Date of this Version
Promastigotes of Leishmania donovani transformed into amastigotes and survived in cultured macrophages. Promastigotes of Leptomonas costoris, a kinetoplastid parasite of water striders, transformed into amastigotes but did not survive in cultured macrophages. A significant difference in size between the two species of parasitic protozoa allowed development of a bioassay in which the in vitro digestion rate of L. costoris was taken as a measure of macrophage digestive capacity following preinfection with L. donovani. In a 12- to 24-hr period, macrophage cultures infected with the 2S strain of L. donovani retained 50% more L. costoris than did control macrophages infected with L. costoris alone. The 2S strain has been in culture since 1967 and has retained its infectivity for hamsters, although its virulence is attenuated. In similar experiments with promastigotes of the Khartoum strain of L. donovani, a strain that evidently has lost its infectivity for hamsters, no such suppression of macrophage ability to digest L. costoris was observed. The results suggest that infective strains of Leishmania donovani suppress macrophage function beyond the time at which initial lysosomal fusion with the parasitophorous vacuole occurs.