Time course and intensity of infection in vitro in the resident peritoneal macrophages of resistant and susceptible mice exposed to different doses of Leishmania donovani promastigotes.

A study was made of the proliferation dynamics in vitro of Leishmania donovani amastigotes in the resting peritoneal macrophages of C57BL/6 (Lshs) and C57L/J (Lshr) mice. Monolayers were inoculated with 5, 50 or 500 promastigotes per macrophage and the number of infected cells and the number of parasites per cell were determined 1, 3, 5, 7 and 14 days following inoculation. Results indicate that, irrespective of the phenotype of the donor mouse and of the inoculum, only 50-65% of the cells became infected initially. Expansion of the infection proceeded more rapidly in monolayers of Lshs cells and may have involved the "recruitment" of non-susceptible macrophages, perhaps by the action of a soluble factor. Also irrespective of the inoculum and phenotype, only 3-6 amastigotes were present in each macrophage initially, suggesting a limited number of ligands for the attachment of the parasite to the cell. Amastigotes did not proliferate for 3-4 days and then divided actively until day 7, when more parasites were present in the macrophages of the susceptible phenotype. Differential expansion of the infection and the proliferation of amastigotes in vitro suggest that resting peritoneal macrophages may, indeed, express the Lsh gene.