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

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

Interleukin-18 deficiency and its long-term behavioural and cognitive impacts in a murine model of pneumococcal meningitis

Pneumococcal meningitis often results in death or neurological sequelae, but the underlying pathogenetic mechanisms remain poorly understood. In C57BL/6J mice subjected to intracerebroventricular (icv) challenge with Streptococcus pneumoniae, the chemokine CCL2 and cytokines interferon-γ, interleukin (IL)-1β, IL-6 and tumour necrosis factor were prominently expressed in the brain during the acute phase of the disease. The upregulation of these immune mediators was markedly diminished in IL-18-deficient mice. Uninfected IL-18(-/-) mice exhibited decreases in anxiety phenotype and licking behaviour, and an increase in behavioural habituation, in an automated monitoring system (the IntelliCage). Without antibiotic intervention, a majority of IL-18(+/+) mice developed irreversible disease after icv S. pneumoniae but this was significantly improved by deleting IL-18 gene function. IL-18(+/+) mice cured of pneumococcal meningitis with four doses of ceftriaxone, initiated at 20 h post-inoculation, showed enduring sequelae. These included abnormal behavioural phenotypes featuring diurnal hypoactivity and nocturnal hyperactivity, light phobia and disrupted cognitive function. While the hyperactive phenotype was absent in the corresponding IL-18(-/-) survivors, cognitive impairments and behavioural deficits were still present. Overall, the results suggest that the high levels of cytokines and/or chemokines released after pneumococcal challenge provoked a series of pathological events, ultimately causing acute death. Furthermore, since only a subset of behavioural phenotypes were ameliorated in the pneumococcus-infected IL-18(-/-) mice, the pathological pathways causing mortality may be, at least in part, distinct from those leading to long-term neurological sequelae.

A novel automated test battery reveals enduring behavioural alterations and cognitive impairments in survivors of murine pneumococcal meningitis

Pneumococcal meningitis, caused by Streptococcus pneumoniae infection, is a major form of lethal bacterial meningitis. Survivors are predisposed to developing lifelong disabling sequelae, including cognitive impairment, psychological problems and motor deficits. In our experimental model, ventricular inoculation of 10(5) colony-forming units of S. pneumoniae type 3 caused 90% of mice to develop life-threatening meningitis within 48 h. Antibiotic treatment with ceftriaxone 20 h post infection reduced the incidence of severe meningitis to <10%. At the time of treatment, upregulation of pro-inflammatory cytokines was detected, including interleukin-1β, interleukin-6 and tumour necrosis factor. We evaluated the long-term behavioural and cognitive sequelae in control mice and those surviving meningitis using an automated system (the IntelliCage) in which mice perform a range of behavioural and spatial tasks to obtain water rewards from conditioning units in their home cage. Surviving mice showed a number of altered behaviours relative to controls, including (i) hypoexploration when first exposed to the IntelliCage, (ii) altered activity patterns (fewer visits to conditioning stations during the light phase and more in the dark phase), (iii) avoidance of light (a constant or flashing LED stimulus), (iv) impaired spatial learning (a complex patrolling task), and (v) impaired discrimination reversal learning. Overall these results suggest photophobia and weakened learning ability in post-meningitic mice, particularly on tasks engaging hippocampal and prefrontal neural substrates. This study also demonstrates a standardised and comprehensive battery of tests that can be readily used to investigate neurological sequelae in undisturbed mice residing in a complex home cage environment.

CD40 and OX40 ligand are differentially regulated on asthmatic airway smooth muscle

BACKGROUND

CD40 and OX40 Ligand (OX40L) are cell-surface molecules expressed on airway smooth muscle (ASM) that can enhance inflammatory cell activation and survival. The aim of this study was to examine the effect of tumour necrosis factor-alpha (TNF-alpha) and interferon-gamma (IFN-gamma) on ASM CD40 and OX40L expression.

METHODS

CD40 and OX40L expression on human ASM cells from asthmatic and nonasthmatic donors following stimulation with TNF-alpha and/or IFN-gamma was measured using cell-surface enzyme-linked immunosorbent assay (ELISA) and flow cytometry. Involvement of signalling pathway was investigated with pharmacological inhibitors. Soluble TNF receptor levels were quantified by ELISA.

RESULTS

Interferon-gamma and TNF-alpha synergistically increased CD40 expression to a greater extent on asthmatic than on nonasthmatic ASM. In contrast, IFN-gamma reduced TNF-alpha-induced OX40L expression to a similar extent in both cell types. TNF-alpha and IFN-gamma induced CD40 via nuclear factor-kappaB (NF-kappaB) and signal transducer and activator of transcription-3 in both cell types and modulated OX40L via NF-kappaB and c-Jun N terminal kinase in nonasthmatic cells. Similar effects on the induction of OX40L in asthmatic cells were seen with NF-kappaB, but these were not statistically significant. The reduced OX40L expression with TNF-alpha and IFN-gamma involved extracellular regulated kinase 1/2 activation.

CONCLUSION

Asthmatic ASM may modulate airway inflammation locally by increasing CD40 and OX40L expression in response to cytokines. IFN-gamma may regulate ASM pro-inflammatory actions by differentially modulating ASM CD40 and OX40L expression.

Control of pathogenic CD8+ T cell migration to the brain by IFN-gamma during experimental cerebral malaria

Previous studies have shown that IFN-gamma is essential for the pathogenesis of cerebral malaria (CM) induced by Plasmodium berghei ANKA (PbA) in mice. However, the exact role of IFN-gamma in the pathway (s) leading to CM has not yet been described. Here, we used 129P2Sv/ev mice which develop CM between 7 and 14 days post-infection with PbA. In this strain, both CD4(+) and CD8(+) T cells were involved in the effector phase of CM. When 129P2Sv/ev mice deficient in the IFN-gamma receptor alpha chain (IFN-gammaR1) were infected with PbA, CM did not occur. Migration of leucocytes to the brain at the time of CM was observed in wild type (WT) but not in deficient mice. However, in the latter, there was an accumulation of T cells in the lungs. Analysis of chemokines and their receptors in WT and in deficient mice revealed a complex, organ-specific pattern of expression. Up-regulation of RANTES/CCL5, IP-10/CCL3 and CCR2 was associated with leucocyte migration to the brain and increased expression of MCP-1/CCL2, IP-10/CCL3 and CCR5 with leucocyte migration to the lung. This shows that IFN-gamma controls trafficking of pathogenic T cells in the brain, thus providing an explanation for the organ-specific pathology induced by PbA infection.

Conventional and experimental treatment of cerebral malaria

The most severe complication of Plasmodium falciparum infection is cerebral malaria (CM). Cerebral malaria implies the presence of neurological features, especially impaired consciousness. The treatment of CM is limited to: (i) a few conventional anti-malarial drugs (quinine or artemisinins), (ii) adjunctive treatments (initial stabilisation, blood exchange transfusion, osmotic diuretics and correction of hypoglycaemia, acidosis and hypovolaemia) and (iii) immunomodulation. There are clear procedures concerning treatment of CM, which include the use of the anti-plasmodial drugs. Adjunctive treatments are permissible but there is no single official guideline and immune intervention is a possibility currently being examined in rodent models only. The suggested immunomodulation approach is based on the strong likelihood that CM is the result of an immunopathological process. P. falciparum initiates the multifactorial chain of events leading to lethal CM and, after a certain stage, it is impossible to stop the progression even by using anti-malarial drugs. We present evidence that CM is a result of a dysregulated immune response. Therefore, it might be prevented by early modulation of discrete factors that participate in this process. In experimental systems, some immunomodulators delay or prevent CM without affecting the parasitaemia. Therefore, in the future the ultimate treatment of CM may be a combination of an anti-malarial and an immunomodulator. However, the overall effect of an immunomodulator would need to be carefully examined in view of concomitant infections, especially in malaria endemic areas.

Phagocyte-derived reactive oxygen species do not influence the progression of murine blood-stage malaria infections

Phagocyte-derived reactive oxygen species have been implicated in the clearance of malaria infections. We investigated the progression of five different strains of murine malaria in gp91(phox-/-) mice, which lack a functional NADPH oxidase and thus the ability to produce phagocyte-derived reactive oxygen species. We found that the absence of functional NADPH oxidase in the gene knockout mice had no effect on the parasitemia or total parasite burden in mice infected with either resolving (Plasmodium yoelii and Plasmodium chabaudi K562) or fatal (Plasmodium berghei ANKA, Plasmodium berghei K173 and Plasmodium vinckei vinckei) strains of malaria. This lack of effect was apparent in both primary and secondary infections with P. yoelii and P. chabaudi. There was also no difference in the presentation of clinical or pathological signs between the gp91(phox-/-) or wild-type strains of mice infected with malaria. Progression of P. berghei ANKA and P. berghei K173 infections was unchanged in glutathione peroxidase-1 gene knockout mice compared to their wild-type counterparts. The rates of parasitemia progression in gp91(phox-/-) mice and wild-type mice were not significantly different when they were treated with l-N(G)-methylarginine, an inhibitor of nitric oxide synthase. These results suggest that phagocyte-derived reactive oxygen species are not crucial for the clearance of malaria parasites, at least in murine models.

Needle in a haystack: microdissecting the proteome of a tissue

Laser-assisted microdissection is a recent technology that enables cells to be harvested from tissue sections. Proteins can be extracted from the dissected cells for molecular analysis. This enables the analysis of proteins in specific cell types in an in vivo system. Although quantities of protein obtained from the dissected material can be small, it is possible to use established methods such as Western Blotting and 2D-PAGE, as well as newer technologies such as SELDI-MS, to analyse the proteins. This review describes the applications and technical considerations for using laser-assisted dissected cells in proteomics research.

Haptoglobin and malaria

Haptoglobin gene knockout mice and wild-type controls were infected with Plasmodium berghei ANKA or Plasmodium chabaudi. The peak parasitaemia and parasite burden were higher in Hp-/- mice than in Hp+/+ mice. The increase in spleen weight following malaria infection was smaller in Hp-/- mice than in Hp+/+ animals. The occurrence of cerebral malaria in P. berghei ANKA infection was not different in Hp gene knockout mice and their controls.

Is ischemia involved in the pathogenesis of murine cerebral malaria?

Sequestration of parasitized erythrocytes in the central nervous system microcirculation and increased cerebrospinal fluid lactate are prominent features of cerebral malaria (CM), suggesting that sequestration causes mechanical obstruction and ischemia. To examine the potential role of ischemia in the pathogenesis of CM, Plasmodium berghei ANKA (PbA) infection in CBA mice was compared to infection with P. berghei K173 (PbK) which does not cause CM (the non-CM model, NCM). Cerebral metabolite pools were measured by (1)H nuclear magnetic resonance spectroscopy during PbA and PbK infections. Lactate and alanine concentrations increased significantly at the terminal stage of CM, but not in NCM mice at any stage. These changes did not correlate with parasitemia. Brain NAD/NADH ratio was unchanged in CM and NCM mice at any time studied, but the total NAD pool size decreased significantly in the CM mice on day 7 after inoculation. Brain levels of glutamine and several essential amino acids were increased significantly in CM mice. There was a significant linear correlation between the time elapsed after infection and small, progressive decreases in the cell density/cell viability markers glycerophosphocholine and N-acetylaspartate in CM, indicative of gradual loss of cell viability. The metabolite changes followed a different pattern, with a sudden significant alteration in the levels of lactate, alanine, and glutamine at the time of terminal CM. In NCM, there were significant decreases with time of glutamate, the osmolyte myo-inositol, and glycerophosphocholine. These results are consistent with an ischemic change in the metabolic pattern of the brain in CM mice, whereas in NCM mice the changes were more consistent with hypoxia without vascular obstruction. Mild obstructive ischemia is a likely cause of the metabolic changes during CM, but a role for immune cell effector molecules cannot be ruled out.

Antioxidants inhibit indoleamine 2,3-dioxygenase in IFN-gamma-activated human macrophages: posttranslational regulation by pyrrolidine dithiocarbamate

Induction of the heme-containing indoleamine 2,3-dioxygenase (IDO) by IFN-gamma is implicated in anti-microbial and pro-inflammatory activities of human macrophages. Antioxidants can modulate the expression of immune and inflammatory genes, and pyrrolidine dithiocarbamate (PDTC) is a frequently used antioxidant to inhibit the transcription factor NF-kappaB. Here we show that IFN-gamma treatment of human monocyte-derived macrophages (hMDMs) increased the proportion of oxidized glutathione. PDTC attenuated this increase and inhibited IDO activity, although it increased IDO protein expression and did not affect IDO mRNA expression and enzyme activity directly. Other antioxidants, 2-ME, ebselen, and t-butyl hydroquinone, inhibited IDO protein expression. Similar to PDTC, the heme biosynthesis inhibitor succinylacetone (SA) and the iron-chelator pyridoxal isonicotinoyl hydrazone inhibited cellular IDO activity without affecting protein expression, whereas addition of hemin or the heme precursor delta-aminolevulinic acid increased IDO activity. Also, incubation of IFN-gamma-activated hMDM with delta-[(14)C]-aminolevulinic acid resulted in the incorporation of label into immunoprecipitated IDO, a process inhibited by PDTC and SA. Furthermore, supplementation of lysates from PDTC- or SA-treated hMDM with hemin fully restored IDO activity to control levels, and hemin also reversed the inhibitory action of SA but not PDTC in intact cells. Together these results establish a requirement for de novo heme synthesis for IDO activity in IFN-gamma-activated hMDM. They show that, similar to other pro-inflammatory proteins, the activity of IDO is modulated by antioxidants though in the case of PDTC this takes place posttranslationally, in part by limiting the availability of heme for the formation of holo-IDO.

NADPH oxidase, Nramp1 and nitric oxide synthase 2 in the host antimicrobial response

Using highly conserved, complex enzyme systems, leukocytes utilize the toxic nature of free radical intermediates, derived from oxygen and nitrogen, to control microbial pathogens as part of the innate immune response. Upon activation, NADPH oxidase generates superoxide anion radicals, which in turn give rise to further reactive oxygen intermediates. Similarly, activated nitric oxide synthase 2 catalyses the production of nitric oxide radicals, which leads to the formation of reactive nitrogen intermediates. Nitrogen- and oxygen-centered reactive intermediates can interact to form further reactive species. In addition, presence of the cationic transporter, Nrampl, may exacerbate the effects of these toxic compounds on invading microbes. While each of these antimicrobial systems can operate independently, the combination of their activities is synergistic in the successful containment of almost all invading pathogens. These systems are activated and modulated by microbial products and a series of temporally expressed cytokines. They also feed directly into the initiation of the adaptive immune response, which culminates in lasting specific immunity. The effector molecules, generated in the early innate immune response, are not specific to the invading pathogen and may also cause damage to the host. It is the critical balance of these processes in the initial stages of infection that determines the outcome of infectious disease.

ERK activation and mitogenesis in human airway smooth muscle cells

Asthmatic airways are characterized by an increase in smooth muscle mass, due mainly to hyperplasia. Many studies suggest that extracellular signal-regulated kinases 1 and 2 (ERK1 and ERK2, respectively), one group of the mitogen-activated protein (MAP) kinase superfamily, play a key role in the signal transduction pathway leading to cell proliferation. PGE(2) and forskolin inhibited mitogen-induced ERK activation. Inhibition of MAP kinase kinases 1 and 2 (MEK1 and MEK2, respectively), which are upstream from ERK, with the specific MEK inhibitor U-0126 blocked both cell proliferation and ERK activation. In addition, U-0126 inhibited mitogen-induced activation of p90 ribosomal S6 kinase and expression of c-Fos and cyclin D1, all of which are downstream from ERK in the signaling cascade that leads to cell proliferation. Antisense oligodeoxynucleotides directed to ERK1 and -2 mRNAs reduced ERK protein and cell proliferation. These results indicate that ERK is required for human airway smooth muscle cell proliferation. Thus targeting the control of ERK activation may provide a new therapeutic approach for hyperplasia seen in asthma.

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.

Dichloroacetate (DCA) reduces brain lactate but increases brain glutamine in experimental cerebral malaria: a 1H-NMR study

Recent findings that levels of brain lactate and alanine were elevated in murine cerebral malaria led us to investigate the effect of dichloroacetate (DCA; 60 mg/kg), an activator of pyruvate dehydrogenase, on the levels of brain metabolites, and on the survival of mice infected with Plasmodium berghei ANKA which normally causes lethal cerebral malaria. DCA significantly reduced brain lactate and alanine levels when administered to infected mice, had no effect on the TCA cycle-related metabolites glutamate, GABA and aspartate and was associated with increased brain glutamine levels: 40% of mice thus treated survived the normally lethal infection.

Tissue distribution of indoleamine 2,3-dioxygenase in normal and malaria-infected tissue

An immunohistochemical method was developed, using a polyclonal antibody, to detect the enzyme indoleamine 2,3-dioxygenase (IDO) in normal and malaria-infected tissue. Plasmodium berghei ANKA, a cerebral malaria (CM) model, and P. berghei K173, a non-cerebral malaria (NCM) model, were used. It was found that vascular endothelial cells were the primary site of IDO expression in both models of malaria infection and that this response was systemic, with the vascular endothelium of brain, heart, lung, spleen and uterus all staining positive. These results suggest that IDO is part of a systemic host response to parasite infection. Although high levels of IDO production alone may not cause pathology, it is possible that when its production is combined with other features of CM, such as breakdown of the blood-brain barrier (BBB), metabolites of the kynurenine pathway may be able to influence the otherwise tightly regulated, immunologically privileged site of the CNS and cause some of the symptoms and pathology observed.

The intracellular oxidation of 2',7'-dichlorofluorescin in murine T lymphocytes

Intracellular reactive oxygen species (ROS) production by activated murine T lymphocytes was investigated by analyzing intracellular dichlorofluorescin (DCFH(2)) oxidation in lymph node cells (LNC). An increase in DCFH(2) oxidation in LNC induced by phorbol myristate acetate (PMA) was detected by flow cytometry. It was confirmed that this increase was present in Thy1(+) LNC. We examined the contribution to intracellular DCFH(2) oxidation of ROS released by leukocytes other than T cells present in the LNC suspension. Superoxide dismutase, catalase, and glutathione/glutathione peroxidase inhibited the PMA-induced increase in intracellular DCFH(2) oxidation. Furthermore, PMA failed to elicit DCFH(2) oxidation in LNC isolated from mice lacking a functional NADPH oxidase (gp91(phox) gene knockout mice), but this response could be restored in these cells by the addition of T cell-depleted LNC from wild-type litter mates. This study highlights the necessity for caution in using the DCFH(2) assay to demonstrate specific intracellular ROS production in heterogeneous cell populations. It also suggests that cells other than T cells in lymph node populations may, through production of ROS, influence the intracellular redox state of T lymphocytes.

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.