Biochemical and Bioinformatic Characterisation of Understudied Erythrocyte Surface Expressed Hypervariable Protein Families in Plasmodium falciparum

Pathogenesis of Plasmodium falciparum (Pf) malaria infection is dependent on parasite and host factors, and geographic and social factors, causing different clinical outcomes and disease severity. Parasite virulence is partly caused by evasion of the human host immune system during the blood stage of infection. Sequestration of parasite infected erythrocytes (IE) through cytoadherence are characteristic Pf virulence factors, enabled by parasite-derived proteins expressed on the surface of IEs. These proteins are antigenic and are associated with acquired immunity to Pf. IE surface-expressed antigens are associated with antigenic variability, called Variant Surface Antigens (VSAs). RIFIN and STEVOR are VSA protein families encoded respectively by approximately 180 rif and 40 stevor gene copies per parasite, expressing a single variant per parasite. Members of each family differ mostly in their hypervariable regions, which are exposed to the circulation and possess antigenic epitopes. Both variable domains are associated with Pf exposure and potentially clinical outcome. Seroreactivity and serorecognition to both protein families are age and exposure dependent, with higher reactivity in adults and higher domain recognition in individuals with clinical disease. This study demonstrates the successful expression of isolated domains from two RIFIN and five STEVOR proteins as recombinant antigens, characterises their antigenicity, and demonstrates age-dependent immunity acquisition to the recombinants. Furthermore, the study reports the development of a specific in-silico model for the characterisation of STEVOR variants into clusters, after exploring other conventional methods for variant grouping. This model is then used to develop a library of eleven STEVOR variants as recombinant antigens and to further explore the breadth of antibody responses to the library in Sub-Saharan African populations, characterised by contrasting endemicity levels: high in Uganda and low in The Gambia and Guinea-Bissau. The overall aim of the study was to develop a library of recombinant antigens from one of the understudied P. falciparum infected erythrocyte hypervariable protein families and explore their immunological profile in populations with contrasting malaria endemicity levels, aiming to investigate whether antibodies to these proteins contribute to the infection immunity to Pf.