The Plasmodium falciparum STEVOR Multigene Family Mediates Antigenic Variation of the Infected Erythrocyte

Abstract

Modifications of the Plasmodium falciparum–infected red blood cell (iRBC) surface have been linked to parasite-associated pathology. Such modifications enable the parasite to establish long-lasting chronic infection by evading antibody mediate immune recognition and splenic clearance. With the exception of the well-demonstrated roles of var-encoded PfEMP1 in virulence and immune evasion, the biological significance of other variant surface antigens (rif and stevor) is largely unknown. While PfEMP1 and RIFIN have been located on the iRBC surface, recent studies have located STEVOR at the iRBC membrane where it may be exposed on the erythrocyte surface. To investigate the role of STEVOR in more detail, we have developed antibodies against two putative STEVOR proteins and used a combination of indirect immunofluorescence assays (IFA), live IFA, flow cytometry, as well as agglutination assays, which enable us to demonstrate that STEVOR is clonally variant at the surface of schizont stage parasites. Crucially, expression of different STEVOR on the surface of the iRBC changes the antigenic property of the parasite. Taken together, our data for the first time demonstrate that STEVOR plays a role in creating antigenic diversity of schizont stage parasites, thereby adding additional complexity to the immunogenic properties of the iRBC. Furthermore, it clearly demonstrates that to obtain a complete understanding of how parasite-induced pathology is linked to variation on the surface of the iRBC, focusing the interactions of multiple multigene families needs to be considered.

Targets of antibodies against Plasmodium falciparum–infected erythrocytes in malaria immunity

“…These antigens may include RIFIN, STEVOR, and SURFIN proteins, which have been identified on the IE surface (21,22,24,35,38). While it remains possible that some of the reactivity to vpkd parasites represents antibodies to residual PfEMP1 on the IE surface (if var gene expression is not completely inhibited in the vpkd lines), our Western blot analysis suggests that residual PfEMP1 is minimal.…”

“…Our standard assays were performed testing human serum samples at a 1:10 dilution; when sera were tested at higher concentrations, the same pattern of reactivity to 3D7 parental and 3D7vpkd parasites was seen (Supplemental Figure 2, A and B). Additionally, we tested sera for reactivity to surface antigens using IEs at the schizont stage of intraerythrocytic development to account for possible differences in the relative expression of surface antigens at a later stage of parasite development, and studies have reported STEVOR expression on schizont-stage IEs (35). Results were very similar to those obtained using mature trophozoite-stage IEs, showing a marked reduction of IgG binding to 3D7vpkd parasites compared with that to 3D7 parental parasites (Supplemental Figure 2, C and D).…”

“…RIFIN, STEVOR, and SURFIN proteins are also encoded by polymorphic multigene families, but their functions and roles in acquired immunity are currently unclear. Recent data suggests that RIFIN and STE-VOR proteins could be important antibody targets (35)(36)(37)(38).…”

Plasmodium falciparum STEVOR phosphorylation regulates host erythrocyte deformability enabling malaria parasite transmission

“…[14][15][16] The encoded STEVOR proteins exhibit a semiconserved N-terminal domain and a central variable domain exposed on the erythrocyte surface, whereas the conserved C-terminal domain is cytoplasmic. 17,18 Besides their role in invasion, adhesion, and rosetting in asexual stages, [19][20][21][22] STEVOR proteins have been shown to impact the deformability of the infected erythrocyte in mature asexual parasites and immature sexual stages. 6,23 However, the mechanism underlying STEVOR-mediated changes in erythrocyte deformability remains unclear.…”

Molecular secrets of a parasite

“…This antigenic variation reflects a fundamental element of parasitism 7 . PfEMP1 belongs to one of several families of variant proteins that are expressed on the surface of erythrocytes infected with P. falciparum [8][9][10] . This is a powerful parasitic defence strategy, and immunity develops slowly in patients with malaria.…”