The Role of an Intercellular Adhesion Molecule 1 Binding Motif in Plasmodium Falciparum Erythrocyte Membrane Protein 1 and Other Determinants Involved in Severe Malaria Pathogenesis
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Date
2017-07
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University Of Ghana
Abstract
A central pathogenic feature in Plasmodium falciparum cerebral malaria (CM) is the sequestration of infected erythrocytes (IEs) which results in the obstruction of blood flow, inflammation and damage to the endothelial cell barrier integrity. Adhesion is mediated by a family of approximately 60 highly variant parasite proteins called P. falciparum erythrocyte membrane protein 1 (PfEMP1), which are displayed on the surface of infected cells. All PfEMP1 contain multiple extracellular domains of Duffy binding like (DBL) and cysteine-rich interdomain region (CIDR) proteins that mediate adhesion to various host endothelial receptors such as intercellular adhesion molecule 1 (ICAM-1). An important unanswered question in malaria pathogenesis has always been whether a specific binding phenotype can be linked to any of the different severe malaria phenotypes. The broad objectives of this study were to investigate the involvement of an ICAM-1 binding motif in PfEMP1 in the development of CM, as well as to evaluate the contributions of the immune response and other determinants driving severe malaria complications.
A highly conserved motif in DBLβ domain of group A PfEMP1 molecules was identified using sequence information. IEs expressing PfEMP1 containing the motif had a dual binding affinity for endothelial protein C receptor (EPCR) and ICAM-1 when tested under in vitro flow adhesion assays. Quantitative real time PCR (qPCR) showed higher transcription of this motif in parasites isolated from children with CM compared with non-cerebral severe malaria (p=0.020). The sequences defining the motif were exceptionally conserved and thus suggested its ability to induce cross-reactive responses. Broadly reactive cross-inhibitory antibodies against the motif were acquired following P. falciparum natural infections, in addition to being experimentally induced. Such cross-reactive antibodies interfered with ICAM-1
binding of recombinant motif-containing DBLβ domains and IEs from paediatric CM patients. These findings make the motif an attractive candidate to be included in a vaccine cocktail that seeks to prevent CM complications.
Markers of endothelial activation (sICAM-1, sEPCR, Ang-2, HMGB1) in Ghanaian children with severe or uncomplicated malaria and their levels following resolution of infection were determined using ELISA. The levels were higher in malaria patients in comparison with healthy controls (p < 0.001). These levels declined significantly in convalescent patients except for high mobility group box 1 protein (HMGB1) which remained high to a similar degree as observed during infection (p > 0.05), an observation reported for the first time in malaria.
Blood outgrowth endothelial cells (BOECs) from peripheral blood mononuclear cells of Ghanaian malaria patients were isolated and characterised as physiologically relevant cells to potentially study falciparum-infected erythrocyte adhesive interactions. Flow cytometry analysis showed that BOECs expressed several endothelial surface markers including malaria parasite adhesion proteins ICAM-1 and EPCR. Thus, future use of these cells could advance our understanding of cytoadhesion involving both receptors.
Very little is known about the frequency of IgM-binding PfEMP1 in the P. falciparum genome, a phenotype that has been associated with severe disease. Quantitative real time PCR was used to assess the var transcript levels of 3D7/NF54 parasites selected for IgM binding. This study uncovered at least six new var/PfEMP1 candidates that could potentially mediate binding to IgM.
Overall, this study expands current knowledge on the contributions made by both host and parasite factors to severe malaria pathogenesis. Detailed knowledge of these factors is crucially important for effective management of malaria.
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Thesis (PHD.)
Keywords
Intercellular Adhesion Molecule 1 Binding Motif, Plasmodium Falciparum, Malaria Pathogenesis, Determinants, Ghana