title: Exposure of the Plasmodium falciparum clonally variant STEVOR proteins on the merozoite surface.
abstract: BACKGROUND
  Plasmodium falciparum merozoites are free invasive forms that invade host erythrocytes in iterative cycles in the presence of different arms of the immune system. Variant antigens are known to play a role in immune evasion and several gene families coding for variant antigens have been identified in P. falciparum. However, none of them have been reported to be expressed on the surface of merozoites.
  
  
  METHODS
  Flow cytometry, immunofluorescence microscopy, and immunoblotting assays were performed to assess surface exposure, membrane association and stage specific expression of the STEVOR family of variants proteins, respectively.
  
  
  RESULTS
  Using a polyclonal antibody (anti-PFL2610w) with a broad specificity towards different STEVOR variants, the STEVOR proteins were identified on the surface of non-permeabilized/non-fixed merozoites in flow cytometry assays. Anti-PFL2610w antibody showed that several STEVORs were expressed in the trophozoite stage of the parasite but only one variant was integrated into the merozoite membrane. Moreover, this antibody failed to identify STEVORs on the surface of the parent schizont infected erythrocytes (IE) although they were readily identified when schizont IE were permeabilized.
  
  
  CONCLUSIONS
  These data suggest for a role for STEVOR in immune evasion by P. falciparum merozoites to allow successful invasion of erythrocytes. Additionally, the expression of STEVORs in the schizont stage may only represent a step in the biogenesis process of the merozoite surface coat.
authors.affiliation: Malaria Research Laboratory, Department of Bacteriology and Immunology, Haartman Institute, University of Helsinki, Helsinki, Finland. ayman.khattab@helsinki.fi
authors.forename: Ayman
authors.initials: A
authors.lastname: Khattab
authors.forename: Seppo
authors.initials: S
authors.lastname: Meri
mesh_terms: D000953:Antigens, Protozoan / Q000032:analysis*; D005434:Flow Cytometry; D020869:Gene Expression Profiling; D006801:Humans; D057131:Immune Evasion; D015151:Immunoblotting; D008565:Membrane Proteins / Q000032:analysis*; D052940:Merozoites / Q000737:chemistry*; D008856:Microscopy, Fluorescence; D010963:Plasmodium falciparum / Q000737:chemistry* / Q000276:immunology / Q000472:pathogenicity*
chemical_list: D000953:Antigens, Protozoan; D008565:Membrane Proteins; C474556:STEVOR antigen, Plasmodium falciparum