Cross-reactivity between apical membrane antgen 1 and rhoptry neck protein 2 in P. vivax and P. falciparum: A structural and binding study

Conformational variability in the D2 loop of Plasmodium Apical Membrane antigen 1

Apical Membrane Antigen 1 (AMA1) plays a vital role in the invasion of the host erythrocyte by the malaria parasite, Plasmodium. It is thus an important target for vaccine and anti-malaria therapeutic strategies that block the invasion process. AMA1, present on the surface of the parasite, interacts with RON2, a component of the parasite's rhoptry neck (RON) protein complex, which is transferred to the erythrocyte membrane during invasion. The D2 loop of AMA1 plays an essential role in invasion as it partially covers the RON2-binding site and must therefore be displaced for invasion to proceed. Several structural studies have shown that the D2 loop is very mobile, a property that is probably important for the function of AMA1. Here we present three crystal structures of AMA1 from P. falciparum (strains 3D7 and FVO) and P. vivax (strain Sal1), in which the D2 loop could be largely traced in the electron density maps. The D2 loop of PfAMA1-FVO and PvAMA1 (as a complex with a monoclonal antibody Fab) has a conformation previously noted in the P. knowlesi AMA1 structure. The D2 loop of PfAMA1-3D7, however, reveals a novel conformation. We analyse the conformational variability of the D2 loop in these structures, together with those previously reported. Three different conformations can be distinguished, all of which are highly helical and show some similarity in their secondary structure organisation. We discuss the significance of these observations in the light of the flexible nature of the D2 loop and its role in AMA1 function.

A broadly cross-reactive i-body to AMA1 potently inhibits blood and liver stages of Plasmodium parasites

Apical membrane antigen-1 (AMA1) is a conserved malarial vaccine candidate essential for the formation of tight junctions with the rhoptry neck protein (RON) complex, enabling Plasmodium parasites to invade human erythrocytes, hepatocytes, and mosquito salivary glands. Despite its critical role, extensive surface polymorphisms in AMA1 have led to strain-specific protection, limiting the success of AMA1-based interventions beyond initial clinical trials. Here, we identify an i-body, a humanised single-domain antibody-like molecule that recognises a conserved pan-species conformational epitope in AMA1 with low nanomolar affinity and inhibits the binding of the RON2 ligand to AMA1. Structural characterisation indicates that the WD34 i-body epitope spans the centre of the conserved hydrophobic cleft in AMA1, where interacting residues are highly conserved among all Plasmodium species. Furthermore, we show that WD34 inhibits merozoite invasion of erythrocytes by multiple Plasmodium species and hepatocyte invasion by P. falciparum sporozoites. Despite a short half-life in mouse serum, we demonstrate that WD34 transiently suppressed P. berghei infections in female BALB/c mice. Our work describes the first pan-species AMA1 biologic with inhibitory activity against multiple life-cycle stages of Plasmodium. With improved pharmacokinetic characteristics, WD34 could be a potential immunotherapy against multiple species of Plasmodium.

Potent AMA1-specific human monoclonal antibody against P. vivax Pre-erythrocytic and Blood Stages

New therapeutics are necessary for preventing Plasmodium vivax malaria due to easy transmissibility and dormancy in the liver that increases the clinical burden due to recurrent relapse. We isolated 12 Pv Apical Membrane Antigen 1 (PvAMA1) specific human monoclonal antibodies from Peripheral Blood Mononuclear Cells of a Pv exposed individual. PvAMA1 is essential for sporozoite and merozoite invasion, making it a unique therapeutic target. HumAb 826827 blocked the invasion of human erythrocytes using Pv clinical isolates and inhibited sporozoite invasion of human hepatocytes in vitro (IC50 of 0.3 to 3.7 ug/mL). It also significantly reduced liver infection of chimeric FRG humHep mice in vivo. The crystal structure of rPvAMA1 bound to 826827 shows that 826827 partially occupies the highly conserved hydrophobic groove in PvAMA1 that binds its known receptor, RON2. We have isolated a potent humAb that is isolate transcendent, blocks both pre erythrocytic and blood stage infection, and could be a new therapy for Pv.

Invasion-inhibitory peptides chosen by natural selection analysis as an antimalarial strategy

Plasmodium vivax's biological complexity has restricted in vitro culture development for characterising antigens involved in erythrocyte invasion and their immunological relevance. The murine model is proposed as a suitable alternative in the search for therapeutic candidates since Plasmodium yoelii uses homologous proteins for its invasion. The AMA-1 protein is essential for parasite invasion of erythrocytes as it is considered an important target for infection control. This study has focused on functional PyAMA-1 peptides involved in host-pathogen interaction; the protein is located in regions under negative selection as determined by bioinformatics analysis. It was found that pyama1 has two highly conserved regions amongst species (>70%) under negative selection. Fourteen synthetic peptides spanning both conserved regions were evaluated; 5 PyAMA-1 peptides having high specific binding (HABP) to murine erythrocytes were identified. The parasite's invasion inhibition capability was analysed through in vitro assays, suggesting that peptides 42681 (43-ENTERSIKLINPWDKYMEKY-62), 42903 (206-RYSSNDANNENQPFSFTPEK-225) and 42904 (221-FTPEKIENYKDLSYLTKNLR-240) had greater than 50% inhibition profile and restricted P. yoelii intra-erythrocyte development. This work proposes that the screening of conserved HABPs under negative selective pressure might be good candidates for developing a synthetic anti-malarial vaccine since they share functionally-relevant characteristics, such as interspecies conservation, specific RBC binding profile, invasion and parasite development inhibition capability, and the predicted B-epitopes within were recognised by sera obtained from experimentally-infected mice.

Structure-based design of a strain transcending AMA1-RON2L malaria vaccine

Apical membrane antigen 1 (AMA1) is a key malaria vaccine candidate and target of neutralizing antibodies. AMA1 binds to a loop in rhoptry neck protein 2 (RON2L) to form the moving junction during parasite invasion of host cells, and this complex is conserved among apicomplexan parasites. AMA1-RON2L complex immunization achieves higher growth inhibitory activity than AMA1 alone and protects mice against Plasmodium yoelii challenge. Here, three single-component AMA1-RON2L immunogens were designed that retain the structure of the two-component AMA1-RON2L complex: one structure-based design (SBD1) and two insertion fusions. All immunogens elicited high antibody titers with potent growth inhibitory activity, yet these antibodies did not block RON2L binding to AMA1. The SBD1 immunogen induced significantly more potent strain-transcending neutralizing antibody responses against diverse strains of Plasmodium falciparum than AMA1 or AMA1-RON2L complex vaccination. This indicates that SBD1 directs neutralizing antibody responses to strain-transcending epitopes in AMA1 that are independent of RON2L binding. This work underscores the importance of neutralization mechanisms that are distinct from RON2 blockade. The stable single-component SBD1 immunogen elicits potent strain-transcending protection that may drive the development of next-generation vaccines for improved malaria and apicomplexan parasite control.

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.

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.

How Apicomplexa Parasites Secrete and Build Their Invasion Machinery

Apicomplexa are obligatory intracellular parasites that sense and actively invade host cells. Invasion is a conserved process that relies on the timely and spatially controlled exocytosis of unique specialized secretory organelles termed micronemes and rhoptries. Microneme exocytosis starts first and likely controls the intricate mechanism of rhoptry secretion. To assemble the invasion machinery, micronemal proteins-associated with the surface of the parasite-interact and form complexes with rhoptry proteins, which in turn are targeted into the host cell. This review covers the molecular advances regarding microneme and rhoptry exocytosis and focuses on how the proteins discharged from these two compartments work in synergy to drive a successful invasion event. Particular emphasis is given to the structure and molecular components of the rhoptry secretion apparatus, and to the current conceptual framework of rhoptry exocytosis that may constitute an unconventional eukaryotic secretory machinery closely related to the one described in ciliates. Expected final online publication date for the Annual Review of Microbiology, Volume 76 is September 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

New highly antigenic linear B cell epitope peptides from PvAMA-1 as potential vaccine candidates

Peptide-based vaccines have demonstrated to be an important way to induce long-lived immune responses and, therefore, a promising strategy in the rational of vaccine development. As to malaria, among the classic vaccine targets, the Apical membrane antigen (AMA-1) was proven to have important B cell epitopes that can induce specific immune response and, hence, became key players for a vaccine approach. The peptides selection was carried out using a bioinformatic approach based on Hidden Markov Models profiles of known antigens and propensity scale methods based on hydrophilicity and secondary structure prediction. The antigenicity of the selected B-cell peptides was assessed by multiple serological assays using sera from acute P.vivax infected subjects. The synthetic peptides were recognized by 45.5%, 48.7% and 32.2% of infected subjects for peptides I, II and III respectively. Moreover, when synthetized together (tripeptide), the reactivity increases up to 62%, which is comparable to the reactivity found against the whole protein PvAMA-1 (57%). Furthermore, IgG reactivity against the tripeptide after depletion was reduced by 42%, indicating that these epitopes may be responsible for a considerable part of the protein immunogenicity. These results represent an excellent perspective regarding future chimeric vaccine constructions that may come to contemplate several targets with the potential to generate the robust and protective immune response that a vivax malaria vaccine needs to succeed.

Plasmodium vivax AMA1: Implications of distinct haplotypes for immune response

In Brazil, Plasmodium vivax infection accounts for around 80% of malaria cases. This infection has a substantial impact on the productivity of the local population as the course of the disease is usually prolonged and the development of acquired immunity in endemic areas takes several years. The recent emergence of drug-resistant strains has intensified research on alternative control methods such as vaccines. There is currently no effective available vaccine against malaria; however, numerous candidates have been studied in the past several years. One of the leading candidates is apical membrane antigen 1 (AMA1). This protein is involved in the invasion of Apicomplexa parasites into host cells, participating in the formation of a moving junction. Understanding how the genetic diversity of an antigen influences the immune response is highly important for vaccine development. In this study, we analyzed the diversity of AMA1 from Brazilian P. vivax isolates and 19 haplotypes of P. vivax were found. Among those sequences, 33 nonsynonymous PvAMA1 amino acid sites were identified, whereas 20 of these sites were determined to be located in predicted B-cell epitopes. Nonsynonymous mutations were evaluated for their influence on the immune recognition of these antigens. Two distinct haplotypes, 5 and 16, were expressed and evaluated for reactivity in individuals from northern Brazil. Both PvAMA1 variants were reactive. Moreover, the IgG antibody response to these two PvAMA1 variants was analyzed in an exposed but noninfected population from a P. vivax endemic area. Interestingly, over 40% of this population had antibodies recognizing both variants. These results have implications for the design of a vaccine based on a polymorphic antigen.

Plasmodium vivax is the most abundant Plasmodium species in Brazil. While this species has been neglected for many years, the recent emergence of drug-resistant strains and the absence of a vaccine intensified the efforts for a better control method. Naturally acquired immune response analysis is a useful tool for understanding the antigenicity of Plasmodium proteins and evaluating the potential of a vaccine candidate. In this study, the genetic variability of one of the leading P. vivax vaccine candidates (PvAMA1) was analyzed. Two distinct variants were expressed and the antibody response was evaluated in infected and noninfected individuals in the Brazilian Amazon. This improved understanding of the magnitude and dynamics of the antibody response will contribute to the knowledge of a vaccine candidate and open new perspectives in vivax malaria vaccine development.

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.

Time-Frequency Approach Applied to Finding Interaction Regions in Pathogenic Proteins

Protein-protein interactions govern all molecular processes for living organisms, even those involved in pathogen infection. Pathogens such as virus, bacteria, and parasites contain proteins that help the pathogen to attach, penetrate, and settle inside the target cell. Thus, it is necessary to know the regions in pathogenic proteins that interact with host cell receptors. Currently, powerful pathogen databases are available and many pathogenic proteins have been recognized, but many pathogenic proteins have not been characterized. This work developed a program in MATLAB environment based on the time-frequency analysis to recognize important sites in proteins. Our program highlights the highest energy patches in proteins from their time-frequency distribution and matches the corresponding frequency. We sought to know if this approach is able to recognize stretches residues related to interaction. Our approach was applied to five study cases from pathogenic co-crystallized structures that have been well characterized. We searched the frequencies that characterize interaction regions in pathogenic proteins and with this information tried to identify new interaction patches in either paralogs or orthologs. We found that our program generates a well-interpretable graphic under several descriptors that can show important regions in proteins even those related to interaction. We propose that this MATLAB program could be used as a tool to explore outstanding regions in uncharacterized proteins.

Blood-Stage Malaria Parasite Antigens: Structure, Function, and Vaccine Potential

Plasmodium parasites are the causative agent of malaria, a disease that kills approximately 450,000 individuals annually, with the majority of deaths occurring in children under the age of 5 years and the development of a malaria vaccine is a global health priority. Plasmodium parasites undergo a complex life cycle requiring numerous diverse protein families. The blood stage of parasite development results in the clinical manifestation of disease. A vaccine that disrupts the blood stage is highly desired and will aid in the control of malaria. The blood stage comprises multiple steps: invasion of, asexual growth within, and egress from red blood cells. This review focuses on blood-stage antigens with emphasis on antigen structure, antigen function, neutralizing antibodies, and vaccine potential.

Functional Conservation of the AMA1 Host-Cell Invasion Ligand Between P. falciparum and P. vivax: A Novel Platform to Accelerate Vaccine and Drug Development

Plasmodium vivax and P. falciparum malaria species have diverged significantly in receptor-ligand interactions and host-cell invasion. One protein common to both is the merozoite invasion ligand AMA1. While the general structure of AMA1 is similar between species, their sequences are divergent. Surprisingly, it was possible to genetically replace PfAMA1 with PvAMA1 in P. falciparum parasites. PvAMA1 complemented PfAMA1 function and supported invasion of erythrocytes by P. falciparum. Genetically modified P. falciparum expressing PvAMA1 evaded the invasion inhibitory effects of antibodies to PfAMA1, demonstrating species specificity of functional antibodies. We generated antibodies to recombinant PvAMA1 that effectively inhibited invasion, confirming the function of PvAMA1 in genetically modified parasites. Results indicate significant molecular flexibility in AMA1 enabling conserved function despite substantial sequence divergence across species. This provides powerful new tools to quantify the inhibitory activities of antibodies or drugs targeting PvAMA1, opening new opportunities for vaccine and therapeutic development against P. vivax.

Structure of Rhoptry Neck Protein 2 is essential for the interaction in vitro with Apical Membrane Antigen 1 in Plasmodium vivax

BACKGROUND

In several Apicomplexa, the formation of moving junctions (MJs) at the interface between the external membranes of the invading parasite and the host cell is essential for the process of parasite invasion. In Plasmodium falciparum and Toxoplasma gondii, the MJ is composed of the Apical Membrane Antigen 1 (AMA1) and Rhoptry Neck Proteins (RONs) complex; specifically, AMA1 interacts with RON2 during host cell invasion.

METHODS

Recombinant proteins based on Plasmodium vivax RON2 (A2033-P2100) and its synthetic peptide fragments, one cyclic and one linear, based on PvRON2 (D2035-T2074) were generated and used to evaluate the interaction with P. vivax AMA1 (PvAMA1) by the far western blot, surface plasmon resonance (SPR), and isothermal titration microcalorimetry (ITC) methods. The structural studies of peptides were performed by circular dichroism, and the structural analysis of the complex of PvAMA1 with peptides based on PvRON2 (D2035-T2074) was conducted with small-angle X-ray scattering (SAXS).

RESULTS

Surface plasmon resonance (KD = 23.91 ± 2.078 μmol/L) and ITC (K = 3 × 10⁵ mol/L) studies conclusively showed an interaction between the cyclic peptide based on PvRON2 and PvAMA1-His₆. In contrast, the linear peptide and recombinant PvRON2 (GST fusion protein) did not show an interaction with PvAMA1. However, the interaction among recombinant proteins PvRON2.2 and PvAMA1-His₆ was possible to show by far western blot.

CONCLUSIONS

The results show that the PvRON2 structure, particularly the S-S bond between C2051 and C2063, is determinant for the existence of the interaction between PvAMA1 and PvRON2.

Genetic sequence characterization and naturally acquired immune response to Plasmodium vivax Rhoptry Neck Protein 2 (PvRON2)

Background

The genetic diversity of malaria antigens often results in allele variant-specific immunity, imposing a great challenge to vaccine development. Rhoptry Neck Protein 2 (PvRON2) is a blood-stage antigen that plays a key role during the erythrocyte invasion of Plasmodium vivax. This study investigates the genetic diversity of PvRON2 and the naturally acquired immune response to P. vivax isolates.

Results

Here, the genetic diversity of PvRON2_1828–2080 and the naturally acquired humoral immune response against PvRON2_1828–2080 in infected and non-infected individuals from a vivax malaria endemic area in Brazil was reported. The diversity analysis of PvRON2_1828–2080 revealed that the protein is conserved in isolates in Brazil and worldwide. A total of 18 (19%) patients had IgG antibodies to PvRON2_1828–2080. Additionally, the analysis of the antibody response in individuals who were not acutely infected with malaria, but had been infected with malaria in the past indicated that 32 patients (33%) exhibited an IgG immune response against PvRON2.

Conclusions

PvRON2 was conserved among the studied isolates. The presence of naturally acquired antibodies to this protein in the absence of the disease suggests that PvRON2 induces a long-term antibody response. These results indicate that PvRON2 is a potential malaria vaccine candidate.

Electronic supplementary material

The online version of this article (10.1186/s12936-018-2543-7) contains supplementary material, which is available to authorized users.

The in Vitro Antigenicity of Plasmodium vivax Rhoptry Neck Protein 2 (PvRON2) B- and T-Epitopes Selected by HLA-DRB1 Binding Profile

Malaria caused by Plasmodium vivax is a neglected disease which is responsible for the highest morbidity in both Americas and Asia. Despite continuous public health efforts to prevent malarial infection, an effective antimalarial vaccine is still urgently needed. P. vivax vaccine development involves analyzing naturally-infected patients' immune response to the specific proteins involved in red blood cell invasion. The P. vivax rhoptry neck protein 2 (PvRON2) is a highly conserved protein which is expressed in late schizont rhoptries; it interacts directly with AMA-1 and might be involved in moving-junction formation. Bioinformatics approaches were used here to select B- and T-cell epitopes. Eleven high-affinity binding peptides were selected using the NetMHCIIpan-3.0 in silico prediction tool; their in vitro binding to HLA-DRB1^*0401, HLA-DRB1^*0701, HLA-DRB1^*1101 or HLA-DRB1^*1302 was experimentally assessed. Four peptides (39152 (HLA-DRB1^*04 and 11), 39047 (HLA-DRB1^*07), 39154 (HLADRB1^*13) and universal peptide 39153) evoked a naturally-acquired T-cell immune response in P. vivax-exposed individuals from two endemic areas in Colombia. All four peptides had an SI greater than 2 in proliferation assays; however, only peptides 39154 and 39153 had significant differences compared to the control group. Peptide 39047 was able to significantly stimulate TNF and IL-10 production while 39154 stimulated TNF production. Allele-specific peptides (but not the universal one) were able to stimulate IL-6 production; however, none induced IFN-γ production. The Bepipred 1.0 tool was used for selecting four B-cell epitopes in silico regarding humoral response. Peptide 39041 was the only one recognized by P. vivax-exposed individuals' sera and had significant differences concerning IgG subclasses; an IgG2 > IgG4 profile was observed for this peptide, agreeing with a protection-inducing role against P. falciparum and P. vivax as previously described for antigens such as RESA and MSP2. The bioinformatics results and in vitro evaluation reported here highlighted two T-cell epitopes (39047 and 39154) being recognized by memory cells and a B-cell epitope (39041) identified by P. vivax-exposed individuals' sera which could be used as potential candidates when designing a subunit-based vaccine.

Bone Marrow Is a Major Parasite Reservoir in Plasmodium vivax Infection

Plasmodium vivax causes heavy burdens of disease across malarious regions worldwide. Mature P. vivax asexual and transmissive gametocyte stages occur in the blood circulation, and it is often assumed that accumulation/sequestration in tissues is not an important phase in their development. Here, we present a systematic study of P. vivax stage distributions in infected tissues of nonhuman primate (NHP) malaria models as well as in blood from human infections. In a comparative analysis of the transcriptomes of P. vivax and Plasmodium falciparum blood-stage parasites, we found a conserved cascade of stage-specific gene expression despite the greatly different gametocyte maturity times of these two species. Using this knowledge, we validated a set of conserved asexual- and gametocyte-stage markers both by quantitative real-time PCR and by antibody assays of peripheral blood samples from infected patients and NHP (Aotus sp.). Histological analyses of P. vivax parasites in organs of 13 infected NHP (Aotus and Saimiri species) demonstrated a major fraction of immature gametocytes in the parenchyma of the bone marrow, while asexual schizont forms were enriched to a somewhat lesser extent in this region of the bone marrow as well as in sinusoids of the liver. These findings suggest that the bone marrow is an important reservoir for gametocyte development and proliferation of malaria parasites.IMPORTANCEPlasmodium vivax malaria continues to cause major public health burdens worldwide. Yet, significant knowledge gaps in the basic biology and epidemiology of P. vivax malaria remain, largely due to limited available tools for research and diagnostics. Here, we present a systematic examination of tissue sequestration during P. vivax infection. Studies of nonhuman primates and malaria patients revealed enrichment of developing sexual stages (gametocytes) and mature replicative stages (schizonts) in the bone marrow and liver, relative to those present in peripheral blood. Identification of the bone marrow as a major P. vivax tissue reservoir has important implications for parasite diagnosis and treatment.

Structural patterns of selection and diversity for Plasmodium vivax antigens DBP and AMA1

BACKGROUND

Plasmodium vivax is a significant contributor to the global malaria burden, and a vaccine targeting vivax malaria is urgently needed. An understanding of the targets of functional immune responses during the course of natural infection will aid in the development of a vaccine. Antibodies play a key role in this process, with responses against particular epitopes leading to immune selection pressure on these epitopes. A number of techniques exist to estimate levels of immune selection pressure on particular epitopes, with a sliding window analysis often used to determine particular regions likely to be under immune pressure. However, such analysis neglects protein three-dimensional structural information. With this in mind, a newly developed tool, BioStructMap, was applied to two key antigens from Plasmodium vivax: PvAMA1 and PvDBP Region II. This tool incorporates structural information into tests of selection pressure.

RESULTS

Sequences from a number of populations were analysed, examining spatially-derived nucleotide diversity and Tajima's D over protein structures for PvAMA1 and PvDBP. Structural patterns of nucleotide diversity were similar across all populations examined, with Domain I of PvAMA1 having the highest nucleotide diversity and displaying significant signatures of immune selection pressure (Tajima's D > 0). Nucleotide diversity for PvDBP was highest bordering the dimerization and DARC-binding interface, although there was less evidence of immune selection pressure on PvDBP compared with PvAMA1. This study supports previous work that has identified Domain I as the main target of immune-mediated selection pressure for PvAMA1, and also supports studies that have identified functional epitopes within PvDBP Region II.

CONCLUSIONS

The BioStructMap tool was applied to leading vaccine candidates from P. vivax, to examine structural patterns of selection and diversity across a number of geographic populations. There were striking similarities in structural patterns of diversity across multiple populations. Furthermore, whilst regions of high diversity tended to surround conserved binding interfaces, a number of protein regions with very low diversity were also identified, and these may be useful targets for further vaccine development, given previous evidence of functional antibody responses against these regions.

Receptor-ligand and parasite protein-protein interactions in Plasmodium vivax: Analysing rhoptry neck proteins 2 and 4

Elucidating receptor-ligand and protein-protein interactions represents an attractive alternative for designing effective Plasmodium vivax control methods. This article describes the ability of P. vivax rhoptry neck proteins 2 and 4 (RON2 and RON4) to bind to human reticulocytes. Biochemical and cellular studies have shown that two PvRON2- and PvRON4-derived conserved regions specifically interact with protein receptors on reticulocytes marked by the CD71 surface transferrin receptor. Mapping each protein fragment's binding region led to defining the specific participation of two 20 amino acid-long regions selectively competing for PvRON2 and PvRON4 binding to reticulocytes. Binary interactions between PvRON2 (ligand) and other parasite proteins, such as PvRON4, PvRON5, and apical membrane antigen 1 (AMA1), were evaluated and characterised by surface plasmon resonance. The results revealed that both PvRON2 cysteine-rich regions strongly interact with PvAMA1 Domains II and III (equilibrium constants in the nanomolar range) and at a lower extent with the complete PvAMA1 ectodomain and Domains I and II. These results strongly support that these proteins participate in P. vivax's complex invasion process, thus providing new pertinent targets for blocking P. vivax merozoites' specific entry to their target cells.