Parasite virulence factors during falciparum malaria: rosetting, cytoadherence, and modulation of cytoadherence by cytokines

The effect of synthetic malaria pigment (beta-haematin) on adhesion molecule expression and interleukin-6 production by human endothelial cells

The effects of synthetic malaria pigment (beta-haematin, BH) on the expression of the intercellular adhesion molecule 1 (ICAM-1) and platelet endothelial cell adhesion molecule 1 (PECAM-1) and the production of interleukin-6 (IL-6) by human microvascular endothelial cells were measured using flow cytometry analysis and immunoenzymatic assay. BH alone did not affect basal levels of ICAM-1, PECAM-1 or IL-6. When added to cell cultures before or with, but not after, lipopolysaccharide or tumour necrosis factor alpha, BH at 1-100 micrograms/mL induced a dose-dependent inhibition of ICAM-1 and PECAM-1 expression and IL-6 production. Cell viability and human leucocyte antigen A,B,C expression remained unaffected. Similar, though more variable, results were obtained using human umbilical vein endothelial cells. These results suggested that accumulation of pigment within endothelial cells following repeated malaria infection reduces local inflammation and parasite sequestration through inhibition of either cytokine production or parasitized erythrocyte receptors on endothelial cells.

Host-parasite interaction and morbidity in malaria endemic areas

Severe morbidity due to Plasmodium falciparum is a major health problem in African children. The patterns of morbidity in endemic areas are modified by the immune response, and vary markedly with transmission intensity. Severe disease falls into three overlapping syndromes: coma, respiratory distress, and severe anaemia. Recently, it has become clear that metabolic acidosis plays a major role in the pathogenesis of severe disease and is particularly important in the overlap between the different clinical syndromes. We propose that the different manifestations of severe malarial morbidity arise from the interaction of a limited number of pathogenic processes: red cell destruction, toxin-mediated activation of cytokine cascades, and infected cell sequestration in tissue microvascular beds. The pattern of severe morbidity varies with age within any one endemic area, with severe anaemia predominating in the youngest children and coma having its highest incidence in older children. Between endemic areas there is a marked variation in mean age of children with severe malaria, and therefore in the importance of different clinical syndromes. The shift in mean age is due to a combination of increased challenge and more rapid development of immunity at higher levels of transmission. Recent comparative studies indicate that at higher levels of transmission the net effect of these shifts may be a paradoxical reduction in total severe malarial morbidity.

Cytoadherence of Plasmodium falciparum to intercellular adhesion molecule 1 and chondroitin-4-sulfate expressed by the syncytiotrophoblast in the human placenta

Late stages of Plasmodium falciparum-infected erythrocytes (IRBCs) frequently sequester in the placentas of pregnant women, a phenomenon associated with low birth weight of the offspring. To investigate the physiological mechanism of this sequestration, we developed an in vitro assay for studying the cytoadherence of IRBCs to cultured term human trophoblasts. The capacity for binding to the syncytiotrophoblast varied greatly among P. falciparum isolates and was mediated by intercellular adhesion molecule 1 (ICAM-1), as binding was totally inhibited by 84H10, a monoclonal antibody specific for ICAM-1. Binding of the P. falciparum line RP5 to the syncytiotrophoblast involves chondroitin-4-sulfate (CSA), as this binding was dramatically impaired by addition of free CSA to the binding medium or by preincubation of the syncytiotrophoblast with chondroitinase ABC. ICAM-1 and CSA were visualized on the syncytiotrophoblast by immunofluorescence, while CD36, E-selectin, and vascular cell adhesion molecule 1 were not expressed even on tumor necrosis factor alpha (TNF-alpha)-stimulated syncytiotrophoblast tissue, and monoclonal antibodies against these cell adhesion molecules did not inhibit cytoadherence. ICAM-1 expression and cytoadherence of wild isolates was upregulated by TNF-alpha, a cytokine that can be secreted by the numerous mononuclear phagocytes present in malaria-infected placentas. These results suggest that cytoadherence may be involved in the placental sequestration and broaden the understanding of the physiopathology of the malaria-infected placenta.

Extensive genetic diversity of Plasmodium falciparum isolates collected from patients with severe malaria in Dakar, Senegal

While some genetic host factors are known to protect against severe Plasmodium falciparum malaria, little is known about parasite virulence factors. We have compared the genetic characteristics of P. falciparum isolates collected from 56 severe malaria patients and from 30 mild malaria patients recruited in Hôpital Principal, Dakar, Senegal. All isolates were typed using polymerase chain reaction amplification of polymorphic genetic loci (MSP-1, MSP-2, HRP1, GLURP, CSP, RESA, and the multigene family Pf60). The complexity of infections was lower in severe than in mild malaria and the parasite genetic diversity in both groups was very large. No specific genetic make-up was associated with severity; there were, however, marked differences in allele frequencies in both groups, with a prevalence up to 60% of MSP-2 alleles specifically observed in the severe malaria isolates. In addition, the presence of MSP-1/RO33 alleles was significantly associated with a higher plasma level of tumour necrosis factor alpha receptor 1 (P < 0.05), a reported indicator of severity in human malaria. These results point to potential differences in the genetic characteristics of parasites inducing severe versus mild pathology.

Plasmodium falciparum gametocyte adhesion to C32 cells via CD36 is inhibited by antibodies to modified band 3

Plasmodium falciparum gametocyte-infected erythrocytes are characterized by their ability to sequester in the microvasculature of various organs, primarily the spleen and bone marrow. This phenomenon is thought to play a critical role in the development and survival of the sexual stages. Little is known, however, about ligands on the gametocyte-infected erythrocyte. Infection of erythrocytes with mature asexual stages of P. falciparum (trophozoites and schizonts) has been shown to induce modification of the erythrocyte anion transporter, band 3, and this has been linked to the acquisition of an adherent phenotype. Here, we demonstrate for the first time that immature gametocyte-infected erythrocytes also express modified band 3. In vitro binding assays demonstrate that gametocyte-infected erythrocytes of the 3D7 strain utilize this surface receptor for adhesion to C32 amelanotic melanoma cells via the host cell receptor CD36 (platelet glycoprotein IIIb). Adhesion of gametocyte-infected erythrocytes to CD36-transfected CHO cells is also dependent on modified band 3. However, modified band 3 does not mediate adhesion of gametocyte-infected erythrocytes to intercellular adhesion molecule 1, a second host receptor for gametocytes expressed on C32 cells.

Adherence of Plasmodium falciparum to chondroitin sulfate A in the human placenta

Women are particularly susceptible to malaria during first and second pregnancies, even though they may have developed immunity over years of residence in endemic areas. Plasmodium falciparum-infected red blood cells (IRBCs) were obtained from human placentas. These IRBCs bound to purified chondroitin sulfate A (CSA) but not to other extracellular matrix proteins or to other known IRBC receptors. IRBCs from nonpregnant donors did not bind to CSA. Placental IRBCs adhered to sections of fresh-frozen human placenta with an anatomic distribution similar to that of naturally infected placentas, and this adhesion was competitively inhibited by purified CSA. Thus, adhesion to CSA appears to select for a subpopulation of parasites that causes maternal malaria.

Novel fibrillar structure confers adhesive property to malaria-infected erythrocytes

Infections with the malaria parasite Plasmodium falciparum are characterized by sequestration of erythrocytes infected by mature forms of the parasite. Sequestration seems critical for the survival of the parasite, but may lead to excessive binding in the microvasculature and death of the human host. We report here that a novel electrondense fibrillar structure, containing immunoglobulins M or M and G, is found at the surface of infected erythrocytes that adhere to host cells. In cases of cerebral malaria, fibrillar strands are also seen in the microvasculature at autopsy. Our findings may explain the adhesive mechanism by which malaria-infected erythrocytes cause the vascular obstruction seen in complicated malaria infections.

Disruption of erythrocyte rosettes and agglutination of erythrocytes infected with Plasmodium falciparum by the sera of Papua New Guineans

People living in areas endemic for Plasmodium falciparum develop humoral responses which may contribute to protection against clinical disease but the specificity of such protective antibody responses remains to be defined. Antibodies disrupting erythrocyte rosettes have been associated with protection against cerebral malaria, and antibodies agglutinating infected erythrocytes with reduced episodes of clinical disease. We have studied the capacity of serum from Papua New Guinean adults and children with a spectrum of malaria exposure, including children and adults at the time of clinical disease, to disrupt erythrocyte rosettes and cause agglutination of infected erythrocytes. Using a single parasite isolate, almost all sera from adults from highly endemic areas agglutinated infected erythrocytes, and the majority disrupted rosettes, in some cases at greater titres than hitherto described. There was a correlation between rosette disruption and agglutination in highly exposed adults. Rosette disrupting antibodies were equally frequent in children with cerebral and uncomplicated malaria. Antibodies causing rosette disruption were frequent only in adults with a long history of malarial exposure. Rosette disrupting antibodies do not appear to protect Papua New Guinean children or adults against cerebral malaria.

Plasma levels of IL-1, TNF alpha, IL-6, IL-8, G-CSF, and IL1-RA during febrile neutropenia: results of a prospective study in patients undergoing chemotherapy for acute myelogenous leukemia

Plasma levels of IL-1, IL-6, IL-8, IL1-RA, TNF alpha, and G-CSF were prospectively studied during 23 chemotherapy cycles of 20 patients suffering from acute myelogenous leukemia. Increased plasma levels of IL-6, IL-8, and G-CSF were observed in patients with febrile neutropenia and/or major infection. Plasma levels of IL-6, IL-1, TNF alpha and IL-1-RA measured 1 day before and 1 day after the onset of febrile episodes did not accurately predict the development of major infection. In contrast, IL-8 plasma levels were significantly higher in those patients who subsequently developed major infection. The question whether IL-8 plasma levels identify high risk or low risk patients with sufficient specificity and sensitivity has to be answered in large scale clinical trials.

Cytoadhesion and Falciparum Malaria: Going with the flow

Sequestration of parasitized red blood cells in the cerebral vasculature is the predisposing event to the development of cerebral malaria during infection with Plasmodium falciparum. The adhesive interaction between these cells and receptors on the endothelial cell (cytoadhesion) occurs in the dynamic environment of the microcirculation, but most studies have neglected this factor and have concentrated on measuring adhesion in static (no flow) assays. Such studies ignore the markedly different rheological properties of parasitized red blood cells that become apparent when adhesion is examined under dynamic, flow conditions that resemble those of the circulation in vivo. Here, Brian Cooke and Ross Coppel review a number of novel aspects of cytoadhesion that have been identified using flow-based assays, and discuss their relevance to the pathophysiology, investigation and clinical management of falciparum malaria.

Plasmodium falciparum rosetting is associated with malaria severity in Kenya

Rosette formation in 154 fresh Plasmodium falciparum isolates from Kenyan children with mild (n = 54), moderate (n = 64), or severe (n = 36) malaria was studied to determine whether the ability to form rosettes in vitro is correlated with malaria severity. There was a wide distribution of rosette frequencies within each clinical category; however, a clear trend towards higher rosette frequency with increasing severity of disease was seen, with the median rosette frequency of the mild-malaria group (1%; range, 0 to 82%) being significantly lower than those of the moderate-malaria group (5%; range, 0 to 45%; Mann-Whitney U test, P < 0.02) and the severe-malaria group (7%; range, 0 to 97%; Mann-Whitney U test, P < 0.003). Within the severe-malaria category there was no difference in rosetting among isolates from cerebral malaria patients or those with other forms of severe malaria. We also examined the ABO blood groups of the patients from whom isolates were obtained and found that isolates from group O patients (median rosette frequency, 2%; range 0 to 45%) rosetted less well than those from group A (median, 7%; range 0 to 82%; Mann-Whitney U test, P < 0.01) or group AB (median, 11%; range 0 to 94%; Mann-Whitney U test, P < 0.03). We therefore confirm that rosetting is associated with severe malaria and provide further evidence that rosetting is influenced by ABO blood group type. Whether rosetting itself plays a direct role in the pathogenesis of severe malaria or is a marker for some other causal factor remains unknown.

Immune Evasion by Babesia bovis and Plasmodium falciparum: Cliff-dwellers of the Parasite World

Erythrocyte-dwelling parasites, such as Babesia bovis and Plasmodium falciparum, are not accessible to the host immune system during most of their asexual reproductive cycle because they are intracellular. While intracellular, the host immune response must be directed toward the surface of the infected erythrocyte. Immune individuals mount protective antibody and cell-mediated responses which eliminate most of the parasites, yet some survive to establish chronic infections. In this review, David Allred discusses some of the mechanisms used by these parasites to evade individual immune mechanisms targeting the infected erythrocyte to survive in the hostile environment of an effective immune response.