Loss of endothelial protein C receptors links coagulation and inflammation to parasite sequestration in cerebral malaria in African children

Plasma Ang2 and ADAM17 levels are elevated during clinical malaria; Ang2 level correlates with severity and expression of EPCR-binding PfEMP1

The pathogenesis of Plasmodium falciparum malaria involves a complex interplay between parasite adhesion and inflammatory response that includes release of cytokines and activation of the endothelium with accompanying release of Angiopoitin 2 (Ang2) to the plasma. A-disintegrin and metalloproteinase 17 (ADAM17) is a protein responsible for releasing cytokines, including Tumor Necrosis Factor α (TNFα), and shedding of adhesion proteins. In this study, we show that plasma levels of ADAM17 are increased in Tanzanian children hospitalized with a malaria infection compared with asymptomatic children but similar to children hospitalized with other infectious diseases. The plasma levels of ADAM17 decreased during recovery after an acute malaria episode. Plasma levels of Ang2 were associated with markers of malaria severity and levels of var transcripts encoding P. falciparum Erythrocyte Membrane Protein 1 (PfEMP1) containing Cysteine Rich Inter Domain Region α1 (CIDRα1) domains predicted to bind Endothelial Protein C receptor (EPCR). ADAM17 levels were not associated with expression of var genes encoding different PfEMP1 types when controlling for age. These data are the first to report ADAM17 plasma levels in malaria-exposed individuals, and support the notion that parasite sequestration mediated by EPCR-binding PfEMP1 is associated with endothelial activation and pathology in severe paediatric malaria.

Triggering receptor expressed on myeloid cells 1 (TREM-1) and cytokine gene variants in complicated and uncomplicated malaria

BACKGROUND

Malaria elicits inflammatory responses, which, if not well regulated, may exert detrimental effects. When activated, triggering receptor expressed on myeloid cells 1 (TREM-1) enhances inflammatory responses by increasing secretion of IL-8 and other Th1 cytokines. In contrast, TREM-like transcript 1 (TREML-1) promotes anti-inflammatory responses by binding to TREM-1 ligands and competing with TREM-1, thus antagonizing TREM-1 activation to reduce inflammation. Endothelial protein C receptor (EPCR) also mediates anti-inflammatory responses by activating endothelial protein C (PC). Upon microbial stimulation, soluble forms of TREM-1 (sTREM-1) and soluble EPCR (sEPCR) are released. Their plasma levels reflect the degree of inflammation and the severity of infection.

METHODS

In a cross-sectional study comparing patients with severe with uncomplicated malaria, sTREM-1, soluble TREML-1 (sTREML-1) and sEPCR plasma levels as well as plasma levels of sEPCR derived from convalescent patients were quantified. Samples were collected on admittance of paediatric patients infected with Plasmodium falciparum to hospitals in Accra, Ghana. Distinct genetic regions of the genes encoding TREM-1, EPCR, interleukin (IL)-8 and IL-18 encompassing known genetic polymorphisms that influence plasma levels underwent DNA sequencing.

RESULTS

Higher sTREM-1 levels were observed among children suffering from severe malaria compared to those with uncomplicated malaria (P = 0.049). Low TREM-1 to TREML-1 ratios were associated with uncomplicated malaria (P = 0.033). The TREM1 rs2234237T variant causing the amino acid exchange Thr25Ser, which has been associated with higher TREM-1 plasma levels, was significantly more frequent among patients with severe malaria than in those with uncomplicated malaria (P = 0.036). Low levels of sEPCR were observed in severe and uncomplicated malaria, while variant genotypes of IL8, IL18 and EPCR did not show any association.

CONCLUSION

Higher plasma levels of sTREM-1 alone or relative to sTREML-1 during malaria predispose to the phenotype of severe malaria. Carriage of the TREM1 rs2234237T allele appears to be a risk factor for the development of severe malaria.

Proteinases, Their Extracellular Targets, and Inflammatory Signaling

Given that over 2% of the human genome codes for proteolytic enzymes and their inhibitors, it is not surprising that proteinases serve many physiologic-pathophysiological roles. In this context, we provide an overview of proteolytic mechanisms regulating inflammation, with a focus on cell signaling stimulated by the generation of inflammatory peptides; activation of the proteinase-activated receptor (PAR) family of G protein-coupled receptors (GPCR), with a mechanism in common with adhesion-triggered GPCRs (ADGRs); and by proteolytic ion channel regulation. These mechanisms are considered in the much wider context that proteolytic mechanisms serve, including the processing of growth factors and their receptors, the regulation of matrix-integrin signaling, and the generation and release of membrane-tethered receptor ligands. These signaling mechanisms are relevant for inflammatory, neurodegenerative, and cardiovascular diseases as well as for cancer. We propose that the inflammation-triggering proteinases and their proteolytically generated substrates represent attractive therapeutic targets and we discuss appropriate targeting strategies.

Severe malaria: what's new on the pathogenesis front?

Plasmodium falciparum causes the most severe and fatal form of malaria in humans with over half a million deaths each year. Cerebral malaria, a complex neurological syndrome of severe falciparum malaria, is often fatal and represents a major public health burden. Despite vigorous efforts, the pathophysiology of cerebral malaria remains to be elucidated, thereby hindering the development of adjunctive therapies. In recent years, multidisciplinary and collaborative approaches have led to groundbreaking progress both in the laboratory and in the field. Here we review the latest breakthroughs in severe malaria pathogenesis, with a specific focus on new pathogenetic mechanisms leading to cerebral malaria. The most recent findings point towards specific parasite phenotypes targeting brain microvasculature, endothelial dysfunction and subsequent oedema-induced brain swelling.

Breaking down brain barrier breaches in cerebral malaria

Recent findings have linked brain swelling to death in cerebral malaria (CM). These observations have prompted a number of investigations into the mechanisms of this pathology with the goal of identifying potential therapeutic targets. In this issue of the JCI, Gallego-Delgado and colleagues present evidence that implicates angiotensin receptors and the relocation of β-catenin to the endothelial cell nucleus in CM. This study provides a renewed focus on infected erythrocyte debris as the cause of endothelial damage and challenges previous work implicating direct effects of infected erythrocyte sequestration in the brain as the major driver of disease. While this work provides potential therapeutic avenues for CM, it leaves a number of questions unanswered.

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.

Plasmodium falciparum var genes expressed in children with severe malaria encode CIDRα1 domains

Most severe Plasmodium falciparum infections are experienced by young children. Severe symptoms are precipitated by vascular sequestration of parasites expressing a particular subset of the polymorphic P. falciparum erythrocyte membrane protein 1 (PfEMP1) adhesion molecules. Parasites binding human endothelial protein C receptor (EPCR) through the CIDRα1 domain of certain PfEMP1 were recently associated with severe malaria in children. However, it has remained unclear to which extend the EPCR‐binding CIDRα1 domains epitomize PfEMP1 expressed in severe malaria. Here, we characterized the near full‐length transcripts dominating the var transcriptome in children with severe malaria and found that the only common feature of the encoded PfEMP1 was CIDRα1 domains. Such genes were highly and dominantly expressed in both children with severe malarial anaemia and cerebral malaria. These observations support the hypothesis that the CIDRα1‐EPCR interaction is key to the pathogenesis of severe malaria and strengthen the rationale for pursuing a vaccine or adjunctive treatment aiming at inhibiting or reducing the damaging effects of this interaction.

Plasma levels of eight different mediators and their potential as biomarkers of various clinical malaria conditions in African children

BACKGROUND

Plasmodium falciparum infection can lead to several clinical manifestations ranging from asymptomatic infections (AM) and uncomplicated malaria (UM) to potentially fatal severe malaria (SM), including cerebral malaria (CM). Factors implicated in the progression towards severe disease are not fully understood.

METHODS

In the present study, an enzyme-linked immunosorbent assay (ELISA) method was used to investigate the plasma content of several biomarkers of the immune response, namely Neopterin, sCD163, suPAR, Pentraxin 3 (PTX3), sCD14, Fractalkine (CX3CL1), sTREM-1 and MIG (CXCL9), in patients with distinct clinical manifestations of malaria. The goal of this study was to determine the relative involvement of these inflammatory mediators in the pathogenesis of malaria and test their relevance as biomarkers of disease severity.

RESULTS

ROC curve analysis show that children with AM were characterized by high levels of Fractalkine and sCD163 whereas children with UM were distinguishable by the presence of PTX3 in their plasma. Furthermore, principal component analysis indicated that the combination of Fractalkine, MIG, and Neopterin was the best predictor of AM condition, while suPAR, PTX3 and sTREM-1 combination was the best indicator of UM when compared to AM. The association of Neopterin, suPAR and Fractalkine was strongly predictive of SM or CM compared to UM.

CONCLUSIONS

The results indicate that the simultaneous evaluation of these bioactive molecules as quantifiable blood parameters may be helpful to get a better insight into the clinical syndromes in children with malaria.

The endothelial protein C receptor rs867186-GG genotype is associated with increased soluble EPCR and could mediate protection against severe malaria

The endothelial protein C receptor (EPCR) appears to play an important role in Plasmodium falciparum endothelial cell binding in severe malaria (SM). Despite consistent findings of elevated soluble EPCR (sEPCR) in other infectious diseases, field studies to date have provided conflicting data about the role of EPCR in SM. To better define this role, we performed genotyping for the rs867186-G variant, associated with increased sEPCR levels, and measured sEPCR levels in two prospective studies of Ugandan children designed to understand immunologic and genetic factors associated with neurocognitive deficits in SM including 551 SM children, 71 uncomplicated malaria (UM) and 172 healthy community children (CC). The rs867186-GG genotype was more frequent in CC (4.1%) than SM (0.6%, P = 0.002). The rs867186-G variant was associated with increased sEPCR levels and sEPCR was lower in children with SM than CC (P < 0.001). Among SM children, those who had a second SM episode showed a trend toward lower plasma sEPCR both at initial admission and at 6-month follow-up compared to those without repeated SM (P = 0.06 for both). The study findings support a role for sEPCR in severe malaria pathogenesis and emphasize a distinct role of sEPCR in malaria as compared to other infectious diseases.

Disseminated intravascular coagulation

Disseminated intravascular coagulation (DIC) is an acquired syndrome characterized by widespread intravascular activation of coagulation that can be caused by infectious insults (such as sepsis) and non-infectious insults (such as trauma). The main pathophysiological mechanisms of DIC are inflammatory cytokine-initiated activation of tissue factor-dependent coagulation, insufficient control of anticoagulant pathways and plasminogen activator inhibitor 1-mediated suppression of fibrinolysis. Together, these changes give rise to endothelial dysfunction and microvascular thrombosis, which can cause organ dysfunction and seriously affect patient prognosis. Recent observations have pointed to an important role for extracellular DNA and DNA-binding proteins, such as histones, in the pathogenesis of DIC. The International Society on Thrombosis and Haemostasis (ISTH) established a DIC diagnostic scoring system consisting of global haemostatic test parameters. This scoring system has now been well validated in diverse clinical settings. The theoretical cornerstone of DIC management is the specific and vigorous treatment of the underlying conditions, and DIC should be simultaneously managed to improve patient outcomes. The ISTH guidance for the treatment of DIC recommends treatment strategies that are based on current evidence. In this Primer, we provide an updated overview of the pathophysiology, diagnosis and management of DIC and discuss the future directions of basic and clinical research in this field.

Severe adult malaria is associated with specific PfEMP1 adhesion types and high parasite biomass

The interplay between cellular and molecular determinants that lead to severe malaria in adults is unexplored. Here, we analyzed parasite virulence factors in an infected adult population in India and investigated whether severe malaria isolates impair endothelial protein C receptor (EPCR), a protein involved in coagulation and endothelial barrier permeability. Severe malaria isolates overexpressed specific members of the Plasmodium falciparum var gene/PfEMP1 (P. falciparum erythrocyte membrane protein 1) family that bind EPCR, including DC8 var genes that have previously been linked to severe pediatric malaria. Machine learning analysis revealed that DC6- and DC8-encoding var transcripts in combination with high parasite biomass were the strongest indicators of patient hospitalization and disease severity. We found that DC8 CIDRα1 domains from severe malaria isolates had substantial differences in EPCR binding affinity and blockade activity for its ligand activated protein C. Additionally, even a low level of inhibition exhibited by domains from two cerebral malaria isolates was sufficient to interfere with activated protein C-barrier protective activities in human brain endothelial cells. Our findings demonstrate an interplay between parasite biomass and specific PfEMP1 adhesion types in the development of adult severe malaria, and indicate that low impairment of EPCR function may contribute to parasite virulence.

Activated Neutrophils Are Associated with Pediatric Cerebral Malaria Vasculopathy in Malawian Children

Most patients with cerebral malaria (CM) sustain cerebral microvascular sequestration of Plasmodium falciparum-infected red blood cells (iRBCs). Although many young children are infected with P. falciparum, CM remains a rare outcome; thus, we hypothesized that specific host conditions facilitate iRBC cerebral sequestration. To identify these host factors, we compared the peripheral whole-blood transcriptomes of Malawian children with iRBC cerebral sequestration, identified as malarial-retinopathy-positive CM (Ret+CM), to the transcriptomes of children with CM and no cerebral iRBC sequestration, defined as malarial-retinopathy-negative CM (Ret-CM). Ret+CM was associated with upregulation of 103 gene set pathways, including cytokine, blood coagulation, and extracellular matrix (ECM) pathways (P < 0.01; false-discovery rate [FDR] of <0.05). Neutrophil transcripts were the most highly upregulated individual transcripts in Ret+CM patients. Activated neutrophils can modulate diverse host processes, including the ECM, inflammation, and platelet biology to potentially facilitate parasite sequestration. Therefore, we compared plasma neutrophil proteins and neutrophil chemotaxis between Ret+CM and Ret-CM patients. Plasma levels of human neutrophil elastase, myeloperoxidase, and proteinase 3, but not lactoferrin or lipocalin, were elevated in Ret+CM patients, and neutrophil chemotaxis was impaired, possibly related to increased plasma heme. Neutrophils were rarely seen in CM brain microvasculature autopsy samples, and no neutrophil extracellular traps were found, suggesting that a putative neutrophil effect on endothelial cell biology results from neutrophil soluble factors rather than direct neutrophil cellular tissue effects. Meanwhile, children with Ret-CM had lower levels of inflammation, higher levels of alpha interferon, and upregulation of Toll-like receptor pathways and other host transcriptional pathways, which may represent responses that do not favor cerebral iRBC sequestration.

There were approximately 198 million cases of malaria worldwide in 2013, with an estimated 584,000 deaths occurring mostly in sub-Saharan African children. CM is a severe and rare form of Plasmodium falciparum infection and is associated with high rates of mortality and neurological morbidity, despite antimalarial treatment. A greater understanding of the pathophysiology of CM would allow the development of adjunctive therapies to improve clinical outcomes. A hallmark of CM is cerebral microvasculature sequestration of P. falciparum-infected red blood cells (iRBCs), which results in vasculopathy in some patients. Our data provide a global analysis of the host pathways associated with CM and newly identify an association of activated neutrophils with brain iRBC sequestration. Products of activated neutrophils could alter endothelial cell receptors and coagulation to facilitate iRBC adherence. Future studies can now examine the role of neutrophils in CM pathogenesis to improve health outcomes.

Emerging roles for hemostatic dysfunction in malaria pathogenesis

Severe Plasmodium falciparum malaria remains a leading cause of mortality, particularly in sub-Saharan Africa where it accounts for up to 1 million deaths per annum. In spite of the significant mortality and morbidity associated with cerebral malaria (CM), the molecular mechanisms involved in the pathophysiology of severe malaria remain surprisingly poorly understood. Previous studies have demonstrated that sequestration of P falciparum-infected erythrocytes within the microvasculature of the brain plays a key role in the development of CM. In addition, there is convincing evidence that both endothelial cell activation and platelets play critical roles in the modulating the pathogenesis of severe P falciparum malaria. In this review, we provide an overview of recent studies that have identified novel roles through which hemostatic dysfunction may directly influence malaria pathogenesis. In particular, we focus on emerging data suggesting that von Willebrand factor, coagulation cascade activation, and dysfunction of the protein C pathway may be of specific importance in this context. These collective insights underscore a growing appreciation of the important, but poorly understood, role of hemostatic dysfunction in malaria progression and, importantly, illuminate potential approaches for novel therapeutic strategies. Given that the mortality rate associated with CM remains on the order of 20% despite the availability of effective antimalarial therapy, development of adjunctive therapies that can attenuate CM progression clearly represents a major unmet need. These emerging data are thus not only of basic scientific interest, but also of direct clinical significance.

Fighting the Monster: Applying the Host Damage Framework to Human Central Nervous System Infections

The host damage-response framework states that microbial pathogenesis is a product of microbial virulence factors and collateral damage from host immune responses. Immune-mediated host damage is particularly important within the size-restricted central nervous system (CNS), where immune responses may exacerbate cerebral edema and neurological damage, leading to coma and death. In this review, we compare human host and therapeutic responses in representative nonviral generalized CNS infections that induce archetypal host damage responses: cryptococcal menigoencephalitis and tuberculous meningitis in HIV-infected and non-HIV-infected patients, pneumococcal meningitis, and cerebral malaria. Consideration of the underlying patterns of host responses provides critical insights into host damage and may suggest tailored adjunctive therapeutics to improve disease outcome.

Marked elevation in plasma osteoprotegerin constitutes an early and consistent feature of cerebral malaria

Adherence of infected erythrocytes to vascular endothelium causes acute endothelial cell (EC) activation during Plasmodium falciparum infection. Consequently, proteins stored in Weibel-Palade (WP) bodies within EC are secreted into the plasma. Osteoprotegerin (OPG) binds to VWF and consequently is stored within WP bodies. Given the critical role of EC activation in the pathogenesis of severe malaria, we investigated plasma OPG levels in children with P. falciparum malaria. At presentation, plasma OPG levels were significantly elevated in children with cerebral malaria (CM) compared to healthy controls (means 16.0 vs 0.8 ng/ml; p<0.01). Importantly, OPG levels were also significantly higher in children with CM who had a fatal outcome, compared to children with CM who survived. Finally, in children with CM, plasma OPG levels correlated with other established prognostic indices (including plasma lactate levels and peripheral parasite density). To further investigate the relationship between severe malaria and OPG, we utilised a murine model of experimental CM in which C57BL/6J mice were infected with P. berghei ANKA. Interestingly, plasma OPG levels were increased 4.6 fold within 24 hours following P. berghei inoculation. This early marked elevation in OPG levels was observed before any objective clinical signs were apparent, and preceded the development of peripheral blood parasitaemia. As the mice became increasingly unwell, plasma OPG levels progressively increased. Collectively, these data suggest that OPG constitutes a novel biomarker with prognostic significance in patients with severe malaria. In addition, further studies are required to determine whether OPG plays a role in modulating malaria pathogenesis.

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.

Alterations of blood coagulation in controlled human malaria infection

Background

Alterations of blood coagulation are thought to be involved in malaria pathogenesis. This study had the aim to investigate changes of blood coagulation under the standardized conditions of controlled human malaria infection.

Methods

In a clinical trial aseptic, purified, cryopreserved Plasmodium falciparum sporozoites were intravenously (n = 24) or intradermally (n = 6) injected into 30 healthy volunteers. Twenty-two participants developed parasitaemia. Serial blood samples before and during prepatent period and at parasitaemia, diagnosed by microscopic assessment of thick blood smear, were obtained. Biomarkers of blood coagulation (thrombin generation potential, D-dimer, prothrombin fragment 1 + 2, von Willebrand factor, ADAMTS13 activity and soluble P-selectin) were determined.

Results

At first detection of P. falciparum parasitaemia, 72.7 % of volunteers had peak thrombin generation 10 % above their baseline. Overall, peak thrombin generation was 17.7 % higher at parasitaemia compared to baseline [median (25th–75th percentile): 225.4 nM (168.1–295.6) vs. 191.5 nM (138.2–231.9); p = 0.026]. There were no significant changes of other coagulation parameters.

Conclusions

The thrombin generation potential, an in vitro blood coagulation test, which reflects an individual´s global coagulation status, was increased by 17.7 % at very early stages of P. falciparum malaria, suggesting a hypercoagulable state may be induced, even when parasite density is low.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-015-1079-3) contains supplementary material, which is available to authorized users.

Differential Plasmodium falciparum surface antigen expression among children with Malarial Retinopathy

Retinopathy provides a window into the underlying pathology of life-threatening malarial coma ("cerebral malaria"), allowing differentiation between 1) coma caused by sequestration of Plasmodium falciparum-infected erythrocytes in the brain and 2) coma with other underlying causes. Parasite sequestration in the brain is mediated by PfEMP1; a diverse parasite antigen that is inserted into the surface of infected erythrocytes and adheres to various host receptors. PfEMP1 sub-groups called "DC8" and "DC13" have been proposed to cause brain pathology through interactions with endothelial protein C receptor. To test this we profiled PfEMP1 gene expression in parasites from children with clinically defined cerebral malaria, who either had or did not have accompanying retinopathy. We found no evidence for an elevation of DC8 or DC13 PfEMP1 expression in children with retinopathy. However, the proportional expression of a broad subgroup of PfEMP1 called "group A" was elevated in retinopathy patients suggesting that these variants may play a role in the pathology of cerebral malaria. Interventions targeting group A PfEMP1 may be effective at reducing brain pathology.

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.

Beninese children with cerebral malaria do not develop humoral immunity against the IT4-VAR19-DC8 PfEMP1 variant linked to EPCR and brain endothelial binding

Background

Malaria is still one of the most prevalent infectious diseases in the world. Sequestration of infected erythrocytes (IEs) is the prime mediator of disease. Cytoadhesion of IEs is mediated by members of the highly diverse Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1). A restricted sub-set of var genes encoding for PfEMP1s possessing the domain cassettes DC8 and DC13 were found to bind to the endothelial protein C receptor (EPCR). These var genes were shown to be highly expressed by parasites from patients with severe malaria clinical outcomes compared to those from patients with uncomplicated symptoms.

Methods

In order to further study the molecular mechanisms underlying DC8/DC13 expressing IEs adhesion to EPCR, a method was developed to produce highly pure recombinant EPCR. The IT4 parasite strain was selected on either anti-IT4-VAR19 purified IgG, EPCR or human brain endothelial cell line and their var gene expression profiles as well as their binding phenotypes were compared. The N-terminal region of IT4-VAR19 comprising a full-length DC8 cassette as well as the single EPCR binding CIDRα1.1 domain were also produced, and their immune recognition (IgG) was assessed using plasma samples from Beninese children presenting acute mild malaria, severe malaria or cerebral malaria at the time of their admission to the clinic, and from convalescent-phase plasma collected 30 days after anti-malarial treatment.

Results

The multi-domain VAR19-NTS-DBLγ6 binds to EPCR with a greater affinity than the CIDRα1.1 domain alone and this study also demonstrates that VAR19-NTS-DBLγ6 binding to the EPCR-expressing endothelial cell line (HBEC5i) is more pronounced than that of the CIDRα1.1 domain alone. IT4-VAR19 represents the preferentially expressed-PfEMP1 when FCR3-IEs are selected based on their capability to bind EPCR. Notably, no significant difference in the levels of antibodies towards IT4-VAR19 antigens was observed within all clinical groups between plasma samples collected during the acute malaria phase compared to samples collected 30 days after anti-malaria treatment.

Conclusions

These data indicate that even being the preferentially selected IT4-EPCR-binding variant, the IT4-VAR19-DC8 region does not appear to be associated with the acquisition of antibodies during a single severe paediatric malaria episode in Benin.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-015-1008-5) contains supplementary material, which is available to authorized users.

Plasmodium falciparum adhesion domains linked to severe malaria differ in blockade of endothelial protein C receptor

Cytoadhesion of Plasmodium falciparum-infected erythrocytes to endothelial protein C receptor (EPCR) is associated with severe malaria. It has been postulated that parasite binding could exacerbate microvascular coagulation and endothelial dysfunction in cerebral malaria by impairing the protein C-EPCR interaction, but the extent of binding inhibition has not been fully determined. Here we expressed the cysteine-rich interdomain region (CIDRα1) domain from a variety of domain cassette (DC) 8 and DC13 P. falciparum erythrocyte membrane protein 1 proteins and show they interact in a distinct manner with EPCR resulting in weak, moderate and strong inhibition of the activated protein C (APC)-EPCR interaction. Overall, there was a positive correlation between CIDRα1-EPCR binding activity and APC blockade activity. In addition, our analysis from a combination of mutagenesis and blocking antibodies finds that an Arg81 (R81) in EPCR plays a pivotal role in CIDRα1 binding, but domains with weak and strong APC blockade activity were distinguished by their sensitivity to inhibition by anti-EPCR mAb 1535, implying subtle differences in their binding footprints. These data reveal a previously unknown functional heterogeneity in the interaction between P. falciparum and EPCR and have major implications for understanding the distinct clinical pathologies of cerebral malaria and developing new treatment strategies.

Haplotypes of the endothelial protein C receptor (EPCR) gene are not associated with severe malaria in Tanzania

Background

Endothelial protein C receptor (EPCR) was recently identified as a key receptor for Plasmodium falciparum erythrocyte membrane protein 1 mediating sequestration of P. falciparum-infected erythrocytes in patients suffering from severe malaria. Soluble EPCR (sEPCR) inhibits binding of P. falciparum to EPCR in vitro and increased levels of sEPCR have been associated with the H3 haplotype of the EPCR encoding PROCR gene. It has been hypothesized that elevated sEPCR levels, possibly linked to the PROCR H3 genetic variant, may confer protection against severe forms of malaria. This study determined the frequencies of PROCR haplotypes H1–4 and plasma levels of sEPCR in a Tanzanian study population to investigate a possible association with severe malaria.

Methods

Study participants were children under 5 years of age admitted at the Korogwe District Hospital (N = 143), and diagnosed as having severe malaria (N = 52; including cerebral malaria N = 17), uncomplicated malaria (N = 24), or an infection other than malaria (N = 67). In addition, blood samples from 71 children living in nearby villages were included. The SNPs defining the haplotypes of PROCR gene were determined by post-PCR ligation detection reaction-fluorescent microsphere assay.

Results

Individuals carrying at least one H3 allele had significantly higher levels of sEPCR than individuals with no H3 alleles (P < 0.001). No difference in the frequency of H3 was found between the non-malaria patients, malaria patients or the village population (P > 0.1). Plasma levels of sEPCR differed between these three groups, with higher sEPCR levels in the village population compared to the hospitalized patients (P < 0.001) and higher levels in malaria patients compared to non-malaria patients (P = 0.001). However, no differences were found in the distribution of H3 (P = 0.2) or levels of sEPCR (P = 0.8) between patients diagnosed with severe and uncomplicated malaria.

Conclusion

Frequencies of SNPs determining PROCR haplotypes were in concordance with other African studies. The PROCR H3 allele was associated with higher levels of sEPCR, confirming earlier findings, however, in this Tanzanian population; neither PROCR haplotype nor level of sEPCR was associated with severe malaria, however, larger studies are needed to confirm these findings.

Antigenic Variation in Plasmodium falciparum

Plasmodium falciparum is the protozoan parasite that causes most malaria-associated morbidity and mortality in humans with over 500,000 deaths annually. The disease symptoms are associated with repeated cycles of invasion and asexual multiplication inside red blood cells of the parasite. Partial, non-sterile immunity to P. falciparum malaria develops only after repeated infections and continuous exposure. The successful evasion of the human immune system relies on the large repertoire of antigenically diverse parasite proteins displayed on the red blood cell surface and on the merozoite membrane where they are exposed to the human immune system. Expression switching of these polymorphic proteins between asexual parasite generations provides an efficient mechanism to adapt to the changing environment in the host and to maintain chronic infection. This chapter discusses antigenic diversity and variation in the malaria parasite and our current understanding of the molecular mechanisms that direct the expression of these proteins.

Pathogenesis of cerebral malaria: new diagnostic tools, biomarkers, and therapeutic approaches

Cerebral malaria is a severe neuropathological complication of Plasmodium falciparum infection. It results in high mortality and post-recovery neuro-cognitive disorders in children, even after appropriate treatment with effective anti-parasitic drugs. While the complete landscape of the pathogenesis of cerebral malaria still remains to be elucidated, numerous innovative approaches have been developed in recent years in order to improve the early detection of this neurological syndrome and, subsequently, the clinical care of affected patients. In this review, we briefly summarize the current understanding of cerebral malaria pathogenesis, compile the array of new biomarkers and tools available for diagnosis and research, and describe the emerging therapeutic approaches to tackle this pathology effectively.

PfEMP1's magical embrace of EPCR

Since discovering PfEMP1 virulence proteins in Plasmodium falciparum, malariologists have struggled to reconcile their limitless sequence diversity with their binding to relatively few host receptors. In this issue of Cell Host & Microbe, Lau et al. (2015) explore how diverse PfEMP1s embrace EPCR, promoting parasite survival and killing African children.

Fatal Pediatric Cerebral Malaria Is Associated with Intravascular Monocytes and Platelets That Are Increased with HIV Coinfection

Cerebral malaria (CM) is a major contributor to malaria deaths, but its pathophysiology is not well understood. While sequestration of parasitized erythrocytes is thought to be critical, the roles of inflammation and coagulation are controversial. In a large series of Malawian children hospitalized with CM, HIV coinfection was more prevalent than in pediatric population estimates (15% versus 2%, P < 0.0001, chi-square test), with higher mortality than that seen in HIV-uninfected children (23% versus 17%, P = 0.0178, chi-square test). HIV-infected (HIV⁺) children with autopsy-confirmed CM were older than HIV-uninfected children (median age, 99 months versus 32 months, P = 0.0007, Mann-Whitney U test) and appeared to lack severe immunosuppression. Because HIV infection is associated with dysregulated inflammation and platelet activation, we performed immunohistochemistry analysis for monocytes, platelets, and neutrophils in brain tissue from HIV⁺ and HIV-uninfected children with fatal CM. Children with autopsy-confirmed CM had significantly (>9 times) more accumulations of intravascular monocytes and platelets, but not neutrophils, than did children with nonmalarial causes of coma. The monocyte and platelet accumulations were significantly (>2-fold) greater in HIV⁺ children than in HIV-uninfected children with autopsy-confirmed CM. Our findings indicate that HIV is a risk factor for CM and for death from CM, independent of traditional measures of HIV disease severity. Brain histopathology supports the hypotheses that inflammation and coagulation contribute to the pathogenesis of pediatric CM and that immune dysregulation in HIV⁺ children exacerbates the pathological features associated with CM.

Importance  There are nearly 1 million malaria deaths yearly, primarily in sub-Saharan African children. Cerebral malaria (CM), marked by coma and sequestered malaria parasites in brain blood vessels, causes half of these deaths, although the mechanisms causing coma and death are uncertain. Sub-Saharan Africa has a high HIV prevalence, with 3 million HIV-infected (HIV⁺) children, but the effects of HIV on CM pathogenesis and mortality are unknown. In a study of pediatric CM in Malawi, HIV prevalence was high and CM-attributed mortality was higher in HIV⁺ than in HIV-uninfected children. Brain pathology in children with fatal CM was notable not only for sequestered malaria parasites but also for intravascular accumulations of monocytes and platelets that were more severe in HIV⁺ children. Our findings raise the possibility that HIV⁺ children at risk for malaria may benefit from targeted malaria prophylaxis and that adjunctive treatments targeting inflammation and/or coagulation may improve CM outcomes.

There are nearly 1 million malaria deaths yearly, primarily in sub-Saharan African children. Cerebral malaria (CM), marked by coma and sequestered malaria parasites in brain blood vessels, causes half of these deaths, although the mechanisms causing coma and death are uncertain. Sub-Saharan Africa has a high HIV prevalence, with 3 million HIV-infected (HIV⁺) children, but the effects of HIV on CM pathogenesis and mortality are unknown. In a study of pediatric CM in Malawi, HIV prevalence was high and CM-attributed mortality was higher in HIV⁺ than in HIV-uninfected children. Brain pathology in children with fatal CM was notable not only for sequestered malaria parasites but also for intravascular accumulations of monocytes and platelets that were more severe in HIV⁺ children. Our findings raise the possibility that HIV⁺ children at risk for malaria may benefit from targeted malaria prophylaxis and that adjunctive treatments targeting inflammation and/or coagulation may improve CM outcomes.

Vascular dysfunction as a target for adjuvant therapy in cerebral malaria

Cerebral malaria (CM) is a life-threatening complication of Plasmodium falciparum malaria that continues to be a major global health problem. Brain vascular dysfunction is a main factor underlying the pathogenesis of CM and can be a target for the development of adjuvant therapies for the disease. Vascular occlusion by parasitised red blood cells and vasoconstriction/vascular dysfunction results in impaired cerebral blood flow, ischaemia, hypoxia, acidosis and death. In this review, we discuss the mechanisms of vascular dysfunction in CM and the roles of low nitric oxide bioavailability, high levels of endothelin-1 and dysfunction of the angiopoietin-Tie2 axis. We also discuss the usefulness and relevance of the murine experimental model of CM by Plasmodium berghei ANKA to identify mechanisms of disease and to screen potential therapeutic interventions.

Laboratory evidence of disseminated intravascular coagulation is associated with a fatal outcome in children with cerebral malaria despite an absence of clinically evident thrombosis or bleeding

BACKGROUND

A procoagulant state is implicated in cerebral malaria (CM) pathogenesis, but whether disseminated intravascular coagulation (DIC) is present or associated with a fatal outcome is unclear.

OBJECTIVES

To determine the frequency of overt DIC, according to ISTH criteria, in children with fatal and non-fatal CM.

METHODS/PATIENTS

Malawian children were recruited into a prospective cohort study in the following diagnostic groups: retinopathy-positive CM (n = 140), retinopathy-negative CM (n = 36), non-malarial coma (n = 14), uncomplicated malaria (UM), (n = 91), mild non-malarial febrile illness (n = 85), and healthy controls (n = 36). Assays in the ISTH DIC criteria were performed, and three fibrin-related markers, i.e. protein C, antithrombin, and soluble thrombomodulin, were measured.

RESULTS AND CONCLUSIONS

Data enabling assignment of the presence or absence of 'overt DIC' were available for 98 of 140 children with retinopathy-positive CM. Overt DIC was present in 19 (19%), and was associated with a fatal outcome (odds ratio [OR] 3.068; 95% confidence interval [CI] 1.085-8.609; P = 0.035]. The levels of the three fibrin-related markers and soluble thrombomodulin were higher in CM patients than in UM patients (all P < 0.001). The mean fibrin degradation product level was higher in fatal CM patients (71.3 μg mL(-1) [95% CI 49.0-93.6]) than in non-fatal CM patients (48.0 μg mL(-1) [95% CI 37.7-58.2]; P = 0.032), but, in multivariate logistic regression, thrombomodulin was the only coagulation-related marker that was independently associated with a fatal outcome (OR 1.084 for each ng mL(-1) increase [95% CI 1.017-1.156]; P = 0.014). Despite these laboratory derangements, no child in the study had clinically evident bleeding or thrombosis. An overt DIC score and high thrombomodulin levels are associated with a fatal outcome in CM, but infrequently indicate a consumptive coagulopathy.

Investigating the Pathogenesis of Severe Malaria: A Multidisciplinary and Cross-Geographical Approach

More than a century after the discovery of Plasmodium spp. parasites, the pathogenesis of severe malaria is still not well understood. The majority of malaria cases are caused by Plasmodium falciparum and Plasmodium vivax, which differ in virulence, red blood cell tropism, cytoadhesion of infected erythrocytes, and dormant liver hypnozoite stages. Cerebral malaria coma is one of the most severe manifestations of P. falciparum infection. Insights into its complex pathophysiology are emerging through a combination of autopsy, neuroimaging, parasite binding, and endothelial characterizations. Nevertheless, important questions remain regarding why some patients develop life-threatening conditions while the majority of P. falciparum-infected individuals do not, and why clinical presentations differ between children and adults. For P. vivax, there is renewed recognition of severe malaria, but an understanding of the factors influencing disease severity is limited and remains an important research topic. Shedding light on the underlying disease mechanisms will be necessary to implement effective diagnostic tools for identifying and classifying severe malaria syndromes and developing new therapeutic approaches for severe disease. This review highlights progress and outstanding questions in severe malaria pathophysiology and summarizes key areas of pathogenesis research within the International Centers of Excellence for Malaria Research program.

Cell painting with an engineered EPCR to augment the protein C system

The protein C (PC) system conveys beneficial anticoagulant and cytoprotective effects in numerous in vivo disease models. The endothelial protein C receptor (EPCR) plays a central role in these pathways as cofactor for PC activation and by enhancing activated protein C (APC)-mediated protease-activated receptor (PAR) activation. During inflammatory disease, expression of EPCR on cell membranes is often diminished thereby limiting PC activation and APC's effects on cells. Here a caveolae-targeting glycosylphosphatidylinositol (GPI)-anchored EPCR (EPCR-GPI) was engineered to restore EPCR's bioavailability via "cell painting." The painting efficiency of EPCR-GPI on EPCR-depleted endothelial cells was time- and dose-dependent. The EPCR-GPI bioavailability after painting was long lasting since EPCR surface levels reached 400 % of wild-type cells after 2 hours and remained > 200 % for 24 hours. EPCR-GPI painting conveyed APC binding to EPCR-depleted endothelial cells where EPCR was lost due to shedding or shRNA. EPCR painting normalised PC activation on EPCR-depleted cells indicating that EPCR-GPI is functional active on painted cells. Caveolin-1 lipid rafts were enriched in EPCR after painting due to the GPI-anchor targeting caveolae. Accordingly, EPCR painting supported PAR1 and PAR3 cleavage by APC and augmented PAR1-dependent Akt phosphorylation by APC. Thus, EPCR-GPI painting achieved physiological relevant surface levels on endothelial cells, restored APC binding to EPCR-depleted cells, supported PC activation, and enhanced APC-mediated PAR cleavage and cytoprotective signalling. Therefore, EPCR-GPI provides a novel tool to restore the bioavailability and functionality of EPCR on EPCR- depleted and -deficient cells.

From the blood to the brain: avenues of eukaryotic pathogen dissemination to the central nervous system

Infection of the central nervous system (CNS) is a significant cause of morbidity and mortality, and treatments available to combat the highly debilitating symptoms of CNS infection are limited. The mechanisms by which pathogens in the circulation overcome host immunity and breach the blood-brain barrier are active areas of investigation. In this review, we discuss recent work that has significantly advanced our understanding of the avenues of pathogen dissemination to the CNS for four eukaryotic pathogens of global health importance: Toxoplasma gondii, Plasmodium falciparum, Trypanosoma brucei, and Cryptococcus neoformans. These studies highlight the remarkable diversity of pathogen strategies for trafficking to the brain and will ultimately contribute to an improved ability to combat life-threatening CNS disease.

The influence of host genetics on erythrocytes and malaria infection: is there therapeutic potential?

As parasites, Plasmodium species depend upon their host for survival. During the blood stage of their life-cycle parasites invade and reside within erythrocytes, commandeering host proteins and resources towards their own ends, and dramatically transforming the host cell. Parasites aptly avoid immune detection by minimizing the exposure of parasite proteins and removing themselves from circulation through cytoadherence. Erythrocytic disorders brought on by host genetic mutations can interfere with one or more of these processes, thereby providing a measure of protection against malaria to the host. This review summarizes recent findings regarding the mechanistic aspects of this protection, as mediated through the parasites interaction with abnormal erythrocytes. These novel findings include the reliance of the parasite on the host enzyme ferrochelatase, and the discovery of basigin and CD55 as obligate erythrocyte receptors for parasite invasion. The elucidation of these naturally occurring malaria resistance mechanisms is increasing the understanding of the host-parasite interaction, and as discussed below, is providing new insights into the development of therapies to prevent this disease.

Diverse functional outcomes of Plasmodium falciparum ligation of EPCR: potential implications for malarial pathogenesis

Plasmodium falciparum-infected erythrocytes (IRBC) expressing the domain cassettes (DC) 8 and 13 of the cytoadherent ligand P. falciparum erythrocyte membrane protein 1 adhere to the endothelial protein C receptor (EPCR). By interfering with EPCR anti-coagulant and pro-endothelial barrier functions, IRBC adhesion could promote coagulation and vascular permeability that contribute to the pathogenesis of cerebral malaria. In this study, we examined the adhesion of DC8- and DC13-expressing parasite lines to endothelial cells from different microvasculature, and the consequences of EPCR engagement on endothelial cell function. We found that IRBC from IT4var19 (DC8) and IT4var07 (DC13) parasite lines adhered to human brain, lung and dermal endothelial cells under shear stress. However, the relative contribution of EPCR to parasite cytoadherence on different types of endothelial cell varied. We also observed divergent functional outcomes for DC8 cysteine-rich interdomain region (CIDR)α1.1 and DC13 CIDRα1.4 domains. IT4var07 CIDRα1.4 inhibited generation of activated protein C (APC) on lung and dermal endothelial cells and blocked the APC-EPCR binding interaction on brain endothelial cells. IT4var19 CIDRα1.1 inhibited thrombin-induced endothelial barrier dysfunction in lung endothelial cells, whereas IT4var07 CIDRα1.4 inhibited the protective effect of APC on thrombin-induced permeability. Overall, these findings reveal a much greater complexity of how CIDRα1-expressing parasites may modulate malaria pathogenesis through EPCR adhesion.

Mycolactone-Dependent Depletion of Endothelial Cell Thrombomodulin Is Strongly Associated with Fibrin Deposition in Buruli Ulcer Lesions

A well-known histopathological feature of diseased skin in Buruli ulcer (BU) is coagulative necrosis caused by the Mycobacterium ulcerans macrolide exotoxin mycolactone. Since the underlying mechanism is not known, we have investigated the effect of mycolactone on endothelial cells, focussing on the expression of surface anticoagulant molecules involved in the protein C anticoagulant pathway. Congenital deficiencies in this natural anticoagulant pathway are known to induce thrombotic complications such as purpura fulimans and spontaneous necrosis. Mycolactone profoundly decreased thrombomodulin (TM) expression on the surface of human dermal microvascular endothelial cells (HDMVEC) at doses as low as 2ng/ml and as early as 8hrs after exposure. TM activates protein C by altering thrombin’s substrate specificity, and exposure of HDMVEC to mycolactone for 24 hours resulted in an almost complete loss of the cells’ ability to produce activated protein C. Loss of TM was shown to be due to a previously described mechanism involving mycolactone-dependent blockade of Sec61 translocation that results in proteasome-dependent degradation of newly synthesised ER-transiting proteins. Indeed, depletion from cells determined by live-cell imaging of cells stably expressing a recombinant TM-GFP fusion protein occurred at the known turnover rate. In order to determine the relevance of these findings to BU disease, immunohistochemistry of punch biopsies from 40 BU lesions (31 ulcers, nine plaques) was performed. TM abundance was profoundly reduced in the subcutis of 78% of biopsies. Furthermore, it was confirmed that fibrin deposition is a common feature of BU lesions, particularly in the necrotic areas. These findings indicate that there is decreased ability to control thrombin generation in BU skin. Mycolactone’s effects on normal endothelial cell function, including its ability to activate the protein C anticoagulant pathway are strongly associated with this. Fibrin-driven tissue ischemia could contribute to the development of the tissue necrosis seen in BU lesions.

Buruli ulcer (BU) is a neglected tropical disease that is most common in West Africa and parts of Australia, but has been reported from over 30 countries worldwide. The symptoms are painless ulcers of the skin caused by a bacterial infection. The bacteria, Mycobacterium ulcerans, produce a macrolide toxin called mycolactone. In this manuscript, we have studied the effects of mycolactone on endothelial cells, specialised cells that line blood vessels and form capillaries. One of the most important functions of these cells is to prevent blood from clotting inside the vessels. We show that mycolactone reduces the ability of cultured endothelial cells to anticoagulate blood, by blocking the expression of a protein called thrombomodulin. We went on to examine samples of BU patient skin and found that thrombomodulin is also reduced here, and that in contrast to normal skin large amounts of fibrin (one of the main constituents of blood clots) were present. This means that it may be useful to consider whether anticoagulants might improve the response to antibiotics and thereby improve treatment outcomes for BU patients.

Protein C system defects inflicted by the malaria parasite protein PfEMP1 can be overcome by a soluble EPCR variant

The Endothelial Protein C receptor (EPCR) is essential for the anticoagulant and cytoprotective functions of the Protein C (PC) system. Selected variants of the malaria parasite protein, Plasmodium falciparum Erythrocyte Membrane Protein 1 (PfEMP1) associated with severe malaria, including cerebral malaria, specifically target EPCR on vascular endothelial cells. Here, we examine the cellular response to PfEMP1 engagement to elucidate its role in malaria pathogenesis. Binding of the CIDRα1.1 domain of PfEMP1 to EPCR obstructed activated PC (APC) binding to EPCR and induced a loss of cellular EPCR functions. CIDRα1.1 severely impaired endothelial PC activation and effectively blocked APC-mediated activation of protease-activated receptor-1 (PAR1) and associated barrier protective effects of APC on endothelial cells. A soluble EPCR variant (E86A-sEPCR) bound CIDRα1.1 with high affinity and did not interfere with (A)PC binding to cellular EPCR. E86A-sEPCR used as a decoy to capture PfEMP1, permitted normal PC activation on endothelial cells, normal barrier protective effects of APC, and greatly reduced cytoadhesion of infected erythrocytes to brain endothelial cells. These data imply important contributions of PfEMP1-induced protein C pathway defects in the pathogenesis of severe malaria. Furthermore, the E86A-sEPCR decoy provides a proof-of-principle strategy for the development of novel adjunct therapies for severe malaria.

α2-Macroglobulin Can Crosslink Multiple Plasmodium falciparum Erythrocyte Membrane Protein 1 (PfEMP1) Molecules and May Facilitate Adhesion of Parasitized Erythrocytes

Rosetting, the adhesion of Plasmodium falciparum-infected erythrocytes to uninfected erythrocytes, involves clonal variants of the parasite protein P. falciparum erythrocyte membrane protein 1 (PfEMP1) and soluble serum factors. While rosetting is a well-known phenotypic marker of parasites associated with severe malaria, the reason for this association remains unclear, as do the molecular details of the interaction between the infected erythrocyte (IE) and the adhering erythrocytes. Here, we identify for the first time a single serum factor, the abundant serum protease inhibitor α2-macroglobulin (α2M), which is both required and sufficient for rosetting mediated by the PfEMP1 protein HB3VAR06 and some other rosette-mediating PfEMP1 proteins. We map the α2M binding site to the C terminal end of HB3VAR06, and demonstrate that α2M can bind at least four HB3VAR06 proteins, plausibly augmenting their combined avidity for host receptors. IgM has previously been identified as a rosette-facilitating soluble factor that acts in a similar way, but it cannot induce rosetting on its own. This is in contrast to α2M and probably due to the more limited cross-linking potential of IgM. Nevertheless, we show that IgM works synergistically with α2M and markedly lowers the concentration of α2M required for rosetting. Finally, HB3VAR06+ IEs share the capacity to bind α2M with subsets of genotypically distinct P. falciparum isolates forming rosettes in vitro and of patient parasite isolates ex vivo. Together, our results are evidence that P. falciparum parasites exploit α2M (and IgM) to expand the repertoire of host receptors available for PfEMP1-mediated IE adhesion, such as the erythrocyte carbohydrate moieties that lead to formation of rosettes. It is likely that this mechanism also affects IE adhesion to receptors on vascular endothelium. The study opens opportunities for broad-ranging immunological interventions targeting the α2M--(and IgM-) binding domains of PfEMP1, which would be independent of the host receptor specificity of clinically important PfEMP1 antigens.

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.

Activated protein C: biased for translation

The homeostatic blood protease, activated protein C (APC), can function as (1) an antithrombotic on the basis of inactivation of clotting factors Va and VIIIa; (2) a cytoprotective on the basis of endothelial barrier stabilization and anti-inflammatory and antiapoptotic actions; and (3) a regenerative on the basis of stimulation of neurogenesis, angiogenesis, and wound healing. Pharmacologic therapies using recombinant human and murine APCs indicate that APC provides effective acute or chronic therapies for a strikingly diverse range of preclinical injury models. APC reduces the damage caused by the following: ischemia/reperfusion in brain, heart, and kidney; pulmonary, kidney, and gastrointestinal inflammation; sepsis; Ebola virus; diabetes; and total lethal body radiation. For these beneficial effects, APC alters cell signaling networks and gene expression profiles by activating protease-activated receptors 1 and 3. APC's activation of these G protein-coupled receptors differs completely from thrombin's activation mechanism due to biased signaling via either G proteins or β-arrestin-2. To reduce APC-associated bleeding risk, APC variants were engineered to lack >90% anticoagulant activity but retain normal cell signaling. Such a neuroprotective variant, 3K3A-APC (Lys191-193Ala), has advanced to clinical trials for ischemic stroke. A rich data set of preclinical knowledge provides a solid foundation for potential translation of APC variants to future novel therapies.

Brain swelling and death in children with cerebral malaria

BACKGROUND

Case fatality rates among African children with cerebral malaria remain in the range of 15 to 25%. The key pathogenetic processes and causes of death are unknown, but a combination of clinical observations and pathological findings suggests that increased brain volume leading to raised intracranial pressure may play a role. Magnetic resonance imaging (MRI) became available in Malawi in 2009, and we used it to investigate the role of brain swelling in the pathogenesis of fatal cerebral malaria in African children.

METHODS

We enrolled children who met a stringent definition of cerebral malaria (one that included the presence of retinopathy), characterized them in detail clinically, and obtained MRI scans on admission and daily thereafter while coma persisted.

RESULTS

Of 348 children admitted with cerebral malaria (as defined by the World Health Organization), 168 met the inclusion criteria, underwent all investigations, and were included in the analysis. A total of 25 children (15%) died, 21 of whom (84%) had evidence of severe brain swelling on MRI at admission. In contrast, evidence of severe brain swelling was seen on MRI in 39 of 143 survivors (27%). Serial MRI scans showed evidence of decreasing brain volume in the survivors who had had brain swelling initially.

CONCLUSIONS

Increased brain volume was seen in children who died from cerebral malaria but was uncommon in those who did not die from the disease, a finding that suggests that raised intracranial pressure may contribute to a fatal outcome. The natural history indicates that increased intracranial pressure is transient in survivors. (Funded by the National Institutes of Health and Wellcome Trust U.K.).

Inflammasome in platelets: allying coagulation and inflammation in infectious and sterile diseases?

Platelets are crucial effector cells in hemostasis. In addition, platelets are increasingly recognized as major inflammatory cells with key roles in innate and adaptive immune responses. Activated platelets have key thromboinflammatory activities linking coagulation to inflammatory response in a variety of coagulation disorders and vasculopathies. Recently identified inflammatory activities of platelets include the synthesis of IL-1β from spliced pre-RNA, as well as the presence and assembly of inflammasome which intermediate IL-1β secretion. Here we review the mechanisms by which platelets activate translation machinery and inflammasome assembly to synthesize and release IL-1β. The contributions of these processes to protective and pathogenic responses during infectious and inflammatory diseases are discussed.

The pathogenesis of pediatric cerebral malaria: eye exams, autopsies, and neuroimaging

Several advances in our understanding of pediatric cerebral malaria (CM) have been made over the past 25 years. Accurate clinical diagnosis is enhanced by the identification of a characteristic retinopathy, visible by direct or indirect ophthalmoscopy, the retinal changes (retinal whitening, vessel color changes, white-centered hemorrhages) being consistently associated with intracerebral sequestration of parasites in autopsy studies. Autopsies have yielded information at tissue levels in fatal CM, but new insights into critical pathogenetic processes have emerged from neuroimaging studies, which, unlike autopsy-based studies, permit serial observations over time and allow comparisons between fatal cases and survivors. Brain swelling has emerged as the major risk factor for death, and, among survivors, brain volume diminishes spontaneously over 24-48 hours. Studies of life-threatening and fatal malaria are suggesting new approaches to identifying and caring for those at highest risk; potential adjuvants should be evaluated and implemented where they are most needed.

Endothelial protein C receptor gene variants not associated with severe malaria in ghanaian children

Background

Two recent reports have identified the Endothelial Protein C Receptor (EPCR) as a key molecule implicated in severe malaria pathology. First, it was shown that EPCR in the human microvasculature mediates sequestration of Plasmodium falciparum-infected erythrocytes. Second, microvascular thrombosis, one of the major processes causing cerebral malaria, was linked to a reduction in EPCR expression in cerebral endothelial layers. It was speculated that genetic variation affecting EPCR functionality could influence susceptibility to severe malaria phenotypes, rendering PROCR, the gene encoding EPCR, a promising candidate for an association study.

Methods

Here, we performed an association study including high-resolution variant discovery of rare and frequent genetic variants in the PROCR gene. The study group, which previously has proven to be a valuable tool for studying the genetics of malaria, comprised 1,905 severe malaria cases aged 1–156 months and 1,866 apparently healthy children aged 2–161 months from the Ashanti Region in Ghana, West Africa, where malaria is highly endemic. Association of genetic variation with severe malaria phenotypes was examined on the basis of single variants, reconstructed haplotypes, and rare variant analyses.

Results

A total of 41 genetic variants were detected in regulatory and coding regions of PROCR, 17 of which were previously unknown genetic variants. In association tests, none of the single variants, haplotypes or rare variants showed evidence for an association with severe malaria, cerebral malaria, or severe malaria anemia.

Conclusion

Here we present the first analysis of genetic variation in the PROCR gene in the context of severe malaria in African subjects and show that genetic variation in the PROCR gene in our study population does not influence susceptibility to major severe malaria phenotypes.

Differential PfEMP1 expression is associated with cerebral malaria pathology

Plasmodium falciparum is unique among human malarias in its ability to sequester in post-capillary venules of host organs. The main variant antigens implicated are the P. falciparum erythrocyte membrane protein 1 (PfEMP1), which can be divided into three major groups (A–C). Our study was a unique examination of sequestered populations of parasites for genetic background and expression of PfEMP1 groups. We collected post-mortem tissue from twenty paediatric hosts with pathologically different forms of cerebral malaria (CM1 and CM2) and parasitaemic controls (PC) to directly examine sequestered populations of parasites in the brain, heart and gut. Use of two different techniques to investigate this question produced divergent results. By quantitative PCR, group A var genes were upregulated in all three organs of CM2 and PC cases. In contrast, in CM1 infections displaying high levels of sequestration but negligible vascular pathology, there was high expression of group B var. Cloning and sequencing of var transcript tags from the same samples indicated a uniformly low expression of group A-like var. Generally, within an organ sample, 1–2 sequences were expressed at dominant levels. 23% of var tags were detected in multiple patients despite the P. falciparum infections being genetically distinct, and two tags were observed in up to seven hosts each with high expression in the brains of 3–4 patients. This study is a novel examination of the sequestered parasites responsible for fatal cerebral malaria and describes expression patterns of the major cytoadherence ligand in three organ-derived populations and three pathological states.

One of the most severe forms of malarial disease is cerebral malaria, which disproportionally affects young children. In this disease, the parasite places proteins on the red blood cell surface, providing a “smokescreen” by which they evade host immunity and hide in organ blood vessels, blocking them and causing tissue damage. It is impossible to study parasites in the organs during life and autopsy studies on children with malaria are exceedingly rare. In Malawi, we examined parasites from the brain, heart and intestine of twenty cases of fatal malaria including controls with low numbers of malaria parasites but another identified cause of death. We found little difference in the category of proteins the parasites used in controls and cerebral malaria, although a small number of specific proteins were detected in multiple infections. In an alternative form of malaria in which the brain is heavily infected but shows no evidence of damage, we found a different set of proteins at high proportion. However, as these children were typically older and most were infected with HIV, we could not determine which of these factors was most important. Interactions between host and parasite have the potential to influence disease outcomes.

Structural conservation despite huge sequence diversity allows EPCR binding by the PfEMP1 family implicated in severe childhood malaria

The PfEMP1 family of surface proteins is central for Plasmodium falciparum virulence and must retain the ability to bind to host receptors while also diversifying to aid immune evasion. The interaction between CIDRα1 domains of PfEMP1 and endothelial protein C receptor (EPCR) is associated with severe childhood malaria. We combine crystal structures of CIDRα1:EPCR complexes with analysis of 885 CIDRα1 sequences, showing that the EPCR-binding surfaces of CIDRα1 domains are conserved in shape and bonding potential, despite dramatic sequence diversity. Additionally, these domains mimic features of the natural EPCR ligand and can block this ligand interaction. Using peptides corresponding to the EPCR-binding region, antibodies can be purified from individuals in malaria-endemic regions that block EPCR binding of diverse CIDRα1 variants. This highlights the extent to which such a surface protein family can diversify while maintaining ligand-binding capacity and identifies features that should be mimicked in immunogens to prevent EPCR binding.

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EPCR binding is retained by PfEMP1 CIDRα1 domains despite huge sequence variation

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Diverse CIDRα1 domains retain structural and chemical features to bind to EPCR

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CIDRα1 domains mimic features of a natural ligand of EPCR and block its binding

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Patient sera contain neutralizing antibodies that prevent parasite binding to EPCR

High plasma levels of soluble endothelial protein C receptor are associated with increased mortality among children with cerebral malaria in Benin

Loss of endothelial protein C receptor (EPCR) occurs at the sites of Plasmodium falciparum-infected erythrocyte sequestration in patients with or who died from cerebral malaria. In children presenting with different clinical syndromes of malaria, we assessed the relationships between endogenous plasma soluble EPCR (sEPCR) levels and clinical presentation or mortality. After adjustment for age, for treatment before admission, and for a known genetic factor, sEPCR level at admission was positively associated with cerebral malaria (P = .011) and with malaria-related mortality (P = .0003). Measuring sEPCR levels at admission could provide an early biological marker of the outcome of cerebral malaria.

Fatal cerebral malaria: a venous efflux problem

Most Plasmodium falciparum-infected children with cerebral malaria (CM) die from respiratory arrest, but the underlying pathology is unclear. Here we present a model in which the ultimate cause of death from CM is severe intracranial hypertension. Dynamic imaging of mice infected with P. berghei ANKA, an accepted model for experimental CM, revealed that leukocyte adhesion impairs the venous blood flow by reducing the functional lumen of postcapillary venules (PCV). The resulting increase in intracranial pressure (ICP) exacerbates cerebral edema formation, a hallmark of both murine and pediatric CM. We propose that two entirely different pathogenetic mechanisms-cytoadherence of P. falciparum-infected erythrocytes in pediatric CM and leukocyte arrest in murine CM-result in the same pathological outcome: a severe increase in ICP leading to brainstem herniation and death from respiratory arrest. The intracranial hypertension (IH) model unifies previous hypotheses, applies to human and experimental CM alike, eliminates the need to explain any selective recognition mechanism Plasmodium might use to target multiple sensitive sites in the brain, and explains how an intravascular parasite can cause so much neuronal dysfunction.

Severity of retinopathy parallels the degree of parasite sequestration in the eyes and brains of malawian children with fatal cerebral malaria

Background. Malarial retinopathy (MR) has diagnostic and prognostic value in children with Plasmodium falciparum cerebral malaria (CM). A clinicopathological correlation between observed retinal changes during life and the degree of sequestration of parasitized red blood cells was investigated in ocular and cerebral vessels at autopsy.

Methods. In 18 Malawian children who died from clinically defined CM, we studied the intensity of sequestration and the maturity of sequestered parasites in the retina, in nonretinal ocular tissues, and in the brain.

Results. Five children with clinically defined CM during life had other causes of death identified at autopsy, no MR, and scanty intracerebral sequestration. Thirteen children had MR and died from CM. MR severity correlated with percentage of microvessels parasitized in the retina, brain, and nonretinal tissues with some neuroectodermal components (all P < .01). In moderate/severe MR cases (n = 8), vascular congestion was more intense (ρ = 0.841; P < .001), sequestered parasites were more mature, and the quantity of extraerythrocytic hemozoin was higher, compared with mild MR cases (n = 5).

Conclusions. These data provide a histopathological basis for the known correlation between degrees of retinopathy and cerebral dysfunction in CM. In addition to being a valuable tool for clinical diagnosis, retinal observations give important information about neurovascular pathophysiology in pediatric CM.

Endothelial cell protein C receptor: a multiliganded and multifunctional receptor

Endothelial cell protein C receptor (EPCR) was first identified and isolated as a cellular receptor for protein C on endothelial cells. EPCR plays a crucial role in the protein C anticoagulant pathway by promoting protein C activation. In the last decade, EPCR has received wide attention after it was discovered to play a key role in mediating activated protein C (APC)-induced cytoprotective effects, including antiapoptotic, anti-inflammatory, and barrier stabilization. APC elicits cytoprotective signaling through activation of protease activated receptor-1 (PAR1). Understanding how EPCR-APC induces cytoprotective effects through activation of PAR1, whose activation by thrombin is known to induce a proinflammatory response, has become a major research focus in the field. Recent studies also discovered additional ligands for EPCR, which include factor VIIa, Plasmodium falciparum erythrocyte membrane protein, and a specific variant of the T-cell receptor. These observations open unsuspected new roles for EPCR in hemostasis, malaria pathogenesis, innate immunity, and cancer. Future research on these new discoveries will undoubtedly expand our understanding of the role of EPCR in normal physiology and disease, as well as provide novel insights into mechanisms for EPCR multifunctionality. Comprehensive understanding of EPCR may lead to development of novel therapeutic modalities in treating hemophilia, inflammation, cerebral malaria, and cancer.

The systemic pathology of cerebral malaria in African children

Pediatric cerebral malaria carries a high mortality rate in sub-Saharan Africa. We present our systematic analysis of the descriptive and quantitative histopathology of all organs sampled from a series of 103 autopsies performed between 1996 and 2010 in Blantyre, Malawi on pediatric cerebral malaria patients and control patients (without coma, or without malaria infection) who were clinically well characterized prior to death. We found brain swelling in all cerebral malaria patients and the majority of controls. The histopathology in patients with sequestration of parasites in the brain demonstrated two patterns: (a) the “classic” appearance (i.e., ring hemorrhages, dense sequestration, and extra-erythrocytic pigment) which was associated with evidence of systemic activation of coagulation and (b) the “sequestration only” appearance associated with shorter duration of illness and higher total burden of parasites in all organs including the spleen. Sequestration of parasites was most intense in the gastrointestinal tract in all parasitemic patients (those with cerebral malarial and those without).