Surface changes to human erythrocytes on infection by Plasmodium falciparum malaria
Of the four Plasmodium species which cause malaria in humans, P. falciparum is responsible for the majority of the morbidity and mortality associated with this disease. The surface expression of parasite-derived proteins in the middle of the asexual cycle coincides with two important modifications of the host erythrocyte. First, a protective immune response is directed against a family of variant antigens, known as P. falciparum Erythrocyte Membrane Protein-1 (PfEMPl). Second, ligands are detected at the surface which mediate the specific cytoadherence of infected erythrocytes to vascular endothelium, such that infected cells are sequestered away from the peripheral circulation in deep vascular beds. The potentially fatal syndrome known as cerebral malaria can ensue when infected cells sequester at high density in the brain. Indirect studies have shown that the antigenic and adhesive phenotypes at the surface are linked to the expression of PfEMPl. However, there is a paucity of biochemical data which relate to PfEMPl, and this problem is addressed in this thesis. This study has confirmed, at the biochemical level, inferences from serology that clonal antigenic variation occurred rapidly. Variation produced a number of novel antigenic and adhesive phenotypes which were associated with unique forms of PfEMPl. Further insights into the mechanism of sequestration were possible because of the finding that single infected erythrocytes had the capacity to bind to at least three putative endothelial cell receptors; CD36, Intercellular Adhesion Molecule-1 (ICAM1), and Thrombospondin (TSP). It was demonstrated for the first time that PfEMPl was responsible for cytoadherence to CD36 and ICAM1, but was probably not involved in adhesion to TSP. Extensive analysis with sequence-specific proteases proved that adhesive interactions with each receptor were separable properties of the surface, and facilitated the proposal of a domain model for PfEMPl. Detailed analysis of the antigenic and adhesive phenotypes of a series of clonally-derived parasites demonstrated that infected cells expressing all variant antigenic types could adhere to CD36 whereas adhesion to ICAM1 was seen in a restricted subset. This may be clinically relevant if, as current data suggests, adhesion of infected cells to ICAM1 is important in the development of cerebral malaria. Identification of all ICAM1 binding phenotypes could lead to the design of novel therapeutic strategies for this life-threatening condition.