Differential survival of Leishmania donovani amastigotes in human monocytes

Leishmania donovani is an important intracellular protozoal pathogen of man; it is found solely within macrophages in its amastigote stage in humans, and exists in its extracellular, flagellated promastigote stage in the sandfly, its arthropod vector. To determine if either stage of L. donovani was capable of surviving within monocytes--the oxidatively active precursors of tissue macrophages--interactions of the parasite with human monocytes were studied in vitro. Amastigotes and promastigotes were ingested to a comparable degree by monocytes; whereas 79% of promastigotes were killed within 48 hr, however, amastigotes survived and multiplied threefold over 5 days. Promastigotes, which have been shown to be sensitive to hydrogen peroxide-peroxidase-halide microbicidal mechanisms, elicited a phagocytic oxidative burst that was 49% of the response to serum-opsonized zymosan, as assessed by luminol-enhanced chemiluminescence. NBT was reduced to formazan in 71% of monocytes exposed to promastigotes. The death of promastigotes within monocytes could be attributed at least in part to oxidative microbicidal mechanisms because there was no significant decrease in the number of cell-associated parasites in monocytes from donors with chronic granulomatous disease of childhood. In contrast to promastigotes, amastigotes survived within monocytes, despite eliciting an oxidative response that was 27% of the response produced by serum-opsonized zymosan; this response was not significantly different from that produced by promastigotes. In a phagocyte-free system, amastigotes were found to be sevenfold more resistant than were promastigotes to the lethal effects of hydrogen peroxide. The survival of L. donovani in human monocytes is thus dependent on the parasite stage; promastigotes are ingested, they elicit an oxidative burst, and the majority are killed by oxidative microbicidal mechanisms, whereas amastigotes are ingested and survive to parasitize human monocytes successfully, despite eliciting a phagocytic oxidative burst.