The asexual erythrocytic cycle responsible for the clinical manifestations of malaria is perpetuated by merozoites which rupture out of infected erythrocytes and invade uninfected ones. Interaction between the merozoites and receptors on the erythrocyte surface initiate invasion (1-3). Chemical or immunologic interference between the merozoite and its receptor could block the infection at this point in the life cycle. Previously, we studied the characteristics of the erythroycte surface that are required for interaction and subsequent invasion by Plasmodium knowlesi merozoites (3, 4). Erythrocytes which lacked Duffy blood group determinants (Duffy negative erythrocytes) (3) or had been treated with chymotrypsin (4) were resistant to invasion. With the recent advances in in vitro cultivation ofPlasmodium falciparum (5, 6), it was possible to compare the erythrocyte surface determinants required for invasion by P. falciparum, the major malaria of man, and P. knowlesi, a monkey malaria that infects man.
Materials and MethodsHuman erythrocytes (test cells) which either lacked a blood group determinant (Table I) or had been treated with various enzymes (Table II) were exposed to P. falciparum and P. knowlesi merozoites in vitro, and the percentage of test cells infected was determined. Chimpanzee erythrocytes infected with P. falciparum (Camp strain) and frozen in liquid nitrogen were thawed (7) on the day of the experiment. They contained approximately 50% infected erythroeytes and varied in viability from experiment to experiment. The infected cells were centrifuged in microhematocrit tubes. Cells from the upper half of the tubes, containing about 90% infected erythrocytes were mixed with test erythrocytes in a ratio of 1 to 25 and cultured as previously described (6). Erythrocytes were removed from culture 48 h later, spread in a thin film on a slide, stained with Giemsa, and the percent of erythrocytes infected with young trophozoites (ring forms) determined.Rhesus erythrocytes that are refractory to invasion by P. falciparum were included in each experiment (Table I). In parallel experiments, samples of test erythrocytes were cultured with P.knowlesi-infected cells (3). Since P. falciparum merozoites were released after 48 h in culture and P. knowlesi within 1 h, test erythrocytes were incubated at 35°C for 48 h before addition of P. knowlesi-infected cells.
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