Time-dependent loss of invasive ability of Plasmodium berghei merozoites in vitro.

UNLABELLED: The invasive ability of Plasmodium berghei merozoites in vivo was studied following their artificial removal from parasitized mouse red cells using complement-mediated immune lysis in vitro and in vivo. Time-course experiments revealed that lysed preparations contained two components contributing to the parasites' infectivity in mice. One component, presumed to be free merozoites released from mature schizont-infected cells, rapidly lost infectivity with time at 1 to 2 C. A second minor component appeared to have more stability at this temperature, and could be accounted for as intact parasitized cells containing mature schizonts not lysed by the complement in vitro, but lysed by the recipients' plasma complement in vivo. Further experiments revealed that suspension of parasitized cells in an isotonic diluent and centrifugation at moderate speeds substantially removes the number of invasive free merozoites insolable from a given sample of infected blood by immune hemolysis.
CONCLUSIONS: merzoites, either contained within the confines of mature schizont-infected cells, or artificially removed from host cells, rapidly lose the ability to invade susceptible erythrocytes in vivo when suspended in an isotonic medium and held at 1 to 2 C in vitro.