The Plasmodium vivax merozoite surface protein 1 paralog is a novel erythrocyte-binding ligand of P. vivax

Plasmodium vivax serological exposure markers: PvMSP1-42-induced humoral and memory B-cell response generates long-lived antibodies

Plasmodium vivax serological exposure markers (SEMs) have emerged as promising tools for the actionable surveillance and implementation of targeted interventions to accelerate malaria elimination. To determine the dynamic profiles of SEMs in current and past P. vivax infections, we screened and selected 11 P. vivax proteins from 210 putative proteins using protein arrays, with a set of serum samples obtained from patients with acute P. vivax and documented past P. vivax infections. Then we used a murine protein immune model to initially investigate the humoral and memory B cell response involved in the generation of long-lived antibodies. We show that of the 11 proteins, especially C-terminal 42-kDa region of P. vivax merozoite surface protein 1 (PvMSP1-42) induced longer-lasting long-lived antibodies, as these antibodies were detected in individuals infected with P. vivax in the 1960-1970s who were not re-infected until 2012. In addition, we provide a potential mechanism for the maintenance of long-lived antibodies after the induction of PvMSP1-42. The results indicate that PvMSP1-42 induces more CD73+CD80+ memory B cells (MBCs) compared to P. vivax GPI-anchored micronemal antigen (PvGAMA), allowing IgG anti-PvMSP1-42 antibodies to be maintained for a long time.

A dot-blot ELISA preliminary evaluation using PvMSP1-42 recombinant protein as antigen for serological diagnosis of Plasmodium vivax infection in Thailand

Plasmodium vivax is the most prevalent cause of malaria in Thailand and is predominant in malarial endemic areas worldwide. P. vivax infection is characterized by low parasitemia, latent liver-stage parasites, or asymptomatic infections leading to underreported P. vivax cases. These are significant challenges for controlling and eliminating P. vivax from endemic countries. This study developed and evaluated a dot-blot enzyme-linked immunosorbent assay (ELISA) using PvMSP1-42 recombinant antigen for serological diagnosis based on the detection of antibodies against P. vivax. The optimal PvMSP1-42 concentration and dilutions of anti-human IgG horseradish peroxidase (HRP)-conjugated antiserum were tested on 88 serum samples from P. vivax, Plasmodium falciparum and bacterial infection, including healthy individuals. A cut-off titer of 1:800 produced optimal values for sensitivity and specificity of 90.9 and 98.2%, respectively, with an accuracy of 95.5%. The positive and negative predictive values were 96.8 and 94.7% respectively. The results from microscopic examination and dot-blot ELISA showed strong agreement with the 0.902 kappa index. Thus, the dot-blot ELISA using PvMSP1-42 antigen provided high sensitivity and specificity suitable for serodiagnosis of P. vivax infection. The test is a simple and quick diagnostic assay suitable for field testing as it does not require specific equipment or particular skills.

The Plasmodium vivax MSP1P-19 is involved in binding of reticulocytes through interactions with the membrane proteins band3 and CD71

The parasite Plasmodium vivax preferentially invades human reticulocytes. Its merozoite surface protein 1 paralog (PvMSP1P), particularly the 19-kDa C-terminal region (PvMSP1P-19), has been shown to bind to reticulocytes, and this binding can be inhibited by antisera obtained by PvMSP1P-19 immunization. The molecular mechanism of interactions between PvMSP1P-19 and reticulocytes during P. vivax invasion, however, remains unclear. In this study, we analyzed the ability of MSP1P-19 to bind to different concentrations of reticulocytes and confirmed its reticulocyte preference. LC-MS analysis was used to identify two potential reticulocyte receptors, band3 and CD71, that interact with MSP1P-19. Both PvMSP1P-19 and its sister taxon Plasmodium cynomolgi MSP1P-19 were found to bind to the extracellular loop (loop 5) of band3, where the interaction of MSP1P-19 with band3 was chymotrypsin sensitive. Antibodies against band3-P5, CD71, and MSP1P-19 reduced the binding activity of PvMSP1P-19 and Plasmodium cynomolgi MSP1P-19 to reticulocytes, while MSP1P-19 proteins inhibited Plasmodium falciparum invasion in vitro in a concentration-dependent manner. To sum up, identification and characterization of the reticulocyte receptor is important for understanding the binding of reticulocytes by MSP1P-19.

Revisiting the antigen markers of vector-borne parasitic diseases identified by immunomics: identification and application to disease control

INTRODUCTION

Protein microarray is a promising immunomic approach for identifying biomarkers. Based on our previous study that reviewed parasite antigens and recent parasitic omics research, this article expands to include information on vector-borne parasitic diseases (VBPDs), namely, malaria, schistosomiasis, leishmaniasis, babesiosis, trypanosomiasis, lymphatic filariasis, and onchocerciasis.

AREAS COVERED

We revisit and systematically summarize antigen markers of vector-borne parasites identified by the immunomic approach and discuss the latest advances in identifying antigens for the rational development of diagnostics and vaccines. The applications and challenges of this approach for VBPD control are also discussed.

EXPERT OPINION

The immunomic approach has enabled the identification and/or validation of antigen markers for vaccine development, diagnosis, disease surveillance, and treatment. However, this approach presents several challenges, including limited sample size, variability in antigen expression, false-positive results, complexity of omics data, validation and reproducibility, and heterogeneity of diseases. In addition, antigen involvement in host immune evasion and antigen sensitivity/specificity are major issues in its application. Despite these limitations, this approach remains promising for controlling VBPD. Advances in technology and data analysis methods should continue to improve candidate antigen identification, as well as the use of a multiantigen approach in diagnostic and vaccine development for VBPD control.

Complement receptor 1 is the human erythrocyte receptor for Plasmodium vivax erythrocyte binding protein

The discovery that Africans were resistant to infection by Plasmodium vivax (P. vivax) led to the conclusion that P. vivax invasion relied on the P. vivax Duffy Binding Protein (PvDBP) interacting with the Duffy Antigen Receptor for Chemokines (DARC) expressed on erythrocytes. However, the recent reporting of P. vivax infections in DARC-negative Africans suggests that the parasite might use an alternate invasion pathway to infect DARC-negative reticulocytes. To identify the parasite ligands and erythrocyte receptors that enable P. vivax invasion of both DARC-positive and -negative erythrocytes, we expressed region II containing the Duffy Binding-Like (DBL) domain of P. vivax erythrocyte binding protein (PvEBP-RII) and verified that the DBL domain binds to both DARC-positive and -negative erythrocytes. Furthermore, an AVidity-based EXtracelluar Interaction Screening (AVEXIS) was used to identify the receptor for PvEBP among over 750 human cell surface receptor proteins, and this approach identified only Complement Receptor 1 (CR1, CD35, or C3b/C4b receptor) as a PvEBP receptor. CR1 is a well-known receptor for P. falciparum Reticulocyte binding protein Homology 4 (PfRh4) and is present on the surfaces of both reticulocytes and normocytes, but its expression decreases as erythrocytes age. Indeed, PvEBP-RII bound to a subpopulation of both reticulocytes and normocytes, and this binding was blocked by the addition of soluble CR1 recombinant protein, indicating that CR1 is the receptor of PvEBP. In addition, we found that the Long Homology Repeat A (LHR-A) subdomain of CR1 is the only subdomain responsible for mediating the interaction with PvEBP-RII.

Merozoite surface protein 1 paralog is involved in the human erythrocyte invasion of a zoonotic malaria, Plasmodium knowlesi

The zoonotic malaria parasite Plasmodium knowlesi is an important public health concern in Southeast Asia. Invasion of host erythrocytes is essential for parasite growth, and thus, understanding the repertoire of parasite proteins that enable this process is vital for identifying vaccine candidates and how some species are able to cause zoonotic infection. Merozoite surface protein 1 (MSP1) is found in all malaria parasite species and is perhaps the most well-studied as a potential vaccine candidate. While MSP1 is encoded by a single gene in P. falciparum, all other human infective species (P. vivax, P. knowlesi, P. ovale, and P. malariae) additionally encode a divergent paralogue known as MSP1P, and little is known about its role or potential functional redundancy with MSP1. We, therefore, studied the function of P. knowlesi merozoite surface protein 1 paralog (PkMSP1P), using both recombinant protein and CRISPR-Cas9 genome editing. The recombinant 19-kDa C-terminus of PkMSP1P (PkMSP1P-19) was shown to bind specifically to human reticulocytes. However, immunoblotting data suggested that PkMSP1P-19-induced antibodies can recognize PkMSP1-19 and vice versa, confounding our ability to separate the properties of these two proteins. Targeted disruption of the pkmsp1p gene profoundly impacts parasite growth, demonstrating for the first time that PkMSP1P is important in in vitro growth of P. knowlesi and likely plays a distinct role from PkMSP1. Importantly, the MSP1P KO also enabled functional characterization of the PkMSP1P-19 antibodies, revealing clear immune cross-reactivity between the two paralogues, highlighting the vital importance of genetic studies in contextualizing recombinant protein studies.

Population-based genomic study of Plasmodium vivax malaria in seven Brazilian states and across South America

Background

Brazil is a unique and understudied setting for malaria, with complex foci of transmission associated with human and environmental conditions. An understanding of the population genomic diversity of P. vivax parasites across Brazil can support malaria control strategies.

Methods

Through whole genome sequencing of P. vivax isolates across 7 Brazilian states, we use population genomic approaches to compare genetic diversity within country (n = 123), continent (6 countries, n = 315) and globally (26 countries, n = 885).

Findings

We confirm that South American isolates are distinct, have more ancestral populations than the other global regions, with differentiating mutations in genes under selective pressure linked to antimalarial drugs (pvmdr1, pvdhfr-ts) and mosquito vectors (pvcrmp3, pvP45/48, pvP47). We demonstrate Brazil as a distinct parasite population, with signals of selection including ABC transporter (PvABCI3) and PHIST exported proteins.

Interpretation

Brazil has a complex population structure, with evidence of P. simium infections and Amazonian parasites separating into multiple clusters. Overall, our work provides the first Brazil-wide analysis of P. vivax population structure and identifies important mutations, which can inform future research and control measures.

Funding

AI is funded by an MRC LiD PhD studentship. TGC is funded by the Medical Research Council (Grant no. MR/M01360X/1, MR/N010469/1, MR/R025576/1, MR/R020973/1 and MR/X005895/1). SC is funded by Medical Research Council UK grants (MR/M01360X/1, MR/R025576/1, MR/R020973/1 and MR/X005895/1) and Bloomsbury SET (ref. CCF17-7779). FN is funded by The Shloklo Malaria Research Unit - part of the Mahidol Oxford Research Unit, supported by the Wellcome Trust (Grant no. 220211). ARSB is funded by São Paulo Research Foundation - FAPESP (Grant no. 2002/09546-1). RLDM is funded by Brazilian National Council for Scientific and Technological Development - CNPq (Grant no. 302353/2003-8 and 471605/2011-5); CRFM is funded by FAPESP (Grant no. 2020/06747-4) and CNPq (Grant no. 302917/2019-5 and 408636/2018-1); JGD is funded by FAPESP fellowships (2016/13465-0 and 2019/12068-5) and CNPq (Grant no. 409216/2018-6).

FT-GPI, a highly sensitive and accurate predictor of GPI-anchored proteins, reveals the composition and evolution of the GPI proteome in Plasmodium species

BACKGROUND

Protozoan parasites are known to attach specific and diverse group of proteins to their plasma membrane via a GPI anchor. In malaria parasites, GPI-anchored proteins (GPI-APs) have been shown to play an important role in host-pathogen interactions and a key function in host cell invasion and immune evasion. Because of their immunogenic properties, some of these proteins have been considered as malaria vaccine candidates. However, identification of all possible GPI-APs encoded by these parasites remains challenging due to their sequence diversity and limitations of the tools used for their characterization.

METHODS

The FT-GPI software was developed to detect GPI-APs based on the presence of a hydrophobic helix at both ends of the premature peptide. FT-GPI was implemented in C ++and applied to study the GPI-proteome of 46 isolates of the order Haemosporida. Using the GPI proteome of Plasmodium falciparum strain 3D7 and Plasmodium vivax strain Sal-1, a heuristic method was defined to select the most sensitive and specific FT-GPI software parameters.

RESULTS

FT-GPI enabled revision of the GPI-proteome of P. falciparum and P. vivax, including the identification of novel GPI-APs. Orthology- and synteny-based analyses showed that 19 of the 37 GPI-APs found in the order Haemosporida are conserved among Plasmodium species. Our analyses suggest that gene duplication and deletion events may have contributed significantly to the evolution of the GPI proteome, and its composition correlates with speciation.

CONCLUSION

FT-GPI-based prediction is a useful tool for mining GPI-APs and gaining further insights into their evolution and sequence diversity. This resource may also help identify new protein candidates for the development of vaccines for malaria and other parasitic diseases.

Plasmodium vivax vaccine: What is the best way to go?

Malaria is one of the most devastating human infectious diseases caused by Plasmodium spp. parasites. A search for an effective and safe vaccine is the main challenge for its eradication. Plasmodium vivax is the second most prevalent Plasmodium species and the most geographically distributed parasite and has been neglected for decades. This has a massive gap in knowledge and consequently in the development of vaccines. The most significant difficulties in obtaining a vaccine against P. vivax are the high genetic diversity and the extremely complex life cycle. Due to its complexity, studies have evaluated P. vivax antigens from different stages as potential targets for an effective vaccine. Therefore, the main vaccine candidates are grouped into preerythrocytic stage vaccines, blood-stage vaccines, and transmission-blocking vaccines. This review aims to support future investigations by presenting the main findings of vivax malaria vaccines to date. There are only a few P. vivax vaccines in clinical trials, and thus far, the best protective efficacy was a vaccine formulated with synthetic peptide from a circumsporozoite protein and Montanide ISA-51 as an adjuvant with 54.5% efficacy in a phase IIa study. In addition, the majority of P. vivax antigen candidates are polymorphic, induce strain-specific and heterogeneous immunity and provide only partial protection. Nevertheless, immunization with recombinant proteins and multiantigen vaccines have shown promising results and have emerged as excellent strategies. However, more studies are necessary to assess the ideal vaccine combination and test it in clinical trials. Developing a safe and effective vaccine against vivax malaria is essential for controlling and eliminating the disease. Therefore, it is necessary to determine what is already known to propose and identify new candidates.

The direct binding of Plasmodium vivax AMA1 to erythrocytes defines a RON2-independent invasion pathway

We used a transgenic parasite in which Plasmodium falciparum parasites were genetically modified to express Plasmodium vivax apical membrane antigen 1 (PvAMA1) protein in place of PfAMA1 to study PvAMA1-mediated invasion. In P. falciparum, AMA1 interaction with rhoptry neck protein 2 (RON2) is known to be crucial for invasion, and PfRON2 peptides (PfRON2p) blocked the invasion of PfAMA1 wild-type parasites. However, PfRON2p has no effect on the invasion of transgenic parasites expressing PvAMA1 indicating that PfRON2 had no role in the invasion of PvAMA1 transgenic parasites. Interestingly, PvRON2p blocked the invasion of PvAMA1 transgenic parasites in a dose-dependent manner. We found that recombinant PvAMA1 domains 1 and 2 (rPvAMA1) bound to reticulocytes and normocytes indicating that PvAMA1 directly interacts with erythrocytes during the invasion, and invasion blocking of PvRON2p may result from it interfering with PvAMA1 binding to erythrocytes. It was previously shown that the peptide containing Loop1a of PvAMA1 (PvAMA1 Loop1a) is also bound to reticulocytes. We found that the Loop1a peptide blocked the binding of PvAMA1 to erythrocytes. PvAMA1 Loop1a has no polymorphisms in contrast to other PvAMA1 loops and may be an attractive vaccine target. We thus present the evidence that PvAMA1 binds to erythrocytes in addition to interacting with PvRON2 suggesting that the P. vivax merozoites may exploit complex pathways during the invasion process.

Plasmodium vivax blood stage invasion pathways: Contribution of omics technologies in deciphering molecular and cellular mechanisms

Vivax malaria is an infectious disease caused by Plasmodium vivax, a parasitic protozoan transmitted by female Anopheline mosquitoes. Historically, vivax malaria has often been regarded as a benign self-limiting infection due to the observation of low parasitemia in Duffy-positive patients in endemic transmission areas and the virtual absence of infections in Duffy-negative individuals in Sub Saharan Africa. However, the latest estimates show that the burden of the disease is not decreasing in many countries and cases of vivax infections in Duffy-negative individuals are increasingly reported throughout Africa. This raised questions about the accuracy of diagnostics and the evolution of interactions between humans and parasites. For a long time, our knowledge on P. vivax biology has been hampered due to the limited access to biological material and the lack of robust in vitro culture methods. Consequently, little is currently known about P. vivax blood stage invasion mechanisms. The introduction of omics technologies with novel and accessible techniques such as third generation sequencing and RNA sequencing at single cell level, two-dimensional electrophoresis, liquid chromatography, and mass spectrometry, has progressively improved our understanding of P. vivax genetics, transcripts, and proteins. This review aims to provide broad insights into P. vivax invasion mechanisms generated by genomics, transcriptomics, and proteomics and to illustrate the importance of integrated multi-omics studies.

Zoonotic Malaria: Non-Laverania Plasmodium Biology and Invasion Mechanisms

Malaria, which is caused by Plasmodium parasites through Anopheles mosquito transmission, remains one of the most life-threatening diseases affecting hundreds of millions of people worldwide every year. Plasmodium vivax, which accounts for the majority of cases of recurring malaria caused by the Plasmodium (non-Laverania) subgenus, is an ancient and continuing zoonosis originating from monkey hosts probably outside Africa. The emergence of other zoonotic malarias (P. knowlesi, P. cynomolgi, and P. simium) further highlights the seriousness of the disease. The severity of this epidemic disease is dependent on many factors, including the parasite characteristics, host-parasite interactions, and the pathology of the infection. Successful infection depends on the ability of the parasite to invade the host; however, little is known about the parasite invasion biology and mechanisms. The lack of this information adds to the challenges to malaria control and elimination, hence enhancing the potential for continuation of this zoonosis. Here, we review the literature describing the characteristics, distribution, and genome details of the parasites, as well as host specificity, host-parasite interactions, and parasite pathology. This information will provide the basis of a greater understanding of the epidemiology and pathogenesis of malaria to support future development of strategies for the control and prevention of this zoonotic infection.

Two 20-Residue-Long Peptides Derived from Plasmodium vivax Merozoite Surface Protein 10 EGF-Like Domains Are Involved in Binding to Human Reticulocytes

Plasmodium parasites’ invasion of their target cells is a complex, multi-step process involving many protein-protein interactions. Little is known about how complex the interaction with target cells is in Plasmodium vivax and few surface molecules related to reticulocytes’ adhesion have been described to date. Natural selection, functional and structural analysis were carried out on the previously described vaccine candidate P. vivax merozoite surface protein 10 (PvMSP10) for evaluating its role during initial contact with target cells. It has been shown here that the recombinant carboxyl terminal region (rPvMSP10-C) bound to adult human reticulocytes but not to normocytes, as validated by two different protein-cell interaction assays. Particularly interesting was the fact that two 20-residue-long regions (³⁸⁸DKEECRCRANYMPDDSVDYF⁴⁰⁷ and ⁴¹⁵KDCSKENGNCDVNAECSIDK⁴³⁴) were able to inhibit rPvMSP10-C binding to reticulocytes and rosette formation using enriched target cells. These peptides were derived from PvMSP10 epidermal growth factor (EGF)-like domains (precisely, from a well-defined electrostatic zone) and consisted of regions having the potential of being B- or T-cell epitopes. These findings provide evidence, for the first time, about the fragments governing PvMSP10 binding to its target cells, thus highlighting the importance of studying them for inclusion in a P. vivax antimalarial vaccine.

Alternative Invasion Mechanisms and Host Immune Response to Plasmodium vivax Malaria: Trends and Future Directions

Plasmodium vivax malaria is a neglected tropical disease, despite being more geographically widespread than any other form of malaria. The documentation of P. vivax infections in different parts of Africa where Duffy-negative individuals are predominant suggested that there are alternative pathways for P. vivax to invade human erythrocytes. Duffy-negative individuals may be just as fit as Duffy-positive individuals and are no longer resistant to P.vivax malaria. In this review, we describe the complexity of P. vivax malaria, characterize pathogenesis and candidate invasion genes of P. vivax, and host immune responses to P. vivax infections. We provide a comprehensive review on parasite ligands in several Plasmodium species that further justify candidate genes in P. vivax. We also summarize previous genomic and transcriptomic studies related to the identification of ligand and receptor proteins in P. vivax erythrocyte invasion. Finally, we identify topics that remain unclear and propose future studies that will greatly contribute to our knowledge of P. vivax.

Cross-species reactivity of antibodies against Plasmodium vivax blood-stage antigens to Plasmodium knowlesi

Malaria is caused by multiple different species of protozoan parasites, and interventions in the pre-elimination phase can lead to drastic changes in the proportion of each species causing malaria. In endemic areas, cross-reactivity may play an important role in the protection and blocking transmission. Thus, successful control of one species could lead to an increase in other parasite species. A few studies have reported cross-reactivity producing cross-immunity, but the extent of cross-reactive, particularly between closely related species, is poorly understood. P. vivax and P. knowlesi are particularly closely related species causing malaria infections in SE Asia, and whilst P. vivax cases are in decline, zoonotic P. knowlesi infections are rising in some areas. In this study, the cross-species reactivity and growth inhibition activity of P. vivax blood-stage antigen-specific antibodies against P. knowlesi parasites were investigated. Bioinformatics analysis, immunofluorescence assay, western blotting, protein microarray, and growth inhibition assay were performed to investigate the cross-reactivity. P. vivax blood-stage antigen-specific antibodies recognized the molecules located on the surface or released from apical organelles of P. knowlesi merozoites. Recombinant P. vivax and P. knowlesi proteins were also recognized by P. knowlesi- and P. vivax-infected patient antibodies, respectively. Immunoglobulin G against P. vivax antigens from both immune animals and human malaria patients inhibited the erythrocyte invasion by P. knowlesi. This study demonstrates that there is extensive cross-reactivity between antibodies against P. vivax to P. knowlesi in the blood stage, and these antibodies can potently inhibit in vitro invasion, highlighting the potential cross-protective immunity in endemic areas.

In recent years, malaria initiatives have increasingly shifted focus from achieving malaria control to achieving malaria elimination. However, the interventions used are leading to drastic changes in the proportions of different Plasmodium species causing clinical infection, particularly within Southeast Asia. Little is known about how these different parasite species interact/compete in nature or whether exposure to one species could cause some level of protection against another. We examined cross-reactive antibody responses to key parasite proteins with roles in red blood cell invasion and identified novel cross-species reactivity among the closest of malaria affecting the human population (P. vivax and P. knowlesi). This comprehensive analysis provides evidence that cross-reactive immunity could play an important role in areas where species distributions are perturbed by malaria control measures, and future efforts to identify the specific cross-reactive epitopes involved would be invaluable both to our understanding of malaria immunity and vaccine development.

The immunology of Plasmodium vivax malaria

Plasmodium vivax infection, the predominant cause of malaria in Asia and Latin America, affects ~14 million individuals annually, with considerable adverse effects on wellbeing and socioeconomic development. A clinical hallmark of Plasmodium infection, the paroxysm, is driven by pyrogenic cytokines produced during the immune response. Here, we review studies on the role of specific immune cell types, cognate innate immune receptors, and inflammatory cytokines on parasite control and disease symptoms. This review also summarizes studies on recurrent infections in individuals living in endemic regions as well as asymptomatic infections, a serious barrier to eliminating this disease. We propose potential mechanisms behind these repeated and subclinical infections, such as poor induction of immunological memory cells and inefficient T effector cells. We address the role of antibody-mediated resistance to P. vivax infection and discuss current progress in vaccine development. Finally, we review immunoregulatory mechanisms, such as inhibitory receptors, T regulatory cells, and the anti-inflammatory cytokine, IL-10, that antagonizes both innate and acquired immune responses, interfering with the development of protective immunity and parasite clearance. These studies provide new insights for the clinical management of symptomatic as well as asymptomatic individuals and the development of an efficacious vaccine for vivax malaria.

Low Levels of Polymorphisms and Negative Selection in Plasmodum knowlesi Merozoite Surface Protein 8 in Malaysian Isolates

Human infections due to the monkey malaria parasite Plasmodium knowlesi is increasingly being reported from most Southeast Asian countries specifically Malaysia. The parasite causes severe and fatal malaria thus there is a need for urgent measures for its control. In this study, the level of polymorphisms, haplotypes and natural selection of full-length pkmsp8 in 37 clinical samples from Malaysian Borneo along with 6 lab-adapted strains were investigated. Low levels of polymorphism were observed across the full-length gene, the double epidermal growth factor (EGF) domains were mostly conserved, and non-synonymous substitutions were absent. Evidence of strong negative selection pressure in the non-EGF regions were found indicating functional constrains acting at different domains. Phylogenetic haplotype network analysis identified shared haplotypes and indicated geographical clustering of samples originating from Peninsular Malaysia and Malaysian Borneo. This is the first study to genetically characterize the full-length msp8 gene from clinical isolates of P. knowlesi from Malaysia; however, further functional characterization would be useful for future rational vaccine design.

Identification and characterization of Pv50, a novel Plasmodium vivax merozoite surface protein

Background

Plasmodium vivax contains approximately 5400 coding genes, more than 40% of which code for hypothetical proteins that have not been functionally characterized. In a previous preliminary screening using pooled serum samples, numerous hypothetical proteins were selected from among those that were highly transcribed in the schizont-stage of parasites, and highly antigenic P. vivax candidates including hypothetical proteins were identified. However, their immunological and functional activities in P. vivax remain unclear. From these candidates, we investigated a P. vivax 50-kDa protein (Pv50, PVX_087140) containing a highly conserved signal peptide that shows high transcription levels in blood-stage parasites.

Results

Recombinant Pv50 was expressed in a cell-free expression system and used for IgG prevalence analysis of patients with vivax malaria and healthy individuals. Immune responses were analyzed in immunized mice and mouse antibodies were used to detect the subcellular localization of the protein in blood-stage parasites by immunofluorescence assay. A protein array method was used to evaluate protein-protein interactions to predict protein functional activities during the invasion of parasites into erythrocytes. Recombinant Pv50 showed IgG prevalence in patient samples with a sensitivity of 42.9% and specificity of 93.8% compared to that in healthy individuals. The non-cytophilic antibodies IgG1 and IgG3 were the major components involved in the antibody response in Pv50-immunized mice. Pv50 localized on the surface of merozoites and a specific interaction between Pv50 and PvMSP1 was detected, suggesting that Pv50-PvMSP1 forms a heterodimeric complex in P. vivax.

Conclusions

Increased immune responses caused by native P. vivax parasites were detected, confirming its immunogenic effects. This study provides a method for detecting new malaria antigens, and Pv50 may be a vivax malaria vaccine candidate with PvMSP1.

Electronic supplementary material

The online version of this article (10.1186/s13071-019-3434-7) contains supplementary material, which is available to authorized users.

Inhibition of parasite invasion by monoclonal antibody against epidermal growth factor-like domain of Plasmodium vivax merozoite surface protein 1 paralog

The Plasmodium vivax merozoite surface protein 1 paralog (PvMSP1P), which has epidermal growth factor (EGF)-like domains, was identified as a novel erythrocyte adhesive molecule. This EGF-like domain (PvMSP1P-19) elicited high level of acquired immune response in patients. Antibodies against PvMSP1P significantly reduced erythrocyte adhesion activity to its unknown receptor. To determine PvMSP1P-19-specific antibody function and B-cell epitopes in vivax patients, five monoclonal antibodies (mAbs) and 18-mer peptides were generated. The mAb functions were determined by erythrocyte-binding inhibition assay and invasion inhibition assay with P. knowlesi. B-cell epitopes of PvMSP1P-19 domains were evaluated by peptide microarray. The pvmsp1p-19 sequences showed limited polymorphism in P. vivax worldwide isolates. The 1BH9-A10 showed erythrocyte binding inhibitory by interaction with the N-terminus of PvMSP1P-19, while this mAb failed to recognize PkMSP1P-19 suggesting the species-specific for P. vivax. Other mAbs showed cross-reactivity with PkMSP1P-19. Among them, the 2AF4-A2 and 2AF4-A6 mAb significantly reduced parasite invasion through C-terminal recognition. The linear B-cell epitope in naturally exposed P. vivax patient was identified at three linear epitopes. In this study, PvMSP1P-19 N-terminal-specific 1BH9-A10 and C-terminal-specific 2AF4 mAbs showed functional activity for epitope recognition suggesting that PvMSP1P may be useful for vaccine development strategy for specific single epitope to prevent P. vivax invasion.

Contribution of Plasmodium immunomics: potential impact for serological testing and surveillance of malaria

Introduction: Plasmodium vivax (Pv) and P. knowlesi account together for a considerable share of the global burden of malaria, along with P. falciparum (Pf). However, inaccurate diagnosis and undetectable asymptomatic/submicroscopic malaria infections remain very challenging. Blood-stage antigens involved in either invasion of red blood cells or sequestration/cytoadherence of parasitized erythrocytes have been immunomics-characterized, and are vital for the detection of malaria incidence. Areas covered: We review the recent advances in Plasmodium immunomics to discuss serological markers with potential for specific and sensitive diagnosis of malaria. Insights on alternative use of immunomics to assess malaria prevalence are also highlighted. Finally, we provide practical applications of serological markers as diagnostics, with an emphasis on dot immunogold filtration assay which holds promise for malaria diagnosis and epidemiological surveys. Expert commentary: The approach largely contributes to Pf and Pv research in identifying promising non-orthologous antigens able to detect malaria incidence and to differentiate between past and recent infections. However, further studies to profiling naturally acquired immune responses are expected in order to help discover/validate serological markers of no cross-seroreactivity and guide control interventions. More so, the application of immunomics to knowlesi infections would help validate the recently identified antigens and contribute to the discovery of additional biomarkers of exposure, immunity, or both.

Molecular and cellular interactions defining the tropism of Plasmodium vivax for reticulocytes

Plasmodium vivax is uniquely restricted to invading reticulocytes, the youngest of red blood cells. Parasite invasion relies on the sequential deployment of multiple parasite invasion ligands. Correct targeting of the host reticulocyte is mediated by two families of invasion ligands: the reticulocyte binding proteins (RBPs) and erythrocyte binding proteins (EBPs). The Duffy receptor has long been established as a key determinant for P. vivax invasion. However, recently, the RBP protein PvRBP2b has been shown to bind to transferrin receptor, which is expressed on reticulocytes but lost on normocytes, implicating the ligand-receptor in the reticulocyte tropism of P. vivax. Furthermore there is increasing evidence for P. vivax growth and sexual development in reticulocyte-enriched tissues such as the bone marrow.

Erythrocyte-binding assays reveal higher binding of Plasmodium knowlesi Duffy binding protein to human Fya+/b+ erythrocytes than to Fya+/b- erythrocytes

Background

The merozoite of the zoonotic Plasmodium knowlesi invades human erythrocytes via the binding of its Duffy binding protein (PkDBPαII) to the Duffy antigen on the eythrocytes. The Duffy antigen has two immunologically distinct forms, Fy^a and Fy^b. In this study, the erythrocyte-binding assay was used to quantitatively determine and compare the binding level of PkDBPαII to Fy^a+/b+ and Fy^a+/b- human erythrocytes.

Results

In the erythrocyte-binding assay, binding level was determined by scoring the number of rosettes that were formed by erythrocytes surrounding transfected mammalian COS-7 cells which expressed PkDBPαII. The assay result revealed a significant difference in the binding level. The number of rosettes scored for Fy^a+/b+ was 1.64-fold higher than that of Fy^a+/b- (155.50 ± 34.32 and 94.75 ± 23.16 rosettes, respectively; t₍₆₎ = -2.935, P = 0.026).

Conclusions

The erythrocyte-binding assay provided a simple approach to quantitatively determine the binding level of PkDBPαII to the erythrocyte Duffy antigen. Using this assay, PkDBPαII was found to display higher binding to Fy^a+/b+ erythrocytes than to Fy^a+/b- erythrocytes.

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.

Immunological characterization of Plasmodium vivax Pv32, a novel predicted GPI-anchored merozoite surface protein

Background

The development of an effective malarial vaccine is an urgent need. Most glycosylphosphatidylinositol (GPI)-anchored proteins of Plasmodium parasites are exposed to neutralizing antibodies, and several are advanced vaccine candidates. In the present study, Plasmodium vivax Pv32 (PVX_084815) as a hypothetical, predicted GPI-anchored and cysteine-rich motif was identified from our previous findings with a focus on its antigenic profiling. The orthologue gene pv32, a predicted GPI anchor of P. falciparum PF3D7_1434400, has still not been well studied.

Methods

The gene information of pv32 was obtained from PlasmoDB. Recombinant Pv32 protein was expressed and purified using a wheat germ cell-free expression system and a glutathione-Sepharose column. Naturally acquired immune response to recombinant Pv32 protein was evaluated using a protein microarray with 96 parasite-infected patients and 96 healthy individuals. Antibodies against recombinant Pv32 proteins from immune animals were produced, used and analyzed for the subcellular localization of native Pv32 protein by an immunofluorescence assay. A total of 48 pv32 sequences from 11 countries retrieved from PlasmoDB were used to determine the genetic diversity, polymorphisms and genealogical relationships with DNAsp and NETWORK software packages.

Results

Pv32 is encoded by a conserved gene with two introns that are located on chromosome 13 and expressed as a 32 kDa protein in mature asexual stage parasites. Immunofluorescence data showed that Pv32 localized on the merozoite surface in schizont-stage parasites. The recombinant Pv32 was recognized by 39.6% of antibodies from P. vivax-infected individuals compared with healthy individuals. Low levels of nucleotide diversity (π = 0.0028) and polymorphisms of pv32 were detected within worldwide isolates.

Conclusions

This study shows the identification and characterization of the hypothetical protein, Pv32. Pv32 provides important characteristics, including a merozoite surface protein, a predicted GPI motif and Cysteine-rich motif among Plasmodium species. These results suggested that Pv32 is immunogenic with a merozoite surface pattern to antibodies during natural infection in humans.

Electronic supplementary material

The online version of this article (10.1186/s12936-018-2401-7) contains supplementary material, which is available to authorized users.

Immunogenicity of the Plasmodium vivax merozoite surface protein 1 paralog in the induction of naturally acquired antibody and memory B cell responses

Background

The Plasmodium vivax merozoite surface protein 1 paralog (PvMSP1P-19) is a glycosylphosphatidylinositol (GPI)-anchored blood-stage protein that is expressed on the merozoite surface. It is proposed as a blood-stage vaccine candidate against P. vivax because of its ability to induce immune responses upon natural P. vivax exposure and in immunized animals. This study aimed to demonstrate the presence of inhibitory antibodies and memory B cell responses to the PvMSP1P-19 antigen during acute P. vivax infection and after recovery from infection.

Methods

To evaluate the antibody responses to PvMSP1P-19 during and after recovery from P. vivax infection, heparinized blood was collected from P. vivax-infected patients and recovered subjects to detect the total IgG response. The seropositive samples were defined into high and low responders, according to their optical density (OD) values obtained from ELISA. High responders were the subjects who had OD values above the OD of antisera from non-exposed controls plus 4× standard deviations, whereas low responders were the subjects who had OD values less than OD of antisera from non-exposed controls plus 4× standard deviations. The plasma from high and low responders were taken for testing the inhibitory activity against PvMSP1P-19-erythrocyte binding by in vitro EBIA. The sustainability of PvMSP1P-19-specific memory B cell responses after recovery from infection was analysed by ELISPOT.

Results

The anti-PvMSP1P-19 antibody levels were significantly higher in acutely infected P. vivax patients compared to healthy controls (P <  0.0001). Monitoring of the anti-PvMSP1P-19 antibody titre showed that the antibody was maintained for up to 9 months after recovery. Almost all high-responder groups strongly inhibited PvMSP1P-19 binding to erythrocytes, whereas no inhibition was shown in most low-responder samples. Interestingly, the inhibitory activity of the antibodies in some individuals from high-responder samples were stable for at least 12 months. The longevity of the antibody response was associated with the presence of PvMSP1P-19-specific memory B cells at 9 months after recovery from infection.

Conclusions

The PvMSP1P-19 antigen has immunogenicity during the induction of the antibody response, in which both the levels and inhibitory activity are maintained after the patient recovered from P. vivax infection. The maintenance of the antibody response was associated with the response of PvMSP1P-19-specific memory B cells. Therefore, the PvMSP1P-19 antigen should also be considered as a reliable vaccine candidate to develop a blood-stage vaccine against P. vivax.

Immunogenicity of glycosylphosphatidylinositol-anchored micronemal antigen in natural Plasmodium vivax exposure

Background

Plasmodium vivax is the most geographically widespread malaria species and codominates with Plasmodium falciparum, the deadliest form of the malaria parasite. For the last few years, the number of vivax malaria cases has increased, but vivax malaria is still considered a neglected disease. During the blood stages of their life cycle, P. vivax parasites export several hundred proteins into host red blood cells. Some of these exported proteins have been discovered and studied for use as a blood-stage malaria vaccine. The P. vivax glycosylphosphatidylinositol (GPI)-anchored micronemal antigen (PvGAMA) was identified in previous study, which plays an important role in parasite invasion. To support the hypothesis that PvGAMA can induce an immune response in natural exposure, the antibody responses and cellular immunity against this antigen was demonstrated during and post-infection.

Methods

The recombinant protein PvGAMA was expressed and purified by wheat germ cell-free (WGCF) system. The analysis of humoral and cellular immune responses to the PvGAMA antigen during infection and post-infection with the P. vivax parasite were done by enzyme-linked immunosorbent assay (ELISA) techniques.

Results

During P. vivax infection, 95% of patients showed significant antibody responses to PvGAMA antigen. The cytophilic IgG1 and IgG3 isotypes were the major isotypes produced in response to PvGAMA. A cross-sectional study of anti-PvGAMA responses during and post-infection with P. vivax found that the majority of individuals, approximately 54% of patients, were shown to maintain a positive anti-PvGAMA titre at 3 months post-infection, and some patients had the ability to maintain an antibody response for up to 12 months post-infection. Moreover, PvGAMA had the ability to stimulate a cellular immune response that was characterized by the production of the cytokines IL-2, IFN-γ and IL-10. The levels of the cytokines IFN-γ and IL-10 were significantly increased in PvGAMA-stimulated lymphocyte cultures.

Conclusions

Taken together, PvGAMA had potential to induce an immune response both humoral and cellular immunity in naturally acquired P. vivax infection individuals during infection and post-infection. Therefore, PvGAMA could be as a vaccine candidate to stimulate immune response against P. vivax infection.

Naturally acquired humoral and cellular immune responses to Plasmodium vivax merozoite surface protein 8 in patients with P. vivax infection

Background

Thirty-one glycosylphosphatidylinositol (GPI)-anchored proteins of Plasmodium vivax, merozoite surface protein 1 (MSP1), MSP1 paralogue, MSP4, MSP5, MSP8, and MSP10 have been reported from homologs of Plasmodium falciparum by gene annotation with bioinformatics tools. These GPI-anchored proteins contain two epidermal growth factor (EGF)-like domains at its C-terminus. Here, P. vivax merozoite surface protein 8 (PvMSP8) are considered as potential targets of protective immunity.

Methods

Recombinant PvMSP8 (rPvMSP8) was expressed, purified, and used for the assessment of humoral and cellular immune responses in P. vivax-infected patients and immune mice. Moreover, the target epitope of ant-PvMSP8 antibodies and subcellular localization of PvMSP8 was also determined.

Results

The rPvMSP8 was successfully expressed and purified as soluble form as ~55 kDa. PvMSP8 was localized to the outer circle of pigments associated with the food vacuole. The rPvMSP8 protein had a high antigenicity (73.2% in sensitivity and 96.2% in specificity) in patients infected with P. vivax. IgG2 antibody subtype was the predominantly responses to this antigen. Antibody response to PvMSP8 increased up to day 7 and after that slightly decreased within a month. The longevity of anti-PvMSP8 antibody was stably sustained up to 12-year recovery patient samples. Most anti-PvMSP8 antibodies recognized two epitopes that were located outside the C-terminal EGF-like domain. The cellular immune response in P. vivax-exposed individuals produced high levels of IFN-γ and IL-10 upon PvMSP8 antigen stimulation in vitro.

Conclusions

All data in this study suggest that PvMSP8 antigen has a potential to induce both humoral and cellular immune responses in patients with P. vivax infection. The subcellular localization of PvMSP8 confirmed that it was associated with the parasite food vacuole in blood-stage parasites. A further characterization of this protein will be useful for blood stage P. vivax vaccine development.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-017-1837-5) contains supplementary material, which is available to authorized users.

Identification of a novel merozoite surface antigen of Plasmodium vivax, PvMSA180

Background

Although a number of Plasmodium vivax proteins have been identified, few have been investigated as potential vaccine candidates. This study characterized the Plasmodium vivax merozoite surface antigen 180 (PvMSA180, PVX_094920), a novel P. vivax antigenic protein.

Methods

The target gene was amplified as four overlapping domains (D1, D2, D3 and D4) to enable expression of the recombinant protein using cell-free and bacterial expression systems. The recombinant PvMSA180 proteins were used in protein microarrays to evaluate the humoral immune response of 72 vivax-infected patients and 24 vivax-naïve individuals. Antibodies produced in mice against the PvMSA180-D1 and -D4 domains were used to assess the subcellular localization of schizont-stage parasites with immunofluorescence assays. A total of 51 pvmsa180 sequences from 12 countries (41 sequences from PlasmoDB and 6 generated in this study) were used to determine the genetic diversity and genealogical relationships with DNAsp and NETWORK software packages, respectively.

Results

PvMSA180 consists of 1603 amino acids with a predicted molecular mass of 182 kDa, and has a signal peptide at the amino-terminus. A total of 70.8% of patients (51/72) showed a specific antibody response to at least one of the PvMSA180 domains, and 20.8% (15/72) exhibited a robust antibody response to at least three of the domains. These findings suggest that PvMSA180 is targeted by the humoral immune response during natural infection with P. vivax. Immunofluorescence analysis demonstrated that PvMSA180 is localized on the merozoite surface of schizont-stage parasites, and pvmsa180 sequences originating from various geographic regions worldwide showed low genetic diversity. Twenty-two haplotypes were found, and haplotype 6 (Hap_6, 77%) of pvmsa180 was detected in isolates from six countries.

Conclusions

A novel P. vivax surface protein, PvMSA180, was characterized in this study. Most of P. vivax-infected patients had specific antibodies against particular antigenic domains, indicating that this protein is immunogenic in naturally exposed populations. Genetic analysis of worldwide isolates showed that pvmsa180 is less polymorphic than other well-known candidates and that some haplotypes are common to several countries. However, additional studies with a larger sample size are necessary to evaluate the antibody responses in geographically separated populations, and to identify the function of PvMSA180 during parasite invasion.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-017-1760-9) contains supplementary material, which is available to authorized users.

What Is Known about the Immune Response Induced by Plasmodium vivax Malaria Vaccine Candidates?

Malaria caused by Plasmodium vivax continues being one of the most important infectious diseases around the world; P. vivax is the second most prevalent species and has the greatest geographic distribution. Developing an effective antimalarial vaccine is considered a relevant control strategy in the search for means of preventing the disease. Studying parasite-expressed proteins, which are essential in host cell invasion, has led to identifying the regions recognized by individuals who are naturally exposed to infection. Furthermore, immunogenicity studies have revealed that such regions can trigger a robust immune response that can inhibit sporozoite (hepatic stage) or merozoite (erythrocyte stage) invasion of a host cell and induce protection. This review provides a synthesis of the most important studies to date concerning the antigenicity and immunogenicity of both synthetic peptide and recombinant protein candidates for a vaccine against malaria produced by P. vivax.

Plasmodium vivax GPI-anchored micronemal antigen (PvGAMA) binds human erythrocytes independent of Duffy antigen status

Plasmodium vivax, a major agent of malaria in both temperate and tropical climates, has been thought to be unable to infect humans lacking the Duffy (Fy) blood group antigen because this receptor is critical for erythrocyte invasion. Recent surveys in various endemic regions, however, have reported P. vivax infections in Duffy-negative individuals, suggesting that the parasite may utilize alternative receptor-ligand pairs to complete the erythrocyte invasion. Here, we identified and characterized a novel parasite ligand, Plasmodium vivax GPI-anchored micronemal antigen (PvGAMA), that bound human erythrocytes regardless of Duffy antigen status. PvGAMA was localized at the microneme in the mature schizont-stage parasites. The antibodies against PvGAMA fragments inhibited PvGAMA binding to erythrocytes in a dose-dependent manner. The erythrocyte-specific binding activities of PvGAMA were significantly reduced by chymotrypsin treatment. Thus, PvGAMA may be an adhesion molecule for the invasion of Duffy-positive and -negative human erythrocytes.

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.

Identification of Immunodominant B-cell Epitope Regions of Reticulocyte Binding Proteins in Plasmodium vivax by Protein Microarray Based Immunoscreening

Plasmodium falciparum can invade all stages of red blood cells, while Plasmodium vivax can invade only reticulocytes. Although many P. vivax proteins have been discovered, their functions are largely unknown. Among them, P. vivax reticulocyte binding proteins (PvRBP1 and PvRBP2) recognize and bind to reticulocytes. Both proteins possess a C-terminal hydrophobic transmembrane domain, which drives adhesion to reticulocytes. PvRBP1 and PvRBP2 are large (> 326 kDa), which hinders identification of the functional domains. In this study, the complete genome information of the P. vivax RBP family was thoroughly analyzed using a prediction server with bioinformatics data to predict B-cell epitope domains. Eleven pvrbp family genes that included 2 pseudogenes and 9 full or partial length genes were selected and used to express recombinant proteins in a wheat germ cell-free system. The expressed proteins were used to evaluate the humoral immune response with vivax malaria patients and healthy individual serum samples by protein microarray. The recombinant fragments of 9 PvRBP proteins were successfully expressed; the soluble proteins ranged in molecular weight from 16 to 34 kDa. Evaluation of the humoral immune response to each recombinant PvRBP protein indicated a high antigenicity, with 38-88% sensitivity and 100% specificity. Of them, N-terminal parts of PvRBP2c (PVX_090325-1) and PvRBP2 like partial A (PVX_090330-1) elicited high antigenicity. In addition, the PvRBP2-like homologue B (PVX_116930) fragment was newly identified as high antigenicity and may be exploited as a potential antigenic candidate among the PvRBP family. The functional activity of the PvRBP family on merozoite invasion remains unknown.

Characterization of Plasmodium vivax Early Transcribed Membrane Protein 11.2 and Exported Protein 1

In Plasmodium, the membrane of intracellular parasites is initially formed during invasion as an invagination of the red blood cell surface, which forms a barrier between the parasite and infected red blood cells in asexual blood stage parasites. The membrane proteins of intracellular parasites of Plasmodium species have been identified such as early-transcribed membrane proteins (ETRAMPs) and exported proteins (EXPs). However, there is little or no information regarding the intracellular parasite membrane in Plasmodium vivax. In the present study, recombinant PvETRAMP11.2 (PVX_003565) and PvEXP1 (PVX_091700) were expressed and evaluated antigenicity tests using sera from P. vivax-infected patients. A large proportion of infected individuals presented with IgG antibody responses against PvETRAMP11.2 (76.8%) and PvEXP1 (69.6%). Both of the recombinant proteins elicited high antibody titers capable of recognizing parasites of vivax malaria patients. PvETRAMP11.2 partially co-localized with PvEXP1 on the intracellular membranes of immature schizont. Moreover, they were also detected at the apical organelles of newly formed merozoites of mature schizont. We first proposed that these proteins might be synthesized in the preceding schizont stage, localized on the parasite membranes and apical organelles of infected erythrocytes, and induced high IgG antibody responses in patients.

Immunoprofiling of the tryptophan-rich antigen family in Plasmodium vivax

Tryptophan-rich antigens (TRAgs) are an antigen family that has been identified in human and rodent malaria parasites. TRAgs have been proposed as candidate antigens for potential vaccines. The Plasmodium vivax TRAg (PvTRAg) family includes 36 members. Each PvTRAg contains a tryptophan-rich (TR) domain in the C-terminal region. In this study, we recombinantly expressed all 36 PvTRAgs using a cell-free expression system, and, for the first time, profiled the IgG antibody responses against all PvTRAgs in the sera from 96 vivax malaria patients and 40 healthy individuals using protein microarray technology. The mean seropositive rate for all PvTRAgs was 60.3%. Among them, nine PvTRAgs were newly identified in this study and showed a seropositive rate of >50%. Five of them, PvTRAg_13, PvTRAg_15, PvTRAg_16, PvTRAg_26, and PvTRAg_29, produced higher levels of IgG antibody, even in low-endemicity countries. In addition, the results of an immunofluorescence analysis suggest that PvTRAgs are, at least in part, associated with caveola-vesicle complexes, a unique structure of P. vivax-infected erythrocytes. The mechanism of formation and the function of these abundant membrane structures are not known. Further investigation aimed at determining the functions of these proteins would lead to a better understanding of the blood-stage biology of P. vivax.

Antigenicity and immunogenicity of PvRALP1, a novel Plasmodium vivax rhoptry neck protein

BACKGROUND

Proteins secreted from the rhoptry in Plasmodium merozoites are associated with the formation of tight junctions and parasitophorous vacuoles during invasion of erythrocytes and are sorted within the rhoptry neck or bulb. Very little information has been obtained to date about Plasmodium vivax rhoptry-associated leucine (Leu) zipper-like protein 1 (PvRALP1; PVX_096245), a putative rhoptry protein. PvRALP1 contains a signal peptide, a glycine (Gly)/glutamate (Glu)-rich domain, and a Leu-rich domain, all of which are conserved in other Plasmodium species.

METHODS

Recombinant PvRALP1s were expressed as full-length protein without the signal peptide (PvRALP1-Ecto) and as truncated protein consisting of the Gly/Glu- and Leu-rich domains (PvRALP1-Tr) using the wheat germ cell-free expression system. The immunoreactivity to these two fragments of recombinant PvRALP1 protein in serum samples from P. vivax-infected patients and immunized mice, including analysis of immunoglobulin G (IgG) subclasses, was evaluated by enzyme-linked immunosorbent assay or protein microarray technology. The subcellular localization of PvRALP1 in blood stage parasites was also determined.

RESULTS

Recombinant PvRALP1-Ecto and PvRALP1-Tr proteins were successfully expressed, and in serum samples from P. vivax patients from the Republic of Korea, the observed immunoreactivities to these proteins had 58.9% and 55.4% sensitivity and 95.0% and 92.5% specificity, respectively. The response to PvRALP1 in humans was predominantly cytophilic antibodies (IgG1 and IgG3), but a balanced Th1/Th2 response was observed in mice. Unexpectedly, there was no significant inverse correlation between levels of parasitaemia and levels of antibody against either PvRALP1-Ecto (R2=0.11) or PvRALP1-Tr (R2=0.14) antigens. PvRALP1 was localized in the rhoptry neck of merozoites, and this was the first demonstration of the localization of this protein in P. vivax.

CONCLUSIONS

This study analysed the antigenicity and immunogenicity of PvRALP1 and suggested that PvRALP1 may be immunogenic in humans during parasite infection and might play an important role during invasion of P. vivax parasites.

Naturally-acquired cellular immune response against Plasmodium vivax merozoite surface protein-1 paralog antigen

BACKGROUND

Plasmodium vivax merozoite surface protein-1 paralog (PvMSP1P) is a glycosylphosphatidylinositol-anchored protein expressed on the merozoite surface. This molecule is a target of natural immunity, as high anti-MSP1P-19 antibody levels were detected during P. vivax infection and the antibody inhibited PvMSP1P-erythrocyte binding. Recombinant PvMSP1P antigen results in production of a significant Th1 cytokine response in immunized mice. The present study was performed to characterize natural cellular immunity against PvMSP1P-19 and PvDBP region II in acute and recovery P. vivax infection.

METHODS

Peripheral blood mononuclear cells (PBMCs) from acute and recovery P. vivax infection were obtained for lymphocyte proliferation assay upon PvMSP1P-19 and PvDBP region II antigen stimulation. The culture supernatant was examined for the presence of the cytokines IL-2, TNF, IFN-γ and IL-10 by enzyme-linked immunosorbent assay (ELISA). To determine whether Th1 or Th2 have a memory response against PvMSP1P-19 and PvDBPII protein antigen, PBMCs from subjects who had recovered from P. vivax infection 8-10 weeks prior to the study were obtained for lymphocyte proliferation assay. Cytokine-producing cells were analysed by flow cytometry.

RESULTS

IL-2 was detected at high levels in lymphocyte cultures from acutely infected P. vivax patients upon PvMSP1P-19 stimulation. Analysis of the Th1 or Th2 memory response in PBMC cultures from subjects who had recovered from P. vivax infection showed significantly elevated levels of PvMSP1P-19 and PvDBPII-specific IFN-γ-producing cells (P  <  0.05). Interestingly, the response of IFN-γ-producing cells in PvMSP1P stimulation was fourfold greater in recovered subjects than that in acute-infection patients. CD4(+) T cells were the major cell phenotype involved in the response to PvMSP1P-19 and PvDBPII antigen.

CONCLUSIONS

PvMSP1P-19 strongly induces a specific cellular immune response for protection against P. vivax compared with PvDBPII as the antigen induces activation of IFN-γ-producing effector cells following natural P. vivax exposure. Upon stimulation, PvMSP1P-19 has the potential to activate the recall response of Th1 effector memory cells that play a role in killing the parasite.

Host-parasite interaction: selective Pv-fam-a family proteins of Plasmodium vivax bind to a restricted number of human erythrocyte receptors

BACKGROUND

Plasmodium vivax synthesizes the largest number of 36 tryptophan-rich proteins belonging to the Pv-fam-a family. These parasite proteins need to be characterized for their biological function because tryptophan-rich proteins from other Plasmodium species have been proposed as vaccine candidates.

METHODS

Recombinant P. vivax tryptophan-rich antigens (PvTRAgs) were used to determine their erythrocyte-binding activity by a cell-based enzyme-linked immunosorbent assay, flow cytometry, and a rosetting assay.

RESULTS

Only 4 (PvTRAg26.3, PvTRAg34, PvTRAg36, and PvTRAg36.6) of 21 PvTRAgs bind to host erythrocytes. The cross-competition data indicated that PvTRAg36 and PvTRAg34 share their erythrocyte receptors with previously described proteins PvTRAg38 and PvTRAg33.5, respectively. On the other hand, PvTRAg26.3 and PvTRAg36.6 cross-compete with each other and not with any other PvTRAg, indicating that these 2 proteins bind to the same but yet another set of erythrocyte receptor(s). Together, 10 of 36 PvTRAgs possess erythrocyte-binding activity in which each protein recognizes >1 erythrocyte receptor. Further, each erythrocyte receptor is shared by >1 PvTRAg.

CONCLUSIONS

This redundancy may be useful for the parasite to invade red blood cells and cause disease pathogenesis, and it can be exploited to develop therapeutics against P. vivax malaria.

Determining the Plasmodium vivax VCG-1 strain blood stage proteome

Plasmodium vivax is the second most prevalent parasite species causing malaria in humans living in tropical and subtropical areas throughout the world. There have been few P. vivax proteomic studies to date and they have focused on using clinical isolates, given the technical difficulties concerning how to maintain an in vitro culture of this species. This study was thus focused on identifying the P. vivax VCG-1 strain proteome during its blood lifecycle through LC-MS/MS; this led to identifying 734 proteins, thus increasing the overall number reported for P. vivax to date. Some of them have previously been related to reticulocyte invasion, parasite virulence and growth and others are new molecules possibly playing a functional role during metabolic processes, as predicted by Database for Annotation, Visualization and Integrated Discovery (DAVID) functional analysis. This is the first large-scale proteomic analysis of a P. vivax strain adapted to a non-human primate model showing the parasite protein repertoire during the blood lifecycle. Database searches facilitated the in silico prediction of proteins proposed for evaluation in further experimental assays regarding their potential as pharmacologic targets or as component of a totally efficient vaccine against malaria caused by P. vivax.

BIOLOGICAL SIGNIFICANCE

P. vivax malaria continues being a public health problem around world. Although considerable progress has been made in understanding genome- and transcriptome-related P. vivax biology, there are few proteome studies, currently representing only 8.5% of the predicted in silico proteome reported in public databases. A high-throughput proteomic assay was used for discovering new P. vivax intra-reticulocyte asexual stage molecules taken from parasites maintained in vivo in a primate model. The methodology avoided the main problem related to standardising an in vitro culture system to obtain enough samples for protein identification and annotation. This study provides a source of potential information contributing towards a basic understanding of P. vivax biology related to parasite proteins which are of significant importance for the malaria research community.

Genetic diversity and natural selection of three blood-stage 6-Cys proteins in Plasmodium vivax populations from the China-Myanmar endemic border

Pv12, Pv38 and Pv41, the three 6-Cys family proteins which are expressed in the blood-stage of vivax malaria, might be involved in merozoite invasion activity and thus be potential vaccine candidate antigens of Plasmodium vivax. However, little information is available concerning the genetic diversity and natural selection of these three proteins. In the present study, we analyzed the amino acid sequences of P. vivax blood-stage 6-Cys family proteins in comparison with the homologue proteins of Plasmodium cynomolgi strain B using bioinformatic methods. We also investigated genetic polymorphisms and natural selection of these three genes in P. vivax populations from the China-Myanmar endemic border. The three P. vivax blood-stage 6-Cys proteins were shown to possess a signal peptide at the N-terminus, containing two s48/45 domains, and Pv12 and Pv38 have a GPI-anchor motif at the C-terminus. Then, 22, 21 and 29 haplotypes of pv12, pv38 and pv41 were identified out of 45, 38 and 40 isolates, respectively. The dN/dS values for Domain II of pv38 and pv41 were 3.33880 and 5.99829, respectively, suggesting positive balancing selection for these regions. Meanwhile, the C-terminus of pv41 showed high nucleotide diversity, and Tajima's D test suggested that this fragment could be under positive balancing selection. Overall, our results have significant implications, providing a genetic basis for blood-stage malaria vaccine development based on these three 6-Cys proteins.

Immunogenicity and antigenicity of Plasmodium vivax merozoite surface protein 10

Among the proteins involved in the invasion by merozoite, the glycosylphosphatidylinositol-anchored proteins (GPI-APs) are suggested as potential vaccine candidates because of their localization to apical organelles and the surface; these candidates are predicted to play essential roles during invasion. As a GPI-AP, Plasmodium vivax merozoite surface protein 10 (PvMSP-10) induces high antibody titers. However, such high antibody titers have shown no protective efficacy for animals challenged with P. vivax parasites in a previous study. To adequately evaluate the immunogenicity and further characterize PvMSP-10 in order to understand its vaccine potential, we assessed its immunogenicity by immunizing BALB/c mice with cell-free expressed recombinant PvMSP-10 protein. The antigenicity of MSP-10 was analyzed, and we found 42% sensitivity and 95% specificity using serum samples from P. vivax-infected Korean patients. The IgG1 and IgG3 were the predominant immunoreactive antibodies against PvMSP-10 in vivax patient sera, and IgG1 and IgG3 and Th1-type cytokines were predominantly secreted in PvMSP-10-immunized mice. We conclude that the immunogenicity and antigenicity of MSP-10 may serve as a potential vaccine against vivax malaria.

Antigenicity studies in humans and immunogenicity studies in mice: an MSP1P subdomain as a candidate for malaria vaccine development

The newly identified GPI-anchored Plasmodium vivax merozoite surface protein 1 paralog (MSP1P) has a highly antigenic C-terminus that binds erythrocytes. To characterize the antigenicity and immunogenicity of two regions (PvMSP1P-19 and -33) of the highly conserved C-terminus of MSP1P relative to PvMSP1-19, 30 P. vivax malaria-infected patients and two groups of mice (immunized with PvMSP1P-19 or -33) were tested for IgG subclass antibodies against PvMSP1P-19 and -33 antigens. In the patients infected with P. vivax, IgG1 and IgG3 levels were significantly higher than those levels in healthy individuals, and were the predominant response to the two C-terminal fragments of PvMSP1P (p < 0.05). In mice immunized with PvMSP1P-19, IgG1 levels were the highest while IgG2b levels were similar to IgG1 levels. The levels of Th1 cytokines in mice immunized with PvMSP1P-19 or -33 were significantly higher than those in mice immunized with PvMSP1-19 (p < 0.05). Our results indicate that: (i) IgG1 and IgG3 (IgG2b in mice) are predominant IgG subclasses in both patients infected with P. vivax and mice immunized with PvMSP1P-19 or -33; (ii) the C-terminus of MSP1P induces a Th1-cytokine response. This immune profiling study provides evidence that MSP1P may be a potential candidate for vivax vaccine.

N-terminal Plasmodium vivax merozoite surface protein-1, a potential subunit for malaria vivax vaccine

The human malaria is widely distributed in the Middle East, Asia, the western Pacific, and Central and South America. Plasmodium vivax started to have the attention of many researchers since it is causing diseases to millions of people and several reports of severe malaria cases have been noticed in the last few years. The lack of in vitro cultures for P. vivax represents a major delay in developing a functional malaria vaccine. One of the major candidates to antimalarial vaccine is the merozoite surface protein-1 (MSP1), which is expressed abundantly on the merozoite surface and capable of activating the host protective immunity. Studies have shown that MSP-1 possesses highly immunogenic fragments, capable of generating immune response and protection in natural infection in endemic regions. This paper shows humoral immune response to different proteins of PvMSP1 and the statement of N-terminal to be added to the list of potential candidates for malaria vivax vaccine.