Digestive vacuole of Plasmodium falciparum released during erythrocyte rupture dually activates complement and coagulation

Significance of Plasmodium berghei Amino Acid Transporter 1 in Food Vacuole Functionality and Its Association with Cerebral Pathogenesis

The food vacuole plays a central role in the blood stage of parasite development by digesting host hemoglobin acquired from red blood cells and detoxifying the host heme released during hemoglobin digestion into hemozoin. Blood-stage parasites undergo periodic schizont bursts, releasing food vacuoles containing hemozoin. Clinical studies in malaria-infected patients and in vivo animal studies have shown the association of hemozoin with disease pathogenesis and abnormal host immune responses in malaria. Here, we perform a detailed in vivo characterization of putative Plasmodium berghei amino acid transporter 1 localized in the food vacuole to understand its significance in the malaria parasite. We show that the targeted deletion of amino acid transporter 1 in Plasmodium berghei leads to a swollen food vacuole phenotype with the accumulation of host hemoglobin-derived peptides. Plasmodium berghei amino acid transporter 1-knockout parasites produce less hemozoin, and the hemozoin crystals display a thin morphology compared with wild-type parasites. The knockout parasites show reduced sensitivity to chloroquine and amodiaquine by showing recrudescence. More importantly, mice infected with the knockout parasites are protected from cerebral malaria and display reduced neuronal inflammation and cerebral complications. Genetic complementation of the knockout parasites restores the food vacuole morphology with hemozoin levels similar to that of wild-type parasites, causing cerebral malaria in the infected mice. The knockout parasites also show a significant delay in male gametocyte exflagellation. Our findings highlight the significance of amino acid transporter 1 in food vacuole functionality and its association with malaria pathogenesis and gametocyte development. IMPORTANCE Food vacuoles of the malaria parasite are involved in the degradation of red blood cell hemoglobin. The amino acids derived from hemoglobin degradation support parasite growth, and the heme released is detoxified into hemozoin. Antimalarials such as quinolines target hemozoin formation in the food vacuole. Food vacuole transporters transport hemoglobin-derived amino acids and peptides from the food vacuole to the parasite cytosol. Such transporters are also associated with drug resistance. Here, we show that the deletion of amino acid transporter 1 in Plasmodium berghei leads to swollen food vacuoles with the accumulation of hemoglobin-derived peptides. The transporter-deleted parasites generate less hemozoin with thin crystal morphology and show reduced sensitivity to quinolines. Mice infected with transporter-deleted parasites are protected from cerebral malaria. There is also a delay in male gametocyte exflagellation, affecting transmission. Our findings uncover the functional significance of amino acid transporter 1 in the life cycle of the malaria parasite.

Brain endothelial STING1 activation by Plasmodium-sequestered heme promotes cerebral malaria via type I IFN response

CM results from loss of blood–brain endothelial barrier function caused by unrestrained inflammatory response in the natural course of infection by Plasmodium parasites. However, the role of brain endothelium in triggering inflammatory mechanisms is still undetermined. We found that the innate immune sensor STING1 is crucial for production of IFNβ by brain endothelial cells in Plasmodium-infected mice. This in turn stimulates CXCL10-mediated recruitment of leukocytes and subsequent brain inflammation and tissue damage. We identified within extracellular particles released from Plasmodium-infected erythrocytes, a fraction containing products of hemoglobin degradation, namely, heme, which we show can bind STING1. Our results unravel a mechanism of CM immunopathogenesis: Heme contained in extracellular particles triggers the STING/IFNβ/CXCL10 axis in brain endothelial cells.

Cerebral malaria (CM) is a life-threatening form of Plasmodium falciparum infection caused by brain inflammation. Brain endothelium dysfunction is a hallmark of CM pathology, which is also associated with the activation of the type I interferon (IFN) inflammatory pathway. The molecular triggers and sensors eliciting brain type I IFN cellular responses during CM remain largely unknown. We herein identified the stimulator of interferon response cGAMP interactor 1 (STING1) as the key innate immune sensor that induces Ifnβ1 transcription in the brain of mice infected with Plasmodium berghei ANKA (Pba). This STING1/IFNβ-mediated response increases brain CXCL10 governing the extent of brain leukocyte infiltration and blood–brain barrier (BBB) breakdown, and determining CM lethality. The critical role of brain endothelial cells (BECs) in fueling type I IFN–driven brain inflammation was demonstrated in brain endothelial–specific IFNβ-reporter and STING1-deficient Pba-infected mice, which were significantly protected from CM lethality. Moreover, extracellular particles (EPs) released from Pba-infected erythrocytes activated the STING1-dependent type I IFN response in BECs, a response requiring intracellular acidification. Fractionation of the EPs enabled us to identify a defined fraction carrying hemoglobin degradation remnants that activates STING1/IFNβ in the brain endothelium, a process correlated with heme content. Notably, stimulation of STING1-deficient BECs with heme, docking experiments, and in vitro binding assays unveiled that heme is a putative STING1 ligand. This work shows that heme resultant from the parasite heterotrophic activity operates as an alarmin, triggering brain endothelial inflammatory responses via the STING1/IFNβ/CXCL10 axis crucial to CM pathogenesis and lethality.

Methods Used to Investigate the Plasmodium falciparum Digestive Vacuole

Plasmodium falciparum malaria remains a global health problem as parasites continue to develop resistance to all antimalarials in use. Infection causes clinical symptoms during the intra-erythrocytic stage of the lifecycle where the parasite infects and replicates within red blood cells (RBC). During this stage, P. falciparum digests the main constituent of the RBC, hemoglobin, in a specialized acidic compartment termed the digestive vacuole (DV), a process essential for survival. Many therapeutics in use target one or multiple aspects of the DV, with chloroquine and its derivatives, as well as artemisinin, having mechanisms of action within this organelle. In order to better understand how current therapeutics and those under development target DV processes, techniques used to investigate the DV are paramount. This review outlines the involvement of the DV in therapeutics currently in use and focuses on the range of techniques that are currently utilized to study this organelle including microscopy, biochemical analysis, genetic approaches and metabolomic studies. Importantly, continued development and application of these techniques will aid in our understanding of the DV and in the development of new therapeutics or therapeutic partners for the future.

Hemozoin in Malarial Complications: More Questions Than Answers

Plasmodium parasites contain various virulence factors that modulate the host immune response. Malarial pigment, or hemozoin (Hz), is an undegradable crystalline product of the hemoglobin degradation pathway in the parasite and possesses immunomodulatory properties. An association has been found between Hz accumulation and severe malaria, suggesting that the effects of Hz on the host immune response may contribute to the development of malarial complications. Although the immunomodulatory roles of Hz have been widely investigated, many conflicting data exist, likely due to the variability between experimental set-ups and technical limitations of Hz generation and isolation methods. Here, we critically assess the potential immunomodulatory effects of Hz, its role in malarial complications, and its potential effects after parasite clearance.

The malaria parasite Plasmodium falciparum in red blood cells selectively takes up serum proteins that affect host pathogenicity

Background

The malaria parasite Plasmodium falciparum is a protozoan that develops in red blood cells (RBCs) and requires various host factors. For its development in RBCs, nutrients not only from the RBC cytosol but also from the extracellular milieu must be acquired. Although the utilization of host nutrients by P. falciparum has been extensively analysed, only a few studies have reported its utilization of host serum proteins. Hence, the aim of the current study was to comprehensively identify host serum proteins taken up by P. falciparum parasites and to elucidate their role in pathogenesis.

Methods

Plasmodium falciparum was cultured with human serum in vitro. Uptake of serum proteins by parasites was comprehensively determined via shotgun liquid chromatography–mass spectrometry/mass spectrometry and western blotting. The calcium ion concentration in serum was also evaluated, and coagulation activity of the parasite lysate was assessed.

Results

Three proteins, vitamin K-dependent protein S, prothrombin, and vitronectin, were selectively internalized under sufficient Ca²⁺ levels in the culture medium. The uptake of these proteins was initiated before DNA replication, and increased during the trophozoite and schizont stages, irrespective of the assembly/disassembly of actin filaments. Coagulation assay revealed that prothrombin was activated and thereby induced blood coagulation.

Conclusions

Serum proteins were taken up by parasites under culture conditions with sufficient Ca²⁺ levels. This uptake phenomenon was associated with their pathogenicity.

Cutting Edge: FHR-1 Binding Impairs Factor H-Mediated Complement Evasion by the Malaria Parasite Plasmodium falciparum

Human complement is the first line of defense against invading pathogens, including the malaria parasite Plasmodium falciparum We previously demonstrated that human complement represents a particular threat for the clinically relevant blood stages of the parasite. To evade complement-mediated destruction, the parasites acquire factor H (FH) via specific receptors. We now report that the FH-related protein FHR-1 competes with FH for binding to the parasites. FHR-1, which is composed of five complement control protein domains with variable homology to FH but lacks C3b regulatory activity, accumulates on the surfaces of intraerythrocytic schizonts and free merozoites. Although binding of FH to schizont-infected RBCs and merozoites is increased in FHR-1-deficient human serum, the addition of recombinant FHR-1 decreases FH binding. The presence of FHR-1 consequently impairs C3b inactivation and parasite viability. We conclude that FHR-1 acts as a protective factor in human immunity by counteracting FH-mediated microbial complement evasion.

Loss of complement regulatory proteins on uninfected erythrocytes in vivax and falciparum malaria anemia

Anemia is a major complication of malaria, driven largely by loss of uninfected RBCs during infection. RBC clearance through loss of complement regulatory proteins (CRPs) is a significant contributor to anemia in Plasmodium falciparum infection, but its role in Plasmodium vivax infection is unknown. CRP loss increases RBC susceptibility to macrophage clearance, a process that is also regulated by CD47. We compared CRPs and CD47 expression on infected and uninfected RBCs in adult patients with vivax and falciparum malaria and different anemia severities from Papua, Indonesia. Complement activation and parasite-specific complement-fixing antibodies were measured by ELISA. Levels of CR1 and CD55 were reduced in severe anemia in both falciparum and vivax malaria. Loss of CRPs and CD47 was restricted to uninfected RBCs, with infected RBCs having higher expression. There was no association among complement-fixing antibodies, complement activation, and CRP loss. Our findings demonstrate that CRP loss is a pan-species, age-independent mechanism of malarial anemia. Higher levels of CRP and CD47 expression on infected RBCs suggest that parasites are protected from complement-mediated destruction and macrophage clearance. Lack of associations between protective antibodies and CRP loss highlight that complement pathogenic and protective pathways are distinct mechanisms during infection.

Evaluating antidisease immunity to malaria and implications for vaccine design

Immunity to malaria could be categorized broadly as antiparasite or antidisease immunity. While most vaccine research efforts have focused on antiparasite immunity, the evidence from endemic populations suggest that antidisease immunity is an important component of natural immunity to malaria. The processes that mediate antidisease immunity have, however, attracted little to no attention, and most interests have been directed towards the antibody responses. This review evaluates the evidence for antidisease immunity in endemic areas and discusses the possible mechanisms responsible for it. Given the key role that inflammation plays in the pathogenesis of malaria, regulation of the inflammatory response appears to be a major mechanism for antidisease immunity in naturally exposed individuals.

Pathogens' toolbox to manipulate human complement

The surveillance and pathogen fighting functions of the complement system have evolved to protect mammals from life-threatening infections. In turn, pathogens have developed complex molecular mechanisms to subvert, divert and evade the effector functions of the complement. The study of complement immunoevasion by pathogens sheds light on their infection drivers, knowledge that is essential to implement therapies. At the same time, complement evasion also acts as a discovery ground that reveals important aspects of how complement works under physiological conditions. In recent years, complex interrelationships between infection insults and the onset of autoimmune and complement dysregulation diseases have led to propose that encounters with pathogens can act as triggering factors for disease. The correct management of these diseases involves the recognition of their triggering factors and the development and administration of complement-associated molecular therapies. Even more recently, unsuspected proteins from pathogens have been shown to possess moonlighting functions as virulence factors, raising the possibility that behind the first line of virulence factors there be many more pathogen proteins playing secondary, helping and supporting roles for the pathogen to successfully establish infections. In an era where antibiotics have a progressively reduced effect on the management and control of infectious diseases worldwide, knowledge on the mechanisms of pathogenic invasion and evasion look more necessary and pressing than ever.

IgG opsonization of merozoites: multiple immune mechanisms for malaria vaccine development

Global eradication of the human-infecting malaria parasite Plasmodium falciparum, the major cause of malaria mortality, is unlikely to be achieved without an effective vaccine. However, our limited understanding of how protective immune responses target malaria parasites in humans, and how to best elicit these immune responses through vaccination, has hampered vaccine development. The red blood cell invading stage of the parasite lifecycle (merozoite) displays antigens that are attractive vaccine candidates as they are accessible to antibodies and raise high antibody titres in naturally immune individuals. The number of merozoite antigens that elicit an immune response, and their structural and functional diversity, has led to a large number of lead antigens being pursued as vaccine candidates. Despite being seemingly spoilt for choice in terms of vaccine candidates, there is still a lack of consensus on exactly how merozoite antibodies reduce parasitemia and malaria disease. In this review we describe the various immune mechanisms that can result from IgG opsonization of merozoites, and highlight recent developments that support a role for these functional antibodies in naturally acquired and vaccine-induced immunity.

Dynamic interactions of Plasmodium spp. with vascular endothelium

Plasmodial species are protozoan parasites that infect erythrocytes. As such, they are in close contact with microvascular endothelium for most of the life cycle in the mammalian host. The host-parasite interactions of this stage of the infection are responsible for the clinical manifestations of the disease that range from a mild febrile illness to severe and frequently fatal syndromes such as cerebral malaria and multi-organ failure. Plasmodium falciparum, the causative agent of the most severe form of malaria, is particularly predisposed to modulating endothelial function through either direct adhesion to endothelial receptor molecules, or by releasing potent host and parasite products that can stimulate endothelial activation and/or disrupt barrier function. In this review, we provide a critical analysis of the current clinical and laboratory evidence for endothelial dysfunction during severe P. falciparum malaria. Future investigations using state-of-the-art technologies such as mass cytometry and organs-on-chips to further delineate parasite-endothelial cell interactions are also discussed.

Thrombin Cleavage of Plasmodium falciparum Erythrocyte Membrane Protein 1 Inhibits Cytoadherence

Plasmodium falciparum malaria remains one of the most deadly infections worldwide. The pathogenesis of the infection results from the sequestration of infected erythrocytes (IRBC) in vital organs, including the brain, with resulting impairment of blood flow, hypoxia, and lactic acidosis. Sequestration occurs through the adhesion of IRBC to host receptors on microvascular endothelium by Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1), a large family of variant surface antigens, each with up to seven extracellular domains that can bind to multiple host receptors. Consequently, antiadhesive therapies directed at single endothelial adhesion molecules may not be effective. In this study, we demonstrated that the serine protease thrombin, which is pivotal in the activation of the coagulation cascade, cleaved the major parasite adhesin on the surface of IRBC. As a result, adhesion under flow was dramatically reduced, and already adherent IRBC were detached. Thrombin cleavage sites were mapped to the Duffy binding-like δ1 (DBLδ1) domain and interdomains 1 and 2 in the PfEMP1 of the parasite line IT4var19. Furthermore, we observed an inverse correlation between the presence of thrombin and IRBC in cerebral malaria autopsies of children. We investigated a modified (R67A) thrombin and thrombin inhibitor, hirugen, both of which inhibit the binding of substrates to exosite I, thereby reducing its proinflammatory properties. Both approaches reduced the barrier dysfunction induced by thrombin without affecting its proteolytic activity on PfEMP1, raising the possibility that thrombin cleavage of variant PfEMP1 may be exploited as a broadly inhibitory antiadhesive therapy.

Plasmodium falciparum malaria is the third leading cause of mortality due to a pathogen, with 214 million people infected and 438,000 deaths annually. The adhesion of Plasmodium falciparum-infected erythrocytes (IRBC) to microvascular endothelium is a major pathological process in severe malaria. While the recent implementation of artemisinin-based antimalarial therapy for severe malaria improves patient survival by targeting all parasite stages, antiparasite drugs alone may not immediately reverse pathophysiological processes in occluded vessels. Here we show that thrombin, an enzyme intimately involved in the clotting process, cleaves the main parasite adhesin expressed on the surface of IRBC, thereby preventing and reversing the binding of IRBC to endothelial cells. This beneficial effect of thrombin can be achieved by modified thrombins that cause significantly less clotting and vessel leakage while preserving the ability to cleave the parasite protein. Our results provide the basis for using modified thrombins as adjunctive therapy in severe malaria.

Compstatin Cp40 blocks hematin-mediated deposition of C3b fragments on erythrocytes: Implications for treatment of malarial anemia

During malarial anemia, 20 uninfected red blood cells (RBCs) are destroyed for every RBC infected by Plasmodium falciparum (Pf). Increasing evidence indicates an important role for complement in destruction of uninfected RBCs. Products of RBC lysis induced by Pf, including the digestive vacuole and hematin, activate complement and promote C3 fragment deposition on uninfected RBCs. C3-opsonized cells are then subject to extravascular destruction mediated by fixed tissue macrophages which express receptors for C3 fragments. The Compstatin family of cyclic peptides blocks complement activation at the C3 cleavage step, and is under investigation for treatment of complement-mediated diseases. We demonstrate, that under a variety of stringent conditions, second-generation Compstatin analogue Cp40 completely blocks hematin-mediated deposition of C3 fragments on naïve RBCs. Our findings indicate that prophylactic provision of Compstatin for malaria-infected individuals at increased risk for anemia may provide a safe and inexpensive treatment to prevent or substantially reduce malarial anemia.

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.

Recruitment of Factor H as a Novel Complement Evasion Strategy for Blood-Stage Plasmodium falciparum Infection

The human complement system is the frontline defense mechanism against invading pathogens. The coexistence of humans and microbes throughout evolution has produced ingenious molecular mechanisms by which microorganisms escape complement attack. A common evasion strategy used by diverse pathogens is the hijacking of soluble human complement regulators to their surfaces to afford protection from complement activation. One such host regulator is factor H (FH), which acts as a negative regulator of complement to protect host tissues from aberrant complement activation. In this report, we show that Plasmodium falciparum merozoites, the invasive form of the malaria parasites, actively recruit FH and its alternative spliced form FH-like protein 1 when exposed to human serum. We have mapped the binding site in FH that recognizes merozoites and identified Pf92, a member of the six-cysteine family of Plasmodium surface proteins, as its direct interaction partner. When bound to merozoites, FH retains cofactor activity, a key function that allows it to downregulate the alternative pathway of complement. In P. falciparum parasites that lack Pf92, we observed changes in the pattern of C3b cleavage that are consistent with decreased regulation of complement activation. These results also show that recruitment of FH affords P. falciparum merozoites protection from complement-mediated lysis. Our study provides new insights on mechanisms of immune evasion of malaria parasites and highlights the important function of surface coat proteins in the interplay between complement regulation and successful infection of the host.

The immunological balance between host and parasite in malaria

Coevolution of humans and malaria parasites has generated an intricate balance between the immune system of the host and virulence factors of the parasite, equilibrating maximal parasite transmission with limited host damage. Focusing on the blood stage of the disease, we discuss how the balance between anti-parasite immunity versus immunomodulatory and evasion mechanisms of the parasite may result in parasite clearance or chronic infection without major symptoms, whereas imbalances characterized by excessive parasite growth, exaggerated immune reactions or a combination of both cause severe pathology and death, which is detrimental for both parasite and host. A thorough understanding of the immunological balance of malaria and its relation to other physiological balances in the body is of crucial importance for developing effective interventions to reduce malaria-related morbidity and to diminish fatal outcomes due to severe complications. Therefore, we discuss in this review the detailed mechanisms of anti-malarial immunity, parasite virulence factors including immune evasion mechanisms and pathogenesis. Furthermore, we propose a comprehensive classification of malaria complications according to the different types of imbalances.

More than just immune evasion: Hijacking complement by Plasmodium falciparum

Malaria remains one of the world's deadliest diseases. Plasmodium falciparum is responsible for the most severe and lethal form of human malaria. P. falciparum's life cycle involves two obligate hosts: human and mosquito. From initial entry into these hosts, malaria parasites face the onslaught of the first line of host defence, the complement system. In this review, we discuss the complex interaction between complement and malaria infection in terms of hosts immune responses, parasite survival and pathogenesis of severe forms of malaria. We will focus on the role of complement receptor 1 and its associated polymorphisms in malaria immune complex clearance, as a mediator of parasite rosetting and as an entry receptor for P. falciparum invasion. Complement evasion strategies of P. falciparum parasites will also be highlighted. The sexual forms of the malaria parasites recruit the soluble human complement regulator Factor H to evade complement-mediated killing within the mosquito host. A novel evasion strategy is the deployment of parasite organelles to divert complement attack from infective blood stage parasites. Finally we outline the future challenge to understand the implications of these exploitation mechanisms in the interplay between successful infection of the host and pathogenesis observed in severe malaria.

Assessment of the neutrophilic antibody-dependent respiratory burst (ADRB) response to Plasmodium falciparum

Semi-immunity against Pf malaria is based on a combination of cellular and humoral immune responses. PMNs and IgGs are considered important components of this process, but the underlying mechanisms are unclear. We investigated the neutrophilic ADRB by analyzing the production of ROS in response to Pf antigen-specific IgGs bound to solid-phase immobilized antigens (sADRB) or whole merozoites (mADRB). We found that the PMN stimulations in each assay were based on different underlying mechanisms, demonstrating the importance of the assay set-up for the evaluation of antibody-triggered PMN responses. In the sADRB assay, ROS were produced externally, and by specific blocking of CD32(a)/FcγRII(a), the immediate neutrophilic response was abolished, whereas the removal of CD16(b)/FcγRIII(b) had no substantial effect. The key role of CD32(a) was confirmed using CD16(b)-deficient PMNs, in which similar changes of neutrophilic ADRB profiles were recorded after treatment. In the mADRB assay, ROS were produced almost exclusively within the cell, suggesting that the underlying mechanism was phagocytosis. This was confirmed using an additional phagocytosis assay, in which PMNs specifically ingested merozoites opsonized with Ghanaian plasma IgGs, seven times more often than merozoites opsonized with European plasma IgGs (P<0.001). Our data show that assay set-ups used to evaluate the responses of PMNs and perhaps other effector cells must be chosen carefully to evaluate the appropriate cellular responses. Our robust, stable, and well-characterized methods could therefore be useful in malaria vaccine studies to analyze the antimalarial effector function of antibodies.

Malarial anemia: digestive vacuole of Plasmodium falciparum mediates complement deposition on bystander cells to provoke hemophagocytosis

The digestive vacuole (DV) of Plasmodium falciparum, which is released into the bloodstream upon rupture of each parasitized red blood cell (RBC), was recently discovered to activate the alternative complement pathway. In the present work, we show that C3- and C5-convertases assembling on the parasitic organelle are able to provoke deposition of activated C3 and C5b-9 on non-infected bystander erythrocytes. Direct contact of DVs with cells is mandatory for the effect, and bystander complement deposition occurs focally, possibly at the sites of contact. Complement opsonization promotes protracted erythrophagocytosis by human macrophages, an effect that is magnified when ring-stage infected RBCs with reduced CD55 and CD59, or paroxysmal nocturnal hemoglobinuria (PNH)-RBCs lacking these complement inhibitors are employed as targets. Bystander attack can also directly induce lysis of PNH-RBCs. Direct evidence for complement activation and bystander attack mediated by DVs was obtained through immunohistochemical analyses of brain paraffin sections from autopsies of patients who had died of cerebral malaria. C3d and the assembled C5b-9 complex could be detected in all sections, colocalizing with and often extending locally beyond massive accumulations of DVs that were identified under polarized light. This is the first demonstration that a complement-activating particle can mediate opsonization of bystander cells to promote their antibody-independent phagocytosis. The phenomenon may act in concert with other pathomechanisms to promote the development of anemia in patients with severe malaria.

Tempol, an intracellular antioxidant, inhibits tissue factor expression, attenuates dendritic cell function, and is partially protective in a murine model of cerebral malaria

Background

The role of intracellular radical oxygen species (ROS) in pathogenesis of cerebral malaria (CM) remains incompletely understood.

Methods and Findings

We undertook testing Tempol—a superoxide dismutase (SOD) mimetic and pleiotropic intracellular antioxidant—in cells relevant to malaria pathogenesis in the context of coagulation and inflammation. Tempol was also tested in a murine model of CM induced by Plasmodium berghei Anka infection. Tempol was found to prevent transcription and functional expression of procoagulant tissue factor in endothelial cells (ECs) stimulated by lipopolysaccharide (LPS). This effect was accompanied by inhibition of IL-6, IL-8, and monocyte chemoattractant protein (MCP-1) production. Tempol also attenuated platelet aggregation and human promyelocytic leukemia HL60 cells oxidative burst. In dendritic cells, Tempol inhibited LPS-induced production of TNF-α, IL-6, and IL-12p70, downregulated expression of co-stimulatory molecules, and prevented antigen-dependent lymphocyte proliferation. Notably, Tempol (20 mg/kg) partially increased the survival of mice with CM. Mechanistically, treated mice had lowered plasma levels of MCP-1, suggesting that Tempol downmodulates EC function and vascular inflammation. Tempol also diminished blood brain barrier permeability associated with CM when started at day 4 post infection but not at day 1, suggesting that ROS production is tightly regulated. Other antioxidants—such as α-phenyl N-tertiary-butyl nitrone (PBN; a spin trap), MnTe-2-PyP and MnTBAP (Mn-phorphyrin), Mitoquinone (MitoQ) and Mitotempo (mitochondrial antioxidants), M30 (an iron chelator), and epigallocatechin gallate (EGCG; polyphenol from green tea) did not improve survival. By contrast, these compounds (except PBN) inhibited Plasmodium falciparum growth in culture with different IC_50s. Knockout mice for SOD1 or phagocyte nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (gp91^phox–/–) or mice treated with inhibitors of SOD (diethyldithiocarbamate) or NADPH oxidase (diphenyleneiodonium) did not show protection or exacerbation for CM.

Conclusion

Results with Tempol suggest that intracellular ROS contribute, in part, to CM pathogenesis. Therapeutic targeting of intracellular ROS in CM is discussed.

Complement activation in malaria: friend or foe?

Complement is activated during malaria infection, but there is little evidence that it benefits the host. On the contrary, growing evidence points to the central role of complement activation in the pathogenesis of complicated malaria. Recent evidence suggests a critical role for C5a and the membrane attack complex in the pathogenesis of cerebral malaria, and for C5a in the pathogenesis of placental malaria. In addition, erythrocytes of children with severe malarial anemia have increased deposition of C3b and decreased capacity to regulate complement activation, that probably increase their susceptibility to destruction by liver and splenic macrophages. These observations justify further investigation of the role of complement in malaria and the testing of complement inhibitors as adjunctive treatment for severe malaria.

Severity of anaemia is associated with bone marrow haemozoin in children exposed to Plasmodium falciparum

There are no large-scale ex vivo studies addressing the contribution of Plasmodium falciparum in the bone marrow to anaemia. The presence of malaria parasites and haemozoin were studied in bone marrows from 290 anaemic children attending a rural hospital in Mozambique. Peripheral blood infections were determined by microscopy and polymerase chain reactions. Bone marrow parasitaemia, haemozoin and dyserythropoiesis were microscopically assessed. Forty-two percent (123/290) of children had parasites in the bone marrow and 49% (111/226) had haemozoin, overlapping with parasitaemia in 83% (92/111) of cases. Sexual and mature asexual parasites were highly prevalent (62% gametocytes, 71% trophozoites, 23% schizonts) suggesting their sequestration in this tissue. Sixteen percent (19/120) of children without peripheral infection had haemozoin in the bone marrow. Haemozoin in the bone marrow was independently associated with decreased Hb concentration (P = 0·005) and was more common in dyserythropoietic bone marrows (P = 0·010). The results of this ex vivo study suggest that haemozoin in the bone marrow has a role in the pathogenesis of malarial-anaemia through ineffective erythropoiesis. This finding may have clinical implications for the development of drugs targeted to prevent and treat malarial-anaemia.

Immunopathological effects of malaria pigment or hemozoin and other crystals

Blood-stage malaria parasites produce insoluble hemozoin (Hz) crystals that are released in the blood circulation upon schizont rupture. In general, endogenous crystal formation or inhalation of crystalline materials is often associated with pathology. As the immune system responds differently to crystalline particles than to soluble molecules, in this review, the properties, immunological recognition, and pathogenic responses of Hz are discussed, and compared with two other major pathogenic crystals, monosodium urate (MSU) and asbestos. Because of the size and shape of MSU crystals and asbestos fibers, phagolysosomal formation is inefficient and often results in leakage of lysosomal content in the cell cytoplasm and/or in the extracellular environment with subsequent cell damage and cell death. Phagolysosomal formation after Hz ingestion is normal, but Hz remains stored inside these cells for months or even longer without any detectable degradation. Nonetheless, the different types of crystals are recognized by similar immune receptors, involving Toll-like receptors, the inflammasome, antibodies, and/or complement factors, and through similar signaling cascades, they activate both proinflammatory and anti-inflammatory immune responses that contribute to inflammation-associated pathology.

Functional roles for C5a and C5aR but not C5L2 in the pathogenesis of human and experimental cerebral malaria

The host immune response plays an important role in the onset and progression of cerebral malaria (CM). The complement system is an essential component of the innate immune response to malaria, and its activation generates the anaphylatoxin C5a. To test the hypothesis that C5a signaling contributes to the pathogenesis of CM, we investigated a causal role for the C5a receptors C5aR and C5L2 in a mouse model of experimental CM (ECM) induced by Plasmodium berghei ANKA infection, and using a case-control design, we examined levels of C5a in plasma samples from Ugandan children presenting with CM or uncomplicated malaria (UM). In the ECM model, C5aR(-/-) mice displayed significantly improved survival compared to their wild-type (WT) counterparts (P = 0.004), whereas C5L2(-/-) mice showed no difference in survival from WT mice. Improved survival in C5aR(-/-) mice was associated with reduced levels of the proinflammatory cytokines tumor necrosis factor (TNF) and gamma interferon (IFN-γ) and the chemokine, monocyte chemoattractant protein 1 (MCP-1) (CCL2). Furthermore, endothelial integrity was enhanced, as demonstrated by increased levels of angiopoietin-1, decreased levels of angiopoietin-2 and soluble ICAM-1, and decreased Evans blue extravasation into brain parenchyma. In the case-control study, the median levels of C5a at presentation were significantly higher in children with CM versus those in children with UM (43.7 versus 22.4 ng/ml; P < 0.001). These findings demonstrate that C5a is dysregulated in human CM and contributes to the pathogenesis of ECM via C5aR-dependent inflammation and endothelial dysfunction.

Imaging Plasmodium immunobiology in the liver, brain, and lung

Plasmodium falciparum malaria is responsible for the deaths of over half a million African children annually. Until a decade ago, dynamic analysis of the malaria parasite was limited to in vitro systems with the typical limitations associated with 2D monocultures or entirely artificial surfaces. Due to extremely low parasite densities, the liver was considered a black box in terms of Plasmodium sporozoite invasion, liver stage development, and merozoite release into the blood. Further, nothing was known about the behavior of blood stage parasites in organs such as the brain where clinical signs manifest and the ensuing immune response of the host that may ultimately result in a fatal outcome. The advent of fluorescent parasites, advances in imaging technology, and availability of an ever-increasing number of cellular and molecular probes have helped illuminate many steps along the pathogenetic cascade of this deadly tropical parasite.

Pathogenesis of malaria revisited

Plasmodium falciparum malaria claims 1 million lives around the globe every year. Parasitemia can reach remarkably high levels. The developing parasite digests hemoglobin and converts the waste product to hemozoin alias malaria pigment. These processes occur in a vesicular compartment named the digestive vacuole (DV). Each parasitized cell releases one DV upon rupture. Myriads of DVs thus gain entry into the blood, but whether they trigger pathobiological events has never been investigated. We recently discovered that the DV membrane simultaneously activates the two major enzyme cascades in blood, complement and coagulation. Activation of both is known to occur in patients with severe malaria, so discovery of the common trigger has large consequences. The DV membrane but not the merozoite has the capacity to spontaneously activate the alternative complement and intrinsic clotting pathway. Ejection of merozoites and the DV into the bloodstream, therefore, results in selective opsonization and phagocytosis of the DV, leaving merozoites free to invade new cells. The DV membrane furthermore has the capacity to assemble prothrombinase, the key convertase of the intrinsic clotting pathway. The dual capacity of the DV to activate both complement and coagulation can be suppressed by low-molecular-weight dextran sulfate. This agent protects experimental animals from the detrimental consequences, resulting from intravenous application of purified DVs. Phagocytosis of DVs not only deploys PMN away from merozoites, but also drives the cells into a state of functional exhaustion. This may be one reason for the enhanced susceptibility of patients with severe malaria toward systemic bacterial infections. Together, these findings indicate that the DV may represent a hitherto unrecognized, important determinant of parasite pathogenicity.

The C5 convertase is not required for activation of the terminal complement pathway in murine experimental cerebral malaria

Cerebral malaria (CM) is the most severe manifestation of clinical malaria syndromes and has a high fatality rate especially in the developing world. Recent studies demonstrated that C5(-/-) mice are resistant to experimental CM (ECM) and that protection was due to the inability to form the membrane attack complex. Unexpectedly, we observed that C4(-/-) and factor B(-/-) mice were fully susceptible to disease, indicating that activation of the classical or alternative pathways is not required for ECM. C3(-/-) mice were also susceptible to ECM, indicating that the canonical C5 convertases are not required for ECM development and progression. Abrogation of ECM by treatment with anti-C9 antibody and detection of C5a in serum of C3(-/-) mice confirmed that C5 activation occurs in ECM independent of C5 convertases. Our data indicate that activation of C5 in ECM likely occurs via coagulation enzymes of the extrinsic protease pathway.