Role of Plasmodium vivax Duffy-binding protein 1 in invasion of Duffy-null Africans

Plasmodium vivax in vitro continuous culture: the spoke in the wheel

Understanding the life cycle of Plasmodium vivax is fundamental for developing strategies aimed at controlling and eliminating this parasitic species. Although advances in omic sciences and high-throughput techniques in recent years have enabled the identification and characterization of proteins which might be participating in P. vivax invasion of target cells, exclusive parasite tropism for invading reticulocytes has become the main obstacle in maintaining a continuous culture for this species. Such advance that would help in defining each parasite protein’s function in the complex process of P. vivax invasion, in addition to evaluating new therapeutic agents, is still a dream. Advances related to maintenance, culture medium supplements and the use of different sources of reticulocytes and parasites (strains and isolates) have been made regarding the development of an in vitro culture for P. vivax; however, only some cultures having few replication cycles have been obtained to date, meaning that this parasite’s maintenance goes beyond the technical components involved. Although it is still not yet clear which molecular mechanisms P. vivax prefers for invading young CD71⁺ reticulocytes [early maturation stages (I–II–III)], changes related to membrane proteins remodelling of such cells could form part of the explanation. The most relevant aspects regarding P. vivax in vitro culture and host cell characteristics have been analysed in this review to explain possible reasons why the species’ continuous in vitro culture is so difficult to standardize. Some alternatives for P. vivax in vitro culture have also been described.

Diversity pattern of Duffy binding protein sequence among Duffy-negatives and Duffy-positives in Sudan

Background

Vivax malaria is a leading public health concern worldwide. Due to the high prevalence of Duffy-negative blood group population, Plasmodium vivax in Africa historically is less attributable and remains a neglected disease. The interaction between Duffy binding protein and its cognate receptor, Duffy antigen receptor for chemokine plays a key role in the invasion of red blood cells and serves as a novel vaccine candidate against P. vivax. However, the polymorphic nature of P. vivax Duffy binding protein (DBP), particularly N-terminal cysteine-rich region (PvDBPII), represents a major obstacle for the successful design of a DBP-based vaccine to enable global protection. In this study, the level of pvdbpII sequence variations, Duffy blood group genotypes, number of haplotypes circulating, and the natural selection at pvdbpII in Sudan isolates were analysed and the implication in terms of DBP-based vaccine design was discussed.

Methods

Forty-two P. vivax-infected blood samples were collected from patients from different areas of Sudan during 2014–2016. For Duffy blood group genotyping, the fragment that indicates GATA-1 transcription factor binding site of the FY gene (− 33T > C) was amplified by PCR and sequenced by direct sequencing. The region II flanking pvdbpII was PCR amplified and sequenced by direct sequencing. The genetic diversity and natural selection of pvdbpII were done using DnaSP ver 5.0 and MEGA ver 5.0 programs. Based on predominant, non-synonymous, single nucleotide polymorphisms (SNPs), prevalence of Sudanese haplotypes was assessed in global isolates.

Results

Twenty SNPs (14 non-synonymous and 6 synonymous) were identified in pvdbpII among the 42 Sudan P. vivax isolates. Sequence analysis revealed that 11 different PvDBP haplotypes exist in Sudan P. vivax isolates and the region has evolved under positive selection. Among the identified PvDBP haplotypes five PvDBP haplotypes were shared among Duffy-negative as well as Duffy-positive individuals. The high selective pressure was mainly found on the known B cell epitopes (H3) of pvdbpII. Comparison of Sudanese haplotypes, based on 10 predominant non-synonymous SNPs with 10 malaria-endemic countries, demonstrated that Sudanese haplotypes were prevalent in most endemic countries.

Conclusion

This is the first pvdbp genetic diversity study from an African country. Sudanese isolates display high haplotype diversity and the gene is under selective pressure. Haplotype analysis indicated that Sudanese haplotypes are a representative sample of the global population. However, studies with a large number of samples are needed. These findings would be valuable for the development of PvDBP-based malaria vaccine.

Electronic supplementary material

The online version of this article (10.1186/s12936-018-2425-z) contains supplementary material, which is available to authorized users.

Plasmodium vivax-like genome sequences shed new insights into Plasmodium vivax biology and evolution

Although Plasmodium vivax is responsible for the majority of malaria infections outside Africa, little is known about its evolution and pathway to humans. Its closest genetic relative, P. vivax-like, was discovered in African great apes and is hypothesized to have given rise to P. vivax in humans. To unravel the evolutionary history and adaptation of P. vivax to different host environments, we generated using long- and short-read sequence technologies 2 new P. vivax-like reference genomes and 9 additional P. vivax-like genotypes. Analyses show that the genomes of P. vivax and P. vivax-like are highly similar and colinear within the core regions. Phylogenetic analyses clearly show that P. vivax-like parasites form a genetically distinct clade from P. vivax. Concerning the relative divergence dating, we show that the evolution of P. vivax in humans did not occur at the same time as the other agents of human malaria, thus suggesting that the transfer of Plasmodium parasites to humans happened several times independently over the history of the Homo genus. We further identify several key genes that exhibit signatures of positive selection exclusively in the human P. vivax parasites. Two of these genes have been identified to also be under positive selection in the other main human malaria agent, P. falciparum, thus suggesting their key role in the evolution of the ability of these parasites to infect humans or their anthropophilic vectors. Finally, we demonstrate that some gene families important for red blood cell (RBC) invasion (a key step of the life cycle of these parasites) have undergone lineage-specific evolution in the human parasite (e.g., reticulocyte-binding proteins [RBPs]).

Plasmodium vivax Infections of Duffy-Negative Erythrocytes: Historically Undetected or a Recent Adaptation?

Plasmodium vivax is the main cause of malarial disease in Asia and South America. Plasmodium vivax infection was thought to be absent in African populations who are Duffy blood group antigen negative (Duffy-negative). However, many cases of P. vivax infection have recently been observed in Duffy-negative Africans. This raises the question: were P. vivax infections in Duffy-negative populations previously missed or has P. vivax adapted to infect Duffy-negative populations? This review focuses on recent P. vivax findings in Africa and reports views on the parasite ligands that may play a role in Duffy-negative P. vivax infections. In addition, clues gained from studying P. vivax infection of reticulocytes are presented, which may provide possible avenues for establishing P. vivax culture in vitro.

Easy and fast method for expression and native extraction of Plasmodium vivax Duffy binding protein fragments

Background

The Plasmodium vivax Duffy binding protein (PvDBP) has been the most studied ligand binding human reticulocytes to date. This molecule has a cysteine-rich domain in region II (RII) which has been used as control for evaluating the target cell binding activity of several parasite molecules. However, obtaining rPvDBP-RII in a soluble form using the Escherichia coli expression system usually requires laborious and time-consuming steps for recovering the molecule’s structure and function, considering it is extracted from inclusion bodies. The present study describes an easy and fast method for expressing and obtaining several PvDBP fragments which should prove ideal for use in protein–cell interaction assays.

Results

Two PvDBP encoding regions (rii and riii/v) were cloned in pEXP5-CT vector and expressed in E. coli and extracted from the soluble fraction (rPvDBP-RII_S and rPvDBP-RIII/V_S) using a simple freezing/thawing protocol. After the purification, dichroism analysis enabled verifying high rPvDBP-RII_S and rPvDBP-RIII/V_S secondary structure α-helix content, which was lowered when molecules were extracted from inclusion bodies (rPvDBP-RII_IB and rPvDBP-RIII/V_IB) using a denaturing step. Interestingly, rPvDBP-RII_S, but not rPvDBP-RII_IB, bound to human reticulocytes, while rPvDBP-RIII/V_S and rPvDBP-RIII/V_IB bound to such cells in a similar way to negative control (cells incubated without recombinant proteins).

Conclusions

This research has shown for the first time how rPvDBP-RII can be expressed and obtained in soluble form using the E. coli system and avoiding the denaturation and refolding steps commonly used. The results highlight the usefulness of the rPvDBP-RIII/V_S fragment as a non-binding control for protein-cell target interaction assays. The soluble extraction protocol described is a good alternative to obtain fully functional P. vivax proteins in a fast and easy way, which will surely prove useful to laboratories working in studying this parasite’s biology.

Microbial Pre-exposure and Vectorial Competence of Anopheles Mosquitoes

Anopheles female mosquitoes can transmit Plasmodium, the malaria parasite. During their aquatic life, wild Anopheles mosquito larvae are exposed to a huge diversity of microbes present in their breeding sites. Later, adult females often take successive blood meals that might also carry different micro-organisms, including parasites, bacteria, and viruses. Therefore, prior to Plasmodium ingestion, the mosquito biology could be modulated at different life stages by a suite of microbes present in larval breeding sites, as well as in the adult environment. In this article, we highlight several naturally relevant scenarios of Anopheles microbial pre-exposure that we assume might impact mosquito vectorial competence for the malaria parasite: (i) larval microbial exposures; (ii) protist co-infections; (iii) virus co-infections; and (iv) pathogenic bacteria co-infections. In addition, significant behavioral changes in African Anopheles vectors have been associated with increasing insecticide resistance. We discuss how these ethological modifications may also increase the repertoire of microbes to which mosquitoes could be exposed, and that might also influence their vectorial competence. Studying Plasmodium–Anopheles interactions in natural microbial environments would efficiently contribute to refining the transmission risks.

Molecular interactions governing host-specificity of blood stage malaria parasites

Non-human primates harbor diverse species of malaria parasites, including the progenitors of Plasmodium falciparum and Plasmodium vivax. Cross-species transmission of some malaria parasites-most notably the macaque parasite, Plasmodium knowlesi-continues to this day, compelling the scientific community to ask whether these zoonoses could impede malaria control efforts by acting as a source of recurrent human infection. Host-restriction varies considerably among parasite species and is governed by both ecological and molecular variables. In particular, the efficiency of red blood cell invasion constitutes a prominent barrier to zoonotic emergence. Although proteins expressed upon the erythrocyte surface exhibit considerable diversity both within and among hosts, malaria parasites have adapted to this heterogeneity via the expansion of protein families associated with invasion, offering redundant mechanisms of host cell entry. This molecular toolkit may enable some parasites to circumvent host barriers, potentially yielding host shifts upon subsequent adaptation. Recent studies have begun to elucidate the molecular determinants of host-specificity, as well as the mechanisms that malaria parasites use to overcome these restrictions. We review recent studies concerning host tropism in the context of erythrocyte invasion by focusing on three malaria parasites that span the zoonotic spectrum: P. falciparum, P. knowlesi, and P. vivax.

An engineered vaccine of the Plasmodium vivax Duffy binding protein enhances induction of broadly neutralizing antibodies

Plasmodium vivax invasion into human reticulocytes is a complex process. The Duffy binding protein (DBP) dimerization with its cognate receptor is vital for junction formation in the invasion process. Due to its functional importance, DBP is considered a prime vaccine candidate, but variation in B-cell epitopes at the dimer interface of DBP leads to induction of strain-limited immunity. We believe that the polymorphic residues tend to divert immune responses away from functionally conserved epitopes important for receptor binding or DBP dimerization. As a proof of concept, we engineered the vaccine DEKnull to ablate the dominant Bc epitope to partially overcome strain-specific immune antibody responses. Additional surface engineering on the next generation immunogen, DEKnull-2, provides an immunogenicity breakthrough to conserved protective epitopes. DEKnull-2 elicits a stronger broadly neutralizing response and reactivity with long-term persistent antibody responses of acquired natural immunity. By using novel engineered DBP immunogens, we validate that the prime targets of protective immunity are conformational epitopes at the dimer interface. These successful results indicate a potential approach that can be used generally to improve efficacy of other malaria vaccine candidates.

Plasmodium vivax ligand-receptor interaction: PvAMA-1 domain I contains the minimal regions for specific interaction with CD71+ reticulocytes

The malarial parasite’s invasion is complex, active and coordinated, involving many low and high affinity interactions with receptors on target cell membrane. Proteomics analysis has described around 40 proteins in P. vivax which could be involved in reticulocyte invasion; few have been studied with the aim of elucidating how many of them establish specific interactions with their respective host cells. Given the importance of knowing which of the parasite’s protein regions are functionally important for invasion, minimum regions mediating specific interaction between Plasmodium vivax apical membrane antigen 1 (PvAMA-1) and its host cell were here elucidated. The region covering PvAMA-1 domains I and II (PvAMA-DI-II) specifically bound to the CD71⁺ red blood cell subpopulation. A 20 residue-long region (⁸¹EVENAKYRIPAGRCPVFGKG¹⁰⁰) located in domain I was capable of inhibiting PvAMA-DI-II recombinant protein binding to young reticulocytes (CD71⁺CD45⁻) and rosette formation. This conserved peptide specifically interacted with high affinity with reticulocytes (CD71⁺) through a neuraminidase- and chymotrypsin-treatment sensitive receptor. Such results showed that, despite AMA-1 having universal functions during late Plasmodium invasion stages, PvAMA-1 had reticulocyte-preferring binding regions, suggesting that P. vivax target cell selection is not just restricted to initial interactions but maintained throughout the erythrocyte invasion cycle, having important implications for designing a specific anti-P. vivax vaccine.

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 Human Antibodies Against Reticulocyte-Binding Domains of Plasmodium vivax Proteins, PvRBP2c and PvRBP1a, Exhibit Binding-Inhibitory Activity

Background

Crucial gaps in our understanding of Plasmodium vivax reticulocyte invasion and protective immunity have hampered development of vivax vaccines. P. vivax exclusively invades reticulocytes that is mediated by the P. vivax reticulocyte-binding proteins (PvRBPs) specifically PvRBP2c and PvRBP1a. Vivax infections in Duffy-null individuals have suggested the evolution of alternate invasion pathways that may be mediated by the PvRBPs. Thus, PvRBPs appear as potential targets for efficacious P. vivax neutralization. However, there are limited data validating their vaccine efficacy. In the absence of vivax invasion assays, binding-inhibitory activity of antibodies has been reported to be associated with protection and a measure of vaccine potential.

Methods

-based analysis was performed of the PvRBP reticulocyte-binding properties and binding-inhibitory activity of specific anti-PvRBP2c/PvRBP1a human antibodies.

Results

PvRBP2c and PvRBP1a displayed a distinct reticulocyte-binding specificity, and their specific reticulocyte-binding domains were mapped within their N-terminal regions. Importantly, naturally acquired antibodies against the reticulocyte-binding domains efficaciously blocked reticulocyte binding of native PvRBPs, suggesting that the human immune system produced functional binding-inhibitory antibodies through exposure to vivax malaria.

Conclusions

Reticulocyte-binding domains of PvRBP2c/PvRBP1a are targets of naturally acquired binding-inhibitory antibodies, substantiating their promise as candidate antigens against which vaccine-inducible immunity could potentially be boosted through natural infections.

Malaria

Malaria is caused in humans by five species of single-celled eukaryotic Plasmodium parasites (mainly Plasmodium falciparum and Plasmodium vivax) that are transmitted by the bite of Anopheles spp. mosquitoes. Malaria remains one of the most serious infectious diseases; it threatens nearly half of the world's population and led to hundreds of thousands of deaths in 2015, predominantly among children in Africa. Malaria is managed through a combination of vector control approaches (such as insecticide spraying and the use of insecticide-treated bed nets) and drugs for both treatment and prevention. The widespread use of artemisinin-based combination therapies has contributed to substantial declines in the number of malaria-related deaths; however, the emergence of drug resistance threatens to reverse this progress. Advances in our understanding of the underlying molecular basis of pathogenesis have fuelled the development of new diagnostics, drugs and insecticides. Several new combination therapies are in clinical development that have efficacy against drug-resistant parasites and the potential to be used in single-dose regimens to improve compliance. This ambitious programme to eliminate malaria also includes new approaches that could yield malaria vaccines or novel vector control strategies. However, despite these achievements, a well-coordinated global effort on multiple fronts is needed if malaria elimination is to be achieved.

Plasmodium vivax Infections over 3 Years in Duffy Blood Group Negative Malians in Bandiagara, Mali

Plasmodium vivax was thought to infect only the erythrocytes of Duffy blood group positive people. In the last decade, P. vivax has appeared throughout Africa, both in areas where Duffy positive and negative people live side by side as in Madagascar and Ethiopia and in areas where people are primarily Duffy negative, such as in western Kenya. We performed quantitative polymerase chain reaction on blood samples dried onto filter paper to determine the prevalence of P. vivax and Plasmodium falciparum in a cohort of 300 children (newborn to 6 years of age) in Bandiagara, a Sahelian area of Mali, west Africa, where the people are Duffy negative. We report 1-3 occurrences of P. vivax in each of 25 Duffy-negative children at six time points over two rainy seasons and the beginning of the third season. The prevalence of P. vivax infection was 2.0-2.5% at every time point (June 2009 to June 2010). All children with P. vivax infections were asymptomatic and afebrile, and parasite densities were extremely low. Anemia, however, was the main burden of infection. Plasmodium vivax could become a burden to sub-Saharan Africa, and the evidence of P. vivax existence needs to be taken into consideration in designing malaria control and elimination strategies in Africa.

P. falciparum malaria prevalence among blood donors in Bamako, Mali

AIM

Malaria parasite is usually transmitted to humans by Anopheles mosquitoes but it can also be transmitted through blood transfusion. Usually malaria transmission is low in African urban settings. In West Africa where the P. falciparum is the most predominant malaria species, there are limited measures to reduce the risk of blood transfusion malaria. The aim of this study was to evaluate the prevalence of P. falciparum malaria carriage among blood donors in the National Blood Center of Bamako, capital city of Mali.

METHODS

The study was conducted using a random sample of 946 blood donors in Bamako, Mali, from January to December 2011. Screening for malaria was performed by thick smear and rapid diagnostic test (RDT). Blood group was typed by Beth-Vincent and Simonin techniques.

RESULTS

The frequency of malaria infection was 1.4% by thick smear and 0.8% by the RDT. The pick prevalence of P. falciparum malaria was in rainy season, indicating a probable high seasonal risk of malaria by blood transfusion, in Mali. The prevalence of P. falciparum infection was 2% among donors of group O the majority being in this group.

CONCLUSION

There is a seasonal prevalence of malaria among blood donors in Bamako. A prevention strategy of transfusion malaria based on the combination of selection of blood donors through the medical interview, promoting a voluntary low-risk blood donation and screening all blood bags intended to be transfused to children under 5, pregnant women and immune-compromised patients during transmission season using thick smear will reduce the risk of transfusion malaria in Mali.

Molecular basis of the Duffy blood group system

ACKR1, located on chromosome 1q23.2, is the gene that encodes a glycoprotein expressing the Duffy blood group antigens. This gene is transcribed in two mRNA variants yielding two isoforms, encoding proteins with 338 and 336 amino acids. This review provides a general overview of the Duffy blood group to characterise and elucidate the genetic basis of this system. The Fy^a and Fy^b antigens are encoded by co-dominant FY*A (FY*01) and FY*B (FY*02) alleles, which differ by c.125G>A (rs12075), defining the Fy(a+b-), Fy(a-b+) and Fy(a+b+) phenotypes. The Fy(a-b-) phenotype that occurs in Africans provides an explanation for the apparent absence of Plasmodium vivax in this region: this phenotype arises from homozygosity for the FY*B allele carrying a point mutation c.1-67T>C (rs2814778), which prevents Fy^b antigen expression only in red blood cells. The same mutation has also been found on the FY*A allele, but it is very rare. The Fy(a-b-) phenotype in Europeans and Asians arises from mutations in the coding region of the FY*A or FY*B allele, preventing Duffy antigen expression on any cell in the body and thus are true Duffy null phenotypes. According to the International Society for Blood Transfusion, ten alleles are associated with the null expression of the Fy antigens. Furthermore, different allelic forms of FY*B modify Fy^b antigen expression, which may result in very weak or equivocal serology results. The mostly common found variants, c.265C>T (rs34599082) and c.298G>A (rs13962) -previously defined in combination only with the FY*B allele - have already been observed in the FY*A allele. Thus, six alleles have been recognised and associated with weak expression of the Fy antigens. Considering the importance of the Duffy blood group system in clinical medicine, additional studies via molecular biology approaches must be performed to resolve and clarify the discrepant results that are present in the erythrocyte phenotyping.

Basigin Interacts with Plasmodium vivax Tryptophan-rich Antigen PvTRAg38 as a Second Erythrocyte Receptor to Promote Parasite Growth

Elucidating the molecular mechanisms of the host-parasite interaction during red cell invasion by Plasmodium is important for developing newer antimalarial therapeutics. Recently, we have characterized a Plasmodium vivax tryptophan-rich antigen PvTRAg38, which is expressed by its merozoites, binds to host erythrocytes, and interferes with parasite growth. Interaction of this parasite ligand with the host erythrocyte occurs through its two regions present at amino acid positions 167-178 (P₂) and 197-208 (P₄). Each region recognizes its own erythrocyte receptor. Previously, we identified band 3 as the chymotrypsin-sensitive erythrocyte receptor for the P₄ region, but the other receptor, binding to P₂ region, remained unknown. Here, we have identified basigin as the second erythrocyte receptor for PvTRAg38, which is resistant to chymotrypsin. The specificity of interaction between PvTRAg38 and basigin was confirmed by direct interaction where basigin was specifically recognized by P₂ and not by the P₄ region of this parasite ligand. Interaction between P₂ and basigin is stabilized through multiple amino acid residues, but Gly-171 and Leu-175 of P₂ were more critical. These two amino acids were also critical for parasite growth. Synthetic peptides P₂ and P₄ of PvTRAg38 interfered with the parasite growth independently but had an additive effect if combined together indicating involvement of both the receptors during red cell invasion. In conclusion, PvTRAg38 binds to two erythrocyte receptors basigin and band 3 through P₂ and P₄ regions, respectively, to facilitate parasite growth. This advancement in our knowledge on molecular mechanisms of host-parasite interaction can be exploited to develop therapeutics against P. vivax malaria.

Breaking the Mold: Partnering with the National Institutes of Health Intramural Research Program to Accelerate PhD Training

Immunology is an increasingly interdisciplinary field. Here we describe a new model for interinstitutional graduate training as partnerships between complementary laboratories. This collaborative model reduces time to graduation without compromising productivity or alumni outcomes. We offer our experience with one such program and thoughts on the ingredients for their success. Despite tremendous recent advances in technology, communications, and the translation of basic scientific discoveries into new diagnostics and therapies for human diseases, graduate training in immunology and other areas of biomedical research in the United States has remained remarkably unchanged since the early 20th century, with coursework and laboratory rotations taking up much of the first 2 years, and a single mentor shepherding the student through a research project over 3 or more subsequent years. The time to graduation still averages more than 6 years in the biomedical sciences field (http://www.nsf.gov/statistics/2016/nsf16300/), with uncertain benefit of this extended time to research productivity and career advancement.

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.

The role of the human Duffy antigen receptor for chemokines in malaria susceptibility: current opinions and future treatment prospects

The Duffy antigen receptor for chemokine (DARC) is a nonspecific receptor for several proinflammatory cytokines. It is homologous to the G-protein chemokine receptor superfamily, which is suggested to function as a scavenger in many inflammatory-and proinflammatory-related diseases. G-protein chemokine receptors are also known to play a critical role in infectious diseases; they are commonly used as entry vehicles by infectious agents. A typical example is the chemokine receptor CCR5 or CXCR4 used by HIV for infecting target cells. In malaria, DARC is considered an essential receptor that mediates the entry of the human and zoonotic malaria parasites Plasmodium vivax and Plasmodium knowlesi into human reticulocytes and erythrocytes, respectively. This process is mediated through interaction with the parasite ligand known as the Duffy binding protein (DBP). Most therapeutic strategies have been focused on blocking the interaction between DBP and DARC by targeting the parasite ligand, while strategies targeting the receptor, DARC, have not been intensively investigated. The rapid increase in drug resistance and the lack of new effective drugs or a vaccine for malaria constitute a major threat and a need for novel therapeutics to combat disease. This review explores strategies that can be used to target the receptor. Inhibitors of DARC, which block DBP-DARC interaction, can potentially provide an effective strategy for preventing malaria caused by P. vivax.

A Novel Erythrocyte Binding Protein of Plasmodium vivax Suggests an Alternate Invasion Pathway into Duffy-Positive Reticulocytes

Erythrocyte invasion by malaria parasites is essential for blood-stage development and an important determinant of host range. In Plasmodium vivax, the interaction between the Duffy binding protein (DBP) and its cognate receptor, the Duffy antigen receptor for chemokines (DARC), on human erythrocytes is central to blood-stage infection. Contrary to this established pathway of invasion, there is growing evidence of P. vivax infections occurring in Duffy blood group-negative individuals, suggesting that the parasite might have gained an alternative pathway to infect this group of individuals. Supporting this concept, a second distinct erythrocyte binding protein (EBP2), representing a new member of the DBP family, was discovered in P. vivax and may be the ligand in an alternate invasion pathway. Our study characterizes this novel ligand and determines its potential role in reticulocyte invasion by P. vivax merozoites. EBP2 binds preferentially to young (CD71^high) Duffy-positive (Fy⁺) reticulocytes and has minimal binding capacity for Duffy-negative reticulocytes. Importantly, EBP2 is antigenically distinct from DBP and cannot be functionally inhibited by anti-DBP antibodies. Consequently, our results do not support EBP2 as a ligand for invasion of Duffy-negative blood cells, but instead, EBP2 may represent a novel ligand for an alternate invasion pathway of Duffy-positive reticulocytes.

For decades, P. vivax infections in humans have been defined by a unique requirement for the interaction between the Duffy binding protein ligand of the parasite and the Duffy blood group antigen receptor (DARC). Recent reports of P. vivax infections in Duffy-negative individuals challenge this paradigm and suggest an alternate pathway of infection, potentially using the recently discovered EBP2. However, we demonstrate that EBP2 host cell specificity is more restricted than DBP binding and that EBP2 binds preferentially to Duffy-positive, young reticulocytes. This finding indicates that this DBP paralog does mediate a Duffy-independent pathway of infection.