Direct activation of human endothelial cells by Plasmodium falciparum-infected erythrocytes

Dyslipidemia and hyperglycemia induce overexpression of Syndecan-3 in erythrocytes and modulate erythrocyte adhesion

Erythrocytes are important vascular components that play vital roles in maintaining vascular homeostasis, in addition to carrying oxygen. Previously, we reported that the changes in the internal milieu (e.g. hyperglycemia or hypercholesterolemia) increase erythrocyte adhesion to various extracellular matrix components, potentially through altering glycosaminoglycans (GAGs). In this study, we have investigated the expression of syndecan (Sdc) family members that could be involved in mediating cytoadherence under conditions of dyslipidemia and hyperglycemia. Among the Sdc family members analysed, we found significant overexpression of Sdc-3 in erythrocyte membranes harvested from high-fat-fed control and diabetic animals. Animal studies revealed a positive correlation between Sdc-3 expression, blood sugar levels and erythrocyte adhesion. In the human study, diabetic cohorts with body mass index >24.9 showed significantly increased expression of Sdc-3. Interestingly, blocking the Sdc-3 moiety with an anti-Sdc-3 antibody revealed that the core protein might not be directly involved in erythrocyte adhesion to fibronectin despite the GAGs bringing about adhesion. Lastly, Nano liquid chromatography-mass spectrometry/MS verified the presence of Sdc-3 in erythrocyte membranes. In conclusion, the high-fat diet and diabetes modulated Sdc-3 expression in the erythrocyte membrane, which may alter its adhesive properties and promote vascular complications.

Cerebral malaria pathogenesis: Dissecting the role of CD4+ and CD8+ T-cells as major effectors in disease pathology

Cerebral malaria (CM) is a severe complication of Plasmodium falciparum (P. falciparum) infection, with complex pathogenesis involving multiple factors, including the host's immunological response. T lymphocytes, specifically CD4+ T helper cells and CD8+ cytotoxic T cells, are crucial in controlling parasite growth and activating cells for parasite clearance via cytokine secretion. Contrary to this, reports also suggest the pathogenic nature of T lymphocytes as they are often involved in disease progression and severity. CD8+ cytotoxic T cells migrate to the host's brain vasculature, disrupting the blood-brain barrier and causing neurological manifestations. CD4+ T helper cells on the other hand play a variety of functions as they differentiate into different subtypes which may function as pro-inflammatory or anti-inflammatory. The excessive pro-inflammatory response in CM can lead to multi-organ failure, necessitating a check mechanism to maintain immune homeostasis. This is achieved by regulatory T cells and their characteristic cytokines, which counterbalance the pro-inflammatory immune response. Maintaining a critical balance between pro and anti-inflammatory responses is crucial for determining disease outcomes in CM. A slight change in this balance may contribute to a disease severity owing to an extreme inflammatory response or unrestricted parasite growth, a potential target for designing immunotherapeutic treatment approaches. The review briefly discusses the pathogenesis of CM and various mechanisms responsible for the disruption of the blood-brain barrier. It also highlights the role of different T cell subsets during infection and emphasizes the importance of balance between pro and anti-inflammatory T cells that ultimately decides the outcome of the disease.

Different PfEMP1-expressing Plasmodium falciparum variants induce divergent endothelial transcriptional responses during co-culture

The human malaria parasite Plasmodium falciparum is responsible for the majority of mortality and morbidity caused by malaria infection and differs from other human malaria species in the degree of accumulation of parasite-infected red blood cells in the microvasculature, known as cytoadherence or sequestration. In P. falciparum, cytoadherence is mediated by a protein called PfEMP1 which, due to its exposure to the host immune system, undergoes antigenic variation resulting in the expression of different PfEMP1 variants on the infected erythrocyte membrane. These PfEMP1s contain various combinations of adhesive domains, which allow for the differential engagement of a repertoire of endothelial receptors on the host microvasculature, with specific receptor usage associated with severe disease. We used a co-culture model of cytoadherence incubating human brain microvascular endothelial cells with erythrocytes infected with two parasite lines expressing different PfEMP1s that demonstrate different binding profiles to vascular endothelium. We determined the transcriptional profile of human brain microvascular endothelial cells (HBMEC) following different incubation periods with infected erythrocytes, identifying different transcriptional profiles of pathways previously found to be involved in the pathology of severe malaria, such as inflammation, apoptosis and barrier integrity, induced by the two PfEMP1 variants.

Nanocarrier based active targeting strategies against erythrocytic stage of malaria

The Global Technical Strategy for Malaria 2016-2030 aims to achieve a 90% reduction in malaria cases, and strategic planning and execution are crucial for accomplishing this target. This review aims to understand the complex interaction between erythrocytic receptors and parasites and to use this knowledge to actively target the erythrocytic stage of malaria. The review provides insight into the malaria life cycle, which involves various receptors such as glycophorin A, B, C, and D (GPA/B/C/D), complement receptor 1, basigin, semaphorin 7a, Band 3/ GPA, Kx, and heparan sulfate proteoglycan for parasite cellular binding and ingress in the erythrocytic and exo-erythrocytic stages. Synthetic peptides mimicking P. falciparum receptor binding ligands, human serum albumin, chondroitin sulfate, synthetic polymers, and lipids have been utilized as ligands and decorated onto nanocarriers for specific targeting to parasite-infected erythrocytes. The need of the hour for treatment and prophylaxis against malaria is a broadened horizon that includes multiple targeting strategies against the entry, proliferation, and transmission stages of the parasite. Platform technologies with established pre-clinical safety and efficacy should be translated into clinical evaluation and formulation scale-up. Future development should be directed towards nanovaccines as proactive tools against malaria infection.

Cerebral Malaria Model Applying Human Brain Organoids

Neural injuries in cerebral malaria patients are a significant cause of morbidity and mortality. Nevertheless, a comprehensive research approach to study this issue is lacking, so herein we propose an in vitro system to study human cerebral malaria using cellular approaches. Our first goal was to establish a cellular system to identify the molecular alterations in human brain vasculature cells that resemble the blood-brain barrier (BBB) in cerebral malaria (CM). Through transcriptomic analysis, we characterized specific gene expression profiles in human brain microvascular endothelial cells (HBMEC) activated by the Plasmodium falciparum parasites. We also suggest potential new genes related to parasitic activation. Then, we studied its impact at brain level after Plasmodium falciparum endothelial activation to gain a deeper understanding of the physiological mechanisms underlying CM. For that, the impact of HBMEC-P. falciparum-activated secretomes was evaluated in human brain organoids. Our results support the reliability of in vitro cellular models developed to mimic CM in several aspects. These systems can be of extreme importance to investigate the factors (parasitological and host) influencing CM, contributing to a molecular understanding of pathogenesis, brain injury, and dysfunction.

The endothelial glycocalyx in critical illness: A pediatric perspective

The vascular endothelium is the interface between circulating blood and end organs and thus has a critical role in preserving organ function. The endothelium is lined by a glycan-rich glycocalyx that uniquely contributes to endothelial function through its regulation of leukocyte and platelet interactions with the vessel wall, vascular permeability, coagulation, and vasoreactivity. Degradation of the endothelial glycocalyx can thus promote vascular dysfunction, inflammation propagation, and organ injury. The endothelial glycocalyx and its role in vascular pathophysiology has gained increasing attention over the last decade. While studies characterizing vascular glycocalyx injury and its downstream consequences in a host of adult human diseases and in animal models has burgeoned, studies evaluating glycocalyx damage in pediatric diseases are relatively few. As children have unique physiology that differs from adults, significant knowledge gaps remain in our understanding of the causes and effects of endothelial glycocalyx disintegrity in pediatric critical illness. In this narrative literature overview, we offer a unique perspective on the role of the endothelial glycocalyx in pediatric critical illness, drawing from adult and preclinical data in addition to pediatric clinical experience to elucidate how marked derangement of the endothelial surface layer may contribute to aberrant vascular biology in children. By calling attention to this nascent field, we hope to increase research efforts to address important knowledge gaps in pediatric vascular biology that may inform the development of novel therapeutic strategies.

Cytoadherence Properties of Plasmodium knowlesi-Infected Erythrocytes

Plasmodium knowlesi is responsible for zoonotic malaria infections that are potentially fatal. While the severe pathology of falciparum malaria is associated with cytoadherence phenomena by Plasmodium falciparum-infected erythrocytes (IRBC), information regarding cytoadherence properties of P. knowlesi-IRBC remained scarce. Here, we characterized the cytoadherence properties of RBC infected with the laboratory-adapted P. knowlesi A1-H.1 strain. We found that late-stage IRBC formed rosettes in a human serum-dependent manner, and rosettes hampered IRBC phagocytosis. IRBC did not adhere much to unexposed (unstimulated) human endothelial cell lines derived from the brain (hCMEC/D3), lungs (HPMEC), and kidneys (HRGEC). However, after being "primed" with P. knowlesi culture supernatant, the IRBC-endothelial cytoadherence rate increased in HPMEC and HRGEC, but not in hCMEC/D3 cells. Both endothelial cytoadherence and rosetting phenomena were abrogated by treatment of P. knowlesi-IRBC with trypsin. We also found that different receptors were involved in IRBC cytoadherence to different types of endothelial cells. Although some of the host receptors were shared by both P. falciparum- and P. knowlesi-IRBC, the availability of glycoconjugates on the receptors might influence the capacity of P. knowlesi-IRBC to cytoadhere to these receptors.

Mast cells-derived exosomes worsen the development of experimental cerebral malaria

Cerebral malaria (CM) is the most severe neurological complication caused by Plasmodium falciparum infection. The accumulating evidence demonstrated that mast cells (MCs) and its mediators played a critical role in mediating malaria severity. Earlier studies identified that exosomes were emerging as key mediators of intercellular communication and can be released from several kinds of MCs. However, the potential functions and pathological mechanisms of MCs-derived exosomes (MCs-Exo) impacting on CM pathogenesis remain largely unknown. Herein, we utilized an experimental CM (ECM) model (C57BL/6 mice infected with P. berghei ANKA strain), and then intravenously (i.v.) injected MCs-Exo into P. berghei ANKA-infected mice to unfold this mechanism and investigate the effect of MCs-Exo on ECM pathogenies. We also used an in vitro model by investigating the pathogenesis development of brain microvascular endothelial cells line (bEnd.3 cells) co-cultured with P. berghei ANKA blood-stage soluble antigen (PbAg) after MCs-Exo treatment. The higher numbers of MCs and levels of MCs degranulation were observed in skin, cervical lymph node, and brain of ECM mice than those of the uninfected mice. Exosomes were successfully isolated from culture supernatants of mouse MCs line (P815 cells) and characterized by spherical vesicles with the diameter of 30-150 nm, and expression of typical exosomal markers (e.g., CD9, CD63, and CD81). The i.v. injection of MCs-Exo dramatically elevated incidence of ECM in the P. berghei ANKA-infected mice, exacerbated liver and brain histopathological damage, promoted Th1 cytokine response, aggravated brain vascular endothelial activation and blood brain barrier breakdown in ECM mice. In addition, the treatment of MCs-Exo led to the decrease of cells viability and mRNA levels of Ang-1, ZO-1, and Claudin-5, but increase of mRNA levels of Ang-2, CCL2, CXCL1, and CXCL9 in bEnd.3 cells co-cultured with PbAg in vitro. Taken together, our data indicated that MCs-Exo could worsen pathogenesis of ECM in mice.

Plasmodium falciparum transcription in different clinical presentations of malaria associates with circulation time of infected erythrocytes

Following Plasmodium falciparum infection, individuals can remain asymptomatic, present with mild fever in uncomplicated malaria cases, or show one or more severe malaria symptoms. Several studies have investigated associations between parasite transcription and clinical severity, but no broad conclusions have yet been drawn. Here, we apply a series of bioinformatic approaches based on P. falciparum's tightly regulated transcriptional pattern during its ~48-hour intraerythrocytic developmental cycle (IDC) to publicly available transcriptomes of parasites obtained from malaria cases of differing clinical severity across multiple studies. Our analysis shows that within each IDC, the circulation time of infected erythrocytes without sequestering to endothelial cells decreases with increasing parasitaemia or disease severity. Accordingly, we find that the size of circulating infected erythrocytes is inversely related to parasite density and disease severity. We propose that enhanced adhesiveness of infected erythrocytes leads to a rapid increase in parasite burden, promoting higher parasitaemia and increased disease severity.

Altered Cytokine Response of Human Brain Endothelial Cells after Stimulation with Malaria Patient Plasma

Infections with the deadliest malaria parasite, Plasmodium falciparum, are accompanied by a strong immunological response of the human host. To date, more than 30 cytokines have been detected in elevated levels in plasma of malaria patients compared to healthy controls. Endothelial cells (ECs) are a potential source of these cytokines, but so far it is not known if their cytokine secretion depends on the direct contact of the P. falciparum-infected erythrocytes (IEs) with ECs in terms of cytoadhesion. Culturing ECs with plasma from malaria patients (27 returning travellers) resulted in significantly increased secretion of IL-11, CXCL5, CXCL8, CXCL10, vascular endothelial growth factor (VEGF) and angiopoietin-like protein 4 (ANGPTL4) if compared to matching controls (22 healthy individuals). The accompanying transcriptome study of the ECs identified 43 genes that were significantly increased in expression (≥1.7 fold) after co-incubation with malaria patient plasma, including cxcl5 and angptl4. Further bioinformatic analyses revealed that biological processes such as cell migration, cell proliferation and tube development were particularly affected in these ECs. It can thus be postulated that not only the cytoadhesion of IEs, but also molecules in the plasma of malaria patients exerts an influence on ECs, and that not only the immunological response but also other processes, such as angiogenesis, are altered.

Acute Phase Responses Vary Between Children of HbAS and HbAA Genotypes During Plasmodium falciparum Infection

Purpose

Haemoglobin genotype S is known to offer protection against Plasmodium falciparum infections but the mechanism underlying this protection is not completely understood. Associated changes in acute phase proteins (APPs) during Plasmodium falciparum infections between Haemoglobin AA (HbAA) and Haemoglobin AS (HbAS) individuals also remain unclear. This study aimed to evaluate changes in three APPs and full blood count (FBC) indices of HbAA and HbAS children during Plasmodium falciparum infection.

Methods

Venous blood was collected from three hundred and twenty children (6 months to 15 years) in Begoro in Fanteakwa District of Ghana during a cross-sectional study. Full blood count (FBC) indices were measured and levels of previously investigated APPs in malaria patients; C-reactive protein (CRP), ferritin and transferrin measured using Enzyme-Linked Immunosorbent Assays.

Results

Among the HbAA and HbAS children, levels of CRP and ferritin were higher in malaria positive children as compared to those who did not have malaria. The mean CRP levels were significantly higher among HbAA children (p=0.2e-08) as compared to the HbAS children (p=0.43). Levels of transferrin reduced in both HbAA and HbAS children with malaria, but the difference was only significant among HbAA children (p=0.0038), as compared to the HbAS children. No significant differences were observed in ferritin levels between HbAA and HbAS children in both malaria negative (p=0.76) and positive (p=0.26) children. Of the full blood count indices measured, red blood cell count (p=0.044) and haemoglobin (Hb) levels (p=0.017) differed between HbAA and HbAS in those without malaria, with higher RBC counts and lower Hb levels found in HbAS children. In contrast, during malaria, lymphocyte and platelet counts were elevated, whilst granulocytes and Mean Cell Haematocrit counts were reduced among children of the HbAS genotypes.

Conclusion

Significant changes in APPs were found in HbAA children during malaria as compared to HbAS children, possibly due to differences in malaria-induced inflammation levels. This suggests that the HbAS genotype is associated with better control of P. falciparum infection-induced inflammatory response than HbAA genotype.

Dimethyl fumarate reduces TNF and Plasmodium falciparum induced brain endothelium activation in vitro

Background

Cerebral malaria (CM) is associated with morbidity and mortality despite the use of potent anti-malarial agents. Brain endothelial cell activation and dysfunction from oxidative and inflammatory host responses and products released by Plasmodium falciparum-infected erythrocytes (IE), are likely the major contributors to the encephalopathy, seizures, and brain swelling that are associated with CM. The development of adjunctive therapy to reduce the pathological consequences of host response pathways could improve outcomes. A potentially protective role of the nuclear factor E2-related factor 2 (NRF2) pathway, which serves as a therapeutic target in brain microvascular diseases and central nervous system (CNS) inflammatory diseases such as multiple sclerosis was tested to protect endothelial cells in an in vitro culture system subjected to tumour necrosis factor (TNF) or infected red blood cell exposure. NRF2 is a transcription factor that mediates anti-oxidant and anti-inflammatory responses.

Methods

To accurately reflect clinically relevant parasite biology a unique panel of parasite isolates derived from patients with stringently defined CM was developed. The effect of TNF and these parasite lines on primary human brain microvascular endothelial cell (HBMVEC) activation in an in vitro co-culture model was tested. HBMVEC activation was measured by cellular release of IL6 and nuclear translocation of NFκB. The transcriptional and functional effects of dimethyl fumarate (DMF), an FDA approved drug which induces the NRF2 pathway, on host and parasite induced HBMVEC activation was characterized. In addition, the effect of DMF on parasite binding to TNF stimulated HBMVEC in a semi-static binding assay was examined.

Results

Transcriptional profiling demonstrates that DMF upregulates the NRF2-Mediated Oxidative Stress Response, ErbB4 Signaling Pathway, Peroxisome Proliferator-activated Receptor (PPAR) Signaling and downregulates iNOS Signaling and the Neuroinflammation Signaling Pathway on TNF activated HBMVEC. The parasite lines derived from eight paediatric CM patients demonstrated increased binding to TNF activated HBMVEC and varied in their binding and activation of HBMVEC. Overall DMF reduced both TNF and CM derived parasite activation of HBMVEC.

Conclusions

These findings provide evidence that targeting the NRF2 pathway in TNF and parasite activated HBMVEC mediates multiple protective pathways and may represent a novel adjunctive therapy to improve infection outcomes in CM.

Cerebral Plasmodium falciparum malaria: The role of PfEMP1 in its pathogenesis and immunity, and PfEMP1-based vaccines to prevent it

Malaria, a mosquito-borne infectious disease caused by parasites of the genus Plasmodium continues to be a major health problem worldwide. The unicellular Plasmodium-parasites have the unique capacity to infect and replicate within host erythrocytes. By expressing variant surface antigens Plasmodium falciparum has evolved to avoid protective immune responses; as a result in endemic areas anti-malaria immunity develops gradually over many years of multiple and repeated infections. We are studying the role of Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) expressed by asexual stages of P. falciparum responsible for the pathogenicity of severe malaria. The immunopathology of falciparum malaria has been linked to cyto-adhesion of infected erythrocytes to specific host receptors. A greater appreciation of the PfEMP1 molecules important for the development of protective immunity and immunopathology is a prerequisite for the rational discovery and development of a safe and protective anti-disease malaria vaccine. Here we review the role of ICAM-1 and EPCR receptor adhering falciparum-parasites in the development of severe malaria; we discuss our current research to understand the factors involved in the pathogenesis of cerebral malaria and the feasibility of developing a vaccine targeted specifically to prevent this disease.

New Insights into Malaria Pathogenesis

Malaria remains a major public health threat in tropical and subtropical regions across the world. Even though less than 1% of malaria infections are fatal, this leads to about 430,000 deaths per year, predominantly in young children in sub-Saharan Africa. Therefore, it is imperative to understand why a subset of infected individuals develop severe syndromes and some of them die and what differentiates these cases from the majority that recovers. Here, we discuss progress made during the past decade in our understanding of malaria pathogenesis, focusing on the major human parasite Plasmodium falciparum.

Sticking for a Cause: The Falciparum Malaria Parasites Cytoadherence Paradigm

After a successful invasion, malaria parasite Plasmodium falciparum extensively remodels the infected erythrocyte cellular architecture, conferring cytoadhesive properties to the infected erythrocytes. Cytoadherence plays a central role in the parasite's immune-escape mechanism, at the same time contributing to the pathogenesis of severe falciparum malaria. In this review, we discuss the cytoadhesive interactions between P. falciparum infected erythrocytes and various host cell types, and how these events are linked to malaria pathogenesis. We also highlight the limitations faced by studies attempting to correlate diversity in parasite ligands and host receptors with the development of severe malaria.

Malaria infection promotes a selective expression of kinin receptors in murine liver

BACKGROUND

Malaria represents a worldwide medical emergency affecting mainly poor areas. Plasmodium parasites during blood stages can release kinins to the extracellular space after internalization of host kininogen inside erythrocytes and these released peptides could represent an important mechanism in liver pathophysiology by activation of calcium signaling pathway in endothelial cells of vertebrate host. Receptors (B1 and B2) activated by kinins peptides are important elements for the control of haemodynamics in liver and its physiology. The aim of this study was to identify changes in the liver host responses (i.e. kinin receptors expression and localization) and the effect of ACE inhibition during malaria infection using a murine model.

METHODS

Balb/C mice infected by Plasmodium chabaudi were treated with captopril, an angiotensin I-converting enzyme (ACE) inhibitor, used alone or in association with the anti-malarial chloroquine in order to study the effect of ACE inhibition on mice survival and the activation of liver responses involving B1R and B2R signaling pathways. The kinin receptors (B1R and B2R) expression and localization was analysed in liver by western blotting and immunolocalization in different conditions.

RESULTS

It was verified that captopril treatment caused host death during the peak of malaria infection (parasitaemia about 45%). B1R expression was stimulated in endothelial cells of sinusoids and other blood vessels of mice liver infected by P. chabaudi. At the same time, it was also demonstrated that B1R knockout mice infected presented a significant reduction of survival. However, the infection did not alter the B2R levels and localization in liver blood vessels.

CONCLUSIONS

Thus, it was observed through in vivo studies that the vasodilation induced by plasma ACE inhibition increases mice mortality during P. chabaudi infection. Besides, it was also seen that the anti-malarial chloroquine causes changes in B1R expression in liver, even after days of parasite clearance. The differential expression of B1R and B2R in liver during malaria infection may have an important role in the disease pathophysiology and represents an issue for clinical treatments.

Brain Endothelium: The "Innate Immunity Response Hypothesis" in Cerebral Malaria Pathogenesis

Cerebral malaria (CM) is a life-threatening neurological syndrome caused by Plasmodium falciparum infection afflicting mainly children in Africa. Current pathogenesis models implicate parasite and host-derived factors in impairing brain vascular endothelium (BVE) integrity. Sequestration of Plasmodium-infected red blood cells (iRBCs) in brain microvessels is a hallmark of CM pathology. However, the precise mechanisms driving loss of blood-brain barrier (BBB) function with consequent brain injury are still unsettled and it is plausible that distinct pathophysiology trajectories are involved. Studies in humans and in the mouse model of CM indicate that inflammatory reactions intertwined with microcirculatory and coagulation disturbances induce alterations in vascular permeability and impair BBB integrity. Yet, the role of BVE as initiator of immune responses against parasite molecules and iRBCs is largely unexplored. Brain endothelial cells express pattern recognition receptors (PRR) and are privileged sensors of blood-borne infections. Here, we focus on the hypothesis that innate responses initiated by BVE and subsequent interactions with immune cells are critical to trigger local effector immune functions and induce BBB damage. Uncovering mechanisms of BVE involvement in sensing Plasmodium infection, recruiting of immune cells and directing immune effector functions could reveal pharmacological targets to promote BBB protection with potential applications in CM clinical management.

Endothelial glycocalyx regulates cytoadherence in Plasmodium falciparum malaria

Malaria is associated with significant microcirculation disorders, especially when the infection reaches its severe stage. This can lead to a range of fatal conditions, from cerebral malaria to multiple organ failure, of not fully understood pathogenesis. It has recently been proposed that a breakdown of the glycocalyx, the carbohydrate-rich layer lining the vascular endothelium, plays a key role in severe malaria, but direct evidence supporting this hypothesis is still lacking. Here, the interactions between Plasmodium falciparum infected red blood cells ( PfRBCs) and endothelial glycocalyx are investigated by developing an in vitro, physiologically relevant model of human microcirculation based on microfluidics. Impairment of the glycocalyx is obtained by enzymatic removal of sialic acid residues, which, due to their terminal location and net negative charge, are implicated in the initial interactions with contacting cells. We show a more than twofold increase of PfRBC adhesion to endothelial cells upon enzymatic treatment, relative to untreated endothelial cells. As a control, no effect of enzymatic treatment on healthy red blood cell adhesion is found. The increased adhesion of PfRBCs is also associated with cell flipping and reduced velocity as compared to the untreated endothelium. Altogether, these results provide a compelling evidence of the increased cytoadherence of PfRBCs to glycocalyx-impaired vascular endothelium, thus supporting the advocated role of glycocalyx disruption in the pathogenesis of this disease.

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

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

A Plasmodium Cross-Stage Antigen Contributes to the Development of Experimental Cerebral Malaria

Cerebral malaria is a complex neurological syndrome caused by an infection with Plasmodium falciparum parasites and is exclusively attributed to a series of host–parasite interactions at the pathological blood-stage of infection. In contrast, the preceding intra-hepatic phase of replication is generally considered clinically silent and thereby excluded from playing any role in the development of neurological symptoms. In this study, however, we present an antigen PbmaLS_05 that is presented to the host immune system by both pre-erythrocytic and intra-erythrocytic stages and contributes to the development of cerebral malaria in mice. Although deletion of the endogenous PbmaLS_05 prevented the development of experimental cerebral malaria (ECM) in susceptible mice after both sporozoite and infected red blood cell (iRBC) infections, we observed significant differences in contribution of the host immune response between both modes of inoculation. Moreover, PbmaLS_05-specific CD8⁺ T cells contributed to the development of ECM after sporozoite but not iRBC-infection, suggesting that pre-erythrocytic antigens like PbmaLS_05 can also contribute to the development of cerebral symptoms. Our data thus highlight the importance of the natural route of infection in the study of ECM, with potential implications for vaccine and therapeutic strategies against malaria.

Cytokines and Chemokines in Cerebral Malaria Pathogenesis

Cerebral malaria is among the major causes of malaria-associated mortality and effective adjunctive therapeutic strategies are currently lacking. Central pathophysiological processes involved in the development of cerebral malaria include an imbalance of pro- and anti-inflammatory responses to Plasmodium infection, endothelial cell activation, and loss of blood-brain barrier integrity. However, the sequence of events, which initiates these pathophysiological processes as well as the contribution of their complex interplay to the development of cerebral malaria remain incompletely understood. Several cytokines and chemokines have repeatedly been associated with cerebral malaria severity. Increased levels of these inflammatory mediators could account for the sequestration of leukocytes in the cerebral microvasculature present during cerebral malaria, thereby contributing to an amplification of local inflammation and promoting cerebral malaria pathogenesis. Herein, we highlight the current knowledge on the contribution of cytokines and chemokines to the pathogenesis of cerebral malaria with particular emphasis on their roles in endothelial activation and leukocyte recruitment, as well as their implication in the progression to blood-brain barrier permeability and neuroinflammation, in both human cerebral malaria and in the murine experimental cerebral malaria model. A better molecular understanding of these processes could provide the basis for evidence-based development of adjunct therapies and the definition of diagnostic markers of disease progression.

Association of Plasmodium falciparum with Human Endothelial Cells in vitro

Endothelial abnormalities play a critical role in the pathogenesis of malaria caused by the human pathogen, Plasmodium falciparum. In serious infections and especially in cerebral malaria, red blood cells infected with the parasite are sequestered in small venules in various organs, resulting in endothelial activation and vascular occlusion, which are believed to be largely responsible for the morbidity and mortality caused by this infection, especially in children. We demonstrate that after incubation with infected red blood cells (iRBCs), cultured human umbilical vein endothelial cells (HUVECs) contain parasite protein, genomic DNA, and RNA, as well as intracellular vacuoles with apparent parasite-derived material, but not engulfed or adherent iRBCs. The association of this material with the HUVECs is observed over 96 hours after removal of iRBCs. This phenomenon may occur in endothelial cells in vivo by the process of trogocytosis, in which transfer of material between cells depends on direct cell contact. This process may contribute to the endothelial activation and disruption involved in the pathogenesis of cerebral malaria.

Plasmodium falciparum isolates from patients with uncomplicated malaria promote endothelial inflammation

The ability of Plasmodium falciparum infected erythrocytes (Pf-IEs) to activate endothelial cells has been described; however, the interaction of the endothelium with Pf-IEs field isolates from patients has been less characterized. Previous reports have shown that isolates alter the endothelial permeability and apoptosis. In this study, the adhesion of 19 uncomplicated malaria isolates to Human Dermal Microvascular Endothelial Cells (HDMEC), and their effect on the expression of ICAM-1 and proinflammatory molecules (sICAM-1, IL-6, IL-8, and MCP-1) was evaluated. P. falciparum isolates adhered to resting and TNFα-activated HDEMC cells at different levels. All isolates increased the ICAM-1 expression on the membrane (mICAM-1) of HDMEC and increased the release of its soluble form (sICAM-1), as well the production of IL-6, IL-8 and MCP-1 by HDMEC with no signs of cell apoptosis. No correlation between parasite adhesion and production of cytokines was observed. In conclusion, isolates from uncomplicated malaria can induce a proinflammatory response in endothelial cells that may play a role during the initial inflammatory response to parasite infection; however, a continuous activation of the endothelium can contribute to pathogenesis.

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.

Possible roles of amyloids in malaria pathophysiology

The main therapeutic and prophylactic tools against malaria have been locked for more than a century in the classical approaches of using drugs targeting metabolic processes of the causing agent, the protist Plasmodium spp., and of designing vaccines against chosen antigens found on the parasite's surface. Given the extraordinary resources exhibited by Plasmodium to escape these traditional strategies, which have not been able to free humankind from the scourge of malaria despite much effort invested in them, new concepts have to be explored in order to advance toward eradication of the disease. In this context, amyloid-forming proteins and peptides found in the proteome of the pathogen should perhaps cease being regarded as mere anomalous molecules. Their likely functionality in the pathophysiology of Plasmodium calls for attention being paid to them as a possible Achilles' heel of malaria. Here we will give an overview of Plasmodium-encoded amyloid-forming polypeptides as potential therapeutic targets and toxic elements, particularly in relation to cerebral malaria and the blood-brain barrier function. We will also discuss the recent finding that the genome of the parasite contains an astonishingly high proportion of prionogenic domains.

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).

Brain endothelial cells increase the proliferation of Plasmodium falciparum through production of soluble factors

We here describe the novel finding that brain endothelial cells in vitro can stimulate the growth of Plasmodium falciparum through the production of low molecular weight growth factors. By using a conditioned medium approach, we show that the brain endothelial cells continued to release these factors over time. If this mirrors the in vivo situation, these growth factors potentially would provide an advantage, in terms of enhanced growth, for sequestered parasitised red blood cells in the brain microvasculature. We observed this phenomenon with brain endothelial cells from several sources as well as a second P. falciparum strain. The characteristics of the growth factors included: <3 kDa molecular weight, heat stable, and in part chloroform soluble. Future efforts should be directed at identifying these growth factors, since blocking their production or actions might be of benefit for reducing parasite load and, hence, malaria pathology.

Ultrastructure of the lung in a murine model of malaria-associated acute lung injury/acute respiratory distress syndrome

Background

The mechanisms through which infection with Plasmodium spp. result in lung disease are largely unknown. Recently a number of mouse models have been developed to research malaria-associated lung injury but no detailed ultrastructure studies of the disease in its terminal stages in a murine model have yet been published. The goal was to perform an ultrastructural analysis of the lungs of mice that died with malaria-associated acute lung injury/acute respiratory distress syndrome to better determine the relevancy of the murine models and investigate the mechanism of disease.

Methods

DBA/2 mice were infected with Plasmodium berghei strain ANKA. Mice had their lungs removed immediately after death, processed using standard methods and viewed by transmission electron microscopy (TEM).

Results

Infected red blood cell:endothelium contact, swollen endothelium with distended cytoplasmic extensions and thickening of endothelium basement membrane were observed. Septa were thick and filled with congested capillaries and leukocytes and the alveolar spaces contained blood cells, oedema and cell debris.

Conclusion

Results show that the lung ultrastructure of P. berghei ANKA-infected mice has similar features to what has been described in post-mortem TEM studies of lungs from individuals infected with Plasmodium falciparum. These data support the use of murine models to study malaria-associated acute lung injury.

The role of chemokines in severe malaria: more than meets the eye

Plasmodium falciparum malaria is responsible for over 250 million clinical cases every year worldwide. Severe malaria cases might present with a range of disease syndromes including acute respiratory distress, metabolic acidosis, hypoglycaemia, renal failure, anaemia, pulmonary oedema, cerebral malaria (CM) and placental malaria (PM) in pregnant women. Two main determinants of severe malaria have been identified: sequestration of parasitized red blood cells and strong pro-inflammatory responses. Increasing evidence from human studies and malaria infection animal models revealed the presence of host leucocytes at the site of parasite sequestration in brain blood vessels as well as placental tissue in complicated malaria cases. These observations suggested that apart from secreting cytokines, leucocytes might also contribute to disease by migrating to the site of parasite sequestration thereby exacerbating organ-specific inflammation. This evidence attracted substantial interest in identifying trafficking pathways by which inflammatory leucocytes are recruited to target organs during severe malaria syndromes. Chemo-attractant cytokines or chemokines are the key regulators of leucocyte trafficking and their potential contribution to disease has recently received considerable attention. This review summarizes the main findings to date, investigating the role of chemokines in severe malaria and the implication of these responses for the induction of pathogenesis and immunity to infection.

Cytoadherence of Plasmodium berghei-infected red blood cells to murine brain and lung microvascular endothelial cells in vitro

Sequestration of infected red blood cells (iRBC) within the cerebral and pulmonary microvasculature is a hallmark of human cerebral malaria (hCM). The interaction between iRBC and the endothelium in hCM has been studied extensively and is linked to the severity of malaria. Experimental CM (eCM) caused by Plasmodium berghei ANKA reproduces most features of hCM, although the sequestration of RBC infected by P. berghei ANKA (PbA-iRBC) has not been completely delineated. The role of PbA-iRBC sequestration in the severity of eCM is not well characterized. Using static and flow cytoadherence assays, we provide the first direct in vitro evidence for the binding of PbA-iRBC to murine brain and lung microvascular endothelial cells (MVEC). We found that basal PbA-iRBC cytoadherence to MVECs was significantly higher than that of normal red blood cells (NRBC) and of RBC infected with P. berghei K173 (PbK173-iRBC), a strain that causes noncerebral malaria (NCM). MVEC prestimulation with tumor necrosis factor (TNF) failed to promote any further significant increase in mixed-stage iRBC adherence. Interestingly, enrichment of the blood for mature parasites significantly increased PbA-iRBC binding to the MVECs prestimulated with TNF, while blockade of VCAM-1 reduced this adhesion. Our study provides evidence for the firm, flow-resistant binding to endothelial cells of iRBC from strain ANKA-infected mice, which develop CM, and for less binding of iRBC from strain K173-infected mice, which develop NCM. An understanding of P. berghei cytoadherence may help elucidate the importance of sequestration in the development of CM and aid the development of antibinding therapies to help reduce the burden of this syndrome.

Artesunate Exerts a Direct Effect on Endothelial Cell Activation and NF-κB Translocation in a Mechanism Independent of Plasmodium Killing

Artemisinin and its derivates are an important class of antimalarial drug and are described to possess immunomodulatory activities. Few studies have addressed the effect of artesunate in the murine malaria model or its effect on host immune response during malaria infection. Herein, we study the effect of artesunate treatment and describe an auxiliary mechanism of artesunate in modulating the inflammatory response during experimental malaria infection in mice. Treatment with artesunate did not reduce significantly the parasitemia within 12 h, however, reduced BBB breakdown and TNF-α mRNA expression in the brain tissue of artesunate-treated mice. Conversely, mefloquine treatment was not able to alter clinical features. Notably, artesunate pretreatment failed to modulate the expression of LFA-1 in splenocytes stimulated with parasitized red blood cells (pRBCs) in vitro; however, it abrogated the expression of ICAM-1 in pRBC-stimulated endothelial cells. Accordingly, a cytoadherence in vitro assay demonstrated that pRBCs did not adhere to artesunate-treated vascular endothelial cells. In addition, NF-κB nuclear translocation in endothelial cells stimulated with pRBCs was impaired by artesunate treatment. Our results suggest that artesunate is able to exert a protective effect against the P. berghei-induced inflammatory response by inhibiting NF-κB nuclear translocation and the subsequent expression of ICAM-1.

The crossroads of neuroinflammation in infectious diseases: endothelial cells and astrocytes

Homeostasis implies constant operational defence mechanisms, against both external and internal threats. Infectious agents are prominent among such threats. During infection, the host elicits the release of a vast array of molecules and numerous cell-cell interactions are triggered. These pleiomorphic mediators and cellular effects are of prime importance in the defence of the host, both in the systemic circulation and at sites of tissue injury, for example, the blood-brain barrier (BBB). Here, we focus on the interactions between the endothelium, astrocytes, and the molecules they release. Our review addresses these interactions during infectious neurological diseases of various origins, especially cerebral malaria (CM). Two novel elements of the interplay between endothelium and astrocytes, microparticles and the kynurenine pathway, will also be discussed.

Signal transduction in Plasmodium-Red Blood Cells interactions and in cytoadherence

Malaria is responsible for more than 1.5 million deaths each year, especially among children (Snow et al. 2005). Despite of the severity of malaria situation and great effort to the development of new drug targets (Yuan et al. 2011) there is still a relative low investment toward antimalarial drugs. Briefly there are targets classes of antimalarial drugs currently being tested including: kinases, proteases, ion channel of GPCR, nuclear receptor, among others (Gamo et al. 2010). Here we review malaria signal transduction pathways in Red Blood Cells (RBC) as well as infected RBCs and endothelial cells interactions, namely cytoadherence. The last process is thought to play an important role in the pathogenesis of severe malaria. The molecules displayed on the surface of both infected erythrocytes (IE) and vascular endothelial cells (EC) exert themselves as important mediators in cytoadherence, in that they not only induce structural and metabolic changes on both sides, but also trigger multiple signal transduction processes, leading to alteration of gene expression, with the balance between positive and negative regulation determining endothelial pathology during a malaria infection.

Cerebral malaria pathogenesis: revisiting parasite and host contributions

Cerebral malaria is one of a number of clinical syndromes associated with infection by human malaria parasites of the genus Plasmodium. The etiology of cerebral malaria derives from sequestration of parasitized red cells in brain microvasculature and is thought to be enhanced by the proinflammatory status of the host and virulence characteristics of the infecting parasite variant. In this article we examine the range of factors thought to influence the development of Plasmodium falciparum cerebral malaria in humans and review the evidence to support their role.

Amplification of P. falciparum Cytoadherence through induction of a pro-adhesive state in host endothelium

This study examined the ability of P.falciparum-infected erythrocytes (IE) to induce a pro-adhesive environment in the host endothelium during malaria infection, prior to the systemic cytokine activation seen in the later phase of disease. Previous work had shown increases in receptor levels but had not measured to actual impact on IE binding. Using a co-culture system with a range of endothelial cells (EC) and IE with different cytoadherent properties, we have characterised the specific expression of adhesion receptors and subsequent IE binding by FACS and adhesion assays. We have also examined the specific signalling pathways induced during co-culture that are potentially involved in the induction of receptor expression. The results confirmed that ICAM-1 is up-regulated, albeit at much lower levels than seen with TNF activation, in response to co-culture with infected erythrocytes in all three tissue endothelial cell types tested but that up-regulation of VCAM-1 is tissue-dependent. This small increase in the levels of EC receptors correlated with large changes in IE adhesion ability. Co-culture with either RBC or IE increased the potential of subsequent adhesion indicating priming/modulation effects on EC which make them more susceptible to adhesion and thereby the recruitment of IE. Trypsin surface digestion of IE and the use of a Pfsbp1-knockout (ko) parasite line abrogated the up-regulation of ICAM-1 and reduced IE binding to EC suggesting that PfEMP-1 and other molecules exported to the IE surface via the PfSBP1 pathway are major mediators of this phenotype. This was also supported by the higher induction of EC adhesion receptors by adherent IE compared to isogenic, non-adherent lines.

Platelets alter gene expression profile in human brain endothelial cells in an in vitro model of cerebral malaria

Platelet adhesion to the brain microvasculature has been associated with cerebral malaria (CM) in humans, suggesting that platelets play a role in the pathogenesis of this syndrome. In vitro co-cultures have shown that platelets can act as a bridge between Plasmodium falciparum-infected red blood cells (pRBC) and human brain microvascular endothelial cells (HBEC) and potentiate HBEC apoptosis. Using cDNA microarray technology, we analyzed transcriptional changes of HBEC in response to platelets in the presence or the absence of tumor necrosis factor (TNF) and pRBC, which have been reported to alter gene expression in endothelial cells. Using a rigorous statistical approach with multiple test corrections, we showed a significant effect of platelets on gene expression in HBEC. We also detected a strong effect of TNF, whereas there was no transcriptional change induced specifically by pRBC. Nevertheless, a global ANOVA and a two-way ANOVA suggested that pRBC acted in interaction with platelets and TNF to alter gene expression in HBEC. The expression of selected genes was validated by RT-qPCR. The analysis of gene functional annotation indicated that platelets induce the expression of genes involved in inflammation and apoptosis, such as genes involved in chemokine-, TREM1-, cytokine-, IL10-, TGFβ-, death-receptor-, and apoptosis-signaling. Overall, our results support the hypothesis that platelets play a pathogenic role in CM.

Dysregulation of coagulation in cerebral malaria

Cerebral malaria (CM) is a life-threatening complication of Plasmodium falciparum infection and represents a major cause of morbidity and mortality worldwide. The nature of the pathogenetic processes leading to the cerebral complications remains poorly understood. It has recently emerged that in addition to their conventional role in the regulation of haemostasis, coagulation factors have an inflammatory role that is pivotal in the pathogenesis of a number of acute and chronic conditions, including CM. This new insight offers important therapeutic potential. This review explores the clinical, histological and molecular evidence for the dysregulation of the coagulation system in CM, looking at possible underlying mechanisms. We discuss areas for future research to improve understanding of CM pathogenesis and for the development of new therapeutic approaches.

Host response to cytoadherence in Plasmodium falciparum

Cytoadherence of PRBCs (Plasmodium falciparum-infected red blood cells) to host endothelium has been associated with pathology in severe malaria, but, despite extensive information on the primary processes involved in the adhesive interactions, the mechanisms underlying the disease are poorly understood. Endothelial cells have the ability to mobilize immune and pro-adhesive responses when exposed to both PRBCs and TNF (tumour necrosis factor). In addition, there is also an up-regulation by PRBCs and TNF and a concurrent down-regulation of a range of genes involved in inflammation and cell death, by PRBCs and TNF. We propose that the balance between positive and negative regulation will contribute to endothelial pathology during malarial infection. Apposition of PRBCs has been shown by a number of groups to activate signalling pathways. This is dependent, at least in part, on the cytoadherence characteristics of the invading isolate, such that the avidity of the PRBC for the receptor on host endothelium is proportional to the level of activation of the signalling pathways. An understanding of the post-adhesive processes produced by cytoadherence may help us to understand the variable pathology seen in malaria and to design appropriate therapies to alleviate severe disease.

Parasite burden and CD36-mediated sequestration are determinants of acute lung injury in an experimental malaria model

Although acute lung injury (ALI) is a common complication of severe malaria, little is known about the underlying molecular basis of lung dysfunction. Animal models have provided powerful insights into the pathogenesis of severe malaria syndromes such as cerebral malaria (CM); however, no model of malaria-induced lung injury has been definitively established. This study used bronchoalveolar lavage (BAL), histopathology and gene expression analysis to examine the development of ALI in mice infected with Plasmodium berghei ANKA (PbA). BAL fluid of PbA-infected C57BL/6 mice revealed a significant increase in IgM and total protein prior to the development of CM, indicating disruption of the alveolar–capillary membrane barrier—the physiological hallmark of ALI. In contrast to sepsis-induced ALI, BAL fluid cell counts remained constant with no infiltration of neutrophils. Histopathology showed septal inflammation without cellular transmigration into the alveolar spaces. Microarray analysis of lung tissue from PbA-infected mice identified a significant up-regulation of expressed genes associated with the gene ontology categories of defense and immune response. Severity of malaria-induced ALI varied in a panel of inbred mouse strains, and development of ALI correlated with peripheral parasite burden but not CM susceptibility. Cd36^−/− mice, which have decreased parasite lung sequestration, were relatively protected from ALI. In summary, parasite burden and CD36-mediated sequestration in the lung are primary determinants of ALI in experimental murine malaria. Furthermore, differential susceptibility of mouse strains to malaria-induced ALI and CM suggests that distinct genetic determinants may regulate susceptibility to these two important causes of malaria-associated morbidity and mortality.

Acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) can occur in adult malaria infections with a case fatality rate of 70%–100%. ALI and ARDS are characterized by protein-rich fluid in the lungs, with reduced gas exchange, and in malaria, often accompany high parasite levels and severe or cerebral disease. In this work we have examined lung physiology, pathology and genomics in mouse malaria—Plasmodium berghei ANKA—to show that mice develop malaria-induced ALI. Infected mice have proteinaceous fluid in their lungs, have a migration of inflammatory cells from the blood into the lung walls, and express immune response–related genes. We also found that severity of ALI depended on high parasite levels, both overall and specifically in the lung tissue, but was not consistent with whether the mice developed cerebral malaria. ALI due to Plasmodium berghei ANKA infection models prominent characteristics of human malaria-associated ALI, and we have better defined this model of malaria ALI so it may be used to further explore disease mechanisms and eventual treatment.

Apoptosis: a potential triggering mechanism of neurological manifestation in Plasmodium falciparum malaria

Plasmodium falciparum infection can lead to a life threatening disease and the pathogenetic mechanisms of severe manifestations are not fully understood. Here, we investigated the capacity of P. falciparum-parasitized red blood cells (PRBC) from 45 children with clinical malaria to induce endothelial cell (EC) apoptosis. In all subjects, PRBC that cytoadhered to ECs could be found albeit to a variable degree. By contrast, PRBC that induce EC apoptosis were found only in nine (20%) subjects. Interestingly, children with neurological manifestations were significantly more likely to harbour apoptogenic strains. There was no quantitative relationship between the capacity of these isolates to cytoadhere and apoptosis induction. We hypothesize that P. falciparum-encoded molecules could be responsible for apoptosis induction and therefore suggest new insights in the pathogenesis of P. falciparum malaria. Further investigations are currently in progress to determine whether these results can be confirmed and to identify putative parasite apoptogenic factors.

Blood coagulation, inflammation, and malaria

Malaria remains a highly prevalent disease in more than 90 countries and accounts for at least 1 million deaths every year. Plasmodium falciparum infection is often associated with a procoagulant tonus characterized by thrombocytopenia and activation of the coagulation cascade and fibrinolytic system; however, bleeding and hemorrhage are uncommon events, suggesting that a compensated state of blood coagulation activation occurs in malaria. This article (i) reviews the literature related to blood coagulation and malaria in a historic perspective, (ii) describes basic mechanisms of coagulation, anticoagulation, and fibrinolysis, (iii) explains the laboratory changes in acute and compensated disseminated intravascular coagulation (DIC), (iv) discusses the implications of tissue factor (TF) expression in the endothelium of P. falciparum infected patients, and (v) emphasizes the procoagulant role of parasitized red blood cells (RBCs) and activated platelets in the pathogenesis of malaria. This article also presents the Tissue Factor Model (TFM) for malaria pathogenesis, which places TF as the interface between sequestration, endothelial cell (EC) activation, blood coagulation disorder, and inflammation often associated with the disease. The relevance of the coagulation-inflammation cycle for the multiorgan dysfunction and coma is discussed in the context of malaria pathogenesis.

CD44, a signal receptor for the inhibition of the cytoadhesion of CD36-binding Plasmodium falciparum-infected erythrocytes by CSA-binding infected erythrocytes

The cytoadhesion of Plasmodium falciparum-infected erythrocytes (IEs) in organ microvessels is a key event in the pathogenesis of cerebral malaria and pulmonary edema. Identification of the molecules involved in the interaction between IEs and endothelial cells has been a major goal of research into severe forms of malaria. In contrast, the consequences of cytoadhesion for endothelial cells have been largely ignored. By combining phenotypic selection, cytoadhesion assays and flow cytometry, we demonstrated that the cytoadhesion of CSA-binding IEs inhibited the cytoadhesion of CD36-binding IEs. We identified CD44 as a signal receptor for CSA-binding IEs cytoadhesion, and demonstrated that the signal was transduced to CD36 through a pathway involving the Src-kinase family and MEK. CD36-mediated cytoadhesion was modulated independently of changes in CD36 expression. These results provide the first evidence that some IEs can downregulate the cytoadhesion of IEs of another phenotype, by modifying endothelial cells via a signaling pathway relating CD44 to CD36. Mimicking this phenomenon may constitute an interesting therapeutic strategy for inhibiting the adhesion of CD36-binding IEs -- the most abundant phenotype among field isolates -- and promoting their degradation in the spleen.

Altered phenotype and gene transcription in endothelial cells, induced by Plasmodium falciparum-infected red blood cells: pathogenic or protective?

Severe malaria is associated with sequestration of Plasmodium falciparum-infected red blood cells (PRBC) in the microvasculature and elevation of intercellular adhesion molecule-1 (ICAM-1) and TNF. In vitro co-culture of human umbilical vein endothelial cells (HUVEC), with either PRBC or uninfected RBC, required the presence of low level TNF (5pg/ml) for significant up-regulation of ICAM-1, which may contribute to increased cytoadhesion in vivo. These effects were independent of P. falciparum erythrocyte membrane protein-1 (PfEMP-1)-mediated adhesion but critically dependent on cell-cell contact. Further changes included increases in IL8 release and soluble TNF receptor shedding. Microarray analysis of HUVEC transcriptome following co-culture, using a human Affymetrix microarray chip, showed significant differential regulation of genes which defined gene ontologies such as cell communication, cell adhesion, signal transduction and immune response. Our data demonstrate that endothelial cells have the ability to mobilise immune and pro-adhesive responses when exposed to both PRBC and TNF. In addition, there is also a previously un-described positive regulation by RBC and TNF and a concurrent negative regulation of a range of genes involved in inflammation and cell-death, by PRBC and TNF. We propose that the balance between positive and negative regulation demonstrated in our study will determine endothelial pathology during a malaria infection.

Plasmodium falciparum-infected erythrocytes induce tissue factor expression in endothelial cells and support the assembly of multimolecular coagulation complexes

BACKGROUND

Plasmodium falciparum malaria infects 300-500 million people every year, causing 1-2 million deaths annually. Evidence of a coagulation disorder, activation of endothelial cells (EC) and increase in inflammatory cytokines are often present in malaria.

OBJECTIVES

We have asked whether interaction of parasitized red blood cells (pRBC) with EC induces tissue factor (TF) expression in vitro and in vivo. The role of phosphatidylserine-containing pRBC to support the assembly of blood coagulation complexes was also investigated.

RESULTS

We demonstrate that mature forms of pRBC induce functional expression of TF by EC in vitro with productive assembly of the extrinsic Xnase complex and initiation of the coagulation cascade. Late-stage pRBC also support the prothrombinase and intrinsic Xnase complex formation in vitro, and may function as activated platelets in the amplification phase of the blood coagulation. Notably, post-mortem brain sections obtained from P. falciparum-infected children who died from cerebral malaria and other causes display a consistent staining for TF in the EC.

CONCLUSIONS

These findings place TF expression by endothelium and the amplification of the coagulation cascade by pRBC and/or activated platelets as potentially critical steps in the pathogenesis of malaria. Furthermore, it may allow investigators to test other therapeutic alternatives targeting TF or modulators of EC function in the treatment of malaria and/or its complications.

Malaria: mechanisms of erythrocytic infection and pathological correlates of severe disease

Malaria is an ancient disease that continues to cause enormous human morbidity and mortality. The life cycle of the causative parasite involves multiple tissues in two distinct host organisms, mosquitoes and humans. However, all the clinical symptoms of malaria are a consequence of infection of human erythrocytes. An understanding of the basic mechanisms that govern parasite invasion, remodeling, growth, and reinvasion of erythrocytes and the complex events leading to tissue pathology may yield new diagnostics and treatments for malaria. This approach is revealing a more complete picture of the most serious syndrome associated with this infection-cerebral malaria. We focus on the most recent understanding of the molecular basis of infection, summarize our finding from an ongoing pediatric cerebral malaria autopsy study in Malawi, and integrate these insights to malarial pathology.

Plasmodium falciparum-infected erythrocytes increase intercellular adhesion molecule 1 expression on brain endothelium through NF-kappaB

Sequestration of Plasmodium falciparum-infected erythrocytes (Pf-IRBC) in postcapillary brain endothelium is a hallmark of cerebral malaria (CM) pathogenesis. There is a correlation between adherent Pf-IRBC and increased expression of intercellular cell adhesion molecule 1 (ICAM-1), which is also a receptor for Pf-IRBC on human brain microvascular endothelial cells (HBMEC). The underlying mechanism for the increased ICAM-1 expression has not been clearly defined. Therefore, we investigated the mechanisms of ICAM-1 expression on isolated HBMEC after exposure to Pf-IRBC. Ultrastructural characterization of the model confirmed that there was attachment through both Pf-IRBC knobs and HBMEC microvillus protrusions. Pf-IRBC induced a dose- and time-dependent increase in ICAM-1 expression on HBMEC that was specific for human brain endothelium and was not observed with human umbilical vein endothelium. Involvement of both membrane-associated Pf-IRBC proteins and parasite-derived soluble factors with the increase in ICAM-1 expression was demonstrated by surface trypsinization and fractionation. Pf-IRBC exposure induced nuclear translocation of NF-kappaB in HBMEC, which was linked to ICAM-1 expression, as shown by use of specific inhibitors of the transcription factor NF-kappaB and immunocytochemistry. In addition, inhibition of reactive oxygen species decreased Pf-IRBC-induced ICAM-1 expression on HBMEC. Parasite-induced ICAM-1 expression explains the localization of this molecule on brain endothelium in postmortem CM brain samples. By increasing ICAM-1 expression, Pf-IRBC may increase their sequestration, thereby perpetuating CM.

Down-regulation of tight junction mRNAs in human endothelial cells co-cultured with Plasmodium falciparum-infected erythrocytes

To understand the mechanism of sequestration in the microvasculature of patients with falciparum malaria, we examined the patterns of expression of mRNAs for adhesion molecules (ICAM-1, VCAM-1, and E-selectin) and tight junction molecules (occludin, vinculin, and ZO-1) in human umbilical vein endothelial cells (HUVECs) co-cultured with Plasmodium falciparum-parasitized red blood cells (PRBCs) in vitro. The PRBCs were collected from patients with uncomplicated, severe, or cerebral malaria (CM). Patterns of mRNA expression in HUVECs co-cultured with PRBCs were examined by real-time quantitative reverse transcription-polymerase chain reaction (RT-PCR). Levels of mRNAs for all the three adhesion molecules increased with increased culture time within 3 h, regardless of the source of the PRBCs. In contrast, the patterns of mRNA expression for the tight junction molecules varied between the different co-cultures. When HUVECs were cultured with PRBCs from uncomplicated malaria patients, levels of mRNAs for tight junction molecules increased according to the culture time. HUVECs co-cultured with PRBCs from severe malaria patients showed no change in the mRNAs levels during 3 h of observation. When HUVECs were cultured with PRBCs from CM patients, levels of mRNAs for tight junction proteins decreased according to the culture time. Although the mechanisms underlying these phenomena are not clear, our results suggest that PRBCs can alter expression of tight junction proteins in endothelial cells at the site of sequestration and thereby influence disease severity.

Human cerebral malaria and the blood-brain barrier

Malaria represents a continuing and major global health challenge and our understanding of how the Plasmodium parasite causes severe disease and death remains poor. One serious complication of the infection is cerebral malaria, a clinically complex syndrome of coma and potentially reversible encephalopathy, associated with a high mortality rate and increasingly recognised long-term sequelae in survivors. Research into the pathophysiology of cerebral malaria, using a combination of clinical and pathological studies, animal models and in vitro cell culture work, has focussed attention on the blood-brain barrier (BBB). This represents the key interface between the brain parenchyma and the parasite, which develops within an infected red cell but remains inside the vascular space. Studies of BBB function in cerebral malaria have provided some evidence for parasite-induced changes secondary to sequestration of parasitised red blood cells and host leukocytes within the cerebral microvasculature, such as redistribution of endothelial cell intercellular junction proteins and intracellular signaling. However, the evidence for a generalised increase in BBB permeability, leading to cerebral oedema, is conflicting. As well as direct cell adhesion-dependent effects, local adhesion-independent effects may activate and damage cerebral endothelial cells and perivascular cells, such as decreased blood flow, hypoxia or the effects of parasite toxins such as pigment. Finally, a number of systemic mechanisms could influence the BBB during malaria, such as the metabolic and inflammatory complications of severe disease acting 'at a distance'. This review will summarise evidence for these mechanisms from human studies of cerebral malaria and discuss the possible role for BBB dysfunction in this complex and challenging disease.