Parameterization of high magnetic field gradient fractionation columns for applications with Plasmodium falciparum infected human erythrocytes

Plasmodium falciparum Gametocyte Enrichment in Peripheral Blood Samples by Magnetic Fractionation: Gametocyte Yields and Possibilities to Reuse Columns

Gametocytes are sexual stage malaria parasites responsible for transmission to mosquitoes. Multiple gametocyte-producing clones may be present in natural infections, but the molecular characterization of gametocytes is challenging. Because of their magnetic properties, gametocyte enrichment can be achieved by magnetic fractionation. This increases detection sensitivity and allows specific genotyping of clones that contribute to malaria transmission. Here, we determined the percentage of Plasmodium falciparum gametocytes successfully bound to magnetic activated cell sorting (MACS) LS columns during magnetic fractionation and assessed whether columns can be reused without risking contamination or affecting column binding efficiency. Bound column fractions were quantified using multiplex quantitative reverse transcription polymerase chain reaction (qRT-PCR) for male (pfMGET) and female (CCp4) gametocytes and ring-stage asexual parasites (SBP1). To investigate cross contamination between columns, parasite strain identity was determined by merozoite surface protein 2 genotyping followed by capillary electrophoresis fragment sizing. A reproducible high percentage of gametocytes was bound to MACS LS columns with < 5% gametocytes appearing in the flow-through and < 0.6% asexual ring-stage parasites appearing in the gametocyte fraction. A high yield (> 94%) of gametocyte enrichment was achieved when columns were used up to five times with lower binding success after eight times (79%). We observed no evidence for cross contamination between columns.

Biosynthesis of heme O in intraerythrocytic stages of Plasmodium falciparum and potential inhibitors of this pathway

A number of antimalarial drugs interfere with the electron transport chain and heme-related reactions; however, the biosynthesis of heme derivatives in Plasmodium parasites has not been fully elucidated. Here, we characterized the steps that lead to the farnesylation of heme. After the identification of a gene encoding heme O synthase, we identified heme O synthesis in blood stage parasites through the incorporation of radioactive precursors. The presence of heme O synthesis in intraerythrocytic stages of Plasmodium falciparum was confirmed by mass spectrometry. Inabenfide and uniconazole-P appeared to interfere in heme synthesis, accordingly, parasite growth was also affected by the addition of these drugs. We conclude that heme O synthesis occurs in blood stage-P. falciparum and this pathway could be a potential target for antimalarial drugs.

The paramagnetic properties of malaria pigment, hemozoin, yield clues to a low-cost system for its trapping and determination

The binding of malaria pigment, hemozoin, by a gradient magnetic field has been investigated in a manual trapping column system. Two types of magnetic filling have been tested to produce field gradients: nickel-plated steel wires, wrapped around a steel core, and superparamagnetic microbeads. The latter system allows an efficient trapping (> 80%) of β-hematin (a synthetic pigment with physical and paramagnetic properties analogous to those of hemozoin). Tests with a Plasmodium falciparum 3D7 culture indicate that hemozoin is similarly trapped. Off-line optical spectroscopy measurements present limited sensitivity as the hemozoin we detected from in vitro cultured parasites would correspond to only a theoretical 0.02% parasitemia (1000 parasites/µL). Further work needs to be undertaken to reduce this threshold to a practical detectability level. Based on these data, a magneto-chromatographic on-line system with reduced dead volumes is proposed as a possible low-cost instrument to be tested as a malaria diagnosis system.

Plasmodium falciparum uses vitamin E to avoid oxidative stress

BACKGROUND

Plasmodium falciparum is sensitive to oxidative stress in vitro and in vivo, and many drugs such as artemisinin, chloroquine and cercosporin interfere in the parasite's redox system. To minimize the damage caused by reactive radicals, antioxidant enzymes and their substrates found in parasites and in erythrocytes must be functionally active. It was shown that P. falciparum synthesizes vitamin E and that usnic acid acts as an inhibitor of its biosynthesis. Vitamin E is a potent antioxidant that protects polyunsaturated fatty acids from lipid peroxidation, and this activity can be measured by detecting its oxidized product and by evaluating reactive oxygen species (ROS) levels.

RESULTS

Here, we demonstrated that ROS levels increased in P. falciparum when vitamin E biosynthesis was inhibited by usnic acid treatment and decreased to basal levels if exogenous vitamin E was added. Furthermore, we used metabolic labelling to demonstrate that vitamin E biosynthesized by the parasite acts as an antioxidant since we could detect its radiolabeled oxidized product. The treatment with chloroquine or cercosporin of the parasites increased the ratio between α-tocopherolquinone and α-tocopherol.

CONCLUSIONS

Our findings demonstrate that vitamin E produced endogenously by P. falciparum is active as an antioxidant, probably protecting the parasite from the radicals generated by drugs.

Temporal changes in Plasmodium falciparum anti-malarial drug sensitivity in vitro and resistance-associated genetic mutations in isolates from Papua New Guinea

Background

In northern Papua New Guinea (PNG), most Plasmodium falciparum isolates proved resistant to chloroquine (CQ) in vitro between 2005 and 2007, and there was near-fixation of pfcrt K76T, pfdhfr C59R/S108N and pfmdr1 N86Y. To determine whether the subsequent introduction of artemisinin combination therapy (ACT) and reduced CQ-sulphadoxine-pyrimethamine pressure had attenuated parasite drug susceptibility and resistance-associated mutations, these parameters were re-assessed between 2011 and 2013.

Methods

A validated fluorescence-based assay was used to assess growth inhibition of 52 P. falciparum isolates from children in a clinical trial in Madang Province. Responses to CQ, lumefantrine, piperaquine, naphthoquine, pyronaridine, artesunate, dihydroartemisinin, artemether were assessed. Molecular resistance markers were detected using a multiplex PCR ligase detection reaction fluorescent microsphere assay.

Results

CQ resistance (in vitro concentration required for 50% parasite growth inhibition (IC₅₀) >100 nM) was present in 19% of isolates. All piperaquine and naphthoquine IC₅₀s were <100 nM and those for lumefantrine, pyronaridine and the artemisinin derivatives were in low nM ranges. Factor analysis of IC₅₀s showed three groupings (lumefantrine; CQ, piperaquine, naphthoquine; pyronaridine, dihydroartemisinin, artemether, artesunate). Most isolates (96%) were monoclonal pfcrt K76T (SVMNT) mutants and most (86%) contained pfmdr1 N86Y (YYSND). No wild-type pfdhfr was found but most isolates contained wild-type (SAKAA) pfdhps. Compared with 2005–2007, the geometric mean (95% CI) CQ IC₅₀ was lower (87 (71–107) vs 167 (141–197) nM) and there had been no change in the prevalence of pfcrt K76T or pfmdr1 mutations. There were fewer isolates of the pfdhps (SAKAA) wild-type (60 vs 100%) and pfdhfr mutations persisted.

Conclusions

Reflecting less drug pressure, in vitro CQ sensitivity appears to be improving in Madang Province despite continued near-fixation of pfcrt K76T and pfmdr1 mutations. Temporal changes in IC₅₀s for other anti-malarial drugs were inconsistent but susceptibility was preserved. Retention or increases in pfdhfr and pfdhps mutations reflect continued use of sulphadoxine-pyrimethamine in the study area including through paediatric intermittent preventive treatment. The susceptibility of local isolates to lumefantrine may be unrelated to those of other ACT partner drugs.

Trial registration

Australian New Zealand Clinical Trials Registry ACTRN12610000913077.

Comparison of three methods for detection of gametocytes in Melanesian children treated for uncomplicated malaria

Background

Gametocytes are the transmission stages of Plasmodium parasites, the causative agents of malaria. As their density in the human host is typically low, they are often undetected by conventional light microscopy. Furthermore, application of RNA-based molecular detection methods for gametocyte detection remains challenging in remote field settings. In the present study, a detailed comparison of three methods, namely light microscopy, magnetic fractionation and reverse transcriptase polymerase chain reaction for detection of Plasmodium falciparum and Plasmodium vivax gametocytes was conducted.

Methods

Peripheral blood samples from 70 children aged 0.5 to five years with uncomplicated malaria who were treated with either artemether-lumefantrine or artemisinin-naphthoquine were collected from two health facilities on the north coast of Papua New Guinea. The samples were taken prior to treatment (day 0) and at pre-specified intervals during follow-up. Gametocytes were measured in each sample by three methods: i) light microscopy (LM), ii) quantitative magnetic fractionation (MF) and, iii) reverse transcriptase PCR (RTPCR). Data were analysed using censored linear regression and Bland and Altman techniques.

Results

MF and RTPCR were similarly sensitive and specific, and both were superior to LM. Overall, there were approximately 20% gametocyte-positive samples by LM, whereas gametocyte positivity by MF and RTPCR were both more than two-fold this level. In the subset of samples collected prior to treatment, 29% of children were positive by LM, and 85% were gametocyte positive by MF and RTPCR, respectively.

Conclusions

The present study represents the first direct comparison of standard LM, MF and RTPCR for gametocyte detection in field isolates. It provides strong evidence that MF is superior to LM and can be used to detect gametocytaemic patients under field conditions with similar sensitivity and specificity as RTPCR.

Electronic supplementary material

The online version of this article (doi:10.1186/1475-2875-13-319) contains supplementary material, which is available to authorized users.

Magnetic isolation of Plasmodium falciparum schizonts iRBCs to generate a high parasitaemia and synchronized in vitro culture

BACKGROUND

The establishment of methods for an in vitro continuous culture of Plasmodium falciparum is essential for gaining knowledge into its biology and for the development of new treatments. Previously, several techniques have been used to synchronize, enrich and concentrate P. falciparum, although obtaining cultures with high parasitaemia continues being a challenging process. Current methods produce high parasitaemia levels of synchronized P. falciparum cultures by frequent changes of culture medium or reducing the haematocrit. However, these methods are time consuming and sometimes lead to the loss of synchrony.

METHODS

A procedure that combines Percoll and sorbitol treatments, the use of magnetic columns, and the optimization of the in vitro culture conditions to reach high parasitaemia levels for synchronized Plasmodium falciparum cultures is described.

RESULTS

A new procedure has been established using P. falciparum 3D7, combining previous reported methodologies to achieve in vitro parasite cultures that reach parasitaemia up to 40% at any intra-erythrocytic stage. High parasitaemia levels are obtained only one day after magnetic column purification without compromising the parasite viability and synchrony.

CONCLUSIONS

The described procedure allows obtaining a large scale synchronized parasite culture at a high parasitaemia with less manipulations than other methods previously described.

Identification of MMV malaria box inhibitors of plasmodium falciparum early-stage gametocytes using a luciferase-based high-throughput assay

The design of new antimalarial combinations to treat Plasmodium falciparum infections requires drugs that, in addition to resolving disease symptoms caused by asexual blood stage parasites, can also interrupt transmission to the mosquito vector. Gametocytes, which are essential for transmission, develop as sexual blood stage parasites in the human host over 8 to 12 days and are the most accessible developmental stage for transmission-blocking drugs. Considerable effort is currently being devoted to identifying compounds active against mature gametocytes. However, investigations on the drug sensitivity of developing gametocytes, as well as screening methods for identifying inhibitors of early gametocytogenesis, remain scarce. We have developed a luciferase-based high-throughput screening (HTS) assay using tightly synchronous stage I to III gametocytes from a recombinant P. falciparum line expressing green fluorescent protein (GFP)-luciferase. The assay has been used to evaluate the early-stage gametocytocidal activity of the MMV Malaria Box, a collection of 400 compounds with known antimalarial (asexual blood stage) activity. Screening this collection against early-stage (I to III) gametocytes yielded 64 gametocytocidal compounds with 50% inhibitory concentrations (IC50s) below 2.5 μM. This assay is reproducible and suitable for the screening of large compound libraries, with an average percent coefficient of variance (%CV) of ≤5%, an average signal-to-noise ratio (S:N) of >30, and a Z' of ∼0.8. Our findings highlight the need for screening efforts directed specifically against early gametocytogenesis and indicate the importance of experimental verification of early-stage gametocytocidal activity in the development of new antimalarial candidates for combination therapy.

Whole-cell in vitro screening for gametocytocidal compounds

The discovery of new chemical starting points with the ability to inhibit Plasmodium falciparum sexual stages, and therefore block the disease transmission, is urgently required. These will form the basis for the development of new therapeutic combinations for the treatment and elimination of malaria and the ultimate goal of global eradication. Recent screening of large chemical libraries against the parasite asexual stages has resulted in the public availability of focused subsets of known antimalarial actives, which represent an excellent starting point for the identification of new gametocytocidal compounds. New stage-specific methodologies aimed at increasing the throughput for assessing compound activity against in vitro cultured gametocytes have recently been published. This article discusses the challenges of assay-oriented large-scale gametocyte culturing and reviews the state-of-the art in gametocytocidal assay development and outcomes.

Ingested human insulin inhibits the mosquito NF-κB-dependent immune response to Plasmodium falciparum

We showed previously that ingested human insulin activates the insulin/IGF-1 signaling pathway in Anopheles stephensi and increases the susceptibility of these mosquitoes to Plasmodium falciparum. In other organisms, insulin can alter immune responsiveness through regulation of NF-κB transcription factors, critical elements for innate immunity that are also central to mosquito immunity. We show here that insulin signaling decreased expression of NF-κB-regulated immune genes in mosquito cells stimulated with either bacterial or malarial soluble products. Further, human insulin suppressed mosquito immunity through sustained phosphatidylinositol 3-kinase activation, since inhibition of this pathway led to decreased parasite development in the mosquito. Together, these data demonstrate that activation of the insulin/IGF-1 signaling pathway by ingested human insulin can alter NF-κB-dependent immunity, and ultimately the susceptibility, of mosquitoes to P. falciparum.