Abstract:
Plasmodium vivax cannot be maintained in a continuous culture. Evidently,
reticulocytes are the main targets for merozoite interaction and subsequent invasion
to sustain the blood-stage cycles. In this study, we produced newly differentiated red
blood cells (RBC) in vitro from freshly isolated human cord-blood stem cells.
Hematopoietic stem cells (HSCs) were activated with erythropoietin and derived to
become erythroid cells. Differentiation and maturation of erythroid cells were
analyzed by determination of erythroid cell surface markers (CD 71, 36, glycophorin
A and Duffy blood group Fy6). Duffy+ reticulocytes appeared after 10 to 16 erythroid
cell cultures. These erythroid cells called growing RBCs (gRBCs), were then used to
culture vivax parasites isolated from patients confirmed by parasite detection using
blood smear stained with Giemsa. After co-culture of vivax parasites and gRBCs,
parasite-infected gRBCs were determined by Giemsa staining and
immunofluorescence assay (IFA) using anti-Duffy binding protein antibody and
monoclonal antibody 3F9. The results showed that infected gRBCs had reacted with
these two antibodies. Schizont and early stages of parasites in culture were illustrated
by Giemsa staining. This indicates that vivax merozoites could invade and develop to
schizont stage in gRBCs. Eleven of 14 vivax parasite isolates could be continuously
cultured by adding fresh gRBCs. These parasite cultures could be maintained for
more than 2 weeks and one culture could grow continuously for up to 85 days.
Therefore, the production of gRBCs from HSCs is an alternative method for
providing Duffy+ reticulocytes, which are specific target cells for in vitro continuous
culture of P. vivax. Interestingly, it was discovered that vivax parasites could invade
nucleated erythrocytes or erythroblasts. Direct or indirect destruction of these cells by
vivax parasites may be an underlying pathogenetic mechanism contributing to
anemia in vivax malaria.