Abstract
Characterization of virulence factors in P. falciparum malaria is essential in order to identify new therapeutic and prophylactic targets. Rosette formation, the binding of uninfected red blood cells to parasite-infected red blood cells, is a P. falciparum virulence phenotype associated with severe clinical manifestations, e. g. cerebral malaria and severe anemia. Humoral responses to rosetting epitopes seem to confer protective immunity and we have shown in these studies that individuals living in malaria endemic areas build up age-related humoral responses to rosetting epitopes that parallel the development of clinical immunity. Furthermore, epitopes that mediate the rosetting phenotype appear to be diverse but conserved between distant regions in Africa.
We have performed a detailed study of the cell-cell binding mechanisms that govern rosetting. Polysaccharides belonging to the glycosaminoglycan family, especially heparin and heparan sulfate, inhibit rosette formation and treatments of host cells indicate that heparan sulfate-like glycans support adhesion. In addition, the ABO blood group phenotype of the infected host modulates rosetting and blood group A and B antigens have been shown to function as co-receptors to other receptors in rosetting.
The parasite-derived adhesion molecule that mediates rosetting has been identified as a Plasmodium falciparum erythrocyte membrane protein I (PfEMP1) variant, a product of the vast family of var genes. PfEMP1 mediates adhesion by interacting with host cell glycan receptors, such as heparan sulfate and blood group A antigen. We have characterized the binding of heparan sulfate and heparin to the rosetting domain of PfEMP1. Important molecular features of oligosaccharides required for optimal binding, such as molecular size (12-mer oligosaccharide chain or larger) and N-sulfation, have thus been identified. In field studies, the heparin-binding phenotype of P. falciparum was found to be more common among patients with severe malaria. Furthermore, the clinical isolates had the ability to adhere to multiple receptors. We have characterized the binding properties of PfEMP1. to multiple host receptors, which may explain the poly-adhesive P. falciparum phenotype associated with severe disease in these studies.
The present investigation contributes to the molecular elucidation of virulent adhesive phenotypes in P. falciparum malaria. The identified molecules involved in the cell-cell adherence may serve as targets for prophylactic and therapeutic measures.
