Effective and cheap removal of leukocytes and platelets from Plasmodium vivax infected blood

Pyrimidine Azepine Targets the Plasmodium bc 1 Complex and Displays Multistage Antimalarial Activity

Malaria control and elimination efforts would benefit from the identification and validation of new malaria chemotherapeutics. Recently, a transgenic Plasmodium berghei line was used to perform a series of high-throughput in vitro screens for new antimalarials acting against the parasite sexual stages. The screens identified pyrimidine azepine chemotypes with potent activity. Here, we validate the activity of PyAz90, the most potent pyrimidine azepine chemotype identified, against P. falciparum and P. vivax in the asexual and sexual stages. PyAz90 blocked parasite transmission to the mosquito vector at nanomolar concentrations and inhibited in vitro asexual parasite multiplication with a fast-action profile. Through the generation of P. falciparum PyAz90-resistant parasites and in vitro assays of mitochondrial activity, we identified cytochrome b as a molecular target of PyAz90. This work characterizes a promising chemotype that can be explored for the future development of new antimalarials targeting the Plasmodium cytochrome bc 1 complex.

The C-terminal region of the Plasmodium berghei gamete surface 184-kDa protein Pb184 contributes to fertilization and male gamete binding to the residual body

BACKGROUND

Malaria, a global health concern, is caused by parasites of the Plasmodium genus, which undergo gametogenesis in the midgut of mosquitoes after ingestion of an infected blood meal. The resulting male and female gametes fuse to form a zygote, which differentiates into a motile ookinete. After traversing the midgut epithelium, the ookinete differentiates into an oocyst on the basal side of the epithelium.

METHODS

Membrane proteins with increased gene expression levels from the gamete to oocyst stages in P. berghei were investigated utilizing PlasmoDB, the functional genomic database for Plasmodium spp. Based on this analysis, we selected the 184-kDa membrane protein, Pb184, for further study. The expression of Pb184 was further confirmed through immunofluorescence staining, following which we examined whether Pb184 is involved in fertilization using antibodies targeting the C-terminal region of Pb184 and biotin-labeled C-terminal region peptides of Pb184.

RESULTS

Pb184 is expressed on the surface of male and female gametes. The antibody inhibited zygote and ookinete formation in vitro. When mosquitoes were fed on parasite-infected blood containing the antibody, oocyst formation decreased on the second day after feeding. Synthesized biotin-labeled peptides matching the C-terminal region of Pb184 bound to the female gamete and the residual body of male gametes, and inhibited differentiation into ookinetes in the in vitro culture system.

CONCLUSIONS

These results may be useful for the further studying the fertilization mechanism of Plasmodium protozoa. There is also the potential for their application as future tools to prevent malaria transmission.

Selective whole-genome sequencing of Plasmodium parasites directly from blood samples by nanopore adaptive sampling

IMPORTANCE

Malaria is caused by parasites of the genus Plasmodium, and reached a global disease burden of 247 million cases in 2021. To study drug resistance mutations and parasite population dynamics, whole-genome sequencing of patient blood samples is commonly performed. However, the predominance of human DNA in these samples imposes the need for time-consuming laboratory procedures to enrich Plasmodium DNA. We used the Oxford Nanopore Technologies' adaptive sampling feature to circumvent this problem and enrich Plasmodium reads directly during the sequencing run. We demonstrate that adaptive nanopore sequencing efficiently enriches Plasmodium reads, which simplifies and shortens the timeline from blood collection to parasite sequencing. In addition, we show that the obtained data can be used for monitoring genetic markers, or to generate nearly complete genomes. Finally, owing to its inherent mobility, this technology can be easily applied on-site in endemic areas where patients would benefit the most from genomic surveillance.

A new Plasmodium vivax reference genome for South American isolates

BACKGROUND

Plasmodium vivax is the second most important cause of human malaria worldwide, and accounts for the majority of malaria cases in South America. A high-quality reference genome exists for Papua Indonesia (PvP01) and Thailand (PvW1), but is lacking for South America. A reference genome specifically for South America would be beneficial though, as P. vivax is a genetically diverse parasite with geographical clustering.

RESULTS

This study presents a new high-quality assembly of a South American P. vivax isolate, referred to as PvPAM (P. vivax Peruvian AMazon). The genome was obtained from a low input patient sample from the Peruvian Amazon and sequenced using PacBio technology, resulting in a highly complete assembly with 6497 functional genes. Telomeric ends were present in 17 out of 28 chromosomal ends, and additional (sub)telomeric regions are present in 12 unassigned contigs. A comparison of multigene families between PvPAM and the PvP01 genome revealed remarkable variation in vir genes, and the presence of merozoite surface proteins (MSP) 3.6 and 3.7. Three dhfr and dhps drug resistance associated mutations are present in PvPAM, similar to those found in other Peruvian isolates. Mapping of publicly available South American whole genome sequencing (WGS) data to PvPAM resulted in significantly fewer variants and truncated reads compared to the use of PvP01 or PvW1 as reference genomes. To minimize the number of core genome variants in non-South American samples, PvW1 is most suited for Southeast Asian isolates, both PvPAM and PvW1 are suited for South Asian isolates, and PvPAM is recommended for African isolates. Interestingly, non-South American samples still contained the least subtelomeric variants when mapped to PvPAM, indicating high quality of the PvPAM subtelomeric regions.

CONCLUSIONS

Our findings show that the PvPAM reference genome more accurately represents South American P. vivax isolates in comparison to PvP01 and PvW1. In addition, PvPAM has a high level of completeness, and contains a similar number of annotated genes as PvP01 or PvW1. The PvPAM genome therefore will be a valuable resource to improve future genomic analyses on P. vivax isolates from the South American continent.

Molecular and immunological characterization of the calcyclin binding protein in rodent malaria parasite

Malaria remains as a global life-threatening disorder due to the emergence of resistance against standard antimalarials. Consequently, there is a serious need to better understand the biology of the malaria parasite in order to determine appropriate targets for new interventions. Calcyclin binding protein (CacyBP) is a multi-functional and multi-ligand protein that is not well characterized in malaria disease. In this study, we have cloned CacyBP from rodent species Plasmodium yoelii nigeriensis and purified the recombinant protein to carry out its detailed molecular, biophysical and immunological characterization. Molecular characterization indicates that PyCacyBP is a ∼27 kDa protein in parasite lysate and exists in monomer and dimer forms. Bioinformatic analysis of CacyBP showed significant sequence and structural similarities between rodent and human malaria parasites. CacyBP is expressed in all blood stages of P. yoelii nigeriensis parasite. In silico studies proposed the immunogenic potential of CacyBP. The rPyCacyBP immunized mice exhibited elevated levels of IgG1, IgG2a, IgG2b and IgG3 in their serum. Notably, cellular immune response in splenocytes from immunized mice showed increased expression of pro-inflammatory cytokines such as IL-12, IFN-γ and TNF-α. This CacyBP exhibited pro-inflammatory immune response in rodent host. These finding revealed that CacyBP may have the potential to boost the host immunity for protection against malaria infection. The present study provides basis for further exploration of the biological function of CacyBP in malaria parasite.

Band 3-mediated Plasmodium vivax invasion is associated with transcriptional variation in PvTRAg genes

The Plasmodium vivax reticulocyte invasion process is still poorly understood, with only a few receptor-ligand interactions identified to date. Individuals with the Southeast Asian ovalocytosis (SAO) phenotype have a deletion in the band 3 protein on the surface of erythrocytes, and are reported to have a lower incidence of clinical P. vivax malaria. Based on this observation, band 3 has been put forward as a receptor for P. vivax invasion, although direct proof is still lacking. In this study, we combined functional ex vivo invasion assays and transcriptome sequencing to uncover a band 3-mediated invasion pathway in P. vivax and potential band 3 ligands. Invasion by P. vivax field isolates was 67%-71% lower in SAO reticulocytes compared with non-SAO reticulocytes. Reticulocyte invasion was decreased by 40% and 27%-31% when blocking with an anti-band 3 polyclonal antibody and a PvTRAg38 peptide, respectively. To identify new band 3 receptor candidates, we mRNA-sequenced schizont-stage isolates used in the invasion assays, and observed high transcriptional variability in multigene and invasion-related families. Transcriptomes of isolates with low or high dependency on band 3 for invasion were compared by differential expression analysis, which produced a list of band 3 ligand candidates with high representation of PvTRAg genes. Our ex vivo invasion assays have demonstrated that band 3 is a P. vivax invasion receptor and confirm previous in vitro studies showing binding between PvTRAg38 and band 3, although the lower and variable inhibition levels observed suggest the involvement of other ligands. By coupling transcriptomes and invasion phenotypes from the same isolates, we identified a list of band 3 ligand candidates, of which the overrepresented PvTRAg genes are the most promising for future research.

RNAseq of Infected Erythrocyte Surface Antigen-Encoding Genes

Massive parallel sequencing technology has greatly increased the breadth and depth of transcriptomic data that can be captured from P. falciparum samples. This has revolutionized in vitro studies but uptake has been slower in the analysis of clinical samples. The principal barriers are the removal of contaminating white blood cells in a malaria endemic setting and preservation of the RNA. We provide here detailed methods for the collection of purified infected erythrocytes and the preservation and extraction of RNA. We also provide methods for assessing and addressing contaminating RNA from erythroid cells, and a protocol for RNAseq library preparation optimized to maximize yield from low amounts of parasite mRNA. Finally, we provide some examples of RNAseq library characteristics that may fail quality control for other species but are in fact satisfactory for P. falciparum RNAseq.

MEFAS, a hybrid of artesunate-mefloquine active against asexual stages of Plasmodium vivax in field isolates, inhibits malaria transmission

Human malaria continues to be a public health problem and an important cause of morbidity and mortality in the world. Malaria control is achieved through both individual protection against mosquito bites and drug treatment, which is hampered by the spread of Plasmodium falciparum resistance to most antimalarials, including artemisinin derivatives. One of the key pharmacological strategies for controlling malaria is to block transmission of the parasites to their mosquito vectors. Following this rational, MEFAS, a synthetic hybrid salt derived from artesunate (AS) and mefloquine has been previously reported for its activity against asexual P. falciparum parasites in vitro, in addition to a pronounced reduction in the viability of mature gametocytes. Herein, MEFAS was tested against asexual forms of Plasmodium vivax and for its ability to block malaria transmission in Anopheles darlingi mosquitoes in a membrane feeding assay using P. vivax field isolates. MEFAS demonstrated high potency, with a IC50 of 6.5 nM against asexual forms of P. vivax. At 50 μM, MEFAS completely blocked oocyst formation in mosquitoes, regardless of the oocyst number in the control group. At lower doses, MEFAS reduced oocyst prevalence by greater than 20%. At equivalent doses, AS irregularly reduced oocyst formation and caused only slight inhibition of mosquito infections. These results highlight the potential of MEFAS as a novel transmission-blocking molecule, as well as its high blood schizonticidal activity against P. vivax and P. falciparum field isolates, representing a starting point for further development of a new drug with dual antimalarial activity.

Plasmodium vivax binds host CD98hc (SLC3A2) to enter immature red blood cells

More than one-third of the world's population is exposed to Plasmodium vivax malaria, mainly in Asia¹. P. vivax preferentially invades reticulocytes (immature red blood cells)^2-4. Previous work has identified 11 parasite proteins involved in reticulocyte invasion, including erythrocyte binding protein 2 (ref. ⁵) and the reticulocyte-binding proteins (PvRBPs)^6-10. PvRBP2b binds to the transferrin receptor CD71 (ref. ¹¹), which is selectively expressed on immature reticulocytes¹². Here, we identified CD98 heavy chain (CD98), a heteromeric amino acid transporter from the SLC3 family (also known as SLCA2), as a reticulocyte-specific receptor for the PvRBP2a parasite ligand using mass spectrometry, flow cytometry, biochemical and parasite invasion assays. We characterized the expression level of CD98 at the surface of immature reticulocytes (CD71⁺) and identified an interaction between CD98 and PvRBP2a expressed at the merozoite surface. Our results identify CD98 as an additional host membrane protein, besides CD71, that is directly associated with P. vivax reticulocyte tropism. These findings highlight the potential of using PvRBP2a as a vaccine target against P. vivax malaria.

Viability and Infectivity of Plasmodium vivax Gametocytes in Short-Term Culture

The control and elimination of malaria caused by Plasmodium vivax both represent a great challenge due to the biological aspects of the species. Gametocytes are the forms responsible for the transmission of the parasite to the vector and the search for new strategies for blocking transmission are essential in a scenario of control and elimination The challenges in this search in regard to P. vivax mainly stem from the lack of a long-term culture and the limitation of studies of gametocytes. This study evaluated the viability and infectivity of P. vivax gametocytes in short-term culture. The samples enriched in gametocytes using Percoll (i), using magnetic-activated cell sorting (MACS®) (ii), and using non-enriched samples (iii) were evaluated. After the procedures, gametocytes were cultured in IMDM medium for up to 48 h. Cultured P. vivax gametocytes were viable and infectious for up to 48 h, however differences in viability and infectivity were observed in the samples after 12 h of culture in relation to 0 h. Percoll-enriched samples were shown to be viable in culture for longer intervals than those purified using MACS®. Gametocyte viability after enrichment procedures and short-term culture may provide new avenues in the development of methods for evaluating P. vivax TB.

Monitoring Plasmodium vivax resistance to antimalarials: Persisting challenges and future directions

Emerging antimalarial drug resistance may undermine current efforts to control and eliminate Plasmodium vivax, the most geographically widespread yet neglected human malaria parasite. Endemic countries are expected to assess regularly the therapeutic efficacy of antimalarial drugs in use in order to adjust their malaria treatment policies, but proper funding and trained human resources are often lacking to execute relatively complex and expensive clinical studies, ideally complemented by ex vivo assays of drug resistance. Here we review the challenges for assessing in vivo P. vivax responses to commonly used antimalarials, especially chloroquine and primaquine, in the presence of confounding factors such as variable drug absorption, metabolism and interaction, and the risk of new infections following successful radical cure. We introduce a simple modeling approach to quantify the relative contribution of relapses and new infections to recurring parasitemias in clinical studies of hypnozoitocides. Finally, we examine recent methodological advances that may render ex vivo assays more practical and widely used to confirm P. vivax drug resistance phenotypes in endemic settings and review current approaches to the development of robust genetic markers for monitoring chloroquine resistance in P. vivax populations.

A suitable RNA preparation methodology for whole transcriptome shotgun sequencing harvested from Plasmodium vivax-infected patients

Plasmodium vivax is a world-threatening human malaria parasite, whose biology remains elusive. The unavailability of in vitro culture, and the difficulties in getting a high number of pure parasites makes RNA isolation in quantity and quality a challenge. Here, a methodological outline for RNA-seq from P. vivax isolates with low parasitemia is presented, combining parasite maturation and enrichment with efficient RNA extraction, yielding ~ 100 pg.µL⁻¹ of RNA, suitable for SMART-Seq Ultra-Low Input RNA library and Illumina sequencing. Unbiased coding transcriptome of ~ 4 M reads was achieved for four patient isolates with ~ 51% of transcripts mapped to the P. vivax P01 reference genome, presenting heterogeneous profiles of expression among individual isolates. Amongst the most transcribed genes in all isolates, a parasite-staged mixed repertoire of conserved parasite metabolic, membrane and exported proteins was observed. Still, a quarter of transcribed genes remain functionally uncharacterized. In parallel, a P. falciparum Brazilian isolate was also analyzed and 57% of its transcripts mapped against IT genome. Comparison of transcriptomes of the two species revealed a common trophozoite-staged expression profile, with several homologous genes being expressed. Collectively, these results will positively impact vivax research improving knowledge of P. vivax biology.

Genetic Markers of Adaptation of Plasmodium falciparum to Transmission by American Vectors Identified in the Genomes of Parasites from Haiti and South America

The malaria parasite, Plasmodium falciparum, was introduced into Hispaniola and other regions of the Americas through the slave trade spanning the 16th through the 19th centuries. During this period, more than 12 million Africans were brought across the Atlantic to the Caribbean and other regions of the Americas. Since malaria is holoendemic in West Africa, a substantial percentage of these individuals carried the parasite. St. Domingue on Hispaniola, now modern-day Haiti, was a major port of disembarkation, and malaria is still actively transmitted there. We undertook a detailed study of the phylogenetics of the Haitian parasites and those from Colombia and Peru utilizing whole-genome sequencing. Principal-component and phylogenetic analyses, based upon single nucleotide polymorphisms (SNPs) in protein coding regions, indicate that, despite the potential for millions of introductions from Africa, the Haitian parasites share an ancestral relationship within a well-supported monophyletic clade with parasites from South America, while belonging to a distinct lineage. This result, in stark contrast to the historical record of parasite introductions, is best explained by a severe population bottleneck experienced by the parasites introduced into the Americas. Here, evidence is presented for targeted selection of rare African alleles in genes which are expressed in the mosquito stages of the parasite's life cycle. These genetic markers support the hypothesis that the severe population bottleneck was caused by the required adaptation of the parasite to transmission by new definitive hosts among the Anopheles (Nyssorhynchus) spp. found in the Caribbean and South America.IMPORTANCE Historical data suggest that millions of P. falciparum parasite lineages were introduced into the Americas during the trans-Atlantic slave trade, which would suggest a paraphyletic origin of the extant isolates in the Western Hemisphere. Our analyses of whole-genome variants show that the American parasites belong to a well-supported monophyletic clade. We hypothesize that the required adaptation to American vectors created a severe bottleneck, reducing the effective introduction to a few lineages. In support of this hypothesis, we discovered genes expressed in the mosquito stages of the life cycle that have alleles with multiple, high-frequency or fixed, nonsynonymous mutations in the American populations which are rarely found in African isolates. These alleles appear to be in gene products critical for transmission through the anopheline vector. Thus, these results may inform efforts to develop novel transmission-blocking vaccines by identifying parasite proteins functionally interacting with the vector that are important for successful transmission. Further, to the best of our knowledge, these are the first whole-genome data available from Haitian P. falciparum isolates. Defining the genome of these parasites provides genetic markers useful for mapping parasite populations and monitoring parasite movements/introductions.

On the survival of 48 h Plasmodium vivax Aotus monkey-derived ex vivo cultures: the role of leucocytes filtration and chemically defined lipid concentrate media supplementation

Background

Filtration of leukocytes (WBCs) is a standard practice of malaria ex vivo cultures. To date, few studies have considered the effect of filtration or the lack thereof on the survival of Plasmodium vivax ex vivo cultures through one cycle of maturation. This study investigates the effect of WBC filtration and culture media supplementation on the survival of 48–72 h ex vivo cultures.

Methods

Using parasitaemia density, the study compares the survival of Plasmodipur^® filtered, filter-retained or washed ex vivo cultures, maintained with McCoy’s5A medium supplemented with 25% serum alone or 20% in combination with 5% chemically defined lipid concentrate (CDLC), and in washed ex vivo cultures plus GlutaMAX™, benchmarked against IMDM™ or AIM-V™ media; also, assessed the survival of ex vivo cultures co-cultivated with human red blood cells (hRBCs).

Results

After 48 h of incubation a statistically significant difference was detected in the survival proportions of filtered and the filter-retained ex vivo cultures supplemented with serum plus CDLC (p = 0.0255), but not with serum alone (p = 0.1646). To corroborate these finding, parasitaemias of washed ex vivo cultures maintained with McCoy’s5A complete medium were benchmarked against IMDM™ or AIM-V™ media; again, a statistically significant difference was detected in the cultures supplemented with CDLC and GlutaMAX™ (p = 0.03), but not when supplemented with either alone; revealing a pattern of McCoy’s5A medium supplementation for Aotus-derived P. vivax cultures as follows: serum < serum + GlutaMAX™ < serum + CDLC < serum + CDLC + GlutaMAX™; confirming a key role of CDLC in combination with GlutaMAX™ in the enhanced survival observed. Lastly, results showed that co-cultivation with malaria-naïve hRBCs improved the survival of ex vivo cultures.

Conclusions

This study demonstrates that WBC filtration is not essential for the survival of P. vivax ex vivo cultures. It also demonstrates that McCoy’s5A complete medium improves the survival of Aotus-derived P. vivax ex vivo cultures, with no significant difference in survival compared to IMDM and AIM-V media. Finally, the study demonstrates that co-cultivation with hRBCs enhances the survival of ex vivo cultures. These findings are expected to help optimize seeding material for long-term P. vivax in vitro culture.

A Multistage Formulation Based on Full-Length CSP and AMA-1 Ectodomain of Plasmodium vivax Induces High Antibody Titers and T-cells and Partially Protects Mice Challenged with a Transgenic Plasmodium berghei Parasite

Infections with Plasmodium vivax are predominant in the Americas, representing 75% of malaria cases. Previously perceived as benign, malaria vivax is, in fact, a highly debilitating and economically important disease. Considering the high complexity of the malaria parasite life cycle, it has been hypothesized that an effective vaccine formulation against Plasmodium should contain multiple antigens expressed in different parasite stages. Based on that, we analyzed a recombinant P. vivax vaccine formulation mixing the apical membrane antigen 1 ectodomain (PvAMA-1) and a full-length circumsporozoite protein (PvCSP-All_FL) previously studied by our group, which elicits a potent antibody response in mice. Genetically distinct strains of mice (C57BL/6 and BALB/c) were immunized with the proteins, alone or in combination, in the presence of poly(I:C) adjuvant, a TLR3 agonist. In C57BL/6, high-antibody titers were induced against PvAMA-1 and the three PvCSP variants (VK210, VK247, and P. vivax-like). Meanwhile, mixing PvAMA-1 with PvCSP-All_FL had no impact on total IgG antibody titers, which were long-lasting. Moreover, antibodies from immunized mice recognized VK210 sporozoites and blood-stage parasites by immunofluorescence assay. However, in the BALB/c model, the antibody response against PvCSP-All_FL was relatively low. PvAMA-1-specific CD3⁺CD4⁺ and CD3⁺CD8⁺ T-cell responses were observed in C57BL/6 mice, and the cellular response was impaired by PvCSP-All_FL combination. More relevant, the multistage vaccine formulation provided partial protection in mice challenged with a transgenic Plasmodium berghei sporozoite expressing the homologous PvCSP protein.

Gene knockdown in malaria parasites via non-canonical RNAi

The lack of endogenous RNAi machinery in the malaria parasite Plasmodium hampers gene annotation and hence antimalarial drug and vaccine development. Here, we engineered rodent Plasmodium berghei to express a minimal, non-canonical RNAi machinery that solely requires Argonaute 2 (Ago2) and a modified short hairpin RNA, so-called AgoshRNA. Using this strategy, we achieved robust and specific gene knockdown throughout the entire parasite life cycle. We also successfully silenced the endogenous gene perforin-like protein 2, phenocopying a full gene knockout. Transcriptionally restricting Ago2 expression to the liver stage further enabled us to perform a stage-specific gene knockout. The RNAi-competent Plasmodium lines reported here will be a valuable resource for loss-of-function phenotyping of the many uncharacterized genes of Plasmodium in low or high throughput, without the need to engineer the target gene locus. Thereby, our new strategy and transgenic Plasmodium lines will ultimately benefit the discovery of urgently needed antimalarial drug and vaccine candidates. Generally, the ability to render RNAi-negative organisms RNAi-competent by mere introduction of two components, Ago2 and AgoshRNA, is a unique paradigm that should find broad applicability in other species.

Protective Immunity against Severe Malaria in Children Is Associated with a Limited Repertoire of Antibodies to Conserved PfEMP1 Variants

Extreme diversity of the major Plasmodium falciparum antigen, PfEMP1, poses a barrier to identifying targets of immunity to malaria. Here, we used protein microarrays containing hundreds of variants of the DBLα domain of PfEMP1 to cover the diversity of Papua New Guinean (PNG) parasites. Probing the plasma of a longitudinal cohort of malaria-exposed PNG children showed that group 2 DBLα antibodies were moderately associated with a lower risk of uncomplicated malaria, whereas individual variants were only weakly associated with clinical immunity. In contrast, antibodies to 85 individual group 1 and 2 DBLα variants were associated with a 70%-100% reduction in severe malaria. Of these, 17 variants were strong predictors of severe malaria. Analysis of full-length PfEMP1 sequences from PNG samples shows that these 17 variants are linked to pathogenic CIDR domains. This suggests that whereas immunity to uncomplicated malaria requires a broad repertoire of antibodies, immunity to severe malaria targets a subset of conserved variants. These findings provide insights into antimalarial immunity and potential antibody biomarkers for disease risk.

Plasmodium vivax transcriptional profiling of low input cryopreserved isolates through the intraerythrocytic development cycle

Approximately one-third of the global population is at risk of Plasmodium vivax infection, and an estimated 7.51 million cases were reported in 2017. Although, P. vivax research is currently limited by the lack of a robust continuous in vitro culture system for this parasite, recent work optimizing short-term ex vivo culture of P. vivax from cryopreserved isolates has facilitated quantitative assays on synchronous parasites. Pairing this improved culture system with low-input Smart-seq2 RNAseq library preparation, we sought to determine whether transcriptional profiling of P. vivax would provide insight into the differential survival of parasites in different culture media. To this end we probed the transcriptional signature of three different ex vivo P. vivax samples in four different culture media using only 1000 cells for each time point taken during the course of the intraerythrocytic development cycle (IDC). Using this strategy, we achieved similar quality transcriptional data to previously reported P. vivax transcriptomes. We found little effect with varying culture media on parasite transcriptional signatures, identified many novel gametocyte-specific genes from transcriptomes of FACS-isolated gametocytes, and determined invasion ligand expression in schizonts in biological isolates and across the IDC. In total, these data demonstrate the feasibility and utility of P. vivax RNAseq-based transcriptomic studies using minimal biomass input to maximize experimental capacity.

Plasmodium vivax is the most prevalent malaria-causing parasite species outside of Sub-Saharan Africa and has many unique and poorly understood biological characteristics that make it particularly challenging to study and combat. Transcriptomic profiling of P. vivax under various conditions has the potential to unlock new experimental abilities and aid in elucidating biology and the development of clinical interventions. However, a lack of a robust in vitro culture system for this parasite has restricted transcriptomic studies to researchers with timely access to fresh human isolates from clinics, which often are in resource-poor settings, as well as nearby, well-equipped laboratories for sample processing. This study aimed to gain insight into the differential survival of P. vivax in various culture media from the parasites transcriptional signature in each media. By implementing low-input RNA library preparation strategies, this study obtains robust transcriptomic data at various parasite development stages and in different culture conditions from just 1000 FACS-purified, P. vivax-infected erythrocytes from viable cryopreserved patient isolates. With these data, we find culture media has little effect on transcriptional profile, we characterize invasion ligand expression across intraerythrocytic development and between clinical isolates, and we define the transcriptome of sexual, transmissible stages of the P. vivax parasite. These results highlight the establishment and utility of a powerful platform for studying the transcriptomic biology of this particularly challenging parasite.

Next-Generation Molecular Surveillance of TriTryp Diseases

Elimination programs targeting TriTryp diseases (Leishmaniasis, Chagas' disease, human African trypanosomiasis) significantly reduced the number of cases. Continued surveillance is crucial to sustain this progress, but parasite molecular surveillance by genotyping is currently lacking. We explain here which epidemiological questions of public health and clinical relevance could be answered by means of molecular surveillance. Whole-genome sequencing (WGS) for molecular surveillance will be an important added value, where we advocate that preference should be given to direct sequencing of the parasite's genome in host tissues instead of analysis of cultivated isolates. The main challenges here, and recent technological advances, are discussed. We conclude with a series of recommendations for implementing whole-genome sequencing for molecular surveillance.

From marginal to essential: the golden thread between nutrient sensing, medium composition and Plasmodium vivax maturation in in vitro culture

Historically neglected, due to its biological peculiarities, the absence of a continuous long-term in vitro blood stage culture system and a propensity towards high morbidity rather than mortality, Plasmodium vivax was put back on the agenda during the last decade by the paradigm shift in the fight against malaria from malaria control to malaria eradication. While the incidence of the deadliest form of malaria, Plasmodium falciparum malaria, has declined since this paradigm shift took hold, the prospects of eradication are now threatened by the increase in the incidence of other human malaria parasite species. Plasmodium vivax is geographically the most widely distributed human malaria parasite, characterized by millions of clinical cases every year and responsible for a massive economic burden. The urgent need to tackle the unique biological challenges posed by this parasite led to renewed efforts aimed at establishing a continuous, long-term in vitro P. vivax blood stage culture. Based on recent discoveries on the role of nutrient sensing in Plasmodium’s pathophysiology, this review article critically assesses the extensive body of literature concerning Plasmodium culture conditions with a specific focus on culture media used in attempts to culture different Plasmodium spp. Hereby, the effect of specific media components on the parasite’s in vitro fitness and the maturation of the parasite’s host cell, the reticulocyte, is analysed. Challenging the wide-held belief that it is sufficient to find the right parasite isolate and give it the right type of cells to invade for P. vivax to grow in vitro, this review contends that a healthy side-by-side maturation of both the parasite and its host cell, the reticulocyte, is necessary in the adaptation of P. vivax to in vitro growth and argues that culture conditions and the media in particular play an essential role in this maturation process.

A simple, ex vivo phagocytosis assay of Plasmodium vivax merozoites by flow cytometry

As phagocytosis is the first line of defense against malaria, we developed a phagocytosis assay with Plasmodium vivax (P. vivax) merozoites that can be applied to evaluate vaccine candidates. Briefly, after leukocyte removal with loosely packed cellulose powder in a syringe, P. vivax trophozoites matured to the merozoite-rich schizont stages in the presence of the E64 protease inhibitor. The Percoll gradient-enriched schizonts were chemically disrupted to release merozoites that were submitted to merozoite opsonin-dependent phagocytosis in two phagocytic lines with human and mouse antibodies against the N- and C-terminus of P. vivax Merozoite Surface Protein-1 (Nterm-PvMSP1 and MSP1₁₉). The resulting assay is simple and efficient for use as a routine phagocytic assay for the evaluation of merozoite stage vaccine candidates.

Clonal tracking of erythropoiesis in rhesus macaques

The classical model of hematopoietic hierarchies is being reconsidered on the basis of data from in vitro assays and single cell expression profiling. Recent experiments suggested that the erythroid lineage might differentiate directly from multipotent hematopoietic stem cells / progenitors or from a highly biased subpopulation of stem cells, rather than transiting through common myeloid progenitors or megakaryocyte-erythrocyte progenitors. We genetically barcoded autologous rhesus macaque stem and progenitor cells, allowing quantitative tracking of the in vivo clonal output of thousands of individual cells over time following transplantation. CD34⁺ cells were lentiviral-transduced with a high diversity barcode library, with the barcode in an expressed region of the provirus, allowing barcode retrieval from DNA or RNA, with each barcode representing an individual stem or progenitor cell clone. Barcode profiles from bone marrow CD45^-CD71⁺ maturing nucleated red blood cells were compared with other lineages purified from the same bone marrow sample. There was very high correlation of barcode contributions between marrow nucleated red blood cells and other lineages, with the highest correlation between nucleated red blood cells and myeloid lineages, whether at earlier or later time points post transplantation, without obvious clonal contributions from highly erythroid-biased or restricted clones. A similar profile occurred even under stressors such as aging or erythropoietin stimulation. RNA barcode analysis on circulating mature red blood cells followed over long time periods demonstrated stable erythroid clonal contributions. Overall, in this nonhuman primate model with great relevance to human hematopoiesis, we documented continuous production of erythroid cells from multipotent, non-biased hematopoietic stem cell clones at steady-state or under stress.

Frequent expansion of Plasmodium vivax Duffy Binding Protein in Ethiopia and its epidemiological significance

Plasmodium vivax invasion of human erythrocytes depends on the Duffy Binding Protein (PvDBP) which interacts with the Duffy antigen. PvDBP copy number has been recently shown to vary between P. vivax isolates in Sub-Saharan Africa. However, the extent of PvDBP copy number variation, the type of PvDBP multiplications, as well as its significance across broad samples are still unclear. We determined the prevalence and type of PvDBP duplications, as well as PvDBP copy number variation among 178 Ethiopian P. vivax isolates using a PCR-based diagnostic method, a novel quantitative real-time PCR assay and whole genome sequencing. For the 145 symptomatic samples, PvDBP duplications were detected in 95 isolates, of which 81 had the Cambodian and 14 Malagasy-type PvDBP duplications. PvDBP varied from 1 to >4 copies. Isolates with multiple PvDBP copies were found to be higher in symptomatic than asymptomatic infections. For the 33 asymptomatic samples, PvDBP was detected with two copies in two of the isolates, and both were the Cambodian-type PvDBP duplication. PvDBP copy number in Duffy-negative heterozygotes was not significantly different from that in Duffy-positives, providing no support for the hypothesis that increased copy number is a specific association with Duffy-negativity, although the number of Duffy-negatives was small and further sampling is required to test this association thoroughly.

Plasmodium vivax invasion of human erythrocytes relies on interaction between the Duffy antigen and P. vivax Duffy Binding Protein (PvDBP). Whole genome sequences from P. vivax field isolates in Madagascar identified a duplication of the PvDBP gene and PvDBP duplication has also been detected in non-African P. vivax-endemic countries. Two types of PvDBP duplications have been reported, termed Cambodian and Malagasy-type duplications. Our study used a combination of PCR-based diagnostic method, a novel quantitative real-time PCR assay, and whole genome sequencing to determine the prevalence and type of PvDBP duplications, as well as PvDBP copy number on a broad number of P. vivax samples in Ethiopia. We found that over 65% of P. vivax isolated from the symptomatic infections were detected with PvDBP duplications and PvDBP varied from 1 to >4 copies. The majority of PvDBP duplications belongs to the Cambodian-type while the Malagasy-type duplications was also detected. For the asymptomatic infections, despite a small sample size, the majority of P. vivax were detected with a single-copy based on both PCR and qPCR assays. There was no significant difference in PvDBP copy number between Duffy-null heterozygote and Duffy-positive homozygote/heterozygote. Further investigation is needed with expanded Duffy-null homozygotes to examine the functional significance of PvDBP expansion.

Complement Receptor 1 availability on red blood cell surface modulates Plasmodium vivax invasion of human reticulocytes

Plasmodium vivax parasites preferentially invade reticulocyte cells in a multistep process that is still poorly understood. In this study, we used ex vivo invasion assays and population genetic analyses to investigate the involvement of complement receptor 1 (CR1) in P. vivax invasion. First, we observed that P. vivax invasion of reticulocytes was consistently reduced when CR1 surface expression was reduced through enzymatic cleavage, in the presence of naturally low-CR1-expressing cells compared with high-CR1-expressing cells, and with the addition of soluble CR1, a known inhibitor of P. falciparum invasion. Immuno-precipitation experiments with P. vivax Reticulocyte Binding Proteins showed no evidence of complex formation. In addition, analysis of CR1 genetic data for worldwide human populations with different exposure to malaria parasites show significantly higher frequency of CR1 alleles associated with low receptor expression on the surface of RBCs and higher linkage disequilibrium in human populations exposed to P. vivax malaria compared with unexposed populations. These results are consistent with a positive selection of low-CR1-expressing alleles in vivax-endemic areas. Collectively, our findings demonstrate that CR1 availability on the surface of RBCs modulates P. vivax invasion. The identification of new molecular interactions is crucial to guiding the rational development of new therapeutic interventions against vivax malaria.

Divergent roles for the RH5 complex components, CyRPA and RIPR in human-infective malaria parasites

Malaria is caused by Plasmodium parasites, which invade and replicate in erythrocytes. For Plasmodium falciparum, the major cause of severe malaria in humans, a heterotrimeric complex comprised of the secreted parasite proteins, PfCyRPA, PfRIPR and PfRH5 is essential for erythrocyte invasion, mediated by the interaction between PfRH5 and erythrocyte receptor basigin (BSG). However, whilst CyRPA and RIPR are present in most Plasmodium species, RH5 is found only in the small Laverania subgenus. Existence of a complex analogous to PfRH5-PfCyRPA-PfRIPR targeting BSG, and involvement of CyRPA and RIPR in invasion, however, has not been addressed in non-Laverania parasites. Here, we establish that unlike P. falciparum, P. knowlesi and P. vivax do not universally require BSG as a host cell invasion receptor. Although we show that both PkCyRPA and PkRIPR are essential for successful invasion of erythrocytes by P. knowlesi parasites in vitro, neither protein forms a complex with each other or with an RH5-like molecule. Instead, PkRIPR is part of a different trimeric protein complex whereas PkCyRPA appears to function without other parasite binding partners. It therefore appears that in the absence of RH5, outside of the Laverania subgenus, RIPR and CyRPA have different, independent functions crucial for parasite survival.

Malaria is one of the most devastating infectious diseases, causing significant human suffering and death. It is caused by parasites of the genus Plasmodium proliferating in the bloodstream. Understanding the mechanism of erythrocyte invasion is key for developing novel intervention strategies. P. falciparum, the cause of the most severe form of malaria, requires the interaction of a trimeric protein complex RH5-CyRPA-RIPR with the host receptor BSG for successful invasion. We show here that the BSG receptor is not essential for invasion by two other major causes of human malaria, P. vivax and P. knowlesi. Furthermore, we analyzed the role of CyRPA and RIPR in the absence of an RH5-like molecule in P. knowlesi and show that these molecules do not associate to form a protein complex unlike in the presence of RH5 in P. falciparum. PkRIPR is part of a different protein complex. Despite this difference CyRPA and RIPR still have essential functions during host cell invasion in other important human malaria-causing parasites.

A defined mechanistic correlate of protection against Plasmodium falciparum malaria in non-human primates

Malaria vaccine design and prioritization has been hindered by the lack of a mechanistic correlate of protection. We previously demonstrated a strong association between protection and merozoite-neutralizing antibody responses following vaccination of non-human primates against Plasmodium falciparum reticulocyte binding protein homolog 5 (PfRH5). Here, we test the mechanism of protection. Using mutant human IgG1 Fc regions engineered not to engage complement or FcR-dependent effector mechanisms, we produce merozoite-neutralizing and non-neutralizing anti-PfRH5 chimeric monoclonal antibodies (mAbs) and perform a passive transfer-P. falciparum challenge study in Aotus nancymaae monkeys. At the highest dose tested, 6/6 animals given the neutralizing PfRH5-binding mAb c2AC7 survive the challenge without treatment, compared to 0/6 animals given non-neutralizing PfRH5-binding mAb c4BA7 and 0/6 animals given an isotype control mAb. Our results address the controversy regarding whether merozoite-neutralizing antibody can cause protection against P. falciparum blood-stage infections, and highlight the quantitative challenge of achieving such protection.

Enhanced Ex Vivo Plasmodium vivax Intraerythrocytic Enrichment and Maturation for Rapid and Sensitive Parasite Growth Assays

Plasmodium vivax chloroquine resistance has been documented in nearly every region where this malaria-causing parasite is endemic. Unfortunately, P. vivax resistance surveillance and drug discovery are challenging due to the low parasitemias of patient isolates and poor parasite survival through ex vivo maturation that reduce the sensitivity and scalability of current P. vivax antimalarial assays. Using cryopreserved patient isolates from Brazil and fresh patient isolates from India, we established a robust enrichment method for P. vivax parasites. We next performed a medium screen for formulations that enhance ex vivo survival. Finally, we optimized an isotopic metabolic labeling assay for measuring P. vivax maturation and its sensitivity to antimalarials. A KCl Percoll density gradient enrichment method increased parasitemias from small-volume ex vivo isolates by an average of >40-fold. The use of Iscove's modified Dulbecco's medium for P. vivax ex vivo culture approximately doubled the parasite survival through maturation. Coupling these with [³H]hypoxanthine metabolic labeling permitted sensitive and robust measurements of parasite maturation, which was used to measure the sensitivities of Brazilian P. vivax isolates to chloroquine and several novel antimalarials. These techniques can be applied to rapidly and robustly assess the P. vivax isolate sensitivities to antimalarials for resistance surveillance and drug discovery.

Genomic Analyses Reveal the Common Occurrence and Complexity of Plasmodium vivax Relapses in Cambodia

Plasmodium vivax parasites have a unique dormant stage that can cause relapses weeks or months after the initial infection. These dormant parasites are among the main challenges of vivax malaria control as they constitute a reservoir that is difficult to eliminate. Since field studies are confounded by reinfections and possible recrudescence of drug-resistant parasites, most analyses of P. vivax relapses have focused on travelers returning from regions of malaria endemicity. However, it is not clear whether these individuals accurately recapitulate the relapse patterns of repeatedly infected individuals residing in areas of endemicity. Here, we present analyses of vivax malaria patients enrolled in a tightly controlled field study in Cambodia. After antimalarial drug treatment was administered, we relocated 20 individuals to a nontransmission area and followed them for 60 days, with blood collection performed every second day. Our analyses reveal that 60% of the patients relapsed during the monitoring period. Using whole-genome sequencing and high-throughput genotyping, we showed that relapses in Cambodia are often polyclonal and that the relapsing parasites harbor various degrees of relatedness to the parasites present in the initial infection. Our analyses also showed that clone populations differed dynamically, with new clones emerging during the course of the relapsing infections. Overall, our study data show that it is possible to investigate the patterns, dynamics, and diversity of P. vivax relapses of individuals living in a region of malaria endemicity and reveal that P. vivax relapses are much more pervasive and complex than previously considered. (This study has been registered at ClinicalTrials.gov under registration no. NCT02118090)IMPORTANCEP. vivax parasites can remain dormant in the liver and relapse weeks or months after the initial infection, greatly complicating malaria control and elimination efforts. The few investigations of this dormant stage have relied on travelers and military personnel returning from areas of malaria endemicity. However, it is not clear whether these individuals, exposed to a limited number of infections, accurately represent the patterns of relapses of individuals living in areas of endemicity, who are repeatedly infected by P. vivax parasites. Our study combined tightly controlled fieldwork with comprehensive genomic analyses, and our report provides a first opportunity to investigate the patterns, dynamics, and diversity of P. vivax relapses directly with individuals living in areas of endemicity.

In Vivo and In Vitro Activities and ADME-Tox Profile of a Quinolizidine-Modified 4-Aminoquinoline: A Potent Anti-P. falciparum and Anti-P. vivax Blood-Stage Antimalarial

Natural products are a prolific source for the identification of new biologically active compounds. In the present work, we studied the in vitro and in vivo antimalarial efficacy and ADME-Tox profile of a molecular hybrid (AM1) between 4-aminoquinoline and a quinolizidine moiety derived from lupinine (Lupinus luteus). The aim was to find a compound endowed with the target product profile-1 (TCP-1: molecules that clear asexual blood-stage parasitaemia), proposed by the Medicine for Malaria Venture to accomplish the goal of malaria elimination/eradication. AM1 displayed a very attractive profile in terms of both in vitro and in vivo activity. By using standard in vitro antimalarial assays, AM1 showed low nanomolar inhibitory activity against chloroquine-sensitive and resistant P. falciparum strains (range IC₅₀ 16–53 nM), matched with a high potency against P. vivax field isolates (Mean IC₅₀ 29 nM). Low toxicity and additivity with artemisinin derivatives were also demonstrated in vitro. High in vivo oral efficacy was observed in both P.berghei and P. yoelii mouse models with IC₅₀ values comparable or better than those of chloroquine. The metabolic stability in different species and the pharmacokinetic profile in the mouse model makes AM1 a compound worth further investigation as a potential novel schizonticidal agent.

Advancing Research Models and Technologies to Overcome Biological Barriers to Plasmodium vivax Control

Malaria prevalence has declined in the past 10 years, especially outside of sub-Saharan Africa. However, the proportion of cases due to Plasmodium vivax is increasing, accounting for up to 90-100% of the malaria burden in endemic regions. Nonetheless, investments in malaria research and control still prioritize Plasmodium falciparum while largely neglecting P. vivax. Specific biological features of P. vivax, particularly invasion of reticulocytes, occurrence of dormant liver forms of the parasite, and the potential for transmission of sexual-stage parasites prior to onset of clinical illness, promote its persistence and hinder development of research tools and interventions. This review discusses recent advances in P. vivax research, current knowledge of its unique biology, and proposes priorities for P. vivax research and control efforts.

Comparative transcriptomics of female and male gametocytes in Plasmodium berghei and the evolution of sex in alveolates

BACKGROUND

The clinical symptoms of malaria are caused by the asexual replication of Plasmodium parasites in the blood of the vertebrate host. To spread to new hosts, however, the malaria parasite must differentiate into sexual forms, termed gametocytes, which are ingested by a mosquito vector. Sexual differentiation produces either female or male gametocytes, and involves significant morphological and biochemical changes. These transformations prepare gametocytes for the rapid progression to gamete formation and fertilisation, which occur within 20 min of ingestion. Here we present the transcriptomes of asexual, female, and male gametocytes in P. berghei, and a comprehensive statistically-based differential-expression analysis of the transcriptional changes that underpin this sexual differentiation.

RESULTS

RNA-seq analysis revealed numerous differences in the transcriptomes of female and male gametocytes compared to asexual stages. Overall, there is net downregulation of transcripts in gametocytes compared to asexual stages, with this trend more marked in female gametocytes. Our analysis identified transcriptional changes in previously-characterised gametocyte-specific pathways, which validated our approach. We also detected many previously-unreported female- and male-specific pathways and genes. Transcriptional biases in stage and gender were then used to investigate sex-specificity and sexual dimorphism of Plasmodium in an evolutionary context. Sex-related gene expression is well conserved between Plasmodium species, but relatively poorly conserved in related organisms outside this genus. This pattern of conservation is most evident in genes necessary for both male and female gametocyte formation. However, this trend is less pronounced for male-specific genes, which are more highly conserved outside the genus than genes specific to female development.

CONCLUSIONS

We characterised the transcriptional changes that are integral to the development of the female and male sexual forms of Plasmodium. These differential-expression patterns provide a vital insight into understanding the gender-specific characteristics of this essential stage that is the primary target for treatments that block parasite transmission. Our results also offer insight into the evolution of sex genes through Alveolata, and suggest that many Plasmodium sex genes evolved within the genus. We further hypothesise that male gametocytes co-opted pre-existing cellular machinery in their evolutionary history, whereas female gametocytes evolved more through the development of novel, parasite-specific pathways.

Hemolytic Potential of Tafenoquine in Female Volunteers Heterozygous for Glucose-6-Phosphate Dehydrogenase (G6PD) Deficiency (G6PD Mahidol Variant) versus G6PD-Normal Volunteers

Tafenoquine is an 8-aminoquinoline under investigation for the prevention of relapse in Plasmodium vivax malaria. This open-label, dose-escalation study assessed quantitatively the hemolytic risk with tafenoquine in female healthy volunteers heterozygous for the Mahidol^487A glucose-6-phosphate dehydrogenase (G6PD)-deficient variant versus G6PD-normal females, and with reference to primaquine. Six G6PD-heterozygous subjects (G6PD enzyme activity 40-60% of normal) and six G6PD-normal subjects per treatment group received single-dose tafenoquine (100, 200, or 300 mg) or primaquine (15 mg × 14 days). All participants had pretreatment hemoglobin levels ≥ 12.0 g/dL. Tafenoquine dose escalation stopped when hemoglobin decreased by ≥ 2.5 g/dL (or hematocrit decline ≥ 7.5%) versus pretreatment values in ≥ 3/6 subjects. A dose-response was evident in G6PD-heterozygous subjects (N = 15) receiving tafenoquine for the maximum decrease in hemoglobin versus pretreatment values. Hemoglobin declines were similar for tafenoquine 300 mg (-2.65 to -2.95 g/dL [N = 3]) and primaquine (-1.25 to -3.0 g/dL [N = 5]). Two further cohorts of G6PD-heterozygous subjects with G6PD enzyme levels 61-80% (N = 2) and > 80% (N = 5) of the site median normal received tafenoquine 200 mg; hemolysis was less pronounced at higher G6PD enzyme activities. Tafenoquine hemolytic potential was dose dependent, and hemolysis was greater in G6PD-heterozygous females with lower G6PD enzyme activity levels. Single-dose tafenoquine 300 mg did not appear to increase the severity of hemolysis versus primaquine 15 mg × 14 days.

An ELISA for the early diagnosis of acute canine babesiosis detecting circulating antigen of large Babesia spp

Babesia canis is the predominant Babesia species in dogs in Europe and is responsible for a severe and fatal disease. An increase in global pet tourism and a widening of the geographic distribution of the tick vector has led to the emergence of infections in areas where previously only imported cases have been reported. Due to the potential for rapid and serious disease progression, direct parasite detection by stained blood smears and light microscopy or DNA-based methods have traditionally been used for the diagnosis of acute infections. This study describes the production of a murine monoclonal antibody ('mAb BcFIII 7/1/2') that reacts to a 65kDa corpuscular epitope present in B. canis-infected erythrocytes and can be used in an ELISA to detect circulating Babesia antigen during acute infections. The sensitivity of the ELISA was 100% (95%CI: 84.5-100) as determined using blood lysate samples from 27 dogs with acute B. canis infections. Sensitivity was reduced to 53.8% in 13 patent Babesia vogeli infections (95%CI: 26.1-79.6) based on the current test design using convalescent serum from a B. canis-infected dog. The specificity was determined to be 86.4% (95%CI: 64-96.4) using 22 samples from healthy canine blood donors. In the course of acute B. canis infections, the ELISA showed a positive result at the same time as a positive PCR result was recorded. This was 24-48h before parasites could be detected by light microscopy. Convalescent samples collected from 6 B. canis-infected dogs at least 14days post treatment resulted in negative ELISA reactions. The hyper-acute to acute phase of a B. canis infection represents an emergency situation with high mortality. To increase the chances of survival, a fast and accurate diagnosis and immediate treatment is required. The current study demonstrates the opportunity of an early and specific detection of acute infections by an AgELISA that is potentially translatable to a rapid diagnostic test design.

Genome-wide analysis of gene expression and protein secretion of Babesia canis during virulent infection identifies potential pathogenicity factors

Infections of dogs with virulent strains of Babesia canis are characterized by rapid onset and high mortality, comparable to complicated human malaria. As in other apicomplexan parasites, most Babesia virulence factors responsible for survival and pathogenicity are secreted to the host cell surface and beyond where they remodel and biochemically modify the infected cell interacting with host proteins in a very specific manner. Here, we investigated factors secreted by B. canis during acute infections in dogs and report on in silico predictions and experimental analysis of the parasite’s exportome. As a backdrop, we generated a fully annotated B. canis genome sequence of a virulent Hungarian field isolate (strain BcH-CHIPZ) underpinned by extensive genome-wide RNA-seq analysis. We find evidence for conserved factors in apicomplexan hemoparasites involved in immune-evasion (e.g. VESA-protein family), proteins secreted across the iRBC membrane into the host bloodstream (e.g. SA- and Bc28 protein families), potential moonlighting proteins (e.g. profilin and histones), and uncharacterized antigens present during acute crisis in dogs. The combined data provides a first predicted and partially validated set of potential virulence factors exported during fatal infections, which can be exploited for urgently needed innovative intervention strategies aimed at facilitating diagnosis and management of canine babesiosis.

Further evaluation of the NWF filter for the purification of Plasmodium vivax-infected erythrocytes

BACKGROUND

Isolation of Plasmodium-infected red blood cells (iRBCs) from clinical blood samples is often required for experiments, such as ex vivo drug assays, in vitro invasion assays and genome sequencing. Current methods for removing white blood cells (WBCs) from malaria-infected blood are time-consuming or costly. A prototype non-woven fabric (NWF) filter was developed for the purification of iRBCs, which showed great efficiency for removing WBCs in a pilot study. Previous work was performed with prototype filters optimized for processing 5-10 mL of blood. With the commercialization of the filters, this study aims to evaluate the efficiency and suitability of the commercial NWF filter for the purification of Plasmodium vivax-infected RBCs in smaller volumes of blood and to compare its performance with that of Plasmodipur^® filters.

METHODS

Forty-three clinical P. vivax blood samples taken from symptomatic patients attending malaria clinics at the China-Myanmar border were processed using the NWF filters in a nearby field laboratory. The numbers of WBCs and iRBCs and morphology of P. vivax parasites in the blood samples before and after NWF filtration were compared. The viability of P. vivax parasites after filtration from 27 blood samples was examined by in vitro short-term culture. In addition, the effectiveness of the NWF filter for removing WBCs was compared with that of the Plasmodipur^® filter in six P. vivax blood samples.

RESULTS

Filtration of 1-2 mL of P. vivax-infected blood with the NWF filter removed 99.68% WBCs. The densities of total iRBCs, ring and trophozoite stages before and after filtration were not significantly different (P > 0.05). However, the recovery rates of schizont- and gametocyte-infected RBCs, which were minor parasite stages in the clinical samples, were relatively low. After filtration, the P. vivax parasites did not show apparent morphological changes. Culture of 27 P. vivax-infected blood samples after filtration showed that parasites successfully matured into the schizont stage. The WBC removal rates and iRBC recovery rates were not significantly different between the NWF and Plasmodipur^® filters (P > 0.05).

CONCLUSIONS

When tested with 1-2 mL of P. vivax-infected blood, the NWF filter could effectively remove WBCs and the recovery rates for ring- and trophozoite-iRBCs were high. P. vivax parasites after filtration could be successfully cultured in vitro to reach maturity. The performance of the NWF and Plasmodipur^® filters for removing WBCs and recovering iRBCs was comparable.

Cellulose filtration of blood from malaria patients for improving ex vivo growth of Plasmodium falciparum parasites

BACKGROUND

Establishing in vitro Plasmodium falciparum culture lines from patient parasite isolates can offer deeper understanding of geographic variations of drug sensitivity and mechanisms of malaria pathogenesis and immunity. Cellulose column filtration of blood is an inexpensive, rapid and effective method for the removal of host factors, such as leucocytes and platelets, significantly improving the purification of parasite DNA in a blood sample.

METHODS

In this study, the effect of cellulose column filtration of venous blood on the initial in vitro growth of P. falciparum parasite isolates from Tanzanian children admitted to hospital was tested. The parasites were allowed to expand in culture without subcultivation until 5 days after admission or the appearance of dead parasites and parasitaemia was determined daily. To investigate whether the filtration had an effect on clonality, P. falciparum merozoite surface protein 2 genotyping was performed using nested PCR on extracted genomic DNA, and the var gene transcript levels were investigated, using quantitative PCR on extracted RNA, at admission and 4 days of culture.

RESULTS

The cellulose-filtered parasites grew to higher parasitaemia faster than non-filtered parasites seemingly due to a higher development ratio of ring stage parasites progressing into the late stages. Cellulose filtration had no apparent effect on clonality or var gene expression; however, evident differences were observed after only 4 days of culture in both the number of clones and transcript levels of var genes compared to the time of admission.

CONCLUSIONS

Cellulose column filtration of parasitized blood is a cheap, applicable method for improving cultivation of P. falciparum field isolates for ex vivo based assays; however, when assessing phenotype and genotype of cultured parasites, in general, assumed to represent the in vivo infection, caution is advised.

Infection of laboratory colonies of Anopheles mosquitoes with Plasmodium vivax from cryopreserved clinical isolates

Plasmodium vivax is the most geographically widespread malaria parasite. Unique features of transmission biology complicate P. vivax control. Interventions targeting transmission are required for malaria eradication. In the absence of an in vitro culture, transmission studies rely on live isolates from non-human primates or endemic regions. Here, we demonstrate P. vivax gametocytes from both India and Brazil are stable during cryopreservation. Importantly, cryopreserved gametocytes from Brazil were capable of infecting three anopheline mosquito species in feedings done in the United States. These findings create new opportunities for transmission studies in diverse locales.

Neutralizing Antibodies against Plasmodium falciparum Associated with Successful Cure after Drug Therapy

An effective antibody response can assist drug treatment to contribute to better parasite clearance in malaria patients. To examine this, sera were obtained from two groups of adult patients with acute falciparum malaria, prior to drug treatment: patients who (1) have subsequent recrudescent infection, or (2) were cured by Day 28 following treatment. Using a Plasmodium falciparum antigen library, we examined the antibody specificities in these sera. While the antibody repertoire of both sera groups was extremely broad and varied, there was a differential antibody profile between the two groups of sera. The proportion of cured patients with antibodies against EXP1, MSP3, GLURP, RAMA, SEA and EBA181 was higher than the proportion of patients with recrudescent infection. The presence of these antibodies was associated with higher odds of treatment cure. Sera containing all six antibodies impaired the invasion of P. falciparum clinical isolates into erythrocytes. These results suggest that antibodies specific against EXP1, MSP3, GLURP, RAMA, SEA and EBA181 in P. falciparum infections could assist anti-malarial drug treatment and contribute to the resolution of the malarial infection.

Invasion characteristics of a Plasmodium knowlesi line newly isolated from a human

Plasmodium knowlesi is extensively used as an important malaria model and is now recognized as an important cause of human malaria in Malaysia. The strains of P. knowlesi currently used for research were isolated many decades ago, raising concerns that they might no longer be representative of contemporary parasite populations. We derived a new P. knowlesi line (University Malaya line, UM01), from a patient admitted in Kuala Lumpur, Malaysia, and compared it with a human-adapted laboratory line (A1-H.1) derived from the P. knowlesi H strain. The UM01 and A1-H.1 lines readily invade human and macaque (Macaca fascicularis) normocytes with a preference for reticulocytes. Whereas invasion of human red blood cells was dependent on the presence of the Duffy antigen/receptor for chemokines (DARC) for both parasite lines, this was not the case for macaque red blood cells. Nonetheless, differences in invasion efficiency, gametocyte production and the length of the asexual cycle were noted between the two lines. It would be judicious to isolate and characterise numerous P. knowlesi lines for use in future experimental investigations of this zoonotic species.

Expression of Plasmodium vivax crt-o Is Related to Parasite Stage but Not Ex Vivo Chloroquine Susceptibility

Chloroquine (CQ)-resistant Plasmodium vivax is present in most countries where P. vivax infection is endemic, but the underlying molecular mechanisms responsible remain unknown. Increased expression of P. vivaxcrt-o (pvcrt-o) has been correlated with in vivo CQ resistance in an area with low-grade resistance. We assessed pvcrt-o expression in isolates from Papua (Indonesia), where P. vivax is highly CQ resistant. Ex vivo drug susceptibilities to CQ, amodiaquine, piperaquine, mefloquine, and artesunate were determined using a modified schizont maturation assay. Expression levels of pvcrt-o were measured using a novel real-time quantitative reverse transcription-PCR method. Large variations in pvcrt-o expression were observed across the 51 isolates evaluated, with the fold change in expression level ranging from 0.01 to 59 relative to that seen with the P. vivax β-tubulin gene and from 0.01 to 24 relative to that seen with the P. vivax aldolase gene. Expression was significantly higher in isolates with the majority of parasites at the ring stage of development (median fold change, 1.7) compared to those at the trophozoite stage (median fold change, 0.5; P < 0.001). Twenty-nine isolates fulfilled the criteria for ex vivo drug susceptibility testing and showed high variability in CQ responses (median, 107.9 [range, 6.5 to 345.7] nM). After controlling for the parasite stage, we found that pvcrt-o expression levels did not correlate with the ex vivo response to CQ or with that to any of the other antimalarials tested. Our results highlight the importance of development-stage composition for measuring pvcrt-o expression and suggest that pvcrt-o transcription is not a primary determinant of ex vivo drug susceptibility. A comprehensive transcriptomic approach is warranted for an in-depth investigation of the role of gene expression levels and P. vivax drug resistance.

Comparison between Flow Cytometry, Microscopy, and Lactate Dehydrogenase-Based Enzyme-Linked Immunosorbent Assay for Plasmodium falciparum Drug Susceptibility Testing under Field Conditions

Flow cytometry is an objective method for conducting in vitro antimalarial sensitivity assays with increasing potential for application in field sites. We examined in vitro susceptibility to seven anti-malarial drugs for 40 fresh P. falciparum field isolates via a flow cytometry method (FCM), a colorimetric LDH-based ELISA : DELI), and standard microscopic slide analysis of growth. For FCM, 184/280 (66%) assays met analytical acceptance criteria, compared to 166/280 (59%) for DELI. There was good agreement between FCM and microscopy, while DELI tended to produce higher half-maximal inhibition constants (IC50s) than FCM, with an overall bias of 2.2-fold (Bland-Altman comparison). Values for artesunate and dihydroartemisinin were most affected. Paradoxical increases in signal at very high concentrations of mefloquine and related compounds were more marked with the DELI assay, suggesting that off-target effects on LDH production may be responsible. Loss of FCM signal due to reinvasion or slow growth was observed in a small number of samples. These results extend previous work on use of flow cytometry to determine antimalarial susceptibility in terms of the number of samples, range of drugs, and comparison with other methods.

Unambiguous determination of Plasmodium vivax reticulocyte invasion by flow cytometry

The invasion of CD71+ reticulocytes by Plasmodium vivax is a crucial yet poorly characterised event. The application of flow cytometry to ex vivo invasion assays promises to facilitate the quantitative analysis of P. vivax reticulocyte invasion. However, current protocols suffer from a low level of sensitivity due to the absence of a particular design for P. vivax cell tropism. Importantly, merozoite invasion into contaminating red blood cells from the schizont inoculum (auto-invasion) may confound the analysis. Here we present a stable two-color flow cytometry assay for the accurate quantification of P. vivax merozoite invasion into intracellularly labelled CD71+ reticulocytes. Various enzymatic treatments, antibodies and invasion inhibitory molecules were used to successfully demonstrate the utility of this method. Fluorescent labelling of red blood cells did not affect the invasion and early intra-erythrocytic development of P. vivax. Importantly, this portable field assay allows for the economic usage of limited biological material (parasites and reticulocytes) and the intracellular labeling of the target cells reduces the need for highly purified schizont inoculums. This assay will facilitate the study of P. vivax merozoite biology and the testing of vaccine candidates against vivax malaria.

Purification Methodology for Viable and Infective Plasmodium vivax Gametocytes That Is Compatible with Transmission-Blocking Assays

Significant progress toward the control of malaria has been achieved, especially regarding Plasmodium falciparum infections. However, the unique biology of Plasmodium vivax hampers current control strategies. The early appearance of P. vivax gametocytes in the peripheral blood and the impossibility of culturing this parasite are major drawbacks. Using blood samples from 40 P. vivax-infected patients, we describe here a methodology to purify viable gametocytes and further infect anophelines. This method opens new avenues to validate transmission-blocking strategies.

Immunization with the MAEBL M2 Domain Protects against Lethal Plasmodium yoelii Infection

Malaria remains a world-threatening disease largely because of the lack of a long-lasting and fully effective vaccine. MAEBL is a type 1 transmembrane molecule with a chimeric cysteine-rich ectodomain homologous to regions of the Duffy binding-like erythrocyte binding protein and apical membrane antigen 1 (AMA1) antigens. Although MAEBL does not appear to be essential for the survival of blood-stage forms, ectodomains M1 and M2, homologous to AMA1, seem to be involved in parasite attachment to erythrocytes, especially M2. MAEBL is necessary for sporozoite infection of mosquito salivary glands and is expressed in liver stages. Here, the Plasmodium yoelii MAEBL-M2 domain was expressed in a prokaryotic vector. C57BL/6J mice were immunized with doses of P. yoelii recombinant protein rPyM2-MAEBL. High levels of antibodies, with balanced IgG1 and IgG2c subclasses, were achieved. rPyM2-MAEBL antisera were capable of recognizing the native antigen. Anti-MAEBL antibodies recognized different MAEBL fragments expressed in CHO cells, showing stronger IgM and IgG responses to the M2 domain and repeat region, respectively. After a challenge with P. yoelii YM (lethal strain)-infected erythrocytes (IE), up to 90% of the immunized animals survived and a reduction of parasitemia was observed. Moreover, splenocytes harvested from immunized animals proliferated in a dose-dependent manner in the presence of rPyM2-MAEBL. Protection was highly dependent on CD4(+), but not CD8(+), T cells toward Th1. rPyM2-MAEBL antisera were also able to significantly inhibit parasite development, as observed in ex vivo P. yoelii erythrocyte invasion assays. Collectively, these findings support the use of MAEBL as a vaccine candidate and open perspectives to understand the mechanisms involved in protection.

Contrasting ex vivo efficacies of "reversed chloroquine" compounds in chloroquine-resistant Plasmodium falciparum and P. vivax isolates

Chloroquine (CQ) has been the mainstay of malaria treatment for more than 60 years. However, the emergence and spread of CQ resistance now restrict its use to only a few areas where malaria is endemic. The aim of the present study was to investigate whether a novel combination of a CQ-like moiety and an imipramine-like pharmacophore can reverse CQ resistance ex vivo. Between March to October 2011 and January to September 2013, two "reversed chloroquine" (RCQ) compounds (PL69 and PL106) were tested against multidrug-resistant field isolates of Plasmodium falciparum (n = 41) and Plasmodium vivax (n = 45) in Papua, Indonesia, using a modified ex vivo schizont maturation assay. The RCQ compounds showed high efficacy against both CQ-resistant P. falciparum and P. vivax field isolates. For P. falciparum, the median 50% inhibitory concentrations (IC50s) were 23.2 nM for PL69 and 26.6 nM for PL106, compared to 79.4 nM for unmodified CQ (P < 0.001 and P = 0.036, respectively). The corresponding values for P. vivax were 19.0, 60.0, and 60.9 nM (P < 0.001 and P = 0.018, respectively). There was a significant correlation between IC50s of CQ and PL69 (Spearman's rank correlation coefficient [r s] = 0.727, P < 0.001) and PL106 (rs = 0.830, P < 0.001) in P. vivax but not in P. falciparum. Both RCQs were equally active against the ring and trophozoite stages of P. falciparum, but in P. vivax, PL69 and PL106 showed less potent activity against trophozoite stages (median IC50s, 130.2 and 172.5 nM) compared to ring stages (median IC50s, 17.6 and 91.3 nM). RCQ compounds have enhanced ex vivo activity against CQ-resistant clinical isolates of P. falciparum and P. vivax, suggesting the potential use of reversal agents in antimalarial drug development. Interspecies differences in RCQ compound activity may indicate differences in CQ pharmacokinetics between the two Plasmodium species.

Characterization of G6PD genotypes and phenotypes on the northwestern Thailand-Myanmar border

Mutations in the glucose-6-phosphate dehydrogenase (G6PD) gene result in red blood cells with increased susceptibility to oxidative damage. Significant haemolysis can be caused by primaquine and other 8-aminoquinoline antimalarials used for the radical treatment of Plasmodium vivax malaria. The distribution and phenotypes of mutations causing G6PD deficiency in the male population of migrants and refugees in a malaria endemic region on the Thailand-Myanmar border were characterized. Blood samples for G6PD fluorescent spot test (FST), G6PD genotyping, and malaria testing were taken from 504 unrelated males of Karen and Burman ethnicities presenting to the outpatient clinics. The overall frequency of G6PD deficiency by the FST was 13.7%. Among the deficient subjects, almost 90% had the Mahidol variant (487G>A) genotype. The remaining subjects had Chinese-4 (392G>T), Viangchan (871G>A), Açores (595A>G), Seattle (844G>C) and Mediterranean (563C>T) variants. Quantification of G6PD activity was performed using a modification of the standard spectrophotometric assay on a subset of 24 samples with Mahidol, Viangchan, Seattle and Chinese-4 mutations; all samples showed a residual enzymatic activity below 10% of normal and were diagnosed correctly by the FST. Further studies are needed to characterise the haemolytic risk of using 8-aminoquinolines in patients with these genotypes.

Histone methyltransferase inhibitors are orally bioavailable, fast-acting molecules with activity against different species causing malaria in humans

Current antimalarials are under continuous threat due to the relentless development of drug resistance by malaria parasites. We previously reported promising in vitro parasite-killing activity with the histone methyltransferase inhibitor BIX-01294 and its analogue TM2-115. Here, we further characterize these diaminoquinazolines for in vitro and in vivo efficacy and pharmacokinetic properties to prioritize and direct compound development. BIX-01294 and TM2-115 displayed potent in vitro activity, with 50% inhibitory concentrations (IC50s) of <50 nM against drug-sensitive laboratory strains and multidrug-resistant field isolates, including artemisinin-refractory Plasmodium falciparum isolates. Activities against ex vivo clinical isolates of both P. falciparum and Plasmodium vivax were similar, with potencies of 300 to 400 nM. Sexual-stage gametocyte inhibition occurs at micromolar levels; however, mature gametocyte progression to gamete formation is inhibited at submicromolar concentrations. Parasite reduction ratio analysis confirms a high asexual-stage rate of killing. Both compounds examined displayed oral efficacy in in vivo mouse models of Plasmodium berghei and P. falciparum infection. The discovery of a rapid and broadly acting antimalarial compound class targeting blood stage infection, including transmission stage parasites, and effective against multiple malaria-causing species reveals the diaminoquinazoline scaffold to be a very promising lead for development into greatly needed novel therapies to control malaria.

Plasmodium vivax: restricted tropism and rapid remodeling of CD71-positive reticulocytes

Plasmodium vivax merozoites only invade reticulocytes, a minor though heterogeneous population of red blood cell precursors that can be graded by levels of transferrin receptor (CD71) expression. The development of a protocol that allows sorting reticulocytes into defined developmental stages and a robust ex vivo P vivax invasion assay has made it possible for the first time to investigate the fine-scale invasion preference of P vivax merozoites. Surprisingly, it was the immature reticulocytes (CD71(+)) that are generally restricted to the bone marrow that were preferentially invaded, whereas older reticulocytes (CD71(-)), principally found in the peripheral blood, were rarely invaded. Invasion assays based on the CD71(+) reticulocyte fraction revealed substantial postinvasion modification. Thus, 3 to 6 hours after invasion, the initially biomechanically rigid CD71(+) reticulocytes convert into a highly deformable CD71(-) infected red blood cell devoid of host reticular matter, a process that normally spans 24 hours for uninfected reticulocytes. Concurrent with these changes, clathrin pits disappear by 3 hours postinvasion, replaced by distinctive caveolae nanostructures. These 2 hitherto unsuspected features of P vivax invasion, a narrow preference for immature reticulocytes and a rapid remodeling of the host cell, provide important insights pertinent to the pathobiology of the P vivax infection.