Virulence and transmission success of the malarial parasite Plasmodium falciparum

Plasmodium falciparum rosetting protects schizonts against artemisinin

Background

Artemisinin (ART) resistance in Plasmodium falciparum is thought to occur during the early stage of the parasite's erythrocytic cycle. Here, we identify a novel factor associated with the late stage parasite development that contributes to ART resistance.

Methods

Rosetting rates of clinical isolates pre- and post- brief (one hour) exposure to artesunate (AS, an ART derivative) were evaluated. The effects of AS-mediated rosetting on the post-AS-exposed parasite's replication and survival, as well as the extent of protection by AS-mediated rosetting on different parasite stages were investigated. The rosetting ligands, mechanisms, and gene mutations involved were studied.

Findings

Brief AS exposure stimulated rosetting, with AS-resistant isolates forming more rosettes in a more rapid manner. AS-mediated rosetting enabled infected erythrocytes (IRBC) to withstand AS exposure for several hours and protected the IRBC from phagocytosis. When their rosetting ability was blocked experimentally, the post-AS exposure survival advantage by the AS-resistant parasites was abrogated. Deletions in two genes coding for PfEMP1 exon 2 (PF3D7_0200300 and PF3D7_0223300) were found to be associated with AS-mediated rosetting, and these mutations were significantly selected through time in the parasite population under study, along with the K13 mutations, a molecular marker of ART-resistance.

Interpretation

Rapid ART parasite clearance is driven by the direct oxidative damages on IRBC by ART and the phagocytic destruction of the damaged IRBC. Rosetting serves as a rapid ‘buying time’ strategy that allows more parasites to complete schizont maturation, reinvasion and subsequent development into the intrinsically less ART-susceptible ring stage.

Funding

A*STAR, NMRC-OF-YIRG, HRC e-ASIA, Wellcome.

Mechanism of Action of N-Acyl and N-Alkoxy Fosmidomycin Analogs: Mono- and Bisubstrate Inhibition of IspC from Plasmodium falciparum, a Causative Agent of Malaria

Malaria is a global health threat that requires immediate attention. Malaria is caused by the protozoan parasite Plasmodium, the most severe form of which is Plasmodium falciparum. The methylerythritol phosphate (MEP) pathway of isoprenoid biosynthesis is essential to the survival of many human pathogens, including P. falciparum, but is absent in humans, and thus shows promise as a new antimalarial drug target. The enzyme 1-deoxy-d-xylulose 5-phosphate reductoisomerase (IspC) catalyzes the first committed step in the MEP pathway. In addition to a divalent cation (Mg²⁺), the enzyme requires the substrates 1-deoxy-D-xylulose 5-phosphate (DXP) and NADPH to catalyze its reaction. We designed N-alkoxy and N-acyl fosmidomycin analogs to inhibit the activity of P. falciparum IspC in a bisubstrate manner. Enzyme assays reveal that the N-alkoxy fosmidomycin analogs have a competitive mode of inhibition relative to both the DXP- and NADPH-binding sites, confirming a bisubstrate mode of inhibition. In contrast, the N-acyl fosmidomycin analogs demonstrate competitive inhibition with respect to DXP but uncompetitive inhibition with respect to NADPH, indicating monosubstrate inhibitory activity. Our results will have a positive impact on the discovery of novel antimalarial drugs.

Evolutionary analyses of the major variant surface antigen-encoding genes reveal population structure of Plasmodium falciparum within and between continents

Malaria remains a major public health problem in many countries. Unlike influenza and HIV, where diversity in immunodominant surface antigens is understood geographically to inform disease surveillance, relatively little is known about the global population structure of PfEMP1, the major variant surface antigen of the malaria parasite Plasmodium falciparum. The complexity of the var multigene family that encodes PfEMP1 and that diversifies by recombination, has so far precluded its use in malaria surveillance. Recent studies have demonstrated that cost-effective deep sequencing of the region of var genes encoding the PfEMP1 DBLα domain and subsequent classification of within host sequences at 96% identity to define unique DBLα types, can reveal structure and strain dynamics within countries. However, to date there has not been a comprehensive comparison of these DBLα types between countries. By leveraging a bioinformatic approach (jumping hidden Markov model) designed specifically for the analysis of recombination within var genes and applying it to a dataset of DBLα types from 10 countries, we are able to describe population structure of DBLα types at the global scale. The sensitivity of the approach allows for the comparison of the global dataset to ape samples of Plasmodium Laverania species. Our analyses show that the evolution of the parasite population emerging out of Africa underlies current patterns of DBLα type diversity. Most importantly, we can distinguish geographic population structure within Africa between Gabon and Ghana in West Africa and Uganda in East Africa. Our evolutionary findings have translational implications in the context of globalization. Firstly, DBLα type diversity can provide a simple diagnostic framework for geographic surveillance of the rapidly evolving transmission dynamics of P. falciparum. It can also inform efforts to understand the presence or absence of global, regional and local population immunity to major surface antigen variants. Additionally, we identify a number of highly conserved DBLα types that are present globally that may be of biological significance and warrant further characterization.

Host Cytoskeleton Remodeling throughout the Blood Stages of Plasmodium falciparum

The asexual intraerythrocytic development of Plasmodium falciparum, causing the most severe form of human malaria, is marked by extensive host cell remodeling. Throughout the processes of invasion, intracellular development, and egress, the erythrocyte membrane skeleton is remodeled by the parasite as required for each specific developmental stage. The remodeling is facilitated by a plethora of exported parasite proteins, and the erythrocyte membrane skeleton is the interface of most of the observed interactions between the parasite and host cell proteins. Host cell remodeling has been extensively described and there is a vast body of information on protein export or the description of parasite-induced structures such as Maurer's clefts or knobs on the host cell surface. Here we specifically review the molecular level of each host cell-remodeling step at each stage of the intraerythrocytic development of P. falciparum We describe key events, such as invasion, knob formation, and egress, and identify the interactions between exported parasite proteins and the host cell cytoskeleton. We discuss each remodeling step with respect to time and specific requirement of the developing parasite to explain host cell remodeling in a stage-specific manner. Thus, we highlight the interaction with the host membrane skeleton as a key event in parasite survival.

Transmission-Blocking Vaccines: Old Friends and New Prospects

In the progression of the life cycle of Plasmodium falciparum, a small proportion of asexual parasites differentiate into male or female sexual forms called gametocytes. Just like their asexual counterparts, gametocytes are contained within the infected host's erythrocytes (RBCs). However, unlike their asexual partners, they do not exit the RBC until they are taken up in a blood meal by a mosquito. In the mosquito midgut, they are stimulated to emerge from the RBC, undergo fertilization, and ultimately produce tens of thousands of sporozoites that are infectious to humans. This transmission cycle can be blocked by antibodies targeting proteins exposed on the parasite surface in the mosquito midgut, a process that has led to the development of candidate transmission-blocking vaccines (TBV), including some that are in clinical trials. Here we review the leading TBV antigens and highlight the ongoing search for additional gametocyte/gamete surface antigens, as well as antigens on the surfaces of gametocyte-infected erythrocytes, which can potentially become a new group of TBV candidates.

Immune Responses to the Sexual Stages of Plasmodium falciparum Parasites

Malaria infections remain a serious global health problem in the world, particularly among children and pregnant women in Sub-Saharan Africa. Moreover, malaria control and elimination is hampered by rapid development of resistance by the parasite and the vector to commonly used antimalarial drugs and insecticides, respectively. Therefore, vaccine-based strategies are sorely needed, including those designed to interrupt disease transmission. However, a prerequisite for such a vaccine strategy is the understanding of both the human and vector immune responses to parasite developmental stages involved in parasite transmission in both man and mosquito. Here, we review the naturally acquired humoral and cellular responses to sexual stages of the parasite while in the human host and the Anopheles vector. In addition, updates on current anti-gametocyte, anti-gamete, and anti-mosquito transmission blocking vaccines are given. We conclude with our views on some important future directions of research into P. falciparum sexual stage immunity relevant to the search for the most appropriate transmission-blocking vaccine.

A 4-Day Incubation Period of Plasmodium falciparum Infection in a Nonimmune Patient in Ghana: A Case Report

Plasmodium falciparum can cause severe infection and has the shortest incubation period compared with all the other Plasmodium species. Incubation periods of 9–14 days for the immune and 6–14 days for the nonimmune have been reported for P. falciparum. However, an incubation period of less than 5 days has not been reported, as of yet. This report presents a case of a 23-year-old nonimmune female who presented with signs and symptoms 4 days after being bitten by mosquitoes while visiting Ghana. The patient was successfully treated with a 1-day course of parenteral artesunate, followed by a 3-day course of oral artemisinin combination therapy.

Out of the net: An agent-based model to study human movements influence on local-scale malaria transmission

Though malaria control initiatives have markedly reduced malaria prevalence in recent decades, global eradication is far from actuality. Recent studies show that environmental and social heterogeneities in low-transmission settings have an increased weight in shaping malaria micro-epidemiology. New integrated and more localized control strategies should be developed and tested. Here we present a set of agent-based models designed to study the influence of local scale human movements on local scale malaria transmission in a typical Amazon environment, where malaria is transmission is low and strongly connected with seasonal riverine flooding. The agent-based simulations show that the overall malaria incidence is essentially not influenced by local scale human movements. In contrast, the locations of malaria high risk spatial hotspots heavily depend on human movements because simulated malaria hotspots are mainly centered on farms, were laborers work during the day. The agent-based models are then used to test the effectiveness of two different malaria control strategies both designed to reduce local scale malaria incidence by targeting hotspots. The first control scenario consists in treat against mosquito bites people that, during the simulation, enter at least once inside hotspots revealed considering the actual sites where human individuals were infected. The second scenario involves the treatment of people entering in hotspots calculated assuming that the infection sites of every infected individual is located in the household where the individual lives. Simulations show that both considered scenarios perform better in controlling malaria than a randomized treatment, although targeting household hotspots shows slightly better performance.

Determinants of Malaria Transmission at the Population Level

Transmission of malaria from man to mosquito defines the human infectious reservoir of malaria. At the population level this is influenced by a variety of human, parasite, and mosquito vector factors some or all of which may vary depending on the epidemiological setting. Here, we review our current state of knowledge related to human infectiousness to mosquitoes and how current malaria control strategies might be adapted to focus on reducing this. While much progress has been made in malaria control, we argue that an improved understanding of human infectivity will allow more effective use of current control tools and make elimination a more feasible goal.

Plasmodium falciparum gametocyte transit through the cutaneous microvasculature: A new target for malaria transmission blocking vaccines?

Malaria remains one of the most significant infectious diseases worldwide. Concordant with scaled intervention efforts and the emphasis of elimination and eradication on the agenda of many malaria control programs, the development of a malaria vaccine that reduces transmission of the parasite from human host to mosquito vector has been incorporated as an important new strategic goal. Transmission of malaria from man to mosquito relies on gametocytes, highly specialized sexual-stage parasites, that once mature, circulate in the peripheral vasculature of the human host. The complex interplay between mature gametocytes, their uptake in the mosquito bloodmeal and forward maturation/fertilization events provide unique opportunities for intervention. Although recent advances have yielded greater understanding into the mechanisms that mediate sequestration of immature gametocytes in the human host, the spatial dynamics of circulating mature gametocytes in the cutaneous microvaculature remains far less defined, which is the focus of this review.

Naturally acquired immunity to sexual stage P. falciparum parasites

Gametocytes are the specialized form of Plasmodium parasites that are responsible for human-to-mosquito transmission of malaria. Transmission of gametocytes is highly effective, but represents a biomass bottleneck for the parasite that has stimulated interest in strategies targeting the transmission stages separately from those responsible for clinical disease. Studying targets of naturally acquired immunity against transmission-stage parasites may reveal opportunities for novel transmission reducing interventions, particularly the development of a transmission blocking vaccine (TBV). In this review, we summarize the current knowledge on immunity against the transmission stages of Plasmodium. This includes immune responses against epitopes on the gametocyte-infected erythrocyte surface during gametocyte development, as well as epitopes present upon gametocyte activation in the mosquito midgut. We present an analysis of historical data on transmission reducing immunity (TRI), as analysed in mosquito feeding assays, and its correlation with natural recognition of sexual stage specific proteins Pfs48/45 and Pfs230. Although high antibody titres towards either one of these proteins is associated with TRI, the presence of additional, novel targets is anticipated. In conclusion, the identification of novel gametocyte-specific targets of naturally acquired immunity against different gametocyte stages could aid in the development of potential TBV targets and ultimately an effective transmission blocking approach.

The role of PfEMP1 as targets of naturally acquired immunity to childhood malaria: prospects for a vaccine

The Plasmodium falciparum erythrocyte membrane protein 1 antigens that are inserted onto the surface of P. falciparum infected erythrocytes play a key role both in the pathology of severe malaria and as targets of naturally acquired immunity. They might be considered unlikely vaccine targets because they are extremely diverse. However, several lines of evidence suggest that underneath this molecular diversity there are a restricted set of epitopes which may act as effective targets for a vaccine against severe malaria. Here we review some of the recent developments in this area of research, focusing on work that has assessed the potential of these molecules as possible vaccine targets.

A validated agent-based model to study the spatial and temporal heterogeneities of malaria incidence in the rainforest environment

BACKGROUND

The Amazon environment has been exposed in the last decades to radical changes that have been accompanied by a remarkable rise of both Plasmodium falciparum and Plasmodium vivax malaria. The malaria transmission process is highly influenced by factors such as spatial and temporal heterogeneities of the environment and individual-based characteristics of mosquitoes and humans populations. All these determinant factors can be simulated effectively trough agent-based models.

METHODS

This paper presents a validated agent-based model of local-scale malaria transmission. The model reproduces the environment of a typical riverine village in the northern Peruvian Amazon, where the malaria transmission is highly seasonal and apparently associated with flooding of large areas caused by the neighbouring river. Agents representing humans, mosquitoes and the two species of Plasmodium (P. falciparum and P. vivax) are simulated in a spatially explicit representation of the environment around the village. The model environment includes: climate, people houses positions and elevation. A representation of changes in the mosquito breeding areas extension caused by the river flooding is also included in the simulation environment.

RESULTS

A calibration process was carried out to reproduce the variations of the malaria monthly incidence over a period of 3 years. The calibrated model is also able to reproduce the spatial heterogeneities of local scale malaria transmission. A "what if" eradication strategy scenario is proposed: if the mosquito breeding sites are eliminated through mosquito larva habitat management in a buffer area extended at least 200 m around the village, the malaria transmission is eradicated from the village.

CONCLUSIONS

The use of agent-based models can reproduce effectively the spatiotemporal variations of the malaria transmission in a low endemicity environment dominated by river floodings like in the Amazon.

PfEMP1 - A Parasite Protein Family of Key Importance in Plasmodium falciparum Malaria Immunity and Pathogenesis

Plasmodium falciparum causes the most severe form of malaria and is responsible for essentially all malaria-related deaths. The accumulation in various tissues of erythrocytes infected by mature P. falciparum parasites can lead to circulatory disturbances and inflammation, and is thought to be a central element in the pathogenesis of the disease. It is mediated by the interaction of parasite ligands on the erythrocyte surface and a range of host receptor molecules in many organs and tissues. Among several proteins and protein families implicated in this process, the P. falciparum erythrocyte membrane protein 1 (PfEMP1) family of high-molecular weight and highly variable antigens appears to be the most prominent. In this chapter, we aim to provide a systematic overview of the current knowledge about these proteins, their structure, their function, how they are presented on the erythrocyte surface, and how the var genes encoding them are regulated. The role of PfEMP1 in the pathogenesis of malaria, PfEMP1-specific immune responses, and the prospect of PfEMP1-specific vaccination against malaria are also covered briefly.

Surface antigens of Plasmodium falciparum-infected erythrocytes as immune targets and malaria vaccine candidates

Understanding the targets and mechanisms of human immunity to malaria caused by Plasmodium falciparum is crucial for advancing effective vaccines and developing tools for measuring immunity and exposure in populations. Acquired immunity to malaria predominantly targets the blood stage of infection when merozoites of Plasmodium spp. infect erythrocytes and replicate within them. During the intra-erythrocytic development of P. falciparum, numerous parasite-derived antigens are expressed on the surface of infected erythrocytes (IEs). These antigens enable P. falciparum-IEs to adhere in the vasculature and accumulate in multiple organs, which is a key process in the pathogenesis of disease. IE surface antigens, often referred to as variant surface antigens, are important targets of acquired protective immunity and include PfEMP1, RIFIN, STEVOR and SURFIN. These antigens are highly polymorphic and encoded by multigene families, which generate substantial antigenic diversity to mediate immune evasion. The most important immune target appears to be PfEMP1, which is a major ligand for vascular adhesion and sequestration of IEs. Studies are beginning to identify specific variants of PfEMP1 linked to disease pathogenesis that may be suitable for vaccine development, but overcoming antigenic diversity in PfEMP1 remains a major challenge. Much less is known about other surface antigens, or antigens on the surface of gametocyte-IEs, the effector mechanisms that mediate immunity, and how immunity is acquired and maintained over time; these are important topics for future research.

Expression of a type B RIFIN in Plasmodium falciparum merozoites and gametes

BACKGROUND

The ability of Plasmodium falciparum to undergo antigenic variation, by switching expression among protein variants encoded by multigene families, such as var, rif and stevor, is key to the survival of this parasite in the human host. The RIFIN protein family can be divided into A and B types based on the presence or absence of a 25 amino acid motif in the semi-conserved domain. A particular type B RIFIN, PF13_0006, has previously been shown to be strongly transcribed in the asexual and sexual stages of P. falciparum in vitro.

METHODS

Antibodies to recombinant PF13_0006 RIFIN were used in immunofluorescence and confocal imaging of 3D7 parasites throughout the asexual reproduction and sexual development to examine the expression of PF13_0006. Furthermore, reactivity to recombinant PF13_0006 was measured in plasma samples collected from individuals from both East and West African endemic areas.

RESULTS

The PF13_0006 RIFIN variant appeared expressed by both released merozoites and gametes after emergence. 7.4% and 12.1% of individuals from East and West African endemic areas, respectively, carry plasma antibodies that recognize recombinant PF13_0006, where the antibody responses were more common among older children.

CONCLUSIONS

The stage specificity of PF13_0006 suggests that the diversity of RIFIN variants has evolved to provide multiple specialized functions in different stages of the parasite life cycle. These data also suggest that RIFIN variants antigenically similar to PF13_0006 occur in African parasite populations.

Early gametocytes of the malaria parasite Plasmodium falciparum specifically remodel the adhesive properties of infected erythrocyte surface

In Plasmodium falciparum infections the parasite transmission stages, the gametocytes, mature in 10 days sequestered in internal organs. Recent studies suggest that cell mechanical properties rather than adhesive interactions play a role in sequestration during gametocyte maturation. It remains instead obscure how sequestration is established, and how the earliest sexual stages, morphologically similar to asexual trophozoites, modify the infected erythrocytes and their cytoadhesive properties at the onset of gametocytogenesis. Here, purified P. falciparum early gametocytes were used to ultrastructurally and biochemically analyse parasite-induced modifications on the red blood cell surface and to measure their functional consequences on adhesion to human endothelial cells. This work revealed that stage I gametocytes are able to deform the infected erythrocytes like asexual parasites, but do not modify its surface with adhesive 'knob' structures and associated proteins. Reduced levels of the P. falciparum erythrocyte membrane protein 1 (PfEMP1) adhesins are exposed on the red blood cell surface by these parasites, and the expression of the var gene family, which encodes 50-60 variants of PfEMP1, is dramatically downregulated in the transition from asexual development to gametocytogenesis. Cytoadhesion assays show that such gene expression changes and host cell surface modifications functionally result in the inability of stage I gametocytes to bind the host ligands used by the asexual parasite to bind endothelial cells. In conclusion, these results identify specific differences in molecular and cellular mechanisms of host cell remodelling and in adhesive properties, leading to clearly distinct host parasite interplays in the establishment of sequestration of stage I gametocytes and of asexual trophozoites.

A switch in infected erythrocyte deformability at the maturation and blood circulation of Plasmodium falciparum transmission stages

Achievement of malaria elimination requires development of novel strategies interfering with parasite transmission, including targeting the parasite sexual stages (gametocytes). The formation of Plasmodium falciparum gametocytes in the human host takes several days during which immature gametocyte-infected erythrocytes (GIEs) sequester in host tissues. Only mature stage GIEs circulate in the peripheral blood, available to uptake by the Anopheles vector. Mechanisms underlying GIE sequestration and release in circulation are virtually unknown. We show here that mature GIEs are more deformable than immature stages using ektacytometry and microsphiltration methods, and that a switch in cellular deformability in the transition from immature to mature gametocytes is accompanied by the deassociation of parasite-derived STEVOR proteins from the infected erythrocyte membrane. We hypothesize that mechanical retention contributes to sequestration of immature GIEs and that regained deformability of mature gametocytes is associated with their release in the bloodstream and ability to circulate. These processes are proposed to play a key role in P falciparum gametocyte development in the host and to represent novel and unconventional targets for interfering with parasite transmission.

Host cell deformability is linked to transmission in the human malaria parasite Plasmodium falciparum

Gametocyte maturation in Plasmodium falciparum is a critical step in the transmission of malaria. While the majority of parasites proliferate asexually in red blood cells, a small fraction of parasites undergo sexual conversion and mature over 2 weeks to become competent for transmission to a mosquito vector. Immature gametocytes sequester in deep tissues while mature stages must be able to circulate, pass the spleen and present themselves to the mosquito vector in order to complete transmission. Sequestration of asexual red blood cell stage parasites has been investigated in great detail. These studies have demonstrated that induction of cytoadherence properties through specific receptor-ligand interactions coincides with a significant increase in host cell stiffness. In contrast, the adherence and biophysical properties of gametocyte-infected red blood cells have not been studied systematically. Utilizing a transgenic line for 3D live imaging, in vitro capillary assays and 3D finite element whole cell modelling, we studied the role of cellular deformability in determining the circulatory characteristics of gametocytes. Our analysis shows that the red blood cell deformability of immature gametocytes displays an overall decrease followed by rapid restoration in mature gametocytes. Intriguingly, simulations suggest that along with deformability variations, the morphological changes of the parasite may play an important role in tissue distribution in vivo. Taken together, we present a model, which suggests that mature but not immature gametocytes circulate in the peripheral blood for uptake in the mosquito blood meal and transmission to another human host thus ensuring long-term survival of the parasite.

Gametocytogenesis in malaria parasite: commitment, development and regulation

Malaria parasites have evolved a complicated life cycle alternating between two hosts. Gametocytes are produced in the vertebrate hosts and are obligatory for natural transmission of the parasites through mosquito vectors. The mechanism of sexual development in Plasmodium has been the focus of extensive studies. In the postgenomic era, the advent of genome-wide analytical tools and genetic manipulation technology has enabled rapid advancement of our knowledge in this area. Patterns of gene expression during sexual development, molecular distinction of the two sexes, and mechanisms underlying subsequent formation of gametes and their fertilization have been progressively elucidated. However, the triggers and mechanism of sexual development remain largely unknown. This article provides an update of our understanding of the molecular and cellular events associated with the decision for commitment to sexual development and regulation of gene expression during gametocytogenesis. Insights into the molecular mechanisms of gametocyte development are essential for designing proper control strategies for interruption of malaria transmission and ultimate elimination.

Origin, composition, organization and function of the inner membrane complex of Plasmodium falciparum gametocytes

The most virulent of the human malaria parasites, Plasmodium falciparum, undergoes a remarkable morphological transformation as it prepares itself for sexual reproduction and transmission via mosquitoes. Indeed P. falciparum is named for the unique falciform or crescent shape of the mature sexual stages. Once the metamorphosis is completed the mature gametocyte releases from sequestration sites and enters the circulation making it accessible to feeding mosquitoes. Early ultrastructural studies showed that gametocyte elongation is driven by the assembly of a system of flattened cisternal membrane compartments underneath the parasite plasma membrane and a supporting network of microtubules. Here we describe the molecular composition and origin of the sub-pellicular membrane complex, and show that it is analogous to the inner membrane complex, an organelle with structural and motor functions that is well conserved across the apicomplexa. We identify novel cross-linking elements that may help stabilize the inner membrane complex during gametocyte development. We show that changes in gametocyte morphology are associated with an increase in cellular deformability and postulate that this enables the gametocytes to circulate in the blood stream without being detected and removed by the mechanical filtering mechanisms in the host's spleen.

Functional analysis of the exported type IV HSP40 protein PfGECO in Plasmodium falciparum gametocytes

During Plasmodium falciparum infection, host red blood cell (RBC) remodeling is required for the parasite's survival. Such modifications are mediated by the export of parasite proteins into the RBC that alter the architecture of the RBC membrane and enable cytoadherence. It is probable that some exported proteins also play a protective role against the host defense response. This may be of particular importance for the gametocyte stage of the life cycle that is responsible for malaria transmission, since the gametocyte remains in contact with blood as it proceeds through five morphological stages (I to V) during its 12-day maturation. Using microarray analysis, we identified several genes with encoded secretory or export sequences that were differentially expressed during early gametocytogenesis. One of these, PfGECO, encodes a predicted type IV heat shock protein 40 (HSP40) that we show is expressed in gametocyte stages I to IV and is exported to the RBC cytoplasm. HSPs are traditionally induced under stressful conditions to maintain homeostasis, but PfGECO expression was not increased upon heat shock, suggesting an alternate function. Targeted disruption of PfGECO indicated that the gene is not essential for gametocytogenesis in vitro, and quantitative reverse transcriptase PCR (RT-PCR) showed that there was no compensatory expression of the other type IV HSP40 genes. Although P. falciparum HSP40 members are implicated in the trafficking of proteins to the RBC surface, removal of PfGECO did not affect the targeting of other exported gametocyte proteins. This work has expanded the repertoire of known gametocyte-exported proteins to include a type IV HSP40, PfGECO.

The stability and complexity of antibody responses to the major surface antigen of Plasmodium falciparum are associated with age in a malaria endemic area

Individuals that are exposed to malaria eventually develop immunity to the disease with one possible mechanism being the gradual acquisition of antibodies to the range of parasite variant surface antigens in their local area. Major antibody targets include the large and highly polymorphic Plasmodium falciparum Erythrocyte Membrane Protein 1 (PfEMP1) family of proteins. Here, we use a protein microarray containing 123 recombinant PfEMP1-DBLα domains (VAR) from Papua New Guinea to seroprofile 38 nonimmune children (<4 years) and 29 hyperimmune adults (≥15 years) from the same local area. The overall magnitude, prevalence and breadth of antibody response to VAR was limited at <2 years and 2–2.9 years, peaked at 3–4 years and decreased for adults compared with the oldest children. An increasing proportion of individuals recognized large numbers of VAR proteins (>20) with age, consistent with the breadth of response stabilizing with age. In addition, the antibody response was limited in uninfected children compared with infected children but was similar in adults irrespective of infection status. Analysis of the variant-specific response confirmed that the antibody signature expands with age and infection. This also revealed that the antibody signatures of the youngest children overlapped substantially, suggesting that they are exposed to the same subset of PfEMP1 variants. VAR proteins were either seroprevalent from early in life, (<3 years), from later in childhood (≥3 years) or rarely recognized. Group 2 VAR proteins (Cys2/MFK-REY+) were serodominant in infants (<1-year-old) and all other sequence subgroups became more seroprevalent with age. The results confirm that the anti-PfEMP1-DBLα antibody responses increase in magnitude and prevalence with age and further demonstrate that they increase in stability and complexity. The protein microarray approach provides a unique platform to rapidly profile variant-specific antibodies to malaria and suggests novel insights into the acquisition of immunity to malaria.

Epidemiology and infectivity of Plasmodium falciparum and Plasmodium vivax gametocytes in relation to malaria control and elimination

Malaria remains a major cause of morbidity and mortality in the tropics, with Plasmodium falciparum responsible for the majority of the disease burden and P. vivax being the geographically most widely distributed cause of malaria. Gametocytes are the sexual-stage parasites that infect Anopheles mosquitoes and mediate the onward transmission of the disease. Gametocytes are poorly studied despite this crucial role, but with a recent resurgence of interest in malaria elimination, the study of gametocytes is in vogue. This review highlights the current state of knowledge with regard to the development and longevity of P. falciparum and P. vivax gametocytes in the human host and the factors influencing their distribution within endemic populations. The evidence for immune responses, antimalarial drugs, and drug resistance influencing infectiousness to mosquitoes is reviewed. We discuss how the application of molecular techniques has led to the identification of submicroscopic gametocyte carriage and to a reassessment of the human infectious reservoir. These components are drawn together to show how control measures that aim to reduce malaria transmission, such as mass drug administration and a transmission-blocking vaccine, might better be deployed.

A molecular epidemiological study of var gene diversity to characterize the reservoir of Plasmodium falciparum in humans in Africa

BACKGROUND

The reservoir of Plasmodium infection in humans has traditionally been defined by blood slide positivity. This study was designed to characterize the local reservoir of infection in relation to the diverse var genes that encode the major surface antigen of Plasmodium falciparum blood stages and underlie the parasite's ability to establish chronic infection and transmit from human to mosquito.

METHODOLOGY/PRINCIPAL FINDINGS

We investigated the molecular epidemiology of the var multigene family at local sites in Gabon, Senegal and Kenya which differ in parasite prevalence and transmission intensity. 1839 distinct var gene types were defined by sequencing DBLα domains in the three sites. Only 76 (4.1%) var types were found in more than one population indicating spatial heterogeneity in var types across the African continent. The majority of var types appeared only once in the population sample. Non-parametric statistical estimators predict in each population at minimum five to seven thousand distinct var types. Similar diversity of var types was seen in sites with different parasite prevalences.

CONCLUSIONS/SIGNIFICANCE

Var population genomics provides new insights into the epidemiology of P. falciparum in Africa where malaria has never been conquered. In particular, we have described the extensive reservoir of infection in local African sites and discovered a unique var population structure that can facilitate superinfection through minimal overlap in var repertoires among parasite genomes. Our findings show that var typing as a molecular surveillance system defines the extent of genetic complexity in the reservoir of infection to complement measures of malaria prevalence. The observed small scale spatial diversity of var genes suggests that var genetics could greatly inform current malaria mapping approaches and predict complex malaria population dynamics due to the import of var types to areas where no widespread pre-existing immunity in the population exists.

Interrupting malaria transmission: quantifying the impact of interventions in regions of low to moderate transmission

Malaria has been eliminated from over 40 countries with an additional 39 currently planning for, or committed to, elimination. Information on the likely impact of available interventions, and the required time, is urgently needed to help plan resource allocation. Mathematical modelling has been used to investigate the impact of various interventions; the strength of the conclusions is boosted when several models with differing formulation produce similar data. Here we predict by using an individual-based stochastic simulation model of seasonal Plasmodium falciparum transmission that transmission can be interrupted and parasite reintroductions controlled in villages of 1,000 individuals where the entomological inoculation rate is <7 infectious bites per person per year using chemotherapy and bed net strategies. Above this transmission intensity bed nets and symptomatic treatment alone were not sufficient to interrupt transmission and control the importation of malaria for at least 150 days. Our model results suggest that 1) stochastic events impact the likelihood of successfully interrupting transmission with large variability in the times required, 2) the relative reduction in morbidity caused by the interventions were age-group specific, changing over time, and 3) the post-intervention changes in morbidity were larger than the corresponding impact on transmission. These results generally agree with the conclusions from previously published models. However the model also predicted changes in parasite population structure as a result of improved treatment of symptomatic individuals; the survival probability of introduced parasites reduced leading to an increase in the prevalence of sub-patent infections in semi-immune individuals. This novel finding requires further investigation in the field because, if confirmed, such a change would have a negative impact on attempts to eliminate the disease from areas of moderate transmission.

Var gene expression and human Plasmodium pathogenesis

Plasmodium falciparum is responsible for most of the morbidity and mortality associated with malaria and is unique in its ability to sequester in organ postcapillary venules. Specific host-parasite interactions mediate this phenomenon and the P. falciparum erythrocyte membrane protein 1 is the predominant ligand responsible for adhering to host endothelial receptors. This review focuses on the current knowledge regarding this protein family, evidence for its role in various pathogenic mechanisms and on insights that have been gained in this area from field studies.

Heritability of the human infectious reservoir of malaria parasites

BACKGROUND

Studies on human genetic factors associated with malaria have hitherto concentrated on their role in susceptibility to and protection from disease. In contrast, virtually no attention has been paid to the role of human genetics in eliciting the production of parasite transmission stages, the gametocytes, and thus enhancing the spread of disease.

METHODS AND FINDINGS

We analysed four longitudinal family-based cohort studies from Senegal and Thailand followed for 2-8 years and evaluated the relative impact of the human genetic and non-genetic factors on gametocyte production in infections of Plasmodium falciparum or P. vivax. Prevalence and density of gametocyte carriage were evaluated in asymptomatic and symptomatic infections by examination of Giemsa-stained blood smears and/or RT-PCR (for falciparum in one site). A significant human genetic contribution was found to be associated with gametocyte prevalence in asymptomatic P. falciparum infections. By contrast, there was no heritability associated with the production of gametocytes for P. falciparum or P. vivax symptomatic infections. Sickle cell mutation, HbS, was associated with increased gametocyte prevalence but its contribution was small.

CONCLUSIONS

The existence of a significant human genetic contribution to gametocyte prevalence in asymptomatic infections suggests that candidate gene and genome wide association approaches may be usefully applied to explore the underlying human genetics. Prospective epidemiological studies will provide an opportunity to generate novel and perhaps more epidemiologically pertinent gametocyte data with which similar analyses can be performed and the role of human genetics in parasite transmission ascertained.

Identification of a major rif transcript common to gametocytes and sporozoites of Plasmodium falciparum

Background

The Plasmodium falciparum parasite is transmitted in its sexual gametocyte stage from man to mosquito and as asexual sporozoites from mosquito to man. Developing gametocytes sequester preferentially in the bone marrow, but mature stage gametocytes are released to the bloodstream. Sexual stage parasite surface proteins are of interest as candidate target antigens for transmission blocking vaccines.

Methods

In this study, the transcript profiles of rif and var genes, known to encode surface antigens in asexual blood stage parasites, were investigated at different stages of 3D7/NF54 gametocytogenesis and in sporozoites.

Results

Gametocytes exhibited a rif transcript profile unlinked to the rif and var transcript profile of the asexual progenitors. At stage V, mature gametocytes produced high levels of a single rif gene, PF13_0006, which also dominated the rif transcript profile of sporozoites. All var genes appeared to be silenced in sporozoites.

Conclusions

The most prominent variant surface antigen transcribed in both gametocytes and sporozoites of 3D7/NF54 is a single variant of the RIFIN protein family. This discovery may lead to the identification of the parasites binding ligands responsible for the adhesion during sexual stages and potentially to novel vaccine candidates.

Genetic variation in human HBB is associated with Plasmodium falciparum transmission

Genetic factors are known to have a role in determining susceptibility to infectious diseases, although it is unclear whether they may also influence host efficiency in transmitting pathogens. We examine variants in HBB that have been shown to be protective against malaria and test whether these are associated with the transmission of the parasite from the human host to the Anopheles vector. We conducted cross-sectional malariological surveys on 3,739 human subjects and transmission experiments involving 60 children and 6,446 mosquitoes in Burkina Faso, West Africa. Protective hemoglobins C (HbC, beta6Glu-->Lys) and S (beta6Glu-->Val) are associated with a twofold in vivo (odds ratio 2.17, 95% CI 1.57-3.01, P = 1.0 x 10(-6)) and a fourfold ex vivo (odds ratio 4.12, 95% CI 1.90-9.29, P = 7.0 x 10(-5)) increase of parasite transmission from the human host to the Anopheles vector. This provides an example of how host genetic variation may influence the transmission dynamics of an infectious disease.

Surface antigens of Plasmodium falciparum gametocytes--a new class of transmission-blocking vaccine targets?

The re-establishment of elimination and eradication on the malaria control agenda has led to calls for renewed effort in the development of parasite transmission-blocking interventions. Vaccines are ideally suited to this task, but progress towards an anti-gamete transmission-blocking vaccine, designed to act on parasites in blood-fed mosquitoes, has been slow. Recent work has confirmed that the surface of the gametocyte-infected erythrocyte presents antigens to the host immune system, and elicits specific humoral immune responses to these antigens, termed gametocyte surface antigens (GSAs). Likely candidate molecules, including antigens encoded by sub-telomeric multi-gene families, are discussed, and a hypothetical group of parasite molecules involved in spatial and temporal signal transduction in the human host is proposed, the tropins and circadins. The next steps for development of anti-gametocyte transmission-blocking vaccines for P. falciparum and the other human malaria species are considered.

Diverse expression patterns of subgroups of the rif multigene family during Plasmodium falciparum gametocytogenesis

Background

The maturation of Plasmodium falciparum gametocytes in the human host takes several days, during which the parasites need to efficiently evade the host immune system. Like asexual stage parasites, immature gametocytes can sequester at various sites in the human body, and only mature sexual stages are found in the circulation. Although the fundamental mechanisms of gametocyte immune evasion are still largely unknown, candidate molecules that may be involved include variant antigens encoded by multigene families in the P. falciparum genome, such as the PfEMP1, STEVOR and RIFIN proteins. While expression of the former two families in sexual stages has been investigated earlier, we report here RIFIN expression during gametocytogenesis.

Methodology/Principal Findings

Variants of two previously characterized RIFIN subfamilies (A- and B-type RIFINs) were found to be synthesized in gametocytes. Immunofluorescence experiments showed A-type RIFINs to be accumulated in a crescent-shaped pattern of discrete punctate structures at the infected erythrocyte membrane, while members of the B-type family were associated with the parasite. Transcription analysis demonstrated the existence of diverse transcriptional regulation patterns during sexual differentiation and indicated variant-specific regulation of B-type RIFINs, in contrast to group-specific regulation for A-type RIFINs. Phylogenetic analysis of 5′-upstream regions showed that the rif–gene family falls into five defined clusters, designated rups (rifupstream) A1, A2, AB, B and C. In trophozoites and early gametocytes, rif variants of the rupsA2-type were preferentially expressed.

Conclusions/Significance

In this work we demonstrate the expression dynamics of the rif-gene family during sexual differentiation and present indications for subgroup specific regulation patterns. Therefore, our data provide a first foundation and point to new directions for future investigations of the potential role of RIFINs in gametocyte immune evasion.

On the evolution of reproductive restraint in malaria

Malaria is one of the leading causes of death among infectious diseases in the world, claiming over one million lives every year. By these standards, this highly complex parasite is extremely successful at generating new infections. Somewhat surprisingly, however, many malaria species seem to invest relatively little in gametocytes, converting only a small percentage of circulating asexual parasite forms into this transmissible form. In this article, we use mathematical models to explore three of the hypotheses that have been proposed to explain this apparent 'reproductive restraint' and develop a novel, fourth hypothesis. We find that only one of the previous three hypotheses we explore can explain such low gametocyte conversion rates, and this hypothesis involves a very specific form of density-dependent transmission-blocking immunity. Our fourth hypothesis also provides a potential explanation and is based on the occurrence of multiple infections and the resultant within-host competition between malaria strains that this entails. Further experimental work is needed to determine which of these two hypotheses provides the most likely explanation.

Population genomics of the immune evasion (var) genes of Plasmodium falciparum

Var genes encode the major surface antigen (PfEMP1) of the blood stages of the human malaria parasite Plasmodium falciparum. Differential expression of up to 60 diverse var genes in each parasite genome underlies immune evasion. We compared the diversity of the DBLα domain of var genes sampled from 30 parasite isolates from a malaria endemic area of Papua New Guinea (PNG) and 59 from widespread geographic origins (global). Overall, we obtained over 8,000 quality-controlled DBLα sequences. Within our sampling frame, the global population had a total of 895 distinct DBLα “types” and negligible overlap among repertoires. This indicated that var gene diversity on a global scale is so immense that many genomes would need to be sequenced to capture its true extent. In contrast, we found a much lower diversity in PNG of 185 DBLα types, with an average of approximately 7% overlap among repertoires. While we identify marked geographic structuring, nearly 40% of types identified in PNG were also found in samples from different countries showing a cosmopolitan distribution for much of the diversity. We also present evidence to suggest that recombination plays a key role in maintaining the unprecedented levels of polymorphism found in these immune evasion genes. This population genomic framework provides a cost effective molecular epidemiological tool to rapidly explore the geographic diversity of var genes.

Malaria parasites live in red blood cells of the human host for part of the life cycle, during which a family of diverse antigens known as PfEMP1 are placed on the surface. PfEMP1 variants switch by sequential expression of up to 60 var genes. This allows the parasite to evade immune detection within an individual host, enhancing its chances to be transmitted to the mosquito vector in situations where mosquitoes are seasonally available. Methods to rapidly assess var gene diversity in parasite populations are needed to measure antigenic diversity and define relationships with malaria transmission. Using a specialized framework, we completed the first systematic sampling of var genes from parasite genomes obtained from the same (Papua New Guinea [PNG]) and different (global) populations. Globally, there was no limit to the number of var genes because parasites rarely shared var genes. In PNG, var gene numbers were restricted due to high levels of sharing, and most were only found in that population. Recombination was important to the evolution of var genes in PNG. The data suggest there are distinct var genes in different populations, which may have consequences for the spread of malarial disease from one geographic area to another.

Programmed transcription of the var gene family, but not of stevor, in Plasmodium falciparum gametocytes

The var genes encode Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) proteins, a set of highly diverse surface-expressed proteins that mediate adhesion of erythrocytes infected with asexual blood-stage parasites to host endothelium. Switching among expressed PfEMP1 variants in the course of a blood-stage infection is a key component of antigenic variation, and thus immune evasion, by the parasite. The majority of var loci are found in the subtelomeric regions of P. falciparum chromosomes associated with members of other multigene families, including stevor. Both PfEMP1 and STEVOR are expressed in gametocytes, the transmissible parasite stage, but the role of these proteins in the biology of sexual-stage parasites remains unknown. PfEMP1 may continue to mediate antigenic variation in gametocytes, which need to persist in the host for many days before reaching maturity. Using quantitative reverse transcription-PCR and Northern hybridization, we demonstrate that transcription of a defined subset of type C var loci occurs during gametocyte development in vitro. This transcriptional program occurs in gametocytes regardless of the var expression phenotype of their asexual progenitors and therefore is subject to regulatory processes distinct from those that manage antigenic variation in the asexual parasite. In contrast, the same stevor variants are transcribed in both gametocytes and their asexual progenitors. We also provide evidence that for both asexual parasites and gametocytes, var and stevor transcription patterns are not linked to each other.

Plasmodium falciparum infection dynamics and transmission potential following treatment with sulfadoxine-pyrimethamine

OBJECTIVES

To investigate the overall efficacy of sulfadoxine-pyrimethamine (SP) treatment and the corresponding transmission potential for patients infected with SP-resistant Plasmodium falciparum.

METHODS

A mathematical model of the in-host dynamics of P. falciparum infections was used to simulate infections with parasites having different numbers of mutations in the dhfr and dhps genes and their responses to SP treatment. The treatment outcome and transmission potential of each simulated infection following SP treatment was assessed by tracking asexual parasite density and the number of days with sufficient mature gametocytes to give a > 95% probability of infecting a mosquito.

RESULTS

The results show treatment failure only occurring in patients infected with parasites having two mutations in dihydrofolate reductase (DHFR) combined with at least two mutations in dihydropteroate synthetase (DHPS) or with parasites having a triple mutation in DHFR. Highly mutated parasites (three mutations in each gene) caused the highest clinical failure rate, while moderately mutated parasites (three mutations in DHFR plus one mutation in DHPS) produced a high rate of asymptomatic parasitological failure following SP treatment. This high rate of asymptomatic recrudescence caused the transmission potential of infections with moderately resistant parasites to exceed that of highly resistant parasites.

CONCLUSIONS

The model output suggests that infection dynamics following SP treatment and the overall transmission potential are inherently linked. The combination of prolonged asymptomatic parasitaemia and increased transmission potential allows parasites having three mutations in DHFR, but fewer mutations in DHPS, to expand largely unnoticed.

Distinct trafficking and localization of STEVOR proteins in three stages of the Plasmodium falciparum life cycle

The genome of Plasmodium falciparum harbors three extensive multigene families, var, rif, and stevor (for subtelomeric variable open reading frame), located mainly in the subtelomeric regions of the parasite's 14 chromosomes. STEVOR variants are known to be expressed in asexual parasites, but no function has as yet been ascribed to this protein family. We have examined the expression of STEVOR proteins in intraerythrocytic sexual stages, gametocytes, and extracellular sporozoites isolated from infected Anopheles mosquitoes. In gametocytes, stevor transcripts appear transiently early in development but STEVOR proteins persist for several days and are transported out of the parasite, travel through the host cell cytoplasm, and localize to the erythrocyte plasma membrane. In contrast to asexual parasites, gametocytes move STEVOR to the periphery via a trafficking pathway independent of Maurer's clefts. In sporozoites, STEVOR appear dispersed throughout the cytoplasm in vesicle-like structures. The pattern of STEVOR localization we have observed in gametocytes and sporozoites differs significantly from that in asexual parasite stages. STEVOR variants are therefore likely to perform different functions in each stage of the parasites life cycle in which they occur.

Virulence in malaria: an evolutionary viewpoint

Malaria parasites cause much morbidity and mortality to their human hosts. From our evolutionary perspective, this is because virulence is positively associated with parasite transmission rate. Natural selection therefore drives virulence upwards, but only to the point where the cost to transmission caused by host death begins to outweigh the transmission benefits. In this review, we summarize data from the laboratory rodent malaria model, Plasmodium chabaudi, and field data on the human malaria parasite, P. falciparum, in relation to this virulence trade-off hypothesis. The data from both species show strong positive correlations between asexual multiplication, transmission rate, infection length, morbidity and mortality, and therefore support the underlying assumptions of the hypothesis. Moreover, the P. falciparum data show that expected total lifetime transmission of the parasite is maximized in young children in whom the fitness cost of host mortality balances the fitness benefits of higher transmission rates and slower clearance rates, thus exhibiting the hypothesized virulence trade-off. This evolutionary explanation of virulence appears to accord well with the clinical and molecular explanations of pathogenesis that involve cytoadherence, red cell invasion and immune evasion, although direct evidence of the fitness advantages of these mechanisms is scarce. One implication of this evolutionary view of virulence is that parasite populations are expected to evolve new levels of virulence in response to medical interventions such as vaccines and drugs.

Gametocytogenesis: the puberty of Plasmodium falciparum

The protozoan Plasmodium falciparum has a complex life cycle in which asexual multiplication in the vertebrate host alternates with an obligate sexual reproduction in the anopheline mosquito. Apart from the apparent recombination advantages conferred by sex, P. falciparum has evolved a remarkable biology and adaptive phenotypes to insure its transmission despite the dangers of sex. This review mainly focuses on the current knowledge on commitment to sexual development, gametocytogenesis and the evolutionary significance of various aspects of gametocyte biology. It goes further than pure biology to look at the strategies used to improve successful transmission. Although gametocytes are inevitable stages for transmission and provide a potential target to fight malaria, they have received less attention than the pathogenic asexual stages. There is a need for research on gametocytes, which are a fascinating stage, responsible to a large extent for the success of P. falciparum.

The 3D7var5.2 (var COMMON) type var gene family is commonly expressed in non-placental Plasmodium falciparum malaria

Relapse variants in chronic Plasmodium falciparum infections are antigenically distinct from the parental parasites. The variable antigen PfEMP1 expressed at the surface of the infected erythrocyte (IE) is encoded by the var gene family with approximately 60 copies per haploid genome. Placental isolates commonly express DBLgamma containing subtypes of var genes with homology to either 3D7var5.2 (var(COMMON)) or FCR3var(CSA). Here we report that var(COMMON) related genes are constitutively transcribed in approximately 60% of malaria infected children in Gabon. var(COMMON) is conserved in field isolates over at least 2.1kb. In 3D7 parasites var(COMMON) is present on chromosome 5 (var5.2) and constitutively transcribed in the opposite direction to most other var genes. It lacks a regulatory intron, an acidic terminal segment and ends in telomeric repeat sequences. var(COMMON) encodes a large, hypothetical PfEMP1 of a structure similar to previous placenta-binding PfEMP1s but it is not present at the IE-surface. IE of a 3D7 clone (3D7S8) transcribe var(COMMON) but express a PfEMP1 distinct from var(COMMON) at the surface and adhere to placental tissues through var(COMMON) independent novel mechanisms. Our report suggests that expression of var(COMMON) type genes is not restricted to placental malaria.

Widespread functional specialization of Plasmodium falciparum erythrocyte membrane protein 1 family members to bind CD36 analysed across a parasite genome

Plasmodium falciparum-infected erythrocytes sequester from blood circulation by binding host endothelium. A large family of variant proteins mediates cytoadherence and their binding specificity determines parasite sequestration patterns and potential for disease. The aim of the present study was to understand how binding properties are encoded into family members and to develop sequence algorithms for predicting binding. To accomplish these goals computational approaches and a binding assay were used to characterize adhesion across Plasmodium falciparum erythrocyte membrane 1 (PfEMP1) proteins in the 3D7 parasite genome. We report that most family members encode the capacity to bind CD36 in the protein's semi-conserved head structure and describe the sequence characteristics of a group of PfEMP1 proteins that do not. Structural and functional grouping of PfEMP1 proteins based upon head structure and additional domain architectural properties provide new insights into the protein family. These can be used to investigate the role of proteins in malaria pathogenesis and potentially tailor vaccines to recognize particular binding variants.

CD36-mediated nonopsonic phagocytosis of erythrocytes infected with stage I and IIA gametocytes of Plasmodium falciparum

Gametocytes, the sexual stages of malaria parasites (Plasmodium spp.) that are transmissible to mosquitoes, have been the focus of much recent research as potential targets for novel drug and vaccine therapies. However, little is known about the host clearance of gametocyte-infected erythrocytes (GEs). Using a number of experimental strategies, we found that the scavenger receptor CD36 mediates the uptake of nonopsonized erythrocytes infected with stage I and IIA gametocytes of Plasmodium falciparum by monocytes and culture-derived macrophages (Mphis). Light microscopy and immunofluorescence assays revealed that stage I and IIA gametocytes were readily internalized by monocytes and Mphis. Pretreating monocytes and Mphis with a monoclonal antibody that blocked CD36 resulted in a significant reduction in phagocytosis, as did treating GEs with low concentrations of trypsin to remove P. falciparum erythrocyte membrane protein 1 (PfEMP-1), a parasite ligand for CD36. Pretreating monocytes and Mphis with peroxisome proliferator-activated receptor gamma-retinoid X receptor agonists, which specifically upregulate CD36, resulted in a significant increase in the phagocytosis of GEs. Murine CD36 on mouse Mphis also mediated the phagocytosis of P. falciparum stage I and IIA gametocytes, as determined by receptor blockade with anti-murine CD36 monoclonal antibodies and the lack of uptake by CD36-null Mphis. These results indicate that phagocytosis of stage I and IIA gametocytes by monocytes and Mphis appears to be mediated to a large extent by the interaction of PfEMP-1 and CD36, suggesting that CD36 may play a role in innate clearance of these early sexual stages.

Malaria pathogenesis: a jigsaw with an increasing number of pieces

Plasmodium falciparum malaria remains as one of the most devastating global health problems of today. It is estimated that around 150 million individuals get the disease every year and of these 2-3 million die from it. Our knowledge of the mechanisms underlying the pathology has expanded greatly over the last decades, but many aspects of the molecular biology, immunology and epidemiology that govern the pathogenesis and spread of this parasite are still unclear. As new insights are gained we are also revealing a challenging biological complexity. Piecing this information together is the key to vaccine development and production of new antimalarial drugs.

Sexual differentiation and sex determination in the Apicomplexa

Protozoan parasites of the phylum Apicomplexa have complex life cycles involving various types of asexual division that allow rapid proliferation of parasites within one or more hosts. Such replication is punctuated by obligate sexual differentiation that produces male and female gametocytes. These stages are transmissible to haematophagous vectors or are necessary ultimately to form resistant cysts that are released into the environment. This article examines the sexual differentiation of apicomplexan parasites as it relates to the timing of commitment and the mechanism of the switch from asexual proliferation to the development of male and female sexual stages.

Stage-specific transcription of distinct repertoires of a multigene family during Plasmodium life cycle

Members of a multigene family in the rodent malaria parasite Plasmodium yoelii yoelii code for 235-kilodalton proteins (Py235) that are located in the merozoite apical complex, are implicated in virulence, and may determine red blood cell specificity. We show that distinct subsets of py235 genes are expressed in sporozoites and hepatic and erythrocytic stages. Antibodies to Py235 inhibited sporozoite invasion of hepatocytes. The switch in expression profile occurred immediately after transition from one stage to another. The results suggest that this differential expression is driven by strong biological requirements and provide evidence that hepatic and erythrocytic merozoites differ.

Reduction of transmission stages concomitant with increased host immune responses to hypervirulent Sarcocystis singaporensis, and natural selection for intermediate virulence

Parasite virulence (pathogenicity depending on inoculum size) and host immune reactions were examined for the apicomplexan protozoan Sarcocystis singaporensis. This parasite is endemic in southeastern Asia and multiplies as a proliferation (merozoite) and transmission stage (bradyzoite) in rats. Virulence in wild brown rats of parasites freshly isolated in the wild (wild-type) was surprisingly constant within the endemic area and showed an intermediate level. In contrast, serially passaged parasites either became avirulent or virulence increased markedly (hypervirulence). Production of transmission stages was maximal for the wild-type whereas numbers were significantly reduced for hypervirulent and avirulent (shown in a previous study) parasites. Analyses of B and T cell immunity revealed that immune responses of WKY rats to the transmission stage were significantly higher for hypervirulent than for wild-type parasites. These results suggest that it is the immune system of the host that is not only responsible for reduction of transmission stages in individual rats, but also could act as a selective force that maintains intermediate virulence at the population level because reduction of muscle stages challenges transmission of S. singaporensis to the definitive host. Collectively, the presented data support evolutionary theory, which predicts intermediate rates of parasite growth in nature and an 'arms race' between host immunity and parasite proliferation.

The effect of partial host immunity on the transmission of malaria parasites

Experiments were carried out to determine the effect of partial host immunity against the rodent malaria parasite Plasmodium chabaudi on the transmission success of the parasite. There was a fourfold reduction in both the blood-stage, asexually replicating parasite density and the gametocyte (transmissable stage) density in immunized hosts. Some of the reduction in asexual parasite densities was due to strain-specific immunity, but there was no evidence that strain-specific immunity affected gametocyte densities. However, immunity did affect transmission in a strain-specific manner, with a fivefold reduction in gametocyte infectivity to mosquitoes in homologous challenges compared with heterologous challenges or non-immunized controls. This implies the existence of a mechanism of strain-specific infectivity-reducing immunity that does not affect the density of gametocytes circulating in peripheral blood. The proportion of asexual parasites that produced gametocytes increased during the course of infection in both non-immunized and in immunized hosts, but immunity increased gametocyte production early in the infection.