Selection and spread of artemisinin-resistant alleles in Thailand prior to the global artemisinin resistance containment campaign

The recent emergence of artemisinin resistance in the Greater Mekong Subregion poses a major threat to the global effort to control malaria. Tracking the spread and evolution of artemisinin-resistant parasites is critical in aiding efforts to contain the spread of resistance. A total of 417 patient samples from the year 2007, collected during malaria surveillance studies across ten provinces in Thailand, were genotyped for the candidate Plasmodium falciparum molecular marker of artemisinin resistance K13. Parasite genotypes were examined for K13 propeller mutations associated with artemisinin resistance, signatures of positive selection, and for evidence of whether artemisinin-resistant alleles arose independently across Thailand. A total of seven K13 mutant alleles were found (N458Y, R539T, E556D, P574L, R575K, C580Y, S621F). Notably, the R575K and S621F mutations have previously not been reported in Thailand. The most prevalent artemisinin resistance-associated K13 mutation, C580Y, carried two distinct haplotype profiles that were separated based on geography, along the Thai-Cambodia and Thai-Myanmar borders. It appears these two haplotypes may have independent evolutionary origins. In summary, parasites with K13 propeller mutations associated with artemisinin resistance were widely present along the Thai-Cambodia and Thai-Myanmar borders prior to the implementation of the artemisinin resistance containment project in the region.

The Plasmodium falciparum parasites that cause malaria are evolving resistance to our most effective and potent anti-malarial drugs available, called artemisinins. Currently, artemisinin resistance is emerging in a number of countries in the Greater Mekong Subregion, including Cambodia, Thailand, Myanmar, and Vietnam. Historically, the Thai-Cambodia border region has been an epicenter of resistance to several anti-malarial drugs. To prevent the spread of artemisinin resistant parasites from the Greater Mekong Subregion, a global artemisinin resistance project was initiated in 2009. Here, we show that artemisinin resistance associated mutation in the K13 gene were widely present throughout Thailand, as early as 2007, primarily along the Thai-Cambodia and Thai-Myanmar border regions. Additional data based on microsatellite markers suggests that the most commonly found K13 C580Y allele may have two recent independent origins in Thailand, on the borders of Cambodia and Myanmar.

No more monkeying around: primate malaria model systems are key to understanding Plasmodium vivax liver-stage biology, hypnozoites, and relapses

Plasmodium vivax is a human malaria parasite responsible for significant morbidity worldwide and potentially death. This parasite possesses formidable liver-stage biology that involves the formation of dormant parasites known as hypnozoites. Hypnozoites are capable of activating weeks, months, or years after a primary blood-stage infection causing relapsing bouts of illness. Elimination of this dormant parasitic reservoir will be critical for global malaria eradication. Although hypnozoites were first discovered in 1982, few advancements have been made to understand their composition and biology. Until recently, in vitro models did not exist to study these forms and studying them from human ex vivo samples was virtually impossible. Today, non-human primate (NHP) models and modern systems biology approaches are poised as tools to enable the in-depth study of P. vivax liver-stage biology, including hypnozoites and relapses. NHP liver-stage model systems for P. vivax and the related simian malaria species P. cynomolgi are discussed along with perspectives regarding metabolite biomarker discovery, putative roles of extracellular vesicles, and relapse immunobiology.

Pan-Plasmodium band sensitivity for Plasmodium falciparum detection in combination malaria rapid diagnostic tests and implications for clinical management

Background

Malaria rapid diagnostic tests (RDTs) are appropriate for case management, but persistent antigenaemia is a concern for HRP2-detecting RDTs in endemic areas. It has been suggested that pan-pLDH test bands on combination RDTs could be used to distinguish persistent antigenaemia from active Plasmodium falciparum infection, however this assumes all active infections produce positive results on both bands of RDTs, an assertion that has not been demonstrated.

Methods

In this study, data generated during the WHO-FIND product testing programme for malaria RDTs was reviewed to investigate the reactivity of individual test bands against P. falciparum in 18 combination RDTs. Each product was tested against multiple wild-type P. falciparum only samples. Antigen levels were measured by quantitative ELISA for HRP2, pLDH and aldolase.

Results

When tested against P. falciparum samples at 200 parasites/μL, 92% of RDTs were positive; 57% of these on both the P. falciparum and pan bands, while 43% were positive on the P. falciparum band only. There was a relationship between antigen concentration and band positivity; ≥4 ng/mL of HRP2 produced positive results in more than 95% of P. falciparum bands, while ≥45 ng/mL of pLDH was required for at least 90% of pan bands to be positive.

Conclusions

In active P. falciparum infections it is common for combination RDTs to return a positive HRP2 band combined with a negative pan-pLDH band, and when both bands are positive, often the pan band is faint. Thus active infections could be missed if the presence of a HRP2 band in the absence of a pan band is interpreted as being caused solely by persistent antigenaemia.

Comparative analysis of field-isolate and monkey-adapted Plasmodium vivax genomes

Significant insights into the biology of Plasmodium vivax have been gained from the ability to successfully adapt human infections to non-human primates. P. vivax strains grown in monkeys serve as a renewable source of parasites for in vitro and ex vivo experimental studies and functional assays, or for studying in vivo the relapse characteristics, mosquito species compatibilities, drug susceptibility profiles or immune responses towards potential vaccine candidates. Despite the importance of these studies, little is known as to how adaptation to a different host species may influence the genome of P. vivax. In addition, it is unclear whether these monkey-adapted strains consist of a single clonal population of parasites or if they retain the multiclonal complexity commonly observed in field isolates. Here we compare the genome sequences of seven P. vivax strains adapted to New World monkeys with those of six human clinical isolates collected directly in the field. We show that the adaptation of P. vivax parasites to monkey hosts, and their subsequent propagation, did not result in significant modifications of their genome sequence and that these monkey-adapted strains recapitulate the genomic diversity of field isolates. Our analyses also reveal that these strains are not always genetically homogeneous and should be analyzed cautiously. Overall, our study provides a framework to better leverage this important research material and fully utilize this resource for improving our understanding of P. vivax biology.

In this study we compare the genome sequences of Plasmodium vivax collected directly from patients with those of parasites propagated in laboratory monkeys. We show that the adaptation and continuous propagation of Plasmodium vivax in monkeys does not induce systematic changes in the genome and, therefore, that these parasites constitute an unbiased resource for studying this important pathogen. Our analyses also reveal that some monkey-adapted Plasmodium vivax strains are not genetically homogenous and retain multiple genetically different parasites present in the original patient infection. Overall, our study confirms the utility of monkey-adapted Plasmodium vivax strains for malaria research but also shows that this resource should be analyzed cautiously as different samples of the same strain might provide different biological material.

Prevalence of pfhrp2 and pfhrp3 gene deletions in Puerto Lempira, Honduras

Background

Recent studies have demonstrated the deletion of the histidine-rich protein 2 (PfHRP2) gene (pfhrp2) in field isolates of Plasmodium falciparum, which could result in false negative test results when PfHRP2-based rapid diagnostic tests (RDTs) are used for malaria diagnosis. Although primary diagnosis of malaria in Honduras is determined based on microscopy, RDTs may be useful in remote areas. In this study, it was investigated whether there are deletions of the pfhrp2, pfhrp3 and their respective flanking genes in 68 P. falciparum parasite isolates collected from the city of Puerto Lempira, Honduras. In addition, further investigation considered the possible correlation between parasite population structure and the distribution of these gene deletions by genotyping seven neutral microsatellites.

Methods

Sixty-eight samples used in this study, which were obtained from a previous chloroquine efficacy study, were utilized in the analysis. All samples were genotyped for pfhrp2, pfhrp3 and flanking genes by PCR. The samples were then genotyped for seven neutral microsatellites in order to determine the parasite population structure in Puerto Lempira at the time of sample collection.

Results

It was found that all samples were positive for pfhrp2 and its flanking genes on chromosome 8. However, only 50% of the samples were positive for pfhrp3 and its neighboring genes while the rest were either pfhrp3-negative only or had deleted a combination of pfhrp3 and its neighbouring genes on chromosome 13. Population structure analysis predicted that there are at least two distinct parasite population clusters in this sample population. It was also determined that a greater proportion of parasites with pfhrp3-(and flanking gene) deletions belonged to one cluster compared to the other.

Conclusion

The findings indicate that the P. falciparum parasite population in the municipality of Puerto Lempira maintains the pfhrp2 gene and that PfHRP2-based RDTs could be considered for use in this region; however continued monitoring of parasite population will be useful to detect any parasites with deletions of pfhrp2.

PET-PCR method for the molecular detection of malaria parasites in a national malaria surveillance study in Haiti, 2011

Background

Recently, a real-time PCR assay known as photo-induced electron transfer (PET)-PCR which relies on self-quenching primers for the detection of Plasmodium spp. and Plasmodium falciparum was described. PET-PCR assay was found to be robust, and easier to use when compared to currently available real-time PCR methods. The potential of PET-PCR for molecular detection of malaria parasites in a nationwide malaria community survey in Haiti was investigated.

Methods

DNA from the dried blood spots was extracted using QIAGEN methodology. All 2,989 samples were screened using the PET-PCR assay in duplicate. Samples with a cycle threshold (CT) of 40 or less were scored as positive. A subset of the total samples (534) was retested using a nested PCR assay for confirmation. In addition, these same samples were also tested using a TaqMan-based real-time PCR assay.

Results

A total of 12 out of the 2,989 samples screened (0.4%) were found to be positive by PET-PCR (mean CT value of 35.7). These same samples were also found to be positive by the nested and TaqMan-based methods. The nested PCR detected an additional positive sample in a subset of 534 samples that was not detected by either PET-PCR or TaqMan-based PCR method.

Conclusion

While the nested PCR was found to be slightly more sensitive than the PET-PCR, it is not ideal for high throughput screening of samples. Given the ease of use and lower cost than the nested PCR, the PET-PCR provides an alternative assay for the rapid screening of a large number of samples in laboratory settings.

Protective efficacy of a Plasmodium vivax circumsporozoite protein-based vaccine in Aotus nancymaae is associated with antibodies to the repeat region

We have previously reported that Vivax Malaria Protein 001 (VMP001), a vaccine candidate based on the circumsporozoite protein of Plasmodium vivax, is immunogenic in mice and rhesus monkeys in the presence of various adjuvants. In the present study, we evaluated the immunogenicity and efficacy of VMP001 formulated with a TLR9 agonist in a water-in-oil emulsion. Following immunization, the vaccine efficacy was assessed by challenging Aotus nancymaae monkeys with P. vivax sporozoites. Monkeys from both the low- and high-dose vaccine groups generated strong humoral immune responses to the vaccine (peak median titers of 291,622), and its subunits (peak median titers to the N-term, central repeat and C-term regions of 22,188; 66,120 and 179,947, respectively). 66.7% of vaccinated monkeys demonstrated sterile protection following challenge. Protection was associated with antibodies directed against the central repeat region. The protected monkeys had a median anti-repeat titer of 97,841 compared to 14,822 in the non-protected monkeys. This is the first report demonstrating P. vivax CSP vaccine-induced protection of Aotus monkeys challenged with P. vivax sporozoites.

Plasmodium vivax is responsible for causing malaria in large parts of the globe, including regions with temperate climates not suited for the transmission of other Plasmodium species. In addition, P. vivax has the propensity to form dormant forms, known as hypnozoites, that can remain latent for weeks to months and reactive periodically to cause recurrent infections. Prevention of P. vivax malaria, more than any other form, will require a vaccine-based intervention due to limitations in treatment options. To this end, we tested the efficacy in non-human primates, of a vaccine based on circumsporozoite protein, a preerythrocytic stage antigen, of P. vivax. Aotus monkeys were immunized with clinical-grade antigen, combined with two immunomodulators, and then challenged with P. vivax sporozoites. Following challenge 66.7% of monkeys were protected. Analysis of serum samples indicated that protection was associated with antibodies to the central repeat region of the molecule, and that protection was lost upon waning of these antibodies. This is the first report demonstrating that active immunization with a recombinant protein can lead to complete protection in monkeys following sporozoite challenge, while also demonstrating a protective associate. Our data can help serve as a benchmark for down-selection of future vaccine formulations for P. vivax.

Novel Mutation in Cytochrome B of Plasmodium falciparum in One of Two Atovaquone-Proguanil Treatment Failures in Travelers Returning From Same Site in Nigeria

Background

 Atovaquone-proguanil (AP) is the most commonly used treatment for uncomplicated Plasmodium falciparum malaria in the United States. Apparent AP treatment failures were reported 7 months apart in 2 American travelers who stayed in the same compound for foreign workers in Rivers State, Nigeria.

Methods

 We analyzed pretreatment (day 0) and day of failure samples from both travelers for mutations in the P falciparum cytochrome B (pfcytb) and dihydrofolate reductase (pfdhfr) genes associated with resistance to atovaquone and cycloguanil, the active metabolite of proguanil, respectively. We genotyped the parasites and sequenced their mitochondrial genomes.

Results

 On day 0, both travelers had proguanil-resistant genotypes but atovaquone-sensitive cytb sequences. Day of failure samples exhibited mutations in cytb for both travelers. One traveler had the common Y268S mutation, whereas the other traveler had a previously unreported mutation, I258M. The travelers had unrelated parasite genotypes and different mitochondrial genomes.

Conclusions

 Despite the infections likely having been contracted in the same site, there is no evidence that the cases were related. The mutations likely arose independently during the acute infection or treatment. Our results highlight the importance of genotyping parasites and sequencing the full cytb and dhfr genes in AP failures to rule out transmission of AP-resistant strains and identify novel mechanisms of AP resistance.

The evolution and diversity of a low complexity vaccine candidate, merozoite surface protein 9 (MSP-9), in Plasmodium vivax and closely related species

The merozoite surface protein-9 (MSP-9) has been considered a target for an anti-malarial vaccine since it is one of many proteins involved in the erythrocyte invasion, a critical step in the parasite life cycle. Orthologs encoding this antigen have been found in all known species of Plasmodium parasitic to primates. In order to characterize and investigate the extent and maintenance of MSP-9 genetic diversity, we analyzed DNA sequences of the following malaria parasite species: Plasmodium falciparum, Plasmodium reichenowi, Plasmodium chabaudi, Plasmodium yoelii, Plasmodium berghei, Plasmodium coatneyi, Plasmodium gonderi, Plasmodium knowlesi, Plasmodium inui, Plasmodium simiovale, Plasmodium fieldi, Plasmodium cynomolgi and Plasmodium vivax and evaluated the signature of natural selection in all MSP-9 orthologs. Our findings suggest that the gene encoding MSP-9 is under purifying selection in P. vivax and closely related species. We further explored how selection affected different regions of MSP-9 by comparing the polymorphisms in P. vivax and P. falciparum, and found contrasting patterns between these two species that suggest differences in functional constraints. This observation implies that the MSP-9 orthologs in human parasites may interact differently with the host immune response. Thus, studies carried out in one species cannot be directly translated into the other.

Plasmodium vivax: modern strategies to study a persistent parasite's life cycle

Plasmodium vivax has unique attributes to support its survival in varying ecologies and climates. These include hypnozoite forms in the liver, an invasion preference for reticulocytes, caveola-vesicle complex structures in the infected erythrocyte membrane and rapidly forming and circulating gametocytes. These characteristics make this species very different from P. falciparum. Plasmodium cynomolgi and other related simian species have identical biology and can serve as informative models of P. vivax infections. Plasmodium vivax and its model parasites can be grown in non-human primates (NHP), and in short-term ex vivo cultures. For P. vivax, in the absence of in vitro culture systems, these models remain highly relevant side by side with human clinical studies. While post-genomic technologies allow for greater exploration of P. vivax-infected blood samples from humans, these come with restrictions. Two advantages of NHP models are that infections can be experimentally tailored to address hypotheses, including genetic manipulation. Also, systems biology approaches can capitalise on computational biology combined with set experimental infection periods and protocols, which may include multiple sampling times, different types of samples, and the broad use of "omics" technologies. Opportunities for research on vivax malaria are increasing with the use of existing and new methodological strategies in combination with modern technologies.

Historical shifts in Brazilian P. falciparum population structure and drug resistance alleles

Previous work suggests that Brazilian Plasmodium falciparum has limited genetic diversity and a history of bottlenecks, multiple reintroductions due to human migration, and clonal expansions. We hypothesized that Brazilian P. falciparum would exhibit clonal structure. We examined isolates collected across two decades from Amapá, Rondônia, and Pará state (n = 190). By examining more microsatellites markers on more chromosomes than previous studies, we hoped to define the extent of low diversity, linkage disequilibrium, bottlenecks, population structure, and parasite migration within Brazil. We used retrospective genotyping of samples from the 1980s and 1990s to explore the population genetics of SP resistant dhfr and dhps alleles. We tested an existing hypothesis that the triple mutant dhfr mutations 50R/51I/108N and 51I/108N/164L developed in southern Amazon from a single origin of common or similar parasites. We found that Brazilian P. falciparum had limited genetic diversity and isolation by distance was rejected, which suggests it underwent bottlenecks followed by migration between sites. Unlike Peru, there appeared to be gene flow across the Brazilian Amazon basin. We were unable to divide parasite populations by clonal lineages and pairwise FST were common. Most parasite diversity was found within sites in the Brazilian Amazon, according to AMOVA. Our results challenge the hypothesis that triple mutant alleles arose from a single lineage in the Southern Amazon. SP resistance, at both the double and triple mutant stages, developed twice and potentially in different regions of the Brazilian Amazon. We would have required samples from before the 1980s to describe how SP resistance spread across the basin or describe the complex internal migration of Brazilian parasites after the colonization efforts of past decades. The Brazilian Amazon basin may have sufficient internal migration for drug resistance reported in any particular region to rapidly spread to other parts of basin under similar drug pressure.

Real-time loop-mediated isothermal amplification (RealAmp) for the species-specific identification of Plasmodium vivax

Plasmodium vivax infections remain a major source of malaria-related morbidity and mortality. Early and accurate diagnosis is an integral component of effective malaria control programs. Conventional molecular diagnostic methods provide accurate results but are often resource-intensive, expensive, have a long turnaround time and are beyond the capacity of most malaria-endemic countries. Our laboratory has recently developed a new platform called RealAmp, which combines loop-mediated isothermal amplification (LAMP) with a portable tube scanner real-time isothermal instrument for the rapid detection of malaria parasites. Here we describe new primers for the detection of P. vivax using the RealAmp method. Three pairs of amplification primers required for this method were derived from a conserved DNA sequence unique to the P. vivax genome. The amplification was carried out at 64°C using SYBR Green or SYTO-9 intercalating dyes for 90 minutes with the tube scanner set to collect fluorescence signals at 1-minute intervals. Clinical samples of P. vivax and other human-infecting malaria parasite species were used to determine the sensitivity and specificity of the primers by comparing with an 18S ribosomal RNA-based nested PCR as the gold standard. The new set of primers consistently detected laboratory-maintained isolates of P. vivax from different parts of the world. The primers detected P. vivax in the clinical samples with 94.59% sensitivity (95% CI: 87.48-98.26%) and 100% specificity (95% CI: 90.40-100%) compared to the gold standard nested-PCR method. The new primers also proved to be more sensitive than the published species-specific primers specifically developed for the LAMP method in detecting P. vivax.

The malaria parasite Plasmodium vivax exhibits greater genetic diversity than Plasmodium falciparum

We sequenced and annotated the genomes of four P. vivax strains collected from disparate geographic locations, tripling the number of genome sequences available for this understudied parasite and providing the first genome-wide perspective of global variability in this species. We observe approximately twice as much SNP diversity among these isolates as we do among a comparable collection of isolates of P. falciparum, a malaria-causing parasite that results in higher mortality. This indicates a distinct history of global colonization and/or a more stable demographic history for P. vivax relative to P. falciparum, which is thought to have undergone a recent population bottleneck. The SNP diversity, as well as additional microsatellite and gene family variability, suggests a capacity for greater functional variation in the global population of P. vivax. These findings warrant a deeper survey of variation in P. vivax to equip disease interventions targeting the distinctive biology of this neglected but major pathogen.

Plasmodium cynomolgi genome sequences provide insight into Plasmodium vivax and the monkey malaria clade

Plasmodium cynomolgi, a malaria parasite of Asian Old World monkeys, is the sister taxon of Plasmodium vivax, the most prevalent human malaria species outside Africa. Since P. cynomolgi shares many phenotypic, biologic and genetic characteristics of P. vivax, we generated draft genome sequences of three P. cynomolgi strains and performed comparative genomic analysis between them and P. vivax, as well as a third previously sequenced simian parasite, Plasmodium knowlesi. Here we show that genomes of the monkey malaria clade can be characterized by CNVs in multigene families involved in evasion of the human immune system and invasion of host erythrocytes. We identify genome-wide SNPs, microsatellites, and CNVs in the P. cynomolgi genome, providing a map of genetic variation for mapping parasite traits and studying parasite populations. The P. cynomolgi genome is a critical step in developing a model system for P. vivax research, and to counteract the neglect of P. vivax.

Dried Plasmodium falciparum-infected samples as positive controls for malaria rapid diagnostic tests

BACKGROUND

Rapid diagnostic tests (RDTs) are central to fulfilling the WHO's recommendation for parasitologic confirmation of all suspected cases of malaria. RDT performance may be compromised when exposed to the high temperature conditions typical of most malaria endemic regions. However, a systematic method to monitor RDT quality and performance in endemic countries is lacking at the present time. Current methods to monitor RDT performance in the field include comparing results from RDTs to diagnoses made by light microscopy and observing health workers perform tests. These methods are not substitutes for direct quality control. In this study, the suitability of dried Plasmodium falciparum-infected blood as quality control samples for malaria RDTs was evaluated.

METHODS

Three cultured strains of P. falciparum at 200 and 2,000 parasites/μl were tested on 10 brands of RDT. After baseline testing to determine initial reactivity, aliquots of parasite-infected blood were air dried, stored at 35°C, room temperature (~25°C) or 4°C for one, four and 12 weeks and were then tested on the 10 RDTs after rehydration. Extended stability testing of dried blood stored at 4°C was done using P. falciparum strain 3D7 at 1,000 and 2,000 parasites/μl.

RESULTS

All dried blood samples at 2,000 parasites/μl retained reactivity (100% sensitivity) at all three temperatures and time points for all nine RDT brands that detect histidine-rich protein-2 (HRP2). The dried blood samples with 200 parasites/μl were detected by six of the nine HRP2-based RDTs at all storage temperatures and time points. The sensitivity for two of the three remaining HRP2-based RDTs was 100% up to four weeks of storage at all temperatures but dropped to 87.5% at week 12. Of the four RDTs that detect plasmodium lactate dehydrogenase (pLDH) in a pan-specific manner, alone or in combination with HRP2, the detection of pLDH in samples with 2,000 parasites/μL was 100% for two RDTs and 80% for the other two RDTs. The mean level for detection of pLDH at 200 parasites/μl was low (29%), with a range of 0% to100%, which was partly attributable to weak initial baseline reactivity. Reactivity of dried 3D7 at 1,000 and 2,000 parasites/μl stored at 4°C was retained at 100% for up to 52 weeks for both HRP2 and pLDH.

CONCLUSIONS

In the absence of native or recombinant positive control antigens, well-standardized P. falciparum-infected dried blood samples can be used as positive control samples for monitoring RDT performance, particularly with HRP2-detecting tests.

Two functional reticulocyte binding-like (RBL) invasion ligands of zoonotic Plasmodium knowlesi exhibit differential adhesion to monkey and human erythrocytes

BACKGROUND

Plasmodium knowlesi is a monkey malaria species that is becoming a serious public health concern infecting hundreds and perhaps thousands of humans in Southeast Asia. Invasion of erythrocytes by merozoites entails a cascade of molecular interactions. One step involves the adhesion of Plasmodium reticulocyte binding-like (RBL) proteins. Plasmodium knowlesi merozoites express only two RBL invasion ligands, known as Normocyte Binding Proteins (PkNBPXa and PkNBPXb).

METHODS

Overlapping N-terminal regions of PkNBPXa and PkNBPXb were expressed in COS7 cells and tested for surface expression and adhesion to rhesus monkey erythrocytes. Subsequent tests to study specific receptor ligand interactions included adhesion to a panel of human and non-human primate erythrocytes, enzymatic treatment, and site directed mutagenesis.

RESULTS

An N-terminal cysteine-rich region of PkNBPXb (PkNBPXb-II) exhibited specific adhesion to rhesus monkey erythrocytes. Mutation of four of five cysteines in PkNBPXb-II interfered with its surface expression on COS7 cells, suggesting disulphide bond conformation is critical for intracellular trafficking. Binding of PkNBPXb-II was abolished when rhesus erythrocytes were pre-treated with chymotrypsin, but not trypsin or neuraminidase. PkNBPXb-II also bound other Old World monkey species and gibbon erythrocytes. However, erythrocytes from other primate species including humans did not bind to PkNBPXb-II or native PkNBPXb. Importantly, unlike PkNBPXb, PkNBPXa bound human erythrocytes, and this binding was independent of the Duffy blood group determinant.

CONCLUSIONS

The data reported here begins to clarify the functional domains of the P. knowlesi RBLs. A binding domain has been identified and characterized in PkNBPXb. Notably, this study demonstrates that unlike PkNBPXb, PkNBPXa can bind to human erythrocytes, suggesting that PkNBPXa may function as a ligand to enable the invasion of P. knowlesi merozoites into human cells.

Influence of HLA-DRB1 and HLA-DQB1 alleles on IgG antibody response to the P. vivax MSP-1, MSP-3α and MSP-9 in individuals from Brazilian endemic area

BACKGROUND

The antibody response generated during malaria infections is of particular interest, since the production of specific IgG antibodies is required for acquisition of clinical immunity. However, variations in antibody responses could result from genetic polymorphism of the HLA class II genes. Given the increasing focus on the development of subunit vaccines, studies of the influence of class II alleles on the immune response in ethnically diverse populations is important, prior to the implementation of vaccine trials.

METHODS AND FINDINGS

In this study, we evaluated the influence of HLA-DRB1* and -DQB1* allelic groups on the naturally acquired humoral response from Brazilian Amazon individuals (n = 276) against P. vivax Merozoite Surface Protein-1 (MSP-1), MSP-3α and MSP-9 recombinant proteins. Our results provide information concerning these three P. vivax antigens, relevant for their role as immunogenic surface proteins and vaccine candidates. Firstly, the studied population was heterogeneous presenting 13 HLA-DRB1* and 5 DQB1* allelic groups with a higher frequency of HLA-DRB1*04 and HLA-DQB1*03. The proteins studied were broadly immunogenic in a naturally exposed population with high frequency of IgG antibodies against PvMSP1-19 (86.7%), PvMSP-3 (77%) and PvMSP-9 (76%). Moreover, HLA-DRB1*04 and HLA-DQB1*03 alleles were associated with a higher frequency of IgG immune responses against five out of nine antigens tested, while HLA-DRB1*01 was associated with a high frequency of non-responders to repetitive regions of PvMSP-9, and the DRB1*16 allelic group with the low frequency of responders to PvMSP3 full length recombinant protein.

CONCLUSIONS

HLA-DRB1*04 alleles were associated with high frequency of antibody responses to five out of nine recombinant proteins tested in Rondonia State, Brazil. These features could increase the success rate of future clinical trials based on these vaccine candidates.

A 95 kDa protein of Plasmodium vivax and P. cynomolgi visualized by three-dimensional tomography in the caveola-vesicle complexes (Schüffner's dots) of infected erythrocytes is a member of the PHIST family

Plasmodium vivax and P. cynomolgi produce numerous caveola-vesicle complex (CVC) structures within the surface of the infected erythrocyte membrane. These contrast with the electron-dense knob protrusions expressed at the surface of Plasmodium falciparum-infected erythrocytes. Here we investigate the three-dimensional (3-D) structure of the CVCs and the identity of a predominantly expressed 95 kDa CVC protein. Liquid chromatography - tandem mass spectrometry analysis of immunoprecipitates by monoclonal antibodies from P. cynomolgi extracts identified this protein as a member of the Plasmodium helical interspersed subtelomeric (PHIST) superfamily with a calculated mass of 81 kDa. We named the orthologous proteins PvPHIST/CVC-81(95) and PcyPHIST/CVC-81(95) , analysed their structural features, including a PEXEL motif, repeated sequences and a C-terminal PHIST domain, and show that PHIST/CVC-81(95) is most highly expressed in trophozoites. We generated images of CVCs in 3-D using electron tomography (ET), and used immuno-ET to show PHIST/CVC-81(95) localizes to the cytoplasmic side of the CVC tubular extensions. Targeted gene disruptions were attempted in vivo. The pcyphist/cvc-81(95) gene was not disrupted, but parasites containing episomes with the tgdhfr selection cassette were retrieved by selection with pyrimethamine. This suggests that PHIST/CVC-81(95) is essential for survival of these malaria parasites.

Mosquito infection studies with Aotus monkeys and humans infected with the Chesson strain of Plasmodiun vivax

Oocyst counts were compared between mosquitoes that fed on humans versus mosquitoes that fed on Aotus monkeys, both of which were infected with the Chesson strain of Plasmodium vivax. Oocyst counts obtained from mosquitoes fed on humans were almost 10-fold higher in number. Mosquitoes were more likely to be infected and with a higher rate of infection when they fed on monkeys before the peak in the asexual parasite count. Mosquitoes that fed on humans were more likely to be more heavily infected when fed after the peak in the asexual count. Of several species of owl monkeys, Aotus vociferans was infected at a higher frequency. On the basis of oocyst counts, Anopheles dirus were the most susceptible and An. maculatus were the least susceptible of the mosquito species tested.

Naturally acquired antibodies to Plasmodium vivax blood-stage vaccine candidates (PvMSP-1₁₉ and PvMSP-3α₃₅₉₋₇₉₈ and their relationship with hematological features in malaria patients from the Brazilian Amazon

An important step when designing a vaccine is identifying the antigens that function as targets of naturally acquired antibodies. We investigated specific antibody responses against two Plasmodium vivax vaccine candidates, PvMSP-1₁₉ and PvMSP-3α₃₅₉₋₇₉₈. Moreover, we assessed the relationship between these antibodies and morbidity parameters. PvMSP-1₁₉ was the most immunogenic antigen and the frequency of responders to this protein tended to increase in P. vivax patients with higher parasitemia. For both antigens, IgG antibody responses tended to be lower in patients who had experienced their first bout of malaria. Furthermore, anemic patients presented higher IgG antibody responses to PvMSP-3α₃₅₉₋₇₉₈. Since the humoral response involves a number of antibodies acting simultaneously on different targets, we performed a Principal Component Analysis (PCA). Anemic patients had, on average, higher first principal component scores (IgG1/IgG2/IgG3/IgG4 anti-MSP3α), which were negatively correlated with hemoglobin levels. Since antibodies against PfMSP-3 have been strongly associated with clinical protection, we cannot exclude the possibility of a dual role of PvMSP-3 specific antibodies in both immunity and pathogenesis of vivax malaria. Our results confirm the high immunogenicity of the conserved C terminus of PvMSP-1 and points to the considerable immunogenicity of polymorphic PvMSP-3α₃₅₉₋₇₉₈ during natural infection.

Evidence of purifying selection on merozoite surface protein 8 (MSP8) and 10 (MSP10) in Plasmodium spp

Evidence for natural selection, positive or negative, on gene encoding antigens may indicate variation or functional constraints that are immunologically relevant. Most malaria surface antigens with high genetic diversity have been reported to be under positive-diversifying selection. However, antigens with limited genetic variation are usually ignored in terms of the role that natural selection may have in generating such patterns. We investigated orthologous genes encoding two merozoite proteins, MSP8 and MSP10, among several mammalian Plasmodium spp. These antigens, together with MSP1, are among the few MSPs that have two epidermal growth factor-like domains (EGF) at the C-terminal. Those EGF are relatively conserved (low levels of genetic polymorphism) and have been proposed to act as ligands during the invasion of RBCs. We use several evolutionary genetic methods to detect patterns consistent with natural selection acting on MSP8 and MSP10 orthologs in the human parasites Plasmodium falciparum and P. vivax, as well as closely related malarial species found in non-human primates (NHPs). Overall, these antigens have low polymorphism in the human parasites in comparison with the orthologs from other Plasmodium spp. We found that the MSP10 gene polymorphism in P. falciparum only harbor non-synonymous substitutions, a pattern consistent with a gene under positive selection. Evidence of purifying selection was found on the polymorphism observed in both orthologs from P. cynomolgi, a non-human primate parasite closely related to P. vivax, but it was not conclusive in the human parasite. Yet, using phylogenetic base approaches, we found evidence for purifying selection on both MSP8 and MSP10 in the lineage leading to P. vivax. Such antigens evolving under strong functional constraints could become valuable vaccine candidates. We discuss how comparative approaches could allow detecting patterns consistent with negative selection even when there is low polymorphism in the extant populations.

A new single-step PCR assay for the detection of the zoonotic malaria parasite Plasmodium knowlesi

Background

Recent studies in Southeast Asia have demonstrated substantial zoonotic transmission of Plasmodium knowlesi to humans. Microscopically, P. knowlesi exhibits several stage-dependent morphological similarities to P. malariae and P. falciparum. These similarities often lead to misdiagnosis of P. knowlesi as either P. malariae or P. falciparum and PCR-based molecular diagnostic tests are required to accurately detect P. knowlesi in humans. The most commonly used PCR test has been found to give false positive results, especially with a proportion of P. vivax isolates. To address the need for more sensitive and specific diagnostic tests for the accurate diagnosis of P. knowlesi, we report development of a new single-step PCR assay that uses novel genomic targets to accurately detect this infection.

Methodology and Significant Findings

We have developed a bioinformatics approach to search the available malaria parasite genome database for the identification of suitable DNA sequences relevant for molecular diagnostic tests. Using this approach, we have identified multi-copy DNA sequences distributed in the P. knowlesi genome. We designed and tested several novel primers specific to new target sequences in a single-tube, non-nested PCR assay and identified one set of primers that accurately detects P. knowlesi. We show that this primer set has 100% specificity for the detection of P. knowlesi using three different strains (Nuri, H, and Hackeri), and one human case of malaria caused by P. knowlesi. This test did not show cross reactivity with any of the four human malaria parasite species including 11 different strains of P. vivax as well as 5 additional species of simian malaria parasites.

Conclusions

The new PCR assay based on novel P. knowlesi genomic sequence targets was able to accurately detect P. knowlesi. Additional laboratory and field-based testing of this assay will be necessary to further validate its utility for clinical diagnosis of P. knowlesi.

Genetic backgrounds of the Plasmodium falciparum chloroquine resistant transporter (pfcrt) alleles in Pakistan

Chloroquine (CQ) resistance in Plasmodium falciparum has been associated with point mutations in the P. falciparum CQ resistance transporter gene (pfcrt). Previous studies have shown 4-5 independent origins for CQ resistant pfcrt alleles globally, two in South America, one each in Southeast Asia, Papua New Guinea (PNG) and Philippines. In Asia, at least two different alleles corresponding to amino acids 72-76 (CVIET and SVMNT) have been found. The CVIET allele originated in Southeast Asia and then spread to Asia and Africa as well. The SVMNT allele, originating from PNG, has been found in India. This study was undertaken to investigate the genetic background of the CQ resistant pfcrt haplotypes in Pakistan. We genotyped microsatellite markers surrounding the pfcrt gene (six different markers at -12.3, -4.8, -1, 1.5, 3.9, 18.8 kb) in 114 clinical isolates of P. falciparum collected from different regions in Pakistan. Microsatellite analysis showed a significant reduction in genetic variation among the mutant SVMNT pfcrt alleles when compared to wild type alleles. The predominant SVMNT haplotype found in this study shared the same microsatellite haplotype found in both PNG and India. Two isolates with CVIET haplotypes showed similar microsatellite background to those found in Africa and Asia. In conclusion, this study suggests that CQ resistant SVMNT haplotypes in India and Pakistan have a common ancestral origin similar to that of Papua New Guinean isolates.