Immunochemotherapy in experimental leishmaniasis

The proliferation dynamics in vitro of the obligate intracellular protozoan parasite Leishmania donovani was studied for 14 days in resting monolayers of peritoneal macrophages of C57BL/6 $(Lsh sp{ rm s})$ mice inoculated with 5, 50, or 500 promastigotes/cell. Irrespective of the inoculum, only 50 to 65% of the cells became infected initially; only 3 to 6 amastigotes were present in each macrophage initially, suggesting a limited number of parasite ligands on the host cell. The amastigotes did not divide in the first 3 or 4 days after infection and then they actively proliferated from day 5 to day 8; the number of parasites was then reduced. Infection with L. donovani down-regulates immunity and parasite clearance by macrophages, but IL-2-stimulated splenocytes activate leishmanicidal action in vitro in infected peritoneal macrophages and in vivo in C57BL/6 (Lsh$ sp{ rm s})$ mice. IL-2-stimulated splenocytes were most effective when used in the non-proliferating phase of the infection, whereas the anti-leishmanial drug Pentostam was most effective when used during the proliferative phase. Immunochemotherapy was more effective than either treatment alone. Infection with L. donovani abolishes the ability of macrophages to produce the superoxide and INO microbicidal responses; curative treatment with IL-2-stimulated splenocytes restores the ability of infected macrophages to secrete inorganic nitrogen oxides, but not to produce a superoxide response. Pentostam had no effect to stimulate either microbicidal mechanism in infected cells; the drug is, therefore, probably directly toxic to the parasite. These studies have indicated, among other things, that IL-2-stimulated splenocytes rescue infected cells and infected animal from the immunological deficit which L. donovani induces, allowing the re-establishment of those mechanisms that make the macrophage an essential component of the host's protective immune system