Endothelial cytoadherence, rosetting and virulence in Plasmodium falciparum malaria

Abstract

Excessive sequestration of Plasmodium falciparum-infected erythrocytes (pRBC) and uninfected erythrocytes directly blocks the microcirculation and precipitates severe symptoms from the affected organ and is thereby thought to be responsible for the severe complications which may occur during P. falciparum infection. Binding of infected erythrocytes to the microvascular endothelium (cytoadherence), to uninfected erythrocytes (rosetting) and to infected erythrocytes (autoagglutination, giant rosetting) participate in the blockade. These features displayed by P. falciparum separate it from the other Plasmodiae causing non-lethal disease in humans.

The investigations included in this thesis were undertaken with the objective to further elucidate the molecular background underlying cytoadherence and rosetting and also to put these findings within a clinical frame.

Platelet endothelial cell adhesion molecule 1 (PECAM-1/CD31) is here described as a novel endothelial receptor for pRBC and the binding was mapped to the N-terminal domains of the receptor. An assay to score PECAM-1/CD31 binding was developed where the soluble receptor was labelled with a fluorophore and detected bound to the surface of the pR13C. PECAM-1/CD31 binding was found to be a common feature among field isolates where up to 88% recognised the receptor, suggesting that the receptor may be of functional relevance.

We studied the importance of non-immune immunoglobulins for rosette formation and developed an assay for the enrichment of infected erythrocytes that bind immunoglobulins to the red cell surface. The adhesive ligand Plasmodium falciparum Erythrocyte Membrane Protein-1 (PfEMP-1) was cloned from the multiadhesive parasite line FCR3S1.2 and characterised for adhesion at the molecular level. The conserved head structure of the molecule was found to mediate binding to several distinct receptors (Heparan sulfatelike GAG, blood group A, IgM, PECAM-1/CD31 and CD36). It was also demonstrated that isolates taken from children suffering from severe malaria infection displayed the ability to adhere to multiple receptors, form rosettes, giant rosettes and bind to blood group A and heparin to a higher extent than did those from patients with uncomplicated disease.

Taken together, these findings increase the understanding of the molecular pathophysiology underlying severe malaria and may be useful in the development of new strategies to combat the disease.