title: Mechanistic Studies of the Negative Epistatic Malaria-protective Interaction Between Sickle Cell Trait and α+thalassemia.
abstract: BACKGROUND
  Individually, the red blood cell (RBC) polymorphisms sickle cell trait (HbAS) and α+thalassemia protect against severePlasmodium falciparummalaria. It has been shown through epidemiological studies that the co-inheritance of both conditions results in a loss of the protection afforded by each, but the biological mechanisms remain unknown.
  
  
  METHODS
  We used RBCs from >300 donors of various HbAS and α+thalassemia genotype combinations to study the individual and combinatorial effects of these polymorphisms on a range of putativeP. falciparumvirulence phenotypesin-vitro, using four well-characterisedP. falciparumlaboratory strains. We studied cytoadhesion of parasitized RBCs (pRBCs) to the endothelial receptors CD36 and ICAM1, rosetting of pRBCs with uninfected RBCs, and pRBC surface expression of the parasite-derived adhesion moleculeP. falciparumErythrocyte Membrane Protein-1 (PfEMP1).
  
  
  RESULTS
  We confirmed previous reports that HbAS pRBCs show reduced cytoadhesion, rosetting and PfEMP1 expression levels compared to normal pRBC controls. Furthermore, we found that co-inheritance of HbAS with α+thalassemia consistently reversed these effects, such that pRBCs of mixed genotype showed levels of cytoadhesion, rosetting and PfEMP1 expression indistinguishable from those seen in normal pRBCs. However, pRBCs with α+thalassemia alone showed parasite strain-specific effects on adhesion, and no consistent reduction in PfEMP1 expression.
  
  
  CONCLUSIONS
  Our data support the hypothesis that the negative epistasis between HbAS and α+thalassemia observed in epidemiological studies might be explained by host genotype-specific changes in the pRBC-adhesion properties that contribute to parasite sequestration and disease pathogenesisin vivo. The mechanism by which α+thalassemia on its own protects against severe malaria remains unresolved.
authors.affiliation: Kenya Medical Research Institute-Wellcome Trust Research Programme, 80108 Kilifi, Kenya ; Centre for Immunity, Infection and Evolution, Institute of Immunology and Infection Research, School of Biological Sciences, University of Edinburgh, EH9 3FL, United Kingdom.
authors.forename: D Herbert
authors.initials: DH
authors.lastname: Opi
authors.affiliation: Kenya Medical Research Institute-Wellcome Trust Research Programme, 80108 Kilifi, Kenya.
authors.forename: Lucy B
authors.initials: LB
authors.lastname: Ochola
authors.affiliation: Kenya Medical Research Institute-Wellcome Trust Research Programme, 80108 Kilifi, Kenya.
authors.forename: Metrine
authors.initials: M
authors.lastname: Tendwa
authors.affiliation: Kenya Medical Research Institute-Wellcome Trust Research Programme, 80108 Kilifi, Kenya.
authors.forename: Bethsheba R
authors.initials: BR
authors.lastname: Siddondo
authors.affiliation: Kenya Medical Research Institute-Wellcome Trust Research Programme, 80108 Kilifi, Kenya.
authors.forename: Harold
authors.initials: H
authors.lastname: Ocholla
authors.affiliation: Kenya Medical Research Institute-Wellcome Trust Research Programme, 80108 Kilifi, Kenya.
authors.forename: Harry
authors.initials: H
authors.lastname: Fanjo
authors.affiliation: Centre for Immunity, Infection and Evolution, Institute of Immunology and Infection Research, School of Biological Sciences, University of Edinburgh, EH9 3FL, United Kingdom.
authors.forename: Ashfaq
authors.initials: A
authors.lastname: Ghumra
authors.affiliation: Nuffield Department of Clinical Laboratory Science, University of Oxford, John Radcliffe Hospital, OX3 9DU, Oxford, United Kingdom.
authors.forename: David J P
authors.initials: DJ
authors.lastname: Ferguson
authors.affiliation: Centre for Immunity, Infection and Evolution, Institute of Immunology and Infection Research, School of Biological Sciences, University of Edinburgh, EH9 3FL, United Kingdom.
authors.forename: J Alexandra
authors.initials: JA
authors.lastname: Rowe
authors.affiliation: Kenya Medical Research Institute-Wellcome Trust Research Programme, 80108 Kilifi, Kenya ; Department of Medicine, Imperial College, St Mary's Hospital, Praed Street, London W21NY, United Kingdom.
authors.forename: Thomas N
authors.initials: TN
authors.lastname: Williams
keywords: PfEMP1; cytoadhesion; epistasis; malaria; red blood cells; sickle cell trait; α–thalassemia