Variation in use of erythrocyte invasion pathways by Plasmodium falciparum mediates evasion of human inhibitory antibodies

Identification of Potential Drug Targets in Erythrocyte Invasion Pathway of Plasmodium falciparum

The erythrocyte invasion phase plays a critical role in multiplication, sexual determination, and drug resistance in Plasmodium falciparum. In order to identify the critical genes and pathways in the erythrocyte invasion phase, the gene set (GSE129949) and the RNA-Seq count data for the W2mef strain were used for further analysis. An integrative bioinformatics study was performed to scrutinize genes as potential drug targets. 487 differentially expressed genes (DEGs) with an adjusted P value < 0.001 enriched 47 Gene Ontology (GO) terms that were over-represented based on hyper-geometric analysis P value < 0.01. Protein-Protein interaction network analysis was done using DEGs with higher confidence interactions (PPI score threshold = 0.7). MCODE and cytoHubba apps were utilized to define the hub proteins and rank them based on multiple topological analyses and MCODE scores. Furthermore, Gene Set Enrichment Analysis (GSEA) was carried out by using 322 gene sets from the MPMP database. The genes involved in multiple significant gene sets were determined by leading-edge analysis. Our study identified six genes encoding proteins that could be potential drug targets involved in the erythrocyte invasion phase related to merozoites motility, cell-cycle regulation, G-dependent protein kinase phosphorylation in schizonts, control of microtubule assembly, and sexual commitment. The druggability of those proteins was calculated based on the DCI (Drug Confidence Index) and predicted binding pockets' values. The protein that showed the best binding pocket value was subjected to deep learning-based virtual screening. The study identified the best small molecule inhibitors in terms of drug-binding score against the proteins for inhibitor identification.

Autoantibodies against red blood cell antigens are common in a malaria endemic area

Plasmodium falciparum malaria can cause severe anemia. Even after treatment, hematocrit can decrease. The role of autoantibodies against erythrocytes is not clearly elucidated and how common they are, or what they are directed against, is still largely unknown. We have investigated antibodies against erythrocytes in healthy adult men living in a highly malaria endemic area in Uganda. We found antibodies in more than half of the individuals, which is significantly more than in a non-endemic area (Sweden). Some of the Ugandan samples had a broad reactivity where it was not possible to determine the exact target of the autoantibodies, but we also found specific antibodies directed against erythrocyte surface antigens known to be of importance for merozoite invasion such as glycophorin A (anti-En^a, anti-M) and glycophorin B (anti-U, anti-S). In addition, several autoantibodies had partial specificities against glycophorin C and the blood group systems Rh, Diego (located on Band 3), Duffy (located on ACKR1), and Cromer (located on CD55), all of which have been described to be important for malaria and therefore of interest for understanding how autoantibodies could potentially stop parasites from entering the erythrocyte. In conclusion, specific autoantibodies against erythrocytes are common in a malaria endemic area.

Defining species-specific and conserved interactions of apical membrane protein 1 during erythrocyte invasion in malaria to inform multi-species vaccines

Plasmodium falciparum and P. vivax are the major causes of human malaria, and P. knowlesi is an important additional cause in SE Asia. Binding of apical membrane antigen 1 (AMA1) to rhoptry neck protein 2 (RON2) was thought to be essential for merozoite invasion of erythrocytes by Plasmodium spp. Our findings reveal that P. falciparum and P. vivax have diverged and show species-specific binding of AMA1 to RON2, determined by a β-hairpin loop in RON2 and specific residues in AMA1 Loop1E. In contrast, cross-species binding of AMA1 to RON2 is retained between P. vivax and P. knowlesi. Mutation of specific amino acids in AMA1 Loop1E in P. falciparum or P. vivax ablated RON2 binding without impacting erythrocyte invasion. This indicates that the AMA1-RON2-loop interaction is not essential for invasion and additional AMA1 interactions are involved. Mutations in AMA1 that disrupt RON2 binding also enable escape of invasion inhibitory antibodies. Therefore, vaccines and therapeutics will need to be broader than targeting only the AMA1-RON2 interaction. Antibodies targeting AMA1 domain 3 had greater invasion-inhibitory activity when RON2-loop binding was ablated, suggesting this domain is a promising additional target for vaccine development. Targeting multiple AMA1 interactions involved in invasion may enable vaccines that generate more potent inhibitory antibodies and address the capacity for immune evasion. Findings on specific residues for invasion function and species divergence and conservation can inform novel vaccines and therapeutics against malaria caused by three species, including the potential for cross-species vaccines.

Acquisition of antibodies to Plasmodium falciparum and Plasmodium vivax antigens in pregnant women living in a low malaria transmission area of Brazil

BACKGROUND

Pregnant women have increased susceptibility to Plasmodium falciparum malaria and acquire protective antibodies over successive pregnancies. Most studies that investigated malaria antibody responses in pregnant women are from high transmission areas in sub-Saharan Africa, while reports from Latin America are scarce and inconsistent. The present study sought to explore the development of antibodies against P. falciparum and Plasmodium vivax antigens in pregnant women living in a low transmission area in the Brazilian Amazon.

METHODS

In a prospective cohort study, plasma samples from 408 pregnant women (of whom 111 were infected with P. falciparum, 96 had infections with P. falciparum and P. vivax, and 201 had no Plasmodium infection) were used to measure antibody levels. Levels of IgG and opsonizing antibody to pregnancy-specific variant surface antigens (VSAs) on infected erythrocytes (IEs), 10 recombinant VAR2CSA Duffy binding like (DBL domains), 10 non-pregnancy-specific P. falciparum merozoite antigens, and 10 P. vivax antigens were measured by flow cytometry, ELISA, and multiplex assays. Antibody levels and seropositivity among the groups were compared.

RESULTS

Antibodies to VSAs on P. falciparum IEs were generally low but were higher in currently infected women and women with multiple P. falciparum episodes over pregnancy. Many women (21%-69%) had antibodies against each individual VAR2CSA DBL domain, and antibodies to DBLs correlated with each other (r ≥ 0.55, p < 0.0001), but not with antibody to VSA or history of infection. Infection with either malaria species was associated with higher seropositivity rate for antibodies against P. vivax proteins, adjusted odds ratios (95% CI) ranged from 5.6 (3.2, 9.7), p < 0.0001 for PVDBPII-Sal1 to 15.7 (8.3, 29.7), p < 0.0001 for PvTRAg_2.

CONCLUSIONS

Pregnant Brazilian women had low levels of antibodies to pregnancy-specific VSAs that increased with exposure. They frequently recognized both VAR2CSA DBL domains and P. vivax antigens, but only the latter varied with infection. Apparent antibody prevalence is highly dependent on the assay platform used.

Phenotypic characterization of Ghanaian P. falciparum clinical isolates reveals a homogenous parasite population

Background

Erythrocyte invasion by P. falciparum involves functionally overlapping interactions between the parasite's ligands and the erythrocyte surface receptors. While some P. falciparum isolates necessarily engage the sialic acid (SA) moieties of the erythrocytes during the invasion, others use ligands whose binding is independent of SA for successful invasion. Deciphering the major pathway used by P. falciparum clinical isolates represent a key step toward developing an efficient blood stage malaria vaccine.

Methods

We collected a total of 156 malaria-infected samples from Ghanaian children aged 2 to 14 years and used a two-color flow cytometry-based invasion assay to assess the invasion phenotype diversity of Ghanaian P. falciparum clinical isolates. Anti-human CR1 antibodies were used to determine the relative contribution of the PfRh4-CR1 interaction in the parasites invasion phenotype and RT-qPCR was used to assess the expression levels of key invasion-related ligands.

Results

Our findings show no clear association between demographic or clinical data and existing reports on the malaria transmission intensity. The complete invasion data obtained for 156 isolates, showed the predominance of SA-independent pathways in Ghanaian clinical isolates. Isolates from Hohoe and Navrongo had the highest diversity in invasion profile. Our data also confirmed that the PfRh4-CR1 mediated alternative pathway is important in Ghanaian clinical isolates. Furthermore, the transcript levels of ten invasion-related genes obtained in the study showed little variations in gene expression profiles within and between parasite populations across sites.

Conclusion

Our data suggest a low level of phenotypic diversity in Ghanaian clinical isolates across areas of varying endemicity and further highlight its importance in the quest for new intervention strategies, such as the investigation of blood-stage vaccine targets, particularly those targeting specific pathways and able to trigger the stimulation of broadly neutralizing invasion antibodies.

A Deadly Embrace: Hemagglutination Mediated by SARS-CoV-2 Spike Protein at Its 22 N-Glycosylation Sites, Red Blood Cell Surface Sialoglycoproteins, and Antibody

Rouleaux (stacked clumps) of red blood cells (RBCs) observed in the blood of COVID-19 patients in three studies call attention to the properties of several enveloped virus strains dating back to seminal findings of the 1940s. For COVID-19, key such properties are: (1) SARS-CoV-2 binds to RBCs in vitro and also in the blood of COVID-19 patients; (2) although ACE2 is its target for viral fusion and replication, SARS-CoV-2 initially attaches to sialic acid (SA) terminal moieties on host cell membranes via glycans on its spike protein; (3) certain enveloped viruses express hemagglutinin esterase (HE), an enzyme that releases these glycan-mediated bindings to host cells, which is expressed among betacoronaviruses in the common cold strains but not the virulent strains, SARS-CoV, SARS-CoV-2 and MERS. The arrangement and chemical composition of the glycans at the 22 N-glycosylation sites of SARS-CoV-2 spike protein and those at the sialoglycoprotein coating of RBCs allow exploration of specifics as to how virally induced RBC clumping may form. The in vitro and clinical testing of these possibilities can be sharpened by the incorporation of an existing anti-COVID-19 therapeutic that has been found in silico to competitively bind to multiple glycans on SARS-CoV-2 spike protein.

Ricin B lectin-like proteins of the microsporidian Encephalitozoon cuniculi and Anncaliia algerae are involved in host-cell invasion

Microsporidia are obligate intracellular pathogens capable of infecting a wide variety of hosts ranging from invertebrates to vertebrates. The infection process requires a step of prior adherence of Microsporidia to the surface of host cells. A few studies demonstrated the involvement of proteins containing a ricin-B lectin (RBL) domain in parasite infection. In this study Anncalia algerae and Encephalitozoon cuniculi genomes were screened by bioinformatic analysis to identify proteins with an extracellular prediction and possessing RBL-type carbohydrate-binding domains, being both potentially relevant factors contributing to host cell adherence. Three proteins named AaRBLL-1 and AaRBLL-2 from A. algerae and EcRBLL-1 from E. cuniculi, were selected and comparative analysis of sequences suggested their belonging to a multigenic family, with a conserved structural RBL domain despite a significant amino acid sequence divergence. The production of recombinant proteins and antibodies against the three proteins allowed their subcellular localization on the spore wall and/or the polar tube. Adherence inhibition assays based on pre-treatments with recombinant proteins or antibodies highlighted the significant decrease of the proliferation of both E. cuniculi and A. algerae, strongly suggesting that these proteins are involved in the infection process.

Multi-functional antibody profiling for malaria vaccine development and evaluation

INTRODUCTION

A vaccine would greatly accelerate current global efforts toward malaria elimination. While a partially efficacious vaccine has been achieved for Plasmodium falciparum, a major bottleneck in developing highly efficacious vaccines is a lack of reliable correlates of protection, and the limited application of assays that quantify functional immune responses to evaluate and down-select vaccine candidates in pre-clinical studies and clinical trials.

AREAS COVERED

In this review, we describe the important role of antibodies in immunity against malaria and detail the nature and functional activities of antibodies against the malaria-causing parasite. We highlight the growing understanding of antibody effector functions against malaria and in vitro assays to measure these functional antibody responses. We discuss the application of these assays to quantify antibody functions in vaccine development and evaluation.

EXPERT OPINION

It is becoming increasingly clear that multiple antibody effector functions are involved in immunity to malaria. Therefore, we propose that evaluating vaccine candidates needs to move beyond individual assays or measuring IgG magnitude alone. Instead, vaccine evaluation should incorporate the full breadth of antibody response types and harness a wider range of assays measuring functional antibody responses. We propose a 3-tier approach to implementing assays to inform vaccine evaluation.

Strain-Dependent Inhibition of Erythrocyte Invasion by Monoclonal Antibodies Against Plasmodium falciparum CyRPA

The highly conserved Plasmodium falciparum cysteine-rich protective antigen (PfCyRPA) is a key target for next-generation vaccines against blood-stage malaria. PfCyRPA constitute the core of a ternary complex, including the reticulocyte binding-like homologous protein 5 (PfRh5) and the Rh5-interacting protein (PfRipr), and is fundamental for merozoite invasion of erythrocytes. In this study, we show that monoclonal antibodies (mAbs) specific to PfCyRPA neutralize the in vitro growth of Ghanaian field isolates as well as numerous laboratory-adapted parasite lines. We identified subsets of mAbs with neutralizing activity that bind to distinct sites on PfCyRPA and that in combination potentiate the neutralizing effect. As antibody responses against multiple merozoite invasion proteins are thought to improve the efficacy of blood-stage vaccines, we also demonstrated that combinations of PfCyRPA- and PfRh5 specific mAbs act synergistically to neutralize parasite growth. Yet, we identified prominent strain-dependent neutralization potencies, which our results suggest is independent of PfCyRPA expression level and polymorphism, demonstrating the importance of addressing functional converseness when evaluating blood-stage vaccine candidates. Finally, our results suggest that blood-stage vaccine efficacy can be improved by directing the antibody response towards defined protective epitopes on multiple parasite antigens.

Naturally Acquired Antibodies against Plasmodium falciparum: Friend or Foe?

Antibodies are central to acquired immunity against malaria. Plasmodium falciparum elicits antibody responses against many of its protein components, but there is also formation of antibodies against different parts of the red blood cells, in which the parasites spend most of their time. In the absence of a decisive intervention such as a vaccine, people living in malaria endemic regions largely depend on naturally acquired antibodies for protection. However, these antibodies do not confer sterile immunity and the mechanisms of action are still unclear. Most studies have focused on the inhibitory effect of antibodies, but here, we review both the beneficial as well as the potentially harmful roles of naturally acquired antibodies, as well as autoantibodies formed in malaria. We discuss different studies that have sought to understand acquired antibody responses against P. falciparum antigens, and potential problems when different antibodies are combined, such as in naturally acquired immunity.

Diverse Roles of TgMIC1/4/6 in the Toxoplasma Infection

Toxoplasma gondii microneme is a specialized secretory organelle that discharges its contents at the apical tip of this apicomplexan parasite in a sequential and regulated manner. Increasing number of studies on microneme proteins (MICs) have shown them as a predominant and important role in host cell attachment, invasion, motility and pathogenesis. In this review, we summarize the research advances in one of the most important MICs complexes, TgMIC1/4/6, which will contribute to improve the understanding of the molecular mechanism of T. gondii infection and provide a theoretical basis for the effective control against T. gondii.

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 towards the development of a P. vivax vaccine

INTRODUCTION

Plasmodium vivax causes significant public health problems in endemic regions. A vaccine to prevent disease is critical, considering the rapid spread of drug-resistant parasite strains, and the development of hypnozoites in the liver with potential for relapse. A minimally effective vaccine should prevent disease and transmission while an ideal vaccine provides sterile immunity.

AREAS COVERED

Despite decades of research, the complex life cycle, technical challenges and a lack of funding have hampered progress of P. vivax vaccine development. Here, we review the progress of potential P. vivax vaccine candidates from different stages of the parasite life cycle. We also highlight the challenges and important strategies for rational vaccine design. These factors can significantly increase immune effector mechanisms and improve the protective efficacy of these candidates in clinical trials to generate sustained protection over longer periods of time.

EXPERT OPINION

A vaccine that presents functionally-conserved epitopes from multiple antigens from various stages of the parasite life cycle is key to induce broadly neutralizing strain-transcending protective immunity to effectively disrupt parasite development and transmission.

Designing peptide-based vaccine candidates for Plasmodium falciparum erythrocyte binding antigen 175

Plasmodium falciparum leads to a virulent form of malaria. Progress has been achieved in understanding the mechanisms involved in the malarial infection, still there is no effective vaccine to prevent severe infection. An effective vaccine against malaria should be one which can induce immune responses against multiple epitopes in the context of predominantly occurring HLA alleles. In this study, an integrated approach was employed to identify promiscuous peptides of a well-defined sequence of erythrocyte binding antigen-175 and promiscuous peptides for HLA alleles were designed using bioinformatics tools. A peptide with 15 amino acids (ILAIAIYESRILKRK) was selected based on its high binding affinity score and synthesized. This promiscuous peptide was used as stimulating antigen in lymphoproliferative responses to evaluate the cellular immune response. It was observed this peptide evokes lymphoproliferative and cytokine responses in individuals naturally exposed to the malaria parasite. The intensity of PBMCs proliferation was observed to be higher in sera obtained from P. falciparum exposed as compared to unexposed healthy individuals, suggesting earlier recognition of peptide of this region by T cells. Furthermore, the binding mode of HLA-peptide complex and their interaction may lead to a rational and selective peptide-based vaccine candidate design approach which can be used as a malaria prophylaxis.

Non-synonymous amino acid alterations in PfEBA-175 modulate the merozoite ligand's ability to interact with host's Glycophorin A receptor

The pathological outcome of malaria due to Plasmodium falciparum infection depends largely on erythrocyte invasion by blood-stage merozoites which employ a cascade of interactions occurring between parasite ligands and RBC receptors. In a previous study exploring the genetic diversity of region-II of PfEBA-175, a ligand that plays a crucial part in parasite's RBC entry through Glycophorin A (GPA) receptor, we demonstrated that F2 domain of region-II underwent positive selection in Indian P. falciparum population through the accumulation of non-synonymous polymorphisms. Here, we examine the functional impact of two highly prevalent non-synonymous alterations in F2, namely Q584E & E592A, using a battery of molecular, biophysical and in-silico techniques. Application of circular dichroism, FTIR, fluorescence spectroscopy reveals that secondary and three-dimensional folding of recombinant-F2 protein carrying 584E and 592A residues (F2-Mut) differs significantly from that carrying 584Q and 592E (F2-3D7). A comparison of spectroscopic and thermodynamic parameters shows that F2-Mut is capable of forming a complex with GPA with higher efficiency compared to F2-3D7. In silico docking predicts both artemisinin and artesunate possess the capacity of slipping into the GPA binding crevices of PfEBA-175 and disrupt PfEBA-GPA association. However, the estimated affinity of artesunate towards PfEBA-175 with 584E and 592A residues is higher than that of artemisinin. Thermodynamic parameters computed using isotherms are concordant with this in-silico prediction. Together, our data suggest that the presence of amino acid alterations in F2 provide structural and functional stability favoring PfEBA-GPA interaction and artesunate can efficiently disrupt the interaction between GPA and PfEBA-175 even carrying altered amino acid residues. The present study alerts the malaria research community by presenting evidence that the parasite is gaining evolutionary fitness by cultivating genetic alterations in many of its proteins.

The role of complement immune response on artemisinin-based combination therapy in a population from malaria endemic region of Western Kenya

Background

Naturally acquired immunity (NAI), which is characterized by protection against overt clinical disease and high parasitaemia, is acquired with age and transmission intensity. The role of NAI on the efficacy of anti-malarial drugs, including artemisinin-based combinations used as the first-line treatment for uncomplicated Plasmodium falciparum, has not been fully demonstrated. This study investigated the role of NAI in response to artemisinin-based combination therapy (ACT), in symptomatic patients living in western Kenya, a high malaria transmission area.

Methods

Sera samples from malaria immune participants (n = 105) in a therapeutic efficacy study were assessed for in vitro growth inhibitory activity against the 3D7 strain of P. falciparum using a fluorescent-based growth inhibition assay (GIA). Participants’ age and parasite clearance parameters were used in the analysis. Pooled sera from malaria naïve participants (n = 6) with no Plasmodium infection from malaria non-endemic regions of Kenya was used as negative control.

Results

The key observations of the study were as follows: (1) Sera with intact complement displayed higher GIA activity at lower (1%) serum dilutions (p < 0.0001); (2) there was significant relationship between GIA activity, parasite clearance rate (p = 0.05) and slope half-life (p = 0.025); and (3) age was a confounding factor when comparing the GIA activity with parasite clearance kinetics.

Conclusion

This study demonstrates for the first time there is synergy of complement, pre-existing immunity, and drug treatment in younger patients with symptomatic malaria in a high-transmission area.

Investigating a Plasmodium falciparum erythrocyte invasion phenotype switch at the whole transcriptome level

The central role that erythrocyte invasion plays in Plasmodium falciparum survival and reproduction makes this process an attractive target for therapeutic or vaccine development. However, multiple invasion-related genes with complementary and overlapping functions afford the parasite the plasticity to vary ligands used for invasion, leading to phenotypic variation and immune evasion. Overcoming the challenge posed by redundant ligands requires a deeper understanding of conditions that select for variant phenotypes and the molecular mediators. While host factors including receptor heterogeneity and acquired immune responses may drive parasite phenotypic variation, we have previously shown that host-independent changes in invasion phenotype can be achieved by continuous culturing of the W2mef and Dd2 P. falciparum strains in moving suspension as opposed to static conditions. Here, we have used a highly biologically replicated whole transcriptome sequencing approach to identify the molecular signatures of variation associated with the phenotype switch. The data show increased expression of particular invasion-related genes in switched parasites, as well as a large number of genes encoding proteins that are either exported or form part of the export machinery. The genes with most markedly increased expression included members of the erythrocyte binding antigens (EBA), reticulocyte binding homologues (RH), surface associated interspersed proteins (SURFIN), exported protein family 1 (EPF1) and Plasmodium Helical Interspersed Sub-Telomeric (PHIST) gene families. The data indicate changes in expression of a repertoire of genes not previously associated with erythrocyte invasion phenotypes, suggesting the possibility that moving suspension culture may also select for other traits.

Cell trace far-red is a suitable erythrocyte dye for multi-color Plasmodium falciparum invasion phenotyping assays

Plasmodium falciparum erythrocyte invasion phenotyping assays are a very useful tool for assessing parasite diversity and virulence, and for characterizing the formation of ligand–receptor interactions. However, such assays need to be highly sensitive and reproducible, and the selection of labeling dyes for differentiating donor and acceptor erythrocytes is a critical factor. We investigated the suitability of cell trace far-red (CTFR) as a dye for P. falciparum invasion phenotyping assays. Using the dyes carboxyfluorescein diacetate succinimidyl ester (CFDA-SE) and dichloro dimethyl acridin one succinimidyl ester (DDAO-SE) as comparators, we used a dye-dilution approach to assess the limitations and specific staining procedures for the applicability of CTFR in P. falciparum invasion phenotyping assays. Our data show that CTFR effectively labels acceptor erythrocytes and provides a stable fluorescent intensity at relatively low concentrations. CTFR also yielded a higher fluorescence intensity relative to DDAO-SE and with a more stable fluorescence intensity over time. Furthermore, CTFR did not affect merozoites invasion of erythrocytes and was not toxic to the parasite’s intraerythrocytic development. Additionally, CTFR offers flexibility in the choice of combinations with several other DNA dyes, which broaden its usage for P. falciparum erythrocyte invasion assays, considering a wider range of flow cytometers with various laser settings.

Receptor Heterodimerization and Co-Receptor Engagement in TLR2 Activation Induced by MIC1 and MIC4 from Toxoplasma gondii

The microneme organelles of Toxoplasma gondii tachyzoites release protein complexes (MICs), including one composed of the transmembrane protein MIC6 plus MIC1 and MIC4. In this complex, carbohydrate recognition domains of MIC1 and MIC4 are exposed and interact with terminal sialic acid and galactose residues, respectively, of host cell glycans. Recently, we demonstrated that MIC1 and MIC4 binding to the N-glycans of Toll-like receptor (TLR) 2 and TLR4 on phagocytes triggers cell activation and pro-inflammatory cytokine production. Herein, we investigated the requirement for TLR2 heterodimerization and co-receptors in MIC-induced responses, as well as the signaling molecules involved. We used MICs to stimulate macrophages and HEK293T cells transfected with TLR2 and TLR1 or TLR6, both with or without the co-receptors CD14 and CD36. Then, the cell responses were analyzed, including nuclear factor-kappa B (NF-κB) activation and cytokine production, which showed that (1) only TLR2, among the studied factors, is crucial for MIC-induced cell activation; (2) TLR2 heterodimerization augments, but is not critical for, activation; (3) CD14 and CD36 enhance the response to MIC stimulus; and (4) MICs activate cells through a transforming growth factor beta-activated kinase 1 (TAK1)-, mammalian p38 mitogen-activated protein kinase (p38)-, and NF-κB-dependent pathway. Remarkably, among the studied factors, the interaction of MIC1 and MIC4 with TLR2 N-glycans is sufficient to induce cell activation, which promotes host protection against T. gondii infection.

Temporal changes in Plasmodium falciparum reticulocyte binding protein homolog 2b (PfRh2b) in Senegal and The Gambia

BACKGROUND

The Plasmodium falciparum reticulocyte binding protein homolog 2b (PfRh2b) is an important P. falciparum merozoite ligand that mediates invasion of erythrocytes by interacting with a chymotrypsin-sensitive "receptor Z". A large deletion polymorphism is found in the c-terminal ectodomain of this protein in many countries around the world, resulting in a truncated, but expressed protein. The varying frequencies by region suggest that there could be region specific immune selection at this locus. Therefore, this study was designed to determine temporal changes in the PfRh2b deletion polymorphism in infected individuals from Thiès (Senegal) and Western Gambia (The Gambia). It was also sought to determine the selective pressures acting at this locus and whether prevalence of the deletion in isolates genotyped by a 24-SNP molecular barcode is linked to background genotype or whether there might be independent selection acting at this locus.

METHODS

Infected blood samples were sourced from archives of previous studies conducted between 2007 and 2013 at SLAP clinic in Thiès and from 1984 to 2013 in Western Gambia by MRC Unit at LSHTM, The Gambia. A total of 1380 samples were screened for the dimorphic alleles of the PfRh2b using semi-nested Polymerase Chain Reaction PCR. Samples from Thiès were previously barcoded.

RESULTS

In Thiès, a consistent trend of decreasing prevalence of the PfRh2b deletion over time was observed: from 66.54% in 2007 and to 38.1% in 2013. In contrast, in Western Gambia, the frequency of the deletion fluctuated over time; it increased between 1984 and 2005 from (58.04%) to (69.33%) and decreased to 47.47% in 2007. Between 2007 and 2012, the prevalence of this deletion increased significantly from 47.47 to 83.02% and finally declined significantly to 57.94% in 2013. Association between the presence of this deletion and age was found in Thiès, however, not in Western Gambia. For the majority of isolates, the PfRh2b alleles could be tracked with specific 24-SNP barcoded genotype, indicating a lack of independent selection at this locus.

CONCLUSION

PfRh2b deletion was found in the two countries with varying prevalence during the study period. However, these temporal and spatial variations could be an obstacle to the implementation of this protein as a potential vaccine candidate.

The lectin-specific activity of Toxoplasma gondii microneme proteins 1 and 4 binds Toll-like receptor 2 and 4 N-glycans to regulate innate immune priming

Infection of host cells by Toxoplasma gondii is an active process, which is regulated by secretion of microneme (MICs) and rhoptry proteins (ROPs and RONs) from specialized organelles in the apical pole of the parasite. MIC1, MIC4 and MIC6 assemble into an adhesin complex secreted on the parasite surface that functions to promote infection competency. MIC1 and MIC4 are known to bind terminal sialic acid residues and galactose residues, respectively and to induce IL-12 production from splenocytes. Here we show that rMIC1- and rMIC4-stimulated dendritic cells and macrophages produce proinflammatory cytokines, and they do so by engaging TLR2 and TLR4. This process depends on sugar recognition, since point mutations in the carbohydrate-recognition domains (CRD) of rMIC1 and rMIC4 inhibit innate immune cells activation. HEK cells transfected with TLR2 glycomutants were selectively unresponsive to MICs. Following in vitro infection, parasites lacking MIC1 or MIC4, as well as expressing MIC proteins with point mutations in their CRD, failed to induce wild-type (WT) levels of IL-12 secretion by innate immune cells. However, only MIC1 was shown to impact systemic levels of IL-12 and IFN-γ in vivo. Together, our data show that MIC1 and MIC4 interact physically with TLR2 and TLR4 N-glycans to trigger IL-12 responses, and MIC1 is playing a significant role in vivo by altering T. gondii infection competency and murine pathogenesis.

Toxoplasmosis is caused by the protozoan Toxoplasma gondii, belonging to the Apicomplexa phylum. This phylum comprises important parasites able to infect a broad diversity of animals, including humans. A particularity of T. gondii is its ability to invade virtually any nucleated cell of all warm-blooded animals through an active process, which depends on the secretion of adhesin proteins. These proteins are discharged by specialized organelles localized in the parasite apical region, and termed micronemes and rhoptries. We show in this study that two microneme proteins from T. gondii utilize their adhesion activity to stimulate innate immunity. These microneme proteins, denoted MIC1 and MIC4, recognize specific sugars on receptors expressed on the surface of mammalian immune cells. This binding activates these innate immune cells to secrete cytokines, which promotes efficient host defense mechanisms against the parasite and regulate their pathogenesis. This activity promotes a chronic infection by controlling parasite replication during acute infection.

Antibody Reactivity to Merozoite Antigens in Ghanaian Adults Correlates With Growth Inhibitory Activity Against Plasmodium falciparum in Culture

Background

Plasmodium falciparum uses a repertoire of merozoite-stage proteins for invasion of erythrocytes. Antibodies against some of these proteins halt the replication cycle of the parasite by preventing erythrocyte invasion and are implicated as contributors to protective immunity against malaria.

Methods

We assayed antibody reactivity against a panel of 9 recombinant antigens based on erythrocyte-binding antigen (EBA) and reticulocyte-like homolog (Rh) proteins in plasma from children with malaria and healthy adults residing in 3 endemic areas in Ghana using enzyme-linked immunosorbent assay. Purified immunoglobulin (Ig)G from adult plasma samples was also tested for invasion inhibition against 7 different P falciparum culture lines, including clinical isolates.

Results

Antibodies against the antigens increased in an age-dependent manner in children. Breadth of reactivity to the different antigens was strongly associated with in vitro parasite growth inhibitory activity of IgG purified from the adults. The strongest predictors of breadth of antibody reactivity were age and transmission intensity, and a combination of reactivities to Rh2, Rh4, and Rh5 correlated strongly with invasion inhibition.

Conclusions

Growth inhibitory activity was significantly associated with breadth of antibody reactivity to merozoite antigens, encouraging the prospect of a multicomponent blood-stage vaccine.

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.

Parasite-Related Genetic and Epigenetic Aspects and Host Factors Influencing Plasmodium falciparum Invasion of Erythrocytes

Malaria, a disease caused by Plasmodium parasites, is widespread throughout tropical and sub-tropical regions worldwide; it mostly affects children and pregnant woman. Eradication has stalled despite effective prevention measures and medication being available for this disease; this has mainly been due to the parasite's resistance to medical treatment and the mosquito vector's resistance to insecticides. Tackling such resistance involves using renewed approaches and techniques for accruing a deep understanding of the parasite's biology, and developing new drugs and vaccines. Studying the parasite's invasion of erythrocytes should shed light on its ability to switch between invasion phenotypes related to the expression of gene sets encoding proteins acting as ligands during target cell invasion, thereby conferring mechanisms for evading a particular host's immune response and adapting to changes in target cell surface receptors. This review considers some factors influencing the expression of such phenotypes, such as Plasmodium's genetic, transcriptional and epigenetic characteristics, and explores some host-related aspects which could affect parasite phenotypes, aiming at integrating knowledge regarding this topic and the possible relationship between the parasite's biology and host factors playing a role in erythrocyte invasion.

The impact of early life exposure to Plasmodium falciparum on the development of naturally acquired immunity to malaria in young Malawian children

Background

Antibodies targeting malaria blood-stage antigens are important targets of naturally acquired immunity, and may act as valuable biomarkers of malaria exposure.

Methods

Six-hundred and one young Malawian children from a randomized trial of prenatal nutrient supplementation with iron and folic acid or pre- and postnatal multiple micronutrients or lipid-based nutrient supplements were followed up weekly at home and febrile episodes were investigated for malaria from birth to 18 months of age. Antibodies were measured for 601 children against merozoite surface proteins (MSP1 19kD, MSP2), erythrocyte binding antigen 175 (EBA175), reticulocyte binding protein homologue 2 (Rh2A9), schizont extract and variant surface antigens expressed by Plasmodium falciparum-infected erythrocytes (IE) at 18 months of age. The antibody measurement data was related to concurrent malaria infection and to documented episodes of clinical malaria.

Results

At 18 months of age, antibodies were significantly higher among parasitaemic than aparasitaemic children. Antibody levels against MSP1 19kD, MSP2, schizont extract, and IE variant surface antigens were significantly higher in children who had documented episodes of malaria than in children who did not. Antibody levels did not differ between children with single or multiple malaria episodes before 18 months, nor between children who had malaria before 6 months of age or between 6 and 18 months.

Conclusions

Antibodies to merozoite and IE surface antigens increased following infection in early childhood, but neither age at first infection nor number of malaria episodes substantially affected antibody acquisition. These findings have implications for malaria surveillance during early childhood in the context of elimination.

Trials registration Clinical Trials Registration: NCT01239693 (Date of registration: 11-10-2010). URL: http://www.ilins.org

KILchip v1.0: A Novel Plasmodium falciparum Merozoite Protein Microarray to Facilitate Malaria Vaccine Candidate Prioritization

Passive transfer studies in humans clearly demonstrated the protective role of IgG antibodies against malaria. Identifying the precise parasite antigens that mediate immunity is essential for vaccine design, but has proved difficult. Completion of the Plasmodium falciparum genome revealed thousands of potential vaccine candidates, but a significant bottleneck remains in their validation and prioritization for further evaluation in clinical trials. Focusing initially on the Plasmodium falciparum merozoite proteome, we used peer-reviewed publications, multiple proteomic and bioinformatic approaches, to select and prioritize potential immune targets. We expressed 109 P. falciparum recombinant proteins, the majority of which were obtained using a mammalian expression system that has been shown to produce biologically functional extracellular proteins, and used them to create KILchip v1.0: a novel protein microarray to facilitate high-throughput multiplexed antibody detection from individual samples.

The microarray assay was highly specific; antibodies against P. falciparum proteins were detected exclusively in sera from malaria-exposed but not malaria-naïve individuals. The intensity of antibody reactivity varied as expected from strong to weak across well-studied antigens such as AMA1 and RH5 (Kruskal–Wallis H test for trend: p < 0.0001). The inter-assay and intra-assay variability was minimal, with reproducible results obtained in re-assays using the same chip over a duration of 3 months. Antibodies quantified using the multiplexed format in KILchip v1.0 were highly correlated with those measured in the gold-standard monoplex ELISA [median (range) Spearman's R of 0.84 (0.65–0.95)]. KILchip v1.0 is a robust, scalable and adaptable protein microarray that has broad applicability to studies of naturally acquired immunity against malaria by providing a standardized tool for the detection of antibody correlates of protection. It will facilitate rapid high-throughput validation and prioritization of potential Plasmodium falciparum merozoite-stage antigens paving the way for urgently needed clinical trials for the next generation of malaria vaccines.

Plasmodium vivax Merozoite Surface Protein 1 Paralog as a Mediator of Parasite Adherence to Reticulocytes

Plasmodium vivax parasites preferentially invade reticulocytes in human beings. P. vivax merozoite surface protein 1 (PvMSP1) and PvMSP1 paralog (PvMSP1P) may have important functions in reticulocyte adherence during invasion. These proteins share similar structures, including the presence of two epidermal growth factor (EGF)-like and glycosylphosphatidylinositol (GPI)-anchored domains at the C terminus. However, there have been no reports concerning the functional activity of PvMSP1P in reticulocyte adherence during P. vivax invasion. In this study, the ability of PvMSP1P-19 to bind to reticulocytes and normocytes was analyzed. The reticulocyte binding activity of PvMSP1P-19 was 4.0-fold higher than its normocyte binding activity. The binding of PvMSP1P-19 to reticulocytes and normocytes was inhibited in a dose-dependent manner by antibodies from immunized rabbits and by antibodies from vivax parasite-infected patients. Consistently, antibodies against PvMSP1P inhibited parasite invasion during short-term in vitro cultivation. Similar to the case for PvDBPII binding activity, PvMSP1P-19 binding activity was reduced in chymotrypsin-treated reticulocytes. However, no significant difference between the binding of PvMSP1P-19 to Duffy-positive and Duffy-negative erythrocytes was found. The minimal binding motif of PvMSP1P-19 was characterized using synthetic peptides. The results showed that the residues at amino acid positions 1791 to 1808 may have an important function in mediating merozoite adherence to reticulocytes. The positively charged residues within the EGF-like domain were shown to constitute a key binding motif. This work presents strong evidence supporting the role of PvMSP1P in host target cell selection and invasion of Duffy-independent pathway in P. vivax Moreover, PvMSP1P-19-specific antibodies may confer protection against P. vivax reinvasion.

Epigenetic Variation and Regulation in Malaria Parasites

Eukaryotic pathogens must survive in different hosts, respond to changing environments, and exploit specialized niches to propagate. Plasmodium parasites cause human malaria during bloodstream infections, where they must persist long enough to be transmitted. Parasites have evolved diverse strategies of variant gene expression that control critical biological processes of blood-stage infections, including antigenic variation, erythrocyte invasion, innate immune evasion, and nutrient acquisition, as well as life-cycle transitions. Epigenetic mechanisms within the parasite are being elucidated, with discovery of epigenomic marks associated with gene silencing and activation, and the identification of epigenetic regulators and chromatin proteins that are required for the switching and maintenance of gene expression. Here, we review the key epigenetic processes that facilitate transition through the parasite life cycle and epigenetic regulatory mechanisms utilized by Plasmodium parasites to survive changing environments and consider epigenetic switching in the context of the outcome of human infections.

Factors influencing the induction of high affinity antibodies to Plasmodium falciparum merozoite antigens and how affinity changes over time

Understanding the functional characteristics of naturally acquired antibodies against P. falciparum merozoite antigens is crucial for determining the protective functions of antibodies. Affinity (measured as k_d) of naturally acquired antibodies against two key targets of acquired immunity, EBA175 and PfRh2, was determined using Surface Plasmon Resonance (SPR) in a longitudinal survey in Nigeria. A majority of the participants, 79% and 67%, maintained stable antibody affinities to EBA175 and PfRh2, respectively, over time. In about 10% of the individuals, there was a reciprocal interaction with a reduction over time in antibody affinity for PfRh2 and an increase for EBA175. In general, PfRh2 elicited antibodies with higher affinity compared to EBA175. Individuals with higher exposure to malaria produced antibodies with higher affinity to both antigens. Younger individuals (5–15 years) produced comparable or higher affinity antibodies than adults (>15 years) against EBA175, but not for PfRh2. Correlation between total IgG (ELISA) and affinity varied between individuals, but PfRh2 elicited antibodies with a higher correlation in a majority of the participants. There was also a correlation between antibody inhibition of erythrocyte invasion by merozoites and PfRh2 affinity. This work gives new insights into the generation and maintenance of antibody affinity over time.

Plasmodium falciparum strains spontaneously switch invasion phenotype in suspension culture

The extensive redundancy in the use of invasion ligands by Plasmodium falciparum, and its unique ability to switch between invasion pathways have hampered vaccine development. P. falciparum strains Dd2 and W2mef have been shown to change from sialic acid (SA)-dependent to SA-independent phenotypes when selected on neuraminidase-treated erythrocytes. Following an observation of increasing ability of Dd2 to invade neuraminidase-treated cells when cultured for several weeks, we systematically investigated this phenomenon by comparing invasion phenotypes of Dd2, W2mef and 3D7 strains of P. falciparum that were cultured with gentle shaking (Suspended) or under static (Static) conditions. While Static Dd2 and W2mef remained SA-dependent for the entire duration of the investigation, Suspended parasites spontaneously and progressively switched to SA-independent phenotype from week 2 onwards. Furthermore, returning Suspended cultures to Static conditions led to a gradual reversal to SA-dependent phenotype. The switch to SA-independent phenotype was accompanied by upregulation of the key invasion ligand, reticulocyte-binding homologue 4 (RH4), and the increased invasion was inhibited by antibodies to the RH4 receptor, CR1. Our data demonstrates a novel mechanism for inducing the switching of invasion pathways in P. falciparum parasites and may provide clues for understanding the mechanisms involved.

Effect of nutrient supplementation on the acquisition of humoral immunity to Plasmodium falciparum in young Malawian children

Background

There is evidence that suggests that undernutrition has a detrimental effect on malarial immunity in children. The aim of the study was to discover whether nutrient supplementation improved development of malarial antibody immunity in children up to 18 months of age.

Methods

The study was conducted with a subset of 432 Malawian children from a randomized controlled trial of nutritional supplements. The arms included pre- and postnatal small-quantity lipid-based nutrient supplements for both mother and child; prenatal supplementation with iron and folic acid; and pre- and postnatal supplementation with multiple micronutrients. Paired plasma samples were collected at 6 and 18 months of age. The levels of antibodies against merozoite surface protein 1 (MSP1 19kD) and MSP2, erythrocyte binding antigen 175 (EBA175), reticulocyte binding protein homologue 2A (Rh2A9), schizont extract and variant antigens expressed on the surface of infected erythrocytes were measured.

Results

At 18 months of age, 5.4% of children were parasitaemic by microscopy and 49.1% were anaemic. Antibodies to the tested merozoite antigens and schizont extract increased between 6 and 18 months and this increase was statistically significant for MSP1, MSP2 and EBA175 (p < 0.0001) whereas IgG to variant surface antigens decreased with increasing age (p < 0.0001). However, the supplementation type did not have any impact on the prevalence or levels of antibodies at either 6 or 18 months of age to any of the tested malaria antigens in either univariate analysis or multivariate analysis after adjusting for covariates.

Conclusions

Pre- and postnatal lipid-based nutrient supplementation did not alter malaria antibody acquisition during infancy, compared to prenatal supplementation with iron and folic acid or pre- and postnatal supplementation with multiple micronutrients.

Trail registeration Clinicaltrials.gov registration number NCT01239693

Declining Malaria Transmission Differentially Impacts the Maintenance of Humoral Immunity to Plasmodium falciparum in Children

Background

We investigated the poorly understood impact of declining malaria transmission on maintenance of antibodies to Plasmodium falciparum merozoite antigens and infected erythrocytes (IEs), including functional immunity.

Methods

In a 3-year longitudinal cohort of 300 Kenyan children, antibodies to different AMA1 and MSP2 alleles of merozoites, IE surface antigens, and antibody functional activities were quantified.

Results

Over a period in which malaria transmission declined markedly, AMA1 and MSP2 antibodies decreased substantially; estimated half-lives of antibody duration were 0.8 year and 1-3 years, respectively. However, 69%-74% of children maintained their seropositivity to AMA1 alleles and 42%-52% to MSP2 alleles. Levels and prevalence of antimerozoite antibodies were consistently associated with increasing age and concurrent parasitemia. Antibodies promoting opsonic phagocytosis of merozoites declined rapidly (half-life, 0.15 years). In contrast, complement-fixing antibodies to merozoites did not decline and antibodies to IE surface antigens expressing virulent phenotypes were much better maintained (half-life, 4-10 years).

Conclusions

A decline in malaria transmission is associated with reduction in naturally acquired immunity. However, loss of immunity is not universal; some key functional responses and antibodies to IEs were better maintained and these may continue to provide some protection. Findings have implications for malaria surveillance and control measures and informing vaccine development.

Genetic structure of two erythrocyte binding antigens of Plasmodium falciparum reveals a contrasting pattern of selection

Erythrocyte binding antigens 175 (EBA-175) and 140 (EBA-140) play key roles in erythrocyte invasion by binding to glycophorin A (GPA) and C (GPC) respectively in human malaria. Since antigenic variation in malaria endemic region is a major barrier to development of effective vaccine, we explore the nature and pattern of sequence diversity of these two vaccine candidates in Kolkata, India. Population genetic parameters based on parasite sequences representing region II of Pfeba-175 and Pfeba-140 genes were estimated using DnaSP V.5.10 and MEGA version 6.0. A novel molecular docking approach was implemented to assess the binding affinities of Kolkata Pfeba-175 variants with GPA. P. falciparum Kolkata isolates experienced a recent population expansion as documented by negative Tajima's D, Fu & Li's statistics, unimodal mismatch distribution and star-like median-joining network for both loci. Positive selection seemed to play a major role in shaping the diversity of Pfeba-175 (d_N/d_S=2.45, and McDonald-Kreitman P-value=0.04) with successive accumulation of Q584K/E, E592A and R664S deriving high frequency haplotypes designated here as F2KH3 and F2KH1. In silico molecular docking demonstrated that polypeptides encoded by F2KH1 and F2KH3 were capable of engaging the parasite ligand into energetically favorable interaction with GPA. Our data demonstrated emergence of Pfeba-175 sequences harboring selectively advantageous nonsynonymous substitutions on Pf3D7 sequence background in the Kolkata parasite isolates. A contrasting pattern of Pf3D7-centric expansion of parasite sequences was noted for Pfeba-140. Together, this study provides a firm genetic and biological support favoring a dominant role of EBA-175 in erythrocyte invasion.

Host Cell Tropism and Adaptation of Blood-Stage Malaria Parasites: Challenges for Malaria Elimination

Plasmodium falciparum and Plasmodium vivax account for most of the mortality and morbidity associated with malaria in humans. Research and control efforts have focused on infections caused by P. falciparum and P. vivax, but have neglected other malaria parasite species that infect humans. Additionally, many related malaria parasite species infect nonhuman primates (NHPs), and have the potential for transmission to humans. For malaria elimination, the varied and specific challenges of all of these Plasmodium species will need to be considered. Recent advances in molecular genetics and genomics have increased our knowledge of the prevalence and existing diversity of the human and NHP Plasmodium species. We are beginning to identify the extent of the reservoirs of each parasite species in humans and NHPs, revealing their origins as well as potential for adaptation in humans. Here, we focus on the red blood cell stage of human infection and the host cell tropism of each human Plasmodium species. Determinants of tropism are unique among malaria parasite species, presenting a complex challenge for malaria elimination.

The use of a P. falciparum specific coiled-coil domain to construct a self-assembling protein nanoparticle vaccine to prevent malaria

Background

The parasitic disease malaria remains a major global public health concern and no truly effective vaccine exists. One approach to the development of a malaria vaccine is to target the asexual blood stage that results in clinical symptoms. Most attempts have failed. New antigens such as P27A and P27 have emerged as potential new vaccine candidates. Multiple studies have demonstrated that antigens are more immunogenic and are better correlated with protection when presented on particulate delivery systems. One such particulate delivery system is the self-assembling protein nanoparticle (SAPN) that relies on coiled-coil domains of proteins to form stable nanoparticles. In the past we have used de novo designed amino acid domains to drive the formation of the coiled-coil scaffolds which present the antigenic epitopes on the particle surface.

Results

Here we use naturally occurring domains found in the tex1 protein to form the coiled-coil scaffolding of the nanoparticle. Thus, by engineering P27A and a new extended form of the coiled-coil domain P27 onto the N and C terminus of the SAPN protein monomer we have developed a particulate delivery system that effectively displays both antigens on a single particle that uses malaria tex1 sequences to form the nanoparticle scaffold. These particles are immunogenic in a murine model and induce immune responses similar to the ones observed in seropositive individuals in malaria endemic regions.

Conclusions

We demonstrate that our P27/P27A-SAPNs induce an immune response akin to the one in seropositive individuals in Burkina Faso. Since P27 is highly conserved among different Plasmodium species, these novel SAPNs may even provide cross-protection between Plasmodium falciparum and Plasmodium vivax the two major human malaria pathogens. As the SAPNs are also easy to manufacture and store they can be delivered to the population in need without complication thus providing a low cost malaria vaccine.

Acquisition, maintenance and adaptation of invasion inhibitory antibodies against Plasmodium falciparum invasion ligands involved in immune evasion

Erythrocyte-binding antigens (EBAs) and P. falciparum reticulocyte-binding homologue proteins (PfRhs) are two important protein families that can vary in expression and utilization by P. falciparum to evade inhibitory antibodies. We evaluated antibodies at repeated time-points among individuals living in an endemic region in Nigeria over almost one year against these vaccine candidates. Antibody levels against EBA140, EBA175, EBA181, PfRh2, PfRh4, and MSP2, were measured by ELISA. We also used parasites with disrupted EBA140, EBA175 and EBA181 genes to show that all these were targets of invasion inhibitory antibodies. However, antigenic targets of inhibitory antibodies were not stable and changed substantially over time in most individuals, independent of age. Antibodies levels measured by ELISA also varied within and between individuals over time and the antibodies against EBA181, PfRh2 and MSP2 declined more rapidly in younger individuals (≤15 years) compared with older (>15). The breadth of high antibody responses over time was more influenced by age than by the frequency of infection. High antibody levels were associated with a more stable invasion inhibitory response, which could indicate that during the long process of formation of immunity, many changes not only in levels but also in functional responses are needed. This is an important finding in understanding natural immunity against malaria, which is essential for making an efficacious vaccine.

Functional Analysis Reveals Geographical Variation in Inhibitory Immune Responses Against a Polymorphic Malaria Antigen

Background

Plasmodium falciparum reticulocyte-binding protein homologue 2b (PfRh2b) is an invasion ligand that is a potential blood-stage vaccine candidate antigen; however, a naturally occurring deletion within an immunogenic domain is present at high frequencies in Africa and has been associated with alternative invasion pathway usage. Standardized tools that provide antigenic specificity in in vitro assays are needed to functionally assess the neutralizing potential of humoral responses against malaria vaccine candidate antigens.

Methods

Transgenic parasite lines were generated to express the PfRh2b deletion. Total immunoglobulin G (IgG) from individuals residing in malaria-endemic regions in Tanzania, Senegal, and Mali were used in growth inhibition assays with transgenic parasite lines.

Results

While the PfRh2b deletion transgenic line showed no change in invasion pathway utilization compared to the wild-type in the absence of specific antibodies, it outgrew wild-type controls in competitive growth experiments. Inhibition differences with total IgG were observed in the different endemic sites, ranging from allele-specific inhibition to allele-independent inhibitory immune responses.

Conclusions

The PfRh2b deletion may allow the parasite to escape neutralizing antibody responses in some regions. This difference in geographical inhibition was revealed using transgenic methodologies, which provide valuable tools for functionally assessing neutralizing antibodies against vaccine-candidate antigens in regions with varying malaria endemicity.

Host age and expression of genes involved in red blood cell invasion in Plasmodium falciparum field isolates

Plasmodium falciparum proteins involved in erythrocyte invasion are main targets of acquired immunity and important vaccine candidates. We hypothesized that anti-parasite immunity acquired upon exposure would limit invasion-related gene (IRG) expression and affect the clinical impact of the infection. 11 IRG transcript levels were measured in P. falciparum isolates by RT-PCR, and IgG/IgM against invasion ligands by Luminex®, in 50 Mozambican adults, 25 children with severe malaria (SM) and 25 with uncomplicated malaria (UM). IRG expression differences among groups and associations between IRG expression and clinical/immunologic parameters were assessed. IRG expression diversity was higher in parasites infecting children than adults (p = 0.022). eba140 and ptramp expression decreased with age (p = 0.003 and 0.007, respectively) whereas p41 expression increased (p = 0.022). pfrh5 reduction in expression was abrupt early in life. Parasite density decreased with increasing pfrh5 expression (p < 0.001) and, only in children, parasite density increased with p41 expression (p = 0.007), and decreased with eba175 (p = 0.013). Antibody responses and IRG expression were not associated. In conclusion, IRG expression is associated with age and parasite density, but not with specific antibody responses in the acute phase of infection. Our results confirm the importance of multi-antigen vaccines development to avoid parasite immune escape when tested in malaria-exposed individuals.

Differing rates of antibody acquisition to merozoite antigens in malaria: implications for immunity and surveillance

Antibodies play a key role in acquired human immunity to Plasmodium falciparum (Pf) malaria and target merozoites to reduce or prevent blood-stage replication and the development of disease. Merozoites present a complex array of antigens to the immune system, and currently, there is only a partial understanding of the targets of protective antibodies and how responses to different antigens are acquired and boosted. We hypothesized that there would be differences in the rate of acquisition of antibodies to different antigens and how well they are boosted by infection, which impacts the acquisition of immunity. We examined responses to a range of merozoite antigens in 2 different cohorts of children and adults with different age structures and levels of malaria exposure. Overall, antibodies were associated with age, exposure, and active infection, and the repertoire of responses increased with age and active infection. However, rates of antibody acquisition varied between antigens and different regions within an antigen following exposure to malaria, supporting our hypothesis. Antigen-specific responses could be broadly classified into early response types in which antibodies were acquired early in childhood exposure and late response types that appear to require substantially more exposure for the development of substantial levels. We identified antigen-specific responses that were effectively boosted after recent infection, whereas other responses were not. These findings advance our understanding of the acquisition of human immunity to malaria and are relevant to the development of malaria vaccines targeting merozoite antigens and the selection of antigens for use in malaria surveillance.

Biological roles of glycans

Simple and complex carbohydrates (glycans) have long been known to play major metabolic, structural and physical roles in biological systems. Targeted microbial binding to host glycans has also been studied for decades. But such biological roles can only explain some of the remarkable complexity and organismal diversity of glycans in nature. Reviewing the subject about two decades ago, one could find very few clear-cut instances of glycan-recognition-specific biological roles of glycans that were of intrinsic value to the organism expressing them. In striking contrast there is now a profusion of examples, such that this updated review cannot be comprehensive. Instead, a historical overview is presented, broad principles outlined and a few examples cited, representing diverse types of roles, mediated by various glycan classes, in different evolutionary lineages. What remains unchanged is the fact that while all theories regarding biological roles of glycans are supported by compelling evidence, exceptions to each can be found. In retrospect, this is not surprising. Complex and diverse glycans appear to be ubiquitous to all cells in nature, and essential to all life forms. Thus, >3 billion years of evolution consistently generated organisms that use these molecules for many key biological roles, even while sometimes coopting them for minor functions. In this respect, glycans are no different from other major macromolecular building blocks of life (nucleic acids, proteins and lipids), simply more rapidly evolving and complex. It is time for the diverse functional roles of glycans to be fully incorporated into the mainstream of biological sciences.

Plasmodium vivax vaccine research - we've only just begun

Plasmodium vivax parasites cause the majority of malaria cases outside Africa, and are increasingly being acknowledged as a cause of severe disease. The unique attributes of P. vivax biology, particularly the capacity of the dormant liver stage, the hypnozoite, to maintain blood-stage infections even in the absence of active transmission, make blood-stage vaccines particularly attractive for this species. However, P. vivax vaccine development remains resolutely in first gear, with only a single blood-stage candidate having been evaluated in any depth. Experience with Plasmodium falciparum suggests that a much broader search for new candidates and a deeper understanding of high priority targets will be required to make significant advances. This review discusses some of the particular challenges of P. vivax blood-stage vaccine development, highlighting both recent advances and key remaining barriers to overcome in order to move development forward.

Malaria Parasites: The Great Escape

Parasites of the genus Plasmodium have a complex life cycle. They alternate between their final mosquito host and their intermediate hosts. The parasite can be either extra- or intracellular, depending on the stage of development. By modifying their shape, motility, and metabolic requirements, the parasite adapts to the different environments in their different hosts. The parasite has evolved to escape the multiple immune mechanisms in the host that try to block parasite development at the different stages of their development. In this article, we describe the mechanisms reported thus far that allow the Plasmodium parasite to evade innate and adaptive immune responses.

A novel approach to identifying patterns of human invasion-inhibitory antibodies guides the design of malaria vaccines incorporating polymorphic antigens

BACKGROUND

The polymorphic nature of many malaria vaccine candidates presents major challenges to achieving highly efficacious vaccines. Presently, there is very little knowledge on the prevalence and patterns of functional immune responses to polymorphic vaccine candidates in populations to guide vaccine design. A leading polymorphic vaccine candidate against blood-stage Plasmodium falciparum is apical membrane antigen 1 (AMA1), which is essential for erythrocyte invasion. The importance of AMA1 as a target of acquired human inhibitory antibodies, their allele specificity and prevalence in populations is unknown, but crucial for vaccine design.

METHODS

P. falciparum lines expressing different AMA1 alleles were genetically engineered and used to quantify functional antibodies from two malaria-exposed populations of adults and children. The acquisition of AMA1 antibodies was also detected using enzyme-linked immunosorbent assay (ELISA) and competition ELISA (using different AMA1 alleles) from the same populations.

RESULTS

We found that AMA1 was a major target of naturally acquired invasion-inhibitory antibodies that were highly prevalent in malaria-endemic populations and showed a high degree of allele specificity. Significantly, the prevalence of inhibitory antibodies to different alleles varied substantially within populations and between geographic locations. Inhibitory antibodies to three specific alleles were highly prevalent (FVO and W2mef in Papua New Guinea; FVO and XIE in Kenya), identifying them for potential vaccine inclusion. Measurement of antibodies by standard or competition ELISA was not strongly predictive of allele-specific inhibitory antibodies. The patterns of allele-specific functional antibody responses detected with our novel assays may indicate that acquired immunity is elicited towards serotypes that are prevalent in each geographic location.

CONCLUSIONS

These findings provide new insights into the nature and acquisition of functional immunity to a polymorphic vaccine candidate and strategies to quantify functional immunity in populations to guide rational vaccine design.

Functional Antibodies and Protection against Blood-stage Malaria

Numerous efforts to understand the functional roles of antibodies demonstrated that they can protect against malaria. However, it is unclear which antibody responses are the best correlates of immunity, and which antibody functions are most important in protection from disease. Understanding the role of antibodies in protection against malaria is crucial for antimalarial vaccine design. In this review, the specific functional properties of naturally acquired and vaccine-induced antibodies that correlate to protection from the blood stages of Plasmodium falciparum malaria are re-examined and the gaps in knowledge related to antibody function in malarial immunity are highlighted.

Recent insights into humoral immunity targeting Plasmodium falciparum and Plasmodium vivax malaria

Recent efforts in malaria control have led to marked reductions in malaria incidence. However, new strategies are needed to sustain malaria elimination and eradication and achieve the World Health Organization goal of a malaria-free world. The development of highly effective vaccines would contribute to this goal and would be facilitated by a comprehensive understanding of humoral immune responses targeting Plasmodium falciparum and Plasmodium vivax malaria. New tools are required to facilitate the identification of vaccine candidates and the development of vaccines that induce functional and protective immunity. Here we discuss recent published findings, and unpublished work presented at the 2016 Molecular Approaches to Malaria conference, that highlight advancements in understanding humoral immune responses in the context of vaccine development. Highlights include the increased application of 'omics' and 'Big data' platforms to identify vaccine candidates, and the identification of novel functions of antibody responses that mediate protection. The application of these strategies and a global approach will increase the likelihood of rapid development of highly efficacious vaccines.

Measuring Plasmodium falciparum Erythrocyte Invasion Phenotypes Using Flow Cytometry

Having the ability to rapidly, accurately, and robustly measure Plasmodium falciparum merozoite invasion is a critical component in effective assessment of a blood stage vaccine's mechanism of action. Being able to measure invasion of erythrocytes accurately, objectively and in a high throughput fashion is of critical importance. Here, we describe a simple and robust flow cytometry method that allows for the measurement of the key invasion parameters of parasite multiplication rate and erythrocyte selectivity-both important determinants of disease severity-from the schizont to the ring stage of the parasite's life-cycle, thus separating invasion from growth of the parasite. Importantly, this method is able to accurately detect low levels of parasitemia and heterogeneity within the population that can be missed by enzymatic methods. Lastly, this method has been successfully adapted and employed in field based research settings for parasitemia measurements in vivo, ex vivo, and in vitro and to measure invasion inhibition by antibodies and the use of alternative pathways for invasion.

Merozoite surface proteins in red blood cell invasion, immunity and vaccines against malaria

Malaria accounts for an enormous burden of disease globally, with Plasmodium falciparum accounting for the majority of malaria, and P. vivax being a second important cause, especially in Asia, the Americas and the Pacific. During infection with Plasmodium spp., the merozoite form of the parasite invades red blood cells and replicates inside them. It is during the blood-stage of infection that malaria disease occurs and, therefore, understanding merozoite invasion, host immune responses to merozoite surface antigens, and targeting merozoite surface proteins and invasion ligands by novel vaccines and therapeutics have been important areas of research. Merozoite invasion involves multiple interactions and events, and substantial processing of merozoite surface proteins occurs before, during and after invasion. The merozoite surface is highly complex, presenting a multitude of antigens to the immune system. This complexity has proved challenging to our efforts to understand merozoite invasion and malaria immunity, and to developing merozoite antigens as malaria vaccines. In recent years, there has been major progress in this field, and several merozoite surface proteins show strong potential as malaria vaccines. Our current knowledge on this topic is reviewed, highlighting recent advances and research priorities.

The authors summarize current knowledge of merozoite surface proteins of malaria parasites; their function in invasion, processing of surface proteins before, during and after invasion, their importance as targets of immunity, and the current status of malaria vaccines that target merozoite surface proteins.

Contrasting Patterns of Serologic and Functional Antibody Dynamics to Plasmodium falciparum Antigens in a Kenyan Birth Cohort

IgG antibodies to Plasmodium falciparum are transferred from the maternal to fetal circulation during pregnancy, wane after birth, and are subsequently acquired in response to natural infection. We examined the dynamics of malaria antibody responses of 84 Kenyan infants from birth to 36 months of age by (i) serology, (ii) variant surface antigen (VSA) assay, (iii) growth inhibitory activity (GIA), and (iv) invasion inhibition assays (IIA) specific for merozoite surface protein 1 (MSP1) and sialic acid-dependent invasion pathway. Maternal antibodies in each of these four categories were detected in cord blood and decreased to their lowest level by approximately 6 months of age. Serologic antibodies to 3 preerythrocytic and 10 blood-stage antigens subsequently increased, reaching peak prevalence by 36 months. In contrast, antibodies measured by VSA, GIA, and IIA remained low even up to 36 months. Infants sensitized to P. falciparum in utero, defined by cord blood lymphocyte recall responses to malaria antigens, acquired antimalarial antibodies at the same rate as those who were not sensitized in utero, indicating that fetal exposure to malaria antigens did not affect subsequent infant antimalarial responses. Infants with detectable serologic antibodies at 12 months of age had an increased risk of P. falciparum infection during the subsequent 24 months. We conclude that serologic measures of antimalarial antibodies in children 36 months of age or younger represent biomarkers of malaria exposure rather than protection and that functional antibodies develop after 36 months of age in this population.

Investigating the Pathogenesis of Severe Malaria: A Multidisciplinary and Cross-Geographical Approach

More than a century after the discovery of Plasmodium spp. parasites, the pathogenesis of severe malaria is still not well understood. The majority of malaria cases are caused by Plasmodium falciparum and Plasmodium vivax, which differ in virulence, red blood cell tropism, cytoadhesion of infected erythrocytes, and dormant liver hypnozoite stages. Cerebral malaria coma is one of the most severe manifestations of P. falciparum infection. Insights into its complex pathophysiology are emerging through a combination of autopsy, neuroimaging, parasite binding, and endothelial characterizations. Nevertheless, important questions remain regarding why some patients develop life-threatening conditions while the majority of P. falciparum-infected individuals do not, and why clinical presentations differ between children and adults. For P. vivax, there is renewed recognition of severe malaria, but an understanding of the factors influencing disease severity is limited and remains an important research topic. Shedding light on the underlying disease mechanisms will be necessary to implement effective diagnostic tools for identifying and classifying severe malaria syndromes and developing new therapeutic approaches for severe disease. This review highlights progress and outstanding questions in severe malaria pathophysiology and summarizes key areas of pathogenesis research within the International Centers of Excellence for Malaria Research program.