The density of knobs on Plasmodium falciparum-infected erythrocytes depends on developmental age and varies among isolates

PLoS One. 2012;7(9):e45658. doi: 10.1371/journal.pone.0045658. Epub 2012 Sep 20.


Background: The virulence of Plasmodium falciparum malaria is related to the parasite's ability to evade host immunity through clonal antigenic variation and tissue-specific adhesion of infected erythrocytes (IEs). The P. falciparum erythrocyte membrane protein 1 (PfEMP1) family expressed on dome-shaped protrusions called knobs on the IE surface is central to both. Differences in receptor specificity and affinity of expressed PfEMP1 are important for IE adhesiveness, but it is not known whether differences in the number and size of the knobs on which the PfEMP1 proteins are expressed also play a role. Therefore, the aim of this study was to provide detailed information on isolate- and time-dependent differences in knob size and density.

Methodology/principal findings: We used atomic force microscopy to characterize knobs on the surface of P. falciparum-infected erythrocytes. Fourteen ex vivo isolates from Ghanaian children with malaria and 10 P. falciparum isolates selected in vitro for expression of a particular PfEMP1 protein (VAR2CSA) were examined. Knob density increased from ∼20 h to ∼35 h post-invasion, with significant variation among isolates. The knob density ex vivo, which was about five-fold higher than following long-term in vitro culture, started to decline within a few months of culture. Although knob diameter and height varied among isolates, we did not observe significant time-dependent variation in these dimensions.

Conclusions/significance: The density of knobs on the P. falciparum-IE surface depends on time since invasion, but is also determined by the infecting isolate in a time-independent manner. This is the first study to quantitatively evaluate knob densities and dimensions on different P. falciparum isolates, to examine ex vivo isolates from humans, and to compare ex vivo and long-term in vitro-cultured isolates. Our findings contribute to the understanding of the interaction between P. falciparum parasites and the infected host.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Child
  • Erythrocytes / parasitology*
  • Humans
  • Malaria, Falciparum
  • Microscopy, Atomic Force
  • Plasmodium falciparum / pathogenicity*

Grant support

The research leading to these results was funded by Danish Medical Research Council Grant 271-07-0301, Danish Consultative Research Committee for Development Research (FFU) Grant 104.Dan.8 l/306, European Community’s Seventh Framework Programme (FP7/2007–2013) under grant agreement 242095, Rigshospitalet Grant 9615.05330, and the University of Copenhagen Program of Excellence in membrane topology and quaternary structure of key membrane proteins involved in P. falciparum malaria pathogenesis and immunity. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.