAR2CSA is involved in placental sequestration and causes pregnancy-associated malaria (PAM). Primigravidae are most susceptible to PAM whereas antibodies associated with protection are often present at higher levels in multigravid women. However, HIV co-infection with malaria has been shown to alter this parity-dependent acquisition of immunity, with more severe symptoms as well as more malaria episodes in HIV positive women versus HIV negative women of a similar parity.

Methods

Using VAR2CSA DBL-domains expressed on the surface of CHO-745 cells we quantified levels of DBL-domain specific IgG in sera from pregnant Malawian women by flow cytometry. Dissociations constants of DBL5ε specific antibodies were determined using a surface plasmon resonance technique, as an indication of antibody affinities.

Results

VAR2CSA DBL5ε was recognized in a gender and parity-dependent manner with anti-DBL5ε IgG correlating significantly with IgG levels to VSA-PAM on the iRBC surface. HIV positive women had lower levels of anti-DBL5ε IgG than HIV negative women of similar parity. In primigravidae, antibodies in HIV positive women also showed significantly lower affinity to VAR2CSA DBL5ε.

Conclusions

Pregnant women from a malaria-endemic area had increased levels of anti-DBL5ε IgG by parity, indicating this domain of VAR2CSA to be a promising vaccine candidate against PAM. However, it is important to consider co-infection with HIV, as this seems to change the properties of antibody response against malaria. Understanding the characteristics of antibody response against VAR2CSA is undoubtedly imperative in order to design a functional and efficient vaccine against PAM.

Sir2 paralogues cooperate to regulate virulence genes and antigenic variation in Plasmodium falciparum

Cytoadherance of Plasmodium falciparum-infected erythrocytes in the brain, organs and peripheral microvasculature is linked to morbidity and mortality associated with severe malaria. Parasite-derived P. falciparum Erythrocyte Membrane Protein 1 (PfEMP1) molecules displayed on the erythrocyte surface are responsible for cytoadherance and undergo antigenic variation in the course of an infection. Antigenic variation of PfEMP1 is achieved by in situ switching and mutually exclusive transcription of the var gene family, a process that is controlled by epigenetic mechanisms. Here we report characterisation of the P. falciparum silent information regulator's A and B (PfSir2A and PfSir2B) and their involvement in mutual exclusion and silencing of the var gene repertoire. Analysis of P. falciparum parasites lacking either PfSir2A or PfSir2B shows that these NAD⁺-dependent histone deacetylases are required for silencing of different var gene subsets classified by their conserved promoter type. We also demonstrate that in the absence of either of these molecules mutually exclusive expression of var genes breaks down. We show that var gene silencing originates within the promoter and PfSir2 paralogues are involved in cis spreading of silenced chromatin into adjacent regions. Furthermore, parasites lacking PfSir2A but not PfSir2B have considerably longer telomeric repeats, demonstrating a role for this molecule in telomeric end protection. This work highlights the pivotal but distinct role for both PfSir2 paralogues in epigenetic silencing of P. falciparum virulence genes and the control of pathogenicity of malaria infection.

The unicellular parasite Plasmodium falciparum is the cause of the most severe form of malaria and is responsible for 300 million infections and ∼2 million deaths a year. Infected erythrocytes clump and block capillaries in the peripheral circulation, the brain, and placenta and are a major contributor to the pathology of malaria. A parasite-derived protein displayed on the surface of the infected erythrocyte is responsible for erythrocyte clumping in capillaries. Although 60 subtelomeric var genes can encode different versions of this “sticky” capillary-binding protein, only one protein is expressed at a time, and switches in expression between these genes causes variation of this pathogenic molecule enabling the parasite to evade the immune system. Here we identify two chromatin-modifying proteins that cooperate to mediate silencing and mutual exclusive expression of var genes. These proteins are thus important virulence factors of the malaria-causing parasite.

Investigation into two Sir2 histone deacetylases in the malaria-causing parasite revealstrans-acting epigenetic factors control mutually exclusive expression of a major subtelomeric virulence gene family.

A var gene promoter implicated in severe malaria nucleates silencing and is regulated by 3' untranslated region and intronic cis-elements

Questions surround the mechanism of mutually exclusive expression by which Plasmodium falciparum mediates activation and silencing of var genes. These encode PfEMP1 proteins, which function as cytoadherent and immunomodulatory molecules at the surface of parasitised erythrocytes. Current evidence suggests that promoter silencing by var introns might play a key role in var gene regulation. To evaluate the impact of cis-acting regulatory regions on var silencing, we generated P. falciparum lines in which luciferase was placed under the control of an UpsA var promoter. By utilising the Bxb1 integrase system, these reporter cassettes were targeted to a genomic region that was not in apposition to var subtelomeric domains. This eliminated possible effects from surrounding telomeric elements and removed the variability inherent in episomal systems. Studies with highly synchronised parasites revealed that the UpsA element possessed minimal activity in comparison with a heterologous (hrp3) promoter. This may result from the integrated UpsA promoter being largely silenced by the neighbouring cg6 promoter. Our analyses also revealed that the DownsA 3' untranslated region further decreased the luciferase activity from both cassettes, whereas the var A intron repressed the UpsA promoter specifically. By applying multivariate analysis over the entire cell cycle, we confirmed the significance of these cis-elements and found the parasite stage to be the major factor regulating UpsA-promoter activity. Additionally, we observed that the UpsA promoter was capable of nucleating reversible silencing that spread to a downstream promoter. We believe these studies are the first to analyse promoter activity of Group A var genes, which have been implicated in severe malaria, and support the model that var introns can further suppress var expression. These data also suggest an important suppressive role for the DownsA terminator. Our findings imply the existence of multiple levels of var gene regulation in addition to intrinsic promoter-dependent silencing.

Ectopic recombination of a malaria var gene during mitosis associated with an altered var switch rate

The Plasmodium falciparum var multigene family encodes P. falciparum erythrocyte membrane protein 1, which is responsible for the pathogenic traits of antigenic variation and adhesion of infected erythrocytes to host receptors during malaria infection. Clonal antigenic variation of P. falciparum erythrocyte membrane protein 1 is controlled by the switching between exclusively transcribed var genes. The tremendous diversity of the var gene repertoire both within and between parasite strains is critical for the parasite's strategy of immune evasion. We show that ectopic recombination between var genes occurs during mitosis, providing P. falciparum with opportunities to diversify its var repertoire, even during the course of a single infection. We show that the regulation of the recombined var gene has been disrupted, resulting in its persistent activation although the regulation of most other var genes is unaffected. The var promoter and intron of the recombined var gene are not responsible for its atypically persistent activity, and we conclude that altered subtelomeric cis sequence is the most likely cause of the persistent activity of the recombined var gene.

C5a enhances dysregulated inflammatory and angiogenic responses to malaria in vitro: potential implications for placental malaria

Background

Placental malaria (PM) is a leading cause of maternal and infant mortality. Although the accumulation of parasitized erythrocytes (PEs) and monocytes within the placenta is thought to contribute to the pathophysiology of PM, the molecular mechanisms underlying PM remain unclear. Based on the hypothesis that excessive complement activation may contribute to PM, in particular generation of the potent inflammatory peptide C5a, we investigated the role of C5a in the pathogenesis of PM in vitro and in vivo.

Methodology and Principal Findings

Using primary human monocytes, the interaction between C5a and malaria in vitro was assessed. CSA- and CD36-binding PEs induced activation of C5 in the presence of human serum. Plasmodium falciparum GPI (pfGPI) enhanced C5a receptor expression (CD88) on monocytes, and the co-incubation of monocytes with C5a and pfGPI resulted in the synergistic induction of cytokines (IL-6, TNF, IL-1β, and IL-10), chemokines (IL-8, MCP-1, MIP1α, MIP1β) and the anti-angiogenic factor sFlt-1 in a time and dose-dependent manner. This dysregulated response was abrogated by C5a receptor blockade. To assess the potential role of C5a in PM, C5a plasma levels were measured in malaria-exposed primigravid women in western Kenya. Compared to pregnant women without malaria, C5a levels were significantly elevated in women with PM.

Conclusions and Significance

These results suggest that C5a may contribute to the pathogenesis of PM by inducing dysregulated inflammatory and angiogenic responses that impair placental function.

VAR2CSA and protective immunity against pregnancy-associated Plasmodium falciparum malaria

People living in areas with stable transmission of P. falciparum parasites acquire protective immunity to malaria over a number of years and following multiple disease episodes. Immunity acquired this way is mediated by IgG with specificity for parasite-encoded, clonally variant surface antigens (VSA) on the surface of infected erythrocytes (IEs). However, women in endemic areas become susceptible to P. falciparum infection when they become pregnant, particularly for the first time, regardless of previously acquired protective immunity. This conundrum was resolved when it was observed that the selective placental accumulation of IEs that characterizes pregnancy-associated malaria (PAM) is caused by an immunologically and functionally unique subset of VSA (VSAPAM) that is only expressed by parasites infecting pregnant women, and that protective immunity to PAM is mediated by IgG with specificity for VSAPAM. In this review we summarize the research leading to the identification of the distinctly structured PfEMP1 variant VAR2CSA as the dominant PAM-type VSA and as the clinically most important target of the protective immune response to placental P. falciparum infection.

Default Pathway of var2csa switching and translational repression in Plasmodium falciparum

Antigenic variation is a subtle process of fundamental importance to the survival of a microbial pathogen. In Plasmodium falciparum malaria, PfEMP1 is the major variable antigen and adhesin expressed at the surface of the infected erythrocyte, which is encoded for by members of a family of 60 var-genes. Peri-nuclear repositioning and epigenetic mechanisms control their mono-allelic expression. The switching of PfEMP1 depends in part on variable transition rates and short-lived immune responses to shared minor epitopes. Here we show var-genes to switch to a common gene that is highly transcribed, but sparsely translated into PfEMP1 and not expressed at the erythrocyte surface. Highly clonal and adhesive P. falciparum, which expressed distinct var-genes and the corresponding PfEMP1s at onset, were propagated without enrichment or panning. The parasites successively and spontaneously switched to transcribe a shared var-gene (var2csa) matched by the loss of PfEMP1 surface expression and host cell-binding. The var2csa gene repositioned in the peri-nuclear area upon activation, away from the telomeric clusters and heterochromatin to transcribe spliced, full-length RNA. Despite abundant transcripts, the level of intracellular PfEMP1 was low suggesting post-transcriptional mechanisms to partake in protein expression. In vivo, off-switching and translational repression may constitute one pathway, among others, coordinating PfEMP1 expression.

Characterization of VAR2CSA-deficient Plasmodium falciparum-infected erythrocytes selected for adhesion to the BeWo placental cell line

Background

Malaria in pregnancy is characterized by accumulation of infected erythrocytes (IE) in the placenta. The key ligand identified as mediating this process is a Plasmodium falciparum erythrocyte membrane protein 1 family member, termed VAR2CSA. VAR2CSA appears to be the main ligand responsible for adhesion to chondroitin sulphate A (CSA). Whether other PfEMP1 molecules can also mediate placental adhesion, independent of CSA binding, is unclear.

Methods

The parasite line CS2 carrying a disrupted var2csa gene (CS2KO) was selected for adhesion to the BeWo choriocarcinoma cell line, which has been proposed as a model for placental malaria. The selected and control IE were tested for adhesion to placental sections and flow cytometry was used to measure recognition of IE by three serum sets from malaria-exposed men and women.

Results

Wild-type CS2 adhere to BeWo and placental tissue via CSA. CS2KO IE were successfully selected for adhesion to BeWo, and adhered by a CSA-independent mechanism. They bound to immobilized ICAM-1 and CD36. BeWo-selected CS2KO bound at moderate levels to placental sections, but most binding was to placental villi rather than to the syncytiotrophoblast to which IE adherence occurs in vivo. This binding was inhibited by a blocking antibody to CD36 but not to ICAM-1. As expected, sera from malaria-exposed adults recognized CS2 IE in a gender and parity dependent manner. In one serum set, there was a similar but less pronounced pattern of antibody binding to selected CS2KO IE, but this was not seen in two others. One var gene, It4var19, was particularly abundant in the selected line and was detected as full length transcripts in BeWo-selected IE, but not unselected CS2KO.

Conclusion

This study suggests that IE with characteristics similar to the CS2KO have a limited role in the pathogenesis of placental malaria. VAR2CSA appear to be the major ligand for placental adhesion, and could be the basis for a vaccine against pregnancy malaria.

Plasmodium falciparum transcriptome analysis reveals pregnancy malaria associated gene expression

BACKGROUND

Pregnancy-associated malaria (PAM) causing maternal anemia and low birth weight is among the multiple manifestations of Plasmodium falciparum malaria. Infected erythrocytes (iEs) can acquire various adhesive properties that mediate the clinical severity of malaria. Recent advances on the molecular basis of virulence and immune evasion have helped identify var2csa as a PAM-specific var gene.

METHODOLOGY/PRINCIPAL FINDINGS

The present study presents a genome-wide microarray transcript analysis of 18 P. falciparum parasite isolates freshly collected from the placenta. The proportion of PAM over-expressed genes located in subtelomeric regions as well as that of PAM over-expressed genes predicted to be exported were higher than expected compared to the whole genome. The identification of novel parasite molecules with specificity to PAM and which are likely involved in host-pathogen interactions and placental tropism is described. One of these proteins, PFI1785w, was further characterized as the product of a two-exon PHIST gene, and was more often recognized by serum samples from P. falciparum-exposed women than from men.

CONCLUSIONS/SIGNIFICANCE

These findings suggest that other parasite proteins, such as PFI1785w, may contribute beside VAR2CSA to the pathogenesis of PAM. These data may be very valuable for future vaccine development.

Var transcription profiling of Plasmodium falciparum 3D7: assignment of cytoadherent phenotypes to dominant transcripts

BACKGROUND

Cytoadherence of Plasmodium falciparum-infected red blood cells is mediated by var gene-encoded P. falciparum erythrocyte membrane protein-1 and host receptor preference depends in most cases on which of the 50-60 var genes per genome is expressed. Enrichment of phenotypically homogenous parasites by panning on receptor expressing cells is fundamental for the identification of the corresponding var transcript.

METHODS

P. falciparum 3D7 parasites were panned on several transfected CHO-cell lines and their var transcripts analysed by i) reverse transcription/PCR/cloning/sequencing using a universal DBLalpha specific oligonucleotide pair and ii) by reverse transcription followed by quantitative PCR using 57 different oligonucleotide pairs.

RESULTS

Each cytoadherence selected parasite line also adhered to untransfected CHO-745 cells and upregulation of the var gene PFD995/PFD1000c was consistently associated with cytoadherence to all but one CHO cell line. In addition, parasites panned on different CHO cell lines revealed candidate var genes which reproducibly associated to the respective cytoadherent phenotype. The transcription profile obtained by RT-PCR/cloning/sequencing differed significantly from that of RT-quantitative PCR.

CONCLUSION

Transfected CHO cell lines are of limited use for the creation of monophenotypic cytoadherent parasite lines. Nevertheless, 3D7 parasites can be reproducibly selected for the transcription of different determined var genes without genetic manipulation. Most importantly, var transcription analysis by RT-PCR/cloning/sequencing may lead to erroneous interpretation of var transcription profiles.

Normal establishment of virus-specific memory CD8 T cell pool following primary infection during pregnancy

Suppression of cell-mediated immunity has been proposed as a mechanism that promotes maternal tolerance of the fetus but also contributes to increased occurrence and severity of certain infections during pregnancy. Despite decades of research examining the effect of pregnancy on Ag-specific T cell responses, many questions remain. In particular, quantitative examination of memory CD8 T cell generation following infection during pregnancy remains largely unknown. To examine this issue, we evaluated the generation of protective immunity following infection during pregnancy with a nonpersistent strain of lymphocytic choriomeningitis virus (LCMV) in mice. The CD8 T cell response to LCMV occurred normally in pregnant mice compared with the nonpregnant cohort with rapid viral clearance in all tissues tested except for the placenta. Despite significant infiltration of CD8 T cells to the maternal-fetal interface, virus persisted in the placenta until delivery. Live pups were not infected and generated normal primary immune responses when challenged as adults. Memory CD8 T cell development in mice that were pregnant during primary infection was normal with regards to the proliferative capacity, number of Ag-specific cells, cytokine production upon re-stimulation, and the ability to protect from re-infection. These data suggest that virus-specific adaptive memory is normally generated in mice during pregnancy.

Disruption of var2csa gene impairs placental malaria associated adhesion phenotype

Infection with Plasmodium falciparum during pregnancy is one of the major causes of malaria related morbidity and mortality in newborn and mothers. The complications of pregnancy-associated malaria result mainly from massive adhesion of Plasmodium falciparum-infected erythrocytes (IE) to chondroitin sulfate A (CSA) present in the placental intervillous blood spaces. Var2CSA, a member of the P. falciparum erythrocyte membrane protein 1 (PfEMP1) family is the predominant parasite ligand mediating CSA binding. However, experimental evidence suggests that other host receptors, such as hyaluronic acid (HA) and the neonatal Fc receptor, may also support placental binding. Here we used parasites in which var2csa was genetically disrupted to evaluate the contribution of these receptors to placental sequestration and to identify additional adhesion receptors that may be involved in pregnancy-associated malaria. By comparison to the wild-type parasites, the FCR3Δvar2csa mutants could not be selected for HA adhesion, indicating that var2csa is not only essential for IE cytoadhesion to the placental receptor CSA, but also to HA. However, further studies using different pure sources of HA revealed that the previously observed binding results from CSA contamination in the bovine vitreous humor HA preparation. To identify CSA-independent placental interactions, FCR3Δvar2csa mutant parasites were selected for adhesion to the human placental trophoblastic BeWo cell line. BeWo selected parasites revealed a multi-phenotypic adhesion population expressing multiple var genes. However, these parasites did not cytoadhere specifically to the syncytiotrophoblast lining of placental cryosections and were not recognized by sera from malaria-exposed women in a parity dependent manner, indicating that the surface molecules present on the surface of the BeWo selected population are not specifically expressed during the course of pregnancy-associated malaria. Taken together, these results demonstrate that the placental malaria associated phenotype can not be restored in FCR3Δvar2csa mutant parasites and highlight the key role of var2CSA in pregnancy malaria pathogenesis and for vaccine development.

Six genes are preferentially transcribed by the circulating and sequestered forms of Plasmodium falciparum parasites that infect pregnant women

In areas of stable malaria transmission, susceptibility to Plasmodium falciparum malaria increases during first pregnancy. Women become resistant to pregnancy malaria over successive pregnancies as they acquire antibodies against the parasite forms that sequester in the placenta, suggesting that a vaccine is feasible. Placental parasites are antigenically distinct and bind receptors, like chondroitin sulfate A (CSA), that are not commonly bound by other parasites. We used whole-genome-expression analysis to find transcripts that distinguish parasites of pregnant women from other parasites and employed a novel approach to define and adjust for cell cycle timing of parasites. Transcription of six genes was substantially higher in both placental parasites and peripheral parasites from pregnant women, and each gene encodes a protein with a putative export sequence and/or transmembrane domain. This cohort of genes includes var2csa, a member of the variant PfEMP1 gene family previously implicated in pregnancy malaria, as well as five conserved genes of unknown functions. Women in East Africa acquire antibodies over successive pregnancies against a protein encoded by one of these genes, PFD1140w, and this protein shows seroreactivity similar to that of VAR2CSA domains. These findings suggest that a suite of genes may be important for the genesis of the placental binding phenotype of P. falciparum and may provide novel targets for therapeutic intervention.

Structural Basis for Binding of Plasmodium falciparum Erythrocyte Membrane Protein 1 to Chondroitin Sulfate and Placental Tissue and the Influence of Protein Polymorphisms on Binding Specificity

Chondroitin sulfate (CS) A is a key receptor for adhesion of Plasmodium falciparum-infected erythrocytes (IEs) in the placenta and can also mediate adhesion to microvascular endothelial cells. IEs that adhere to CSA express var2csa-type genes, which encode specific variants of the IE surface antigen P. falciparum erythrocyte membrane protein 1 (PfEMP1). We report direct binding of native PfEMP1, isolated from IEs and encoded by var2csa, to immobilized CSA. Binding of PfEMP1 was dependent on 4-O-sulfated disaccharides and glucuronic acid rather than iduronic acid, consistent with the specificity of intact IEs. Using immobilized CS oligosaccharides as neoglycolipid probes, the minimum chain length for direct binding of PfEMP1 was eight monosaccharide units. Similarly for IE adhesion to placental tissue there was a requirement for 4-O-sulfated GalNAc and glucuronic acid mixed with non-sulfated disaccharides; 6-O-sulfation interfered with the interaction between placental CSA and IEs. The minimum chain length for maximal inhibition of adhesion was 10 monosaccharide residues. Partially 4-O-sulfated CS oligosaccharides (45-55% sulfation) were highly effective inhibitors of placental adhesion (IC(50), 0.15 microg/ml) and may have potential for therapeutic development. We used defined P. falciparum isolates expressing different variants of var2csa in adhesion assays and found that there were isolate-specific differences in the preferred structural motifs for adhesion to CSA that correlated with polymorphisms in PfEMP1 encoded by var2csa-type genes. This may influence sites of IE sequestration or parasite virulence. These findings have significant implications for understanding the pathogenesis and biology of malaria, particularly during pregnancy, and the development of targeted interventions.

Serum lipoproteins promote efficient presentation of the malaria virulence protein PfEMP1 at the erythrocyte surface

The virulence of the malaria parasite Plasmodium falciparum is related to its ability to express a family of adhesive proteins known as P. falciparum erythrocyte membrane protein 1 (PfEMP1) at the infected red blood cell surface. The mechanism for the transport and delivery of these adhesins to the erythrocyte membrane is only poorly understood. In this work, we have used specific immune reagents in a flow cytometric assay to monitor the effects of serum components on the surface presentation of PfEMP1. We show that efficient presentation of the A4 and VAR2CSA variants of PfEMP1 is dependent on the presence of serum in the bathing medium during parasite maturation. Lipid-loaded albumin supports parasite growth but allows much less efficient presentation of PfEMP1 at the red blood cell surface. Analysis of the serum components reveals that lipoproteins, especially those of the low-density lipoprotein fraction, promote PfEMP1 presentation. Cytoadhesion of infected erythrocytes to the host cell receptors CD36 and ICAM-1 is also decreased in infected erythrocytes cultured in the absence of serum. The defect appears to be in the transfer of PfEMP1 from parasite-derived structures known as the Maurer's clefts to the erythrocyte membrane or in surface conformation rather than a down-regulation or switching of particular PfEMP1 variants.

Chondroitin sulfate proteoglycan but not hyaluronic acid is the receptor for the adherence of Plasmodium falciparum-infected erythrocytes in human placenta, and infected red blood cell adherence up-regulates the receptor expression

A low-sulfated chondroitin sulfate proteoglycan (CSPG) has been shown to be the receptor for the adherence of Plasmodium falciparum-infected red blood cells (IRBCs) in human placenta. Recently, hyaluronic acid (HA) has been suggested as an additional receptor even though IRBC binding to HA and the presence of HA at locations where IRBCs adhere in the placenta have not been established. In this study, we investigated whether HA is also a receptor for IRBC binding. IRBCs from infected placentas as well as those from different laboratory strains could bind to CSPG but not to HA. In a cell depletion assay, IRBCs from infected placentas could bind quantitatively to CSPG. Although CSPG is present both in the intervillous space and on the syncytiotrophoblast surface, HA is absent in these locations. These data conclusively demonstrate that CSPG, but not HA, is a receptor for IRBC adherence in the placenta. Our data also show, for the first time, that the IRBC-binding CSPG in the placenta is of fetal origin and that, in P. falciparum-infected placentas, the CSPG level is significantly increased, which could exacerbate IRBC adherence and placental pathogenesis. These results have important implications for the development of anti-IRBC adhesion-based vaccine for pregnancy-associated malaria.

Differential var gene expression in the organs of patients dying of falciparum malaria

Sequestration of parasitized erythrocytes in the microcirculation of tissues is thought to be important in the pathogenesis of severe falciparum malaria. A major variant surface antigen, var/Plasmodium falciparum erythrocyte membrane protein 1, expressed on the surface of the infected erythrocyte, mediates cytoadherence to vascular endothelium. To address the question of tissue-specific accumulation of variant types, we used the unique resource generated by the clinicopathological study of fatal paediatric malaria in Blantyre, Malawi, to analyse var gene transcription in patients dying with falciparum malaria. Despite up to 102 different var genes being expressed by P. falciparum populations in a single host, only one to two of these genes were expressed at high levels in the brains and hearts of these patients. These major var types differed between organs. However, identical var types were expressed in the brains of multiple patients from a single malaria season. These results provide the first evidence of organ-specific accumulation of P. falciparum variant types and suggest that parasitized erythrocytes can exhibit preferential binding in the body, supporting the hypothesis of cytoadherence-linked pathogenesis.