A cytochemical study of cerebrovascular lesions in mice infected with Plasmodium berghei

Elevated cell-specific microparticles are a biological marker for cerebral dysfunctions in human severe malaria

Cerebral malaria (CM) and severe anemia (SA) are the most severe complications of Plasmodium falciparum infections. Although increased release of endothelial microparticles (MP) correlates with malaria severity, the full extent of vascular cell vesiculation remains unknown. Here, we characterize the pattern of cell-specific MP in patients with severe malaria. We tested the hypothesis that systemic vascular activation contributes to CM by examining origins and levels of plasma MP in relation to clinical syndromes, disease severity and outcome. Patients recruited in Douala, Cameroon, were assigned to clinical groups following WHO criteria. MP quantitation and phenotyping were carried out using cell-specific markers by flow cytometry using antibodies recognizing cell-specific surface markers. Platelet, erythrocytic, endothelial and leukocytic MP levels were elevated in patients with cerebral dysfunctions and returned to normal by discharge. In CM patients, platelet MP were the most abundant and their levels significantly correlated with coma depth and thrombocytopenia. This study shows for the first time a widespread enhancement of vesiculation in the vascular compartment appears to be a feature of CM but not of SA. Our data underpin the role of MP as a biomarker of neurological involvement in severe malaria. Therefore, intervention to block MP production in severe malaria may provide a new therapeutic pathway.

Cytokines and adhesion molecules expression in the brain in human cerebral malaria

Although the role of systemic proinflammatory cytokines, IL-1β and TNF-α, and their up-regulation of adhesion molecules, ICAM-1, VCAM-1 and E-Selectin, in the pathogenesis of cerebral malaria (CM) is well established, the role of local cytokine release remain unclear. Immunohistochemistry (IHC) was used to compare the expression of ICAM-1, VCAM-1, E-Selectin, IL-1β, TNF-α and TGF- β at light microscopic level in cerebral, cerebellar and brainstem postmortem cryostat sections from 10 CM, 5 severe malarial anemia (SMA), 1 purulent bacterial meningitis (PBM), 2 non-central nervous system infections (NCNSI) and 3 non-infections (NI) deaths in Ghanaian children. Fatal malaria and Salmonella sepsis showed significantly higher vascular expression of all 3 adhesion molecules, with highly significant co-localization with sequestration in the malaria cases. However, there was negligible difference between CM and SMA. TGF-β showed intravascular and perivascular distribution in all cases, but expression was most intense in the PBM case and CM group. TNF-α and IL-1β showed prominent brain parenchymal staining, in addition to intravascular and perivascular staining, in only the PBM case and CM group. The maximal expression of all 6 antigens studied was in the cerebellar sections of the malaria cases. Endothelial activation is a feature of fatal malaria and Salmonella sepsis, with adhesion molecule expression being highly correlated with sequestration. IL-1β and TNF-α are upregulated in only cases with neurodegenerative lesions, whilst TGF-β is present in all cases. Both cytokines and adhesion molecules were maximally upregulated in the cerebellar sections of the malaria cases.

High-level cerebellar expression of cytokines and adhesion molecules in fatal, paediatric, cerebral malaria

Although the roles played by systemic tumour necrosis factor (TNF) and interleukin 1beta (IL-1beta), and their upregulation of intercellular adhesion molecule 1 (ICAM-1), vascular cell adhesion molecule 1 (VCAM-1) and E-selectin, in the pathogenesis of human cerebral malaria (CM) are well established, the role of local cytokine release, in the brain, remains unclear. Immunohistochemistry was therefore used to compare the expression of ICAM-1, VCAM-1, E-selectin, IL-1beta, TNF and transforming growth factor beta (TGF-beta) at light-microscope level, in cryostat sections of cerebral, cerebellar and brainstem tissues collected, post-mortem, from Ghanaian children. Among the 21 children investigated were 10 cases of CM, five of severe malarial anemia (SMA), one of purulent bacterial meningitis (PBM), two of non-central-nervous-system infection (NCNSI) and three children who had no infection (NI) when they died. Parasitised erythrocytes were detected in all of the sections from the cases of fatal malaria (CM and SMA), and sequestered leucocytes were present in most of the sections from the CM cases (but none of the sections from the SMA cases). Significantly elevated vascular expression of all three adhesion molecules investigated was detected in the brains of the 15 cases of fatal malaria and one of the cases of NCNSI (a child with Salmonella septicaemia), and in the malaria cases this showed highly significant co-localization with the areas of erythrocyte sequestration. In terms of the levels of expression of ICAM-1, VCAM-1 and E-selectin, there were, however, negligible differences between the CM and SMA cases. Although TGF-beta showed intravascular and perivascular distribution in all the subjects, its expression was most intense in the PBM case and the CM group. Only in the sections from the PBM and CM cases did TNF and IL-1beta show prominent brain parenchymal staining, in addition to the intravascular and perivascular staining seen in all subjects. The highest observed expression of each of the six antigens studied was in the cerebellar sections of the malaria cases. Endothelial activation in the brain therefore appears to be a feature of fatal malaria and Salmonella sepsis, and in cases of fatal malaria is closely associated with leucocyte sequestration. In the present study, IL-1beta and TNF were only up-regulated in the brains of children with neurodegenerative lesions, whereas TGF-beta was present in all cases.

Isolating vessels from the mouse brain for gene expression analysis using laser capture microdissection

Studies of gene expression often examine a pool of RNA extracted from the diverse cell types making up a tissue. We have developed a method for isolating vessels from the brain in order to understand the changes occurring in the vessels during the pathogenesis of cerebral malaria. Vessels were visualised by incubating sections of mouse brain with a substrate for alkaline phosphatase. Vessels were collected by laser capture microdissection and the specificity was monitored by measuring the expression of cell-specific markers. RNA from the captured vessels was highly enriched in mRNA for genes associated with endothelial cells and pericytes. Measurement of indoleamine 2,3-dioxygenase mRNA indicated it was possible to detect changes in gene expression, due to malaria infection, occurring specifically within the vessels. Laser capture microdissection can be used to study changes in gene expression occurring at the blood-brain barrier.

Redistribution and degeneration of retinal astrocytes in experimental murine cerebral malaria: relationship to disruption of the blood-retinal barrier

To determine whether astrocytes play a critical role in the pathogenesis of experimental murine cerebral malaria (EMCM), we examined changes in astrocyte morphology and distribution, using retinal wholemounts, in three models: a fatal cerebral malaria (CM) model, in which mice die showing cerebral symptoms; a "resolving" model, in which mice exhibit mild cerebral symptoms, but then recover; and a non-CM model, in which cerebral symptoms are not seen. In the fatal model, retinal astrocytes lost their even distribution from day 3 post-inoculation (p.i.) with malaria parasites, progressing to gliosis (day 5 p.i.), well before the onset of cerebral symptoms on day 6-7 p.i. At the terminal stage of the disease there was a loss of astrocyte processes contacting retinal vessels, often along vessel segments containing adherent monocytes. These features occurred in a mild form in the resolving model and were absent in the non-CM models. To investigate the mechanisms underlying these astrocytic changes, we carried out two experimental manipulations. Firstly, since dexamethasone ameliorates cerebral complications in the fatal CM model, the astrocytic response was monitored after dexamethasone treatment on days 0 and 1 p.i., or days 3 and 4 p.i. Second, to determine whether increased blood-retinal barrier (BRB) permeability initiates the astrocyte changes, breakdown of the BRB was induced experimentally by intra-carotid injection of arabinose and astrocyte morphology and distribution were examined 12, 24, and 48 h later. Retinal astrocytes in both the dexamethasone- and the arabinose-treated groups showed loss of even astrocyte distribution but no loss of astrocyte ensheathment of vessels. It is concluded that: i) astrocytes are involved in the pathogenesis of EMCM, since these changes are only prominent in the fatal model and occur substantially before the onset of cerebral symptoms; ii) the initial changes in astrocyte distribution may be a consequence of the increase in BRB permeability; and iii) the immune response triggered by the malaria parasite may be responsible for the loss of astrocyte ensheathment of vessel segments.

Neurological manifestations of malaria

The involvement of the nervous system in malaria is reviewed in this paper. Cerebral malaria, the acute encephalopathy which complicates exclusively the infection by Plasmodium falciparum commonly affects children and adolescents in hyperendemic areas. Plugging of cerebral capillaries and venules by clumped, parasitized red cells causing sludging in the capillary circulation is one hypothesis to explain its pathogenesis. The other is a humoral hypothesis which proposes nonspecific, immune-mediated, inflammatory responses with release of vasoactive substances capable of producing endothelial damage and alterations of permeability. Cerebral malaria has a mortality rate up to 50%, and also a considerable longterm morbidity, particularly in children. Hypoglycemia, largely in patients treated with quinine, may complicate the cerebral symptomatology. Other central nervous manifestations of malaria include intracranial hemorrhage, cerebral arterial occlusion, and transient extrapyramidal and neuropsychiatric manifestations. A self-limiting, isolated cerebellar ataxia, presumably caused by immunological mechanisms, in patients recovering from falciparum malaria has been recognized in Sri Lanka. Malaria is a common cause of febrile seizures in the tropics, and it also contributes to the development of epilepsy in later life. Several reports of spinal cord and peripheral nerve involvement are also available. A transient muscle paralysis resembling periodic paralysis during febrile episodes of malaria has been described in some patients. The pathogenesis of these neurological manifestations remains unexplored, but offers excellent perspectives for research at a clinical as well as experimental level.

Morphological characteristics of intracerebral arterioles in clinical (Plasmodium falciparum) and experimental (Plasmodium berghei) cerebral malaria

Spastic constriction of intracerebral arterioles was identified in clinical (P. falciparum) and experimental (P. berghei) cerebral malaria. Morphological criteria were used to characterize pathologically spastic constriction of arterioles. The significance of spastic constriction of intracerebral arterioles for microcirculatory disturbance in relation to development of cerebral malaria is discussed.