Purification of mature-stage Plasmodium falciparum by gelatine flotation

Acquisition of antibodies to Plasmodium falciparum and Plasmodium vivax antigens in pregnant women living in a low malaria transmission area of Brazil

BACKGROUND

Pregnant women have increased susceptibility to Plasmodium falciparum malaria and acquire protective antibodies over successive pregnancies. Most studies that investigated malaria antibody responses in pregnant women are from high transmission areas in sub-Saharan Africa, while reports from Latin America are scarce and inconsistent. The present study sought to explore the development of antibodies against P. falciparum and Plasmodium vivax antigens in pregnant women living in a low transmission area in the Brazilian Amazon.

METHODS

In a prospective cohort study, plasma samples from 408 pregnant women (of whom 111 were infected with P. falciparum, 96 had infections with P. falciparum and P. vivax, and 201 had no Plasmodium infection) were used to measure antibody levels. Levels of IgG and opsonizing antibody to pregnancy-specific variant surface antigens (VSAs) on infected erythrocytes (IEs), 10 recombinant VAR2CSA Duffy binding like (DBL domains), 10 non-pregnancy-specific P. falciparum merozoite antigens, and 10 P. vivax antigens were measured by flow cytometry, ELISA, and multiplex assays. Antibody levels and seropositivity among the groups were compared.

RESULTS

Antibodies to VSAs on P. falciparum IEs were generally low but were higher in currently infected women and women with multiple P. falciparum episodes over pregnancy. Many women (21%-69%) had antibodies against each individual VAR2CSA DBL domain, and antibodies to DBLs correlated with each other (r ≥ 0.55, p < 0.0001), but not with antibody to VSA or history of infection. Infection with either malaria species was associated with higher seropositivity rate for antibodies against P. vivax proteins, adjusted odds ratios (95% CI) ranged from 5.6 (3.2, 9.7), p < 0.0001 for PVDBPII-Sal1 to 15.7 (8.3, 29.7), p < 0.0001 for PvTRAg_2.

CONCLUSIONS

Pregnant Brazilian women had low levels of antibodies to pregnancy-specific VSAs that increased with exposure. They frequently recognized both VAR2CSA DBL domains and P. vivax antigens, but only the latter varied with infection. Apparent antibody prevalence is highly dependent on the assay platform used.

Deletion of the Plasmodium falciparum exported protein PTP7 leads to Maurer’s clefts vesiculation, host cell remodeling defects, and loss of surface presentation of EMP1

Presentation of the variant antigen, Plasmodium falciparum erythrocyte membrane protein 1 (EMP1), at knob-like protrusions on the surface of infected red blood cells, underpins the parasite’s pathogenicity. Here we describe a protein PF3D7_0301700 (PTP7), that functions at the nexus between the intermediate trafficking organelle, the Maurer’s cleft, and the infected red blood cell surface. Genetic disruption of PTP7 leads to accumulation of vesicles at the Maurer’s clefts, grossly aberrant knob morphology, and failure to deliver EMP1 to the red blood cell surface. We show that an expanded low complexity sequence in the C-terminal region of PTP7, identified only in the Laverania clade of Plasmodium, is critical for efficient virulence protein trafficking.

We describe a malaria parasite protein, PTP7, involved in virulence factor trafficking that is associated with Maurer’s clefts and other trafficking compartments. Upon disruption of the PTP7 locus, the Maurer’s clefts become decorated with vesicles; the knobby protrusions on the host red blood cell surface are fewer and distorted; and trafficking of the virulence protein, EMP1, to the host red blood cell surface is ablated. We provide evidence that a region of PTP7 with low sequence complexity plays an important role in virulence protein trafficking from the Maurer’s clefts.

Static Adhesion of Plasmodium falciparum-Infected Erythrocytes to Purified and Recombinant Receptors

The pathology of Plasmodium falciparum malaria syndromes, such as cerebral malaria, severe anemia, respiratory distress, and malaria in pregnancy are associated with the cytoadherence of P. falciparum-infected erythrocytes (IEs) to host receptors. To investigate binding of laboratory strains or patient isolates to specific receptors, a relatively simple but informative method is a static binding assay. Purified protein receptors are absorbed onto polystyrene dishes, overlaid with a trophozoite IE suspension and incubated for a fixed time. After washing to remove unbound cells, the plates are fixed, stained, and adherent IEs counted by microscopy. Although simple, this assay requires careful implementation to provide reproducible results, but it is deliverable in relatively low-resource settings and so well matched to using fresh patient isolates for adhesion assays.

Immunomagnetic Selection of Plasmodium falciparum-Infected Erythrocytes Expressing Particular PfEMP1 Variants

Plasmodium falciparum expresses a broad range of proteins on the surface of infected erythrocytes (IEs), including members of the P. falciparum erythrocyte membrane protein 1 (PfEMP1) family. This protocol describes an immunomagnetic selection method using PfEMP1-specific antibodies to obtain a parasite clone homogenously expressing a particular PfEMP1 protein. The expression of the corresponding PfEMP1 is later tested by flow cytometry, and the selected parasites can be used for further analysis.

Receptor-Functionalized Lipid Membranes as Biomimetic Surfaces for Adhesion of Plasmodium falciparum-Infected Erythrocytes

Here, we describe a detailed protocol of how to manufacture biomimetic, host receptor-functionalized membranes and how to use them in adhesion assays. Receptor-functionalized membranes have the advantage that the receptor identity and the receptor density can be controlled, which, in turn, enables studies on the kinetics, dynamics and biomechanics of receptor/ligand interactions. Such information is difficult to obtain from currently used in vitro systems, including cultured primary human microvascular endothelial cells or receptor-coated surfaces, which often display either multiple receptors or receptors with uncertain density and arrangement.

Establishing and Maintaining In Vitro Cultures of Asexual Blood Stages of Plasmodium falciparum

In vitro culture of asexual blood stages of Plasmodium falciparum is essential to study the parasite biology, and several aspects need to be addressed to successfully cultivate the parasites, including the requirements for red blood cells and specific nutrients. Since Trager and Jensen established the technique in 1976, some modifications have been introduced to improve the growth rate and yield. In brief, the method is based on the use of human red blood cells suspended in RPMI-1640 culture medium supplemented with a source of lipids and maintained at 37 °C in a low-oxygen atmosphere. In this protocol, a description of thawing, culturing, and cryopreservation of asexual blood stages of P. falciparum is presented.

Co-chaperone involvement in knob biogenesis implicates host-derived chaperones in malaria virulence

The pathology associated with malaria infection is largely due to the ability of infected human RBCs to adhere to a number of receptors on endothelial cells within tissues and organs. This phenomenon is driven by the export of parasite-encoded proteins to the host cell, the exact function of many of which is still unknown. Here we inactivate the function of one of these exported proteins, PFA66, a member of the J-domain protein family. Although parasites lacking this protein were still able to grow in cell culture, we observed severe defects in normal host cell modification, including aberrant morphology of surface knobs, disrupted presentation of the cytoadherence molecule PfEMP1, and a total lack of cytoadherence, despite the presence of the knob associated protein KAHRP. Complementation assays demonstrate that an intact J-domain is required for recovery to a wild-type phenotype and suggest that PFA66 functions in concert with a HSP70 to carry out host cell modification. Strikingly, this HSP70 is likely to be of host origin. ATPase assays on recombinant protein verify a functional interaction between PFA66 and residual host cell HSP70. Taken together, our data reveal a role for PFA66 in host cell modification, strongly implicate human HSP70s as being essential in this process and uncover a new KAHRP-independent molecular factor required for correct knob biogenesis.

To survive in the human body, the malaria parasite invades and lives within human red blood cells. Once within the red blood cell, the parasite renovates the host cell to its own needs. Here we have studied which factors from both parasite and host cell are required for this renovation process, and discover that human chaperone proteins, referred to as HSP70, are required. It appears that a particular parasite-derived protein, PFA66, recruits and modifies the function of the human HSP70. As this interaction between a parasite and human protein is novel and essential for parasite survival, our study identifies a potential Achilles’ Heel which may be targeted for development of new anti-malaria therapies.

Functionalized supported membranes for quantifying adhesion of P. falciparum-infected erythrocytes

The pathology of Plasmodium falciparum malaria is largely defined by the cytoadhesion of infected erythrocytes to the microvascular endothelial lining. The complexity of the endothelial surface and the large range of interactions available for the infected erythrocyte via parasite-encoded adhesins make analysis of critical contributions during cytoadherence challenging to define. Here, we have explored supported membranes functionalized with two important adhesion receptors, ICAM1 or CD36, as a quantitative biomimetic surface to help understand the processes involved in cytoadherence. Parasitized erythrocytes bound to the receptor-functionalized membranes with high efficiency and selectivity under both static and flow conditions, with infected wild-type erythrocytes displaying a higher binding capacity than do parasitized heterozygous sickle cells. We further show that the binding efficiency decreased with increasing intermolecular receptor distance and that the cell-surface contacts were highly dynamic and increased with rising wall shear stress as the cell underwent a shape transition. Computer simulations using a deformable cell model explained the wall-shear-stress-induced dynamic changes in cell shape and contact area via the specific physical properties of erythrocytes, the density of adhesins presenting knobs, and the lateral movement of receptors in the supported membrane.

Developing a multivariate prediction model of antibody features associated with protection of malaria-infected pregnant women from placental malaria

Background

Plasmodium falciparum causes placental malaria, which results in adverse outcomes for mother and child. P. falciparum-infected erythrocytes that express the parasite protein VAR2CSA on their surface can bind to placental chondroitin sulfate A. It has been hypothesized that naturally acquired antibodies towards VAR2CSA protect against placental infection, but it has proven difficult to identify robust antibody correlates of protection from disease. The objective of this study was to develop a prediction model using antibody features that could identify women protected from placental malaria.

Methods

We used a systems serology approach with elastic net-regularized logistic regression, partial least squares discriminant analysis, and a case-control study design to identify naturally acquired antibody features mid-pregnancy that were associated with protection from placental malaria at delivery in a cohort of 77 pregnant women from Madang, Papua New Guinea.

Results

The machine learning techniques selected 6 out of 169 measured antibody features towards VAR2CSA that could predict (with 86% accuracy) whether a woman would subsequently have active placental malaria infection at delivery. Selected features included previously described associations with inhibition of placental binding and/or opsonic phagocytosis of infected erythrocytes, and network analysis indicated that there are not one but multiple pathways to protection from placental malaria.

Conclusions

We have identified candidate antibody features that could accurately identify malaria-infected women as protected from placental infection. It is likely that there are multiple pathways to protection against placental malaria.

Funding

This study was supported by the National Health and Medical Research Council (Nos. APP1143946, GNT1145303, APP1092789, APP1140509, and APP1104975).

Innate immune responses to malaria-infected erythrocytes in pregnant women: Effects of gravidity, malaria infection, and geographic location

BACKGROUND

Malaria in pregnancy causes maternal, fetal and neonatal morbidity and mortality, and maternal innate immune responses are implicated in pathogenesis of these complications. The effects of malaria exposure and obstetric and demographic factors on the early maternal immune response are poorly understood.

METHODS

Peripheral blood mononuclear cell responses to Plasmodium falciparum-infected erythrocytes and phytohemagglutinin were compared between pregnant women from Papua New Guinea (malaria-exposed) with and without current malaria infection and from Australia (unexposed). Elicited levels of inflammatory cytokines at 48 h and 24 h (interferon γ, IFN-γ only) and the cellular sources of IFN-γ were analysed.

RESULTS

Among Papua New Guinean women, microscopic malaria at enrolment did not alter peripheral blood mononuclear cell responses. Compared to samples from Australia, cells from Papua New Guinean women secreted more inflammatory cytokines tumor necrosis factor-α, interleukin 1β, interleukin 6 and IFN-γ; p<0.001 for all assays, and more natural killer cells produced IFN-γ in response to infected erythrocytes and phytohemagglutinin. In both populations, cytokine responses were not affected by gravidity, except that in the Papua New Guinean cohort multigravid women had higher IFN-γ secretion at 24 h (p = 0.029) and an increased proportion of IFN-γ+ Vδ2 γδ T cells (p = 0.003). Cytokine levels elicited by a pregnancy malaria-specific CSA binding parasite line, CS2, were broadly similar to those elicited by CD36-binding line P6A1.

CONCLUSIONS

Geographic location and, to some extent, gravidity influence maternal innate immunity to malaria.

Different Life Cycle Stages of Plasmodium falciparum Induce Contrasting Responses in Dendritic Cells

Dendritic cells are key linkers of innate and adaptive immunity. Efficient dendritic cell activation is central to the acquisition of immunity and the efficacy of vaccines. Understanding how dendritic cells are affected by Plasmodium falciparum blood-stage parasites will help to understand how immunity is acquired and maintained, and how vaccine responses may be impacted by malaria infection or exposure. This study investigates the response of dendritic cells to two different life stages of the malaria parasite, parasitized red blood cells and merozoites, using a murine model. We demonstrate that the dendritic cell responses to merozoites are robust whereas dendritic cell activation, particularly CD40 and pro-inflammatory cytokine expression, is compromised in the presence of freshly isolated parasitized red blood cells. The mechanism of dendritic cell suppression by parasitized red blood cells is host red cell membrane-independent. Furthermore, we show that cryopreserved parasitized red blood cells have a substantially reduced capacity for dendritic cell activation.

The impact of early life exposure to Plasmodium falciparum on the development of naturally acquired immunity to malaria in young Malawian children

Background

Antibodies targeting malaria blood-stage antigens are important targets of naturally acquired immunity, and may act as valuable biomarkers of malaria exposure.

Methods

Six-hundred and one young Malawian children from a randomized trial of prenatal nutrient supplementation with iron and folic acid or pre- and postnatal multiple micronutrients or lipid-based nutrient supplements were followed up weekly at home and febrile episodes were investigated for malaria from birth to 18 months of age. Antibodies were measured for 601 children against merozoite surface proteins (MSP1 19kD, MSP2), erythrocyte binding antigen 175 (EBA175), reticulocyte binding protein homologue 2 (Rh2A9), schizont extract and variant surface antigens expressed by Plasmodium falciparum-infected erythrocytes (IE) at 18 months of age. The antibody measurement data was related to concurrent malaria infection and to documented episodes of clinical malaria.

Results

At 18 months of age, antibodies were significantly higher among parasitaemic than aparasitaemic children. Antibody levels against MSP1 19kD, MSP2, schizont extract, and IE variant surface antigens were significantly higher in children who had documented episodes of malaria than in children who did not. Antibody levels did not differ between children with single or multiple malaria episodes before 18 months, nor between children who had malaria before 6 months of age or between 6 and 18 months.

Conclusions

Antibodies to merozoite and IE surface antigens increased following infection in early childhood, but neither age at first infection nor number of malaria episodes substantially affected antibody acquisition. These findings have implications for malaria surveillance during early childhood in the context of elimination.

Trials registration Clinical Trials Registration: NCT01239693 (Date of registration: 11-10-2010). URL: http://www.ilins.org

The sickle cell trait affects contact dynamics and endothelial cell activation in Plasmodium falciparum-infected erythrocytes

Sickle cell trait, a common hereditary blood disorder, protects carriers from severe disease in infections with the human malaria parasite Plasmodium falciparum. Protection is associated with a reduced capacity of parasitized erythrocytes to cytoadhere to the microvascular endothelium and cause vaso-occlusive events. However, the underpinning cellular and biomechanical processes are only partly understood and the impact on endothelial cell activation is unclear. Here, we show, by combining quantitative flow chamber experiments with multiscale computer simulations of deformable cells in hydrodynamic flow, that parasitized erythrocytes containing the sickle cell haemoglobin displayed altered adhesion dynamics, resulting in restricted contact footprints on the endothelium. Main determinants were cell shape, knob density and membrane bending. As a consequence, the extent of endothelial cell activation was decreased. Our findings provide a quantitative understanding of how the sickle cell trait affects the dynamic cytoadhesion behavior of parasitized erythrocytes and, in turn, endothelial cell activation.

Resistance to Plasmodium falciparum in sickle cell trait erythrocytes is driven by oxygen-dependent growth inhibition

Sickle cell trait (AS) confers partial protection against lethal Plasmodium falciparum malaria. Multiple mechanisms for this have been proposed, with a recent focus on aberrant cytoadherence of parasite-infected red blood cells (RBCs). Here we investigate the mechanistic basis of AS protection through detailed temporal mapping. We find that parasites in AS RBCs maintained at low oxygen concentrations stall at a specific stage in the middle of intracellular growth before DNA replication. We demonstrate that polymerization of sickle hemoglobin (HbS) is responsible for this growth arrest of intraerythrocytic P. falciparum parasites, with normal hemoglobin digestion and growth restored in the presence of carbon monoxide, a gaseous antisickling agent. Modeling of growth inhibition and sequestration revealed that HbS polymerization-induced growth inhibition following cytoadherence is the critical driver of the reduced parasite densities observed in malaria infections of individuals with AS. We conclude that the protective effect of AS derives largely from effective sequestration of infected RBCs into the hypoxic microcirculation.

Extracellular vesicles from early stage Plasmodium falciparum-infected red blood cells contain PfEMP1 and induce transcriptional changes in human monocytes

Pathogens can release extracellular vesicles (EVs) for cell-cell communication and host modulation. EVs from Plasmodium falciparum, the deadliest malaria parasite species, can transfer drug resistance genes between parasites. EVs from late-stage parasite-infected RBC (iRBC-EVs) are immunostimulatory and affect endothelial cell permeability, but little is known about EVs from early stage iRBC. We detected the parasite virulence factor PfEMP1, which is responsible for iRBC adherence and a major contributor to disease severity, in EVs, only up to 12-hr post-RBC invasion. Furthermore, using PfEMP1 transport knockout parasites, we determined that EVs originated from inside the iRBC rather than the iRBC surface. Proteomic analysis detected 101 parasite and 178 human proteins in iRBC-EVs. Primary human monocytes stimulated with iRBC-EVs released low levels of inflammatory cytokines and showed transcriptomic changes. Stimulation with iRBC-EVs from PfEMP1 knockout parasites induced more gene expression changes and affected pathways involved in defence response, stress response, and response to cytokines, suggesting a novel function of PfEMP1 when present in EVs. We show for the first time the presence of PfEMP1 in early stage P. falciparum iRBC-EVs and the effects of these EVs on primary human monocytes, uncovering a new mechanism of potential parasite pathogenesis and host interaction.

Effect of nutrient supplementation on the acquisition of humoral immunity to Plasmodium falciparum in young Malawian children

Background

There is evidence that suggests that undernutrition has a detrimental effect on malarial immunity in children. The aim of the study was to discover whether nutrient supplementation improved development of malarial antibody immunity in children up to 18 months of age.

Methods

The study was conducted with a subset of 432 Malawian children from a randomized controlled trial of nutritional supplements. The arms included pre- and postnatal small-quantity lipid-based nutrient supplements for both mother and child; prenatal supplementation with iron and folic acid; and pre- and postnatal supplementation with multiple micronutrients. Paired plasma samples were collected at 6 and 18 months of age. The levels of antibodies against merozoite surface protein 1 (MSP1 19kD) and MSP2, erythrocyte binding antigen 175 (EBA175), reticulocyte binding protein homologue 2A (Rh2A9), schizont extract and variant antigens expressed on the surface of infected erythrocytes were measured.

Results

At 18 months of age, 5.4% of children were parasitaemic by microscopy and 49.1% were anaemic. Antibodies to the tested merozoite antigens and schizont extract increased between 6 and 18 months and this increase was statistically significant for MSP1, MSP2 and EBA175 (p < 0.0001) whereas IgG to variant surface antigens decreased with increasing age (p < 0.0001). However, the supplementation type did not have any impact on the prevalence or levels of antibodies at either 6 or 18 months of age to any of the tested malaria antigens in either univariate analysis or multivariate analysis after adjusting for covariates.

Conclusions

Pre- and postnatal lipid-based nutrient supplementation did not alter malaria antibody acquisition during infancy, compared to prenatal supplementation with iron and folic acid or pre- and postnatal supplementation with multiple micronutrients.

Trail registeration Clinicaltrials.gov registration number NCT01239693

Atypical activation of dendritic cells by Plasmodium falciparum

Dendritic cells (DCs) are activated by pathogens to initiate and shape immune responses. We found that the activation of DCs by Plasmodium falciparum, the main causative agent of human malaria, induces a highly unusual phenotype by which DCs up-regulate costimulatory molecules and secretion of chemokines, but not of cytokines typical of inflammatory responses (IL-1β, IL-6, IL-10, TNF). Similar results were obtained with DCs obtained from malaria-naïve US donors and malaria-experienced donors from Mali. Contact-dependent cross-talk between the main DC subsets, plasmacytoid and myeloid DCs (mDCs) was necessary for increased chemokine and IFN-α secretion in response to the parasite. Despite the absence of inflammatory cytokine secretion, mDCs incubated with P. falciparum-infected erythrocytes activated antigen-specific naïve CD4⁺ T cells to proliferate and secrete Th1-like cytokines. This unexpected response of human mDCs to P. falciparum exhibited a transcriptional program distinct from a classical LPS response, pointing to unique P. falciparum-induced activation pathways that may explain the uncharacteristic immune response to malaria.

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.

Rolling Adhesion of Schizont Stage Malaria-Infected Red Blood Cells in Shear Flow

To avoid clearance by the spleen, red blood cells infected with the human malaria parasite Plasmodium falciparum (iRBCs) adhere to the vascular endothelium through adhesive protrusions called "knobs" that the parasite induces on the surface of the host cell. However, the detailed relation between the developing knob structure and the resulting movement in shear flow is not known. Using flow chamber experiments on endothelial monolayers and tracking of the parasite inside the infected host cell, we find that trophozoites (intermediate-stage iRBCs) tend to flip due to their biconcave shape, whereas schizonts (late-stage iRBCs) tend to roll due to their almost spherical shape. We then use adhesive dynamics simulations for spherical cells to predict the effects of knob density and receptor multiplicity per knob on rolling adhesion of schizonts. We find that rolling adhesion requires a homogeneous coverage of the cell surface by knobs and that rolling adhesion becomes more stable and slower for higher knob density. Our experimental data suggest that schizonts are at the border between transient and stable rolling adhesion. They also allow us to establish an estimate for the molecular parameters for schizont adhesion to the vascular endothelium and to predict bond dynamics in the contact region.

Comparison of apoptosis in human primary pulmonary endothelial cells and a brain microvascular endothelial cell line co-cultured with Plasmodium falciparum field isolates

BACKGROUND

Plasmodium falciparum infection can progress unpredictably to severe forms including respiratory distress and cerebral malaria. The mechanisms underlying the variable natural course of malaria remain elusive.

METHODS

The cerebral microvascular endothelial cells-D3 and lung endothelial cells both from human were cultured separately and challenged with P. falciparum field isolates taken directly from malaria patients or 3D7 strain (in vitro maintained culture). The capacity of these P. falciparum isolates to induce endothelial cell apoptosis via cytoadherence or not was then assessed.

RESULTS

Overall, 27 P. falciparum isolates were collected from patients with uncomplicated malaria (n = 25) or severe malaria (n = 2). About half the isolates (n = 17) were able to bind brain endothelial cells (12 isolates, 44%) or lung endothelial cells (17 isolates, 63%) or both (12 isolates, 44%). Sixteen (59%) of the 27 isolates were apoptogenic for brain and/or lung endothelial cells. The apoptosis stimulus could be cytoadherence, direct cell-cell contact without cytoadherence, or diffusible soluble factors. While some of the apoptogenic isolates used two stimuli (direct contact with or without cytoadherence, plus soluble factors) to induce apoptosis, others used only one. Among the 16 apoptogenic isolates, eight specifically targeted brain endothelial cells, one lung endothelial cells, and seven both.

CONCLUSION

These results indicate that the brain microvascular cell line was more susceptible to apoptosis triggered by P. falciparum than the primary pulmonary endothelial cells and may have relevance to host-parasite interaction.

Oxidative insult can induce malaria-protective trait of sickle and fetal erythrocytes

Plasmodium falciparum infections can cause severe malaria, but not every infected person develops life-threatening complications. In particular, carriers of the structural haemoglobinopathies S and C and infants are protected from severe disease. Protection is associated with impaired parasite-induced host actin reorganization, required for vesicular trafficking of parasite-encoded adhesins, and reduced cytoadherence of parasitized erythrocytes in the microvasculature. Here we show that aberrant host actin remodelling and the ensuing reduced cytoadherence result from a redox imbalance inherent to haemoglobinopathic and fetal erythrocytes. We further show that a transient oxidative insult to wild-type erythrocytes before infection with P. falciparum induces the phenotypic features associated with the protective trait of haemoglobinopathic and fetal erythrocytes. Moreover, pretreatment of mice with the pro-oxidative nutritional supplement menadione mitigate the development of experimental cerebral malaria. Our results identify redox imbalance as a causative principle of protection from severe malaria, which might inspire host-directed intervention strategies.

The Plasmodium falciparum blood stages acquire factor H family proteins to evade destruction by human complement

The acquisition of regulatory proteins is a means of blood-borne pathogens to avoid destruction by the human complement. We recently showed that the gametes of the human malaria parasite Plasmodium falciparum bind factor H (FH) from the blood meal of the mosquito vector to assure successful sexual reproduction, which takes places in the mosquito midgut. While these findings provided a first glimpse of a complex mechanism used by Plasmodium to control the host immune attack, it is hitherto not known, how the pathogenic blood stages of the malaria parasite evade destruction by the human complement. We now show that the human complement system represents a severe threat for the replicating blood stages, particularly for the reinvading merozoites, with complement factor C3b accumulating on the surfaces of the intraerythrocytic schizonts as well as of free merozoites. C3b accumulation initiates terminal complement complex formation, in consequence resulting in blood stage lysis. To inactivate C3b, the parasites bind FH as well as related proteins FHL-1 and CFHR-1 to their surface, and FH binding is trypsin-resistant. Schizonts acquire FH via two contact sites, which involve CCP modules 5 and 20. Blockage of FH-mediated protection via anti-FH antibodies results in significantly impaired blood stage replication, pointing to the plasmodial complement evasion machinery as a promising malaria vaccine target.

Type of in vitro cultivation influences cytoadhesion, knob structure, protein localization and transcriptome profile of Plasmodium falciparum

In vitro cultivation of Plasmodium falciparum is critical for studying the biology of this parasite. However, it is likely that different in vitro cultivation conditions influence various aspects of the parasite’s life cycle. In the present study two P. falciparum isolates were cultivated using the two most common methods, in which AlbuMAX or human serum as additives are used, and the results were compared. The type of cultivation influenced the knob structure of P. falciparum-infected erythrocytes (IEs). IEs cultivated with AlbuMAX had fewer knobs than those cultivated with human serum. Furthermore, knob size varied between isolates and is also depended on the culture medium. In addition, there was a greater reduction in the cytoadhesion of IEs to various endothelial receptors in the presence of AlbuMAX than in the presence of human serum. Surprisingly, cytoadhesion did not correlate with the presence or absence of knobs. Greater numbers of the variant surface antigen families RIFIN, STEVOR, and PfMC-2TM were found at the IE membrane when cultivated in the presence of AlbuMAX. Moreover, the type of cultivation had a marked influence on the transcriptome profile. Compared with cultivation with human serum, cultivation with AlbuMAX increased the expression of approximately 500–870 genes.

A comparative study of the localization and membrane topology of members of the RIFIN, STEVOR and PfMC-2TM protein families in Plasmodium falciparum-infected erythrocytes

Background

Variant surface antigens (VSA) exposed on the membrane of Plasmodium falciparum infected erythrocytes mediate immune evasion and are important pathogenicity factors in malaria disease. In addition to the well-studied PfEMP1, the small VSA families RIFIN, STEVOR and PfMC-2TM are assumed to play a role in this process.

Methods

This study presents a detailed comparative characterization of the localization, membrane topology and extraction profile across the life cycle of various members of these protein families employing confocal microscopy, immunoelectron microscopy and immunoblots.

Results

The presented data reveal a clear association of variants of the RIFIN, STEVOR and PfMC-2TM proteins with the host cell membrane and topological studies indicate that the semi-conserved N-terminal region of RIFINs and some STEVOR proteins is exposed at the erythrocyte surface. At the Maurer’s clefts, the semi-conserved N-terminal region as well as the variable stretch of RIFINs appears to point to the lumen away from the erythrocyte cytoplasm. These results challenge the previously proposed two transmembrane topology model for the RIFIN and STEVOR protein families and suggest that only one hydrophobic region spans the membrane. In contrast, PfMC-2TM proteins indeed seem to be anchored by two hydrophobic stretches in the host cell membrane exposing just a few, variable amino acids at the surface of the host cell.

Conclusion

Together, the host cell surface exposure and topology of RIFIN and STEVOR proteins suggests members of these protein families may indeed be involved in immune evasion of the infected erythrocyte, whereas members of the PfMC-2TM family seem to bear different functions in parasite biology.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-015-0784-2) contains supplementary material, which is available to authorized users.

An automated method for determining the cytoadhesion of Plasmodium falciparum-infected erythrocytes to immobilized cells

Background

Plasmodium falciparum exports antigens to the surface of infected erythrocytes causing cytoadhesion to the host vasculature. This is central in malaria pathogenesis but in vitro studies of cytoadhesion rely mainly on manual counting methods. The current study aimed at developing an automated high-throughput method for this purpose utilizing the pseudoperoxidase activity of intra-erythrocytic haemoglobin.

Methods

Chinese hamster ovary (CHO) cells were grown to confluence in chamber slides and microtiter plates. Cytoadhesion of co-cultured P. falciparum, selected for binding to CHO cells, was quantified by microscopy of Giemsa-stained chamber slides. In the automated assay, binding was quantified spectrophotometrically in microtiter plates after cell lysis using tetramethylbenzidine as peroxidase-catalysed substrate. The relevance of the method for binding studies was assessed using: i) binding of P. falciparum-infected erythrocytes to CHO cells over-expressing chondroitin sulfate A and ii) CHO cells transfected with CD36. Binding of infected erythrocytes including field isolates to primary endothelial cells was also performed. Data was analysed using linear regression and Bland-Altman plots.

Results

The manual and automated quantification showed strong, positive correlation (r² = 0.959, p <0.001) and with similar detection limit and precision. The automated assay showed the expected dose-dependent reduction in binding to CHO cells when blocking with soluble chondroitin sulfate A or anti-CD36 antibody. Quantification of binding to endothelial cells showed clear distinction between selected vs. non-selected parasite lines. Importantly, the assay was sufficiently sensitive to detect adhesion of field isolates to endothelial cells.

Conclusions

The assay is simple and in a reproducible manner quantifies erythrocyte adhesion to several types of immobilized cells.

Hemoglobin S and C affect protein export in Plasmodium falciparum-infected erythrocytes

Malaria is a potentially deadly disease. However, not every infected person develops severe symptoms. Some people are protected by naturally occurring mechanisms that frequently involve inheritable modifications in their hemoglobin. The best studied protective hemoglobins are the sickle cell hemoglobin (HbS) and hemoglobin C (HbC) which both result from a single amino acid substitution in β-globin: glutamic acid at position 6 is replaced by valine or lysine, respectively. How these hemoglobinopathies protect from severe malaria is only partly understood. Models currently proposed in the literature include reduced disease-mediating cytoadherence of parasitized hemoglobinopathic erythrocytes, impaired intraerythrocytic development of the parasite, dampened inflammatory responses, or a combination thereof. Using a conditional protein export system and tightly synchronized Plasmodium falciparum cultures, we now show that export of parasite-encoded proteins across the parasitophorous vacuolar membrane is delayed, slower, and reduced in amount in hemoglobinopathic erythrocytes as compared to parasitized wild type red blood cells. Impaired protein export affects proteins targeted to the host cell cytoplasm, Maurer's clefts, and the host cell plasma membrane. Impaired protein export into the host cell compartment provides a mechanistic explanation for the reduced cytoadherence phenotype associated with parasitized hemoglobinopathic erythrocytes.

High-Throughput Testing of Antibody-Dependent Binding Inhibition of Placental Malaria Parasites

The particular virulence of Plasmodium falciparum manifests in diverse severe malaria syndromes as cerebral malaria, severe anemia and placental malaria. The cause of both the severity and the diversity of infection outcome, is the ability of the infected erythrocyte (IE) to bind a range of different human receptors through Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) on the surface of the infected cell. As the var genes encoding the large PfEMP1 antigens are extensively polymorphic, vaccine development strategies are focused on targeting the functional binding epitopes. This involves identification of recombinant fragments of PfEMP1s that induce antibodies, which hinder the adhesion of the IE to a given receptor or tissue. Different assays to measure the blocking of adhesion have been described in the literature, each with different advantages. This chapter describes a high-throughput assay used in the preclinical and clinical development of a VAR2CSA based vaccine against placental malaria.

Plasmodium falciparum avoids change in erythrocytic surface expression of phagocytosis markers during inhibition of nitric oxide synthase activity

Nitric oxide (NO) accumulates in Plasmodium falciparum-infected erythrocytes. It may be produced by a parasite NO synthase (NOS) or by nitrate reduction. The parasite's benefit of NO accumulation is not understood. We investigated if inhibiting the P. falciparum NOS with specific and unspecific NOS inhibitors led to a decrease in intraerythrocytic NO accumulation and if this was associated with a change in surface expression of the phagocytosis markers CD47 and phosphatidyl serine. The specific inducible NOS inhibitors l-canavanine and GW274150 dose-dependently decreased intraerythrocytic NO while l-NMMA (an unspecific NOS inhibitor) and caveolin-1 scaffolding domain peptide (a specific endothelial NOS inhibitor) did not affect NO levels. Phosphatidyl serine externalization markedly increased upon P. falciparum infection. l-canavanine did not modify this whereas caveolin-1 scaffolding domain peptide increased the fraction of phosphatidyl serine exposing cells significantly. The infection did not change the level of expression of neither total CD47 nor its oxidized form. Unrelated to NOS inhibition, incubation with caveolin-1 scaffolding domain peptide lead to a decrease in oxidized CD47. In conclusion, the data imply that NOS inhibitors decrease NO accumulation in P. falciparum-infected erythrocytes but this does not correlate with the level of two major erythrocytic phagocytosis markers.

Label-free microfluidic enrichment of ring-stage Plasmodium falciparum-infected red blood cells using non-inertial hydrodynamic lift

BACKGROUND

Understanding of malaria pathogenesis caused by Plasmodium falciparum has been greatly deepened since the introduction of in vitro culture system, but the lack of a method to enrich ring-stage parasites remains a technical challenge. Here, a novel way to enrich red blood cells containing parasites in the early ring stage is described and demonstrated.

METHODS

A simple, straight polydimethylsiloxane microchannel connected to two syringe pumps for sample injection and two height reservoirs for sample collection is used to enrich red blood cells containing parasites in the early ring stage (8-10 h p.i.). The separation is based on the non-inertial hydrodynamic lift effect, a repulsive cell-wall interaction that enables continuous and label-free separation with deformability as intrinsic marker.

RESULTS

The possibility to enrich red blood cells containing P. falciparum parasites at ring stage with a throughput of ~12,000 cells per hour and an average enrichment factor of 4.3 ± 0.5 is demonstrated.

CONCLUSION

The method allows for the enrichment of red blood cells early after the invasion by P. falciparum parasites continuously and without any need to label the cells. The approach promises new possibilities to increase the sensitivity of downstream analyses like genomic- or diagnostic tests. The device can be produced as a cheap, disposable chip with mass production technologies and works without expensive peripheral equipment. This makes the approach interesting for the development of new devices for field use in resource poor settings and environments, e.g. with the aim to increase the sensitivity of microscope malaria diagnosis.

High guanidinium permeability reveals dehydration-dependent ion selectivity in the plasmodial surface anion channel

Malaria parasites grow within vertebrate erythrocytes and increase host cell permeability to access nutrients from plasma. This increase is mediated by the plasmodial surface anion channel (PSAC), an unusual ion channel linked to the conserved clag gene family. Although PSAC recognizes and transports a broad range of uncharged and charged solutes, it must efficiently exclude the small Na⁺ ion to maintain infected cell osmotic stability. Here, we examine possible mechanisms for this remarkable solute selectivity. We identify guanidinium as an organic cation with high permeability into human erythrocytes infected with Plasmodium falciparum, but negligible uptake by uninfected cells. Transport characteristics and pharmacology indicate that this uptake is specifically mediated by PSAC. The rank order of organic and inorganic cation permeabilities suggests cation dehydration as the rate-limiting step in transport through the channel. The high guanidinium permeability of infected cells also allows rapid and stringent synchronization of parasite cultures, as required for molecular and cellular studies of this pathogen. These studies provide important insights into how nutrients and ions are transported via PSAC, an established target for antimalarial drug development.

P. falciparum isolate-specific distinct patterns of induced apoptosis in pulmonary and brain endothelial cells

The factors implicated in the transition from uncomplicated to severe clinical malaria such as pulmonary oedema and cerebral malaria remain unclear. It is known that alterations in vascular integrity due to endothelial cell (EC) activation and death occur during severe malaria. In this study, we assessed the ability of different P. falciparum clinical isolates to induce apoptosis in ECs derived from human lung and brain. We observed that induction of EC apoptosis was sensitive to the environmental pH and required direct contact between the parasite and the cell, though it was not correlated to the ability of the parasite to cytoadhere. Moreover, the extent of induced apoptosis in the two EC types varied with the isolate. Analysis of parasite genes transcript led us to propose that the activation of different pathways, such as Plasmodium apoptosis-linked pathogenicity factors (PALPF), PALPF-2, PALPF-5 and PF11_0521, could be implied in EC death. These observations provide an experimental framework to decipher the molecular mechanism implicated in the genesis of severe malaria.

Adherence to human lung microvascular endothelial cells (HMVEC-L) of Plasmodium vivax isolates from Colombia

BACKGROUND

For years Plasmodium vivax has been considered the cause of benign malaria. Nevertheless, it has been observed that this parasite can produce a severe disease comparable to Plasmodium falciparum. It has been suggested that some physiopathogenic processes might be shared by these two species, such as cytoadherence. Recently, it has been demonstrated that P. vivax-infected erythrocytes (Pv-iEs) have the capacity to adhere to endothelial cells, in which intercellular adhesion molecule-1 (ICAM-1) seems to be involved in this process.

METHODS

Adherence capacity of 21 Colombian isolates, from patients with P. vivax mono-infection to a microvascular line of human lung endothelium (HMVEC-L) was assessed in static conditions and binding was evaluated at basal levels or in tumor necrosis factor (TNF) stimulated cells. The adherence specificity for the ICAM-1 receptor was determined through inhibition with an anti-CD54 monoclonal antibody.

RESULTS

The majority of P. vivax isolates, 13 out of 21 (61.9%), adhered to the HMVEC-L cells, but P. vivax adherence was at least seven times lower when compared to the four P. falciparum isolates. Moreover, HMVEC-L stimulation with TNF led to an increase of 1.6-fold in P. vivax cytoadhesion, similar to P. falciparum isolates (1.8-fold) at comparable conditions. Also, blockage of ICAM-1 receptor with specific antibodies showed a significant 50% adherence reduction.

CONCLUSIONS

Plasmodium vivax isolates found in Colombia are also capable of adhering specifically in vitro to lung endothelial cells, via ICAM-1 cell receptor, both at basal state and after cell stimulation with TNF. Collectively, these findings reinforce the concept of cytoadherence for P. vivax, but here, to a different endothelial cell line and using geographical distinct isolates, thus contributing to understanding P. vivax biology.

A simple protocol for platelet-mediated clumping of Plasmodium falciparum-infected erythrocytes in a resource poor setting

P. falciparum causes the majority of severe malarial infections. The pathophysiological mechanisms underlying cerebral malaria (CM) are not fully understood and several hypotheses have been put forward, including mechanical obstruction of microvessels by P. falciparum-parasitized red blood cells (pRBC). Indeed, during the intra-erythrocytic stage of its life cycle, P. falciparum has the unique ability to modify the surface of the infected erythrocyte by exporting surface antigens with varying adhesive properties onto the RBC membrane. This allows the sequestration of pRBC in multiple tissues and organs by adhesion to endothelial cells lining the microvasculature of post-capillary venules ¹. By doing so, the mature forms of the parasite avoid splenic clearance of the deformed infected erythrocytes ² and restrict their environment to a more favorable low oxygen pressure ³. As a consequence of this sequestration, it is only immature asexual parasites and gametocytes that can be detected in peripheral blood.

Cytoadherence and sequestration of mature pRBC to the numerous host receptors expressed on microvascular beds occurs in severe and uncomplicated disease. However, several lines of evidence suggest that only specific adhesive phenotypes are likely to be associated with severe pathological outcomes of malaria. One example of such specific host-parasite interactions has been demonstrated in vitro, where the ability of intercellular adhesion molecule-1 to support binding of pRBC with particular adhesive properties has been linked to development of cerebral malaria ^4,5. The placenta has also been recognized as a site of preferential pRBC accumulation in malaria-infected pregnant women, with chondrotin sulphate A expressed on syncytiotrophoblasts that line the placental intervillous space as the main receptor ⁶. Rosetting of pRBC to uninfected erythrocytes via the complement receptor 1 (CD35)^7,8 has also been associated with severe disease ⁹.

One of the most recently described P. falciparum cytoadherence phenotypes is the ability of the pRBC to form platelet-mediated clumps in vitro. The formation of such pRBC clumps requires CD36, a glycoprotein expressed on the surface of platelets. Another human receptor, gC1qR/HABP1/p32, expressed on diverse cell types including endothelial cells and platelets, has also been shown to facilitate pRBC adhesion on platelets to form clumps ¹⁰. Whether clumping occurs in vivo remains unclear, but it may account for the significant accumulation of platelets described in brain microvasculature of Malawian children who died from CM ¹¹. In addition, the ability of clinical isolate cultures to clump in vitro was directly linked to the severity of disease in Malawian ¹² and Mozambican patients ¹³, (although not in Malian ¹⁴).

With several aspects of the pRBC clumping phenotype poorly characterized, current studies on this subject have not followed a standardized procedure. This is an important issue because of the known high variability inherent in the assay ¹⁵. Here, we present a method for in vitro platelet-mediated clumping of P. falciparum with hopes that it will provide a platform for a consistent method for other groups and raise awareness of the limitations in investigating this phenotype in future studies. Being based in Malawi, we provide a protocol specifically designed for a limited resource setting, with the advantage that freshly collected clinical isolates can be examined for phenotype without need for cryopreservation.

Life cycle-dependent cytoskeletal modifications in Plasmodium falciparum infected erythrocytes

Plasmodium falciparum infection of human erythrocytes is known to result in the modification of the host cell cytoskeleton by parasite-coded proteins. However, such modifications and corresponding implications in malaria pathogenesis have not been fully explored. Here, we probed the gradual modification of infected erythrocyte cytoskeleton with advancing stages of infection using atomic force microscopy (AFM). We reported a novel strategy to derive accurate and quantitative information on the knob structures and their connections with the spectrin network by performing AFM-based imaging analysis of the cytoplasmic surface of infected erythrocytes. Significant changes on the red cell cytoskeleton were observed from the expansion of spectrin network mesh size, extension of spectrin tetramers and the decrease of spectrin abundance with advancing stages of infection. The spectrin network appeared to aggregate around knobs but also appeared sparser at non-knob areas as the parasite matured. This dramatic modification of the erythrocyte skeleton during the advancing stage of malaria infection could contribute to the loss of deformability of the infected erythrocyte.

Separation of Plasmodium falciparum late stage-infected erythrocytes by magnetic means

Unlike other Plasmodium species, P. falciparum can be cultured in the lab, which facilitates its study (1). While the parasitemia achieved can reach the ≈40% limit, the investigator usually keeps the percentage at around 10%. In many cases it is necessary to isolate the parasite-containing red blood cells (RBCs) from the uninfected ones, to enrich the culture and proceed with a given experiment. When P. falciparum infects the erythrocyte, the parasite degrades and feeds from haemoglobin (2, 3). However, the parasite must deal with a very toxic iron-containing haem moiety (4, 5). The parasite eludes its toxicity by transforming the haem into an inert crystal polymer called haemozoin (6, 7). This iron-containing molecule is stored in its food vacuole and the metal in it has an oxidative state which differs from the one in haem (8). The ferric state of iron in the haemozoin confers on it a paramagnetic property absent in uninfected erythrocytes. As the invading parasite reaches maturity, the content of haemozoin also increases (9), which bestows even more paramagnetism on the latest stages of P. falciparum inside the erythrocyte. Based on this paramagnetic property, the latest stages of P. falciparum infected-red blood cells can be separated by passing the culture through a column containing magnetic beads. These beads become magnetic when the columns containing them are placed on a magnet holder. Infected RBCs, due to their paramagnetism, will then be trapped inside the column, while the flow-through will contain, for the most part, uninfected erythrocytes and those containing early stages of the parasite. Here, we describe the methodology to enrich the population of late stage parasites with magnetic columns, which maintains good parasite viability (10). After performing this procedure, the unattached culture can be returned to an incubator to allow the remaining parasites to continue growing.

Transcription sites are developmentally regulated during the asexual cycle of Plasmodium falciparum

Increasing evidence shows that the spatial organization of transcription is an important epigenetic factor in eukaryotic gene regulation. The malaria parasite Plasmodium falciparum shows a remarkably complex pattern of gene expression during the erythrocytic cycle, paradoxically contrasting with the relatively low number of putative transcription factors encoded by its genome. The spatial organization of nuclear subcompartments has been correlated with the regulation of virulence genes. Here, we investigate the nuclear architecture of transcription during the asexual cycle of malaria parasites. As in mammals, transcription is organized into discrete nucleoplasmic sites in P. falciparum, but in a strikingly lower number of foci. An automated analysis of 3D images shows that the number and intensity of transcription sites vary significantly between rings and trophozoites, although the nuclear volume remains constant. Transcription sites are spatially reorganized during the asexual cycle, with a higher proportion of foci located in the outermost nuclear region in rings, whereas in trophozoites, foci are evenly distributed throughout the nucleoplasm. As in higher eukaryotes, transcription sites are predominantly found in areas of low chromatin density. Immunofluorescence analysis shows that transcription sites form an exclusive nuclear compartment, different from the compartments defined by the silenced or active chromatin markers. In conclusion, these data suggest that transcription is spatially contained in discrete foci that are developmentally regulated during the asexual cycle of malaria parasites and located in areas of low chromatin density.

Plasmodium falciparum malaria elicits inflammatory responses that dysregulate placental amino acid transport

Placental malaria (PM) can lead to poor neonatal outcomes, including low birthweight due to fetal growth restriction (FGR), especially when associated with local inflammation (intervillositis or IV). The pathogenesis of PM-associated FGR is largely unknown, but in idiopathic FGR, impaired transplacental amino acid transport, especially through the system A group of amino acid transporters, has been implicated. We hypothesized that PM-associated FGR could result from impairment of transplacental amino acid transport triggered by IV. In a cohort of Malawian women and their infants, the expression and activity of system A (measured by Na⁺-dependent ¹⁴C-MeAIB uptake) were reduced in PM, especially when associated with IV, compared to uninfected placentas. In an in vitro model of PM with IV, placental cells exposed to monocyte/infected erythrocytes conditioned medium showed decreased system A activity. Amino acid concentrations analyzed by reversed phase ultra performance liquid chromatography in paired maternal and cord plasmas revealed specific alterations of amino acid transport by PM, especially with IV. Overall, our data suggest that the fetoplacental unit responds to PM by altering its placental amino acid transport to maintain adequate fetal growth. However, IV more profoundly compromises placental amino acid transport function, leading to FGR. Our study offers the first pathogenetic explanation for FGR in PM.

Malaria infection during pregnancy can cause fetal growth restriction and low birthweight associated with high infant mortality and morbidity rates. The pathogenesis of fetal growth restriction in placental malaria is largely unknown, but in other pathological pregnancies, impaired transplacental amino acid transport has been implicated. In a cohort of Malawian women and their infants, we found that placental malaria, especially when associated with local inflammation, was associated with decreased expression and activity of an important group of amino acid placental transporters. Using an in vitro model of placental malaria with local inflammation, we discovered that maternal monocyte products could impair the activity of amino acid transporters on placental cells. Amino acid concentrations in paired maternal and cord plasmas revealed specific alterations of amino acid transport by placental malaria, especially with local inflammation. Overall, our data suggest that, more than malaria infection per se, the local inflammation it triggers compromises placental amino acid transport function, leading to fetal growth restriction. Greater understanding of the mechanisms involved, combined with interventions to improve fetal growth in malaria, are important priorities in areas of the world where the co-existence of malaria and maternal malnutrition threatens the health and lives of millions of young babies.

Quantification of Plasmodium falciparum malaria from complex infections in the Peruvian Amazon using quantitative PCR of the merozoite surface protein 1, block 2 (PfMSP1-B2): in vitro dynamics reveal density-dependent interactions

The majority of Plasmodium falciparum field isolates are defined as complex infections because they contain multiple genetically distinct clones. Studying interactions between clones in complex infections in vivo and in vitro could elucidate important phenomena in malaria infection, transmission and treatment. Using quantitative PCR (qPCR) of the P. falciparum merozoite surface protein 1, block 2 (PfMSP1-B2), we provide a sensitive and efficient genotyping method. This is important for epidemiological studies because it makes it possible to study genotype-specific growth dynamics. We compared 3 PfMSP1-B2 genotyping methods by analysing 79 field isolates from the Peruvian Amazon. In vivo observations from other studies using these techniques led to the hypothesis that clones within complex infections interact. By co-culturing clones with different PfMSP1-B2 genotypes, and measuring parasitaemia using qPCR, we found that suppression of clonal expansion was a factor of the collective density of all clones present in a culture. PfMSP1-B2 qPCR enabled us to find in vitro evidence for parasite-parasite interactions and could facilitate future investigations of growth trends in naturally occurring complex infections.

In vitro culturing Plasmodium falciparum erythrocytic stages

The in vitro cultivation of Plasmodium falciparum is absolutely essential for the molecular dissection of parasite biology and still poses several challenges. The dependence on, and interaction with host red blood cells, the tightly regulated stage-specific expression of proteins, and the parasite peculiar demands on nutrients and gaseous environments are only a few aspects that need to be addressed to successfully cultivate P. falciparum in vitro. In this chapter, we present techniques for normal maintenance of the erythrocytic stages of P. falciparum cultures, their synchronization and the generation of clonal cell lines.

Association of severe malaria outcomes with platelet-mediated clumping and adhesion to a novel host receptor

Introduction

Severe malaria has been attributed partly to the sequestration of Plasmodium falciparum-infected erythrocytes (IEs) in the microvasculature of vital host organs. Identification of P. falciparum cytoadherence phenotypes that are associated with severe malaria may lead to the development of novel strategies against life-threatening malaria.

Methods and Findings

Forty-six P. falciparum isolates from Mozambican children under 5 years of age with severe malaria (cases) were examined and compared to 46 isolates from sex and age matched Mozambican children with uncomplicated malaria (controls). Cytoadherence properties such as platelet-mediated clumping, rosetting and adhesion to purified receptors (CD36, ICAM1 and gC1qR), were compared between these matched pairs by non-parametric tests. The most common clinical presentation associated with severe malaria was prostration. Compared to matched controls, prevalence of platelet-mediated clumping was higher in cases (P = .019), in children presenting with prostration (P = .049) and in children with severe anaemia (P = .025). Prevalence of rosetting and gC1qR adhesion were also higher in isolates from cases with severe anemia and multiple seizures, respectively (P = .045 in both cases), than in controls.

Conclusions

These data indicate a role for platelet-mediated clumping, rosetting and adhesion to gC1qR in the pathogenesis of severe malaria. Inhibition of these cytoadherence phenotypes may reduce the occurrence or improve the prognosis of severe malaria outcomes.

Atorvastatin prevents Plasmodium falciparum cytoadherence and endothelial damage

Background

The adhesion of Plasmodium falciparum parasitized red blood cell (PRBC) to human endothelial cells (EC) induces inflammatory processes, coagulation cascades, oxidative stress and apoptosis. These pathological processes are suspected to be responsible for the blood-brain-barrier and other organs' endothelial dysfunctions observed in fatal cases of malaria. Atorvastatin, a drug that belongs to the lowering cholesterol molecule family of statins, has been shown to ameliorate endothelial functions and is widely used in patients with cardiovascular disorders.

Methods

The effect of this compound on PRBC induced endothelial impairments was assessed using endothelial co-culture models.

Results

Atorvastatin pre-treatment of EC was found to reduce the expression of adhesion molecules and P. falciparum cytoadherence, to protect cells against PRBC-induced apoptosis and to enhance endothelial monolayer integrity during co-incubation with parasites.

Conclusions

These results might suggest a potential interest use of atorvastatin as a protective treatment to interfere with the pathophysiological cascades leading to severe malaria.

Inhibition of Plasmodium falciparum field isolates-mediated endothelial cell apoptosis by Fasudil: therapeutic implications for severe malaria

Plasmodium falciparum infection can abruptly progress to severe malaria, a life-threatening complication resulting from sequestration of parasitized red blood cells (PRBC) in the microvasculature of various organs such as the brain and lungs. PRBC adhesion can induce endothelial cell (EC) activation and apoptosis, thereby disrupting the blood-brain barrier. Moreover, hemozoin, the malarial pigment, induces the erythroid precursor apoptosis. Despite the current efficiency of antimalarial drugs in killing parasites, severe malaria still causes up to one million deaths every year. A new strategy targeting both parasite elimination and EC protection is urgently needed in the field. Recently, a rho-kinase inhibitior Fasudil, a drug already in clinical use in humans for cardio- and neuro-vascular diseases, was successfully tested on laboratory strains of P. falciparum to protect and to reverse damages of the endothelium. We therefore assessed herein whether Fasudil would have a similar efficiency on P. falciparum taken directly from malaria patients using contact and non-contact experiments. Seven (23.3%) of 30 PRBC preparations from different patients were apoptogenic, four (13.3%) acting by cytoadherence and three (10%) via soluble factors. None of the apoptogenic PRBC preparations used both mechanisms indicating a possible mutual exclusion of signal transduction ligand. Three PRBC preparations (42.9%) induced EC apoptosis by cytoadherence after 4 h of coculture (“rapid transducers”), and four (57.1%) after a minimum of 24 h (“slow transducers”). The intensity of apoptosis increased with time. Interestingly, Fasudil inhibited EC apoptosis mediated both by cell-cell contact and by soluble factors but did not affect PRBC cytoadherence. Fasudil was found to be able to prevent endothelium apoptosis from all the P. falciparum isolates tested. Our data provide evidence of the strong anti-apoptogenic effect of Fasudil and show that endothelial cell-P. falciparum interactions are more complicated than previously thought. These findings may warrant clinical trials of Fasudil in severe malaria management.

Plasmodium falciparum adhesion on human brain microvascular endothelial cells involves transmigration-like cup formation and induces opening of intercellular junctions

Cerebral malaria, a major cause of death during malaria infection, is characterised by the sequestration of infected red blood cells (IRBC) in brain microvessels. Most of the molecules implicated in the adhesion of IRBC on endothelial cells (EC) are already described; however, the structure of the IRBC/EC junction and the impact of this adhesion on the EC are poorly understood. We analysed this interaction using human brain microvascular EC monolayers co-cultured with IRBC. Our study demonstrates the transfer of material from the IRBC to the brain EC plasma membrane in a trogocytosis-like process, followed by a TNF-enhanced IRBC engulfing process. Upon IRBC/EC binding, parasite antigens are transferred to early endosomes in the EC, in a cytoskeleton-dependent process. This is associated with the opening of the intercellular junctions. The transfer of IRBC antigens can thus transform EC into a target for the immune response and contribute to the profound EC alterations, including peri-vascular oedema, associated with cerebral malaria.

Use of magnetically purified Plasmodium falciparum parasites improves the accuracy of erythrocyte invasion assays

Merozoite invasion of erythrocytes is a crucial step for the asexual cycle of Plasmodium falciparum. Multiple invasion pathways, which involve different ligand-receptor interactions, have been identified in P. falciparum by examining the entry of purified parasite into erythrocytes with different surface receptors, either mutant or under different enzyme treatments. The most critical step for a successful invasion assay is the isolation of erythrocytes infected with viable schizonts. Here, we applied a magnetic column to purify the schizonts for the erythrocyte invasion assay. Comparing to Percoll-sorbitol purification method, this modified approach showed great improvement on reproducibility and reliability of invasion assay, particularly for short-term, culture-adapted parasite isolates. The magnetic purification method is an excellent alternative for parasite isolates that do not tolerate or with unknown sensitivity to Percoll-sorbitol exposure.

Optimized high gradient magnetic separation for isolation of Plasmodium-infected red blood cells

BACKGROUND

Highly purified infected red blood cells (irbc), or highly synchronized parasite cultures, are regularly required in malaria research. Conventional isolation and synchronization rely on density and osmotic fragility of irbc, respectively. High gradient magnetic separation (HGMS) offers an alternative based on intrinsic magnetic properties of irbc, avoiding exposure to chemicals and osmotic stress. Successful HGMS concentration in malaria research was previously reported using polymer coated columns, while HGMS depletion has not been described yet. This study presents a new approach to both HGMS concentration and depletion in malaria research, rendering polymer coating unnecessary.

METHODS

A dipole magnet generating a strong homogenous field was custom assembled. Polypropylene syringes were fitted with one-way stopcocks and filled with stainless steel wool. Rbc from Plasmodium falciparum cultures were resuspended in density and viscosity optimized HGMS buffers and HGMS processed. Purification and depletion results were analysed by flow cytometer and light microscopy. Viability was evaluated by calculating the infection rate after re-culturing of isolates.

RESULTS

In HGMS concentration, purity of irbc isolates from asynchronous cultures consistently ranged from 94.8% to 98.4% (mean 95.7%). With further optimization, over 90% of isolated irbc contained segmented schizonts. Processing time was less than 45 min. Reinfection rates ranged from 21.0% to 56.4%. In HGMS depletion, results were comparable to treatment with sorbitol, as demonstrated by essentially identical development of cultures.

CONCLUSION

The novel HGMS concentration procedure achieves high purities of segmented stage irbc from standard asynchronous cultures, and is the first HGMS depletion alternative to sorbitol lysis. It represents a simple and highly efficient alternative to conventional irbc concentration and synchronization methods.

Antibodies to variant surface antigens of Plasmodium falciparum-infected erythrocytes are associated with protection from treatment failure and the development of anemia in pregnancy

BACKGROUND

In pregnancy-associated malaria (PAM), Plasmodium falciparum-infected erythrocytes (IEs) express variant surface antigens (VSA-PAM) that evade existing immunity and mediate placental sequestration. Antibodies to VSA-PAM develop with gravidity and block placental adhesion or opsonize IEs for phagocytic clearance, helping to prevent maternal anemia and low birth weight in infants.

METHODS

Using serum samples from 141 pregnant Malawian women with parasitemia enrolled in a randomized trial of antimalarials and VSA-PAM-expressing CS2 IEs, we quantified levels of immunoglobulin (Ig) G to VSA-PAM by flow cytometry and levels of opsonizing antibodies by measuring uptake of IEs by THP1 promonocytes.

RESULTS

After controlling for gravidity and antimalarial treatment, higher levels of IgG to VSA-PAM were associated with decreased anemia at delivery (odds ratio [OR], 0.66 [95% confidence interval {CI}, 0.46-0.93]; P = .018) and were weakly associated with decreased parasitological failure (OR, 0.78 [95% CI, 0.60-1.03]; P = .075), especially reinfection (OR, 0.73 [95% CI, 0.53-1.01]; P = .057). Higher levels of opsonizing antibodies to CS2 IEs were associated with less maternal anemia (OR, 0.31 [95% CI, 0.13-0.74]; P = .008) and treatment failure (OR, 0.48 [95% CI, 0.25-0.90]; P = .023), primarily because of recrudescent infection (OR, 0.49 [95% CI, 0.21-1.12]; P = .089).

CONCLUSION

Higher levels of both IgG antibodies to VSA-PAM and opsonizing antibodies, a functional measure of immunity, correlate with parasite clearance and less anemia in pregnancy malaria.

Preerythrocytic, live-attenuated Plasmodium falciparum vaccine candidates by design

Falciparum malaria is initiated when Anopheles mosquitoes transmit the Plasmodium sporozoite stage during a blood meal. Irradiated sporozoites confer sterile protection against subsequent malaria infection in animal models and humans. This level of protection is unmatched by current recombinant malaria vaccines. However, the live-attenuated vaccine approach faces formidable obstacles, including development of accurate, reproducible attenuation techniques. We tested whether Plasmodium falciparum could be attenuated at the early liver stage by genetic engineering. The P. falciparum genetically attenuated parasites (GAPs) harbor individual deletions or simultaneous deletions of the sporozoite-expressed genes P52 and P36. Gene deletions were done by double-cross-over recombination to avoid genetic reversion of the knockout parasites. The gene deletions did not affect parasite replication throughout the erythrocytic cycle, gametocyte production, mosquito infections, and sporozoite production rates. However, the deletions caused parasite developmental arrest during hepatocyte infection. The double-gene deletion line exhibited a more severe intrahepatocytic growth defect compared with the single-gene deletion lines, and it did not persist. This defect was assessed in an in vitro liver-stage growth assay and in a chimeric mouse model harboring human hepatocytes. The strong phenotype of the double knockout GAP justifies its human testing as a whole-organism vaccine candidate using the established sporozoite challenge model. GAPs might provide a safe and reproducible platform to develop an efficacious whole-cell malaria vaccine that prevents infection at the preerythrocytic stage.

Cellular tumor necrosis factor, gamma interferon, and interleukin-6 responses as correlates of immunity and risk of clinical Plasmodium falciparum malaria in children from Papua New Guinea

The role of early to intermediate Plasmodium falciparum-induced cellular responses in the development of clinical immunity to malaria is not well understood, and such responses have been proposed to contribute to both immunity and risk of clinical malaria episodes. To investigate whether P. falciparum-induced cellular responses are able to function as predictive correlates of parasitological and clinical outcomes, we conducted a prospective cohort study of children (5 to 14 years of age) residing in a region of Papua New Guinea where malaria is endemic Live, intact P. falciparum-infected red blood cells were applied to isolated peripheral blood mononuclear cells obtained at baseline. Cellular cytokine production, including production of interleukin-2 (IL-2), IL-4, IL-6, IL-10, tumor necrosis factor (TNF) (formerly tumor necrosis factor alpha), and gamma interferon (IFN-gamma), was measured, and the cellular source of key cytokines was investigated. Multicytokine models revealed that increasing P. falciparum-induced IL-6 production was associated with an increased incidence of P. falciparum clinical episodes (incidence rate ratio [IRR], 1.75; 95% confidence interval [CI], 1.20 to 2.53), while increasing P. falciparum-induced TNF and IFN-gamma production was associated with a reduced incidence of clinical episodes (IRR for TNF, 0.55 [95% CI, 0.38 to 0.80]; IRR for IFN-gamma, 0.71 [95% CI, 0.55 to 0.90]). Furthermore, we found that monocytes/macrophages and gammadelta-T cells are important for the P. falciparum-induced production of IL-6 and TNF. Early to intermediate cellular cytokine responses to P. falciparum may therefore be important correlates of immunity and risk of symptomatic malaria episodes and thus warrant detailed investigation in relation to the development and implementation of effective vaccines.