Reticulocyte-prone malaria parasites predominantly invade CD71hi immature cells: implications for the development of an in vitro culture for Plasmodium vivax

Background: The lack of a continuous in vitro culture system for blood stages of malarial parasites with a unique tropism for reticulocytes, such as Plasmodium vivax and the Plasmodium yoelii 17X reticulocyte-prone strain, hinders research in these organisms. The maturation of reticulocytes into erythrocytes is a complex process involving the selective removal of membrane proteins such as the transferrin receptor, CD71. In order to advance in the characterization of infected cells during experimental infections of BALB/c mice with P. yoelii 17X, CD71 expression in erythroid cells (TER119(+)) was assessed and in vitro culture of P. yoelii 17X was attempted by adding reticulocytes highly expressing CD71. Methods: BALB/c mice were infected with P. yoelii 17X-GFP transgenic parasites and erythroid cells (TER119(+)) were analysed in blood, spleen and bone marrow cells. TER119, CD71 and GFP expression was assessed at different points post-infection by flow cytometry. Moreover, in vitro culture of P. yoelli 17X was attempted by adding red blood cells (RBCs) from mice with a pyruvate kinase deficiency, which contain high percentages of CD71(hi) cells in peripheral blood as compared to healthy animals. Results: A predominance of erythroid cells lacking expression of CD71 (CD71(-)) was observed in peripheral blood and spleen in normal and infected animals up to ten days post-infection (pi). At ten days pi, however, a dramatic temporal switch to erythroid cells highly expressing CD71 (CD71(hi)) was observed in the spleen and at day 15 pi in peripheral blood of the infected cells. A distribution of erythroid cells expressing differently CD71 was noticed in the bone marrow. Yet, similar to peripheral blood and spleen, a predominance of CD71(hi) cells was observed at 15 days pi. Remarkably, CD71(hi) cells were the cells predominantly infected in these organs as well as in peripheral blood. Attempts were thus made to culture in vitro the P. yoelli 17X strain by adding RBCs from pyruvate kinase-deficient mice containing high percentages of CD71(hi) cells in peripheral blood. Conclusions: The parasite preference for immature cells that are rare in normal peripheral blood could have important implications for the development of an in vitro culture system for P. vivax.