Susceptibility of 3H-thymidine-prelabelled Leishmania donovani amastigotes and promastigotes to the cytotoxic activity of peritoneal, splenic and liver macrophages.

C57BL/6 macrophage populations from spleen and liver, the main organs for the manifestation of visceral leishmaniasis, were investigated for their ability to perform spontaneous phagocytosis-associated killing of 3H-thymidine (3H-TdR)-prelabelled L. donovani amastigotes and promastigotes. The results showed that organ macrophages from spleen and liver killed L. donovani amastigotes and promastigotes spontaneously with high efficiency. This consistent finding was first detectable at 2-3 h, and the reaction was completed at 12 h. This type of killing was strongly enhanced when spleen and liver macrophages were activated. This phagocytosis-associated killing mechanism may contribute, to a large extent, in maintaining the infection under control in vivo, by drastically reducing the amount of parasites that is required to establish intracellular parasitism. To be able to assay phagocytosis-associated destruction of both promastigotes and amastigotes, a reproducible system for the production in vitro of Leishmania donovani amastigotes by the macrophage cell-line J774 was developed. The DNA of the Leishmania amastigotes was labelled with 3H-TdR with high efficiency. The spontaneous label release of prelabelled L. donovani amastigotes was comparable to that of prelabelled promastigotes over an assay period of 24 h.