In Vitro Variant Surface Antigen Expression in Plasmodium falciparum Parasites from a Semi-Immune Individual Is Not Correlated with Var Gene Transcription

PLoS One. 2016 Dec 1;11(12):e0166135. doi: 10.1371/journal.pone.0166135. eCollection 2016.

Abstract

Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) is considered to be the main variant surface antigen (VSA) of Plasmodium falciparum and is mainly localized on electron-dense knobs in the membrane of the infected erythrocyte. Switches in PfEMP1 expression provide the basis for antigenic variation and are thought to be critical for parasite persistence during chronic infections. Recently, strain transcending anti-PfEMP1 immunity has been shown to develop early in life, challenging the role of PfEMP1 in antigenic variation during chronic infections. In this work we investigate how P. falciparum achieves persistence during a chronic asymptomatic infection. The infected individual (MOA) was parasitemic for 42 days and multilocus var gene genotyping showed persistence of the same parasite population throughout the infection. Parasites from the beginning of the infection were adapted to tissue culture and cloned by limiting dilution. Flow cytometry using convalescent serum detected a variable surface recognition signal on isogenic clonal parasites. Quantitative real-time PCR with a field isolate specific var gene primer set showed that the surface recognition signal was not correlated with transcription of individual var genes. Strain transcending anti-PfEMP1 immunity of the convalescent serum was demonstrated with CD36 selected and PfEMP1 knock-down NF54 clones. In contrast, knock-down of PfEMP1 did not have an effect on the antibody recognition signal in MOA clones. Trypsinisation of the membrane surface proteins abolished the surface recognition signal and immune electron microscopy revealed that antibodies from the convalescent serum bound to membrane areas without knobs and with knobs. Together the data indicate that PfEMP1 is not the main variable surface antigen during a chronic infection and suggest a role for trypsin sensitive non-PfEMP1 VSAs for parasite persistence in chronic infections.

MeSH terms

  • Antigenic Variation / genetics
  • Antigenic Variation / immunology
  • Antigens, Protozoan / genetics*
  • Antigens, Protozoan / immunology
  • Antigens, Surface / genetics
  • Antigens, Surface / immunology
  • Erythrocytes / parasitology
  • Flow Cytometry
  • Gene Expression Regulation
  • Gene Knockdown Techniques
  • Genotype
  • Humans
  • Malaria, Falciparum / genetics*
  • Malaria, Falciparum / immunology
  • Malaria, Falciparum / parasitology
  • Malaria, Falciparum / pathology
  • Plasmodium falciparum / genetics*
  • Plasmodium falciparum / immunology
  • Plasmodium falciparum / pathogenicity
  • Protozoan Proteins / biosynthesis
  • Protozoan Proteins / genetics*

Substances

  • Antigens, Protozoan
  • Antigens, Surface
  • Protozoan Proteins
  • erythrocyte membrane protein 1, Plasmodium falciparum

Grants and funding

his work was funded by BMBF-grant 01KA110 of the German ministry for education and research (BMBF) and GIF-Research Project No. 997/2008 of the German Israeli Foundation to MFrank. SD, EB and MFrank received salary support through the BMBF-grant 01KA1101. CE was the recipient of a PhD scholarship by the Landesgraduiertenförderung Baden-Württemberg. CE and SD received additional salary support through GIF-Research Project No. 997/2008. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.