Structure-guided identification of a family of dual receptor-binding PfEMP1 that is associated with cerebral malaria

Cerebral malaria is a deadly outcome of infection by Plasmodium falciparum, occurring when parasite-infected erythrocytes accumulate in the brain. These erythrocytes display parasite proteins of the PfEMP1 family that bind various endothelial receptors. Despite the importance of cerebral malaria, a binding phenotype linked to its symptoms has not been identified. Here, we used structural biology to determine how a group of PfEMP1 proteins interacts with intercellular adhesion molecule 1 (ICAM-1), allowing us to predict binders from a specific sequence motif alone. Analysis of multiple Plasmodium falciparum genomes showed that ICAM-1-binding PfEMP1s also interact with endothelial protein C receptor (EPCR), allowing infected erythrocytes to synergistically bind both receptors. Expression of these PfEMP1s, predicted to bind both ICAM-1 and EPCR, is associated with increased risk of developing cerebral malaria. This study therefore reveals an important PfEMP1-binding phenotype that could be targeted as part of a strategy to prevent cerebral malaria

Structure of the PfEMP1:EPCR complex and inhibition of the interaction

The Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) family is central for Plasmodium falciparum infection. This family mediates interactions between the infected erythrocyte and the host endothelial surface, making it an obvious vaccine target. However this family is very diverse, with sixty different variants in the genome of each parasite. Despite the diversity within the PfEMP1 family, only around ten ligands have been identified. This suggests that there must be conservation of ligand binding amongst the sequence variety, which could be targeted in vaccine design strategies. This thesis explores this possibility, using the interaction between the CIDR&alpha;1 domain within the PfEMP1 and endothelial protein C receptor (EPCR) as an example, as this interaction has been associated with severe childhood malaria. This thesis details the solution of the first PfEMP1:ligand crystal structure. The two CIDR:EPCR complex structures show the binding surface of the CIDR domains to consist of a central hydrophobic residue which protrudes into a hydrophobic groove in EPCR, surrounded by a ring of hydrophilic residues. This surface mimics features of the natural EPCR ligand, protein C, and can block this ligand interaction. Combination of this structure with sequence data from 885 CIDR&alpha;1 domains show that the EPCR-binding surfaces of CIDR&alpha;1 domains are conserved in shape and bonding potential, despite dramatic sequence diversity. Having identified the conserved features of this interaction, three synthetic immunogens were designed to elicit cross-inhibitory antibodies against this interaction site. The work within this thesis shows for the first time how the malarial PfEMP1 proteins manage to simultaneously diverge whilst maintaining the capacity to bind to their ligand, and explores vaccine design strategies to raise a cross-inhibitory response.</p

Structure-guided design of a synthetic mimic of an endothelial protein C receptor-binding PfEMP1 protein

Vaccines train our immune systems to generate antibodies which recognize pathogens. Some of these antibodies are highly protective, preventing infection, while others are ineffective