Under the Microscope

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Human malaria is caused by four different species of Plasmodium: P. falciparum, P. malariae, P. ovale and P. vivax. Prevalent in Southeast Asia and Latin America, P. vivax is the second most significant species in terms of infected patients in the world. Malaria parasites invade red blood cells and most Plasmodium species prefer to invade younger erythrocytes (reticulocytes) as they have a longer remaining life span. This allows for uninterrupted asexual development and the extended circulation of sexual forms of the parasite.

The process of parasite invasion needs a succession of steps associated with the recognition of host-protein by parasite protein-ligands but the number of interaction ligand-receptors and the temporal sequence of these events are not known. The first step probably happens when free P. vivax merozoites use P. vivax Merozoite Surface Protein 1 (PvMSP1) to identify the surface of erythrocytes. The second step involves proteins expressed at the surface of young reticulocytes (CD71 positive) only. For the past 40 years, the best known receptor in P. vivax biology is the Duffy Antigen Chemokine Receptor (DARC or Duffy receptor) but this receptor cannot explain cell tropism of P. vivax because it is also expressed on mature erythrocytes. However, recent study has identified CD71 as a P. vivax reticulocyte receptor and confirmed the CD71+ reticulocyte cell tropism. The expression of reticulocyte-specific receptor(s) on mature erythrocytes can be an approach to facilitate the development of P. vivax continuous culture.

Malleret B, Rénia L, Russell B (2017) The unhealthy attraction of Plasmodium vivax to reticulocytes expressing transferrin receptor 1 (CD71). Int J Parasitol, 47(7):379-383.