While the mechanisms by which hemoglobinopathies confer mala

While the mechanisms by which hemoglobinopathies confer malaria protection are debated (), with the notable exception of HbAS, they do not seem to protect by suppressing parasite densities. Epidemiological observations, together with in-vitro data, support alternative mechanisms of protection that enable individuals to “tolerate” parasitemia without developing symptoms. In one mechanism, HbS and αthalassemia each impairs binding of parasitized red blood sirtuin 1 (pRBCs) to microvascular endothelial cells (MVECs, cytoadherence) and uninfected RBCs (rosetting), two virulence traits mediated by interactions between erythrocyte membrane protein 1 (PfEMP1) ligands on pRBCs and various receptors (CD36, ICAM1, endothelial protein C receptor [EPCR]) on host cells. We have always believed that future experimental data consistent with this mechanism are more likely to be biologically and clinically relevant if they reconcile with robust epidemiological findings. In this issue of , meet exactly this type of criterion in proposing a mechanism to explain their original 2005 report of negative epistasis between αthalassemia and HbAS in Kenya (). Specifically, they hypothesized that if αthalassemia antagonizes malaria-protective effects of HbAS in the field, then it should also antagonize cytoadherence-weakening effects of HbAS in the laboratory. Using RBCs from individuals with HbAS, αthalassemia, or both, they performed experiments that measure cytoadherence, rosetting, and PfEMP1 expression of four lines. While their reductionist approach of using few parasite strains is a minor limitation of this study, they made up for this by using a large panel of RBCs from hemoglobinopathic donors (10–20 of each genotype) (). Their findings largely confirm earlier reports that HbAS and αthalassemia are each associated with impaired cytoadherence, rosetting, and PfEMP1 expression (), with two exceptions. For one parasite strain, αthalassemia increased PfEMP1 expression without affecting rosetting; for another, αthalassemia increased PfEMP1 expression but paradoxically decreased cytoadherence. These findings contrast with those of , who generally found that αthalassemia reduced both PfEMP1 expression and cytoadherence (αthalassemia increased the cytoadherence of parasite isolates) (see Fig. 1A–B of reference ). These apparent discrepancies may result from marked differences in the authors\' use of (i) antibodies specific for PfEMP1 variants expressed by parasite lines ITvar9 and TM284var1 (vs. FVO, A4tres, and FCR3); (ii) four laboratory-adapted parasite lines (vs. multiple naturally-circulating parasite isolates from Malian children with malaria); and (iii) purified CD36 and ICAM1 proteins as binding substrates (vs. CD36-expressing MVECs and monocytes). As emphasize, future studies should test large numbers of both RBC donors and strains to help resolve these differences, which will hopefully provide more consistent experimental data that adequately explain well-established field observations in Africa (reviewed in ). These differences notwithstanding, the major new finding of this study is that αthalassemia consistently reverses HbAS-mediated reductions in cytoadherence, rosetting, and PfEMP1 expression in vitro. This solid result now provides impetus for exploring how αthalassemia antagonizes the effects of HbAS on PfEMP1-mediated phenomena. One possibility is that αthalassemia selectively reduces the amount of HbS in HbAS RBCs, for example, by reducing the number of α-globins that bind β globins to form stable hemoglobin αβ tetramers. provide preliminary evidence to support this, showing that the proportion of intracellular HbS significantly decreases as the degree of αthalassemia increases. How well these HbS proportions correlate with cytoadherence, rosetting, PfEMP1 expression, and “knob” densities () remains to be determined. Nevertheless, this study further highlights in-vitro binding assays as valuable tools in defining how hemoglobinopathies confer malaria protection. Additional experiments using naturally-circulating isolates and hemoglobinopathic RBCs from areas where αthalassemia is common are needed to confirm this study\'s findings, to investigate the lack of epistasis between αthalassemia and HbC in clinical studies (reviewed in ), and the effects of αthalassemia and HbAS on pRBC binding to EPCR — a phenotype strongly associated with cerebral malaria syndromes that are rarely seen in HbAS children.