The Duffy binding protein as a key target for a Plasmodium vivax vaccine: lessons from the Brazilian Amazon

High-throughput genotyping of Plasmodium vivax in the Peruvian Amazon via molecular inversion probes

Plasmodium vivax transmission occurs throughout the tropics and is an emerging threat in areas of Plasmodium falciparum decline, causing relapse infections that complicate treatment and control. Targeted sequencing for P. falciparum has been widely deployed to detect population structure and the geographic spread of antimalarial and diagnostic resistance. However, there are fewer such tools for P. vivax. Leveraging global variation data, we designed four molecular inversion probe (MIP) genotyping panels targeting geographically differentiating SNPs, neutral SNPs, putative antimalarial resistance genes, and vaccine candidate genes. We deployed these MIP panels on 866 infections from the Peruvian Amazon and identified transmission networks with clonality (IBD[identity by descent]>0.99), copy number variation in Pvdbp and multiple Pvrbps, mutations in antimalarial resistance orthologs, and balancing selection in 13 vaccine candidate genes. Our MIP panels are the broadest genotyping panel currently available and are poised for successful deployment in other regions of P. vivax transmission.

High-Throughput Genotyping of Plasmodium vivax in the Peruvian Amazon via Molecular Inversion Probes

Plasmodium vivax transmission occurs throughout the tropics and is an emerging threat in areas of Plasmodium falciparum decline, causing relapse infections that complicate treatment and control. Targeted sequencing for P. falciparum has been widely deployed to detect population structure and the geographic spread of antimalarial and diagnostic resistance. However, there are fewer such tools for P. vivax . Leveraging global variation data, we designed four molecular inversion probe (MIP) genotyping panels targeting geographically differentiating SNPs, neutral SNPs, putative antimalarial resistance genes, and vaccine candidate genes. We deployed these MIP panels on 866 infections from the Peruvian Amazon and identified transmission networks with clonality (IBD>0.99), copy number variation in Pvdbp and multiple Pvrbps , fixation of putative antimalarial resistance, and balancing selection in 13 vaccine candidate genes. Our MIP panels are the broadest genotyping panel currently available and are poised for successful deployment in other regions of P. vivax transmission.

Genetic differentiation of Plasmodium vivax duffy binding protein in Ethiopia and comparison with other geographical isolates

BACKGROUND

Plasmodium vivax Duffy binding protein (PvDBP) is a merozoite surface protein located in the micronemes of P. vivax. The invasion of human reticulocytes by P. vivax merozoites depends on the parasite DBP binding domain engaging Duffy Antigen Receptor for Chemokine (DARC) on these red blood cells (RBCs). PvDBPII shows high genetic diversity which is a major challenge to its use in the development of a vaccine against vivax malaria.

METHODS

A cross-sectional study was conducted from February 2021 to September 2022 in five study sites across Ethiopia. A total of 58 blood samples confirmed positive for P. vivax by polymerase chain reaction (PCR) were included in the study to determine PvDBPII genetic diversity. PvDBPII were amplified using primers designed from reference sequence of P. vivax Sal I strain. Assembling of sequences was done using Geneious Prime version 2023.2.1. Alignment and phylogenetic tree constructions using MEGA version 10.1.1. Nucleotide diversity and haplotype diversity were analysed using DnaSP version 6.12.03, and haplotype network was generated with PopART version 1.7.

RESULTS

The mean age of the participants was 25 years, 5 (8.6%) participants were Duffy negatives. From the 58 PvDBPII sequences, seven haplotypes based on nucleotide differences at 8 positions were identified. Nucleotide diversity and haplotype diversity were 0.00267 ± 0.00023 and 0.731 ± 0.036, respectively. Among the five study sites, the highest numbers of haplotypes were identified in Arbaminch with six different haplotypes while only two haplotypes were identified in Gambella. The phylogenetic tree based on PvDBPII revealed that parasites of different study sites shared similar genetic clusters with few exceptions. Globally, a total of 39 haplotypes were identified from 223 PvDBPII sequences representing different geographical isolates obtained from NCBI archive. The nucleotide and haplotype diversity were 0.00373 and 0.845 ± 0.015, respectively. The haplotype prevalence ranged from 0.45% to 27.3%. Two haplotypes were shared among isolates from all geographical areas of the globe.

CONCLUSIONS

PvDBPII of the Ethiopian P. vivax isolates showed low nucleotide but high haplotype diversity, this pattern of genetic variability suggests that the population may have undergone a recent expansion. Among the Ethiopian P. vivax isolates, almost half of the sequences were identical to the Sal-I reference sequence. However, there were unique haplotypes observed in the Ethiopian isolates, which does not share with isolates from other geographical areas. There were two haplotypes that were common among populations across the globe. Categorizing population haplotype frequency can help to determine common haplotypes for designing an effective blood-stage vaccine which will have a significant role for the control and elimination of P. vivax.

Human monoclonal antibodies inhibit invasion of transgenic Plasmodium knowlesi expressing Plasmodium vivax Duffy binding protein

BACKGROUND

Plasmodium vivax has been more resistant to various control measures than Plasmodium falciparum malaria because of its greater transmissibility and ability to produce latent parasite forms. Therefore, developing P. vivax vaccines and therapeutic monoclonal antibodies (humAbs) remains a high priority. The Duffy antigen receptor for chemokines (DARC) expressed on erythrocytes is central to P. vivax invasion of reticulocytes. P. vivax expresses a Duffy binding protein (PvDBP) on merozoites, a DARC ligand, and the DARC: PvDBP interaction is critical for P. vivax blood stage malaria. Therefore, PvDBP is a leading vaccine candidate for P. vivax and a target for therapeutic human monoclonal antibodies (humAbs).

METHODS

Here, the functional activity of humAbs derived from naturally exposed and vaccinated individuals are compared for the first time using easily cultured Plasmodium knowlesi (P. knowlesi) that had been genetically modified to replace its endogenous PkDBP orthologue with PvDBP to create a transgenic parasite, PkPvDBPOR. This transgenic parasite requires DARC to invade human erythrocytes but is not reticulocyte restricted. This model was used to evaluate the invasion inhibition potential of 12 humAbs (9 naturally acquired; 3 vaccine-induced) targeting PvDBP individually and in combinations using growth inhibition assays (GIAs).

RESULTS

The PvDBP-specific humAbs demonstrated 70-100% inhibition of PkPvDBPOR invasion with the IC50 values ranging from 51 to 338 µg/mL for the 9 naturally acquired (NA) humAbs and 33 to 99 µg/ml for the 3 vaccine-induced (VI) humAbs. To evaluate antagonistic, additive, or synergistic effects, six pairwise combinations were performed using select humAbs. Of these combinations tested, one NA/NA (099100/094083) combination demonstrated relatively strong additive inhibition between 10 and 100 µg/mL; all combinations of NA and VI humAbs showed additive inhibition at concentrations below 25 µg/mL and antagonism at higher concentrations. None of the humAb combinations showed synergy. Invasion inhibition efficacy by some mAbs shown with PkPvDBPOR was closely replicated using P. vivax clinical isolates.

CONCLUSION

The PkPvDBPOR transgenic model is a robust surrogate of P. vivax to assess invasion and growth inhibition of human monoclonal Abs recognizing PvDBP individually and in combination. There was no synergistic interaction for growth inhibition with the humAbs tested here that target different epitopes or subdomains of PvDBP, suggesting little benefit in clinical trials using combinations of these humAbs.

Host genetic backgrounds: the key to determining parasite-host adaptation

Parasitic diseases pose a significant threat to global public health, particularly in developing countries. Host genetic factors play a crucial role in determining susceptibility and resistance to infection. Recent advances in molecular and biological technologies have enabled significant breakthroughs in understanding the impact of host genes on parasite adaptation. In this comprehensive review, we analyze the host genetic factors that influence parasite adaptation, including hormones, nitric oxide, immune cells, cytokine gene polymorphisms, parasite-specific receptors, and metabolites. We also establish an interactive network to better illustrate the complex relationship between host genetic factors and parasite-host adaptation. Additionally, we discuss future directions and collaborative research priorities in the parasite-host adaptation field, including investigating the impact of host genes on the microbiome, developing more sophisticated models, identifying and characterizing parasite-specific receptors, utilizing patient-derived sera as diagnostic and therapeutic tools, and developing novel treatments and management strategies targeting specific host genetic factors. This review highlights the need for a comprehensive and systematic approach to investigating the underlying mechanisms of parasite-host adaptation, which requires interdisciplinary collaborations among biologists, geneticists, immunologists, and clinicians. By deepening our understanding of the complex interactions between host genetics and parasite adaptation, we can develop more effective and targeted interventions to prevent and treat parasitic diseases. Overall, this review provides a valuable resource for researchers and clinicians working in the parasitology field and offers insights into the future directions of this critical research area.

Glycan masking in vaccine design: Targets, immunogens and applications

Glycan masking is a novel technique in reverse vaccinology in which sugar chains (glycans) are added on the surface of immunogen candidates to hide regions of low interest and thus focus the immune system on highly therapeutic epitopes. This shielding strategy is inspired by viruses such as influenza and HIV, which are able to escape the immune system by incorporating additional glycosylation and preventing the binding of therapeutic antibodies. Interestingly, the glycan masking technique is mainly used in vaccine design to fight the same viruses that naturally use glycans to evade the immune system. In this review we report the major successes obtained with the glycan masking technique in epitope-focused vaccine design. We focus on the choice of the target antigen, the strategy for immunogen design and the relevance of the carrier vector to induce a strong immune response. Moreover, we will elucidate the different applications that can be accomplished with glycan masking, such as shifting the immune response from hyper-variable epitopes to more conserved ones, focusing the response on known therapeutic epitopes, broadening the response to different viral strains/sub-types and altering the antigen immunogenicity to elicit higher or lower immune response, as desired.

Naturally-acquired and Vaccine-induced Human Monoclonal Antibodies to Plasmodium vivax Duffy Binding Protein Inhibit Invasion of Plasmodium knowlesi (PvDBPOR) Transgenic Parasites

The Duffy antigen receptor for chemokines (DARC) expressed on erythrocytes is central to Plasmodium vivax (Pv) invasion of reticulocytes. Pv expresses a Duffy binding protein (PvDBP) on merozoites, a DARC ligand, and their protein-protein interaction is central to vivax blood stage malaria. Here we compared the functional activity of humAbs derived from naturally exposed and vaccinated individuals for the first time using easily cultured P. knowlesi (Pk) that had been genetically modified to replace its endogenous PkDBP orthologue with PvDBP to create a transgenic parasite, PkPvDBPOR. This transgenic parasite requires DARC to invade human erythrocytes but is not reticulocyte restricted. Using this model, we evaluated the invasion inhibition potential of 12 humAbs (9 naturally acquired; 3 vaccine-induced) targeting PvDBP individually and in combinations using growth inhibition assays (GIAs). The PvDBP-specific humAbs demonstrated 70-100% inhibition of PkPvDBPOR invasion with the IC50 values ranging from 51 to 338 μg/mL for the 9 naturally acquired (NA) humAbs and 33 to 99 μg/ml for the 3 vaccine-induced (VI) humAbs. To evaluate antagonistic, additive, or synergistic effects, six pairwise combinations were performed using select humAbs. Of these combinations tested, one NA/NA (099100/094083) combination demonstrated relatively strong additive inhibition between 10-100 μg/mL; all combinations of NA and VI humAbs showed additive inhibition at concentrations below 25 μg/mL and antagonism at higher concentrations. None of the humAb combinations showed synergy. This PkPvDBPOR model system enables efficient assessment of NA and VI humAbs individually and in combination.

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.

Variable immunogenicity of a vivax malaria blood-stage vaccine candidate

Relapsing malaria caused by Plasmodium vivax is a neglected tropical disease and an important cause of malaria worldwide. Vaccines to prevent clinical disease and mosquito transmission of vivax malaria are needed to overcome the distinct challenges of this important public health problem. In this vaccine immunogenicity study in mice, we examined key variables of responses to a P. vivax Duffy binding protein vaccine, a leading candidate to prevent the disease-causing blood-stages. Significant sex-dependent differences were observed in B cell (CD80+) and T cell (CD8+) central memory subsets, resulting in significant differences in functional immunogenicity and durability of anti-DBP protective efficacy. These significant sex-dependent differences in inbred mice were in the CD73+CD80+ memory B cell, H2KhiCD38hi/lo, and effector memory subsets. This study highlights sex and immune genes as critical variables that can impact host responses to P. vivax antigens and must be taken into consideration when designing clinical vaccine studies.

Global distribution of single amino acid polymorphisms in Plasmodium vivax Duffy-binding-like domain and implications for vaccine development efforts

Plasmodium vivax (Pv) malaria continues to be geographically widespread with approximately 15 million worldwide cases annually. Along with other proteins, Duffy-binding proteins (DBPs) are used by plasmodium for RBC invasion and the parasite-encoded receptor binding regions lie in their Duffy-binding-like (DBL) domains-thus making it a prime vaccine candidate. This study explores the sequence diversity in PvDBL globally, with an emphasis on India as it remains a major contributor to the global Pv malaria burden. Based on 1358 PvDBL protein sequences available in NCBI, we identified 140 polymorphic sites within 315 residues of PvDBL. Alarmingly, country-wise mapping of SAAPs from field isolates revealed varied and distinct polymorphic profiles for different nations. We report here 31 polymorphic residue positions in the global SAAP profile, most of which map to the PvDBL subdomain 2 (α1-α6). A distinct clustering of SAAPs distal to the DARC-binding sites is indicative of immune evasive strategies by the parasite. Analyses of PvDBL-neutralizing antibody complexes revealed that between 24% and 54% of interface residues are polymorphic. This work provides a framework to recce and expand the polymorphic space coverage in PvDBLs as this has direct implications for vaccine development studies. It also emphasizes the significance of surveying global SAAP distributions before or alongside the identification of vaccine candidates.

Extensive genetic diversity of Plasmodium vivax dbp-II in Rio de Janeiro Atlantic Forest and Brazilian Amazon Basin: evidence of positive selection

Background

Plasmodium vivax is the most widespread human malaria parasite outside Africa and is the predominant parasite in the Americas. Increasing reports of P. vivax disease severity, together with the emergence of drug-resistant strains, underscore the urgency of the development of vaccines against P. vivax. Polymorphisms on DBP-II-gene could act as an immune evasion mechanism and, consequently, limited the vaccine efficacy. This study aimed to investigate the pvdbp-II genetic diversity in two Brazilian regions with different epidemiological patterns: the unstable transmission area in the Atlantic Forest (AF) of Rio de Janeiro and; the fixed malaria-endemic area in Brazilian Amazon (BA).

Methods

216 Brazilian P. vivax infected blood samples, diagnosed by microscopic examination and PCR, were investigated. The region flanking pvdbp-II was amplified by PCR and sequenced. Genetic polymorphisms of pvdbp-II were estimated based on the number of segregating sites and nucleotide and haplotype diversities; the degree of differentiation between-regions was evaluated applying Wright’s statistics. Natural selection was calculated using the rate of nonsynonymous per synonymous substitutions with the Z-test, and the evolutionary distance was estimated based on the reconstructed tree.

Results

79 samples from AF and 137 from BA were successfully sequenced. The analyses showed 28 polymorphic sites distributed in 21 codons, with only 5% of the samples Salvador 1 type. The highest rates of polymorphic sites were found in B- and T cell epitopes. Unexpectedly, the nucleotide diversity in pvdbp-II was higher in AF (0.01) than in BA (0.008). Among the 28 SNPs detected, 18 are shared between P. vivax isolates from AF and BA regions, but 8 SNPs were exclusively detected in AF—I322S, K371N, E385Q, E385T, K386T, K411N, I419L and I419R—and 2 (N375D and I419M) arose exclusively in BA. These findings could suggest the potential of these geographical clusters as population-specific-signatures that may be useful to track the origin of infections. The sample size should be increased in order to confirm this possibility.

Conclusions

The results highlight that the pvdbp-II polymorphisms are positively selected by host’s immune pressure. The characterization of pvdbp-II polymorphisms might be useful for designing effective DBP-II-based vaccines.

Cross-Species Immune Recognition Between Plasmodium vivax Duffy Binding Protein Antibodies and the Plasmodium falciparum Surface Antigen VAR2CSA

Background

In pregnancy, Plasmodium falciparum parasites express the surface antigen VAR2CSA, which mediates adherence of red blood cells to chondroitin sulfate A (CSA) in the placenta. VAR2CSA antibodies are generally acquired during infection in pregnancy and are associated with protection from placental malaria. We observed previously that men and children in Colombia also had antibodies to VAR2CSA, but the origin of these antibodies was unknown. Here, we tested whether infection with Plasmodium vivax is an alternative mechanism of acquisition of VAR2CSA antibodies.

Methods

We analyzed sera from nonpregnant Colombians and Brazilians exposed to P. vivax and monoclonal antibodies raised against P. vivax Duffy binding protein (PvDBP). Cross-reactivity to VAR2CSA was characterized by enzyme-linked immunosorbent assay, immunofluorescence assay, and flow cytometry, and antibodies were tested for inhibition of parasite binding to CSA.

Results

Over 50% of individuals had antibodies that recognized VAR2CSA. Affinity-purified PvDBP human antibodies and a PvDBP monoclonal antibody recognized VAR2CSA, showing that PvDBP can give rise to cross-reactive antibodies. Importantly, the monoclonal antibody inhibited parasite binding to CSA, which is the primary in vitro correlate of protection from placental malaria.

Conclusions

These data suggest that PvDBP induces antibodies that functionally recognize VAR2CSA, revealing a novel mechanism of cross-species immune recognition to falciparum malaria.

Identification and Characterization of Functional Human Monoclonal Antibodies to Plasmodium vivax Duffy-Binding Protein

Plasmodium vivax invasion of reticulocytes relies on distinct receptor-ligand interactions between the parasite and host erythrocytes. Engagement of the highly polymorphic domain II of the P. vivax Duffy-binding protein (DBPII) with the erythrocyte's Duffy Ag receptor for chemokines (DARC) is essential. Some P. vivax-exposed individuals acquired Abs to DBPII that block DBPII-DARC interaction and inhibit P. vivax reticulocyte invasion, and Ab levels correlate with protection against P. vivax malaria. To better understand the functional characteristics and fine specificity of protective human Abs to DBPII, we sorted single DBPII-specific IgG⁺ memory B cells from three individuals with high blocking activity to DBPII. We identified 12 DBPII-specific human mAbs from distinct lineages that blocked DBPII-DARC binding. All mAbs were P. vivax strain transcending and targeted known binding motifs of DBPII with DARC. Eleven mAbs competed with each other for binding, indicating recognition of the same or overlapping epitopes. Naturally acquired blocking Abs to DBPII from individuals with high levels residing in different P. vivax-endemic areas worldwide competed with mAbs, suggesting broadly shared recognition sites. We also found that mAbs inhibited P. vivax entry into reticulocytes in vitro. These findings suggest that IgG⁺ memory B cell activity in individuals with P. vivax strain-transcending Abs to DBPII display a limited clonal response with inhibitory blocking directed against a distinct region of the molecule.

Blood-stage Plasmodium vivax antibody dynamics in a low transmission setting: A nine year follow-up study in the Amazon region

Plasmodium vivax remains a global health problem and its ability to cause relapses and subpatent infections challenge control and elimination strategies. Even in low malaria transmission settings, such as the Amazon basin, where progress in malaria control has caused a remarkable reduction in case incidence, a recent increase in P. vivax transmission demonstrates the continued vulnerability of P.vivax-exposed populations. As part of a search for complementary approaches to P.vivax surveillance in areas in which adults are the majority of the exposed-population, here we evaluated the potential of serological markers covering a wide range of immunogenicity to estimate malaria transmission trends. For this, antibodies against leading P. vivax blood-stage vaccine candidates were assessed during a 9 year follow-up study among adults exposed to unstable malaria transmission in the Amazon rainforest. Circulating antibody levels against immunogenic P. vivax proteins, such as the Apical Membrane Antigen-1, were a sensitive measure of recent P. vivax exposure, while antibodies against less immunogenic proteins were indicative of naturally-acquired immunity, including the novel engineered Duffy binding protein II immunogen (DEKnull-2). Our results suggest that the robustness of serology to estimate trends in P.vivax malaria transmission will depend on the immunological background of the study population, and that for adult populations exposed to unstable P.vivax malaria transmission, the local heterogeneity of antibody responses should be considered when considering use of serological surveillance.

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

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

The Presence, Persistence and Functional Properties of Plasmodium vivax Duffy Binding Protein II Antibodies Are Influenced by HLA Class II Allelic Variants

Background

The human malaria parasite Plasmodium vivax infects red blood cells through a key pathway that requires interaction between Duffy binding protein II (DBPII) and its receptor on reticulocytes, the Duffy antigen/receptor for chemokines (DARC). A high proportion of P. vivax-exposed individuals fail to develop antibodies that inhibit DBPII-DARC interaction, and genetic factors that modulate this humoral immune response are poorly characterized. Here, we investigate if DBPII responsiveness could be HLA class II-linked.

Methodology/Principal Findings

A community-based open cohort study was carried out in an agricultural settlement of the Brazilian Amazon, in which 336 unrelated volunteers were genotyped for HLA class II (DRB1, DQA1 and DQB1 loci), and their DBPII immune responses were monitored over time (baseline, 6 and 12 months) by conventional serology (DBPII IgG ELISA-detected) and functional assays (inhibition of DBPII–erythrocyte binding). The results demonstrated an increased susceptibility of the DRB1*13:01 carriers to develop and sustain an anti-DBPII IgG response, while individuals with the haplotype DRB1*14:02-DQA1*05:03-DQB1*03:01 were persistent non-responders. HLA class II gene polymorphisms also influenced the functional properties of DBPII antibodies (BIAbs, binding inhibitory antibodies), with three alleles (DRB1*07:01, DQA1*02:01 and DQB1*02:02) comprising a single haplotype linked with the presence and persistence of the BIAbs response. Modelling the structural effects of the HLA-DRB1 variants revealed a number of differences in the peptide-binding groove, which is likely to lead to altered antigen binding and presentation profiles, and hence may explain the differences in subject responses.

Conclusions/Significance

The current study confirms the heritability of the DBPII antibody response, with genetic variation in HLA class II genes influencing both the development and persistence of IgG antibody responses. Cellular studies to increase knowledge of the binding affinities of DBPII peptides for class II molecules linked with good or poor antibody responses might lead to the development of strategies for controlling the type of helper T cells activated in response to DBPII.

Vaccines are a crucial component of the current efforts to eliminate malaria, and much of the vaccine-related research on P. vivax has been focused on the Duffy binding protein II (DBPII), a ligand for human blood stage infection. A high proportion of individuals who are naturally exposed to P. vivax fail to develop neutralizing antibodies, but the host genetic factors modulating this immune response are poorly characterized. We investigated whether DBPII responsiveness was dependent on the variability of human leucocyte antigen (HLA) class II cell surface proteins involved in the regulation of immune responses. To obtain a reliable estimate of DBPII antibodies, we carried out a longitudinal study, collecting serum from the same individuals over a period of 12-months. The results confirmed the heritability of the DBPII immune response, with genetic variation in HLA class II genes influencing both the development and persistence of the antibody response. HLA class II genotype also influenced the ability of DBPII antibodies to block the ligand-receptor interaction in vitro. Computational approaches identified structural specificity between HLA variants, which we propose as an explanation for differences between a good or poor antibody responder. These results may have implications for vaccine development, and might lead to strategies for controlling the type of immune response activated in response to DBPII.

Analysis of Pvama1 genes from China-Myanmar border reveals little regional genetic differentiation of Plasmodium vivax populations

Background

With the premise of diminishing parasite genetic diversity following the reduction of malaria incidence, the analysis of polymorphic antigenic markers may provide important information about the impact of malaria control on local parasite populations. Here we evaluated the genetic diversity of Plasmodium vivax apical membrane antigen 1 (Pvama1) gene in a parasite population from the China-Myanmar border and compared it with global P. vivax populations.

Methods

We performed evolutionary analysis to examine the genetic diversity, natural selection, and population differentiation of 73 Pvama1 sequences acquired from the China-Myanmar border as well as 615 publically available Pvama1 sequences from seven global P. vivax populations.

Results

A total of 308 Pvama1 haplotypes were identified among the global P. vivax isolates. The overall nucleotide diversity of Pvama1 gene among the 73 China-Myanmar border parasite isolates was 0.008 with 41 haplotypes being identified (Hd = 0.958). Domain I (DI) harbored the majority (26/33) of the polymorphic sites. The McDonald Kreitman test showed a significant positive selection across the ectodomain and the DI of Pvama1. The fixation index (F_ST) estimation between the China-Myanmar border, Thailand (0.01) and Myanmar (0.10) showed only slight geographical genetic differentiation. Notably, the Sal-I haplotype was not detected in any of the analyzed global isolates, whereas the Belem strain was restricted to the Thai population. The detected mutations are mapped outside the overlapped region of the predicted B-cell epitopes and intrinsically unstructured/disordered regions.

Conclusions

This study revealed high levels of genetic diversity of Pvama1 in the P. vivax parasite population from the China-Myanmar border with DI displaying stronger diversifying selection than other domains. There were low levels of population subdivision among parasite populations from the Greater Mekong Subregion.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-016-1899-1) contains supplementary material, which is available to authorized users.

Plasmodium simium, a Plasmodium vivax-related malaria parasite: genetic variability of Duffy binding protein II and the Duffy antigen/receptor for chemokines

Plasmodium simium is a parasite from New World monkeys that is most closely related to the human malaria parasite Plasmodium vivax; it also naturally infects humans. The blood-stage infection of P. vivax depends on Duffy binding protein II (PvDBPII) and its cognate receptor on erythrocytes, the Duffy antigen receptor for chemokines (hDARC), but there is no information on the P. simium erythrocytic invasion pathway. The genes encoding P. simium DBP (PsDBP_II) and simian DARC (sDARC) were sequenced from Southern brown howler monkeys (Alouatta guariba clamitans) naturally infected with P. simium because P. simium may also depend on the DBPII/DARC interaction. The sequences of DBP binding domains from P. vivax and P. simium were highly similar. However, the genetic variability of PsDBPII was lower than that of PvDBPII. Phylogenetic analyses demonstrated that these genes were strictly related and clustered in the same clade of the evolutionary tree. DARC from A. clamitans was also sequenced and contained three new non-synonymous substitutions. None of these substitutions were located in the N-terminal domain of DARC, which interacts directly with DBPII. The interaction between sDARC and PvDBPII was evaluated using a cytoadherence assay of COS7 cells expressing PvDBPII on their surfaces. Inhibitory binding assays in vitro demonstrated that antibodies from monkey sera blocked the interaction between COS-7 cells expressing PvDBPII and hDARC-positive erythrocytes. Taken together, phylogenetic analyses reinforced the hypothesis that the host switch from humans to monkeys may have occurred very recently in evolution, which sheds light on the evolutionary history of new world plasmodia. Further invasion studies would confirm whether P. simium depends on DBP/DARC to trigger internalization into red blood cells.

Distinct genetic difference between the Duffy binding protein (PkDBPαII) of Plasmodium knowlesi clinical isolates from North Borneo and Peninsular Malaysia

BACKGROUND

Plasmodium knowlesi is one of the monkey malaria parasites that can cause human malaria. The Duffy binding protein of P. knowlesi (PkDBPαII) is essential for the parasite's invasion into human and monkey erythrocytes. A previous study on P. knowlesi clinical isolates from Peninsular Malaysia reported high level of genetic diversity in the PkDBPαII. Furthermore, 36 amino acid haplotypes were identified and these haplotypes could be separated into allele group I and allele group II. In the present study, the PkDBPαII of clinical isolates from the Malaysian states of Sarawak and Sabah in North Borneo was investigated, and compared with the PkDBPαII of Peninsular Malaysia isolates.

METHODS

Blood samples from 28 knowlesi malaria patients were used. These samples were collected between 2011 and 2013 from hospitals in North Borneo. The PkDBPαII region of the isolates was amplified by PCR, cloned into Escherichia coli, and sequenced. The genetic diversity, natural selection and phylogenetics of PkDBPαII haplotypes were analysed using MEGA5 and DnaSP ver. 5.10.00 programmes.

RESULTS

Forty-nine PkDBPαII sequences were obtained. Comparison at the nucleotide level against P. knowlesi strain H as reference sequence revealed 58 synonymous and 102 non-synonymous mutations. Analysis on these mutations showed that PkDBPαII was under purifying (negative) selection. At the amino acid level, 38 different PkDBPαII haplotypes were identified. Twelve of the 28 blood samples had mixed haplotype infections. Phylogenetic analysis revealed that all the haplotypes were in allele group I, but they formed a sub-group that was distinct from those of Peninsular Malaysia. Wright's FST fixation index indicated high genetic differentiation between the North Borneo and Peninsular Malaysia haplotypes.

CONCLUSIONS

This study is the first to report the genetic diversity and natural selection of PkDBPαII of P. knowlesi from Borneo Island. The PkDBPαII haplotypes found in this study were distinct from those from Peninsular Malaysia. This difference may not be attributed to geographical separation because other genetic markers studied thus far such as the P. knowlesi circumsporozoite protein gene and small subunit ribosomal RNA do not display such differentiation. Immune evasion may possibly be the reason for the differentiation.