Plasmodium falciparum variant STEVOR antigens are expressed in merozoites and possibly associated with erythrocyte invasion.
BACKGROUND
Plasmodium falciparum STEVOR proteins, encoded by the multicopy stevor gene family have no known biological functions. Their expression and unique locations in different parasite life cycle stages evoke multiple functionalities. Their abundance and hypervariability support a role in antigenic variation.


METHODS
Immunoblotting of total parasite proteins with an anti-STEVOR antibody was used to identify variant antigens of this gene family and to follow changes in STEVOR expression in parasite populations panned on CSA or CD36 receptors. Immunofluorescence assays and immunoelectron microscopy were performed to study the subcellular localization of STEVOR proteins in different parasite stages. The capacity of the antibody to inhibit merozoite invasion of erythrocytes was assessed to determine whether STEVOR variants were involved in the invasion process.


RESULTS
Antigenic variation of STEVORs at the protein level was observed in blood stage parasites. STEVOR variants were found to be present on the merozoite surface and in rhoptries. An insight into a participation in erythrocyte invasion was gained through an immunofluorescence analysis of a sequence of thin slides representing progressive steps in erythrocyte invasion. An interesting feature of the staining pattern was what appeared to be the release of STEVORs around the invading merozoites. Because the anti-STEVOR antibody did not inhibit invasion, the role of STEVORs in this process remains unknown.


CONCLUSIONS
The localization of STEVOR proteins to the merozoite surface and the rhoptries together with its prevalence as a released component in the invading merozoite suggest a role of these antigens in adhesion and/or immune evasion in the erythrocyte invasion process. These observations would also justify STEVORs for undergoing antigenic variation. Even though a role in erythrocyte invasion remains speculative, an association of members of the STEVOR protein family with invasion-related events has been shown.
Khattab
Ayman
A
Department of Molecular Medicine, Bernhard-Nocht Institute for Tropical Medicine, Hamburg, Germany. ayman.khattab@helsinki.fi
Bonow
Insa
I
Schreiber
Nadine
N
Petter
Michaela
M
Schmetz
Christel
C
Klinkert
Mo-Quen
MQ
D000818:Animals; D000913:Antibodies, Protozoan / Q000276:immunology; D000940:Antigenic Variation / Q000276:immunology; D000953:Antigens, Protozoan / Q000276:immunology / Q000378:metabolism*; D002478:Cells, Cultured; D004912:Erythrocytes / Q000166:cytology / Q000469:parasitology*; D005455:Fluorescent Antibody Technique; D006801:Humans; D015151:Immunoblotting; D020066:Merozoite Surface Protein 1 / Q000378:metabolism; D052940:Merozoites / Q000276:immunology / Q000378:metabolism*; D008856:Microscopy, Fluorescence; D010963:Plasmodium falciparum / Q000276:immunology / Q000378:metabolism*
D000913:Antibodies, Protozoan; D000953:Antigens, Protozoan; D020066:Merozoite Surface Protein 1; C474556:STEVOR antigen, Plasmodium falciparum