The Synaptic Leap

PLoS Pathog, Vol. 4, No. 5. (16 May 2008), e1000068.

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.
Fiona Lovegrove, Sina Gharib, Lourdes Peña-Castillo, Samir Patel, John Ruzinski, Timothy Hughes, Conrad Liles, Kevin Kain

Malaria Journal, Vol. 6, No. 1. (16 March 2007), 32.

BACKGROUND:Adhesion of Plasmodium-infected red blood cells (iRBC) to different host cells, ranging from endothelial to red blood cells, is associated to malaria pathology. In vitro studies have shown the relevance of CD36 for adhesion phenotypes of Plasmodium falciparum iRBC such as sequestration, platelet mediated clumping and non-opsonic uptake of iRBC. Different adhesion phenotypes involve different host cells and are associated with different pathological outcomes of disease. Studies with different human populations with CD36 polymorphisms failed to attribute a clear role to CD36 expression in human malaria. Up to the present, no in vivo model has been available to study the relevance of different CD36 adhesion phenotypes to the pathological course of Plasmodium infection.METHODS:Using CD36-deficient mice and their control littermates, CD36 bone marrow chimeric mice, expressing CD36 exclusively in haematopoietic cells or in non-haematopoietic cells, were generated. Irradiated CD36-/- and wild type mice were also reconstituted with syngeneic cells to control for the effects of irradiation. The reconstituted mice were infected with Plasmodium berghei ANKA and analysed for the development of blood parasitaemia and neurological symptoms.RESULTS:All mice reconstituted with syngeneic bone marrow cells as well as chimeric mice expressing CD36 exclusively in non-haematopoietic cells died from experimental cerebral malaria between day 6 and 12 after infection. A significant proportion of chimeric mice expressing CD36 only in haematopoietic cells did not die from cerebral malaria.CONCLUSION:The analysis of bone marrow chimeric mice reveals a dual role of CD36 in P. berghei ANKA infection. Expression of CD36 in haematopoietic cells, most likely macrophages and dendritic cells, has a beneficial effect that is masked in normal mice by adverse effects of CD36 expression in non-haematopoietic cells, most likely endothelial cells.
Margarida Rodrigues, Silvia Portugal, Maria Febbraio, Maria Mota

Immunology letters, Vol. 65, No. 1-2. (January 1999), pp. 29-33.

IgE, the immunoglobulin instrumental in atopic diseases is also elevated in many infections. This paper reports on the occurrence and possible pathogenic role of IgE in human Plasmodium falciparum malaria, one of the most widely spread and severe infectious diseases world wide. Plasmodial infections induce IgE elevation in the blood of the majority of people living in malaria endemic areas and up to 5% of this IgE constitutes anti-malaria antibodies. Production of IgE is controlled by T cells and elevated IgE concentrations in the blood of malaria patients are the result of an increased ratio of T-helper 2 (Th2) over T-helper 1 (Th1) cells. The underlying Th1 to Th2 switch is controlled by a variety of environmental and genetic factors. The importance of the latter is demonstrated by the IgE levels occurring in monozygotic or dizygotic twins originating from malarious areas of Africa. While these levels were indistinguishable within monozygotic twin pairs, they were different within the dizygotic pairs. Comparison of the levels of total IgE or IgE anti-malaria antibodies in patients with uncomplicated malaria with those in patients with the severe form of the disease (cerebral malaria or severe malaria without cerebral involvement) indicate that these levels are significantly higher in the cases with severe disease. This is the reverse with IgG and suggests that IgE plays a role in malaria pathogenesis. An important pathogenic mediator causing malaria fever and tissue lesions is tumor necrosis factor (TNF), generally believed to be induced by toxins released from the parasite. However, sera from malaria patients can also cause TNF release from monocytes in a reaction dependent on the presence of IgE containing immune complexes or aggregates. This results in induction and cross-linking of Fcepsilon receptor II (CD23) and by binding to and activating these cells, IgE will contribute to a local over-production of TNF in capillaries and post-capillary venules where P. falciparum parasites or their products accumulate in the severe forms of this disease.
P Perlmann, H Perlmann, G ElGhazali, MT Blomberg

Clinical & Experimental Immunology, Vol. 112, No. 2. (1998), pp. 303-307.

To assess the eosinophil response to Plasmodium falciparum infection a cohort of initially parasite-free Ghanaian children was followed for 3 months. Seven of nine children who acquired an asymptomatic P. falciparum infection showed increase in eosinophil counts, while a decrease was found in seven of nine children with symptomatic malaria, and no change was observed in 14 children who remained parasite-free. In a hospital-based study, paediatric patients with cerebral malaria (CM), severe anaemia (SA), or uncomplicated malaria (UM) had uniformly low eosinophil counts during the acute illness followed by eosinophilia 30 days after cure. Plasma levels of eosinophil cationic protein (ECP) and eosinophil protein X (EPX) were measured as indicators of eosinophil activation. In spite of the low eosinophil counts, ECP levels were increased on day 0 and significantly higher in patients with CM (geometric mean (95% confidence interval) 8.5 ng/ml (6.8201310.7 ng/ml)) than in SA (4.7 ng/ml (3.020137.5 ng/ml)) and UM patients (4.3 ng/ml (3.620135.3 ng/ml), P < 0.001). A similar pattern was found for EPX. It thus appears that the low eosinophil counts may be due to tissue sequestration and destruction rather than decreased production. The plasma levels of the granule proteins correlated with levels of tumour necrosis factor and soluble IL-2 receptor, implicating inflammatory responses and T cell activation as causes of the eosinophil activation. By contrast, the eosinophil induction did not appear to be part of a Th2-like response. Eosinophil granule proteins may be important in both control of malaria infection and the pathogenesis of severe malaria.
Kurtzhals, Reimert, Tette, Dunyo, Koram, Akanmori, Nkrumah, Hviid

Clinical & Experimental Immunology, Vol. 109, No. 1. (1997), pp. 84-89.

People living in Plasmodium falciparum-endemic areas frequently have elevated levels of total as well as P. falciparum-specific serum IgE. This study aimed at investigating whether the elevated serum IgE levels reflect a shift in the balance between CD4+ T helper 1 (Th1) and T helper 2 (Th2) cells in individuals naturally exposed to the P. falciparum parasite. To investigate the role of Th1 and Th2 cells in the human P. falciparum system we used the ELISPOT assay to determine the ratio of IFN-03B3- and IL-4-producing cells after specific antigen or mitogen activation in vitro. The donors were individuals who had acquired immunity through natural exposure to the parasite. In response to the specific malaria antigens, very few IL-4-producing cells were seen. However, in the response of individual donors to the polyclonal T cell activator, leucoagglutinin (La), the anti-malarial IgE levels in plasma were correlated with an increased ratio of IL-4/IFN-03B3 producing cells. Thus, donors with ratios of IL-4/IFN-03B3 > 1 exhibited mean plasma anti-malarial IgE levels significantly greater than those with ratios < 1. In individuals not living in P. falciparum-endemic areas the ratio of IL-4/IFN-03B3 was always < 1. Taken together, our data suggest a shift in the balance between Th1 and Th2 cells in naturally P. falciparum-primed individuals, associated with elevated anti-P. falciparum plasma IgE levels. The role and biological significance of IgE (Th2-type immune response) for protection against P. falciparum and/or pathogenesis of malaria require further study.
G Elghazali, H Perlmann, ASM Rutta, P Perlmann, M troye-Blomberg

The Southeast Asian journal of tropical medicine and public health, Vol. 23, No. 4. (December 1992), pp. 795-797.

Eosinophilia was a frequently detected incidental finding during a prospective study of malaria seroepidemiology in Thailand. Blood eosinophil counts were performed every 3 months for a year in 823 Thai soldiers on border guard duty in a malaria endemic area. Soldiers developing malaria were admitted to hospital and more frequent eosinophil counts were done. P. falciparum parasitemia suppressed preexisting eosinophilia but eosinophilia returned following treatment. P. vivax and mixed infections had a similar but less marked effect on the peripheral blood eosinophil count. Eosinophilia in persons from a malaria endemic area may represent a normal late response to malaria infection.
GD Shanks, C Wilairatanaporn

J. Exp. Med., Vol. 205, No. 2. (18 February 2008), pp. 395-408.

From the inoculation of Plasmodium sporozoites via Anopheles mosquito bites to the development of blood-stage parasites, a hallmark of the host response is an inflammatory reaction characterized by elevated histamine levels in the serum and tissues. Given the proinflammatory and immunosuppressive activities associated with histamine, we postulated that this vasoactive amine participates in malaria pathogenesis. Combined genetic and pharmacologic approaches demonstrated that histamine binding to H1R and H2R but not H3R and H4R increases the susceptibility of mice to infection with Plasmodium. To further understand the role of histamine in malaria pathogenesis, we used histidine decarboxylase-deficient (HDC-/-) mice, which are free of histamine. HDC-/- mice were highly resistant to severe malaria whether infected by mosquito bites or via injection of infected erythrocytes. HDC-/- mice displayed resistance to two lethal strains: Plasmodium berghei (Pb) ANKA, which triggers cerebral malaria (CM), and Pb NK65, which causes death without neurological symptoms. The resistance of HDC-/- mice to CM was associated with preserved blood-brain barrier integrity, the absence of infected erythrocyte aggregation in the brain vessels, and a lack of sequestration of CD4 and CD8 T cells. We demonstrate that histamine-mediated signaling contributes to malaria pathogenesis. Understanding the basis for these biological effects of histamine during infection may lead to novel therapeutic strategies to alleviate the severity of malaria. 10.1084/jem.20071548
Walid Beghdadi, Adeline Porcherie, Bradley Schneider, David Dubayle, Roger Peronet, Michel Huerre, Takeshi Watanabe, Hiroshi Ohtsu, Jacques Louis, Salaheddine Mecheri

Infect. Immun., Vol. 55, No. 4. (1 April 1987), pp. 877-881.

The multiplication of two strains of Plasmodium falciparum in culture, as measured by [3H]hypoxanthine incorporation, was inhibited in a dose-dependent manner by granule proteins secreted by purified eosinophils obtained from patients with the hypereosinophilic syndrome. Morphological examination revealed the presence of abnormal parasites inside erythrocytes, indicating that they were killed in situ, and the later stages of the developmental cycle were found to be most susceptible to these toxic effects. A monoclonal antibody against eosinophil cationic protein partially blocked the inhibitory effect, suggesting that it was caused by more than one of the eosinophil granule proteins. Thus some of the antimalarial effects of molecules such as the tumor necrosis factor, which activates eosinophils, may be mediated through the enhanced production of eosinophil secretion products.
LS Waters, J Taverne, PC Tai, CJ Spry, GA Targett, JH Playfair

Transactions of the Royal Society of Tropical Medicine and Hygiene, Vol. 70, No. 1. (1976), pp. 36-38.

Histamine, serum complement factor 3 (C'3) and platelets were studied in 33 Plasmodium falciparum patients. Elevation of histamine level in the blood was found during acute infection but was more marked in the group of patients with systemic complications. A correlation between histamine changes, clinical complications, reduction of C'3 and degree of thrombocytopenia was observed. The possible role of histamine in the development of complications in P. falciparum infection and the possible release of this substance through the activation of complement system and immune destruction of platelets were discussed.
T Srichaikul, N Archararit, T Siriasawakul, T Viriyapanich

Proceedings of the National Academy of Sciences of the United States of America, Vol. 102, No. 32. (9 August 2005), pp. 11468-11473.

10.1073/pnas.0503386102 Sequestration of malaria-parasite-infected erythrocytes in the microvasculature of organs is thought to be a significant cause of pathology. Cerebral malaria (CM) is a major complication of infections, and PfEMP1-mediated sequestration of infected red blood cells has been considered to be the major feature leading to CM-related pathology. We report a system for the real-time imaging of sequestration using transgenic luciferase-expressing parasites of the rodent malaria parasite These studies revealed that: () as expected, lung tissue is a major site, but, unexpectedly, adipose tissue contributes significantly to sequestration, and () the class II scavenger-receptor CD36 to which PfEMP1 can bind is also the major receptor for sequestration, indicating a role for alternative parasite ligands, because orthologues of PfEMP1 are absent from rodent malaria parasites, and, importantly, () cerebral complications still develop in the absence of CD36-mediated sequestration, dissociating parasite sequestration from CM-associated pathology. Real-time imaging of parasitic processes may be used to evaluate the molecular basis of pathology and develop strategies to prevent pathology.
Blandine Franke-Fayard, Chris Janse, Margarida Cunha-Rodrigues, Jai Ramesar, Philippe Büscher, Ivo Que, Clemens Löwik, Peter Voshol, Marion den Boer, Sjoerd van Duinen, Maria Febbraio, Maria Mota, Andrew Waters

Science, Vol. 323, No. 5915. (6 February 2009), pp. 797-800.

Platelets play a critical role in the pathogenesis of malarial infections by encouraging the sequestration of infected red blood cells within the cerebral vasculature. But platelets also have well-established roles in innate protection against microbial infections. We found that purified human platelets killed Plasmodium falciparum parasites cultured in red blood cells. Inhibition of platelet function by aspirin and other platelet inhibitors abrogated the lethal effect human platelets exert on P. falciparum parasites. Likewise, platelet-deficient and aspirin-treated mice were more susceptible to death during erythrocytic infection with Plasmodium chabaudi. Both mouse and human platelets bind malarial-infected red cells and kill the parasite within. These results indicate a protective function for platelets in the early stages of erythrocytic infection distinct from their role in cerebral malaria. 10.1126/science.1166296
Brendan McMorran, Vikki Marshall, Carolyn de Graaf, Karen Drysdale, Meriam Shabbar, Gordon Smyth, Jason Corbin, Warren Alexander, Simon Foote

Blood, Vol. 115, No. 7. (18 February 2010), pp. 1317-1318.

In this issue of Blood, Bridges and colleagues report that Plasmodium falciparum-infected erythrocytes are able to attach to the endothelial surface by binding platelet-decorated VWF strands in a CD36-dependent fashion, revealing a new mechanism for erythrocyte cytoadherence in malaria.1 10.1182/blood-2009-12-254060
Jose Lopez

Blood, Vol. 115, No. 7. (18 February 2010), pp. 1472-1474.

During Plasmodium falciparum malaria infections, von Willebrand factor (VWF) levels are elevated, postmortem studies show platelets colocalized with sequestered infected erythrocytes (IEs) at brain microvascular sites, whereas in vitro studies have demonstrated platelet-mediated IE adhesion to tumor necrosis factor-activated brain endothelium via a bridging mechanism. This current study demonstrates how all these observations could be linked through a completely novel mechanism whereby IEs adhere via platelet decorated ultra-large VWF strings on activated endothelium. Using an in vitro laminar flow model, we have demonstrated tethering and firm adhesion of IEs to the endothelium specifically at sites of platelet accumulation. We also show that an IE pro-adhesive state, capable of supporting high levels of binding within minutes of induction, can be removed through the action of the VWF protease ADAMTS-13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13). We propose that this new mechanism contributes to sequestration both independently of and in concert with current adhesion mechanisms. 10.1182/blood-2009-07-235150
Daniel Bridges, James Bunn, Jan van Mourik, Georges Grau, Roger Preston, Malcolm Molyneux, Valery Combes, James O'Donnell, Bas de Laat, Alister Craig

The American journal of pathology, Vol. 152, No. 6. (June 1998), pp. 1477-1487.

Fatal Plasmodium falciparum malaria is accompanied by systemic endothelial activation. To study endothelial activation directly during malaria and sepsis in vivo, the expression of cell adhesion molecules on dermal microvascular endothelium was examined in skin biopsies and correlated with plasma levels of soluble (circulating) ICAM-1, E-selectin, and VCAM-1 and the cytokine tumor necrosis factor (TNF)-alpha. Skin biopsies were obtained from 61 cases of severe malaria, 42 cases of uncomplicated malaria, 10 cases of severe systemic sepsis, and 17 uninfected controls. Systemic endothelial activation, represented by the up-regulation of inducible cell adhesion molecules (CAMs) on endothelium and increased levels of soluble CAMs (sCAMs), were seen in both severe and uncomplicated malaria and sepsis when compared with uninfected controls. Plasma levels of sICAM-1, sVCAM-1, and sE-selectin correlated positively with the severity of malaria whereas TNF-alpha was raised nonspecifically in malaria and sepsis. Immunohistochemical evidence of endothelial activation in skin biopsies did not correlate with sCAM levels or disease severity. This indicates a background of systemic endothelial activation, which occurs in both mild and severe malaria and sepsis. The levels of sCAMs in malaria are thus not an accurate reflection of endothelial cell expression of CAMs in a particular vascular bed, and other factors must influence their levels during disease.
GD Turner, VC Ly, TH Nguyen, TH Tran, HP Nguyen, D Bethell, S Wyllie, K Louwrier, SB Fox, KC Gatter, NP Day, TH Tran, NJ White, AR Berendt

Parasitology Research, Vol. 102, No. 4. (1 March 2008), pp. 571-576.

Abstract  Falciparum malaria infection influences blood coagulation by various interacting pathobiological mechanisms, the most important being the overwhelming response of the host to sepsis resulting in a cytokine storm. In addition, the parasite infects the red cells leading to changes in the red cell phospholipid composition which supports blood coagulation. Red cells infected with Plasmodium falciparum also adhere to deeper tissue capillary endothelium leading to profound damage to endothelial cells leading to further activation. This results in widespread consumption of platelets and activation of blood coagulation which at times culminates in a clinically and pathologically detectable disseminated intravascular coagulation (DIC). Monocyte–macrophage system also gets activated in this infection compounding the hypercoagulable state. Heavy parasitaemia leading to occlusion of hepatic microcirculation leads to abnormalities in synthesis and secretion of coagulation factors and their inhibitors. Drugs used in the treatment for falciparum malaria can cause thrombocytopaenia, bone marrow suppression and haemolytic anaemia, all of which can interfere indirectly with blood coagulation. Microparticle formation from platelets, red cells and macrophages also causes widespread activation of blood coagulation, and this recently observed mechanism is the focus of intense research in many other inflammatory and neoplastic conditions where there is activation of blood coagulation system. Thus, in severe falciparum malaria, there is activation of blood coagulation system along with thrombocytopaenia, even before widespread DIC and coagulation failure occur.
Kanjaksha Ghosh, Shrimati Shetty

Microcirculation, Vol. 15, No. 2. (1 January 2008), pp. 81-107.

doi: 10.1080/10739680701451516 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. [Supplementary materials are available for this article. Go to the publisher's online edition of Microcirculation to access this free supplemental resource]
Ivo Francischetti, Karl Seydel, Robson Monteiro

Molecular and Biochemical Parasitology, Vol. 166, No. 2. (August 2009), pp. 99-108.

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.
Christopher Moxon, Robert Heyderman, Samuel Wassmer

Trends in Parasitology, Vol. 24, No. 6. (6 June 2008), pp. 258-263.

Plasmodium falciparum infection is often associated with a procoagulant state. Recent identification of tissue factor in the brain endothelium of patients who have died from cerebral malaria casts new light on our understanding of the coagulation disorder found in P. falciparum infection. It has also been revealed that parasitized red blood cells support the assembly of multimolecular coagulation complexes. Tissue factor expression by the endothelium and amplification of the coagulation cascade by parasitized red blood cells and/or activated platelets (particularly at sequestration sites) have crucial roles in mounting and sustaining a coagulation–inflammation cycle which contributes to organ dysfunction and coma in falciparum malaria.
Ivo Francischetti

Infect. Immun., Vol. 68, No. 7. (1 July 2000), pp. 4135-4144.

Adherence of erythrocytes infected with Plasmodium falciparum to microvascular endothelial cells (sequestration) is considered to play an important role in parasite virulence and pathogenesis. However, the real importance of sequestration for infection and disease has never been fully assessed. The absence of an appropriate in vivo model for sequestration has been a major barrier. We have examined the rodent malaria parasite Plasmodium chabaudi chabaudi AS in mice as a potential model. Erythrocytes infected with this parasite adhere in vitro to purified CD36, a critical endothelium receptor for binding P. falciparum-infected erythrocytes. P. c. chabaudi-infected erythrocytes adhere in vitro to endothelial cells in a gamma interferon-dependent manner, suggesting the involvement of additional adhesion molecules in the binding process, as is also the case with P. falciparum-infected cells. Furthermore, plasma or sera from infected and hyperimmune mice, respectively, have the ability to block binding of infected erythrocytes to endothelial cells. In vivo, erythrocytes containing mature P. c. chabaudi parasites are sequestered from the peripheral circulation. Sequestration is organ specific, occurring primarily in the liver, although intimate contact between infected erythrocytes and endothelial cells is also observed in the spleen and brain. The results are discussed in the context of the use of this model to study (i) the relationship between endothelial cell activation and the level of sequestration and (ii) the primary function of sequestration in malaria infection. 10.1128/IAI.68.7.4135-4144.2000
Maria Mota, William Jarra, Elizabeth Hirst, Pradeep Patnaik, Anthony Holder