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Query: UMLS:C0024530 (malaria)
 44,886 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Immunity to P. falciparum malaria is developed as a result of long term exposure to the parasite and depends on immunological memory. The key directors in immune recognition and regulation of the immunological responses are the T-cells. It seems reasonable to propose that immunity is acquired when a critical mass of T-cells, recognizing relevant malaria antigens, has been developed. These T-cells mediate immunity by regulating macrophage and B-cell activity, but they may also act directly as cytotoxic cells on infected hepatocytes and through production of parasite-toxic cytokines. The potential immune effector mechanisms against P. falciparum are many. The relative importance of each in protection is unknown and protection seems to be mediated through different mechanisms according to the degree of exposure to malaria and the pattern of malaria transmission. Since immunity to malaria is not an absolute phenomenon, many effector mechanisms are probably working together in (partially) protected individuals. Immunity to P. falciparum is acquired after years of exposure to the parasite and several disease episodes. The protracted course to clinical immunity indicates that the parasite interfere with development of immunity. Several mechanisms seem to be operating. 1) Induction of the immune response to some macromolecules is avoided because the parasites are living inside host cells during part of their life cycle, and the reaction to other molecules is apparently avoided by mimicry of host molecules. 2) Immune recognition is hampered by the extraordinary diversity of antigen phenotypes in the parasite population. 3) Immune regulation is obstructed by immune suppression. During P. falciparum malaria such suppression is characterized by a profoundly diminished in vitro proliferative response to malaria antigens, which probably is precipitated by defects in the early events of T-cell activation and inhibition of IL-2 function elucidated, but soluble factors secreted either by the parasites, or by host cells as a result of exposure to the parasite, seem to be involved. 4) Immune effector mechanisms in the liver and the spleen are avoided by sequestration of the mature parasites to the vascular endothelium. The interplay between the human defence system and the malaria parasite governs the symptomatology, the pathology and the development of immunity to the disease. These interactions are extremely complex, and only partly understood. Figure 1 summarizes my view on how these interactions could explain the characteristics of acquired immunity to P. falciparum.(ABSTRACT TRUNCATED AT 400 WORDS)
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PMID:Defence mechanisms and immune evasion in the interplay between the humane immune system and Plasmodium falciparum. 156 95

Cerebral malaria is the most important manifestation of severe Plasmodium falciparum infection. The clinical picture in South East Asian adults differs from that in African children. The children are more likely to have abnormal brain stem reflexes, signs suggestive of cerebral herniation, and raised CSF opening pressure, and to suffer persistent neurological sequelae. The mortality remains high at about 20%. The diagnosis must be considered in all patients with fever and impaired consciousness who may have been exposed to the infection. The pathophysiology of cerebral malaria may involve mechanical obstruction of the cerebral circulation by parasitized erythrocytes which have adhered to the vascular endothelium. Cytokines such as tumor necrosis factor may also contribute. The most important element of treatment is early, optimal chemotherapy with quinine, but artemisinine derivatives may prove even more effective.
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PMID:Cerebral malaria. 161 97

Adhesion of parasitized red blood cells (RBCs) to vascular endothelium is thought to be a key factor in the pathology of falciparum malaria. However, quantitative analyses of the intercellular forces and of the effects of flow on adhesion have been lacking. We have characterized cytoadhesion of RBCs parasitized by the strains ITO4 (which can bind to receptors ICAM-1 or CD36) and FCR3A2 (which can bind to CD36 only) using micropipette manipulation and flow chamber techniques. Target cells were unfixed or glutaraldehyde-fixed human umbilical vein endothelial cells (HUVEC, bearing ICAM-1 only) or human amelanotic melanoma cells (C32, bearing CD36 and ICAM-1). In the static, micropipette assay, 60% to 70% of parasitized cells would adhere when tested at up to three successive sites. The percentage of cells adhering and the force required for their detachment (approximately 10(-10) N) were similar for each combination of parasite strain and adhesion target (ITO4/HUVEC, ITO4/C32, FCR3A2/C32). In the flow chamber, efficiency of initial adhesion of parasitized cells was essentially constant (at about 1%) up to a stress of 0.1 Pa, and then decreased rapidly with increasing stress. Either receptor (ICAM-1 or CD36) could immobilize flowing cells at a physiologic flow stress (0.1 Pa), but the numbers of cells adhering varied for the different combinations (ITO4/C32 greater than ITO4/HUVEC greater than FCR3A2/C32). When flow was increased in steps, adhered cells were gradually washed off but many could withstand stresses at which they would not initially adhere. The force for detachment estimated in this way was similar to the pipette value, and again, was similar for the different combinations of strains and targets. Adhesion from flow depends on the affinity between surfaces being above a critical level, and once adhesion is established, the fracture energy determines resistance to disruption of adhesion. The results show that the fracture energy is greater than the affinity (ie, that adhesion becomes stabilized after it is initially established) and that the ratio of affinity to fracture energy is different for different receptor/ligand pairs, with ICAM-1 appearing to be the more efficient immobilizing receptor. Also, static and flow-based assays of adhesion clearly differ; the affinity is less critical in the static situation, so that most parasitized cells were capable of adhering in a static assay, but fewer did so under flow. Adhesiveness varied markedly from cell to cell, both for targets and parasitized cells.(ABSTRACT TRUNCATED AT 400 WORDS)
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PMID:Rheological analysis of the adhesive interactions of red blood cells parasitized by Plasmodium falciparum. 173 18

Membrane lipid peroxidation by reactive oxygen species leading to increased capillary permeability is considered an important event in the pathogenesis of severe malaria. A significant decrease in plasma albumin and increases in cerebrospinal fluid (CSF) protein and malondialdehyde (MDA) were observed in 73 patients with cerebral malaria, compared to values in 23 control patients. The greatest effect was noticed in the most severely ill patients. The ratio of CSF protein to plasma albumin was increased in the patients compared to the controls, and in fatal cases of cerebral malaria compared to non-fatal cases. Brain necropsies showed oedema, fibrin deposits and mononuclear cell infiltration. It is proposed that cerebral oedema due to enhanced permeability of vascular endothelium induced by increased lipid peroxidation plays a crucial role in the causation of cerebral malaria.
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PMID:Increased cerebrospinal fluid protein and lipid peroxidation products in patients with cerebral malaria. 180 39

The ability of red cells to deform is essential to allow their circulation. However the degree of rheological abnormality which can be tolerated before flow is impaired is not so clear. Red cell rheology has been characterised in a number of physiological, pathological and genetic conditions, and some inferences can be drawn. In vivo aging causes a small loss of cell deformability attributable to increased membrane and internal viscosity; volume and surface area are also lost. These changes cannot be sufficient to cause cellular removal, since the cells sampled had continued to circulate. In sickle cell disease, the oxygenated blood contains dense cells that are more severely abnormal than dense, aged cells from normal individuals. Melanesian ovalocytes have comparable rigidity to dense SS cells, but this condition has no marked circulatory pathology. Thus circulatory problems in SS disease probably stem from deoxygenation-induced sickling which causes extreme loss of deformability, rather than from the abnormal cells in oxygenated blood. In falciparum malaria, immature parasites cause appreciable loss of deformability but continue to circulate. Maturation of the parasites causes much greater rheological changes, including attachment to vascular endothelium, and the cells cease to circulate. In summary, quite marked changes in cell mechanics can occur without loss of ability to circulate. It thus seems that slight rheological alterations reported in some clinical studies are unlikely to cause appreciable flow disruption.
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PMID:Red cell mechanics: what changes are needed to adversely affect in vivo circulation. 193 15

To determine whether isolates of Plasmodium falciparum have intrinsically different cytoadherent properties and whether these differences contribute to the clinical severity of human falciparum malaria, we studied the cytoadherence to C32 melanoma cells in vitro of 59 parasite isolates from patients with naturally acquired infections in Thailand. Parasitized erythrocytes adhere to these melanoma cells principally via the glycoprotein CD36, which is also expressed on most vascular endothelium. In vitro cytoadherence was significantly greater for isolates from patients with biochemical evidence of severe malaria. The cytoadherent properties of P. falciparum parasites may thus be a virulence factor in human falciparum malaria. However, there was no correlation between the degree of in vitro cytoadherence and cerebral symptoms, which suggests that other receptors and/or host factors may be important in the adherence of malaria parasites to cerebral vascular endothelium. The cytokines tumor necrosis factor, interleukin-1, and gamma interferon, which have been implicated in the pathogenesis of cerebral malaria and are known to promote intercellular adhesion in other systems, did not enhance the cytoadherence of P. falciparum isolates to C32 melanoma cells.
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PMID:Clinical correlates of in vitro Plasmodium falciparum cytoadherence. 199 37

The malaria-induced surface antigens on Plasmodium falciparum-infected erythrocytes from West African patients were characterized by agglutination of infected cells by human sera, surface immunofluorescence of live infected cells, inhibition of cytoadherence to C32 melanoma cells by human sera, immunoelectron microscopy (immunoEM), and immunoprecipitation. In a nonimmune individual, serum antibody reactivity to surface antigens of infected cells was acquired during convalescence, as tested by all five methods, and was generally parasite isolate-specific. By contrast, adult hyperimmune West African sera reacted with many isolates, including isolates from geographically distinct regions. A quantitative correlation was established between agglutination and surface immunofluorescence assay titers, and between surface immunofluorescence assay and immunoEM reactivity, suggesting that a single antigen or a set of coexpressed antigens is being detected. Surface iodination of infected cells identified trypsin-sensitive high M, antigens in the sodium dodecyl sulfate extract. All sera tested that agglutinated infected cells also immunoprecipitated these antigens. The same surface antigens were immunoprecipitated by the homologous convalescent serum as by adult sera. By immunoEM these antigens were localized exclusively at the knob-like protrusions of infected cells, where they may participate in adherence to vascular endothelium.
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PMID:Characterization and localization of Plasmodium falciparum surface antigens on infected erythrocytes from west African patients. 207 55

Plasmodium falciparum-infected erythrocytes (IE) specifically adhere to vascular endothelium in vivo and to human endothelial cells, some human melanoma cell lines, and human monocytes in vitro. The tissue cell receptor for a ligand on the surface of the infected erythrocytes is an Mr 88,000 glycoprotein (GP88) recognized by the MAb OKM5, which also blocks cytoadherence of IE. Isolated, affinity-purified GP88 (CD36) competitively blocks cytoadherence and when absorbed to plastic surfaces, specifically binds P. falciparum IE. Additionally, monoclonal and polyclonal antibodies to GP88 block cytoadherence to both target cells and immobilized GP88. Binding to GP88 by IE is unaffected by the absence of calcium or the absence of thrombospondin, a putative mediator for cytoadherence of P. falciparum IE. Thus, GP88 (CD36), which has been demonstrated to be the same as platelet glycoprotein IV, interacts directly with P. falciparum IE, presumably via a parasite-induced ligand exposed on the surface of the infected erythrocytes. CD36 is shown to be present on brain endothelium in both individuals without malaria and individuals with cerebral malaria. This would suggest that factors other than just cerebral sequestration of IE play an initiating role in the genesis of cerebral malaria.
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PMID:A human 88-kD membrane glycoprotein (CD36) functions in vitro as a receptor for a cytoadherence ligand on Plasmodium falciparum-infected erythrocytes. 247 74

The brains of mice with established symptoms of Plasmodium berghei erebral malaria were investigated histochemically and histologically. The activity of mitochondrial, lysosomal, glyco/glycogenolytic, hydrolytic and oxidizing enzymes as well as enzymes of the Krebs cycle and the pentose cycle was studied during the course of the infection. For comparison cryostate sections were also stained with haematoxylin and eosin, and according to Kluver-Barrera. Changes in enzyme activity, particularly of the vascular endothelium suggest a functional alteration of the blood-brain barrier which precedes the histochemically detectable lesions of the brain parenchyma. Decrease and total loss of enzyme activity in circumscript areas, also of ependymal cells were indicative of an early ischemic lesion. A population of small, non-phagocytozing, granuloma-like cells frequently accumulating in the frontobasal regions and in the subependymal zones were probably immature astrocytes. During early infection, these cells apparently fail to differentiate and turn to necrosis at the end of the second week. The results of this study support the concept of a triggering role of an initial vascular lesion and a functional breakdown of the blood-brain barrier in susceptible areas of the brain, in the pathogenesis of experimental malaria.
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PMID:Histochemistry of cerebral lesions in mice infected with Plasmodium berghei. 306 19

Research since the World War II has confirmed that, apart from the production of haemozoin from haemoglobin, most of the pathological processes in the evolution of malaria are nonspecific. A few of these nonspecific host reactions are discussed, including the production of inflammatory stasis in certain areas (including the brain) where the vascular endothelium is normally highly impermeable to heavy molecules. This production of stasis is regarded as the basic phenomenon in local obstruction to blood flow. So-called "plugging" of small vessels with "sticky" infected erythrocytes is discussed in relation to stasis and to deep intravascular schizogony. Nonspecific vasomotor effects including shock and renal and hepatic failure are also discussed. Intravascular coagulation is not regarded as a potentially important host response despite demonstrable consumption coagulopathy. The disease malaria is regarded as an example of a chain reaction of physiological-pathological responses in the host, which in the early stages are reversible.
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PMID:Other pathological processes in malaria. 421 9


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