Severe malaria: what's new on the pathogenesis front?

Plasmodium falciparum causes the most severe and fatal form of malaria in humans with over half a million deaths each year. Cerebral malaria, a complex neurological syndrome of severe falciparum malaria, is often fatal and represents a major public health burden. Despite vigorous efforts, the pathophysiology of cerebral malaria remains to be elucidated, thereby hindering the development of adjunctive therapies. In recent years, multidisciplinary and collaborative approaches have led to groundbreaking progress both in the laboratory and in the field. Here we review the latest breakthroughs in severe malaria pathogenesis, with a specific focus on new pathogenetic mechanisms leading to cerebral malaria. The most recent findings point towards specific parasite phenotypes targeting brain microvasculature, endothelial dysfunction and subsequent oedema-induced brain swelling

Vascular endothelial cells cultured from patients with cerebral or uncomplicated malaria exhibit differential reactivity to TNF.

Plasmodium falciparum malaria is a major cause of morbidity and mortality in African children, and factors that determine the development of uncomplicated (UM) versus cerebral malaria (CM) are not fully understood. We studied the ex vivo responsiveness of microvascular endothelial cells to pro-inflammatory stimulation and compared the findings between CM and UM patients. In patients with fatal disease we compared the properties of vascular endothelial cells cultured from brain tissue to those cultured from subcutaneous tissue, and found them to be very similar. We then isolated, purified and cultured primary endothelial cells from aspirated subcutaneous tissue of patients with CM (EC(CM) ) or UM (EC(UM) ) and confirmed the identity of the cells before analysis. Upon TNF stimulation in vitro, EC(CM) displayed a significantly higher capacity to upregulate ICAM-1, VCAM-1 and CD61 and to produce IL-6 and MCP-1 but not RANTES compared with EC(UM) . The shedding of endothelial microparticles, a recently described parameter of severity in CM, and the cellular level of activated caspase-3 were both significantly greater in EC(CM) than in EC(UM) . These data suggest that inter-individual differences in the endothelial inflammatory response to TNF may be an additional factor influencing the clinical course of malaria

Platelet-Induced Clumping of Plasmodium falciparum–Infected Erythrocytes from Malawian Patients with Cerebral Malaria—Possible Modulation In Vivo by Thrombocytopenia

Platelets may play a role in the pathogenesis of human cerebral malaria (CM), and they have been shown to induce clumping of Plasmodium falciparum–parasitized red blood cells (PRBCs) in vitro. Both thrombocytopenia and platelet-inducedPRBCclumping are associated with severe malaria and, especially, withCM.In the present study, we investigated the occurrence of the clumping phenomenon in patients with CM by isolating and coincubating their plasma and PRBCs ex vivo. Malawian children with CM all had low platelet counts, with the degree of thrombocytopenia directly proportional to the density of parasitemia. Plasma samples obtained from these patients subsequently induced weak PRBC clumping. When the assays were repeated, with the plasma platelet concentrations adjusted to within the physiological range considered to be normal, massive clumping occurred. The results of this study suggest that thrombocytopenia may, through reduction of platelet-mediated clumping of PRBCs, provide a protective mechanism for the host during CM

Platelets Alter Gene Expression Profile in Human Brain Endothelial Cells in an In Vitro Model of Cerebral Malaria

Platelet adhesion to the brain microvasculature has been associated with cerebral malaria (CM) in humans, suggesting that platelets play a role in the pathogenesis of this syndrome. In vitro co-cultures have shown that platelets can act as a bridge between Plasmodium falciparum-infected red blood cells (pRBC) and human brain microvascular endothelial cells (HBEC) and potentiate HBEC apoptosis. Using cDNA microarray technology, we analyzed transcriptional changes of HBEC in response to platelets in the presence or the absence of tumor necrosis factor (TNF) and pRBC, which have been reported to alter gene expression in endothelial cells. Using a rigorous statistical approach with multiple test corrections, we showed a significant effect of platelets on gene expression in HBEC. We also detected a strong effect of TNF, whereas there was no transcriptional change induced specifically by pRBC. Nevertheless, a global ANOVA and a two-way ANOVA suggested that pRBC acted in interaction with platelets and TNF to alter gene expression in HBEC. The expression of selected genes was validated by RT-qPCR. The analysis of gene functional annotation indicated that platelets induce the expression of genes involved in inflammation and apoptosis, such as genes involved in chemokine-, TREM1-, cytokine-, IL10-, TGFβ-, death-receptor-, and apoptosis-signaling. Overall, our results support the hypothesis that platelets play a pathogenic role in CM

Type I Interferon in Malaria: A Balancing Act.

Type I interferons (IFN-Is) can now be considered as the wedge that balances clinical protection to malaria. New studies recently highlighted a central role for IFN-Is in orchestrating an immunoregulatory network leading to the dampening of proinflammatory responses, expansion of type 1 regulatory (Tr1) cells, and restriction of humoral immunity during malaria blood stage infection. Plasmacytoid dendritic cells (pDCs) were identified as the major source of IFN-Is. Here, we integrate the findings and provide a model for the mechanisms involved

Pathogenèse du neuropaludisme: Faits et hypothèses

Cerebral malaria (CM) is one of the most serious complications of Plasmodium falciparum infection. It is characterized by sequestration of parasitized red blood cells (PRBC) in cerebral capillaries and venules. Although the exact cause of CM remains unclear, current evidence has clearly implicated metabolic disturbances and host immune responses. Studies on mouse CM models suggest the involvement of host cells and in particular platelets. These results led us to study the role of platelets in human CM. Our findings demonstrated that significantly greater accumulation of platelets occurred in capillaries and venules of Malawian patients who died from CM than from other diseases. We also assessed the role of platelets in cytoadherence of PRBCs using PRBC adhering only on CD36, platelets and endothelial cells (EC) constitutively devoid of CD36. Cultures using the three components showed that platelets played a role in inducing cytoadherence of PRBC on EC via a cellular bridging resistant to physiological flow conditions. Having established the link between platelets and sequestration, the next step will be to examine the link between platelets and CM. A combination of approaches from different disciplines will be needed to gain further insight into the mechanisms underlying the complications of malaria.info:eu-repo/semantics/publishe

Cerebral malaria: Role of microparticles and platelets in alterations of the blood-brain barrier

Brain lesions of cerebral malaria (CM) are characterised by a sequestration of Plasmodium falciparum-parasitised red blood cells (PRBC), leucocytes and platelets within brain microvessels, by an excessive release of pro-inflammatory cytokines as well as by disruption of the blood-brain barrier (BBB). We evaluated the possibility that PRBC and platelets interact and induce functional alterations in brain endothelium. Using an in vitro model of endothelial lesion, we showed that platelets can act as bridges between PRBC and endothelial cells (EC) allowing the binding of PRBC to endothelium devoid of cytoadherence receptors. Furthermore, platelets potentiated the cytotoxicity of PRBC for brain EC by inducing an alteration of the integrity of their monolayer and increasing their apoptosis. These findings provide insights into the mechanisms by which platelets can be deleterious to the brain endothelium during CM. Another aspect of inflammatory and infectious diseases is that they often lead to activation of vascular and blood cells. Such activation results in an enhanced vesiculation, i.e. the release of circulating microparticles (MP). We thus explored plasma levels of endothelial MP in Malawian children with malaria. Plasma MP numbers were markedly increased on admission only in patients with severe malaria complicated with coma. Using the experimental mouse model of CM, we evaluated the pathogenic implications of MP using genetically deficient mice in which the capacity to vesiculate is impaired. Such mice, lacking the ABCA-1 gene, upon infection by Plasmodium berghei ANKA, showed complete resistance to CM. When purified from infected susceptible animals, MP were able to reduce normal plasma clotting time and to significantly enhance tumour necrosis factor release from naïve macrophages. Altogether these data provide a novel insight into the pathogenic mechanisms leading to the neurological syndrome. The finding that ABCA-1 gene deletion confers complete protection against cerebral pathology, linked to an impaired MP production, provides new potential targets for therapeutic amelioration of severe malaria.info:eu-repo/semantics/publishe

Bridging and Clumping: Investigating Platelet Interactions with P. falciparum-Infected Red Blood Cells and Endothelial Cells in Cerebral Malaria

The methods presented in this chapter describe how to perform ex vivo clumping and in vitro bridging assays in the context of cerebral malaria. Both the protocols are detailed, and emphasis is made on how to prepare platelet suspensions suitable to each technique, including description of specific buffers and reagents to minimize the risk of aggregation while maintaining the platelet properties

Platelets reorient Plasmodium falciparum-infected erythrocyte cytoadhesion to activated endothelial cells.

Severe malaria is characterized by the sequestration of Plasmodium falciparum-infected erythrocytes (IEs). Because platelets can affect tumor necrosis factor (TNF)-activated endothelial cells (ECs), we investigated their role in the sequestration of IEs, using IEs that were selected because they can adhere to endothelial CD36 (IE(CD36)), a P. falciparum receptor that is expressed on platelets. The results of coincubation studies indicated that platelets can induce IE(CD36) binding to CD36-deficient brain microvascular ECs. This induced cytoadhesion resisted physiological shear stress, was increased by EC stimulation with TNF, and was abolished by anti-CD36 monoclonal antibody. Immunofluorescence and scanning electron microscopy results showed that platelets serve as a bridge between IEs and the surface of ECs and may therefore provide receptors for adhesion to microvascular beds that otherwise lack adhesion receptors. This novel mechanism of cytoadhesion may reorient the sequestration of different parasite phenotypes and play an important role in the pathogenesis of severe malaria