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

cAMP-Signalling Regulates Gametocyte-Infected Erythrocyte Deformability Required for Malaria Parasite Transmission

Blocking Plasmodium falciparum transmission to mosquitoes has been designated a strategic objective in the global agenda of malaria elimination. Transmission is ensured by gametocyte-infected erythrocytes (GIE) that sequester in the bone marrow and at maturation are released into peripheral blood from where they are taken up during a mosquito blood meal. Release into the blood circulation is accompanied by an increase in GIE deformability that allows them to pass through the spleen. Here, we used a microsphere matrix to mimic splenic filtration and investigated the role of cAMP-signalling in regulating GIE deformability. We demonstrated that mature GIE deformability is dependent on reduced cAMP-signalling and on increased phosphodiesterase expression in stage V gametocytes, and that parasite cAMP-dependent kinase activity contributes to the stiffness of immature gametocytes. Importantly, pharmacological agents that raise cAMP levels in transmissible stage V gametocytes render them less deformable and hence less likely to circulate through the spleen. Therefore, phosphodiesterase inhibitors that raise cAMP levels in P. falciparum infected erythrocytes, such as sildenafil, represent new candidate drugs to block transmission of malaria parasites.

Malaria transmission is ensured by deformable mature gametocyte-infected erythrocytes being taken up when a mosquito bites. Non-deformable immature gametocyte stages are sequestered in the bone marrow, as their lack of deformability would lead to their splenic clearance. In the present study, we apply nano-filtration technology to mimic splenic retention and demonstrate that deformability of transmissible mature stage V gametocytes is regulated by parasite cyclic AMP-dependent kinase signalling. Importantly, when we used drugs to raise cAMP levels we render transmissible mature gametocytes as stiff as non-transmissible gametocytes. In contrast, when we inhibit the cAMP-dependent kinase we render immature gametocytes more deformable. Thus, by two different approaches we confirm that the drop in cAMP levels in mature gametocytes leads to an increase in their deformability and hence more likely to circulate through the spleen. Our molecular observations have the potential to be translated into therapies for blocking malaria transmission by demonstrating that raising cAMP levels with sildenafil also known as “Viagra” renders mature gametocytes rigid. These findings provide the proof of principle that deformability of circulating gametocytes is targetable by pharmacological agents and as such, it provides a novel approach to prevent the spread of parasites. PDE inhibitors therefore represent novel drug leads potentially capable of blocking transmission and improving the worldwide fight to eliminate malaria from the human population.

Ensuring transmission through dynamic host environments: host-pathogen interactions in Plasmodium sexual development

A renewed global commitment to malaria elimination lends urgency to understanding the biology of Plasmodium transmission stages. Recent progress toward uncovering the mechanisms underlying Plasmodium falciparum sexual differentiation and maturation reveals potential targets for transmission-blocking drugs and vaccines. The identification of parasite factors that alter sexual differentiation, including extracellular vesicles and a master transcriptional regulator, suggest that parasites make epigenetically controlled developmental decisions based on environmental cues. New insights into sexual development, especially host cell remodeling and sequestration in the bone marrow, highlight open questions regarding parasite homing to the tissue, transmigration across the vascular endothelium, and maturation in the parenchyma. Novel molecular and translational tools will provide further opportunities to define host-parasite interactions and design effective transmission-blocking therapeutics.

Erythrocyte remodeling by Plasmodium falciparum gametocytes in the human host interplay

The spread of malaria critically relies on the presence of Plasmodium transmission stages - the gametocytes - circulating in the blood of an infected individual, which are taken up by Anopheles mosquitoes. A striking feature of Plasmodium falciparum gametocytes is their long development inside the erythrocytes while sequestered in the internal organs of the human host. Recent studies of the molecular and cellular remodeling of the host erythrocyte induced by P. falciparum during gametocyte maturation are shedding light on how these may affect the establishment and maintenance of sequestration of the immature transmission stages and the subsequent release and circulation of mature gametocytes in the peripheral bloodstream.

Plasmodium falciparum transmission stages accumulate in the human bone marrow

Transmission of Plasmodium falciparum malaria parasites requires formation and development of gametocytes, yet all but the most mature of these sexual parasite forms are absent from the blood circulation. We performed a systematic organ survey in pediatric cases of fatal malaria to characterize the spatial dynamics of gametocyte development in the human host. Histological studies revealed a niche in the extravascular space of the human bone marrow where gametocytes formed in erythroid precursor cells and underwent development before reentering the circulation. Accumulation of gametocytes in the hematopoietic system of human bone marrow did not rely on cytoadherence to the vasculature as does sequestration of asexual-stage parasites. This suggests a different mechanism for the sequestration of gametocytes that could potentially be exploited to block malaria transmission.

Protein export into malaria parasite-infected erythrocytes: mechanisms and functional consequences

Phylum Apicomplexa comprises a large group of obligate intracellular parasites of high medical and veterinary importance. These organisms succeed intracellularly by effecting remarkable changes in a broad range of diverse host cells. The transformation of the host erythrocyte is particularly striking in the case of the malaria parasite Plasmodium falciparum. P. falciparum exports hundreds of proteins that mediate a complex cellular renovation marked by changes in the permeability, rigidity, and cytoadherence properties of the host erythrocyte. The past decade has seen enormous progress in understanding the identity and function of these exported effectors, as well as the mechanisms by which they are trafficked into the host cell. Here we review these advances, place them in the context of host manipulation by related apicomplexans, and propose key directions for future research.

Longitudinal study on Plasmodium falciparum gametocyte carriage following artemether-lumefantrine administration in a cohort of children aged 12-47 months living in Western Kenya, a high transmission area

BACKGROUND

The effects that artemether-lumefantrine (AL) has on gametocyte dynamics in the short-term have recently been described. However there is limited long-term longitudinal data on the effect of AL on gametocyte dynamics in asymptomatic children.

METHODS

An epidemiological study was conducted in Kombewa, Western Kenya, in which 270 asymptomatic children aged between 12 and 47 months were enrolled. The subjects were randomized to receive either a course of AL or placebo at enrolment. Active follow-up was conducted for one year.

RESULTS

The gametocyte prevalence and density dynamics throughout the study period mirrored that of the asexual forms. The proportion of initially parasitaemic subjects becoming gametocytaemic was significantly lower in the AL arm for the first 12 weeks following randomization. The geometric mean gametocyte density was lower in the AL arm for 2 weeks following randomization. None of the variables of interest had a statistically significant effect on the duration of gametocytaemia. There is no effect seen in subjects who are not parasitaemic at the time of drug administration.

CONCLUSIONS

The treatment of asymptomatic parasitaemic subjects with AL results in a significant reduction in the proportion of subjects who become gametocytaemic for at least 12 weeks.

Endoplasmic reticulum stress triggers gametocytogenesis in the malaria parasite

The malaria parasite experiences a significant amount of redox stress during its growth in human erythrocytes and heavily relies on secretory functions for pathogenesis. Most certainly, the parasite is equipped with machinery to tackle perturbations in the secretory pathway, like the unfolded protein response pathway in higher eukaryotes. Our bioinformatics analysis revealed the complete absence of genes involved in the canonical unfolded protein response pathway in Plasmodium falciparum. Accordingly, the parasite was unable to up-regulate endoplasmic reticulum (ER) chaperones or ER-associated degradation in response to DTT-mediated ER stress. Global profiling of gene expression upon DTT treatment revealed a network of AP2 transcription factors and their targets being activated. The overall outcome was up-regulation of genes involved in protein export and the sexual stage of the parasite life cycle culminating in gametocytogenesis. Our results suggest that the malaria parasite uses ER stress as a cue to switch to the transmissible sexual stages.

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.

Feeling at home from arrival to departure: protein export and host cell remodelling during Plasmodium liver stage and gametocyte maturation

Obligate intracellular pathogens actively remodel their host cells to boost propagation, survival, and persistence. Plasmodium falciparum, the causative agent of the most severe form of malaria, assembles a complex secretory system in erythrocytes. Export of parasite factors to the erythrocyte membrane is essential for parasite sequestration from the blood circulation and a major factor for clinical complications in falciparum malaria. Historic and recent molecular reports show that host cell remodelling is not exclusive to P. falciparum and that parasite-induced intra-erythrocytic membrane structures and protein export occur in several Plasmodia. Comparative analyses of P. falciparum asexual and sexual blood stages and imaging of liver stages from transgenic murine Plasmodium species show that protein export occurs in all intracellular phases from liver infection to sexual differentiation, indicating that mammalian Plasmodium species evolved efficient strategies to renovate erythrocytes and hepatocytes according to the specific needs of each life cycle phase. While the repertoireof identified exported proteins is remarkably expanded in asexual P. falciparum blood stages, the putative export machinery and known targeting signatures are shared across life cycle stages. A better understanding of the molecular mechanisms underlying Plasmodium protein export could assist in designing novel strategies to interrupt transmission between Anopheles mosquitoes and humans.

Molecular evidence for the localization of Plasmodium falciparum immature gametocytes in bone marrow

Plasmodium falciparum immature gametocytes are not observed in peripheral blood. However, gametocyte stages in organs such as bone marrow have never been assessed by molecular techniques, which are more sensitive than optical microscopy. We quantified P falciparum sexual stages in bone marrow (n = 174) and peripheral blood (n = 70) of Mozambican anemic children by quantitative polymerase chain reaction targeting transcripts specific for early (PF14_0748; PHISTa), intermediate (PF13_0247; Pfs48/45), and mature (PF10_0303; Pfs25) gametocytes. Among children positive for the P falciparum housekeeping gene (PF08_0085; ubiquitin-conjugating enzyme gene) in bone marrow (n = 136) and peripheral blood (n = 25), prevalence of immature gametocytes was higher in bone marrow than peripheral blood (early: 95% vs 20%, P < .001; intermediate: 80% vs 16%; P < .001), as were transcript levels (P < .001 for both stages). In contrast, mature gametocytes were more prevalent (100% vs 51%, P < .001) and abundant (P < .001) in peripheral blood than in the bone marrow. Severe anemia (3.57, 95% confidence interval 1.49-8.53) and dyserythropoiesis (6.21, 95% confidence interval 2.24-17.25) were independently associated with a higher prevalence of mature gametocytes in bone marrow. Our results highlight the high prevalence and abundance of early sexual stages in bone marrow, as well as the relationship between hematological disturbances and gametocyte development in this tissue.

Host immune constraints on malaria transmission: insights from population biology of within-host parasites

Background

Plasmodium infections trigger complex immune reactions from their hosts against several life stages of the parasite, including gametocytes. These immune responses are highly variable, depending on age, genetics, and exposure history of the host as well as species and strain of parasite. Although the effects of host antibodies that act against gamete stages in the mosquito (due to uptake in the blood meal) are well documented, the effects of host immunity upon within-host gametocytes are not as well understood. This report consists of a theoretical population biology-based analysis to determine constraints that host immunity impose upon gametocyte population growth. The details of the mathematical models used for the analysis were guided by published reports of clinical and animal studies, incorporated plausible modalities of immune reactions to parasites, and were tailored to the life cycl es of the two most widespread human malaria pathogens, Plasmodium falciparum and Plasmodium vivax.

Results

For the same ability to bind and clear a target, the model simulations suggest that an antibody attacking immature gametocytes would tend to lower the overall density of transmissible mature gametocytes more than an antibody attacking the mature forms directly. Transmission of P. falciparum would be especially vulnerable to complete blocking by antibodies to its immature forms since its gametocytes take much longer to reach maturity than those of P. vivax. On the other hand, antibodies attacking the mature gametocytes directly would reduce the time the mature forms can linger in the host. Simulation results also suggest that varying the standard deviation in the time necessary for individual asexual parasites to develop and produce schizonts can affect the efficiency of production of transmissible gametocytes.

Conclusions

If mature gametocyte density determines the probability of transmission, both Plasmodium species, but especially P. falciparum, could bolster this probability through evasion or suppression of host immune responses against the immature gametocytes. However, if the long term lingering of mature gametocytes at low density in the host is also important to ensure transmission, then evasion or suppression of antibodies against the mature stages would bolster probability of transmission as well.