Deciphering Plasmodium vivax invasion pathway(s) in Duffy-negative patients

Vivax malaria is an acute debilitating illness caused by the parasitic protozoan Plasmodium vivax and transmitted by female Anopheline mosquitoes. It is mainly prevalent in America, South-East Asia, Middle East, Western Pacific, Eastern Africa, and Southern Africa but considered rare in Sub-Saharan Africa. Historically, the disease has often been regarded as a benign self-limiting infection. This is due to the observation of low parasitemia in Duffy-positive patients and virtual absence of infections in Duffy-negative individuals. Indeed, previous studies showed that individuals with African and African American origins were naturally resistant to P. vivax. Parasitologists thought that this resistance was due to the absence of Duffy Antigen Receptor for Chemokines (DARC) on the surface of their erythrocytes. Subsequently, the identification of the ligand P. vivax Duffy Binding Protein (PvDBP) specific to DARC and the evidence that the interaction PvDBP- DARC was crucial for invasion led to a scientific paradigm by which P. vivax merozoites exclusively invade Duffy-positive erythrocytes. Consequently, research on vivax malaria has long been neglected and many knowledge and tool gaps remain to be filled. However, since a few years, the stagnating burden of the disease in many countries associated with the increasing report of P. vivax infections in Duffy-negative patients raised questions about P. vivax invasion pathways. A first hypothesis was that P. vivax may have evolved to a new invasion pathway that would overcome Duffy-negativity. Alternatively, a second hypothesis was that Duffy-negative populations may have always been a silent and insidious reservoir of infection which had previously gone unnoticed. My thesis project aimed at deciphering Plasmodium vivax invasion pathways in Duffy-negative patients. 1.Bibliographical study: understand the global context and key issues of the presence of vivax malaria in Sub-Saharan Africa and review current knowledge on P. vivax invasion pathways through the lens of omics technologies. 2.Molecular epidemiological study: understand vivax malaria epidemiology in Ethiopia, dissect the genome of P. vivax strains circulating in Sub-Saharan Africa and search for potential molecular signature in genes encoding proteins involved in P. vivax invasion pathways using next generation sequencing techniques. 3.Functional study: characterize Duffy-negative erythroblasts during terminal erythroid differentiation and assess the ability of P. vivax merozoites to invade Duffy-negative erythroblasts by developing reproducible in vitro functional assays. Findings of the bibliographical study showed that despite the recent advances of omics technologies, key knowledge and tool gaps remain in vivax malaria research. Besides, findings of the molecular epidemiological studies revealed that P. vivax strains cluster in distinct geographic clusters. However, these findings did not reveal any association between genetic diversity and adaptation to Duffy-negative human hosts. This suggests that the parasites did not evolve towards an alternative invasion pathway. In fact, findings of the in vitro functional study demonstrated that a subset of Duffy-negative erythroblasts can express functional DARC during terminal erythroid differentiation and that P. vivax merozoites can invade them. Taken together, these findings contrast with the established scientific paradigm by which P. vivax merozoites exclusively invade Duffy-positive erythrocytes and support the assumption that African Duffy-negative populations may be a silent and insidious reservoir of infection, with all the consequences that this entails for the control and eradication of vivax malaria in Sub-Saharan Africa.