Evidence of blood-brain barrier dysfunction in human cerebral malaria

Pathogenesis, clinical features, and neurological outcome of cerebral malaria

Cerebral malaria is the most severe neurological complication of Plasmodium falciparum malaria. Even though this type of malaria is most common in children living in sub-Saharan Africa, it should be considered in anybody with impaired consciousness that has recently travelled in a malaria-endemic area. Cerebral malaria has few specific features, but there are differences in clinical presentation between African children and non-immune adults. Subsequent neurological impairments are also most common and severe in children. Sequestration of infected erythrocytes within cerebral blood vessels seems to be an essential component of the pathogenesis. However, other factors such as convulsions, acidosis, or hypoglycaemia can impair consciousness. In this review, we describe the clinical features and epidemiology of cerebral malaria. We highlight recent insights provided by ex-vivo work on sequestration and examination of pathological specimens. We also summarise recent studies of persisting neurocognitive impairments in children who survive cerebral malaria and suggest areas for further research.

Phagocytosis of hemozoin enhances matrix metalloproteinase-9 activity and TNF-alpha production in human monocytes: role of matrix metalloproteinases in the pathogenesis of falciparum malaria

Matrix metalloproteinase-9 (MMP-9), secreted by activated monocytes, degrades matrix proteins, disrupts basal lamina, and activates TNF-alpha from its precursors. In turn, TNF-alpha enhances synthesis of MMP-9 in monocytes. We show here that trophozoite-parasitized RBCs/hemozoin-fed adherent human monocytes displayed increased MMP-9 activity and protein/mRNA expression, produced TNF-alpha time-dependently, and showed higher matrix invasion ability. MMP-9 activation was specific for trophozoite/hemozoin-fed monocytes, was dependent on TNF-alpha production, and abrogated by anti-TNF-alpha Ab and by a specific inhibitor of MMP-9/MMP-13 activity. Hemozoin-induced enhancement of MMP-9 and TNF-alpha production would have a 2-fold effect: to start and feed a cyclic reinforcement loop in which hemozoin enhances production of TNF-alpha, which in turn induces both activation of MMP-9 and shedding of TNF-alpha into the extracellular compartment; and, second, to disrupt the basal lamina of endothelia. Excess production of TNF-alpha and disruption of the basal lamina with extravasation of blood cells into perivascular tissues are hallmarks of severe malaria. Pharmacological inhibition of MMP-9 may offer a new chance to control pathogenic mechanisms in malaria.

Inflammation and brain edema: new insights into the role of chemokines and their receptors

Brain edema is associated with a variety of neuropathological conditions such as brain trauma, ischemic and hypoxic brain injury, central nervous system infection, acute attacks of multiple sclerosis, and brain tumors. A common finding is an inflammatory response, which may have a significant impact on brain edema formation. One critical event in the development of brain edema is blood-brain barrier (BBB) breakdown, which may be initiated and regulated by several proinflammatory mediators (oxidative mediators, adhesion molecules, cytokines, chemokines). These mediators not only regulate the magnitude of leukocyte extravasation into brain parenchyma, but also act directly on brain endothelial cells causing the loosening of junction complexes between endothelial cells, increasing brain endothelial barrier permeability, and causing vasogenic edema. Here we review junction structure at the BBB, the effects of pro-inflammatory mediators on that structure, and focus on the effects of chemokines at the BBB. New evidence indicates that chemokines (chemoattractant cytokines) do not merely direct leukocytes to areas of injury. They also have direct and indirect effects on the BBB leading to BBB disruption, facilitating entry of leukocytes into brain, and inducing vasogenic brain edema formation. Chemokine inhibition may be a new therapeutic target to reduce vasogenic brain edema.

In vitro study of malaria parasite induced disruption of blood–brain barrier

The mechanism of blood-brain barrier breakdown in the complex pathogenesis of cerebral malaria is not well understood. In this study, primary cultures of porcine brain capillary endothelial cells (PBCEC) were used as in vitro model. Membrane-associated malaria antigens obtained from lysed Plasmodium falciparum schizont-infected erythrocytes stimulated human peripheral blood mononuclear cells (PBMC) to secrete tumor necrosis factor alpha. In co-cultivation with the brain endothelial cell model, the malaria-activated PBMC stimulated the expression of E-selectin and ICAM-1 on the PBCEC. Using electric cell-substrate impedance sensing, we detected a significant decrease of endothelial barrier function within 4h of incubation with the malaria-activated PBMC. Correspondingly, immunocytochemical studies showed the disruption of tight junctional complexes. Combination of biochemical and biophysical techniques provides a promising tool to study changes in the blood-brain barrier function associated with cerebral malaria. Moreover, it is shown that the porcine endothelial model is able to respond to human inflammatory cells.

Blood-brain barrier alterations in both focal and diffuse abnormalities on postmortem MRI in multiple sclerosis

Postmortem MRI-guided tissue sampling significantly enhances the yield of MS lesions in autopsy material, but so far it is unknown whether abnormalities concur with blood-brain barrier alterations. Here we sampled MS lesions with focal and diffuse abnormalities (diffusely abnormal white matter; DAWM) on MRI; both were coupled to the presence of MS lesions upon neuropathological examination. Extravascular distribution of fibrinogen, indicating BBB disturbance, was observed in so-called (p)reactive lesions that reflect discrete areas of microglial activation without demyelination within an otherwise normal appearing white matter. Leakage became more extensive in active demyelinating MS lesions to chronic inactive lesions. An enlargement of the perivascular (Virchow-Robin) space containing infiltrated leukocytes was associated with both DAWM and focal abnormalities on postmortem MRI. This study shows for the first time that in MS brain changes in the vasculature take place not only in focal lesions but also in DAWM as detected by postmortem MRI.

Monocyte chemoattractant protein-1 regulation of blood-brain barrier permeability

The present study was designed to elucidate the effects of the chemokine monocyte chemoattractant protein (MCP-1) on blood-brain barrier (BBB) permeability. Experiments were conducted under in vitro conditions (coculture of brain endothelial cells and astrocytes) to study the cellular effects of MCP-1 and under in vivo conditions (intracerebral and intracerebroventricular administration of MCP-1) to study the potential contribution of MCP-1 to BBB disruption in vivo. Our results showed that MCP-1 induces a significant increase in the BBB permeability surface area product for fluorescein isothiocyanate (FITC)-albumin under in vivo conditions, particularly during prolonged (3 or 7 days) exposure (0.096+/-0.008 versus 0.031+/-0.005 microL/g min in controls at 3 days, P<0.001). Monocyte chemoattractant protein-1 also enhanced (17-fold compared with control) the permeability of the in vitro BBB (coculture) model. At the cellular level, MCP-1 causes alteration of tight junction (TJ) proteins in endothelial cells (redistribution of TJ proteins determined by Western blotting and loss of immunostaining for occludin, claudin-5, ZO-1, ZO-2). Monocyte chemoattractant protein-1-induced alterations in BBB permeability are mostly realized through the CCR2 receptor. Absence of CCR2 diminishes any effect of MCP-1 on BBB permeability in vitro and in vivo. The permeability surface area product for FITC-albumin after 3 days exposure to MCP-1 was 0.096+/-0.006 and 0.032+/-0.007 microL/g min, in CCR2+/+ and CCR2-/- mice, respectively (P<0.001). Monocytes/macrophages also participate in MCP-1-induced alterations in BBB permeability in vivo. Monocytes/macrophages depletion (by clodronate liposomes) reduced the effect of MCP-1 on BBB permeability in vivo approximately 2 fold. Our results suggest that, besides its main function of recruiting leukocytes at sites of inflammation, MCP-1 also plays a role in 'opening' the BBB.

Molecular targets in radiation-induced blood-brain barrier disruption

Disruption of the blood-brain barrier (BBB) is a key feature of radiation injury to the central nervous system. Studies suggest that endothelial cell apoptosis, gene expression changes, and alteration of the microenvironment are important in initiation and progression of injury. Although substantial effort has been directed at understanding the impact of radiation on endothelial cells and oligodendrocytes, growing evidence suggests that other cell types, including astrocytes, are important in responses that include induced gene expression and microenvironmental changes. Endothelial apoptosis is important in early BBB disruption. Hypoxia and oxidative stress in the later period that precedes tissue damage might lead to astrocytic responses that impact cell survival and cell interactions. Cell death, gene expression changes, and a toxic microenvironment can be viewed as interacting elements in a model of radiation-induced disruption of the BBB. These processes implicate particular genes and proteins as targets in potential strategies for neuroprotection.

Review Article: Blood-brain barrier in falciparum malaria*

Plasmodium falciparum malaria is the most important parasitic disease infecting the central nervous system of humans worldwide. The pathogenesis of the neurological complications of falciparum malaria remains unclear. In particular, how do asexual parasites confined to the vascular space of the brain cause neuronal impairment? The evidence for a breakdown in the blood-brain barrier (BBB) is conflicting. In some animal models of malaria, there is evidence of breakdown of the BBB, but the data from humans suggests the BBB is mildly impaired only, with few morphological changes. Whether these changes in the BBB are sufficient to account for the neurological complications remains to be determined.

Randomized Trial of Volume Expansion with Albumin or Saline in Children with Severe Malaria: Preliminary Evidence of Albumin Benefit

BACKGROUND

Metabolic acidosis is the best predictor of death in children with severe falciparum malaria; however, its treatment presents a therapeutic dilemma, because acidosis and hypovolemia may coexist with coma, which can be associated with elevated intracranial pressure. We postulated that volume resuscitation with albumin might correct acidosis and hypovolemia with a lower risk of precipitating cerebral edema than crystalloid. In an open-label, randomized, controlled trial, we compared the safety of resuscitation with albumin to saline in Kenyan children with severe malaria.

METHODS

We randomly assigned children with severe malaria and metabolic acidosis (base deficit, >8 mmol/L) to receive fluid resuscitation with either 4.5% albumin or normal saline. A control (maintenance only) group was only included for patients with a base deficit of <15 mmol/L. The primary outcome measure was the percentage reduction in base deficit at 8 h. Secondary end points included death, the requirement for rescue therapies, and neurological sequelae in survivors.

RESULTS

Of 150 children recruited for the trial, 61 received saline, 56 received albumin, and 33 served as control subjects. There was no significant difference in the resolution of acidosis between the groups; however, the mortality rate was significantly lower among patients who received albumin (3.6% [2 of 56 patients]) than among those who received saline (18% [11 of 61]; relative risk, 5.5; 95% confidence interval, 1.2-24.8; P=.013).

CONCLUSIONS

In high-risk children with severe malaria and acidosis, fluid resuscitation with albumin may reduce mortality. Our study design did not enable us to determine whether saline administration is preferable to fluid restriction or whether saline administration is actually hazardous. Further studies are needed to confirm our findings before definitive treatment recommendations can be made.

Induction of the CD23/nitric oxide pathway in endothelial cells downregulates ICAM-1 expression and decreases cytoadherence of Plasmodium falciparum-infected erythrocytes

Cytoadherence of parasitized red blood cells (PRBCs) to postcapillary venules and cytokine production are clearly involved in the pathogenesis of cerebral malaria. Nitric oxide and TNF-alpha have been proposed as major effector molecules both in protective and physiopathological processes during malaria infections. Nitric oxide production has been shown to be induced by engagement of CD23 antigen. This study aimed to investigate the potential role of the CD23/nitric oxide pathway in the control of the cytoadherence of PRBCs on human endothelial cells. We demonstrate that normal human lung endothelial cells (HLECs) are able to express the low affinity receptor for IgE (Fc in RII/CD23), following cell incubation with interleukin 4 or PRBCs. Ligation of the CD23 antigen by a specific anti-CD23 monoclonal antibody at the cell surface of HLECs was found to induce iNOS mRNA and protein expression, NO release and P. falciparum killing. In addition, the specific CD23-engagement on these cells also induced a significant decrease in ICAM-1 expression, an adhesion molecule implicated in PRBCs cytoadherence. These data not only described for the first time the expression of a CD23 antigen at the cell surface of endothelial cells but also suggest a possible new regulatory mechanisms via the CD23/NO pathway during malaria infection.

Comparative study of brain CD8+ T cells induced by sporozoites and those induced by blood-stage Plasmodium berghei ANKA involved in the development of cerebral malaria

To obtain insight into the mechanisms that contribute to the pathogenesis of Plasmodium infections, we developed an improved rodent model that mimics human malaria closely by inducing cerebral malaria (CM) through sporozoite infection. We used this model to carry out a detailed study on isolated T cells recruited from the brains of mice during the development of CM. We compared several aspects of the immune response related to the experimental model of Plasmodium berghei ANKA infection induced by sporozoites in C57BL/6 mice and those related to a blood-stage infection. Our data show that in both models, oligoclonal TCRVbeta4(+), TCRVbeta6(+), TCRVbeta8.1(+), and TCRVbeta11(+) major histocompatibility complex class I-restricted CD8 T cells were present in the brains of CM(+) mice. These CD8(+) T cells display an activated phenotype, do not undergo apoptosis, secrete gamma interferon or tumor necrosis factor alpha, and are associated with the development of the neurological syndrome.

Molecular aspects of malaria pathogenesis

Plasmodium falciparum being the most lethal plasmodiae is still a major cause of the disease burden and mortality in malaria endemic areas. Due to the wide spread drug resistance in combination with poor socio-economic situation in the vast majority of the endemic countries, malaria is today a great global challenge. The scientific community is, however, progressing. The 23 Mb genome of P. falciparum has been decoded and publicly available. Data of transcriptional profiling at certain developmental stages have already been generated. More than 50% of P. falciparum genes are transcribed constitutively in all the developmental stages of parasite life cycle. Functional disruption of these genes might have implications for parasite growth and development. Available microarray data indicate that P. falciparum preferentially expresses rif and stevor gene families at gametocyte and sporozoite stages while var genes are predominantly expressed at the erythrocytic stage. Gene regulation mechanisms of the variant gene families in P. falciparum are still not understood though some regulatory elements have been proposed. The occurrence of severe malaria is determined by both parasite and human host factors. Sequestration and antigenic variation are two of the evasion mechanisms utilized by P. falciparum in order to escape the human host defences. Understanding the molecular mechanisms underlying these phenomena is of a major importance and interest in malaria research. Here, we summarize and highlight the recent progress in molecular aspects of severe malaria.

Mice lacking insulin or insulin-like growth factor 1 receptors in vascular endothelial cells maintain normal blood–brain barrier

The blood-brain barrier (BBB) is created by a combination of endothelial cells with tight junctions and astrocytes. One of the key tight junction proteins, zona occludens-1 (ZO-1), has been reported to be stimulated in its expression by insulin and IGF-1. To assess the role of insulin and IGF-1 in endothelial cells in the BBB we have utilized mice with a vascular endothelial cell-specific knockout of the insulin receptor (VENIRKO) and IGF-1 receptor (VENIFARKO). Both of these mice show a normal BBB based on no increase in leakage of Evans blue dye in the brain of these mice basally or after cold injury. Furthermore, the structural integrity of the BBB and blood-retinal barrier (BRB) was intact using the vascular markers lectin B-4 and ZO-1, and both proteins were properly co-localized in both brain and retinal vascular tissue of these mice. These observations indicate that neither insulin nor IGF-1 signaling in vascular endothelial cells is required for development and maintenance of BBB or BRB.

Malaria

PURPOSE OF REVIEW

This review addresses recent developments that relate to the pathogenesis of severe malaria and its treatment, and also highlights the increase in the global burden of malaria and provides a summary of clinical trials of malaria vaccines.

RECENT FINDINGS

Malaria, one of the world's most important parasitic infections, is on the increase globally. This has resulted in an increase in the morbidity and mortality from malaria in endemic areas, a resurgence in areas where it was previous eradicated, and an increase in imported malaria in Europe and North America. Mortality from severe malaria continues to be high, even when effective drugs are available, because most deaths occur within hours of admission to hospital. In severe malaria, the presence of acidosis is the most important prognostic factor in children and adults. A number of therapies have resulted in clinical improvements and the correction of acidosis in phase I and II studies, but larger trials are required to examine the effect on mortality. More malaria vaccines are now in phase I or II trials; however, available data do not yet promise an imminent impact on malaria control.

SUMMARY

Recent developments include a better understanding of the pathogenesis of severe malaria, and have given rise to a number of novel therapeutic strategies that should be examined in larger phase III trials. Similarly, there has been considerable progress in the field of vaccine development.

Angiogenic proteins in brains of patients who died with cerebral malaria

In cerebral malaria (CM), microvascular activation accompanies blood-brain barrier dysfunction which in turn represents the pathophysiological basis of neurological impairments in affected patients. To dissect the molecular basis of this process, we analyzed localization of proangiogenic vascular endothelial growth factor (VEGF), its receptor vascular endothelial growth factor receptor-1 (VEGFR-1, Flt-1), of downstream VEGF effectors matrix-metalloproteinase-1 (MMP-1) and connective tissue growth factor (CTGF), and of VEGF-interacting antiangiogenic thrombospondin-1 and -independent angiostatin in brains of patients who died with CM and controls by immunohistochemistry and Western blotting experiments. Most prominently, we detected more VEGF(+) astrocytes in CM patients and deposition of Flt-1 in Dürck's granulomas. MMP-1 and thrombospondin-1 accumulated in macrophages/microglial cells in Dürck's granulomas. In one CM patient, massive amounts of CTGF were detected as perivascular paracellular deposits. Angiostatin was observed in the serum of 2/7 control but in no CM patients. These data demonstrate the activation of the proangiogenic VEGF signaling cascade in patients with CM, probably reflecting compensatory mechanisms of general and focal brain hypoxia observed in these patients.

The pathophysiology of falciparum malaria

Falciparum malaria is a complex disease with no simple explanation, affecting organs where the parasite is rare as well as those organs where it is more common. We continue to argue that it can best be understood in terms of excessive stimulation of normally useful pathways mediated by inflammatory cytokines, the prototype being tumor necrosis factor (TNF). These pathways involve downstream mediators, such as nitric oxide (NO) that the host normally uses to control parasites, but which, when uncontrolled, have bioenergetic failure of patient tissues as their predictable end point. Falciparum malaria is no different from many other infectious diseases that are clinically confused with it. The sequestration of parasitized red blood cells, prominent in some tissues but absent in others with equal functional loss, exacerbates, but does not change, these overriding principles. Recent opportunities to stain a wide range of tissues from African pediatric cases of falciparum malaria and sepsis for the inducible NO synthase (iNOS) and migration inhibitory factor (MIF) have strengthened these arguments considerably. The recent demonstration of bioenergetic failure in tissue removed from sepsis patients being able to predict a fatal outcome fulfils a prediction of these principles, and it is plausible that this will be demonstrable in severe falciparum malaria. Understanding the disease caused by falciparum malaria at a molecular level requires an appreciation of the universality of poly(ADP-ribose) polymerase-1 (PARP-1) and Na(+)/K(+)-ATPase and the protean effects of activation by inflammation of the former that include inactivation of the latter.

Pharmacokinetics and clinical effects of phenytoin and fosphenytoin in children with severe malaria and status epilepticus

AIMS

Status epilepticus is common in children with severe falciparum malaria and is associated with poor outcome. Phenytoin is often used to control status epilepticus, but its water-soluble prodrug, fosphenytoin, may be more useful as it is easier to administer. We studied the pharmacokinetics and clinical effects of phenytoin and fosphenytoin sodium in children with severe falciparum malaria and status epilepticus.

METHODS

Children received intravenous (i.v.) phenytoin as a 18 mg kg-1 loading dose infused over 20 min followed by a 2.5 mg x kg(-1) 12 hourly maintenance dose infused over 5 min (n = 11), or i.v. fosphenytoin, administered at a rate of 50 mg x min(-1) phenytoin sodium equivalents (PE; n = 16), or intramuscular (i.m.) fosphenytoin as a 18 mg x kg(-1) loading dose followed by 2.5 mg x kg(-1) 12 hourly of PE (n = 11). Concentrations of phenytoin in plasma and cerebrospinal fluid (CSF), frequency of seizures, cardiovascular effects (respiratory rate, blood pressure, trancutaneous oxygen tension and level of consciousness) and middle cerebral artery (MCA) blood flow velocity were monitored.

RESULTS

After all routes of administration, a plasma unbound phenytoin concentration of more than 1 microg x ml(-1) was rapidly (within 5-20 min) attained. Mean (95% confidence interval) steady state free phenytoin concentrations were 2.1 (1.7, 2.4; i.v. phenytoin, n = 6), 1.5 (0.96, 2.1; i.v. fosphenytoin, n = 11) and 1.4 (0.5, 2.4; i.m. fosphenytoin, n = 6), and were not statistically different for the three routes of administration. Median times (range) to peak plasma phenytoin concentrations following the loading dose were 0.08 (0.08-0.17), 0.37 (0.33-0.67) and 0.38 (0.17-2.0) h for i.v. fosphenytoin, i.v. phenytoin and i.m. fosphenytoin, respectively. CSF: plasma phenytoin concentration ratio ranged from 0.12 to 0.53 (median = 0.28, n = 16). Status epilepticus was controlled in only 36% (4/11) following i.v. phenytoin, 44% (7/16), following i.v. fosphenytoin and 64% (7/11) following i.m. fosphenytoin administration, respectively. Cardiovascular parameters and MCA blood flow were not affected by phenytoin administration.

CONCLUSIONS

Phenytoin and fosphenytoin administration at the currently recommended doses achieve plasma unbound phenytoin concentrations within the therapeutic range with few cardiovascular effects. Administration of fosphenytoin i.v. or i.m. offers a practical and convenient alternative to i.v. phenytoin. However, the inadequate control of status epilepticus despite rapid achievement of therapeutic unbound phenytoin concentrations warrants further investigation.

CCR5 deficiency decreases susceptibility to experimental cerebral malaria

Infection of susceptible mouse strains with Plasmodium berghei ANKA (PbA) is a valuable experimental model of cerebral malaria (CM). Two major pathologic features of CM are the intravascular sequestration of infected erythrocytes and leukocytes inside brain microvessels. We have recently shown that only the CD8+ T-cell subset of these brain-sequestered leukocytes is critical for progression to CM. Chemokine receptor-5 (CCR5) is an important regulator of leukocyte trafficking in the brain in response to fungal and viral infection. Therefore, we investigated whether CCR5 plays a role in the pathogenesis of experimental CM. Approximately 70% to 85% of wild-type and CCR5+/- mice infected with PbA developed CM, whereas only about 20% of PbA-infected CCR5-deficient mice exhibited the characteristic neurologic signs of CM. The brains of wild-type mice with CM showed significant increases in CCR5+ leukocytes, particularly CCR5+ CD8+ T cells, as well as increases in T-helper 1 (Th1) cytokine production. The few PbA-infected CCR5-deficient mice that developed CM exhibited a similar increase in CD8+ T cells. Significant leukocyte accumulation in the brain and Th1 cytokine production did not occur in PbA-infected CCR5-deficient mice that did not develop CM. Moreover, experiments using bone marrow (BM)-chimeric mice showed that a reduced but significant proportion of deficient mice grafted with CCR5+ BM develop CM, indicating that CCR5 expression on a radiation-resistant brain cell population is necessary for CM to occur. Taken together, these results suggest that CCR5 is an important factor in the development of experimental CM.

Loss and recovery of the blood-nerve barrier in the rat sciatic nerve after crush injury are associated with expression of intercellular junctional proteins

The blood-nerve barrier in peripheral nerves is important for maintaining the environment for axons. Breakdown of the barrier by nerve injury causes various pathologies. We hypothesized that the breakdown and recovery of the blood-nerve barrier after injury are associated with the changes in the expression of intercellular junctional proteins. To test this hypothesis, we induced crush injuries in the rat sciatic nerve by ligation and analyzed spatiotemporal changes of claudin-1, claudin-5, occludin, VE-cadherin, and connexin43 by immunoconfocal microscopy and morphometry and compared them with changes in the permeability of the blood-nerve barrier by intravenous and local administration of Evans blue-albumin (EBA). On day 1 after removal of the ligature EBA leaked into the connective tissue in the endoneurium and then the leakage gradually decreased and disappeared on day 7. On day 1 claudin-1, claudin-5, occludin, VE-cadherin, and connexin43 had totally disappeared from the perineurium and endoneurium. Thereafter, claudin-1, claudin-5, occludin, and VE-cadherin recovered from day 2, whereas connexin43 was redetected on day 5. These results indicate that the breakdown and following recovery of the blood-nerve barrier are closely associated with changes in the expression of claudins, occludin, VE-cadherin, and connexin43 and that the recovery time course is similar but nonidentical.

Cerebral malaria: optimising management

Cerebral malaria is one of the most common nontraumatic encephalopathies in the world. Children living in sub-Saharan Africa bear the brunt of the disease, but cerebral malaria is being seen increasingly in adults throughout the world, including outside malarious areas. There are differences in the clinical presentation and pathophysiology between African children and nonimmune adults from any region. Mortality is high (10-20%). Parenteral antimalarials are the only interventions that have been shown to affect outcome. The cinchona alkaloids (quinine and quinidine) are the mainstay of antimalarial treatment, but the artemisinin derivatives are increasingly being used. Aggressive treatment and prevention of convulsions may be important, particularly in children. Other ancillary treatments that can be used to augment standard antimalarial drugs, such as exchange blood transfusions, osmotic diuretics and pentoxifylline, may improve outcome but have not been subjected to rigorous clinical trials. There is little support for corticosteroids or deferoxamine (desferrioxamine) in cerebral malaria. Other adjuncts have not been adequately tested. Further research is required on drugs that interfere with the pathophysiological processes to prevent neurological complications and death.

Effect of efflux inhibition on brain uptake of itraconazole in mice infected with Cryptococcus neoformans

Itraconazole is a fungistatic agent that, although highly lipophilic, shows poor transport through the blood brain barrier that may be due to efflux proteins. The combined administration of an efflux inhibitor with itraconazole should increase cerebral itraconazole concentrations and therefore, improve the treatment of Cryptococcus neoformans meningitis with this antifungal agent. To test this hypothesis, we have studied the influence of murine cerebral infection with C. neoformans and the inhibition of efflux by intraperitoneal injection of a P-glycoprotein inhibitor, GF120918 [N-(4-[2-(1,2,3,4-tetrahydro-6,7-dimethoxy-2-isoquinolinyl)-ethyl]-phenyl)9,10-dihydro-5-methoxy-9-oxo-4-acridine carboxamide], on the pharmacokinetics of itraconazole in plasma and brain after a single intraperitoneal itraconazole injection. We also investigated the influence of efflux inhibition on the efficacy of repeated doses of itraconazole in this murine model. The results showed that in healthy and infected mice pretreated or not with GF120918, plasma itraconazole values of area under the curve (AUC) were similar. In contrast, cerebral values of AUC were higher in infected mice compared with healthy mice. Moreover, the pretreatment of infected mice with GF120918 significantly increased cerebral itraconazole values of area under the curve and decreased weight loss in the treatment with itraconazole of a cerebral infection with C. neoformans.

Quinine distribution in mice withplasmodium berghei malaria

The disposition of a single 80 mg/kg injection of quinine base was compared in control and Plasmodium berghei-infected mice. Pharmacokinetic parameters were determined on repeated whole blood samples from caudal vein (experiment 1) and quinine distribution was evaluated in tissues and blood fractions from mice sacrificed two hours post dosing (experiment 2). Quinine concentrations were assessed by high performance liquid chromatography with fluorometric detection. Whole blood concentrations and AUC(0 - infinity) of quinine increased in a parasitaemia-dependent manner. Quinine blood clearance and peak blood concentrations of metabolites negatively correlated with the parasitaemia. The apparent distribution volume of quinine only decreased in severely ill mice. Quinine concentrations rise in a parasitaemia-dependent manner in homogenates of spleen, lungs and kidney and in erythrocyte pellets. The negative relationship, observed between the parasitaemia and the tissue-to-whole blood ratio for muscle, heart, liver and brain, contributes to the reduction of the blood distribution volume. Quinine uptake by muscle and heart was dependent on the free fraction of plasma quinine. The liver and brain concentrations of quinine were similar in control and infected mice. The tissue-to-plasma free fraction ratios decrease when the parasitaemia rises suggesting a restrictive uptake of quinine by these tissues. In conclusion. P. berghei malaria decreases both total clearance and apparent volume of distribution with a heterogeneous redistribution of quinine between the tissues.

Tight junctional abnormality in multiple sclerosis white matter affects all calibres of vessel and is associated with blood-brain barrier leakage and active demyelination

Blood-brain barrier (BBB) hyperpermeability in multiple sclerosis (MS) is associated with lesion pathogenesis and has been linked to pathology in microvascular tight junctions (TJs). This study quantifies the uneven distribution of TJ pathology and its association with BBB leakage. Frozen sections from plaque and normal-appearing white matter (NAWM) in 14 cases were studied together with white matter from six neurological and five normal controls. Using single and double immunofluorescence and confocal microscopy, the TJ-associated protein zonula occludens-1 (ZO-1) was examined across lesion types and tissue categories, and in relation to fibrinogen leakage. Confocal image data sets were analysed for 2198 MS and 1062 control vessels. Significant differences in the incidence of TJ abnormalities were detected between the different lesion types in MS and between MS and control white matter. These were frequent in oil-red O (ORO)(+) active plaques, affecting 42% of vessel segments, but less frequent in ORO(-) inactive plaques (23%), NAWM (13%), and normal (3.7%) and neurological controls (8%). A similar pattern was found irrespective of the vessel size, supporting a causal role for diffusible inflammatory mediators. In both NAWM and inactive lesions, dual labelling showed that vessels with the most TJ abnormality also showed most fibrinogen leakage. This was even more pronounced in active lesions, where 41% of vessels in the highest grade for TJ alteration showed severe leakage. It is concluded that disruption of TJs in MS, affecting both paracellular and transcellular paths, contributes to BBB leakage. TJ abnormality and BBB leakage in inactive lesions suggests either failure of TJ repair or a continuing pathological process. In NAWM, it suggests either pre-lesional change or secondary damage. Clinically inapparent TJ pathology has prognostic implications and should be considered when planning disease-modifying therapy.

Accumulation of endostatin/collagenXVIII in brains of patients who died with cerebral malaria

Endostatin is a 20 kDa C-terminal fragment of collagenXVIII that, when added exogenously, inhibits angiogenesis by inducing apoptosis of endothelial cells. In cerebral malaria (CM), blood-brain barrier dysfunction is a hallmark alteration in the formation of edema, inflammation, hemorrhage and Dürck's granulomas that are thought to represent the histopathological basis of neurological impairments observed in CM patients. We now analyzed endostatin/collagenXVIII expression in brains of seven patients who died with CM and in seven control patients by immunohistochemistry double-labeling experiments. Endostatin/collagenXVIII immunoreactive macrophages/microglial cells accumulated predominantly in Dürck's granulomas. Some immunoreactivity was observed in macrophages located in cerebral capillaries with deposition of malarial pigment and sequestration, but almost no immunoreactivity was detected in ring hemorrhages. Focal accumulation of endostatin/collagenXVIII in granulomas but not in ring hemorrhages of CM brains suggests a novel process that is involved in the destruction of endothelial cells at the time of Dürck's granuloma formation.

Breaking down the blood-brain barrier: signaling a path to cerebral malaria?

Cerebral malaria is a major killer in the developing world, but we still know very little about the causes of this disease. How does Plasmodium falciparum cause such a devastating neurological disease while it is in the brain vasculature? Why do some patients die, whereas others survive? What processes contribute to disease in the brain, and can we reverse them? Here, the latest evidence from post-mortem, in vitro and animal studies is reviewed to highlight the role of blood-brain barrier breakdown in cerebral malaria. Blood-brain barrier integrity is disturbed during severe malaria, causing leakage of cerebral vessels. Understanding how this happens and how it contributes to the pathogenesis of coma may provide new opportunities for the treatment of cerebral malaria.

The adhesion molecule ICAM-1 and its regulation in relation with the blood-brain barrier

The blood-brain barrier (BBB) is formed by high resistance tight junctions within the capillary endothelium perfusing the vertebrate brain. Normal BBB maintains a unique microenvironment within the central nervous system (CNS). In neurodegenerative disorders (for example multiple sclerosis, MS), the BBB becomes impaired. Perivascular cells (astrocytes, macrophages and microglial cells) and brain microvascular endothelial cells (BMEC) produce various inflammatory factors that affect the BBB permeability and the expression of adhesion molecules. Indeed, cytokines can stimulate the expression of several adhesion molecules on brain microvascular endothelial cells. Among these adhesion molecules, the intercellular adhesion molecule-1 (ICAM-1) binds to its leukocyte ligands and allows activated leukocytes entry into the CNS. This review is dealing with the expression and regulation of ICAM-1 in relation with several properties of the BBB. Particularly, the role of ICAM-1 in the control of the leukocyte traffic into the CNS, as well as in cerebral malaria and in CNS infection by viruses, is discussed.

CD36, a class B scavenger receptor, is expressed on microglia in Alzheimer's disease brains and can mediate production of reactive oxygen species in response to beta-amyloid fibrils

A pathological hallmark of Alzheimer's disease is the senile plaque, composed of beta-amyloid fibrils, microglia, astrocytes, and dystrophic neurites. We reported previously that class A scavenger receptors mediate adhesion of microglia and macrophages to beta-amyloid fibrils and oxidized low-density lipoprotein (oxLDL)-coated surfaces. We also showed that CD36, a class B scavenger receptor and an oxLDL receptor, promotes H(2)O(2) secretion by macrophages adherent to oxLDL-coated surfaces. Whether CD36 is expressed on microglia, and whether it plays a role in secretion of H(2)O(2) by microglia interacting with fibrillar beta-amyloid is not known. Using fluorescence-activated cell sorting analysis and immunohistochemistry, we found that CD36 is expressed on human fetal microglia, and N9-immortalized mouse microglia. We also found that CD36 is expressed on microglia and on vascular endothelial cells in the brains of Alzheimer's disease patients. Bowes human melanoma cells, which normally do not express CD36, gained the ability to specifically bind to surfaces coated with fibrillar beta-amyloid when transfected with a cDNA encoding human CD36, suggesting that CD36 is a receptor for fibrillar beta-amyloid. Furthermore, two different monoclonal antibodies to CD36 inhibited H(2)O(2) production by N9 microglia and human macrophages adherent to fibrillar beta-amyloid by approximately 50%. Our data identify a role for CD36 in fibrillar beta-amyloid-induced H(2)O(2) production by microglia, and imply that CD36 can mediate binding to fibrillar beta-amyloid. We propose that similar to their role in the interaction of macrophages with oxLDL, class A scavenger receptors and CD36 play complimentary roles in the interactions of microglia with fibrillar beta-amyloid.

The dynamics of blood-brain barrier breakdown in an experimental model of glial cell degeneration

This study was undertaken to investigate the dynamics of blood-brain barrier breakdown in an in vivo rat model of selective CNS vulnerability. 1,3-Dinitrobenzene was used to induce rapid glial degeneration in highly defined areas of the brainstem. Leakage of fluorescent dextran was used to demonstrate the breakdown of the blood-brain barrier, and antibodies to glial and neuronal specific proteins to assess the accompanying cell changes. Beginning 18 h after a toxic dose of dinitrobenzene and before loss of glial ensheathment, a sub-population of blood vessels became permeable to fluorescent dextrans below 500,000 mol. wt in size. By 24h most macroglial cells had been lost from within susceptible areas and vascular leakage had reached peak levels. Macrophage invasion was detected three days following dinitrobenzene. Vessels continued to leak up to four days after the lesion was formed, but by six days blood-brain barrier integrity was largely re-established. Multiple tracer injections over time demonstrated that a single sub-population of vessels was leaking during the experimental period. From these findings we conclude that blood-brain barrier breakdown in this model system is highly selective, graded in extent and molecular weight specificity and not a direct consequence of astrocyte degeneration or microglial activation. This system could be useful in modeling human CNS pathological processes with a vascular component and for understanding in vivo glial blood-brain barrier interactions.

Central nervous system in cerebral malaria: 'Innocent bystander' or active participant in the induction of immunopathology?

Cerebral malaria (CM) is a major life-threatening complication of Plasmodium falciparum infection in humans, responsible for up to 2 million deaths annually. The mechanisms underlying the fatal cerebral complications are still not fully understood. Many theories exist on the aetiology of human CM. The sequestration hypo-thesis suggests that adherence of parasitized erythrocytes to the cerebral vasculature leads to obstruction of the microcirculation, anoxia or metabolic disturbances affecting brain function, resulting in coma. This mechanism alone seems insufficient to explain all the known features of CM. In this review we focus on another major school of thought, that CM is the result of an over-vigorous immune response originally evolved for the protection of the host. Evidence in support of this second hypothesis comes from studies in murine malaria models in which T cells, monocytes, adhesion molecules and cytokines, have been implicated in the development of the cerebral complications. Recent studies of human CM also indicate a role for the immune system in the neurological complications. However, it is likely that multiple mechanisms are involved in the induction of cerebral complications and both the presence of parasitized erythrocytes in the central nervous system (CNS) and immunopathological processes contribute to the pathogenesis of CM. Most studies examining immunopathological responses in CM have focused on reactions occurring primarily in the systemic circulation. However, these also do not fully account for the development of cerebral complications in CM. In this review we summarize results from human and mouse studies that demonstrate morphological and functional changes in the resident glial cells of the CNS. The degree of immune activation and degeneration of glial cells was shown to reflect the extent of neurological complications in murine cerebral malaria. From these results we highlight the need to consider the potentially important contribution within the CNS of glia and their secreted products, such as cytokines, in the development of human CM.

Trypanosoma brucei brucei crosses the blood-brain barrier while tight junction proteins are preserved in a rat chronic disease model

African trypanosomiasis, sleeping sickness in humans, is caused by the systemic infection of the host by the extracellular parasite, the African trypanosome. The pathogenetic mechanisms of the severe symptoms of central nervous system involvement are still not well understood. The present study examined the routes of haematogenous spread of Trypanosoma brucei brucei (Tbb) to the brain, in particular on the question whether parasites can cross the blood-brain barrier, as well as their effect on tight junction proteins. Rats were infected with Tbb and at various times post-infection, the location of the parasite in the central nervous system was examined in relation to the brain vascular endothelium, visualized with an anti-glucose transporter-1 antibody. The tight junction-specific proteins occludin and zonula occludens 1, and the possible activation of the endothelial cell adhesion molecules ICAM-1 and VCAM-1 were also studied. At 12 and 22 days post-infection, the large majority of parasites were confined within blood vessels. At this stage, however, some parasites were also clearly observed in the brain parenchyma. This was accompanied by an upregulation of ICAM-1/VCAM-1. At later stages, 42, 45 and 55 days post-infection, parasites could still be detected within or in association with blood vessels. In addition, the parasite was now frequently found in the brain parenchyma and the extravasation of parasites was more prominent in the white matter than the cerebral cortex. A marked penetration of parasites was seen in the septal nuclei. In spite of this, occludin and zonula occludens 1 staining of the vessels was preserved. The results indicate that the Tbb parasite is able to cross the blood-brain barrier in vivo, without a generalized loss of tight junction proteins.

Cerebral malaria in mice: interleukin-2 treatment induces accumulation of gammadelta T cells in the brain and alters resistant mice to susceptible-like phenotype

In this study, we report that infection with Plasmodium yoelii 17XL, a lethal strain of rodent malaria, does not result in death in the DBA/2 strain of mice. In contrast to BALB/c mice, DBA/2 mice developed significantly less parasitemia and never manifested symptoms of cerebral malaria (CM) on infection with this parasite. Moreover, the histological changes evident in the brain of susceptible BALB/c were absent in DBA/2 mice. Interestingly, the resistant DBA/2 mice when treated with recombinant interleukin (IL)-2, were found to develop CM symptoms and the infection became fatal by 6 to 8 days after infection. This condition was associated with an augmented interferon-gamma and nitric oxide production. Unexpectedly, IL-10 levels were also elevated in IL-2-treated DBA/2 mice during late stage of infection (at day 6 of infection) whereas the inverse relationship between IL-10 and interferon-gamma or nitric oxide was maintained in the early stage of infection (at day 3 after infection). The level of tumor necrosis factor-alpha production was moderately increased in the late phase of infection in these mice. Histology of brain from IL-2-treated mice demonstrated the presence of parasitized erythrocytes and infiltration of lymphocytes in cerebral vessels, and also displayed some signs of endothelial degeneration. Confocal microscopical studies demonstrated preferential accumulation of gammadelta T cells in the cerebral vessels of IL-2-treated and -infected mice but not in mice treated with IL-2 alone. The cells recruited in the brain were activated because they demonstrated expression of CD25 (IL-2R) and CD54 (intercellular adhesion molecule 1) molecules. Administration of anti-gammadelta mAb prevented development of CM in IL-2-treated mice until day 18 after infection whereas mice treated with control antibody showed CM symptoms by day 6 after infection. The information concerning creating pathological sequelae and death in an otherwise resistant mouse strain provides an interesting focus for the burden of pathological attributes on death in an infectious disease.

Lesion associated expression of urokinase-type plasminogen activator receptor (uPAR, CD87) in human cerebral malaria

Blood-brain barrier disintegration and inflammatory cell recruitment are key processes in the pathogenesis of cerebral malaria (CM). Recent data provide convincing evidence that the serine protease urokinase-type plasminogen activator receptor (uPAR) is a key molecule in promoting cell adhesion and spreading. We have now analyzed expression of urokinase-type plasminogen activator receptor (uPAR, CD87), which is part of a cell surface associated proteolytic system, in brains of eight CM patients and seven neuropathologically unaltered and diseased controls by immunohistochemistry. Double labeling experiments with antibodies directed against CD68 (macrophages/microglial cells), myeloid-related protein (MRP8), and glial fibrillary acid protein (GFAP) confirmed the nature of uPAR expressing cells. We observed focal accumulation of uPAR expressing macrophages/microglial cells in Dürck's granulomas and adjacent to petechial hemorrhages, in astrocytes, and in endothelial cells. In contrast, focal uPAR expression in macrophages/microglial cells but not in astrocytes was found in microglial nodules of toxoplasmic encephalitis and in the cellular infiltrate of bacterial meningitis. Normal brains showed only faint uPAR expression in endothelial cells. We conclude from these data that lesion-associated uPAR expression at least in part contributes to blood-brain barrier alteration and immunologic dysfunction in CM patients.

Cerebral malaria

Cerebral malaria may be the most common non-traumatic encephalopathy in the world. The pathogenesis is heterogeneous and the neurological complications are often part of a multisystem dysfunction. The clinical presentation and pathophysiology differs between adults and children. Recent studies have elucidated the molecular mechanisms of pathogenesis and raised possible interventions. Antimalarial drugs, however, remain the only intervention that unequivocally affects outcome, although increasing resistance to the established antimalarial drugs is of grave concern. Artemisinin derivatives have made an impact on treatment, but other drugs may be required. With appropriate antimalarial drugs, the prognosis of cerebral malaria often depends on the management of other complications-for example, renal failure and acidosis. Neurological sequelae are increasingly recognised, but further research on the pathogenesis of coma and neurological damage is required to develop other ancillary treatments.

Blood-brain barrier function in cerebral malaria and CNS infections in Vietnam

BACKGROUND

The intraerythrocytic parasite Plasmodium falciparum induces the life-threatening neurologic syndrome of cerebral malaria (CM) from within cerebral blood vessels, without entering the brain parenchyma.

OBJECTIVES

1) To assess the use of CSF as an indicator of specific pathologic processes occurring in the brain during CM; 2) to compare this with other neurologic and infectious diseases to understand the distinct pathogenic features of CM; 3) to test the hypothesis that CM involves a specific functional breakdown of the blood-brain barrier (BBB).

METHODS

1) Radial immunodiffusion assays to detect albumin and IgG in matched plasma and CSF samples as indicators of BBB integrity and intrathecal IgG production; and 2) ELISA for soluble intracellular adhesion molecule-1 and sE-selectin, the cytokines tumor necrosis factor-alpha and transforming growth factor-beta1, and the matrix metalloproteinase MMP-9, to detect cellular activation and inflammatory responses within the brain.

RESULTS

Albumin and IgG indices implied only minimal degree of BBB breakdown in a few cases of CM, with most remaining within the normal range. In contrast, cryptococcal, tubercular, and acute bacterial meningitis produced detectable changes in the composition of the CSF and evidence of BBB breakdown.

CONCLUSIONS

CM appears to involve only subtle functional changes in BBB integrity with minimal intraparenchymal inflammatory responses compared with other neurologic infections. This focuses attention on local events within and around the cerebral microvasculature in CM, rather than indicating widespread parenchymal disease.

Reactive changes of retinal microglia during fatal murine cerebral malaria: effects of dexamethasone and experimental permeabilization of the blood-brain barrier

Microglial activation and redistribution toward blood vessels are some of the earliest observable events occurring within the central nervous system (CNS) during fatal murine cerebral malaria (FMCM). To investigate stimuli that might modulate microglial reactivity during FMCM we have performed two experimental manipulations and observed microglial responses in retinal whole mounts. First, to determine whether increased blood-brain barrier (BBB) permeability in the absence of the malaria parasite initiates the microglial changes, BBB function was compromised experimentally by intracarotid injection of arabinose and retinae were examined 12, 24, or 36 hours later. Second, to determine whether the immune response against the malaria parasite modulates microglial reactivity, infected mice were treated with dexamethasone before day 4 postinoculation. This treatment regime ameliorates cerebral complications without affecting parasite growth. We observed that increased BBB permeability was sufficient to elicit thickening of microglial processes and redistribution of microglia toward the vasculature, characteristic of the early stages of FMCM. However, despite the presence of plasma constituents in the CNS for up to 36 hours, microglia with amoeboid and vacuolated morphology were not observed. Dexamethasone treatment inhibited the up-regulation of alpha-D-galactose expression and reactive morphological changes in microglia during FMCM. These results suggest that disruption of the CNS milieu by entry of plasma constituents, or circulating malaria parasites in the absence of an immune response, by themselves are insufficient to induce the reactive microglial changes that are characteristic of FMCM. In addition, dexamethasone-sensitive event(s), presumably associated with immune system activation, occurring within the first few days of malaria infection are essential for the development of reactive microglia and subsequent fatal neurological complications.