Mechanistic Studies of the Negative Epistatic Malaria-protective Interaction Between Sickle Cell Trait and α⁺thalassemia.

Abstract

Individually, the red blood cell (RBC) polymorphisms sickle cell trait (HbAS) and α⁺thalassemia protect against severe <i>Plasmodium falciparum</i> malaria. 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. 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 putative <i>P. falciparum</i> virulence phenotypes <i>in-vitro</i>, using four well-characterised <i>P. falciparum</i> laboratory 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 molecule <i>P. falciparum</i> Erythrocyte Membrane Protein-1 (PfEMP1). 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. 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 pathogenesis <i>in vivo</i>. The mechanism by which α⁺thalassemia on its own protects against severe malaria remains unresolved.