Angiogenic proteins in brains of patients who died with cerebral malaria

Beyond cuts and scrapes: plasmin in malaria and other vector-borne diseases

Plasmodium and other vector-borne pathogens have evolved mechanisms to hijack the mammalian fibrinolytic system to facilitate infection of the human host and the invertebrate vector. Plasmin, the effector protease of fibrinolysis, maintains homeostasis in the blood vasculature by degrading the fibrin that forms blood clots. Plasmin also degrades proteins from extracellular matrices, the complement system, and immunoglobulins. Here, we review some of the mechanisms by which vector-borne pathogens interact with components of the fibrinolytic system and co-opt its functions to facilitate transmission and infection in the host and the vector. Further, we discuss innovative strategies beyond conventional therapeutics that could be developed to target the interaction of vector-borne pathogens with the fibrinolytic proteins and prevent their transmission.

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.

Elevated Levels of the Endothelial Molecules ICAM-1, VEGF-A, and VEGFR2 in Microscopic Asymptomatic Malaria

Background

In malaria, clinical disease has been associated with increased levels of endothelial activation due to the sequestration of infected erythrocytes. However, the levels and impact of endothelial activation and pro-angiogenic molecules such as vascular endothelial growth factor (VEGF)–A and its receptor vascular endothelial growth factor receptor 2 (VEGFR2) in asymptomatic malaria have not been well characterized.

Methods

Blood samples were obtained from community children for malaria diagnosis using microscopy and polymerase chain reaction. A multiplex immunoassay was used to determine the levels of intracellular adhesion molecule (ICAM)–1, vascular endothelial growth factor (VEGF)–A, and VEGFR2 in the plasma of children with microscopic or submicroscopic asymptomatic parasitemia and compared with levels in uninfected controls.

Results

Levels of ICAM-1, VEGF-A, and VEGFR2 were significantly increased in children with microscopic asymptomatic parasitemia compared with uninfected controls. Also, levels of VEGF-A were found to be inversely associated with age. Additionally, a receiver operating characteristic analysis revealed that plasma levels of ICAM-1 (area under the curve [AUC], 0.72) showed a moderate potential in discriminating between children with microscopic malaria from uninfected controls when compared with VEGF-A (AUC, 0.67) and VEGFR2 (AUC, 0.69).

Conclusions

These data imply that endothelial activation and pro-angiogenic growth factors could be one of the early host responders during microscopic asymptomatic malaria and may play a significant role in disease pathogenesis.

Induction of Angiogenesis by Malarial Infection through Hypoxia Dependent Manner

Malarial infection induces tissue hypoxia in the host through destruction of red blood cells. Tissue hypoxia in malarial infection may increase the activity of HIF1α through an intracellular oxygen-sensing pathway. Activation of HIF1α may also induce vascular endothelial growth factor (VEGF) to trigger angiogenesis. To investigate whether malarial infection actually generates hypoxia-induced angiogenesis, we analyzed severity of hypoxia, the expression of hypoxia-related angiogenic factors, and numbers of blood vessels in various tissues infected with Plasmodium berghei. Infection in mice was performed by intraperitoneal injection of 2×106 parasitized red blood cells. After infection, we studied parasitemia and survival. We analyzed hypoxia, numbers of blood vessels, and expression of hypoxia-related angiogenic factors including VEGF and HIF1α. We used Western blot, immunofluorescence, and immunohistochemistry to analyze various tissues from Plasmodium berghei-infected mice. In malaria-infected mice, parasitemia was increased over the duration of infection and directly associated with mortality rate. Expression of VEGF and HIF1α increased with the parasitemia in various tissues. Additionally, numbers of blood vessels significantly increased in each tissue type of the malaria-infected group compared to the uninfected control group. These results suggest that malarial infection in mice activates hypoxia-induced angiogenesis by stimulation of HIF1α and VEGF in various tissues.

Thrombospondin-1 Production Regulates the Inflammatory Cytokine Secretion in THP-1 Cells Through NF-κB Signaling Pathway

Thrombospondin-1 (TSP-1) is upregulated in several inflammatory diseases. Recent data have shown that macrophages from TSP-1-deficient mice have a reduced inflammatory phenotype, suggesting that TSP-1 plays a part in macrophage activation. DNA microarray approach revealed that Porphyromonas gingivalis lipopolysaccharide (P. gingivalis LPS) may induce the enhanced TSP-1 expression in human monocytes, suggesting a role of TSP-1-mediated pathogenesis in periodontitis. Until recently, the function of TSP-1 has been a matter of debate. In this study, we explored the role of TSP-1 in inflammatory cytokine secretions and its putative mechanism in pathogenesis of periodontitis. We demonstrated that TSP-1 expression was significantly upregulated in gingival tissues with periodontitis and in P. gingivalis LPS-stimulated THP-1 cells. Deficiency of TSP-1 by transfecting siRNAs decreased IL-6, IL-1β, and TNF-α secretions in THP-1 cells, whereas overexpression of TSP-1 resulted in an upregulation of IL-6, IL-1β, and TNF-α productions. Additional experiments showed that Pyrrolidine dithiocarbamate (PDTC) inhibited IL-6, IL-1β, and TNF-α expression induced by overexpression of TSP-1, accompanying with downregulation of phosphorylated p65 and IκBα protein levels in response to P. gingivalis LPS. These results indicated that TSP-1 played a significant role in P. gingivalis LPS-initiated inflammatory cytokines (IL-6, IL-1β, and TNF-α) secretions of THP-1 cells, and the NF-κB signaling is involved in its induction of expression. Thus, TSP-1 effectively elevated P. gingivalis LPS-induced inflammation mediated by the NF-κB pathway and may be critical for pathology of periodontitis.

Nanoformulations of albendazole as effective anticancer and antiparasite agents

Initially emerging as a widely used clinical antiparasitic drug, albendazole (ABZ) has been increasingly recognized as an effective anticancer agent due to its outstanding advantage, in other words, low toxicity to normal cells but high effectiveness against parasites and some tumors. The major challenge is its poor water solubility and subsequently low bioavailability. This article thus first reviews the brief achievements in using ABZ to treat parasites and cancers, and summarizes the basic mechanisms of action of ABZ. Then this article critically reviews recent nanotechnological strategies, in other words, formulating/conjugating it with carriers into nanoformulations, in practices of improving aqueous solubility and efficacy in treatment of tumors and parasites. Our expert opinions in this field are provided for more effective delivery of ABZ to treat tumors and parasites in vivo.

Pathogenic CD8+ T Cells Cause Increased Levels of VEGF-A in Experimental Malaria-Associated Acute Respiratory Distress Syndrome, but Therapeutic VEGFR Inhibition Is Not Effective

Malaria is a severe disease and kills over 400,000 people each year. Malarial complications are the main cause of death and include cerebral malaria and malaria-associated acute respiratory distress syndrome (MA-ARDS). Despite antimalarial treatment, lethality rates of MA-ARDS are still between 20 and 80%. Patients develop pulmonary edema with hemorrhages and leukocyte extravasation in the lungs. The vascular endothelial growth factor-A (VEGF-A) and the placental growth factor (PlGF) are vascular permeability factors and may be involved in the disruption of the alveolar-capillary membrane, leading to alveolar edema. We demonstrated increased pulmonary VEGF-A and PlGF levels in lungs of mice with experimental MA-ARDS. Depletion of pathogenic CD8⁺ T cells blocked pulmonary edema and abolished the increase of VEGF-A and PlGF. However, neutralization of VEGF receptor-2 (VEGFR-2) with the monoclonal antibody clone DC101 did not decrease pulmonary pathology. The broader spectrum receptor tyrosine kinase inhibitor sunitinib even increased lung pathology. These data suggest that the increase in alveolar VEGF-A and PlGF is not a cause but rather a consequence of the pulmonary pathology in experimental MA-ARDS and that therapeutic inhibition of VEGF receptors is not effective and even contra-indicated.

Microbial Translocation Associated with an Acute-Phase Response and Elevations in MMP-1, HO-1, and Proinflammatory Cytokines in Strongyloides stercoralis Infection

Microbial translocation, characterized by elevated levels of lipopolysaccharide (LPS) and related markers, is a common occurrence in HIV and some parasitic infections. This is usually associated with extensive inflammation and immune activation. To examine the occurrence of microbial translocation and the associated inflammatory response in asymptomatic Strongyloides stercoralis infection, we measured the plasma levels of LPS and other microbial translocation markers, acute-phase proteins, inflammatory markers, and proinflammatory cytokines in individuals with (infected [INF]) or without (uninfected [UN]) S. stercoralis infections. Finally, we also measured the levels of all of these markers in INF individuals following treatment of S. stercoralis infection. We show that INF individuals exhibit significantly higher plasma levels of microbial translocation markers (LPS, soluble CD14 [sCD14], intestinal fatty acid-binding protein [iFABP], and endotoxin core IgG antibody [EndoCAb]), acute-phase proteins (α-2 macroglobulin [α-2M], C-reactive protein [CRP], haptoglobin, and serum amyloid protein A [SAA]), inflammatory markers (matrix metalloproteinase 1 [MMP-1] and heme oxygenase 1 [HO-1]), and proinflammatory cytokines (interleukin-6 [IL-6], IL-8, monocyte chemoattractant protein 1 [MCP-1], and IL-1β) than do UN individuals. INF individuals exhibit significantly decreased levels of tissue inhibitor of metalloproteinases 4 (TIMP-4). Following treatment of S. stercoralis infection, the elevated levels of microbial translocation markers, acute-phase proteins, and inflammatory markers were all diminished. Our data thus show that S. stercoralis infection is characterized by microbial translocation and accompanying increases in levels of acute-phase proteins and markers of inflammation and provide data to suggest that microbial translocation is a feature of asymptomatic S. stercoralis infection and is associated with an inflammatory response.

Systemic and Cerebral Vascular Endothelial Growth Factor Levels Increase in Murine Cerebral Malaria along with Increased Calpain and Caspase Activity and Can be Reduced by Erythropoietin Treatment

The pathogenesis of cerebral malaria (CM) includes compromised microvascular perfusion, increased inflammation, cytoadhesion, and endothelial activation. These events cause blood-brain barrier disruption and neuropathology and associations with the vascular endothelial growth factor (VEGF) signaling pathway have been shown. We studied this pathway in mice infected with Plasmodium berghei ANKA causing murine CM with or without the use of erythropoietin (EPO) as adjunct therapy. ELISA and western blotting was used for quantification of VEGF and relevant proteins in brain and plasma. CM increased levels of VEGF in brain and plasma and decreased plasma levels of soluble VEGF receptor 2. EPO treatment normalized VEGF receptor 2 levels and reduced brain VEGF levels. Hypoxia-inducible factor (HIF)-1α was significantly upregulated whereas cerebral HIF-2α and EPO levels remained unchanged. Furthermore, we noticed increased caspase-3 and calpain activity in terminally ill mice, as measured by protease-specific cleavage of α-spectrin and p35. In conclusion, we detected increased cerebral and systemic VEGF as well as HIF-1α, which in the brain were reduced to normal in EPO-treated mice. Also caspase and calpain activity was reduced markedly in EPO-treated mice.

Effects of the vascular endothelial growth factor receptor-2 (VEGFR-2) inhibitor SU5416 on in vitro cultures of Plasmodium falciparum

BACKGROUND

Vascular endothelial growth factor (VEGF) is taken up by parasitized red blood cells during malaria and stimulates intra-erythrocytic growth of Plasmodium falciparum in vitro. The cause and consequence of this uptake is not understood.

METHODS

Plasmodium falciparum was cultured in vitro. Parasite growth and intracellular VEGF levels were assessed using flow cytometry. Intracellular VEGF was visualized by fluorescence immunocytochemistry. Phosphorylated tyrosine was measured by western blotting. In vivo assessment of intra-erythrocytic VEGF was performed in Plasmodium berghei ANKA-infected C57BL/6 mice.

RESULTS

VEGF accumulated intracellularly in infected red blood cells, particularly in schizonts. In vitro growth of P. falciparum was unchanged when co-cultured with the anti-VEGF antibody bevacizumab or with an anti-VEGF receptor-1 peptide. In contrast, the VEGF receptor-2 inhibitor, SU5416, dose-dependently inhibited growth. None of the treatments reduced intracellular VEGF levels. Thus, the anti-parasitic effect of SU5416 seemed independent of VEGF uptake. SU5416 reduced phosphorylated tyrosine in parasitized red blood cells. Similarly, the broad-spectrum tyrosine kinase inhibitor genistein dose-dependently inhibited P. falciparum growth and reduced tyrosine phosphorylation. Neither bevacizumab nor anti-VEGF receptor-1 peptide affected tyrosine kinase activity. Finally, in vivo uptake of VEGF in P. berghei ANKA was demonstrated, analogous to the in vitro uptake in P. falciparum, making it a possible model for the effects of VEGF signalling in vivo during malaria.

CONCLUSIONS

Inhibition of VEGFR-2 signalling reduces intra-erythrocytic growth of P. falciparum, likely due to tyrosine kinase inhibition. Internalisation of VEGF in P. falciparum-infected red blood cells does not rely on VEGF receptors. The function of in vivo uptake of VEGF can be studied in rodent malaria models.

Affinity proteomics reveals elevated muscle proteins in plasma of children with cerebral malaria

Systemic inflammation and sequestration of parasitized erythrocytes are central processes in the pathophysiology of severe Plasmodium falciparum childhood malaria. However, it is still not understood why some children are more at risks to develop malaria complications than others. To identify human proteins in plasma related to childhood malaria syndromes, multiplex antibody suspension bead arrays were employed. Out of the 1,015 proteins analyzed in plasma from more than 700 children, 41 differed between malaria infected children and community controls, whereas 13 discriminated uncomplicated malaria from severe malaria syndromes. Markers of oxidative stress were found related to severe malaria anemia while markers of endothelial activation, platelet adhesion and muscular damage were identified in relation to children with cerebral malaria. These findings suggest the presence of generalized vascular inflammation, vascular wall modulations, activation of endothelium and unbalanced glucose metabolism in severe malaria. The increased levels of specific muscle proteins in plasma implicate potential muscle damage and microvasculature lesions during the course of cerebral malaria.

Why do some malaria-infected children develop severe and lethal forms of the disease, while others only have mild forms? In order to try to find potential answers or clues to this question, we have here analyzed more than 1,000 different human proteins in the blood of more than 500 malaria-infected children from Ibadan in Nigeria, a holoendemic malaria region. We identified several proteins that were present at higher levels in the blood from the children that developed severe malaria in comparison to those that did not. Some of the most interesting identified proteins were muscle specific proteins, which indicate that damaged muscles could be a discriminatory pathologic event in cerebral malaria compared to other malaria cases. These findings will hopefully lead to an increased understanding of the disease and may contribute to the development of clinical algorithms that could predict which children are more at risks to severe malaria. This in turn will be of high value in the management of these children in already overloaded tertiary-care health facilities in urban large densely-populated sub-Saharan cities with holoendemic malaria such as in the case of Ibadan and Lagos.

Protective or pathogenic effects of vascular endothelial growth factor (VEGF) as potential biomarker in cerebral malaria

Cerebral malaria (CM) is the major lethal complication of Plasmodium falciparum infection. It is characterized by persistent coma along with symmetrical motor signs. Several clinical, histopathological, and laboratory studies have suggested that cytoadherence of parasitized erythrocytes, neural injury by malarial toxin, and excessive inflammatory cytokine production are possible pathogenic mechanisms. Although the detailed pathophysiology of CM remains unsolved, it is thought that the binding of parasitized erythrocytes to the cerebral endothelia of microvessels, leading to their occlusion and the consequent angiogenic dysregulation play a key role in the disease pathogenesis. Recent evidences showed that vascular endothelial growth factor (VEGF) and its receptor-related molecules are over-expressed in the brain tissues of CM patients, as well as increased levels of VEGF are detectable in biologic samples from malaria patients. Whether the modulation of VEGF is causative agent of CM mortality or a specific phenotype of patients with susceptibility to fatal CM needs further evaluation. Currently, there is no biological test available to confirm the diagnosis of CM and its complications. It is hoped that development of biomarkers to identify patients and potential risk for adverse outcomes would greatly enhance better intervention and clinical management to improve the outcomes. We review and discuss here what it is currently known in regard to the role of VEGF in CM as well as VEGF as a potential biomarker.

Host matrix metalloproteinases in cerebral malaria: new kids on the block against blood-brain barrier integrity?

Cerebral malaria (CM) is a life-threatening complication of falciparum malaria, associated with high mortality rates, as well as neurological impairment in surviving patients. Despite disease severity, the etiology of CM remains elusive. Interestingly, although the Plasmodium parasite is sequestered in cerebral microvessels, it does not enter the brain parenchyma: so how does Plasmodium induce neuronal dysfunction? Several independent research groups have suggested a mechanism in which increased blood–brain barrier (BBB) permeability might allow toxic molecules from the parasite or the host to enter the brain. However, the reported severity of BBB damage in CM is variable depending on the model system, ranging from mild impairment to full BBB breakdown. Moreover, the factors responsible for increased BBB permeability are still unknown. Here we review the prevailing theories on CM pathophysiology and discuss new evidence from animal and human CM models implicating BBB damage. Finally, we will review the newly-described role of matrix metalloproteinases (MMPs) and BBB integrity. MMPs comprise a family of proteolytic enzymes involved in modulating inflammatory response, disrupting tight junctions, and degrading sub-endothelial basal lamina. As such, MMPs represent potential innovative drug targets for CM.

Microglia development and function

Proper development and function of the mammalian central nervous system (CNS) depend critically on the activity of parenchymal sentinels referred to as microglia. Although microglia were first described as ramified brain-resident phagocytes, research conducted over the past century has expanded considerably upon this narrow view and ascribed many functions to these dynamic CNS inhabitants. Microglia are now considered among the most versatile cells in the body, possessing the capacity to morphologically and functionally adapt to their ever-changing surroundings. Even in a resting state, the processes of microglia are highly dynamic and perpetually scan the CNS. Microglia are in fact vital participants in CNS homeostasis, and dysregulation of these sentinels can give rise to neurological disease. In this review, we discuss the exciting developments in our understanding of microglial biology, from their developmental origin to their participation in CNS homeostasis and pathophysiological states such as neuropsychiatric disorders, neurodegeneration, sterile injury responses, and infectious diseases. We also delve into the world of microglial dynamics recently uncovered using real-time imaging techniques.

Effects of Plasmodium falciparum-infected erythrocytes on matrix metalloproteinase-9 regulation in human microvascular endothelial cells

OBJECTIVE

To investigate the regulation of matrix metalloproteinases (MMPs) and tissue inhibitors of metalloproteinases (TIMPs) in human microvascular endothelium (HMEC-1) exposed to erythrocytes infected by different strains of Plasmodium falciparum (P. falciparum).

METHODS

HMEC-1 cells were co-incubated for 72 h with erythrocytes infected by late stage trophozoite of D10 (chloroquine-sensitive) or W2 (chloroquine-resistant) P. falciparum strains. Cell supernatants were then collected and the levels of pro- or active gelatinases MMP-9 and MMP-2 were evaluated by gelatin zymography and densitometry. The release of pro-MMP-9, MMP-3, MMP-1 and TIMP-1 proteins was analyzed by western blotting and densitometry.

RESULTS

Infected erythrocytes induced de novo proMMP-9 and MMP-9 release. Neither basal levels of proMMP-2 were altered, nor active MMP-2 was found. MMP-3 and MMP-1 secretion was significantly enhanced, whereas basal TIMP-1 was unaffected. All effects were similar for both strains.

CONCLUSIONS

P. falciparum parasites, either chloroquine-sensitive or -resistant, induce the release of active MMP-9 protein from human microvascular endothelium, by impairing balances between proMMP-9 and its inhibitor, and by enhancing the levels of its activators. This work provides new evidence on MMP involvement in malaria, pointing at MMP-9 as a possible target in adjuvant therapy.

Involvement of p38 MAPK in haemozoin-dependent MMP-9 enhancement in human monocytes

The lipid moiety of natural haemozoin (nHZ, malarial pigment) was previously shown to enhance expression and release of human monocyte matrix metalloproteinase-9 (MMP-9), and a major role for 15-(S,R)-hydroxy-6,8,11,13-eicosatetraenoic acid (15-HETE), a nHZ lipoperoxidation product, was proposed. Here, the underlying mechanisms were investigated, focusing on the involvement of mitogen-activated protein kinases (MAPKs). Results showed that nHZ promoted either early or late p38 MAPK phosphorylation; however, nHZ did not modify basal phosphorylation/expression ratios of extracellular signal-regulated kinase-1/2 and c-jun N-terminal kinase-1/2. 15-HETE mimicked nHZ effects on p38 MAPK, whereas lipid-free synthetic (s)HZ and delipidized (d)HZ did not. Consistently, both nHZ and 15-HETE also promoted phosphorylation of MAPK-activated protein kinase-2, a known p38 MAPK substrate; such an effect was abolished by SB203580, a synthetic p38 MAPK inhibitor. SB203580 also abrogated nHZ-dependent and 15-HETE-dependent enhancement of MMP-9 mRNA and protein (latent and activated forms) levels in cell lysates and supernatants. Collectively, these data suggest that in human monocytes, nHZ and 15-HETE upregulate MMP-9 expression and secretion through activation of p38 MAPK pathway. The present work provides new evidence on mechanisms underlying MMP-9 deregulation in malaria, which might be helpful to design new specific drugs for adjuvant therapy in complicated malaria.

The significance of matrix metalloproteinases in parasitic infections involving the central nervous system

Matrix metalloproteinases (MMPs) represent a large family of over twenty different secreted or membrane-bound endopeptidases, involved in many physiological (embryogenesis, precursor or stem cell mobilization, tissue remodeling during wound healing, etc.), as well as pathological (inflammation, tumor progression and metastasis in cancer, vascular pathology, etc.) conditions. For a long time, MMPs were considered only for the ability to degrade extracellular matrix (ECM) molecules (e.g., collagen, laminin, fibronectin) and to release hidden epitopes from the ECM. In the last few years, it has been fully elucidated that these molecules have many other functions, mainly related to the immune response, in consideration of their effects on cytokines, hormones and chemokines. Among others, MMP-2 and MMP-9 are endopeptidases of the MMP family produced by neutrophils, macrophages and monocytes. When infection is associated with leukocyte influx into specific organs, immunopathology and collateral tissue damage may occur. In this review, the involvement of MMPs and, in particular, of gelatinases in both protozoan and helminth infections will be described. In cerebral malaria, for example, MMPs play a role in the pathogenesis of such diseases. Also, trypanosomosis and toxoplasmosis will be considered for protozoan infections, as well as neurocysticercosis and angiostrongyloidosis, as regards helminthiases. All these situations have in common the proteolytic action on the blood brain barrier, mediated by MMPs.

Preserved vascular integrity and enhanced survival following neuropilin-1 inhibition in a mouse model of CD8 T cell-initiated CNS vascular permeability

BACKGROUND

Altered permeability of the blood-brain barrier (BBB) is a feature of numerous neurological conditions including multiple sclerosis, cerebral malaria, viral hemorrhagic fevers and acute hemorrhagic leukoencephalitis. Our laboratory has developed a murine model of CD8 T cell-initiated central nervous system (CNS) vascular permeability in which vascular endothelial growth factor (VEGF) signaling plays a prominent role in BBB disruption.

FINDINGS

In this study, we addressed the hypothesis that in vivo blockade of VEGF signal transduction through administration of peptide (ATWLPPR) to inhibit neuropilin-1 (NRP-1) would have a therapeutic effect following induction of CD8 T cell-initiated BBB disruption. We report that inhibition of NRP-1, a co-receptor that enhances VEGFR2 (flk-1) receptor activation, decreases vascular permeability, brain hemorrhage, and mortality in this model of CD8 T cell-initiated BBB disruption. We also examine the expression pattern of VEGFR2 (flk-1) and VEGFR1 (flt-1) mRNA expression during a time course of this condition. We find that viral infection of the brain leads to increased expression of flk-1 mRNA. In addition, flk-1 and flt-1 expression levels decrease in the striatum and hippocampus in later time points following induction of CD8 T cell-mediated BBB disruption.

CONCLUSION

This study demonstrates that NRP-1 is a potential therapeutic target in neuro-inflammatory diseases involving BBB disruption and brain hemorrhage. Additionally, the reduction in VEGF receptors subsequent to BBB disruption could be involved in compensatory negative feedback as an attempt to reduce vascular permeability.

Macrophage inflammatory protein-1alpha mediates matrix metalloproteinase-9 enhancement in human adherent monocytes fed with malarial pigment

OBJECTIVE

To investigate the role of macrophage inflammatory protein-1alpha (MIP-1alpha) in the detrimental enhancement of matrix metalloproteinase-9 (MMP-9) expression, release and activity induced by phagocytosis of malarial pigment (haemozoin, HZ) in human monocytes.

METHODS

Human adherent monocytes were unfed/fed with native HZ for 2 h. After 24 hours, MIP-1alpha production was evaluated by ELISA in cell supernatants. Alternatively, HZ-unfed/fed monocytes were treated in presence/absence of anti-human MIP-1alpha blocking antibodies or recombinant human MIP-1alpha for 15 h (RNA studies) or 24 h (protein studies); therefore, MMP-9 mRNA expression was evaluated in cell lysates by Real Time RT-PCR, whereas proMMP-9 and active MMP-9 protein release were measured in cell supernatants by Western blotting and gelatin zymography.

RESULTS

Phagocytosis of HZ by human monocytes increased production of MIP-1 alpha, mRNA expression of MMP-9 and protein release of proMMP-9 and active MMP-9. All the HZ-enhancing effects on MMP-9 were abrogated by anti-human MIP-1alpha blocking antibodies and mimicked by recombinant human MIP-1alpha.

CONCLUSIONS

The present work suggests a role for MIP-1alpha in the HZ-dependent enhancement of MMP-9 expression, release and activity observed in human monocytes, highlighting new detrimental effects of HZ-triggered proinflammatory response by phagocytic cells in falciparum malaria.

Matrix metalloproteinases as therapeutic targets in protozoan parasitic infections

Matrix metalloproteinases (MMPs) are associated with processes of tissue remodeling and are expressed in all infections with protozoan parasites. We here report the status of MMP research in malaria, trypanosomiasis, leishmaniasis and toxoplasmosis. In all these infections, the balances between MMPs and endogenous MMP inhibitors are disturbed, mostly in favor of active proteolysis. When the infection is associated with leukocyte influx into specific organs, immunopathology and collateral tissue damage may occur. These pathologies include cerebral malaria, sleeping sickness (human African trypanosomiasis), Chagas disease (human American trypanosomiasis), leishmaniasis and toxoplasmic encephalitis in immunocompromised hosts. Destruction of the integrity of the blood-brain barrier (BBB) is a common denominator that may be executed by leukocytic MMPs under the control of host cytokines and chemokines as well as influenced by parasite products. Mechanisms by which parasite-derived products alter host expression of MMP and endogenous MMP inhibitors, have only been described for hemozoin (Hz) in malaria. Hence, understanding these interactions in other parasitic infections remains an important challenge. Furthermore, the involved parasites are also known to produce their own metalloproteinases, and this forms an extra stimulus to investigate MMP inhibitory drugs as therapeutics. MMP inhibitors (MMPIs) may dampen collateral tissue damage, as is anecdotically reported for tetracyclines as MMP regulators in parasite infections.

Natural haemozoin modulates matrix metalloproteinases and induces morphological changes in human microvascular endothelium

Severe malaria, including cerebral malaria (CM), is characterized by the sequestration of parasitized erythrocytes in the microvessels after cytoadherence to endothelial cells. Products of parasite origin, such as haemozoin (HZ), contribute to the pathogenesis of severe malaria by interfering with host inflammatory response. In human monocytes, HZ enhanced the levels of matrix metalloproteinase-9 (MMP-9), a protease involved in neuroinflammation. Here the effects of HZ on the regulation of MMPs by the human microvascular endothelial cell line HMEC-1 were investigated. Cells treated with natural (n)HZ appeared elongated instead of polygonal, and formed microtubule-like vessels on synthetic basement membrane. nHZ enhanced total gelatinolytic activity by inducing proMMP-9 and MMP-9 without affecting basal MMP-2. The level of the endogenous tissue inhibitor of MMP-9 (TIMP-1) was not altered by nHZ, while TIMP-2, the MMP-2 inhibitor, was enhanced. Additionally, nHZ induced MMP-1 and MMP-3, two enzymes sequentially involved in collagenolysis and proMMP-9 proteolytic activation. Lipid-free HZ did not reproduce nHZ effects. Present data suggest that the lipid moiety of HZ alters the MMP/TIMP balances and promotes the proteolytic activation of proMMP-9 in HMEC-1, thereby enhancing total gelatinolytic activity, cell activation and inflammation. These findings might help understanding the mechanisms of blood brain barrier damage during CM.

Matrix Metalloproteinase-9 and Haemozoin: Wedding Rings for Human Host and Plasmodium falciparum Parasite in Complicated Malaria

It is generally accepted that the combination of both Plasmodium falciparum parasite and human host factors is involved in the pathogenesis of complicated severe malaria, including cerebral malaria (CM). Among parasite products, the malarial pigment haemozoin (HZ) has been shown to impair the functions of mononuclear and endothelial cells. Different CM models were associated with enhanced levels of matrix metalloproteinases (MMPs), a family of proteolytic enzymes able to disrupt subendothelial basement membrane and tight junctions and shed, activate, or inactivate cytokines, chemokines, and other MMPs through cleavage from their precursors. Among MMPs, a good candidate for targeted therapy might be MMP-9, whose mRNA and protein expression enhancement as well as direct proenzyme activation by HZ have been recently investigated in a series of studies by our group and others. In the present paper the role of HZ and MMP-9 in complicated malaria, as well as their interactions, will be discussed.

Combinations of host biomarkers predict mortality among Ugandan children with severe malaria: a retrospective case-control study

Background

Severe malaria is a leading cause of childhood mortality in Africa. However, at presentation, it is difficult to predict which children with severe malaria are at greatest risk of death. Dysregulated host inflammatory responses and endothelial activation play central roles in severe malaria pathogenesis. We hypothesized that biomarkers of these processes would accurately predict outcome among children with severe malaria.

Methodology/Findings

Plasma was obtained from children with uncomplicated malaria (n = 53), cerebral malaria (n = 44) and severe malarial anemia (n = 59) at time of presentation to hospital in Kampala, Uganda. Levels of angiopoietin-2, von Willebrand Factor (vWF), vWF propeptide, soluble P-selectin, soluble intercellular adhesion molecule-1 (ICAM-1), soluble endoglin, soluble FMS-like tyrosine kinase-1 (Flt-1), soluble Tie-2, C-reactive protein, procalcitonin, 10 kDa interferon gamma-induced protein (IP-10), and soluble triggering receptor expressed on myeloid cells-1 (TREM-1) were determined by ELISA. Receiver operating characteristic (ROC) curve analysis was used to assess predictive accuracy of individual biomarkers. Six biomarkers (angiopoietin-2, soluble ICAM-1, soluble Flt-1, procalcitonin, IP-10, soluble TREM-1) discriminated well between children who survived severe malaria infection and those who subsequently died (area under ROC curve>0.7). Combinational approaches were applied in an attempt to improve accuracy. A biomarker score was developed based on dichotomization and summation of the six biomarkers, resulting in 95.7% (95% CI: 78.1–99.9) sensitivity and 88.8% (79.7–94.7) specificity for predicting death. Similar predictive accuracy was achieved with models comprised of 3 biomarkers. Classification tree analysis generated a 3-marker model with 100% sensitivity and 92.5% specificity (cross-validated misclassification rate: 15.4%, standard error 4.9%).

Conclusions

We identified novel host biomarkers of pediatric severe and fatal malaria (soluble TREM-1 and soluble Flt-1) and generated simple biomarker combinations that accurately predicted death in an African pediatric population. While requiring validation in further studies, these results suggest the utility of combinatorial biomarker strategies as prognostic tests for severe malaria.

Role of the NF-κB transcription pathway in the haemozoin- and 15-HETE-mediated activation of matrix metalloproteinase-9 in human adherent monocytes

Haemozoin (HZ, malarial pigment) is a crystalline ferriprotoporphyrin IX polymer derived from undigested host haemoglobin haem, present in late stages of Plasmodium falciparum-parasitized RBCs and in residual bodies shed after schizogony. It was shown previously that phagocytosed HZ or HZ-containing trophozoites increased monocyte matrix metalloproteinase-9 (MMP-9) activity and enhanced production of MMP-9-related cytokines TNF and IL-1beta. Here we show that in human monocytes the HZ/trophozoite phagocytosis effects and their recapitulation by 15(S,R)-hydroxy-6,8,11,13-eicosatetraenoic acid (15-HETE), a potent lipoperoxidation derivative generated by HZ from arachidonic acid via haem catalysis, were mediated via activation of NF-κB transcription pathway. After phagocytosis of HZ/trophozoites or treatment with 15-HETE, the NF-κB complex migrated to the nuclear fraction while the inhibitory cytosolic IκBalpha protein was phosphorylated and degraded. All HZ/trophozoite/15-HETE effects on MMP-9 activity and TNF/IL-1beta production were abrogated by quercetin, artemisinin and parthenolide, inhibitors of IκBalpha phosphorylation and subsequent degradation, NF-κB nuclear translocation, and NF-κB-p65 binding to DNA respectively. In conclusion, enhanced activation of MMP-9, and release of pro-inflammatory cytokines TNF and IL-1beta, a triad of effects involved in malaria pathogenesis, elicited in human monocytes by trophozoite and HZ phagocytosis and recapitulated by 15-HETE, appear to be causally connected to persisting activation of the NF-κB system.

Induction of the vascular endothelial growth factor pathway in the brain of adults with fatal falciparum malaria is a non-specific response to severe disease

Medana I M, Day N P J, Roberts R, Sachanonta N, Turley H, Pongponratn E, Hien T T, White N J. & Turner G D H (2010) Histopathology57, 282–294

Induction of the vascular endothelial growth factor pathway in the brain of adults with fatal falciparum malaria is a non-specific response to severe disease

Artemisone effective against murine cerebral malaria

Background

Artemisinins are the newest class of drug approved for malaria treatment. Due to their unique mechanism of action, rapid effect on Plasmodium, and high efficacy in vivo, artemisinins have become essential components of malaria treatment. Administration of artemisinin derivatives in combination with other anti-plasmodials has become the first-line treatment for uncomplicated falciparum malaria. However, their efficiency in cases of cerebral malaria (CM) remains to be determined.

Methods

The efficacy of several artemisinin derivatives for treatment of experimental CM was evaluated in ICR or C57BL/6 mice infected by Plasmodium berghei ANKA. Both mouse strains serve as murine models for CM.

Results

Artemisone was the most efficient drug tested, and could prevent death even when administered at relatively late stages of cerebral pathogenesis. No parasite resistance to artemisone was detected in recrudescence. Co-administration of artemisone together with chloroquine was more effective than monotherapy with either drug, and led to complete cure. Artemiside was even more effective than artemisone, but this substance has yet to be submitted to preclinical toxicological evaluation.

Conclusions

Altogether, the results support the use of artemisone for combined therapy of CM.

Elevated levels of vascular endothelial growth factor (VEGF) and soluble vascular endothelial growth factor receptor (VEGFR)-2 in human malaria

In cerebral malaria, the binding of parasitized erythrocytes to the cerebral endothelium and the consequent angiogenic dysregulation play a key role in pathogenesis. Because vascular endothelial growth factor (VEGF) is widely regarded as a potent stimulator of angiogenesis, edema, inflammation, and vascular remodeling, the plasma levels of VEGF and the soluble form of the VEGF receptor (sVEGFR)-1 and -2 in uncomplicated malaria patients and healthy adults were measured by enzyme-linked immunosorbent assay (ELISA) to examine their roles in malaria. The results showed that VEGF and sVEGFR-2 levels were significantly elevated in malaria patients compared with healthy adults. Moreover, it was confirmed that malarial parasite antigens induced VEGF secretion from the human mast cell lines HMC-1 or KU812 cell. This is the first report to suggest that the interaction of VEGF and sVEGFR-2 is involved in the host immune response to malarial infection and that malarial parasites induce VEGF secretion from human mast cells.

CD8 T cell-initiated vascular endothelial growth factor expression promotes central nervous system vascular permeability under neuroinflammatory conditions

Dysregulation of the blood-brain barrier (BBB) is a hallmark feature of numerous neurologic disorders as diverse as multiple sclerosis, stroke, epilepsy, viral hemorrhagic fevers, cerebral malaria, and acute hemorrhagic leukoencephalitis. CD8 T cells are one immune cell type that have been implicated in promoting vascular permeability in these conditions. Our laboratory has created a murine model of CD8 T cell-mediated CNS vascular permeability using a variation of the Theiler's murine encephalomyelitis virus system traditionally used to study multiple sclerosis. Previously, we demonstrated that CD8 T cells have the capacity to initiate astrocyte activation, cerebral endothelial cell tight junction protein alterations and CNS vascular permeability through a perforin-dependent process. To address the downstream mechanism by which CD8 T cells promote BBB dysregulation, in this study, we assess the role of vascular endothelial growth factor (VEGF) expression in this model. We demonstrate that neuronal expression of VEGF is significantly upregulated prior to, and coinciding with, CNS vascular permeability. Phosphorylation of fetal liver kinase-1 is significantly increased early in this process indicating activation of this receptor. Specific inhibition of neuropilin-1 significantly reduced CNS vascular permeability and fetal liver kinase-1 activation, and preserved levels of the cerebral endothelial cell tight junction protein occludin. Our data demonstrate that CD8 T cells initiate neuronal expression of VEGF in the CNS under neuroinflammatory conditions, and that VEGF may be a viable therapeutic target in neurologic disease characterized by inflammation-induced BBB disruption.

Phagocytosis of haemozoin (malarial pigment) enhances metalloproteinase-9 activity in human adherent monocytes: role of IL-1beta and 15-HETE

BACKGROUND

It has been shown previously that human monocytes fed with haemozoin (HZ) or trophozoite-parasitized RBCs displayed increased matrix metalloproteinase-9 (MMP-9) enzyme activity and protein/mRNA expression and increased TNF production, and showed higher matrix invasion ability. The present study utilized the same experimental model to analyse the effect of phagocytosis of: HZ, delipidized HZ, beta-haematin (lipid-free synthetic HZ) and trophozoites on production of IL-1beta and MMP-9 activity and expression. The second aim was to find out which component of HZ was responsible for the effects.

METHODS

Native HZ freshly isolated from Plasmodium falciparum (Palo Alto strain, Mycoplasma-free), delipidized HZ, beta-haematin (lipid-free synthetic HZ), trophozoites and control meals such as opsonized non-parasitized RBCs and inert latex particles, were fed to human monocytes. The production of IL-1beta by differently fed monocytes, in presence or absence of specific MMP-9 inhibitor or anti-hIL-1beta antibodies, was quantified in supernatants by ELISA. Expression of IL-1beta was analysed by quantitative real-time RT-PCR. MMP-9 activity and protein expression were quantified by gelatin zymography and Western blotting.

RESULTS

Monocytes fed with HZ or trophozoite-parasitized RBCs generated increased amounts of IL-1beta and enhanced enzyme activity (in cell supernatants) and protein/mRNA expression (in cell lysates) of monocyte MMP-9. The latter appears to be causally related to enhanced IL-1beta production, as enhancement of both expression and enzyme activity were abrogated by anti-hIL-1beta Abs. Upregulation of IL-1beta and MMP-9 were absent in monocytes fed with beta-haematin or delipidized HZ, indicating a role for HZ-attached or HZ-generated lipid components. 15-HETE (15(S,R)-hydroxy-6,8,11,13-eicosatetraenoic acid) a potent lipoperoxidation derivative generated by HZ from arachidonic acid via haem-catalysis was identified as one mediator possibly responsible for increase of both IL-1beta production and MMP-9 activity.

CONCLUSION

Results indicate that specific lipoperoxide derivatives generated by HZ may play a role in modulating production of IL-1beta and MMP-9 expression and activity in HZ/trophozoite-fed human monocytes. Results may clarify aspects of cerebral malaria pathogenesis, since MMP-9, a metalloproteinase able to disrupt the basal lamina is possibly involved in generation of hallmarks of cerebral malaria, such as blood-brain barrier endothelium dysfunction, localized haemorrhages and extravasation of phagocytic cells and parasitized RBCs into brain tissues.

Plasma IP-10, apoptotic and angiogenic factors associated with fatal cerebral malaria in India

Background

Plasmodium falciparum in a subset of patients can lead to cerebral malaria (CM), a major contributor to malaria-associated mortality. Despite treatment, CM mortality can be as high as 30%, while 10% of survivors of the disease may experience short- and long-term neurological complications. The pathogenesis of CM is mediated by alterations in cytokine and chemokine homeostasis, inflammation as well as vascular injury and repair processes although their roles are not fully understood. The hypothesis for this study is that CM-induced changes in inflammatory, apoptotic and angiogenic factors mediate severity of CM and that their identification will enable development of new prognostic markers and adjunctive therapies for preventing CM mortalities.

Methods

Plasma samples (133) were obtained from healthy controls (HC, 25), mild malaria (MM, 48), cerebral malaria survivors (CMS, 48), and cerebral malaria non-survivors (CMNS, 12) at admission to the hospital in Jabalpur, India. Plasma levels of 30 biomarkers ((IL-1β, IL-1ra, IL-2, IL-4, IL-5, IL-6, IL-8, IL-9, IL-10, IL-12 (p70), IL-13, IL-15, IL-17, Eotaxin, FGF basic protein, G-CSF, GM-CSF, IFN-γ, IP-10, MCP-1 (MCAF), MIP-1α, MIP-1β, RANTES, TNF-α, Fas-ligand (Fas-L), soluble Fas (sFas), soluble TNF receptor 1 (sTNF-R1) and soluble TNF receptor 2 (sTNFR-2), PDGF bb and VEGF)) were simultaneously measured in an initial subset of ten samples from each group. Only those biomarkers which showed significant differences in the pilot analysis were chosen for testing on all remaining samples. The results were then compared between the four groups to determine their role in CM severity.

Results

IP-10, sTNF-R2 and sFas were independently associated with increased risk of CM associated mortality. CMNS patients had a significantly lower level of the neuroprotective factor VEGF when compared to other groups (P < 0.0045). The ratios of VEGF to IP-10, sTNF-R2, and sFas distinguished CM survivors from non survivors (P < 0.0001).

Conclusion

The results suggest that plasma levels of IP-10, sTNF-R2 and sFas may be potential biomarkers of CM severity and mortality. VEGF was found to be protective against CM associated mortality and may be considered for adjunctive therapy to improve the treatment outcome in CM patients.

High levels of erythropoietin are associated with protection against neurological sequelae in African children with cerebral malaria

Cerebral malaria (CM) in children is associated with a high mortality and long-term neurocognitive sequelae. Both erythropoietin (Epo) and vascular endothelial growth factor (VEGF) have been shown to be neuroprotective. We hypothesized that high plasma and cerebrospinal fluid (CSF) levels of these cytokines would prevent neurological sequelae in children with CM. We measured Epo, VEGF, and tumor necrosis factor in paired samples of plasma and CSF of Kenyan children admitted with CM. Logistic regression models were used to identify risk and protective factors associated with the development of neurological sequelae. Children with CM (n = 124) were categorized into three groups: 76 without sequelae, 32 with sequelae, and 16 who died. Conditional logistic regression analysis matching the 32 patients with CM and neurological sequelae to 64 patients with CM without sequelae stratified for hemoglobin level estimated that plasma Epo (>200 units/liter) was associated with >80% reduction in the risk of developing neurological sequelae [adjusted odds ratio (OR) 0.18; 95% C.I. 0.05-0.93; P = 0.041]. Admission with profound coma (adjusted OR 5.47; 95% C.I. 1.45-20.67; P = 0.012) and convulsions after admission (adjusted OR 16.35; 95% C.I. 2.94-90.79; P = 0.001) were also independently associated with neurological sequelae. High levels of Epo were associated with reduced risk of neurological sequelae in children with CM. The age-dependent Epo response to anemia and the age-dependent protective effect may influence the clinical epidemiology of CM. These data support further study of Epo as an adjuvant therapy in CM.

Cerebrospinal fluid and serum biomarkers of cerebral malaria mortality in Ghanaian children

Background

Plasmodium falciparum can cause a diffuse encephalopathy known as cerebral malaria (CM), a major contributor to malaria associated mortality. Despite treatment, mortality due to CM can be as high as 30% while 10% of survivors of the disease may experience short- and long-term neurological complications. The pathogenesis of CM and other forms of severe malaria is multi-factorial and appear to involve cytokine and chemokine homeostasis, inflammation and vascular injury/repair. Identification of prognostic markers that can predict CM severity will enable development of better intervention.

Methods

Postmortem serum and cerebrospinal fluid (CSF) samples were obtained within 2–4 hours of death in Ghanaian children dying of CM, severe malarial anemia (SMA), and non-malarial (NM) causes. Serum and CSF levels of 36 different biomarkers (IL-1β, IL-1ra, IL-2, IL-4, IL-5, IL-6, IL-7, IL-8, IL-9, IL-10, IL-12 (p70), IL-13, IL-15, IL-17, Eotaxin, FGF basic protein, CRP, G-CSF, GM-CSF, IFN-γ, TNF-α, IP-10, MCP-1 (MCAF), MIP-1α, MIP-1β, RANTES, SDF-1α, CXCL11 (I-TAC), Fas-ligand [Fas-L], soluble Fas [sFas], sTNF-R1 (p55), sTNF-R2 (p75), MMP-9, TGF-β1, PDGF bb and VEGF) were measured and the results compared between the 3 groups.

Results

After Bonferroni adjustment for other biomarkers, IP-10 was the only serum biomarker independently associated with CM mortality when compared to SMA and NM deaths. Eight CSF biomarkers (IL-1ra, IL-8, IP-10, PDGFbb, MIP-1β, Fas-L, sTNF-R1, and sTNF-R2) were significantly elevated in CM mortality group when compared to SMA and NM deaths. Additionally, CSF IP-10/PDGFbb median ratio was statistically significantly higher in the CM group compared to SMA and NM groups.

Conclusion

The parasite-induced local cerebral dysregulation in the production of IP-10, 1L-8, MIP-1β, PDGFbb, IL-1ra, Fas-L, sTNF-R1, and sTNF-R2 may be involved in CM neuropathology, and their immunoassay may have potential utility in predicting mortality in CM.

Expression microarray analysis implicates apoptosis and interferon-responsive mechanisms in susceptibility to experimental cerebral malaria

Specific local brain responses, influenced by parasite sequestration and host immune system activation, have been implicated in the development of cerebral malaria. This study assessed whole-brain transcriptional responses over the course of experimental cerebral malaria by comparing genetically resistant and susceptible inbred mouse strains infected with Plasmodium berghei ANKA. Computational methods were used to identify differential patterns of gene expression. Overall, genes that showed the most transcriptional activity were differentially expressed in susceptible mice 1 to 2 days before the onset of characteristic symptoms of cerebral malaria. Most of the differentially expressed genes identified were associated with immune-related gene ontology categories. Further analysis to identify interaction networks and to examine patterns of transcriptional regulation within the set of identified genes implicated a central role for both interferon-regulated processes and apoptosis in the pathogenesis of cerebral malaria. Biological relevance of these genes and pathways was confirmed using quantitative RT-PCR and histopathological examination of the brain for apoptosis. The application of computational biology tools to examine systematically the disease progression in cerebral malaria can identify important transcriptional programs activated during its pathogenesis and may serve as a methodological approach to identify novel targets for therapeutic intervention.

Glial activation and matrix metalloproteinase release in cerebral malaria

Although neurological symptoms associated with cerebral malaria (CM) are largely reversible, recent studies suggest that lasting neurological sequelae can occur. This may be especially true for children, in whom persistent deficits include problems with memory and attention. Because the malaria parasite is not thought to enter the brain parenchyma, lasting deficits are likely related to factors including the host response to disease. Studies with a rodent model, and with human postmortem tissue, suggest that glial activation occurs with CM. In this review, the authors will highlight studies focused on such activation in CM. Likely causes will be discussed, which include ischemia and activation of blood brain barrier endothelial cells. The potential consequences of glial activation will also be discussed, highlighting the possibility that glial-derived proteinases contribute to structural damage of the central nervous system (CNS). Of note, for the purposes of this focused review, glial activation will refer to the activation of astrocytes and microglial cells; discussion of oligodendroglial cells will not be included. In addition, although events thought to be critical to the pathogenesis of CM and glial activation will be covered, a comprehensive review of cerebral malaria will not be presented. Excellent reviews are already available, including Coltel et al (2004; Curr Neurovasc Res 1: 91-110), Medana and Turner (2006; Int J Parasitol 36: 555-568), and Hunt et al (2006; Int J Parasitol 36: 569-582).

Current perspectives on the mechanism of action of artemisinins

Artemisinin derivatives are the most recent single drugs approved and introduced for public antimalarial treatment. Although their recommended use is for treatment of Plasmodium falciparum infection, these drugs also act against other parasites, as well as against tumor cells. The mechanisms of action attributed to artemisinin include interference with parasite transport proteins, disruption of parasite mitochondrial function, modulation of host immune function and inhibition of angiogenesis. Artemisinin combination therapies are currently the preferred treatment for malaria. These combinations may prevent the induction of parasite drug resistance. However, in view of the multiple mechanisms involved, especially when additional drugs are used, the combined therapy should be carefully examined for antagonistic effects. It is now a general theory that the crucial mechanism is interference with plasmodial SERCA. Therefore, future development of resistance may be associated with overproduction or mutations of this transporter. However, a general mechanism, such as alterations in general drug transport pathways, is feasible. In this article, we review the evidence for each mechanism of action suggested.

Hypertension and maternal-fetal conflict during placental malaria

BACKGROUND

Malaria and hypertension are major causes of maternal mortality in tropical countries, especially during first pregnancies, but evidence for a relationship between these syndromes is contradictory.

METHODS AND FINDINGS

In a cross-sectional survey of Tanzanian parturients, the rate of hypertension was similar in placental malaria (PM)-positive (11/85 = 13%) and PM-negative (73/602 = 12%) individuals. However, we found that PM was associated with hypertension in first-time mothers aged 18-20 y but not other mothers. Hypertension was also associated with histologic features of chronic malaria, which is common in first-time mothers. Levels of soluble vascular endothelial growth factor receptor 1 (sVEGFR1), a preeclampsia biomarker, were elevated in first-time mothers with either PM, hypertension, or both, but levels were not elevated in other mothers with these conditions. In first-time mothers with PM, the inflammatory mediator vascular endothelial growth factor (VEGF) was localized to maternal macrophages in the placenta, while sVEGFR1, its soluble inhibitor, was localized to the fetal trophoblast.

CONCLUSIONS

The data suggest that maternal-fetal conflict involving the VEGF pathway occurs during PM, and that sVEGFR1 may be involved in the relationship between chronic PM and hypertension in first-time mothers. Because placental inflammation causes poor fetal outcomes, we hypothesize that fetal mechanisms that promote sVEGFR1 expression may be under selective pressure during first pregnancies in malaria-endemic areas.

Matrix metalloproteinases, tissue inhibitors of MMPs and TACE in experimental cerebral malaria

Cerebral malaria (CM) is a life-threatening disorder and a major medical problem in developing countries. It is caused by the sequestration of malaria-infected erythrocytes onto brain endothelia, followed by blood-brain barrier (BBB) damage and neurological deficit. In the present study, matrix metalloproteinases (MMPs) were analysed in a mouse model of CM with Plasmodium berghei ANKA. Increased numbers of gelatinase B (MMP-9)-positive cells, which were also CD11b(+), were detected in the brain. In addition, activation of gelatinase B occurred in CM brains, and not in brains of mice with non-CM. However, selective genetic knockout of gelatinase B did not alter the clinical evolution of experimental CM. To study other protease balances, the mRNA expression levels of nine matrix metalloproteinases (MMPs), five membrane-type MMPs, TNF-alpha converting enzyme (TACE) and the four tissue inhibitors of metalloproteinases (TIMPs) were analysed during CM in different organs. Significant alterations in expression were observed, including increases of the mRNAs of MMP-3, -8, -13 and -14 in the spleen, MMP-8, -12, -13 and -14 in the liver and MMP-8 and -13 in the brain. Net gelatinolytic activity, independent of gelatinase B and inhibitable with EDTA, was detected in situ in the endothelia of blood vessels in CM brains, but not in brains of mice with non-CM, suggesting that metalloproteases, different from gelatinase B, are active in the BBB environment in CM. The increase in MMP expression in the brain was significantly less pronounced after infection of C57Bl/6 mice with the noncerebral strain P. berghei NK65, but it was similar in CM-susceptible C57Bl/6 and CM-resistant Balb/C mice upon infection with P. berghei ANKA. Furthermore, in comparison with C57Bl/6 mice, a larger increase in TIMP-1 and a marked, >30-fold induction in MMP-3 were found in the brains of Balb/C mice, suggesting possible protective roles for TIMP-1 and MMP-3.

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.