Abstract
Sickle hemoglobin (Hb) S and HbC may protect against malaria by reducing the expression of Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) on the surface of parasitized red blood cells (RBCs), thereby weakening their cytoadherence to microvascular endothelial cells (MVECs) and impairing their activation of MVECs to produce pathological responses. Therefore, we hypothesized that parasites causing malaria in HbAS or HbAC heterozygotes have overcome this protective mechanism by expressing PfEMP1 variants which mediate relatively strong binding to MVECs. To test this hypothesis, we performed 31 cytoadherence comparisons between parasites from HbAA and HbAS (or HbAC) Malian children with malaria. Ring-stage parasites from HbAA and HbAS (or HbAC) children were cultivated to trophozoites, purified, and then inoculated in parallel into the same wildtype uninfected RBCs. After one cycle of invasion and maturation to the trophozoite stage expressing PfEMP1, parasite strains were compared for binding to MVECs. In this assay, there were no significant differences in the binding of parasites from HbAS and HbAC children to MVECs compared to those from HbAA children (HbAS, fold-change = 1.46, 95% CI 0.97–2.19, p = 0.07; HbAC, fold-change = 1.19, 95% CI 0.77–1.84, p = 0.43). These data suggest that in-vitro reductions in cytoadherence by HbS and HbC may not be selecting for expression of high-avidity PfEMP1 variants in vivo. Future studies that identify PfEMP1 domains or amino-acid motifs which are selectively expressed in parasites from HbAS children may provide further insights into the mechanism of malaria protection by the sickle-cell trait.
Highlights
The significant morbidity and mortality of Plasmodium falciparum malaria has selected for red blood cell (RBC) polymorphisms, including sickle hemoglobin (Hb) S, HbC, a-thalassemia, and G6PD deficiency [1,2,3,4,5,6,7,8,9,10]
Parasites from HbAS children showed increased binding to microvascular endothelial cells (MVECs) compared to those from HbAA children, but this increase was not significant (Figure 2, Table 2)
We hypothesized that P. falciparum parasites in HbAS and HbAC children with malaria overcome abnormal Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) display by expressing PfEMP1 variants which bind relatively strongly to MVECs
Summary
The significant morbidity and mortality of Plasmodium falciparum malaria has selected for red blood cell (RBC) polymorphisms, including sickle hemoglobin (Hb) S, HbC, a-thalassemia, and G6PD deficiency [1,2,3,4,5,6,7,8,9,10]. These malaria protective polymorphisms have reached high frequencies in tropical areas despite the spectrum of deleterious consequences associated with their homozygous (or hemizygous) states. One proposed mechanism, which would protect through alterations in host pathogenesis, involves the abnormal display of P. falciparum erythrocyte membrane protein 1 (PfEMP1), the parasite’s variant surface antigen and cytoadherence ligand, on ‘knobs’ on the surface of parasitized HbAS and HbAC RBCs [13,14]
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